ARTICLE pubs.acs.org/Langmuir
Growth of Mesoporous Silica Nanoparticles Monitored by Time-Resolved Small-Angle Neutron Scattering Martin J. Hollamby,*,†,|| Dimitriya Borisova,† Paul Brown,‡ Julian Eastoe,‡ Isabelle Grillo,§ and Dmitry Shchukin† †
Max Planck Institute for Colloids and Interfaces, Wissenschaftspark Potsdam-Golm, Am M€uhlenberg 1 OT Golm, 14476 Potsdam, Germany ‡ School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. § Institute Max-von-Laue-Paul-Langevin, BP 156-X, F-38042 Grenoble, Cedex, France
bS Supporting Information ABSTRACT: Since the first development of surfactanttemplated mesoporous silicas, the underlying mechanisms behind the formation of their structures have been under debate. Here, for the first time, time-resolved small-angle neutron scattering (tr-SANS) is applied to study the complete formation of mesoporous silica nanoparticles. A distinct advantage of this technique is the ability to detect contributions from the whole system, enabling the visualization not only of particle genesis and growth but also the concurrent changes to the coexistent micelle population. In addition, using contrast-matching tr-SANS, it is possible to highlight the individual contributions from the silica and surfactant. An analysis of the data agrees well with the previously proposed “current bun” model describing particle growth: Condensing silica oligomers adsorb to micelles, reducing intermicellar repulsion and resulting in aggregation to form initial particle nuclei. From this point, the growth occurs in a cooperative manner, with condensing silica filling the gaps between further aggregating micelles. The mechanistic results are discussed with respect to different reaction conditions by changing either the concentration of the silica precursor or the temperature. In doing so the importance of in situ techniques is highlighted, in particular, tr-SANS, for mechanism elucidation in the broad field of materials science.
’ INTRODUCTION Mesoporous silica has found widespread application in a range of industrial and laboratory-based methods, particularly in catalysis and separation technology. Bulk mesoporous silica has historically been prepared by solgel-based methods13 in which rodlike templates formed in the early stages of the reaction direct the growth of ordered arrays of similarly sized pores.4 Depending on the conditions, different pore structures can be obtained (hexagonal, cubic, etc.), and their formation mechanisms have been studied by techniques including small-angle X-ray and neutron scattering (SAXS and SANS, respectively) and X-ray diffraction (XRD).510 In some cases, phase diagrams have been assembled,11 equating synthesis parameters to the final pore structure. Such detailed knowledge not only allows for an accurate reproduction of existing materials but also aids in the design of new structures. More recently, similar surfactant-templated solgel methods were refined, permitting the generation of nanoscale (diameter of less than 100 nm) particles of mesoporous silica (MSNs).1218 The high colloidal stability of the MSNs, in addition to their large pore volume has led to increased interest in MSN use for a variety of delivery applications.1923 The concentration of surfactant employed in their preparation is typically small (∼102 mol dm3) compared to that used to form the bulk material (which can be as r 2011 American Chemical Society
much as 50 wt %)4 and far lower than concentrations known to form liquid-crystal phases in an aqueous dispersion. It is therefore clear that the mechanism behind the formation of MSNs might differ from the bulk case.12,13,18,24 Prior studies have proposed several different routes, typically based on the silicate ions either preferentially adsorbing and condensing at the micelle interface or forming short prepolymer units that then bind micelles.24,25 In this initial step, the formation of larger structures occurs by either micelle aggregation, to form a “current bun” structure, or by cooperative self-assembly, whereby silica adsorption provokes micelle elongation and subsequent aggregation (Scheme 1). Such mechanistic evidence has typically been obtained by the use of imaging techniques (particularly high-resolution transmission electron microscopy), the early-stage interpretation of which can suffer inherent difficulties (e.g., low resolution) plus further crystallization or reorganization upon drying. Techniques that are able to visualize the process in situ (e.g., small-angle scattering) have clear advantages in observing these early reaction stages. Because of the continuing improvement in beamline Received: August 8, 2011 Revised: October 25, 2011 Published: November 01, 2011 4425
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Langmuir Scheme 1. Previously Proposed Mechanisms for MSN Formation (Based on Diagrams and Descriptions in References 2224)a
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Table 1. Summary of the Important Experimental Parameters for Reactions AFa case T/°C [TMOS]/g dm3
SANS contrast
A
40
14
D2O/h-CTAB/h-TEA/h-TMOS
B C
40 40
14 14
cms-water/h-CTAB, cms-TEA/cms-TMOS H2O/h-CTAB/h-TEA/h-TMOS
D
40
7
D2O/h-CTAB/h-TEA/h-TMOS
E
40
28
D2O/h-CTAB/h-TEA/h-TMOS
F
60
14
D2O/h-CTAB/h-TEA/h-TMOS
a
cms denotes contrast-matched to silica. In case B, the surfactant reorganization is followed, but in case C, the silica assembly is studied.
