Effects of Metal-Doping on Hydrogen Evolution Reaction Catalyzed by

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Functional Nanostructured Materials (including low-D carbon)

Effects of Metal-Doping on Hydrogen Evolution Reaction Catalyzed by MAu24 and M2Au36 Nanoclusters (M = Pt, Pd) Woojun Choi, Guoxiang Hu, Kyuju Kwak, Minseok Kim, De-en Jiang, Jai-Pil Choi, and Dongil Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16178 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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ACS Applied Materials & Interfaces

Effects of Metal-Doping on Hydrogen Evolution Reaction Catalyzed by MAu24 and M2Au36 Nanoclusters (M = Pt, Pd) Woojun Choi,† Guoxiang Hu,‡ Kyuju Kwak,† Minseok Kim,† De-en Jiang,*‡ Jai-Pil Choi* †§ and Dongil Lee*†

†

Department of Chemistry, Yonsei University, Seoul 03722, Korea

‡

Department of Chemistry, University of California, Riverside, California 92521, USA

§

Department of Chemistry, California State University–Fresno, Fresno, California 93740, USA

KEYWORDS

Nanocluster, Hydrogen Evolution Reaction, Catalyst, Metal-Doping, Hydrogen

Adsorption Energy

ACS Paragon Plus Environment

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ABSTRACT This paper describes the effects of doped metals on hydrogen evolution reaction (HER) electrocatalyzed by atomically controlled MAu24 and M2Au36 nanoclusters, where M = Pt and Pd. HER performances, such as onset potential (Eonset), catalytic current density and turnover frequency (TOF), are comparatively examined with respect to the doped metals. Doping Pt or Pd into gold nanoclusters not only changes the electrochemical redox potentials of nanoclusters but also considerably improves the HER activities. Eonset is found to be controlled by the nanocluster’s reduction potential matching with the reduction potential of H+. The higher catalytic current and TOF are observed with the doped nanoclusters, in order of PtAu24 > PdAu24 > Au25. The same trend is observed with the Au38 group (Pt2Au36 > Pd2Au36> Au38). Density functional theory calculations have revealed that the hydrogen adsorption free energy (∆GH) becomes significantly lowered by metal-doping, in order of Au25 > PdAu24 > PtAu24 and Au38 > Pd2Au36 > Pt2Au36, indicating hydrogen adsorption on the active site of nanocluster is thermodynamically favored by Pd-doping and further by Pt-doping. The doped metals, albeit buried in the core of the nanoclusters, have profound impact on their HER activities by altering their reduction potentials and hydrogen adsorption free energies.


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INTRODUCTION

Hydrogen (H2) is widely considered as the fuel of the future.1-2 However, the majority of hydrogen is currently being produced from fossil fuels, which inevitably produce the environmentally hazardous byproducts, such as CO2.3-4 One of the sustainable technologies for the hydrogen production with less damage to the environment is to use surplus electric energy for the electrolytic conversion of water into hydrogen.5 Many types of homogeneous6-8 and heterogeneous9-12 catalyst materials have been developed to electrocatalyze hydrogen evolution reaction (HER) from water. The recent in silico design of active solid catalysts for HER has disclosed the importance of hydrogen adsorption free energy (∆GH) in the design of active electrocatalysts.13-15 A plot of catalytic activity for a wide range of catalyst materials against ∆GH typically gives a volcano relationship, illustrating that the highest catalytic activity can be achieved by having a thermodynamically neutral catalyst (i.e., ∆GH = 0).14-15 It remains challenging, however, to experimentally realize this concept in solid catalysts because of their inherent heterogeneity in the surface structure and composition.16-17 A highly homogeneous catalyst with well-defined structure would be indispensable to precisely tune the catalytic property at the atomic level. Atomically precise gold nanoclusters, such as Au25(SR)18,18-20 Au38(SR)24,21-22 and Au102(SR)44,23 where SR is thiolate ligand, display well-defined atomic and electronic structures.18-25 They have attracted growing attention for the last decade because of their unique electrochemical, optical, and catalytic properties, which are distinctly different from the plasmonic gold nanoparticles.18 Recently, doping one or more foreign metals into the Au25 and Au38 nanoclusters have been extensively explored as a powerful tool to tune their electronic,


