Langmuir 2007, 23, 3509-3512
3509
Role of Urea in the Preparation of Highly Porous Nanocomposite Aerogels Maria F. Casula,* Danilo Loche, Sergio Marras, Giorgio Paschina, and Anna Corrias Dipartimento di Scienze Chimiche and INSTM, UniVersita` di Cagliari, S.P. Monserrato-Sestu Km 0.700, I-09042 Monserrato, Cagliari, Italy ReceiVed December 11, 2006. In Final Form: February 1, 2007 The preparation of highly porous CoFe2O4-SiO2 nanocomposite aerogels was successfully achieved by a novel sol-gel procedure involving urea-assisted co-gelation of the precursor phases. This method allows fast gelation, giving rise to an aerogel with 97% porosity. The formation of CoFe2O4 nanocrystals homogeneously distributed within the matrix occurs after calcination at 750 °C and is complete at 900 °C. Despite the high temperature required for the formation of the CoFe2O4 nanocrystals, the high porosity typical of aerogels is still retained.
1. Introduction Aerogel materials display unique insulating, optical, and catalytic properties thanks to their highly extended and open porosity, low densities, and high inner surface areas.1,2 They are prepared by the sol-gel process through special drying techniques that avoid the capillary forces at the liquid/vapor interface responsible for shrinkage and cracking of the original porous structure of the parent alcogels.1-3 From a chemical standpoint, aerogel properties can be tailored by varying the preparative variables because they affect the microstructure of aerogels. For example, the apparent densities of silica aerogels can vary between 0.003 and 0.35 g/cm3 mainly depending on the volume of solvent entrapped in the wet gel, which is in turn related to the sol-gel precursors and to the preparation conditions, particularly the pH. Typically, the strategy adopted for obtaining silica aerogels involves either basic catalysis or prehydrolysis of the alkoxide precursor under acidic conditions, followed by a second step under basic catalysis.4,5 Significant effort is focused on broadening the range and viability of SiO2 aerogel applications by dispersing a nanophase within the aerogel matrix. A strategy for obtaining homogeneous nanocomposite aerogels containing metal oxides and metal or alloy nanoparticles dispersed in amorphous silica is based on co-gelation under all-acidic catalysis.6-9 A drawback of these conditions is that as a result of a mainly microporous network a relatively dense structure is obtained, leading to very high surface areas but apparent densities at the upper limit of the typical values for aerogels. Because basic catalysis might cause the precipitation of metal hydroxides, co-gelation of the precursors for both the nanophase and the silica cannot be performed using the typical preparation conditions for a silica aerogel. In this letter, we approach the preparation of low-density silicabased nanocomposite aerogels by a modification of the two-step * Corresponding author. E-mail:
[email protected]. (1) Hu¨sing, N.; Schubert, U. Angew. Chem., Int. Ed. 1998, 37, 22. (2) Pierre, A. C.; Pajonk, G. M. Chem. ReV. 2002, 102, 4243. (3) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990. (4) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S. J. Non-Cryst. Solids 1982, 48, 47. (5) Tillotson, T. M.; Hrubesh, L. W. J. Non-Cryst. Solids 1992, 145, 44. (6) Moreno, E. M.; Zayat, M.; Morales, M. P.; Serna, C. J.; Roig, A.; Levy, D. Langmuir 2002, 18, 4972. (7) Casula, M. F.; Corrias, A.; Paschina, G. J. Mater. Chem. 2002, 12, 1505. (8) Casula, M. F.; Corrias, A.; Paschina, G. J. Non-Cryst. Solids 2001, 293295, 25. (9) Casula, M. F.; Corrias, A.; Paschina, G. J. Mater. Res. 2000, 15, 2187.
acid-base-catalyzed sol-gel procedure that uses urea, CO(NH2)2, as the basic catalyst. The decomposition of urea follows a complex pathway; in particular, it has been recognized that in acidic aqueous media urea decomposes to ammonia and carbonate ions, releasing hydroxide ions. Such decomposition is concentrationand temperature-dependent, becoming significant over 75 °C.10,11 In the past, the controlled decomposition of urea has been exploited for a number of applications ranging from the preparation of water-soluble colloidal cobalt ferrite nanocrystals12 to the precipitation of ammonia-intercalated nickel hydroxides and cobalt hydroxides13 to the controlled gelation of perovskites such as manganates10 and pure alumina.14 Moreover, chromia aerogels have been obtained by urea-assisted precipitation and supercritical drying.15 In this work, we demonstrate that urea can also be successfully used for the preparation of highly homogeneous and porous CoFe2O4-SiO2 nanocomposite aerogels that exhibit interesting magnetic properties.16 The key point is that the pH can vary smoothly as a consequence of the slow and temperature-dependent basic decomposition. To clarify the relative importance of factors correlating the chemical parameters of the synthesis to microstructural features of the final nanocomposite, we report the textural and microstructural features of the aerogel and the effect of its structural evolution on calcination by means of N2 physisorption at 77 K, X-ray diffraction (XRD), and transmission electron microscopy (TEM).
