Temperature Effects on the Composition and Microstructure of Spray

3798

Langmuir 2006, 22, 3798-3806

Temperature Effects on the Composition and Microstructure of Spray-Dried Nanocomposite Powders D. Sen,†,‡ O. Spalla,‡ L. Belloni,‡ T. Charpentier,§ and A. Thill*,‡ CEA Saclay, Direction des Sciences de la Matie` re, Laboratoire Interdisciplinaire sur l’Organisation Nanome´ trique et Supramole´ culaire and Laboratoire de Structure et Dynamique par Re´ sonance Magne´ tique, CEA/CNRS URA 331, 91191 Gif-sur-YVette, France, and Solid State Physics DiVision, Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed October 14, 2005. In Final Form: January 23, 2006 Porous composite powders, prepared by spray drying of silica and polybromostyrene nanoparticles, were calcined at various temperatures up to 750 °C. The structure in these powders are quantitatively investigated by ultra small-angle X-ray scattering, thermogravimetric analysis, and nuclear magnetic resonance measurements. It has been found that the polybromostyrene latex is efficient in templating mesopores. However, polybromostyrene remains almost completely in the interstitial micropores in the grain after the spray-drying process. A post thermal treatment of the powders has been applied from 250 up to 750 °C. We found that the hydrocarbon part of the polybromostyrene is decomposed and leaves the micropores at around 350 °C. However, it is demonstrated that a significant amount of bromine remains in the interstitial micropores between the silica particles. At around 600 °C, the silica nanoparticles start to fuse with each other and a coalescence of the micropores takes place. At still higher temperature, around 750 °C, the micropore network totally disappears, and the growth in pore size occurs due to the coalescence of the mesopores with a significant decrease of the total porosity. During this process, the silica network densification is accompanied by a lowering of the specific surface area.

1. Introduction Many materials, both natural and synthetic, possess a significant porosity. The porosity imparts some functional properties to these materials, for instance, lightweight insulating and protective materials or photonic band gap materials. The wider field of applications and research certainly regards the chemical properties in which filtration and catalysis are the main focus. For all of these applications, the aim is to achieve some specific functions through the proper control of the porosity characteristics: (i) the pores must be connected such that they are accessible by both liquid and reactants (closed pores are not useful for that sort of applications), (ii) the pores must be well organized in a quasicrystalline structure, (iii) their size must be easily controllable, and (iv) the surface of the pores must be functionalized to a specific chemical function (for captors for instance). Finally, one wants to obtain these properties with a wide range of materials making the solid skeleton of the porous structures (for photonic materials for instance one needs a high refractive index and for filtration membranes working in extreme media one needs a good chemical durability). In that regard, spray drying is an interesting process to synthesize materials with a controlled porosity. For example, the spray drying of colloidal suspensions gives nanostructured powders with controllable morphologies in a one step process.1-4 The crystallization of nanoparticles in a confined medium allows * To whom correspondence should be addressed. E-mail: antoine.thill@ cea.fr. † Bhabha Atomic Research Centre. ‡ Laboratoire Interdisciplinaire sur l′Organisation Nanome ´ trique et Supramole´culaire. § Laboratoire de Structure et Dynamique par Re ´ sonance Magne´tique. (1) Cagnol, F.; Grosso, D.; Soler-Illia,G. J.de A. A.; Crepaldi, E. L.; Babonneau, F.; Amenitsch, H.; Sanchez, C. J. Mater. Chem. 2003, 13, 61-66. (2) Wang, D.; Mo¨hwald, H. J. Mater. Chem. 2004, 14, 459-468. (3) Iskandar, F.; Mikrajuddin; Okuyama, K. Nano Lett. 2001, 1, 231-234. (4) Lyonnard, S.; Bartlett, J. R.; Sizgek, E.; Finnie, K. S.; Zemb, T.; Woolfrey, J. L. Langmuir 2002, 18, 10386-10397.

the formation of a wide variety of microstructures,3,5,6 and Abdullah and co-workers have demonstrated that it is possible to prepare, in a single step, organized porous grains with different compositions and porosity.7 Beside the drying conditions,3 the interactions between nanoparticles inside the drying droplet have a significant impact on the final powder structure,4 and thus, it is worth understanding quantitatively the physicochemical mechanisms controlling the final morphology of a dried grain. In this spirit, it is interesting to investigate quantitatively the structure modifications of spray-dried powders at the mesoscopic scale during different steps of the process. To achieve this goal, we have already worked on two aspects.8 On one hand, we have introduced an experimental setup to produce spray-dried grains. This setup allows controlled and versatile conditions of drying for a very wide range of complex mixtures of nanoparticles. On the other hand, a quantitative model to analyze the Porod regime of small-angle X-ray scattering data from porous materials with multi level structures9 has been extended to explain the scattering data from composite spraydried grains. This quantitative model was already applied to analyze the structure of spray-dried latex/silica composite grains in detail,8 and it is worth recalling briefly the main results of it. In a spraydrying experiment, when a drop in the supporting inert gas flow is entering the hot tube, different successive phases influence its structure and composition. During the first stage of drying, the water is evaporated and all of the moieties contained in the droplet are concentrated. When the silica particles (which are the major component in the solution) reach the jamming limit (their volume (5) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 22402243. (6) Kulak, A.; Davis, S. A.; Dujardin, E.; Mann, S. Chem. Mater. 2003, 15, 528-535. (7) Abdullah, M.; Iskandar, F.; Shibamoto, S.; Ogi, T.; Okuyama, K. Acta Mater. 2004, 52, 5151. (8) Thill, A.; Spalla, O. J. Colloid Interface Sci. 2005, 291, 477-488. (9) Spalla, O.; Lyonnard, S.; Testard, F. J. Appl. Cryst. 2003, 36, 338-347.

