Effects of hydrothermal etching and conversion on photocatalytic hydrogen evolution and overall water splitting with nanoparticulate and mesoporous TiO2 and SrTiO3/TiO2 composites

Abstract

TiO₂ and SrTiO₃ are well-studied semiconductors for photocatalytic H₂ evolution and overall water splitting, respectively. Hydrothermal conversion of different TiO₂ starting materials enables the synthesis of SrTiO₃/TiO₂ composites with various morphological designs, which have so far shown increases in photocatalytic H₂ evolution activity. Herein, we address this phenomenon and the underlying influences of alkaline and pH-neutral media during hydrothermal conversion reactions by detailed material characterization of SrTiO₃/TiO₂ composites and etched TiO₂. Trends in photocatalytic H₂ evolution activities can be related directly to morphological factors, which in turn determine the influence of hydrothermal treatment. TiO₂ nanoparticles are mainly subjected to agglomeration, while mesoporous TiO₂ benefits from increasing hydrophilicity and a broadening of pore size distributions. Taken together, both influences lead to “pore activation” that strongly enhances photocatalytic H₂ evolution activities. Simple hydrothermal treatment in diluted NaOH solution or even plain H₂O might be a promising strategy to enhance photocatalytic activities of mesoporous TiO₂ and potentially other mesoporous semiconductors, but more importantly, it also needs to be taken into account in the case of hydrothermal conversion reactions, since the subtle impact of the reaction medium may be overlooked or misinterpreted as a composite effect.