Langmuir 2002, 18, 6453-6457
Synthesis of AgBr and SnO2 Microwires Induced by Mixed Surfactant Nematic Liquid Crystalline Phases Tsuyoshi Kijima,*,† Takayuki Ikeda,† Mitsunori Yada,‡ and Masato Machida† Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki, 889-2193, Japan, and Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan Received December 17, 2001. In Final Form: April 30, 2002
Introduction Inorganic synthesis using surfactants as a templating agent has attracted increasing attention because it affords nanostructured materials such as mesoporous silica,1-3 alumina,4-6 zirconia,7 and rare earth oxides,8 nanotubular vanadia,9 titania10 and lanthanoid oxides,11 and nanowires of gold12 and titania.13 The liquid crystalline phases of nonionic oligoethylene alkyl ether surfactant with water have also successfully served as a templating agent for the synthesis of monolithic mesoporous silicas,14,15 platinum16,17 and cadmium and zinc sulfides.18,19 Such nanostructured materials would be applicable to catalysis, membrane and separation technology, microelectronics, and molecular device engineering. The previous syntheses have unexceptionally used a single surfactant, cationic, anionic, or nonionic, as templating agents. A combined use of any two types of surfactants, on the other hand, might be applicable to the synthesis of nano- or microstructured materials with specific properties. If oligo* To whom correspondence should be addressed. Tel: +81-985-58-7311. Fax: +81-985-58-2876. E-mail: t0g102u@ cc.miyazaki-u.ac.jp. † Miyazaki University. ‡ Saga University. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (3) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317. (4) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (5) Yada, M.; Hiyoshi, H.; Ohe, K.; Machida, M.; Kijima, T. Inorg. Chem. 1997, 36, 5565. (6) Bagshaw, S. A.; Pinnavia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (7) Terns, P.; Hudson, M. J.; Denoyel, R. J. Mater. Chem. 1998, 8, 2147. (8) Yada, M.; Ichinose, A.; Kitamura, H.; Machida, H.; Kijima, T. Angew. Chem., Int. Ed. Engl. 1999, 38, 3506. (9) Spahr, M. E.; Bitterli, P.; Nesper, R.; Muller, M.; Krumeich, F.; Nissen, H. U. Angew. Chem., Int. Ed. Engl. 1998, 37, 1263. (10) Harada, M.; Adachi, M. Adv. Mater. 2000, 12, 839. (11) Yada, M.; Mihara, M.; Mouri, S.; Kuroki, M.; Kijima, T. Adv. Mater. 2002, 14, 309. (12) Kondo, Y.; Takayanagi, K. Science 2000, 289, 606. (13) Zhu, Y.; Li, H.; Koltypin, Y.; Hacohen, Y. R.; Gedanken, A. Chem Commun. 2001, 2616. (14) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366. (15) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (16) Attard, G. S.; Goltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 1315. (17) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (18) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (19) Braun, P. V.; Osenar, P.; Tohver, V.; Kennedy, S. B.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 7302.
