Cation Vacancy-Initiated CO2 Photoreduction over ZnS for Efficient

http://pubs.acs.org/journal/aelccp

Cation Vacancy-Initiated CO2 Photoreduction over ZnS for Efficient Formate Production Hong Pang,†,‡,∇ Xianguang Meng,†,§,∇ Peng Li,⊥ Kun Chang,† Wei Zhou,¶ Xin Wang,∥,# Xueliang Zhang,∥,# Wipakorn Jevasuwan,† Naoki Fukata,† Defa Wang,∥,# and Jinhua Ye*,†,‡,∥,#

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†

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0814, Japan ∥ TJU-NIMS International Collaboration Laboratory, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China # Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China § Hebei Provincial Key Laboratory of Inorganic Nonmetallic Materials, North China University of Science and Technology, Tangshan 063210, People’s Republic of China ⊥ Jiangsu Key Laboratory of Electrochemical Energy Storage Technologies, Department of Applied Chemistry, College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, People’s Republic of China ¶ Department of Applied Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics, Preparing Technology Faculty of Science, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: Vacancies have been demonstrated to be significant for CO2 reduction reaction (CO2RR) over ZnS, but anion vacancies were easily refilled with oxygen species and could work as both H2 and CO evolution sites, aggravating the competition between hydrogen evolution reaction (HER) and CO2RR. In this study, cation vacancies (VZn) were proposed as new active sites on the ZnS surface. With no cocatalyst, the VZn-rich ZnS acquired a high selectivity of formate production (>85%) in inorganic aqueous solution. In situ attenuated total reflection-infrared (ATR-IR) spectroscopy and first-principle calculations have clarified the CO2RR pathways into formate and proved that the surface VZn could greatly lower the barrier of CO2RR and suppress the proton adsorption, elucidating the origin of the highly selective CO2RR in the presence of competitive HER. This work gives an in-depth understanding of the cation vacancies and inspiration to develop efficient photocatalysts.

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capable of showing CO2RR performance due to inert surface lack of active sites.7 It is no wonder that utilizing a small stoichiometric deviation can alter the physicochemical properties of materials significantly.9−11 Numerous studies have applied the anion vacancies in various catalytic reactions.12−17 The sulfur vacancies in ZnS have been claimed to be effective active sites for H2 and CO.18 However, as water reduction proceeds much easier than CO2RR, the hydrogen evolution reaction (HER) occurs in severe competition if there is no cocatalyst loaded on.7,19,20 The selectivity could be boosted close to unity

hotocatalytic CO2 reduction reaction (CO2RR) toward chemical feedstocks relying on sunlight and suitable catalysts stands out as an attractive approach to CO2 sequestration.1−5 A critical issue worth noting in CO2RR is the use of carbon-containing photocatalysts, hole scavengers, and organic chemicals for synthesis. The involvement of carboncontaining materials makes it ambiguous to tell whether the seemingly identified products come from CO 2 RR or decomposition of the organics, hindering assessment of the products.3,4 More importantly, the simple inorganic aqueous system is more in line with the concept of artificial photosynthesis. ZnS, with a relatively high conduction band situated at −1.04 V vs NHE (pH = 0),6 is an excellent host material for CO2RR in all-inorganic aqueous solution.7,8 Nonetheless, commercial ZnS with high crystallinity was rarely © XXXX American Chemical Society

Received: April 2, 2019 Accepted: May 20, 2019

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DOI: 10.1021/acsenergylett.9b00711 ACS Energy Lett. 2019, 4, 1387−1393

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Cite This: ACS Energy Lett. 2019, 4, 1387−1393

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Figure 1. (a) Room-temperature ESR spectra. (b) Calculated intensity of ESR signals at g2 = 1.978 for the estimated relationship between VZn and the pH value of the acidic solutions. (c) Optimal cell structures of VZn-ZnS (Zn53S54) and perfect ZnS (Zn54S54) with charge density. High-resolution XPS spectra of (d) Zn 2p and (e) S 2p of the pristine ZnS and the representative acid-etched sample. (f) PL spectra of the pristine ZnS and the acid-etched ZnS samples.

