Sol-Gel Synthesis of Titanate Phases from Aurivillius And

Journal of the UniversityA. Yoleva, of ChemicalS. Djambazov, Technology Y. Ivanova, and E.Metallurgy, Kashchieva 46, 3, 2011, 255-260

SOL-GEL SYNTHESIS OF TITANATE PHASES

FROM AURIVILLIUS AND SILLENITE TYPE (Bi4Ti3O12 AND Bi12TiO20) A.Yoleva, S. Djambazov, Y. Ivanova, E. Kashchieva

University of Chemical Technology and Metallurgy Received 25 May 2011 8 Kliment Ohridsky, 1756 Sofia, Bulgaria Accepted 12 July 2011 E-mail: [email protected]

ABSTRACT

We studied the application of the sol-gel method for synthesis of powders from the titanate phases Bi Ti O and 4 3 12 Bi TiO on the basis of hydrolysis-condensation reactions in acetic and nitric acid medium. The phase formation was 12 20 traced during the heat treatment in temperature range from 25 to 600oC. The used precursors for the synthesis were bismuth nitrate Bi(NO ) .5H O and titanium butoxide Ti(OBu) . Two different routes of sol-gel synthesis were realized 3 3 2 4 according to the used solvents for Bi(NO ) .5H O - acetic or nitric acid. The results of X-ray diffraction analysis show that 3 2 2 the phase formation mechanism is similar for both routes of synthesis. Complete crystallization takes place at 600oCwith main crystalline phases corresponding to the stoichiometrical compounds Bi Ti O or Bi TiO . These data are con- 4 3 12 12 20 firmed by IR spectroscopy and scanning electron microscopy. Conclusions are drawn that the proposed options for sol- gel synthesis are appropriate routes for obtaining the phases Bi Ti O and Bi Ti O at lower temperature (600oC) as 4 3 12 4 3 12 compared to other methods. Keywords: hydrolysis-condensation reactions, sol-gel, phase formation, Bi Ti O Bi TiO . 4 3 12, 12 20

interesting phase in the system Bi O -TiO is the sillenite INTRODUCTION 2 3 2 phase Bi TiO . Its practical application in photocatalys 12 20 The continuously increasing interest towards the is due to the high electro optical coefficient, low optical bismuth-titanate ceramics is due to their specific pi- activity and high optical sensitivity in the visible range ezoelectric and electro-optical properties. These mate- [4-6]. In order to find new effective technical applica- rials possess layered structures, described for the first tions, it is established that the properties of the titanate time by Aurivillius. They are build by alternate phases are strongly influenced by the method of their perovskite units of the type (Ìe R O ), where R = production. In many publications discussions are made m-1 m 3m-1 Ti4+, Nb5+, Ta5+ and others, were Me are univalent, biva- on the low and high temperature synthesis of similar lent or three valent ions, or their mixture with bismuth phases using solid state reactions, hot pressing, hydro- oxygen layers of the type (Bi O )2+ [1 - 3]. One of the thermal synthesis, decomposing of metalorganic com- 2 2 main characteristics of the Aurivillius family phases is pounds, precipitation of oxalate solutions, the method that they keep their ferroelectric properties up to rela- of super cooled melts and others. [7 - 11] Recently spe- tively high temperatures. For example, in the case of the cial attention is paid on the sol-gel technologies for phase Bi Ti O , this temperature is 675oC, while the lead- producing ceramic powders and thin films. Among the 4 3 12 zirconium titanate is ferroelectric up to 380oC. The other advantages of these materials are their nanosizes and

