Templating behavior of a triblock copolymer surfactant with very long hydrophilic PEO chains...

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Microporous and Mesoporous Materials 113 (2008) 352–361

Templating behavior of a triblock copolymer surfactant with verylong hydrophilic PEO chains (PEO140PPO39PEO140) for the synthesis

of cubic mesoporous silica with large cage-like cavities

Ligia Sierra a,b,*, Sabine Valange a,*, Joel Barrault a, Jean-Louis Guth c

a Groupo Ciencias de los Materiales, Instituto de Quımica, Universidad de Antioquia, A.A. 1226, Medellın, Colombiab LACCO, UMR CNRS 6503, ESIP, Universite de Poitiers, 40 Av. Recteur Pineau, F-86022 Poitiers, France

c Laboratoire de Materiaux a Porosite Controlee, ENSCMu, UMR CNRS 7016, 3 rue A. Werner, F- 68093 Mulhouse Cedex, France

Received 31 August 2007; received in revised form 26 November 2007; accepted 27 November 2007Available online 4 December 2007

Abstract

A series of cubic mesoporous silica materials exhibiting large cage-like cavities (up to 12.5 nm) interconnected through windows(Im3m symmetry group) and with a mesopore volume as high as 1.30 cm3 g�1 have been synthesized for the first time by using a newpolyalkylene oxide-based triblock copolymer (PEO140PPO39PEO140). The ability of this new triblock copolymer surfactant to act as atemplate for well-defined multidirectional pore network cubic silica particles, exhibiting tailored porous characteristics, was shown todepend on the synthesis conditions, in particular the acidity and temperature, as well as the presence of a cationic co-surfactant. Theseparameters influence the aggregation of nanoparticles either in micron-sized particles with different nanoparticles coalescence degrees orin blocks of aggregated nanoparticles. The aforementioned materials were characterized by small angle X-ray powder diffraction, nitro-gen adsorption, scanning and transmission electronic microscopy, while the hydrodynamic diameters of the new triblock copolymer spe-cies were determined in situ by dynamic light scattering.� 2007 Elsevier Inc. All rights reserved.

Keywords: Mesoporous silica; Triblock copolymer surfactant; Cage-like structure; Particle morphology; Dynamic light scattering

1. Introduction

Ordered cage-like mesoporous silicas [1,2] are attractingwide current interest because of their fascinating features,such as (i) high adsorption capacity, (ii) high structural sta-bility [3], (iii) controlled pore volume, pore cage andentrance dimensions (pore connectivity) [4–7], (iv) con-trolled pore topology and (iv) availability in forms ofrhombododecahedron and spherical micron-sized particles,

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.11.033

* Corresponding authors. Address: Groupo Ciencias de los Materiales,Instituto de Quımica, Universidad de Antioquia, A.A. 1226, Medellın,Colombia. Tel.: +57 42106547; fax: +57 42106550 (L. Sierra); tel.: +33 549 45 40 48; fax: +33 5 49 45 33 49 (S. Valange).

E-mail addresses: [email protected] (L. Sierra), [email protected] (S. Valange).

thin films, fibers or monoliths [8–14]. Precise control ofthese structural and textural properties is highly desirablefor their use and ultimately commercial applications asselective sorbents, catalyst supports [15,16], templates fornanostructured materials synthesis [17,18], matrixes forbiomolecules [19], and so forth. This has brought theresearchers in this area to look for the way of designingthe synthesis conditions from diverse surfactant systemsallowing the elaboration of nanostructured materials withthe desired characteristics. In this respect, the use of pol-yalkylene oxide-based triblock copolymers, such as theinexpensive ecological pluronic-type triblock copolymersurfactants (PEOmPPOnPEOm) offers possibilities of tailor-ing mesoporous silicas properties. Among these polymer-templated ordered mesoporous silicas, cubic materialsconsisting of a multidirectional pore network are particu-

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L. Sierra et al. / Microporous and Mesoporous Materials 113 (2008) 352–361 353

larly attractive because they are likely to be more resistantto local pore blockage than channel-like pores. Orderedcubic materials with spherical cage-like cavities intercon-nected through smaller windows are synthesized using tri-block copolymer surfactants with long polyethyleneoxidechains in strong or medium acidic conditions [2]. Numer-ous papers have been published on the synthesis and char-acterization of SBA-16 silica, in which each cage isconnected with neighboring cages via eight cylindricalnecks, arranged in cubic body-centered Im3m symmetry[1,20]. For other cage-like mesoporous silicas such asFDU-1, FDU-12 and KIT-5 synthesized under differentconditions, the cavities distributed on the nodes of a face-centered cubic cell (Fm3m symmetry) have a connectivityof 12 [4,21–23]. To date SBA-16 materials have been syn-thesized mostly by employing Pluronic F127 as well as Plu-ronic F108 [2,6,24]. Other triblock surfactants with shorterhydrophilic PEO chains seem to lead to noncubic struc-tures, although the behavior of Pluronic P123 has beenadjusted in the presence of butanol as a co-solute to pro-duce a cubic silica with pore system formed of two inter-penetrating bicontinuous networks of channels (Ia3dsymmetry) [17]. There are also a few reports on the useof copolymer blends of P123 and F127 [6] or even nonionicoligomeric surfactants [25], which lead to small pore sizes.By using the more hydrophobic poly(butylene oxide) con-taining triblock copolymer PEO39PBO47PEO37 as a tem-plate in a 2 M HCl solution, Yu et al. [26] obtained at100 �C FDU-1 type silica exhibiting an Im3m symmetrystructure with a large pore size (12.0 nm).

