Atomically Precise Noble Metal Nanoclusters as Efficient Catalysts: A

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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Atomically Precise Noble Metal Nanoclusters as Efficient Catalysts: A Bridge between Structure and Properties Yuanxin Du,†,§ Hongting Sheng,†,§ Didier Astruc,‡ and Manzhou Zhu*,† †

Chem. Rev. Downloaded from pubs.acs.org by YORK UNIV on 03/22/19. For personal use only.

Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui 230601, China ‡ Université de Bordeaux, ISM, UMR CNRS 5255, Talence 33405 Cedex, France ABSTRACT: Improving the knowledge of the relationship between structure and properties is fundamental in catalysis. Recently, researchers have developed a variety of well-controlled methods to synthesize atomically precise metal nanoclusters (NCs). NCs have shown high catalytic activity and unique selectivity in many catalytic reactions, which are related to their ultrasmall size, abundant unsaturated active sites, and unique electronic structure different from that of traditional nanoparticles (NPs). More importantly, because of their definite structure and monodispersity, they are used as model catalysts to reveal the correlation between catalyst performance and structure at the atomic scale. Therefore, this review aims to summarize the recent progress on NCs in catalysis and provide potential theoretical guidance for the rational design of highperformance catalysts. First a brief summary of the synthetic strategies and characterization methods of NCs is provided. Then the primary focus of this reviewthe model catalyst role of NCs in catalysisis illustrated from theoretical and experimental perspectives, particularly in electrocatalysis, photocatalysis, photoelectric conversion, and catalysis of organic reactions. Finally, the main challenges and opportunities are examined for a deep understanding of the key catalytic steps with the goal of expanding the catalytic application range of NCs.

CONTENTS 1. Introduction 2. Brief Summary of the Synthetic Methods and Characterization of NCs 2.1. Synthesis Methods 2.1.1. Direct Synthesis Method 2.1.2. Size-Focusing Method 2.1.3. Ligand-Exchange Method 2.1.4. Chemical Etching Method 2.1.5. Reduction Method 2.1.6. Metal-Exchange Method 2.1.7. Separation Method 2.1.8. Intercluster Reaction Method 2.1.9. Anion-Template-Assisted Method 2.2. Characterization 3. Theoretical Prediction of the Catalytic Performances of NCs 4. Noble Metal NCs in Catalysis 4.1. Electrocatalysis 4.1.1. Electrocatalytic Reactions in Fuel Cells 4.1.2. Electrocatalytic Reaction in Water Splitting 4.1.3. Electrocatalytic CO2 Reduction 4.1.4. Other Electrocatalytic Reactions 4.2. Photocatalysis 4.2.1. Photocatalytic Decomposition of Pollutants 4.2.2. Photocatalytic Water Splitting © XXXX American Chemical Society

4.2.3. Selective Photocatalytic Organic Transformation Reactions 4.3. NCs in Photoelectrocatalysis of Energy Conversion 4.4. Catalysis of Organic Reactions 4.4.1. Catalytic Selective Oxidation 4.4.2. Catalytic Selective Reduction 4.4.3. Catalysis of Coupling Reactions 4.4.4. Other Catalytic Reactions 5. Concluding Remarks and Future Prospects 5.1. Multimetallic NCs To Investigate the Composition Effect 5.2. Rational Design of Ligands To Optimize Catalytic Activity and Selectivity 5.3. Designing NCs with Inherent Uncoordinated Active Sites 5.4. Functionalizing the Support To Construct Highly Efficient and Stable NC-Based Composite Catalysts 5.5. Developing Chiral Catalysis by NCs 5.6. Capturing the Intermediate in Catalytic Reactions 5.7. Building More Realistic Theoretical Models in Catalysis

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Special Issue: Nanoparticles in Catalysis

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Received: November 28, 2018

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DOI: 10.1021/acs.chemrev.8b00726 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 5.8. Developing Non-Noble-Metal NC Catalysts 5.9. Some Technical Difficulties Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments Abbreviations References

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were synthesized, including Moiseev’s cluster [Pd570±30(phen)63±3(OAc)190±10] that catalyzed the oxidative acetoxylation of various substrates with high turnover numbers and selectivities, but their charge state and exact structure were not known with certainty.4 The synthesis of various noble metal NPs (mixtures of NCs) upon stabilization by polymers and solvents for efficient homogeneous catalysis burst in the early 1990s, in particular by the groups of Reetz5 and Bönnemann.6 Such Pd and Pt NPs on the 2−3 nm scale were very active catalysts in, inter alia, Heck cross-coupling reactions7 and enantioselective hydrogenations,8 respectively. In contrast to metal carbonyl NCs, which are robust, these NCs and NPs are weakly stabilized by solvents and polymers. Finke’s group reported Ir and Rh NPs (Ir∼300,neutral) stabilized by polyoxometalate salts, and these NPs were excellent olefin9 and arene10 hydrogenation catalysts. In summary, all of these NCs and NPs (NPs were often and are still sometimes misleading called NCs) were either well-defined with uncertain catalytically active species or very active catalysts with uncertain structure and definition. Under these conditions, it was not possible to establish a clear relationship between their structure and their catalytic activity. Extensive efforts have been devoted by nanoscientists generation after generation toward a clear knowledge of the correlation between structure and activity of the catalysts. Thanks to the development of nanomaterial growth theory and synthesis technology, more and more nanomaterials with adjustable shape (e.g., nanocubes, nanooctahedra, nanotetrahedra, nanodecahedra, nanoicosahedra) and single size have recently been synthesized in a controllable manner,11 providing a good platform for elucidation of the relationship between the structure and chemical behavior of the catalyst during the catalytic process. Xiong et al. prepared Pd nanocubes and nanooctahedra with exposure of (100) and (111) facets, respectively, and compared the O2 activation processes on the different surface facets. They found that singlet oxygen (1O2) was preferentially formed on the (100) facets, and thus, particles with (100) facet exposure are more favorable for the catalytic reaction involving O2.12 In benzene hydrogenation, Pt nanocuboctahedra covered by a mixture of (111) and (100) facets showed worse selectivity than Pt nanocubes enclosed by (100) facets.13 Kim et al. prepared a series of different-sized Au nanodot arrays and utilized the localized surface plasmon resonance (LSPR) effect to enhance the photoelectrochemical water-splitting performance of TiO2. They found that the catalytic activity was related to the quality of plasmonic metal.14 The above cases are examples showing that uniform nanocatalysts are of major importance in correlating the structure with the performance. However, uniform nanocatalysts (i.e., regular morphology and monodispersity) are still limited to the 10−100 nm size range, and obviously this is not enough for a fine understanding of the relationship between the catalyst structure and performance at the molecular or atomic scale. In general, the smaller the size, the larger is the specific surface area, and hence the higher is the catalytic activity and the more efficient the atom-utilization rate. Therefore, the desire for ultrasmall nanocatalysts (e.g., 46) with precise control of size and coverage.329

Figure 12. Schematic diagram of the instrument used for the preparation of size-selected NCs in ultrahigh vacuum. Reprinted from ref 89. Copyright 2016 American Chemical Society.

Because of the independent and precise regulation of the size and distribution of the clusters, the system was very suitable for study of the structure−activity relationship. Compared with the standard Tanaka TKK catalysts, Pt20 and Pt>46 showed significant improvement, i.e., 2- and 3.5-fold enhancements in surface-area-normalized specific activities and more than 2- and 6-fold improvements in mass-normalized specific activities, respectively, in the electrocatalytic ORR (Figure 13A,B). At first, however, the absence of a correlation between the activity and the cluster size made them confused, so they considered that another factor may be the key point in influencing the ORR activity; specifically, they found that if the edge-to-edge interparticle distance was smaller than 1 nm (which means that the coverage of clusters becomes higher), the ORR activity became higher than that of commercial Pt/C whatever the size of the cluster. To eliminate the erroneous judgment resulting from the aggregation of clusters during catalysis, they performed HRTEM in order to observe the distribution of clusters before and after the ORR. They concluded that the interparticle distance should not be the fundamental cause. On the basis of previous experimental and theoretical reports, it was estimated that the ORR activity was influenced by either the specific anion adsorption or the position of the metal d-band center. They concluded that these factors originated from the electric field effect, i.e., the distribution of the electric potential in the electric double layer located between the NCs (the potential at the compact layer plane, not the conducting surface) was the P


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Figure 13. (A, B) Specific activity and mass activity of various Pt NCs for the ORR. (C) Relationship between the ORR specific activity and interparticle edge-to-edge distance for different Pt NCs. (D) Simulated potential distributions of 4 nm Pt NCs with various edge-to-edge distances. Reproduced with permission from ref 329. Copyright 2013 Springer Nature.

for carbon oxidation during the in situ ORR was ascribed to the higher core-level binding energy (Figure 14). This was taken into account by the higher Pt 4f binding energy in Pt7 that was still unclear and needed further research.

determining factor for the electrode coverage with oxygenated species and ORR activity (Figure 13C,D).329 If the distance between clusters was smaller than the Debye length, the electric double layers would overlap, and then the potential at the compact layer would increase and the potential drop would decrease, which is positively correlated with the charge density of adsorbed ions. Anderson et al. also prepared mass-selected Ptn NCs (n ≤ 11) on an electrode in ultrahigh vacuum and employed them to carry out the electrocatalytic ORR in situ or ex situ without or with air.330 A typical glassy carbon electrode (GCE) was chosen for deposition of the clusters because of its chemical inertness under common conditions. These authors found that during the in situ ORR, Pt7 showed ORR behavior obviously similar to that of polycrystalline Pt foil (Ptpoly) and nanometer-sized Pt particles (Ptnano), but the Pt7 activity (per Pt atom) was twice as high as that of larger Pt particles. This enhancement results from the fact that for Pt7 all of the Pt atoms are located on the surface. In contrast, Pt4 showed a strange change of current, i.e., in a potential range impossible for water electrolysis, a huge current appeared, gas evolved, and electrode damage occurred (color changed from light gray to black) in O2-saturated solution, which was attributed to carbon oxidation at a low overpotential. During the ex situ ORR with air exposure, Pt7 still showed a normal ORR current with no unusual carbon oxidation or electrode degradation. Compared with the in situ ORR without air exposure, not only the carbon oxidation but also the ORR on Pt4 were suppressed under air exposure, and the electrode performed like Pt-free glassy carbon. This phenomenon was called the “air exposure passivation effect”. A series of detailed experiments were conducted to investigate the effect of size on the ORR activity. The reason that Pt7 and Pt10,11 were inactive

Figure 14. Relationship between the binding energy of Pt 4f7/2 and the Pt cluster size. Reprinted from ref 330. Copyright 2013 American Chemical Society.

After two years this group used the same approach to deposit mass-selected Ptn NCs (n = 1 to 14) on ITO in ultrahigh vacuum.331 Considering the carbon erosion in the previous work, these authors chose ITO as a non-oxidizable substrate. Among the size series, Pt10/ITO exhibits the lowest mass activity, but this activity is still an order of magnitude higher than that of 5 nm Pt particles on ITO. On the basis of the successful inverse correlation between the EOR activity and Pt 4d binding energy in the previous work,323 they tried to obtain a rule Q


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layer, resulting in precise control of the number of metal atoms wrapped in the cagelike dendritic architecture. The important advantage of this approach is the “site-isolation effect”, which prevents clusters from aggregation via fusion. Pt clusters with three sizes were applied in the ORR. The smallest cluster, Pt12, showed the highest catalytic activity, i.e., a 13-fold enhancement compared with that of the commercial Pt/C (Figure 17). These clusters with small sizes show high activity in the ORR in the region of high overpotentials but no electron transfer enhancement in the region of low overpotentials. Yamamoto et al. explained the phenomena by the number of atoms on the surface, assuming a face-centered-cubic or hexagonal closepacked structure in subnanometer clusters. Considering the two aspects of ORR, these authors further introduced a foreign metal (Sn) into Ptn in synthesizing bimetallic clusters. As a result of the synergistic effect, the PtnSn(28−n) exhibited a higher onset potential and larger kinetic current compared with a mixture of Pt and Sn clusters. The authors proposed a hypothesis involving enhanced activity in bimetallic clusters depending on the energy of the d-band center or the surface coordination. The reaction and electron-transfer processes were independent of the cluster size and composition, respectively. For the chemical reaction, the determining factor is the high ratio of surface atoms of the cluster. In the beginning of the decade there were two kinds of conflicting viewpoints about the size effect in ORR electrocatalysis. On one hand, Pt particles with sizes smaller than 2 nm were reported to be unsuitable for the ORR because of toostrong Pt−O binding.336 On the other hand, NCs were reported to have better catalytic activities in the ORR.332,337 Therefore, the Yamamoto’s group thought that there eventually was another factor influencing the ORR activity besides the size effect. Thus, they continued to apply their dendrimer approach to synthesize two kinds of Pt clusters with different numbers of atoms, one with the “magic” number 13, and the other with the number 12. Interestingly, Pt12 showed better activity than Pt13, with a 2-fold enhancement in mass activity, even though there was only a one atom difference (Figure 18A).70 The high ratio of surface atoms theory was not applicable here any longer. Compared with the activities of Pt28 and Pt60 reported previously, Pt13 showed a lower activity. Thus, the atomnumber-specific geometric structure of the cluster possibly also influences the ORR activity, which is no longer dependent only on the size. In contrast to the high symmetry and stability of icosahedral Pt13, Pt12 present several structural possibilities. The relatively stable structures with C2v, D3h, and D3v symmetry were regarded as Pt12 configurations following DFT calculations. These structures all have atomic coordination behaviors different from that of Pt13. As reported earlier, the structural transition occurs between n = 12 and n = 13.338 After n = 13, adding more atoms does not cause a structural transition, and the larger clusters all have similar structures based on icosahedral Pt13. Pt13 presents a high oxygen binding energy (ΔE0 = −0.1 eV) based on the known “volcano-shaped” relationship, and Pt12 has an ideal ΔE0 (0.2−0.3 eV), which explains why Pt12 has a higher activity than Pt13 (Figure 18B). These authors synthesized a fourth-generation phenylazomethine dendrimer with a triphenylpyridylmethane core (DPAG4-PyTPM) acting as template to form Ptn NCs (n = 12− 24).339 Taking Pt19 as an example, the structure was confirmed on the basis of icosahedral Pt13 (with a Pt13 core and three Pt dimers at edge sites) by DFT calculations and aberrationcorrected high-magnification HAADF-STEM images. Pt19

concerning the Pt 4d binding energy and ORR activity, but there was no obvious relationship. The ORR onset potential increased with increasing cluster size, but the Pt 4d binding energy fluctuated with the size change. The authors disclosed another interesting rule concerning the size-dependent branching between water and hydrogen peroxide production in the ORR. The oxidative peak near 1.2 V is attributed to the oxidation of hydrogen peroxide in the positive sweep mode, so the peak intensity is a probe of the amount of H2O2 produced. The H2O2/H2O ratio gradually decreased from 0.53 (for Pt1/ ITO) to 0.24 (for Pt14/ITO) and to 0.04 (for Ptnano/ITO). The origin of this effect is either the electronic structure or the geometric structure. The O2 dissociation is affected by the electronic structure of the cluster, but the Pt 4d binding energy has nothing to do with the H2O2/H2O ratio, so the electronic structure factor is eliminated; the most reliable reason is the geometric structure of the cluster. The smaller cluster with higher H2O2/H2O ratio has no appropriate sites to dissociate O2; thus, the ORR process occurs according to the two-electron pathway. The larger cluster with lower H2O2/H2O ratio has relatively efficient sites for O2 dissociation. Although the H2O2/ H2O ratio of the larger cluster is lower, it is still higher than that in Pt particles and bulk Pt. This means that the efficiency of O2 dissociation on large clusters is still inefficient compared with Pt particles and bulk Pt. Thus, the four-electron process on large clusters is not thorough compared with that on Pt particles and bulk Pt. However, H2O2 branching was 20% for 5 nm Pt particles and 100% for the smallest Ptn/ITO, suggesting another useful strategy to control the catalytic selectivity, i.e., to tune the size of the active site (Figure 15).

Figure 15. Relationship between the H2O2/H2O ratio in the ORR and the Pt cluster size. Reprinted from ref 331. Copyright 2015 American Chemical Society.

Although these mass-selected clusters with uniform and exact numbers of atoms have been successfully prepared using the gasphase synthesis approach in ultrahigh vacuum, the sophisticated equipment, complex operation steps, and extremely low yield are realistic bottlenecks for real-world catalytic applications of such an approach. Yamamoto et al. synthesized a series of ultrafine Pt NCs with 12, 28, and 60 atoms using spherical phenylazomethine dendrimer templates (Figure 16).332 As early as in 1998, polyamidoamine (PAMAM) dendrimers had been known as macromolecular ligands for the synthesis of NPs, in which the number of atoms is predetermined by the initial dendrimer loading.333−335 In order to obtain clusters at the subnanometer scale, Yamamoto’s phenylazomethine dendrimers were ideally designed, however. The complexation of this latter type of dendrimer is stepwise, going from the inner layer to the outer R


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Figure 16. Synthesis of Pt NCs with 12, 28, or 60 atoms using the spherical macromolecular template. Reproduced with permission from ref 332. Copyright 2009 Springer Nature.

Figure 18. (A) Relationship between kinetic current density and the cluster weight of Pt12 and Pt13 NCs. (B) “Volcano-shaped” relationship between kinetic current density and oxygen adsorption energy for Pt12, Pt13, and Pt(111). Reprinted from ref 70. Copyright 2013 American Chemical Society.

the high activity of Pt19 was attributed to the unique structure with edge sites (Figure 19B). Although Pt-based materials have been regarded as the most active catalysts in fuel cells, their high cost has restricted their applications on a large scale. Therefore, many studies have focused on non-Pt- or little-Pt-based catalysts. Chen et al. transferred attention to Au. They believed that small Au clusters might be active in the ORR340 because earlier reports had shown that small Au particles were active in catalysis of CO oxidation.341−343 These authors prepared a series of Au clusters (Aun, n = 11, 25, 55, 140) by the solution-phase thiolate ligand protection method and evaluated their ORR activities (Figure 20A). In a comparison of the onset potentials and peak current densities, Au11 showed the best activity in the ORR, and the ORR activity decreased with increasing core size (Figure 20B). The size effect was ascribed to the large fraction of surface Au atoms that have low coordination numbers in smaller clusters. Furthermore, theoretical calculations demonstrated that the smaller Au clusters had narrower d bands, energies closer to the Fermi level,344,345 and therefore easier O2 adsorption. Besides, the smaller clusters (n = 11, 25, 55) underwent the four-electron pathway, whereas only the larger Au140 NCs proceeded

Figure 17. Relationship between the kinetic current density for the ORR and the cluster weight. Reproduced with permission from ref 332. Copyright 2009 Springer Nature.

exhibited the highest activity among these Ptn NCs (n = 12−24), with 3.7-fold higher mass activity than Pt13. They also investigated the ORR activities of Ptn NCs protected by PAMAM to explore the dendrimer-related origin of the enhanced activity. The clusters surrounded by PAMAM generally showed lower activities than those protected by DPAG4-PyTPM because of the tight surroundings, and the trend was almost the same in both kinds of NCs with other atom numbers (Figure 19A). Thus, the huge difference in activity between Pt19 and Pt13 did not reveal a dendrimer dependence but was caused by the unique specific structure related to the number of atoms. By calculations, the edge sites of the NCs were considered as having ideal oxygen binding energy, and the binding energy of the icosahedral kernel was too strong. Thus, S


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Jeyabharathi et al. utilized an electrodeposition method to synthesize cetyltrimethylammonium bromide (CTAB)−Au clusters and in situ spectroscopy to characterize the evolution process from cluster to particle. 346 When the CTAB concentration was 50 mM, the clusters exhibited typical molecular-like adsorptions without surface plasmon resonance bands. Analysis by MALDI-TOF-MS showed that the clusters were mixtures with various numbers of atoms, including Au5, Au6, Au9, Au11, and Au13, and among them Au5 was the main kind. The CTAB−Au clusters exhibited the direct four-electron pathway in the ORR process, whereas the bulk polycrystalline Au went through the indirect two-electron pathway. When the CTAB concentration was 0.1 mM (lower than the critical micelle concentration), the clusters showed an interesting phenomenon in multiple linear-sweep voltammograms (LSVs) during the ORR process. Transfer was observed from the initial direct four-electron pathway to the indirect two-electron pathway after the first scan. Therefore, questions appear involving formation of the cluster phase, the transformation from clusters to particles, and stability. In view of the observation of positively charged Au&+ as the active site for CO oxidation,347 charged active sites may be suggested to play an important role in catalytic processes, including reactant adsorption, intermediate binding, and product desorption. Despite reports on charged Au species in electrocatalytic reactions, the influence of a charged active site was usually ignored, contrary to other key parameters, including in particular the particle size and d-band energy.286,340,348,349 Therefore, Kauffman et al. investigated the role of charged active sites in electrocatalytic reactions by using Au25 NCs with different charge states (Au25q).350 Au25q NCs were supported on

Figure 19. (A) Relationship between mass-specific activity and number of Pt atoms for NCs with different dendrimer ligands. (B) Atomic structure of Pt NCs with various numbers of atoms. Reproduced with permission from ref 339. Copyright 2015 Wiley-VCH.

according to the two-electron pathway, as calculated from the Koutecky−Levich plots (Figure 20C,D).

