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Article Cite This: Chem. Mater. 2017, 29, 10178−10186
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Generalized Synthesis of a Family of Highly Heteroatom-Doped Ordered Mesoporous Carbons Zhe Qiang,†,§,∥ Yanfeng Xia,‡,∥ Xuhui Xia,† and Bryan D. Vogt*,† †
Department of Polymer Engineering, The University of Akron, 250 South Forge Street, Akron, Ohio 44325, United States Department of Polymer Science, Goodyear Polymer Center, The University of Akron, 170 University Circle, Akron, Ohio 44325, United States
‡
S Supporting Information *
ABSTRACT: High concentrations of heteroatom can be doped into ordered mesoporous carbon by infiltration of molten dopants into silica-reinforced mesoporous cross-linked polymer (resol) and subsequent carbonization. The high concentration of dopants relative to polymer enables a high probability of heteroatoms to be dynamically integrated into the framework through carbonization, while the silica in the framework prevents loss of the ordered structure. This method is demonstrated to generate ordered mesoporous carbons with high heteroatom content (up to 26 atom % N, 15 atom % B, 7 atom % P, or 4 atom % S) for a wide variety of elements through melt infusion of the appropriate dopant (melamine, boric anhydride, ammonium dihydrogen phosphate, or dibenzyl sulfide). The ratio of the solid dopants to mesoporous silica−resol in a physical mixture during the melt infusion provides a simple methodology to precisely tune the doping content in the carbonized material. Etching of the silica postcarbonization generates additional micropores to produce porous doped carbons with high surface areas that can exceed 2000 m2/g. Increasing the doping of the mesoporous carbon leads to a decrease in the surface area as the framework is swollen during the incorporation of the heteroatoms that leads to an increase in the d-spacing of the mesostructure, but the ordered structure is maintained with well-defined mesopores. With the advantages of high surface area, well-defined pore size, and tunable doping concentration through a straightforward methodology, it is expected that this family of mesoporous carbons will provide model materials for fundamental studies in diverse applications from energy storage to catalysis to adsorption for separations.
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INTRODUCTION Carbonaceous materials are ubiquitous in modern technologies, including the graphitic anodes in most Li ion batteries,1 carbon black in tires,2 and activated carbon for water filtration.3 This wide diversity of existing applications for carbon is enabled by its unique set of chemical and physical characteristics, including electric conductivity, thermal conductivity, excellent chemical stability, low density, wide availability, and low cost.4 The significant research activity in low-dimensional carbons5 has driven the consideration of carbonaceous materials for a variety of additional applications in catalysts (including catalyst supports),6 gas storage materials,7 sorbents for separation processes,8 and electrode materials for batteries,9 fuel cells,10 and supercapacitors.11 In a number of these applications, modification of the physicochemical properties of carbon is desired to modulate interactions with other species in the system,12 the band gap,13 or other properties. Doping of the carbon framework with heteroatoms provides a facile route to readily modulate the properties of carbon.14 The incorporation of heteroatoms into the carbon framework provides the opportunity for material property enhancement.15 The catalysis of the oxygen reduction reaction (ORR) © 2017 American Chemical Society
represents one common application for doped carbons, where the reaction can be catalyzed without requiring expensive metals such as Pt or Pd.16 The most commonly used doping element is nitrogen, which can be readily incorporated due to its similar atomic size compared to that of carbon, and nitrogen-doped carbon provides enhanced ORR performance.17 However, other heteroatoms, including boron,18 phosphorus,19 and sulfur,20 have been shown to enhance the ORR. The ability of multiple heteroatoms to promote the ORR suggests that the key parameter for the efficacy is associated with the electronic band structure of the material.21 In this respect, the performance is determined by the combination of the selection of the heteroatom, the specifics of its incorporation (bonding), and its concentration.15 Similar to the ORR, doping of carbon has been demonstrated to be an effective route to improve the performance for electrochemical energy storage.22 These studies have demonstrated the need for well-defined heteroatom-doped carbon materials. Received: September 24, 2017 Revised: November 20, 2017 Published: November 20, 2017 10178