2. Results and Discussion The sol-gel route developed for the preparation of the multicomponent gel makes use of commercially available precursors and was optimized to achieve a highly reproducible and fast procedure. The detailed preparation route is given in the Supporting Information. In the first step, the sol was prepared by mixing the metal precursors to the tetraethoxysilane, which (10) Va`zques-Va`zques, C.; Blanco, M. C.; Lo`pez-Quintela, M. A.; Sa`nchez, R. D.; Rivas, J.; Oseroff, S. B. J. Mater. Chem. 1998, 8, 991. (11) Peland, R. B.; Mizushima, S.; Curran, C.; Quagliano, J. V. J. Am. Chem. Soc. 1957, 79, 1575. (12) Cao, X.; Gu, L. Nanotechnology 2005, 16, 180. (13) Dixit, M.; Subbana, G. N.; Kamath, P. V. J. Mater. Chem. 1996, 6, 1429. (14) Macedo, M. I. I.; Osawa, C. C.; Bertran, C. A. J. Sol-Gel Sci. Technol. 2004, 30, 135. (15) Abecassis-Wolfovich, M.; Rotter, H.; Landau, M. V.; Korin, E.; Erenburg, A. I.; Mogilyansky, D.; Gartstein, E. J. Non-Cryst. Solids 2003, 318, 95. (16) Casu, A.; Casula, M. F.; Corrias, A.; Falqui, A.; Loche, D.; Marras, S. J. Phys. Chem. C 2007, 111, 916.
10.1021/la0635799 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007
3510 Langmuir, Vol. 23, No. 7, 2007
Letters
Figure 1. Nitrogen physisorption isotherm (solid line - adsorption branch; dotted line - desorption branch) of the aerogel as obtained by supercritical drying and the pore size distribution calculated by the Barrett-Joyner-Halenda (BJH)19 method from the desorption branch (inset).
was prehydrolyzed under acidic conditions, so that the pH was about 0.6. The composition was adjusted to obtain nanocomposites having a nominal ratio of 10 wt % (CoFe2O4/CoFe2O4 + SiO2). In the second step, the gelling solution containing urea was added, and heating at 85 °C for 2 h under reflux resulted in a very viscous sol. The sol was poured into a closed glass vial and kept at 40 °C to allow for gelation, which occurred in about 40 h, and finally the as-obtained alcogel was submitted to high-temperature supercritical drying in an autoclave to produce the aerogel. The effectiveness of the supercritical drying was confirmed by thermal analysis of the aerogel (reported in the Supporting Information). The volume variation was monitored throughout the preparation of the aerogel composites because it affects the apparent density of the resulting aerogel. Thanks to the occurrence of fast gelation in a closed vessel, a very limited volume contraction of the alcogel with respect to the parent sol was observed. In addition, the formation of the aerogel by supercritical drying of the alcogel was accompanied by a limited volume reduction, whereas the weight loss of the alcogel during supercritical drying is about 93%. As a consequence, the resulting aerogel is very light and has an apparent density of about 0.07 g‚cm-3. The preparation protocol was optimized to achieve very low density aerogels by taking advantage of the temperature dependence of the urea hydrolysis rate. For example, performing the second step at room temperature resulted in gelation times of more than 5 days and corresponding aerogels that were 4 times denser than those obtained by performing the second step at 85 °C. Figure 1 shows the N2 physisorption isotherm of the aerogel, which can be classified as type IV with an H1-type hysteresis loop associated with cylindrical mesopores.17 The surface area as assessed by the BET method18 is 350 m2‚g-1, and the total pore volume VP is 3.39 cm3‚g-1. The hysteresis lies in a narrow range of high relative pressures, indicating a quite monodisperse pore size at the upper limit of mesoporosity. This is confirmed by the pore size distribution as obtained by the Barret-JoynerHalenda (BJH) method19 reported in the inset of Figure 1, which indicates that pore size ranges between 40 and 50 nm and is (17) Rouquerol, F.; Rouquerol, J.; Sing, K. S. W. Adsorption by Powders and Porous Solids: Principles, Methodology and Applications; Academic Press: London, 1999. (18) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (19) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.
