Formation of Carbon Nanofibers from Supported Pt Catalysts through

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Formation of Carbon Nanofibers from Supported Pt Catalysts through Catalytic Chemical Vapor Deposition from Acetylene Qilong Chen, Jia Wang, and Feng Li* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P.O. Box 98, Beijing, 100029, P.R. China ABSTRACT: Carbon nanofibers (CNFs) were synthesized by catalytic chemical vapor deposition from acetylene from a series of supported Pt catalysts derived from Pt-containing MgAl layered double hydroxide precursors. The materials were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, temperature programmed reduction, low-temperature N2 adsorptiondesorption experiments, X-ray photoelectron spectroscopy, and Raman spectroscopy. The effects of reaction temperature and Pt content on the morphologies and microstructures of CNFs were investigated. The results revealed that the reaction temperature of 600 °C was appropriate for the growth of uniform CNFs with regular shape. Furthermore, the structural defects and the diameters of CNFs were reduced with the increasing Pt content, which is attributable to the high dispersion of smaller Pt nanoparticles as well as the quick deposition rate of carbon atoms on active metal particles. The present work developed an additional approach to optimize the growth of CNFs.

1. INTRODUCTION Since carbon nanofibers (CNFs) have unique physicochemical properties including high strength, good electrical conductivity, and excellent resistance to strong acids and bases,13 they can be used as matrix composites,4,5 catalyst supports,6 and other functional materials.7,8 At present, catalytic pyrolysis of carboncontaining gases via catalytic chemical vapor deposition (CCVD) has been widely investigated in the production of various CNFs,9,10 based on the advantages of a good control of the experimental parameters with low-cost equipments and raw materials, which makes it attractive for bulk synthesis at the industrial scale.1115 In the CCVD, carbon atoms from hydrocarbon decomposition are deposited on the surfaces of small transition metal nanoparticles including Fe,1517 Ni,18 and Cu,19,20 which are prepared by the most widely used impregnation of porous supports such as alumina and silica with aqueous or aqueous alcohol solutions of metal salts followed by reduction.2123 Finally, metal nanoparticles may be located at the tips or in the bodies of the carbon nanostructures, leading to the formation of nanocomposites for some advanced applications.24 The particular shapes of CNFs differ with the introduction of different catalysts, which definitely influence their physicochemical properties and thus remarkably determine the applicability to various functions. To date, there are few reports on the catalytic performance of noble metal catalysts (Pd,25 Pt,26 Au,27 etc.) for the growth of carbon nanostructures. The main reason is that they cannot form metal carbides with carbon atoms at high temperature. Moreover, during the growth of carbon nanostructures, noble metal nanoparticles easily aggregate, which is due to sintering effects, thus giving rise to the formation of irregular carbon nanostructures with a large amount of defects and low yields of carbon. As a class of highly ordered two-dimension layered materials,28 layered double hydroxides (LDHs, [M1x2+Mx3+(OH)2]x+(An)x/n 3 mH2O) are potential precursors for metal catalysts in virtue of their structural versatility achieved by varying divalent r 2011 American Chemical Society

or trivalent metal cations on the layers and the anions intercalated into the interlayer space.2931 Thus, highly dispersed active metal nanoparticles on metal oxide matrix can be obtained by reducing calcined LDHs containing desired metals either in the form of metal cations within the layers or in the form of metal complexes in the interlayers.3234 Recently, we found that carbon nanostructures including CNFs could be synthesized by CCVD from acetylene from cobalt and nickel nanoparticle catalysts derived from LDH precursors.24,3537 In the present work, we demonstrated for the first time that platinum was an effective catalyst for the growth of CNFs via CCVD from acetylene. Supported Pt catalysts were prepared from Pt-containing MgAl LDH precursors. Furthermore, the effects of growth temperature and Pt content in LDH precursors on the morphologies and microstructures of CNFs were investigated. The results revealed that supported Pt catalysts had excellent catalytic performance for the growth of uniform CNFs with a narrow range of diameters. To the best of our knowledge, the study of the growth of CNFs from Pt catalysts has not been reported before.

