Polypyrrole-Based Nitrogen-Doped Carbon Replicas of SBA-15 and

Aug 6, 2008 - Polypyrrole-based ordered mesoporous carbons (OMCs) were synthesized via chemical vapor infiltration of pyrrole into pores of the SBA-15...
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J. Phys. Chem. C 2008, 112, 13126–13133

Polypyrrole-Based Nitrogen-Doped Carbon Replicas of SBA-15 and SBA-16 Containing Magnetic Nanoparticles Pasquale F. Fulvio,† Mietek Jaroniec,*,† Chengdu Liang,‡ and Sheng Dai‡ Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242, and Chemical Sciences DiVision and Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: June 22, 2008

Polypyrrole-based ordered mesoporous carbons (OMCs) were synthesized via chemical vapor infiltration of pyrrole into pores of the SBA-15 and SBA-16 silica templates containing iron(III) chloride catalyst (FeCl3). After carbonization of polypyrrole at 800 °C and etching of the silica templates with hydrofluoric acid solution, nitrogen-doped and graphitic OMCs with incorporated magnetic nanoparticles were obtained. These materials were analyzed by CHNS elemental analysis, thermogravimetry (TG), nitrogen adsorption, small and wide angle powder X-ray diffraction (XRD), Raman spectroscopy, scanning, and transmission electron microscopy (TEM). The resulting carbon replicas retained the crystallographic symmetry of the silica templates: namely, P6mm in the case of the SBA-15 template, and Im3m in the case of the SBA-16 template. The uniformity, size, and volume of ordered mesopores in the carbon replicas were affected by structural properties of the templates used as shown by analysis of nitrogen adsorption isotherms and pore size distributions. A better infiltration of carbon precursor was achieved for the templates with larger pores, which resulted in the carbon replicas of improved adsorption and structural properties. Elemental analysis revealed the presence of nitrogen in the carbon replicas studied in the range of 3-8 wt %, whereas TG analysis of the replica samples in air gave about 2-5% residue, which was identified as hematite (Fe2O3). The presence of graphitic domains was confirmed by characteristic TG oxidation profile above 400 °C, the D and G bands on the Raman spectra, and the intense reflections on the wide angle XRD patterns. Powder XRD also showed the presence of extraframework magnetic iron (R-Fe) and iron carbide (Fe3C) nanoparticles having crystallite size in the ranges of 40-80 and 20-40 nm, respectively. TEM images also revealed that these nanoparticles were larger than the carbon rods and pore widths of the SBA-15 carbon replica, which is in good agreement with the XRD-based estimation. The in situ EDS analysis of carbon rods and spheres showed that iron was present in the carbonaceous framework, which does not exclude the existence of much smaller nanoparticles, below 5 nm. Introduction Ordered mesoporous carbons (OMCs) templated by ordered mesoporous silicas (OMSs) have been extensively studied, especially CMK-11 and CMK-32 carbons templated by MCM483 and SBA-154 silicas, respectively. The templating synthesis of OMCs involves the introduction of carbon precursor into pores of the template followed by carbonization and template dissolution. While carbon replicas of MCM-48 often undergo a symmetry change1 and the MCM-41 replication results in disordered carbon rods, the SBA-15 replication permits an exact inverse replica to be obtained.2 The observed differences in replication of the aforementioned OMSs are caused by structural differences of these templates. The MCM-41 (2D hexagonal; P6mm symmetry) and MCM-48 (3D cubic; Ia3d symmetry) OMSs are synthesized by using cationic surfactants as soft templates, which afford purely mesoporous materials. In the case of SBA-15 (P6mm), which also possesses a 2D hexagonal arrangement of cylindrical mesopores, a specific poly(ethylene oxide)--poly(propylene oxide)-poly(ethylene oxide) triblock copolymer is used as a soft template. This material possesses ordered mesopores interconnected with irregular micropores created due to the interpenetration of hydrophilic blocks of the * Corresponding author. Phone: 330-672-3790. Fax: 330-672-3816. E-mail: [email protected]. † Kent State University. ‡ Oak Ridge National Laboratory.

template into siliceous pore walls.5 Thus, the resulting silica material exhibits a 3D porous system, which after a complete filling of pores with a carbon precursor affords a stable and exact inverse replica of SBA-15, known as CMK-3, being a hexagonal structure composed of ordered carbon rods interconnected by irregular carbon threads. An analogous structure to CMK-3 is CMK-5, which consists of ordered carbon pipes instead of carbon rods formed due to an incomplete filling of the ordered mesopores of SBA-15 with a carbon precursor.6 Other OMSs obtained in the presence of triblock copolymers, which were successfully used as hard templates for the synthesis of carbons, were SBA-167 with Im3m symmetry and KIT-68 with Ia3d symmetry. While the latter is a large pore analogue of MCM-48, SBA-16 is composed of a 3D array of ordered spherical cages, in which each cage is connected with eight neighboring cages through small apertures. Similarly to SBA15 both mesostructures possess additional irregular micropores in the siliceous mesopore walls. In comparison to a 3D cubic bicontinuous arrangement of cylindrical mesopores in KIT-6, the replication process of SBA-16 due to its cage-like structure is challenging. In the case of SBA-16, its carbon inverse replica (Im3m symmetry) consists of interconnected spherical carbon particles. While several carbon precursors have been successfully used for the replication of SBA-15 and KIT-6, some difficulties were reported for the SBA-16 nanocasting.9-11 For instance,