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ethylene alkyl ethers and long-chain alkylammonium salts were employed as such mixed surfactant components, nano- or microstructured metal halides, metal oxides, and so forth would be obtainable by the hydrolysis of the metal cations reserved by the former surfactant or their reaction with counteranions for the latter. The mixed surfactant approach was thus applied to the synthesis of AgBr and SnO2 as their typical cases. Silver halide microcrystals have been used as a highquality photosensitive and imaging material for photography. Many efforts have been made for control in size, shape, defects, and other structural properties of silver halide microcrystals to improve their photographic capabilities.20-23 In addition to aqueous gel crystallization techniques early reported,24 the crystallization of silver bromide in nonaqueous gels was also reported to be effective for the fine control of crystal morphology.25 Tin oxide is widely used for potential applications such as semiconductor gas sensors, catalysts for oxidation of organic compounds, and indium-tin oxide transparent conductive glasses. Extensive studies have been made on the morphological control or surface modification of tin oxide with the intention to improve its sensor performance in sensitivity and selectivity toward gases.26,27 This intention was also focused on the synthesis of mesoporous tin oxides using either cationic, anionic, or neutral surfactants28-30 and the characterization of their gas sensor properties.31 In this paper, we report the synthesis of AgBr and SnO2 microwires by mixing cetyltrimethylammonium bromide with oligoethylene alkyl ether/water smectic liquid crystals containing AgNO3 or SnF2 species, demonstrating a novel method for the synthesis of microwire materials using mixed surfactant nematic phases as a reaction medium. Experimental Section Materials. The reagent-grade silver nitrate AgNO3, tin difluoride SnF2, cetyltrimethylammonium bromide (CTAB), and CnH2n+1(OCH2CH2)mOH, hereafter abbreviated as CnEOm (n, m ) 12, 4; 12, 8; 12, 9; 12, 23; 16, 20) were purchased from Wako Co. Ltd. and used without further purification. AgNO3/CnEOm/CTAB/H2O System. Silver nitrate, CnEOm (n, m ) 12, 4; 12, 8; 12, 9; 12, 23; 16, 20), and H2O (adjusted to pH 1.4 with HNO3) were mixed at a molar ratio of 1:1:60 with stirring and then heated to 80 °C to obtain a clear mixed solution. Nitric acid was added to avoid the precipitation of Ag2O. The mixed solution was cooled to 60 °C and then remixed with the same molar amount of CTAB as that of CnEOm, followed by being allowed to stand at that temperature for 24 h. The resulting solid was separated, washed several times with ethanol, and then dried in air. SnF2/C12EO9/CTAB/H2O System. The surfactant C12EO9 was added to an aqueous solution of tin difluoride at 60 °C with (20) Berry, C. R. Photogr. Sci. Eng. 1976, 21, 1. (21) Sugimoto, T. Photogr. Sci. Eng. 1984, 28, 137. (22) Takada, H.; Nozoe, H. Langmuir 1993, 9, 3305. (23) Makasky, J. E. J. Imaging Sci. Technol. 1996, 40, 79. (24) Blank, Z.; Speyer, D. M.; Brenner, W.; Okamoto, Y. Nature 1967, 216, 1103. (25) Doxsee, K. M.; Chang, R. C.; Chen, E. J. Am. Chem. Soc. 1998, 120, 585. (26) Ando, M.; Suto, S.; Suzuki, T.; Tsuchida, T.; Nakayama, C.; Miura, N.; Yamazoe, N. J. Ceram. Soc. Jpn. 1996, 104, 409. (27) Wada, K.; Egashira, M. Sens. Actuators, B 1998, 53, 147. (28) Chen, F.; Liu, M. Chem Commun. 1999, 1829. (29) Ulagappan, N.; Rao, C. N. R. Chem Commun. 1996, 1685. (30) Severin, K. G.; Abdel-Fattah, T. M.; Pinnavaia, T. J. Chem Commun. 1998, 1471. (31) Li, G.-J.; Kawi, S. Talanta 1998, 45, 759.
10.1021/la0118154 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/12/2002
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Figure 1. SEM images of the AgBr crystals grown in the AgNO3/CnEOm/CTAB/H2O (1:1:1:60) system; n, m ) (a) 12, 4 (r ) 1), (b) 12, 8 (r ) 2), (c) 16, 20 (r ) 3.75), and (d) 12, 23 (r ) 5.75). stirring, followed by addition of CTAB. The mixture of SnF2, CTAB, C12EO9, and H2O at a total molar ratio of 1:1:1:60 was kept at that temperature for 72 h. The resulting solid was separated, washed several times with distilled water, and then dried in air. Some portion of the as-grown sample was calcined at 500 °C for 1 h. Characterization. Powder X-ray diffraction (XRD) measurements were conducted on a Shimadzu XD-D1 diffractometer with Cu KR radiation at a scanning rate of 2° min-1. Observations by polarizing and nonpolarizing optical microscopy were performed using an Olimpus BX50 equipped with a temperature-controlled stage. Transmission and scanning electron microscopic (TEM and SEM) images were taken by a Hitachi H-800MU and a Hitachi H-4100M, respectively. Energy-dispersive X-ray (EDX) microanalysis was carried out on a HORIBA EMAX-5770. Thermogravimetric and differential thermal analyses (TG/DTA) were conducted by a Seiko TG/DTA320U with a heating rate of 10 °C/min in air. Fourier transform infrared (FT-IR) spectra were measured by the KBr pellet method using a Nippon Bunko FT/ IR-300.
