The porous network in carbon aerogels investigated by small angle neutron scattering

Carbon 45 (2007) 1185–1192 www.elsevier.com/locate/carbon

The porous network in carbon aerogels investigated by small angle neutron scattering N. Cohaut b

a,*

, A. Thery a, J.M. Guet a, J.N. Rouzaud b, L. Kocon

c

a CRMD, UMR 6619, CNRS-Universite´ d’Orle´ans, 1b Rue de la Fe´rollerie, 45071 Orle´ans Cedex 2, France Laboratoire de Ge´ologie, UMR 8538, Ecole Normale Supe´rieure – CNRS, 24 Rue Lhomond, 75231 Paris Cedex 5, France c CEA le Ripault, BP 16, 37260 Monts, France

Received 15 September 2006; accepted 9 February 2007 Available online 22 February 2007

Abstract Small angle neutron scattering treated with the Porod approach has been applied to compare the influence of catalysts (C = NaOH, Na2CO3 and Ca(OH)2) on the porous structure of resorcinol-formaldehyde (RF) carbon aerogels. Investigated parameters are the molar ratio (R/C varies from 10 to 800 mol/mol) and the pyrolysis temperature (1050 °C, 1700 °C and 2600 °C). At 1050 °C, carbon aerogels based on NaOH and Na2CO3 catalysts provide denser materials than with Ca(OH)2-based one, due to a three-dimensional network of smaller particles. The density of particles decreases with the amount of catalyst. At 2600 °C the development of an intraparticle microporosity, which is quantified, leads to a slight decrease of the interparticle mesoporosity noticed at 1050 °C. This effect is induced by a stiffness of carbon layers in polyhedral pore walls as illustrated by the feature of the chords length distribution g(r) and TEM micrographs. Ó 2007 Elsevier Ltd. All rights reserved.

1. Introduction Carbon aerogels have received considerable attention in the literature since the synthesis of organic aerogels performed 15 years ago [1,2]. Some reviews have already described the effect of synthesis and processing conditions on the structure of carbon aerogels [3–6]. These are promising materials in the field of energy storage as electrodes for supercapacitors [7–10] or in the environmental protection as adsorbents of the removal of heavy metals [11,12]. For these applications, the interesting feature of carbon aerogels is to form a network of interconnected carbon particles. Micropores are generated by the spatial disorder of carbon layers inside particles whereas the spatial distribution of particles generates mesopores. These textures can be directly imaged by transmission electron microscopy

*

Corresponding author. Fax: +33 2 38 25 53 76. E-mail address: [email protected] (N. Cohaut).

0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.02.020

but are difficult to quantify because of the projection of 3D structure on 2D images. Neutron or X-ray small angle scattering (SAS) techniques were extensively used to quantify the structure of carbon aerogels over a size range from 1 nm up to 80 nm [13–22]. SAS and diffraction profiles were even regarded as the target function for fitting in modeling approach [13,23,24]. SAS profiles of aerogels clearly display two distinct structural levels, corresponding to the intraparticle and the interparticle porosity which can be observed on TEM high resolution images. Each structural level can be ideally described by the surface area, porous volume and a mean size of particles. Concerning the mesoporous texture, the surface area is easily obtained from the Porod law [14,15]. Size parameters, deduced by different methods, usually need assumptions on the morphology and on the existence or not of interparticle interaction. For instance, the radius of gyration corresponding to the particle size is obtained by the Guinier law assuming a diluted system of particles i.e without interparticle interaction [16–18], this

