Graphitic mesoporous carbons synthesised through mesostructured silica templates

Carbon 42 (2004) 3049–3055 www.elsevier.com/locate/carbon

Graphitic mesoporous carbons synthesised through mesostructured silica templates Antonio B. Fuertes *, Sonia Alvarez Instituto Nacional del Carbo´n (CSIC), Department of Chemistry and Materials, P.O. Box 73, 33080 Oviedo, Spain Received 10 April 2004; accepted 11 June 2004 Available online 13 September 2004

Abstract In this paper the fabrication and characterization of graphitizable and graphitized porous carbons with a well-developed mesoporosity is described. The synthetic route used to prepare the graphitizable carbons was: (a) the infiltration of the porosity of mesoporous silica with a solution containing the carbon precursor (i.e. poly-vinyl chloride, PVC), (b) the carbonisation of the silica– PVC composite and (c) the removal of the silica skeletal. Carbons obtained in this way have a certain graphitic order and a good electrical conductivity (0.3 S cm 1), which is two orders larger than that of a non-graphitizable carbon. In addition, these materials have a high BET surface area (>900 m2 g 1), a large pore volume (>1 cm3 g 1) and a bimodal porosity made up of mesopores. The pore structure of these carbons can be tailored as a function of the type of silica selected as template. Thus, whereas a graphitizable carbon with a well-ordered porosity is obtained from SBA-15 silica, a carbon with a wormhole pore structure results when MSU-1 silica is used as template. The heat treatment of a graphitizable carbon at a high temperature (2300 °C) allows it to be converted into a graphitized porous carbon with a relatively high BET surface area (260 m2 g 1) and a porosity made up of mesopores in the 2–15 nm range. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Porous carbon; B. Carbonisation; C. Adsorption, X-ray diffraction; D. Electrical properties

1. Introduction Carbons which combine a good electronic conductivity with a large and accessible surface area are of great interest in certain emergent applications, e.g. as electrodes in double-layer electrical capacitors or as catalytic supports in low-temperature fuel cell systems. However, materials with these characteristics are difficult to synthesise. Thus, porous carbons (i.e. active carbons) are obtained from non-graphitizable materials and consequently they cannot attain a high electrical conductivity even after heat treatment at high temperatures. Although, graphitizable carbons such as those de*

Corresponding author. Tel.: +34 985 11 9090; fax: +34 985 29 7662. E-mail address: [email protected] (A.B. Fuertes). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.06.020

rived from certain polymers (i.e. poly-vinyl chloride), aromatic hydrocarbons (i.e. naphthalene, phenantrene, etc.) or mesophase pitch, do exhibit good electrical conductivities, they are unsuitable for developing porosity by means of the activation procedures used to prepare activated carbons. This is because these carbons are impervious to gasifying agents, which prevent the creation of pores. Recently several papers that describe the preparation of porous carbons with a graphitic framework have appeared. These materials were synthesised using non-conventional techniques such as: (a) catalytic graphitisation of carbons obtained from polymeric gels [1,2], (b) imprinting of silica nanoparticles into the matrix of a graphitizable carbon precursor [3] or (c) infiltration of a graphitizable carbon precursor into the porosity of mesostructured silica which is used as template [4].

