Effect of length of anodized TiO₂ tubes on photoreactivity: Photocurrent, Cr(VI) reduction and H₂ evolution

Anodized tubular TiO₂ electrodes (ATTEs) are prepared using an organic additive consisting of either (i) ethylene glycol (EG) or (ii) glycerol (Gly) to make various photoanodes with different length of TiO₂ tubes and thereby to investigate the effect of their length on the photo-driven activity for hydrogen evolution and Cr(VI) reduction, as well as on the photocurrent. The ATTEs with EG have longer TiO₂ tubes (3.42-15.6 μm) than those with Gly (0.26-1.95, 6.82 μm). The former samples exhibit higher photocurrent densities (22.8-32.8 mA cm^-2) than the latter (8.0-19.4, 20.3 mA cm^-2). The latter samples (tube length of less than 7 μm) clearly exhibit a change of the rate-determining step from electron migration to photohole capture as the scanned applied bias increases, since the photocurrent shows a plateau for tube lengths above 2 μm. Meanwhile, the samples with EG remain in the electron migration step up to a tube length of 16 μm and is due to the difference of the morphology, crystal phase and crystallinity. This favourable characteristic is also applied to and well matched with the results from the reactions of Cr(VI) reduction and hydrogen evolution (up to ca. 250 μmol h^-1).