WITH THE discovery of the liquid-crystal templating

490 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 4, NO. 5, SEPTEMBER 2005 Self-Assembly of Uniform Nanoporous Silica Fibers Yaroslav Kievsky and Igor Sokolov Abstract Self-assembly of nanoporous silica shapes is of great interest for modern nanotechnology because of uniform pore size, simplicity, and low cost of production. However, there are two major problems that prevent broad use of the self-assembly process. First, the process brings too broad a variety of the assembled shapes. Secondly, the yield of the desired shapes is far from 100%. Here, we describe a process of acidic self-assembly of silica shapes that is free of both of these problems. The process described results in virtually a 100% of very uniform fibers. Each fiber has a hexagonal cross section of approximately 2 m and a length of approximately 5 m. The highly uniform pores with periodicity of 3.8 nm are unidirectional along the fiber. These new fibers can be used in chromatography, drug delivery, manufacturing nanowires, nanoreactors for one-dimensional chemistry, etc. Index Terms Fibers, mesoporous silica colloids, nanoporous, self-assembly. I. INTRODUCTION WITH THE discovery of the liquid-crystal templating (see, e.g., [1] [3]) of hexagonal, cubic, and lamellar meso(nano)structured silica, materials chemistry has moved into the realm of design and synthesis of inorganics with complex form. It becomes possible to synthesize inorganics with structural features of a few nanometers and architectures over such large length scales up to hundreds of micrometers. This nanochemistry is inspiring research in materials science, solid-state chemistry, semiconductor physics, biomimetics, and biomaterials [3] [26]. Manipulation of surfactant packing parameter, headgroup charge, co-surfactants, solvents, co-solvents, and organic additives have been used to template particular nanostructures. Dimension of the pores can be tuned with angstrom precision over the size range of 20 100 (see, e.g., [8]). It was shown that the use of cationic surfactants in the presence of a silica precursor can result in synthesizing a variety of well-defined nanoporous silica shapes. A cationic assembly can exchange its counteranion, say, chloride, with a mineralizable inorganic anion protonated silicate. Synthesis of mesoporous thin films [7], [14], [18], [21] [23], [31], spheres [17], [32], [33], [38], curved-shaped solids [16], [18], tubes [24], [34], rods and fibers [18], [35], membranes [36], and other monoliths [37] has been recently reported. Despite demonstrated success Manuscript received July 28, 2004; revised December 17, 2004. This work was supported in part by the Center of Advanced Material Processing, Clarkson University under the New York Office of Science, Technology, and Academic Research. Y. Kievsky is with the Department of Physics, Clarkson University, Potsdam, NY 13699-5820 USA (e-mail: kievskyy@clarkson.edu). I. Sokolov is with the Department of Physics and Department of Chemistry, Clarkson University, Potsdam, NY 13699-5820 USA (e-mail: isokolov@clarkson.edu). Digital Object Identifier 10.1109/TNANO.2005.851425 of this approach, there are two problems that prevent broader use of the assembled porous shapes, which are: 1) the self-assembly process brings too large a variety of assembled shapes and 2) the yield of the desired shapes is far from 100%. These factors make it difficult to extract and, subsequently, to use the desired shapes. In this paper, we describe the self-assembly templating of a cationic surfactant in the presence of a silica precursor that is free of both of the above-mentioned problems. The reported synthesis shows surprisingly high yield (virtually 100%) of nanoporous fibers with a rather narrow size distribution. II. EXPERIMENTAL The acidic synthesis of nanoporous silica fibers was described previously in [9]. Here, we used the same ideas of liquid-crystal templating and condensation of silica precursor tetraethylorthosilicate (TEOS, 99.999%, Aldrich). The protonated silicate alkoxysilane precipitates on the surfactant head groups due to complimentarity of charges. We used cetyltrimethylammonium chloride (CTACl) (25 wt.% aqueous solution, Aldrich) as a surfactant template. The required acidity was created by means of hydrochloric acid (HCl) (37.6 wt.