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High-surface area mesoporous carbons from gel templating and inorganic-organic hybrid gel formation
Journal of Solid State Chemistry 281 (2020) 121040
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High-surface area mesoporous carbons from gel templating and inorganic-organic hybrid gel formation Alex M. Volosin, Shaojiang Chen, Dong-Kyun Seo * School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287-1604, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous carbon Sol-gel Thixotropic gel Template method
Mesoporous carbon materials with high surface areas and a high mesoporosity have been prepared using an interpenetrating inorganic-organic hybrid gel with hydrous alumina gel as soft template via two synthetic routes. In the single mix route, the hybrid gel was prepared with all the precursors are mixed at beginning, the inorganic alumina gel forms and then catalyzes the polymerization of a carbon precursor polymer. The final carbon product has the surface area over 1500 m2/g. In the alternative thixotropic mixing route, the hybrid gel was prepared by sequential but separate addition of inorganic and organic precursors. The alumina nanoparticles can reversibly self-assemble to form a three-dimensional gel network structure that ensures an open and connected network of the mesopores in the mesoporous carbon. The highest surface area was observed to be 1138 m2/g based on Brunauer-Emmett-Teller (BET) analysis on the N2 sorption isotherms, while the mesopores have relatively uniform pore sizes around 5 nm in the Barrett-Joyner-Halenda (BJH) pore size distributions. Only 4% of the surface area was attributed to micropores for the high-surface area mesoporous carbons. Scanning electron microscopic studies reveal smooth surfaces without microscopic cracks in the material, indicating a macroscopic homogeneity of the materials.
1. Introduction Porous carbon is an important industrial material with many applications in energy storage, such as batteries and supercapacitors [1,2], as well as other applications including filtration, decontamination and catalysis. Carbon is particularly appealing for large scale applications due to its natural abundance, chemical robustness, and ecological friendliness. While porous carbon has been produced in a large scale through pyrolysis of abundant biomass [3,4], such methods generate predominantly microporous and/or macroporous carbons and lack fine control of the type and extent of porosity. Other synthetic techniques that demonstrate a greater control over the porosity makes use of a sacrificial template material whose negative replica becomes the empty space (pores) in the carbon product after the template is removed. Templates can be “soft” like self-assembled molecules such as block co-polymer systems and surfactants [5–8], or “hard” components which are pre-prepared solids [9–14]. Most common templates for porous carbon are hard templates, such as metal oxides, that are preformed in a desired shape. Metal oxide nanoparticles can be dispersed as a colloid throughout a carbon precursor solution and subsequently removed chemically after pyrolysis [15,16]. Additional surface modification may
be necessary to stabilize the colloidal to prevent settling [9]. Instead, the particle precipitation can be avoided by careful monitoring of gelation times to predict the appropriate moment of template addition to a solution on the verge of gelling [10] to obtain well distributed template nanoparticles. However, the colloidal templates, being discretely dispersed in the medium, do not warrant the resulting pores to be all connected and thus open for access of molecules in the later use of the porous carbon. Alternatively, a preformed porous metal oxide like MCM-48 and SBA-15 can be infiltrated as a porous hard template with a carbon precursor, pyrolyzed, and etched to generate a negative replica of the original non-carbonaceous solid [11–13]. For preparation of the porous hard template, soft templates are often used by proxy [12]. While it can provide well-controlled pore structures with a very high surface area, the method is often tedious and not scalable due to the required preparation of the sacrificial template and multiple repetitions of infiltration step. Herein, we report new synthetic routes to prepare high-surface area mesoporous carbons by using hydrous alumina gel as the soft template. In our synthesis, the alumina template provides a three-dimensional continuous network of mesopores as its negative replica in the mesoporous carbon product. Controlled and sequential formation of the
* Corresponding author. E-mail address: [email protected] (D.-K. Seo). https://doi.org/10.1016/j.jssc.2019.121040 Received 15 July 2018; Received in revised form 25 October 2019; Accepted 30 October 2019 Available online 2 November 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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Journal of Solid State Chemistry 281 (2020) 121040
lowest speed setting for about 3 min. The blends were then poured into molds and they re-gelled within a few minutes. The gels were left at room temperature for 24 h followed by heating at 70 C for 3 days to promote the polymerization of the resorcinol and formaldehyde. The translucent red gel monoliths were then removed from their molds and cut into centimeter-sized pieces and left to dry in air for overnight to produce hybrid xerogel pieces. The prepared xerogels were placed in a tube furnace and purged with argon gas for 15 min then heated at a rate of 2 C/min and held at 110 C for 1 h under flowing argon. The furnace was then heated to 1000 C at a rate of 6 C/min and held for 3 h under flowing argon. The furnace was then allowed to cool ambiently and the metal oxide-carbon hybrid (MOCH) pieces were removed from the furnace. The five MOCH samples were designated as M-1 to M-5, in the increasing order of R–F precursor concentrations. To produce mesoporous carbon products (correspondingly, designated as C-1 through C-5), the alumina component was removed from the MOCHs by hydrothermal treatment in H2SO4. Our preliminary experiment showed that using HCl under the refluxing condition could not remove alumina component from MOCHs completely. For the alumina removal, each MOCH sample was loaded into a Teflon lined Parr bomb charged with 9 g of 32% w/w H2SO4, sealed and heated at 160 C for 4 days. The mesoporous carbon samples were removed from the Parr bombs and rinsed with 32 wt% H2SO4 several times and then with deionized water until the wash solution reached pH of 7. The different alumina removal routes do not allow a full comparison of the products of the two methods in terms of their porosity. However, it is clear that the single mix route is advantageous in terms of the convenience of alumina template removal. As shown below, materials characterization and analysis establish that the single mix route is superior in terms of porosity and surface areas, in addition to the advantage in template removal.
alumina network and polymer network guarantees the interpenetration of the two different gel networks which is important for the open and connected mesopores in the mesoporous carbon. The first route (hereafter called “single mix route”) involves the mixing of organic and inorganic components at beginning. The inorganic alumina gel form first and then catalyze the polymerization of a carbon precursor polymer in a one-pot preparation. In an alternative “thixotropic mixing route”, the inorganic and organic precursor are mixed separately and sequentially that utilizes the thixotropic property of the alumina gel that can be reversibly disassembled and reassembled. Thixotropic materials exhibit a time-dependent reduction in viscosity at the application of shear forces. At the removal of the shear force, the viscosity recovers after a certain time to return to a solid-like gel state [17]. Alumina gels are a thixotropic material such that the initial bulk gel can be liquefied, and given time, can reform into a bulk gel [18]. In addition to the open porosity, this synthetic route may allow additional flexibility in synthetic design, such as generation of hierarchically porous carbons by homogeneously incorporating large pore templates to introduce macroporosity in the final carbon materials. 2. Experimental 2.1. Single mix route of synthesis An interpenetrating gel network of alumina gel and resorcinol (R)formaldehyde (F) polymer was prepared in bulk by first using epoxidedriven gelation for the hydrous alumina gel formation [19,20]. First, 142 g of AlCl3⋅6H2O (Sigma-Aldrich, 99%) was dissolved in 1.2 L of 50 vol% aqueous ethanol (Decon Laboratories, Inc.) in a large jar. Then 20 g of resorcinol (Sigma-Aldrich) was added and stirred until dissolution was complete. 30 g of formaldehyde aqueous solution (37 wt%; Sigma-Aldrich) was added to the solution with stirring. This precursor solution was then constantly stirred while propylene oxide (1,2-epoxypropane, Sigma-Aldrich, 99%) was added in approximately 100 ml aliquots every 4 min until a total of 412 ml was added. This solution was stirred for about 20 min, covered with a lid, and then left undisturbed. The concentrations of Al3þ ion, resorcinol, formaldehyde and propylene oxide were 0.49, 0.15, 0.30 and 4.9 M, respectively, in the aqueous ethanol solution. The solution gelled into a clear colorless gel in less than 1 h. The gel was left at room temperature for about 18 h and then placed in an oven at 70 C for 4 days to promote the polymerization of the R–F precursors. The dark red gel was then removed from the oven, chopped into pieces less than 1 cm in dimension, and air dried under a fume hood for about 2 weeks. The dried hybrid xerogels were pyrolyzed in a tube furnace purged with argon. The furnace was ramped at a rate of 6 C/min to 1000 C and held for 3 h, all under flowing argon gas. The resulting metal oxide-carbon hybrids (MOCHs) were then treated to etch the alumina component. The MOCHs were placed in a 1 L round-bottom flask with a water-cooled reflux condenser and refluxed in 425 ml of 19% hydrochloric acid for 5 h. The resulting mesoporous carbon pieces were taken out, rinsed with water using a vacuum filter until the pH of the filtrate was neutral, and then placed in an oven at 110 C for 24 h to dry. Some portion of the mesoporous carbon samples was ball-milled for 45 min at 120 rpm using steel balls to produce a fine powder.
