Effect of Framework Polymerization on the Phase Stability of Periodic

5448

J. Phys. Chem. B 2000, 104, 5448-5461

Effect of Framework Polymerization on the Phase Stability of Periodic Silica/Surfactant Nanostructured Composites Adam F. Gross, E. Janette Ruiz, and Sarah H. Tolbert* Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095-1569 ReceiVed: December 20, 1999; In Final Form: February 24, 2000

Real time X-ray diffraction is used to examine kinetic barriers and metastability in periodic silica/surfactant nanophase composites. These materials, which are the precursors to ordered mesoporous silicas, are synthesized in a P6mm hexagonal structure using a range of base concentrations. When the composites are heated under hydrothermal conditions, they are observed first to anneal into a more ordered hexagonal structure and then to undergo a hexagonal-to-lamellar phase transition. The transition and annealing are followed in situ through low-angle X-ray diffraction as the materials are heated under a linear temperature ramp. From the variation in diffraction peak intensities with time as a function of ramp rate, it is possible to determine activation energies for the phase transition and for annealing using the Ozawa method. Activation energies both for annealing and for the hexagonal-to-lamellar phase transformation are found to correlate with the base concentration used to synthesize the composites. Materials made at the lowest pH show an activation energy of 163 ( 3 kJ/mol for the hexagonal-to-lamellar phase transition, whereas materials made at the highest pH show an activation energy of only 106 ( 3 kJ/mol. This result can be explained by a more condensed framework in materials synthesized at lower pH and, thus, the need to hydrolyze more silioxane linkages in order for the material to rearrange. The annealing process shows the opposite dependence on pH, with the highest activation energies observed for those materials synthesized at the highest pH (Ea ) 58 ( 12 kJ/mol for materials synthesized at the lowest pH and 80 ( 16 kJ/mol for materials synthesized at the highest pH). This result can be explained by postulating that the activation energy for annealing is related to silica condensation, rather than silica hydrolysis. Higher-level kinetic analyses allow us to extract an activation energy from each temperature ramp and, thus, to examine the chemical changes that occur during the heating process. Composites made at the highest synthesis pHs have activation energies that increase with slower ramp rates, suggesting that silica condensation occurs during heating, which, in turn, increases the activation energies. Composites made at the lowest synthesis pHs, in contrast, have activation energies that decrease with slower ramp rates, suggesting that silica hydrolysis occurs during heating, which, in turn, lowers the activation energies. These ideas are corroborated by ex situ 29Si MAS NMR experiments, which show that materials made at the lowest pH start out with the most condensed silica framework. During annealing, the degree of condensation increases, but at the start of the phase transformation, the degree of framework condensation again decreases. This work provides a basis for understanding the relationship between synthetic parameters, silica chemistry, and the stability of silica/surfactant nanostructured composites.

Introduction Ordered silica/surfactant composites and the mesoporous silicates that are derived from them represent an exciting new class of self-organized nanostructured materials.1 These materials consist of an inorganic framework that surrounds periodic arrays of nanometer-scale organic domains. The organic domains can be removed by calcination to produce ordered arrays of pores with sizes ranging from 20 to 300 Å, arranged in a geometric array.2,3 Potential applications for these materials include uses as catalysts and catalytic supports,4-9 separation materials and membranes,10-12 and hosts for composite optical materials.13-17 Progress has been made toward these goals by tailoring the pore sizes, wall thicknesses, and pore periodicities of these materials.1-3,18 Although much progress has been made in the range of materials that can be produced, less work has been performed to investigate their robustness. The failure of these materials under aqueous conditions has been documented,19-22 but a systematic study of the relationship between synthetic conditions and durability has not been undertaken for these silica/surfactant

composites. An understanding this relationship, however, is of significant importance for future applications. Failure, or disordering in this case, is frequently a hard process to quantify, as it does not have a well-defined endpoint. Similar information, however, can be obtained by forcing a material to undergo a controlled structural rearrangement, or phase change. A phase change represents the local failure of the framework to internal and external stress. A variety of phase transformations have been observed in ordered silica/surfactant composites under reactive conditions, suggesting that this type of well-defined structural change can be readily produced.26,27 If synthetic procedures can be related to the kinetics of these phase transitions, an understanding of the variables that most affect the strength of these materials will be gained. Particularly for nanostructured materials, which are often intrinsically metastable phases, having a well-defined starting and ending structure is crucial for the interpretation of kinetic data. The material studied in this work is a composite formed by the hydrolysis of tetraethylorthosilicate in the presence of a 20carbon alkane tailed quaternary ammonium surfactant.2 The

10.1021/jp9944379 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/19/2000

Periodic Silica/Surfactant Nanostructured Composites

J. Phys. Chem. B, Vol. 104, No. 23, 2000 5449

Figure 1. X-ray diffraction patterns of high (left), middle (center), and low (right) synthesis pH composites. Solid lines are for as-made silica/ surfactant composites, and dotted lines are for calcined porous silicas. All syntheses produce nanoscale P6mm hexagonal order that survives calcination.

composite (with the surfactant included, not the porous inorganic material) was previously found to go through a transition from a hexagonal honeycomb type structure to a layered lamellar phase under hydrothermal conditions.2,27 The driving force for the transformation can be understood in terms of thermally induced conformational disorder of the surfactant tails. As the tail motion increases with increasing temperature, the effective surfactant volume and shape change. As the volume increases, the shape goes from something that looks like a truncated cone (big headgroup, smaller tail) to something that looks like a cylinder (big headgroup, big tail).28 Both the volume change and the shape change produce an expansive force that drives the transformation. To understand the relationship between synthetic variables and the ease of this phase transition, it is necessary to synthesize similar materials under a range of conditions. Base (in this case NaOH) can be used to control the polymerization of the inorganic phase. Changing pH changes the condensation rate of the solution-phase silica oligomers.29 This condensation rate increases from silica’s isoelectric point at pH 2.5 toward higher and lower pH. The rate is at a maximum at pH 5-7, but does not begin to rapidly fall off until pH 8-9. The rate of condensation should be important to the framework of these composites because it can control inorganic bonding. A slow condensation rate should condense the framework less rapidly, resulting in a final composite that is not highly cross-linked. With lower synthesis pH in the regime above pH 8, the silicates should condense more quickly, resulting in a higher wall density for a fixed reaction time. Over a moderate range of hydroxide concentrations (about a factor of 1.5), similar hexagonal composites can be formed, presumably with different framework bonding. We can thus produce a range of materials with almost identical nanometer-scale structure but with different inorganic bonding. By examining the ease or difficulty with which these materials undergo phase transitions, we can learn about how inorganic bonding controls stability. Although these experiments address stability in only one specific geometry of silica/surfactant composites, the results of this work should be relevant to a much broader range of composite types. Silica framework condensation should have similar mechanisms under all basic conditions, regardless of

the surfactant type. The pHs used in this work are similar to the levels of alkalinity found in most base-catalyzed silica/ surfactant composites.1,2 Our conclusions can help provide an understanding of the factors that influence framework strength. The role of annealing in synthetic processes and the mechanism by which it occurs are also discussed. The roles of silica hydrolysis and condensation in annealing and phase transitions are analyzed. The end result is a more complete picture of the route by which stronger composites are formed and the importance of initial conditions in achieving this objective. Experimental Section Composites form through the cooperative self-organization of silicate oligomers with a basic solution of quaternary ammonium surfactants containing a 20-carbon alkane tail. CH3(CH2)19N(CH3)3Br was synthesized from N(CH3)3 and CH3(CH2)19Br by established methods.30 Synthesis concentrations of 0.150, 0.195, and 0.235 M NaOH were used in combination with 0.41 M tetraethylorthosilicate (TEOS) and 0.024 M surfactant. The synthesis mixtures were stirred for 1 h; composites were filtered, washed, and dried at room temperature. All composites synthesized in this way undergo a hexagonalto-lamellar hydrothermal phase transition. For the purpose of this paper, we will refer to 0.150, 0.195, and 0.235 M NaOH as low, medium, and high synthesis pHs, respectively. Note that, in reality, all reaction mixtures correspond to a fairly high pH. To produce porous materials, samples were calcined at 500 °C under flowing N2 followed by flowing O2 at the same temperature. The composites (both before and after calcination) were characterized with X-ray diffraction using a Rigaku rotating anode Mo X-ray source and a Roper Scientific 1242 × 1152 cooled X-ray CCD detector. The sample-to-detector distance was calibrated with Tolmetin (Sigma). Figure 1 shows X-ray diffraction patterns of as-made and calcined composites synthesized for each of the three synthesis pHs. In all cases, the peaks can be easily indexed to P6mm hexagonal phases in both the composite and porous silica forms. Once calcination and characterization was complete, N2 absorption/desorption data were collected to determine pore size and surface area using a Micrometrics ASAP 2000 porosimeter.