% aqueous solution, SafeCote). All chemical were used as received. The surfactant, acid, and ultrapure water (18 M cm, MilliQ Ultrapure) were mixed in a plastic bottle (HDPE), stirred first at room temperature (25 C) for 1 min, and then cooled down to 4 C in a refrigerator for 15 min. Cooled TEOS was added to the acidic solution of surfactant, stirred for 30 s. The final molar composition of the reactants was 100 H O:9 HCl : 0.22 CTACl : 0.13 TEOS. The resulting solution was kept under quiescent conditions for 3 h at 4 C. The material was collected by centrifugation (5 min, 6000 r/min, IEC Clinical Centrifuge), then resuspended in distilled water, and centrifuge again. The resuspension procedure was repeated. Collected powder was dried in ambient conditions and calcined at 550 C for 4 h to remove the surfactant from the pores. To demonstrate the importance of the self-assembly at low temperatures, we repeated the synthesis as described above, but done at room temperature (25 C). A scanning electron microscope (SEM) (JEOL JSM-6300) was used to characterize morphology of the synthesized particles. A thin layer of gold was spattered on the particle surface to improve the SEM contrast. The pore periodicity was found by using a low-angle powder X-ray diffraction (XRD) technique (1050X, Ordela). The particle size distribution was measured by using light-scattering technique (ALV-NIBS high-performance particle sizer). The particles were suspended in ultrapure water, ultrasonicated to disperse the particles and remove any bubbles. 1536-125X/$20.00 2005 IEEE

KIEVSKY AND SOKOLOV: SELF-ASSEMBLY OF UNIFORM NANOPOROUS SILICA FIBERS 491 Fig. 1. SEM images of the fibers assembled at the: (a), (a ) cold and (b), (b ) room temperatures. The horizontal bar is 11 m. Fig. 2. 10 m. Zoo of shapes assembled at room temperature. The horizontal bar is III. RESULTS AND DISCUSSION Using optical microscopy (not shown), as well as the SEM, we see that the low-temperature (cold) synthesis results in a surprisingly high virtually 100% yield of hexagonal fibers (here, yield denotes the volume percentage of well-defined shapes in the collected batch versus shapeless junk ). Fig. 1(a) shows a representative SEM image of the fibers obtained in the cold synthesis, while Fig. 1(b) demonstrates the best part (closest to the straight fibers) of the batch synthesized at room temperature. To clearly see the difference, Fig. 1(a ) and (b ) show a higher resolution image of the particles assembled at cold and room (best part) temperatures, respectively. One can see that the fibers in Fig. 1(b) and (b ) are not that uniform as assembled in a cold environment, i.e., Fig. 1(a) and (a ). There are number of round shapes, discoids, and the fibers are bent. In contrast, the cold synthesis results in the shapes of only one type, almost straight fibers of a hexagonal cross section. Furthermore, the yield of the fibers synthesized at room temperature is hard to estimate due to high variability of the shapes. Fig. 2 presents a typical zoo of such shapes obtained at room temperature. One can see a large variety of fibers of different size, discoids, including somewhat close to haired fibers (right-hand-side bottom image), which are hard to distinguish from shapeless junk. It should be stressed that there is no variation of the fibers assembled in the cold synthesis. Fig. 1(a) and (a ) represent all fibers in the batch. This makes this material very attractive for various applications (see Section IV). Hereafter, we will discuss the fibers obtained in the cold synthesis. A higher magnification SEM image of the fibers is shown in Fig. 3. Hexagonal cross section is clearly seen. Below we present quantitative proof that the images obtained by the SEM [as shown in Fig. 1(a) and (a )] are indeed representative. First we measure statistical distribution of the fiber diameters (we define diameter for a hexagonal cylinder as the diameter of circumference inside the hexagonal cross section) and lengths. Note that it cannot be done with such a popular technique as TEM because the cross section is not very visible in TEM. Such measurements can be easily done with the SEM. To estimate the diameters of the fibers and their length, we measured approximately 80 fibers in the SEM images. Fig. 4 shows histograms of distributions of the fiber diameter, length, and the length diameter aspect ratio. The average diameter of the fibers is 2.0 m (standard deviation is 0.3 m), the average length is 4.8 m (standard deviation is 0.5 m), and the average aspect ratio is 2.4 (standard deviation is 0.4 m). Based on these numbers, we can say that the dispersions of the diameter, length, and ratio are 16%, 11%, and 16%, respectively. Let us show now that these statistics are robust, and the analyzed SEM images