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J. Phys. Chem. C 2008, 112, 722-731
CMK-5 Mesoporous Carbon Synthesized via Chemical Vapor Deposition of Ferrocene as Catalyst Support for Methanol Oxidation Zhibin Lei,*,† Shiying Bai,† Yi Xiao,† Liqin Dang,† Lizhen An,† Guangning Zhang,† and Qian Xu‡ Institute of Chemistry for Functionalized Materials, Faculty of Chemistry and Chemical Engineering, Liaoning Normal UniVersity, Dalian, Liaoning, 116029, China, and State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China ReceiVed: September 12, 2007; In Final Form: October 31, 2007
A novel and effective method was described in this work to prepare two-dimensional hexagonally ordered mesoporous CMK-5 carbon materials. This method is based on the chemical vapor deposition (CVD) of ferrocene in the mesopores of SBA-15 at 500 °C, followed by graphitization at different temperatures. Both the silica/carbon composite and the resulting CMK-5 were characterized by N2 adsorption, powder X-ray diffraction, Raman spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy (HRTEM), and thermogravimetric analysis. It was found that the ferrocene could be used as a new precursor to prepare CMK-5 nanopipes, with pipe thicknesses varying from 0.8 to 2.6 nm, by increasing the CVD time from 20 to 120 min. The resulting CMK-5 exhibits high Brunauer-Emmett-Teller (BET) surface area (1044-2449 m2/g) and large pore volume (1.13-2.20 cm3/g). The graphitization degree of the resulting CMK-5 was investigated by pyrolyzing the corresponding silica/carbon composite at different temperatures. Pyrolysis temperatures below 850 °C led to gradually improved graphitization degrees of CMK-5 nanopipes. Pyrolysis temperatures above 850 °C resulted in the partial collapse of ordered CMK-5 nanopipes accompanied by the appearance of a considerable amount of entangled graphitic ribbons. The structural evolution process of CMK-5 from ordered nanopipes to the final entangled graphitic ribbons was observed by HRTEM. The obtained CMK-5 was applied as a catalyst support of Pt for methanol oxidation. The electrochemical activities of Pt nanoparticles loaded on the CMK-5 carbon materials were investigated by cyclic voltammograms and compared with the commercial Pt/Vulcan XC-72 catalyst. It was found that the specific mass activity of Pt/CMK-5 was much higher than Pt/Vulcan XC-72.
1. Introduction Porous carbon materials have attracted extensive attention in recent years because of their potential applications as catalyst supports,1,2 adsorbents,3 hydrogen storage materials,4-6 and electrode materials.7 The conventional method for the preparation of porous carbons is based on the methodology of nanocasting, generally using colloid silica or mesoporous silica as a hard template and hydrocarbon as precursors.8 Of all the reported carbon materials created from silica materials, mesoporous carbon materials generated from SBA-15 have attracted the most attention because the final carbon materials possess uniform pore size, high surface area, and adequate pore volume.9 Mesoporous CMK-3, which was created by sufficiently filling the nanopores of SBA-15 with different carbon precursors, exhibits two-dimensional (2-D) hexagonal ordered nanorods consisting of either amorphous9,10 or graphitic carbon,11 depending on the selection of different carbon precursors. By reducing the filling degree of carbon precursor in SBA-15 mesopores and controlling the synthetic condition, ordered mesoporous carbons featuring CMK-5 structure could be obtained.12 The difference between CMK-5 and CMK-3 is that CMK-5 is * Corresponding author. Address: Institute of Chemistry for Functionalized Materials, Faculty of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian, Liaoning, 116029, China. Tel: 86-41182159736. Fax: 86-411-82156858. E-mail:
[email protected]. † Liaoning Normal University. ‡ Chinese Academy of Sciences.
essentially a surface-templated process, giving rise to the 2-D hexagonally ordered carbon nanopipes, which were rigidly interconnected by the carbon spacers that were formed inside the complementary pores between the adjacent cylinders.13-15 The attractive property of CMK-5 is its uniform nanopipes and extremely high specific surface area, which make it a good protonic acid catalyst16 for the esterification reaction or catalyst support for the dispersion of platinum nanoparticles in a lowtemperature fuel cell.12 The conventional method for the synthesis of CMK-5 involves the pyrolysis of poly(furfuryl alcohol) under vacuum, which requires the additional metal Al to catalyze the polymerization of furfuryl alcohol.13-15,17 Moreover, the structural parameter of CMK-5 was restrictedly controlled by SBA-15 templates with different pore sizes13 or changing the concentration of furfuryl alcohol.18 Alternatively, hexagonally ordered mesoporous CMK-5 was also synthesized by catalytic chemical vapor deposition (CVD) using ethylene as a carbon precursor.19 The catalyst cobalt was also required to be incorporated into SBA-15. additionally, the graphitization degree of CMK-5 prepared by using furfuryl alcohol,12 acenaphthene,14 or ethylene19 as a carbon precursor was comparatively low, possibly related to the inherent properties of the carbon precursor. The carbon sources suitable for the synthesis of graphitized carbon generally contain graphitic building blocks. Although polyaromatic hydrocarbons,11,20 polyacrylonitrile,21 acetonitrile,22
10.1021/jp077322a CCC: $40.75 © 2008 American Chemical Society Published on Web 01/01/2008
Synthesis of CMK-5 Mesoporous Carbon benzene,23 and styrene24 had been extensively investigated as carbon precursors for the generation of mesoporous graphitic carbon, the final carbons templated from SBA-15 generally consist of 2-D hexagonal ordered nanorods, namely, CMK-3 rather than the CMK-5 structure. Although acenaphthene was also applied to synthesize graphitic carbon with CMK-5 structure, the resulting carbon material displayed low specific surface area and small pore volume.14 For the electrodes of electrochemical devices such as fuel cells, porous carbon with improved graphitic structure is highly desirable.7,25,26 Therefore, exploration of other carbon precursors suitable for the synthesis of CMK-5 with large surface area and improved graphitization degree is still an interesting but very challenging task in the materials sciences. The catalytic graphitization had been demonstrated to be an effective method to prepare graphitic carbon with higher crystallinity in mild conditions.27-31 With the aid of catalysts (Fe, Co, Ni), porous graphitic carbon could be facilely replicated frommesoporoussilicaatarelativelylowpyrolysistemperature.32-34 