Figure 20. (A) MALDI-TOF-MS spectra and (B) rotating disk electrode (RDE) polarization curves for the ORR at a rotation rate of 3600 rpm for Au NCs with various numbers of atoms. (C) RDE polarization curves of Au11 NCs with various rotation rates. (D) Koutecky−Levich plots of Au11 NCs at various potentials. Reproduced with permission from ref 340. Copyright 2009 Wiley-VCH. T


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Figure 21. RDE polarization curves for (A) CO2 electroreduction, (B) CO electrooxidation, and (C) O2 electroreduction at a rotation rate of 2500 rpm for Au25 NCs with various charges (on the left) and corresponding TOF and reactant binding energy relationships (on the right). Reproduced with permission from ref 350. Copyright 2014 Royal Society of Chemistry.

series and the lowest ORR TOF. The stability and purity of Au25q before and after the catalytic reactions were confirmed by in situ X-ray absorption spectroscopy. Because of the precise and robust structure of Au25, the correlation between active-site charge and catalytic activity was first successfully established. The effects of active-site charge in other electrocatalytic reactions were also studied (they are described in the corresponding sections). Lu et al. also reported three charge states Au25 NCs for the electrocatalytic ORR.351 Au25 showed an catalytic activity trend similar to that reported by Kauffman et al. The Au25− NCs displayed the most effective two-electron reduction pathway with maximum H2O2 production among the three different charged Au25 NCs (Figure 22). The catalyst ink was drop-cast onto a polished glassy carbon electrode. The

carbon black, and the mixed suspension was drop-cast onto a glassy carbon electrode. They first performed a series of careful experiments to exclude spontaneous charge transfer caused by physisorption between reactants (e.g., CO2, CO, or O2) and Au25q (Figure 21). The Au25q NCs with fully ligand-covered configurations were used in DFT calculations to meet with real cases. The calculation results also indicated weak reactant adsorption on the surface of Au25q. Three different charged Au25 NCs (with identical surface structure) produced equivalent electron transfer numbers of 3.0 ± 0.3 in the ORR and showed an activity trend of Au25− > Au250 > Au25+ (Figure 21C). There was an antidependence relationship between the calculated OH− binding energy and the measured turnover frequency (TOF), and Au25+ showed the largest OH− binding energy in the U


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Figure 22. (A) H2O2 percentages and (B) electron transfer numbers for Au25 NCs with various charges at various potentials. Reproduced with permission from ref 351. Copyright 2014 Royal Society of Chemistry.

authors suggested that the high activity of Au25− in H2O2 production was due to electron transfer from the Au25− core into the LUMO (π*) of O2 to form the superoxide radical anion O2•−. This charge-state-induced catalytic activity opens up new horizons in the investigation of the mechanisms of such catalytic reactions. The catalytic activity in electrocatalytic processes depends on the intrinsic activity of the catalytically active metal center, but the conductivity of the electrode material also plays a critical role. Graphene is a chemically inert material with good conductivity that is very suitable to combine with metal nanomaterials to form composite electrocatalysts with improved activity and stability. Kwak et al. prepared Au25 and reduced graphene oxide (rGO) composites and controlled the number of Au25 layers from 0 to 15 by tuning the cluster:rGO ratio.352 In the redox process, Au25−rGO showed a significantly enhanced current (13.4 μA at the oxidation peak) compared with that of Au25 (4.5 μA), and the redox potential difference decreased from 127 to 60 mV. Both of these values indicated that the electron transfer process was enhanced by combining Au25 with rGO (Figure 23A). As the number of Au25 layers increased, the redox peak current increased, but the redox potential difference almost maintained the 60 mV value expected for a reversible singleelectron-transfer step (Figure 23B). The authors used Au25− rGO to catalyze the ORR in a neutral environment. The onset potential showed a positive shift, and the catalytic current increased increasing Au25 film thickness (Figure 23C). Besides the catalytic activity, the effect of the number of Au25 layers on the electron transfer dynamics during the ORR process was investigated by chronoamperometry and electrochemical impedance spectroscopy (EIS) analyses. The ORR electrocatalytic rate constant drastically increased with increasing number of Au25 layers (Figure 23D). This was attributed to the porous structure of the composites, which provided a confined space to increase the residence time of O2 on the electrode surface (Figure 23E).353,354 The charge transfer resistance (Rct) decreased with increasing Au25 film thickness, which also confirmed the phenomenon (Figure 23D). Furthermore, in the rotating ring-disk electrode (RRDE) experiments, the Au25− rGO composite showed a more positive onset potential (−0.4 V) and a higher limiting current density (311.3 μA/cm2) compared with Au25 (−0.51 V, 193.8 μA/cm2) and rGO (−0.52 V, 194.7 μA/cm2). The calculated H2O2 percentage produced by Au25−rGO was only 8%, while for Au25 and rGO the numerical was up to >95%. The Au25−rGO composites underwent a four-electron pathway, as measured by RRDE

and RDE voltammograms, whereas for Au25 and rGO the reaction pathway involved a two-electron transfer mechanism, which means that Au25−rGO provided a much more efficient electron-transfer process (Figure 23F). Besides Pt and Au NCs, Yang et al. reported 2mercaptobenzothiazole-protected Ag NCs with various numbers of atoms (from 2 to 5, as confirmed by MALDI-TOF-MS) to catalyze the ORR.355 These Ag NCs showed a dramatic activity with a more positive onset potential and a higher current density compared with commercial Pt/C. The NCs mentioned above used in electrocatalysis for fuel cells are mostly singlecomponent (Pt, Au or Ag). In terms of economy, developing other non-noble-metal and multicomponent NCs is very essential for applications. Because of the intrinsically low activity of non-noble NCs in both anode and cathode reactions, related research is still in its infancy.270 As mentioned above, PtnSn28−n NCs have shown better performance in the ORR than Ptn NCs,332 suggesting that synergistic effects are very important. This was also confirmed by Mahata et al. as mentioned in section 3 concerning the theoretical predictions.310 These authors calculated the energetic stability, thermal stability, and dissolution limit of cuboctahedral Ti19@Pt60 core−shell NCs and pure Pt79 NCs to evaluate their applicability for the ORR. The Ti19@Pt60 NCs showed a significantly improved ratedetermining step in comparison with Pt79. Therefore, on the basis of synergistic effects for multicomponent NCs, studies based on such catalysts need to be developed. 4.1.2. Electrocatalytic Reaction in Water Splitting. With the continuous development and progress of society, humans’ demand for energy is becoming higher and higher. The excessive reliance on energy has resulted in the depletion of fossil energy and environmental pollution. Therefore, developing efficient, clean, and renewable energy is a key challenge that must be solved as soon as possible in this century. During the past 50 years, renewable resources such as solar, geothermal, and wind power have been vigorously developed. These energy sources are relatively unpredictable and somewhat unable to meet the actual demand, however. Hydrogen, an ideal clean energy carrier, has many advantages such as high energy density, recyclability, and pollution-free properties of the combustion products, and therefore, the development and utilization of hydrogen energy has attracted widespread attention. At present, hydrogen is mainly produced from fossil resources by a steam reforming process, which not only consumes fossil energy but also releases carbon dioxide. On the other hand, water splitting by electrolysis or photolysis is considered a V


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theoretical voltage. The lowest voltage that goes beyond the thermodynamic voltage is called the overpotential. Thus, there is a great demand to develop high-performance catalysts to decrease the overpotential. Even though various materials have recently been synthesized to apply in catalytic water splitting, they either are constrained by high price (e.g., Pt, RuO2, IrO2) or present trouble with poor stability (e.g., transition metal compounds). Therefore, gaining insight into the relationship between catalyst structure and activity is essential to guide researchers toward the design of better and more practical catalysts. Small NCs with precise structure and components, as new materials, provide us an opportunity to bridge the structures and properties of these catalysts. This part mainly summarizes the NC electrocatalytic HER and OER in water splitting. 4.1.2.1. Electrocatalytic HER. As of now, the electrocatalytic HER has been acknowledged as a two-step reaction, including intermediate electrochemical hydrogen adsorption (the Volmer reaction: H3O+ + * + e− → H* + H2O in acid solution, H2O + e− + * → H* + OH− in alkaline solution) and electrochemical/ chemical desorption of H2 (the Heyrovsky reaction, H* + H3O+ + e− → H2 + H2O, or the Tafel reaction, H* + H* → H2).360,361 There are two mechanisms, the Volmer−Tafel mechanism and the Volmer−Heyrovsky mechanism, and the operative mechanism is estimated from the Tafel slope in the experiment. In the first case, if the Volmer reaction occurs quickly, the Tafel reaction is the rate-determining step, and the Tafel slope is ∼30 mV/dec. In the second case, if the Volmer reaction occurs quickly, the Heyrovsky reaction is the rate-determining step, and the Tafel slope is ∼40 mV/dec. In the third case, if the Volmer reaction occurs slowly, the effect of the reaction rate can be ignored. In this case, whether the desorption step is the Heyrovsky reaction or the Tafel reaction, the Tafel slope is ∼120 mV/dec. To date, Pt-based materials are still the state-of-the-art catalysts for the HER. The high price and scarce reserves of Pt make its large-scale commercial application unattainable, however. On the premise of the guaranteed catalytic activity, a considerable number of strategies have been proposed to reduce the usage of Pt. One is to decrease the particle size in order to expose more active sites, and the other is to introduce relatively cheaper metals to form alloys with enhanced activity. Recently, Kwak et al. reported the NC catalyst Pt1Au24(SC6H13)18 for highly efficient electrocatalytic HER.143 SWV was used to obtain the redox potential of the NCs. These Pt1Au24 NCs showed a positive shift of ∼1 V in reduction potential compared with that of Au25(SC6H13)18, which means that it has the possibility to decrease the overpotential for reductive electrocatalysis (Figure 24A). At first, the authors applied NCs in a tetrahydrofuran (THF) electrolyte containing 1.0 M trifluoroacetic acid (TFA) to investigate the HER performance. Pt1Au24 showed better catalytic activity with a higher onset potential (−0.89 V) than that of Au25 (−1.10 V) (Figure 24B). The thermodynamic reduction potential of protons in this electrolyte is about −0.82 V; thus, Pt1Au24 greatly reduces the difference between the actual and theoretical potentials. Moreover, the pseudo-firstorder rate constant (kobs) of Pt1Au24 for the HER (121 000 s−1 at 1.5 V; Figure 24C) was higher than those of molecular electrocatalysts reported earlier.362−364 LSVs were measured with increasing concentration of TFA (Figure 24D). The first reduction peak of Pt1Au24 ([Pt1Au24]0/−), located at −0.76 V, did not significantly change with increasing TFA, while the second reduction peak ([Pt1Au24]−/2−), located at −1.10 V, was

Figure 23. (A) CV curves of rGO (black), Au25 (blue), and Au25−rGO (red) in 0.1 M KCl. (B) CV curves and (C) LSVs for the ORR of Au25− rGO with different numbers of layers in the Au25 film in 0.1 M KCl. (D) Relationship between electrocatalytic rate constant and the number of layers in the Au25 film. The inset shows the relationship between Rct and the number of layers in the Au25 film. (E) Schematic diagram of Au25− rGO-catalyzed [Ru(NH3)6]3+ reduction (top) and ORR (bottom). (F) RRDE curves of rGO (black), Au25 (blue), and Au25−rGO (red). The inset shows the relationship between the generated H2O2 proportion and the applied potential. Reproduced with permission from ref 352. Copyright 2016 Wiley-VCH.

promising method to obtain high-purity hydrogen and oxygen at the same time.356−359 The whole process of water splitting is divided into two half-cell reactions, namely, the HER at the cathode and the oxygen evolution reaction (OER) at the anode. At standard atmospheric pressure and temperature, the required ΔG° to decompose a water molecule into hydrogen and oxygen is +237.2 kJ/molH2, corresponding to a thermodynamic voltage of 1.23 V. In practice, however, because of the effects of electrode polarization, electrolyte resistance, ion migration rate, and conductivity, the actual voltage is much higher than the W


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the proton preferred to adsorb on Pt1Au24 and that the HER activity was higher than that of Au25. Then the second step (H2 desorption) on Pt1Au24 was estimated to be the Heyrovsky process by comparison of the required energies for the two cases (−0.155 eV for the Heyrovsky step and 0.369 eV for the Tafel step; Figure 24E). For Au25, the H2 desorption step was considered to be the Tafel process (the required energies are 1.21 eV for the Heyrovsky step and −0.624 eV for the Tafel step). For molecular-like NCs, the thermoneutral principle is still suitable. Subsequently, Pt1Au24 was loaded onto carbon black for immobilization on a gas diffusion layer (GDL) electrode, and Pt1Au24/C/GDL was used as a heterogeneous catalyst to check the HER activity in aqueous solution. The TOF of Pt1Au24/C/GDL reached 34 mol H2 molcat−1 s−1. Pt1Au24/C/ GDL has an onset potential similar to that of commercial Pt/C/ GDL, but it showed a higher H2 production rate (25 molH2 g−1 h−1) than Pt/C/GDL (11 molH2 g−1 h−1). The hydrogen−metal interaction is a fundamental problem in many technologies, especially in hydrogenation catalysis and water splitting. The preceding example shows that changing only one atom in ultrasmall NCs can have a great influence on the properties. Therefore, Hu et al. utilized first-principles DFT to predict the HER activities of a series of M1Au24 NCs (M = Pt, Pd, Ag, Cu, Hg, Cd).312 They also thought that since the HER activity of Pt1Au24 is higher than that of Au25, Pd1Au24 and center-doped Cu1Au24 should be regarded as good catalysts for HER as well. Detailed information was given in section 3 involving the theoretical predictions. In addition to their use as homogeneous HER electrocatalysts with their own molecular characteristics, ligand-protected NCs were loaded on functional support materials used as cocatalysts to improve the HER activity. MoS2, a relatively cheap and highly active HER electrocatalyst, is considered to be an alternative candidate for Pt and has received much attention.365−369 Its intrinsic poor conductivity and little active site restrict its application, however. Zhao et al. loaded Au25 NCs (with −SCH2CH2Ph as the ligand) on MoS2 nanosheets to tune the catalytic activity of MoS2.69 The Au25/MoS2 composites showed improved HER activity with a positive shift of 40 mV in the onset potential, 1.79-fold enhanced current density at −0.4 V, and smaller charge-transfer resistance (Figure 25C). XPS analysis was performed to study the reason from the perspective of electronic interactions. The binding energy of Mo 3d exhibited a negative shift of ∼0.4 eV after Au25 deposition, and the Au 4f binding energy showed an opposite positive shift after Au25/ MoS2 formation (Figure 25A, B). This suggested that electron density was transferred from Au25 to MoS2 and that the interactions between the NCs and MoS2 were the main reason for the enhancement of the HER activity. Furthermore, Zhao et al. compared the HER activities of Au25(SCH2CH2Ph)18/MoS2 and Au25(SePh)18/MoS2 and tried to investigate the interfacial effect. Au25 with the −SePh ligand showed worse HER activity than Au25 with −SCH2CH2Ph because of its weaker electronrelaying capability compared with the latter, which was confirmed by the smaller positive shift of the Au 4f binding energy in Au25(SePh)18/MoS2 (Figure 25D). Even though the −SePh ligand is more conductive than −SCH2CH2Ph, the HER activity of Au25(SePh)18/MoS2 was worse, which indicated that the dual interfacial effect is the critical influential factor for the HER activity in this system (Figure 25E). However, there are two differences between the −SCH2CH2Ph and −SePh ligands, due to the missing compound −SPh for comparison, and it is

Figure 24. (A) SWVs of Au25 and Pt1Au24 NCs in CH2Cl2, (B) LSVs of Au25, Pt1Au24 NCs, and without NCs in TFA. (C) Plots of kobs vs potential for the Au25 and Pt1Au24 NCs. (D) LSVs of the Pt1Au24 NCs in solutions containing various concentrations of TFA. (E) Calculations of reaction energies on the Pt1Au24 NC catalyst for the HER. Reproduced with permission from ref 143. Copyright 2017 Springer Nature.

greatly enhanced, which suggested that the HER started by the formation of [Pt1Au24]2−. Besides, the onset potential of the second reduction peak of Pt1Au24 was the same as that for the HER (−0.89 V), indicating that the HER property relies on the charge state of the NC. The charge-state-dependent catalytic activity of Pt1Au24 demonstrates that NC catalysts have a behavior similar to those of molecules with discrete charges, which is the key point differing from large metal NP catalysts. In this reaction, an important question needs to be further addressed, namely, the acidic environment due to the electrolyte content of 1.0 M TFA. Indeed whether the NCs are intact and whether the ligand is protonated during the reaction are key points that require attention. Therefore, the UV−vis and mass spectra of the NCs during and after the reaction should be measured in order to identify the actual catalyst. Theoretical simulations were carried out with sufficient accuracy given the exact structure of Pt1Au24 and the homogeneous medium of the catalyst system. The first step (Volmer reaction) involved proton abstraction from THF by [Pt1Au24]2− to form [H−Pt1Au24]−. The required energy was −0.059 eV (i.e., nearly 0), whereas the energy required to change [Au25]− to [H−Au25]0 was 0.539 eV. This suggested that X


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Figure 25. (A, B) XPS spectra of Au25 and Au25/MoS2. (C) LSVs of MoS2 and MoS2 combined with Au25 including different ligands for the HER. (D) Au 4f XPS spectra of MoS2 combined with Au25 including different ligands. (E) Proposed dual interfacial effect in Au25/MoS2. Reproduced with permission from ref 69. Copyright 2017 Wiley-VCH.