10.1021/jp8045164 CCC: $40.75  2008 American Chemical Society Published on Web 08/06/2008

Polypyrrole-Based Ordered Mesoporous Carbons the carbon replicas of SBA-16 prepared from sucrose could be obtained only for pores exceeding 6 nm.10 Other carbon precursors such as furfuryl alcohol,9,11-14 pitch,15-18 polyacrylonitrile (PAN),19-21 and polyaniline22,23 have been employed to obtain graphitic carbons, which could retain, at least partially, the original ordered mesostructures during graphitization (∼2000 °C). In a further attempt to improve the conductive properties of these carbons, doping with a heteroatom such as nitrogen19-23 was used. For instance, nitrogen-doped carbons with partial graphitic ordering were obtained from acetonitrile19,20 after thermal treatments at relatively low temperatures (below 1000 °C). Sulfur-doped carbons obtained by polymerization and carbonization of thiophene in SBA-15 also have been studied;24 S-doping led to some enhancement of conductivity of these carbons. Graphitic carbons were also obtained by in situ oxidative polymerization of pyrrole in the SBA-15 pores in the presence of iron chloride (FeCl3).25-28 The resulting iron species after polymerization acted as a catalyst for the formation of graphitic domains;25-27 the amount of nanocasted carbon and the extent of graphitization were found to be dependent on the catalyst loading within the silica host.25 Further studies of this system showed that some doped amounts of nitrogen remained in the graphitic framework after thermal treatments up to 1000 °C.26,27 Nevertheless, the presence of iron species was not reported in the resulting carbons. Other studies suggested that only a small fraction of the iron catalyst was deeply embedded in the carbon matrix and consequently protected from leaching.28 The same work reports that carbonization and silica template removal with a sodium hydroxide solution affords an amorphous carbon containing super paramagnetic iron oxide (Fe3O4) nanoparticles with traces of nonoxidized R-Fe (ferrite). In all of the aforementioned studies involving polypyrrole, the iron catalyst loadings inside the SBA-15 template were similar, while differences appeared in the synthetic conditions such as solvents (ethanol26,27 or water25,28) or the method of polymerization (CVD,25,26 incipient wetness impregnation,27 or stirring in an aqueous solution of the catalyst28). Some results were surprisingly different and raised questions about the possibility of generating a graphitic and nitrogen-doped carbon with embedded magnetic nanoparticles in its framework. Also, to the best of our knowledge, there are no previous reports on the use of large pore SBA-16 materials as templates for the synthesis of pyrrole-based carbons. Thus, in this work the polypyrrole-based carbon inverse replicas of SBA-15 and SBA16 are investigated. These replicas are nitrogen doped and partially graphitic ordered carbon mesostructures with embedded magnetic nanoparticles. As revealed by N2 adsorption measurements at -196 °C, small angle XRD, and STEM, the high quality carbon replicas were obtained. The pore size, pore volume, and carbon wall thickness of these replicas were largely influenced by the structural properties of the silica template used.29 While elemental analysis provided an estimation of the nitrogen content, powder X-ray diffraction analysis showed the presence of ferrite (R-Fe) and iron carbide (Fe3C) nanoparticles in the carbon replicas studied as well as confirmed their partially graphitic nature. The latter property was also confirmed by TG experiments and Raman spectroscopy. The STEM images and the XRD analysis confirmed the existence of large nanoparticles (20-80 nm), whereas in situ energy dispersive spectroscopy (EDS) proved the presence of iron in the aforementioned carbon replicas. This study concurs with a recent report on the synthesis of magnetically separable CMK-3 carbon by using furfuryl

