Growth Behavior And Photocatalytic Property Of TiO₂ Films On Polyimide Prepared By SILAR At Room Temperature

Development in the fields of flexible components, aerospace industry, new energy and environmental protection raises huge applications and requirements for high-quality polymers. Meanwhile, it is also essential to improve the environmental stability of the polymer materials to be multifunctional in the utilization. Due to the polymorphic form of TiO₂ and its wide application, the research objects were taken using polyimide（Kapton） as substrate materials and TiO₂ film deposited by successive ion layer absorption and reaction technique（SILAR）. The growth behavior and crystallization processes of TiO₂ film and optical catalyst properties were studied using some complementary analytical techniques such as electron paramagnetic resonant spectrometer, Transmission Electron Microscope, X-ray Diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, Atomic Force Microscope. On the basis the mechanisms on the film formation and continuous surface reactions on Kapton were observed, while the TiO₂ film crystallization mechanisms was also demonstrated.The results indicate that, before totally covering the flexible Kapton substrate, TiO₂ film grows in an island mode within 10 depositing cycles; afterward the film grows in a planar one due to completely covering the substrate. The deposited TiO₂ appears roughening within 10 depositing cycles, and afterward the surface roughness decreases down to 2.5 nm. The average grown rate is 0.5nm/cycle. The results on EPR and water contact angle tests demonstrate that there products lots of hydroxyl free radicals on the substrate during the activated process and then on the film being-deposited film during each SILAR reactions. These free radicals result in smaller water contact angle of the deposited surface, and thus enhancing the ionic absorption capability and reaction activity of metallic precursor ions（Ti4+）.The results from structure investigation analysis show that, there include certain of anatase/rutile phases in amorphous matrix in TiO₂ films. However the films show completely amorphous structure as deposited on vitreous silica, Kapton with pre-deposited amorphous SiO₂ film and amorphous polyethylene. Thus the possible reason for the room-temperature crystallisition of TiO₂ film on Kapton was thought as that there forms a kind of graphite-like structure forming on part-crystal Kapton surface due to ultra-violet/ozone exposed activation. Furthermore, crystallizations of TiO₂ films could be drastically enhanced to form mainly anatase structure as some kinds of amino-surfactants were added in cation precursor（H2O） solution. The grain size was abserved as 5 nm in average diameter while the surface roughness of the TiO₂ film increases intensively as TEAH template was used. Correspondingly, the film roughness after 40 cycle depositions could reach up to about 191.76 nm when complex template was used combined TEAH and CTAC. Furthermore, the rough surface shows some self-organized order-morphology. Thus, one can obtain a kind of rough TiO₂ film with ultra-large specific surface area with anatase phase structure. The main mechanisms on the formation of crystallized TiO₂ films enhanced by amino-template on Kapton are from some combined effects as surface absorption of the template micellar and self-organization reaction.It is important to be noted that proton irradiation could promote the crystallization of TiO₂ films on Kapton. On one hand, proton irradiation on Kapton substrate would cause more surface graphitization on part-crystal Kapton to include crystallization of TiO₂ films to be anatase structure. On the other hand, for part-crystallizing TiO₂ films on the as-received Kapton, the post proton irradiation after deposition could enhance the TiO₂ film to be more crystallized with anatase phase. This irradiation-induced phase transformation is due to the energy implantation, accelerating atomic diffusion and re-arrangement to be anatase phase.TiO₂ film on Kapton presents only weak opto-degraded capability for soluble methylene blue in water. However, the TiO₂ films deposited on Kapton using amino-templates shows excellent opto-catalytic performance to degrade methylene blue. Under ultra-violet irradiation with wavelength 254 nm and 1.5m W power for 120 mins 40 cycle TiO₂ films on Kapton can only get 11.3% degradation rate, while 40cycle-TEAH-TiO₂ films can reach 80.0% degradation rate, and even up to 91.9% degradation rate as 40cycle-CTAC+TEAH-TiO₂ films used. EPR analysis indicates that photocatalysis of TiO₂ films is due to the formation of hydroxyl free radicals（?OH） under ultra-violet radiation. The population of hydroxyl free radicals（?OH） increases reasonably with the radiation time, but also increases with the anatase content and surface roughness of the films.Based on the structural characteristics of TiO₂ films, it could be determined that one can utilize a few pathways to promote crystallization of TiO₂ film, and then to enchance its photocatalytic degradation performance. They are to make use of amino-template during SILAR deposition, proton irradiation of Kapton substrate or proton irradiation of the deposited TiO₂ film.