Time-based investigation of the growth of VO2(B) micro- and nanostructures on vanadium by hydrothermal synthesis

In an autoclave partly-filled with water wherein a V metal strip is placed 70° to the horizontal we observed that VO2 nanorods and exfoliated microspheres were generally grown on the portion of the strip above the water surface while parallelepiped-like structures were generally grown on the portion below. XRD, HRTEM, showed that VO2 structures produced were monoclinic and base centred in crystal structure. FT-IR, Raman Spectroscopy and XPS suggested that V metal was oxidized by hydrothermal synthesis in water to yield two oxides, V2O3, and VO2, with VO2 forming the bulk of the product. A time-based investigation of the synthesis of VO2 micro-nanostructures on V metal by hydrothermal synthesis suggests that the nucleation and growth of VO2/V2O3 micro-nanostructures is temperature-controlled and not kinetics/diffusion-controlled, with optimal temperatures for the synthesis being in the vicinity of 185–190 °C and growth of these oxides taking place in 2 h as opposed to 7 days in previously reported work [L. Jiang et al., VO2 rods with a rectangular cross-section, Journal of Crystal Growth, 310 (2008) 4301–4304]. We propose here a mechanism for the growth of the two types of micro/nanorods above water and the parallelepiped-like structures grown on V metal found below the water level. -- Graphical abstract: Schematic representation of the growth mechanism of VO2 micro/nanorods from (a) exfoliated microspheres and (b) parallelepipeds respectively formed above and below water. Highlights: ► Hydrothermal synthesis of VO2(B) from V metal in H2O occurred in 2 h instead of 7 days. ► V metal strip inclined 70° to the vertical in a partly filled autoclave gave 2 kinds of micro/nanostructures. ► Nanostructured VO2 exfoliated microspheres grew on V strip above H2O, VO2 parallelipeds grew under H2O. ► FT-IR and XPS suggest existence of VO2 and V2O3 in samples. ► A mechanism of growth is proposed.