Electrochemistry of Atomically Precise Metal Nanoclusters - Accounts

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Electrochemistry of Atomically Precise Metal Nanoclusters Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Kyuju Kwak and Dongil Lee*

Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV OF RHODE ISLAND on 11/30/18. For personal use only.

Department of Chemistry, Yonsei University, Seoul 03722, Korea

ABSTRACT: Thiolate-protected metal nanoparticles containing a few to few hundred metal atoms are interesting materials exhibiting unique physicochemical properties. They encompass the bulk-to-molecule transition region, where discrete electronic states emerge and electronic band energetics yield to quantum confinement effects. Recent progresses in the synthesis and characterization of ultrasmall gold nanoparticles have opened up new avenues for the isolation of extremely monodispersed nanoparticles with atomically precision. These nanoparticles are also called nanoclusters to distinguish them from other regular metal nanoparticles with core diameter >2 nm. These nanoclusters are typically identified by their actual molecular formulas; prominent among these are Au25(SR)18, Au38(SR)24, and Au102(SR)44, where SR is organothiolate. A number of single crystal structures of these nanoclusters have been disclosed. Researchers have effectively utilized density functional theory (DFT) calculations to predict their atomic and electronic structures, as well as their physicochemical properties. The atomically precise metal nanoclusters have been the focus of recent studies owing to their novel size-specific electrochemical, optical, and catalytic properties. In this Account, we highlight recent advances in electrochemistry of atomically precise metal nanoclusters and their applications in electrocatalysis and electrochemical sensing. Compared with gold nanoclusters, much less progress has been made in the electrochemical studies of other metal nanoclusters, and thus, we mainly focus on the electrochemistry and electrochemical applications of gold-based nanoclusters. Voltammetry has been extremely powerful in investigating the electronic structure of metal nanoclusters, especially near HOMO and LUMO levels. A sizable opening of HOMO−LUMO gap observed for Au25(SR)18 gradually decreases with increasing nanocluster size, which is in line with the change in the optical gap. Heteroatomdoping has been a powerful strategy to modify the optical and electrochemical properties of metal nanoclusters at the atomic level. While the superatom theory predicts 8-electron configuration for [Au25(SR)18]− and many doped nanoclusters thereof, Ptand Pd-doped [PtAu24(SR)18]0 and [PdAu24(SR)18]0 nanoclusters show dramatically different electronic structures, as manifested in their optical spectra and voltammograms, suggesting the occurrence of the Jahn−Teller distortion in these doped nanoclusters. Furthermore, metal-doping may alter their surface binding properties, as well as redox potentials. Metal nanoclusters offer great potential for attaining high activity and selectivity in their electrocatalytic applications. The well-defined core−shell structure of a metal nanocluster is of special advantage because the core and shell can be independently engineered to exhibit suitable binding properties and redox potentials. We discuss recent progress made in electrocatalysis based upon metal nanoclusters tailored for water splitting, CO2 conversion, and electrochemical sensing. A well-defined model nanocatalyst is absolutely necessary to reveal the detailed mechanism of electrocatalysis and thereby to lead to the development of a new efficient electrocatalyst. We envision that atomically controlled metal nanoclusters will enable us to systematically optimize the electrochemical and surface properties suitable for electrocatalysis, thus providing a powerful platform for the discovery of finely tuned nanocatalysts.

1. INTRODUCTION

catalytic activities of the ultrasmall nanoparticles arise from the presence of low-coordination environment of metal surface

During the last decades, metal nanoparticles have received much attention as electrocatalysts for low-temperature fuel cells and water splitting reactions.1,2 Theoretical and experimental studies have disclosed that the extraordinary © XXXX American Chemical Society

Received: July 31, 2018

A

DOI: 10.1021/acs.accounts.8b00379 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Figure 1. (a) MALDI mass spectra and (b) UV−vis absorption spectra of Au25, Au38, Au67, Au102, Au144, and Au333 nanoclusters. SWVs of (c) Au25, Au38, and Au67 and (d) Au102, Au144 and Au333 nanoclusters in dichloromethane. Reproduced with permission from ref 16. Copyright 2017 American Chemical Society.