Exhibited in Figure 1a, two discernible lines with a g-factor value located at 2.021 and 1.978 belong to the reported six hyperfine lines of ZnS, while the other four fall beyond the measured range (313.5−328.5 mT). The other two symmetric peaks in the spectra are the reference of Mn markers (third and fourth).27 Moreover, the absence of the ESR peak at g = 2.003, which is usually observed in the middle of the Mn third and fourth makers and considered as the signature of sulfur vacancy (VS),20,28 also indicates the generation of VZn instead of VS. Because the ESR signal intensity is correlated with the density of vacancies,29 the enhanced intensities of ZnS (pH = 0.7) and ZnS (pH = 1.8), in contrast with the pristine ZnS, can give a rough estimation of the VZn amount tendency, as displayed in Figure 1b. In Figure 1c, density functional theory (DFT) calculations on (001) surface slabs of pristine and VZn-containing ZnS (VZn-ZnS) predict the lower charge density of S atoms in the vicinity of VZn than that of the normal site, which is further confirmed by XPS. The survey spectra can be found in Figure S7. Compared with the high-resolution spectra of Zn 2p over the pristine ZnS, the binding energies of Zn 2p1/2 and Zn 2p3/2 (see Figure 1d) at 1021.3 and 1044.4 eV shift about 0.1 eV toward the higher binding energy, suggesting a decrease of the Zn valence in the acid-etched sample. As VZn decreases the electron density at the neighboring S sites, the binding energy of S is supposed to decrease concomitantly. In the spectra, the deconvoluted peaks at binding energies of 162.82 and 162.03 eV, assigned to S 2p3/2 and S 2p1/2 of the normal S atoms, respectively, almost remain at the same position but with a lower proportional ratio. The peaks at 161.60 and 162.78 eV,

in aqueous solution only when the ZnS was loaded with Cd, which actually is not expected as a toxic element.7,19,20 In addition, anion vacancies are susceptibly deactivated by the oxygen species from the reaction environment. In this work, instead of an anion vacancy, cation vacancyrich ZnS has been developed by an acid-etching method, which can create more sustainable imperfections on the surface of ZnS and avoid the detrimental effect of the bulk defects on charge transport. By adjusting the pH value of the acidic solution, a series of samples were prepared. The SEM/TEM images (Figures S1 and S2), the XRD patterns (Figure S3), and the Raman spectra (Figure S4) indicate that ZnS well maintained a typical morphology and wurtzite structure after acid etching. However, the acid-etched sample possesses three times higher CO2 adsorption capacity than the pristine counterpart despite a similar BET surface area, partly indicating the increased surface sites (Figures S5 and S6). To confirm the type of vacancy species existing on the surface, the Zn/S ratios of acid-etched ZnS samples were determined by energy-dispersive spectroscopy (EDS) and Xray photoelectron spectroscopy (XPS), as shown in Table S1. A stoichiometric excess of sulfur is seen over the acid-etched sample, suggesting that acid etching promoted the Zn deficiency on the surface. Electron spin resonance (ESR) spectroscopy was further employed to gain more direct evidence of the electron behavior associated with the vacancies. The signals originated from the nonaxial singly ionized VZn site with one hole localized on the three S neighbors in a trigonal symmetry split into six hyperfine lines in the ZnS polycrystalline form.21−26 1388

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Figure 2. (a) Product (HCOOH, CO, H2) evolution rates and (b) selectivity over the as-prepared ZnS samples etched with sulfuric acid of different pH values (experiment period: 100 h).

