Deposition of multifunctional TiO2 and ZnO top-protective coatings for CSP application

• Deposition of TiO2 and ZnO thin films with homogeneous thickness and flower like morphology using ILGAR deposition technique.
• Deposited thin films behave as multifunctional coating combining photocatalytic activity and transparency.
• Wettability switching behavior was pointed out for both ZnO and TiO2 deposited thin films under UV radiation.
• Deposited multifunctional thin films are potential top-coating candidates for CSP reflectors with self-cleaning properties.

In this work, high quality, transparent multifunctional and smart anatase TiO2 and wurtzite ZnO top protective coatings used for CSP silver-based first-surface thick glass mirrors are studied. The TiO2 and ZnO top-coatings are deposited using chemical spray pyrolysis technique which conserves a very good specular reflectance of the mirrors (94% reflectance in the Near Infrared range). The photocatalytic activity of the prepared top-coatings is evaluated using the methylene blue dye (MB) as a model molecule, and the highest activity is observed for ZnO top-coating, with a degradation efficiency of 42%. Moreover, the effect of post-annealing treatment on the photocatalytic degradation of the top-coatings is pointed out, and is explained in terms of crystallinity and surface morphology. The increase of the photocatalytic activity can be achieved by increasing surface roughness, which is a conflicting requirement with optical transparency needed for concentrated solar power (CSP) top-coating application. Furthermore, it is pointed out that TiO2 and ZnO top protective thin films exhibit a photo-induced switching of their wetting properties only after 30 min of UV exposure, and their usability as multifunctional top-protective layers for CSP application is discussed.