2.86 and 10.22 times higher current density at −0.423 V than ligand-on Pd6/AC and Pt/C (Figure 26A,B). Moreover, ligandoff Pd6/AC showed almost no change in 10 000 cycled accelerated durability tests. However, ligand-on Pd6/AC and Pt/C displayed gradually decreased current density during only 5000 cycles. These results indicated that the ultrasmall Pd6 NCs had better HER activity than the commercial Pt catalyst. The performance improvement was even more marked without ligand. The size influence factor was excluded because the size of the NCs observed by TEM was almost unchanged. Gao et al. attributed the superior HER activity to two reasons: (1) more active site exposure by removal of the ligand and (2) easier release of H atoms due to the lower electronic state of Pd. However, only the MALDI-TOF-MS spectrum of the obtained Pd6 NCs was available in order to establish an estimation of the composition. Because of the lack of the actual crystal structure, it was not possible to conduct a well-defined theoretical simulation of the NC-catalyzed HER process. Therefore, the exact buildup of the Pd6 structure−HER activity relationship should be the subject of further research.

therefore difficult to provide the accurate reason because of the duality of variable parameters. The precise structure and high purity of the Au25 NCs offer an excellent opportunity to explain the internal mechanism in nanogold-induced HER enhancement at the atomic level. Because of the complexity of the heterogeneous catalyst system, it is possibly difficult to build a model based on calculations to simulate the HER process from theory. Zhao et al. discussed only the electronic interaction influence factor by XPS study. A more accurate simulation needs to be conducted to further investigate the dual interfacial effect. Besides the gold-based NCs, especially the most studied Au25 and M1Au24 NCs, many other new NCs with precise structures and HER-active components should also receive attention. Gao and Chen synthesized Pd6(SC12H26)12 NCs.370 These authors loaded these Pd6 NCs on activated carbon (AC) and obtained ligand-off or -on Pd6/AC composites by annealing at 200 °C or not, respectively. From LSV analysis, ligand-off Pd6/AC showed an onset potential closer to that of commercial Pt/C (−0.043 V) and exhibited a positive shift of 30 mV compared with the ligand-on composite. Meanwhile, ligand-off Pd6/AC showed Y


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Figure 26. (A, C) LSVs of Pd6/AC composites before and after ligand removal and the Pt/C reference for (A) the HER and (C) the OER and (B, D) corresponding current density comparisons at different potentials. Reproduced with permission from ref 370. Copyright 2017 Royal Society of Chemistry.

Wang et al. developed a novel transition-metal-based coordination wheel with abundant pores in which the central metal is Ni and the ligand is a calixarene derivative.371 The molecular formula of the coordination wheel is Ni18Cl6(TC4A)6(MNA)6 (the abbreviation is CIAC-123; H4TC4A = p-tert-butylthiacalix[4]arene, H2MNA = 2-mercaptonicotinic acid). These authors used it as a template to synthesize a series of metal NCs (i.e., Ir, Ru, Rh, Pt, Pd, and Au monometallic NCs and AuPd alloy NCs) by utilizing its confined internal environment and ample S atoms on the inner surface. Among them, AuPd@CIAC-123 showed higher activity than commercial Pt/C and Au@CIAC-123 in the HER, which was attributed to the electronic regulation effect of Au and Ni (Figure 27).371 Considering the synergistic effect in the compositions of bimetallic NCs, Zhu et al. synthesized Au2Pd6 NCs as catalysts for the HER and compared them with Pd3 and Au2 NCs.372 The structure of the Au2Pd6 NC can be regarded as two Pd3 triangles linked by a Au2 unit. They chose a popular HER electrocatalyst, MoS2, as a support to deposit NCs, and the NCs greatly improved the HER performance of MoS2 (Figure 28). The electronic interaction between the NCs and MoS2 in the composite was the reason for the high HER activity, which was confirmed by the XPS analysis. Au2Pd6 NCs caused a larger shift than Pd3 NCs in the Mo 3d and S 2p binding energies of MoS2. Besides, the DFT calculations also demonstrated that Au2Pd6/ MoS2 has the best ideal ΔGH * for the best HER activity among Pd3/MoS2, Au2/MoS2, and MoS2. Furthermore, the Au2Pd6/ MoS2 composite has more HER active sites than a single component of the composite. 4.1.2.2. Electrocatalytic OER. For water splitting, the OER occurs at the anode. It is a complex multistep reaction involving a four-electron transfer process with O−H bond breaking and O−O bond formation. To date, the OER reaction mechanisms that have been widely accepted are as follows:

Figure 27. (A) Schematic diagram of AuPd@CIAC-123 as a catalyst for the HER. (B) LSV curves for the HER for various samples. Reprinted from ref 371. Copyright 2018 American Chemical Society.

in acid solution: * + H 2O → *OH + H+ + e−

*OH → *O + H+ + e−

*O + H 2O → *OOH + H+ + e−

*OOH → *O2 + H+ + e− *O2 → * + O2

in alkaline solution: * + OH− → *OH + e− Z


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Figure 28. (A) LSV curves of samples for the HER. (B) Current density and overpotential comparison among these samples. (C) Tafel curves, (D) EIS curves, and (E) electron double capacity calculations for these samples. (F) Stability tests for Au2Pd6/MoS2. Reproduced with permission from ref 372. Copyright 2018 the Chinese Chemical Society (CCS), Peking University (PKU), and the Royal Society of Chemistry.

*OH + OH− → H 2O + *O + e−

Ultrananocrystalline diamond (UNCD) was chosen to act as the working electrode to load NCs for OER testing because of its ultrathin shape, good conductivity, and excellent stability in an extreme environment. From LSV analysis, Pd17 had the highest anodic current density, and Pd6 also had a significantly enhanced current after UNCD background removal. Pd4 showed a decreased anodic current, however, which suggested that the OER activity increased with increasing Pd NC size, but Pd4 had no OER activity. The accurate composition of the cluster provided the possibility to calculate the turnover rate. The turnover rates per atom of Pd6 and Pd17 were 0.68 and 0.60 atom−1 s−1, respectively, which are close to that of the most active OER metal, Ir (0.64 atom−1 s−1). To investigate the potential reason, grazing-incidence X-ray absorption spectroscopy (GIXAS) was performed to test the state change of the three clusters with unimmersed, immersed, and cycled styles (Figure 29). All three clusters existed in a stable form on the electrode during electrochemical tests, and all of them had oxidized Pd species. For Pd4, there was no difference in valence state during electrochemical cycles. Pd6 and Pd17 displayed some change in oxidation state, however. Then grazing-incidence small-angle x-ray scattering (GISAXS) was conducted to study the underlying change in cluster size and morphology of the UNCD film. The almost identical spectra suggested that the cluster size and UNCD morphology were stable without change

*O + OH− → *OOH + e− *OOH + OH− → *O2 + e− *O2 → * + O2

The OER usually includes multistep reactions with single electron transfer as described above because a single-step reaction with multielectron transfer is kinetically difficult. Thus, compared with that of the HER at the cathode (a two-electron process), the OER kinetics is more sluggish, and it is the ratedetermining step for the whole water-splitting reaction.360,373−376 Therefore, there is a strong demand to develop high-efficiency OER catalysts for the realization of rapid water splitting. As can be seen from the reaction pathway, the OER activity of a catalyst mainly depends on the binding energies of OER intermediates (e.g., *OH, *O, *OOH) on the surface.376,377 Previous reports have concluded that an appropriate binding energy of oxygen species (neither too high nor too low) is beneficial to the OER.377,378 In this way, an appropriate binding energy of adsorbed oxygen species can be used to predict the OER activity of the catalyst. Kwon et al. prepared a series of size-selected Pdn clusters (Pd4, Pd6, Pd17) using the laser vaporization deposition approach.20 AA


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of the absence of the precise structure, Gao et al. only conducted XPS measurements to search for the reason thereof. Pd6/AC exhibited a higher electron density than that without ligand, suggesting easier desorption of the oxygen atom, which was beneficial to the OER. This result indicates that the presence of ligands does not always have a negative effect in catalysis; it can eventually be tuned to control the electronic state of the metal core, thus promoting catalysis reactions. Because of the polydispersity of NPs, it is difficult to connect the structure and the properties. For instance, in the catalysis field, opinions vary for nanogold-induced OER activity. Zhao et al. deposited structurally precise Au25 NCs on functional CoSe2 nanosheets. These authors obtained high OER activity and presented cluster structure−OER activity correlations (Figure 30).71 Among Au25/CoSe2, Au25/C, Pt/C, and CoSe2, the Au25/ CoSe2 nanocomposite exhibited the best OER activity with the smallest onset potential (∼1.406 V), the highest current density (11.78 mA/cm2 at 1.68 V), and the most stable cycling performance (only ∼11 mV change in potential at 10 mA/cm2 after 1000 cycles). A study of the ligand effect was also conducted by removing the ligands at 300 °C upon annealing. After ligand removal, the cluster became larger and displayed a little better OER activity due to the direct interaction with support. It is worth noting that after ligand removal, the intact NC is the precatalyst, whereas the real catalyst is the NC without ligand. A series of different-sized Au NCs (Au25, Au144, Au333) with the same ligand (−SCH2CH2Ph) were supported on CoSe2 to study the effect of size on the OER activity. The result was that the OER activity was enhanced with increasing NC size. XPS and Raman analyses were performed to investigate the interfacial interaction between the NCs and CoSe2. After combination with the Au25 NCs, a negative shift of ∼1 eV in the Co 2p binding energy and a little positive shift in the Raman peak of CoSe2 at 657 cm−1 were observed, indicating a significant synergistic electronic interaction between the NCs and the support. DFT calculations were used to further explain the catalysis mechanism. A simplified model with one Au atom in place of the whole Au25 NC was used to reduce the calculation amount. The formation of *OOH, an important intermediate for the OER, is regarded as a descriptor to indicate the OER activity. The calculation results showed that it was similarly complex to form *OOH for both catalysts (CoSe2 and CoSe2− Au). However, the concentration of *O (an intermediate produced in the previous step) was higher at the CoSe2−Au interface. This accelerated the *OOH production and promoted the OER for CoSe2−Au. The experimental and theoretical results all demonstrate that Au25 improved the OER activity of CoSe2. Research on ultrasmall clusters with accurate composition and structure applied in electrocatalytic water splitting is still in the initial stage, and a lot of work needs to be conducted in order to obtain high-performance catalysts. For example, (1) the intrinsic electrocatalytic water-splitting mechanism of NCs as molecularlike homogeneous catalysts is still unknown; (2) real efficient NC-based electrocatalysts to be used in actual applications are scarce; (3) all of the catalysts mentioned above are noble metal NCs, and their high price is a question worth pondering; and (4) although water splitting was first discovered in acid solution, it is more feasible to electrochemically decompose water in alkaline solution in industry because of the corrosion resistance of industrial equipment. The HER catalysts mentioned above all performed in acid solution, however, and the performance of

Figure 29. (left) GIXAS spectra and corresponding enlarged areas (right) for different electrode states (unimmersed, immersed, and cycled) for loading of the three different Pd clusters: (A) Pd4; (B) Pd6; (C) Pd17. Reprinted from ref 20. Copyright 2013 American Chemical Society.

during the electrocatalytic OER. The thermochemistry of each OER step performed by the three clusters was calculated to obtain insight into the great activity difference between them, and Pd−Pd sites in Pd6O6 were found to be active species for the OER. Gao and Chen also reported that the Pd6 cluster was highly active for the OER.370 Here the Pd6 cluster with 12 −SC12H26 ligands was prepared by the wet chemical method. To investigate the ligand effect, two kinds of Pd6/AC composites were prepared by annealing. Pd6/AC showed a larger current density, lower onset potential, higher mass-specific activity, and better stability for the OER than that without ligand and commercial Pt/C (Figure 26C,D). The trend was just the opposite for the HER performance, as described above. Because AB


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Figure 30. (A) UV−vis absorption spectrum and (B) MALDI-TOF-MS spectrum of the NC Au25. The inset shows its crystal structure. (C) LSVs for the OER of these samples. (D) Comparison of the overpotentials and current densities of these samples. Reprinted from ref 71. Copyright 2017 American Chemical Society.

eight-electron reactions. The thermodynamic potentials and corresponding reactions for the various products (pH 7, aqueous solution, vs normal hydrogen electrode (NHE), 25 °C) are as follows:382,383

these NC-based catalysts in alkaline solution needs to be verified. 4.1.3. Electrocatalytic CO2 Reduction. Since the industrial revolution, the large-scale application of fossil fuels has brought about many inconveniences to people and caused excessive emission of carbon dioxide. The soaring concentration of CO2 in the atmosphere and oceans is considered to be the main reason for the destruction of the natural carbon cycle. The resulting global ecological environment changes, including the greenhouse effect and ocean acidification, will greatly affect the survival and development of human beings. On the other hand, fossil fuels as nonrenewable sources of energy have limited reserves, leading researchers to seek and develop a variety of new and renewable sources of energy and reduce their dependence on fossil fuels. CO2 is an abundant, cheap, and nontoxic source of carbon and oxygen available for the synthesis of various chemicals with added value such as formic acid, urea, and hydrocarbon fuels. Consequently, it can alleviate the environmental and energy crises at the same time and thus has application prospects.379−381 The transformation and utilization of carbon dioxide has now become a research hotspot in the fields of the environment, energy, and materials. To achieve sustainable development of human society, researchers have put forward the use of renewable energy sources such as bioenergy, solar light, and electricity to drive carbon dioxide reduction. Among these sources, electrocatalytic CO2 reduction has good application prospects because it has the following three advantages: (1) mild reaction conditions without high temperature and high pressure, (2) high energy efficiency, and (3) easy control of the reaction rate and product selectivity by changes in the electrode potential. The products of CO2 reduction are numerous and common, like formic acid, carbon monoxide, and ethylene, and also include methanol and methane obtained through six- or

CO2 + e− → CO2•− E = − 1.90 V CO2 + 2H+ + 2e− → CO + H 2O E = − 0.53 V CO2 + 2H+ + 2e− → HCOOH E = − 0.61 V CO2 + 4H+ + 4e− → HCHO + H 2O E = −0.48 V CO2 + 6H+ + 6e− → CH3OH + H 2O E = − 0.38 V CO2 + 8H+ + 8e− → CH4 + 2H 2O E = − 0.24 V

From the above equations, electrocatalytic CO2 reduction, including CO2 activation and the hydrogenation process, is a complex multistep interface reaction involving electron gain or loss, adsorption, and desorption. Meanwhile, CO2, the most stable oxidation state of carbon, is thermodynamically very stable, and thus, high energy is required to activate it. Thus, reducing the overpotential to activate CO2 to form CO2̇− is the bottleneck in real applications. Many experimental and theoretical studies based on the exploration of reaction mechanisms and optimization of catalysts to obtain high energy efficiency and reaction rate have been conducted. This section is mainly about NCs as structure-precise electrocatalysts for CO2 reduction. Research in this area is in its initial stage. Most such studies were reported by Kauffman et al. using Au25 as a model catalyst. Up to 2012, although many studies on NCs as catalysts had been conducted, the catalytic reaction types were limited to simple redox reactions. Because of the lack of NCs participating AC


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Figure 31. (A) UV−vis absorption and PL spectra, (B) corresponding difference spectra, and (C) SWVs of Au25 NCs in different gases. (D) DFT simulation of a stable CO2 adsorption model on the Au25 NC and (E) corresponding Bader charge analysis. Reprinted from ref 296. Copyright 2012 American Chemical Society.

oxidation, electron-withdrawing-ligand-caused Au250/− peak shift). The weak and easily reversed change caused by CO2 suggested a relatively weak interaction between Au25 and CO2. Meanwhile, DFT simulations were performed to find the stable configuration of CO2 adsorbed on Au25. The authors used the fully ligand-covered NC as a model to mimic the real case. The three predicted adsorption configurations all indicated that CO2 adsorption only slightly disturbed the Au25 density of states, just resulting in charge redistribution within Au25, which suggested a weak interaction between them consistent with the experimental phenomenon. Kauffman et al. then used three different charge states of Au25 NCs to further investigate the effect of charge on the electrocatalytic CO2 reduction activity (Figure 21A).350 The kinds and amounts of catalytic products, the onset potentials, and the Tafel slopes of all three Au25 NCs were almost the same. Au25− showed higher cathodic current density, TOF, and FE than Au25+ and Au250, however, suggesting that the three charge states of Au25 had the same catalytic pathway but that the catalytic rate of Au25− was the highest one. The reason invoked for that was the charge state, because the three Au25 NCs with different charges have nearly the same size, shape, and surface structure. On the basis of DFT calculations, a positive correlation was found between the TOF values and the CO2 + H+ coadsorption binding energies on Au25 with the three diferent charge states. Au25− displayed stronger reactant adsorption ability and higher catalytic activity. Although the complete structure of the Au25 NC was applied without simplification in the DFT calculations, the simulation environment was in vacuo, not under the electrocatalytic conditions, and thus was still far from real cases. Detailed electronic structure information monitoring during the electrocatalysis process was lacking, although optical spectra confirmed the stability and purity of the Au25 catalysts before and after the reaction. These aspects need to be investigated in future studies. Besides the charge state effect, the interface chemistry effect is another important factor in highly active catalysts. Andrews et al.

in challenging catalytic reactions, Kauffman et al. utilized negatively charged Au25(SC2H4Ph)18− (Au25− for short) to test electrocatalytic CO2 reduction.296 CO and H2 were the only products of CO2 electroreduction when Au25− was used as the catalyst. Au25− showed a lower onset overpotential for CO formation (90 mV) than that observed (∼200−300 mV) with larger Au NPs (2 and 5 nm). Furthermore, CO production using Au25− as the catalyst involved nearly 100% Faradaic efficiency (FE), which was 7−700-fold higher than those of larger 2−5 nm Au particles. The TOF for Au25− was estimated to be 87 CO molecules site−1 s−1, which is 10−100 times higher than those of state-of-the-art electrocatalysts. The optical spectra of Au25− did not show any significant change after the electrocatalysis process, which was attributed to the unique −Au−S−Au−S− motif on the NC shell protecting the NC from destruction. From potential-dependent product distribution analysis combined with DFT results, Kauffman et al. deduced that the high electrocatalytic activity of Au25− for CO2 reduction benefited from the three following aspects. 1. the negative charge of Au25−; 2. appropriate CO2 adsorption on Au25− (described below); 3. the excellent reactive site on Au25− promoting CO bond activation and Hads formation. Kauffman et al. also investigated the electronic interaction between CO2 and the Au25 NCs (Figure 31). Because the optical absorbance and PL of Au25 are sensitive to the charge state, a benchmark for adsorbate-induced change was established by in situ spectroelectrochemistry. The weak changes in the optical and PL spectra of Au25 in N2- and CO2-saturated solution were attributed to the interaction between Au25 and CO2. Other possible factors (e.g., signal drift, gas purity, solvent evaporation and polarity) were all excluded through careful experiments. Moreover, the electrochemical properties of Au25 were affected by CO2 with a small but apparent positive shift in the Au250/− redox peak. These CO2-induced spectral or electrochemical changes were similar but smaller than in those cases (e.g., Au25 AD


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Figure 32. Electrocatalytic CO2 reduction instruments using (A) a solar cell and (B) a rechargeable battery (top) and corresponding CO selectivity and TON (bottom). Reprinted from ref 72. Copyright 2015 American Chemical Society.

