Preparation of Bi-Based Ternary Oxide Photoanodes, BiVO 4,

Transcription

1 Preparation of Bi-Based Ternary Oxide Photoanodes, BiVO 4, Bi 2 WO 6 and Bi 2 Mo 3 O 12, Using Dendritic Bi Metal Electrodes Donghyeon Kang, a, Yiseul Park, a, James C. Hill, b and Kyoung-Shin Choi a,* a Department of Chemistry, University of Wisconsin-Madison, Madison, WI b Department of Chemistry, Purdue University, West Lafayette, IN Supporting Information These authors contributed equally to this work. * Corresponding author. kschoi@chem.wisc.edu 1

2 Experimental Methods Electrodeposition of Bi metal The deposition was carried out in an undivided cell using a VMP2 multichannel potentiostat (Princeton Applied Research). A typical 3-electrode system composed of an FTO working electrode, a Ag/AgCl (4M KCl) reference electrode, and a Pt counter electrode was used. The Pt counter electrode was prepared by e-beam, evaporating 20 nm of titanium followed by 100 nm of platinum on clean glass slides. An ethylene glycol solution containing 20 mm of Bi(NO 3 ) 3 5H 2 O (Sigma-Aldrich) was prepared as the plating solution. The deposition was carried out by passing 0.04 C/cm 2 at E = -1.8 V vs. Ag/AgCl, followed by a resting time of 2 s. This cycle was repeated for 5 to 10 times to pass a total charge of 0.20 to 0.40 C/cm 2. During the resting time Bi 3+ ions were replenished to the interface, which increased the nucleation density of Bi on the working electrode, resulting in better coverage. The thickness of the Bi metal electrode was optimized to achieve the highest photocurrent when it was converted to BiVO 4. The optimum thickness was approximately 1.3 µm, which was obtained by passing 0.32 C/cm 2 (8 cycles of the pulsed deposition). Bi electrodes with the same thickness were used to prepare Bi 2 WO 6 and Bi 2 Mo 3 O 12 electrodes. After electrodeposition, the films were carefully washed with ethanol and dried by blowing air. For comparison, Bi metal was also deposited using a 1 M HNO 3 aqueous solution containing 20 mm of Bi(NO 3 ) 3 5H 2 O as the plating solution. All other deposition conditions (potential, temperature, duration) were the same except distilled water was used for washing the films after deposition. Conversion of Bi metal to BiVO 4, Bi 2 WO 6, and Bi 2 Mo 3 O 12 In order to convert Bi to BiVO 4 electrodes, 100 µl of 150 mm VO(acac) 2 dimethylsulfoxide (DMSO) solution were placed onto the as-deposited Bi electrode (geometric area = 1.8 cm 2 ) to fully cover its surface. The film was then heated at 450 C for 2 hours in air (ramping rate = 1.8 C/min). During the heating procedure, Bi metal and VO 2+ were oxidized to Bi 2 O 3 and V 2 O 5, respectively, and reacted with each other to form BiVO 4. Excess VO 2+ ions 2

3 were used to ensure the complete conversion of Bi to BiVO 4 because any residual V 2 O 5 can be easily removed by soaking the electrode in a 1 M NaOH solution for 30 min. while stirring. The conversion of Bi to Bi 2 WO 6 was achieved by adding 50 µl of 20 mm (NH 4 ) 2 WS 4 DMSO solution on the Bi electrode and annealing it at 600 C for 2 hours in air (ramping rate = 2.4 C/min). After annealing, excess WO 3 was removed by soaking the electrode in a 1 M NaOH solution for 3 min at 80 C while stirring. To prepare Bi 2 Mo 3 O 12 electrodes, 100 µl of 50 mm (NH 4 ) 2 MoS 4 DMSO solution were placed onto the Bi electrode and the electrode was annealed at 550 C for 4 hours in air (ramping rate = 2.2 C/min). After annealing, excess MoO 3 was removed by soaking the electrode in a 1 M NaOH solution for 5 min while stirring. Characterization The purity and crystallinity of the Bi metal, BiVO 4, Bi 2 WO 6 and Bi 2 Mo 3 O 12 electrodes were examined by powder X-ray diffraction (PXRD) (Bruker D8 Advanced PXRD, Ni-filtered Cu Kα-radiation λ = Å) at room temperature. The surface morphologies and thicknesses of the electrodes were examined with Scanning Electron Microscopy (SEM) using a LEO 1530 at an accelerating voltage of 5 kv. The atomic Bi:V, Bi:W, and Bi:Mo ratios in BiVO 4, Bi 2 WO 6 and Bi 2 Mo 3 O 12 electrodes, respectively, were obtained by using a Hitachi S3400-N scanning electron microscope equipped with an energy dispersive X-ray spectrometer (EDS) (Thermo Fisher Scientific Inc.) at an accelerating voltage of 20 kv and the results are summarized in Table S1. UV-vis absorption spectra were recorded using a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent), in which the sample electrode was placed in the center of an integrating sphere to measure all light reflected and transmitted. Photoelectrochemical and electrochemical characterization Photocurrent measurements were carried out using a SP-200 potentiostat/eis (BioLogic Science Instrument) and simulated solar illumination obtained by passing light from a 300 W Xe arc lamp through neutral density filters, an AM 1.5G filter, and a water filter into an optical fiber. The light power density was calibrated to 100 mw/cm 2 before the light passes through FTO by 3

