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Photocathode characteristics of a spray-deposited CuZnGeS thin film for CO reduction in a CO-saturated aqueous solution 2

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Shigeru Ikeda, Shotaro Fujikawa, Takashi Harada, Thi Hiep Nguyen, Shuji Nakanishi, Tomoaki Takayama, Akihide Iwase, and Akihiko Kudo ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01418 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on September 1, 2019

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ACS Applied Energy Materials

Photocathode characteristics of a spray-deposited Cu2ZnGeS4 thin film for CO2 reduction in a CO2saturated aqueous solution Shigeru Ikeda,*,† Shotaro Fujikawa,‡ Takashi Harada,‡ Thi Hiep Nguyen,‡,|| Shuji Nakanishi,‡ Tomoaki Takayama,§ Akihide Iwase,§ and Akihiko Kudo§ †Department

of Chemistry, Konan University, 9-1 Okamoto, Higashinada, Kobe, Hyogo 6588501, Japan

‡Research

Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

§Department

of Applied Chemistry, Faculty of Science, Tokyo University of Science,1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan.

ABSTRACT

A kesterite Cu2ZnGeS4 (CZGS) thin film was prepared on an Mo-coated glass substrate by spray deposition of an aqueous solution containing constituent elements followed by heat treatment in a sulfur atmosphere. The results of analysis of the ionization potential of the CZGS thin film by

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photoelectron spectroscopy (PES) and the external quantum efficiency (EQE) spectrum of a solar cell device based on the CZGS thin film showed that this compound had ca. 0.7 eV more negative conduction band minimum (CBM) than that of the well-studied kesterite Cu2ZnSnS4 (CZTS) but had almost the same energy level of the valence band maximum (VBM) as that of CZTS. Owing to the relatively shallow CBM, the CZGS film worked as a photocathode for photoelectrochemical (PEC) CO2 reduction in a neutral aqueous solution (0.1 M KHCO3) saturated with CO2 by applying a bias potential lower than that for inducing the reaction under a dark condition, whereas the CZTS film could not work under the same condition. Enhancement of PEC CO2 reduction activity by surface coverage of the CZGS thin film with a ZnS layer is also discussed on the basis of the electronic structure of the thus-formed CZGS-ZnS interface.

KEYWORDS Cu2ZnGeS4 thin film; spray deposition; electron energy structure; photoelectrochemical CO2 reduction; surface modification; carrier injection and recombination

INTRODUCTION In order to achieve production of storable and transportable energy sources from sunlight, photocatalytic or photoelectrochemical (PEC) conversion of sunlight into a chemical energy of hydrogen (H2) through water splitting has been studied extensively.1-6 Reduction of carbon dioxide (CO2) using water (H2O) as an electron source into a more reduced chemical species, such as carbon monoxide (CO), formic acid, formaldehyde, and methanol, is also an important subject for


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utilization of solar energy because of its possible production of energy-rich carbonaceous compounds without the use of fossil resources.7-13 Among the various possible systems, a coupling of two different semiconductors referred to as a Z-scheme system, in which photoexcited electrons from the semiconductor having a less negative conduction band minimum (CBM) transfer to the valence band maximum (VBM) of the coupled semiconductor and combine with photoexcited holes, has been studied as an attractive concept in recent years.14-18 In a powder photocatalytic system, some of the co-authors have demonstrated photocatalytic properties of modified CuGaS2 powders for CO2 reduction: the CuGaS2 powder induced CO2 reduction into CO when combined with an O2 evolution photocatalyst powder (CoOx-loaded BiVO4) and a reduced graphene oxide as an electron mediator.19,20 In a similar powder-based Z-scheme system, Jung et al. showed activity of carbon-coated Cu2O for CO2 reduction into CO.21 Instead of these powder systems, the use of photoelectrodes, i.e., PEC systems, is also interesting because of the possible separate productions of reduced energy-rich chemicals from oxygen (O2) produced from H2O. However, there have been few examples of the realization of a bias-free system for CO2 reduction using H2O as the electron source by using a UV-light-driven photoanode (TiO2 or SrTiO3)14 or an extremely expensive triple-junction solar cell,15 whereas several examples have been shown for water reduction into H2 (i.e., overall water splitting) using simple semiconductor-liquid junctions under visible light.22-27 One of the difficulties in inducing CO2 reduction compared to H2O reduction is the requirement of a high voltage derived from the relatively negative equilibrium potentials of CO2 compared to H2O reduction and a large overpotential of the reduction, leading to preferential H2 evolution in an aqueous solution. In order to lower the high overpotential to induce CO2 reduction, efficient mediators having selective catalytic activity for CO2 reduction have been


