TiO₂ nanotubes formed in aqueous media: Relationship between morphology, electrochemical properties and photoelectrochemical performance for water oxidation

Departamento de Qúimica, Universidad Autonoma Metropolitana-Iztapalapa, C.P. 09340 Ciudad de Mexico, Mexico This paper studies the effect of voltage and electrolyte composition on the morphology of TiO₂ nanotube films, their semiconductor properties and photoelectrochemical performance forwater oxidation. TiO₂ nanotube films were formed by potentiostatic anodization in fluoride containing electrolytes (using either HF or NH₄F) during 1 h at four different voltages and were heat treated at 450°C to obtain anatase polymorph. SEM images showed a more significant chemical attack when NH₄F was employed, which was evidenced by larger tube diameters and thinner films. UV-vis reflectance spectra showed that the TiO₂ films formed in aqueous electrolyte were highly defective, presenting various broad absorption peaks in the visible region attributed to the sub-bandgap energy states. The semiconducting properties were modified by the nature of the counter-ion utilized during film growth, resulting in films with a higher donor density and a lower flatband potential in NH₄F electrolytes. The films formed in NH₄F showed higher photopotentials and photocurrents related to the modification of their morphology and semiconducting properties, attaining a maximum at a formation voltage of 15 V.