© Copyright 2007 American Chemical Society
OCTOBER 23, 2007 VOLUME 23, NUMBER 22
Letters Preparation of Highly Monodispersed Hybrid Silica Spheres Using a One-Step Sol-Gel Reaction in Aqueous Solution Yong-Geun Lee, Jae-Hyung Park, Chul Oh, Seong-Geun Oh,* and Young Chai Kim Department of Chemical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea ReceiVed March 7, 2007. In Final Form: September 17, 2007 The successful one-step preparation method of monodisperse hybrid silica particles was studied using organosilane chemicals in aqueous solution. In general, almost all of the hybrid silica materials were made by a complex method where organic materials were coated on the surface of silica substrate via chemical reaction. However, our novel method can be applied to prepare colloidal hybrid particles without using substrate material. This method has three advantages: (i) this simple method gives the opportunity to prepare hybrid particles with high monodispersity through the self-hydrolysis of various organosilane monomers in aqueous solution, (ii) this efficient method can be applied to load lots of organic functional groups on the surface of silica particles through a one-step preparation method using only organosilane, and (iii) this effective method can be used to control the particle size of the product by changing the experimental conditions such as the concentration of the precursor or the reaction temperature. Detailed characterization of the hybrid particles by scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis (TGA) was performed to elucidate the morphologies and properties of the hybrid silica particles.
1. Introduction Nowadays, the development of hybrid materials is a subject of powerful interest in the chemistry and physics fields. Especially, surface modification1 is important to immobilize useful materials in substrate materials by a chemical reaction. This significant progress has been used in silica particles including thiol, amine, vinyl, and other organic groups. A wide range of wet-chemistry methods have been employed for these organic groups to modify or coat colloids, aiming for application in catalysis,2 luminescence,3,4 and biosensors.5,6 However, most of these procedures * To whom correspondence should be addressed. E-mail: seongoh@ hanyang.ac.kr. (1) Badley, R. D.; Ford, W. T.; McEnroe, F. J.; Assink, R. A. Langmuir 1990, 6, 792. (2) Lee, J. M.; Kim, D. W.; Jun, Y. D.; Oh, S. G. Mater. Res. Bull. 2006, 41, 1407. (3) Rossi, L. M.; Shi, L.; Quina, F. H.; Rosenzweig, Z. Langmuir 2005, 21, 4277. (4) Wang, L.; Li, Y. Small 2007, 3, 1218. (5) Oh, C.; Lee, J. H.; Lee, Y. G.; Lee, Y. H.; Kim, J. W.; Kang, H. H.; Oh, S. G. Colloids Surf., B 2006, 53, 225.
unfortunately require inconvenient complex processing steps. Silica particles as a substrate were prepared and redispersed in reaction medium. And then, using an organosilane, the organic source was attached to the surfaces of silica spheres. Furthermore, when the bare silica particles are used as substrates for the deposition of organic groups, the coverage of organic groups on the silica surface is low because silica spheres with a nonporous characteristic are formed during the Sto¨ber-Fink-Bohn (SFB) process.7 Monodisperse silica materials are one of the most attractive issues in colloidal science. After the discovery of monodisperse silica particles in 1968, many researchers were dedicated to creating silica materials with a narrow size distribution, controlled size and shape, and various surface properties via the SFB process because they are ideal candidates for applications in several fields such as ceramics, magnets, semiconductors, and chromatographic adsorbents. Unfortunately, the SFB synthesis needs large (6) Kim, J. B.; Grate, J. W. Nano Lett. 2003, 3, 1219. (7) Sto¨ber, W.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62.
10.1021/la702462b CCC: $37.00 © 2007 American Chemical Society Published on Web 09/29/2007
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Figure 1. Scanning electron microscopy (SEM) images of highly monodisperse and micrometer sized silica particles modified with thiol organic groups. The samples were prepared using (A) 1 g of MPTMS, (B) 10 g of MPTMS, and (C) 20 g of MPTMS in 100 g of aqueous solution, and the reaction was conducted at room temperature.
