Preparation, Characterization, And Properties Of Hollow Structure Materials Of Cerium Compounds

Hollow nanostructures with remarkable interior space and shell in nanosize have attracted fascinating interest owing to their higher specific surface area,lower density and better permeation,and widespread potential applications in chemical reactors,drug delivery,catalysis,sensors,and various new application fields.Hollow nanostructured ceria is a promising catalytic material owing to its oxygen stroge capacity and oxygen ions conductivity.This paper focused on controlled synthesis of hollow nanostructured cerium compounds through liquid-chemical routes.In addition,controlled synthesis,formation mechanism,and properties are also investigated.The detailed information of the dissertation is listed as follows.1.Template-free synthesis of single-crystalline CeO₂ hollow nanocubesAll of these current CeO₂ hollow structures were prepared under the assistance of templates and were all polycrystalline character.Single-crystalline CeO₂ hollow nanocubes were synthesized through a solvothermal method using CeCl₃·7H₂O as cerium source and peroxyacetic acid(PAA) as oxidant.It is believed that both oriented attachment and Ostwald ripening should be the main formation mechanisms for the hollow nanocubes through the TEM images at different time.In the first stage,initial nanoparticles are assumed to act as the molecules of Brownian motion under the solvothermal conditions.So it is expected that the growth of nanoparticles via oriented attachment shares some characteristics with the collision reactions of molecules.As a result,the oriented attachment process finishes so fast that a loose structure is formed.In the second stage,the Ostwald ripening is dominant with "solid-solution-solid" mass transportation.Crystallites located in the outermost surface of aggregates are larger and would grow at the expense of smaller ones inside,so the solid evacuation occurred.To explore the key factors for the hollow nanocubes formation,H₂O₂ instead of peroxyacetic acid was used because a certain amount of H₂O₂ is retained in the peroxyacetic acid.When H₂O₂ instead of peroxyacetic acid was used,the resulting product was mainly small crystallites and no hollow cubes observed.In contrast,hollow particles appeared when the CeCl₃ ethanol solution was primarily acidified by concentrated H₂SO₄,HCl,and HNO₃ before adding H₂O₂, respectively.Both the peroxide(peroxyacetic acid or H₂O₂) and the acidic condition played crucial roles in determining the final morphology of the products.The catalytic activity of CeO₂ hollow nanocubes towards CO oxidation was studied,and it was found the as-prepared hollow nanocubes have better catalytic property,excellent stability and recycling performance.2.Synthesis of hollow nanoparticles in the mixed solvents with the assistance of PVPWe have synthesized CeO₂ hollow nanoparticles in the mixed solvents with the assistance of PVP.In the same way,we studied the formation mechanism through the evolution process observation from TEM/SEM images at different time,and found it was different from that of the hollow nanocubes.The nanoparticles almost retain their size, while the surface of nanoparticles becomes rough and the hollowing gradually enlarges with extending reaction time.The initial tiny nanocrystallines on the shell develop into bigger nanoparticles with increasing reaction time from the SEM observations.Therefore, we concluded that the "dissolution and recrystallization" mechanism was responsible for the formation of hollow CeO₂ nanoparticles.The composition of ethanol-water solvent also influences the sample finally morphology.The as-prepared samples incline to assembly when increasing the ethanol volume ratio owing to the change of interaction force between nanoparticles and solvent results from the alternation ofε.It was found that the cerium source has some effect on the final morphology of CeO₂.The resulting samples was mainly octahedral when using Ce(NO₃)₃ or(NH₄)₂Ce(NO₃)₆ as reactants.The possible reason lies in the different reaction routes result in different crystal plan growth rate,and influences final morphology.Because the nitrate species could possess enough potential to oxidize ethanol and OH^-as a product produces.However,there is no OH^-were involved in the reaction when using CeCl₃ as cerium precursor,and the formation of CeO₂ depends on the slow hydrolysis of Ce⁴⁺.3.Interface reaction route to three different types of CeO₂ nanotubesThree types of CeO₂ nanotubes were prepared in alkaline solutions by employing Ce(OH)CO₃ nanorods as precursors just slightly tuning the post processing conditions. Firstly,Ce(OH)CO₃ nanorods dissociate slowly;The coupled reaction/diffusion at the solid-liquid interface lead to the quick formation of an interconnected Ce(OH)₃ shell; Ce³⁺/Ce⁴⁺ oxidation is favored at higher pH value and the conversion from Ce(OH)₃ to Ce(OH)₄(CeO₂·2H₂O) can be realized easily;Finally,the Ce(OH)₄ can be dehydrated to CeO₂ at high temperature.Although the reaction principle is similar,the formation mechanisms of three types of nanotubes are different.For T-type nanotubes,the Ce(OH)₃ shell formed at the early stages can be converted into CeO₂ by aging with concentrated NaOH.Subsequently,the unreacted Ce(OH)CO₃ nanorod cores washed away by diluted HNO₃ result in a large interior space in the T-type nanotubes.For L-nanotubes,the hydrothermal condition favors the anisotropic growth of 1D Ce(OH)₃ structures,and the lamellar rolling of Ce(OH)₃ nanosheets occurs to form tubular structures,which will be transformed into CeO₂ nanotubes by hydroxide assisted hydrothermal treatment.For K-type nanotubes,different diffusivities of ions in the diffusion couple(Ce³⁺ and OH^-) would give the necessary condition for the Kirkendall effect diffusion in the solid-liquid interface reaction.In addition,we compare T-type nanotubes with K-type nanotubes and L-type nanotubes,respectively.Finally,the catalytic activity of different types of nanotubes was studied.4.Template synthesis of CePO₄:Tb nanotubesWe designed a solid-liquid interface reaction between Ce(OH)CO₃:Tb nanorods and H₃PO₄ in ethanol-water mixed solvent,using Tb-doped Ce(OH)CO₃ nanorods as precursors.Combining XRD and EDS measurements,we confirm the Tb³⁺ ion has been successfully doped into Ce(OH)CO₃ precursors and the doping is unaffected by the solvothermal conversion from Ce(OH)CO₃:Tb to CePO₄:Tb.The as-prepared nanotubes show strong green luminescence owing to energy transfer from Ce³⁺ to Tb³⁺.Upon the addition of KMnO₄ solution to the as-prepared colloidal dispersion of CePO₄:Tb³⁺ nanotubes,the emission spectrum disappears,and subsequent reduction of Ce⁴⁺ by adding aqueous ascorbic acid solution to the oxidized solution induces an increase in the luminescence.These phenomena demonstrate the nanotubes possess redox switch function.5.Au nanoparticles-decorated CeO₂ nanotubesIn the synthesis of T-type CeO₂ and CePO₄:Tb nanotubes,we used HNO₃ to wash the unreacted core precursors.Concerning the acidity of HAuCl₄,we have also fabricated the nanotubes by using HAuCl₄ to wash the unreacted Ce(OH)CO₃.Furthermore,the AuCl₄^-ions easily diffuse into the interior space of nanotubes owing to the static electric force, and the Au NPs were decorated successfully in the CeO₂ nanotubes.We also found that there exists different Au loading amount when we design different pretreatment process, which possible results from the formation of Au-Cl-OH complex on the CeO₂ surface. There is no Au NPs when the Ce(OH)CO₃-CePO₄ core-shell nanostructures were washed by HAuCl₄.Based on the experiment results,we concluded that the IEP of oxides and pH influence the Au loading amount.The Au NPs decorated CeO₂ nanotubes show high catalytic performance towards CO oxidation.