nanoparticles synthesis

Nanomaterials: a review of synthesis methods, properties, recent progress, and challenges

First published on 24th February 2021

Nanomaterials have emerged as an amazing class of materials that consists of a broad spectrum of examples with at least one dimension in the range of 1 to 100 nm. Exceptionally high surface areas can be achieved through the rational design of nanomaterials. Nanomaterials can be produced with outstanding magnetic, electrical, optical, mechanical, and catalytic properties that are substantially different from their bulk counterparts. The nanomaterial properties can be tuned as desired via precisely controlling the size, shape, synthesis conditions, and appropriate functionalization. This review discusses a brief history of nanomaterials and their use throughout history to trigger advances in nanotechnology development. In particular, we describe and define various terms relating to nanomaterials. Various nanomaterial synthesis methods, including top-down and bottom-up approaches, are discussed. The unique features of nanomaterials are highlighted throughout the review. This review describes advances in nanomaterials, specifically fullerenes, carbon nanotubes, graphene, carbon quantum dots, nanodiamonds, carbon nanohorns, nanoporous materials, core–shell nanoparticles, silicene, antimonene, MXenes, 2D MOF nanosheets, boron nitride nanosheets, layered double hydroxides, and metal-based nanomaterials. Finally, we conclude by discussing challenges and future perspectives relating to nanomaterials.

1. Introduction

The term nanometer was first used in 1914 by Richard Adolf Zsigmondy. 5 The American physicist and Nobel Prize laureate Richard Feynman introduced the specific concept of nanotechnology in 1959 in his speech during the American Physical Society's annual meeting. This is considered to be the first academic talk about nanotechnology. 5 He presented a lecture that was entitled “There's Plenty of Room at the Bottom”. During this meeting, the following concept was presented: “why can’t we write the entire 24 volumes of the Encyclopedia Britannica on the head of a pin?” The vision was to develop smaller machines, down to the molecular level. 6,7 In this talk, Feynman explained that the laws of nature do not limit our ability to work at the atomic and molecular levels, but rather it is a lack of appropriate equipment and techniques that limit this. 8 Through this, the concept of modern technology was seeded. Due to this, he is often considered the father of modern nanotechnology. Norio Taniguchi might be the first person who used the term nanotechnology, in 1974. Norio Taniguchi stated: “nano-technology mainly consists of the processing of, separation, consolidation, and deformation of materials by one atom or one molecule.” 5,9 Before the 1980s, nanotechnology remained only an area for discussion, but the concept of nanotechnology was seeded in the minds of researchers with the potential for future development.

The invention of various spectroscopic techniques sped up research and innovations in the field of nanotechnology. IBM researchers developed scanning tunneling microscopy (STM) in 1982, and with STM it became feasible to attain images of single atoms on “flat” ( i.e. , not a tip) surfaces. 10 Atomic force microscopy (AFM) was invented in 1986, and it has become the most crucial scanning probe microscope technique. 11 The motivation to develop hard discs with high storage density stimulated the measurement of electrostatic and magnetic forces. This led to the development of Kelvin-probe-, electrostatic-, and magnetic-force microscopy. 12 Currently, nanotechnology is rapidly evolving and becoming part of almost every field related to materials chemistry. The field of nanotechnology is evolving every day, and now powerful characterization and synthesis tools are available for producing nanomaterials with better-controlled dimensions.

Nanotechnology is an excellent example of an emerging technology, offering engineered nanomaterials with the great potential for producing products with substantially improved performances. 13 Currently, nanomaterials find commercial roles in scratch-free paints, surface coatings, electronics, cosmetics, environmental remediation, sports equipment, sensors, and energy-storage devices. 14 This review attempts to provide information in a single platform about the basic concepts, advances, and trends relating to nanomaterials via covering the related information and discussing synthesis methods, properties, and possible opportunities relating to the broad and fascinating area of nanomaterials ( Scheme 1 ). It is not easy to cover all the literature related to nanomaterials, but important papers from history and the current literature are discussed in this review. This review provides fundamental insight for researchers, quickly capturing the advances in and properties of various classes of nanomaterials in one place.

2. Descriptions of terms associated with nanomaterials

3. Approaches for the synthesis of nanomaterials

3.1. Top-down approaches

The conditions under which arc discharge takes place play a significant role in achieving new forms of nanomaterials. The conditions under which different carbon-based nanomaterials are formed via the arc discharge method are explained in Fig. 6 . Various carbon-based nanomaterials are collected from different positions during the arc discharge method, as their growth mechanisms differ. 44 MWCNTs, high-purity polyhedral graphite particles, pyrolytic graphite, and nano-graphite particles can be collected from either anode or cathode deposits or deposits at both electrodes. 46–48 Apart from the electrodes, carbon-based nanomaterials can also be collected from the inner chamber. Different morphologies of single-wall carbon nanohorns (SWCNHs) can be obtained under different atmospheres. For example, ‘dahlia-like’ SWCNHs are produced under an ambient atmosphere, whereas ‘bud-like’ SWCNHs are generated under CO and CO 2 atmospheres. 49 The arc discharge method can be used to efficiently achieve graphene nanostructures. The conditions present during the synthesis of graphene can affect its properties. Graphene sheets prepared via a hydrogen arc discharge exfoliation method are found to be superior in terms of electrical conductivity and have good thermal stability compared to those obtained via argon arc discharge. 50

3.2. Bottom-up approaches

The hard template method is also called nano-casting. Well-designed solid materials are used as templates, and the solid template pores are filled with precursor molecules to achieve nanostructures for required applications ( Fig. 10 ). 78 The selection of the hard template is critical for developing well-ordered mesoporous materials. It is desirable that such hard templates should maintain a mesoporous structure during the precursor conversion process, and they should be easily removable without disrupting the produced nanostructure. A range of materials has been used as hard templates, not limited to carbon black, silica, carbon nanotubes, particles, colloidal crystals, and wood shells. 85 Three main steps are involved in the synthetic pathway for obtaining nanostructures via templating methods. In the first step, the appropriate original template is developed or selected. Then, a targeted precursor is filled into the template mesopores to convert them into an inorganic solid. In the final step, the original template is removed to achieve the mesoporous replica. 86 Via using mesoporous templates, unique nanostructured materials such as nanowires, nanorods, 3D nanostructured materials, nanostructured metal oxides, and many other nanoparticles can be produced. 87 From this brief discussion, it can be seen that a wide range of unique structured nanomaterials can be produced using soft and hard template methods.

4. Unique nanomaterial features

The electronic properties of semiconductors in the 1–10 nm range are controlled by quantum mechanical considerations. Thus, nanospheres with diameters in the range of 1–10 nm are known as quantum dots. The optical properties of nanomaterials such as quantum dots strongly depend upon their shape and size. 96 A photogenerated electron–hole pair has an exciton diameter on the scale of 1–10 nm. Thus, the absorption and emission of light by semiconductors could be controlled via tuning the nanoparticle size in this range. However, in the case of metals, the mean free path of electrons is ∼10–100 nm and, due to this, electronic and optical effects are expected to be observed in the range of ∼10–100 nm. The colors of aqueous solutions of metal nanoparticles can be changed via changing the aspect ratio. Aqueous solutions of Ag NPs show different colors at different aspect ratios. A red shift in the absorption band appears with an increase in the aspect ratio ( Fig. 12 ). 21

Among a range of unique properties, the following key properties can be obtained upon tuning the sizes and morphologies of nanomaterials.

4.1. Surface area

4.2. magnetism, 4.3. quantum effects, 4.4. high thermal and electrical conductivity, 4.5. excellent mechanical properties, 4.6. excellent support for catalysts, 4.7. antimicrobial activity.

Overall, these features have made nanoscale materials valuable for a wide range of applications, substantially boosting the performances of various devices and materials in a number of fields. Details of various nanomaterials, their properties, and applications in various fields will be discussed below.

5. Nanomaterials, characteristics, and applications

5.1. special carbon-based nanomaterials.

In the carbon-based nanomaterial family, fullerenes were the first symmetric material, and they provided new perspectives in the nanomaterials field. This led to the discovery of other carbon-based nanostructured materials, such as carbon nanotubes and graphene. 110 Fullerenes are present in nature and interstellar space. 111 Interestingly, fullerenes were the molecule of the year in 1991 and attracted the most research projects compared to other scientific subjects during that period. 112 Fullerenes possess several unique features that make them attractive for applications in different fields. Fullerenes display solubility to some extent in a range of solvents, and these characteristics make them unique compared to the other allotropes of carbon. 108

The chemical modification of fullerenes is an exciting subject, improving their effectiveness for applications. There are two main ways to modify fullerenes: 113 fullerene inner-space modification, and fullerene outer-surface modification.

Endohedral and exohedral doping examples are shown in Fig. 13 . 114 Fullerenes are hollow cages, and the interior acts as a robust nano-container for host target species when forming endohedral fullerene. 115 Endohedral fullerenes do not always follow the isolated pentagon rule (IPR). 116 To date, fullerene nanocages have received substantial consideration in the materials chemistry field due to their useful potential applications. Neutral and charged single atoms in free space are highly reactive and unstable. In the confined environment of fullerenes, these reactive species can be stabilized; for example, the LaC 60 + ion does not react with the NH 3 , O 2 , H 2 , or NO. Thus, reactive metals can be protected from the surrounding environment by trapping them inside fullerene cages. 117 Another emerging carbon nanomaterial is endohedral fullerene containing lithium (Li@C 60 ). 118 Lithium metal is very reactive, and extreme controlled environmental conditions are required to preserve or use it. In other words, secure structures are required for lithium storage. Li-Based endohedral fullerene shows unique solid-state properties. The encapsulation of lithium atoms in fullerene helps to protect lithium atoms from external agents. Li-Based endohedral fullerenes have the potential to open the door to nanoscale lithium batteries. 119 For the development of endohedral metallofullerenes, larger fullerenes are generally required, as they possess large cages to accommodate lanthanide and transition metal atoms more smoothly. 118 Fullerene nanocages are useful for the storage of gases. Fullerene is under consideration for hydrogen storage. 120,121

Exohedral fullerenes carry more potential for applications as outer surfaces can be more easily modified or functionalized. The exohedral doping of metals into fullerenes strongly affects the electronic properties via shifting electrons from the metal to the fullerene nanocage. 122 The practical application of fullerenes can be achieved with tailor-made fullerene derivatives via chemical functionalization. As fullerene chemistry has matured, a wide range of functionalized fullerenes has been realized through simple synthetic routes. 123 The combination of hydrogen-bonding motifs and fullerenes has allowed the modulation of 1D, 2D, and 3D fullerene-based architectures. 124 The excellent electron affinities of fullerenes have shown great potential for eliminating reactive oxygen species. The presence of excess reactive oxygen species can cause biological dysfunction or other health issues. The surfaces of fullerenes have been functionalized via mussel-inspired chemistry and Michael addition reactions for the fabrication of C 60 –PDA–GSH. The developed C 60 –PDA–GSH nanoparticles demonstrated excellent potential for scavenging reactive oxygen species. 125

Amphiphiles have great importance in industrial processes and daily life applications. Amphiphilic molecules consist of hydrophilic and hydrophobic parts, and they perform functions in water via forming two- and three-dimensional assemblies. Recently, conical fullerene amphiphiles 126 have emerged as a new class of amphiphiles, in which a nonpolar apex is supplied by fullerenes and a hydrophilic part is achieved through functionalization. The selective functionalization of the fullerene on one side helps to achieve a supramolecule due to unique interfacial behavior. The unique supramolecular structure formed via the spontaneous assembly of one-sided selectively functionalized fullerenes through strong hydrophobic interactions between the fullerene apexes and polar functionalized portions is soluble in water. Conical fullerene amphiphiles are mechanically robust. Via upholding the structural integrity, conical fullerene amphiphiles can be readily aggregated with nanomaterials and biomolecules to form multicomponent agglomerates with controllable structural features. 127 Fullerenes, after suitable surface modification, have excellent potential for use in drug delivery, but there have only been limited explorations of their drug delivery applications. 128,129 Fullerene-based nano-vesicles have been developed for the delayed release of drugs. 130 Water-soluble proteins have great potential in the field of nanomedicine. The water-soluble cationic fullerene, tetra(piperazino)[60] fullerene epoxide (TPFE), has been used for the targeted delivery of DNA and siRNA specifically to the lungs. 131 For diseases in lungs or any other organ, efficient treatment requires the targeted delivery of active agents to a targeted place in the organ. The accumulation of micrometer-sized carriers in the lung makes lung-selective delivery difficult, as this may induce embolization and inflammation in the lungs. Size-controlled blood vessel carrier vehicles have been developed using tetra(piperazino)fullerene epoxide (TPFE). TPFE and siRNA agglutinate in the bloodstream with plasma proteins and, as a result, micrometer-sized particles are formed. These particles clog the lung capillaries and release siRNA into lungs cells; after siRNA delivery, they are immediately cleared from the lungs ( Fig. 14 ). 132

The supramolecular organization of fullerene (C 60 ) is a unique approach for producing shape-controlled moieties on the nano-, micro-, and macro-scale. Nano-, micro-, and macro-scale supramolecular assemblies can be controlled via manipulating the preparation conditions to achieve unique optoelectronic properties. 133 The development of well-ordered and organized 1D, 2D, and 3D fullerene assemblies is essential for achieving advanced optical and organic-based electronic devices. 134 Fullerene-based nanostructured materials with new dimensions are being developed from zero-dimensional fullerene and tuned to achieve the desired characteristics. 1D C 60 fullerene nanowires have received substantial attention over other crystalline forms due to their excellent features of potential quantum confinement effects, low dimensionality, and large surface areas. 135

Carbon nanomaterials are also used as supports for catalysts, and the main reasons to use them are their high surface areas and electrical conductivities. Carbon supports strongly influence the properties of metal nanoparticles. In fuel cells, the carbon support strongly affects the stability, electronic conductivity, mass transport properties, and electroactive surface area of the supported catalyst. 136 In fuel cells, the degradation of some catalysts, such as platinum-based examples, and carbon is correlated and reinforced as a result of both being present. Carbon support oxidation is catalyzed by platinum and the oxidation of carbon accelerates platinum-catalyst release. Overall, this results in a loss of catalytically active surface area. 137 Fullerenes are considered suitable support materials due to their excellent electrochemical activities and stability during electrochemical reactions. 138 Due to their high stability and good conductivity, fullerenes can replace conventional carbon as catalyst support materials. Fullerenes are also used for the development of efficient solar cells. 139

Apart from the applications mentioned above, fullerenes have a broader spectrum of applications where they can be used to improve outcomes considerably. Fullerenes have the potential to be used in the development of superconductors. 140 The strong covalent bonds in fullerenes make them useful nanomaterials for improving the mechanical properties of composites. 141 The combination of fullerenes with polymers can result in good flame-retardant and thermal properties. 142 Fullerenes and their derivatives are used for the development of advanced lubricants. They are used as modifiers for greases and individual solid lubricants. 138 Fullerenes have tremendous medicinal importance due to their anticancer, antioxidant, anti-bacterial, and anti-viral activities. 104

Fullerenes are vital members of the carbon-based nanomaterial family and they certainly possess exceptional properties. This discussion further emphasizes their importance for advanced applications. However, the discovery of other carbon-based nanomaterials has put fullerenes in the shade, and the pace of their exploration has been reduced. As fullerenes are highly symmetrical molecules with unique properties, they can act as performance boosters, but more attention is needed from researchers for their practical expansion. 110

Single-walled carbon nanotubes consist of a seamless one-atom-thick graphitic layer, in which carbon atoms are connected through strong covalent bonds. 146 Double-walled carbon nanotubes consist of two single-walled carbon nanotubes. One carbon nanotube is nested in another nanotube to construct a double-walled carbon nanotube. 147 In multi-walled carbon nanotubes, multiple sheets of single-layer carbon atom are rolled up. In other words, many single-walled carbon nanotubes are nested within each other. From different types of nanotubes, it can be concluded that the nanotubes may consist of one, tens, or hundreds of concentric carbon shells, and these shells are separated from each other with a distance of ∼0.34 nm. 148 Carbon nanotubes can be synthesized via chemical vapor deposition, 149 laser ablation, 150 arc-discharge, 143 and gas-phase catalytic growth. 151

Single-walled carbon nanotubes display a diameter of 0.4 to 2 nm. The inner wall distance between double-walled carbon nanotubes was found to be in the range of 0.33 to 0.42 nm. MWCNT diameters are usually in the range of 2–100 nm, and the inner wall distance is about 0.34 nm. 147,152 However, it is essential to note that the diameters and lengths of carbon nanotubes are not well defined, and they depend on the synthesis route and many other factors. The electrical conductivities of SWCNTs and MWCNTs are about 10 2 –10 6 S cm −1 and 10 3 –10 5 S cm −1 , respectively. SWCNTs and MWCNTs also display excellent thermal conductivities of ∼6000 W m −1 K −1 and ∼2000 W m −1 K −1 , respectively. CNTs remain stable in air at temperatures higher than 600 °C. 153 These properties indicate that CNTs have obvious advantages over graphite.

Single-walled carbon nanotubes can display metallic or semiconducting behavior. Whether carbon nanotubes show metallic or semiconducting behavior depends on the diameter and helicity of the graphitic rings. 154 The rolling of graphene sheets leads to three different types of CNTs: chiral, armchair, and zigzag ( Fig. 15 ). 155

Carbon nanotubes demonstrate some amazing characteristics that make them valuable nanomaterials for possible practical applications. Theoretical and experimental studies of carbon nanotubes have revealed their extraordinary tensile properties. J. R. Xiao et al. used an analytical molecular structural mechanics model to predict SWCNT tensile strengths of 94.5 (zigzag nanotubes) and 126.2 (armchair nanotubes) GPa. 156 In another study, the Young's modulus and average tensile strength of millimeter-long multi-walled carbon nanotubes were analyzed and found to be 34.65 GPa and 0.85 GPa, respectively. 157 Carbon nanotubes possess a high aspect ratio. Due to their high tensile strength, carbon nanotubes are used to enhance the mechanical properties of composites.

Carbon nanotubes have become an important industrial material and hundreds of tonnes are produced for applications. 158 Their high tensile strength and high aspect ratio have made carbon nanotubes an ideal reinforcing agent. 159 Carbon nanotubes are lightweight in nature and are used to produce lightweight and biodegradable nanocomposite foams. 160 The structural parameters of carbon nanotubes define whether they will be semiconducting or metallic in nature. This property of carbon nanotubes is considered to be effective for their use as a central element in the design of electronic devices such as rectifying diodes, 161 single-electron transistors, 162 and field-effect transistors. 163 The chemical stability, nano-size, high electrical conductivity, and amazing structural perfection of carbon nanotubes make them suitable for electron field emitter applications. 164 The unique set of mechanical and electrochemical properties make CNTs a valuable smart candidate for use in lithium-ion batteries. 165 CNTs have the full potential to be used as a binderless free-standing electrode for active lithium-ion storage. CNT-based anodes can have reversible lithium-ion capacities exceeding 1000 mA h g −1 , and this is a substantial improvement compared with conventional graphite anodes. In short, the following factors play a role in controlling and optimizing the performances of CNT-based composites: 166 (i) the volume fraction of carbon nanotubes; (ii) the CNT orientation; (iii) the CNT matrix adhesion; (iv) the CNT aspect ratio; and (iv) the composite homogeneity.

For some applications, a proper stable aqueous dispersion of CNTs at a high concentration is pivotal to allow the system to perform its function efficiently and effectively. 167 One of the major issues associated with carbon nanotubes is their poor dispersion in aqueous media due to their hydrophobic nature. Clusters of CNTs are formed due to van der Waals attraction, π–π stacking, and hydrophobicity. The CNT clusters, due to their strong interactions, hinder solubility or dispersion in water or even organic-solvent-based systems. 168 This challenging dispersion associated with CNTs has limited their use for promising applications, such as in biomedical devices, drug delivery, cell biology, and drug delivery. 167 Carbon nanotube applications and inherent characteristics can be further tuned via suitable functionalization. The functionalization of carbon nanotubes helps scientists to manipulate the properties of carbon nanotubes and, without functionalization, some properties are not attainable. 169 The functionalization of nanotubes can be divided into two main categories: covalent functionalization and non-covalent functionalization.

The heating of CNTs under strongly acidic and oxidative conditions results in the formation of oxygen-containing functionalities. These functional groups, such as carboxylic acid, react further with other functional groups, such as amines or alcohols, to produce amide or ester linkages on the carbon nanotubes. 172 One of the main issues preventing the utilization of CNTs for biomedical applications is their toxicity. The cytotoxicity of pristine carbon nanotubes can be reduced via introducing carbonyl, –COOH, and –OH functional groups. Apart from functionalization through oxidized CNTs, the direct functionalization of CNTs is also possible. However, direct functionalization requires more reactive species to directly react with the CNTs, such as free radicals. Addition reactions to CNTs can cause a transformation from sp 2 hybridization to sp 3 hybridization at the point of addition. At the point where functionalization has taken place, the local bond geometry is changed from trigonal planar to tetrahedral geometry. Some addition reactions to the sidewalls of CNTs are shown in Fig. 16 . 155

It is important to discuss how the covalent functionalization of carbon nanotubes comes at the price of the degradation of the carbon sp 2 network. This substantially affects the electronic, thermal, and optoelectronic properties of the carbon nanotubes. 169 Efforts are being made to introduce a new method of covalent functionalization that can keep the π network of CNTs intact. Antonio Setaro et al. introduced a new [2+1] cycloaddition reaction for the non-destructive, covalent, gram-scale functionalization of single-walled carbon nanotubes. The reaction rebuilds the extended π-network, and the carbon nanotubes retained their outstanding quantum optoelectronic properties ( Fig. 17 ). 173

Polymers are frequently combined with CNTs to enhance their dispersion capabilities. Polymers interact with CNTs through CH–π and π–π interactions. 174 Hexanes and cycloalkanes are poor CNT solvents but the good solubility or dispersion of CNTs in these solvents is required for surface coating applications. Poly(dimethylsiloxane) (PDMS) macromer-grafted polymers have been prepared using PDMS macromers and pyrene-containing monomers that strongly adsorb on CNTs, thus improved the solubility of CNTs in chloroform and hexane. 176 The use of head–tail surfactants is another attractive way to achieve a fine dispersion of CNTs in an aqueous medium. In head–tail surfactants, the tail is hydrophobic and interacts with the CNT sidewalls, and the hydrophilic head groups interact with the aqueous environment to provide a fine dispersion. 177

For electrical applications, non-covalently functionalized CNTs are more preferred because the electrical properties of the CNTs are not compromised. CNTs have been non-covalently functionalized with a variety of biomolecules for the fabrication of electrochemical biosensors. 175 Non-covalently functionalized SWCNTs are used for energy applications. Single-walled carbon nanotubes (SWCNTs) have been non-covalently functionalized with 3d transition metal( II ) phthalocyanines, lowering the potential of the oxygen evolution reaction by approximately 120 mV compared with unmodified SWCNTs. 178 The toxicity of pristine CNTs toward living organisms can be lessened via using surfactant-functionalized CNTs. 170 However, in some cases, during polymer non-covalent functionalization, the polymer may wrap CNT bundles and make it difficult to separate the CNTs from each other. Polymers can develop into insulating wrapping that affects the CNT conductivity.

