Photocathode for Water Electrolysis Applications

Transcription

1 Supporting Information Efficient and Stable Pt/TiO 2 /CdS/Cu 2 BaSn(S,Se) 4 Photocathode for Water Electrolysis Applications Yihao Zhou 1#, Donghyeop Shin 1,2,4#, Edgard Ngaboyamahina 3#, Qiwei Han 1,2, Charles B. Parker 3, David B. Mitzi 1,2 *, Jeffrey T. Glass 1,3 * 1 Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA 2 Department of Chemistry, Duke University, Durham, NC 27708, USA 3 Department of Electrical & Computer Engineering, Duke University, Durham NC, USA 4 Photovoltaic Laboratory, Korea Institute of Energy Research, Daejeon, 34129, Korea

2 Materials synthesis First, copper (Cu), tin (Sn) and barium sulfide (BaS) targets were co-sputtered onto a molybdenum (Mo) coated glass substrate. After annealing in excess sulfur atmosphere at 570 o C for 5 min, the as-formed Cu 2 BaSnS 4 films were annealed at 570 o C in excess selenium atmosphere for 5 min to form the Cu 2 BaSnS 4-x Se x (CBTSSe) films 1. A 50 nm-thick CdS layer was deposited on the CBTSSe(x 3) films using a CBD approach reported in the literature 2. For some of the electrodes examined, a TiO 2 layer was deposited on the CdS/CBTSSe(x 3) films using a home-made hot-wall viscous flow atomic layer deposition (ALD) reactor at 200 o C. TiCl 4 and deionized water were used as the precursor and co-reactant for the deposition, respectively. A pulse sequence of 1.5 s/5 s TiCl 4 dose/purge and 1.5 s/5 s H 2 O dose/purge was applied for 700 cycles to deposit ~30 nm TiO 2 on the CdS/CBTSSe(x 3). The thickness was measured by ellipsometry using a reference silicon substrate. Materials characterization To verify the microstructure of the CBTSSe films, SEM images were obtained using a FEI XL30 system. X-ray diffraction (XRD) patterns were measured to characterize the crystal structure of the CBTSSe films, using a PANalytical Empyrean powder X-ray diffractometer and Cu Kα radiation. To estimate the band gap of the CBTSSe film, photoluminescence measurements were performed at room temperature with a 442 nm HeCd laser excitation using a Horiba Jobin Yvon LabRAM ARAMIS system. Optical absorption measurements were carried out using a Shimadzu UV-3600 spectrophotometer.

3 Photocathode fabrication One edge of the as-prepared TiO 2 /CdS/CBTSSe(x 3) film was scratched and electrically contacted to a copper wire. A glass tube was used to protect the copper wire from reacting with the electrolyte. Side surfaces were covered by an insulating resin (Loctite ). The surface area of the TiO 2 /CdS/CBTSSe(x 3) electrodes was measured using the ImageJ package. The tested electrodes have surface areas ranging from 0.25 to cm 2 (the surface area of the electrode with best performance is 0.642cm 2 ). Before the measurements, Pt was electrochemically deposited on the TiO 2 /CdS/CBTSSe(x 3) surface as an electron-catalyst, by polarizing the electrode in 1 mm H 2 PtCl 6 and 0.1 M Na 2 SO 4 at -0.4 V vs Ag/AgCl for 2 min. After the Pt deposition, the as-prepared Pt/TiO 2 /CdS/CBTSSe(x 3) was tested in 0.5 M Na 2 SO 4, 0.5 M KH 2 PO 4 electrolyte (ph=4.3). The fabrication process was the same for all the Pt/TiO 2 /CdS/CBTSSe(x 3), Pt/CBTSSe(x 3) and Pt/CdS/ CBTSSe(x 3) electrodes. Electrochemical measurement All electrochemical measurements were performed using a Bio-Logic SP-200 potentiostat. Measurements were performed at room temperature in a standard three-electrode setup. Ag/AgCl in saturated KCl was used as reference electrode and a platinum mesh was used as counter electrode. The potential was converted to the reversible hydrogen electrode scale according to the following equation 3 : E RHE = E Ag/AgCl ph ,

