Preparation of feedstocks from nano/submicron-sized TiO2 particles to obtain photocatalytic coatings by atmospheric plasma spraying

Atmospheric plasma spraying (APS) has been proved useful in obtaining TiO2 coatings with effective photocatalytic activity. However, the influence of feedstock characteristics on photocatalytic activity has hardly been addressed. This study was undertaken to prepare TiO2 photocatalytic coatings by APS from different feedstocks. The feedstocks were obtained by spray drying suspensions of a nano- and a submicron-sized TiO2 powder, with different solids contents and/or ratios of the nano- to submicron-sized particles. All the resulting powders were fully characterised to assess their suitability for use in APS processes. Feedstock powders were then deposited on steel coupons by APS using hydrogen or helium as secondary plasma gas. Coating microstructure and phase composition were characterised. Coating photocatalytic activity was determined by measuring the degradation of methylene blue dye in an aqueous solution. Powder characterisation showed that all feedstocks met plasma spray operation requirements with regard to agglomerate size distribution and density, as well as to powder flowability. Optimum agglomerate density was obtained when a well-balanced mixture of nano- and submicron-sized particles was used. All coatings displayed a bimodal microstructure with partially melted agglomerates that retained the initial nano- or submicron-sized structure and composition (anatase phase) of the feedstock, surrounded by a fully melted matrix mainly formed by rutile. As expected, coating porosity as well as the amount of partially melted areas depended strongly on plasma spray conditions and on feedstock characteristics. With regard to photocatalytic activity, a reasonably good fit of a first-order kinetic model to the experimental data was found for all coatings. The kinetic constants obtained displayed higher values than those of a commercial sol–gel coating. The values of these constants were related to feedstock characteristics as well as to plasma spraying conditions on the basis of anatase content. The contribution of other factors to the resulting coating surface is also discussed