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J. Phys. Chem. C 2008, 112, 1028-1033
Highly Dispersed Pt Nanoparticles on Mesoporous Carbon Nanofibers Prepared by Two Templates Guiwang Zhao, Jianping He,* Chuanxiang Zhang, Jianhua Zhou, Xiu Chen, and Tao Wang College of Material Science and Technology, Nanjing UniVersity of Aeronautics and Astronautics, Nanjing 210016, P. R. China ReceiVed: July 1, 2007; In Final Form: October 17, 2007
Ordered mesoporous carbon nanofibers (MCNFs) were synthesized through the hard template of AAO (anodic aluminum oxide) membrane together with the soft template of block copolymer surfactant F127 (PEO106PPO70-PEO106). Meanwhile, ordered mesoporous carbons (OMCs) were prepared through the soft template during the same procedure. The TEM images of the MCNFs show highly ordered parallel channels and regularly circular pore structure. The MCNFs and OMCs as supports for loading Pt nanoparticles were prepared via microwave synthesis process. The electrochemical properties of the two catalysts and the commercial catalyst Pt/C(E-TEK), which is used for comparison, were studied by cyclic voltammogram at room temperature. The results reveal that the catalyst using mesoporous carbon nanofibers as support has excellent electrochemical performance for hydrogen and methanol electro-oxidation. The electrochemical active surface of Pt particles for the Pt/MCNFs in sulfuric acid solution reaches a peak value of 235.2 m2/g, which is about four times that of Pt/OMCs and twice that of Pt/C(E-TEK). Besides, the Pt/MCNFs catalyst with higher current density, higher ia/ib in the methanol solution, exhibits higher activity than the other two catalysts for methanol electrooxidation.
1. Introduction Since their first discovery,1-3 ordered mesoporous carbons (OMCs) with high surface areas, large pore volumes, tunable pore sizes, and good gas penetrability have attracted considerable interest in many areas, such as catalyst supports, electrode materials, and adsorbents for gas separation. Meanwhile, the one-dimensional (1D) materials are interesting because of their organized structure and potential application,4 for their diameter and length could be controlled by dimensions of the cylindrical pore in the template. Accordingly, ordered mesoporous carbon nanofibers that combined the ordered mesoporous structure with the 1D morphology are rather fascinating. Recently, great efforts have been made to develop mesoporous materials with the 1D structure. Among them, several 1D materials with unique mesophase structures have been successfully prepared within the channel of the AAO (anodic aluminum oxide) membrane. These obtained materials are hierarchically ordered both in the arrangement of channels in the templates and in the assembly of the mesophase counterparts within these channels. Yang et al.4 have synthesized 1D hierarchically mesostructured silica materials and their arrays in AAO in a controlled way by adjusting the wettability of the alumina pore wall. Compared with silica materials, carbon materials offer greater electrical conductivity and chemical functionality. On this basis, Chae and co-workers5 have attained mesoporous carbon nanofibers using a mesoporous silica nanofiber template that was prepared within AAO support. Cott et al.6 have obtained two distinct arrangements of mesoporous carbon when using AAO as template. But an extra step is necessary to prepare silica nanofiber template during the process of preparing ordered * To whom correspondence should be addressed. E-mail: jianph@ nuaa.edu.cn.
meosoporous carbon. Therefore, the whole procedure is complicated and time-consuming, which limits further application. In this paper, we report a direct preparation of mesoporous carbon nanofibers through the hard template of AAO membrane together with the soft template of block copolymer surfactant F127 (PEO106-PPO70-PEO106). The highly ordered AAO membrane with tunable structure was utilized to control the ratio of the length to the diameter. The AAO template was immerged into the nanocomposite sol, which was mixed well and further self-assembled by the surfactant with the low molecular weight resol. After carbonization under nitrogen flow, the ordered mesoporous carbon nanofibers were obtained and applied as support to load with Pt particles. For comparison, we also prepared ordered mesoporous carbon (OMC) powder in the same procedure without AAO template. The two catalysts were then compared with the commercially available catalyst E-TEK 20 wt % Pt/C [denoted as Pt/C(E-TEK)] in the electrocatalytic activity. 2. Experimental Section For further experiments, the through-channel AAO template with the diameter of about 75 nm was fabricated by the wellknown two-step anodization in our previous work.7 2.1. Synthesis of MCNFs and OMCs. Before the synthesis, the low molecular weight resol (phenol-formaldehyde) as a carbon precursor was prepared by the method reported by the Zhao group.8 Two grams of triblock copolymer F127 was completely dissolved in 20.0 g of ethanol, and 5 g of resol was then added with vigorous stirring. The solution was stirred at 42 °C for 2 h and then divided into two equal parts. The AAO template was immerged into one part for 5-8 h at room temperature to evaporate ethanol, while the other part was also evaporated under the room temperature. The two parts were
10.1021/jp075116x CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008
Highly Dispersed Pt Nanoparticles on MCNFs
J. Phys. Chem. C, Vol. 112, No. 4, 2008 1029
Figure 1. Small-angle XRD patterns of the OMCs and the MCNFs.
