Metal clusters: New era of hydrogen production

Renewable and Sustainable Energy Reviews 79 (2017) 878–892

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Metal clusters: New era of hydrogen production a,⁎

Yasser Attia , Mohamed Samer a b

MARK

b,⁎

National Institute of Laser Enhanced Sciences (NILES), Cairo University, 12613 Giza, Egypt Department of Agricultural Engineering, Faculty of Agriculture, Cairo University, 12613 Giza, Egypt

A R T I C L E I N F O

A BS T RAC T

Keywords: Clusters Nanomaterials Photocatalysts Hydrogen production Water splitting

Clusters show intermediate properties between the isolated atoms and the bulk metals and represent the most elemental building blocks in nature (after atoms). They are characterized by their size, which establish a bridge between atomic and nanoparticle performances, with properties completely different from these two size regimes. If particle size becomes comparable to the Fermi wavelength of an electron, i.e. < 2 nm, then this is a cluster. Reducing the size from the bulk material to nanoparticles displays a scaling behavior in physical properties in the later ones, due to the large surface-to-volume portion. Through further size reduction, entering into the subnanometric cluster region, physical properties are largely affected by strong quantum confinement. These quantum size effects (HOMO-LUMO gap), the small size and the specific geometry grants subnanometric clusters with entirely novel properties, including cluster photoluminescence, enhanced catalytic activity, etc. In this literature review, an introduction to the physical properties of clusters is reported; the controlled synthesis methods and the catalytic properties in hydrogen evolution. Hydrogen (H2) production by water splitting is hindered mainly by the lack of low-cost and efficient photocatalysts. Here, we show that sub-nanometric metal clusters can be used as photocatalysts for H2 production in the presence of holes or electrons scavengers by water splitting. This illustrates the considerable potential of very small zerovalent, metallic clusters as novel atomic-level photocatalysts.

1. Introduction Recently, fossil energy resources have depleted fast as well as serious environmental problems caused by the ever increasing carbon dioxide (CO2) content of the atmosphere. To reduce CO2 emissions and fulfill the ever-increasing energy demands, much research efforts have been devoted to develop sustainable and renewable energy sources, which are environmentally friendly, cost-effective and clean. In the recent years, photoelectrochemical water splitting which could produce hydrogen (H2) using sunlight and semiconducting photoelectrodes has attracted great interest of scientists due to its cleanness and renewable characteristics. Hydrogen is a chemical fuel with high energy density, and it is an environmentally clean energy source since it does not emit greenhouse gases (GHGs) when burned. Therefore, hydrogen is recognized as a clean energy since no additional CO2 is emitted when used. Furthermore, hydrogen can be obtained directly from water and solar radiation, both of which are the abundant, widespread, renewable resources. Therefore, hydrogen has attracted intensive interest as a promising and sustainable energy supply for replacing fossil energy resources in the future. Investigations of H2 production from the solar water splitting have

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increased in recent researches due to its environmental cleanness and unlimited utilization of solar energy and renewable characteristics in converting solar energy into chemical energy. The combustion of H2 is not accompanied by greenhouse gases (GHGs) emission such as CO2 and CO. Therefore, one of its enormous benefits is zero-emission of GHGs if H2 is used as the fuel for power generation through fuel cells or turbines. Photocatalytic system to generate hydrogen from water provides us a green and renewable way to generate hydrogen fuel. The splitting of water into hydrogen and oxygen has been studied continuously in the years following the discovery of photocatalytic water splitting using a semiconductor photoelectrode, TiO2 in a Photoelectrochemical cell (PEC), as was demonstrated by Fujishima and Honda [1]. Clusters display intermediate properties between the isolated atoms and the bulk metals and represent the most elemental building blocks in nature (after atoms). They are characterized by their size, which set up an overpass between atomic and nanoparticle behaviors, with properties entirely different from these two size regimes. If particle size becomes comparable to the Fermi wavelength of an electron, i.e. < 2 nm, then this is a cluster. The objectives of this review are:

Corresponding authors. E-mail addresses: [email protected] (Y. Attia), [email protected] (M. Samer).

http://dx.doi.org/10.1016/j.rser.2017.05.113 Received 16 May 2016; Received in revised form 23 February 2017; Accepted 18 May 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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1. Defining clusters and discussing their properties and role in hydrogen production. 2. Investigating the charactaristics of clusters based on their size, and the subsequent properties difference based on the size. 3. Studying the role of clusters as photocatalysts in hydrogen production by water splitting. 2. Photocatalysts and co-catalysts for H2 production In the twenty-first century, more and more greenhouse gas emissions from fossil fuels have caused the obvious greenhouse effect and created a serious impact on the environment. Hence, looking for alternative fuels and environmentally friendly green energy have become a major concern. Recently, H2 energy is expected to be the ideal alternative for green energy owing to its cleanliness and high efficiency. Particularly, the photocatalytic H2 production from water splitting using solar energy is a most potentially clean and renewable source for H2 fuel, where Fujishima and Honda firstly reported the photo-oxidation of water on a semiconductor TiO2 electrode [1,2]. To achieve this goal, many efforts have been devoted to the development of highly active photocatalysts, including metal oxides (Ta2O5 and SiTiO3) [3,4], metal sulfides (CdS, PdS/CdS and AgInZn7S9) [5–7] and metal oxynitrides or oxysulfides (TaON, GaN:ZnO and Sm2Ti2O5S2) [8–10]. At present, the perovskite-type tantalate, NaTaO3, has been widely studied because of its outstanding performance in photocatalytic water splitting into H2 and O2 under ultraviolet-light irradiation [11,12]. However, because of the relatively wide energy band-gap (Ebg = 4.0 eV and λ < 310 nm), as a photocatalyst the NaTaO3 only absorbs the ultraviolet-light to perform the photocatalytic reaction. That is, the NaTaO3 cannot be active under visiblelight irradiation. Therefore, it is necessary to provide more UV-light in order to uphold higher photocatalytic hydrogen evolution activity of NaTaO3. Unfortunately, ultraviolet-light only accounts for less activities than 5.0% of the solar light, which leads to the solar energy utilization rate being extremely low. In order to obtain a high solar energy utilization rate it must try to utilize the visible-light (~48%) and infrared-light (~44%) in solar light [13–16]. Recently, some research groups reported the application of the photocatalysts combined with up-conversion luminescence agents in photocatalytic degradation of organic pollutants [17–20] and photocatalytic hydrogen production from water splitting [21]. The technology combining with up-conversion luminescence agents can not only keep a high photocatalytic activity of wide bandwidth TiO2 photocatalysts (Ebg = =3.2 eV and λ < 380 nm), but also effectively uses solar light to undertake photocatalytic reaction. Accordingly, it can also be considered to combine upconversion luminescence agent (Er3+:YAlO3) with NaTaO3 photocatalyst (Ebg = 4.0 eV and λ < 310 nm), which may attain the purpose of making the wide bandwidth of NaTaO3 effectively utilize solar light to carry out photocatalytic reaction. In this combined photocatalyst, the Er3+:YAlO3, which was reported to be able to generate one high-energy photon by absorbing two or more incident low-energy photons [22], has been employed for converting infrared and visible lights to ultraviolet light that can effectively activate the NaTaO3 photocatalyst. In order to obtain the best catalytic activity of photocatalyst the use of some co-catalysts is inseparable. The co-catalysts loaded on semiconductor photocatalysts play an essential role in the production of H2 and O2 [23–26]. For photocatalytic reactions caused by semiconductor materials, the co-catalysts can promote the separation of photogenerated electrons (e-) and holes (h+). Moreover, the co-catalysts can also offer the low activation potentials for H2 or O2 evolution and be often served as the active sites [27–29]. Therefore, the loading of proper cocatalysts can greatly enhance the activities of photocatalysts [30–33]. In most work reported so far, mainly some noble metals (for example: Pt and Au) or metal oxides (for example: NiO and RuO2) have been used as the co-catalysts, while some inexpensive inorganic compounds such as sulfides of transition metals can also be used as co-catalysts in