Results and Discussion AgNO3/CnEOm/CTAB/H2O System. All the resulting solids separated at reaction times of 0.1 and 24 h in all the CnEOm-based systems were identified as AgBr since they gave XRD peaks at 2θ ) 26.79° (d ) 0.333 nm), 31.04° (d ) 0.288 nm), 44.43° (d ) 0.204 nm), and 55.14° (d ) 0.166 nm) assignable to the 111, 200, 220, and 222 reflections for the AgBr crystal, respectively. Infrared spectra also showed that no organic species are contained in any of these solids. SEM observations indicated that all the solids separated at a reaction time of as short as 0.1 h in all the systems used are an aggregate of irregularshaped granular AgBr particles. The morphological properties of the 24-h reaction products, on the other hand, were found to remarkably change depending on the chemical composition of CnEOm molecules contained in the reaction systems, as shown in Figure 1. The nonionic surfactant molecules are composed of an oligoethylene chain as the hydrophilic group and an alkyl chain as the
Table 1. Characterization of AgBr crystals Grown in the AgNO3/CnEOm/CTAB/H2O System product CnEOm
ra
morphologies
C12EO4 C12EO8 C12EO9 C16EO20 C12EO23
1 2 2.25 3.75 5.75
granular & wire granular & wire granular & wire granular & wire granular & platelike
max length of wire/µm ∼20 ∼100 ∼100 ∼200
a The quantity r () 3m/n) indicates the degree of hydrophilicity for the CnEOm molecule defined by the chain length ratio of ethyleneoxide to alkyl group.
hydrophobic one. Since the sizes of oxygen and carbon atoms are very close to each other, the parameter r, defined by r ) 3m/n, can be employed as a measure for the degree of hydrophilicity of the CnEOm molecule. In the (n, m) ) (12, 4) (r ) 1) system, the 24-h reaction product was a mixture of rodlike and granular AgBr particles, as shown in Figure 1a. The size of the AgBr wires was ∼0.5 µm in diameter and up to ∼20 µm long. Both the (12, 8) (r ) 2) and (12, 9) (r ) 2.25) systems yielded AgBr wires ∼0.5 µm in diameter and up to 100 µm long (Figure 1b). Similar diameter but much longer AgBr wires with a maximum length of ∼200 µm were observed to form for the (16, 20) (r ) 3.75) system (Figure 1c). In the (12, 23) (r ) 5.75) system, in contrast, hexagonal platelike AgBr crystals appeared along with granular particles (Figure 1d). Precipitation of similar crystals was observed when aqueous solutions of AgNO3 and HBr were mixed without any surfactant. The above results are summarized in Table 1. To clarify the markedly r-dependent crystallization behavior of AgBr leading to microwires, mainly the C12EO8 (r ) 2) or C12EO9 (r ) 2.25) based systems and additionally the other ones were studied in further detail. The C12EO8/H2O sample was identified as a smectic phase
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Figure 2. Optical microscope images under the cross Nicol field (a,c,e,f) and X-ray diffraction patterns (b,d) for different liquid crystalline phases: (a,b) C12EO8/H2O (1:60) at room temperature, (c,d) C12EO8/CTAB/H2O (1:1:60) at 60 °C, (e) C12EO4/CTAB/H2O (1:1:60) at 60 °C, and (f) C12EO8/CTAB/H2O (1:1:60) at 60 °C.
