Irreversible phase transition in BiVO4 nanostructures synthesized by a polyol method and enhancement in photo degradation of methylene blue

Abstract

Structure and bonding in metal oxides has been studied extensively using Raman vibrational spectroscopy and is found to provide complementary information to the crystallographic observations. In this study, BiVO₄ nanostructures with pure and stable crystal phases were synthesized selectively by reaction between Bi(NO₃)₃·5H₂O and NH₄VO₃ in ethylene glycol at temperatures between 80 °C to 140 °C. A high temperature tetragonal scheelite (t-s) phase was obtained directly at synthesis temperature, whereas calcination at 600 °C resulted in phase transformation to a monoclinic scheelite (m-s) crystal structure. Distortion of the VO₄³⁻ tetrahedron, being the reason for the phase transition that results in two sets of V–O bonds, was confirmed by bond length and bond strength calculations with the help of Raman spectroscopy. Irreversibility of the phase transition was observed through Differential Scanning Calorimetric (DSC) analysis and X-Ray Diffraction (XRD). The band gap was found to vary between 1.97 to 2.33 eV with respect to synthesis temperature as well as calcination temperature. Nanostructures synthesized at 110 °C and calcined at 600 °C for 3 h had a band gap of 2.25 eV and were found to bring about the highest photocatalytic degradation of MB under visible light irradiation. Increase in the distortion of the VO₄³⁻ tetrahedron in terms of increase in the difference between bond lengths of two sets of V–O bonds was found to be the reason for the enhancement in the photodegradation of MB over BiVO₄ catalysts with a monoclinic scheelite crystal structure.