Hydrothermal synthesis of single-crystal CeCO3OH and their thermal conversion to CeO2

OriginalarticleHydrothermal synthesis of single-crystal CeCO3OH and their thermal conversion to CeO2Kun Gao a,Yi-Yang Zhu a,Da-Qing Tong a,Li Tian a,Zhao-Hui Wang a,b,Xiao-Zu Wang a,*a College of Chemistry and Chemical Engineering,Nanjing University of Technology,Nanjing210009,Chinab State Key Laboratory of Materials-Oriented Chemical Engineering,Nanjing University of Technology,Nanjing210009,China1.IntroductionIn recent years,cerium compounds have been widely used incatalysis[1–4],fuel cells[5]and chemical materials[6–8]due totheir specific4f energy levels of the Ce-element[9,10].Among allthe cerium compounds,cerium carbonate hydroxide,as animportant functional material,has been attracted much attentionbecause of its novel electronic properties,optical properties andchemical characteristics arising from their4f electrons[9–12].Recently,cerium carbonate hydroxide with different morphol-ogies was synthesized by different methods,such as self-assembly,sonochemical[13,14],hydrothermal[15–18],and microwave-assisted hydrothermal route[19].Among all the preparationmethods,the hydrothermal process is considered to be an effectiveand economical route due to its merits of low synthesistemperature,high powder reactivity and versatile shape control[20–23].In a hydrothermal system,CeCO3OH with differentstructures corresponding to distinct morphologies have beensynthesized[18,24,25].There have been sufficient studies report-ing on the synthesis of different morphologies,for example,Guoet al.reported the synthesis of triangular micro-plate,bundle-like,shuttle-like andflower-like structures of CeCO3OH by hydrother-mal method[15,16,18].Li and Zhao synthesized single-crystallineCeCO3OH with dendrite-like structures through a facile hydro-thermal method and obtained CeO2by heating CeCO3OH at5008Cfor6h[26].Zhang et al.synthesized CeCO3OH rhombic micro-plates by the precipitation method in the presence of3-aminopropyltriethoxysilane[28].However,most of these reportson the synthesis of CeCO3OH micro/nanoparticles were preparedusing CO(NH2)2or HMT as the alkaline and carbon resource[13–18]and added surfactant or template to adjust the nucleation andcrystal growth of CeCO3OH particles[13,15–18,28],which makesthe process complex and raw materials more costly.So it isimportant to explore a facile method to synthesize morphology-controlled CeCO3OH micro/nanomaterials.In this paper,we report a simple method to synthesize dendrite-like CeCO3OH crystallites using CeCl3Á7H2O as the cerium source,triethylenetetramine as both an alkaline and carbon source.Thepolycrystalline CeO2was obtained by calcination of the precursor at5008C for4h,partly maintaining the dendrite-like morphology.Theoptical absorption properties of CeO2were also investigated.2.ExperimentalAll chemical reagents were of analytical grade without furtherpurification.In a typical synthesis,0.001mol of CeCl3Á7H2O wasdissolved in60mL deionized water to form a clear solution,andthen0.30mL triethylenetetramine was added to the transparentsolution in order to completely react with Ce3+at258C for about0.5h with continued stirring.The resulting homogenous solutionChinese Chemical Letters25(2014)383–386A R T I C L E I N F OArticle history:Received10August2013Received in revised form8September2013Accepted26September2013Available online1December2013Keywords:CeCO3OHHydrothermalCerium carbonate hydroxideNanostructuresA B S T R A C THexagonal single-crystalline cerium carbonate hydroxide(CeCO3OH)precursors with dendritemorphologies have been synthesized by a facile hydrothermal method at1808C using CeCl3Á7H2O asthe cerium source,triethylenetetramine as both an alkaline and carbon