Structural, photocatalytic, and photophysical properties of perovskite MSnO 3 (M = Ca, Sr, and Ba) photocatalysts
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The photophysical properties of MSnO3 (M ⳱ Ca, Sr, and Ba) including optical absorption, photoluminescence, and energy band structure including band edge positions were investigated experimentally and theoretically in association with their photocatalytic properties. Photocatalytic reactions for H2 and O2 evolution in the case of sacrificial reagents were performed under ultraviolet (UV) light irradiation. The order of the activities of H2 evolution was CaSnO3 > SrSnO3 > BaSnO3, agreeing not only with that of the conduction-band edges (or band gaps) but also with that of the transferred excitation energy, while that of O2 evolution was CaSnO3 < SrSnO3 < BaSnO3, consistent with that of the angle of the Sn–O–Sn bonds as well as the delocalization of excited energy. When loaded with RuO2 cocatalyst, both CaSnO3 and SrSnO3 can efficiently split pure water into hydrogen and oxygen in a stoichiometric ratio under UV light irradiation. In addition, RuO2-loaded SrSnO3 showed higher water splitting activity than RuO2-loaded CaSnO3 did. This is attributed to the suitable conduction and valence band edges and to high mobility of the photogenerated charge carriers caused by the proper distortion of SnO6 connection in SrSnO3. The RuO2-loaded BaSnO3 photocatalyst cannot split pure water, which might be because of a high concentration of defect centers such as Sn2+ ions and the probability of radiative recombination in BaSnO3.
I. INTRODUCTION
Photocatalytic water splitting has received extensive attention from the viewpoint of photon energy conversion since the Honda–Fujishima effect was found.1 In the past three decades, considerable efforts have been invested in developing semiconductor photocatalysts for water splitting2–4 and in understanding the reaction mechanism on the surface of a photocatalyst and the charge migration between the photocatalyst and the aqueous electrolyte.5,6 Although the photon energy conversion with photocatalysts is far from practical use at the present stage, photocatalytic water splitting will be an important project in the long view. Semiconductors have a band structure in which the conduction band (CB) is separated from the valence band (VB) by a band gap with a suitable width, which should be larger than 1.23 eV for photocatalytic water splitting. Generally, the photocatalytic water splitting includes the following basic processes: (i) the formation of electron–hole pairs inside the semiconductor via photoexcitation with energy equal to or greater than the band gap energy: h → e− (CB) + h+ a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0259 J. Mater. Res., Vol. 22, No. 7, Jul 2007
(VB), (ii) the charge separation and migration of photogenerated carriers with adequately long lifetime and large mobility, and (iii) the redox reactions of water molecules on the active sites of the photocatalyst surface: H2O + 2h+ (VB) → 1⁄2O2 + 2H+ and 2H2O + 2e− (CB) → H2 + 2OH −. In principle, the band structure of a photocatalyst should satisfy the potential fo
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