| Anti-reflecting film is applied to decrease reflecting loss on the surfaces of optical elements and increase luminous flux. It is a kind of most widely applied optical films. Accordingly, anti-reflecting film is a very important research topic in optical films. The research is focused on searching new materials, designing new film system, improving depositing technics to realize ideal anti-reflecting effect. At present, Titanium dioxide (TiO2) has attracted much attention from both fundamental and industry research since its photocatalytic and hydrophicilic characteristics under ultraviolet (UV) irradiation, accordingly, widely applied in many fields such as environment purification, medical treatment and electric devices. In particular, the most actively pursued industrial application is in self-cleaning. TiO2 coating can decompose organic contaminants and microorganisms on the surfaces. In addition, stains on the superhydrophilic surface are easily washed away by water.Nowadays, the self-cleaning coating has been used on the glasses of theater, mall and office building. Comparing with usual glasses, the self-cleaning glasses have the advantages of saving cleaning expenses and solving the cleaning difficulties for walls and roofs, showing perfect style of modern architecture. However, the transparent TiO2 coatings with high refractive index deposited on glass induce strong unwanted reflection, haven't been solved so far. Apparently, application scope of self-cleaning glasses will be widened by combining with AR property. Many reports have been published about antireflection coating, but few papers propose to deposit TiO2 on glasses as AR coatings. Here, we firstly proposed a double-layer TiO2 coating with AR property coated on glassby sol-gel method. This is a feasible method to prepare self-cleaning glasses with antireflection property for Industrial Production.According to theory of Interference, the double-layer TiO2 coating with AR property is prepared. The principle of AR coating is based on the destructive interference of reflected light from interfaces of air-film, film-film and film-substrate. In this double-layer AR coating design, the optical thickness of each layer should be equal to a quarter wavelength of the incident light. The inner film material is anatase phase TiO2, which is a high refractive index material (n~2.52). The outer layer is nanoporous TiO2 film adjusted by the pore-maker.The double-layer TiO2 antireflection (AR) coating with self-cleaning property was fabricated by the sol-gel method. The method of preparing TiO2 film particularly porous TiO2 film was concluded. Different organic compound such as polyethyleneglycol (PEG2000), triblock-copolymer (EO20PO70EO20, Pluronic P123), cetyl-trimethyl-ammonium-bromide (CTAB) and polyvinylpyrrolidone (PVP) were used to make the porosity of material. In this experiment, polyethylene glycol (PEG2000) was selected to add into 20ml TiO2 precursor solution as pore-maker for its good controllability. The research found that the larger the amount of PEG added into the precursor solution, the larger the size and number of pores produced in the TiO2 film accompanying the increasing of transmissivity of the coating. But too big pores would cause scattering and affect the light transmission. Transmittance of the substrate covered with coating TiO2/0.15g PEG+TiO2 is higher than that of untreated substrate in the range of 514~602 nm, with a maximum of 94.1% at 539 nm.According to the Wenzel's theory, roughness enhances the wettability,making the hydrophilic one more hydrophilic or superhydrophilic for high surface-energy surface. Water can enter the pores when a drop of water was applied to the surface of the nanoporous TiO2, it penetrates into the film and spreads. The surface structure with bigger porosity will lead to bigger hydrophilic area, better hydrophilic effect and shorter spreading time. Consequently, the nanoporous structure of film can induce faster spreading behavior and keep excellent hydrophilicity for long without long-term UV illumination.The SiO2 film coated on glass could realize transmittance of 99.7% in the whole visible range for the morphology of SiO2 film similar to the corneal nipple of moth's eyes. Another way to increase the transmittance of TiO2 film was researched. The TiO2 film with SiO2 particles was prepared by the precursor solution that TiO2 sol and SiO2 sol were mixed in different proportions. The rougher structure of surface, higher transmittance and flatter transmission spectrum in the whole visible range were obtained with the increasing volume ratio of SiO2 sol. However, the decreasing of TiO2 sol would affect the effect of self-cleaning.Characterization of nanoporous films:Photos about the dynamic process of wettability were captured by the OCA20 dataphysics contact angle system (Germany) at ambient temperature. Transmission measurement was performed using a U-4100 UV-Vis Spectrophotometer (Hitachi, Japan). Scanning electron microscopy (SEM) and pore size distribution of the films were obtained with a field-emission scanning electron microscopy (JEOL JSM-6700, Japan).
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