surface, which is 1.12 J/m2, also testifies that the defective ZnS is more reactive than pristine ZnS. Last, photocatalytic CO2RR was carried out in the aqueous solution with no cocatalyst. When the catalyst was pristine commercial ZnS, only H2 and a small amount of CO could be detected (Figure S12). No detectable liquid product was identified by NMR. In contrast, the ZnS samples treated by different sulfuric acid all showed markedly enhanced activity (Figure 2a) with a paramount formate production when the acid solution pH reached 1.2. As an important indicator to assess the photocatalytic activity of catalysts,31,32 the corresponding apparent quantum yield (AQY) was also measured under a monochromatic wavelength (λ = 300 nm, λ1/2= 16.7 nm) and calculated as 1.24% as a lower limit (see the equation in the Supporting Information). The selectivities of HCOOH, CO, H2 are shown in Figure 2b. The main product of HCOOH over the acid-etched ZnS (pH = 0.2) attained a selectivity up to 86.6% in the absence of any rare, toxic, and expensive elements and cocatalysts. Using the XPS/ EDS Zn/S ratio to semiquantitatively estimate the VZn contents at different pHs in Table S3, the relationship between the production rate of carbonaceous species and the VZn were correlated in Figure S13. The linear curve within a certain range shows that the production rates of HCOOH and CO are well-correlated with the VZn amount trend given by XPS/EDS and ESR estimation, demonstrating that VZn initiates the CO2 photoreduction over the ZnS surface. Also, as excessive VZn is detrimental to the charge carriers, as shown by PL results, the optimal production rate is a balance between the active site amounts and photogenerated carriers’ properties. Under longterm irradiation of 60 h, both H2 and CO were generated in a linear manner (Figure S14) without any deactivation, indicating that the VZn-ZnS samples were fairly stable during CO2RR. The isotope experiment using 13CO2 as the carbon source confirmed that the gaseous product of CO originated from CO2 instead of other carbon source on gas chromatographymass spectrometry (GC-MS), with the peak at m/z = 29 assigned to 13 CO (Figure S15). The liquid product (HCOOH) was validated by both 1H and 13C NMR spectra. As shown in Figure S16a, due to J-coupling between the 1H atom and 13C atom, the H13COO− signal in the 1H spectrum split into two peaks at 8.1 and 8.6 ppm. Together with the only peak of H12COO− at 8.3 ppm in the 1H spectrum of the normal 12CO2 reduction (Figure S16b) and the peak at 170.5

representing the disordered S, shift toward lower binding energy to 161.22 and 162.38 eV, further validating the formation of VZn after acid etching (Figure 1e). The luminescent property and the electron transfer behavior of the acid-etched ZnS were examined by steady-state photoluminescence (PL) spectroscopy. As shown in Figure 1f, excitation of 320 nm induces two major emission peaks at 390 and 580 nm. The emission peak near 390 nm lies close to the bandgap, while the lower-energy peak at 580 nm is consistent with the reported donor−acceptor luminescence band associated with VZn, which is additional evidence of VZn formation.30 PL studies also reveal that the intensities of the 580 nm peak increase with the pH value of the acid decreasing, indicating that more VZn were generated by more acidic etching. The most significant quenching of the blue emission occurs over ZnS (pH = 1.2), reflecting its suppressed recombination of photocharged carriers and highest charge separation efficiency of the electron−hole pairs. When timeresolved PL spectroscopy (tr-PL) was performed and the fluorescence decay curves were fitted with a biexponential function (Figure S8 and Table S2), the average lifetime of the ZnS (pH = 1.2) showed the longest lifetime up to 33.7 ns, accounting for the most effective charge separation and optimal formate production performance via suppling longerlived electrons to the adsorbed CO2.3,4 To disclose the correlation between surface VZn and electronic structure, DFT calculations were employed to predict the band structure. On the basis of the models in Figure S9, the band structure of pristine ZnS and VZn-ZnS were acquired. As shown in Figure S10, after incorporation with a VZn, the bandgap of the ZnS is narrowed down, consistent with the slight red shift of the absorption edge in the UV−visible spectra (Figure S11). Noting that the electronic states of the VZn-ZnS above Fermi level display a higher density than that of the pristine ZnS, it is reasonable to deduce that the defective ZnS can supply photoexcited electrons to the adsorbates in a more kinetically favorable manner, beneficial for the CO2RR. Furthermore, as the electronic states of density increase at the Fermi level of the VZn-ZnS (marked by the blue ribbon in Figure S10a), the defective ZnS possesses better conductivity, facilitating electron transport and charge separation. When atomic and energetic disorder occurs, it may provide a possible reactive site for the activation of CO2. On the basis of the same models, the surface energy was also calculated. The higher surface energy (γ) of 1.93 J/m2 for the VZn-containing (001) surface than that of the perfect w-ZnS 1389

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Figure 3. Time-dependent in situ ATR-IR spectra over VZn-ZnS under photoirradiation in a CO2 atmosphere in the ranges of 1000−1800 and 2800−3600 cm−1.

