MgO Catalysts Prepared under

Nov 1, 2006 - Hydrothermal treatments are used to reform Fe/MgO catalysts into lamella-like Fe/Mg(OH)2 catalysts, resulting in specific surface areas ...
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J. Phys. Chem. C 2007, 111, 1969-1975

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Porous and Lamella-like Fe/MgO Catalysts Prepared under Hydrothermal Conditions for High-Yield Synthesis of Double-Walled Carbon Nanotubes Guoqing Ning, Yi Liu, Fei Wei,* Qian Wen, and Guohua Luo Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China ReceiVed: July 14, 2006; In Final Form: NoVember 1, 2006

Hydrothermal treatments are used to reform Fe/MgO catalysts into lamella-like Fe/Mg(OH)2 catalysts, resulting in specific surface areas (SSAs) of more than 100 m2/g. Porous and lamella-like Fe/MgO catalyst can be obtained under the reaction condition. In the materials prepared by CH4 cracking at 850 °C over the hydrothermtreated catalysts, more than 70% of the nanotubes are found to be double-walled carbon nanotubes (DWNTs). With the catalyst after a 180-220 °C hydrothermal treatment, a high purity of DWNTs and a yield up to nearly 3 times that obtained with the original catalyst are achieved at the same reaction condition, as characterized by scanning electron microscope, thermogravimetric, and Raman analysis. The porous structure with high surface area of the hydrotherm-treated catalysts and the good dispersion of Fe species on Mg(OH)2 layers are considered as the main factors for the improvement of nanotube yield. Considering the good hydrophilicity of MgO, hydrothermal treatments are promising in being applied to other metal catalysts supported by hydrophilic oxides.

Introduction Carbon nanotubes (CNTs) have received much attention over the past decade, due to their unique physical and chemical properties1 and potential applications. Significant progress has been accomplished in the chemical vapor deposition (CVD) synthesis of single-walled carbon nanotubes2-10 (SWNTs) and double-walled carbon nanotubes (DWNTs)11-15 in recent years. MgO-supported catalysts have been demonstrated to be efficient for synthesis of SWNTs5-7 and DWNTs.12-14 As compared to other support materials, MgO is easier to be removed from the as-prepared materials and is promising in realizing the continuous large-scaled production of SWNTs and DWNTs at a lower cost. It has been well-established that metal particles with a size of 5 nm will lead to carbon nanofibers (CNFs).16,17 However, it is still not easy for such powder-supported catalysts to obtain a good size control of catalytic metal particles at a nanometer scale,18 which is crucial for the growth of SWNTs and DWNTs. Many works have been done to precisely control metal particle size and obtain a good dispersion of metal species on support materials, for example, by varying the urea/nitrate ratio in a combustion method,16 using size-controlled iron oxide particles,11 or applying calcination treatments on Fe/MgO catalysts.19 In this work, we have noticed that MgO has good hydrophilicity and can be easily reformed by hydrothermal treatment, as reported in the literature.20-22 However, as far as we know, there is still no work reported on such hydrothermal treatment to reform Fe/MgO catalysts for CNT growth. Here, we report a hydrothermal treatment method to reform Fe/MgO catalysts with cubic particle morphology into lamella-like Fe/ Mg(OH)2 catalysts. Good dispersion of iron species was found on lamella-like Mg(OH)2 substrates several hundred nanometers * To whom correspondence may be addressed. Fax: +86-1062772051. E-mail: [email protected].