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studied.28,29 The other strategy is surveys of photocathode materials having a highly negative CBM to gain sufficient potential of photogenerated electrons to induce CO2 reduction. The quaternary compound semiconductor Cu2ZnSnS4 (CZTS) crystallized in a kesterite structure has attracted much attention for its potential applications as a solar-cell absorber30-36 and a thermoelectric material37,38 due to its earth-abundance of constituent elements and low toxicity. Regarding PEC applications, we have shown that a CZTS thin film obtained by electrodeposition followed by annealing could be used as a photocathode for PEC water splitting by applying appropriate surface modifications to the film: bias-free water splitting was demonstrated upon combination with a BiVO4-based photoanode.22,27 Ohno et al. also applied a CZTS thin film obtained by spray deposition for PEC CO2 reduction: appreciable amounts of CO were produced when a bias voltage higher than that for inducing the reaction under a dark condition was applied.39 In order to facilitate CO2 reduction without applying such a high bias voltage, therefore, a p-type semiconductor material having a CBM more negative than that of CZTS would be promising. Cu2ZnGeS4 (CZGS) belongs to the family with the same kesterite structure as that of CZTS. The compound is experimentally and theoretically known to be a p-type semiconductor with a relatively wide bandgap (Eg = 1.9-2.2 eV).40-42 Results of recent theoretical and experimental studies have shown that the band offset of the valence band maximum (VBM) between CZGS and CZTS is almost constant (the offset being less than 0.2 eV), while the conduction band minimum (CBM) shifts up from CZTS to CZGS.42,43 Therefore, we can expect a significant upward shift of the CBM of CZGS to that of CZTS, leading to possible induction of PEC CO2 reduction by using a CZGS thin film-based electrode. In the present study, we aimed to fabricate a phase-pure CZGS thin film through a facile spray pyrolysis technique as reported previously for fabrication of several p-type compound thin films


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such as CZTS27,31,33,35 and Cu(In,Ga)(Se,S)2.44-49 Structural, electric, and solar cell properties of thus-obtained CZGS thin films were examined. We then demonstrated for the first time the PEC performance for CO2 reduction using the CZGS thin film. Significant improvements of PEC properties induced by coverage with a ZnS layer are also reported.

EXPERIMENTAL SECTION Thin Film Preparation. Spray deposition was carried out by using a home-made apparatus composed of a Lumina HMP 6RX automatic spray gun and a HAKKO ELECTRIC custom-made hot plate in a fume hood. The source materials used for the CZGS thin film were Cu(NO3)2, Zn(NO3)2, GeO2 and thiourea (SC(NH2)2). A solution for spray deposition was prepared as follows. Firstly, GeO2 powder was dissolved in aqueous ammonia at pH 10.0. After the addition of conc. HNO3 to adjust pH to 2.8, an aqueous solution of Cu(NO3)2 and Zn(NO3)2 and an aqueous SC(NH2)2 solution containing a small amount of hexadecyltrimethylammonium bromide were added. The final composition of the solution was 0.02 M Cu(NO3)2, 0.01 M Zn(NO3)2, 0.01 M GeO2 and 0.06 M SC(NH2)2. A 1.5-times excess amount of thiourea from the stoichiometry of Cu2ZnGeS4 was necessary to compensate its evaporation loss during the spray deposition. The asobtained solution (6 cm3) was sprayed twice onto an Mo-coated glass substrate (Mo-glass). The temperature of the Mo-glass was adjusted to 320 °C. Nitrogen gas was used as a carrier gas during the deposition in order to avoid oxidation of the Mo film. The precursor film on Mo-glass was then put in an evacuated borosilicate glass ampoule together with 10 mg of sulfur powder; the ampoule was heated at different temperatures (520-600 °C) for 30 min. Thus-obtained CZGS thin films on Mo-glass are labeled CZGS_temp., e.g., CZGS_520 denotes a CZGS thin film on Mo-glass obtained by 520 °C heat treatment. For PEC CO2 reduction (see below), loading of Au or Ag