of this work on electrocatalytic CO2 reduction was very simple and needs to be further detailed. Recently, Zhao et al. utilized two different geometric structures of Au25 NCs (nanospheres and nanorods) for electrocatalytic CO2 reduction (Figure 33).386 The Au25 nanospheres showed higher total current density, FE, and CO formation rate than Au25 nanorods at all potentials (Figure 33B−D). Considering previous calculation results confirming that partially ligand-protected Au25 NCs exhibited high CO2 electroreduction activity,297 Zhao et al. first compared the difficulty of losing one ligand for these two forms of Au25 NCs (Figure 33E). Then they calculated the process for the formation of the important intermediate *COOH (Figure 33F). Furthermore, they performed a Bader charge analysis of the two forms of Au25 NCs because previous reports had claimed that anionic Au25 had better activity in CO2 electroreduction than neutral and positive Au25.350 From careful calculations, they found that sphere-shaped Au25 had better activity than nanorodshaped Au25 for the following reasons: (1) these NCs easily lose one ligand to expose an active site; (2) they stabilize *COOH well; (3) they are more electron-rich than Au25 nanorods. 4.1.4. Other Electrocatalytic Reactions. Because of the unique molecule−metal transition property of NCs, they can be used as electronic conductors and redox mediators at the same time in electrochemisty. Au25 was also applied in electrocatalytic redox reactions of small molecules. Kumar et al. made a Au25 film on a glassy carbon electrode (Au25 GCE) for the electrocatalytic oxidation of small biological molecules (ascorbic acid and uric acid).387 Only the anodic peak current changed because the oxidation of these molecules was irreversible. The anodic peak current density of the Au25 GCE showed a significant increase upon addition of just 1 μM reactants. On the other hand, it only slightly increased on a bare GCE upon addition of 5-fold

reported that Au25 NCs immobilized in Nafion showed better performance in electrocatalytic CO2 reduction than those in in poly(vinylidene fluoride) (PVDF). The reason for that is the presence of sulfonates, which changed the interface environment and the reactant adsorption binding energy.384 In real catalytic systems, all fossil-fuel-powered electrocatalytic CO2 reductions produce more CO2 than they consume. Therefore, Kauffman et al. utilized cost-effective consumergrade renewable energy and Au25 catalysts to drive CO2 electroconversion on a large scale.72 The loading and dispersion of catalysts are critical factors for catalytic activity and product selectivity, and they were first optimized. With the optimal catalyst loading, a 1.5 W, 6 V solar panel and a solar-rechargeable 6 V battery were used to simulate day and night operation, respectively, and an initial estimation of the CO2 conversion performance powered by renewable energy was made. The turnover numbers (TONs) of the renewable energy with the optimal Au25 NC system reached up to 4 × 106 molCO 2 molcatalyst−1, indicating that the present renewable energy technology was able to realize large-scale (ton per day) CO2 conversion (Figure 32). NCs applied in electrocatalytic CO2 reduction are scarce, as studies have almost exclusively concentrated on Au25 model catalysts. It is necessary to develop more NCs with high activities for CO2 reduction. Jupally et al. synthesized a series of Au137based NCs such as Au137(SCH2CH2Ph)56, Au137(SC6H13)56, A u 1 3 7 ( S C 4 H 9 ) 5 6 , A u 1 3 7 − x Ag x ( S CH 2 CH 2 P h ) 5 6 , an d Au137−xPdx(SCH2CH2Ph)56, with precise compositions confirmed by MALDI-TOF-MS and ESI-MS.385 Au137(SCH2CH2Ph)56 showed nearly 6-fold increased current density with CO2 in dimethylformamide (DMF) compared to that without CO2. The onset overpotential for converting CO2 to CO was less than 200 mV for Au137(SCH2CH2Ph)56. The part AE


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Figure 33. (A) Crystal structures of a Au25 nanosphere and nanorod. (B) Total current density, (C) CO formation rate, and (D) CO FE for the two kinds of Au25 at various potentials. (E, F) ΔG values for (E) removal of the ligand and (F) CO2 electroreduction to CO for the two forms of Au25. Reprinted from ref 386. Copyright 2018 American Chemical Society.

between Au25 and reactants. Kwak et al. utilized an electrostatic attraction strategy to synthesize Au25 ionic liquid films and applied them in highly effective electrocatalytic glucose oxidation.389

reactant. The oxidation potentials of the Au25 GCE were lower than those of a Au electrode, suggesting high electrocatalytic activity of Au25. Kumar et al. deduced the catalytic mechanism as follows: Au 25− → Au 250

4.2. Photocatalysis

Because of the rapid economic development of human society and the increase in people’s living standard, the energy shortage and environmental pollution have become two major worldwide problems threatening human survival, influencing social progress, and attracting more and more attention. Indeed, people are paying a painful price for increasing the living environment and atmospheric pollution. World energy presently mainly depends on coal, oil, and other nonrenewable fossil resources by unlimited energy consumption to achieve rapid development of the social economy. Now we are faced with double pressure on economic development and environmental protection. Photocatalytic technology is a green environmental protection technique that utilizes solar energy to realize energy conversion and environmental purification. It has involved rapid

Au 250 + molecule → Au 25− + analyte (reduced)

(oxidized)

The unique electronic structure of Au25 causes a nonuniform distribution between the Au13 core and Au12 shell. The electrocatalytic activity of Au25 was attributed to the electrondeficient Au12 shell and low-coordinate surface Au atoms. Furthermore, the authors reported water-soluble glutathioneprotected Au25 with pH-dependent electrocatalytic activity for ascorbic acid and dopamine oxidation.388 Because of the presence of two −COOH groups and one −NH2 group in the glutathione ligand, the Au25 NCs protected by glutathione induce pH-dependent protonation and deprotonation processes. The different catalytic activities of Au25 in different pH environments were due to the different electrostatic interactions AF


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Figure 34. (A) Production of 1O2 by photoexcited Au25 under visible/near-infrared light and corresponding 1O2 emission spectrum and visible detection. Reprinted from ref 390. Copyright 2014 American Chemical Society. (B) Formula of the azobenzene thiolate derivative (S-Az) (top) and changes in photoinduced absorbance of [Au25(S-Az)18]− (bottom). Reproduced with permission from ref 391. Copyright 2012 Royal Society of Chemistry.

recent development and has important research significance and broad application prospects. NCs, a new kind of nanomaterials bridging the molecule state and plasmonic nanoparticle state with quantum energy levels and unique optical properties, have unexpected interesting photoreactivity related to their structure. For example, Kauffman et al. reported that room light had a large impact on Au25− chemistry. Au25− is not spontaneously oxidized by O2, but under room-light irradiation, electron transfer occurs between Au25− and O2.392 Kawasaki et al. found that 1O2, which is highly reactive in catalytic reactions, is produced by photoexcitation of Au25 under visible/near-infrared (NIR) light, whatever the solubility and the charge state of Au25. However, Au38 with a HOMO−LUMO gap of 0.9 eV was unable to produce 1O2 under light irradiation because of its low triplet-state energy (Figure 34A).390 The authors used indirect spectroscopic methods to obtain the 1O2 quantum yield. It is worth noting that in the air atmosphere, Au25− was not converted to Au250 subsequent to 1O2 formation, but the authors believed that in a pure O2 environment this might occur. Negishi et al. reported that the azobenzene thiolate (S-Az)-protected Au25 NCs were converted between cis and trans forms back and forth with 100% efficiency under photoirradiation in five cycles. This provided a potential opportunity for photoinduced chiral catalysis (vide infra and Figure 34B).391 Stamplecoskie et al. investigated the size effect of the GSH-protected Au NCs on their lightharvesting ability. The larger the Au NC size, the less dominant the short-lifetime relaxation component. The NCs with a longlived excited state acted as efficient light-harvesting antennas, which was beneficial to the photocatalytic reaction.264 Xiao et al. deposited Aux NCs (where x signifies that the number of Au atoms is not accurately known; the diameter of the Aux NCs is ∼1.51 nm) onto TiO2 nanotube arrays (TNAs) by electrostatic self-assembly and obtained an enhanced activity in the photocatalytic decomposition of pollutants and water splitting (Figure 35).393 These results demonstrate that NCs may play specific roles in photocatalytic reactions. Research in this area only is in the initial stage, however. This section focuses on recent studies of NC-based photocatalytic reactions. 4.2.1. Photocatalytic Decomposition of Pollutants. In order to meet the food needs due to the rapid growth of the world population, the agricultural use of pesticides and fertilizers

Figure 35. Aux NCs deposited on a TiO2 nanotube array for photoinduced water-splitting, oxidation, and reduction reactions. Reproduced with permission from ref 393. Copyright 2014 WileyVCH.

has been too extensively utilized, which has caused serious soil and water pollution. In addition, the industrialization process of towns has provoked heavy-metal pollution. Finally, the production and use of chemical products, including fuel combustion, have caused fearful air pollution. As a consequence, nowadays the world is facing serious environmental pollution problems. Preventing and controlling environmental pollution and protecting the ecosystem have become urgent tasks of the whole society. Solar energy has been considered as an abundant, AG


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Figure 36. (A) UV−vis absorption spectra of methyl orange treated with Au25/TiO2 under visible-light irradiation and (B) corresponding concentration changes of methyl orange after different reaction times. (C) Photocatalytic mechanism of Au25/TiO2. Reprinted from ref 73. Copyright 2013 American Chemical Society.

OH− + h+ → ·OH

economical, and clean resource. Therefore, a large number of studies have shown that photocatalytic technology can deal with all kinds of organic pollutants, effects very good sterilization, suppresses virus activities, and does not cause harm to the human body and organisms. Compared with traditional pollutant treatment technology, photocatalytic degradation technology has many advantages: (1) it can completely decompose the pollutant; (2) it does not cause secondary pollution; (3) the catalytic material can be used continuously, with high stability and low cost; (4) the reaction can be performed at normal temperature and pressure. Therefore, the decomposition of harmful pollutants driven by photocatalysis has important practical significance. At present, the principle of semiconductor photocatalytic degradation of pollutants is generally regarded as follows:394,395 when a semiconductor is excited by incident light with an energy larger than or equal to the band gap (Eg), an electron in the valence band (VB) of the semiconductor undergoes an interband transition, producing a photogenerated electron− hole (e−−h+) pair. The e−−h+ pair is transferred onto the semiconductor surface, where the electron is captured by dissolved oxygen to form superoxide radical anion (O2•−) while the hole oxidizes hydroxide anion (OH−) and H2O to afford neutral hydroxyl radical (·OH). The high-energy ·OH and O2•− radicals decompose the pollutant to CO2 and H2O. The main reactions involved are as follows:

O2 + e− → O2•−

O2•− + H+ → HO2 · 2HO2 · → O2 + H 2O2 H 2O2 + O2•− → ·OH + OH− + O2

·OH + organic compounds → CO2 + H 2O O2•− + organic compounds → CO2 + H 2O

Given the low efficiencies of solar energy utilization and light quantum in semiconductor photocatalysts, a series of strategies have been proposed to expand the photoresponse range and separate the electron and hole, including noble metal deposition, element doping, surface photosensitization, and combination with another semiconductor. NCs, representing a transition state between atoms and the bulk, have unique discrete electronic structures and optical properties. They are considered as a good model to correlate the structure with high activity in many catalytic reactions. However, NCs with precise numbers of atoms and structure have been little applied in photocatalysis. This section summarizes the recent research on the use of NCs in photocatalytic decomposition of pollutants. Kogo et al. prepared and deposited GSH-protected Au25/ anatase TiO2 films on electrodes.396 In the presence of a donor (phenol derivative or ferrocyanide), the composite system displayed much higher photocurrent density (11.5 μA cm−2) under visible-light irradiation than bare TiO2 (0.18 μA cm−2) or the composite without donors (0.16 μA cm−2). This difference

semiconductor + hν → semiconductor(h+ + e−) H 2O + h+ → ·OH + H+

H + + e− → H 2 AH


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Figure 37. (A, B) Changes in the concentrations of (A) TH and (B) R6G with photocatalysis time upon using various ZnO−Au NCs. (C) Decay profiles of excitation emission of different ZnO−Au NCs. Reprinted from ref 398. Copyright 2011 American Chemical Society.

(Figure 36C). Hu et al. reported similar results in the photocatalytic degradation of rhodamine B (RhB). These authors deposited Au25 into porous ZnO397 and optimized the loading of Au25 in the composite to balance the e−−h+ separation efficiency and the active surface area. When the loading amount was 1.16%, the composite showed the highest catalytic activity, with a DR of 95% in 15 min. Lee et al. first reported that the photocatalytic activity and electron transfer were controlled by the size of the Au NCs.398 They synthesized three different-sized GSH-protected Au NCs (1.1 nm Au25, 1.6 nm Au144, and 2.8 nm Au807) and deposited them on a ZnO surface via carboxylic binding to degrade thionine (TH) under light irradiation. ZnO−Au807 showed the shortest photolysis time (t1/2, defined as the time required to reach 50% DR) of 34 s, compared with 112 s for ZnO−Au25 and 47 s for ZnO−Au144, suggesting that the photocatalytic activity was enhanced with the Au NC size increase (Figure 37A). The experiment was conducted in a CH3OH/H2O medium, with CH3OH acting as a hole scavenger. The possible catalytic mechanism of the ZnO−NC composite was surmised as follows: the Au NC acts as an electron acceptor, transferring the electron generated by the photoexcited ZnO to reduce TH, while the hole is consumed by the hole scavenger. The optical spectra of ZnO exhibited no obvious change before and after Au NC deposition, suggesting that the introduction of the Au NC does not expand light harvesting. The higher photocatalytic activity of larger-sized Au NCs compared to that of smaller-sized ones was attributed to the enhanced charge separation efficiency, which was confirmed by another photocatalysis test, namely, oxidation of rhodamine 6G (R6G). In this test, the trend in the catalytic activities of these ZnO−NC catalysts was similar to that already observed in TH photoreduction. This indicated that the Au NCs played a charge-separation role in this composite system. The separation efficiency increased with increasing NC size (Figure 37B). The PL spectra were measured to monitor the effect of size on the dynamics of the photogenerated electrons. After the deposition of Au NCs, the band-edge emission of ZnO at 380 and 520 nm drastically decreased. Upon testing of the timeresolved PL spectra, the composite with larger-sized Au NCs showed a shorter lifetime, indicating that larger Au NC size induced higher electron transfer efficiency (Figure 37C). Han et al. reported the same catalytic activity trend in Nile blue photoreduction (a single-electron-transfer reaction). In the photoreduction of azobenzene, however, the conclusion was no longer suitable because the reaction is a multielectron/ multiproton process involving proton-coupled electron transfer that was no longer directly related with the charge transfer.399 In the case of azobenzene photoreduction, there were clear size and

in photoactivity suggests that an electron is transferred from the photoexcited Au25 to TiO2 and from the donor to oxidized Au25. The visible light could excite an electron of Au25 from the HOMO to the LUMO, LUMO+1, or LUMO+2. As the potential of the HOMO is +1.52 V vs NHE, donors with more negative E° values are oxidized in this system. If the E° of the acceptor is more positive than the observed photopotential (+0.4 V), according to the thermodynamics the acceptor can be reduced by the system. Under visible-light irradiation in the presence of an acceptor (Ag+, Cu+, or oxygen), the system showed higher photocurrent (3.7 μA cm−2) than it did without the acceptor (0.06 μA cm−2). The increase in the in situ absorbance of the composite system also confirmed the photocatalytic reduction of Ag+ to Ag NPs. Furthermore, the Au25/TiO2 system can even catalyze a steeper uphill reaction such as phenol oxidation coupled with Cu2+ reduction (with a +0.53 eV energy difference) under visible-light irradiation. Yu et al. deposited the TOA+ salt of the Au25(SCH2CH2Ph)18 anion on TiO2 with two mixed phases (anatase and rutile) and found that it displayed light-dependent catalytic activity in methyl orange degradation.73 The HRTEM image and UV−vis absorption spectra were obtained to confirm that the NCs were successfully loaded on TiO2 without changes in size and charge state. As a result of the introduction of Au25, the composite has expanded light absorption from the UV to NIR range, as confirmed by UV−vis diffuse-reflectance spectra. Under UV and visible-light irradiation, the composite (with an NC loading of only 0.94%) showed higher catalytic activity in methyl orange degradation (69% decomposition ratio (DR)) compared with that obtained using bare TiO2 (only 27% DR) (Figure 36A,B). The composite has relatively good stability, with the degradation being kept at ∼50% after three cycles. A series of careful control experiments were conducted to investigate the catalytic mechanism. Under UV irradiation, the composite showed no significantly increased activity compared with bare TiO2, indicating that a role of Au25 as an electron trap to facilitate the e−−h+ pair separation was excluded. However, under visiblelight irradiation, the composite showed greater activity than TiO2 without Au25. The speculated reason is that Au25 (with a small Eg of ∼1.3 eV) is excited by the incident visible light and then produces a photogenerated e−−h+ pair, which provokes injection of an electron into the conduction band (CB) of TiO2 (due to the energy band difference). This results in successful separation of the e−−h+ pair and suppression of e−−h+ recombination. The hole is then transformed to ·OH to decompose the pollutant. In addition, Au25 generates 1O2 under irradiation with visible and NIR light, which also promotes the photocatalytic methyl orange degradation activity AI


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Figure 38. (A) EDS patterns of a long TNA before and after loading of Au NCs. The inset shows photos of a long TNA before treatment (left), after heat treatment (middle), and after loading of Au NCs (right). (B) Tetracycline removal efficiency with various TNA−Au NC composites treated. (C) Mechanism of photoelectrocatalytic organic compound oxidation by TNA−Au NC composites. Reproduced with permission from ref 400. Copyright 2016 Royal Society of Chemistry.

term photostability.401 These authors used a metal oxide (i.e., SiO2, TiO2, ZnO, ZrO2, etc.) as the core and polyethylenimine (BPEI) with positive charge as the surface modifier to assemble a nanocomposite with negatively charged Au25(SG)18 NCs. The size and structure of the metal oxide−BPEI−Au GSH cluster did not change over at least 24 h of light irradiation (λ > 420 nm). On the other hand, in other cases (i.e., without BPEI or upon changing BPEI to another positively charged surface modifier, 3aminopropyltriethoxysilane), the size of the Au GSH cluster significantly increased. Therefore, these authors speculated that BPEI here is not only the connector but also acts as the reducing and stabilizing agent. The multiple functions of BPEI (with a highly reducing and branched structure) protects the ligands of the Au cluster from oxidation and keeps the Au cluster unchanged (Figure 39). In order to obtain better photocatalytic activity, the authors continued to grow different thicknesses of TiO2 on the surface of the NCs. In the SiO2−BPEI−Au GSH cluster−TiO2 system, the degradation rate of RhB under light irradiation was dependent on the thickness of TiO2, the optimal thickness being 0.15 nm. Furthermore, the system has good stability, and in 10 recycle tests the degradation rate did not increase. 4.2.2. Photocatalytic Water Splitting. Photocatalytic water splitting is considered to be a green, efficient, and lowenergy-consumption mode of hydrogen production. Its pathway mainly involves three steps: (1) e−−h+ pair generation by the photoexcited semiconductor catalyst; (2) electron and hole separation and migration onto the surface of the photocatalyst; (3) production of H2 and O2 by the redox reaction between H2O and e−−h+. In theory, the energy conversion system for

excitation intensity thresholds in the ZnO−Au NC composite systems, in which ZnO−Au807 showed the highest activity. Higher excitation intensity led to a four electron/hole process reducing azobenzene to aniline, whereas lower excitation intensity resulted in a two electron/hole process reducing azobenzene to hydrazobenzene. Liu et al. prepared 6-mercaptohexanoic acid (MHA)protected Au25 NCs that acted as photosensitizers, and they loaded them on highly ordered TNAs for photocatalytic decomposition of the antibiotic tetracycline.400 They changed the protecting ligand from the previously mostly reported GSH to MHA on the basis of a consideration of two aspects: (1) the structure of MHA is simpler than that of GSH, which is better for the synthesis of Au25, and (2) MHA is negatively charged, which is better for combining with positively charged TiO2. Color change, EDS, and XPS tests confirmed the successful deposition of Au25 onto TNAs (Figure 38A). After Au25 deposition, the diffuse-reflectance spectra (DRS) of TNAs showed a significant decrease in Eg from 3.25−3.35 eV to 2.51−2.25 eV, suggesting efficient visible-light harvesting. In the photodecomposition of tetracycline, Au25−TNA composites showed a 1.6-fold enhanced activity and a 1.67−1.75-fold enhanced kinetic constant compared with bare TNAs (Figure 38B). A series of photoelectrochemical measurements (photocurrent test, EIS, and linear sweep voltammogram) were conducted, showing that the Au25 NCs were an effective photosensitizer to facilitate charge transfer and suppress charge recombination (Figure 38C). Considering the poor photostability of Au NCs, Weng et al. proposed a combinatorial strategy to design Au NCs with longAJ


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Figure 39. (A) Schematic diagram of the metal oxide−BPEI−Au GSH cluster synthetic procedure. (B, C) TEM images of SiO2−BPEI−Au GSH cluster (B) before and (C) after photoirradation. (D, E) XPS spectra of SiO2−BPEI−Au GSH NCs before and after photoirradiation. (F, G) TEM images of a SiO2−Au GSH cluster without BPEI (F) before and (G) after photoirradation. The photoirradation conditions were λ > 420 nm for 10 h. Scale bar: 5 nm. Reproduced with permission from ref 401. Copyright 2018 Springer Nature.

