Photocatalytic VOCs Removal Under UV Or Visible Light Over Anatase TiO₂-based Thin Films

As a cause for the formation of haze,volatile organic compounds?VOCs?are also the main pollutants of indoor air.Photocatalysis technology can be suitable for the degradation of low-concentration VOCs,a complete mineralization to CO₂ for air purification improvement.?1?Toluene,a typical VOC pollutant,features stable chemical structure,for which its degradation involves multiple steps,maybe accompanied by its intermediates of the same toxicity.Hence,a complete oxidation of toluene to CO₂ is of significance for atmospheric environmental treatment and human settlements environment.TiO₂,one of the most widely used photocatalysts,currently is mostly in powder,however which in thin film is an active subject in the future.Although there are literatures related to TiO₂ thin films for photocatalytic oxidation of toluene and the mechanism,those are not a complete oxidation of toluene but only a partial oxidation?degradation?.Humidity is crucial to the photocatalytic?in particular complete?oxidation of toluene.So far,there are no works related to TiO₂-based thin films for the photocatalytic complete oxidation of toluene,and the effect of humidity on the reaction mechanism of such a complete oxidation.?2?Due to the limitation by its wide band gap of TiO₂?3.0³.2 eV?,TiO₂ can only absorb ultraviolet?UV?light which constitutes unfavorably less than 5%of the solar spectrum.More recently,it seems reasonable to modify TiO₂ with shifting the absorption band toward the visible light region,which is a very active subject of visible-light photocatalysis.However,in the literatures it is rarely reported that photocatalytic degradation of VOCs over Au/TiO₂ thin films under visible light.In this work,optical anatase TiO₂ thin film was prepared on a quartz substrate using atmospheric-pressure chemical vapor deposition?CVD?followed by calcination at 500°C,which was characterized in terms of morphology by scanning electron micro-scope?SEM?,crystalline structure by X-ray diffraction?XRD?,optical absorption by UV-Vis absorption,and photo-induced properties by photocurrent approaches under UV irradiation.In a single-pass continuous flow photocatalytic reactor,under conditions of 25°C and an irradiation by 365 nm UV LED,the effects of humidity on the activity of complete oxidation of toluene to CO₂ in a synthetic air and the reaction mechanism were investigated.The conversion curve of toluene to CO₂ with humidity shows a peak shape.At 53%relative humidity,the conversion of toluene to CO₂ reaches 61%,and remains relatively stable.In situ Fourier transform infrared spectroscopy?DRIFT?was used to study the adsorbed toluene and its reaction mechanism for the surface photocatalytic reaction.The results show that the adsorbed toluene on TiO₂ can be oxidized to benzyl alcohol under dark.The humidity not only accelerates the intermediates that can be further converted to benzaldehyde and benzoic acid,but also accelerates the decomposition of carbonate into CO₂,which is beneficial to the continuous and stable oxidation of toluene.In parallel,to explore visible-light driven activity,Au nanoparticles were deposited on optical anatase TiO₂ thin films by deposition precipitation method?DP?.Such Au/TiO₂ thin films after the activation treatment were obtained.With tuning the Au deposition conditions,Au/TiO₂ thin films with various Au loading and Au nanoparticle size were prepared,and characterized by SEM,XRD,X-ray photoelectron spectroscopy?XPS?,photoluminescence spectra?PL,Corresponding electron-hole recombination?,UV-Vis absorption and visible light photocurrent techniques.It is observed that an increase in Au loading brings an increase in the absorption of visible light,hence a decrease in the recombination of photo-generated an pair of electron and hole.The activity of photocatalytic oxidation of formaldehyde under visible light is not only related to Au loading,but also to the size of Au nanoparticles.Under the same loading condition,the smaller size of Au nanoparticles,the higher its activity under visible light.Moreover,Au/TiO₂ thin film can enable photocatalytic degradation of formaldehyde not only under visible light?of above 420 nm wavelength?,but also under the LED light?of 521 nm?.