The catalysts used for the graphitization of carbons are intentionally introduced into the mesoporous silica or the carbon matrix, which makes the synthetic route time-consuming and even more complicated. In the past few years, a large number of reports were focused on the synthesis of carbon nanotubes using ferrocene as the precursor or as the Fe catalyst. For example, pyrolysis of the mixture of ferrocene and anthracene with different molar ratios would yield carbon microspheres or carbon nanotubes with different diameters.35-37 Ferrocene was also used as a carbon source and a catalyst for single-walled carbon nanotube growth, with the diameter controlled by the pressure of the carrier gas.38 Recently, it is interesting that ferrocene encapsulated inside the carbon nanotubes was used as the carbon precursor to grow new carbon nanotubes with the layer number precisely controlled from one to two to three.39 Moreover, an anodic alumina membrane could also be used as a hard template to grow amorphous carbon nanotubes, using ferrocene as the carbon precursor.40 This approach involves the sublimation of ferrocene into the nanopores of anodic alumina and sequent pyrolysis of the confined ferrocene in the argon atmosphere. These reports inspire us to explore the possibility of preparing carbon nanotubes using the pore confinement of mesoporous silica. In this work, we report a facile synthetic route to 2-D hexagonally ordered mesoporous CMK-5 materials based on the CVD of ferrocene in nanopores of SBA-15 and subsequent graphitization of the silica/carbon composite at higher temperatures. The effect of CVD time and the graphitization temperature of silica/carbon composite on the textural structure of CMK-5 was systematically investigated. The structural evolution of CMK-5 from nanopipe to the disordered graphitic ribbons during the graphitization process was observed by high-resolution transmission electron microscopy (HRTEM). The obtained CMK-5 carbons were also applied as supports for Pt loading, and their electrochemical activity on methanol oxidation was investigated and compared with that of Pt/Vulcan XC-72. 2. Experimental Section 2.1. Sample Preparation. Mesoporous SBA-15 was synthesized according to the previously reported method.41 Typically, 4.0 g of Pluronic P123, 105 mL of H2O and 20 mL of 37% HCl were mixed and magnetically stirred at 40 °C. Then 8.5 g of tetraethylorthosilicate (TEOS) was added into the solution, followed by further stirring for 4 h. The white solution was then transferred into an autoclave and aged at 110 and 90 °C
J. Phys. Chem. C, Vol. 112, No. 3, 2008 723 SCHEME 1. Illustration of the Apparatus Used for the Synthesis of CMK-5 Carbon Materials
for 48 h, respectively. The silica products were filtered without washing, dried at 90 °C, and calcinated at 550 °C for 5 h. The calcinated SBA-15 mesopores aged at 110 and 90 °C were denoted as SBA-15-A and SBA-15-B, respectively. For the synthesis of CMK-5 carbon materials, about 2.0 g of ferrocene and 0.3 g of calcinated SBA-15 were put in two separated quartz boats and inserted into the quartz tube, as illustrated in Scheme 1. Under a flow rate of 10 mL/min of Ar, the temperature of furnace II was raised to 500 °C. Afterward, the temperature of furnace I was increased to 120 °C, the ferrocene was sublimed, and its vapor was carried to the hot zone of furnace II with flowing Ar gas. After CVD of ferrocene at 500 °C for different times, the obtained silica/carbon composite was allow to cool to room temperature to estimate the weight variation of SBA15 before and after CVD. The silica/carbon composite was then subjected to different temperatures to facilitate the graphitization. The silica was removed by dispersing the silica/carbon composites into 20% hydrofluoric acid (HF) solution for 24 h, followed by washing with copious deionized water and ethanol, and vacuum-dried at room temperature to yield the CMK-5 carbon materials. The silica/carbon composite and the final CMK-5 carbon materials were denoted as SBA-15/CMK-5-X-Y and CMK-5-X-Y, respectively, where X is the graphitization temperature of the silica/carbon and Y is the CVD time. 2.2. Preparation of Pt/Carbon Electrocatalysts and the Electrochemical Measurements. Platinum supported on CMK-5 carbon materials were prepared by ethylene glycol reduction. Typically, 20 mg of carbon and the desired amount of H2PtCl6‚ H2O (3.77 mg Pt/mL) aqueous solution were dispersed in 30 mL of ethylene glycol, followed by refluxing at 140 °C in Ar for 4 h under continuous stirring. For comparison, Pt loaded on the carbon black (Vulcan XC-72) support was also prepared by the same method. The Pt loading on both CMK-5 and XC72 was kept at 20 wt % for comparison. The catalyst performance for the room-temperature methanol oxidation reaction was evaluated by cyclic voltammetry (CV) using IM6ex potentiostat/galvanostat. Pt gauze and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes, respectively. All potentials in this work are quoted against the SCE. The working electrode was prepared by casting catalyst ink consisting of catalyst and 5% Nafion solution onto a glassy carbon disk electrode with 3.0 mm diameter. The electrolyte solution consists of 1.0 M CH3OH and 0.5 M H2SO4, and the CVs were recorded at a scanning rate of 20 mV‚s-1 2.3. Characterization. Powder X-ray diffraction (XRD) measurements were carried out on Rigaku D/max-2500 diffractometer with Cu KR radiation. Large and small angles were obtained at a scanning rate of 5°/min and 2°/min and an accelerating current of 100 mA and 30 mA, respectively. Transmission electron microscope (TEM) images were obtained with an FEI Tecnai G2 Spirit microscope at an accelerating voltage of 300 kV. The sample morphology was also observed on an FEI TECNAI G2 F30 field-emission HRTEM, working at a 300 KV accelerating voltage. Prior to observation, the
