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Hexagonal Mesostructure in Powders Produced by Evaporation-Induced Self-Assembly of Aerosols from Aqueous Tetraethoxysilane Solutions Mangesh T. Bore, Shailendra B. Rathod, Timothy L. Ward,* and Abhaya K. Datye Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131 Received August 8, 2002. In Final Form: November 6, 2002 Spherical submicron mesoporous particles possessing two-dimensional hexagonal order have been produced by evaporation-induced self-assembly in aerosols generated from acidic (pH ≈ 2) aqueous solutions of tetraethoxysilane (TEOS) and the surfactant cetyltrimethylammonium bromide (CTAB). Particle internal structure consists of multiple regions of hexagonally ordered tubular pore bundles with various orientations. Exploration of a broad range of CTAB and TEOS content showed that hexagonally ordered particles with Brunauer-Emmett-Teller surface areas of 700-1300 m2/g are produced when the CTAB/Si ratio is in the range 0.09 < CTAB/Si < 0.28. The mean pore diameter in ordered material is nearly constant at 2.8-3.0 nm, while wall thickness decreases from approximately 1 to 0.6 nm as the CTAB/Si ratio increases over that range. Hexagonal order is lost very abruptly for CTAB/Si < 0.09, with a corresponding rapid loss of surface area. Similar results were found for a solution pH of 1-3; however, a solution pH above 3.5 led to particles that appeared to be agglomerates of precipitated nanoparticles. Aerosol reactor temperatures from 30 to 550 °C were explored, with ordered particles produced under appropriate conditions at all temperatures. Lower reactor temperatures produced more highly ordered particles; however, reactor temperatures below approximately 125 °C can lead to particle coalescence on the collection filter. Coalescence occurs when silica condensation reactions are not sufficiently promoted to fix the individual particle structure before collection.
Introduction Since the seminal work of Mobil scientists,1 there has been extensive research utilizing self-assembly of amphiphilic molecules to synthesize highly ordered mesoporous and composite materials. These materials possess structures that reflect the liquid crystalline phases of the assembled amphiphilic molecules, with periodic spacings or phase domains in the 1-100 nm size range. The highly controllable and monodisperse nature of the pore sizes, controllable pore surface chemistry, and periodic nanoscale pore spacings make these materials intriguing for applications such as heterogeneous catalyst supports, chromatography, and controlled release. A variety of strategies have been explored to tailor the internal chemistry of mesoporous materials, many of which have direct implications for catalytic applications. These strategies have been thoroughly reviewed2 and include modification of pore surface acid-base chemistry, grafting of species to pore surfaces, substitution of other elements into the silica framework, and incorporation of metals, semiconductors, or oxides into the mesoporous hosts by various means. The catalytic application of mesoporous materials has also been reviewed thoroughly through 1997,3 and extensive investigation continues. The vast majority of the previous research into mesoporous materials and mesoporous catalyst hosts has used synthetic methods similar to that of the Mobil work. This relies on spontaneous self-assembly of amphiphilic molecules from a bulk solution, with concurrent templating * To whom correspondence should be addressed. Ph: 505-2772067. Fax: 505-277-5433. E-mail:
[email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (3) Corma, A. Chem. Rev. 1997, 97, 2373.
of inorganic precursor species. While self-assembly can occur quickly, the process is usually conducted over a time scale of days or longer to ensure a high degree of order in the final inorganic product. An alternative synthetic approach has been reported that exploits solvent evaporation from films or droplets to drive the self-assembly process.4,5 This “evaporation-induced self-assembly” (EISA), when used with aerosols, can produce highly ordered mesostructured silica particles with a total process time of only several seconds or less,5,6 much shorter than the typical time scales used for the conventional approach. For the synthesis of mesostructured or mesoporous powders, EISA of aerosols or sprays has several other very attractive features compared to traditional bulk solution methods. The process is a continuous scalable process that can make particles over a fairly wide size range. Particles produced are generally spherical, which frequently has advantages for subsequent powder processing. Perhaps most interesting from the materials synthesis perspective is the flexibility of the method, which provides for uniform incorporation into every particle of any chemical species that can be dissolved or dispersed into a precursor solution or dispersion. The first report on structured particles made by this concept was conducted using a spray drying procedure, which led to 10-20 µm diameter hollow particles with thin ordered silica walls.7 More recently, it was demonstrated that for EISA of aerosol droplets in the micron (4) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Adv. Mater. 1999, 11, 579. (5) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (6) Rama Rao, G. V.; Lopez, G. P.; Bravo, J.; Pham, H.; Datye, A. K.; Xu, H.; Ward, T. L. Adv. Mater. 2002, 14, 1301. (7) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran. S. Chem. Mater. 1997, 9, 2507.
