Accepted Manuscript Triple Hierarchical Micro-Meso-Macroporous Carbonaceous Foams Bearing Highly Monodisperse Macroporosity Simona Ungureanu, Marc Birot, Hervé Deleuze, Véronique Schmitt, Nicolas Mano, Rénal Backov PII: DOI: Reference:
S0008-6223(15)00383-8 http://dx.doi.org/10.1016/j.carbon.2015.04.092 CARBON 9891
To appear in:
Received Date: Accepted Date:
5 January 2015 28 April 2015
Please cite this article as: Ungureanu, S., Birot, M., Deleuze, H., Schmitt, V., Mano, N., Backov, R., Triple Hierarchical Micro-Meso-Macroporous Carbonaceous Foams Bearing Highly Monodisperse Macroporosity, Carbon (2015), doi: http://dx.doi.org/10.1016/j.carbon.2015.04.092
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Triple Hierarchical Micro-Meso-Macroporous Carbonaceous Foams Bearing Highly Monodisperse Macroporosity
Simona Ungureanu,1 Marc Birot,2,* Hervé Deleuze, 2 Véronique Schmitt,1 Nicolas Mano1, Rénal Backov1,*
Centre de Recherche Paul Pascal, UPR 8641-CNRS, 115 Avenue Albert Schweitzer, 33600
Pessac, France. 2
Université de Bordeaux, Institut des Sciences Moléculaires, UMR 5255-CNRS, 351 cours de la
Libération, 33405 Talence, France.
Abstract The limited coalescence phenomenon occurring in Pickering emulsions stabilized by solid particles allows preparing monolithic silica foams with nearly monodisperse macroscopic voids and also meso- and microporosities. After soaking of these foam hard templates with a phenolic resin, partially graphitized interconnected porous carbon monoliths can be obtained easily. Like the silica templates, these carbon monoliths possess a hierarchical, triple porosity. Mercury intrusion porosimetry reports a macropore volume fraction of 45–70 % with a narrow pore size distribution, while their BET specific surface area values lie between 700 and 900 m2·g–1and their BJH mesopore specific surface area between 200 and 500 m2·g–1. This approach allows preparing
Corresponding authors: [email protected]
(Renal Backov); [email protected]
hierarchical porous carbon whose mesoporosity does not require using any soft template to organize the resin precursor.
1. Introduction The design of materials holding hierarchical porosity is a strong and competitive field of research. Indeed, hierarchical porosity is encompassing several advantages; macropores provide fast fluid motion through a convection mode, while micro-mesopores, where fluid hydrodynamics is addressed via diffusion and dispersion modes respectively, account for high surface area and reactivity. In this vein, porous carbon materials, providing both chemical inertness and thermal stability, are attractive candidates as adsorbents  catalyst supports [2,3], electrodes for batteries [4-6], double-layer capacitors [7-9], or host sites for hydrogen storage [10-14]. For the last decade, advances in soft and hard-templating techniques have offered tunable pore sizes and structures. Powdered ordered mesoporous carbons (OMC) have been obtained through insertion of carbon precursors into various inorganic hard templates such as zeolites , MCM-48 , SBA-15 [18-20], or HMS [21,22]. Hierarchical porous carbon monoliths have also been prepared using colloidal crystals [23,24], meso-macroporous synthetic silica molds [5,25,26], or bio-templates . Other soft-templating approaches such as HIPE [28,29], (HIPE acronym refers to the high internal phase emulsion process employed to texture polymers ) or spinodal decomposition [31,32], have allowed the generation of hierarchical structures without the use of sacrificial harmful chemicals. Moreover, the synthesis of highly ordered OMC from evaporationinduced self-assembly (EISA) of polymers has opened new ways of texturing carbon materials [33-35]. Recently, several synergetic synthetic paths using both soft and hard templates have emerged. Mesoporous carbons have been prepared by EISA from the sol-gel polymerization of
