Wet chemical synthesis of redispersible crystalline BaTiO3 in organic medium

Journal of Alloys and Compounds449(2008)77–81Wet chemical synthesis of redispersible crystallineBaTiO3in organic mediumNimai Chand Pramanik,Sang Il Seok∗,Bok Yeop AhnAdvanced Materials Division,Korea Research Institute of Chemical Technology,100Jang-Dong,Yuseong-Gu,Daejeon305-600,South KoreaReceived4November2005;received in revised form28December2005;accepted10January2006Available online16January2007AbstractRedispersible crystalline BaTiO3nanoparticles were prepared by wet chemical method from alkoxide-hydroxide precursor in the presence of ethylene diamine.Crystal structure,crystallite size and morphology of BaTiO3nanoparticles were characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The crystallite sizes of the BaTiO3nanoparticles were found to be in the range24–43nm,while the average particle sizes,measured by dynamic light scattering method and SEM were in the range55–120nm. The dispersibility of BaTiO3nanoparticles in organic media was remarkably enhanced by preparing them under the presence of ethylene diamine. The dispersion stability of the nanoparticles was higher in the organic media such as methanol and N,N-dimethyl foramide with relatively high dielectric constants than organic media such as isopropyl alcohol,methyl isobutyl ketone and cyclohexanone with low dielectric constants.©2007Published by Elsevier B.V.Keywords:Ferroelectric;Chemical synthesis1.IntroductionBaTiO3is a well known multi-component ceramic mate-rial that exhibited several outstanding chemical and physical properties,such as catalytic,oxygen-transport,ferroelectric, pyroelectric,piezeoelectric and dielectric behavior[1–3].Very high value of dielectric constant of BaTiO3[4–7]makes it an especially desirable material from which capacitors,condensers and other electronic components can be fabricated[3,8–10].There are many reports available on the preparation of well crystalline BaTiO3by different methods,such as homogeneous co-precipitation[11–13],hydrothermal synthesis[5,14,15],the sol–gel method[16–19],etc.Recently,with the development of polymer science,the preparation of ceramic-polymer compos-ite(BaTiO3-polymer in particular)has received much current interest because of the fact that polymer matrix exhibited enough flexibility in designing,while the inorganic component enhances the dielectric property of the composite.Many attempts were made to prepare BaTiO3-polymer nanocomposites[10,20–22] for the fabrication of microelectronic components,such as ∗Corresponding author.Tel.:+82428607314;fax:+82428614245.E-mail address:seoksi@krict.re.kr(S.I.Seok).embedded capacitors in printed circuit boards(PCB),multi-layer ceramic capacitors(MLCC),insulating layers in organic thinfilm transistors,etc.Simple incorporation of crystalline particles,prepared by conventional methods into the organic matrices was also attempted[23–25],but the results were not sat-isfactory for the applications point of view[26,27].This might be due to the interaction between the organic moiety and the nanoparticles having very high surface and interfacial energies, whichfinally,lead to the phase separation.Therefore,the redispersion of the crystalline particles into the organic media is considered to be most important.Several approaches have been made to overcome this problem,such as surface modification of the nanoparticles by using oleic acid, acetic acid,etc.[28–30],synthesis of mono-dispersed crys-talline particles using Ba-Ti-single alkoxide precursor[31]and so on.Thus,considering the current trend of microelectronic industries,and increasing interest of BaTiO3based nanocom-posites[10,20–25],the dispersion of crystalline nanoparticles into organic media requires further attention to improve the dis-persibility of crystalline particles into organic media,because of the fact that most of the organic polymers are soluble in organic solvents.In this research,redispersible crystalline BaTiO3is prepared by wet chemical method and its dispersion phenomena in0925-8388/$–see front matter©2007Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.01.12478N.C.Pramanik et al./Journal of Alloys and Compounds449(2008)77–81different organic