source,with triethylenete-tramine also playing an important role in the formation of the dendrite structure.Polycrystalline ceria(CeO2)have been obtained by calcining the precursor at5008C for4h.The morphology of the precursorwas partly maintained during the heating process.The optical absorption spectra indicate the CeO2nano/microstructures have a direct band gap of2.92eV,which is lower than values of the bulk powderdue to the quantum size effect.The high absorption in the UV region for CeO2nano/microstructureindicated that this material was expected to be used as UV-blocking materials.ß2013Xiao-Zu Wang.Published by Elsevier B.V.on behalf of Chinese Chemical Society.All rightsreserved.*Corresponding author.E-mail address:wangxiaozu@(X.-Z.Wang).Contents lists available at ScienceDirectChinese Chemical Lettersj o u rn a l h om e p a g e:w w w.e l s e v i e r.c o m/l o c a t e/c c l e t1001-8417/$–see front matterß2013Xiao-Zu Wang.Published by Elsevier B.V.on behalf of Chinese Chemical Society.All rights reserved./10.1016/let.2013.11.047was transferred to a 100mL Teflon-line stainless steel autoclave,which was sealed and maintained at 1808C for 24h,and cooled to room temperature naturally.The white precipitate was collected by centrifugation,washed several times with distilled water and ethanol,and dried at 708C for 6h.The as-synthesized CeCO 3OH was calcined to produce straw-yellow CeO 2in air at 5008C for 4h.The XRD measurements were performed on a Bruker-D8Advanced X-ray diffractometer,equipped with graphite-monochromatizedhigh-intensity Cu K a radiation (l =1.5418A˚).The morphologies and sizes of the resulting products were examined by field-emission scanning electron microscopy (FESEM,Hitachi S-4800)and transmission electron microscopy (TEM,JEM2000EX),respec-tively.The thermal behavior of the resulting products was carried out by differential scanning calorimetric analysis (DSC)and thermogravimetric analysis (TG)with a Netzsch-449C simulta-neous TG/DSC apparatus heating from room temperature to 6008C (108C/min)in flowing air.UV/vis absorption spectra were acquired on a spectrophotometer (Shimadzu)and the analyzed range was 200–800nm.3.Results and discussionFig.1presents the typical XRD pattern of the as-synthesized CeCO 3OH products.All of the diffraction peaks in Fig.1can be exactly indexed to the pure hexagonal crystalline phase ofCeCO 3OH with lattice constants a =7.2382A˚,c =9.9596A ˚,which are in good agreements with the literature values (JCPDS 32-0189).No impurity peaks are detected,indicating the high purity of the final product.The strong and sharp diffraction peaks suggest that the products are highly crystallined.Fig.2shows a typical SEM image of the CeCO 3OH dendrite structure synthesized at 1808C for 24h.As shown in Fig.2,it reveals most of the as-prepared CeCO 3OH products display twofold-symmetric structures with a length of 1–2m m along the trunk.The detailed morphology of the structures of CeCO 3OH dendrites is further studied using TEM and SAED (Select-area electron diffraction).A typical high magnification TEM image of the structure of CeCO 3OH dendrites is shown in Fig.3.It reveals that the product is composed of a long central trunk with secondary and tertiary sharp branches,which are parallel to each other and emerge at 608angles with respect to the central trunk.The SAED pattern in the inset of Fig.3taken from an individual dendrite-like nanostructure is indexed to hexagonal CeCO 3OH,indicating that the individual is a single crystal.The diffraction pattern indicates the individual dendrite-like CeCO 3OH is well crystallized.The typical TG pattern of the as-prepared CeCO 3OH dendrite structure is shown in Fig.4a.The TG curve shows that CeCO 3OH begins to decompose at about 