Influences of Silver and Zinc Contents in the Stannite Ag2ZnSnS4

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Influences of Silver and Zinc Contents in the Stannite Ag2ZnSnS4 Photoelectrodes on Their Photoelectrochemical Performances in the Salt-Water Solution Kong-Wei Cheng, and Shu-Wei Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04849 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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ACS Applied Materials & Interfaces

Influences of Silver and Zinc Contents in the Stannite Ag2ZnSnS4 Photoelectrodes on Their Photoelectrochemical Performances in the Salt-Water Solution

Kong-Wei Cheng a,b,*, Shu-Wei Honga

a

b

Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, 333, Taiwan

Department of Orthopaedic Surgery, Chang Gung Memorial Hospital, Keelung Branch, Taoyuan, 204, Taiwan

*Corresponding author Address: 259 Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan E-mail : [email protected] Tel.: +886-3-2118800-3353; Fax: +886-3-2118668

Keywords: Metal sulphides; salt-water splitting; electrochemical impedance spectra; sputtering; photoelectrochemical response; photoanode. 1

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Abstract: The multicomponent metal sulphide (stannite Ag2ZnSnS4) samples were grown onto the conductive metal oxide coated glass substrates by using the sulfurization of co-sputtering silver-zinc-tin precursors. Several [Ag]/[Zn+Sn] and [Zn]/[Sn] ratios were set in the metal precursors to investigate their influences on the crystal phases, microstructures and physical properties of the stannite Ag2ZnSnS4 samples. The results of the crystal phases and compositions of samples showed that the stannite Ag2ZnSnS4 phase can be obtained using the two-step sulfurization process, which maintained the silver-zinc-tin precursors at 160°C for 1 hour and then kept them at 450°C for 30 minutes under sulfur/nitrogen atmosphere. N-type stannite Ag2ZnSnS4 samples with the carrier concentrations of 5.54×1012 - 9.11×1012 cm-3 can be obtained. High resistivities of Ag2ZnSnS4 samples were observed due to the low values of carrier concentration. Increasing the silver content in the sample can improve its PEC performance due to the decrease in the sample resistivity. The ratio of [Ag]/[Zn+Sn] kept at 0.8 and ratio of [Zn]/[Sn] set at 0.90 in the stannite Ag2ZnSnS4 sample had the highest photoelectrochemical performance of 0.31 mA⋅cm-2 with the potential set at 1.23 V vs. relative hydrogen electrode applied on the sample because of it having the lowest charge transfer resistance in electrolyte.

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1. Introduction: Water splitting using a semiconductor photo-absorber as the electrode in the photoelectrochemical (PEC) cell is a clean and interesting process for the hydrogen production1-4. In a PEC cell, a semiconductor photoanode or photocathode absorbs the light with the photon energy of greater than its energy band gap and the light-excited carriers are formed in the bulk photoelectrode. The electrolyte react with these light-excited carriers to form the H2 and O2 gases at the photoelectrode and counter electrode, respectively, depending on the conduction type of the photoelectrode. To achieve the good PEC water splitting efficiency, the development of the photoelectrode with high solar-light absorption coefficient is thus necessary4-6. Therefore, the visible-light-active semiconductors with low carrier recombination rate and high carrier moving rate have to be developed and many possible candidate photo-absorbers such as metal oxide2,7-8, metal sulphides2, 9-10 and metal selenides11-12 or their solid solutions6, 13-15 were reported for further PEC water splitting applications. Although the PEC performance of greater than 10 % using the multi-layer photoelectrode has been reported, the main problems for the large-scale industrial PEC water splitting process are the costs for material issues and photocorrosion of these photoelectrodes taking place in electrolytes under light irradiation5,16. The electrolytes applied for the testing of sample’s PEC activity in the literatures are

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aqueous Na2SO417-18, K2SO410, Na2S + K2SO311,19, or LiClO4 + trietanolamine20 solutions. However, the seawater is the nature available electrolyte with high ionic conductivity but seldom used for the evaluation of the PEC activities of these samples under light irradiation. Application of seawater as the electrolyte in the PEC cell can make PEC water splitting become a cheap way for the hydrogen production. In 1997, Ichikawa21 reported a PEC cell composed with the titanium oxide and high surface area of platinum plate as the photoelectrode and counter electrode in the seawater bath, respectively. His results showed that the seawater can be decomposed into gases output with the hydrogen production rate of around 0.4 L·h-1m-2 under outdoor sunlight illumination. Our previous study also reported that the ternary Ag8SnS6 photoelectrode in the salt-water solution had superior PEC performance than those in the traditional electrolytes22. Although the salt-water bath can be applied as the electrolyte for the PEC hydrogen production, the numbers of the reports about the PEC salt-water or seawater splitting using the semiconductor photoelectrodes are not enough to find out the major factors that affect the PEC performances of these candidate photoelectrodes in the salt-water bath. Another important factor that influences the industrial PEC applications is the long-term instability of the photoelectrode in the electrolyte, which was caused due to the light-excited holes accumulated in an n-type sample surfaces or the electrons accumulated in the p-type

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sample surfaces16. The formation of surface states, which are caused by surface defects (ex. anion vacancies and surface disorder layer) formed at the manufacturing process or during the reaction process, results in the photo-corrosion effect occurred at the sample surface under light irradiation. Therefore, the influence of sample’s composition on its optical and electronic properties has to be further examined to eliminate the possible influence of surface states during the PEC salt-water splitting. Recently, the Cu-based multicomponent semiconductors such as Cu–Zn–Sn-S (named Cu-II-IV-VI) have been applied in the solar cells and PEC water splitting23-25. These multicomponent Cu-II-IV-VI solid solution semiconductors are composed with the suitable ratio

of Cu-IV-VI metal sulphide/selenide and the

II-VI metal

sulphide/selenide. Their physical properties can be adjusted with the change of ratio between the Cu-IV-VI/II-VI metal sulphides/selenides. The band positions (valence or conduction band), electrical and the PEC properties of these quaternary Cu-II-IV-VI systems can be controlled by applying the technology of semiconductor band engineering, which were reported in the literatures15,19,26. In Cu-II-IV-VI thin film solar cells, some reports showed that the silver ions doped into the bulk Cu2ZnSnSe4/Cu2ZnSnS4 photo-absorbers can improve the microstructures and electronic properties of these Cu-II-IV-VI photo-absorbers27-28. It indicates that the Ag-II-IV-VI semiconductors such as Ag2ZnSnS4 (AZTS) may also have potentials

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applied in the thin film solar cells or photocatalytic hydrogen production. Tsuji et al.19 and Sasamura et al.20 reported the PEC activities of quaternary AZTS powders and photoelectrodes in the aqueous SO32- + S2- (sacrifice agents) solution and LiClO4 + triethanolamine (sacrifice agent), respectively. The photocatalytic activity of 482 µmol⋅hr-1 for AZTS powders and the PEC activity of 40 µA⋅cm-2 at the potential of 0 V vs. Ag/AgCl reference electrode applied on the AZTS photoelectrode in an aqueous solution containing sacrifice agents were reported, respectively. Li et al.29 also reported that the hydrothermal synthesis AZTS powders with the powder size distribution of 100-200 nm have the maximum photocatalytic activity of 580 µmol⋅hr-1 and stability of greater than 16 hours in the electrolyte containing sacrifice agents. The electronic structures and photocatalytic water splitting properties of Ag2ZnSn(S1-xSex)4 samples calculated using the first principle density function theory were also reported by Jing et al.30. They suggest that the control of lattice growth direction for the AZTS sample improves its photocatalytic activity for hydrogen production. For the report proposed by the Yuan et al.31, the conduction type of AZTS sample is the n-type semiconductor. From their calculation results, high formation energy of AgZn acceptors (the silver ions occupied in the Zn sites for samples) is observed. Therefore, the dominant donors for the SnZn (tin ions occupied the Zn sites in sample), Vs (vacancies of S ions), and ZnAg (Zn ions occupied in the silver sites)

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are the major contributions for the conductivity of n-type AZTS sample. However, for the (Zn-poor, Sn-rich) samples, the formation energy of AgZn acceptors is slightly lower than those for donor energy levels. The compensation of acceptor and donor defects occurs for the Sn-rich AZTS samples and it makes the AZTS sample as early intrinsic semiconductor. For the (S-poor and Zn-rich) samples, the donor energy levels are a little lower than those for the acceptors and it makes the number of donors of greater than that for acceptors, which make the samples become the n-type semiconductors with low carrier concentrations. These reports indicated that the compositions of AZTS samples influence their physical and PEC properties. However, few reports for the experimental investigations about the relationships between the compositions and electrical properties for these quaternary Ag-II-IV-VI metal sulphides were discussed. In this study, we developed a systemic investigation for the influences of the Ag and Zn contents in the AZTS samples on their PEC performances in electrolyte containing sodium chloride. Samples’ electrochemical impedance spectra (EIS) in the electrolyte (0.5 M NaCl solution) were carried out to examine the influence of the compositions on their PEC salt-water splitting activities.