J. Phys. Chem. C, Vol. 112, No. 34, 2008 13127 alcohol and cobalt nitrate in the SBA-15 template30 as well as with other reports on the catalytic synthesis of graphite,31 core-shell nanostructures,32 and carbon nanotubes (CNTs).33,34 Experimental Section Synthesis of SBA-15 Materials. For the synthesis of SBA15 from TEOS (Fluka) 4.00 g of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (PEO20PPO70-PEO20; Pluronic 123 from BASF) was added to 144 mL of a 1.7 M aqueous solution of hydrochloric acid and the mixture was stirred for 4 h at 40 °C. Next, TEOS was added dropwise (mass ratio of TEOS/P123 ) 2) and the polymer-TEOS synthesis mixture was stirred for 2 or 6 h. The synthesis gels were transferred to the Teflon-lined sealed containers and the gel stirred for 2 h was kept at 100 °C for 48 h, whereas the gel stirred for 6 h was kept at 140 °C for 24 h under static conditions. The final products were filtered, washed with water, and dried for 12 h at 80 °C. The as-synthesized sample obtained under hydrothermal conditions at 100 °C (∼1.0 g) was extracted with a mixture of 200 mL of EtOH 95% (v/v) and 4 mL of concentrated HCl at 60-70 °C for 24 h twice, while the material prepared at 140 °C was calcined at 540 °C for 3 h under flowing air and a heating rate of 5 deg per min. The samples obtained at 100 and 140 °C were labeled as SBA-15 and SBA-15#, respectively, where # refers to the hydrothermal treatment temperature of 140 °C. Synthesis of SBA-16 Materials. The large pore SBA-16 silica samples were prepared according to the previously reported procedure (Zhao et al. 2004). The triblock copolymer Pluronic F127 (EO106PO70EO106, BASF Corporation) was used as the structure directing agent, TEOS (Acros, 98%) as the silica source, and sodium chloride at low hydrochloric acid concentrations as an additive. In a typical synthesis, 4.00 g of the F127 polymer and 14.10 g of NaCl were dissolved in 160 mL of 0.5 M HCl at 40 °C under stirring for 4 h. Next, 16.8 g of TEOS was added dropwise to the polymer solution. The resulting mixture was stirred for 20 h followed by hydrothermal treatment at 100 °C for 24 h under static conditions, using Teflon-lined sealed containers. The precipitated product was filtered, washed with deionized water, and dried under air at 80 °C. The resulting solid product (∼1.0 g) was extracted with a mixture of 200 mL of EtOH 95% (v/v) and 4 mL of concentrated HCl at 60-70 °C for 24 h. After a second stage of drying at 80 °C the sample was calcined for 4 h at either 350 or 550 °C in flowing air with a heating rate of 3 deg per min. The resulting template-free SBA-16 samples were denoted as SBA-16 and SBA-16*, where / stands for the temperature of calcination of 550 °C. Synthesis of Carbon Replicas. For the synthesis of the silica (SBA-15 and SBA-16) supported iron catalyst, approximately 1.6 g of anhydrous FeCl3 (98% Acros Organics) was quickly dissolved in 50 mL of ethanol (200 proof, Acros Organics). Approximately 1.0 g of template-free silica (vacuum dried at 80 °C for 1 h) was added to the resulting solution giving a sol, which was stirred for ∼4 h at room temperature in a sealed container. The system was then transferred to an oven at 60 °C and the solvent was allowed to evaporate. The dry yellow solid product was later transferred to a Petri plate and dried under vacuum at 80 °C for ∼1 h. This final silica-supported iron catalyst was exposed to pyrrole vapors in a sealed container, which was quickly evacuated to facilitate the evaporation and vapor infiltration of the pyrrole monomer within the silicacatalyst pores. This system was kept at room temperature for 24 h and the final black product was thermally treated under flowing N2 at 800 °C for 3 h, using a heating rate of 3 deg per