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carbon sample was sonicated in absolute ethanol for 10 min and then dropped onto a holey carbon film supported on a copper grid. The Raman spectra were collected on an Acton Raman spectrometer using a 532 nm laser line as the excitation source. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer Diamond TG analyzer. The measurement was carried out under air atmosphere from room temperature to 900 °C, with a heating rate of 10 °C/min and an air flowing rate of 20 mL/min. An autosorb-1 analyzer from Quantachrome was used for the nitrogen adsorption experiments at -196 °C. Prior to measurement, the samples were degassed at 180 °C for about 6 h. The surface area of samples was evaluated by the Brunauer-Emmett-Teller (BET) method using adsorption data in the relative pressure (P/P0) range from 0.05 to 0.2. The pore size distributions (PSDs) of the samples were calculated from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method. The total pore volume was estimated from the adsorbed amount at a relative pressure of 0.99. The pore volume of structural pores (Vstr) and textural pores (Vtex) were estimated using the V-t plot method. 3. Results and Discussions 3.1. Formation of SBA-15/CMK-5 Composite. In the past few years, ferrocene has proved to be a good precursor and/or catalyst in the growth of carbon nanotube precursors.37-39 In this work, we use ferrocene as both precursor and catalyst for the synthesis of CMK-5 mesoporous carbon. The procedure of the CVD of ferrocene is illustrated in Scheme 1. The temperature of furnace I and II was independently controlled by two temperature-program controllers. Ferrocene is a highly volatile organometallic compound with excellent vaporizability. When the temperature of furnace I is above the sublimation temperature, ferrocene is vaporized and the gaseous ferrocene molecules enter the reaction zone of furnace II, where the temperature was kept at 500 °C to allow the ferrocene to be decomposed and deposited in the nanochannels of SBA-15. In the generation of CMK-5 using furfuryl alcohol as the carbon source, the strong interaction of the precursor with the silica walls is crucial for the nanopipe formation.14 Recently, the incorporation of ferrocene molecules into porous glass and subsequent pyrolysis generated the silica/carbon composite.42,43 The oxidation of ferrocene into ferricinium was essential to the formation of composite, because of the strong interaction between the ferricinium and the silanol groups on the surface of porous glass. Moreover, the aligned carbon nanotubes could selectively grow on the surface of silica from the Si/SiO2 patterns by pyrolyzing the mixture of xylene and ferrocene at 800 °C.44 The selective growth of aligned carbon nanotubes on silica suggested the strong interaction between pyrolytic carbon and silica substrate. This result was recently supported by the fact that pyrolysis of pure ferrocene in the absence of silica generates carbon microspheres.35,36 In our experiment, no carbon microspheres or nanotubes were found when SBA-15 was used as the template. Our result suggests that there may exist strong interaction between the pyrolytic carbon clusters and the pore wall of SBA-15. Otherwise, the obtained mesoporous carbon would exhibit CMK-3 (for example, graphitic CMK-3 synthesized by CVD of acetonitrile or styrene22) instead of CMK-5 structure. This interaction allows the carbon clusters to be preferentially deposited on the inner surface of SBA-15 nanopores and rearranged into the carbon films. Thus, by varying the CVD time, it is possible to control the carbon amount in the silica/carbon composite and change the thickness of the resulting nanopipes of CMK-5.
Figure 1. Nitrogen adsorption isotherms (A) and the corresponding BJH PSD (B) of the silica/carbon composite synthesized by CVD of ferrocene for different times at 500 °C, followed by pyrolyzation at 700 °C for 2 h. The isotherms of SBA-15/CMK-5-700-20, SBA-15/ CMK-5-700-60, SBA-15/CMK-5-700-90 and SBA-15/CMK-5-700-120 were offset vertically by 450, 700, 900, and 1100 cm3 g-1 at standard temperature and pressure (STP), respectively.
The adsorption isotherms of the silica/carbon composites are shown in Figure 1A, and the structural parameters derived from them are listed in Table 1. The shape of the nitrogen adsorption isotherms for the composite is similar to that of SBA-15, and the hysteresis can be ascribed to the capillary condensation from the mesopore interiors of the carbon nanopipes. However, both the surface area and pore volume were sharply reduced when the carbon deposited in the mesopores of SBA-15. The carbon content in the composite calculated from the TGA increased from 20.2 wt % to 32 wt % as the CVD time increased from 20 to 120 min, indicating the reduced pore volume and surface area arose from the occupation of SBA-15 nanopores by the pyrolytic carbon from ferrocene. Moreover, the hysteresis loops corresponding to the capillary condensation in the silica/carbon composites shifted to the lower relative pressure as the CVD time increased, suggesting the gradually decreased pore size. The BJH PSD derived from the adsorption branch of silica/ carbon composites is shown in Figure 1B. It is evident that the pore size of the silica/carbon composite systematically shifts from 8.9 nm for SBA-15 to 3.7 nm as the CVD time is increased to 120 min. The broad PSD of the silica/carbon composite compared to that of SBA-15 indicates that carbon films deposited on the inner surface of the template are more or less nonuniform. Additionally, all the isotherms in Figure 1A levels off at higher relative pressures, suggesting that the carbon clusters exclusively deposited in the nanopores rather than on the external surface of SBA-15. The microporous structure of the silica/carbon composites was also evaluated by t-plot microporous analysis. It was found that the composites, regardless of CVD time, exhibit no micropore adsorption. It is wellknown that SBA-15 possesses a considerable amount of micropores in the pore wall. These complementary pores connect the adjacent nanorods or nanopipes to form the highly ordered carbon replica. In our case, the silica/carbon composites without micropores suggested that the pyrolytic carbon from the ferrocene could not only fill the whole complementary pores but also fill the partial primary mesopores of SBA-15. The
Synthesis of CMK-5 Mesoporous Carbon
J. Phys. Chem. C, Vol. 112, No. 3, 2008 725
TABLE 1: Structural Parameters of SBA-15/CMK-5-A Composite Synthesized via CVD of Ferrocene for Different Timesa
composite
CVD time (min)
SBET (m2/g)
Vt (cm3/g)
WBJH (nm)
thickness of nanopipe (nm)
content of carbon (%)
SBA-15-A-700 SBA-15/CMK-5-700-20 SBA-15/CMK-5-700-60 SBA-15/CMK-5-700-90 SBA-15/CMK-5-700-120
0 20 60 90 120
662.3 358.1 264.1 230.6 222.5
1.18 0.58 0.35 0.24 0.20
8.9 7.4 5.0 4.5 3.7
0 0.8 2.0 2.2 2.6
0 20.2 27.7 29.2 32.0
a SBET is the specific surface area deduced from the isotherm analysis in the relative pressure of 0.05-0.2; Vt is the total volume calculated at a relative pressure of 0.99; WBJH is the pore diameter at maximum of the PSD calculated by the BJH method from the adsorption branch. The thickness of nanopipe is determined based on the difference between pore size of SBA-15-A (calcinated at 700 °C) and SBA-15/CMK-5 composite. The content of carbon is obtained from the TGA. Parameters of SBA-15-700: a0 ) 11.1 nm, WBJH ) 8.9 nm, wall thickness ) 2.2 nm.
