TiO2 Nanotubes Formed in Aqueous Media: Relationship between Morphology, Electrochemical Properties and Photoelectrochemical Performance for Water Oxidation

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This paper studies the effect of voltage and electrolyte composition on the morphology of TiO2 nanotube films, their semiconductor properties and photoelectrochemical performance for water 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.