Received: January 16, 2001 Final version: July 3, 2001
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[1] S. Ijima, Nature 1991, 354, 54. [2] L. D. Zhang, G. W. Meng, F. Phillipp, Mater. Sci. Eng. A 2000, 286, 34. [3] G. S. Cheng, S. H. Chen, X. G. Zhu, Y. Q. Mao, L. D. Zhang, Mater. Sci. Eng. A 2000, 286, 165. [4] D. W. Wang, A. J. Millis, S. D. Sarma, Phys. Rev. Lett. 2000, 85, 4570. [5] M. Q. He, I. Minus, P. Z. Zhou, N. Mohammed, Appl. Phys. Lett. 2000, 77, 3731. [6] T. Tenne, L. Margulis, M. Genet, G. Hodes, Nature 1992, 360, 444. [7] L. Marguis, R. Tenne, S. Ijima, Microsc. Microanal. Microstruct. 1996, 7, 87. [8] D. H. Galvan, R. Rangel, G. Alonso, Fullerene Sci. Technol. 1998, 6, 1025. [9] Y. Feldman, E. Wasserman, D. J. Srolovitz, R. Tenne, Science 1995, 267, 222. L. Margulis, G. Salitra, R. Tenne, M. Talianker, Nature 1993, 365, 113. [10] M. Terrones, W. K. Hsu, H. Terrones, A. K. Cheetham, H. W. Kroto, D. R. M. Walton, Chem. Phys. Lett. 1996, 259, 568. [11] N. G. Chopra, R. J. Luyken, K. Cherry, V. H. Crespi, M. L. Cohen, S. G. Louie, A. Zettl, Science 1995, 269, 966. [12] O. Stephan, P. Bernier, P. Lefin, Science 1994, 266, 1683. [13] Z. Sieh, K. Weng, N. G. Cherrey, X. Chopra, Y. Blase, A. Miyampto, M. L. Rubio, S. G. Cohen, A. Louie, R. Zettl, Phys. Rev. B 1995, 51, 11 229. [14] H. Dai, E. W. Wong, Y. Z. Liu, S. S. Fan, C. M. Lieber, Nature 1995, 374, 769. [15] P. D. Yang, C. M. Lieber, Science 1996, 273, 1836. [16] R. S. Wagner, W. C. Ellis, Appl. Phys. Lett. 1964, 4, 89. [17] E. I. Givargizov, J. Cryst. Growth 1975, 32, 20. [18] R. S. Wanger, in Whisker Technology (Ed: A. P. Levitt), Wiley, New York 1970, p. 47. [19] T. C. Yeh, K. M. Hickman, S. C. Goel, A. M. Viano, P. C. Gibbons, W. E. Buhro, Science 1995, 270, 1791. [20] S. A. Sapp, B. B. Lakshmi, C. R. Martin, Adv. Mater. 1999, 11, 402. [21] K. H. Yoon, Y. S. Cho, D. H. Kang, J. Mater. Sci. 1998, 33, 2977. [22] H. Wada, K. Sakane, T. Kitamura, J. Mater. Sci. Lett. 1991, 10, 1076. [23] S. Hashimoto, A. Yamaguchi, J. Eur. Ceram. Soc. 2000, 20, 397. [24] M. Kajiwara, J. Mater. Sci. 1987, 22, 1223. [25] A. C. Tas, J. Am. Ceram. Soc. 2001, 84, 295. [26] Y. Hayashi, T. Kimura, T. Yamaguchi, J. Mater. Sci. 1986, 21, 757. [27] S. Nakamura, Science 1998, 281, 956. [28] C. Liu, Y. Y. Fan, M. Liu, H. T. Cong, H. M. Cheng, M. S. Dresselhaus, Science 1999, 286, 1127. [29] X. L. Chen, J. K. Liang, Y. P. Xu, T. Xu, P. Z. Jiang, Y. D. Yu, K. Q. Lu, Mod. Phys. Lett. B 1999, 13, 286. [30] G. J. Li, X. H. Zhang, S. Kawi, Sens. Actuators B 1999, 60, 64. [31] G. Sberveglieri, Sens. Actuators B 1992, 6, 64. [32] C. M. Lampert, Sol. Energy Mater. 1991, 6, 1. [33] L. Aello, B. Bochu, A. Gaskov, S. Koudryavtseva, G. Lucazeau, M. Roumyantseva, J. Solid State Chem. 1998, 135, 78. [34] R. S. Katiyar, P. Dawson, M. M. Hargreave, G. R. Wilkinson, J. Phys. C: Solid State Phys. 1971, 4, 2421. [35] R. Sato, T. Asari, J. Phys. Soc. Jpn. 1995, 64, 1193. [36] T. Azuhata, T. Sota, K. Suzuki, S. Nakamura, J. Phys.: Condens. Matter 1995, 7, L129. [37] G. S. Cheng, S. H. Chen, X. G. Zhu, Y. Q. Mao, L. D. Zhang, Mater. Sci. Eng. A 2000, 286, 165. [38] M. Nagano, J. Cryst. Growth 1984, 66, 377. [39] G. Onori, A. Santucci, J. Phys. Chem. 1999, 57, 5430.
