Viscosity Controlled Processing of TiO2@SiO2 Green Sol-gel Coatings for Environmental Photocatalysis

This research presents a green and sustainable approach to synthesize TiO2 and SiO2 photocatalyst coatings for environmental cleanup, following the European Green directives on eco-friendly materials development [1]. Both TiO2 and SiO2 sol-gels were prepared by eco-friendly reagents including water as solvent/hydrolyzing agent and acetic acid as catalyst/chelating agent and respective precursor (TTIP and TEOS). TiO2 was the primary photocatalyst, while SiO2 was used as a functional interlayer to enhance the adhesion of TiO2 onto glass substrates, prevent ion diffusion from the substrate, and increase the surface area available for photocatalytic reactions. The initial sols were prepared in summer and subjected to aging to evaluate their photocatalytic activity over time, using Rhodamine B (RhB) degradation as a model reaction for water purification. The properties sols and corresponding coatings were analyzed through DLS, XRD, and SEM. The aging process consistently improved the viscous appearance of sols, with significant photocatalytic efficiency correlating with sol stability over time. Challenges arose during resynthesis in winters, where low viscosity ranging in 1-2 mPa.s of sols over aging resulted in non-uniform and poor photocatalyst loading over substrate (~ 0.016 mg/cm2). This was addressed by a post-aging thermal incubation at 50 °C for around 30 hours after two weeks of aging. This process induced sufficient viscosity to sols within the range of 6-8 mPa.s, enabling uniform and homogeneous deposition with substantial loading (~ 0.065 mg/cm2 for single layer) of material. Viscosity was monitored weekly throughout the aging process to optimize the sols behavior. The TiO2 sol was stored at 4 °C to assess their long-life, but it showed thermoreversible behaviour leading to a consistent decrease in viscosity form 6 mPa·s to 2.7 mPa·s within one week [2]. The optimized coatings were tested for gas-phase formaldehyde (FMD) degradation in a continuous-flow photoreactor and for self-cleaning activity according to ISO 27448:2009 standard by the photodegradation of oleic acid under UV-A exposure. A single-layer coating of optimized TiO2 sol achieved degradation of FMD (10 ppm) under UV-A irradiance upto 100%, 73%, and 56% at 300, 500, and 700 mL.min-1 flow rates, respectively. TiO2@SiO2 coatings showed 20 % higher efficiency compared to pure TiO2 coatings confirming the SiO2 contribution towards improved surface area and higher degradation efficacy by inhibiting the ions diffusion from substrate. Similarly, optimized TiO2 coatings showed a photoinduced super hydrophilic behaviour with the water contact angle measurements with values below 10° after 180 min under UV-A exposure, confirming effective surface activation and strong self-cleaning nature [3]. This study highlights the critical influence of sol aging, viscosity control, thermoreversibility in green synthesis and processing of TiO2 based photocatalytic coatings along with functional role of SiO2 interlayer for efficient water and air purification. References. [1]. EU Commission. (2023). COM/2023/166 final. https://eur-lex. europa.eu/legal-content/EN/TXT. [2]. Jeong, B., et. al., (2012). Advanced drug delivery reviews, 64, 154-162. [3]. Mateos, M. J., et. al., (2024). J. Photochem. Photobiol. A: Chem. 457, 115880.