In the literature, several graphene-related materials have been reported, such as graphene oxide and reduced graphene oxide. 187 Among graphenoids, graphene oxide is a more reported and explored graphene-related material as a precursor for chemically modified graphene. The synthetic route to graphene oxide is straightforward, and it is synthesized from inexpensive graphite powder that is readily available. 188 Graphene oxide has many oxygen-containing functional groups, such as epoxy, hydroxyl, carboxyl, and carbonyl groups. The basal plane of graphene oxide is generally decorated with epoxide and hydroxyl groups, whereas the edges presumably contain carboxyl- and carbonyl-based functional groups. 189 The presence of active functional groups in graphene oxide allows its further functionalization with different polymers, small organic compounds, or other nanomaterials to realize several applications. 190

Graphene oxide, due to its oxygen functionality, is insulating in nature and displays poor electrochemical performance. The presence of oxygen functionalities in graphene oxide breaks the conjugated structure and localizes the π-electron network, resulting in poor carrier mobility and carrier concentration. 196 Its electrochemical performance is improved substantially after removing the oxygen-containing functional groups. 197 These functional groups can be removed or reduced via thermal, electrochemical, and chemical means. The product obtained after removing or reducing oxygen moieties is called reduced graphene oxide. The properties of reduced graphene oxide depend upon the effective removal of oxygen moieties from graphene oxide. The process used to remove oxygen-containing functionalities from graphene oxide will determine the extent to which reduced the properties of graphene oxide resemble pristine graphene. 198

Reduced graphene oxide is extensively used to improve the performances of various electrochemical devices. 199 It is essential to mention that even after reducing graphene oxide, some residual sp 3 carbon bonded to oxygen still exists, which somehow disturbs the movement of charge through the delocalized electronic cloud of the sp 2 carbon network. 200 Apart from this, the electrochemical activity of reduced graphene oxide is substantially high enough to manufacture electrochemical devices with improved performances. Recently, the demand for super-performance electrochemical devices has increased to overcome modern challenges relating to electronics and energy-storage devices. 201 Graphene-based materials are considered to be excellent electrode materials, and they can be proved to be revolutionary for use in energy-storage devices such as supercapacitors (SC) and batteries. Graphene-based electrodes improve the performances of existing batteries (lithium-ion batteries) and they are considered useful for developing next-generation batteries such as sodium-ion batteries, lithium–O 2 batteries, and lithium–sulfur batteries ( Fig. 18 ). Being flat in nature, each carbon atom of graphene is available, and ions can easily access the surface due to low diffusion resistance, which provides high electrochemical activity. 202

Graphene and its derivatives are extensively used for the development of electrochemical sensors. 203 The surfaces of bare electrodes are usually not able to sense analytes at trace levels and they cannot differentiate between analytes that have close electrooxidation properties due to their poor surface kinetics. The addition of graphene layers to the surfaces of electrodes can substantially improve the electrocatalytic activity and surface sensitivity towards analytes. 204 Graphene has definite advantages over other materials that are used as electrode materials for sensor applications. Graphene has a substantially high surface-to-volume ratio and atomic thickness, making it extremely sensitive to any changes in its local environment. This is an essential factor in developing advanced sensing tools, as all the carbon atoms are available to interact with target species.

Consequently, graphene exhibits higher sensitivity than its counterparts such as CNTs and silicon nanowires. 205 Graphene has two main advantages over CNTs for the development of electrochemical sensors. First, graphene is mostly produced from graphite, which is a cost-effective route, and second, graphene does not contain metallic impurities like CNTs can. Graphene offers many other advantages when developing sensors and biosensors, such as biocompatibility and π–π stacking interactions with biomolecules. 206 Graphene-based materials are ideal for the construction of nanostructured sensors and biosensors.

The mechanical properties of graphene are used to fabricate highly desired stretchable and flexible sensors. 207 Graphene can be utilized to develop transparent electrodes with excellent optical transmittance. It displays good piezoresistive sensitivity. Researchers are making efforts to replace conventional brittle indium tin oxide (ITO) electrodes with flexible graphene electrodes in optoelectronic devices such as liquid-crystal displays and organic light-emitting diodes. 208 For human–machine interfaces, transparent and flexible tactile sensors with high sensitivity have become essential. Graphene film (GF) and PET have been applied to develop transparent tactile sensors that exhibit outstanding cycling stability, fast response times, and excellent sensitivity ( Fig. 19 ). 209 Similarly, graphene is applied for the fabrication of pressure sensors. 210 Overall, graphene is an excellent material for developing transparent and flexible devices. 211,212

The use of graphene-based materials is an effective way to deal with a broad spectrum of pollutants. 213 There are many ways to deal with environmental pollution; among these, adsorption is an effective and cost-effective method. 214,215 Graphene-based adsorbents are found to be useful in the removal of organic, 216 inorganic, and gaseous contaminants. Graphene-based materials have some obvious advantages over CNT-based adsorbents. For example, graphene sheets offer two basal planes for contaminant adsorption, enhancing their effectiveness as an adsorbent. 192 GO contains several oxygen functional groups that impart hydrophilic features. Due to appropriate hydrophilicity, GO-based adsorbents can efficiently operate in water to remove contaminants. Moreover, graphene-oxide-based materials can be functionalized further through reactive moieties with various organic molecules to enhance their adsorption capacities. 217

In short, extensive research must continue in order to develop graphene-based materials with high performance and bring them to the market. Massive focus on graphene research is also justified due to the extraordinary features described in extensive theoretical and experimental research works.

Nanodiamonds possess a core–shell-like structure and display rich surface chemistry, and numerous functional groups are present on their surface. Several functional groups, such as amide, aldehyde, ketone, carboxylic acid, alkene, hydroperoxide, nitroso, carbonate ester, and alcohol groups, are present on nanodiamond surfaces, assisting in their further functionalization for desired applications ( Fig. 20 ). 226

Furthermore, nanodiamond surfaces can be homogenized with a single type of functional group according to the application requirements. 227 The use of nanodiamond particles as a reinforcing material in polymer composites has attracted great attention for improving the performance of polymer composite materials. The superior mechanical properties and rich surface chemistry of nanodiamonds have made them a superior material for tuning and reinforcing polymer composites. Nanodiamonds might operate via changing the interphase properties and forming a robust covalent interface with the matrix. 228 Nanodiamond (ND)-reinforced polymer composites have shown superior thermal stabilities, mechanical properties, and thermal conductivities. Nanodiamonds have shown great potential for energy storage applications. 229 Nanodiamonds and their composites are also used in sensor fabrication, environmental remediation, and wastewater treatment. 230,231 Their stable fluorescence and long fluorescence lifetimes have made nanodiamonds useful for imaging and cancer treatment. For biomedical applications, the rational engineering of nanodiamond particle surfaces has played a crucial role in the carrying of bioactive substances, target ligands, and nucleic acids, resisting their aggregation. 232,233 Nanodiamonds have a great future in nanotechnology due to their amazing surface chemistry and unique characteristics.

Carbon quantum dots can be synthesized through several chemical routes. 241–245 Some methodologies for synthesizing carbon dots are described in Fig. 21 . 246–248 Carbon itself is a black material and displays low solubility in water. In contrast, carbon quantum dots are attractive due to their excellent solubility in water. They contain a plethora of oxygen-containing functional groups on their surface, such as carboxylic acids. These functional moieties allow for further functionalization with biological, inorganic, polymeric, and organic species.

Carbon quantum dots are also called carbon nano-lights due to their strong luminescence. 248 In particular, carbon quantum dots offer enhanced chemiluminescence, 249,250 fluorescent emission, 251 two-photon luminescence under near-infrared pulsed-laser excitation, 252 and tunable excitation-dependent fluorescence. 253 The luminescence characteristics of carbon quantum dots have been used to develop highly sensitive and selective sensors. In most cases, a simple principle is involved in sensing with luminescent carbon quantum dots: their photoluminescence intensity changes upon the addition of an analyte. 254 Based on this principle, several efficient sensors have been developed using carbon quantum dots. 255–257 They can be used as sensitive and selective tools for sensing explosives such as TNT. Recognition molecules on the surfaces of carbon quantum dots can help to sense targeted analytes. Amino-group-functionalized carbon quantum dot fluorescence is quenched in the presence of TNT through a photo-induced electron-transfer effect between TNT and primary amino groups. This quenching phenomenon can help to sense the target analyte ( Fig. 22 ). 258 Chiral carbon quantum dots (cCQDs) can exhibit an enantioselective response. The PL responses of cCQDs were evaluated toward 17 amino acids and it was found that the PL intensity of the cCQDs was only substantially enhanced in the presence of L -Lys ( Fig. 22 ). 254

Carbon quantum dots have received significant interest in the fields of biological imaging and nanomedicine ( Fig. 23 ). 239 Direct images of RNA and DNA are essential for understanding cell anatomy. Due to the limitations of current imaging probes, tracking the dynamics of these biological macromolecules is not an easy job. Recently, membrane-penetrating carbon quantum dots have been developed for the imaging of nucleic acids in live organisms. 259 It is important to note that most of the carbon quantum dots utilized to attain cell imaging under UV excitation emit blue radiation. Some biological tissue also emits blue light, specifically that involving carbohydrates, and this interferes with cell imaging carried out with blue-emitting CQDs. This seriously hinders their potential in the field of biomedical imaging. Due to this reason, researchers are focusing on tuning CQDs in a way that their emission peak is red-shifted to avoid interference. 260 Carbon quantum dots with yellow and green fluorescence have been reported for bioimaging purposes. 261,262 The suitable doping of carbon quantum dots can red-shift the emission to enhance the bioimaging effectiveness. 263 Doped carbon quantum dots are capable of biological imaging and display advanced capabilities for scavenging reactive oxygen species. 264

Carbon quantum dots demonstrate photo-induced electron transfer properties 265 that make them valuable for photocatalytic, light-energy conversion, and other related applications. 266 Carbon quantum dots enhance the activities of other photocatalysts to which they are attached. Carbon quantum dots, along with photocatalysts, provide better charge separation and suppress the regeneration of photogenerated electron–hole pairs. Moreover, the proper implantation of carbon quantum dots into photocatalysts can broaden the photo-absorption region. Implanted carbon quantum dots form micro-regional heterostructures that facilitate photo-electron transport. 267 The implantation of carbon quantum dots into g-C 3 N 4 can substantially enhance charge transfer and separation efficiencies, prevent photoexcited carrier recombination, narrow the bandgap, and red shift the absorption edge. 268 The intrinsic catalytic activity of polymeric carbon nitride is improved as a result of the nano-frame heterojunctions formed with the help of CQDs. 269

Carbon quantum dots offer many advantages over conventional semiconductor-based QDs and, thus, they have attracted considerable researcher attention. 244 Due to their remarkable features, they have shown importance in recent years in the fields of light-emitting diodes, nanomedicine, solar cells, sensors, catalysis, and bioimaging. 236

The production of carbon nanohorns has some obvious advantages over carbon nanotubes, such as the ability for toxic-metal-catalyst-free synthesis and large-scale production at room temperature. Carbon nanotube synthesis involves metal particles, and harsh conditions, such as the use of strong acids, are required to remove metallic catalysts. This process introduces many defects into CNT structures and may cause a loss of carbon material. 270 Carbon nanohorns possess a wide diameter compared to CNTs. CNHs possess good absorption capabilities and their interiors are also available after partial oxidation, which provides direct access to their internal parts. Heat treatment under acidic or oxidative conditions facilitates the facile introduction of holes into carbon nanohorns. Holes in graphene sheets of single-walled carbon nanohorns can be produced with O 2 gas at high temperatures. A large quantity of material can be stored inside CNH tubes. 274 The surface area of CNHs is substantially enhanced upon opening the horns to make their interiors accessible. 275 Carbon nanohorns have great potential for energy storage, 275 electrochemiluminescence, 276 adsorption, 277 catalyst support, 278 electrochemical sensing, 279 and drug delivery system 273 uses. CNHs as catalyst supports can provide a homogeneous dispersion of Pt nanoparticles ( Fig. 25 ). The current density of Pt supported on single-walled carbon nanohorns is double compared to a fuel cell made from Pt supported on carbon black. 280 Thus, carbon nanohorns provide a better uniform dispersion that facilitates a high surface area and better catalyst performance.

5.2. Nanoporous materials

In nanoporous materials, the size distributions, volumes, and shapes of the pores directly affect the performances of porous materials for particular applications. It has become a hot area of research to develop materials with precisely controlled pores and arrangements. Recent research has focused more on the precise control of the shapes, sizes, and volumes of pores to produce nanoporous materials with high performance. Several state-of-the-art reviews are present in the literature that focus explicitly on the synthesis, properties, advances, and applications of nanoporous materials. 85,287–289 Based on the materials used, nanoporous materials can be divided into three main groups: inorganic nanoporous materials; carbonaceous nanoporous materials; and organic polymeric nanoporous materials.

Inorganic nanoporous materials include porous silicas, clays, porous metal oxides, and zeolites. The generation of pores in the material can introduce striking features into the material that are absent in non-porous materials. Nanoporous materials offer rich surface compositions with versatile characteristics. Nanoporous materials exhibit high surface-to-volume ratios. Their outstanding features and nanoporous framework structures have made these materials valuable in the fields of environmental remediation, adsorption, catalysis, sensing, energy conversion, purification, and medicine. 284,290

Porous silica is a crucial member of the inorganic nanoporous family. Over the decades, it has generated significant research interest for use in fuel cells, chemical engineering, ceramics, and biomedicine. It is essential to note that specific morphologies and pore size diameters are required for each application, and these can be achieved via tuning during the synthesis process. Nanoporous silica offers two functional surfaces: one is the cylindrical pore surfaces, and the second is the exterior surfaces of the nanoporous silica particles. The surfaces of nanoporous silica can be easily functionalized for the desired applications. The nanoporous silica surface is heavily covered with many silanol groups that act as reactive sites for functionalization ( Fig. 26 ). 291,292 For biomedical applications, mesoporous silica has emerged as a new generation of inorganic platform materials compared to other integrated nanostructured materials. Several factors make it a unique material for biomedical applications: 293,294 (a) its ordered porous structure; (b) its tunable particle size; (c) its large pore volume and surface area; (d) its biocompatibility; (e) its biodegradation, biodistribution, and excretion properties; and (f) its two functional surfaces. For instance, ordered MCM-48 nanoporous silica was used for the delivery of the poorly soluble drug indomethacin. It has been found that surface modification can control drug release. 295 Mesoporous silica-based materials have emerged as excellent materials for use in sustained drug delivery systems (SDDSs), immediate drug delivery systems (IDDSs), targeted drug delivery systems (TDDSs), and stimuli-responsive controlled drug delivery systems (CDDSs). The drug release rate from mesoporous silica can also be controlled via introducing appropriate polymers or functional groups, such as CN, SH, NH 2 , and Cl. Researchers are currently focusing on developing MSN-based (MSN = mesoporous silica nanoparticle) multifunctional drug delivery systems that can release antitumor drugs on demand in a targeted fashion via minimizing the premature release of the drug ( Fig. 27 ). 296

Hierarchically nanoporous zeolites are a vital member of the nanoporous material family. They are crystalline aluminosilicate minerals whose structures comprise uniform, regular arrays of nanopores with molecular dimensions. The microporous structures of zeolites contain pores that are usually below 1 nm in diameter. In zeolites, the micropores are uniform in shape and size, and these pores can effectively discriminate between molecules based on shape and size. 297 Currently, based on crystallography, more than 200 zeolites have been classified. 298 Zeolites have been proved to be useful materials in the field of host–guest chemistry. In solid catalysis, about 40% of the entire solid catalyst field is taken up by zeolites in chemical industry. The excellent catalysis success of zeolites is based on their framework stability, shape-selective porosity, solid acidity, and ion-exchange capacity. Oxygen tetrahedrally coordinates with the Al atoms in most zeolite crystalline silicate frameworks, resulting in charge mismatch between the oxide framework and Al. Extra-framework Na + ions compensate for this charge mismatch. The Na + ions are exchangeable for other cations such as H + and K + . 298 The zeolite crystalline networks are remarkable in that they provide high mechanical and hydrothermal stabilities. The most crucial task facing the zeolite community is to find new structures with desired functions and apply them more effectively for different applications.

Apart from these inorganic porous materials, several other metal- and metal-oxide-based nanoporous materials have been introduced that are more prominent for use in electrode material, catalyst, photodegradation, energy storage, and energy conversion applications. 299–302 Nanoporous metal-based materials are famous due to the nanosized crystalline walls, interconnected porous networks, and numerous surface metal sites that provide them with unique physical/chemical properties compared with their bulk counterparts and other nanostructured materials. 303 For example, nanoporous WO 3 films were developed via tuning the anodization conditions for photoelectrochemical water oxidation. It has been observed that the morphology of the film strongly affected the photoelectrochemical performance. 304 Nanoporous alumina is also a unique material in the inorganic nanoporous family due to several aspects. Nanoporous alumina can be prepared in a controlled fashion with any size and shape in polyprotic aqueous media via the anodic oxidation of the aluminum surface. The parallel arrangement of pores on alumina can easily be controlled from 5 nm to 300 nm, and alumina is stable in the range of 1000 °C. The anodizing time plays a significant role in controlling the pore length. Nanoporous alumina membranes offer various unique properties, such as pores of variable widths/lengths, temperature stability, and optical transparency. Nanoporous alumina pores can be filled with magnetically and optically active elements to produce the desired applications at the nanoscale level. Photoluminescent alumina membranes can be produced via introducing cadmium sulfide, gallium nitride, and siloxenes inside nanoporous alumina using appropriate precursors. 305 Porous alumina also acts as an efficient support and template for the designing of other nanomaterials. Palladium nanowires, 306 high aspect ratio cobalt nanowires, 307 and highly aligned Cu nanowires 308 were developed using porous alumina as a template. Ni–Pd as a catalyst was supported on porous alumina for hydrogenation and oxidation reactions. 309 Nanoporous anodic alumina is also considered to be an efficient material for the development of biosensors due to the ease of fabrication, tunable properties, optical/electrochemical properties, and excellent stability in aqueous environments. 310

Nanoporous carbon-based materials are a hot topic in the field of materials chemistry. Nanoporous carbon materials have become ubiquitous choices in the environmental, energy, catalysis, and sensing fields due to their unique morphologies, large pore volumes, controlled porous structures, mechanical, thermal, and chemical stabilities, and high specific surface areas ( Fig. 28A ). 311 Nanoporous materials are found to be useful in the treatment of water. The separation of spilled oil and organic pollutants from water has emerged as a significant challenge. 312–314 The design of materials that can allow the efficient separation of organic, dye, and metal contaminants from water has become a leading environmental research area. 315,316 Nanoporous carbon can be derived from different natural and synthetic sources. 317–319 Nanoporous carbon foam can be derived from natural sources, such as flour, pectin, and agar, via table-salt-assisted pyrolysis. The agar-derived nanoporous carbon foam showed high absorption capacities, a maximum of 202 times its own weight, for oil and organic solvents. Air filtration paper developed from carbon nanoporous materials and non-woven fabrics has shown a filtration efficiency of greater than 99% ( Fig. 28B ). 320 Nanoporous carbon can also be produced from other porous frameworks, such as metal–organic frameworks. MOF- and COF-based materials are promising precursors for nanoporous carbon-based materials. The direct carbonization of amino-functionalized aluminum terephthalate metal–organic frameworks has produced nitrogen-doped nanoporous carbon that shows an adequate removal capacity of 98.5% for methyl orange under the optimum conditions. 321 Fe 3 O 4 /nanoporous carbon was also produced with Fe salts as a magnetic precursor and MOF-5 as a carbon precursor for removing the organic dye methylene blue (MB) from aqueous solutions. 322 The mesoporous carbon removal efficiency could be further enhanced via modifying or functionalizing the surface with various materials. Unmodified mesoporous carbon has shown a mercury removal efficiency of 54.5%. This efficiency can be substantially improved to 81.6% and 94% upon modification with the anionic surfactant sodium dodecyl sulfate and cationic surfactant cetyltrimethyl ammonium bromide (CTAB), respectively. 323

Ordered nanoporous carbon, CNTs, and fullerenes are extensively applied for energy and environmental applications. The complicated synthesis routes required for fullerenes and CNTs have slowed down the full exploitation of their potential for highly demanding applications. In comparison, the synthesis of highly ordered nanoporous carbon is facile, and the properties of ordered nanoporous carbon are also appealing for energy and environmental applications. 311 CO 2 is a greenhouse gas, and its sustainable conversion into value-added products has become the subject of extensive research. A nitrogen-doped nanoporous-carbon/carbon-nanotube composite membrane is a high-performance gas-diffusion electrode applied for the electrocatalytic conversion of CO 2 into formate. A faradaic efficiency of 81% was found for the production of formate. 324 Nanoporous carbon materials modified with the non-precious elements P, S, N, and B have emerged as efficient electrode materials for use in the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), batteries, and fuel cells. 311,325–327

Nanoporous polymers, including nanoporous coordination polymers and crystalline nanoporous polymers, have emerged as impressive nanoporous materials. 328 Nanoporous polymers have many applications, and these materials are extensively being evaluated for gas separation and gas storage. The great interest in these applications arises from the presence of pores providing an exceptionally high Brunauer–Emmett–Teller (BET) surface area. Recently, new classes of metal organic framework and covalent organic framework porous materials have been reported that have shown exceptionally high and unprecedented surface areas. For instance, in 2010, a MOF was reported with a surface area of 6143 m 2 g −1 ; 329 in 2012, a MOF was reported with a surface area greater than 7000 m 2 g −1 ; 330 and in 2018, a MOF (DUT-60) was reported with a record surface area of 7836 m 2 g −1 . 331 Mesoporous DUT-60 has also shown a high free volume of 90.3% with a density of 0.187 g cm −3 . 331

Due to their exceptionally high surface areas and porous networks, these MOFs and COFs are ideal for gas storage. Air separation and post-combustion CO 2 capture have become integral parts of mainstream industries related to the energy sector in order to avoid substantial economic penalties. Due to the inefficiencies of available technology and the critical importance of this area, earnest efforts are being made to design gas-selective porous materials for the selective adsorption of desired gases. Nanoporous MOF- and COF-based materials can significantly capture CO 2 and help reach zero or minimum CO 2 emission levels. For instance, nanoporous fluorinated metal–organic frameworks have shown the selective adsorption of CO 2 over H 2 and CH 4 . 332 Hasmukh A. Patel et al. developed N 2 -phobic nanoporous covalent organic polymers for the selective adsorption of CO 2 over N 2 . The azo groups in the framework rejected N 2 , leading to CO 2 selectivity. 333 Nanoporous polymers that are superhydrophobic in nature can also be used for volatile organic compounds and organic contaminants. 334 Nanoporous polymers, due to the presence of a porous network, have been considered as highly suitable materials for catalyst supports. Furthermore, organocatalytic functional groups can be introduced pre-synthetically and post-synthetically into solid catalysts. 335

Nanoporous polymeric materials are amazingly heading towards being extremely lightweight with exceptionally high surface areas. These high surface areas and the fine-tuning of the nanopores has made these nanoporous materials, specifically MOFs and zeolites, ideal support materials for encapsulating ultrasmall metal nanoparticles inside void spaces to produce nanocatalysts with exceptionally high efficiencies. 336 In the coming years, more exponential growth of nanoporous materials is expected in the energy, targeted drug delivery, catalysis, and water treatment fields.

