Journal of Colloid and Interface Science 297 (2006) 578–583www.elsevier.com/locate/jcis

Nanofibers of barium strontium titanate (BST) by sol–gel processingand electrospinning

Santi Maensiri a,!, Wiwat Nuansing a, Jutharatana Klinkaewnarong a, Paveena Laokul a,Jinda Khemprasit b

a Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailandb Department of Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand

Received 21 June 2005; accepted 1 November 2005

Available online 5 December 2005

Abstract

This paper describes the fabrication of barium strontium titanate (Ba0.6Sr0.4TiO3 or BST) nanofibers by electrospinning method using a solutionthat contained poly(vinylpyrrolidone) and a sol–gel solution of BST. The as-spun and calcined BST/PVP composite nanofibers were characterizedby TG-DTA, X-ray diffraction, FT-IR, SEM and TEM, respectively. After calcination of the as-spun BST/PVP composite nanofibers at above700 "C in air for 2 h, BST nanofibers of 188 ± 25 nm in diameter having well-developed cubic-perovskite structure were successfully obtained.The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. Calcination at below 700 "C resulted inamorphous phase whereas BST with second phase such as barium titanate were formed at above 700 "C. Diameters of the nanofibers decreasedfrom 208 ± 35 to 161 ± 18 nm with increasing calcination temperature between 600 and 800 "C.! 2005 Elsevier Inc. All rights reserved.

Keywords: Barium strontium titanate; Ferroelectrics; Perovskite; Nanofibers; Sol–gel processing; Electrospinning; Transmission electron microscopy; X-raydiffraction

1. Introduction

Barium strontium titanate (i.e., Ba1#xSrxTiO3 or BST) is awell-known ferroelectric material having a ferroelectric transi-tion temperature that can be tuned by varying the Ba/Sr ratio.Solid solutions with x = 0.2 to 0.5 are normally used to shiftthe transition temperature to, or just below, room temperature.These materials have unique combination of large dielectricconstant, high tenability, low dc leakage, low loss tangent, andstable operation at high temperature [1–3]. Thus, BST bulk ce-ramics and thin films have been considered to be importantmaterials for tunable microwave devices such as microwavetunable phase shifters, tunable filters, and high-Q resonatorsfor radar and communication applications [4–16].

Due to their interesting properties such as a highly crys-tal axis orientation of the grains and a high thermal stability,

* Corresponding author. Fax: +66 43 202374.E-mail address: sanmae@kku.ac.th (S. Maensiri).

ferroelectric fibers have gained considerable interest for theirpotential use as functional ceramic fibers for reinforcing ceram-ics and metals [17,18]. Various ceramic fibers such as bariumtitanate (BT) [17,19], lead titanate (PT) [20], lead zirconatetitanate (PZT) [21], La-doped PZT [22] and (Mn, Sm)-dopedPT [23] have been recently fabricated. However, there are onlya few reports on the preparation of barium strontium titanatefibers. Garcia et al. [24] reported a growth of Ba0.9Sr0.1TiO3single crystal fibers with 10 mm in length and 0.8 mm in diam-eter using a laser heated pedestal growth (LHPG) technique. Luet al. [25] synthesized polycrystalline Ba1#xSrxTiO3 (x = 0.1,0.2) fibers with a diameter of 6–10 µm by a sol–gel process.Most of the ferroelectric fibers mentioned above have dimen-sion in microscale or even larger. Nanofiberization of functionalceramics will expand their technological application in manyareas including nanocomposites, nanosensors, nanoelectronicsand photonics.

Electrospinning represents a simple and convenient methodfor preparing polymer fibers and ceramic fibers with both solidand hollow interiors that are exceptionally long, uniform in

0021-9797/$ – see front matter ! 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2005.11.005

S. Maensiri et al. / Journal of Colloid and Interface Science 297 (2006) 578–583 579

diameter ranging from tens of nanometers to several microm-eters, and diversified in compositions [26–29]. Although somenovel metal oxide nanofibers, e.g., titania (TiO2) [30], vana-dium pentoxide (V2O5) [31], and zinc oxide [32], and ox-ide ceramic compounds, e.g., nickel cobaltite (NiCo2O4) [33],magnesium titanate (MgTiO3) [34], lithium manganese oxide(LiMn2O4) [35], PZT (Pb(Zr0.52Ti0.48O3)) [36] and sodiumcobalt oxide (NaCo2O4) [37], have been successfully preparedby electrospinning process followed by calcination at high tem-perature, to our knowledge there is no previous report on thefabrication of barium strontium titanate nanofibers by electro-spinning.

