For the inactivation of mold spores by UVC irradiation, with ozone acting as a promoter, TiO₂ nanoparticles may act better as a "sun block" than as a photocatalytic disinfectant

Fungal spores are known as critical indoor allergens, and indoor air purification techniques including photocatalytic disinfection using titanium dioxide (TiO₂), ultraviolet germicidal irradiation (UVGI) and ozonation, have been considerably investigated. However, most of the research is in regard to photocatalytic disinfection, focused on the anti-bacterial efficacy of TiO₂ nanoparticles (NPs). Furthermore, some research even showed that the photocatalytic antifungal efficacy of TiO₂ NPs may not be that significant. Thus, investigating the reasons behind the non-significant antifungal efficacy of TiO₂ photocatalytic disinfection and enhancing the antifungal efficacy is indispensable. In this study, ozone was employed to improve the photocatalytic antifungal efficacy of the TiO₂ NPs and nano-metal supported on TiO₂ NPs. The commercial TiO₂ NPs (Degussa (Evonik) P25) served as a good support, and incipient wetness impregnation was successfully exploited to prepare oxidized nano-metals (Ag, Cu and Ni) in this study. There were two surfaces (quartz and putty) used in the inactivation experiments of Aspergillus niger spores which were manipulated under two conditions: exposed to ultraviolet (UVC) light, and exposed to UVC and ozone simultaneously. The SEM images demonstrated that the spores were sheltered from UVC light in the microcracks between TiO₂ agglomerates. When irradiating with UVC, the A. niger spores on the two testing surfaces, without TiO₂ NPs, were inactivated faster than those with TiO₂ NPs, implying a "sun block" effect of this material and a lower photocatalytic antifungal efficacy than UVGI. On both surfaces, the inactivation rate constants (k) of A. niger spores exposed to UVC and ozone simultaneously (on quartz: k = 2.09-6.94 h^-1, on putty: k = 3.17-6.66 h^-1) were better than those exposed to only UVC (on quartz: k = 1.80-5.89 h^-1; on putty: k = 2.97-3.98 h^-1), indicating ozone can enhance the UVGI antifungal efficacy.