quantities of alcohol to generate the homogeneous phase for producing a monodisperse shape. Additionally, although silica particles formed by the SFB process present a high monodispersity, it is difficult to obtain a high yield of silica spheres. Here, we report a facile, reproductive one-step synthetic approach in which silica particles have many organic groups on their surfaces with a mean diameter larger than 1 µm. The remodeled sol-gel process was used because it is particularly well suited to create hybrid silica materials and because the particles are formed at low temperature, at which organic groups are not degraded. There are several major advantages in our method as follows. First, self-hydrolysis is employed in the fabrication of silica particles having a highly monodispersity. It is an effective method to obtain monomodal particles in a nonalcohol solution. Second, it is a really simple mechanism for preparing particles loading a lot of organic materials. The efficient one-step preparation of hybrid silica particles just needs an organosilane without the other precursors such as tetramethyl orthosilicate (TMOS), TEOS, or tetrapropyl orthosilicate (TPOS). Through this method, a high yield of monodiperse hybrid particles was obtained under the basic conditions. Moreover, the size of the particles can be effectively well-defined and controlled depending upon the synthesis conditions such as the concentration of organosilane and the reaction temperature. 2. Experimental Section Materials. 3-Mercaptopropyl trimethoxysilane (MPTMS), 3-mercaptopropyl triethoxysilane (MPTES), vinyltrimethoxysilane (VTMS), octyltrimethoxysilane (OTMS), and Tween 20 (nonionic surfactant) were purchased from Sigma-Aldrich Chemical Co. Ammonium hydroxide (NH4OH, 25%) was purchased from Wako Pure Chemical Industry. All chemicals were used as received without further
purification. Water was obtained from a Milli-Q water purification system (Millipore). Synthesis. Each amount of MPTMS (1, 10, and 20 g) was added into 100 g of water under vigorous stirring until the oil (MPTMS) droplets completely disappeared and a transparent solution was obtained. NH4OH (0.1 mL) was added to the mixture solution (pH 11), and then the reaction progressed during 12 h at room temperature. After completion of the reaction, the solution was kept at room temperature. The resulting precipitate was centrifuged and washed several times using alcohol. Also, MPTMS silica spheres (functionalized by thiol groups) were fabricated at room temperature (around 27 °C), 40, 60, and 80 °C. VTMS silica spheres (functionalized by vinyl groups) were fabricated by adding 10 g of VTMS at room temperature. OTMS silica particles were prepared using 10 g of OTMS with 1 g of Tween 20 at room temperature. The procedures for preparing VTMS and OTMS silica particles are based on the synthetic method of MPTMS silica spheres. Contrary to the common sol-gel reaction for the formation of silica particles, ethanol was not added to dissolve the precusors in this study.
3. Results and Discussion SEM images of silica particles fabricated by 3-mercaptoropyl trimethoxysilane (MPTMS) are shown in Figure 1. It is clear from these images that the silica particles have a monodisperse shape and are micrometer sized. The particle size is systematically controlled by adjusting the concentration of the precursor, MPTMS, in the presence of NH4OH. When the amount of MPTMS precursor is 1 g, highly monodisperse silica particles are obtained with a diameter of 1.2 µm. A further increase in the concentration of MPTMS to 10 and 20 g resulted in 2.5 and 3.7 µm particle sizes, respectively. Interestingly, the size of the silica particles tends to increase linearly depending on the amount of MPTMS as shown in Figure 1. As the concentration of MPTMS
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Figure 2. Typical SEM images of the as-prepared particles obtained at reaction temperatures of (A) 40 °C, (B) 60 °C, and (C) 80 °C. Each sample reveals (A) 2.2, (B) 1.7, and (C) 1.6 µm diameters. The 10 g amount of MPTMS precursor was used. Table 1. Mean Diameters of Silica Colloid Particles Synthesized under Various Conditions reaction mixture composition organosilane water (g) (g) sample 1 sample 2 sample 3 sample 4 sample 5 sample 6 sample 7 sample 8
1 g of MPTMS 10 g of MPTMS 20 g of MPTMS 10 g of MPTMS 10 g of MPTMS 10 g of MPTMS 10 g of VTMS 10 g of OTMS
100 100 100 100 100 100 100 100
reaction temp (°C)
mean diameter (µm)
room temp room temp room temp 40 60 80 room temp room temp
1.2 ( 0.04 2.5 ( 0.05 3.7 ( 0.14 2.2 ( 0.04 1.7 ( 0.04 1.6 ( 0.05 1.0 ( 0.02 5.2/1.3
was altered from 1 to 20 g, the particle diameter presented a noticeable change from 1.2 to 3.7 µm, respectively (Table 1). Additionally, various sizes of particles can be selectively obtained at different reaction temperatures because temperature directly influences the particle size. At temperatures below 60 °C (room temperature and 40 °C), the particle size decreases when the reaction temperature increases (Figure 2). At a reaction temperature higher than 60 °C (80 °C), the particle size decreased to 1.6 µm (Table 1). The growth mechanism and final structure of these silica particles can be interpreted by the research of LaMer and Dinegar.8 According to their literature, nuclei particles that are less than 20 nm are formed during an initial stage of the sol-gel reaction. As the reaction temperature increases, the reaction rate increases so that more nuclei particles are formed. Thus, when the temperature increases, the growth process of the nuclei is limited and the average particle size is decreased. There is an important factor to explain the formation mechanism of highly monodisperse particles functionalized by organosilanes. (8) LaMer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4847.
