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High resolution transmission electron microscope observation of α-TiCl3
Applied Surface Science 214 (2003) 272–277
High resolution transmission electron microscope observation of a-TiCl3 Tomohiro Higuchia, Boping Liua, Hisayuki Nakatanib, Nobuo Otsukaa, Minoru Teranoa,* a
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan b Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan Received 23 August 2002; received in revised form 17 February 2003; accepted 7 March 2003
Abstract High resolution transmission electron microscopy (HRTEM) was used to observe the structure of a-TiCl3 in order to investigate the nature of active sites in Ziegler–Natta catalyst. The crystalline structures of the a-TiCl3 surface were directly observed in the HRTEM image and the electron diffraction pattern. The ordered arrangement of the crystal lattices in a-TiCl3 particles was clearly revealed. The rapid deterioration of the crystalline structure of the a-TiCl3 due to the exposure in ambient conditions was also observed on the atomic scale, reflecting its ultra-high sensitivity to the impurities in the air. # 2003 Elsevier Science B.V. All rights reserved. PACS: 82.65.J Keywords: Ziegler–Natta catalyst; HRTEM; a-TiCl3
1. Introduction Ziegler–Natta catalysts are extensively used to polymerize ethylene and propylene for the production of various homo- and co-polymers. The elucidation of many unanswered questions existing in olefin polymerization with these catalysts, such as the states of the titanium species and its correlation to the catalyst performance, is still an intensive research target both from the industrial and academic aspects. Particularly, unsatisfactory point is the poor understanding of the *
Corresponding author. Tel.: þ81-761-51-1620; fax: þ81-761-51-1625. E-mail address: [email protected] (M. Terano).
surface features of the catalysts as well as the existing states and the atomic structure of the active sites in the catalysts, because of the experimental difficulty encountered in such studies. One of the main reasons for the uncertainty is that the majority of the experimental information available in the literature comes mainly from the detailed analysis of the structure of the polymer product. The difficulty of direct observation of the titanium species in the catalysts is due to the complexity of the constitution, low content of the active sites in the catalysts, high sensitivity to oxygen and moisture in the ambient conditions, etc. Electron microscope studies of Ziegler–Natta catalysts have been carried out by several research groups [1–10]. The objectives of these studies were mainly
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00517-8
T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277
the characterization of the resulting polymer, aiming at elucidation of the process during olefin polymerization on the catalysts. Indirect techniques, such as carbon replica, gold sputtering, staining techniques, were also used for the observation of the polymer samples, but the spatial resolution of these techniques was insufficient for the atomic-scale investigation of the active sites in Ziegler–Natta catalysts. In our previous study, it was demonstrated that the high resolution transmission electron microscope (HRTEM) technique was capable of direct observation of real space images of the atomic structures of MgCl2-supported high performance Ziegler–Natta catalysts [11,12]. These results suggest that the distribution of MgCl2 crystalline regions sensitively depends on the catalyst preparation methods and, hence, the catalyst performance. However, atomic structures of the active sites including the location of surface active titanium species has not been clarified yet. Due to the complexity of the industrial supported catalyst system derived from the complicated interactions between the multicomponents including support and electron donors. Thus, model catalyst system with more simple and well-defined surface structure has to be considered. Recently, new understanding in Ziegler–Natta catalysis had been achieved by surface science approaches using various planar model catalyst systems with well-defined surface structures by Somorjai and coworkers [13–15]. Here, we choose TiCl3 Ziegler–Natta catalyst. The non-supported TiCl3 catalyst without donor compound has been known to be the first generation Ziegler– Natta catalyst which can produce polypropylene with different level of isotacticity depending on the crystalline forms of the TiCl3 [16–20], similar to the newest generation MgCl2-supported donor-contained counterparts. For this reason the TiCl3 with well-defined crystalline structure can be simple but adequate target for the direct observation of Ziegler–Natta catalysts, which has been preliminarily demonstrated in our recent reports based on Raman spectroscopy [21,22]. In this study, TiCl3 produced by electrolysis of TiCl4 was observed by HRTEM to analyze the catalyst surfaces and its crystalline structures. The direct observation of deterioration of highly reactive and moisture sensitive Ziegler–Natta catalysts by HRTEM is also presented in this paper.
