Importance of Au nanostructures in CO2 electrochemical reduction reaction

Journal Pre-proofs Article Importance of Au nanostructures in CO2 electrochemical reduction reaction Dong-Rui Yang, Ling Liu, Qian Zhang, Yi Shi, Yue Zhou, Chungen Liu, FengBin Wang, Xing-Hua Xia PII: DOI: Reference:

S2095-9273(20)30028-1 https://doi.org/10.1016/j.scib.2020.01.015 SCIB 939

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Science Bulletin

Received Date: Revised Date: Accepted Date:

14 November 2019 30 December 2019 15 January 2020

Please cite this article as: D-R. Yang, L. Liu, Q. Zhang, Y. Shi, Y. Zhou, C. Liu, F-B. Wang, X-H. Xia, Importance of Au nanostructures in CO2 electrochemical reduction reaction, Science Bulletin (2020), doi: https://doi.org/ 10.1016/j.scib.2020.01.015

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© 2020 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

Received 14-November-2019; Revised 30-December-2019; Accepted 15-January-2020

Importance of Au nanostructures in CO2 electrochemical reduction reaction Dong-Rui Yang1†, Ling Liu2†, Qian Zhang1†, Yi Shi1, Yue Zhou1, Chungen Liu2*, Feng-Bin Wang1, Xing-Hua Xia1* 1

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing 210023, China 2

Key Laboratory of Mesoscopic Chemistry of MOE, Institute of Theoretical and Computational

Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China

Corresponding Authors [email protected] (C.G. Liu); [email protected] (X. H. Xia). †

Dong-Rui Yang, Ling Liu and Qian Zhang contributed equally to these work.

ABSTRACT Electrochemical conversion of CO2 into fuels is a promising means to solve greenhouse effect and recycle chemical energy. However, the CO2 reduction reaction (CO2RR) is limited by the high overpotential, slow kinetics and the accompanied side reaction of hydrogen evolution reaction. Au nanocatalysts exhibit high activity and selectivity toward the reduction of CO 2 into CO. Here, we explore the Faradaic efficiency (FE) of CO2RR catalyzed by 50 nm gold colloid and trisoctahedron. It is found that the maximum FE for CO formation on Au trisoctahedron reaches 88.80% at -0.6 V, which is 1.5 times as high as that on Au colloids (59.04% at -0.7 V). The particle-size effect of Au trisoctahedron has also been investigated, showing that the FE for CO decreases almost linearly to 62.13% when the particle diameter increases to 100 nm. The X-ray diffraction characterizations together with the computational hydrogen electrode (CHE) analyses reveal that the (221) facets on Au trisoctahedron are more feasible than the (111) facets on Au colloids in stabilizing the critical intermediate COOH*, which are responsible for the higher FE and lower overpotential observed on Au trisoctahedron.

Keywords Gold nanostructures Electrocatalysis CO2 reduction reaction (CO2RR)

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Morphological effect Size effect