2. Experimental process The detail preparation process for the quaternary AZTS samples on the

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substrates is similar with those for the preparation of ternary n-Ag8SnS6 and p-Cu2SnS3 samples reported in our previous studies22,32, but the sputtering targets changed from two sources (Ag and Sn) to three sources (Ag, Zn, and Sn). Reactive sulfurization of co-sputtering silver-zinc-tin metal precursors on the conductive metal oxide (fluorine tin oxide, FTO)-coated glass substrates provide Union-Chemicals Co. (Taiwan, 10 Ω⋅cm-2) was employed to prepare the AZTS samples. However, in order to eliminate the influence of tin component in the FTO-coated glass substrate, the soda-lime glass was used as the substrate for the growth of some metal precursor (silver-zinc-tin precursor) for the measurements of their compositions, crystal phases, and physical properties. A three-sputtering guns system is applied in this work and is shown in Figure S1 (supporting file). Various [Ag]/[Zn+Sn] and [Zn]/[Sn] ratios in these metal precursors were set to obtain the AZTS samples with various Ag and Zn contents after the sulfurization process. The AZTS sample were then used to study the influence of the compositions on their physical properties and PEC activities in the electrolyte. Three 2-inch metal targets (Ag, Sn and Zn with their purity of greater than 99.99 %) provided from the Summit tech. Co. (Taiwan) were applied for the co-sputtering of the metal precursors. When the base pressure of sputtering chamber was decrease of less than 5×10-6 Torr, Ar gas with purity of greater than 99.995 % was injected into the chamber. The pressure of Ar gas in the chamber of

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5×10-3 Torr was set to generate the plasma for the co-sputtering of the metal precursors. Loading another sputtering source into the sputtering chamber would make a change of sputtering rate for the metal layer on substrate compared with those for the preparation of ternary metal sulfide samples. Therefore, detail calibration curves for the sputtering rates of metal targets (Ag, Sn and Zn) have to be re-do again in order to obtain the correct sputtering rate of metal layer on substrates during co-sputtering. First, three individual calibration curves for the average sputtering rate of the each metal layer on substrate and the corresponding sputtering power were obtained using the metal layer thickness obtained at various sputtering powers applied on the metal target with the sputtering time of 20 min, respectively. Then the average sputtering rate of each metal target on substrate was be modified using the digital rate/thickness monitor during co-sputtering process with the suitable calibration factor. The theoretical amount of each metal (Ag, Zn or Sn) on substrates can be obtained using the relationship for the average sputtering rate and the corresponding sputtering power applied on the each metal target with the calibration factor. Then the possible sputtering powers applied on the metal targets for the co-sputtering of metal precursors can be set using the above calibration factors in order to obtain the silver-zinc-tin metal precursors with various [Ag]/[Ag+Sn] and [Zn]/[Sn] ratios.

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The sputtering powers applied on the silver, zinc and tin metal targets were set in the ranges of 10-16, 19-25 and 57-50 W, respectively, for the deposition of these silver-zinc-tin metal precursors. Total silver-zinc-tin metal precursor thickness was set at 1000 nm and measured using a digital rate/thickness monitor (Fil-Tech, SQM-180). After co-sputtering of these metal precursors, a post-annealing process with the annealing time of 1 hours were carried out for these metal precursors. The detail discussions were shown later. The sulfurization process for the obtaining of AZTS sample is similar with our previous studies22,32, but the temperature profiles were changed to obtain the high crystalline AZTS samples. The S powders (purity > 96 %, Simga-Aldrich Co.) with total amount of 2.5 g and the two pieces of silver-zinc-tin metal precursor were put together into the graphite box and loaded into the sulfurizaion apparatus to obtain the AZTS samples. The pressure in the sulfurization apparatus was set at 7.6 Torr with the injection of nitrogen gases (purity > 99,995%) during the sulfurization process. Several possible sulfurization temperature profiles were carried out to find the optimal sulfurization parameters and we will discuss them later. Sample’s crystal phase, composition, surface morphology, physical properties were measured using the Bruker made X-ray diffracometer (D2-Phaser, A26-X1-1), Raman spectroscopy made by Protrustech Co. (UniRaman, 532 nm YAG laser), Hitachi made scanning electron

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microscope (SEM, S-3000N) connected with Horiba made energy dispersive analysis of X-ray (EDAX, 7021-H), Hitachi Co. made field-emission scanning electron microscope

(FE-SEM,

S-4800),

ultraviolet-visible-near-infrared

(UV-Vis-NIR)

spectrophotometer provided by JASCO Co. (V-670) and Ecopica made Hall effect measurement system (HMS-3000, magnetic field of 0.57 T), respectively. The working distances of 15 mm were set for SEM and FE-SEM with the acceleration voltage of 15 kV for SEM and 1 kV for FE-SEM, respectively. An X-ray photoelectron spectroscopy (XPS, thermo VG 350) using MgKα exciting X-ray source was employed for the analysis of the surface compositions and oxidation states of the samples. The binding energy of C1s shown in the XPS spectra of samples was calibrated to 285 eV, as the hydrocarbon content present in the sample. Quantitative analysis was carried out using standard curve fitting procedure with convolution of Gaussian and Lorentzian functions with Shirley background. The AZTS sample was also analyzed using the transmission electron microscope (TEM, JEOL JEM-2100F). The TEM specimens were prepared mechanically scratching the AZTS sample on substrate. The electrochemical tank for the testing of PEC activities of samples in electrolyte (0.5 M NaCl solution) is the same with our previous study33. The reference and counter electrodes in this study were the Ag/AgCl electrode (Metrohm 6.0733.100)

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in the saturate KCl solution and a high surface Pt plate electrode, respectively. The white light with a light intensity of 100 mW⋅cm-2 was provided form a 300-W Xe lamp (Perkin Elmer PE300BF). The light intensity for the PEC tests of samples was calibrated using a photometer (Newport, 818P-015-19). Samples’ PEC activities and EIS spectra in electrolyte (0.5 M NaCl solution) were obtained using a potentiostat (CHI 600C) equipped with a frequency response analyzer. Their EIS spectra were recorded with an amplitude perturbation of 10 mV in the range of 10 to 100 kHZ at the potential of 0.4 V vs. Ag/AgCl electrode applied on samples under illumination. Samples’ EIS spectra were then fitted with the reasonable equivalent circuit models reported by Klahr et al. (2012)34.

3. Results and discussion From the simulation results proposed by Yuan et al.31, the resistivity of AZTS sample decreases with increasing in the Zn content in sample. Therefore, we kept the silver amount in all metal precursors the same and changed the Zn and Sn contents in the metal precursors to investigate the influence of Zn content in the AZTS samples on their PEC activities in salt-water solutions. The silver-zinc-tin precursor with the total metal precursor thickness of 1000 nm was prepared using the co-sputtering technology. The Zn and Sn contents in the metal precursors can be obtained by the

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change of sputtering powers on each metal target (ex. Zn or Sn). The ratios of [Ag]/[Ag+Sn] and [Zn]/[Sn] in the metal precursors obtained from the EDAX analysis are shown in Table 1. According to the EDAX analysis results of metal precursors in the Table 1, metal precursors (A)-(E) have almost the same [Ag]/[Zn+Sn] ratio, while the [Zn]/[Sn] ratios in metal precursor lied between the values of 1.13 – 2.11. The low [Ag]/[Zn+Sn] ratio set in the metal precursors is due to relatively low melting points of Zn and Sn metals, which may result in the tin or zinc loss during the annealing process and increase in the Ag content in the samples. After co-sputtering of these silver-zinc-tin metal precursors, a post annealing process has to be carried out to decrease the possibility of impurity formation such as ZnO when the metal precursors were moved from the sputtering apparatus to the sulfurization apparatus22,32. From the phase diagrams of the Ag-Sn and Ag-Zn systems35, the Ag3Sn phase can be observed at the temperature of around 221°C and the Ag5Zn8 phase can be detected at the temperature of below of 631°C. Pure tin metal become the liquid phase at the temperature of around 231°C35. For the obtaining the silver-zinc-tin metal alloys, we tested two possible annealing temperatures (200°C and 225°C) in this study. Figure 1 reports the XRD patterns of metal precursor (A) after the post-annealing process in the sputtering chamber with the temperature of 200°C and 225°C, respectively. With the metal precursor (A) annealed at 200°C, two peaks corresponded to the (2 1 1)