5.3. Ultrathin two-dimensional nanomaterials beyond graphene

However, from a material synthesis standpoint, a graphite-like layered form of Si does not exist in nature and there is no conventional exfoliation process that can generate 2D silicene, although single-walled 351 and multi-walled 352 silicon nanotubes and even monolayers of silicon have been synthesized via exfoliation methods. 353 Forming honeycomb Si nanostructures on substrates like Ag(001) and Ag(110) via molecular beam deposition, so-called “epitaxial growth”, was then proposed as a method for the architectural design of silicene sheets. 354–356 The successful synthesis of a silicene monolayer was first achieved on Ag(111) and ZrB 2 (0001) substrates in 2012; 357,358 later, various demonstrations were made using Ir(111), ZrB 2 (001), ZrC(111), and MoS 2 surfaces as the silicene growth substrates. 359–361 Despite various extensive studies to date involving the “epitaxial growth” of silicene on different substrates and investigations of the electronic properties, 357,362–364 the limited nanometer size, difficulties relating to substrate removal, and air stability issues have substantially impeded the practical applications of silicene. Bearing in mind all these known difficulties, Akinwande and co-workers recently reported a growth–transfer–fabrication process for novel silicene-based field-effect transistor development that involved silicene-encapsulated delamination with native electrodes. 365 An etch-back approach was used to define source/drain contacts in Ag film. Without causing any damage to the silicene, a novel potassium-iodide-based iodine-containing solution was used to etch Ag, avoiding rapid oxidation, unlike other commonly used Ag etchants. The results demonstrated that this was the first proof-of-concept study confirming the Dirac-like ambipolar charge transport predictions made about silicene devices. Comparative studies with a graphene system, the low residual carrier density, and the high gate modulation suggested the opening of a small bandgap in the experimental devices, proving that silicene can be considered a viable 2D nanomaterial beyond graphene.

Nonetheless, the synthesis of silicene on a large-scale is greatly limited, as “epitaxial growth” is the only promising method for obtaining high-quality silicene, and this presents an enduring challenge in relation to silicene research and development. Xu and co-workers recently introduced liquid oxidation and the exfoliation of CaSi 2 as a means for the first scalable preparation of high-quality silicene nanosheets. 366 This new synthetic strategy successfully induced the exfoliation of stacked silicene layers via the mild oxidation of the (Si 2 n ) 2 n layers in CaSi 2 into neutral Si 2 n layers without damage to the pristine silicene structure ( Fig. 29 ). The selective oxidation of pristine CaSi 2 into free-standing silicene sheets without any damage to the original Si framework was carried out via exfoliation in the presence of I 2 in acetonitrile solvent. Furthermore, the obtained silicene sheets yielded ultrathin monolayers or layers with few-layer thickness and exhibited excellent crystallinity. This 2D silicene nanosheet material was extensively explored as a novel anode, which was unlike previously developed silicon-based anodes for lithium-ion batteries. It displayed a theoretical capacity of 721 mA h g −1 at 0.1 A g −1 and superior cycling stability of 1800 cycles. Overall, during the last decade, silicene has been widely accepted as an ideal 2D material with many fascinating properties, suggesting great promise for a future beyond graphene.

Like other 2D materials, MXenes exhibit crystal geometry with a hexagonal close-packed structure based on the equivalent MAX-phase precursor, and the close-packed structure is formed from M atoms with X atoms occupying octahedral sites. 371 According to the formula, there are three representative structures of MXenes: M 2 XT x , M 3 X2T x , and M 4 X3T x . In these combinations, X atoms are formed with n layers, whereas M atoms have n + 1 layers ( Fig. 30 ). 372 Apart from graphene, MXenes are considered the most dynamic developing material, and they have incredible innovation potential amongst typical 2D nanomaterials because of their remarkable properties, such as hydrophilicity, conductivity, considerable adsorption abilities, and catalytic activity. These vital properties of MXenes suggest their use for various potential applications, including in the photocatalysis, electrocatalysis, 373,374 energy, 375 membrane-based separation, 376,377 and biological therapy 378 fields. In this section, we focus on describing new developments relating to MXenes that are utilized for electrocatalytic and energy storage applications, competing as alternatives to graphene materials.

Interestingly, due to the presence of abundant terminal groups, mainly –O, –OH, and –F, and their modifying nature, MXenes can exhibit outstanding hydrophilic properties and high conductivity and charge carrier mobility, making them a very attractive material for various electrocatalytic applications, such as the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, and CO 2 reduction reaction. To further increase their electrocatalytic activities, recent works involving MXenes have included incorporation with CNTs, 379 g-C 3 N 4 , 380 FeNi-LDH, 381 NiFeCo-LDH, 382 and metal–organic frameworks. 383

Cho and co-workers designed and developed MXene–TiO 2 2D nanosheets via the surface oxidation of MXene with defect-free control. These MXene–TiO 2 2D nanosheets were successfully implemented in nano-floating-gate transistor memory (NFGTM) providing a floating gate ( i.e. , multilayer MXene) and tunneling dielectric ( i.e. , the TiO 2 layer). A process of oxidation in water further represented a cost-effective and environmentally benign method, as depicted in Fig. 31 . The MXene NFGTM with an optimal oxidation process displayed exceptional nonvolatile memory features, having a great memory window, high programming/erasing current ratio, long term retention, and high durability. 384

There have been some exciting reports on 2D materials from the pnictogen family, particularly phosphorene. Recently, more attention has also been given to the remaining group 15 elements, 390 with the novel 2D materials arsenene, antimonene, and bismuthene being obtained from the key elements arsenic, antimony, and bismuth, respectively. It is reported that 2D monolayers of group 15 elements, including phosphorene allotropes, have five distinct honeycomb (α, β, γ, δ, and ε) and four distinct non-honeycomb (ζ, η, θ, and ι) structures, as depicted in Fig. 32 . Dissimilar crystal orientations were found for single-layered As, Sb, and Bi. Zeng and co-workers also reported comprehensive density functional theory (DFT) computations that proved the energetic stability and broad-range application of these materials in 2D semiconductors. 391 Previously, following theoretical predictions, Wu and co-workers successfully demonstrated that α-phosphorene showed lowest energy configurations in both honeycomb and non-honeycomb nanosheets. 392 In contrast, Zeng and co-workers proved that the buckled forms of 2D sheets of As, Sb, and Bi allotropes are the most stable structures, particularly their β phases. 391

Among monolayer group 15 family materials, 2D sheets of arsenic (As) and antimony (Sb) have gained considerable attention from researchers. 393,394 Studies have shown that As and Sb exhibit better stability than black phosphorus; they are highly stable at room temperature and less reactive to air, likely inhibiting the oxidization process. 395–398 Nevertheless, it has been demonstrated that the oxidation process is perhaps favorable for fine-tuning the electronic properties; increases in the indirect band gaps ranging from 0 to a maximum of 2.49 eV are found in free-standing arsenene and antimonene semiconductors. 399–403 Simultaneously, arsenene and antimonene can also be transformed into semiconductors with direct band gaps. These two 2D nanosheets can be used to design mechanical sensors, moving beyond common electronic and optoelectronic applications. These two extraordinary 2D nanosheets have been studied for their structural–property relationships via first-principles methods. 403–405

Continuing the characterization and structural property studies of arsenene carried out by Kamal 404 et al. and Zhang 403 et al. , Anurag Srivastava and co-workers analyzed applications of arsenene to explore the possibility of improving sensor devices that can be utilized to detect ammonia (NH 3 ) and nitrogen dioxide (NO 2 ) molecules. 406,407 They investigated the affinities of NH 3 and NO 2 molecules for pristine arsenene sheets, examining the binding energies, bonding distances, density distributions, and current–voltage features. The results showed that arsenene 2D sheets are highly durable, with significant electronic charge transfer. They also considered germanium-doped arsenene and characterized the 2D lattice based on molecular affinity relationships with respect to the dopant.

However, the incorporation of any dopants into 2D nanomaterials not only results in experimental difficulty but it also lowers the stability of 2D materials. 408 Recently, Dameng Liu and co-workers reported the electronic structures, focusing on band structures, band offsets, and intrinsic defect properties, of few-layer arsenic and antimony. 409 The spontaneous oxide passivation layer that is formed naturally on pristine antimonene provides excellent stability. 410 Very recently, Stefan Wolff and co-workers conducted DFT calculations on various single or few-layer antimony oxide structures to describe the stoichiometry and bonding type. Interestingly, the samples exhibited various structural stabilities and electronic properties with a wide range of direct and indirect band gaps. Showing band gaps between 2.0 and 4.9 eV, these 2D layers of antimonene exhibited the potential to be used as insulators or semiconductors. 411 The same group also analyzed Raman spectra and discussed identifying the predicted antimonene oxide structures experimentally. The enduring task of exploring the utility of antimonene has boosted recent research interest in 2D nanomaterials due to the broad range of potential applications, such as their use in electrochemical sensors, 412,413 stable organic solar cells, 414 and supercapacitors 415 to name a few.

The 2D MOF nanosheets are also evaluated for the development of high-performance power-storage devices. For example, Li et al. 427 recently reported two novel Mn-2D MOFs and Ni-2D MOFs as anode materials for rechargeable lithium batteries. The Mn-based ultrathin metal–organic-framework nanosheets, due to thinner nanosheets, a higher specific surface area, and smaller metal ion radius, had structural advantages over Ni-based ultrathin metal–organic-framework nanosheets. Due to these features, the Mn-based ultrathin metal–organic-framework nanosheets displayed a high reversible capacity of 1187 mA h g −1 at 100 mA g −1 for 100 cycles and a rate capability of 701 mA h g −1 even at 2 A g −1 .

The expensive metal oxides utilized in the catalytic process can be replaced in due course by 2D-MOF-based nanosheets with exposed metal sites that impart an adjustable pore structure, ultrathin thickness, a high surface-to-volume atom ratio, and high design flexibility. As a result, 2D-MOFs have extensively been explored for various electrocatalytic applications, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and carbon dioxide reduction reaction (CO 2 RR). For example, Marinescu et al. 428 combined cobalt dithiolene species with benzenehexathiol (BHT) and yielded 2D-MOFs capable of acting as electrocatalysts for the HER in water ( Fig. 34 ). In the presence of 2D-MOF sheets, a high current density of 41 mA cm −2 , at −0.8 V vs. SHE and a pH value of 1.3, is observed. Similarly, Feng et al. 429 also developed single-layer Ni-based 2D-MOF sheets that are highly effective for electrocatalytic hydrogen evolution. Later, Patra et al. 430 reported similar 2D sheets from covalent organic frameworks (2D-COFs) as metal-free catalysts for HER applications. 2D-MOFs are also being explored as active catalysts for the OER process. For example, Xu et al. 431 reported the preparation of 2D Co-MOF sheets using polyvinylpyrrolidone as a surfactant under mild solvothermal conditions. These novel 2D Co-MOFs displayed ultrathin nanosheets with many surface-based metal active sites, improving the overall OER performance.

Interestingly, experimental electrochemical measurement data showed that Co-MOF sheets offer a low overpotential ( i.e. , 263 mV at 10 mA cm −2 ). Similarly, Wang et al. 432 also reported that double-metal 2D-sheets (2D NiFe MOFs) consisting of a very ultrathin structure with a thickness of ∼10 nm further offer a low overpotential of 260 mV at 10 mA cm −2 . In other reports, Zhang et al. 433 successfully performed the OER process with ultrathin 2D-MOF sheets prepared via electrochemical and chemical exfoliation strategies.

Recent work on the catalytic activity of 2D-MOFs has also been reported in relation to the ORR and CO 2 RR because of their layered crystal structures and high-volume modifiable porous structures. For example, Dincă et al. 434 demonstrated that ultrathin layered conductive sheets of the 2D-MOF Ni 3 (HITP) 2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) could actively be utilized as a catalyst in an alkaline medium for the ORR process. These 2D-MOF sheets show high stability while retaining 88% of the initial current density over 8 h at 0.77 V vs. RHE. In another report, through fabricating Co x Zn 2− x (bim) 4 2D-sheets as precursors, Zhao et al. 435 successfully synthesized cobalt nanodots (Co-NDs) with bimetallic Co x Zn 2− x (bim) 4 nanosheets encapsulating few-layer graphene (Co@FLG). For the CO 2 RR, a cobalt–porphyrin-containing 2D-MOF was achieved for the selective electrochemical reduction of CO 2 to CO with enhanced stability by Peidong Yang and co-workers. 436 The results further proved that these thin-film catalysts have the highest selectivity for CO ( i.e. , 76%) at −0.7 V vs. RHE with the little-to-no substantial decrease in activity over 7 h at −0.7 V vs. RHE, and 16 mL of CO was produced. Besides, like many other porous materials, 2D-MOFs were also shown to be a supporting platform for catalytic nanoparticles because of their high specific surface areas and favorable porosity distributions. To this end, an example can be noted from Wang et al. 437 reporting that fine porous MOF-5 nanosheets can be utilized to immobilize Pd nanoparticles.

5.4. Metal-based nanostructured materials

As discussed, catalysis is one of the main uses of metal-based nanostructured materials. A continuous increase in the demand for energy, the rapid depletion of conventional energy reservoirs, and rising concerns over the emission of CO 2 have increased the challenges and urgency in the energy field. 460 Metal-based nanostructured materials are extensively being explored to produce alternative clean and renewable energy sources. A range of metal-based nanomaterials has been evaluated and is under consideration for developing robust electrodes that can be effectively applied to water splitting, batteries, and solar cells.

High energy demands have led to more pressure to improve the performances of existing highly demanded lithium-ion batteries. Researchers have focused on improving their lifetimes, sizes, and safety. 462 Nanostructured metal-oxide-based materials are promising electrode materials for use in high-performance charge-storage devices. A metal-based nanostructured electrode is evaluated as both the anode and cathode to overcome the challenges of conventional electrodes. 463 In a conventional LIB, LiCoO 2 was used as the cathode material. Controlled morphology plays a crucial role in determining the performance of a material. Powder composed of spherical particles of LiNi 0.8 Co 0.2 O 2 showed a higher tap density compared to irregular particles and the material substantially improved the power density of secondary lithium batteries. 464 Hierarchical nanostructures of metal-based oxides (such as 3D hierarchical ZnCo 2 O 4 nanostructures) have emerged as a new trend for the development of high-capacity electrodes for lithium-ion batteries. 465 Since their commercialization by Sony in the early 1990s, LIBs have achieved tremendous success in bringing portable electronic devices to the market. However, their sustainable development on the grid-scale is hampered due to limited Li resources in nature, and this is causing a continuous increase in cost. 466 Sodium-ion batteries are in the spotlight to replace powerful lithium-ion batteries due to the widespread availability of sodium and its lower cost compared with lithium. 467 It is essential to note that, in terms of energy densities for SIBs, it is difficult to bypass LIBs because of the low standard electrochemical potential and higher weight of Na. SIBs could be proved to be ideal for those applications where cost is a critical factor compared to energy density. 466

SIBs also operate similarly to LIBs, based on an intercalation mechanism. SIBs also consist of cathode and anode electrodes separated through an electrolyte. During the charging process, sodium ions are extracted from the cathode and inserted into the anode via the electrolyte. In the discharging process, the electrons leave the anode through an external circuit to reach the cathode, providing electricity to the load, whereas Na + moves to the cathode during this process. The radius of Na + (1.02 Å) is greater than that of Li + (0.76 Å), making it challenging to intercalate into electrode materials. 468 Thus, appropriate electrode materials are required in which fast Na-ion insertion and extraction is possible. However, SIBs are suffering from a lack of appropriate electrode materials. It is important to develop electrode materials that have enough interstitial space within their crystallographic structures and better electrochemical performance. Among the various proposed electrode materials, Na x MO 2 layered transition-metal oxides (M = V, Fe, Cu, Co, Ni, Cr, Mn, and their combinations) are considered to be promising electrode materials for SIBs. Layered metal oxides are considered to be promising electrode materials due to their facile scalable synthesis, simple structures, appropriate operating potentials, and high capacities. 469,470 Large volume expansion and poor kinetics during the charge–discharge process can severely affect the cyclability and performance of SIBs. One of the effective strategies to deal with the mechanical stress triggered by large volume changes is the design of hollow or porous structures. In response, three-dimensional network-based Sb 2 O 3 @Sb composite anode materials can help to relieve the volume-change-related stress through their uniform porous networks and provide better transportation channels for Na + . 471

The large volume expansion of electrodes can also be buffered via designing 2D metal-oxide materials with large interlayer spacing. The ultrathin nanosheets provide high reversible capacity with enhanced cycling stability and contribute to providing reaction sites for electrons/ions, decreasing the diffusion distance, providing effective diffusion channels, and facilitating fast charge/discharge for sodium and lithium. 2D SnO nanosheet anodes were evaluated for SIBs. The capacity and cyclic stability improved, as the number of atomic SnO layers is decreased in the sheets. 472 Sb is a promising anode material, but during the sodiation/desodiation processes, huge volume expansion of 390% is observed, which hinders its practical use. Nanostructured Sb in the form of nanorod arrays with large interval spacing displays the great capacity to accommodate volume changes during cycling. 473 A comparison of various nanostructured metal-based electrodes for various charge storage purposes is shown in Table 3 . Overall, well-structured metal or metal-based oxide nanomaterials have the capacity to resolve current issues relating to charge storage devices.

Recently, an immense focus of research has been to produce H 2 fuel via water-splitting to replace conventional fossil fuels. This will help to eliminate emissions from the use of carbonaceous species. 484 Electrochemical method are considered simple water splitting approaches, as these methods only require an applied voltage and water as inputs to produce hydrogen fuel. 485 The coupling of solar irradiation to electrochemical water splitting has enhanced the performance and reduced the process cost. Due to these reasons, this has become a hot area of research. 486 During water electrolysis, H 2 is produced through the hydrogen evolution reaction at the cathode and O 2 is produced through the oxygen evolution reaction at the anode. However, water splitting is not so straightforward, and it requires an efficient catalyst that can facilitate the splitting of water. Metal- and metal-oxide-based catalysts are extensively being explored for water splitting. For the HER reaction, Pt-based catalysts are found to be suitable, whereas for OER reactions, Ir-/Ru-based compounds are found to be benchmark catalysts. Scarcity and high cost have limited the widespread use of these metals. The barrier of noble-metal cost can be mitigated through developing noble-metal nanostructured surfaces that produce more active sites or via depositing monolayers of noble metals on low-cost materials. The alloying of noble metals with other metals has enhanced site-specific activity. 484 At present, more focus is being placed on developing noble-metal-free catalysts for water splitting. 485 Usually, an efficient electrocatalyst is characterized by: 487 a low overpotential; high stability; low production costs; and high electrocatalytic activity.

The nano-structuring of catalysts is an effective tool to boost their surface areas. The electrolysis of water occurs at the surface of a catalyst, and nanostructured catalysts provide more active sites and the better diffusion of ions and electrolytes. 484 Non-noble metals that are under observation for the development of HER electrocatalysts include nickel (Ni), tungsten (W), iron (Fe), molybdenum (Mo), cobalt (Co), and copper (Cu). 487 For instance, a noble metal-free catalyst, carbon-decorated Co 3 O 4 nanoarrays on carbon paper, required a small overpotential of 370 mV to reach a current density of 10 mA cm −2 . It can maintain a current density of 100 mA cm −2 for 413.8 h and 86.8 h under alkaline and acidic conditions, respectively. 488

Metal-based semiconductor materials play a crucial role in a range of applications. For photoelectrochemical water splitting, the semiconductor material plays a central role in the solar-to-hydrogen conversion efficiency. Some critical features are prerequisites when it comes to selecting the right semiconductor material for the photoelectrochemical splitting of water: 489 an extraordinary capacity to absorb visible light; an appropriate bandgap; suitable valence and conduction band positions; commercial feasibility; and chemical stability.

For an ideal semiconductor for water splitting, the valence band and conduction band edge positions must straddle the oxidation and the reduction potentials of water. Metal oxides have received significant attention among semiconductors due to their wide band gap distributions, remarkable photo-electrochemical stabilities, and favorable band edge positions. 490 Semiconductor-based photoelectrodes become excited upon light irradiation, and electrons from the valence band move to the unoccupied conduction band. Some of the generated electrons at the cathode surface reduce protons to hydrogen gas, whereas holes at the photoanode produce oxygen gas via water splitting. 490 As a result, various nanostructured metal oxides can be used as photoelectrode materials, such as WO 3 , 491 Cu 2 O, 492 TiO 2 , 493 ZnO, 494 SnO 2 , 495 BiVO 4 , 496 and α-Fe 2 O 3 , 490 for the efficient splitting of water. As discussed, the nano-structuring of semiconductors can significantly impact the electrode photoelectrochemical performance during water splitting.

Metal-based nanomaterials have been used for the development of sensitive sensors. These metal-based sensors can replace the complex and expensive instruments that are conventionally used for the sensing of analytes. Metal-oxide-based sensors have the interesting characteristics of low detection limits, low cost, high sensitivity, and facile operation. 497 Mostly, semiconducting metal-oxide-based sensors are used for the sensing of toxic, flammable, and exhaust gases. Semiconductor metal oxides with a size in the range of 1–100 nm have been significantly investigated as gas sensors due to their size-dependent properties. The geometry and size of a nanomaterial can considerably affect the hole and electron movement in semiconductors. 498 The surface-to-volume ratio and surface area are substantially enhanced at the nanoscale level, and this is amazingly beneficial for sensing. Chemiresistive semiconducting metal oxides are potential candidates for gas sensing due to the following features: 499 rapid response times; fast recovery times; low cost; simple electronic interfaces; user-friendliness and low maintenance; and abilities to sense a wide range of gases.

Electrode materials decorated with metal- or metal-oxide-based nanostructured materials have shown better responses and selectivity for determining various analytes over conventional electrode materials. The nano-sized metal structures act as an electrocatalyst and electronic wires to provide rapid electron transfer between the transducers and analyte molecules. 500 The electrochemical redox reaction of H 2 O 2 can be improved via the thermally controlled anchoring of Pt NPs on the electrode surface. 501

Currently, researchers are not just concentrating on the development of randomly shaped nanomaterials; instead, they are very focused on and interested in the rational design of materials with controlled nano-architectures for boosting their performances for specific applications. As a result, extensive research has been carried out to develop metal-based materials with controlled dimensions to achieve better catalytic responses. Particle morphology is a crucial factor in the performance of nanomaterials for specific applications. Laifa Shen et al. rationally designed an electrode architecture via growing mesoporous NiCo 2 O 4 nanowire arrays on carbon textiles, which boosted the electrode performance ( Fig. 37 ). 474

The same materials with different morphologies can produce different outcomes. For instance, MnO 2 nanoflowers have provided high initial sodium-ion storage capacity compared with MnO 2 nanorods. 481 Radha Narayanan and Mostafa A. El-Sayed have analyzed various nanoscale morphologies of Pt, such as tetrahedral, cubic, and near-spherical nanoparticles. The highest rate constant is observed with tetrahedral nanoparticles and the lowest rate constant was observed with cubic nanoparticles, whereas spherical nanoparticles exhibited an intermediate rate constant during catalysis. 502 Xiaowei Xie et al. found that Co 3 O 4 nanorods show high activity compared to conventional Co 3 O 4 nanoparticles for the low-temperature oxidation of CO. 503 The catalytic activity of metal-based nanomaterials is strongly affected by their shape. 504 Shape-defined mesoporous materials (TiO 2 ) have shown superior photoanode activities ( Fig. 38 ). 505 As a result, in the literature, several nanostructured morphologies of metal-based materials, such as nanotubes, 506,507 nanorods, 508,509 nanoflowers, 510 nanosheets, 511 nanowires, 512 nanocubes, 513 nanospheres, 514,515 nanocages, 516 and nanoboxes, 517 have been reported for a range of applications.

Hollow nanostructures have surfaced as an amazing class of nanostructured material, and they have received significant attention from researchers. Hollow nanostructures have the unique features of: 518,519 low density; abundant inner void spaces; large surface areas; and the ability to act as nanoscale containers with high loading capacity, nanoreactors, and nanocarriers.