10.1021/la020704h CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002
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Langmuir, Vol. 19, No. 2, 2003 257 liters per minute (slpm)) carried the droplets through a threezone furnace (Lindbergh) with a heated length of 0.6 m. The furnace tube inside diameter was 2.5 cm, which provided a mean residence time of approximately 4 s for droplets/particles in the heated zone of the reactor at 125 °C. Experiments in which pH was varied were conducted using a temperature-controlled heating tape to heat the tube wall. For these experiments, the heated length was 41 cm, and the axial temperature profile varied from approximately 25 °C below the set point (150 °C) at the entrance to 25 °C above the set point at the exit. The gas flow and tube diameter were the same as for the furnace experiments, providing a mean residence time of approximately 2 s for a set point of 150 °C. The powder product was collected on a filter whose housing was maintained at ∼80 °C to avoid water condensation. After powder collection, calcination was done at 500 °C for 12 h in air to remove the surfactant template from the particles. For the precursor solution, CTAB was mixed with water and stirred for 15 min to get a clear solution. To this solution, TEOS and 1 N HCl were added and stirred for 10 min. CTAB content was varied over the range of 0.05-0.44 mol % for one set of experiments in which the other molar ratios were fixed at TEOS/ H2O/HCl ) 1:63.3:0.022. For another set of experiments, TEOS content was varied from 0.78 to 5.9 mol %, with the remaining ratios at CTAB/H2O/HCl ) 1:405:0.15. In a third set of experiments, pH was varied by way of the HCl content while maintaining the other molar ratios at CTAB/TEOS/H2O ) 0.15: 1:620. Reported pH values were calculated based on moles of HCl and total solution volume, assuming additive volumes for all liquid components. Reactor temperatures in the 125-550 °C range were investigated. X-ray powder diffraction (XRD) patterns were obtained on a Scintag diffractometer (Cu KR radiation). Adsorption isotherms of nitrogen were carried out on a Micromeritics Gemini 2360. Transmission electron microscopy (TEM) was conducted on JEOL 2010 and 2010F instruments, and scanning electron microscopy (SEM) was done on a Hitachi S-800 instrument.
Figure 1. Schematic diagram of the aerosol powder synthesis apparatus.
size range, fully structured (nonhollow) submicron particles could be obtained.5 In that work, several different amphiphilic molecules were utilized with an acid-catalyzed tetraethoxysilane (TEOS) sol to produce a variety of ordered mesoporous silica particles. While a variety of intriguing structures were demonstrated, little systematic investigation into the nature and control of the pore structure and size was reported. In this paper, we report in detail on the effect of synthesis parameters on the mesoporous nature of silica particles produced by the EISA aerosol method using an acidic aqueous TEOS solution with the surfactant cetyltrimethylammonium bromide (CTAB). Our previously reported work utilized a prehydrolyzed TEOS sol with high alcohol content. The nature of inorganic species in the precursor solution for the experiments reported here is thus different, and the lack of alcohol impacts the evaporation rate as well as, presumably, the self-assembly chemistry. For this aqueous precursor solution, we have explored in detail the nature and control of particle pore structure. Results demonstrate the role of surfactant and inorganic precursor concentrations, pH, and reactor temperature in determining structural organization, pore size, and wall thickness.
