Temperature dependence in the synthesis of ...

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Microporous and Mesoporous Materials 74 (2004) 213–220 www.elsevier.com/locate/micromeso

Temperature dependence in the synthesis of hexagonal MSU-3 type mesoporous silica synthesized with Pluronic P123 block copolymer Marco A.U. Martines 1, Erica Yeong 2, Andre´ Larbot, Eric Prouzet

*

Institut Europeén des Membranes (CNRS UMR 5635), C.N.R.S., 1919 Route de Mende, F-34293 Montpellier Cedex 5, France Received 23 July 2003; received in revised form 28 June 2004; accepted 29 June 2004

Abstract MSU-type mesoporous silica prepared with nonionic surfactants according to the two-step pathway, exhibits a specific feature that makes its structure sensitive to the synthesis temperature. Until now, this factor had been demonstrated to affect the pore structure in the mesoporous range, between 2 and 5 nm. We describe in the present report that this parameter affects drastically syntheses of MSU-3 silica made with the block copolymer Pluronic 123 within a temperature range of 5–75 C. Depending on the synthesis temperature, the pore size can vary from microporous (below 2 nm) to mesoporous (close to 9 nm) in a steady-like evolution. However, two properties of block copolymers must be addressed for a full understanding of this evolution: the solubility limit of nonionic templates at higher temperatures, which prevents the formation of micelles because a too high hydrophobicity, and the Critical Micelle Temperature (CMT) at lower temperatures, which prevents the formation of micelles because a too high hydrophilicity. These results confirm that the assembly mechanism that leads to mesostructured materials requires a fine balance of interactions, not only between inorganic and organic reagents but also between the organic templates themselves. 2004 Published by Elsevier Inc. Keywords: MSU; Pluronic P123; CMT; Mesoporous silica; Mechanism

1. Introduction As researches in advanced materials are more and more focused on controlling both nanostructure and morphology, they get their inspiration from nature, especially from bio-mineralization processes where weak interactions in aqueous medium at room temperature, between dissolved inorganic precursors and organic molecules lead to complex topologies [1,2]. Even if attempts to imitate living world mechanisms run quickly * Corresponding author. Tel.: +33-(0)-467-613-398; fax: +33-(0)467-613-385. E-mail address: [email protected] (E. Prouzet). 1 Present address: Instituto Militar de Engenharia, Dept de Engenharia Quimica (DE/5), Praça General Tiburcio 80, Praia Vermelha, CEP 22290-000 Rio de Janeiro, RJ, Brazil. 2 Present address: 1, Carnation Close, East Malling, West Malling, ME 19 6EZ Kent, England.

1387-1811/$ - see front matter 2004 Published by Elsevier Inc. doi:10.1016/j.micromeso.2004.06.021

against their intrinsic complexity, they have allowed researchers to develop new ‘‘bio-mimicked’’ or ‘‘bio-inspired’’ processes [3] that were perfectly illustrated by the tremendous quickly growing field of mesostructured materials, discovered almost in the same time, by two groups [4–7]. These porous materials are based on weak non-covalent assembly mechanisms (electrostatic or Hbonding interactions) between surfactants and inorganic species with resulting morphologies governed by oriented growth based on localized defects [8–11]. Depending on the assembly agent and the synthesis conditions, different geometries of the porous framework––hexagonal, cubic, or 3D wormhole––were obtained [12,13]. In the curse of hierarchical control of the mesostructure, research was also extended to the control of particle morphology and since then, monolithic films [14–18], spheres [19–23] or fibers [11,24–30] were successfully prepared.

