Comparison of photocatalytic activities of two kinds of lead magnesium niobate for decomposition of organic compounds un
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Two new oxide photocatalysts in the Pb–Mg–Nb ternary system, Pb3MgNb2O9 (PMN) and Pb1.83Mg0.29Nb1.71O6.39, have been synthesized, and their photophysical and photocatalytic properties were investigated for the first time. The band gaps of PMN and Pb1.83Mg0.29Nb1.71O6.39 were estimated to be 3.2 and 2.8 eV, respectively, from their absorption spectra. The photocatalytic activity evaluated from the decomposition of 2-propanol into acetone under visible-light irradiation indicated that Pb1.83Mg0.29Nb1.71O6.39 exhibited at least 20 times higher activity than PMN and has a strong oxidizing potential to mineralize organic compounds. I. INTRODUCTION
A TiO2 photocatalyst is applied to many products, such as self-cleaning walls and antifogging mirrors, due to the strongly oxidizing holes that are generated by ultraviolet (UV) light irradiation.1,2 However, the absorption characteristics of TiO2 limits its indoor application because the intensity of UV light is very weak in indoor illuminations, the main component of which is visible light. Therefore, much research has been devoted to the development of visible-light-sensitive photocatalysts. TiO2 doped with a foreign element, such as Cr, V, N, or C, has been reported as one of the predominant visiblelight-sensitive photocatalysts.3–8 Apart from the TiO2-based materials, Bi-, Pb-, or Ag-containing complex niobium oxides, such as PbBi2Nb2O9 and ABi2Nb5O16 (A ⳱ Cs, Rb, and K), have been reported to be promising visible-light-sensitive photocatalysts used for the decomposition of organic compounds.9–11 It is known that simple alkaline niobates, such as NaNbO3, the band structures of which consist of O 2p at the top of the valence band (VB) [potential versus standard hydrogen electrode (SHE) ∼ +3.0 eV] and Nb 4d orbitals at the bottom of the conduction band (CB) (potential versus SHE approximately −0.4 eV), have band gaps as wide as 3.4 eV and cannot absorb visible light.12 In contrast, in the Bi- and Pb-containing complex niobium oxides, it turns out that the top of the VB consists of the hybridized orbitals of O 2p, Bi 6s, and Pb 6s.9
This hybridization narrows the band gaps of the oxides, resulting in their visible-light absorption. The Bi- and Pb-containing complex oxide, PbBi2Nb2O9 was reported to show a relatively high photocatalytic activity for organic decomposition under visible-light irradiation.9 However, the bottoms of the CB of some Bi3+-containing oxides consist of the Bi 6p orbitals.13 Because of the relatively low energy of the Bi 6p orbitals, the Bi3+-containing oxides may be easily reduced by the photogenerated electrons, suggesting that the oxides are less stable.14,15 Based on the above analysis, we considered that Bi3+ ions may suppress activities for some photocatalytic reactions, and we attempted to prepare Bi-free photocatalysts for higher activities. Instead of the Bi ions, Mg ions were selected to construct the framework of the crystal structure because in some complex oxides, Mg orbitals were reported to contribute little to the top of the VB and the
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