2.3. Material characterization Nitrogen sorption isotherms were collected on a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer at 77 K. Samples were degassed under vacuum at 200 C for 8 h. For surface area calculation, the Brunauer-Emmett-Teller (BET) model was applied to the adsorption branch in the partial pressure range of 0.05 – 0.2 [21]. For pore size distributions, the Barrett-Joyner-Halenda (BJH) model was applied to the adsorption branch with the Carbon Black STSA thickness equation and Faass correction [22] to account for multilayer desorption in adsorbed layer thickness estimation. Total pore volume was estimated from the total quantity of gas adsorbed at the data point closest to P/Po ¼ 0.98 on the desorption branch. Transmission electron microscopy (TEM) was performed on a JEOL JEM 2000FX operating at 200 kV. The TEM samples were prepared by grinding them in an agate mortar in ethanol and dispersing the ground particles in ethanol. A copper grid covered with a holey carbon film was dipped into the solution, removed, and dried in air. Scanning electron microscopy (SEM) studies were carried out with ground samples using an FEI XL-30 Environmental SEM using 10 keV electrons. The SEM samples were prepared by grinding and dusting the sample onto sticky carbon tape on an SEM sample stage. Thermogravimetric analysis (TGA) studies were carried out using a Mettler-Toledo TGA/DSC 1 STARe system. All analyses were carried out under an air flow at 50 ml/min with 70 μL alumina crucibles. Powder X-ray diffraction data were collected using a Siemens D5000 diffractometer with a Cu Kα radiation. Raman spectral data were collected using a custom-built Raman spectrometer in a 180 geometry. The sample was excited using a 100 mW Compass 532 nm laser. The laser power was controlled using neutral density filters and reduced to a power of 5 mW at the sample surface. The laser was focused onto the sample using a 50X super long working distance Mitutoyo objective with a numerical aperture of 0.42. The signal was discriminated from the laser excitation using a Kaiser laser band pass filter followed by a Semrock edge filter. The data were
2.2. Thixotropic mixing route of synthesis A bulk alumina wet gel was prepared following the epoxide-driven gelation method in the same way in the previous section. The precursors were mixed together and gelled in approximately 1 h after which the gel was aged for an additional hour. This bulk gel was divided into 5 portions and each portion was blended with an aqueous ethanol solution of resorcinol and formaldehyde with varying final concentrations of resorcinol, 0.110, 0.324, 0.533, 0.752 and 0.990 M (at fixed molar ratio of R:F ¼ 1:2) but with a fixed final concentration of Al3þ, 0.49 M. The blending was carried out using a handheld kitchen blender (Oster) on the 2
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Journal of Solid State Chemistry 281 (2020) 121040
with the final mesoporous carbon samples Fig. 1a. In an alternative route (“thixotropic mixing”) shown in Fig. 1b, the alumina/R–F hybrid gels were prepared by sequential but separate addition of the alumina gel precursors and R–F polymer precursors, by taking advantage of the thixotropic property of the hydrous alumina gel. Namely, a hydrous alumina gel becomes fluidic upon agitation, as the 3D gel network connection breaks up, but once the mechanical force is removed, the gel can regain its rigidity, with a reestablished network connection. In the thixotropic mixing route, a hydrous alumina gel was prepared first only with the alumina gel precursors. The R–F polymer precursors were then slowly added to the gel, while the gel was liquified in a blender. After the mixing, the blender was stopped and the mixture regelated within a few minutes. The gel was subsequently treated following the same steps described in the single mix route to produce a mesoporous carbon.
collected using an Acton 300i spectrograph and a back thinned Princeton Instruments with a liquid nitrogen-cooled CCD detector. 3. Results and discussion 3.1. Synthesis Fig. 1 illustrates the overall synthetic procedures in which mesoporous carbon is produced by creating, drying and calcining inorganicpolymer hybrid gels that exhibit a three-dimensional (3D) interpenetrating network structure. The hybrid gels are produced by forming a hydrous alumina gel network followed by promoting polymeric R–F gel formation. In Fig. 1a, the synthesis was carried out by first mixing all the reaction precursors in the first step (“single mix route”). The solution was acidic because of the hydrolysis of AlCl3⋅6H2O precursor. The precursor solution turned into a translucent solid gel monolith within about 1 h and then became red-orange after one day at room temperature. At an ambient temperature, the propylene oxide effectively undergoes an addition reaction with the hydrochloric acid to form 1-chloro-2-propanol and a smaller amount of 2-chloro-1-propanol [23]. The consumption of the acid increases the pH of the solution and induces a uniform formation of an inorganic gel network [24,25]. The R–F polymeric gelation can be catalyzed by an acid but the catalysis is effective only at elevated temperatures typically above 60 C [26–28]. When catalyzed by an acid, the formation of R–F gels is indicated by a distinctive red-brown color which is due to the formation of o-methide quinones as a byproduct [29]. Therefore, the red-orange color of the gel observed after a day may indicate that the R–F polymerization can take place even at room temperature but only slowly in the current experimental condition. This is in contrast to the rapid formation of the hydrous alumina gel that takes place in about 1 h. Upon subsequent heating at 70 C, the R–F polymerization fully occurred in the hydrous alumina gels to give a unique red-brown color. It is plausible that unreacted metal precursors may catalyze the polymerization, although the pH of the gel is somewhat higher (pH ~ 5) than the typical values used for the effective catalysis of the R–F polymerization [29]. More importantly, it is possible that the inorganic gel network itself may also promote the polymerization, as high valent metal oxides can act as a Lewis acid [30,31]. The alumina/R–F hybrid gels were then dried in air to form hybrid xerogels. The R–F polymer in the hybrid xerogels were carbonized by heating the xerogels at 700 C in an inert gas atmosphere, and subsequently the alumina in the hybrid was removed by acid-etching to result in mesoporous carbon, the final product. The porous materials prepared were analyzed at several stages during their preparation. Nitrogen gas sorption isotherms were collected and pore characteristics were examined for the MOCHs prior to the extraction step for comparison
3.2. Mesoporous carbon from single mix route The alumina removal by dissolution in hydrochloric acid was shown to be complete through thermogravimetric analysis. The sample was shown to leave essentially no inorganic mass after combustion in air as seen in Fig. 2. The dissolution of alumina in excess hydrochloric acid generates an acidic solution of aluminum chloride [32]. This particular aspect is indicative of the recyclability of the alumina template, if necessary. The gel precursor solution consists of an acidic solution of
Fig. 2. TGA thermogram of mesoporous carbon product from single mix route in air atmosphere.