silica in the presence of triblock copolymer soft-templates and resol-type carbon precursors [36,37]. Also, confinement of a concentrated precursor solution in a colloidal-crystal template , or evaporation-induced coating on polyurethane foams can be used to obtain mesomacroporous carbonaceous architectures [39,40]. Lately, the formation of silica colloids, generated through the Stöber process, allows obtaining macroporous carbons based on a selfassembly approach . From this interdisciplinary approach to rationally conceive functional materials has emerged the concept of “integrative chemistry” [42-44], combining general chemistry with physico-chemistry of complex fluids, as for instance air-liquid foams [45-47] or emulsions. Recently, hierarchical macro-mesoporous carbon foams have been obtained from silicone oil/resorcinol-formaldehyde precursors HIPE templates . Using emulsions as macroscopic molds and lyotropic mesophases as mesoscopic templates, our research group has developed a process to obtain mesoporous macrocellular silica foams, labeled Si(HIPE) , with a good control on the final macroscopic cell diameter and functionalities [50-52]. Later, these Si(HIPE) materials used as hard exotemplates have led to micro-macroporous carbonaceous C(HIPE) foams presenting outstanding properties . Recently, additional mesoporosity has been created in C(HIPE) foams by means of a cooperative templating approach with Pluronic mesophases as soft templates and Si(HIPE) foams as hard molds . One drawback of the Si(HIPE) foams is the wide distribution of the cells size, feature being circumvented by using Pickering-based HIPE process to obtain silica foams that we have labeled Si(PHIPE) . Here we report the synthesis and characterization of hierarchical macro/meso/microporous carbon foams from a hard template approach using Si(PHIPE) monoliths as molds and without employing mesophase soft templating agent to generate mesoporosity.
2. Experimental 2.1 Materials Hexadecane (99.9%) and tetraethylorthosilicate (TEOS) were purchased from Sigma-Aldrich, and the surfactant cetyltrimethylammonium bromide (CTAB) from Fluka. Aerosil silica nanoparticles R816 (diameter 12 nm) were provided by Evonik. Hydrofluoric acid (HF, 48 %) and hydrochloric acid (HCl, 37 %) were obtained from Prolabo. Ablaphene® RS101 (a resol-type formophenolic prepolymer in a hydroalcoholic solution) was purchased from Rhodia. All reactants were used as received without further purification.
2.2 Syntheses Si(PHIPE) synthesis . Aqueous phases were prepared by adding equal volumes of the particles dispersion (at various concentrations), HCl (37%) solution and TEOS under magnetic stirring. Then, hexadecane was emulsified at a 64%-80% volume fraction using 2.6, 4.8 or 8 mg of CTAB-modified silica particles per gram of oily phase. Emulsification was performed by means of an Ultra-Turrax homogenizer (T25 Janke & Kunkel) equipped with a S25 KV-25F rotor head operating at 12 000 rpm. The O/W emulsions were then left at rest for a 10 day period in order to fully complete the sol–gel process. In a second step, the monoliths were washed by immersion in a THF/acetone (7:3 v/v) mixture three times over a 24 h period in order to remove the aqueous and oily phases, and then dried slowly by air. To remove any remaining organic traces and to mechanically strengthen by sintering the silica macrocellular foams, the materials were thermally treated as follows: the first step consisted of a heating ramp at 2 °C· min–1 to 200 °C. Then, this temperature was held for 2 h before a second ramp at 2 °C· min-1 was imposed to reach 650 °C where the materials were left for 6 h.
Carbon foams synthesis. A monolith of Si(PHIPE) (0.6 g) was put into a solution of Ablaphene RS101 (60 wt% or 80 wt%) in THF. For a good soaking, the suspension was placed under vacuum until the effervescence disappeared. After 24 h aging at room temperature, the solution was filtered. The monolith was then quickly washed with THF and dried in an air oven at 80 °C for 24 h in order to initiate the thermally-induced reticulation of the resin. A second thermal treatment was performed at 155 °C for 5 h under air (heating rate of 2 °C· min–1), with a first plateau at 80 °C for 12 h, and a second one at 110 °C for 3 h. The cooling process was uncontrolled and directed by the oven inertia. Carbonaceous monoliths were synthesized by beginning with a pyrolysis step, carried out at 900 °C for 1 h under N2 flow (heating rate of 1 °C· min–1). Then, silica was removed by immersing the monolith three times in a 10 wt% HF solution, followed by extensive washing with deionized water. Finally, the monoliths were dried in an air oven at 90 °C overnight. Final carbonaceous foams are labeled
where "x" is the quantity of modified
silica particles per gram of oily phase, "y" is the quantity of oil phase (vol %) and "z" is the resin concentration (wt %).