Figure 2. Small angle XRD patterns of mesoporous CMK-5-A carbons synthesized via CVD of ferrocene for different times.
complementary pore filled with pyrolytic carbon connects adjacent nanopipes and lead to the formation of 2-D hexagonally ordered CMK-5 structure after removal of SBA-15 template. The thickness of the carbon nanopipes can be calculated on the basis of the difference between the radius of mesopores of SBA15 in the composite and the radius of the mesopores of the silica/ carbon composite.13 However, in this work, SBA-15-A calcinated in air at 700 °C instead of burned from the silica/carbon composite was used to roughly estimate the thickness of the carbon nanopipes because, after calcination of silica/carbon in air, the obtained SBA-15 contained a considerable amount of Fe2O3 nanoparticles, which make the pore size of SBA-15 smaller than the actual value in the composite. The thickness of nanopipes was also summarized in Table 1 under the assumption that the carbon films are uniformly deposited on the nanopores of the silica template. The thickness of the carbon film was increased to 2.6 nm as the CVD time increase up to 120 min, a value larger than that previously reported 0.6-1.3 nm.13,14 3.2. CMK-5 Carbons Isolated from the Composites. The small XRD patterns of the carbon materials after removal of silica from the composite are shown in Figure 2. The obtained carbon materials synthesized with CVD time increased from 60 to 120 min showed five well-resolved diffraction peaks, characteristic of mesoporous CMK-5 structure.12-15 Quite different from both the CMK-3 and SBA-15 templates, the (100) reflection, which is the strongest peak for SBA-15, becomes very weak, while the (110) reflection accompanied by a weak (200) reflection is dramatically increased in intensity. The low intensity of the (100) diffraction peak in comparison to that of the (110) peak was ascribed to the diffraction interference between the pipe walls and the spacers interconnecting adjacent pipes.12,13 This observation corroborates the idea that the carbon materials isolated from the silica/carbon composite feature typical CMK-5 structure. From the XRD pattern of Figure 2, it is evident that the structure of 2-D hexagonally ordered CMK-5 is highly dependent on the CVD time of ferrocene. When the CVD time is shorter than 20 min, the structure of the CMK-5 carbon is partially collapsed, as the consequence of a lower
Figure 3. TEM images of CMK-5-A carbon synthesized via CVD of ferrocene for 20 min (A), 60 min (B), and 120 min viewed perpendicular to the channel (C) and along the channel (D).
carbon content in the silica/carbon composite. The 2-D hexagonally ordered CMK-5 could be obtained when the CVD time varied in the range of 60 to 120 min. TEM images of CMK-5 carbon with different CVD times are shown in Figure 3. A CVD time of 20 min leads to inadequate coverage of the inner surface of SBA-15 mesopores by pyrolytic carbon, which results in the formation of poorly ordered CMK-5 carbon with many defects on the pipes (Figure 3A). With the CVD time increased to 60 min, the obtained CMK-5 materials exhibit hexagonally ordered structure (Figure 3B), and this periodically ordered structure was further improved when increasing the CVD time up to 120 min, consistent with the XRD in Figure 2. A TEM image of CMK-5 (CVD time of 120 min) viewed along the carbon nanopipes axis and the corresponding Fourier diffractogram (inset) is shown in Figure 3D. It is worth noting that two types of pores can be clearly identified. The circular traces are images of the pores originated from the dissolution of the SBA-15 framework, where the circular white areas at the center of circles correspond to the pores generated in the inner part of the SBA15 channels that were not filled with the carbon. The adsorption isotherms of CMK-5 are shown in Figure 4A, and the structural parameters derived from them are summarized in Table 2. It is evident that the CMK-5 materials, after removing silica from the composite, display significantly high adsorption capacity. For example, the surface area and total pore volume for the silica/carbon composite is 264.1 m2/g and 0.35
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Figure 5. Large-angle powder XRD patterns of CMK-5-A carbon synthesized by CVD of ferrocene at 500 °C for different times, followed by graphitization at 700 °C for 2 h.
Figure 4. Nitrogen adsorption isotherms (A) and the corresponding BJH PSD (B) of the CMK-5-A carbon synthesized via CVD of ferrocene for different times. The isotherms of CMK-5-700-90 and CMK-5-700-120 were offset vertically by 600 and 1200 cm3 g-1 at STP, respectively.
TABLE 2: Structural Parameters of Mesoporous CMK-5-A Synthesized by CVD of Ferrocene for Different Timesa WBJH (nm) CMK-5-A
CVD time (min)
SBET (m2/g)
Vt (cm3/g)
I
II
CMK-5-700-20 CMK-5-700-60 CMK-5-700-90 CMK-5-700-120
20 60 90 120
1357 1755 1365 1044
1.54 2.10 1.50 1.13
3.3 3.5 3.3 3.3
5.7 5.0 4.4 3.6
a
See notation in Table 1.