5450 J. Phys. Chem. B, Vol. 104, No. 23, 2000

Gross et al.

Figure 2. Evolution of X-ray diffraction patterns for a hexagonal silica/surfactant composite made at the middle synthesis pH and heated at a rate of 5.9 °C/min. At low temperature, four peaks indicative of a hexagonal phase are observed. Toward the end of the transformation, two peaks that index to lamellar order are present. The intensity axis is plotted on a log scale for ease of viewing. The temperature axis is converted from time to temperature using the known linear heating rate. The rise in intensity of the (100)hexagonal and (110)hexagonal peaks at low temperatures is due to annealing; the dip in intensity at high temperatures is caused by the phase transformation.

Composites were characterized by 29Si MAS NMR using a 7.05T Bruker MSL 300 spectrometer with a standard one-pulse acquisition using a 5.2 µs π/2 pulse, an 8.8 µs dead time delay, and a 240 s recycle delay. Five hundred to 800 signal acquisitions were utilized. Samples for NMR spectroscopy either were used as-synthesized or were hydrothermally treated in an oil bath (ex situ NMR). The ex situ samples were prepared by sealing a slurry of the composite and water in a glass tube. The sample was then heated in a PID-feedback temperaturecontrolled silicone oil bath (Sigma) at a rate of 1.42 °C/min. At various temperatures, samples were removed from the bath, rapidly quenched to room temperature, and filtered for NMR analysis as dry powders. Elemental analysis was performed by Desert Analytics, Tuscon, Az. Phase transitions were followed using real-time X-ray powder diffraction. Data were collected at the Stanford Synchrotron Radiation Laboratory on wiggler beamline 10-2 using an X-ray energy of 9 keV. Samples were prepared by mixing composite with water in a ratio of 1:5. The resulting slurry was injected into a quartz capillary tube, which was sealed with a graphite ferrule to withstand a dynamic N2 back-pressure of 330 psi.

The assembly was rocked continually in the X-ray beam to mix the sample, increase the uniformity of heating, and average the diffraction pattern.31 Thermal energy for the transformation was provided by a calibrated PID-feedback-controlled air stream that utilized an Omega CN3251 temperature controller. The distance between the heat source and the sample was controlled to be the same for all samples. The temperature drop between the thermocouple and the sample was calibrated using a 2-point linear correction based on the melting points of sulfur (mp ) 117 °C) and ascorbic acid (mp ) 190 °C). As the air temperature was ramped at a constant rate, data were collected in 20-30 s acquisitions using a Roper Scientific 1242 × 1152 cooled X-ray CCD detector. Multiple ramp rates were used for each composite. Data were analyzed by integrating all frames and fitting the 100, 110, and 200 hexagonal and lamellar peaks with Voigt functions; a linear baseline correction was employed in all cases. Results Figure 2 shows the progress of a hexagonal-to-lamellar phase transition under a linear temperature ramp. It can be seen that the four hexagonal peaks initially grow in intensity and then


Figure 3. Integrated area of the (100)hexagonal and (100)lamellar diffraction peaks as a function of temperature for a 3.5 °C/min heating ramp. This sample was synthesized at the highest synthesis pH. The area of the (100)hexagonal peak (solid line) increases because of annealing and then plummets as the hexagonal phase disappears during the phase transition. As the hexagonal phase disappears, the lamellar phase (dashed line) forms. The second rise of the lamellar peak may be due to annealing.

Figure 4. Normalized integrated areas of the (100)hexagonal and (100)lamellar peaks as a function of temperature for various ramp rates. For all ramps, the sample was a silica/surfactant composite synthesized at the highest pH. A ramp-rate legend is presented on the graph. At faster ramp rates, the kinetically slow transition cannot keep up with the changing temperature. The faster the ramp, the more the transition lags behind the changing temperature, and thus, the higher the temperature at which the drop in the (100)hexagonal area appears.

begin to lose intensity at higher temperatures. As they lose intensity, the peak positions move slightly toward higher q. As the hexagonal peaks disappear, a set of two evenly spaced lamellar peaks begins to appear. The total intensity of the fundamental diffraction peak is lowest when there are equal amounts of both phases. The lamellar peaks continue growing as the temperature increases, and by the end of the run, only lamellar peaks exist. Figure 3 presents a one-dimensional view of Figure 2 by showing how the areas of both hexagonal and lamellar fundamental diffraction peaks change during the transition. As discussed above, the initial rise in the hexagonal peak area is assigned to annealing. Once annealing is complete, we can use the hexagonal and lamellar peak areas as a measure of the extent of transformation. In Figure 3, the hexagonal peak area begins to decrease rapidly at 109 °C. As the hexagonal peak loses intensity, the lamellar peak grows in. For the purpose of these experiments, we define the point at which the hexagonal peak has decreased by 50% as the transition midpoint, or simply the transition point. The transition point in Figure 3 is 126.4 °C. Because the decay of the hexagonal peak tends to be more reproducible than the rise of the lamellar peak, numerical fits to the transition use the data for the decay of the hexagonal peak only. Tracking the area of the hexagonal peak thus makes it possible to follow the progress of the transition. Figure 4 shows the effect of changing the rate at which the temperature is ramped. As the ramp rate increases (ramp time


Figure 5. Temperature of the midpoint of the drop in the (100)hexagonal peak area for all composite types at various ramp rates. The legend is shown on the graph. Composites made at the lowest pH have the highest transition temperatures, and those made at the highest pH have the lowest transition temperatures. All composites show a trend of increasing transition temperature with increasing ramp rate, indicating that all the phase transformations are kinetically limited.

decreases), the rapid decrease in the hexagonal peak area is observed to shift to higher temperature. This effect can be understood with the postulate of a slow, kinetically limited transition. For fast ramp rates, the transition cannot keep up with the ramp rate, and thus, the transition appears to occur at a higher temperature. As the ramp rate is decreased, the transition is better able to track the changing temperature, and thus, the transition appears to occur at a lower temperature. The dependence of the transition midpoint temperature on the ramp rate can be used to quantify this change. Figure 5 shows a plot of transition temperature versus ramp rate for silica/surfactant composites synthesized under a range of hydroxide concentrations. For all composites, the transition temperatures are observed to occur at higher temperatures as the ramp rate increases. As discussed above, this trend is indicative of a kinetically limited transformation. From the shift in transition temperature with ramp rate, we will extract kinetic parameters for these transitions. Figure 5 also shows that all of the temperatures at which the low synthesis pH composite undergoes a phase transition are significantly higher than the temperatures at which the high pH composite transforms. The phase transition of the middle pH composite occurs between those of the higher and lower pH composites, and none of the transition regions overlap. The cause of this phenomenon will be discussed in the following section. Beyond the obvious phase transitions, other changes can also be observed in Figure 2. The higher-order hexagonal diffraction peaks, (110)hexagonal and (200)hexagonal, begin with approximately the same areas. As time progresses, however, the (110)hexagonal peak area increases, whereas there is little change in the area of the (200)hexagonal peak. In calculations of ideal hexagonal diffraction patterns, the (110)hexagonal peak area is observed to be greater than the (200)hexagonal peak area,32 and so, this increase of the (110)hexagonal peak area relative to the (200)hexagonal area is believed to be indicative of annealing to a more perfect hexagonal structure. The change in the (110)hexagonal/(200)hexagonal ratio can be seen in Table 1. All peaks are normalized to the initial (100)hexagonal peak area from the first X-ray diffraction pattern taken in an experimental ramp. Data shown are the average for the high, middle, and low synthesis pH composites heated at 3.51, 3.52, and 3.96 °C/minute, respectively. The ratio of (110)hexagonal/(200)hexagonal can thus be used to follow the annealing process under hydrothermal conditions. For these experiments, we define the point at which the (110)hexagonal/ (200)hexagonal ratio begins to increase as the annealing temperature. Figure 6 shows the change in this ratio with temperature.


