Hydrothermal Soft Chemical Synthesis and Particle MorphologyControl of BaTiO3 in Surfactant Solutions

Qi Fengw

Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, Takamatsu 761-0396, Japan

Manabu Hirasawa, Koji Kajiyoshi,* and Kazumichi Yanagisawa

Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi 780-8520, Japan

Barium titanate (BaTiO3) particles with book-like and sphericalmorphology were prepared by using a hydrothermal soft chem-ical process in the presence of a cationic surfactant. A layeredtitanate of H1.07Ti1.73O4 with a lepidocrocite-like structure andplate-like particle morphology was used as the precursor. Thelayered titanate was hydrothermally treated in a Ba(OH)2–(HTMA-OH) (n-hexadecyltrimethylammonium hydroxide)solution or a Ba(OH)2–(HTMA-Br) (n-hexadecyltrimethylam-monium bromide) solution in a temperature range of 801–2501Cto prepare BaTiO3. The intercalation reaction of HTMA

1

with the layered titanate promotes the structural trans-formation reaction from the layered titanates to BaTiO3, whileit inhibits the structural transformation reaction to anataseunder the hydrothermal conditions. The particle morphologyof BaTiO3 prepared by this method dramatically changes withchanging reaction conditions. HTMA

1plays an important

role in changing particle morphology in the hydrothermal softchemical process.

I. Introduction

BARIUM TITANATE (BaTiO3) is a well-known electroceramicmaterial widely utilized in the manufacture of multilayer

capacitors, thermistors, and electro-optic devices because of itshigh dielectric permittivity and ferroelectric properties.1–3 Alarge number of studies on the preparation of BaTiO3 particleshave been carried out since the discovery of its ferroelectricproperties in the 1940s. Recently, most of the studies have fo-cused on low-temperature synthesis and particle morphologycontrol of BaTiO3 by using the hydrothermal method,4–7 sol–gelmethod,8–10 and metal-organic precursor decomposition meth-od,11,12 and also on thin film preparations.13–16 The high-purityBaTiO3 with controlled particle size can be prepared by us-ing the hydrothermal and sol–gel methods at low temperatures.1

These methods, however, cannot offer BaTiO3 particles with aparticular shape for crystal-axis-oriented ceramic materials. Thesynthesis of crystal-axis-oriented BaTiO3 particles with particu-lar shape, such as plate-like and fibrous particles, is importantfor the preparations of high-performance ceramic materials,because the dielectric materials show crystal-axis anisotropicproperties,17 and the plate-like and fibrous particles can be ori-

ented easily by the mechanical method, such as the doctor blademethod.

Soft chemical synthesis using host–guest reactions is a uniqueand useful method for inorganic material synthesis. This methodcan be used for the low-temperature synthesis, modification, andcontrol of the product structure, preparation of inorganic–or-ganic nano-composite crystal, and also for the morphology con-trol of product particles.18–21 In the soft chemical synthesis,a compound with a layered structure or open structure can beused as the precursor. The layered structure of the precursor canbe transformed to a desired structure by an in situ topotacticstructural transformation reaction, and the morphology of theprecursor can be retained after the reaction. It means that themorphology of the product is dependent on that of the precur-sor, which is different from normal methods like sol–gel andsolid-state reaction methods, where the crystal particle morphol-ogy is almost independent of the morphology of the precursor.By the normal methods, crystal morphology is controlled main-ly in the crystal growth process, meaning control of the growingplane of the crystal, which is not easy in normal cases.

Hydrothermal reaction is useful for structural transformationin the soft chemical synthesis, and we call the method using thehydrothermal reaction for the soft chemical synthesis a hydro-thermal soft chemical process.22 For crystal structure control,we have prepared a series of tunnel metal oxides and sandwichlayered compounds by using the hydrothermal soft chemicalprocess.22–26 The structures of tunnel metal oxides and sandwichlayered compounds can be controlled by the guest ions in theinterlayer space of the layered compounds used as the precursor.