10.1021/la052775x CCC: $33.50 © 2006 American Chemical Society Published on Web 03/14/2006

Spray-Dried Nanocomposite Powders

Langmuir, Vol. 22, No. 8, 2006 3799

Table 1. Parameters Used for the Powder Synthesis QN2 ) 6 L/min, Toven ) 500 °C, Ts ) 125 °C, and P ) 0.99 Bar name

φl(0)

φS(0)

Qw (ml/min)

NC

7.82 × 10-4

1.44 × 10-3

1.7

fraction ∼ 0.64), the grain stops shrinking and accordingly the ongoing evaporation implies that water de-wets the material which can afterward be called a grain. Finally, as the heating continues to irradiate the grain, the latex particles melt and escape from their initial locations, leaving behind them the mesopores. One important contribution of the former paper was to show that the latex does not escape from the grains but indeed fills the interstitial microporous regions between the silica particles. This result, which was counterintuitive, was already due to the coupling between SAXS analysis and chemical analysis. The aim of the present paper is studying the elimination of templating moieties and associated structural modifications in detail during post thermal treatment of spray-dried composites grains. It will be shown that the elimination of the templating latex is not achieved just after the spray-drying stage even when it is initially driven at 500 °C (oven temperature). A further thermal treatment at a minimum of 350 °C but for a longer time is required for achieving template calcination. It will also be shown that this thermal treatment has to be below 600 °C since beyond this threshold temperature the structure of the porous material is altered significantly by solid reorganizations. It will be seen that these conclusions could be drawn only by proper analysis of the ultra small-angle X-ray scattering (USAXS) data over a wide q range. In parallel and in complement to the earlier model,8 the interparticle structure factors have been incorporated in the analysis, and by this way, the present model can explain the USAXS data in the entire experimental wave vector (q) range. Extra chemical information, such as thermal gravimetric analysis (TGA) or nuclear magnetic resonance (NMR), allows the grain average composition and thus a true scale in intensity to be obtained. Therefore, the only parameters in the model concern the repartition of matter inside the grains. It will be demonstrated that the treatment of the USAXS data in true scale helps in proper quantifications of the changes in the grain structure and its composition during calcination.

The measured scattering profile Im(q) for each specimen has been corrected for the background and the smearing effects.11 If Id(q) represents the count rate per unit solid angle after desmearing and background correction, then the differential scattering cross section per unit solid volume of the samples can be written as I1(q) )

Id(q) 1 dσ (q) ) Vsolid ∆Ω Vsolidφ0T

(1)

where Vsolid is the volume of solid in the scattering sample, I1(q) is the scattered intensity at q expressed in (cm-1), φ0 is the incoming flux per unit area (cps‚cm-2), T is the sample transmission, and (dσ/∆Ω) is the differential scattering cross section per unit solid angle (cm2). It is noteworthy that classically the “absolute scale” is taken for a unit volume of sample. However, here we use the normalization with respect to a unit volume of solid (excluding the empty space) since this has been shown, in the case of powders,9 to be an effective and robust way to extract absolute numbers such as specific surface, volume fraction, etc. It is worth mentioning that the volume of solid crossed by X-ray multiplied by the flux of X-ray per unit area reduces to the equivalent thickness of solid times the total X-ray count rate across the sample, i.e. N0 Vsolidφ0 ) esSbeam ) e s N0 Sbeam

(2)

Id(q) Id(q) ) esN0T esNT

(3)

So I1(q)(cm-1) )

where NT is the transmitted flux. From eq 3, it is evident that, in order to express the measured count rates of the USAXS experiments as an absolute scattering cross section per unit solid volume, the knowledge of the equivalent solid thickness es of the sample is required. To estimate the equivalent thickness of the powder sample from the transmission value of X-rays through the sample, one needs to know the linear X-ray absorption coefficient (µg) of the solid, as the transmission T and the solid thickness es are related by T ) e-µges. Indeed, µg depends on the composition of the solid grains, and one has µg )

m m (φm l µl/Fl + φS µS/FS + φWµw/Fw) 1/Fg

(4)

2. Materials and Methods 2.1. Spray Drying. The silica nanoparticles for the spray drying are commercial silica nanoparticles (Ludox SM30) (Aldrich). The organic polybromostyrene particles were synthesized in the laboratory through the copolymerization of bromostyrene and styrene sulfonate. The physical parameters of the nanoparticles used for the spray drying have been characterized beforehand using the X-ray absorption and small angle scattering study. The details of the characterization of these nanoparticles are already reported.8 A spray-dried composite of silica and polybromostyrene has been prepared using an ultrasonic spray-drying generator. The initial volume fraction of polybromostyrene particles φl(0) and SiO2 φS(0) used in the feeding solution are given in Table 1 with the solution flow rate Qw; however, the details of the preparation of the spray-dried samples before calcination are already reported.8 Afterward, the virgin (noncalcined) spraydried powder (NC) was calcined for 4 h at various temperatures namely, 250 (specimen 2), 350 (specimen 3), 400 (specimen 4), 500 (specimen 5), 600 (specimen 6), and 750 °C (specimen 7), respectively. 2.2. Ultra-Small Angle X-ray Scattering (USAXS) Experiments. The calcined powder samples have been investigated using USAXS camera10 within a q range going from 3 × 10-4 to 0.1 Å-1. (10) Lambard, J.; Lesieur, P.; Zemb, T. J. Phys. I 1992, 2, 1191-.