cm3/g when CVD time is 60 min, which increased to 1755 m2/g and 2.1 cm3/g, respectively, after complete dissolution of silica template. By comparing the pore volume before and after silica dissolution, it was found that about 83% pore volume of CMK-5 was contributed from the pore between the nanopipes, in accordance with previously reported results.13 Although the surface area in our work is comparable to that of previous CMK-5 investigations using furfuryl alcohol as the precursor, the total volume under the optimized condition is larger than Ryoo’s result.13 The adsorption branch for all CMK-5 synthesized with different CVD times levels off at higher relative pressure, indicating the contribution from the secondary mesoporosity is negligible. From Table 2, it is observed that the surface area and pore volume for CMK-5 materials reached the maximum value at a CVD time of 60 min and then declined when the CVD time increased to 120 min. This variation is reasonable because the longer CVD time tends to increase carbon content, and yields carbon nanopipes with small diameter. The low surface area and small pore volume of CMK-5 at a CVD time of 20 min is, however, associated with the partially collapsed pore structure, which was also confirmed by XRD and TEM. Although the capillary condensation steps related to the adsorption of two different types of pores of CMK-5 were not distinctly separated, the existence of two types of pores in the CMK-5 materials was more easily observed from the PSD as shown in Figure 4B. The PSDs for all CMK-5 materials demonstrated two overlapped peaks. The size of pore
I remain essentially invariable, regardless of CVD time, and the size of pore II decreases as the CVD time increased. This result allows one to conclude that pore I was created by the dissolution of silica, whereas pore II was formed by the nanopores of SBA-15 not filled with the carbon. By comparing the wall thickness of SBA-15 and the pore size of the silica/ carbon composite with that of corresponding CMK-5 materials, it is found that the size of pore I is slightly larger than the wall thickness of 2.2 nm of SBA-15-A (calcinated at 700 °C), while the size of pore II of CMK-5 is generally smaller than the pore size determined from the corresponding silica/carbon composite. This result can be attributed to the structural shrinkage during the template dissolution, which is more pronounced for CMK-5 at a CVD of 20 min because of the low carbon loading in the composite. The powder XRD patterns of the CMK-5 using SBA-15-700 as the template are shown in Figure 5. All the CMK-5 materials synthesized at 500 °C, followed by graphitization at 700 °C for 2 h, showed two broad peaks at 2θ ∼ 24.7° and 43.5°, corresponding to the (002) and (101) diffraction peaks of graphite, respectively. The (002) peaks showed comparable intensity for all the CMK-5, regardless of the CVD time. This observation is possibly related to the same pore size of SBA15 as well as the identical graphitization temperature (700 °C). On the basis of the Bragg equation, the interlayer distance value (d002) between graphene planes of CMK-5 materials synthesized under the present conditions was calculated to be 0.367 nm (2θ ∼ 24.9°), which is smaller than the 0.405 nm of CMK-5 prepared using furfuryl alcohol as the precursor (2θ ∼ 22.3°),15 suggesting that CMK-5 using ferrocene as the carbon source showed an improved graphitization degree. Nevertheless, the lattice spacing of our CMK-5 materials is still much larger than that of graphite (2θ ∼ 26.4°, d(002) ∼ 0.338 nm) because of the lower graphitization degree of CMK-5. 3.3. CMK-5 Carbons Graphitized at Different Temperatures. In this section, the SBA-15 silica aged at 90 °C (denoted as SBA-15-B) was used as a template to investigate the effect of the pore size of SBA-15 on the PSD and the graphitization degree of the final CMK-5. We pyrolyzed ferrocene in the nanochannels of SBA-15-B at 500 °C for 60 min, followed by graphitization of the silica/carbon composite at different temperatures to investigate the graphitization degree of the final CMK-5-B materials. The nitrogen adsorption and the corresponding BJH PSD are shown in Figure 6. As shown in Table 3, CMK-5-B materials templated from SBA-15-B generally showed increased BET surface area and pore volume in comparison with that templated from SBA-15-A, which correlated well with previously reported CMK-5 investigations using furfuryl alcohol as precursor.13,14 The adsorption branches of CMK-5-B level off at higher relative pressures at lower
Synthesis of CMK-5 Mesoporous Carbon
Figure 6. Nitrogen adsorption isotherms (A) and the corresponding BJH PSD (B) of the CMK-5-B carbon fabricated by CVD of ferrocene at 500 °C, followed by graphitization at different temperatures. The isotherms of CMK-5-700-60, CMK-5-800-60, CMK-5-850-60, and CMK-5-900-60 were offset vertically by 400, 700, 1600, and 2400 cm3 g-1 at STP, respectively.
graphitization temperatures, whereas, at higher graphitization temperatures (above 850 °C), the adsorption branches ascend at higher relative pressures, suggesting the appearance of a considerable amount of secondary porosity. Although the surface area of CMK-5-B in Table 3 was reduced at higher temperatures, the total pore volume remained invariable, even at a graphitization temperature of 900 °C. However, by applying t-plot micropore analysis, it is possible to distinguish the pore volume of textural pores from the total pore volume. As shown in Table 3, at graphitization temperatures lower than 850 °C, the structural mesopore volume mainly contributes to the total pore volume of CMK-5, and the textural pore volume is comparable to the value of 0.25 cm3/g for SBA-15-B. However, this case inversed when the graphitization temperature increased up to 900 °C. The structural mesopore volume sharply decreased to 0.3 cm3/g, and textural pore volume significantly increased to 2.3 cm3/g. This variation indicated that most of the ordered CMK-5 structure was replaced by the secondary porosity at higher graphitization temperatures. The PSDs shown in Figure 6B display two better-resolved peaks, with the diameter of pore I in the range of 2.7-2.9 nm and pore II varying from 4.4 to 4.7 nm. The CMK-5 templated from SBA-15 with smaller pore size seems to display a smaller nanopipe diameter (for example, 5.0 nm of CMK-5-700-60-A vs 4.4 nm of CMK-5-700-60-B). This result is rationalized by considering that the same CVD time would produce approximately equal amounts of carbon in the composite. The 2-D hexagonal structure of CMK-5-B during the graphitization process was also investigated by small-angle XRD. Figure 7 shows the XRD patterns of CMK-5-B carbon obtained by CVD of ferrocene at 500 °C, followed by graphitization at different temperatures. The well-resolved five diffraction peaks of CMK-5-B can be observed at a graphitization temperature as high as 850 °C, indicating that the CMK5-B materials synthesized under the present conditions can preserve their hexagonal ordered structure. If the temperature
