pubs.acs.org/Langmuir © 2009 American Chemical Society
Silica Nanoparticles Grown and Stabilized in Organic Nonalcoholic Media Nancy El Hawi,† Celine Nayral,*,† Fabien Delpech,*,† Yannick Coppel,§ Alfonso Cornejo,† Annie Castel,‡ and Bruno Chaudret§ †
Universit e de Toulouse, INSA, UPS, CNRS, LPCNO (Laboratoire de Physique et Chimie des Nano-Objets), 135 avenue de Rangueil, F-31077 Toulouse, France, ‡Laboratoire H et erochimie Fondamentale et Appliqu ee, UMR-CNRS 5069, 118 route de Narbonne, 31062 Toulouse cedex 9, France, and §Laboratoire de Chimie de Coordination, UPR-CNRS 8241, 205 route de Narbonne, 31077 Toulouse Cedex, France Received July 18, 2008. Revised Manuscript Received May 12, 2009
This work features an alternative approach to the well-documented preparation of silica nanoparticles in protic media. We present here the one-pot synthesis of silica nanoparticles of adjustable size (between 18 and 174 nm), prepared and stabilized in organic nonalcoholic solvents. This novel route is based on hydrolysis and condensation of tetraethoxysilane, using water as reactant and different primary amines (butylamine, octylamine, dodecylamine, hexadecylamine) as catalysts in tetrahydrofuran or dimethoxyethane. The growth rate can be finely adjusted, and the first stages of the formation are observed by transmission electronic microscopy, revealing a silicated network in which the silica particles are formed and then released in solution. The amine plays not only a catalyst role but is also implied, as well as the solvent, in the stabilization process and the size control of the particles. A detailed NMR study demonstrates a core-shell structure in which the silica core is surrounded by a layer of alkylammonium ions together with solvent.
Introduction Elaboration of silica nanoparticles has attracted a steady interest in connection with a wide range of applications, from catalysis1 to biolabeling,2,3 nanopatterning,4 humidity sensors,5 polymeric composites,6 or also mechanical polishing.7 Regarding chemical methods in liquid phases, such nanoparticles can be made in water in oil microemulsions8-11 or by Stober’s method which has been widely exploited since 1968.12 Synthesis in water in oil microemulsions is an advantageous method compared to Stober processes to reach particles of diameters from 30 to 60 nm with a very high monodispersity. At the same time, these processes involve current difficulties in sample homogeneity inherent to heterogeneous synthesis media and require extensive cleaning due to the large amounts of surfactant used.13 The success of the Stober process relies essentially on an easy and inexpensive production of silica particles, exempt of contaminant, with a narrow size distribution. The procedure is based on hydrolysis and condensation of a tetraalkoxysilane (mainly tetraethoxysilane *Corresponding authors. (1) Zou, J.-L.; Chen, X.-L. Microchem. J. 2007, 86, 42. (2) Drake, T. J.; Zhao, X. J.; Tan, W. In Nanobiotechnology Concepts Applications and Perspectives; Niemeyer, C.M., Mirkin, C.A., Eds.; Wiley, 2004; Chap. 27, p 444. (3) Tan, M.; Ye, Z.; Wang, G.; Yuan, J. Chem. Mater. 2004, 16, 2494. (4) Leichle, T.; Silvan, M. M.; Belaubre, P.; Valsesia, A.; Ceccone, G.; Rossi, F.; Saya, D.; Pourciel, J.-B.; Nicu, L.; Bergaud, C. Nanotechnology 2005, 16, 525. (5) Wang, C.-T.; Wu, C.-L.; Chen, I.-C.; Huang, Y.-H. Sens. Actuators, B 2005, 107, 402. (6) Castrillo, P. D.; Olmos, D.; Amador, D. R.; Gonzalez-Benito, J. J. Colloid Interface Sci. 2007, 308, 318. (7) Yasuhiko, A.; Hiroyo, S.; Kazuaki, Y. J. Sol-Gel Sci. Technol. 2004, 32, 79. (8) Finnie, K. S.; Bartlett, J. R.; Barbe, C. J. A.; Kong, L. Langmuir 2007, 23, 3017. (9) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1999, 211, 210. (10) Esquena, J.; Tadros, Th. F.; Kostarelos, K.; Solans, C. Langmuir 1997, 13, 6400. (11) Venditti, F.; Angelico, R.; Palazzo, G.; Colafemmina, G.; Ceglie, A.; Lopez, F. Langmuir 2007, 23, 10063. (12) Stober, W.; Fink, A. J. Colloid Interface Sci. 1968, 26, 62. (13) Hartlen, K. D; Athanasopoulos, A. P. T.; Kitaev, V. Langmuir 2008, 24, 1714.
