Angewandte. A Journal of the Gesellschaft Deutscher Chemiker. Accepted Article

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1 Angewandte A Journal of the Gesellschaft Deutscher Chemiker International Edition Chemie Accepted Article Title: Oxygen Vacancy Enhances CO Production over ZnO Nanosheets towards CO2 Electrochemical Reduction Authors: Zhigang Geng, Xiangdong Kong, Weiwei Chen, Hongyang Su, Yan Liu, Fan Cai, Guoxiong Wang, and Jie Zeng This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Angew. Chem. Int. Ed. Angew. Chem /ange Link to VoR:

2 Revised ms # R2 Oxygen Vacancies in ZnO Nanosheets Enhance CO 2 Electrochemical Reduction into CO Zhigang Geng, Xiangdong Kong, Weiwei Chen, Hongyang Su, Yan Liu, Fan Cai, Guoxiong Wang,* and Jie Zeng* Z. Geng, [ ] X. Kong, [ ] W. Chen, [ ] H. Su, Y. Liu, Prof. J. Zeng Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui , P. R. China zengj@ustc.edu.cn F. Cai, Prof. G. Wang State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , China wanggx@dicp.ac.cn [ ] These authors contributed equally to this work. [**]This work was supported by Collaborative Innovation Center of Suzhou Nano Science and Technology, MOST of China (2014CB932700), NSFC ( and ), Key Research Program of Frontier Sciences of the CAS (QYZDB-SSW-SLH017), Anhui Provincial Key Scientific and Technological Project (1704a ), Major Program of Development Foundation of Hefei Center for Physical Science and Technology (2017FXZY002), and Fundamental Research Funds for the Central Universities.

3 Abstract As electron transfer to CO 2 is generally considered as the critical step during the activation of CO 2, it is of significant importance to develop approaches to engineer the electronic properties of catalysts to improve their performance in CO 2 electrochemical reduction. Herein, we developed an efficient strategy to facilitate CO 2 activation by introducing oxygen vacancies into electrocatalysts with electronic-rich surface. ZnO nanosheets with rich oxygen vacancies exhibited a current density of ma cm -2 with the Faradaic efficiency of 83% for CO production. Based on the density functional theory calculations, the introduction of oxygen vacancies increased the charge density of ZnO around the valence band maximum, resulting in the enhanced activation of CO 2. Mechanistic study further revealed that the enhancement of CO production by introducting oxygen vacancies into ZnO nanosheets originated from the promoted binding strength of CO 2 and the facilitated CO 2 activation. Keywords: Oxygen vacancies; ZnO nanosheets; Electronic properties; CO 2 activation; CO 2 electrochemical reduction Table of Contents 2

4 As CO 2 is an abundant and inexpensive carbon resource in the chemical industry, recycling CO 2 into fuels and high value-added chemical stocks via an electrochemical process represents a promising path to mitigate the energy crisis. [1,2] Due to the high thermodynamic stability of CO 2, [3] the standard potential of CO 2 /CO - 2 couple is as negative as -1.9 V versus reversible hydrogen electrode (vs RHE) in water for the formation of highly energetic CO 2 anion radicals. [4] As such, the effective activation of CO 2 plays a key role in CO 2 electrochemical reduction. [5] From both theoretical and experimental perspectives, the electron transfer to CO 2 is generally considered as the critical step during the activation of CO 2. [6,7] To overcome this bottleneck, it is especially important to rationally design a highly active and robust electrocatalyst which possesses assistant electron-donation centers to capture CO 2 molecules and activate the stable C=O non-polar bonds. Recently, the introduction of oxygen vacancies provides a powerful method to enhance the performance of CO 2 conversion in heterogenous catalysis by promoting CO 2 adsorption and activation. For instance, the introduction of oxygen vacancies defects into TiO 2 surface gives rise to highly stable adsorption configurations with a stronger activation of the C=O bonds, which leads to the exothermic dissociation of CO 2 with barriers up to 22.2 kcal mol -1. [8] In addition, the oxygen vacancies decorated on the BiOCl nanoplates made the surface electronic-rich, and - served as electron trap centers for transformation of CO 2 into CO 2 by one-electron transfer process. [9] Moreover, previous studies have theoretically proved that engineering the surface vacancy of electrochemical catalysts also pave a new avenue to regulate the catalytic performance for ammonia synthesis and oxygen reduction by modifying their electronic structures. [10] However, to the best of our knowledge, implementing this strategy in electrochemical reduction of CO 2 is still at the rudimentary state. Herein, we developed an efficient strategy to facilitate CO 2 activation by introducing oxygen vacancies into ZnO nanosheets. In CO 2 electrochemical reduction, ZnO nanosheets with rich oxygen vacancies exhibited a current density of ma cm -2 with the Faradaic efficiency (FE) of 83% for CO production at -1.1 V vs RHE. Based on density functional theory (DFT) calculations, the introduction of oxygen vacancies into ZnO increased the charge density around the valence band maximum, resulting in the enhancement of CO 2 activation. Mechanistic study further revealed that the enhanced CO production by introducing oxygen vacancies into ZnO nanosheets originated from the facilitated CO 2 activation. 3