Fig. 1. Synthetic schemes of (a) single mix route and (b) thixotropic mixing route. 3
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Journal of Solid State Chemistry 281 (2020) 121040
aluminum chloride salt and could be regenerated from the etching solution produced during template removal. Powder X-ray diffraction (XRD) patterns were collected for both the mesoporous carbon and the ball-milled mesoporous carbon (Fig. 3). Both products show broad Bragg peaks near 23 and 43 , which can be assigned to (002) and (011) peaks of graphite, respectively. The apparent peak shifts to a lower angle in comparison the graphite peaks are likely due to an increase in the d-spacing which may indicate that the graphitic layers in the products are spaced further apart compared to highly crystalline graphite. The presence of these peaks indicates that the graphite layers are not randomly rotated relative to one another as in turbostratic graphite [33], but there exists a regular ABAB stacking to some extent. Additionally, the mesoporous carbon also shows a diffuse hump near 80 that is near the graphite (110) and (112) peaks (Fig. S1), which also indicates regular stacking of graphite sheets. These peaks are less pronounced in the ball-milled carbon and could suggest a disruption in the ABAB stacking of hexagonal graphite which has been observed previously in ground graphite materials [34]. The graphitic nature of the prepared carbons was also investigated using Raman spectroscopy. Fig. 4 shows the collected Raman spectra for the mesoporous carbon and the ball-milled mesoporous carbon. Peaks near 1350 and 1600 cm-1 are the so-called D and G peaks, respectively. The presence of the G peak indicates existence of sp2-carbon atoms in graphitic arrangement, the appearance of such significant G bands is remarkable and yet is not new for a mesoporous carbon [35-36]. For example, ordered mesoporous carbon from template synthesis using
Fig. 3. Powder X-ray diffraction patterns of mesoporous carbon and ball-milled mesoporous carbon as compared to a calculated pattern of graphite. The peaks in calculated pattern of graphite are at 26.5 (002), 42.3 (010), 44.5 (011), 50.7 (012), 54.7 (004), 59.8 (013), 77.4 (110) and 83.5 (112).
Fig. 4. (a) Raman spectra of mesoporous carbon and ball-milled mesoporous carbon and (b) a fitted line shape (dashed line) of mesoporous carbon for determining relative peak intensities.
Fig. 5. (a) Nitrogen gas sorption isotherms and (b) BJH pore size distribution plots of the MOCH hybrid (▴), mesoporous carbon (■) and the ball-milled mesoporous carbon (●). 4
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carbon a capacity somewhat smaller than that. The porosity of the MOCH is due to the nature of xerogel (i.e., air-dried gel), which is consistent with the small pore size and small pore volume (Table 1). Upon removal of the oxide template, there is a large increase in pore volume from pores between 6 and 10 nm, as seen in the mesoporous carbon sample (Fig. 5b). The pore size distributions in Fig. 5b show that both carbon samples have approximately the same pore distributions, but the ball-milled samples show a large reduction in mesopore volume between 4 and 8 nm. The ball milling process is probably destroying the thin mesopore shells (see below). Upon a close inspection, the pore size distribution for the ballmilled carbon shows a slight increase in large mesopore volume between 10 and 50 nm, relative to the mesoporous carbon before milling. There is also a suggestion of the generation of macropores caused by the ball-milling step due to the slight upturn in the nitrogen isotherm for the ball-milled carbon at P/Po close to 1. This occurs in the presence of macropores that cannot be completely filled by capillary condensation [39]. Fig. 6a shows a TEM image of the mesoporous carbon in which the carbon materials appear as highly tortuous networks of thin shells. The nanostructure extends homogeneously well into a micrometer scale without crevices or coarse structural features, as seen in the SEM image (Fig. 6b). The morphology of the mesoporous carbon is very different from those of the porous carbon materials from typical R–F xerogels, cryogels and aerogels where the basic building blocks are circular nanoparticles in the range of 50 – 100 nm [40].
ordered mesoporous silica SBA-15 has partially ordered graphitic structure in its pore wall. Meanwhile, the strong intensity of the D peak implies that the sp2-carbon atoms are highly disordered. The degree of the disordering was compared between the two products, by using Tuinstra-Koenig formula, IðDÞ=IðGÞ ¼ C’ðλÞ=La , where C’ðλÞ ¼ ð2:4 1010 nm3 Þ λ4 and La is considered as the lower bound of the graphitic domain [37]. The domain sizes of the mesoporous carbon and the ball-milled mesoporous carbon are estimated to be 3.6 and 3.5 nm, respectively. The peak intensities were determined following the suggestions of Ferrari and Robertson by fitting a Breit-Wigner-Fano line to the G peak and a Lorentzian line to the D peak as shown in Fig. 4b [38]. In nitrogen gas sorption isotherms (Fig. 5a), all the materials show a hysteresis loop, indicating a mesoporosity. The surface areas, pore volumes and average pore sizes were calculated from the isotherms, as shown in Table 1. The MOCH material shows the lowest adsorption capacity because both inorganic and organic networks remain within the porous structure, but it is still highly porous. The mesoporous carbon shows the highest adsorption capacity and the ball-milled mesoporous
Table 1 Surface area and pore characteristics of mesoporous carbon and ball-milled mesoporous carbon. Sample
BET surface areaa (m2/g)
Pore volumeb (cm3/g)
Average pore sizec (nm)
Porosityd (%)
MOCH Mesoporous Carbon Ball-milled mesoporous carbon
442 1561
0.46 2.1
4 5
– 82
1380
1.6
5
78
a b c d
3.3. Mesoporous carbon from thixotropic mixing route After preliminary experiments, five different concentrations of R–F precursors were chosen to examine the effect of the polymer concentration on the porosity of the mesoporous carbon products as well as the intermediate materials during the synthesis. Although not shown here, all the mesoporous carbon products from the thixotropic mixing route showed the XRD patterns and Raman spectra that are identical to those from the single mix route. This is not surprising as the different methods would not lead to carbon materials with different nature in chemical structure. Nitrogen gas sorption isotherms of the MOCHs in Fig. 7a show a decrease in sorption as the polymer precursor concentration is increased. For all the samples except M-1, the isotherms exhibit a low pressure hysteresis at P/Po less than 0.4. This typically indicates the entrapment of nitrogen molecules within pores of comparable size to the adsorptive or the swelling of the porous structure during the measurement [39]. Since the material is rigid, it is highly unlikely that the latter is the case. Because the hysteresis loops are type H2 and they lack a hysteresis loop closure, the adsorption branch was used for all BJH analyses for the MOCH samples. The BJH pore size distributions for the MOCH samples in Fig. 7b show the presence of pores in the more or less same size range. It is interesting and yet puzzling that the pore sizes do not change
Pressure range P/Po ¼ 0.05-0.2. Single point desorption nearest P/Po ¼ 0.98. 4V/A (BET). Calculated based on the assumption ρcarbon ¼ 2.15 g/cm.3.
Fig. 6. (a) TEM and (b) SEM images of the mesoporous carbon. Scale bar ¼ 10 nm in (a) and 500 nm in (b).