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Novel Ultrasonically Assisted Templated Synthesis of Palladium and Silver Dendritic Nanostructures** By Jianping Xiao, Yi Xie,* Rui Tang, Meng Chen, and Xiaobo Tian In recent years, nanostructured noble metal particles have been the subject of much intensive research due to their fundamental significance for addressing some basic issues of the quantum size effect derived from the dramatic reduction of the number of free electrons[1] as well as their potential applications as advanced materials with intriguing chemical, electronic, magnetic, optical, thermal, and catalytic properties.[2] In particular, fractal structures of noble metals, including metal±polymer composites, have attracted much attention in the past two decades.[3] Fractals are generally observed in nonequilibrium growth phenomena; therefore they can provide a natural framework for the study of disordered systems. Examples are many and varied, ranging from the growth of a snowflake to the aggregation of a soot particle, from oil recovery by fluid injection to solidification of metals, and from the formation of a coral reef to cell differentiation during embryonic development. The diffusion-limited aggregation (DLA) model[4] and cluster±cluster aggregation (CCA) model[5] are widely used to explain and analyze these fractal phenomena. Of the various methods available to prepare nanoscale materials, the template method in which the desired materials are encapsulated into the channels or pores of a host has a number of interesting and useful features for the preparation of nanostructures, since the size and shape of the desired materials can be easily adjusted using a well-defined template matrix.[6] Many templates have emerged, such as carbon nanotubes,[7] porous anodic alumina,[8] ªtrack-etchº polycarbonate membranes,[9] micelles,[10] block copolymers,[11] mesoporous silica,[12] and hybrid organic±inorganic mesoporous materials.[13] Raney materials are often manufactured from alloys that contains an active metal, such as Ni, Co, or Fe, and a soluble metal, e.g., Al, Si, or Sn. For example, a typical Raney nickel is derived from Ni±Al alloy. Aluminum is dissolved from the material in alkaline solution and a porous structure is obtained. The residue after leaching of the Al component consists mainly of Ni in a highly dispersed state (surface areas of 50 to 120 m2 g±1);[14] thus Raney nickel is also named skeleton
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[*] Prof. Y. Xie, Dr. J. P. Xiao, Dr. M. Chen Structure Research Laboratory University of Science and Technology of China Hefei, Anhui 230026 (P. R. China) E-mail:
[email protected] Prof. Y. Xie, Dr. J. P. Xiao, R. Tang, Dr. M. Chen, X. B. Tian Laboratory of Nanochemistry & Nanomaterials Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026 (P. R. China)
[**] This work was supported by the National Science Foundation (NSF) of Natural Science Research of China and the Ministry of Education of China.
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per grid coated with a layer of amorphous carbon. Crystal structures were determined by SAED (using a JEM-200CX TEM), using an accelerating voltage of 200 kV and a double-tilt sample holder. Measurements of Raman spectra were performed on a Spex 1403 Raman spectrometer under a back scattering geometry. A blue line (514.5 nm) of Ar+ laser was taken as the excitation source. SnO2 nanorods were weighed and compressed into discs 12 mm in diameter and 0.5 mm in thickness under an identical pressure of 3 108 Pa.