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. Mg(NO3)2 3 6H2O, Al(NO3)3 3 9H2O and H2PtCl6 3 6H2O with the Mg2+/Al3+ molar ratio of 3.0 and the PtCl62-/Al3+ molar ratios of 0.15, 0.3, and 0.5 were dissolved in 80 mL of deionized water with the total cation concentration of 0.05 M. Subsequently, the 100 mL of alkali solution of NaOH (0.08 M) and Na2CO3 (0.25 M) was added dropwise into salt solution under vigorous stirring at room temperature. The pH value of solution was adjusted to 9.5 by further titration of alkali solution. The suspension was aged at Received: February 19, 2011 Accepted: June 23, 2011 Revised: June 22, 2011 Published: June 23, 2011 9034

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Figure 1. XRD patterns of LDHs (A) and calcined LDHs (B): (a) Pt0.15-LDH, (b) Pt0.3-LDH, (c) Pt0.5-LDH, (d) Pt0.15-LDO, (e) Pt0.3-LDO, and (f) Pt0.5-LDO.

60 °C for 12 h, and then filtered and washed with distilled water until the pH of the filtrates was nearly 7.0. The solid was dried at 70 °C for 10 h in a vacuum oven. The obtained product is denoted as Ptx-LDH, where x means the PtCl62-/Al3+ molar ratio in the synthesis mixture. At last, the Ptx-LDH sample was calcined in air in a muffle furnace at 600 °C for 2 h, which was denoted as Ptx-LDO. CNFs were synthesized in a quartz tube inside a horizontal furnace equipped by CCVD from acetylene. About 50 mg Ptx-LDO was loaded in a ceramic boat and placed in the middle of a horizontal furnace, which was heated to a certain temperature (500, 600, or 700 °C) at a rate of 5 °C/min under a flowing N2 (as protecting gas, flow rate: 60 standard-state cm3/min, sccm). Subsequently, C2H2 was switched into the furnace with a flow rate of 6 sccm, and the furnace temperature was maintained at the previously installed level for 90 min. After the reaction C2H2 was switched off, and N2 was continued until the furnace was cooled to room temperature. The resultant black powder (denoted as Ptx-CNF) was collected from the ceramic boat. The yield of carbon is defined as yield % ¼

M1 M2 100 M2

where M1 is the mass of Ptx-CNF and M2 is the initial mass of PtxLDO. 2.2. Characterization. Powder X-ray diffraction (XRD) data were collected from a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu KR source (λ = 0.15418 nm). The samples were step-scanned in steps of 0.04° (2θ) using a count time of 10 s/step. Elemental analysis for metal ions in samples was performed using a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-ES). Scanning electron microscopy (SEM) observations were investigated using a Hitachi S-4700 apparatus with an applied voltage of 20 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were carried out on a JEOL JEM-2100 electron microscope operated at 120 and 200 kV, respectively. Temperature programmed reduction (TPR) of the samples was characterized by using a Micromeritics ChemiSorb 2720. The sample (100 mg) was placed in a quartz U-tube reactor. Before reduction, the precursor was degassed under flowing argon at 200 °C for 2 h. Then the sample was reduced in a stream of 10% v/v

Table 1. Analytical and Structural Data for the Ptx-LDH and Ptx-CNF Samples sample

a

Pt0.15-LDH

Pt0.3-LDH

Pt0.5-LDH

d003 (nm)

0.756

0.772

0.780

crystallite size in a direction (nm)

41.5

36.5

33.8

crystallite size in c direction (nm) Pt/Al molar ratioa

33.6 0.136

27.5 0.271

19.8 0.448

Pt (wt %)a

2.21

4.41

7.28

yield of carbon (%)b

180

230

235

mean diameter of fibers (nm)

65

55

35

ID/IG for CNFsb

2.12

1.96

1.47

Determined by ICP-ES. b At the reaction temperature of 600 °C.

