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Synthesis and Size Control of Tetragonal Barium Titanate Nanopowders by Facile Solvothermal Method

Synthesis and Size Control of Tetragonal Barium Titanate Nanopowders

by Facile Solvothermal Method

Hyun-Wook Lee,?San Moon,?Chang-Hak Choi,§and Do Kyung Kim*,?,??Department of Materials Science and Engineering,Korea Advanced Institute of Science and Technology(KAIST),

Daejeon305-701,Korea §LCR Material Development Group,Samsung Electro-Mechanics,Suwon443-743,Korea

A facile synthetic strategy was implemented to obtain nano-sized barium titanate(BaTiO3)powders with tetragonal struc-ture.The nanoparticles were synthesized using solvothermal process employing diethanolamine and triethanolamine to sup-press the particle growth and the as-prepared nanopowders were characterized using X-ray di?raction,scanning electron microscopy,and high-resolution dispersive Raman spectros-copy.It was found that the particle size can be easily tuned by adjusting the experimental parameters while retaining the tetragonality.The average diameters of the particles prepared with and without the organic amines were found to be80and 100nm,respectively.All the synthesized BaTiO3nanopowders exhibit a narrow size distribution with a uniform morphology. Rietveld re?nement of the XRD patterns and Raman spectra revealed that the synthesized BaTiO3nanopowders have tetrag-onal asymmetry dominant structures.A slight decrease in the tetragonality of the prepared powders with decrease in particle size is attributed to the presence of cubic shell layer and inner defects.The tetragonal-dominant structure was also con?rmed by normalizing the peak area of the Raman spectra.

I.Introduction

B ARIUM titanate(BaTiO3)has been utilized extensively in

various electroceramic areas,including thermistors,1 electroluminescence,2electro-optical devices,3–5and multi-layer ceramic capacitors(MLCC)6,7due to its ferroelectric response and high dielectric constant.8,9As technological advances demand more complex portable devices with vari-ous functions,the fabrication of thinner dielectric layers of MLCCs becomes a more signi?cant issue as these devices becomes more miniaturized.This requires reduction in BaTiO3particle size to nanoscale dimensions.

As the sizes of BaTiO3particles decrease below100nm, the formidable challenge related to the fabrication of minia-turized MLCCs lies in the gradual decline in the dielectricity of the layers.10It is well known that BaTiO3exists in several polymorphic phases,such as tetragonal and cubic structures with ferroelectric and paraelectric features,respectively.In tetragonal BaTiO3,a structural deformation due to the dis-placement of the Ti atom from the octahedron center causes the ferroelectric stabilization of tetragonal BaTiO3.11How-ever,nanosized BaTiO3reduces the spontaneous polarization of Ti atoms at the surface region,resulting in a cubic shell layer.Consequently,the main challenges lie in the synthesis of highly dielectric BaTiO3nanopowders with decrease in particle size.Moreover,various defects can exist inside the particles,such as oxygen vacancies12and lattice impuri-ties.13,14Therefore,to downsize the dielectric layers in MLCCs further,thinner cubic shell layers and a method that reduces the number of defects inside the BaTiO3nanopow-ders should be considered to acquire the highest possible dielectricity.

Another issue pertaining to BaTiO3nanopowders involves controlling the particle size distribution.Conventional solid-state synthesis can yield larger quantities,but it is di?cult to achieve a narrow particle size distribution when the particle sizes are less than100nm.Particle dispersity a?ects the fab-rication of MLCCs when using tape-casting and sintering processes.Nonuniform particle sizes can lead to vacancies inside BaTiO3dielectric layers and abnormal grain growth after tape-casting and sintering.In this regard,various soft chemistry techniques have been explored to prepare BaTiO3 nanoparticles,such as spraying15and electrophoretic deposi-tion16,17as well as the catecholate,18sol–gel,19–21pyrolysis,22 hydrothermal synthesis using gel powders,23and supercritical hydrothermal reaction methods.24Although uniform BaTiO3 nanopowders are obtained using these methods,synthetic procedures are generally complex and expensive.Also,these nanoparticles exhibit cubic structure instead of required tetragonal structure,which makes them unsuitable for minia-turized MLCC applications.Hence,a facile and cost-e?ective method is highly recommended to fabricate?ne tetragonal BaTiO3nanopowders to meet current demands.

