Solar water oxidation in sputter-deposited nanocrystalline WO3 photoanodes via tuning of Ar:O2 flow rate combinations

• WO3 photoanodes were prepared by reactive sputtering in Ar and O2 mixtures.
• Furnace annealed films were monoclinic WO3 with nanocrystalline grain sizes.
• Photocurrent of 4.1 mA cm−2 obtained from combination of high Ar and O2 flow rates.
• Flow rate combinations affect structural and electrical properties of photoanodes.

Thin film WO3 photoanodes were prepared by reactive sputtering in Ar and O2 gas mixtures of various flow rate combinations. Furnace annealed films were nanocrystalline monoclinic WO3 with (002), (020) and (200) plane orientations. Water oxidation in 0.33 M H2SO4 electrolyte under simulated solar illumination showed that photoanodes deposited in highest Ar and O2 flow rate combinations exhibited highest photocurrent of 4.1 mA cm−2 (at 1.3 V vs Ag/AgCl) compared to 3–3.8 mA cm−2 for photoanodes deposited in lower flow rate combinations. The higher photocurrents were ascribed to lower bulk resistivity and charge transfer resistance at the WO3/electrolyte interface. These photoanodes consisted of randomly oriented (002), (020) and (200) planes in contrast to the preferentially orientated (002) and (200) planes of photoanodes which were highly resistive with poorer photocurrent responses. These results were interpreted in terms of the effects of Ar:O2 flow rate combinations on the distribution of oxygen vacancies and formation of crystallographic shear planes in the sputtered films.