Suspension plasma sprayed TiO2 coatings using different injectors and their photocatalytic properties

Stable water- and water/ethanol suspensions of TiO2 were plasma sprayed on stainless steel substrates. The suspensions were injected using two different systems: external, using an atomizing injector, and internal, performed with a continuous-stream injector inside the plasma torch anode. In order to find the optimal spray parameters, seven experimental runs were performed and the resulted deposits were mainly characterized by means of scanning electron microscopy and X-ray diffraction analysis. The microstructural examinations revealed that the coatings obtained by internal injection of the suspension show two principal characteristics: dense zones with well molten big lamellae and porous zones containing fine nanometric and submicrometric grains. When applying the internal continuous-stream injector, the amount of anatase varied from about 1.6 vol.% to 8.8 vol.%. The amount was with 9.4 vol.% to 15.4 vol.% considerably lower when an external atomizing injector was used. The photocatalytic activity of selected coatings was determined by measuring the degradation of an aqueous solution of dye methylene blue. The photon efficiency of titania deposits, which is a measure of the photocatalytic performance, varied from 0.022% up to 0.032% and did not seem to depend on the amount of anatase phase.