| Development of new renewable energy sources has become a research hotspot.The photocatalytic reaction would be an ideal technology for environmental pollution control and the production of clean energy.Due to its unique properties,photocatalytic technology can be driven under mild reaction conditions and directly use of solar energy.Cerium is the most abundant element in rare earth elements.Therefore,it provides an rich raw materials for the preparation of cerium dioxides.In addition,cerium dioxide owns non-toxicity,simple preparation method,high oxygen storage capacity,chemical stability and other characteristics,so as to become a new type photocatalytic material.Compared with the traditional titanium dioxide photocatalyst,cerium dioxide has more active sites due to its activie oxidation-reduction characteristics,which is in favor of the photocatalytic reaction.In this dissertation,a series of ceria-based nanomaterials were synthesized by a sol-gel method,solvothermal method,condensation reflux-calcination method,and planetary milling method.The formation mechanism,surface properties and optimization of different morphologies of cerium dioxides were explored by electron microscopy,infrared spectroscopy,UV-Vis spectroscopy,Raman spectroscopy,X-ray diffraction,X-ray photoelectron spectroscopy,thermogravimetry and simulation calculations technologies.The synthesis conditions and mechanism of improving photocatalytic activity were discussed.The synthesized ceria-based nanomaterials were utilized as the photocatalysts,the degradation of indoor air pollutants(acetaldehyde)and the photocatalytic performance of isopropanol conversion would be investigated:(1)Bulk,multi-sized octahedral,nanorod-like,and flaky cerium dioxide nanomaterials were prepared by the solvothermal method.The solvothermal method is easy to operate and is easy to be prepared in batch.It is beneficial to the progress of the experiment.And the properties of ceria nanomaterials can be adjusted by the reaction temperature,duration,precursors,solvents and other parameters.Octahedral and flaky cerium oxide are more likely to adsorb reactants relative to bulk materials due to their larger specific surface areas,improving their photocatalytic activities.Special structures of cerium oxide nanomaterials,anisotropic sheet,and spindle-like ceria were prepared by sol-gel method and condensation-reflux-calcination method.Compared with the sol-gel method and condensation reflux-calcination method,we finally choose solvothermal method as the main preparation method of this dissertation due to their advantages.In addition,the thread of this dissertation has been proposed:Morphology and light absorption are the key points of photocatalysis.The excellent absorption capacity for reactant could be obtained by morphology controlling;strong absorption capacity for light could be procured by tuning the content of Ce3+ and be hybrid with other materials.(2)We prepared the hollow structured CeO2 with high surface area via solvothermal method.Under the system of water,ethanol and ethylene glycol,cerium oxide microspheres with mesoporous hollow structures were synthesized by solvothermal method.Ostwald ripening theory is the main theoretical basis for the formation of mesoporous hollow microspheres.The activity of CeO2-PH sample was about 4-times higher than that of the contrast sample for CeO2-NP,and the 18-times higher than that of P25.(3)We prepared the hollow structured CeO2 with high surface area via a more environmentally friendly waymethod.A large amount of carbon dioxide gas is bubbled into the precursor solution so that the solvothermal reaction is carried out in the atmosphere of carbon dioxide.Since the ceria grains themselves contain a large amount of alkaline active sites and the carbon dioxide is an acidic gas,carbon dioxide can be easily adsorbed on the ceria grains to participate in the grain growth process and eventually form an octahedron hollow structure.At the same time,we also studied the effects of other atmospheres on the products.Compared with the products without an atmosphere,inert gas argon-atmosphere makes the size of the cerium dioxide particles smaller and the surface smoother.The surface of cerium oxide particles are destroyed and hollow structure is beginning to form under an oxygen atmosphere.The main advantage of the hollow structure is that it provides more reactive sites,improving the absorptive capacity of the photocatalyst to the reactants,and then improves the photocatalytic activity.In order to futher improved the photocatalytic activiy under visible light condiotion,stronger light absorption should be obtained.(4)Under the premise of preserving the morphology,the cerium dioxide itself was modified to increase its visible light absorption,and then improving the photocatalytic performance.Surface plasmon resonance is a phenomenon of resonance between the light and the electron gas of the material itself,which can improve the light absorption ability of the material around the specific band.By controlling the content of trivalent cerium ions in ceria,we control the content of free carriers in the ceria to produce a range of visible light absorption simlar to surface plasmon resonance absoption,which in turn increase the absorption of visible light.With the increase of ethylene glycol content,the content of trivalent cerium ions also increases,and the absorptive capacity of visible light is also changed.Here,we have revealed the relationship between the content of trivalent cerium ions and photocatalytic activity.When the addition of ethylene glycol is up to 30 ml,the photocatalytic activity reaches the optimum value.In addition,a confeito-like cerium oxide nanoparticles were obtained,and its special morphology also had a certain influence on the photocatalytic activity.The transition state found by theoretical simulation(the intermediate state of isopropanol conversion to acetone)reveals the possibility of further improving the photocatalytic activity,indicating the direction for the next experimental study.(5)In order to further improve the photocatalytic activity of visible light response,compounding with other materials is a simple and direct method.Here,we have found a kind of g-C3N4 semiconductor with double doping of sulfur and oxygen through theoretical simulation.In the case of sulfur and oxygen co-doping,the band gap of synthesized g-C3N4 will be greatly reduced,which is beneficial to the absorption of visible light.Graphitic carbon nitride(g-C3N4)was synthesized with a new precursor(thiourea oxide)by a simple one-pot calcination method.Sulphur-oxygen co-doping could modulate the band structure of pristine g-C3N4,resulting in an absorption edge of up to 600 nm,which is well consistent with DFT(Density Functional Theory)calculation.Thus,its photocatalytic properties were enhanced,which is about 5-times higher than that of pristine g-C3N4. |