Gross et al. Discussion

Figure 6. Integrated area ratio (110)hexagonal/(200)hexagonal versus temperature from experiments performed on the high synthesis pH composites heated at different ramp rates. All data show a flat portion at lower temperature followed by a rise in the area ratio at higher temperatures. Both of these sections of the graph are found to be linear. Lines are fit to both the flat and the rising sections of the graph, and the intersection of the two lines shows the onset of annealing. Like the phase transition midpoints, annealing points also show an increase in annealing onset point with increasing ramp rate. This indicates that annealing is a kinetically limited process.

Figure 7. Temperature of the onset of annealing for all composites at various ramp rates. The legend on the graph identifies all composite types. Composites made at the lowest synthesis pH have the highest annealing temperatures, and those made at the highest pH have the lowest annealing temperatures. A rise in annealing temperature at faster ramp rates is observed, similar to the trend found in transformation midpoint temperatures.

TABLE 1 peak area base conc (M)

frame

(100)

(110)

(200)

(210)

0.235

hex-start hex-annealed lamellar hex-start hex-annealed lamellar hex-start hex-annealed lamellar

1 1.74 2.18 1 1.89 1.98 1 1.91 1.79

0.03 0.11

0.02 0.04 0.11 0.03 0.05 0.09 0.03 0.05 0.08

0.01 0.02

0.195 0.150

0.02 0.13 0.03 0.13

0.01 0.04 0.01 0.04

The annealing point is found by fitting a line to the slowly varying data before annealing and another to the increasing data after the onset of annealing. The intersection of these two lines is defined as the onset of annealing or the annealing point. The dependence of annealing temperature on ramp rate for composites synthesized under a range of hydroxide concentrations is shown in Figure 7. As with the phase transition data, annealing temperatures increase as ramp rate increases, indicating a kinetically limited process. Also, a trend is again observed in which composites synthesized at lower pH anneal at higher temperatures than those materials synthesized at middle or higher pH.

Calorimetric studies on a range of crystalline and amorphous porous silicas have shown that the thermodynamic stability of all of these phases is about the same.33 A total difference of less than 6 kJ/mol is found between the enthalpies of hexagonal mesoporous silicas, dense amorphous silica glass, and a range of pure silica zeolite phases.33 This leads to the conclusion that kinetic barriers play a dominant role in controlling phase stability in these nanostructure materials. This idea is exemplified in our phase transition data: although a clean hexagonal-to-lamellar transition is observed with increasing temperature, the newly created lamellar phase is entirely metastable, with no reverse transition occurring upon cooling to room temperature. Ideally, such a reverse transition could be driven by cooling the sample sufficiently. At such low temperatures, however, there would be insufficient thermal energy to overcome the ever-present activation barriers, and thus, no reverse transformation is predicted at any temperature. To quantify kinetic barriers, we can calculate activation energies for the hexagonal-to-lamellar phase transformation and for the annealing processes described above. Two methods will be applied. The first, developed by Ozawa in the 1960s to fit differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) data, utilizes the change in characteristic temperature with ramp rate (Figures 4 and 6) to extract a single activation energy for transition. The other method, presented in a concise form by Kennedy and Clark only a few years ago,34 fits the actual time-dependent area curves (Figure 3) to extract an activation energy from each experimental ramp. Comparison of these two methods allows us to learn both about the effect of initial synthesis conditions on metastability and about changes that occur during the hydrothermal heating process. Driving Force for the Hexagonal-to-Lamellar Phase Transformation. The most obvious trend observed in Figures 5 and 7 is the fact that all of the points for composites made at high pH are below all of the points for composites made at low pH. This result can be understood in terms of the driving force for this structural phase transformation. As discussed above, the transformation appears to be driven by thermally induced conformational disorder in the surfactant tails. This disorder can be thought of as an expansive pressure that drives the phase transition. The idea is supported by synthetic experiments in which composites are made using 20-, 18-, and 16-carbon tailed trimethylammonium surfactants. Whereas hexagonal-to-lamellar phase transitions are almost always observed with 20-carbon surfactants, only partial transformation is seen with 18-carbon surfactants, and composites made with 16-carbon surfactants can be heated for long periods of time at high temperatures without undergoing a phase transition. Below, we will show that the gross variation in phase transition temperature with synthesis pH observed in Figures 5 and 7 is another manifestation of the effect of surfactant pressure. Composite formation is driven by electrostatic interactions between the positively charged surfactant micelles and the negatively charged silicate oligomers.35 Charge neutrality and thus charge density matching have been identified as primary factors in controlling composite structure.35 For composites synthesized at high pH, the silicate oligomers should carry a high charge. This necessitates a large amount of surfactant to maintain charge neutrality. For composites synthesized at low pH, in contrast, a lower charge on the silica requires a smaller amount of surfactant to maintain charge neutrality. Returning to the hypothesis of an expansive pressure produced by the motion of the surfactant tails, we can now explain the trend in


J. Phys. Chem. B, Vol. 104, No. 23, 2000 5453 TABLE 3 base conc (M)

% excess O

% excess C

% excess H

0.235 0.195 0.150

21.68 17.21 17.82

1.54 2.69 3.51

9.56 13.34 13.37

TABLE 4

Figure 8. NMR spectra of as-made silica surfactant composites synthesized at high, middle, and low synthesis pHs. Q4 indicates a Si bonded to four other Si atoms through O bridges, whereas the Q3 peak indicates three Si-O-Si bonds and one terminal Si-O- or Si-OH. A sample is most polymerized when it has the largest Q4/Q3 integrated area ratio. High synthesis pH composites have the least polymerized framework, as indicated by the low Q4/Q3 area ratio. Composites made with the lowest pH are the most polymerized, as shown by the large Q4 peak.

TABLE 2 base conc (M)

%C

%H

%N

%Si

% other

0.235 0.195 0.150

42.84 40.49 37.75

8.44 8.16 7.55

2.14 2.00 1.85

18.08 19.69 21.11

28.50 29.66 31.75

Figures 5 and 7. Composites synthesized at higher pH contain more surfactant, which can, in turn, produce enough expansive pressure to drive the phase transformation at a fairly low temperature. For samples made at lower pH, where the composite contains less surfactant, significantly more conformational disorder of the surfactant tail (and thus higher temperature) is needed to provide sufficient driving force for transformation. The fact that the same trends are observed in both Figures 5 and 7 indicates that this surfactant pressure is the driving force for both the hexagonal-to-lamellar phase transformation and the annealing process. Evidence for this type of phenomenon can be obtained using elemental analysis. Mass percentages of C, H, N, and Si for each of our three composites are shown in Table 2. A systematic decrease in the carbon percentage and an increase in the Si percentage are observed as the synthesis pH is reduced (Table 2). A more detailed analysis of the data can be performed by making a few assumptions. Because these composites should contain only silica and surfactant, it is sensible to assume that all of the material that is not C, N, H, or Si is oxygen. The amount of oxygen needed to satisfy the Si present can be calculated from the NMR Q4/Q3 ratio (Figure 8). The large chemical shift observed for 29Si results in a clear separation of peaks from Si bonded to four other Si atoms though oxygen bridges (Q4 Si) compared to peaks from Si bonded to three other Si atoms through oxygen bridges with one Si-O- or Si-OH bond (Q3 Si). Q4 Si corresponds to a stoichiometry of SiO2; Q3 Si corresponds to SiO2.5. Using the NMR data presented in Figure 8, we calculate that there is an average of approximately 20% more moles of oxygen present than are necessary to satisfy the silica bonding in all composite types (Table 3). In addition, theoretically, the only source of carbon in the composite should be from surfactant, which has a 23:1 C:N ratio. Carbon was thus found to be in excess (Table 3). To explain these results, we must consider the other components that could be included in the composites. The silica source for these materials was TEOS. When TEOS is hydro-