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deposits on the CZGS_540 thin film was performed by thermal evaporation to form AuCZGS_540 and Ag-CZTS_540 thin films on Mo-glass. Modifications of the CZGS_540 thin film surface with constituent metallic elements were also prepared by sequential spray deposition of the CZGS precursor layer and 0.01 M Cu(NO3)2, 0.01 M GeO2, or 0.01 M Zn(NO3)2 solution followed by heat treatment at 540 °C for 30 min in an evacuated glass ampoule with sulfur. Thusobtained thin films on Mo-glass are labeled MxCZGS_540, where Mx denotes the charged molar content of excess metals). A CZTS thin film on Mo-glass was also prepared by a process similar to that for the CZGS deposition using Cu(NO3)2, Zn(NO3)2, tin methanesulfonate (Sn(CH3SO3)2), and SC(NH2)2 as source materials. Details of the CZTS deposition were reported previously.27,31,33,35 Characterization. Crystallographic structures were determined by X-ray diffraction (XRD) analyses using a Rigaku Mini Flex X-ray diffractometer (Cu Kα, Ni filter) and Raman spectroscopy using a JASCO NRS 3100 laser Raman spectrophotometer. Ionization potentials of CZGS and CZTS thin films were analyzed by photoelectron spectroscopy (PES) using a Riken Keiki AC-3 surface analyzer after calibration by measuring the work function of an Au film (4.83 eV).47,50 In order to determine an effective photoabsorption onset, an external quantum efficiency (EQE) spectrum of a solar cell device based on the CZGS_600 thin film on Mo-glass was analyzed with a Bunkoh-Keiki CEP-015 photovoltaic measurement system. To complete the solar cell device, a CdS buffer layer was deposited on the CZGS_600 film by chemical bath deposition, followed by sequential deposition of an intrinsic ZnO-indium tin oxide (ITO) double layer and an Al top contact by radio frequency magnetron sputtering and thermal evaporation, respectively. The active area of the thus-obtained solar cell was 0.03 cm2. Surface compositions of ZnS-modified CZGS films were examined by X-ray photoelectron spectroscopy (XPS) using a Shimadzu AXIS


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ULTRA X-ray photoelectron spectrometer. Observation of surface morphologies was performed by using a Hitachi S-5000 field emission scanning electron microscope (SEM). PEC Reactions. PEC reactions were performed using a conventional three-electrode setup. Three electrodes, namely, Pt wire, Ag/AgCl, and CZGS (or CZTS)-based films, as counter, reference, and working electrodes, respectively, were inserted into a sealed vessel with a flat window. For evaluation of water reduction (H2 evolution) properties, the surface of the CZGS (or CZTS) thin film was modified with a chemical-bath-deposited CdS layer and Pt deposits to form a PtCdS/CZGS (or CZTS) structure.22,47,52 A pH 6.5 phosphate buffer solution (0.2 M Na2HPO4/NaH2PO4) was used as an electrolyte. PEC CO2 reduction was performed by the same setup as that for the H2 evolution using a 0.1-M KHCO3 solution saturated with CO2 (pH 6.8) as an electrolyte. In order to quantitatively evaluate gas-phase components during PEC CO2 reduction, the PEC cell was connected to an online gas analysis system based on an Agilent 490 Micro GC Gas Analyzer equipped with a PoraPLOT Q column and a thermal conductivity detector. Simulated AM 1.5G solar irradiation from an Asahi Spectra HAL320 solar simulator was used as a light source for all PEC measurements. Potentials referred to the Ag/AgCl electrode (VAg/AgCl) were converted to a reversible hydrogen electrode (VRHE) using the Nernst equation (VRHE = VAg/AgCl + 0.059 × pH + 0.199). The active area of photocathodes was fixed at 0.35 cm2.