Results and Discussion Effect of CTAB/Si Ratio. The effect of the relative amounts of surfactant and TEOS on the order of the particles was investigated by varying both the CTAB concentration at fixed TEOS content and the TEOS content at fixed CTAB concentration. Analysis from the results of these two independent sets of experiments revealed that the ratio of CTAB/Si (or CTAB/TEOS) was the most important and appropriate variable to use for correlating
Experimental Section The powder synthesis process is shown schematically in Figure 1. An aerosol was generated from the precursor solution using a TSI model 3076 aerosol generator which produces droplets in the submicron to micron size range. A gas flow (air, 3.6 standard
Table 1. Summary of Data and Calculated Pore Diameters and Wall Thickness for Powders Produced at 425 °C Using Variable CTAB Contenta CTAB (mol %)
CTAB/Si ratio
d100b (Å)
ac (Å)
Vpd (cm3/g)
SBETe (m2/g)
dBJHf (Å)
dhg (Å)
dph (Å)
twi (Å)
0.05 0.15 0.24 0.34 0.44
0.031 0.094 0.16 0.22 0.28
35.4 32.8 30.9 29.0
40.8 37.9 35.7 33.5
0.037 0.467 0.659 0.728 0.765
50 670 1010 1389 1343
27.2 25.6 25.1 29.1
29.4 27.9 26.1 21.0 22.8
30.5 30.6 29.4 27.9
10.3 7.3 6.3 5.6
a Molar ratios for other solution components were TEOS/H O/HCl ) 1:63.3:0.023. b d spacing between (100) planes. c Hexagonal unit 2 cell parameter (a ) 2d100/3). d Single point pore volume determined at P/P0 ≈ 0.975. e BET surface area. f Pore diameter based on BJH theory. g Hydraulic mean pore diameter (eq 6). h Pore diameter based on hexagonal configuration (eq 7). i Wall thickness (eq 8, based on eq 7 for pore diameter).
Table 2. Summary of Data and Calculated Pore Diameters and Wall Thickness for Powders Produced at 425 °C Using Variable TEOS Contenta TEOS (mol %)
CTAB/Si ratio
d100 (Å)
a (Å)
Vp (cm3/g)
SBET (m2/g)
dBJH (Å)
dh (Å)
dp (Å)
tw (Å)
0.78 1.6 2.3 3.1 4.5 5.9
0.31 0.16 0.10 0.078 0.052 0.039
29.2 32.8 36 39.2 41.5 42.7
33.7 37.9 41.6 45.3 47.9 49.3
0.405 0.659 0.452 0.373 0.025 0.022
892 1010 916 529 276 19
39.2 25.6 26.2 26.6
18.2 26.1 19.7 28.2
24.3 30.6 30.8 31.9
9.41 7.27 10.7 13.3
a
1.
Molar ratios for other solution components were CTAB/H2O/HCl ) 1:405:0.15. All symbol definitions are consistent with those of Table
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Figure 2. Scanning electron micrograph of mesoporous silica particles after calcination. Precursor solution molar ratios were TEOS/H2O/CTAB/HCl ) 1:63.3:0.219:0.022 (pH ≈ 2), and the furnace temperature was 425 °C.
the data. Thus, the detailed results from both sets of experiments are presented separately in Tables 1 and 2, but the results from those two series are combined and discussed in terms of the CTAB/Si ratio in this section. For experiments in which CTAB was varied, the molar percentage of CTAB in the precursor solution was varied from 0.05 to 0.44 mol % while keeping the synthesis temperature and pH fixed at 425 °C and approximately 2, respectively. This effectively varied the CTAB/Si ratio from 0.03 to 0.28 while keeping the TEOS mole percentage constant at 1.6 mol % TEOS (Table 1). For experiments in which TEOS content was varied, the TEOS concentration ranged from 0.78 to 5.9 mol %, which provided CTAB/ Si ratios of 0.04-0.31 while the CTAB content was constant at 0.15 mol % (Table 2). All particles produced at temperatures above 100 °C were typically spherical without agglomeration, with most particles falling in the 0.2-1 µm range (Figure 2). Figure 3 shows the X-ray diffraction patterns of samples prepared with different CTAB/Si ratios. At the lowest CTAB/Si ratio
Bore et al.