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Following the first approach, Hyeon and co-workers prepared hollow graphitic carbon nanoparticles [1] and carbon nanocoils [2] by carbonising resorcinol–formaldehyde gels doped with cobalt and nickel salts. These metals act as catalysts for graphitisation and they facilitate the formation of graphite crystallites at relatively low temperatures (900 °C). The XRD patterns of these carbons show that a certain degree of graphitisation has been attained (d0 0 2  0.34 nm). The materials exhibit surface areas of 318 m2 g 1 (carbon coils) and 88 m2 g 1 (hollow carbon particles) and they are shown to be excellent supports for Pt/Ru electrocatalysts in DMFC (direct methanol fuel cells) systems. The second method has been explored by Li et al. [3], who prepared a graphitic mesoporous carbon by imprinting colloidal silica particles into mesophase pitch. The carbon obtained after carbonisation and subsequent heat treatment at 2400 °C has a specific surface area of 240 m2 g 1 and a pore volume of 0.7 cm3 g 1 (pore size 17 nm). These authors do not provide details about the degree of graphitisation attained by this material. In the template approach, the pore network of the carbon is an inverse replica of the skeletal of the inorganic material used as template. This technique, which permits a precise control of the carbon porosity has been widely used to fabricate non-graphitizable mesoporous carbon using different types of mesoporous silica materials (MSM) as templates [5–15]. The structural characteristics (i.e. surface area, pore volume, pore size, particle size, morphology, etc.) of carbons synthesised in this way can be modulated by selecting an appropriate template. With this technique porous carbons with a graphitic framework can be obtained if the porosity of the template is infiltrated by a graphitizable carbon precursor. Recently, Kim et al. [4] analysed this method and prepared ordered graphitic carbons by infiltrating the porosity of mesostructured silica materials (i.e. MCM-48, SBA-1 and SBA-15) with acetanaphthene. The infiltrated material is treated at 750 °C under pressure in an autoclave and then heated at 900 °C under vacuum. The resulting carbon shows a turbostratic structure with a d0 0 2 spacing of 0.36 nm. The textural properties (SBET, pore volume, pore size, etc.) of such carbons were not reported, so their porous characteristics cannot be evaluated. In this work, we report a novel route, based on the template approach, for the fabrication of porous carbons with a graphitic framework and a large surface area with an accessible porosity made up of mesopores. The synthesis of these materials was carried out by simply infiltrating, under ambient conditions (temperature and pressure), the porosity of a mesostructured silica with a solution containing a polymer, which was converted into a graphitizable carbon after the carbonisation step. Heat treatment of the graphitizable carbon at high temperature (2300 °C) gives rise to a porous car-

bon with a well-developed graphitic order. The polymeric material chosen to be infiltrated was a poly(vinyl chloride) (PVC). As templates, two different MSM were used, i.e. a well-ordered silica (SBA-15) and a silica with a wormhole pore structure (MSU-1).

2. Experimental section 2.1. Synthesis of the silica templates The SBA-15 silica was synthesised, under acid pH, as reported by Zhao et al. [16] by using a non-ionic oligomeric alkyl-ethylene oxide surfactant as structure-directing agent, Pluronic P123 (BASF). In a typical synthesis, the silica source (tetraethyl orthosilicate, TEOS, Aldrich) is added to an aqueous solution containing HCl and a surfactant (starting mole ratio: TEOS/surfactant/ HCl/H2O = 1/0.017/5.7/193). The mixture was magnetically stirred until the TEOS was dissolved. Then, it was placed in a closed Teflon vessel and stirred for 20 h at 35 °C. Next, it was subjected to aging in a temperature range of 105 °C for one day. The solid product was filtered, washed with distillate water, dried at 40 °C and calcined in air at 600 °C (2 °C/min) for 4 h. The MSU-1 silica materials were prepared in a twostep pathway as reported by Boissiere et al. [17]. Briefly, the silica source (TEOS, Aldrich) was added under stirring to a 0.02 M solution of a non-ionic polyethylene oxide surfactant Tergitol 15-S-12 (CH3(CH2)14(EO)12, Sigma). The pH of the solution was adjusted to around 2 by the addition of HCl 0.25 M. The solution was kept in a closed Teflon vessel for 18 h at 20 °C without stirring. Afterwards, a small amount of NaF was added and as a result the condensation of the silica occurred suddenly. The mole ratio of the synthesis mixture was: TEOS:T-15-S-12:NaF:HCl:H2O = 1:0.125:0.06:0.064:375. The mixture was hydrothermally treated for three days at 100 °C. The white precipitate obtained was filtered, dried and calcined at 600 °C for 4 h. 2.2. Preparation of porous carbons The carbons were synthesised by filling the silica porosity with a solution of PVC (MW  43,000; Aldrich) in 1-methyl-2-pyrrolidinone (15 wt.%) until incipient wetness. In order to facilitate the filling of the silica porosity, both the silica and the PVC solution were maintained at a temperature of around 70 °C during the impregnation step. The impregnated sample was carbonised under N2 at 800 °C (2 °C/min). The sample was held for 1 h at this temperature. In order to obtain a substantial amount of infiltrated carbon, the impregnation–carbonisation cycle was repeated several times. Finally, the resulting carbon–silica composite was im-