2.3 Characterizations Scanning electron microscopy (SEM) observations were performed with a Hitachi TM-1000 apparatus at 15 kV. Specific surface areas were determined by N2 adsorption measurements on a Micromeritics ASAP 2010 analyzer. The samples (100–120 mg) were degassed for 30 h at 300 °C under vacuum (1–2 Pa) prior to analysis. The collected data were subjected to the BET  and
intrusion/extrusion mercury measurements using a Micromeritics Autopore IV 9500 porosimeter with the following parameters: contact angle = 130 °, mercury surface tension = 485 mN· m–1, 5
maximum intrusion pressure = 124 MPa. Transmission electron microscopy (TEM) experiments were performed with a Jeol 2000 FX microscope (accelerating voltage of 200 kV). Highresolution TEM (HR-TEM) micrographs were obtained with a Jeol 2200 FS microscope. The samples were prepared as followed: carbonaceous powders were deposited on a copper grid coated with a Formvar/carbon membrane. Raman spectra were recorded at 297 K on a Xplora confocal micro-Raman spectrometer (Horiba Jobin-Yvon), in backscattering geometry at 2.33 eV laser energy (532 nm) with a typical spectral resolution of 1.7 cm–1 (lens 50 ×). We managed to irradiate below an incident power of 1 mW in order to avoid sample damaging. Fouriertransformed infrared (FTIR) spectra were obtained using a Nicolet 750 FTIR spectrometer on KBr pellets of ground samples. Conductivity measurements were obtained through the four-point van der Paw technique, using direct current. A 2000 Keithley multimeter was used for conductivity (resistance) measurements while employing a Keithley 2220 precision current source apparatus. As final carbonaceous foams can be cut at ease, each sample has been cut into four discs (thickness 1.5 to 2 mm and constant diameter of 5 mm). Elemental analyses have been done by flame spectrophotometry (PF5000 spectrophotometer) in order to quantify residual silica (found to be lower than 0.5 wt%).
3. Results and discussion 3.1 Characterizations of carbonaceous foams at the macroscopic length scale At first, we can notice the persistence of the monolithic character when going from the starting Si(PHIPE) foam used as sacrificial hard template, through the intermediate material where the resin is pre-polymerized, to the final carbonaceous foam (Figure 1).
Figure 1 - Typical monolithic materials: a) the raw hard silica template
material after resin imbibition and polymerization and c) after thermal treatment at 900 °C and HF washing (example of 4.8/75C(PHIPE)60 material).
When looking at the macroporosity (Figure 2), the first characteristics to underline are the narrow distributions of the cells diameters regardless of the wt% of resin in use (60 wt% or 80 wt%), and that these diameters diminish when increasing the amount of silica nanoparticles employed during the synthesis of the Si(PHIPE) foams. These results rely on the limited coalescence scenario used to synthesize the Si(PHIPE) foams . Indeed, as the nanoparticles anchoring energy is very high (around 1500 kT), this anchoring is irreversible. In the particle poor regime, the droplet surface areas are not entirely protected, and so oil droplets coalesce. Therefore, once the nanoparticles surface covering of the oil/water interface is optimized, coalescence stops, promoting thereby a very high monodispersity of the oil droplets.
Figure 2 - SEM images of porous carbonaceous foams synthesized with increasing amounts of silica particles, internal phase volume fraction and carbon precursor. a)2.6/70C(PHIPE)60; b) 8
c)2.6/80C(PHIPE)60; d)4.8/70C(PHIPE)60; e)4.8/75C(PHIPE)60; f)4.8/80C(PHIPE)60; g)
represent 500 µm.
