Formation and properties of copper(II) divanadate(V)

Photoelectrochemical properties of copper pyrovanadate (Cu2V2O7) thin films synthesized by pulsed laser deposition

Polymorphic phases of copper pyrovanadate (α- and β-Cu₂V₂O₇) were synthesized by solid state reaction and the mechanisms governing the phase transitions have been highlighted by the ThermoGravimetric Analysis (TGA) and the Differential Scanning Calorimetry (DSC). The thermal evolution of the lattice parameters was determined by high temperature X-ray Diffraction revealing negative thermal expansion coefficients. The thermogravimetric analysis coupled with differential scanning calorimetry was also used to determine the optimal conditions to obtain a dense target in order to produce thin films by the Pulsed Laser Deposition (PLD) technique. Thin films elaborated under different oxygen pressures and temperatures exhibit a β-Cu₂V₂O₇ polycrystalline phase and their band gap indicates absorption in the visible range. These oxides can be used as photoanodes and their photoelectrochemical properties were studied for both bulk (α-Cu₂V₂O₇) and thin films (β-Cu₂V₂O₇), as a function of the wavelength and/or intensity of the luminous flux. The best photocurrent efficiency was obtained under 450 nm illumination. Moreover, in the case of thin films, we have observed a linear evolution of the current density with the luminous flux. Finally, the photostability of thin films was measured and shows a reduction in the photocurrent of 8% after 1 h of measurement. This photocorrosion phenomenon was also highlighted by the elemental mapping performed on thin films by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectrometry (EDS).

Pressure-Induced Phase Transition and Band Gap Decrease in Semiconducting β-Cu2V2O7

The understanding of the interplay between crystal structure and electronic structure in semiconductor materials is of great importance due to their potential technological applications. Pressure is an ideal external control parameter to tune the crystal structures of semiconductor materials in order to investigate their emergent piezo-electrical and optical properties. Accordingly, we investigate here the high-pressure behavior of the semiconducting antiferromagnetic material β-Cu2V2O7, finding it undergoes a pressure-induced phase transition to γ-Cu2V2O7 below 4000 atm. The pressure-induced structural and electronic evolutions are investigated by single-crystal X-ray diffraction, absorption spectroscopy and ab initio density functional theory calculations. β-Cu2V2O7 has previously been suggested as a promising photocatalyst for water splitting. Now, these new results suggest that β-Cu2V2O7 could also be of interest with regards to barocaloric effects, due to the low phase -transition pressure, in particular because it is a multiferroic material. Moreover, the phase transition involves an electronic band gap decrease of approximately 0.2 eV (from 1.93 to 1.75 eV) and a large structural volume collapse of approximately 7%.

New Solid Solution and Phase Equilibria in the Subsolidus Area of the Three-Component CuO-V2O5-Ta2O5 Oxide System

The results of the study of the three-component system of CuO–V₂O₅–Ta₂O₅ oxides showed, inter alia, that in the air atmosphere in one of its cross-sections, i.e., in the CuV₂O₆–CuTa₂O₆ system, a new substitutional solid solution with the general formula CuTa_2−xV_xO₆ and homogeneity range for x > 0.0 and x ≤ 0.3 is formed. The influence of the degree of incorporation of V⁵⁺ ions into the CuTa₂O₆ crystal lattice in place of Ta⁵⁺ ions on the unit cell volume, thermal stability and IR spectra of the obtained solid solution was determined. Moreover, the value of the band gap energy of the CuTa_2−xV_xO₆ solid solution was estimated in the range of 0.0 < x ≤ 0.3, and on this basis, the new solid solution was classified as a semiconductor. On the basis of the research results, the studied system of CuO–V₂O₅–Ta₂O₅ oxides was also divided into 12 subsidiary subsystems.

Chemical transformations at the nanoscale: nanocrystal-seeded synthesis of β-Cu2V2O7 with enhanced photoconversion efficiencies

Nanocrystal-seeded synthesis relies on the reaction of nanocrystal seeds with a molecular precursor and it can be regarded as the link between sol–gel and solid-state chemistries.

Nanocrystal-seeded synthesis relies on the reaction of nanocrystal seeds with a molecular precursor and it can be regarded as the link between sol–gel and solid-state chemistries. This synthesis approach aims at accessing compositionally complex materials, yet to date its full potential remains unexploited. Herein, surface oxidized Cu nanocrystal seeds with diameters from 6 nm to 70 nm are reacted with vanadium acetylacetonate to form β-Cu₂V₂O₇ with a tunable grain size ranging from 29 nm to 63 nm. In situ X-ray diffraction measurements evidence the occurrence of a solid-state reaction between the NC seeds and the vanadium oxide formed during the annealing. The variation of the ion diffusion lengths, the homogeneity of the precursor solution and the number of nucleation sites with the NC seed size explains the lower formation temperature, the smaller grain size and the higher grain size monodispersity of β-Cu₂V₂O₇ as the seed size decreases. Finally, the tunability afforded by the nanocrystal-seeded synthesis provides a unique opportunity to correlate the photoelectrochemical performance with the grain size in a size regime close to the charge carrier diffusion length of β-Cu₂V₂O₇ (20–40 nm). The net photocurrent density peaks when the grain size is 39 nm by reaching 0.23 mA cm^–2 at 1.23 V vs. RHE in the presence of a hole scavenger. While still far from the theoretical limit, this result overcomes the current state-of-the-art for β-Cu₂V₂O₇. An interesting double fold increase in the photocurrent is found in mixed phase β-Cu₂V₂O₇/CuV₂O₆ samples, suggesting that nanostructuring and heterostructuring are beneficial to the performance.