Water oxidation by P25 TiO photoanodes in acidic solution.

P25 TiO photoanodes are used to photo-oxidise water in two different acids, 0.5 M HSO and 1 M HClO. In the former acid, the linear sweep voltammogram, LSV, appears to exhibit two photocurrent waves, whilst only one in the latter. In 0.5 M HSO, the recorded LSV coupled with a low faradaic efficiency (0.58) for the photooxidation of water to O, f, and a significant level of persulfate, f = 0.12, shows that the electrochemical kinetics are not simply those for water oxidation. In 1 M HClO, the LSV coupled with a high f value (0.91) suggest that the photocurrent is due to water oxidation. Photo-induced absorption spectroscopy, PIAS, measurements made using the P25 TiO photoanode reveal a steady state absorbance change, ΔAbs, associated with the steady-state concentration of surface accumulated holes, [h], which varies with: (i) monitoring wavelength, with a peak at ca. 500 nm, and (ii) applied potential, flattening off at ca. 0.7 V vs Ag/AgCl. PIAS measurements, coupled with concomitant transient photocurrent (TC) measurements, on the P25 TiO photoanode polarised at 1.3 v vs Ag/AgCl, in 1 M HClO, show that the oxidation of water is second order with respect the concentration of the surface-accumulated, photogenerated holes, [h], which have a calculated turnover frequency of 19 s, under 1 sun irradiation. This is the first reported example of the use of PIAS/TC to probe the photoelectrochemical kinetics exhibited by a mesoporous semiconductor photoanode derived from a powder, for water oxidation and the significance of such is discussed briefly.