since the mixture with a jellylike character appeared birifringent or anisotropic under the cross Nicol field and gave an XRD peak attributable to a long period of d ) 3.9 nm (Figure 2a,b). On addition of CTAB to the smectic phase, the resulting mixture was converted into a nematic phase in which the smectic-order-related XRD peak was significantly weakened with keeping its anisotropic and jellylike characters (Figure 2c,d). The transition from smectic to nematic order is probably because the CTA+ cationic molecules accompanying counter Br- anions are inserted in the bilayered assembly of C12EO8 molecules in such a way that both surfactant molecules are arranged side by side with their alkyl chains oriented uniaxially through their van der Waals interaction. A similar transition from smectic to nematic order was observed for both the C12EO4/CTAB/H2O and C16EO20/CTAB/H2O systems (Figure 2e,f). The smectic domain size optically determined is in the order of below ∼50 µm for (12, 4) < ∼300 µm for (12, 8) < ∼1500 µm for (16, 20), being in agreement with that for the length of AgBr microwires observed above. Furthermore, when the polarizing and the nonpolarizing images of the C12EO9-based sample as prepared without washing, for instance, were viewed under an optical microscope, the AgBr microwire crystals were observed to extend straightly or inflectionally along the birifringent streak characteristic of the nematic liquid crystalline phase, as shown in Figure 3. On the basis of the above optical, SEM, and X-ray observations for the r ) 1-3.75 systems, we can assume that AgBr clusters would occur along the intermolecular narrow space formed within the nematic domain of uniaxially oriented surfactant molecular assemblies through the intradomain cleavage along the domain axis, followed by the growth and linkage of the bromide clusters into straight or partly
Figure 3. Nonpolarization (a) and polarization (b) images of AgNO3/C12EO9/CTAB/H2O (1:1:60). A wirelike crystal indicated by the arrow A in the rectangular area of each image is schematically shown by a solid line in the right-top copied field.
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Scheme 1. Schematic Model Proposed for the Transition from Smectic to Nematic Structure and Formation of AgBr Microwires in the AgNO3/CnEOm/CTAB/H2O Systema
a
(a) Addition of CTAB; (b) aging.
Figure 4. SEM images of the resulting solids in the SnF2/ C12EO9/CTAB/H2O system: (a) as grown and (b) calcined at 500 °C.
inflexed AgBr wires, as schematically shown in Scheme 1. This crystallization scheme definitely explains the fact that the reaction system with larger domain size yields longer microwires of AgBr. For example, in the C16EO20 and CTAB mixed system, the resulting nematic structure would be much increased in both structural stability and domain size through the amplified interaction between their hexadecyl chains with the same length leading to the formation of a long straight path more advantageous for the uniaxial growth and linkage of AgBr clusters or crystallites into long wires. On the other hand, the C12EO23 molecules with much longer hydrophilic chains were observed to yield an isotropic liquid crystalline phase on mixing with acidic water, followed by its conversion into a sol on further addition of CTAB. The spherical surfactant micelles thus formed would not serve as a specific medium for crystallization, leading to the occurrence of hexagonal platelike AgBr crystals, similar to the surfactant-free system. The AgBr microwires thus obtained in the present study might be useful as imaging materials with anisotropic character for special applications such as photographic films for art. SnF2/C12EO9/CTAB/H2O System. The resulting solid as grown was white and gave a TEM image indicative of an aggregate of ∼0.5 µm diameter beads-linked chain particles 20-30 µm long, as shown in Figure 4. The XRD pattern of the sample showed a very weak band at 2θ ) 1-2°, along with two weak bands at 2θ ) ∼27° and ∼52° attributable to the 110 and 211 reflections for the cassiterite form of SnO2, respectively (Figure 5). The poorly
Figure 5. XRD patterns of the resulting solids in the SnF2/ C12EO9/CTAB/H2O system: (a) as grown and (b) calcined at 500 °C (Cu KR).
