Partially graphitic, high-surface-area mesoporous carbons from polyacrylonitrile templated by ordered and disordered mesoporous silicas
Introduction
Graphitic mesoporous carbons (that is, those with pores of diameter 2–50 nm) are attractive materials for many applications, including liquid chromatography [1], [2], [3], adsorption, and manufacturing of electrochemical double-layer capacitors and Li-ion batteries. Graphitic carbons exhibit enhanced thermal stability and electrical conductivity, and have unique adsorption properties related to their highly homogeneous surfaces [1], [2], [3], in addition to their high degree of chemical inertness. Carbon structures with an appreciable degree of ordering of graphene sheets typically form as a result of a heat treatment at 2300–3300 K under inert atmosphere (argon, nitrogen) [4]. This high-temperature treatment is often referred to as “graphitization”, although its outcome depends not only on temperature (and other conditions), but also on the properties of the carbon sample subjected to heating. The addition of a catalyst to the carbon precursor or chemical vapor deposition of carbon can induce the formation of graphitic structures at much lower temperatures (818–1523 K) [2], [5], [6], [7], [8].
Some important beneficial features of graphitic carbons can be harnessed in materials with high surface area and readily accessible pores. However, except for the cases of graphitic carbon nanotubes (CNTs) [5], [9], [10], [11], [12], [13] and certain other carbon nanostructures composed of curved, closed graphitic surfaces [14], creation of high surface area inherently involves an increase in the surface heterogeneity through the formation of edges and defects in the stacked graphene sheets. Another challenge is the formation of graphitic carbons with permanent porous structures, especially those with controlled pore size and shape.
In the early 1980s, Knox et al. [1] reported a silica-templated synthesis of graphitized porous carbon with a specific surface area of 150 m2 g−1 using a phenol–formaldehyde resin as a carbon precursor. Since then, this carbon has been commercially available as a packing material for high-performance liquid chromatography (HPLC). More recently, carbon suitable for application in monolithic HPLC columns was prepared using a polymer prepared from resorcinol/iron(III) complex and formaldehyde [2]. An appreciable degree of graphitization was achieved with iron catalyst at temperature as low as 1523 K. This graphitic material had a specific surface area of ∼200 m2 g−1 with a contribution from micropores, and it had a rather low total pore volume. Li and Jaroniec [3] prepared a graphitized carbon from mesophase pitch, which was suitable for HPLC separations and had a specific surface area of 52 m2 g−1 and pore volume of 0.25 cm3 g−1. There are also numerous commercially available graphitized carbon blacks [15] with specific surface areas ranging from several to more than two hundred square meters per gram. Recently, there was some success in the synthesis of graphitized carbons with uniform pores of diameter in the mesopore range (2–50 nm) [16] or slightly above this size range. These carbons were prepared using the silica particles or silica colloidal crystals as templates. In particular, Li et al. synthesized mesoporous carbon from mesophase pitch using monodisperse silica particles to generate uniform pore voids with diameter of 24 nm, and graphitized this carbon under argon at ∼2670 K [17]. The pore size decreased to 16 nm and the pore size distribution (PSD) broadened, but appreciable pore volume (0.72 cm3 g−1) and specific surface area (239 m2 g−1) were retained. Low-pressure nitrogen adsorption measurements suggested that the surface of this carbon was as homogeneous as that of Carbopack X (225 m2 g−1) graphitized carbon black [15], although PSD of the silica-particle-templated carbon was much narrower. More recently, an ordered macroporous carbon replica of colloidal-crystal silica was synthesized using a mesophase pitch as a precursor and was graphitized at 2773 K [18]. The resulting material was highly graphitic, as seen from X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). The pore diameter decreased from ∼74 to ∼65 nm, but the periodic pore structure was retained.