both thermopolymerized at 100 °C for 24 h in an oven and then calcined in a tubular furnace at 900 °C for 2 h under nitrogen flow to get rid of the template F127 and to carbonize further. Then the part containing AAO template was immersed in 5 wt % HF solutions for 24 h to remove the template and the acquired product was named MCNFs. The other part without AAO was denoted as OMCs. 2.2. Catalyst Preparation. The synthesis was carried out with the aid of a domestic microwave oven (LG MG-5021MW1, 700 W, 2450 MHz). Platinum (intake 20 wt %) was deposited as follows: 1.4 mL of 0.038 M hexachloroplatinic acid in ethylene glycol was added to 20 mL of ethylene glycol to produce a yellowish solution, followed by the addition of a 2.5 M ethylene glycol solution of sodium hydroxide to adjust the pH to 9.0. Then 40 mg of MCNFs was mixed with the solution containing hexachloroplatinic acid. After ultrasonicating for 30 min, the suspension was exposed in the middle of a microwave oven for 60 s at 700 W and cooled in air, and then the black precipitate was gathered by centrifugation and washed with acetone. After being dried at 80 °C in a vacuum oven overnight, the catalyst denoted as Pt/MCNFs has been fabricated. For comparison, the same amount of Pt was also deposited on the OMCs, denoted as Pt/OMCs, which was prepared following the same method. 2.3. Characterization. X-ray diffraction measurement was carried out with a Bruker D8 ADVANCE diffractometer from 2θ ) 0.7° to 10° at a counter rate of 2°/min to reveal the periodic array of carbon support and from 2θ ) 10° to 90° at a counter rate of 5°/min to reveal the crystalline configuration of the catalysts. Morphologies, sizes, and distributions of the samples were observed with a transmission electron microscope (TEM, 200 kV, FEI Tecnai G2). Nitrogen adsorption-desorption isotherms were measured at -196 °C using a Micromeritics ASAP 2010 system. Surface areas of the samples were calculated using the Bruauer-Emmett-Teller (BET) equation, while the mesopore size distributions were determined by the Barret-Hoyner-Halenda (BJH) method using the adsorption branch. The electrical conductivity of the samples was determined by a four-point probe meter (Wentworth Laboratories probe station) at room temperature in conjunction with a multimeter (Keithley 6514 system electrometer). Inductively coupled plasma atomic emission spectroscopy (ICP-AES, JarrellAsh 1100) was used to analyze the contents of Pt on the catalysts. 2.4. Electrochemical Measurement. The cyclic voltammogram (CV) for the catalyst was determined with a Solartron 1287. A conventional three-electrode system was used in the
Figure 2. TEM images of MCNFs (a-c) and TEM images of OMCs (d, e).
experiment. The work electrode was manufactured as follows: 5 mg of catalyst was dispersed in a mixed solution of 1 mL of ethanol and 50 µL of Nafion solution (5 wt %). After ultrasonicating for 30 min, approximately 25 µL of the mixture was dropped on to the glassy carbon substrate to form a thin layer of ca. 0.1256 cm2 in geometrical area, which was then dried at 80 °C to vaporize the ethanol. A platinum foil and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. All potentials in this experiment are quoted against SCE, which is 0.2412 V versus normal hydrogen electrode at 25 °C. Under the room temperature, the cyclic voltammograms were collected in 0.5 M H2SO4 between -0.22 and 0.98 V at a scan rate of 20 mV/s and in 2.0 M CH3OH + 1.0 M H2SO4 between 0 and 1 V at the same scan rate. 3. Results and Discussion 3.1. Physicochemical Characterization of the Carbon Support. Small-angle XRD is utilized to obtain the structural information on the samples. The XRD patterns of the carbon supports obtained after carbonization under nitrogen at 900 °C are shown in Figure 1. We can observe that the XRD patterns of the OMCs are patterns typical of a high-quality 2D hexagonal mesostructure. Three resolved diffraction peaks, which can be indexed as 10, 11, and 20 reflections, are easily noticed.9 Meanwhile, the sample MCNFs give only a single peak at low diffraction pattern, which is consistent with the circular pore architecture of the framework.10
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Figure 3. N2 adsorption-desorption isotherms (a) and the BJH pore diameter distribution curves (b) of the OMCs and the MCNFs.
