- Email: [email protected]
X-ray, SEM, Raman and IR studies of Bi2W2O9 prepared by Pechini method
Vibrational Spectroscopy 53 (2010) 199–203
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
X-ray, SEM, Raman and IR studies of Bi2 W2 O9 prepared by Pechini method a,∗ a ˛ ˛ nski ´ M. Maczka , M. Ptak a , L. Kepi , P.E. Tomaszewski a , J. Hanuza b a b
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.Nr 1410, 50-950 Wrocław 2, Poland Department of Bioorganic Chemistry, Faculty of Industry and Economics, Wrocław University of Economics, ul. Komandorska 118/120, 53-345 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 17 February 2010 Accepted 17 February 2010 Available online 25 February 2010 Keywords: Bi2 W2 O9 Aurivillius phase Raman Infrared Pechini method
a b s t r a c t Formation of Bi2 W2 O9 has been monitored by X-ray powder diffraction, SEM, and Raman and IR spectra. These methods show that more stable Bi2 WO6 tungstate is formed at low temperatures. Significant amount of Bi2 W2 O9 starts to form at about 650 ◦ C and synthesis of Bi2 W2 O9 is completed after firing the material during 1 h at 850 ◦ C. The crystallites have plate-like shape with the thickness of about 60–300 nm for the samples synthesized at 800 ◦ C. Raman and IR studies revealed weak decrease of the WO6 tilting with decreasing crystallite size. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Bismuth layered compounds (Aurivillius family), of general formula (Bi2 O2 )(Am−1 Bm O3m+1 ), where A = Na, K, Ca, Sr, Ba, Pb, etc. and B = W, Ta, Nb, Ti, etc., consist of alternating perovskite-like and fluorite-like layers. The perovskite-like layer may contain 1–8 layers (m = 1–8) [1]. This family of compounds have received much attention for device applications. For instance, these compounds are important candidates for the development of ferroelectric random access memories [2–4]. They constitute also an important class of oxide anion conductors [1,5,6]. Recently, Bi2 W2 O9 was found to be a photocatalytic material for H2 and O2 evolution as well as photodegradation of organic compounds [7,8]. It was also found to be a promising material for microwave applications [9]. Similarly as the other Aurivillius compounds, Bi2 W2 O9 consists of alternating perovskite-like and fluorite-like layers. However, no voluminous A cation is present on the cubooctahedral sites of the W2 O7 slabs [10,11]. Therefore, Bi2 W2 O9 can be considered as the m = 2 member of the family (Bi2 O2 )(Bm VI O3m+1 ) of cation-deficient Aurivillius phases [10,11]. This compound crystallizes in the Pna21 structure with a = 5.440 Å, b = 5.413 Å and c = 23.740 Å [10,11]. Although its structure is orthorhombic, it can be regarded as a slight modification of tetragonal structure, I4/mmm [11]. In the present paper, we report preparation of Bi2 W2 O9 by the Pechini method. The formation process of this material has been
∗ Corresponding author. Tel.: +48 71 3435021; fax: +48 71 3441029. ˛ E-mail address: [email protected] (M. Maczka). 0924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.02.008
studied in detail by X-ray, scanning electron microscopy (SEM), infrared (IR) and Raman techniques. 2. Experimental procedure Bismuth nitrate hydrate (Bi(NO3 )3 ·5H2 O, Fluka) and ammonium metatungstate ((NH4 )6 W12 O41 , Fluka) were used as source of metallic cations. At first citric acid was dissolved in distilled water (the amount of citric acid corresponded to the 1:5 molar ratio of the citric acid and the cations). Then half of the citric acid solution was added to the bismuth nitrate and another half to ammonium metatungstate. After complete homogenization, the both solutions were mixed together. To the resulting clear solution of citric acid complexed metal ions, the appropriate amount of ethylene glycol was added (the glycol to citric acid molar ratio was 1:1), and this mixture was then heated to about 70 ◦ C on a hot plate with constant stirring. After half an hour the mixture was transferred to the dryer and kept at 80 ◦ C for 7 days. The obtained resin was then heat-treated in a platinum crucible for 2 h at 300 ◦ C temperature in order to promote decomposition of the organic material. Then the obtained powder was re-heated for 1 h at different temperatures ranging from 600 to 850 ◦ C in order to evaluate the formation of Bi2 W2 O9 . X-ray diffraction powder patterns were recorded at room temperature by using STADI-P powder diffractometer (STOE, Germany) working in the transmission geometry, equipped with a linear 140◦ -PSD detector and using Cu K␣ radiation in the 2 range from 0.008◦ to 133.988◦ with a step of 0.03◦ . Morphology of the samples was monitored using a FEI Nova NanoSEM 230 microscope. Composition of the samples was
200
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203
Fig. 1. X-ray powder diffraction patterns of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , the Pechini samples annealed at various temperatures and the mixture of the bulk Bi2 WO6 and Bi2 W2 O9 in a ratio 1:1.
