Band positions and photoelectrochemical properties of solution

Subscriber access provided by Queen Mary, University of London

Article

Band positions and photoelectrochemical properties of solution-processed Ag-substituted Cu2ZnSnS4 Photocathode Zhiqiang Xu, Zhongjie Guan, Jianjun Yang, and Qiuye Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00116 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 16 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

ACS Applied Energy Materials

Band Positions and Photoelectrochemical Properties of Solution-Processed Ag-Substituted Cu2ZnSnS4 Photocathode Zhiqiang Xua,b, Zhongjie Guana*, Jianjun Yanga, Qiuye Lia* a

Engineering Research Center for Nanomaterials, Henan University, Kaifeng 475004,

China bSchool

of Pharmacy, Jining Medical University, Rizhao 276826, China

* E-mail: [email protected]; [email protected]; Abstract: Ag substituted Cu2ZnSnS4 (ACZTS) thin films were fabricated using a solution-processed method. The effects of Ag-substituted Cu sites on the crystallinity, band gap, band positions and photoelectrochemical water splitting performance of Cu2ZnSnS4 (CZTS) were investigated. Compared to pure CZTS, the grain sizes and band gap of ACZTS are gradually increased as the Ag content increases. More importantly, the valence band maximum position of ACZTS is deeper than pure CZTS, which is beneficial to improve the onset potential. When the Ag/(Ag+Cu) molar ratio is 30%, the highest photocurrent of Pt/CdS/ACZTS photocathode reaches to 3.78 mA/cm2 at 0 VRHE and the most positive onset potential is 0.33 VRHE, which are much higher than pure CZTS. The study will deepen understanding of the band structure of Ag substituted CZTS and its effect on the photoelectrochemical performance of CZTS. Keywords: Cu2ZnSnS4 photocathode; Ag substitution; band positions; onset potential; solar water splitting 1. Introduction Producing clean hydrogen using solar energy provide a feasible method for solving the increasing serious energy crisis.1-3 Photoelectrochemical water splitting is considered to be one of the promising techniques to convert solar energy into hydrogen energy.4 Since Fujishima et al. reported that TiO2 photoanode could split water into hydrogen and oxygen under solar irradiation, intensive efforts have been made to develop the low-cost and efficient photoelectrode materials.5 Among several types of photoelectrochemical cells, the p-n tandem cell shows a great advantage because water splitting can be realized without an external bias.2-3 In a p-n tandem cell, a photoanode directly connects with a photocathode for water oxidation and proton reduction, respectively. Recently, the photocurrent of some n-type

ACS Paragon Plus Environment

ACS Applied Energy Materials 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 2 of 16

photoanodes, such as Ta3N5 and BiVO4, has been close to their theoretical maximun photocurrent, but a large external bias is still need for water splitting.6-7 A photocathode can be employed to offer a bias in the p-n tandem cell. High performance has been obtained over the p-type Si, InP or Cu(In,Ga)Se2 photocathodes.8-10 However, the insufficient bias of Si photocathode or the scarcity of In/Ga limit their practical application. Therefore, a low-cost and efficient photocathode is desired to match with these photoanodes for solar water splitting under zero external bias. Lately, Cu2ZnSnS4 (CZTS) has attracted intensive interest due to its abundance, non-toxicity, suitable band gap and high absorption coefficient.11-14 CZTS as a lightabsorption material has been intensively researched for a thin-film solar cell or CO2 reduction.15-17 Meanwhile, CZTS as a photocathode for photoelectrochemical water splitting has also been studied in detail in recent years.18-21 Photoelectrochemical water splitting via CZTS photocathode was firstly investigated by Domen et al. and the Pt/TiO2/CdS/CZTS photocathode showed a promising photocurrent of

9.0

mA/cm2 at 0 VRHE.19 After that, the highest photocurrent of 17.0 mA/cm2 at 0 VRHE was obtained on the Pt/TiMo/CdS/Cu2Cd0.4Zn0.6SnS4 photocathode.22 The onset potential

of

CZTS

is

an

another

important

performance

indicator

for

photoelectrochemical water splitting. Among previous reports, the onset potential of CZTS is about 0.4-0.6 VRHE after coating an TiO2/CdS or TiMo/CdS double layer and Pt electrocatalyst.18-19, 22 High positive onset potential will provide a large bias in a pn tandem cell. The onset potential of CZTS can be further enhanced if making the valence band maximum (VBM) deeper. Element substitution is a useful method to modify the VBM of semiconductor photoelectrode.23 In previous studies, the photocurrent of CZTS can be remarkably improved by Ge or Cd substituted.20,