and the quantum size effects.3−5 Remarkable catalytic performance has been observed in a number of heterogeneous reactions upon downsizing particle size.1−3 However, these metal nanoparticles are generally polydisperse in size and morphology and, thus, include multiple active sites showing different catalytic activities. The heterogeneity would considerably affect the catalytic activity and selectivity for a desired product. Systematic optimization of a metal catalyst at the atomic level remains a daunting challenge. Recent synthetic innovations have opened the possibility of isolating ultrasmall metal nanoclusters with atomic precision.6,7 These atomically precise metal nanoclusters comprising a fewto-few hundred metal atoms encompass the bulk-to-molecule transition region where unusual size-specific electrochemical, optical and catalytic properties are observed.8−11 Much progress has been made toward understanding their atomic and electronic structures and the quantum size effects, which are manifested by their molecule-like properties; for example, opening of HOMO−LUMO gap,10 exciton dynamics,9,12 and photoluminescence.13 However, significantly less progress has been made in the applications of metal nanoclusters with unique electrochemical properties in technological areas such as electrocatalysis, and electrochemical and biological sensing. In this Account, we focus on recent progress made in electrochemistry of the atomically precise metal nanoclusters and the applications in electrocatalysis and electrochemical sensing. The electronic structures and redox properties of the molecule-like nanoclusters are critically determined by their

size, shape, and chemical composition, which can be strategically utilized in the design of tailored electrocatalysts and electrochemical sensors.

2. ELECTROCHEMISTRY OF ATOMICALLY PRECISE METAL NANOCLUSTERS Recent advances in the experimental and theoretical characterizations of metal nanoclusters have established their structures and chemical compositions with atomic precision.7,14 Further details on the syntheses and characterizations of metal nanoclusters can be found in other reviews.7,15 Figure 1a shows matrix-assisted laser desorption ionization (MALDI) mass spectra of Au nanoclusters. As the figure shows, there is solely a pointy peak discovered for every nanocluster, suggesting the extreme purity of the separated nanoclusters. The peaks observed at around m/z 7034, 10298, 17300, 25249, 35397, and 74852 Da are associated with the intact nanocluster ions with chemical formulas of Au25(SC6H13)18, Au 3 8 (SC 6 H 1 3 ) 2 4 , Au 6 7 (SC 6 H 1 3 ) 3 5 , Au 1 0 2 (SC 6 H 1 3 ) 4 4 , Au144(SC6H13)60, and Au333(SC6H13)79, respectively. The Au nanocluster with the formula of AuN(SR)L, where N and L denote the respective number of Au atoms and ligands, will be abbreviated as AuN. In Figure 1b, the optical spectra show that absorbance seems to be extinguished as an absorbance edge in the low energy region that links to the optical gap of a nanocluster for electronic transition and decreases with a growing nanocluster size; for instance, 1.3, 0.9, and 0.7 eV for Au25, Au38, and Au67, B


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Figure 2. (a) MALDI mass spectra of PdAu24 and PtAu24. The matching between the simulated (black bars) and experimental (lines) isotope patterns is shown in insets. (b) UV−vis absorption spectra of Au25, PdAu24, and PtAu24 in trichloroethene. (c) SWVs of Au25, PdAu24, and PtAu24 in dichloromethane. (d) Electronic structures of the superatomic 6e and 8e systems. Optical transitions occurring in the superatom systems are denoted by α, β, and γ. The MAu12 core of the Jahn−Teller distorted 6e [MAu24(SR)18]0 (left) undergoes structural conversion to approximately spherical 8e [MAu24(SR)18]2− (M = Pd, Pt) upon reduction (right). Reproduced with permission from ref 26. Copyright 2015 American Chemical Society.

HOMO−LUMO gaps match reasonably with the optical gaps determined from the absorption edges (Figure 1b). Size-dependent O1−R1 gaps are discovered for Au25, Au38, and Au67 nanoclusters while those observed for larger nanoclusters are relatively small; 0.49, 0.39, and 0.22 V for Au102, Au144, and Au333, respectively. In Figure 1d, evenly spaced current peaks are observed for larger nanocluster (Au102−Au333), suggesting that these nanoclusters act as quantum capacitors. In other words, the double layer capacitances of these nanoclusters become so small that quantized double layer (QDL) charging behavior can be experimentally observed. The capacitances (CCLU) of Au102, Au144, and Au333 were determined to be 0.49, 0.57, and 0.88 aF by

respectively. The absorption edge becomes less prominent for larger nanoclusters like Au102, Au144, and Au333, which all exhibit the optical energy gap of smaller than 0.5 eV. 2.1. Size-Dependent Electrochemistry

