controlled synthesis and assembly of ceria-based...

Journal of Colloid and Interface Science 335 (2009) 151–167

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Journal of Colloid and Interface Science

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Feature Article

Controlled synthesis and assembly of ceria-based nanomaterials

Quan Yuan, Hao-Hong Duan, Le-Le Li, Ling-Dong Sun, Ya-Wen Zhang, Chun-Hua Yan *

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials andBioinorganic Chemistry, Peking University, Beijing 100871, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 January 2009Accepted 1 April 2009Available online 14 April 2009

Keywords:CeriaCeria–zirconia solid solutionsNanomaterialsAssembly

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.04.007

* Corresponding author. Fax: +86 10 6275 4179.E-mail address: [email protected] (C.-H. Yan).

The nanoscience and nanotechniques have brought with new chance for new applications of some tradi-tional materials, for instance, ceria-based materials, which are of great interest due to their wide appli-cations, in particular, as redox or oxygen storage promoters in the three-way catalysts, catalysts for H2production from fuels, and solid state conductors for fuel cells. We highlight here current research activ-ities focused on the controlled synthesis and assembly of ceria-based nanomaterials. We begin with abrief introduction to the urgency for research of ceria-based nanomaterials and our different consider-ation. Typical synthesis is then discussed with examples of nanosized ceria, ceria–zirconia solid solutions,and doped ceria developed by our group and the others. Controlled synthesis to manipulate the shape,crystal plane, and size is the topic of this article, with approaches elaborated for the assembly of ceria-based materials. Finally, we conclude this article with personal understandings and perspectives on thisexciting realm.

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1. Introduction

Cerium is the most abundant element in rare earth family. Cer-ia-based materials are attractive due to their widespread applica-tions in catalysis [1–3],fuel cells [4], optical films [5], polishingmaterials, gas sensors [6], and other fields. Particularly these mate-rials are used as redox or oxygen storage promoters in the three-way catalysts (TWCs) and catalysts for H2 production from fuelsand solid state conductors for fuel cells. Pure stoichiometric CeO2has the calcium fluoride (fluorite) type of structure with spacegroup Fm3m over the temperature range from room temperatureto the melting point, and it can maintain its structure under theintensive reduction at elevated temperatures. The color of CeO2is pale yellow, probably due to Ce(IV)–O(II) charge transfer, whilenonstoichiometric CeO2�d is blue and turns almost black. Ceria inthe fluorite structure exhibits several defects depending on partialpressure of oxygen, which is the intrinsic property for its potentialsin catalysis, energy conversion, and others.

In the past decade, controlled synthesis of ceria-based nanom-aterials with pure phase, desirable composition, uniform mor-phology, and tunable surfaces has become one of the essentialtopics in materials science, since these materials exhibit uniqueproperties when their sizes are reduced to nanometer scale, forexample, the band gap of nano-ceria increases, concomitant theblue-shifting appearing in the adsorption spectra owing to thequantum size effect [7]. Not only are physical properties exploited

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with nanoscale ceria, but are special chemical properties investi-gated with modern techniques [8,9]. In most of the applications,the catalytic performance is strongly dependent on the particlesize, and ceria nanoparticles show high catalytic activity [10].Grain size-dependent electrical conductivity was also observedin electrical application [11]. Recently, besides the conventionalapplication fields mentioned above, ceria-based nanomaterialsare exploited for nanodevice as well, such as for single nano-wire-based gas sensor [12].

The discussion of the structures [13,14] and properties [15–17]of these materials have been reviewed. A comprehensive featurearticle by Kašpar gave a rationale of aspects concerning the variousfactors and the inter-relations to the structure, texture, and particlesize of the CeO2–ZrO2 solid solutions, and the concurrent effects onthe chemical properties [13]. In our recent review of rare earthnanostructures, ceria-based nanostructures are only included asone examples of nanostructure [18]. In the present feature article,our different consideration is to highlight recent development ofceria-based nanomaterials, paying special attentions to the syn-thetic strategies, morphological manipulation, and assemblybehaviors of the nano-building blocks. The synthetical strategies,such as alcohothermal, hydrothermal, and thermolysis, based onsolution-phase methods, are introduced in detail. Then morphol-ogy evolution, especially the formation of nanowires and naotubes,shape and size control, and the shape-enabled properties are dis-cussed. The design of assembled structures through two- or one-step strategies is clarified using typical examples reported by ourgroup and other researchers. Finally, a near future is prospectedbriefly in this article.

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152 Q. Yuan et al. / Journal of Colloid and Interface Science 335 (2009) 151–167

2. Synthesis of ceria-based nanomaterials

Ceria-based materials include a wide range of materials such asceria, ceria–zirconia solid solution, ceria–titania, doped ceria, andothers. Conventionally, many ceria-based materials are synthe-sized at high temperatures. In contrast, solution-based routes havebeen developed as the most important strategies, with advantagesof low temperatures, simple apparatus, easy control, and versatilepost-treatments. Hydro- and solvothermal methods and precipita-tion in high-boiling solvents, selected from a great many ofsolution-based routes, are widely applied in the synthesis ofhigh-quality ceria-based nanomaterials. The experimental condi-tions, such as temperature, acidity, surfactant, concentration, andreaction time, are all flexible to tailor the nanostructures.

2.1. Ceria nanomaterials

As a well-known functional rare earth oxide, nanostructuredceria has been extensively studied. As early as in 1988, Matij-ević et al. reported the preparation of the dispersions contain-ing polycrystalline CeO2 spheres with a uniform size inmicrometers via forced hydrolysis of cerium salts in acidic solu-tions [19]. Chane-Ching obtained aqueous dispersions of nano-metric CeO2 particles by hydrolysis of a Ce4+ salt at a verylow pH in 1987 [20]. The ultrafine CeO2 powders have beensynthesised by using techniques such as flash combustion[21], mechanochemical processing [22], coprecipitation [23],homogeneous precipitation[24], electrochemical synthesis [25],hydrothermal [26], and solvothermal processes [27,28]. Re-cently, it has been advanced intensively to develop environ-mentally friendly and economically efficient pathways for

Fig. 1. Transmission electron microscope (TEM) images of ceria nanocrystals synthespermission from Ref. [29]. Copyright 2003, American Chemical Society) and (b) alkylamCopyright 2006, American Chemical Society), (c) alcohothermal mechanism for syntheCopyright 2006, American Chemical Society).

fabrication of controlled ceria nanostructures via aqueous-phasesynthesis method.