Figure 4. (a) Adsorption site of H* on the VZn-ZnS surface (left panel: top view; right panel: side view) (b) Proton adsorption free energy versus the reaction coordinate of HER (c) Free energy diagram for the pathways of CO2 conversion into formate on a perfect ZnS and VZnZnS surface. The simplified surface structures correspond to the various species along the pathway, indicating the electron/proton transfer process at the (001) surface of w-ZnS.

ppm in the 13C spectrum (Figure S17), it is confirmed that the main product formate stemmed from the starting 13CO2. In order to probe the catalytic transformation over the VZnZnS, the in situ attenuated total reflection-infrared (ATR-IR) spectra were collected. As shown in Figure S18, the FTIR spectra monitored the peaks in the range of 800−4000 cm−1 during the CO2 transformation. The doublet bands centered at 2340 cm−1 indicate the chemisorbed gaseous CO2 molecules.33 As the range from 1000 to 1750 cm−1 contains a lot of carbonaceous species information, a zoomed-in perspective of

this region is shown in Figure 3. Peaks at 1684, 1637, 1386, and 1018 cm−1 indicate the stretching vibration of CO [ν(CO)], asymmetric and symmetric vibration of CO3 stretching [νas(CO3) and νs(CO3)], as well as the stretching vibration of C−O [ν(C−O)] of bicarbonate, respectively. The shift of 1637 cm−1 to 1652 cm−1 indicates generation of the monodentate bicarbonate (m-HCO3*) intermediate after bicarbonate attaches to the catalyst surface in the initial stage. It is generally accepted that the chemically adsorbed CO2* initially accepts one electron to form the CO2•− intermediate. 1390

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in Figure S21 in the Supporting Information). In contrast, CO2RR starting with CO2 to formate requires a lower energy barrier, which is dominant and displayed in Figure 4c. When the reaction starts from CO2, the free energy of adsorption on both surfaces is exothermic, indicating that CO2 adsorption is fairly easy. In this step, the free energy changed from −0.26 eV on the VZn-free surface to −0.53 eV on the VZncontaining surface, indicating a much stronger capacity of CO2. When CO2 accepts one electron to COO−*, the free energy increases, suggesting that the first electron transfer from the ZnS surface to CO2 is the rate-determining step. The energy barrier of 0.11 eV on perfect ZnS is reduced to 0.06 eV on the VZn-containing surface. Compared with the defect-free surface, the presence of VZn helps stabilize the COO−* and thus accelerate the overall catalytic reduction rate. Because the conversion of COO−* to HCOO* is exergonic (downhill in free energy) along the pathway, the second electron transfer to the adsorbed species is easily surmountable. The intermediate HCOO* finds its optimized position attached to the ZnS surface with an O bridging between the intermediate and VZn, in accordance with our interpretation of the in situ ATR-IR results, implying that the pathway through m-HCOO* is dominant. In this step, more negative free energy (−0.94 eV) is observed on the VZn-containing surface, signifying that the stability of m-HCOO* species is encouraged on the VZncontaining surface. This step should be responsible for more production of formate. In the next step, on either the defective or perfect surface, this reaction barrier is much small, suggesting the weakly bound conditions of HCOO− to the reactive sites in both cases, which enables desorption of HCOO− from the active sites. In a word, the key holding for efficient CO2RR to formate over ZnS lies in the formation of the intermediate COO−* and HCOO* from CO2. With VZn being exposed on the surface, the activation barrier to form the key intermediate of COO−* can be significantly reduced and thus beneficial for the overall process of CO2 reduction. In summary, VZn created by acid etching on the surface enabled well-crystalline ZnS to be active for CO2RR and significantly improved photocatalytic CO2 conversion into formate. The lower energy barrier of CO2RR, the higher surface energy, and the enhanced charge separation caused by the VZn contributed to the pronounced efficiency of the CO2RR over the cation vacancy-rich surface of ZnS. In situ ATR-IR and DFT calculations elucidated the process of CO2RR over ZnS via HCOO* and the origin of the high selectivity of CO2RR in the presence of competitive HER. The current work proposes a new highly selective active site for CO2RR and discloses the importance of cation vacancies on ZnS. For a broad perspective, either the strategy of acid etching or surface cation vacancy should be applicable to the other visible light-responsive sulfide photocatalysts and nanomaterials.