Voltammograms of these gold nanoclusters show size-specific electrochemical properties and have been extremely useful in unlaveling their electronic structures.8 The square wave voltammograms (SWVs) in Figure 1c display well-resolved oxidation and reduction peaks of gold nanoclusters. As can seen in the figure, the opening of the electrochemical energy gap, estimated by the potential difference between the first oxidation (O1) and the first reduction (R1) potentials, is clearly observed for these small nanoclusters. For Au25, the electrochemical energy gap is observed to be 1.65 V. Charging energy correction (approximated by potential difference between the first (O1) and second (O2) oxidation peaks) for the electrochemical gap led to a corrected energy gap that constitutes an electrochemical determination of the HOMO− LUMO gap (1.32 eV) for Au25. Similarly, the HOMO−LUMO gaps of 0.99, 0.61, 0.18, and 0.15 eV were electrochemically estimated for Au38, Au67, Au102, and Au144, respectively. Uncertainties in the electrochemical determination of HOMO−LUMO gaps are related with the charging energy estimation by and large. The electrochemically determined

Ez°, z − 1

1 z − 2 )e ( =

CCLU

+ E PZC

(1)

where z, E°z,z−1, and EPZC are the charge state, the formal potential of z/z − 1 change, and the potential of zero charge of a nanocluster, respectively. It is demonstrated that CCLU increases as the size of nanocluster increases, which is expected for a capacitor that displays QDL charging pattern. Comparing with the charging behavior of smaller nanoclusters, it can be concluded that the nanocluster molecule with salient HOMO− C


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Figure 3. (a) SWVs of Au38, Pt2Au36, and Pd2Au36 in dichloromethane. The solution open-circuit potentials are denoted by arrows. (b) Atomic and electronic structures of [Au38(SR)24]0, [Pt2Au36(SR)24]2−, and [Pd2Au36(SR)24]0. Reproduced with permission from ref 29. Copyright 2018 American Chemical Society.

using a variety of synthetic methods,20−23 whereas multiply doped nanoclusters were predominantly obtained from doping of Ag or Cu.20,24,25 At first glance, the superatomic 8e model appears to prevail in these doped nanoclusters; for example, multiply doped 8-electron [AgxAu25−x(SR)18]− (x = 1−5)24 and [CuxAu25−x(SR)18]− (x = 1−5)25 show similar electronic structures and voltammograms to [Au25(SR)18]−. The 8e configuration of charge neutral [HgAu24(SR)18]0 also exhibited similar voltammetric pattern to [Au25(SR)18]−.23 Doping of Pd or Pt into Au25(SR)18 produced, however, fundamentally different nanoclusters. In Figure 2a, MALDI mass spectra exhibit that the Pd- and Pt-doped nanoclusters were isolated in extremely pure forms and were observed at m/ z ∼6943 and ∼7032 (Figure 2a inset), which agree well with the simulated patterns of PdAu24(SR)18 and PtAu24(SR)18, respectively. Further analyses found that Pd and Pt dopants were placed in the central position of the core. In addition, the doped products were proved to be charge neutral nanoclusters, which are explicitly different from the 8-electron systems predicted from the superatom model. Figure 2b shows that the absorption profile of Au25 became markedly altered upon doping of Pd (or Pt). Figure 2c also shows that both PdAu24 and PtAu24 exhibit drastically changed SWVs that are fundamentally different from that of Au25. The electrochemical energy gap (O1−R1) observed for PdAu24 and PtAu24 were respectively 0.75 and 0.73 V drastically reduced compared with that of the undoped Au25. After correcting the charging energy term, the HOMO−LUMO gaps of PdAu24 and PtAu24 were identified as 0.32 and 0.33 V, respectively, dramatically reduced from 1.32 V of Au25. These results explicitly suggest that the electronic structure of Au25 is drastically altered by Pd(Pt)-doping. The voltammetrically estimated HOMO−LUMO gaps were in excellent agreement with those predicted for PdAu24 and PtAu24 by DFT calculations.26 The origin of the changes observed for the Pd(Pt)-doped nanoclusters was explained by the Jahn−Teller distortion. In the superatom model, [Au25(SR)18]− is an 8e system, corresponding to a noble gas-like superatom electron configuration (1S21P6). DFT calculations predicted that the HOMO level is triply degenerate 1P orbitals whereas the LUMO is doubly degenerate 1D orbitals with a HOMO− LUMO gap of 1.32 eV (Figure 2d). Considering [PdAu24(SR)18]0 and [PtAu24(SR)18]0, they are 6e systems

