4 Chalcogenide for Photoelectrochemical Cell Application

Letter pubs.acs.org/JPCL

Synthesis and Characterization of an Earth-Abundant Cu2BaSn(S,Se)4 Chalcogenide for Photoelectrochemical Cell Application Donghyeop Shin,†,‡,∥ Edgard Ngaboyamahina,§,∥ Yihao Zhou,†,∥ Jeffrey T. Glass,*,†,§ and David B. Mitzi*,†,‡ †

Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States Department of Chemistry, Duke University, Durham, North Carolina 27708, United States § Department of Electrical & Computer Engineering, Duke University, Durham, North Carolina 27708, United States ‡

S Supporting Information *

ABSTRACT: Cu2BaSnS4−xSex films consisting of earth-abundant metals have been examined for photocathode application. Films with different Se contents (i.e., Cu2BaSnS4−xSex with x ≤ 2.4) were synthesized using a cosputter system with postdeposition sulfurization/selenization annealing treatments. Each film adopts a trigonal P31 crystal structure, with progressively larger lattice constants and with band gaps shifting from 2.0 to 1.6 eV, as more Se substitutes for S in the parent compound Cu 2 BaSnS 4 . Given the suitable bandgap and earth-abundant elements, the Cu2BaSnS4−xSex films were studied as prospective photocathodes for water splitting. Greater than 6 mA/cm2 was obtained under illumination at −0.4 V versus reversible hydrogen electrode for Pt/Cu2BaSnS4−xSex films with ∼60% Se content (i.e., x = 2.4), whereas a bare Cu2BaSnS4−xSex (x = 2.4) film yielded ∼3 mA/cm2 at −0.4 V/RHE.

P

of 4 mA/cm2 at 0 V/RHE for bare CIS. Additionally, CdS/ TiO2/Pt coating further increased the photocurrent at 0 V/ RHE to 8 mA/cm2.6 CuInS2 photocathodes incorporated within a Pt/TiO2/CdS/CIS PEC structure also yielded a photocurrent density of ∼13 mA/cm2 at 0 V/RHE and an onset potential of 0.6 V/RHE.5 For CIS and CIGSSe materials, In and Ga are less abundant elements and widely used for display and IT applications, perhaps limiting broad scalability and reduction in fabrication cost. As an effort to develop earthabundant materials, Cu2ZnSnS4 (CZTS) materials have been studied as a photocathode for PEC applications. The photocurrent of the CdS/CZTS-based PEC showed 6 mA/cm2 at −0.4 V/RHE was achieved. Additionally, electrochemical impedance spectroscopy (EIS) was used to provide information about the interfacial properties of the photocathode and to estimate both its flat band potential and charge carrier density. To determine the structural quality of the Cu2BaSnS4 and sulfur/selenium mixed Cu2BaSnS4−xSex films, we performed Xray diffraction (XRD) measurements, confirming the singlephase and well-crystallized nature of the samples (Figure 1a). The X-ray diffraction pattern of the pure sulfide Cu2BaSnS4 sample matches the trigonal P31 structure previously reported, with lattice constants a = 6.334(1) Å and c = 15.810(1) Å.12 By introducing Se into the film, the XRD peaks of the Cu2BaSnS4−xSex phase shift toward lower 2θ angles, while the peak for the Mo film (which can serve as a reference) remains centered at 2θ = 40.5° (Figure 1a). For each Se-incorporated 4555

DOI: 10.1021/acs.jpclett.6b02010 J. Phys. Chem. Lett. 2016, 7, 4554−4561

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The Journal of Physical Chemistry Letters composition, Cu2BaSnS4−xSex, the diffraction pattern has been Pawley fit to the same trigonal structure type, with progressively larger lattice constants as the amount of Se incorporation increases (Table 1). On the basis of the reported lattice Table 1. Summary of Lattice Parameters (from XRD Data) and Band Gaps (from Photoluminescence Data) for Cu2BaSnS4−xSex Films lattice parameters (Å) x

space group

a

c

band gap (eV)

0 0.4 2.4

P31 P31 P31

6.334(1) 6.384(1) 6.532(1)

15.810(1) 15.954(2) 16.344(2)

