*2.3. Photocatalytic Activity*

The photocatalytic behavior of the TiO2 coatings was assessed from decomposing methylene blue (MB) in a home-made setup. The coated samples were immersed in a glass reactor containing 50 mL of MB solution (5 ppm), and then illuminated with a UV-lamp (λ = 370 nm, *I* = 2.5 mW/cm2). The concentration of MB was determined at certain intervals by measuring the solution absorbance with a UV-visible spectrometer at 664 nm wavelength, which was the maximum absorption peak of MB.

#### **3. Results and Discussion**

#### *3.1. Crystal Structure and Phase Composition*

The crystal structure and phase composition of as-deposited coatings were mainly analyzed by XRD. Figure 2 illustrates the patterns, where the dominant phase is rutile. The minor phase is anatase. It is interesting that there is some preferred orientation in XRD patterns. This may be resulted from the lamellar structure of as-deposited TiO2 coatings stacked by flat granules, but it should be investigated further. Additionally, Ti8O15 phases are observed in all of as-prepared samples, which is consistent with other works [13,19]. For TiO2 coatings sprayed by thermal plasma, Magneli phases were widely reported [20]. This is mainly ascribed to the reduction of TiO2, which results from the high temperature of the thermal plasma jet. There may be other reduced phases, but the analysis is inconclusive because the crystallite size is very small.

As shown in Figure 2, the peak intensity of 2θ = 25.25◦ (101) becomes stronger as distilled water is injected into the plasma jet. The XRD results illustrate that the total amount of anatase phase is increased by modifying the solidification pathway. Moreover, the anatase content and crystallite size are calculated by the Berger-Keller relationship and Scherrer formula, respectively. The results are demonstrated in Table 2.

At atmospheric pressure, anatase and brookite are two metastable phases. They can transform to rutile at the temperature range of 573–1073 K. However, from a thermodynamics view, the nucleation from the melt could be modified by the temperature and quenching rate [12]. Though anatase is a

metastable phase, it preferentially solidifies with a high quenching rate due to its lower surface energy in comparison to rutile (γAnatase = 0.38 J·m<sup>−</sup>2, γRutli = 0.93 J·m<sup>−</sup>2) [11,21].

**Figure 2.** XRD patterns of TiO2 coatings: (**a**) TiO2-12-0, (**b**) TiO2-12-30, and (**c**) TiO2-20-30.


During the plasma spraying process, the plasma temperature is higher than 14,000 K. The speed can reach about 500 m/s [22]. TiO2 particles are fully heated by the plasma jet. Consequently, these molten particles with high temperature deposit on the substrate. As the solidification temperature is close to the melting point of TiO2, the particles are apt to nucleate into stable rutile. Therefore, the rutile phase (96%) in the coating is obtained, and the crystallite size is 73 nm.

However, when distilled water is injected into the plasma jet, the stream is fragmented into smaller drops and then evaporated. The evaporation of the droplet solvent can carry off a grea<sup>t</sup> deal of heat from melted particles, and those quenching particles with moderate temperature are deposited on the substrate. Thus, the particles solidify with the temperature lower than the melting point of TiO2 and generate the anatase phase. As a result, the anatase content is 11% and 19.9% for TiO2-12-30 and TiO2-20-30, and the crystallite size is 22.21 and 19.9 nm.