22

However, Ge or Cd substituted CZTS has not or slight effect on its VBM because the VBM of CZTS is mainly dominated by the antibonding s-d coupling between Cu and S atom. Theoretical calculation indicates that partial substitution of Cu by Ag in CZTS can make the VBM deeper.24 As far as we know, the effects of Ag-substituted Cu sites on the VBM and photoelectrochemical water splitting performance of CZTS has not experimentally studied in detail. In this study, Ag substituted CZTS (ACZTS) thin films were prepared using a facile solution-processed method and the facile Mott-Schottky method was employed to measure the band positions. The results show that the VBM of CZTS becomes


Page 3 of 16 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


deeper as the Ag content increases, which is benefited to improve the onset potential. When the Ag/(Ag+Cu) molar ratio is 30%, the highest photocurrent of Pt/CdS/ACZTS photocathode reaches to 3.78 mA/cm2 at 0 VRHE and the most positive onset potential is 0.33 VRHE, which are much higher than pure CZTS. The mechanism of enhanced performance of CZTS photocathode was also investigated in detail. 2. Experimental section 2.1 Preparation of CZTS and ACZTS thin films Pure CZTS thin film was prepared using the modified method as follow14: 6.1 mmol Cu(CH3COO)2·H2O, 2.4 mmol ZnCl2, 2.0 mmol of SnCl2·2H2O and 20 mmol thiourea were sequentially added into 20 ml of 2-methoxyethanol under vigorous stirring. After aging at the humidity of 60% for several hours, the prepared precursor solution was converted into a viscous sol-solution. After that, 3 mL ethanolamine (C2H7NO) was added the above viscous sol-solution in order to obtain a uniform film. Then the prepared precursor solution was spin-coated onto the Mo-coated soda lime glass substrate at 3000 rpm for 30 s and carried out several times. The prepared thin films were first calcined at 400 °C for 5 min in air and then sulfurized at 580 °C for 60 min in sulfur vapor with a continuous nitrogen flow (200 mL min-1) as a carrier gas. In order to investigate the effects of Ag-substituted Cu sites on properties of CZTS, the ACZTS thin films prepared by replacing some Cu(CH3COO)2·H2O with a certain amount of AgNO3 using the above same procedure. The Ag substituted CZTS was spin-coated four times to achieve same thickness with pure CZTS. Ag-substituted CZTS are labeled as A(x)CZTS, where x represents the molar ratio of Ag to (Ag+Cu). 2.2 Surface coating of CdS and Pt electrocatalyst A CdS layer was coated on the surface of thin films using a chemical bath deposition method.14,

25

The chemical bath solution consisted with 2.5 mL thiourea

(1.5 mol/L), 6.52 mL NH3·H2O, 5 mL CdSO4 (0.015 mol/L) and 36.6 mL distilled water. The deposition process was carried out at 50 °C for 70 s. After deposition, the CdS modified thin films were annealed at 200 °C for 60 min at nitrogen atmosphere in order to achieve an intimate interface between the CdS and the thin films. Pt cocatalyst was electrodeposited on the surface of CdS/CZTS or CdS/ACZTS in a H2PtCl6 (0.001 mol/L) aqueous solution under visible light (λ420 nm) irradiation. The electrodeposition potential of -0.3 V vs SCE was chose. 2.3 Characterization of samples