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(TEOS)) catalyzed by ammonia in a water/ethanol medium. Using the same main principle, a number of “Stober-modified” silica nanoparticle syntheses have then emerged, by varying the experimental parameters (time, temperature, concentrations and ratios of reactants14-17), using tetraalkyl ammonium18 or an amine19-21 as catalyst or different alcohols as solvents.14 All these strategies have in common the necessary involvement of protic solvents (water or alcohols) for the preparation and the stabilization of the silica nanoparticles. Therefore, this requirement excludes a direct association with chemistry processes developed in aprotic media such as many organometallic syntheses, oxidizable metallic nanoparticles, or domains of polymer chemistry. Representative examples of this potential limitation are found in the nanocomposites field for which compatibility between the organic matrix (polymer) and the inorganic fillers is of critical importance for the dispersion of silica within the polymer and, thus, for the resulting mechanical properties.22 Consequently, in order to prepare silica nanoparticles soluble in nonprotic and/or nonpolar organic solvents, various and numerous strategies have been developed to modify the surface of the particles. Examples have been described by grafting trialkoxysilanes in a second separate step23,22 or by including trialkoxysilanes during the synthesis to (14) Wang, H.-C.; Wu, C.-Y.; Chung, C.-C.; Lai, M.-H.; Chung, T.-W. Ind. Eng. Chem. Res. 2006, 45, 8043. (15) Rao, K. S.; El Hami, K.; Kodaki, T.; Matsushige, K.; Makino, K. J. Colloid Interface Sci. 2005, 289, 125. (16) Kim, K. D.; Kim, H. T. Mater. Lett. 2003, 57, 3211. (17) Rahman, I. A.; Vejayakumaran, P.; Sipaut, C. S; Ismail, J.; Abu Bakar, M.; Adnan, R.; Chee, C. K. Colloids Surf., A 2007, 294, 102. (18) Rimer, J. D.; Vlachos, D. G.; Lobo, R. F. J. Phys. Chem. B 2005, 109, 12762. (19) Davis, T. M.; Snyder, M. A.; Krohn, J. E.; Tsapatsis, M. Chem. Mater. 2006, 18, 5814. (20) Mine, E.; Hirose, M.; Kubo, M.; Kobayashi, Y.; Nagao, D.; Konno, M. J. Sol-Gel Sci. Technol. 2006, 38, 91. (21) Yokoi, T.; Sakamoto, Y.; Terasaki, O.; Kubota, Y.; Okubo, T.; Tatsumi, T. J. Am. Chem. Soc. 2006, 128, 13664. (22) Du, M.; Zheng, Y. Polym. Compos. 2007, 28, 198. (23) Suratwala, T. I.; Hanna, M. L.; Whitman, P. K.; Thomas, I. M.; Ehrmann, P. R.; Maxwell, R. S.; Burnham, A. K. J. Non-Cryst. Solids 2003, 316, 349.
Published on Web 06/04/2009
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29 Si solid-state NMR spectrum of silica nanoparticles prepared in THF. Spectrum was fully deconvoluted by Topspin software, and the single lines are reported for Q2, Q3, and Q4.
