**2. Experimental**

#### *2.1. Atmospheric Plasma Spraying*

An atmospheric plasma spraying system (MF-P1000, GTV, Luckenbach, Germany) was used to deposit TiO2 coatings. A carrier gas with a flow rate of 3.5 L/min radially injected feedstock powder into the plasma jet, while distilled water was radially injected into the plasma jet through a nozzle with average diameter of 0.3 mm by a suspension feeder (GTV, Luckenbach, Germany). The powder injector was 8.0 mm apart from the gun outlet, while the solution injector was 15.0 mm away from the gun outlet. Three typical coatings were obtained with different feeding parameters. As seen in Table 1, the speed of the feed disc regulated the mass of the feedstock powder injected into the plasma jet, and the flow rate controlled the mass of distilled water injected into the jet.


**Table 1.** The details of plasma spraying parameters for TiO2 coatings.

Micron-sized commercial TiO2 powders ( −45 + 15 μm, Sunspraying Science and Technology Co. Ltd., Beijing, China) were adopted as feedstock powders (Figure 1). Stainless steel plates (30 mm in diameter, STS316) were employed as substrates. Prior to the deposition, the substrates were cleaned with ethanol to remove contaminants, followed by blasting with Al2O3 abrasives.

## *2.2. Coating Characterization*

Scanning electron microscopy (SEM, Nova-Nona-430, Thermo Fisher Scientific, Waltham, MA, USA) was utilized to investigate the microstructure of TiO2 coatings. A UV-Vis spectrophotometer (HP8453, Agilent, Santa Clara, CA, USA) was used to record spectra in the wavelength range of 250 to 750 nm. Microhardness was obtained with a load of 100 g for 10 s using Vickers (2100B, Instron Tukon, Shanghai, China), and the value was averaged from five indents per specimen.

An X-ray diffractometer (XRD, D8-Advance, Bruker, Billerica, MA, USA) equipped with copper radiation X-ray was employed to assess the phase composition of the proposed coatings. The 2θ was acquired in the range of 20◦ to 90◦ with a 0.05◦ step size. The anatase content was simply determined by the use of the relationship given by Berger-Keller et al. [17]. Meanwhile, crystallite size was calculated using the Scherrer formula [18].

**Figure 1.** (**a**) SEM morphology and (**b**) XRD pattern of feedstock powders.