checked with energy dispersive spectroscopy (EDS) standard-less analysis. EDS spectra were acquired and analysed using an EDAX Pegasus XM4 spectrometer (with SDD Apollo 40 detector) mounted on a FEI Nova NanoSEM 230 microscope. Infrared spectra were measured with a Biorad 575C FT-IR spectrometer in KBr pellet for the 1200–400 cm−1 region and in Nujol suspension for the 500–50 cm−1 region. FT-Raman spectra were measured using BRUKER 110/S spectrometer with the YAG:Nd3+ excitation. Both IR and Raman spectra were recorded with a spectral resolution of 2 cm−1 . 3. Results and discussion 3.1. X-ray diffraction and scanning electron microscopy Fig. 1 shows representative X-ray diffraction patterns of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , and the Pechini samples annealed at various temperatures. The XRD pattern of the Pechini sample annealed at 600 ◦ C shows peaks at 2 approximately 28.3◦ , 32.9◦ , 47.2◦ , 55.9◦ and 58.6◦ . The positions of these peaks agree with those observed for bulk Bi2 WO6 indicating that annealing at 600 ◦ C lead to crystallization of Aurivillius type Bi2 WO6 but Bi2 W2 O9 is not formed at this temperature. Significant broadening of diffraction lines indicates that this sample contains nanocrystalline Bi2 WO6 . This spectrum also shows a few weak peaks at 2 approximately 23.1◦ , 25.6◦ , 26.3◦ , 26.9◦ , 31.2◦ , 36.8◦ , which probably correspond to some tungsten-rich compounds. The sample annealed at 650 ◦ C also exhibits peaks characteristic for Bi2 WO6 but some additional peaks appear at 23.5◦ , 27.7◦ , 29.9◦ , 57.2◦ , 59.6◦ . Positions of these peaks are in good agreement with the peaks observed for the bulk Bi2 W2 O9 indicating that annealing at this temperature initiated formation of Aurivillius type Bi2 W2 O9 . The amount of Bi2 W2 O9 increases rapidly at the expense of Bi2 WO6
when the samples are annealed at higher temperatures and nearly pure material is obtained after annealing at 850 ◦ C. In order to estimate the concentrations of Bi2 W2 O9 and Bi2 WO6 in the synthesized samples, we have measured the XRD pattern for the mixture of these materials in a ratio 1:1 prepared from the bulk materials. Using the ratio of the integrated areas for the peaks at 2 approximately 27.7◦ and 28.3◦ , we have estimated that the concentration of Bi2 W2 O9 is about 41, 77, 89, 90 and 98% for the samples synthesized at 650, 700, 750, 800 and 850 ◦ C, respectively. Fig. 1 also shows that bandwidth of the peaks characteristic for Bi2 WO6 decreases quickly with increasing annealing temperature. This result indicates fast growth of the Bi2 WO6 crystallites with increasing temperature. As far as peaks characteristic for Bi2 W2 O9 are concerned, it is difficult to estimate the broadening of lines for the sample synthesized at 650 ◦ C since the peaks are weak. The samples synthesized at 700 and 750 ◦ C show relatively weak broadening that implies that the particle size is large. The average size of coherently diffracting crystallites perpendicular to the diffraction (h k l) plane can be calculated from the full width at half maximum (FWHM) of the diffraction peak under the Scherrer approximation: D = K/ˇ cos , where D is the mean crystallite size along the [h k l] direction, is the X-ray wavelength (in our study = 1.54056 Å), ˇ is the FWHM of the diffraction line (in radians), is the angle of diffraction, and the Scherrer constant K is conventionally set to 1.0 [12–15]. Since the actual profiles are neither purely Gauss nor purely Cauchy (Lorentz), we have used the Halder–Wagner parabolic approximate relation for the true diffraction profile: ˇ = B − b2 /B, where B and b are the measured FWHM of the equivalent diffraction lines in the specimen and the reference sample, respectively [14–16]. In our case the data from the powdered single crystal (bulk sample) is used as a reference. The calculated average size of the Bi2 W2 O9 crystallites along the [1 1 5] direction, calculated from broadening of the line 114 at 2 = 27.7◦ , is about 61 and 114 nm for the samples synthesized at 700 and 750 ◦ C, respectively. This size corresponds approximately to the thickness of the plates because the [1 1 5] direction is nearly perpendicular to the surface of the plates. For the samples synthesized at 800 and 850 ◦ C the broadening is negligible, indicating very large size of the crystallites. The morphology of the samples synthesized at 700 and 800 ◦ C was investigated by SEM, as shown in Fig. 2. SEM and EDS analysis of the sample synthesized at 700 ◦ C shows that it contains at least two phases with composition corresponding to Bi2 W2 O9 and Bi2 WO6 , in agreement with X-ray diffraction pattern. Lateral size of the Bi2 W2 O9 crystallites is about 100–400 nm. The thickness cannot be clearly observed. However, Fig. 2a allows to estimate the thickness for one plate as about 50 nm. Bi2 WO6 crystallites are much bigger (0.5–3 m) and they have poorly defined faces. The sample synthesized at 800 ◦ C contains well-crystallized plate-like particles of Bi2 W2 O9 and we could not find any Bi2 WO6 crystallites by SEM. The lateral size of these crystallites is about 0.4–2 m and their thickness 60–300 nm. 3.2. Raman and IR studies Detailed analysis of Raman and IR spectra for bulk Bi2 W2 O9 was presented in our previous paper [17]. We showed that for the room temperature Pna21 structure of Bi2 W2 O9 , which consists of alternating (Bi2 O2 )2+ and (W2 O7 )2− layers, the number of optic modes should be 38A1 + 39A2 + 38B1 + 38B2 [17]. However Raman and IR spectra of polycrystalline spectra showed smaller number of bands due to weak Davydov splitting. Our previous study allowed us to propose assignment of modes [17] and here we report this assignment in Table 1, which lists the Raman and IR modes observed for the polycrystalline (bulk) Bi2 W2 O9 sample and the Pechini sample prepared at 700 ◦ C.