RESULTS AND DISCUSSION Crystallographic and Electric Structures. Figure 1 shows XRD patterns and Raman spectra of an as-sprayed precursor film and that heated at 520-600 °C in an evacuated glass ampoule with sulfur. As shown in the XRD results (Figure 1a), the as-sprayed precursor film on Mo-glass showed broad reflections assignable to the Cu2-xS compound in addition to reflections of the Mo


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bottom layer. No appreciable reflection related to Zn- and Ge-related compounds was observed in the precursor film, suggesting amorphous features of these components in the as-sprayed film. The diffraction pattern of Cu2-xS was changed when the precursor film was heated at 520 °C: several peaks at 2θ of 29.0°, 48.0°, 48.5°, and 57.1° assignable to (112), (220), (204), and (312) reflections of CZGS with a kesterite structure40-42 were observed. Heat treatment at a higher temperature (>540 °C) resulted in sharpening and increase in these reflections, indicating grain growth of CZGS without any structural change. It should be noted that the observation of broad reflection at 2θ of ca. 32°-35° in the film was due to the overlap of (200) and (004) reflections of CZGS as well as a reflection derived from MoS2 formed by sulfurization of the Mo bottom layer.33,47 The corresponding Raman spectra in Figure 1b also showed a structural alteration from Cu2-xS to CZGS after the heat treatment. A weak broad band at 479 cm−1 derived from Cu2-xS observed on the asdeposited film disappeared after the heat treatment; instead, several bands at 299, 319, 362, 381, and 407 cm−1, all of which correspond to the CZGS structure, were observed.40,41,51 The fact that no other appreciable peak appeared in Raman spectra of samples after the heat treatment indicates successful formation of a phase-pure CZGS film by the present spray process. In order to examine the band offset between CZGS and CZTS, energy levels of VBMs from the vacuum level for CZGS and CZTS films were estimated from their ionization potentials measured directly by PES. Figure 2a shows cube root of photoemission yield (Y1/3)-energy (hν) plots of CZGS_600 and CZTS films. It is clear that onsets of both of the films, estimated by extrapolations of linear slopes of these plots to baselines of them,49 showed almost the same values of 5.26 eV for CZGS_600 and 5.27 eV for CZTS, indicating that the depth of the VBM of CZGS does not change significantly by substitution of Ge for Sn in CZGS as was found for a powder sample42 as well as by theoretical prediction.43


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Figure 2b shows representative current density and voltage (J-V) curves of a solar cell based on the CZGS_600 film under a dark condition and under a condition of irradiation of simulated sunlight (AM 1.5G). The device showed appreciable rectification behavior and photoresponse. As determined from the J-V curve under irradiation, short-circuit current density, open circuit voltage, fill factor, and power conversion efficiency (PCE) were 3.1 mA cm−2, 0.58 V, 0.41, and 0.74%, respectively. Corresponding series resistance (26 Ω cm2) and shunt resistance (570 Ω cm2) calculated by using the J-V curve indicated poor film quality, specifically for significantly large series resistance, leading to the low PCE of the present device. However, this is the first demonstration of solar cell properties of pure sulfide CZGS without containing Sn components produced by a facile non-vacuum deposition technique. Figure 2c shows wavelength dependence of external quantum efficiency (EQE) for the CZGS_600-based solar cell. The EQE value of the solar cell reached a maximum value of ca. 50% in the range of 520-540 nm but decreased gradually with a decrease in wavelength; a steeper drop in EQE was also observed in relatively long wavelength regions (> 540 nm). The former decrease was due to photoabsorption of the CdS buffer layer and the latter is transmission loss. By applying the incident photon energy (hν)-vs-(hν × ln(1 − EQE))2 plot (inset of Figure 2c), the Eg value of the sample was estimated to 2.16 eV, being in good agreement with values reported previously.40-42 Based on the above electrostructural analyses, the band offset between CZGS and CZTS was determined as shown in Figure 2d. As mentioned above, VBMs of CZGS and CZTS were almost constant, and thereby a significant upward shift of the CBM of CZGS was induced by widening of Eg compared to that of CZTS (1.48 eV32). Since VBMs of both compounds primarily consist of S 3p and Cu 3d atomic orbitals, the VBM potential was not affected by the substitution of Sn to Ge.40,43 On the other hand, CBMs of these compounds are antibonding states formed by


