【作者】 陈国柱；

【导师】 孙思修；

【作者基本信息】 山东大学 ， 无机化学， 2009， 博士

【摘要】 纳米空心结构是在纳米尺度范围内具有较大内部空间和一定厚度壳层的一类特殊结构,具有低密度、高比表面、优良渗透性等特点,在化学反应器、催化剂、传感器、药物载体等方面有着重要应用,是纳米化学领域中引人注目的问题之一。由于CeO₂具有很好的储氧性及氧离子传导性,所以铈基化合物纳米空心结构是很有应用前景的纳米催化材料。本论文利用溶液化学法对这类纳米空心结构进行了控制合成,并分别从材料制备、形成机理以及性质表征等几个方面进行了系统的研究。1.无模板法制备单晶CeO₂空心立方体目前制备的CeO₂纳米空心结构（纳米管除外）都是借助于硬模板,形貌为球形并且为多晶结构。本文中我们以CeCl₃·7H₂O为铈源、过氧乙酸为氧化剂,通过醇热反应制备了单晶空心纳米立方体。根据不同时间段的TEM图像观察,我们认为,定向聚集（Oriented Attachment）和奥氏熟化（Ostwald Ripening）是空心立方体形成的机理。即:起始形成的小晶粒类似溶液中分子的布朗运动,这些小粒子通过碰撞,并按照相同的晶面接合为大的粒子以降低粒子之间的界面能。由于起始聚集速度迅速,导致立方体内部具有疏松结构。在反应的后期,进入“固-液-固”质量传输的奥氏熟化控制阶段,小的粒子不断溶解,大的粒子不断长大。因此,位于内部中心的小粒子由于曲率大容易溶解继而在颗粒表面再结晶,因此形成空心立方体结构。由于过氧乙酸中含有一定量的H₂O₂,因此我们用H₂O₂代替过氧乙酸研究空心结构形成的影响因素。对比实验表明,单独利用H₂O₂时得到的是不成形的小晶粒;而如果铈的醇溶液在加入H₂O₂之前首先用浓酸（HCl、HNO₃、H₂SO₄）酸化,最后得到的产品具有空心结构。其中,过氧化物氧化Ce³⁺为Ce⁴⁺,Ce⁴⁺和乙醇、-OOH生成络合物,而溶液的酸性能防止铈的醇溶液中加入H₂O₂时迅速地沉淀。尽管详细的机理目前还不清楚,但可以肯定过氧化物和酸是形成空心立方体CeO₂的关键因素。对所得空心立方体进行CO催化氧化实验,发现空心CeO₂立方体具有很好的催化性能以及良好的热稳定性。2.PVP辅助、醇水混合溶剂下制备空心CeO₂纳米颗粒借助PVP,利用CeCl₃在醇水混合溶剂中制备空心CeO₂纳米颗粒。我们同样借助不同时间段的TEM/SEM图像进行分析,发现所制备的空心颗粒形成过程明显与空心立方体的不同。制备的空心颗粒随着反应时间的延长,其粒径基本无变化,但是颗粒的表面由起始的光滑变得粗糙。SEM图像分析,起始纳米颗粒表面由很多小的晶粒组成,但随着反应的进行,表面小晶粒明显变大。因此,我们推断“溶解—再结晶”是空心颗粒的形成机制。对比实验表明,醇水混合溶剂的组成对产品的最后形貌也有影响。当醇的体积比增大时,由于溶液介电常数的变化,导致沉淀颗粒与溶剂之间作用力的变化,纳米颗粒倾向聚集。改变不同的铈盐前驱物,如利用Ce（NO₃）₃或者（NH₄）₂Ce（NO₃）₆作为反应物,最后得到具有八面体结构的CeO₂。其可能原因是NO₃^-氧化乙醇,产生OH^-;而CeCl₃为铈源时,没有碱源（如OH^-）的情况下,仅依靠Ce⁴⁺缓慢的水解。不同的反应途径可能影响各晶面的生长速率,从而导致最后纳米颗粒的形貌不同。3.同一界面反应下三种不同形成机制的CeO₂纳米管的制备利用Ce（OH）CO₃与NaOH水溶液之间的固液界面反应,仅仅改变后处理条件,便得到三种不同形成机制的纳米管。