Microporous and Mesoporous Materials 173 (2013) 53–63

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Microporous and Mesoporous Materials

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Synthesis of texturally biphasic mesoporous carbon-silica composites and carbons

Cristiam F. Santa a, Maguy Jaber b, Jean L. Guth c, Ligia Sierra a,⇑a Grupo Ciencias de los Materiales, Sede de Investigaciones Universitarias (SIU), Universidad de Antioquia, Calle 62 No. 52-59, Lab 310, Medellín, Colombiab UPMC Univ. Paris 06, Laboratoire de Réactivité de Surface – CNRS UMR 7197, Tour 54-55, 4 Place Jussieu, 75252 Paris Cedex 05, Francec Institut de Science des Matériaux de Mulhouse (IS2M-LRC CNRS 7228), 15 rue Jean Starcky, 68057 Mulhouse Cedex, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 May 2012Received in revised form 29 January 2013Accepted 31 January 2013Available online 8 February 2013

Keywords:Block copolymers self-assemblyMicellar stabilizationSilica stabilized vesiclesTexturally biphasic mesoporous materialsMesoporous carbons

1387-1811/$ - see front matter � 2013 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.micromeso.2013.01.033

⇑ Corresponding author. Tel./fax: +57(4) 2196546 2E-mail address: ligia.sierra@gmail.com (L. Sierra).

We report a new synthesis of texturally biphasic carbon-silica composites and carbons that exhibit highsurface areas (up to 1500 m2 g�1), pore volumes (up to 1.9 cm3 g�1) and mesopores (4.6–6.9 nm). Thecomposite materials have ordered mesoporous carbon domains embedded in a disordered mesoporouscomposite carbon-silica phase. This biphasic texture is preserved for the carbons obtained after silicaetching. The synthesis involves the addition of tetraethoxysilane (TEOS) into an alkaline triblock copoly-mer (PEO140PPO39PEO140) aqueous solution, followed by addition of phenol and formaldehyde. The resol-silica composites, obtained after several days under stirring and heating, are pyrolyzed at 500 �C or800 �C. TEOS and resin monomers effects on the copolymer self-assembly to form micelles and vesicleswere studied by dynamic light scattering (DLS) before flocculation. We propose a formation mechanism,where siliceous species, are localized mainly in micelles outside the vesicles and in the vesicles bilayers,modifying their properties. The hydrophilic resol monomers are absorbed by micelles outside the vesiclesas well as by those inside the vesicles core. After flocculation, biphasic materials, with a resol containingphase embedded in a resol–silica composite phase, are obtained. The in-situ formation of silica reducesthe mesopores shrinking occasioned by pyrolysis in both the hybrid carbon-silica and carbon domainsalthough causes disorder in the pores of the hybrid domain.

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1. Introduction

The synthesis of ordered mesoporous carbons has technologicalinterest for many applications such as adsorbents, catalysts, cata-lysts supports and electrodes materials due to their high surfacearea, narrow pore size distribution and regular framework [1–7].

After the synthesis of ordered mesoporous silicas in 1992 [8–11], they were used as hard templates for producing mesoporouscarbons. Ryoo and co-workers first fabricated ordered mesoporouscarbon by a nanocasting method [12]. These methods have highproduction costs for large scale fabrication and limit the possibili-ties to modulate the desired porous characteristics. To overcomethese difficulties, direct templated syntheses of mesoporous car-bon frameworks, which employ organic polymer precursors andsurfactants assembly strategies such as the evaporation-inducedself-assembly (EISA) [13,14] and the aqueous surfactant–polymerself-assembly or hydrothermal approach [15–17] have been ex-plored. The EISA method can produce organized mesoporous car-bons in form of films or monoliths but with difficulties forimplementation at large commercial scale. By contrast, a facileaqueous route was developed by Zhao et al. with phenol and form-

ll rights reserved.

191049.

aldehyde as builders of the polymer precursor and carbon frame-work [18–20]. This aqueous route is generally carried out in aslightly alkaline medium (pH of the final mixture between 8 and9).

In spite of the large number of articles about mesoporous car-bon syntheses, still more research is needed to understand the roleof the different parameters on the mesoporous structure andtherefore to control them in order to modulate and get larger porediameters and volumes. Specifically, the behavior of the Pluronicsurfactants under the mesoporous carbons synthesis conditionshas not been reported. Also the adjustment of the polymer poly-condensation to a value able to stabilize such behavior is an impor-tant factor that the literature does not take into consideration [21].