2.04 1.98 1.62

constants for Cu2BaSnS4−xSex bulk powders over the range 0 ≤ x ≤ 3,12 the concentration of Se incorporation can be estimated (Table 1), and these x values agree with the energy-dispersive spectroscopy (EDS) data provided in Table S1. Top-view and cross-sectional scanning electron microscopy (SEM) images reveal that all Cu2BaSnS4−xSex films with small x are continuous (no pinholes), with grain sizes in the range of 100−500 nm (Figure 1b). However, when larger amounts of Se are incorporated into the films, grain sizes >1 μm are achieved, which could be helpful for reducing recombination at grain boundaries. Additionally, atomic force microscopy (AFM) images for Cu 2 BaSnS 4 and mixed sulfur/selenium Cu2BaSnS4−xSex films were collected to determine root-meansquare surface roughness values (Figure S1), yielding 50, 73, and 75 nm for x = 0, 0.4 and 2.4, respectively. There is no significant difference in the overall surface roughness for these films, despite the larger grain size as x (i.e., incorporated amount of Se) increases. To further confirm the absence of impurity phases,14,15 such as binary Cu(S,Se) and ternary Cu2Sn(S,Se)3, a Raman spectrum of the Cu2BaSnS4−xSex film with x = 2.4 has been measured. Impurity peaks reported in the literature for the binary/ternary phases are not seen in the spectrum (Figure S2), which is consistent with the XRD pattern. Photoluminescence (PL) measurements were carried out to verify the band gap, Eg, of Cu2BaSnS4 and Cu2BaSnS4−xSex films and to examine radiative recombination. Note that previously12 the authors showed that the PL peak position is within ∼10 meV of the band gap value and therefore the PL peak provides a good estimate of Eg. Using these measurements, Eg of the Cu2BaSnS4 (x = 0) sample is found to be 2.04 eV (Figure 2 and Table 1), consistent with a previous report.12 As Se atoms are incorporated into the Cu2BaSnS4−xSex films, Eg decreases from 1.98 (x = 0.4) to 1.62 eV (x = 2.4). The decrease in Eg follows the change in composition, as determined by XRD and EDS data, and it is useful to note that the band gap for the x = 2.4 sample falls near the lowest value achievable (∼1.55 eV) before the system shifts to the orthorhombic Ama2 structure for x > 3. These band gaps also fall into the relevant range for PEC cell application (1.5 to 2.5 eV), thereby prompting the further investigation, as described later in the paper. The strong and sharp PL features also indicate effective radiative recombination and may reflect a less defective lattice (i.e., less cationic antisite disorder) compared with analogous Cu2ZnSnS4−xSex films.12 Slight broadening of the PL peak for increased Se content (Figure 2) likely indicates modest inhomogeneity in the S:Se ratio throughout the film thickness.

Figure 2. Photoluminescence spectra for Cu 2 BaSnS 4 , and Cu2BaSnS4−xSex (x = 0.4 and 2.4) films obtained with 442 nm laser excitation at room temperature.

The Cu2BaSnS4−xSex films were utilized as PEC electrodes after the materials characterization. The electrode structure used for the electrochemical tests is shown in Scheme 1. In this Scheme 1. Cu2BaSnS4−xSex Electrode Structure

configuration, the incident excitation light promotes electrons from the valence band to the conduction band of the Cu2BaSnS4−xSex and leaves holes in the valence band. The electrons are then transferred at the semiconductor−electrolyte interface to reduce water into H2. The holes flow in the opposite direction through the external circuit to generate oxidation reactions at the counter electrode. It has been reported that the addition of selenium into CZTS results in a lower bandgap, which ultimately enables a higher photocurrent density in prospective PEC devices.16,17 We therefore performed photocurrent measurements by linear sweep voltammetry (LSV) for different compositions of the Cu2BaSnS4−xSex films (Figure 3a−c). As expected, the 4556


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Figure 3. Linear sweep voltammetry for Cu2BaSnS4−xSex with (a) x = 0, (b) x = 0.4, and (c) x = 2.4 without a Pt catalyst and (d) x = 2.4 with a Pt catalyst.