A second observation in Figure 2 is that porosity is open, with windows connecting adjacent cells. Qualitatively, we can observe at ease the connecting windows when performing HR-SEM investigations. Examples of connecting windows can be seen in the supplemental section (Figure S1). To address this feature more quantitatively, we have performed mercury porosimetry analyses (Figure 3).
Log Differential Intrusion (mL/g)
Pore size diameter (µm)
Figure 3 - Example of window size distributions obtained from mercury intrusion porosimetry for a constant amount of particles while increasing the oil volume fraction:
(empty squares); 4.8/75C(PHIPE)60 (empty triangles); 4.8/80C(PHIPE)60 (empty circles).
Figure 3 depicts the evolution of the windows diameter for a constant amount of particles while increasing the oil volume fraction. As for the cell diameters, the distributions are narrow. Secondly, we can see that the windows diameter is increasing when increasing the oil volume fraction at constant silica particles amount. This effect was expected because, as observed previously for the Si(PHIPE) series , when the oil volume fraction is increased, the surface of contact zone between adjacent droplets is increased too, promoting finally after calcination higher diameters of the windows connecting adjacent cells. Figure 4 shows the evolution of the windows diameter at a constant oil volume fraction while varying the amount of silica nanoparticles. Once again, we notice the narrow distributions of the diameters. Here we can see that the windows diameter is diminishing when increasing the silica nanoparticles amount, because it follows the same geometric trend as the cells diameter. Only examples of window diameter distributions are displayed in Figures 3 and 4 to clear the figures. The full set of windows size distributions is proposed within the supplemental section, Figure S2.
Log Differential Intrusion (mL/g)
Pore size diameter (µm) Figure 4 - Example of windows size distributions obtained from mercury intrusion porosimetry for a constant oil volume fraction while increasing the amount of nanoparticles used to prepare the silica hard templates: 8/70C(PHIPE)60
Beyond the macroscopic voids morphology, mercury porosimetry provides prior parameters: the foams' porous volume, as well as their bulk and skeleton densities displayed in Table 1.
Table 1 - Mercury intrusion porosimetry data of the x/yC(PHIPE)z carbonaceous foams Porosity
The results in Table 1 have to be compared with those of our previous study of C(HIPE)s , obtained from conventional emulsions as soft templating molds and not Pickering ones. As for the C(HIPE) series, C(PHIPE) foams present porosity between 40 and 70%. As it was the case previously, the higher the concentration of the resin solution (80 wt%), the lower the final porous fraction. The bulk and skeleton densities are roughly 30% lower for this C(PHIPE) series 12
than for the C(HIPE) one. This decrease of densities may be due to the presence of mesoporosity in the C(PHIPE)s, whereas C(HIPE) foams were essentially micro-macroporous , because the THF solution of the resin did not wet the Si(HIPE) walls to reach their internal vermicular mesoporosity. As already mentioned, to obtain mesoporous C(HIPE) foams, it was essential to use Pluronic mesophases as soft templates .
3.2 Characterizations of carbonaceous foams at the meso- and microscopic length scales The characteristics of the foams at the mesoscopic length scale have been examined through nitrogen physisorption measurements. Figure 5 depicts examples of adsorption-desorption isotherms as well as pores size distributions. Again, only typical examples are proposed to clear the figures, the whole set of physisorption curves can be found within the supplemental section as Figure S3. Where the C(HIPE)s series exhibited type-I isotherms associated to microporosity only , we can see in Figure 5 that C(PHIPE) isotherms are all mixed I-IV type with hysteresis loops.