Various metal-based hollow nanostructures, such as hollow SnO 2 , 520 hollow palladium nanocrystals, 521 Co–Mn mixed oxide double-shell hollow spheres, 521 hollow Cu 2 O nanocages, 522 three-dimensional hollow SnO 2 @TiO 2 spheres, 523 hollow ZnO/Co 3 O 4 nano-heterostructure, 524 triple-shell hollow α-Fe 2 O 3 , 525 and hierarchical hollow Mn-doped Ni(OH) 2 nanostructures, 526 have been developed for various applications. The presence of nanoscale hollow interiors and functional shells imparts them with great potential for gas sensing, catalysis, biomedicine, energy storage, and conversion. 519

From this discussion, it can be concluded that metal-based nanostructured materials have great potential compared to their bulk counterparts. The conversion of materials to the nanoscale is not enough to achieve high performance with better selectivity. Now, research is switching from conventional nanomaterials to more advanced and smartly designed nanomaterials. In modern research, nanomaterials are being designed with better-controlled morphologies and regulated features.

5.5. Core–shell nanoparticles

A spherical nanoparticle core–shell nanostructure is a practical way to introduce multiple functionalities on the nanoscopic length scale. 528 The properties arising from the core or shell can be different, and these properties can be tuned via controlling the ratio of the constituent materials. The shape, size, and composition play a critical role in tuning the core–shell nanoparticle properties. 529 The shell material can help to improve the chemical and thermal stabilities of the core material. The core–shell design has become effective where an inexpensive material cannot be used directly due to its instability or easily oxidizable nature. The core can consist of an easily oxidizable inexpensive metal, whereas the shell might consist of noble metals, oxides, polymers, or silica. 530 For instance, magnetic nanoparticles when prepared can be sensitive toward air, acids, and bases. Magnetic nanoparticles can be protected via coating with organic or inorganic shells. 528

Core–shell metal nanoparticles are an emerging nanostructured material with great potential in the fields of energy and catalysis. 531 The first report of core–shell nanoparticles (2007) for supercapacitor applications consisted of a polyaniline/multi-walled-carbon-nanotube composite (PANI/MWNTs). 532 Metal-based core–shell structured nanoparticles have shown enhanced catalytic performance due to their shape-controlled properties. 533 Ming-Yu Kuo et al. developed Au@Cu 2 O core–shell particles with controllable shell thicknesses that acted as a dual-functional catalyst. The shell thickness of Cu 2 O increased with an increasing concentration of Cu 2+ precursor. The thicknesses of the shells of Au@Cu 2 O-1.5 (12.2 ± 1.7 nm), Au@Cu 2 O-2 (13.2 ± 1.8 nm), Au@Cu 2 O-3 (18.2 ± 2.2 nm), and Au@Cu 2 O-4 (20.8 ± 2.5 nm) due to various concentrations are shown in Fig. 40 . 534 A NiO@SiO 2 core–shell catalyst provided a higher yield of acrylic acid from acetylene hydroxycarbonylation. 535 Core–shell architecture can be used to prevent active metal nanoparticles from oxidation during operation. For instance, a plasmonic photocatalyst was developed that consisted of silver nanoparticles embedded in titanium dioxide. The direct contact of Ag with TiO 2 could lead to its oxidization; this is prevented via developing core–shell architecture in which Ag is used as the core and SiO 2 is used as a shell to protect it. 536 Another excellent option is to replace an expensive core with a non-noble metal to reduce the core–shell cost while using a thin layer of a noble metal that consumes a small amount of metal as the shell. This will ensure the prolonged stability of the catalyst during operation. 533 Overall, core–shell morphologies provide better catalytic activity due to the synergistic effect of the metallic core–shell components. 152

Among the several classes of nanomaterials, core–shell nanoparticles are found to be more promising for different biomedical applications. For instance, magnetic nanoparticles are considered to be useful for biomedical applications due to the following reasons: (a) aggregation is prevented due to superparamagnetism; (b) delivery and separation can be controlled using an external magnetic field; (c) they can be appropriately dispersed; and (d) there is the possibility of functionalization. A range of magnetic nanoparticles is available, such as NiO, Ni, Co, and Mn 3 O 4 . The most famous example is iron oxide, but uncoated iron oxides are unstable under physiological conditions. This may result in controlled drug delivery failure due to improper ligand surface binding and the promotion of the formation of harmful free radicals. Therefore, the formation of shells around magnetic nanoparticles has tremendous significance for biomedical applications. 537 One of the approaches is to use gold shells on magnetic nanoparticles. Au NPs are also called surface plasmons and they substantially enhanced the absorption of light in the visible and near-infrared regions. Thus, coating magnetic nanoparticles with a Au shell can result in a core–shell nanostructure that displays both optical and magnetic functionality in combination. 529

Numerous biocompatible core–shell nanoparticles are being developed for photothermal therapy, as core–shell materials are found to be useful for photothermal therapy. Hui Wang et al. have developed bifunctional core–shell nanoparticles for dual-modal imaging-guided photothermal therapy. The core–shell nanoparticles consist of a magnetic ∼9.1 nm core of Fe 3 O 4 covered by an approximately 3.4 nm fluorescent carbon shell. The Fe 3 O 4 core leads to superparamagnetic behavior, whereas the carbon shell provides near-infrared (NIR) fluorescence properties. The bifunctional nanoparticles have shown dual-modal imaging capacity both in vivo and in vitro . The iron oxide–carbon core–shell nanoparticles absorbed and converted near-infrared light to heat, facilitating photothermal therapy. 538 Au-Based core–shell structures are also being prepared for photothermal therapy. Bulk gold is biocompatible, but Au NPs can accumulate in the spleen and liver, causing severe toxicity. Koo Chul Kwon et al. have developed Au-NP-based core–shell structures that did not result in any gross or histological lesions in the major organs of mice, which revealed that this is a potent and safe agent for photothermal cancer therapy. The core–shell nanoparticles consisted of proteinticle/gold (PGCS-NP) and were developed via proteinticle surface engineering. PGCS-NP was injected intravenously into mice with tumors, and the injected core–shell nanoparticles successfully reached the EGFR-expressing tumor cells. The tumor size was significantly reduced upon exposure to near-infrared laser irradiation ( Fig. 41 ). No accumulation of Au NPs was observed in the mice organs, which indicated that PGCS-NP disassembled into many tiny gold dots, which were easily excreted by the kidneys and liver without causing any toxicity. 539 In another example, multifunctional Au@graphene oxide nanocolloid core@shell nanoparticles were developed, in which the core and shell consisted of gold and a graphene oxide nanocolloid, respectively. The developed core–shell structure showed multifunctional properties, allowing Raman bioimaging and photothermal/photodynamic therapy with low toxicity. 540 Apart from this, numerous other core–shell nanoparticles, such as polydopamine–mesoporous silica core–shell nanoparticles, 541 AuPd@PVP core–shell nanoparticles, 542 Au@Cu 2− x S core–shell nanoparticles, 543 bismuth sulfide@mesoporous silica core–shell nanoparticles, 544 and Ag@S-nitrosothiol core–shell nanoparticles, have been used for photothermal therapy. 545

Due to their unique features and the combination of properties from the shell and core, these core–shell nanoparticles have received considerable interest in many fields, ranging from materials chemistry to the biomedical field. For electrochemical reactions, the core–shell structure conductivity can be enhanced via conducting polymers, carbon materials, and metals. Core–shell nanoparticles as electrode materials showed better performance compared to single components. Most of the core materials are prepared via hydrothermal methods, and shells can be prepared via hydrothermal or electrodeposition methods. 546 Even though significant progress has been made relating to the synthesis methods of core–shell materials, a major challenge is the high-quality production of core–shell materials in more effective ways for required applications, specifically biomedical applications.

6. Challenges and future perspectives

(a) The presence of defects in nanomaterials can affect their performance and their inherent characteristics can be compromised. For instance, carbon nanotubes are one of the strongest materials that are known. However, impurities, discontinuous tube lengths, defects, and random orientations can substantially impair the tensile strength of carbon nanotubes. 547

(b) The synthesis of nanomaterials through cost-effective routes is another major challenge. High-quality nanomaterials are generally produced using sophisticated instrumentation and harsh conditions, limiting their large-scale production. This issue is more critical for the synthesis of 2D nanomaterials. Most of the methods that have been adopted for large-scale production are low cost, and these methods generally produce materials with defects that are of poor quality. The controlled synthesis of nanomaterials is still a challenging job. For example, a crucial challenge associated with carbon nanotube synthesis is to achieve chiral selectivity, conductivity, and precisely controlled diameters. 548,549 Obtaining structurally pure nanomaterials is the only way to achieve the theoretically calculated characteristics described in the literature. More focused efforts are required to develop new synthesis methods that overcome the challenges associated with conventional methods.

(c) The agglomeration of particles at the nanoscale level is an inherent issue that substantially damages performance in relevant fields. Most nanomaterials start to agglomerate when they encounter each other. The process of agglomeration may be due to physical entanglement, electrostatic interactions, or high surface energy. 550 CNTs undergo van der Waals interactions and form bundles, making it difficult to align or properly disperse them in polymer matrices. 159 Similarly, graphene agglomeration is triggered by the basal planes of graphene sheets due to π–π interactions and van der Waals forces. Due to severe agglomeration, the high surface areas and other unique graphene features are compromised. These challenges hinder the practical application of high-throughput electrode materials or composite materials for various applications. 551

(d) The efficiency of nanomaterials can be tuned via developing 3D architectures. 3D architectures have been tried with several nanomaterials, such as graphene, to improve their inherent features. 3D architectures of 2D graphene have provided high specific surface areas and fast mass and electron transport kinetics. This has become possible due to the combination of the exceptional intrinsic properties of graphene and 3D porous structures. 194,552 The combination of graphene and CNT assemblies into 3-D architectures has emerged as the most investigated nanotechnology research area. Porous architectures of other nanomaterials can be developed to enhance their catalysis performance through providing nanomaterial interior availability.

(e) 2D ultrathin materials are an outstanding class of nanomaterial with promising theoretical properties; however, very little experimental evaluation of these materials has been done, apart from the case of graphene. The synthesis and stability of 2D ultrathin materials are some of the major challenges associated with them. In the future, more focus is anticipated to be placed on their synthesis and practical utilization.

(f) Nanomaterial utilization in industry is being increased, and there is also demand for nanoscale material production at higher rates. Moreover, nanotechnology research has vast horizons; the exploration of new nanomaterials with fascinating features will continue and, in the future, more areas will be discovered. One of the significant concerns relating to nanomaterials that cannot be overlooked is their toxicity, which is still poorly understood, and this is a serious concern relating to their environmental, domestic, and industrial use. The extent to which nanoparticle-based materials can contribute to cellular toxicity is unclear. 553 There is a need for the scientific community to put efforts into reducing the knowledge gap between the rapid development of nanomaterials and their possible in vivo toxicity. A proper and systematic understanding of the interaction of nanomaterials with cells, tissues, and proteins is critical for the safe design and commercialization of nanotechnology. 14

The future of advanced technology is linked with advancements in the field of nanotechnology. The dream of clean energy production is becoming possible with the advancement of nanomaterial-based engineering strategies. These materials have shown promising results, leading to new generations of hydrogen fuel cells and solar cells, acting as efficient catalysts for water splitting, and showing excellent capacity for hydrogen storage. Nanomaterials have a great future in the field of nanomedicine. Nanocarriers can be used for the delivery of therapeutic molecules.

7. Conclusions

Conflicts of interest, acknowledgements.

• N. Taniguchi, Proc. Int. Conf. Prod. Eng. Issue PART II , 1974, 18–23.
• Nat. Nanotechnol. , 2019, 14 , 193  Search PubMed .
• M. S. Khan, V. Ranjan and A. Srivastava, Proc. – 2015 IEEE Int. Symp. Nanoelectron. Inf. Syst. iNIS 2015 , 2016, 248–251.

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New Green Approaches in Nanoparticles Synthesis: An Overview

Bogdan andrei miu, anca dinischiotu.

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Correspondence: [email protected]

Received 2022 Aug 14; Accepted 2022 Sep 26; Collection date 2022 Oct.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( https://creativecommons.org/licenses/by/4.0/ ).

Nanotechnology is constantly expanding, with nanomaterials being more and more used in common commercial products that define our modern life. Among all types of nanomaterials, nanoparticles (NPs) occupy an important place, considering the great amount that is produced nowadays and the diversity of their applications. Conventional techniques applied to synthesize NPs have some issues that impede them from being appreciated as safe for the environment and health. The alternative to these might be the use of living organisms or biological extracts that can be involved in the green approach synthesis of NPs, a process that is free of harmful chemicals, cost-effective and a low energy consumer. Several factors, including biological reducing agent concentration, initial precursor salt concentration, agitation, reaction time, pH, temperature and light, can influence the characteristics of biologically synthesized NPs. The interdependence between these reaction parameters was not explored, being the main impediment in the implementation of the biological method on an industrial scale. Our aim is to present a brief review that focuses on the current knowledge regarding how the aforementioned factors can control the size and shape of green-synthesized NPs. We also provide an overview of the biomolecules that were found to be suitable for NP synthesis. This work is meant to be a support for researchers who intend to develop new green approaches for the synthesis of NPs.

Keywords: metal nanoparticle, metal oxide nanoparticle, green synthesis, biological method

1. Introduction

The global market for nanomaterials has been on a growing trend in the past few years, and economic analysts expect this growth to accelerate in the future. Among all types of nanomaterials, NPs are the most widely produced [ 1 , 2 ]. NPs are included in several products, ranging from nanocomposites in the aerospace and automotive industries to daily products, such as food packaging and additives, sporting goods and textiles, cosmetics, or electronics and IT products. Considering the current progress in research, in the near future, NPs for biomedical [ 3 , 4 , 5 ] and agricultural [ 6 , 7 ] purposes might become widespread in the global market. Not least, developing countries might be interested in NPs, due to the benefits they bring to the economy [ 8 ].

The major reason for which the potential of nanotechnology could be limited is related to the concerns regarding the safety of NPs when they are in contact with human and animal organisms [ 9 ]. Despite the evidence that NPs might be harmful, a lot of products incorporating them are already used and, therefore, NPs are produced on an industrial scale. The conventional synthesis techniques usually involve physical and chemical methods, which are disadvantageous in terms of energy consumption and the use and release of pollutant chemicals [ 10 ].

Biological methods implemented for the synthesis of NPs could be a feasible and more sustainable alternative for the future. Their benefits consist of eco-friendliness, cost-effectiveness and low-energy consumption, in comparison with the conventional methods [ 10 , 11 ]. Green synthesis of NPs uses the reducing potential of the compounds within living organisms. Plants and microorganisms are cosmopolitan, accessible and safe-handling resources. Moreover, the involvement of biomass waste in the green synthesis process can facilitate the transition of NPs production to the principles of a circular economy.

Bio-compounds coat the surface of NPs during green synthesis [ 12 ]. Therefore, this additional layer contributes to the enhanced biological properties of green NPs compared to the ones produced using chemical reduction. Moreover, the involvement of bio-compounds prevents the contamination of green NPs with toxic by-products. These can be hazardous compounds that attach to the NPs during the physical and chemical synthesis [ 13 ]. However, one of the reasons for which bio-assisted synthesis has remained, so far, in the research stage is that the process involves a set of interrelated parameters (the biological reducing agent composition and concentration, initial precursor salt concentration, agitation speed, reaction time, pH, temperature and light). These impact the characteristics of the formulated NPs. Therefore, the optimization of a biological method is a complex process that needs considerable time and resource. Also, the organisms’ involvement makes the protocols hard to standardize and reproduce, as the internal environment of living beings differs from one species to another and between individuals from the same species.

The different review articles have focused on the green synthesis of NPs, considering that research interest in this subject has continuously increased in the past ten years [ 14 ]. However, some of the published reviews have not referred to the factors affecting the biological synthesis of NPs [ 15 , 16 , 17 ], while others briefly discussed them [ 13 , 18 ]. Also, the more exhaustive analyses of the green reaction parameters were limited to a single type of NP, such as silver [ 19 ], copper [ 20 ] or zinc oxide [ 21 , 22 ]. There are no recent in-depth investigations regarding the general conditions that guide the green synthesis of NPs. Therefore, our study was conducted to provide a better understanding of the processes that drive the modulation of NPs’ characteristics during green synthesis. We highlighted the remarkable diversity of biological agents that are involved in nanobiotechnology. Our main aim was to discuss the recently achieved knowledge regarding the factors that influence the biological synthesis of metal and metal oxide NPs. Furthermore, this work could contribute to the drawing-up of guidelines for the development of novel approaches dealing with the synthesis of green NPs.

2. Mechanism of Green Synthesis

There are three ways in which living organisms can be involved in NP synthesis: intracellular (endogenous) synthesis, extracellular (exogenous) synthesis and the use of specific biochemicals.

The endogenous NPs’ biosynthesis is based on the ability of certain organisms to extract metals from the growing medium and hyperaccumulate them. These microorganisms and plants have been used in biomining activities to extract economically important metals from lands where conventional mining would not have been effective [ 23 , 24 ]. The in-depth research on the bioaccumulation mechanism has shown that plants retain metals in the form of nanometric particles [ 25 ]. The synthesis occurs within the cell cytosol, due to its high reductive capacity. Cellular enzymes and other biomolecules take part in the process. For example, Dahoumane et al. set up an innovative approach using living cells of the Chlamydomonas reinhardtii microalga to obtain silver–gold alloy NPs. Their study showed that adding silver and gold salts to the algal culture would cause cell sedimentation, due to the internalization of the metallic ions in the cytoplasm. Thus, the reduction process in this case occurs intracellularly; the formed NPs are then released into the extracellular matrix. The last event is very important for the stability of silver–gold NPs, as the extracellular matrix is rich in polysaccharides that act as capping agents [ 26 ].

Phytomined metals are conventionally extracted by biomass combustion, followed by sintering or smelting. However, these methods are inappropriate for applications that need to maintain the nanometer size of the metallic NPs, because of the extreme temperature and pressure used in these cases [ 27 , 28 ]. In order to maintain the NPs’ structure and properties, methods using mild conditions have been developed. For example, Abdallah et al. developed a method involving different steps of filtration and centrifugation. After the plant material was ground and mixed with 1% SDS, the resulting mixture was filtrated using a sieve. Following decantation, a centrifugation step was performed and the obtained pellet was subjected to lyophilization. Finally, ultracentrifugation in sucrose density gradients was performed in order to separate the NPs’ fraction from the plant debris [ 25 ]. Moreover, Marshall et al. proposed enzymatic digestion for the concentration of gold NPs intracellularly accumulated into Brassica juncea . They used 1-β-endoglucanase from Trichoderma viridae to solubilize the plant biomass for obtaining NPs at a concentration that is suitable for catalysis. However, only 55–60% of the biomass was converted into a soluble form. Even though the NPs were not purified by this enzymatic treatment, their size was maintained [ 29 ].

It is worth mentioning that the separation of NPs from cells is not always necessary when an endogenous synthesis is performed for catalytic purposes [ 30 , 31 ].

The intracellular NP synthesis imposes some limitations which make it unsuitable for implementation at an industrial scale. The NPs’ morphology cannot be controlled and there are also difficulties regarding the efficiency of NP extraction, isolation and purification [ 32 ]. In order to avoid these impediments, the researchers have focused on the development of what we know as in vitro approaches. The mechanism of in vitro NP green synthesis is broadly based on the internal biomolecules from plants and microorganisms able to act as reducing and capping agents. In vitro approaches can involve extracts containing the total biocomposition of the organism used. In vitro synthesis of NPs was also performed by using a single biomolecule, such as quercetin [ 33 ], resveratrol [ 34 ] or curcumin [ 35 ].

Living plants can release biomolecules into the environment in response to metal stress. The exogenous synthesis of NPs uses the ability of secondary metabolites released by plant roots to chelate metallic ions. These are secreted to reduce the toxicity of metallic ions, converting them into less toxic nanoscale particles. For example, Shabnam et al. showed that gold NPs were produced when parts of germinated seeds and roots of seedlings of Vigna unguiculata were immersed in different concentrations of chloroauric acid. The secretion of phenolic compounds during seed germination and the early development of seedlings might explain the occurrence of the NPs [ 36 ]. Similarly, silver NPs have been produced when peanut ( Arachis hypogaea ) seedlings’ roots were exposed to 1 mM silver nitrate [ 37 ]. Among all the aforementioned approaches, whole composition extract-based synthesis is used on a large scale. The preparation of extracts generally follows the same steps ( Figure 1 A). Referring to plant extracts, the protocol usually involves the collection of the selected plant part (1), followed by washing (2) and drying (3). After that, the plant parts are ground (4) and mixed with a solvent at a relatively high temperature (5). The last step consists of filtration of the obtained solvent extract (6) [ 38 , 39 ].

The typical approach of green synthesis of NPs: ( A ) Preparation of the plant extract; ( B ) factors affecting the reaction of green synthesis and the characteristics of the resulted NPs.

Referring to microorganisms, they can be grown in a liquid medium and centrifuged or filtered in order to separate the bio-compounds released into the external environment [ 40 , 41 , 42 ].

The typical green synthesis reaction simply consists of the mixture between the biological extract and a metal salt solution, which represents the precursor of NPs ( Figure 1 B). Biomolecules, such as polyphenols (hydroxybenzoic and hydroxycinnamic acids, flavonoids, stilbenes and lignanes) and terpenoids, can transfer electrons to metallic ions, leading to their reduction [ 33 , 43 ]. As a result, the reduced ions start to form ordered arrangements resembling a crystalline structure named nucleus. The nuclei provide support for the reduced ions, which will continue to settle on their surface, leading to the enlargement of the particles [ 44 ]. The growth is stopped by biomolecules that act as capping agents, because they attach to the surface of particles, stabilizing their size [ 45 ].

The mechanism involving the bio-reduction of ions is generally valid for the formation of metallic NPs. However, this cannot explain the formation of metallic oxide NPs. After bio-reduction, metallic ions such as Zn 2+ , Cu 2+ or Ni 2+ could acquire a zero-valent state. Therefore, their conversion to oxides can only be explained by an additional step. Several of the papers have attempted to explain the origin of the oxygen atoms from the metallic oxides produced through green synthesis. Osuntokun et al. proposed that the synthesis of zinc oxide, mediated by Brassica oleracea extract, involved two steps. In the first one, chelation of Zn 2+ cation to polyphenols or flavonoids occurred by the bonding of hydroxyl groups from biomolecules, leading to the generation of zinc hydroxide. In the second step, one molecule of water is released by drying at 70 °C, leading to the formation of zinc oxide [ 46 ]. The generation of an intermediate in the green synthesis of metallic oxide NPs was supported also by Velsankar et al., who hypothesized that diethyl phthalate and 1,2-benzenedicarboxylic acid from a Capsicum frutescens leaf can interact with the Cu 2+ cations from copper nitrate to form copper hydroxide. Similar to the mechanism described above, probably the thermal treatment led to the release of one molecule of water and the copper hydroxide being converted to copper oxide [ 47 ]. Alternatively, Singh et al. proposed that bio-reduction can occur during the biological synthesis of zinc oxide NPs. They considered that, after Zn 2+ ions have been reduced to the zero-valent state by phytochemicals from Eclipta alba , the Zn 0 reacted with the molecular oxygen dissolved in the reaction mixture, leading to the formation of zinc oxide [ 48 ]. As nucleation and particle growth kinetics are controllable, the formation of particles that fit into the nanoscale can be achieved. Probably the biological extract influences, to the greatest extent, the green synthesis of NPs as its composition and biomolecules’ concentration have a direct impact on the first and last steps of the reaction, i.e., the ion reduction and capping, respectively. Other factors such as the pH value of the mixture can change the electrical charge of the biomolecules [ 49 ], while the temperature applied impacts their kinetic energy [ 50 ].

