A mechanistic study into the catalytic effect of Ni(OH)2 on hematite for photoelectrochemical water oxidation.

We report a mechanistic study of the catalytic effect of Ni(OH)2 on hematite nanowires for photoelectrochemical water oxidation. Ni compounds have been shown to be good catalysts for electrochemical and photoelectrochemical water oxidation. While we also observed improved photocurrents for Ni-catalyst decorated hematite photoanodes, we found that the photocurrents decay rapidly, indicating the photocurrents were not stable. Importantly, we revealed that the enhanced photocurrent was due to water oxidation as well as the photo-induced charging effect. In addition to oxidizing water, the photoexcited holes generated in hematite efficiently oxidize Ni(2+) to Ni(3+) (0.35 V vs. Ag/AgCl). The instability of photocurrent was due to the depletion of Ni(2+). We proposed that the catalytic mechanism of the Ni(II) catalyst for water oxidation is a two-step process that involves the fast initial oxidation of Ni(2+) to Ni(3+), and followed by the slow oxidation of Ni(3+) to Ni(4+), which is believed to be the active catalytic species for water oxidation. The catalytic effect of the Ni(II) catalyst was limited by the slow formation of Ni(4+). Finally, we elucidated the real catalytic performance of Ni(OH)2 on hematite for photoelectrochemical water oxidation by suppressing the photo-induced charging effect. This work could provide important insights for future studies on Ni based catalyst modified photoelectrodes for water oxidation.