Different strategies, based on the presence of silica precursors inthe reaction mixtures for the direct synthesis of mesoporous hy-brid and carbon materials, can be found in the literature. For in-stance mesoporous carbon–silica nanocomposites and carbonsare obtained from polymerization of a phenolic resin in the pres-ence of Pluronic F127 and TEOS by an aerosol process [22], andby the EISA method [23]. The silica plays the role of co-template,increasing micelle stability, as well as of framework modifier orhardener. After carbonization of the organic matter in the compos-ites, the silica is dissolved in HF, leaving a mesoporous carbonwhich has interesting properties as a component material in sup-ercapacitor applications [24].

54 C.F. Santa et al. / Microporous and Mesoporous Materials 173 (2013) 53–63

New macroporous and large mesoporous silica materials withpromising applications were obtained by vesicular templating[25–28] and especially by using polymersomes [11,29–34]. Morerecently, the same strategy was applied to search new porous poly-mers, carbon–silica composites and carbon materials [35–38]. Por-ous carbon materials, with a multilayer vesicle structure, weresynthesized from an aqueous mixture of resol, prehydrolyzedTEOS, Pluronic F127 and 1,3,5-trimethylbenzene [38]. The obtainedpolymer-silica composite could be transformed into carbon—silica,carbon and silica materials by usual procedures.

We explore a method to prepare porous carbons and carbon—silica composites in an alkaline medium, using TEOS and phenoland formaldehyde as silica and carbon sources, respectively. Thetriblock copolymer PEO140PPO39PEO140, with long hydrophilicchains, was used as pore template. The effects of TEOS, phenoland formaldehyde on the self-assembly of the triblock copolymerat the synthesis conditions, were studied using DLS. The effectsof TEOS/copolymer molar ratio, and the carbonization temperatureon the morphology, texture and structure (as studied by TEM andSEM) and pore characteristics (evaluated from their N2 adsorptionisotherms) of the materials were studied. The results indicate thatthe materials obtained exhibit a new mesoporous biphasic texture.A formation mechanism that explains the origin of this mesopor-ous biphasic texture is proposed.

2. Experimental section

2.1. Chemicals

All chemicals were used as received. The large hydrophilic(LH) triblock copolymer named here LH-14,600 correspondsto poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), average Mn �14,600, PEG, 84.4 wt.%,PEO140PPO39PEO140, catalog product number 542342, batch12924ME, from Sigma–Aldrich Chemical Co. Tetraethoxysilane(TEOS, 98%) was also purchased from Sigma–Aldrich ChemicalCo. A commercial formaldehyde solution (37%) was used. SodiumHydroxide and Phenol were purchased from Merck and Co.Millipore water (18.2 MX cm) was used in all experiments.

1 Unpublished results.

2.2. Synthesis of composite and carbon materials

Three aqueous alkaline solutions of LH-14,600 were preparedby dissolving the surfactant in 30 mL 0.05 M NaOH solution(H2O/LH-14,600M ratio of 16,900). The pH of these solutions was12.7. Then TEOS was added to the three solutions, under stirring,in order to obtain TEOS/LH-14,600M ratios of 30 (solution S30),50 (solution S50) and 70 (solution S70). The solutions were heatedby an oil bath at 45 �C, under magnetic stirring (200–300 rpm),for 18 h. Then phenol and formaldehyde were added to obtainthree solutions with molar ratios: X TEOS/1surfactant/60phenol/292formaldehyde, where X = 30, 50 or 70. The reaction mixtureswere initially heated, under stirring, at 45 �C for 1 h, and then at67 �C for 143 h. The solids obtained in the bottom of the flaskswere dispersed and recovered by centrifugation, washed withwater and dried at room temperature overnight (as-synthesizedmaterials RSi30, RSi50 and RSi70). The porous composite carbon–sil-ica materials were obtained by carbonization (pyrolysis) of the as-synthesized samples at 500 �C (samples CSi30–500, CSi50–500 andCSi70–500) or at 800 �C (samples CSi30–800, CSi50–800 and CSi70–800)for 3 h, in a horizontal tubular furnace under a N2 flow rate of100 cm3/min and a heating rate of 2 �C/min. The silica in thesecomposites was removed by acid treatment in a 9% HF solutionduring 3 days to obtain the carbon materials C30–500, C50–500 andC70–500 or C30–800, C50–800 and C70–800.

For comparison, two materials named C0–500 and C0–800 weresynthesized under the same experimental conditions but withoutTEOS addition.

2.3. Effects of TEOS and the resol monomers on the behavior of thesurfactant

The copolymer assemblies, present in solutions S30, S50 and S70,were observed for 18 h following their preparation by dynamiclight scattering (DLS) experiments keeping the DLS cell at 45 �C.After these 18 h, phenol and formaldehyde were added to obtainthe solutions S30R, S50R and S70R with 1surfactant/60phenol/292formaldehyde molar ratio. The reaction mixtures were kept un-der stirring for 1 h at 45 �C, and then for 72 h at 67 �C. The copoly-mer assemblies present in each solution were observed by DLSduring this time keeping the DLS cell at 60 �C.