contribution to the observed charge recombination (i.e., process (iii), listed above). Pt coating increased the photocurrent (i.e., interfacial charge transfer) but did not improve the onset potential, which suggests that some charge recombination traps are localized in the bulk film. Process (ii), hole transport limitation, also cannot be dismissed from the data shown in Figure 3. Chronoamperometry (CA) at −0.2 V versus RHE was performed to determine Cu2BaSnS4−xSex stability under illumination, as shown in Figure S3. The photocurrent for the Cu2BaSnS4−xSex (x = 2.4) film dropped rapidly from 3 to ∼1 mA/cm2 due to associated charge recombination and was followed by a quasi-plateau for almost 10 min. A gradual photocurrent decrease was observed and is assumed to be the contribution of photocorrosion, as evidenced by SEM images shown in Figure S3c and d. However, bare Cu2BaSnS4−xSex (x = 2.4) appears to be more stable than electrodeposited CZTS, whose stability vanishes within 3 mA/cm2 at −0.4 V/RHE compared with 0.2 mA/cm2 for x = 0. Even though the onset potential seems to be more positive than 0.6 V/RHE, photocurrents of at least several hundreds of microamperes are only observed for voltages more negative than 0.2 V/RHE. The presence of electron traps in the space charge region likely accounts for this observation.18 Grain boundaries and intragrain defects have been found to induce carrier recombination in CZTS-based solar cells. However, given the variety of preparation methods, experimental setups, and alloy compositions used in investigating their effects, a straightforward conclusion on the origin of these traps cannot be drawn.19 To further improve the performance of Cu2BaSnS4−xSex electrodes, we deposited 1.2 nm Pt as a catalyst using e-beam evaporation. The LSV of Cu2BaSnS4−xSex electrodes (x = 2.4) with and without Pt coating is shown in Figure 3d. The electrode with Pt coating exhibits photocurrent value >6 mA/cm2, while the electrode without Pt exhibits ∼3 mA/cm2 at −0.4 V versus RHE, indicating that Pt acts as an adequate catalyst for Cu2BaSnS4−xSex photocathodes. Further analysis shows that in all cases without the Pt catalyst voltammograms exhibit transients when the light is switched on and off, indicating (i) accumulation of electrons near the surface, (ii) accumulation of holes in the bulk (due to slow transport), or (iii) trapping of electrons or holes at surface states.20 More precisely, when light is turned off, the appearance of anodic current spikes pertains to electron accumulation near the surface, that is, process (i). It is assumed that some photogenerated electrons get accumulated at the photocathode surface and recombine with bulk holes, inducing a back electron flow from the bulk toward the external circuit. Given the small amplitude of these spikes, this process is unlikely to be the main recombination mechanism. Additionally, current transients seem to be voltage-dependent, suggesting that trapping at surface states is the main 4557


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Figure 4. (a) Bode representation of the EIS experimental results for bare Cu2BaSnS4−xSex. (b) Mott−Schottky representation obtained for selected low frequencies using the Randles model. (c) Suggested equivalent circuit used for EIS fitting. (d) Mott−Schottky representation obtained using the equivalent circuit in panel c over three frequency decades.

permittivity of free space, εr is the semiconductor dielectric constant, A is the electrode surface area in contact with the electrolyte, k is the Boltzmann constant, T is the temperature, N is the majority charge carrier density, and Efb is the flat band potential

worth noting that the thickness and quality of the CdS layer has not been systematically studied; therefore, an improved stability is expected in future work. Illuminated open-circuit potential measurements were performed to further explore the charge separation and surface quality of bare Cu2BaSnS4−xSex (x = 2.4) films. The light was switched on and off every 5 min, and results are shown in Figure S5. The low photopotential is likely to originate from low band bending under dark conditions and a high recombination rate at the surface.21 Incident photon to current efficiency (IPCE) of bare Cu2BaSnS4−xSex (x = 2.4) was measured at −0.4 V versus RHE to verify the onset of the photoresponse and the efficiency of energy conversion for photons at different wavelength. The onset wavelength of photoresponse (Figure S6) is 755 nm (i.e., 1.64 eV), which is consistent with the PL spectrum. For a deeper insight into the electrochemical behavior of Cu2BaSnS4−xSex, EIS measurements were performed in the dark to avoid the contribution of the charge-transfer resistance related to photocurrent. From the Bode representation of the EIS results (Figure 4a), multiple time constants can be observed and likely correspond to the contribution of: (i) the semiconductor space charge layer, (ii) the charge of the double layer at the semiconductor−electrolyte interface, (iii) slow faradaic reactions, and (iv) eventually the relaxation of charges via surface states.22 The Mott−Schottky analysis, described by eq 1, is used to determine the flat band potential and the charge carrier density of the semiconductor. It is usually assumed that the capacitance at the semiconductor−electrolyte interface is dominated by that of the semiconductor.23 In that case, eq 1 describes an ideal semiconductor where C is the space charge layer capacitance, q is the charge of one electron, ε0 is the

