Photoelectrochemical manifestation of intrinsic photoelectron transport properties of vertically aligned {001} faceted single crystal TiO2 nanosheet films

Abstract

In this work, vertically aligned anatase TiO₂ single crystal nanosheets with laterally exposed {001} facets onto a conducting FTO substrate (VATN) were successfully synthesised using hydrofluoric acid (40 wt%) as a crystal facet controlling agent by a simple hydrothermal method. The as-synthesised VATN without calcination exhibited a good crystalline structure, and was used as a photoanode showing superior photoelectrocatalytic activity toward water oxidation under UV irradiation. After thermal treatment at 550 °C for 2 h, the photoelectrocatalytic activity of the VATN photoanode was almost 2.6 times that for unsintered VATN under the same experimental conditions, which could be mainly due to the surface passivation role of surface fluorine in unsintered VATN to decrease the photoelectrocatalytic activity. A photoelectrochemical method was used to manifest the photoelectron transport properties inside VATN photoanodes and concurrently quantify the inherent resistances (R₀) of UV illuminated photoanodes before and after calcination. The results demonstrated that the determined R₀ values were respectively 155 Ω and 66 Ω for VATN photoanodes before and after calcination, inversely proportional to their photoelectrocatalytic activities. Compared to VATN before calcination, a significantly decreased R₀ value of VATN after calcination further confirmed the presence of surface fluorine in VATN unfavorable for photoelectron transport inside a photocatalyst film. This work provided direct evidence to prove the intrinsic photoelectron transport properties of {001} faceted anatase TiO₂ nanosheet array film photoanodes in the presence and absence of surface fluorine.