A salt-assisted approach for the pore-size-tailoring of the ionic-liquid-templated TiO2 photocatalysts exhibiting high activity

In this study, we develop a novel salt-assisted approach to adjusting the pore size of ionic-liquid-templated TiO2 photocatalysts by simply changing the concentrations and types of the inorganic salts. Four types of salts, including NaCl, CaCl2, NH4Cl, and NH4NO3, are selected in order to investigate the effects of the ionic radii and the charges of the constituent ions on the resulting textures. In addition, a mechanism that the salts assist the self-assembly of hydrophilic 1-butyl-3-methylimidazolium chloride (C(4)mimCl) molecules in hydrophobic benzyl alcohol is proposed. The templated TiO2 powder, which incorporates phosphate species to enhance its structural stability, exhibits a small pore size of 4.2 nm and a high surface area of 164 m(2) g(-1). Small amounts of the salts increase the ionic strength, slightly shrinking the template and the pore size, and pore expansion results when the salt/Ti ratio is above 0.2. The salts at a salt/Ti ratio of 1.0 remarkably extend the pore size to 8.7-16.8 nm in the order of NH4NO3 (16.8 nm)>NH4Cl (13.7 nm)>CaCl2 (12.4 nm)>NaCl (8.7 nm). Moreover, high surface areas of 154-199 m(2) g(-1) are still achieved. The ions that have large radii and high valence numbers are more capable of expanding the pores because of strong volume exclusion and Coulomb repulsion. The critical pore size for the adsorption of hydrated bisphenol A molecules within the pores is ca. 4.2 nm. Pore expansion facilitates mass diffusion in the channels and turns internal surface areas available for reactions, thus greatly enhancing the activity of the mesoporous photocatalyst by 2.7-5.5 times. (C) 2012 Elsevier B.V. All rights reserved.