4 where E Ag/AgCl is the experimentally measured potential and E 0 Ag/AgCl= V at roomtemperature. In the linear sweep voltammetry, the potential was swept from V to V/RHE with a sweep rate of 10 mv/s. In the chronoamperometry (CA), the potential was set at 0 V/RHE. A 150 W Xe lamp with AM 1.5G filter was used as solar simulator. A silicon photodetector was used to calibrate the light intensity to 100 mw/cm 2. The half-cell solar-to-hydrogen (HC-STH) efficiency was calculated according to the following equation: HC-STH = ( J ph (E RHE - E H+/H2 ) /P sun ) 100%, where J ph is the photocurrent density obtained at an applied bias E RHE under illumination (AM 1.5G) and E H+/H2 is the equilibrium potential of hydrogen evolution (0 V/RHE). The Incident Photon-to-Current efficiency (IPCE) was measured using a customized Newport-Oriel system powered by a 300 W Xe (Ozone free) lamp. A Cornerstone 130 1/8M monochromator was used to produce light from 300 to 900 nm. The irradiance was measured with a Newport 1918-R power meter and calibrated using a standard silicon photodetector. The IPCE is calculated according to the following equation 4 : IPCE = ( J ph 1240/P light /λ) 100% Electrochemical impedance spectroscopy was performed on bare CBTSSe(x 3) electrodes. Fitting with different equivalent circuits did not yield reasonable results in terms of flat band potential and doping density values.

5 Dissolved hydrogen measurement The dissolved hydrogen was measured using a Unisense hydrogen microsensor. Before the experiment, the microsensor was calibrated using 0.5 M Na 2 SO 4, 0.5 M KH 2 PO 4 electrolyte saturated with hydrogen and deionized water. Figure S1. Direct band gap fit of the Tauc plot obtained from the absorbance data for a CBTSSe(x 3) film on glass substrate.

6 Figure S2. Linear sweep voltammetry of Pt/CBTSSe(x 3) in light and dark. The scan rate is 10 mv/s from anodic to cathodic potentials. The light is chopped on and off every 50 mv. Figure S3. (a) Photopotential of Pt/TiO 2 /CdS/CBTSSe(x 3) and Pt/CdS/CBTSSe(x 3) electrodes at open circuit voltage. (b) Photopotential of Pt/CBTSSe(x 3) at open circuit voltage.

7 Figure S4. Derivative of the IPCE spectrum as a function of wavelength. The inflection point indicates the bandgap of CBTSSe(x 3) at ~1.53 ev. The electrode structure is Pt/TiO 2 /CdS/CBTSSe(x 3).

8 Figure S5. Representative top-view SEM of Pt/TiO2/CdS/CBTSSe(x 3) electrode before (a) and after (b) the 10-hour stability test. No change is observed in the SEM images. Figure S6. (a) Linear Sweep Voltammetry of Pt/TiO2/CdS/CBTSSe(x 3) before and after the 10 hours stability test. (b) The half-cell solar-to-hydrogen efficiency of Pt/TiO2/CdS/CBTSSe(x 3) before and after the 10-hour stability test.

9 Figure S7. Photocurrent ratio of Pt/TiO 2 /CdS/CBTSSe(x 3) versus the Linear Sweep Voltammetry cycles (in light) after the 10-hour stability test. Photocurrent ratio is defined as the photocurrent after the stability test divided by the photocurrent before the 10-hour stability test at 0 V/RHE.

10 REFERENCE (1) Shin, D.; Ngaboyamahina, E.; Zhou, Y.; Glass, J. T.; Mitzi, D. B. Synthesis and Characterization of an Earth-Abundant Cu 2 BaSn(S,Se) 4 Chalcogenide for Photoelectrochemical Cell Application. J. Phys. Chem. Lett. 2016, 7, (2) Shin, D.; Saparov, B.; Zhu, T.; Huhn, W. P.; Blum, V.; Mitzi, D. B. BaCu 2 Sn(S,Se) 4 : Earth-Abundant Chalcogenides for Thin-Film Photovoltaics. Chem. Mater. 2016, 28, (3) Wang, L.; Lee, C.-Y.; Schmuki, P. Solar Water Splitting: Preserving the Beneficial Small Feature Size in Porous α-fe 2 O 3 Photoelectrodes During Annealing. J. Mater. Chem. A 2013, 1, (4) Chen, Z.; Jaramillo, T. F.; Deutsch, T. G.; Kleiman-Shwarsctein, A.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M. Accelerating Materials Development for Photoelectrochemical Hydrogen Production: Standards for Methods, Definitions, and Reporting Protocols. J. Mater. Res. 2010, 25, 3-16.