Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
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Enabling Solar Water Oxidation by BiVO4 Photoanodes in Basic Media Dongho Lee,† Alexander Kvit,‡ and Kyoung-Shin Choi*,† †
Department of Chemistry and ‡Materials Science Center, University of WisconsinMadison, Madison, Wisconsin 53706, United States
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ABSTRACT: Titanium dioxide (TiO2) deposited by atomic layer deposition (ALD) has been the most commonly used protection layer to enhance chemical and photoelectrochemical stabilities of photoelectrodes. In this study, we report a new electrochemical deposition method that can place a thin, conformal TiO2 coating layer on a photoelectrode. This method takes 11). Enabling the operation of BiVO4 photoanodes in strongly basic conditions can be advantageous as water oxidation kinetics of typical oxidebased OECs improve considerably in strongly basic conditions.17,18 Also, operating in basic conditions makes it possible to pair a BiVO4 photoanode with photocathodes composed of a p-type semiconductor and a hydrogen evolution catalyst that perform optimally in strongly basic conditions.19,20 There have been a few studies on extending the stability of BiVO4-based photoanodes to basic media using a protection layer that prevents direct contact between the BiVO4 electrode and a strongly alkaline solution.21−23 Titanium dioxide (TiO2) has been the most commonly used compound as a protection layer to enhance chemical and photoelectrochemical stabilities of photoelectrodes owing to its chemical and electrochemical
N-type bismuth vanadate (BiVO4) has received great attention as one of the most promising metal-oxide-based photoanodes for a water splitting photoelectrochemical cell (PEC).1−8 The conduction band minimum of BiVO4 is located close to 0 V vs reversible hydrogen electrode (RHE), making it possible to achieve a photovoltage gain for water oxidation greater than 1 V.5 In addition, BiVO4 can utilize a significant portion of visible light and does not suffer from extremely fast bulk or surface electron−hole recombination unlike typical oxidebased photoelectrodes. These features together enabled BiVO4 to generate a photocurrent density for water oxidation greater than 3 mA/cm2 at a potential as low as 0.5 V vs RHE when coupled with proper oxygen evolution catalysts (OECs).6 In addition, BiVO4 is one of the most stable photoanodes when prepared with high crystallinity and high purity (i.e., absence of easily soluble, amorphous V-rich impurities at the grain boundaries and at the back contact). For example, immersion of BiVO4 in a pH 9 solution over 50 days showed no sign of chemical dissolution.6 Furthermore, its photostability for solar water splitting when paired with proper OECs was also demonstrated at pH 9 for more than several hundred hours.6,7 © XXXX American Chemical Society
Received: April 5, 2018 Revised: June 19, 2018
A
DOI: 10.1021/acs.chemmater.8b01405 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials stability in a wide range of pHs.24−27 The protection layer should cover the surface of the photoelectrode conformally and completely while being thin enough not to interfere with the transport of charge carriers from the photoelectrode to the electrolyte. To satisfy these requirements, the TiO2 protection layer has almost always been deposited by atomic layer deposition (ALD).28−30 However, considering the slow deposition time and economic viability of ALD for largescale deposition, it would be highly beneficial to develop a simple, facile, and inexpensive solution-based method that can produce a TiO2 coating whose quality is as good as, or even superior to, an ALD-deposited TiO2 coating. The purpose of this study is 2-fold. The first is to report a new electrochemical deposition method that can place a thin, conformal TiO2 layer on a nanoporous BiVO4 electrode. This method takes 18 MΩ·cm). Electrodeposition was carried out using a standard three-electrode cell where a nanoporous BiVO4 electrode was used as the working electrode (WE), a Ag/AgCl (4 M KCl) electrode was used as the reference electrode (RE), and a platinum electrode was used as the counter electrode (CE). The platinum CE was prepared by depositing a 100 nm platinum layer on top of a 30 nm titanium adhesion layer on a cleaned glass slide by e-beam evaporation. A VMP2 multichannel potentiostat (Princeton Applied Research) was used to perform the electrodeposition. The potentiostatic deposition was carried out at −0.45 V vs Ag/AgCl (4 M KCl) at room temperature with various amounts of total charge passed to control film thicknesses. Asdeposited films were then rinsed in a water/ethanol = 3:1 mixture and dried with a gentle stream of air. Finally, the films were annealed at 350 °C for 1 h in air (ramping rate: 2 °C/min). We note that there is nothing unique about the BiVO4 surface that allowed for the B
DOI: 10.1021/acs.chemmater.8b01405 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials All electrodes were tested in an undivided three-electrode cell using a BioLogic SP-200 potentiostat. A Pt CE and a Ag/AgCl (4 M KCl) RE were used to record the potential. The potential vs the RE was converted to the potential vs reversible hydrogen electrode (RHE) by the equation below.