However, as the vibrational modes are in the same region with carbonate or bicarbonate species, it is difficult to identify its adsorbed configuration in the IR spectra.34−36 From our observation and literature, the 1396 cm−1 peak contains the symmetric vibration of the intermediate COO−*, which is considered visible within the spectra. The peak alterations at 1712, 1341, 1049, and 2884 cm−1 are assigned to ν(CO), ν(COO), ν(C−O), and C−H deformation [δ(C−H)] of formate. Together with the peak shifted from 1400 to 1396 cm−1 , the generation of monodentate formate (m-HCOO*) was confirmed.37 Closer examination reveals that there are two types of formate intermediates. The peak at 1646 cm−1 between 1637 and 1652 cm−1 is characteristic of O−C−O stretching of bidentate bicarbonate (b-HCO3*).38 The growth bands at 1735, 1352, and 1065 cm−1, attributed to ν(CO), νs(COO), and ν(C− O), respectively, also demonstrate the generation of bHCOO*. Accordingly, in addition to the m-HCOO* pathway, bidentate formate (b-HCOO*) existed in the reaction. With the prolongation of photoirradiation time, the increasing peak at 1699 cm−1 associated with the ν(C−O) at 1104 cm−1 and the broad peak at 2700−3000 cm−1 arising from carboxylic acid hydroxide confirm the formate ion generation. The characteristic peak at 3120 cm−1 suggests that the final product is the stable conformer trans-HCOOH.39 Compared with the production of HCOOH, CO is negligible as a minor pathway in the CO2RR. A combination thereof suggests a plausible major pathway for CO2 reduction on the surface of ZnS. The reaction pathways consist of three elementary steps.3,4 In the first step, the chemisorbed CO2 or HCO3− is attached to the ZnS surface. When the semiconductor is excited and electron transfer occurs, HCOO* is generated. Then the second electron transfer takes place to form HCOO−, which desorbs from the catalyst surface subsequently. To clarify the impact of VZn on the kinetics of CO2RR over the defective ZnS surface, the free energy configuration was calculated based on the adsorption process of the intermediates. First, we optimized the proton (H) adsorption to elucidate why the VZn-ZnS could suppress HER and give high selectivity to CO2RR. Shown in Figures 4a and S19, it is found that the H is simultaneously bound to the middle site of three Zn atoms, far away from the VZn. Furthermore, the adsorption energy of CO2 is −0.53 eV, lower than that of the proton adsorption energy of −0.41 eV (see Figure 4b), suggesting that CO2 is more stabilized than the proton. When the proton adsorption energy on the defective surface is compared with that on the pristine surface, the former stays further from zero, indicating that the VZn-containing surface is not as favorable as the perfect one for HER. Then, we threw the corresponding intermediates of CO2RR and the main product of HCOO− and CO on the surface with and without VZn. The free energy diagrams coupled with the models of CO2, HCOO*, and HCOO− on the w-ZnS(100) surface were figured out. In the case of bicarbonate ion (see Figure S20), the large energy barrier occurs where a proton− electron pair couples with bicarbonate to form HCOO*, demonstrating that the pathway via HCO3− is difficult, consistent with what Guzman et al. implied.3 Instead, the HCO3− is suggested to enhance the concentration of effective CO2.7,34 In addition, CO cannot be the dominant product in CO2 reduction due to a higher adsorption energy of the *CO intermediate than that of HCOO− desorption (see the details