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nickel. The scanning electron microscopy (SEM) image of Raney nickel can be found in the literature.[15] It indicates that Raney nickel is composed of pores arranged in a crisscross pattern with a supporting skeleton. Raney nickel, as a porous material, has found widespread applications in the field of hydrogenation of organic compounds[15] and especially as an anode in liquid fuel cells.[16] Theoretically speaking, Raney nickel is also attractive as a host framework for the preparation of nanoscale materials in addition to its applications in catalysis. However, to the best of our knowledge, there has been no previous report about the use of Raney nickel as a template for fabricating nanoscale materials. In this communication, we attempt to use Raney nickel as the template and reducing agent to prepare elegant palladium and silver dendritic nanostructures with the assistance of ultrasonic waves. This strategy has two distinctive advantages: the first is that the whole reaction process can be performed at room temperature and ambient pressure; the second is that the removal of Raney nickel template is much easier compared to removal of other templates, as it reacts rapidly with dilute HCl. Herein, palladium is chosen as the case study material due to its catalytic activity, economic advantage over gold, as well as its potential applications in hydrogen storage and advanced electronics.[17] It is well known that catalytic activity and selectivity are dependent on the particle size and shape of the metal nanoparticles. Therefore, the synthesis of palladium with well-controlled shape and size could be critical for its applications. X-ray diffraction (XRD) was used to examine the crystal structure of the sample. Figure 1a shows a typical XRD pattern of palladium dendrites, indicating that the sample is of high crystallinity. The three diffraction peaks can be in-
by X-ray photoelectron spectroscopy (XPS) measurements. The XPS survey spectrum of the sample is shown in Figure 2a, indicating the presence of Pd as well as C from reference and O impurity from absorbed gaseous molecules. A higher resolution spectrum was also recorded in the Pd 3d
Fig. 2. a) XPS survey spectrum of as-obtained palladium dendrites. b) Higher resolution spectrum of Pd 3d region.
Fig. 1. XRD patterns of a) palladium dendrites, b) silver dendrites.
dexed to the (111), (200), and (220) planes of the face centered cubic (fcc) structure of palladium.[18] The refined lattice parameter a = 3.874 ± 0.004 is extracted from the XRD data, which is in good agreement with the literature value of a = 3.889 .[18] No impurity peaks from Ni or PdO were found in the experimental range. Further evidence for the purity and composition of palladium dendrites was obtained
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region. Figure 2b shows the XPS spectrum in the Pd 3d5/2 and Pd 3d3/2 binding energy region with a spin-orbit separation of 5.24 eV. The Pd 3d5/2 peak is centered at 335.34 eV, which is close to that of pure metallic Pd and no peaks due to palladium oxides (~336.5±338.0 eV) were observed.[19] In order to investigate the morphology of the product, it was mounted on the copper grid of a transmission electron microscope (TEM). A typical TEM image is shown in Figure 3a, indicating that well-defined nanoscale palladium dendrites are observed. Furthermore, the highly crystalline nature of the palladium dendrites was confirmed by selected area electron diffraction (SAED) measurement (Fig. 3b). The diffraction rings/spots can again be indexed as (111), (200), (220), and (311) reflections, according to the cubic structure of polycrystalline palladium, which further supports the XRD result. The energy dispersive X-ray (EDX) spectrum recorded on the palladium dendrites is illustrated in Figure 4, indicating the presence of a Pd peak as well as a Cu peak from the TEM grid. No peaks from Ni and O are observed, which reveals that there is no residual impurity in the product. Therefore, both XPS and EDX analyses show that the palladium den-
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Fig. 4. EDX analysis of palladium dendrites. Cu is from the TEM grid.
drites are pure (without contamination) within the limits of instrumental error. In our experiment, Pd2+ firstly diffuses into the pores of Raney nickel under the action of ultrasonic waves. Then, Pd2+ in the pores is reduced by Raney nickel to metallic Pd. At the same time, interspaces in the skeleton of Raney nickel form due to the reaction of Raney nickel with Pd2+. Later, Pd2+ most certainly re-diffuses into these interspaces and is again reduced by Raney nickel. This process lasts until Pd2+ disappears completely. Thus, the palladium that is formed in the pores becomes the trunk of the dendritic structure and the palladium that forms in the interspaces becomes the branches. Finally, well-defined nanoscale palladium dendrites
Scheme 1. Schematic illustration of the growth process of palladium dendrites.