Our group has developed some techniques for synthesis of ceriananomaterials. As a robust technique, solvothermal/hydrothermalmethod possesses extraordinary advantages of single step, low-temperature, controlled composition and morphology, and highpowder reactivity because of low aggregation and high crystalli-nity of as-obtained nanocrystals. During the solvothermal treat-ment, the pressure in the autoclave is able to be readily adjustedto some extent by using different solvents, for different solventshave different vapor pressures at a given temperature. An alco-hothermal method with the addition of bases (KOH or NaOH)[29] is used to fabricate uniform ceria nanocrystals with sizes ran-ging from 2.6 to 6.9 nm, a relatively high degree of crystallinity,and higher surface areas than that in previous reports (Fig. 1a).The alcohothermal mechanism is investigated to follow a hydroly-tic route: (i) the formation of Ce3+ and Ce4+ ions when Ce(III) andCe(IV) nitrates are dissolved; (ii) the formation of polymers of cer-ium hydroxide coordinated with ethanol; (iii) the transition of hy-droxides to hydrated and alcoholized CeO2�d; (iv) CeO2�dnanocrystals obtained through alcohothermal treatment. Waterproduced during the synthesis through the reaction of the basewith the solvent molecule (ethanol) affects the crystallization ofCeO2�d nanocrystals via dissolution and recrystallization. Thismethod is then improved by using alkylamines as base and poly(-vinylpyrrolidone) (PVP) as the stabilizer (Fig. 1b) [30], and the col-loidal CeO2 nanocrystals are achieved under basic media in theethanol solvent which has an appropriate dielectric constant. Theresult is different from the previous work in that aqueous disper-sions of CeO2 particles were generally prepared in a high concen-tration of nitric acid (pH = 0.5–2) with or without the aid of PVP

ized via alcohothermal method using (a) KOH or NaOH as bases (reprinted withines as base and PVP as the stabilizer (reprinted with permission from Ref. [30].

sis of 4 nm colloidal CeO2 nanocrystals (reprinted with permission from Ref. [30].

Q. Yuan et al. / Journal of Colloid and Interface Science 335 (2009) 151–167 153

[19,31]. A tentative mechanism responsible for the formation ofthe CeO2 colloids is depicted in Fig. 1c. Initially, the PVP moleculeis adsorbed onto the solvated Ce3+ ions in ethanol via the coordina-tion between cerium cations and the O atoms in PVP [31b], thenOH� ions are released by the hydrolysis of the dropped alkylaminewith water from Ce(NO3)3�6H2O and ethanol ( {100} > {110}. Generally, the face with a higher densityof surface atoms is blocked by the surfactants adsorbed duringthe crystal growth of colloidal nanocrystals, and the growth alongthis facet is therefore considerably restricted. Since OM is charac-terized by its nonselective adsorption on these faces, however,the CeO2 nuclei grows in a 3D mode to generate nanopolyhedra en-closed by both the (111) and (200) planes. To construct a typicalthermal decomposition synthesis system of monodisperse inor-ganic nanocrystals, metal precursors, surfactant ligands, and sol-vents are three basic elements. Upon heating a reaction mediumto a sufficiently high temperature, the precursors chemically trans-form into active atomic or molecular species (monomers) and sub-sequently, the monomers evolve into nanocrystals with a certainsize and shape. With carefully adjusting the type and amount ofmetal precursor and surfactant ligand, reaction temperature andtime, and feeding mode of the metal precursor, we also obtainedceria monodisperse nanocrystals through the thermolysis of cer-ium octylate precursor (Ce(OC8H17)4) at 250–280 �C in a combinedsolvent of tri-n-octylphosphine oxide and dioctylether [47].

2.2. CeO2–ZrO2 solid solution nanomaterials

Ceria incorporated with zirconia is well-known to distinctly en-hance oxygen storage capacity (OSC) [48] and improve the thermalstability, surface area, and reducibility of the TWCs [2,49]. Despitethe apparent simplicity of this system, both the phase diagram andstructural properties of the CeO2–ZrO2 are still a matter of investi-gation. The phase transition and the structure of Ce1�xZrxO2 arecomplicated, for the reason that even X-ray diffraction and Ramanspectra are impotent to detect microdomain heterogeneity of thesesolid solutions. Pulsed neutron diffraction technique, a more pow-erful tool for studying the oxygen positions in oxides than X-raydiffraction because of the better scattering contrast of oxygen com-pared to metal ions, is then adapted in studying atomic structures

of CeO2–ZrO2 [50]. The atomic pair-distribution function structuralanalysis, carried out by neutron diffraction, revealed a seeminglyhomogeneous ceria–ziconia solid solution had unusual nanoscaleintracrystallite compositional heterogeneities associated with theformation of zirconia-enriched domains in the ceria-enriched ma-trix [51]. Using methanol as a surface probe, Fourier transforminfrared spectroscopy is also applied to investigate the surfacestate of CeO2–ZrO2 with the variance of O concentration [52].Methoxy species, from methanol dissociative adsorption, compen-sate the coordinative unsaturation of surface cations. Methanoladsorption over pure ceria and zirconia surfaces gives methoxyspecies characterized by well-defined spectra, being quantitativeprobes for the evaluation of amounts of surface Ce and Zr atoms[53].

The Ce1�xZrxO2 phase diagram may be summarized as follows:when x < 0.15 Ce1�xZrxO2 exhibits cubic, fluorite-type phase; whilex > 0.85, monoclinic phase. At intermediate compositions, variousphases (t, t0, t00, j and t*) have been identified [54–56], amongwhich a metastable cubic phase, named as j [57], possesses an or-dered arrangement of cations; it is prepared from a precursor witha pyrochlore-type structure, Ce2Zr2O7+d, which is obtained in areductive atmosphere and possesses respective arrays of Ce andZr ions along the h110i direction in its unit cell [58]. Fornasieroet al. [59] have systematically investigated the structure ofCe1�xZrxO2 after redox treatments which lead to low temperaturereduction. Bernal and coworkers [60] recently reported the nano-analytical features of ceria–zriconia during phase transition pro-cess, finding that OSC of ceria–zirconia mixed oxides criticallydepend on their nanostructure and, more specifically, on the or-der–disorder structure induced by their cationic sublattice. It is aconsensus that solid solution of a single-phase is preferable forthe TWC application compared with the microdomain or phase-segregated CeO2–ZrO2 mixed oxides, for the former generally leadsto better textural stability and redox properties [61]. Nevertheless,the homogeneity of the composition and structure of Ce1�xZrxO2nanomaterials and their thermal stability are great challenges tothe chemists. Meeyoo et al. synthesized nanostructured Ce1�xZrxO2(x = 0–1) mixed oxides via urea hydrolysis-based coprecipitationusing Ce(NO3)3 as a starting material, and the as-derived CeO2–ZrO2 mixed oxides were not homogeneous but segregated inmicrodomains [62].