H2/Ar (40 mL/min total flow) with a heating rate of 10 °C/min up to 800 °C. The hydrogen consumption was monitored continuously by a thermal conductivity detector (TCD). Low-temperature N2 adsorptiondesorption isotherms of the samples were obtained on a Micromeritics ASAP 2020 sorptometer apparatus. All samples were outgassed prior to analysis at 200 °C for 12 h under 104 Pa vacuum. The total specific surface areas were evaluated from the multipoint BrunauerEmmett Teller (BET) method, the mesopore size distribution and average pore diameter were determined by the BarrettJoynerHalenda (BJH) method applied to desorption isotherms. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo VG ESCALAB2201-XL X-ray photoelectron spectrometer at a base pressure of 2 109 Pa using Al KR X-ray (1486.6 eV) as the excitation source. The binding energy calibration of all spectra was referenced to the C1s signal at 284.6 eV. Raman spectra of the samples were recorded at room temperature on a microscopic confocal Raman spectrometer (JobinYvon Horiba HR800) using an Ar+ laser of 514 nm wavelength as excitation source. The laser with 10 mW output power was focused on the surface of the samples.

3. RESULTS AND DISCUSSION 3.1. Characterization of Materials. To investigate the effect of Pt content on the morphology and microstructure of CNFs, different LDH precursors were prepared. Figure 1A shows the 9035

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Figure 2. SEM micrographs of LDHs and calcined LDHs: (a) Pt0.15-LDH, (b) Pt0.3-LDH, (c) Pt0.5-LDH, (d) Pt0.15-LDO, (e) Pt0.3-LDO, and (f) Pt0.5-LDO.

XRD patterns of Ptx-LDH samples obtained, while Table 1 summarizes the structural and analytical data of samples. Obviously, the XRD patterns are typical of hydrotalcite-like layered double hydroxide (JCPDS no. 38-0487) materials in three cases.37 All samples exhibit the characteristic reflections appearing as symmetric lines at low 2θ angle, which are indexed to reflections by (003), (006), and (009) planes corresponding to the basal spacing and its higher order diffractions, indicative of a crystallized single phase. Meanwhile, it was noted that with the increasing Pt content, LDH samples showed a slight degree displacement of (003) reflection line (inset in Figure 1A), denoting an increase in the basal spacing (d003) from 0.756 to 0.780 nm. It is well-known that intercalation of the flat bivalent carbonate ions between the platelets of LDHs always predominates over that of other ions.29 Under the present synthesis conditions, initial H2PtCl6 could decompose into PtCl4 and HCl in the solution. With the increasing amount of H2PtCl6, some substitution of CO32 anions by Cl anions in the interlayer could take place due to the reaction of interlayer CO32 anions with HCl, thus leading to the significantly lower intensity of the XRD pattern of Pt0.5LDH sample and the shift of the maximum of the very much broadened (003) diffraction. In addition, with the increasing amount of H2PtCl6, the greatly reduced intensities of (00l) reflections of LDH phase should be attributed to the decreased structural integrality of LDHs caused by the enhanced adsorption of the negatively charged platinum hexachloride ions (PtCl62-) on the edges of the LDH platelets through interfacial

electrostatic interaction and the enhanced deposition of PtCl4 on the LDH platelets. On the other side, the average crystallite size in c direction (the stacking direction, perpendicular to the layers), which may be estimated from the values of the full-width at half-maximum (fwhm) of the (003) reflection by means of the Scherrer equation, decreases from 33.6 to 19.8 nm (Table 1). With the increasing Pt content, the average crystallite size in the a direction estimated from the fwhm of the (003) peak also shows a decreasing trend from 41.5 to 33.8 nm (Table 1). Such decrease in the crystallite size of LDH samples suggests that the introduction of Pt inhibits the growth of LDH nanocrystallites to a certain extent, probably owing to the strong adsorption of PtCl62 ions on the edges of LDH platelets. Figure 2 panels ac illustrate SEM micrographs of PtxLDH samples. It can be seen that in each case the sample is composed of densely packed plate-like particles with visible edges and a thickness of 2030 nm. Especially, the hexagonal plate-like nature of the crystallites is clearly apparent in the Pt0.15-LDH sample, indicative of uniformity of the product with a well-crystalline structure. With increasing Pt loading, the smaller roundish plate-like crystallites with lower aggregation are observed in the Pt0.5 -LDH sample due to the adsorption of more platinum hexachloride ions and the deposition of more PtCl4 on the LDH platelets. Further increasing Pt content leads to the decrease in the lateral dimension from about 5030 nm. Elemental analysis by ICPES reveals the Pt/Al molar ratios in the LDH samples are 9036