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optical and electrochemical properties.26-32 Depending on the dopant, multiply doped MxAu25-x nanoclusters (M = Ag, Cu) and mono-metal doped MAu24 (M = Pd, Pt, Cd, Hg) nanoclusters were obtained from various synthetic procedures.26, 29-31 Pt(Pd)-doped MAu24 nanoclusters are of particular interest because they undergo Jahn-Teller distortion, accompanying splitting of the superatomic 1P orbitals. The consequences of the Jahn-Teller distortion occurred in PtAu24 and PdAu24 nanoclusters were visibly observed in their altered electrochemical and optical properties that were distinctively different from those of the undoped Au25 nanocluster.28 More recently, we showed that Pt(Pd)-doped M2Au36 nanoclusters can be prepared and the two Pt(Pd) atoms are located in the two central positions of the face-fused bi-icosahedral M2Au21 core.27 In this case, however, their electrochemical and optical properties were distinctly different from each other and the Jahn-Teller distortion was only observed for Pd2Au36 nanocluster. In a previous study, we have shown that PtAu24 nanocluster exhibits remarkable HER activities in both homogeneous and heterogeneous electrocatalytic conditions.33 The origin of the extraordinary catalytic activity was ascribed to the nearly neutral hydrogen binding step on PtAu24 as predicted by density functional theory (DFT) calculations. In other words, an efficient thermoneutral HER catalyst could be prepared by simple doping of a platinum atom into the Au25 nanocluster. In another study, we have shown that both PtAu24 and PdAu24 nanoclusters have nearly isoelectronic structure and exhibit very similar redox properties.28 Would PdAu24 exhibit the high catalytic activity for HER as PtAu24 does? This has motivated us to synthesize highly pure PtAu24 and PdAu24 nanoclusters and compare their HER activities. The comparison of the electrocatalytic HER activities has revealed that PtAu24 shows much higher HER activity than PdAu24, indicating the important role of the dopant in HER. The comparison was also extended to Pt2Au36 and Pd2Au36 nanoclusters that exhibit distinctly different electronic


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structures and redox properties. Comparative experimental and DFT investigations evidently unveil the effects of the dopant at the atomic level that controls the hydrogen adsorption step and the resulting HER activity.

RESULTS AND DISCUSSION

MAu24 and M2Au36 Nanoclusters (M = Au, Pt, Pd). MAu24(SC6H13)18 and M2Au36(SC6H13)24 (M= Au, Pt, Pd) were synthesized according to the procedures reported elsewhere.27-28 The MAu12 and M2Au21 core structures of MAu24 and M2Au36 nanoclusters are shown in Figures 1a and 1b, respectively. The formulas of the synthesized nanoclusters were determined by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Figures 1c and 1d show the MALDI spectra of the isolated MAu24 and M2Au36 nanoclusters (M= Au, Pt, Pd), respectively. In Figure 1c, there is only one peak observed in the m/z range of 2,500 – 11,500 Da for each nanocluster, indicating the isolated nanoclusters are highly pure. The peaks observed at m/z ~7,034 (black), ~7,032 (red), and ~6,943 Da (blue) correspond to the intact ions of Au25(SC6H13)18, PtAu24(SC6H13)18, and PdAu24(SC6H13)18, respectively. Indeed, the isotope patterns shown in the insets of Figure 1c match well with the simulated ones of PtAu24(SC6H13)18 and PdAu24(SC6H13)18, indicating that a single gold atom in Au25(SC6H13)18 was cleanly replaced by a Pt(Pd) atom. Similarly, a single peak is observed for each nanocluster in the m/z range of 5,000 – 14,500 Da as can be seen in Figure 1d. The peaks at m/z ~10,298 (green), ~10,294 (purple), and ~10,117 Da (orange) were assigned to Au38(SC6H13)24, Pt2Au36(SC6H13)24, and Pd2Au36(SC6H13)24, respectively. The isotope patterns shown in the insets of Figure 1d are superimposed with the simulated ones of


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Pt2Au36(SC6H13)24 and Pd2Au36(SC6H13)24, respectively, indicating that extremely pure nanoclusters were obtained. Hereafter, Au25(SC6H13)18, MAu24(SC6H13)18, Au38(SC6H13)24, and M2Au36(SC6H13)24 will be abbreviated as Au25, MAu24, Au38, and M2Au36 (M = Pt, Pd), respectively. In addition, the charge states of the isolated nanoclusters were found to be [Au25]-, [PtAu24]0, [PdAu24]0, [Au38]0, [Pt2Au36]2-, and [Pd2Au36]0 as determined in previous studies.18, 2728, 31-32, 34-35

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Pt2Au36(SC6H13)24

Pd2Au36(SC6H13)24

Photon energy (eV)

Photon energy (eV)

Figure 1. Core structures of (a) MAu24 and (b) M2Au36 nanoclusters (M = Pt, Pd). The doped metals (blue) are located in the central position(s) of (a) MAu12 and (b) M2Au21 cores. MALDI mass spectra of (c) MAu24(SC6H13)18 and (d) M2Au36(SC6H13)24 (M = Au, Pt, Pd). The insets show the comparison between the experimental data (colored line) and the simulated isotope patterns (grey bars). UV-Vis-NIR absorption spectra of (e) MAu24(SC6H13)18 and (f)


