Thin films of hematite (α-Fe_2O_3) were deposited by atomic layer deposition (ALD), and the effects of metal oxide underlayers on the photocatalytic water oxidation performance were investigated. It was found that a Ga_2O_3 underlayer dramatically enhances the water oxidation performance of the thinnest hematite films. The performance enhancement is attributed to the increased crystallinity of the ultrathin films induced by the oxide underlayers. The degree of crystallinity was examined by Raman line shape analysis of the characteristic hematite phonon modes. It was found that multiple metal oxide underlayers, including Nb_2O_5, ITO, and WO_3, increase the film crystallinity compared to hematite deposited on bare FTO. The increased crystallite size was also clearly evident from the high resolution SEM images. The degree of crystallinity was found to correlate with absorbance and the photocatalytic water oxidation performance. These findings shed light on the origin of the dead layer at the interface of the FTO substrate and ultrathin hematite films and elucidate strategies at overcoming it
