Fundamental Characterization Studies of Advanced Photocatalytic Materials

Solar powered photocatalytic water splitting has been proposed as a method for the production of sustainable, non-carbon hydrogen fuel. Although much technological progress has been achieved in recent years in the discovery of advanced photocatalytic materials, the progress in the fundamental scientific understanding of such novel, complex mixed oxide and oxynitride photocatalysts has significantly lagged. One of the major reasons for this slow scientific progress is the limited number of reported surface characterization studies of the complex bulk mixed oxide and oxynitride photocatalyst systems. Although photocatalytic splitting of water by bulk mixed oxide and oxynitride materials involves both bulk (generation of excited electrons and holes) and surface phenomena (reaction of H2O with excited electrons and holes at the surface), the photocatalysis community has almost completely ignored the surface characteristics of such complex bulk photocatalysts and correlates the photocatalytic properties with bulk properties.;Some of the most promising photocatalyst systems (NaTaO3, GaN, (Ga1-xZnx)(N1-xOx) and TaON) were investigated to establish fundamental bulk/surface structure photoactivity relationships. The bulk molecular and electronic structures of the photocatalysts were determined with Raman and UV-vis spectroscopy. Photoluminescence (PL) and transient PL spectroscopy were provided insight into how recombination of photogenerated electrons is related to the photocatalysis activity. The chemical states and atomic compositions of the surface region of the photocatalysts were determined with high resolution X-ray photoelectron spectroscopy (∼1-3 nm) and high sensitivity-low energy ion scattering spectroscopy (∼0.3 nm).;The new insights obtained from surface characterization clarified the role of La and Ni promoters species for the NaTaO3 photocatalyst system. The La2O3 additive was found to be a structural promoter that stabilizes small NaTaO3 nanoparticles (NPs) and increases the surface area, but not affecting the specific photoactivity. Only the NiO additive was found to enhance the photoactivity due to the ability of surface NiOx species to trap photogenerated electrons. The supported Rh-Cr NPs on GaN and (Ga1-xZnx)(N1-xOx) photocatalysts consist of Rh+3 which is the catalytic photoactive sites for H2 along with Cr+3, GaZnO or at their contact points being possible sites for O2 production. The RuO2 promoted TaON photocatalyst system was found to consist of dissolved Ru+4 cations in the TaOx thin film and not as RuO2 NPs as previously proposed.;In summary, these current studies for the first time revealed the surface nature of mixed oxide and oxynitride photocatalysts and stress the importance of establishing fundamental bulk/surface structure--photoactivity relationships for complex, multicomponent photocatalyst systems.