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crystal plane for Ag3Sn phase (JCPDS no. 71-530) and (3 3 0) crystal plane for Ag5Zn8 phase (JCPDS no.65-1794) can be observed. However, the peak intensities for the XRD patterns of metal precursor (A) annealed in 200°C were still low. Then we changed the post annealing temperature from the 200°C to 225°C. The XRD patterns of metal precursor (A) shown in Figure 1 indicated that the crystal phase for metal precursor (A) changed to the Ag3Sn, An5Zn8 and Sn mixing phases because this annealing temperature approached to the temperature of phase change (221°C) from Ag3Sn metal alloy to the Ag3Sn and S mixing phases35. Because the metal precursor (A) can form the binary Ag3Sn and Ag5Zn8 metal alloys at the post-annealing temperature of 225°C, the post annealing temperature of 225°C was set for all metal precursors. Figure 2 reports the XRD patterns of metal precursors with several Zn contents after the post annealing process. Two major mixing phases for metal precursor (A) are the Ag3Sn and Sn phases. With increasing the Zn contents in the metal precursors (metal precursors (A)-(E)), the peak intensities for Sn and Ag3Sn phases decreased, while the peak intensities for the AgZn (JCPDS no. 65-3210) and Ag5Zn8 phases increased. Increasing the Zn amount in the metal precursors resulted in the increase in the intensities of AgZn and Ag5Zn8 phases. For metal precursors (E), the crystal phases became the Sn, AgZn, Ag5Zn8 mixing phase with some Ag3Sn phase.

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Because these co-sputtering metal precursors had transferred to the solid mixtures of tin metal /metal alloys (AgZn, Ag3Sn, Ag5Zn8) after the post-annealing treatment, the possibility of metal oxide formed at the metal precursor surface decreased when they were moved to the sulfurization apparatus. The possible temperature profiles used for the sulfurization of these metal alloys therefore are important to get the AZTS samples with high PEC performances. First, we used the three–stage temperature profiles sulfurizaion process provided in our previous study22. The XRD patterns of samples (sample (A) as the example) after the above sulfurization process are shown in supporting files (Figure S2). The first stage of 160°C for 60 min and second stage of 450°C for 30 min were set for this three-stage sulfurization process, which made the sulphur vapor release from the sulphur powders in the graphite box and the react with metal precursor to form the AZTS sample, respectively. Finally, the temperatures in the third stage were changed from 480 to 560°C in order to obtain the high crystalline AZTS samples. Form the XRD patterns of samples shown in Figure S2, the three-stage sulfurization process was very difficult to obtain the pure AZTS sample. With increasing the temperature at the third stage for this sulfurization process, the samples decomposed and became the ZnS and Ag4SnS3 mixing phases. This is due to some tin or zinc losses at the high temperature sulfurization process23. Then we deleted the third stage in the sulfurizaion process

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reported in our previous study22 in order to obtain the pure AZTS samples. Detail sulfurization processes with two-stage temperature profiles are shown in Figure S3. Figure S3 also shows the samples’ crystal phases after the two-stage sulfurization process with different sulfurizaion times. When the metal precursor was maintained at the first stage kept at 160°C for 30 min and second stage kept at 450°C for 30 min in the two-stage sulfurizaion process, the intensities for the XRD peaks of AZTS sample are very poor. Only low crystalline Ag8SnS6 and the sulfur mixing phases were observed. The possible reason may be due to the insufficient time for the formation of S vapor, which make the sample become the low-crystalline ternary Ag8SnS6 and some sulfur mixing phase. Then we changed the time from 30 min to 60 min at the first stage for the two-stage sulfurization process. The XRD patterns of sample shows that the intensities of XRD peaks for AZTS phase increase although there are still some small amounts of binary or ternary metal sulphides detected in the XRD patterns. Then we changed the time for the second stage of the sulfurizaion process from 30 min to 60 min in order to make these binary or ternary metal sulphides form the pure quaternary AZTS samples. From the results shown in Figure S3, more binary and ternary metal sulphides formed in the samples. Long sulfurization time or high sulfurization temperature is not suitable for obtaining the AZTS samples. Similar results can also found for the thermodynamic analysis of the formation of ternary

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Ag8SnS6 sample36 and the phase diagram of CZTS system37. The detail temperature profiles for the sulfurization process is shown in Figure S4. Figure 3 (I) displays the changes of crystal phases of the samples with several Zn contents in metal precursors after the two-stage sulfurizaion process. XRD patterns of samples showed that the major phase of all samples after the sulfurizaion process is the AZTS phase. The peaks at 2θ degree of 27.3°, 30.6°, 44.0°, 45.9° and 52.5° are the signals of the crystal planes of (1 1 2), (2 0 0), (2 2 0), (2 0 4) and (1 3 2) of stannite AZTS sample (JCPDS no. 35-544). The XRD patterns of samples are similar with those reported in the literatures

19,29,38

. Sample (A) has some impurities such as SnSx due to it having

lowest Zn content in metal precursor. Increasing the Zn content in the metal precursors, the SnSx phase disappeared and some minor impurities such as ZnS phase formed because of the increasing in the Zn content in the samples. Figure 3 (II) reports the variation of the XRD peak for (1 1 2) crystal plane of the samples (A)-(E). The locations of the (1 1 2) crystal plane in the XRD patterns for samples (A)-(D) match with that for standard AZTS phase in the JCPDS card. The position of (1 1 2) crystal plane for sample (E) slightly moved to lower angle compared with that for sample (D) due to the difference of Zn and Sn ion radii. The ion radius for Zn2+ and Sn4+ ions is 0.74 and 0.69 Å, respectively39. Some tin ions in the AZTS samples were replaced by the zinc ions, which make the location of peak for (1 1 2) crystal plane of

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sample (E) move to lower angle. From results shown in Figure 3, samples (A)-(E) are the stannite AZTS samples with minor binary impurities such as SnSx or ZnS phase. These minor impurities may become the recombination centers and result in the low PEC activities under light illumination. Although the results reported in Figure 3 indicated that there are some minor impurities in the samples, some possible binary metal sulphides such as Ag2S may not be detected just using the X-ray diffracometer. We also used the Raman spectroscopy to check the crystal phases of samples (A)-(E). Figure 4 shows the Raman shift spectra for samples (A)-(E) and the major Raman shift peaks for all samples are similar with those reported by Ma et al.38, which also confirmed that the major crystal phase is the AZTS phase for all sample. We also checked the peaks of the Raman shifts for the possible binary or ternary metal sulphides such as Ag2S40, SnS41, Sn2S341, SnS242, ZnS43 and ternary Ag8SnS622 reported in the literatures. We found that samples (A)-(C) have some binary impurities such as SnS2. Although no peak for the ZnS phase observed in the Raman shift spectra for all samples, the ZnS phase was detected in the XRD patterns of samples (D) and (E). From these results, all samples confirmed that the major phase is the AZTS sample with some metal sulphides impurities. Figure 5 shows the microstructures of samples (A)-(E) obtained from the FE-SEM. The grain sizes for sample (A) is in the range of several hundred

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nanometers to several micrometers. Similar grain sizes and microstructures for sample (A) are observed with those reported by Tsuji et al.19. With increasing in the Zn content in the metal precursors, the larger grain sizes of samples were observed (samples (B)-(E)) and the small grains observed in sample (A) disappeared. The increase in the grain size of samples would result in the less numbers of grain boundary, which may enhance the carrier moving rate and increase in the PEC activities. However, some cracks or pinholes were observed for samples (C)-(E) due to the tin or zinc losses during the sulfurization process. The recombination of the light-excited carriers may occur at the cracks or pinholes. From the SEM images of samples, the Zn ratio in the samples influences the surface morphologies of samples. Therefore, the PEC performance of the AZTS samples in electrolyte may be influenced with their compositions. The EDAX spectra for the AZTS samples were then analyzed to obtain their compositions. Figure S5 shows the EDAX spectra of samples (A)-(E). The EDAX spectra of samples (A)-(E) indicated that the silver, zinc, tin and sulphur spectra can be detected by EDAX analysis. The atomic ratios of each element in the AZTS samples were converted into the [Ag]/[Zn+Sn], [Zn]/[Sn] and [S]/[Ag+Zn+Sn] ratios, respectively, and they are given in Table 1. Samples (A)-(E) have almost the same [Ag]/[Ag+Sn] ratio, which are in the range of 0.69-0.71. The smaller [Zn]/[Sn] molar ratios in the samples (A)-(E) were observed compared with