For morphology control, fibrous BaTiO3 and PbTiO3

have been prepared by hydrothermal treatment of a fibrouslayered hydrous potassium titanate (2K2O � 11TiO2 � 3H2O) withBa(OH)2 and Pb2O(OH), respectively.27–31 The fibrous BaTiO3

and PbTiO3 particles show high-degree crystal-axis orientationand anisotropic dielectric properties. Our previous study has in-dicated that a fibrous H1-form tetratitanate of H2Ti4O9 with alayered structure showed higher reactivity than the fibrous hy-drous potassium titanate for the hydrothermal soft chemicalsynthesis.32 The fibrous particles of anatase and ATiO3 (A5Ba,Sr, and Ca) can be prepared by hydrothermal treatment of thefibrous H2Ti4O9 in distilled water and A(OH)2 solutions, re-spectively. We have also prepared plate-like anatase and BaTiO3

particles by hydrothermal treatment of an H1-form lepidocro-cite-like layered titanate of H1.07Ti1.73O4 (HTO) with plate-likeparticle morphology.33 The plate-like anatase and BaTiO3 showhigh-degree crystal-axis orientation and are easy to orient on asubstrate.

In the present paper, we describe preparation, particle mor-phology control, and formation mechanism of BaTiO3 from theplate-like HTO precursor by using the hydrothermal soft chem-ical process in cationic surfactant solutions. The morphology ofthe product particles can be changed dramatically by adding thecationic surfactant in the reaction system.

1415

JournalJ. Am. Ceram. Soc., 88 [6] 1415–1420 (2005)

DOI: 10.1111/j.1551-2916.2005.00298.x

R. E. Riman—contributing editor

Presented in part at Joint 6th International Symposium onHydrothermal Reactions and4th International Conference on Solvo-Thermal Reactions, Kochi, Japan, July 25–28, 2000(paper no. 1P-34).

Supported by grants-in-aid for scientific research (C) (no. 13650894) from Japan Societyfor the Promotion of Science, and The Murata Science Foundation.

*Member, American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: feng@eng.kagawa-

u.ac.jp

Manuscript No. 10522. Received September 29, 2004; approved December 9, 2004.

II. Experimental Procedure

(1) Sample Preparation

A layered titanate of K0.8Ti1.73Li0.27O4 (KTLO) with a lepid-ocrocite-like layered structure, which was used as a precursor,was prepared by a flux method. Stoichiometric K2CO3 and TiO2

(anatase) and 10% excess of Li2CO3 were mixed and groundtogether. The mixture of the starting materials was mixed withK2MoO4 flux in a mole ratio of K0.8Ti1.73Li0.27O4/K2MoO453/7, and then heated to 11001C at a heating rate of 1501C/h andkept at the temperature for 5 h. After the sample cooled down inthe furnace naturally, the product was washed with boilingwater to remove the K2MoO4 flux and was dried at room tem-perature. The layered H1-form titanate HTO was prepared bytreatment of KTLO (10 g) with a 1M HNO3 solution (1 L) for1 day to exchange K1 and Li1 in the layered structure with H1.The acid treatment was repeated twice to complete the ion-ex-change reaction. The ion-exchanged sample was washed withdistilled water and dried at room temperature.

The H1-form titanate (0.147 g) was added to a 0.1M cationicsurfactant solution (15 mL) of n-hexadecyltrimethylammoniumhydroxide (HTMA-OH) or n-hexadecyltrimethylammoniumbromide (HTMA-Br), and stirred at room temperature for 2h, and then a desired amount of Ba(OH)2 � 8H2O was added tothe solution to adjust the concentration of Ba(OH)2 to 0, 0.1,0.2, and 0.3M, respectively, by assuming that all added Ba(OH)2can dissolve in the solution. Thus, the Ba/Ti mole ratios in thereaction system were controlled to be 0, 1, 2, and 3, respectively.The mixture (15 mL) was placed in a Teflon-lined, sealed stain-less-steel vessel (30 mL of inner volume), and then hydrother-mally treated in a temperature range of 801–2501C for 1 dayunder stirring or stationary conditions. The product was filtered,washed with hot distilled water, and then dried at 801C for1 day. A pHmeasurement indicated that the pH values of 0, 0.1,0.2, and 0.3M Ba(OH)2–(HTMA-OH) solutions were 13.1, 13.5,13.8, 13.9, respectively, and those of 0, 0.1, 0.2, and 0.3MBa(OH)2–(HTMA-Br) solutions were 6.8, 13.4, 13.7, 13.8, re-spectively, at room temperature.