m m where φm l , φS , and φW are respectively the mass fractions of polybromostyrene, silica, and water in the grain and µl/Fl, µS/FS, and µw/Fw are their X-ray absorption cross section per unit mass (in cm2/g). Fg represents the density of the grain and can be expressed as

Fg )

(FSφS + Flφl) 1 - φm w

(5)

where φS and φl represent the volume fraction of the silica and the latex, respectively. It is clear from the SAXS treatment that an external measure of the composition is mandatory to scale the intensity to the correct unit. In the present case, the mass fractions of all of the constituents have been measured using TGA. For instance, the presence of a slight residual amount of water, as shown by TGA, has also been taken into account to deduce the correct thickness and then the correct scaling. It will be seen in the later part of the article that expressing the scattering profile in real scale (cm-1) allows one to quantify the structure and the composition modifications significantly more than it would have been for the profiles in arbitrary unit. (11) Strobl, G. R. Acta. Cryst. A 1970, 26, 367-371.

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Sen et al. Table 2. Results for the Fits of the 29Si MAS NMR Spectra of Samples NC, Sp-3, and Sp-7 name

Q4

Q3

Q2 or Q3-Br

NC 77% (-110.6 ppm) 23% (-100.4 ppm) nd (virgin) Sp-3 71% (-111.4 ppm) 25% (-100.6 ppm) 4% (-89.3 ppm) (350 °C) Sp-7 87% (-111.1 ppm) 13% (-97.2 ppm) nd (750 °C) Table 3. Mass Fraction of Water, Polybromostyrene, and Silica (or Silica + Bromine) Obtained with TGA Measurements and Percentage of Noncalcined Polymer [QN2 ) 6 L/min, Toven ) 500 °C, Ts ) 125 °C, and P ) 0.99 bar]

Figure 1. Results from TGA measurements. 2.3. Powders Composition by Thermogravimetric Analysis. TGA measurements has been performed on a NETZSCH STA 409 for the powders after equilibrium with ambient atmosphere. The analyses were performed in an Al2O3 crucible with an oxygen gas flow on initial mass varying from 24 to 36 mg. The temperature was increased from ambient temperature to 1000 °C with a rate of 3 °C/min. Three stages of mass decrease are observed. A first decrease is observed between ambient temperature and about 250 °C. It is attributed to adsorbed water in the powders. The TGA experiments were driven after the thermal treatment. Therefore, the powder is brought back to ambiant temperature before the start of the TGA. During this cooling stage, it readsorbs an amount of water depending on its hydrophilicity. The NC sample readsorbs less water because the inner-silica surface is covered by polybromostyrene which is an hydrophobic material. The second and third stages correspond to the polybromostyrene mass loss. To estimate the water quantity in powders, the relative mass loss difference between ∼25 and 300 °C (i.e., ∆m(25 °C) - ∆m(300 °C)) was used. The polybromostyrene quantity is defined as ∆m(300 °C) - ∆m(500 °C), the mass fraction of Silica being the remaining. It will be seen afterward from the detail USAXS analysis that the remaining mass for the samples calcined at a temperature above 500 °C is not only due to the silica but due to the presence of both silica and Bromine that remains after decomposition of polybromostyrene. 2.4. 29Si, 13C, and 1H MAS NMR Analysis. NMR experiments were performed at room temperature on a Bruker Avance 500 WB spectrometer (magnetic field 11.75 T) using a 4 mm o.d. Bruker MAS (magic angle spinning) probe and a sample spinning frequency of 12.5 kHz. 1H (I ) 1/2) MAS spectra were recorded using a recycle delay of 2 s. For 29Si (I ) 1/2), both MAS and 1H-29Si cross-polarization MAS (CPMAS) (contact time 12 ms) were collected using recycle delays of 20 and 2 s, respectively. Because of the stronger relative signal enhancement of Q2 and Q3 silicon species, MAS and CPMAS spectra were fitted together using Gaussian line shapes of the same width, to improve the accuracy of our measurements. For MAS, increasing the recycle delay (up to 80 s) only increase the intensity of the NMR spectra without changing its shape. 13C (I ) 1/2) CPMAS spectra were recorded using a recycle delay of 2 s. Attempts to directly detect 79Br (I ) 3/2) NMR signal were made but remained unsuccessful. This is probably due to very strong quadrupolar coupling constants, as generally encountered for bromide in organic compounds. 1H (respectively 29Si and 13C) chemical shifts were referenced using external sample of tetra(trimethyl)silane (TMS) 0.2 ppm (respectively TMS -9.9 ppm and the carbonyl resonance of glycine 176.04 ppm).