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Fig. 3. a) TEM image of palladium dendrites. b) The corresponding SAED pattern.
appear after the removal of unreacted Raney nickel by HCl solution. The whole process is illustrated diagrammatically in Scheme 1, in which the pore structure of Raney nickel is represented by two end-open flat structures for simplification.[20] From Scheme 1, we can see that the formation of the interspaces in the skeleton leads to the fractals of palladium. The formation of interspaces indicates that Raney nickel indeed plays an important role not only as the template but also as the reducing agent for the growth of palladium dendrites. In general other templates such as mesoporous silica and carbon nanotubes do not participate in the reaction itselfÐthey only play the role of template. It is found that the template and the ultrasonic waves play important roles in the formation of well-defined nanoscale palladium dendrites. When this reaction was carried out without Raney nickel as the template and reducing agent, only irregular palladium nanoparticles were obtained using ethanol or methanol as the reductant and keeping other reaction conditions unchanged. A TEM image of thus-formed irregular palladium nanoparticles is shown in Figure 5a. On the other hand, ill-defined palladium dendrites, the TEM image of which is shown in Figure 5b, were prepared without the action of ultrasonic waves. We presume that the growth mechanism of palladium dendrites should be considered within the framework of a DLA model,[4] which involves cluster formation by the adhesion of a particle with random path to a selected seed on contact and allows the particle to diffuse and stick to the growing structure. The above contrast experiments further support the DLA mechanism. Without the template, palladium nanoparticles instead of palladium dendrites were obtained. In the absence of ultrasonic waves, ill-defined palladium dendrites formed, since Pd2+ could not fully diffuse into the pores of the Raney nickel. It is well known that ultrasonic waves can accelerate the diffusion of Pd2+ into the pores of Raney nickel,[21] which leads to the formation of well- defined nanoscale palladium dendrites. To prove the generality of this method using Raney nickel as the template for the growth of dendritic nanostructures, we have also prepared Ag dendrites using AgNO3 as the silver source by the same route as that used for palladium dendrites. The XRD pattern of silver dendrites is shown in Figure 1b. All the reflections can be identified as cubic Ag.[22] However, an unusually strong (200) diffraction peak in the pattern indicates a preferential orientation of [100] in the product. The TEM image (Fig. 6a) indicates that well-defined Ag dendrites are also prepared in this way. The corresponding SAED pat-
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Experimental
Fig. 5. a) TEM image of palladium nanoparticles without the use of Raney nickel. b) TEM image of ill-defined palladium dendrites without the use of ultrasonic waves.
Synthesis of Raney Nickel: As described elsewhere [14], Raney nickel is obtained by leaching the aluminum from Ni±Al alloy (%Ni » 40±50 wt.-%) in sodium hydroxide (NaOH) solution. Details of the procedure are as follows: approximately 2 g of the ground alloy was covered with an excess of NaOH solution in a beaker. After addition of NaOH, a violent reaction and heating of the suspension was observed. Finally, the reaction product was rinsed several times with distilled water until no further evolution of hydrogen could be observed. In addition, the neutrality of the suspension was checked using pH indicator paper. The as-prepared Raney nickel was kept immersed in distilled water. Synthesis of Dendritic Nanostructures: Analytically pure PdCl2 (0.3 g) was put into a 100 mL conical flask containing the above-obtained Raney nickel, which was then filled with distilled water up to 75 % of the total volume. The flask was immersed in a commercial ultrasonic cleaner (WuXi, H-66025, 220 V, 100 W). The whole reaction was performed at room temperature and ambient pressure and lasted for 2 h. Then, the precipitate was filtered, washed with diluted HCl and distilled water in sequence, and dried under vacuum at room temperature for 4 h. Finally, the product was collected for characterization. For the synthesis of silver dendrites, AgNO3 instead of PdCl2 was used, keeping all other procedures the same as those for the synthesis of palladium dendrites. Characterization of Dendritic Nanostructures: XRD analysis was carried out with a Japan Rigaku D/max-rA X-ray diffractometer with graphite monochromatized Cu Ka radiation (k = 1.54178 ). The scan rate of 0.06 /s was used to record the patterns in the 2h range of 20±80 . TEM images, SAED patterns, and EDX analysis were performed on a Hitachi Model H-800 transmission electron microscope with an accelerating voltage of 200 kV. Samples for the electron microscope were prepared by ultrasonic dispersion for 1 h of 0.1 g of the as-prepared powder with 10 mL of ethanol in a 30 mL conical flask. Then, the suspension was dropped onto a conventional carbon-coated copper grid and dried in air before analysis. In order to obtain composition information about the product, XPS was recorded on an ESCALab MKII X-ray photoelectron spectrometer, using Mg Ka X-rays as the excitation source. The binding energies obtained in the XPS analysis were calibrated against the C 1s peak at 284.6 eV. Received: June 18, 2001 Final version: August 13, 2001
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Fig. 6. a) TEM image of silver dendrites. b) The corresponding SAED pattern.