Figure 2. HP DEC
Figure 1. TEM images of silica nanoparticles obtained in THF after 7 days considering the following conditions: [TEOS] = 0.12 M, [BA] = 0.06 M, and [H2O] = 0.48 M.
form organosilica nanoparticles.24 Thus, synthesis, characterization, and surface modifications of silica nanoparticles have been fully optimized during the past decades to produce methods and materials perfectly controlled and adaptable for various applications. However, such approaches still exclude the use of a same synthesis medium to form such composites, ruling out the whole area of chemistry in nonprotic solvents. To the best of our knowledge, there has been, to date, no description of silica nanoparticles directly synthesized and stabilized in a homogeneous organic nonalcoholic medium. Interestingly, some aprotic solvents as tetrahydrofuran (THF) or dioxane are known as suitable media for the solubilization of alkylsubstituted or partially hydrolyzed alkoxysilicated compounds;25 thus, their use in the context of sol-gel chemistry has been only focused on the synthesis of hybrid organic-inorganic26,27 or of silica films.28 Herein, we present the one-pot synthesis in organic nonalcoholic media of stable silica nanoparticles with adjustable mean diameter. In addition, several challenging issues such as surface chemistry and first stages of the synthesis of these nanoparticles are also addressed.
Results and Discussion Our process is derived from the Stober method and is based on hydrolysis and condensation of TEOS in THF or dimethoxyethane (DME), using water as reactant and different primary amines (butylamine (BA), octylamine (OA), dodecylamine (DDA), hexadecylamine (HDA)) as catalysts. Water is miscible with THF and DME, and thus, the medium is homogeneous. In a typical example, the nanoparticles were synthesized in THF at 50 °C in the presence of butylamine. The relative ratios of the reactants TEOS/BA/H2O are 1/0.5/4 (with [TEOS] = 0.12 M). For a reaction time of 7 days, transmission electron microscopy (TEM) images of the as-synthesized materials showed particles with a mean diameter of 88 ((20) nm (Figure 1). No porosity has been evidenced by Brunauer-Emmett-Teller (BET) surface measurement (specific area is 43 m2/g), and this material presents a density about 1.77 g/cm3. Magic angle (24) Noguchi, A.; Kikuchi, M.; Kamiya, H.; Maeda, K. J. Mater. Chem. 2006, 16, 2170. (25) Brinker, C. J.; Scherrer, G. W. In Sol-gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: London, 1990. (26) Valle, K.; Belleville, P.; Pereira, F.; Sanchez, C. Nat. Mater. 2006, 5, 107. (27) Shimojima, A.; Kuroda, K. Angew. Chem., Int. Ed. 2003, 42, 4057. (28) Shibuya, T.; Hagiwara, Y. U.S. Patent 6,875,262 B1, 2005. (29) Bonhomme, C.; Coelho, C.; Baccile, N.; Gervais, C.; Azais, T.; Babonneau, F. Acc. Chem. Res. 2007, 40, 738.
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spinning (MAS) solid-state NMR has been shown to provide valuable information about silica structure.29 The 29Si high power decoupling (HP DEC) NMR spectrum (Figure 2) of the silica nanoparticles displays only the Q-type silicon signals (i.e., corresponding to silicon atoms forming four Si-O bonds): Q2 (Si(OSi)2(OR)2, -91 ppm), Q3 (Si(OSi)3OR, -101 ppm), and Q4 (Si(OSi)4, -110 ppm with R = H, Et, or negative charge). The proportions of the Q species (Q2, 2%; Q3, 29%; Q4, 69%) are obtained from deconvolution of the 29Si HP DEC NMR spectrum and indicate that the as-prepared nanoparticles are highly condensed (slightly more than those prepared using Stober’s method in aqueous ammonia solution in ethanol),30 with Q3 and Q4 being the main cross-linking network structures in the nanoparticles. Similar Q ratios (Q2, 2%; Q3, 28%; Q4, 70%) were obtained when synthesis was done in DME. The Fourier transform infrared (FT-IR) spectrum of the as-synthesized nanoparticles shows very strong absorption peaks between 1100 and 1000 cm-1 which are assigned to the siloxane (Si-O-Si) backbone. Additionally to Si-O stretching absorption of the SiOR moiety at 963 cm-1, weak bands from residual organic species are also detectable between 3000 and 2850 cm-1. Consistently, the elemental analysis confirms the presence of C, H, and N elements in the nanoparticles despite thorough washing and drying. The size of the nanoparticles can be adjusted by controlling the reaction parameters. In the case of Stober’s derived processes,14,15,17 it has been pointed out