Since the first synthesis of SBA-16 using Pluronic F127as structure directing agent [27], more information is cur-rently available in the literature regarding the pore volumeand pore size of cubic mesoporous silicas with connectedcage-like cavities (Im3m or Fm3m structure symmetry).Unfortunately, different methods are used to determinethe size of the cavities, some of them being not adaptedto that pore topology (for instance the estimation of thepore size from the adsorption isotherm according to theBJH method). Moreover, before the presence of intrawallmicropores and secondary mesopores was evidenced[28,29], all the pore volume was attributed to the mesop-ores. It is thus very difficult to analyze and to comparethe effect of the synthesis variables (nature of the surfac-tant, temperature, acidity, dilution, presence of a co-surfac-tant and/or a co-solute). Nevertheless, all the results agreewith a pronounced increase of the pore volume andenlargement of the pore diameter when increasing the tem-perature or aging of the synthesis mixture above 80 �C[6,7,9]. Among all the papers related to the synthesis andmodification of the pore topology of cubic mesoporous sil-icas, the results reported by Kleitz et al. [7,30,31] provide adetailed and accurate characterization of large-pore cage-like mesoporous SBA-16 silica materials with tailored poredimensions. The materials are synthesized in a TEOS–EO106PO70EO106 (F127)–butanol–HCl–H2O system undermildly acidic conditions with the molar composition

1:0.0035:1.3–1.8:0.88:117 ([HCl] = 0.4 mol L�1). Synthesesare carried out in two steps: 24 h at 45 �C and then 24 hat a temperature varying between 45 and 130 �C. The poresize analysis is determined by appropriate methods such asa new nonlocal density functional theory (NLDFT) or acombination of gas physisorption and comprehensiveXRD modeling (calculated from the unit-cell parameters,mesopore volume and number of cavities). The diameterof the cavities is shown to vary from 8.2 nm at 45 �C to13 nm at 130 �C, while the mesopore volume varies from0.17 to 0.66 cm3 g�1.

In this study, a new triblock copolymer surfactant char-acterized by very long PEO chains (m = 140 and n = 39) isused for the first time with the aim of investigating its abil-ity to act as template for mesostructured cubic silica (forPluronic F127 and F108, values of m and n are, respec-tively, equal to 106 and 132, and to 70 and 50). The effectsof various synthesis conditions (acidity, synthesis tempera-ture and time, addition of co-surfactant) on the structure,porosity and morphology of the resulting materials are sys-tematically examined. In general, the syntheses of cubicmesoporous silica with long PEO chains triblock surfac-tants reported in the literature are carried out at HCl con-centrations around 0.4 mol L�1. Higher concentrations(�2 mol L�1) are used for syntheses performed at low tem-perature [24]. In this respect, we have selected the range0.2–0.8 mol L�1 HCl for the syntheses carried out withthe new triblock copolymer between 80 and 125 �C.

The present work is an advance in the synthesis of mul-tidirectional pore network cubic mesoporous silica frompolyalkylene oxide-based triblock copolymer exhibitingvery long EO group moieties.

2. Experimental

2.1. Synthesis of cubic mesoporous silica with large cage-like

cavities

A new polyalkylene oxide-based triblock copolymer sur-factant is used, characterized by a molecular weight of14,600 and a composition in polyethyleneoxide (PEO) of84.4 wt% (Sigma–Aldrich) and named ‘‘triblock” hereun-der. The syntheses are carried out according to the methoddescribed for SBA-16 [9] where Pluronic F127 (PEO106-PPO70PEO106) is replaced by the new triblock copolymersurfactant. Briefly, an aqueous solution of this new copoly-mer template is prepared by dissolving it under mild stir-ring into a diluted HCl solution at room temperature.TEOS (99%, Sigma–Fluka) is further added to this solutionunder strong stirring and kept for 15 min at this tempera-ture. In some syntheses a co-surfactant, cetyltrimethyl-ammonium bromide (CTMABr, 99%, Sigma–Fluka) isadded. The temperature and synthesis duration are variedbetween 80 and 125 �C and between 2 and 4 days, respec-tively. The molar composition of the reaction mixture is1 TEOS; 0.005 triblock; 0 or 0.02 CTMABr; 0.05–6.9HCl; 247 H2O. The resulting white solid precipitates are

354 L. Sierra et al. / Microporous and Mesoporous Materials 113 (2008) 352–361

isolated by filtration, washed with a less amount of waterand dried at 60 �C overnight. They are further calcined inair at 500 �C and held at this temperature for 2.5 h toremove the copolymer template.