base conc (M)

silica

surfactant

ethanol

water

0.235 0.195 0.150

1 1 1

0.24 0.21 0.18

0.04 0.06 0.07

0.45 0.33 0.33

lyzed, ethanol is released. Excess carbon may thus be in the form of ethanol that did not leave the structure. It is also likely that water resides in the structure because these composites were only dried in air before analysis. This idea is supported by the fact that the mass of a composite does decrease slightly when it is more thoroughly dried under vacuum. We therefore predict that oxygen that is not part of the silica framework is in the form of ethanol or water. Excess hydrogen results from both of these sources, as well as from some terminal hydroxides on the surface of the silica framework. Because hydrogen is so light, however, the analysis numbers for this element are not very accurate, and quantitative analysis is not dependable. Using the above assumptions for the types of constituents present and the calculated mole excesses of carbon and oxygen, it is possible to find the mole ratios of all species for each composite type (Table 4). A systematic decrease in the surfactant-to-silica ratio is observed with decreasing synthesis pH, supporting the idea of a reduced driving force for the phase transition in low synthesis pH composites, as discussed above. To fill some of the pore volume that is not occupied by surfactant in these lower pH composites, an increase in the amount of included ethanol is observed with decreasing synthesis pH. Water hydrogen bonds well to silica surfaces. The data showing some water remaining in all composites as a result of this favorable interaction is thus understandable. With an understanding of the changes in driving force for transformation, we can now begin to analyze the changes in transition kinetics. Kinetic Analysis of the Hexagonal-to-Lamellar Transition from A Series of Temperature Ramps. It has been shown from a variety of measurements that it is possible to relate the midpoints of phase transitions occurring at different ramp rates to the activation energy for transformation.36 The method integrates the Arrhenius equation over the variable T, using a linear temperature ramp to convert time to temperature. The equation does not assume a specific rate law, but requires simply that the rate law be a single-valued function involving only of the extent of reaction. Ozawa expressed the simplified relationship as

ln(b) ) -

Ea + int RT

(1)

where b is the ramp rate, Ea is the activation energy, R is the gas constant, T is the absolute temperature at the midpoint of the transition, and int is the intercept of the plot (which is not important for determining the activation energy). Each experimental run gives a (b, Tmidpoint) point that can be used with other points from other runs to find the best equation for a line. The slope of this line is -Ea/R. Improvement in the statistical error can be made by choosing points other than the midpoint of transition (i.e., the 50% transformed point) to calculate the activation energy.36 In this work, activation energies were


Gross et al. TABLE 6 pore wall base conc a uncalcined a calcined shrinkage diameter thickness (M) (Å) Å) (%) (Å) (Å) 0.235 0.195 0.150

Figure 9. Ozawa method activation energy of phase transformation for composites made at different synthesis pHs. Low synthesis pH composites have the largest activation energy, and high synthesis pH composites have the smallest activation energy. This trend suggests that activation energies are determined by the amount of silica hydrolysis needed for the structure to rearrange.

TABLE 5 base conc (M)

Q4/Q3 ratio

0.235 0.195 0.150

0.86 0.95 1.07

calculated from the 40%, 50%, and 60% transformed points and averaged to generate the final values. The activation energies for the hexagonal-to-lamellar phase transition for low, medium, and high synthesis pH composites are plotted in Figure 9. A variety of trends can be observed in these data. In the first place, the activation energies are large: they range from 106 ( 3 kJ/mol for the high synthesis pH composites to 163 ( 3 kJ/mol for the low synthesis pH composites. The Ozawa method has been shown to produce reasonable agreement with both isothermal36 and other nonisothermal methods,37 and thus, we consider these differences to be significant. In addition, our large activation energies are reasonably consistent with phase transitions in other cross-liked inorganic materials.38,39 Importantly, Figure 9 indicates that activation energies can be increased by almost 50% simply by making small changes in synthesis pH. This result emphasizes that the stability of these materials is a sensitive function of the reaction conditions. Finally, a systematic trend is observed whereby samples made with less base have a higher activation energy compared to samples made with more base. The increased activation energy for samples made at lower pH indicates a greater resistance to change. An understanding of the trend with pH can be gained through a range of supporting experiments. In this case, we combine 29Si MAS NMR spectroscopy and nitrogen adsorption/desorption to paint a more complete picture of the local bonding in our composites at the start of hydrothermal heating. 29Si solid-state MAS NMR spectroscopy provides a powerful and direct method for analyzing the local bonding in these materials.2,22-24,40-42 As discussed above, Q4 indicates silica in a fully cross-linked environment. Q3 silica corresponds to either interfaces (which are plentiful in these materials) or defect sites. NMR spectra for our three composites are shown in Figure 8, and numerical values for the ratio of the Q3 to Q4 peak areas are given in Table 5. In agreement with our calculated activation energies, as synthesis pH decreases, the Q3 peak area decreases, while the Q4 peak increases in area. This suggests a stronger, more interbonded framework for materials made at lower pH. Combining these data with the activation energies plotted in Figure 9, we can postulate that the rate-limiting step for

53.3 53.1 53.7

44.6 45.3 46.5

16.3 14.6 13.4

28.7 28.2 28.5

15.9 17.1 18.0

transformation is the hydrolysis of the Si-O-Si bond to form two Si-OH groups. Transformation cannot begin until sufficient hydrolysis has occurred that the framework becomes flexible and is able to alter its long-range structure in response to surfactant pressure. Materials with a more cross-linked initial structure will require more hydrolysis before a flexible framework is produced and, thus, will show a higher activation energy. Note that these data only explore the variations in the Q4/Q3 ratio in as-synthesized materials; further experimental results examining the change in the Q4/Q3 ratio with temperature for the high and low synthesis pH composites are included later in this work (Figures 11 and 12). The same conclusions can be drawn in a more indirect manner using nitrogen adsorption/desorption measurements (Table 6). In these experiments, the pore center-to-center distance is calculated using X-ray diffraction (XRD) both before and after calcination. The pore diameter is then calculated from gas adsorption/desorption experiments, and the difference in these two numbers is used to determine the wall thickness. We note that the pore diameter calculation is done assuming no microporosity in the walls. Initially, all composites have approximately the same XRD pore center-to-center distance (a), but this distance shrinks unequally after calcination (Table 6). Because the pore sizes determined by the BET method from nitrogen absorption isotherms are approximately equal after calcination, the final pore sizes must be approximately the same in all composites. This leads to a systematic increase in calcined wall thickness with decreasing synthesis pH. Because calcination is a high-temperature process, all composites should have approximately the same degree of framework polymerization after calcination, regardless of synthesis pH. The more a composite shrinks, the less polymerized it was before calcination. The fact that low synthesis pH composites have a thicker wall after calcination but the same pore center-to-center distance before calcination, suggests that these composites had a denser, more cross-linked wall before calcination. These conclusions are in good agreement with the 29Si MAS NMR results (Figure 8) presented above. Higher activation energies in materials made at lower pH are thus the result of more cross-linked, denser silica walls. The degree of silica hydrolysis needed to produce a mobile framework appears to correlate directly with the measured activation energies. Kinetic Analysis of Annealing from A Series of Temperature Ramps and ex Situ 29Si MAS NMR. Activation energies for annealing can be found using the same method as was used to determine activation energies for transformation. By fitting the data presented in Figure 7 using the Ozawa method, we calculate the activation energies presented in Figure 10. These values are all found to be smaller than the activation energies for the hexagonal-to-lamellar phase transition. That result makes intuitive sense: less structural rearrangement should be required for annealing compared to a full phase transition. Contrary to intuition, however, the activation energy for annealing decreases as the synthesis pH decreases. This result is counter to the trend found for phase transformations. One way to understand this result is to postulate that the rate-limiting step for annealing is silica condensation, rather than silica hydrolysis. If one assumes