2.4. Measurements

DLS measurements of the precursor solutions were carried outin a DLS particle size analyzer Horiba LB-550.

The carbon–silica composite and carbon materials were charac-terized by scanning electron microscopy (SEM) (JEOL, JSM 6490LV20 kV), and nitrogen sorption (Micromeritics ASAP2010) at 77 K.TEM experiments and EDX analysis (JEOL JEM-100 CXII electronicmicroscope operating at 200 keV) were carried out as follows: afew mg of powder was mixed in a Beem capsule with Agar 100embedding resin. After polymerization at 60 �C during one night,the blocks were cut using a microtome equipped with a diamondknife. The ultra-fine slices, having a thickness of about 50 nm, wererecovered on copper grids.

3. Results and discussion

3.1. LH-14,600 block copolymer behavior in alkaline medium

A previous study shows that this copolymer in alkaline mediumat temperatures P40 �C, can form micelles accompanied by med-ium and large vesicles1. The medium vesicles can be formed bythe insertion of unimers to the micelles [39] or by the fusion of mi-celles [39–41] and are transformed continuously with stirring timeby means of fusion processes to form larger vesicles, which in turncan suffer fission to form smaller vesicles [41,42].

At this pH there is probably a cosmotropic dehydration effect ofthe OH� anion [43,44] on the LH-14,600 micelles. This type of an-ions will compete with the ether groups, in the large hydrophilicchains of the surfactant micelles, for water molecules causing a de-crease in both the cloud point and the CMC and an increase in theattractive interactions between the micelles, which can lead to theformation of aggregates (vesicles) [43,44]. On the other hand thespontaneous transformation of these aggregates is influenced bythe high solventphilicity degree, which is derived from the highhydrophilic/lipophilic balance due to the long hydrophilic PEOblocks of LH-14,600 [44,45]. Probably the apparition of vesicles,seen at this pH value, is related to these phenomena.

The hydrodynamic diameters of the copolymer LH-14,600assemblies, present in the synthesis solutions, were determinedby DLS during the first 90 h. The aim was to show that, under theseconditions, the copolymer LH-14,600, associated to TEOS and tothe monomers of the resol, forms the ‘‘building blocks’’ (micellesand vesicles) proposed in the formation mechanism of thematerials.

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3.2. TEOS effect on the LH-14,600 block copolymer self-assembly inalkaline medium

Table 1 and Fig. 1 show the hydrodynamic diameter and the fre-quency of the copolymer assemblies present in solution S0 (reac-tion mixture before TEOS addition) and in the solutions S30, S50

and S70 (reaction mixtures after TEOS addition).The solutions S0, S30 and S50 show the presence of micelles (9–

12 nm) beside medium vesicles (180–280 nm). On the other handfor the solution S70, the micelles are not detected and new aggre-gates with larger size (>6 lm) are formed (Fig. 1d). In this solutionthe population of micelles could be much smaller and therefore itsdetection more difficult. According to these observations, it can beconcluded that the ability of the surfactant to form vesicles is pre-served in the presence of TEOS. Moreover by controlling the TEOS/LH-14,600M ratio, it is possible to stabilize the LH-14,600 micelles[46] or to promote their transformation to vesicles (optical andconfocal images of the vesicles are shown in supplementary infor-mation SI1).

3.3. Phenol and formaldehyde effect on the LH-14,600 block copolymerself-assembly in alkaline medium

After the solutions S30, S50 and S70 were stirred for 18 h at 45 �Cin the presence of TEOS, resol monomers were added to them in

Table 1Dh of the species present in the surfactant solutions at pH of 12.4 and with addition of TE

Solution Temperature (�C) Time (h) Micelles

Dh (nm) Frequency

S0 45 0 9 60S30 45 18 12 32S50 45 18 8 67S70 45 18S30R 67 41 40* 85

90S50R 67 41 38* 68

90 67* 77S70R 67 41 38* 76

90

* Aggregates of micelles.

Fig. 1. Dh of the species present in: (a) surfactant solution before the addition of TEOS.

the same way as it is done for the syntheses of the materials. Thesize of the copolymer assemblies, present in the solutions S30R,S50R and S70R, were measured by DLS until the flocculation started.As shown in Fig. 2 and in Table 1 the micelles are transformed intolarger entities, which are assigned tentatively to micelle clusters(40–70 nm), and the medium vesicles grow up. These results showthat the addition of resol monomers also preserves, as TEOS does,the ability of the surfactant to self-assembly into micelles andvesicles.