DOI: 10.1021/acs.chemmater.7b04061 Chem. Mater. 2017, 29, 10178−10186
Article
Chemistry of Materials Several strategies have been developed to fabricate doped carbons. One common strategy is chemical vapor deposition (CVD) with heteroatom-containing precursors to generate doped carbon nanotubes,23 graphene,24 and carbon fibers.25 The high-vacuum requirements and limited productivity lead to some challenges in the scale-up of these processes for commercial applications. Alternatively, solid heteroatomcontaining precursors, such as melamine and phytic aciddoped polypyrrole, can be directly converted to doped carbon through pyrolysis processing.26,27 One drawback to the direct conversion is the limited efficiency for the incorporation of the heteroatom, leading to limited doping levels.28 Alternatively, physical mixing of the heteroatom precursors with carbon precursors or carbon materials provides a reasonable route to fabricate doped carbons. For instance, melamine,29 benzyl disulfide,30 boric anhydride,31 and organophosphonic acids32 can be used to fabricate nitrogen-, sulfur-, boron-, and phosphorus-doped carbon, respectively. The concentration of heteroatoms in the final carbon material is generally limited by the metastability of the heteroatom doping in the framework relative to graphitic carbon.33 In addition, many applications for heteroatom-doped carbons require a high surface area,15 so the collapse of soft templated mesoporous materials at high concentrations of added dopant precursors generally provides an additional limitation to obtaining high heteroatom content in carbons.33 In this work, an alternative generalizable route to highly doped ordered mesoporous carbons (OMCs) is demonstrated that exploits the mechanical reinforcement of silica in mesoporous silica−polymer precursors. The heteroatom dopants are melt infused into the mesopores to provide intimate contact with the thin pore walls to promote the incorporation of the desired heteroatoms into the framework during carbonization of the mixture. We demonstrate the ability to use this method to fabricate ordered mesoporous carbons that are doped with high levels of nitrogen, boron, phosphorus, and sulfur with maximum heteroatom contents of 26, 15, 7, and 4 atom %, respectively. The doping level in the product can be simply controlled by the physical mixture ratio of the mesoporous silica−polymer and the heteroatom dopant. The incorporation of the heteroatoms tends to decrease the specific surface area due to the added mass from the doping as the dspacing increases and the pore size decreases with increasing dopant content. The mechanical reinforcement of the framework during the doping and carbonization by the silica enabled the ordered structure to be maintained. This methodology appears generalizable and can be applied to fabricate high surface area carbons with controlled heteroatom content, with the doping levels exceeding those readily obtainable by other methods.
Scheme 1. Schematic Illustration of the Synthetic Route to Heteroatom (Nitrogen/Sulfur/Boron/Phosphorus) Doped Ordered Mesoporous Carbons that First Entails (A) Cooperative Assembly To Form Micelle-Templated CrossLinked Resol−Silica, (B) Calcination at 350°C To Yield Mesoporous Polymer−Silica, (C) Dopant Melt Diffusion for Infiltration of Mesopores and Subsequent Carbonization To Form Mesoporous Doped Carbon−Silica, and (D) Etching of Silica To Yield Mesoporous Doped Carbon with Micropores in the Framework