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Among the different families of surfactants used to achieve these syntheses, polyethylene oxide (PEO) nonionic surfactants as well as polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) triblock-copolymers were used at first as assembly agents for the preparation of materials named as MSU-X [31,32]. These materials were sorted as a function of the nature of the assembling agent: MSU-1, -2, -3 and -4 were assigned to alkyl-, alkylaryl-, blockcopolymers- and Tween-based surfactants, respectively. They exhibit usually a 3D wormhole porous structure but various structures including stick-like particles with a hexagonal symmetry were also obtained [31–38]. However, these nonionic template-based materials were further developed by the successful synthesis of large pores hexagonal silica synthesized with triblock-copolymer Pluronic P123 (EO20PO70EO20). in acidic medium (SBA-15) [39–41]. Within this field, Stucky and coworkers showed that the preparation of highly ordered hexagonal silica with Pluronic P123 in mild acidity was enhanced if one worked near the isoelectric point of silica (pH 2) and if the relative rates of hydrolysis and condensation of silica species were controlled through the use of fluoride as catalyst and tetramethoxysilane as silica precursor [42]. This provided one of the first examples that the fine tuning of final morphology and nanostructure of MTS silica, always based on weak non-covalent interactions, may require a fine control of the reaction parameters. In this domain, we developed from our side a new approach for the synthesis of MSU-X silica, based on the intermediary formation of hybrid micelles, that are further condensed by the addition of fluorine as a silica condensation catalyst [22,32,37,43,44]. This two-step synthesis process allowed us to understand how changing slightly the temperature of the condensation step could govern the final structure of the material, especially the tuning of the pore size in the mesopore range (between 2 and 5 nm) [32,45]. However, we observed also that some MSU-3 silica prepared with P123 or MSU-4 prepared with Tween 60 could exhibit an hexagonal structure that is not encountered with other nonionic templates [37]. These observations can be linked with the synthesis of SBA-15 silica that is mostly based on P123 copolymer. Regarding the specific behavior of Pluronic P123, we undertook a systematic study of the preparation of the mesoporous silica prepared with Pluronic P123 (EO20PO70EO20) triblock-copolymer according to our synthesis process, in order to fully understand the conditions of formation of the hexagonal structure that we had observed previously [37]. The present report describes how the influence of synthesis temperature, even in a narrow range, can modify drastically the final nanostructure in a much broader range than previously described for other nonionic surfactants, as well as the final morphology of particles. Even if it can be linked

with our previous results, new parameters such as the Critical Micelle Temperature (CMT) that is a specific feature of tribloc copolymers, must be addressed to fully understand this mechanism.

2. Experimental MSU-3 hexagonal mesoporous silica was prepared according to the two-step reaction process [22,37,44]. All reagents including the assembly agent (Pluronic P123 (EO20PO70EO20) from Aldrich Chemicals), the silica source (TEOS: Si(OCH2CH3)4) from Fluka, conc. hydrochloric acid (SDS) and sodium fluoride (Aldrich) were used as received. In a typical synthesis, 1.5 g (2.58 · 103 mol) of Pluronic P123 was dissolved in 100 mL of deionized water previously acidified at pH 2 with concentrated hydrochloric acid (SDS). After full dissolution, 6.66 g (0.32 mol) of TEOS was added at room temperature under moderate magnetic stirring. The solution, initially cloudy upon the TEOS addition, became quickly colorless, due to the hydrolysis of TEOS and the formation of the hybrid micelles [44]. After a 12 h aging at room temperature without stirring, this solution was heated or cooled at the desired temperature (5, 10, 15, 25, 35, 45, 55 and 65 C) and the final condensation step was induced by the addition of 5.1 mL of sodium fluoride (0.25 M) (NaF/TEOS mol ratio = 4%) into the solution kept in a thermostated shaking bath that was let under slow shaking (40 rpm) for 3 days. The final product was filtered off and washed with water, air-dried at 100 C, and calcined in air at 620 C for 6 h with a 6 h preliminary step at 200 C (heating rate of 3 C min1). All materials were characterized by scanning electron microscopy (SEM), X-ray diffraction and nitrogen adsorption at 77 K. SEM micrographs were obtained on a Hitachi S-5400 FEG microscope operating at 5 kV. The samples were covered with Au to increase conductivity. The X-ray diffraction patterns were recorded with a Bruker D5000 diffractometer in Bragg-Brentano reflection geometry. Cu L3,2 radiation monochromatized by a graphite single crystal, was employed. Nitrogen adsorption isotherms were measured at 77 K on a Micromeritics 2010 sorptometer using standard continuous procedures, and samples were first degassed at 150 C for 15 h. Surface area were determined by BET method in the 0.05–0.2 relative pressure range and pore diameter distribution by a polynomial relationship based on the Kelvin equation developed by Broekhoff and Boer (BdB) [36,46]. All analyses refer to calcined materials.

