**1. Introduction**

Titanium dioxide (TiO2) has been extensively investigated due to its high photocatalytic activity [1,2]. There are three normal crystal phases for TiO2 material: brookite, anatase, and rutile. Under atmosphere pressure and temperature, rutile is the stable phase. Calcined at the temperature about 573 to 1073 K, anatase and brookite will irreversibly transform into rutile. However, the photocatalytic activity of rutile is lower than that of anatase due to an increased electron-hole recombination rate [3,4].

The deposition of TiO2 coating well attached on the surface of a substrate can be more efficient and as compared to TiO2 P25 powder or similar products, owing to the easy recovery of the photocatalyst from the water [5]. Several methods have been reported to prepare TiO2 coatings, including vapor deposition [6,7], electrophoretic deposition [8,9], sol-gel [10], and thermal spraying [11]. In comparison to other methods, thermal spraying has several highlighted features to make it particularly attractive, which include flexibility and high efficiency.

Unfortunately, during the thermal spraying of a plasma jet with a temperature of 14,000 K, the powder feedstock is melted. These molten particles are then driven to deposit on a substrate. Compared with initial feedstock powders, the anatase transformation to rutile is clearly observed in sprayed coatings. Even given P25/20 TiO2 granulated nanopowders as the starting powder, an anatase content of only 1.7%–5% were obtained in the coatings [12]. Then, retaining the metastable anatase phase in thermally sprayed TiO2 coatings becomes a grea<sup>t</sup> challenge.

Bozorgtabar et al. [12] demonstrated that photocatalytic activity was strongly related to the process conditions of thermal spraying. Thus, Colmenares-Angulo et al. [13] adjusted spraying conditions to reduce the heat input, and expected to reduce the transformation of anatase TiO2 to rutile. Ctibor et al. [14] reported that most of the reduced phases were formed in TiO2 coatings. The stoichiometry was further found to be a function of process parameters. Zhang et al. [15] reported that oxygen vacancies at the surface remarkably prolonged the life of the photon-generated carrier. Thus, it significantly increased the activity. However, others have recently observed that oxygen vacancies caused a significant raise to the charge carrier recombination and resulted into a depressed photocatalytic activity [16].

Herein, modifying the solidification pathway of inflight melted particles was investigated by injecting distilled water into the plasma jet rather than adjusting the spray conditions, and photocatalytic TiO2 coatings with enhanced activity were obtained. The elaborated coating microstructure was mainly characterized by SEM, while phase composition and crystallite size were investigated by XRD. Finally, the photocatalytic activity of the proposed TiO2 coatings was evaluated by decomposing an aqueous solution of methylene blue.