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M2Au36(SC6H13)24 in trichloroethylene. The wavelength-scale absorption spectrum, Abs(λ), was converted to the energy-scale spectrum by using Abs(E) ∝Abs(λ)×λ2 relationship. FT-NIR spectrum of [Pd2Au36]0 (dashed orange) in trichloroethylene is displayed together to show the NIR bands. UV-Vis-NIR absorption spectra of the doped PtAu24 and PdAu24 show distinctly different absorption profiles from that of Au25 as can be seen in Figure 1e. In particular, the appearance of the NIR band at 1.1 and 1.2 eV for respectively PtAu24 and PdAu24 suggests that the electronic structures of the nanoclusters were significantly altered upon doping. In addition, the UV-VisNIR absorption profiles of PtAu24 and PdAu24 are consistent with those of neutral [PtAu24]0 and [PdAu24]0, in which the central Au atom in the nanocluster core was replaced with Pt and Pd, respectively.28 The UV-Vis-NIR absorption spectra of doped Au38 shown in Figure 1f are also significantly altered upon metal-doping. In this case, however, the absorption profile of Pt2Au36 is drastically different from that of Pd2Au36. That is, whereas Pt2Au36 exhibits a rather similar absorption profile to that of Au38 with the first absorption peak at 1.4 eV, Pd2Au36 shows a NIR band at around 0.8 eV along with the peak at 1.2 eV. This result suggests that the doped Pt2Au36 and Pd2Au36 have very different electronic structures. Furthermore, the absorption profiles of Pt2Au36 and Pd2Au36 match well with those of [Pt2Au36]2− and [Pd2Au36]0, where the two Pt(Pd) are located in the two central positions of the face-fused bi-icosahedral M2Au21 core (Figure 1b).27 To further investigate the electronic structures and redox properties of doped nanoclusters, we carried out square wave voltammetry of the nanoclusters in CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6). Figure 2a shows the square wave voltammograms (SWVs) of Au25 (black), PtAu24 (red), and PdAu24 (blue) obtained at room


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temperature. In Figure 2a (black), the highly resolved current peaks labelled as O2, O1, and R1 are observed at the formal potentials of Au25. The open circuit potential (OCP) measured in the solution of Au25 nanoclusters was −0.52 V vs. ferrocene (Fc+/0). Therefore, the O1 and O2 peaks found at −0.40 and −0.03 V, respectively, correspond to the first ([Au25]0/−) and second oxidations ([Au25]+/0), while the R1 peak at −2.08 V represents the first reduction ([Au25]−/2−). As can be seen in Figure 2a, the redox potentials are drastically altered upon doping. That is, when Au25 is doped with a Pt atom, the first reduction peak (R1) is found at –0.76 V, dramatically shifted by more than 1.20 V to the positive direction. Unlike Au25, PtAu24 (red) clearly shows the second reduction (R2) as well in the potential window investigated. The HOMO-LUMO gap of PtAu24 can then be determined from the electrochemical gap between O1 and R1 (O1-R1). Since this gap includes the charging energy term, the HOMO-LUMO gap was determined after subtracting the charging energy from the electrochemical gap.24, 29 In this work, the charging energy term was approximated by taking the potential difference between O1 and O2. The HOMO-LUMO gap determined for PtAu24 is 0.30 eV, significantly reduced compared with that of Au25 (1.31 eV). This is the result of the Jahn-Teller distortion occurring in [PtAu24]0 that possess two less electrons compared with the superatomic 8-electron [Au25]-.28 Because of the unequal occupation of the superatomic 1P orbitals, PtAu24 undergoes Jahn-Teller distortion and the triply degenerated HOMO splits into doubly degenerated HOMO and LUMO. Figure 2a (blue line) shows that the SWV of PdAu24 is very similar to that of PtAu24, indicating that both PdAu24 and PtAu24 have very similar electronic structures (nearly isoelectronic) and redox properties. The formal potentials, electrochemical gaps and HOMO-LUMO gaps obtained for Au25, PtAu24, and PdAu24 are listed in Table S1 (Supporting Information). Table S1 also shows that the electrochemically predicted HOMO-LUMO gaps match very well with those predicted


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from the DFT calculations.28 We found that the geometric difference between Pd and Pt doped nanoclusters is rather small. Table S2 compares the average Au(shell)-Au(shell) and M(center)Au(shell) distances between PtAu24 and PdAu24. One can see that the difference is less than 0.5%.

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O3

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Figure 2. SWVs of (a) 1.0 mM MAu24 nanoclusters (M = Au, Pt, Pd): Au25 (black), PtAu24 (red), and PdAu24 (blue) and (b) 1.0 mM M2Au36 nanoclusters: Au38 (green), Pt2Au36 (purple) and Pd2Au36 (orange) in CH2Cl2 containing 0.1 M Bu4NPF6. The measured potentials were corrected using ferrocene (Fc+/0) as an internal standard. The arrows denote the OCPs of the nanocluster solutions. Square wave voltammetry of Au38, Pt2Au36 and Pd2Au36 nanoclusters were conducted in the same condition and their SWVs are shown in Figure 2b. The OCP of Au38 (green) was found to be −0.50 V vs. Fc+/0. Therefore, the peaks at −1.32 (R1) and −1.52 V (R2) correspond respectively to the first and second reductions, while the peaks at −0.10 (O1), 0.25 (O2), and 0.47 (O3) V are for the first, second, and third oxidations, respectively. Pt2Au36 (purple, OCP = −0.58 V) exhibits the similar oxidation patterns (three oxidation peaks, O1 – O3) to Au38, but one reduction peak (R1). In contrast, Pd2Au36 (orange, OCP = −0.50 V) displays completely