(2) Characterization

The crystal structures of samples were investigated by usinga powder X-ray diffractometer (Rigaku Rotaflex RAD-RC,Tokyo, Japan). The particle size and morphology were charac-terized by scanning electron microscopy (Model S-530, HitachiLtd., Tokyo, Japan).

III. Results and Discussion

(1) Ion-Exchange Reaction of HTO

Figure 1 shows XRD patterns of the H1-form titanate (HTO),and the samples obtained by treatment of HTO in 0.3MBa(OH)2, and 0.1M HTMA-OH solutions at room tempera-ture, respectively. HTO has a lepidocrocite-like layered struc-ture,34 as shown in Fig. 2, with a basal spacing of 0.923 nm. Thecrystal water and H3O

1 occupy its interlayer space, and H3O1

can be exchanged with other cations.35 After treatment withBa(OH)2 solution, the basal spacing of the layered titanate de-creased from 0.923 to 0.918 nm, indicating that H3O

1 in theinterlayer space were exchanged with Ba21, but retained thelayered structure at room temperature. When HTO was treatedwith an HTMA-OH solution, three-layered phases were ob-served (Fig. 1(c)). Two phases have large basal spacings of 2.98and 3.32 nm, respectively, corresponding to those phases withHTMA1 in the interlayer spaces, and another phase had a smallbasal spacing of 0.899 nm corresponding to that withoutHTMA1 in the interlayer spaces. When the HTMA-OH solu-tion-treated sample was treated further with a 0.3M Ba(OH)2solution, one-layered phase with a basal spacing of 0.905 nmwas observed (Fig. 1(d)), suggesting that HTMA1 in the inter-layer space can be exchanged easily with Ba21 owing to its highpositive charge.

(2) Hydrothermal Reaction in Ba(OH)2–(HTMA-OH)Solution

Figure 3 shows XRD patterns of samples obtained by hydro-thermal treatment of HTO in Ba(OH)2–(HTMA-OH) solutionsat 801C under stirring conditions. The layered titanate retainedthe layered structure after reaction at 801C in Ba21-free HTMA-OH solution (Fig. 3(a)). Most of the layered titanate phase wastransformed to a BaTiO3 phase, and only a very small amountof unreacted layered titanate phase remained after the reactionat 801C in the 0.1M Ba(OH)2–(HTMA-OH) solution (Fig. 3(b)).In the 0.2 and 0.3M Ba(OH)2–(HTMA-OH) solutions, allthe layered titanate phase was transformed completely to theBaTiO3 phase. This result suggests that a single BaTiO3 phasecan be obtained at a much lower temperature in the Ba(OH)2–(HTMA-OH) solution system than that in Ba(OH)2 solutionsystem where a small amount of the unreacted layered titanateremained even at 2001C in a 0.3M Ba(OH)2 solution.

33 A smallamount of BaCO3 phase was also observed in the BaTiO3 sam-ples, because of the reaction of Ba(OH)2 with CO2 gas in air.

The BaTiO3 phase was also formed at 1501 and 2501C in theBa(OH)2–(HTMA-OH) solutions in a manner similar to that at801C, except that the crystallinity increased with increasing re-action temperature. The BaTiO3 phase obtained in the tem-perature range below 1501C belongs to the cubic system. Abroadening of (200) diffraction of the BaTiO3 phase, however,was observed for the samples obtained at 2501C (Fig. 4). The

0 10 20 30 40 50 60

(a)

(b)

(c)

(d)

2θ (°)

Fig. 1. X-ray diffraction patterns of (a) H1-form layered titanateH1.07Ti1.73O4 (HTO), samples obtained by treatment of HTO in (b)0.3M Ba(OH)2 solution and (c) 0.1M HTMA-OH solution, and (d)sample after treatment of sample (c) in 0.3M Ba(OH)2 solution, for 1 dayat 251C. J, HTO phase; ., Ba21-exchanged layered phase; & , �, &,HTMA1-exchanged layered phases with basal spacings of 3.32, 2.98,and 0.899 nm, respectively; ,, BaCO3 phase.