Figure 40. (A) Trend of photocatalytic hydrogen evolution activities of Ptn clusters. (B) Energy level position diagram of the CdS CB, Ptn LUMO, and H+/H2. (C) Photocatalytic HER and side-reaction scheme catalyzed by PtO NCs and Pt NPs. Reproduced with permission from refs 407 and 408. Copyright 2013 American Chemical Society and Springer Nature, respectively.

cally by photocatalysis. Usually, semiconductor photocatalysts suffer from low solar energy utilization and photoquantum efficiency. Typically, coupling with another semiconductor with matched energy levels, adding and electron or hole scavenger, and combining with metal NPs are efficient methods to separate charges and expand light adsorption. Some reports have appeared on small clusters as cocatalysts for photocatalytic water splitting.393,404,405 Herein, we mainly discuss NCs in this area.

photocatalytic water splitting must meet the following thermodynamic requirements: (1) the energy required for photocatalytic water splitting is 237 kJ/mol, so the energy of the adsorbed photons energy must be larger than or equal to this (237 kJ/mol or 1.23 eV); (2) photocatalytic H2 generation requires that the CB minimum energy level of the photocatalyst be higher than that of H+/H2 (0 V vs NHE), and photocatalytic O2 generation requires that the VB maximum energy level of the photocatalyst be lower than that of O2/H2O (1.23 V vs NHE).402,403 It is difficult to generate H2 and O2 stoichiometriAK


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Figure 41. TEM images of various sizes of Au−GSH NCs before treatment (left), after combining with BaLa4Ti4O15 (middle), and after removal of GSH ligands (right). The corresponding diameter distributions of the Au NCs are also shown. Reprinted from ref 74. Copyright 2015 American Chemical Society.

generation, hole consumption, electron trapping, and proton reduction. For this Pt cluster/CdS nanorod system, it was obvious that the e−−h+ pair generation and recombination sequence was not related to the cluster. Only the electron capture and transfer to the hydrogen atom was strongly dependent on the cluster size. The LUMO position of the cluster was very important for an effective electron capture and transfer process (Figure 40B). It was necessary to have the LUMO lower than the CB of the semiconductor and higher than the reduction potential of H+/H2 at the same time. Among these NCs, Pt46 has the most suitable LUMO position, and thus, it showed the highest QE. This example tells us that the NC size strongly affects its electronic structure. Besides the size of the NC, its valence state also plays a critical role in photocatalytic hydrogen evolution. Pt NCs have been widely used as photocatalysts for the HER, although their efficiency is low because they can also catalyze a side reaction,

In order to investigate photocatalysis at the atomic level, Berr et al. utilized an ultrahigh-vacuum approach to build a platform by decorating CdS nanorods with Pt NCs of various sizes.406 By tuning the coverage of NCs, they determined the minimum loading amount (∼30 NCs/CdS nanorod) to reach the highest quantum efficiency (QE) for hydrogen evolution. Larger Pt nanoparticles showed lower QEs, although the coverage was high, up to ∼3000 nanoparticles/CdS nanorod. In terms of size, Pt46 (each cluster accurately contained 46 atoms) showed higher efficiency than Ptn≥36 (i.e., small Pt NPs with a size distribution peaking at Pt46), and the authors underwent deeper investigations to search for the reason for that. Then they studied the effect of cluster size on the photocatalytic activity of the same system by maintaining the cluster coverage constant (∼23 NCs/ CdS nanorod).407 The trend in the photocatalytic activity was Pt8 ≈ Pt22 < Pt34 < Pt68 < Pt46 (Figure 40A). Photocatalytic hydrogen evolution consists of the sequence of e−−h+ pair AL


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Figure 42. (A) Division of these Au−GSH NCs into two groups on the basis of their stabilities. (B) UV−vis absorption spectra of these Au− BaLa4Ti4O15 composites after removal of the GSH ligands. Reprinted from ref 74. Copyright 2015 American Chemical Society.

Figure 43. (A) Scheme of photocatalytic water splitting using Au NCs as cocatalysts. (B) Relationship between the size of the Au NCs and the activity for H2 and O2 production. Reprinted from ref 74. Copyright 2015 American Chemical Society.

under UV−vis light, the generated amount of H2 is wellretained. In photocatalytic water splitting, small NPs are usually used as cocatalysts to promote the catalytic activity. Typical synthetic methods are direct photodeposition and impregnation. Both methods suffer from nonuniform size distribution problems, however. Negishi et al. prepared size-uniform NC-based catalysts using the front synthesis approach.409 They mixed GSH−Au25 and BaLa4Ti4O15 together to ensure full adsorption and then removed the ligand by calcination at 300 °C in vacuo. To ensure the dispersibility and catalytic activity of the NCs, the NC loading was controlled to be less than 0.2%. The optical spectra, TEM image, and XPS measurements confirmed the successful deposition of Au25 on BaLa4Ti4O15 with good dispersibility without size change. The Au25−BaLa4Ti4O15 composite (with only 0.1% Au25 NCs) produced both H2 and O2 in stoichiometric proportions under light irradiation and displayed 2.6-fold higher catalytic activity than Au NP− BaLa4Ti4O15 (with 0.5% 10−30 nm Au NPs). Besides, the photocatalytic activity of Au25−BaLa4Ti4O15 was dependent on

namely, the hydrogen oxidation reaction (HOR). Li et al. found that Pt NCs with a higher oxidation state effectively suppress the side reaction (Figure 40C).408 First, these authors confirmed by XPS and EXAFS studies that the Pt cations in PtO/TiO2 were in a high oxidation state. Then they checked the HOR suppression efficiency of PtO/TiO2. There was a significant stoichiometric increase in both H2 and O2 under UV irradiation in the case of PtO/TiO2, compared with a decrease in H2 and O2 evolution in the presence of Pt/TiO2. In a dark environment, in methanol aqueous solution or in pure water, the H2 pressure of Pt/TiO2 decreased quickly, and that of PtO/TiO2 only showed a little decrease. All of these compared experiments demonstrated that PtO/TiO2 has the ability to suppress the HOR side reaction. By comparing the calculated ΔG values and activation energies at the Pt/TiO2 and PtO/TiO2 surfaces during the HER and HOR processes, the authors speculated that the reason that PtO/TiO2 suppresses the HOR is that O2 dissociation on PtO/TiO2 is much more difficult than on metallic Pt/TiO2. Furthermore, PtO/TiO2 has good stability, and after 6 days of irradiation AM


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Figure 44. (A) EIS spectra, (B) photocurrent curves, and (C) transient open-circuit voltage decay curves of g-C3N4, Ag25/g-C3N4, and Pt1Ag24/gC3N4. The inset of (C) shows the average lifetimes of photogenerated carriers. (D) Proposed photocatalytic water-splitting mechanism. Reproduced with permission from ref 410. Copyright 2017 Royal Society of Chemistry.

the same loading of Au NP (8−22 nm)−BaLa4Ti4O15 to examine the correlation of the size with the activity. Au10− BaLa4Ti4O15 showed 4.2-fold higher activity than Au NP− BaLa4Ti4O15 because the number of surface Au atoms in Au10− BaLa4 Ti4 O15 was 17−28-fold higher than in Au NP− BaLa4Ti4O15. This means that the activity per surface Au atom in Au10−BaLa4Ti4O15 was lower than that in Au NP− BaLa4Ti4O15. The ultrasmall size caused an increase in the number of surface Au atoms at a rate that overcame the degree of reduction in their activities. Besides, the ligand-off system showed a better catalytic activity than the ligand-on system. For the ligand-off system, the intact NC is the precatalyst, whereas the NC without ligand is the actual catalyst. Given the precisely controllable structure of NCs, Du et al. investigated the effect of the heteroatom on the photocatalytic activity by comparing Ag25 and Pt1Ag24 NCs (the photocatalyst formulas are Ag25(SPhMe2)18PPh4 and [Pt1Ag24(SPhMe2)18](PPh4)2, respectively).410 Pt1Ag24/g-C3N4 showed the highest activity for photocatalytic H2 generation, with 330- and 4-fold enhancements relative to bare graphitic carbon nitride (g-C3N4) and Ag25/g-C3N4, respectively. In addition, after three cycles the amount of H2 generated by catalysis with Pt1Ag24/g-C3N4 was still retained. In the NC/g-C3N4 composites used here, the ligands were also removed from the NCs, so the intact NCs are the precatalyst. Upon low-temperature annealing, the size of the NCs usually remained without a significant increase. XPS and inductively coupled plasma atomic emission spectrometry (ICPAES) were often used to confirm the ligand removal. XRD and FTIR were performed to characterize the structure of g-C3N4 before and after annealing. The light-harvesting efficiency of gC3N4 was almost unchanged after NC deposition. Most of the Ag and Pt atoms were considered to be in the metallic state (Ag0 and Pt0 atoms) by XPS and EXAFS analyses. A series of photoelectrochemical characterizations were conducted to

the dispersibility. When the loading was less than 0.2%, the activity was well-correlated with the loading. When the loading was larger than 0.2%, the activity decreased with increasing loading. The authors controlled the loading and deposited a series of GSH-protected Au NCs (Au10(SG)10, Au15(SG)13, Au18(SG)14, Au 22 (SG) 16 , Au 25 (SG) 18 , Au 29 (SG) 20 , Au 33 (SG) 22 , and Au39(SG)24) on BaLa4Ti4O15 to reveal the effect of the size of the Au NCs on the photocatalytic water-splitting activity.74 ICPMS was used to examine the Au NC adsorption efficiency and guarantee the same loading (0.1%) of these Au NCs of various sizes. The XPS test was used to confirm ligand removal from the Au NCs after calcination. TEM images were obtained to characterize the size change before and after ligand removal (Figure 41). After calcination, for group 1 (Au10, Au15, Au18, Au25, and Au39), the size of the Au NCs displayed no significant change with a slight increase. On the other hand, for group 2 (Au22, Au29, and Au33), these Au NCs showed a clear size increase (Figure 42A). The optical spectra of Au−BaLa4Ti4O15 with Au NCs in group 2 showed a distinct plasmonic peak at ∼520 nm after calcination, whereas it was not observed for group 1 NCs (Figure 42B). Therefore, the authors speculated that the numbers of atoms in the Au NCs of group 1 remained the same, whereas those of group 2 changed. This phenomenon may be ascribed to the stabilities of these Au NCs. The Au NC− BaLa4Ti4O15 system with more stable Au NCs (those in group 1) was further used to investigate the effect of size on the photocatalytic water-splitting activity. All of the Au NC− BaLa4Ti4O15 composites with Au NCs of various sizes produced H2 and O2 in the stoichiometric ratio, but the activity increased with decreasing size, which was attributed to the increase in the number of surface atoms with the decreasing size (Figure 43). At variance with the previous operation, in this study Negishi et al. used the same “presynthesis postdeposition” method to obtain AN


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Table 3. Au25 Catalysis of Sulfoxidation under Visible-Light Irradiationa,390

selectivity (%)b entry 1 2 3 4 5 6d

temp. (°C) 25 55 25 55 25 40

time (h) 2 2 2 2 24 12

catalyst

conv. (%)b

Au25(PET)18 (no light) Au25(PET)18 (no light) Au25(PET)18 (532 nm) Au25(PET)18 (532 nm) Au25(PET)18 (532 nm) none

trace 99.5 >99.5 >99.5 87

trace trace trace 13

a Conditions: 50 μM reactant, 2 mL of solvent, 0.2 μM catalyst bMeasured by NMR analysis with an error of ∼3%. cNot determined because the conversion was too low. dThe data cited are from ref 412.

explore the origin of the better activity of Pt1Ag24/g-C3N4. First, Pt1Ag24/g-C3N4 showed a smaller barrier for photocarrier transfer (Figure 44A). Second, it displayed a higher photocurrent, meaning improved charge separation ability (Figure 44B). Finally, it showed a longer photocarrier lifetime (Figure 44C). Following the comparison with Ag25/g-C3N4, the higher activity of Pt1Ag24/g-C3N4 was attributed to the single Pt atom (Figure 44D). This work provides a strategy using precise atomic engineering to enhance metal activity that is inherently sluggish. 4.2.3. Selective Photocatalytic Organic Transformation Reactions. Selective organic transformations are important for the controllable synthesis of chemicals with few byproducts. For example, the selective oxidation of sulfide to sulfoxide has been actively investigated because sulfoxides are important chemical intermediates that are usually produced along with the sulfone byproduct. Au NCs displayed excellent activities in selective oxidation of sulfide with an organic oxidant (PhIO).411,412 Organic oxidants are usually toxic and environmentally harmful, however. Thus, selective catalytic oxidation is more practical using dioxygen as a stoichiometric oxidant because it is cheap and environmentally friendly. Au NCs as a new material have promising potential to produce 1O2. Considering that the Au25 NCs have a triplet excited state with a relatively long lifetime, Kawasaki et al. utilized Au25 NCs as a photosensitizer to obtain highly reactive 1O2 by direct visible/NIR photoexcitation.390 These authors used various selective probes (3,3′-diaminobenzidine, 1,3-diphenylisobenzofuran) and a scavenger (histidine) to confirm the existence of 1 O2 from both the positive and negative sides, respectively. The photoexcited 1O2 generated was used as an oxidant in selective photocatalytic oxidation of sulfides. Under visible/NIR-light irradiation, Au25 catalyzed sulfoxidation with nearly 100% selective production of sulfoxide (Table 3). Li et al. also utilized 1O2 generated by photoexcited Au NCs to selectively oxidize sulfides and amines (Figure 45).413 These authors developed a new synthetic method to produce Au38 NCs (with 1-adamantanethiolate (S-Adm) as the ligand) in improved yield (10%). Compared with Au38 (with thiolate ligands), SAdm-protected Au38 displayed a larger HOMO−LUMO gap (∼1.57 eV), which was appropriate to generate 1O2. Because of the higher efficiency of 1O2 production using photoexcited Au38, Au38 showed a higher activity than Au25 in selective photocatalytic sulfide oxidation. In addition, the Au38 NCs have good reusability, and after three cycles the catalytic activity and

Figure 45. Au38-generated 1O2 under photoexcitation and its catalyzed selective aerobic oxidation of sulfide to sulfoxide and benzylamine to imine. Reprinted from ref 413. Copyright 2017 American Chemical Society.

selectivity are retained. The same Au38 core with various ligands showed different activities for selective sulfide or amine oxidation. Au38S2(S-Adm)20 showed significant activity, whereas Au38(PET)20 (PET = SCH2CH2Ph) displayed no activity under light irradiation. This suggests an effect of the ligand on the photoreactivity of Au NCs. Furthermore, Zhu et al. utilized an alloy, Au25−xAgx(PET)18−, to achieve 100% conversion and selectivity in the catalytic transformation of benzylamine to Nbenzylidenebenzylamine under visible-light irradiation.414 The advantages of the alloy NCs in this reaction (complete conversion and resolute selectivity) were evidently shown by a series of comparison experiments with Au25−, Au250, and Ag25− acting as catalysts (Table 4). The mechanism was supposed to involve oxidation of benzylamine to N-benzylidenebenzylamine by 1O2, which was generated by photoirradiation of the NCs, as confirmed by EPR and chemical probe tests. Selective oxidation of amines to imines is important for the synthesis of fine chemicals and pharmaceuticals. Chen et al. utilized Au25/TiO2 composites to realize highly efficient (TOF = 1522 h−1) and selective oxidation of amines under mild conditions (light irradiation, 30 °C, O2 as the oxidant).75 Bare TiO2 (both anatase and rutile phases) showed only 12−14% yield but 99% selectivity in photoxidation of benzylamine to the imine. After Au25 deposition, the conversion was significantly enhanced to 45% with rutile TiO2 and 80% with anatase TiO2. In addition, the composite presents good recyclability. After three cycles, the selectivity is still retained, and the catalytic activity loses only 10%. Under the same conditions, the Au NP/TiO2 composite (with ∼2.0 nm Au NPs) displayed just 14% conversion with no increase compared with bare TiO2. After AO


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Table 4. Photoinduced Catalysis by These Samples414

selectivity (%) entry

catalyst

conv. (%)

1

2

1 2 3 4 5 6 7 8 9 10 11

Au(I) Ag(I) Au NPs Ag NPs Au25−xAgx−a Au25−xAgx−b Ag25− Au25− Au250 Au25−xAgx− Au25−xAgx−c

0.9 1.2 10.5 7.4 3 0 100 80.3 82.1 100 99

>99 >99 87.7 35.5 >99 0 29.1 73.6 71.9 100 100

0 0 12.3 64.5 0 0 70.9 26.4 28.1 0 0

Figure 46. Mechanism involving TiO2−Au25 NCs for the photocatalytic oxidation of benzylamine. Reprinted from ref 75. Copyright 2017 American Chemical Society.

a

In the dark. bWithout oxygen. cIn an oxygen atmosphere (1 atm) for 4.5 h.

single-crystal analysis. This needs the development of measurement technology to meet the experimental requirements.

annealing, Au25/TiO2 (with Au NC ligands removed) showed conversion similar to those of Au NP/TiO2 and bare TiO2 because of the destruction of the Au NCs and the formation of plasmonic Au NPs. A series of control experiments were conducted to investigate the Au25/TiO 2 photocatalytic oxidation mechanism. After addition of either electron or hole scavengers, the catalytic activity decreased in both cases, which indicated that photogenerated electrons and holes both drove the reaction. Au25 NCs protected by various ligands and of different sizes were used to study the ligand and size effects. The results demonstrated that the PPh3 ligand and the size of the Au NCs do not influence the catalytic activity. Besides, the reaction was regarded as involving Au−H species and carbocation intermediates. The active sites were considered to be the exposed Au atoms with partial ligand removal. The following mechanism was speculated to proceed: first, Au25 as a smallband-gap semiconductor generated an e−−h+ pair under appropriate light irradiation. The electron was injected into the CB of TiO2, thus facilitating charge separation. Second, the electron reduced O2 to O2̇−, and the hole interacted with benzylamine to form benzylamine radical cation. Third, the exposed Au atom abstracted the α-H atom of the benzylamine radical cation to form a Au−H intermediate along with a carbocation intermediate. At last, O2̇− abstracted the H atom from the Au−H and the amine to produce the Ph−CHNH intermediate, which was then oxidized to the imine (Figure 46). Although there have been some successful cases of promotion of photocatalytic activity by NCs, the application of NCs in photocatalysis has yet to be expanded. The roles that NCs play in different catalyst systems still need to be understood, and the relationship between structure and activity needs to be established. In addition, optimal catalytic activity of NCs is usually obtained upon combination with a functional support to form a composite by removal of a ligand, which complicates understanding. It is desirable to know whether ligand removal is accompanied by structural changes and how to characterize the structure and composition. HRTEM images, diffuse-reflectance UV−vis spectra, and XPS valence analysis are used to check the changes in size and electronic structure, although direct evidence of the absence of structural changes needs MS or

4.3. NCs in Photoelectrocatalysis of Energy Conversion

The demand for energy is increasing in the modern society, and therefore, in order to avoid the depletion of traditional fossil energy and to protect the ecological environment, the development of renewable energy is a general trend (vide supra). Solar energy, as a new renewable resource with abundant resources that is clean, pollution-free, and absolutely safe, will occupy a very important position in the future energy strategy. Solar photovoltaic conversion is used to convert light energy directly into electricity through solar cells, which is a major way of solar energy utilization. With the development of solar cell technology, third-generation solar cells have been developed, mainly based on dye-sensitized solar cells (DSSCs). Compared with the previous two generations of solar cells, the thirdgeneration solar cells have the advantages of wide selection of electrode materials and low cost of raw materials. Sensitizers in DSSCs are very important, directly influencing the current applications. At present, the widely used sensitizers are organic dyes (e.g., ruthenium complexes), semiconductor quantum dots (e.g., II−VI and III−V compounds), etc. However, because of the high production cost, high toxicity, poor stability, and narrow absorption range, their light utilization, photoelectric conversion efficiency, and practical applications are limited. NCs with controllable energy band structure and light absorption ability and good stability are used as a new kind of sensitizer and play a critical role in solar light absorption and photoinduced carrier generation. In return, photovoltaic tests potentially are another experimental approach to estimate fundamental information on NCs, such as the HOMO energy level.415 This section will summarize the recent progress in NCs for photoelectrocatalytic energy conversion applications. Chen et al. decorated a mesoscopic TiO2 film with GSHprotected Au NCs and used the molecular-like properties of Au NCs as sensitizers to build metal-cluster-sensitized solar cells (MCSCs) (Figure 47A).77 The significant fluorescence emission quenching in the Au NC/TiO2 system suggested a strong interaction between the components. This interaction may be caused by charge injection from a photoexcited Au NC to the TiO2 CB due to the appropriate energy match between AP


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Figure 47. (A) Working principle of a TiO2−Au NCs MCSC. (B) IPCE spectra of solar cells with various photoanode components. Reprinted from ref 77. Copyright 2013 American Chemical Society.