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assumption is only relevant with highly porous carbon aerogels. Assuming that the structure of carbon aerogels is described by a model of connected array of clusters, each resulting from the aggregation of primary particles according to the model diffusion-limited cluster aggregation (DLCA) [25], the features of simulated SAS profiles allows to determine the mean cluster size from the position of the maximum observed on some profiles. The size and the degree of polydispersity of spherical particles can be determined from the position of oscillations noticed on the Porod plot [13,19,20,26]. In other publications [21,26], SAS curves were fitted assuming interference model of oblate globules to obtain the particle volume fraction, mean particle radius, interference distance and the local packing fraction. Otherwise, maximum entropy model permit to fit profiles with adequately assumed particle shape to obtain the particle size distribution [27]. The intraparticle structure is not obvious to be quantified by SAS since the two phase media model with a smooth interface is not available because of the disordered structure of carbon matter at 1050 °C, as displayed by TEM images [28]. Thus, the Porod law (q4 dependence) is not strictly observed on SAS plot. Nevertheless, some attempts were done to separate the intraparticle micropore surface area created during the pyrolysis from the mesoporous surface area using a model for inhomogeneous solid (Debye–Anderson–Brumberger model) [19,29] or a model assuming spherical voids or particles [17,30]. When a comparison is made with adsorptiometry results, SAS surface area and porous volume are often higher than values obtained by N2 or even CO2 adsorption, as only a part of the microporosity is accessible to molecules [17,22]. This paper presents a small angle neutron scattering (SANS) study of RF carbon aerogels based on the Porod analysis, not only to obtain the surface area and the porous volume fraction but also to determine a mean size of pores and particles without assumption on the shape. An analysis of the chord length distribution is proposed in relation with the type of catalyst and with the pyrolysis temperature. A comparison is done with structural and textural features obtained from the observation of TEM high resolution images. 2. Experimental 2.1. Aerogels synthesis The organic aerogels were synthesized using a sol–gel process which is derived from the methods developed by Pekala et al. [1]. Briefly, resorcinol (1,3-dihydroxybenzenze: C6H4(OH)2) and formaldehyde (HCHO) were mixed in 1:2 molar ratio with deionised water solvent (W) and with different basic catalysts (Ca(OH)2, NaOH and Na2CO3) to obtain a resorcinol-formaldehyde (RF) gel. The molar ratio between resorcinol and catalyst (R/C) has been fixed at different values (10, 45, 100, 200, 300 and 800). The gelation and ageing were performed during a few days at different temperatures (from about 25 °C to 55 °C), giving dark-red-transparent gels. The gels were rinsed with ethanol to eliminate water trapped within the pores. Ethanol was then exchanged with carbon dioxide in an autoclave, and the RF gel was transformed in RF aerogel by

supercritical drying, i.e. by increasing temperature and pressure above the critical point of carbon dioxide (Tc = 31 °C, Pc = 7.4 MPa). This method allows limiting shrinkage of the samples during solvent elimination, hence the porous structure of the gel is preserved. The gel is also less brittle and it keeps its macroscopic shape; that is important for some applications, such as shaping of electrodes. Argon atmosphere and a heating rate of 120 °C/h were selected for the elaboration of carbon aerogels, the soaking time at HTT (high temperature of treatment) of 1050 °C, 1700 °C and 2600 °C was half an hour.

2.2. TEM characterization The multiscale organization was directly imaged from micrometer to nanometer length scales by TEM (Philips CM20). After grinding in a small agate mortar, the carbon aerogel (few milligrams) was dispersed in anhydrous ethanol. A drop of this suspension was deposited on a TEM grid covered by a lacey amorphous carbon film. To get quantitative data from HRTEM images, the raw HRTEM images were first skeletonised, following an original procedure, and structural and microtextural data were extracted with user-friendly software developed in our laboratory [31,32].

2.3. SANS techniques SANS experiments were performed at the LLB laboratory on the PAXY line of the ORPHEE reactor (CEA Saclay). Samples were prepared by cutting aerogels into slices 1 mm thick at most. Three configurations (wavelength, sample-to-detector distance) allow to cover whole q-scattering vectors ranging from 0.04 to 4 nm. Recorded intensities are wavelength independent and thus not affected by multiple scattering effect. After normalization corrections, intensities (cm1) are expressed versus the q values (nm1), usually in a logarithmic scale. Because RF carbon aerogels contain hydrogen (<9 at%), we verified that incoherent scattering was negligible towards the high q coherent scattering. Definitions of parameters used in the scattering study are detailed below.