investigated (0.03), there was no detectable order by XRD. At all higher CTAB/Si ratios, the diffraction peaks were consistent with the two-dimensional (2-D) hexagonal phase, although there was a very marked decrease in intensity above CTAB/Si ) 0.28, presumably associated with a lower degree of order. As the CTAB/Si ratio varied from 0.05 to 0.28, the d100 spacing decreased from 41.5 to 29.0 Å (see Tables 1 and 2). At CTAB/Si ) 0.094, our d spacing (3.5 nm) is very similar to that reported by others for MCM-41 material.8 The d spacing is effectively the pore center separation distance in a hexagonal arrangement and can be related to pore size and wall thickness, as described further below. TEM images of calcined samples at different CTAB/Si ratios are shown in Figure 4. For particles less than approximately 0.5 µm, electron transmission is sufficient to produce these images without any special sample preparation. The particles appeared well-ordered with areas of apparent hexagonal structure at CTAB/Si ratios in the 0.094-0.28 range, consistent with the inferences made above from the XRD data. At CTAB/Si ) 0.04, there was no apparent order, but the particles were mesoporous with a wormlike structure. TEM of particles produced at CTAB/Si ) 0.03 showed no apparent order or porosity (not shown in Figure 4). At the highest CTAB/Si ratio investigated (0.31, Figure 4e), there is no apparent order but some limited regions of wormlike mesoporosity appear to exist, consistent with the substantial decrease in the (100) XRD peak (Figure 3). In the well-ordered particles in the CTAB/Si ) 0.094-0.22 range, the hexagonal order is revealed by hexagonally arranged bright or dark spots in some regions (e.g., region A in Figure 4b) or as channels typically running parallel to the particle surface (region B in Figure 4b). The interface between these two types of domains often appears mottled or wormlike in the TEM images. Pores may appear dark or light in phase contrast images, depending on subtle changes in the focus. We interpret these observations to indicate that the hexagonally structured particles are made up of numerous regions of hexagonally ordered tubular pore bundles which lie at various angles relative to one another. Figure 5 depicts schematically how this model could look in the
Figure 3. X-ray diffraction patterns for powders produced using various CTAB/Si ratios. Traces are labeled with the CTAB/Si ratio, which can be related to other specific experimental details using Tables 1 and 2. The furnace temperature was 425 °C, and pH ≈ 2.
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Figure 4. Transmission electron micrographs for particles produced using various CTAB/Si ratios: (a) 0.04, (b) 0.094, (c) 0.16, (d) 0.28, and (e) 0.31. CTAB/Si ratios can be related to other specific experimental details using Tables 1 and 2. The furnace temperature was 425 °C, and pH ≈ 2.
focal plane of the TEM. Some tubular pores are oriented end-on relative to the electron beam, leading to hexagonally arranged spots such as those in region A in Figures 4b and 5. In other regions, pore bundles may lie in the plane perpendicular to the beam, leading to apparently parallel channels in TEM such as those in region B in (8) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834.
Figures 4b and 5. In other regions, pore bundles may lie at some oblique angle, providing no clear orderly pattern in TEM. This interpretation has been confirmed by observing transitions between pore orientations by tilting of the TEM sample stage within the microscope. The differently oriented domains are analogous to grains in a polycrystalline structure, and the mottled regions in the boundaries between differently oriented domains are analogous to grain boundaries. The detailed structural nature of these boundaries is not clear, but there is
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Figure 7. BET surface area and pore volume obtained by N2 adsorption for particles produced at varying CTAB/Si ratios in experiments in which CTAB and TEOS contents were varied.
Figure 5. Schematic diagram of pore orientations in hexagonally ordered particles synthesized by the aerosol route. The particle consists of small domains of hexagonally ordered tubular pores that are oriented end-on in some regions (A) and lie in the focal plane (normal to the electron beam) in other areas, appearing as parallel channels (B).
Figure 6. Nitrogen adsorption isotherms for particles produced using various CTAB/Si ratios. Traces are labeled with the CTAB/ Si ratio, which can be related to other specific experimental details using Tables 1 and 2. The furnace temperature was 425 °C, and pH ≈ 2.
apparently uninhibited connectivity between the different porous domains, based on the pore volume and lack of hysteresis in the nitrogen adsorption data (see below). The presence of a three-dimensional interconnected network of pores is a very useful property for many applications where uninhibited access to the pore volume is important. The presence of multiple ordered domains with different orientations within individual particles is notably different from what is typically seen in mesoporous material prepared by the conventional solution synthesis, where pores normally possess a single orientation within one particle. The limited size of the ordered domains in the aerosol-derived powders accounts for the less welldefined higher order XRD reflections seen for these powders compared to those from conventional synthesis. The nitrogen adsorption isotherms (Figure 6) after calcination showed type IV features with essentially no hysteresis, typical of relatively monodisperse pores,9 for samples with 0.09 e CTAB/Si e 0.22. Slight hysteresis is seen for CTAB/Si ) 0.28, and substantial hysteresis with (9) Tanev, P. T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, 2068.