3. Most Common Techniques Used for Characterization of Biologically Synthesized Nanoparticles

Usually, the reduction of metallic ions determines a color change of the reaction mixture [ 51 , 52 ], thus visual observation can confirm that green synthesis has occurred. A more accurate analysis of NPs can be performed through UV-Vis spectroscopy, which is one of the facile and indispensable techniques for most studies dealing with green nano-chemistry. UV-Vis spectroscopy is mainly used for the primary characterization of biologically synthesized NPs, confirming their formation [ 53 ]. The optical extinction spectrum of colloidal solutions of green NPs exhibits an absorption peak that fits into a specific range, depending on the particle composition ( Table 1 ).

The maximum absorption peak of different NPs synthesized via green chemistry.

Besides UV-Vis spectroscopy, a series of other techniques is involved in the characterization of novel green NPs. Electron microscopy is widely used to directly observe the shape and size of NPs, while dynamic light scattering (DLS) can estimate their hydrodynamic diameter. Also, the features regarding the crystalline structure of the particles are analyzed by X-ray diffraction. Fourier-transform infrared spectroscopy (FTIR) is another commonly utilized technique, providing information about the functional groups that are present in NPs after green synthesis. The principle of FTIR consists of exposing the sample to infrared radiation, which is then absorbed by the functional groups within its structure. As a result, different types of vibrations, i.e., bending, deformation or stretching, occur. The FTIR spectrum is calculated through a mathematical algorithm based on the initial interferogram. The spectrum displays the absorbance depending on wavenumbers, which are inversely proportional to wavelength. Based on the value of the wavenumber, the functional groups are identified. For example, the FTIR spectra of ZnO NPs, synthesized using Deverra tortuosa extract, showed a wide peak at 3434 cm −1 , suggesting the presence of hydroxyl groups within the structure. Other peaks corresponded to the amines functional groups specific for proteins [ 92 ].

High-performance liquid chromatography (HPLC) is a useful technique when the mechanism of the formation of green NPs is investigated, especially when plant extracts are involved. The HPLC provides information regarding the composition of the biological agent used in the green synthesis. Different studies have suggested that proteins present in the cell-free supernatants obtained from microbial cultures can act as coating agents [ 93 , 94 , 95 ]. The proteins implicated in the functionalization of NPs can be identified and characterized by electrophoresis, immunoblotting or sequencing. The knowledge of the sequence of proteins is useful for the understanding of the capping mechanism.

Biomolecules that act as capping agents are important, as they remain attached to the surface of particles, conferring them stability over time. A key measure of the stability of a NP suspension is the zeta potential, which quantifies the electrical charge on the surface of NPs. Generally, suspensions of NPs that have a zeta potential value under –25 and over +25 mV are stable, due to the electrostatic repulsion between strongly charged surfaces. The repulsion between lower-charged NPs is weak, therefore the van der Waals attraction forces can lead to particle aggregation [ 53 ].

4. Organisms Involved in the Biological Synthesis of Nanoparticles

4.1. bacteria.

A vast variety of bacterial species has been used in the synthesis of metallic and/or metal oxide NPs. Several papers have presented strains of different genera with most promising results, such as: Aeromonas , Bacillus , Desulfovibrio , Enterobacter , Escherichia , Klebsiella , Lactobacillus , Pseudomonas , Rhodobacter , Rhodopseudomonas , Shewanella , Ureibacillus or Weissella [ 96 ]. On this list, other strains of genera isolated from marine environments have been added, including Alcaligenes , Alteromonas , Ochrobactrum , and Stenotrophomonas [ 97 ]. Different species of Bacillus, including B. subtilis , B. pumilus , B. persicus and B. licheniformis, have been used extensively to produce silver or gold NPs [ 41 , 42 ], while B. amyloliquefaciens has been used for the synthesis of cadmium sulfide NPs [ 98 ].

Studies are still in progress, thus it is not known yet how many types of metallic ions a bacterial cell can reduce. So far, some bacterial strains have been involved in the production of only one or a few types of NP, while others can mediate the formation of many more types. It is the case of Pseudomonas aeruginosa , which is able to synthesize extracellularly seven types of NP, including silver, palladium, iron, nickel, platinum, rhodium and ruthenium. The same microorganism also has the ability to produce large cobalt and lithium particles involving intracellular mechanisms [ 99 ].

Most of the bacterial strains involved in NP synthesis are terrestrial organisms. Reference strains, as well as isolates from specific environments (e.g., mines) where bacteria may enhance their native ability to bioleach and bioaccumulate metals, have been used. However, the recent studies have reported the use of marine bacterial cultures in the synthesis of silver, gold, copper and cadmium sulfide NPs [ 97 ]. For example, the extracellular polymeric substances (EPS) extracted from a marine strain (JP-11) of P. aeruginosa were successfully used to produce cadmium sulfide NPs. The sulfhydryl functional groups of the EPS play a major role in the mechanism of NP formation, as they represent binding sites for metallic ions [ 100 ].

The manipulation of genetic material is easier when it comes from bacterial organisms, rather than other living beings. Bacterial genes have already been used in transgenic approaches aiming to obtain eukaryotic cell cultures able to generate NPs. Therefore, the mms6 gene, which contributes to the formation of magnetosome in Magnetospirillum magneticum bacteria, was inserted in human mesenchymal stem cells, and the intracellular formation of iron NPs with magnetic properties was revealed. NPs with sizes between 10 and 500 nm were accumulated in vacuoles [ 101 ].

4.2. Microfungi and Actinomycetes

In recent years, many fungal strains were studied for their ability to reduce metallic ions. Some of the literature reviews summarized the genera with the most important results, such as: Aspergillus , Cladosporium , Colletotrichum , Fusarium , Penicillium , Phoma , Trichoderma and Trichothecium [ 102 ]. Other studies have focused on biosynthesis, using ascomycetes of the Brevibacterium , Corynebacterium , Kocuria and Neurospora genera [ 96 ].

The group of fungi that produce NPs is completed by some actinomycetes, also known as mycelial bacteria, as they share some characteristics with prokaryotes. The NPs synthesized by actinomycete-mediated approaches appear to be stable, with good polydisperse distribution and high bactericidal activity. However, the potential of actinomycetes to be involved in NP production has not been explored at a high capacity. These microorganisms have been used extensively in the biosynthesis of silver and gold NPs, while a few of them have been involved in the formation of zinc and some bimetallic NPs. The most studied actinomycetes in nanobiotechnology belong to the Nocardia , Nocardiopsis , Rhodococcus , Streptomyces , Thermoactinomyces and Thermomonospora genera [ 103 ].

4.3. Yeasts

Yeast-mediated approaches have been used extensively to synthesize cadmium sulfide or lead sulfide NPs. However, the ability of yeasts to synthesize gold, titanium dioxide, antimony trioxide (Sb 2 O 3 ) or other NPs has also been proved [ 104 ]. The production of silver NPs has been performed using different strains of Candida , including C. albicans , C. glabrata , C. lusitaniae and C. utilis [ 97 ]. The most important studies performing yeast-mediated synthesis of different NPs were cited by Narayanan and Sakthivel [ 102 ]. These works used yeast organisms such as Pichia jadinii , Saccharomyces cerevisiae , Schizosaccharomyces pombe or Yarrowia lipolytica [ 102 ]. The recent papers cited by Gahlawat and Choudhury [ 97 ] complete this list. These include organisms belonging to the Cryptococcus , Magnusiomyces , Phaffia , Rhodosporidium and Rhodotorula genera [ 97 ].

4.4. Microalgae and Cyanobacteria

Microalgae can be used for various biotechnological applications. They were first studied for the removal of nutrients and organic carbon, as well as heavy metals, from wastewater. Also, their potential to be used in the manufacture of commercial products or biofuels has been the subject of research studies over time [ 105 , 106 ]. The advantages of algal cultures have recently granted them the status of candidates for the greener synthesis of NPs. Not only their high tolerance to metallic ions [ 107 ], but also their high growth rate and efficiency in producing different biomolecules [ 108 ], has led to greater attention paid to microalgae in the nanobiotechnological research.

Chlorella vulgaris seems to be the most studied microalga for its ability to reduce metal ions. So far, this has been involved in the green synthesis of many types of NPs [ 97 , 109 ]. Other Chlorella species, namely C. pyrenoidosa and C. kessleri , have also proved to be effective as nanofactories. They have been involved in the synthesis of silver and/or copper NPs [ 110 , 111 , 112 ]. Also, Botryococcus sp., Chlamydomonas sp., Coelastrum sp., Scenedesmus sp., Neochloris oleoabundans , Galdieria sp., Dunaliella tertiolecta or Tetraselmis suecica [ 106 , 110 ] represent other examples of microalgae used recently to produce metallic NPs. Diatoms represent a group of unicellular algae distinguishable by micro- and nanopatterned siliceous cell walls. These structures are formed by the deposition of silica, biogenerated from orthosilicic acid taken-up from the environment [ 113 ]. Different species of diatoms, such as Stephanopyxis turris [ 114 ], Amphora copulate [ 115 ], Navicula atomus [ 116 ] and Diadesmis gallica [ 117 ], have been involved in the production of gold NPs, while Navicula sp. [ 118 ], Chaetoceros sp., Skeletonema sp. and Thalassiosira sp. [ 119 ] have been used for the generation of silver nanostructures. Due to their unique 3D structure, diatoms can be used as scaffolds for the anchoring of metallic NPs, hence the occurrence of biosilica-based nanocomposites for drug delivery [ 120 ], biosensing [ 121 ] and catalysis [ 117 ] purposes.

Cyanobacteria represent a large taxonomic group containing photosynthetic organisms. As with other microorganisms, these green bacteria were found to be suitable for the biological synthesis of nanoscale particles. Even though their nanotechnological potential has not been well explored, some studies focused on cyanobacteria-mediated NP synthesis have emerged recently. The ability to reduce metallic ions was found in cyanobacterial species belonging to the Anabaena , Cylindrospermopsis , Limnothrix , Lyngbya , Synechococcus , Synechocystis and Arthrospira genera [ 110 , 122 ].

4.5. Plants

Several living plants have been explored extensively for their ability to accumulate heavy metals, thus remedying contaminated environments. The researchers in nanobiotechnology have paid more attention to the plants with phytomining potential, after it was discovered that they store metals in the form of nanoscale particles inside their tissues. Mohammadinejad et al. recently listed the most important plants involved in the endogenous mediated synthesis of NPs, including Sesbania drummondii , Ipomoea lacunosa , Festuca rubra and Arabidopsis thaliana . The most studied plant species for the production of NPs inside its tissues is Medicago sativa, followed by Brassica juncea . These species have been involved, so far, in the synthesis of silver, gold, copper and platinum NPs [ 123 ]. Even though the list of plants studied for endogenous NP synthesis is not so wide, there are many plants with phytomining potential that have not yet been involved in nanotechnological approaches (e.g., Acanthopanax sciadophylloides , Maytenus founieri and Clethra barbinervis ) [ 123 , 124 ].

Regarding the synthesis of NPs using plant extracts, the research teams have focused on plants belonging to the angiosperm taxonomic group. The diversity of plant extracts that have been involved so far in NP synthesis is remarkably high. For example, extracts from more than 30 different plant species have been used only for the synthesis of titanium dioxide NPs [ 125 ]. It seems that medicinal plants represent a priority, as their biomolecules already possess healing properties that may be transferred to NPs through the capping process. In a recent review, Agarwal and Gayathri [ 126 ] identified at least 25 studies exploring the potential of medicinal plants to produce different types of NPs, most of them composed of silver or zinc oxide. The most explored property was their antibacterial activity, while a few studies focused on the anticancer, antileishmanial, antioxidant or anticoagulant potential of green NPs. Examples of medicinal plants used as nanofactories are: Prunella vulgaris , Suaeda maritima , Bauhinia acuminata , Taraxacum laevigatum , Carum copticum and many others [ 126 ].

Besides the species with medical importance, some plants used as food have received attention in nanotechnological research. For example, different types of fruit trees, including Malus domestica (apple tree), Citrus sinensis (orange tree), Juglans regia (walnut tree) and Prunus persica (peach tree), have been involved in the biological synthesis of silver, titanium dioxide and zinc oxide, respectively, and iron oxide NPs [ 127 , 128 , 129 , 130 ]. The synthesis of iron oxide NPs has also been performed using beet ( Beta vulgaris ) and pumpkin ( Cucurbita moschata ) extracts [ 131 ], while spice plants such as rosemary ( Rosmarinus officinalis ) and ginger ( Zingiber officinale ), respectively, have been used to produce iron- and copper-containing bimetallic NPs [ 132 , 133 ]. Not least, several angiosperms with no specific importance to humans have proved to be effective as mediators of nanoscale particle synthesis (e.g., Eichhornia crassipes , Linaria maroccana ) [ 134 , 135 ].

Besides the angiosperms, the Kingdom Plantae also includes gymnosperms, pteridophytes (ferns), bryophytes (mosses) and macroalgae. All of these groups contain at least one representative that has been explored for its ability to mediate the production of NPs, especially the ones made of silver, gold, platinum or palladium. Das et al. [ 136 ] have recently managed to include all these organisms in a broad and comprehensive literature review. They found that the most studied gymnosperms for the green synthesis of NPs are: Cycas circinalis , Ginkgo biloba (also used for medicinal properties), different species of Pinus , Thuja orientalis and Torreya nucifera . Regarding the pteridophytic plants, there are at least three genera that have been explored so far for nanobiotechnological applications: Adiantum , Pteris and Nephrolepis . The bryophytes are also less explored. To date, some approaches trying to obtain silver NPs using extracts from Anthoceros sp., Riccia sp. or Fissidens minutus have been reported. Compared to the mosses and ferns, the macroalgae are better represented in nanobiotechnological research. The studies dealing with the green synthesis of NPs have used macroalgal organisms classified as: Colpomenia sinuosa , Pterocladia capillacea , Jania rubens , Ulva fasciata , Ulva intestinalis and different species of Sargassum [ 136 ].

5. Factors Affecting the Biological Synthesis of Nanoparticles

5.1. reducing agent and precursor salt nature and concentration.

Increasing the concentration of the reducing agent would normally accelerate the growth of particles. Thus, highly concentrated biological extracts would lead to the formation of larger NPs. For example, the study of Kumari et al. [ 137 ] showed that increasing the concentration of fungal filtrate of Trichoderma viride from 10% to 100% would increase the size of gold NPs by almost six times. The same tendency was indicated by the absorption spectra analyses of silver NPs, when different concentrations of Ocimum sanctum [ 43 ] or Plantago major [ 138 ] extracts were used.

The increased size of NPs might be explained by a secondary reduction process that occurs on the metal nuclei, because of the excess of reducing phytochemicals. Nagar and Devra [ 139 ] observed that exceeding a concentration of 20% of Azadirachta indica plant extract causes the agglomeration of copper NPs, thus larger structures are formed. However, a low reductant concentration (<5%) does not provide enough biomolecules to start the conversion of ions [ 139 ]. Even if a low level of biomolecules could be enough to trigger the reduction step, they would be rapidly depleted by the metallic ions. This would cause an insufficient amount of biochemicals to be involved in the capping process, and so the aggregation of particles and precipitation could occur [ 140 ]. On the contrary, an excessive amount of biochemicals can impair the nucleation step [ 141 ].

The reducing power of biological extracts is linked to their composition and amount of biochemicals. Therefore, the solvent used for obtaining the extract can impact the green synthesis of NPs. A recent study showed that the average size of silver NPs, synthesized using the ethanolic extract of Acacia cyanophylla , almost reached 200 nm. On the contrary, NPs obtained in the same conditions, but with the aqueous extract, had an average size of around 87 nm [ 142 ]. As ethanol is less polar than water, the biomolecules are expected to be more concentrated and also diverse in the ethanolic extracts compared with the aqueous one. The variety and higher content of molecules in the ethanolic extracts from different plants were also confirmed experimentally [ 143 , 144 ].

The synthesis of palladium NPs prepared via a methanolic extract of Eryngium caeruleum took place in 60 min, while the ethanolic extract-mediated synthesis needed 100 min to be completed. Also, the formation of the same NPs took a couple of hours when the aqueous extract was used [ 145 ].

The biomolecules involved in the green synthesis of NPs differ, depending on the organism used for their extraction. However, this is not valid for the precursors, which are simply salts containing the ion of interest. Most studies describe a decrease in the average size of NPs, which is proportional to the increase in the precursor concentration [ 137 , 139 , 146 ]. When the precursor amount is high, more nuclei were formed and the capping agents acted quickly to stabilize them. However, when the concentration of the precursor was too high, the level of phytochemicals became insufficient to stabilize a large number of nuclei.

Thus, the larger NPs are formed due to the aggregation of nuclei. Nagar and Devra showed that the increasing of the concentration of copper chloride (CuCl 2 ) up to 7.5 mM decreases the NPs size to almost 45 nm. Over the aforementioned concentration value, the size of the copper NPs exceeds 75 nm [ 139 ]. Also, in the case of silver NP synthesis, the increase of silver nitrate (AgNO 3 ) amount in the reaction mixture leads to the formation of larger NPs, as revealed by spectroscopic analyses [ 146 , 147 ].

Similar results were obtained for gold NP synthesis. Using chloroauric acid (HAuCl 4 ) mixed with T. viride fungal extract, Kumari et al. [ 137 ] showed that the growth of particles is promoted by a high precursor concentration. For example, at 30 °C and a concentration of 250 mg/L HAuCl 4 , the average size of the gold NPs was 34 nm, while at 500 mg/L HAuCl 4 , the NPs were 85.2 nm in size. At 50 °C and the same concentrations of gold salt previously mentioned, the mean size of the particles increased from 273.6 nm to 699 nm. In both cases, the average size increased by about 150%, proving that the metal salt concentration works together with other reaction parameters to mediate the size of NPs [ 137 ].

Different precursor salts can be utilized to obtain a single type of NP ( Table 2 ). Therefore, the properties of NPs produced via green approaches can be influenced simply by the chosen precursor. However, the information regarding the effect of different precursor salts on green NP characteristics is limited. Droepenu et al. revealed that ZnO nanospheres with a diameter of approximately 107 nm have been obtained when zinc acetate and Anacardium occidentale leaf extract were mixed. The use of zinc chloride has led to the synthesis of ZnO nanorods that were 167 nm in length and 68 nm in width. However, both samples faced aggregation [ 148 ]. The ZnO NPs synthesized from zinc sulphate had a nanorod morphology and an average size of 30 nm when Justicia adhatoda leaf extract was used. Contrariwise, the NPs obtained by the same method, but starting from zinc nitrate or zinc acetate, were cubic and 15–20 nm in size. Moreover, their tendency to form agglomerates was observed. Interestingly, the use of different precursors can have an impact also on the biological activity of NPs. The antimicrobial activity of ZnO NPs obtained from zinc nitrate was more pronounced against bacterial strains, while the ones produced using zinc sulphate were more efficient against different strains of Aspergillus [ 149 ]. The influence of different precursors on ZnO NP morphology was also confirmed by Fakhari et al., using Laurus nobilis leaf extract as a reducing agent [ 150 ]. Fatima et al. revealed the effect of different precursors on the shape of iron (III) oxide particles using synthetic reagents. Cubic and octahedral particles were formed from ferrous sulphate, solubilized in ethylene glycol and capped with potassium hydroxide. On the contrary, spherical particles occurred when ferric chloride was used instead [ 151 ].

Different precursors for green synthesized NPs.

5.2. Agitation Speed

Agitation is important to keep reactants in motion. The chance of metal ions to come into contact with biomolecules is higher in a continuously stirred mixture. Therefore, the relationship between the rate of the green synthesis reaction and the agitation speed should be directly proportional. This role of agitation was confirmed when iron NPs were synthesized using fungal biomass. The UV-Vis analyses revealed that absorption of the agitated mixture was double in comparison with that corresponding to the static one [ 174 ]. The fact that agitation speed can increase the reaction rate is also supported by the study of Selvakumar et al., in which a silver NP synthesis was based on the molecules found in the leaves of Acalypha hispida . The formation of NPs was faster as the stirring speed increased, reaching 5–7 min at 700 rpm. However, the results pointed out that excessive speed could affect the biomolecules involved in the green process. This could be the reason for which the reaction rate was negatively affected at an agitation speed over 700 rpm [ 175 ].

Agitation speed may also have an impact on the characteristics of NPs. Chan and Don revealed that stirring the reaction mixture at 100 rpm led to the formation of silver NPs with sizes over 80 nm, while increasing the speed to 250 rpm caused the formation of NPs with a diameter around 15 nm [ 176 ]. Some computational statistical optimizations also confirmed that increasing the agitation speed led to a decrease in the size of the NPs [ 177 , 178 ].

5.3. Reaction Time

The variation in reaction times can be used to control the size and shape of green-synthesized NPs. During the biological synthesis of NPs, three main events occur: (i) the initiation of the ion reduction process, (ii) the nucleation and growing of the NPs and (iii) the complete reduction of ions. There is evidence that the initiation of the reduction process requires between 5 and 15 min [ 146 , 179 ] while, in some cases, it occurs immediately after the precursor salt is mixed with the biological extract [ 180 , 181 ]. A complete reaction can occur in between 45 and 120 min, depending on the reducing agent’s effectiveness [ 141 , 146 , 179 ]. However, some studies have proved that the green reactions may reach equilibrium after one week [ 182 , 183 ]. For example, Wei et al. [ 184 ] reported that silver NPs needed five hours to be completely formed in a mixture of organic residues while, in Hibiscus cannabinus leaf extract, they became stable after five days [ 185 ].

The increase in reaction time is directly proportional to particle size and number of generated nuclei. If the complete reduction time is exceeded, the NPs start to aggregate and form larger structures. For example, after 24 h, Kumari et al. observed that T. viride fungal extract-mediated gold NPs had a spherical shape and a size between 7 nm and 24 nm. Increasing the incubation time to 72 h led to the formation of NPs with a size between 20 nm and 400 nm. Moreover, the gold NPs changed their shape, becoming triangular, and also nanoprisms were observed. It was stated that the crystal growth was enhanced by time. The initial nanospheres fused and formed triangular-shaped NPs, which further fused and, as a result, the nanoprisms were formed [ 137 ]. Similarly, silver NPs obtained from a 24-h green reaction had a size of around 62 nm, while their diameter exceeded 100 nm after 72 h [ 142 ].

5.4. Reaction pH

Biomolecules involved in the green synthesis of NPs could have different reducing activity levels based on the pH value of the reaction mixture. Alkaline pH conditions cause the deprotonation and activation of phytochemicals, while lower pH values constrain them to remain mostly protonated, leading to a decreased reducing or capping activity [ 49 , 186 , 187 ]. The inactivation of phytochemicals in an acidic environment is supported by some studies reporting no NP formation at a very low pH [ 49 , 139 , 188 , 189 ]. For example, the formation of silver NPs via biological methods cannot take place at pH 2 [ 188 ] or 3 [ 189 ]. Also, the green synthesis of copper NPs cannot occur in a reaction mixture with a pH of 4.7. However, the synthesis of green copper NPs is more efficient in a mildly acidic environment (pH 6—6.6), because of the agglomeration that takes place at a higher pH value [ 139 ]. On the contrary, Din et al. reported that, at pH values between 2 and 8, the leaf extract of Calotropis gigantea was not able to reduce the Ni 2+ ions provided by the nickel nitrate. The formation of nickel and nickel oxide NPs occurred only at a strong basic pH, with values between 10 and 12 [ 49 ].