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those in the metal precursors. The EDAX results in Table 1 indicated that some zinc loss occurred during sulfurization process, although the melting point of tin (231°C) is lower than that of zinc metal (419°C)35. The possible reason is due to low crystallinity of Ag5Zn8 phases formed in the metal precursors. Low crystallinity of Ag5Zn8 metal alloy indicates that the few amounts of Zn in the metal precursors form as the Ag5Zn8 metal alloys at the post-annealing process. According to the CZTS formation mechanisms reported in the literatures37, 44-45, binary Cu2S and SnS2 form at 270°C and then react to form the ternary Cu2SnS3 samples during the sulfurization process. The ternary Cu2SnS3 samples then react with Zn or ZnS to form the CZTS samples at around 470°C. It indicates that the Zn maintains as the metal form until the ternary Cu-Sn-S samples formed. The formation of AZTS samples may be similar with the reaction mechanisms of CZTS during the sulfurization process. Because the zinc metal maintains until the ternary Ag-Sn-S sample forms, the possibility of zinc loss increases compared with that for tin loss during the sulfurization process. The zinc loss in the CZTS samples can be also observed in the report provided from the Araki et al.46. The values of [S]/[Ag+Zn+Sn] ratios for samples (A)-(E) are less than one, which indicated that the samples have S vacancies. Sample (E) is the Zn-rich and S-poor AZTS sample while others are the Sn-rich and S-poor samples. From the calculation results reported by Yuan et al.31, samples (A)-(E) are the n-type

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semiconductors with high resistivity. The thickness of samples (A)-(E) shown in Table 1 are in the range of 1.9-2.2 µm, which are about two time higher than the metal precursors set during the co-sputtering process. Optical properties of AZTS samples influence the sample’s PEC salt-water splitting activities under illumination. For the obtaining of the energy band gaps of AZTS samples, samples’ transmittance and reflectance spectra were recorded using the UV-Vis-NIR spectrophotometer. They are shown in Figure S6. Almost zero transmittances for all AZTS samples were observed at the photon wavelength of less than 500 nm and become 50-60 % transmittance when wavelength of the incident photon is greater than 800 nm. The reflectance spectra of AZTS samples of less than 20 % can be observed with the photon wavelength of less than 600 nm and become around 30-40% with the incident photon wavelength of greater than 700 nm. The absorption coefficients (α) of AZTS samples can be calculated using the Manifacier model47. The direct energy band gap (Eg) of AZTS samples can be obtained using calculation procedures reported in the literatures32-33,36. The plots of (αhν)2 vs. hν for the samples are displayed in Figure 6. The values of Eg for samples (A)-(E) obtained from the Figure 6 are also given in Table 1. Almost the same Eg for the AZTS samples were obtained. The values of Eg for the AZTS samples in this study agree well with those for the AZTS samples (~ 2.0 eV) reported in the literatures19,29. The value of

21



energy band gap for AZTS samples indicates that the maximum light wavelength of around 600 nm can be absorbed by the AZTS samples and generated the light-excited electron-holes pairs for PEC salt-water splitting. Then we analyzed the electronic properties such as the carrier concentration, mobility and resistivity of AZTS samples using the Hall measurements at the room temperature. The samples’ carrier concentrations and mobilities are shown in Table 1. The resistivity of samples was measured using the van der Pauw method with four-point probe. Four Au tips were connected to the sample with the symmetrical square shape. The average resistivity of the sample can be obtained with relationship of current across the tips and the corresponding voltage applied on the samples. The resistivities of 6.0×104 to 9.5×104 Ω-cm were obtained for the samples (A)-(E) and decreased with increasing the [Zn]/[Sn] ratio in the samples. N-type conduction with high resistivity can be observed for all samples. The results agree well with the theoretical calculations proposed by Yuan et al.31. Theoretically, each atom has to be at its designated position in an ideal lattice. However, the defects (vacancy or interstitial) form in the samples due to the deviation from the ideal structure. From the calculation results proposed by Yuan et al.31, the carrier concentration of AZTS samples can slightly increase with increasing the [Zn]/[Sn] ratios in the samples due to the increase amount of donor defects (the Ag sites in AZTS sample were replaced by the Zn ions , ZnAg). However,

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the increase in the carrier concentration of AZTS samples is limited because the increase in the Zn content causes the spontaneous formation of the acceptors (the Ag ions occupied in the Zn site in ATZTS sample, AgZn). Because the numbers of acceptors (AgZn) also increase, the donor concentrations of AZTS sample only slightly increase even the large increase in Zn contents in the samples. Our results agree with the calculation results reported in the literature31. The mobility of samples increases with increasing the Zn content in samples when the [Zn]/[Sn] molar ratio in samples of less than 0.85. Then the mobility of samples decrease with an increase in the Zn content in samples. From the sample’s SEM images, the grain size of AZTS samples increases with increasing in the Zn content in samples. The increase in the grain size resulted in the decrease of grain boundaries, which increase the carrier mobility. However, there are some cracks and pinholes observed in samples (C)-(E) due to the zinc loss or S vacancies formed in the samples. The cracks and pinholes at the sample surface would form the defects (ex. S vacancies or Zn loss), which decrease the average mobility of samples. Therefore, there is an optimal [Zn]/[Sn] ratio for the AZTS sample with good mobility. High carrier mobility results in the low possibility of recombination for light-excited carriers, which may increase in the PEC response of sample in electrolyte36. The electrochemical properties of samples in electrolyte influence their PEC performances. The well-known Mott-Schottky equation is often

23



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employed to obtain the flat-band potentials of samples in the electrolyte48:

1 C = ±[2 (εε eN A )][E − E − (kT e)] 2

2

0

D

(1)

FB

where ε is the sample’s dielectric constant, A is the sample surface area contacts with electrolyte, ND is the sample’s carrier concentration, EFB is the its flat-band potential, e is the electronic charge, and ε0 is the vacuum permittivity. N-type sample shows a positive slope of Mott-Schottky plot, while it is in negative value for p-type semiconductor. The calculation procedures were reported in our previous studies32,36. Figure 7 shows the Mott-Schottky plots for samples (A)-(E) in aqueous 0.5 M NaCl solution. The positive slopes for samples (A)-(E) are observed and conclude that all samples are n-type semiconductors. We can obtain the positions of EFB for samples using of the intersection (E0) at the C-2 vs. E plot at the E-axis:

E 0 = E FB + kT e

(2)

The flat-band potentials of samples (A)-(E) obtained from the Figure 7 are -0.83, -0.87, -0.89, -0.87, -0.88 V vs. Ag/AgCl. Their flat-band potential are almost at the

24


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same value. Because some physical properties such as dielectric constant of AZTS sample are not available in the literature, we assumed the difference between their conduction band and flat-band potential of 0.3 V for all samples. The band positions for the AZTS samples estimated using their energy band gaps and flat-band potentials are shown in Figure S7. All samples have higher conduction bands than the H+/H2 reduction potential in the salt-water solution (-0.413 V vs. NHE), while their valence bands are lower than the H2O/O2 oxidation potential (+0.817 V vs. NHE). The band diagrams of AZTS samples match with those calculated using density function theory19. The band diagrams of AZTS samples in electrolyte indicate that the AZTS sample can directly split salt-water solution into H2 and O2 gases under light illumination. However, the high resistivities of AZTS samples may decrease their PEC performances. Figure 8 displays the PEC performances of AZTS samples in the 0.5 M NaCl solution using the chopping method at the potentials set in the range of -0.4 to 1.4 V vs. (relative hydrogen electrode, RHE). Larger current density of sample was observed under light illumination compared with that in the dark at the same external bias because of the light-excited carriers generated in the bulk sample under illumination. The external bias vs. the Ag/AgCl reference electrode is changed to the RHE scale by using the equation22:

25



0 E RHE = E Ag / AgCl + 0.059 pH + E Ag / AgCl

(3)

where ERHE is the external bias vs. RHE, E0Ag/AgCl is 0.1976 V, and EAg/AgCl is the experimentally external bias vs. the Ag/AgCl reference electrode. The anodic enhanced current densities for all samples were observed with the more positive external bias applied on the sample under light irradiation, which confirmed that the samples are the n-type semiconductor. From results displayed in Figure 8, the photo-enhanced current densities of samples (A)-(E) are 0.08, 0.15, 0.16, 0.19, and 0.09 mA⋅cm-2 with the potential of 1.23 V vs. RHE applied on the samples, respectively. The differences between the current density under light irradiation and in the dark at the same external bias are set as the photo-enhanced current density of the sample. Some spike peaks were observed for samples (B)-(E). The spike peaks in the Figure 8 are based on the effect of surface states or low reaction rate of oxygen evaluation reaction49-50. Increasing the external bias makes the spike peaks disappear, which corresponds to the stable carrier transferring rate into the electrolyte and results in the PEC salt-water splitting occurring at the sample surface. Sample (D) has the highest PEC performance in the salt-water solution but it is still poor. The possible reason is due to high resistivities or