J. Phys. Chem. C, Vol. 112, No. 3, 2008 727 increased up to 900 °C, a very weak 110 diffraction peak characteristic of CMK-5 is ambiguously observed, suggesting that most of the CMK-5-B structure is likely to be lost at such high temperature. In comparison with conventional CMK-5 prepared at a carbonization temperature of 900 °C,12-15 the collapsed CMK-5 at a graphitization temperature of 900 °C may be attributed to the existence of a large amount of Fe/Fe2O3 nanoparticles, which facilitate the graphitization of CMK-5 and lead to the formation of graphitic ribbons, which will be discussed later. The structure of CMK-5-B graphitized at different temperatures was observed by TEM. Figure 8 displays TEM images of CMK-5-B graphitized at 800 and 900 °C. Consistent with the XRD of Figure 7, CMK-5-B at a graphitization temperature of 800 °C exhibited ordered nanopipe structure (Figure 8A). Wide light stripes can be considered as projections of interiors of nanopipes, while the narrow light stripes correspond to the mesopores between the adjacent nanopipes. In Figure 8B, two different morphologies of CMK-5 can be distinguished. The bundles of rod-like primary particles with a diameter of 500 nm and a length of about 1.5 µm in the left area is analogous to the original morphology of SBA-15, while the particles in the right area exhibited peculiar morphology consisting of many entangled ribbons. The magnified images of the left and right areas are shown in Figure 8C,D, respectively. It is observed that the left area of Figure 8B exhibits well-ordered CMK-5 structure (Figure 8C), whereas the right area of Figure 8B displays partially ordered CMK-5 accompanied by a large amount of tangled ribbon-like structures (Figure 8D), which were proven to be graphite by HRTEM (not shown) and Raman spectra in Figure 9. The formation of a large amount of entangled graphitic ribbons at 900 °C correlated well with the appearance of the upward adsorption branch in Figure 6 and the increased textural pore volume listed in Table 3. On the basis of TEM images and N2 adsorption, it may be envisioned that the structural evolution of CMK-5-B from ordered nanopipes to disordered structure probably takes place between 800 and 900 °C. We pyrolyzed the silica/carbon composite at 850 °C in order to track the structural evolution process of CMK-5-B by HRTEM, XRD, and TGA. As shown in Figure 10A, most of CMK-5-B carbon graphitized at 850 °C exhibited ordered nanopipe structure, in accordance with the XRD patterns of Figure 7. The curved and concentric graphene layers are evident in the inset of Figure 10A, indicating that the nanopipes consist of graphitic carbon. It is interesting that the ordered CMK-5-B area (area I), the transition area from ordered to semi-ordered (area II), and the disordered area (area III) coexist in Figure 10B, where the three types of different structures associated with CMK-5 during the graphitization process are well preserved and distinctly observed. The microstructure of area I was magnified in Figure 10C. The graphene layers were generally loosely stacked parallel to the axis of nanopipes, suggesting that the CMK-5 itself consisted of carbon with a limited graphitization degree. By comparing the microstructure of the CMK-5-B in area II with that in area I, it is evident that the CMK-5-B in area II consists of slightly fused carbon nanopipes, which still remains the long-ranged arrangement similar to CMK-5-B. However, in area III, the nanopipes characteristic of CMK-5 were completely lost, and a large amount of deformed carbon nanocoils were observed, as shown in Figure 10D. These nanocoils would eventually transform into the graphitic ribbons at a higher graphitization temperature of 900 °C, as shown in Figure 8D. From the TEM images in Figure 8 and the HRTEM
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TABLE 3: Structural Parameters of Mesoporous CMK-5-B Synthesized at Different Graphitization Temperaturesa WBJH (nm) CMK-5-B
graphitization temperature (°C)
SBET (m2/g)
Vt (cm3/g)
Vstr (cm3/g)
Vtex (cm3/g)
I
II
CMK-5-600-60 CMK-5-700-60 CMK-5-800-60 CMK-5-850-60 CMK-5-900-60
600 700 800 850 900
2418 2369 2449 1749 942
2.40 2.41 2.68 2.36 2.60
2.01 2.01 2.20 1.08 0.30
0.30 0.30 0.48 1.28 2.30
2.7 2.9 2.9 2.9 2.7
4.7 4.4 4.4 4.7 4.4
a CMK-5-B materials are templated from SBA-15-B via CVD of ferrocene at 500 °C for 60 min, followed by graphitization at different temperatures. Parameters of SBA-15-B: a0 ) 10.2 nm, WBJH ) 8.0 nm, wall thickness ) 2.2 nm. SBET, Vt, and WBJH are BET surface area, total pore volume, and the pore diameter at the maximum of PSDs, respectively. Vstr and Vtex are determined from the t-plot analysis.
Figure 7. Small-angle XRD patterns of CMK-5-B carbon synthesized via CVD of ferrocene at 500 °C, followed by graphitization at different temperature.s
Figure 8. TEM images of CMK-5-B carbon synthesized via CVD of ferrocene at 500 °C, followed by graphitization at 800 °C (A) and 900 °C (B-D).
images in Figure 10, it may be concluded that, if the graphitization temperature of the composite is raised to 850 °C, the CMK5-B starts a structural evolution from hexagonally ordered nanopipe to a semi-ordered structure, which is displayed as sintered carbon films in the nanochannels of SBA-15. Simultaneously, the sintered carbon films could escape from the pore confinement of SBA-15 and transform into disordered graphitic nanocoils and finally into graphitic ribbons with the assistance of Fe nanoparticles. The lost of nanopipes resulted in the evident decrease of surface area. The formation of a large number of nanocoils seemed to not vary the total pore volume of carbon materials. The evolution of CMK-5-B from nanopipe to the final
Figure 9. Raman spectra of CMK-5-B synthesized via CVD of ferrocene at 500 °C, followed by graphitization at different temperatures.
Figure 10. HRTEM images of CMK-5-B carbon synthesized via CVD of ferrocene at 500 °C for 60 min, followed by graphitization at 850 °C.
disordered graphitic ribbons may be accomplished by direct transformation or via the transition of semi-ordered fused nanopipes. Both routes proceed along the nanopipe direction and finally lead to the formation of disordered graphitic ribbons at higher temperatures of 900 °C. The XRD patterns of CMK-5-B graphitized at different temperatures are shown in Figure 1S (Supporting Information). The lower temperature (below 850 °C) results in CMK-5-B with broad diffraction peaks, suggesting its lower graphitization degree. CMK-5-B materials graphitized at higher temperatures (above 850 °C) displayed intensive diffraction peaks at 2θ ∼ 26°, which were mainly ascribed to the appearance of a considerable amount of graphitic nanocoils. The Raman spectra
Synthesis of CMK-5 Mesoporous Carbon
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Figure 11. TG curves of silica/carbon composite and CMK-5-B synthesized via CVD of ferrocene at 500 °C for 60 min, followed by graphitization at different temperatures.