2.4. Porous volume fraction /p If /p and /s are, respectively the pore volume fraction and the solid volume fraction. /p may theoretically be obtained from SAS profiles with the so-called invariant Q (in cm4) [33]: Z 1 dr ðqÞdq ¼ 2p2 /p ð1  /p ÞðDqÞ2 q2  ð1Þ Q¼ dX 0 dr ðqÞ is expressed at the absolute scale where the scattered intensity dX 1 (cm ). Experimentally, integration is performed over a limited range (qmin, qmax). qmin is the lower experimental q-value and qmax = 10 nm1 is the upper limit after extrapolation assuming a q4 dependence [34] so that truncation effects are negligible. In Eq. (1), the mean fluctuation of neutronic density (Dq)2 supposed that the elemental composition and the density of the solid phase (qs), are known with a good accuracy since this parameters are related by the following equation [15]: P bi Dq ¼ N a  qs  P i ð2Þ iM i

where Na is the Avogadro’s number and Mi and bi, respectively, the atomic weight and the scattering length of atom i. If (2) is combined with (1), we obtain a new expression of the invariant Q which may be useful to determine qs, a value difficult to estimate for porous media. The solid volume fraction (1  P) is consequently obtained from SANS data and from qb, the bulk density of the aerogel measured from Hg porosimetry: qs ¼ qb þ qs ð1  /s Þ ¼ qb þ Q=ð2  p2  qb  C 2 Þ P bi with C ¼ N a  P i iM i

ð3Þ

N. Cohaut et al. / Carbon 45 (2007) 1185–1192

2.5. Mean chord length and chord length distribution

Sv is also dependent on the mean chord length named Porod length which is independent on the pore volume fraction:

The mean chord lp called the Porod length is obtained from the limit value of the Porod plot and from the invariant Q by the relation [34]:   p dr 4 ðqÞ ¼  lim q4  ð4Þ Qq!1 dX lp lp is then used to calculate g(r), the chord length distribution (CLD) [35,36] gðrÞ ¼

Z

1 0

00    lp 2 sin q  r 2  q4  IðqÞ    dq p qr 4p

ð5Þ

g(r) calculated from the small angle scattering describes statistically the pore-solid phases distribution in the medium. This function gives the probability of finding randomly located segments with a length r, in intersecting, the solid phase with the length ‘s the porous phase with the length ‘p or two (‘p + ‘s), three (‘p + ‘s + ‘p) or more consecutive chords according the relation: gðrÞ ¼ uð‘p Þ þ uð‘s Þ þ uð‘p ; ‘s Þ þ uð‘p ; ‘s ; ‘p Þ þ . . .

ð6Þ

If a dilute system in pores (or solid phase) is assumed, terms in Eq. (6) related to probabilities of finding two, three or more consecutive chords are so low that g(r) is directly related to the chord length distribution in pores u(‘p) (or solid phase u(‘s)) and the mean chord length may be estimated: Z 1 ‘pðsÞ ¼ ‘pðsÞ :uð‘pðsÞ Þ  d‘pðsÞ ð7Þ 0

The value of u(‘s) and u(‘p) at the origin provides information about the topology of the phase boundary, such as curvature and angularity whereas power law dependence of the distribution is observed with surface or mass fractals. In the case of a concentrated system, g(r) in Eq. (6) is equivalent to a CLD u(‘). The u(‘) first moment provides a mean chord length ‘CLD , with the same signification of the Porod length lP and related to ‘p and ‘s with: ‘CLD ¼ lP ¼ ‘p  ð1  /p Þ ¼ ‘s  /p