273
2. Experimental part 2.1. Sample and X-ray diffraction (XRD) analysis TiCl3 (donated by Toho Titanium Co. Ltd., prepared by the electrolysis of TiCl4 with metal-Ti) was used after removing large particles. TiCl3 used as a HRTEM specimen was ground using a mortar with a pestle at room temperature in a nitrogen atmosphere to reduce the sample size and thickness. The XRD analysis was performed using a Philips MRD diffractometer with the Cu Ka radiation. The sample was placed on the Si(1 1 1) wafer and covered by a mayler film in the polyethylene glove bag under a nitrogen atomsphere. AA-type TiCl3 (d-TiCl30.33AlCl3, donated by Toho Titanium Co. Ltd.) was employed as a scavenger for the traces of moisture existing in the glove bag. 2.2. HRTEM analysis HRTEM observation was performed using a Hitachi H-9000NAR operated at an accelerating voltage of 300 kVaccording to the procedure reported previously [11,12]. The spherical aberration coefficient of the objective lens is 0.7 mm which gives rise to a point resolution of 0.18 nm at the Scherzer focus. Sample damages caused by electron beam radiation during the HRTEM observation is a major problem for nonconducting samples such as Ziegler–Natta catalysts which mainly consists of metallic chloride compounds. HRTEM images of thin and small specimens were quickly recorded in this study in order to minimize sample damage. Low magnification images of the samples were also obtained by Hitachi H-7000 TEM operated at an accelerating voltage of 100 kV. All TEM images were recorded on particles which protruded over openings in the carbon microgrids.
3. Results and discussion Ziegler–Natta catalyst is known to be very sensitive to moisture and oxygen in the atmosphere. The reactions of the catalyst with these impurities proceed resulting in the loss of the activity for olefin polymerization. The structure of active sites in the catalyst seems to change quickly by these deactivation reactions. In this study, all samples are treated strictly
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T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277
Fig. 1. X-ray diffraction pattern of the TiCl3 produced by electrolysis of TiCl4. The labels ‘‘mayler’’ indicate that the labeled peak is from the material for the sample holder.
under nitrogen in a polyethylene glove bag to prevent the sample from interaction with moisture and oxygen. The sample introduction chamber of HRTEM was covered by the glove bag filled with nitrogen and AA-type TiCl3 as a moisture scavenger. a-Form crystalline structure of the TiCl3 used in this study was confirmed by XRD analysis. As shown in Fig. 1, six reflection lines were identified with the aphase crystal whose structure belongs to the space group R 3 with lattice parameters being a ¼ 0:612 nm, c ¼ 1:75 nm [22,23]. In the ordered structure Titanium atoms occupy the (a) positions with the z parameter being 0.33, while chlorine atoms occupy the (c) positions with the x, y and z parameters being 0.025, 0.33 and 0.58, respectively. A low-magnification TEM image of a a-TiCl3 particle, observed at 100 kV, is shown in Fig. 2a. Irregular shape particles and polyhedron particles with various sizes were observed in the low-magnification images. The ratio of the number of irregular shape particles to that of polyhedron particles was ca. 90%. Fig. 2b shows an electron diffraction pattern with the incident beam direction parallel to the [0 0 1] axis of the hexagonal structure of the a-TiCl3. It was obtained with a selected area aperture in order to include only the a-TiCl3 particle shown in Fig. 2a. In the diffraction pattern shown in Fig. 2b, the reflection spots marked with 1 1 0, 1 2 0 and 2 1 0 were ascribed to the lattice planes with the spacing of ca. 0.2 nm. On the other
Fig. 2. (a) A bright field image and (b) an electron diffraction pattern of the a-TiCl3 particle. The reflection spots marked by arrows indicate the superstructure reflection spots of the ordered structure of the a-TiCl3.
hand, the reflection spots marked by arrows near the direct spot marked with 0 0 0 correspond to a larger lattice spacing ca. 0.6 nm than the former spacing. Hence, the reflection spots marked by arrows are considered to result from a superstructure having lattice planes with the three times spacing of the (1 1 0) plane. This was also confirmed by the HRTEM observations as explained below. The radius of the objective aperture used for HRTEM observations is greater than the distance of the 1 1 0 spot from the direct spot, but
T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277
the lattice spacing of (1 1 0) planes is close to the resolution limit of the microscope and, hence, is hardly visible in the observed image. As shown in Fig. 3a, the HRTEM image of the interior region of the a-TiCl3 particle clearly demonstrated that the observed spacing of lattice fringes is approximately 0.6 nm. The electron diffraction patterns and HRTEM images suggest that
Fig. 3. (a) A HRTEM image of an interior region of the a-TiCl3 particle, (b) a schematic projection of the atomic arrangement of the a-TiCl3 crystal (0 0 1): open and solid circles are Cl and Ti atoms, respectively, and (c) a calculated HRTEM image of the a-TiCl3 crystal. The incident beam direction is [0 0 1].
