*3.2. Morphology Characterization*

The photocatalytic performance of TiO2 coatings is directly correlated to phase compositions and microstructural characteristics. Thus, the morphology and microstructure of TiO2 coatings are investigated by SEM. The surface micrographs of TiO2 coatings are shown in Figure 3. It can be observed that there is no significant discrepancy among these coated samples. They are all composed of fully molten areas and insufficient molten areas; the latter exhibit a rough surface. During the spraying process, the plasma jet of high enthalpy heats the injected particles and accelerates the melting to form solidified droplets (or splats) onto the substrate. Meanwhile, small voids are observed in coating surface. This is mainly ascribed to un-melted particles. As a result, the pores and roughness can increase the reaction area between the catalyzer and the solution during the degradation of the MB dye.

**Figure 3.** SEM images of TiO2 coatings: (**a1**–**a3**) TiO2-12-0, (**b1**–**b3**) TiO2-12-30, and (**c1**–**c3**) TiO2-20-30.

The corresponding cross-sections are illustrated in Figure 3 as well. Along with the morphology, those coatings all exhibit lamella structure. Meanwhile, the TiO2 coatings are firmly bonded to the stainless steel substrates, and there is no crack at the interface. For the TiO2-12-0 coating, some voids are observed, which may be resulted from the relaxation of the thermal stress of inter-laminar or incomplete contact between the plates and un-melted particles. When modifying the solidification pathway, the cross-section of the TiO2-12-30 coating becomes fine and the porosity decreases obviously. On the other hand, the coating of TiO2-20-30 presents a rough and porous cross-section that is caused by the lower melting point of the particles.

In addition, the thicknesses of TiO2-12-0, TiO2-12-30, and TiO2-20-30 are about 150, 180, and 200 μm, respectively. The increased thicknesses of TiO2-20-30 can be ascribed to the augmented speed of the feed disc (from 1.2 to 2.0 rpm). As the speed increases, more TiO2 particles are injected into the plasma jet and take part in the coating deposition. However, it is interesting that the thickness of TiO2-12-30 also becomes thicker when compared with TiO2-12-0, because the speed of feed disc is 1.2 rpm. The results imply that distilled water injected into the plasma jet can not only modify the solidification pathway of inflight melted TiO2 particles, but also can increase the deposition efficiency. Further study will be carried out to clarify this in future.