In this article,we report the preparation of BaTiO3nano-powders via hydrothermal and solvothermal synthesis meth-ods and describe their structural characteristics.To control the particle size,organic solvents of diethanolamine(DEA)and triethanolamine(TEA)were employed as part of a solvother-mal reaction.The resulting BaTiO3structure and morphology were characterized using X-ray di?raction,scanning electron microscopy,and Raman spectroscopy to con?rm the tetrago-nal phase and particle size with narrow dispersity.Rietveld re?nement of the XRD patterns and Raman spectroscopy were carried out to evaluate the tetragonality of the nanopow-ders.The results showed that the particle size could be tuned simply by adjusting the experimental parameters.The average sizes of particles prepared with and without DEA and TEA were found to be80and100nm,respectively.

II.Experimental Procedure

The synthesis of BaTiO3nanopowders was based on a solvo-thermal process using barium hydroxide monohydrate(Ba (OH)2·H2O(Ba-hydroxide),98%,Aldrich,Milwaukee,WI) and titanium butoxide(Ti[O(CH2)3·CH3]4(Ti-butoxide),

97%,Aldrich)as starting materials.In a typical BaTiO3 nanopowder fabrication procedure,17.018g(50mmol)of Ti-

S.-J.Kang—contributing editor

Manuscript No.30582.Received October31,2011;approved January06,2012.

*Member,The American Ceramic Society.

This study was presented at the International Conference on Sintering2011,Jeju, Korea,August29,2011(Electroceramics I,Presentation No.MoC1-1).

?Author to whom correspondence should be addressed.e-mail:dkkim@kaist.ac.kr

2429J.Am.Ceram.Soc.,95[8]2429–2434(2012) DOI:10.1111/j.1551-2916.2012.05085.x

?2012The American Ceramic Society

J ournal

butoxide was mixed with20mL of high-purity ethanol,after which7mL of ammonium hydroxide solution(25%NH3in H2O,Aldrich)was added to the solution mixture.A quantity of14.204g(75mmol)of Ba-hydroxide was dissolved in 25mL of preheated DI-water to prepare clear Ba-hydroxide solution in parallel.The aqueous Ba-hydroxide solution was then added to the solution mixture.To control the particle size,5mL of DEA(HN(CH2CH2OH)2,!98%,Aldrich)or TEA((HOCH2CH2)3N,!98%,Aldrich)was added to the solution mixture.The?nal suspension was transferred into a 100mL Te?on-lined stainless-steel autoclave and heat-treated at200°C for48h.After the reaction,the resultant product was washed repeatedly using water and high-purity ethanol and then dried at80°C for24h in an oven.Also,BaTiO3 nanoparticles were prepared without organic amines as a control sample using similar procedure.

The structural characteristics and phase purity of the syn-thesized BaTiO3nanopowders were investigated using an X-ray di?ractometer(XRD;Rigaku D/Max-RB(12kW), Tokyo,Japan)with Cu K a radiation(k=1.5418A)operat-ing at40kV and100mA.The unit cell parameters were obtained via Rietveld re?nement of the XRD data.The morphology of the nanopowders was observed using a?eld-emission scanning electron microscope(FE-SEM;Hitachi S-4800,Tokyo,Japan)and a?eld-emission transmission electron microscope(FE-TEM;Tecnai G2F30S-Twin, Eindhoven,the Netherlands).The Raman spectra were recorded by a high-resolution dispersive Raman microscope (LabRAM HR UV/Vis/NIR,Horiba Jobin Yvon,Longju-meau,France)equipped with three laser sources(UV,Vis, and NIR),a confocal microscope,and a liquid-nitrogen-cooled charge-coupled device(CCD)multichannel detector (256pixels91024pixels).The measurements were per-formed using a514.5nm line of an argon ion laser at room temperature.A509objective lens was used,and the acqui-sition time for each Raman spectrum was approximately 10min depending on the sample.The measured Raman shift was in the range100–1000cmà1.

III.Results and Discussion

The as-synthesized BaTiO3nanopowders were characterized using XRD and SEM.Figure1shows the XRD pattern,a SEM image,and the particle size histogram of the hydrother-mally synthesized nanopowders,in which organic amines were not added.The XRD pattern indicates that the synthe-sized material shows good agreement with the conventional tetragonal BaTiO3structure with the P4mm space group (JCPDS data No.05-0626),with no impurity peak appearing in the di?ractogram.Rietveld re?nement gives lattice param-