loss of pore volume is seen at CTAB/Si ) 0.31. This corresponds to the loss of hexagonal order indicated by the XRD and TEM results. Similarly, there is a clear appearance of hysteresis between CTAB/Si ) 0.09 and CTAB/Si e 0.08, which also corresponds to the loss of order and mesoporosity. For samples with CTAB/Si in the range corresponding to hexagonally ordered material, the adsorption data indicated a gradual increase in total pore volume and Brunauer-Emmett-Teller (BET) surface area with increasing CTAB/Si ratio up to CTAB/Si ) 0.28, above which a decline occurred (Tables 1 and 2, Figure 7). It is apparent that substantial internal porosity disappears for CTAB/Si ratios below approximately 0.09 under these conditions, and surface area and pore volume reach their maximum levels in the vicinity of CTAB/Si ) 0.22-0.28, declining sharply above that. These results indicate that the hexagonal ordering occurs over a relatively narrow CTAB/Si compositional range and that the wormlike mesoporous structure commonly displays adsorption hysteresis, indicative of bottlenecks in the pore network. The magnitudes and trends in surface area (and pore volume) with respect to the CTAB/Si ratio can be rationalized by a simple model that assumes all CTAB is present in cylindrical micelles at the time when the mesostructure is fixed by silica condensation reactions. If each CTAB molecule is assumed to contribute an area a0 to the total specific surface area (S), where a0 is the surfactant headgroup cross-sectional area, then the total specific surface area of the mesoporous material can be written as
S)
(
)
[CTAB] NAVa0 2 m /g SiO2 [TEOS] MSiO2
(1)
where [CTAB]/[TEOS] is the ratio of molar concentrations, NAV is Avogadro’s number, and MSiO2 is the molecular weight of SiO2. For cylindrical micellar structures, geometric packing requires
a0 )
2v lc
(2)
where v is the volume occupied by the surfactant hydrocarbon tail, and lc is a critical length that is roughly equal to the extended length of the hydrocarbon chain. Israelachvili10 suggested the following equations for v and lc, both slight modifications to equations determined by Tanford:11
v ) 27.4 + 26.9(n - 2) Å3
(3)
lc ) 1.5 + 1.265(n - 2) Å
(4)
where n is the number of carbons in the surfactant tail.
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For CTAB (n ) 16), these equations give v ) 404 Å3 and lc ) 19.21 Å. This yields a0 ) 42.06 Å2, which leads to the following simple relation for specific surface area:
S ) 3956
(
CTAB 2 m /g Si
)
(5)
This result, shown as a dashed line in Figure 7, has a lower slope than the BET data in the CTAB/Si < 0.2 range but confirms the magnitude and trends of the experimental data reasonably well, especially considering the simplicity and assumptions of the approach. The data are consistent with a simple structural model where variation of the CTAB/Si ratio effectively changes the surface area and pore volume in 2-D hexagonally ordered material by changing the number of cylindrical micellar structures per unit mass of silica. This simple model does not represent the data at very low CTAB levels, since it cannot account for a threshold level of CTAB needed to induce structural ordering, or at high CTAB/Si where hexagonal order breaks down. Estimates of mesopore size can be derived in several different manners, and results of several approaches are included in Tables 1 and 2. Application of the commonly used Barrett-Joyner-Halenda (BJH) method12 to the adsorption data indicates that mean pore size (dBJH) decreases from 27.2 to 25.1 Å as CTAB/Si increases from 0.094 to 0.22 (CTAB experiments, Table 1) and then increases substantially to 29.1 Å at CTAB/Si ) 0.28. A similar large increase was indicated for dBJH between CTAB/Si ) 0.16 and 0.31 in the TEOS experiments (Table 2). Though some deviations from the hexagonal ordering are apparent in the TEM for CTAB/Si ) 0.28 (Figure 4d), it is nevertheless difficult to physically explain the large jump in dBJH for that sample based on the relatively subtle structural changes revealed in the TEM. One alternative is to calculate a “hydraulic” diameter from mesopore volume (Vp) and BET surface area (SBET) values using the equation13,14
dh )
4Vp SBET
(6)
Values calculated from eq 6 agree well with the BJH values for CTAB/Si in the 0.08-0.16 range; however, the agreement is poor outside this range (Tables 1 and 2). For example, there is a substantial decrease in dh between CTAB/Si ) 0.16 and 0.22 in the CTAB experiments that is difficult to physically rationalize based on the minimal structural changes revealed in the TEM. Similarly, there is a large difference in dh values between the CTAB/Si ) 0.16 and 0.10 samples in the TEOS experiments (Table 2) that is difficult to explain or rationalize. For material with 2-D hexagonal order, it is also possible to calculate a pore size based only on the mesopore volume and geometric factors associated with perfect hexagonal packing of cylinders using the XRD-derived dimensions. This geometric approach yields the following equation for pore diameter:13,14 (10) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (11) Tanford, C. The Hydrophobic Effect; John Wiley and Sons: New York, 1973. (12) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (13) Kruk, M.; Jaroniec, M.; Sayari, A. Chem. Mater. 1999, 11, 492. (14) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267.