Figure 3. XPS spectra of samples.

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Figure 5. N2 adsorptiondesorption isotherms of calcined LDHs: (a) Pt0.15-LDO, (b) Pt0.3-LDO, (c) Pt0.5-LDO. Inset shows the pore size distributions.

Table 2. Textural Properties of Ptx-LDO Samples

Figure 4. TPR curves of calcined samples: (a) Pt0.15-LDO, (b) Pt0.3LDO, (c) Pt0.5-LDO.

slightly smaller than those in the initial synthesis mixtures (Table 1). Figure 1B shows the XRD patterns of calcined Ptx-LDH samples at 600 °C. Obviously, calcination has destroyed the layered structure since no characteristic reflections of LDH phase are present. Two broad reflections are observed in each case, which are almost at the same positions as (200) and (220) reflections of the crystalline MgO phase (periclase). No characteristic reflections of Pt-containing oxide phases are detected from the XRD patterns. Interestingly, it is found that the morphology of the primary LDH crystals is retained even after their thermal decomposition at 600 °C (Figure 2df). Since LDHs can decompose to magnesium oxide crystallites that are oriented in parallel, the ordering of the resulting magnesium oxide crystallites within the initial platelet is maintained.38,39 However, the particle size of the initial platelets slightly decreases due to a release of volatile components (H2O and CO2) on heating at elevated temperature. As a result, collapse of the lamellar structure of LDH samples results in the formation of MgO-like metal oxides with smaller particles, and the resulting

BET surface

pore volume

average pore

sample

area (m2/g)

(cm3/g)

diameter (nm)

Pt0.15-LDO

224

0.56

7.9

Pt0.3-LDO

209

0.44

6.1

Pt0.5-LDO

196

0.40

6.0

Pt-containing species can highly disperse in the metal oxide phases. The surface/near-surface chemical states of representative samples were analyzed by XPS measurements (Figure 3). It can be noted that the Pt 4f region of the XPS spectrum for Pt0.3LDH can be deconvoluted into two pairs of doublets. For each doublet, the binding energy (BE) of Pt 4f5/2 is about 3.23.3 eV higher than that of Pt 4f7/2.40 Two smaller peaks at about 75.0 and 78.3 eV are assigned to the Pt 4f7/2 and Pt 4f5/2 doublet of Pt4+ species in the form of PtCl4, while another two larger peaks at higher BE values of about 76.7 and 80.8 eV should be assigned to the Pt 4f7/2 and Pt 4f5/2 doublet of Pt4+ species in the form of adsorbed PtCl62 onto the LDH surfaces due to the strong interaction between the adsorbed PtCl62 and the LDH matrix. In the Pt 4f7/2 region for Pt0.3-LDO sample (Figure 3) there are two intense peaks at around 74.5 and 76.4 eV corresponding to Pt2+ and Pt4+ species in PtO and PtO2, respectively. The BE values for Pt species in PtO and PtO2 reported in the literature (around 72.6 and 74.5 eV for Pt2+ and Pt4+, respectively),4143 however, are much lower than those of Pt0.3-LDO. This may be explained reasonably by the charge transfer from Pt to the surface of support, due to the strong electronic interaction between welldispersed PtOx and MgO-like metal oxides support. TPR results of Ptx-LDO samples (Figure 4) demonstrate clearly that in each case the reduction regions cover a wide temperature range from 150 to 750 °C. Since the shapes of the reduction process are not symmetrical and possess two shoulders, the reduction actually may be divided into three steps. The peak centered in the range of 279308 °C is assigned to the reduction of Pt4+ species to Pt2+ species, whereas the peak centered at around 394 500 °C is attributed to the reduction of Pt2+ to metallic Pt. Additionally, the reduction peak above 500 °C is probably due to the reduction of Pt2+ species dissolved 9037



