Towards the design of oxychalcogenide materials as candidates for solar water-splitting photocatalysis

Photocatalysts, capable of splitting water for clean hydrogen production, are needed if we are to realize the potential of fuel cells (which combine hydrogen and oxygen to give electrical energy) to help meet the world's energy demands. The use of solar energy is promising and the search for photocatalysts that are active under solar irradiation is growing. Whilst numerous materials have been found to be effective for water splitting, many oxide materials have band gaps too large for excitation by visible light, and many sulfides have smaller band gaps or lower stability. On the other hand, the electronic structure (including band gap nature and magnitude) can be optimized by tuning the chemical composition, including substitutions on the anion sublattice (e.g. with H-, N3− or S2−) to give mixed-anion materials. By appropriate anion-substitution, this can raise the valence band maximum which will decrease the band gap, moving it to the visible range. This substitution can also promote polarity which can lead to a better charge carrier's (e-/h+) separation. In particular, having heteroleptic environments around an active site may lead to enhanced polarity. This innovative strategy has been applied to give optically active materials and has been highlighted by theory work to give improved photocatalytic performance. Thus, the aim of this thesis is to test these hypotheses by combined experimental and computational studies on known oxychalcogenide materials.

In this context, a series of iron-based oxychalcogenides was investigated in which the roles of polar structure, the connectivity and transition metal coordination are highlighted. Polar CaFeOSe demonstrated its activity as photocathode with fast e-/h+ separation compared to the non-polar La2O2Fe2OQ2 (Q = S, Se), but on the other hand, these transition metals could suffer from an oxidation reaction which was seen with CaFeOS. Then, investigations of the polar oxysulfide Sr6Cd2Sb6S10O7 indicate its potential for photocatalytic applications with efficient electron-hole separation. This work demonstrates the importance of lone pairs in designing photocatalytic materials, and the balance between the cation site polarity and energies of cation valence states that can be tuned by anion substitution. Lastly, the study of Sr2Sb2O2Q3 (Q = S, Se) phases revealed exceptionally low effective masses for both electrons and holes: this is attributed to the presence of a stereochemically active lone pair and a 1D building block of SbOS4 with ~180° bond angles. Experimentally, this was observed in the photocurrent measurements of Sr2Sb2O2Se3 with even lower effective masses. These studies gave us insight into the structure-property relationships for those families and suggest a potential design strategy for functional oxychalcogenide photocatalysts in the visible light.