reinforcement to the framework during the doping reaction and carbonization. This reinforcement has been shown to inhibit collapse of the nanostructure during carbonization in thin films 36 and also limits the distortion of the ordered mesostructure during carbonization.37 The surface area of these composites can be dramatically increased by etching of the silica to generate micropores in the nanostructure framework. The separation of the self-assembly and inclusion of the dopants allows a much higher concentration of heteroatoms during the carbonization compared to previous methods that required templating of heteroatom-containing carbon precursors.38−41 The carbonization and reactions between the dopant (or its decomposition product) and carbon precursor occur in a single process in this work, so the exact mechanisms associated with the doping are not well understood. However, the phenolic resin precursor is polyaromatic with many oxygen defects, so the doping of graphene oxide29 is likely an appropriate analogue. The decomposition of the precursors will generate reactive species that can be incorporated into the phenolic resin likely through hydrogen abstraction, dehydration, or deoxidation. The details of the reactions involved in the fabrication of the final carbon product will depend on the selection of the dopant. Figure 1 illustrates the efficacy of this protocol for the incorporation of the heteroatom into the carbon framework as determined by X-ray photoelectron spectroscopy (XPS). Due to the similarity in size between N and C, the nitrogen doping should be the most effective. As shown in Figure 1A, inclusion of melamine during the carbonization of the ordered mesoporous cross-linked resol−silica leads to nitrogen-doped carbon with clear N 1s peaks from 396 to 402 eV in the XPS spectra. At low loadings of melamine (nitrogen precursor), the N 1s peak is faint due to the limited nitrogen content (2.8 atom %) when the mass ratio of the dopant (melamine to polymer− silica ratio (md/mp) is 0.1. By increasing this ratio, the N 1s peak intensity increases with 26.8 atom % N when md/mp reaches 30. This systematic increase in intensity as the organic
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RESULTS AND DISCUSSION A common combined soft−hard template of mesoporous crosslinked resol−silica was used as the starting material for all of the doped mesoporous carbons as illustrated in Scheme 1. The first step is the synthesis of the mesoporous cross-linked resol−silica using cooperative self-assembly and calcination at 350 °C.34 To control the heteroatom and extent of doping of the carbon product, the mesoporous resol−silica is physically mixed with the appropriate dopant that can infiltrate into pores on heating. Carbonization of the mixture yields heteroatom-doped mesoporous carbon−silica composites. The embedded silica nanoparticles that are dispersed in the cross-linked resol35 act as 10179
DOI: 10.1021/acs.chemmater.7b04061 Chem. Mater. 2017, 29, 10178−10186
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Chemistry of Materials
polymeric framework (resol) or silica. As shown in Figure 1B, benzyl disulfide, ammonium dihydrogen phosphate, and boric anhydride can be used to introduce sulfur, phosphorus, and boron atoms into the carbon framework. These dopants were selected due to their availability and prior success in doping carbons, but other dopants that can be infiltrated into the mesopores should also work for the doping process. For Figure 1B, a single heteroatom dopant loading of 10 (md/mp) is used. The peaks associated with these heteroatoms are much reduced in comparison to that obtained for nitrogen, but the carbon was still doped under these conditions. At md/mp = 10, the carbons contain 24.8 atom % N with melamine as the dopant, but the doping content decreases to 8.9 atom % B from boric anhydride, 4.8 atom % P from ammonium dihydrogen phosphate, and 2.9 atom % S from benzyl disulfide. The fabrication mass ratio (md/mp) can also be used to control the doping level of these heteroatoms as shown in Figure S1 and Tables S1−S4 (Supporting Information). Analysis of the XPS data provides the relationship between the heteroatom concentration in the mesoporous carbon and the ratio of the dopant to the mesoporous polymeric−silica precursor as shown in Figure 1C. For nitrogen doping, the N content in the mesoporous carbon increases continuously as md/mp increases from 0.1 to 5, but further increasing md/mp does not statistically change the heteroatom content with a plateau in the N content at approximately 25 atom % N. This ease in the incorporation of N is likely associated with the similarity of the size of N (65 pm) and C (70 pm).48 With the other dopant materials examined here, a higher md/mp ratio was required to achieve substantial heteroatom doping in the mesoporous carbon. At md/mp = 1, an order of magnitude lower concentration of heteroatoms is incorporated into the