Photo-induced hydrophilic properties of reactive RF magnetron sputtered TiO2 thin films

TiO2 thin films were deposited by reactive radio frequency magnetron sputtering of Ti target at low working pressure (1 mTorr) and various O2 and Ar flow ratios (RO2 = O2/(O2 + Ar)). Properties of TiO2 films were measured by field emission scanning electron microscopy, atomic force microscopy, X-ray diffractometry, UV-VIS transmission spectroscopy, X-ray photoelectron spectroscopy and evaluated by contact angle analysis. Well defined columnar structure with rough surface was obtained at relatively lower RO2. As RO2 increased, the oxygen vacancies in the films decreased and the stoichiometric ratio of O/Ti changed from 1.79 to 1.98. Transition from anatase to rutile phase was observed with increase in RO2. The optical bandgap showed a red shift with increase in RO2. In addition, the film with co-existing of rutile and anatase showed a high Urbach tail. Lower RO2 resulted in higher photo-hydrophilicizing efficiency due to the change in morphology, structure and composition. However, very low RO2 (less than 10%), which led to transition mode, resulted in poor photo-hydrophilicizing efficiency.