ð8Þ

Few authors used the chord length distribution function to study the morphology of porous carbon [37,38]. In some cases, the CLD may also be calculated from a binary thresholded images and compared with CLD obtained by SAS [39]. 2.5.1. Surface area The surface area per volume Sv (m2/cm3) is extracted from one of these equivalent expressions of the Porod law [34]: 1 dr ðqÞ lim q4  dX 2pðDqÞ2 q!1 p  /p ð1  /p Þ p  /p ð1  /p Þ dr ðqÞ ¼ or S v ¼ lim q4  K q!1 dX Q Q

1187

Sv ¼

ð9Þ

Sv ¼

4/p ð1  /p Þ lp

ð10Þ

3. Results and discussion 3.1. Aerogels carbonized at 1050 °C 3.1.1. Qualitative interpretation HRTEM micrographs in Fig. 1 clearly show that aerogels carbonized at 1050 °C display a two-scale organization. At low magnification (left image), the texture of aerogels seems to be composed by a juxtaposition of pseudo-spheroidal particles named ‘‘string-of-pearls structure’’ but a more localized observation reveals (middle image) that the strong interconnection of particles form nodules with rounded and irregular contours and generate well-opened and accessible mesopores. At the same time, a second electronic contrast arises from the organization of graphene inside particles. The higher magnification (right image) reveals that polyaromatic carbon layers are short and weakly stacked, either single or stacked usually by 2 or 3. This mutual disorientation of carbon layers generates micropores (<2 nm) and ultramicropores (<0.4 nm). Like in TEM, these two structural scales are also observed on SANS curves of aerogels observable in Fig. 2. From the lowest q-values, most scattering profiles show first a plateau and a less or more pronounced maximum, then an asymptotic power law decay in q4 and at least a slighter decrease with a power law exponent close to 2, with exact values reported in Table 1. Constant intensity in the vicinity of the origin proves that the largest dimensions of the carbon domains are reached by the range of the experiment and therefore a quantification is possible. Alternatively, the occurrence of a maximum at qm value (see Table 1) is noticed on some aerogels (NaOH R/C = 45, Na2CO3 R/C = 100, 200) and gives evidence for a correlated system described by a correlation length which is characteristic of the alternation between mesopores and carbon domains. In the intermediate range of curves, the decrease according to a power law in q4 arises from a sharp interface

Fig. 1. Transmission electron microscopy images of RF carbon aerogels synthesized with C = Ca(OH)2, R/C = 45 and HTT = 1050 °C. Left: texture at low magnification. Middle and right: texture at high resolution. Interparticle pores are designed by a black arrow, intraparticles by a white arrow.

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The q-position corresponding to the beginning of the Porod law is related to the size of scattering domains. The shift to the lower q-value reveals an increase of the carbon domains/pores as the R/C ratio. For the aerogel (Ca(OH)2, R/C = 800), the SANS curve does not reach a constant value because of the too large size of particles or pores. For q-values larger than 1.5 nm1, intensities decrease slighter because of this local scale, where particles are no longer regarded as having an homogeneous electronic density as previously seen on HRTEM images (Fig. 1).

4

10

3

-1

dσ/dΩ [cm ]

10

2

10

1

10

0

10

-1

10

4

5 6 7 8 9

2

3

4

5 6 7 8 9

0.1

2

3

4

1 -1

q [nm ]

Fig. 2a. SANS data of RF carbon aerogels (HTT = 1050 °C) in a log–log plot. C = NaOH: s R/C = 45. C = Na2CO3: - - - - R/C = 100; n, R/C = 200; m, R/C = 800.

10

-1 dσ/dΩ [cm ]

10

10

10

10

4

3

2

1

0

-1

10

4

5 6 7 8

2

3

4

5 6 7 8

0.1

2

3

4

1 -1

q [nm ]

Fig. 2b. SANS data of RF carbon aerogels (HTT = 1050 °C) in a log–log plot: C = Ca(OH)2 ___, R/C = 45; h, R/C = 100; , R/C = 800.

between mesopores and carbon domains. In this range, the Porod law is observed and the surface area may be deduced.