3.4. Biphasic mesoporous carbon-silica composites and carbons

3.4.1. Effect of TEOS/surfactant ratio, pyrolysis temperature and silicaremoval on the morphology and textural characteristics of thematerials

The SEM micrographs of the composite carbon–silica materialsCSi30–500 and CSi50–500 show aggregates or blocks without texturalporosity (size c.a 10 lm, Fig. 3a and b). Besides these aggregates,micron and submicron sized spherical particles are seen. The com-posite carbon–silica material CSi70–500 (Fig. 3c and d) exhibitsmainly large blocks (size >100 lm) with numerous macrocavitiesof different shapes and sizes. The resol–silica and the carbon–silicacomposites RSi70 and CSi70–500 present similar morphologies(Fig. 3c–e). The material C0–500, obtained without TEOS addition,shows only spherical particles (Fig. 3f).

OS (S30, S50, and S70) and resol monomers (S30R, S50R, and S70R).

Medium aggregates Large aggregates

(%) Dh (nm) Frequency (%) Dh (nm) Frequency (%)

180 40237 68280 33146 52 >6000 48443 15137 87 >6000 13751 32290 14 >6000 9600 19 >6000 5598 100

(b)–(d) Solution S30, S50 and S70, respectively, after 18 h at 45 �C of TEOS addition.

Fig. 2. Dh of the present species at 67 �C in: (a) S30R at 41 h, (b)S30R at 90 h, (c) S50R at 41 h, (d) S50R at 90 h, (e) S70R at 41 h and (f) S70R at 90 h.

Fig. 3. SEM images for (a) CSi30–500, (b) CSi50–500, (c) and (d) CSi70–500, (e) RSi70 and (f) C0–500.

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The aspect of the particles or the blocks does not change whenthe pyrolysis temperature is raised at 800 �C or when the silica isremoved with HF (SEM micrographs not shown).

The TEM micrographs of CSi30–500 (Fig. 4a and b), CSi50–500

(Fig. 5a and b) and CSi70–500 (Fig. 6a and b) show the presenceof ordered and disordered mesoporous domains (see also sup-plementary information SI2). The ordered domains are spheroi-dal inclusions embedded in the disordered phase (in somerare cases these inclusions are seen as disordered domains).The ‘‘waves’’ seen in the TEM micrographs come from the cut-ting with the diamond knife of the microtome. For the materialCSi30–500 it seems that the adhesion of the ordered inclusions isnot very strong: on several microtome slices there are holescorresponding to inclusions which were lost during the slicing(Fig. 4b).

The EDX analyses (Fig. 4c and d, Fig. 5c and d and Fig. 7b and c)show that the ordered embedded domains have much lower silicacontent than the disordered domains. The degree of pore organiza-tion in the embedded domains is affected by the amount of silicaformed in them. In the material prepared with TEOS/surfactantmolar ratio of 70, all of the inclusions exhibit by EDX a negligibleamount of silica and have the highest pore organization. In theembedded domains of the materials prepared with TEOS/surfac-tant molar ratio of 30 and 50 more presence of silica and thereforeless pore order is seen.

The biphasic texture and composition of the samples pyrolyzedat 800 �C are similar to those of the samples obtained at 500 �C(Figs. 6 and 7, and SI2). After treatment with HF, the biphasic tex-ture is preserved but the embedded domains tend to loss poreorganization (Fig. 6 and SI2).

Fig. 4. TEM micrographs of thin slices of sample CSi30–500: (a) disordered embedding phase (II) and ordered spheroidal embedded phase (I), (b) missing ordered phase. EDXanalysis (c) of the disordered embedding domain in b (circled area), (d) of the ordered phase in a (circled area).

Fig. 5. (a) and (b) TEM micrographs of thin slices of sample CSi50–500: ordered spheroidal inclusions (I) and disordered embedding domain (II), (c) and (d) EDX analyses of theordered embedded phase (area circled in Fig. 4b) and of the disordered embedding phase (area circled in Fig. 4a), respectively.

C.F. Santa et al. / Microporous and Mesoporous Materials 173 (2013) 53–63 57

The TEM micrograph of the reference material C0–800, synthe-sized in the absence of TEOS, shows only the presence of a singlenon-mesoporous phase (Fig. 8). This indicates that the TEOS playsan important role on both the formation of mesopores and the bi-phasic texture of the materials.

