Visible-Light Photocurrent in Nanostructured High-Pressure TiO₂-II (Columbite) Phase

Titanium oxide (TiO2), with the anatase and rutile structures, has been widely studied for photovoltaic and solar cell applications, but its main drawback is large bandgap, which limits its activity to the UV light region. In this study, a high-pressure TiO2-II (columbite) phase, which has already been suggested to have low bandgap with downward-shifted Fermi level, is stabilized by the high-pressure torsion (HPT) method and its photovoltaic activity is examined. The TiO2-II formation by HPT processing leads to photocurrent generation under visible light, while the visible-light photocurrent is enhanced further after the recovery of oxygen vacancies by thermal annealing.