Structural and photocatalytic characteristics of TiO₂ coatings produced by various thermal spray techniques

Titanium dioxide (TiO₂) was elaborated by four different thermal spray techniques—(i) plasma spraying using a water-stabilized torch, (ii) plasma spraying using a gas-stabilized torch, (iii) high velocity oxy–fuel gun, and (iv) oxy–acetylene flame. The porosity of the coatings was studied by optical microscopy, nano-structural features by scanning electron microscopy (SEM), phase composition by X-ray diffraction (XRD); the microhardness, surface roughness and wear resistance were evaluated. The diffuse reflectance was measured by ultra-violet/visible/near-infrared (UV/Vis/NIR) scanning spectrophotometer. The kinetics of photocatalytic degradation of gaseous acetone was measured under a UV lamp with 365 nm wavelength. After all the applied spray processes, the transformation of anatase phase from the initial powders to rutile phase in the coatings occurred. In spite of this transformation, all the coatings exhibited certain photocatalytic activity, which correlated well with their band gap energy calculated from reflectivity. All the coatings offer relatively good mechanical properties and can serve as robust photocatalysts.