Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting

Carbon modified (CM)-n-TiO2 nanotube arrays were successfully synthesized by anodization of Ti metal sheet in fluoride solution and subsequent annealing in air and natural gas flame oxidation. Both nanotube structure and carbon doping contributed to the enhancement of photoresponse of n-TiO2. About two fold increase in photocurrent density was observed at undoped n-TiO2 nanotube film compared to that at its undoped n-TiO2 flat thin film. Also, about eight fold increase in photocurrent density was observed at carbon modified (CM)-n-TiO2 nanotube film compared to that at undoped n-TiO2 flat thin film. The sample prepared by anodization at 20 V cell voltage for 20 h followed by annealing in air at 500 °C for 1 h and natural gas flame oxidation at 820 °C for 18 min produced highest photocurrent density. It was found that the bandgap of n-TiO2 was reduced to 2.84 eV and an additional intragap band was introduced in the gap at 1.30 eV above the valence band. The bandgap reduction and the new intragap band formation in CM-n-TiO2 extended its utilization of solar energy up to the visible to infrared region.