An X-ray diffractometer (D8-Adanced, Bruker) was used to record the X-ray diffraction (XRD) patterns of samples. The morphologies of samples were measured using a Field-emission scanning electron microscope (JSM-7001F, JEOL) with an acceleration voltage of 200 kV. The Raman spectra of samples was recorded on a micro-Raman spectrometer (InVia, Renishaw). An excitation wavelength of 532 nm was chose. The UV-visible diffuse reflectance spectra (UV-vis DRS) of samples were investigated on a UV-vis spectrophotometer with BaSO4 as a reference (Lambda 950, PerkinElmer ). The Ag content in the A(30%)CZTS sample was measured using an inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICAP6000, Thermo). 2.4 Electrochemical and Photoelectrochemical measurements The Mott-Schottky curves were measured using an electrochemical analyzer (CHI660E, Chenhua) in an Na2SO4 aqueous solution (0.5 mol/L, pH=5.93). The amplitude perturbation was 10 mV and the frequencies of 200, 500 and 1000 Hz were chose. Photoelectrochemical measurements were carried out on an electrochemical analyzer (CHI660E, Chenhua) using a three-electrode mode in an Na2HPO4 aqueous solution (0.2 mol/L, pH=10). The scan rate is 10 mV s-1. The photocathode, a saturated calomel electrode (SCE) and a Pt slice were employed as the working, reference and counter electrodes, respectively. Before using, the electrolyte was bubbled with N2 for 30 min to remove the dissolved O2. AM 1.5G simulated light (100 mW/cm2) was used to measure the photocurrent and stability, which was provided by a solar simulator (Oriel 94011A, Newport) and calibrated by a standard Si solar cell. The incident photonto-current efficiency (IPCE) was evaluated under a Xe lamp illumination equipped with different monochromatic filters. The intensities of monochromatic lights were measured using a photometer (843-R, Newport). The Faradaic efficiencies of H2 and O2 were carried out in a sealed cell. The amounts of H2 and O2 were measured by a gas chromatography (GC-7920, TCD, Ar carrier). 3. Results and discussion 3.1 Effect of Ag substitution on the structure and morphology of CZTS Figure 1 (a) shows the XRD patterns of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag + Cu). Four main peaks at about 2θ = 28.5°, 32.9°, 47.3° and 56.2° are corresponding to the (112), (200), (220) and (312) planes of kesterite-type CZTS (JCPDS No.026-0575). No impurity diffraction peaks can be detected. The enlarged view of (112) diffraction peak for pure CZTS and A(x)CZTS with different


Page 4 of 16

Page 5 of 16 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 1. (a) XRD patterns of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag + Cu). (b) Enlarged view of (112) diffraction peak for pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag + Cu).

Figure 2. Top-view SEM images of pure CZTS (a), A(10%)CZTS(b), A(30%)CZTS(c) and A(40%)CZTS(d). Cross-sectional SEM images of pure CZTS (e), A(10%)CZTS(f), A(30%)CZTS(g) and A(40%)CZTS(h). molar ratios of Ag/(Ag + Cu) are showed in Figure 1 (b). As the Ag content increases, the intensity of (112) diffraction peak become stronger, which suggests that the crystalline quality is improved. In addition, the (112) peak gradually shifts low diffraction angle, which indicates the lattice structure are expanded since the Cu atoms are replaced by the large Ag atoms.26-28 The result suggests that the Ag atoms are successfully incorporated into the CZTS lattices. The Raman spectrum of CZTS and ACZTS with different molar ratios of Ag/(Ag + Cu) were also collected to further investigate the purity of phase (see Figure S1 in the Supporting Information). Two main peaks of 288 cm-1 and 336 cm-1 are attributed to the kesterite CZTS phase. No Raman peaks of the impurity phase are observed. Furthermore, the intensity of Raman



peaks increases as the Ag content increases, which further confirmed that the crystalline quality of CZTS is improved after Ag substituted. The top-view and cross-sectional SEM images of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag+Cu) are shown in Figure 2. Pure CZTS consists of small crystalline grains. The thickness of pure CZTS film is about 800 nm. After introduced with Ag, large crystalline grains are formed on the surface of ACZTS thin films. As the Ag content increases, the upper crystalline grains size gradually increases and the thickness of lower layer with small crystalline grains is gradually reduced, indicating the improved crystalline quality. The improved crystalline quality is consistent with the XRD and Raman results, which may be come from the Cu-AgSn liquid alloy enhanced crystalline grains growth.29-30 The improved crystalline quality can reduce the grain boundary carrier recombination, which will be benefited to enhance the photoelectrochemical performance. A bilayer structure of the ACZTS films could caused by the carbon residual. Similar phenomena are usually observed in a solution method in previous studies.14, 31 The element mapping of Cu, Zn, Sn, S and Ag for the A(30%)CZTS sample are shown in Figure S2. The element of Cu, Zn, Sn, S are evenly distributed and the Ag element is homogeneously incorporated into the CZTS thin films. The actual Ag content in the A(30%)CZTS sample was measured using an ICP-AES method. The molar ratio of Ag to (Ag+Cu) is 28.58%, which is close to the amount in preparation solution.