eters a=3.99454A and c=4.02411A,which are very close to the reported values pertaining to this tetragonal structure (JCPDS data No.05-0626).XRD pattern of as-synthesized BaTiO3powders show peak splitting at45°.In general,the XRD patterns of the tetragonal BaTiO3show split peaks at 45°corresponding to the(hkl)Miller index(002)and(200), whereas cubic BaTiO3(JCPDS data No.31-0174)has one single peak at45°corresponding to(002).Therefore,we can conclude that the hydrothermally synthesized BaTiO3pow-ders show a tetragonal-or tetragonal-dominant structure. The SEM image con?rms that the BaTiO3powders have a nonagglomerated shape with an average diameter of 106.69nm.The particle-size distribution histogram in Fig.1(c)shows that the nanopowders have narrow size dis-tribution ranges with the average particle dispersity(D SEM99/ D SEM50)of1.57.The structural and morphological character-izations converge in demonstrating that the hydrothermal synthesis process leads to the formation of the tetragonal BaTiO3phase with100-nm grade nanopowders and a narrow size distribution.

In comparison,the BaTiO3nanopowders synthesized using the solvothermal process show di?erences both in terms of the particle size and tetragonality.The XRD patterns of the as-synthesized BaTiO3created by means of the DEA and TEA processes are shown in Fig.2.Both patterns indicate a tetragonal BaTiO3structure with no impurity peak.How-ever,evidence of peak splitting at45°is not very distinct. This may be due to the peak broadening e?ect of nano-crys-talline structures or to the cubic-dominant BaTiO3structure. The tetragonalities of the two samples as determined by Riet-veld re?nement were1.0068(DEA)and1.0072(TEA).These values are comparable to the tetragonality of nanopowders synthesized using a hydrothermal process.To compare the particle size,the morphologies of BaTiO3nanopowders cre-ated using the DEA and TEA processes were characterized by SEM.These results are presented in Fig.3.The average sizes(D SEM50)of the two powders are78.72nm(DEA)and 84.78nm(TEA).Micrograph images of the nanopowders revealed particle size reductions of24.3%(DEA)and18.5% (TEA)compared with a hydrothermally synthesized sam-ple.Thus,organic amines serve to reduce the particle size of synthesized BaTiO3nanopowders while retaining their tetragonality.

(a)

(b)

(c)

Fig.1.(a)XRD pattern,(b)typical SEM image,and(c)particle size distribution histogram of hydrothermally synthesized BaTiO3 nanopowders.

2430Journal of the American Ceramic Society—Lee et al.Vol.95,No.8

The tetragonality values determined by Rietveld re?ne-ment of the XRD data are di?cult to assign the crystal sym-metry due to the peak broadening e?ect and low intensity.

Therefore,an alternate analysis method is required to con-?rm the tetragonalities of BaTiO 3nanopowders.Raman spectroscopy is capable of measuring the lattice vibrational spectra in an investigation of the tetragonal-cubic symmetry of BaTiO 3samples.25,26The Raman spectra of di?erent BaTiO 3particles (denoted by BTR1to BTR7based on tetragonality)are given in Fig.4and their corresponding size,lattice parameters,and tetragonality values are given in Table I.The Raman spectra of BTR1to BTR6samples,which have a tetragonal feature according to the XRD data,are consistent with the spectrum of tetragonal BaTiO 3as reported elsewhere.26,27It is known that tetragonal BaTiO 3has Raman scattering bands of A 1(TO)at 250cm à1,B 1,E (TO +LO)at 307cm à1,[E(TO),A 1(LO)]at 520cm à1,and [E(LO),A 1(LO)]at 720cm à1.However,a mixture of the Raman scattering characteristics of both cubic and tetrago-nal asymmetry can be observed at around 180,307,520,and 720cm à1.In the present experiment,the scattering intensities at 250and 307cm à1gradually declined as the tetragonalities of the BaTiO 3nanopowders decreased.It is interesting to note that there is a relationship between the peak intensity and the tetragonal features in the Raman scattering results.Figure 5demonstrates the tetragonality of BaTiO 3nanopowders as a function of the normalized area of peaks at 180,250,307,520,and 720cm à1.The normal-ized peak areas are in very good agreement with the tetrago-nalities determined by the Rietveld re?nement of the XRD data.These results indicate that Raman spectroscopy can serve as an alternate tool with which to evaluate nanosized BaTiO 3.

To enhance our understanding of the roles of the organic solvents of DEA and TEA,the amount of DEA was

varied

Fig.2.XRD patterns of BaTiO 3nanopowders synthesized by addition of organic solvents of (a)DEA and (b)TEA.

(a)

(b)

Fig.3.SEM images of BaTiO 3nanopowders prepared by addition of organic solvents of (a)DEA and (b)

TEA.

