TiO2 nanotube (T_NT) surface treatment revisited: Implications of ZnO, TiCl4, and H2O2 treatment on the photoelectrochemical properties of T_NT and T_NT-CdSe.

The surface treatment of an anodized TiO(2) nanotube (T_NT) is very desirable for enhancing its photoelectrochemical properties and often is a prerequisite to deposition of any overlying layer for photoactivity efficiency improvement. This study provides a comparative analysis of the effects of such surface treatments and the mechanistic insights behind the observed improvements in the performance of the treated T_NTs. T_NT surface treatment using three approaches, viz., TiCl(4), Zn(NH(3))(4)(2+), and H(2)O(2) is examined. TiCl(4) and Zn(NH(3))(4)(2+) treatment results in the formation of discontinuous islands of the respective oxides with 5-10 nm and 15-20 nm diameter particles. TiCl(4) treatment demonstrates an increase of 7.4% in photovoltage and is the most effective of the three approaches. Zn(NH(3))(4)(2+) treatment also results in an ~2% increase in photovoltage. However, a surface treatment of T_NT using H(2)O(2) results only in a favourable shift in flatband potential (80 mV). The T_NTs are rendered ineffective as H(2)O(2) treatment causes the destabilization of the T_NT at the base. Finally, the activity of an overlying chalcogenide layer is improved with the TiCl(4) and Zn(NH(3))(4)(2+) treatment (and not with H(2)O(2)) as evident from the photoelectrochemical responses: (J(T_NT-TiO(2)-CdSe) > J(T_NT-ZnO-CdSe) > J(T_NT-CdSe) > J(T_NT-H(2)O(2)-CdSe)).