that temperature was a parameter affecting the rate of the reaction. However, depending on the reactant concentration ranges, the effect of rising temperature can be an increase or a decrease of particle size with no clear rationalization proposed. In our conditions (not directly comparable to the ones previously reported), the range of temperature tested is limited to 25-50 °C (because of the boiling points of the solvents used) and no significant effect can be evidenced. More interestingly, the concentration of the reactants is of critical importance (as also noted in the alcoholic Stober’s process12,14,17), since the mean diameter varies from 29 ((11) to 154 ((30) nm for a reaction time of 7 days when increasing the TEOS concentration from 0.04 to 0.2 M (while keeping constant relative ratios between reactants) (Figure 3). When the TEOS concentration reaches 0.4 M, the monodispersity cannot be controlled anymore and two populations of particles appear. The role of the amine has also been studied, and four primary amines with different lengths of alkyl chain (C4, BA; C8, OA; C12, DDA; C16, HDA) have been examined ([TEOS] = 0.04 M). Whatever the length, a minimal ratio [amine]/[TEOS] is required to ensure the formation of silica. After 7 days of reaction, (30) Lee, K.; McCormick, A. V. J. Sol-Gel Sci. Technol. 2005, 33, 255.
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Figure 3. Effect of the concentration of TEOS on the silica nanoparticle diameter after 7 days of reaction in THF.
Figure 4. Effect of the length of the amine chain on the silica nanoparticle diameter after 14 days of reaction in THF.
when the ratio [amine]/[TEOS] is lower than 1/50, no particle or aggregate can be detected by TEM and this shows that the formation of silica occurs very slowly or does not occur at all. This result is consistent with the expected and well-documented catalytic role played by the amine19-21 which is attributed to the formation of the corresponding ammonium hydroxide species. However, interestingly, while silica aggregates of undefined shape can be observed for a ratio [amine]/[TEOS] ranging from 1/50 to 1/5, the use of a ratio higher than 1/3 allowed the synthesis of well-dispersed nanoparticles. This suggests that the role of the amine goes beyond that of base catalyst and, thus, that it is involved in the stabilization process of silica particles. This assessment is further confirmed when comparing the syntheses using BA, OA, DDA, and HDA ([TEOS] = 0.04 M with the ratio of the reactants TEOS/amine/H2O being 1/0.5/4). The mean diameter of particles clearly increases when the length of the chain decreases (Figure 4). This could be imputed to the higher mobility of the shorter chains (see table in the Diffusion-Ordered NMR Spectroscopy (DOSY) Experiments section the Supporting Information) leading to a lower steric hindering around the particles. In addition, it has to be noted that the formation of individual nanoparticles is also closely related to the nature of the solvent used for the reaction. Among the following aprotic solvents toluene, mesitylene, anisole, dioxane, THF, and DME and in the same conditions ([TEOS] = 0.04 M, with the ratio of the reactants TEOS/BA/H2O being 1/0.5/4), only the last two (the most polar ones) have led to individual silica nanoparticles. In the case of toluene and mesitylene in which water is not soluble, the synthesis medium is an emulsion which leads to formation of large silica blocks. For the other solvents, hydrolysis and condensation occur but tend to form aggregates with undefined shape and very large size distribution. Unexpectedly, a protic and polar solvent such as ethanol is also totally inappropriate to form particles, and only reticulated agglomerates are obtained. Interestingly, as it can be seen in Figure 5, the reaction leads to significantly larger particles in DME than in THF, with the slope in DME being, indeed, more than 3 times the one measured in THF. Thus, in the same conditions, after 7 days of reaction, the mean diameter of silica nanoparticles prepared in THF is around 29 nm whereas in DME it is around 62 nm. The physical constants of THF and DME are quite similar and, thus, provide no clear evidence to rationalize the solvent-dependent behavior. However, as will 7542 DOI: 10.1021/la9011789
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Figure 5. Effect of the solvent on the silica nanoparticle diameter.