mesoporous carbon in comparison to N. However, as md/mp is further increased, higher contents of B, P, and S can be obtained. For the dopants used here, the highest heteroatom content obtainable for nitrogen-, boron-, phosphorus-, and sulfur-doped mesoporous carbon is 26.3, 15.5, 6.9, and 4.2 atom %, respectively, as shown in Figure 1C. These XPS results demonstrate the capability to dope heteroatoms into mesoporous carbon at high concentrations through this approach, but the maximum doping concentration is dependent on the selection of heteroatoms. To explain this difference, the heteroatom content in these selected dopants needs to be considered first as the density of the heteroatom can vary significantly with the precursor selection. Melamine contains the highest heteroatom concentration, with approximately 67 wt % nitrogen; while it decreases to 31.4 wt % B for boric anhydride, 27 wt % P for ammonium dihydrogen phosphate, and 26 wt % S for benzyl disulfide. Thus, at the same md/mp, the concentration of heteroatoms present may vary by a factor of 2.5. Correcting for the elemental availability, the inset in Figure 1C shows how the heteroatom content in the carbon framework depends on the mass of the doping element in the precursor to the mass of the carbon precursor (resol) (me/mr). A plateau in the heteroatom content (maximum) occurs at approximately me/mr = 1 for N, B, P, and S, which indicates a common limitation for the incorporation of heteroatoms with this method. However, the concentration for this plateau depends on the heteroatom. As the relative difference in atomic size between the heteroatom and carbon increases, the maximum concentration of the heteroatoms in carbons decreases due to the increased energetic penalty associated with the incorporation of the
Figure 1. XPS survey scan spectra of doped ordered mesoporous carbons (A) as a function of the mass ratio of the melamine (md) to the mesoporous polymer−silica (mp) for nitrogen-doped carbons and (B) with fixed md:/mp = 10 using melamine (N), benzyl disulfide (S), ammonium dihydrogen phosphate (P), and boric anhydride (B) as the dopants. (C) Impact of md/mp during fabrication on the heteroatom content in the ordered mesoporous carbon (inset as a function of the mass of the dopant element to mp).
N functional dopants increases in the precursor indicates that the N-doping content of the carbons can be controlled simply by this ratio. It is important to note that soft templated mesoporous carbon tends to exhibit loss of ordering of the mesostructure by the addition of the excessive dopants during assembly, such that the maximum N content in the ordered systems is usually less than 10 atom %,42,43 so the high concentration of N in the doped carbon at high md/mp with the approach illustrated in Scheme 1 is greater than can typically be obtained with soft templating. Similarly, the N content obtained from the direct doping of carbon also tends to be limited to 1, the fraction of pyrrolic N remains almost constant at ∼8 atom %, while the pyridinic N increases from 9.5 to around 14 atom % (Table S6). This may be associated with defect (five-element ring) density in the polymeric framework, which sets the upper limit for the content of pyrrolic N in the doped carbon. For the case of boron-doped mesoporous carbon (Figure S5 and Table S7, Supporting Information), binding energies of 189, 190.4, and 191.9 eV correspond to BC3, BC2O, and BCO2, respectively. As the B doping increases, there is a prominent increase in the BC3 component from 0.01% to 1.89% and the BC2O component from 0.67% to 4.04%, while the increase in the BCO2 component is more modest from 0.11% to 0.97% as shown in Figure S4 and Table S4. This result shows that BC2O is the most favorable component, which is consistent with previous work on B-doped carbons.59 However, this work yields a higher B content than previously reported. For the phosphorus-doped mesoporous carbon, the binding energies of 133.3 and 132.3 eV correspond to P−C and P−O. As shown in Figure S6 and Table S8 (Supporting Information), the fraction of P−O component increases from 0.8 to 6.9 atom % as md/mp increases from 1 to 50, while the P−C component increases from 2.6 to 8.5 atom %. For sulfur-doped ordered mesoporous carbon, the binding energies of 163.5, 164.9, and 164.3 eV correspond to S 2p3/2, S 2p1/2, and S−O. As md/mp increases from 1 to 50, the ratio of S 2p3/2 to S 2p1/2 