13128 J. Phys. Chem. C, Vol. 112, No. 34, 2008 min. A portion of this composite material was treated with HF 48% (wt) for 24 h. The final silica-free carbon was filtered, washed with water several times, and vaccuum dried at 80 °C for at least 12 h. The final carbon replicas were labeled as C-15 and C-15# in the case of the SBA-15 and SBA-15# templates, respectively, and as C-16 and C-16* in the case of the SBA-16 and SBA-16* templates, respectively. The C-15 replica was obtained after two impregnations of SBA-15 with pyrrole vapor; the second impregnation was preceded by a thermal treatment of the composite at 800 °C and catalyst loading. Characterization. The calcined SBA-15 and SBA-16, carbon-silica nanocomposites, and carbon replicas were characterized by CHNS elemental analysis, thermogravimetric analysis (TG), nitrogen adsorption at -196 °C, powder X-ray diffraction, Raman spectroscopy, and transmission electron microscopy (TEM). The nitrogen content of the carbon replicas was estimated by using a LECO CHNS-932 elemental analyzer (St. Joseph, MI). The TG measurements for as-synthesized silica-triblock copolymer composites and related carbon materials were performed by using a high resolution mode of the TA Instruments TGA 2950 thermogravimetric analyzer. The TG profiles were recorded up to 800 °C in flowing air with a heating rate of 10 deg per min. Nitrogen adsorption isotherms were measured at -196 °C with use of ASAP 2010 and 2020 volumetric adsorption analyzers manufactured by Micromeritics (Norcross, GA). Before adsorption measurements the silica, carbon-silica, and carbon samples were outgassed under vacuum for at least 2 h at 200 °C. The specific surface area of the samples was calculated by using the BET method within the relative pressure range of 0.05 to 0.2.35 The pore size distributions (PSDs) were determined by using the BJH method calibrated for cylindrical pores according to the improved KJS method.36 The small and wide angle XRD patterns were recorded on a PANanalytical. Inc. X’Pert Pro (MPD) Multi Purpose Diffractometer with Cu KR radiation (0.1540 nm), using an operating voltage of 40 kV and 40 mA, 0.01° step size, and 20 s step time for small angle measurements (0.40° < 2θ < 5.00°) and 0.02° step size (6 s step time) for wide angle measurements (5.00° < 2θ < 70.00°). Microscope glass slides were used as sample supports for small angle measurements and for wide angle measurements of silica-carbon composites, whereas Al holders (PANanalytical PW1172, 15 mm × 20 mm × 1.8 mm) were used for wide angle measurements of the final carbon replicas. The samples were manually ground prior to the XRD analysis and all measurements were performed at room temperature. Crystallite dimensions (D) of the iron species were estimated by using the Scherrer equation D ) 0.90λ/β cosθ, where 0.90 is a constant, λ is 0.1540 nm, and β is the full width at half-maximum of the peak at the diffraction angle θ. Raman spectroscopy measurements were made on a HORIBA Jobin Yvon HR800 with a microscope attachment. The laser wavelength was of 633 nm focused by using a diffraction limited spot and the scan time was 10 s for each sample. For the STEM analysis the sample powders were dispersed in ethanol by a moderate sonication at concentrations of 5 wt % of solids. A Lacy carbon coated 200-mesh copper TEM grid was first dipped into a sample suspension and then dried under vacuum at 80 °C for 12 h prior to the microscopic analysis. All samples were imaged by a Hitachi HD-2000 Scanning and Transmission Electron Microscope (STEM). The unit was operated at an accelerating voltage of 200 kV and an emission current at 30 mA. The samples were imaged by a secondary

Fulvio et al.

Figure 1. Small angle XRD patterns for the polymer-free OMSs and the corresponding carbon replicas prepared by thermal treatment of pyrrole-silica composites at 800 °C in flowing nitrogen followed by silica template removal: (A) patterns for the SBA-15 samples obtained by hydrothermal treatment at 100 and 140 °C and their inverse carbon replicas; (B) patterns for the SBA-16 samples obtained by extraction and calcinations at 350 and 550 °C and their inverse carbon replicas.

electron detector, an annular disk Z-contrast detector, and a transmission electron detector. The same sample spots were used to take simultaneously SEM, Z-contrast (dark-field images), and TEM images. The energy dispersive spectra (EDS) were collected by a detector controlled through another computer that acquires signals from the selected area of samples. The EDS spectra were collected at 30% of detector dead-time and 3 min of the acquisition time. Results and Discussion Characterization of the SBA-15 and SBA-16 Materials. The small angle XRD patterns for the template-free SBA-15 and SBA-16 silica materials (Figure 1) are characteristic for the P6mm and Im3m symmetry groups, respectively. For all aforementioned samples, at least 3 reflections are present indicating a good structural quality of these materials. For the SBA-15 samples, the most intense reflection is the (100) peak with the d100 spacing in the range of ∼9.3-10.0 nm (Table 1). The unit cell parameters calculated from these d-spacing values are about 11.0 nm, being slightly larger for the SBA-15 sample obtained by solvent extraction of the P123 template. For the SBA-16 materials, the most intense reflection is the (110) peak, giving the d110-spacings in the range of ∼9.8-10.7 nm. The unit cell parameters of the SBA-16 samples studied (13-15 nm) are larger than those of the SBA-15 materials (Table 1). The observed difference in the unit cell parameters of SBA-16 and SBA-16* is in agreement with previous studies of large pore cage-like materials prepared under analogous conditions.37

Polypyrrole-Based Ordered Mesoporous Carbons

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TABLE 1: Parameters Obtained from N2 Adsorption at -196 °C and Small Angle XRD Data for the SBA-15 and SBA-16 Silica Samples and the Corresponding Inverse Carbon Replicas PSD sample

SBET (m2/g)a

VSP (cm3/g)b

d (nm)c

a (nm)d

w (nm)e

b (nm) f

wKJSg (nm)

V h (cm3/g)

Vci (cm3/g)