其反应原理:Ce（OH）CO₃前驱体在溶液中首先缓慢发生解离;解离的Ce³⁺与NaOH水溶液反应在前驱体表面生成Ce（OH）₃;Ce（OH）₃在碱性条件下容易氧化生成Ce（OH）₄;Ce（OH）₄干燥脱水得到CeO₂。尽管反应原理一样,但是采取不同的后处理,得到的纳米管形貌和反应尺寸都不尽相同。对于T-type纳米管来说,起先形成的Ce（OH）₃室温下在NaOH溶液中老化转化为CeO₂,然后未反应的Ce（OH）CO₃利用HNO₃溶解;对于L-type纳米管来说,水热条件有利于一维Ce（OH）₃的生长,不同于室温下无序的小颗粒的生长。随之,一维的Ce（OH）₃在水热条件下,卷曲成管,继而氧化转化为CeO₂纳米管;对于K-type纳米管来说,首先生成的Ce（OH）₃覆盖在Ce（OH）CO₃前驱体表面阻碍了OH^-、Ce³⁺的继续反应。进一步的反应就依靠Ce³⁺、OH^-的扩散来进行。由于两种离子半径不同,导致Ce³⁺从Ce（OH）CO₃向外迁移速率大于OH向内迁移速率,于是晶格空位聚集形成空心结构。为了进一步区别三种不同形成机制的纳米管,我们分别区别了T-type、L-type纳米管,T-type、K-type纳米管。对比实验表明,溶解/结晶速率驱动着Ce（OH）₃的各向异性生长,并且发现在T-type纳米管的形成过程中,Kirkendall扩散同样存在,但K-type纳米管和T-type纳米管具有相反的的变化趋势:即在K-type纳米管形成过程中,其内部空心随着反应时间的延长而增大,而T-type纳米管相反。最后测试了三种纳米管的催化性质。4.模板法制备CePO₄:Tb纳米管以掺杂Tb的Ce（OH）CO₃为前驱体代替Ce（OH）CO₃,在醇水混合溶剂中与H₃PO₄缓慢的发生固液界面反应,未反应的Ce（OH）CO₃:Tb前驱体利用HNO₃洗涤得到结晶良好的CePO₄:Tb纳米管。XRD、EDS表明Tb不仅成功地掺杂于Ce（OH）CO₃,而且经过溶剂热反应后依然掺杂于CePO₄产物中。由于Ce³⁺、Tb³⁺之间存在能量传递,所制备的CePO₄:Tb纳米管表现出很强的绿色荧光。实验发现,当CePO₄:Tb纳米管被KMnO₄氧化后,由于Ce⁴⁺不再向Tb³⁺传递能量,因此荧光发射消失;但加入还原剂（抗坏血酸）时使Ce⁴⁺还原至Ce³⁺,绿色荧光发射光谱再次出现,说明CePO₄:Tb纳米管具有很好的荧光开关效应。5.金纳米颗粒修饰CeO₂纳米管在T-type CeO₂和CePO₄:Tb的纳米管制备过程中,我们都是利用稀HNO₃洗涤未反应的前驱体。考虑到HAuCl₄较强的酸性,我们改用HAuCl₄洗涤Ce（OH）CO₃-CeO₂核壳结构,同样也可以得到纳米管。更重要的是由于电荷的静电吸引作用,AuCl₄^-很容易地进入纳米管,成功地将金纳米颗粒负载于纳米管内腔。为了验证影响因素,我们设计不同的前处理条件,即:分别用HNO₃洗涤、高温煅烧Ce（OH）CO₃-CeO₂前驱体然后再负载,结果发现Au负载量发生明显变化。我们初步认为这与CeO₂表面Au-Cl-OH形成有关。如利用Ce（OH）CO₃-CePO₄核壳结构为研究对象,经过HAuCl₄洗涤、处理,在纳米管内没有发现金颗粒。根据实验结果,初步推断氧化物的等电电位（IEP）以及溶液的pH影响金的负载量。同时发现负载金的CeO₂纳米管在催化CO性能上有了明显的提高。更多还原

【Abstract】 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.更多还原