275
the TiCl3 used in this study forms an ordered superstructure pattern with three times the periodicity in the [1 1 0] directions of the a-TiCl3 crystal structure. Therefore, reflection spots marked by arrows in Fig. 2b near the direct spots are reflection of the ordered superstructure which are considered too weak to be observed in a XRD pattern. A specific structure model, in which the arrangement of Ti and Cl atoms in the ordered superstructure of the a-TiCl3 crystal viewed in the [0 0 1] direction is schematically shown in Fig. 3b. Image simulations were also performed to further strengthen the validity of this structural model. The calculated HRTEM image of the a-TiCl3 crystal is shown in Fig. 3c. The calculation was carried out by using the multi-slice method with the same parameters applied during the observation of HRTEM images. As seen in the calculated image, clear hexagonal lattice fringes similar to the observed ones appear at the amount of defocus being from 80 to 100 nm for the sample thickness up to 6 nm. A HRTEM image of a surface region of the a-TiCl3 particle is shown in Fig. 4. Crystal lattices are clearly observed to near the surface of the a-TiCl3. The surface structure is resembling the step-terrace structure and the plane of each step is expected to be cleaved into the (0 0 1) plane. In the image, the thickness of the crystal decreases at each step and, hence, the contrast of the image changes due to the change of the thickness. The a-TiCl3 is described as staking of Ti and Cl atomic planes in the c-axis, and staking sequence is –Cl–Ti–Cl–Cl–Ti–Cl–Cl–Ti–Cl–. The
Fig. 4. A HRTEM image of a surface region of the a-TiCl3 particle. The incident beam direction is [0 0 1].
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T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277
sites produced in Ziegler–Natta catalysts after contact with Al-alkyl cocatalyst.
4. Conclusion
Fig. 5. A bright field image of the a-TiCl3 particle on exposure to air for short period (ca. 5 s).
Cl–Cl bond is most weaker than the Ti–Cl bond, and therefore, surface of terraces are terminated with Cl planes. In the next stage, the sample was allowed to stand for a short period (ca. 5 s) in air followed by TEM observation in order to clarify the sensitivity of the aTiCl3 to air. A low-magnification TEM image is shown in Fig. 5. Polyhedron particles shown in Fig. 2a cannot be observed any more. Spherical particles observed in the exposed sample (as shown in Fig. 5) might be due to the reaction between the TiCl3 and moisture in air. By HRTEM observation, it was clearly show that the destruction of the crystalline structure occurs after the exposure of the a-TiCl3 to the atmosphere even within extremely short period (ca. 5 s). In almost all the exposed particles crystalline structure cannot be observed except that region in very few particles (ca. 1%) were observed with crystalline lattice fringes surrounded by amorphous layer. Exposure of the a-TiCl3 to air even for ca. 5 s resulted in being covered with continuous amorphous layer, which is considered to be due to the destruction of the crystal lattice by contacting with moisture in air. It is confirmed that special care has to be taken to prevent the sample from moisture in air for the direct TEM observation of highly reactive and moisture sensitive Ziegler–Natta catalysts. The study on the topic concerning the surface atomic structure and morphology of the a-TiCl3 are expected to provide a good basis for the understanding of the real active
HRTEM was applied for the observation of highly reactive and moisture sensitive Ziegler–Natta catalyst like TiCl3. Information about the surface crystalline structures of the a-TiCl3 was obtained by HRTEM. Existence of the crystal lattices was confirmed in the surface of the a-TiCl3. Moreover, the deterioration with respect to the crystalline structure of the a-TiCl3 due to short exposure in the air was demonstrated at an atomic-scale indicating the ultra-high sensitivity of the a-TiCl3 to the moisture. It becomes clear that the HRTEM can provide the unique information through the direct observation of the surface atomic-scale crystalline structure of the aTiCl3.
Acknowledgements The authors are grateful to Prof. Tominaga Keii for his valuable advices for this work. Thanks are due to Toho Titanium Co. Ltd., for the support and donation to this work.
