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1、Members:Xianhong Rui Yu Chen Litao Yan Huamin Yao Liangjun Yi,Synthesis of ceria nano-particles,Department of Materials Science and EngineeringUniversity of Science and Technology of ChinaJan 3,2008,1,Simply introduce the structure and applications of CeO2,3.Future works,2.Synthesis of nanocrystalli
2、ne CeO2 by different methods,Outline,2,Brief introduction,CeO2属于萤石型氧化物。CeO2晶胞中的Ce4+按面心立方点阵排列，O2-占据所有的四面体位置，每个Ce4+被8个O2-包围，而每个O2-则与4个Ce4+配位。,1.Structure of CeO2,2.功能特性,CeO2的结构中有1/2立方体空隙，可称之为敞型结构。敞型结构允许离子快速扩散。经高温(T950)还原后，CeO2转化为具有氧空位、非化学计量比的CeO2-X氧化物(0 x0.5)，而在低温下(T450)CeO2可形成一系列组成各异的化合物。值得注意的是，即使从晶格
3、上失去相当数量的氧，形成大量氧空位之后，CeO2仍然能保持萤石型晶体结构，这种亚稳氧化物暴露于氧化环境时又易被氧化为CeO2，因而CeO2具有优越的储存和释放氧功能及氧化还原反应能力，同时CeO2也有着良好的化学稳定性和高温快速氧空位扩散能力。,3,Applications of CeO2,此外，CeO2还用作催化材料、高温氧敏材料、pH传感材料、电化学池中膜反应器材料、燃料电池的中间材料、中温固体氧化物燃料电池(SOFC)用电极材料,4,Synthesis of CeO2,1.Direct precipitation,Nitrate:Ce(NO3)3 or(NH4)2Ce(NO3)6,Pre
4、cipitant:ammonia or NH4HCO3,Surface active agent:PEG-4000,Process:nitrate and PEG-4000 were dissolved in distilled wate.Then ammonia or NH4HCO3 solution was added dropwise under vigorous stirring till the pH reached 9.The precipitate was filtered,washed thrice with distilled water and alcohol and dr
5、ied at 80 over night.,5,Results and discussion,SEM photoes of precursor,XRD of precursor,(a):Ce(NO3)3+NH3H2O(b):(NH4)2Ce(NO3)6+NH3H2O(c):Ce(NO3)3+NH4HCO3(d):(NH4)2Ce(NO3)6+NH4HCO3,6,7,8,Microwave homogeneous precipitation,Microwave reaction equipment,Nitrate:Ce(NO3)3 or(NH4)2Ce(NO3)6,Precipitant:ure
6、a,Surface active agent:PEG-4000,CO(NH2)2+H2O CO2+2NH3NH3+H2O NH4+OH-CO2+H2O CO32-+2H+,水解生成的构晶离子OH-、CO32-,在微波辐照作用下,与Ce3+、Ce4+等结合生成不溶前驱物,9,Results and discussion,XRD of precursor(a),(a)Ce(NO3)3+urea,without PEG-4000(b)Ce(NO3)3+urea+PEG-4000(c)(NH4)2Ce(NO3)6+urea+PEG-4000,10,SEM photo of CeO2 calcined
7、at 600,SEM photo of precursor(a),11,Hydrothermal synthesis of CeO2 nano-particles,1.Cerium(IV)hydroxide precursor,A.I.Y.Tok,et al(Nanyang Technological University),Journal of Materials Processing Technology 190(2007)217222,12,2.Ceria acetate precursor,13,Fig.1.DTA/TG of Ce(OH)4 precursor,Results and
8、 discussion,The total measured weight loss from 25 to 900 was 11.64%,while the theoreticalweight loss for the decomposition of cerium hydrate oxide is 17.3%,i.e.Ce(OH)4/CeO22H2O to CeO2,The decomposition of the precursor is a form of dehydration process of the hydrated CeO2,the difference in weight
9、loss observed could be due to the following reasons:(a)precipitate consisting of a partially hydrated form of ceria,(i.e.CeO2xH2O),for which a 11.64%weight loss on decomposition corresponds tox=1.35 or(b)the precipitate consisted of a mixture of phaseslike CeO22H2O+CeO2,14,Fig.2 DTA/TG of ceria acet
10、ate precursor,The precursor measured a total weight loss of 12.55%with four distinct temperature peaks,The first endothermic peak was detected at around 100.This is attributed to the release of the water molecules present in the precursor,From 100 to 200,the weight loss was attribute to the removal
11、of the surface acetate groups and later the formation of the acetic acid when surface acetate hydrolysis occurs.This also explains the very weak endothermic peak detected at 200,There was a sharp weight loss from 200 to 400 and a corresponding exothermic peak.This exothermic peak suggests the format
12、ion of oxyacetate and dioxocarbonate complexes with cerium,Ce(OH)(CH3COO)andCe2O2CO3,As temperature increased to 700,the Ce2O2CO3decomposed endothermally to produce the final product CeO2,15,Fig.3 DTA/TG for CeO2 synthesized from ceria acetate:(a)after 6 h treatment;(b)after 24 h treatment,after 6 a
