Relationship between crystal orientation and energy storage capacity in WO3 photoelectrodes for water splitting

For photoelectrochemical water splitting, tungsten trioxide (WO3) films with a monoclinic structure were synthesized on fluorine-doped tin oxide coated glass substrates with a vacuum evaporation method. To control the WO3 film thickness from 0.42 to 5.6 μm, the crystal orientation was intergraded from the (200) crystal plane to the (002) one. In x-ray diffraction measurements, the intensity ratio of the (002) crystal plane to the (200) one was defined as rp. In the WO3 photoelectrode with higher (002)-preferred orientation, the photocurrent continued to flow even when the incident light against the photoelectrode was completely blocked after the irradiation for 60 s. This suggests that hydrogen continues to be produced owing to the electrons charged by the formation of a hydrogen tungsten bronze (HxWO3) phase. After 72 s, the photocurrent density of the WO3 photoelectrode with rp = 42 became one-tenth of the value before the incident light was blocked. This charge release time was remarkably long compared to those of the WO3 photoelectrodes with (200)-preferred orientation and non-orientation. Therefore, it was considered that the hydrogen ion diffusion through the defects in the WO3 crystals tends to occur in the [002] crystal direction. However, the improvement in the (002)-preferred orientation can facilitate the structural change from the WO3 phase to the HxWO3 one for the entire film.