Figure 48. (A) Working principle of TiO2−SQ and TiO2−SQ−Au NC DSSCs. (B) UV−vis absorption and (C) IPCE spectra of different photoanode components. Reprinted from ref 416. Copyright 2014 American Chemical Society.

In order to further improve the photoelectrocatalytic energy conversion efficiency, a strategy utilizing synergistic effects was proposed by combining the NCs with another dye to form cosensitizers. The same group used Au−GSH NCs and squaraine (SQ) dye as cosensitizers in DSSCs (Figure 48A). Since the absorption ranges of the Au NCs and SQ are complementary (the absorption region is below 500 nm for Au NCs and from 500 to 800 nm for SQ), the absorption range can be extended to the full spectral range by combining the two of them.416 The selection of electrode material and electrolyte and the preparation approach were similar to the previous report. The IPCEs of the single-sensitizer DSSCs showed photocurrent response only in the regions of the absorption ranges of the sensitizers, while the dual-sensitizer DSSC displayed a broader photoresponse region (Figure 48B,C). This suggested that both the photoexcited Au NC and SQ can inject electrons into TiO2. The cosensitizer DSSC showed a PCE of 4%, which is close to the sum of the corresponding PCEs of the single-sensitizer DSSCs (the PCE was 2% for the Au NC DSSC and 2.4% for the SQ DSSC), indicating that the dual sensitizers cooperatively capture photons in a series mode. The Voc value of the dualsensitizer DSSC was a little higher than that of the single Au NC DSSC, indicating that Au NCs mainly contributed the

them. The photogenerated electrons that are injected into TiO2 are then transported to the conductive substrate (here fluorinedoped tin oxide (FTO)) and reach the counter electrode (here Pt-deposited FTO) through the external circuit. The holes left in the Au NC HOMO oxidize the electrolyte redox couple (here Co(bpy)3(PF6)2/Co(bpy)3(PF6)3). The oxidized form of the electrolyte diffuses to the counter electrode and is reduced by the electrons transported from the external circuit. At this point, the whole circuit is successfully formed, and the photocurrent is generated in the external circuit. These MCSCs exhibited a high external QE or incident photon to photocurrent generation efficiency (IPCE) of 70% at 400−425 nm, which was closer to that of CdS/ZnS−TiO2 quantum dot solar cells (QDSCs) (Figure 47B). Furthermore, the IPCE of these MCSCs matched well with the absorption of the Au NCs that were utilized. More parameters were compared through the J−V characteristics. The Au NC MSCSs showed a higher open-circuit voltage (Voc), short-circuit current (Jsc), and power conversion efficiency (PCE) compared with the CdS-based QDSCs. In addition, the photocurrent generation remained steady during 4 h of irradiation. These results indicated Au NCs could act as newgeneration high-performance sensitizers in DSSCs. AQ


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photovoltage in the dual-sensitizer DSSC. To check the universality of the Au NC- and dye-cosensitized DSSC, another commonly used dye, Ru(II)−polypyridyl-complex-based N719, was also applied in a dual-sensitizer DSSC with Au NCs. This cosensitizer DSSC also displayed better photoelectrocatalytic energy conversion efficiency. The Au NCs efficiently improve the Voc value. Upon testing of the Voc decay time profile, the dual-sensitizer DSSC showed a lower rate of voltage decay (50% decay in ∼30 s) compared with the single-sensitizer DSSCs (90% decay in ∼5 s for the Au NC DSSC and 90% decay in 0.7 (Figure 51B and Table 5). Among them, the Au18 MCSC had the highest PCE (3.8%). The larger the cluster size, the higher was the MCSC PCE, until the size grew to Au18. Au25 showed a lower PCE. The trend in PCE with cluster size change was similar to that in the photocurrent, suggesting that the photocurrent may be the main governing factor for the MCSC performance. Since Au25 showed better light absorption ability than Au18, the light absorption factor was excluded, indicating that there were other factors that might have influenced the MCSC performance. Through deep analysis of the EIS results and fitting formula, these authors concluded that Voc depended on the cluster size and was determined by two factors, the quantum yield of photogenerated electrons (ηQY) and the rate constant for recombination (ket).422−424 Voc is positively correlated with ηQY, which is related to the IPCE. Au18 had the highest IPCE, which means that it had the highest ηQY. Voc is negatively correlated with ket, which is related to the recombination resistance (Rr). From the EIS test, the Au NCs showed the following trend in Rr: Au15 > Au18 > Au10−12 > Au25. Although Au18 did not have the largest Rr value, a better balance between Rr and ηQY made it become the most beneficial sensitizer. Besides, the β recombination model was also applied to evaluate the

Figure 50. (A) UV−vis absorption and (B) photocurrent spectra of ITO/Au NP/TiO2/Au NC, ITO/TiO2/Au NC, and ITO/Au NP/ TiO2. (C) Dependence of the electric field intensity on the TiO2 thickness. The inset shows the electric field distribution. Reproduced with permission from ref 419. Copyright 2013 Royal Society of Chemistry.

Since the QD size and electrolyte have extremely important effects on the performance of DSSCs, these two factors may also play critical roles in MCSCs. The Bang group systematically investigated how the cluster size and electrolyte influence factors

Figure 51. (A) Structures of Au NCs of different sizes. (B) J−V curves of MCSCs with various sizes of Au NCs. (C) J−V curves of MCSCs with Au18 NCs in different electrolytes. Reprinted from ref 421. Copyright 2015 American Chemical Society. AS


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Table 5. Parameters of Au NC MCSCs with Different Sizes421 Au NC

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

ΔE1 (eV)a

ΔE2 (eV)b

β

Au10−12(SR)10−12 Au15(SR)13 Au18(SR)14 Au25(SR)18 Au18(SR)14 with Co2+/Co3+

2.18 6.70 8.18 6.34 4.53

0.632 0.654 0.672 0.625 0.758

0.743 0.692 0.727 0.720 0.809

1.02 3.02 3.80 2.81 2.69

1.08 0.56 0.51 0.12 0.51

1.77 1.64 1.42 0.63 1.18

0.35 0.37 0.48 0.37 −

a

Energy gap between the Au NC LUMO and the TiO2 CB. bEnergy gap between the electrolyte redox potential and the Au NC HOMO.

Figure 52. (A) Working principle of MCSCs. (B) J−V curves and (C) corresponding IPCE spectrum of Au30@BSA MCSCs. The inset of (C) is the UV−vis absorption spectrum of Au30@BSA NCs. (D) Energy level diagram of TiO2 and Au30@BSA, Ag44MBA30, and Au25SBB18 NCs. Reproduced with permission from ref 76. Copyright 2017 Wiley-VCH.

recombination kinetics. A β value of the sensitizer lower than 0.5 suggested that it is active in recombination.425,426 Among these Au NCs, only Au18 showed a β value close to 0.5; other values were all below 0.4, which confirmed the good photovoltaic performance of the Au18 MCSC. Furthermore, the ratios of the effective diffusion length to the photoanode thickness for all of the Au NC MCSCs were larger than 7, indicating 100% charge collection in these MCSCs and no transport limit using the electrolyte of the I−/I3− redox couple.427 Through step-by-step analysis, Jsc was related to the IPCE, which was correlated with the light-harvesting efficiency (LHE(λ)), the charge separation efficiency (ηsep(λ)), and the collection efficiency of photogenerated charge carriers (ηcoll). LHE(λ) was related to the sensitizer amount, the absorption capability, the photoanode film thickness, and the light scattering.428−430 The LHE(λ) value for the cluster also increased with increasing cluster size. For all of the Au NCs, the ηcoll values were 100%, as mentioned above. The last factor, ηsep(λ), is the product of the charge injection efficiency (ηinj(λ)) and the sensitizer regeneration efficiency (ηreg(λ)). Au18 and Au15 showed high ηsep(λ), whereas Au25 showed relatively low ηsep(λ). The reason that Au25 had a lower ηsep(λ) value is that Au25 had a relatively small energy offset. ηinj(λ) is related to the

electron injection driving force, which is the energy difference between the Au NC LUMO and the TiO2 CB.431 For DSSCs, usually an energy difference larger than 0.2 eV effectively drives the electron injection into the semiconductor.432−436 All of these Au NC MCSCs had a high energy difference (larger than 0.5 eV), except for Au25, which presented a low energy difference (only 0.12 eV). It seemed difficult to drive electron injection given such a low energy difference. ηreg(λ) is related to the hole scavenging driving force, which is the energy difference between the Au NC HOMO and the electrolyte redox level. For DSSCs, an energy difference of about 0.8−0.9 eV results in more than 95% photoexcited sensitizer regeneration.437−440 Almost all of these Au NC MCSCs had large energy differences (1.3−1.7 eV), with only Au25 having a smaller one (0.63 eV), which suggested that it had a poorer ηreg(λ) value. Au25 had a poorer ηsep(λ) due to both its lower ηinj(λ) and ηreg(λ) values. Considering all of the factors mentioned above, although Au25 had relatively better light absorption, because of its lower ket and ηsep(λ), its relative photovoltaic performance was poor. Concerning the Au18 MCSC, its higher synthesis score made it become the best among these MCSCs with various cluster sizes. Besides the sensitizer size effect, the electrolyte effect also had a great influence on the MCSC performance. Abbas et al.421 AT


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only Au30@BSA can easily inject electrons from the photoexcited state of the NC into the CB of TiO2 because its LUMO is higher in energy than the TiO2 CB (Figure 52D). For Ag44MBA30, electron injection was unable to occur because of its lower LUMO energy. For Au25SBB18, although the energy levels matched well, the electronic coupling between the ligand SBB and TiO2 was poor because of the absence of chemical binding between the thiolate ligand and TiO2. Thus, the integral factors caused the medium performance. From the band-gap alignments, the Ag44MBA30 MCSC should have the lowest performance, but actually it showed better performance than Au25SBB18. The reason will be further explained with detailed characterizations. Besides the use of Au or Ag NCs as sensitizers for MCSCs, Pd and Pt NCs were also applied to the construction of MCSCs. The Tatsuma group utilized mercaptosuccinic acid (MSA) and meso-2,3-dimercaptosuccinic acid (DMSA) as protecting ligands to synthesize NCs with various metals (e.g., Ptx1MSAy1, Pt x 2 DMSA y 2 , Pd x 3 MSA y 3 , Pd x 4 DMSA y 4 , Au x 5 MSA y 5 , and Aux6DMSAy6, where xi and yi were unknown).443 From the optical spectra of these clusters, the absorption edge wavelength was related only to the metal type (increasing in the order Pt < Pd < Au), regardless of the protecting ligand and cluster size. Because of the electrostatic attraction between the negatively charged MSA or DMSA and the positively charged TiO2 surface, these clusters were efficiently stabilized onto TiO2. After NC deposition onto TiO 2 , the composites showed optical absorption behavior similar to that of the corresponding NCs. The MSA-protected NCs displayed higher absorption than those with DMSA protection. All of these cluster MCSCs showed obvious photovoltatic performances (Jsc = 0.24−4.2 μA/cm2 and Voc = 0.27−0.46 V vs only Jsc = 0.04 μA/cm2 and Voc = −0.098 V for bare TiO2), suggesting that all of these clusters can be used as sensitizers to inject electrons from photoexcited clusters into TiO2. Pt and Pd NCs also showed better performance than Au NCs, although they are relatively weaker light absorbers. Pd and Pt NCs are not destabilized in the presence of either triethanolamine or iodide ions as electron donors, whereas Au NCs are corroded by iodine. In the presence of iodide ions, Pd and Pt NC MCSCs showed increased photocurrents, with more than 5-fold enhancement relative to those observed in the presence of triethanolamine. NCs have many advantages such as nontoxicity, good stability, high light utilization, precisely regulable energy levels, and optical absorption, and therefore, they are very suitable sensitizers for DSSCs. To date, although there have been some reports on metal NCs to build MCSCs, the field is still under development. More research must be conducted in order to improve the performance of MCSCs and understand the influential factors and operational mechanism of MCSCs. For example, one might tune the optical and electronic structure of the NCs to obtain a broader light absorption region and appropriate energy level matching with the semiconductor by accurate control of the size, ligand, and composition of the NCs. One can change the type of semiconductor and transparent conductive substrate or coat an insulating layer to make more suitable photoanodes for MCSCs. It is necessary to investigate how to enhance the electron injection efficiency and reduce the electron back-transfer. The optimal supporting electrolyte, electron donor, and NC loading for each MCSC need to be explored. It is also important for the use of synergistic effects to combine other functional materials in order to construct

used the Au18 MCSC with the best performance in various electrolytes (Co2+/Co3+ and I−/I3− redox couples) to compare the photovoltaic parameters and investigate the electrolyte effect (Figure 51C). The Au18 MCSC showed a higher Voc and lower Jsc in the Co-based electrolyte than in the I-based electrolyte. For the Co-based electrolyte, the main governing factors were a lower Rr value and a shorter carrier diffusion length. For the long-term perspective, it is better to use Co-based electrolytes in Au NC MCSCs because of its good stability and lack of reactivity with Au NCs. Stamplecoskie et al. also synthesized a series of Co-based complexes (i.e., bipyridine and phenanthroline derivatives) and used them to replace the I−/I3− electrolyte. In this way, improved long-term stability of Au NC MCSCs was achieved.415 In addition to Au NC MCSCs, several Ag-NC-based MCSCs were also reported. Tatsuma’s group synthesized GSHprotected Ag NCs of three different sizes (Ag15, Ag25, and Ag29).441 Ag15, Ag25, and Ag29 have HOMO−LUMO gaps of 2.1, 1.8, and 1.4 eV, respectively, indicating that the larger the cluster size, the narrower the HOMO−LUMO gap. All of the Ag NC/ TiO2 MCSCs showed significant photovoltaic performances in the presence of [Co(bpy)3]2+ acting as the electron donor, which was consistent with the corresponding absorption spectra of Ag NCs. The photocurrent generation mechanism was similar to that of Au NC MCSCs (vide supra). The internal quantum yield of Ag NC MCSCs was lower than that of Au NC MCSCs, however, which was ascribed to the lower efficiency of photogenerated electron injection due to the shorter lifetime of the Ag NC excited state. Besides the Ag NC size effect, the selection of the electron donor also had a great influence on the Ag NC MCSC performance. The authors compared three Cobased electron donors: [Co(dtb)3](PF6)2, [Co(bpy)3](PF6)2, and [Co(phen)3](PF6)2. The Voc values of Ag NC MCSCs were enhanced with increasing redox potential of the electron donor. The highest Jsc value was obtained using [Co(bpy)3]2+, which was attributed to the higher electron transfer rate, which benefited from the larger energy difference between the Ag NC HOMO and the donor redox potential and the shorter distance between the Ag NC and the donor. Bang’s group also used Ag16(SG)9 (in short Ag16) NCs as a sensitizer to build Ag NC MCSCs with the I−/I3− redox couple electrolyte.442 From PL quenching and lifetime tests, the Ag16 NC LUMO was estimated to be located higher than the TiO2 CB. Thus, the photoexcited Ag16 NCs can inject electrons into TiO2. The Ag16 MCSC exhibited better photovoltaic performance, with a Voc value of 650 mV and a Jsc value of 617 μA/cm2. The authors conjectured that the reason for the better performance of the Au16 MCSC compared with the Au29 MCSC was the lower recombination rate of the smaller Ag NC. Considering the poor stability of GSH-protected Au or Ag NCs in the presence of electrolytes, it is essential to build other ligand-protected NC MCSCs with better stability. Pradeep and co-workers compared the photovoltatic performances of MCSCs with different metal clusters, numbers of atoms in the cluster, and protecting ligands (e.g., Au30@BSA, Au25SBB18, and Ag44MBA30) (Figure 52A).76 Among these MCSCs, Au30@BSA exhibited the best photovoltaic performance with PCE = 0.35%, Jsc = 0.98 mA/cm2, and Voc = 0.71 (Figure 52B,C). The reason that the Au30@BSA MCSC showed the best performance is the energy level relative position. The HOMO−LUMO relative energy position of these clusters was estimated by ultraviolet photoelectron spectroscopy (UPS) and UV−vis absorption spectroscopy measurements. From the energy level diagram, AU


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amount of work has focused on the critical subject of CO oxidation by NCs in order to clarify the detailed mechanism at the molecular level. Jin et al. found that the system Au25(SR)18/ CeO2 exhibits excellent activity for CO oxidation, whereas the related system Au25(SR)18/TiO2 has no measurable activity. The drastic effect of the support indicated that the NC−support interface should constitute the catalytic active sites (Figure 53).

cosensitizers or multicomponent photoanode materials. Since the performance of MCSCs is affected by various factors, many parameters remain to be further studied. 4.4. Catalysis of Organic Reactions

Heterogeneous catalysis of organic reactions is crucial in many industrial areas. Deep understanding of the reaction mechanisms in heterogeneous catalysis benefits the rational design of catalysts, avoiding trial-and-error approaches. The underlying reaction mechanism is more elusive when conventional metal NPs are used as catalysts because the surface structure of NPs is complex and reaction intermediates are not easily obtained or most often not observed at all. Although model catalysts and theoretical calculations have also been studied to reveal the key aspects of reaction mechanisms, model catalysts are very different from actual catalysts, and the conclusions may not be completely transferred. This is part of the reason why there have not been unified conclusions over the years concerning NP catalysis. Fortunately, NCs with catalysis as their main application provide an ideal platform to decode the “black box reaction mechanism” by means of controlled experiments and DFT calculations. Here, we briefly review the catalytic applications of NCs for organic reactions, including oxidation, reduction, and coupling reactions. 4.4.1. Catalytic Selective Oxidation. 4.4.1.1. CO Oxidation. CO oxidation is a simple but important reaction in practical processes such as removal of CO impurities from H2 in fuel cells, which is the most intensively studied reaction catalyzed by NPs.444 However, there is no specific clarification of the activation process of NP catalysts, and controversies focus on the following issues: the interaction between the support and the NPs (binding sites, charge transfer, interfacial effect, etc.), the active charge state of supported nanoparticles (cationic, anionic, or metallic), and the active site of supported NPs and its role in the reaction process. Therefore, the detailed mechanism is best explored at the atomic level using NCs. As early as 1999, Heiz, Landman, and co-workers utilized sizeselected Aun clusters (n ≤ 20) to catalyze CO oxidation at the nanometer size range. Au8 was the minimum size for active species in this reaction.445 Soon afterward, Anderson et al. used Aun of different sizes (n = 1, 2, 3, 4, 7) to investigate the size effect. For n ≥ 3, the clusters began to be active in CO oxidation.16 In 2008, Kiely and Hutchings reported the use of aberration-corrected STEM to identify the actual active catalyst as ∼0.5 nm Au clusters.343 Recently, they used a new counting protocol to check the size distribution and activity contribution of various Au species. Not only 0.5 nm Au clusters but also 1−3 nm Au NCs were active species.446 Besides, other factors were also carefully studied, including the composition and thickness of the support,447 the charging effect,448 structural fluxionality,449 and the water promotion effect.450 Besides Au NCs, size-selected Pdn and Ptn NCs were also used to catalyze CO oxidation. A lot of experiments and theoretical studies were systematically conducted to investigate the effects of the electronic structure,451 size,452−454 support,455−457 oxidation state,458,459 and geometric structure.460 CO oxidation catalyzed by these size-selected clusters prepared in ultrahigh vacuum has already been reviewed by Liu and Corma.461 Here we mainly introduce NCs with defined crystal structures prepared by liquid chemical methods. Since the pioneering work on Au25(SCH2CH2Ph)18 NCs in supported form for the CO oxidation reaction,462 a tremendous

Figure 53. Proposed active site for CO oxidation at the interfacial sites of the Au25(SR)18/CeO2 catalyst. Reprinted from ref 462. Copyright 2012 American Chemical Society.