Figure 10. Ozawa method activation energy of annealing for all composite types. Low synthesis pH composites have the lowest activation energy, and high synthesis pH composites have the highest activation energy. This trend is opposite to that for phase transitions (Figure 9) and is explained by the hypothesis that annealing is controlled by silica condensation rather than hydrolysis of silica bonds.

that a significant amount of condensation is required to form an “annealed” structure, the observed trend in activation energies can be explained. 29Si NMR experiments (Figure 8) indicate that materials formed at high pH do not have a well cross-linked framework. Significant condensation is thus required to produce a well-annealed structure, potentially resulting in a high activation energy for annealing. Materials formed at lower pH, in contrast, already have a well-condensed framework, and thus, less condensation is required for annealing, resulting in a lower activation energy for the process. Evidence for this idea can again be found using 29Si solid state NMR data. The slow relaxation rates of solid silica, combined with the low natural abundance of this element, mean that the typical integration time for a single spectrum is 2 days. This completely excludes the possibility of running in situ NMR experiments. At room temperature, however, dry silica is almost completely unreactive, so ex situ quench experiments are a viable option. In these experiments, samples are heated in a sealed vessel under a linear temperature ramp. This ramp rate (1.42 °C/min) is slightly slower than the slowest rate used for in situ X-ray diffraction. Once the desired temperature is reached, samples are rapidly cooled to room temperature, filtered, and dried. Slightly higher transition temperatures are observed in these experiments compared to those observed in the in situ X-ray diffraction experiments. Unlike the small samples used for diffraction, the NMR samples are very large, and so heat transfer may cause the sample temperature to slightly lag the set temperature. Figure 11 shows an example of the type of data that can be obtained for a sample made at high pH. In the initial material (25 °C), a poorly condensed framework is found with Q3 greater than Q.4 About 8 °C beyond the onset of annealing (76 °C), the Q4/Q3 ratio increases dramatically, with the Q4 peak area now significantly greater than the Q3 area. This result indicates that condensation of silanol groups (Si-OH) to form siloxane linkages (Si-O-Si) is, in fact, occurring as the material anneals (see Table 7 for Q4/Q3 ratios). Others have also observed the increase in Q4/Q3 ratios in samples heated hydrothermally.2,24,40,42 A temperature of 110 °C corresponds to the start of the hexagonal-to-lamellar transition. Beyond this point, the hexagonal peak width broadens, corresponding to disordering of the hexagonal phase and transition to the lamellar phase. In agreement with the trend in activation energies discussed above, the Q4/Q3 ratio is observed to decrease at the start of the transition. This supports the idea that significant silica hydrolysis is required to provide flexibility before the framework can begin


Figure 11. Ex situ NMR spectra of composites synthesized at the highest pH. The number on top of each spectrum indicates the temperature to which the sample was heated. All samples were heated at a rate of 1.42 °C/min. Samples were not heated (held at 25 °C), stopped 8 °C into annealing (76 °C), stopped near the start of transition (110 °C), stopped 40% through the phase transition (142 °C), and allowed to become completely lamellar (175 °C). Initially, the sample anneals, as shown by the high Q4/Q3 integrated area ratio at 76 °C. However, hydrolysis occurs in preparation for the phase transition, as indicated by a lower Q4/Q3 ratio at 110 °C. Once the composite transforms to the lamellar phase, new bonds are created, and the Q4/Q3 ratio increases again at 142 °C. The Q4/Q3 ratio decreases slightly in the fully formed lamellar phase because of increased hydrolysis at 175 °C.

TABLE 7 high synthesis pH (0.235 M NaOH)

low synthesis pH (0.150 M NaOH)

final temp (°C)

Q4/Q3

final temp (°C)

Q4/Q3

25 76 110 142 175

0.91 1.54 1.08 1.37 1.22

25 89 98 126 176

1.07 1.03 1.18 1.12 1.47

to rearrange. By the midpoint of the phase transition (142 °C), the framework has already begun to recondense, and thus, the Q4/Q3 ratio has begun to increase again. At the highest temperatures achieved in these ex situ NMR experiments (175 °C), the sample is fully lamellar. The Q4/Q3 ratio is slightly lower at this temperature than at 142 °C, probably because the silica network in the lamellar framework is less stable than the hexagonal network under hydrothermal conditions, and thus, the silica begins to break up or dissolve at very high temperatures. This process is observed as a loss of lamellar peak intensity at very high temperature (near 200 °C). Table 7 shows all of the Q4/Q3 integrated peak area ratios for these experiments. Similar trends can be seen in materials made at lower pH (Figure 12 and Table 7 above), although the trends are slightly less clear in these data. Because of the lower synthesis pH, the initial Q4/Q3 ratio is much larger than for materials made at high pH. The first treated temperature (89 °C) corresponds to a temperature that is below the onset of structural annealing. In agreement with the idea that condensation is the chemical process associated with annealing of the nanoscale architecture, no increase in the Q4/Q3 ratio is observed at this temperature. The small decrease in the Q4/Q3 ratio observed at this temperature may not be statistically significant. In contrast, at 98 °C, a temperature that is above the onset of structural annealing, the Q4/Q3 ratio clearly increases, again indicating that silica condensation is the chemical process associated with annealing. We note, however, that the net increase in the Q4/Q3 ratio upon annealing is much lower in samples made at low pH than in those made at high pH. The activation energy for this process is therefore low because little bonding change occurs.


Figure 12. Ex situ NMR spectra of hydrothermally heated composites synthesized at the lowest pH. The number on top of each spectrum indicates the temperature to which the sample was heated at a rate of 1.42 °C/min. Samples were not heated (held at 25 °C), stopped before annealing commences (89 °C), stopped 8 °C into annealing (98 °C), stopped at the start of transition (126 °C), and stopped 60% through the phase transition (176 °C). The sample Q4/Q3 ratio does not begin to change until the annealing temperature is reached, as shown by the similarity between the 25 °C and 89 °C spectra. Some increase in the Q4 area is observed after the onset of annealing at 98 °C. Hydrolysis occurs in preparation for the phase transition, as indicated by a slightly lower Q4/Q3 ratio at 126 °C. During the transformation to the lamellar phase, new bonds are created, and the Q4/Q3 ratio increases.

In contrast to the results for materials made at high pH, only a very small drop in the Q4/Q3 ratio is observed at the start of the transition (126 °C). This may be because the total range of variability is smaller in these materials compared to the samples shown in Figure 11. By the midpoint of the transition (176 °C), however, the Q4/Q3 ratio does increase, indicating increased condensation in the newly forming lamellar phase. It is interesting to note that, despite the fact that materials made at high pH show more condensation upon annealing than those made at low pH, the Q4/Q3 ratio at the start of the phase transformation is still slightly higher for high pH materials than for those made at low pH. This suggests that the siloxane bonds formed during annealing are more readily hydrolyzed than those formed during initial synthesis. That idea is corroborated by our experimental results that show that, despite significant annealing, the activation energy for transformation is still lower in the materials formed at high pH. One explanation for this phenomenon could be that bonds formed during hydrothermal annealing are more strained than those formed during initial synthesis and thus are easier to break. For instance, the dramatic change in Q4/Q3 ratio observed upon annealing (Figure 11 and Table 7) for materials made at high synthesis pH would likely produce some internal strain. Therefore, these results suggest that, although rapid hydrothermal annealing can be used to make more ordered phases, initial synthesis conditions may play the dominant role in determining whether materials are robust under aqueous conditions. The many-day annealing processes used for some materials may also be successful in making robust materials because they allow sufficient time for this strain to relax.2,41 The contrast between the silica chemistry associated with the phase transitions (hydrolysis) and that observed upon annealing (condensation) emphasizes the fact that the inorganic framework can control composite stability in a complex fashion and that many factors must be addressed to design optimal materials for specific applications. Kinetic Analysis of the Hexagonal-to-Lamellar Transition from Individual Temperature Ramps. The fact that both silica condensation and silica hydrolysis are observed by ex situ 29Si NMR indicates that silica chemistry can occur during heating.