This paper reports for the first time on the fabrication of bar-ium strontium titanate (Ba0.6Sr0.4TiO3) nanofibers by electro-spinning using a solution that contained poly(vinylpyrrolidone)and a sol–gel solution of BST. The structure and morphologyof electrospun BST nanofibers were investigated.

2. Experimental

Titanium(diisopropoxide) bis(2,4-pentanedionate) 75 wt%in 2-propanol abbreviated TIAA (Acros Organics, 99%), bar-ium acetate (Aldrich, 99%), strontium acetate (Aldrich, 99%),1,3-propanediol (Acros, 98%), poly(vinylpyrrolidone) (PVP)(Mn = 1,300,000, Aldrich), acetic acid (BDH, 100%) andethanol (BDH, 100%) were used as the starting chemicals. Inthe preparation of the solution for electrospinning, we used asolution that contained poly(vinylpyrrolidone) and sol–gel so-lution of Ba0.6Sr0.4TiO3. A PVP/ethanol solution was preparedusing a ratio of 0.45 g PVP to 7.5 ml ethanol. In the preparationof the Ba0.6Sr0.4TiO3 solution, barium acetate and strontiumacetate were refluxed separately with 1,3-propanediol and asmall amount of distilled water to provide barium precursor andstrontium precursor, respectively. TIAA was then added to themixture of barium precursor and strontium precursor, followedby refluxing the mixture at 80–90 "C for 4 h, to obtain the cleargolden yellowish sol–gel solution. This sol–gel solution (8 ml)was then dissolved in the prepared PVP/ethanol solution. Themixed solution was vigorously stirred at room temperature untilit became a homogeneous polymer solution.

The electrospinning process was performed using the BST/polymer solution prepared as discussed above. The solution wasloaded into a plastic syringe equipped with a 23-gauge needlemade of stainless steel. The needle was connected to a high-voltage supply (DEL High Voltage (0–100 kV), DEL Electron-ics Corp., USA). The solution was fed at a rate of 0.5 ml/husing a syringe pump (TERUMO Terufusion Syringe pumpTE-331, Japan). A piece of flat aluminum foil was placed 12 cmbelow the tip of the needle, and used to collect the nanofibers.The voltage for electrospinning was 12.5 kV. All electrospin-ning processes were carried out at room temperature.

The as-spun BST/PVP composite nanofibers were sub-jected to thermogravimetric-differential thermal analysis (TG-DTA) using Pyris Diamond TG/DTA (Perkin–Elmer Instru-ment, USA). This was done to determine the temperatures ofpossible decomposition and crystallization (or phase changes)of the as-spun nanofibers. The analyses were performed with

a heating rate of 15 "C/min in static air up to 1000 "C. Thecomposite nanofibers were calcined at 600–800 "C for 2 h inair in box furnace (Lenton Furnaces, UK), using heating andcooling rates of 5 "C/min. The as-spun and calcined nanofiberswere characterized by means of X-ray diffraction (XRD) usingCuKa radiation with ! = 0.15418 nm (PW3710 mpd control,The Netherlands), FT-IR spectroscopy (Spectrum One FT-IRSpectrometer, Perkin–Elmer Instruments, USA), scanning elec-tron microscopy (SEM) (LEO VP1450, UK) and transmissionelectron microscopy (TEM) (JEOL TEM 2010, Japan).

A measurement of $100 randomly selected fibers takenfrom SEM micrographs was used to determine average fiberdiameter and distribution.

