*3.4. Photocatalytic Properties*

The UV-Vis diffuse reflectance spectra of TiO2 coatings were firstly recorded in the wavelength range of 250–750 nm. The results are illustrated in Figure 4. All coatings have an absorbance edge in the range of 390–400 nm, which is characteristic of semiconductor coatings. The absorbance edge is ascribed to the charge transfer from the valence band to the conduction band. For TiO2 materials, the valence band is largely caused by 2*p* orbitals of the oxide anions, and the conduction band is constituted with 3*d t2g* orbitals of the Ti4+ cations [23]. These charge carriers present high reactive activity and can act with different paths. One is the recombination of electron-hole pairs without taking part in the degradation. The other involves gathering at the surface, and reacting with the pollutant [24]. In comparison, all of the spectra show similar curve shapes without new spectrum phenomena. Although the solidification pathway is modified, no notable shift to longer or shorter wavelengths was observed in the TiO2-12-30 and TiO2-20-30 coatings. Perhaps the main reason for this is that the rutile phase still dominates in both coatings, though the anatase content increases.

The photocatalytic performances are shown in Figure 5. All coatings present different degrees of activity in decomposing methylene blue with the irritation of UV light. The coating of TiO2-12-0 shows depressed photocatalytic activity. This is consistent with other works [12,13]. Without modifying the solidification pathway, the anatase content in this coating is as low as 4%. For the TiO2-12-30 and TiO2-20-30 coatings, they both exhibit a significant increase in photocatalytic activity. Obviously, the enhanced activity is ascribed to the augmented anatase content in both TiO2 coatings because they present similar structure. As the TiO2-20-30 coating has a higher anatase content than TiO2-12-30, it presents higher activity.

Furthermore, the variation in MB concentration with irradiation time is coincident with Langmuir-Hinshelwood model. The kinetics equation can be depicted by −ln *C/C*0 *= kt*, where *C* and *C*0 are the measured concentration of MB and the initial concentration, respectively, and *k* is the time constant of the activity (*h*−*<sup>1</sup>*). By a first-order fitting of the exponential term to the time *t*, the values of *k* are obtained for all coatings and summarized in Table 2. As presented in Table 2, the *k* value for the TiO2-12-30 or TiO2-20-30 coating is higher than that of the TiO2-12-0 coating, as the former is

deposited with the modification of the solidification pathway. As clarified by the equation, the bigger value of *k* implies a higher photocatalytic activity.

**Figure 4.** UV-Vis reflectance spectra of TiO2 coatings: (**a**) TiO2-12-0, (**b**) TiO2-12-30, and (**c**) TiO2-20-30.

**Figure 5.** Photocatalytic activity in decomposing methylene blue of TiO2 coatings: (**a**) TiO2-12-0, (**b**) TiO2-12-30, and (**c**) TiO2-20-30.