Fig.4.Plot showing dependence of Raman spectra on tetragonality of BaTiO 3

nanopowders.

Fig.5.Correlation between the normalized peak area from the Raman spectra in Fig.4and tetragonality determined from XRD data.

August 2012Synthesis and Size Control of BaTiO 3Nanopowders 2431

in the experimental procedure.Table II presents a compari-son of the particle size,distributions,and lattice parameters of various BaTiO 3nanopowders synthesized using hydrother-mal and solvothermal processes by varying the DEA content.The notation of the sample numbers indicates the di?erent ratios of organic solvents and the addition of ethanol.When we added DEA of 2(BT 4)or 10mL (BT 5)to the solvo-thermal reaction,the average particle size decreased by 20%–30%compared with hydrothermally synthesized BaTiO 3(BT 1).Although the volume ratio of organic amines was increased or decreased by 12.5%(BT 5)or 2.5%(BT 3),the particle size decreased in a similar manner by about 75or 80nm.Although the DEA process decreased the average particle size,the in?uence of the amount of DEA on the particle size was insigni?cant,whereas a reduction in the average size did not occur in BT 6or 7despite the addition of DEA.This phenomenon can be explained based on the kinetics of the reaction in relation to ethanol and DEA.28,29During the solvothermal reaction,Ti-O bonds are broken via hydrolytic attack to form hydroxytitanium complexes (Ti(OH)x (4àx )+),which are soluble and react further with barium ions or com-plexes (Ba 2+or BaOH +)in a solution.30In the present study,ethanol is a solvent with weaker polarity than water;thus,the formation of Ti(OH)x (4àx )+can occur at a signi?-cantly faster rate.Simultaneously,DEA plays a role in the formation of hydrogen bonds with hydrated water of barium salts and exhibits a strong inclination to take Ba 2+or BaOH +away from the hydroxide complexes.This process using DEA is advantageous,as more rapid formation of Ti (OH)x (4àx )+and the strong attraction with Ba 2+or BaOH +can lead to an increase in the number of nucleation sites and

can suppress the particle growth by means of the dissolution-precipitation reaction mechanism.30This phenomenon requires both DEA and ethanol to suppress the growth reac-tion,otherwise a size reduction does not occur,as in the case of the BT 8sample.Accordingly,with organic amines of DEA and TEA,smaller BaTiO 3particles with a uniform size distribution can be synthesized than hydrothermally synthesized samples.

The theoretical tetragonal asymmetry of BaTiO 3is 1.011

(a =3.992A

,c =4.036A ),but in the present nanoparticles,the tetragonalities are generally about 1.006or 1.007.It is known that BaTiO 3nanoparticles contain two general defects 14attributable to the reduction in the tetragonality of the nanoparticles.The ?rst defect is the presence of a cubic shell layer in BaTiO 3.31,32In a bulk system,a cubic shell layer with a thickness of a few nanometers is essentially neg-ligible,whereas in nano-sized particles,a few nanometers become a relatively critical value,decreasing the tetragonality of the nanopowders.In addition,hydrothermally synthesized BaTiO 3contains some degree of internal pores due to the presence of OH àgroups in the BaTiO 3nanopowders.14These two defects can be presented as a model,as shown in Fig.6(a).The presence of an outer shell and inner pores in our synthesized BaTiO 3nanopowders were con?rmed using scanning transmission electron microscopy (STEM),as shown in Fig.6(b).Similarly,Zhu et al.has observed the outer shell of BaTiO 3nanocrystals by HR-TEM.32Therefore,the shell layer and inner pores inevitably in?uenced the tetragonality of the BaTiO 3nanopowder.

Based on the BaTiO 3model and on the HR-TEM obser-vations by Zhu et al.,32if the cubic shell layer is assumed to have a thickness of 3nm thick and if inner defects do not exist in the nanoparticles,the tetragonality as a function of the particle size is plotted as a solid line,as shown in Fig.7.The calculated line was assumed that the nanoparticle per-tain as spheral morphology.There is a drastic decrease in the tetragonality value,from 1.009to 1.008,when the particle size is less than 80nm.These values indicate that at an aver-age particle size of 80nm,the maximum attainable tetrago-nality of BaTiO 3nanopowders is 1.0087.In a comparison with our synthesized results,all of the tetragonal asymmetry values of the synthesized BaTiO 3nanopowders are presented in Fig.7.The tetragonality values of BT 1to 8are plotted with the white circles,and the black circles represent those values of the reproduced BaTiO 3nanopowders.The tetrago-nality values of our data show very good agreement with their ?tting line (dashed line)and show a declining trend according to the decrease in the particle size.As a result,the tetragonal decline can likely be attributed to the increase in the ratio of the outer cubic shell layer in the BaTiO 3nano-powders,which assumes a more critical value at a smaller

Table I.Average Particle Size,Lattice Parameter,and Tetragonality of BaTiO 3Nanopowders for Raman

Spectroscopy

Sample no.