Figure 3. (a) UV-vis absorption spectra of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag+Cu). (b) Band gaps of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag+Cu). The UV-vis absorption spectra of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag + Cu) are shown in Figure 3 (a). Pure CZTS exhibits a strong absorption in the whole visible light range. The absorption band edge of pure CZTS is


Page 6 of 16

Page 7 of 16 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


about 880 nm. The absorption band edges gradually shift toward short wavelength when the Ag content increased. The band gaps of pure CZTS, A(10%)CZTS, A(30%)CZTS and A(40%)CZTS are 1.41 eV, 1.43 eV, 1.53 eV and 1.72 eV, respectively, which are calculated from the DRS spectra (see Figure 3 (b)). The band gaps of CZTS increase with the Ag content increased, which is consistent with previous reported results.24,

29

Deeper VBM is the mainly reason for the increased

band gap according to previous theoretical calculations and our measurements32 (see below). The reduced light absorption will be harmful for the photoelectrochemical performance. 3.2 Effect of Ag substitution on the band positions of CZTS The Mott-Schottky method was a facile and valid method to measure the band positions of photoelectrodes.33-34 Therefore, the Mott-Schottky plots of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag+Cu) were recorded and the results are shown in Figure 4. The Mott-Schottky plots with negative slope imply that all samples are a p-type semiconductor character. The flat band potential of all samples is independent on the test frequencies. When the Ag content increases to 40%, the flat band potential of CZTS from 0.78 VRHE positively shift to 1.02 VRHE. Based on the flat band potential (EFB) results, the conduction band position (ECB) and valence band position (EVB) can be calculated using the following formulas34: EVB = EFB +

κT e

NV

ln (NA)

Eg = EVB ― ECB

(1) (2)

where κ is the Boltzmann constant (1.38065×10-23 J/K), T is room temperature (298.15 K), e is the electronic charge (1.60217×10-19 C), Eg is the band gap of samples. NV is the effective density of states (typically~1019 cm-3) at the valence band edge. The acceptor concentration NA is obtained from the Mott-Schottky plot. The calculated ECB and EVB are shown in Table 1 and the band positions diagram of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag + Cu) are plotted in Figure 5. For pure CZTS, the ECB and EVB are -0.44 VRHE and 0.97 VRHE, respectively. The ECB of CZTS is slight low than that of CdS (-0.45 VRHE), which will form a type-I band alignment. Similar results are reported in previous study.35 Compared to pure CZTS, the ECB of A(10%)CZTS is nearly unchanged and the EVB of A(10%)CZTS shows a slight shift to positive potential. This is mainly due to the fact that the VBM



Figure 4. Mott-Schottky plots of pure CZTS (a), A(10%)CZTS(b), A(30%)CZTS(c) and A(40%)CZTS(d).

Figure 5. Band positions diagram of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag+Cu). of CZTS is mainly dominated by the antibonding s-d coupling between Cu and S atom, while the conduction-band minimum (CBM) of CZTS is greatly influenced by the antibonding s-p coupling between Sn and S atom. As the Ag content further increases, the VBM of ACZTS become more positive than pure CZTS. The increased in the VBM potential and flat band potential can improve the onset potential. The


Page 8 of 16

Page 9 of 16 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


conduction-band position potential become slightly low, which may be due to the crystal structural distortion.32 Table 1. Band positions of pure CZTS and A(x)CZTS with different molar ratios of Ag/(Ag+Cu). thin films

