Nanocasting Route to Ordered Mesoporous Carbon with FePt

23 Mar 2009 - N2 adsorption−desorption isotherms were obtained on a Nova 2000 pore analyzer at 77 K under continuous adsorption conditions...
7 downloads 0 Views 867KB Size
Download PDF
5998

J. Phys. Chem. C 2009, 113, 5998–6002

Nanocasting Route to Ordered Mesoporous Carbon with FePt Nanoparticles and Its Phenol Adsorption Property Yufang Zhu,*,† Emanuel Kockrick,‡ Stefan Kaskel,‡ Toshiyuki Ikoma,† and Nobutaka Hanagata† ICYS-Sengen, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan, and Institut fu¨r Anorganische Chemie, Technische UniVersita¨t Dresden, Mommsenstrasse 6, Dresden, 01069, Germany ReceiVed: NoVember 30, 2008; ReVised Manuscript ReceiVed: February 20, 2009

Magnetic ordered mesoporous carbon with superparamagnetic FePt nanoparticles has been successfully prepared via a simple nanocasting route. Polyfurfuryl alcohol was formed in the channels of mesoporous silica; FePt sources (iron(III) acetylacetonate (Fe(acac)3 and platinum(II) acetylacetonate (Pt(acac)2) were located in the framework of polyfurfuryl alcohol. After the carbonization process in argon, cubic FePt nanoparticles were formed in the carbon matrix. The FePt@C composites were characterized by X-ray diffraction, transmission electron microscopy, N2 adsorption-desorption technique, and SQUID magnetometer. The superparamagnetic FePt@C sample can be obtained at the lower FePt loading level; while at higher FePt loading levels, the FePt@C sample exhibits the ferromagnetic property. By use of phenol as a model pollutant, the adsorption capacities of FePt@C-L-700 and FePt@C-H-700 can reach 139 and 114 mg/g, respectively. The magnetic separation can also be realized using an external magnetic field. Therefore, the magnetic ordered mesoporous carbon composite has potential application as a catalyst support and adsorbent. Introducion Ordered mesoporous materials have been gained much attention for their application perspectives in heterogeneous catalysis, host-guest chemistry, environmental technology, adsorption, and biomedical field because of their large specific surface area, uniform pore size distribution, tunable porosity, and well-defined surface properties.1-6 Among them, ordered mesoporous carbon is an important member.7 Mesoporous carbon materials combine chemical inertness, biocompatibility, and thermal stability and are thus suitable for many applications, such as catalyst supports, adsorbents, and electrode materials for supercapacitors and fuel cells.8-11 Generally, it is difficult to obtain mesoporous carbon materials with an ordered structure via a sol-gel process involving a surfactant templating strategy, owning to the complexity of the carbon-structure evolution. Only a few reports are available until now.12-15 However, as an alternative to cooperative surfactant templating in solution, the nanocasting pathways, which use hard templates to create ordered replicas, provide promising routes for the preparation of mesoporous carbons.16 Since Ryoo et al. first reported the synthesis of mesoporous carbon materials (CMK-1,4) via a nanocasting route using mesoporous silica MCM-48 as a hard template,7 various types of mesoporous silica and carbon precursors have been adapted to synthesize mesoporous carbons, such as CMK-1 from MCM-48,7 CMK-2 from SBA-1,17 CMK-3 and CMK-5 from SBA-15,18,19 and so on. Actually, mesoporous carbons are notoriously difficult to separate from solutions, especially for carbons with small particle sizes, which cause some problems in practical applications. * To whom correspondence should be addressed. Phone: +81-298513354 ext 6194. Fax: +81-29-8592200. E-mail: [email protected]. † National Institute for Materials Science. ‡ Technische Universita¨t Dresden.

At present, magnetic separation as a promising strategy has been paid more and more attention due to the fact that it can be easily separated under an applied magnetic field. Therefore, introduction of ferromagnetism in mesoporous carbon particles has been attempted to obtain magnetic mesoporous carbons. Although mesoporous carbon is easy to prepare by the nanocasting route, it is also difficult to load magnetic nanoparticles into the mesopores because of its open pore structure between carbon rods and the hydrophibic nature of carbon surface. Lu et al. prepared cobalt nanoparticles deposited on CMK-3 (CoOMC) and successfully applied mesoporous carbon to magnetically separable adsorbent and hydrogenation catalyst supports.20 But the synthetic procedure for Co-OMC is rather complex, which hampers broad application of the magnetically separable mesoporous carbon, despite its many important characteristics. Dong et al. reported a new cocasting method to synthesize mesostructured γ-Fe2O3/C composites with γ-Fe2O3 nanoparticles embedded in the wall of ordered mesoporous carbon materials using furfuryl alcohol and FeCl3 · 6H2O as carbon and γ-Fe2O3 sources.21 Cao et al. and Tian et al. prepared a magnetic nanocomposite of CMK-3 decorated with Ni and Fe3O4 nanoparticles on the outer surface of CMK-3, respectively.22,23 FePt nanoparticles are promising materials for magnetic data storage and for biomagnetic applications.24-26 They are known to have a chemically disordered face-centered cubic (fcc) structure or a chemically ordered face-centered tetragonal (fct) structure.27 The fcc-structured FePt nanoparticles have superparamegnatic and good acid-resistant property, which is suitable for separation application. Several publications have reported that FePt nanoparticles can be incorporated in the channels of mesoporous silica materials without aggregation and particle growth.28-30 To date, no report can be found on ordered mesoporous carbon with FePt nanoparticles. From the viewpoint of applications in adsorption and separation, the design and synthesis of magnetic mesoporous carbons with magnetic

10.1021/jp8105309 CCC: $40.75  2009 American Chemical Society Published on Web 03/23/2009

