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Synthesis of New Mixed-Metal Ammonium Vanadates: Cation Order versus Disorder, and Optical and Photocatalytic Properties

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journal contribution
posted on 06.09.2016, 00:00 by Lan Luo, Erkang Ou, Tatyana I. Smirnova, Paul A. Maggard
Two new ammonium vanadate hydrates, i.e., M3(H2O)2V8O24­·2NH4 (M = Mn and Co, I and II, respectively) were synthesized using hydrothermal reaction conditions, and their structures were determined by single crystal X-ray diffraction [I: P2/m (No. 10), Z = 1, a = 8.2011(2) Å, b = 3.5207(1) Å, c = 9.9129(3) Å, β = 110.987(2)°; II: C2/m (No. 12), Z = 2, a = 19.4594(6) Å, b = 6.7554(2) Å, c = 8.4747(3) Å, β = 112.098(2)°]. Interestingly, the two structures are homeotypic, with the structure of I exhibiting an uncommon type of structural disorder between locally-bridging Mn­(H2O)22+ (i.e., part of the oxide framework) and nonbridging NH4+ cations over the same site (1:2 ratio), wherein two NH4+ ions occupy the same site as the two H2O molecules when Mn­(II) is vacant. The amount of Mn­(II) in the formula of I was determined by a combination of techniques, including electron paramagnetic resonance, while the relative amounts of NH4+/H2O in its structure were determined by combined thermogravimetric-mass spectrometry analyses as well as confirmed by infrared spectroscopy. In contrast, this site disorder is absent in the crystal structure of II, which contains a fully ordered arrangement of locally-bridging Co­(H2O)22+ and NH4+ cations that alternate down its c-axis within a larger superstructure related to I by (ac, b → 2b, c → 2a). Within both structures, the respective Mn2+/Co2+ cations bridge to neighboring edge-sharing chains of distorted VO5 square pyramids, forming a three-dimensional network that contains channels of H2O and NH4+ molecules. Hydrogen bonding distances in I are significantly longer and weaker than in II and leading to the disordered structure of I. Both show the loss of all H2O and NH4+ molecules, by ∼300 °C for I and a slightly higher ∼325 °C for II, in each case yielding V2O5 and MV2O6 (M = Co or Ni) as the final products. Both I and II exhibit visible-light bandgap sizes of ∼1.55 and ∼1.77 eV, respectively, owing to low-energy metal-to-metal electronic transitions. Further, I shows a temperature-dependent photocatalytic activity (at 40 °C) for the production of hydrogen from the reduction of water under irradiation by UV–vis or only visible light at respective rates of ∼314 μmol H2 g–1 h–1 and ∼54 μmol H2 g–1 h–1 (irradiant power density of ∼1.0 W/cm2). Thus, these first two known ammonium vanadate hydrates provide new insights into the structural driving forces for ordered versus disordered structures as well as into their resulting physical properties.