a
Triangles represent small silicate nuclei (either individual ions or small prepolymer units).
technology and accessibility, time-resolved SANS studies are now becoming more routine.26,27 A significant advantage of SANS is contrast variation, which allows the effective “switching off ” of contributions to the overall scattering from different system components by selective deuteration. SANS is therefore particularly appropriate for interrogating the formation of small MSNs because of its ability to monitor both micelle interactions and silica particle growth selectively. Here, for the first time, time-resolved SANS is used to determine the growth mechanism of small MSNs (d < 20 nm). A benefit of the chosen system is that the final nanoparticles remain within the size range of SANS using the same instrument configuration on the powerful diffractometer at D22, ILL, France. For any application, the system could easily be concentrated by centrifugation. By matching the SANS signal from the solvent and other reagents to the silica or surfactant, information about the growth of the MSNs with and around the surfactant template is deduced. Fitting the data gives insight into the changes not only in MSN size but also in the development of porosity and the reduction in size, population, and interactions of coexisting micelles.7,8,28 The effect of changing the silica precursor concentration and temperature on the reaction dynamics and mechanism is additionally investigated. In doing so a mechanism for MSN formation is proposed and the broad applicability of SANS as a technique for mechanistic elucidation in materials science is tested.
’ EXPERIMENTAL SECTION Chemicals. Cetyltrimethylammonium bromide (h-CTAB, g99%), tetramethoxysilane (h-TMOS, g99%), triethanolamine (h-TEA, g99%), deuterated TMOS (d-TMOS, SiC4D12O4, 99 atom % D), and D2O (99 atom % D) were purchased from Aldrich and used as received. Deuterated TEA (d-TEA, C6D15NO3) was purchased from QMX Laboratories and used as received. Perdeuterated CTAB (d-CTAB, C19D42BrN) was received from Dr. R. Thomas through the Oxford Isotope Facility. Time-Resolved SANS. SANS data were obtained on the D22 diffractometer at the ILL (Grenoble, France) using a neutron wavelength of λ = 10 Å at a single detector distance to cover a Q range of 0.0150.326 Å1. Absolute intensities were determined by normalization to the incident flux. Time-resolved experiments were carried out using a flow cell (1 mm path length) connected to a reaction vessel by tubing and cycled using a peristaltic pump.8 The synthesis of mesoporous
silica nanoparticles (MSNs) was an optimized method based on a previously reported synthesis.14 In a typical experiment, CTAB (0.55 mmol) and TEA (0.28 mmol) were dissolved in water (24 mL) in a stirred round-bottomed flask and heated to 80 °C for 30 min. The solution was then allowed to cool to the required temperature (Table 1), and the pipes connecting the flow cell were added to the mixture. Thermostatted baths were used to control both the temperature of the reaction vessel and the flow cell. The contents were cycled through the system by means of a peristaltic pump set to a flow rate of 0.4 mL s1. A figure showing a setup similar to the one used here can be found in ref 8. SANS was detected through the flow cell of the cycling precursor solution for 10 min, allowing for good statistics but also for complete temperature equilibration prior to the kinetic run. For each kinetic run, the required quantity of TMOS (Table 1) was quickly added to the stirred reaction vessel, the safety gate was closed, and the run was immediately started. This process was practiced prior to the first run and took on average 10 s, with a variation of no more than 3 s. Taking into account the tube length (3 m), diameter (1 mm), and the flow rate (as above), complete mixing (judged as four complete cycles) ought to have taken place within 10 s (i.e., the first run commenced at almost exactly the time at which complete mixing was achieved). In all cases, SANS data were collected from this time, for 10 s every 10 s, for 900 s. This was to provide the best signal resolution while keeping the time averaging to a minimum, which is especially important in the growth period. In the text, the times given are taken from the moment that SANS collection began and do not include the mixing time. The different reactions that were