Visible Light-Driven Electron Transfer from a Dye-Sensitized <i>p</i>‑Type NiO Photocathode to a Molecular Catalyst in Solution: Toward NiO-Based Photoelectrochemical Devices for Solar Hydrogen Production

The photoelectrochemical activity of a mesoporous NiO electrode sensitized by a ruthenium complex was investigated with several rhodium and cobalt H₂-evolving catalysts. Photocurrent as high as 80 μA/cm² was produced by irradiation of such photocathode in the presence of the Rh­(III) polypyridyl complexes, while cobalt complexes gave almost no photocurrent. Photolysis experiments led to the two-electron reduced form of the Rh­(III) complexes into Rh­(I) complexes and demonstrate the occurrence of an electron transfer chain from NiO to the catalyst. Mott–Schottky experiments evidenced the pH dependence of the NiO flat band potential, explaining the dramatic drop of the photocurrent in acidic conditions (cyanoanilinium). By contrast, in weaker acid conditions (formic acid) the photocurrent increases and the key Rh­(III) hydride intermediate was efficiently generated. In acetonitrile solution, Rh­(III)-H slowly reacts with HCOOH to generate H₂. However, this process was not catalytic, because the reduction potential of the Ru sensitizer is not sufficiently negative to reduce the Rh­(III)-H into Rh­(II)-H