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Figure 6. (a) XRD patterns of Pt0.3-CNF obtained at different growth temperature and (bd) SEM micrographs of Pt0.3-CNF obtained at different growth temperature: (b) 500, (c) 600, and (d) 700 °C.

Figure 7. (a) XRD patterns of carbon products obtained at a growth temperature of 600 °C and (bd) corresponding SEM micrographs of Pt0.15-CNF (b), P0.3-CNF (c), and Pt0.5-CNF (d).

in MgO-like metal oxides, interacting in a different way with the support. It is worthwhile to note that with the increase in Pt content, the reduction peaks of Pt species shift toward the lower temperatures and the relative amounts of the H2 consumption of the former two reduction steps in the total amount of H2 consumption of the reduction process increases gradually. In our study, it is further explained reasonably that the shift toward lower temperatures for Pt0.5-LDO may be ascribed to the higher dispersion of active Pt-containing species on the surface of the MgO-like metal oxide, although a higher Pt loading can lead to the evolution of more water upon reduction. Since the higher dispersion of active Pt-containing species led to the exposure of

more defect sites, the dissociation of dihydrogen in the reduction proceeded more easily at the surface of the active Pt-containing oxides, and consequently the hydrogen consumption slightly shifted to lower temperatures. Figure 5 shows the N2 adsorptiondesorption isotherms and BJH pore size distribution curves of Ptx-LDO samples with different Pt content. All samples present type IV isotherms with pronounced H2 type hysteresis loops, which are characteristic of mesoporous materials.44 The N2 adsorption jump in the range P/ P0 = 0.550.9 is due to the capillary condensation in the mesopores.45 The slight monotonic shift of the closure points to higher relative pressure in the isotherms of samples Pt0.5-LDO 9038



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Figure 8. TEM micrographs and the diameter distribution of CNFs obtained at growth temperature of 600 °C: (a,d) Pt0.15-CNF, (b,e) P0.3-CNF, and (c,f) Pt0.5-CNF.

and Pt0.3-LDO to Pt0.15-LDO is indicative of an increase in the pore diameter of the samples. From the BJH pore size distributions (inset in Figure 5), it can be observed that the corresponding Pt0.5-LDO and Pt0.3-LDO exhibit only a narrow pore size distribution in the range below 12 nm. This might be associated with the existence of a large quantity of small nanoparticles as well as the aggregation of particles. The pore diameter distribution of Pt0.15-LDO sample covers a wide range (around 220 nm),

with a maximum at 12.8 nm, as a consequence of the retained plate-like structure. As shown in Table 2, the BET specific surface area decreases slightly from 224 to 196 m2/g with increasing Pt content in samples, which is consistent with the increase in the agglomerated particles as evidenced by SEM micrographs in the previous discussion. To sum up, combined with SEM, TPR, XPS, and N2 adsorption desorption experiment results, it is concluded that with the increasing 9039



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Figure 10. Raman spectra of CNFs obtained at growth temperature of 600 °C: (a) Pt0.15-CNF, (b) P0.3-CNF and (c) Pt0.5-CNF.

Figure 9. HRTEM micrographs of CNFs from Pt0.3-LDO: (a) micrograph showing microstructure of a CNF, (b) micrograph showing an individual catalyst particle at a tip of fiber, (c) micrograph showing an elliptical catalyst particle encapsulated by several graphitic flakes, and (d) micrograph showing an interlayer spacing of 0.263 nm for the (111) plane distance of metal Pt.

