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

Manganese (II)-vanadate (V)/organic hybrids were prepared in high purity using four different N-donor organic ligands (2,6:2',2 ''-terpyridine = terpy, 2,2'-bipyrimidine = bpym, o-phenanthroline = o-phen, and 4,4'-bipyridine = 4,4'-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 center dot H2O (I), Mn-2(bpym)V4O12 center dot 0.6H(2)O (II), Mn(H2O) (o-phen)V2O6 (III), and Mn(4,4'-bpy)V2O6 center dot 1.16H(2)O (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 MnV2O6 chains and layers, respectively, While the bridging ligands in II (bpym) and IV (4,4'-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 similar to 7 m(2)/g for III to similar to 41 m(2)/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 similar to 1.7 to similar to 2.0 eV, decreasing with the increased dimensionality of the [MnV2O6] network in the order of I > II approximate to III > IV. These bandgap sizes are smaller by similar to 0.1-0.4 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 similar to 30 degrees C for III, similar to 36 degrees C for I, and similar to 40 degrees C for II, IV, and V4O10(o-phen)(2). Hydrogen production rates are similar to 142 mu mol H-2 g(-1).h(-1), similar to 673 mu mol H-2 g(-1).h(-1), similar to 91 mu mol H-2 g(-1).h(-1), and similar to 218 mu mol H-2 g(-1).h(-1) at 40 degrees 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 similar to 36 H-2 g(-1).h(-1). Electronic structure calculations based on density-functional theory methods show that the valence band edges are primarily derived from the half-filled Mn 3d(5) orbitals in each, while the conduction band edges are primarily comprised of contributions from the empty V 3d(0) orbitals in I and II and from ligand pi* 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 photo catalytic activities, visible-light bandgap sizes, electronic structures, and thermal stabilities.