EFB/VRHE

Eg/eV

ECB/ VRHE

EVB/ VRHE

pure CZTS

0.78

1.41

-0.44

0.97

A(10%)CZTS

0.81

1.43

-0.43

1.00

A(30%)CZTS

0.95

1.53

-0.22

1.31

A(40%)CZTS

1.02

1.72

-0.33

1.39

3.3 Enhanced photoelectrochemical performance of CZTS by Ag substitution

Figure 6. Top-view (a) and cross-sectional (b) SEM images of Pt/CdS/A(30%)CZTS thin films. The photocurrents of pure CZTS and A(30%)CZTS photocathodes are shown in Figure S3. The photocurrent of A(30%)CZTS is higher than pure CZTS, indicating the favorable of Ag-substitution for improving the photocurrent performance of CZTS. An n-type CdS layer coating on the surface of pure CZTS or ACZTS photocathode can form a p-n heterojunction to further improving the charge separation.36-37 In addition, the Pt as a co-catalyst is usually employed to increase the hydrogen reaction.38-39 Figure 6 shows the morphologies of the Pt/CdS/A(30%)CZTS photocathode. An n-type CdS layer is covered on the surface of (30%)CZTS thin film and some Pt particles are homo-dispersed. Figure 7 (a) shows the photocurrents of Pt/CdS/CZTS and Pt/CdS/A(x)CZTS photocathodes with different molar ratios of Ag/(Ag+Cu). Pure CZTS exhibits a low photocurrent of 0.55 mA/cm2 at 0 VRHE and the onset potential is about -0.07 VRHE. Poor crystal quality and large amount of CuZn antisite defects are responsible for the low photoelectrochemical performance of pure CZTS. After incorporated with Ag, the photoelectrochemical performance of photocathodes gradually increases. When the



Ag/(Ag+Cu) molar ratio reaches to 30%, the photocurrent increases from 0.55 mA/cm2 to 3.78 mA/cm2 and the onset potential improves from -0.07 VRHE to 0.33 VRHE (see Figure S4). The increased photocurrent can be attributed to the enhanced crystalline quality and decreased the amount of CuZn antisite defects. While the increased in the VBM potential and flat band potential are another two important reasons for the improved onset potential. Coating a TiO2 or TiMo layer can further

Figure 7. (a) Photocurrents of Pt/CdS/CZTS and Pt/CdS/A(x)CZTS photocathodes with different molar ratios of Ag/(Ag+Cu). (b) Incident photon to current efficiency (IPCE) spectra of Pt/CdS/CZTS and Pt/CdS/A(x)CZTS photocathodes with different molar ratios of Ag/(Ag+Cu) at 0 VRHE. (c) Photocurrent-time curve of Pt/CdS/A(30%)CZTS photocathode at 0 VRHE. (d) Time courses of hydrogen and oxygen evolution on the Pt/CdS/A(30%)CZTS photocathode. The theoretical number of hydrogen and oxygen molecules is denoted by e-/2 and e-/4, respectively. improve the performance of Pt/CdS/A(x)CZTS photocathode.18, 22 If further increased the Ag content, the photoelectrochemical performance of ACZTS photocathode decreases, which could be assigned to the deterioration of light absorption. The incident photon to current efficiency (IPCE) spectra of Pt/CdS/CZTS and Pt/CdS/A(x)CZTS photocathodes with different molar ratios of Ag/(Ag+Cu) at 0