Fig. 7. (a) Nitrogen gas sorption isotherms and (b) BJH pore size distributions of MOCHs M-1 (●), M-2 (■), M-3 (▴), M-4 (★), M-5 (◆). 5
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process. The increase in R–F polymer component, as in M-2 to M-5, does not improve the gel rigidity but rather clog up the pores in the gel network, reducing the porosity of the xerogels and MOCHs. Although not detailed here, TGA analysis indicated that the carbon contents in M-1 to M-5 were 39, 67, 76, 82 and 85 wt%, respectively. Based on TGA analysis, furthermore, the final mesoporous carbon product (C-5) from M-5 contained 15 wt% alumina after the acid etching, while all the rest of the mesoporous carbons from M-1 to M-4 did not include alumina. In other words, the large carbon content and consequent pore clogging in M-5 hampered etching of the alumina component in mesoporous carbon production within the time period of the acid-etching step. Fig. 8a shows a TEM image of the M-1, while the alumina and mesoporous carbon (C-1) from the M-1 are shown in Fig. 8b and 8c/d, respectively. The alumina was obtained after removing the carbon component by calcining the M-1 in air. The alumina exhibits the typical morphology of a gel which is made up of 5 – 10 nm-sized primary particles that are fused together, forming a 3D gel network structure. The anisotropic shape of the alumina primary particles is characteristic of the alumina gels prepared from aluminum chloride precursors [24,25]. The mesoporous carbon in Fig. 8c and 8d clearly displays its porosity with textural domains around 10 nm. The porosity in the mesoporous carbon is originated both from the sacrificial alumina network and from the innate porosity of the MOCH hybrid gel. The pores in the mesoporous carbon are interconnected as the sacrificial alumina has a continuous inorganic gel structure. Furthermore, at a low R–F polymer concentration, the pores in the MOCH hybrid are likely to be open and connected as expected in a dried and pyrolyzed gel. Fig. 9a shows the nitrogen gas sorption isotherms of the mesoporous carbon materials C-1 to C-5. The isotherm for C-1 shows a hysteresis loop of type H1, while samples C-2 through C-5 show a H2 type. This indicates that the mesoporous C-2 through C-5 likely experience pore blocking or percolation effects with a larger extent for a higher carbon precursor concentration. The adsorption capacity is reduced with increasing carbon precursor concentration, which follows the same trend seen for the MOCH samples. The calculated BET surface areas and pore volumes of the mesoporous carbons echo this trend in that decreases in BET surface area and total pore volume are observed with increasing carbon precursor concentration (Table 3). This reduction in pore space is shown for the mesoporous carbons in the BJH pore size distribution in Fig. 9b, as well as the continuous reduction of the average pore size for samples C-1 through C-4 (Table 3). The t-plot method was also used to calculate the surface area from micropores in the mesoporous carbons (Table 3). Both the specific micropore surface area and its contribution to the total surface area increase with carbon precursor concentration. Since the micropores form by pyrolysis throughout the body of the carbon, the microporosity would increase as the carbon content increases, which was indeed the case among our samples. For the C-5, the pore clogging does not allow a proper measurement of both microporosity and mesoporosity.
significantly with the changes in R–F polymer concentration. However, the specific surface area and pore volume do show a decrease with increasing carbon precursor concentration (Table 2) among the MOCH samples as well as among the corresponding hydrous alumina/R–F polymer xerogels (shown in the parentheses in Table 2). This suggests that even at a low concentration, R–F polymer component in the gel improves the rigidity of the gel network and thus effectively minimizes the pore collapsing caused by capillary pressure during the drying Table 2 Surface area and pore characteristics of MOCH products after pyrolysis. The corresponding properties of the samples before pyrolysis are given in parentheses. Sample
Resorcinol concentration (M)
BET surface areaa (m2/g)
Pore volumeb (cm3/g)
Average pore sizec (nm)
M-1 M-2 M-3 M-4 M-5
0.110 0.324 0.533 0.752 0.990
391 365 301 275 124
0.53 0.27 0.18 0.17 0.08
5.4 2.9 2.4 2.5 2.7
a b c
(268) (182) (128) (76) (40)
(0.28) (0.20) (0.16) (0.10) (0.05)
(4.2) (4.3) (5.0) (5.1) (4.7)
Pressure range P/Po ¼ 0.05 – 0.2. Single point desorption nearest P/Po ¼ 0.97. 4V/A (BET).
Fig. 8. TEM images of (a) hybrid M-1, (b) porous alumina from burning M-1, and mesoporous carbon C-1 in (c) high magnification and (d) low magnification.
Fig. 9. (a) Nitrogen gas sorption isotherms and (b) BJH pore size distributions of the mesoporous carbon samples C-1 (●), C-2 (■), C-3 (▴), C-4 (★), C-5 (◆). 6
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Tillotson, J.H. Satcher Jr., J.F. Poco, L.W. Hrubesh, R.L. Simpson, Use of epoxides in the sol gel synthesis of porous iron (III) oxide monoliths from Fe (III) salts, Chem. Mater. 13 (3) (2001) 999–1007. [20] A.E. Gash, T.M. Tillotson, J.H. Satcher Jr., L.W. Hrubesh, R.L. Simpson, New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors, J. Non-Cryst. Solids 285 (1–3) (2001) 22–28. [21] J. Rouquerol, P. Llewellyn, F. Rouquerol, Is the BET equation applicable to microporous adsorbents, Stud. Surf. Sci. Catal. 160 (07) (2007) 49–56. [22] G.S. Faass, Correlation of Gas Adsorption, Mercury Intrusion, and Electron Microscopy Pore Property Data for Porous Glasses, Georgia Institute of Technology, 1981. [23] R.E. Phillips, R.L. Soulen, Propylene oxide addition to hydrochloric acid: a textbook error, J. Chem. Educ. 72 (7) (1995) 624. [24] T.F. Baumann, A.E. Gash, S.C. Chinn, A.M. Sawvel, R.S. Maxwell, J.H. Satcher, Synthesis of high-surface-area alumina aerogels without the use of alkoxide precursors, Chem. Mater. 17 (2) (2005) 395–401. [25] D.M. Ladd, A. Volosin, D.-K. Seo, Preparation of highly porous γ-alumina via combustion of biorenewable oil, J. Mater. Chem. 20 (28) (2010) 5923–5929. [26] T.F. Baumann, M.A. Worsley, T.Y.-J. Han, J.H. Satcher Jr., High surface area carbon aerogel monoliths with hierarchical porosity, J. Non-Cryst. Solids 354 (29) (2008) 3513–3515. [27] R. Brandt, J. Fricke, Acetic-acid-catalyzed and subcritically dried carbon aerogels with a nanometer-sized structure and a wide density range, J. Non-Cryst. Solids 350 (2004) 131–135. [28] M. Reuß, L. Ratke, Subcritically dried RF-aerogels catalysed by hydrochloric acid, J. Sol. Gel Sci. Technol. 47 (1) (2008) 74–80. [29] S. Mulik, C. Sotiriou-Leventis, N. Leventis, Time-efficient acid-catalyzed synthesis of resorcinolformaldehyde aerogels, Chem. Mater. 19 (25) (2007) 6138–6144. [30] N. Leventis, N. Chandrasekaran, A.G. Sadekar, C. Sotiriou-Leventis, H. Lu, One-pot synthesis of interpenetrating inorganic/organic networks of CuO/resorcinolformaldehyde aerogels: nanostructured energetic materials, J. Am. Chem. Soc. 131 (13) (2009) 4576–4577. [31] N. Leventis, N. Chandrasekaran, A.G. Sadekar, S. Mulik, C. Sotiriou-Leventis, The effect of compactness on the carbothermal conversion of interpenetrating metal oxide/resorcinol-formaldehyde nanoparticle networks to porous metals and carbides, J. Mater. Chem. 20 (35) (2010) 7456–7471. [32] F. Roelofs, W. Vogelsberger, Dissolution kinetics of nanodispersed γ-alumina in aqueous solution at different pH: unusual kinetic size effect and formation of a new phase, J. Colloid Interface Sci. 303 (2) (2006) 450–459. [33] J. Biscoe, B. Warren, An X-ray study of carbon black, J. Appl. Phys. 13 (6) (1942) 364–371. [34] K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, 1988.
Table 3 Surface area and pore characteristics for mesoporous carbon samples. Sample
C-1 C-2 C-3 C-4 C-5 a b c d
Resorcinol concentration (M)
0.110 0.323 0.530 0.746 0.980
BET surface area (m2/g)a
Pore volume (cm3/g)b
1138 752 572 501 80
1.5 0.8 0.5 0.4 0.07
Average pore size (nm)c
5.3 4.0 3.4 3.1 3.4
Micropore surface aread (m2/ g)
(%)
50 107 214 257 52
4 14 37 51 65
Pressure range P/Po ¼ 0.05 – 0.2. Single point desorption nearest P/Po ¼ 0.97. 4V/A (BET). t-plot fit for thickness range of 0.35 – 0.5 nm.
