Low-temperature fabrication of nanocrystalline photocatalytic TiO2 thin films on polymeric substrates

Abstract

This work investigates the use of different sol–gel routes for fabricating photocatalytic thin TiO2 films on polymeric substrates. Systematic investigation of the sol–gel process was conducted which involved the preparation of TiO2 suspensions by different sol–gel routes, such as: (1) non-aqueous processing (well-controlled hydrolysis rate), (2) alkoxide hydrolysis with excess water + HNO3 + aging at 35°C, (3) sol–gel with excess water + varying HNO3 concentrations (molar ratio of HNO3/Ti = 0.05–1.00) plus refluxing at 80°C, and (4) sol–gel with excess water + HNO3, followed by hydrothermal treatment at 150°C.

The major findings are as follows: (1) the first route resulted in amorphous TiO2 with BET surface area close to zero and pore volume which is not detectable, (2) the second and third routes produced nanocrystalline TiO2 with degree of crystallinity of ~30–35% and ~45–55%, respectively, crystallite sizes of ~4.0 nm and ~4.0–5.0 nm, respectively, BET surface area of 170 m2/g and 236 m2/g, respectively, and pore volumes of 0.047 cm3/g and 0.076 cm3/g, respectively, (3) in the case of crystalline TiO2 films, the films with larger BET surface area displayed higher photoactivity, (4) the fourth route produced nanocrystalline TiO2 with degree of crystallinity ~70% and crystallite sizes of ~8.5 nm, (5) the TiO2 samples derived from the non-aqueous sol–gel route and aqueous sol–gel route behaved differently during thermally-induced crystallisation, (6) refluxing at 80°C promotes the crystallisation of TiO2 and the formation of homogeneous TiO2 colloids. The high colloidal homogeneity is a base for suspension stability and results in films of higher porosity and thus higher photoactivity, (7) hydrothermal treatment of TiO2 suspensions resulted in highly crystalline TiO2 with crystallite interfaces; which led to films with optical band-gaps comparable to bulk crystalline TiO2, and high photoactivities, and (8) a novel route for production of stable TiO2–Ag nanocomposite suspensions and TiO2–Ag nanocomposite films was developed.

The combination of photoactive TiO2 and polymeric substrates (flexible, lightweight and inexpensive) has the potential for harvesting direct solar energy for applications such as self-cleaning and self-sterilising materials, pollutant degradation, and flexible photovoltaics.