Pt content, Pt species are more highly dispersed on the mixed metal oxides, which are composed of smaller nanoparticles, in spite of the aggregation of particles. 3.2. The Effect of Reaction Temperature on the Growth of CNFs. To investigate the effect of reaction temperature in CCVD process on the morphology and microstructure of carbon nanostructures, different reaction temperatures (500, 600, and 700 °C) were applied to Pt0.3-LDO sample. In the XRD patterns of carbon materials obtained (Figure 6a), the (002) reflection of graphite carbon centered at about 25.4° confirms the existence of carbon species. Compared to those at the reaction temperatures of 600 and 700 °C, the intensity of the (002) reflection of graphite carbon at 500 °C is slightly reduced, indicative of a lower yield of carbon. Meanwhile, the characteristic (111) reflection of metallic Pt phase (JCPDS no. 4-802) appears at about 39.6° in all products. XPS of Pt 4f for the Pt0.3-CNF sample (Figure 3) also reveals there are two intense peaks at around 71.8 and 74.1 eV corresponding to metallic Pt, indicating that the Pt species exist in the form of Pt0 in Pt0.3-CNF. The aforementioned results demonstrate that PtO2 and PtO in mixed metal oxides were reduced into metallic Pt. Figures 6 panels bd show SEM micrographs of carbon products obtained at different reaction temperatures. Observation of SEM reveals that very few fibrous carbons are distributed on the solid particles at the reaction temperature of 500 °C (Figure 6b), which agrees with the XRD result. In the present reaction system, C2H2 is decomposed to hydrogen and carbon atoms. The generated hydrogen atoms can reduce Pt-containing oxide species in the Ptx-LDO into metallic Pt, while carbon atoms provide the carbon source for the growth of carbon products. The low catalytic activity for the growth of CNFs may be attributable to the slow rate of C2H2 decomposition, which is due to the relatively low reaction temperature. At the reaction temperature of 600 °C, a larger amount of CNFs with the uniform diameters of about 5070 nm are observed (Figure 6c). However, with the increasing reaction temperature to 700 °C, the yield of

carbon decreases and the resulting CNFs present a wide diameter distribution from 50 to 200 nm (Figure 6d), probably resulting from the sintering of Pt particles at the relatively higher temperature. In this case, coalescence of Pt particles makes it difficult for the carbon to deposit on individual catalyst particles toward fiber growth. As a result, the growth temperature in CCVD process plays an important role on the growth rate of CNFs and the sintering of catalysts, and the appropriate growth temperature of 600 °C facilitates the formation of uniform CNFs. 3.3. The Effect of Pt Content on the Growth of CNFs. The XRD patterns of carbon products synthesized with different Pt content in precursors at 600 °C show that the (002) reflection of graphite carbon appears at about 25.4° in all cases (Figure 7a). With the increasing Pt content, the (111) reflection of metallic Pt is enhanced obviously, due to the formation of a more metallic Pt phase. Figure 7 panels bd show the typical SEM micrographs of CNFs formed with different Pt contents. In the case of low Pt content, a large quantity of contorted nanofibers with rough surface are formed (Figure 7b), while a few solid nanoparticles are found to mix together with CNFs. The SEM study shows that further increasing Pt content does not lead to the distinct change in the morphology of CNFs (Figure 7c). With the increasing Pt content, more uniform CNFs with smooth external surfaces are formed (Figure 7d). Meanwhile, the yield of carbon from the catalysts increases from 180 to 235% (Table 1). Such a slight increase in the yield of carbon for Pt0.5-CNF should be attributable to the aggregation of smaller catalyst precursor particles as shown in the SEM micrograph (Figure 2f). TEM micrographs of carbon materials grown from catalysts confirm the formation of CNFs. It is observed from Figure 8ac that the one-dimensional carbon nanostructures present the fibrous character of CNFs in all cases and some hollow cores can be sparsely observed in Pt0.3-CNF sample. Such a fibrous microstructure was also reported in the literature.9,13,20 In addition, Pt nanoparticles are found to locate at the tips of fibers or in the bodies in the Pt0.3-CNF and Pt0.5-CNF, and the diameters of Pt particles are similar to those of CNFs and decrease with the Pt loadings. With the addition of Pt ions, more uniform and regular CNFs with more smooth external surfaces are formed, suggestive of the higher degree of graphitization. The diameter distribution curves of CNFs further reveal that with the increasing Pt content, the mean diameter of fibers obtained decreases gradually from about 65 to 55 and 35 nm (Figure 8df), suggesting 9040