3.1.2. Quantitative interpretation Table 1 gathers the main parameters used to calculate the SANS structural data reported on Table 2. The bulk density qb of aerogels lies in the range 0.170–0.445 g/cm3 and tends to decrease with increasing R/C ratio, i.e. with the decrease of catalyst concentration, giving more porous aerogels, formed by larger carbon domains, and consequently with lower surface areas. The density of the solid phase which is equivalent to the density of a particle is very low, and decreases as the amount of catalyst. These values are quite lower than true densities reported on literature for RF carbon aerogels. Most true densities measured by helium replacement [14,22], or by benzene replacement [40] are ca. 1.9 g/cm3. Skeletal densities of RF carbon aerogels is also assumed to be equal to 2 g/cm3 [19]. On the contrary, densities of carbon particles determined from SAXS data [15] are lower than 1 g/cm3 and thus in the same order of our results. In fact, as helium and benzene enters into carbon particles microporosity, densities measured by helium pycnometry are always higher than densities, measured by SAS. Above q > 1.5 nm1, the increase of the neutronic contrast, i.e. the deviation from the Porod law (q4) is stronger for aerogels with lower particle densities qs. The porosity arising from this scale is well related to a weak association of aromatic carbon layers. The local maximum reached by the intensity on some spectra (NaOH R/C = 45, Na2CO3 R/C = 100, 200) always corresponds to the denser aerogels qb > 0.3 g/cm3, i.e. with the lower porous fraction

Table 1 SANS parameters used for the determination of structural data Catalyst-R/C

Temperature (°C)

Q invariant (1024 cm4)

K Porod limit (1028 cm5)

qm (101 nm1)

1/qm (nm)

Final slope

NaOH-45

1050 2600 1050 1050 1050 1050 1050 2600 1050 1050

0.0133 0.0246 0.0128 0.0089 0.0069 0.0139 0.0073 0.0127 0.0055 0.0034

4.4 35.0 4.0 2.0 0.7 1.7 1.5 11.8 0.8 0.2

2.3 ± 0.1 1.9 ± 0.2 1.8 ± 0.3 1.7 ± 0.3 No No No No No No

4.3 ± 0.2 5.3 ± 0.5 5.7 ± 1.0 6±1 – – –

– 3.7 – – – – – 3.9 – –

Na2CO3-100 Na2CO3-200 Na2CO3-800 Ca(OH)2-10 Ca(OH)2-45 Ca(OH)2-200 Ca(OH)2-800

No, not observed. Uncertainty in the parameters Q and K are estimated to be 7%.

– –

N. Cohaut et al. / Carbon 45 (2007) 1185–1192

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Table 2 Structural data obtained by SANS (T = 1050 °C) T = 1050 °C Catalyst-R/C

M (g)

qb (g/cm3)

qs (g/cm3)

/p

Sv (m2/cm3)

Sm (m2/g)

‘P ðnmÞ

‘p ðnmÞ

‘s ðnmÞ

‘CLD ðnmÞ

NaOH-45 Na2CO3-100 Na2CO3-200 Na2CO3-800 Ca(OH)2-45 Ca(OH)2-200 Ca(OH)2-800

11.073 11.54 11.73 12 11.16 11.16 11.16

0.445 0.418 0.308 0.230 0.200 0.170 0.263

1.00 0.97 0.81 0.76 0.92 0.79 0.49

0.56 0.57 0.62 0.70 0.78 0.79 0.47

255 239 166 71 113 77.5 50

575 571 540 309 565 456 190

3.8 4.4 6.0 12.2 6.2 8.3 14.4

8.8 9.5 14.9 38.9 27.6 40.7 39.0

6.8 7.2 9.1 17.3 7.7 10.9 40.8

4.4 4.1 5.6 11.9 6.1 8.6 20.0

Uncertainty in the experimental values is estimated to be 10%.

