TiO2 nanocone photoanodes by Ar ion-beam etching and physics-based electrochemical impedance spectroscopy

Abstract

TiO2 nanocone (NC) arrays were produced by argon ion beam etching of nanotube (NT) arrays prepared by Ti anodizing. The anti-reflection performance of NC arrays was enhanced by anodizing a 1 μm-thick Ti film deposited on a p-type silicon substrate. The NC morphology was maintained upon heat treatment up to 400 °C, where the anatase phase was formed. Higher temperature treatments resulted in agglomeration and the formation of the rutile phase. The photoelectrochemical performances of the NC and NT arrays on Ti/Si substrates and the NTs on the Ti disk, both annealed at 400 °C, were compared. The photocurrent curves of the samples on Ti/Si were qualitatively different from NT on Ti. Impedance spectroscopy revealed that the response is dominantly from the p-Si substrates, which can be almost ideally described by the diffusion-recombination governed by the minority electronic carriers. NT on Ti can be characterized by the transmission line model for nanoporous electrodes where the transverse and interfacial capacitances are described by Havriliak-Negami capacitance functions with fixed exponents of either 1 and ½ at high- and low-frequency limits. The model then powerfully deconvolutes TiO2 response from the limited information at the low frequencies for NT and NC on Ti/Si, which can be related to the photoelectrochemical performance. Mott-Schottky analysis of well-defined capacitance parameters from the Havriliak-Negami function allows the physics-based description of the spatially and energetically distributed dopant defects in nanostructured materials.