SBA-15 SBA-15# SBA-16 SBA-16* C-15 C-15# C-16 C-16*

567 623 1179 954 487 690 775 560

1.11 1.35 0.75 0.63 0.61 0.70 0.92 0.92

9.97 9.34 10.66 9.77 8.51 8.40 9.72 9.05

11.51 10.78 15.08 13.02 9.8 9.7 13.8 12.8

9.95 9.81 9.03 8.17 3.3 4.1

1.56 0.97 4.05 3.78 7.2 6.8

10.05 10.25 7.53 7.12 4.42 5.22 5.03 6.86

1.11 1.37 0.75 0.65 0.37 0.59 0.73 0.62

0.05 0.00 0.48 0.41 0.04 0.04 0.10 0.05

a SBET: specific surface area calculated within the relative pressures of 0.05-0.20. b VSP: single-point pore volume corresponding to the amount adsorbed at the relative pressure of 0.98. c d: interplanar spacing for the most intense reflection peaks, (100) for the samples with P6mm symmetry group and (110) for the samples with Im3m symmetry group. d a: unit cell parameter calculated for the most intense reflection peak on each small angle XRD pattern. e w: pore width calculated by using the geometrical relationship between pore volume and the unit cell parameter. f b: pore wall thickness calculated by using the w and a values according to previously reported relations for SBA-15 (ref 38), for the diameter of rods in the C-15 carbons (ref 29), and for the minimal pore wall thickness in SBA-16 (ref 39). g wKJS: width of primary mesopores at the maximum of PSD obtained according to the improved KJS method reported in ref 36. h V: pore volume calculated by integration of the PSD curves up to ∼16 nm. i Vc: volume of complementary pores calculated by integration of the PSD curves up to 5 nm for all silica samples, and up to 3 nm for all carbon inverse replicas.

As shown for SBA-16, the larger pore size was obtained for the sample after initial extraction followed by calcination at lower temperature because its shrinkage was smaller. Nitrogen adsorption isotherms measured at -196 °C are characteristic for the channel-like and cage-like materials such as SBA-15 and SBA-16 samples, respectively (see Figure 2). The BET surface areas for the SBA-15 materials were considerably smaller (about 40-50%) than those for SBA-16. The larger surface areas of the SBA-16 samples are due to the higher amount of fine pores, which include the regular apertures interconnecting spherical cages as well as irregular complementary pores present in the mesopore walls. The steep capillary condensation steps visible on the adsorption isotherms for the SBA-15 samples indicate a high uniformity of mesopores in these materials. Since these steps occur at similar relative pressures, the diameter of mesopores in both SBA-15 samples is similar too. While for the SBA-16 samples the steepness of the capillary condensation steps indicates narrow pore size distributions of spherical cages, a sudden drop in the desorption branches, which appears at p/p0 ≈ 0.45, shows that the size of the apertures interconnecting ordered spherical cages is below 5 nm. The corresponding PSD curves for the SBA-15 and SBA16 samples are shown in panels C and D of Figure 2, respectively; in addition, the basic adsorption parameters are summarized in Table 1. Note that in the case of the SBA-15 sample, a small volume (∼0.05 cm3 g-1) of the complementary pores was found by integration of PSD up to 5 nm, while this volume was negligible for SBA-15#. The volume of micropores is usually larger for the SBA-15 materials; a very small volume obtained for the SBA-15 samples studied can be due to the inaccuracy of pore size analysis in the range of micropores. The hydrothermal synthesis of SBA-15# at a higher temperature (140 °C) afforded material with larger interconnections, which resulted in about 20% higher volume of large mesopores in comparison to those present in SBA-15. The PSD curves for the SBA-16 samples show a bimodal distribution of pores. For the larger pores, which reflect ordered spherical cages, these distributions are very narrow. Even though the KJS method was calibrated for cylindrical mesopores, it is still useful for a comparison of the samples studied. For the reason previously stated, the volume of complementary pores in SBA-16 is much larger than that in the case of SBA-15. The volume of mesopores for SBA-16 obtained by calcination at