investigated are shown in Table 1. SANS is sensitive to the scattering length of nuclei present in a sample and as such is isotope-dependent. One of the largest and most useful differences between isotopes is that between hydrogen and deuterium (e.g., FH2O = 0.5 1010 cm2, FD2O, 99% D = 6.4 1010 cm2). The scattering intensity, I(Q) ≈ jVΔF2, of a population i with volume V and volume fraction j is related to the scattering length through the contrast factor ΔF2 = (Fi Fbulk)2. Changing ΔF2 by altering either Fbulk or Fi through selective deuteration (contrast variation) can therefore have a strong effect on SANS and allow different parts of a system to be highlighted. Cases AC (Table 1) aimed to exploit contrast matching to investigate the mechanism behind MSN formation by highlighting different parts of the system using deuterated components. In this work, H2O, h-methanol, h-CTAB, and h-TEA have F ≈ 0.5 1010, 0.4 1010, 0.3 1010, and 0.5 1010 cm2, respectively. Silica, with a density in this case of approximately 2 g cm3, was assumed to have F ≈ 3.4 1010 cm2. D2O, d-methanol, d-CTAB, and d-TEA were calculated to have F ≈ 6.4 1010, 5.3 1010, 7.0 1010, and 7.3 1010 cm2, respectively. An experiment mainly highlighting the silica contribution (case C) to the overall scattering could therefore be carried out using fully deuterated or hydrogenated initial components. Fully deuterated components were initially preferred because of the lower resulting incoherent scattering background. Unfortunately, it was found 4426
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Figure 1. (A) SANS data for cases AC at t = 0 and 840 s. In all cases, solid lines represent model fits to the data. (See the following text.) For case C at t = 0 s, a dotted line is given as a guide to the eye and represents the incoherent background due to excess H components in the mixture. (B) Time-resolved change in detected SANS for case A. Details of the conditions and sample compositions are given in Table 1. that reactions using d-CTAB would not proceed as with h-CTAB, and large precipitates of silica were observed upon disassembling the apparatus. Despite known slight differences in behavior between H- and D-substituted materials (e.g., ref 29), this result was surprising given the proven nearly interchangeable use of deuterated and hydrogenated surfactants in countless other SANS studies over the past 20 years (e.g., refs 4, 7, 8, 25, 26, and 30). The quality of d-CTAB was assessed by looking at the scattering from d-CTAB in a H2O/h-TEA mixture at the same concentration as that used in the kinetic experiments. The fitted profile is shown in the Supporting Information (SI, Figure S1) and shows similar characteristics to that of h-CTAB in D2O (shown for comparison), so the purity is not in doubt. We can only speculate that in this complicated system the combination of thermodynamic (e.g., slightly different chain-chain and chain-solvent interactions)29 and kinetic (slightly heavier, bulkier molecule) factors adds up to render d-CTAB ineffectual as a templating surfactant. As a consequence, contrasts using d-CTAB were not studied, necessitating the use of a fully hydrogenated mixture (case C) to highlight the contributions from the silica. Unfortunately, because of the high incoherent scattering of hydrogen, the background scattering was high for this sample and the signal-to-noise ratio was poor. To highlight CTAB (case B), solutions of the various reactants contrast matched to silica (by mixing d and h components) were prepared. In this case, no adverse effect of using the deuterated components was noted. Contrast matching experiments are described in Table 1. Off-Line Experiments. To compare the parameters obtained by fitting the SANS data, dynamic light scattering (DLS, Malvern Zetasizer 4) and transmission electron microscopy (TEM, Zeiss EM912) measurements were performed on some of the particles. Additionally, for further comparison, small-angle X-ray scattering (SAXS) data was obtained using a Bruker AXS NanoStar (high flux configuration) with a sealed tube, Cu Kα radiation source, crossed G€obel mirrors, HiStar multiwire proportional counter, and two detector distances (26 and 103 cm). The results are summarized in Figure S2 and are discussed in the text. Average hydrodynamic radii were calculated from at least five measurements by employing the widely used CONTIN model.