3. Results The SEM observations of the MSU-3 silica prepared at different temperatures, are displayed in Fig. 1 (T = 5–

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Fig. 1. SEM observation of calcined MSU-3 silica prepared with P123, synthesized at 5 C (a, b), 10 C (c, d), 15 C (e, f) and 25 C (g, h).

25 C) and Fig. 2 (T = 35–65 C). The particles morphology is highly dependent on the synthesis temperature and three domains can be discriminated: below 25 C, the particles exhibit the same spherical shape as most of the MSU-X silica [22,37], from 35 to 45 C, the powder is totally made of stick-like particles (there is an intermediary stage at 25 C), and finally, above 45 C, the particles loose their structure and evolve toward a fluffy powder with a growing amorphous part made of very small aggregates of silica. A parallel trend is also observed in the X-ray diffraction patterns where

the nanostructure evolves from an amorphous state (below 25 C) to another amorphous state (above 45 C) with a long-range order described by a 3D wormhole structure and a hexagonal one for the intermediary temperatures (Fig. 3). Samples synthesized at 35 and 45 C exhibit a clear set of diffraction peaks that can be assigned to a hexagonal symmetry. For the 35 C sample, the main peak points out at 1.17, which can be assigned ˚ (a = 2.d1 0 0/p 3 = 87.0 A ˚ ). Two weak to a d1 0 0 = 75.4 A ill-defined peaks appear also at 1.86 and 2.15, which may be assigned to the d1 1 0 and d0 2 0 distances, respec-



Fig. 2. SEM observation of calcined MSU-3 silica prepared with P123, synthesized at 35 C (a, b), 45 C (c, d), 55 C (e, f) and 65 C (g, h).

tively. Sample at 45 C exhibits the best diffraction pattern characteristic of a well ordered hexagonal structure, with three peaks pointing out at 0.96, 1.62 and 1.84, ˚ (a = 106 A ˚ ), d1 1 0 = corresponding to d1 0 0 = 91.9 A ˚ , and d2 0 0 = 48.0 A ˚ , respectively. One sees that 54.5 A from 35 to 45 C, the cell parameter has expanded from ˚ . The nitrogen adsorption/ 92.2(5.2) to 108.5(2.5) A desorption isotherms are reported in Fig. 4. As the synthesis temperature increases, the adsorption step shifts toward higher pressure, characteristic of an increasing pore size. Isotherms from 5 to 15 C exhibit

a Type I shape, which is characteristic of microporous materials [47] (the microporosity is confirmed by the measurement of the microporous surface area that equals the BET estimation––see white squares in Fig. 5). From 25 to 45 C, the isotherms present a Type IV shape, characteristic of mesopores, with a desorption loop due to the nitrogen condensation within mesopores. Finally, above 45 C the isotherm exhibits a Type II shape, characteristic of macroporous materials. A closer observation of the specific surface area, the total porous volume (calculated for P/P0 < 0.98)


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Fig. 3. X-ray diffraction patterns of calcined MSU-3 silica prepared at different temperatures ranging between 5 and 65 C.

show that variations in particle morphology, X-ray diffraction patterns and N2 isotherms arise actually from a continuous evolution: the surface area varies continuously from 300 to 700 m2 g1 and the porous volume from 0.1 to almost 1.0 mL g1, when the synthesis temperature varies from 5 to 65 C (Fig. 5). The evolution of the pore size distribution (Fig. 6) is also meaningful, since it can vary from less than 2 nm to more than 10 nm, only by varying the synthesis temperature between 5 and 55 C. with a linear variation of the pore size in the 25–55 C temperature range (see inset in Fig. 6).

