Sol-gel nanoparticulate mesoporous films with enhanced self-cleaning properties

This paper presents a brief overview of the work done in our group concerning a versatile approach for the elaboration of TiO ₂-based photocatalytic coatings through the use of preformed TiO ₂ colloidal particles dispersed within a surfactant-templated mesoporous silica binder. Investigation of the influence of porosity on the photocatalytic efficiency of these films showed that the major impact arises from diffusion of the pollutant molecules (stearic acid) within the volume of the film, thus improving the action of the radicals photogenerated at the surface of the TiO ₂ particles. Starting from standard films elaborated using a commercial colloid, we showed that our approach allows a systematic investigation of any systems available as a colloidal suspension. A first example is given on titanium dioxide nanopowders (calcined Millennium or Degussa P25 particles). A grinding process was developed allowing the recovery of these powders as suspension of 120 nm aggregates that can be subsequently incorporated within mesoporous silica films. The obtained films exhibited photocatalytic performances similar to those of the standard films directly synthesized with the Millennium colloid. This result implies that the influence of the microstructure on photocatalytic efficiency is mainly governed by the silica matrix, allowing the investigation of almost any compounds even those synthesized through conventional solid state reaction at high temperature. We also developed an original process to prepare colloidal solutions of N-doped TiO ₂ particles obtained by nitridation treatment at high temperature. Investigation of the doping and photocatalytic efficiency under visible light irradiation evidenced the competition between nitrogen insertion and detrimental Ti ⁴⁺ reduction. The latter effect may be somewhat limited by a post-treatment under air that re-oxidize the Ti ³⁺ while preserving part of the inserted nitrogen.