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different voltammetric behaviors (two oxidations and two reductions) from Au38 and Pt2Au36. While Au38 and Pt2Au36 have their first reduction peaks (R1) at −1.32 and −1.68 V, respectively, R1 of Pd2Au36 is observed at a much more positive potential, −0.76 V (Figure 2b). The dramatic shift of the reduction potential is the consequence of the Jahn-Teller distortion occurring in [Pd2Au36]0 that has two less electrons.27 Formal potentials observed from SWVs of Au38, Pt2Au36, and Pd2Au36 are summarized in Table S1 in the Supporting Information. The estimated electrochemical gaps and HOMO-LUMO gaps of Au38 and Pt2Au36 were rather close to each other but distinctly different from those of Pd2Au36. In addition, the electrochemically determined HOMO-LUMO gaps of Au38, Pt2Au36, and Pd2Au36 reasonably agreed with the DFTpredicted HOMO-LUMO gaps.27 Interestingly, the geometric difference between Pd and Pt doped nanoclusters is again quite small. As summarized in Table S2, the average Au(shell)Au(shell) and M(center)-Au(shell) distances between Pt2Au36 and Pd2Au36 differ by less than 0.5%.

Electrocatalytic HER Activities of MAu24 Nanoclusters. The voltammetric investigations in the previous section have shown that PtAu24 and PdAu24 exhibit very similar electronic structures and redox properties, which are plainly different from those of Au25. In a previous study, we reported the remarkably high HER activity of PtAu24 in both homogenous and heterogeneous conditions.33 It would thus be interesting to compare the HER activities of PtAu24 and PdAu24 and to investigate the role of dopant in their electrocatalytic HER. The catalytic HER performances of Au25, PtAu24 and PdAu24 were examined by linear sweep voltammetry (LSV) and constant potential electrolysis (CPE). The composite electrodes were fabricated by dropcasting a nanocluster suspension containing carbon black (C) and Nafion


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on a glassy carbon electrode (GCE) and a gas diffusion layer (GDL) electrode for LSV and CPE experiments, respectively. Figure 3a shows LSVs obtained from MAu24/C/GCE, where MAu24 = Au25, PtAu24 and PdAu24, in 1.0 M Britton-Robinson buffer and 2.0 M KCl solution (pH 3.0). As can be seen in the figure, PtAu24 shows much higher HER activity than PdAu24, even though they have almost identical electronic structures and redox properties. In comparison, the HER current densities were found in order of: PtAu24 (15.3 mA/cm2) > PdAu24 (8.7 mA/cm2) > Au25 (4.2 mA/cm2) at −0.6 V vs. RHE.

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Figure 3. (a) LSVs of MAu24/C/GCE and (b) plots of TOF vs. potential obtained from CPE of MAu24/C/GDL electrodes, where MAu24 = Au25, PtAu24, and PdAu24, in 1.0 M Britton-Robinson buffer solution and 2.0 M KCl (pH 3). Insets show the enlarged graphs in the low overpotential region. In Figure 3a inset, one can see that the first reduction peak of PtAu24 is clearly observed in the LSV and the catalytic HER current starts to increase significantly near the second reduction potential of PtAu24. Similar LSV is observed for PdAu24, but the HER current is much smaller. The fact that the HER current dramatically increases near the second reduction potential suggests


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that HER is greatly enhanced by the electrochemically produced [PtAu24]2− and [PdAu24]2-. Therefore, the PtAu24 and PdAu24 can be considered as a redox mediator that shuttles electrons from GCE to protons for HER.36-37 The onset potential (Eonset) for the catalytic HER was roughly estimated by LSV and then confirmed by CPE in which Eonset was determined by the potential where the generation of H2 gas begins to be detected by gas chromatography after 3h electrolysis. CPE was conducted at 0.0, −0.07, −0.1, −0.2, −0.3, −0.4, −0.5, and −0.6 V vs. RHE. Eonset found for Au25 was −0.20 V vs. RHE. Eonset found for PtAu24 was −0.07 V, significantly lowered compared to Au25. For PdAu24, the H2 production was found to be much smaller than PtAu24, but the Eonset was clearly found at -0.07 V as can be seen in the inset of Figure 3b. This is remarkable in that the overpotential (η) for electrocatalytic HER can be greatly reduced by a simple doping of Pt or Pd. It is noteworthy that the overpotentials found for both PtAu24 and PdAu24 (η = 0.07 V) are quite close to the thermodynamic reduction potential of proton and also comparable to that of natural hydrogenase enzymes (η = ~0.1 V).38 The effect of the reduction potential of catalyst on HER has been investigated in a number of molecular catalysts.39-40 For example, Co(II) complexes prepared with pentadentate polypyridyl ligands showed tunable reduction potentials that could be controlled by the moieties covalently attached to the ancillary scaffold of polypyridyl ligand or the ligands directly coordinated to the Co(II) ion.40 Also, Ni(II) complexes showed diversified reduction potentials with various substituents on cyclic diphosphine ligands.39 However, the relationship between the reduction potential and Eonset was not clearly established in these systems and the potential shifts were often unpredictable. In the present system, tuning the reduction potential by doping a foreign metal atom into Au25 appears to directly influence the Eonset. This is especially the case for PtAu24 and PdAu24, which act as an efficient electron transfer mediator36-37, 41-42 and have the