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high charge transfer resistances of these AZTS samples, which results in the low PEC activities. Considering the Ag contents in the AZTS samples set before is the low [Ag]/[Zn+Sn] ratio in sample (around 0.7). We tried to increase the Ag content in the metal precursor (D) (the [Zn]/[Sn] ratio of 1.73 in metal precursor) to examine the influence of Ag content on their PEC salt-water splitting activities under illumination. The increase in the Ag content in the silver-zinc-tin metal precursors was adjusted by changing the sputtering power applied on the Ag target, which was 13 W (metal precursor (F)) and 16 W (metal precursor G), respectively. The compositions of metal precursors (F) and (G) obtained using EDAX analysis are shown in Table 1, respectively. Because the total metal precursor thickness was still kept at 1000 nm, increasing the Ag content resulted in the decrease in [Zn]/[Sn] ratio in metal precursors. After the two-stage sulfurization process, we can obtain the AZTS samples with higher Ag contents compared with that for sample (D). The comparisons of XRD patterns of samples (D), (F) and (G) are given in Figure S8. Sample (D) has minor ZnS impurity and the ZnS impurity almost disappear with increasing in the Ag content in AZTS sample. However, the ternary Ag4Sn3S8 phase appeared in the sample if the [Ag]/[Zn+Sn] ratio in metal precursor increased to 0.62 (sample (G)). The EDAX spectra and compositions of samples (F) and (G) are also given in Figure S5 and Table 1, respectively. The [Ag]/[Zn+Sn] ratio for samples (F) and (G) are 0.80 and 0.89,

27



respectively, which indicated the Ag content in the AZTS samples increased but the [Zn]/[Sn] ratio in AZTS samples decrease due to total thickness of metal precursor set for 1000 nm. The sample thickness and energy band gap obtained from the optical transmittance/reflectance spectra shown in Figure S9, and electronic properties of samples (F) and (G) are also given in Table 1. The resistivity of sample (F) is 5.71×104 Ω-cm, while the sample (D) is 6.74×104 Ω-cm, respectively. However, the resistivity of sample (G) become 1.05×105 Ω-cm due to the decrease in the carrier concentration and mobility. According the results proposed by Yuan et al.31, the increase in the Ag content in the samples results in the increase in the number of acceptors (AgZn). The [Zn]/[Sn] ratio in the sample (G) also decrease compared with that of sample (F). The decrease in [Zn]/[Sn] ratio results in the decrease in the number donors (ZnAg), which decreases in the carrier concentration of sample (G). Then we measured the PEC responses of samples (D), (F) and (G) in the salt-water solution. Figure 9 displays the comparisons of PEC performances of samples (D), (F) and (G) in the salt-water solution under light illumination. The PEC performances for sample (F) is around 0.31 mA⋅cm-2 when the external bias was set at 1.23 V vs. RHE. It seems that the improvement of conductivity of AZTS samples can make an increase in the PEC response of AZTS sample in electrolyte. However, high Ag content (sample (G)) in the AZTS samples results in the low PEC performance due to low

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carrier concentration and mobility. Because the increase in the Ag content in the AZTS samples with suitable value can improve its PEC performance in electrolyte, the influence of their silver contents at the sample surface on the reaction mechanisms at the AZTS sample surface is important. Therefore, the high-resolution XPS surveys of samples (D), (F) and (G) were carried out in order to estimate the possible surface defects formed at the samples. Figure S10 (I)-(IV) shows the high-resolution XPS surveys for Ag3d, Zn2p, Sn3d and S2p spectra in sample (F). The binding energies of Ag ion in sample (F) (368.2 and 374.3 eV for Ag 3d5/2 and Ag 3d3/2, respectively) approach to those of Ag2S reported by Jiang et al. (2015)51. The binding energies of Zn ions (1022.1 and 1045.1 eV for Zn 2p3/2 and Zn 2p1/2, respectively) are close to the ZnS reported by Wu et al. (2009)33 and Hayter et al. (2002)52. The binding energies of Sn ions (487.0 and 495.4 eV for Sn 3d5/2 and 3d3/2, respectively) approach to those for SnS2 phase53. The typical peaks in Figure S10 (IV) at the binding energy of 162 and 163.2 eV are assigned to those for S 2p3/2 and S 2p1/2, respectively51-53. The AZTS sample is the solid solution of binary Ag2S, SnS2 and ZnS phase with the molar ratio of 1:1:1. The oxidation states of sample (F) are matched with those for the binary metal sulphides and the AZTS powder samples reported in the literature29, 51-53. Same oxidation states for samples (D) and (F) can be observed using the XPS surveys. Although the

29



oxidation states for samples cannot give enough information about the surface defects. The variations of surface compositions of samples by using the integrations of the peak areas reported in the Figure S10 show the interesting results. The molar ratios of [Ag]/[Zn+Sn]:[Zn]/[Sn]:[S]/[Ag+Zn+Sn] for samples (D), (F) and (G) are 0.41: 1.67: 0.73, 0.61: 1.08: 0.85, and 1.68: 0.32: 1.36. The Zn rich AZTS can be observed in the surface of sample (D), which indicated that the ZnS phase was easily formed at the sample (D). The XPS data for sample (G) showed that the surface of sample (G) was the Zn-poor sample, which indicated that the ternary Ag4Sn3S8 was easily formed due to the low Zn content at its surface. The XPS data for sample (F) showed that the surface composition of sample (F) was the Ag-poor and Zn-rich AZTS samples, which may be the major contributions for its PEC performance in aqueous solution under light irradiation. From the results using CZTS samples as the photoelectrode for water splitting, the secondary phase such as CuxS is easily formed at the Cu-rich sample surface and decrease its PEC performance18. The KCN solution has to be employed to remove the CuxS phase formed in the CZTS sample. The secondary phase such as Ag2S was difficult formed in sample (F) due to Ag-poor condition. Therefore its PEC performance can be improved. From the XPS data shown in the Figure S10, the impurities such as ZnS or Ag4Sn3S8 phase are easily formed in the samples (D) and (G). The impurities observed in the XRD samples located at the sample surfaces will

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influence their PEC performances. Figure S11 also shows the high-resolution TEM image of sample (G). Two interplanar spaces of around 0.31 nm and 0.33 nm were observed, which are consistence with (2 2 2) crystal plane for Ag4Sn3S8 and (1 1 2) crystal plane for AZTS sample. From the XPS data and the TEM image for samples, we can conclude that the sample (D) and (G) are the AZTS samples with some impurity and while that for sample (F) is the pure AZTS sample. These impurities may act as the surface recombination centers and lower their PEC performances. Then, we used the EIS analysis to examine the reaction kinetic of these AZTS samples in electrolyte. Suitable model for the samples in the aqueous solution was proposed by Klahr et al. (2012)34 and Dias et al. (2017)54. It was shown in Figure 10 (I). Figure 10 (II) shows the EIS spectra for samples (D), (F) and (G) with the external bias kept at 1.0 V vs. RHE under light irradiation. The effect of carriers in the sample’s surface states moving to the electrolyte is contributed at the EIS spectrum in the high Z’ value region (low frequency). The influence of carriers moving in the bulk samples is observed at the EIS spectrum in the low Z’ value region (high frequency). The resistances of FTO-coated substrate, electrolyte and external contact resistance in the Figure 10 (I) is named as the Rs (total sheet resistance)34,54. The factor that affects the carriers for moving to/leaving from the surface states in the sample is named as Rtapping and the bulk sample capacitance is named as Cbulk. The value of Rct,trap

31



influences the charge transfer resistance of the carriers moving from the surface states to the electrolyte. The capacitance of the surface state was in parallel with the Rct,trap in the model shown in the Figure 10 (I). A constant phase element (CPE) was used instead of a capacitance for the surface states due to the un-uniform composition distribution at the sample surface. The model shown in Figure 10 (I) considers the recombination occurred at the surface states, the carriers kinetic moving to/leaving from the bulk sample to the surface states and the contribution of the charge transfer kinetic for carriers from the surface states to the electrolyte. Figures 10 (II) shows the fitting results of the EIS spectra for samples in electrolyte using the model shown in Figure 10 (I) with the external potential of 1.0 V vs. RHE under light illumination. Table 2 reports the values for the parameters in the model shown in Figure 10 (I) for samples (D), (F) and (G). The values of Rs for samples (D), (F) and (G) are around 10 Ω, which shows the ohmic contact behavior formed at the FTO/sample interface. The value of Rtrapping for samples (D), (F) and (G) are in the range of 17.5 – 35.7 Ω. The value of Rtrapping influences the charge transfer kinetic in the bulk sample to the surface states. Low Rtrapping value results in the increase in the number of light-excited holes moving to the surface states, which may increase the PEC performances or increase the recombination for these light-excited carriers at the surface states54. The value of Rtrapping for sample (D) is highest, while that for sample (F) is the lowest. It