Remarkably for the C-1, only 4% of the high surface area (1138 m2/g) is contributed from micropores, while the rest is from only mesopores with the average pore size around 5 nm. The elemental analysis of the carbon products from X-ray photoelectron spectroscopy indicates that the materials have contain mostly carbon (> 98 at%) with a small amount of oxygen (< 2 at%) (Fig. S2). The deconvolution of the high-resolution O1s spectrum indicates that most of the oxygen atoms are present as a hydroxyl group, which may be due to the aqueous etching process. 4. Concluding remarks We have successfully developed new synthetic routes to prepare highsurface area mesoporous carbons by using hydrous alumina gel as a soft template. The alumina gel soft template provides a three-dimensional continuous network of mesopores as its negative replica in the mesoporous carbon product. Controlled and sequential formation of the alumina network and polymer network guarantees the interpenetration of the two different gel networks which is important for ensuring the open and connected mesopores in the mesoporous carbon. Between the single mix and thixotropic mixing routes, it is found that the products from the latter have lower surface areas and that the template removal requires a harsher etching condition. Nevertheless, it is speculated that further optimization of the latter route may be beneficial. Namely, by taking advantage of the thixotropic nature of the hydrous alumina gel, the method may allow additional flexibility in synthetic design, such as production of hierarchically porous carbons by homogeneously incorporating large pore templates to introduce macroporosity in the final carbon materials. Acknowledgement This work was originally supported by the Center for Bio-Inspired Solar Fuel Production, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001016. We gratefully acknowledge the use of facilities within the Eyring Materials Center at Arizona State University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.121040. References [1] R. Pekala, J. Farmer, C. Alviso, T. Tran, S. Mayer, J. Miller, B. Dunn, Carbon aerogels for electrochemical applications, J. Non-Cryst. Solids 225 (1998) 74–80. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer Science & Business Media, 2013.
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Journal of Solid State Chemistry 281 (2020) 121040 [38] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (20) (2000) 14095. [39] S. Lowell, J.E. Shields, M.A. Thomas, M. Thommes, Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, Springer Science & Business Media, 2012. [40] J. Shen, J. Hou, Y. Guo, H. Xue, G. Wu, B. Zhou, Microstructure control of RF and carbon aerogels prepared by sol-gel process, J. Sol. Gel Sci. Technol. 36 (2) (2005) 131–136.
[35] Y. Gogotsi, A. Nikitin, H. Ye, W. Zhou, J.E. Fischer, B. Yi, H.C. Foley, M.W. Barsoum, Nanoporous carbide-derived carbon with tunable pore size, Nat. Mater. 2 (9) (2003) 591. [36] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles, Nature 412 (6843) (2001) 169. [37] A.C. Ferrari, Raman spectroscopy of graphene and graphite: disorder, electron–phonon coupling, doping and nonadiabatic effects, Solid State Commun. 143 (1–2) (2007) 47–57.
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High-surface area mesoporous carbons from gel templating and inorganic-organic hybrid gel formation Alex M. Volosin, Shaojiang Chen, Dong-Kyun Seo * School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287-1604, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous carbon Sol-gel Thixotropic gel Template method
Mesoporous carbon materials with high surface areas and a high mesoporosity have been prepared using an interpenetrating inorganic-organic hybrid gel with hydrous alumina gel as soft template via two synthetic routes. In the single mix route, the hybrid gel was prepared with all the precursors are mixed at beginning, the inorganic alumina gel forms and then catalyzes the polymerization of a carbon precursor polymer. The final carbon product has the surface area over 1500 m2/g. In the alternative thixotropic mixing route, the hybrid gel was prepared by sequential but separate addition of inorganic and organic precursors. The alumina nanoparticles can reversibly self-assemble to form a three-dimensional gel network structure that ensures an open and connected network of the mesopores in the mesoporous carbon. The highest surface area was observed to be 1138 m2/g based on Brunauer-Emmett-Teller (BET) analysis on the N2 sorption isotherms, while the mesopores have relatively uniform pore sizes around 5 nm in the Barrett-Joyner-Halenda (BJH) pore size distributions. Only 4% of the surface area was attributed to micropores for the high-surface area mesoporous carbons. Scanning electron microscopic studies reveal smooth surfaces without microscopic cracks in the material, indicating a macroscopic homogeneity of the materials.
1. Introduction Porous carbon is an important industrial material with many applications in energy storage, such as batteries and supercapacitors [1,2], as well as other applications including filtration, decontamination and catalysis. Carbon is particularly appealing for large scale applications due to its natural abundance, chemical robustness, and ecological friendliness. While porous carbon has been produced in a large scale through pyrolysis of abundant biomass [3,4], such methods generate predominantly microporous and/or macroporous carbons and lack fine control of the type and extent of porosity. Other synthetic techniques that demonstrate a greater control over the porosity makes use of a sacrificial template material whose negative replica becomes the empty space (pores) in the carbon product after the template is removed. Templates can be “soft” like self-assembled molecules such as block co-polymer systems and surfactants [5–8], or “hard” components which are pre-prepared solids [9–14]. Most common templates for porous carbon are hard templates, such as metal oxides, that are preformed in a desired shape. Metal oxide nanoparticles can be dispersed as a colloid throughout a carbon precursor solution and subsequently removed chemically after pyrolysis [15,16]. Additional surface modification may
be necessary to stabilize the colloidal to prevent settling [9]. Instead, the particle precipitation can be avoided by careful monitoring of gelation times to predict the appropriate moment of template addition to a solution on the verge of gelling [10] to obtain well distributed template nanoparticles. However, the colloidal templates, being discretely dispersed in the medium, do not warrant the resulting pores to be all connected and thus open for access of molecules in the later use of the porous carbon. Alternatively, a preformed porous metal oxide like MCM-48 and SBA-15 can be infiltrated as a porous hard template with a carbon precursor, pyrolyzed, and etched to generate a negative replica of the original non-carbonaceous solid [11–13]. For preparation of the porous hard template, soft templates are often used by proxy [12]. While it can provide well-controlled pore structures with a very high surface area, the method is often tedious and not scalable due to the required preparation of the sacrificial template and multiple repetitions of infiltration step. Herein, we report new synthetic routes to prepare high-surface area mesoporous carbons by using hydrous alumina gel as the soft template. In our synthesis, the alumina template provides a three-dimensional continuous network of mesopores as its negative replica in the mesoporous carbon product. Controlled and sequential formation of the
* Corresponding author. E-mail address: [email protected] (D.-K. Seo). https://doi.org/10.1016/j.jssc.2019.121040 Received 15 July 2018; Received in revised form 25 October 2019; Accepted 30 October 2019 Available online 2 November 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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lowest speed setting for about 3 min. The blends were then poured into molds and they re-gelled within a few minutes. The gels were left at room temperature for 24 h followed by heating at 70 C for 3 days to promote the polymerization of the resorcinol and formaldehyde. The translucent red gel monoliths were then removed from their molds and cut into centimeter-sized pieces and left to dry in air for overnight to produce hybrid xerogel pieces. The prepared xerogels were placed in a tube furnace and purged with argon gas for 15 min then heated at a rate of 2 C/min and held at 110 C for 1 h under flowing argon. The furnace was then heated to 1000 C at a rate of 6 C/min and held for 3 h under flowing argon. The furnace was then allowed to cool ambiently and the metal oxide-carbon hybrid (MOCH) pieces were removed from the furnace. The five MOCH samples were designated as M-1 to M-5, in the increasing order of R–F precursor concentrations. To produce mesoporous carbon products (correspondingly, designated as C-1 through C-5), the alumina component was removed from the MOCHs by hydrothermal treatment in H2SO4. Our preliminary experiment showed that using HCl under the refluxing condition could not remove alumina component from MOCHs completely. For the alumina removal, each MOCH sample was loaded into a Teflon lined Parr bomb charged with 9 g of 32% w/w H2SO4, sealed and heated at 160 C for 4 days. The mesoporous carbon samples were removed from the Parr bombs and rinsed with 32 wt% H2SO4 several times and then with deionized water until the wash solution reached pH of 7. The different alumina removal routes do not allow a full comparison of the products of the two methods in terms of their porosity. However, it is clear that the single mix route is advantageous in terms of the convenience of alumina template removal. As shown below, materials characterization and analysis establish that the single mix route is superior in terms of porosity and surface areas, in addition to the advantage in template removal.