4. Discussion Since the first synthesis of mesoporous silica with nonionic templates [31], three pathways have been mainly explored [48]. The first one, which is not a true templating mechanism, used a ‘‘nanocasting’’ of metal or oxide material by a liquid crystal [49–53] the second being based on electrostatic interactions in highly acidic medium [39,40] and the third one based on H-bondings [31,38] with possibly a preliminary assembly step of hybrid micelles [44]. Synthesis of MSU-X type mesoporous silica based on the last process illustrates perfectly the versatility of this approach since the final material structure (pore diameter, particle size) can be readily modified by adjusting several synthesis parameters, with the same reaction medium [32,37,45]. From the beginning, Pluronic P123 exhibited a special behavior, both for the preparation of MSU and SBA-type silica. The present study confirms this fact, especially for the synthesis temperature that affects the final structure in a larger way than observed with other classes of MSU-X silica (MSU-1 and -2, especially). This special behavior can be explained only on the light of the specific properties of P123.

The self-assembly properties of block-copolymers differ by various aspects from those of nonionic surfactants [54,55]. First of all, PEO–PPO–PEO copolymers are rather less hydrophobic than comparable PEO-based surfactants because the PO segments exhibit a relative hydrophilic trend, at least at low temperature. As the temperature increases, these PO segments become more hydrophobic and micelles can form at higher temperatures without changing the concentration. Hence, the critical micelle concentration (CMC) required for the micelle formation is a temperature-dependent parameter. An additional parameter is therefore required for the block-copolymers: the Critical Micelle Temperature (CMT) is the temperature needed for the micelle formation at a given concentration. Thus, for lower concentrations in copolymer, one may observe successively as one increases the temperature, a continuous shift from single molecules to spherical micelles, structured mesophase, then phase separation due to the correlative hydrophobic character of PEO chains with the temperature increasing. To understand why Pluronic P123 leads to the formation of hexagonally mesostructured silica, one must bear in mind an additional factor: the behavior described above will appear if the copolymer is mostly hydrophobic. Two families of copolymers were defined according to their enthalpy of micellization DH [56] There are relatively hydrophilic copolymers such as L64, P65, P84 and P85 (DH = 180–230 kJ mol1) and relatively hydrophobic ones (DH = 300–350 kJ mol1) such as P104, P105 and P123. This hydrophobic character of P123 (P123 presents a rather low hydrophilic–lipophilic balance (HLB) close to 7, to be compared with that of most nonionic surfactants templates that is always above 10) is an additional parameter to bear in mind. Finally, one must take into account how the synthesis temperature may control the global reaction. It can act



Fig. 5. Evolution of the specific surface area (j) and total porous volume (n) of calcined MSU-3 silica prepared at different temperatures ranging between 5 and 65 C. The micropore surface area was also determined for some samples (h). Dashed lines are for visual help.

Fig. 4. Nitrogen adsorption/desorption isotherms of calcined MSU-3 silica prepared at different temperatures ranging between 5 and 65 C.

through various mechanisms like changing the solubility of organics, increasing dynamics by Brownian motion, providing additional energy to overcome energy thresholds between different shape configurations and modifying the reactions kinetics, especially those between the organic/inorganic species assembly and the silica condensation. Our process provides a unique feature that allows us to overcome this last factor. All the syntheses described in this study start from the same aqueous solution of hybrid micelles (H.M.), prepared and let overnight at room temperature. This solution is then placed in the thermostated bath and the catalyst (sodium fluoride) is added as soon as the solution is equilibrated with the bath temperature. The self-induced

Fig. 6. Pore size distribution calculated by the BdB method of calcined MSU-3 silica prepared at different temperatures ranging between 5 and 65 C. Inset: evolution of the pore size with temperature, between 25 and 55 C.