LUMO gap acts as a molecular capacitor and becomes a QDL charging capacitor as size increases. Another interesting observation in Figure 1c and 1d is the metal-to-molecule transition in Au nanoclusters. The QDL charging observed in Au102, Au144, and Au333 suggests that they are small but metallic. By contrast, voltammograms of Au25, Au38, and Au67 with distinct HOMO−LUMO gap indicate that these are molecule-like. The metal-to-molecule transitions have been found in various spectroscopic and structural studies of Au nanoclusters.10,12 In a combined femtosecond and nanosecond transient absorption study, we also showed that exciton dynamics is observed in Au nanoclusters ranging from Au25 to Au144 as opposed to the electron dynamics.16 In these molecular nanoclusters, the exciton lifetime increases with increasing HOMO−LUMO gap, demonstrating the energy gap law of exciton dynamics. In addition to the size effects, the difference in atomic structure is also sensitively detected by voltammetry. In the voltammetric study of rod-shaped [Au25(PPh3)10(SR)5Cl2]2+ having a vertex-shared biicosahedral Au25 core, Park et al.17 found that the voltammetric peaks are all positively shifted relative to the spherical Au25 nanocluster and the electrochemical energy gaps are all about 1.54 V regardless the thiolate ligands employed. This is plainly smaller than that of the Au25(SR)18 nanocluster (1.65 V). 2.2. Metal-Doped MxAu25−x Nanoclusters

Structural analysis of the anionic [Au25(SR)18]− revealed that it is composed of a Au13 core and six protecting Au2(SR)3 semirings.18,19 The extraordinary stability of [Au25(SR)18]− was attributed to the closure of the electronic shell of the superatomic nanocluster. That is, the number of superatomic electrons, n*, for a spherical metal nanocluster protected with thiolate ligands, [MN(SR)L]z, is given by n* = NVA − L − z

(2)

where VA is the valence of the metal atom. For [Au25(SR)18]− nanocluster, n* = (25 × 1) − 18 − (−1) = 8. The superatomic electron shell (1S21P6) is completely filled with the eight electrons. Doping of heteroatoms into a stable gold nanocluster is a powerful strategy to tune their physicochemical characteristics.20 Many researchers have explored heteroatom doping of the stable Au 25 nanocluster. Monometal-doping of MAu24(SR)18 was carried out with M = Hg, Cd, Pt, Pd) D


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Figure 4. (a) ESI mass spectrum of PT-Au25. Each peak corresponds to [Au25(MPS)18Nay(TOA)z]n− containing different numbers of Na+, TOA+, and charge and is assigned by (y, z, n). (b) UV−vis absorption spectra of PT-Au25 (blue) in dichloromethane and MPS-Au25 in water (red). (c) SWVs of C6S-Au25 and PT-Au25 in dichloromethane. Reproduced with permission from ref 31. Copyright 2012 American Chemical Society.

with a 1S21P4 superatom electron configuration in which the triply degenerate HOMO splits into a doubly degenerate HOMO and LUMO to be symmetrically occupied by electrons. The dramatically reduced HOMO−LUMO gaps found are thus the consequence of the Jahn−Teller distortion of the 6e [MAu24(SR)18]0 nanoclusters, which concurrently accompanies splitting of the 1P orbitals. Voltammetric studies of doped silver nanoclusters were also conducted. The atomic structure of the Ag25(SPhMe2)18 nanocluster was proved to be quite similar to Au25(SR)18,27 but the electronic structures of the center-doped MAg24(SPhMe2)18 nanoclusters were substantially different from MAu24(SR)18, where M is Pt or Pd.28 That is, the voltammograms of dianionic [PtAg24(SR)18]2− and [PdAg24(SR)18]2− were quite comparable with that of the undoped nanocluster, monoanionic [Ag25(SR)18]-. The electrochemical energy gaps determined for PtAg24 and PdAg24 were in the range of 1.71−1.96 V, rather similar to that of Ag25 (1.82 V), suggesting that the Jahn−Teller distortion is absent in these systems.

analysis were 0.86, 0.95, and 0.26 eV for Au38, Pt2Au36, and Pd2Au36, respectively, which agreed reasonably well with the DFT results in Figure 3b. DFT calculations predicted that the 14-electron system, [Au38(SCH3)24]0 and [Pt2Au36(SCH3)24]2−, with 1S21P61D6 superatom electron configuration has a prolate shape. The neutral [Pd2Au36(SCH3)24]0 has 12 electrons, two fewer electrons than [Au38(SCH3)24]0 and [Pt2Au36(SCH3)24]2−, and the doubly degenerate HOMO would break into HOMO and LUMO as shown in Figure 3b. The splitting was also attributed to the Jahn−Teller distortion. The origin of the Jahn−Teller distortion observed only for [Pd2Au36(SCH3)24]0 (not for [Pt2Au36(SCH3)24]2−) was explained by size mismatch between dopant and host and by the role of the shared face in the biicosahedral M2Au21 core that could keep the core from distortion when the size mismatch was insignificant. 2.4. Voltammetry of Water-Soluble Au Nanoclusters