4 using both a thermopile detector (International Light) and an NREL certified reference cell (Photo Emission Tech. Inc.). Illumination through the FTO (back-side illumination) was used. All oxide electrodes were masked with epoxy resin to make the exposed geometrical area (0.05 cm 2 ) smaller than the illuminated area (0.12 cm 2 ). An undivided three-electrode cell composed of a working electrode (BiVO 4, Bi 2 WO 6, or Bi 2 Mo 3 O 12 ), a Pt counter electrode, and a Ag/AgCl (4 M KCl) reference was used. All photocurrent was measured either while sweeping the potential to the positive direction with a scan rate of 10 mv/s (for J-V plots) or while applying a constant bias (for J-t plots) in a 0.1 M potassium phosphate buffer solution (ph 7.6) with or without 1 M sodium sulfite (Na2SO3) as a hole scavenger. While all measurements were carried out using a Ag/AgCl (4 M KCl) reference electrode, all results in this work were presented against the reversible hydrogen electrode (RHE) for ease of comparison with other reports that used electrolytes with different ph conditions. The conversion between potentials vs. Ag/AgCl and vs. RHE is performed using the equation below. E (vs. RHE) = E (vs. Ag/AgCl) + E Ag/AgCl (reference) V ph (E Ag/AgCl (reference) = V vs. NHE at 25 ºC) Incident photon-to-current efficiency (IPCE) at each wavelength was measured using AM 1.5G illumination from a 300 W Xe arc lamp through neutral density filters. Monochromatic light was generated by using Oriel Cornerstone 130 monochromator with a 10-nm bandpass, and the output was measured with a photodiode detector. IPCE was measured at 0.6 V vs. RHE in 0.1 M phosphate buffer (ph 7.6) containing 1 M of sodium sulfite using the same threeelectrode setup for above photocurrent measurements. Absorbed photon-to-current efficiency (APCE) was obtained by dividing the IPCE by the light harvesting efficiency (LHE) at each wavelength using the equations below. APCE (%) = IPCE (%) / LHE LHE = A(λ) (A(λ) : absorbance at wavelength λ) Capacitances were measured to obtain Mott-Schottky plots using a SP-200 potentiostat/eis (BioLogic Science Instrument). The same three-electrode cell used for the photocurrent measurement was used with a 0.1 M phosphate buffer containing 1 M sodium 4

5 sulfite (ph 7.6). All electrodes were masked with epoxy resin to expose the same geometrical area (0.05 cm 2 ). A sinusoidal modulation of 10 mv was applied at frequencies of 0.5 and 1 khz. The electron-hole separation yield (Φ sep ) for BiVO 4 was obtained by dividing the experimentally obtained photocurrent for sulfite oxidation by the calculated photocurrent assuming 100% absorbed photon-to-current efficiency (J abs ) based on the UV-vis spectrum, which was calculated to be 4.32 ma/cm 2 for the BiVO 4 electrode prepared in this study. Detailed procedures to calculate Φ sep and J abs can be found elsewhere. 1,2 References 1. Park, Y.; Kang, D.; Choi, K.-S. Marked enhancement in electron-hole separation achieved in the low bias region using electrochemically prepared Mo-doped BiVO 4 photoanodes. Phys. Chem. Chem. Phys. 2013, 16, Kim, T. W.; Choi, K.-S. Nanoporous BiVO 4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting. Science 2014, 343,

6 Table S1. Average atomic metal ratios in the BiVO 4, Bi 2 WO 6 and Bi 2 Mo 3 O 12 electrodes obtained by EDS analysis. Results obtained from multiple locations on three different samples were averaged for each compound. BiVO 4 Bi 2 WO 6 Bi 2 Mo 3 O 12 Average ratio Bi : V = 1 : 0.98 ± 0.06 Bi : W = 1.99 ± 0.40 : 1 Bi : Mo = 2 : 3.00 ± 0.12 Table S2. Average flatband potentials with standard deviations obtained from Mott-Schottky plots using five different samples for each electrode. BiVO 4 Bi 2 WO 6 Bi 2 Mo 3 O 12 E FB 0.13 ± 0.02 V ± 0.03 V 0.36 ± 0.02 V 6

7 Figure S1. (A) Top-view SEM image of Bi metal electrode deposited from an aqueous acidic medium. Side-view SEM images of (B) BiVO 4, (C) Bi 2 WO 6, and (D) Bi 2 Mo 3 O 12 electrodes. Figure S2. Tauc plots obtained from UV-vis spectra shown in Figure 3 in the main text assuming that BiVO 4 (red), Bi 2 WO 6 (blue), and Bi 2 Mo 3 O 12 (black) have direct bandgaps. 7

8 Figure S3. Separation yield (Φ sep ) vs. V plot of a nanoporous BiVO 4 electrode obtained based on the UV-vis spectrum and the J-V plot for sulfite oxidation. Figure S4. Typical Mott-Schottky plots for (A) BiVO 4, (B) Bi 2 WO 6, and (C) Bi 2 Mo 3 O 12 obtained from a 0.1 M phosphate buffer solution (ph 7.6) containing 1 M sodium sulfite (squares: 500 Hz, circles: 1 khz). 8

9 Figure S5. J-t plots of BiVO 4 obtained at 0.6 V vs. RHE for (A) sulfite and (B) water oxidation under AM 1.5G illumination (100 mw/cm 2 ) in a 0.1 M phosphate buffer solution (ph 7.6) with and without 1 M sodium sulfite. Figure S6. J-t plots of Bi 2 WO 6 obtained at 0.6 V vs. RHE for (A) sulfite and (B) water oxidation under AM 1.5G illumination (100 mw/cm 2 ) in a 0.1 M phosphate buffer solution (ph 7.6) with and without 1 M sodium sulfite. 9