solvents are studied.It is noteworthy to mention here that the enthylene diamine(EDA)is a well known bidentate ligand,which can coordinate with the transition metals forming a tris-complex.In addition,EDA liberated after the decomposition of Ti–EDA complex can also act as the in situ capping agent to prevent further agglomeration among the nanoparticles.Furthermore,the concept of using Ti–EDA on the preparation of BaTiO3and the role of EDA on the dispersion into organic media has not been explored so far.Therefore,the present work focused on the preparation of the redispersible crystalline BaTiO3nanoparticles and the effect of EDA on its dispersion phenomenon in different organic media.2.Experimental2.1.MaterialsTitanium isopropoxide(Ti(OPr i)4,97%,Aldrich),barium hydroxide mono-hydrate(Ba(OH)2·H2O,98+%,Aldrich),ethylene diamine(EDA,>98%, Daejung Chemicals,Japan)were used as received without further purification. All other chemicals and solvents were reagent grade commercial materials from Sigma–Aldrich Co.and were also used as-received without further purification. Deionized water was used after elimination of CO2by purging dry N2.2.2.Experimental procedure2.2.1.Preparation of crystalline BaTiO3Ba(OH)2·H2O(1.67g,Ba/Ti molar ratio=1.0)was dissolved in50g of CO2 free deionized water by stirring at50◦C under N2atmosphere.2.5g of tita-nium(IV)isopropoxide and ethylene diamine(EDA/Ti molar ratio=0–10)were added to the10g of isopropyl alcohol(IPA)and they were stirred at50◦C for 30min.This Ti–EDA mixed solution was then slowly injected into to the aque-ous barium hydroxide solution,and then the mixture was stirred at70◦C for 2h.All the reactions were carried out in a Teflon coated polypropylene reac-tion vessel under N2atmosphere to avoid introduction of unwanted impurities as well as formation of BaCO3.Crystalline nanoparticles formed were then collected by centrifugation(17,000rpm,20min),followed by thorough wash-ing with CO2free deionized water and then product was dried in air at room temperature.2.2.2.Dispersion of crystalline BaTiO3into organic mediumThe wet particles obtained from the above experiment(Section2.2.1),was again dispersed in N,N-dimethyl formamide(DMF)using ultrasonic treatment for30min and then centrifuged again.The process was repeated twice to elim-inate water from the surface of the nanoparticles.Finally,we obtained the wet nanoparticles solvent exchanged by DMF.Then calculated amount of solid (2wt%)was added into different organic solvents such as DMF,methanol (MeOH),isopropyl alcohol(IPA),methyl isobutyl ketone(MIBK),cyclohex-anone(CHN)using ultrasonic treatment for1h and they were used for further study.2.3.CharacterizationCrystal structures of all air-dried samples were determined by X-ray diffrac-tion technique(XRD,Rigaku D/Max-2200V)using Cu K␣radiation(1.5418˚A). Bio-Rad Win-IR FTS-165spectrometer was used to record the FT-IR spectra for all air-dried samples in the form of pellet using KBr.Morphologies of the nanoparticles were studied byfiled emission scanning electron microscopy(FE-SEM,JEOL JSM6700F)and transmission electron microscopy(TEM,Tecnai G2,FEI)operating with200kV.The dispersion properties of the crystalline BaTiO3nanoparticles in organic media were studied by dynamic light scatter-ing analyzer(DLS,ELS-800,Otsuka Electronics Ltd.)using633nm laser as a light source at roomtemperature.Fig.1.XRD patterns of the air-dried BaTiO3prepared with different EDA/Ti molar ratio:(a)1.0,(b)3.0,(c)5.0and(d)10.0.3.Results and discussion3.1.Preparation of crystalline BaTiO3Crystalline BaTiO3nanoparticles were prepared from solu-tion by chemical decomposition of Ti–EDA complex with aqueous barium hydroxide solution.Ti–EDA complex was formed in situ during the reaction between Ti(OPr)4and EDA,as ethylene diamine(H2N-CH2CH2-NH2)is a well known biden-tate ligand,which can coordinate with the transition metal ions (Ti4+in this case)by the donation of lone-pair of electrons of the nitrogen.Although,attempts were not made to isolate and char-acterize the Ti–EDA complex,but different EDA/Ti molar ratio (EDA/Ti=1.0–10.0)were used to study the effect of EDA on dispersion of crystalline BaTiO3nanoparticles in organic solvent (discussed in