Although voltammetry of small nanoclusters has proven to be very instructive for the electronic properties, it has not been widely used for water-soluble nanoclusters. Voltammograms obtained in aqueous media are generally poor-resolved because of the high dielectric ligand shell swollen by water. Moreover, the electrochemical potential window is generally limited by the oxidation and reduction potentials of water. In fact, voltammetric investigations of glutathione-protected Au25 nanoclusters in water exhibited only a broad current peak for the oxidation of Au25 nanoclusters in water and the first and second oxidation peaks were totally unresolved.32 A strategy developed by our group31 to investigate the electronic structure of water-soluble gold nanoclusters was transferring them into organic phase by ion-pairing with hydrophobic counterions. A water-soluble Au25 nanocluster protected with (3-mercaptopropyl)sulfonate (MPS-Au25) was transferred into toluene phase by ion-pairing with tetraoctylammonium cation (TOA+). Electrospray ionization (ESI) mass spectrometry in Figure 4a confirmed that the phase-

2.3. M2Au36 Nanoclusters (M = Pt, Pd)

Doped Au38 nanoclusters were investigated to gain further insights into the Jahn−Teller distortion in nanocluster systems. In the structural investigation of Au38(SC2H4Ph)24,30 Jin and co-workers reported that it is composed of a face-fused biicosahedral Au23 core and three monomeric and six dimeric protecting staples. Detailed characterizations of the doped products revealed that whereas the Pd-doped nanocluster is neutral [Pd2Au36(SR)24]0, the Pt-doped nanocluster is dianionic [Pt2Au36(SR)24]2−.29 DFT calculations confirmed that the two Pt(Pd) atoms are placed at the two center positions of the M2Au21 core of both nanoclusters.29 Figure 3a shows distinctly different SWVs depending on the dopant. Whereas Pt2Au36 exhibits a rather similar SWV to Au38, the SWV of Pd2Au36 displays distinctly different voltammetric pattern. HOMO−LUMO gaps determined from voltammetric E


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Figure 5. (a) kobs-potential plots for Au25 and PtAu24 in the presence of 1.0 M trifluoroacetic acid. (b) HER mass activity comparison between PtAu24/C and Pt/C electrodes. (c) Calculated reaction energies for H2 evolution on PtAu24. In step 1, a proton solvated by THF molecules is transferred to [PtAu24]2− to form [H-PtAu24]−; in step 2a, a second solvated proton is transferred to [H-PtAu24]− to form [2H-PtAu24]0; in step 2b, a second solvated proton reacts with H in [H-PtAu24]− to produce H2. Reproduced with permission from ref 34. Copyright 2017 Nature Publishing Group.

electrochemical and electrocatalytic properties have been utilized in various applications, such as electrocatalysis,11,33,34 electrochemical sensing,32,35,36 and electrochemiluminescence.37 Herein, we focus on our recent efforts for the advancement nanocluster-based electrocatalysis that includes hydrogen evolution reaction,34 CO2 reduction reaction (CO2RR),38 and electrochemical sensing.32,35,36

transferred Au25 (PT-Au25) retained the original composition after the phase-transfer reaction. Figure 4b shows that the absorption spectrum of the PT-Au25 nanocluster was almost identical to that of the MPS-Au25. Like the hexanethiolateprotected Au25 (C6S-Au25), the PT-Au25 exhibits well-resolved current peaks; that is, two oxidation peaks (O1 and O2) and one reduction peak (R1) as can be seen in Figure 4c. The electrochemical gap (O1−R1), however, was found to be only 1.39 V, substantially smaller than that typically found for the C6S−Au25 (1.65 V). The positive shift of the first reduction potential (R1) was clearly explained by the electrostatic field effect of the MPS ligand on the gold core. In other words, a positive shift of the redox potential of Au25 would be caused as more positive charge is induced on the gold core by the polarized −SO3− anions.