2808C and finishes at 6008C.Thetotal weight loss between 2808C and 6008C is measured at about 21.70%,which is close to the results in the theoretical value calculated from following reaction:4CeCO 3OH þO 2!4CeO 2þ2H 2O þ4CO 2(1)The DSC curve (Fig.4b)shows one endothermic peak with a maximum at 300.08C.The temperature range of the endothermic peak in the DSC curve agrees well with the weight loss in the TG curve,corresponding to endothermic behavior during the thermal decomposition/oxidation of CeCO 3OH to CeO 2.Fig.5shows the XRD pattern of CeO 2obtained by calcinations of as-prepared CeCO 3OH.All of the peaks are well assigned to pure face-centered cubic (fcc)structure of CeO 2with lattice constantsa =5.412A˚,which is in good agreement with the JCPDS card (No.43-1002).No obvious peaks for other elements or impurities were observed.The strong and sharp reflection peaks suggest that the as-prepared products are well crystallized.After the CeCO 3OH dendrites are calcined in air at 5008C for 4h,CeO 2dendrites are formed.As shown in Fig.6a,SEM image of CeO 2reveals that the dendrite morphology was partly sustained after thermal decomposition/oxidation to CeO 2.Fig.6b presents a typical TEM image of CeO 2dendrite and its corresponding ED pattern (Fig.6b inset).The discontinuous rings in ED pattern indicate that it could consist of CeO 2polycrystals with an oriented crystallographic axis.In our experiment,since triethylenetetramine was not used,noting products were obtained.Fig.7shows the SEM image of raw material (CeCl 3Á7H 2O),only erose particles were observed,indicating the triethylenetetramine plays an important role in the formation of CeCO 3OH dendrite structures.As is well known,triethylenetetramine at the room tempera-ture will release OH Àin the aqueous solution.Meanwhile,triethylenetetramine has large average capacities for the absorp-tion CO 2[27].So the CO 32Àanions in the solution may result from1020304050602θ (° )(002)(110)(112)(004)(300)(114)(302)(220)(222)(304)Fig.1.XRD patternof the CeCO 3OH dendrite-like nanostructure.Fig.2.SEM image of the as-synthesized CeCO 3OH.Fig.3.TEM image of the as-synthesized CeCO 3OH.K.Gao et al./Chinese Chemical Letters 25(2014)383–386384the possible oxidation of triethylenetetramine,the absorption andslight dissolution of CO 2from air.In the hydrothermal process,the C—N bond in triethylenete-tramine is the easiest bond to break,so upon heating to a certain temperature,triethylenetetramine hydrolyzes to form NH 4+and CO 32À.The cerium ions can exist in the form of [Ce(H 2O)n ]3+in aqueous solution,and then [Ce(H 2O)n ]3+is changed into [Ce(OH)(H 2O)n +]2+,and finally CeCO 3OH is obtained by the reaction between [Ce(OH)(H 2O)n À1]2+and CO 32À.It is proposed that dendrite structures are obtained through a seed-mediated growth in the presence of micelles of triethylenetetramine.The CeCO 3OH nuclei were created and used as seed center,these random moving nuclei in the environment can aggregate with each other to form anisotropic morphology.In our experiment,we consider that triethylenetetramine used as the alkaline and surfactant in the hydrothermal process.Therefore,existing triethylenetetramine,as a capping agent in the reaction system,is absorbed selectively on the different planes of CeCO 3OH seeds,helps to lower the surface energy and results in the different growth rate of different planes to form the dendrite structures.The real formation mechanism of CeCO 3OH dendrite structure needs further investigation.Fig.8shows the UV/vis diffuse absorption spectra of CeCO 3OH and CeO 2.Fig.8a shows the UV/vis absorption spectra for CeCO 3OH.The spectra displayed a strong absorption band below 400nm in the spectra.As seen in Fig.8b and 8c,when the synthesized