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Materials Letters 61 (2

Synthesis of barium titanate nanopowder by a soft chemical process

Sushmita Ghosh, Subrata Dasgupta ⁎, Amarnath Sen, Himadri Sekhar Maiti

Central Glass and Ceramic Research Institute, Kolkata 700 032, India

Received 6 December 2005; accepted 4 May 2006Available online 26 May 2006

Abstract

A simple soft chemical method of synthesizing barium titanate nanopowders and nanorods is described here, where titanium dioxide/titaniumisopropoxide was taken as a source of titanium, tartaric acid was taken as a template material, nitric acid as an oxidizing agent. The synthesizedpowders and rods were characterized by XRD, TG and DTA, SEM and IR spectroscopy. In this process phase pure barium titanate nanopowdersand nanorods can be prepared at a temperature of 900 °C and the process is simple and cost-effective.© 2006 Elsevier B.V. All rights reserved.

Keywords: Nanorod; Solution evaporation; Template

1. Introduction

The rapid growth of the electronic component industry has ledto a demand forminiaturedmultilayer ceramic capacitor (MLCC),where BaTiO3 is used due to its superior frequency characteristics,higher reliability, high breakdown voltage, excellent volumetricefficiency of the capacitance and reduced cost [1]. MLCCs with adielectric thickness of 2 μm have already been commercializedbut the next generation components demand a thickness of 1 μm.Such requirement demands nanosized dielectric powders withuniform composition and size distribution, and weak agglomer-ation to allow low temperature sintering with minimum graingrowth. Various methods of preparation of BaTiO3 is available inthe literature. The solid state route needs high calcinationtemperature to get perovskite phase and often results in theformation of multiphase and inhomogeneous powders [2–5].High energy ball milling [6] is also reported to produce 10nmparticle size but the approach suffers from small batch size, highprocessing time and energy consumption. The complex doublemetal salts methods [7–9] involve the use of solid precursors forthe manufacture of pure BaTiO3. The process suffers from the useof costly materials, multisteps, uncontrolled particle size andinterparticle agglomeration. Pechini's autocombustion method[10] is also reported to produce 10 nm particle size where costlymaterials are used but the approach suffers from small batch

⁎ Corresponding author.E-mail address: sdasgupta@cgcri.res.in (S. Dasgupta).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.05.006

volume. Hydrothermal process [11–14] involves the incorpora-tion of costly materials and the optimization of it has often been amatter of empiricism.

In the present paper, we report the formation of phase pureBaTiO3 in the form of powders (mostly rod shaped) by a softchemical technique where the TiO2/titanium isopropoxide and Ba(NO3)2 were used as starting materials, tartaric acid as a chelatingagent and HNO3 is used as an oxidizing agent.

Fig. 1. X-ray diffraction patterns of BaTiO3, calcined at 600 °C, 700 °C, 800 °Cand 900 °C.

Fig. 2. (a). Scanning electron micrograph of powder BaTiO3 synthesized from titanium isopropoxide at 900 °C. (b). Scanning electron micrograph of powder BaTiO3

synthesized from titanium dioxide at 900 °C.

539S. Ghosh et al. / Materials Letters 61 (2007) 538–541

2. Experimental

BaTiO3 powders and nanorods were synthesized by a softchemical method where 0.25 M Ba(NO3)2 solution and 0.25 MTiO(NO3)2 solution, dissolved in 2 N nitric acid, were mixedtogether in a beaker. Tartaric acid (0.6 M) solution was thenadded to the resulting solution under constant stirring. Thesolution was then heated on a hot plate under continuous stirringcondition to its boiling temperature until all the liquidevaporated. A 7 g solid ammonium nitrate was added towardsthe end to avoid slurry formation. There was an immenseevolution of brown fumes towards the end of the reactionleaving a fluffy mass at the base of the beaker. This fine powderwas dried on a hot plate at 130 °C for 30 min. The powderobtained was calcined at 900 °C for 2 h to get phase pureBaTiO3. The yield was 90%.

The TiO(NO3)2 solution used for making of BaTiO3 powderwas prepared in two different ways. In the first method, 1.99 gof TiO2 (AR Grade), and 10 g of ammonium sulphate (ARGrade) were added to 80 mL of concentrated H2SO4 and themixture was stirred on a hot plate until clear solution wasobtained. The formed Ti-oxysulphate was then treated withammonia in cold condition. The precipitated TiO2·xH2O wasfiltered and washed free from the sulphate solution. This preci-

Fig. 3. TGA trace of green BaTiO3 powder.

pitate was then treated with cold 1:1 nitric acid to get TiO(NO3)2solution. In the second method, 8.90 g of titanium isopropoxidewas first hydrolyzed with very slow addition of dilute ammoniain ice-cold condition with vigorous stirring, as the reaction washighly exothermic. The precipitated TiO2·xH2O was thenfiltered and washed thoroughly. This was further nitrated withthe addition of 1:1 nitric acid. A change in color was observedas the white mass changed to a yellowish green transparentsolution.