255 Journal of the University of Chemical Technology and Metallurgy, 46, 3, 2011 respectively higher grain surface, leading to increased ture, is the main reason for some deviations in the di- chemical activity, lower temperature of synthesis and electric properties. improved specific properties [12-13]. From scientific and practical points of view, it is In a very informative review concerning the low important to perform comparative studies on the appli- temperature sol-gel methods for producing powders of cation of the sol-gel method for synthesis of powders titanate phases with perovskite and sellenite structure from titanium phases with composition Bi Ti O and 4 3 12 the attention is attracted to the main technological ad- Bi TiO on the basis of hydrolysis-condensation reac- 12 20 vantages of these methods for sintering and solid state tions in acetic and nitric acid medium and to follow the chemical interaction at temperatures lower that in other phase formation during the heat treatment in the tem- techniques. In on other publication, related to the sol- perature range from 25 to 600oC. gel synthesis of bismuth titanates, the influence of the different factors (type of the used precursors, ðÍ of the EXPERIMENTAL medium, firing temperature, heating rate and others) on the composition and properties of the final product Table 1 and Table 2 present the experimented is discussed. Thin films of Bi Ti O , based on hydroly- stoichiometrical compositions. The used precursors are 4 3 12 sis-condensation reactions between bismuth nitrate and bismuth nitrate Bi(NO ) .5H O and titanium butoxide 3 3 2 titanium isopropoxide, and dissolved respectively in Ti(OBu) . In order to achieve homogeneous mixtures, 4 acetic acid and methoxyetanol, were obtained by Joschi the titanium ions are connected in stable complexes with and Desu [12]. It is proved that up to 350oC the layer is acetylacetate (ÅtAcAc) in ratio 1:3. Two options are amorphous, and after additional treatment at 700oC the applied in relation to the bismuth nitrate: 1) dissolving phase Bi Ti O crystallizes. The ferroelectric proper- in CH COOH and H O in ratio 1:1,6 and 1) dissolving 4 3 12 3 2 ties are influenced by the rate of the secondary thermal in a mixture of concentrate HNO and H O in ratio 1:1. 3 2 treatment. Asimilar technology of synthesis is reported The procedure of gel production according to these two by Du and Chen [13 - 15]. They used precursors are options is shown in Figs. 1 and 2. After gelling and Bi(C H COO) and Ti(OC H ) (C H COO) and the drying at 60oC for 8 hours, the obtained powders are 7 15 3 2 5 2 7 15 3 solvent is p-xylene (C H4CH CH ). A textured poly crys- thermally treated at 250oC, 330oC and 600oC with 2 hours 6 3 3 talline film with crystal orientation to crystallographic retaining at every mentioned temperature. The phase and axe ñ is obtained. Monophase finely dispersed structural changes in the gels are followed by X-ray dif- Bi Ti O powder is produced by Bi(NO ) , dissolved in fraction (XRD - Bruker Diffractometer, D8 Advance, 4 3 12 3 3 glacial acetic acid and Ti(OC H ) , stabilized in Cu-K radiation), IR spectroscopy (Bruker IFS 25 spec- 4 9 4 α acetylacetone after treatment of the gel up to 650oC. It trometer), scanning electron microscopy (SEM - 525M, is established that the change of the crystallographic Philips) and energy dispersive spectroscopy (EDS - cell parameters depending on the treatment tempera- EDAX 9900).

Table 1. Composition 1 2Bi O .3TiO [Bi Ti O ]. 2 3 2 4 3 12

Oxides Mol % Mass % Batch Bi(NO3)3 .5H2O Ti(OC4H9)4 10 g g ml

Bi2O3 40 80 8 14,52 - TiO2 60 20 2 - 8,51

Table 2. Composition 2 6Bi O .TiO [Bi TiO ]. 2 3 2 12 20

Oxides Mol % Mass % Batch Bi(NO3)3 .5H2O Ti(OC4H9)4 10 g g ml

Bi2O3 85,8 97 9,7 17,61 - TiO2 14,2 3 0,3 - 1,28

256 A. Yoleva, S. Djambazov, Y. Ivanova, E. Kashchieva

Fig. 1. Scheme of sol-gel procedure according variant 1.

Fig. 2. Scheme of sol-gel procedure according variant 1’.

RESULTS AND DISCUSSION obtained by the second variant of the synthesis. Decom- position of the nitrate and crystallization of Bi O (tet- 2 3 Figs. 3 and 4 show the phase formation during ragonal) take place in both cases at 330oC but still a the thermal treatment in the two types of gels (1 and 1) significant amount of amorphous phase exists. At maxi- with composition Bi Ti O . It should be pointed out mal thermal treatment temperature (600oC) in \both 4 3 12 that up to 250oC characteristic diffraction maxima of cases (1 and 1) full crystallization is present, the prod- the precursor bismuth nitrate are still observed and the ucts are mono phase and contain the stoichiometrical amount of the amorphous phase is more in the sample compound Bi Ti O as a main crystalline phase. A 4 3 12 257 Journal of the University of Chemical Technology and Metallurgy, 46, 3, 2011

ones in the spectrum at 600oC which corresponds to the already made conclusion for the complete mono phase crystallization at this temperature. The SEM mi- crographs, coupled with EDS data for the local composition of the crystals in different points of the sample, are given in Fig. 6. The EDS data (mass %) also prove the presence of the stoichiometrical phase Bi Ti O 4 3 12 (Table 1). At 600oC the crystallization process is fin- ished, the crystals are with sizes between 0.5 and 5 ìm. Concerning the phase formation of gels with composition Bi TiO (Fig. 7), obtained by the two variants 12 20 of synthesis, the XRD data establish a similar of the discussed mechanism. The only difference is that the

Fig. 3. XRD patterns of composition 1 prepared by sol-gel (variant 1), thermal treated at 330oC and 600oC.

