Controlled Synthesis of Mesoporous Carbon Nanostructures via a

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Letter pubs.acs.org/NanoLett

Controlled Synthesis of Mesoporous Carbon Nanostructures via a “Silica-Assisted” Strategy Zhen-An Qiao,† Bingkun Guo,† Andrew J. Binder,‡ Jihua Chen,§ Gabriel M. Veith,∥ and Sheng Dai*,†,‡ †

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States § The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056, United States ‡

S Supporting Information *

ABSTRACT: We have established a facile and generalizable “silica-assisted” synthesis for diverse carbon spheresa category that covers mesoporous carbon nanospheres, hollow mesoporous carbon nanospheres, and yolk-shell mesoporous carbon nanospheresby using phenolic resols as a polymer precursor, silicate oligomers as an inorganic precursor, and hexadecyl trimethylammoniumchloride as a template. The particle sizes of the carbon nanospheres are uniform and easily controlled in a wide range of 180−850 nm by simply varying the ethanol concentrations. All three types of mesoporous carbon nanospheres have high surface areas and large pore volumes and exhibit promising properties for supercapacitors with high capacitance and favorable capacitance retention. KEYWORDS: Mesoporous carbon nanospheres, hollow nanospheres, yolk-shell, silica-assisted, supercapacitors

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hydrothermal route, which was developed by Zhao and coworkers.24 In this work, a hydrothermal treatment with a controlled low concentration of a surfactant (approximately 10−7 mol L−1 F127) was carried out to obtain the nanostructure and to confine the particle size. This technique results in a low yield and difficult procedures for isolating the product. Hence it remains a great challenge to develop a general, effective approach for preparing high-yield MCNs, especially with highly controlled nanostructure and particle size. Herein we describe a new method, referred to as a “silica-assisted” strategy, for the facile synthesis of discrete and highly dispersible MCNs, hollow MCNs (HMCNs), and yolk-shell MCNs (YSMCNs) that have tailorable particle size and cavity structure. The process uses phenolic resols as a polymer precursor, TEOS as an inorganic precursor, and hexadecyl trimethylammonium chloride (CTAC) as a template (Scheme S1, Supporting Information). Carbonization was followed by etching of the silica in the carbon/silica nanocomposite, resulting in the formation of MCNs. The mesostructures were retained while the spherical diameters were tuned from 180 to 850 nm by simply varying the ethanol concentration for these three types of MCNs. Additional porosity is produced by removing the silica present in the walls, and shrinkage is reduced during carbonization;

n recent years, monodispersed carbon nanospheres have received considerable attention because of their potential applications in drug delivery, catalysis, energy storage, and active material encapsulation.1−10 Mesoporous carbon nanospheres (MCNs) have many advantages over solid materials including ordered mesostructure, large surface area and porosity, and open-framework structuresthat enable them to be widely used in absorbents, drug/gene carriers, supercapacitors, fuel cells, lithium batteries, and particle templates.11−14 Although many efforts have been made to fabricate MCNs,15−17 most pathways for MCN production rely on hard templating with mesoporous silica nanoparticles or colloidal crystals.18−21 However, the size, mesostructure, and morphology of the replicated MCNs are limited to the parent template.22 For example, Nazar et al. reported a two-step casting process to obtain MCNs with a hierarchical porosity.23 An opal structure of PMMA spheres was cast with a silica precursor solution to form a silica inverse opal, which was then used as a template for a triconstituent precursor solution containing resol as the carbon precursor, tetraethylorthosilicate (TEOS) as the silica precursor, and the block copolymer Pluronic F127 as a structure-directing agent. After hightemperature carbonization to yield a silica−carbon composite material, the silica template and the silica in the carbon/silica nanocomposite were later removed under a hydrogen fluoride (HF) solution. More recently, MCNs with diameters ranging from 20 to 140 nm were synthesized by a low-concentration © 2012 American Chemical Society

Received: October 22, 2012 Revised: December 14, 2012 Published: December 20, 2012 207

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Figure 1. SEM (a, b, d, e, g, and h) and TEM (c, f, and i) images of mesoporous carbon nanospheres with different particle sizes. (a−c) MCNs-180 with a diameter of 180 nm; (d−f) MCNs-420 with a diameter of 420 nm, and (g−i) MCNs-850 with a diameter of 850 nm.