In general, NPs produced in an alkaline reaction mixture have a smaller size and are more stable in time. The reason might be the capping process that occurs earlier and is more efficient at alkaline pH values, due to a large quantity of activated phytochemicals. Using the Parachlorella kessleri microalgae, Velgosová et al. have reported a wide histogram of the size distribution for silver NPs produced at pH 4, with a minimum of 20 nm and a maximum of 60 nm. By comparison, silver NPs had an average diameter of 15 nm and a size range between 10 and 20 nm when their formation took place at pH 10 [ 188 ]. The same tendency was observed when silver NPs were synthesized using green tea ( Camellia sinensis ) extract. The average size of NPs formulated at pH 5.8 was around 50 nm. It successively reduced when the pH value increased, the NPs reaching ≈28 nm in diameter [ 190 ]. Similar results were obtained in the case of Amomum sp. synthesized gold NPs. At pH 3, the obtained NPs were larger (90–100 nm) than those produced at a pH value of around 7 (20–40 nm) [ 180 ].

On the contrary, there is also evidence that NPs synthesized in strong acidic environments can have smaller diameters than the ones produced at mildly acid pH values. Prabhakar et al. reported the aqueous leaf extract of Eichhornia crassipes led to the formation of iron NPs with a size between 20 and 60 nm [ 134 ]. Almost the same NP morphology was reported when R. officinalis leaf extract was used [ 132 ]. However, when a more alkaline leaf extract was used, the analyses showed that larger iron NPs (60–200 nm) were formed. For comparison, larger NPs were obtained using the leaf extract of Mimosa pudica , which has an average pH of 6, compared to E. crassipes that has an average pH of 4 [ 134 ]. The pH value of aqueous plant extracts is mainly dependent on the amount of solubilized organic acids. As R. officinalis is rich in rosmarinic and carnosic acids [ 191 ], probably its pH value is also situated in the same range with that of E. crassipes extract.

Even if the pH value can be used to adjust particle size, other properties of NPs can be affected. As expected, the NPs produced at an acidic pH are usually less stable because of the inefficient capping process. Velgosová et al. [ 188 ] showed that silver NPs produced at an alkaline pH were unchanged three weeks after their formation. By comparison, the size of the NPs formed in acidic solutions continued to increase over time, and some aggregates were also observed [ 188 ]. Probably, the pH applied during the green synthesis influences the electric charge on the NPs’ surface, affecting their stability. The experiments performed by Manosalva et al. [ 192 ] show that, at a high pH, the zeta potential of green Ag NPs is lower than −30 mV. Normally, the surface charge of these NPs is strongly anionic, giving them high and long-lasting stability.

The influence of pH on NPs’ shape is not well understood so far. Singh and Srivastava showed that there is no correlation between the pH and bio-inspired gold NPs shape, even though, at an acidic pH, the NPs had a spherical flower shape and, at an alkaline pH, some triangular NPs were observed [ 180 ]. Kumari et al. also reported that gold NPs changed their morphology when the pH value increased. While the NPs obtained at pH 5 had a variable shape, they became triangular when the applied pH had a value between 6 and 8, and spherical at pH 9 [ 137 ].

5.5. Reaction Temperature

There are two stages within green synthesis methods when the temperature applied can impact the physicochemical characteristics of the formed NPs: the reaction itself and the drying of the particles. A reaction temperature increase leads to a higher kinetic energy of the biomolecules involved in the reducing process, thus the metallic ions are consumed faster. Therefore, the high temperature causes the formation of smaller particles; this fact has been confirmed experimentally by a series of studies. There is evidence that the biological synthesis of Ag NPs cannot occur in Ocimum sanctum leaf extract at 5 °C [ 43 ]. Also, the Hygrophila spinosa extract-mediated synthesis of gold NPs was not effective at room temperature [ 141 ]. However, gold NPs were obtained at room temperature when other plant extracts were used [ 193 , 194 ]. Absorbance spectra analyses suggested that an increase of temperature from 15 °C to 35 °C could cause a decrease in the silver NP size [ 43 ]. Shankar et al. obtained a similar result, analyzing the effect of temperature on the synthesis of silver NPs mediated by Rhodomyrtus tomentosa extract. The synthesis performed at 28 °C resulted in the formation of NPs that were 30% larger in diameter than those obtained at 50 °C, according to the DLS analysis. Furthermore, the quantity of recovered NPs from the reaction medium was higher and the capping process was more efficient at the high temperature [ 195 ]. However, extreme temperatures could affect the properties of NPs, probably due to the degradation of the phytochemicals involved in the synthesis process. Using the extract of Hygrophila spinosa to synthesize gold NPs, Satpathy et al. found that the optimum temperature of the reduction process is around 80 °C. UV-Vis spectra showed that, at temperatures above 80 °C, the NPs would form aggregates, suggesting an enhanced surface activity of the formed nuclei and an inefficient capping process. Also, it seemed that the formation of gold NPs was not completed at temperatures below 80 °C [ 141 ].

The same parameters are valid for biologically synthesized copper NPs. The conversion of Cu 2+ ions to copper NPs is significantly higher when the temperature increases in the range between 60 °C and 85 °C. At temperatures above 85 °C, the copper nuclei would turn into agglomerates because of the growth rate of NPs, which is surpassed by the nucleation rate [ 139 ].

The temperature applied for the drying of metallic oxide NPs can also have an impact on the particle morphology, and it is crucial for their crystalline structure. Bala et al. showed that the drying temperature of 30 °C is not high enough for a complete formation of the crystalline structure of ZnO; higher temperatures (60 °C or 100 °C) are necessary for a complete crystalline structure [ 196 ]. The same condition is valid for TiO 2 NPs, which exhibit different crystalline phases during calcination. Usually, the transformation of the anatase phase into rutile begins at temperatures between 600 and 700 °C [ 197 ]. However, it is considered that the transition to rutile needs higher temperatures if the TiO 2 NPs are synthesized using Peltophorum pterocarpum extract [ 198 ]. The crystallinity of green nickel oxide NPs was also increased at higher temperatures, between 300 °C and 500 °C [ 199 ].

The drying temperature can also have an impact on the final size of NPs. Dried at 60 °C, ZnO NPs presented a spherical shape and a size between 16 and 60 nm. If the drying process occurred at 100 °C, the particles exceeded the size of 100 nm [ 196 ]. This fact was also observed by Maensiri et al., who developed a simple laboratory method which used Aloe vera leaves extract and indium acetylacetonate as a precursor for the synthesis of indium oxide NPs. The control of the NPs’ size by calcination at different temperatures in the post-synthesis step was achieved. Therefore, three sets of indium oxide NPs were obtained: (1) within the size range 5–10 nm calcinated at 400 °C, (2) within the size range 10–25 nm calcinated at 500 °C, and (3) within the size range 30–50 nm calcinated at 600 °C [ 200 ]. The dimensions of the NPs seemed to increase in a temperature-dependent manner.

Scanning electron microscopy (SEM) images of cobalt oxide NPs synthesized using Piper nigrum extract reveal that excessive temperatures cause cluster formation. Therefore, the size of cobalt oxide NPs successively increased from around 22 nm (at 100 °C calcination) to ≈ 77 nm (at 900 °C calcination) [ 201 ].

5.6. Light Exposure

There is evidence that the biological synthesis of some photoactive NPs, such as silver or gold, is a light-dependent reaction. The necessity of light exposure in green synthesis was demonstrated by studies reporting no NPs formation in dark conditions. Srikar et al. showed that the formation of silver NPs did not occur in dark conditions when Prunus amygdalus fruit extract was used as a reducing agent [ 202 ]. This result was confirmed by the study of Kumar et al., who used the leaf extract of Erigeron bonariensis [ 147 ].

Light radiation can have an influence on the green synthesis of NPs due to its intensity and wavelength. Light intensity is proportional to the NPs’ rate of formation. Placing the reaction tubes in direct sunlight will speed up the production of silver NPs 60 times compared to the case when the reaction takes place in diffuse light [ 202 ]. A more detailed study has confirmed that the increase in light intensity leads to the maximum production also for gold NPs. Using Shewanella oneidensis bacteria as the reducing agent source, the amount of gold NPs increased fast in bright light (>10,000 lux) conditions. Furthermore, the concentration of gold NPs in the test tube exposed to the highest light intensity was almost double compared to that maintained at low light after 3 h of exposure. The reaction did not start in dark conditions, showing that the presence of light is essential for gold NPs formation [ 203 ].

There is less knowledge about the light wavelength impact on the synthesis efficiency of silver or gold NPs. Srikar et al. found that violet light (380–450 nm) contributes the most to accelerating the biosynthesis of silver NPs. However, better results were reported when all the visible spectrum was applied [ 202 ]. Blue light (425–490 nm) stimulated the largest number of functional groups on the surface of S. oneidensis , enhancing the formation of gold NPs [ 203 ]. Also, blue light significantly reduced the time needed for the complete formation of silver NPs within the Prunus serotina fruit extract [ 204 ]. According to these results, the biosynthesis of specific NPs seems to be more efficient when the reaction occurs at low wavelength visible light.

Recently it was reported that the biofabrication of silver and gold NPs can take place also under low UV light [ 205 ]. Moreover, Lomeli-Rosales et al. proved that microwaves are an appropriate source of radiation for the green synthesis of gold NPs [ 206 ].

Regarding exposure time, the reaction starts within the first 5 min in a high-intensity light source, and there is evidence that the formation of NPs cannot be completed in less than half an hour. The increase in exposure time leads to the formation of more particles, but a longer time would cause the aggregation of the NPs [ 147 ].

Some studies have tried to explain the mechanism by which light irradiation promotes the green synthesis of some NPs. It has been shown that only the conversion of metallic ions to NPs can be light-dependent. The adsorption of ions to phytochemicals or the capping of NPs are processes that can occur even under dark conditions [ 207 ]. Most probably, the light-induced synthesis mechanism is based on photon flux, which may activate the chemical groups of biomolecules involved in electron transfer [ 203 ]. NPs containing silver or gold possess photocatalytic activity. After the formation of the first NPs, these may be photoexcited, acting as catalysts for ion reduction. This hypothesis may explain the enhancement of NPs formation when light is applied [ 207 ].

5.7. Biomolecules Involved in Green Synthesis

Even though the reaction between biological extracts and ionic compounds is not fully understood, some studies provide insight into the biomolecules that drive the green synthesis of NPs. Arsiya et al. revealed that chemicals containing amide and polyol functional groups from the unicellular organism C. vulgaris were involved in the reduction of Pd 2+ ions, leading to spherical NPs with an average size of 15 nm [ 109 ]. FTIR measurements showed that biomolecules containing carbonyl and hydroxyl groups from Solanum nigrum leaf extract were involved in the green synthesis of palladium NPs. Kaempferol, luteolin and gentisic acid, which are present in S. nigrum , were possibly responsible for the reduction of Pd 2+ ions. The formed palladium NPs had a spherical shape and a size between 3 and 35.7 nm, but they were not effectively capped by the respective phytochemicals [ 152 ]. The approach of Ismail led to the production of copper NPs with a size between 7 and 10 nm, and, most probably, the hydroxyl and carbonyl groups from the Rhus coriaria fruit were involved in the capping process. As revealed by the FTIR analysis, the structure of these functional groups is especially found in a series of glucosides of antocyanidins (cyanidin, peonidin, pelargonidin) and flavonols (myricetin, quercetin) [ 155 ].

The study of Ghidan et al. showed that copper oxide NPs were capped mainly with proteins linked through the hydroxyl and carbonyl groups when the aqueous extract of Punica granatum fruit peel was used [ 208 ]. Moreover, Singh et al. have also highlighted that the capping process of gold NPs synthesized using the aqueous extract of Dunaliella salina was most probably managed by proteins [ 209 ].

Furthermore, different polyphenols were involved in the reduction of Au 3+ during a plant-mediated synthesis. Ascorbic, gallic and caffeic acids are polyphenolic compounds normally found in the Sansevieria roxburghiana leaf extract. HPLC analyses detected the amount of ascorbic and gallic acids significantly decreased after the reduction of the gold ions, while the caffeic acid was completely consumed. The morphology of the produced gold NPs was assessed by transmission electron microscopy images that showed mostly spherical particles with an average diameter of 17.48 nm [ 210 ]. It is considered that polyphenols of pumpkin leaves ( C. moschata ) and beet stalks ( B. vulgaris ) form a complexation system through which the Fe 2+/3+ ions are bound. This system stabilizes the iron nuclei in the oxide form, arranging around them and forming a protective layer [ 131 ].

Even though FTIR measurements have highlighted the role of small molecules such as phenols and proteins in the synthesis process, it seems that NADPH-dependent reductases from fungi are crucial in the reduction of AgNO 3 . Experiments have shown that the dialyzed fungal filtrate of C. cladosporioides containing cellular proteins cannot reduce silver salt, and the same situation happens in the case of NADPH alone. But when both solutions were applied, the formation of silver NPs took place [ 211 ]. Hulikere et al. demonstrated the involvement of NADPH-dependent enzymes also in the case of Au 3+ ions’ reduction [ 212 ]. Regarding the stabilization of silver NPs, more important are phenols, tannins and flavonoids. These biomolecules can bind metals, with their presence on the surface of the silver NPs being suggested by the FTIR analysis performed by Jini and Sharmila [ 213 ]. Silver NPs have been generated using an aqueous onion ( Allium cepa ) extract, with the whole process being carried out at room temperature. A SEM analysis has revealed highly stable and uniformly distributed silver NPs with a diameter size between 49 and 73 nm [ 213 ].

In general, whole organism extracts are preferred among the majority of the studies, probably due to their simple way of implementation and low cost. However, variation in the composition of biological extracts could be an impediment to the standardization of the green synthesis of NPs and the reproducibility of the developed approaches. Moreover, the utilization of whole organism extracts does not allow the analysis of the impact of different molecules on the green synthesis of NPs. Some one-molecule green approaches that have recently emerged might provide an insight into the suitability of biochemicals for green synthesis of NPs ( Table 3 ).

Studies exploring metallic/metallic oxide NP synthesis through one-molecule green approaches.

Among the phytochemicals used in the green synthesis of NPs, eugenol is one of the terpenoids that has proved to be helpful in the production of NPs. Tekin et al. reported the obtaining of silver NPs with a cubic shape and dimensions between 20 and 30 nm [ 224 ]. Also, lycopene, a major terpenoid present in tomatoes, can act as a ligand for titanium ions when combined with titanium tetrabutoxide. Baskar and Nallathambi proposed that the mechanism of green titanium dioxide NPs formation involves the cleavage of a double bond C = C within the structure of the lycopene. Afterwards, one titanium ion interacts with one of the C atoms, while O interacts with another one, thus a titanium dioxide-lycopene complex is generated [ 225 ].

Moreover, it seems that other phytochemicals, such as organic acids, are involved in the reduction of metallic ions into NPs, due to the release of hydrogen when the enol group undergoes transformation to the keto group. Thereby, organic acids act as electron donors for the reduction of ions. This is the case of gallic acid within some hydroxyl groups passing through keto–enol tautomerization, while other hydroxyls are involved in the formation of hydrogen bonds, stabilizing the structure of the NPs. The keto–enol tautomerization occurs through two electrons oxidation at a normal pH [ 217 ]. The same mechanism is valid for the tannic acid-mediated synthesis of NPs [ 219 ].

Besides the organic acids, some studies have proved that hydroxyl groups are also involved in the ion reduction process driven by flavonoids and other polyphenols. Probably, the reactivity of biomolecules is dependent on the number of hydroxyls within their structure. By comparing the FTIR measurements, Das et al. confirmed that the hydroxyl groups within epigallocatechin-3-gallate, a major polyphenol in green tea, were the ones involved in the reduction of Ag + [ 215 ]. The FTIR analyses also revealed that phenolic hydroxyl groups of silymarin are involved in the stabilization of gold NPs, being attached to their surface [ 230 ], probably by electrostatic attraction.

Curcumin, originating from turmeric, is another polyphenolic compound that has proved to be effective in synthesizing different types of NPs. Iron oxide NPs synthesized via curcumin had a relatively high stability, considering their zeta potential of –35 ± 2.5 mV, while their size was less than 20 nm [ 239 ]. There is evidence that curcumin is efficient in the synthesis of copper oxide NPs when its concentration, at most, equals the precursor’s one [ 240 ].

The impact of the concentration of biomolecules on the characteristics of the formed NPs should follow the general rule presented above (see Section 5.1 .). As the concentration of gallic acid increased from 0.1 to 5 mM, the silver NPs increased from 35 to 79 nm and the same tendency was reported in the case of the gold NPs [ 217 ]. However, by studying the efficiency of different extracts in synthesizing cerium oxide NPs, Iqbal et al. found that the particle size was smaller when a higher level of quercetin in plants was noticed [ 241 ]. However, this result could be explained by the interference of the reducing power of the other biomolecules present in the extracts (e.g., eugenol, furfural and caffeine).

6. Conclusions and Future Prospects

Environmental issues force researchers to develop new eco-friendly strategies to produce the goods we need in human society. The future of NPs in industry, medicine and agriculture is expected to be extraordinary, but also uncertain if their environmental issues are taken into consideration. The disadvantages and unsustainability of conventional techniques for the synthesis of NPs have not attracted the public’s attention so far. However, the constant growth of the use of NPs in day-to-day goods require us to think about better alternatives to these. At the moment, the research has demonstrated that bringing living organisms into the nanotechnology area is possible and useful. Progress is constantly being made and researchers have identified new suitable organisms for NP synthesis, taking into account the extremely large diversity of our biosphere. Further efforts to decipher how to control NP characteristics must be made, because this is how green NPs could become attractive for industry, avoiding the negative impact of the growing nanotechnological processes on the environment.

We consider that the future research should focus less on discovering new organisms suitable for the green synthesis of NPs, and more on the improvement of the reaction itself. In order to achieve this objective, the understanding of the interrelation between the factors is required. Broadly, it is known that an alkaline pH or raising the temperature leads to smaller NPs, but there are also in-depth mechanisms that need to be explained. In the ideal scenario, we would need to know how to manipulate all the parameters involved in the biological synthesis process to adapt the characteristics of the NPs. Therefore, we would be able to design NPs with a suitable activity, according to the application in which they are meant to be used. We assume that one-molecule approaches will be more used due to some advantages. The utilization of a single biomolecule into the synthesis of NPs prevents the variability of the natural extracts, encouraging the standardization of biological methods and their reproducibility. Moreover, these approaches are expected to contribute significantly to the understanding of the function of specific molecules in the mechanism of synthesis. However, one-molecule approaches eliminate the possible synergism between different biomolecules of living organisms. This could affect the biological activity of NPs. Future research may also investigate the fragility of the biocompound layer stabilizing the NPs. Laboratory processes such as centrifugation might affect this coating.

We also presume that, in the near future, computational modelling will significantly contribute to the deciphering of the complex relationships between all the interrelated variables that influence the formation of NPs. Research involving artificial neural networks for the prediction of bio-inspired NP characteristics has already emerged [ 242 , 243 ] and will possibly boost the scaling of NPs green synthesis to the industrial level. Referring to all the aspects discussed, we consider that the potential of green NPs is a long way from being fully explored.

Author Contributions

Conceptualization, B.A.M. and A.D.; methodology, B.A.M. and A.D.; formal analysis, B.A.M.; investigation, B.A.M.; resources, B.A.M. and A.D.; data curation, B.A.M.; writing—original draft preparation, B.A.M.; writing—review and editing, A.D.; visualization, B.A.M.; supervision, A.D. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Funding Statement

B.A.M. acknowledges the support of UEFISCDI within the project no. 81TE/2022 (PN-III-P1-1_1-TE-2021-1375-TRANS-NANO-BIO).

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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• Published: 16 February 2024

Continuous synthesis of high-entropy alloy nanoparticles by in-flight alloying of elemental metals

• Keun Su Kim   ORCID: orcid.org/0000-0003-3358-5159 1 , 2 , 3 ,
• Martin Couillard 4 ,
• Ziqi Tang   ORCID: orcid.org/0009-0000-1951-8625 3 ,
• Homin Shin 1 ,
• Daniel Poitras   ORCID: orcid.org/0000-0003-4403-8214 5 ,
• Changjun Cheng   ORCID: orcid.org/0000-0002-2720-8683 6 ,
• Olga Naboka 7 ,
• Dean Ruth 1 ,
• Mark Plunkett 1 ,
• Lixin Chen 6 ,
• Liliana Gaburici 1 ,
• Thomas Lacelle   ORCID: orcid.org/0000-0002-4224-8413 1 ,
• Michel Nganbe   ORCID: orcid.org/0000-0002-2240-9099 3 &
• Yu Zou   ORCID: orcid.org/0000-0002-0179-3642 6  

Nature Communications volume  15 , Article number:  1450 ( 2024 ) Cite this article

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• Nanoparticles

High-entropy alloy (HEA) nanoparticles (NPs) exhibit unusual combinations of functional properties. However, their scalable synthesis remains a significant challenge requiring extreme fabrication conditions. Metal salts are often employed as precursors because of their low decomposition temperatures, yet contain potential impurities. Here, we propose an ultrafast (< 100 ms), one-step method that enables the continuous synthesis of HEA NPs directly from elemental metal powders via in-flight alloying. A high-temperature plasma jet ( > 5000 K) is employed for rapid heating/cooling (10 3  − 10 5  K s −1 ), and demonstrates the synthesis of CrFeCoNiMo HEA NPs ( ~ 50 nm) at a high rate approaching 35 g h −1 with a conversion efficiency of 42%. Our thermofluid simulation reveals that the properties of HEA NPs can be tailored by the plasma gas which affects the thermal history of NPs. The HEA NPs demonstrate an excellent light absorption of > 96% over a wide spectrum, representing great potential for photothermal conversion of solar energy at large scales. Our work shows that the thermal plasma process developed could provide a promising route towards industrial scale production of HEA NPs.

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Introduction.

High-entropy alloys (HEAs), consisting of five or more principal elements with a near equimolar ratio, have become one of the most transformative concepts in current alloy design 1 , 2 , 3 . Homogenous mixing of a large number of elements induces synergistic effects among different elements, thus resulting in an unusual combination of functional properties appealing to a broad range of applications such as structural alloys, catalysis, sensing, and energy storage 4 . While the synthesis of bulk HEAs has been the main focus in the past decade, nano-sized HEA particles are emerging as a new class of multifunctional materials due to their more fascinating properties 5 . Scalable and economically viable synthetic methods for HEA nanoparticles (NPs) are of particular interest, yet the controlled incorporation of multiple elements into a tiny particle (<100 nm) remains a significant challenge 5 .

In 2018, a carbothermal shock (CTS) technique was developed to incorporate multiple immiscible metal elements into a single NP and successfully demonstrated the synthesis of HEA NPs containing up to eight elements uniformly dispersed on a conductive carbon support 6 . Although the CTS technique has demonstrated a remarkable potential in synthesizing HEA NPs in a controlled manner, the process is limited to electrically conductive supports and operated in a batch mode, thus not suitable for industrial-scale production of HEA NPs. To overcome this challenge, new methods building on the CTS technique were reported, such as microwave heating 7 , fast moving bed pyrolysis 8 , and aerosol methods 9 . Very recently, a pulsed/scanning laser ablation method was also reported and demonstrated synthesis of high-entropy materials on various substrates 10 . However, the feedstock is typically limited to metal salts. The vapor-source technique also provides a very effective way to form alloy NPs. In this approach, a well-mixed vapor comprising multiple elements is formed from the vaporization of pure metal feedstock, and then rapidly quenched to form crystal solids. High-temperature (>4000 K) environments are usually created by using arc discharge 11 and oscillatory spark discharge 12 . Similar to the CTS, this approach has demonstrated great potential for rapid synthesis of various HEA NPs. However, most processes are operated in a batch mode. The current state-of-the-art technology is still lacking a scalable synthesis method that enables the continuous synthesis of HEA NPs directly from a mixture of pure elemental metal feedstock.