Fig. 1. Photocatalytic hydrogen production principle and process of co-catalysts under visible-light irradiation (Amended and redrawn after Lu et al. [2]).

photocatalytic H2 production in recent years. Kudo et al. [34] proposed that the NaTaO3 doped with 2.0 mol% La and modified by a NiO cocatalyst under UV-light can improve the efficiency of water-splitting hydrogen production. Lin et al. [35] prepared a Nb2O5 catalyst enclosing pure metal particles, such as Pt and Au, to investigate the photochemical hydrogen production. Xu et al. [20] reported that the MoS2 as a co-catalyst loaded on CdS for photocatalytic H2 production. These results showed that the addition of metal particles, metal oxides or metal sulfide into the photocatalyst materials effectively enhanced the photocatalytic activity and hydrogen production efficiency [35]. Additionally, it pointed out that the use of ultraviolet-light sources and higher concentrations of sacrificial reagents can effectively enhance the hydrogen production rate [36–38]. However, the researches on the different factors affecting co-catalyst materials in photocatalytic hydrogen production under visible-light conditions remained insufficient. Particularly, the comparison of different co-catalyst materials in hydrogen production efficiency from low-concentration methanol solution (methanol as sacrificial reagents) and even long-term hydrogen production assessments have not gotten much attention. Fig. 1 shows visible-light photocatalytic hydrogen production principle and process of co-catalysts (CuO, MoS2 and Pt) loading Er3+:YAlO3/NaTaO3 under visible-light irradiation. 3. Solar reactors for H2 production Thermochemical cycles are known as appropriate processes to generate hydrogen in a sustainable way, utilizing water as input and concentrated sunlight as heat source. In common two-step metal oxide based cycles, a metal oxide (MO) is reduced at high temperature liberating oxygen (MOox = MOred +½O2) and successively re-oxidized by water at lower temperature (MOred + H2O = MOox + H2). The complete process splits water, producing hydrogen and oxygen in separate steps [39]. Numerous active chemical substrates were investigated, together with different reactor concepts [40–46], for cycles operating in the 1000–2000 °C temperature range. Such temperature levels are challenging and pose severe demands on materials and reactor design [47,48]. Varsano et al. [39] described a solar concentration facility (~1 kW), where this facility is a solar furnace, composed of one heliostat (2.2×2.0 m2), with azimuthal and vertical sun tracking system driven by photocells, and a parabolic reflector (diameter 1.5 m, horizontal axis), which has a focal distance of 0.64 m and a concentration factor of about 600 (Fig. 2). The concentrated solar radiation is directed towards the reactor-receiver cavity, placed in the focus of the concentrating dish. A pyranometer is positioned between the heliostat and the parabolic mirror to quantify the reflected radiation from the heliostat. The incident solar radiation has been constantly monitored and recorded. Although no automatic control of the incoming solar 879

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Fig. 2. Scheme of the solar concentration facility: an heliostat (1) is driven by a photocell (2) to reflect the incoming radiation on a concentrating parabolic mirror (3); The receiver cavity of the reactor (4) is placed on the focus of the concentrated radiation. The analytical system consists in a water-ice trap (5) and silica gel trap (6) utilized to capture unreacted water, a gas chromatograph (7) to detect hydrogen and carbon dioxide fluxes output. A pyranometer (8) is placed on the incoming light path to measure the radiation intensity (Amended and redrawn after Varsano et al. [39]).

for all studied metal oxides. The highest efficiency is observed for relatively large micrometric particles of ThO2 which is assigned to ultrasonically-driven particle fragmentation accompanied by mechanochemical water molecule splitting. The nanosized metal oxides do not exhibit particle size reduction under ultrasonic treatment but nevertheless yield higher quantities of H2. The enhancement of sonochemical water splitting in this case is most likely resulting from better bubble nucleation in heterogeneous systems. At high-frequency ultrasound (362 kHz), the effect of metal oxide particles results in a combination of nucleation and ultrasound attenuation. In contrast to 20 kHz, micrometric particles slowdown the sonolysis of water at 362 kHz due to stronger attenuation of ultrasonic waves while smaller particles show a relatively weak and various directional effects [53].

power is available in the facility, undesired excessive heating can be avoided by blinding the parabolic mirror. Direct thermal splitting of water by solar energy requires a temperature of over 2000 °C to obtain sufficient amounts of hydrogen [41]. 4. Recent developments in photocatalysts Zhu et al. [49] stated that the experimental results showed that the Pt/C–HS–TiO2 with hollow sphere structure displayed excellent photocatalytic activity and stability under visible light radiation at room temperature. Accordingly, Zhu et al. [49] suggested a mechanism of formation of hollow sphere Pt/C–HS–TiO2 and of photocatalytic water splitting on hollow sphere structured Pt/C–HS–TiO2. Lin and Shih [50] prepared series of metal ion (M = Cr, Ni, Cu, Nb) and nitrogen co-doped TiO2 with high hydrogen production by photocatalytic water splitting by microwave-assisted hydrothermal method using titanium sulfate as precursor in the presence of urea. The catalysts were characterized by powder X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy analysis (XPS), N2 sorption and Ultraviolet–visible spectroscopy (UV–vis). The photocatalytic water splitting was investigated in the presence of methanol as sacrificial hole scavengers under UV light irradiation and visible light irradiation. The rate of H2 evolution significantly changed with the variation of doped metal atoms, and their doping concentrations. The Cu/N co-doped TiO2 catalyst prepared by microwave-assisted hydrothermal showed the highest photoactivity, with a rate of H2 production of 27.4 mmol g−1 h−1 under UV light irradiation and 283 μmol g−1 h−1 under visible light irradiation. On the other hand, Gokon et al. [51] produced hydrogen using 30mol%-Fe-, Co-, Ni-, Mn-doped CeO2-δ via a thermochemical two-step water-splitting cycle in the temperature range of 1200–1500 °C. Further, Gokon et al. [52] succeeded to produce hydrogen by thermochemical reactivity of 5–15 mol% Fe, Co, Ni, Mn-doped cerium oxides via a two-step water-splitting cycle in the temperature range of 800– 1150 °C.

5.2. Photoanodes for photoelectrocatalytic water splitting Monfort et al. [54] deposited nanocrystalline bismuth vanadate (BiVO4) by simple wet chemistry procedure on fluorine doped tin oxide (FTO) electrodes in order to construct visible light responsive photoanodes, which were implemented for photoelectrochemical H2 production by water splitting. The photoanodes were employed in photoelectrochemical cells for water splitting and H2 production under electric and chemical bias. Maximum hydrogen production rate was 0.15 mmol/ h under electric bias of 1.4 V vs Ag/AgCl plus 0.37 V chemical bias. Fàbrega et al. [55] presented a systematic study on the synthesis of monoclinic γ-WO3 obtained using pulsed laser deposition (PLD). A photocurrent of 2.4 mA/cm2 (60% of the optical maximum for a 2.7 eV gapmaterial) was obtained for films as thick as 18 µm. FE-SEM images revealed that tungsten oxide (WO3) films were actually formed by an array of oriented columns. Efficient hole extraction to the electrolyte was observed and imputed to a possible accommodation of the electrolyte between the WO3 columns, even for relatively compact films. This feature, combined with the detailed optical absorption and IPCE characterization, allowed Fàbrega et al. [55] to implement a double-stack configuration of WO3 photoanodes which resulted in a remarkable photocurrent density of 3.1 mA/cm2 with 1 sun AM 1.5 G illumination in 0.1 M H2SO4 electrolyte. Faradaic efficiencies of more than 50% were obtained without co-catalyst, which is one the highest values reported for pure WO3. By adding a 3 nm layer of Al2O3 by ALD, a faradaic efficiency of 80% was reached without diminishing the photocurrent density.