silica condensation is then totally prohibited and the silica condensation will freeze a structure that arises from the concomitant influence of the H.M. structure evolution with temperature and the growth of the silica framework interacting with the micelles. At lowest temperatures, the hydrophobicity of PPO chains is low. The hybrid micelles are destabilized and all species (silica oligomers and copolymer molecules) dissolve. The addition of fluoride will induce the condensation of the silica oligomers containing dispersed


single copolymers. These latter generate the microporosity observed at least up to 10 C. Since there is no longrange order, the X-ray diffraction pattern is flat. As the temperature increases, the hydrophobicity of PPO increases and the hybrid micelles are stable in solution. The formation of mesostructured silica is then observed from 25 to 55 C. Above this temperature, the parallel trend to the hydrophobicity of the PEO chains increases with temperature. Once again, the hybrid micelles are destroyed but for an opposite reason due to the phase separation between hydrophobic copolymer and water [57,58]. In this latter case, the silica will condense alone and form small particles covered by the hydrophobic copolymer adsorbed onto the silica surface [59,60]. Unlike the synthesis at lower temperature where the pore maker is a single molecule––given a narrow microporosity––at higher temperature, the pore makers are the voids between these particles––given a broad mesoporosity . These observations confirm that the long-range order formation requires the existence of stable micellar objects (see X-ray diffraction patterns in Fig. 3) and the correct association strength between silica species and copolymers. The progressive evolution of this system, from 5 to 65 C, is then marked by a first step where there is no structuration because the copolymers are too hydrophilic, an intermediate stage where the correct HLB of the copolymer favors this interaction, hence the formation of a long-range order, and finally a last step where the progressive lack of long-range order is explained by the hydrophobic character of copolymers at high temperature. The morphological modifications are also explained by the variation of the interactions between silica and copolymer. Below 25 C, the material is amorphous, microporous and presents a spherical particle shape. The addition of fluoride creates silica seeds that will grow in a homogeneous medium as they are fed with silica oligomers and single P123 molecules dissolved in water. P123 molecules become trapped in the silica network because their hydrophilic character promotes strong interactions with silica (amorphous microporous silica gels were obtained with PEO polymers). The spherical shape arises from a nucleation and growth process of silica promoted by the fluoride ions that was described previously [37]. The microporosity in this temperature range is thus created by the departure upon calcination of single P123 molecules trapped in the silica gel. The temperature of 25 C seems to be the borderline between two systems. The particle shape starts changing with the formation of relatively facetted particles whose nanostructure is mesoporous (Fig. 4) but still ill-ordered (Fig. 3). Above this temperature and up to 45 C, the structure of the hybrid micelles is maintained. One observes a progressive long-range ordering of the structure leading to the parallel alignment of pores that gives stick-like hexagonal silica as previously reported [37].

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The relative hydrophobicity of P123 (compared with other nonionic surfactants and copolymers) might be the origin of a higher stiffness that favors the formation of elongated rigid hybrid micelles. Let remind that hexagonal structures were obtained with Tween 60 that is also a relatively hydrophobic molecule [37]. Such a hypothesis means that hexagonal structures should be obtained with P104 and P105 copolymers that present the same relative hydrophobicity [55]. Finally, above 55 C, the particles size progressively decreases and presents a loose shape that evolves toward a fluffy morphology (Fig. 2). Their nanostructure becomes also disordered and the diffractions peaks characteristics of the ordered structure disappear. This final evolution arises from the trend to phase separation encountered with nonionic surfactants. As the EO chains become too hydrophobic, the copolymers that become non-soluble, will tend to adsorb onto these articles and help to limit their growth [59]. The porosity measured in this case arises from textural (intergranular) porosity between dense particles.

5. Conclusion Synthesis of MSU-X type silica through the two-step process that involves the preliminary preparation of stable hybrid micelles allowed us to modify its pore size through the monitoring of the synthesis temperature [44,45]. This property applies also on preparation of MSU-3 with P123 block-copolymer. However, if the nature of P123 allows a pore size adjustment in a broader range (from 2 to almost 12 nm) than other templates, its additional property marked by the CMT, limits its use in lower temperature whereas the intrinsic trend of EO groups to become hydrophobic at high temperature limits for the opposite reason, the synthesis at too high temperatures. It appears that the formation of well-defined stick-like particles of hexagonal mesoporous silica with pore diameters close to 9 nm, requires a fine tuning of the synthesis parameters, especially the temperature. In the pH range used for this work, the assembly of silica with micelles of copolymers is also controlled by a charge of silica oligomers. The influence of this parameter will be addressed in a next report.

Acknowledgement M.M. thanks CAPES for scholarship funding.

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