(
dp ) cd100
)
FVp 1 + FVp
0.5
(7)
where F is the density of the walls, which has been shown to be near that of amorphous silica (2.2 g/cm3) for MCM41 materials,14 and c ) 8/πx3 for cylindrical pores.14 Assuming that the Vp values reported in Table 1 are primary mesopore volume, eq 7 leads to values of dp that decrease steadily from 30.6 to 27.9 Å with increasing CTAB concentration, showing no trend reversal (Table 1). Similarly, consistent values of dp showing a gradual decreasing trend with increasing CTAB/Si were calculated for the TEOS experiments (Table 2). We therefore conclude that these calculated pore diameters are the most reliable for the samples that possess hexagonal order because of the physical reasonableness of calculated pore size magnitudes and trends, as well as the magnitudes of derived wall thicknesses (see below). For powders not possessing substantial hexagonal order, eq 7 is inappropriate, and values derived from eq 6 or the BJH theory may be better. For perfect hexagonal cylindrical packing, the wall thickness can be calculated by taking the difference between the lattice parameter and the pore diameter:
tw ) a - dp
(8)
The lattice parameter is related to d100 by14
a ) 2d100/x3
(9)
Using the pore diameters calculated using eq 7, wall thicknesses were calculated, and results are presented in Tables 1 and 2 and in Figure 8. We note that all calculated pore diameters and the wall thickness are omitted for the two highest TEOS levels in Table 2 because the values broke dramatically from the trend of the lower TEOS levels and were physically unrealistic. Figure 8 shows how d100 and calculated dp and wall thickness values vary with the CTAB/Si ratio for all samples produced by varying CTAB or TEOS content. Here, dp and tw values (from Tables 1 and 2) are presented only for powders with a significant extent of hexagonal ordering. The data for CTAB-varied and TEOS-varied experiments are quite consistent with each other. For CTAB/Si values between approximately 0.08 and 0.28, all three parameters have a smooth decreasing dependence on CTAB/Si, with pore size changing only slightly. TEM and XRD indicate reasonably well-ordered hexagonal structure for 0.09 < CTAB/Si < 0.28. The sample with CTAB/Si ) 0.28 is ordered and consistent with the trends of hexagonally ordered material in Figure 8, but TEM indicates that only some portions are hexagonally ordered (Figure 4d). The decreasing wall thickness with increasing CTAB/Si ratio is consistent with a simple physical picture in which a roughly constant amount of silica must encapsulate a larger number of liquid crystalline structures, leading to thinner walls. There is presumably a minimum wall thickness that can be tolerated before the ordered structure is destabilized. In fact, at the highest CTAB/Si ratios employed (0.28 and 0.31) high pore volume and BET surface areas were maintained, but the XRD (Figure 3) showed a lowered intensity and broadening of the (100) peak that is consistent with a decrease in longrange order. For traditionally produced MCM-41 material, a wall thickness of approximately 0.8-1 nm is considered typical.9,15 Our powder produced with the CTAB/Si ratio near 0.1 is in good agreement with this, while thinner walls were calculated for higher CTAB/Si. Traditional
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Figure 8. Hexagonal (100) d spacing, wall thickness, and pore diameter of particles as a function of the CTAB/Si ratio for experiments in which CTAB and TEOS contents were varied.