We developed the urea hydrolysis-based hydrothermal methodto synthesize nanostructured Ce1�xZrxO2 (x = 0–0.8) solid solutionswith better structural homogeneity and higher thermal stability[63]. A strong correlation between OSC and lattice strain in theas-obtained Ce1�xZrxO2 (x = 0–0.8) solid solutions has been ob-served and explained based on the qualitative analyses of theircrystal structures and lattice defects. It is expected that this corre-lation can be applied to the assessment and design of better-per-forming CeO2–ZrO2 catalysts. However, CexZr1�xO2 (x = 0.4–0.6)solid solutions are not stable upon long-term (P100 h) sinteringat about 1000 �C when phase separation is induced, as alsoreported by many other groups [14,49,61]. As a result, a lick oftrivalent rare earths (RE) ions is doped into CexZr1�xO2 nanocrys-tals, forming ternary CeO2–ZrO2–RE2O3 solid solutions(CexZr1�x�yREyO2�z, x = 0.4–0.6, y = 0–0.16, and z is the number ofoxygen vacancy) to intensify their stability [64]. The superior goodcompositional homogeneity of CeO2–ZrO2–RE2O3 solid solutions inboth bulk and microdomain is confirmed by measurements of X-ray fluorescence and energy-filtered transmission electron micros-copy. High structural homogeneity is demonstrated after calcina-tion even at 1000 �C for 100 h. It can be concluded that thealiovalent substitution of the RE3+ ion for the Ce4+ or Zr4+ ion isnot only able to stabilize the pseudo cubic t00-structure, but alsobeneficial to improve the surface area and enhance the OSC forthe binary CeO2–ZrO2 system.


Because of uncontrollable nuclei formation and aggregationrate, it is difficult to control the particle size accurately in the prep-aration of CeO2–ZrO2 solid solutions, thus it brings difficulties infully addressing fundamental issues in catalytic reactions such assize effects. The monotony of morphology is another main problemthat should be addressed. The most common morphology of CeO2–ZrO2 composite is irregular nanoparticles, therefore, subtle fabrica-tion of CeO2–ZrO2 composites with tunable sizes, shapes, and com-positions remains as a big challenge. Wang and co-authorsreported Ce1�xZrxO2 and CeO2@Ce1�xZrxO2 nanocages with differ-ent outer and interior morphologies using the colloid ceria clustersas both chemical precursors and physical templates by Kirkendalleffect [65]. If spherical ceria clusters are used as precursors, spher-ical nanocages are obtained; whereas cubic nanocages are formedwhen nearly cubic ceria clusters are used as precursors (Fig. 2).This approach has shown great flexibilities in controlling the sizes,shapes, and compositions of the solid solution. The penetrability ofthe shell of these nanocages dissipates the mist of the controllabledeposition of noble metals on the inside or outside of these nano-structures, which may be served as novel nanoreactors for catalysisapplications.

2.3. Other ceria-based nanocrystals

Metal-doped CeO2 has also received much attention due to itsconsiderable number of applications in catalysis and as a solidelectrolyte for solid oxide fuel cells (SOFCs) [66]. Versatile metals,such as Ca [67,68], Sr [69,70], Bi [71], Y(III) [72], Gd(III) [73,74], andSm(III) [75,76], have been used as dopants. Nanosized powders ofyttria-stabilized ceria have been synthesized by using CeCl3 andYCl3 as precursors with thermophoresis-assisted CVD [77]. In addi-tion to a large surface to volume ratio, these materials contain alarge density of oxygen-ion vacancies, therefore, they have beenstudied as catalyst for the reduction of SO2 by CO, the oxidationof CO by O2, and the development of moderate-temperature SOFCs.

Fig. 2. Representative TEM images of (a) nearly monodispersed spherical ceria nanocnanostructures obtained by shortening the reaction time, (c) spherical Ce–Zr–O nanocagon Kirkendall effect (reprinted with permission from Ref. [65]. Copyright 2008, America

Leite et al. prepared gadolinium-doped ceria nanorods with a nom-inal composition of Gd0.2Ce0.8O2�x by hydrothermal treatment of acolloidal suspension of Gd-doped CeO2 nanocrystals in an aqueoussolution (Fig. 3a) [78]. A interesting work shows that the undopedCeO2 nanocrystals display a very weak emission band peaking at501 nm, which is remarkably enhanced by doping additional lan-thanide ions (Eu3+, Tb3+, Sm3+) in the CeO2 nanocrystals (Fig. 3b–d) [79]. The greatly increased emission peak of CeO2:Eu3+ (Tb3+,Sm3+) nanocrystals (located at 500 nm) is not attributed to thecharacteristic emission of the lanthanide ions (Eu3+, Tb3+, Sm3+)but arised from the presence of oxygen vacancy in the lattices ofCeO2 nanocrystals.

CuO–CeO2 mixed-metal oxides are another important materialswidely applied as electrolytes in fuel cells [80], gas sensors [81],and efficient catalysts for various reactions such as oxidation ofCO and methane [82–84], the water–gas shift (WGS) reaction[85], methanol synthesis [86], and the wet oxidation of phenol[87]. The previous research proved that the Cu–Ce–O compositecatalyst exhibited high catalytic activity for CO oxidation, whichwas ascribed to the combined effect of CuO and CeO2 [88,89].Nanoparticles of Ce1�xCuxO2 solid solutions prepared following anovel procedure [90,91] show unusual physical and chemicalbehaviors, as is reported by Fernández-Garcı́a et al. [92]. In princi-ple, a Cu cation embedded in a fluorite lattice can have up to eightoxygen neighbors rather than four in CuO and two in Cu2O, and Cudoes not adopt oxidation states of ‘‘+4” and ‘‘+3” like Ce does; theseenables occasional substantial structural perturbations (stress,point and line defects, and O vacancies) [93] within the ceria lat-tice. The lattice of the Ce1�xCuxO2 systems therefore maintains afluorite-type structure, but highly distorted with the presence oflocal order defects and multiple cation-oxygen distances, introduc-ing a large strain into the oxide lattice to favor the formation of Ovacancies.

Due to the apparent difference in the ionic radius between Ceand Ti ions, phases of CeO2 and TiO2 can coexist in the mixed

rystal clusters obtained at low water concentration, (b) of the hollow core-shelles, (d) the illustration about the formation process of the Ce–Zr–O nanocages basedn Chemical Society).

