Article pubs.acs.org/JPCC
Improved Stability of Polycrystalline Bismuth Vanadate Photoanodes by Use of Dual-Layer Thin TiO2/Ni Coatings Matthew T. McDowell,†,‡ Michael F. Lichterman,†,‡ Joshua M. Spurgeon,†,‡ Shu Hu,†,‡ Ian D. Sharp,⊥ Bruce S. Brunschwig,§ and Nathan S. Lewis*,†,‡,§,∥ †
Division of Chemistry and Chemical Engineering, ‡Joint Center for Artificial Photosynthesis, §Beckman Institute, and ∥Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States ⊥ Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Ultrathin dual layers of TiO2 and Ni have been used to stabilize polycrystalline BiVO4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte. Conformal, amorphous TiO2 layers were deposited on BiVO4 thin films by atomic-layer deposition, with Ni deposited onto the TiO2 by sputtering. Under simulated air mass 1.5 illumination, the dual-layer coating extended the lifetime of the BiVO4 photoanodes during photoelectrochemical water oxidation from minutes, for bare BiVO4, to hours, for the modified electrodes. X-ray photoelectron spectroscopy showed that these layers imparted chemical stability to the semiconductor/electrolyte interface. Transmission electron microscopy revealed the structure and morphology of the polycrystalline BiVO4 film as well as of the thin coating layers. This work demonstrates that protection schemes based on ultrathin corrosion-resistant overlayers can be applied beneficially to polycrystalline photoanode materials under conditions relevant to efficient solar-driven water-splitting systems.
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INTRODUCTION The conversion of solar energy into storable chemical fuels via the electrolysis of water is a promising route to deal with the locally intermittent nature of terrestrial solar radiation.1−4 An elegant and attractive approach to solar-driven water splitting involves a photoelectrochemical cell that directly utilizes charge carriers generated within photoactive semiconductors to oxidize and reduce water at semiconductor/liquid interfaces.2,5 Because of better utilization of the solar spectrum, a cell comprising a separate photoanode and photocathode, arranged in series with respect to the incident light, can yield higher efficiency than a device based on a single semiconductor. Accordingly, several candidate materials for photoanodes and photocathodes are currently being studied.6−14 Many semiconductors of interest, including technologically important materials such as Si and GaAs, photocorrode in aqueous solution. Furthermore, the rate of corrosion is usually accelerated in the alkaline or acidic electrolytes in which an intrinsically safe, efficient, solar-driven water-splitting system can be constructed.15,16 Significant research efforts have therefore been dedicated to interfacing candidate semiconductors with protective coatings and catalytic layers to promote the desired reactions relative to the photocorrosion reaction.6,17−23 A number of n-type oxide semiconductors, such as BiVO4, WO3, and Fe2O3, have been studied as photoanodes for the oxygen-evolution reaction (OER).9,24−27 Oxides are generally more stable than other types of photoanode materials, and oxides can be fabricated using inexpensive and scalable methods. Although most known oxides have band gaps that © 2014 American Chemical Society
are larger than optimal for a tandem device with a Si photocathode,28 valuable insight can be gained by studying the influence of catalytic and protective layers on the photoelectrochemical performance of known oxide materials. BiVO4 has a lower band gap energy (Eg = 2.4 eV), and thus absorbs more visible light than many other metal oxides (Eg ≈ 3 eV), and has therefore received much research attention.26,29−36 Furthermore, the potential of the conductionband edge (Ecb) of BiVO4 is more negative with respect to the formal potential for water oxidation, E°′(O2/H2O), than that of most other metal oxides, potentially allowing for large photovoltages from such semiconductor/liquid interfaces.26 BiVO4 has been interfaced with various OER catalysts to improve photoanode performance.21,30,31,33,34,36−38 For example, nanoporous BiVO4 combined with an FeOOH/NiOOH catalyst has been reported to exhibit an open-circuit potential of 0.2 V vs RHE as well as relatively high current densities of 3−4 mA cm−2 at 1.23 V vs RHE, under neutral pH conditions.39 BiVO4 is prone to photocorrosion, which can be mitigated at near-neutral pH values (pH = 5−9) by integration of appropriate catalyst layers. At high pH, photocorrosion is more severe. Nanometer-thick CoOx films deposited on BiVO4 by ALD have recently been shown to provide catalytic activity as well as stability for tens of minutes at pH = 13.21 Received: June 20, 2014 Revised: August 6, 2014 Published: August 7, 2014 19618
dx.doi.org/10.1021/jp506133y | J. Phys. Chem. C 2014, 118, 19618−19624
The Journal of Physical Chemistry C
Article