3. Results and discussion

The results of simultaneous TG and DTA analyses of theas-spun BST/PVP composite nanofibers are shown in Fig. 1.With increasing temperature, significant weight loss was ob-served below 350 "C, which indicates the loss of moisture andtrapped solvent (water, ethanol and carbon dioxide) in the as-spun nanofibers. The DTA curve depicted three exothermicpeaks at $330, 380 and 580 "C. Exothermic peaks at $330 and380 "C were due to the decomposition of TIAA along with thedegradation of PVP by a dehydration on the polymer side chain,which was confirmed by a dramatic weight loss in TG curveat the corresponding temperature range (300–400 "C). Anotherexothermic peak at $580 "C possibly corresponded to the de-composition of main chain of PVP and the formation of metaloxide phase of perovskite BST [11,38].

Fig. 2 shows the XRD patterns of BST/PVP compositenanofibers calcined at different temperature. By the comparisonof the peaks to those of the powder diffraction pattern of BSTin the JCPDS card No. 34-0411 and Refs. [4,7,9,13,14,16], the(100), (110), (111), (200), (210) and (211) peaks confirmed theformation of cubic-perovskite BST structure in the nanofiberscalcined at 700 and 800 "C, although peaks of cubic bariumtitanate phase (i.e., BT(200), JCPDS card No. 05-0626) and or-thorhombic barium carbonate (JCPDS card No. 05-0378), andanother peak of unknown phase (labeled as +) were observedin the nanofibers calcined at 800 "C. The carbonate contamina-tion is a regular feature of acetate synthesis and the presence of

Fig. 1. TG-DTA curves of the thermal decomposition of as-spun BST/PVPcomposite nanofibers at a heating rate of 15 "C in static air.

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Fig. 2. The XRD patterns of electrospun BST/PVP composite nanofibers cal-cined in air for 2 h at different temperatures. (BST, barium strontium titanate;BT, barium titanate; BC, barium carbonate; +, unknown phase.)

Fig. 3. The FT-IR spectra of electrospun BST/PVP composite nanofibers cal-cined in air for 2 h at different temperatures.

barium carbonate phase observed in the nanofibers calcined at800 "C reveals the fact that the carbonate decomposes abovethis temperature [25]. When calcined at temperatures below700 "C, the nanofibers were amorphous in nature, and no dif-fraction pattern was seen.

The formation of BST nanofibers was further supportedby FT-IR spectra as shown in Fig. 3. The FT-IR spectra ofthe nanofibers calcined at 600, 700 and 800 "C showed mainbands at around 3420, 2960, 1420 and 570 cm#1 correspond-ing to O–H, C–H, O–H, and Ti–O stretching vibrations, re-spectively [10,14,38,39]. The intensities of the O–H stretch-ing vibration bands (at 3420 and 1420 cm#1) became weakeras calcination temperature increased. This corresponded to themuch lower content of organics in the BST/PVP compositenanofibers. A broad band at about 570 cm#1, which is typicalof Ti–O vibrations in BST started to develop after calcinationat 600 "C, while the band become sharper and narrower aftercalcination at 700 "C. After calcinations at 800 "C, the band isseparated into two bands at 1420 and 570 cm#1, and this maybe due to the formation of metal oxide bonds as suggested inthe XRD results. Note the bands at around 850 cm#1 are dueto formation of carboxylate during oxidative decomposition ofthe alkoxide radicals [39].

The morphology of the as-spun and calcined BST/PVP com-posite nanofibers was revealed by SEM. Fig. 4 shows the SEMmicrographs with the respective diameter histograms of the as-

prepared and calcined TiO2/PVP composite nanofibers. The as-spun composite nanofibers appeared quite smooth due to theamorphous nature of BST/PVP composite (Fig. 4a). Each indi-vidual nanofiber was quite uniform in cross section, and theaverage diameter of the fibers was 298 ± 45 nm. The PVPwas selectively removed by calcinations the as-spun compos-ite nanofibers in air at above 600 "C. After calcination, thenanofibers remained as continuous structures (Figs. 4b–4d).Their diameter appeared to be decreased from 208 ± 35 to161 ± 18 nm with increasing calcination temperature between600 and 800 "C. The reduction in size of the nanofibers couldbe attributed to the loss of PVP from the nanofibers and thecrystallization of BST. After calcinations at above 600 "C, thenanofibers consisted of the structure of packed particles or crys-tallites (Figs. 4c, 4d), of which their sizes can be estimated fromX-ray line broadening of (111) peak using Scherrer’s equa-tion [40] (i.e., D = 0.9!/(" cos #), where ! is the wavelengthof the X-ray radiation (0.154056 nm), K is a constant takenas 0.9, # is the diffraction angle, and " is the full width at halfmaximum (FWHM)). The particle sizes were obtained to be 72and 36 nm for the BST/PVP composite nanofibers calcined at700 and 800 "C, respectively. These changes in the morphologyare related to a dramatic change in crystal structure as observedin electrospun vanadium pentoxide [31].