can describe the whole batch. This can be proven, for example, by measuring the particle size distribution with the light-scattering technique. The experimental setup used in this study allows one to measure effective radius of the particles. Since the size of the particles is relatively large comparing to the wavelength of light, we can assume that the signal in the light scattering setup is proportional to the particle geometrical cross-sectional area. However, in the SEM images, we were only able to analyze the fibers lying down on the substrate. Therefore, we need to take the fibers from the SEM, and process them through a computer algorithm that will rotate the fibers in all possible directions, and calculate the cross section of each particle s position. These cross sections will be compared with what is observed in the light-scattering experiments. Specifically, the cross section is measured as a radius of effective sphere that has area equal to the area of the cross section. Let us describe such a computer algorithm. Rod-like fibers of the diameters (as define above for the fibers with hexagonal cross section) and length can be oriented with an angle with

492 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 4, NO. 5, SEPTEMBER 2005 Fig. 4. Distributions of the fiber diameters, lengths, and length diameter aspect ratios. Fig. 3. Higher magnification SEM images of fibers assembled in the cold synthesis. Hexagonal cross section of the fiber is clearly seen. The horizontal bar is 5 m. respect to the laser beam. This results in effective change of the projected area of the particle so that the corresponding effective sphere of the same area will have the radius of (1) It should be noted that possible rotation of the hexagonal fibers around their axes should also be taken into account. This results in a modification of the above equation. To avoid a heavy algebraic equation, we will not describe it here. Nevertheless, the final FORTRAN code does this simulation. The code can be freely obtained directly from the authors. Using the diameter and length that were measured with the SEM, and taking the randomness of the angle distribution, we can now simulate the distribution of the particles measured with the SEM as it should be seen in the particle-size analyzer. Fig. 5 shows both distributions, simulated, as described above, Fig. 5. Distribution of the effective particle radii. The simulated distribution (the histogram) is based on SEM statistics (Fig. 3). The measured distribution with the help of the light-scattering setup is denoted by a solid line. and measured, with the help of the light-scattering technique. One can see a rather good agreement. This proves that the results of the SEM analysis are robust, and described the whole batch. A low-angle powder XRD pattern is shown in Fig. 6. The observed (100) peak corresponds to the pore-to-pore periodicity of 3.8 nm. Comparing the observed peak with those reported in literature, we can conclude that we have observed rather sharp distribution. According to the classification of the mesoporous ma-

KIEVSKY AND SOKOLOV: SELF-ASSEMBLY OF UNIFORM NANOPOROUS SILICA FIBERS 493 Uniform silica shapes will allow to assembly various types of nanowires in their pores of uniform length. Example of filling the pores with semiconductors is described in [40]. In addition, it is worth mentioning that these shapes can be a model system to study one-dimensional chemistry chemical reactions in the channels. Fig. 6. Low-angle powder XRD pattern of the synthesized fibers. terial, our synthesized shapes are an MCM-41 type. From TEM analysis of similar shapes synthesized previously [9] (without uniformity of shapes), we can conclude that the pores are running along the fiber. The mechanism of formation of the assembled fibers is presumably the same as described in [25]. The surfactant template is growing and rigidifying with adsorption of the hydrolyzed silica precursor. As was noted in [25], the higher temperature of synthesis, which should be equilibrated with the free energy of the assembling shapes, should lead to development of curvature of the fibers. This was observed in the synthesis described here (see Fig. 1). However, it is not still clear why the length of the fibers becomes noticeably smaller in the cold-temperature versus room-temperature synthesis. Further study of the formation mechanism is needed. IV. CONCLUSION Here, we have reported a novel modification of acidic synthesis of nanoporous silica fibers. The assembled fibers are rather uniform in size and