(369d) Hydrogen By Photocatalysis with Silver-Loaded Perovskites and Charge Transfer

Titanium dioxide TiO₂ is a benchmark photocatalyst which, however, suffers from a low activity under near-UV and visible light. TiO₂ is not very effective in the photocatalytic water reduction, since electrochemical potential of its conduction band minimum (CBM) is only slightly more negative than the potential of water reduction half-reaction. Certain alkaline earth titanates MTiO₃ (M=Ca, Sr) feature the more negative potentials of the CBM vs. that in TiO₂, while their band gaps are about the same. Plasmonic photocatalysis utilizes the phenomenon of surface plasmon resonance (SPR) in supported metal nanoparticles (NPs) for “sensitization” of semiconductors towards near-UV/visible light. We prepared nanocrystalline and mesoporous perovskites MTiO₃ (M=Ca, Sr) and characterized them by the BET, XRD, UV-Vis Diffuse Reflectance Spectroscopy (UV-Vis DRS), ATR-FTIR, Raman spectroscopy, and TEM. Then, we prepared composite photocatalysts Ag/MTiO₃ (M=Ca, Sr) by the in-situ deposition, and characterized them by UV-Vis DRS, X-ray fluorescence, and photoluminescence (PL) spectroscopy. The UV-Vis DRS show optical absorption due to the SPR in supported silver NPs. We have tested Ag/MTiO₃ in hydrogen generation from water with and without organic sacrificial donor under near-UV/visible light. Activity of Ag/MTiO₃ was optimized against experimental conditions, and it follows a bell shape vs. amount of noble metal at 0.1-10 %. Composite photocatalysts Ag/MTiO₃ are more active in hydrogen generation than the respective MTiO₃ under UV/visible light (λ>254 nm), near-UV light (λ>365 nm), and visible light (λ>400 nm); an enhancement of hydrogen generation rate is >4-fold, up to 340 μmol/g*h. Further, we have utilized defect states in perovskites as intrinsic spectroscopic probe, in order to determine the direction of the photoinduced charge transfer in the Ag/MTiO₃ photocatalysts. As shown by the front-face photoluminescence spectroscopy (FF-PL), under photoexcitation of MTiO₃ with near-UV light, electron transfer proceeds to supported metal; work is in progress to determine the direction of charge transfer under visible light. Acknowledgments: A.S. thanks Research Corporation for Science Advancement (RCSA) for his Cottrell College Science grant, and Rutgers University for his Research Council Award.