3. Results 3.1. Grain Composition. The TGA results obtained for the virgin spray-dried powder and the calcined powders are presented in Figure 1. For the uncalcined virgin specimen the TGA analysis

name

φm w

φm 1

NC (virgin) Sp-2 (250 °C) Sp-3 (350 °C) Sp-4 (400 °C) Sp-5 (500 °C) Sp-6 (600 °C) Sp-7 (750 °C)

3.3 × 3.3 × 10-2 7.7 × 10-2 6.9 × 10-2 5.9 × 10-2 2.7 × 10-2 6.21 × 10-3

2.22 × 2.22 × 10-1 1.90 × 10-2 9.01 × 10-3 9.00 × 10-3 4.00 × 10-3 0.00

10-2

φm S 10-1

knc

7.45 × 7.45 × 10-1 9.04 × 10-1 9.21 × 10-1 9.32 × 10-1 9.69 × 10-1 9.94 × 10-1 10-1

0.982 0.982 0.077 0.036 0.035 0.015 0.000

displays a three stages mass loss pattern. However, for the samples calcined at higher temperatures, the three stages are no longer apparent. The mass decrease of two stages between ∼300 and 500 °C is known to be due to the decomposition of the polybromostyrene.12 The mass fractions of water, polybromostyrene, and silica are summarized in Table 3. In addition, the fraction of uncalcinated polybromostyrene (knc) can be defined as the initial mass fraction of polybromostyrene (φl(0)) to silica (φS(0)) compared to the measured ratio of mass fractions (φm). Then knc is the fraction of polybromostyrene in the dried grain that has not been calcined

knc )

φm l φS(0)FS φl(0)Fl φm S

(6)

Values of knc are given for all powders in Table 3. For the virgin sample (NC), it is seen from the TGA analysis that almost the whole polybromostyrene remains in the grain. Furthermore, TGA analysis displays significant differences in the water content of the powders. The water content is 3.3% of the total mass for sample NC. This water content increases up to almost 8% for sample Sp-3 (350 °C), and it decreases again strongly to less than 1% after thermal treatment at 750 °C for Sp-6. This trend illustrates first the loss of polybromostyrene that allows some water adsorption on silica and then the loss of the specific surface which reduces the amount of adsorbed water. The 13C MAS NMR spectra of NC and Sp-3 (350 °C) are presented as Supporting Information. The presence of the carbon atoms of the polymer in the NC specimen is clearly observed with three distinct peaks at chemical shifts of 148, 130, and 40 ppm corresponding respectively to aromatic carbons, aliphatic carbons, and finally to Br-linked carbon atoms. The fact that no 13C NMR signal could be detected (CPMAS and MAS) for Sp-3 and Sp-7 confirms the TGA results that already at 350 °C the polybromostyrene is completely removed from the sample. 1H MAS NMR spectra are measured on samples NC, Sp-3 (350 °C), and Sp-7 (750 °C). The results are presented as Supporting Information. (12) Bertini, F.; Audisio, G.; Kiji, J.; Fujita, M. J. Anal. Appl. Pyrol. 2003 68/69, 61-81.

Spray-Dried Nanocomposite Powders

Figure 2. 29Si MAS spectra of Sp-1, Sp-3, and Sp-7 samples. Data are normalized to the same quantity of sample.

Figure 3. Experimental and simulation of the 29Si MAS and CPMAS spectra of Sp-3 sample. Contribution of each unit (Q2 or Q3-Br (see text), Q3, and Q4) is shown (dotted line). Both spectra were fitted together with same NMR spectral parameters and different intensities.

Sp-1 1H MAS spectra have a significant broad resonance around 7.5 ppm, a typical value for aromatic protons. It disappears in other spectra (Sp-3 and Sp-7). Sp-3 spectrum displays a strong narrow resonance at 5 ppm, corresponding to adsorbed (weakly physisorbed) water molecules. All of these resonances disappear in the 1H Sp-7 spectrum, where the residual signal is coming from protons at the surface of particles. As water is adsorbed on the surface of the silica particles, this indicates a great decrease of the specific surface. Silica network modifications during the thermal treatment were followed by 29Si MAS and CPMAS NMR measurements on samples NC, Sp-3, and Sp-7 (see Figure 2). Data and fit are shown in Figure 3 for the Sp-3 sample. The contribution of three silicon species is clearly visible on Sp-3 spectra, and fitted values of chemical shifts are reported in Table 2. These resonances can be attributed to silicon species21 in Q2 (SiO2(OH)2),Q3 (SiO3(13) Pedersen, J. S. J. Appl. Cryst. Sci. 1994, 7, 595-608. (14) Aschroft, N. W.; Lekner, J. Phys. ReV. 1966, 145, 83-90. (15) Kinning, D. J.; Thomas, E. L. Phys. ReV. 1984, 17, 1712-1718. (16) Baxter, R. J. J. Chem. Phys. 1968, 49, 2770-2774. (17) Menon, S. V. G.; Manohar, C.; Srinivas Rao, K. J. Chem. Phys. 1991, 95, 9186-9190. (18) Iskandar, F.; Mikrajuddin; Okuyama, K. Nano Lett. 2002, 2, 389-392. (19) Sen, D.; Patra, A. K.; Mazumder, S.; Ramanathan, S. J. Alloys Compd. 2003, 361, 270-275. (20) Ninham, B. W., Yaminski, V. Langmuir 1997, 13, 2097-2108.

Langmuir, Vol. 22, No. 8, 2006 3801

Figure 4. Variation of the USAXS profiles with thermal treatment.

