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1、Members: Xianhong Rui Yu Chen Litao Yan Huamin Yao Liangjun YiDepartment of Materials Science and EngineeringUniversity of Science and Technology of ChinaJan 3, 200811.Simply introduce the structure and applications of CeO23. Future works2. Synthesis of nanocrystalline CeO2 by different methods2Brie
2、f introductionCeO2属于萤石型氧化物。属于萤石型氧化物。 CeO2晶胞中的晶胞中的Ce4+按面心按面心立方点阵排列，立方点阵排列，O2-占据所占据所有的四面体位置，每个有的四面体位置，每个Ce4+被被8个个O2-包围，而每个包围，而每个O2-则与则与4个个Ce4+配位。配位。1. Structure of CeO22.功能特性功能特性 CeO2的结构中有的结构中有1/2立方体空隙，可称之为敞立方体空隙，可称之为敞型结构。敞型结构允许离子快速扩散。经高温型结构。敞型结构允许离子快速扩散。经高温(T950)还原后，还原后，CeO2转化为具有氧空位、非化转化为具有氧空位、非化学计量比
3、的学计量比的CeO2-X氧化物氧化物(0x0.5)，而在低温下，而在低温下(T450) CeO2可形成一系列组成各异的化合物。可形成一系列组成各异的化合物。 值得注意的是，即使从晶格上失去相当数量值得注意的是，即使从晶格上失去相当数量的氧，形成大量氧空位之后，的氧，形成大量氧空位之后，CeO2仍然能保持萤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时又易被氧化为CeO2，因而，因而CeO2具有优越的储存具有优越的储存和释放氧功能及氧化还原反应能力，同时和释放氧功能及氧化还原反应能力，同时CeO2也也有着良好的化学稳定性和高温快速
4、氧空位扩散能有着良好的化学稳定性和高温快速氧空位扩散能力。力。 3Applications of CeO2 玻璃脱色剂氧化铈大颗粒氧化铈磨料氧化铈抛光粉/液晶显示屏氧化铈抛光粉氧化铈抛光轮CeO2 Slurry 此此外外， CeO2还还用用作作催催化化材材料料、高高温温氧氧敏敏材材料料、 pH传传感感材材料料、电电化化学学池池中中膜膜反反应应器器材材料料、燃燃料料电电池池的的中中间间材材料料、中中温温固固体体氧氧化化物物燃燃料料电电池池(SOFC)用用电电极极材材料料4Synthesis of CeO21. Direct precipitationprecipitationStir and a
5、geing stageScouring and dryingto calcine precursorThe power of CeO2Ce3+ or Ce4+technology of direct precipitationprecipitantNitrate: Ce(NO3)3 or (NH4)2Ce(NO3)6Precipitant: ammonia or NH4HCO3 Surface active agent: PEG-4000Process: nitrate and PEG-4000 were dissolved in distilled wate.Then ammonia or
6、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 dried at 80 over night.5(a)(b)(c)(d)Results and discussion(a)(b)(c)(d)SEM photoes of precursorXRD of precursor(a): Ce(NO3)3 + NH3H2O(b):
7、 (NH4)2Ce(NO3)6 + NH3H2O(c): Ce(NO3)3 + NH4HCO3 (d): (NH4)2Ce(NO3)6 + NH4HCO36(a)(c)XRD of CeO2 synthesized at 700(a)(b)(c)(d)XRD of CeO2 synthesized at 500(a)(c)XRD of CeO2 synthesized at 6007(a)(c)SEM photoes of CeO2 calcined at 6008Microwave homogeneous precipitationMicrowave reaction equipmentNi
8、trate: Ce(NO3)3 or (NH4)2Ce(NO3)6Precipitant: urea Surface active agent: PEG-4000CO(NH2)2 + H2O CO2 + 2NH3NH3 + H2O NH4+ + OH-CO2 + H2O CO32- + 2H+ 水解生成的构晶水解生成的构晶离子离子OH-、CO32-, ,在微波辐照作用在微波辐照作用下下, ,与与Ce3+、Ce4+等结合生成不溶等结合生成不溶前驱物前驱物 9Results and discussionXRD of precursor calcined at 500(a)(b)(c)XRD of
9、precursor (a)Mean:(a)0.093um(b)0.171um(c)0.210umLS of CeO2 calcined at 600 (a) Ce(NO3)3 + urea, without PEG-4000 (b) Ce(NO3)3 + urea + PEG-4000(c) (NH4)2Ce(NO3)6 + urea + PEG-400010600700XRD of CeO2 synthesized at 600、700 SEM photo of CeO2 calcined at 600SEM photo of precursor(a)11Hydrothermal synth
10、esis of CeO2 nano-particles1. Cerium(IV) hydroxide precursorA.I.Y. Tok ,et al (Nanyang Technological University), Journal of Materials Processing Technology 190 (2007) 217222H2O2 + cerium(III) nitrate , stirred for 5 min under heat to convert Ce3+ to Ce4+ammonia (pH =8.8), stir continuously at 80 fo
11、r 1 hthe pale yellow precipitates (Ce(OH)4) were washed ,the conductivity of the supernatant =2ms30 ml of the washed precipitates (pH=10) were placed into the Teflon vessel of the hydrothermal bomb, then placed in the oven and heated at the respective durations (024 h)The final products were re-wash
12、ed, conductivity=2ms, dried at 75 122. Ceria acetate precursorhydrous cerium oxide stabilized by acetate ions (cerium acetate gel) was dissolved in deionized water to yield acetate stabilized colloidal ceria and will be identified as ceria acetateceria acetate was diluted, placing 30 ml of the solut
