Optimized synthetic conditions of structural colored TiO₂/SiO₂ multilayers

Abstract

Sol–gel synthesis of TiO₂/SiO₂ multilayers was studied for application to structural color materials. The purpose of this study is to achieve crack-free thick films, which are indispensable for high reflectance and color vividness. We investigated the effect of polymer additives and heating conditions on the fracture of TiO₂ gel films. The critical cracking thickness increased more than three-fold at maximum by adding poly(vinyl pyrrolidone) (PVP) to precursor sols and by using a low heating rate. From stress measurement based on Stoney’s model, we found that the addition of PVP extensively reduced residual stress, proving the mechanism suggested so far that PVP hinders the evolution of the Ti–O network and reduces drying stress. Under the restriction of the critical thickness, TiO₂/SiO₂ multilayered films and flakes were prepared as a feasibility test of structural coloration. A high-order interference condition was used for the optical design, which allows us to obtain narrow spectral peaks and the primary colors of light. Theoretically expected reflection spectra that corresponded to blue and green colors were attained for five-layered films.