sealed to the glass tube using epoxy (Loctite Hysol 9460). The Cu wire, Ag paint, and FTO were also covered with epoxy. The exposed active area of each electrode was measured using an optical scanner (Epson Perfection V370 in conjunction with ImageJ software), and the active areas ranged between 0.5 and 1 cm2. Photoelectrochemistry. Photoelectrochemical measurements were performed using a three-necked, round-bottom, 50 mL flask that was equipped with a planar quartz window to facilitate illumination of the photoelectrode. A three-electrode configuration was used, with the oxide photoanode as the working electrode, a Pt gauze (Aldrich) counter electrode separated from the main compartment by a glass frit, and a Ag/ AgCl (1 M KCl) reference electrode (CH Instruments). In most experiments, the electrolyte was 0.10 M KOH(aq) (pH = 13) that was prepared by mixing KOH pellets (Macron Chemicals, >88%) with H2O (18 MΩ·cm resistivity), but in certain experiments the electrolyte was a pH = 10.4 potassium borate buffer made by mixing 200 mL of 1.0 M KOH(aq) solution with 100 mL of 2.0 M boric acid (Sigma-Aldrich, 99.5%). A 150 W xenon lamp (Newport 6255) coupled with an AM1.5 global filter (Newport 81094) and a quartz diffuser was utilized for illumination of the working electrode through the quartz window of the three-necked flask. A Si photodiode was placed in the electrolyte in the cell before testing, and it indicated that the light intensity was ∼135 mW cm−2 at the position of the photoelectrode. Although this light intensity will lead to larger photocurrents than are obtained under 100 mW cm−2 of AM1.5G illumination, this increased intensity was useful for accelerated stability testing because higher light intensities will generally lead to more rapid photocorrosion of insufficiently protected electrodes. Cyclic voltammetry was performed by sweeping the potential of the working electrode at a rate of either 25 or 40 mV s−1 using a Biologic SP-200 potentiostat. Oxygen Measurements. A fluorescence-based oxygen sensor (Neofox, Ocean Optics) was used to measure the amount of O2 produced by the BiVO4 electrodes. The probe was held within a glass tube that had an O2-sensitive fluorescent patch on the outer tip. This tube was then placed in the electrolyte. The sensor was calibrated in an air-saturated 0.10 M KOH(aq) solution using an O2 solubility of 7.7 mg L−1 and 0.21 atm of O2 partial pressure. The BiVO4 working electrode, Ag/AgCl (1 M KCl(aq)) reference, fritted Pt gauze counter electrode, and O2 probe were sealed in a four-necked flask with O-ring thermometer adapters, and the cell was completely filled with electrolyte. The cell was then purged with ultrahigh purity Ar for at least 30 min. For measurement under illumination, the cell was sealed and the working electrode was first held at open circuit for 10 min, then at the formal potential for water oxidation to O2, E°′(O2/H2O) (1.23 V vs RHE) for 30 min, and finally at open circuit for another 10 min. The 10 min periods before and after the oxygen-production step were used to calculate the O2 leak rate of the cell. Materials Characterization. Scanning-electron microscopy (SEM) was performed with a Nova NanoSEM 450 (FEI) with a 15 kV accelerating voltage. Transmission electron microscopy (TEM) imaging was performed with an FEI Tecnai F30 with a 300 kV accelerating voltage. Scanning TEM (STEM) in conjunction with energy-dispersive spectroscopy (EDS) was used for elemental analysis in the TEM. TEM samples were fabricated with conventional cross-section thinning techniques, including manual polishing, dimpling,
In contrast to work on oxides, a variety of protection schemes have been investigated for single-crystal Si photoanodes in alkaline or acidic solutions.17−20,22,23,40 Some of these protection schemes rely on an ultrathin layer of an otherwise insulating, corrosion-resistant oxide to protect the surface of the photoanode while allowing tunneling of holes to the electrolyte.17,20 Recently, thick (up to 143 nm) amorphous TiO2 films deposited by atomic-layer deposition (ALD), in conjunction with Ni-based OER electrocatalysts, have been shown to conduct anodic current and to protect n-Si for over 100 h under >100 mW cm−2 of illumination at pH = 14.22 Single-crystalline photoanodes have atomically flat surfaces that readily allow deposition of the ultrathin conformal films needed to produce tunnel barriers. We explore herein whether a protection scheme based on ultrathin corrosion-resistant oxide layers can be implemented in conjunction with a polycrystalline oxide photoanode. Specifically, we have evaluated the effectiveness of an ultrathin amorphous TiO2 coating, combined with Ni- or Co-based OER catalysts, for preventing the corrosion of polycrystalline BiVO4 photoanodes at high pH. We have also investigated the chemistry, structure, and degradation mechanisms of the protective layers on the electrode surface.