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00711. Additional experimental data and results including experimental methods, SEM/TEM images, XRD spectra, Raman spectra, N2 adsorption−desorption isotherms, CO2 adsorption, XPS results, PL results, UV−visible 1391

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(12) Wang, S.; Chen, P.; Bai, Y.; Yun, J.-H.; Liu, G.; Wang, L. New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar-Driven Water Splitting. Adv. Mater. 2018, 30, 1800486. (13) Zhang, B.; Wang, L.; Zhang, Y.; Ding, Y.; Bi, Y. Ultrathin FeOOH Nanolayers with Abundant Oxygen Vacancies on BiVO4 Photoanodes for Efficient Water Oxidation. Angew. Chem., Int. Ed. 2018, 57, 2248−2252. (14) Mei, Z.; Zhang, B.; Zheng, J.; Yuan, S.; Zhuo, Z.; Meng, X.; Chen, Z.; Amine, K.; Yang, W.; Wang, L. W.; Wang, W.; Wang, S.; Gong, Q.; Li, J.; Liu, F. S.; Pan, F. Tuning Cu Dopant of Zn0.5Cd0.5S Nanocrystals Enables High-Performance Photocatalytic H2 Evolution from Water Splitting under Visible-Light Irradiation. Nano Energy 2016, 26, 405−416. (15) Wu, Q.; van de Krol, R. Selective Photoreduction of Nitric Oxide to Nitrogen by Nanostructured TiO2 Photocatalysts: Role of Oxygen Vacancies and Iron Dopant. J. Am. Chem. Soc. 2012, 134, 9369−9375. (16) Li, C.; Wang, T.; Zhao, Z. J.; Yang, W.; Li, J. F.; Li, A.; Yang, Z.; Ozin, G. A.; Gong, J. Promoted Fixation of Molecular Nitrogen with Surface Oxygen Vacancies on Plasmon-Enhanced TiO2 Photoelectrodes. Angew. Chem., Int. Ed. 2018, 57, 5278−5282. (17) Zhang, N.; Jalil, A.; Wu, D.; Chen, S.; Liu, Y.; Gao, C.; Ye, W.; Qi, Z.; Ju, H.; Wang, C.; Wu, X.; Song, L.; Zhu, J.; Xiong, Y. Refining Defect States in W18O49 by Mo Doping: A Strategy for Tuning N2 Activation towards Solar-Driven Nitrogen Fixation. J. Am. Chem. Soc. 2018, 140, 9434−9443. (18) Hosokawa, H.; Murakoshi, K.; Wada, Y.; Yanagida, S.; Satoh, M. Extended X-ray Absorption Fine Structure Analysis of ZnS Nanocrystallites in N, N-Dimethylformamide. An Effect of Counteranions on the Microscopic Structure of a Solvated Surface. Langmuir 1996, 12, 3598−3603. (19) Meng, X.; Zuo, G.; Zong, P.; Pang, H.; Ren, J.; Zeng, X.; Liu, S.; Shen, Y.; Zhou, W.; Ye, J. A Rapidly Room-TemperatureSynthesized Cd/ZnS:Cu Nanocrystal Photocatalyst for Highly Efficient Solar-Light-Powered CO2 Reduction. Appl. Catal., B 2018, 237, 68−73. (20) Pang, H.; Meng, X.; Song, H.; Zhou, W.; Yang, G.; Zhang, H.; Izumi, Y.; Takei, T.; Jewasuwan, W.; Fukata, N.; Ye, J. Probing the Role of Nickel Dopant in Aqueous Colloidal ZnS Nanocrystals for Efficient Solar-Driven CO2 Reduction. Appl. Catal., B 2019, 244, 1013−1020. (21) Kasai, P. H.; Otomo, Y. Electron Paramagnetic Resonance Studies of the ZnS-A and -B Centers. J. Chem. Phys. 1962, 37, 1263− 1275. (22) Schneider, J.; Räuber, A.; Dischler, B.; Estle, T. L.; Holton, W. C. Direct Confirmation of the A-Center Model in ZnS by Observation of Hyperfine Structure in EPR Spectra. J. Chem. Phys. 1965, 42, 1839−1841. (23) Schneider, J.; Räuber, A. Electron Spin Resonance of the Fcentre in ZnS. Solid State Commun. 1967, 5, 779−781. (24) Li, H.; Bredas, J. L. Comparison of the Impact of Zinc Vacancies on Charge Separation and Charge Transfer at ZnO/ Sexithienyl and ZnO/Fullerene Interfaces. Adv. Mater. 2016, 28, 3928−3936. (25) Djurišić, A. B.; Choy, W. C. H.; Roy, V. A. L.; Leung, Y. H.; Kwong, C. Y.; Cheah, K. W.; Gundu Rao, T. K.; Chan, W. K.; Fei Lui, H.; Surya, C. Photoluminescence and Electron Paramagnetic Resonance of ZnO Tetrapod Structures. Adv. Funct. Mater. 2004, 14, 856−864. (26) Son, N. T.; Isoya, J.; Ivanov, I. G.; Ohshima, T.; Janzén, E. Hydrogen at Zinc Vacancy of ZnO: An EPR and ESEEM study. AIP Conf. Proc. 2013, 1583, 341−344. (27) Schneider, J.; Dischler, B.; Räuber, A. Defects in ZnS/ZnSe Mixed Crystals  II. The Paramagnetic A-Centers in Cubic ZnS1−xSex. J. Phys. Chem. Solids 1970, 31, 337−353. (28) Fang, Z.; Weng, S.; Ye, X.; Feng, W.; Zheng, Z.; Lu, M.; Lin, S.; Fu, X.; Liu, P. Defect Engineering and Phase Junction Architecture of Wide-Bandgap ZnS for Conflicting Visible Light Activity in