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hybridization between the group IV (Sn , Ge) s orbitals and S 3p orbital.40,42 Because of the shorter Ge-S bond than the Sn-S bond as expected from lattice parameters of CZGS and CZTS,42 the antibonding character of the CBM for CZGS is enhanced, leading to increment of Eg by replacing Sn to Ge. PEC Water Reduction and CO2 Reduction. Figure 3a shows current density-potential curves in a pH 6.5 phosphate buffer solution (0.2 M Na2HPO4/NaH2PO4) obtained by CZGS and CZTS films modified with a CdS layer and Pt deposits under chopped illumination of simulated sunlight (AM 1.5G). CZGS films on Mo-glass obtained at different heat-treatment temperatures were used to optimize PEC properties. All of the modified films exhibited appreciable photocurrents due to water reduction at VRHE more negative than ca. 0.5. Among the CZGS-based electrodes, the optimum PEC property for H2 evolution was obtained for the CZGS_540-based electrode, though the obtained photocurrent on this electrode was still much lower than that on the CZTS-based electrode. As discussed above for the photovoltaic results, there should be room for improvement of the film quality for the present CZGS thin film. Further studies are now in progress. Figure 3b shows photocurrent profiles over CZGS_540 and CZTS films on Mo-glass immersed in a 0.1-M KHCO3 solution saturated with CO2 (pH 6.8) using the three-electrode setup at −0.2 VRHE under illumination of simulated sunlight (AM 1.5G). Based on the above-discussed results for PEC H2 evolution, the CZGS_540 thin film on Mo-glass was used for this experiment. Results for ZnxCZGS_540 films on Mo-glass are also shown in this figure (see below). The CZTS thin film on Mo-glass showed almost no photocurrent under the present conditions. On the other hand, an appreciable photocurrent flow was observed over the CZGS_540 thin film on Mo-glass, though a significant photocurrent drop appeared during photoirradiation. Based on the results of the above band offset analyses (Figure 2d), the shallower CBM energy of CZGS than that of CZTS gains


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sufficient overvoltage to induce CO2 reduction. As was reported previously,39 application of a higher bias voltage should be required for the CZTS thin film on Mo-glass to induce the reaction. It should be noted that the activity of CO2 reduction would depend strongly on solution conditions such as pH and the concentration of KHCO3 used: these optimizations of reaction conditions will be discussed elsewhere. Impacts of Surface Modification to the Cu2ZnGeS4 film. In our previous studies on CuGaS2based photocatalytic and PEC CO2 reduction systems,19,49 CuGaS2 powders and films without any modifications were used; there is no example of a successful impact on CO2 reduction being achieved by loading of a noble metal catalyst. Thus, several trials to enhance activity for PEC CO2 reduction by loading of metallic Au or Ag on the surface of the CZGS_540 thin film on Mo-glass were carried out. As summarized in Table 1, however, such modifications were not successful: significant drops in activity for CO evolution were observed., due probably to enhancement of the carrier recombination at heterointerfaces between these metal deposits and the CZGS surface. Thus, it should be necessary to find the other co-catalyst that can coexist formation of a good heterointerface and catalytic effect for the selective CO2 reduction. As the other strategy, we attempted to control the surface composition of CZGS by addition of constituent metal salts on the top of the CZGS precursor thin film followed by application of heat treatment (540 °C) in a sulfur atmosphere. Typical results for Cu0.3CZGS_540, Ge0.3CZGS_540, and ZnxCZGS_540 (x = 0.30.8) thin films on Mo-glass are summarized in Table 1. It is clear that the addition of excess Cu significantly suppressed the photocurrent, and a similar detrimental effect was observed for the Ge-excess CZGS thin film on Mo-glass, whereas CZGS thin films loaded with appropriate amounts of excess Zn components showed increases in photocurrents, though the main products on these electrodes were still H2. Hence, the effects of Zn addition were further studied.