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indicates more light-excited holes transferring to the surface states for sample (F) compared with other two samples. However, it may also result in the high recombination rate if the high charge transfer resistance is at the surface states. The value of Csc for sample (F) under illumination is also highest, which indicates that the more light-excited carriers are formed in sample (F) under illumination compared with other samples. The lowest value of charger transfer resistance at the surface states for sample (F) is observed, which indicates the reaction kinetic at the surface states for sample (F) is higher than other samples. Because the sample (F) have the lowest values for Rct,ss and Rtrapping, which indicates the fast hole moving rate to the surface states and fast reaction kinetic at the interface of sample/electrolyte, the PEC activity of sample (F) in the electrolyte is thus higher than other AZTS samples. The values of Css (the capacitance at the surface state of samples) are also interesting for the discussion of the PEC responses of samples. Sample (D) has the highest Css and Rct values compared with other two samples. Many light-excited holes are trapped at the surface states for sample (D) and thus the photo-corrosion may take place. Compared with those for sample (D), both charge transfer resistance (Rct,ss) and capacitance at surface state (Css) of sample (F) decrease. The value of Css for sample (F) is only around half of that for sample (D). The low Css value for sample (F) showed the decrease in the accumulation of light-excited holes stayed in the surface

33



state. The above results indicated that more light-excited holes at the surface of sample (F) took part in the PEC salt-water splitting with external bias of 1.0 V vs. RHE under light irradiation and hence increased the PEC performances of sample (F). The charge transfer resistance for sample (G) is a little higher than sample (F) but lower value for capacitance at the surface state compared with sample (F). We also checked the value of capacitance at the semiconductor (Csc) for sample (G). The lower value of Csc was found compared with that for sample (F). It seems that the number of light-excited carriers in sample (G) is lower than that for sample (F). Due to low number of light-excited carriers generated in sample (G), the relatively poor PEC response of sample (G) can be expected. Finally, we measured the current density-time curve of samples (D) and (F) at the external bias kept at 1.0 V vs. RHE and they are shown in Figure 11. Almost stable current density for sample (F) in the aqueous solution was obtained during the 1000 sec test but the current density decay for sample (D) was observed. The stability test can confirm that the photo-excited holes are accumulated at the surface states and low reaction kinetic for sample (D) make the photo-corrosion taking places at the sample surface during the test. The gas productions rate from the PEC cell at an applied voltage of 1.0 V vs. RHE electrode in salt-water solution under light irradiation were also analyzed by using a gas chromatograph (GC-14B, Shimadzu) with nitrogen as the carrier gas. The molar ratio

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of hydrogen and oxygen gases was around 4: 1 obtained from the gas chromatograph. The amount of H2 gas measured by a gas chromatograph was around 90 % of theoretical value calculated using Faraday's laws during the stability test, which was similar with that reported in our previous study55. The low oxygen production rate in the PEC cell is due to the side-reaction such as chloride ions reacted with the photo-excited holes to form chloride gases at the photoanode. The chlorine gas then reacts with water to form hydrochloric and hypochlorite acids22,32. Table 3 listed the comparisons of various preparation methods and PEC performances for the I-II-VI-IV samples reported in the literatures17-20,56. The electrolytes used for the tests of the PEC performances of AZTS samples in the literatures are the aqueous solutions containing sacrifice regents (Na2S + K2SO3 or trientylamine). The aqueous solutions containing sacrifice regents (ex. Na2S + K2SO3) for the PEC tests are the down-hill reaction (△ G0)2, which belong to the non-spontaneous reaction. Our study showed that pristine AZTS sample with suitable Ag and Zn contents in the samples has the comparable PEC performance (0.31 mA⋅cm-2 at 1.23 V vs. RHE ) in the salt-water solution with those for the

35



AZTS/ZnS tandem photoelectrode in the aqueous solution containing sacrifice regents (ex. around 0.3 mA/cm2 at 0.7 V vs. RHE)56. The stability test for AZTS samples also showed no current decay during the 1000 sec test. It indicates that the Ag content in the AZTS sample play an important role for the PEC activities in the salt-water solution and can have further industrial application in sea-water splitting process if the resistivity of the AZTS can be further improved.

Conclusions In this study, a systemic investigation for the Ag and Zn contents in the AZTS samples was carried out to understand their influences on the PEC performances in the salt-water solution. It was found that the two-stage sulfurizaion process with the first stage of 160°C for 60 min and second stage of 450°C for 30 min is suitable for obtaining the AZTS samples. Increasing the Zn content in the AZTS samples has no sufficient influence on their energy band gaps and flat-band potentials. The samples’ carrier concentrations increase with increasing in the Zn content in samples because of the increasing number of donor defects [ZnAg]. Sample with [Zn]/[Sn] ratio of 0.97 had relatively higher PEC response compared with other samples, however, it is still poor. After we increased the Ag content in the AZTS samples, the PEC activity of AZTS improved and approached to 0.31 mA⋅cm-2 at the external bias kept at 1.23 V

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vs. RHE. From the EIS analysis results, the AZTS sample with [Ag]/[Zn+Sn] of 0.80 and [Zn]/[Sn] molar ratio of 0.90 has best PEC performance in the salt-water solution under light illumination because it has the lowest charge transfer resistance and highest number of light-excited carriers in the salt-water solutions under light irradiation.

Supporting information: The co-sputtering system used in this study; the XRD patterns of samples after two-stages and three-stage sulfurization processes; the two-stage sulfurization process used in the study; EDAX spectra of samples; transmittance and reflectance spectra of samples as a function of light wavelength; the band diagrams for AZTS samples in the electrolyte; the XRD patterns and the plots of of (αhν)2 vs. hν for the samples (F) and (G) in this study; XPS data and TEM image for samples.

Acknowledgements

This work was sponsored by the Ministry of Science and Technology of Taiwan,

with

the

grants

numbers

of

103-2221-E-182-059-MY3

(NEPRD2D0203), 106-2628-E-182-002-MY3 (NERPD2G0291) and Chang Gung Memorial Hospital under grant no. BMRP948. The authors also thanks

37



the Prof. Ming-Chung Wu in Chang Gung University and Dr. Andy Ting for the assistances for the XPS and TEM analysis.

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and performance of Cu2ZnSnS4 semiconductor as photocathode for solar water splitting. J. Alloy. Comp. 2016, 688, 923-932. 25. Guo Q; Hilhouse H. W.; Agrawal R. Synthesis of Cu2ZnSnS4 nanocrystal ink and its use for solar cells. J. Am. Chem. Soc. 2009, 131, 11672-11673. 26. Tsuji I.; Kato, H.; Kudo, A. Visible-Light-Induced H2 Evolution from an Aqueous Solution

Containing

Sulfide

and

Sulfite

over

a

ZnS-CuInS2-AgInS2

Solid-Solution Photocatalyst. Angew. Chem. Int. Ed. 2005, 44, 3565-3568. 27. Qi Y.; , Tian Q.; Meng, Y.; Kou, D.; Zhou, Z.; Zhou, W.; Wu, S.; Elemental Precursor Solution Processed (Cu1-xAgx)2ZnSn(S,Se)4 Photovoltaic Devices with over 10% Efficiency. ACS Appl. Mater. & Interface 2017, 9, 21243-21250. 28. Hages C. J.; Koeper M. K.; Agrawal, R. Optoelectronic and Material Properties of Nanocrystal-based CZTSe Absorbers with Ag-alloying. Solar Energ. Mater. Solar Cells 2016, 145, 342-348. 29. Li, K.; Chai, B.; Peng, T.; Mao, J.;Zan, L. Synthesis of multicomponent sulfide Ag2ZnSnS4 as an efficient photocatalyst for H2 production under visible light irradiation. RSC Adv. 2013, 3, 253-258. 30. Jing, T.; Dai, Y.; Ma, X.; Wei, W.; Huang, B. Electronic Structure and Photocatalytic

Water-Splitting Properties of Ag2ZnSn(S1-xSex)4. J. Phys. Chem.

C 2015, 119, 27900-27908.

