Femtosecond laser mediated fabrication of micro/nanostructured TiO_2-x photoelectrodes: Hierarchical nanotubes array with oxygen vacancies and their photocatalysis properties

Titanium dioxide (TiO₂₎ photoelectrodes that offer high light absorption and efficient charge separation hold great promise in photocatalysis. In this study, a simple and controllable method for fabricating hierarchical TiO_2−x photoelectrodes by coupling femtosecond laser processing and anodization is proposed. The fabricated photoelectrodes consist of microcones (approximately 150,000 per square centimetre) covered with large quantities of nanotubes. The hierarchical structures possess significant light-trapping effect, meanwhile, oxygen vacancies were simultaneously introduced through laser processing with anodization, which can be attributed to the fs-laser mediated lattice phase transformation (polycrystalline and amorphous layer) of titanium. With the synergistic effects of hierarchical structures and oxygen vacancies, the hierarchical TiO_2−x electrode exhibited a narrowed bandgap (1.95 eV) and remarkable light absorption. Especially under visible light, this photoelectrode demonstrated 15-fold photocurrent enhancement and double the usual photodegradation rate of methylene blue. This morphological control and defect introduction method may be extensively used in efficient solar utilisation.