To address the above challenge, we propose an ultrafast (<100 ms), one-step method for the continuous synthesis of HEA NPs directly from a mixture of pure elemental metal powders by using a thermal plasma jet. Thermal plasma jets are partially ionized gases with high temperature (>8000 K) and high speed 13 . They are capable of rapid heating of feedstock to produce an atomically mixed multi-component vapor, and also rapid cooling of the vapor at an ultrahigh cooling rate of 10 5  − 10 6  K s −1 to form solid-solution particles. Feedstock can be injected continuously, thus this technology is well suited for scalable synthesis of HEA NPs 14 . Despite this unique potential, thermal plasma jets have only been used for the processing of bulk HEAs 15 or spheroidization of pre-alloyed HEA powders 16 . The main challenge would be the existence of a large nucleation temperature gap among different metal vapors; upon plasma jet cooling, the element with the lowest saturation vapor pressure reaches the supersaturated state first and segregates.

Here, in a proof-of-concept demonstration, we report the synthesis of CrFeCoNiMo HEA NPs with an average size of 50 nm at a high rate approaching 35 g h −1 using an inductively coupled plasma jet (ICPJ). Despite the low saturation pressure of Mo, all metal elements in a particle are homogeneously mixed at the atomic level with good crystallinity. It is discussed that mixing multiple elements at a near equimolar ratio not only increases the mixing entropy but also decreases the partial pressure of the constituent element (i.e., P i   =   P total   /   N , where P i is the partial pressure of i th constituent element, P total is the total pressure, and N is the total number of the constituent elements) in the vapor, which prohibits the continuous growth of nuclei into pure metal particles via homogenous condensation. This renders the HEA NPs formation feasible even in the presence of a large nucleation temperature gap. We have also investigated the effects of reactor geometry and plasma gases on the NP growth using homogenous nucleation theory and thermofluid simulation. This synthetic route presents considerable potential in the commercial scale applications of HEA NPs where a large amount of powders are desirable. The as-synthesized HEA NPs are exploited as high-performance photothermal materials for solar energy harvest, and achieve a high absorptance of >96% over a wide spectrum without noble metals.

Thermal plasma synthesis of HEA NPs

Figure  1a shows the schematics of the plasma process developed for the continuous synthesis of HEA NPs from a mixture of pure elemental metal powders (see also Supplementary Fig.  1 ). In a typical synthesis run, a mixture of five elemental metal powders at an equimolar ratio is continuously fed into a high-temperature plasma jet generated by an induction plasma torch (45 kW, ~3 MHz, 66.7 kPa). The feed rate typically ranges from 1.2 g min −1 to 2.0 g min −1 . The injected feedstock evaporates within a few tens of milliseconds in the core of the plasma jet (>8000 K) releasing elemental metal vapors. The metal vapors are mixed together upon plasma jet expansion and form a homogeneous multicomponent vapor in the reactor zone. As a rapid quenching is applied, nuclei are formed from the multicomponent vapor and particles grow through the co-condensation of metal monomers. The final HEA NPs formed are continuously collected by a porous filter unit at a high yield rate of ~35 g h −1 , while unvaporized feedstock is removed from the reaction stream by a cyclone separator.

a Schematic of an inductively coupled plasma jet (ICPJ) process developed for the continuous synthesis of HEA NPs directly from a mixture of pure elemental metal powders via in-flight alloying. b , c Schematic diagrams illustrating the formation mechanism of ( b ) conventional alloy NPs and ( c ) HEA NPs by a thermal plasma jet.

To ensure the compositional uniformity, unlike the conventional alloy (Fig.  1b ), it would be ideal that all the constituent elements in the vapor nucleates simultaneously by co-nucleation process (i.e., nucleation by multicomponent), followed by co-condensation of metal monomers onto the nuclei formed (Fig.  1c ). However, the nucleation temperature of a metal vapor strongly depends on its saturation vapor pressure - an intrinsic property of an element. This presents a potential challenge in the synthesis of HEA NPs from the direct vaporization of metal powders by a plasma jet. As a proof-of-concept demonstration, we have studied a Cr-Fe-Co-Ni-Mo system where Mo exhibits a significantly different nucleation temperature compared with other elements, and discussed the growth mechanism. To investigate the effects of the heating/cooling rate, we have also employed plasma gases with high thermal conductivity such as hydrogen and helium. These NPs are denoted as HEA-H 2 and HEA-He, respectively in the following discussion. A plasma gas comprising 100% Ar is also of interest for the comparison; however, it was not considered in this work because pure argon gas is limited to operations at low plasma powers (<25 kW) to avoid damage to the plasma torch.

Morphology, composition, and structure characterization

Figure  2a shows a photo of the HEA-H 2 sample collected from the cyclone separator (42 g) and the filter unit (84 g) after a 150-min operation. In total about 200 g of powder was fed and the productivity approaches 35 g h −1 (with a conversion rate of 42%, Supplementary Table  1 ), which is >10 times higher than what has been reported in the literature to date 17 . The productivity or the conversion rate can be further improved by minimizing powder deposition on the reactor walls (74 g), such as by employing porous reactor walls that allow continuous gas flow through the wall surfaces. Figure  2b presents scanning electron microscopy (SEM) images of the as-synthesize powders collected from the filter unit, showing the effect of the plasma gases (Supplementary Fig.  2 shows more SEM images). In both cases, the samples consist of spherical NPs, dispersed uniformly without serious physical aggregation/agglomeration. However, the HEA-He sample exhibits a smaller average size with a narrow size distribution. The average diameter is estimated as 39.7 ± 20.3 nm for the HEA-He sample while 60.8 ± 30.1 nm for the HEA-H 2 sample from transmission electron microscopy (TEM) images (Supplementary Fig.  3 ). This may be attributed to the different cooling rate and residence time achieved by the different plasma gases.

a Photo of the HEA NP samples produced after a 150-min synthesis experiment. b SEM images of the HEA NPs produced with different plasma gases of hydrogen and helium, showing morphology change. Scale bar, 100 nm. c Size distribution of the HEA NPs produced with different plasma gases. d XRD pattern of the feedstock mixture of Cr, Fe, Co, Ni, and Mo. e XRD patterns of the HEA NPs (single FCC) produced with different plasma gases which confirm the in-situ alloying of pure elemental metals by the ICPJ strategy.

The thermal plasma process is usually accompanied by the production of impurities from incomplete vaporization of feedstock because a fraction of the feedstock injected typically bounces back from the plasma core. This has also been prevalent in the ICPJ process due to the elevated viscosity near the plasma core (e.g., 10 times higher than at room temperature) 18 and the presence of recirculation eddies formed by the magnetic pinch 19 . Unprocessed feedstock powders are larger than NPs, and thus can be removed from the reaction stream by means of centrifugal force. SEM images of the sample collected from the cyclone separator confirmed the presence of large particulates up to a few tens of μm (Supplementary Fig.  2a ). However, such particulates are not observable from the samples collected from the filter unit (Supplementary Fig.  2b, c ), suggesting that the cyclone separator is highly effective in removing the unprocessed powder in the current ICPJ process.

The X-ray diffraction (XRD) pattern of the feedstock in Fig.  2d confirms the existence of the five elements, showing the peaks corresponding to their different crystal structures (Cr: BCC, Fe: BCC, Co: HCP, Ni: FCC, and Mo: BCC); however, the patterns of the reaction products exhibit a single FCC structure with diffraction peaks at 43.3° 50.4° and 74.1°, corresponding to the (111), (200) and (220) planes of a FCC structure, respectively (Fig.  2e ). An additional small peak also showed up around 41°, but this is possibly attributable to the oxide formation at the surface. This result clearly supports that the mixture of metal powders injected was successfully alloyed in the ICPJ process. The sharp peak at 43.3° reveals the high crystallinity of the HEA samples produced. The d -spacing of each sample was estimated and summarized in Supplementary Tables  2 , 3 with calculated lattice constants. While both samples exhibit a similar average lattice constant around 3.58 Å ( a 0  = 3.63 Å for Cr 0.2 Fe 0.2 Co 0.2 Ni 0.2 Mo 0.2 from the density functional theory (DFT) simulation, Supplementary Fig.  4 ), the HEA-He exhibits a larger crystallite size of L a  = 17.93 nm compared to HEA-H 2 ( L a  = 17.08 nm), probably due to the different cooling rate employed. The XRD patterns of the samples collected from the cyclone are shown in Supplementary Fig.  5 . Co-existence of HEA NPs and feedstock material is evident from the patterns, which is consistent with the SEM observation. Among various metal peaks, the Mo peak still remains comparable to those of HEA, indicating Mo might have been evaporated less compared with other elements. This may present a challenge to reuse those powders because the ratio among different elements has changed. The vaporization efficiency should be improved through optimization of the processing parameters (e.g., feed rate, plasma power, powder injection geometry and so on) so that the amount of powders collected in the cyclone can be minimized.

Figure  3 presents representative high-resolution TEM (HR-TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images of the samples with energy dispersive X-ray (EDX) spectroscopy elemental mapping and EDX line scans of individual particles. Regardless of the plasma gases employed, homogenous mixing of five elements in a single particle (~60 nm) was observed without significant elemental segregation or phase separation; however, a slight segregation of Cr is visible from the HEA-He sample (see also Supplementary Fig.  6 ), implying that the two samples were grown under different environments or via different growth mechanisms. The presence of oxygen in the HEA NP was also evident from the elemental mapping (Supplementary Fig.  7 ); however, the presence of oxygen is limited to the surface of the NP (~5 nm thick). The oxide layers might have been formed upon exposure to air. EDX line scans of individual particles for each HEA sample are presented in Fig.  3g, h , which confirm the spatial uniformity in their compositions throughout the particle. A statistical study has been performed to evaluate the composition of the HEA NPs produced (Supplementary Figs.  8 – 10 and Supplementary Table  4 ). The average composition was estimated as Cr (19.2%), Fe (21.5%), Co (22.3%), Ni (22.1%), and Mo (14.7%) for HEA-H 2 , while Cr (23.9%), Fe (23.3%), Co (25.6%), Ni (22.0%), and Mo (5.2%) for HEA-He, which differ from that of the feedstock, especially for its Mo composition. This is likely due to the relatively low evaporation pressure of Mo (e.g., 0.0107 kPa at 3000 K) compared to those of the other four elements (e.g., Cr: 132 kPa, Fe: 67.3 kPa, Co: 40.4 kPa, and Ni: 52.8 kPa at 3000 K) 20 . Molybdenum might have evaporated less in the plasma jet, especially in the helium plasma case. A similar result has been reported on the HEA NPs synthesis (e.g., Co-Cr-Cu-Fe-Ni system) with the arc discharge technique 11 , where elements with higher evaporation pressures (e.g., Cu) become rich in the products. To achieve the targeted composition of a near equimolar ratio, we could increase the plasma power to ensure the complete vaporization of Mo in feedstock or increase the Mo content in feedstock to compensate its low evaporation pressure 7 . The composition also varied among NPs with a variation up to about 8.4% for HEA-H 2 and 2.4% for HEA-He, respectively. This may be attributable to the relatively large particle size distribution. Further process optimization in this regard is needed to achieve good uniformity.

a , b TEM, HR-TEM and HAADF-STEM images of the HEA NPs produced with different plasma gases of hydrogen and helium by the ICJP. Scale bar, 10 nm (HR-TEM) and 100 nm (HAADF). c – f EDX elemental maps of single and multiple HEA NPs, showing homogenous distribution of the five metals in particles. Scale bar, 25 nm (single NP) and 100 nm (multiple NPs). g , h EDX line scans of individual NPs showing the spatial uniformity in their compositions; ( g ) HEA-H 2 case and ( h ) HEA-He case. i , j Phase stability calculations by DFT simulation for the HEA NPs; ( i ) Cr 0.19 Fe 0.22 Co 0.22 Ni 0.22 Mo 0.15 (HEA-H 2 case) and ( j ) Cr 0.24 Fe 0.23 Co 0.26 Ni 0.22 Mo 0.05 (HEA-He case), demonstrating a higher stability of a FCC structure over a BCC.

Crystal structure of the HEA NPs was also investigated by atomically-resolved HAADF-STEM image and the corresponding Fast Fourier Transform (FFT) analysis (Supplementary Fig.  11 ). Although most of the constituent elements have a BCC structure (Supplementary Table  5 ), the analysis reveals that the HEA NPs were stabilized with a FCC structure. DFT calculations were performed to investigate the phase structures for the two different compositions that were identified by HR-TEM analysis: Cr 0.19 Co 0.22 Fe 0.22 Ni 0.22 Mo 0.15 (HEA-H 2 ) and Cr 0.24 Co 0.23 Fe 0.26 Ni 0.22 Mo 0.05 (HEA-He). As shown in Fig.  3i, j , the relative stability of the BCC and FCC phases were estimated by evaluating the total energy differences between the BCC and FCC phases, \(\Delta {E}_{t}={E}^{{{{{{\rm{BCC}}}}}}}-{E}^{{{{{{\rm{FCC}}}}}}}\) , and the equilibrium lattice constants were obtained from the calculation of the total energy as a function of lattice constant a for each phase. For both compositions, \(\Delta {E}_{{t}}\) was found to be positive, indicating that the FCC structure is more stable. Atomic lattice distortion appeared more notable for the BCC phase as presented in the relaxed structures in Fig.  3i, j . This observation was also quantitatively confirmed by the lattice distortion energy \(\Delta {E}_{{dist}}\) (defined as the difference between the energies calculated with and without relaxation) estimated as \(\Delta {E}_{{dist}}^{{{{{{\rm{BCC}}}}}}}=60.7\) and \(\Delta {E}_{{dist}}^{{{{{{\rm{FCC}}}}}}}=37.9\)  meV atom −1 for HEA-H 2 and \(\Delta {E}_{{dist}}^{{{{{{\rm{BCC}}}}}}}=30.9\) and \(\Delta {E}_{{dist}}^{{{{{{\rm{FCC}}}}}}}=21.0\)  meV atom −1 for HEA-He. It was found that as the atomic fraction of Mo decreases from 15 to 5%, the FCC phase stability increases from \(\Delta {E}_{t}=18.5\) to 34.2 meV atom −1 , while the equilibrium lattice constant decreases from a  = 3.60 to 3.56 Å. Note that the VEC value ( \({{{{{\rm{VEC}}}}}}=\mathop{\sum}\limits_{i}{c}_{i}{{{{{{\rm{VEC}}}}}}}_{i}\) , where c i is the atomic fraction of the alloy component i ) increases from 7.98 to 8.12, in agreement with the DFT calculation indicating that the FCC phase becomes more stable for the smaller Mo content. The FCC formation enthalpy, defined as the difference between the total energy of HEA and the ground-state energy of the i component E i , \(\Delta H={E}_{{{{{{\rm{HEA}}}}}}}-\mathop{\sum}\limits_{i}{c}_{i}{E}_{i}\) , were calculated as 114.6 and 86.4 meV atom −1 for the HEA-H 2 and HEA-He, respectively. The BCC formation enthalpy was as estimated 133.1 and 120.6 meV atom −1 for the HEA-H 2 and HEA-He, respectively. The mixing entropy ( \(\Delta {S}_{{{{{{\rm{mix}}}}}}}=-{k}_{B}\mathop{\sum}\nolimits_{i}{c}_{i}{{{{\mathrm{ln}}}}}{c}_{i}\,\) ) contributions for the HEA-H 2 and HEA-He compositions are 1.60 \({k}_{B}\) and 1.51 \({k}_{B}\) , where \({k}_{B}\) is the Boltzmann constant. The temperature that gives the HEA FCC phase formation (i.e., the negative Gibbs free energy \(G=\Delta H-T\Delta {S}_{{{{{{\rm{mix}}}}}}}\) ) is 832 K and 662 K for HEA-H 2 and HEA-He, respectively. The calculation implies that though the HEA formation is driven by mixing entropy as the formation enthalpy is positive for both composition cases, the FCC phase stability over the BCC phase was mainly driven by enthalpy.

Thermal stability of HEA NPs

To investigate the thermal stability of the HEA NPs produced, the samples were annealed at 1173 K (900 °C) for 72 h using a tube furnace with a continuous flow of argon. Their XRD patterns are shown in Supplementary Fig.  12 (Supplementary Note  1 ). After the annealing, the main phase (single FCC structure) of the HEA NPs still remained unchanged without peak splitting or new peak appearances, suggesting that dealloying or segregation of the constituent elements is minimal in both samples. However, a few minor phases with crystalline structures newly appeared. Based on the peak identification, it seems that those are most probably various oxide phases such as Cr 2 O 3 . Other minor phase is also observable with HEA-H 2 sample which might be from formation of Co 7 Mo 6 or Fe 7 Mo 6 intermetallic phase while this phase is absent in HEA-He sample suggesting different stabilities of the HEA NPs. This is attributable to the higher crystallinity of HEA-He sample. Overall, the HEA NPs produced by the plasma process are thermally stable even in a high-temperature environment of above 1000 K for an extended period of time.

Growth mechanism of HEA NPs in a thermal plasma

To understand the in-situ plasma jet alloying mechanism, we have performed optical emission spectroscopy (OES), homogenous nucleation temperature calculations, and thermofluid simulations. Figure  4a shows an emission spectrum measured at Z = 0.23 m from the top of the plasma torch for the HEA-H 2 case. Atomic emission lines from the five elements are evident 21 , indicating that the plasma jet temperature is high enough to vaporize the metal powders injected. Our numerical simulation predicts a temperature of above 8000 K at the plasma core (Fig.  4c ). We note that the thermofluid simulations were performed without considering the injection of metals and thus the actual plasma temperature is expected to be slightly lower than the predicted one due to the powder loading effect.

a , b Optical emission spectra measured at ( a ) Z  = 0.23 m and ( b ) Z  = 0.49 from the top of the plasma torch for the HEA-H 2 case. c – g Thermofluid simulation showing the effect of the reactor geometry ( D r  =  D t v.s. D r  = 3 D t where D r is the reactor diameter and D t is the torch diameter) on the turbulence intensity. h , i Calculated ( h ) saturation vapor pressures and ( i ) nucleation temperatures of each element in a vapor mix of Cr:Fe:Co:Ni:Mo = 1:1:1:1:1 produced at a feed rate of 1.5 g min −1 , showing the existence of a large nucleation temperature gap.

A spectrum measured at Z = 0.49 m is presented in Fig.  4b . In this region, the plasma jet undergoes a rapid cooling by the jet expansion and the turbulence, caused by an abrupt diameter change at the entrance of the reactor (e.g., D r  > 3 D t , where D r is the reactor diameter and D t is the torch diameter). In this study, a reactor geometry that promotes turbulence has been employed to improve the intermixing of vapors produced from different metals (Fig.  4d ). In Fig.  4e–g , the ratios of turbulent to laminar viscosity ( μ t / μ l ) are calculated for the different reactor geometries, and showed that the turbulence effect is enhanced compared to the D r  =  D t case. Although the emission intensity in this region has decreased due to the cooling, all the atomic emission lines from the five elements are still evident. At the same time, the continuum emission from the formation of nanoparticles is negligible, suggesting that the temperature is still high to trigger either homogenous nucleation or co-nucleation. The temperature predicted at the center is around 4200 K. A similar behavior was observed with the spectra measured with the helium gas (Supplementary Fig.  13 ).

As the temperature decreases further, the saturation pressure of each metal vapor decreases and becomes comparable to its partial pressure (i.e., supersaturated, Fig.  4h ), which leads to homogenous nucleation of vapors. For the given feed rate of 1.5 g min −1 , the homogeneous nucleation temperature of each metal was calculated using the self-consistent classical theory (Supplementary Note  2 ) 22 , 23 , 24 . Figure  4i presents the calculated nucleation temperature of each metal with melting temperatures of their bulks. Owing to their low partial pressures, most of the nucleation temperatures are close to or lower than the melting points of the bulk metals. Due to the low saturation vapor pressure of Mo (Fig.  4h ) 20 , Mo becomes supersaturated earlier than other metals and its nucleation temperature is estimated to be 2655 K and 2668 K for Ar-H 2 and Ar-He cases, respectively (see Supplementary Fig.  14 for the Ar-He case). The other metals nucleate between 1800 and 1500 K; thus, there exists a large nucleation temperature gap, over 1000 K, between Mo and the other metals. This gap may present a challenge in the formation of HEA particles via co-nucleation process. The melting temperature of the expected HEA NP was also estimated by using the mixing rule and considering the melting point depression effect because of the nanometer size (Supplementary Note  3 and Supplementary Table  6 ) 25 , 26 . For the given composition, it turned out to be 1865 K for case Ar-H 2 and 1919 K for the Ar-He case, which is close to the nucleation temperatures of most metal elements except for Mo. Thus, in this ICPJ process, the growth mechanism of HEA NPs can be predominated either by VLS (vapor-liquid-solid) or VS (vapor-solid) transformation, depending on the size of the temperature zone of ~2000−2550 K. With a long residence time in this zone, it is proposed that Mo vapor forms nuclei first via homogenous nucleation around 2650 K, followed by co-condensation (i.e., heterogeneous condensation) of metal monomers (including Mo) to grow HEA NPs. However, it is unlikely that Mo nuclei continue to grow into pure Mo particles by homogenous condensation of Mo monomers due to its low partial pressure, which was decreased by mixing multiple elements at a near equimolar ratio (i.e., P i   =   P total   /   N ). In addition, the particles formed reside in their liquid phase for enough time, and thus the different metal elements condensated can diffuse around in a particle creating homogenous mixing of the elements (i.e., alloying) driven by the high-entropy effect, until the temperature reaches the solidification limit of ~1890 K (i.e., VLS transformation). On the other hand, a small size of this temperature zone may result in a co-nucleation of five metals in the temperature range of 1800−1500 K, followed by co-condensation of their monomers. In this case, the direct vapor to solid transformation (i.e., VS transformation) would be the predominant NP growth mechanism as the co-nucleation temperature is lower than the melting temperature of HEA NPs estimated.

Effect of plasma gases

The plasma jet temperature and the resulted thermofluid field can be controlled by carefully choosing the plasma gas composition because different gases have different heat transfer capabilities. Figure  5a shows the temperature fields inside the reactor calculated for different plasma gases of Ar (100%), Ar-H 2 (H 2 : 8.3%), and Ar-He (He: 77.4%) (See Supplementary Fig.  15 for the velocity and species fields). Adding hydrogen or helium in the plasma gas alters the thermofluid field substantially; it causes rapid cooling of the gas and consequently the plasma jet becomes shorter. The rapid quenching of the plasma jet can be explained by the higher thermal conductivity of hydrogen or helium over argon (Supplementary Fig.  16 ). Such gases can enhance the heat exchange with the surroundings downstream, resulting in rapid cooling of the plasma jet. Figure  5b shows the contours of thermal conductivities calculated. More enhanced heat transfer with the helium case is attributed to its higher concentration (77.4%) in the plasma gas compared with the hydrogen case (8.3%). The hydrogen content can be increased up to 20%; however, excessive hydrogen may have adverse effects on NPs such as hydrogen-driven embrittlement 27 .

a Temperature fields calculated for different plasma gases of Ar (100%), Ar-H 2 (H 2 : 8.3%), and Ar-He (He: 77.4%), showing different cooling rates of the plasma jet. b Thermal conductivity ( κ eff ) distributions calculated which show the effect of the plasma gas on the heat transfer rate. c Temperature zones where HEA NPs nucleated are expected to be in a liquid phase (i.e., liquid zone) for Ar-H 2 and Ar-He cases. d – f Axial temperature profiles with local heating and cooling rates calculated for different plasma gases. The arrows represent the estimated liquid zones. g Residence times of HEA NPs calculated for different plasma gases. h Growth mechanisms of HEA NPs under different plasma gases of Ar-H 2 and Ar-He.