Fig. 2. Lepidocrocite-like layered structure of H1.07Ti1.73O4.

1416 Journal of the American Ceramic Society—Feng et al. Vol. 88, No. 6

broadening became remarkable with increasing Ba(OH)2 con-centration, and the (200) diffraction split into two peaks in the0.3M Ba(OH)2–(HTMA-OH) solution at 2501C (Fig. 4(d)).These results suggest that the cubic BaTiO3 phase distorts tothe tetragonal phase under high-temperature and high alkalineconditions. Usually, the cubic BaTiO3 phase is formed undernormal hydrothermal conditions.4–6 It has been reported thatthe tetragonal BaTiO3 phase can be obtained in a very high al-kaline solution (3M KOH) at 2201C for 3 days,7 or in the pres-ence of chloride ion conditions at 2401C for 1 week36–38 by thenormal hydrothermal reaction. The present results indicate thatthe tetragonal BaTiO3 phase can be obtained in a low alkalinesolution and short reaction time by using the hydrothermal softchemical process described here.

In the Ba21-free HTMA-OH solution, the layered titanatephase was stable up to 1501C, but transformed to the anatasephase at 2501C (Fig. 4(a)). In distilled water without HTMA-OH, however, the layered titanate phase transforms to the an-atase phase completely at 1501C. These facts indicate that theHTMA1-form titanate is more stable that the H1-form titanateunder hydrothermal conditions.

The particle morphology of hydrothermal products is de-pendent on the alkaline concentration, the stirring conditions,and reaction temperature (Fig. 5). At 1501C under the stirringconditions, the product in the Ba21-free HTMA-OH solutionretained the plate-like particle morphology of the layered titan-ate precursor, but the plate-like particles aggregated together(Fig. 5(a)). In the solution containing 0.1M Ba(OH)2, plate-like particles and small particles of BaTiO3 were observed(Fig. 5(b)). The morphology of the plate-like particles corre-sponded to that of the layered titanate precursor, and the smallparticles were formed by destruction of the plate-like particles.In the solutions containing 0.2 and 0.3M Ba(OH)2, sphericalparticles of BaTiO3 were obtained (Fig. 5(c)). The spherical par-ticles were constructed by stacking small plate-like particles. Thespherical BaTiO3 particles were observed in a temperature rangeof 801–1501C in the 0.2 and 0.3M Ba(OH)2–(HTMA-OH) so-lutions under stirring conditions. The spherical BaTiO3 particlestended to be formed in high Ba(OH)2 concentration solutions.At 2501C, the situation was different from that at the lowertemperature, where no spherical particle was observed. Most ofthe particles showed plate-like morphology, while the particlesize was smaller than that of the layered titanate precursor(Fig. 5(d)), indicating that size breakdown occurred under thereaction conditions.

XRD studies indicated that the formations of BaTiO3 andanatase phases from the layered titanate in the Ba(OH)2–

0 10 20 30 40 50 60

(100

)

(110

)

(111

)

(200

)

(210

) (211

)

(a)

(b)

(c)

(d)

2θ (°)

Fig. 3. X-ray diffraction patterns of products obtained by hydrother-mal treatment of H1.07Ti1.73O4 in the Ba(OH)2–(HTMA-OH) solutionscontaining (a) 0M, (b) 0.1M, (c) 0.2M, and (d) 0.3M Ba(OH)2, respec-tively, at 801C for 1 day under stirring conditions. & , &, layered ti-tanate phases; J, BaTiO3 phase; ,, BaCO3 phase.