be seen thereafter, a detailed NMR study will allow elucidation of this issue. This array of observations suggests that the amine stabilizing behavior is related to the interplay between several factors such as the water solubility, the polarity of the medium (less polar solvents being inappropriate, but a polar and protic solvent such as ethanol being also unsuitable), but also the length of the alkyl chain of the amine. Thus, playing on the different parameters examined here, we can prepare silica nanoparticles of chosen size, by selecting the appropriate amine, solvent, concentration, and reaction time. For example, using BA in THF and a reaction time of 2 days, tuning the concentrations leads to sizes adjustable from 18 to 174 nm. The fine control of the reaction parameters also offers the opportunity to slow down the reaction time and to study the first stages of the synthesis. In aqueous systems, this issue still remains very controversial and is a source of debates.31-34 In our case (nonalcoholic media), observation of the temporal evolution has been monitored by TEM (Figure 6) using (i) HDA as amine and (ii) diluted solutions ([TEOS] = 0.04 M, [HDA] = 0.02 M, [H2O] = 0.20 M) in THF at 50 °C. After 24 h of reaction, we do not observe primary isolated seeds but instead a polymeric silicated network. Then, the particle growth occurs inside the network, by a progressive condensation around seeds included in the network. Indeed, after 7 days, particles connected to each other by fine links can be distinguished inside the network (Figure 7). Progressively, theses links are then broken and the particles are released from the network. When choosing conditions leading to a faster growth of particles, for instance, by using butylamine instead of hexadecylamine or a higher concentration of reactants, these steps cannot be evidenced anymore. Our observations are in accordance with the study by Harris and co-workers33 (coupling NMR, SAXS, and DLS techniques for monitoring silica nanoparticle formation in aqueous ethanol catalyzed by ammonia), showing that the first nuclei result from the build-up of singly hydrolyzed monomers leading to a fractal and open structure. In order to gain some insight into the precise role of the amine in our systems, a detailed NMR study has been undertaken. Indeed, a comprehensive knowledge of the nature of its potential interaction with silica and its location is of particular interest but remains a highly challenging issue. For instance, in the case of aqueous synthesis of periodic arrangement of silica nanoparticles catalyzed by lysine, the silica core/lysine shell description initially proposed21 to account for the ordered structure is subject to controversy and has been recently ruled out.35 In the aqueous Stober process involving tetraalkylammonium hydroxide, the (31) Provis, J. L.; Vlachos, D. J. Phys. Chem. B 2006, 110, 3098. (32) Boukari, H.; Long, G. G.; Harris, M. T. J. Colloid Interface Sci. 2000, 229, 129. (33) Green, D. L.; Lin, J. S.; Lam, Y.-F.; Hu, M. Z.-C.; Schaefer, D. W.; Harris, M. T. J. Colloid Interface Sci. 2003, 266, 346. (34) Masse, S.; Laurent, G.; Chuburu, F.; Cadiou, C.; Dechamps, I.; Coradin, T. Langmuir 2008, 24, 4026. (35) Snyder, M. A.; Lee, J. A.; Davis, T. M.; Scriven, L. E.; Tsapatsis, M. Langmuir 2007, 23, 9924.
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Figure 6. TEM images showing the temporal evolution of the silica nanoparticles in THF.
Figure 7. TEM images of the particles linked to each other inside the network after 7 days of reaction in THF.