’ RESULTS AND DISCUSSION Contrast Variation Study. Three differently contrasted samples were initially investigated, referred to here as cases A, B, and C, which highlight contributions from different parts of the sample. The parts of interest are specifically the developing mesoporous
silica nanoparticles (MSNs), which consist of both surfactant and silica, and the micelle population. Case A provides an overview of the whole system, and cases B and C highlight the surfactant and silica parts, respectively, through contrast matching (Table 1). Reactions were monitored by SANS, and data for times t = 0 s (precursor micelle solution) and t = 840 s (fully formed MSNs), respectively, for cases AC are shown in Figure 1a. As can be seen, at t = 0 s for cases A and B similar SANS is noted, representative of a dilute solution of charged micelles due to the clear decay at mid/high Q and the strong repulsive peak at low Q. The only differences between these profiles lie in the absolute intensities, which arise from the differences in scattering lengths of the solvent, Fsolv, in the two cases. Interestingly, in case C no scattering could be detected at t = 0 s, suggesting that the sample was well contrast-matched. The development of SANS over time is summarized in Figure 1b for case A and in Figure S3 for cases B and C. During the reaction, several changes can be noted in the SANS. An initial reduction of the repulsive peak occurs concurrently with increased intensity at the lowest Q values, suggesting the development of a second population of larger objects (MSNs) while surfactant micelles are consumed to form the template. This is additionally indicated by the decrease in intensities for Q > 0.06 Å 1. (See also Figure 1a.) At longer times (e.g., > 400 s), only slight changes in the SANS are detected, suggesting that the structural changes have reached completion on this nanometer length scale, in line with previous results for dilute basic systems.24 A summary of the scattering after this (t = 840 s) for cases AC is shown in Figure 1a. The decay at low Q is clearly observed alongside fluctuations in the mid/high Q range that are especially apparent in case A. The second of these is likely to be a Bragg peak at Q = 0.11 Å 1 (d = 57 Å), in line with findings by SAXS (Figure S2a). This peak probably indicates MSN porosity but is not apparent in the scattering from case C because of the high incoherent background (see the Experimental Section). Interestingly, the SANS for cases A and B do not differ greatly, apart from the reduced intensity in the latter case, showing that the surfactant is strongly involved in the assembly of the MSNs. Model Fitting. The studied system is very complex and potentially includes micelle aggregation or elongation, particle growth, 4427
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Figure 2. (A) SANS data for case A at t = 0, 40, and 80 s. Solid lines represent the complete fits to the data, which take into account both P(Q) and S(Q) contributions and where the complete fit is P(Q) S(Q). Dashed lines represent the P(Q) contributions to the overall scattering from t = 0 s () and t = 40s (---). Note that no S(Q) was required to fit the t = 80 s data, hence the P(Q) plot is the same as the complete fit. (Inset, A) The graph of S(Q) vs Q summarizes the proportional contribution of S(Q) to the complete fit for t = 0, 40, and 80 s. (B) SANS data for case A at t = 900 s, with lines showing how the multiform model fit is built up from P(Q) contributions corresponding to the MSNs, micelles, and a Bragg peak describing the MSN porosity. In all cases, the contributions are shown and include a flat background (0.013 cm1).