⎛ 1 2 kT ⎞ = * − E + E − ⎟ ⎜ fb q ⎠ C2 qε0εrNA2 ⎝

(1)

The typical Randles model, which consists of an RC circuit in series with the electrolyte resistance, was first tested to evaluate the behavior of the semiconductor space-charge layer. The model did not fit the experimental data over a wide frequency range. Results at selected low frequencies, where the contribution of the space charge is assumed to be the main component, are displayed in a Mott−Schottky representation in Figure 4b. The plots have different slopes and yield flat band potentials of between 0.7 and 0.9 V (intercept of the fitted straight line with the x axis). The dependence of the flat band potential on the selected frequency indicates that the different electrochemical processes contributing to the visible capacitance and cited above have either similar RC time constants or a frequency dispersion that complicates their contribution separation.18 Alternatively, a second equivalent circuit model is proposed to model the EIS results (Figure 4c) and consists of: (i) R and C, the resistance and capacitance at the semiconductor−electrolyte interface, respectively, (ii) an R-constant phase element (CPE) additional branch to account for “parasitic” contributions, and (iii) a Warburg element to reflect an eventual delay in the trapping/detrapping of surface states.22 The suggested alternative equivalent circuit fits the EIS response of the electrode over three frequency decades and is 4558


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reported in literature.30 A 50 nm thick CdS layer was deposited on selected Cu2BaSnS4−xSex films using a chemical bath approach described in literature.12 Pt as a catalyst was deposited via e-beam evaporation on selected Cu2BaSnS4−xSex films used in the study. The Pt film thickness estimated by a quartz crystal microbalance (QCM) was found to be ∼1.2 nm. Characterization Methods. The phase purity for Cu2BaSnS4−xSex films was investigated using a PANalytical Empyrean powder X-ray diffractometer using Cu Kα radiation. To verify the microstructure of the Cu2BaSnS4−xSex films, SEM images were taken using a FEI XL30 system. PL and Raman analyses were carried out at room temperature with a 442 and 633 nm laser excitation, respectively, using a Horiba Jobin Yvon LabRAM ARAMIS system. Electrochemical Tests. Electrochemical measurements were performed at room temperature in a standard three-electrode cell. All potentials were measured against a KCl-saturated silver/silver chloride reference electrode (Ag/AgCl), with a platinum mesh counter electrode. However, for easy comparison to the hydrogen evolution, results were converted to the reversible hydrogen electrode (RHE) scale according to the following equation31

therefore believed to be more appropriate to the electrochemical system investigated in the current work.24 Fitting results at potentials ranging from 0.3 to 0.7 V versus RHE are available in the Supporting Information (Figures S7 and S8 and Tables S3 and S4). In Figure 4d, the negative Mott−Schottky slope confirms the p-type nature of the material. From the same slope, the charge carrier density (N) for Cu2BaSnS4−xSex has been evaluated to be ∼1019 cm−3 for x = 2.4 and ∼1020 for x = 0. Comparable values ranging from 1018 to 1021 cm−3 are reported elsewhere for Cu 2 ZnSnS 4 and Cu 2 ZnSn(S,Se)4.16,25−27 The preparation methods described in literature vary and obviously lead to different morphologies (e.g., nanoflakes, nanoworms, and nanoparticles), which may partially explain the dispersion of the calculated carrier concentration in these previous studies. Also, as mentioned above, the choice of the equivalent circuit used for fitting EIS data is critical to the associated calculated carrier densities. The model must be carefully determined to reduce the errors from frequency dispersion. This phenomenon originates from several factors, such as changing resistivity throughout the film thickness, doping inhomogeneity, or microroughness.24,28 Further evaluation of the circuit model will be undertaken to reduce uncertainty in the current carrier density numbers. As for the flat band potential, for x = 2.4, it is estimated to be ∼0.84 V, whereas it is ∼0.70 V for x = 0. An increase in the flat band potential with the introduction of Se was also observed by Patil et al. with respect to CZTS films.16 It can be explained by an improved charge carrier separation efficiency at the semiconductor−electrolyte interface in the absence of band bending.29 In conclusion, Cu2BaSnS4−xSex films comprising earthabundant metals, with trigonal P31 crystal structure and different concentrations of Se (x ≤ 2.4), were successfully synthesized. The introduction of Se in Cu2BaSnS4−xSex lowers the band gap, leading to better charge separation and a higher photocurrent, thereby rendering Cu2BaSnS4−xSex a better candidate for PEC application compared with its parent (x = 0) structure. Cu2BaSnS4−xSex with x = 2.4 was found to deliver ∼3 mA/cm2 at −0.4 V/RHE without Pt and >6 mA/cm2 at −0.4 V/RHE with the metal catalyst. Bare Cu2BaSnS4−xSex showed improved stability when coated with Pt/CdS layers. However, optimized surface and interface engineering (i.e., catalyst, buffer layers, and protective layers) may offer a pathway to achieve more stable and efficient Cu2BaSnS4−xSex photocathodes.

