TiO2nanotube (T_NT) surface treatment revisited: Implications of ZnO, TiCl4, and H2O2 treatment on the photoelectrochemical properties of T_NT and T_NT–CdSe

Abstract

The surface treatment of an anodized TiO₂ 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., TiCl₄, Zn(NH₃)₄²⁺, and H₂O₂ is examined. TiCl₄ and Zn(NH₃)₄²⁺ treatment results in the formation of discontinuous islands of the respective oxides with 5–10 nm and 15–20 nm diameter particles. TiCl₄ treatment demonstrates an increase of 7.4% in photovoltage and is the most effective of the three approaches. Zn(NH₃)₄²⁺ treatment also results in an ∼2% increase in photovoltage. However, a surface treatment of T_NT using H₂O₂ results only in a favourable shift in flatband potential (80 mV). The T_NTs are rendered ineffective as H₂O₂ treatment causes the destabilization of the T_NT at the base. Finally, the activity of an overlying chalcogenide layer is improved with the TiCl₄ and Zn(NH₃)₄²⁺ treatment (and not with H₂O₂) as evident from the photoelectrochemical responses: (J_{T_NT–TiO2–CdSe} > J_{T_NT–ZnO–CdSe} > J_{T_NT–CdSe} > J_{T_NT–H2O2–CdSe}).