pore formation, and other chemical factors such as methanol release from hydrolyzing TMOS. The following SANS fitting model was chosen because it is relatively simple, physically realistic, and represents the best fit, not to any single piece of data but to the whole data set. No clear evidence of anisotropic particles (e.g., a region where I(Q) scales as Q1) is observed in any of the data for cases AC (cf. Figure 1), and both CTAB micelles and MSNs are known to be broadly spherical.14 A multiform model was therefore employed to fit the data in case A, consisting of two sets of polydisperse spheres30 with an additional Bragg peak and a flat background to account for extra incoherent scattering from any additional H in the system (see below). During the early stages of the reaction, repulsive interactions were also included, as represented by a model describing a dilute dispersion of charged spheres.31 This allowed the contributions to the scattering from the individual populations of larger species (MSNs) and smaller species (micelles) and the peak (porosity) to be separately assessed. An attractive structure factor, which might arise from aggregating micelles during the early reaction stages, was deemed unnecessary, primarily because of the high quality of the form-factor-only fit and also because there was no evidence of it in the measured I(Q) curves over this Q range. The MSNs were fitted as spheres with an average value for F as described in the Supporting Information. Typical uncertainties in SANS data are usually considered to be on the order of 10%. Fitted data points therefore may have this level of error associated with them, although this percentage will decrease for the larger particles, for which the error might be on the order of a few angstroms. However, this is clearly a value averaged both over a macroscopic dimension and the time period, which is pertinent in the case of the growth stage. This time period was chosen to be 10 s as a compromise between data resolution and time averaging (see the Experimental Section). However, in the following analysis mainly trends and averaged values are discussed, which puts less weight on the error of any specific point. To justify the models used, Figure 2 shows how each part of the scattering contributed to the overall fit. Figure 2a shows the
contribution of the form factor, P(Q) and the structure factor, S(Q), to the overall scattering for selected short reaction times (0, 40, and 80 s). As can be seen in the inset in Figure 2a, the contribution of repulsive S(Q) decreased over the period of 080 s, and the data is correspondingly better modeled by just a combination of two form factors representing MSNs and micelles (cf. the difference between a complete fit and P(Q) alone). Figure 2b summarizes the contributions from the P(Q) values of different populations and the Bragg peak to the SANS after the completion of the reaction (900 s). The scattering is dominated by the P(Q) representing the MSN population (Figure 2b), the contribution of which provides the initial intensity decay plus the additional maximum at around Q = 0.06 Å1. The second maximum (at around Q = 0.11 Å1) is a Bragg peak, which is probably indicative of the center-to-center distance between micelles within the MSN. Although these two models account for all of the major features in the scattering curve, they do not fully quantify the observed intensity, which was compensated for intuitively by considering a contribution to the SANS from the residual micelle population. Additionally, a flat background is included that arises from extra incoherent scattering due to the introduction of additional small quantities of hydrogenated compounds (methanol, TEA, and CTAB) into the reaction mixture. For case B, the same model was used but without the Bragg peak because its contribution to the overall scattering was too low to be resolved reliably. The micelle radius was generally lower than that for case A. This could be due to similarities between the solvent contrast and that of the hydrated surfactant headgroup or to (disregarded) additional scattering from the Bragg peak affecting the analysis. For case C, because of the poor data resolution at high Q, only the contribution from the large population (MSNs) was modeled. Precise details of the equations used are given in the Supporting Information (eqs S1S6) alongside tables of the fitted parameters (Tables S1S3). Factors Influencing the Fit Parameters. Before analyzing the fitted data, the various changes that occur during the reaction that could potentially affect the fitted parameters must be 4428
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Figure 3. (A) Change in fitted radii values as a function of time for case A: Rmic = small radius (shaded triangles, CTAB micelles) and RMSN = large radius (open circles, mesoporous nanoparticles plus surfactant template). Solid lines are fits to the data as discussed in the text. (B) Change in fitted RMSN values for cases B (open triangles) and C (shaded circles). Here, solid lines are guides to the eye (not fits) but still represent exponential growth.