13、nd 24 h of hydrothermal treatment,weight loss is dramatically reduced to 2.64 and 1.37%,The distinct temperature peaks are similar to that of the precursor.However,the distinct exothermic peak for the hydrothermal treatedsamples is no longer as pronounced as that of the precursor,This could be due t
14、o the amount of acetate complexes formation being reduced considerably after hydrothermal treatment.Traces of cerium acetate complexes were still present in the samples after hydrothermal treatment.The amount is however,significantly lower than that found in the precursor,16,Fig.4 CeO2 using Ce(OH)4
15、 precursor(250)as a function of time,Fig.5 CeO2 using ceria acetate precursor(250)as a function of time,Fig.4,the nano-particles exhibited some degree of crystallinity and displayed all of the major peaks of CeO2 with a cubic structure after 6 h treatment,No significant improvement in crystallinity
16、was observed between 6 and 24 h,and the peaks were broad with weak intensities.This trend is similar with the ceria acetate system,Fig.5,the peaks are significantly narrower with higher intensities suggesting larger crystallite sizes at an average of 15.5 nm as calculated and larger degree of crysta
17、llinity as compared to the cerium(IV)hydroxide system.The peaks at higher 2 angles can also be clearly observed for all samples,17,Fig.6.Lattice constant of CeO2 after hydrothermal treatment at 250 using Ce(OH)4 precursor,Fig.7 Lattice constant of CeO2 after hydrothermal treatment at 250 using ceria
18、 acetate precursor,the lattice parameter decreased by about 0.2%after hydrothermal treatment at 250 for 6 h.From 6 to 12 h at the same temperature,the lattice expanded.The lattice constant only varied within a narrow range(|a|/a0.03%)after 12 h,indicating that the structure became stable.,The lattic
19、e constant decreased by about 0.5%after hydrothermal treatment at 250 C for 6 h.Further changes of lattice constant were very small when treatment duration was increased.The variation of lattice constant was less than 0.03%,18,Fig.8 CeO2 from Ce(OH)4(24 h)heat treated at(a)500,(b)1000,Fig.9 CeO2 fro
20、m ceria acetate(24 h)heat treated at(a)500,(b)1000,In both figures,it can be seen that the characteristic peaks are sharper and narrower,The higher 2 peaks for the hydroxide system can also be observed after heat treatment.This crystallite size after heat treatment at 500 and 1000 grew to 8.8 and 47
21、.4 nm,respectively,The samples from the ceria acetate system exhibited a larger degree of crystallinity than cerium hydroxide system.The crystallite size for the ceria acetate system after heat treatment was 17.7 and 53.6 nm at 500 and 1000,respectively,19,Fig.10 TEM and electron diffraction pattern
22、 of CeO2 from cerium(IV)hydroxide(a)and ceria acetate(b)after 24 h hydrothermal treatment.,Fig.10(a)exhibited very fine particles,which were agglomerated.Crystallinity could be observed based on the particles and its corresponding electron diffraction pattern.Its crystallite size is about 56 nm as e
23、stimated from the TEM micrographs.The particles generally shown rounded edges but they are not well-defined due to its small size,Fig.10(b),particles are very well-defined and relatively dispersed.Good crystalline faces and crystallinity state could be observed,The particle sizes,at about 1015 nm,ar
24、e slightly bigger compared to the cerium(IV)hydroxide system.ceria acetate system appears to be less agglomerated than the cerium(IV)hydroxide system.However,agglomeration of the particles still appears to be a problem.,20,Salt-assisted ultrasonic aerosol decomposition,Salt-assisted aerosol decompos