(OH)), and Q4 (SiO4) tetrahedra. However, the contribution at -90 ppm also may be due to Q3-Br species (SiO3(OH) f SiO3(Br)) instead of Q2 species. NC and Sp-7 spectra were fitted using only two components (Q3 and Q4). Under the same acquisition conditions, no CPMAS signal for Sp-7 could be detected, as expected. From NC to Sp-3, Q4 and Q3 chemical shifts are very close and relative quantities are of same order of magnitude. Thus, the thermal treatment at 350 °C, does not significantly affect the silica network structure. For Sp-7, as expected, a significant increase of Q4 population and a lowering of Q3 chemical shift are observed. The latter can be attributed to the creation of Si-Br bonds, leading to a decrease of the mean chemical shift. Thus, for this sample, Q3 is probably partly due to SiO3(Br) species. From the measured mass fractions and X-ray transmissions, one can estimate the equivalent solid thickness for all powders using eq 4 and can use it to scale the scattered intensities in absolute unit. The exact thickness will depend on the hypothesis made about the final masses. It is noteworthy that if it is only silica or silica with residual bromine the thickness will not be the same. 3.2. Scattering after Temperature Treatment. The scattering profile of the virgin powder and after the temperature treatments is given in Figure 4 on a Porod plot of I(q)q4 vs q. In the virgin grain, there exists three length scales corresponding to the overall grain structure, latex and silica, respectively. From the figure, it is also evident that the three length scales are reflected in the scattering profile for the virgin spray-dried sample (zones I, II, and III, respectively). Indeed, three different regimes are apparent on the first diagram. Below q ) 0.001 Å-1 (zone I), where the observed scale is the largest, a decrease in I(q)q4 reveals that one is sensitive to the external size of the grain. Between q ) 0.001 Å-1 and q ) 0.02 Å-1 (zone II), the oscillations in a Porod plot are the signature of a rather monodisperse form, and finally, at higher q (zone III) starts the signature of another quasimonodisperse form but with an order of magnitude smaller length scale. It is logical to put in correspondence the three length scales and direct structures in the following manner: the small q (zone I) corresponds to the outer size of the grain, whereas the intermediate oscillations (zone II) are the fingerprint of the latex (ghost of latex) in the material, and the large q (zone III) oscillations are attributed to the presence of the silica spheres. A further temperature treatment (of longer time) influences the three regimes in the scattering profiles at different stages. Up (21) Engelhardt, G.; Michel, D. High-Resolution Solid State NMR of silicates and zeolites; Wiley: New York, 1987.

3802 Langmuir, Vol. 22, No. 8, 2006

Sen et al.

Figure 5. Grain schematic.

to 500 °C, the main influence regards the high q part (zone III) where essentially the signal is largely increased with a small deformation. This reveals an increase of contrast between the silica particles toward its surrounding medium. The detail analysis in the discussion section will reveal that this is due to the real elimination of the polymer increasing the contrast between the silica particle and the micropores. Indeed, from the TGA measurements, it is seen that the knc value for the specimens starts decreasing significantly with calcination. This implies that at higher temperatures the polybromostyrene starts to be calcined and major parts of the polymer leave the grain leaving empty micropores. Finally, the small deformation in shape, Figure 4, also shows that calcination at temperatures higher than 300 °C modifies significantly the nature of the interaction between the silica particles. Below 500 °C, the middle range of q is not modified by the thermal treatment, which is consistent with the absence of the latex in the mesopores after the spray-drying stage.8 The very low q part is weakly affected by these changes at the local scale. At 600 °C, the Porod plateau of the silica particles again decreases to a lower value (as indicated by a dashed line in Figure 4) because of the fusion of the silica spheres. The scattering is therefore due to coalesced micropores embedded in the silica matrix. Nevertheless, the amount of micropores is still quite high, as the Porod limit at high q (zone III) is still significantly different from the one in the middle q range (zone II) associated with the specific surface of mesopores only. At 750 °C, the silica have totally fused and the micropores have disappeared, but there still exists a significant amount of the mesopores. However, the oscillations are largely reduced because of the modifications of the monodisperse nature of the mesopores. Therefore, the silica is dense but the grain is mesoporous. This shows that a thermal treatment is required not only to eliminate the latex but also to produce mesoporous grains.

4. Discussion Our main results regard the evolution of the structure during the post thermal treatment. A direct model being able to fit the scattering profiles is constructed to quantify the evolution of the different types of porosity present in the grains. 4.1. Model for SAXS Data Fitting. As discussed in the results section, the scattering by the grains evolves drastically with postdrying thermal treatment both in intensity and in functionality. The aim of this section is to extract, from this scattering evolution, the direct space structure evolution. As classically, the basic scheme that will follow is therefore to construct a direct space

structure model able to fit all of the evolutions of the scattering diagrams. As a start, it is important to present schematically the structure of a grain in a spray-dried powder. The schematic diagram of a typical droplet is shown in Figure 5 before the drying and after the rapid spray drying (T1) following the conclusions of the former article8 already recalled in the Introduction. The schematic also shows a possible scheme for the further modifications of the structure with temperature (T2, T3) according to the results obtained by SAXS. Nevertheless, to go further, a procedure regarding the full fitting of the scattering profiles will be discussed now. In the present case, the spray-dried grains (mostly spherical with few toroidal grain as evident from SEM but not shown here) are composed of silica and latex particles. The size distributions of the constituent silica and the latex particles were estimated separately in solution scattering measurements.8 The average value of the radius of silica and the latex were found to be 50 ( 8 and 420 ( 50 Å, respectively for silica and polybromostyrene8 showing that the average size of the silica particle is almost 10 times less than those of the latex one. These values serve as an initial guess for the size distribution and the particle volume terms, for silica and latex. The scattering length density are respectively FeS and Fel . The interstitial space between the silica and the latex particles is considered to be filled with a homogeneous material with a scattering length density Feint. The droplets are suspended in a medium in which the scattering length density is Fem (in the present case it is air), and we note ∆FeS ) FeS - Fem, ∆Fel ) Fel - Fem, and ∆Feint ) Feint - Fem. The differential scattering cross section of Ng droplets per unit of solid volume without interactions was given by8 I1(q b) )