References [1] L.A.M. Rodriguez, H.M. van Looy, J. Poly. Sci.: Polym. Part A-1 4 (1996) 1971. [2] J.Y. Gutterman, J.E. Guillet, Macromolecules 1 (1968) 461. [3] J.Y. Gutterman, J.E. Guillet, Macromolecules 3 (1970) 470. [4] T. Keii, Kinetics of Ziegler–Natta Polymerization, Kodansha, Tokyo, 1972, p. 238. [5] R.T. Murry, R. Pearce, D. Platt, J. Polym. Sci.: Polym. Lett. Ed. 16 (1978) 303. [6] M. Kakugo, H. Sadatoshi, J. Sakai, M. Yokoyama, Macromolecules 22 (1989) 3172. [7] M. Kakugo, H. Sadatoshi, M. Yokoyama, K. Kojima, Macromolecules 22 (1989) 547. [8] M. Kakugo, H. Sadatoshi, J. Sakai, in: T. Keii, K. Soga (Eds.), Catalytic Olefin Polymerization, Kodansha, Elsevier, Tokyo, 1990, p. 345. [9] L. Noristi, E. Marchetti, G. Baruzzi, P. Sgarzi, J. Polym. Sci.: Polym. Chem. Ed. 32 (1994) 3047. [10] P.C. Babe, L. Noristi, G. Baruzzi, E. Marchetti, Macromol. Chem. Rapid Commun. 4 (1983) 249.
T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277 [11] H. Mori, M. Sawada, T. Higuchi, K. Hasebe, N. Otsuka, M. Terano, Macromol. Rapid Commun. 20 (1999) 245. [12] H. Mori, T. Higuchi, N. Otsuka, M. Terano, Macromol. Chem. Phys. 201 (2000) 2789. [13] E. Magni, G.A. Somorjai, Appl. Surf. Sci. 89 (1995) 187. [14] S.H. Kim, G.A. Somorjai, Appl. Surf. Sci. 161 (2000) 333. [15] S.H. Kim, G.A. Somorjai, Surf. Interf. Anal. 31 (2001) 701. [16] G. Natta, J. Polym. Sci. 34 (1959) 21. [17] G. Natta, I. Pasquon, A. Zambelli, G. Gatti, J. Polym. Sci. 51 (1961) 387. [18] F. Danusso, J. Polym. Sci., Part C 4 (1963) 1497.
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[19] H.W. Coover Jr., J. Polym. Sci., Part C 4 (1963) 1511. [20] K. Soga, H. Yanagihara, Makcromol. Chem. 189 (1988) 2839. [21] H. Miyaoka, K. Hasebe, M. Sawada, H. Sano, H. Mori, G. Mizutani, S. Ushioda, N. Otsuka, M. Terano, Vibr. Spectrosc. 17 (1998) 183. [22] H. Miyaoka, T. Kuze, H. Sano, H. Mori, G. Mizutani, S. Ushioda, N. Otsuka, M. Terano, J. Luminescence 87–89 (2000) 709. [23] G. Natta, P. Corradini, I.W. Bassi, L. Porri, Atti. Accad. Nazl. Lincei. Rend. 24 (1958) 121.
High resolution transmission electron microscope observation of a-TiCl3 Tomohiro Higuchia, Boping Liua, Hisayuki Nakatanib, Nobuo Otsukaa, Minoru Teranoa,* a
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan b Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan Received 23 August 2002; received in revised form 17 February 2003; accepted 7 March 2003
Abstract High resolution transmission electron microscopy (HRTEM) was used to observe the structure of a-TiCl3 in order to investigate the nature of active sites in Ziegler–Natta catalyst. The crystalline structures of the a-TiCl3 surface were directly observed in the HRTEM image and the electron diffraction pattern. The ordered arrangement of the crystal lattices in a-TiCl3 particles was clearly revealed. The rapid deterioration of the crystalline structure of the a-TiCl3 due to the exposure in ambient conditions was also observed on the atomic scale, reflecting its ultra-high sensitivity to the impurities in the air. # 2003 Elsevier Science B.V. All rights reserved. PACS: 82.65.J Keywords: Ziegler–Natta catalyst; HRTEM; a-TiCl3
1. Introduction Ziegler–Natta catalysts are extensively used to polymerize ethylene and propylene for the production of various homo- and co-polymers. The elucidation of many unanswered questions existing in olefin polymerization with these catalysts, such as the states of the titanium species and its correlation to the catalyst performance, is still an intensive research target both from the industrial and academic aspects. Particularly, unsatisfactory point is the poor understanding of the *
Corresponding author. Tel.: þ81-761-51-1620; fax: þ81-761-51-1625. E-mail address: [email protected] (M. Terano).