0 10 20 30 40 50 60

(100

) (110

)

(111

)

(200

)

(210

)

(211

)

(a)

(b)

(c)

(d)

2θ (˚)

Fig. 4. X-ray diffraction patterns of products obtained by hydrother-mal treatment of H1.07Ti1.73O4 in the Ba(OH)2–(HTMA-OH) solutionscontaining (a) 0M, (b) 0.1M, (c) 0.2M, and (d) 0.3M Ba(OH)2, respec-tively, at 2501C for 1 day under stirring conditions. J, BaTiO3 phase;n, anatase phase.

Fig. 5. Scanning electron microscopy photographs of products ob-tained by hydrothermal treatment of H1.07Ti1.73O4 in Ba(OH)2–(HTMA-OH) solutions for 1 day. (a–c) Samples in the solutions con-taining 0, 0.1, and 0.3M Ba(OH)2, respectively, at 1501C under stirringconditions; (d) sample in the solution containing 0.3M Ba(OH)2 at2501C under stirring conditions; (e, f) sample in the solution containing0.2M Ba(OH)2 at 1501C under stationary conditions. (a) Layered titan-ate phase; (b–f) BaTiO3 phase.

June 2005 Synthesis and Particle Morphology Control of BaTiO3 1417

(HTMA-OH) solutions under stationary hydrothermal condi-tions are similar to those under stirring conditions. However, theparticle morphology was different, where no spherical particlesof BaTiO3 were observed under the stationary conditions. TheBaTiO3 particles almost retained the plate-like particle mor-phology of the precursor, but they had many cracks along thebasal plane of the plate-like particles (Figs. 5(e) and (f)). In theparticles, thin sheets stacked together, forming a book-like mor-phology.

(3) Hydrothermal Reaction in Ba(OH)2–(HTMA-Br)Solution

The hydrothermal reactions in the Ba(OH)2–(HTMA-Br) solu-tions were investigated to compare with those in the Ba(OH)2–(HTMA-OH) solutions. In the 0.1, 0.2, and 0.3M Ba(OH)2–(HTMA-Br) solutions, most of the layered titanate phase wastransformed to the BaTiO3 phase, and a small amount of thelayered titanate phase remained after the reaction at 1001C(Fig. 6). The amount of residual layered titanate phase decreasedwith increasing Ba(OH)2 concentration. This indicates that theBa(OH)2–(HTMA-Br) solution has a lower reactivity than theBa(OH)2–(HTMA-OH) solution for the formation of BaTiO3,because of the lower OH� concentration in the Ba(OH)2–(HTMA-Br) solution than that in the Ba(OH)2–(HTMA-OH)solution. The layered titanate phase was partially transformedto the anatase phase after reaction in the Ba21-free HTMA-Brsolution at 1001C (Fig. 6(a)), suggesting that the anatase phasecan be formed more easily in the HTMA-Br solution than in theHTMA-OH solution. The layered titanate phase was completelytransformed to the BaTiO3 phase in the Ba(OH)2–(HTMA-Br)solutions, and partially remained in the Ba21-free HTMA-Brsolution after the hydrothermal reaction at 1501C. The layeredtitanate phase completely transformed to the anatase phase afterthe hydrothermal reaction in the Ba21-free HTMA-Br solutionat 2501C.

The above results indicate that the solution reactivity forthe formation of BaTiO3 phase increases in an order ofBa(OH)2–(HTMA-OH) solution4Ba(OH)2–(HTMA-Br) solu-tion4Ba(OH)2 solution, and for the formation of the anatasephase in an order of distilled water4HTMA-Br solution4HTMA-OH solution. These facts indicate that both OH� and

HTMA1 promote the formation reaction of BaTiO3, andinhibit the formation reaction of anatase from the layeredtitanate. The promotion of the formation reaction of BaTiO3

by OH� is generally also observed in a normal hydrothermalreaction system.6,7 The promotion of the formation reaction ofBaTiO3 by HTMA1, however, has been found for the first time .This may be because of the intercalation reaction of HTMA1

with the layered titanate precursor. Since the intercalation ofHTMA1 increases the interlayer space of the layered titanate,Ba21 in the solution can migrate easily into the crystal bulkof the layered titanate through the interlayer space, and thenreact with the layered titanate in its crystal bulk to form BaTiO3.Since the HTMA1-form layered titanate is more stable thanthe H1-form layered titanate under hydrothermal conditions,the intercalation of HTMA1 inhibits the transformation reac-tion from the layered titanate to anatase under the Ba21-freeconditions.