-3

Chord length distribution g(r)

20x10

15

10

5

0 10

20

30

40

r[nm]

Fig. 3. Chord length distribution g(r) of carbon aerogels at HTT = 1050 °C. C = NaOH: -s-, R/C = 45. C = Na2CO3 3, R/C = 100; n, R/C = 200; m, R/C = 800. C = Ca(OH)2, 3, R/C = 45; h, R/ C = 200; , R/C = 800.

(/p < 0.62) and close values of mean chords in pores and solid ‘p and ‘s . The functions g(r) obtained by all aerogels are plotted in Fig. 3. As previously said, in the general case, the shape of g(r) combines morphological features of both phases. But in case of two phase media, where the amount of one phase is very low, g(r) can be regarded as the distribution of chords in the dilute phase. In that particular case, the value of the distribution at the origin, g(0) = 0 or g(0) > 0, gives information on the morphology of the dilute phase, spheroidal or angular shape, respectively. According to HRTEM images of aerogels, pores possess rather angular edges generated by the curved morphology of carbon domains. In the mean time pore volume fractions given in Table 2 indicate that carbon particles form the dilute phase. In that case, g(r) features depend on the morphology of the carbon domains and thus should tend to 0 at the origin because of the rounded form of particles. On the contrary, as g(r) plotted on Fig. 3 are positive at r = 0, they are also sensitive to the angular morphology of pores. We conclude that none of the studied aerogels verify the

assumption of a diluted system in particles, so that the CLD g(r) combines the morphological characteristics of both phases. Consequently, the mean chord length obtained from the first moment of the CLD g(r) gives ‘ and not directly ‘s and ‘p which have to be calculated from ‘ or lP (Eq. 8). ‘s and ‘p values reported in Tables 2 and 3 are calculated from ‘p . On denser aerogels (NaOH R/C = 45, Na2CO3 R/C = 100, 200), the CLD in particles are so narrow that morphological features of this phase have a predominant effect on g(r) leading to a well defined maximum. It is noticeable that the qm position of the maximum gives a distance 1/qm very close to the mean chords ‘P and ‘CLD , respectively calculated from the Porod law or from the first derivative of the distribution g(r). Finally, with similar R/C ratio, aerogels obtained from Na2CO3 and NaOH show similar structural characteristics, approximately the same size for mean chords in pores and in matter and a narrow PSD of particles. The use of Ca(OH)2 as catalyst leads to a larger PSD in particles and a 3D organization involving larger pores and subsequently lower surface area. 3.2. Aerogels carbonized at 1700 °C and 2600 °C 3.2.1. Qualitative interpretation The modification of the texture as the carbonization temperature rises was followed with NaOH and Ca(OH)2 catalysts. In HRTEM images obtained at 2600 °C (Fig. 4), carbon layers are even longer and form planar walls of polyhedral pores. Walls are formed by a limited number of layers from 2 up to 6. Log–log and Porod plots obtained at 1050 °C, 1700 °C and 2600 °C are compared in Figs. 5 and 6, respectively. The increase of the pyrolysis temperature causes changes mainly in the highest q region: in the log–log plot, the deviation of the Porod law noticed at ˚ 1 turns into a well defined hump, 1050 °C from q > 0.15 A slightly translated towards smaller q values and with a final slope close to 4 (or a plateau in the Porod plot). This feature shows that new interfaces are observed at 2600 °C by the parallel alignment of a defined number of carbon sheets, inside domains regarded as highly disordered at 1050 °C.

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Table 3 Structural data obtained by SANS (T = 2600 °C) Catalyst-R/C ratio

qb (g/cm3)

qs (g/cm3)

/p

Sv (m2/cm3)

Sm (m2/g)

‘P ðnmÞ

NaOH-45 Whole porosity Interparticle porosity

0.44 0.44

1.53 0.87

0.67 0.49

772 275

1735 618

1.0 3.6

Ca(OH)2-45 Whole porosity Interparticle porosity

0.20 0.20

1.40 0.73

0.84 0.73

313 146

1563 730

1.5 5.4

‘p ðnmÞ 3.7 7.2 11 20

‘s ðnmÞ

‘CLD ðnmÞ

1.5 7.4

1.5 4.2

1.8 7.5

1.5 5.4

Uncertainty in the experimental values is estimated to be 10%.