BiVO4 layer remained intact. The as-deposited hydrated TiO2 layer was converted to a TiO2 layer by annealing at 350 °C for 1 h in air.34 The annealing step to dehydrate the TiO2 layer was critical to use the TiO2 layer as the protection layer because the as-deposited hydrated TiO2 layer was not stable in basic solutions.
E (vs RHE) = E (vs Ag/AgCl) + EAg/AgCl (reference)
[TiO(C2O4 )2 ]2 − + 2OH−
+ 0.0591 V × pH
→ TiO(OH)2 + 2NH4 + + 2C2O4 2 −
(EAg/AgCl (reference) = 0.1976 V vs NHE at 25°C)
In this study, nanoporous BiVO4 electrodes possessing intricate surface morphologies (Figure 1a) were used as the
A 300 W Xe arc lamp (Ushio, UXL-302-O) was used to generate light, which was passed through a water (IR) filter (Newport), neutral density filters, and an AM 1.5 G filter to reach a fiber optic cable and finally illuminated on the BiVO4 film (back-side illumination). The light intensity at the back surface of the FTO was calibrated to 100 mW/cm2 using a NREL-certified Si reference solar cell (Photo Emission Tech, Inc.). Photoelectrochemical measurements were taken in 50 mL 0.1 M phosphate buffer (pH 12) and 0.1 M KOH (pH 13) solutions. For sulfite oxidation, 1 M Na2SO3 was added to both solutions. J−V plots were obtained by sweeping the potential from the open-circuit potential of the photoanode to the positive direction with a scan rate of 10 mV/s. J−t plots were obtained at 0.6 V vs RHE. O2 gas generated during water oxidation was detected using an Ocean Optics fluorescence-based oxygen sensor (Neofox, FOSPOR-R). For O2 detection, a custom-built, airtight, twocompartment cell divided by a frit was used. The cathode compartment held a Pt CE while the anode compartment held the photoanode (i.e., BiVO4/TiO2/FeOOH/NiOOH) along with a Ag/ AgCl (3 M NaCl) RE. The oxygen sensor measured the O2 content in the headspace as mole %. This was converted to μmol after first adjusting for the O2 dissolved in solution using Henry’s Law. The Faradaic efficiency (FE) for O2 production was calculated by the equation below, FE (%) =
4 × nO2 (mol) × F (C mol−1) charge passed through WE (C)
(2)
× 100 Figure 1. Top-view SEM images of (a) BiVO4 and (b) BiVO4/TiO2 electrodes; high-resolution TEM images of (c) BiVO4 and (d) BiVO4/TiO2; insets in d are FFT images of the TiO2 layer (top) and BiVO4 (bottom).
where nO2 is moles of evolved O2 gas and F is the Faraday constant (96485.33 C mol−1).
3. RESULTS AND DISCUSSION The key requirement for electrodeposition of a TiO2 layer on a BiVO4 electrode is that BiVO4 used as the WE must be inert to the electrodeposition condition. We found that previously reported electrodeposition methods for TiO 2 are not compatible with BiVO4 because either the plating solution is strongly acidic (pH 2−2.5),32 which dissolves BiVO4, or the required deposition potential is too negative, which can reduce Bi3+ in BiVO4 to Bi0 during TiO2 deposition.33−36 In this study, a mildly acidic plating solution (pH ≈ 4.5) was used where Ti4+ ions were solubilized by oxalate ions in the form of [TiO(C2O4)2]2−.33,34 BiVO4 does not readily dissolve in this plating solution. We employed cathodic deposition where the reduction of p-benzoquinone increases the local pH at the WE (eq 1) and induces the precipitation of the Ti4+ ions as an
WE, and TiO2 layers with various thicknesses were deposited on the BiVO4 electrode. The thickness of the TiO2 layer could be easily tuned by controlling the amount of charge passed during deposition. The goal was to deposit a TiO2 layer that is thick enough to completely cover the BiVO4 surface to prevent the dissolution of BiVO4 in basic media but thin enough not to interfere considerably with hole transport from BiVO4 to the electrolyte. We identified that a ∼6 nm thick TiO2 layer deposited on the BiVO4 electrode, which required passing 0.1 C/cm2 and took only ∼35−45 s, was optimum to satisfy this requirement. As the thickness of the TiO2 layer increased further, a considerable decrease in photocurrent generation by BiVO4 resulted (Figure S1). Therefore, the rest of this study focuses on investigating the properties and stabilities of the BiVO4/TiO2 electrode with a ∼6 nm thick TiO2 layer in comparison with the pristine BiVO4 electrode. The SEM image of the BiVO4/TiO2 electrode is shown in Figure 1b. The TiO2 layer on BiVO4 is too thin to show any noticeable change in the SEM image when compared with that of the BiVO4 electrode. However, when the BiVO4 and BiVO4/TiO2 samples were scraped from the substrate and examined by high-resolution transmission electron microscopy (HRTEM), the presence of a ∼6 nm thick TiO2 layer that is uniform and conformal was revealed (Figure 1c and d). The fast Fourier transform (FFT) images of the BiVO4 and TiO2
amorphous hydrated TiO2 film on the WE (eq 2). The potential required for the reduction of p-benzoquinone is more positive than the potential that can reduce Bi3+ in BiVO4. As a result, a TiO2 layer could be deposited while the underlying C
DOI: 10.1021/acs.chemmater.8b01405 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 2. SEM and XRD of (a) BiVO4 and (b) BiVO4/TiO2 electrodes before and after 30 days of immersion in 0.1 M phosphate buffer (pH 12) and (c) BiVO4 and (d) BiVO4/TiO2 electrodes before and after 30 days of immersion in 0.1 M KOH (pH 13); first row, SEM images before immersion; second row, SEM images after immersion; third row, changes in XRD.
