Controlled Hydrothermal Synthesis Of CeO₂ Micro/nanostructures With Different Dimensions And The Evolution Of Their Properties

For the past few years,there has been growing interest in the controlled synthesis of higher ordered inorganic crystals with specific dimensions and well-defined shapes because of the unique shape-dependent materials properties.CeO₂ nanomaterials with remarkable properties due to its abundant oxygen vacancy defects,high oxygen storage capacity and ability to uptake and release oxygen via the transformation between Ce³⁺ and Ce⁴⁺ has attracted considerable attention in recent years.Therefore,it has been extensively studied and applied in many fields,such as three-way catalysts?TWC?for the elimination of vehicle exhaust gases,ultraviolet?UV?blocking materials,photocatalysts for water oxidation,oxygen sensors and oxygen ion conductors in solid oxide fuel cells.This paper focused on controlled synthesis of CeO₂ nanomaterials with different morphology by facile hydrothermal routes.Besides,morphology controlled synthesis,detailed formation mechanism,and properties are also investigated.The main results achieved in this paper are listed as follows:1.Monocrystalline CeO₂ tablet-like nanostructures and triangular prism-like nanotubes were synthesized by thermal conversion of cerium carbonate hydroxide?CeOHCO₃?precursors prepared by a simple template-free hydrothermal method using Ce?NO₃?₃·6H₂O as cerium source,CO?NH₂?2 as precipitator and polyvinylpyrrolidone?PVP?as surfactant.The morphology of samples changes from tablet-like to triangular prism-like structures,and then to triangular prism-like nanotubes with the increase of temperature from 120 up to 200?.The oriented attachment and Oswald ripening was proposed to explain the form of triangular prism-like nanotubes.The morphology change of the nanostructures has caused their optical properties different.There is a red-shifting in the band gap of the material compared to buIk one,which is mainly attributed to the influences of the Ce³⁺ions,oxygen vacancies and the change of morphology.2.The triangular nanosheets of 50-150 nm in thickness and 0.5-?m in side-length have been successfully synthesized via controlling the morphology of CeCO₃0H precursors by a facile hydrothermal technique using CeCl₃·7H₂O as cerium source,NH4HCO₃ as precipitator and ethanediamine as surfactant.The reaction temperature,reaction time and the amount of ethylenediamine were systematically investigated.The results showed that the triangular CeO₂ nanosheets owned a fluorite cubic structre and mesocrystal structre,the bundle-like structures gradually transform into nanosheets with the increase of the reaction time,and the number of sides of nanosheets decreases with the increase of reaction temperature.The possible formation mechanism related to a defect driven dissolution-recrystallization was proposed.There is a red-shifting in the band gap of the material compared to bulk one,which is mainly attributed to the influences of the CeC³⁺ ions,oxygen vacancies and the change of morphology.The triangular CeO₂ nanosheets and bundle-like structures exhibited similar emission peaks of room temperature photoluminescence and the emission intensity increases with the increase of concentration of oxygen vacancies.3.The monodisperse lotus-like CeO₂ hierarchical structures with an average of 4-6?m have been synthesized by a facile hydrothermal strategy via controlling CeCO₃OH precursors using CeCl₃·7H₂O as cerium source,NH4HCO₃ as precipitator,H₂O₂ as an oxidant and ethanediamine as surfactant.Based on the time-dependent morphology evolution evidences,a nucleation-dissolution-recrystallization mechanism has been proposed.It is found that there are Ce³⁺ ions and oxygen vacancies in surface of samples.The magnetic and photoluminescence measurements indicated that all CeO₂ samples exhibit excellent ferromagnetism and optical properties at room temperature,and while increasing the reaction time,the ferromagnetism and optical properties increase more,which can be reasonably explained for the influence of the different morphology of the particles and the concentration of oxygen vacancies and Ce³⁺ ions.4.Monodisperse flowerlike porous CeO₂ micropheres with the diameters of 2-5?m composed of numerous nanosheets as the petals with variable thickness of 40-200 nm have been successfully synthesized via a facile hydrothermal technique using CeCl₃·7H₂O as cerium source,NH4HCO₃ as precipitator,H₂O₂ as an oxidant and ethanediamine as surfactant.The reaction temperature,reaction time,reaction solution,the amount of H₂O₂ and ethylenediamine were systematically investigated.Based on the time-dependent morphology evolution evidence,a possible mechanism involves assemble and self-assemble of nanoparticle followed by the dissolution and reconstruction of blocky and rod-like structures is proposed.The as-synthesized CeO₂ samples showed enhanced optical properties with the decrease of reaction temperature.Furthermore,flowerlike CeO₂ micropheres obtained at 12?C for 48 h displayed a high surface area of 83.81 m2g-1 and narrow pore size of 3.86 nn by N2 adsorption and desorption measurement.The superior catalytic reduction of NO with NH₃ for the flowerlike CeO₂ micropheres can be attributed to the much larger surface areas,fancy morphology and the best redox behaviour of surface oxygen on the CeO₂ surface.