Ng dσ dσ 1 (q b) ) (q b) Vsolid dΩ g Vg(kncφl + φS + φw) dΩ g

( )

( (

φlVl ∆F˜ el -

)

( )

)

)

2 φS ∆F˜ e P (q b) + φSVS∆F˜ eS 2PS(q b) + Vg(∆Feg)2 Pg(q b) 1 - φl S l

(kncφl + φS + φw)

(7) where P(q b) is the normalized form factors and V the associated volumes. The contrasts are ∆F˜ el ) ∆Fel - ∆Feint and ∆F˜ eS ) ∆FeS - ∆Feint. ∆Feg ) φl∆F˜ el + φS∆F˜ eS + ∆Feint represents the average contrast of the whole grain. Equation 7 describes the scattering by Ng grains made of three different components (silica, latex, and solvent). As it is shown in ref 8, it includes the excluded volume effects between latex

Spray-Dried Nanocomposite Powders

Langmuir, Vol. 22, No. 8, 2006 3803 Table 4. Parameters From the USAXS Analysis

name

rg(Å)

σg

τl

τS

NC 250°C 350°C 400°C 500°C 600°C 750°C

1470 1470 1765 1760 1790 1660 1530

0.391 0.391 0.390 0.390 0.390 0.430 0.369

0.10 0.10 0.10 0.10 0.10 0.10 1.00

0.24 0.24 0.10 0.10 0.10 0.10

φBr

rmeso (Å)

φmeso

φmicro

Σmeso (m2/g)

Σmicro (m2/g)

(1 - φmeso - φS)/(1 - φmeso)

0.09 0.09 0.09 0.09 0.09

420 425 425 430 430 430 790

0.24 0.24 0.24 0.24 0.24 0.26 0.33

0.30 0.30 0.30 0.30 0.30 0.27

7.42 7.42 7.42 7.42 7.18 7.54 4.67

115.98 115.98 115.51 112.94 112.39 46.95

0.40 0.40 0.40 0.40 0.40 0.36

and silica. To make it more usable to the present case, the correlations will be introduced in the following. Before that, we will start by defining properly the first contrast term in eq 7. Indeed, it has been shown that, after the spray-drying stage, the latex particles have liquefied, lost their spherical shape, and invaded part of the microporosity between silica particles. Therefore, in this case, the first term in (7) accounts for the presence of the mesopores left by the departure of the latex (their ghost in fact). Accordingly, one has

φmeso ) φl

(8)

since mesopores are created by the departure of latex. Since the departure of latex occurrs at a temperature where the grains are already almost dry, the inorganic skeleton around the latex (becoming mesopores) is hardly affected by the process. Therefore, as a first approximation, we will use

Vmeso ) Vl

(9)

Pmeso(q b) ) Pl(q b)

(10)

and

These initial values will be modified in order to get the best fit of the data. It will be seen that the approximation is indeed correct until the higher temperatures where the silica starts to fuse (see Table 4). Nevertheless, in the denominator of eq 7, kncφl does not have a structural meaning since it accounts for the mass fraction of polymer material left in the grain after thermal treatment. Thus, we will keep the initial notation accordingly in the following equations. Finally, the prefactor term ∆F˜ el (φS/1 - φl) ∆F˜ eS of the first form factor in eq 7 (see the Supporting Information) is nothing but the contrast between the mesopores (or latex) and their average environment. We note, hereafter this contrast ∆Fmeso. Equation 7 can be rewritten in a more readable way for the present study I1(q b) ) b) + φSVS ∆FeS 2PS(q b) + Vg(∆Feg)2Pg(q b) φmesoVmeso∆Fmeso2Pmeso(q (kncφl + φS + φw)

(11) When the latex is not melted, ∆Fmeso can be developed as

∆Fmeso ) ∆Fl -

φS∆FeS + (1 - φmeso - φS)∆Feint (12) 1 - φmeso

When the latex melts, the scattering length density of the mesopores becomes

φS∆ FeS + (1 - φmeso - φS)∆Feint ∆Fmeso ) 0 1 - φmeso

(13)

with ∆Feint being the scattering length density of the interstitial

medium. This medium will depend on the drying temperature and will be expressed for every case treated. Equation 11 does not take into account the self-correlations. The cross correlations between mesopores and silica is accounted for through an uncorrelated excluded volume effect (see the Supporting Information and ref 8). In a previous article,8 only relative compositions and contrasts of the three length scales were discussed through the comparison of Porod plateau and, thus, leading to correct quantitative results since the Porod plateau ratios do not depend on self-correlation structure factors (indeed, they are only counting the extension of interfaces). Nevertheless, in the present paper, the objective is to produce a complete fitting of the q dependence, and as the concentrations of silica and the latex ghost (or holes) are quite significant in the spray-dried grain, the scattering profiles are strongly dependent on the interparticle structure factor, and these correlations have to be introduced. The rather subtle demonstration is given in the Supporting Information where it is shown that in the approximation where rs , rl (0.1 in our case) the final result reduces exactly to

