Surface modification of TiO2 nanotubes by low temperature thermal treatment in C2H2 atmosphere

Nanotube layers of TiO2 were formed by electrochemical anodization of Ti substrates in a mixture of glycerol and water containing NH4F. The tubes were annealed in air to anatase and then modified in an acetylene atmosphere at 200–500 °C. In contrast to earlier reports, we show that the nanotube layers treated at 300 °C show a significant photoresponse in the UV as well as the visible spectral range. The present treatment leads to the formation of a thin carbon layer on the surface that is considered to sensitize the TiO2 nanotubes similar to the well established case of dye sensitization – i.e. the effect cannot be ascribed to a true bulk doping effect.