alumina network and polymer network guarantees the interpenetration of the two different gel networks which is important for the open and connected mesopores in the mesoporous carbon. The first route (hereafter called “single mix route”) involves the mixing of organic and inorganic components at beginning. The inorganic alumina gel form first and then catalyze the polymerization of a carbon precursor polymer in a one-pot preparation. In an alternative “thixotropic mixing route”, the inorganic and organic precursor are mixed separately and sequentially that utilizes the thixotropic property of the alumina gel that can be reversibly disassembled and reassembled. Thixotropic materials exhibit a time-dependent reduction in viscosity at the application of shear forces. At the removal of the shear force, the viscosity recovers after a certain time to return to a solid-like gel state [17]. Alumina gels are a thixotropic material such that the initial bulk gel can be liquefied, and given time, can reform into a bulk gel [18]. In addition to the open porosity, this synthetic route may allow additional flexibility in synthetic design, such as generation of hierarchically porous carbons by homogeneously incorporating large pore templates to introduce macroporosity in the final carbon materials. 2. Experimental 2.1. Single mix route of synthesis An interpenetrating gel network of alumina gel and resorcinol (R)formaldehyde (F) polymer was prepared in bulk by first using epoxidedriven gelation for the hydrous alumina gel formation [19,20]. First, 142 g of AlCl3⋅6H2O (Sigma-Aldrich, 99%) was dissolved in 1.2 L of 50 vol% aqueous ethanol (Decon Laboratories, Inc.) in a large jar. Then 20 g of resorcinol (Sigma-Aldrich) was added and stirred until dissolution was complete. 30 g of formaldehyde aqueous solution (37 wt%; Sigma-Aldrich) was added to the solution with stirring. This precursor solution was then constantly stirred while propylene oxide (1,2-epoxypropane, Sigma-Aldrich, 99%) was added in approximately 100 ml aliquots every 4 min until a total of 412 ml was added. This solution was stirred for about 20 min, covered with a lid, and then left undisturbed. The concentrations of Al3þ ion, resorcinol, formaldehyde and propylene oxide were 0.49, 0.15, 0.30 and 4.9 M, respectively, in the aqueous ethanol solution. The solution gelled into a clear colorless gel in less than 1 h. The gel was left at room temperature for about 18 h and then placed in an oven at 70 C for 4 days to promote the polymerization of the R–F precursors. The dark red gel was then removed from the oven, chopped into pieces less than 1 cm in dimension, and air dried under a fume hood for about 2 weeks. The dried hybrid xerogels were pyrolyzed in a tube furnace purged with argon. The furnace was ramped at a rate of 6 C/min to 1000 C and held for 3 h, all under flowing argon gas. The resulting metal oxide-carbon hybrids (MOCHs) were then treated to etch the alumina component. The MOCHs were placed in a 1 L round-bottom flask with a water-cooled reflux condenser and refluxed in 425 ml of 19% hydrochloric acid for 5 h. The resulting mesoporous carbon pieces were taken out, rinsed with water using a vacuum filter until the pH of the filtrate was neutral, and then placed in an oven at 110 C for 24 h to dry. Some portion of the mesoporous carbon samples was ball-milled for 45 min at 120 rpm using steel balls to produce a fine powder.
2.3. Material characterization Nitrogen sorption isotherms were collected on a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer at 77 K. Samples were degassed under vacuum at 200 C for 8 h. For surface area calculation, the Brunauer-Emmett-Teller (BET) model was applied to the adsorption branch in the partial pressure range of 0.05 – 0.2 [21]. For pore size distributions, the Barrett-Joyner-Halenda (BJH) model was applied to the adsorption branch with the Carbon Black STSA thickness equation and Faass correction [22] to account for multilayer desorption in adsorbed layer thickness estimation. Total pore volume was estimated from the total quantity of gas adsorbed at the data point closest to P/Po ¼ 0.98 on the desorption branch. Transmission electron microscopy (TEM) was performed on a JEOL JEM 2000FX operating at 200 kV. The TEM samples were prepared by grinding them in an agate mortar in ethanol and dispersing the ground particles in ethanol. A copper grid covered with a holey carbon film was dipped into the solution, removed, and dried in air. Scanning electron microscopy (SEM) studies were carried out with ground samples using an FEI XL-30 Environmental SEM using 10 keV electrons. The SEM samples were prepared by grinding and dusting the sample onto sticky carbon tape on an SEM sample stage. Thermogravimetric analysis (TGA) studies were carried out using a Mettler-Toledo TGA/DSC 1 STARe system. All analyses were carried out under an air flow at 50 ml/min with 70 μL alumina crucibles. Powder X-ray diffraction data were collected using a Siemens D5000 diffractometer with a Cu Kα radiation. Raman spectral data were collected using a custom-built Raman spectrometer in a 180 geometry. The sample was excited using a 100 mW Compass 532 nm laser. The laser power was controlled using neutral density filters and reduced to a power of 5 mW at the sample surface. The laser was focused onto the sample using a 50X super long working distance Mitutoyo objective with a numerical aperture of 0.42. The signal was discriminated from the laser excitation using a Kaiser laser band pass filter followed by a Semrock edge filter. The data were
2.2. Thixotropic mixing route of synthesis A bulk alumina wet gel was prepared following the epoxide-driven gelation method in the same way in the previous section. The precursors were mixed together and gelled in approximately 1 h after which the gel was aged for an additional hour. This bulk gel was divided into 5 portions and each portion was blended with an aqueous ethanol solution of resorcinol and formaldehyde with varying final concentrations of resorcinol, 0.110, 0.324, 0.533, 0.752 and 0.990 M (at fixed molar ratio of R:F ¼ 1:2) but with a fixed final concentration of Al3þ, 0.49 M. The blending was carried out using a handheld kitchen blender (Oster) on the 2
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with the final mesoporous carbon samples Fig. 1a. In an alternative route (“thixotropic mixing”) shown in Fig. 1b, the alumina/R–F hybrid gels were prepared by sequential but separate addition of the alumina gel precursors and R–F polymer precursors, by taking advantage of the thixotropic property of the hydrous alumina gel. Namely, a hydrous alumina gel becomes fluidic upon agitation, as the 3D gel network connection breaks up, but once the mechanical force is removed, the gel can regain its rigidity, with a reestablished network connection. In the thixotropic mixing route, a hydrous alumina gel was prepared first only with the alumina gel precursors. The R–F polymer precursors were then slowly added to the gel, while the gel was liquified in a blender. After the mixing, the blender was stopped and the mixture regelated within a few minutes. The gel was subsequently treated following the same steps described in the single mix route to produce a mesoporous carbon.