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1. Introduction Electrochemical CO2 reduction reaction (CO2RR) at ambient environments is a promising means to counteract the substantial CO2 emission, and recycle chemical energy [1-7]. However, this reaction is limited by the high overpotential, slow kinetics and the accompanied side reaction of hydrogen evolution reaction (HER), which result in low conversion efficiency and poor selectivity. Using proper metal nanocatalysts can effectively improve the CO2RR performance [8,9]. Au nanocatalysts exhibit high activity and good selectivity for the electrochemical reduction of CO 2 to CO [10-12]. It has been suggested that the different crystal planes of Au nanostructures could have different impacts in the adsorption of CO2 and the related reactive intermediates, and the desorption of final products, thus strongly affecting the overall CO2 conversion efficiency [13,14]. This is possibly determined by the low-coordinated Au sites exposed at the edges and corners of the nanocatalysts that might have distinct activity or selectivity for CO2RR comparing to the ones at basal planes. Recent efforts have been focused on tuning the shapes, sizes and compositions of Au nanostructures to control their crystal planes, and improved CO2RR performance has been achieved [15-18]. Particularly, the shapes of metallic nanostructures should primarily determine the arrangement of surface metal atoms [19,20]. Hori et al. [21] reported that in Cu nanostructures based CO2RR catalysis, the Cu (111) facets favored the conversion of CO2 into CH4, while the Cu (100) facets generated C2H4. Both theoretical and experimental investigations on Cu based CO 2RR catalysis showed that in various elementary steps, Cu (111) facets directly hydrogenate CO to form an intermediate, leading to the formation of CH4, while the Cu (100) facets dimerize neighboring adsorbed CO to generate C 2H4. In addition, the sizes of metallic nanostructures determine the catalytic performance of CO2RR through affecting the proportion of lower-coordinated metal atoms [22-24]. Gao et al. [25] investigated the CO2RR performance on Pd nanoparticles with varying sizes. They found that the Faradaic efficiency (FE) of CO2 to CO conversion inversely correlated with the particle diameters varying from 2.4 to 10.3 nm. The smaller Pd nanoparticle (3.7 nm) was reported to give high CO FE of 91.2%, which is 18.4 times that of the 10.3 nm Pd nanoparticle. They attributed this phenomenon to the fact that varying the particle size changed the distribution of face, edge, corner on the surface of the Pd nanoparticle, while distinct activities for binding and hydrogenating CO2 should be expected at these different sorts of surface sites. Hemma et al. [26] synthesized Au nanoparticles with sizes ranging from 1 to 8 nm by the micelle method. They found that the FE of CO product decreased with decreasing the size of Au nanoparticles. Here, we explore the structural effects of Au nanoparticle for electrochemical conversion of CO 2 to CO using 50 nm Au colloids and trisoctahedron [27-29]. It is found that the Faradaic conversion efficiency for CO formation on Au trisoctahedron (Au TOH) is much higher than that on Au colloids. In

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addition, it is also found that the FE for CO product is determined by the ratio of edge sites to surface sites on Au (221) facet as revealed by the study of CO2RR on Au TOH with different sizes. Density functional theory (DFT) calculations confirm that the edge-site Au atoms are superior to the face-sites in catalyzing CO2RR under electrochemical conditions. The edge atoms with lower coordination character are chemically more unsaturated and feasible for binding CO 2 and hydrogenating it, forming more stabilized intermediate, COOH*. The present work would promise to design high performance catalysts for CO2RR. 2. Materials and methods 2.1. Reagents HAuCl4·4H2O was purchased from Shanghai Jiuyue Chemical Co., Ltd. Sodium borohydride (NaBH4), potassium bicarbonate (KHCO3), ascorbic acid (AA) and hexadecyl trimethylammonium bromide (CTAB) were provided by Sigma. Hexadecyle trimethylammonium chloride (CTAC) was bought from Shanghai Aladdin Biochemical Technology Co., Ltd. Trisodium citrate dihydrate was purchased from Nanjing Chemical Reagent Co., Ltd. All solutions were prepared with ultrapure water (18.2 MΩ cm, Millipore, USA) 2.2. Preparation of Au colloid We adopted the Frens method to synthesize 50 nm Au colloids [30]. A 100 mL of 0.01% HAuCl4 solution was heated to boil when 1 mL of 1% trisodium citrate solution was added. The color of the solution changed gradually from a little blue, to light blue, blue, and finally to red for 7−10 min. Finally, the prepared Au colloid solutions were centrifuged at 6000 r min−1 for 10 min, while the precipitates were re-dispersed in ultrapure water. This process was repeated twice to remove the excess reactants and capping agents. 2.3. Preparation of Au TOH In a typical synthesis [31], 46 µL of 20 mmol L−1 HAuCl4 solution was added to 7 mL of 75 mmol L−1 CTAB solution at 30 °C. Then, 0.42 mL of 10 mmol L−1 NaBH4 solution taken from a refrigerator was injected into the mixture quickly under vigorous mixing, forming brown Au seeds solution, which was stirred within the next 2−5 h. In the synthesis of Au TOH, 0.125 mL of 20 mmol L−1 HAuCl4 solution was mixed with 9 mL of 22 mmol L−1 CTAC solution in a clean glass bottle at ambient temperature, followed by adding 3.06 mL of 38.8 mmol L−1 ascorbic acid solution. Then, 50 µL of the Au seeds solution diluted 100 fold with ultrapure water was added to the above growth solution and mixed thoroughly. The color of the mixed solution changed into pinkish red, indicating the formation of Au TOH with 50 nm size. Finally, the prepared 50 Au TOH solutions were centrifuged at 6000 r min−1 for 10 min, while the precipitates were re-dispersed in ultrapure water. This process was repeated twice to remove the excess reactants and