5 To gain insight into the effect of oxygen vacancies on electronic property, we conducted DFT calculations by using ZnO slab with/without oxygen vacancy as the models. As shown in Figure 1, a and b, a new defect level appeared around the valence band maximum for ZnO slab with oxygen vacancy. [11] Besides, from the partial charge density in Figure 1, c and d, the electrons tended to accumulate around the valence band maximum of ZnO slab with oxygen vacancy than those without oxygen vacancy. These abundant localized electrons around the valence band maximum are expected to be transferred into the anti-bonding orbitals of CO 2 molecules for CO 2 activation. [12] The DFT calculations inspired us to prepare ZnO with oxygen vacancies. Pristine ZnO nanosheets with wrinkly morphology were synthesized according to a literature with slight modification (Fig. S1). [13] Generally, H 2 plasma treatment serves as a facile strategy to generate oxygen vacancies in metal oxides, where a positive correlation was found between concentration of oxygen vacancies and treatment time. [14] In this case, pristine ZnO nanosheets were treated using H 2 plasma for 30 s and 120 s to synthesize ZnO nanosheets with poor oxygen vacancies (V o -poor ZnO nanosheets) and rich oxygen vacancies (V o -rich ZnO nanosheets), respectively. The corresponding transmission electron microscopy (TEM) images demonstrated similar morphologies of the three ZnO nanosheets (Fig. S1). Figure 2a shows the high-resolution transmission electron microscopy (HRTEM) image of an individual V o -rich ZnO nanosheet. The lattice fringes with an interplanar spacing of 0.26 nm and 0.16 nm were ascribed to the (0002) and facets of wurtzite ZnO, respectively. In XRD patterns, the main peaks of the plasma-treated ZnO nanosheets were located at the same positions as those of pristine ZnO nanosheets (Figure 2b and Table S1). Moreover, the interplanar spacing of (0002) facet for V o -rich ZnO nanosheets corresponded to that of pristine ZnO nanosheets (Fig. S2). To prove the existence of oxygen vacancies in the as-prepared ZnO nanosheets, we conducted a series of characterizations. In O 1s X-ray photoelectron spectroscopy (XPS) spectra (Fig. 2c), the peaks at ev and ev corresponded to the O atoms in the vicinity of oxygen vacancies and O bond of Zn-O-Zn, respectively. [15] As shown in Figure S3, the integral-area ratios of the peak at ev to the peak at ev were calculated to be 0.30, 0.57, and 0.76 for pristine, V o -poor, and V o -rich ZnO nanosheets, respectively. Meanwhile, the binding energies of Zn 2p 1/2 and 2p 3/2 in the three ZnO nanosheets showed a decreasing trend with the increased concentration of oxygen vacancies (Fig. 2d). The relative concentration of 4

