Materials Science and Engineering B 110 (2004) 11–17

Formation mechanisms of SrTiO3 nanoparticlesunder hydrothermal conditions

Shicheng Zhanga,∗, Jiaxiang Liub, Yuexin Hanc, Bingchen Chenc, Xingguo Lia

a College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR Chinab The Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education,

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR Chinac School of Resource and Civil Engineering, Northeastern University, Shenyang 110004, PR China

Received 5 November 2003; accepted 21 January 2004

Abstract

Formation mechanisms of SrTiO3 particles from TiO2·nH2O gel under hydrothermal condition have been investigated. It was found thatthe mechanisms conformed to a new model, i.e. dissolution–crystallization–aggregative growth–recrystallization mechanism model. Thedissolution of TiO2·nH2O gel was a slow process and controlled the whole reaction. The crystallinity of TiO2·nH2O gel was an importantfactor, influencing the solubility of TiO2·nH2O gel, the formation mechanism of the SrTiO3 nucleus, and the stability of the primary particlesof aggregative growth. The particle size of hydrothermally synthesized SrTiO3 could also be controlled by modifying the crystallinity of theTiO2·nH2O gel; under certain condition, nanoparticles of SrTiO3 could be synthesized hydrothermally.© 2004 Elsevier B.V. All rights reserved.

Keywords: SrTiO3 particles; Titanium oxide; Chemical synthesis; Transmission electron microscopy; X-ray diffraction

1. Introduction

Strontium titanate (SrTiO3) has been extensively studiedand widely used as a ceramic material for electronic appli-cations[1–5]. It is usually synthesized by solid-state reac-tion of strontium carbonate and titanium dioxide, typicallyat temperatures >900◦C. The lack of control over the phys-ical and/or chemical characteristics of commercial stron-tium titanate powders results in micro-structural variationsthat lead to poor electrical property optimization and repro-ducibility. The hydrothermal method seems to be promisingdue to the ability to control chemical homogeneity, purity,morphology, shape and phase composition of the powdersunder moderate conditions[6–8].

There have been many investigations concerning thehydrothermal synthesis of strontium titanate, generally fo-cusing on processing parameters[9–13]. For preparationof high quality particles, it is essential to understand theformation mechanisms for hydrothermal synthesis. Such

∗ Corresponding author. Present address: Analysis Center, Departmentof Chemistry, Tsinghua University, Beijing 100084, PR China.Tel.: +86-10-6277-6886.

E-mail address: s.c.zhang@263.net (S. Zhang).

studies on mechanisms for hydrothermal synthesis of par-ticles mostly focused on BaTiO3 [14,15], PbTiO3 [16] andZrO2 [17,18]. The proposed mechanisms were commonly(1) in situ transformation mechanism and (2) dissolution–precipitation mechanism. In contrast, we have investigatedthe crystallization process of SrTiO3 from amorphousTiO2·nH2O under hydrothermal conditions, and found thatthe crystallization process of SrTiO3 particles includesthree stages: nucleation, coagulation and recrystallization[19]. In another work, we have found that the crystallinityof TiO2·nH2O had a great influence on the particle sizeof SrTiO3 [20]. In this paper, we investigated the crystal-lization process of SrTiO3 from crystallized TiO2·nH2Ounder hydrothermal conditions. Combined with the previ-ous results, we put forward a new mechanism model for thehydrothermal synthesis of SrTiO3 from TiO2·nH2O gel andhave indicated strategies for the control of particle sizes.

2. Experimental procedures

For the preparation of the precursor solution, TiCl4 andSrCl2·6H2O (analytical reagents grade, AR) were used asthe starting materials. The pure TiCl4 was dissolved in cold

0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.mseb.2004.01.017

12 S. Zhang et al. / Materials Science and Engineering B 110 (2004) 11–17

distilled water to form a TiOCl2 solution, in which the Ticoncentration was 0.5 mol l−1. While stirring, a KOH so-lution (4 mol l−1) was added dropwise to the TiOCl2 solu-tion in which the molar ratio of Ti/KOH was 1:4 yieldingTiO2·nH2O gel. After the TiO2·nH2O gel was crystallizedby stirring in a 80◦C water bath for 2 h, a SrCl2 solution(2.5 mol l−1) and a KOH solution (4 mol l−1) was added,forming the precursor solution, with the concentration of Ti0.25 mol l−1, Sr 0.25 mol l−1 and KOH 1.5 mol l−1.

