To illustrate the structure of TiO2–SiO2 stack coatings, in which thickness of each layer with different SiO2 content was measured by spectroscopic ellipsometer. The plot of thickness versus content of SiO2 is shown in Fig. 4, and the results are listed in Table 1. Each layer has a thickness less than 120 nm and that of top layer is approximate one quarter wavelength of incident light (550 nm). Landau–Levich equation, d¼0:94ððηU0Þ2=3=γ1=6 LV ðρgÞ1=2Þ, gives the relation of thickness of coatings, parameters of preparation and properties of solution [44], where η, ρ, g, γLV and U0 are viscosity, density, gravitational acceleration, surface tension at air/sol inter- face and dip-coating speed, respectively. Among the parameters of preparation, the density, which decreases with increasing content of SiO2, may mainly affect composite layer resulting in increased thickness of the layer.