6 oxygen vacancies was also verified by photoluminescence (PL) spectra (Fig. 2e). Specifically, the peak at 380 nm originated from a direct band excitation, whereas the peak at 580 nm corresponded to the recombination of two-electron-trapped oxygen vacancies with photogenerated holes. [16] In evidence, V o -rich ZnO nanosheets exhibited the highest concentration of oxygen vacancies among the three ZnO nanosheets. To further determine the concentration of oxygen vacancies, we conducted positron annihilation spectrometry to measure the lifetime of the positron. Figure 2f shows that both V o -poor and V o -rich ZnO nanosheets exhibited longer average positron lifetime than pristine ZnO nanosheets due to the trapping of positron by oxygen vacancies. The positron lifetime spectra of ZnO nanosheets exhibited two lifetime components, τ 1 and τ 2, with corresponding relative intensities, I 1 and I 2 (Table S2). The shorter component (τ 1, 0.25~0.28 ns) observed in the positron lifetime spectra was assigned to bulk defects in the perfect crystal, whereas the longer one (τ 2, about 0.43 ns) was attributed to the positron annihilation trapped by oxygen vacancies. [17] V o -rich ZnO nanosheets exhibited the largest I 2 value of 70.8% among the three ZnO nanosheets, indicating the concentration of oxygen vacancies in V o -rich ZnO nanosheets is much larger than those in the other two counterparts. The three as-prepared ZnO nanosheets were applied in CO 2 electrochemical reduction. Linear sweep voltammograms (LSV) were measured from 0 to -1.1 V vs RHE in N 2 -saturated and CO 2 -saturated 0.1 M KHCO 3 electrolyte, respectively (Figs. 3a and S4). In N 2 -saturated electrolyte, an obvious reduction peak appeared around -0.8 V vs RHE, indicating that the oxidized Zn species in V o -rich ZnO nanosheets were reduced into the metallic state during LSV measurements. Similar peaks for ZnO reduction were also observed for the other two counterparts (Fig. S4). In CO 2 -saturated electrolyte, the peaks for the reduction of ZnO were absent in LSV curves, which revealed that the CO 2 reduction restrained the self-reduction of ZnO nanosheets. Figure 3b shows the geometric current density at all applied potentials. V o -rich ZnO nanosheets displayed the largest current density, indicating the highest activity towards CO 2 electrochemical reduction among the three ZnO nanosheets. This result was consistent with IR-corrected LSV curves of the three ZnO nanosheets in CO 2 -saturated electrolyte (Fig. S5). Figure 3c shows the current density for CO production of the three ZnO nanosheets. At all applied potentials, V o -rich ZnO nanosheets exhibited the highest current density for CO production. At -1.1 V vs RHE, the current density of V o -rich ZnO nanosheets for CO production 5

7 was up to ma cm -2, which was 5.1 and 1.4 times as high as that of pristine and V o -poor ZnO nanosheets, respectively. Figure 3d shows the FE of the three ZnO nanosheets for CO production. V o -rich ZnO nanosheets exhibited a higher FE for CO with respect to the other two counterparts. Especially, when the potential was set as -1.1 V vs RHE, the FE of V o -rich ZnO nanosheets for CO production reached 83%, whereas the FE of pristine and V o -poor ZnO nanosheets was 44% and 73%, respectively. The catalytic performance of V o -rich ZnO nanosheets was comparable to that of the state-of-the-art catalysts for electrochemical reduction CO 2 into CO production (Table S3). The total FE for CO and H 2 production of the three ZnO nanosheets approximately equaled to 100%, demonstrating that the self-reduction of ZnO rarely occurred during CO 2 electrochemical reduction (Fig. S6). In durability tests, both the pristine and V o -poor ZnO nanosheets showed about 8% decay for current density and 5% decay for FE (Figure 3, e and f). In contrast, V o -rich ZnO nanosheets exhibited negligible decay in current density and the FE for CO was always greater than 80%. TEM and O 1s XPS further revealed that the morphology and oxygen-vacancy concentration of V o -rich ZnO nanosheets were perfectly preserved after durability tests (Figs. S7 and S8). To gain further insights into the reaction kinetics on the three ZnO nanosheets, we analyzed the Tafel plots and electrochemical impedance spectroscopy (EIS). It is well known that the whole process of CO 2 electrochemical reduction to CO mainly includes the following three steps: [18] Step 1: CO 2 + * + H + + e - COOH* (Activation process) Step 2: COOH* + H + + e - CO* + H 2 O (Surface reaction) Step 3: CO* CO + * (Desorption process) Theoretically, the Tafel slopes of activation process and surface reaction are calculated to be approximately 118 and 59 mv dec -1, respectively. [19] As shown in Figure 4a, the Tafel slopes of pristine, V o -poor, and V o -rich ZnO nanosheets were 125, 92, and 72 mv dec -1, respectively. As such, the initial CO 2 activation process served as the rate-limiting step for pristine and V o -poor ZnO nanosheets. As for V o -rich ZnO nanosheets, the rate-limiting step switched to surface reaction instead of activation process. Figure 4b shows the Nyquist plots and the corresponding equivalent circuit. Based on radius of the Nyquist plots, the charge transfer resistance (R ct ) of 6