The precursor solution, with an initial temperature of20◦C, was heated in an open autoclave to 60◦C over aperiod of about 40 min, during which time small sampleswere extracted by pipette at 20, 40, and 60◦C. The solwas then kept at 60◦C for 370 min, with small samplesextracted with a pipette after 20, 40, 70, 120, 240, and370 min. A drop of each of these samples was transferredto a small specimen tube containing ethanol to dispersethe sol particles. A carbon coated transmission electron mi-croscopy (TEM) Cu grid was dipped into the suspension,air-dried, and stored for further use. The remaining sam-ples were filtered, washed with ethanol, and dried at roomtemperature.

Fig. 1. X-ray diffraction patterns for samples from crystallization process ((�) anatase; (�) SrCO3; (�) SrTiO3): (a) TiO2·nH2O gel; (b) 20◦C; (c)40◦C; (d) 60◦C, 0 min; (e) 60◦C, 20 min; (f) 60◦C, 40 min; (g) 60◦C, 70 min; (h) 60◦C, 120 min; (i) 60◦C, 240 min; (j) 60◦C, 370 min.

The particles on the supported carbon films were thenexamined using: a Philips EM400T to investigate theirmorphology and crystalline structure; a Philips EM420Tequipped with a EDAX9100 to analyze the chemical com-position of the particles.

The dry powder of these samples were characterized byX-ray diffraction (XRD, Rigaku D/Max-� A diffractometer)using Cu k� radiation (λ = 1.5405) and a scan rate of4◦ min−1 from 20 to 70◦ 2θ to investigate the formation ofthe SrTiO3 phase during the reaction sequence and the phasepurity of the powders.

3. Results

3.1. The phase transformation

Fig. 1 shows the XRD patterns for samples from thedifferent reaction stages. Before the reaction, there wasan anatase phase in the TiO2·nH2O gel. After adding theSrCl2, the SrCO3 phase occurred, for the Sr2+ ion couldreact with OH− to form Sr(OH)2 precipitate, and then be

S. Zhang et al. / Materials Science and Engineering B 110 (2004) 11–17 13

Fig. 2. Morphology of the samples from SrTiO3 crystallization process: (a) TiO2·nH2O gel, dark field; (b) 20◦C, dark field; (c) 40◦C; (d) 60◦C, 0 min;(e) 60◦C, 20 min; (f) 60◦C, 40 min; (h) 60◦C, 120 min; (j) 60◦C, 370 min.

transformed to SrCO3 during the washing and drying pro-cess by reacting with CO2 in the air. With the increase ofthe reaction temperature, the intensity of diffraction of theanatase phase and SrCO3 phase decreased gradually. At40◦C, the SrTiO3 phase occurred and with longer reactiontime, the SrTiO3 phase increased gradually, which showedthat the formation process was a gradual change.

3.2. Morphological evolution

Before the reaction, the TiO2·nH2O gel was network-likeand there were some small crystallites with a particle size of6 nm from dark field micrograph (Fig. 2a). With the increaseof reaction temperature, the network-like TiO2·nH2O gelreduced gradually, and the size and quantity of crystallites

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Fig. 3. Electron diffraction pattern of SrTiO3 single crystalline at 60◦C.

were enlarged. At 60◦C, there were many big particlesinlaid in the network-like TiO2·nH2O gel, with a parti-cle size of 25 nm. The particle shape was raspberry-like,near spheric or tetragonal with unfilled corners. The elec-tron diffraction pattern of a single particle showed that itwas SrTiO3 (Fig. 3). The SrTiO3 particles were dispersedwell for the coating of the network-like TiO2·nH2O gel.With an extension of thermostatic time, the content of thenetwork-like TiO2·nH2O gel reduced and subsequently thedispersibility of the particles decreased. The particle sizeincreased, and the particle shape became regular.