Pore Width (nm)
Quantity Adsorbed (cm3/g STP)
Quantity Adsorbed (cm /g STP)
Differential Pore Volume (cm /g)
Quantity Adsorbed (cm /g STP)
Pore Width (nm)
Relative Pressure (P/P°)
Pore Width (nm)
Relative Pressure (P/P°)
Differential Pore Volume (cm /g)
Quantity Adsorbed (cm /g STP)
D ifferential Pore V olume (cm 3 /g)
Relative Pressure (P/P°)
Quantity Adsorbed (cm /g STP)
Pore Width (nm)
Relative Pressure (P/P°)
Figure 5 - Example of nitrogen adsorption-desorption isotherms and pore size distributions for the foams: a)2.6/75C(PHIPE)60; b)4.8/75C(PHIPE)60; c)2.6/75C(PHIPE)80; d)4.8/75C(PHIPE)80. Filled circles correspond to the adsorption curves and empty circles correspond to the desorption curves. Embedded figures represent the pore size distributions calculated by DFT.
This type of isotherms is distinctive of porous materials possessing both micro-and mesoporosity. This mesoporosity is poorly organized, vermicular-like (see TEM micrographs, supplemental section, Figure S4) similar to the Si(PHIPE) solid mold . This similarity of textural mesoporosities between the final carbon material and the starting silica template demonstrates that the THF solution of resin impregnates the Si(PHIPE) completely, i.e., both the macroscopic voids and the micro-mesoporous walls. Consequently, mesoporosity is now far from being negligible, representing between 30% and 60% of the BET specific surface area, as 14
depicted in Table 2. Furthermore, this mesoporosity does not required any sacrificial mesophase soft templating agent to be obtained. Table 2. Nitrogen physisorption data of the x/yC(PHIPE)z carbonaceous foams. BJH surface area are calculated from the desorption curves. BET surface area
BJH surface area
Total pore volume
Also, when deducing the BJH surface area values from the BET ones, we observe that the micropore surface areas are quite high, around 400 m2·g-1, resulting from both the THF departure during carbonization and the templating effect of the silica intrinsic microporosity (the SiO4 tetrahedra spatial repartition is statistic/random). Overall, we can state that these new carbonaceous foams possess a hierarchical triple micro-, meso-, and macroporosity. Because of the presence of silica nanoparticles at the surface of the macropores of the Si(PHIPE) hard templates and as wetting by the resin is complete, we also have investigated the walls roughness of this novel set of C(PHIPE) foams. Surface roughness is related with the surface fractal dimension (Ds) that can be deduced from the nitrogen adsorption isotherms. Ds is calculated according to the procedure described previously [58,59]. θ = K·[log(P0/P)]–ν
where ν = 3– Ds, θ is the relative adsorption calculated by normalizing the curve with the highest adsorption value, and K is a constant. An easy way to obtain Ds is to transform Eq. (1) in: log(θ) = log(K) – ν·log(log[P0/P])
The experimental adsorption data are replotted according to Eq. (2) and Ds is deduced from the slope of the line. It must vary between 2 (flat surface) and 3 (any value higher than 2 describes an increasing surface roughness). The adsorption range to be used for this analysis has to be taken within the partial pressure range 0.05 < P/P0 < 0.3 (corresponding to –0.28 < log(log[P0/P] < 0.11) and limited to below the Kelvin condensation step that corresponds to the pore filling [58,59]. Examples of resulting curves are proposed in the Figure 6a,b.
Figure 6 - Example of surface fractal analysis of the nitrogen adsorption isotherms: (a)2.6/75C(PHIPE)60; (b)2.6/75C(PHIPE)80. HR–SEM micrographs for (c)2.6/75C(PHIPE)60 and (d)2.6/75C(PHIPE)80.
Considering the Figures 6a,b we can extract the slopes of the fitted straight lines (red lines) from which the surface fractal dimension Ds is obtained: around 2.7 and 2.8 for the x/yC(PHIPE)60 and
series respectively. That is to say the walls surface should be very rough.
Roughness becomes evident when considering the SEM observations shown in the Figure 6c,d.
3.3 Characterizations of carbonaceous foams at the nanoscopic length scale First, it is important to verify that silica has been removed by the treatment with HF. This has been performed using infrared spectroscopy, as illustrated in the Figure 7. 17
Wavenumber (cm )
Figure 7 - Example of FTIR spectra: a) raw silica template
after carbidization at 900 °C and HF washing. Embedded is a zoom of the b spectrum in the region 1700-1050 cm–1. The black arrows indicate weak aromatic and sp3 ν(C–C) stretching modes present around 1650 cm–1 and 1150 cm–1 respectively.