Gross et al. To understand the effect of this chemistry on activation energies, we need a higher-level kinetic model for our data. In particular, the data in Figure 4 show actual time versus percent transformation traces. Taking only a few points from these data (as in the Ozawa method) does not make full use of the data. The Ozawa method is ideally suited for differential data, such as DTA or DSC, for which only the midpoint of the transition is taken from the differential curve peak.43,44 Recently, a new method has been developed by Kennedy and Clark that is designed to be used with powder X-ray diffraction under nonisothermal conditions.34 This method relies on directly following the phase transition using the area of the diffraction peaks to find the decimal percentage transformed (R). In this method, one experimental run can provide many (T, R) points. It is thus possible to find the activation energy from a single experimental run. By heating at multiple ramp rates, we are then able to determine how the silica chemistry that occurs during heating affects the activation energy. Three equations are necessary to derive the functional form for Kennedy and Clark’s method. The first is a rate equation for the transformation. Like many others, we use the Avrami equation to describe our transformation.45 The Avrami equation is probably the most straightforward rate law for a solid-tosolid phase transition. It assumes a simple volume-to-volume transformation and then considers the way in which a volume of starting phase can evolve into a volume of transformed phase. The basic functional form is

-ln(1 - R) ) (kt)n

(2)

where R is again the decimal percentage transformed, k is the reaction rate, t is the time since the beginning of the reaction, and n is the dimensionality, which describes the way in which one volume transforms into another. The n parameter will be discussed in more detail below. The Avrami rate law can be used directly with the Arrhenius equation to calculate activation energies, Ea, from isothermal time/transformation data.

k ) Ae-Ea/RT

(3)

Here, A is the frequency factor or maximum possible rate, R is the gas constant, and T is the temperature in Kelvin. To modify these equations for nonisothermal data, time must be replace by temperature. This can be done using the known temperature ramp rate (b).

T ) bt + To

(4)

Here, T is again the temperature in Kelvin, t is the time, and To is the temperature at which the phase change begins. As with the Ozawa method, eqs 2 and 4 can be substituted into the Arrhenius eq 3 to convert a time-dependent equation to a temperature-dependent form. In this case, however, we do not integrate over T, and so detailed information about the temperature progress of the reaction is not lost. By keeping this detailed information, however, we also retain all of the parameters of the rate law, such as n, which conveniently cancel in the Ozawa method. The final equation, as derived by Kennedy and Clark, is

ln

{

}

Ea b[-ln(1 - R)]1/n + ln(A) )T - To RT

(5)

All variables are defined as above. If To and n are known, the left-hand side of the equation can be plotted against 1/T and fit with a line of slope -Ea/R.


J. Phys. Chem. B, Vol. 104, No. 23, 2000 5457 TABLE 8 base conc (M)

n

error

0.235 0.195 0.150

1.3 1.1 1.0

0.1 0.3 0.1

2 can be solved isothermally47 or nonisothermally to find n. Ozawa showed that, using nonisothermal data, n can be found if multiple experiments at different ramp rates are performed.36 The relevant form of the equation is

ln[-ln(1 - R)] ) n ln b + int Figure 13. Full width at half-maximum (fwhm) (in Å-1) of the (100)hexagonal diffraction peak as a function of temperature for a 3.5 °C/ min heating ramp for a high synthesis pH composite (transition midpoint ) 126.4 °C). The fwhm is found by subtracting a linear baseline and fitting the diffraction data with a Voigt function. The fwhm of the (100)hexagonal peak decreases because of annealing and then increases as the composite becomes disordered as it approaches the phase transformation. The minimum in fwhm is used as To for the Kennedy and Clark method.

Determining To experimentally is not a straightforward process. Kennedy and Clark suggest an iterative method in their paper,34 but that did not produce meaningful results with these data. To is theoretically the start of the phase transition. From the point of view of silica chemistry, that should be the point at which the framework stops condensing and starts hydrolyzing. Alternately, To should be the point at which the framework periodicity stops improving and starts to become disordered in preparation for the phase transformation. One measure of longrange periodicity is the width of a diffraction peak. In these materials, the hexagonal diffraction peaks narrow with increasing temperature as the composite anneals. At temperatures a number of degrees before the first appearance of lamellar diffraction peaks, the hexagonal peak width begins to increase again. For these analyses, we thus choose To as the point at which the hexagonal (100) diffraction peak stops narrowing and begins to broaden. Figure 13 shows a representative graph of full width at half-maximum for a high synthesis pH composite heated at 5.9 °C/min. At least for materials made at the high synthesis pH, ex situ NMR experiments (Figure 11) indicate that this temperature correspond to the point at which silica condensation becomes overshadowed by silica hydrolysis. Errors in choosing To will affect the value of Ea that we calculate. Much analysis, however, has shown that any consistent method of choosing To produces the same trends in Ea. The general trends in Ea calculated using the Kennedy and Clark method are, therefore, much more reliable than the numerical values of Ea. The value of n can be determined in a much more reliable manner directly from the experimental data. As discussed above, n is the overall dimensionality of a reaction.46 The n parameter is the sum of the dimensionality by which the reaction actually proceeds and the number of steps needed for nuclei formation (0 or 1). One-dimensional growth results in the phase-transition boundary that propagates uniaxially. Two-dimensional growth is planar growth in which the new phase grows in like a disk or a cylinder. Three-dimensional growth is when the transformation grows out of the nuclei spherically or hemispherically. Zero steps to nuclei formation indicate that the reaction nucleates off preexisting defects and does not need extra defects to be created during the course of a transformation. In the case in which one nucleation step is necessary, the transition can only proceed once new defects are created. Finding the overall dimensionality is best accomplished by directly fitting data to the Avrami equation (eq 2).45 Equation

(6)

where, again, b is the ramp rate and int is the intercept. To find n, a temperature at which all transitions are partially complete is chosen. At this temperature, one (b, R) point is determined for each experiment. The parameter R is calculated by first determining the maximum (100)hexagonal area. This is done by fitting a sigmoid to the (100)hexagonal peak area versus temperature data in the region of the phase transition to determine the maximum area before the phase transition has begun. We determine R by dividing the (100)hexagonal peak area at a given temperature during the transformation, T, by this maximum area. A least-squares analysis of the slope from a plot of ln[-ln(1 - R)] versus ln(b) yields n. Data from all composite types were analyzed to find n (Table 8). Figure 14 (top) shows an example of a least-squares fit to our data that was used to find n for the low synthesis pH composite. Similar values of n are calculated from a range of data as long as all temperatures used in these fits are near the middle of all transformations. For interpretation purposes, n is usually rounded to the nearest integer. We found that n ≈ 1 regardless of composite type. Most materials, in contrast, have n ) 2 or n ) 3 as their Avrami exponent.46 Transformations with n ) 2 or n ) 3, however, almost always involve at least one three-dimensionally periodic crystalline phase. That three-dimensional periodicity tends to drive transformations in three dimensions, producing the common result of n ) 3. The result of n ) 1 is reasonable upon examination of the P6mm hexagonal structure. Unlike most materials, a hexagonal mesophase has order in only two dimensions. The lamellar phase is periodic in only one dimension. For the transformation to progress in a diffusionless manner, it must connect a symmetry axis of the hexagonal structure to one in the lamellar structure, which necessitates a one-dimensional transformation. Note also that a value of n ) 1 indicates that the production of defects during transformation is not required for nucleation. Therefore, we postulate that the transformation occurs as shown in Figure 14 (bottom). Heating of the hydrothermal mixture excites thermal motion of the surfactant. The tails take up more area, creating an internal pressure that is relaxed by rearrangement of the framework into a layered lamellar structure along a (100)hexagonal direction. Because the d spacing of the lamellar phase is less than that of the hexagonal structure, a discontinuous transformation is observed. The transition spreads uniaxially through the material. Using a value of n ) 1 and determining To as described above, we can use eq 5 to calculate an activation energy from each experimental run. Data and linear fits for the low synthesis pH composites are shown in Figure 15. The slowest ramp rate composite forms the series of points that have the lowest slope. As the ramp rate increases, the slope also increases, corresponding to changes in the activation energy. This result indicates that silica chemistry may be taking place during the heating


