Kinetics of Coloration in Photochromic Tungsten(VI) Oxide/Silicon Oxycarbide/Silica Hybrid Xerogel: Insight into Cation Self-diffusion Mechanisms

depended exclusively on a variety of alkali-metal cations present in the hybrid xerogel system. Furthermore, a detailed analysis of the self-diffusion mechanism confirmed that the alkali-metal cations electrostatically interact with a layer of unreacted silanol groups on the TESPMA/TEOS matrix surface,and subsequently pass through the interconnected pore network of the hybrid xerogel. More interestingly, in the context of an Arrhenius analysis, we found a good coincidence between the activation energies for alkali-metal cation self-diffusion and UV-induced coloration in the WO3/TESPMA/TEOS hybrid xerogel system containing the corresponding alkali-metal sulfate. It is experimentally obvious that the photochromic properties are dominated by the diffusion process of alkali-metal cations in the WO3/TESPMA/TEOS hybrid xerogel system. Such hybrid materials with cation controlled photochromic properties will show promising prospects' in applications demanding energy-efficient "smart windows" and "smart glasses". Silicon oxycarbide/silica composites with well-dispersed tungsten(VI) oxide (WO3) nanoparticles were obtained as transparent hybrid xerogels via an acid-catalyzed sol gel process (hydrolysis/condensation polymerization) of 3-(triethoxysilyl)propyl methacrylate (TESPMA) and tetraethoxysilane (TEOS). The self-diffusion mechanism of alkali-metal cations and the kinetics of the photochromic-coloration process in WO3/TESPMA/TEOS hybrid xerogel systems have-been, systematically investigated. Under continuous UV. illumination, a gradual color change (colorless -> blue) corresponding to the reduction of W6+ into W5+ states in WO3 nanoparticles can be confirmed from the WO3/TBSPMA/TEOS hybrid xerogels containing alkali-metal sulfates, although:no coloration of the hybrid xerogel without alkali-metal sulfate was observed. The coloration behavior