considered. In particular, the value of the scale factor, scale = 1024ϕ(ΔF)2 is crucial because it gives the relative contribution of each partial P(Q) to the overall fit. This value is very sensitive to the size, polydispersity, and volume fraction ϕ of the population that it represents as well as to the contrast step, ΔF = Fsolvent Fspecies. It is therefore of particular importance to identify the possible factors that would be expected to influence it, which are outlined below. Methanol. The hydrolysis of TMOS releases methanol. For cases AC (and F), the final concentration of methanol in the mixture after complete hydrolysis will be approximately 1.5 vol %. This is likely to affect the scattering as follows. First, because of the change in its composition, Fsolvent will change slightly. For cases B and C, where the solvent is contrast matched to the methanol in the TMOS, this effect will be negligible, but for case A (and cases DF), the effect will lead to a reduction in Fsolvent, lowering ΔF and consequently the scale factor. This reduction, however, will be only a few percent and is therefore unlikely to greatly affect any of the conclusions that are drawn from the data. A more significant effect of methanol might be on the micelle size. It has often been found that even small additives of methanol or other polar organic solvents can reduce the micelle size and increase the cmc.32,33 For example, under basic aqueous conditions, increasing the concentration of methanol to 10 wt % (i.e., 12.6 vol %) was found to increase the cmc by a few percent but to reduce the micelle radii from approximately 23 to 20 Å.33 On this basis, throughout the reaction a decrease in micelle size might be expected even in the more complicated system studied here, alongside a decrease in the micelle population due to their incorporation into the MSNs and the small increase in the cmc. Changing Contrast Due to Particle Growth. Given that FMSN is composed of averaged contributions from silica and CTAB, the SANS contrast of a nascent particle, consisting mainly of CTAB, is likely to be quite different from that of the fully formed particle. The quantity of CTAB involved in the final MSNs can be roughly estimated from the fitted micelle scale factor. The initial concentration of CTAB (cmc ≈ 1 mM) is 23 mM; therefore, approximately 96 mol % is present within the micelles. Because the quantity of methanol released throughout the reaction is relatively small, it is unlikely to have a large effect on the cmc value.33
In case A at t > 800 s, the average scale factor is on the order of 1.1 105 cm1 for the micelle population. This suggests that the remaining solution surfactant volume fraction is ϕsurf, soln ≈ 0.0026. Given that ϕsurf, t=0 ≈ 0.008, the formed MSNs have sequestered 70% of the surfactant into the pore structure (ϕsurf, MSN ≈ 0.0054), with the remainder of the MSN volume being silica. Assuming complete conversion, 0.4 g of silica can be produced per 1 g of TMOS, or a volume on the order of 0.2 mL (density ≈ 2 g cm3). Because 14 g dm3 TMOS was added, the final volume of silica would be expected to be approximately 2.8 mL dm3 (ϕsilica, MSN ≈ 0.0028). Therefore, an average MSN consists of 35 vol % silica and 65 vol % CTAB, with FMSN ≈ 1.0 1010 cm2. Theoretically, therefore, for a fully formed particle in case A, scaleMSN ≈ 2.2 105 cm1, which is similar to the obtained value of 1.8 105 cm1 (Table S1). The significance of this calculation is that FMSN changes throughout the reaction, from as low as 0.3 1010 cm2 (pure CTAB) to 1.0 1010 cm2 (fully formed MSN). Given that Fsolvent in case A varies from 6.4 1010 to 6.3 1010 cm2 because of the release of methanol, the effect of these changes is to lower the value of ΔF and therefore lower the scale value attributed to the MSNs. This effect is particularly apparent in case E, where a maximum is noted in the fitted scale values, suggesting an initial aggregation of micelles followed by filling in with silica. Discussion of Fitted Data. Fitting the data gave parameters as summarized in Tables S1S3 for cases AC, respectively. In general, increases in the nanoparticle + template size (RMSN) and peak amplitude (peak scale) and decreases in the micelle size (Rmic), volume fraction (via scalemic), and interactions S(Q) are noted with time. The change in fitted radii values (Rmic and RMSN) for case A is given in Figure 3a. Rmic can be seen to decrease from an initial value on the order of 23 Å (t = 0 s) to < 20 Å (t = 400 s). As already discussed, this is probably due to the influx of methanol into the reaction mixture. The observed reduction in intermicellar repulsion is probably due to the adsorption of silica oligomers, which are likely to be negatively charged under these basic conditions, to the micelle surface. SANS from the MSN dispersion was not detected until 40 s. The first fitted value for RMSN is 50 Å, which increases to 84 Å during the period of 0 < t < 400 s, although much of the detected growth (i.e., to RMSN > 80 Å) 4429
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Langmuir Scheme 2. Proposed Growth Schemea
a
The adsorption of silica oligomers triggers the aggregation of CTAB micelles, followed by further aggregation and (slow) filling in to form the solid MSNs.
