Matsunaga et al.(1985) first discovered the bactericidal activity of photocatalytic TiO₂ reactions. Using TiO₂ powder particles in the bactericidal action and photocatalytic degradation for the oxidation of organic and inorganic water pollutants were extensively studied (Michael et al., 1999). However, conventional TiO₂ powder catalysts were characterized by difficult separation and reproduction after reaction, so the application of titanium dioxide thin film has attracted much attention in recently years. We investigated the relationship between the antibacterial activity and the properties of the N-TiO₂ thin films, and found that the crystal structure (anatase, rutile) and morphology of particles size are important factors affecting the photocatalytic bactericidal action. The viability of Pseudomonas aeruginosa strain (AS1.50) and Bacillus subtilis strain (AS1.439) during the N-TiO₂ photocatalytic reactions are also discussed.

Fig. 1 Preparation schematic of N-TiO2 powder and films

The mesh grid (156.2 mm×25.4 mm×1.5 mm), which was pretreated as the loaded basis for thin films, was dipped into the N-TiO₂ colloidal sol solution for 2 min, then withdraw from the solution at rate of 1 mm/s. The film was formed on mesh grid and dried at ambient temperature for 24 h. The mesh grid coated with gel films was treated at different temperature (film A at 500 °C and film B at 350 °C/500 °C) for 1 h in a muffle furnace.

For film A, the mesh grid coated with the gel was sintered at 500 °C for 1 h with temperature raising rate of 3 °C/min in the muffle furnace, then repeatedly coated and calcined several times and finally sintered at 500 °C. For film B, the mesh grid coated with the gel films was sintered at 350 °C for 1 h, then repeatedly coated and calcined several times, with the last coating film being sintered at 500 °C for 1 h. Different thickness films were obtained by repeating the above process several times.

Fig. 2 Diagram of photo-degradation reactor

The CFU were determined by plating aliquots of serially diluted suspensions on duplicate nutrient agar plates. All plates were incubated for 24 h at 30 °C, the viable number of cells were counted. The loss of cell viability was examined by the viable count procedure. All experiments were performed at least in triplicate.

Fig. 3 AFM images of the N-TiO2 thin films. (a) Film A (at 500 °C, five layers); (b) Film B (at 350 °C/500 °C, five layers)

Fig.3 is the AFM images of the N-TiO₂ thin films with five layers in film A and film B. N-TiO₂ film B has more uniform granular microstructure and thinner flat texture than N-TiO₂ film A (Fig.3). The thickness and the average diameter of the particle size of the film were 12.17 nm and 27.50 nm for film A, 8.18 nm and 25.15 nm for film B respectively determined by AFM and XRD technique. The granular microstructure is the key factor influencing the photocatalytic activity (Jang et al., 2001). In the N-TiO₂ film B, the mass fraction of crystallitic anatase present in granular nanostructure are mostly up to 94.8% higher than 65.2% of mass fraction in the N-TiO₂ film A.

Zarzychi et al.(1982) explained the mechanism of the particles growth of gel films in the calcining process. From Fig.4 shows that during calcining treatment for the film A process, there are many H₂O moleculars and hydroxyl groups forming hydrogen bonds on the sol-gel surface of the N-TiO₂ film coated on mesh grid, when N-TiO₂ gel films were treated at 500 °C five times, hydrogen bonds between the two molecules would turn into chemical covalent bond during sintering process, then link close N-TiO₂ crystallitic particles tightly, result in the particles growing and solid cluster conglomeration, consequently, lead to the rapidly increasing number of larger diameter particles. In film B, N-TiO₂ thin film (8.18 nm thickness, 2.760 nm height and 25.15 nm average nanoparticle diameter) has more uniform granular microstructure and thinner flat texture than N-TiO₂ thin film A (12.17 nm thickness, 3.578 nm height and 27.50 nm average nanoparticle diameter) because low sintering temperature decreases the coagulation and growth rate of particles, hence, the mass fraction of N-TiO₂ anatase nanostructure has more reactive center in N-TiO₂ thin film B than in N-TiO₂ thin film A, which is probably due to the smaller size of anatase nanoparticle.

Fig. 4 The size of nanoparticle on N-TiO2 thin film for (a) film A (at 500 °C, five layers) and (b) film B (at 350 °C/500 °C, five layers)

Fig.5 shows the effect of illuminated N-TiO₂ thin films B (8.18 nm thickness) on the sterilization of Pseudomonas aeruginosa strain (AS1.50) and Bacillus subtilis strain (AS1.439) in comparison with N-TiO₂ thin films A (12.17 nm thickness) under the conditions of initial concentration of 10⁶ CFU/ml, 18 W/m² of UV intensity. The viability of cells is determined by colonies counting. The survival curves are shown in Fig.5 for Pseudomonas aeruginosa strain (AS1.50) and Bacillus subtilis strain (AS1.439), respectively.

Fig. 5 Effect of the N-TiO2 films on the viability of (a) Pseudomonas aeruginosa strain (AS1.50) and (b) Bacillus subtilis strain (AS1.43)

More than 95% photocatalytic sterilization rate for Pseudomonas aeruginosa strain (AS1.50) and 75% for Bacillus subtilis strain (AS1.43) were achieved by using N-TiO₂ thin films B for 70~80 min of irradiation during the photo-bactericidal experimental process. The results indicated that the antibactericidal effect of N-TiO₂ film was associated directly with the thickness of films (Trapalis et al., 2003). The photo-bactericidal action no Bacillus subtilis strain (AS1.439) and Pseudomonas aeruginosa strain (AS1.50) treated by N-TiO₂ thin films B was shown to be more effectively than N-TiO₂ thin films A. Bacillus subtilis strain (AS1.439) showed stronger resistance to photocatalytic sterilization than the Pseudomonas aeruginosa strain (AS1.50) (Fig.5).

The photocatalytic chemical reaction mainly occurs on the surface of the films, indicating that the photocatalytic reaction mainly occurs on the specific particles size, the photocatalytic activity is enhanced with decreasing nanoparticles diameter and increasing contact area between the photocatalyst and target material (Maness et al., 1999). In addition, the higher temperature treatment could not only cause the aggregation of N-TiO₂ particles, but also partially change crystalline phase from the anatase to rutile which probably result in losing some active sites of the films for photocatalytic reaction during different thermal treatment (Trapalis et al., 2003). The experimental result suggested photo-induced bactericidal efficiency of N-TiO₂ was affected by the morphology and thickness of N-TiO₂ films, the grain size of nanoparticles, surface area, mass anatase composition of N-TiO₂.