Figure 2. TEM bright-field micrograph (a) and corresponding darkfield and selected-area electron diffraction (b) of the aerogel sample after supercritical drying.
centered at 46 nm. The porosity of the aerogel obtained by taking into account the pore volume determined by physisorption analysis is 88%, whereas the porosity calculated by the apparent density of the aerogel is 97%.20 This disagreement could be due to the presence of inaccessible porosity and to the inaccuracy in the pore volume determination caused by the fact that most of the adsorption takes place near a relative pressure of 1.21 In particular, the substantial amount of the porosity not revealed by physisorption has been ascribed to the macroporosity not being measured effectively by this technique.22,23 In addition, it should be taken into account that the BJH calculation method underestimates the pore size for materials that do not possess cylindrical or well-defined pores.24,25 However, methods based upon nonlocal density functional theory, which provide more accurate pore size calculations, are not readily available.25,26 More information on the highly porous and open texture of the aerogel was obtained by TEM observations (Figure 2a), which show the presence of a branched structure with mesopores between 40 and 50 nm that are visible near the edge of the particles and (20) Porosity was determined from physisorption data as follows (P ) VP/ VTOT), where VP is the pore volume as obtained by BET analysis and VTOT is the total volume, given by the sum of VP and VS (the skeletal volume). Porosity was also determined taking into account F′, the apparent density, and Fs, the skeletal density, according to P ) 1 - (F′/Fs). (21) Kim, D.-Y.; Du, H.; Johnson, D. W., Jr.; Bhandarkar, S. J. Am. Ceram. Soc. 2004, 87, 1789. (22) Reichenauer, G.; Scherer, G. W. J. Non-Cryst. Solids 2001, 285, 167. (23) Tastevin, G.; Nacher, P.-J. J. Chem. Phys. 2005, 123, article 064506. (24) Lukens, W. W.; Schmidt-Winkel, P.; Zhao, D.; Feng, J.; Stucky, G. D. Langmuir 1999, 15, 5403. (25) Ravikovitch, P. I.; O’Domhnaill, S. C.; Neimark, A. V.; Schuth, F.; Unger, K. K. Langmuir 1995, 11, 4765. (26) Ravikovitch, P. I.; Wei, D.; Chueh, W. T.; Haller, G. L.; Neimark, A. V. J. Phys. Chem. B 1997, 101, 3671.
Letters
Langmuir, Vol. 23, No. 7, 2007 3511
Table 1. N2 Physisorption Results for the Aerogel Sample after Supercritical Drying and Heat Treatment at Increasing Temperature: Surface Area and Pore Volumea sample
SBET/m2‚g-1
VP/cm3‚g-1
uncalcined 450 °C (1 h) 750 °C (1 h) 750 °C (6 h) 900 °C (1 h) 1000 °C (1 h) 1200 °C (1 h)
350 600 407 403 397 192 11
3.39 5.64 5.23 5.15 3.11 0.92 n.d.
a Differences between Repeated runs were found to be less than 5% (n.d. - not determined.).
Figure 4. TEM bright-field (a) and dark-field (b) images and corresponding electron diffraction of the aerogel after calcination at 900 °C for 1 h.
macropores that cannot be detected by physisorption measurements. Some needlelike features are also observed, as shown in the inset, which arise from the morphology of the aerogel and are also observed in a composite aerogel containing only cobalt, which was prepared as a reference. In the dark-field TEM image reported in Figure 2b, very small spherical crystallites (2 to 3 nm) are observed as bright spots, and the selected-area electron diffraction shows only diffuse rings due to the amorphous matrix. As a consequence of the small crystal size, X-ray diffraction shows only some extra broad and weak peaks superimposed on the typical halos due to the amorphous silica matrix; these peaks are difficult to attribute to specific nanocrystalline phases. The comparison of the XRD patterns of the aerogels with those containing only one metal, which were prepared as reference materials (Supporting Information), indicates that the broad peaks superimposed on the silica haloes are due to two separate nanophases of iron and cobalt. In particular, iron is present as ferrihydrite, an iron oxide hydroxide that was already found as an intermediate phase in the formation of denser nanocomposite aerogels,7,27 whereas cobalt is present in a phase that is fairly different from Co3O4, the cobalt oxide phase found as an intermediate phase in the formation of denser nanocomposite aerogels,7,27 and also from CoO. An X-ray absorption spectroscopy investigation is currently being carried out to gain further insight into the cobalt phase. Preliminary results confirm that iron is present as ferrihydrite and suggest that cobalt is present
in the form of a poorly crystalline Co(II) nanophase with a layered structure, as suggested by the observed morphology.28 After supercritical drying, the aerogel samples were powdered and calcined at 450 °C in static air for 1 h in order to eliminate the organics. Further calcination treatments at increasing temperature up to 1200 °C were performed to study the structural and textural evolution upon sintering. The physisorption isotherms of all samples treated up to 1000 °C (reported in the Supporting Information) indicate that the texture is quite similar to that of the uncalcined aerogel and that mesoporosity is retained up to 1000 °C. Table 1 summarizes the evolution of the surface area and pore volume of the aerogels as a function of the heat treatment. The sample calcined at 450 °C shows a larger surface area and pore volume with respect to the uncalcined aerogel as a consequence of the removal of organics, as revealed by thermal analysis, which improves the accessibility of the pore structure. For calcination temperatures between 450 and 900 °C, the surface area and pore volume decrease quite gradually, indicating the large thermal stability of the aerogels. At 1000 °C, a marked collapse of the pore network is observed, although the calcined aerogel still retains a relevant porosity. Only at 1200 °C does densification occur, as indicated by the sudden drop in the value of the surface area. At this stage, the sample also changes its appearance, losing the lightness and transparency typical of aerogel materials. The XRD patterns of the samples submitted to thermal treatment at increasing temperature (Figure 3) indicate that no significant changes are observed in comparison to the uncalcined sample until treatment at 750 °C is protracted for 6 h. At this
(27) Corrias, A.; Casula, M. F.; Ennas, G.; Marras, S.; Navarra, G.; Mountjoy, G. J. Phys. Chem. B 2003, 107, 3030.