surface features of the catalysts as well as the existing states and the atomic structure of the active sites in the catalysts, because of the experimental difficulty encountered in such studies. One of the main reasons for the uncertainty is that the majority of the experimental information available in the literature comes mainly from the detailed analysis of the structure of the polymer product. The difficulty of direct observation of the titanium species in the catalysts is due to the complexity of the constitution, low content of the active sites in the catalysts, high sensitivity to oxygen and moisture in the ambient conditions, etc. Electron microscope studies of Ziegler–Natta catalysts have been carried out by several research groups [1–10]. The objectives of these studies were mainly
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00517-8
T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277
the characterization of the resulting polymer, aiming at elucidation of the process during olefin polymerization on the catalysts. Indirect techniques, such as carbon replica, gold sputtering, staining techniques, were also used for the observation of the polymer samples, but the spatial resolution of these techniques was insufficient for the atomic-scale investigation of the active sites in Ziegler–Natta catalysts. In our previous study, it was demonstrated that the high resolution transmission electron microscope (HRTEM) technique was capable of direct observation of real space images of the atomic structures of MgCl2-supported high performance Ziegler–Natta catalysts [11,12]. These results suggest that the distribution of MgCl2 crystalline regions sensitively depends on the catalyst preparation methods and, hence, the catalyst performance. However, atomic structures of the active sites including the location of surface active titanium species has not been clarified yet. Due to the complexity of the industrial supported catalyst system derived from the complicated interactions between the multicomponents including support and electron donors. Thus, model catalyst system with more simple and well-defined surface structure has to be considered. Recently, new understanding in Ziegler–Natta catalysis had been achieved by surface science approaches using various planar model catalyst systems with well-defined surface structures by Somorjai and coworkers [13–15]. Here, we choose TiCl3 Ziegler–Natta catalyst. The non-supported TiCl3 catalyst without donor compound has been known to be the first generation Ziegler– Natta catalyst which can produce polypropylene with different level of isotacticity depending on the crystalline forms of the TiCl3 [16–20], similar to the newest generation MgCl2-supported donor-contained counterparts. For this reason the TiCl3 with well-defined crystalline structure can be simple but adequate target for the direct observation of Ziegler–Natta catalysts, which has been preliminarily demonstrated in our recent reports based on Raman spectroscopy [21,22]. In this study, TiCl3 produced by electrolysis of TiCl4 was observed by HRTEM to analyze the catalyst surfaces and its crystalline structures. The direct observation of deterioration of highly reactive and moisture sensitive Ziegler–Natta catalysts by HRTEM is also presented in this paper.
273
2. Experimental part 2.1. Sample and X-ray diffraction (XRD) analysis TiCl3 (donated by Toho Titanium Co. Ltd., prepared by the electrolysis of TiCl4 with metal-Ti) was used after removing large particles. TiCl3 used as a HRTEM specimen was ground using a mortar with a pestle at room temperature in a nitrogen atmosphere to reduce the sample size and thickness. The XRD analysis was performed using a Philips MRD diffractometer with the Cu Ka radiation. The sample was placed on the Si(1 1 1) wafer and covered by a mayler film in the polyethylene glove bag under a nitrogen atomsphere. AA-type TiCl3 (d-TiCl30.33AlCl3, donated by Toho Titanium Co. Ltd.) was employed as a scavenger for the traces of moisture existing in the glove bag. 2.2. HRTEM analysis HRTEM observation was performed using a Hitachi H-9000NAR operated at an accelerating voltage of 300 kVaccording to the procedure reported previously [11,12]. The spherical aberration coefficient of the objective lens is 0.7 mm which gives rise to a point resolution of 0.18 nm at the Scherzer focus. Sample damages caused by electron beam radiation during the HRTEM observation is a major problem for nonconducting samples such as Ziegler–Natta catalysts which mainly consists of metallic chloride compounds. HRTEM images of thin and small specimens were quickly recorded in this study in order to minimize sample damage. Low magnification images of the samples were also obtained by Hitachi H-7000 TEM operated at an accelerating voltage of 100 kV. All TEM images were recorded on particles which protruded over openings in the carbon microgrids.
