Article pubs.acs.org/cm
All First Row Transition Metal Oxide Photoanode for Water Splitting Based on Cu3V2O8 Jason A. Seabold and Nathan R. Neale* Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: Identification of viable photoanode candidates for use in a tandem photoelectrochemical water splitting system remains a significant challenge to the realization of efficient solar-driven hydrogen production. Herein, copper vanadate (Cu3V2O8) is introduced as a new, all first row transition metal oxide with a band gap of near 2 eV that makes it suitable as a photoanode candidate in such a solar water splitting system. In this work, many of the key physical and photoelectrochemical properties of Cu3V2O8 are established including band gap, doping type, ability to extrinsically dope, flat-band potential, band positions, electron diffusion length, chemical stability, and O2 evolution faradaic efficiency. This study provides a key initial step in identifying the features that can lead to a complete understanding of this new ternary metal oxide and motivate discovery of related photoanodes comprised of multicomponent oxides.
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Eg = 1.9−2.2 eV).18−20 Researchers have been making significant progress on addressing the challenges presented by this binary oxide semiconductor such as a short carrier diffusion length, significant recombination, and indirect absorption.19 In parallel with these efforts on α-Fe2O3, work to discover new photoanodes is now delving into more complex oxides that could exhibit suitable properties for water oxidation such as a band gap ≤2 eV and a long majority carrier lifetime. Indeed, over the past several years a few multicomponent oxides with band gaps near 2 eV have been identified toward these ends. The first candidate, spinel ZnFe2O4, has properties similar to Fe2O3 including a band gap of 2 eV.21 Second, the intercalation of molecular dinitrogen into WO3 results in a N2:WO3 clathrate with a significantly reduced band gap (1.9 eV) compared with the parent WO3 (2.6 eV).22 Finally, CuW1−xMoxO4 (0.4 ≤ x ≤ 0.65), an alloy of CuWO4 and a thermodynamically unstable CuMoO4 phase, exhibits an absorption onset (2.0 eV) lower than that of CuWO4 (2.3 eV).23 Further advances in our understanding of how to control band structure in multicomponent oxides will come from the discovery of additional candidates. It is in this context that copper vanadate (Cu3V2O8), an ntype ternary oxide with a band gap of ∼2 eV, is introduced as another potential photoanode candidate for a tandem PEC water splitting system. Copper vanadate is a naturally occurring mineral known as pseudolyonsite24 and has been given cursory examination in Li-ion battery25,26 and dye degradation applications26 but has never been considered for water splitting. In this work, many of the key physical and PEC properties of Cu3V2O8 are established including band gap, doping type,
INTRODUCTION The production of hydrogen via photoelectrochemical (PEC) water splitting is extremely challenging for a single junction semiconductor due to very exacting requirements including adequate band positions (thermodynamics), substantial overpotentials for the proton reduction and oxygen evolution reactions (kinetics), and a sufficiently small band gap to provide absorption overlap with the terrestrial solar spectrum.1−5 Given these rigid requirements and the fact that greater solar-tohydrogen conversion efficiencies can be achieved using a tandem device, much research is being devoted to photoabsorbers that can accomplish either the proton reduction or water oxidation half-reactions for use in a tandem configuration.2,3,5−10 The optimal band gap combination in a tandem device utilizes a top electrode with a band gap of 1.6−1.8 eV, and a bottom electrode with a band gap near 1.0 eV, resulting in a solar-to-hydrogen efficiency of >25%.4,11−13 The theoretical efficiency drops off with a larger gap top absorber, and thus configurations with a top electrode of 2 eV would achieve maximum efficiencies near 18.5%.4,12 Silicon photocathodes, with a band gap of 1.1 eV, have recently advanced as some of the best candidates for the bottom electrode,9,14−16 leaving wider band gap photoanodes as the necessary top electrode in these designs. Thus, discovery and development of compatible photoanodes is currently one of the biggest challenges to practicable water splitting based on a dual photoelectrochemical system.3,5,9 Metal oxide semiconductors are promising for photoanodes because of their relative stability to oxidative photocorrosion and their low-cost fabrication.3,5,17 After extensive investigation, however, only one binary oxide (hematite, α-Fe2O3) has been identified as a photoanode with a suitable band gap (α-Fe2O3 © 2015 American Chemical Society
Received: November 24, 2014 Published: January 12, 2015 1005
DOI: 10.1021/cm504327f Chem. Mater. 2015, 27, 1005−1013
Article
Chemistry of Materials ability to extrinsically dope, flat-band potential, band positions, electron diffusion length, chemical stability, and O2 evolution faradaic efficiency. These studies are a key initial step in identifying the features that can lead to a complete understanding of Cu3V2O8 and discovery of related photoanodes comprised of multicomponent oxides suitable for use in a tandem water splitting system.
