Study On Recovering Of Hydrophilicity Of Titanium Dioxide Thin Films

In the n-type semiconductors, titanium dioxide （TiO₂） is the best known and mostwidely applied photocatalyst. UV irradiation onto the TiO₂surface induces photocatalyticreactions and photoinduced hydrophilicity. TiO₂thin films have been extensively studieddue to their practical applications in various industrial areas such as production ofself-cleaning and anti-fogging surfaces. The freshly prepared anatase TiO₂thin films showsuper-hydrophilicity, but it will deteriorate during storage and normal usage, which willinfluence the applications of films. This paper firstly presents the classification ofself-cleaning materials and the research progress of hydrophilic films, and then gives adetailed review of the structure, main performance, as well as photocatalytic andhydrophilic mechanism and the influencing factors of TiO₂. In this study, TiO₂thin filmswere deposited onto glass substrates by DC reactive magnetron sputtering Ti target, andthe deposited films were characterized by x-ray diffraction analysis （XRD）. This paper triedto recover the super-hydrophilicity by （1） post-annealing in air,（2） O₂plasma treatment and（3） UV irradiation, and the reasons for the inactivity and recovery of the hydrophilicity areinvestigated by XPS.XRD shows that after annealing in450℃air for2hr, TiO₂films turned into anatasestructure, and the diameter of1μl water drop on the surface of TiO₂films was about15mm. After the super-hydrophilicity of TiO₂disappeared, annealing in400℃air for10minutes can recover the hydrophilicity. XPS analyses indicated that carbon pollution wasreduced and the content of oxygen vacancies increased due to desorption of theadsorbed water and hydroxyl on the surface. The hydrophilicity of TiO₂thin films can alsobe recovered by O₂plasma treatment. With the increasing of rf power of plasma treatment,the recovery degree of the hydrophilicity decreased. After low energy O₂plasma treatment,the surface of TiO₂was oxidized and carbon pollution was reduced, meanwhile a lot ofneutral oxygen atoms from RF plasma directly chemisorbed on5-fold-coordinated titaniumatoms, which will improve the hydrophilicity. Finally with the increasing of UV irradiationtime, the diameter of1μl water drop on TiO₂surfaces firstly increases and keepssaturated after it reachesthe maximum.Each way can effectively restore the super-hydrophilicity of TiO₂thin films.Annealing in air and irradiation with UV light are easy to handle but oxygen plasma treatment can keep visible light induced super-hydrophilicity for longertime.