A novel method, Snow Globe Coating (ref1 and ref2), is formed from high refractive index titania particles without containing binder and gives close to 100% reflectance for wavelengths above 400 nm. Snow globe coating is a physicochemical coating method executable in pH neutral media.
Figure 1 Snow globe coating method: (a) dispersed titania particles form a binder free coating (SG coating) after settling by gravity. Scanning electron micrograph of titania particles, (b) scale bar is 10 mm; (c) scale bar is 200 nm.
The Snow Globe Coating method (see Figure 1) uses the fact that thick, uniform and porous coatings of relatively large particles can be achieved by dispersion followed by settling by gravity, as in a children’s snow globe. The technique allows large particles, which provide highly effective light scattering to be used. To form a uniform coating, the particles must first be dispersed in a liquid medium. Depending on the charge on the particles, they may either form a stable dispersion in the liquid, or may coagulate too fast, preventing the formation of a uniform coating. Dispersion is stable when the charge on the particles is sufficiently high that repulsive electrostatic forces exceed the attracting van der Waals forces. The surface charge can be tuned by adsorption of surfactants or ions, by varying the pH, or changing the concentration of ions.
ref1) Angelika Basch, Fiona Beck, Thomas Söderström, Sergey Varlamov and Kylie R. Catchpole, Enhanced light trapping in solar cells using snow globe coating DOI: 10.1002/pip.2240
ref2) Patent AT 512 111 B1 2013-09-15 "Methode zur Beschichtung eines Substrates", WO 2013/067560, PCT/AT2012/000283