The Chloroalkoxide Route to Transition Metal Oxides. Synthesis of V2O5 Thin Films and Powders from a Vanadium Chloromethoxide

Vanadium chloromethoxide was prepared by the methanolysis Of VCl4. The addition of water with a H2O:vanadium molar ratio ranging between 0 and 16 changed the solution color from green to deep blue at a rate increasing with water concentration. UV-vis, Fourier transform infrared, Raman, and electron paramagnetic resonance spectroscopy investigations showed that VCl4 precursor is initially methanolyzed to (VO)Cl-x(OCH3)(y) (x + y = 4) monomers. Upon water addition, methoxo ligands undergo hydrolysis. The presence of chlorine ligands and of the vanadyl group explains the long-term stability of the solutions. V2O5 thin films and powders were prepared, respectively, by spin-coating onto silicon substrates and drying of the solutions, followed by heat-treatments at temperatures up to 500 degrees C. Films and powder precursors were characterized by X-ray diffraction, FTIR, Raman, UV-vis, and EPR spectroscopy, transmission electron microscopy, and thermal analyses. The structural evolution begins by condensation of the initial monomers to form amorphous structures with poor extension of the three-dimensional network. Heat-treatment at higher temperatures induces further condensation and the elimination of the chlorine ligands, which leave an oxygen-defective structure and cause the initial formation of substoichiometric V3O7 phase. When the samples were annealed at T >= 400 degrees C, only the V2O5 phase was found. V2O5 powders annealed at 500 degrees C were used in chemoresistive gas sensing devices and were able to detect reducing gases such as ammonia and ethanol over a broad range of concentrations by stable and reliable signal.