occurs in the first 200 s. The fitted polydispersity (pMSN) changes only slightly with t, with a maximum value of 0.22 at 70 e t e 130 s. The fitted scale values, scaleMSN and scalemic, increased and decreased, respectively, during the growth of the MSNs, in line with the incorporation of surfactant micelles into the enlarging mesoporous particle structure. The Bragg peak was initially observed at 80 s, after which its position remained fairly constant. Its amplitude increased with t to a maximum value at ∼ 300 s, indicating that the pore structure was subsequently not excessively altered. Figure 3b shows the time-resolved growth in RMSN for cases B and C. Although fitted radii values are somewhat more scattered in case B, indicating more error in the individual values, MSN growth apparently occurs more quickly than in case A, although both cases still attain a similar final size (85 Å in case B). Because of the inability to use d-CTAB (Experimental Section), the data for case C suffered from a high signal-to-noise ratio (cf. Figure 1a), although it was possible to fit the MSN contribution to the scattering. The initial value for RMSN ≈ 50 Å is indicative of the formation of surfactantsilica hybrid aggregates, with a compound scale factor (as described above) and being of just sufficient size to be resolved above the background. The growth period in case C is noted to be longer than that for case B and similar to that in case A, with the maximum value of RMSN of approximately 70 Å reached after 300 s. Complementary SAXS (Figure S2a) and TEM measurements (Figure S2c) were also performed to visualize the particles. Although it was difficult to resolve the TEM images precisely because of the presence of excess CTAB, it was possible to fit the obtained SAXS data to give RMSN = 68 Å. SAXS is sensitive to the differences in electron density between the studied species and solvent and is therefore more likely to be dominated by contributions related to the silica structure (Si, O) than those from the surfactant (C, H, and N). In this case, it is likely that some incorporated micelles in the MSN protrude out of the silica structure. In both case C and SAXS measurements, RMSN is equivalent to the radius of the condensed silicasurfactant hybrid, whereas in cases A and B, RMSN includes the additional micellar contributions. DLS data for case A (Figure S2b), with an average hydrodynamic radius of Rh = 91 Å, supports the latter model. These additional structures may still be coated with a small amount of silica, although the concentration of these is clearly too low to be observed in the scattering profiles for case C and cannot be detected in the other measurements. Proposed Mechanism. From these results, a reaction mechanism can be proposed, which is described in Scheme 2. After initial TMOS hydrolysis, at t ≈ 40 s condensing silicate ions and short oligomers form and adsorb to the CTAB micelles. This lowers the intermicellar repulsion, as evidenced by the declining charge per micelle (Table S1). The oligomers remain small
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because of the dilute basic conditions (1 mol of base per 8 mols of TMOS). The decreased repulsion allows the formation of small aggregates of silica/surfactant hybrid structures with an initial radius of approximately 50 Å and a low polydispersity, as noted in cases AC (Scheme 2). This initial step is similar to those preceding the formation of the current bun-type structure already discussed; therefore, further growth might proceed by the successive absorption of silica-coated micelles to the nucleus. The data supports this, with a lack of any anisotropy in the scattering suggesting no extensive micelle elongation, alongside a relatively smooth increase in the MSN radius. The micelle aggregation is accompanied by the condensation of silica into the gaps, which occurs on a slightly longer timescale, as evidenced by the longer growth periods noted in cases A and C than for case B. At early stages, the MSNs are composed of both aggregated inner micelles surrounded by silica and covering outer micelles with proportionally less adsorbed silica. Therefore, the contrast is not constant throughout the particle. This explains the difference in observed growth rates between case A, where D2O has a much higher F than either silica or CTAB and both of these species are strongly detected, and case B, where the solvent is contrast matched to the silica and the outer micelles proportionally dominate the scattering. The mechanism of micelle aggregation, followed by the condensation of silica into the available volume, is also backed up by results in case E, as discussed in the next section. The