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203
201
Fig. 2. SEM images of the samples synthesized at 700 (panels a and b) and 800 ◦ C (panels c and d). EDS analysis shows that the large crystallite visible in panel (a) corresponds to Bi2 WO6 and the remaining conglomerates of smaller crystallites have Bi2 W2 O9 composition.
Raman and IR spectra confirm that the sample synthesized at 600 ◦ C is composed of mainly Bi2 WO6 whereas the sample synthesized at 650 ◦ C contains already large concentration of Bi2 W2 O9 (see Figs. 3 and 4). Closer inspection of the spectra indicates that the bands of Bi2 W2 O9 exhibit some changes with decreasing firing temperature. First, bandwidth of the observed bands increases. Second, many Raman bands exhibit shift towards lower wavenumbers. For instance, the negative shift is 2 cm−1 for the 852, 323,
Fig. 3. Raman spectra of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , and the Pechini samples annealed at various temperatures.
and 309 cm−1 Raman bands and 5 cm−1 for the 143 cm−1 band (see Table 1). Third, some bands exhibit shift towards higher wavenumbers. This behaviour is especially pronounced for the IR bands at 938 and 549 cm−1 , which shift to 947 and 558 cm−1 for the sample synthesized at 700 ◦ C (see Table 1). It is well known that when particle size decreases, phonon properties are significantly affected due to a few factors such as creation of defects, distribution of crystallite size, phonon confinement effect, etc. [18,19]. Since the crystallites of Bi2 W2 O9 are relatively large, in particular in the lateral dimensions, the contribution due to phonon confinement effect is probably weak. Therefore, the observed broadening and shifts of bands towards lower wavenumbers can be most likely attributed mainly to strains and defects in the synthesized samples. However, the observed up shifts of some modes for the synthesized Bi2 W2 O9 indicates that also some other weak changes occur with decreasing particle size for this material. Our former studies of other layered Aurivillius type compounds such as Bi2 WO6 and Bi2 MoO6 indicated symmetry enhancement with decreasing particle size [20,21]. It is therefore reasonable to assume that also in case of Bi2 W2 O9 some slight decrease of the orthorhombic distortion occurs for the synthesized crystallites. In the Aurivillius family of compounds the symmetry decrease from a high symmetry prototype structure (space group symmetry I4/mmm) is related to three different displacive distortions, e.g. antiphase displacement of the Bi3+ ions and the perovskite blocks, rotations of the rigid octahedra about the pseudo-tetragonal axis and rotations about [1 1 0] direction of the tetragonal phase [11,22,23]. The two later distortions are observed at much lower
202
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203
Table 1 Raman and IR wavenumbers of polycrystalline Bi2 W2 O9 and Bi2 W2 O9 prepared by the Pechini method (at 700 ◦ C) together with the proposed assignment. S, m, w, vw and sh denote strong, medium, weak, very weak and shoulder, respectively. T denotes translational modes.
temperatures than the former one [23,24]. It was shown that for Bi2 W2 O9 the driving forces for the distortions correspond to displacement of W atoms from the center of the WO6 octahedra and to the attraction of the apical oxygen atoms (Oap ) of the W2 O7 network by bismuth atoms of the Bi2 O2 layers [11]. As a result an important tilting is observed with respect to the b-axis, slight tilting with respect to the a-axis and about 9◦ rotation around the c-axis [11]. As discussed above, our results showed strong shifts towards higher wavenumbers for the bands at 938 and 549 cm−1 . These bands were attributed to stretching motions of the W–Oap bonds and bending vibration of the WO6 octahedra involving large motions of atoms in the direction perpendicular to the layers, respectively [17]. Such vibrations are expected to be strongly affected if the tilting of the WO6 octahedra changes. We suppose, therefore, that the observed