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31. Yuan, Z. K.; Chen, S.; Xiang, H.; Gong, X. G.; Walsh, A.; Park, J. S.; Repins, I.; Wei S. H. Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu- and Ag-Based Kesterite Compounds. Adv. Fun. Mater. 2015, 25, 6733-6743. 32. Cheng, K. W. Influence of [Cu]/[Cu+Sn] Molar Ratios in p-type Cu-Sn-S Photoelectrodes on their Photoelectrochemical Performances in Water and Salt-Water Solutions. J. Taiwan Inst. Chem. Engineer 2017, 75, 209-219. 33. Wu, C. C.; Cheng, K. W.; Chang, W. S.; Lee, T. C. Preparation and Characterizations of Visible Light-responsive (Ag-In-Zn)S Thin-Film Electrode by Chemical Bath Deposition. J. Taiwan Inst. Chem. Engineers 2009, 40, 180-187. 34. Klahr, B.; Gimenez, S.; Fabregt-Santiago, F.; Hamann, T.; Bisquert, J. Water Oxidation at Hematite Photoelectrodes: the Role of Surface States. J. Am. Chem. Soc. 2012, 134, 4294-4302. 35. D. R. Lide, CRC Handbook of Chemistry and Physics, 75th ed. CRC PRESS, 1994. 36. Yeh, L. Y.; Cheng, K. W. Preparation of Chemical Bath Synthesized Ternary Ag-Sn-S Thin Films as the Photoelectrodes in Photoelectrochemical Cell, J. Power Sources 2015, 275, 750-759.

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37. Olekseyuk, I. D.; Dudchak, I. V. Piskach, L. V. Phase Equilibria in the Cu2S–ZnS– SnS2 System. J. Alloys Comp., 2004, 368,135-143. 38. Ma, C.; Guo, H.; Zhang, K.; Yuan, N.; Ding, J. Fabrication of p-type Kesterite Ag2ZnSnS4 Thin Films with a High Hole Mobility. Materials Letters, 2017, 186, 390-393. 39. Tsega, M.; Kuo, D. H. The Performance of the Donor and Acceptor Doping in the Cu-rich

Cu2ZnSnSe4

Bulks

with Different Zn/Sn

Ratios.

Solid

State

Communications, 2013, 164, 42-46. 40. Martina, I.; Wiesinger, R.; Jembrih-Simburger, D.; Schreiner M. Micro-Raman Characterization of Silver Corrosion Products: Instrumental Set Up and Reference Database. e-Preservation Science, 2012, 9, 1-8

41. Kawano, Y.; Chantana, J.;T. Minemoto, T. Impact of Growth Temperature on The Properties of SnS Film Prepared by Thermal Evaporation and its Photovoltaic Performance. Curr. Appl. Phys., 2015, 15, 897-901. 42. Sun, K.; Liu, F.; Yan, C.; Zhou, F.; Huang, J.; Shen, Y.; Liu, R.; Hao, X. Influence of Sodium Incorporation on Kesterite Cu2ZnSnS4 Solar Cells Fabricated on Stainless Steel Substrates. Solar Energy Materials and Solar Cells, 2016, 157, 565-571. 43.Lin, Y. P.; Chi, Y. F.; Hsieh, T. E.; Chen, Y. C.; Huang, K. P. Preparation of Cu2ZnSnS4 (CZTS) Sputtering Target and its Application to the Fabrication of CZTS Thin-film Solar cells. J. Alloys Comps. 2016, 654, 498-508.

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44. Yoo, H.;Kim, J.;Zhang, L. Sulfurization Temperature Effects on the Growth of Cu2ZnSnS4 Thin Film. Current Appl. Phys., 2012, 12, 1052-1057. 45. Chalapathy, R. V. B.; Jung, G. S.; Ann, B. T. Fabrication of Cu2ZnSnS4 Films by Sulfurization of Cu/ZnSn/Cu Precursor Layers in Sulfur Atmosphere for Solar Cells. Solar Energy Mater. Solar Cells 2011, 95, 3216-3221. 46. Araki, H.; Mikaduki, A.; Kubo, Y.; Sato, T.; Jimbo, K.; Maw, W. S.; Katagiri, H.; Yamazaki, M.; Oishi, K.;Takeuchi, A. Preparation of Cu2ZnSnS4 Thin Films by Sulfurization of Stacked Metallic Layers. Thin Solid Films 2008, 517, 1457-1460. 47. Manifacier, J. C.; De Murcia, M.; Fillard, J. P.; Vicario, E. Optical and electrical properties of SnO2 thin films in relation to their stoichiometric deviation and their crystalline structure, Thin Solid Films 1977, 41, 127-135. 48. Xu, F.; Mei, J.; Zheng, M.; Bai, D.; Wu, D.; Gao, Z.; Jinag, K. Au nanoparticles modified branched TiO2 nanorod array arranged with ultrathin nanorods for enhanced photoelectrochemical water splitting, J. Alloy. Comp. 2017,693, 1124-1132. 49. Kamimura, J.; Bogdanoff, P.; Abdi, F. F.; Lähnemann, J.; van de Krol, R.; Riechert, H.; Geelhaar, L. Photoelectrochemical properties of GaN photoanodes with cobalt phosphate catalyst for solar water splitting in neutral electrolyte. J. Phys. Chem. C 2017, 121, 12540-12545.

45



50. Dunn, H. K.; Feckl, J. M.; Müller, A.; Fattakhova-Rohlfing, D.; Morehead, S. G.; Roos, J.; Peter, L.; Scheu, M. C.; Bein, T. Tin doping speeds up hole transfer during light-driven water oxidation at hematite photoanode. Phys. Chem. Chem. Phys. 2014, 16, 24610-24620. 51. Jiang, W; Wu, Z.; Yue, X.; Yuan, S.; Lu, H.; Liang, B. Photocatalytic performance of Ag2S under irradiation with visible and near-infrared light and its mechanism of degradation, RSC Advance 2015, 5, 24064-24071. 52. Hayter, C. E.; Evans, J.; Corker, J. M.; Oldman, R. J.;Williams, B. P. Sulfur K-edge X-Ray absorption spectroscopy study of the reaction of zinc oxide with hydrogen sulfide, J. Mater. Chem. 2002, 3172-3177. 53. Liu J.; Jing. L.; Gao, G.; Xu, Y.; Xie, M.; Huang, L.; Ji, H.; Xie, J.; Li H. Ag2S quantum dots in situ coupled to hexagonal SnS2 with enhanced photocatalytic activity for MO and Cr(VI) removal, RSC Advance 2017, 7, 46823-46831. 54. Dias, P.; Andrade, L.; Mendes, A. Hematite based photoelectrode for solar water splitting with very high photovoltage. Nano Energy 2017, 38, 218-231. 55. Cheng, K. W.; Hinaro, K.; Antony, M. P. Photoelectrochemical water splitting using Cu(In,Al)Se2 photoelectrodes developed via selenization of sputtered Cu-In-Al metal precursors, Solar Energy. Mater. Solar Cells 2015, 151, 120-130.

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56. Ikeda, S; Nakamura T.; Harada, T; Matsumura, M. Multicomponent sulfide as a narrow gap hydrogen evolution photocatalysts, Phys. Chem. Chem. Phys 2010, 12, 13943-13949. 57. Bühler N.; Meier K.; Reber J. F. Photochemical hydrogen production with cadmium sufide suspension. J. Phys. Chem. 1984, 88, 3261-3268.

47



Figure captions Figure 1 XRD patterns of metal precursor (A) after the posting annealing process with various annealing temperatures. Figure 2 XRD patterns of metal precursors with various [Zn]/[Sn] ratios after the post annealing process. Figure 3 (I) XRD patterns of samples and (II) variation of the XRD peak for (1 1 2) crystal plane after the two-stage sulfurizaion process. Figure 4 Raman shift spectra for all samples. Figure 5 SEM images of AZTS samples. (A) sample (A), (B) sample (B), (C) sample (C), (D) sample (D) and (E) sample (E). Figure 6 Plots of (αhν)2 vs. hν for the samples in this study. Figure 7 Mott-Schottky plots of samples (A)-(E) in aqueous 0.5 M NaCl solutions. Figure 8 The comparisons of PEC performances of samples with various [Zn]/[Sn] ratios at the external bias of -0.4 to 1.4 V vs. RHE in electrolyte. Figure 9 The comparisons of PEC performances of samples (D), (F) and (G) at the external bias of -0.2 to 1.4 V vs. RHE in electrolyte. Figure 10 (I) The equivalent circuit used in the study and (II) the fitting results using the equivalent circuit model shown in figure 10 (I) at the applied potential of 1.0 V vs. RHE in 0.5 M NaCl solution under light illumination. Figure 11 Stability test for the AZTS sample in the salt-water solution with the applied voltage of 1.0 V vs. RHE.