Industrial & Engineering Chemistry Research the decreases in the size of Pt particles formed. In the representative HRTEM micrograph of Pt0.3-CNF sample (Figure 9a), some short-range ordered graphitic layers with a d-spacing of about 0.35 nm for the (002) reflection of graphitic structures are oriented in parallel with the fiber axis. Additionally, an individual catalyst grain is present at the tip of a fiber (Figure 9b,c), suggesting a tipgrowth mode for CNFs. An enlarged HRTEM micrograph indicates clearly an interplanar distance of about 0.263 nm for catalyst grain, corresponding to the (111) plane of metallic Pt phase with face-centered structure (Figure 9d). The structural nature of as-synthesized CNFs also was achieved by Raman spectra (Figure 10). The two distinct peaks are observed at 1343 and 1578 cm1, which can be assigned to the ‘‘D’’ band and the ‘‘G’’ band of carbon materials, respectively. The ‘‘G’’ band is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, indicative of the formation of a crystalline graphite phase.46 The ‘‘D’’ band is associated with vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite or the presence of amorphous carbon.47 Commonly, the intensity ratio of the D band to the G band (ID/IG) gives the surface defect and the degree of lattice distortion of a graphite layer within carbon material. The smaller this value, the lower the surface defects and the higher the crystalline order of the graphite phase. In our case, the ID/IG ratio of CNFs is 2.12, 1.96, and 1.47 corresponding to Pt0.15-CNF, Pt0.3-CNF, and Pt0.5-CNF, respectively. Such large values of the ID/IG ratio indicate that a large amount of disordered carbon or other structural imperfections exist within the resultant carbon products. Interestingly, the variation of the ID/IG ratio for the three samples presents a decreasing tendency, indicative of an improvement in the degree of graphitization with Pt content in catalysts. This result is in good agreement with the above SEM and TEM results. Consequently, increasing the Pt content has a positive impact on the graphitization of the carbon nanostructures or on the ordered arrangement of graphitic flakes. The influence of Pt content on the morphology and microstructure of CNFs may be explained as follows: (1) It is wellknown that the size of carbon fibers is dependent on the catalyst particle size.4851 In the present system, it has been proved that with the increasing Pt content, the mean particle size of Ptx-LDO catalyst precursor decreased gradually. Correspondingly, smaller Pt nanoparticles are more easily formed and more highly dispersed on the MgO-like metal oxide supports by an in situ reduction of smaller Ptx-LDO precursors. High dispersion of Pt nanoparticles reduces the likelihood of metalmetal interaction leading to the coalescence of particles, and thereby stabilizes the particle maintaining consistent particle size at high temperature during the growth of the CNFs. Therefore, the smaller is the particle sizes of Pt nanoparticles, the smaller is the diameter of CNFs formed. (2) The growth of CNFs also is related to the deposition rate of carbon atoms on metal surfaces from acetylene. As found in other work,48 the increase in the growth rate of nanofibers leads to a larger number of smaller fibers growing at a faster rate than the number of large fibers. In the case of higher Pt content, the availability of smaller catalyst particles makes the improved deposition rate of carbon atoms on the surface of a metal particle and the subsequent growth of uniform CNFs with smaller diameters a quicker process.