tern (Fig. 6b) is consistent with the single-crystal nature of the Ag dendrites. In addition, EDX analysis indicates the presence of Ag as well as Cu from the grid. From the TEM images of palladium and silver dendrites, we can see that silver dendrites are of larger size compared with palladium dendrites. This phenomenon is also observed through the comparison of XRD patterns of silver and palladium dendrites. One probable explanation is that the reduction rate of Ag+ to metallic Ag is faster than that of Pd2+ to metallic Pd, leading to the more rapid nucleation and growth of Ag clusters in the pores of Raney nickel. In conclusion, we have achieved the first synthesis of welldefined palladium and silver dendritic nanostructures using Raney nickel as the template and reducing agent with the assistance of ultrasonic waves at room temperature and ambient pressure. It is found that the template and ultrasonic waves play important roles in the formation of well-defined dendritic structures. The DLA mechanism is proposed for the nucleation and growth of dendritic structures. We believe this work may be of interest in that it provides the first example of the use of Raney nickel as the template for the fabrication of nanoscale materials and also suggests a way to prepare a similar structure for other noble metals.
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[1] a) T. Teranishi, M. Miyake, Chem. Mater. 1998, 10, 594. b) W. P. Halperin, Rev. Mod. Phys. 1986, 58, 533. c) S. W. Chen, K. Huang, J. A. Stearns, Chem. Mater. 2000, 12, 540. d) M. Antonietti, C. Göltner, Angew. Chem. Int. Ed. Engl. 1997, 36, 910. e) S. Förster, M. Antonietti, Adv. Mater. 1998, 10, 195. f) M. Antonietti, E. Wenz, L. Bronstein, M. Seregina, Adv. Mater. 1995, 7, 1000. [2] a) V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature 1994, 370, 354. b) G. Schmid, Chem. Rev. 1992, 92, 1709. c) R. P. Andres, J. D. Bielefeld, J. I. Henderson, D. B. Janes, V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney, R. J. Osifchin, Science 1996, 273, 1690. [3] a) L. M. Sander, Nature 1986, 322, 789. b) J. Nittmann, H. E. Stanley, Nature 1986, 321, 663. c) E. Ben-Jacob, P. Garik, Nature 1990, 343, 523. d) S. T. Selvan, Chem. Commun. 1998, 351. e) Y. Zhou, S. H. Yu, C. Y. Wang, X. G. Li, Y. R. Zhu, Z. Y. Chen, Adv. Mater. 1999, 11, 850. f) S. Z. Wang, H. W. Xin, J. Phys. Chem. B 2000, 104, 5681. [4] a) T. A. Witten Jr., L. M. Sander, Phys. Rev. Lett. 1981, 47, 1400. b) S. R. Forrest, T. A. Witten Jr., J. Phys. A 1979, 12, L109. [5] a) P. Meakin, Phys. Rev. Lett. 1983, 51, 1119. b) M. Kolb, R. Botet, R. Jullien, Phys. Rev. Lett. 1983, 51, 1123. [6] a) C. R. Martin, Chem. Mater. 