E RHE = EAg/AgCl + 0.059 pH + 0.197

(2)

where EAg/AgCl is the experimentally measured value and E0Ag/AgCl = 0.197 V at 298 K. Electrodes were immersed in a phosphate-buffered saline electrolyte (PBS, pH 7). For analysis under illumination, the films were illuminated with a 150 W xenon lamp using a solar spectrum filter. The light source was placed a distance of 15 cm from the working electrode, and the beam was perpendicular to the working electrode. A silicon photo detector was used to calibrate the light intensity to 100 mW/cm2. The error was estimated to be within 5%. Side and back surfaces of the working electrode were covered with an insulating resin (Loctite). The nominal surface areas of the samples were measured using the ImageJ software package. For electrical contact, a conductive copper wire was attached on the Mo layer using a liquid indium/gallium alloy. A glass tube was used to protect the copper wire from electrochemical reactions. The contact resistance was tested to be around 100−200 Ω using a multimeter (Fluke). LSV was used to characterize the photoelectrochemical performance of the films. In the LSV technique, the potential was varied from 0.7 to −0.4 V versus RHE to prevent any chemical reaction beyond this range, and the sweep rate was 5 mV/s. In the measurement, the light was chopped every 50 mV. CA was used to characterize the stability of the films. In the CA technique, the potential was set to be −0.2 V versus RHE. IPCE was measured at −0.4 V versus RHE using a customized Newport-Oriel system powered by a 300 W Xe (Ozone-free) lamp. A Cornerstone 130 1/8 M monochromator was used to produce monochromatic lights at different wavelengths from 800 to 320 nm. The irradiance was measured with a power meter (Newport 1918-R) and calibrated by a standard silicon photodetector. An electrochemical interface equipped with a frequency response analyzer (Biologic SP-200) was used for EIS measurements under potentiostatic conditions over frequencies ranging from 0.1 Hz to 2 MHz using a 10 mV sinusoidal potential modulation. The potential range was from 0.7 to 0.3 V versus RHE with an interval of 50 mV. The selected potential was applied for 2 min to reach a steady state before EIS

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EXPERIMENTAL METHODS Thin Film Deposition. Cu−Ba−Sn−S precursor layers were codeposited at room temperature with a background pressure of 3 mTorr using Cu, Sn, and BaS targets on Mo-coated substrates.12 For pure Cu2BaSnS4 films, the precursors were annealed at 560−580 °C for 5−10 min in the presence of excess sulfur. To prepare S-rich Cu2BaSnS4−xSex films with ∼10% Se content, a sulfur/selenium mixed source was used instead of excess sulfur during postannealing. To introduce a high concentration of Se atoms, sulfurized films were annealed at 550−560 °C for 5 min in the presence of excess selenium. All postannealing processes took place in a nitrogen-filled drybox. The thicknesses of the pure Cu2BaSnS4 and sulfur/selenium mixed films were all approximately 900−1000 nm, as determined by cross-sectional SEM imaging. After the sulfurization or selenization treatment, the films were annealed at 280 °C for 10 min in air to passivate grain boundaries, as 4559