5. Further advancements in water splitting 5.1. Sonochemical water splitting The study of H2 generation revealed that at low-frequency ultrasound (20 kHz) the sonochemical water splitting is greatly improved 880

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5.3. MIEC membranes for H2 production from water splitting Recently, H2 production from water splitting reaction in mixed ionic-electronic conducting (MIEC) membrane reactors are in spot, where process intensification can be realized via water splitting coupling with other catalytic reactions in membrane reactors for both H2 and chemicals production. MIEC membranes are a kind of dense ceramic membranes composed of oxides that can simultaneously conduct electrons and oxygen ions. Therefore, only oxygen can permeate through the membranes. Li et al. [56] stated that significant amount of hydrogen can be produced at moderate temperature if a mixed ionic-electronic conducting (MIEC) membrane is used to remove the produced oxygen from water splitting reaction, although the equilibrium constant for water splitting is small. 6. Nanomaterials as photocatalysts for water splitting Strategies to design novel nanomaterials as photocatalysts for efficient production of hydrogen by water splitting includes: codoping, hydrogenation, defect engineering, sensitization, formation of heterojunction, metal decoration, band-edge-states modification, and designs of cell structures (tandem cell). Nanomaterials include: (1) oxides such as TiO2, Ta2O5, Fe2O3 and SrTiO3; (2) and nitrides such as GaN, graphitic carbon nitride, and Ta2N3. Additionally, these strategies can generally pertain to all materials, such as oxide and nitride semiconductors. The maximal conversion efficiency could be achieved by optimizing the electronic structures of photocatalysts and engineering the structures of cells [57]. Further nanomaterials were successfully prepared and implemented such as: NaTaO3 nanoparticles [58] and nickel nanoparticles [59].

Fig. 3. Evolution of the band gap as the number of atoms in a system decrease (from left to right), where δ is the so-called Kubo gap.

7.1. Clusters band-gap One of the consequences of the quantum-size regime (with the presence of discrete states in metal clusters) is the appearance of a sizable HOMO-LUMO band gap similar to that of semiconductors, where HOMO-LUMO is the highest occupied orbital and the lowest unoccupied orbital. As shown in Fig. 4, such a semiconductor like behavior is particularly significant for smaller clusters with band gaps widely exceeding of 1 eV. Zheng et al. [69] established a correlation of the emission energy with the number of atoms (N) for small gold clusters by observing that the energy emission (Egap) decreases with the increasing number of cluster atoms. They quantitatively fitted this correlation by the expression: Eemission=Egap= EFermi/N1/3, where EFermi is the Fermi energy of bulk gold (5.32 eV), as predicted by the Jellium model [65], this being a very good approximation for small metal clusters with N < 20. However, for larger clusters, it was observed that a small harmonic distortion term is required for N≥25. Fig. 4 shows characteristics band gaps of different metal clusters. It is known that, in general, band gap decreases when the cluster size increases.

7. Clusters as photocatalysts for water splitting Although, nanotechnology has received everyone's attention since the beginnings of XXI century, it was Michael Faraday on the 1850s [60] who started these studies on metal colloidal particles. Nanotechnology focuses on the synthesis, characterization, design, applications and manipulation of the matter at low levels, including: “nanoparticles” (aggregates of metal atoms where at least one dimension has a size from two to several tens of nanometers) and “metal clusters” (when particle size becomes comparable to the Fermi wavelength of an electron, < 2 nm). Due to their unique physical and chemical properties, metal clusters have become an interesting topic of research in recent years. This section focuses on the synthesis, characterization, study of the physical, catalytic and optical properties of metal clusters and their application for hydrogen production. Clusters display intermediate properties between the isolated atoms and the bulk metals and represent the most elemental building blocks in nature (after atoms). They are characterized by their size, which establish an overpass between atomic and nanoparticle behaviors, with properties entirely different from these two size systems. The percentage of atoms present on the surface of clusters increases with a decrease of the core size, which can strongly affect their properties. Bulk metal has a continuous band structure with free electrons oscillate, however when the size decreases to nanoparticle a splitting of the energies at the Fermi level is observed and therefore the valence density of states and the conduction band will be affected; changing from a continuous density of states (on the bulk metal) to discrete energy levels (Fig. 3). As the size becomes smaller and approaches the nanoscale, the wave character becomes more important and quantum mechanics becomes necessary to explain its behavior. The metal clusters dramatically exhibit, unique electronic and optical properties smoothly size dependent, such as molecule-like energy gaps [61–63], strong photoluminescence [64–66] and high catalytic properties [67,68].

7.2. Optical properties of metal clusters Significantly different from large nanoparticles, optical absorption spectra of clusters exhibit molecular like optical transitions (between the discrete energy levels corresponding to the last occupied orbital and the first unoccupied orbitals of the delocalized conduction electrons) [70,71]. Bands at lower energies will appear with the increase of the cluster size. The UV–Vis spectrum of a gold cluster solution left for reaction during 5 min can be observed in Fig. 1A in [73], showing that samples do not display any Surface Plasmon Resonance (SPR) band. For comparison purposes, Fig. 1A in [73] shows further the UV–Vis spectrum of a seed sample after a reaction time of 10 min (needed for the formation of seeds), showing the incipient appearance of the SPR band and after 24 h clearly shows the presence of the SPR band of Au particles. Therefore, the very small size of the particles formed and the absence of plasmon band points to the formation of small Au clusters with less than 12 atoms, as it was previously reported for Au [72,73]. The presence of such clusters can also be detected because of their luminescence properties [74], as shown in Fig. 1B in [73]. Assuming the simple spherical Jellium model, the number of atoms in the cluster can be calculated by the simple expression N=(EF/Eg)3, where EF and Eg represent the Fermi level (5.5 eV for bulk Au) and the HOMO881

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Fig. 4. Schematic comparison between band-gaps of some silver, gold and copper clusters (MN, where M = metal and N = number of atoms) and those of well-known semiconductors. Band-gaps (Eg) were calculated from the spherical Jellium model (Eg = EF/N1/3; EF = Fermi level) and the position of the conduction band, ECB, was estimated by the formula ECB = χ – EF – ½ Eg, where χ is the electronegativity.

Fig. 5. UV-V is spectra of seed solution at 5 min, 10 min and 24 h (A), luminescence spectra (B) [73].

The synthesized Au5, Au8, Au13, Au23 and Au31 clusters show UV (385 nm), blue (455 nm), green (510 nm), red (760 nm) and near IR (866 nm) fluorescence, respectively. One can see that the excitation and emission bands shift to longer wavelengths (low energy) with increasing cluster size. Fluorescence properties of metal clusters are very sensitive to their chemical environment, including the cluster size, solvent and surface protecting ligands [78,79]. By increasing the charge transfer from the surface ligands to the metal core an enhancement of the fluorescence can be observed [79]. This enhancement can be achieved by (1) increasing the electron donation capability of the ligands, (2) improving the electropositivity of the metal core and (3) using the protecting ligands with electron-rich atoms and groups.