Page 10 of 16

Page 11 of 16 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


VRHE are shown in Figure 7 (b). The IPCE values of Pt/CdS/A(30%)CZTS photocathode are above 10% at the 400-600 nm range and are the highest among the samples with different contents of Ag, which is in good agreement with the photocurrents. The photocurrent of Pt/CdS/A(30%)CZTS photocathode was also investigated by integrating IPCE with the standard solar spectrum according to previous report (see Figure S5)25,40. The integrated solar photocurrent of 3.5 mA/cm2 is close to the measured value, which suggests that the solar photocurrent and IPCE are reliable. The highest solar energy to hydrogen efficiency of 0.32% is obtained at 0.16 VRHE (see Figure S6). Figure 7 (c) shows the photocurrent-time curve of Pt/CdS/A(30%)CZTS photocathode at 0 VRHE. A slightly decay of photocurrent is observed after one-hour test. However, obvious photocurrent decrease occur after one-hour test (see Figure S7). Similar results are observed in previous studies.18,19 Time courses of hydrogen and oxygen evolution on the Pt/CdS/A(30%)CZTS photocathode are given in Figure 7 (d). The Faradaic efficiency of hydrogen and oxygen are about 95% and 82%, respectively. The result indicates that the photocurrent are come from proton reduction, not self-corrosion of the A(30%)CZTS photocathode. 4. Conclusions Ag substituted CZTS thin films with different Ag/(Ag+Cu) molar rations were successfully prepared using a solution-processed method. When the Ag/(Ag+Cu) molar ratio is 30%, the highest photocurrent of Pt/CdS/ACZTS photocathode is 3.78 mA/cm2 at 0 VRHE and the most positive onset potential reaches to 0.33 VRHE, which are much higher than pure CZTS. The enhanced crystalline quality and decreased the amount of CuZn antisite defects are two reasons for the increased photocurrent. While the increased in the VBM potential and flat band potential are another two important reasons for the improved onset potential. The study suggests that the Ag substituted CZTS is an effective way to improve the photocurrent and onset potential of CZTS photocathode. Notes The authors declare no competing financial interest. Acknowledgments The work was supported by the National Natural Science Foundation of China (51702087, 21673066 and 21703054) and the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University.



References (1) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338-344. (2) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (3) Sivula, K. Solar-to-Chemical Energy Conversion with Photoelectrochemical Tandem Cells. Chimia 2013, 67, 155-161. (4) Li, Z.; Luo, W.; Zhang, M.; Feng, J.; Zou, Z. Photoelectrochemical Cells for Solar Hydrogen Production: Current State of Promising Photoelectrodes, Methods to Improve their Properties, and Outlook. Energy Environ. Sci. 2013, 6, 347-370. (5) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (6) Liu, G.; Ye, S.; Yan, P.; Xiong, F.; Fu, P.; Wang, Z.; Chen, Z.; Shi, J.; Li, C. Enabling an Integrated Tantalum Nitride Photoanode to Approach the Theoretical Photocurrent Limit for Solar Water Splitting. Energy Environ. Sci. 2016, 9, 1327-133. (7) Kim, T. W.; Choi, K. S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990-994. (8) Oh, I.; Kye, J.; Hwang, S. Enhanced Photoelectrochemical Hydrogen Production from Silicon Nanowire Array Photocathode. Nano Lett. 2012, 12, 298-302. (9) Gao, L.; Cui, Y.; Wang, J.; Cavalli, A.; Standing, A.; Vu, T. T. T.; Verheijen, M. A.; Haverkort, J. E. M.; Bakkers, E. P. A. M.; Notten, P. H. L. Photoelectrochemical Hydrogen Production on InP Nanowire Arrays with Molybdenum Sulfide Electrocatalysts. Nano Lett. 2014, 14, 3715-3719. (10) Jackson, P.; Wuerz, R.; Hariskos, D.; Lotter, E.; Witte, W.; Powalla, M. Effects of Heavy Alkali Elements in Cu(In,Ga)Se2 Solar Cells with Efficiencies up to 22.6%. Phys. Status Solidi RRL 2016, 10, 583-586. (11) Yu, K.; Carter, E. A. A Strategy to Stabilize Kesterite CZTS for HighPerformance Solar Cells. Chem. Mater. 2015, 27, 2920-2927. (12) Repins, I.; Beall, C.; Vora, N.; DeHart, C.; Kuciauskas, D.; Dippo, P.; To, B.; Mann, J.; Hsu, W. C.; Goodrich, A.; Noufi, R. Co-evaporated Cu2ZnSnSe4 Films and Devices. Sol. Energy Mater. Sol. Cells 2012, 101, 154-159. (13) Katagiri, H.; Jimbo, K.; Yamada, S.; Kamimura, T.; Maw, W. S.; Fukano, T.; Ito, T.; Motohiro, T. Enhanced Conversion Efficiencies of Cu2ZnSnS4-Based Thin Film Solar Cells by Using Preferential Etching Technique. Appl. Phys. Express 2008, 1, 041201-041202.


