Sol-gel synthesis of highly reproducible WO 3 photoanodes for solar water oxidation

Although monoclinic WO3 is widely studied as a prototypical photoanode material for solar water splitting, limited success, hitherto, in fabricating WO3 photoanodes that simultaneously demonstrate high efficiency and reproducibility has been realized. The difficulty in controlling both the efficiency and reproducibility is derived from the ever-changing structures/compositions and chemical environments of the precursors, such as peroxytungstic acid and freshly prepared tungstic acid, which render the fabrication processes of the WO3 photoanodes particularly uncontrollable. Herein, a highly reproducible sol-gel process was developed to establish efficient and translucent WO3 photoanodes using a chemically stable ammonium metatungstate precursor. Under standard simulated sunlight of air mass 1.5 G, 100 mW cm−2, the WO3 photoanode delivered photocurrent densities of ca. 2.05 and 2.25 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (RHE), when tested in 1 mol L−1 H2SO4 and CH3SO3H, respectively. Hence, the WO3 photoanodes fabricated herein are one of the WO3 photoanodes with the highest performance ever reported. The reproducibility of the fabrication scheme was evaluated by testing 50 randomly selected WO3 samples in 1 mol L−1 H2SO4, which yielded an average photocurrent density of 1.8 mA cm−2 at 1.23 VRHE with a small standard deviation. Additionally, the effectiveness of the ammonium metatungstate precursor solution was maintained for at least 3 weeks, when compared with the associated upper-limit values of peroxytungstic and tungstic acid based precursors after 3 d. This study presents a key step to the future development of WO3 photoanodes for efficient solar water splitting.