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

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 (AH) 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.