Assessing the Suitability of Iron Tungstate (Fe<sub>2</sub>WO<sub>6</sub>) as a Photoelectrode Material for Water Oxidation

Orthorhombic iron tungstate (Fe<sub>2</sub>WO<sub>6</sub>), 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 Fe<sub>2</sub>WO<sub>6</sub> 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 Fe<sub>2</sub>WO<sub>6</sub> 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 Fe<sub>2</sub>WO<sub>6</sub> as a novel photoelectrode material for photoelectrochemical and photocatalytic applications.