2.2. Characterization

Powder XRD patterns of the samples are recorded on aBruker D5005 diffractometer with monochromatized CuKa radiation, operating voltage of 40 kV, 30 mA, 0.03� stepsize and 10 s step time over a range of 0.6� < 2h < 6� at roomtemperature. Hydrodynamic diameters DH of the new tri-block copolymer species are determined in situ by DLS (Mal-vern, Zeta Sizer NanoZS instrument), in aqueous HClsolutions with the same concentrations than those used forthe syntheses. DH values are calculated from the measureddiffusion coefficient by using the Stokes–Einstein model withthe appropriate values of the solvent viscosity. The size andmorphology of the synthesized solids are studied with a scan-ning electron microscope (Philips XL30) operated at 5 kV.TEM micrographs of some selected samples are taken witha transmission electron microscope (Philips CM120) oper-ated at 120 kV to determine the organization of the porousstructure. Samples were dispersed in ethanol (99.9 vol%)using the ultrasonic method and the suspension was subse-quently dropped onto a carbon microgrid. The surface areaand pore size analysis of the samples are carried out byadsorption–desorption of nitrogen using a TRISTAR volu-metric adsorption analyser manufactured by Micromeritics(�196 �C). Prior to N2 adsorption, the samples are degassedunder vacuum at 90 �C for 1 h, followed by a further heatingat 300 �C for 6 h. The BET equation is used to calculate thesurface area from adsorption data obtained at p/p0 between0.05 and 0.2. The diameter of the cavities is estimated fromthe adsorption isotherm according to the IDBdB method[32], while the pore diameter (windows entrance) is evaluatedat the maximum of the pore size distribution calculated bythe BJH method (desorption).

3. Results and discussion

3.1. Characterization of the new triblock copolymer

surfactant

The formula of the new triblock copolymer surfactant isestablished from its molecular weight (14,600) and its com-

Table 1Hydrodynamic diameter of the micelles of the new triblock copolymer surfactan[HCl] = 0.4 and 1.2 mol L�1

Temperature (�C) Triblock ‘‘14600” micelles (nm)

HCl 0.4 mol L�1 HCl 1.2 m

25 – –30 – –35 36 41.540 28.7 30.450 25.4 26.8

position in polyethyleneoxide groups (84.4% PEO). Thismolecule consists of very long EO chains and shorter POblocks than the currently Pluronic copolymers used forthe synthesis of cubic mesoporous silica. The correspond-ing formula, PEO140PPO39PEO140, differs from the one ofPluronic F108 triblock copolymer (PEO132PPO50PEO132),which has the same molecular weight and was until nowthe nonionic surfactant with the longest PEO chains usedfor the synthesis of mesoporous silica. With a molar ratioPEO/PPO of 7.2, compared to the value of 5.8 for PluronicF108, the new surfactant becomes the most hydrophilic oneof the polyalkylene oxide-based triblock copolymer series,justifying the study of its ability to act as a template formesostructured cubic silica since a different templatingbehavior is expected.

A DLS study performed in acidic solution clearly showssignificant differences between these two polyalkyleneoxide-based triblock copolymers. The evolution of the sizeof the micelles was determined in situ in aqueous solutionswith the same acid (0.4 and 1.2 mol L�1 HCl) and surfac-tant concentrations than those used for the silica synthesis(Table 1 and Fig. 1a and b). For both surfactants, nomicelles are present at a temperature lower than 35 �C.At higher temperature, only micelles are present in the0.4 mol L�1 HCl of the new triblock copolymer, whilethose of Pluronic F108 are always accompanied by largeraggregates. The hydrodynamic diameters (DH) of both sur-factant micelles decrease as the temperature increases from35 to 50 �C and increase with the acid concentration.Excepting the micelle DH value of Pluronic F108 at 50 �Cin the 0.4 mol L�1 HCl solution, all other micelle DH valuesof the new triblock copolymer are systematically higher.Monomer, micelles and clusters hydrodynamic diametersof the two triblock copolymers are in agreement with thoseobtained for neutral aqueous solutions of other triblockcopolymers with long polyethylene oxide chains like Plu-ronic P85, F87, F88 and F127 surfactants [33–35].

3.2. Effect of the acidity and synthesis temperature on thesilica particle morphology

The effects of the amount (195 < HCl/triblock < 779)and concentration of the HCl solution ([HCl = 0.2, 0.4 or0.8 mol L�1], synthesis temperature (80–125 �C) and possi-bly the presence of CTMABr as a co-surfactant on theporosity and morphology characteristics of the mesosilicas,

t and Pluronic F108, determined as a function of temperature by DLS with

Pluronic F108 micelles (nm)

ol L�1 HCl 0.4 mol L�1 HCl 1.2 mol L�1

– –– –34.8 36.127.2 27.933.6 25.6

1

10

100

1000

10000

25 30 35 40 50 25*Temperature (°C)

Hyd

rody

nam

ic d

iam

eter

(nm

)

1

10

100

1000

10000

25 30 35 40 50 25*Temperature (°C)

Hyd

rody

nam

ic d

iam

eter

(nm

)a b

Fig. 1. Hydrodynamic diameters DH of the surfactant species present in a 0.4 mol L�1 HCl solution of 0.001 mol L�1 (a) new triblock copolymer and (b)Pluronic F108 between 25 and 50 �C. Column 25*: species present at 25 �C after cooling of the solution heated at 50 �C.


are studied for a starting molar ratio composition: 1 TEOS;0.005 triblock; 0 or 0.020 CTMABr; 0.90, 1.75 or 3.5 HCl;247 H2O.