in size. The materials prepared with the hydrotherm-treated catalysts were found to mainly contain DWNTs and retain a high degree of cleanliness without any other carbon impurities. The yield of DWNTs can be up to 3 times that obtained with the Fe/MgO catalyst without hydrothermal treatment. The significant promotion of nanotube yield achieved after hydrothermal treatment indicates that the current work is probably of high value in the field of CVD growth of CNTs with MgOsupported catalysts. The mechanism for such significant improvement of catalysts has been discussed. The current work also provides a universal approach to reform metal catalysts supported by hydrophilic oxide, such as MgO, Bi2O3, and so on, resulting in a high catalyst surface and a good dispersion of metal species. Experimental Section The MgO-supported Fe catalyst was prepared by a typical impregnation method.7 Analysis pure commercial MgO powder with a particle size of 20-30 nm and Fe(NO3)3‚9H2O were respectively dispersed in deionized water, and then mixed together. The Fe content was 0.03 mol/mol MgO. The resulting sol was disturbed and ultrasonicated for 10 min followed by overnight drying. The as-prepared Fe/MgO catalyst was labeled as C1. Further hydrothermal treatments were conducted by ultrasonically dispersing the original catalyst C1 in deionized water for 5 min, resulting in a viscous suspension. Two different hydrothermal treatments were performed over the suspension at 100 °C and a higher temperature (180-220 °C), respectively. The 100 °C treatment was carried out in a glass beaker heated by an electric heater. The suspension was boiled and dried in air. Then the solid material was crushed into fine powder for use (labeled C2). The 180-220 °C treatment was preformed in a sealed iron autoclave. The temperature of the furnace was accurately controlled to avoid any temperature runaway. After being cooled to room temperature, the suspension was boiled

10.1021/jp064483q CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007

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TABLE 1: Samples Prepared with Fe/MgO Catalysts Treated by Different Proceduresa

no.

treatment procedure of catalysts

reaction time (min)

M1 M2 M3 M4

No hydrothermal treatment 100 °C boiled for 20 min 180-220 °C hydrothermal treatment for 2 h Same as M3

20 20 20 60

SSA-C (m2/g)

SSA-M (m2/g)

∆A/mC (m2/g)

Raman D/G

yield (wt %)

65.5 195.3 103.3 103.3

140.7 296.7 398.8

656.5 671.4 950.0

0.11 0.08 0.06 0.07

12.7 21.3 34.9 49.1

a SSA-C: specific surface area of catalysts; SSA-M: specific surface area of the as-prepared materials; ∆A/mC: increment of sample area/mass of the carbon deposit (measured by TGA), i.e., the SSA of the carbon deposit.

Figure 1. TEM images of C1 (a), C2 (b) and C3 (c,d,e, with EDS spectrum), and the catalyst prepared by 850 °C dehydration of C3 (f).

in air, as was that of the 100 °C treatment, and crushed into fine powder (labeled C3). CNT growth was carried out in a vertically set quartz tube.23 After being heated to 850 °C in Ar, a CH4 flow of 20 mL/min in atmospheric pressure was introduced to about 30 mg of catalyst for 20 or 60 min. The as-grown CNTs were cooled to room temperature in Ar atmosphere. The materials prepared by CH4 cracking for 20 min with C1, C2, and C3 are labeled M1, M2, and M3, respectively. The sample prepared by CH4 cracking for 60 min over catalyst C3 is labeled as M4. The sample information is listed in Table 1. Transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET) specific surface area (SSA) measurement, and thermogravimetric (TG) analysis were conducted to characterize both the catalysts and the CNTs. BET analysis was carried out by online quick measurement using the one-point method, as described previously.23 Heat treatment at 300 °C for 30 min in Ar was performed to get rid of vapor or gas adsorbed on samples before BET measurements. TG analysis was carried out in an air flow of 100 mL/min with a slope of 10 °C/min. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were applied to obtain information on the substance structure and surface element composition of the catalysts. Scanning electron microscope (SEM) and Raman analysis at 514 nm were used to describe CNTs in a relatively wide scope.

Results and Discussion Characterization of Fe/Mg(OH)2 Catalysts Prepared under Hydrothermal Conditions. Catalyst morphology is found to be significantly changed by hydrothermal treatments. As shown in Figure 1a, Fe/MgO without treatment (C1) is composed of cubic particles 20-30 nm in size. However, as shown in Figure 1b,c,d, the catalysts prepared with thermal treatments (C2 and C3) are mainly in lamella-like layers with a thickness of 5-15 nm and a lateral dimension of several hundred nanometers (also see Figure 3 for SEM images of catalysts), which is of very different morphology compared to the starting material. The BET SSAs of C2 and C3 are 195.3 and 103.3 m2/g, respectively, which is much larger than that of C1, 65.5 m2/g (Table 1). Such an increase in SSA is very consistent with the TEM observation (Figure 1). In Figure 1a, there are some obvious iron particles with an appearance of dark contrasts as well as iron-free MgO particles with an appearance of light contrasts. Such a gathering of iron species in Fe/MgO catalysts has also been reported in the literature.17 However, no obvious iron particles can be found in the lamella-like plates of the hydrotherm-treated catalyst (Figure 1b,c,d). A high-resolution TEM (HRTEM) image and an energy-dispersive (EDS) spectrum of the same site are shown in Figure 1e. The iron content of the lamella-like plates measured by EDS is very consistent with the total iron content (0.03 mol/ mol MgO), suggesting that the iron species has been well dispersed in the catalyst.