Interestingly, the fresh Au25(SR)18/CeO2 catalyst was not active for CO conversion, whereas the O2-pretreated Au25(SR)18/ CeO2 catalyst was good for it, especially upon addition of water vapor to the feed gas. This indicated that the high catalytic activity of O2-pretreated Au25(SR)18/CeO2 was not due to ligand desorption (which occurs at >200 °C), as confirmed by TGA. Instead, the pretreatment process involved adsorption and activation of O2 on the catalyst and generation of such active oxygen in the perimeter and low-coordinated corner of the Au sites. This article developed a new method for activation of supported NC/metal oxide catalysts by mild strategies without removal of the protecting ligands upon high-temperature calcination. In order to confirm the effect of O2 thermal pretreatment, Nie et al. compared the activities of ligand-on and -off Au38(SC12H25)24/CeO2 catalysts.463 The authors found that the ligand-on and ligand-off catalysts exhibited opposite responses to water vapor. In addition, the O2-pretreated ligand-on Au38(SC12H25)24/CeO2 at lower temperature (200 °C) gave rise to a somewhat lower activity. Moreover, O2-pretreated ligand-on Au38(SC12H25)24/CeO2 did not agglomerate and exhibited high activity and long durability for 20 h, whereas ligand-off Au38 was unstable under the same conditions. The self-promoting CO oxidation mechanism in the Au25(SCH2CH2Ph)18/CeO2 and Au38(SC12H25)24/CeO2 systems was identified by the theoretical results. Indeed, triangular Au3 sites in both NCs were the key for CO oxidation.464 Scheme 1 shows that the O2 pretreatment process involves O2 activation on the catalyst surface and produces active oxygen species. Then O2 from the feed gas readily replenishes active oxygen, forming footprints of the active oxygen species and leading to high catalytic activity at a lower reaction temperature. According to Scheme 1. Activation of O2 on the Surface of the Au38(SR)24 NC463

AV


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Pei’s theoretical work, the dihapto peroxo complex is most probably formed by oxidative addition of the O−O bond at the superoxo stage.464 (Scheme 2).

By means of similar activating methods, Spivey et al. reported that the reductively pretreated composite Au38(SC12H25)24/ TiO2 was an efficient catalyst for CO oxidation, whereas the airdried catalyst Au38(SC12H25)24/TiO2 showed almost no activity for CO oxidation under the same conditions (Figure 55).466

Scheme 2. CO Oxidation via CO-Induced OO Oxidative Addition to Dihapto Peroxo on the AuNC Surfacea

a Yellow balls, Au; gray balls, C; red balls, O. Reprinted from ref 464. Copyright 2013 American Chemical Society.

Using various technologies, including reaction kinetic tests, in situ FTIR and XAS, and DFT calculations, Overbury et al. offered a different perspective on the role of ligands: thiolate ligands would act as a double-edged sword for the Au25 NCs in CO oxidation.301 They found that the intact catalyst Au25(SR)18 is not active for CO oxidation because of the lack of ability of all Au sites to adsorb CO, whereas bare Au25 without protecting ligands is only active for CO oxidation at elevated temperatures. High-temperature calcination treatment (423 K or above) provokes ligands to fall between Au25(SR)18 and CeO2 interface, as evidenced by IR and EXAFS. In addition, isotopic labeling experiments clearly indicated that CO oxidation on the Au25(SR)18/CeO2 catalyst predominantly proceeds via the Mars−Van Krevelen (MvK) mechanism rather than the Languir−Hinshewood (L−H) mechanism (Figure 54) at low temperature. This means that CO adsorbed on the Au sites of the Au25(SR)18/CeO2 catalyst reacts with the lattice oxygen of CeO2 to form CO2, whereas O2 replenishes the consumed lattice oxygen. Thus, partial removal of thiolate ligands (generating partial positively charged Au sites) is required for lowtemperature CO oxidation on the Au25(SR)18/CeO2 catalyst. These results provided guidance for further study of ligandprotected NCs as effective catalysts for gas reactions. For the well-defined NCs, stripping off the ligands inevitably provokes significant changes in the electronic and geometric structures and impedes fundamental understanding. A mild strategy for activating NCs for CO oxidation was established by Jin’s team. For example, a highly efficient catalyst was obtained by adding a reducing gas during O2 pretreatment under various atmospheres, such as O 2 , CO, O 2 /H 2 , or O 2 /CO. 465 Furthermore, they found by Raman spectroscopy and pulse experiments that activation of the catalyst was closely related to the production of active oxygen species on CeO2

Figure 55. Comparison of catalytic activities of H2-treated and air-dried catalysts. Reproduced with permission from ref 466. Copyright 2012 The PCCP Owner Societies.

Results from FTIR, EXAFS, and XPS revealed that H2/Hetreated Au38/TiO2 did not show detectable sulfur in the catalyst, indicating that the thiol ligands prevented contact of Au sites with CO. In addition, they discovered that the electropositive sulfide species strongly interacted with Au+, likely at the Au− TiO2 support interface, suggesting that the interfacial sites play an important role in catalyzing CO oxidation. Overbury’s group indicated a promising direction for catalytic applications of intact NCs by creating in situ coordinatively unsaturated (cus) Au atoms (Figure 56). These authors first

Figure 56. Low-temperature CO oxidation by intact Au22(L8)6/CeO2 with uncoordinated Au sites: (top) front views; (bottom) side views. Reprinted from ref 467. Copyright 2016 American Chemical Society.

Figure 54. Proposed oxidation mechanism of carbon monoxide by intact, partially, and completely dethiolated Au25(SR)18/CeO2 catalysts. Reprinted from ref 301. Copyright 2014 American Chemical Society. AW


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Figure 57. CO oxidation catalyzed by Au22 NCs supported on CeO2 with various shapes. Reproduced with permission from ref 468. Copyright 2018 Elsevier.

anionic Au NCs to produce superoxo species (Figure 58). On the basis of these results, the authors proposed that the

reported that a ligand-protected intact NC, Au22(L8)6 (L = 1,8bis(diphenylphosphino)octane) with eight cus Au atoms, is highly active for low-temperature catalytic CO oxidation without the need for ligand removal.467 The eight cus Au atoms were barely affected by charge transfer from the phosphine ligands to the Au NCs and thus were close to neutral charge, a situation quite different from that of the cus sites on slightly dethiolated Au25 clusters. Wu et al. used the Au22(L8)6 (L = 1,8-bis(diphenylphosphino)octane) NCs to study the effect of the CeO2 support shape on the CO oxidation reaction.468 By a combination of techniques including in situ FTIR, EXAFS, and STEM, they found that the CeO2 shape impacted the nature and quantity of surface exposure of Au sites as well as the efficiency of organic ligand dissociation. All in all, the activity of the Au22(L8)6 NCs supported on rod-shaped CeO2 in low-temperature CO oxidation was always higher than that of NCs supported on cube-shaped CeO2 (Figure 57). This work offered a design strategy for the synthesis of more active catalysts by the choice of shape-controlled supports. Li et al. further found that the catalytic performance of the NCs was influenced by the electronic and steric effects of the protecting ligands. It was also disclosed that heteroatom-doped NCs are important to modify the catalytic performance of NCs in CO oxidation reactions.469,470 For example, CeO2-supported Au25(SR)18 (SR = SPh, S-Nap, SC2H4Ph) and MnAu25−n(SCH3)18 (M = Cu, Ag) NCs exhibited various catalytic performances in CO oxidation. Good et al. recently inferred that the coordination of the lattice oxygen with CO is the rate-limiting step in CO oxidation by the Au38/CeO2 catalyst, which was identified by regulating the Ce3+/Ce4+ ratio.471 4.4.1.2. Alcohol Oxidation. Alcohol oxidation is industrially important because its products (aldehydes, ketones, or acids) are essential raw materials for the synthesis of fine chemicals. As early as 2009, Tsukuda et al. first discovered that very small ( CeO2 > MgO. This indicated that the MSI effect existed in this catalytic system. A plausible mechanism for sulfide oxidation by PhIO as the oxidant is illustrated in Figure 70. First, PhIO is adsorbed onto the pocket site of the Au25(SR)18 cluster,296,497 and then the sulfide coordinates to an Au atom of Au25(SR)18. Finally, the activated

oxidant is expected to transfer an oxygen atom to the sulfide, yielding the sulfoxide product. With the optimized support, Li et al. further exploited sulfide oxidation by Au NC catalysts of different sizes (see Table 8 for a summary of these results).411 The experimental results showed that the Au144(SCH2Ph)60/TiO2 catalyst exhibited the highest catalytic performance (92% conversion of methyl phenyl sulfide with 99% selectivity for sulfoxide). Interestingly, an unusual size dependence of the Au NC catalyst was observed: Au144 > Au99 > Au38 > Au25. The catalytic activity of TiO 2-supported Au102(SPh)44 is similar to that of Au99(SPh)44/TiO2 (Table 8), probably because the two clusters had similar core sizes.498 4.4.1.5. Hydrocarbon Oxidation. Hydrocarbon oxidation has been widely studied as a prototype reaction because it is an important industrial process for the removal of volatile organic compounds. Extensive studies have shown that the size (diameter) of traditional NP catalysts has a significant effect on their catalytic performance: the catalytic activity increases with decreasing size. Many fundamental questions remained unanswered concerning the relationship between the activity BC


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Figure 66. Proposed mechanism of styrene oxidation over Au25(SR)18. For clarity, the thiolate ligands are not shown. Dark gray, Au atoms in the core; light gray, Au atoms in the shell. Reproduced with permission from ref 486. Copyright 2010 Wiley-VCH.

were more difficult to oxidize than those located on the open surface. This property has been used to explore and regulate the oxidation characteristics of other nanocatalysts. Wang, Zheng, and co-workers surprisingly observed for the first time the promoting effect of surface ligands on catalysis. These authors reported that acetylene-protected Au34Ag28 NCs exhibited better activity than those with surface ligands partially or completely removed in the hydrolytic oxidation of triethylsilane.124 From Figure 73, the untreated Au34Ag28/XC72 catalyst shows high conversion (40% in 4 min and 100% in 12 min) for the hydrolytic oxidation of organosilanes to silanols, whereas the Au34Ag28/XC-72 catalyst calcinated at 200 °C gave conversion below 3% after 30 min, showing the promoting effect of the surface ligands. Ultrasmall NCs (around 1 nm) have potential as catalysts for oxidation reactions because in most cases the catalytic activity of conventional nanoparticles is known to increase as the particle size decreases.21,502 Yamamoto et al. reported that by the use of a dendrimer template with coordination sites on a gradient of alkalinity, the synthesis of a series of finely controlled NC catalysts was achieved (the alloy NCs were around 1 nm in diameter) (Scheme 3).503 As expected, the catalytic performance of Cu32Pt16Au12@TPM-DPA-G4/GMC (TPM-DPA-G4 = fourth-generation polyphenylazomethine dendrimer with a tetraphenylmethane core; GMC = graphitized mesoporous carbon) was 24 times greater than that of a commercially available Pt catalyst for aerobic oxidation of hydrocarbons. The mechanistic investigation indicated that the high activity of Cu32Pt16Au12 was due to synergistic effects on the interface between Cu(0)/Cu(I) and other precious metals in the conversion of the peroxide intermediate to the ketone (Figure 74). In addition to the clusters made using the dendrimer template, size-selected clusters prepared in ultrahigh vacuum presented property−structure correlations at the atomic level, as mentioned above in connection with CO oxidation (section

and the catalyst structure, and the Au NCs helped in understanding these correlations. Tsukuda et al. first revealed the size effect of Aun/HAP-300 (n = 10, 18, 25, 39) on cyclohexane oxidation.499 In addition, these authors also synthesized a larger Au NC (∼1.4 nm) by a conventional adsorption method, which was called Au∼85 on the basis of the average diameter of the Au NCs. Figure 71 summarizes the catalytic performance of Aun/HAP for the oxidation of cyclohexane under 1 MPa O2 at 150 °C. The TOF increases with the size of Au NCs, reaching the maximum value (18 500 h−1 (Au atom)−1) for n = 39. From then, the values started to drop with further increases in size. These results provided a basic insight into size-related effects on catalysis by NCs (diameter < 2 nm). The Barrabés group investigated the MSI effect of Au38(SC2H4Ph)24 with two diverse supports, CeO2 and Al2O3, in cyclohexane oxidation.500 These authors found that Au38/ CeO2 was more active and selective than Au38/Al2O3 after pretreatment at 300 °C, which was caused by the state of Au (cationic or metallic) on the surfaces of Au38/CeO2 and Au38/ Al2O3, respectively (Table 9). It was also unexpected to find the formation of cyclohexanethiol especially in the Au38/Al2O3 system. Bao’s group found that encapsulated ultrasmall Pt clusters within the CNT channels (Pt@CNT) exhibited higher activity and stability for toluene oxidation than those loaded on the outer surface of the CNTs (Pt/CNT), although contact with reactants was easier with Pt/CNT.501 The explanation for this phenomenon was that the CNT channels not only restricted the cluster size but also protected the Pt species at the active reduced state in the presence of oxygen, which was further validated by DFT calculations and in situ EXAFS. Figure 72 shows that the average oxidation reaction energy was approximately −1.72 eV/ O for Pt43O4/CNT. This energy decreased to −1.63 eV/O for Pt43O4@CNT. This indicated that the encapsulated Pt NCs BD


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Table 7. Epoxidation of Styrene by NC Catalysts

selectivity (%) catalyst Au25(SCH2CH2Ph)18 Au25(SC6H13)18 Au38(SCH2CH2Ph)24 Au38(SC12H25)24 Au144(SCH2CH2Ph)60 Au144(SC12H25)60 0.5Au25−HAP 0.5Au25−HAP Au25(SR)18/TiO2 Pt1Au24(SR)18/TiO2 Au25−CeO2 NRs Au25−CeO2 NPs Au25−CeO2 NRs Au25−CeO2 NPs Ext-SBA-15-SH-Au25 Ext-SBA-15-SH-Au25-1 Ext-SBA-15-SH-Au25-5 Ext-SBA-15-SH-Au144 Ext-SBA-15-SH-Au144-1 Ext-SBA-15-SH-Au144-5 Au25/HAP Au25/HAP-300 Au25/P25 Au25/P25−300 Au25/AC Au25/AC-300 Au25/PGO Au25/PGO-300 Au25/SiO2 Au25/SiO2-300 Au25/CNT Ag44/CNT Ag46Au24/CNT Ag32Au12/CNT Au25(SR)18/TiO2 AgxAu25−x(SR)18/TiO2 CuxAu25−x(SR)18/TiO2 Au25(SR)18/CeO2 AgxAu25−x(SR)18/CeO2 CuxAu25−x(SR)18/CeO2 Au25(SR)18/SiO2 AgxAu25−x(SR)18/SiO2 CuxAu25−x(SR)18/SiO2 Au25@SiO2-250 Au25@SiO2-650 Au25/SiO2-250 Au25/SiO2-650 Au25/CNT Ag44/CNT AgxAu25−x/CNT Ag32Au12/CNT

temp. (°C)

time (h)

conv. (%)

1

2

3

ref

O2

oxidant

100

24

6 5 4 4 trace trace

485

100 80 70

24 12 10

O2

80

24

TBHP

80

24

24 26 24 25 20 16 36 92 44.3 9.8 trace trace 44 14

70 69 72 71 80 84

O2 TBHP PhI(OAc)2

TBHP

80

24

TBHP

80

16

TBHP

65

16

PhI(OAc)2

70

10

TBHP

82

24

TBHP

65

24

27 25 14 15 12 11 22 100 58.9 90.8 5 5 99 98 30 38 100 32 68 100 70.6 75.4 74.6 73.7 35.3 44.6 44.8 63.5 52.3 46.9 72.8 43.6 68.2 69.7 58.9 60.1 41.9 64.2 82.1 66.3 54.5 81.5 65.7 70.0 62.3 75.6 15.1 78.3 45.1 76.5 74.4

4.4.1.1). Vajda et al.21 and Anderson et al.504 used size-selected Pt clusters to investigate the effect of size in the propane and

34.5 37.9 27.0 9.2 4.3 14.0 5.6 28.9 22.2 14.7 20.3 6.1 1.2 57.6 44.3 12.4 11.7 51.9 53.6 47.1 48.7 28.2 24.4 92.3 91.8 93.7 93.4 19.3 5.1 3.1 56.8

273 54 89.9 100 100 32 36 100

1.7 0.3 trace trace 22 50

142 487

488

489

66.4 92.6 96.3 37.6 54 86.9 88.3 46.7 46.4 52.9 50.3 71.2 74.9 7.6 8.3 6.3 6.6 73.9 94.5 95.9 41.2

