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

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