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modification with CdS and TiO2 on porous CuInS2 photocathodes prepared by an electrodeposition−sulfurization method. Angew. Chem., Int. Ed. 2014, 53, 11808−11812. (6) Guijarro, N.; Prévot, M. S.; Yu, X.; Jeanbourquin, X. A.; Bornoz, P.; Bourée, W.; Johnson, M.; Le Formal, F.; Sivula, K. A bottom-up approach toward all-solution-processed high-efficiency Cu(In,Ga)S2 photocathodes for solar water splitting. Adv. Energy Mater. 2016, 6, 1501949. (7) Rovelli, L.; Tilley, S. D.; Sivula, K. Optimization and stabilization of electrodeposited Cu2ZnSnS4 photocathodes for solar water reduction. ACS Appl. Mater. Interfaces 2013, 5, 8018−8024. (8) Jiang, F.; Gunawan; Harada, T.; Kuang, Y.; Minegishi, T.; Domen, K.; Ikeda, S. Pt/In2S3/CdS/Cu2ZnSnS4 thin film as an efficient and stable photocathode for water reduction under sunlight radiation. J. Am. Chem. Soc. 2015, 137, 13691−7. (9) Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Adv. Energy Mater. 2014, 4, 1301465. (10) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar cell efficiency tables (version 48). Prog. Photovoltaics 2016, 24, 905−913. (11) Gokmen, T.; Gunawan, O.; Todorov, T. K.; Mitzi, D. B. Band tailing and efficiency limitation in kesterite solar cells. Appl. Phys. Lett. 2013, 103, 103506. (12) Shin, D.; Saparov, B.; Zhu, T.; Huhn, W. P.; Blum, V.; Mitzi, D. B. BaCu2Sn(S,Se)4: Earth-abundant chalcogenides for thin-film photovoltaics. Chem. Mater. 2016, 28, 4771−4780. (13) Hong, F.; Lin, W.; Meng, W.; Yan, Y. Trigonal Cu2-II-Sn-VI4 (II = Ba, Sr and VI = S, Se) quaternary compounds for earth-abundant photovoltaics. Phys. Chem. Chem. Phys. 2016, 18, 4828−34. (14) Ge, J.; Yu, Y.; Yan, Y. Earth-Abundant Orthorhombic BaCu2Sn(SexS1−x)4(x≈ 0.83) Thin Film for Solar Energy Conversion. ACS Energy Lett. 2016, 1, 583−588. (15) Fontané, X.; Izquierdo-Roca, V.; Fairbrother, A.; EspíndolaRodríguez, M.; López-Marino, S.; Placidi, M.; Jawhari, T.; Saucedo, E.; Pérez-Rodríguez, A. In Selective Detection of Secondary Phases in Cu2ZnSn(S, Se)4 Based Absorbers by Pre-Resonant Raman Spectroscopy, 2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); IEEE: 2013; pp 2581−2584. (16) Patil, S. J.; Lokhande, V. C.; Lee, D.-W.; Lokhande, C. D. Electrochemical impedance analysis of spray deposited CZTS thin film: Effect of Se introduction. Opt. Mater. 2016, 58, 418−425. (17) Ahmed, S.; Reuter, K. B.; Gunawan, O.; Guo, L.; Romankiw, L. T.; Deligianni, H. A high efficiency electrodeposited Cu2ZnSnS4 solar cell. Adv. Energy Mater. 2012, 2, 253−259. (18) Chen, Z.; Dinh, H. N.; Miller, E. Photoelectrochemical Water Splitting; SpringerBriefs in Energy: New York, 2013. (19) Ito, K. Copper Zinc Tin Sulfide-Based Thin Film Solar Cells; John Wiley & Sons: Nagano, 2014. (20) Van de Krol, R.; Grätzel, M. Photoelectrochemical Hydrogen Production; Springer: New York, 2012. (21) Su, R.; Bechstein, R.; Kibsgaard, J.; Vang, R. T.; Besenbacher, F. High-quality Fe-doped TiO2 films with superior visible-light performance. J. Mater. Chem. 2012, 22, 23755−23758. (22) Pu, P.; Cachet, H.; Ngaboyamahina, E.; Sutter, E. Relation between morphology and conductivity in TiO2 nanotube arrays: an electrochemical impedance spectrometric investigation. J. Solid State Electrochem. 2013, 17, 817−828. (23) Albery, W. J.; O’Shea, G. J.; Smith, A. L. Interpretation and use of Mott−Schottky plots at the semiconductor/electrolyte interface. J. Chem. Soc., Faraday Trans. 1996, 92, 4083−4085. (24) Vanýsek, P.; Hansen, D.; Orazem, M. Impedance in Electrochemistry−from Analytical Applications to Mechanistic Speculation 2; The Electrochemical Society: San Francisco, 2009. (25) Huang, Y.; Li, G.; Fan, Q.; Zhang, M.; Lan, Q.; Fan, X.; Zhou, Z.; Zhang, C. Facile solution deposition of Cu2ZnSnS4 nano-worm films on FTO substrates and its photoelectrochemical property. Appl. Surf. Sci. 2016, 364, 148−155.

