Exploring the mechanism of a pure and amorphous black-blue TiO2:H thin film as a photoanode in water splitting

The use of amorphous disordered surface layers on TiO₂ nanocrystals prepared by hydrogenated post-treatment has been proven to be an effective approach for enhancing the light absorption and photocatalytic activity of TiO₂ photoanodes. However, the mechanism underlying the enhanced optical-electrical performance caused by the hydrogenated amorphous shell has still not been fully unveiled. Herein, to explore the mechanism without the effect of the crystalline structure, a pure, amorphous hydrogen-doped TiO₂ (a-TiO₂:H) film was prepared using a magnetron sputtering technique under reactive hydrogen plasma. We propose that the cooperative effects of the extended mid-gap states and valence band tail are responsible for the enhanced visible and near-infrared optical absorption. In addition, the doped H acts as a shallow donor to provide carriers and shift the Fermi level to the conduction band, ultimately accelerating charge transport and transfer at the semiconductor/electrolyte interface. Finally, the photoconversion efficiency of a-TiO₂:H was improved one order of magnitude compared to undoped a-TiO₂:H. Through the investigation of a-TiO₂:H, we gained further insight into black TiO₂. In addition, we believe our a-TiO₂:H film grown at room temperature opens new opportunities for a broad range of applications, including PEC water splitting, supercapacitors, dye-sensitized solar cells, and perovskite solar cells.