Manganese鈥揤anadate Hybrids: Impact of Organic Ligands on Their Structures, Thermal Stabilities, Optical Properties, and Photocatalytic Activities

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• 作者：Lan Luo ; Yuhan Zeng ; Le Li ; Zhixiang Luo ; Tatyana I. Smirnova ; Paul A. Maggard
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：August 3, 2015
• 年：2015
• 卷：54
• 期：15
• 页码：7388-7401
• 全文大小：824K
• ISSN：1520-510X

文摘

Manganese(II)鈥搗anadate(V)/organic hybrids were prepared in high purity using four different N-donor organic ligands (2,6:2鈥?2鈥?terpyridine = terpy, 2,2鈥?bipyrimidine = bpym, o-phenanthroline = o-phen, and 4,4鈥?bipyridine = 4,4鈥?bpy), and their crystalline structures, thermal stabilities, optical properties, photocatalytic activities and electronic structures were investigated as a function of the organic ligand. Hydrothermal reactions were employed that targeted a 1:2 molar ratio of Mn(II)/V(V), yielding four hybrid solids with the compositions of Mn(terpy)V₂O₆路H₂O (I), Mn₂(bpym)V₄O₁₂路0.6H₂O (II), Mn(H₂O)(o-phen)V₂O₆ (III), and Mn(4,4鈥?bpy)V₂O₆路1.16H₂O (IV). The inorganic component within these hybrid compounds, that is, [MnV₂O₆], forms infinite chains in I and layers in II, III, and IV. In each case, the organic ligand preferentially coordinates to the Mn(II) cations within their respective structures, either as chelating and three-coordinate (mer isomer in I) or two-coordinate (cis isomers in II and III), or as bridging and two coordinate (trans isomer in IV). The terminating ligands in I (terpy) and III (o-phen) yield nonbridged 鈥淢nV₂O₆鈥?chains and layers, respectively, while the bridging ligands in II (bpym) and IV (4,4鈥?bpy) result in three-dimensional, pillared hybrid networks. The coordination number of the ligand, that is, two- or three-coordinate, has the predominant effect on the dimensionality of the inorganic component, while the connectivity of the combined metal-oxide/organic network is determined by the chelating versus bridging ligand coordination modes. Each hybrid compound decomposes into crystalline MnV₂O₆ upon heating in air with specific surface areas from 鈭? m²/g for III to 鈭?1 m²/g for IV, depending on the extent of structural collapse as the lattice water is removed. All hybrid compounds exhibit visible-light bandgap sizes from 鈭?.7 to 鈭?.0 eV, decreasing with the increased dimensionality of the [MnV₂O₆] network in the order of I > II 鈮?III > IV. These bandgap sizes are smaller by 鈭?.1鈥?.4 eV in comparison to related vanadate hybrids, owing to the addition of the higher-energy 3d orbital contributions from the Mn(II) cations. Each compound also exhibits temperature-dependent photocatalytic activities for hydrogen production under visible-light irradiation in 20% methanol solutions, with threshold temperatures of 鈭?0 掳C for III, 鈭?6 掳C for I, and 鈭?0 掳C for II, IV, and V₄O₁₀(o-phen)₂. Hydrogen production rates are 鈭?42 渭mol H₂ g^鈥?路h^鈥?, 鈭?73 渭mol H₂ g^鈥?路h^鈥?, 鈭?1 渭mol H₂ g^鈥?路h^鈥?, and 鈭?18 渭mol H₂ g^鈥?路h^鈥? at 40 掳C, for I, II, III, and IV, respectively, increasing with the oxide/organic network connectivity. In contrast, the related V₄O₁₀(o-phen)₂ exhibits a much lower photocatalytic rate of 鈭?6 H₂ g^鈥?路h^鈥?. Electronic structure calculations based on density-functional theory methods show that the valence band edges are primarily derived from the half-filled Mn 3d⁵ orbitals in each, while the conduction band edges are primarily comprised of contributions from the empty V 3d⁰ orbitals in I and II and from ligand 蟺* orbitals in III. Thus, the coordinating organic ligands are shown to significantly affect the local and extended structural features, which has elucidated the underlying relationships to their photocatalytic activities, visible-light bandgap sizes, electronic structures, and thermal stabilities.