shape. Each fiber is a hexagonal cylinder of approximately 2.0 m in diameter and approximately 4.8 m in length. The yield of the assembled fibers is virtually 100%. It shows no junk, which is typical for silica self-assembly. A very narrow XRD peak indicates high uniformity of pores. Uniformity in distribution denotes uniformity of properties. Therefore, the use of the assembled shapes will have advantages in any areas in which such properties are desirable. There are areas were uniformity is required. It includes, for example, filtering applications for chromatography. More nontrivial use of these shapes would be in drug delivery. Well-controlled shapes will allow reliably controlled drug release by diffusion out through the pores [39]. When being taken inside the organism, silica has a serious advantage here by being very chemically resistive and biocompatible. REFERENCES [1] T. Yanagisawa, T. Shimizu, K. Kuroda, and C. Kato, The preparation of alkyltrimethylammonium kanemite complexes and their conversion to microporous materials, Bull. Chem. Soc. Jpn., vol. 63, pp. 988 992, 1990. [2] C. T. Kresge, M. Leonowicz, W. J. Roth, J. C. Vartuli, and J. C. Beck, Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature, vol. 359, pp. 710 712, 1992. [3] U. Ciesla and F. Schüth, Ordered mesoporous materials, Microporous Mesoporous Mater., vol. 27, pp. 131 149, 1999. [4] S. Mann and G. A. Ozin, Synthesis of inorganic materials with complex form, Nature, vol. 382, pp. 313 318, 1996. [5] D. M. Antonelli and J. Y. Ying, Mesoporous materials, Curr. Opin. Coll. Interf. Sci., vol. 1, pp. 523 529, 1996. [6] T. Jiang and G. A. Ozin, New directions in tin sulfide materials chemistry, J. Mater. Chem., vol. 8, pp. 1099 1100, 1998. [7] G. S. Attard, P. N. Bartlett, N. R. B. Coleman, J. M. Elliott, J. R. Owen, and J. H. Wang, Mesoporous platinum films from lyotropic liquid crystalline phases, Science, vol. 278, pp. 838 840, 1997. [8] D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, and G. D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores, Science, vol. 279, pp. 548 552, 1998. [9] H. Yang, N. Coombs, and G. A. Ozin, Morphogenesis of shapes and surface patterns in mesoporous silica, Nature, vol. 386, pp. 692 695, 1997. [10] H. Yang, G. A. Ozin, and C. T. Kresge, Defects in a hexagonal lyotropic silicate mesophase: Role in morphogenesis of hexagonal mesoporous silica, Adv. Mater., vol. 10, pp. 883 887, 1998. [11] L. Brousseau, K. Aoki, M. E. Garcia, G. Cao, and T. E. Mallouk, Shape Selective Intercalation Reactions and Chemical Sensing in Layered Metal Phosphates and Phosphonates. ser. ASI C Multifunctional Mesoporous Inorganic Solids, C. Sequeira and M. J. Hudson, Eds: NATO, 1993, vol. 400, pp. 225 236. [12] J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, and J. L. Schlenker, A new family of mesoporous molecular sieves prepared with liquid crystal templates, J. Amer. Chem. Soc., vol. 114, pp. 10 834 10 843, 1992. [13] G. A. Ozin, C. T. Kresge, S. M. Yang, H. Yang, I. Y. Sokolov, and N. Coombs, Morphokinetics: Growth of mesoporous silica curved shapes, Adv. Mater., vol. 11, pp. 52 57, 1999. [14] I. Y. Sokolov, G. A. Ozin, G. S. Henderson, H. Yang, and N. Coombs, Beyond the hemimicellar cylindrical monolayer on graphite: AFM evidence for surface lyotropic liquid crystals, Adv. Mater., vol. 9, pp. 917 921, 1997. [15] G. A. Ozin, H. Yang, and N. Coombs, Morphogenesis of shapes and surface patterns in mesoporous silica, Nature, vol. 386, pp. 692 695, 1997. [16] G. A. Ozin, N. Coombs, I. Y. Sokolov, and H. Yang, Shell mimetics, Adv. Mater., vol. 9, pp. 662 667, 1997. [17] G. A. Ozin, H. Yang, N. Coombs, and I. Y. Sokolov, Synthesis of mesoporous silica spheres under quiescent aqueous acidic conditions, J. Mater. Chem., vol. 8, pp. 743 750, 1998. [18] G. A. Ozin, C. T. Kresge, and H. Yang, Defects in silicate liquid crystals: Role in the formation of mesoporous silica fibers, films and curved shapes, Adv. Mater., vol. 10, pp. 883 887, 1998. [19], Nucleation growth and form of mesoporous silica: Role of defects and language of shape, Stud. Surf. Sci. Catal., vol. 117, pp. 119 127, 1998. [20] G. A. Ozin, D. Khushalani, S. Oliver, G. C. Shen, I. Y. Sokolov, and H. Yang, Blueprints for organic materials with natural form: Inorganic liquid crystals and a language of shape, J. Chem. Soc. (Special Issue), pp. 3941 3952, 1997. [21] H. Yang, I. Y. Sokolov, N. Coombs, O. Dag, and G. A. Ozin, Freestanding mesoporous silica films: Morphogenesis of channel and surface patterns, J. Mater. Chem., vol. 7, pp. 1755 1761, 1997.