collected using an Acton 300i spectrograph and a back thinned Princeton Instruments with a liquid nitrogen-cooled CCD detector. 3. Results and discussion 3.1. Synthesis Fig. 1 illustrates the overall synthetic procedures in which mesoporous carbon is produced by creating, drying and calcining inorganicpolymer hybrid gels that exhibit a three-dimensional (3D) interpenetrating network structure. The hybrid gels are produced by forming a hydrous alumina gel network followed by promoting polymeric R–F gel formation. In Fig. 1a, the synthesis was carried out by first mixing all the reaction precursors in the first step (“single mix route”). The solution was acidic because of the hydrolysis of AlCl3⋅6H2O precursor. The precursor solution turned into a translucent solid gel monolith within about 1 h and then became red-orange after one day at room temperature. At an ambient temperature, the propylene oxide effectively undergoes an addition reaction with the hydrochloric acid to form 1-chloro-2-propanol and a smaller amount of 2-chloro-1-propanol [23]. The consumption of the acid increases the pH of the solution and induces a uniform formation of an inorganic gel network [24,25]. The R–F polymeric gelation can be catalyzed by an acid but the catalysis is effective only at elevated temperatures typically above 60 C [26–28]. When catalyzed by an acid, the formation of R–F gels is indicated by a distinctive red-brown color which is due to the formation of o-methide quinones as a byproduct [29]. Therefore, the red-orange color of the gel observed after a day may indicate that the R–F polymerization can take place even at room temperature but only slowly in the current experimental condition. This is in contrast to the rapid formation of the hydrous alumina gel that takes place in about 1 h. Upon subsequent heating at 70 C, the R–F polymerization fully occurred in the hydrous alumina gels to give a unique red-brown color. It is plausible that unreacted metal precursors may catalyze the polymerization, although the pH of the gel is somewhat higher (pH ~ 5) than the typical values used for the effective catalysis of the R–F polymerization [29]. More importantly, it is possible that the inorganic gel network itself may also promote the polymerization, as high valent metal oxides can act as a Lewis acid [30,31]. The alumina/R–F hybrid gels were then dried in air to form hybrid xerogels. The R–F polymer in the hybrid xerogels were carbonized by heating the xerogels at 700 C in an inert gas atmosphere, and subsequently the alumina in the hybrid was removed by acid-etching to result in mesoporous carbon, the final product. The porous materials prepared were analyzed at several stages during their preparation. Nitrogen gas sorption isotherms were collected and pore characteristics were examined for the MOCHs prior to the extraction step for comparison
3.2. Mesoporous carbon from single mix route The alumina removal by dissolution in hydrochloric acid was shown to be complete through thermogravimetric analysis. The sample was shown to leave essentially no inorganic mass after combustion in air as seen in Fig. 2. The dissolution of alumina in excess hydrochloric acid generates an acidic solution of aluminum chloride [32]. This particular aspect is indicative of the recyclability of the alumina template, if necessary. The gel precursor solution consists of an acidic solution of
Fig. 2. TGA thermogram of mesoporous carbon product from single mix route in air atmosphere.
Fig. 1. Synthetic schemes of (a) single mix route and (b) thixotropic mixing route. 3
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aluminum chloride salt and could be regenerated from the etching solution produced during template removal. Powder X-ray diffraction (XRD) patterns were collected for both the mesoporous carbon and the ball-milled mesoporous carbon (Fig. 3). Both products show broad Bragg peaks near 23 and 43 , which can be assigned to (002) and (011) peaks of graphite, respectively. The apparent peak shifts to a lower angle in comparison the graphite peaks are likely due to an increase in the d-spacing which may indicate that the graphitic layers in the products are spaced further apart compared to highly crystalline graphite. The presence of these peaks indicates that the graphite layers are not randomly rotated relative to one another as in turbostratic graphite [33], but there exists a regular ABAB stacking to some extent. Additionally, the mesoporous carbon also shows a diffuse hump near 80 that is near the graphite (110) and (112) peaks (Fig. S1), which also indicates regular stacking of graphite sheets. These peaks are less pronounced in the ball-milled carbon and could suggest a disruption in the ABAB stacking of hexagonal graphite which has been observed previously in ground graphite materials [34]. The graphitic nature of the prepared carbons was also investigated using Raman spectroscopy. Fig. 4 shows the collected Raman spectra for the mesoporous carbon and the ball-milled mesoporous carbon. Peaks near 1350 and 1600 cm-1 are the so-called D and G peaks, respectively. The presence of the G peak indicates existence of sp2-carbon atoms in graphitic arrangement, the appearance of such significant G bands is remarkable and yet is not new for a mesoporous carbon [35-36]. For example, ordered mesoporous carbon from template synthesis using
Fig. 3. Powder X-ray diffraction patterns of mesoporous carbon and ball-milled mesoporous carbon as compared to a calculated pattern of graphite. The peaks in calculated pattern of graphite are at 26.5 (002), 42.3 (010), 44.5 (011), 50.7 (012), 54.7 (004), 59.8 (013), 77.4 (110) and 83.5 (112).
Fig. 4. (a) Raman spectra of mesoporous carbon and ball-milled mesoporous carbon and (b) a fitted line shape (dashed line) of mesoporous carbon for determining relative peak intensities.
Fig. 5. (a) Nitrogen gas sorption isotherms and (b) BJH pore size distribution plots of the MOCH hybrid (▴), mesoporous carbon (■) and the ball-milled mesoporous carbon (●). 4
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carbon a capacity somewhat smaller than that. The porosity of the MOCH is due to the nature of xerogel (i.e., air-dried gel), which is consistent with the small pore size and small pore volume (Table 1). Upon removal of the oxide template, there is a large increase in pore volume from pores between 6 and 10 nm, as seen in the mesoporous carbon sample (Fig. 5b). The pore size distributions in Fig. 5b show that both carbon samples have approximately the same pore distributions, but the ball-milled samples show a large reduction in mesopore volume between 4 and 8 nm. The ball milling process is probably destroying the thin mesopore shells (see below). Upon a close inspection, the pore size distribution for the ballmilled carbon shows a slight increase in large mesopore volume between 10 and 50 nm, relative to the mesoporous carbon before milling. There is also a suggestion of the generation of macropores caused by the ball-milling step due to the slight upturn in the nitrogen isotherm for the ball-milled carbon at P/Po close to 1. This occurs in the presence of macropores that cannot be completely filled by capillary condensation [39]. Fig. 6a shows a TEM image of the mesoporous carbon in which the carbon materials appear as highly tortuous networks of thin shells. The nanostructure extends homogeneously well into a micrometer scale without crevices or coarse structural features, as seen in the SEM image (Fig. 6b). The morphology of the mesoporous carbon is very different from those of the porous carbon materials from typical R–F xerogels, cryogels and aerogels where the basic building blocks are circular nanoparticles in the range of 50 – 100 nm [40].
ordered mesoporous silica SBA-15 has partially ordered graphitic structure in its pore wall. Meanwhile, the strong intensity of the D peak implies that the sp2-carbon atoms are highly disordered. The degree of the disordering was compared between the two products, by using Tuinstra-Koenig formula, IðDÞ=IðGÞ ¼ C’ðλÞ=La , where C’ðλÞ ¼ ð2:4 1010 nm3 Þ λ4 and La is considered as the lower bound of the graphitic domain [37]. The domain sizes of the mesoporous carbon and the ball-milled mesoporous carbon are estimated to be 3.6 and 3.5 nm, respectively. The peak intensities were determined following the suggestions of Ferrari and Robertson by fitting a Breit-Wigner-Fano line to the G peak and a Lorentzian line to the D peak as shown in Fig. 4b [38]. In nitrogen gas sorption isotherms (Fig. 5a), all the materials show a hysteresis loop, indicating a mesoporosity. The surface areas, pore volumes and average pore sizes were calculated from the isotherms, as shown in Table 1. The MOCH material shows the lowest adsorption capacity because both inorganic and organic networks remain within the porous structure, but it is still highly porous. The mesoporous carbon shows the highest adsorption capacity and the ball-milled mesoporous
Table 1 Surface area and pore characteristics of mesoporous carbon and ball-milled mesoporous carbon. Sample
BET surface areaa (m2/g)
Pore volumeb (cm3/g)
Average pore sizec (nm)
Porosityd (%)
MOCH Mesoporous Carbon Ball-milled mesoporous carbon
442 1561
0.46 2.1
4 5
– 82
1380
1.6
5
78
a b c d
3.3. Mesoporous carbon from thixotropic mixing route After preliminary experiments, five different concentrations of R–F precursors were chosen to examine the effect of the polymer concentration on the porosity of the mesoporous carbon products as well as the intermediate materials during the synthesis. Although not shown here, all the mesoporous carbon products from the thixotropic mixing route showed the XRD patterns and Raman spectra that are identical to those from the single mix route. This is not surprising as the different methods would not lead to carbon materials with different nature in chemical structure. Nitrogen gas sorption isotherms of the MOCHs in Fig. 7a show a decrease in sorption as the polymer precursor concentration is increased. For all the samples except M-1, the isotherms exhibit a low pressure hysteresis at P/Po less than 0.4. This typically indicates the entrapment of nitrogen molecules within pores of comparable size to the adsorptive or the swelling of the porous structure during the measurement [39]. Since the material is rigid, it is highly unlikely that the latter is the case. Because the hysteresis loops are type H2 and they lack a hysteresis loop closure, the adsorption branch was used for all BJH analyses for the MOCH samples. The BJH pore size distributions for the MOCH samples in Fig. 7b show the presence of pores in the more or less same size range. It is interesting and yet puzzling that the pore sizes do not change
Pressure range P/Po ¼ 0.05-0.2. Single point desorption nearest P/Po ¼ 0.98. 4V/A (BET). Calculated based on the assumption ρcarbon ¼ 2.15 g/cm.3.