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mersed in 48% HF at room temperature for 15 h in order to remove the silica template. The carbon obtained as an insoluble fraction was washed with distilled water and then dried in air at 120 °C. The carbon samples were graphitized by heat treatment under Ar atmosphere up to 2300 °C (0.5 h). 2.3. Sample characterization X-ray diffraction (XRD) patterns at small (2h = 0.5– 5°) and wide (2h = 10–90°) angles were obtained on a Siemens D5000 instrument operating at 40 kV and 20 mA and using Cu Ka radiation (k = 0.15406 nm). Nitrogen adsorption and desorption isotherms were performed at 196 °C in a Micromeritics ASAP 2010 volumetric adsorption system. The BET surface area was deduced from the isotherm analysis in the relative pressure range of 0.04–0.20. The total pore volume was calculated from the amount adsorbed at a relative pressure of 0.99. The pore size distribution (PSD) was calculated by means of the Kruk–Jaroniec–Sayari method [18] applied to the adsorption branch. The structure of the silica and carbon materials was characterised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SBA-15 and MSU-1

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silica samples were identified as S and M respectively and the corresponding carbons were denoted as C–S and C–M. The graphitized carbon prepared from the C–S sample was labelled as CG–S. A non-graphitizable mesoporous carbon was used as reference material. This was prepared by using SBA-15 silica as template and polyfurfuryl alcohol as carbon precursor according to a method described elsewhere [14]. This carbon was denoted as C–S-ng. The electrical conductivity of the carbons was measured in a 1260 Solartron gain-phase analyser (frequency range 5Hz–1MHz) using the four-probe method.

3. Results and discussion Two mesostructured silica materials (i.e. SBA-15 and MSU-1), with different structural characteristics, were selected as templates in order to fabricate the porous carbons. The SBA-15 silica has a well-ordered porosity made up of uniform mesopores as can be deduced from the XRD diffraction pattern of SBA-15 in Fig. 1a. In contrast, the porosity in the MSU-1 silica is not as ordered and it has a wormhole-like structure. In consequence, this material exhibits a single XRD peak (Fig.

Fig. 1. XRD patterns in the low-angle region for (a) SBA-15 silica and the corresponding templated carbons and (b) MSU-1 silica and the corresponding templated graphitizable carbon.

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1b), which reflects the pore–pore correlation distance. Firmer evidence of the porous structure of the silica samples is provided by the N2 sorption isotherms. The pore characteristics of both silica samples, as deduced from the analysis the N2 isotherms, are listed in Table 1. The SBA-15 silica exhibits a porosity made up of mesopores of uniform size (maximum: 8.2 nm; full width at the half maximum: 0.65 nm). In contrast, the MSU-1 sample shows a heterogeneity in the size of the pores (maximum: 7.2 nm; full width at the half maximum: 5 nm). The SBA-15 and MSU-1 silica samples were used as templates to make mesoporous carbons. The morphology of the silica particles and their structural characteristics are preserved in the templated carbons. This can be deduced from the SEM and TEM images, and the XRD diffraction patterns obtained in the low-angle range (2h < 5°). Thus, the carbon replica C–M, like the MSU-1 silica, is made up of perfect spherical particles with diameters of 3 ± 1 lm (Fig. 2b). The XRD patterns of the templated carbons are shown in Fig. 1. The C–S sample obtained from the SBA-15 silica exhibits a diffraction pattern similar to that of the silica with three well-resolved diffraction peaks that are characteristic of a hexagonally ordered material. In contrast, the C– M carbon, like the MSU-1 silica, exhibits a single XRD peak, which is distinctive of materials with a wormhole pore structure (i.e. HMS, MSU) [19,20]. TEM microphotographs of the templated carbons are shown in Fig. 2. The images obtained for the C–S sample (Fig. 2c and d) evidence a hexagonally ordered framework and confirm that the structural order characteristic of SBA-15 silica is preserved in the templated carbon. Furthermore, TEM images of the C–S sample reveal the existence of voids between the carbon rods. This suggests that the SBA-15 silica porosity was not fully and uniformly filled by the carbon precursor. Moreover, the TEM image obtained for the C–M sample (Fig. 2f) clearly shows that this material has a disordered porosity with a wormhole structure. Fig. 3 shows the N2 sorption isotherms and the PSDs (inset) corresponding to the C–S and C–M carbons. The sorption isotherms of both samples exhibit broad capil-

