Optical properties of sol-gel derived vanadium oxide films

Vanadium oxide gels can be made from vanadate aqueous solutions or from vanadium alkoxides. The condensation of vanadic acid gives long ribbon-like oxide particles which macroscopically orient in the same direction in aqueous sols when their concentration is larger than 0.12 mol·l−1. These anisotropic sols and gels should be considered as lyotropic nematic liquid crystals. Thick films in which ribbons align along the same direction can be deposited. These oriented coatings exhibit improved electrochemical properties as reversible cathodes for lithium batteries. Amorphous oxo-polymers are formed via the controlled hydrolysis of vanadium alkoxides. They allow the deposition of optically transparent thin films that exhibit interesting electrochromic properties and turn reversibly from yellow to green upon electrochemical reduction. Moreover these alkoxide derived films can be easily reduced into vanadium dioxide. These VO2 thin films exhibit thermochromic properties and could be used as optical switches in the infrared. The transition temperature of these VO2 films can be modified by doping the vanadium oxide with other cations.