of the CMK-5-B sample are shown in Figure 9. The two main Raman modes were observed at 1339 and 1589 cm-1, respectively. The band at 1339 cm-1 is referred to as the D band and is mainly attributed to vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite. The band at 1589 cm-1 is referred to as the G band and is ascribed to the E2g vibration mode of graphite layers. At graphitization temperatures higher than 850 °C, both the D band and the G band became narrow, and their intensities evidently increased. Moreover, the shifts of the G band from 1589 cm-1 to a lower wave number of 1570 cm-1, the decreased ratio of ID/IG as well as the decreased width at half-maximum of the G band suggested the increase in the graphitization degree of the carbon materials at higher pyrolysis temperatures.45 The higher graphitization degree of carbon materials was mainly ascribed to the formation of a large amount of graphitic nanoribbons, which showed a much higher graphitization degree than the CMK-5 itself and contributed to the intensive peaks of the XRD patterns in Figure 1S (Supporting Information). The oxidation resistance property of CMK-5-B was also measured by TGA in Figure 11. The TGA curve of the silica/ carbon composite revealed an obvious weight increment in the temperature range from 160 to 300 °C, corresponding to the oxidation of Fe nanoparticles. In comparison with the corresponding CMK-5 carbon materials, the oxidation temperature of carbon in the silica/carbon composite was decreased by 60 °C, as a consequence of carbon oxidation catalyzed by Fe2O3 nanoparticles.23,31 For all the CMK-5-B carbon, about a 5-7 wt % weight loss below 200 °C was observed, which was likely to be caused by the adsorption of water or organic molecules on the carbon samples. The negligible residue above 750 °C indicates that most of the silica template together with the Fe/ Fe2O3 nanoparticles in the carbon matrix was leached out by HF aqueous solution, which was also confirmed by energydispersive X-ray spectroscopy (not shown). Concomitant with the variation of graphitization temperature from 700 to 800 °C, the complete oxidation temperature of CMK-5-B was found to slightly increase from 605 to 620 °C. When the graphitization temperature is further increased to 850 °C, the weight loss curve exhibits two obvious steps, with one step emerging at a lower temperature range (520-630 °C) and the other step emerging at a higher temperature range (630-720 °C). These two steps can be assigned to the combustion of carbon nanopipes and the graphitic nanocoils, respectively. The continuously increased oxidation temperature of CMK-5-B from 605 to 630 °C suggests the improved graphitization degree of nanopipes. In comparison with previous graphitic carbon studies,23,24,46 the relatively low oxidation temperature of nanopipes is ascribed to the microcrystal graphite with a low graphitization degree. On the basis of the different oxidation temperatures, it is possible to estimate
Figure 12. TEM images of Pt catalyst loaded on XC-72 (A), CMK5-700-60 (B), CMK-5-850-60 (C), and CMK-5-900-60 (D).
the content of graphitic ribbons from the TG curves.33 The content of graphitic nanocoils was calculated to be 24.9 wt % in the CMK-5-850 sample. This content was found to dramatically increase to 67.0 wt % when graphitized at 900 °C, which was corroborated by TEM images and XRD. Although SBA-15 with different pore sizes can lead to a final CMK-5 carbon with a more distinguishable PSD, this template effect seems to have no noticeable effects on the graphitization degree of the final CMK-5 carbon by comparing the intensity of XRD patterns of CMK-5 templated from SBA-15-A and SBA-15-B. This result is possibly related to the similar pore sizes of SBA-15-A (WBJH ) 8.9 nm) and SBA-15-B (WBJH ) 8.0 nm). Thus, CMK-5 with a higher graphitization degree may be expected by further increasing the pore size of SBA-15 and carefully controlling the synthesis conditions. 3.4. Electrochemical Activity of Pt/CMK-5. The obtained CMK-5-B was applied as a catalyst support for Pt loading. The powder XRD patterns of Pt loaded on the commercial support XC-72 and the CMK-5-B mesoporous carbon are present in Figure 2S (Supporting Information). The peaks at about 26° were due to the 002 diffraction of graphite. The diffraction peaks observed at 39.8°, 46.5°, 67.8°, and 81.2° corresponded to the (111), (200), (220), and (311) facets of Pt nanocrystals. According to the Scherrer equation, the average size of Pt particles supported on Vulcan XC-72 was estimated to be 5.3 nm. However, this value decreased to 3.1, 3.0, and 3.8 nm when loaded on CMK-5 mesoporous carbon synthesized at graphitization temperatures of 700, 850, and 900 °C, respectively. The smaller particle size of Pt is ascribed to the much higher surface area of the CMK-5-B carbon support. Figure 12 shows the TEM images of Pt nanoparticles loaded on XC-72 and CMK-5-B carbon. It was observed that the Pt particles were uniformly dispersed on both XC-72 and CMK-5-B mesoporous carbon. The particle size of Pt loaded on CMK-5-B mesoporous carbon is much smaller than that on XC-72, consistent with the results calculated from the Scherrer equation. The different Pt particle sizes may be related to the different surface areas, as well as the different surface chemistries of the carbon support.47 The CV curves of Pt/XC-72 and Pt/CMK-5 in 0.5 M H2SO4 aqueous solution are presented in Figure 3S (Supporting
730 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Figure 13. CVs of methanol oxidation on Pt/XC-72 and Pt/CMK5-B catalyst in 0.5 M H2SO4 + 1 M CH3OH solution at room temperature.
