Pressure Dependence of Electrical Conductivity of Black Titania Hydrogenated at Different Temperatures

Temperature can control the degree of hydrogenation of titania (i.e., black TiO ₂ ), determining the defect chemistry and hence its optical absorption and electrical conductivity that are key to the photocatalytic activity. However, how pressure affects the two key factors is unknown. In this work, we used a diamond anvil cell to produce the required high pressure (HP) and studied the pressure dependences of the structure change, the electrical conductivity, and the light absorption of black titania using HP X-ray diffraction, Raman/UV-vis spectroscopy, and electrical transport measurements. Results reveal that accompanying the HP phase transition the electrical conductivity exhibits complex variation with pressure, in good accord with the band gap changes of involved HP phases as a function of pressure. This confirms the assumption that pressure affects the electrical conductivity of black titania via controlling the number of free electrons (holes) distributed in the conduction (valence) band, which is inversely proportional to the exponent of the band gap that scales almost linearly with the pressure. This work provides a fundamental understanding of the pressure-induced structure-property relationship in black titania and will have important implications for tuning the photocatalytic activity via pressure and for developing new applications such as pressure sensors.