In the spectrum of Si(PHIPE) (Figure 7a), a strong absorption centered at 1076 cm–1 corresponds to the ν(Si-O) stretching mode. Considering the spectrum of the C(PHIPE) (Figure 7b), we can notice that this absorption mode vanished, thereby proving that silica has been removed from the porous matrix via HF-treatment. At that stage, elemental analyses detect 3–5 wt% of Si still entrapped within the carbonaceous foams. Overall, the spectrum of the carbon foam is almost flat, with weak aromatic and sp3 ν(C–C) stretching modes present around 1650 cm–1 and 1150 cm–1 respectively (Figure 7, embedded). Second, graphitization has been addressed by Raman spectroscopy (Figure 8).
Intensity (u.a.) 2000
-1 Wave number (cm )
Figure 8 - Example of Raman spectra:
(open circles) and
In Raman spectrum of partially graphitized carbon, two bands around 1590 cm–1 and 1350 cm–1 correspond respectively to the graphite sp2 (E2g) carbon band (usually labeled G band) and to the defect sp3 carbon band (usually labeled D band) . We observe in Figure 8 the G band at 1593 cm–1 and the D band at 1355 cm–1, proving without any ambiguity the dual amorphous/crystalline nature of the carbonaceous foams obtained. Beyond, from the intensity ratio ID band/IG band, it is possible to estimate the graphitized crystallite coherence length [61,62] at around 7 nm. As a direct consequence of cooperative effects between this partial crystallization and the mesoporosity of the walls, these carbonaceous monoliths possess an electronic conductivity of ~ 0.5 S·cm–1, 102 times lower than that of C(HIPE) free of mesoporosity , but in the same range as mesoporous C(HIPE) . Indeed, the charge transport does not vary much from one sample to another with a standard deviation of ± 0.2 S·cm–1. This demonstrates that the charge transport is not triggered by the bulk density (the active mass). As the graphitization rate is rather constant from one sample to another, the only one parameter that tunes the quite low conductivity obtained is certainly the mesoporosity that is both minimizing the graphitization 19
process while enhancing grain boundaries within the carbonaceous foams, contrary to what was observed with carbonaceous foams free of mesoporosity . Good conductivity is an important characteristic for porous carbonaceous foams as they can be nice candidates as novel electrodes for energy storage or energy conversion devices, as for instance Li-ions [53,54], Li-Sulfides batteries [63-64] biofuel cells [65-67] or electrochemical capacitors . As porous carbons play an imperative role in advanced energy storage devices, the electrochemical performances of these foams as electrodes for electrochemical capacitors will be investigated. The mesoscopic roughness may increase the electrolyte Debye double layer thickness, favoring thereby the capacity effects at electrodes-electrolytes interfaces.
4. Conclusions In this work silica foams with hierarchical triple porosity were used as hard templates to prepare C(PHIPE) porous carbon monoliths with a hierarchical triple porosity. Unlike the previous C(HIPE) foams, no mesophase soft templating agent was necessary to create mesoporosity. Mercury intrusion revealed an open macroporosity of 45-70 % with narrow pore size distribution. By changing the silica nanoparticle/oil/resin proportions, we were able to vary the macropores size and the pore throats size. Nitrogen sorption are characteristic of micro/mesoporous materials and showed BET specific surface area values of 700-900 m2·g–1 with 30-60% mesopores. Raman spectroscopy indicated that the foams were partially graphitized. Surface fractal dimension and SEM proved the cell walls to be rough.
Acknowledgments We wish to thank Isabelle Ly (CRPP) and Elisabeth Sellier from Placamat-ICMCB (Bordeaux) for HR-TEM and HR-SEM investigations, Alain Derré (CRPP) for the thermal treatments and Alain Pénicaud (CRPP) for Raman spectroscopy measurements. The authors gratefully acknowledge funding from the Région Aquitaine and the Agence Nationale de la Recherche through the project RATIOCELLS (ANR-12-BS08-0011-01).
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