The pore size values estimated from the TEM micrographs(Table 2, see also SI2) of the embedded domains are between 4and 6 nm. The influence of the TEOS/surfactant molar ratio onthese pore sizes is weak. In most of the materials, the dissolutionof the silica with HF and the increase of the pyrolysis temperature

produce larger pores but less organized pore structure. The HFtreatment during silica removal not only changes the pore volumeand size but generates carbon roughness [47] in both the embed-ding and embedded part. This effect can be responsible of the de-crease in the pore order, of the embedded domains of thematerial C70–800, since its precursor CSi70–800 can offer more perme-ability to the acid during dissolution.

In the disordered embedding hybrid matter, which separatesthe inclusions, the pore size values are in average larger and exhi-bit a broader size distribution than in the embedded domains.

Fig. 6. (a) and (b) TEM micrographs of thin slices of CSi70–500 (c) and (d) TEM micrographs of microtomes of C70–500. The embedded ordered phase and the embeddingdisordered phase are identified, respectively, by the I and II.

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These values increase, in most of the materials, with TEOS/surfac-tant ratio and pyrolysis temperature and decrease with the silicadissolution.

3.4.2. Porosity characteristics of the materials pyrolyzed at 500 �C:effect of the TEOS/surfactant ratio and silica removal

The nitrogen adsorption–desorption isotherms for the referencematerial C0–500 (not shown here), correspond to a microporousmaterial (Table 3). Materials CSi30–500 and CSi50–500, present typeIV isotherms (Fig. 9) and have a significant mesoporous volume.The mesopore diameter is the same (5.5–5.6 nm) in both samples,but the BET surface area and the pore volume are larger in materialCSi50–500. After HF treatment, the type of the adsorption–desorp-tion isotherms does not change and a more significant increaseof the pore volume and surface area is seen for the materialC50–500, where more silica is involved in its preparation. Since theembedding material is formed by an interpenetrated network ofcarbon and silica, the silica removal allows better nitrogen adsorp-tion (less restricted pore opening) and therefore an increased porevolume and surface area. On the other hand, after silica etching thepore sized observed by N2 adsorption decreases. According to theTEM information (see Table 2), when silica is eliminated the porediameter increases in the embedded domain but decreases in theembedding domain for both samples. The unexpected pore size de-crease can be explained as a result of some slight carbon wall col-lapse resulting from the silica dissolution, which will partiallyocclude the cavities or pores (falling in effect of carbonnanoparticles).

Material CSi70–500 does not show mesoporosity by N2 adsorp-tion although it exhibits, by TEM, domains of organized mesopores

Fig. 7. a) TEM micrograph of thin slices of sample CSi70–800. (b) and (c) EDX analysis o

inside the carbon inclusions and non-organized ones in the sur-rounding hybrid matter. C70–500, which is obtained by extractingthe silica, does not exhibit either nitrogen adsorption mesoporosi-ty, indicating that even after silica removal the nitrogen does notaccess the mesopores. To explain these results one can suggestthat, in materials with 70 TEOS/surfactant ratio pyrolyzed at500 �C, there is a high concentration of still hydroxylated siliceousspecies in the hybrid domain that prevents the formation ofopened ways for having a complete decomposition and eliminationof the carbonaceous matter. This can also be the origin of theempty macropores (seen only in this material Fig. 3c and d), whichcan be formed by the pressure of the volatile decomposition prod-ucts. The treatment with HF eliminates the siliceous species butnot the remaining non decomposed carbonaceous matter. Thusthere is no significant improvement of the porosity after silicaetching.

3.4.3. Porosity characteristics of the materials pyrolyzed at 800 �C:effect of the TEOS/surfactant ratio and of the silica removal

The reference material C0–800 (synthesized without TEOS addi-tion) obtained by pyrolysis at 800 �C has lower microporous andtotal volume, and BET surface than C0–500 (Table 3). On the otherhand, for the materials synthesized with TEOS addition there is astrong increase in the mesopore volume accompanied by a weakincrease in mesopore size when they are pyrolyzed at 800 �C in-stead 500 �C. For example the material CSi70–800 exhibits a 3-foldincrement in the pore volume and a significant amount of mesop-ores. The results indicate that the higher pyrolysis temperature hasa positive effect on the porous characteristics for all materialsespecially for those prepared with TEOS/surfactant molar ratio of

f the ordered embedded (I) and disordered embedding domains (II), respectively.

Fig. 8. TEM micrograph of a thin slice of the C0–800 reference material.