High-Yield DWNT Synthesis Using Fe/MgO Catalysts

Figure 2. XRD patterns of C1 (a), C2 (b), C3 (c), and the catalyst obtained by heating C3 in Ar at 850 °C for 30 min (d).

TABLE 2: Crystallite Size Information from the XRD Patterns of Fe/MgO Catalyst C1, and Fe/Mg(OH)2 Catalyst C3 Miller indices (hkl)

full-width at half-maximum (deg)

crystallite size (nm) (by Scherrer eq)

Fe/MgO C1

200 220

0.50 0.50

21.92 23.84

Fe/Mg(OH)2 C3

001 101 102 110 111

1.97 0.96 2.68 0.76 0.96

4.29 9.83 3.39 13.98 10.85

The reason for such a significant reformation of catalyst morphologies has been investigated by XRD and TG analysis. The catalyst without hydrothermal treatment (C1) displays an XRD pattern of cubic structure MgO (Figure 2a). However, for C2 and C3 (Figure 2b,c), most peaks of MgO have disappeared, and an XRD pattern of the hexagonal structure Mg(OH)2 can be indexed,20 which indicates that most of the MgO has been converted into Mg(OH)2 by the hydrothermal treatments. The significant peak broadening indicates that Mg(OH)2 has a very small grain size. It is noted that the crystallite size, estimated by means of the Debye-Scherrer formula based on the fullwidth at half-maximum of different diffraction peaks, has different values (Table 2). For C1, the crystallite sizes of 21.92 nm (200) and 23.84 nm (220) indicate that the particles have a cubic morphology. It is noticed that the calculated value of the crystallite sizes is exactly consistent with the TEM observation shown in Figure 1. For the Fe/Mg(OH)2 catalyst C3, the crystallite sizes in different directions are quite different, which indicates that the particles have a thin-plate morphology with layers in the (001) and (102) direction. In the XRD pattern of C2, there is still a weak peak of the (200) plane of MgO, which implies that the 100 °C boiling treatment did not fully convert MgO into Mg(OH)2. It is validated by TG analysis of C2, which shows the weight loss from 200 to 600 °C as 24.5 wt %, smaller than the water content of 31 wt % in Mg(OH)2. The XRD pattern of C3 has no MgO peak but displays much weaker intensity compared with that of C1 and C2. It implies that the 180-220 °C hydrothermal treatment has fully converted MgO into Mg(OH)2 and has more efficiently destroyed the crystallinity of the catalyst, which

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Figure 3. SEM images of the as-prepared materials: M1 (a), M2 (b,c), M3 (d), and M4 (e,f).

behaves like a polycrystal in XRD. It is interesting to note that such a slight difference between catalysts C2 and C3 can be clearly pointed out in TEM observation. For the 100 °C boilingtreated catalyst (Figure 1b), some cubic particles with smooth surfaces can be observed, which might indicate that there are still some MgO particles remaining. However, for the 180220 °C treated catalyst, such cubic particles have been converted into a porous and irregular morphology (Figure 1c). In the TG curve of catalyst C2, the weight loss of 24.5 wt % from 200 to 600 °C can be ascribed to the dehydration of Mg(OH)2.20 The XRD pattern (Figure 2d) of the catalyst obtained by heating C3 in Ar at 850 °C for 30 min contains only MgO peaks, indicating that, during the heat process before nanotube growth (up to 850 °C), Mg(OH)2 will be dehydrated and converted into MgO. In Figure 1f, one can see that lamellalike morphology has been fully retained, and many micropores of 5-35 nm have formed24 after the heat treatment. Figure 3 also shows a similar retainment of morphology in the asprepared CNT materials. These results indicate that Fe/MgO with a porous and lamella-like morphology and a large SSA will be formed in the reaction condition to catalyze the growth of nanotubes. The As-Prepared CNTs. Figure 3 shows the SEM images of the as-prepared materials (see Table 1). SEM observations in a wide field show that the materials prepared with the hydrotherm-treated catalysts (M2, M3, and M4) contain much more nanotubes compared to the original Fe/MgO catalyst (M1). The nanotubes are straight and stretchy in appearance, and are mainly DWNTs, which will be demonstrated later. In the SEM observation of M1, a few CNFs were sporadically observed. However, for the materials M2, M3, and M4, no CNFs or multiwalled carbon nanotubes (MWNTs) were found in all observations, indicating a high purity of the as-prepared materials. The lamella-like morphology of the catalysts was well maintained after the high-temperature reaction. In Figure 3c, many porous catalyst layers can be observed, which is consistent with the TEM observation of the dehydrated catalyst (Figure 1f). In the nanotube-abundant areas (Figure 3b,d,f), most surfaces of the catalyst plates are covered by nanotubes, and only the side profile can be observed, which implies that the nanotubes have grown from the surface of the catalyst plates. The SSAs of the as-prepared materials (SSA-M) are listed in