(28) Carta, D.; Mountjoy, G.; Navarra, G.; Casula, M. F.; Locke, D.; Marras, S.; Corrias, A. J., submitted for publication.
Figure 3. X-ray diffraction patterns of the aerogel sample subjected to calcination at increasing temperature.
3512 Langmuir, Vol. 23, No. 7, 2007
stage, the characteristic peaks due to the CoFe2O4 spinel appear,29 although a shoulder at 2θ ∼26° due to phases present at lower temperature is still detectable. After thermal treatment at 900 °C for 1 h, only broad peaks due to CoFe2O4 are visible in addition to the halos due to the silica matrix. Upon further heat treatment at 1000 °C, the structure of the aerogel varies only slightly (XRD peaks of CoFe2O4 become slightly sharper) whereas the diffraction pattern of the sample calcined at 1200 °C, leading to complete densification of the sample as observed by physisorption measurements, shows sharper peaks due to CoFe2O4 (mean crystallite size 12 nm) and some additional peaks, indicating that the matrix undergoes partial crystallization. Calcination at 900 °C is therefore the most favorable heat treatment for obtaining a pure CoFe2O4-SiO2 aerogel nanocomposite, with ferrite crystallites having an average size of 6 nm as derived by diffraction line broadening. It should be noted that the aerogel calcined at 900 °C still retains its high porosity. Typical TEM images of the aerogel calcined at 900 °C are reported in Figure 4 a,b, indicating that the sample meso- and macroporosity are retained and that nanocrystals, which can be identified as being due to the CoFe2O4 phase from the electron diffraction pattern, are present, in agreement with XRD results.
3. Conclusions
Letters
the two-step hydrolysis-condensation reaction (generally used for preparing pure silica aerogels) involving the use of urea as a gelation agent. The main advantages of our approach are (i) the homogeneous co-gelation of the silica and ferrite precursors thanks to a smooth increase in pH due to the addition of urea and (ii) the achievement of fast gelation by hydrolyzing urea at the appropriate temperature (85 °C). The formation of single-crystalline CoFe2O4 nanocrystals starts after a prolonged heat treatment at 750 °C and is complete after treatment at 900 °C. Thanks to the homogeneity and porosity of the aerogel, the dispersed phase undergoes limited growth during calcination treatment and the nanocrystals are well spaced even after heat treatment at high temperature. In fact, the aerogel is stable up to high temperature because only at 1200 °C crystallization of the matrix do condensation of the network and coarsening of the ferrite nanocrystals occur. The thermal stability of the porous network, small nanocrystal size, and homogeneous dispersion of the nanophase within the matrix have positive implications for potential applications in gas-phase catalysis.30 Moreover, preliminary results demonstrate the synthetic flexibility of the procedure that has been successfully extended to the preparation of CoFe2O4-SiO2 aerogels with different ratios16 as well as pure silica and single metal and other metal ferrite nanocomposites.
A sol-gel procedure involving urea-assisted gelation, supercritical drying, and post-gelation heat treatment was successful in preparing highly porous CoFe2O4-SiO2 nanocomposite aerogels. The initial aerogel has a very low density (porosity 97%) with a very homogeneous distribution of the dispersed phase (made of 2 to 3 nm nanocrystals) within the amorphous silica matrix. It was shown for the first time that aerogels with improved characteristics can be obtained by a modification of
Acknowledgment. Funding was provided by the Italian Ministero dell’Istruzione, Universita` e Ricerca (MIUR PRIN).
(29) PDF-2 File, JCPDS International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA, card 22-1086.
(30) Mirzaei, A. A.; Habibpour, R.; Faizi, M.; Kashi, E. Appl. Catal., A 2006, 301, 272.
Supporting Information Available: Details of the experimental procedures, TG/DTA curves of the aerogel, XRD patterns of the composite aerogels, additional porosity data, and a TEM image of the sample after condensation. This material is available free of charge via the Internet at http://pubs.acs.org. LA0635799
