Preparation Of GO-Bi₂WO₆/Starch Composite Membranes And Photocatalytic Degradation Of Ethylene With Visible Light

The total output of fruits and vegetables is very high in China every year,but the fruits and vegetables themselves are perishable,resulting in a large loss of fresh fruits and vegetables and making a great waste.One of the reasons for the loss of fruits and vegetables is that ethylene is produced during storage and transportation,and ethylene will promote the maturity and aging of post-harvest fruits and vegetables,thus affecting the nutritional quality and taste of fruits and vegetables during storage.Therefore,it is a great significance to control the ethylene content in the storage and transportation of fruits and vegetables.At present,the main methods for removing ethylene are vacuum degassing,adsorption separation,catalyst and heating,oxidant oxidation,etc.These methods still have defects and deficiencies in practical applications.As a new technology,the photocatalytic oxidation of semiconductors has the advantages of simple operation,low energy consumption,mild reaction conditions and no secondary pollution.It has potential applications for removing ethylene in the storage environment of fruits and vegetables.Bi₂WO₆ as a narrow bandgap semiconductor has visible light photocatalytic activity and can degrade ethylene.However,Bi₂WO₆ has some practical problems that need to be solved to remove ethylene in the storage environment of fruits and vegetables.One is that the electron-hole pairs generated by photoexcitation of Bi₂WO₆ are easy to recombine,and the quantum efficiency is low.Second,the direct use of powdery Bi₂WO₆ for gas phase photocatalysis is difficult,and it is necessary to increase the contact chance and adsorption capacity of gas with photocatalyst.In this paper,Bi₂WO₆ was modified by graphene oxide（GO）to solve the above problems.With the large specific surface area and electron transport capability of GO,the photo-quantum efficiency of the material was improved.The starch film was used as the substrate to make the photocatalytic material on the starch film which can increase the adsorption and exposure to ethylene.Bi₂WO₆ photocatalyst was prepared by hydrothermal method.GO-Bi₂WO₆ composite photocatalyst was prepared by solution mixing method.Bi₂WO₆（GO-Bi₂WO₆）/starch composite film was prepared by loading photocatalyst on starch film substrate by means of internal doping and surface penetration in the process of casting film formation.In this article,it used hydrothermal method to get Bi₂WO₆ powder,added oxidized graphene（GO）to prepare GO-Bi₂WO₆ composite material,and the powder material is added into starch film by the composite method.In the experiment,Changing the pH of the precurd liquid,the amount of photocatalyst addition,the amount of GO doped,etc.It were evaluated by SEM,XRD,UV-Vis and other series of characterization,photocatalytic degradation of ethylene experiment and membrane performance test,and evaluated the Bi₂WO₆ powder,GO-Bi₂WO₆ powder,Bi₂WO₆/Starch film,GO-Bi₂WO₆/Starch film,and obtained the Bi₂WO₆/Starch film,GO-Bi₂WO₆/Starch film,which was the best photocatalytic activity.The paper systematic studies of the influence of preparation conditions on the photocatalytic degradation of ethylene by photocatalyst-loaded starch composite membrane.The structure of the photocatalyst and the photocatalyst-loaded starch composite membrane were characterized by scanning electron microscope（SEM）,atomic force microscopy（AFM）,X-ray diffraction（XRD）and ultraviolet-visible（UV-Vis）analysis techniques.The membrane properties（film thickness,mechanical properties,water permeability）of the composite membrane were investigated.The results of the study are as follows:（1）Bi₂WO₆ was prepared by hydrothermal method using Bi（NO₃）₃·5H₂O and Na₂WO₆·2H₂O as precursors.The results showed that the Bi₂WO₆ powder had a hollow flower-like structure with a particle size of about 3μm when the pH of the flooding solution was 2,and its crystal form was orthorhombic.The average size of the crystal grain（131）crystal face was 49.1 nm.The band gap was 2.57 eV.（2）The Bi₂WO₆/starch composite membrane was prepared by loading Bi₂WO₆synthesized on the precursor solution at pH=2 onto the starch membrane.The results showed that when the amount of Bi₂WO₆ was 5.5%,the composite film exhibited the fastest degradation rate of ethylene under visible light,the reaction rate constant K’value was 9.38×10^-4 min^-1,and the ethylene degradation rate was 19.4%.At this time,the degradation rate of ethylene on the composite membrane under simulated sunlight was the highest,and the reaction rate constant K’value was 9.75×10^-4 min^-1,it was 1.04 times that of visible light irradiation.The performance of the composite membrane was studied.The results showed that the membrane thickness was 250.73μm,the elongation at break was51.87%,the tensile strength was 2.519 MPa,and the water permeability coefficient was5.05×10^-7 g/m²·h·Pa.（3）The Bi₂WO₆ prepared by the hydrothermal method is thoroughly mixed with the GO dispersion to obtain the GO-Bi₂WO₆ photocatalyst.When the GO content was 0.5%,the GO and Bi₂WO₆ can form a good composite.The lamellar GO adheres to the hollow flower-like Bi₂WO₆ surface.The GO doping modification does not affect the Bi₂WO₆crystal structure.However,the oxygen vacancies on the surface increase after the combination of GO and Bi₂WO₆,and C,Bi,and W share some of the oxygen atoms,forming C-O-Bi bonds and C-O-W bonds,which are beneficial to the improvement of photocatalytic activity.When GO dosage was 0.5%,GO-Bi₂WO₆/starch composite membrane has the best photocatalytic degradation of ethylene,and the reaction rate constant K’value was the largest,which is 9.91×10^-4 min^-1,and the ethylene degradation rate was 19.6%.It was 1.06times that of the Bi₂WO₆/starch composite film.