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Figure 4. TG and DTG curves of the as-prepared materials: M1 (a), M2 (b), M3 (c), and M4 (d).

Table 1. M2 and M3 have larger SSAs than M1, showing the existence of abundant DWNTs. The value of ∆A/mC can be considered as the SSA of carbon species. In previous work, it has been concluded that, in our system, a high SSA of carbon species corresponds to a high SWNT or DWNT purity.23 The ∆A/mC value of M3 is 950.0 m2/g, significantly larger than that of M2 and M1, indicating a high purity of DWNTs in M3. Comparison of Raman D-to-G ratios of the as-prepared materials (Table 1) shows that M3 has the lowest D/G value, implying that it has the best quality. TG analysis (Figure 4) shows that the nanotube yields of the materials prepared by hydrotherm-treated catalysts (M2, M3, and M4) are much higher than that of the material prepared by the original Fe/MgO catalyst (M1). The nanotube yield of M3 is nearly 3 times that obtained with the original catalyst under the same reaction conditions. M3 has a larger nanotube yield than M2 at the same reaction time of 20 min, indicating that the 180-220 °C hydrothermal treatment is better for enhancing nanotube yield compared to the 100 °C hydrothermal treatment. The material M4, which is prepared by CH4 cracking for 60 min over C3, has a yield as high as 49.1 wt %. If the weight of support material is taken out, the yield relative to Fe in the Fe/ MgO catalyst is more than 2434 wt %, revealing a significantly high yield of CNTs. In the differential thermogravimetric (DTG) curve of M4 (Figure 4d), the only peak occurs at 597 °C, which is a higher temperature compared to other materials. Herrera et al.25 pointed out that the catalytic oxidation caused by metal catalysts would result in a lower burning temperature of CNTs. In their work, the peak for SWNTs in DTG curves moved from 460 to 630 °C after metal catalysts were removed from the SWNT material. As the relation between burning temperature and carbon content is considered, one can see that the higher carbon content of the material corresponds to the larger burning temperature, as shown in Figure 4. The increase in burning temperatures might be attributed to the less catalytic oxidation at a lower content of Fe catalyst. The possibility of the presence of MWNTs or CNFs, which might account for both the increase in the overall amount of carbon and the shift in temperatures in the TGA profiles,

Figure 5. TEM images of the sample M3.

has also been considered. However, in SEM observations of sample M4 in a wide field (Figure 3e,f), no MWNTs or CNFs have been observed, which can be easily distinguished from SWNTs and DWNTs. The high purity of DWNTs relative to MWNTs or CNFs can also be confirmed by Raman spectra, which show lower D-to-G ratios for the M4 sample compared to M1 (Table 1). According to our experience, when a sample contains MWNTs or CNFs as well as SWNTs and DWNTs, the DTG curve would possibly contain two or more obvious peaks in the temperature range from 500 to 700 °C, showing a quite different peak shape in comparison with that shown in Figure 4d. Hence, we believe the change of the peak position shown in Figure 4 should not be ascribed to the presence of MWNTs or CNFs. By TEM observation, we have found that most of the nanotubes in the as-prepared materials are DWNTs. Figure 5 shows the TEM images of M3. Figure 5b,c shows HRTEM images of individual DWNTs and DWNT bundles, respectively. In TEM observation, it has been found that the DWNTs in a large diameter (e.g., more than 3 nm) are not easy to combine into bundles in comparison with SWNTs and often remain as individual tubes. However, those with small diameters are often found to exist in bundles. The inset of Figure 5c shows the open tips of a bundle. This bundle is composed of DWNTs with diameters of no more than 2 nm and three SWNTs with a diameter of 0.9 nm. Statistics of about 100 nanotubes in TEM images show that DWNTs make up more than 70% of the total (Figure 6a). The outer diameter distribution of DWNTs shows that most DWNTs have diameters of 1-4 nm, with centraliza-