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(FESEM) operated at 5 kV and a working distance of 4 mm. Energy dispersive X-ray spectroscopy (EDX) also made use of the JEOL JSM 7000F FESEM, with a 15 kV accelerating voltage and a 10 mm working distance. UV−vis absorption data were obtained using a Cary 6000i UV−vis-NIR with an integrating sphere attachment. Absorbance was calculated as A = − log[(%T + %R )/100]
(1)
Chemical stability tests involved soaking 1 cm2 portions of 500 nm thick Cu3V2O8 films on FTO in the dark for 48 h in sealed vials with 20 mL of aqueous solutions containing various electrolytes relevant to PEC testing: pH 6.2 sodium nitrate, pH 6.5 sodium sulfate, pH 7.0 sodium phosphate, pH 9.2 sodium borate, pH 10.3 sodium carbonate, pH 12.3 sodium phosphate, pH 12.3 sodium hydroxide, and pH 13.6 sodium hydroxide. All solutions were prepared with concentrations of 0.1 M, except the sodium hydroxide solutions (NaOH concentration ≈10[pH − 14]). All solutions were adjusted to the desired pH with NaOH except sodium nitrate and sulfate, which were unmodified. Ultrapure deionized water (Millipore, Milli-Q, 18.2 MΩ cm) was used to prepare solutions for all chemical stability, electrochemical, and photoelectrochemical tests. Electrochemical Measurements. Mott−Schottky analysis utilized a flat-faced glass cell with a three-electrode configuration. The 2.0 cm2 Cu3V2O8 films were swept from 0.4 to 0.0 V at 5 mV/s vs a Ag/ AgCl (saturated KCl) reference electrode with a Pt foil counter electrode (3 cm2) positioned approximately 1 cm away. The solution contained 0.1 M H3BO3 (≥99.5%, Sigma-Aldrich), adjusted to pH 9.2 using KOH. Potential was applied by a Solartron 1287 Electrochemical Interface using an ac amplitude of 10 mV and frequencies of 0.5, 1, or 2 kHz. Capacitance was measured by a Solartron 1260 Frequency Response Analyzer, and the 1/C2 output (calculated by the CorrView software using a simple parallel RC circuit model) was plotted versus potential. All potentials in this study are reported versus RHE for ease of comparison with the literature and with the redox potentials of H+/ H2 and O2/H2O. The conversion equation is
EXPERIMENTAL SECTION
Synthesis of Cu3V2O8. Cu3V2O7(OH)2·2H2O nanoparticles were synthesized as a precursor to Cu3V2O8 following a modified literature procedure.27 Two 20 mL aqueous solutions were prepared as follows: NH4VO3 (0.5 g, 4.3 mmol of NH4VO3, lot no. 032979, Alfa) was dissolved in deionized water heated to ∼80 °C for 5 min (2 NH4VO3 + H2O → V2O74− + 2 NH4+ + 2 H+); Cu(CH3COO)2·H2O (1.2 g, 6.0 mmol, ACS Reagent, 98+%, Alrich) was dissolved similarly (∼80 °C,