25、ition(SAD),Conventional aerosol decomposition(CAD),the same operating conditions,the same experimental apparatus,without the salts,B.Xia,I.W.Lenggoro and K.Okuyama,Hiroshima University,Japan,J.Mater.Chem.,2001,11,29252927,21,Results and discussion,Fig.1 Submicron to micron CeO2 particles synthesized
26、 by the CAD method at 800:(a)lower magnification image;(b)higher magnification image of the particle marked A,comprising sintered nano-crystallites.,The particles(Fig.1a)are solid and nearly spherical with a mean particle size of 0.74 um,Fig.1 shows the TEM images of the CeO2 particles,which were sy
27、nthesized by theCAD method at 800,Consist of nanosized crystallites(Fig.1b)with mean size of 13.8 nm determined by the X-ray diffraction(XRD)technique.These nanosized crystallites are virtually inseparable due tosintering,22,Fig.2 Nanometer nanosized CeO2 particles synthesized by the SAD method at(a
28、)800,and(b)a typical high resolution TEM image of sample(a),showing the crystal lattice of a particle,Important differences between the CAD and the SAD products are indicated below:,First,the SAD product(Fig.2a)is composed of isolated nanoparticles(mean size 51 nm),while the CAD(mean size 0.74 mm)co
29、ntaining sintered nano-crystallites,Second,the SAD CeO2 particles are single crystals while the CAD CeO2 particles are polycrystalline(as shown in Fig.1b)The single crystals are evidenced by the agreement between the particle sizes and the crystallite ones at all synthesis temperatures,as shown in T
30、able 1.The typical crystal latticeimage shown in Fig.2b confirms the presence of singlecrystalline particlesClearly,the particle sizedistribution of the SAD product has been remarkably narrowed in comparison to the CAD product,23,Table 1 Comparison of particle and crystallite diameters(in anometers)
31、of CeO2 synthesized by the CAD and the SAD processes,Fig.3 Powder XRD patterns of roducts synthesized at(a)CAD,800(CeO2);(b)SAD,800(CeO2),Third,the SAD product has a much higher crystallinity than the CAD product,as shown from the sharp peaks in Fig.3b.The crystallite size of the SAD 800 sample is 5
32、4.4 nm,as shown in Table 1.This is much larger than the corresponding CAD sample,Details of the SAD process:CeO2 can participate in dissolution and precipitation in the liquid-state salt media,which can greatly facilitate mass transfer and thus material formation and crystallization processes A crys
33、tallite grows by depleting its adjacent crystallites and is then isolated from others due to theenergy-favorable interaction of the oxide surface with the salts,24,Fig.2 CeO2 particles synthesized by the SAD method at(c)900,and(d)800 with addition of acetic acid to the solution prior to aerosol deco
34、mposition,As is shown in Table 1,the CAD CeO2 particle sizes change slightly with synthesis temperature,because it is well understood that the CAD particle size is primarily determined by the droplet size and concentration of the precursor solution.,In the SAD process:many factors such as precursor(
35、s),inert salts,additives and process parameters can be used to control particle size and morphology,For example,Fig.2c and Table 1:the mean particle size of the SAD product increases from 51 nm(Fig.2a)to 119 nm when the synthesis temperature is increased from 800 to 900,Fig.2d shows that the addition of acetic acid as an additive to the precursor solution resulted in a reduction in the final particle size from 51 nm to 21 nm.The broadened XRDpeak is clearly seen in the inset of Fig.3,25,Future works,1.改善颗粒间的团聚问题,2.对纳米CeO2合成过程中其它影响因素(如：反应时间、反应温度、搅拌速率、表面活性剂和热处理条件等)进行优化,26,The end,Thank you,27,
