Water oxidation by P25 TiO2 photoanodes in acidic solution

Abstract

P25 TiO2 photoanodes are used to photo-oxidise water in two different acids, 0.5 M H2SO4 and 1 M HClO4. 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 H2SO4, the recorded LSV coupled with a low faradaic efficiency (0.58) for the photooxidation of water to O2, fO2, and a significant level of persulfate, fS2O8 = 0.12, shows that the electrochemical kinetics are not simply those for water oxidation. In 1 M HClO4, the LSV coupled with a high fO2 value (0.91) suggest that the photocurrent is due to water oxidation. Photo-induced absorption spectroscopy, PIAS, measurements made using the P25 TiO2 photoanode reveal a steady state absorbance change, ΔAbsss, associated with the steady-state concentration of surface accumulated holes, [h+]ss, 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 TiO2 photoanode polarised at 1.3 v vs Ag/AgCl, in 1 M HClO4, show that the oxidation of water is second order with respect the concentration of the surface-accumulated, photogenerated holes, [h+]ss, which have a calculated turnover frequency of 19 s−1, 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.