LUMO energy band gap, respectively. The value of the band gap can be estimated by using the emission peak energy (412 nm, i.e. 3.0 eV), from which N = 6.2 can be obtained. Accordingly, it can be assumed that Au6 clusters can be kinetically trapped during the first 5 min of the reaction of the seed solution (Fig. 5). 7.3. Photoluminescence properties of metal clusters Luminescence is one of the major properties of metal clusters. Fluorescence of Au, Ag and Cu clusters has been extensively studied during the past decade. Several studies have demonstrated that the photoluminescence could be assigned to the electronic transitions between the highest occupied orbital and the lowest unoccupied orbital (HOMO-LUMO) [75,76]. Photoluminescence was first observed in noble metal by Mooradian [77], who observed visible emission from copper and gold films with a quantum efficiency of ~10−1 °. Sizedependent fluorescence properties of Au quantum dots were studied by the Dickson group [69]. They found that excitation and emission bands shift to smaller wavelengths with decreasing cluster size.

7.4. Synthesis of metal clusters Due to the highly necessary control of experimental conditions and a suitable purification and isolation method for the synthesis of metal clusters, more accurate synthesis techniques will be required than for nanoparticles. Metal clusters can generally be prepared through 882

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particles [98]. Additionally, Park et al. reported a synthetic method of uniform Ag clusters by reducing AgNO3 in the presence of oleylamine and oleic acid [99]. The process is greatly simplified compared to the previously reported accounts involving mild oleylamine as a reductant. This simple procedure could easily be scaled to achieve gram quantities. Platinum group metals of ultrasmall size have also been reported [100]. Teranish et al. reported a simple synthetic method for obtaining Pt clusters [100]. This method involves the reduction of simple alcohols in the presence of PVP as a protective polymer in a refluxing aqueous system. The size of the clusters could then be controlled from 1.9 to 3.3 nm by changing the alcohol or the concentration of the reagents. Specifically, smaller NPs could be synthesized by increasing the concentration of alcohol in water or increasing the amount of PVP. Li et al. reported the synthesis of monodisperse Pt clusters stabilized with peptides in aqueous solution at room temperature [101]. The specifically selected peptide molecule, P7A, was able to bind to the surface of the Pt clusters and regulate the nucleation and growth rates to obtain monodisperse Pt NPs with sizes in the 1.7–3.5 nm range. Pt clusters have also been synthesized with hydrophobic ligands. Uniform 2 nm-sized Pt clusters were obtained by the decomposition of platinum dibenzylideneacetone under mild conditions in the presence of n-octylsilane [102]. Pd clusters containing 7–8 atom shells have been synthesized by hydrogen reduction of Pd(II) acetate in acetic acid in the presence of imidazolium functionalized bipyridines [103]. Alternatively, using a simple redox-controlled method, 2.0–2.5 nm sized Pd clusters have been obtained [104].

Fig. 6. Schematic diagram for the chemical reduction synthesis of metal clusters.

“bottom-up” or “top-down” synthesis. The “bottom-up’’ approaches involve metal precursors which are reduced to atoms with a reducing reagent forming the metal clusters by the nucleation of the zero valent metal atoms. The ‘‘top-down’’ methods consist of a ligand etching process from nanoparticles to clusters. The different synthesis techniques of metal clusters are: electrochemical synthesis, photoreduction, microemulsion technique, chemical reduction, templating techniques and etching methods. 7.4.1. Chemical reduction by modified Brust-Schiffrin method metal precursors are first dissolved in an aqueous solution, subsequently, organic protecting ligands and reducing reagents are added into the solution to generate metal clusters and then they are transferred to an organic solvent by phase-transferring reagents such as tetraoctylammonium bromide. The core size and the surface properties of the metal clusters can be controlled effectively by adjusting the experimental parameters, such as the metal to ligand ratio, chemical structure of the protecting ligands, the nature of the reducing agent, reaction temperature and time, pH of the solution, etc. By using this method different metal clusters such as Au [73,80,81], Pt [82], Ag [83] and Cu [84] clusters have been successfully synthesized (Fig. 6).

7.4.3. Ligand induced etching of metal nanoparticles The etching capability of some ligands (like thiols) is used to synthesized clusters by removing the surface atoms from metal nanoparticles leading to stable quantum clusters (shell-closing magic numbers) [105]. Examples of this technique are the synthesis of AuCLs (Au8 or Au25) from mercaptosuccinic acid (MSA)-protected Au nanoparticles (4– 5 nm core diameter) by etching with excess glutathione varying the etching pH (~7–8 to Au8 and ~3 for Au25) [105]. Although the detailed mechanisms of the ligand- and precursorinduced nanocrystal etching have not yet been specified clearly (Fig. 7), this new synthetic strategy holds promise for preparing new types of sub-nanometer sized clusters containing only a few metal atoms. Bare Au clusters with closed shell structures such as Au13, Au55 and Au147 were obtained by the cluster beam method [106]. Schmid et al. developed one of the first synthetic methods for phosphine-stabilized Au

7.4.2. Template-based synthesis Template-based synthesis provides a predetermined environment for the clusters formation, which is favorable to produce clusters with well-controlled size and shape. Over the past few decades, templatebased methods have proved to be efficient synthetic techniques for preparing fluorescent metal clusters, using polymers [85], proteins [86–89], dendrimers [90,91] or even DNA [92–94] as templates. Copper clusters have been synthesized by a modified poly(amidoamine) (PAMAM) dendrimer as a template [95]. Another method to obtain nearly monodisperse Au clusters reported by Kim et al. involves using a dendrimer as a template [96]. The dendrimer-encapsulated Au NPs of 1.3 or 1.6 nm in size were prepared within G4, G6, and G8 poly(amidoamine) (PAMAM) dendrimers. The dendrimers were hydroxyterminated and thus uncharged although synthesis of the Au clusters was also done in cationic G8 dendrimers resulting in a greater polydispersity. Briefly, Au ions interact within the dendrimer core and are reduced by NaBH4. The particles are then coated with hydrophobic alkanethiol and extracted from the dendrimer. Using similar chemistries to Au, several synthetic methods have also been developed to synthesize Ag clusters. Ag clusters were stabilized by 3-mercaptophenylboronic acid (3-MPB) following the reduction of AgNO3 by NaBH4 [97]. As opposed to NaBH4, highly fluorescent Ag clusters were also synthesized via the reduction of Ag ions by photogenerated ketyl radicals which were optimized to allow a minimum exposure time to prevent quenching by the formed nano-

Fig. 7. Schematic representation of two possible routes for the formation of gold clusters by etching of mercaptosuccinic acid-capped gold nanoparticles (Amended and redrawn after Muhammed et al. [105]).

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balanced hydrophilic-lipophilic properties [129]. These colloidal systems are optically isotropic, with a very small characteristic size (2– 50 nm). Microemulsion is a commonly deployed technique for preparing metallic nanoparticles, because it allows one to precisely control the droplet size, and consequently, this droplet acts as a nanoreactor for preparing metallic nanoparticles [130,131]. Therefore, the two more interesting systems are water-in-oil microemulsions (water-swollen inverse micelles dispersed in oil) and oil-in-water microemulsions (oil-swollen direct micelles dispersed in water). There are several reviews in the literature describing the preparation of nanomaterials and, in particular, metallic nanoparticles in water-in-oil microemulsions [132,133], water-in-supercritical fluid microemulsions [133] and oil-in-water microemulsions [134]; however, there are a few publications about the synthesis of metallic clusters in microemulsions, due to the difficulties related to their characteristics.