absorption spectra, photocatalytic activity, GC-MS spectrum, NMR spectra, ATR-IR spectra, and DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xueliang Zhang: 0000-0001-5037-5781 Naoki Fukata: 0000-0002-0986-8485 Defa Wang: 0000-0001-7196-6898 Jinhua Ye: 0000-0002-8105-8903 Author Contributions ∇

H.P. and X.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work received financial support from the World Premier International Research Center Initiative (WPI Initiative) on Materials Nanoarchitectonics (MANA), MEXT (Japan), the National Natural Science Foundation of China (201633004 and 21703065), JSPS KAKENHI Grant Number JP18H02065, Photoexcitonix Project in Hokkaido University, the Natural Science Foundation of Hebei Province (B2018209267), and the Natural Science Foundation of Jiangsu Province (BK20180438).

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REFERENCES

(1) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Nano- Photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229−251. (2) Chang, X.; Wang, T.; Gong, J. CO2 Photo-Reduction: Insights into CO2 Activation and Reaction on Surfaces of Photocatalysts. Energy Environ. Sci. 2016, 9, 2177−2196. (3) Zhou, R.; Guzman, M. I. CO2 Reduction under Periodic Illumination of ZnS. J. Phys. Chem. C 2014, 118, 11649−11656. (4) Zhou, R.; Guzman, M. I. Photocatalytic Reduction of Fumarate to Succinate on ZnS Mineral Surfaces. J. Phys. Chem. C 2016, 120, 7349−7357. (5) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazabal, G. O.; Perez-Ramirez, J. Status and Perspectives of CO2 Conversion into Fuels and Chemicals by Catalytic, Photocatalytic and Electrocatalytic Processes. Energy Environ. Sci. 2013, 6, 3112−3135. (6) Xu, Y.; Schoonen, M. A. A. The Absolute Energy Positions of Conduction and Valence Bands of Selected Semiconducting Minerals. Am. Mineral. 2000, 85, 543−556. (7) Meng, X.; Yu, Q.; Liu, G.; Shi, L.; Zhao, G.; Liu, H.; Li, P.; Chang, K.; Kako, T.; Ye, J. Efficient Photocatalytic CO 2 Reduction in All-Inorganic Aqueous Environment: Cooperation between Reaction Medium and Cd(II) Modified Colloidal ZnS. Nano Energy 2017, 34, 524−532. (8) Guzman, M. I.; Martin, S. T. Photo-Production of Lactate from Glyoxylate: How Minerals Can Facilitate Energy Storage in a Prebiotic World. Chem. Commun. 2010, 46, 2265−2267. (9) Pan, L.; Wang, S.; Mi, W.; Song, J.; Zou, J.-J.; Wang, L.; Zhang, X. Undoped ZnO Abundant with Metal Vacancies. Nano Energy 2014, 9, 71−79. (10) Pacchioni, G. Oxygen Vacancy: The Invisible Agent on Oxide Surfaces. ChemPhysChem 2003, 4, 1041−1047. (11) Li, G.; Blake, G. R.; Palstra, T. T. Vacancies in Functional Materials for Clean Energy Storage and Harvesting: The Perfect Imperfection. Chem. Soc. Rev. 2017, 46, 1693−1706. 1392