1


Page 48 of 64

Page 49 of 64

Figure 1

(211)

JCPDS no. 71-530 Ag3Sn JCPDS no. 65-1794 Ag5Zn8

(420)

(220)

(032)

(203)

(221)

(211)

(020)

(330)

(200) (101)

(201)

JCPDS no. 86-2265 Sn

Intensity (arb.units)

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


Metal precursor annealed at 225°C

metal precursor annealed at 200°C 20

30

40

50

60

2θ θ (degrees)

2


70

80


Figure 2

♦ JCPDS 71-530 Ag3Sn JCPDS 65-1794 Ag5Zn8 JCPDS 65-3210 AgZn JCPDS 86-2265 Sn

(211) (110) (330) (220) (211)

(200) (101)

(020)

(201)


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

Page 50 of 64

(E)

(D)

(C)

(B)

(A) 20

30

40

50

60

2θ θ (degrees)

3


70

80

Page 51 of 64

(112)

Figure 3


(132)

(220) (204)

(008) (200)

JCPDS 35-544 Ag2ZnSnS4 JCPDS 39-1363 ZnS JCPDS 40-1467 SnS2 SnSx (E)

(101)

(100)

(D)

(C)

(110)

(B)

20

(A)

30

40

50

60

70

80

2θ θ (degrees)

(112)

(I)

Stannite Ag2ZnSnS4

(E)


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


(D)

(C)

(B)

(A) 26.5

27.0

27.5

28.0

2θ θ (degrees)

(II)

4


28.5


Figure 4

Stannite Ag2ZnSnS4

• SnS2

(E)


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

Page 52 of 64

(D)

(C)

(B)

(A) 200

250

300

350

400 -1

Raman shift (cm )

5


450

500

Page 53 of 64 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5

(A)

(B)

(C)

(D)

(E)

6



Figure 6

1.6x109 9

1.4x10

9

1.2x10

(α ν )2 (eV cm-1)2 α hν

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

Page 54 of 64

(A) (B) (C) (D) (E)

1.0x109 8.0x108 6.0x108 8

4.0x10

2.0x108 0.0 1.90

1.95

2.00

2.05

2.10

hν ν (eV)

7


2.15

Page 55 of 64

Figure 7

4x1011

3x1011

1/C2s (cm4/F2)

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


(A) (B) (C) (D) (E)

2x1011

11

1x10

0 -1.0

-0.9

-0.8

-0.7

-0.6

Applied potential (V vs. Ag/AgCl)

8


-0.5

-0.4


Figure 8

0.2 mA/cm2 (E) 0.09 mA⋅ cm-2 at 1.23 V vs. RHE

Current density (mA⋅⋅ cm-2)

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

Page 56 of 64

light (D) 0.19 mA⋅ cm-2 at 1.23 V vs. RHE

(C) 0.16 mA⋅ cm-2 at 1.23 V vs. RHE

dark

(B) 0.15 mA⋅ cm-2 at 1.23 V vs. RHE

(A) 0.08 mA⋅cm-2 at 1.23 V vs. RHE

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Applied voltage (V vs. RHE)

9


1.0

1.2

1.4

Page 57 of 64

Figure 9

0.2 mA/cm2 -2

(G) 0.22 mA⋅ cm at 1.23 V vs. RHE

Current density (mA⋅⋅ cm-2)

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


light (F) 0.31 mA⋅ cm-2 at 1.23 V vs. RHE

dark (D) 0.19 mA⋅cm-2 at 1.23 V vs. RHE

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Applied voltage (V vs. RHE)

10


1.2

1.4


Figure 10

(I)

1000

800

-Z'' (Ω/cm2)

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

Page 58 of 64

600

400 Sample (D) Sample (F) Sample (G) Fitting results

200

0

0

200

400

600 ''

800

2

Z (Ω/cm )

(II)

11


1000

Page 59 of 64

Figure 11

1.0

(D) (F) Current density (mA/cm2)

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


0.8

0.6

Light 0.4

0.2

Dark 0.0 0

200

400

600

Time (s)

12


800

1000


Table Captions Table 1 Physical properties of the stannite Ag2ZnSnS4 photoelectrodes on substrates. Table 2 EIS parameters for samples (D), (F) (G). Table 3 Summaries of preparation methods and PEC performances AZTS and Cu2ZnSnS4 (CZTS) samples reported in the literatures.

13


Page 60 of 64

Page 61 of 64 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 1

Sample

[Ag]/[Zn+Sn] ratio in

[Zn]/[Sn] ratio in

Ag-Zn-Sn metal Ag-Zn-Sn precursor metal precursor

Ratios in AZTS samples (Obtained from EDAX analysis)

AZTS sample

Eg

Carrier

[Ag]/ [Zn+Sn]

[Zn]/[Sn]

[S]/ [Ag+Zn+Sn]

Thickness (nm)

(eV)

concentration (cm-3)

Mobility (cm ⋅V s )

Conduction type

2

-1 -1

(A)

0.29

1.13

0.71

0.61

0.80

1941

2.01

5.54×1012

7.1

n

(B)

0.32

1.30

0.70

0.78

0.87

2250

2.02

6.25×1012

12.5

n

(C)

0.28

1.44

0.70

0.85

0.84

1963

2.03

6.69×1012

37.3

n

(D)

0.31

1.73

0.70

0.97

0.85

2082

2.03

7.20×1012

26.6

n

(E)

0.32

2.11

0.69

1.21

0.84

2161

2.04

9.11×1012

5.8

n

(F)

0.40

1.61

0.80

0.90

0.85

2088

2.04

7.53×1012

14.5

n

(G)

0.62

0.72

0.89

0.80

0.82

2019

2.04

5.71×1012

9.7

n

14


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 62 of 64

Table 2

Sample

PEC performance

Rtrapping

Csc

Rct,ss

Css

at 1.23 V vs. RHE

(Ω)

(µF)

(Ω)

(µF)

-2

(mA⋅cm ) (D)

0.19

35.7

0.0153

12528

20.02

(F)

0.31

17.5

0.0626

4892

9.29

(G)

0.22

30.1

0.0372

5908

4.07

15


Page 63 of 64 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 3 Samples

Synthesis method

Thermal treatments °

Cu2ZnSnS4

Spin coating with

420 C for 20 min. and

(CZTS) thin film

precursor solution

repeat 6 times

CZTS

Electrodeposition

585-600°C for one hours

thin film

electrolyte

PEC performances 2

Stability test

Reference

0.5 M Na2SO4 (pH = 9.5)

-0.7mA/cm at 0 V vs. RHE

Around 4 hours

17

0.5 M Na2SO4(pH = 9.5)

1µA/cm2 for pure CZTS thin film and

Around 900

18

2

and etching with KCN

increase to 0.8-1.2 mA/cm at 0 V vs RHE

seconds

with CdS/CZTS photocathode °

AZTS powders

Solution growth method

450 C for 10 hours

0.5 M K2SO3 + 0.1M Na2S

482 µmol/g-hr with Pt co-catalyst loading

Around 5 hours

19

AZTS thin film

Layer deposition with

250-350°C for ten min.

0.1 M LiClO4 + 0.01M

30 µA/cm2 at 0 V vs. Ag/AgCl electrode

Not available

20

0.1 M Na2SO4+ 0.05

AZTS powders show poor photocatalytic

Not available

57

triethanolamine

activity.

This work

AZTS/CZTS/ZnS

°

hydrothermal

(powders)/200 C for 10

method

min for thin film

Co-precipitation

550°C for ten min.

powder and thin

triethanolamine

Around 0.3 mA/cm2 for ZnS/AZTS thin

film

film electrode at 0.7 V vs. RHE

AZTS thin film

Sulfurization of sputtering

160°C for 30 min and

0.5 M NaCl aqueous

Around 0.23mA/cm2 at 0.7 V vs. RHE

At least 1000

metal precursors

450°C for 30 min.

solution

Around 0.31 mA/cm2 at 1.23 V vs.RHE

sec.

16



Influences of Silver and Zinc Contents in the Stannite Ag2ZnSnS4 photoelectrodes on Their Photoelectrochemical Performances in the Salt-Water Solution

Kong-Wei Chenga,b,*,Su-Wei Honga a

Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan

b

Department of Orthopaedic Surgery, Chang Gung Memorial Hospital, Keelung Branch, Taoyuan, Taiwan

With optimal Ag content in sample

17


Page 64 of 64

Influences of Silver and Zinc Contents in the Stannite Ag2ZnSnS4

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