Freeze-drying Preparation Of Tungsten Oxide Based Porous Heterojunction And Its Photocatalytic Properties

Tungsten trioxide（WO₃）is a typical n type semiconductor material with band gap width of 2.7 eV.It is widely used in electrochromic and photochromism,chemical sensors and photocatalysis.Since the properties of the materials are strongly dependent on their microstructure and morphology,the WO₃ with various micro/nano structures has been synthesized in recent years.Three dimensional porous WO₃ has attracted wide attention due to its high specific surface area,abundant pore structure and three-dimensional self-supporting structure.At present,the technology for preparing three-dimensional porous WO₃ mainly includes template,the anodic oxidation and the self-assembly of solvent evaporation induction method.However,these methods often faced with many problems such as complex preparation process,harsh reaction conditions and difficult mass production.Therefore,in view of the above problems,this paper uses a new three-dimensional porous material preparation method—vacuum freeze drying to prepare three-dimensional WO₃porous materials.The main results are as follows:（1）The polymer polyvinyl alcohol（PVA）/phosphotopystic heteropoly acid(H₃PW₁₂O₄₀)aqueous solution was used as precursor and the composite porous material of PVA/H₃PW₁₂O₄₀ was obtained by vacuum freeze drying technology.The porous WO₃ materials with macroscopic three-dimensional structure and uniform channel were prepared by calcination process.In the experiment,the influence of synthetic conditions on the structure of three-dimensional porous WO₃ was tailored by varying the polymer content and calcination temperature.Then,the three-dimensional porous WO₃ was used as the photocatalyst,and the photocatalytic degradation ability of Rhodamine B（RhB）was investigated under the visible light condition.Results revealed that the porous structure have channels of 3–10μm and wall thicknesses of about 0.68μm.When the PVA content is high,the specific surface area of WO₃is larger,and its photocatalytic activity is better.When calcining temperature was changed,it was found that the photocatalytic activity of samples obtained at 800°C was the best.Through systematic research,we have found that the enhancement of the photocatalytic performance of porous WO₃ may be attributed to its larger specific surface area,interconnected macroporous structure as well as higher crystal quality.（2）The three dimensional porous TiO₂/WO₃ heterojunction materials were prepared by vacuum freeze drying and subsequent calcination by using the aqueous solution of PVA/H₃PW₁₂O₄₀/P25 as precursor,and the effect of the load of WO₃ on the photocatalytic properties of TiO2/WO3 heterojunction was investigated.The results showed that the photocatalytic properties of the samples gradually improved with the increase of WO₃ loading.On the one hand,the construction of heterojunction can effectively separate the electron hole pair,so that the photogenerated charge will spread to the surface of the material and participate in the surface reaction.On the other hand,the enhancement of photocatalytic properties of TiO₂/WO₃ heterojunction materials based on the macroporous structure may also be due to the increase of the specific surface area and the increase of the active site.