3.3. Compositions

X-ray energy dispersive spectroscopy (EDAX) analysisof the gel and the single SrTiO3 particle is shown inFig. 4.The horizontal axis represents the total time including theheating and thermostatic stage. All quantified EDAX results

Fig. 4. The variation of element composition in selected zone during crystallization procedure.

are quoted in atomic percent of Sr and Ti; oxygen is notquantified since it is difficult to obtain reliable results.

With the increase of reaction time, the content of Ti de-creased and the content of Sr increased. At 60◦C, SrTiO3particles occurred; the content of Ti in TiO2·nH2O gel de-creased rapidly. After 20 min hold at 60◦C, the TiO2·nH2Ogel was rare, and the content of Ti could only be detectedin a single particle. The content of Sr in a single particleincreased fast in the initial stage and reached equilibriumin about 120 min. The equilibrium content of Sr in a singleparticle was about 44 atom %, but the chemical analysis ofthe product showed that the Sr/Ti atom ratio was 0.99. Thedeviation may be caused from the system error of the equip-ment. Although the EDAX results did not have absolute ac-curacy, it was possible to use for analyzing the variationtendency of the Sr/Ti ratio in TiO2·nH2O gel and a singleparticle.

4. Discussions

4.1. Formation mechanism model of SrTiO3 particlesunder hydrothermal condition

From the TEM (Fig. 2) and EDAX (Fig. 4) results, itcan be shown that, with the extension of reaction time,the strontium penetrated into the interior of TiO2·nH2Ogel, and at the same time the TiO2·nH2O gel dissolvedgradually, which confirmed the possibility of the existenceof dissolution–precipitation mechanism. But just fromthis, it is difficult to exclude the possibility of the in situtransformation mechanism. If the SrTiO3 system underhydrothermal condition followed the in situ transforma-tion mechanism, SrTiO3 would nucleate on the TiO2·nH2Oparticles. Since almost each TiO2·nH2O particle is a po-tential nucleation site of SrTiO3, this mechanism renders it

S. Zhang et al. / Materials Science and Engineering B 110 (2004) 11–17 15

nearly impossible to grow larger SrTiO3 than TiO2·nH2Oparticles, just as in the case of BaTiO3 reported by Chun[21]. Fig. 1 showed that the particle size of product SrTiO3was about 50 nm in diameter, greater than the initialTiO2·nH2O particle size 6 nm, which confirmed that it wasimpossible to explain the formation mechanism of SrTiO3under hydrothermal condition by in situ transformationmechanism.

According to the literature[15], the growth of SrTiO3particles by the dissolution–precipitation mechanism be-longed to the classical LaMer model, i.e. after the nucle-ation, growth on the existing nuclei continued until theconcentration was reduced to the equilibrium solubility, sothat the amount of products was the same as the amountof nucleus and the growth speed of the bigger particlewas larger than that of smaller one. The experiment re-sults show that the big particle was formed almost at once

Fig. 5. The dissolution–crystallization–aggregative growth–recrystallization mechanism model.

and with the extension of reaction time, the particle sizechanged little (Fig. 2). It could not be explained by thetraditional dissolution–precipitation mechanism, but wassimilar to the aggregative growth reported in literatures[21–23]. The particles aggregated due to van der Waalsforces. This agglomeration process leads to the growth ofprimary particle clusters which exhibit a “raspberry-like”appearance[21], or some crystallographic orientation withunfilled corners which was shown inFig. 2. In later stagesof growth, the aggregates were seen to have the samecrystallographic orientation and more regular shape. Themechanism leading to the observed crystallographic align-ment is not fully understood but factors such as the di-rectionality of the Hamaker constant and inhomogeneouscharge distribution may be responsible for alignment ofthe primary particles such as that for BaTiO3 [21]. Sothe aggregative growth of SrTiO3 may be due to two

16 S. Zhang et al. / Materials Science and Engineering B 110 (2004) 11–17

possibilities. One is that the primary particles aggregateto raspberry-like aggregate, and subsequently rearrange tothe same crystallographic orientation. The other way iscoordinate aggregative growth, which means that the pri-mary particles aggregate by the crystallographic orientationdirectly.

