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

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 the 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/TESPMA/TEOS hybrid xerogels containing alkali-metal sulfates, although no coloration of the hybrid xerogel without alkali-metal sulfate was observed. The coloration behavior 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”.