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XRD patterns of ZnxCZGS_540 (x = 0.3-0.8) thin films on Mo-glass showed the same diffraction pattern assigned to the kesterite Cu2ZnGeS4 crystal (data not shown). As can be seen in expanded parts of the XRD result at 2θ of 28.0-30.0° (Figure 4a), an additional reflection due to the (111) plane of zincblende ZnS was also observed for the ZnxCZGS_540 thin film on Moglass. In addition, atomic compositions of the surface regions of these thin films determined by XPS analyses showed a monotonic increase in Zn contents in these thin films with an increase in the amount of Zn added to the source solutions, as shown in Figure 4b. Corresponding surface SEM images of these samples shown in Figure 4c clearly indicate coverage of granular grains on reticular grains of the pristine surface of the CZGS_540 thin film. Based on the results of these structural analyses, these Zn-modified CZGS thin films are expected to have a layered structure, i.e., CZGS films were decorated with ZnS layers with different thicknesses depending on the amounts of loaded Zn. Photocurrent profiles over ZnxCZGS_540 (x = 0.3-0.8) thin films on Mo-glass were compared with the CZGS thin film on Mo-glass obtained from the stoichiometric source solution (CZGS_540), as shown in the above Figure 3b. Significant improvement of photocurrents was achieved with the exception for samples with the largest amounts of Zn (Zn0.8CZGS_540). Corresponding time course curves of CO evolution over these ZnS-modified CZGS thin films on Mo-glass are shown in Figure 5a together with those over the CZGS_540 thin films on Mo-glass. The best sample of the Zn0.5CZGS_540 thin film on Mo-glass showed a ca. 5-times lager rate of CO evolution than those over the bare CZGS_540 thin films on Mo-glass. It should be noted that there was no detectable product such as formaldehyde in the aqueous phase as confirmed by ionchromatography for the reaction solution after 2-h photoirradiation.


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As shown in the above results for PEC H2 evolution, surface modification of n-type metal sulfide materials, specifically CdS, with a thickness in a scale of nanometers deposited by the chemical bath deposition (CBD) method has been reported to be a good surface modification for p-type photoelectrodes for water reduction.22,24-27,52-54 The effects of such modification would include control of built-in potential to facilitate charge separation at the semiconductor-electrolyte interface, suppression of the formation of interfacial defects to avoid recombination losses, and induction of a potentially downward gradient of band alignment to transfer photoexcited charge carriers smoothly. We proposed a similar function of surface modified ZnS in the present system. Figure 5b shows the estimated band alignment between CZGS and ZnS with the application of a bias potential of −0.2 VRHE. Potentials of the CBM and VBM for ZnS were calculated to be −3.60 V and −7.19 V vs vacuum, respectively.55 Although the carrier concentration and relative permittivity of the CZGS thin film on Mo-glass were not clarified, a space charge layer of ca. 120 nm would be formed on the basis of the assumption of them being 1017 cm−3 and 10, respectively. An appropriate CBM offset between CZGS and ZnS (0.35 eV) and an appropriate VBM offset between CZGS and ZnS (1.93 eV) would be achieved by using the significantly large band gap of ZnS (3.59 eV)55 as the surface modifier even with the use of the present wide-gap CZGS absorber. This results in efficient injection of photoexcited electrons from CZGS to ZnS as well as suppression of hole injection from CZGS to the electron-rich ZnS layer. It is notable that the low lattice mismatch (2.4%) between the (100) plane of CZGS 42 and (100) plane of zincblende ZnS56 induces smaller amounts of defects between the CZGS-ZnS interface, leading to a decrease in the probability of carrier recombination at the heterointerface. Moreover, ZnS has been studied for a long time as an active photocatalyst for CO2 reduction.55,57 Hence, the surface of this material should have a relatively high catalytic function to induce the reaction compared to that of the


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CZGS surface. These specific characteristics would be responsible for enhancement of the activity in the present system.

