Design, Preparation And Performance Study Of Carbon Quantum Dot Doped Composite Photocatalyst

The high energy consumption and pollution of traditional industrial processes have led to three global problems:resource shortage,environmental pollution and ecological damage.The increase of population and and social progress aggravate the above problems and hence it is urgent to reform technology to break through energy consumption barriers and solve the environmental crisis.Solar energy is a kind of clean and renewable energy.The highly efficient development and utilization of solar energy has received worldwide attention and is also an important part of China's sustainable development strategy.As a kind of green technology,the photocatalytic technology can realize the direct utilization of solar energy and hence have shown great application potential in the fields of energy and environment.Thereinto,Using photocatalytic technology to treat organic pollutants in water not only solves the problem of water pollution,but also avoids the consumption of fossil energy and emission of new pollutants in the traditional treatment process,which is attracting more and more attention of people.The traditional photocatalysts usually possess some disadvantages such as wide enegy band,weak light absorption and quick recombination of photon-generated carriers and hence their photocatalytic degradation efficiencies need to be further improved.As a new kind of carbon nanomaterials,carbon quantum dots(CDs)have many outstanding optical and electrical properties including up-conversion fluorescence,strong light absorption,and excellent photogenic carrier transfer acceptance and transfer properties.In recent years,people found that the photocatalytic performance of photocatalysts were greatly enhanced when the CDs were introduced.The related researches have become the hotspot of photocatalytic field.Herein,we constructed two kinds of hybrid photocatalysts by introducing CDs onto the CoFe2O4 and graphite phase carbon nitride(g-C3N4)microspheres.The effects of CDs loading content on the activity of visible light degradation of organic dyes of hybrid photocatalysts were investigated.The photocatalytic mechanism was discussed.The specific research cotent was listed as follows.(1)CDs were stripped and purified from graphite electrode by alkali assisted electrochemistry using graphite rod as carbon source.The results of photoluminescence(PL)test showed that CDs have excellent up-conversion fluorescence properties.Using ethylene glycol(EG)as soft template,cobalt ferrite nanoparticles were synthesized in the PTFE reactor by solvent thermal method,and a series of CDs/CoFe2O4 composite catalysts with different CDs contents were constructed by loading different amounts of CDs onto cobalt ferrite microspheres.The phase,structure,morphology and composition of the catalyst were analyzed by X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),N2 adsorption-desorption,energy dispersive X-ray spectrometer(EDX)and X-ray photoelectron spectroscopy(XPS).In the experiment of catalytic degradation of the simulated organic pollutant methylene blue(MB)under visible light driving,the catalytic activity of cobalt ferrite with a CDs load of 4%was 1.64 times higher than that of pure cobalt ferrite,and it was 3.65 times higher when a little hydrogen peroxide(H2O2)was added.Based on the characterization of the photoelectric properties of the catalyst,the mechanism of the photocatalytic process in the system was discussed.(2)Using chromic acid and terephthalic acid as raw materials,the metal organic framework materials(MOF,MIL-101(Cr))were synthsized by means of the coordination between Cr(VI)and terephthalic acid.The carbon source(citric acid)and modifier(ionic liquid of 1-aminopropyl 3-methyl imidazole bromide)were loaded into the cages of MIL-101(Cr)in a certain molar ratio.Then the ionic liquid modified CDs were synthesized by the facile pyrolysis method with unifonn particle size distribution and an average particle size of 4 nm.The g-C3N4 microspheres composed of stacking sheets with a thickness of about 12 nm were prepared by high temperature calcination of the supermolecules formed by self-assembly of cyanuric acid and melamine in DMSO.SEM and TEM images showed that there are a lot of interstitial structures among the sheets of g-C3N4 microspheres.The specific surface area of g-C3N4 microspheres was up to 95.1 m2/g according to the N2 adsorption-desorption test.To tune the photocatalytic performance of g-C3N4 microspheres,the CDs were introduced onto the surfaces of g-C3N4 microspheres by electrostatic,?-electron stacking and amide bond interactions.The strong interactions between CDs and g-C3N4 microspheres were confirmed by Zeta potential,XRD and XPS tests.With increasing loading of CDs,the surface charges of g-C3N4 microspheres were gradually transformed from being negative to being positive.Meanwhile,the heterojunction structures between CDs and g-C3N4 microspheres were formed.To distinguish the effects of surface charges and heterojunction structures on the visible light catalytic degradation performance of CDs/g-C3N4 composite catalysts,the photodegradability of catalysts towards three kinds of organic pollutants including anionic dyes(methyl orange and methyl blue),cationic dye(Rhodamine B)and neutral pollutant(p-nitrophenol)was investigated.The photocatalytic mechanism was analyzed by the results of active component capture experiments combined with the characterization results of photoluminescence(PL),electrochemical impedance(EIS),photocurrent,XPS valence band spectrum and ultraviolet-visible diffuse reflectance(UV-Vis).The results demonstrated that the photodegradation activity of composite catalysts towards anionic dyes and neutral pollutant was outperformed the g-C3N4 microspheres.However,the photodegradation activity of composite catalysts towards cationic dye was inferior to the g-C3N4 microspheres.These phenomena reflected that the surface charges could enhance or suppress the photodegradation activity of composite catalysts by electrostatic attraction or repulsion of polluant molecules despite the heterojunctions played a positive role in improving the photodegradation activity of composite catalysts by promoting the separation of photon-generated carriers.Sometimes,the surface charges even played a dominant role.