Stydies On The Fabrication Of Cerium Dioxide With Different Morphologies And Their Performance Of Composite Photocatalysts Based On The Cerium Dioxide

In recent years,photocatalytic technology has been widely studied in the fields of degradation of water pollutants and carbon dioxide reduction due to its green,clean,high-efficiency and convenient characteristics,and has promising application in environmental protection and energy storage.Cerium dioxide is one of the most abundant rare earth oxides in China.Due to its excellent oxygen storage capacity and having abundant oxygen vacancies,it process excellent catalytic oxidation activity and is also widely used in photocatalytic oxidation to eliminate pollutants.However,cerium dioxide semiconductor with a wide band gap exhibits poor photocatalytic activity and limits its application due to low solar energy utilization and rapid photogenerated electron-hole pair recombination.In this thesis,cerium oxide with exposed high-energy crystal planes was synthesized by controlling morphology,and a higher concentration of Ce3+was obtained.The doping of Ce3+would narrow the band gap of the cerium oxide,thereby enhancing its light absorption ability and improving its photocatalytic activity.By combining CeO2 with narrow-band gap of AgI and g-C3N4,AgI/CeO2 and H-CeO2/g-C3N4 composite photocatalysts with higher visible light photocatalytic activity were prepared,and their structure,performance and photocatalytic mechanism were systematically studied.The main work and results of the thesis are listed as follows:1、By using simple hydrothermal method,we chose trivalent cerium(Ce(NO3)3·6H2O)as the cerium source and sodium hydroxide(NaOH)as the precipitating agent,through the control of alkali concentration and reaction temperature,the cubic-shaped,rod-shaped,hexagonal-shaped and nanoparticle shaped CeO2 was fabricated.The study found that trivalent cerium reacts with sodium hydroxide to form the precursor cerous hydroxide(Ce(OH)3).Ce(OH)3 is formed at low alkali concentration(0.1-0.5M)and low reaction temperature(100-120℃)),first obtain nanoparticle ceria;when the reaction temperature is fixed,when the alkali concentration is increased to 3-10 M,rod-shaped ceria is formed;continue to keep the concentration of strong alkali,and increase the reaction temperature.When the temperature is as high as 150-180℃,cube-shaped ceria will be obtained;while maintaining the reaction temperature unchanged,the alkali source is replaced with 1.5-3M aqueous ammonia,and when PVP is introduced as a surfactant,a hexagonal shape CeO2 will be obtained.Through the experimental investigation,it is found that the grain growth mode changes from isotropy to anisotropy with the increase of alkalinity under different temperature conditions,and CeO2 products with different morphologies was obtained.2、The XRD and HRTEM characterization results showed the synthesized samples with various morphologies have good crystallinity and stability.Among them,cubic ceria mainly exposes {100} crystal planes,and rod-shaped ceria mainly exposes {110} and {100} crystal planes,nanoparticle ceria mainly exposes {111} crystal planes,while hexagonal ceria exposes {100} and {111} crystal planes.According to the results of XPS characterizations,the Ce3+concentration on the surface of cubic,rod,hexagon and nano-particle ceria are 30.2%,25.6%,21.2%,and 17.5%,respectively.Ce3+ions will produce more surface oxygen vacancies,and the presence of oxygen vacancies can not only increase the light absorption of the sample,but also facilitate the transfer of photo-induced carriers,inhibit photoelectron-hole recombination,and improve photocatalytic activity.Under visible light irradiation,the degradation rates of cube,rod,hexagon and nano-particle ceria to RhB are 42.1%,28.2%,17.5%and 8.5%,respectively.The difference in photocatalytic activity depends on that on the one hand,the exposed crystal faces of ceria with different morphologies are different,and the higher the surface energy of the crystal faces,the stronger the photocatalytic activity;on the other hand,the different Ce3+concentrations of the ceria surface lead to the difference of its photocatalytic activity.3、By using the most active cubic CeO2 and AgI as raw materials,ethylene glycol as the dispersant,AgNO3 was mixed into silver ammonia solution,and AgI nanoparticles were deposited on the cubic CeO2 surface to prepare AgI/C-CeO2 heterojunction photocatalyst.Through various characterization and analysis of the as-prepared samples,it is shown that the loading of AgI nanoparticles increases the visible light absorption of cubic CeO2 and reduces the recombination of electron-hole pairs.Under visible light irradiation,the prepared AgI/C-CeO2 composite photocatalyst showed good catalytic activity for RhB,and the degradation rate reached 97%within 150 minutes.The degradation rate is 3.5 times higher that of pure AgI.4、By using methanol as the dispersant,hexagonal H-CeO2 and ultrathin g-C3N4 nanosheets as raw materials,the ultrasonic assisted dispersion method was used to deposit H-CeO2 on the surface of ultrathin g-C3N4 nanosheets to prepare H-CeO2/g-C3N4 heterojunction photocatalyst.The results show that more active sites are exposed on the surface of ultrathin g-C3N4 nanosheets.After loading hexagonal CeO2,a tight heterojunction is formed,which significantly promotes the separation of photoinduced electrons and holes,thereby improve the photocatalytic activity.The degradation rate of H-CeO2/g-C3N4 composite photocatalyst for RhB under visible light is 7.8 times higher than that of pure g-C3N4.