by us shifts of the 938 and 549 cm−1 bands towards higher wavenumbers can be attributed to a slight decrease of the WO6 tilting. It is worth to mention that studies of nanosized effect on structural transition in WO3 also support our conclusion that decrease of Bi2 W2 O9 crystallite size may lead to some symmetry enhancement. As emphasized by Champarnaud-Mesjard et al., the W2 O7 slabs in Bi2 W2 O9 are similar to those observed in WO3 and the octahedral network distortion is nearly as important in the monoclinic WO3 as in the Bi2 W2 O9 structure [11]. And studies of WO3 showed that the nanosized effect is very strong for this material [25]. For instance, the stability field of the tetragonal phase decreased by about 470 K for the 35 nm sample, when compared to the bulk material [25]. Structural similarities between WO3 and W2 O7 slabs in the Bi2 W2 O9 structure suggest, therefore, that also in case of Bi2 W2 O9 some symmetry enhancement can be expected even for relatively large crystallites. In Bi2 W2 O9 case this effect can be observed due to plate-like shape of the crystallites with the thickness often well below 100 nm, as evidenced from images presented in Fig. 2. 4. Conclusions X-ray diffraction, SEM, Raman and IR studies showed that very significant amount of Bi2 W2 O9 (about 41 mol%) is formed in the Pechini method already at approximately 650 ◦ C. Unfortunately, at this low annealing temperature the main phase is Bi2 WO6 . Further increase of annealing temperature promotes synthesis of Bi2 W2 O9 at the expense of Bi2 WO6 but the size of Bi2 W2 O9 crystallites also increases. Our results show, therefore, that in order to obtain a sample containing relatively small concentration of Bi2 WO6 impurity phase and simultaneously relatively small Bi2 W2 O9 crystallites, the annealing temperature should be near 750 ◦ C. The Pechini method is, however, not suitable for synthesis of relatively pure samples containing Bi2 W2 O9 nanocrystallites. Vibrational studies show that phonon properties of the Bi2 W2 O9 are very similar to the properties of the bulk material. This result can be attributed to large size of the crystallites obtained by this method. Nevertheless, Raman and IR studies suggest that there might be some weak decrease of the WO6 tilting in the synthesized Bi2 W2 O9 crystallites. Acknowledgements We would like to thank Mrs. E. Bukowska for X-ray diffraction measurements. This work was supported by Ministry of Science and Higher Education in the frame of grant No. N N209 097335. References
Fig. 4. IR spectra of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , and the Pechini samples annealed at various temperatures in the mid-IR (a) and far-IR regions (b).
[1] M.S. Islam, S. Lazure, R.N. Vannier, G. Nowogrocki, G. Mairesse, J. Mater. Chem. 8 (1998) 655. [2] X. Mao, W. Wang, X. Chen, Y. Lu, Appl. Phys. Lett. 95 (2009) 082901.
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203 [3] C.A. Paz de Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott, J.F. Scott, Nature 374 (1995) 627. [4] Y. Noguchi, K. Murata, M. Miyayama, Appl. Phys. Lett. 89 (2006) 242916. [5] K.R. Kendall, C. Navas, J.K. Thomas, H.C. zur Loye, Chem. Mater. 8 (1996) 642. [6] N. Kim, R.N. Vannier, C.P. Grey, Chem. Mater. 17 (2005) 1952. [7] A. Martinez-de la Cruz, S.O. Alfrao, L.M. Torres-Martinez, I.J. Ramirez, J. Ceram. Proc. Res. 9 (2008) 490. [8] J. Tang, J. Ye, J. Mater. Chem. 15 (2005) 4246. [9] A. Feteira, D.C. Sinclair, J. Am. Ceram. Soc. 91 (2008) 1338. [10] Y. Bando, A. Watanabe, Y. Sekikawa, M. Goto, S. Horiuchi, Acta Crystallogr. A 35 (1979) 142. [11] J.C. Champarnaud-Mesjard, B. Frit, A. Watanabe, J. Mater. Chem. 9 (1999) 1319. [12] J.I. Langford, A.J.C. Wilson, J. Appl. Crystallogr. 11 (1978) 102. [13] M. Crosa, V. Boero, M. Franchini-Angela, Clays Clay Miner. 47 (1999) 742. [14] S. Chattopadhyay, P. Ayyub, V.R. Palkar, M. Multani, Phys. Rev. B 52 (1995) 13177. [15] J.M. Amigó, F.J. Serrano, M.A. Kojdecki, J. Bastida, V. Esteve, M.M. Reventós, F. Marti, J. Eur. Ceram. Soc. 25 (1995) 1479.