Complete evaporation of feedstock is a prerequisite to the formation of a homogeneous multicomponent vapor, which is largely affected by the heat exchange rate between plasma and feedstock and the residence time. Pure argon plasma presents the largest area of the hot zone which is favourable for feedstock evaporation while the heat exchange between feedstock and Ar plasma is expected to be slow due to the low thermal conductivity of Ar, resulting in a low vaporization efficiency. The hydrogen plasma seems more promising to achieve a good evaporation efficiency thanks to its relatively high thermal conductivity and large hot zone, which is in line with the composition analysis.

In Fig.  5c , the temperature contour above the Mo nucleation temperature and below the HEA NP solidification temperature were cut-off. In this zone, Mo nuclei start to form from the homogenous nucleation and continue to grow into HEA NPs via the co-condensation of metal monomers. The particles are in the molten phase and thus can be alloyed by diffusion process inside particles. The size of this zone is larger for the hydrogen plasma case due to its moderate cooling rate. Hydrogen is also found to extend this zone towards the reactor exit, maintaining the gas temperature above the solidification limit by releasing the recombination heat of H atoms. The axial temperature profiles for each case are plotted in Fig.  5d–f with the corresponding local heating and cooling rates. In all cases, overall high heating and cooling rates up to 10 6  K s −1 were observed, which is ideal for the formation of HEA NPs. The liquid droplet zone is also indicated by an arrow and it appears earlier and lasts shorter in the helium plasma because of the faster cooling. The residence times along the centreline were calculated and presented in Fig.  5g . As expected, the longest residence time (100 ms) in the liquid droplet zone was observed with the hydrogen case while the shortest time (22 ms) was observed with the helium case. This may imply that NPs were grown via different pathways for the two different plasma gases.

From the thermofluid simulation, it is probable that the VLS transformation would be the predominant growth mechanism in the hydrogen case, while particles would grow via the direct VS transformation mechanism in the helium case (Fig.  5h ). This could explain some differences in the structural and morphological properties between HEA-H 2 and HEA-He observed from the XRD, SEM, and TEM analysis. For the HEA-H 2 case, NPs grow through the liquid droplet phase, followed by rapid quenching. This allows for more formation of an amorphous or glass state (i.e., kinetically frozen liquid) 12 , which is evident from the XRD analysis; the peak of (111) plane is broader than that of the HEA-He sample. The distribution of elements in a NP also seems more homogeneous and diffusive which is evident from the comparison of elemental maps. In the VLS growth, NPs can grow further through interparticle coagulation until they solidify. This increases the particle size as well as broadens the size distribution, which is in line with the SEM observation. For the HEA-He case, it is suggested that NP growth mostly occurs in the solidification limit through the direct VS transformation. Therefore, only stable phases, such as segregate phases or random alloys can be formed with less amorphous state 12 . This would be the reason why the HEA-He sample exhibit a higher crystallinity or crystallite size from the XRD patterns. The slight segregation of element observed (see Supplementary Fig.  6 ) also supports the VS growth mechanism. In the VS growth, NPs cannot grow via interparticle coagulation, resulting in size reduction with a narrow size distribution, which is in line with the SEM analysis. However, the HEA NPs produced via the VS path (i.e., the helium plasma case) exhibit a higher Mo loss in the composition due to the low vaporization efficiency of feedstock. Although the helium plasma provides a higher maximum temperature (9740 K) and gas conductivity compared to those of the hydrogen plasma, it cools down rapidly through the enhanced heat exchange with the surroundings. To achieve the VS growth without the large Mo losses, a lower feed rate of feedstock or nano-sized Mo powders can be employed to ensure a better evaporation Mo without altering the thermofluidic field significantly. Our study demonstrates that the structural or morphological properties of HEA NPs can be tailored in this plasma process by changing plasma gases which controls the thermal history of NPs effectively. It should be noted that the thermal history of NPs can also be controlled by other processing parameters, such as reactor geometry. For instance, a reactor geometry with a rapid jet expansion promotes the quenching of the reaction stream resulting in size reduction with a narrow size distribution. The presence of thermal insulators or active heating units inside the reactor can increase the residence time of NPs in the liquid zone reducing element segregation.

Synthesis of other HEA NPs

The main purpose of the development of the ICPJ process is to demonstrate the scalable synthesis of non-precious metal based HEA NPs for their cost-effective applications. Therefore, in our study, we considered HEA systems mainly consisting of earth-abundant metals (e.g., Cr, Mn, Fe, Co, Ni, Cu, Mo) rather than precious metals. Molybdenum and Cu were included specifically because Mo has different physical properties compared with other elements (e.g., low saturation pressure) while Cu is known to be hardly alloyed with other elemental metals due to its positive binary mixing enthalpies (Supplementary Table  7 ) 28 . We further demonstrated HEA NPs of CrMnFeCoNi, MnFeCoNiCu, and CrFeCoNiCu, and their structural and compositional properties were studied by XRD and HR-TEM analyses including EDX elemental mapping and EDX line-scanning (Fig.  6 , Supplementary Figs.  17 – 26 ). All the XRD patterns of the HEA NPs produced exhibit a single FCC structure (Supplementary Figs.  17 , 20 , 23 ; Supplementary Tables  8 – 10 ), indicating successful alloying of the feedstock materials. Two minor peaks show up around 46.4° and 48.6° in the CrMnFeCoNi sample synthesized with hydrogen and this would be attributable to the formation of metal hydrides. Those peaks are absent in the sample produced with helium. Figure  6 shows EDX elemental maps of a single and multiple HEA NPs synthesized with different plasma gases. For CrMnFeCoNi (Fig.  6a–d ) and MnFeCoNiCu HEA NPs (Fig.  6e–h ), homogenous mixing of five elements in a single particle was observed without significant elemental segregation or phase separation regardless of the plasma gases employed. The elements in CrMnFeCoNi and MnFeCoNiCu HEA NPs also have near equimolar ratios (Supplementary Table  11 ): Cr (20.2%), Mn (20.4%), Fe (20.7%), Co (20.3%), and Ni (18.4%) for CrMnFeCoNi HEA-H 2 ; Mn (17.8%), Fe (22.4%), Co (21.6%), Ni (19.2%), and Cu (18.9%) for MnFeCoNiCu HEA-H 2 . On the other hand, the EDX elemental maps of CrFeCoNiCu HEA NPs indicate Cu segregation at the edges of particles while other elements of Cr, Fe, Co, and Ni are homogeneously distributed (Fig.  6i–l ). This is attributable to the high positive binary mixing enthalpies between Cu and other elements, especially Cr and Fe (Supplementary Table  7 ). Similar observations have been reported in other HEA synthesis methods (e.g., arc discharge) 11 and other HEA systems of AlCrFeCoCu 29 and CoCrFeNiCuAl 30 .

a , b ( a ) a single and ( b ) multiple CrMnFeCoNi HEA NPs produced with hydrogen plasma. c , d ( c ) a single and ( d ) multiple CrMnFeCoNi HEA NPs produced with helium plasma. e , f ( e ) a single and ( f ) multiple MnFeCoNiCu HEA NPs produced with hydrogen plasma. g , h ( g ) a single and ( h ) multiple MnFeCoNiCu HEA NPs produced with helium plasma. i , j ( i ) a single and ( j ) multiple CrFeCoNiCu HEA NPs produced with hydrogen plasma. k , l ( k ) a single and ( l ) multiple CrFeCoNiCu HEA NPs produced with helium plasma. The elements in CrMnFeCoNi and MnFeCoNiCu HEA NPs have near equimolar ratios while the Cu content in CrFeCoNiCu HEA NPs (12~14%) is deviated from that of feedstock due to local segregation (Supplementary Figs.  18 , 21 , 24 and Supplementary Table  11 ). Scale bar, 50 nm (single NP) and 100 nm (multiple NPs).

To study the uniformity of compositions in the ICPJ process, we have performed EDX analysis on 10~20 particles with different sizes for each HEA NP system (Supplementary Figs.  18 , 21 , 24 and Supplementary Table  11 ). The composition of the HEA NPs synthesized by the ICPJ process varies up to 8.4%. To study the spatial uniformity of compositions inside a single particle, we also performed EDX line scanning of individual particles (Supplementary Figs.  19 , 22 , 25 ). For CrMnFeCoNi and MnFeCoNiCu HEA NPs, uniform distribution of five elements across the particle was observed without significant elemental segregation, confirming the spatial uniformity of compositions throughout the particle. On the other hand, the EDX line scans of CrFeCoNiCu HEA NPs indicate Cu segregation at the edges of the particles (Supplementary Fig.  25a ). We also found that the Cu segregation seems reduced as the particle size decreases (Supplementary Fig.  26 ). We speculate that a large particle may take more time to cool down and provide more time for the Cu segregation. In this case, the size of particle may affect the uniformity of compositions; a smaller particle size with a narrow size distribution would be favorable to achieve a good compositional uniformity. A torch or reactor geometry that allows a better control of the particle size and size distribution could be considered for further improvement.

Optical absorption performance as a photothermal conversion material

Nanoparticles comprising multiple 3 d transient-metal elements (e.g., Ti, V, Cr, Fe, Co, Ni, Cu) have been proven that their optical absorption can be broadened through the d-d interband transitions (IBTs), presenting a great potential as efficient photothermal conversion materials 31 . Such property of HEA NPs is attributable to the reinforcement of IBTs by fully filling energy regions around the Fermi level, upon introducing more 3 d transient-metals in a nanoparticle. Very recently, this approach has also been extended to HEA NPs comprising beyond 3 d transient-metals (e.g., Mo, Ta, W) 32 . Despite this new opportunity, the current synthesis method is intrinsically operated in a batch mode and not suitable for the preparation of HEA NPs for large-scale/area photothermal conversions 31 , 32 . Since our plasma jet process promises the commercial scale production of HEA NPs, the optical absorption performances of the as-produced HEA NPs have been investigated through a diffuse reflectance spectroscopic measurement in a wavelength region of 250 to 2500 nm (UV-Vis-NIR).

Figure  7 shows the absorptance spectra of the HEA NP samples along with that of the feedstock mixture. Both HEA NP samples exhibit an average absorptance greater than 96%, which is a significant improvement from that of the feedstock mixture. The strong absorption for the HEA NP samples is mainly contributable to d - d IBTs as reported previously 31 , 32 . Overall HEA-He sample shows better performance than HEA-H 2 . Its higher crystalline structure or smaller size may be responsible for the enhanced performance; however, further understanding is needed to elucidate the mechanism for the improvement.

Absorptance spectra of the HEA NPs (CrFeCoNiMo) prepared by the ICPJ strategy with different plasma gases. Both HEA NPs exhibit an excellent light absorption performance of >96%. The grey area presents the solar radiation spectrum (Air mass 1.5). The noisy signal above 2000 nm is due to water absorption bands, mostly in air. The discontinuity of the signal around 800 nm is caused by a detector change in the spectrophotometer.

We develop an ultrafast (<100 ms), one-step method that enables the continuous synthesis of HEA NPs directly from a mixture of pure elemental metal powders using a high-temperature plasma jet (>5000 K). Highly crystalline HEA NPs (CrFeCoNiMo) were continuously produced at a high rate approaching 35 g h −1 via in-flight alloying of elemental metal powders. This plasma process can be easily scaled up as the core technology is mature with high-power plasma torches up to ~MW levels available. Our study also indicates that the structural or morphological properties of HEA NPs can be tailored by changing the plasma gas which controls the thermal history of NPs. The yield demonstrated represents a seminal milestone towards the exploitation of HEA NPs in real-world applications. The HEA NPs produced exhibit an excellent light absorption performance of >96%, and thus represent great potential in the cost-effective, large-area solar energy harvesting for thermophotovoltaics, photocatalysis, and water desalination. In this regard, a direct solar energy absorption experiment with the HEA NPs produced in this work is of particular interest as future research.

Chromium (Cr, <10 μm, 99.2%), manganese (Mn, <10 μm, 99.6%), iron (Fe, 6−10 μm, 99.5%), cobalt (Co, 1.6 μm, 99.8%), nickel (Ni, 3−7 μm, 99.9%), copper (Cu, 0.5−1.5 μm, 99%), and molybdenum (Mo, 3−7 μm, 99.95%) powders were purchased from Alfa Aesar. The as-received elemental metal powders were mixed at an equal ratio (1:1:1:1:1), and the mixtures (i.e., Cr-Fe-Co-Ni-Mo, Cr-Mn-Fe-Co-Ni, Mn-Fe-Co-Ni-Cu, Cr-Fe-Co-Ni-Cu) were employed for the synthesis experiments without further treatment.

Plasma synthesis system

HEA NPs were synthesized by using an RF thermal plasma technology. The plasma processing system was originally developed for the synthesis of nanotubes (e.g., carbon and boron nitride nanotubes) 33 and further modified for an effective synthesis of HEA NPs. The current synthesis system consists of five major parts: an induction plasma torch, a reaction chamber, a cyclone separator, a filtration chamber, and feedstock delivery (see Supplementary Fig.  1 ). For the plasma generation, a commercial RF induction plasma torch (Tekna PS-50, Tekna Systems, Inc.) composed of a five-turn coil and a ceramic tube with an internal diameter of 50 mm was employed. A 1-m long, double-walled stainless-steel chamber was employed as the reaction chamber. Its diameter was designed to be at least 3 times larger (e.g., 150 mm) than that of the plasma torch to enhance the mixing of metal vapors by the increased turbulence effect. The chamber walls were cooled by water to increase the cooling rate of the HEA NPs produced. To selectively remove unvaporized feedstock particles, a cyclone separator (inner diameter: 0.034 m; outer diameter: 0.097 m; length: 0.6 m) was employed at the bottom of the reactor. The nano-size final products were collected from four porous metal filter units (surface area = 20 × 50 cm, 2.8 μm pore size) inside a filtration chamber connected to the end of the cyclone separator.

HEA NP synthesis

In the synthesis experiment, the plasma power was fixed at 45 kW at an RF frequency of ~3 MHz (Lepel Co.) while two different plasma gas compositions were used to investigate the effect of the plasma gas: 5 slpm of carrier gas (Ar), 30 slpm of central gas (Ar), and 120/14 slpm of sheath gas (Ar/H 2 ); 5 slpm of carrier gas (Ar), 30 slpm of central gas (Ar), and 120 slpm of sheath gas (He). The feedstock was continuously fed by a vibrating powder feeder (PFR200 feeder, Tekna Systems, Inc.) and delivered to an injection probe located on the top of the plasma torch by Ar carrier gas. The feed rate of the powder mix was about 1.2−2.0 g min −1 . During the synthesis, the reactor pressure was kept constant at 66.7 kPa. The reaction products were collected from the cyclone separator and the filtration chamber in an open environment and characterized without further purification or treatment.

Optical emission spectroscopy

To verify the formation of metal vapors from feedstock vaporization and investigate their spatial evolution, optical emission spectra were measured at Z = 0.23 m and Z = 0.49 m from the top of the plasma torch during the synthesis. A modular spectrometer (JAZ-EL200-XR1, Ocean Optics, with 1.7 nm FWHM resolution) was employed for the measurement over a wavelength range from 200 to 1025 nm. The emission light was collected through a quartz window and transported to the spectrometer by an optical fiber (QMMJ-55-UVVIS-200/240-2PCBL-0.25, OZ Optics Ltd., with a core size of 200 μm). It was confirmed that the emission from atomic oxygen (e.g., 777.4 nm) was not observable in our process, implying that oxygen contamination was not significant.

Materials characterization

Morphology, structure and composition of the HEA NP samples produced under different conditions were analyzed by XRD, SEM and HR-TEM. SEM analysis was performed with a field emission scanning electron microscope (Hitachi S-4800). Samples were mounted on the sample holders with double-sided conductive glue tape and were imaged in a secondary electron mode. To avoid dislodgement of magnetic particles by magnetic objective lens, the working distance was chosen to be more than 9 mm. X-ray powder diffraction was performed using D8 DISCOVER (Bruker) with Cu K α 1 radiation ( λ  = 1.5406 Å, 8.0478 keV). The 2-theta range was set from 20 to 90 degree with a scan speed of 0.02 degree per second. TEM specimens were prepared by dispersing the solid powder in ethanol, and sonicating for 5 min. One drop of the solution was then placed onto a 400 mesh TEM copper/gold grid coated with an ultrathin (<3 nm) carbon film supported on lacey carbon (Ted Pella) and dried in air. A FEI Titan3 80–300 TEM operated at 300 keV, and equipped with a CEOS aberration corrector for the probe forming lens and a monochromated field-emission gun was used for the analytical TEM analysis. HAADF images were collected using a Fischione detector in scanning transmission electron microscopy (STEM) mode. The TEM instrument is also equipped with an energy-dispersive X-ray (EDX) spectrometer (EDAX Analyzer, DPP-II). To optimize the signal intensity, EDX spectra were acquired with the specimen tilted at 15 degrees.

Light absorption measurement

The light absorptance spectra of the HEA NP samples were extracted from diffuse reflectance measurements. Diffuse reflectance spectroscopic (DFS) measurements were done with a Perkin Elmer Lambda-900 spectrophotometer equipped with a 15 cm diameter integrating sphere (Labsphere). A home-made mortar and pestle system was used to crush powder samples into 1 cm diameter non-translucent pellets with a near Lambertian scattering behaviour, following a procedure similar to refs [ 34 , 35 ]. The reflectance was measured with the pellet still in the mortar, using a 1 cm diameter aperture accessory to expose only the surface of the pellet to the incident light beam. With the light beam cross-section being larger than the aperture, the reflectance of the aperture without samples ( I 0 ) was subtracted from all measurements, and the samples measurements ( I s ) were compared to a white standard ( I 100 , spectralon material, labsphere), given the diffuse reflectance as R d  = ( I s - I 0 )/( I 100 - I 0 ). The absorptance A was directly extracted using A  = 1- R d  +  T d  = 1- R d , where T d is the transmittance, null is our case (the pellets were opaque). To reduce the spectral noise above 2000 nm and around 810 nm (detector change), we decrease the scan rate from 240 nm min −1 to 6 nm min −1 over these regions.

Thermofluid simulations

To investigate the effects of reactor geometry and plasma gas composition on the thermofluid field (e.g., turbulence intensity, quenching rate, and residence time), computational fluid dynamics (CFD) simulations were performed inside the reactor using an in-house plasma code coupled with ANSYS-FLUENT 36 . The plasma generation model was based on the magneto-hydrodynamic (MHD) theory and consists of conservation equations for mass, momentum, and energy coupled with a set of Maxwell’s equations. To investigate turbulence effects with reasonable computational costs, we adopted the Reynolds stress model which has been widely employed in the modeling of various nanomaterials synthesis by thermal plasmas 37 . Thermodynamic and transport properties of plasma gases (e.g., density, specific heat at constant pressure, viscosity, thermal conductivity, electrical conductivity, and radiation losses) were calculated under the local thermodynamic equilibrium (LTE) assumption 18 . Although powder injection and its vaporization are important phenomena to study, the vaporization of feedstock was not taken into account due to the lack of thermodynamic and transport data of some metal vapors for a wide range of temperature of 1000 − 10,000 K. More details on the governing equations, computational domain, and boundary conditions can be found in the Supplementary Note  4 (see Supplementary Fig.  27 and Supplementary Table  12 ).

Density functional theory (DFT) simulations

The structure relaxation and total energy calculation were performed by using DFT implemented in the Vienna ab initio simulation package (VASP). The projector augmented wave (PAW) method and generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerhof (PBE) were used 38 , 39 , 40 . The basis set consists of plane waves with a cutoff energy of 520 eV. Monkhorst-Pack method with 1 × 1 × 2 was used. Spin-polarized calculation were performed. For both BCC and FCC phases, 100-atom BCC and FCC supercell (5 × 5 × 4 unit cells) models were constructed. The random solid solution structures were generated by using the hybrid Cuckoo search (CS) code as implemented in ref [ 41 ]. Ten different random configurations for a given phase and composition were generated and examined. All the calculations were performed at 8 different lattice constants around the equilibrium, in which the unit cell shape was fixed and the atomic positions have been fully relaxed using conjugated-gradient algorithm.

Data availability

All the supporting data are provided in the main text and Supplementary Information. The data sources that support the findings of this study are available from the corresponding author upon request.

Code availability

The source code for the thermofluid simulation used in this study is managed by the National Research Council Canada (NRC-CNRC). It could be made available from the corresponding author for disclosure upon acceptance of NRC-CNRC’s terms and conditions.

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Acknowledgements

This work was supported by the National Research Council Canada-University of Toronto Collaboration Centre in Green Energy Materials (CC-GEM) within the Materials for Clean Fuels (MCF) Challenge program at the National Research Council Canada (A1-018124, recipients K.S.K. and Y.Z.). The authors are grateful to R. Iannitto and G. Li for their assistant in the powder preparation and sharing their experiences on handling nano-sized metal powders.

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Security and Disruptive Technologies Research Centre, National Research Council Canada, Ottawa, ON, K1A 0R6, Canada

Keun Su Kim, Homin Shin, Dean Ruth, Mark Plunkett, Liliana Gaburici & Thomas Lacelle

Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, ON, M5S 3G8, Canada

Keun Su Kim

Department of Mechanical Engineering, University of Ottawa, Ottawa, ON, K1N 6N5, Canada

Keun Su Kim, Ziqi Tang & Michel Nganbe

Energy, Mining and Environment Research Centre, National Research Council Canada, Ottawa, ON, K1A 0R6, Canada

Martin Couillard

Advanced Electronics and Photonics Research Centre, National Research Council Canada, Ottawa, ON, K1A 0R6, Canada

Daniel Poitras

Department of Materials Science and Engineering, University of Toronto, Toronto, ON, M5S 3G8, Canada

Changjun Cheng, Lixin Chen & Yu Zou

Construction Research Centre, National Research Council Canada, Ottawa, ON, K1A 0R6, Canada

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Contributions

K.S.K. and H.S. conceived the idea and designed the present work. K.S.K., Z.T., D.R. and M.P. performed the HEA synthesis experiments. M.C., O.N. and L.G. carried out the detailed microscopic analyses. Z.T. and M.N. carried out the nucleation temperature calculation and XRD data interpretation. H.S. performed the DFT simulation on the phase stability. K.S.K. carried out the thermofluid simulation. D.P. conducted the light absorption measurement and extracted the absorptance spectra. T.L., O.N., C.C., L.C. and Y.Z. performed XRD characterization and electrochemical tests. All authors discussed the results together and commented on the manuscript.

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Correspondence to Keun Su Kim .

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Competing interests.

An international patent (PCT) application has been applied for via National Research Council Canada (No. PCT/CA2023/051556). The authors K.S.K, M.C, H.S, D.R, and M.P. are involved in the patent application. The remaining authors declare no competing interests.

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Kim, K.S., Couillard, M., Tang, Z. et al. Continuous synthesis of high-entropy alloy nanoparticles by in-flight alloying of elemental metals. Nat Commun 15 , 1450 (2024). https://doi.org/10.1038/s41467-024-45731-z

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Received : 21 December 2022

Accepted : 01 February 2024

Published : 16 February 2024

DOI : https://doi.org/10.1038/s41467-024-45731-z

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