β-VO2—a V(IV) or a mixed-valence V(III)–V(V) oxide—studied by 51V MAS NMR spectroscopy

Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO₂, whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO₂ from 1.86 to 2.44 on the band structure and insulator-metal transitions are presented for the first time. Results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO₂ results in the formation of e^- and h⁺ pairs that lead to delocalized polaronic V³⁺ and V⁵⁺ cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V-O bond length, localization of V³⁺ electrons at V⁵⁺ sites, which results in the formation of V⁴⁺-V⁴⁺ dimers, and removal of [Formula: see text] screening electrons. It is shown that the nature of phase transitions is linked to the lattice V³⁺/V⁵⁺ concentrations of stoichiometric VO₂ and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition.

Insights into the Exfoliation Process of V2O5·nH2O Nanosheet Formation Using Real-Time 51V NMR

Nanostructured hydrated vanadium oxides (V₂O₅·nH₂O) are actively being researched for applications in energy storage, catalysis, and gas sensors. Recently, a one-step exfoliation technique for fabricating V₂O₅·nH₂O nanosheets in aqueous media was reported; however, the underlying mechanism of exfoliation has been challenging to study. Herein, we followed the synthesis of V₂O₅·nH₂O nanosheets from the V₂O₅ and VO₂ precursors in real time using solution- and solid-state ⁵¹V NMR. Solution-state ⁵¹V NMR showed that the aqueous solution contained mostly the decavanadate anion [H₂V₁₀O₂₈]^4- and the hydrated dioxovanadate cation [VO₂·4H₂O]⁺, and during the exfoliation process, decavanadate was formed, while the amount of [VO₂·4H₂O]⁺ remained constant. The conversion of the solid precursor V₂O₅, which was monitored with solid-state ⁵¹V NMR, was initiated when VO₂ was in its monoclinic forms. The dried V₂O₅·nH₂O nanosheets were weakly paramagnetic because of a minor content of isolated V⁴⁺. Its solid-state ⁵¹V signal was less than 20% of V₂O₅ and arose from diamagnetic V⁴⁺ or V⁵⁺.This study demonstrates the use of real-time NMR techniques as a powerful analysis tool for the exfoliation of bulk materials into nanosheets. A deeper understanding of this process will pave the way to tailor these important materials.

Multiple Redox Modes in the Reversible Lithiation of High-Capacity, Peierls-Distorted Vanadium Sulfide

Vanadium sulfide VS4 in the patronite mineral structure is a linear chain compound comprising vanadium atoms coordinated by disulfide anions [S2](2-). (51)V NMR shows that the material, despite having V formally in the d(1) configuration, is diamagnetic, suggesting potential dimerization through metal-metal bonding associated with a Peierls distortion of the linear chains. This is supported by density functional calculations, and is also consistent with the observed alternation in V-V distances of 2.8 and 3.2 Å along the chains. Partial lithiation results in reduction of the disulfide ions to sulfide S(2-), via an internal redox process whereby an electron from V(4+) is transferred to [S2](2-) resulting in oxidation of V(4+) to V(5+) and reduction of the [S2](2-) to S(2-) to form Li3VS4 containing tetrahedral [VS4](3-) anions. On further lithiation this is followed by reduction of the V(5+) in Li3VS4 to form Li3+xVS4 (x = 0.5-1), a mixed valent V(4+)/V(5+) compound. Eventually reduction to Li2S plus elemental V occurs. Despite the complex redox processes involving both the cation and the anion occurring in this material, the system is found to be partially reversible between 0 and 3 V. The unusual redox processes in this system are elucidated using a suite of short-range characterization tools including (51)V nuclear magnetic resonance spectroscopy (NMR), S K-edge X-ray absorption near edge spectroscopy (XANES), and pair distribution function (PDF) analysis of X-ray data.

Using hyperbolic secant pulses to assist characterization of chemical shift tensors for half-integer spin quadrupolar nuclei in MAS powder samples

Determination of the NMR anisotropic magnetic shielding parameters from magic angle spinning, MAS, powder samples containing half-integer spin quadrupolar nuclei is achieved by analysis of the difference spectrum obtained with and without application of a hyperbolic secant pulse. Application of a hyperbolic secant pulse to any spinning sideband associated with the central transition, m(I) = 1/2 to m(I) = - 1/2, results in 'saturation' of the entire central transition manifold. Similarly, if one spinning sideband associated with the m(I) = 3/2 to m(I) = 1/2 and m(I) = - 1/2 to m(I) = - 3/2 satellite transitions is perturbed, the entire satellite manifold associated with these transitions is 'saturated' while the central transition is enhanced by population transfer. Three 'difference spectrum' techniques are employed to selectively yield the spinning sidebands associated predominantly from the central transition. The success of these difference techniques is first demonstrated by examining (51)V NMR spectra of three metavanadate salts and (59)Co NMR spectra of Co(acac)(3). The vanadium and cobalt chemical shift tensors in these compounds have spans between 400 and 1400 ppm. Because the hyperbolic secant techniques proposed here yielded results that are in good agreement with earlier reports, they have been applied to characterize the (51)V chemical shift tensor of the dimer of bis(N, N-dimethylhydroxamido)-hydroxooxovanadate, {V(O)(ONMe(2))(2)}(2)O, whose chemical shift tensor has not been previously reported.

The complete 51V MAS NMR spectrum of surface vanadia nanoparticles on anatase (TiO2): vanadia surface structure of a DeNOx catalyst

The first observations of the complete manifold of spinning sidebands (ssbs) including both the central and satellite transitions in (51)V MAS NMR spectra of surface vanadia nanoparticles on titania in DeNO(x) catalysts are presented. (51)V quadrupole coupling and chemical shift anisotropy parameters for the dominating vanadia structure are determined from (51)V MAS NMR spectra recorded at 9.4 and 14.1 T. Based on correlations previously established between (51)V NMR parameters and crystal structure data for inorganic vanadates, the NMR data are consistent with vanadium in a distorted octahedral oxygen coordination environment for the so-called strongly bonded vanadia species on the surface. The investigation includes two vanadia-titania model catalysts and six industrial-type DeNO(x) catalysts.