Photo-induced effects on self-organizedTiO2 nanotube arrays: the influence of surface morphology

Abstract

Self-organized TiO2 nanotubes with packed, vertically aligned morphology and different lateralcharacteristics were grown on Ti metal substrates by controlled electrochemicalanodization in phosphate/HF and ethylene glycol/HF electrolytes. The wetting,photo-induced superhydrophilicity, and photocatalytic activity of the nanotubularmaterials were investigated under ultraviolet irradiation. The photoactivity of theTiO2 nanotube arrays was analysed in terms of their morphological characteristicsthat were determined by means of scanning electron microscopy and atomicforce microscopy in conjunction with geometrical modelling. The wetting andthe UV-induced superhydrophilicity could be accordingly modelled by theCassie–Baxter mode arising from the large scale roughness of the nanotubulararrays in combination with the Wenzel mode due to the small scale roughnessinduced by ridges at the outer tube surface. The photocatalytic activity of theTiO2 nanotube arrays was further found to correlate quantitatively with thevariation of the geometric roughness factor, verifying the strong impactof morphology on the photo-induced properties of the vertically orientedTiO2 tubular architecture.