350 °C is higher by about 12% than that for the SBA-16* sample calcined at 550 °C, which is due to the greater structure shrinkage in the latter case. In addition to the mesopore widths estimated at the maximum of the PSD curves, these quantities were also obtained by using the geometrical formulas for the P6mm38 and Im3m39 symmetries, which relate the pore width with the d-spacing and the pore volume. As can be seen from Table 1 the latter pore width values for the SBA-16 samples are higher than those obtained by the KJS analysis, which is applicable for cylindrical pores; note that the KJS pore analysis of the SBA-16 samples was done to get information mainly about complementary porosity in these materials. Also, geometrical relations were used to estimate the pore wall thickness for the SBA-15 samples38 and the minimal pore wall thickness for SBA-1639 (see Table 1). These values were found to be about 4 nm for the SBA-16 samples, while those for SBA-15 were about 1-1.5 nm; the latter values are underestimated due to the known difficulty of determining the volume of complementary pores in SBA-15. Carbon-Silica Nanocomposites and Carbon Inverse Replicas. The nitrogen adsorption isotherms at -196 °C for the carbon-silica nanocomposites and inverse carbon replicas are shown in Figure 2, panels A and B, in comparison to the isotherms for the corresponding silica templates. These isotherms reveal that the pores of the SBA-15# template were almost completely filled during a single cycle of vapor deposition; the unfilled pore volume was ∼0.15 cm3 g-1. For the other nanocomposites, the presence of the condensation steps and hysteresis loops show that the ordered mesopores were partially filled with the carbon precursor. The unfilled pore volumes were in the range of 0.23-0.27 cm3 g-1, which corresponds to ∼87% of the total pore volume of SBA-15# filled with the carbon precursor and to ∼77% of the pore volume of SBA-15 filled after two cycles of the vapor deposition. A similar percentage was found for the cage-like samples with ∼70% of the filled pores in SBA-16 and ∼57% in the case of SBA-16*. The small angle XRD patterns for the carbon inverse replicas were indexed by using the same symmetry groups as those for the corresponding silica templates; namely, P6mm for C-15 and C-15# and Im3m for C-16 and C-16*. The d-spacing values and the corresponding unit cell parameters for the carbon replicas studied are listed in Table 1. As can be seen from Table 1 the d-spacing values for the carbon replicas are smaller than the

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Figure 3. SEM images for the C-15# and C-16 carbons: (top panel) SEM image of the C-15# sample showing a long-range ordering of this nanostructure composed of interconnected carbon rods; (bottom panel) SEM image of C-16 showing the interconnected spherical carbon particles as expected for the inverse replica of the SBA-16 silica. Figure 2. Nitrogen adsorption isotherms measured at -196 °C for the silica templates, carbon-silica composites and carbon replicas: (A) SBA-15 and SBA-15# samples, C-SiO2 nanocomposites, and carbon replicas C-15 and C-15# (adsorption isotherms for the samples coded with # index were shifted by 800 cm3 STP g-1); (B) SBA-16 and SBA16* samples and the corresponding composites and carbon replicas (adsorption isotherms for the samples coded with / index were shifted by 500 cm3 STP g-1); (C and D) pore size distribution (PSD) curves for the SBA-15 and SBA-16 samples in comparison with the PSD curves for the corresponding carbon replicas, respectively [the PSD curves for the samples labeled with # were shifted by 1.5 cm3 g-1 nm-1 (panel C) and for the samples labeled with / by 0.4 cm3 g-1 nm-1 (panel D)]. The PSD curves were calculated by the improved KJS method reported in ref 36.

d-values for the corresponding silica templates due to the structure shrinkage during the carbonization process; this difference is smaller for the carbon replicas of SBA-16. It appears that the 2D hexagonal structures shrunk more during carbonization than the 3D cubic structures studied. The nitrogen adsorption isotherms for the C-15# and C-15 inverse carbon replicas of the SBA-15# and SBA-15 samples exhibit the well-defined condensation steps in the relative pressure range of 0.4-0.5; this step for the former carbon is more pronounced. Also, the total pore volume for this carbon ()0.7 cm3/g) is higher by about 15% than that for C-15. About 30% higher values of the pore volume were obtained for the C-16 and C-16* carbon replicas of the SBA-16 and SBA-16* samples. The adsorption isotherm for the C-16 sample reveals slightly steeper capillary condensation and evaporation steps than those for C-16*. Consequently, the PSD curve for the latter sample is broader (see Figure 2D). As can be seen from panels C and D of Figure 2, the PSD curves for C-15# and C-16 are narrower than PSDs for C-15 and C-16*, respectively. The pore

widths at the maximum of the PSD curves are listed in Table 1. For comparison, the diameters of the carbon rods and the pore widths for the C-15 and C-15# samples were also calculated according to the previously reported geometrical relations for the inverse carbon replicas (CMK-3) of SBA-15.29 The pore widths predicted by using the aforementioned geometrical relations are about 1 nm smaller than those obtained by the KJS method. Note that the KJS method is applicable for cylindrical pores but the pores in CMK-3 are not cylindrical; they represent the space between hexagonally ordered carbon rods. The SEM and TEM images of the carbon replicas studied are shown in Figures 3 and 4, respectively. These images confirm a good structural ordering in the carbon replicas studied. The TEM and Z-contrast TEM images for the C-15# sample (top panel in Figure 4 and panel a in Figure 1S, Supporting Information) reveal also the presence of nanoparticles. Even though large nanoparticles were not observed for C-16, all the carbon samples studied possessed magnetic properties as illustrated in the Supporting Information (see Figure 2S). Isolated CNTs formed on the surface of nanoparticles were not observed for the systems studied as in previous reports.28 The EDS in situ analysis of these nanoparticles revealed a high content of iron as can be seen in the Supporting Information, Figure 3S. Also, the EDS analysis of the carbon rods and carbon spheres revealed some iron too. A quantitative analysis was not possible due to the contribution arising from the carbon-coated grid used for the STEM imaging. The wide angle powder XRD patterns recorded for the carbon inverse replicas studied reveal the composition of the aforementioned nanoparticles; they were identified as R-Fe (Ferrite) and Fe3C (Cementite), whereas the visible strong reflections in Figure 5 are mainly assigned to graphitic carbon. The crystallite

Polypyrrole-Based Ordered Mesoporous Carbons

Figure 4. TEM images for the carbon materials C-15# and C-16 carbons: (top panel) TEM image of C-15# showing the presence of iron-containing nanoparticles; (bottom panel) TEM image of C-16.