1996, 8, 1739. b) C. Schönenberger, B. M. I. Van der Zande, L. G. J. Fokkink, M. Henny, C. Schmid, M. Krüger, A. Bachtold, R. Huber, H. Birk, U. Staufer, J. Phys. Chem. B 1997, 101, 5497. c) L. Sun, P. C. Searson, C. L. Chien, Appl. Phys. Lett. 1999, 74, 2803. d) S. Shingubara, O. Okino, Y. Sayama, H. Sakaue, T. Takahagi, Solid State Electronics 1999, 43, 1143. e) S. A. Sapp, B. B. Lakshmi, C. R. Martin, Adv. Mater. 1999, 11, 402. [7] a) H. J. Dai, E. W. Wong, Y. Z. Lu, S. S. Fan, C. M. Lieber, Nature 1995, 375, 769. b) W. Q. Han, S. S. Fan, Q. Q. Li, Y. D. Hu, Science 1997, 277, 1287. c) B. K. Pradhan, T. Kyotani, A. Tomita, Chem. Commun. 1999, 1317. [8] a) C. K. Preston, M. Moskovits, J. Phys. Chem. 1993, 97, 8495. b) D. Moutkevitch, T. Bigioni, M. Moskovits, J. M. Xu, J. Phys. Chem. 1996, 100, 14 037. c) Z. Zhang, J. Y. Ying, M. S. Dresselhaus, J. Mater. Res. 1998, 13, 1745. [9] C. R. Martin, Science 1994, 266, 1961. [10] R. Schöllhorn, Chem. Mater. 1996, 8, 1747. [11] H. L. Frisch, J. E. Mark, Chem. Mater. 1996, 8, 1735.
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Optically Aligned Ferroelectric Liquid Crystals** By Yue Zhao* and Nadine Paiement Ferroelectric liquid crystals (FLCs) have a chiral smectic-C phase, S*C, in which each layer has a permanent electric polarization that is confined in the plane of the layer and perpendicular to the director (molecular orientation direction) tilted by an angle from the layer normal. Normally in a bulk FLC, the chirality leads to rotation of the directors of the successive layers, and the induced helical structure cancels the polarization vectors from each other over a pitch length. Therefore, to observe and make use of the spontaneous polarization of FLCs, the key condition is to suppress the helical structure in the S*C phase by aligning molecules in one direction. There has been a considerable interest in developing FLC technologies over the last two decades, because FLCs have important advantages over other liquid crystals (LCs) for use in electronic and photonic devices, mainly due to the fast switching speed of FLCs in an electric field. However, until now, the only technique for the suppression of the helix in the S*C
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[*] Prof. Y. Zhao, N. Paiement DØpartement de chimie, UniversitØ de Sherbrooke Sherbrooke, QuØbec, J1K 2R1 (Canada) E-mail:
[email protected] and Centre de recherche en science et ingØnierie des macromolØcules (CERSIM) UniversitØ Laval, QuØbec, G1K 7P4 (Canada)
[**] The authors thank R. Lemieux (Queen's University, Canada) for the measurements of spontaneous polarization. Financial support from the Natural Sciences and Engineering Research Council of Canada and the Fonds pour la Formation de Chercheurs et l'Aide à la Recherche of QuØbec is also acknowledged.