Section3.3).All air-dried samples were characterized by X-ray diffraction technique and the diffraction patters corresponded to the cubic phase of BaTiO3.Fig.1shows the X-ray diffraction(XRD)pat-terns of the air-dried samples with different EDA/Ti molar ratios. The XRD patterns appeared to be almost similar in all the cases. However,the determination of crystallite sizes,calculated by using Scherer’s equation[D=kλ/h1/2cosθ,where k is a constant (0.9),λthe wavelength of the X-ray radiation(1.5418˚A),h1/2 the broadening of XRD peak at half-height at an angle ofθ], showed interesting results.It was observed that the crystallite sizes were in the range24–43nm depending on the EDA/Ti molar ratio.The variation of crystallite sizes with the EDA/Ti molar ratio is presented in Fig.2.It was observed that the crystal-lite size decreased with increasing the EDA/Ti molar ratio.This is because of the fact that with increasing the EDA/Ti molar ratio,the nanoparticles formed in solution were remained in situ capped by the EDA,which prevented the agglomeration as well as growth of the particles.In addition,the alkalinity of the solu-tion also increased with increasing EDA/Ti molar ratio,which may also cause lowering of crystallite sizes as explained in our previous study[32].N.C.Pramanik et al./Journal of Alloys and Compounds449(2008)77–8179Fig.2.Crystallite size of BaTiO3prepared with different EDA/Ti molar ratio.3.2.Microstructure and morphological study of the crystalline BaTiO3nanoparticlesMicrostructure and morphology of the nanoparticles were studied by transmission electron microscopy(TEM)andfield emission scanning electron microscopy(FESEM).Fig.3shows the representative FE-SEM images of the BaTiO3nanoparticles prepared with and without EDA.FE-SEM images showed ran-dom shaped morphology and a broad size distribution of the nanoparticles.The TEM images are shown in the inset of the respectivefigures and they also showed crystalline nature of the nanoparticles were polycrystalline in nature.The particle sizes determined from TEM and FE-SEM images were in the range55–120nm depending on the content of EDA,which were found to be bigger as determined by using Scherer’s equation (24–43nm).This is because of the fact that nanoparticles were agglomerated when the solvent was removed.The agglomera-tion was found to be more prominent in the case of EDA/Ti=0. Fig.3.FE-SEM images of the sample prepared with and without using EDA:(a)EDA/Ti=0and(b)EDA/Ti=10.The insetfigure shows the TEM image of the respectivesamples.Fig.4.(A)Particle size distribution of the crystalline BaTiO3nanoparticles in DMF,prepared with different EDA/Ti molar ratios and(B)particle size distribution of the crystalline nanoparticles dispersed in different organic media(EDA/Ti=10).80N.C.Pramanik et al./Journal of Alloys and Compounds449(2008)77–81 This indicated that the presence of EDA definitely played animportant role to prevent the particle agglomeration.3.3.Dispersion of crystalline BaTiO3into organic mediumWet particles of the BaTiO3nanoparticles obtained using dif-ferent EDA/Ti molar ratios(EDA/Ti=0–10)were dispersed intothe DMF using ultrasonic instrument for1h.The solid contentof the dispersed solution wasfixed to2wt%for all the cases(EDA/Ti=0–10)and the dispersion behavior of the crystallineBaTiO3nanoparticles was studied in terms of particle size dis-tribution by DLS.Fig.4(A)shows the particle size distributionof the nanoparticles with different molar ratios of EDA/Ti.Itwas observed that the particles in the dispersed solution hadtwo broad size distributions with average particle size120and1200nm along with relatively bigger particles in the case ofEDA/Ti molar ratio=0.The distribution was found to be shiftedtowards the smaller particle sizes with increasing the EDA/Timolar ratio.This indicates that EDA play an important role on theredispersion of the nanoparticles into organic media.For exam-ple,a narrow size distribution with average particle size about55nm was observed in the case of EDA/Ti=10,while about80nm average particles size with relatively broader distributionwas observed when EDA/Ti=3was used.It was assumed thatthe crystalline nanoparticles were in situ capped by the EDAliberated during