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capping agents. 2.4. Synthesis of 75 and 100 nm Au TOH For the synthesis of 75 and 100 nm Au trisoctahedron, 6 and 2 mL of prepared 50 nm Au TOH were mixed with 9 mL of 22 mmol L−1 CTAC solution, respectively. Then, 3.06 mL of 38.8 mmol L−1 ascorbic acid solution and 0.123 mL of 20 mmol L−1 HAuCl4 successively were added into the above solution, forming the 75 and 100 nm Au TOH, respectively. Finally, the prepared Au TOH solutions were centrifuged at 5000 r min−1 for 10 min, while the precipitates were re-dispersed in ultrapure water. This process was repeated twice to remove the excess reactants and capping agents. 2.5. Characterization Transmission electron microscope (TEM, JEM-2100, Japan) and scanning electron microscope (SEM, S-4800, Japan) were used to observe the structure and morphology of samples, respectively. All samples were dispersed in aqueous solution and then dropped on a Cu grid for TEM and a Si substrate for SEM. UV-visible adsorption spectroscopic characterization was performed on a Nanodrop-2000C spectrophotometer (Thermo Fisher Scientific, Inc., USA). X-ray diffraction pattern (XRD) was carried out on a X’TRA (Switzerland). 2.6 Electrochemical Characterization All electrochemical measurements were performed on a CHI 660E electrochemical workstation (CHI, USA) in a three-electrode configuration where carbon paper loaded samples was applied as the working electrode, an Ag/AgCl electrode immersed in saturated KCl solution and a Pt mesh electrode (1.5×1.5 cm2) as the reference and counter electrodes, respectively. The cathode chamber containing the working and reference electrodes was separated from the anode chamber with the counter electrode using a Nafion membrane. Before electrochemical measurements, CO 2 was purged into the cell with working electrode for at least 30 min to ensure CO2-saturated 0.1 mol L−1 KHCO3 solution (pH 6.8) as the electrolyte. Unless specified, chronoamperometry and linear sweep voltammetry were carried out versus Ag/AgCl electrode and the potential versus the reversible hydrogen electrode (RHE) is converted using the equation: Evs. RHE = Evs. Ag/AgCl + 0.059 × pH + 0.197 (V). 2.7. Analysis of reaction products During chronoamperometry, in-line gas chromatograph (GC-2014, Shimadzu, Japan) was used for product analysis, in which 1 mL sample loop was directly connected to the working compartment at a flow rate of 20 sccm in a gas-tight H-type electrochemical cell. The gas products, sampled every 7 min, were analyzed by a TCD (thermal conductivity detector) for H2 and an FID (flame ionization detector) for CO. Gas chromatograph was equipped with Porpack-N 80 (100 mesh 3.2×2.1 mm×1.0 m) and MS-13X 80 (100 mesh 3.2×2.1 mm×2.0 m) for quantification with N2 and H2 as the carrier gas. The chromatography of CO and H2 is shown in Fig. S1 (online).

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2.8. Calculation of Faradaic efficiency According to the Faraday’s law of electrolysis, the Faradaic efficiency is calculated by the following equation, FE(CO) =

Q(CO) Q(total)

× 100% =

n(CO)zF 𝑡

∫0 𝑖d𝑡

× 100%.