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phase, which leads to successful commercialization of FLC devices, is the surface-stabilized FLC (SSFLC) discovered by Clark and Lagerwall in 1980.[1] In this communication we report a new strategy that needs no rubbed surfaces to align and stabilize FLCs. An azobenzene diacrylate monomer is dissolved in a FLC host, polymerized and irradiated with linearly polarized light; the photoalignment of azobenzene results in an anisotropic polymer network that subsequently accomplishes both the induction and stabilization of the FLC orientation in the S*C phase. To make SSFLCs, the FLC compound (or mixture) is introduced into an electrooptic cell whose inner surfaces, coated with indium tin oxide (ITO), are parallely rubbed, and whose gap is less than the helical pitch of the S*C phase. Under these conditions, the helical structure cannot develop in the S*C phase, while rubbed surfaces can impose a uniaxial orientation on the FLC molecules. With a desirable bookshelf geometry, where the layers are perpendicular to the binding plates and the molecules are oriented at the tilt angle H with respect to the surface rubbing direction, the spontaneous polarization is normal to the binding plates. The direction of this polarization may switch between two (up and down) states in response to change in the polarity of the electric field applied across the cell, leading to the in-plane switching of the FLC molecules by an angle of 2H. Under crossed polarizers, such switching may create the dark and bright states, which is the basis for FLC devices. The lack of major breakthroughs in the commercialization of SSFLCs is mainly explained by complications in the fabrication process and the difficulty of inducing and sustaining a uniform alignment;[2] their weak shock resistance is another drawback.[3] Since the discovery of SSFLCs, considerable efforts have been made to develop new techniques for the alignment and stabilization of FLCs by external effects other than rubbed surfaces. Representative studies include polymer-dispersed FLCs,[4] microphase-stabilized FLCs,[5] and ferroelectric elastomers,[6,7] for which a mechanical shear is generally employed to induce the alignment. In the present study, we succeeded in aligning FLCs by light and fixing the molecular orientation by a polymer network, without the use of rubbed surfaces. This work was based on a new optical approach recently developed in our laboratory for nematic LCs,[8±10] which makes use of the well-known photoisomerization-induced orientation of azobenzene molecules when exposed to linearly polarized light.[11,12] Our approach uses a diacrylate monomer bearing an azobenzene group in its structure to accomplish both the induction and stabilization of the LC orientation. One of the findings on nematic LCs is that when an azobenzene diacrylate monomer is dissolved in the LC host, thermally induced polymerization in the isotropic phase of the LC under irradiation with linearly polarized light can result in an aligned azobenzene polymer network,[9] and that on cooling under irradiation, a long-range uniaxial LC orientation is induced by the anisotropic network in the nematic phase. We investigated the usability of this optical approach for FLCs on the basis of the following consideration. For a FLC having a chiral nematic, N*, and a smec-
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[12] a) M. J. MacLachlan, P. Aroca, N. Coombs, I. Manners, G. A. Ozin, Adv. Mater. 1998, 10, 144. b) M. H. Huang, A. Choudrey, P. D. Yang, Chem. Commun. 2000, 1063. c) H. J. Shin, R. Ryoo, Z. Liu, O. Terasaki, J. Am. Chem. Soc. 2001, 123, 1246. d) K. B. Lee, S. M. Lee, J. Cheon, Adv. Mater. 2001, 13, 517. e) H. Kang, Y. W. Jun, J. I. Park, K. B. Lee, J. Cheon. Chem. Mater. 2000, 12, 3530. f) H. Parala, H. Winkler, M. Kolbe, A. Wohlfart, R. A. Fisher, R. Schmechel, H. von Seggern, Adv. Mater. 2000, 12, 1050. g) Z. Liu, Y. Sakamoto, T. Ohsuna, K. Hiraga, O. Terasaki, C. H. Ko, H. J. Shin, R. Ryoo, Angew. Chem. Int. Ed. Engl. 2000, 39, 3107. [13] A. Fukuoka, Y. Sakamoto, S. Guan, S. Inagaki, N. Sugimoto, Y. Fukushima, K. Hirahara, S. Iijima, M. Ichikawa, J. Am. Chem. Soc. 2001, 123, 3373. [14] J. Rothe, J. Hormes, C. Schild, B. Pennemann, J. Catal. 2000, 191, 294. [15] J. P. Mikkola, H. Vainio, T. Salmi, R. Sjöholm, T. Ollonqvist, J. Väyrynen, Appl. Catal. A: Gen. 2000, 196, 143. [16] E. W. Justi, A. W. Winsel, J. Electrochem. Soc. 1961, 108, 1073. [17] S. P. Parker, McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill, New York 1989, p. 1343. [18] JCPDS, No. 5-681. [19] C. D. Wanger, W. M. Riggs, L. E. Davis, J. F. Moulder, G. E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy Perkin-Elmer Corp., Eden Prairie, MN 1978. [20] Catalysis Group of Department of Chemistry in Jilin University, Basis of Catalysis, Science Press, Beijing 1977, p. 63 (in Chinese). [21] a) K. Chatakondu, M. L. H. Green, M. E. Thompson, K. S. Suslick, J. Chem. Soc., Chem. Commun. 1987, 900. b) K. S. Suslick, G. J. Price, Annu. Res. Mater. Sci. 1999, 29, 295. [22] JCPDS, No. 4-783.