to S−O remains almost constant at 4:2:1 (as shown in Figure S7 and Table S9, Supporting Information), which agrees with the typical S doping of carbons in terms of the types of binding in the framework.60−62 A summary of the compositions of the doped mesoporous carbons on an absolute scale is shown in Figure S8 (Supporting Information) along with confirmation by FTIR of the chemistry of the doped mesoporous carbons (Figure S9, Supporting Information). To understand how the doping reaction influences the pore architecture, gas (N2) sorption isotherms are obtained to elucidate information about the pore texture in these mesoporous carbons, as shown in Figure S10 (Supporting Information). From these isotherms, the pore characteristics, including the Brunauer−Emmett−Teller (BET) surface area and pore size, are obtained as summarized in Table S1. For all nitrogen-, boron-, and phosphorus-doped carbons examined, the type IV isotherms transition from an H2 hysteresis loop at low doping content to an H3 hysteresis loop at high doping content. This change in the isotherm corresponds to a change in constricting connections between the mesopores from relatively uniform (H2) to a broader distribution that leads to a more gradual emptying of the pores on desorption. This transition from H2 to H3 occurs at 26.8 atom % N, 13.6 atom % B, and 4.8 atom % P, while the isotherms for the sulfurdoped mesoporous carbons always contain an H2 hysteresis loop. The distortion of the connecting pores between the mesopores is consistent with a change in the nanostructure by the SAXS (Figure S3) for the B- and P-doped carbons; the primary scattering peak associated with the ordered structure becomes ill defined for samples that exhibit an H3 hysteresis loop. From analysis of the adsorption isotherms, information on the average pore size can be obtained (Figure 5). For both N-doped and S-doped mesoporous carbons, the pore size slightly decreases as the doping content increases, while doping the mesoporous carbon with B and P leads to an increase in the mesopore size. These behaviors can be rationalized in terms of knowledge about the doping reactions. Doping would be
Figure 5. (A) Average pore size and (B) BET surface area as a function of the heteroatom content in the doped ordered mesoporous carbons.
expected to swell the carbon framework and thus decrease the pore size as found for S and N. When doping with B and P, there is a concurrent increase in the O content in the carbon. This additional oxygen can also act to etch the carbon. As the B and P contents are increased, the oxygen tends to remove C in addition to the incorporation of the heteroatoms. In addition to the pore size, the adsorption isotherms also provide information about the surface area. Figure 5B shows how the doping chemistry and content influence the BET surface areas. There is a general trend that a higher heteroatom content in the carbon framework leads to lower surface areas. However, a low level of N doping (3.2 atom %) actually increases the surface area (2432 m2/g) in comparison to that of the undoped OMC (2096 m2/g; see Figure S10I). This increased surface area is attributed to activation of carbon63 from the NH3 obtained on decomposition of melamine.64 At higher N doping, the BET surface area decreases to approximately 1500 m2/g with samples containing 25 atom % N content due to the added mass from the increased wall thickness as can be ascertained from the increase in the dspacing and decreased pore size. The surface area decreased more prominently for the other heteroatoms, but relatively high surface areas can be maintained with these highly doped mesoporous carbons. The use of a common soft templated mesoporous polymer− silica enables the facile generation of a family of doped mesoporous carbons through inclusion of heteroatom-containing molecules into the mesopores during the carbonization process. The versatility of this approach to incorporate a wide variety of heteroatoms at relatively high levels in comparison to other methods for fabricating doped mesoporous carbons should enable systematic investigations of the influence of dopants and their concentration for a wide variety of applications ranging from energy storage to catalysis. 10183
DOI: 10.1021/acs.chemmater.7b04061 Chem. Mater. 2017, 29, 10178−10186
