Role of doped titanium species in the enhanced photoelectrochemical properties of iron oxide films: Comparison between water oxidation and iodide ion oxidation

Fumiaki Amano’s and colleagues identified the role of doped titanium species in the enhanced photo electrochemical properties of iron oxide films and compared between water oxidation and iodide ion oxidation. The photo electrochemical properties of titanium-doped iron(III) oxide (α-Fe₂O₃, hematite) nano particulate thin films are investigated by the collaborative research team using two photo electrode reactions, the oxidation of water and iodide ions to elucidate the role of doped Ti⁴⁺ species in the enhanced photocurrent.

Increase in the calcination temperature of titanium doped Fe₂O₃ thin films from 450 °C to 550 °C provides significant enhancement in their efficiency for water oxidation to evolve O₂, which is four-electron transfer slow reaction. In contrast, such a calcination effect is not remarkable for oxidation of I⁻ to I₃⁻, which is two-electron transfer fast reaction. The electrochemical impedance spectroscopy (EIS) study of this team indicates that the surface reaction rates feature heterogeneity only for water oxidation.

This heterogeneity in EIS responses decreased with increasing calcination temperature. This was shown by this team of researchers that calcination creates photoactive surface sites with doped Ti⁴⁺ species especially for kinetically limited water oxidation. Long-lived photoholes might be effectively captured in the vicinity of these sites to suppress surface recombination and induce multi-electron transfer water oxidation.

Their research results are given in a nut shell as follows: As previously reported, Ti⁴⁺ doping decreased bulk recombination in the ca. 200 nm thick ferric oxide (Fe₂O₃) nano particulate film electrodes, which resulted in the observed increase of photocurrent for I⁻ oxidation and water oxidation. Unexpectedly, Ti⁴⁺ doping enhanced the photocurrent of the Fe₂O₃ thin films without increase in donor density in this study. The increase in photocurrent cannot be explained only by the decrease in bulk recombination, because the dependence of the photocurrent of the Ti⁴⁺-doped Fe₂O₃ film electrodes on their calcination temperature differed between I⁻ oxidation (fast, two-electron oxidation) and water oxidation (slow, four-electron oxidation).

Ti⁴⁺ doping was essential to induce water oxidation, but dispensable for I⁻ oxidation. EIS responses clearly indicated the presence of heterogeneity in the water oxidation kinetics. The statistical distribution in EIS response was decreased by increasing the annealing temperature of Ti-Fe₂O₃ thin films with increasing the photocurrent, suggesting the improvement in the heterogeneity of photoactive site by calcination.

The results of this study suggest that photoactive site especially for water oxidation were created on ferric oxide nanoparticles by the dispersion of Ti⁴⁺ species. A long photo generated hole lifetime is required for multi-electron reactions and therefore the photoactive site with Ti⁴⁺ species might effectively capture the four holes to induce O₂ evolution by water oxidation.