The influence of processing conditions on the morphology and thermochromic properties of vanadium oxide films

Vanadium oxides belong to a class of multifunctional materials with a wide range of applications spanning catalysis to optical switches and smart windows for intelligent energy management. The most intriguing oxides of this family are VO2 and V2O5 with peculiar optical properties that can be switched either by temperature variation (thermochromism) or electrical field (electrochromism). In this paper we address aspects of solution processing thin films that were until now ignored. We have established that the morphology of the films obtained depends on the drying temperature (Td) of the films after spin-coating. The resultant morphology is only revealed after final annealing at an optimum temperature of 560 °C. We show that the presence of polyethylene glycol as a film promoting agent, together with sample drying temperature after spin-coating result in drastic changes in film morphologies. When Td is < 40 °C, mesoporous films with equiaxed VO2 grains are observed, whereas for Td ≥ 55 °C self-organized hemispherical micro-discs of V2O5 are observed. This influence of Td is correlated with the ebullition temperature of the solvent used, in this case methanol. We also show that V2O5 can be reduced to VO2, demonstrate how the starting morphology affects the final morphology of the product phase, after reduction, and discuss its formation mechanisms. Finally the optical and thermochromic properties are correlated to the different morphologies and structures, and are shown to reflect very well the effects thereof through absorption phenomena and thermochromic switching characteristics.