C.F. Santa et al. / Microporous and Mesoporous Materials 173 (2013) 53–63 59

70. A more complete decomposition of the surfactant and resolpolymer and removal of the decomposition products, followed bya higher conversion of the resol polymer into carbon during thepyrolysis at 800 �C can explain those improvements. Other reasonscan be proposed, for instance at 500 �C siliceous species that arenot completely dehydroxylated interact strongly with the incom-pletely pyrolyzed resol. This interaction allows both species to re-main mixed. At 800 �C there is probably a local separation (at thescale of the PEO corona of the surfactant) between the dehydroxy-lated silica and the carbon due to the absence of strong interac-tions, allowing the densification of the carbon domains. For thematerials carbonized at 800 �C, the effect of the silica dissolutionis more pronounced. There is a strong increase of the pore volumeand size and BET surface for all carbon materials, especially forC50–800. The volume liberated by the dissolution of the silica besidemore dense carbon walls leads then to a significant increase of thepore volume and size. On the other hand the hysteresis loop of theisotherms also changes after the silica dissolution: the hysteresisloop of the composite samples has a sharp evaporation at P/P0 = 0.45, indicating that desorption takes place via a cavitationmechanism because the exit openings are smaller than the criticalsize of 4 nm. For the carbon materials obtained after silica removal,the desorption branch moves to higher P/P0 values as result of theincrease in the size of the pore entries, causing the mechanism toswitch from cavitation to pore blocking. This indicates that silica,in the composite materials is present at the entrance of the cavi-ties. Sample C70–800 (and in a less degree C50–800) shows a two-step

Table 2Porosity organization and pore diameters from TEM observations.

Samples Embedding matter

Pore arrangement Pore size (nm

CSi30–500 n. o. 2–4CSi30–800 – –C30–500 n. o. 1–2

C30–800 n. o. 1–2CSi50–500 n. o. 2–8CSi50–800 n. o. 3–4C50–500 n. o. 1–2C50–800 n. o. 1–2CSi70–500 n. o. 4–12CSi70–800 n. o. 6–8C70–500 n. o. 4–6C70–800 n. o. 1–4

C0–800 Pore arrangement

n. o.

n. o.: non-organized; o.: organized.

desorption branch which is indicative of different pore entriessizes. This is in agreement with the different pore sizes seen byTEM of the embedding and embedded regions.

The need of a higher carbonization temperature for carrying outa complete decomposition of the organic matter can also be relatedto the interaction of the siliceous species not only with the resolbut with the copolymer, which suggests that hybrid resol—silica—surfactant micelles are formed.

3.5. Formation mechanism

The characteristics obtained in the new hybrid and carbonmaterials can be explained as a result of the effect of TEOS andthe monomers of the phenolic resin on the self-assembly of theblock copolymer LH-14,600 in alkaline medium.

The formation mechanism is based on five steps as already pro-posed for the formation mechanism of mesoporous silica materials[48,49]: (1) incorporation of the TEOS and precursors of the pheno-lic polymer into the micelles and vesicles, (2) transformation intocolloidal particles, (3) flocculation into a liquid like phase, (4) coa-lescence and solidification by polymerization, and (5) transforma-tion into a porous solid by pyrolysis or calcination.

The materials C0, obtained without TEOS addition (Fig. 10), aremainly formed from LH-14,600 micelles covered with phenolic re-sin. These covered micelles are not stable against shrinking pro-cesses and the obtained solids pyrolyzed at 500 �C and 800 �C(C0–500 and C0–800) are microporous carbons with pore diametersaround 1 nm. Unlike the materials obtained in the presence ofTEOS, when the pyrolysis temperature is raised to 800 �C to obtainC0–800, a strong decrease in porous characteristics is observed (Ta-ble 3). This indicates that the porous structured formed in the ab-sence of TEOS, has not enough thermal stability, which causes theporous network to shrink considerably when the carbonization ismade at 800 �C.

C0–500 and C0–800 were prepared with a reaction mixture havingthe same molar composition than the one used by Zhang et al. toprepare the mesoporous carbon FDU16 [19], but with the triblockcopolymer LH-14,600 (PEO140PPO39PEO140) replacing the PluronicF127 (PEO106PPO70PEO106). The larger pore diameter measuredfor FDU-16 (3.2 nm) can be due to a better stability of PluronicF127 micelles but also to some differences in the preparation pro-cedure (for instance absence of the prepolymerization step of thephenol and formaldehyde, mixing temperature with thesurfactant).

Fig. 11 illustrates the formation mechanism of the mesoporousbiphasic carbon—silica composites and carbons, formed in the

Embedded matter

) Pore arrangement Pore size (nm)

o. 3–4– –o. 4–5n. o. 5–6n. o. 5–6o. (weak) 4n. o. 5–6n. o. 4–6n. o. 4–5o. 4o. 3–4o. 4n. o. 4–6

Pore size (nm)

1.5

Table 3Total pore volume (Vp), micropore volume (VM), BET surface area (SBET), micropore and mesopore diameters (Dmeso and Dmicro), of the hybrid andcarbon materials after pyrolysis at 500 or 800 �C.