(1)

In Eq. (1), FE denotes the Faradaic efficiency at a specific potential. n(CO) is the amount of substance of CO. From GC-2014, we can directly get ppm data of CO (a value that represents the CO part of a whole number in units of 1/1000000). After multiplied by the total volume in one experiment, the generated volume of CO is obtained. Here, CO is treated as an ideal gas, so the number of moles n(CO) can be easily calculated by dividing the molar volume of gas. z is the number of electrons transferred, and F is the Faradaic constant. Current (i) can be acquired from the i-t curve, while t (time) is related to the velocity of gas flow (20 sccm). FE(H2) is calculated by a similar way. 2.9. Computational details All calculations were performed with the plane wave density functional theory (DFT) code, VASP 5.4.4 [32,33], based on the projector augmented wave (PAW) method [34]. The exchange and correlation potential was approximated using the generalized gradient approximation (GGA) as formulated by PBE functional. The valence electron was expanded in a 500 eV cutoff basis set and the smearing width was set to 0.1 eV with the first order Methfessel-Paxton scheme. The lattice constant of Au was optimized to be 4.157 Å. The Au (111) and Au (221) surfaces were simulated by the periodic systems, with the Brillouin zone sampled by 3×3×1 Monkhorst-Pack [35] grids of k-points. The 3×4×3 atom slabs were employed to model the Au surfaces, using 15 Å vacuum width and keeping the lower two layers frozen. The representative surfaces on the Au trisoctahedron nanoparticle were simulated by the cluster systems, except the face site, which was modelled by the Au (221) periodic slab. The three different top sites and the edge site were constructed by cutting 3 or 4 atom layers from the Au TOH model. The vacuum surrounded the clusters was set to at least 15 Å to avoid the obvious interaction between the adjacent cells. The Brillouin zone in the top-site models was sampled at the Γ-point, while the edge-site model employed a 3×1×1 Monkhorst-Pack k-points sampling, for its periodicity along the x-direction. Minimum structures were optimized with all forces on free atoms being smaller than 0.05 eV Å -1 and the convergence criterion for the wavefunction was set to 10 -6 eV. Zero-point energies (ZPE) and entropy contributions to Gibbs free energies at room temperature (298.15K) were calculated from vibrational frequency analysis by assuming that vibrations of the Au surface were minimal, and only 3N degrees of freedom of the adsorbates were considered. Low frequency modes were reset to a threshold value of 60 cm-1 to remove the spurious contribution to entropy term. 3. Results and discussion