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matched reduction potentials with that of H+/H2. That is, metal-doping can effectively change the redox potential of nanocluster catalyst that has a direct impact on Eonset. Although PtAu24 and PdAu24 exhibit almost identical Eonset for HER, their overall catalytic HER activities appear to be significantly different; the HER current generated at PtAu24 is much higher than that at PdAu24. To evaluate the actual HER activities of Au25, PtAu24, and PdAu24, the turnover frequencies (TOFs) were determined at various overpotentials. The observed TOF values of Au25, PdAu24, and PtAu24 were respectively 8.2, 13.0 and 33.3 mol H2 (mol catalyst)−1 s−1 at η = 0.6 V. This result shows that the catalytic HER activity is greatly enhanced by Pddoping and further by Pt-doping. It is notable that PtAu24 exhibits much higher TOF than PdAu24 at all potentials, even though they exhibit the same Eonset and very similar reduction potentials. The higher current density and TOF of PtAu24 observed in Figure 3 can be hardly explained by the electronic structure or redox properties of nanocluster.


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a

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[PtAu24 -H]

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1/2 H2

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Figure 4. (a) Calculated Gibbs free energy diagram for hydrogen adsorption (ΔGH) on Au25, PdAu24 and PtAu24. (b) Optimized structures of [PtAu24-H], [PdAu24-H], and [Au25-H]. Protecting motifs are shown in line mode. Pt, pink; Pd, purple; Au, yellow; adsorbed H, blue; S, green; C, grey. To better understand the origin of the different catalytic activities observed for PtAu24 and PdAu24 in Figure 3, we computed their hydrogen adsorption free energies (∆GH) using a reversible hydrogen electrode (H+ + e- → ½H2). Nørskov et al. reported that the ∆GH is the key descriptor for HER and a good HER catalyst is the one having a nearly neutral ∆GH.13-15 In Figure 4a, the ∆GH values computed for Au25(SCH3)18, PdAu24(SCH3)18 and PtAu24(SCH3)18 were respectively 0.43, 0.36, and 0.31 eV. The significantly reduced ∆GH calculated for PtAu24(SCH3)18 clearly explains the higher HER activity observed in Figure 3. That is, by altering ∆GH the central metal atom of MAu24 nanocluster greatly facilitates the catalytic HER.


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In Figure 4b, the optimized structures of the [PtAu24-H], [PdAu24-H] and [Au25-H] intermediates show an interesting trend of H-M distances: for the center atom, H-Pt = 1.787 Å, H-Pd = 1.789 Å, and H-Au = 2.189 Å; for the three Au atoms on the icosahedral shell, H-Au(Pt) = 2.016 Å, HAu(Pd) = 1.991 Å, and H-Au(Au) = 1.937 Å. In other words, the adsorbed H penetrate into the icosahedral shell and interacts directly with the central dopant in PtAu24 and PdAu24. In contrast, H interacts stronger with the Au atoms on the icosahedral shell in Au25. In addition, the structural changes of nanoclusters after H incorporation were investigated. The changes to the nanocluster structures after H incorporation are best reflected in the changes in the Au-Au distances. These results are summarized in Table S3 (Supporting Information). For PtAu24 and PdAu24, the adsorbed H penetrates into the icosahedral shell, interacting with the central dopant and three Au atoms where the average Au-Au bond distance increases from 2.973 Å to 3.438 Å and from 2.968 Å to 3.384 Å for Pt and Pd, respectively. In other words, the Au3 triangle directly interacts with H is pushed open. These analysis manifest that the central dopant, albeit buried in the icosahedral shell, directly interacts with the adsorbed H and thus greatly reduce the ∆GH, playing a crucial role in HER.

Electrocatalytic HER Activities of M2Au36 Nanoclusters. The comparison of HER activities in the previous section has revealed that the dopant located at the center directly affect the onset potentials and HER activities by altering their reduction potentials and ∆GH. To see if the dopant effects are observed in other doped nanoclusters, HER activities of M2Au36 (M = Au, Pt, Pd) were investigated by LSV and CPE (Figure 5). Unlike PtAu24 and PdAu24, Pt2Au36 and Pd2Au36 exhibit distinctly different SWVs (Figure 2b). The different redox behavior of Pd2Au36 is reflected in its HER activity. That is, HER current is


15


clearly noticed near the second reduction potential of Pd2Au36 (Figure 5a inset) while it is silent for Pt2Au36 or Au38, indicating that Pd2Au36 acts as an electron transfer mediator for HER. The Eonset is found at −0.07 V vs. RHE for Pd2Au36, at a lower overpotential than those of Au38 and Pt2Au36 (both −0.20 V vs. RHE). On the other hand, the HER current of Pt2Au36 increases more rapidly than Pd2Au36 and Au38 when η is higher than 0.4 V. Comparing the HER current at η = 0.6 V (Figure 5a), the current density for catalytic HER appears in order: Pt2Au36 (10.9 mA/cm2) > Pd2Au36 (8.7 mA/cm2) > Au38 (6.8 mA/cm2).

