Photocatalysis and hydrogen production from water solution

Abstract Tin is an earth abundant harmless element and its oxides (tin oxide) has been attracted researchers of a wide range of fields in environmentally benign materials development. Photocatalytic hydrogen production is one such promising application of tin oxide for several reasons such as stability in water solution and sufficiently negative band-edge position for the proton reduction. However, two major bulk phases SnO and SiO2 are not suitable for solar-driven photocatalysis reaction due to the fact that too negative valence band edge of SnO disables the oxidation reaction of water solution and too wide bandgap of SnO2 (~ 3.6 eV) hampers the absorption of visible light. Fortunately, bandgap or band-edge engineering is feasible by adjusting relative distance of lone pairs induced by divalent tin ions, Sn2 +, and the demonstration of this strategy is the main focus of this article. In this chapter, we introduce our two approaches toward the band-edge engineering of tin oxides. One is the formation of SnO nanosheets and modulation of band edges by application of strain. Thanks to the quantum confinement effect, SnO nanosheets have much wider bandgaps than of bulk SnO. The bandgap of SnO nanosheet depends both on the number of layers and biaxial strain and these two parameters were successfully used for the design of promising photocatalysts. Second, we seek the possibility of forming mixed valence tin oxides in which both divalent and tetravalent tin ions are coexisting. Using advanced computational technique of evolutional crystal structure search, we have identified stable tin oxides such as Sn3O4, that is, (Sn2 +)2(Sn4 +)O4, and Sn5O6, that is, (Sn2 +)2(Sn4 +)3O6, which possess suitable band-edge positions for visible-light-driven photocatalysis hydrogen evolution. Finally, we discuss experimental realization of Sn3O4 photocatalyst. From hydrothermal method, the mixed valent tin oxide Sn3O4 was successfully synthesized and it exhibits excellent performance for photocatalytic H2 evolution from water solution under visible light irradiation.