The cross-sectional morphology of TiO2–SiO2 stack coatings was analyzed using FE-SEM to investigate their structure as shown in Fig. 3. The stack coatings consisting of six layers seem to be as a whole film, and the interfaces of each layer are schematically marked in white dash lines according to the measured thickness. The size of particles and pores increases from bottom to top layer, which results in gradual decrease of n.