Figure 2. SEM (a, e and h) and TEM (b, c, d, f, g, and i) images of hollow mesoporous carbon nanospheres with different particle sizes. (a−d) HMCNs-180 with a diameter of 180 nm; (e−g) HMCNs-450 with a diameter of 450 nm, and (h, i) HMCNs-800 with a diameter of 800 nm. (j) N2 adsorption−desorption isotherms of the MCNs-420 (A), HMCNs-450 (B), and YSMCNs-450 (C).

conversion and storage and as adsorbents for drug delivery and water treatment processes. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images show that the three representative MCN samples synthesized by the “silica-assisted”

therefore, very high porosity is created. All three MCN types exhibit properties that are promising for use in supercapacitors with high capacitance and favorable capacitance retention and show great potential for prospective applications in energy 208

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Figure 3. SEM (a, c, e, and g) and TEM (b, d, f, and h) images of yolk-shell mesoporous carbon nanospheres with different particle sizes. (a−d) YSMCNs-450 with a diameter of 450 nm and (e−h) YSMCNs-880 with a diameter of 880 nm.

800 nm; the hollow core sizes and shell thickness were in the ranges 100−550 nm and 45−115 nm, respectively. These HMCNs have high BET surface areas of 1098−1312 m2 g−1, large pore sizes from 3.2 to 3.6 nm, and large pore volumes from 1.0 to 1.05 cm3 g−1 (Table S1). Yolk-shell structures are powerful platforms for controlled release, confined nanocatalysis, and optical and electronic applications.26,27 Conventional methods require the preparation of core/shell nanoparticles, and the synthetic procedures are multistep and complex. Using our “silica-assisted” method, it is apparent that uniform and highly monodisperse mesoporous carbon yolk-shell spheres with a size of 450−800 nm are obtained in a one-pot process by hydrothermal treatment of resol−silica nanocomposites for hollow nanospheres (Figure 3 and Figure S2). The yield of the product is very high (>85%), and each is encapsulated by a thin mesoporous carbon shell. As with MCNs and HMCNs, varying the ethanol/water volume ratio from 1:2.71 to 1:2.38 for synthesis solutions has a dramatic effect upon the particle size: the average diameter of the YSMCNs could be tailored from 450 to 880 nm, and the core sizes and shell thickness were in the ranges of 190−420 nm and 85−110 nm, respectively. These spheres have high BET surface areas of 629−777 m2 g−1, with total pore volumes of 0.73−0.79 cm3 g−1 (Table S1). Although it is difficult to determine if both the cores and shells have mesopores, we think the carbon shells have wormhole-like mesopores with diameters of about 4.1−4.7 nm, as confirmed by the TEM and N2 sorption analyses. The yolk carbon particles should be solid because of the lower BET surface areas compared with those of hollow spheres with the same diameter (Table S1). All of the MCNs, HMCNs, and YSMCNs smaller than 600 nm in diameter that were prepared by this method can be dispersed in water by sonification to form stable colloidal suspensions, which have remained stable over the entire period of this study. The resol−silica nanospheres are synthesized by the polymerization of phenolic resols and silica in a mixture of alcohol, CTAC, and aqueous ammonia, a process similar to the synthesis of solid carbon spheres by the sol−gel method.3 Emulsion droplets are first formed through the hydrogen bonding of water, alcohol, resorcinol, formaldehyde, and silicates. The polymerization of phenolic resols and silicate oligomers takes place from the outside of droplets by ammonia