High-Yield DWNT Synthesis Using Fe/MgO Catalysts

Figure 6. Nanotube type (a) and outer diameter of DWNTs (b) distribution of M3.

Figure 7. Raman spectra of the as-prepared material M3.

tion on 2-3 nm (Figure 6b). The radial breathing mode (RBM) of the Raman spectrum is used to calculate diameters of small CNTs.26 Figure 7 shows a close-up view of the RBM of M3. Due to the tube-tube interaction in a bundle, we used the formula ω (cm-1) ) 246/d (nm)27 to determine both of the inner and the outer diameters of DWNTs. In Figure 7, the RBM peaks at 373.9 and 180.6, 308.9 and 164.7, 261.4 and 150.4, and 221.8 and 137.8 cm-1 can be taken as corresponding to the diameters of the inner and outer tubes of the DWNTs, that is, 0.66 and 1.36, 0.80 and 1.49, 0.94 and 1.64, and 1.11 and 1.79 nm, respectively, with an interlayer distance of about 0.34-0.35 nm. The selectivity of DWNTs can be ascribed to the control of Fe content in catalysts. In our former research, at an Fe content of about 0.01 mol/mol MgO, much more SWNTs were obtained.19 These results are consistent with the conclusion made by Ago et al.28 Mechanism of the Thermodynamic Treatment. Yu et al.20 prepared porous magnesium hydroxide nanoplates from commercial bulk magnesium oxide crystals by a hydrothermal treatment at 160 °C for 24 h. They concluded that the formation process of porous magnesium hydroxide nanoplates occurred as follows: first, bulk MgO crystals are dissolved in water and hydroxylated into Mg(OH)2 primary particles, and then these particles are aggregated to produce large plate-like particles through a mechanism of oriented attachment of nanocrystals.29 In our case, the hydrothermal treatment and the product morphology are similar to those in their work. We believe there is also a similar formation mechanism for our lamella-like catalysts. In comparison with the Mg(OH)2 nanoplates reported by Yu et al., which have a thickness of 50-110 nm, the catalyst layers in the current work are of a much smaller thickness (515 nm). We believe the bubbling process in the 100 °C boiling treatment is a key factor for reducing the thickness of catalyst layers. Many bubbles containing catalyst sol in their surfaces have been observed growing up and expanding to bursting when

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Figure 8. SEM image (a) and TG curve (b) of the material prepared by C2H4 cracking at 600 °C over the catalyst C3.