I1(q b) ) b) + φS∆F˜ eS2IS(q b) + Vg(∆Feg)2P ˜ g(q b) φmeso∆Fmeso2Imeso(q (kncφl + φS + φw)

(14)

where

∫Pi(q, r) Vi(r)2 Di(r) Si(q, r) dr Ii(q) ) ∫Vi(r) Di(r) dr

(15)

where i ) S or meso, Si(q) represents the interparticle structure factor, and DS(r) and Dl(r) represent the radius distribution of the silica and the latex particles. It is important to note that eq 14 does indeed treat the cross-correlation structure factor between mesopores and silica nanoparticles on an equal footing as the two self-structure factor terms. More precisely, due to the scale decoupling, rs , rl, the cross term is contributing to the prefactor of Imeso and is therefore hidden in the first term of (14). The polydispersities of the nanoparticles and mesopores have also been incorporated in (14) at the level of local monodisperse approximations13 since the polydispersities were small. The mass fractions of the silica or silica + bromine, remaining latex, and water are known from the TGA. Therefore, the unknown parameters in the above equation are the size distribution of the grains and the interparticle structure factors for silica and mesopores, respectively. The size distribution Dg(r) for the grains has been assumed to have a parametric form of a standard lognormal distribution

Dg(r) )

1

x2πσg2r2

(

exp

)

-ln(r/rg)2 2σg2

(16)

For the two interparticle structure factors, the analysis have been

3804 Langmuir, Vol. 22, No. 8, 2006

Sen et al.

Figure 6. Fit of the model to the USAXS profiles.

carried out for both the hard sphere (HS)14,15 and the sticky hard sphere (SHS).16,17 Here, it is interesting to mention that in the case of a HS type interaction the potential consists solely of a repulsive hard core, whereas the same for the SHS consists of a hard core together with an attractive well, which leads to HSs with surface adhesion. The stickiness parameter for the SHS type interaction16,17 will be assigned by τ for the rest of the article. 4.2. Evolution of the Structure. In this section, we discuss the evolution of the structure in light of the USAXS, NMR, and TGA analysis. Direct grains observations with SEM or TEM for both virgin and calcined grains can be found in similar conditions elsewhere.8,18 The grain compositions deduced from TGA were first used to scale the intensity. As a start, this was done for the virgin powder NC. The predetermined size distributions of the silica and latex were used. The size distribution of the grains, the interactions between silica, and the correlation between the latex templated pores have been deduced by fitting eq 14 to the profile of NC. It has been found that almost all of the latex remains in the interstitial spaces between the silica spheres. It has also been found that the SHS type of interaction between the silica particles enables a better representation of the data than the simple HS interaction. Similarly, the SHS type interaction for the mesopores (ghost of latex particles) has been found to be more effective. The term ∆Feint in eq 13 for this case,

where polybromostyrene is in the interstitial space, is given by

∆Feint )

φw ∆Fes + φl∆Fel 1 - φmeso - φS

(17)

where φl refers to the volume fraction of latex in the grain taking into account that knc ) 1. A reasonable agreement between the model (particularly with SHS type interaction) and the data is evident from Figure 6. The distribution used for the silica spheres is the one determined initially. The average size of the mesopores is very close to the one of the latex and is reported in Table 4. The USAXS profile for Sp-2 (250 °C) is almost identical to that of NC. This is quite understandable because of the fact that the decomposition of the polybromostyrene occurs almost suddenly around 350 °C, and so at lower temperature, the grain structure is not modified much. Then, in a second step, we tried to fit the data for the samples Sp-3 (350 °C) to Sp-5 (500 °C) with eq 14 like the way it has been done for NC but with the knc < 1 values obtained from TGA. However, it has been found that in all of these cases the fitted curves (Figure 7) do not agree well at the higher q region if it is assumed that the remaining mass in the TGA mass loss spectrum at very high temperature (please refer the figure TGA)

Spray-Dried Nanocomposite Powders

Langmuir, Vol. 22, No. 8, 2006 3805

Figure 7. Mismatch at the high q region because of not considering Br contribution in the contrast.

is only due to silica. Indeed, the scattering at large q is too high in the model. However, it is useful to note from the figure that the profiles at the lower and intermediate q region agree well with the model. The mismatch at the higher q region guides us to consider the possibility that the remaining mass during TGA measurements at the high temperature region is not due to the silica alone. In fact, one should consider the possibility that the bromine stays in the grain, meanwhile the only hydrocarbon part decomposes and leaves the grain at higher temperature. The 29Si MAS spectra of Sp-3 also display a contribution at a chemical shift of -89.3 ppm that could hardly be attributed to the appearance of Q2 silica species after thermal treatment. These chemical shifts are compatible with Q3 silica with Si-Br links. The maximum amount of Br which can stay inside the grain is fixed and can be estimated (mass fraction ∼ 0.1) from the mass fraction of the latex and the fractional weight (79.904/182.904) of Br in polybromostyrene. It is interesting to note that if the whole Br atoms stay in the grain, we would have 1 Br atom for about 12 Si atoms. This ratio is between the 4% Q3-Br in Sp-3 sample and the 13% of shifted Q3 (-97.2 ppm) species in Sp-7. If one considers this fact, then the value of the mass fraction of silica calculated from TGA is nothing but the mass fraction of silica plus Br with a constraint that the amount of Br cannot exceed a value as discussed above. Considering these facts, the model profile has been re-fitted with the profiles of Sp-3 (350 °C), Sp-4 (400 °C), and Sp-5 (550 °C). So in this case, the term ∆Feint in eq 13 is given by