approach have uniform spherical morphology in large domains (Figure 1). It is remarkable that a large number of mesopores can be clearly observed from the exposed hemispheres, implying an open pore structure on the surface. Individual particles with smooth surfaces are seen in the corresponding TEM images, with pores radially oriented from the center to the outer surface. All of the spheres exhibit uniform projections with the same pore arrangement patterns, indicating that the particles indeed have a spherical morphology and the pores are distributed in a spherically symmetric mode. The pore size is roughly estimated at about 2.4 nm. The diameter of the mesoporous carbon spheres increases from 180 to 850 nm as the ethanol/water volume ratio increases from 1:4.75 to 1:2.38. The N2 adsorption−desorption isotherms of the MCN samples (Figure 2j) exhibit typical type-IV hysteresis, indicative of the presence of mesopores. It can be seen that the adsorption isotherm of each sample shows an apparent capillary condensation step at a relative pressure (P/P0) of 0.14−0.32; this corresponds to a pore size of 2.4 nm, which agrees well with the size evaluated from TEM images, as calculated with the Barrett−Joyner−Halenda (BJH) method. In addition to the pore condensation step, isotherms show increases in adsorption at a low relative pressure, suggesting the existence of a large number of micropores in the carbon nanospheres after the silica is removed from the carbon−silica composites. Moreover, a hysteresis loop at a higher pressure (P/P0 above 0.9) may reflect the interparticle texture between the carbon nanospheres. These MCNs have high BET surface areas in the range of 1044−1129 m2 g−1. The pore sizes are calculated at 2.4−2.7 nm, and the pore volumes are calculated as being as large as 0.66−0.76 cm3 g−1 (Table S1 of the Supporting Information). As the amount of TEOS increased by molar ratio of TEOS/ resorcinol from 0.95 to 1.9, monodisperse hollow carbon nanospheres with radially aligned mesopores in carbon shells are obtained. As shown in Figure 2 and Figure S1 of the Supporting Information, the remarkable feature of the spheres is the obvious contrast between the dark edge and the pale center, as is reported for other hollow particles with a central cavity.25 It is observed that carbon spheres, synthesized at an ethanol/water volume ratio of 1:4.75, have very thin shells with a large number of mesopores (Figure 2c,d). As with MCNs, varying the ethanol/water volume ratio from 1:4.75 to 1:2.38 could tune the average diameter of the HMCNs from 180 to 209

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Figure 4. Cyclic voltammograms for HMCNs-180 (a), HMCNs-450 (b), MCNs-420 (c), and YSMCNs-450 (d) at different scan rates.

carbon spheres with a diameter of 70 nm. This indicates that the silicates are indeed a key factor in generating the mesoporous structure. In the absence of phenolic resols, the reaction mixture is a typical synthesis system of mesoporous silica nanospheres, and discrete mesoporous silica nanospheres of 80 nm in diameter were obtained (Figure S4).31 After carbonizing resol−silica nanospheres, we could obtain carbon−silica composite nanospheres, including mesoporous carbon−silica nanospheres, hollow mesoporous carbon−silica nanospheres, and yolk-shell mesoporous carbon−silica nanospheres (Figure S5). These spheres are characterized by a continuous composition of the two constituents that are “homogeneously” dispersed inside the pore walls. By combustion in air to remove carbon from the three types of mesoporous carbon−silica composite nanospheres, we could obtain mesoporous and hollow mesoporous pure silica frameworks nanospheres (Figure S6), further indicating that the yolk carbon particles of YSMCNs are solid. All the carbon− silica composite nanospheres and pure silica nanospheres have lower BET surface areas than those of carbon nanospheres, in the range of 294−472 m2 g−1 and 456−724 m2 g−1 (Figure S7 and Table S2). The XRD patterns and Raman spectrum reveal that the resultant carbon nanospheres possess an amorphous nature (Figure S8). X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemistry of the carbon spheres. In all cases there was no evidence of Si after etching with HF indicating the complete removal of the silica (Figure S9). The surface chemistry of the carbon spheres were dominated by carbon and oxygen. The O1s data show the presence of two unique oxygen species located at ∼531 and 533 eV, which are attributed to CO and CO, respectively.32 The C1s data reveals the carbon surface chemistry is dominated with C−C or