clusters which resulted in moderately uniform, 1.4 nm-sized clusters by the reduction of Ph3PAuCl with diborane in benzene [107]. However, this synthetic process requires rigorously inert conditions and utilizes highly toxic diborane gas as the reducing agent. To make the conditions milder, the process was modified to use NaBH4 for the reduction and triphenylphosphine to passivate the particles, resulting in similar 1.5 nm particles [108]. In 1994, Brust and Schiffrin developed a more convenient and scalable synthetic method for thiol-stabilized Au clusters where the Au precursor (HAuCl4) is reduced by NaBH4 in the presence of dodecanethiol ligand resulting in relatively uniformly sized Au clusters of < 5 nm [109]. Since this seminal work, a tremendous amount of research has been conducted with focus on thiol-stabilized Au clusters. Au25 [110,111], Au38 [112,113], Au40 [114], Au68 [115], Au102 [116– 118], Au144 [119] and Au333 [120] were obtained through modified protocols of the Brust-Schiffrin method. Thiolstabilized Au NPs were also obtained by exchanging phosphine- stabilized Au particles with thiols [121]. Recently, various synthetic procedures for preparing Au clusters using other ligands have been reported. Highly water soluble nucleotidecapped Au clusters were obtained by reducing HAuCl4 in the presence of adenosine 5'-triphosphate [122]. In addition to small molecules, polymers have also been used as stabilizing agents [123]. Au clusters have also been obtained using imidazolium- based ionic liquids as solvents when the ionic liquids were amino-modified [124]. The neat reduction of HAuCl4 in the ionic liquid (amino-modified methylimidazolium) resulted in 1.7 nm particles capped by the ionic liquid solvent molecules.

7.4.6. Photoreduction synthesis Photoreduction is a simple method where metal salts are reduced by irradiating the system by UV-light in presence of some capping agent. Since the first report of size-dependent fluorescent Ag and Au clusters prepared by Zheng et al. [135] photoreduction synthesis of metal clusters has received more and more attention and copper clusters are under investigation [136]. 7.4.7. Other synthesis methods Nanoparticles of ferromagnetic metals (Fe, Co, Ni) or ferromagnetic metal oxides (iron oxide, ferrites) have widely been investigated because of their superparamagnetic properties. Many synthetic methods for producing iron oxide nanoparticles have been reported, but the sizes are usually larger than 3 nm [137–144]. To reduce the size of iron oxide nanoparticles below 3 nm, nanoparticles must be synthesized within systems that can constrain the size. Bonacchi et al. synthesized 1.8 nm sized maghemite NPs by precipitation in small constrained media [140]. Briefly, this method entails the formation of iron oxide clusters within a cyclodextrin host. Due to the uniform size of the cyclodextrin, the resulting particle size could also be controlled. Ultrasmall iron oxide nanoparticles were also prepared by mineralization inside the cavity of proteins [138]. Kim et al. reported a synthetic method for the large-scale production of monodisperse iron oxide clusters by thermal decomposition of iron-oleate complexes in the presence of oleyl alcohol at a relatively low temperature of about 250 °C [137]. Ex-situ sampling experiments revealed that oleyl alcohol acted as a mild reductant and lowered the reaction temperature producing a large number of nuclei. The large number of nuclei coupled with the limited amount of reduced iron leads to a controlled growth process resulting in uniform clusters. Glaria et al. reported the synthesis of reasonably monodisperse maghemite NPs with a diameter of ~2.8 nm following the hydrolysis and oxidation of an organometallic precursor, [Fe{N(SiMe3)2}2], in the presence of amine ligand as stabilizing agent [139]. The precursor readily decomposes exothermically at room temperature, leading to small-sized particles. The polyol method has also been used to synthesize iron oxide clusters. Park et al. reported the synthesis of nearly monodisperse and highly water dispersible 1.7 nm sized iron oxide NPs by refluxing Fe3+ ions in tripropylene glycol under O2 [141]. The same group also synthesized paramagnetic ~1 nm-sized gadolinium oxide (Gd2O3) NPs using a similar method [142]. Co NPs of 2–3 nm were obtained from organometallic precursors such as Co(η3C8H13), (η4C8H12) and Co[N(SiMe3)2]2 in the presence of diisobutyl aluminum hydride and PVP [145,146]. Similar to other syntheses presented, Co clusters were synthesized within the sizeconstrained nanopores of zeolites [147]. Monodisperse 2 nm-sized Ni clusters were synthesized by the thermal decomposition of nickel acetylacetonateoleylamine complexes in a phosphine solvent [148]. The strong reducing environment of the solvent leads to rapid nucleation which generates small particles. The initially formed Ni NPs were then easily oxidized to form NiO clusters. Monodisperse Ni

7.4.4. Electrochemical synthesis Electrochemical synthesis, first developed by Reetz [125] in 1994, is a very promising technique to produce metal clusters because of their simplicity. Compared to chemical reduction, the electrochemical method exhibits many advantages in the preparation of metal clusters, such as the low reaction temperature, large-scale yield, low cost of the initial materials, and easy manipulation of cluster size by tuning current, voltage, electrolyte, concentration of stabilizers etc. For example, Reetz et al. [126] prepared 1–5 nm palladium nanoparticles just by changing the current density (Fig. 8). 7.4.5. Microemulsion method Nanodroplets of water dispersed in oil using surfactants and/or amphiphilic block copolymers will be used as nanoreactors in order to obtain clusters with well-defined sizes. The cluster size can be tuned by amending the liquid core dimensions of microemulsions. Silver clusters AgN (N ≤ 10) were prepared in this way by Ledo-Suárez et al. [127] achieving novel photoluminescent and magnetic properties. Recently, small copper clusters as CuN, with N ≤ 13, showing UV photoluminescent properties have also been synthesized by this method [128]. Microemulsions are thermodynamically stable colloidal dispersions of two immiscible liquids (typically water and oil) that coexist in one phase due to the presence of a monolayer of surfactant molecules with

Fig. 8. Electrochemical mechanism formation of stabilized metal clusters. (Amended and redrawn after Reetz et al. [126]).

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Fig. 9. Schematic image of different stabilized nanoparticles. A) Ions adsorbed onto the particle surface, creating an electrical double layer which provides Coulombic repulsion and thus stabilization against aggregation. B) Two polymer-protected particles interacting. The region between the two particles becomes crowded as a high local concentration of polymer builds up. C) Electrosteric stabilization of a Pd particle by tetra(octyl) ammonium stabilizer (Amended and redrawn after Aiken and Finke [150]).

trometry, fluorescence spectroscopy, lifetime measurements, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and non contact atomic force microscopy (NC-AFM).

clusters of < 1.2 nm have also been prepared using hydrophobic dendrimers as templates [149]. 7.5. Clusters stabilization

7.7. Physical properties of metal clusters Metal clusters must be stabilized against the aggregation into larger particles. The stabilization of metal clusters can be classified as follows:

There is an important relation between the sizes of the material the number of atoms. When the size of a material decreases to 1–3 nm, the number of atoms constituting the material falls to less than 500. Consequently, clusters can be regarded as large molecules where the majority of the component atoms are located at the interface with the solvent [157]. This means that a greater number of the constituent atoms of clusters are exposed to the outer environment. This tendency is discussed in [158], where the smallest clusters are almost entirely exposed to the solvent; thus, there is essentially no true core. When considering the range of clusters sizes, the percentage of atoms on the surface of a 1.2 nm particle is 96% while a 3.1 nm particle exposes only 31% [158]. Below 1 nm, the particles are almost complete molecular dispersions, which is a partial reason for the differences in the macroscopic properties of clusters compared to clusters. Additionally, as many properties are derived from interfacial interactions of the surface atoms with the solvent, it is easy to see why clusters accentuate these properties compared to their bulk counterparts. Dominant surface states and the surrounding environment in clusters can also lead to unique physical properties. For example, when iron oxide clusters become nearly paramagnetic, it is due to the disordered surface spin. Additionally the surrounding matter, such as surface ligands, can have a dramatic effect on the overall properties of these particles. Au clusters can exhibit ferromagnetism when the surface atoms are coated with thiol ligand. In addition, CdSe clusters modulate their emission spectra based on attached ligand as well. Additionally, clusters have different quantum states compared to larger NPs due to their small volume and small number of atoms [70,159]. The controlled energy state modulates the reactivity compared to larger NPs [160]. Noble metal clusters show attenuated surface plasmon resonance and exhibit molecular-like optical properties due to loss of their metallic properties [71]. Then the generation of an energy gap near the Fermi energy induces the unique optical properties of metal clusters. Similarly, iron oxide clusters possess quantized spin states while larger iron oxide NPs follow a continuum.