6x10

-3

4

4

Porod plot : q .dσ/dΩ

5

Fig. 4. High resolution transmission electron microscopy (HRTEM) images of RF carbon aerogels synthesized with catalyst ratio R/C = 45 and HTT = 2600 °C. C = Ca(OH)2 (left); C = NaOH (right); Intraparticles pores are designed by white arrows.

3

2

1

0 10

0

4

5

10 2

15

-2

q [nm ]

-1

dσ/dΩ [cm ]

10

10

10

10

Fig. 6. SANS data of RF carbon aerogels (R/C = 45) in a Porod plot versus the HTT. C = NaOH: s, 1050 °C; +, 1700 °C; j, 2600 °C; C = Ca(OH)2: 3, 1050 °C; - - - 2600 °C.

3

2

1

0

-1

10

4

5 6 7 8

2

3

4

5 6 7 8

0.1

2

3

4

1 -1

q [nm ]

Fig. 5. SANS data of RF carbon aerogels (R/C = 45) in a log–log plot versus the HTT. C = NaOH: s, 1050 °C; +, 1700 °C; j, 2600 °C; C = Ca(OH)2: 3, 1050 °C - - - 2600 °C.

3.2.2. Quantitative interpretation To take account for new pores developed by the carbonization at 2600 °C, we have to consider the whole scattering pattern to estimate the total porosity /p, the whole surface area Sv and the density of the carbon solid phase qs (Table 3). /p is higher at 2600 °C. Hence, the better ordering of carbon sheets produces a significant increase of the density of the solid phase. Densities are lower than densities of non graphitizable carbons treated at high temperatures [28]. Nevertheless, carbon interlamellar spaces deduced from

measured SANS qs densities are close to 0.5 nm i.e in agreement with the results obtained from HRTEM image analysis. These quite low carbon densities may be explained by the morphology of pores walls : a large amount of edges limits planar extension in pores walls and thus constrain the improvement in ordering in stackings. The appearance of an intraparticle porosity at 2600 °C is responsible for a large increase of the total surface area Sv. The size of new formed pore walls is so small that only 3–4 carbon layers are stacked inside, which is also in agreement with TEM observations. In reason of the bimodal distribution of pores size (inter- and intraparticles), the mean chord in pores ‘p is an average value between size of interparticle mesopores and smaller intraparticle pores, so that we are not able to determine singly the size of the intraparticle pores. Nevertheless, the mesoporous network of aerogels can be quantified as previously made at 1050 °C, i.e. neglecting the formation of the porosity generated by the textural evolution of carbon domains with HTT. This may be done by extrapolation of the first Porod law observed in the small qrange. In Table 3, the description of this primary distribution pores/solid phase is referred as interparticle porosity

N. Cohaut et al. / Carbon 45 (2007) 1185–1192 -3

18x10

Chord length distribution g(r)

16 14 12 10 8 6 4 2 0 0

10

20 r (nm)

30

40

Fig. 7. Chord length distribution g(r) of RF carbon aerogels (R/C = 45) at different HTT. C = NaOH s, 1050 °C; +, 1700 °C; j, 2600 °C. C = Ca(OH)2 3, 1050 °C; - - - - 2600 °C.