SCHEME 1: Synthesis Process of Mesoporous Nanofibers Frameworks
Figure 2 shows the TEM images of the MCNFs and OMCs. From the Figure 2a-c, the MCNFs typically exhibit a size of 75 nm in diameter and more than 1 µm in length. There are two different mesoporous structures in TEM images, highly ordered parallel channel structure (Figure 2b) and the regularly circular pore structures with a diameter of 10 nm (Figure 2c). In Figure 2d,e, the TEM images of the OMCs viewed in the [110] and [001] directions, respectively, together with the smallangle XRD patterns, further confirm an ordered hexagonal arrangement of the mesostructure. The structure of the MCNFs is quite different from that of OMCs. This may be affected by the nanoconfinement effect, which plays dominant roles in a physically confined environment.10-11 On the basis of the TEM images, we propose a possible schematic illustration (Scheme 1) to explain the process that
generates frameworks of mesoporous nanofibers, which correspond to Figure 2b. In brief, the carbon precursor (phenolformaldehyde) was introduced into the AAO template and further self-assembled after mixing well. Then the soft template F127 was easily removed by heating at 350 °C under nitrogen. Upon the calcination at 900 °C in N2 flow, polymers were transformed to carbons. During this procedure, a phase of intergradation existed. Without the influence of the AAO template, the carbon may be arrayed as the intergradation, finally. The resin polymer was strongly attached to the surface of the AAO template because of the hydroxyl groups they both have. The polymer shrank continuously along with the carbonization. The presence of AAO template inhibited the shrinkage of the polymer attached to its surface due to the strong interaction between it and the polymer. Then one part of the
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J. Phys. Chem. C, Vol. 112, No. 4, 2008 1031 TABLE 1: Textural Characteristics and Conductivity Data for the OMCs and the MCNFs
sample
BET area (m2/g)
micropore area (m2/g)
pore volume (cm3/g)
pore size (nm)
conductivity (S/m)
MCNFs OMCs
536 618
124.8 265.7
0.91 0.37
10.2 2.3
71.75 1.24
TABLE 2: Pt Content in the Pt/MCNFs, Pt/OMCs, and Pt/C(E-TEK) Catalysts Pt content (wt %) catalyst sample Pt/MCNFs Pt/OMCs Pt/C(E-TEK) Figure 4. XRD patterns of the catalysts.
nominal
actual
20 20 20
15.4 12.0 16.2
TABLE 3: Electrochemical Parameters of the Catalysts in 0.5 M H2SO4a sample
LPt (mg)
i (mA/cm2)
Q (mC)
S (m2/g)
Pt/MCNFs Pt/OMCs Pt/C(E-TEK)
0.0183 0.0143 0.0193
10.9 1.1 5.0
9.052 1.639 4.555
235.2 54.7 112.5
a LPt, the actual Pt loading on glassy carbon substrate; i, current density of the hydrogen oxidation peak; Q, total charge; S, electrochemical active surface area.
Figure 5. TEM images of the MCNFs (a, b) and the OMCs (c).