Figure 5. Wide angle powder XRD patterns for the carbon replicas of SBA-15 (A) and SBA-16 (B) obtained by thermal treatment of polypyrrole-silica nanocomposites at 800 °C and etching the silica templates with 48% (w/w) HF. These patterns were assigned by using the XRD database for graphite (/, Cliftonite ref 41-1487), R-Fe (R, Ferrite ref 6-0696), and Fe3C (o, Cementite ref 35-0772).

size analysis by means of the Scherrer equation suggests the presence of the Fe3C nanoparticles of 20-40 nm and the R-Fe nanoparticles of 40-80 nm (see Table 2). This analysis is in a

J. Phys. Chem. C, Vol. 112, No. 34, 2008 13131 good agreement with the particle sizes estimated on the basis of the TEM image shown in the top panel of Figure 4 for the C-15# sample. Furthermore, the presence of the crystallites containing iron was confirmed by analysis of the as-prepared polypyrrole-silica nanocomposites, prior to their thermal treatment. These crystallites were identified as iron(II) chloride tetrahydrate (FeCl2 · 4H2O) and they were at least 40 nm in size (see the XRD patterns in Figure 4S (Supporting Information). After pyrolysis at 800 °C the R-Fe and Fe3C crystallites were identified as those present in the carbon inverse replicas obtained by etching of the silica template with HF solution. As regards the graphitic nature of the carbon replicas, the first and most intense reflection on each XRD pattern was assigned as the (002) reflection of graphite, even though it may contain a low-intensity reflection of the Cementite phase. As can be seen from Table 2, the (002) reflection was usually centered at 26.33-26.47° with d-spacing values ranging from 0.3367 to 0.3385 nm. The dimension of the graphitic crystallites along the c-axis, i.e., the stacking height of the graphene layers (Lc), was estimated by using the Scherrer equation with a constant of 0.84 previously reported for graphitic carbons.21 For the carbon samples studied the β values are about 1.2-1.5°, which lead to the Lc-values of about 5.1-6.3 nm. A relatively large broadening of the (002) reflection and the d-spacing values exceeding the value characteristic for graphite (∼0.334 nm) indicate the presence of amorphous carbon in these materials too. However, the in-plane dimension of the graphene layers (La) was not estimated because the (100) and (110) peaks are superimposed with reflections arising from the iron phases. The presence of graphitic domains was also confirmed by oxidation of the carbon samples studied, which is illustrated by the TG profiles recorded in air (see Figure 5S in the Supporting Information). For these samples a part of the carbon was oxidized at ∼400 °C, which is usually related to the oxidation of amorphous carbon; however, the remaining large fraction of carbon was oxidized over a broad temperature interval (from 400 to 700 °C). A small residue of about 2-5% was identified by powder XRD as hematite. The oxidation of each initial polypyrrole-silica nanocomposites prior to the thermal treatment at 800 °C is shown in Figure 5S (Supporting Information). For these samples, the carbon precursor was completely oxidized at 400 °C and the extents of decomposition were about 66% for C-15, ∼60% for C-15#, ∼60% for C-16, and 56% for C-16*. Similar oxidation patterns to those of the final carbon inverse replicas were obtained for the carbon-silica nanocomposites (see Figure 5S in the Supporting Information). The carbon present in these nanocomposites was oxidized above 400 °C, but in these cases the red residues containing hematite and silica were 58% and 51% for C-15# and C-15, respectively, and 67% and 73% for C-16 and C-16*, respectively. Shown in Figure 6 are the Raman spectra for the carbons studied, which provide an additional confirmation about the presence of amorphous (D band) and graphitic carbon (G band) domains.40 The relative intensities of these bands (ID/IG) were in the range of ∼0.8-1.0, which is in agreement with previous reports on similar carbon nanomaterials.21,27 An additional feature of each spectrum was the appearance of the D′ band, indicating the disordered nature of graphene sheets, usually attributed to the plane edges and/or heteroatoms such as nitrogen. The presence of nitrogen (about 3-8 wt %) was confirmed by CHNS elemental analysis (Table 2). In summary, the morphology of silica templates is preserved during hard templating synthesis of carbons as shown in Figure 6S (Supporting Information); particle size distributions

13132 J. Phys. Chem. C, Vol. 112, No. 34, 2008

Fulvio et al.