Fig. 3. High resolution TEM (HRTEM) image of (a) Gd-doped CeO2 nanorod (reprinted with permission from Ref. [78]. Copyright 2008, American Chemical Society) and (b)CeO2: Eu3+ nanocrystals. The photographs of (c) CeO2 nanocrystals and (d) CeO2: Eu3+ nanocrystals (in hexane solution), (e) excitation and emission spectra of CeO2 (red dashline) and CeO2:Eu3+ (black solid line) nanocrystals in hexane solution (reprinted with permission from Ref. [79]. Copyright 2007, American Chemical Society). (Forinterpretation of color mentioned in this figure the reader is referred to the web version of the article.)


oxides. Ti4+ ions, in principle, can be substituted for Ce4+ ions toform Ce1�xTixO2, in which both Ce4+ and Ti4+ ions can be reducedto +3 states, so that reducibility of the solid solution or OSC is high-er than pure CeO2. Hegde’s group reported that Ce1�xTixO2 synthe-sized by the solution combustion method had higher OSC thanpure CeO2 [94]. Yang [95] prepared CeO2/TiO2 nanocompositesvia sol–gel strategy, finding that these nanoparticles had higherphotocatalytic activity than pure TiO2 and the increase in the Ce/Ti molar ratio had a major effect on photocatalytic degradation.Core/shell-type titania nanoparticles containing CeO2 nanoparti-cles were prepared using the colloid seeded deposition reactionof Ti(SO4)2 as titania precursor in a CeO2 nanoparticle suspension[96]. The materials in this work show potential multi-functionalityto be used as a photoactive material as well as a photocatalyst.

The c-Al2O3–CeO2 system can act as excellent support systemfor acid catalysis [97], in which Al2O3 serves as a support material.In nanoscale form, the amount of lattice defects such as oxygen va-cancy and grain boundary increases as the particle size decreases.Moreover, high surface area increases contact area between thecatalyst and reactant (liquid or gas), which promotes the catalyticefficiency of these inorganics. Bosew and Wu have found that mor-phology of nano-structured Al2O3–CeO2 composite can be con-trolled by using surfactant template system [98]. Much to theresearchers’ expectation, a strong interaction between ceria andalumina under oxidizing conditions will result in formation of CeA-lO3 at the interface causing the stabilization of the textural proper-ties of CeO2–Al2O3 oxides [99–101]. Take CeO2–Al2O3 mixed oxidesprepared by wetness impregnation method as an example [102],XPS spectra of the sample uncovered the presence of a strong inter-action between ceria and alumina, implying the formation of

superficial CeAlO3-like phase. In this case, Ce3+ cations probably oc-cupy vacant octahedral coordinated sites on the surface of alumina,blocking the transition of Al3+ cations from tetrahedral to octahe-dral sites during high temperature treatment, and causing the lossof surface area of alumina.

In the synthesis of doped ceria nanomaterials, we have achievedsome attractive results. Using the thermal decomposition method,we have recently obtained fine nanocrystals of RE (Gd, Sm) dopedceria with varied dopant amounts. Ce1�xSnxO2 solid solutions nan-acrysals with different Ce/Sn ratios are also synthesized with thesame method only by substituting stannum precursor for rareearth precursors and these ultrasmall doped ceria nanomaterials(with average particle sizes of 4–5 nm) have large surface/volumeratios and active sites, promising potentials to catalysts and gassensors.

3. Controlled synthesis

3.1. Shape and size control

The last decade has witnessed an unprecedented revolution insynthesis of metal/metal oxide nanocrystals, especially in the con-struction of specific shapes and the exact manipulation thereofexamples include: sphere, cube, octahedron, tetrahedron, rightbipyramid, decahedron, icosahedrons, thin plate with a triangular,hexagonal, or circular profile, and rod or wire with a circular,square, rectangular, pentagonal, or octagonal cross-section [103].Via the control of a few critical synthesis parameters, ceria-basednanomaterials can be obtained with predictable morphologies,namely well-defined crystal planes, and the use of these materials


as model catalysts in laboratory investigations as well as practicalapplications becomes possible.

3.1.1. Nanowire and nanorodOne-dimensional (1D) nanostructures (rods, wires, belts, and

tubes) are nowadays the most popular realm in the synthesis ofCeO2-based nanomaterials. Generally, the growth habit of crystalsplays the most important role in determining their final shape[104]. For 1D nanostructures, the growth direction is often domi-nated by the crystallographic symmetry, for example, it naturallyfollows the unique axis when the material possesses a trigonal,hexagonal or tetragonal symmetry [18,105]. The materials withanisotropic structures, CdSe [106], for example, grow anisotropi-cally, forming 1D nanocrystals in the nontemplated solution-phasesyntheses. For CeO2, however, it is difficult to grow anisotropicallyunder the same condition owing to its isotropic structure whiletruncated octahedral shaped CeO2 nanoparticles are always ob-tained [26a,26c,33,34]. In this respect, template or capping re-agents is crucial to differentiate the surface energy of each facet,thus it is a shortcut to introduce anisotropic growth and directthe formation of 1D structure [107]. Therefore, various templatingmethods have been developed to prepare CeO2 nanorods and nano-wires in solutions [108].

Very recently, ceria nanowires are obtained through severalsynthesis routes, including a sol–gel process within the nanochan-nels of porous anodic alumina templates, nonisothermal precipita-tion, spontaneous self-assembly of cerium oxide nanoparticles tonanorods, and a solution-based hydrothermal method by usingan autoclave [109]. Hyeon et al. [110] synthesized ceria nanowireswith an ultrasmall diameter of 1.2 nm and a length of 71 nm fromthe nonhydrolytic sol–gel reaction of cerium(III) nitrate and diphe-nyl ether in the presence of appropriate surfactants. During thethermolysis of the precursor complexes, the selective adsorptionof capping ligand onto particular crystallographic facets is para-mount in the formation of ultrathin nanowires, whose lengthsare tunable via varying the amount of OA. This strategy is powerfulin tailoring the length/diameter ratio of nanowire, but in the mean-while it introduces an unavoidable drawback that the surface ofthe nanowire is not ‘‘clean”, with large amounts of organic speciescovering it. Our group described an easy solution-based hydrother-mal synthesis of ceria nanorods without addition of any templates.The growth mechanism of single crystal ceria nanorods is attrib-uted to a nucleation and growth process controlled mainly by dis-solution/recrystallization at a high pH (pH > 10) under 100 �C [35].In this condition, the hexagonal Ce(OH)3 is maintained in the dura-tion of the entire hydrothermal treatment, enabling the anisotropicgrowth of the Ce(OH)3. After drying, white Ce(OH)3 nanocrystalsare converted into yellow CeO2�H2O ones by air oxidization, with-out obvious changes of their shape. Kuiry et al. reported the syn-thesis of ceria polycrystalline nanorods and proposed a growthmechanism based on the preferential assembly of ceria nanocrys-tallites in a longitudinal direction [111], which is employed to ex-plain the formation of gadolinium-doped ceria nanorods [78] aswell.

