Investigation of the charge transport kinetics and photoelectrochemical properties of CuInS₂/R-TiO₂ NTs heterostructures photoanode

It is significant to study the role of Ti³⁺ sites in heterojunction and its influence on charge transport kinetics for photoelectrochemical water splitting. Here, by using NaBH₄ reduction treatment, Ti³⁺ self-doped reduced TiO₂ nanotubes (R-TiO₂ NTs) were obtained under mild conditions. CuInS₂, a narrow bandgap semiconductor, was uniformly coated on R-TiO₂ NTs to form a CuInS₂/R-TiO₂ NTs heterostructure photoelectrode through a simple one-step pulse electrodeposition method. The charge transfer and recombination process were characterized by the intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). The results showed that the introduced Ti³⁺ sites formed an intermediate gap between the valence band and the conduction band. What's more, the heterojunction structure effectively enhanced the interface charge separation, which promoted the surface reaction kinetics. The electron transfer time of CuInS₂/R-TiO₂ NTs was about 0.28 ms, which was higher than that of the individually R-TiO₂ NTs and CuInS₂/TiO₂ NTs alone. The electron lifetime was extended from 1.19 ms to 3.77 ms. Due to the reduction treatment and surface loading of CuInS₂ nanoparticles, the carrier collection efficiency and the charge transfer efficiency of the CuInS₂/R-TiO₂ NTs were increased to 92.6% and 10.71%, respectively. This study sheds light on the charge transport kinetics of CuInS₂/R-TiO₂ NTs heterostructures for application in efficient PEC water splitting.