3.1. Electrocatalytic Applications of Metal Nanoclusters

3.1.1. Hydrogen Evolution Reaction (HER). Hydrogen production from water is an important electrocatalytic process and generated the extreme interest in making artificial catalytic systems that can efficiently catalyze HER. The HER is a classic example of a two-electron transfer reaction that may proceed through either the Volmer−Tafel or the Volmer−Heyrovsky mechanism.1 Recent progress in the computational design of solid catalysts for HER has disclosed the importance of the hydrogen adsorption free energy (ΔGH) for an active HER catalyst. A plot of catalytic activity for a wide range of catalyst materials against ΔGH typically gives a volcano relationship, illustrating that the highest catalytic performance is expected when a catalyst with ΔGH = 0 is employed. This simple concept is a quantitative demonstration of the Sabatier principle, which states that an active catalyst should bind the reaction intermediates neither too strongly nor too weakly. Doped nanoclusters with a well-defined core−shell structure may offer special advantage in that the core can be independently engineered to show suitable ΔGH for HER.

3. ELECTROCHEMICAL APPLICATIONS OF METAL NANOCLUSTERS Recent progress in the computational design of solid catalysts for electrocatalysis has revealed the importance of the adsorption free energy of reaction intermediates on the solid surface.1 Metal nanoclusters with a well-defined core−shell structure may be of special advantage because the core and shell can separately be engineered to show suitable binding properties for electrocatalysis. Their redox potentials can additionally be tuned by their size, shape and composition for them to act as an efficient electron transfer mediator for electrocatalysis. The metal nanoclusters exhibiting unique F


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Figure 6. (a) Overall lattice-hydride mechanism of CO2 conversion into HCOOH on Cu32H20L12. The structures of the key intermediates and transition states are shown. Color code: orange, Cu; green, hydride; red, oxygen; gray, carbon; L= S2PH2, not shown. (b) Selectivity-overpotential plots for HCOOH, CO, and H2 produced after 90 min electrolysis at different overpotentials. Constant potential electrolysis experiments were carried out in 0.1 M KHCO3 and 0.4 M KCl (pH 6.8) on a Cu32/C electrode. Reproduced with permission from ref 38. Copyright 2017 American Chemical Society.

PtAu24 nanocluster was first examined as an HER electrocatalyst.34 As shown in Figure 2c, Pt-doping drastically altered the redox properties of the host nanocluster; more specifically, reduction potentials of PtAu24 shifted positively by almost 1.0 V in comparison with that of Au25, which has practical implications in reductive electrocatalysis. Figure 5a indeed shows that the HER catalytic rate constants (kobs) found for PtAu24 are much higher than those for Au25. The turnover frequencies (TOFs) obtained from controlled potential electrolysis (CPE) at various overpotentials (η) for PtAu24 in both homogeneous and heterogeneous conditions were remarkably high; for example, 34 mol H2 (mol cat)−1 s−1 at η= 0.6 V in aqueous media. The mass activity of PtAu24 was even higher than the benchmarking Pt/C as compared in Figure 5b. Figure 5c shows the comparison of the hydrogen adsorption and the HER energetics calculated for PtAu24. The energy change of the Volmer step (step 1) calculated for [PtAu 24 (SR) 18 ] 2− is rather thermodynamically neutral (−0.059 eV), whereas that for [Au25(SR)18]− is 0.539 eV. This result clearly indicated the initial hydrogen binding is energetically favorable on [PtAu24(SR)18]2− but is endothermic on [Au25(SR)18]−, explaining the high HER activity observed for [PtAu24(SR)18]2−. The DFT further supported the heterolytic (Heyrovsky pathway) observed for PtAu24 with an energy change of −0.155 eV (step 2b). The structural analysis of the [H-PtAu24]− intermediate revealed that hydrogen favorably binds on the hollow surface of the PtAu12 core. It was also noted that the bond length from the central Pt to the adsorbed H is 1.788 Å, considerably shorter than the length from the surface Au to the adsorbed H (2.031 Å), suggesting that the adsorbed H atom forms H−Pt chemical bond with the central Pt. This work demonstrated that the redox potentials and binding affinity can be fine-tuned by doping of a Pt atom into gold catalysts. 3.1.2. CO2 Reduction Reaction (CO2RR). Another important energy conversion reaction includes the electrocatalytic CO2 conversion into valuable chemicals and fuels. Unlike the HER, the CO2RR is a multielectron and multiproton process, involving a number of different surfacebound reaction intermediates. A number of CO2 reduction products can be produced in various conditions.1 Additionally, the thermodynamic reduction potentials for most of CO2RR products are close to that of HER, making H2 an additional byproduct.1 Thus, it is of crucial importance to design