Table 2 shows the morphology, structural and porouscharacteristics of the mesoporous silica samples. Since theacidity and synthesis temperature have an interrelatedand predominant influence on the physicochemical andmorphology characteristics of the materials, their effectscannot be separated and are therefore discussed simulta-neously. The materials described here after are identifiedby the synthesis temperature (80, 95 and 125), while A, Band C stand for the acid concentrations equal to 0.2, 0.4and 0.8 mol L�1, respectively. *Refers to the samples syn-thesized in the presence of CTMABr along with the newtriblock copolymer.

3.2.1. Morphology of the materials in the absence of

CTMABr

At very low acid concentration ([HCl] = 0.01 mol L�1

and HCl/triblock = 10), a nonmesoporous gel is obtained,whatever be the synthesis temperature, due to the low inter-action between the weakly protonated EO head groups sur-

Table 2Synthesis conditions and morphology characteristics of the materials

Sample HCl/triblock d (nm) Morphology

80A 195 10.4a Blocks and spheroidal particles (0.5–95A 195 9.6a Spheroidal particles (2–6 lm) and sha125A 195 10.7 Spherical particles (2–10 lm), some f80B 390 10.4a Spheroidal particles (<1 lm) of aggre95B 390 10.0 Spheroidal particles (1 lm) of aggreg125B 390 10.8 Spherical particles (2–8 lm), some fu80C 779 11.8a Spheroidal particles (0.5–1 lm) of ag95C 779 11.6 Spheroidal particles (0.5–1 lm) of ag125C 779 11.7 Blocks of aggregated nanoparticles80A* 195 9.3 Spherical particles (3–12 lm)95B* 390 10.5 Spherical particles (2–12 lm), many f125B* 390 10.4 Spherical particles (2–16 lm), many f80C* 779 9.5 Spherical particles (1–7 lm)95C* 779 10.6 Spherical particles (2–8 lm) and som

Samples are identified by the synthesis temperature (80, 95 or 125) and acidityNotation: *refers to the samples synthesized in the presence of CTMABr alongmeasured for the calcined materials; arefers to several Bragg diffraction reflect

factant and the siliceous species, as well as the lowpolycondensation degree of these species. The triblockcopolymer surfactant lacks its ability for forming hydrogenbonds with the silica oligomers, avoiding the formation ofan organized silicatropic liquid crystal phase and conse-quently the formation of an ordered mesoporous silica.

At low acidity values ([HCl] = 0.2 mol L�1 and HCl/triblock = 195), the size of the silica particles increaseswith the synthesis temperature. These micron-sized parti-cles appear to be constituted by nanoparticles with a sizeof 30–50 nm and show different coalescence degreesdepending on the hydrothermal treatment temperature.The particles of the material synthesized at 80 �C aremade of agglomerates of nanoparticles, while more densespheroidal-like particles are obtained for the sample pre-pared at 95 �C and even more dense spheres exhibit asmooth surface in the one synthesized at 125 �C (Table2). Representative SEM micrographs of the materials syn-thesized at 80 and 125 �C are shown in Fig. 2 (see samples80A and 125A in Table 2). These results indicate that atthis low acid concentration, nanoparticles are probablypresent at the phase separation, whose coalescence degree

1 lm) of aggregated nanoparticlespeless aggregates of aggregated nanoparticles

usedgated nanoparticles and shapeless aggregates of aggregated nanoparticlesated nanoparticles and shapeless aggregates of aggregated nanoparticlessed and few polyhedragregated nanoparticlesgregated nanoparticles in blocks constituted by aggregated nanoparticles

usedused

e aggregates of nanoparticles

with [HCl] equal to 0.2, 0.4 and 0.8 mol L�1 for A, B and C, respectively.with the new triblock copolymer; d: value of the first Bragg diffraction peakions present at 2h angles below 5�.

Fig. 2. SEM micrographs of the mesoporous silicas synthesized at (a) 80 �C at low acidity (sample 80A), (b) 125 �C at low acidity (sample 125A), (c) 80 �Cat high acidity (sample 80C), and (d) 125 �C at high acidity (sample 125C).


into micron-sized particles increases with the synthesistemperature.

