Optical characterization of tandem absorber/reflector systems based on titanium oxide–carbon coatings

The use of chemical methods to produce coatings with good spectrally selective properties is described. Sol–gel and dipping techniques were combined to fabricate thickness-sensitive solar-selective absorbing coatings based on titanium oxide combined with carbon black (TiO2/CB) and carbon nanotubes (TiO2/CNT) on top of stainless-steel substrates. The low to high reflectance transition (λT) observed in the tandem system based on TiO2/CB films was smooth and red shifted when compared to the sharp transition observed in systems based on mesoporous TiO2/CNT films. Mesoporosity of the oxide was controlled by the use of polymeric surfactants with different molecular weights, causing a significant blue shift in λT as the molecular weight increases. These films were characterized by X-ray diffraction and UV–vis light absorption spectroscopy.