Research On The Degradation Of Ethylene Using Titanium Dioxide Nanotube Arrays Co-decorated With CdS Quantum Dots And Reduced Graphene Oxide

The change of ethylene content in fresh and refrigerated environment plays an important role in the life of fruit and vegetable after harvest.In a closed and refrigerated environment,fruit and vegetable wounds,which cause by harvesting and peeling,can induce large amounts of ethylene in tissues.If accumulated ethylene can not be removed,it will stimulate more fruits and vegetables to produce ethylene.Ethylene can increase cell respiration intensity,enhance metabolic enzyme activity,accelerate membrane permeability and cell partition loss,thus promoting fruit softening and vegetable chlorosis.Therefore,seek an effective way for controlling ethylene is very importment for the storage ages and quality of fruits and vegetables.Due to the unique properties,titania?TiO₂?semiconductor photocatalytic technology as the representative of the material have attracted much attention,it is considered to be a promising photocatalyst and expected to achieve effective control ethylene content for new technology development to provide a new path for the preservation in cold storage and storage.Because of the limited optical absorption range of TiO₂ and the high recombination rate of light carrier,the application scope is greatly limited.Therefore,through using effective modification and doping method,we can design a new type of photocatalyst with wide spectral response,high quantum yield and large specific surface area,which also is the key of this strategy.In this paper,we first designed and synthesized titanium dioxide nanotube arrays?TNAs?with high specific surface area.A new type of nano tube structure?^*TNAs?with photocatalytic activity was then obtained by using a new gamma ray irradiation technique to modify the crystal shape of the nanotube.The precursors containing S₂O₃^2-and Cd²⁺ions were selected and the CdS quantum dots?CdS QDs?were also synthesized by the reduction of gamma rays.Graphite oxide?GO?materials with larger specific surface area were selected,and more stable reduced graphene oxide?rGO?structure was obtained by the irradiation of gamma rays as well.Compounding the above synthetic materials?^*TNAs,CdS QDs,rGO?to construct a new three-dimensional nano-heterostructure CdS QDs/rGO-^*TNAs.The structure of the heterostructure was then characterized by modern analytical instruments such as FESEM,XRD,AFM,FTIR,RS,UV/Vis-DRS and XPS.In the simulated refrigerated environment,the degradation of the ethylene gas was studied and the photocatalytic performance of the heterojunction was evaluated.This paper hopes to provide theoretical and experimental basis for the preparation of new photocatalytic materials,and further lays a foundation for the development of new technologies for ethylene removal and preservation in cold storage environment.The main results obtained in this paper are as follows:?1?The structure of titanium dioxide nanotube with large specific surface area was successfully prepared by potentiostatic anodization.The nanotubes were irradiated by gamma rays and analyzed according to the XRD results.It is shown that titanium dioxide can transform from amorphous state to anatase crystal with stronger photocatalytic activity.?2?Sodium thiosulfate hydrate?Na₂S₂O₃·5H₂O?and cadmium acetate hydrate?Cd?CH₃CO₂?₂·2H₂O?are selected as precursor,cadmium sulfide quantum dots?CdS QDs?were successfully prepared by gamma ray irradiation reduction under the condition of adding proper amount of dispersant and stabilizer.Then CdS QDs are combined with irradiated titanium dioxide nanotube to construct a new type of photocatalyst CdS QDs-^*TNAs.The effects of mole ratio of Na₂S₂O₃·5H₂O and Cd?CH₃CO₂?₂·2H₂O,dispersant PVP content,gamma radiation dose and CdS QDs addition on the preparation of CdS QDs-^*TNAs and its photocatalytic degradation of ethylene were investigated.The results showed that CdS QDs was successfully prepared by the reduction of gamma ray,and the CdS QDs-^*TNAs obtained after modifying with^*TNAs showed excellent performance in photocatalytic degradation of ethylene compared with TNAs and^*TNAs.The rate constant of photocatalyst TNAs to ethylene is 1.48×10^-4 min^-1,what's more the degradation rate constant of^*TNAs can reach up to 2.30×10^-4 min^-1.The degradation rate constant of CdS QDs-^*TNAs after CdS QDs and^*TNAs modifying is further