The anatase and BaTiO3 particles obtained by the hydrother-mal reaction in Ba(OH)2–(HTMA-Br) solutions almost retainedthe plate-like particle morphology of the layered titanate pre-cursor (Fig. 7). The BaTiO3 particles were book like, with cracksalong basal plane of the plate-like particles. The spherical par-ticle was not observed in the products even under stirring con-ditions. The above results suggest that except HTMA1 and

0 10 20 30 40 50 60

(a)

(b)

(c)

(d)

2θ (°)

Fig. 6. X-ray diffraction patterns of products obtained by hydrother-mal treatment of H1.07Ti1.73O4 in the Ba(OH)2–(HTMA-Br) solutionscontaining (a) 0M, (b) 0.1M, (c) 0.2M, and (d) 0.3M Ba(OH)2, respec-tively, at 1001C for 1 day under stirring conditions. & , &, layered ti-tanate phases; J, BaTiO3 phase; n, anatase phase.

Fig. 7. Scanning electron microscopy photographs of products ob-tained by hydrothermal treatment of H1.07Ti1.73O4 in the Ba(OH)2–(HTMA-Br) solutions containing (a) 0M and (b, c) 0.3M Ba(OH)2, re-spectively, at 2501C for 1 day under stirring conditions. (a) Anatasephase; (b, c) BaTiO3 phase.

1418 Journal of the American Ceramic Society—Feng et al. Vol. 88, No. 6

stirring, a high alkaline concentration is also necessary for theformation of the spherical BaTiO3 particles.

(4) Crystal-Axis Orientation of Book-Like BaTiO3 Particles

The book-like BaTiO3 particles prepared here show crystal-axisorientation properties. Figure 8 gives XRD patterns of thebook-like particles, which were spread on a silica glass slide insuch a way that the book-like particles were preferred oriented,with the basal plane parallel to the plane of the glass slide. Theoriented book-like BaTiO3 particles show a relatively higher in-tensity for the (110) diffraction peak than that measured by us-ing the normal method, indicating that the book-like BaTiO3

particles show a crystal-axis orientation along the [110] directionthat is vertical to the basal plane of the book-like particle. Theorientation direction is the same as that of the plate-like BaTiO3

particle prepared by the hydrothermal treatment of plate-likeHTO particles in Ba(OH)2 solution.

33 The orientation propertiessuggest that an in situ topotactic structural transformation oc-curs in the formation process of the BaTiO3. The relative inten-sity of (110) diffraction decreased with increasing Ba(OH)2concentration in the reaction solution. This suggests that a dis-solution–deposition reaction also occurs in the formation proc-ess of BaTiO3 under the hydrothermal conditions, whichincreased with increasing alkaline concentration.

(5) Reaction Mechanism for the Formation of BaTiO3

The above results suggest that the morphology of BaTiO3 par-ticles is sensitive to the preparation conditions. We propose areaction model, as shown in Fig. 9, for the formation of BaTiO3

under the hydrothermal conditions. In the solutions containingHTMA1, H3O

1 in HTO were exchanged with HTMA1, andthe basal spacing of the layered titanate was expanded from0.923 to 2.98 and 3.32 nm (Fig. 1). When Ba(OH)2 was added inthe reaction system, HTMA1 in the interlayer space of the lay-ered titanate were exchanged with Ba21, and the basal spacingwas shrunk to 0.905 nm. The basal spacing expanding–shrinkingcaused the formation of cracks along the plane parallel to theplate-like particles, leading to the formation of the book-likeparticles after the ion-exchange reactions.