Figure 14. Top: Fit to the Ozawa equation for determining n, the Avrami dimensionality, for the low synthesis pH composites at 177 °C. The value of n is equal to the slope of this line, which in this case is one. An n value of one indicates a one-dimensionally propagating transition. Bottom: The transition of hexagonal cylinders breaking open and becoming lamellar layers is illustrated. The transition propagates from one pore to another in a direction perpendicular to the axis of the pore. A transition of the sort illustrated here would have an n parameter of 1.

Figure 15. Kennedy and Clark method fits to data for the low synthesis pH composites. Solid circles are the fastest ramp rate (7.9 °C/min), open circles are the middle ramp rate (4.0 °C/min), and solid triangles indicate the slowest ramp rate (2.6 °C/min). The slopes of these lines are equal to -Ea. Note that the composite heated at the fastest ramp rate has the regression line with the steepest slope, and thus the highest activation energy.

ramp. Thus, composites do not experience exactly the same activation energy when they are heated in different ways.

Gross et al.

Figure 16. Kennedy and Clark activation energy versus ramp rate for all composite types. (a) High synthesis pH composites (circles) show activation energies that decrease with faster ramp rates. This trend is probably a result of annealing, which can occur to a greater extent with a slower ramp rate. (b) Middle synthesis pH composites (squares) show no trend with respect to ramp rates. (c) Low synthesis pH composites (triangles) show activation energies that decrease with decreasing ramp rates. Because these composites are better condensed before transformation, hydrolysis is likely to dominate the kinetic behavior. Slower ramp rates allow more time for this hydrolysis to occur.

An overall feel for the type of chemistry that is occurring can be obtained by examining all of the activation energies calculated using eq 5. The data, presented in Figure 16, show activation energies as a function of ramp rate for composites made at (a) high, (b) medium, and (c) low synthesis pH. Figure 16a shows that, for composites synthesized at the highest pH, the activation energy for transformation increases as the ramp rate decreases. In contrast, for composites synthesized at the lowest pH, the activation energy decreases as the ramp rate decreases. For composites synthesized at an intermediate pH, almost no change in activation energy is observed across a range of ramp rates. This trend can be understood by considering the annealing chemistry that is observed in both low-angle X-ray diffraction and ex situ 29Si NMR. Figure 11 shows that, for composites made at high pH, significant condensation of the framework takes place in the first ∼50 °C of heating in water if the temperature is ramped slowly. From X-ray diffraction, we know that the rearrangement of the framework is a kinetically limited process with an activation energy of ∼80 kJ/mol. If, as discussed above, we assume that condensation of the framework is associated with annealing, then this condensation should also be a slow, kinetically limited process, and consequently, there should be less condensation occurring in composites that are heated at faster rates. As a result, phase transformations occurring in materials heated quickly should have fewer bonds

Periodic Silica/Surfactant Nanostructured Composites to be broken before the transformation and a correspondingly smaller activation energy. As the ramp rate decreases, there is more time to anneal, and so, the structure becomes more resistant to change. This is seen as an increase in activation energy with decreasing ramp rate. For well-condensed starting phases (low synthesis pH), Figure 16c shows that the activation energies are lowest when the ramp rate is slow, and that they increase with decreasing ramp rate. This trend can be explained with the hypothesis of net silica hydrolysis with time under hydrothermal condition. As the composites are allowed to sit under hydrothermal conditions for longer times, there is more time for hydrolysis to occur before the phase transition temperature is reached. As more bonds are hydrolyzed, it is easier to re-form the framework in a lamellar structure. Although condensation must also be occurring in these materials, net hydrolysis appears to dominate over net condensation, so activation energies decrease for slower ramp rates (longer ramp times). The dominance of hydrolysis may be caused by two factors. In the first place, low synthesis pH composites start out much more condensed than high synthesis pH materials, so there is less potential for condensation (as seen in Figure 12 compared to Figure 11). In fact, some hydrolysis under hydrothermal conditions may, in part, be responsible for the very small increase in the Q4/Q3 ratio observed upon annealing in materials made at low pH compared to the ratio for those synthesized at high pH (Figures 11 and 12). In addition, Figure 5 shows that all of the phase transitions in low synthesis pH composites take place at much higher temperatures (∼170-180 °C) than those observed for high synthesis pH composites (∼115-125 °C). The different temperature ranges may also affect how in situ chemistry modifies the transition kinetics. In agreement with the ideas discussed above to explain Figure 16a,c, the middle synthesis pH composites do not show a clear pattern of changing activation energies with changing ramp rate. We hypothesize that, in these intermediate pH materials, hydrolysis and condensation rates are approximately balanced so that activation energies do not change significantly with heating rate. Note that, although the trends in activation energies calculated using the Kennedy and Clark method show good agreement with the silica chemistry hypothesized from analysis of the data with the Ozawa equation, the specific numbers are not in agreement. That is, the average activation energy for each type of composite plotted in Figure 16 is not the same as the value given in Figure 9. Although all values appear in the same range (100-200 kJ/mol), there is no specific agreement between Figures 16 and 9, and no monotonic trends in average activation energy with synthesis pH can be gleaned from Figure 16. As discussed above, this is probably because of our inability to accurately determine To. Without an accurate To value, the Kennedy and Clark method activation energies are not accurate. It was found that the trends within composite types remain constant regardless of the method used to pick To, as long as it is picked in a uniform fashion for all runs. Because of differences in the way samples anneal, our choice of To as the minimum (100)hexagonal peak width (Figure 13) may also have produced different systematic errors for different composite types (synthesis pHs). The greatest value of the Kennedy and Clark method is the trends that it reveals. The trends depicted in Figure 16, in fact, suggest that the numbers given in Figure 9 are not entirely accurate. The midpoints of many phase transitions, each heated at a different rate, were used earlier in this work to find the activation energy

