Morphology Control Of CeCO₃OH And Photocatalytic Performance Of Its Thermal Decomposition Product CeO_2-x

Ceria（CeO₂）has a wide range of applications in the field of photocatalysis,and its morphology,defects and element doping have a significant impact on its catalytic performance.This paper intends to obtain CeO₂ photocatalytic materials with excellent photocatalytic performance based on the controllable preparation of CeCO₃OH and its thermal decomposition,and the method and mechanism of the morphology regulation of CeCO₃OH,thermal decomposition behavior,and the photocatalytic performance of the decomposition product CeO_2-x are mainly studied.Moreover,the effect of rare earth element doping CeCO₃OH on the photocatalytic performance of the decomposition products was studied.The cerium carbonate salts with morphologies such as sheet,flocs,almonds,cubes,and small spheres were successfully prepared,which enriched the morphology and structure of cerium carbonate.It was found that cerium compounds gradually transformed from low-energy crystal system to high-energy crystal system through dissolution-recrystallization:orthorhombic Ce（CO₃）₃·6H₂O,orthorhombic CeCO₃OH,hexagonal CeCO₃OH,face-centered cubic CeO₂;Under the action of defects,orthorhombic CeCO₃OH can be reversely transformed to hexagonal CeCO₃OH;crystal form transformation and particle self-assembly are the key factors for morphology regulation and transformation.The thermal decomposition process of CeCO₃OH is clarified.In the air atmosphere,CeCO₃OH will be oxidized to CeO₂;in the argon and hydrogen atmosphere,CeCO₃OH are self-oxidized into CeO_2-x by H₂O and CO₂ generated by the decomposition which confirms the theory of oxygen vacancy formation of CeO_2-x by"incomplete oxidation"of cerium salt at low temperature,that is,CeO_2-x with high oxygen vacancy concentration can be obtained by thermally decomposing trivalent cerium salts at low temperature in the reduction atmosphere.The almond-shaped CeO₂-H₂ obtained by thermal decomposition in H₂ atmosphere has more oxygen vacancies than those obtained by thermal decomposition in Air and Ar.These oxygen vacancies not only reduce the band gap from 3.26 e V to 3.12 e V,but also strengthen the high separation efficiency of photogenerated carriers,thus exhibiting excellent photocatalytic performance.It shows that the"incomplete oxidation"of trivalent cerium salt at low temperature can be used as a better preparation method for high-performance CeO_2-x photocatalytic material.In addition,compared with the almond-shaped and sheet CeO₂-H₂,the small spherical CeO₂-H₂ has a smaller particle size and higher specific surface area,which further promotes the separation efficiency of photogenerated carriers and exhibits the best photocatalysia and the degradation ratio of methylene blue（MB）can reach 87.24%La³⁺and Sm³⁺are uniformly doped into small spherical CeCO₃OH,while Y³⁺transforms the small spherical hexagonal CeCO₃OH into short rod-like orthorhombic CeCO₃OH due to the large lattice distortion.Rare earth ions can be inherited into CeO₂-H₂through"incomplete oxidation”and form an intermediate transition state in the band gap,which reduces the band gap of the undoped sample from 3.23 e V to 3.16 e V.At the same time,many oxygen vacancies are also generated,but these excessively high concentrations of oxygen vacancies easily form bulk oxygen vacancies,which act as photogenerated carrier recombination centers,reducing the separation efficiency of carriers and leading to a decrease in photocatalytic activity.