At medium acidity ([HCl] = 0.4 mol L�1 and HCl/tri-block = 390), the size of the individual particles tends tobe smaller and is likely associated with changes in the coa-lescence of the nanoparticles, depending on the synthesistemperature. For instance, the particles of the sample syn-thesized at 125 �C at medium acidity look less dense alongwith a less smooth surface than those obtained at loweracidity, indicating a lower coalescence degree of the aggre-gated constitutive nanoparticles.

At higher acidity ([HCl] = 0.8 mol L�1 and HCl/tri-block = 779), spheroidal silica particles composed of aggre-gated nanoparticles with size of 0.5–1 lm are obtained forthe sample prepared at 80 �C. However, when increasingthe synthesis temperature, these spheroidal particles tendto form blocks constituted by aggregated nanoparticles.This effect is even more pronounced for the sample synthe-sized at 125 �C. In this case, the tendency to form well-defined individual particles disappears with a decreasingability of the nanoparticles to coalesce (Fig. 2d). With theincrease in acidity at this high temperature, the individualsilica particles are replaced by a continuous assembling ofweakly coalesced nanoparticles. This may be attributed toa partial degradation of the triblock copolymer surfactantdue to the high acidity and synthesis temperature condi-tions, as already established by Mesa et al. [9] for the surfac-tant Pluronic F127. With such severe synthesis conditions,the new triblock copolymer surfactant loses its micellarbehavior, which probably together with a higher polycon-densation degree of the silica species hinders the nanoparti-cles association into micron-sized silica particles exhibitinga well-organized mesoporous structure. At lower synthesistemperature, this effect is not clearly appreciated at leastfrom the morphology of the resulting materials.

In summary, depending on the acidity and synthesistemperature, the nanoparticles aggregate into micron-sized

particles or blocks. The coalescence degree of these nano-particles inside the silica micron-sized particles or blocksincreases with temperature and decreases with the increaseof acidity.

3.2.2. Morphology of the materials in the presence of

CTMABr

Syntheses combining the new triblock copolymer withthe co-surfactant CTMABr result in the formation ofspherical mesoporous silica particles exhibiting a denseand smooth appearance (high coalescence degree), what-ever be the acidity and temperature. The size of thesewell-defined mesoporous silica particles is also larger thanthe one obtained for the samples synthesized in the absenceof CTMABr. Indeed, spherical particles with micron size of2–8 lm are obtained at 95 �C, at high acidity and in thepresence of CTMABr, while 0.5–1 lm particles areobtained with the same synthesis conditions but in theabsence of the co-surfactant. It seems that the presence ofthe cationic co-surfactant along with the triblock copoly-mer in the reaction medium favors the nanoparticles coa-lescence inside the micron-sized silica particles and at thesame time limits the triblock surfactant decomposition.

When a triblock copolymer surfactant having a shorterPEO chain length like Pluronic F127 is used with the samesynthesis conditions (concentration, acidity and tempera-ture) than those of this study, the resulting silica particlesare more often polyhedral [9]. This can be related to alower hydrolysis degree of the PEO chains. The effect ofthe surfactant degradation on the morphology of the silicaparticles only occurs at high temperature through thesmoothing of the polyhedra edges. In the case of the newtriblock copolymer which is characterized by much longerPEO chains, such polyhedral particles are rather uncom-mon. One may suppose that the surfactant micelles irregu-larities originating from an even more pronounceddegradation of the long PEO chains (than that observed


for Pluronic F127) lead to a less regular and periodicarrangement during their assemblage. Moreover, the thick-ness of the micelle PEO corona can also contribute to theirregularity.

3.3. Effect of the acidity, synthesis temperature and presence

of a cationic co-surfactant on the silica porous organization

The XRD patterns of the materials exhibit a low anglereflection peak corresponding to d-spacing values varyingbetween 9.3 and 11.8 nm (Table 2). The intensity and widthof these diffraction peaks, which give an indication of theporous organization degree, are shown to depend on thesynthesis conditions. More well-resolved XRD patternsare obtained for the samples synthesized at moderate tem-perature such as 80 or 95 �C and rather at low (or medium)acidity. For samples 80A, 80B, 80C and 95A, at least threeadditional Bragg diffraction reflections are observed at 2hangles below 5�, which can be indexed in the Im3m spacegroup corresponding to a body-centered cubic structure,as for SBA-16 materials. Fig. 3 shows the Cu Ka XRD pat-tern obtained for sample 80B. Also, Fig. 3 shows represen-tative TEM images for such mesoporous silica materialsand in particular for sample 95B, demonstrating the highlyordered cage-like arrangement of mesopores (viewed from[100] and [111] directions). By contrast, the materials onlyexhibiting the first low angle reflection peak fail to show aperiodic arrangement of the mesoporous organization byTEM. As already discussed in Section 3.2.2, the degrada-tion of the long PEO chains is probably responsible forthe lack of regular and periodic arrangement when synthe-ses are carried out at high temperature and acidity.

The same conclusion can be made for the samples syn-thesized in the presence of the cationic co-surfactant, forwhich a less well-organized mesoporous structure is sys-tematically observed than that of samples prepared withoutCTMABr under the same synthesis conditions.