Material Vpa (cm3/g) VM

b (cm3/g) SBETc (m2/g) Dmeso

d (nm) Dmicroe (nm)

C0–500 0.29 0.27 558 N.p. 1.3C0–800 0.15 0.13 224 N.p. 1.1CSi30–500 0.34 0.09 448 5.5 0.6–1.5C30–500 0.47 0.08 602 4.6 1.0–1.4CSi30–800 0.38 0.14 527 5.7 0.9–1.4C30–800 0.74 0.23 849 5.0–6.9 1.0CSi50–500 0.41 0.09 535 5.6 1.1–1.5C50–500 0.82 0.06 1006 5.2 0.8–1.4CSi50–800 0.43 0.06 516 5.8 1.2–1.5C50–800 1.98 0.04 1508 6.6 1.3CSi70–500 0.29 0.19 515 N.p. 0.7–1.3C70–500 0.32 0.20 576 N.p. 1.2CSi70–800 0.45 0.03 476 5.7 1.4–1.7C70–800 0.89 0.02 746 6.5 1.3

N.p.: pores of size �4–6 nm are not present.a Total volume at P/P0 = 0.9941.b as-Plot micropore volume (using a nonmicroporous reference carbon [45]).c BET specific area.d Mesopore diameter >2.0 nm (BJH theory).e Micropore diameter <2.0 nm (DFT theory).

Fig. 9. Nitrogen adsorption–desorption isotherms and BJH pore size distributions (insets) for the materials stemming from the as-synthesized samples: (a) CSi30, (b) CSi50, (c)CSi70, (d) C30, (e) C50 and (f) C70.

60 C.F. Santa et al. / Microporous and Mesoporous Materials 173 (2013) 53–63

presence of TEOS. According to the DLS results, the LH-14,600 sur-factant solutions contain under the conditions used in this study(45 �C, pH 12.7 and 4.8 wt.%) micelles and vesicles. The micellesare present in the solutions outside (external micelles) and inside(internal micelles) the vesicles. In order to explain the final forma-tion of a biphasic material, we make the assumption that the addedTEOS migrates preferentially into the external micelles and into thevesicular bilayers. Indeed the hydrolysis of the TEOS modifies thebehavior of the vesicles making them more stable, with less dy-namic changes. Moreover through the increase of the TEOS amountand of its hydrolysis, the vesicular bilayer becomes less permeableto the transit of the hydrophobic TEOS into the inner space of thevesicles.

The TEOS, due to its hydrophobic nature, is initially located inthe PPO core of micelles and in the PPO part of the vesicular bi-layer. The oligomeric and polymeric siliceous species resulting

from the subsequent hydrolysis and polycondensation of the TEOSmigrate to the PEO-PPO interfaces and PEO corona [48,49]. Due tothe low Si/surfactant molar ratios, low temperature and high pH,the density of the siliceous species in the corona of the external mi-celles remains low and therefore these incipiently covered micellesdo not flocculate into the precursor of mesoporous silica (the Si/surfactant ratio is around 200 in the reactions mixtures used forthe preparation of mesoporous silica [50]).

When the phenolic resin precursors are added, they are par-tially adsorbed on the micelles containing already siliceous species,but they diffuse also into the inner space of the vesicles helped bypermeability of the bilayer to hydrophilic species [51]. The increaseof the siliceous species concentration in the vesicles bilayers,which depends on the TEOS/surfactant ratio, will increase theirhydrophylicity favoring their permeability towards the phenolicresin precursors. Due to the very low amount of siliceous species

Fig. 10. Formation of microporous carbons as C0 in the absence of TEOS.

C.F. Santa et al. / Microporous and Mesoporous Materials 173 (2013) 53–63 61

covering the internal micelles, these are covered mainly by resol.According to the DLS results, after addition of the phenolic resinprecursors and before the flocculation, the external micelles seemto form small clusters with sizes between 40 and 70 nm.

We propose that during the flocculation the vesicles collapseand the external hybrid micelles surround the internal and morehydrophobic organic micelles without mixing. We do not excludethat after flocculation, the aggregates of the internal micelles cangrowth into larger aggregates by collision. During the transforma-tion of the liquid like phase into a solid phase, by the coalescence ofthe micelles and by the continuation of the polymerization reac-tions, the presence of siliceous species prevents the formation ofa periodically organized and regular embedding hybrid phase. Thisis not the case for the embedded phase formed from the innermicelles covered with resol: the absence of significant amounts

Fig. 11. Formation of mesoporous biphasic

of silica allows the ordering of the structure after pyrolysis at500 �C, especially in the case of the materials prepared with highTEOS/surfactant ratios, where the vesicle permeability to TEOSis very low. This explains why the carbon inclusions for thecomposite materials prepared with the highest amount of TEOS(TEOS/surfactant ratio = 70) exhibit the maximum pore organiza-tion. In the case of the composites with smaller amount of TEOS(TEOS/surfactant ratio = 30 or 50) both ordered and non-orderedporous inclusions can be observed.