4. CONCLUSIONS CNFs were synthesized by CCVD from acetylene from a series of supported Pt catalysts derived from PtMgAl LDH

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precursors. The catalytically active Pt species were achieved by an in situ reduction of calcined LDHs in the CCVD process. It is found that a reaction temperature of 600 °C was appropriate for the growth of CNFs with regular morphology and high yield. Further, the results indicated that CNFs were synthesized uniformly from the supported catalyst with higher Pt content, owing to higher dispersion of smaller Pt particles as well as quicker deposition rate of carbon atoms on metal particles. Undoubtedly, as-synthesized supported Pt catalysts will make it possible to prepare nanocomposites of CNFs and noble metal Pt, which are promising for catalysis applications in view of its characteristics of availability of the complicated geometric shape, the high dispersion of noble metal particles with unique catalytic properties, and the high mechanical strength of CNFs as supports. The catalytic properties of such nanocomposites will be reported in our following publication.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: 8610-64451226. Fax: 8610-64425385. E-mail: lifeng@ mail.buct.edu.cn.

’ ACKNOWLEDGMENT We gratefully thank the 973 Program (2011CBA00506) and National Natural Science Foundation of China (No. 20876010) for financial support. ’ REFERENCES (1) Song, Q. S.; Aravindaraj, G. K.; Sultan, H.; Chan, S. L. I. Performance Improvement of Pasted Nickel Electrodes with Multiwall Carbon Nanotubes for Rechargeable Nickel Batteries. Electrochim. Acta 2007, 53, 1890. (2) Shiraishi, M.; Takenobu, T.; Yamada, A.; Ata, M.; Kataura, H. Hydrogen Storage in Single-Walled Carbon Nanotube Bundles and Peapods. Chem. Phys. Lett. 2002, 358, 213. (3) Endo, M.; Kim, Y. A.; Hayashi, T.; Nishimura, K.; Matusita, T.; Miyashita, K.; Dresselhaus, M. S. Vapor-Grown Carbon Fibers (VGCFs), Basic Properties, and Their Battery Applications. Carbon 2001, 39, 1287. (4) Toebes, M. L.; van der Lee, M. K.; Tang, L. M.; Huis in ’t Veld, M. H.; Bitter, J. H.; Jos van Dillen, A.; de Jong, K. P. Preparation of Carbon Nanofiber Supported Platinum and Ruthenium Catalysts: Comparison of Ion Adsorption and Homogeneous Deposition Precipitation. J. Phys. Chem. B 2004, 108, 11611. (5) Motoyama, Y.; Takasaki, M.; Yoon, S.; Mochida, I.; Nagashima, H. Rhodium Nanoparticles Supported on Carbon Nanofibers as an Arene Hydrogenation Catalyst Highly Tolerant to a Coexisting Epoxido Group. Org. Lett. 2009, 11, 5042. (6) Kvande, I.; Zhu, J.; Zhao, T.; Hammer, N.; Rønning, M.; Raaen, S.; Walmsley, J. C.; Chen, D. Importance of Oxygen-Free Edge and Defect Sites for the Immobilization of Colloidal Pt Oxide Particles with Implications for the Preparation of CNF-Supported Catalysts. J. Phys. Chem. C 2010, 114, 1752. (7) Levy, S. L.; Mannion, J. T.; Cheng, J.; Reccius, C. H.; Craighead, H. G. Entropic Unfolding of DNA Molecules in Nanofluidic Channels. Nano Lett. 2008, 11, 3839. (8) Tsuji, M.; Kubokawa, M.; Yano, R.; Miyamae, N.; Tsuji, T.; Jun, M.; Hong, S.; Lim, S.; Yoon, S.; Mochida, I. Fast Preparation of PtRu Catalysts Supported on Carbon Nanofibers by the Microwave-Polyol Method and Their Application to Fuel Cells. Langmuir 2007, 23, 387. (9) Maldonado, S.; Stevenson, K. J. Influence of Nitrogen Doping on Oxygen Reduction Electrocatalysis at Carbon Nanofiber Electrodes. J. Phys. Chem. B 2005, 109, 4707. 9041


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Formation of Carbon Nanofibers from Supported Pt Catalysts through

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