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(Swagelok). Due to the hazards associated with the decomposition products, care should be taken to avoid uncontrolled release of the purge gas from the furnace. A control sample of undoped OMC was synthesized from the MRS without the addition of any dopant using the same heating procedure. The carbonized powders were subsequently etched with 6 M KOH solution (ethanol:H2O = 50:50, v/v) that was daily refreshed for 3 days to remove the silica and byproducts from the doping reaction. The doped mesoporous carbon was washed repeatedly with deionized water to remove KOH and other byproducts and recovered by centrifugation (Fisher Scientific, accuSpinTM 400) until pH 7. Characterization. The nanostructure of the materials was characterized using small-angle X-ray scattering (SAXS; Rigaku MicroMax 002+) operating at 50 kV and 0.6 mA with Cu Kα Xrays (wavelength (λ) = 0.154 nm). A 2D multiwire area detector was used to collect the 2D scattering patterns. The sample to detector distance was 0.8 m. Silver behenate was used to calibrate the scattering vector (q) on the basis of its primary reflection at q = 1.076 nm−1. The acquisition time for each sample was 15 min, and the 1D averaging was performed using the SAXSGUI v2.13.01 software package in MatLab. From the scattering data, the domain spacing of doped mesoporous carbon, d, was calculated as d = 2π/q*, where q* is the scattering vector for the primary reflection. High-resolution transmission electron micrographs and energy-dispersive X-ray (EDX) chemical maps were obtained using an FEI Tecnai F20ST/STEM instrument equipped with an EDAX SUTW (super ultrathin window) and operated at 200 keV. The TEM sample was prepared from sonicated dispersion of the mesoporous doped carbon powder in the acetone (0.1 mg/10 mL) that was cast on a 400 mesh lacy carbon coated copper grid (CF200-CU, Electron Microscopy Sciences). The residual acetone was removed at 40 °C under vacuum for 30 min. N2 adsorption−desorption isotherms were measured using TriStar II (Micromeritics) at 77 K. The specific surface area was determined by the Brunauer−Emmett−Teller (BET) method and the pore size distributions from the adsorption isotherms using the Barrett− Joyner−Halenda (BJH) model. Details on the chemistry of the samples were elucidated using X-ray photoelectron spectroscopy (XPS; PHI5000 Versa Probe II scanning XPS microprobe, ULVAC-PHI, Inc.) with both survey and highresolution scans. For survey scans from 700 to 0 eV, a passing energy of 117.4 eV was used. For high-resolution spectra associated with N 1s, S 2p, P 2p, and B 1s, a 0.05 eV step size and a pass energy of 11.75 eV were used. The scan ranges for N 1s, S 2p, P 2p, and B 1s are 394− 406, 160.5−166, 129−137, and 186.5−194 eV, respectively. All XPS data were recorded at a takeoff angle of 45°. The peaks were quantitatively analyzed with the MultiPak Data Reduction software using Gaussian−Lorentz peak fitting and Shirley background subtraction. Fourier transform infrared spectroscopy (Thermo Scientific iS50 FT-IR) was used to provide information on the chemical bonds of the doped carbon samples. The specimen was ground into fine powders and loaded onto the sample stage (Praying Mantis Sampling Kit, DRP-SAP, Harrick Scientific Products, Inc.) in a reaction chamber (DRK-3-NI8, Harrick Scientific Products, Inc.) for the FTIR analysis. The measurement was performed in reflection mode, with a resolution of 4 cm−1 and 256 scans for each sample.
CONCLUSIONS A generalized synthetic route to fabricate a family of highly heteroatom-doped ordered mesoporous carbon has been reported. This method is demonstrated to generate ordered mesoporous carbons with up to 26.3 atom % nitrogen, 15.5 atom % boron, 6.9 atom % phosphorus, and 4.2 atom % sulfur. The mass ratio of resol−silica precursor to dopant effectively controls the doping level of the heteroatom. Doping of the framework does adversely impact the ordering and the surface area, but relatively well-defined pores can be maintained along with modest surface areas even at high heteroatom content. This dependence on the surface area suggests a trade-off for some applications of these doped mesoporous carbon materials, such as energy storage and catalysis, where heteroatom doping and surface area are both critical parameters that contribute to the performance. This generalized method provides a convenient strategy for large-scale synthesis of heteroatomdoped carbon with the desired element and heteroatom content.