The powders obtained by using TiO(NO3)2 prepared by theabove two methods were characterized using X-ray diffraction(Phillips, PW 1710) analysis using Cu–Kα radiation. Thepowder morphology was studied using SEM (Leo 430i). Thethermal studies (TGA and DTA) were carried out usingShimadzu (TGA-50) analyzer. FTIR spectra studies weremade with Nicolet Model 5PC FTIR.

3. Results and discussion

Fig. 1 shows the XRD patterns of the synthesized powder calcinedin air at different temperatures. The absence of any peak at 21.6° and

Fig. 4. DTA trace of BaTiO3 calcined at 800 °C.

Fig. 5. FTIR spectra of BaTiO3 calcined at 600 °C, 700 °C, 800 °C, 900 °C.

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26.8° indicates that no intermediate phase like Ba2Ti2O5·CO3 asreported by other workers [15] is present here. Crystalline BaTiO3 isfound to form at a temperature as low as 600 °C along with someimpurities like BaCO3 and TiO2. As the calcination temperatureincreases, the peaks become sharper and the phase pure BaTiO3 isfound to form at 900 °C, which is completely devoid of any impurityphases. The average particle size, of BaTiO3 at different temperatureswere calculated using Scherrer's formula:

D ¼ 0:9kbcosh

where D is the average grain size, λ=1.541 Å (X-ray wavelength), andβ is the width of the diffraction peak at half maximum for thediffraction angle 2θ. The average particle size was found to be around24 nm.

Fig. 2(a) and (b) shows the micrographs of the synthesized powdersformed by two different routes. The presence of considerable amountof nanorods formed by both the routes is clearly visible here. Fig. 3shows the TGA curve of the green synthesized powder, at a heating rateof 10 °C/min. The TGA curve depicts a loss of the weight 8.76% up toa temperature of 600 °C, which is due to the decomposition ofentrapped nitrates, carboxylates, water and unburnt carbon. The weightloss at around 800 °C (5.65%) is due to the decomposition of residualBaCO3. The corresponding DTA curve is shown in Fig. 4 which is inagreement with the TG analysis. The presence of BaCO3 phase is alsoconfirmed from the IR spectrum (Fig. 5) carried out with powderscalcined at different temperatures. The peaks at 1440 cm−1, 852 cm−1,692 cm−1 of the IR spectrum (Fig. 5) corresponds to BaCO3 phase [16].The presence of BaCO3 phase is also seen from the XRD trace ofpowders calcined at different temperatures. The absence of any BaCO3

at a temperature of 900 °C in both IR spectrum and XRD shows thecompletion of reaction. The peak at 2350 cm−1 of the IR spectrum isdue to ambient CO2, and peak at 1575 cm−1 is due to asymmetriccarbonate (COO−) [17]. The peaks in the range of 400–600 cm−1 isdifficult to predict, but the presence of Ti–O bonding around 440 cm−1

for TiO2 is observed in both calcined and uncalcined samples [18]. Thepresence of TiO2 phase is also seen from the XRD data of powderscalcined at different temperatures. The gradual decomposition ofBaCO3 and subsequent formation of phase pure BaTiO3 is clearly seenfrom the IR spectra and XRD traces in samples calcined at differenttemperatures.

Different authors report the uniqueness of tartaric acid as acomplexing agent for titanium and barium. Dhage et al. [19] used

tartaric acid and TiOCl2 to form anatase ultrafine nanopowder of TiO2,which on calcination at higher temperature is converted to rutile form.Yu et al. [20] also prepared spherical nanosized TiO2 by hydrolyzingand calcining a mixture of titanium isopropoxide with tartaric acid. TheSharpless epoxidation [21] gives a dimeric mechanism where titaniumtartarate complex exists in the dimeric form with two Ti–O–Ti bridges.In the present case Ba-tartarate and Ti-tartarate formed undergo in situdecomposition, which occurs towards the end of the reaction whennitric acid concentration is high enough to oxidize the precursorsresulting in the formation of NO2, CO2 and water vapour along withdiscrete nanoparticles BaO and TiO2, which leads to formation of theproduct at a higher temperature. Small amounts of BaCO3 and TiO2

also form (as depicted from IR and XRD) which at a higher temperaturecombine together resulting in the formation of phase pure BaTiO3 [22].Synthesis of well isolated nanostructures of [23] ferroelectric oxideshave been rather scanty. The exact mechanism of formation of nanorodis not known in the present case.

4. Conclusion

The present study describes a simple low temperaturemethod of synthesis of barium titanate nanopowders andnanorods by solution–evaporation method. Barium titanatecan be formed at a low temperature (900 °C) through tartarateprecursor method. Further, the oxidation of tartarate precursorby HNO3 was accompanied by the evolution of CO2, NO2 andwater vapour and the gas evolution helped the product to resultin a fine-grained structure. Tartaric acid and HNO3 present inthe solution play the key role for the synthesis of shaped bariumtitanate at a low temperature.

Acknowledgement

The financial support of the CSIR Networked project CMM0022 is gratefully acknowledged.

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