catalysis, resulting in uniform colloidal spheres. At low molar ratio of TEOS/resorcinol (0.95), the polymerization between phenolic resols and silicates inside of droplets takes place very homogeneous, which lead to the formation of solid resol−silica nanospheres. On increasing the molar ratio of TEOS/ resorcinol to 1.9, not enough phenolic resols could diffuse into the droplets and polymerize with silicates, therefore hollow resol−silica nanospheres are obtained. During the hydrothermal treatment of hollow resol−silica nanospheres, more phenolic resols could diffuse into the resol−silica hollow cores and further polymerize to form smaller resol nanospheres; therefore YSMCNs are obtained after carbonization and removal of the silica (Scheme S1). On the basis of these observations, we propose a silicaassisted assembly process for the formation of MCNs. The choice of TEOS as an inorganic precursor to assist coassembly of inorganic and organic phases is the key to the successful synthesis of MCNs. Because of the strong electrostatic interface interaction between silicate anions and cationic surfactant molecules, CTAC surfactant molecules interact preferentially with silicates, which could drive the assembly and organization of the surfactant and micelles into the framework of the resol nanospheres.28,29 It is well-known that TEOS or oligomer silicate species can react with the hydroxyl group of phenol or phenolic resins. Silicate oligomers undergo a cooperative process, during which they not only assemble with the surfactants but also condense and cross-link together to form the framework.30 In addition, supplementary experiments were carried out in the absence of TEOS while keeping the other reaction conditions the same as those used for MCNs (ethanol/water volume ratio 1:4.75). As a consequence, solid spheres as opposed to mesoporous ones were obtained. As seen in Figure S3, they are mainly composed of aggregated solid 210

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C−H species (284.8 eV), along with a smaller concentration of C−O (286 eV), O−CO (289 eV) which is consistent with the O1s spectra.32 Further investigation of the XPS data reveals the presence of low concentrations of F (689 eV) and N (399 eV) on the surface of the carbon (Figure S9 and Table S3). These species originate from the reaction of F− (from HF) with the carbon, and the decomposition of CTAC during the synthesis of the carbon sphere resulting in the formation of C− F species (291 eV)33 and C−N (286 eV)34 evident in the C1s data. The electrochemical properties of the obtained mesoporous carbon nanomaterials were measured by a cyclic voltammogram (CV). Figure 4 shows the CV curves of the carbon nanosphere samples obtained with a three-electrode cell under a potential window of 0.0−0.8 V (vs Ag/AgCl) at different scan rates. It can be observed that all of the samples present a relatively good rectangular shape at a voltage scan rate of up to 200 mV s−1, indicating typical double-layer capacitance behavior and remarkable high rate capability. The specific capacitances of MCNs-180, MCNs-420, HMCNs-180, HMCNs-450, and YSMCNs-450 are 80, 77, 95, 83, and 78 F g−1, respectively (Figure 4 and Table S4). In addition, the high surface areas of MCNs, as demonstrated above, allow a large amount of electrical charge to accumulate on the electrode/electrolyte interface. The small particle size provides a large additional pseudocapacitance because it shortens the ion transport length and makes ion diffusion in the carbon nanospheres easier than in traditional bulk porous materials. The combination of the two factors leads to the high capacitance of MCNs. Apart from the high capacitance, our porous carbon nanospheres also possess excellent capacitance retention. Even at an extremely high voltage scan rate of 200 mV s−1, the specific capacitance for all carbon spheres remains nearly 63%, superior to that of many previously reported advanced carbon materials such as porous graphitic carbon (Figure S10).35−38 In summary, we have demonstrated a new concept, a silicaassisted method, for producing individual and dispersible MCNs, including MCNs, HMCNs, and YSMCNs. The synthesis with high yield (≥76.6%) can be carried out on a relatively large scale (grams) by using CTAC as a template, phenolic resol as a carbon source, and silicate oligomers as inorganic precursors. The particle sizes of the carbon nanospheres are uniform and easily tunable from 180 to 850 nm by simply varying the ethanol concentrations. The MCNs have high surface areas and large pore volumes. The three types of MCNs obtained using this method exhibit not only promising properties for supercapacitors with high capacitance and favorable capacitance retention but also an ideal model system for advanced energy storage materials, adsorbents, catalyst supports, drug delivery carriers, and templates. Furthermore, it is easy to envisage that this silica-assisted strategy can also be applied to the preparation of other mesoporous metal and mesoporous metal oxide nanomaterials with highly controlled nanostructures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy, under contract no. DEAC05-00OR22725 with Oak Ridge National Laboratory managed and operated by UT-Battelle, LLC. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, and Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy under contract with UT-Battelle, LLC (GMV-XPS).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, Scheme S1, Tables S1, S2, S3, and S4, and Figures S1, S2, S3, S4, S5, S6, S7, S8, S9, and S10. This material is available free of charge via the Internet at http:// pubs.acs.org. 211

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