the suspension of catalyst powder is boiled. Such a surface tension of bubbles is quite strong and probably has reformed the catalysts into thin layers. Popov et al.30 reported that the hydrothermal process over Fe(OH)3 suspension at 160 °C and pH 6-8 led to the predominant formation of rhombic R-Fe2O3 particles. The pH value of our Fe/MgO suspension is measured to be ∼8. According to these results, although the XRD patterns of catalysts have little information for Fe species due to the small content, it is most possible that the iron species remain as oxides during hydrothermal treatments. In Fe/MgO catalysts, Fe species are adsorbed on MgO surfaces or combined in MgO lattices. When Mg(OH)2 layers grow in the (001) and (102) direction, Fe species, in the form of oxides, might have to migrate at the same time. XPS analysis showed that the surface Fe-to-Mg atomic ratios (with a depth of ∼3 nm) of C1 and C3 were 0.046 and 0.079, respectively. The surface Fe-to-Mg atomic ratio of C3 was much higher than the total Fe content of 0.03, which indicated that there existed a process of Fe species redispersion and enrichment on the catalyst surface due to the hydrothermal treatment. The mass transfer of iron species resulted in a good dispersion of Fe species on the support material, as shown above by TEM and EDS analysis (Figure 1d,e). In order to confirm the effect of mass transfer during hydrothermal treatment, we have tried another process for catalyst preparation. First, a hydrothermal treatment was applied on MgO powder, resulting in lamella-like Mg(OH)2 nanoplates. And then a Fe/Mg(OH)2 catalyst was obtained by impregnating the lamella-like Mg(OH)2 in an Fe(NO3)3 aqueous solution. In this catalyst preparation process, Fe species were not redispersed by hydrothermal treatment. The material prepared with such a catalyst resulted in a much lower nanotube yield compared to that prepared with M2 or M3. Such a result indicates that the mass transfer during the reformation of support material (MgO to Mg(OH)2, particles to plates morphology) may be critical to achieve a good dispersion of iron species in the final catalyst. Because Mg is much more than Fe in catalysts (100:3, mol), it is possible that Fe species might be carried into Mg hydroxide thin layers or adsorbed on the surfaces to complete the mass transfer. Although the microcosmic mechanism of iron species mass transfer is still not clear, two possible factors are considered to be responsible for the good dispersion of Fe species: (i) the lamella-like structure of the support material provides a large surface for Fe species dispersion, and (ii) the interaction between iron species and Mg(OH)2 is strengthened due to the formation of hydrates, preventing iron particles from agglomeration.17 Introducing a small amount of water has been reported to be able to carry some significant influence on the growth of CNTs.8 However, in the current work, we have proved it is not the function of water that has improved the yield of nanotubes. When catalyst C2 and C3 are heated in Ar at 850 °C for 30

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Figure 9. Suggested mechanism of hydrothermal treatment and nanotube growth.

min before introducing CH4 for nanotube growth, a similar yield of DWNTs is still obtained. When hydrotherm-treated catalysts are heated to 850 °C for nanotube growth, the formation of a web-like porous structure of support material due to the dehydration of Mg(OH)2 can help to avoid Fe species agglomeration in high-temperature conditions. In the previous work, we used C2H4 cracking to characterize Fe/MgO catalysts.19 The Fe/MgO catalyst with some Fe species particles on the surface can catalyze the growth of MWNTs, but the calcined catalyst, which is composed of Fe-Mg-O solid solution with no obvious Fe species particles on surface, cannot be activated at 600 °C by C2H4 for nanotube growth. Figure 8 is the SEM image and TG curve of the material prepared by C2H4 cracking at 600 °C over the catalyst C3. The as-prepared material is black, but no nanotubes can be found in the SEM observation. TG analysis shows that the carbon deposit is in the form of amorphous carbon, which starts to be burned below 500 °C (Figure 8b). From the above results, it can be concluded that there is no obvious Fe species particle on the surface of the support material, which is usually active to catalyze nanotube growth. SEM observation reveals that the conductivity of the as-prepared material is not good, with a large extent of distortion, as shown in Figure 8a, indicating that the carbon deposit is separately distributed in catalyst pores. Micropores formed by hydroxide decomposition can help to adsorb C2H4 and lead to the deposition of amorphous carbon by thermal cracking of C2H4. It is quite possible that, after dehydration at high-temperatures, Fe species have firmly combined with MgO support material in form of Fe-Mg-O solid solution, keeping the good dispersion of Fe species under the reaction conditions. In the case of CH4 cracking at 850 °C, the porous structure can help CH4 adsorption, and Fe-Mg-O solid solution will be activated by CH4 reduction for nanotube growth.19 In our experiments, CH4 has been proven to be quite stable at 850 °C: no thermal cracking occurred without catalysts or with deactivated catalysts. So the porous structure of the catalyst has less possibility to arouse the deposition of amorphous carbon by CH4 cracking. By the SEM images shown in Figure 3, it has been concluded that the nanotubes have grown from the surface of the catalyst plates. It implies that the large surface provided by the porous and lamella-like structures might be in favor of the formation of more active sites for the growth of CNTs. A suggested mechanism of hydrothermal treatments and nanotube growth is shown in Figure 9. One can see that, after hydrothermal treatments, most Fe species dispersed in Mg(OH)2 layers can be regarded as being on the surface due to the small thickness of Mg(OH)2 layers. Hence, Fe species can be more fully used to catalyze CNT growth, in comparison with the original catalyst. It may be the reason why the yield of nanotubes has significantly increased after the hydrothermal treatments. The hydrothermal treatment is also promising in being applied to reform other kinds of catalysts. After hydrothermal treatment of CoMo/MgO, significant changes of catalyst morphology and nanotube growth have been found in the primary investigation. The good hydrophilic property of MgO is an important factor for hydrothermal reformation. So we consider that the hydro-