Figure 8. Comparison of 1H-13C CP MAS spectra for silica nanoparticles prepared in DME (a), in THF (b), for butylammonium chloride (c), and for butylamine (d).
core-shell structure is commonly admitted while the coverage of the surface and/or the nature of adsorption sites remains unclear.36 In this nonaqueous system, NMR characterizations of the nanoparticles obtained in THF and DME using the experimental conditions of a typical example previously mentioned above (TEOS/butylamine/water in a 1/0.5/4 ratio at 50 °C in THF or DME with [TEOS] = 0.12 M) are presented here. Figure 8 shows the cross-polarization (CP) MAS 1H-13C NMR spectra (of a washed and dried sample of THF- and DME-derived nanoparticles) and allows the identification of solvent resonances at 68 and 25 ppm for THF and at 58 and 71 ppm for DME. The remaining resonances (at 40, 30, 20, and 13 ppm) are assigned to a butyl moiety. Solution 13C NMR analysis of the initial reaction mixture shows that equilibrium between butylamine and butylammonium (36) Rimer, J. D.; Lobo, R. F.; Vlachos, D. G. Langmuir 2005, 21, 8960.
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Figure 9. 1H MAS (14.5 kHz) NMR spectrum of silica nanoparticles prepared in THF.
hydroxide exists but is significantly shifted toward the amine, with the molar fraction of the ammonium being close to 10%. Comparison of the CP MAS 1H-13C NMR spectra of the nanoparticles to the ones of butylamine and of butylammonium chloride (used as reference) strongly supports the sole presence of butylammonium ions in the as-synthesized silica nanoparticles (Figure 8). In the case of nanoparticles prepared in THF, the 1H MAS NMR spectrum (Figure 9) confirms the presence of THF (peaks at 1.8 and 3.7 ppm) and displays the methyl resonances of the alkyl chain at 1.1 ppm, with the other resonances of the butylammonium moiety being hidden by the ones of THF. In addition, a broad signal at 6.2 ppm allows the identification of silanol groups involved in hydrogen bonds.37,38 Similar conclusions (presence of (37) Trebosc, J.; Wiench, J. W.; Huh, S.; Lin, V. S.-Y.; Pruski, M. J. Am. Chem. Soc. 2005, 127, 3057. (38) Saalwachter, K.; Krause, M.; Gronski, W. Chem. Mater. 2004, 16, 4071.
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Figure 10. 1H-29Si HETCOR solid-state NMR spectra for silica nanoparticles synthesized in THF (contact times of 0.5 ms (a) and 5 ms (c)) and in THF-D8 (contact times of 0.5 ms (b) and 5 ms (d)).
Figure 11. Graphical schematic representation of the interactions observed between the Q sites and THF and butylammonium molecule.
residual solvent molecules, of butylammonium, and of silanol) can be deduced from the 1H MAS NMR spectrum of DME-derived nanoparticles (see Supporting Information, Figure S1). The solid-state two-dimensional (2D) 1H-29Si heteronuclear NMR correlation (HETCOR) sequence is a key NMR technique for investigating the organic and inorganic interfaces at a molecular level.29 It relies on dipolar coupling between silicon atoms and protons and has recently provided detailed characterization of the surface of mesoporous silicas.37,39 In particular, the spatial connectivities between species at the interface can be established (on the order of 0.5 nm) by using the dipolar through-space interactions, with the relative vicinity being inferred from the contact time (τC) employed for the HETCOR sequence. The use of short contact time (τC = 0.5 ms) allows the selective observation of silicon and proton nuclei which are spatially very close. In our case for nanoparticles prepared in THF, a broad correlation peak (-100, 6.2 ppm) between the surface OH groups and the Q3 sites is observed and evidences their connectivity (Figure 10a). Strong cross-peaks are also viewed between the THF protons (3.7 and 1.8 ppm) and the surface Q3 groups, showing clearly the presence of adsorbed THF molecules. In contrast to the protons located in the R position to the oxygen atom of THF (3.7 ppm), (39) Baccile, N.; Maquet, J.; Babonneau, F. C. R. Chim. 2006, 9, 478.