3.1.2. NanotubeMuch effort has been devoted to the controlled synthesis of

nanotube materials since the discovery of carbon nanotubes in1991 [112]. Up to now, several strategies have been demonstratedto process a broad range of inorganic materials into nanotubes,such as chemical vapor deposition, template-directed synthesis,and hydrothermal treatment [113]. One synthesis strategy amongthem based on ‘‘rolling-up” mechanism, proposed by Tang et al, iseffective to fabricate nanotube structures of various compounds,such as BN, V2O5, WS2, and NiCl2 [114]. Two-dimensional (2D) lay-ered precursors are considered to ‘‘roll up” when increasing the

temperatures [115]. Ce(OH)3 nanotubes, therefore, can be pro-duced through a simple hydrothermal alkali treatment of anhy-drate cerium chloride under oxygen-free conditions and thenslowly annealed in a reducing atmosphere to convert Ce(OH)3 toCeO2. The abundant defects in the lattice of these nanotubes implythat the nanotube-like structure of CeO2 is a metastable morphol-ogy. In spite of the ‘‘roll up” mechanism, a dissolution mechanismis feasible to guide the fabrication of CeO2 nanotubes as well. Hanet al. reported a production of ceria nanotubes by a two-stage pro-cedure: precipitation at 100 �C and aging at 0 �C for 45 days [116],but this method is time-consuming and side productions such asnanorods and nanowires are also observed in the products. Zhouand his co-workers developed a simple method via hydrothermalroute under mild reaction conditions [117]. The formation mecha-nism is explored in detail and a oxidation–coordination–assisteddissolution is involved in whole process (Fig. 4), where the partiallyoxidized Ce(OH)3 are used as the starting materials. Since most ofthe outer wall has been oxidized into ceria, the oxidation–coordi-nation–assisted dissolution process is limited inside the 1Dnanomaterials; thus, the ceria nanotubes with extended cavitiesare formed finally.

3.1.3. Other shapesOther shapes of ceria nanostructures such as cube [35,118],

sphere [119], tadpole-shaped wire [110], triangular plate [120],and hollow structure [121], have been successfully preparedthrough hydrothermal, OA-assist hydrothermal, and hard templatemethod (Fig. 5). Among them, ceria nanoparticles of tadpole-shaped, consisting of a spherical head with a diameter of 3.5 nmand a wire-shaped tail with a diameter of 1.2 nm and a length of27 nm (Fig. 5b), are obtained by adding relatively excessive OA inthermal decomposition of cerium(III) nitrate hexahydrate. Ceriahollow nanospheres have been synthesized by using carbonaceouspolysaccharide microspheres prepared from saccharide solution astemplates (Fig. 5d).

3.1.4. Shape and size evolutionIn all cases the structures of the productions such as shapes

and sizes are not only dependent on synthetic methods, but alsohighly condition-sensitive to particular solvent or surfactant, pHgradients, environmental conditions of temperature and pres-sure, contaminants, time, etc. Take thermal decomposition meth-od as a case [110], with the addition of OA, the final shape ofceria transfers from sphere (OM as capping agent) through wire(OM and OA both as capping agents) to tadpole-shaped wire.Similar shape evolution was also observed in hydrothermal pro-cedure wherein precursor concentration, additive concentration,reaction time, and other reaction parameters are all involved[121,122]. According to our experience in hydrothermal synthe-sis, the shape-selective mechanism, which is responsible forthe synthesis of CeO2 nanopolyhedra, nanorods, and nanocubes[35], can be schematically described in Fig. 6a. The whole hydro-thermal process has been ruled by the well-known dissolution/recrystallization mechanism [26a,26c]. Anisotropic Ce(OH)3 nu-clei is formed once the Ce3+ ions is mixed with NaOH solution.When both the OH� concentrations and the temperature arelow (CNaOH < 1 mol L�1, 100 �C), chemical potential for the aniso-tropic growth of Ce(OH)3 nuclei is inadequate, and therefore, theas-obtained ceria nanocrystals are of polyhedron, enclosed with{111} and {100} facets (Fig. 6b). When the OH� concentrationis high enough (CNaOH > 1 mol L�1), the dissolution/recrystalliza-tion rate is vigorous, and the anisotropic growth of Ce(OH)3 nu-clei is persistent; thus, pure CeO2 nanorods enclosed with {110}and {100} facets are obtained under high base concentrations(CNaOH > 6 mol L�1) as shown in Fig. 6c. As the hydrothermaltemperature elevated to 180 �C, Ce(OH)3 became unstable and

Fig. 4. (a) TEM image of the CeO2 nanotubes synthesized via oxidation complex dissolution reaction, (b) HRTEM images of the CeO2 nanotubes, (c) illustration of oxidation–coordination–assisted dissolution process all the major morphological changes involved in the synthesis of ceria nanotubes (reprinted with permission from Ref. [117].Copyright 2007, American Chemical Society).


is readily oxidized into CeO2. As a result, CeO2 nanocubes with{100} facets were produced under such a high base concentra-tion (Fig. 6e).

The size of nanoparicle can also be controlled by many factors insynthesis such as, especially, surfactant species, surfactant/precur-sor ratio, and others. Gao and coworkers synthesized monodis-perse nanocubes with the average size of ca. 4 nm under a lowconcentration of Ce(NO3)3 (Fig. 8a) [118]. When the concentrationof Ce(III) is high, the distribution of nanocubes is apparently bimo-dal. However, the amount of large-sized nanocubes in the wholereaction system increases with Ce(III) concentration. The small-sized nanocubes always have an average size of ca. 4 nm, whilethe size of large-sized nanocubes can be controlled by tuning theratio of OA/Ce(III), i.e., the size increases with the rise of the OA/Ce(III) molar ratio (Fig. 7b,c). In addition, the large-sized nanocubescould not be further smoothed at the cost of the small particles.The above evidences imply that Ostwald ripening is absent in thecurrent system because of the low solubility of the OA-capped cer-ia in the water phase, instead, oriented attachment dominates thegrowth and aggregation of the nuclei (Fig. 7d). Recently we haveobtained hydrophilic ceria nanocubes through hydrothermal routewith the assistance of acetate. Particle sizes are also influenced bydifferent cerium precursors (Ce(III) and Ce(IV)). Ceria nanocubes of5 nm are synthesized when (NH4)2Ce(NO3)6 is used as precursorwhereas those of 20 nm with Ce(NO3)3�6 H2O.

