Enhancement of photoelectrochemical activity of TiO electrode by particulate/dense double-layer formation.

Photoelectrochemical water splitting has attracted significant attention as an environment-friendly method to generate H and O. Electrodes composed of powders exhibit a large surface area and are favorable for surface reactions; however, their high resistance prevents charge transportation. In contrast, photoelectrodes composed of a dense film exhibit high conductivity; however, they constitute a small surface area. Therefore, a combination of these two films can lead to higher photoelectrochemical activities. Herein, a particulate/dense TiO hybrid electrode exhibited eightfold and twofold higher activities for water oxidation at sufficiently positive potentials as compared to those of the single particulate and dense film electrodes, respectively. Electrochemical impedance measurements and the light intensity dependence of the photocurrent suggest that the activity enhancement is responsible for the synergistic effects of effective charge separation in the highly conductive dense TiO film and effective hole-consuming reaction at the particulate TiO layer. However, the activity rather decreased near the onset potential of water oxidation (<0.3 V) under the illumination of light near the bandgap energy (375 nm). Such an activity decrease was not observed for the 340 nm illumination; hence, the recombination of charge carriers generated in dense and particulate layers is responsible: the charge transfer resistance at the particulate/dense interface prevents the effective charge separation. These results demonstrate that the combination of particulate matter and dense films can produce a synergistic effect; however, the resistance at the junction remains a significant bottleneck, rendering resistance reduction necessary to maximize the advantages of hybrid electrodes, especially under the illumination of longer wavelength light.