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

Abstract

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·H2O (I), Mn2(bpym)V4O12·0.6H2O (II), Mn(H2O)(o-phen)V2O6 (III), and Mn(4,4'-bpy)V2O6·1.16H2O (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 "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 ∼7 m(2)/g for III 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 ∼1.7 to ∼2.0 eV, decreasing with the increased dimensionality of the [MnV2O6] network in the order of I > II ≈ III > IV. These bandgap sizes are smaller by ∼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 ∼30 °C for III, ∼36 °C for I, and ∼40 °C for II, IV, and V4O10(o-phen)2. Hydrogen production rates are ∼142 μmol H2 g(-1)·h(-1), ∼673 μmol H2 g(-1)·h(-1), ∼91 μmol H2 g(-1)·h(-1), and ∼218 μmol H2 g(-1)·h(-1) at 40 °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 ∼36 H2 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 ...

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

“…Recently, hybrid organic–inorganic materials featured as solar energy transducers for hydrogen production. − Maggard et al developed manganese­(II)–vanadate­(V)/organic hybrids capable of producing hydrogen from water under visible light illumination in the presence of SEDs. , These hybrid photocatalysts show structures assembled from metal-oxide chains or layers that are linked together by organic ligands. In particular, the Mn­(bpy)­V 4 O 11 (bpy) (bpy = 2,2′-bipyridine) with a bandgap of 1.6 eV exhibits a photocatalytic behavior of cleaving water into H 2 and O 2 in the stoichiometric molar ratio of 2:1.…”

MOF‐Derived MnV2O4/C Microparticles with Graphene Coating Anchored on Graphite Sheets: Oxygen Defect Engaged High Performance Aqueous Zinc‐Ion Battery

“…Based on Mn-V MOF 1, [Mn(phen)H Syntheses of two Mn-V-MOF Precursors in the Absence and Presence of Graphite Template: Two Mn-V MOF precursors based on different ligands in the absence and presence of graphite template were prepared according to previous literatures with some modifications. [27][28][29][30] Syntheses of [Mn(phen) O 6 ]/C (MOF 1/C) Precursors: Mn-V MOF 1 was prepared by a facile reflux method in a flask. [27,28] In a typical process, MnCl 2 • 4H 2 O (0.297 g, 1.5 mmol), NH 4 VO 3 (0.351 g, 3 mmol) and 1,10-phenanthroline hydrate (0.297 g, 1.5 mmol) were dissolved in deionized water (30 mL) at room temperature under sonication for 5 min.…”

“…Syntheses of [Mn(bpy)V 2 O 6 ]•1.16H 2 O (bpy = 4,4′-bipyridine) (MOF 2) and [Mn(bpy)V 2 O 6 ]•1.16H 2 O/C (MOF 2/C) Precursors: Mn-V MOF 2 was also synthesized by a simple reflux method in a round-bottom flask. [29,30] Typically, Mn(OAc) 2 • 4H 2 O (0.134 g, 0.545 mmol), NH 4 VO 3 (0.191 g, 1.628 mmol) and 4,4′-bipyridine (0.255 g, 1.612 mmol) were dissolved in deionized water (30 mL) at room temperature under sonication for 5 min, then the mixture was refluxed at 110 °C for 4 h. After cooled to room temperature, the brown-yellow precipitate was collected by centrifugation, and washed three times with water and ethanol, then dried at 70 °C in oven overnight. Yield: ≈98%.…”

“…Furthermore, density functional theory (DFT) calculations were performed to reveal the effect of oxygen defects on conductivity, electron transfer, the affinity toward H 2 O, and the migration barrier of Zn 2+ in MnV 2 O 4 . [27,28] 1c), [29,30] indicating the successful synthesis of the bpy-based MOF 2 in the absence and presence of graphite template. However, the bpy-based MOF 2 displays a completely different 3D architecture in comparison to MOF 1, which is composed of bpy pillars connecting inorganic Mn-V-O 2D layer (Figure 1d).…”

Hydrogen-Bonding Assembly of Coordination Polymers Showing Reversible Dynamic Solid-State Structural Transformations

“…Coordination polymers containing guest molecules are very attractive research subjects due to their flexibilities and tunable functional applications, which provide an opportunity to develop advanced functional materials [1][2][3][4][5]. To design coordination polymers with excellent performance, the synergistic effects between the metal ions, ligands, and guest molecules are essential factors [6][7][8][9]. Weak but significant supramolecular interactions between coordination frameworks and guest molecules, as well as reversible metal-ligand covalent bonds which generate notable flexibility of structural topologies and enhance functionalities, are quite important in supramolecular compounds [10].…”