Preparation of High-Transparency, Superhydrophilic Visible Photo-Induced Photocatalytic Film via a Rapid Plasma-Modification Process

In this study, different amounts of SiO2 nanoparticles (7 nm) were added to simultaneously reach high transmittance, high hardness, and high adhesion for TiO2 film prepared by the sol–gel method and coated on glass through a dip-coating technique. For the film to achieve self-cleaning, anti-fogging, superhydrophilicity, and visible photo-induced photocatalysis, TiO2-SiO2 film was modified via a rapid microwave plasma-nitridation process for efficient N-doping by various N2-containing gases (N2, N2/Ar/O2, N2/Ar). Through nitrogen plasma, the content of N atom reached 1.3% with the ratio of O/Ti atom being 2.04. The surface of the thin films was smooth, homogeneous, and did not crack, demonstrated by the root mean square (RMS) roughness of film surface being 3.29–3.94 nm. In addition, the films were composed of nanoparticles smaller than 10 nm, with a thickness of about 100 nm, as well as the crystal phase of the thin film being anatase. After the plasma-nitridation process, the visible-light transmittance of N-doped TiO2-SiO2 films was 89.7% (clean glass = 90.1%). Moreover, the anti-fogging ability was excellent (contact angle < 5°) even without light irradiation. The degradation of methylene blue showed that the photocatalytic performance of N-doped TiO2-SiO2 films was apparently superior to that of unmodified films under visible-light irradiation. Moreover, the pencil hardness and adhesion rating test of the thin films were 7H and 5B, respectively, indicating that the obtained coatings had great mechanical stability.