Recently, there has been a growing interest in the synthesis of ordered mesoporous carbons (OMCs) [19], [20], [21] with graphitic frameworks using ordered mesoporous silicas (OMSs) as templates [22], [23], [24], [25], [26], [27], [28], [29], [30]. First, Kim et al. [22] reported that OMCs synthesized from a polyaromatic hydrocarbon (acenaphthene) carbonized at 1173 K exhibited some degree of graphitic ordering in their frameworks. A wide-angle XRD pattern of one of these carbons featured quite narrow peaks (the (0 0 2) peak had a full width at half height of ∼3°), and TEM images showed short graphene sheets stacked perpendicular to the surface of the material. Although the perfection of stacking was limited, the formation of semi-graphitic carbon structures as a result of the tendency of the carbon precursor to stack in a uniform way at low temperatures is quite remarkable. Stacking of imperfect graphene sheets perpendicular to the surface was also observed [28] for OMCs synthesized from mesophase pitch [28], [31], whereas some extent of the alignment parallel to the surface was observed for OMCs synthesized from polypyrrole [29]. Fuertes and Centeno [25] synthesized OMC with a small content (∼8 wt.%) of graphitic domains from pyrrole polymerized in the mesopores of OMS template impregnated with FeCl3. The synthesis of OMCs with graphitic ordering in the framework was also pursued by Xia, Mokaya, and coworkers using CVD of acetonitrile at temperatures from 1123 to 1373 K [26], [27], [30]. Higher CVD temperatures allowed them to obtain graphitic carbons with mesoporous structures. The specific surface areas and pore volumes of these carbons were from 280 to 880 m2 g−1, and from 0.26 to 0.70 cm3 g−1, respectively, and they decreased as the CVD temperature was increased. In addition to these partially graphitic carbons synthesized without the high-temperature treatment, Fuertes and Alvarez [24] synthesized OMC from poly(vinyl chloride) (PVC) and heated it at ∼2570 K for 0.5 h under argon atmosphere. The wide-angle XRD pattern for the resulting carbon featured (0 0 2) peak, which was very sharp, but appreciably broadened near the baseline. The graphitized carbon exhibited a single, very broad low-angle XRD peak, indicating some degree of local nanoscale ordering, although TEM showed a disordered structure. The obtained carbon exhibited a specific surface area of 260 m2 g−1, a pore volume of 0.34 cm3 g−1 and broad, perhaps bimodal, PSD.
Herein, it is shown that heat treatment at ∼2470 K under argon atmosphere appreciably improves the atomic-scale ordering in frameworks of high-pore-volume mesoporous carbons [32] synthesized from polyacrylonitrile (PAN) using a templating method [32], [33]. The evidence of the formation of graphitic structures was obtained from TEM, XRD and low-pressure nitrogen adsorption. The obtained partially graphitic carbons exhibited large mesopore volumes and high specific surface areas, although the carbons templated by OMCs largely or completely lost their nanoscale ordering. These results suggest that it will be possible to successfully graphitize carbons derived from PAN using other synthesis approaches, such as a direct conversion of PAN-containing block copolymers [34], [35], [36], [37], [38] to nanostructured carbons [34], [35], [38].
Section snippets
Materials
Ordered and disordered mesoporous carbons were synthesized using ordered mesoporous silicas (SBA-15 [39], [40], FDU-1 [41], [42]) and disordered silica gel Aldrich Si-150 as templates, and PAN as a carbon precursor, as described in detail elsewhere [32]. PAN was introduced to the pores of the templates through surface-initiated atom transfer radical polymerization (ATRP) [43], [44], [45], [46], [47], [48], [49], [50]. The silica/PAN composites were converted to silica/carbon composites through
Nitrogen adsorption, SAXS and TEM
As reported elsewhere [32], the templated mesoporous carbons from PAN exhibited nitrogen adsorption isotherms with prominent adsorption–desorption hysteresis loops and with capillary condensation steps located in the relative pressure interval from 0.6 to 0.95 (see Fig. 1, Fig. 2, Fig. 3). As can be seen in Fig. 1, Fig. 2, the isotherms for the OMCs heat-treated at 2470 K (CG-SBA-15 and CG-FDU-1) still featured pronounced capillary condensation steps, thus indicating the preservation of the
Conclusions
The described results demonstrate the synthesis of partially graphitic high-surface-area carbons with very high mesopore volumes, using PAN as a precursor and mesoporous silicas as templates. The heat treatment of mesoporous carbons from PAN at ∼2470 K under argon induced the formation of graphitic domains, significantly enhanced the surface homogeneity and eliminated the microporosity. This was paralleled by a moderate reduction of the specific surface area (by 30–40%), whereas the pore volume
Acknowledgments
The support from NSF Grant DMR-0304508 is gratefully acknowledged. K.M. also acknowledges support from NSF Grant DMR-0090409. Abigail M. Laurent and Dr. Todd Przybycien (Carnegie Mellon University) are acknowledged for help in the Raman spectroscopy measurements. SAXS measurements conducted at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the National Science Foundation under award DMR-0225180. Dr. Detlef-M. Smilgies (CHESS, Cornell University) is gratefully
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Present address: Department of Chemistry, College of Staten Island and Graduate Center, City University of New York, 2800 Victory Boulevard, Staten Island, NY 10314, USA.
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Present address: Corning SAS, 7bis avenue de Valvins, 77210 Avon, France.