Tunable photocatalytic selectivity of TiO₂/SiO₂ nanocomposites : Effect of silica and isolation approach

TiO₂ nanoparticles have widely been used as a photocatalyst to eliminate organic contaminants from wastewater. However, the lack of selectivity is considered as one of the main drawbacks of TiO₂ nanoparticles in wastewater treatment applications. To overcome this problem, this study aims at developing highly adsorbent photocatalysts with tunable selective photocatalytic activity. To this end, the selective photocatalysts of anatase TiO₂ nanoparticles and TiO₂/SiO₂ nanocomposites with different surface charges were synthesized through a sol-gel method. TiO₂/SiO₂ nanocomposites were prepared based on three molar ratios of Ti:Si, which were 1:0.43, 1:1, and 1:2.33. Two isolation procedures of base-precipitation (BP method) and solvent-evaporation (SE method) were adopted to extract the nanoparticles from the colloids and clarify their impact on selective performance of the photocatalysts. The sequential decolorization processes of aqueous dye solutions consisting of methylene blue (MB) and methyl orange (MO) in the presence of photocatalysts were monitored under UV illumination and used as probes to compare the selective behaviors of TiO₂ and TiO₂/SiO₂ powders. The results revealed that while commercial TiO₂ (Degussa P-25) decomposed both dyes non-selectively, the synthesized photocatalysts induced a selective and sequential decomposition of dyes. The samples prepared through the BP and SE methods showed selective adsorption and photocatalytic activity in favor of cationic MB and anionic MO dye molecules, respectively. The presence of silica significantly increased the MB dye adsorption on photocatalysts prepared through BP method compared with pure TiO₂. The isolation method of nanoparticles had a direct impact on surface charge and selective performance of photocatalysts.