polymer was adhered to the AAO and the other part was ruptured to form the core of the framework, and the core had been lightly moved along the axis of the AAO template. After removal of the AAO template, the frameworks from side-view are just like Figure 2b, and the framework of Figure 2c, which is very similar to the Im3m structure of the Zhao group,8 may be due to the ratio of the F127 to the resol impacted by the AAO template. Besides, the calcination temperature, calcination time, and the diameter of the AAO template also impact the structure of the MCNFs. Figure 3 displays the N2 sorption isotherms and the corresponding BJH pore size distribution of the OMCs and MCNFs. The textural parameters of the OMCs and MCNFs are given in Table 1. The isotherms exhibit a characteristic hysteresis behavior; however, the two isotherms are different. The pore size distribution for the OMCs is narrowly centered at about 2.3 nm, while the pore size distribution for the MCNFs exhibit a sample with bimodal pores. One set of the pores correspond with the TEM image (Figure 2b,c), and the other are consistent with a small quantity of ordered mesoporous carbons which are outside of the hard template. The BET surface areas for the MCNFs and OMCs are 536 and 618 m2/g, respectively. The pore volumes for them are estimated to be 0.91 and 0.37 cm3/
g, respectively. On the basis of the results, the surface area for the MCNFs is lower than the value for the OMCs, which can be attributed to larger pore diameter resulting in lower surface areas and larger pore volumes. The two samples were pressed at 30 MPa for 1 min before the electrical conductivity measurement. The conductivity of the MCNFs was 71.75 S/m, which is much higher than that of OMCs (1.24 S/m). Thus, the MCNFs exhibit more graphite character than the OMCs,12 which will result in better electrochemical properties when using them as catalyst supports. 3.2. Physicochemical Characterization of the Catalysts. The XRD pattern of the Pt/MCNFs compared with the Pt/OMCs in Figure 4 reveals a high degree of crystallinity in both of the prepared catalysts. Both XRD patterns for the two catalysts exhibit strong diffractions at around 2θ ) 39.8°, 46.3°, 67.5°, 81.3°, and 85.9°, which can be indexed as platinum (111), (200), (220), (311) and (222) reflections, respectively, indicating that the catalysts have face-centered-cubic (fcc) structure.13 No diffraction peaks could be attributed to platinic oxides or hydroxides in the XRD. The diffraction peaks of Pt/MCNFs are broader than those of the Pt/OMCs catalyst, showing that the average size of the Pt on MCNFs is smaller than that of the Pt on OMCs. Estimated using the Scherrer equation,14,15 the average sizes of the Pt nanoparticles are 4.5 and 3.9 nm, respectively. The diffraction peak at around 2θ ) 23° of Pt/ OMCs attributed to the amorphous mesoporous carbon are broader than that of the Pt/MCNFs catalyst, indicating that the MCNFs have better crystallization than the OMCs support and thus will have better electronic conductivity than OMCs,16 which has been probed above. It is proverbial that the distribution and the particle size of the catalysts are crucial to the stable and efficient operation of hydrogen electro-oxidation.14,17 So we observed the shape and distribution of Pt nanoparticles from TEM (Figure 5). Figure 5a is the image of the Pt/MCNFs and Figure 5b is the image at higher magnification. It can be found that the mesostructure of the MCNFs is not maintained well. We can only find mesostructure on the left of Figure 5b, which can be also observed in
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Figure 6. Cyclic voltammograms of the catalysts in 0.5 M H2SO4.
papers.12,18
many previous It may be related to the ultrasonication and synthesis method during the Pt loading. The Pt particles are uniformly dispersed on the surface of the MCNFs support with a few agglomerations and have an average diameter of about 5 nm, which are consistent with data obtained by XRD. The Pt/MCNFs catalyst is expected to have a super performance. Meanwhile, under the same Pt nominal content, the number of Pt nanoparticles on OMCs is much less than that on MCNFs because of the poor wettability of the OMCs. The mechanism of improving wettability by adding the AAO template needs further study. ICP-AES was conducted to determine the Pt contents in the Pt/MCNFs, Pt/OMCs, and Pt/C(E-TEK) catalysts. The results are listed in Table 2. The measured loading of Pt on the catalyst Pt/MCNFs is 15.4%, while the loading of Pt on the OMCs is only 12.0%, which is in good agreement with the TEM images. The low Pt content in the Pt/OMCs maye due to the relatively poor wettability of the OMCs and the larger pore volume, which resulted in unsatisfactory performance in hydrogen and methanol electro-oxidation. 3.3. Electrochemical Performance of the Catalysts. To gain more practical performance of the catalysts, a three-electrode system was used to test the cyclic voltammograms in 0.5 M H2SO4 under ambient conditions. Figure 6 shows the curves of the Pt/MCNFs (solid curve), Pt/OMCs (dashed curve), and Pt/ C(E-TEK) (dashed dot dot curve) catalysts. While the MCNFs are used as catalyst support, both the reduction and oxidation peaks of the hydrogen are more resolved than those of the other two catalysts tested and thus MCNFs have a higher activity than them. Generally, the electrochemically active surface of Pt particles is used to reflect the intrinsic electrocatalytic activity.19 Calculated by the formula,20 S ) Q/LPt QHref, where Q is the total charge, LPt is the actual Pt loading on glassy carbon substrate, and QHref is assumed to be 0.21 mC/cm2 corresponding to a surface density of 1.3 × 105 atom/cm2 of Pt, the parameters of the electrochemical performance are listed in Table 3. Obviously, the MCNFs show an excellent performance; the current density of the hydrogen oxidation peak is reached at 10.9 mA/ cm2, and the electrochemical active surface of Pt particles for the Pt/MCNF reaches a peak value of 235.2 m2/g, which is much higher than those of Pt/OMCs (54.7 m2/g) and Pt/C(E-TEK) (112.5 m2/g). The higher activity of Pt/MCNFs catalysts is due to three factors. First, the support MCNFs have a better electrical conductivity than the OMCs, which has been discussed above. Second, the number of Pt particles supported on MCNFs is larger than on OMCs (Figure 5b,c), which was also confirmed
Zhao et al.
