Correlation of structural properties and film thickness to photocatalytic activity of thick TiO2 films coated on stainless steel

A transparent, relatively dense and thick TiO2 film with good mechanical properties and enhanced photocatalytic activity has been successfully immobilized on 304 stainless steel. The study included optimization of structural properties and film thickness. Experiments on the photocatalytic degradation of 4-chlorobenzoic acid (4-CBA) as a model organic contaminant showed that the photocatalytic activity of this kind of thick film can be significantly improved by decreasing calcination temperature (i.e. from 600 to 500 °C), which is due to an increase in BET surface area and porosity, decrease in crystal size, and decrease in foreign metal diffusion from stainless steel support to the surface of the film. The observed apparent rate constant of TiO2-500 (500 °C, two dip coating layers) was found to be approximately 5.2 times that of TiO2-600 (600 °C, two dip coating layers). The results showed that increasing film thickness within a certain range (up to 10 μm) could significantly improve the photocatalytic activity without causing crack formation for the thick TiO2-500 film (500 °C). The optimum film thickness is 10 μm (four dip coating cycles), under which the observed apparent rate constant of TiO2-500 is approximately 2.3 times higher than that of TiO2-500 with 2.5 μm film thickness (one dip coating layer). Based on several advantages of TiO2-500 film (500 °C, four dip coating layers), including high photocatalytic activity, absence of foreign metal ions at the film surface, optimum film thickness, good structural integrity, and excellent adhesion on the stainless steel support, this kind of sol–gel-derived thick film immobilized on stainless steel support is a promising potential photocatalyst for water treatment applications.