thermal treatment might be effective for most catalysts supported by hydrophilic materials, such as MgO, Bi2O3,20 and so forth. More work should be done on the accurate control of metal particle size in these and other similar catalyst systems. Conclusion In summary, we have found a simple approach to prepare porous and lamella-like Fe/MgO catalysts by hydrothermal treatments to achieve a high yield of DWNT growth. Fe species have been well dispersed on the support material due to their migration at the time of Mg(OH)2 layer growth. A high purity and a yield up to 49.1 wt % have been obtained by CH4 cracking for 60 min over the catalyst with the 180-220 °C hydrothermal treatment. A high selectivity of DWNTs has been found in the as-prepared material, with diameters in the range of 1-4 nm. The porous structure with high surface area of the hydrothermtreated catalysts and the good dispersion of Fe species are considered as the main factors for the improvement of nanotube yield. The hydrothermal treatments reported in this paper are also promising in being applied to other metal catalysts supported by hydrophilic oxides. Acknowledgment. The work was supported by the FANEDD (No. 200548), NSFC Key Program (Grant No. 20236020), NSFC (No. 20606020), China National program (No. 2006CB0N0702), THSJZ, and National center for nanoscience and technology of China (Nanoctr). References and Notes (1) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, UK, 1998. (2) Hafner, J. H.; Bronikowski, M. J.; Azamian, B. R.; Nikolaev, P.; Rinzler, A. G.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1998, 296, 195. (3) Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. J. Phys. Chem. B 1999, 103, 6484. (4) Flahaut, E.; Govindaraj, A.; Peigney, A.; Laurent, C.; Rousset, A.; Rao, C. N. R. Chem. Phys. Lett. 1999, 300, 236. (5) Colomer, J. F.; Stephan, C.; Lefrant, S.; Van Tendeloo, G.; Willems, I.; Konya, Z.; Fonseca, A.; Laurent, C.; Nagy, J. B. Chem. Phys. Lett. 2000, 317, 83. (6) Tang, S.; Zhong, Z.; Xiong, Z.; Sun, L.; Liu, L.; Lin, J.; Shen, Z. X.; Tan, K. L. Chem. Phys. Lett. 2001, 350, 19. (7) Li, Q. W.; Yan, H.; Cheng, Y.; Zhang, J.; Liu, Z. F. J. Mater. Chem. 2002, 12, 1179. (8) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362. (9) Maruyama, S.; Kojima, R.; Miyauchi, Y.; Chiashi, S.; Kohno, M. Chem. Phys. Lett. 2002, 360, 229. (10) Lyu, S. C.; Liu, B. C.; Lee, S. H.; Park, C. Y.; Kang, H. K.; Yang, C. W.; Lee, C. J. J. Phys. Chem. B 2004, 108, 1613. (11) Ago, H.; Nakamura, K.; Imamura, S.; Tsuji, M. Chem. Phys. Lett. 2004, 391, 308. (12) Flahaut, E.; Bacsa, R.; Peigney, A.; Laurent, C. Chem. Commun. 2003, 1442. (13) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (14) Lyu, S. C.; Liu, B. C.; Lee, S. H.; Park, C. Y.; Kang, H. K.; Yang, C. W.; Lee, C. J. J. Phys. Chem. B 2004, 108, 2192. (15) Ramesh, P.; Okazaki, T.; Taniguchi, R.; Kimura, J.; Sugai, T.; Sato, K.; Ozeki, Y.; Shinohara, H. J. Phys. Chem. B 2005, 109, 1141. (16) Coquay, P.; Peigney, A.; Grave, E. D.; Vandenberghe, R. E.; Laurent, C. J. Phys. Chem. B 2002, 106, 13199. (17) Ago, H.; Nakamura, K.; Uehara, N.; Tsuji, M. J. Phys. Chem. B 2004, 108, 18908.

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