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

Abstract

Two new ammonium vanadate hydrates, i.e., M 3 (H 2 O) 2 V 8 O 24 •2NH 4 (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(H 2 O) 2 2+ (i.e., part of the oxide framework) and nonbridging NH 4+ cations over the same site (1:2 ratio), wherein two NH 4 + ions occupy the same site as the two H 2 O 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 NH 4 + /H 2 O 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(H 2 O) 2 2+ and NH 4 + cations that alternate down its c-axis within a larger superstructure related to I by (a → c, b → 2b, c → 2a). Within both structures, the respective Mn 2+ /Co 2+ cations bridge to neighboring edge-sharing chains of distorted VO 5 square pyramids, forming a three-dimensional network that contains channels of H 2 O and NH 4 + 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 H 2 O and NH 4 + molecules, by ∼300 °C for I and a slightly higher ∼325 °C for II, in each case yielding V 2 O 5 and MV 2 O 6 (M = Co or Ni) as the final products. Both I and II exhibit visiblelight 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 H 2 g −1 h −1 and ∼54 μmol H 2 g −1 h −1 (irradiant power density of ∼1.0 W/cm 2 ). 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.

One-Dimensional Layered Sodium Vanadate Nanobelts: A Potential Aspirant for High-Performance Supercapacitor Applications

“…into vanadium oxides resulted in a new class of multifunctional vanadium-based ternary compounds known as metal vanadates. − Generally, the metal vanadates have the form of M x V y O z (where M can be a variety of metals including Zn, Cu, Ni, Fe, Mn, etc.) and contain layered structures with large d -spacings, multiple oxidation states, and open structures which support fast ion diffusion and the synergistic effect between the transition metal and vanadium. , Nowadays, due to desirable optical and electrochemical properties owing to their peculiar structures, these metal vanadate structures have been used for catalysts, optoelectronics, metal ion batteries, and SCs. − To date, different kinds of metal vanadate materials, such as MnV 2 O 6 , Cu 3 V 2 O 7 , Zn 3 V 2 O 8 , Ni 3 V 2 O 8 , Co 3 V 2 O 8 , FeVO 4 , etc., have been prepared, and their electrochemical performances were studied. , However, very few attempts have been made to prepare alkali metal (such as Li, Na, K, etc. )-based metal vanadates and investigate their energy storage applications.…”