Under hydrothermal conditions, the layered structure istransformed to the BaTiO3 structure by two simultaneous reac-tions.33 One is the in situ topotactic transformation reaction, in

which Ba21 migrate into the crystal bulk of layered titanatethrough the interlayer space, and then react with the layered ti-tanate to form BaTiO3 by an in situ structural transformationreaction. Another is the dissolution–deposition reaction on thesurface of the particles, similar to the normal hydrothermal re-action. BaTiO3 formed by the topotactic transformation reac-tion retains the morphology of the precursor, and shows thecrystal-axis orientation properties. However, BaTiO3 formed bythe dissolution–deposition reaction changes the morphology ofthe precursor, and does not show the crystal-axis orientationproperties.

The formation of the book-like BaTiO3 particles in Ba(OH)2–(HTMA-Br) solutions or in the Ba(OH)2–(HTMA-OH) solu-tions containing a low concentration of Ba(OH)2 is because ofthe fact that BaTiO3 is formed mainly by the topotactic trans-formation reaction that retains the book-like particle morphol-ogy of the ion-exchanged layered titanate. Since the solubilityof Ti(IV) increases in high alkaline solution, the fraction ofthe dissolution–deposition reaction increases in the Ba(OH)2–(HTMA-OH) solutions containing a high concentration ofBa(OH)2, which causes damage to the book-like morphology.Under the stirring conditions, the book-like particles are brokendown into small plate-like particles, and the small plate-likeparticles assemble together to form the spherical particles. Sincethe experimental results indicate that the high alkaline solution,HTMA1, and stirring conditions are necessary for the forma-tion of the spherical particles, we think HTMA1 adsorbed onthe surface of the small plate-like particles increase the affinitybetween these particles in the assembling process. The affinitywill also be affected by the concentration of Ba(OH)2. The ag-gregates of the small particles assume the spherical particle mor-phology by the stirring operation.

IV. Conclusions

The intercalation reaction of HTMA1 promotes the transfor-mation reaction from the layered titanates to BaTiO3, while itinhibits the transformation reaction to anatase under the hy-drothermal conditions. The cubic BaTiO3 phase is obtained inthe low temperature range below 1501C, while the tetragonal

0 10 20 30 40 50 60

(a)

(b)

(c)

(d)

(100

)

(110

)

(110

)

(200

)

(210

)

(211

)

2θ (°)

Fig. 8. X-ray diffraction patterns of book-like particles oriented on sil-ica glass slide. The samples were prepared by hydrothermal treatment ofH1.07Ti1.73O4 in the Ba(OH)2–(HTMA-Br) solutions containing (a) 0M,(b) 0.1M, (c) 0.2M, and (d) 0.3M Ba(OH)2, respectively, at 1501C for 1day under stirring conditions. & , &, layered titanate phases; J, BaT-iO3 phase; n, anatase phase.

Stirring

Destruction

HTMA+

Ion-Exchange Ion-Exchange

Ba

H Ti OLayered Structure

(Plate-Like)

BaTiO3(Book-Like)

BaTiO3 (Spherical)

HydrothermalReaction

StructuralTransformation

:HTMA+

Layered Structure(Plate-Like)

:Ba

Layered Structure(Book-Like)

Stirring

Assembling

BaTiO3(Small Plate-Like)

Fig. 9. A model for particle morphology change in the formation proc-ess of BaTiO3 from the layered titanate in the presence of the cationicsurfactant.

June 2005 Synthesis and Particle Morphology Control of BaTiO3 1419

BaTiO3 phase is obtained in the high-temperature range over2501C.

The morphology of BaTiO3 particles dramatically changeswith changing reaction conditions. HTMA1 play an importantrole in changing the particle morphology in the hydrothermalsoft chemical process. The book-like BaTiO3 particles show acrystal-axis orientation along the [110] direction. The hydro-thermal soft chemical process is useful for the preparation ofBaTiO3 with a particular morphology.

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