CONCLUSION In this study, a new compound, CZGS, was developed for PEC CO2 reduction. Compared to the CZTS compound, which is known to work as an efficient electrode for water reduction, a CZGS thin film has a relatively high CBM potential: this energy structure is indispensable for facilitating CO2 reduction because of the achievement of sufficient overpotential to induce the reaction. Moreover, surface coverage with a ZnS layer was found to be effective for enhancing the PEC function of CZGS for CO2 reduction. The modification might result in modulation of band alignment, passivation of interfacial defects, and creation of efficient catalytic sites for CO2 reduction, leading to a ca. 5-times larger rate of CO evolution than that when using a CZGS thin film without any modification. One of the critical problems of the present system is the low quality of the CZGS film used. Another significant point to improve the activity is development of an efficient mediator having an appropriate heterointerface structure to prevent carrier recombination for inducing selective CO2 reduction. Although such further optimizations should be required to enhance the activity, the GZGS-based electrode would be a candidate to make a Z-scheme system for CO2 reduction using water as an electron donor upon combination of an appropriate photoanode for water oxidation. Thus, our finding will contribute to develop sunlight-induced artificial photosynthetic systems.


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Figure 1. XRD patterns (a) and Raman spectra (b) of an as-sprayed thin film on Mo-glass and thin films on Mo-glass heated at 520-600 °C in an evacuated glass ampoule containing sulfur powder.


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Figure 2. (a) Photoemission yield (Y1/3)-energy (hν) plots of CZGS_600 and CZTS thin films on Mo-glass. (b) Current density-potential curve of the CZGS_600-based device under illumination (AM1.5G) and that in a dark condition. (c) EQE spectrum of the CZGS_600-based device measured under a short-circuit condition. The inset shows a corresponding photon energy (hν) vs (hν × ln(1 − EQE))2 plot. (d) Band offset between CZGS and CZTS. Redox potentials of H2O/H2 and CO2/CO couples at pH 7.0 are also shown.


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Figure 3. (a) Current density-potential curves of CZGS and CZTS thin films on Mo-glass modified with a chemical-bath-deposited CdS layer and Pt deposits in 0.2 M Na2HPO4/NaH2PO4 solution (pH 6.5) under chopped solar simulated AM 1.5G light irradiation. (b) Current density-time curves for CZGS_540, CZTS, and ZnxCZGS_540 (x = 0.3-0.8) thin films on Mo-glass in 0.1 M KHCO3 (pH 6.8) at −0.2 VRHE under photoirradiation from simulated sunlight (AM 1.5G).


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Figure 4. (a) XRD patterns, (b) surface elemental compositions, and (c) surface SEM images of CZGS_540 and ZnxCZGS_540 (x = 0.3-0.8) thin films on Mo-glass.


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Figure 5. (a) Time course curves of CO liberation over CZGS_540, CZTS, and ZnxCZGS_540 (x = 0.3-0.8) thin films on Mo-glass. (b) Estimated and alignment profiles at the solid-liquid interface for CZGS and that covered with a ZnS thin layer with application of a potential at −0.2 VRHE.

Table 1. Summary of amounts of CO and H2 production over CZTS- and CZGS-based electrodes from CO2-saturated 0.1 M KHCO3 solution under illumination of simulated sunlight (AM 1.5G). CO (μmol)a

H2 (μmol)b

F. eff. (%)c CO

H2

CZTS

0.00

0.00

–

–

CZGS_540

0.06

1.46

3.1

76.2

Au-CZGS_540

ACS Applied Energy Materials - ACS Publications - American

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