13.3 1.3 2.5 4.8 1.7 0.7 trace 1.4 trace trace 1.0 0.6 0.7 0 0 0 0 6.8 Au99 > Au38 > Au25.411 9. In cyclohexane oxidation, the activity increases with the size of Aun (n = 10, 18, 25, 39) NCs, reaching the maximum value for n = 39 and dropping with further increase in size.499 10. In D-glucose oxidation, the catalytic activity of the Au NCs is significantly dependent on the size, following the order Au144(PET)60/AC > Au38(PET)24/AC > Au25(PET)18/AC.505 11. In the hydrogenation of 4-nitrobenzaldehyde, the catalytic performance is enhanced with increasing core size: Au15(SG)13 < Au18(SG)14 < Au25(SG)18 < Au38(SG)24.552 1. In the ORR, among Pt12−Pt24 NCs, Pt19 shows the best activity because of the abundant edge sites.339 2. In electrocatalytic CO2 reduction, spherical Au25 NCs show better activity than rod-shaped Au25 NCs.386 3. In CO oxidation, triangular Au3 sites in both Au25(SCH2CH2Ph)18 and Au38(SC12H25)24 NCs are the key for efficient CO oxidation.464 4. A ligand-protected intact NC, Au22(L8)6 with eight coordinatively unsaturated Au atoms, is highly active for low-temperature catalytic CO oxidation.467 5. In 4-NP reduction, Au13Cu8 NCs exhibit good activity, whereas the Au13Cu2 and Au13Cu4 NCs show no activity. The reason for this is that Au13Cu8 leaves open space for small molecules to access its Au sites.133 1. In the HER, Au25 NCs with −SePh ligands exhibit worse HER activity than those with −SCH2CH2Ph ligands, showing the clear ligand influence on the catalytic activity.69 2. Pd6 NCs with and without ligands show different activity trends in the HER and OER. In the HER, Pd6 NCs without ligands show better activity than those with ligands, whereas in the OER, Pd6 NCs with ligands show a better activity than those without ligands.370 3. In the hydrolytic oxidation of triethylsilane, the acetylene-protected Au34Ag28 NCs exhibit better activity than those with surface ligands partially or completely removed.124 4. In the semihydrogenation of alkynes, alkynyl-protected Au38 NCs are very active for alkenes, whereas thiol-protected Au38 NCs show very low conversion.513 5. In the reduction of 4-NP, the activity of the shorter-chain ligand-protected Au25 NCs is higher than that of those with longer chains, and the activity of aromatic-ligand-protected Au25 NCs is higher than that of NCs with aliphatic ligands.543 1. In FAO and the HER, Pt1Au24 shows better activity than Au25.143,326 2. In photocatalytic water splitting, Pt1Ag24 shows better activity than Ag25.410 3. In alcohol oxidation, Pd1Au24 shows better activity than Au25.473 4. In epoxidation of alkenes, Pt1Au24 shows better activity than Au25.142 5. In the semihydrogenation of acetylene, Au20Cd4 (from doping of Au23 with Cd) exhibits improved selectivity for acetylene reduction but decreased activity.514 6. In olefin hydrogenation, Au34Rh1 exhibits remarkable catalytic activity, much higher than that of Au34 and Au33Pd1.515 7. In the reduction of 4-NP, Au24−xAgxHg1 NCs exhibit remarkably higher catalytic activity than Hg- or Ag-doped Au25 NCs.544 1. In the ORR and electrocatalytic CO2 reduction, Au25 NCs with different charges show different activities, with Au25− being the best among Au25q (q = −1, 0, +1).350 2. In the photocatalytic HER, Pt NCs with higher oxidation state effectively suppress the side reactions.408 3. In styrene oxidation, core−shell Ag32Au12 NCs show lower selectivity than the surface-doped Au24Ag46 catalyst.125 4. In the A3-coupling reaction, both the Auσ+ sites on the Au38(SC2H4Ph)24 NC surface and the Au0 atoms in the core are key to enhance the catalytic performance.589

5.2. Rational Design of Ligands To Optimize Catalytic Activity and Selectivity

different metallic components and geometric configurations will

The stereoelectronic properties of the ligands are an essential part of the catalysis parameters. In NCs, ligands need to be all the stronger protectors of the structure because the cluster core is smaller, whereas in large NPs loose nanoligands or polymers are

be essential for high-performance catalysis in the future. BY


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5.3. Designing NCs with Inherent Uncoordinated Active Sites

As discussed in the previous section, the ligand has two “faces”. A lot of research has been performed on ligand removal (e.g., by calcination) to improve NC catalytic properties. Strategies should be developed toward functionalization of surface ligands, by which ligands need not be removed. Such postsynthetic modification strategies should enhance the catalytic functionality of NCs. The cluster may be viewed as an entity that may carry inherent active sites. For example, Au22 protected by ligands showed high activity in CO oxidation, in which the catalytic activity originates from the inherent uncoordinated metal active sites.467 Pt19 NCs showed higher activity in the ORR because they have a more asymmetric structure and hence more active edge sites than Pt13.339 Therefore, inherent generation of uncoordinated active sites on NCs is an effective way to enhance the catalytic performance, as opposed to or complementary to postsynthetic modification. 5.4. Functionalizing the Support To Construct Highly Efficient and Stable NC-Based Composite Catalysts

Because of the very small size of NCs, they might degrade into larger-sized aggregates during the catalytic process, causing decreased activity in recycling tests. A suitable support immobilizes the clusters and keeps them stable, preventing them from aggregation and loss in catalyst recycling. In earlier studies, supports with large surface areas were used as catalytically inert materials to adsorb clusters without considering the function of the support. The supports are usually cheap, easily produced on a large scale, and eventually inert in some catalytic reactions, such as metal oxides.462 The inert supports are beneficial to investigate the cluster size or electronic effects on the catalytic performance. It is desirable to combine clusters with functional supports, however, in order to achieve higher catalytic efficiency. Cluster-based composite catalysts exhibiting better catalytic performances than the clusters or support alone by utilizing the synergistic effect between the support and the cluster have recently been reported. Zhu’s group prepared Au11@ZIF-8 and Au13Ag12@MIL-101 by the in situ synthesis method to selectively catalyze benzyl alcohol oxidation.477 At the same time, Su’s group also utilized the unique properties of MOF materials to achieve synergistic catalytic activity. Specifically, these authors obtained Au25−ZIF8 composites by precisely controlling the cluster location on the inner (Au25@ZIF-8) or outer surface (Au25/ZIF-8) by two different synthesis strategies, and the composites showed good activity in 4-NP reduction.550 Beside MOFs, Zhu’s group has used polymers to stabilize NCs, such as hyperstar polymer−Au25 composites by ligand exchange to synthesize highly recyclable catalysts.546 Although some initial results have been obtained and the synergism between the support and the cluster has shown promise, there are still and fundamental questions to be addressed, such as how to functionalize the support, develop more strategies to realize the close integration between the support and NCs, and rationally choose appropriate reactions according to the particularity of the support. Besides, the interaction between the support and the active species is important to understand the catalytic mechanism. The unique interface model is meaningful to investigate the surface/ interface effect. Unexpected catalytic performances might be reached upon utilizing the cluster as a monomer and the ligand as a linker to construct composite assemblies such as “clusterMOFs”.

Figure 95. Schematic representation of the future of NC catalysts.

sufficient to maintain the nanostructure stability. Although some size-selected clusters have been produced by laser ablation technology, they only exist stably in ultrahigh vacuum. This does not hamper applications in exploring catalysis mechanisms by the use of in situ tests and characterization methods, but it is a major bottleneck for the use of clusters as practical catalysts on a larger scale. In NCs, because of the coverage by the ligands, the exposure of catalytic active sites and direct possible contact between the catalyst surface and reactant(s) can be very limited or cut off. Although the presence of the ligand lowers the metal site activity,268,474 catalysis is still possible. Recently, promotion of the catalytic efficiency and selectivity in the presence of ligands has been reported, whereby NCs benefit from the ligand steric and electronic effects. As mentioned above, with NCs Jin’s group has found that the presence or absence of ligands can selectively induce catalysis of semihydrogenation of terminal or internal alkynes, respectively.511 Au34Ag28 NCs with full ligand coverage have shown higher activity in hydrolytic oxidation of triethylsilane than the related NCs that have undergone partial or complete ligand removal, although it is possible that some sintering may play a role in the comparison.124 The Wang group has found that Au38 NCs protected by various ligands have shown distinctly different catalytic activities in alkyne semihydrogenation.513 Zheng’s group has investigated the effect of surface coordination environment on the catalytic activity and selectivity.603 Therefore, the presence of ligands is not necessarily detrimental to the catalytic performance. It is essential to optimize the catalytic selectivity by rationally designing and synthesizing functional ligands with specific interactions with the reactant, intermediate, and product. The synthesis of NCs is quite mature, but to date the NCs that have been obtained are the result of natural evolution, yielding the most stable states with fixed ligand numbers. Future work should develop strategies for molecular manipulation to control the number of ligands at designated locations on the cluster in order to open up a specific site for high activity or selectivity. The synthetic methods need further improvements to realize such site-specific and quantitative removal of ligands on the cluster surface. BZ


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5.5. Developing Chiral Catalysis by NCs

5.9. Some Technical Difficulties

The 2001 Nobel Prize winner Ryo̅ji Noyori pointed out that chiral catalytic synthesis is one of the important ways to achieve “perfect synthetic chemistry” (generating the desired product with 100% selectivity in 100% yield without waste). Some chiral NCs have recently been obtained by chiral ligand induction, outer-motif asymmetric arrangement, and inherently chiral packing of the metal core.67,228,604−606 Some chiral NCs that have revealed the origin of chirality from the atomic level have also been used in chiral fluorescence.607 In chiral catalysis, however, NCs have not yet undergone a breakthrough. It is a promising area worth future exploration and efforts. In NCbased chiral catalysis, obtaining a highly pure cluster with a single chirality is crucial. Therefore, for ligand-induced cluster chirality, regulating the chiral ligands is a feasible strategy to achieve enantioselective catalytic activity. For those with chirality coming from the metal core arrangement, effective and fast isolation methods should be developed in future work. The enantioselective catalytic reactions should also be carefully selected.

NCs may exhibit a dynamic structure during a catalytic reaction process, and the change in geometric or electronic structure of NCs can cause reaction pathways that are different from those that are expected. Therefore, in situ monitoring of the dynamic structure of clusters is very important in order to understand the catalytic mechanisms. Such in situ tests should be developed in future efforts. Besides, the possible change of the cluster size or structure after deposition onto the support is an important question to address. Usually, to expose more active sites, a common strategy is to partially remove ligands by annealing at a suitable temperature. Although the purity of the cluster can be characterized by some indirect measurements such as optical absorption spectra, ICP spectra, TEM imaging, etc., more direct evidence such as MS spectra and single-crystal X-ray diffraction are more convincing. Because of technical difficulties, however, it is nontrivial to obtain direct evidence for the structures of the supported clusters. In addition, for the majority of characterization techniques, the results are often averaged, i.e., they cannot give information on a designated spot because of the limitation of the resolution. However, this point is very important for investigating the catalytic mechanism at the atomic level. Therefore, future breakthroughs to overcome the above technical barriers are highly desirable, which should significantly benefit understanding the relationship between the cluster structure and catalytic activity.

5.6. Capturing the Intermediate in Catalytic Reactions

When clusters participate in catalysis, the reactant may connect to the cluster by ligand exchange or the interaction between the reactant and the ligand. Given the well-defined structure of NCs, they provide the possibility of obtaining the structure of catalytic intermediates, enhancing evidence for proposed reaction mechanisms. This task is difficult, but it is a worthy direction for future cluster catalysis research.

AUTHOR INFORMATION Corresponding Author

*E-mail:[email protected].

5.7. Building More Realistic Theoretical Models in Catalysis

ORCID

In catalytic reactions, building theoretical models is an effective way to help understand the catalytic process and mechanism. The catalytic center and pathway can be speculated by calculating the binding energies between the catalysts and reactants, products, or important intermediates, reaction activation energy barriers, and so on. The correctness of simulation is dependent on the rationality and authenticity of the theoretical model, however. Because of the huge size and complexity of the fully ligand-covered clusters, the model usually involves a simplification of the real cluster by the use of H atoms to replace the full −R group in the thiolate ligands. Therefore, if we want to approach closer to the real situation, this is obviously not enough. In consideration of the diversity of the catalytic reaction conditions and the complexity of the reaction environment, building more realistic reaction models is still a long way off.

Didier Astruc: 0000-0001-6446-8751 Manzhou Zhu: 0000-0002-3068-7160 Author Contributions §

Y.D. and H.S. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Yuanxin Du is a lecturer of chemistry and materials at Anhui University (Hefei, China). She received her B.S. in physics from Anhui University in 2010 and her Ph.D. in materials from the University of Science and Technology of China (USTC) (Hefei, China) in 2015. Her interests include the electro- and photocatalysis of nanomaterials. Hongting Sheng is an associate professor of chemistry and materials at Anhui University (Hefei, China). She received her B.S. in chemistry education from Anhui Normal University in 2002 and her Ph.D. in organometallic chemistry from Suzhou University (Suzhou, China) in 2007. Her interests include the synthesis of atomically precise nanoclusters and their application in organic catalysis.

5.8. Developing Non-Noble-Metal NC Catalysts

To date, there are many NCs with precise composition and structure. Although they show good performances in catalytic reactions, they are almost all based on noble metals. The high cost and scarce reserves limit their practical applications. Nonnoble-metal NCs may be promising catalysts in some specific reactions in the same way, such as alloys containing Cu that showed better electrocatalytic CO2 reduction activity than those without Cu.608,609 Likewise, some transition-metal-based oxides, sulfides, and phosphides exhibited good performance in the electrocatalytic water-splitting reaction.610−613 In order to move toward useful productivity, it is necessary to develop non-noblemetal NCs that would be more meaningful for industrial catalysis.

Didier Astruc has been a professor of chemistry at the University of Bordeaux in France since 1984 and visiting professor at Anhui University (Hefei, China). He studied in Rennes, where he obtained doctorate degrees, and then was a NATO postdoc at MIT (Cambridge, MA, USA). His interests are in electron and proton reservoir complexes and their applications to organic syntheses, dendrimers, redox sensors, nanoparticles, and nanocatalysis. Manzhou Zhu is currently the Changjiang Chair Professor of Chemistry at Anhui University. He received his Ph.D. in Chemistry from USTC in 2000. He then conducted postdoctoral research at USTC and Carnegie CA


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Mellon University (Pittsburgh, PA, USA). He joined the chemistry faculty of Anhui University in 2010. His current research interests include atomically precise nanoclusters, structure−property correlation of nanoclusters, and applications.

EXAFS FAO FDTD FTIR

ACKNOWLEDGMENTS We acknowledge financial support provided by the National Natural Science Foundation of China (U1532141, 21631001, 61601001, and 21871001), the Ministry of Education and Education Department of Anhui Province, the Anhui Provincial Natural Science Foundation (1708085QB37), the CNRS, and the University of Bordeaux.

FTO FTP GCE GDL GISAXS

ABBREVIATIONS AC AFM AIMD ATR-IR BDPP BPEI BSA Capt CB CD CIAC-123 CNT CTAB CV DAFC DFT DHLA DMBT DMF DMFC DMSA DPAG4-PyTPM DPPPE DPPM DPPP DPV DR DRS DSSC EDX Eg e−−h+ EIS EOR E1/2 EPR ESI-MS ET

GIXAS GMC GSH HAADF HAP HER H2MNA HOMO HOR HPLC

activated carbon atomic force microscopy ab initio molecular dynamics simulations attenuated total reflection infrared 2,4-bis(diphenylphosphino)pentane polyethylenimine bovine serum albumin captopril conduction band circular dichroism Ni18Cl6(TC4A)6(MNA)6 carbon nanotube cetyltrimethylammonium bromide cyclic voltammetry direct alcohol fuel cell density functional theory dihydrolipoic acid 2,4-dimethylbenzenethiol dimethylformamide direct methanol fuel cell meso-2,3-dimercaptosuccinic acid fourth-generation phenylazomethine dendrimer with a triphenylpyridylmethane core 1,5-bis(diphenylphosphino)pentane bis(diphenylphosphino)methane achiral 1,3-bis(diphenylphosphino)propane differential pulse voltammetry decomposition ratio diffuse reflectance spectra dye-sensitized solar cell energy-dispersive X-ray spectroscopy band gap electron−hole pair electrochemical impedance spectroscopy ethanol oxidation reaction half-wave potential electron paramagnetic resonance electrospray ionization mass spectrometry electron transfer

HRTEM H4TC4A ICP-AES IPCE ITO Jsc ket LDH L−H LHE(λ) LSPR LSV LUMO MALDI-TOF-MS MBA MCNTs MCSC MHA m-MBT MOF MOR MPC MPP MSA MSI MvK NC NHE NIR NMR

CB

X-ray absorption fine structure formic acid oxidation finite-difference time domain Fourier transform infrared spectroscopy fluorine-doped tin oxide 4-fluorothiophenol glassy carbon electrode gas diffusion layer grazing-incidence small-angle Xray scattering grazing-incidence X-ray absorption spectroscopy graphitized mesoporous carbon glutathione high-angle annular dark-field hydroxyapatite hydrogen evolution reaction 2-mercaptonicotinic acid highest occupied molecular orbital hydrogen oxidation reaction high-performance liquid chromatography high-resolution transmission electron microscopy p-tert-butylthiacalix[4]arene inductively coupled plasma atomic emission spectrometry incident photon to photocurrent generation efficiency indium tin oxide short-circuit current rate constant for recombination layered double hydroxide Languir−Hinshewood light harvesting efficiency localized surface plasmon resonance linear-sweep voltammogram lowest unoccupied molecular orbital matrix-assisted laser desorption ionization time-of-flight mass spectrometry 4-mercaptobenzoic acid multiwalled carbon nanotubes metal-cluster-sensitized solar cell 6-mercaptohexanoic acid 3-methylbenzenethiolate metal−organic framework methanol oxidation reaction mesoporous carbon 2-mercapto-5-n-propylpyrimidine mercaptosuccinic acid metal−support interaction Mars−Van Krevelen atomically precise nanocluster with known X-ray crystal structure normal hydrogen electrode near-infrared nuclear magnetic resonance


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XAS XMCD XPS XRD ηcoll

NP

more or less polydisperse nanoparticle (mixture of a number of large clusters) n-type electrons are most carriers OER oxygen evolution reaction ORR oxygen reduction reaction PAGE polyacrylamide gel electrophoresis PAMAM polyamidoamine dendrimer PCE power conversion efficiency PDOS projected density of states PET SCH2CH2Ph PL photoluminescence p-type holes are most carriers PTZ-TCBQ phenothiazine−tetrachloro-pbenzoquinone PVDF poly(vinylidene fluoride) PVP poly(N-vinyl-2-pyrrolidone) QDSC quantum dot solar cell QE quantum efficiency Rct charge transfer resistance RDE rotating disk electrode R6G rhodamine 6G rGO reduced graphene oxide RhB rhodamine B Rr recombination resistance RRDE rotating ring−disk electrode RWGS reverse water-gas shift S-Adm 1-adamantanethiolate SBB 4-(tert-butyl)benzyl mercaptan SEC size-exclusion chromatography SQ squaraine STEM scanning transmission electron microscopy STM scanning tunneling microscopy SWV square-wave voltammetry TBBT 4-tert-butylbenzenethiol TBHP tert-butyl hydroperoxide TEM transmission electron microscopy TEMPO+ 2,2,6,6-tetramethylpiperidin-1-oxoammonium cation TFA trifluoroacetic acid TGA thermogravimetric analysis TH thionine THF tetrahydrofuran TLC thin-layer chromatography TNA nanotube array TNT titanate nanotube TOA tetraoctylammonium TOAX tetraoctylammonium halide TOF turnover frequency TON turnover number TPA two-photon absorption TPF two-photon fluorescence TPM-DPA G4 or TPM G4 fourth-generation polyphenylazomethine dendrimer with a tetraphenylmethane core UNCD ultrananocrystalline diamond UPS ultraviolet photoelectron spectroscopy VB valence band Voc open-circuit voltage XANES X-ray absorption near-edge structure

ηinj(λ) ηQY ηreg(λ) ηsep(λ) 4-AP 4-NP 11-MUA

X-ray absorption spectroscopy X-ray magnetic circular dichroism X-ray photoelectron spectroscopy X-ray diffraction collection efficiency of photogenerated charge carriers charge injection efficiency quantum yield of photogenerated electrons sensitizer regeneration efficiency charge separation efficiency 4-aminophenol 4-nitrophenol mercaptoundecanoic acid

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