Some interesting properties of black hydrogen-treated TiO 2 nanowires and their potential application in solar energy conversion

Hydrogen treatment has been proposed as a simple and effective strategy to enhance the performance of TiO2 nanostructures for applications such as photocatalysis and photoelectrochemical (PEC) water splitting. While some studies have suggested that the black color can be ascribed to surface disorder, other reports have suggested that it is caused by the “oxygen vacancy” states associated with Ti3+ within the bandgap of the TiO2. The chemical nature and potential use of the bandgap states responsible for the black color of increased visible absorption is not yet well understood and subject of strong interest. Here we briefly review current understanding of the possible mechanisms behind the black color of hydrogen-treated TiO2 nanowires and its relevance to photocatalysis and solar water splitting for hydrogen generation. One important conclusion to date is that while hydrogen treatment enhances photocurrent of TiO2 with UV excitation, no noticeable photocurrent can be detected with visible light, which seems to be due to the very short lifetimes of the bandgap oxygen vacancy states arising from hydrogen treatment.