Highly crystallized mesoporous anatase films with enhanced photoactivity derived from sputter deposited TiO2-WO3 composites: The positive role of sodium

Abstract Soda-lime glass is a widely used substrate for TiO2 films, and the diffusion of Na into TiO2 is detrimental to photoactivity as it creates charge recombination center and promotes the formation of less-active rutile. Here, soda-lime glass is used for sputtering coating of TiO2-WO3 films over a wide range of WO3 molar fractions of 0.23, 0.38, and 0.80. Using TiO2-WO3/Na-free glass and TiO2/soda-lime glass as reference reveals that annealing induces peculiar phase transitions and striking film morphological changes only in the TiO2-WO3/soda-lime glass involving segregation and chemical interaction of WO3 and Na. Upon annealing and soaking, only the composition-optimized TiO2-WO3 (0.23) film impacted by Na is converted to 10-nm-grained, highly crystallized mesoporous anatase coupling some residual WO3 crystallites. By contrast, the Na-free TiO2-WO3 reference film is transformed into poorly crystallized, dense rutile and WO3 grains made of acicular and plate-like defective subgrains. The mesoporous film exhibits sustainable photoinduced superhydrophilicity and exceptionally high photocatalytic activity upon exposure to ultraviolet light due to microstructural improvement, porosity generation, and mechanistic role of the incorporated heterogeneous WO3.