lary condensation steps, which suggests that the porosity is made up of pores of a wide range of sizes. This is confirmed by the PSDs (Fig. 3, inset). It can be seen that both carbons have two pore systems made up of mesopores of different sizes. The mesopores network of size 3 nm has been generated by the removal of the silica walls. The other pore system is made up of larger mesopores (i.e. 7 nm for C–S and 16 nm for C–M). These mesopore are probably the result of the unfilled silica pores coalescing with those formed by the removal of the silica framework. The TEM images displayed in Fig. 2c and d provide clear evidence of this kind of mesopores in the C–S sample. The structural characteristics of these templated carbons are presented in Table 1. They have BET surface areas of >900 m2 g 1 and large pore volumes of 1.09 cm3 g 1 (C–S) and 1.6 cm3 g 1 (C–M). It can be observed that the carbons reported in this work have a lower BET surface area and pore volume compared to the materials obtained from a non-graphitizable carbon precursor (i.e. furfuryl alcohol or sucrose). This is evidenced by comparing the structural characteristics of C–S with those of the reference carbon (C–S-ng) (Table 1). The same results are obtained by comparing the physical characteristics of C– M with those corresponding to a carbon fabricated from MSU-1 silica using furfuryl alcohol as carbon precursor [12]. We believe that the differences mentioned above are due to the fact that the furfuryl alcohol, with respect to PVC, infiltrates the silica porosity more easily. This is probably a consequence of the relatively high viscosity of the PVC/NMP solution, which hinders its infiltration into the silica nanopores. The XRD patterns in the wide-angle region (10–90°) allow the graphitic nature of the synthesised carbon to be assessed. As shown in Fig. 4a, the graphitizable porous carbons obtained from PVC have well-defined XRD peaks at around 2h = 26°and 44°, which can be assigned to the (0 0 2) and (1 0) diffractions of the graphitic framework. In contrast, the non-graphitizable porous carbon derived from furfuryl alcohol (C–S-ng) shows very poorly formed XRD peaks, which evidence an amorphous carbon framework. Table 1 contains the structural parameters for these carbons, i.e. d(0 0 2) spacing

Table 1 Physical properties of silica samples and templated carbons Sample

Code

Silica

SBA-15 MSU-1

Carbon

C–S CG–S C–M C–S-ng

a b c

SBET (m2 g 1)

Vp (cm3 g 1)a

dKJS (nm)b

940 770

1.19 1.18

8.2 7.2

930 260 950 1790

1.09 0.34 1.60 1.43

3.0, 7 3, 7 3.2, 16 3.1

Total pore volume from N2 adsorption at p/po = 0.99. Maximum of PSD. Crystallite size along c-axis.

d(0 0 2) (nm)

Lc (nm)c

Electrical conductivity (S cm 1)

0.351 0.342 0.354 –

1.6 19.4 1.2 –

0.3 4.2 – 0.003

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Fig. 2. SEM (a: C–S; b: C–M) and TEM (c: C–S; d: C–S; e: CG–S; f: C–M) images for the templated carbons.