crystalline solid is likely grown within a nematic phase because the product as grown without washing appeared entirely birefringent under the cross Nicol field. Furthermore, the reaction using CTAB alone as the surfactant component yielded spherical SnO2 particles, whereas the C12EO9-based reaction gave no solid products. As additionally suggested by the significant rise of pressure in the reaction vessel, the formation of SnO2 in the CTABcontaining system would proceed through the following oxidation reaction of Sn2+ into Sn4+ induced by the attractive interaction between the CTA+ cation and the hydroxyl group of the water molecule:
Sn2+ + 2H2O f SnO2 + H2 It was also confirmed that the SnO2-based solid contains CTA+ species as the major organic component and C12EO9 species as the minor organic one, as suggested from its infrared spectrum indicating strong νC-H bands at 2910 and 2857 cm-1 due to the -CH2- groups along with a weak νC-O-C band at 1124 cm-1 due to the ethylene oxide groups (Figure 6). The TG/DTA curves for the asgrown solid showed four events accompanying significant weight losses in the wide temperature range up to 450 °C, as presented in Figure 7. The first weight loss around 100 °C accompanying two endothermic peaks is
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Figure 6. FT-IR spectra of (a) CTAB and (b) the as-grown solid in the SnF2/C12EO9/CTAB/H2O system.
Figure 7. TG (a) and DTA (b) curves for the as-grown solid in the SnF2/C12EO9/CTAB/H2O system. The TG curves for (c) CTAB and (d) C12EO9 are also shown.
due to desorption of absorbed water. The second slight weight loss around 220 °C with an accompanying exothermic peak would be attributable to degradation and/or combustion of a small amount of surfactant species in their free state. The third weight loss at temperatures of 250-360 °C with an accompanying pronounced exothermic peak is due to desorption of a major portion of surfactant species through degradation and/or combustion, followed by complete desorption of the remaining surfactant moiety through combustion at higher temperatures. The surfactant desorption temperature for the SnO2-based solid is much higher than ∼200 °C observed for CTAB or C12EO9, suggesting that the former solid is not a mixture of both SnO2 and surfactant phases but is a surfactant-complexed phase. The detailed interior structure of the as-grown particles could not be revealed by transmission electron microscopy because their finely segregated form was unobtainable. A possible explanation, however, for the SnO2-surfactant complexing accompanied by the formation of beads-linked wires would be that low-crystalline SnO2 core materials are linked together by the surfactant bridge to form microbeads leading to their assembling
into a linear chain within the nematic domain effective for one-dimensional crystal growth, as observed in the Ag-based system. Such solidification is likely in common with the assembling of inorganic nanoparticles through the adhesion with surfactant species observed in several systems such as barium chromate32 and magnetite.33 The SnO2 microbead chain produced in the present system also markedly resembles in morphology intracellular elongated magnetite crystals grown within vesicles of a magnetotactic bacterium assembled into a linear chain to maintain a magnetic dipole along the cell axis.34 On calcination at 500 °C after spin coating on a glass plate, the surfactant-complexed SnO2 was deorganized into tin oxide microwires or micronetworks with keeping the essential morphology of their precursory particles, as shown in Figure 4b. The calcination resulted in a slight increase in crystallinity of the SnO2 solid, as judged by its XRD pattern indicating three broad but definite peaks attributable to the 110, 101, and 211 reflections for the cassiterite form of SnO2 along with some other weak bands (Figure 5). The SnO2 microwires thus formed might be available for highly sensitive gas sensors if their precursory wires were placed perpendicular to electrode wires before calcination. In conclusion, the present study has first demonstrated the growth of AgBr and SnO2 microwires using mixed surfactant nematic phases as a reaction medium. The mixed surfactant medium provides a molecular-scale constructional archetype similar to molecular tectonics in biominerallization. We can thus propose the new mediating method as a promising approach to the synthesis of various microwire materials. Further work to apply this method to other inorganic materials such as zinc oxide is under way. LA0118154 (32) Li, M.; Schnablegger, H. Nature 1999, 383, 393. (33) Lin, J.; Iyouda, T.; Cao, C.; Song, Y.; Jiang, L.; Li, T. J.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 40, 2135. (34) Mann, S. Nature 1993, 365, 499.