The BET specific surface area, total pore volume, intra-wall microporous volume, volume of primary mesopore,interparticle volume and diameter of the cavities and con-

0.5 1 1.5 2 2.52 Theta (°)

Inte

nsity

(a.u

.)

hkl d (nm) 110 10.38 200 7.41 211 6.04

a0 = 14.8 nm

ba

Fig. 3. (a) XRD pattern obtained for sample 80B and (b) TEM images of a repr95B), viewed from [100] and [111] directions.

necting windows for these samples are listed in Table 3.Nitrogen adsorption–desorption isotherms of the materialssynthesized at 80 and 95 �C are type IV and are in agree-ment with a mesoporous silica structure having cavitiesinterconnected through smaller windows as for SBA-16materials. The broad hysteresis loop starting at a relativepressure of �0.6–0.7 and abruptly ending at �0.45 indi-cates the presence of good-quality cage-like cavities [36].With the increase of the synthesis temperature, the capil-lary condensation step is shifted to higher relative pres-sures, corresponding to the formation of larger cavities.The associated hysteresis loop becomes less importantwhich indicates that the diameter of the connecting poresor windows increases. Representative nitrogen adsorp-tion–desorption isotherms of the materials synthesized at95 and 125 �C at medium acidity are shown in Fig. 4.

The diameter of the spherical cavities is estimated fromthe adsorption isotherm using the IDBdB method recentlyproposed by Kowalczyk et al. for the characterization ofthe internal structure of cage-like mesoporous silicas [32].The window (or pore) diameter is determined at the maxi-mum of the pore size distribution calculated by the BJHmethod using the desorption isotherm. However, if thewindow diameter is less than 4–4.5 nm, an accurate deter-mination of the pore diameter cannot be reached using thismethod. The hysteresis loop stems from two phenomena:the bottleneck effect produced by the connecting poresand the tensile strength effect TSE [37] which occurs for apore diameter larger than �4.5 nm. According to Thom-mes et al. [38], if the connecting pores (or apertures)between the cavities are larger than 4–4.5 nm, the cavitieswould empty during nitrogen desorption when the relativepressure reaches the value allowing to empty the connect-ing pores (bottleneck effect) during desorption. In thatcase, the desorption branch can be used to determine thediameter of the connecting pores (as for sample 125B pre-sented on Fig. 4b). But if the windows diameter is equal orless than 4–4.5 nm, the cavities empty by the cavitationeffect when p/p0 reaches the value of �0.5 (correspondingto the hysteresis limit at p/p0 TSE) on the desorption

50 nm50 nm

c

esentative mesoporous silica sample synthesized at medium acidity (sample

Table 3Physicochemical characteristics of the materials

Sample SBET (m2 g�1) Vt (cm3 g�1) Vmi (cm3 g�1) Vp (cm3 g�1) Vinterp (cm3 g�1) Dc (Dw) (nm)

80A 769 0.51 0.19 0.26 0.06 7.7 (63.5)95A 932 0.94 0.17 0.48 0.29 8.0 (63.8)125A 610 0.90 0.05 0.85 0.00 11.8 (5.4)80B 781 0.51 0.22 0.23 0.06 7.2 (63.7)95B 926 1.17 0.13 0.58 0.46 8.9 (63.8)125B 785 1.34 0.01 1.30 0.04 12.5 (6.2)80C 807 0.53 0.27 0.18 0.08 7.5 (63.4)95C 983 0.94 0.19 0.50 0.25 9.0 (63.8)125C 676 1.84 0.05 0.73 1.06 8.1 (63.8)80A* 811 0.44 0.21 0.23 0.00 6.3 (63.4)95B* 1116 0.80 0.16 0.64 0.00 8.1 (63.8)125B* 798 1.11 0.00 1.11 0.00 10.3 (5.4)80C* 837 0.51 0.22 0.26 0.03 6.7 (63.4)95C* 1122 0.85 0.11 0.74 0.00 8.5 (63.8)

Samples are identified by the synthesis temperature (80, 95 or 125) and acidity with [HCl] equal to 0.2, 0.4 and 0.8 mol L�1 for A, B and C, respectively.Notation: *refers to the samples synthesized in the presence of CTMABr along with the new triblock copolymer; SBET: BET specific surface area; Vt: totalpore volume; Vmi: microporous volume determined by the t-plot analysis; Vinterp: interparticle volume estimated at p/p0 > 0.6, >0.7, >0.85, respectively, forthe samples synthesized at 80, 95 and 125�C; Vp: mesoporous volume = Vt � Vmi � Vinterp; Dc and Dw: diameter of the spherical cavities and diameter ofthe windows (aperture) interconnecting adjacent cage determined according to Refs. [32,37,38].

0

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1

Relative pressure (P/P0)

N2 v

olum

e ad

sorb

e (c

m3 .g

-1)

0

200

400

600

800

1000

0 0.2 0.4 0.6 0.8 1

Relative pressure (P/P0)

N2 v

olum

e ad

sorb

ed (c

m3 .g

-1)a b

Fig. 4. Nitrogen adsorption–desorption isotherms at �196 �C for (a) sample 95B* synthesized in the presence of CTMABr and (b) sample 125B (mediumacidity).


branch, which hinders a precise determination of the con-necting windows diameter using the BJH method (as forsample 95B* shown in Fig. 4a).