DOI: 10.1021/acsenergylett.9b00711 ACS Energy Lett. 2019, 4, 1387−1393

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ACS Energy Letters Photocatalytic H2 Evolution. ACS Appl. Mater. Interfaces 2015, 7, 13915−13924. (29) Shono, Y.; Oka, T. Complex Defects in Electron-Irradiated ZnS. J. Cryst. Growth 2000, 210, 278−282. (30) Lee, K. M.; O'donnell, K. P.; Watkins, G. D. Optically Detected Magnetic Resonance of the Zinc Vacancy in ZnS. Solid State Commun. 1982, 41, 881−883. (31) Serpone, N.; Emeline, A. V. Semiconductor Photocatalysis Past, Present, and Future Outlook. J. Phys. Chem. Lett. 2012, 3, 673− 677. (32) Hoque, M. A.; Guzman, M. Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis. Materials 2018, 11, 1990. (33) Baruch, M. F.; Pander, J. E.; White, J. L.; Bocarsly, A. B. Mechanistic Insights into the Reduction of CO2 on Tin Electrodes Using in Situ ATR-IR Spectroscopy. ACS Catal. 2015, 5, 3148−3156. (34) Dunwell, M.; Lu, Q.; Heyes, J. M.; Rosen, J.; Chen, J. G.; Yan, Y.; Jiao, F.; Xu, B. The Central Role of Bicarbonate in the Electrochemical Reduction of Carbon Dioxide on Gold. J. Am. Chem. Soc. 2017, 139, 3774−3783. (35) Sheng, H.; Oh, M. H.; Osowiecki, W. T.; Kim, W.; Alivisatos, A. P.; Frei, H. Carbon Dioxide Dimer Radical Anion as Surface Intermediate of Photoinduced CO2 Reduction at Aqueous Cu and CdSe Nanoparticle Catalysts by Rapid-Scan FT-IR Spectroscopy. J. Am. Chem. Soc. 2018, 140, 4363−4371. (36) Amatore, C.; Saveant, J. M. Mechanism and Kinetic Characteristics of the Electrochemical Reduction of Carbon Dioxide in Media of Low Proton Availability. J. Am. Chem. Soc. 1981, 103, 5021−5023. (37) Teramura, K.; Hori, K.; Terao, Y.; Huang, Z.; Iguchi, S.; Wang, Z.; Asakura, H.; Hosokawa, S.; Tanaka, T. Which is an Intermediate Species for Photocatalytic Conversion of CO2 by H2O as the Electron Donor: CO2 Molecule, Carbonic Acid, Bicarbonate, or Carbonate Ions? J. Phys. Chem. C 2017, 121, 8711−8721. (38) Kock, E. M.; Kogler, M.; Bielz, T.; Klotzer, B.; Penner, S. In Situ FT-IR Spectroscopic Study of CO2 and CO Adsorption on Y2O3, ZrO2, and Yttria-Stabilized ZrO2. J. Phys. Chem. C 2013, 117, 17666− 17673. (39) Richter, F.; Carbonniere, P. Vibrational Treatment of the Formic Acid Double Minimum Case in Valence Coordinates. J. Chem. Phys. 2018, 148, No. 064303.

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DOI: 10.1021/acsenergylett.9b00711 ACS Energy Lett. 2019, 4, 1387−1393