increased,and rate constant of 6.25×10^-4 min^-1 can be reached.?3?Graphite oxide?GO?is selected as precursor material.Under the condition of adding appropriate dispersants and stabilizers,a large number of oxygenated functional groups on graphite oxide can be removed by ultrasonic dispersion and gamma irradiation reduction to get more stable reduced graphene oxide?rGO?structure.Combined with irradiated titanium dioxide nanotube to construct a photocatalyst rGO-^*TNAs with a larger specific surface area and the effect of ultrasonic power,GO addition and irradiation dose on the preparation of rGO-^*TNAs catalyst and its photocatalytic degradation of ethylene are invested.The results showed that gamma ray can effectively reduce the large number of functional groups on graphite oxide to obtain more stable rGO structure and combine with^*TNAs to obtain rGO-^*TNAs photocatalyst.The composite rGO-^*TNAs showed good degradation ability for ethylene removal in simulated refrigerated environment.The rate constant of ethylene degradation depending on rGO-^*TNAs can reach up to 3.91×10^-4 min^-1.?4?In order to further promote the photocatalytic degradation performance of the catalyst,the excellent properties of CdS QDs and rGO will be synthesized in this paper.Selection of sodium thiosulfate hydrate,cadmium acetate hydrate and graphite oxide as precursors.The CdS QDs/rGO composites were synthesized by one-step in-situ synthesis using the irradiation reduction of gamma ray,and CdS QDs/rGO was combined with the irradiated titanium dioxide nanotubes to form a new photocatalyst CdS QDs/rGO-^*TNAs using the surface deposition method.Photocatalytic degradation of ethylene in refrigerated environment with a new type of photocatalyst as the main body are then invested.Mranwhile,the effects of preparation conditions,like mass ratio of GO and Cd?CH₃CO₂?₂·2H₂O,ultrasonic treatment power and irradiation dose on the construction of CdS QDs/rGO-^*TNAs and its photocatalytic degradation of ethylene activity,as well as the micromorphology and optical properties of the new nano heterojunction are deeply into studied.The results showed that the CdS QDs/rGO nanomaterials were successfully prepared by one-step reduction with the introduction of gamma rays,and the photocatalytic degradation of ethylene was further promoted by CdS QDs/rGO-^*TNAs.The rate constant of ethylene degradation can reach up to 9.73×10^-4 min^-1,and the rate constant increases by 35.76%and 59.82%,respectively,compared with CdS QDs-^*TNAs and rGO-^*TNAs.?5?Establish the relationship between micromorphology of photocatalyst and macro ethylene degradation for a better understanding of photocatalytic activity play.The ordered arrangement of titanium dioxide nanotube structure can be clearly seen in the FESEM diagram,and the huge specific surface area provides the carrier for the adsorption of ethylene gas.The introduction of rGO enables the CdS QDs to be uniformly distributed without aggregation,and thus promotes the enhancement of the photocatalytic ability of CdS QDs/rGO-^*TNAs.From the diffraction peak of XRD results,we can predict that the transformation of amorphous crystal form to anatase after the introduction of gamma ray,and the appearance of the characteristic diffraction peak of CdS crystal in compound CdS QDs/rGO-^*TNAs,which proves the successful preparation and loading of CdS QDs.In fourier transform infrared spectroscopy,the partial disappearance and decrease of the peak intensity of oxygen-containing functional groups after GO irradiation by gamma rays showed that part of GO was successfully reduced to rGO.Atomic force microscopy?AFM?showed that the reduced rGO was presented with a larger slice structure,but still had a larger roughness.The band gap of TNAs,^*TNAs,CdS QDs-^*TNAs,CdS QDs and CdS QDs/rGO-^*TNAs were about 3.21,3.19,2.34,2.41,2.32 eV,respectively.The narrower band gap of CdS QDs/rGO-^*TNAs shows stronger photocatalytic degradation performance.XPS analysis of the photocatalyst samples showed that the content of hydroxyl radical in CdS QDs/rGO-^*TNAs photocatalyst was about 21.93%,and the number of Ti³⁺was about 1.75%,while the content of hydroxyl radical in TNAs photocatalyst was about 20.12%,and the number of Ti³⁺was about 1.64%.The hydroxyl free radicals with high catalytic oxidation capacity and the increase of Ti³⁺amount may provide an explanation for the enhancement of photocatalytic activity of CdS QDs/rGO-^*TNAs.