15

0.2

0.4

0.6

0.8

12 9 6

Current density (mA cm-2)

0.6

0.4

Pt2Au36

0.2

Pd2Au36

0.0 0.1

Au38 0.0

-0.1

-0.2

-0.3

-0.4


3 0 0.0

-0.2

-0.4

-0.6

-0.8


Overpotential (V)

b 40

0.0

0.2

0.4

0.6

0.8

1.5

30


Overpotential (V) 0.0


a Current density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Pt2Au36

1.0

0.5

20

0.0 0.0

-0.1

-0.2

-0.3


Pd2Au36

10 Au38

0 0.0

-0.2

-0.4

-0.6

-0.8


Figure 5. (a) LSVs of M2Au36/C/GCE and (b) plots of TOF vs. potential obtained from CPE of M2Au36/C/GDL electrodes, where M2Au36 = Au38, Pt2Au36, and Pd2Au36, in 1.0 M BrittonRobinson buffer solution and 2.0 M KCl (pH 3). Insets show the enlarged graphs in the low overpotential region. The HER activities of M2Au36 were further studied by CPE experiments at overpotentials from 0.0 V to 0.6 V. As can be seen in Figure 5b, the H2 was detected at low η of 0.07 V for Pd2Au36, but relatively high η (0.20 V) was needed to first detect H2 for both Au38 and Pt2Au36. Similar to PtAu24 and PdAu24, the lower η observed can be attributed to the mediated effect of Pd2Au36.


16

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Therefore, these results again corroborate that the redox potential matching between the catalyst and proton plays an important role in the determination of Eonset and η. The HER activities of M2Au36 compared in Figure 5b clearly show the effect of the central dopant on HER activities. Pt2Au36 exhibits much higher TOF (28.2 mol H2 (mol catalyst)−1 s−1) at η = 0.6 V than Au38 and Pd2Au36 (10.3 and 13.1 mol H2 (mol catalyst)−1 s−1, respectively). Although Pt2Au36 shows higher overpotential for HER than Pd2Au36, it exhibits much higher TOF than Pd2Au36 and Au38 when the overpotential is higher than 0.4 V as shown in Figure 5b. Comparing with Au38, the TOF of Pt2Au36 was enhanced by more than 2.7-fold at η = 0.6 V. The enhancement upon Ptdoping is smaller than that of PtAu24 (4.1-fold at η = 0.6 V) because of the obvious difference in Eonset. The TOFs of PtAu24 and Pt2Au36 are compared in Figure S1.


17


a

[Au38 -H]

∆GH (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

[Pd2Au36 -H]

[Pt2Au36 -H] H+ + e-

1/2 H2

Reaction coordinate

b

Figure 6. (a) Calculated Gibbs free energy diagram for hydrogen adsorption (ΔGH) on Au38, Pt2Au36, and Pd2Au36. (b) Optimized structures of [Pt2Au36-H], [Pd2Au36-H] and [Au38-H] (upper and lower images show two different angles of the same structure). Protecting motifs are shown in line mode. Pt, pink; Pd, purple, Au, yellow; adsorbed H, blue; S, green; and C, grey. To gain further insight into the origin of the dopant effects on the HER activity, ∆GH of M2Au36(SCH3)24 was calculated by DFT. In Figure 6a, the ∆GH values computed for Au38(SCH3)24, Pd2Au36(SCH3)24 and Pt2Au36(SCH3)24 were respectively 1.04, 0.60, and 0.52 eV. The smallest ∆GH of Pt2Au36 again explains the highest HER activity found for Pt2Au36 in Figure 5 and supports the argument that the dopants located in the central positions of the rod-like core