study had a highly intricate nanoporous morphology. The prevention of dissolution for 30 days confirmed the uniformity of the 6 nm thick TiO2 layer over the entire BiVO4 electrode. This result suggests that this solution-based electrodeposition method can offer an inexpensive, simple, yet fast means to deposit a TiO2 protection layer of an excellent quality. When the pristine BiVO4 electrode was immersed in an even harsher pH 13 solution, a decrease in the XRD peaks was noticed in 7 days, which became more severe in 30 days (Figure 2c). The SEM image after 30 days showed a considerable loss of BiVO4, revealing the surface of the FTO substrate. In contrast, the BiVO4/TiO2 films showed no noticeable difference in SEM images even after 30 days of immersion (Figure 2d). Also, XRD patterns did not show any change up to 7 days. However, a slight decrease in the XRD peaks became noticeable in 30 days. This suggests that, although the TiO2 layer covered the BiVO4 electrode almost uniformly, there existed pinholes, and the dissolution of BiVO4 through these pinholes was not negligible in the harsher pH 13 condition. Nonetheless, the quality of the 6 nm thick TiO2 layer in suppressing the dissolution of BiVO4 in pH 13 solution over 30 days is remarkable. We believe that the pinholes in the TiO 2 layer may be completely eliminated by future optimizations of the electrodeposition conditions. After confirming that the TiO2 layer can effectively suppress chemical dissolution of BiVO4 at pH 12, the effect of the TiO2 layer on photoelectrochemical properties and stability of BiVO4 was investigated in pH 12 phosphate buffer. The J−V and J−t plots of BiVO4 and BiVO4/TiO2 electrodes were first compared for photoelectrochemical sulfite oxidation. Because sulfite oxidation has fast kinetics,5,6,12,37,38 sulfite oxidation can quickly consume surface-reaching holes, preventing the accumulation of holes at the electrode surface. This gives a clear contrast to the case of water oxidation, the rate of which
regions in the bright-field image show that, while BiVO4 is crystalline, TiO2 is amorphous. The XRD pattern of the BiVO4/TiO2 electrode also did not show any crystalline peaks generated by the TiO2 layer (Figure S2). Before examining the effect of the TiO2 layer on the photoelectrochemical properties and photostability of BiVO4, we first investigated the effect of the TiO2 coating on the chemical stability of BiVO4 in two basic media, 0.1 M phosphate buffer (pH 12) and 0.1 M KOH (pH 13) solutions. The stability comparison of the BiVO4 and BiVO4/TiO2 electrodes in basic media can provide the most convincing evidence for the quality of the TiO2 coating throughout the entire electrode (1 cm × 1.2−1.3 cm); even if the TiO2 coating looks uniform under TEM, if it is not completely uniform for the entire electrode, it would fail to suppress the dissolution of BiVO4. In addition, evaluating the effect of the TiO2 layer on chemical stability separately from that on photoelectrochemical stability of BiVO4 makes it possible to reveal any relationship between chemical stability and photoelectrochemical stability. For the chemical stability test, the BiVO4 and BiVO4/TiO2 electrodes were immersed in the aforementioned solutions without stirring for 30 days in the dark, and the changes of these electrodes were monitored by the collection of SEM images and XRD patterns. The SEM images of the BiVO4 electrode after 30 days of immersion in pH 12 phosphate buffer showed a clear indication that chemical dissolution had occurred (Figure 2a). The loss of BiVO4 by dissolution could also be easily confirmed by the decrease in intensity of the major Bragg peaks of BiVO4 in the XRD pattern. In contrast, the SEM and XRD of the BiVO4/TiO2 electrode show no noticeable change after 30 days of immersion (Figure 2b). This result is remarkable considering that the TiO2 layer was deposited in