203
[16] H.H. Tian, M. Atzmon, Philos. Magn. A 79 (1999) 1769. [17] M. Maczka, L. Macalik, J. Hanuza, J. Raman Spectrosc. 40 (2009) 2099. [18] J.E. Spanier, R.D. Robinson, F. Zhang, S.W. Chan, I.P. Herman, Phys. Rev. B 64 (2001) 245407. [19] A.K. Arora, M. Rajalakshmi, T.R. Ravindran, V. Sivasubramanian, J. Raman Spectrosc. 38 (2007) 604. [20] M. Maczka, L. Macalik. K. Hermanowicz, L. Kepinski, P. Tomaszewski, J. Raman Spectrosc., published on-line, doi:10.1002/jrs.2526. [21] M. Maczka, L. Macalik. K. Hermanowicz, L. Kepinski, J. Hanuza, J. Raman Spectrosc., published on-line, doi:10.1002/jrs.2568. [22] Yu.E. Kitaev, M.I. Aroyo, J.M. Perez-Mato, Phys. Rev. B 75 (2007) 64110. [23] N.A. McDowell, K.S. Knight, P. Lightfoot, Chem. Eur. J. 12 (2006) 1493. [24] J.M. Perez-Mato, M. Aroyo, A. Garcia, P. Blaha, K. Schwarz, J. Schweifer, K. Parlinski, Phys. Rev. B 70 (2004) 214111. [25] M. Boulova, G. Lucazeau, J. Solid State Chem. 167 (2002) 425.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
X-ray, SEM, Raman and IR studies of Bi2 W2 O9 prepared by Pechini method a,∗ a ˛ ˛ nski ´ M. Maczka , M. Ptak a , L. Kepi , P.E. Tomaszewski a , J. Hanuza b a b
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.Nr 1410, 50-950 Wrocław 2, Poland Department of Bioorganic Chemistry, Faculty of Industry and Economics, Wrocław University of Economics, ul. Komandorska 118/120, 53-345 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 17 February 2010 Accepted 17 February 2010 Available online 25 February 2010 Keywords: Bi2 W2 O9 Aurivillius phase Raman Infrared Pechini method
a b s t r a c t Formation of Bi2 W2 O9 has been monitored by X-ray powder diffraction, SEM, and Raman and IR spectra. These methods show that more stable Bi2 WO6 tungstate is formed at low temperatures. Significant amount of Bi2 W2 O9 starts to form at about 650 ◦ C and synthesis of Bi2 W2 O9 is completed after firing the material during 1 h at 850 ◦ C. The crystallites have plate-like shape with the thickness of about 60–300 nm for the samples synthesized at 800 ◦ C. Raman and IR studies revealed weak decrease of the WO6 tilting with decreasing crystallite size. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Bismuth layered compounds (Aurivillius family), of general formula (Bi2 O2 )(Am−1 Bm O3m+1 ), where A = Na, K, Ca, Sr, Ba, Pb, etc. and B = W, Ta, Nb, Ti, etc., consist of alternating perovskite-like and fluorite-like layers. The perovskite-like layer may contain 1–8 layers (m = 1–8) [1]. This family of compounds have received much attention for device applications. For instance, these compounds are important candidates for the development of ferroelectric random access memories [2–4]. They constitute also an important class of oxide anion conductors [1,5,6]. Recently, Bi2 W2 O9 was found to be a photocatalytic material for H2 and O2 evolution as well as photodegradation of organic compounds [7,8]. It was also found to be a promising material for microwave applications [9]. Similarly as the other Aurivillius compounds, Bi2 W2 O9 consists of alternating perovskite-like and fluorite-like layers. However, no voluminous A cation is present on the cubooctahedral sites of the W2 O7 slabs [10,11]. Therefore, Bi2 W2 O9 can be considered as the m = 2 member of the family (Bi2 O2 )(Bm VI O3m+1 ) of cation-deficient Aurivillius phases [10,11]. This compound crystallizes in the Pna21 structure with a = 5.440 Å, b = 5.413 Å and c = 23.740 Å [10,11]. Although its structure is orthorhombic, it can be regarded as a slight modification of tetragonal structure, I4/mmm [11]. In the present paper, we report preparation of Bi2 W2 O9 by the Pechini method. The formation process of this material has been
∗ Corresponding author. Tel.: +48 71 3435021; fax: +48 71 3441029. ˛ E-mail address: [email protected] (M. Maczka). 0924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.02.008
studied in detail by X-ray, scanning electron microscopy (SEM), infrared (IR) and Raman techniques. 2. Experimental procedure Bismuth nitrate hydrate (Bi(NO3 )3 ·5H2 O, Fluka) and ammonium metatungstate ((NH4 )6 W12 O41 , Fluka) were used as source of metallic cations. At first citric acid was dissolved in distilled water (the amount of citric acid corresponded to the 1:5 molar ratio of the citric acid and the cations). Then half of the citric acid solution was added to the bismuth nitrate and another half to ammonium metatungstate. After complete homogenization, the both solutions were mixed together. To the resulting clear solution of citric acid complexed metal ions, the appropriate amount of ethylene glycol was added (the glycol to citric acid molar ratio was 1:1), and this mixture was then heated to about 70 ◦ C on a hot plate with constant stirring. After half an hour the mixture was transferred to the dryer and kept at 80 ◦ C for 7 days. The obtained resin was then heat-treated in a platinum crucible for 2 h at 300 ◦ C temperature in order to promote decomposition of the organic material. Then the obtained powder was re-heated for 1 h at different temperatures ranging from 600 to 850 ◦ C in order to evaluate the formation of Bi2 W2 O9 . X-ray diffraction powder patterns were recorded at room temperature by using STADI-P powder diffractometer (STOE, Germany) working in the transmission geometry, equipped with a linear 140◦ -PSD detector and using Cu K␣ radiation in the 2 range from 0.008◦ to 133.988◦ with a step of 0.03◦ . Morphology of the samples was monitored using a FEI Nova NanoSEM 230 microscope. Composition of the samples was
200
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203
Fig. 1. X-ray powder diffraction patterns of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , the Pechini samples annealed at various temperatures and the mixture of the bulk Bi2 WO6 and Bi2 W2 O9 in a ratio 1:1.