final particles, obtained after approximately 400 s, probably consist of walls of condensed silica surrounding micelles in a hexagonally packed structure of mesopores (cf. TEM images in SI and in ref 14). The reaction timescale is in line with other studies under similarly dilute, basic conditions. Within this structure, the average center-to-center intermicellar spacing is on the order of 57 Å, as evidenced by the Bragg peak, which becomes more distinct after 80 s as the number of micelles within the aggregate increases. This value can be used to estimate that the minimum wall thickness is on the order of 11 Å (on the basis of the initial micelle diameter of 2 23 Å = 46 Å). The final radius of the underlying silica structure is less than that of the surfactant template (Scheme 2), indicating that several partially formed pores exist on the particle surface, in line with TEM images shown here (Figure S2) and in reference 14. Parameter Variation: Concentration of TMOS. To investigate the parameters affecting the synthesis and to obtain further insight into the proposed mechanism of formation, the reaction conditions (concentration of TMOS and temperature) were varied and the resulting experiments were monitored by tr-SANS. Case A, which maximizes the available contrast and has a reduced background, was chosen as a basis for these variations. The first condition to be studied was the concentration of TMOS, the silica precursor. Case A used a concentration of 14 g dm3, so the effects of halving and doubling this value were investigated in cases D and E, respectively (Table 1). Plots showing the development of SANS with time are shown in Figures S4a,b. The data were fitted in the same way as for case A, and the parameters are given in Tables S4 and S5. Figure 4a shows the influence of changing the concentration of TMOS on the eventual MSN radius and growth rate. The final radii scale with the amount of TMOS added, as might intuitively be expected. Interestingly, initial growth rates are similar for 7 and 14 g dm3 TMOS (cases D and A) but quicker for 28 g dm3 TMOS (case E), suggesting the former are limited by the quantity of available precursor. To better quantify this growth, the following equation was used to fit the curves (solid lines in Figure 3a) and thereby to establish 4430
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Figure 4. Change in fitted (A) MSN and (B) micelle radii values as a function of time for syntheses carried out with different quantities of added TMOS (7, 14, and 28 g dm3). In all cases, the concentration of all other reactants was held constant and reactions were run at 40 °C. Solid lines represent fits to the data as described in the text.
Table 2. Summary of Calculated t1 Values for Different Reaction Cases A, D, E, and Fa case RMSN,max/Å RMSN,0/Å kMSN/s1 Rmic,min/Å Rmic,0/Å kmic/s1 A
84.1
50.6
0.012
19.7
23.4
D
67.8
49.9
0.029
21.9
23.6
0.007 0.033
E
86.0
50.5
0.035
15.6
24.0
0.017
F
83.1
50.5
0.029
20.5
22.7
0.026
a
The reader is directed to Table 1 for information on the conditions (e.g., temperature and TMOS concentration) used.
approximate growth rates: Rt ¼ RMSN, max þ ðRMSN, 0 RMSN, max ÞekMSN ðt t0 Þ
ð1Þ
Rt is the MSN radius at time t. RMSN,max is the maximum radius, RMSN,0 is the radius of the MSN nuclei first detected at t = t0. Note the difference between t0, the time at which the signal for the MSN was first recorded, and t = 0 s, the time at which the reaction started. Units of radii and time are angstroms (Å) and seconds (s), respectively. kMSN is the growth rate constant with a unit of s1. For the associated decrease in size of the CTAB micelles, eq 2 was used. Here, Rmic,min is the fitted minimum value for the micelle radius, and Rmic,0 is the initial micelle radius found at t = 0 s, hence the lack of t0 in the following equation. kmic is the decay rate constant of the micelles. Fitted values in all cases are given in Table 2. Rt ¼ Rmic, min þ ðRmic, 0 Rmic, min ÞekmicðtÞ
ð2Þ
Theoretical initial MSN radii RMSN,0 for all cases are on the order of 50 Å, which again is likely to represent small clusters of silica oligomer-coated CTAB micelles. It is important to note that in cases A, D, and E the CTAB concentration was kept constant. For case D, RMSN,max is lower than that for case A, in line with halving the particle volume, echoing the halving of the available silica precursor (VMSN = 1.3 106 Å3 for case D and 2.5 106 Å3 for case A). Fitted off-line SAXS and DLS observations (Figure S2) for particles formed in cases A and D showed similar trends in radii values, backing up the SANS fits here. TEM images for case D (Figure S2d) showed well-formed spherical
mesoporous particles with radii
