Effect of Anodization Parameters on the Surface Morphology and Photoelectrochemical Properties of TiO2 Nanotubes

Titanium sheets are anodized to prepare titanium dioxide nanotubes (TiO2 NTs) with varying chemical polishing times, anodizing voltages, anodizing times and NH4F electrolyte concentrations. The surface morphology of the TiO2 NTs is observed by a Scanning Electron Microscope (SEM) and used to characterize the inner diameter (d(i)), centre to centre distance (l) and wall thickness (w). The photoelectrochemical performance of TiO2 NT arrays is described by measurements of the photocurrent density and the incident photon to current efficiency (IPCE). The data show l can be controlled by the anodizing voltage; in contrast, d(i) and w are influenced by the anodizing time and NH4F electrolyte concentration. Furthermore, the effects of the anodization parameters on the TiO2 NTs' surface morphology were determined to influence each other. Using the optimum condition of an anodizing voltage of 60 V at 60 min in 0.125 mass % NH4F ethylene glycol, TiO2 NTs were fabricated that showed a photocurrent density of 90.42 mu A/cm(2) and maximum IPCE of 18.76 % at 300 nm. The results indicate that the effects of the anodization parameters on the photocurrent response of TiO2 NTs are primarily achieved through control of the wall thickness and the ratio of the diameter to the wall thickness (d(i) / w). For a wall thickness that is greater than a critical value, the ratio d(i) / w can reach a range in which the TiO2 NTs show a high photocurrent output.