Solvo-hydrothermal approach for the shape-selective synthesis of vanadium oxide nanocrystals and their characterization.

A new solvo-hydrothermal method has been developed for the synthesis of uniform vanadium oxide nanocrystals (NCs) with various sizes and shapes in aliphatic amine/toluene/water using V(V) diperoxo alkylammonium complexes. The vanadium complex precursors were prepared from an ion exchange reaction of V(V) diperoxo gels and tetraalkylammonium bromide in the water-toluene mixture using H(2)O(2) solution and commercial bulk V(2)O(5) powders as starting vanadium gel source. The obtained VO(2) NC products were characterized by means of transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Fourier transform infrared absorption spectroscopy (FTIR), thermogravimetric differential thermal analysis (TGA-DTA), and nitrogen adsorption/desorption (BET). The size and shape of NCs can be controlled by different synthesis parameters such as water content, steric ligands of complexes, alkyl chain lengths of capping aliphatic amines, as well as nature of solvent. Monodisperse vanadium oxide NCs with various sizes and shapes, nanospheres, nanocubes, nanorices, and nanorods, can be easily achieved. The possible mechanisms for the formation of vanadium complex precursors and vanadium oxide NCs as well as the shape evolution of NCs were also discussed. The as-made vanadium oxide products exhibited the monoclinic rutile VO(2) structure, which was however converted to the orthorhombic V(2)O(4.6) structure after calcination in air. The XPS results also revealed only one V(4+) state for the as-made sample; however, the coexistence of V(5+) and V(4+) states and two components of oxygen associated with OV and O-V for the calcined samples on the vanadium oxide NC surface were observed. The surface chemical composition of both as-made and calcined samples were found to be VO(2) and V(2)O(5-x) (x = 0.4), respectively. Our approach may provide a novel route for the extended synthesis to other inorganic NCs.