As expected, the size of the cavities and the connectingpores or windows increases with the synthesis temperature[4,6,31], but with a maximum increase at medium acidity.As shown in Table 3, the cage and connecting windowsdiameters could be adjusted to 12.5 and 6.2 nm, respec-tively, for the sample synthesized at 125 �C at medium acidconcentration ([HCl] = 0.4 mol L�1).

The same trend is observed for the total pore volume Vt

and mesoporous volume Vp which increase when increas-ing the synthesis temperature, whereas the microporousvolume Vmi decreases accordingly (Table 3). The texturalpore volume Vinterp (or interparticle volume) depends onthe morphology of the samples, in particular on the pres-ence of silica blocks composed of low coalesced nanoparti-cles. As shown in Fig. 5, the textural pore volume estimated

at p/p0 > 0.6, >0.7, or >0.85, respectively, for the samplessynthesized at 80, 95 and 125 �C, reaches the highest valuefor the sample prepared at high temperature and acidityand exhibiting a morphology of blocks (sample 125C).

As can be seen from Fig. 4, sample 125B synthesized atmedium acidity features a much larger volume of mesop-ores than that of sample 95B*. Quite surprisingly, a mes-oporous volume as high as 1.30 cm3 g�1 is obtained forthis sample 125B. Such a high mesoporous volume wasnever mentioned previously for cage-like mesoporous sili-cas prepared at similar synthesis temperatures and exhib-iting Im3m symmetry like SBA-16 materials or Fm3m

space group such as KIT-5 [4,6,31]. The analysis of Table3 indicates that all the samples synthesized at high tem-perature (125 �C) systematically exhibit higher primarymesopore volume, whatever be the acidity conditions,than those obtained for samples synthesized at 80 �C oreven at 95 �C.

0

200

400

600

800

1000

1200

1400

0 0.2 0.4 0.6 0.8Relative pressure (P/P0)

N2 v

olum

e ad

sorb

ed (c

m3 .g

-1)

1

Fig. 5. Nitrogen adsorption–desorption isotherm at �196 �C for sample125C synthesized at high acidity.


Taking into account the porosity a (cavity volume/unit-cell volume) equal to (p

p3)/8 in the case of a cage-like

Im3m cubic structure and the specific mass q for silicaequal to 2.2 g cm�3, the calculated mesoporous volumeVc (Vc = a/q(1 � a)) will be equal to 0.97 cm3 g�1, whichis lower than the experimental value of 1.30 cm3 g�1 mea-sured for sample 125B. This means that the spherical cav-ities are probably overlapping which results in a decrease ofthe volume occupied by the walls [36]. Another possibilitymay be that the structure of this mesoporous silica samplecorresponds to a close packing of spherical cavities withFm3m symmetry, with a porosity a = p/3

p2 and a theoret-

ical value of Vc equal to 1.30 cm3 g�1. Since this sample125B exhibits only one low angle Bragg diffraction peak,this assumption cannot be verified.

Concerning the samples synthesized in the presence ofthe cationic co-surfactant, the evolution of the total porevolume, mesopore volume and sizes of the interconnectedcavities and windows, in function of the synthesis temper-ature, is similar to the observations made previously. How-ever, under identical acidity and synthesis temperatureconditions, the values of the total pore volumes and diam-eters of the cavities and apertures are systematically smallerthan those exhibited by the materials synthesized in the

Table 4Comparison of the porosity characteristics of SBA-16 samples prepared withtriblock copolymer surfactant ([HCl] = 0.4 mol L�1, except 80Aa for which ([H

Synthesistemperature (�C)

Surfactant (m, n) Total volume(cm3 g�1)

80 Pluronic F127 (106, 70) 0.5080 New triblock (140, 39) 0.5180 New triblock (140, 39) 0.4495 New triblock (140, 39) 1.1795 New triblock (140, 39) 0.80

100 Pluronic F127 (106, 70) 0.56120 Pluronic F127 (106, 70) 0.67125 New triblock (140, 39) 1.34125 New triblock (140, 39) 1.11130 Pluronic F127 (106, 70) 0.73

Notation: *refers to the samples synthesized in the presence of CTMABr alon

absence of CTMABr. Moreover, these micron-sized silicaparticles exhibit a spherical morphology with a denseappearance and smooth surface, preventing any additionalinterparticle (textural) pore volume. Recently, we measuredby dynamic light scattering the micelles hydrodynamicdiameter of triblock copolymer surfactants with longPEO chains with or without the addition of CTMABr (atthe same pH and dilution used during the synthesis). Thepresence of CTMABr along with the triblock copolymerinduces a significant decrease of the micelles hydrodynamicdiameter which may explain a lower pore size and pore vol-ume of the resulting silicas.