measurements. The different impedance parameters involved in the selected EIS model were obtained by fitting the experimental data with EC-Lab software (Bio-Logic). In this publication, all electrochemical potentials are expressed with respect to RHE.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02010. Details of composition (obtained from EDS data), AFM morphologies, Raman spectrum for Cu2BaSnS4−xSex films, and XRD peak list for reference Cu2BaSnS4 with a trigonal structure are displayed in Figures S1 and S2 and Tables S1 and S2. Details of stability measurements, photopotential measurement, and IPCE measurement for bare Cu2BaSnS4−xSex and Pt/CdS/Cu2BaSnS4−xSex (x = 2.4) are displayed in Figures S3−S6. Details of EIS fitting and fitted values for bare Cu2BaSnS4−xSex are displayed in Figures S7 and S8 and Tables S3 and S4. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*J.T.G.: E-mail: jeff[email protected]. *D.B.M.: E-mail: [email protected]. Author Contributions ∥

D.S., E.N., and Y.Z. contributed equally. The manuscript was written through contribution of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Charles B. Parker for providing scientific guidance. This material is based on work supported by the National Science Foundation under Grant No. 1511737 and by the Duke University Energy Initiative Research Seed Fund. This work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of the NSF.

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37−38. (2) Sivula, K.; Van De Krol, R. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 2016, 1, 15010. (3) Wang, J.; Yu, N.; Zhang, Y.; Zhu, Y.; Fu, L.; Zhang, P.; Gao, L.; Wu, Y. Synthesis and performance of Cu2ZnSnS4 semiconductor as photocathode for solar water splitting. J. Alloys Compd. 2016, 688, 923−932. (4) Kang, D.; Kim, T. W.; Kubota, S. R.; Cardiel, A. C.; Cha, H. G.; Choi, K.-S. Electrochemical synthesis of photoelectrodes and catalysts for use in solar water splitting. Chem. Rev. 2015, 115, 12839−12887. (5) Zhao, J.; Minegishi, T.; Zhang, L.; Zhong, M.; Gunawan; Nakabayashi, M.; Ma, G.; Hisatomi, T.; Katayama, M.; Ikeda, S.; et al. Enhancement of solar hydrogen evolution from water by surface 4560


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The Journal of Physical Chemistry Letters (26) 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. (27) Saha, S. K.; Guchhait, A.; Pal, A. J. Cu2ZnSnS4 nanoparticle based nontoxic and earth-abundant hybrid pn-junction solar cells. Phys. Chem. Chem. Phys. 2012, 14, 8090−8096. (28) Harrington, S. P.; Devine, T. M. Relation between the semiconducting properties of a passive film and reduction reaction rates. J. Electrochem. Soc. 2009, 156, C154−C159. (29) Liu, M.; Nam, C.-Y.; Black, C. T.; Kamcev, J.; Zhang, L. Enhancing water splitting activity and chemical stability of zinc oxide nanowire photoanodes with ultrathin titania shells. J. Phys. Chem. C 2013, 117, 13396−13402. (30) Kim, J. H.; Choi, S. Y.; Choi, M.; Gershon, T.; Lee, Y. S.; Wang, W.; Shin, B.; Chung, S. Y. Atomic-scale observation of oxygen substitution and its correlation with hole-transport barriers in Cu2ZnSnSe4 thin-film solar cells. Adv. Energy Mater. 2016, 6, 1501902. (31) Wang, L.; Lee, C.-Y.; Schmuki, P. Solar water splitting: preserving the beneficial small feature size in porous α-Fe2O3 photoelectrodes during annealing. J. Mater. Chem. A 2013, 1, 212− 215.

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