From VO2 (B) to VO2 (A) nanobelts: first hydrothermal transformation, spectroscopic study and first principles calculation

Abstract

The phase transition process from VO₂ (B) to VO₂ (A) was first observed through a mild hydrothermal approach, using hybrid density functional theory (DFT) calculations and crystallographic VO₂ topology analysis. All theoretical analyses reveal that VO₂ (A) is a thermodynamically stable phase and has a lower formation energy compared with the metastable VO₂ (B). For the first time, X-ray absorption spectroscopy (XAS) of the V L-edge and O K-edge was performed on different VO₂ phases, and the differences in the electronic structure of the two polymorphic forms provide further experimental evidence of the more stable VO₂ (A). Consequently, transformation from VO₂ (B) to VO₂ (A) is much easier to be realized from a dynamical point of view. Notably, the transformation of VO₂ (B) into VO₂ (A) show the sequence VO₂ (B)-high-temperature VO₂ (A_H) phase-low-temperature VO₂ (A) phase, which was achieved by hydrothermal treatment, respectively. Also, an alternative synthesis route was proposed based on the above hydrothermal transformation, and VO₂ (A) was successfully prepared via the simple one-step hydrothermal method by hydrolysis of VO(acac)₂ (acac = acetylacetonate). Therefore, VO₂ nanostructures with controlled phase compositions can be obtained in high yields. Through elucidating the structural evolution in the crystallographic shear mechanism, we can easily guide the design of other metal oxide nanostructures with controllable phases.