Influence of Surface Defects on WO₃ Photoelectrodes for Catalyzing Chloride Oxidation in Water

Tungsten oxide (WO3) is an n-type semiconductor due to oxygen vacancies (□O•• in Kroger-Vink notation) or surface protonation as HxWO3. It is one of the few acid-stable oxides under large positive bias, which makes WO3 ideal for interrogating the mechanism of the chloride oxidation reaction (COR). The large, positive valence band edge of ∼3 eV provides the overpotential necessary to carry out the COR, but the reaction competes with the oxygen-evolution reaction in water. The □O•• defect density can be controlled by the atmosphere under which the material is annealed, so WO3 films were prepared by a spin-coating method from an ammonium metatungstate precursor annealed at 500 °C under air, flowing O2, and flowing argon. Annealing the films in a flowing O2 atmosphere hinders the formation of □O••, and annealing in Ar leads to greater surface W6+, likely due to expelling intercalated H+. The saturated photocurrent density (jph) is highest in films with the greatest concentration of W5+ and greatest concentration of oxide defects: (0.66 mA/cm2 annealed in air, 0.58 mA/cm2 annealed in Ar, and 0.49 mA/cm2 annealed in O2, reported at 1.5 V vs Ag/AgCl, pH 3 (before the onset of a dark reaction). The defect concentrations are determined by X-ray photoelectron spectroscopy. In all cases the Faradaic efficiency for the COR is near unity. Finally, we demonstrate that W 5d states can be probed by ligand K-edge X-ray absorption near-edge spectroscopy via pre-edge (Cl 1s → W 5d) transitions, lower in energy than the ligand-centered (Cl 1s → 4p) transition. We use this analysis to show the presence of W─Cl covalent bonds on the WO3 films post-COR, corroborated by DFT calculations. This result stands in contrast to the commonly assumed mechanistic proposal invoking outer-sphere electron transfer to a physisorbed chloride ion.