18

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of M2Au36 nanoclusters (M=Au, Pd, Pt) play a key role in HER. To further understand the difference in ΔGH among Pt2Au36(SCH3)24, Pd2Au36(SCH3)24, and Au38(SCH3)24, we compared the optimized geometries after H binding. In Figure 6b, one can see that H binds at the Au3 triangle on the side of the M2Au21 rod-like core of the Pt2Au36 nanocluster. At this site, H has a more direct interaction with one of the Pt centers with a Pt-H distance of 1.791 Å. H binding on Pd2Au36 has very similar structure as Pt2Au36, with a Pd-H distance of 1.795 Å. The interaction between Pt and H is also evidenced from the local density of states (LDOS) plots (Figure S2), which show hybridizations between Pt-5d states and H-1s states. On Au38, however, H binds at the bridge site in the middle or waist of the Au23 rod-like core with a Au-H distance of 1.773 Å. Structural changes of nanoclusters after H incorporation were also observed for Pt2Au36 and Pd2Au36. As summarized in Table S3, the Au-Au bond distance at the Au3 triangle on the side of the M2Au21 rod-like core increases from 2.996 Å to 3.430 Å and from 2.988 Å to 3.362 Å for Pt and Pd, respectively. The Gibbs free energies of H adsorption (∆GH) at different sites for Au38, Pt2Au36, and Pd2Au36 nanoclusters are listed in Table S4. As can be clearly seen, H prefers the bridge site in the middle or waist of the Au23 core for the Au38, but the Au3 triangle on the side of the M2Au21 core for the Pt2Au36 and Pd2Au36 nanoclusters. The DFT results evidently explain the dopant effects on HER catalyzed by M2Au36 nanoclusters. That is, even though the Pt(Pd) dopants are buried in the M2Au21 core, they directly interact with the adsorbed H and dramatically reduce the ∆GH, leading to the enhanced HER activity. Whereas the ∆GH appears to be a key descriptor accounting for the higher HER activity found for Pt2Au36 than Au38, the differences in ∆GH and Au-H distance between Pt2Au36 and Pd2Au36 appear to be rather too small to explain the observed difference in the HER activity. This may suggest that, in addition to ∆GH, the H2 production step is also of crucial importance in


19


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these catalysts. The release of molecular H2 occurs through either homolytic (Tafel pathway) or heterolytic (Heyrovsky pathway) processes.43-44 Considering the localized active site of M2Au21 core, it is likely that the HER occurs via the coupling of the adsorbed hydrogen with another proton from the solution (Heyrovsky pathway), as revealed in our previous study.33 The Gibbs free energy of the second H adsorbed at the bridge site in the middle of the Pt2Au36 was found to be 0.79 eV, which is too positive or uphill, indicating that the Volmer-Tafel mechanism is indeed difficult. The higher HER activity found for Pt2Au36 in Figure 5b therefore suggests that the Heyrovsky pathway more readily occurs on Pt2Au36 than Pd2Au36.

CONCLUSION

We have shown that the metal dopant(s) in MAu24 and M2Au36 nanoclusters have profound effects on their HER activities by altering their reduction potentials and hydrogen adsorption energies. PtAu24 and PdAu24 displayed very similar electronic structures and reduction potentials that were distinctly different from those of the undoped Au25. Their reduction potentials were well matched with the reduction potential of proton, making them an efficient electron transfer mediator for HER. However, the catalytic HER activity of PtAu24 was found to be much higher than that of PdAu24. DFT calculations revealed that the ∆GH is significantly reduced upon doping of Pt that directly interacts with the adsorbed H. Significantly enhanced HER activity was also observed upon doping of Pt atoms into Au38 nanocluster that induced a substantial ∆GH reduction. In this case, however, the significant overpotential decrease was not observed, highlighting the importance of the reduction potential of catalyst that matches with HER. These comparative studies provide clear experimental evidence delineating the factors that control the HER


20

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performance of nanocluster-based electrocatalyst with atomic precision. We expect that the atomically controlled model catalysts will open up new avenues for the understanding of electrocatalytic processes at the atomic level.

EXPERIMENTAL SECTION

Chemicals. Tetrabutylammonim hexafluorophosphate (Bu4NPF6, >99%) and trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) were purchased from SigmaAldrich. Extrapure grade dichloromethane, tetrahydrofuran, and tetrachloroethylene were obtained from Sigma-Aldrich. All chemicals were used without further purification. Deionized water, having resistivity of 18.2 MΩ·cm, was prepared using a Millipore water purification system. Syntheses of Nanoclusters.

Au25, PtAu24, PdAu24, Au38, Pt2Au36, and Pd2Au36 were

synthesized according to the procedures reported elsewhere.27-28 Characterizations of Nanoclusters.

MALDI mass spectra of nanoclusters were obtained

from an AB Sciex MALDI-TOF mass spectrometer (4800 plus) with a standard UV nitrogen laser (337 nm). A refractron positive ion mode with the accelerating voltage of 15 kV was adopted to acquire the spectra. Samples were prepared by dissolving nanoclusters in CH2Cl2 (0.7 mM) saturated with DCTB as a matrix. The mixture solution was spotted onto a target plate and air-dried. Optical absorption spectra were collected from a Shimadzu UV-Vis-NIR spectrophotometer (UV-3600) using freshly prepared nanocluster solutions in trichloroethylene.


21


Page 22 of 28

FT-NIR spectra were collected from a Bruker (MPA) equipped with a Tungsten source in the spectrum range 5300-12500 cm-1. Electrochemical Measurements.

