The Hydrothermal Preparation And Photocatalytic Properties Of WO₃、TiO₂and Their Composites

With the appearance of more and more ecological problems, people start topay close attention to the studies on environmental protection; photocatalytictechnology, as a infinitely potential mean of pollution control, its core is thepreparation of photocatalysts. TiO2has been one of the most widely studiedphotocatalysts since it was found as a material with high chemical stability,catalytic activity and cost-effective. However, it is reported that TiO2is only bephotosensitized under the UV irradiation (less than388nm) with a band gap of3.2eV, largely limiting its practical applications. WO3, as a semiconductor with arelatively narrower energy gap, is considered to effectively improve thephotocatalytic ability of TiO2in various ways. Researchers suggest that thecomposite structure of TiO2and WO3may prevent the recombination of theelectron-hole pairs with broadened spectral response range to enhance thephotocatalytic performance.In this work, WO3microcrystalline, TiO2nanocrystalline and TiO2-WO3composite crystalline were prepared by hydrothermal reaction usingNa2WO4·2H2O as tungsten source, butyl titanate and Ti(SO4)2as titanium source.The samples were characterized by X-ray diffractometer (XRD), scanningelectron microscopy (SEM), transmission electron microscopy (TEM), andenergy dispersive spectrometer (EDS), Thermogravimetry-Differential ScanningCalorimetry (TG-DSC) and Ultraviolet-visible spectrum (UV-Vis) methods. Theinfluence of reaction time, hydrothermal temperature, heat treatment on theforms, morphologies and visible light catalytic properties of WO3microcrystalline was discussed; the influence of different time, temperatures andadditive on crystallinity, micro structure and photocatalytic activities of TiO2nano crystalline was studied; the influence of Ti/W molar ratios in precursors onphases, morphologies, especially photocatalytic properties of the composites was investigated. Results show:Orthorhombic WO3·0.33H2O micro crystalline were prepared under100~180℃for8~24h using Na2WO4·2H2O as the source. Products preparedunder160℃for12h had better crystallinity and relatively completemicrostructure. The as-prepared crystalline exhibited an increase in thediffraction peak intensity and a morphology evolution from sheet-like structureto self-assembled sphere-like structure when increasing hydrothermal reactiontemperature. Products were found to transform into hexagonal WO3aftercalcined under300℃. Corresponding photocatalytic properties of theas-obtained WO3particles before and after calcined were found to increase firstand then decrease with increasing hydrothermal reaction temperatures. Powdersgot under120℃presented the best properties and its degradation rate of RhBunder visible light irradiation reaches87%. Further discussion found that thecrystallinity and the size of specific surface area played crucial roles whencontrolling the product photocatalytic properties. Moreover, comparing theas-prepared particles before and after post heat treatment revealed that hexagonalWO3structure presents better photocatalytic activity than orthorhombicWO3·0.33H2O structure with similar microstructure.Anatase TiO2powders were prepared under160℃for4~24h using butyltitanate as raw material. The crystallinity of products was generally poor and wasfound to increase first and then decrease with increasing hydrothermal reactiontime, which got the highest crystallinity at8h. The crystallinity of samples hadstrongly enhanced and XRD curves became smoother after adding acetylacetoneinto precursor. At the same time, anatase TiO2were also prepared under100~160℃for4~12h using Ti(SO4)2as the source material. Best crystallinityand loosened morphology of products was obtained when hydrothermaltemperature was140℃reacting for8h and the as-prepared products exhibitedbest photocatalytic property, which degradation rate for RhB under visible lightcould reach almost90%.Butyl titanate was used as titanium source to prepare composites with Ti/Wratio of99:1,70:30,50:50,30:70,1:99in precursors. The phase of productstransformed from anatase TiO2to TiO2and WO3coexistence then into WO3finally with the decrease of Ti. Products obtained at a Ti/W ratio of99:1had the highest photocatalytic activities. TiO2-WO3composite powders were preparedwith Ti/W ratios of99.5:0.5~1:99using Ti(SO4)2as titanium source. Productswere anatase TiO2phase with a content of Ti more than75%, and transformedfrom amorphous into WO3with the increasing dosage of W. Photocatalytic testshowed: the photocatalytic activities of composites which were prepared fromprecursor with content of Ti more than92%, were higher than pure TiO2. Thedegradation of products prepared at Ti/W=99:1reached95.8%of RhB undersunlight irradiation for28min.