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METHODS Fabrication of BiVO4 Thin Films. BiVO4 films were fabricated on fluorine-doped tin oxide (FTO)-coated glass (TEC-15, Hartford) via a previously reported spin-coating method.31 A Bi(NO3)3 solution was made by mixing 728 mg of Bi(NO3)3·5H2O (Sigma-Aldrich, 99.99%) in 7.5 mL of 2,4pentanedione, and a VO(acac)2 solution was made by mixing 398 mg of vanadyl acetylacetonate (Sigma-Aldrich, 98%) in 50 mL of 2,4-pentanedione. These two solutions were then combined and mixed thoroughly. Using a spin coater (Laurell Technologies), this combined solution was then cast onto an FTO-coated glass slide that had been partially masked with tape. Each coating cycle consisted of covering the exposed FTO with solution and spinning for 6 s at 1000 rpm two separate times. After each coating cycle, the tape was removed and the slide was annealed in a muffle furnace in air at 500 °C for 10 min. This process was repeated nine times, and the samples were then annealed at 500 °C for 2 h in air. ALD of TiO2. TiO2 films were deposited using a Cambridge Nanotech S200 ALD system. Each ALD cycle consisted of a 0.015 s pulse of distilled, deionized H2O with a resistivity of 18 MΩ·cm, followed by a 0.10 s pulse of tetrakis(dimethylamido)titanium (TDMAT, Sigma-Aldrich, 99.999%). Between each pulse, a 15 s purge was performed under a 20 sccm flow of N2(g). The substrate temperature in the deposition chamber was 150 °C, the TDMAT precursor was heated to 75 °C, and the H2O was maintained at room temperature. Sputtering of Ni and Co. Metals were sputtered at room temperature using an RF magnetron sputtering system (AJA Orion) with a power set to 140 W, an Ar flow rate of 17 sccm, and an Ar pressure of 8.5 mTorr. The Ni and Co targets were purchased from ACI Alloys and were 99.95% pure. Sputtering times ranged from 12 to 30 s. Fabrication of Electrodes. Photoelectrodes were made by sectioning the FTO-coated glass covered with BiVO4 into ∼2.5 cm × 0.5 cm pieces. A Cu wire (Consolidated Electronic Wire and Cable) was electrically contacted with Ag paint to the FTO that was exposed on each piece (SPI, Inc.). The Cu wire was then threaded through a glass tube, and the electrode was 19619
dx.doi.org/10.1021/jp506133y | J. Phys. Chem. C 2014, 118, 19618−19624
The Journal of Physical Chemistry C
Article
Figure 1. Structure and morphology of BiVO4 films and TiO2/Ni dual-layer coatings. (a) Top-view SEM image of a spin-cast BiVO4 film on FTO. (b) Cross-sectional TEM image of a BiVO4 film on FTO. The polycrystalline BiVO4 film is about 50 nm thick. (c) High-resolution cross-sectional TEM image of a TiO2/Ni dual-layer coating on FTO (labeled F:SnO2). The {110} lattice fringes of the tetragonal SnO2 crystal structure are visible in the lower part of the image. The TiO2 layer, which was deposited with 30 ALD cycles, was amorphous. The Ni layer was sputtered for 4 min (longer than used for electrochemical tests) to clearly show the TiO2 interfacial layer. The EDS spectra to the right of the image were collected in STEM mode by placing the converged beam on each portion of the dual-layer structure. The EDS spectrum collected from the TiO2 layer shows signal from Ti as well as Ni and Sn because the electron beam was wider than the thickness of the TiO2 layer.
Figure 2. Photoelectrochemical characterization of BiVO4/TiO2/Ni photoanodes. (a) Cyclic voltammetry of a BiVO4/TiO2/Ni electrode (black) and a bare BiVO4 electrode (red) in 0.10 M KOH(aq) (pH 13) under 135 mW cm−2 of simulated solar illumination. The BiVO4/TiO2/Ni electrode was coated with 30 ALD cycles of TiO2 (∼1 nm) and