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The SEM image of Au TOH with 50 nm size (Fig. 1a) indicates the uniform distribution of nanoparticles with clear edge and corner features. The image presents as inset in Fig. 1a clearly indicates the α, β and γ values of 144°, 94° and 150°, respectively, corresponding to the Au (221) facets. These values show good agreement with the calculated ones (Fig. S2 online). The SEM (Fig. 1b) and TEM images (upper inset) show that the prepared Au colloids are of high monodispersity with size of 50 nm. The high-resolution TEM (HRTEM) images (down inset) shows that the lattice space of the Au colloid is 2.431 Å, corresponding to the Au (111) facet as illustrated in Fig. S3b (online). Fig. 1c indicates the localized surface plasmon resonance absorption of 50 nm Au TOH and 50 nm Au colloids at 540 nm and 530 nm, respectively. The X-ray diffraction (XRD) patterns of the Au TOH and Au colloids (Fig. 1d) show that there are four diffraction peaks for (111), (200), (220) and (311) facets at 38.2°, 44.3°, 64.6°, and 77.6°, respectively. If we set the intensity of Au (111) facet as a standard unit, the diffraction peak intensity of Au (200), Au (220) and Au (311) facets on Au standard cards is 0.45, 0.24, and 0.24; 0.50, 0.31, and 0.24 for Au TOH; 0.35, 0.18, and 0.19 for Au colloid, respectively (Table S1 online). The results reflect that the ratio of high-index facets especially the (220) facet on Au TOH is much higher than the Au standard card, while the ratio of low-index facets especially the (111) facet on Au colloids is higher than the Au standard card. Here, we use the electrochemical surface area (ECSA) to normalize current density. ECSA of Au catalysts was calculated from equation (ECSA=Q/C, C=0.386 mC cm−2) [36] by measuring the charge required for the reduction of Au oxide (red region in Fig. S4 online). Fig. 2a and b show the linear sweep voltammetric (LSV) curves of Au nanocatalysts in 0.1 M KHCO 3 saturated with N2 and CO2, respectively. It is clear that both the gold nanoparticles catalyze the CO 2RR as evidenced by the positive shift of the onset potential when CO2 is available in solution. The onset potential of CO2RR on the two catalysts occurs at -0.4 V. We performed the product analysis at the potentials of -0.4 V, -0.5 V, -0.6 V, -0.7 V, -0.8 V and -0.9 V by in-line GC measurements, and CO and H2 as the reaction products were detected. As shown in Fig. 2c, the CO2RR is significantly affected by the surface structure of gold particles. The FE for CO2RR on Au TOH reaches a maximum value of 88.80% at -0.6 V, and decreases as the potential gets more negative. The decrease in FE for CO could be contributed to the competing HER process, since HER is considerably accelerated at the same potential level (Fig. 2d). Another possible reason would be the rapid consumption of CO2 on the surface of catalysts at potentials in the higher over-potential region (−0.5 to −1.0 V), which cannot be fully compensated from slow dissolution of the CO2 gas and HCO3- to CO2 conversion. On the Au colloids, the FE for CO increases with the potentials to -0.7 V where a maximum value of 59.04% is reached, then it decreases following the trace as in the case of Au TOH. The similar trace for FE of CO at potentials negative to −0.7 V

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indicates that the HER reaction is the dominant process within this potential region (−0.7 to −1.0 V). The significant difference in FE for CO on both Au particles could be attributed to the distinct major exposed surface planes of Au TOH (with mainly exposing the high-index 221 plane) and Au colloid (with mainly exposing the low-index 111 plane). In a word, high-index plane is more liable to the production of CO. In addition, to verify whether the surface structures affect the performance of CO2RR, Au TOH with different sizes (50, 75 and 100 nm) were synthesized (Fig. S5 online). They have the same structure besides the different surface atomic proportion of the surface active sites (surface, edge, corner) (Fig. S5a−c online). Their surface plasmon resonance absorption peaks show red-shift with the increase of size (Fig. S5d online). The catalytic performance of the three different Au TOH is compared in LSVs. As shown in Fig. 3a, the CO2RR catalytic activity of the Au TOH increases with the decrease of the size. The FEs of CO and H2 were calculated based on the detected chromatographic and the chronoamperometric results at different potentials (Fig. S6 online). It is observed that the maximum FE of CO decreases from 88.80% to 62.13% with the increase of particle sizes of Au TOH from 50 to 100 nm (Fig. 3b and c). It is likely that with the decrease of particle size of Au TOH, the population of edge sites with low coordination increases. The small size Au TOH which have a much higher proportion of edge sites is supposed to manifest a better selectivity of CO. During the reaction process, the low-coordinated edge sites contribute more to the reduction of CO2, and thus the decrease in particle size of Au TOH improves the performance of electrocatalytic CO2 reduction. In our experiment, we also considered the influence of surfactants on catalytic performance. We compared with the original catalyst and the catalyst which was cleaned by plasma, and the results show that catalytic performance of the catalysts with or without treatment does not show significant difference (Fig. S7 online). In addition, the catalyst shows high stability during electrochemical testing as revealed by the SEM and TEM morphology characterizations (Fig. S8 online). DFT calculations are applied to understand the activity of CO2 conversion on different facets and active sites. The computational hydrogen electrode (CHE) model [37] is employed to describe the potential effect on the free energy evolution along the proposed CO2RR reaction routes. According to the constructed trisoctahedron models (Fig. S9−S11, online), more than 98% of the surface atoms reside inside the (221) facet, while only less than 2% are edge- and vertex-atoms, which form the junctions between neighboring (221) facets. Therefore, we tentatively suggest that the divergent electrocatalytic behaviors between trisoctahedron and colloid nanoparticles are due to the different catalytic activities between the major exposed (221) surface sites in trisoctahedron and the (111) sites in the colloid nanoparticles.