13、ion into the Teflon vesselthe bomb was then placed in the oven and heated to 250 at different treatment timesthe products were later centrifuged and dried at 75 13Fig. 1. DTA/TG of Ce(OH)4 precursorResults and discussionThe total measured weight loss from 25 to 900 was 11.64%, while the theoreticalw
14、eight loss for the decomposition of cerium hydrate oxide is 17.3%, i.e. Ce(OH)4/CeO22H2O to CeO2The decomposition of the precursor is a form of dehydration process of the hydrated CeO2the difference in weight loss observed could be due to the following reasons: (a) precipitate consisting of a partia
15、lly 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 phasesl i k e C e O2 2 H2O + C e O214Fig. 2 DTA/TG of ceria acetate precursorThe precursor measured a total weight loss of 12.55% with
16、four distinct temperature peaksThe first endothermic peak was detected at around 100. This is attributed to the release of the water molecules present in the precursorFrom 100 to 200, the weight loss was attribute to the removal of the surface acetate groups and later the formation of the acetic aci
17、d when surface acetate hydrolysis occurs. This also explains the very weak endothermic peak detected at 200There was a sharp weight loss from 200 to 400 and a corresponding exothermic peak. This exothermic peak suggests the formation of oxyacetate and dioxocarbonate complexes with cerium, Ce(OH)(CH3
18、COO) andCe2O2CO3As temperature increased to 700, the Ce2O2CO3decomposed endothermally to produce the final product CeO215Fig. 3 DTA/TG for CeO2 synthesized from ceria acetate: (a) after 6 h treatment;(b) after 24 h treatmentafter 6 and 24 h of hydrothermal treatment, weight loss is dramatically r e
19、d u c e d t o 2. 6 4 a n d 1 . 3 7 %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 a s t h a t o f t h e p r e c u r s o rThis could be due to the amount of acetate complexes for
20、mation being reduced considerably after h y d r o t h e r m a l t r e a t m e n t .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 precursor16Fig. 4 CeO2 using Ce(OH)4 precursor (250 )
21、 as a function of timeFig. 5 CeO2 using ceria acetate precursor (250) as a function of timeFig. 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 treatmentNo significant improvement in crystallinity was observed
22、 between 6 and 24 h, and the peaks were broad with weak intensities. This trend is similar with the ceria acetate systemFig. 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 crystallinity a
23、s compared to the cerium(IV) hydroxide system. The peaks at higher 2 angles can also be clearly observed for all samples17Fig. 6. Lattice constant of CeO2 after hydrothermal treatment at 250 using Ce(OH)4 precursorFig. 7 Lattice constant of CeO2 after hydrothermal treatment at 250 using ceria acetat
24、e precursorthe 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 lattice
25、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%18Fig. 8 CeO2 from Ce(OH)4 (24 h) heat treated at (a) 500 , (b) 1000Fig. 9 C
26、eO2 from 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 narrowerThe higher 2 peaks for the hydroxide system can also be observed after heat treatment. This crystallite size after heat treatment at 500 and 1000 gre
27、w to 8.8 and 47.4 nm, respectivelyThe 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 , respectively19Fig. 10 TEM and electron di
28、ffraction pattern 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 crystalli
