Synthesis of Orthorhombic Sn₃O₄ Film and Its Photocatalytic Visible-Light Activity

Although visible-light-responsive Sn₃O₄ is known to be synthesized by a hydrothermal method, its reported crystal structure is limited to the monoclinic phase. Recently, orthorhombic Sn₃O₄ has been reported as a new polymorph of tin oxides; however, its photocatalytic properties have not been examined. This paper reports the thin film synthesis of orthorhombic Sn₃O₄ and its photocatalytic properties under visible-light irradiation. We synthesized films and powders of orthorhombic and conventional monoclinic Sn₃O₄ and evaluated their crystal structures, morphologies, and physical properties by using X-ray diffraction (XRD), scanning electron microscope (SEM), hard X-ray photoemission spectroscopy (HAXPES) for core levels and valence bands, and ultraviolet-visible light (UV-vis) spectroscopy. Further, we compared photoelectrochemical and photocatalytic properties of the monoclinic Sn₃O₄ and those of the orthorhombic Sn₃O₄. A comparison between the film and powder showed that the orthorhombic Sn₃O₄ film had higher crystallinity with a strong crystal orientation and exhibited higher photocatalytic activity than the orthorhombic powder form. By comparison between the orthorhombic film and the monoclinic film, the orthorhombic Sn₃O₄ film was responsive to longer wavelength photons and exhibited higher internal quantum efficiency for hydrogen production than the monoclinic Sn₃O₄ film. Orthorhombic Sn₃O₄ exhibited a rod-like structure with clear crystal facets, where rectangular (110) facets were exposed on the side faces, and the (001) facet was exposed at the edge of the rod. In the orthorhombic Sn₃O₄ rod, photoexcited electron-hole pairs are efficiently separated because of the high crystallinity and different surface energies of the crystal facets, yielding high photocatalytic activity toward hydrogen production. Orthorhombic Sn₃O₄ films are expected to be applied as visible-light-sensitive photocatalysts.