Information). The Pt/CMK-5 showed generally stronger hydrogen adsorption peaks relative to those of Pt/XC-72. The welldeveloped hydrogen adsorption and desorption observed in the potential range of -0.2 to 0.05 V are due to the presence of different Pt facets. The electroactive surface area (ESA) of Pt catalyst was calculated from the charges associated with the hydrogen adsorption on Pt facets in the potential range from -0.2 to 0.05 V, assuming a correspondence value of 0.21 mC/ cm2 Pt.30,48 The ESA of Pt/XC-72 was calculated to be 19.8 m2/g Pt. However, the ESAs of Pt/CMK-5-700, Pt/CMK-5-850, and Pt/CMK-5-900 were increased to 30.0, 33.4, and 37.4 m2/g Pt, respectively, much larger than that of Pt/XC-72. The larger ESA suggests the high utilization of Pt loaded on CMK-5-B carbon, and thus the higher intrinsic electrochemical activity. The electrocatalytic performance of Pt/CMK-5-B was measured in 0.5 M H2SO4 + 1 M CH3OH solution at room temperature and compared with that of Pt/XC-72. The typical CVs of the Pt/CMK-5 are shown in Figure 13. The peaks observed at 0.65 V in the forward scan are typical for methanol oxidation,25,30,49,50 whereas the peaks at 0.47 V in the reverse scan are primarily associated with the removal of the residual carbon species formed in the forward scan.51,52 It is clearly observed that the current density of Pt/CMK-5-B at 0.65 V in the forward scan is significantly higher than that of Pt/XC-72. The specific activity of Pt/XC-72 is 220 mA mg-1 Pt, which was dramatically increased to 344 mA mg-1 Pt when Pt catalyst was loaded on CMK-5-700-60 with a surface area of 2369 m2/g. The higher activity of Pt/CMK-5-700-60 is attributed to the high dispersion of Pt nanoparticles on mesoporous carbon, as shown in Figure 12B. In comparison with CMK-5-700-60, the activity of Pt/CMK-5-900-60 is further increased to 370 mA mg-1 Pt despite its relatively lower surface area (942 m2/g). This observation is ascribed to the formation of a large amount of graphitic ribbon, which showed a higher graphitization degree, as revealed by XRD and Raman spectra. However, the highest specific activity of 433 mA mg-1 Pt for methanol oxidation was achieved on the Pt/CMK-850-60 catalyst. CMK850-60 carbon shows lower graphitic structure but higher surface area than CMK-900-60. This structure property makes CMK850-60 an optimized support for Pt loading. Therefore, the integrated factors, including the uniform pore structure of CMK5, together with the improved graphitization degree in comparison with XC-72 carbon, lead to much higher activity of Pt/ CMK-5 than Pt/XC-72 in room-temperature methanol oxidation. 4. Conclusions In this work, CVD of ferrocene has been demonstrated to be an effective way to synthesize 2-D ordered hexagonal arrays of mesoporous CMK-5. The textural parameter of CMK-5, such as nanopipe diameter, nanopipe wall thickness, surface area,
Lei et al. pore volume, as well as the ordered structure, could be facilely controlled by changing the CVD conditions in combination with the selection of SBA-15 templates with different pore sizes. The SBA-15 template with small pore size generally produces CMK-5 with a small diameter of nanopipes but higher surface area under otherwise identical synthetic conditions. Lower pyrolysis temperature (below 850 °C) produces highly ordered CMK-5 with a relatively low graphitization degree. Higher pyrolysis temperature (above 850 °C) could significantly improve the graphitization of CMK-5 nanopipes, but severely destroy the hexagonally ordered structure due to the formation of a considerable amount of graphitic ribbon. The obtained CMK-5 with different graphitization degrees were applied as catalyst supports for Pt loading toward methanol oxidation. Compared to XC-72 carbon support, platinum nanoparticles loaded on CMK-5 displayed reduced particles size and increased ESA. CVs showed that Pt/CMK-5 exhibited nearly 2 times higher specific mass activities than Pt/XC-72 for methanol oxidation under optimized conditions. The larger surface area, higher graphitization degree, and uniform pore structure of the CMK-5 support were considered to be the main reasons for its enhanced catalytic activity. Since the textural parameters of SBA-15 can be adjusted in a wide temperature range, the variation of synthetic conditions together with the proper selection of SBA-15 would allow the structural parameter of the final CMK-5 to be tailored in a relatively wide range. The obtained CMK-5 could open new possibilities for applications as catalyst supports, electrochemical double-layer capacitors, and sorbents for dye elimination. Acknowledgment. We thank Dr. Xuming Wei, in State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, for HRTEM measurements. We also acknowledge the Natural Science Foundation of Liaoning Province (Grant No. 2050798) and the Natural Science Foundation of Dalian City (Grant No. 2006J23JH019) for financial support of this work. Supporting Information Available: XRD patterns of CMK5-B synthesized via CVD of ferrocene at 500 °C, followed by graphitization at different temperatures; XRD patterns of Pt/ Vulcan XC-72 and Pt/CMK-5-B; and CVs of Pt/XC-72 and Pt/ CMK-5-B in 0.5 M H2SO4 solution. This information is available free of charge via the Internet at http://pubs.acs.org References and Notes (1) Yu, J. S.; Kang, S.; Yoon, S. B.; Chai, G. S. J. Am. Chem. Soc. 2002, 124, 9382. (2) Chai, G. S.; Yoon, S. B.; Kim, J. H.; Yu, J. S. Chem. Commun. 2004, 2766. (3) Han, S. J.; Sohn, K. K.; Hyeon, T. Chem. Mater. 2000, 12, 3337. (4) Kabbour, H.; Baumann, T. F.; Satcher, J. H., Jr.; Saulnier, A.; Ahn, C. C. Chem. Mater. 2006, 18, 6085. (5) Yang, Z. X.; Xia, Y. D.; Mokaya, R. J. Am. Chem. Soc. 2007, 129, 1673. (6) Xia, Y. D.; Mokaya, R. J. Phys. Chem. C 2007, 111, 10035. (7) Hyeon, T.; Han, S.; Sung, Y. E.; Park, K. W.; Kim, T. W. Angew. Chem., Int. Ed. 2003, 42, 4352. (8) Lu, A. H.; Schu¨th, F. AdV. Mater. 2006, 18, 1793. Lee, J. W.; Kim, J. Y.; Hyeon, T. AdV. Mater. 2006, 18, 2073. Zhao, X. S.; Su, F. B.; Yang, Q. F.; Guo, W. P.; Bao, X. P.; Lv, L. Z.; Zhou, C. J. Mater. Chem. 2006, 16, 637. (9) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712. (10) Vinu, A.; Srinivasu, P.; Takahashi, M.; Mori, T.; Balasubramanian, V. V.; Ariga, K. Microporous Mesoporous Mater. 2007, 100, 20. (11) Kim, T. W.; Park, I. S.; Ryoo, R. Angew. Chem., Int. Ed. 2003, 42, 4375. (12) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169.
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