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the ones located in the β position (1.8 ppm) display a strong correlation with the Q4 sites, suggesting a preferential orientation of THF toward silica (Figure 11). When running the same NMR experiment with silica nanoparticles synthesized in THF-D8 (Figure 10b), the resulting spectrum displays only the correlation between the surface OH groups and the Q3 sites. These two experiments confirm the sole presence of THF in the close vicinity of the surface. Using a longer contact time (5 ms), we observe extra correlation peaks, which arise from longer range dipolar through-space interactions (Figure 10c and d). In the case of the synthesis in nondeuterated THF (Figure 10c), definitive proof of a spatial proximity between silica surface and butylammonium is the identification of a cross-peak between the methylene group adjacent to the ammonium head (around 3 ppm; see Figure S2 in the Supporting Information for the 1 H NMR spectrum of butylammonium chloride used as reference) and the surface Q3 sites (Figure 11). The use of the deuterated solvent allows the identification of an extra correlation peak between Q sites and resonances located approximately between 0 and 2 ppm, assigned to the other resonances of the butylammonium ion, that is, the ones of the propyl moiety (Figure 10d). These results support a silica core/organic shell structure for the silica nanoparticles prepared in THF. The shell is essentially composed of adsorbed THF and butylammonium ions, with the former displaying stronger interactions with the silica surface (Figure 11). The presence of amine-derived species at the surface is in perfect agreement with their involvement in the stabilizing process (as demonstrated by the amine length dependence of nanoparticle size), but these results also show that THF acts as a stabilizer. Surprisingly, 1H-29Si HETCOR NMR analysis of silica nanoparticles prepared in DME drives to different conclusions concerning the respective location of the butylammonium ion and the solvent. Using short a contact time (0.5 ms), the 2D NMR spectra displays only correlations between silicon Q sites and butylammonium resonances (Figure 12a): On one hand, the two correlations involving the alkyl chain (methylene adjacent to Langmuir 2009, 25(13), 7540–7546
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Figure 12. 1H-29Si HETCOR solid-state NMR spectra for silica nanoparticles synthesized in DME (contact times of 0.5 ms (a) and 5 ms
(b)). In spectra b, the correlation between Q3 sites and the NH3+ group is still present, but for the sake of clarity the contour plot was put to a higher level where this correlation is not visible anymore.
Figure 13. Graphical schematic representation of the interactions observed between the Q sites and DME and butylammonium molecule.
N atom ammonium signal at ∼3 ppm and remaining alkyl moiety chain resonances around 1 ppm) clearly show the proximity of the butyl fragment with the silica surface. On the other hand, the correlation of the more acidic protons (between 5 and 8.5 ppm) that spreads over the range of the 1H chemical shift observed for the ammonium head NH3+ (∼8 ppm as indicated by arrow; see Figure S2 in the Supporting Information for 1H NMR spectrum of butylammonium chloride used as reference) confirms this spatial arrangement. Examining longer distances by increasing contact times (5 ms) allows the observation of an extra correlation which involves Q signals and DME resonances at δ 3.4 ppm (Figure 12b). This information is illustrated in the schematic representation of the silica surface of the nanoparticles prepared in DME (Figure 13). In summary, this study provides a rare example of a comprehensive description of the coordination sphere of silica nanoparticle surfaces. In the case of THF, the solvent is present on silica nanoparticles in the first coordination sphere, with butylammonium being located in the second coordination sphere. In contrast, in the case of DME, the butylammonium ion is in direct contact with the silica surface while the solvent is located further. As previously reported, THF displays higher affinity for silanol than DME40 thanks to its higher hydrogen bond acceptance,41 and therefore, THF interacts stronger than DME with the silica surface. This ability of THF accounts for the size-dependency reported in this paper and results in the formation of smaller nanoparticles. The solvent-dependent size of the nanoparticles can, thus, be rationalized by the solvent-dependent structure of the organic shell surrounding the silica core.
Conclusion This study presents an alternative and complementary approach to the well-established and versatile synthesis of silica (40) Kagiya, T.; Sumida, Y.; Tachi, T. Bull. Chem. Soc. Jpn. 1970, 43, 3716. (41) Markus, Y. Chem. Soc. Rev. 1993, 409.