FromFig. 2, we can also see that, although the particle sizechanged little after the formation of aggregative particles,there still was some increase of particle size and the amountof small particles reduced, which implied the existence ofOstwald recrystallization. The same results can be foundin the literature for hydrothermal synthesis of SrTiO3 fromamorphous TiO2·nH2O gel [19].

Based on the above analyses, we put forward the disso-lution–crystallization–aggregative growth–recrystallizationmechanism model (Fig. 5), which includes four stages:dissolution of TiO2·nH2O, nucleation, aggregative growth,and recrystallization of SrTiO3. For a single particle, theformation process is in sequence. For a reaction system, thedissolution of TiO2·nH2O is a gradual process, so the dif-ferent stages of SrTiO3 formation process possibly occursat one time.

4.2. Crystallization of TiO2·nH2O and controlling forparticle size of SrTiO3 particles

From Fig. 2, it is obvious that during the most crystal-lization process the TiO2·nH2O gel can be found, which im-plies the dissolution of TiO2·nH2O is a slow process andcontrols the whole reaction. In our previous work, the samephenomenon occurred for hydrothermal synthesis of SrTiO3from amorphous TiO2·nH2O gel [19]. But the SrTiO3 wasformed at a lower temperature and produced a bigger prod-uct particle size from crystallized TiO2·nH2O. Based onour new mechanism model, the main reason is the SrTiO3prefers to heterogeneous nucleation on anatase crystallitesthan homogeneous nucleation.

At the same time, the lower solubility of crystallizedTiO2·nH2O made the network of TiO2·nH2O more stableand hindered the aggregative growth of primary SrTiO3 crys-tallites, so the primary SrTiO3 crystallites grew to a biggersize by the LaMer model. With the dissolving of TiO2·nH2O,the hindering force became smaller, and the SrTiO3 crys-tallites began to grow by aggregation. The driving force ofaggregation was smaller for the bigger primary SrTiO3 crys-tallites, and subsequently the size of the final particles wassmaller. This result was similar to the aggregative growth ofSiO2 [24].

Our previous work had shown that there were two peaksin the box-chart of the particle size distribution of the prod-uct from TiO2·nH2O gel with lower crystallinity[20]. Thereason maybe was that the SrTiO3 nucleus were formedby both heterogeneous nucleation on anatase crystallitesand homogeneous nucleation, and subsequently aggre-gated to final particles with two different particle sizedistributions.

5. Conclusions

The SrTiO3 nanoparticles could be synthesized fromTiO2·nH2O gel and SrCl2 under hydrothermal condition.The hydrothermal synthesis mechanism of SrTiO3 parti-cles submitted to dissolution–crystallization–aggregativegrowth–recrystallization mechanism model. The dissolu-tion of TiO2·nH2O gel is a slow process and controlsthe whole reaction. The thermal-treatment of TiO2·nH2Ogel can make it crystallized to the anatase phase and de-crease its solubility. The SrTiO3 was easy to undergoheterogeneous nucleation on the surface of anatase crys-tallites. For the hindrance of TiO2·nH2O gel, the newformed SrTiO3 nucleus can not aggregate to each otherand form the final particles, until the TiO2·nH2O gel dis-solved to a certain degree. The stability of SrTiO3 nu-cleus were enhanced by the TiO2·nH2O gel surroundingthem. Finally the particle size of final SrTiO3 product wassmaller.

According to the above argument, the particle size of aSrTiO3 product can be controlled by changing the crys-tallinity of TiO2·nH2O gel, and nanometer sized particlescan be obtained under hydrothermal conditions.

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