Photoelectrochemical Properties of Heterojunction CdTe/TiO2 Electrodes Constructed Using Highly Ordered TiO2 Nanotube Arrays

Abstract

A heterojunction CdTe/TiO2 photoelectrode was prepared by electrochemically filling the tubes and tube-to-tube voids of a TiO2 nanotube array with CdTe. The TiO2 nanotube arrays used in this study were prepared by anodizing titanium films, which resulted in closely packed n-type TiO2 tubes with an average inner pore diameter of 50 nm, wall thickness of approximately 13 nm, and length of 250−300 nm. CdTe was cathodically deposited using TiO2 nanotubes as the working electrode at E = −0.4 V vs Ag/AgCl at 85 °C (3H+ + Cd2+ + HTeO2+ + 6e− → CdTe + 2H2O). The resulting electrodes contained three-dimensionally organized CdTe/TiO2 junction structures with significantly enhanced junction areas. Formation of the CdTe/TiO2 junction improved the photocurrent generation and photostability of the CdTe layer when compared with a two-dimensional CdTe layer deposited directly on a conducting substrate (i.e., fluorine-doped tin oxide). A more intimate and conformal CdTe/TiO2 junction was formed via a new deposition technique developed in this study that promotes deposition of CdTe only in the TiO2 tubes while minimizing deposition at the tube entrances, thus preventing pore clogging. The CdTe/TiO2 electrodes prepared by the new technique created a longer path length for light in the CdTe layer as well as increased CdTe/TiO2 and CdTe/electrolyte junction areas. This resulted in enhanced photon absorption and photocurrent generation, achieved using a minimal amount of CdTe.