3. Results and discussion Ziegler–Natta catalyst is known to be very sensitive to moisture and oxygen in the atmosphere. The reactions of the catalyst with these impurities proceed resulting in the loss of the activity for olefin polymerization. The structure of active sites in the catalyst seems to change quickly by these deactivation reactions. In this study, all samples are treated strictly
274
T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277
Fig. 1. X-ray diffraction pattern of the TiCl3 produced by electrolysis of TiCl4. The labels ‘‘mayler’’ indicate that the labeled peak is from the material for the sample holder.
under nitrogen in a polyethylene glove bag to prevent the sample from interaction with moisture and oxygen. The sample introduction chamber of HRTEM was covered by the glove bag filled with nitrogen and AA-type TiCl3 as a moisture scavenger. a-Form crystalline structure of the TiCl3 used in this study was confirmed by XRD analysis. As shown in Fig. 1, six reflection lines were identified with the aphase crystal whose structure belongs to the space group R 3 with lattice parameters being a ¼ 0:612 nm, c ¼ 1:75 nm [22,23]. In the ordered structure Titanium atoms occupy the (a) positions with the z parameter being 0.33, while chlorine atoms occupy the (c) positions with the x, y and z parameters being 0.025, 0.33 and 0.58, respectively. A low-magnification TEM image of a a-TiCl3 particle, observed at 100 kV, is shown in Fig. 2a. Irregular shape particles and polyhedron particles with various sizes were observed in the low-magnification images. The ratio of the number of irregular shape particles to that of polyhedron particles was ca. 90%. Fig. 2b shows an electron diffraction pattern with the incident beam direction parallel to the [0 0 1] axis of the hexagonal structure of the a-TiCl3. It was obtained with a selected area aperture in order to include only the a-TiCl3 particle shown in Fig. 2a. In the diffraction pattern shown in Fig. 2b, the reflection spots marked with 1 1 0, 1 2 0 and 2 1 0 were ascribed to the lattice planes with the spacing of ca. 0.2 nm. On the other
Fig. 2. (a) A bright field image and (b) an electron diffraction pattern of the a-TiCl3 particle. The reflection spots marked by arrows indicate the superstructure reflection spots of the ordered structure of the a-TiCl3.
hand, the reflection spots marked by arrows near the direct spot marked with 0 0 0 correspond to a larger lattice spacing ca. 0.6 nm than the former spacing. Hence, the reflection spots marked by arrows are considered to result from a superstructure having lattice planes with the three times spacing of the (1 1 0) plane. This was also confirmed by the HRTEM observations as explained below. The radius of the objective aperture used for HRTEM observations is greater than the distance of the 1 1 0 spot from the direct spot, but
T. Higuchi et al. / Applied Surface Science 214 (2003) 272–277
the lattice spacing of (1 1 0) planes is close to the resolution limit of the microscope and, hence, is hardly visible in the observed image. As shown in Fig. 3a, the HRTEM image of the interior region of the a-TiCl3 particle clearly demonstrated that the observed spacing of lattice fringes is approximately 0.6 nm. The electron diffraction patterns and HRTEM images suggest that
Fig. 3. (a) A HRTEM image of an interior region of the a-TiCl3 particle, (b) a schematic projection of the atomic arrangement of the a-TiCl3 crystal (0 0 1): open and solid circles are Cl and Ti atoms, respectively, and (c) a calculated HRTEM image of the a-TiCl3 crystal. The incident beam direction is [0 0 1].
275
the TiCl3 used in this study forms an ordered superstructure pattern with three times the periodicity in the [1 1 0] directions of the a-TiCl3 crystal structure. Therefore, reflection spots marked by arrows in Fig. 2b near the direct spots are reflection of the ordered superstructure which are considered too weak to be observed in a XRD pattern. A specific structure model, in which the arrangement of Ti and Cl atoms in the ordered superstructure of the a-TiCl3 crystal viewed in the [0 0 1] direction is schematically shown in Fig. 3b. Image simulations were also performed to further strengthen the validity of this structural model. The calculated HRTEM image of the a-TiCl3 crystal is shown in Fig. 3c. The calculation was carried out by using the multi-slice method with the same parameters applied during the observation of HRTEM images. As seen in the calculated image, clear hexagonal lattice fringes similar to the observed ones appear at the amount of defocus being from 80 to 100 nm for the sample thickness up to 6 nm. A HRTEM image of a surface region of the a-TiCl3 particle is shown in Fig. 4. Crystal lattices are clearly observed to near the surface of the a-TiCl3. The surface structure is resembling the step-terrace structure and the plane of each step is expected to be cleaved into the (0 0 1) plane. In the image, the thickness of the crystal decreases at each step and, hence, the contrast of the image changes due to the change of the thickness. The a-TiCl3 is described as staking of Ti and Cl atomic planes in the c-axis, and staking sequence is –Cl–Ti–Cl–Cl–Ti–Cl–Cl–Ti–Cl–. The
Fig. 4. A HRTEM image of a surface region of the a-TiCl3 particle. The incident beam direction is [0 0 1].