electrocatalysts to possess both high activity and high selectivity for the particular product of interest. Atomically precise nanoclusters with the tunable core−shell structure may offer a well-defined platform for the design and optimization of CO2RR catalyst for selective and efficient transformation of CO2 to a desired product. Earlier work on the selective CO2 conversion to CO electrocatalyzed by Au25 nanoclusters can be found in other review article and research report.33,39 Here, we briefly highlight our recent work on a Cu-hydride nanocluster. Highly selective electroreduction of CO2 was observed with a Cu-hydride nanocluster, namely Cu32H20L12 nanocluster (L= S2P(OiPr)2). On the basis of DFT calculations, Jiang and coworkers38 predicted that the negatively charged hydrides in Cu nanocluster present great significance in deciding the product selectivity in the CO2 conversion reaction, which generates HCOOH over CO at a lower overpotential. HCOOH is produced through the lattice-hydride mechanism as illustrated in Figure 6a; that is, HCOOH is first produced as the surface hydrides reduce CO2, and the hydride vacancies are then remade by the electrochemical proton reduction. CPE results in Figure 6b show that the theoretical prediction is indeed verified experimentally. HCOOH was predominantly produced at low overpotentials (89% at 0.3 V and 83% at 0.4 V), with minor production of CO and H2. The selective conversion of CO2 to HCOOH is noteworthy and highlights the important role of the lattice hydride in the electrocatalytic CO2 conversion. In comparison, CO and H2 are typically produced from Cu nanoparticles and Cu foil, respectively, at lower overpotentials. 3.2. Electrochemical Sensing

Another technological application of metal nanoclusters that takes advantage of their unique redox properties is electrochemical sensing. In fact, gold nanoparticles have been widely used in a vast number of electrochemical sensing applications.40 However, these nanoparticles (>3 nm) are redox inactive, and thus, they were typically immobilized into a modified electrode along with a redox mediator or a redox enzyme. The extraordinary electrochemical characteristics of gold nanoclusters present explicit virtues for the development of a versatile modified electrode in which the redox-active gold nanoclusters can function as an electron transfer mediator as well as an electronic conductor. Kumar et al.35 first demonstrated the Au25-based amperometric sensing of biologically important analytes; for instance, G


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Figure 7. (a) Mechanism illustrating the mediated electrocatalytic oxidation and ensuing electron transport across the entrapped Au25 in a Au25 sol−gel electrode. Cyclic voltammograms showing electrocatalytic oxidations of (b) dopamine (DA) and (c) ascorbic acid (AA) at a GS-Au25 sol− gel electrode. Reproduced with permission from ref 32. Copyright 2012 The Royal Society of Chemistry.

Figure 8. (a) Schematic illustrating processes that occur in a GOx-DMIm-Au25 composite electrode. Glucose-reduced GOx is reoxidized by the Au25-mediated electrooxidation process. The resulting electron injected onto Au25 is then transported by the electron hopping process through Au25 sites in a GOx-DMIm-Au25 composite electrode. (b) Plots showing the dependencies of electronic diffusion coefficient (blue) and detection sensitivity for glucose (red) on the GOx volume fraction. Reproduced with permission from ref 36. Copyright 2014 American Chemical Society.

ascorbic acid and uric acid. A modified electrode was fabricated by immobilizing C6S−Au25 nanoclusters into a sol−gel matrix (Figure 7a). The Au25 nanocluster displayed superior catalytic performance toward the oxidation of various analytes. The increased anodic current was found to linearly augment with increasing the concentration of analyte with sensitivities of 1.56 and 1.50 μAμM−1 for ascorbic acid and uric acid, respectively. [Au 25]− → [Au 25]0

Equations 3 and 4 illustrate the Au25-mediated electrocatalytic oxidation of analyte (Figure 7a). Detailed electron transfer study of Au25 sol−gel electrodes manifested that Au25 plays the dual role as an electronic conductor, as well as an electron transfer mediator. Another advantage of nanoclusterbased sensor is that the ligand shell can be engineered to improve the selectivity. Kwak et al.32 reported selective determination of dopamine using Au25 protected with charge selective ligands, glutathione (GS) that carries negative charges at neutral pH. The GS-Au25 nanoclusters were entrapped in a sol−gel matrix via thiol linkers. The GS-Au25 modified sol−gel electrode exhibited superior electrocatalytic performance for

(3)

[Au 25]0 + analyte (reduced) → [Au 25]− + analyte (oxidized)

(4) H


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Figure 9. Schematic illustrating tailored electrocatalysis by rationally engineered metal nanoclusters.