checked with energy dispersive spectroscopy (EDS) standard-less analysis. EDS spectra were acquired and analysed using an EDAX Pegasus XM4 spectrometer (with SDD Apollo 40 detector) mounted on a FEI Nova NanoSEM 230 microscope. Infrared spectra were measured with a Biorad 575C FT-IR spectrometer in KBr pellet for the 1200–400 cm−1 region and in Nujol suspension for the 500–50 cm−1 region. FT-Raman spectra were measured using BRUKER 110/S spectrometer with the YAG:Nd3+ excitation. Both IR and Raman spectra were recorded with a spectral resolution of 2 cm−1 . 3. Results and discussion 3.1. X-ray diffraction and scanning electron microscopy Fig. 1 shows representative X-ray diffraction patterns of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , and the Pechini samples annealed at various temperatures. The XRD pattern of the Pechini sample annealed at 600 ◦ C shows peaks at 2 approximately 28.3◦ , 32.9◦ , 47.2◦ , 55.9◦ and 58.6◦ . The positions of these peaks agree with those observed for bulk Bi2 WO6 indicating that annealing at 600 ◦ C lead to crystallization of Aurivillius type Bi2 WO6 but Bi2 W2 O9 is not formed at this temperature. Significant broadening of diffraction lines indicates that this sample contains nanocrystalline Bi2 WO6 . This spectrum also shows a few weak peaks at 2 approximately 23.1◦ , 25.6◦ , 26.3◦ , 26.9◦ , 31.2◦ , 36.8◦ , which probably correspond to some tungsten-rich compounds. The sample annealed at 650 ◦ C also exhibits peaks characteristic for Bi2 WO6 but some additional peaks appear at 23.5◦ , 27.7◦ , 29.9◦ , 57.2◦ , 59.6◦ . Positions of these peaks are in good agreement with the peaks observed for the bulk Bi2 W2 O9 indicating that annealing at this temperature initiated formation of Aurivillius type Bi2 W2 O9 . The amount of Bi2 W2 O9 increases rapidly at the expense of Bi2 WO6
when the samples are annealed at higher temperatures and nearly pure material is obtained after annealing at 850 ◦ C. In order to estimate the concentrations of Bi2 W2 O9 and Bi2 WO6 in the synthesized samples, we have measured the XRD pattern for the mixture of these materials in a ratio 1:1 prepared from the bulk materials. Using the ratio of the integrated areas for the peaks at 2 approximately 27.7◦ and 28.3◦ , we have estimated that the concentration of Bi2 W2 O9 is about 41, 77, 89, 90 and 98% for the samples synthesized at 650, 700, 750, 800 and 850 ◦ C, respectively. Fig. 1 also shows that bandwidth of the peaks characteristic for Bi2 WO6 decreases quickly with increasing annealing temperature. This result indicates fast growth of the Bi2 WO6 crystallites with increasing temperature. As far as peaks characteristic for Bi2 W2 O9 are concerned, it is difficult to estimate the broadening of lines for the sample synthesized at 650 ◦ C since the peaks are weak. The samples synthesized at 700 and 750 ◦ C show relatively weak broadening that implies that the particle size is large. The average size of coherently diffracting crystallites perpendicular to the diffraction (h k l) plane can be calculated from the full width at half maximum (FWHM) of the diffraction peak under the Scherrer approximation: D = K/ˇ cos , where D is the mean crystallite size along the [h k l] direction, is the X-ray wavelength (in our study = 1.54056 Å), ˇ is the FWHM of the diffraction line (in radians), is the angle of diffraction, and the Scherrer constant K is conventionally set to 1.0 [12–15]. Since the actual profiles are neither purely Gauss nor purely Cauchy (Lorentz), we have used the Halder–Wagner parabolic approximate relation for the true diffraction profile: ˇ = B − b2 /B, where B and b are the measured FWHM of the equivalent diffraction lines in the specimen and the reference sample, respectively [14–16]. In our case the data from the powdered single crystal (bulk sample) is used as a reference. The calculated average size of the Bi2 W2 O9 crystallites along the [1 1 5] direction, calculated from broadening of the line 114 at 2 = 27.7◦ , is about 61 and 114 nm for the samples synthesized at 700 and 750 ◦ C, respectively. This size corresponds approximately to the thickness of the plates because the [1 1 5] direction is nearly perpendicular to the surface of the plates. For the samples synthesized at 800 and 850 ◦ C the broadening is negligible, indicating very large size of the crystallites. The morphology of the samples synthesized at 700 and 800 ◦ C was investigated by SEM, as shown in Fig. 2. SEM and EDS analysis of the sample synthesized at 700 ◦ C shows that it contains at least two phases with composition corresponding to Bi2 W2 O9 and Bi2 WO6 , in agreement with X-ray diffraction pattern. Lateral size of the Bi2 W2 O9 crystallites is about 100–400 nm. The thickness cannot be clearly observed. However, Fig. 2a allows to estimate the thickness for one plate as about 50 nm. Bi2 WO6 crystallites are much bigger (0.5–3 m) and they have poorly defined faces. The sample synthesized at 800 ◦ C contains well-crystallized plate-like particles of Bi2 W2 O9 and we could not find any Bi2 WO6 crystallites by SEM. The lateral size of these crystallites is about 0.4–2 m and their thickness 60–300 nm. 3.2. Raman and IR studies Detailed analysis of Raman and IR spectra for bulk Bi2 W2 O9 was presented in our previous paper [17]. We showed that for the room temperature Pna21 structure of Bi2 W2 O9 , which consists of alternating (Bi2 O2 )2+ and (W2 O7 )2− layers, the number of optic modes should be 38A1 + 39A2 + 38B1 + 38B2 [17]. However Raman and IR spectra of polycrystalline spectra showed smaller number of bands due to weak Davydov splitting. Our previous study allowed us to propose assignment of modes [17] and here we report this assignment in Table 1, which lists the Raman and IR modes observed for the polycrystalline (bulk) Bi2 W2 O9 sample and the Pechini sample prepared at 700 ◦ C.