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We show in Fig. 4a the calculated Gibbs free energy evolution diagrams of the reaction pathways catalyzed by the Au (221) and the Au (111) facets, at the electrical potential of 0 V vs. RHE. It can be found that converting CO2 to COOH* on the (221) facet requires a reaction potential of 1.04 eV, which is 0.34 eV lower than that on the (111) facet. Since COOH* is characterized as the critical point with the highest free energy along the reaction pathway, the onset potential for generating CO should be determined mostly by the elementary reaction step in forming this surface species. Even though the CHE method cannot provide the reaction barriers, it could still be concluded that the COOH* formation step should have a lower activation barrier on the (221) facet, according to the well-documented empirical Brønsted-Evans-Polanyi (BEP) principle [38,39], which suggests that the reaction barrier for a given reaction is linearly related to the corresponding reaction free energy [40-42]. Therefore, the CO2RR process is more efficiently catalyzed by Au TOH, which reasonably explains the observed higher CO2 reduction activity on the Au TOH than on the Au colloids. In order to assess the role of the edge and vertex atoms in Au TOH, we constructed five model structures to mimic various surface sites of trisoctahedron, including one model (F) for the face-sites, another one (E) for the edge-sites, and the others (VO, VT1 and VT2) for three kinds of vertex-sites, as shown in Fig. 4d. As for the vertex-sites, VO denotes the vertex atoms at the six octahedral corners of trisoctahedron (Fig. S9 online), while VT1 and VT2 denotes two kinds of vertices appear alternately with increasing radius of the particle, which reside at each top of the eight triangular pyramids (Figs. S10 and S11 online). Fig. 4b displays the calculated free energy diagram for the five model clusters. It can be found that the most stabilized COOH* is formed on VT1, however, this reaction pathway is blocked by the succeeding step of CO desorption, for the over-binding of CO to the surface (desorption energy is calculated to be 0.81 eV). Instead, the edge sites (E) are suggested as the most active catalytic centers due to the balanced COOH* stability and CO* desorption free energy (0.37 eV). Even after considering the ratio of the face-site to the edge-site (Fig. 4c), the role of the later is significant and can make an effect on the Faraday efficiency of CO with varying particle sizes. 4. Conclusion In summary, we design precisely the Au nanostructure to control the surface property of the catalyst for effective conversion of CO2 to CO. It is found that the maximum Faradaic conversion efficiency for CO formation on Au TOH is 1.5 times as high as that of Au colloids. In addition, the FE for CO formation on Au TOH decreases with increasing the particle size, indicating a monotonous decline of the catalytic performance of the Au TOH with increasing particle sizes. Theoretical analysis proves that the Au (221) facet has a lower potential barrier than the Au (111) facet in forming the potential-limiting intermediate COOH* along the reaction route to CO. From the studies of various reaction centers on Au TOH, which includes vertex-, edge-, as well as face-sites, it is found that edge-sites should be the most

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active catalytic centers due to balanced COOH* stability and CO* desorption free energy. Accordingly, we tentatively attribute the better catalytic activity of the smaller nanoparticles to the higher edge-sites to face-sites ratio.

Conflict of interest The authors declare that they have no conflict of interest.

Acknowledgments This work was supported by the National Key Research and Development Program of China (2017YFA0206500) and the National Natural Science Foundation of China (21635004 and 21675079). Part of the numerical calculations were carried out in the High Performance Computing Center (HPCC) of Nanjing University.

