Abstract

ABSTRACT
Electrochromic Properties of Vanadium Pentoxide Nanostructured Thin Films
Afaf Almoabadi
The focus of this work is the improvement of the electrochromic properties of vanadium pentoxide thin films in order to expand its use. Indeed, because of its rather poor electrochromic properties, until now, vanadium pentoxide has only been used as a storage material in an electrochromic device, in conjunction with tungsten oxide, molybdenum oxide etc. To this purpose, vanadium pentoxide thin films were prepared under different conditions and characterized by using optical and electrochemical methods. Films were deposited on indium tin oxide (ITO) substrates by dip-coating at both room- and sub-zero temperature (-100C) and porosity in the sol-gel prepared vanadium pentoxide film was created by using templating methods. The morphology, optical and electrochromic properties of the macro- and mesoporous films, prepared in the presence of structure-directing agents such as polystyrene microspheres and triblock copolymer, have been compared with those of dense films. By using various methods to remove the template material, it was shown that the morphology of the vanadium pentoxide film can be controlled and new nanostructures can be created. The transformation of the lamellar into a nanorod structure, observed when the film is heated at 425-500◦C for several hours, resulted in the development of an elegant method for the synthesis of vanadium oxide nanostructures. The electrochromic performance of the nanorods prepared through the thermal treatment was found to be superior to that of the vanadium pentoxide with the layered structure, especially in the near-infrared region, demonstrating their potential for electrochromic applications. The structure, morphology, optical and electrochromic properties of dense and porous vanadium oxide films, coated at low temperature were also determined and compared to those of the corresponding films, deposited under room-temperature conditions. The results indicated that in the films coated at -100C, a residual compressive stress exists that originates from a non-uniformity in depth of the film, most probably, due to the formation of micro voids during the deposition. The micro voids are preserved during the heat-treatment of the films. The “micro void” morphology was found to account for the considerably improved electrochromic properties of the sub-zero dip-coated films. Low-temperature coated films, heated at 4500C for several hours, undergo the transformation from a layered to a highly uniform nanorod structure with important potential optoelectronic applications. The overall aim of this work is thus to evaluate how the morphology of vanadium pentoxide thin films is instrumental in obtaining a material with a high lithium ion intercalation capacity. With an appropriate morphology, the performance of vanadium oxide as electrochromic material and as cathode in lithium ion batteries can be improved significantly. For this purpose, both layered (dense and porous) and nanorod films were prepared and characterized. Scanning electron microscopy, cyclic voltammetry and electrical impedance spectroscopy measurements were used for the characterization of the different V2O5 films.