Fig. 6. (a) TEM and (b) SEM images of the mesoporous carbon. Scale bar ¼ 10 nm in (a) and 500 nm in (b).
Fig. 7. (a) Nitrogen gas sorption isotherms and (b) BJH pore size distributions of MOCHs M-1 (●), M-2 (■), M-3 (▴), M-4 (★), M-5 (◆). 5
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Journal of Solid State Chemistry 281 (2020) 121040
process. The increase in R–F polymer component, as in M-2 to M-5, does not improve the gel rigidity but rather clog up the pores in the gel network, reducing the porosity of the xerogels and MOCHs. Although not detailed here, TGA analysis indicated that the carbon contents in M-1 to M-5 were 39, 67, 76, 82 and 85 wt%, respectively. Based on TGA analysis, furthermore, the final mesoporous carbon product (C-5) from M-5 contained 15 wt% alumina after the acid etching, while all the rest of the mesoporous carbons from M-1 to M-4 did not include alumina. In other words, the large carbon content and consequent pore clogging in M-5 hampered etching of the alumina component in mesoporous carbon production within the time period of the acid-etching step. Fig. 8a shows a TEM image of the M-1, while the alumina and mesoporous carbon (C-1) from the M-1 are shown in Fig. 8b and 8c/d, respectively. The alumina was obtained after removing the carbon component by calcining the M-1 in air. The alumina exhibits the typical morphology of a gel which is made up of 5 – 10 nm-sized primary particles that are fused together, forming a 3D gel network structure. The anisotropic shape of the alumina primary particles is characteristic of the alumina gels prepared from aluminum chloride precursors [24,25]. The mesoporous carbon in Fig. 8c and 8d clearly displays its porosity with textural domains around 10 nm. The porosity in the mesoporous carbon is originated both from the sacrificial alumina network and from the innate porosity of the MOCH hybrid gel. The pores in the mesoporous carbon are interconnected as the sacrificial alumina has a continuous inorganic gel structure. Furthermore, at a low R–F polymer concentration, the pores in the MOCH hybrid are likely to be open and connected as expected in a dried and pyrolyzed gel. Fig. 9a shows the nitrogen gas sorption isotherms of the mesoporous carbon materials C-1 to C-5. The isotherm for C-1 shows a hysteresis loop of type H1, while samples C-2 through C-5 show a H2 type. This indicates that the mesoporous C-2 through C-5 likely experience pore blocking or percolation effects with a larger extent for a higher carbon precursor concentration. The adsorption capacity is reduced with increasing carbon precursor concentration, which follows the same trend seen for the MOCH samples. The calculated BET surface areas and pore volumes of the mesoporous carbons echo this trend in that decreases in BET surface area and total pore volume are observed with increasing carbon precursor concentration (Table 3). This reduction in pore space is shown for the mesoporous carbons in the BJH pore size distribution in Fig. 9b, as well as the continuous reduction of the average pore size for samples C-1 through C-4 (Table 3). The t-plot method was also used to calculate the surface area from micropores in the mesoporous carbons (Table 3). Both the specific micropore surface area and its contribution to the total surface area increase with carbon precursor concentration. Since the micropores form by pyrolysis throughout the body of the carbon, the microporosity would increase as the carbon content increases, which was indeed the case among our samples. For the C-5, the pore clogging does not allow a proper measurement of both microporosity and mesoporosity.
significantly with the changes in R–F polymer concentration. However, the specific surface area and pore volume do show a decrease with increasing carbon precursor concentration (Table 2) among the MOCH samples as well as among the corresponding hydrous alumina/R–F polymer xerogels (shown in the parentheses in Table 2). This suggests that even at a low concentration, R–F polymer component in the gel improves the rigidity of the gel network and thus effectively minimizes the pore collapsing caused by capillary pressure during the drying Table 2 Surface area and pore characteristics of MOCH products after pyrolysis. The corresponding properties of the samples before pyrolysis are given in parentheses. Sample
Resorcinol concentration (M)
BET surface areaa (m2/g)
Pore volumeb (cm3/g)
Average pore sizec (nm)
M-1 M-2 M-3 M-4 M-5
0.110 0.324 0.533 0.752 0.990
391 365 301 275 124
0.53 0.27 0.18 0.17 0.08
5.4 2.9 2.4 2.5 2.7
a b c
(268) (182) (128) (76) (40)
(0.28) (0.20) (0.16) (0.10) (0.05)
(4.2) (4.3) (5.0) (5.1) (4.7)
Pressure range P/Po ¼ 0.05 – 0.2. Single point desorption nearest P/Po ¼ 0.97. 4V/A (BET).
Fig. 8. TEM images of (a) hybrid M-1, (b) porous alumina from burning M-1, and mesoporous carbon C-1 in (c) high magnification and (d) low magnification.
Fig. 9. (a) Nitrogen gas sorption isotherms and (b) BJH pore size distributions of the mesoporous carbon samples C-1 (●), C-2 (■), C-3 (▴), C-4 (★), C-5 (◆). 6
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Table 3 Surface area and pore characteristics for mesoporous carbon samples. Sample
C-1 C-2 C-3 C-4 C-5 a b c d
Resorcinol concentration (M)
0.110 0.323 0.530 0.746 0.980
BET surface area (m2/g)a
Pore volume (cm3/g)b
1138 752 572 501 80
1.5 0.8 0.5 0.4 0.07
Average pore size (nm)c
5.3 4.0 3.4 3.1 3.4
Micropore surface aread (m2/ g)
(%)
50 107 214 257 52
4 14 37 51 65
Pressure range P/Po ¼ 0.05 – 0.2. Single point desorption nearest P/Po ¼ 0.97. 4V/A (BET). t-plot fit for thickness range of 0.35 – 0.5 nm.
Remarkably for the C-1, only 4% of the high surface area (1138 m2/g) is contributed from micropores, while the rest is from only mesopores with the average pore size around 5 nm. The elemental analysis of the carbon products from X-ray photoelectron spectroscopy indicates that the materials have contain mostly carbon (> 98 at%) with a small amount of oxygen (< 2 at%) (Fig. S2). The deconvolution of the high-resolution O1s spectrum indicates that most of the oxygen atoms are present as a hydroxyl group, which may be due to the aqueous etching process. 4. Concluding remarks We have successfully developed new synthetic routes to prepare highsurface area mesoporous carbons by using hydrous alumina gel as a soft template. The alumina gel soft template provides a three-dimensional continuous network of mesopores as its negative replica in the mesoporous carbon product. Controlled and sequential formation of the alumina network and polymer network guarantees the interpenetration of the two different gel networks which is important for ensuring the open and connected mesopores in the mesoporous carbon. Between the single mix and thixotropic mixing routes, it is found that the products from the latter have lower surface areas and that the template removal requires a harsher etching condition. Nevertheless, it is speculated that further optimization of the latter route may be beneficial. Namely, by taking advantage of the thixotropic nature of the hydrous alumina gel, the method may allow additional flexibility in synthetic design, such as production of hierarchically porous carbons by homogeneously incorporating large pore templates to introduce macroporosity in the final carbon materials. Acknowledgement This work was originally supported by the Center for Bio-Inspired Solar Fuel Production, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001016. We gratefully acknowledge the use of facilities within the Eyring Materials Center at Arizona State University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.121040. References [1] R. Pekala, J. Farmer, C. Alviso, T. Tran, S. Mayer, J. Miller, B. Dunn, Carbon aerogels for electrochemical applications, J. Non-Cryst. Solids 225 (1998) 74–80. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer Science & Business Media, 2013.
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