29、te size is about 56 nm as estimated from the TEM micrographs. The particles generally shown rounded edges but they are not well-defined due to its small sizeFig. 10 (b), particles are very well-defined and relatively dispersed.Good crystalline faces and crystallinity state could be observedThe parti
30、cle sizes, at about 1015 nm, are 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 a p p e a r s t o b e a p r o b l e m .20Salt-assisted ultrasonic aero
31、sol decompositionSalt-assisted aerosol decomposition (SAD)Conventional aerosol decomposition (CAD)the same operating conditions , the same experimental apparatus, without the saltsprecursor solution: cerium nitratewas dissolved in distilled watera mixture of potassium and sodium nitrates was added t
32、o the precursor solutionthe solution was misted by an ultrasonic transducer (1.7 MHz) into dropletscarried by air into a hot tubular reactor where they were rapidly heated and decomposed to form particles, heating time was less than five secondsCeO2 were obtained by washing the product in water to r
33、emove the salts or their derivativesB. Xia, I. W. Lenggoro and K. Okuyama, Hiroshima University, Japan, J. Mater. Chem., 2001, 11, 2925292721Results and discussionFig. 1 Submicron to micron CeO2 particles synthesized by the CAD method at 800 : (a) lower magnification image; (b) higher magnification
34、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 umFig. 1 shows the TEM images of the CeO2 particles, which were synthesized by theCAD method at 800 Consist of nanosized crystallites (Fig. 1
35、b) with mean size of 13.8 nm determined by the X-ray diffraction (XRD) technique.These nanosized crystallites are virtually inseparable due tosintering22Fig. 2 Nanometer nanosized CeO2 particles synthesized by the SAD method at (a) 800 , and (b) a typical high resolution TEM image of sample (a), sho
36、wing the crystal lattice of a particleImportant 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) containing sintered nano-crystallitesSecond, the SAD CeO2 par
37、ticles are single crystals while the CAD C e O2 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 Table 1. The typical crystal latticeimage shown in Fig
38、. 2b confirms the presence of singlecrystalline particlesClearly, the particle sizedistribution of the SAD product has been remarkably narrowed in comparison to the CAD product23Table 1 Comparison of particle and crystallite diameters (in anometers) of CeO2 synthesized by the CAD and the SAD process
39、esFig. 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 54.4 nm, as shown in Table 1. This is mu
40、ch larger than the corresponding CAD sampleDetails 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 m a t e r i a l f o r m a t i o n a n d crystallization processes A crystallite grows by dep
41、leting its adjacent crystallites and is then isolated from others due to theenergy-favorable interaction of the oxide surface with the salts24Fig. 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 decompositionAs is
42、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(s), inert salt
43、s, additives and process parameters can be used to control particle size and morphologyFor 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 900Fig. 2d shows that the addition of ac
44、etic 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. 325Future works1. 1. 改善颗粒间的团聚问题改善颗粒间的团聚问题2. 2. 对纳米对纳米CeOCeO2 2合成过程中其它影合成过程中其它影响因素响因素( (如：反应时间、反应温度、如：反应时间、反应温度、搅拌速率、表面活性剂和热处理条搅拌速率、表面活性剂和热处理条件等件等) )进行优化进行优化 2627