(1) Electrostatic stabilization: results from the adsorption of ions to the electrophilic metal surface creating an electrical double layer, which results in a coulombic repulsion force between particles (Fig. 9A) [150]. (2) Steric stabilization: achieved by surrounding the metal center by large organic molecules (such as poly (N-vinyl pyrrolidone) (PVP), which prevent close contact of the metal particle centers (Fig. 9B). (3) By ligands (P, N, S donors): phospines, thiols, etc have been very exploited throughout the years as suitable cluster ligands for the “metallic full shell clusters”. (4) Electrosteric stabilization: strong coordination of bulky molecules such as surfactants at the surface of the particles, which is achieved well using tetralkyl salts, where the negatively charged anion is binded to the metal surface and the alkyl chains shield the metallic core like an umbrella (Fig. 9C). (5) Solvent stabilization: solvents as tetrahydrofurane (THF) or THF/ MeOH can act as stabilizers. Due to quantum effects, metal clusters possess discrete energy levels with the band gap increasing with decreasing cluster size (i.e. they lose the metallic character of metal nanoparticles characterized by the typical plasmon band) [151]. The small clusters, containing less than 50 atoms, behave as stable molecules. Due to their large band gap their stability is very high. At temperatures below approx. 120–150 °C, they are very difficult to be oxidize or reduce [152,153]. This stability is can be enhanced in those clusters with closed electronic shell [154], as a result there is no need for surface ligands to stabilize them. In such cases, clusters remain in solution stable for years, as it has been previously reported in electrocatalytic [155] and, recently, in catalytic studies [156]. It can be note that in the absence of strong ligands, such clusters are suitable for catalytic studies [155,156]. 7.6. Characterization techniques of metal clusters

7.8. Chemical properties of metal clusters

Characterization of metal clusters is essentially developed to deeply understand the structure–properties relationship. It should be noted that the tiny size of metal clusters make the characterization by some techniques difficult. For instance, the transmission electron microscopy (TEM) is a powerful tool in the studies of nanomaterials. However, due to the limited resolution, TEM is actually not a reliable method to obtain accurate core sizes (atom numbers) of sub-nanometer sized metal clusters. In this section, the techniques commonly used for the characterization of the metal clusters are summarized: UV–Vis spectroscopy, TEM, high resolution transmission electron microscopy (HRTEM), mass spec-

Clusters often exhibit unusual chemical properties and their reactivity is distinctly related to their size due to a combination of their large surface area and reformed structure [120,161–164]. For instance, the ability of Au clusters to oxidize CO well below room temperature [162]. Tsunoyama et al. reported that Au clusters of < 1.5 nm stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) presented higher reactivity for the aerobic oxidation of alcohols than larger sized Au NPs [163]. This enhanced catalytic activity was explained by the 885

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numerous studies have focused on non-Pt catalyst like for example Au clusters [68,182]. Ag [183] and Cu [184] clusters. The results showed that the activity of clusters against ORR increases as the cluster core size decreases. The catalytic behavior of sub-nanometric clusters has been rarely studied, due to the scarcity of well-controlled synthetic routes and limitations in the characterization techniques. Even though, recent advancements in these two fields are making it possible to study the catalytic activity and the factors affecting it. In the last few years, the developments in the synthesis of nanomaterials have been taken advantage of by means of microemulsion procedures to extend it for the preparation of metallic quantum clusters. In addition, in recent years, clusters have been reported as surprisingly good and selective catalysts in different reactions.

negatively charged Au cores resulting from electron donation from PVP. Additionally, the crystal structures of clusters are often markedly different from their bulk counterparts. While bulk Au and Au NPs > 3 nm exhibit a face centered cubic (FCC) structure, Au clusters often have non-FCC atomic packing structures [163]. For clusters, even minor atomic changes in particle size can lead to dramatic property differences. For instance, Au38 is smaller than Au40 by only 2 atoms; however, they show completely different Lewis acidities in the chelation of bidentate thiol ligands [165–167]. 7.9. Ferromagnetism of noble metal clusters Kubo developed the theory of paramagnetism of small particle of group 11 metals [168], although those metals (Au, Ag, and Cu) are well-known diamagnetic materials. It is hypothesized that if a metal NP has an odd number of atoms, then one electron in the particle must exist as an unpaired electron in the highest occupied state. When the number of atoms in a metal is small enough, the odd number effect becomes significant and the particles begin exhibiting paramagnetism instead of diamagnetism as a result. In fact, this effect has been observed in both metal and semiconductor clusters [169]. However, ferromagnetic behavior was observed for thiolcapped Au clusters [170–172] with similar results for clusters of Ag and Cu [173]. The Kubo theory then cannot explain these ferromagnetic properties because it is hard to say only one spin per particle induces exchange coupling. According to Crespo et al., this is a ligand dependent phenomenon as 1.4 nm Au NPs stabilized by weakly interacting amine ligands exhibited diamagnetism whereas those stabilized by thiol ligands exhibited ferromagnetism [172]. The aforementioned thiol ligands on the Au surface can induce 5d localized holes. These holes cause localized frozen magnetic moments due to the symmetry reduction from the two types of bonding (Au-Au and Au-S) and strong spinorbit coupling. Consequently, the local structure of the Au-S bond can account for the observed ferromagnetism. Because of the extremely large surface area of metal clusters, the ferromagnetic properties originating from the surface become more dominant than the diamagnetic properties originating from the greatly diminished metal core. Iron oxide NPs typically show size-dependent superparamagnetic properties by the Neel and Brown relaxation effect induced by thermal fluctuation. This occurs when the thermal energy exceeds the anisotropic energy [174,175]. However, the spins of surface atoms are disordered because of the differences between the states of the surface atoms and the bulk atoms. This is called the "spin canting effect," and the thickness at which the effect occurs in maghemite is ~ 0.5−0.9 nm [176–179]. Therefore, magnetic NPs can be considered as core/shell structures composed of a magnetic core and a magnetically disordered shell [137].