The formation of large mesopores during pyrolysis is also re-lated to the presence of silica. In the hybrid phase its mixing withthe polymer forms a stable framework which avoids a strongshrinking of the mesopores.

In the carbon inclusions, a containment effect due to the sur-rounding of the hybrid matter, which leads to a retarded decompo-

carbon/silica composites and carbons.

62 C.F. Santa et al. / Microporous and Mesoporous Materials 173 (2013) 53–63

sition of the polymers during the pyrolysis, can explain the obtain-ing of the mesopores.

The increase of the silica amount strengths these effects (silica–resol mixing in the hybrid phase and polymer retarded decompo-sition in the embedded phase) but prevents a complete pore open-ing in the material with the highest amount of silica after pyrolysisat 500 �C: in CSi70–500, the decomposition of the polymer and theremoval of the decomposition products are incomplete.

Pyrolysis at 800 �C resolves this limitation, allowing the detec-tion of the mesopores inside CSi70–800, and improving the mesopor-ous characteristics of all the materials. As it was already proposed,the improvement can be due to a better decomposition of the or-ganic matter at 800 �C and to a local separation between the dehy-droxylated silica and the carbon, which allows the densification ofthe carbon domains at 800 �C.

The carbon materials, resulting of the dissolution of the silicawith HF, exhibit a significant improvement in mesoporous volumewith an increase in pore size for materials pyrolyzed at 800 �C but adecrease in pore size for materials carbonized at 500 �C. This de-crease can be due to a ‘‘falling in effect’’ resulting from the releaseof carbon species which were held together by siliceous species.

The empty macro pores with variable shapes present in thematerials prepared with the TEOS/surfactant ratio of 70, pyrolyzedat 500 or 800 �C (Fig. 3c and d), have their origin in the formation ofgaseous decomposition products which cannot be evacuated fastenough through the non-connected pores (these empty macrop-ores are absent in the as-synthesized materials).

4. Conclusions

Precursors of the biphasic mesoporous carbon–silica compos-ites and carbons were synthesized at 67 �C in alkaline aqueousmedium using the triblock copolymer LH-14,600 surfactant withlong hydrophilic chains, as pore template and TEOS and resol pre-cursors as silica and carbon sources. Composite materials withspheroidal mesoporous domains of ordered mesoporous carbonsembedded into a disordered mesoporous carbon–silica hybridmatter are obtained after pyrolysis in nitrogen at 500 �C or800 �C. After silica etching, the obtained carbons keep their bipha-sic texture and show a decrease of the pore size in the former hy-brid matter, an increase in the overall pore volume and surfacearea and a tendency to loss pore organization in the embeddedphase. The mesoporous ordering degree of the embedded phaseis improved with the increase of the TEOS/surfactant molar ratio.The porous volume, the pore size and the specific surface area ofthe materials are larger when they are pyrolyzed at 800 �C instead500 �C.

A formation mechanism that explains the biphasic texture,found by TEM and EDX, as well as the porosity of the obtainedmaterials was proposed. The mechanism is based on the study ofthe self-assembly of LH-14,600 copolymer upon the synthesis con-ditions. Especially the effect of alkaline medium, TEOS and resolmonomers on this behavior was studied. The effects of the pyroly-sis temperature on the porosity of the materials were alsorationalized.

The triblock copolymer surfactant used in this study is the ori-gin of the mesoporous characteristics in the pyrolyzed materials bythe formation of micelles, and it is also the origin of the biphasictexture of the materials by the formation of the vesicles. Neverthe-less, without the silica generated by the TEOS, the biphasic texturecannot be formed as it is shown in this study.

Silica plays another role allowing the formation of large sizedmesopores as well in the hybrid matter (anti-shrinking agent) asin the carbon inclusions (delaying the pyrolysis of the phenolicresin).

Acknowledgments

The authors thank Dr. Sandra Casale (Service Commun deMicroscopie, UFR Chimie, UPMC) for kindly carrying out the TEMexperiments. Authors also wish to thank to Programa de Sosten-ibilidad 2013–2014 of the Universidad de Antioquia (Medellín,Colombia) for the financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.micromeso.2013.01.033.

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