494 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 4, NO. 5, SEPTEMBER 2005 [22] H. Yang, N. Coombs, I. Y. Sokolov, and G. A. Ozin, Registered growth of mesoporous silica films on graphite, J. Mater. Chem. (Special Issue), vol. 7, pp. 1285 1290, 1997. [23], Free-standing and oriented mesoporous silica films grown at the interface of air and water, Nature, vol. 381, pp. 589 592, 1996. [24] M. Yang, I. Y. Sokolov, N. Coombs, C. T. Kresge, and G. A. Ozin, Supramolecular origami: Hollow helicoids of mesoporous silica, Adv. Mater., vol. 11, pp. 1427 1431, 1999. [25] I. Y. Sokolov, H. Yang, G. A. Ozin, and C. T. Kresge, Radial patterns in mesoporous silica, Adv. Mater., vol. 11, pp. 636 641, 1999. [26] D. A. Doshi, N. K. Huesing, M. Lu, H. Fan, Y. Lu, K. Simmons-Potter, B. G. Potter, Jr., A. J. Hurd, and C. J. Brinker, Optically defined multifunctional patterning of photosensitive thin-film silica mesophases, Science, vol. 290, pp. 107 111, 2001. [27] D. H. Everett, Basic Principles of Colloid Science. Cambridge, U.K.: Roy. Soc. Chem., 1988. [28] D. F. Evans and H. Wennerström, The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet, 2nd ed. New York: VCH, 1999. [29] J. M. Calvert, E. A. Chandross, T. E. Mallouk, and P. Yager, Editorial: Self-assembly and materials research, Supramolec. Sci., vol. 4, pp. 1 23, 1997. [30] G. A. Ozin, E. Chomski, D. Khushalani, and M. J. Maclachlan, Mesochemistry, Curr. Opin. Coll. Inter. Sci., vol. 3, pp. 181 193, 1998. [31] D. Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka, and G. D. Stucky, Continuous mesoporous silica films with highly ordered large pore structures, Adv. Mater., vol. 10, pp. 1380 1385, 1998. [32] S. Schacht, Q. Huo, I. G. Voigt-Martin, G. D. Stucky, and F. Schuth, Oil water interface templating of mesoporous macroscale structures, Science, vol. 273, pp. 768 771, 1996. [33] M. Grun, I. Lauer, and K. K. Unger, Homogeneous precipitation of siliceous MCM-41, Adv. Mater., vol. 9, pp. 254 257, 1997. [34] F. Kleitz, F. Marlow, G. D. Stucky, and F. Schuth, Mesoporous silica fibers: Synthesis, internal structure, and growth kinetics, Chem. Mater., vol. 15, pp. 3587 3595, 2001. [35] P. Schmidt-Winkel, P. Yang, D. I. Margolese, B. F. Chmelka, and G. D. Stucky, Fluoride-induced hierarchical ordering of mesoporous silica in aqueous acid-syntheses, Adv. Mater., vol. 11, pp. 303 307, 1999. [36] D. Zhao, P. Yang, B. F. Chmelka, and G. D. Stucky, Multiphase assembly of mesoporous macroporous membranes, Chem. Mater., vol. 11, pp. 1174 1178, 1999. [37] Ö. Dag, O. Samarskaya, N. Coombs, and G. A. Ozin, The synthesis of mesostructured silica films and monoliths functionalised by noble metal nanoparticles, J. Mater. Chem., vol. 13, pp. 328 334, 2003. [38] H.-P. Lin and C.-Y. Mou, Structural and morphological control of cationic surfactant-templated mesoporous silica, Acc. Chem. Res., vol. 35, pp. 927 935, 2002. [39] N. K. Mal, M. Fujiwara, and Y. Tanaka, Photocontrolled reversible release of guest molecules from coumarin modified mesoporous silica, Nature, vol. 421, p. 353, 2003. [40] P. Yang, Y. Wu, and R. Fan, Inorganic semiconductor nanowires, Int. J. Nanosci., vol. 1, pp. 1 39, 2002. Yaroslav Kievsky was born in Kiev, Ukraine, on May 16, 1972. He received the B.S. and M.S. degree in applied physics and mathematics from the Moscow Institute of Physics and Technology, Moscow, Russia, in 1993 and 1995, respectively, the M.S. degree in physics from Clarkson University, Potsdam, NY, in 2001, and is currently working toward Ph.D. degree in physics at Clarkson University. Mr. Kievsky is a member of the American Physical Society. Igor Sokolov was born in Saint Petersburg, Russia, on May 1, 1961. He received the B.S. and M.S. in physics and mathematics from St. Petersburg State University, St. Petersburg, Russia, in 1982 and 1984 respectively, and the Ph.D. degree in physics from the D. I. Mendeleev Central Metrology Institute, St. Petersburg, Russia, in 1991. He is currently a Professor Assistant with the Department of Physics and Department of Chemistry, Clarkson University, Potsdam, NY. He is also with the Center of Advanced Material Processing (CAMP), Clarkson University. His research interests include the study of intermolecular forces with atomic force microscopy, biomaterials, biomimetic, and nanomaterials.