Square wave voltammetry was performed on an

electrochemical workstation (model 660D, CH Instruments) with 1 mM of nanocluster in CH2Cl2 containing 0.1 M Bu4NPF6 as a supporting electrolyte. The solution was degassed with highpurity Ar gas at room temperature. Two Pt disk electrodes (0.4 mm diameter) were used as the working and counter electrodes and Ag wire quasi-reference electrode (AgQRE) was used as the reference electrode. Measurement was carried out with a pulse height and a width of 20 mV and 20 ms, respectively, at the scan rate of 100 mV/s. Ferrocene (Fc+/0) was added as an internal reference to correct the potentials measured with AgQRE. Linear sweep voltammetry (LSV) was conducted on an electrochemical workstation (model 660D, CH Instruments) in 1.0 M Brinton-Robinson buffer solution (pH 3) containing 2.0 M KCl as an additional supporting electrolyte. The solution was degassed with high-purity Ar gas at room temperature. The composite electrode formed on a glassy carbon electrode (GCE, 0.071 cm2) was used as the working electrode, while Pt plate (1.44 cm2) and Ag/AgCl (3.0 M NaCl) were used as the counter and reference electrodes, respectively. The catalyst ink was prepared by mixing 2.5 nmol of nanocluster with 200 μg of carbon black (Vulcan XC-72) and 3.5 μL of Nafion solution (5 wt%, Sigma-Aldrich) in 50 μL of tetrahydrofuran. The composite electrode was then fabricated by dropcasting the catalyst ink on the GCE and air-dried for 2 h at room temperature. LSVs were obtained at the scan rate of 2 mV/s. Controlled Potential Electrolysis (CPE) Experiments. CPE was performed for 60 min with a ZIVE MP1 potentiostat (WonATech, Korea) under vigorous stirring. The experiment was carried out in a two-compartment, H-cell (110 mL for


22

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each) that was equipped with a composite working electrode and a Ag/AgCl (3.0 M NaCl) reference electrode in the cathode compartment and a Pt plate (1.44 cm2) in the anode compartment. Two compartments were separated by a proton exchange membrane (Nafion 117, Sigma-Aldrich) and filled with 60 mL of degassed 1.0 M Brinton-Robinson buffer solution (pH 3) containing 2.0 M KCl. The composite electrode was fabricated by dropcasting a catalyst ink prepared by mixing 2.5 nmol of nanocluster with 200 μg of carbon black (Vulcan XC-72) and 3.5 μL of Nafion solution (5 wt%, Sigma-Aldrich) in 50 μL of tetrahydrofuran on a gas diffusion layer (GDL) electrode (N1S1007; CeTech Co., 1 cm2) and air-dried for 2 h at room temperature.

Determination of Turnover Frequency (TOF). TOFs were calculated by dividing moles of H2 produced during CPE by moles of the nanocluster used (2.5 nmol) and electrolysis time (s).45 The amount of H2 produced was corrected by subtracting the amount of H2 generated at the GDL coated with only carbon black and Nafion composite (i.e., no catalyst). TOF (s−1 ) =

produced H2 (mol)

amount of catalyst (mol) × time (s)

DFT Calculations.

(1)

DFT calculations of the nanoclusters were all performed with the

quantum chemistry program Turbomole V6.5.46 To save computation time, we simplified the R groups with –CH3 groups. The def2-SV(P) basis sets were used for C, S and H, while effective core potentials which include scalar relativistic corrections and 19 (18) valence electrons were used for Au (Pd, Pt).47 Geometry optimization was done with the TPSS (Tao, Perdew, Staroverov, and Scuseria) functional.48 Gibbs free energy of hydrogen adsorption, ∆GH, was obtain


23


∆𝐺𝐺𝐻𝐻 = 𝐸𝐸(𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝐻𝐻) − 𝐸𝐸(𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐) −

1 2

𝐸𝐸(𝐻𝐻2 ) + ∆𝐸𝐸𝑍𝑍𝑍𝑍𝑍𝑍 − 𝑇𝑇∆𝑆𝑆𝐻𝐻

Page 24 of 28

(2)

where E(catalyst + H) represents the total energy of the catalyst with one adsorbed H, and E[catalyst] represents the total energy of the catalyst without H, E(H2) is the total energy of one gas phase H2 molecule, ∆EZPE is the difference in zero-point energy between the adsorbed H and H in the gas phase H2 molecule, and ∆SH is the entropy difference between the adsorbed H and 1/2H2 in the gas phase at the standard condition.

ASSOCIATED CONTENT Supporting Information. Figures S1-S2 and Tables S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] ACKNOWLEDGMENT This work was supported by the Korea CCS R&D Center (KCRC) Grant (NRF2014M1A8A1074219) and NRF Grants NRF-2017R1A2B3006651 and NRF-2009-0093823. Computation by DFT (G.H. and D.-e.J.) was sponsored by the U.S. Department of Energy, Oﬃce of Science, Oﬃce of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division.


24

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27


TOC

2-

ee-

e-

eMAu24 (M = Pt, Pd)

ΔGH (eV)

e-

Electrode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

0

e-

2H+

e-

Au25 PdAu24 H+ + e-

PtAu24

1/2 H2

Reaction Coordinate

e-

e-

H2


28

Effects of Metal-Doping on Hydrogen Evolution Reaction Catalyzed by

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