Macroporous silica and titania obtained using poly[styrene-co-(2-hydroxyethyl methacrylate)] as template

Abstract

Ordered macroporous silica and titania have been prepared using poly[styrene-co-(2-hydroxyethyl methacrylate)] (PS-HEMA) latex arrays as templates. Polystyrene (PS) particles have also been used to template macroporous titania. Arrays of spherical latex particles, with average diameters of 260 nm (PS-HEMA) and 390 nm (PS), have been obtained by either evaporation of the solvent at room temperature or by deposition on filtration membranes. In both cases, 3D extended regions with cubic close packing of microspheres are observed, but in the samples obtained after evaporation of the solvent the self-assembled particles show a higher degree of order. The oxides were synthesized by hydrolysis of sodium silicate or titanium isopropoxide (diluted in n-propanol or hexane) which were infused into the cavities of the latex arrays. Two different methods have been used to fill the cavities with the precursor: filtration with suction and capillary forces. When solidification of the ceramic matrix took place, the template was eliminated by heating at 500 °C. As replica of the latex assemblies, periodic porous silica and titania were obtained. Sheets of amorphous silica, as thin as 4–7 nm, form the wall of the interconnected pores whose diameters range from 200 to 220 nm. In macroporous titania obtained from titanium isopropoxide diluted in n-propanol and imprinted on PS-HEMA particles, the pore diameters range from 160–180 nm. The porous network is shaped by walls that are 8–12 nm thick and are formed from anatase nanocrystals with a major dimension of 8–20 nm. Pore diameters, crystallite sizes and thicknesses are similar when hexane is used as the solvent of the inorganic precursor. However, in macroporous titania imprinted on PS particles, nanocrystals with a major dimension of 6–10 nm form the pore wall with a thickness of 6 to 8 nm. Interactions between the titania precursor and the latex particles seem to affect the microstructural parameters. The porous skeleton of titania is preserved after treatment at 600 °C for 16 hours but the pore wall thickness slightly increases with crystallite growth and in some particles disordered areas appear.