J. Phys. Chem. B, Vol. 104, No. 23, 2000 5459 for each class of composite (Figures 5 and 9). An implicit assumption made by Ozawa in developing his method, however, was that only the observed transition point, but not the activation energy, would have a dependence on ramp rate. Figure 16 shows that this is clearly not correct for many transformations in silica/ surfactant composite materials. The results calculated using the Ozawa method thus contain systematic errors that need to be understood. To determine the direction of the error, it is necessary to first determine the change in phase transition points that would allow all activation energies for a given type of composite to be equal. We then examine how those changes in phase transition points would alter the calculated Ozawa activation energies. For the high synthesis pH composites, activation energies increase with decreasing heating rates. In order for all the activation energies in Figure 16a to be the same, the fastest ramp rate transformation would need to have a higher activation energy, and the slowest ramp rate transformation would need to have a lower activation energy. If this were the case, then the midpoint for the fast ramp transition would have occurred at a higher temperature than that actually observed and the midpoint for the slower ramp phase change would have a midpoint at lower temperature than that actually observed. The end result would be a greater temperature difference between the midpoints for the different ramp rates. Thus, in the absence of the silica condensation observed in Figure 16a, the activation energy measured by the Ozawa method (Figure 9) for the high synthesis pH composites would be lower than that currently calculated. Figure 16b indicates that the activation energy for composites synthesized at a middle pH is relatively invariant with ramp rate. Thus, the Ozawa activation energy for this composite does not require modification. For the composites synthesized at the lowest pH, Figure 16c shows a trend opposite to that observed for the high synthesis pH composites. By arguments similar to those presented above, to correct for this trend, the phase transition midpoints would need to be closer together than the measured values. Thus, in the absence of the silica hydrolysis observed in Figure 16c, the activation energy measured by the Ozawa method (Figure 9) for the low synthesis pH composites would be higher than that currently calculated. From this analysis, we conclude that the trend of increasing activation energy with decreasing synthesis pH shown is Figure 9 is correct. The differences in activation energies between composites synthesized under different conditions, however, may in reality be larger than they appear in Figure 9. These results emphasize the importance of modeling kinetic data in a variety of ways so that systematic errors do not mask (or create the appearance of) real physical phenomena. Application of These Results. The use of phase transformations as a synthetic method for producing new types of silica/ surfactant composites has been well established. Researchers have show that the elusive Ia3d cubic phase can be efficiently produced through a phase transformation process that occurs under hydrothermal conditions.25 In the area of silica/surfactant composite films, researchers have shown that a cubic structure with an accessible pore system can be produced via a phase transformation process.48 The phase transformation process in thin films has the important result of producing a film with a crystallographic orientation that is not determined solely by the kinetics of interface-nucleated film growth.48 The work presented here provides insight into the types of conditions that favor this sort of reaction. Activation energies for transformation are lower if the silica framework is poorly condensed. This poorly condensed framework is more likely to be preserved

5460 J. Phys. Chem. B, Vol. 104, No. 23, 2000 when rapid heating to a temperature high enough to drive a transformation is employed. Slower heating can potentially increase activation energies and prevent transformation to the desired phase. More importantly, this work shows that very small changes in synthesis conditions can have a dramatic effect both on the driving force for a structural rearrangement and on the kinetics of the rearrangement. This means that careful experiments are likely to be required to find conditions that favor clean transformations. The results of the annealing experiments also shed significant light on the process of annealing under hydrothermal conditions. Empirically, a number of research groups have found that the quality of the diffraction pattern in a silica/surfactant composite can be improved by annealing the material at high temperature under aqueous conditions.2,23,41 In all of these cases, the best diffraction patterns are produced by heating the composite in pure water or by heating it in a basic solution that has a pH that is lower than the original synthesis solution. Although synthetic methods for annealing were obtained by careful experimental trial, the reasons those conditions worked for annealing are now clearer: High temperatures are needed to overcome the substantial activation energy for annealing. A reduced pH is needed to drive condensation, which is the chemical process associated with annealing. Interest in the mechanism of structural change that occurs in silica/surfactant composites when heated under hydrothermal conditions has been stimulated recently by experiments that show a dramatic increase in pore diameter in materials that are heated in a very high pH solution at high temperature (150 °C).49 The driving force for this change has been ascribed to surfactant chemistry that results in demethylation of the quaternary ammonium surfactant to form a tertiary amine, accompanied by incorporation of a free tetramethylammonium ion to balance the charge.50,51 Although both the initial and final structure in this material is hexagonal, the dramatic changes in pore diameter and repeat distance suggest that this process should be modeled as a phase transition, rather than as an annealing process, from a kinetic point of view. Because the surfactant chemistry is slow (over 16 h induction times), rapid heating to preserve low activation barriers is not possible. As a result, high pH is required to reduce activation barriers in situ by driving silica hydrolysis. When this same pore enlargement is attempted in water, only a small increase in pore diameter is observed for moderate heating times.52 Major restructuring and pore expansion does occur with very long heating times, but the material produced is inhomogeneous. Because of the low pH environment used (i.e., water), we would ascribe the structural refinement and minor pore expansion that occurs at short times to an annealing process driven by the conformational disorder of the surfactant tails. The major changes that occur at long times are likely the result of a phase transition driven by degradation of the surfactant and are, therefore, a fundamentally different process. The inhomogeneity of the material produced from major pore expansion in water is likely the result of very high activation barriers at the low pH used. Conclusions We have shown that synthesis pH affects the phase stability of silica/surfactant composite materials under hydrothermal conditions. Lower synthesis pH results in a mesophase with a more polymerized framework. This has the benefit of making the composite easier to anneal and more able to tolerate the internal, thermally induced expansive force of its surfactant. Low, middle, and high synthesis base concentration composites

Gross et al. have activation energies for a hexagonal-to-lamellar transition of 163 ( 3, 145 ( 15, and 106 ( 3 kJ/mol, respectively, and activation energies for annealing of 58 ( 12, 69 ( 12, and 80 ( 16 kJ/mol, respectively. These trends in activation energy can be understood by postulating that the rate-limiting step for structural phase transitions is the hydrolysis (or breaking) of Si-O-Si bonds. In contrast, the rate-limiting step for annealing appears to be the condensation of silanol groups to form SiO-Si bonds. These conclusions are supported by both transition kinetics and ex situ 29Si NMR spectroscopy. The results emphasize that different types of structural rearrangements can be controlled by different types of framework chemistry. The data also show that, although activation energies can be understood in terms of the synthesis methods used to make composites, in situ chemical changes can also affect activation energies. Both silica condensation and hydrolysis of Si-O-Si bonds can occur during heating. If a composite is already wellannealed before an attempt is made to force it to undergo a phase change, hydrolysis will dominate the relationship between ramp rate and phase stability. However, if the composite is not well-annealed before a phase transition is attempted, a slower ramp rate will give composites more time to anneal, resulting in a more polymerized framework and a higher activation energy for structural phase transitions. This work broadens the fundamental understanding of the ways in which synthesis conditions and in situ chemical reactivity affect stability of silica/surfactant composites under hydrothermal conditions. Although well-defined rearrangements, such as the hexagonal-to-lamellar phase transition described here, cannot be driven in the calcined porous versions of these materials, it is like that structural changes in porous materials (such as collapse) are controlled by similar activation barriers. Using this knowledge, it may become possible to vary synthesis parameter to create more durable composites. Acknowledgment. The authors thank Dr. M. J. Strouse for help with the NMR spectroscopy. Assistance with X-ray diffraction techniques from Mr. J. Wu and Dr X. Liu is appreciated. Help synthesizing silica/surfactant samples for NMR analysis from Ms. V. Le is gratefully acknowledged. This manuscript contains work performed at the Stanford Synchrotron Radiation Laboratory (SSRL), which is operated by the Department of Energy, Office of Basic Energy Sciences. This work was supported by the National Science Foundation under Grants DMR-9807180 and CHE-9805254. References and Notes (1) Kresge, C. T.; Leonowitz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. T.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higggins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (3) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Sayari, A. Chem. Mater. 1996, 8, 1840. (5) Corma, A. Chem. ReV. 1997, 97, 2373 and references therein. (6) Aronson, B. J.; Blanford, C. F.; Stein, A. Chem. Mater. 1997, 9, 2842. (7) Kageyama, K.; Tamazawa, J.-I.; Aida, T. Science 1999, 285, 2113. (8) Johnson, B. F. G.; Raynor, S. A.; Shephard, D. S.; Mashmeyer, T.; Thomas, J. M.; Sankar, G.; Bromley, S.; Oldroyd, R.; Gladden, L.; Mantle, M. D. J. Chem. Soc., Chem. Commun. 1999, 1167. (9) Brown, J.; Mercier, L.; Pinnavaia, T. J. J. Chem. Soc., Chem. Commun. 1999, 69.

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Effect of Framework Polymerization on the Phase Stability of Periodic

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