Self-Cleaning Photocatalytic Coatings on Bricks and Tiles

Aerosol-spraying of sol-gel suspensions was used to produce films of photocatalytic TiO2 on clay bricks and terracotta tiles. Solutions of titanium tetra-isopropoxide and isopropanol were added to a dilute solution of nitric acid, followed by refluxing at 80 degrees C for similar to 8 h in order to coarsen the TiO2 nucleants and so reduce the potential for dissolution and coverage by the glass. The suspensions then were aerosol sprayed onto structural clay substrates and annealed at 100-275 degrees C for 12 h (heating rate 1 degrees C/min). The annealed films were of variable thickness owing to the uneven topography of the bricks, where the depressions and associated pooled areas showed coatings of similar to 1 mu m maximal thickness. The morphological, mineralogical, and photocatalytic properties of the coated substrates were assessed by scanning electron microscopy/profilometry, X-ray diffraction/laser Raman microspectroscopy, and methylene blue decomposition/UV-Vis spectrophotometry, respectively. The results indicated that films annealed at temperatures as low as 100 degrees C were photoactive. However, while bonding of the TiO2 film to the brick substrate after annealing at only 225 degrees C was strong, optimisation of the processing conditions is required in order to obtain the best photocatalytic performance. This requires a compromise between the positive effect of TiO2 recrystallisation (requiring higher annealing temperatures) and the negative effect of glass contamination (requiring lower annealing temperatures).