Page 12 of 16

Page 13 of 16 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


(14) Guan, Z.; Luo, W.; Xu, Y.; Tao, Q.; Wen, X.; Zou, Z. Aging Precursor Solution in High Humidity Remarkably Promoted Grain Growth in Cu2ZnSnS4 Films. ACS Appl. Mater. Interfaces 2016, 8, 5432-5438. (15) Cho, J. W.; Ismail, A.; Park, S. J.; Kim, W.; Yoon, S.; Min, B. K. Synthesis of Cu2ZnSnS4 Thin Films by a Precursor Solution Paste for Thin Film Solar Cell Applications. ACS Appl. Mater. Interfaces 2013, 5, 4162-4165. (16) Yoshida, T. Yamaguchi, A. Umezawa, N. Miyauchi, M. Photocatalytic CO2 Reduction Using a Pristine Cu2ZnSnS4 Film Electrode under Visible Light Irradiation. J. Phys. Chem. C 2018, 122, 21695-21702. (17) Arai, T.; Tajima, S.; Sato, S.; Uemura, K.; Morikawa, T.; Kajino, T. Selective CO2 conversion to formate in water using a CZTS photocathode modified with a ruthenium complex polymer. Chem. Commun. 2011, 47, 12664-12666. (18) Yang, W.; Oh, Y.; Kim, J.; Jeong, M. J.; Park, J. H.; Moon, J. Molecular Chemistry-Controlled Hybrid Ink-Derived Efficient Cu2ZnSnS4 Photocathodes for Photoelectrochemical Water Splitting. ACS Energy Lett. 2016, 1, 1127-1136. (19) Yokoyama, D.; Minegishi, T.; Jimbo, K.; Hisatomi, T.; Ma, G.; Katayama, M.; Kubota, J.; Katagiri, H.; Domen, K. H2 Evolution from Water on Modified Cu2ZnSnS4 Photoelectrode under Solar Light. Appl. Phys. Express 2010, 3, 101202101204. (20) Wen, X.; Luo, W.; Guan, Z.; Huang, W.; Zou, Z. Boosting Efficiency and Stability of a Cu2ZnSnS4 Photocathode by Alloying Ge and Increasing Sulfur Pressure Simultaneously. Nano Energy 2017, 41, 18-26. (21) Wang, K.; Huang, D.; Yu, L.; Gu, H.; Ikeda, S.; Jiang, F. Environmentally Friendly Cu2ZnSnS4-Based Photocathode Modified with a ZnS Protection Layer for Efficient Solar Water Splitting. J. Colloid Interf. Sci. 2019, 536, 9-16. (22) Tay, Y. F.; Kaneko, H.; Chiam, S. Y.; Lie, S.; Zheng, Q.; Wu, B.; Hadke, S. S.; Su, Z.; Bassi, P. S.; Bishop, D.; Sum, T. C.; Minegishi, T.; Barber, J.; Domen, K.; Wong, L. H. Solution-Processed Cd-Substituted CZTS Photocathode for Efficient Solar Hydrogen Evolution from Neutral Water. Joule 2018, 2, 1-12. (23) Kaga, H.; Tsutsui, Y.; Nagane, A.; Iwase, A.; Kudo, A. An Effect of Ag(i)Substitution at Cu Sites in CuGaS2 on Photocatalytic and Photoelectrochemical Properties for Solar Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 21815-21823. (24) 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. Funct. Mater. 2015, 25, 6733-6743. (25) Guan, Z.; Luo, W.; Feng, J.; Tao, Q.; Xu, Y.; Wen, X.; Fu, G.; Zou, Z. Selective Etching of Metastable Phase Induced an Efficient CuIn0.7Ga0.3S2 Nano-Photocathode for Solar Water Splitting. J. Mater. Chem. A 2015, 3, 7840-7848. (26) 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. Interfaces 2017, 9, 21243-21250. (27) Shin, D.; Saparov, B.; Mitzi, D. B. Defect Engineering in Multinary EarthAbundant Chalcogenide Photovoltaic Materials. Adv. Energy Mater. 2017, 7, 1602366-1602394. (28) Hages, C. J.; Koeper, M. J.; Agrawal, R. Optoelectronic and Material Properties of Nanocrystal-Based CZTSe Absorbers with Ag-Alloying. Sol. Energy Mater. Sol. Cells 2016, 145, 342-348. (29) Li, W.; Liu, X.; Cui, H.; Huang, S.; Hao, X. The Role of Ag in (Ag,Cu)2ZnSnS4 Thin Film for Solar Cell Application. J. Alloys. Compd. 2015, 625, 277-283. (30) Gershon, T.; Lee, Y. S.; Antunez, P.; Mankad, R.; Singh, S.; Bishop, D.; Gunawan, O.; Hopstaken, M.; Haight, R. Photovoltaic Materials and Devices Based on the Alloyed Kesterite Absorber (AgxCu1-x)2ZnSnSe4. Adv. Energy Mater. 2016, 6, 1502468-1502474. (31) Chernomordik, B. D.; Béland, A. E.; Deng, D. D.; Francis, L. F.; Aydil, E. S. Microstructure Evolution and Crystal Growth in Cu2ZnSnS4 Thin Films Formed by Annealing Colloidal Nanocrystal Coatings. Chem. Mater. 2014, 26, 3191-3201. (32) Tsuji, I.; Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. Novel Stannite-type Complex Sulfide Photocatalysts AI2-Zn-AIV-S4(AI= Cu and Ag; AIV= Sn and Ge) for Hydrogen Evolution under Visible-Light Irradiation. Chem. Mater. 2010, 22, 14021409. (33) Thimsen, E.; Martinson, A. B. F.; Elam, J. W.; Pellin, M. J. Energy Levels, Electronic Properties, and Rectification in Ultrathin p-NiO Films Synthesized by Atomic Layer Deposition. J. Phys. Chem. C 2012, 116, 16830-16840. (34) Huang, S.; Luo, W.; Zou, Z. Band Positions and Photoelectrochemical Properties of Cu2ZnSnS4 Thin Films by the Ultrasonic Spray Pyrolysis Method. J. Phys. D: Appl. Phys. 2013, 46, 235108-235113.


