Solution Combustion Synthesis and Characterization of Magnesium Copper Vanadates

Abstract

Magnesium vanadate (MgV₂O₆) and its alloys with copper vanadate were synthesized via the solution combustion technique. Phase purity and solid solution formation were confirmed by a variety of experimental techniques, supported by electronic structure simulations based on density functional theory (DFT). Powder X-ray diffraction combined with Rietveld refinement, laser Raman spectroscopy, diffuse reflectance spectroscopy, and high-resolution transmission electron microscopy showed single-phase alloy formation despite the MgV₂O₆ and CuV₂O₆ end members exhibiting monoclinic and triclinic crystal systems, respectively. DFT-calculated optical band gaps showed close agreement in the computed optical bandgaps with experimentally derived values. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements confirmed a systematic variation in the optical bandgap modification and band alignment as a function of stoichiometry in the alloy composition. These data indicated n-type semiconductor behavior for all the samples which was confirmed by photoelectrochemical measurements.

Figure 1

Compositional line diagrams exhibiting the stoichiometric relationship of binary and ternary oxides in the Mg–V–O and Mg–Cu–V–O phase spaces.

Figure 2

Crystal structures for (a) MgV₂O₆ and (b) CuV₂O₆.

Figure 3

(a) PXRD data in the range of 2θ = 10–60° (b) zoomed over 2θ = 25–35° for the four Mg_1–xCu_xV₂O₆ alloys and the two end members. Reference patterns for the two end members are also shown in black at the bottom of each frame for comparison.

Figure 4

(a–d) SAED patterns for the two end members (frames a, f) and the four Mg_1–xCu_xV₂O₆ alloy samples (frames b–e) as x = 1 → 0.

Figure 5

(a) Raman spectra for the four Mg_1–xCu_xV₂O₆ samples and the two end members in the range, 100–1000 cm^–1. (b) Corresponding spectra in the shift range from 100–500 cm^–1.

Figure 6

High-resolution XPS data for (a) Mg 1s in MgV₂O₆, (b) Cu 2p in CuV₂O₆. (c and d) V 2p and O 1s in MgV₂O₆ and CuV₂O₆ respectively. Peak deconvolutions are also shown in selected cases.

Figure 7

(a) UV–visible spectra, (b) direct Tauc plots, (c) indirect Tauc plots, and (d) energy bandgap variation with Cu content for the two end members and the four Mg_1–xCu_xV₂O₆ alloy samples. The lines in frame (d) are drawn for visualization.

Figure 8

Orbital-projected DOS plots for CuV₂O₆ and MgV₂O₆ and three alloys using the PBE formulation of the generalized gradient approximation to the exchange–correlation functional in DFT. The plots depict the DOS for (a) CuV₂O₆, (b) Cu_0.75Mg_0.25V₂O₆, (c) Cu_0.50Mg_0.50V₂O₆, (d) Cu_0.25Mg_0.75V₂O₆, and (e) MgV₂O₆.

Figure 9

Orbital-projected DOS for (a) CuV₂O₆ and (b) MgV₂O₆ using density functional theory with a hybrid density functional.

Figure 10

Linear sweep photovoltammograms for CuV₂O₆ and three Mg_1–xCu_xV₂O₆ alloy samples (a) in 0.1 M borate buffer (pH = 8) and (b) in 0.1 M borate buffer + 4 M formate soln. (pH = 9). (c) Chronoamperometric measurement for 2 h in 0.1 M borate buffer at 1.23 V vs RHE. (d) Raman spectra of deposited films after the photostability test.