Fig. 5. IR spectrum of composition 1 prepared by sol-gel Fig. 4. XRD patterns of composition 1 prepared by sol-gel (variant 1), thermal treated at 600oC. (variant 1’), thermal treated at 330 oC and 600oC. conclusion an be made that independently of the used dissolvent of the precursor bismuth nitrate, the mechanism of the phase formation is one and the same. The XRD results are confirmed by the IR spec- tra (Fig. 5). In the spectrum at 60oC, besides the characteristic bands of the OH groups and of the water (around 3434 and 1630 cm-1) there exist other bands around 1380 cm-1 (NO groups) and 1760, 1040 - 1020, 740, 3 510 cm-1 assigned to the C=O bonds in the acetate complexes [COO-]. The intensity of the mentioned bands strongly decreases at lower temperature (330oC), but the decomposition is not yet completed. It is interesting to pay attention to the bands in the ranges 810-820 -1 -1 cm and 570 - 580 cm related to the crystallization of Fig. 6. SEM and EDS of composition 1 prepared by sol-gel the phase Bi Ti O [16,17]. These bands are the main o 4 3 12 (variant 1), thermal treated at 600 C.

258 A. Yoleva, S. Djambazov, Y. Ivanova, E. Kashchieva

Fig. 7. XRD patterns of composition 2 prepared by sol-gel (variant 1), thermal treated at 330oC and 600oC. Fig. 9. SEM and EDS of composition 2 prepared by sol-gel (variant 1), thermal treated at 600oC.

CONCLUSIONS

Homogeneous powders are obtained in the bi- nary system Bi O -TiO corresponding to the stoichio- 2 3 2 metrical phases of Aurivillius and sillenite type (Bi Ti O 4 3 12 and Bi TiO ) on the basis of hydrolysis-condensation 12 20 reactions between bismuth nitrate and titanium butoxide in acetic and nitric acid medium. By means of XRD, IR spectroscopy, SEM and EDS it is proved that the monophase crystalline products from the stoichiometrical phases Bi Ti O and Bi TiO are obtained at lower 4 3 12 12 20 temperature (600oC) in comparison to other routes of their synthesis. Fig. 8. IR spectrum of composition 2 prepared by sol-gel (variant 1), thermal treated at 600oC. REFERENCES crystallization of the phase Bi TiO , initiated at 330oC, 1. J. Zhua, X. Chen, J. He, J. Shen, Raman scattering 12 20 o investigations on lanthanum-doped Bi Ti O finishes at 600 C and the final product is mono phase. 4 3 12 SrBi Ti O intergrowth ferroelectrics, J. Solid State These suggestions are in accordance with the IR spec- 4 4 15 troscopy and SEM data. The IR spectrum of the sample Chemistry, 178, 9, 2005, 2832-2837. treated at 600oC (Fig. 8) corresponds to the complete 2. S. Fu, Ozoe H., Reaction pathways in the synthesis of photoreactive gamma Bi MO (M=Si, Ge or Ti), J. crystallization and mono phase product. The fourth main 12 20 intensive bands at 457, 525, 586 and 663 cm-1 are as- Amer. Cer. Soc., 80, 1997, 2501. signed to the formation of sillenite phases of the type 3. W. Yao, H. Wang, X. Xu, X. Cheng, J. Huang, S. Bi ÌO , where Ì = Ge, Si, Ti [2,16]. The SEM mi- Shang, X. Yang, M. Wang, Photocatalytic property 12 20 crograph (Fig. 9) show a high crystallization ability with of bismuth titanate crystals, Appl. Catal. A., 243, clearly formed cubic shape of the crystals and strong ag- 2003, 185. gregation tendency. The EDS analysis confirms the pres- 4. I. Vasconcelos, M. Pimenta, A. Sombra, Optical prop- ence of the stoichiometrical phase Bi TiO (Table 2). erties of Bi SiO (BSO) and Bi TiO (BTO) ob- 12 20 12 20 12 20

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