7.11. Implementation of clusters in hydrogen production In order to use metal clusters that have semiconductor properties and drive water splitting reaction with light, the metal clusters must absorb radiant light with photon energies of larger than 1.23 eV (≤ wavelengths of 1000 nm) to convert the energy into H2 and O2 from water. This process must generate two electron-hole pairs per molecule of H2 (2 × 1.23 eV = 2.46 eV). In the ideal case, semiconducting metal clusters having a band gap energy (Eg) large enough to split water and having a conduction band-edge energy (Ecb) and valence band-edge energy (Evb) that straddles the electrochemical potentials E° (H+/H2) and E° (O2/H2O), can drive the hydrogen evolution reaction and oxygen evolution reaction using electrons/holes generated under illumination (Fig. 10) [185,186]. There is growing concern about fossil fuel availability and related environmental effects due to the increasing number of fossil fuels powered vehicles and low efficiency of their utilization. A promising solution for these problems is offered by use of fuel cells, a direct energy conversion device that consumes H2, because they are inherently clean, efficient and compatible with renewable energy sources. Pt is the best catalyst for the hydrogen oxidation reaction (HOR). However, small amount of CO is present in H2 fuel as a byproduct of H2 production from organics, thus causing poisoning of Pt catalyst [187]. Due to the high price and limited resources of Pt, reduction of its contents in catalysts or substitution is needed. Although H2 cannot be dissociated on bulk Au due to its nobleness [188], theoretical and experimental studies showed that H2 can be dissociated on Au clusters

7.10. Catalytic properties Metals including gold, silver and copper, are usually quite inert and show little activity for reactive molecules adsorption, but when their dimensions are diminished to the nanoscale the properties of the materials exhibit a dramatic deviation from those of the bulk. For instance, gold clusters were found to be reactive at room temperature with oxygen and other molecules. Quantum chemical calculations indicate that such high reactivity is due to under coordination of the metal atoms forming the cluster [180]. In recent years, Au clusters have been extensively examined as active catalysts for CO oxidation and oxygen electro-reduction [181]. Even though, a field not explored much so far, these electrocatalytic properties make metal clusters promising materials in fuel cell applications. Oxygen electro-reduction (ORR) represents a critical cathodic reaction in fuel cells. Despite extensive research progress, wide-spread commercialization of fuel cells has been hindered partly because of the sluggish reaction dynamics for the ORR at the cathode and the high cost of Pt- based electrocatalyst. Therefore,

Fig. 10. The mechanism of photocatalytic hydrogen evolution from water (Amended and redrawn after Walter et al. [186]).

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Fig. 11. Schematic representation of the Au photodissolution and H2 photoproduction by Ag clusters (Amended and redrawn after Attia et al. [197]).

consist of no more than 50 atoms to be active for the HOR. The requirements for minimum and maximum band gaps impose opposite trends in activity vs. cluster size. Therefore, a volcano-type dependence of activity vs. cluster size is expected. Given the importance of the size, geometric structure and relative position of the HOMO-LUMO bands, it becomes imperative to analyze the relevance of the absolute value of the band gap. Following the previous example, when adsorbed on a metal surface, a metal cluster retains its semiconductor properties [197]. In the same publication, it is reported that, when irradiated with UV light, Ag3 clusters adsorbed on the tips of Au nanorods are able to oxidize them in the presence of atmospheric oxygen. A scheme of the mechanism can be seen in Fig. 13. The bandgap of Ag3 clusters is around 3.5 eV [198], and therefore, their maximum absorption stands in the UV region. When irradiated with UV light, an electron is excited in the cluster up to the LUMO level, and oxygen, as an electron scavenger, will trap this electron. The Au nanorod will then be oxidized by the highly oxidizing photogenerated holes in the Ag clusters. With the incorporation of a hole scavenger, like ethanol, in the system, this photodissolution is inhibited, since the hole scavenger oxidizes faster than the Au nanorod. It was found that when the electron scavenger (oxygen) is eliminated from the solution, irradiation in the presence of the hole scavenger produces hydrogen with high efficiencies up to 10% without any further optimization of the process [197]. The mechanism of this hydrogen photoproduction is also explained in Fig. 11. When Ag clusters are irradiated in the presence of a hole scavenger without any further electron scavenger, Au nanorods act as an electron sink. Accumulation of electrons causes the Fermi level of such nanorods to increase. When this Fermi level is pinned by the H2/H+ redox

and nanoparticles [189–191]. In addition, the ability of Au clusters to oxidize CO [192], Au's lower price, multiple times higher natural abundance, and greater accessibility compared to Pt make them an interesting candidate as a possible catalyst for HOR. Recently, it was demonstrated that 3.8 nm Au NPs become active for HOR after sonication treatment [193]. This was explained by the sonication-induced enrichment of the Au NP surface with small clusters. Similarly, Au can be activated (formation of small clusters) for electro-oxidation of methanol [194]. According to Buceta et al. [72], Au clusters (25 atoms) are up to 180 times more active for the hydrogen oxidation reaction (HOR) than bulk Au. They were created from catalytically inactive Au clusters (2−5 atoms) by electrochemical activation in the presence of Au ions. The HOR activity depends on the cluster's size and it is related to the fact that the position of the LUMO is below the redox potential for the HOR, while HOMO is below the H 1 s-Au d antibonding resonance. According Buceta et al. [72], the difference in the electrocatalytic behavior of small-, medium-, large- AuAQCs, and Au NPs. It must be considered that there is an upper limit for the cluster size to become electroactive, due to the nobility of Au. Further, H2 cannot be dissociated on bulk Au, because the Au Fermi level is located above the H 1 s-Au d antibonding resonance [188], thus filling H2 antibonding states and so causing repulsion between H2 and Au. Therefore, Au clusters with valence band (VB) above the antibonding resonance (Fig. 11) would not be active for HOR. Thus, the minimum band gap is 0.94 eV which corresponds to clusters larger than 200 atoms. Clearly, this is an overestimate since Au NPs larger than ≈ 100- 150 atoms (depending on the ligand) have bulk properties [195] and knowing that Au55 has band gap of around 1 eV [196]. Therefore, Au clusters should

Fig. 12. Schematic photocatalytic H2 evolution and back-reaction processes. Both PtO-clusters and metallic Pt nanoparticles (m-Pt NPs) cocatalysts can act as H2 evolution site on host photocatalyst surface, whereas the undesirable H2 back-reaction can be suppressed by PtO-clusters cocatalyst but facilitated by m-Pt NPs cocatalyst (Amended and redrawn after Li et al. [199]).

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Fig. 13. Hydrogen production by X/Y/G Y = semiconductor and G = graphene sheets.

nanocomposites

where

for Photocatalytic Hydrogen Evolution rate than that of pure CdS nanoparticles. The hydrogen evolution rate of the nanocomposite with graphene content as 1.0 wt% and Pt 0.5 wt% was about 4.87 times higher than that of pure CdS nanoparticles under visible-light irradiation. Similarly, Gao et al. [202] reported a new sulfonated graphene (SG)/ZnO/Ag composite as a highly efficient photocatalyst for hydrogen production for the first time. SG/ZnO/Ag composites were prepared through a step-wise approach, including growth of ZnO nanorods on SG sheets by the nanocrystal-seed-directed hydrothermal method and deposition of Ag clusters by the polyol-reduction process. The results show that SG/ZnO/Ag composites achieve a significant high hydrogen evolution rate of 2.36 mmol/h g−1, which is around 20 times, 3 times and 2.5 times faster than that of pure ZnO rods, ZnO/Ag and SG/ZnO, respectively. The outstanding hydrogen production activity of SG/ZnO/Ag can be attributed to the positive synergetic effects between SG sheets and Ag clusters, which enhance the light absorption ability and facilitate the charge separation activity. Hence, this study highlights that appropriate combination of co-catalysts with photocatalysts can greatly improve the photocatalytic hydrogen production performance. Fig. 13 shows the hydrogen production from Metal clusters/ Semiconductors/graphene nanocomposites. Using solar energy to produce hydrogen from water splitting metal clusters is believed to be a good choice to solve energy shortage and environmental crisis. However, the practical application of this strategy is limited due to the recombination of photoinduced electron-holes pairs like semiconductors but with high utilization efficiency of visible light as advantage unlike semiconductors. To solve this drawback, sacrificial agents such as methanol [203–205], ethanol [206–208] or sulfide/sulfite [209–211] are often added into the photocatalytic system with the aim to trap photogenerated holes thus improving the photocatalytic activity for hydrogen evolution. The reaction occurred in this case is usually not the water photocatalytic decomposition reaction [212]. For example, overall methanol decomposition reaction will occur in a methanol/water system, which has a lower splitting energy than water [213]:

