Aqueous citrato-oxovanadate(IV) precursor solutions for VO2: synthesis, spectroscopic investigation and thermal analysis
An aqueous precursor solution, containing citrato-VO2+ complexes, is synthesized for the formation of monoclinic VO2. With regard to the decomposition of the VO2+ complexes towards vanadium oxide formation, it is important to gain insights into the chemical structure and transformations of the precursor during synthesis and thermal treatment. Hence, the conversion of the cyclic [V4O12]4− ion to the VO2+ ion in aqueous solution, using oxalic acid as an acidifier and a reducing agent, is studied by 51Vanadium nuclear magnetic resonance spectroscopy. The citrate complexation of this VO2+ ion and the differentiation between a solution containing citrato-oxalato-VO2+ and citrato-VO2+ complexes are studied by electron paramagnetic resonance and Fourier transform infra-red spectroscopy. In both solutions, the VO2+ containing complex is mononuclear and has a distorted octahedral geometry with a fourfold R-CO2− ligation at the equatorial positions and likely a fifth R-CO2− ligation at the axial position. Small differences in the thermal decomposition pathway between the gel containing citrato-oxalato-VO2+ complexes and the oxalate-free gel containing citrato-VO2+ complexes are observed between 150 and 200 °C in air and are assigned to the presence of (NH4)2C2O4 in the citrato-oxalato-VO2+ solution. Both precursor solutions are successfully used for the formation of crystalline vanadium oxide nanostructures on SiO2, after thermal annealing at 500 °C in a 0.1% O2 atmosphere. However, the citrato-oxalato-VO2+ and the oxalate-free citrato-VO2+ solution result in the formation of monoclinic V6O13 and monoclinic VO2, respectively.