Average particle size ?

(nm)

a (A

)c (A

)Tetragonality ?

(c/a )

BTR1115.42 3.99375 4.02962 1.0090BTR2158.57 3.99009 4.02428 1.0086BTR384.78 3.99435 4.02275 1.0072BTR474.65 3.99260 4.01701 1.0061BTR597.98 3.99849 4.02100 1.0056BTR674.39 3.99721 4.01890 1.0054BTR7

36.69 4.02867 4.03245 1.0009

?

Average particle sizes of BTRs were measured using SEM images (D SEM50).?

Tetragonalities (c/a )of BTRs were calculated by Rietveld re?nement of X-ray di?raction data.

Table II.

Comparison of Average Particle Size,Particle Dispersity,and Lattice Parameter of BaTiO 3

Nanopowders Produced from Hydrothermal and Solvothermal Reactions

Sample no.

Amine

Amount of amine (mL)

Ethanol

Average particle size ?(nm)

Standard deviation (nm)

Particle dispersity (D SEM99/D SEM50)

a (A

)c (A

)Tetragonality ?

(c/a )

Reference

BT1X 0X 106.6914.69 1.57 3.99454 4.02411 1.0074Fig.1BT2DEA 5O 78.7210.78 1.43 3.99129 4.01824 1.0068Figs.2(a)and 3(a)BT3TEA 5O 84.7811.07 1.42 3.99435 4.02275 1.0072Figs.2(b)and 3(b)

BT4DEA 2O 81.2413.58 1.50 3.99792 4.02411 1.0066–BT5DEA 10O 76.7617.69 1.75 4.00002 4.02439 1.0061–BT6DEA 5X 103.9715.16 1.39 3.98999 4.02018 1.0076–BT7DEA 10X 103.7215.55 1.41 3.98818 4.01920 1.0078–BT8

X

O

101.97

19.23

1.52

3.99333

4.01765

1.0061–

X,not included;O,included;DEA,diethanolamine (HN(CH 2CH 2OH)2);TEA,triethanolamine ((HOCH 2CH 2)3N).?

Average particle sizes of BTs were measured by SEM images (D SEM50).?

Tetragonalities (c/a )of BTs were calculated by Rietveld re?nement of X-ray di?raction data.

2432Journal of the American Ceramic Society—Lee et al.Vol.95,No.8

size.Moreover,the di?erence between the line of the calcu-lated value and the ?tting line may have been attributed to the presence of inner defects.

IV.

Conclusion

Tetragonal BaTiO 3nanopowders of two di?erent sizes were fabricated using facile and scalable hydrothermal and solvo-thermal methods.The average diameters of the BaTiO 3parti-cles obtained using hydrothermal and solvothermal reaction methods were around 100and 80nm,respectively,with a low particle dispersity (D SEM99/D SEM50)of approximately 1.5.The reduction in particle size can be attributed to the synergistic e?ect of organic amine and ethanol in increasing the number of nucleation sites and suppressing the particle growth via dissolution-precipitation mechanism.The tetrago-nalities of the synthesized powders,as determined by Riet-veld re?nement of the XRD data and by high-resolution dispersive Raman spectra,decrease with the decrease in par-ticle size.It was revealed that the reduced tetragonality fea-tures of BaTiO 3nanopowders are caused by the presence of a cubic shell layer and inner pores,the e?ects of which become relatively critical in nanosized materials.Also,by measuring the normalized peak areas of the Raman spectra,we established a trend according to the tetragonality values of the BaTiO 3.These synthesized BaTiO 3nanoparticles with tetragonal structure could be potentially used for the minia-turization of MLCC devices.

Acknowledgments

The work was supported by the Center for Advanced MLCC Manufacturing Processes of Samsung Electro-Mechanics,the Priority Research Centers Pro-gram (2009-0094041),and Program to Solve Climate Changes through the National Research Foundation of Korea (NRF)funded by the Ministry of Education,Science and Technology (MEST)(NRF-2010-C1AAA001-2010-0029031).

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August 2012Synthesis and Size Control of BaTiO 3Nanopowders

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2434Journal of the American Ceramic Society—Lee et al.Vol.95,No.8

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