∆Feint )

φw∆Fes

kncφl∆Fe1 +

+ 1 - φmeso - φS

φBr ∆FeBr

Figure 8. Size distribution of the mesopores.

than the simple repulsive HS structure. It is interesting to note that the difference between the model with HS and SHS type interaction for silica is more prominent for the specimens Sp-3 (350 °C) to Sp-5 (500 °C) than for NC. At still higher temperature (Sp-6 600 °C), the modifications in the silica structure becomes quite significant. At this temperature, silica nanospheres start fusing with each other and the contribution to the scattering at higher q region comes from the coalesced micropores containing Br. The agreement of the model with the data is shown by comparing the fit of the model to the profile of the sample Sp-6 (600 °C). At 750 °C, the shape of the profile is significantly changed. The oscillations in the intermediate q range and the characteristic features at higher q totally disappears in this case. This implies that after the fusion of the silica, which started at 600 °C, the micropore structures totally collapse which leads to the disappearance of the features at high q. The modifications of the oscillations in the intermediate q range implies that the polydispersity of the mesopores increases significantly. Furthermore, the intermediate part of the profile shifts significantly at lower q value. These happen because of the growth in pore size due to pore coalescence. Here it is interesting to mention that a growth in pore size can occur during sintering in porous ceramics because of the mass transport in order to attain a morphological configuration with low free energy.19 At this temperature, almost 40% of the pores present in the virgin specimen are eliminated. The contrast for the mesopores in this case is given by (13) with

∆Feint ) (18)

where φBr corresponds almost to the maximum volume fraction of Br contained in the 1 - knc fraction of calcinated latex. It thus depends on the temperature and is deduced from TGA. Now the good agreement between the data and the fitted profile for all of the specimens from Sp-3 (350 °C) to Sp-5 (500 °C) is evident from Figure 6. Here, it should be mentioned that the retention of bromine in the grain is the only possibility which conforms to a real situation. Without consideration of this fact, there is a significant mismatch in the scale as shown in the (Figure 7). From the fit, it is evident that, even in these cases, the SHS type of structure for the silica-silica scattering term fits better

φBr ∆FeBr 1 - φmeso - φS

(19)

Figure 6 shows the fit of the model to the data. The estimated size distribution of the coalesced mesopores is compared to the initial latex size distribution in the figure (Figure 8). It is interesting to note from Table 4 that the mode of the size distribution of the grains shifts toward lower size as the pores go out of the grains. The volume fraction of the mesopores (φmeso) and micropores (φmicro) and the respective specific surface area (Σmeso and Σmicro, respectively) are estimated and tabulated in Table 4. It is obvious from the table that the specific surface area of the micropores are significantly more than that of the mesopores and also that it falls significantly at high enough temperatures. From the above USAXS analysis, it is revealed that the calcination gives rise to modifications in the pore and grain

3806 Langmuir, Vol. 22, No. 8, 2006

structure as well as the correlations between silica particles. The calcination beyond 350 °C results in the departure of most hydrocarbon part of the polybromostyrene from the grain. However, the bromine part still remains in the grain. It is found that the correlation between the silica particles is better described by the SHS type rather than the simple HS type. The difference is more prominent at higher temperatures when the polybromostyrene is decomposed. The structure of the inorganic network which is observed comes from a rapid drying of a colloidal suspension. In the suspension, the particles repel each other when the concentration is not too high. At higher concentration, the particles are forced to be close to each other, and eventually, they will overcome the electrostatic repulsion. The fusion of the nanosize silica particles at a temperature of 600 °C takes place. Here it should be mentioned that because of the nanosize of the silica the fusion takes place at lower temperature than it should be for the bulk silica. The heat treatment at 750 °C makes the micropores disappear and the grains become solely mesoporous with coalesced mesopores.

5. Conclusion The production of highly mesoporous inorganic grains can be achieved by the spray-drying setup. From the material point of

Sen et al.

view, it was shown that the elimination of the templating lattice initial material is total only after a post-thermal treatment above 350 °C and that the microporosity is preserved up to a temperature of 500 °C. At 600 °C and above, the microporosity is reduced due to partial fusion of the silica nanoparticles. It is worth noting that the method is not limited to silica in principle and that many other interesting nanoparticles can be used. Accordingly, using a very polarizable element such ZrO2, these types of hierarchically structured materials would be very interesting confining systems regarding selective adsorption through dispersion forces.20 From the process point of view, it is worth emphasizing again that the scattering technique when thoroughly analyzed can yield very important feedback on a process by following in situ the evolution of the structure at a quantitative level. Supporting Information Available: A demonstration of eq 14 is given. The validity of the assumptions is briefly discussed. 13C and 1 H MAS NMR spectra of sample NC and Sp-3 (350 °C) are also shown. This material is available free of charge via the Internet at http:// pubs.acs.org. LA052775X