The essential clue is an elimination of the insoluble impurities which cause the heterogeneous nucleation forming bimodal particle sizes.9 In the preparation of silica particles, it is generally impossible to perfectly mix TEOS and water. Because of the different chemical properties of TEOS and water, a TEOS/water solution is quickly separated into TEOS and water phases (heterogeneous state). To prevent separation of the TEOS/water solution, in general, a large amount of alcohol has been used as a reaction medium to dissolve TEOS in water. Contrary to the case of TEOS, it is interesting to note that MPTMS does not need alcohol to be dissolved in water. MPTMS is easily hydrolyzed into organosilanetriol by self-hydrolysis. Although the MPTMS/water solution appeared turbid at the initial stage, it became a transparent solution through mechanical stirring for 1 h, and the MPTMS droplets completely disappeared due to self-hydrolysis. The unique self-hydrolysis phenomenon of MPTMS is related to its molecular structure. A MPTMS molecule has three short carbon chains suitable for changing from alkoxy groups to hydroxy groups. In addition, MPTMS released methanol, which was generated as a byproduct during the hydrolysis reaction.10 Methanol helps the MPTMS precursor to be fully dissolved into water. Subsequently, MPTMS is polymerized in the alcohol-free sol-gel system, and the traditional cosolvent concept proposed in the SFB method is unnecessary.11 In the case of preparing silica particles by using OTMS, our synthetic method is faced with the problem that it is difficult to dissolve octyltrimethoxysilane (OTMS) in water because it has a long alkyl chain of eight carbons. It is clear that relatively long alkyl chains of molecules have hydrophobic properties. To (9) Fine Particles; Sugimoto, T., Ed.; Marcel Dekker, Inc: New York, 2000. (10) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (11) Avnir, D.; Kaufman, V. R. J. Non-Cryst. Solids 1987, 192, 180.
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Figure 3. Representative SEM image of the non-monodispersive silica particles formed in the non-homogeneous state (OTMS/water mixture).
disperse hydrophobic materials such as OTMS in aqueous solution, a surfactant (Tween 20) was used. However, when the surfactant was added into water, the homogeneous solution was converted into a heterogeneous solution (emulsion phase) including OTMS droplets of various sizes. As a result, silica particles with broad size distributions were obtained in the heterogeneous solution (Figure 3).12 The collected OTMS particles were composed of about 26% small spheres and 74% large spheres. The diameters of the small and large spheres were revealed to be about 1.7-2.3 and 4.6-5.1 µm, respectively. Theoretical and experimental studies were conducted to better understand the relation between the number of carbon chains of organosilane and the size of the resulting products.13 In our study, additional experiments were performed to control the particle size by using VTMS and OTMS. The experiments were conducted under the same experimental conditions as those in the case of MPTMS (concentration, temperature, stirring rate, etc.). Figure 4 shows a SEM image of spheres covered by vinyl organic groups. As shown in this figure, the particles have a uniform diameter. Figure 3 presents the SEM image of silica particles made by using OTMS. Although the particles have a broad size distribution, it has a larger scale compared with that of the VTMS silica particles and the maximum particle diameter is about 5 µm. As the number of carbon chains increases by 2 < 3 < 8 in VTMS (Figure 4) < MPTMS (Figure 1B) < OTMS (Figure 3), the particle sizes also increase by the same order: 1 µm < 2.5 µm < 5 µm. These results confirm that the presence of organosilanes (12) Lee, Y. G.; Oh, C.; Yoo, S. K.; Koo, S. M.; Oh, S. G. Microporous Mesoporous Mater. 2005, 86, 134. (13) Colloids and Colloid Assemblies; Caruso, F., Ed.; Wiley-VCH: Weinheim, 2004.
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Figure 4. SEM image showing that the size of the VTMS silica particles is smaller (1 µm) than that of the MPTMS silica particles due to the different carbon chain length. The VTMS silica particles were made under the same conditions as those for MPTMS (10 g of VTMS and room temperature).
with a diverse number of carbon chains has an influence on silica particle size.
4. Conclusions In summary, we describe a synthetic method for highly monodispersed hybrid silica spheres through a simple and effective approach for the first time. The advantages of the presented method are a one-pot route and size selectivity controlled by the concentration of organosilane and the reaction temperature. It was found that the self-hydrolysis of organosilane plays an important role in preparing the monodisperse shape. However, since an organosilane such as OTMS has a strong hydrophobic characteristic, though a little self-hydrolysis occurs, monodisperse silica particles are not obtained. The organic molecules functionalized on the surface provide silica particles with chemically active sites. This effective approach can be readily extended to the immobilization of other organic or inorganic materials and can open up new potential avenues for the surface functionalization and hybrid materials synthesis. Acknowledgment. This research was supported by a project (2006-N-HY08-P-01-0-000) granted by the Korea Energy Management Corporation sponsored by the Ministry of Commerce, Industry and Energy. Supporting Information Available: Molecular structures of MPTMS, MPTES, OTMS, VTMS, and APTMS; details about why micrometer sized particles cannot be formed using APTMS; and TGA/ DTA and EDX data for measurements of the amount of thiol groups in hybrid silica particles. This material is available free of charge via the Internet at http://pubs.acs.org. LA702462B