particles were calcined to produce straw yellow CeO 2by heating at 5008C for 4h,the CeO 2has a stronger absorption band below 480nm in the spectra,which is originated from change-transfer transition between O 2p and Ce 4f bonds [28–30].The optional band gap E g can be determined based on the absorbance spectrum of the powders by the following equation:E g =1240/l AE ,where l AE is the edge wavelength of absorbance.The onset of absorption for CeO 2is at 425.3nm,which corresponds to the band gap energy (E g )of 2.92eV,lower than the values of bulk100200300400500600700800024681012←b. DSC cur veH e a t f l o w (m W /m g )Temerature (oC)← a . TG curve80859095100W e i g h t l o s s(%)Fig.4.TG-DSC pattern of the as-synthesized CeCO 3OH.10203040506070802θ (° )(111)(200)(220)(311)(222)(400)(311)(420)Fig.5.XRD pattern of the CeO 2sample.Fig.6.The typical SEM and TEM images of CeO 2obtained from thermal decomposition/oxidation of CeCO 3OH dendrite structure.Fig.7.SEM image of CeCl 3Á7H 2O powders.800700600500400300200b A b s o r b a n c e (a .u )Wavelength (nm)a800700600500400300200A b s o r b a n c e (a .u )Wavelength (nm)cFig.8.UV/vis absorption spectra of CeCO 3OH (a)and CeO 2(b),(c).K.Gao et al./Chinese Chemical Letters 25(2014)383–386385powders(3.19eV).In general,reduction in crystal size would increase the band gap width because of the quantum size effect [31].Hence,the high absorption in the UV region for CeO2show that the materials can be used as UV-blocking,shielding materials to avoid damage from ultraviolet rays and optical devices.4.ConclusionIn summary,we have successfully synthesized CeCO3OH dendrite structures by a facile hydrothermal method in the presence of triethylenetetramine.After annealing the CeCO3OH precursor powders at5008C for4h,CeO2nano/microstructures with dendrite morphology could be obtained with the morphology partly kept.It is believed that triethylenetetramine plays an important role in the growth of CeCO3OH dendrite structures.The optical absorption spectra indicate that the CeO2nano/microstruc-ture have a direct band gap of2.92eV,which is lower than the values of bulk powders.It is expected that these materials canfind potential application in catalysis and UV-blocking material. AcknowledgmentsThis work was supportedfinancially by the Program for Innovative Research Team in Jiangsu Province(No.SZK[2011]87), Creative and Innovative Talents Introduction Plan(No.SZT[2011]43) and Special Research Foundation of Young teachers of Nanjing University of Technology(No.39701007).References[1]D.Andreeva,I.Ivanov,L.Ilieva,et al.,Nanosized gold catalysts supported on ceriaand ceria–alumina for WGS reaction:influence of the preparation method, Powder Technol.333(2007)153–160.[2]rese,M.L.Granados,F.C.Galisteo,et al.,TWC deactivation by lead:a study ofthe Rh/CeO2system,Appl.Catal.B62(2006)132–143.[3]Y.Dai,B.D.Li,H.D.Quan,et al.,CeCl3Á7H2O as an efficient catalyst for one-potsynthesis of b-amino ketones by three-component Mannich reaction,Chin.Chem.Lett.21(2010)31–34.[4]M.Hajjami,A.G.Choghamarani,M.A.Zolfigol,et al.,An efficient and versatilesynthesis of aromatic nitriles from aldehydes,Chin.Chem.Lett.23(2012) 1323–1326.[5]G.Jacobs,L.Williams,U.Graham,et al.,Low-temperature water–gas shift:in-situDRIFTS reaction study of a Pt/CeO2catalyst for fuel cell reformer applications,J.Phys.Chem.B107(2003)10398–10404.[6]M.S.Tsai,Powder synthesis of nano grade cerium oxide via homogenous precipi-tation and its polishing performance,Mater.Sci.Eng.B110(2004)132–134. [7]D.S.Lim,J.W.Ahn,H.S.Park,et al.,The effect of CeO2abrasive size on dishing andstep height reduction of silicon oxidefilm in STI–CMP,Surf.Coat.Technol.200 (2005)1751–1754.