Probing the activities of UV and visible-light absorbing photocatalyst powders using a resazurin-based photocatalyst activity indicator ink (Rz Paii)

A variety of semiconductor photocatalyst powdered materials, i.e. TiO2, CdS, WO3 and C3N4, are able to mediate the UV (365 nm) photocatalysed reduction of Rz (blue) to Rf (pink) by glycerol, when used as a dispersion in deaerated, aqueous solution. Under the same conditions, but using visible (455 nm) light, the same reaction is promoted by the visible-light absorbing semiconductors, CdS, WO3 and C3N4. These powders are mixed with SiO2 and PTFE powder and pressed as discs, for assessment using a RzPhotocatalyst Activity Indicator Ink, i.e. Rz paii, coated onto the surface of each disc. Results show that only the TiO2 pressed film, with UV irradiation, is able to photocatalyse the reduction of Rz to Rf, under ambient conditions, due to competition from ambient air. However, when covered with Sellotape™ to exclude O2, all but the C3N4 photocatalyst demonstrated photocatalytic activity, and in the same order as observed in solution for this reduction reaction, using UV or visible light. Irradiations carried out using 617 nm light on the same systems reveal that the dye is slowly bleached, at a rate that this largely independent of the semiconductor/insulator present, and no Rf is generated, suggesting dye degradation via photolysis rather than dye sensitisation. This is the first example of the use of an Rz paii responding to visible light absorbing photocatalysts and the first example of an Rz paii being used to assess the photocatalytic activity of powdered SC photocatalysts.