Figure 7. Cyclic voltammograms of the catalysts in 2.0 M CH3OH and 1.0 M H2SO4 electrolyte.
TABLE 4: Electrochemical Parameters of the Catalysts in 2.0 M CH3OH + 1.0 M H2SO4a sample
Ua (V)
ia (mA/cm2)
ia/ib
Pt/MCNFs Pt/OMCs Pt/C(E-TEK)
0.78 0.75 0.74
69.6 17.2 40.3
1.23 1.21 1.16
a U , potential of the methanol oxidation peak; i , current density of a a the methanol oxidation peak; ib, the reverse anodic peak current density.
by ICP-AES. The large number of Pt particles and high metal dispersion are important design factors, because they control the structure sensitivity of the catalysts.21 In addition, the 1D carbon nanofibers favor the mass transport and the electron transport in the reaction; thus, the high active performance for hydrogen electro-oxidation was achieved. The cyclic voltammogram in the presence of 2 M methanol and 1 M H2SO4 mixture solution under the ambient temperature is shown in Figure 7. We can observe that the Pt/MCNFs have higher activity for oxidation of methanol when compared with the other two catalysts. The calculated parameters of the electrochemical performance according to the Figure 7 are listed in Table 4. The current density of the methanol oxidation for Pt/MCNFs reaches a peak value of 69.6 mA/cm2 (ia), which is much higher than that of Pt/OMCs (17.2 mA/cm2) and Pt/C(ETEK) (40.3 mA/cm2). And the ratio of ia to ib of the Pt/MCNFs, which can be used to describe the catalyst tolerance to carbonaceous species accumulation,14 also reaches 1.23, while the ratio of Pt/OMCs reaches 1.21, and the ratio of Pt/C(ETEK) reaches 1.16. A higher ia/ib indicates better oxidation of methanol to carbon dioxide during the anodic scan and less accumulation of carbonaceous residues on the catalyst surface. Besides, the onset potential of the Pt/MCNFs is relatively more negative than that of the Pt/OMCs. This may be attributed to the proper granularity and high dispersion of Pt nanoparticles loading on the MCNFs.22 Above all, the Pt/MCNFs catalyst could have great potential application in direct methanol fuel cells. 4. Conclusions The work presented a novel MCNFs with 1D structure prepared through the hard template (AAO) together with the soft template (F127). The AAO template was immerged into the nanocomposite sol which was mixed well and further selfassembled by the surfactant with the low molecular weight resol.
Highly Dispersed Pt Nanoparticles on MCNFs Then the carbon nanofibers with ordered mesoporous structure running along the length were gained by subsequent carbonization under nitrogen flow. By using the AAO membrane to control the ratio of the length and the diameter, products were obtained with a uniform diameter and length of about 75 nm and a few micrometers, respectively. From the TEM, we can observe a highly ordered parallel channel structure and regular round pore structure. The carbon nanofibers with 3D mesoporous network structure are propitious to the transfer of the reactants and the resultants. The MCNFs supported Pt electrocatalyst (Pt/MCNFs) has much higher performance than ordered mesoporous carbon supported Pt electrocatalyst (Pt/OMCs) and Pt/C(E-TEK) in sulfuric acid solution, which may be attributed to the higher dispersion of the Pt nanoparticles, better electronic conductivity of the support, and better structure in favor of the mass transport and the electron transport. The current density and the ia/ib of the Pt/MCNFs in methanol solution are also higher than those of the other two catalysts. Besides, the onset potential of the Pt/MCNFs is more negative than that of the Pt/OMCs. Therefore, the MCNFs have potential application as catalysts for fuel cell technology. Acknowledgment. These investigations are supported by the Jiangsu Provincial Hi-Tech Research (No. BG2005009) References and Notes (1) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (2) Ryoo, R.; Joo, S. H.; Kim, J. M. J. Phys. Chem. B 1999, 103, 7435. (3) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z. J. Am. Chem. Soc. 2000, 122, 10712. (4) Yang, Z. L.; Liu, Z. W.; Cao, X. Y.; Yang, Z. Z.; Lu, Y. F.; Hu, Z. B.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 4201.
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