8 V o -rich ZnO nanosheets was smaller than that of the other two counterparts, suggesting a favorable Faradaic process by introducing oxygen vacancies. Based on the analyses above, the introduction of oxygen vacancies in ZnO nanosheets greatly promoted the kinetics of CO 2 activation process. The intrinsic reason that oxygen vacancies enhanced CO 2 activation was further investigated. Derived from cyclic voltammograms (CV) technique under different scan rates, electrochemical surface areas (ECSAs) of the three ZnO nanosheets were determined by measuring double layer capacitance (C dl ) (Fig. S9). [20] As shown in Figure 4c, V o -rich ZnO nanosheets showed a slightly larger C dl than that of the other two counterparts. The total current density and current density for CO production were further normalized by ECSA (Fig. S10), Similar to the geometric current density, the normalized current density of V o -rich ZnO nanosheets was the highest among the three ZnO nanosheets. Figure 4d shows CO 2 temperature-programmed desorption (CO 2 -TPD) profiles. The chemical desorption peaks of pristine, V o -poor, and V o -rich ZnO nanosheets were located at the temperature of 260 o C, 271 o C, and 282 o C, respectively. This result suggested the binding strength of CO 2 on ZnO nanosheets was enhanced after the introduction of oxygen vacancies. As such, the increased activity of ZnO nanosheets was mainly attributed to the enhanced binding strength of CO 2 due to the introduction of oxygen vacancies, rather than the increasing number of active sites. To provide a theoretical insight into the reaction mechanism, we calculated the Gibbs free energy with solvation corrections for each step involved in the CO 2 reduction at 0 V vs RHE on the ZnO slab with/without oxygen vacancy. [21] As shown in the Figure 4e, the Gibbs free energy for the formation of *COOH on ZnO slab with oxygen vacancy was 0.28 ev lower than that on ZnO slab without oxygen vacancy, suggesting the promoted CO 2 activation process by the introduction of oxygen vacancy (Fig. S11, Tables S4 and S5). In addition, the CO 2 activation process held the highest energy barrier among the three steps on ZnO slab without oxygen vacancy, whereas the surface reaction limited the CO 2 reduction on ZnO slab with oxygen vacancy. The transition of rate-limiting step induced by oxygen vacancy was consistent with the previous Tafel analysis. Moreover, the Gibbs free energy for CO 2 activation process on ZnO slab with two oxygen vacancies decreased by 0.20 ev with respect to ZnO slab with an oxygen vacancy, which revealed that more oxygen vacancies on the ZnO surface further enhanced the activation of CO 2 for CO 2 electrochemical reduction (Fig. S12, Tables S4 and S5). The energy 7

9 barrier of surface reaction on the ZnO slab with two oxygen vacancies was 0.38 ev, which was only 0.03 ev lower than that of the ZnO slab with one oxygen vacancy. This point was also verified by the catalytic activation of ZnO nanosheets which was treated by 300-s H 2 plasma for more oxygen vacancies (Fig. S13). In summary, guided by the DFT calculations, we developed V o -rich ZnO nanosheets as efficient catalysts towards CO 2 electrochemical reduction. The concentration of oxygen vacancies was controlled by a facile H 2 plasma treatment. In CO 2 electrochemical reduction, V o -rich ZnO nanosheets exhibited a current density for CO production of ma cm -2 with a FE up to 83% at -1.1 V vs RHE. Mechanistic study revealed that oxygen vacancies facilitated the activation of CO 2, leading to the superior kinetics for CO production in V o -rich ZnO nanosheets. This work would provide a guideline for the rational design of novel and highly efficient catalysts towards CO 2 electrochemical reduction by regulating electronic properties of catalysts. 8

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12 Figure 1. Calculated DOS of (a) ZnO slab and (b) ZnO slab with oxygen vacancy. The partial charge density around the valence band maximum (marked with the orange region) of (c) ZnO slab and (d) ZnO slab with oxygen vacancy. In all partial charge density, yellow parts refer to charge density contour with the isovalue of e/bohr 3. The gray, red, and white spheres represent Zn, O, and H atoms, respectively. 11

13 Figure 2. (a) HRTEM image of an individual V o -rich ZnO nanosheet. (b) XRD patterns, (c) O 1s XPS spectra, (d) Zn 2p XPS spectra, (e) PL spectra, and (f) positron lifetime spectra of the three ZnO nanosheets. 12

14 Figure 3. (a) LSV curves of V o -rich ZnO nanosheets were performed in N 2 -saturated and CO 2 -saturated 0.1 M KHCO 3 (ph = 6.8). (b) Total current densities, (c) Current densities for CO production, and (d) FE for CO production on the three ZnO nanosheets at selected potentials. (e) Current densities and (f) FE for CO production on the three ZnO nanosheets at -1.1 V vs RHE with 8-hour potentiostatic tests. 13

15 Figure 4. (a) Tafel plots of CO production, (b) Nyquist plots, (c) charging current density differences plotted against scan rates, and (d) CO 2 -TPD profiles for three ZnO nanosheets. (e) Gibbs free energy diagrams for CO 2 reduction to CO on ZnO slab with/without oxygen vacancy. * represents an adsorption site. 14

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