Synthesis, characterization and formation mechanism of metastable phase VO2(A) nanorods

• Pure phases of VO2(B) and VO2(A) were prepared by a facile hydrothermal method.
• Belt-like particles prepared at 180 °C was indexed as monoclinic VO2(B) phase.
• Rod-like particles prepared at 230 °C was indexed as tetragonal VO2(A) phase.
• VO2(A) nanorods resulted from VO2(B) nanobelts by assembly and crystal adjustment.

Pure phase VO2(A) nanorods were synthesized via the reduction of V2O5 by oxalic acid during the hydrothermal treatment. Two sets of samples were prepared by varying both system temperature and reaction time under a filling ratio of 0.40 for observing the formation and evolution of VO2(A) nanorods. Structures were characterized by X-ray diffraction, scanning and transmission electron microscopies, respectively. It was found that VO2(B) was firstly formed and then transformed into VO2(A) as the increasing system temperature or extending reaction time. An assembling and following crystal adjustment was proposed for explanation the formation process of VO2(A) from VO2(B). For VO2(A) nanorods, the phase transition temperature of 169.7 °C was higher than that of the VO2(A) bulk, it might be ascribed to the lower crystallinity or nonstoichiometry in VO2(A) nanorods. VO2 nanostructures with controllable phases and properties should find their promising applications in a single VO2 nanodevice.

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