According to the results of the literature, there is no sim-ple relation between the triblock copolymer surfactantcomposition and the pore size of the resulting materials.It seems that the length of the PEO chains plays an impor-tant role in the formation of the mesoporous structure [2](i.e. short PEO chains lead to channel-like porous struc-tures while long PEO chains orient towards porous struc-tures formed by connected cage-like cavities). The size ofthe cavities seems to be more dependent on the size ofthe hydrophobic core of the triblock surfactant (numberof PPO or PBO) than on the number of PEO chains. Thisis due to the fact that the contribution of a volume Dv (forinstance, the volume of a PEO group) to the increase Dr ofthe radius r of triblock surfactant micelles is theoreticallyequal to Dr = (1/4pr2)Dv. This means that the increase ofthe number of the PEO groups inside the corona of themicelle has less effect (high r values) on the cavities diame-ter than the same increase of the number of PPO groups inthe core (low r values). There is also another reason whythe length of the PEO chains has a weak effect on the sizeof the cavities at least at low synthesis temperatures.Indeed, their hydrophilic character favors the presence ofthe siliceous species between the PEO chains through theformation of hydrogen bonds. This leads to the integrationof the PEO chains into the walls during the polycondensa-tion of the silica and results in the formation of microporesduring the removal of the organic matter. The strongincrease of the mesoporous volume and of the cavitiesdiameter with the concomitant decrease of the microporous

Pluronic F127 with those of the cubic silicas synthesized with the newCl] = 0.2 mol L�1)

Mesoporous volume(cm3 g�1)

Cavity diameter(nm)

Reference ofsample

0.39 9.8 [7]0.23 7.2 80B0.23 6.3 80Aa

0.58 8.9 95B0.64 8.1 95B*

0.34 10.5 [30]0.59 12.4 [30]1.30 12.5 125B1.11 10.3 125B*

0.66 13.0 [30]

g with the new triblock copolymer.


volume when increasing the synthesis temperature, derivedfrom a more hydrophobic behavior of the PEO chains pre-venting the intrusion of the siliceous species. Then the con-tribution of the PEO chains to the pore volume and to thecavities size becomes very significant when they are long(Table 4). The comparison between some silica samplessynthesized with Pluronic F127 (PEO106PPO70PEO106)and with the new triblock surfactant (PEO140PPO39-

PEO140) shows that at low synthesis temperature (80 �C),the large hydrophobic PPO core of Pluronic F127 isresponsible of higher values of mesopore volume and cav-ities diameters than those obtained with the triblockcopolymer PEO140PPO39PEO140. However, at high temper-ature, the fact that the PEO chains become more hydro-phobic gives advantage to the samples synthesized withthe new triblock copolymer having the longest PEO chains,as the cubic silicas exhibit very large mesopore volumes.

4. Conclusion

A new triblock copolymer surfactant with large EOgroup moieties (PEO140PPO39PEO140) has been investi-gated in this work to verify its ability to act as a templatefor mesostructured cubic silica. Being the most hydrophilicsurfactant among the polyalkylene oxide-based triblockcopolymer series, the purpose of this study aimed at takingadvantage of the new triblock, very long EO chains to syn-thesize mesoporous silica materials with large sphericalcage-like cavities and window sizes.

The proposed copolymer PEO140PPO39PEO140 gener-ates cage-like micron-sized mesoporous silicas with largecage-like cavities and with much higher values of mesoporevolume than those reported so far for SBA-16 type materi-als. The cage and connecting window diameters could beadjusted at 12.5 and 6.2 nm, respectively, through accu-rately controlling two synthesis parameters, namely acidity(195 < HCl/triblock < 779) and temperature (80–125 �C).Compared to other cage-like cubic mesoporous silica mate-rials (Im3m or Fm3m symmetry), the volume of primarymesopores is significantly increased up to 1.30 cm3 g�1

for syntheses carried out at 125 �C at medium acidity(0.4 mol L�1). The samples synthesized at moderate tem-perature such as 80 or 95 �C and at rather low (or medium)acidity exhibit well-resolved XRD patterns, which can beindexed in the Im3m space group corresponding to abody-centered cubic structure, as for SBA-16. Using highertemperatures and for medium to high acidity values, thematerials exhibit only one low angle reflection peak. Themorphology of the mesoporous silica particles stronglydepends on the acidity and synthesis temperature andwas shown to vary continuously from individual particlesformed by the coalescence of nanoparticles at low acidity,towards blocks composed of agglomerated nanoparticlesat high acidity, for temperature above 95 �C. Well-definedand dense spherical mesoporous silica particles are onlyobtained at 125 �C at low to medium acidity. The presenceof CTMABr as cationic co-surfactant along with the new

triblock copolymer allows the formation of sphericalmicron-sized mesoporous silica particles, exhibiting a denseand smooth morphology (high degree of coalescence),independently on the synthesis conditions.

Acknowledgments

The authors thank the French Ministry of Research andCNRS for their financial supports. L.S. gratefully acknowl-edges the Region Poitou-Charentes for a research Grant(05/RPC-R-143) at the University of Poitiers.

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