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203
201
Fig. 2. SEM images of the samples synthesized at 700 (panels a and b) and 800 ◦ C (panels c and d). EDS analysis shows that the large crystallite visible in panel (a) corresponds to Bi2 WO6 and the remaining conglomerates of smaller crystallites have Bi2 W2 O9 composition.
Raman and IR spectra confirm that the sample synthesized at 600 ◦ C is composed of mainly Bi2 WO6 whereas the sample synthesized at 650 ◦ C contains already large concentration of Bi2 W2 O9 (see Figs. 3 and 4). Closer inspection of the spectra indicates that the bands of Bi2 W2 O9 exhibit some changes with decreasing firing temperature. First, bandwidth of the observed bands increases. Second, many Raman bands exhibit shift towards lower wavenumbers. For instance, the negative shift is 2 cm−1 for the 852, 323,
Fig. 3. Raman spectra of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , and the Pechini samples annealed at various temperatures.
and 309 cm−1 Raman bands and 5 cm−1 for the 143 cm−1 band (see Table 1). Third, some bands exhibit shift towards higher wavenumbers. This behaviour is especially pronounced for the IR bands at 938 and 549 cm−1 , which shift to 947 and 558 cm−1 for the sample synthesized at 700 ◦ C (see Table 1). It is well known that when particle size decreases, phonon properties are significantly affected due to a few factors such as creation of defects, distribution of crystallite size, phonon confinement effect, etc. [18,19]. Since the crystallites of Bi2 W2 O9 are relatively large, in particular in the lateral dimensions, the contribution due to phonon confinement effect is probably weak. Therefore, the observed broadening and shifts of bands towards lower wavenumbers can be most likely attributed mainly to strains and defects in the synthesized samples. However, the observed up shifts of some modes for the synthesized Bi2 W2 O9 indicates that also some other weak changes occur with decreasing particle size for this material. Our former studies of other layered Aurivillius type compounds such as Bi2 WO6 and Bi2 MoO6 indicated symmetry enhancement with decreasing particle size [20,21]. It is therefore reasonable to assume that also in case of Bi2 W2 O9 some slight decrease of the orthorhombic distortion occurs for the synthesized crystallites. In the Aurivillius family of compounds the symmetry decrease from a high symmetry prototype structure (space group symmetry I4/mmm) is related to three different displacive distortions, e.g. antiphase displacement of the Bi3+ ions and the perovskite blocks, rotations of the rigid octahedra about the pseudo-tetragonal axis and rotations about [1 1 0] direction of the tetragonal phase [11,22,23]. The two later distortions are observed at much lower
202
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203
Table 1 Raman and IR wavenumbers of polycrystalline Bi2 W2 O9 and Bi2 W2 O9 prepared by the Pechini method (at 700 ◦ C) together with the proposed assignment. S, m, w, vw and sh denote strong, medium, weak, very weak and shoulder, respectively. T denotes translational modes.
temperatures than the former one [23,24]. It was shown that for Bi2 W2 O9 the driving forces for the distortions correspond to displacement of W atoms from the center of the WO6 octahedra and to the attraction of the apical oxygen atoms (Oap ) of the W2 O7 network by bismuth atoms of the Bi2 O2 layers [11]. As a result an important tilting is observed with respect to the b-axis, slight tilting with respect to the a-axis and about 9◦ rotation around the c-axis [11]. As discussed above, our results showed strong shifts towards higher wavenumbers for the bands at 938 and 549 cm−1 . These bands were attributed to stretching motions of the W–Oap bonds and bending vibration of the WO6 octahedra involving large motions of atoms in the direction perpendicular to the layers, respectively [17]. Such vibrations are expected to be strongly affected if the tilting of the WO6 octahedra changes. We suppose, therefore, that the observed by us shifts of the 938 and 549 cm−1 bands towards higher wavenumbers can be attributed to a slight decrease of the WO6 tilting. It is worth to mention that studies of nanosized effect on structural transition in WO3 also support our conclusion that decrease of Bi2 W2 O9 crystallite size may lead to some symmetry enhancement. As emphasized by Champarnaud-Mesjard et al., the W2 O7 slabs in Bi2 W2 O9 are similar to those observed in WO3 and the octahedral network distortion