Sol-gel synthesis of highly reproducible WO3photoanodes for solar water oxidation

Although monoclinic WO(3)is widely studied as a prototypical photoanode material for solar water splitting, limited success, hitherto, in fabricating WO(3)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 WO(3)photoanodes particularly uncontrollable. Herein, a highly reproducible sol-gel process was developed to establish efficient and translucent WO(3)photoanodes using a chemically stable ammonium metatungstate precursor. Under standard simulated sunlight of air mass 1.5 G, 100 mW cm(-2), the WO(3)photoanode delivered photocurrent densities of ca. 2.05 and 2.25 mA cm(-2)at 1.23 Vversusthe reversible hydrogen electrode (RHE), when tested in 1 mol L(-1)H(2)SO(4)and CH3SO3H, respectively. Hence, the WO(3)photoanodes fabricated herein are one of the WO(3)photoanodes with the highest performance ever reported. The reproducibility of the fabrication scheme was evaluated by testing 50 randomly selected WO(3)samples in 1 mol L-1H2SO4, which yielded an average photocurrent density of 1.8 mA cm(-2)at 1.23 V(RHE)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 WO(3)photoanodes for efficient solar water splitting.