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sites produced in Ziegler–Natta catalysts after contact with Al-alkyl cocatalyst.
4. Conclusion
Fig. 5. A bright field image of the a-TiCl3 particle on exposure to air for short period (ca. 5 s).
Cl–Cl bond is most weaker than the Ti–Cl bond, and therefore, surface of terraces are terminated with Cl planes. In the next stage, the sample was allowed to stand for a short period (ca. 5 s) in air followed by TEM observation in order to clarify the sensitivity of the aTiCl3 to air. A low-magnification TEM image is shown in Fig. 5. Polyhedron particles shown in Fig. 2a cannot be observed any more. Spherical particles observed in the exposed sample (as shown in Fig. 5) might be due to the reaction between the TiCl3 and moisture in air. By HRTEM observation, it was clearly show that the destruction of the crystalline structure occurs after the exposure of the a-TiCl3 to the atmosphere even within extremely short period (ca. 5 s). In almost all the exposed particles crystalline structure cannot be observed except that region in very few particles (ca. 1%) were observed with crystalline lattice fringes surrounded by amorphous layer. Exposure of the a-TiCl3 to air even for ca. 5 s resulted in being covered with continuous amorphous layer, which is considered to be due to the destruction of the crystal lattice by contacting with moisture in air. It is confirmed that special care has to be taken to prevent the sample from moisture in air for the direct TEM observation of highly reactive and moisture sensitive Ziegler–Natta catalysts. The study on the topic concerning the surface atomic structure and morphology of the a-TiCl3 are expected to provide a good basis for the understanding of the real active
HRTEM was applied for the observation of highly reactive and moisture sensitive Ziegler–Natta catalyst like TiCl3. Information about the surface crystalline structures of the a-TiCl3 was obtained by HRTEM. Existence of the crystal lattices was confirmed in the surface of the a-TiCl3. Moreover, the deterioration with respect to the crystalline structure of the a-TiCl3 due to short exposure in the air was demonstrated at an atomic-scale indicating the ultra-high sensitivity of the a-TiCl3 to the moisture. It becomes clear that the HRTEM can provide the unique information through the direct observation of the surface atomic-scale crystalline structure of the aTiCl3.
Acknowledgements The authors are grateful to Prof. Tominaga Keii for his valuable advices for this work. Thanks are due to Toho Titanium Co. Ltd., for the support and donation to this work.
References [1] L.A.M. Rodriguez, H.M. van Looy, J. Poly. Sci.: Polym. Part A-1 4 (1996) 1971. [2] J.Y. Gutterman, J.E. Guillet, Macromolecules 1 (1968) 461. [3] J.Y. Gutterman, J.E. Guillet, Macromolecules 3 (1970) 470. [4] T. Keii, Kinetics of Ziegler–Natta Polymerization, Kodansha, Tokyo, 1972, p. 238. [5] R.T. Murry, R. Pearce, D. Platt, J. Polym. Sci.: Polym. Lett. Ed. 16 (1978) 303. [6] M. Kakugo, H. Sadatoshi, J. Sakai, M. Yokoyama, Macromolecules 22 (1989) 3172. [7] M. Kakugo, H. Sadatoshi, M. Yokoyama, K. Kojima, Macromolecules 22 (1989) 547. [8] M. Kakugo, H. Sadatoshi, J. Sakai, in: T. Keii, K. Soga (Eds.), Catalytic Olefin Polymerization, Kodansha, Elsevier, Tokyo, 1990, p. 345. [9] L. Noristi, E. Marchetti, G. Baruzzi, P. Sgarzi, J. Polym. Sci.: Polym. Chem. Ed. 32 (1994) 3047. [10] P.C. Babe, L. Noristi, G. Baruzzi, E. Marchetti, Macromol. Chem. Rapid Commun. 4 (1983) 249.
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