and qs is the density of a particle. Results show that the mesoporous fraction decreases slightly between 1050 °C and 2600 °C thus leading to a slight decrease in the mean size of pores. At 2600 °C, the density of the so-called solid phase qs is smaller than qs at 1050 °C since the measurement includes the porous intraparticle texture (polyhedral walls/pores). Fig. 7 compares the effect of heat treatments on the shape of g(r). With higher temperature of carbonization, it is noticeable that the presence of a maximum noticed on g(r) at 1050 °C disappeared or in others terms the value of g(r) near the origin increases. As previously explained, this feature arises from the formation of a large amount of angulous interfaces consecutively to the parallel stackings of carbon layers which form pores walls with polyhedral shape as seen on HRTEM images (Fig. 4). 4. Conclusion With the support of a multi-scale observation of textures by TEM, we quantified by SANS the porosity developed by RF carbon aerogels with different catalyst species and different catalyst ratios by SANS technique. For some carbon aerogels, the influence of the pyrolysis temperature is investigated. The porosity and the size of particles increase with R/C ratio. When using NaOH and Na2CO3 catalyst with low R/C ratio, the particle volume fraction is only a few times smaller than the pore volume fraction. The maximum value of the density reached by the carbon particles at 1050 °C is small i.e. below 1 g/cm3 and rises up to 1.53 g/cm3 at 2600 °C. Otherwise, the density of particles tends to decrease as the amount of catalyst. Thus, during the sol–gel polycondensation process of the RF hydrogel, the type of catalyst seems not only affect the reactivity of the resorcinol towards formaldehyde during the addition

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reaction but also the degree of cross-linked polymers in primary particles or clusters. The low bulk density of RF carbon aerogels at 1050 °C is formed by a network of low dense particles developing an interparticle mesoporosity more than 50%. As seen on TEM images, an intraparticle texture is evolved as the temperature increases to 2600 °C. This texture is composed by encapsulated pores, limited by angular pore walls. In addition to provide quantitative features on HTT closed porosity, SAS techniques allow to highlight on the morphology of phases. The pore volume fraction indicates the phase which has a predominant effect on the CLD function and CLD characteristics near the origin allow to conclude about the morphology of the concerned phase. For all aerogels, g(r) shows a slight or strong maximum and thus exhibits features of phases with rounded contours but near the origin, g(r) > 0 s is in favor of angular shape phases. Consequently, g(r) features depend on both phases morphologies even with the more porous one. TEM is then essential to know that pores are most probably the angulous phase pointed out by the CLD. As clearly seen on TEM, the angulous shape of pore walls is enhanced with HTT as the g(r) function rises up near the origin. For the less porous RF carbon aerogels, g(r) reaches a strong maximum for r 5 0 probably due to a narrow distribution of chords in the solid phase. Otherwise, the characteristic distance 1/qm obtained from the q position of the maximum noticed on SANS plots is close to the mean chord length ‘CLD . Acknowledgements We are grateful to the Leon Brilloin Laboratory, CEA (Saclay) for access to the PAXY line on the nuclear reactor Orphee and to the Conseil Re´gional de la Region Centre for partly supporting this research through a PhD grant. References [1] Pekala RW. Organic aerogels from the polycondensation of resorcinol with formaldehyde. J Mater Sci 1989:3221–7. [2] Pekala RW, Alviso CT, Kong FM, Hulsey SS. Aerogels derived from multifunctional organic monomers. J Non-cryst Solids 1992;145:90–8. [3] Fricke J, Emmerling A. Aerogels – recent progress in production techniques and novel applications. J Sol–gel Sci Technol 1998;13:299–303. [4] Shaheen A, Al-Muhtaseb SA, Ritter JA. Preparation and properties of resorcinol-formaldehyde organic and carbon gels. Adv Mater 2003;15:101–14. [5] Moreno-Castilla C, Maldonado-Ho´dar FJ. Carbon aerogels for catalysis applications: an overview. Carbon 2005;43:455–65. [6] Job N, Thery A, Pirard R, Marien J, Kocon L, Rouzaud JN, et al. Carbon aerogels, cryogels and xerogels: influence of the drying method on the textural properties of porous carbon materials. Carbon 2005;43:2481–94. [7] Fang B, Wei YZ, Maruyama K, Kumagai M. High capacity supercapacitors based on modified activated carbon aerogel. High capacity supercapacitors based on modified carbon aerogel. J Appl Electrochem 2005;35:229–33.

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