Ceria nanopolyhedra with truncated octahedral or octahedralshape, are apt to be obtained because of the exposure of the most

stable facets of {10 0} and {11 1}. Feng et al. discovered that dopedceria with a small content of Ti exhibited sphere shapes and singlecrystal structure [119]. The synthesis strategy is based on liquid-phase flame spray pyrolysis of a solution made from cerium andtitanium precursors dissolved in a flammable solvent alcohol.Molecular dynamic (MD) simulation is carried out and demon-strated, in agreement with experiment quantitatively, that Ti(dopant) ions change the shape of CeO2 nanocrystals from polyhe-dron to sphere (Fig. 7a). This result rationally explains the ob-served morphological variation in the atomistic level. Inparticular, CeO2 nanocrystals can be synthesized via crystalliza-tion from melt in MD simulation. Initially presented as a molten,undoped-CeO2 nanoparticle is cooled, and then nucleating seedsspontaneously evolve at the surface and expose energetically sta-ble {1 11} facets to minimize the energy, thus facilitating the for-mation of the polyhedral shape. In contrast, when doped with Ti, aTiO2 shell encapsulates the inner CeO2 core obstructing the evolu-tion of nucleating seeds at the surface thus coercing them to beamorphous shells during cooling. Therefore, crystallization isforced to proceed via the evolution of a nucleating seed in thebulk CeO2 region of the nanoparticle, and as this seed grows, it re-mains surrounded by amorphous ions, which ‘‘wrap” around thecore so that the energies for high-index facets are drastically re-duced. The simulation results show that, for the Ti-doped system,the CeO2 occupies the core region and is encapsulated by a TiO2shell. The {111} and {10 0} facets are enveloped by the amor-phous TiO2 shell, appearing a sphere shape. Herein, we highlight

Fig. 5. TEM images of ceria nanomaterials with different shapes: (a) CeO2 nanorods (reprinted with permission from Ref. [109c]. Copyright 2005, Academic Press Inc., ElsevierScience), (b) 1.2 � 71 nm CeO2 nanowiers (reprinted with permission from Ref. [110]. Copyright 2005, Wiley-VCH), (c) CeO2 nanocubes, (d) CeO2 hollow nanospheres(reprinted with permission from Ref. [121a] . Copyright 2006, Wiley-VCH), (e) CeO2 triangular microplates (reprinted with permission from Ref. [120]. Copyright 2006,American Chemical Society).


that the computer modeling is an effective strategy to explain theshape conversion of ceria during synthesis process, and recom-mend it as a powerful tool to predict the nanostructure producedbefore the experiment carried out. Combined with ‘‘simulationsynthesis”, synthesists can follow the steps ‘‘design-simulation-controlled synthesis” and acquire nanoparticles of various exoticshapes and tailored properties. Sayle and his colleagues per-formed simulation with full atomistic models of pure and tita-

nium-doped ceria nanorods and framework architectures [123].The formation of amorphous nanoparticles firstly, then self-aggregation into nanorods and nanoporous architectures, andlastly the crystallization from the amorphous oxide precursorsare simulated step by step. The simulation results predicted thatTi doping would ‘‘smooth” the surfaces, sleeking the CeO2 hexag-onal prisms exploding {1 11} and {10 0} surfaces to cylindrical,and framework architectures change from facetted pores and

Fig. 6. (a) Mechanism for the shape-selective synthesis of CeO2, RT stands for room temperature, HRTEM images of (b) CeO2 nanopolyhedra, (c) CeO2 nanorods, inset is a fastFourier transform (FFT) analysis, (d) CeO2 nanocubes, inset is a fast Fourier transform (FFT) analysis (reprinted with permission from Ref. [35]. Copyright 2005, AmericanChemical Society).


channels with well-defined {111} and {10 0} surfaces to‘‘smooth” pores and channels as shown in Fig. 8b–e.

3.2. Properties enabled by shape-controlled synthesis

The synthesis of ceria-based nanomaterials with well-con-trolled shapes and sizes has been motivated by the potentials inindustrial applications. The crystal planes show the distinct cata-lytic activity, for example, the Pt(111) surface is found to be threeto seven times more active than the Pt(100) surface for aromatiza-tion reactions [124]. Pt(111) is also a more active surface in meth-anol oxidation of fuel cell anode [125]. Another case is thatammonia synthesis occurs at the Fe(111) [126]. The reactivityand selectivity of a catalyst are able to be tailored by controllingits shape and size.

Experimental and theoretical studies both show that manyproperties of CeO2 nanocrystals are surface structure dependent.The crystal plane of ceria plays an essential role in determiningits catalytic oxidation properties. Previous computer modeling re-sult shows that the low-index (111) surface is of the lowest sur-face energy and thus is the most stable surface, followed by(110) surface, and then the high energy surfaces, (100), (210)and (310) [127,128]. Accordingly, CeO2 crystals generally expose(111) surface, afterward, (110) and (100) surfaces. As revealedby Sayle et al. [129], the (100) terminated surface is inherently

more reactive and catalytically important as compared with(111) and (110) surfaces. They also predicate that (110) and(310) surfaces are more reactive in the oxidation of CO than(111) due to the formation of oxygen vacancies on them [127].Conesa indicated that less energy is required to form oxygenvacancies on (110) and (100) than on (111) [128]. Controlledgrowth of CeO2 nanostructures with defined exposed surface, thus,is of significant importance.

Li and coworkers reported that single-crystalline CeO2 nanorodsare more active for CO oxidation than irregular nanoparticles[109c]. They ascribed the higher CO oxidation activity to the morereactive planes {001} and {110} of nanorods and this result indi-cates that better-performing catalysts with high reactive sitescan be ‘‘designed” rather than ‘‘prepared”. The shape-dependentOSC of ceria nanocrystals with various shapes (nanopolyhedra,{111} and {100}); nanorods, {110} and {100}, and nanocubes,{100}) are explored in our work [35]. In this work we not only con-firm that (100) and (110)-dominated surface structures are morereactive for CO conversion than (110)-dominated, but also revealthat the former surface structures facilitate the migration of latticeoxygen from bulk to surface, that is, oxygen storage takes placeboth at the surface and in the bulk of nanorods and nanocubesbut is restricted on the surface of nanopolyhedra. The observationof the shape-dependent OSC behavior in this work strongly suggestthat high OSC CeO2 materials for TWCs application can be designed

Fig. 7. TEM images of ceria nanocubes with the average sizes of (a) 4.43, (b) 7.76, and (c) 15.65 nm; the insets are SAED patterns, (d) schematic illustration of orientedaggregation mediated precursor growth mechanism of nanocubes (reprinted with permission from Ref. [118]. Copyright 2006, American Chemical Society).