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nanoparticles in alcoholic solution. In addition, it leads to the first precise and unambiguous description of the coordination sphere which is unexpectedly solvent-dependent. Silica nanoparticles of adjustable size (from 18 to 174 nm) stabilized by an ammonium/ solvent shell in THF or DME are synthesized in a one-step reaction. The growth process rate can be easily regulated in such solvents and can be then more easily studied. Initial steps have, indeed, been followed by TEM showing first a silicated network from which particles are released progressively. Favorably, these ammonium stabilized particles are fully compatible with amine stabilized nanoparticles of metal or semiconductor stable in nonprotic solvents. This compatibility opens up new perspectives to control composite (for instance, metal-silica or silica-metal core-shell nano-objects) formation in such organic media. As a first example, this silica synthesis process has been successfully applied for the coating of metallic magnetic nanoparticles42 prepared and stabilized in nonprotic solvents. Further developments related to this unprecedented nonalcoholic synthetic approach are expected, especially the design of heterogeneous catalysts based on supported metallic nanoparticles dispersed onto surfaces of these silica particles.
Experimental Section All manipulations were carried out under argon atmosphere using Fischer-Porter bottle and vacuum line techniques, or in a glovebox. Tetraethoxysilane was purchased from Alfa Aesar, and hexadecylamine (HDA; g99%), dodecylamine, (DDA; g99%), octylamine (OA; g99%), and butylamine (BA; g99.5%) were from Fluka. Dimethoxyethane was purchased from Acros Organics (99.9%). Tetrahydrofuran was purchased from SDS (99.9%), dried, and distilled over sodium/ benzophenone. All reagents and solvents were degassed before use by using three freeze-pump-thaw cycles. Samples for TEM analysis were prepared in a glovebox by slow evaporation of a drop of the colloidal solution deposited onto a carboncovered copper grid. TEM analyses were performed at the “Service Commun de Microscopie Electronique de l’Universite Paul Sabatier” (TEMSCAN) on a JEOL JEM 200 CX electron microscope operating at 200 kV with a point resolution of 4.5 A˚ and on a JEOL JEM 1011 electron microscope operating at 100 kV with a point resolution of 4.5 A˚. The size distributions were determined by measuring ∼150 particles using Image J software (variation range is given by 3σ). IR spectra were recorded on a Perkin-Elmer Spectrum 100 FT-IR spectrometer. Solid-state NMR experiments were performed on a Bruker Avance 400WB instrument equipped with 4 mm probe, with the sample rotation frequency being set at 12 kHz unless otherwise indicated. All 1H, 29Si, and 13C chemical shifts are (42) Delpech, F.; Nayral, C.; El Hawi, N. Fr. Patent 07/08107, 2007.
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Article reported using the δ scale and are referenced to tetramethylsilane (TMS) at 0 ppm. In a typical experiment, tetraethoxysilane (0.53 mL, 2.4 mmol) was added to a solution of butylamine (0.12 mL, 1.2 mmol) in tetrahydrofuran (20 mL) in a Fischer-Porter bottle at room temperature under vigorous stirring. After 10 min, water (0.173 mL, 9.6 mmol) was injected into this mixture which was left stirring at 50 °C for 7 days. The resulting milky suspension was centrifuged at 20 000 rpm for 20 min, yielding silica nanoparticles as a white solid at the bottom of the tubes. The supernatant was eliminated, and the precipitate was then washed three times with tetrahydrofuran. A white powder (195 mg) was obtained after drying the precipitate under vacuum for 6 h. Elemental analysis: 6.3%, C; 2.0%, H; 0.7%, N.
7546 DOI: 10.1021/la9011789
El Hawi et al. The same procedure was followed for the reactions carried out in the different conditions except for the reagent quantities which were adapted to a constant solvent volume of 20 mL.
Acknowledgment. N.E.H. is grateful to the Fond Social Europeen for a fellowship. A.C. is grateful to the Spanish Ministerio de Ciencia e Innovacion for a postdoctoral grant. Supporting Information Available: 1H MAS (30 kHz) NMR
spectrum of silica nanoparticles prepared in DME, 1H MAS (30 kHz) NMR spectrum of butylammonium chloride, experimental procedure for NMR diffusion experiment, and diffusion coefficients table. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2009, 25(13), 7540–7546