TABLE 2: Elemental Analysis and Powder XRD Diffraction Parameters Obtained for the Carbon Inverse Replicas of the Various SBA-15 and SBA-16 Silica Templates sample

Na [w/w %]

wcb [%]

2θ002c [deg]

d002d [nm]

ID/IGe

D(Fe3C)f [nm]

D(R-Fe)g [nm]

C-15 C-15# C-16 C-16*

3.1 7.8 2.8 2.7

97 95 98 98

26.33 26.47 26.41 26.46

0.3385 0.3367 0.3375 0.3368

0.97 0.83 0.90 1.00

24.7 37.0 37.0 24.7

49.1 65.4 78.5 78.5

a N: Nitrogen contents determined by CHNS elemental analysis. b wc: weight change obtained from the TG curve over the entire decomposition range. c 2θ002: diffraction angle for the most intense diffraction peak on the XRD powder patterns for all carbon materials, assigned as the (002) peak for graphite. d d002: d-spacing for the 002 peaks calculated by using Bragg’s equation. e ID/IG: relative intensities of the D and G bands from the Raman spectra obtained for the carbon materials. f D(Fe3C): crystallite size for the Fe3C phases, estimated from the XRD powder patterns by using the Scherrer equation. g D(R-Fe): crystallite size for the R-Fe phases, estimated from the XRD powder patterns by using the Scherrer equation.

tant growth of a protective carbon structure if the carbon precursor is in excess (see particle in the top panel of Figure 4).34 As suggested by others,41,42 the graphitic domains can be primarily formed by condensation of pyrrole rings with elimination of nitrogen,41 or by high temperature inter- and intramolecular aromatization of carbon rings.42 Such reactions may cause a depletion of nitrogen in the resulting carbons. Conclusions

Figure 6. Raman spectra for the carbon replicas (prepared by thermal treatment of polypyrrole-silica composites at 800 °C followed by the silica template removal) possess the D and G bands characteristic for amorphous disordered and graphite-like carbons, respectively. Note the presence of the D′ band characteristic for disorder introduced to the graphene sheets by edges or heteroatoms such as nitrogen. Panel a shows spectra for C-15 and C-15#, whereas panel b shows those for C-16 and C-16* carbons.

of the template and its inverse replica are very similar. The other data agree with the commonly accepted mechanism of the formation of carbon nanostructures under analogous conditions.30-34 The divalent iron was formed during oxidative polymerization of pyrrole; its further reduction occurred during pyrolysis with simultaneous oxidation of the carbon precursor. Furthermore, the catalytic graphitization proceeded via adsorption and decomposition of carbonaceous material by iron particles, which can also lead to the formation of the carbide phase on the surface of these particles. In the latter case, graphitic carbon also can be formed by the decomposition of the Fe3C phase generating more iron, which diffuses throughout the graphitic phase and forms particles up to ∼20n m.30,33 These latter particles, which are located outside pores of the silica template, may coalesce and form larger clusters (about 60-80 nm), with the concomi-

The polypyrrole-based carbon inverse replicas of various SBA-15 and SBA-16 templates were successfully synthesized via a chemical vapor deposition route. The use of silica templates with different widths and volumes of mesopores permitted some tailoring of the adsorption properties of the carbons. Better carbon replicas were obtained for the silica templates having highly interconnected large mesopores as in the case of SBA-15#. The polypyrrole-based carbons synthesized in the presence of iron catalysts are nitrogen-doped carbons possessing extensive graphitic domains as evidenced by the well-resolved powder XRD peaks and Raman spectra. The presence of large magnetic nanoparticles in the resulting carbon materials was evidenced by TEM imaging of C-15# and powder XRD data. These nanoparticles were protected by a carbon coating and preserved during the acid etching of the silica templates. Furthermore, the presence of smaller amounts of iron within the periodic rodtype structure of C-15# and sphere-type structure of C-16* was only confirmed by in situ EDS analysis. This iron was probably homogeneously dispersed, which does not exclude the presence of nanoparticles below 5 nm as previously reported for amorphous carbon inverse replicas of SBA-15.28,43 Acknowledgment. The authors thank the BASF Co. for providing the triblock polymer. The STEM characterization of the carbon samples was conducted at the Center for Nanophase Materials Sciences, which is sponsored at the Oak Ridge National Laboratory, Division of Scientific User Facilities, U.S. Department of Energy. Supporting Information Available: Z-contrast TEM images for two carbon replicas, images showing the magnetic properties of these replicas, EDS spectra for the C-15# and C-16 carbon samples, powder XRD patterns for the polypyrrole-silica and carbon-silica nanocomposites and oxidation residues, TG oxidation curves for all samples, and particle size distributions for SBA-16 and C-16 samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743.

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