Fig. 8. (a) Images taken during a MD simulation for crystallization of Ti-doped CeO2 nanoparticles, showing the initial amorphous precursor (left), evolution, and growth ofthe seed (circled) to the fully crystalline nanoparticle with amorphous shell (right) (reprinted with permission from Ref. [119]. Copyright 2006 the American Association forthe Advancement of Science), atomistic sphere models of (b) the undoped CeO2 nanorod and (c) the Ti-doped CeO2 nanorod, atomistic sphere models of nanoporousframework architectures: (d) CeO2 and (e) Ti–CeO2. Ce is colored white, Ti, blue, and O, red (reprinted with permission from Ref. [123]. Copyright 2007, American ChemicalSociety). (For interpretation of color mentioned in this figure the reader is referred to the web version of the article.)



by tuning their shapes with specific surface crystallographic facets,i.e., how to enhance the fraction of more reactive {100} and {110}planes, together with decreasing the fraction of less-reactive {111}planes in the nanocrystalline catalysts. This work further providesthe possibility in designing high OSC ceria-based nanomaterialsthrough shape-selective synthesis strategy. Following the abovework, Flytzani-Stephanopoulos and co-workers prepared gold–cer-ia catalysts by depositing gold on the different facets of ceria nano-rods, nanocubes, and nanopolyhedra in a two-step process,hydrothermal synthesis of ceria followed by deposition/precipita-tion of gold [130]. A strong shape/crystal plane effect of CeO2 onthe gold–ceria activity for the WGS reaction (CO + H2O ? CO2 + H2)has been identified, and the ceria nonorods, enclosed by {110} and{100} planes, are confirmed to be the most active for gold stabil-ization/activation (Fig. 9).

4. Assembly of ceria-based nanomaterials

Self-organization of entities in 2D and 3D is one of the basicprocesses in nature. For instance, spherical particles having thesame diameter self-organize into compact 2D or 3D ordered struc-tures (face-centered cubic (fcc) or hexagonal close packed (hcp)).The first self-assembly of particles possessing a diameter of a fewnanometers (

Fig. 11. TEM images of CeO2 nanoflowers with (a) cubic and (b) star morphologies. (c) Schematic illustration of the growth stages of CeO2 nanoflowers (reprinted withpermission from Ref. [136]. Copyright 2008, Wiley-VCH).

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Fig. 10. (a) Illustration for the self-assembly process of surface-modified ceria nanoparticles. (b) The small-angle X-ray scattering pattern and (c) TEM image of the assembledCeO2 nanostructured material (reprinted with permission from Ref. [134]. Copyright 2005, Wiley-VCH).



the particles trapped within the nanometer-wide voids in the icecombine with an oriented attachment process, forming ultralongpolycrystalline ceria nanorods with length of ca. 3–3.5 mm anddiameter of ca. 30 nm (Fig. 12a). Constant-volume MD simulationreveals attractive forces acting between the neighbor amorphous/molten nanoparticles (Fig. 12b). The amorphous nanoparticles inthe voids, attracted by their neighbors, move closer to one anotherand began to crystallize. Once crystalline, the nanoparticles con-tinue to be attractive towards one another. MD simulations proveto be a powerful tool for expansion and prediction of not only syn-thesis but assembly as well.

Fig. 12. (a) Image generated by selected masking and inverse FFT of a HRTEM of CeOassembly. (b) Structure of a chain/nanorod comprising CeO2 nanocrystals. Top: after 30green nanoparticles as they attach themselves to one another; bottom: after 1500 psCopyright 2008, Wiley-VCH). (For interpretation of color mentioned in this figure the re

4.2. One-step assembly

One-step assembly means that no previous synthesis of nano-crystals has to be done and the self-assembly process occurs atthe same time when nanocrystals are formed. This assemblybehavior as well as the final assembled structure is strongly af-fected by synthesis condition including precursor sort and concen-tration, solvent composition, surfactant or template concentration,reaction temperature, reaction time, and others.

Our group firstly obtained mesoporous Ce0.2Zr0.8O2 solid solu-tions via non-template hydrothermal route [140]. The mesoporous

2 nanorods showing the dislocations (circles) formed during orientation and self-0 ps of MD simulation; the arrows indicate the trajectory followed by the blue and

showing the attached configuration (reprinted with permission from Ref. [139].ader is referred to the web version of the article.)


structure is fabricated during the nanocrystal growth of nanocrys-tals and exhibits short-range order but long-range disorder, re-sulted from the natural agglomeration of uniform nanoparticlesupon hydrothermal treatment. The as-prepared powders have highsurface areas (232–281 m2 g�1) and narrow pore size distributions(3.5–4.0 nm), and the highest OSC as well as the lowest 50% con-version temperature of CO oxidation are obtained for the sampleswith the highest surface area and the largest pore volume. We fur-ther develop a facile method via sol–gel and evaporation inducedself-assembly process to synthesize highly ordered 2D hexagonalmesoporous Ce1�xZrxO2 solid solutions with crystalline walls andtunable Ce/Zr ratios (Fig. 13) [141]. These novel assembly struc-tured ceria–ziconia solid solutions are good candidates for applica-tion in catalysis due to the high surface area and uniform channel.Very recently, we make a breakthrough in self-assembly towardCeO2 spherical aggregates directed by ionic liquids (ILs)(Fig. 14a,b) [142], which plays dual roles of both template andcosolvent agents in fabricating the ceria spherical aggregates. Wealso synthesized hierarchically macro-/mesoporous structured cer-

Fig. 13. TEM images of the mesoporous Ce0.5Zr0.5O2 recorded along the (a) [001] and (bthe one in (b) is the corresponding SAED pattern. (c) Schematic illustration for the assembmesostructures (reprinted with permission from Ref. [141]. Copyright 2007, American C

ium–tin mixed oxide with nanocrystalline walls, through a simpleone-step sol–gel process involving block copolymer as a single-template [143].

The application of templates is necessary for assembly of CeO2nanoparticle in both two- and one-step processes. Up to now, agreat many of chemicals, especially those having hydrophobicfunctional groups, have been frequently used as templates forassembly of ceria nanostructures. The driven force for orderedassembled structure is mainly ascribed to the van der Waals forcebetween hydrophobic species of template molecules, which can re-main or be removed after assembly process. Accordingly, morecomplicated assembled structures can be designed and thenachieved by choosing proper templates as well as tailoring thechemical properties of template molecules.

5. Remarks and prospective

Ceria-based materials, which have been known for two hun-dred years, are witnessing significant progress in controlled syn-

) [110] orientations. The inset in (a) is the corresponding FFT diffraction image, andly of metal precursors with triblock copolymer P123 as SDAs to form highly orderedhemical Society).

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Fig. 14. (a) TEM image of monodisperse spherical CeO2 aggregates synthesized with ILs. (b) A schematic diagram illustrating the microstructure of aggregated CeO2composed of closely packed nanocrystals (reprinted with permission from Ref. [142]. Copyright 2008, American Chemical Society).