Author contributions Xing-Hua Xia designed experiments. Dong-Rui Yang and Qian Zhang carried out the experiments. Ling Liu and Chungen Liu performed the DFT calculation. Yi Shi and Yue Zhou provided technical support. Dong-Rui Yang, Ling Liu, Qian Zhang, Chungen Liu and Xing-Hua Xia wrote the manuscript.

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Figure captions

Fig. 1. Structures of the as-prepared Au nanostructures. (a) SEM and HRTEM (inset) images of 50 nm Au TOH. (b) SEM, TEM (upper inset) and HRTEM (down inset) images of Au colloids. (c) UV-vis absorption spectra of 50 nm Au TOH and Au colloids. (d) XRD patterns of Au TOH, Au colloids and Au standard card. Fig. 2. Electrochemical measurement of the Au TOH and Au colloid. LSV curves of (a) Au TOH and (b) Au colloids in 0.1 mol L-1 potassium bicarbonate solution saturated with CO2 or N2 at a scan rate of 100 mV s-1, respectively. Faradaic efficiencies for (c) CO and (d) H 2 as function of potential. Fig. 3. Electrochemical measurement of the Au TOH with different size. (a) LSV curves of the Au TOH with different sizes in 0.1 mol L-1 potassium bicarbonate solution saturated with CO2 at a scan rate of 100 mV s-1. (b) Relationship between FE and overpotential for Au TOH with different sizes. Faradaic efficiency of (c) CO and (d) H2 on the Au TOH with size of 50, 75 and 100 nm. Fig. 4. Gibbs free energy profiles at various reactive sites. Gibbs free energy diagram for electrochemical reduction of CO2 to CO (a) on Au (221) and Au (111) facets (b) at face (F), edge (E) and vertex active sites (VO, VT1 and VT2) of Au TOH at 0 V vs. RHE. (c) Proportion of different surface sites in Au TOH with respect to the particle size. (d) Simulation models of the surface sites with the active atoms highlighted.

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Dong-Rui Yang received her B.S. degree from Jilin university. She is now a Ph.D. candidate in Nanjing university under the guidance of Prof. Xing-Hua Xia. Her research mainly focuses on the electrocatalytic and localized surface plasmonic resonance properties of Au nanostructures.

Ling Liu is currently a Ph.D. candidate at the School of Chemistry and Chemical Engineering, Nanjing University. She received her B.S. degree at Nanjing University of Posts and Telecommunications in 2015. Her research mainly focuses on the theoretical simulation on electrochemical catalysis and interfacial water structures.

Qian Zhang received her B.S. degree from Nanjing Normal university and M.S. degree from Nanjing university. Her research focused on electrocatalytic reduction reaction of carbon dioxide.

Chungen Liu received his B.S. degree in Chemistry from Suzhou University in 1987, M.S. degree from East China University of Science and Technology in 1992, and Ph.D. degree from Nanjing University in 1995. He is currently a Professor at School of Chemistry and Chemical Engineering, Nanjing University. His current research interest focuses on developing efficient theoretical methods for the atomic-scale understanding of the heterogeneous electrochemical processes.

Xing-Hua Xia is a Professor at the State Key Laboratory of Analytical Chemistry in Nanjing University. The ongoing researches in Xia’s group include: (1) the understanding of bio- and chemical processes occurring at interfaces/surfaces: kinetics and mechanism of bio/electro-chemical catalysis reactions at functional electrode surfaces using in situ spectroscopic techniques, relationship between conformation and orientation of biomolecules and their functions of direct electron transfer and biocatalytic activity; (2) mass transport in micro/nanofluidic systems; (3) synthesis of bio-inspired functional materials for electrochemical energy conversion and storage.

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Based on experimental and theoretical results, we find that the high-index (221) facets and the low-coordinated edge sites of gold nanoparticles are more favorable for the reduction of CO 2 to CO by comparing the performance of Au colloids with Au trisoctahedron of different sizes.

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