Page 14 of 16

Page 15 of 16 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


(35) Nguyen, T. H.; Kawaguchi, T.; Chantana, J.; Minemoto, T.; Harada, T.; Nakanishi, S.; Ikeda, S. Structural and Solar Cell Properties of a Ag-Containing Cu2ZnSnS4 Thin Film Derived from Spray Pyrolysis. ACS Appl. Mater. Interfaces 2018, 10, 5455-5463. (36) Moriya, M.; Minegishi, T.; Kumagai, H.; Katayama, M.; Kubota, J.; Domen, K. Stable Hydrogen Evolution from CdS-Modified CuGaSe2 Photoelectrode under Visible-Light Irradiation. J. Am. Chem. Soc. 2013, 135, 3733-3735. (37) Zhao, J.; Minegishi, T.; Zhang, L.; Zhong, M.; Gunawan; Nakabayashi, M.; Ma, G.; Hisatomi, T.; Katayama, M.; Ikeda, S.; Shibata, N.; Yamada, T.; Domen, K. Enhancement of Solar Hydrogen Evolution from Water by Surface Modification with CdS and TiO2 on Porous CuInS2 Photocathodes Prepared by an ElectrodepositionSulfurization Method. Angew. Chem. Int. Ed. 2014, 53, 11808-11812. (38) Jacobsson, T. J.; Platzer-Björkman, C.; Edoff, M.; Edvinsson, T. CuInxGa1−xSe2 as an Efficient Photocathode for Solar Hydrogen Generation. Int. J. Hydrogen Energy 2013, 38, 15027-15035. (39) Ikeda, S.; Nonogaki, M.; Septina, W.; Gunawan, G.; Harada, T.; Matsumura, M. Fabrication of CuInS2 and Cu(In,Ga)S2 Thin Films by a Facile Spray Pyrolysis and their Photovoltaic and Photoelectrochemical Properties. Catal. Sci. Technol. 2013, 3, 1849-1854. (40) Tilley, S. D.; Cornuz, M.; Sivula, K.; Grätzel, M. Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis. Angew. Chem. Int. Ed. 2010, 49, 6405-6408.



Graphical abstract The photoelectrochemical water splitting performance of CZTS photocathode was significantly improved by substituted with Ag.


Page 16 of 16

Band positions and photoelectrochemical properties of solution

Recommend Documents