Preparation and characterization of VO2 nanopowders

VO2 powders with sizes of < 30 nm were successfully synthesized by pyrolysi s of the precursor, [NH4](5)[(VO)(6) (CO3)(4)(OH)(9)]<bullet>10H(2)O. The e ffects of various pyrolysis conditions on VO2 stoichiometries and crystal s tates mere investigated in detail. The results of IR measurements show that for the stoichiometric VO2, from micro- to nanocrystals and to amorphous s tate, the absorptions shifted to lower wavenumbers, the numbers of bands de creased gradually, and the widths of the bands broadened. Moreover, the IR spectra of nanocrystals were obviously different from those of microcrystal s and amorphism. In comparison with stoichiometric VO2 crystals, the IR abs orptions of oxygen-rich VO2 crystals clearly were blue shift, and those of oxygen-deficient VO2 crystals lightly red shift. The heats and temperatures of phase transition from VO1.96 to VO2.07 were determined. The results ind icate that the phase transition temperature of VO2.02 is 70.1 degreesC and has the maximum phase transition heat. (C) 2001 Academic Press.