Preparation And Characterization Of The Antireflective And Self Cleaning Functional Coatings

The efficient ways to improve the efficiency of using the solar light include increasing the proportion of the incident light, decreasing light losing on the substrate surface as well as reducing the maintenance cost in the using process. This goal can be achieved by preparing complex functional thin films with proper materials on the substrate and the suitable material is TiO2. In the methods of preparing TiO2 thin films, the direct current magnetron (DMS) technique has excellent features such as the environment-friendly power, convenient and precise controllable parameters, fast deposition speed as well as the strong repeatability. It is convenient to fabricate many kinds of thin films which can be combined closely with the substrates. To make progress of the preparing technique, the energy filtering magnetron sputtering (EFMS) technique was developed on the basis of the traditional DMS technique to enhance the thin film properties in this work. The quality of the thin film is increased by controlling the particle energy and incident angles. In this paper, TiO2 thin films and Al2O3 doped TiO2 thin films were prepared by the DMS and EFMS techniques. The variations of the structure, optical and photocatalytic properties of the thin films with the preparation parameters were studied. The thin films with both antireflective and self cleaning functions were fabricated according to the results of the optical and photocatalytic properties. The research contents are as follows:(1) Optimization of the preparation parameters of the TiO2 thin films. We found that the TiO2 thin films can be prepared in single anatase phase with the deposition parameters of temperature (100℃-400 ℃), pressure (0.75 Pa-3.0 Pa), time (60 min-90 min), ratio of O2 to Ar (1:2-1:10) and sputtering power (148 W-443 W). With the increasing of the deposition temperature, the decreasing of the pressure, the raising of time, the falling of the ratio of O2 to Ar and the increasing of the power, the crystalline of the TiO2 thin film becomes better and the mean crystal size becomes smaller. The deposition rate is the fastest at 300℃. The deposition rate decreases with the increasing of the pressure and the ratio of O2 to Ar. It becomes slower when the power becomes higher.(2) Investigation on the effects of the preparation parameters on the transmittance, refractive indices, extinction coefficients and optical band gaps of the TiO2 thin films. The results show that the average transmittance is the highest at 300℃. The average transmittance increases with the increasing of the pressure and power. It reaches the lowest in the deposition time of 90 min and decreases with the decreasing of the the ratio of O2 to Ar. The refractive index increases with the increasing of the temperature, the ratio of O2 to Ar and the power, while it decreases with the increasing of the pressure. The refractive index reaches the maximum value at the deposition time of 120 min. The optimized preparation parameters for the anatse TiO2 thin film with the lowest refractive indices are as follows. Temperature:100℃; Pressure:3.0 Pa; O2 to Ar ratio:1:6; Deposition time:90 min; Deposition power:295 WThe refractive index of the TiO2 thin film prepared at the above parameter at the wavelength of 550 nm is 2.06. The TiO2 thin films are transparent non-assimilation with 0 extinction coefficient at the wavelength of above 400 nm. The optical band gap is the largest for the thin film deposited at 300℃. The optical band gap increases with the increasing of pressure, time and power. As the ratio of O2 to Ar decreases, the optical band gap also becomes larger. After annealing the TiO2 thin film at 500℃, the crystallinity becomes better and the refractive index and the optical band gap increase, but the mean transmittance decreases.(3) Comparison of the TiO2 thin films prepared by the EFMS and DMS techniques. The effect of the mesh number of the filtrating electrode was investigated. It shows that the thin films prepared by the EFMS had more uniform smooth surface, smaller mean crystal size, larger optical band gap and higher refractive index as compared with the thin films prepared by the DMS technique. The optical band gap of the TiO2 thin film deposited by the DMS and EFMS techniques (60 mesh) are 3.04 eV and 3.22 eV respectively. With the increase of the mesh number for the filtrating electrode, the deposition rate becomes lower, and the crystallinity becomes weaker and its particle size become smaller, as results the refractive index and optical band gap increase while the mean transmittance decreases.(4) Investigation on the effects of the preparation parameters on the photocatalytic properties of the TiO2 thin films. The optimized condition was found and the mechanism was analyzed. TiO2 thin film deposited at temperature 100℃, pressure 3.0 Pa, time 90 min, O2 to Ar ratio 1:6 and power 295 W has the highest degradation rate of-0.0034 min-1 on RhB. The degradation rates of the TiO2 thin films prepared by the EFMS technique at 60 mesh and 120 mesh are increased to -0.00493 min-1 and -0.00486 min-1, respectively. However, if the mesh number is over 120 mesh, the degradation rates decreases dramatically.(5) Investigation on the effects of different doping amount of Al2O3 on the structure, optical and photocatalytic properties of the TiO2 thin film. The results show that the doped TiO2 thin film has worse crystallinity, larger deposition rate, lower refractive index and photocatalytic activity. The reasons were analyzed.(6) TiO2 thin films with refractive indices gradient change were prepared by the DMS and EFMS techniques. By considering their photocatalytic properties, the deposition parameters of the surface layer for the antireflective and self cleaning coatings were selected as follows:temperature 100℃、pressure 3.0 Pa、O2 to Ar ratio 1:6 and power 295 W. The bottom layer with high refractive index was chosen to be fabricated by the EFMS technique using 460 mesh according to the results obtained by the optical design principle. The antireflective coatings meeting the design requirements were designed by the TFC optical design software. According to the designing result, single and double antireflective coating of TiO2 thin film and doped TiO2 thin film were fabricated. The antireflective and self cleaning functions were characterized and analyzed. Results indicated that double layers coating has advantages over single layer. Single material TiO2 double layers antireflective coating has the mean transmittance of 89.1% (400 nm-800 nm), the degradation rate of-0.00337 min-1 and the contact angle of 8° (after 30 min). The antireflective coatings containing the doped surface layer had the mean transmittance of 85.4%, the degradation rate of-0.000496 min-1 and the contact angle of 29.5°. Results of the utility in the solar cell show that single material TiO2 double layers coating has the best photocatalytic activity and lowest impaction on the conversion efficiency.