and the Lc crystallite size. The values of Lc calculated for the C–S and C–M samples indicate that around 4–5 graphene layers are stacked in a more or less parallel form. Table 1 also provides the values of electrical conductivity for the synthesised carbons. The sample obtained from PVC exhibits a conductivity of 0.3 S cm 1 which is two orders larger than that obtained for the nongraphitizable carbon C–S-ng (0.003 S cm 1). Clearly, this difference is a consequence of the fact that the carbon framework in the C–S sample has a certain degree of graphitic order. Carbons with a high degree of graphitisation can be obtained by heat treatment of a graphitizable carbon

at >2000 °C. Thus, the treatment of the C–S sample at 2300 °C gives rise to a carbon (denoted as CG–S) with a high degree of graphitisation. This can be deduced from the XRD pattern shown in Fig. 4b, which contains intense diffraction peaks at 26°, 43°, 53° and 78°. The value obtained for crystallite size along the c-axis (Lc) is as high as 19.4 nm while the electrical conductivity measured is 4.2 S cm 1. Heat treatment of the C–S sample at 2300 °C induces a loss of order in the carbon framework and a reduction in its porosity. Thus, in contrast to the C–S sample, which exhibits, at the XRD low-range angles three well-defined peaks, the X-ray diffraction pattern for

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Fig. 3. Nitrogen sorption isotherms and pore size distributions (inset) of templated carbons. Isotherm for C–M is vertically shifted by 200 cm3 g 1 for clarity.

the CG–S carbon shows a single broad peak centred at 1.5° (Fig. 1a). This indicates that the hexagonal order of C–S is lost after heat treatment but that nevertheless some degree of pore–pore correlation is preserved. The TEM image shown in Fig. 2e illustrates the disordered pore structure of the CG–S sample. Fig. 5 shows the N2 sorption isotherm and PSD (inset) for the CG–S carbon. From the structural characteristics given in Table 1 it is clear that heat treatment causes an important reduction in BET surface area and pore volume. Thus, the values obtained for the CG–S carbon are 260 m2 g 1 and 0.34 cm3 g 1 respectively. However, it is worth noting that the porosity remaining in the CG–S graphitized carbon is made up of mesopores of the same size range as those of the graphitizable C–S carbon. Moreover, CG–S exhibits a bimodal porosity as can be deduced from the PSD (Fig. 5, inset). These data suggest that the reduction in BET surface area and pore volume are probably due more to the pores closing than to pore shrinkage. Likewise, by comparing the PSDs of C–S (Fig. 3, inset) and CG–S (Fig. 5, inset), it can be inferred that the smaller mesopores (3 nm) are preferably closed during heat treatment.

4. Conclusions Graphitizable carbons with a large BET surface area, a high pore volume and a porosity made up of mesopores can be synthesised by means of the template technique by using mesostructured silica materials as templates. Thus, the silica porosity is filled with a carbon

Fig. 4. XRD patterns in the wide-angle region for (a) graphitizable and non-graphitizable carbons and (b) CG–S graphitized carbon.

precursor (PVC), which is converted into graphitizable carbon after the carbonisation step. The pore structure of the graphitizable carbons can be tailored as a function of the silica that is used as template. Thus, a carbon with a well-ordered porosity is prepared from SBA-15 silica, whereas a carbon with a wormhole pore structure is obtained if a MSU-1 silica is used as template. The synthesised graphitizable carbons have a certain graphitic order as can be deduced from the XRD patterns. Consequently, they have an electrical conductivity (0.3 S cm 1), which is two orders larger than that of a nongraphitizable carbon. Heat treatment of the graphitizable carbon at high temperature (2300 °C) gives rise to a porous carbon with a well-developed graphitic order. This treatment leads to a significant reduction in the BET surface area and pore volume with respect to the graphitizable sample. However, instead of this reduction the graphitized carbon

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Fig. 5. Nitrogen sorption isotherm and pore size distribution (inset) of graphitized CG–S carbon.

retains an appreciable BET surface area (260 m2 g 1) and a porosity made up of mesopores.

Acknowledgments The financial support for this research provided by the Spanish MCyT (MAT2002-00059) is gratefully acknowledged. We thank Dr. J.M. Rojo (ICMM, CSIC) for the electrical conductivity measurements.

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