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EXPERIMENTAL SECTION
Materials. Pluronic F127 (Mw = 12600 g/mol, PEO106−PPO70− PEO106), sodium hydroxide (NaOH; >97%), potassium hydroxide (KOH; >85%), acetone (>98%), ethanol (>99%), tetraethyl orthosilicate (TEOS; >98%), phenol (>99%), formaldehyde (ACS reagent, 37 wt % in H2O; contains 10−15% methanol as a stabilizer), hydrochloric acid (HCl; ACS reagent, 37%), melamine (>99%), dibenzyl sulfide (>95%, HPLC), ammonium dihydrogen phosphate (ACS reagent, >98%), and boric anhydride (>98%) were all obtained from Sigma-Aldrich. Thermally stabilized poly(ethylene terephthalate) (PET) sheets (125 μm thick, Terphane, Inc.) were used as substrates for roll-to-roll processing. Low-molecular-weight phenolic resin (resol) was synthesized from condensation of phenol and formaldehyde under basic (NaOH) conditions as previously reported for the synthesis of mesoporous carbons.65 Nitrogen (UN1066, >99.95%) was purchased from Praxair. Synthesis of Mesoporous Resol−Silica Composites (MRS). To prepare the cooperatively assembled materials, 96 g of Pluronic F127 was first dissolved in a mixture of 80 g of 0.2 M HCl and 240 g of ethanol. This solution was heated to 45 °C and stirred for 2 h to ensure complete dissolution. A 120 g portion of resol solution (50 wt % in ethanol) and 150 g of TEOS were added to this solution. After being stirred at 45 °C for another 2 h to hydrolyze the TEOS, the solution was cast onto the PET substrate via doctor blade casting with an initial wetting thickness of 400 μm at 50 cm/min using a roll-to-roll processing line. The residual solvent was evaporated from the as-cast polymeric film at 50 °C after 2 h. Subsequently, the resol was crosslinked at 100 °C for 12 h. The residual stresses that develop during cross-linking enabled the film to be peeled from the PET substrate with ease. The free-standing films were then calcined at 350 °C for 2 h under a N2 atmosphere with a heating ramp of 1 °C/min to obtain mesoporous resol−silica (MRS) composites. The obtained MRS samples were ground into fine powders in 1 min using an electric ginder (MIRA grinder CP-GR-101). Doped Mesoporous Carbon from MRS. The dopants used for introducing nitrogen, sulfur, phosphorus, and boron into the carbon framework were melamine, benzyl disulfide, ammonium dihydrogen phosphate, and boric anhydride, respectively. For synthesis of doped porous carbon, the MRS powders and dopants at the desired mass ratio were physically mixed using the MIRA grinder for 1 min. The mixture was placed in an alumina boat and then carbonized at 800 °C for 3 h in a tube furnace (SentroTech) using a temperature ramp of 5 °C/min under N2 protection. The exit gas from the furnace was plumbed through a large stainless container (Swagelok) containing molecular sieves (Sigma-Aldrich) to remove the byproducts associated with the doping reaction. The exit gas from the sieve-containing container was directed into a hood with perfluoroalkoxy (PFA) tubing
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04061. Additional XPS survey data for doped carbon, TEM image of undoped carbon, additional SAXS data for doped carbon, XPS high-resolution data for doped carbon, FTIR data for doped carbon, and adsorption− desorption isotherms and pore size distribution for doped carbon (PDF) 10184
DOI: 10.1021/acs.chemmater.7b04061 Chem. Mater. 2017, 29, 10178−10186
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Chemistry of Materials
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bryan D. Vogt: 0000-0003-1916-7145 Present Address §
Z.Q.: Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Rd, Evanston, IL 60208. Author Contributions ∥
Z.Q. and Y.X. contributed equally to this work.
Funding
This work was supported by the National Science Foundation under Grant Nos. CBET-1159295 and CBET-1336057. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Min Gao for assistance with the TEM EDX measurements, Mr. Zebin Su for help with the SAXS measurements, and Dr. Nikolov Zhorro for his assistance with the XPS measurements.
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