[8]Y.H.Kim,S.K.Kimb,N.Kimb,et al.,Crystalline structure of ceria particlescontrolled by the oxygen partial pressure and STI CMP performances,Ultramicro-scopy108(2008)1292–1296.[9]A.W.Xu,Y.Gao,H.Q.Liu,The preparation,characterization,and their photo-catalytic activities of rare-earth-doped TiO2nanoparticles,J.Catal.207(2002) 151–157.[10]D.C.Koskenmaki,K.A.Gschneidner Jr.,Handbook on the Physics and Chemistry ofRare Earths,vol.1,North-Holland,Amsterdam,1978,pp.338–340.[11]Y.G.Sun,B.Mayers,Y.N.Xia,Template engaged replacement reaction:a one stepapproach to the large scale synthesis of metal nanostructures with hollow interiors,Nano Lett.3(2003)675–679.[12]A.P.Alivisatos,Semiconductor clusters,nanocrystals,and quantum dots,Science271(1996)933–937.[13]K.Li,P.S.Zhao,Synthesis and characterization of CeCO3OH one-dimensionalquadrangular prisms by a simple method,Mater.Lett.63(2009)2013–2015.[14]Z.Y.Guo,F.F.Jian,F.L.Du,Sonochemical synthesis of luminescent CeCO3OH one-dimensional quadrangular prisms,Mater.Res.Bull.207(2011)35–41.[15]Z.Y.Guo, F.L.Du,G.C.Li,et al.,Synthesis and characterization of single-crystal Ce(OH)CO3and CeO2triangular microplates,Inorg.Chem.45(2006) 4167–4169.[16]Z.Y.Guo,F.L.Du,G.C.Li,et al.,Synthesis and Characterization of bundle-likestructures consisting of single crystal Ce(OH)CO3nanorods,Mater.Lett.61(2007) 694–696.[17]M.Y.Cui,J.X.He,N.P.Lu,et al.,Morphology and size control of cerium carbonatehydroxide and ceria micro/nanostructures by hydrothermal technology,Mater.Chem.Phys.121(2010)314–319.[18]Z.Y.Guo,F.L.Du,G.C.Li,et al.,Hydrothermal synthesis of single-crystallineCeCO3OHflower-like nanostructures and their thermal conversion to CeO2, Mater.Chem.Phys.113(2009)53–56.[19]E.L.Qi,L.Y.Man,S.H.Wang,et al.,Microwave homogeneous synthesis andphotocatalytic property of CeO2nanorods,Chin.J.Mater.Res.25(2011)221–224.[20]L.Yan,R.B.Yu,J.Chen,et al.,Template-free hydrothermal synthesis of CeO2nano-octahedrons and nanorods:investigation of the morphology evolution,Cryst.Growth Des.8(2008)1474–1477.[21]X.J.Yang,X.P.Li,X.T.Bai,et al.,Facile synthesis and characterization of uniformCdS colloidal spheres,Chin.Chem.Lett.23(2012)1091–1094.[22]W.T.Yao,S.H.Yu,Recent advances in hydrothermal syntheses of low dimensionalnanoarchitectures,Int.J.Nanotechnol.4(2007)129–162.[23]D.Zhao,J.S.Tan,Q.Q.Ji,et al.,Mn2O3nanomaterials:facile synthesis andelectrochemical properties,Chin.J.Inorg.Chem.26(2010)832–838.[24]Z.H.Han,N.Guo,K.B.Tang,et al.,Hydrothermal crystal growth and characteri-zation of cerium hydroxycarbonates,J.Cryst.Growth219(2000)315–318. [25]C.H.Lu,H.C.Wang,Formation and microstructural variation of cerium carbonatehydroxide prepared by the hydrothermal process,Mater.Sci.Eng.B90(2002) 138–141.[26]K.Li,P.S.Zhao,Synthesis of single-crystalline Ce(CO3)(OH)with novel dendritemorphology and their thermal conversion to CeO2,Mater.Res.Bull.45(2010) 243–246.[27]Z.Wang,M.X.Fang,Y.L.Pan,et al.,Amine-based absorbents selection for CO2membrane vacuum regeneration technology by combined absorption–desorp-tion analysis,Chem.Eng.Sci.93(2013)238–249.[28]Y.W.Zhang,R.Si,C.S.Liao,et al.,Facile alcohothermal synthesis,size-dependentultraviolet absorption,and enhanced CO conversion activity of ceria nanocrystals, J.Phys.Chem.B107(2003)10159–10167.[29]N.Imanaka,T.Masui,H.Hirai,et al.,Amorphous cerium–titanium solid solutionphosphate as a novel family of band gap tunable sunscreen materials,Chem.Mater.15(2003)2289–2291.[30]S.Tsunekawa,T.Fukuda,A.Kasuya,Blue shift in ultraviolet absorption spectra ofmonodisperse CeO2Àx nanoparticles,J.Appl.Phys.87(1999)1318–1321. [31]Y.B.Yin,X.Shao,L.M.Zhao,et al.,Synthesis and characterization of CePO4nanowires via microemulsion method at room temperature,Chin.Chem.Lett.20(2009)857–860.K.Gao et al./Chinese Chemical Letters25(2014)383–386 386。