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 VCl 4 . The addition of water with a H 2 O: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 VCl 4 precursor is initially methanolyzed to (VO)Cl x (OCH 3 ) 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. V 2 O 5 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 °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 V 3 O 7 phase. When the samples were annealed at T ≥ 400 °C, only the V 2 O 5 phase was found. V 2 O 5 powders annealed at 500 °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.