realization of this concept has been difficult for solid catalysts because of their inherent surface heterogeneity. It is even more challenging for molecular catalysts because of their complicated chemical environment. Metal nanoclusters with a welldefined core−shell structure may offer special advantage in that the core and shell can be independently engineered to show suitable properties for electrocatalysis. No single nanocluster catalyst can have the appropriate bind energy for every reaction. Instead, the core and shell of nanocluster have to be engineered to exhibit suitable properties tailored for a specific reaction (Figure 9). The redox potentials of nanoclusters can be additionally engineered by their composition, shape, and size for them to act as an efficient electron transfer mediator for the targeted electrocatalysis. It was demonstrated that highly active and selective electrocatalysis can be achieved by PtAu24 (HER), Au25 (CO2 → CO), and Cu-hydride (CO2 → HCOOH) nanoclusters. The protecting ligand shell of these ultrasmall nanoclusters appears to be quite open and thus small reactants can readily access to the core surface, as demonstrated in their HER and CO2RR electrocatalysis. Much work remains to achieve advances in revealing active sites and engineering thereof in nanoclusters. A well-defined model nanocatalyst is indispensable to unravel the detailed mechanism of electrocatalysis and to lead to a new efficient electrocatalyst. Atomically controlled nanoclusters are anticipated to provide a powerful platform for the discovery of finely tuned nanocatalysts, which has broad implications for catalysis beyond HER and CO2RR.

the oxidation of dopamine (DA) with a sensitivity of 0.29 μAμM−1 but essentially no activity for the oxidation of ascorbic acid (AA). The electrode additionally presented great selectivity for DA in the presence of an interferent, ascorbic acid. Ionic liquids of a Au25 nanocluster were developed as universal matrix for amperometric enzyme electrodes.36 In this work, MPS-Au25 nanoclusters were ion-paired with 1-decyl-3methylimidazolium (DMIm) to form a stable ionic liquid possessing both ionic and electronic conductivity (Figure 8a). By incorporating glucose oxidase (GOx) into a film of Au25 ionic liquid, composite enzyme electrodes could be readily fabricated. The enzyme electrodes showed great mediated electrocatalytic activity that was successfully exploited for the detection of glucose. It was found that the Au25 nanoclusters in the composite films function as efficient redox mediators, as well as electronic conductors, whose transport dynamics was found to be closely associated with the sensing sensitivity (Figure 8b). Additionally, the Au25 ionic liquid serves as an excellent support for GOx.

4. SUMMARY AND PROSPECTS Atomically precise metal nanoclusters exhibit unique electrochemical properties that are very sensitive to the change in their size, shape, and composition. Understanding how these structural parameters affect the electrochemical properties is a grand challenge in the practical utilizations of them. A prerequisite is to synthesize a series of well-defined metal nanoclusters with various sizes and compositions. Future work is expected to gain more insight into the size-focused synthesis and galvanic and antigalvanic exchange reactions for the preparation of atomically precise nanoclusters with molecular uniformity. Electrochemical methods are very informative about their electronic structures especially near the HOMO and LUMO levels. The total structural determinations and theoretical predictions of atomically precise metal nanoclusters have played critical roles in understanding their optical, electronic, and other physicochemical properties. Combined and concerted efforts of these will permit in-depth understanding of the nanocluster structure−electrochemical property relationship. The theoretically predicted volcano plots for many electrochemical conversion processes, such as oxygen evolution/ reduction reactions (OER/ORR), hydrogen evolution/oxidation reactions (HER/HOR), and CO2RR have revealed that the best catalyst is the one that has appropriate bind energy with reaction intermediates. However, the experimental

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dongil Lee: 0000-0001-7254-1197 Notes

The authors declare no competing financial interest. Biographies Kyuju Kwak received her B.S. in Chemistry (2010) and Ph.D. in Analytical Chemistry (2016) from Yonsei University. She is currently a National Research Foundation Postdoctoral Fellow in the laboratory of Prof. Dongil Lee at Yonsei University. Her research interests include the development of electrochemical methods and novel catalysts for clean energy-related applications. Dongil Lee received his B.S. and M.S. (Inorganic Chemistry) from Yonsei University and Ph.D. (Physical Chemistry) from the I


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University of Cambridge in 2000. Following postdoctoral research at the University of North Carolina at Chapel Hill, he joined the chemistry faculty of Western Michigan University in 2003. In 2008, he moved to Yonsei University, where he is a professor of chemistry. His current research interests include metal nanoclusters and their applications in electrocatalysis, sensing, and solar energy conversion.

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ACKNOWLEDGMENTS This work was supported by the Korea CCS R&D Center (KCRC) Grant (NRF-2014M1A8A1074219) and NRF Grants NRF-2017R1A2B3006651 and NRF-2009-0093823. K.K. acknowledges the support from the NRF Grant (NRF2017R1A6A3A01008549).

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