Assessing the Suitability of Iron Tungstate (Fe₂WO₆) as a Photoelectrode Material for Water Oxidation

Orthorhombic iron tungstate (Fe₂WO₆), with a reported bandgap of ∼1.5–1.7 eV, is a potentially attractive material as the top absorber in a tandem photoelectrochemical (PEC) device. Few studies have been carried out on this material, and most of the important optical, electronic, and PEC properties are not yet known. We fabricated thin film Fe₂WO₆ photoanodes by spray pyrolysis and identified the performance limitations for PEC water oxidation. Poor charge separation is found to severely limit the photocurrent, which is caused by a large mismatch between the light penetration depth (∼300 nm) and carrier diffusion length (<10 nm) of the material. In addition, the conduction band of Fe₂WO₆ lies 0.65 V positive of the reversible hydrogen electrode potential, which means that a large external bias potential is required for water oxidation. On the basis of these observations, we critically discuss the suitability of Fe₂WO₆ as a novel photoelectrode material for photoelectrochemical and photocatalytic applications

The Origin of Slow Carrier Transport in BiVO₄ Thin Film Photoanodes: A Time-Resolved Microwave Conductivity Study

We unravel for the first time the origin of the poor carrier transport properties of BiVO₄, a promising metal oxide photoanode for solar water splitting. Time-resolved microwave conductivity (TRMC) measurements reveal an (extrapolated) carrier mobility of ∼4 × 10^–2 cm² V^–1 s^–1 for undoped BiVO₄ under ∼1 sun illumination conditions, which is unusually low for a photoanode material. The poor carrier mobility is compensated by an unexpectedly long carrier lifetime of 40 ns. This translates to a relatively long diffusion length of 70 nm, consistent with the high quantum efficiencies reported for BiVO₄ photoanodes. Tungsten (W) doping is found to strongly decrease the carrier mobility by introducing intermediate-depth donor defects as carrier traps. At the same time, the increased carrier density improves the overall photoresponse, which confirms that bulk electronic conductivity is one of the main performance bottlenecks for BiVO₄

Comprehensive Evaluation of CuBi₂O₄ as a Photocathode Material for Photoelectrochemical Water Splitting

CuBi₂O₄ is a multinary p-type semiconductor that has recently been identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. It has an optimal bandgap energy (∼1.8 eV) and an exceptionally positive photocurrent onset potential (>1 V vs RHE), making it an ideal candidate for the top absorber in a dual absorber PEC device. However, photocathodes made from CuBi₂O₄ have not yet demonstrated high photoconversion efficiencies, and the factors that limit the efficiency have not yet been fully identified. In this work we characterize CuBi₂O₄ photocathodes synthesized by a straightforward drop-casting procedure and for the first time report many of the quintessential material properties that are relevant to PEC water splitting. Our results provide important insights into the limitations of CuBi₂O₄ in regards to optical absorption, charge carrier transport, reaction kinetics, and stability. This information will be valuable in future work to optimize CuBi₂O₄ as a PEC material. In addition, we report new benchmark photocurrent density and IPCE values for CuBi₂O₄ photocathodes

Unraveling the Carrier Dynamics of BiVO₄: A Femtosecond to Microsecond Transient Absorption Study

Bismuth vanadate (BiVO₄) is a promising semiconductor material for photoelectrochemical water splitting showing good visible light absorption and a high photochemical stability. To improve the performance of BiVO₄, it is of key importance to understand its photophysics upon light absorption. Here we study the carrier dynamics of BiVO₄ prepared by the spray pyrolysis method using broadband transient absorption spectroscopy (TAS), in thin films as well as in a photoelectrochemical (PEC) cell under water-splitting conditions. The use of a dual-laser setup consisting of electronically synchronized Ti:sapphire amplifiers enable us to measure the femtosecond to microsecond time scales in a single experiment. On the basis of this data, we propose a model of carrier dynamics that includes relaxation and trapping rates for electrons and holes. Hole trapping occurs in multiple phases, with the majority of the photogenerated holes being trapped with a time constant of 5 ps and a small fraction of this hole trapping taking place within the instrument response of 120 fs. The induced absorption band that represents the trapped holes is modulated by an oscillation of 63 cm^–1, which is assigned to the coupling of holes to a phonon mode. We find electrons to undergo a relaxation with a time constant of 40 ps, followed by deeper trapping on the 2.5 ns time scale. On time scales longer than 10 ns, trap-limited recombination that follows a power law is found, spanning time scales up to microseconds. Finally, we observe no spectral or kinetic differences by applying a bias voltage to the PEC cell, indicating that the effect of a voltage and the charge transfer processes between BiVO₄ and the electrolyte occurs on longer time scales. Our results therefore provide new insights into the carrier dynamics of BiVO₄ and further expand the application window of TAS as an analytical tool for photoanode materials

Gradient Self-Doped CuBi₂O₄ with Highly Improved Charge Separation Efficiency

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi₂O₄ photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBi₂O₄ photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBi₂O₄, which can facilitate charge separation. Compared to homogeneous CuBi₂O₄ photocathodes, CuBi₂O₄ photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBi₂O₄ photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBi₂O₄ photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBi₂O₄ up to −2.5 mA/cm² at 0.6 V vs RHE with H₂O₂ as an electron scavenger, and they show a charge separation efficiency of 34% for 550 nm light. The gradient self-doping accomplishes this without the introduction of external dopants, and therefore the tetragonal crystal structure and carrier mobility of CuBi₂O₄ are maintained. Lastly, forward gradient self-doped CuBi₂O₄ photocathodes are protected with a CdS/TiO₂ heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of −1.0 mA/cm² at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%