The detailed morphology and crystalline structure of thecalcined BST/PVP composite nanofibers were further investi-gated by transmission electron microscopy (TEM). Figs. 5a–5cshow TEM bright field images with corresponding selected-area electron diffractions (SAED) of the BST/PVP compositenanofibers calcined in air for 2 h at different temperatures. Thediameter of nanofibers calcined at 600 "C was $200–300 nm(Fig. 5a). The diffraction patterns of these nanofibers showedthe central halo and faint diffused rings, revealing that it isessentially amorphous in nature. In contrast, the image of thenanofibers calcined at 700 "C shows that each fiber containssmall BST particles of $60 nm in diameter. The diameter ofthe nanofibers was referred to 180–250 nm (Fig. 5b). The corre-sponding SAED pattern of these nanofibers shows spotty ringsthat can be indexed to highly crystalline cubic-perovskite phaseof BST. The diffraction rings are identified as the (110), (111),(200), (211), and (220) planes. This concurs with the results ofXRD in Fig. 2. For calcination at 800 "C, the crystallinity of thenanofibers was improved, while the diameter of the nanofibersdecreased to <200 nm (Fig. 5c). The TEM image indicates thatthe fibers consist of BST grains of less than 40 nm in diameter.This agrees well with the particle size estimated by XRD linebroadening. The smaller grain size in the nanofibers calcinedat 800 "C may be due to suppression of the grain growth by asecond phase of barium carbonate formed in the sample. Thecorresponding SAED pattern of these nanofibers shows alsospotty rings which their d-values corresponded to the (100),(110), (111), (200), (210), (211), (220) and (310) planes of thecrystalline cubic-perovskite phase of BST.

Since our BST nanofibers are based on nanoparticle net-work, they should have mechanical and electronic proper-ties different from micrometer-size grains and micrometer-size fibers. It is therefore of particular important to look

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Fig. 4. SEM micrographs with the histogram (left) showing the size distribution of electrospun BST/PVP composite nanofibers. (a) As-spun nanofibers. (b), (c), and(d) are BST/PVP composite nanofibers calcined in air for 2 h at 600, 700, and 800 "C, respectively.

into the nanomechanical and also electronic behavior of theBST nanofibers to explore their possible applications suchas nanocomposites and nanoelectronics. Both mechanical andelectronic properties of these nanofibers are under investigationand will be reported in the near future.

4. Conclusion

Nanofibers of BST have been successfully fabricated usingan electrospinning and sol–gel techniques. Polycrystalline BST

nanofibers (diameter of $160–200 nm) as confirmed by XRD,FT-IR and SAED are formed after calcination of the as-spunBST/PVP composite nanofibers in air at above 700 "C for 2 h.The nanofibers consisted of the structure of packed particles orcrystallites of $40–60 nm, as revealed by TEM. The crystalstructure and morphology of the nanofibers were influenced bythe calcination temperature. We believe the electrospun BSTnanofibers could have potential in some new applications asferroelectric BST fibers for nanocomposites and as electronicmaterial for nanodevices.

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Fig. 5. TEM bright-field images with corresponding selected-area electron diffraction (SAED) of electrospun BST nanofibers. (a), (b), and (c) are BST/PVPcomposite nanofibers calcined in air for 2 h at 600, 700, and 800 "C, respectively.

Acknowledgments

The authors thank the Faculty of Science Electron Mi-croscopy Unit for providing SEM facilities, and the Science andTechnology Service Center (Chiang Mai University) for provid-ing TEM facilities. This work is financially supported by theResearch Division, Khon Kaen University, Thailand (ProjectNo. 028/47).

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