is nearly as important in the monoclinic WO3 as in the Bi2 W2 O9 structure [11]. And studies of WO3 showed that the nanosized effect is very strong for this material [25]. For instance, the stability field of the tetragonal phase decreased by about 470 K for the 35 nm sample, when compared to the bulk material [25]. Structural similarities between WO3 and W2 O7 slabs in the Bi2 W2 O9 structure suggest, therefore, that also in case of Bi2 W2 O9 some symmetry enhancement can be expected even for relatively large crystallites. In Bi2 W2 O9 case this effect can be observed due to plate-like shape of the crystallites with the thickness often well below 100 nm, as evidenced from images presented in Fig. 2. 4. Conclusions X-ray diffraction, SEM, Raman and IR studies showed that very significant amount of Bi2 W2 O9 (about 41 mol%) is formed in the Pechini method already at approximately 650 ◦ C. Unfortunately, at this low annealing temperature the main phase is Bi2 WO6 . Further increase of annealing temperature promotes synthesis of Bi2 W2 O9 at the expense of Bi2 WO6 but the size of Bi2 W2 O9 crystallites also increases. Our results show, therefore, that in order to obtain a sample containing relatively small concentration of Bi2 WO6 impurity phase and simultaneously relatively small Bi2 W2 O9 crystallites, the annealing temperature should be near 750 ◦ C. The Pechini method is, however, not suitable for synthesis of relatively pure samples containing Bi2 W2 O9 nanocrystallites. Vibrational studies show that phonon properties of the Bi2 W2 O9 are very similar to the properties of the bulk material. This result can be attributed to large size of the crystallites obtained by this method. Nevertheless, Raman and IR studies suggest that there might be some weak decrease of the WO6 tilting in the synthesized Bi2 W2 O9 crystallites. Acknowledgements We would like to thank Mrs. E. Bukowska for X-ray diffraction measurements. This work was supported by Ministry of Science and Higher Education in the frame of grant No. N N209 097335. References
Fig. 4. IR spectra of the powdered bulk samples of Bi2 WO6 and Bi2 W2 O9 , and the Pechini samples annealed at various temperatures in the mid-IR (a) and far-IR regions (b).
[1] M.S. Islam, S. Lazure, R.N. Vannier, G. Nowogrocki, G. Mairesse, J. Mater. Chem. 8 (1998) 655. [2] X. Mao, W. Wang, X. Chen, Y. Lu, Appl. Phys. Lett. 95 (2009) 082901.
˛ M. Maczka et al. / Vibrational Spectroscopy 53 (2010) 199–203 [3] C.A. Paz de Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott, J.F. Scott, Nature 374 (1995) 627. [4] Y. Noguchi, K. Murata, M. Miyayama, Appl. Phys. Lett. 89 (2006) 242916. [5] K.R. Kendall, C. Navas, J.K. Thomas, H.C. zur Loye, Chem. Mater. 8 (1996) 642. [6] N. Kim, R.N. Vannier, C.P. Grey, Chem. Mater. 17 (2005) 1952. [7] A. Martinez-de la Cruz, S.O. Alfrao, L.M. Torres-Martinez, I.J. Ramirez, J. Ceram. Proc. Res. 9 (2008) 490. [8] J. Tang, J. Ye, J. Mater. Chem. 15 (2005) 4246. [9] A. Feteira, D.C. Sinclair, J. Am. Ceram. Soc. 91 (2008) 1338. [10] Y. Bando, A. Watanabe, Y. Sekikawa, M. Goto, S. Horiuchi, Acta Crystallogr. A 35 (1979) 142. [11] J.C. Champarnaud-Mesjard, B. Frit, A. Watanabe, J. Mater. Chem. 9 (1999) 1319. [12] J.I. Langford, A.J.C. Wilson, J. Appl. Crystallogr. 11 (1978) 102. [13] M. Crosa, V. Boero, M. Franchini-Angela, Clays Clay Miner. 47 (1999) 742. [14] S. Chattopadhyay, P. Ayyub, V.R. Palkar, M. Multani, Phys. Rev. B 52 (1995) 13177. [15] J.M. Amigó, F.J. Serrano, M.A. Kojdecki, J. Bastida, V. Esteve, M.M. Reventós, F. Marti, J. Eur. Ceram. Soc. 25 (1995) 1479.
203
[16] H.H. Tian, M. Atzmon, Philos. Magn. A 79 (1999) 1769. [17] M. Maczka, L. Macalik, J. Hanuza, J. Raman Spectrosc. 40 (2009) 2099. [18] J.E. Spanier, R.D. Robinson, F. Zhang, S.W. Chan, I.P. Herman, Phys. Rev. B 64 (2001) 245407. [19] A.K. Arora, M. Rajalakshmi, T.R. Ravindran, V. Sivasubramanian, J. Raman Spectrosc. 38 (2007) 604. [20] M. Maczka, L. Macalik. K. Hermanowicz, L. Kepinski, P. Tomaszewski, J. Raman Spectrosc., published on-line, doi:10.1002/jrs.2526. [21] M. Maczka, L. Macalik. K. Hermanowicz, L. Kepinski, J. Hanuza, J. Raman Spectrosc., published on-line, doi:10.1002/jrs.2568. [22] Yu.E. Kitaev, M.I. Aroyo, J.M. Perez-Mato, Phys. Rev. B 75 (2007) 64110. [23] N.A. McDowell, K.S. Knight, P. Lightfoot, Chem. Eur. J. 12 (2006) 1493. [24] J.M. Perez-Mato, M. Aroyo, A. Garcia, P. Blaha, K. Schwarz, J. Schweifer, K. Parlinski, Phys. Rev. B 70 (2004) 214111. [25] M. Boulova, G. Lucazeau, J. Solid State Chem. 167 (2002) 425.