the decomposition of Ti–EDA complex.Thepresence of EDA with thefinal product was investigated bystudying the FT-IR spectra of the air-dried sample in the rangebetween400and4000cm−1.Fig.5shows FT-IR spectra of the air-dried samples with dif-ferent EDA/Ti molar ratios.FT-IR spectra of all the samplesdisplayed several characteristic vibrations.Attempts werenotFig.5.FT-IR spectra of the air-dried BaTiO3prepared with different EDA/Ti molar ratio:(a)0,(b)3and(c)10.made to assign each and individual bands.However,the spectra were compared with that of the BaTiO3prepared without using EDA,the reported values[33–37]and that of the pure EDA (spectrum not shown here).Vibrations observed at563,1370and 1447cm−1was found to be common for all the cases and they were assigned to the Ti-O stretching,Ti-OH and Ba-OH,respec-tively.A broad band centered at3440cm−1and another strong band at1640cm−1was assigned to the stretching and in-plane deformation mode of OH,respectively.In addition,several new vibrations were also observed in the case of samples prepared using EDA.The appearance of bands at2930and2850cm−1 corresponded to the C–H stretching mode of the EDA was also observed,while the vibration due the–NH2(1645cm−1)could not be observed due to overlapping with that of the deformation mode of the OH group.Several vibrations due to various modes of C–N of the C–NH2and N–H of NH2were also observed at1057,1208,1285,1370and1500cm−1,respectively.There-fore,the comparative study of the FT-IR spectra further supports the enhancement of redispersibility of the crystalline BaTiO3 nanoparticles by EDA into organic medium.The dispersion behavior of crystalline nanoparticles in differ-ent organic medium was also investigated using various organic solvents,such as DMF,Methanol(MeOH),isopropanol(IPA), cyclohexanone(CHN)and methyl isobutyl ketone(MIBK).The wet particles,prepared using EDA/Ti molar ratio=10were used for this purpose to prepare2wt%solution,and the dispersion behaviors were studied as described earlier.Fig.4(B)shows the distribution of the particles in different organic media.The nanoparticles redispersed in methanol and DMF showed good dispersion stability,but those in IPA,MIBK and CHN exhibited a strong tendency to form agglomeration with phase separation. These results suggest that the interaction between the nanopar-ticles and the organic media have a key role in their dispersion stabilities.The dielectric constants(ε)are the predominant factors governing electrostatic forces of the BaTiO3nanopar-ticles capped by the EDA in organic media,and they gradually decrease with increasing molecular weight in the case of alco-hols[38].Surface charging of BaTiO3nanoparticles might be related with ionization of surface groups,namely EDA capped on BaTiO3.This means that organic media such as methanol and DMF with relatively high dielectric constants are polarized more easily than organic media such as IPA,MIBK and cyclo-hexanone with low dielectric constants,and thus have a higher tendency to form surface charges on the BaTiO3nanocrystals, resulting in stable suspensions.4.ConclusionsRedispersible crystalline BaTiO3nanoparticles were pre-pared by chemical decomposition of the Ti–EDA complex with an aqueous barium hydroxide solution.X-ray diffraction stud-ies showed that products were well crystallized with crystallite sizes ranging from24to43nm depending on the EDA/Ti molar ratios.The dispersibility of the nanoparticles in organic media was remarkably enhanced by the presence of EDA,in which the in situ capping on the surface of the nanoparticles by EDA was assumed.In addition,BaTiO3nanoparticles capped by theN.C.Pramanik et al./Journal of Alloys and Compounds449(2008)77–8181EDA were well dispersed in organic media such as methanol and DMF with relatively high dielectric constants. AcknowledgementsThis research was supported by a grant from the Core Technology Development Program funded by the Ministry of Commerce,Industry and Energy(MOCIE),Republic of Korea. One of the authors(N.C.Pramanik)is thankful to the Korean Federation of Science and Technology Society(KOFST)for inviting him as visiting scientist under Brain pool program. 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