X = clusters,

potential, hydrogen is produced in solution. Since the absolute value of the band-gap is the key point in this reasoning, this allows one to think of the possibility that bigger clusters, with smaller band-gaps, will absorb less energetic radiations, thus being able to present similar activities with visible light. All of these results open up a whole new catalytic source of materials not explored until recent times, where one can “tailor” catalyst a la carte. By selecting the metal and the size of the cluster, one can tune the position of the HUMO-LUMO bands and the value of the band-gap, and therefore, one can select the most efficient catalyst to solve problems in many important reactions that, from the subnanometric world, would not be possible. PtO-clusters were found to have a pivotal role in unidirectional suppression of undesirable H2 oxidation in photocatalytic water cleavage process [199]. More importantly, these PtO-clusters can also demonstrate excellent efficiency in hydrogen evolution rate. On the basis of experimental findings and theoretical models in this work, other high-efficient heterogeneous catalysts or catalytic systems might be developed for clean energy and environment applications. Pt in a higher oxidation state (that is, PtO) demonstrates remarkable HOR suppression ability, while its hydrogen evolution capacity is still comparable to that of the benchmark of conventional metallic Pt cocatalyst through comprehensive experimental and theoretical analysis (Fig. 12). This work confirms the role of PtO cocatalyst in governing the preferred direction of H2 reactions, and the finding may pave the way for developing other high efficiency catalysts for water splitting, water–gas shift reactions and fuel cells. Wu et al. [200] have explored a new concept of substantially increasing photocatalytic activity for H2 production of conventional semiconductors by modifying them with sub-nm Pt particles. By combining both experimental and theoretical approaches we have also developed new mechanistic insights into the 17 times increase in photocatalytic activity of Pt modified CdS catalysts (Fig. 12). Their results revealed strong structural and electronic interaction between clusters and substrate, which may substantially influence the electronic structures of both sub-nm Pt cluster and CdS surface, as well as the local surface potential. The resulting electronic structure of the cluster/ semiconductor interface may be the key component of significant enhancement of photocatalytic hydrogen production activity of this system. Given the difficulty of directly measuring such properties from experiments, our work offered new and important views to understanding the complicated stories behind this exceptional photocatalytic activity. CdS/graphene nanocomposites have also attracted many attentions for photocatalytic hydrogen evolution. Li et al. [201] investigated the visible-light-driven photocatalytic activity of CdS-cluster-decorated graphene nanosheets prepared by a solvothermal method for hydrogen production. These nanosized composites exhibited higher H2-production Graphene/Semiconductor Nanocomposites: Preparation and Application

CH3OH(I)↔ HCHO(g) + H2(g) ΔG1° = 64·1 kJ/mol HCHO(g) + H2O ↔HCO2H(I) + H2(g) ΔG2° = 47·8 kJ/mol HCO2H(I)↔CO2(g) + H2(g) ΔG3° = −95·8 kJ/mol With the overall reaction being: CH2OH(I) + H2O(I)↔CO2(g) + 3H2(g) ΔG0 = 16·1 kJ/mol In addition, graphene can be also coupled with various metal clusters to form graphene-metal clusters nanocomposites due to its unique large surface area, high conductivity and carriers mobility, easy functionalization and low cost. The unique properties of graphene have opened up new pathways to fabricate high-performance photocatalysts. The introduction of graphene into the nanocomposites mainly acts to promote the separation of charge carriers and transport of photogenerated electrons. The performance of photocatalysts is highly dependent on the semiconducting metal clusters photocatalysts and their surface structures such as the morphologies and surface states. Therefore, the development of novel photocatalysts is required. Nevertheless, there are still many challenges and opportunities for graphene-metal clusters nanocomposites and they are still expected to be developed as potential photocatalysts to address various environmental and energy-related issues. 8. Recent advancements Attia and Altalhi [214] controlled the synthesis of Sub-nm titanium dioxide (TiO2) clusters via the hydrolysis of TiCl4 and produced clean and surfactant-free oxide surfaces, where stable TiO2 nanoclusters with well888

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References

defined size distributions were obtained. The rate order of hydrogen photoproduction under UV light irradiation of the samples was increased by decreasing the size of TiO2 nanoparticles from 47 nm to 3 nm. The hydrogen evolution rate of TiO2 nanoclusters with size lower than 5 nm was 3.03 times and 1.96 times faster than that of 47 nm of TiO2 and 12 nm of TiO2, respectively. It was concluded that the ternary TiO2 nanoclusters can serve as a highly efficient catalyst for hydrogen production from water splitting. On the other hand, He et al. [215] incorporated Cu2(OH)2CO3 clusters onto the surface of TiO2, where Cu2(OH)2CO3/ TiO2 exhibited excellent photocatalytic H2 generation activity and Cu2(OH)2CO3 clusters functioned as efficient and stable cocatalysts. Feliz et al. [216] explored the photocatalytic hydrogen evolution reaction from water under homogeneous and heterogeneous conditions for the {Mo6Bri8}4+ cluster core based unit. The catalytic activity of {Mo6Bri8}4+ was enhanced by the in situ generation of [Mo6Bri8Fa5(OH)a]2−, [Mo6Bri8Fa3(OH)a3]2−, and [Mo6Bri8(OH)a6]2−. Full substitution of F− by OH− led to the formation of (H3O)2[Mo6Bri8(OH)a6]⋅10 H2O. The immobilization of the active {Mo6Bri8}4+ onto graphene oxide surfaces enhanced its stability under catalytic conditions. Cheng et al. [217] reported on a practical synthesis method to produce isolated single platinum atoms and clusters using the atomic layer deposition technique, where platinum-based catalysts have been considered as the most effective electrocatalysts for the hydrogen evolution reaction in water splitting. The single platinum atom catalysts were investigated for the hydrogen evolution reaction, where they exhibited significantly enhanced catalytic activity (up to 37 times) and high stability in comparison with the state-of-the-art commercial platinum/carbon catalysts. The X-ray absorption fine structure and density functional theory analyses indicated that the partially unoccupied density of states of the platinum atoms’ 5d orbitals on the nitrogen-doped graphene were responsible for the excellent performance. Yuan et al. [218] demonstrated a feasible strategy of two-dimensional (2D) nanojuctions to enhance solar hydrogen generation of the MoS2/TiO2 system. Loading of 2D MoS2 nanosheets on the surface of 2D anatase TiO2 nanosheets with exposed (001) facets greatly increased the interfacial contact. At an optimal ratio of 0.50 wt% MoS2, the 2D-2D MoS2/TiO2 photocatalyst showed the highest H2 evolution rate of 2145 μmol h–1 g–1, which is almost 36.4 times higher than that of pure TiO2 nanosheets.

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9. Conclusions According to this literature review, it can be concluded that: 1. Clusters display intermediate properties between the isolated atoms and the bulk metals and represent the most elemental building blocks in nature (after atoms). 2. Clusters are characterized by their size, which establish a bridge between atomic and nanoparticle behaviors, with properties entirely different from these two size regimes. 3. Reducing the size from the bulk material to nanoparticles produces a scaling behavior in physical properties in the later ones, due to the large surface-to-volume fraction. By further size reduction, entering into the subnanometric cluster region, physical properties are largely affected by strong quantum confinement. These quantum size effects (HOMO-LUMO gap), the small size and the specific geometry award subnanometric clusters with totally new and fascinating properties, including cluster photoluminescence, and enhanced catalytic activity. 4. Hydrogen production by water splitting is hindered mainly by the lack of low-cost and efficient photocatalysts. 5. Sub-nanometric metal clusters can be used as photocatalysts for hydrogen production in the presence of holes or electrons scavengers by water splitting. This illustrates the considerable potential of very small zerovalent, metallic clusters as novel atomic-level photocatalysts.

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