MCM-41 synthesis is normally conducted under basic conditions where the silicate species are anionic. Under those conditions, charge compensation between the cationic surfactant and anionic silicate is considered to be the major factor in wall thickness.9,16 Self-assembly within droplets undergoing EISA is notably different than the traditional synthesis in several ways. Since all solution species are essentially captive within each droplet (except for neutral solvent molecules that evaporate), the overall charge balance is assured and should not be a factor determining wall thickness, though charge may affect species distribution within the droplet. Furthermore, most of our experiments were carried out with the pH initially near 2, which is near the isoelectric point for silicate species.16,17 Thus, the silicate species are initially relatively neutral or only slightly protonated. However, as evaporation proceeds, the pH should drop and all electrolytes will become more concentrated. Thus the situation inside each droplet probably becomes similar to the S+X-I+ templating route described by Tanev and Pinnavaia,9 in which electrolyte counterions interface and charge compensate between cationic surfactant headgroups and protonated silicate species. Without overall charge compensation limitations, it might be expected that thicker silica walls may be achievable with aerosol EISA compared to the traditional self-assembly synthesis. However, our results indicate that powders with hexagonal order by XRD and TEM are produced only over the 0.09 < CTAB/Si < 0.28 composition range, leading to a wall thickness that varied from about 6 to 10 Å. The pore diameter is determined largely by the geometry of the surfactant molecules, and our results are consistent with other literature using CTAB. We see a slight decrease in pore diameters with increasing CTAB concentration. This indicates that the CTAB/Si ratio influences surfactant packing in the micellar structures somewhat. This could be due to electrostatic interactions between charge compensation layers as the walls become thinner or could be related to lower effective electrolyte (Cl-) concentration available due to larger numbers of micellar structures at higher CTAB content. At the highest CTAB levels where the calculated wall thickness became less than 6 Å (see Table 1), the periodic silica structure starts to collapse. This may be related to destabilization due to interaction of the charge compensation layers around the liquid crystalline structures. Effect of pH. The important effects of pH on the rates of silicate hydrolysis and silica condensation have already (15) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurthy, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (16) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (17) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: Boston, 1990.
Figure 9. X-ray diffraction of powders produced using different solution pH levels. Precursor solution molar ratios were TEOS/ H2O/CTAB ) 1:62.5:0.15, and the furnace temperature was 125 °C.
been mentioned. These rates should be sufficiently high that the mesostructure introduced by surfactant selfassembly is eventually locked in by solid formation but must also be slow enough that the internal structure is fluid enough to accommodate droplet shrinkage as the solvent evaporates. We have conducted the majority of our previous research with an initial solution pH near 2, which has been shown to provide a minimum in the rate of silica condensation for the TEOS-water system.17 Here we have investigated the effect of higher and lower pH on structural order and microstructure of particles. Figure 9 shows XRD data indicating that hexagonally ordered powders were obtained over the pH 1-3 range. No higher order peaks were seen for the pH 3.5 sample, and there is no evidence of mesostructural order by XRD in the pH 4 powder. TEM (Figure 10) was consistent with those results, showing the typical features of hexagonal structure for pH 1 and 3 (not shown). Using pH 3.5, the particles appear mesoporous but there is no apparent hexagonal order, and particles produced using pH 4 appear to be agglomerates of smaller nanoparticles. At even higher pH, precipitation occurred prior to aerosol generation. The BET surface areas for the powders produced at pH 1 and pH 3 were 1353 and 1272 m2/g, respectively, both values that are consistent with a relatively high degree of mesoporosity. The powder produced at pH 4 possessed 685 m2/g. This is still a relatively high surface area, and indicates that the surface area of the interior nanoparticles is accessible. These experiments are consistent with a rapidly increasing condensation rate at pH > 3 and demonstrate the importance of an appropriate balance of the chemical and physical dynamics in EISA. Effect of Reactor Temperature. It was evident from the variable pH experiments that competition between rates of evaporation and silica condensation has a major impact on the extent of ordering. Reactor temperature impacts both evaporation and reaction rates and thus is another important variable in the EISA process. We investigated reactor temperatures from 30 to 550 °C, while keeping the molar ratio of reactants fixed at TEOS/H2O/ CTAB/HCl ) 1:63.3:0.16:0.022 and the initial pH fixed at approximately 2. XRD patterns of calcined samples (Figure 11) showed that particles produced at all temperatures possessed the 2-D hexagonal structure. At higher reactor temperatures, the d100 spacing decreased slightly. This can be attributed to shrinkage due to initiation of sintering or thermal relaxation of the silica structure. It is interest-
Hexagonal Mesostructure in Powders
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Figure 10. Transmission electron micrographs of powders produced using different solution pH levels: pH ) 1 (a), pH ) 3.5 (b), and pH ) 4 (c). Precursor solution molar ratios were TEOS/H2O/CTAB ) 1:62.5:0.15, and the furnace temperature was 125 °C.
various temperatures showed order typical of hexagonal structure at all synthesis temperatures. These experiments also provide some insight into the nature of the self-assembly process in the EISA process. The particles produced at 30 °C are partially coalesced but display ordered structure. Apparently, particles produced at sufficiently low temperatures (
