Title: The Origin of Slow Carrier Transport in BiVO 4 Thin Film Photoanodes: a Time- Resolved Microwave Conductivity Study Authors: Fatwa F. Abdi 1*, Tom J. Savenije 2, Matthias M. May 3, Bernard Dam 1 and Roel van de Krol 3 Affiliations: 1 Materials for Energy Conversion and Storage (MECS), Department of Chemical Engineering, Delft University of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands 2 Opto-electronic Materials (OM), Department of Chemical Engineering, Delft University of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands 3 Helmholtz-Zentrum Berlin für Materialien und Energie, Institute for Solar Fuels, Hahn- Meitner-Platz 1, 14109 Berlin, Germany * Correspondence: f.f.abdi@tudelft.nl Supporting Information
Experimental Methods Synthesis of BiVO 4 thin films Dense thin films of BiVO 4 and 1% W-doped BiVO 4 were prepared by spray pyrolysis. A BiVO 4 precursor solution was prepared by dissolving Bi(NO 3 ) 3.5H 2 O (98%, Alfa Aesar) in acetic acid (98%, Sigma Aldrich) and VO(AcAc) 2 (99%, Alfa Aesar) in absolute ethanol (Sigma Aldrich). The Bi solution was then added to the V solution, and the mixture was diluted to 4 mm with excess ethanol. In the case of 1% W-doped BiVO 4 films, 1 at% of W(OC 2 H 5 ) 6 (5% w/v in C 2 H 5 OH, 99.8% Alfa Aesar) were added to the precursor solution of Bi and V, resulting in a mixture containing Bi:V+W in a 1:1 ratio. 1 vol% of triethyl orthoformate (TEOF, 98%, Fluka Analytical) was added to the mixture to prevent undesired hydrolysis in the presence of small amounts of water. Bare quartz substrates (S1-UV fused silica, Esco Products) were used for the TRMC study, whereas F-doped SnO 2 glass (15 /sq., TEC-15, Hartford Glass Co.) was used for the photocurrent measurements. Prior to the deposition, the substrates were cleaned in successive 15 min. ultrasonic rinsing steps in acetone and isopropanol. The substrates were placed on a heating plate that was set to 450 o C during deposition. The spray nozzle (Quickmist Air Atomizing Spray) was placed 20 cm above the heating plate and driven by an overpressure of 0.6 bar of nitrogen gas. A pulsed deposition mode was used, with one spray cycle consisting of 5 seconds of spray time and 55 seconds of delay time to allow solvent evaporation. Each deposition was done for 200 cycles, with a deposition rate of ~1 nm per cycle. The deposition rate was determined in previous reports on these spray-deposited BiVO 4 films using cross-section scanning electron microscopy. 1,2 The thickness of 200 nm was confirmed by measuring the optical density and using the
known optical absorption coefficient of BiVO 4. 5 After the deposition, both samples were annealed for 2 hours at 450 o C in air to further improve the crystallinity. BiVO 4 photoanodes for the AM1.5 photocurrent-voltage measurement were prepared with the same spray pyrolysis recipe; except that ~80 nm SnO 2 layer was spray deposited prior to BiVO4 deposition according to the previously reported procedure. 1,2 TRMC technique For the TRMC measurements, the samples were mounted in a microwave cavity cell and placed within the setup described elsewhere. 3,4 The X-band (8.2-12.4 GHz) microwaves are generated using a Gunn diode. The presented experiments were carried out at 8.44 and 8.43 GHz for undoped and 1% W-doped BiVO 4, respectively; these frequencies were found to be the resonant frequencies of the loaded cavity. During the measurements, a change in the microwave power reflected by the cavity upon nanosecond pulsed laser excitation (3.5 ns FWHM), ΔP/P, was monitored and correlated to the photoinduced change in the conductance of the sample, ΔG, by P () t K G () t (1) P where K is the sensitivity factor derived from the resonance characteristics of the cavity and the dielectric properties of the medium. The change in conductance is then associated with the product of the ratio of mobile charge carriers normalized to the number of absorbed photons (f) and the sum of their mobility ( µ) according to G (2) I ef 0 A
where I 0 is the incident intensity per pulse, e is the elementary charge, β is a factor related to the inner dimensions of the waveguide, and F A is the fraction of incident photons attenuated within the sample. Other experimental details Photoelectrochemical characterization was carried out using a three-electrode configuration, with an aqueous 0.1 M KPi solution (ph~7) as the electrolyte. The potential of the working electrode was controlled by a potentiostat (EG&G PAR 283). A coiled Pt wire and an Ag/AgCl electrode (XR300, saturated KCl and AgCl solution, Radiometer Analytical) were used as the counter and reference electrodes, respectively. White light photocurrent measurements were performed under simulated AM1.5 solar illumination (100 mw/cm 2 ) with a Newport Sol3A Class AAA solar simulator (type 94023A-SR3). Structural analysis was performed with a Bruker D8 Advance X-ray diffractometer (Co-K α, λ = 0.178897 nm) equipped with a LynxEye detector in a Bragg- Brentano configuration. Scanning electron micrographs were taken with JEOL JSM 6500F Scanning Electron Microscope. The UV-vis absorption was measured inside an integrating sphere using a Perkin Elmer Lambda 900 spectrometer. X-ray photoelectron spectroscopy was carried out with a monochromatic Al K source (Specs Focus 500) and a hemispherical analyzer (Specs Phoibos 100) in a UHV system with a base pressure in the low 10-8 mbar range.
80 Intensity (cps) 60 40 20 * * 0 20 40 60 * 2 (degree) 1%W-BiVO 4 on quartz BiVO 4 on quartz BiVO 4 on FTO * : FTO peak Figure S1. X-ray diffraction spectra for undoped BiVO 4 thin films on fluorine-doped tin oxide (FTO), undoped and 1% W-doped BiVO 4 on quartz. The dashed vertical lines represent the location of monoclinic BiVO 4 peaks, and the FTO peaks are indicated with star (*).
100 80 A (%) 60 40 20 0 1% W-doped undoped 400 500 600 (nm) Figure S2. Optical absorption of undoped and 1% W-doped BiVO 4 on quartz. Inset shows the photographs of (left to right) undoped BiVO 4 on quartz, 1% W-doped BiVO 4 on quartz, and blank quartz substrate.
Figure S3. Scanning electron micrographs of undoped and 1% W-doped BiVO 4 on quartz.
Figure S4. XPS spectrum of a 1% tungsten-doped BiVO 4 film (blue) and a 1% W-doped film covered by an equally-thick undoped BiVO 4 film (red). The samples were measured as-received, which explains the presence of adventitious carbon at the surface. No traces of other impurities were found. The large Sn-3d peaks indicate that part of the underlying SnO 2 layer is exposed, suggesting that the surface coverage is not perfect. The tungsten dopant (main peaks expected at 243 and 256 ev) is not observed due to the toolow concentration (1%).
1.0 0.8 0.6 front back I/I 0 0.4 0.2 0.0 0 50 100 150 200 surface x (nm) Figure S5. Absorption profile (λ = 300 nm) in BiVO 4 as a function of location within interface the film under front- and back-side illumination, calculated based on the reported absorption coefficient of BiVO 4. 5
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