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thesis of nanoscale during the last decade. Different shapes andsizes of ceria have been achieved with various synthesis methodsdeveloped. However, chemical synthesis of ceria-based nanoma-terials now still remains in empirical lack of instructions fromphysics and chemistry, for the atomistic details for the evolutionfrom a precursor to final well-defined nanostructures are far be-yond perception. As a result, high-quality ceria-based nanomate-rials in controlled manners as well as detailed understanding ofthe synthetic mechanisms are still challenges to face in the com-ing years. The development of in situ monitoring techniques isable to open new windows to reveal the mechanisms in the crys-tal nucleation and growth processes. Electrical measurementsdeveloped by our group have shed lights on this way and moretechniques such as optical, spectroscopy, and others are of greaturgency.

Meanwhile, simulation method is an efficient way for us tounderstand the nanocrystal formation from atomic level. The MDsimulation of crystal growth behavior, surface effect inducedphase, and morphology control would be essential topics forrevealing the structural characteristics and formation mechanisms.We have made valuable attempts to simulate surfactant-modifiedshape evolution of ceria nanomaterials. Only an intergration ofexperimental evidence and theoretical modeling will unfurl a vasepanorama of crystal growth, providing more rational strategies toobtain various materials with the aim of application.

In most cases of catalytic reactions, CeO2 acts as an oxygen buf-fer by storing/releasing O2 due to the Ce4+/Ce3+ redox couple,which is a result of intrinsic defect chemistry of ceria (Ce3+ and Ovacancy concentrations). In previous simulation study, the role ofCeO2 in the oxidation of carbon monoxide were investigated, andthe reaction can be described, following Ref. [128] as:

COðgÞ þ CeO2 ! COþ V00o þ 2Ce0Ce

where O2 is extracted from an exposed surface of the CeO2, facil-itated by a reduction from 2Ce4+ to 2Ce3+. The simulations revealthat oxygen vacancy formation, together with Ce4+ reduction toCe3+, is energetically more favorable on the surface of the CeO2compared with the bulk [137,144]. The control of the densityand the nature of oxygen vacancies would be able to tailor thereactivity of ceria-based catalysts. With assistance of high-resolu-tion scanning tunneling microscopy, a detailed atomic-level in-sight into the local defect structure of (111) surface was carriedout [145]. Electrons on the cerium ions, which are left behind byoxygen release, determine that clusters of more than two vacan-

cies exclusively expose the reduced cerium ions, therefore, elec-trons play a crucial role in the process of vacancy clusterformation. In some works, Ce4+/Ce3+ ratio is found to rely on dif-ferent particle size or the content of other components in thenanocrystals [8,12].

A pre-designed synthesis with precisely adjusting of Ce4+/Ce3+

ratio, in other words, the design of defect structure, is requiredto meet the special applications in catalysis, sensors and others.Reversible transformation in ceria Ce4+–Ce3+ will provide a promis-ing access to new biocatalyst, biomedicines and biotherapies. Somepioneer researches have revealed that ceria nanoparticles are use-ful for related blinding diseases therapy, such as spinal cord repairas well as other diseases of the central nervous system [146,147],and even for protection of normal human cell from irradiation[148]. The potentials of vacancy-engineered ceria in therapy dis-eases and anti-aging are fascinating enough to push us deepen re-search in this area. The cytochemistry, biological functions, andcompatibility of nanoceria, and the concomitant mechanisms,however, are indispensable to be further illustrated. For example,with respect to the bio-safety of nanoceria, toxicity of cerium onEscherichia coli was previously investigated in its Ce(III) form[149] and a mechanism of ceria uptake by fibroblasts was then pro-posed by Stark’s group [150]. More recently, Thill et al. [151] foundthat a direct spatial contact between cells and particles is neces-sary so that a cytoxicity of CeO2 nanoparticles can be provokedon E. coli. Not only the ‘‘negative” but the ‘‘positive” effects of ceriananoparticles have been discovered as well. Andre and coworkersdid a mechanistic study to elucidate the physicohemical character-istics of TiO2, ZnO, and CeO2, which determine their toxic effects onbiological processes, and found that antioxidant properties of CeO2protect cells from oxidant injury while other two metal oxides in-duce cytotoxicity [152]. Nevertheless, more investigations are re-quired in this field.

Further demands in this field include green, environment-friendly, economic, high-throughput, and reliability to obtainnanocrystals with precisely controlled shape and size, desired sur-face states. In most cases, especially when surfactant or template isused in synthesis, the surface of ceria-based nanoparticles is cov-ered with organic species, which is a disadvantage for furtherapplication in catalysis. Another embarrassment for the synthesistis that due to different nucleation growth, and hydrolysis behaviorof cerium and zirconium species, the well-controlled synthesis ofceria–zirconia solid solutions nanocrystalls with homogeneouscompositions, tunable shapes, and uniform sizes are hardly ob-


tained. The synthesis of other ceria-based compounds, such asCeO2–TiO2, CeO2–SnO2, in particular their shape control, are alsodeserved to be improved.

In the regard of assembly, new strategies need to be exploitedto generate new assembly structures, and the relationship betweenthe assembly structure and the collecitve properties is critical todevelop new materials and devices from nanoscaled buildingblocks. First, it is expected to expose a mono-facet within uniformceria nanoparticles through the assembly, which is crucial to thefacet selective catalytic activity. Second, multicomponent assem-bly is an efficient way to multi-functionalize the desiged materials,that is, more components with special functions are introduced asbuilding blocks to design ceria nanocrystals and other nanocrys-tals, for example, different components can be linked together ina binary building blocks for applications in magnetic separationof catalyst, or new nanodevice. Third, dimension and morphologycontrol of the aggregations of ceria-based materials is also impor-tant to create collective properties of related materials. On theother aspect, the development of nanodevice calls for the applica-tion of ceria-based assembled nanomaterials for example in thefield of nanosensor.

Acknowledgments

This work is supported by the NSFC (Grant Nos. 20671005 and20821091) and MOST of China (Grant No. 2006CB601104).

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Controlled synthesis and assembly of ceria-based nanomaterialsIntroductionSynthesis of ceria-based nanomaterialsCeria nanomaterialsCeO2–ZrO2 solid solution nanomaterialsOther ceria-based nanocrystals

Controlled synthesisShape and size controlNanowire and nanorodNanotubeOther shapesShape and size evolution

Properties enabled by shape-controlled synthesis

Assembly of ceria-based nanomaterialsTwo-step assemblyOne-step assembly

Remarks and prospectiveAcknowledgmentsReferences

controlled synthesis and assembly of ceria-based...

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