Ordered Mesoporous Carbon with FePt Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 15, 2009 5999

nanoparticles obviously provides the advantages of retaining their pore system and improving the stability of magnetic nanoparticles. In this paper, we have successfully prepared magnetic ordered mesoporous carbon with superparamagnetic FePt nanoparticles via a simple nanocasting route. This kind of magnetic mesoporous carbon has large surface area and pore volume and no blocking in the channels. By use of phenol as a model pollutant, the phenol adsorption ability of the magnetic mesoporous carbon was investigated. The superparamagnetic property of the mesoporous carbon indicated that it can be easily separated by an external magnetic field. Experimental Section Synthesis of Mesoporous Silica KIT-6. KIT-6 was prepared following a previously reported method.31 In a typical synthesis, 6 g of P123 (EO20PO70EO20, MW ) 5800, Aldrich) was dissolved in 217 g of distilled water and 11.8 g of HCl (35%). To this, 6 g of butanol (Aldrich, 99.4%) was added under stirring at 35 °C. After 1 h of stirring, 12.9 g of TEOS (Acros, 98%) was added at 35 °C (TEOS:P123:HCl: H2O:BuOH ) 1:0.017: 1.83:195:1.31 molar ratio). The mixture was left under stirring for 24 h at 35 °C and was subsequently heated for 24 h at 100 °C under static conditions in a closed polypropylene bottle. The solid product obtained after hydrothermal treatment was filtered and dried at 100 °C without washing. The template was removed by extraction in an ethanol-HCl mixture, followed by calcination at 550 °C. Synthesis of Magnetic Mesoporous Carbon with FePt Nanoparticles. The magnetic mesoporous carbon with FePt nanoparticles was prepared using KIT-6 as the hard template, furfuryl alcohol as carbon source, and iron(III) acetylacetonate (Fe(acac)3, 99.9%, Sigma-Aldrich) and platinum(II) acetylacetonate (Pt(acac)2, 98%, Acros) as FePt sources. Typically, 1.2 mL of furfuryl alcohol and a small quantity of oxalic acid were dissolved in 5 mL of ethanol. This solution was incorporated into 1 g of KIT-6 by the wetness impregnation technique. After evaporating the ethanol and polymerizing furfuryl alcohol at 100 °C for 5 h, the residual unpolymerized furfuryl alcohol was evaporated at 150 °C for 2 h. After obtaining polyfurfuryl alcohol/KIT-6 composite, the Fe(acac)3 and Pt(acac)2 were introduced into the composite via the incipient wetness impregnation technique. Equimolar amounts (0.225 mmol) of Fe(acac)3 and Pt(acac)2 were dissolved in 4 mL of chloroform (CHCl3), and then a certain amount of this solution was added to 0.5 g of polyfurfuryl alcohol/KIT-6 composite and subsequently evaporated off chloroform very slowly. The infiltration and evaporation treatment procedure was repeated three times with decreasing amount of the Fe(acac)3/Pt(acac)2/CHCl3-solution (low FePt loading: first, 0.3 mL; second, 0.2 mL; third, 0.1 mL/ high FePt loading: first, 0.6 mL; second, 0.4 mL; third, 0.2 mL). The resulting composite was thermal-treated in argon at 700 °C to carbonize the polyfurfuryl alcohol. The silica template in the composite was removed by twice washing with heated 2 M NaOH solution. The template-free mesoporous FePt/C materials were collected by filtering, washed with water and ethanol, and dried at 100 °C. The samples were named as FePt@C-L-t and FePt@C-H-t, respectively. (L and H are denoted as different amount of FePt loading, t is the carbonization temperature). Phenol Adsorption Tests. A certain amount of dried magnetic mesoporous carbons (FePt@C-L-700 and FePt@CH-700) and 30 mL of phenol solution (0.1 mg/ml) were mixed, followed by stirring at room temperature for 10 h. The magnetic mesoporous carbons with adsorbed phenol were separated by a

Figure 1. SAXRD patterns of the samples of FePt@C-L-700 and FePt@C-H-700.

magnet, and the concentration of the solution was analyzed by UV adsorption at a wavelength of 270 nm. For the recycling experiment, the sample of FePt@C-L-700 was attracted on the bottom of the bottle by using a magnet after adsorption of phenol. The pollutant solution was removed, and ethanol was added in the bottle. The release of pollutants from FePt@C-L-700 in ethanol lasted for 12 h and was followed by removing the ethanol by using a magnet to attract FePt@CL-700 on the bottom of the bottle. When the release operation was repeated for 3 times, these FePt@C-L-700 particles were added in the phenol solution under stirring for adsorption. Characterization. The wide-angle X-ray diffraction (WAXRD) patterns were obtained on a Stoe Stadi P powder diffractometer equipped with a curved germanium (111) monochromator and linear PSD using Cu KR1 radiation (1.5405 Å) in transmission geometry. The small-angle X-ray diffraction (SAXRD) patterns were measured on a Bruker AXS Nanostar using Cu KR1 radiation (1.5405Å) and 105 cm of sample to detector distance. Transmission electron microscopy (TEM) was performed using a JEOL 2100F electron microscope operated at 200 kV. The UV/vis absorbance spectra were measured using a Shimadzu UV-1650PC spectrophotometer. N2 adsorption-desorption isotherms were obtained on a Nova 2000 pore analyzer at 77 K undercontinuousadsorptionconditions.Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) analyses were used to determine the surface area, pore size, and pore volume. Magnetic measurement was carried out on a superconducting quantum interference device (SQUID) magnetometer at room temperature. Results and Discussion Figure 1 shows the SAXRD patterns of the samples of FePt@C-L-700 and FePt@C-H-700. It can be observed that both samples show two well-resolved peaks and several weak higherorder reflections. The two well-resolved peaks can be indexed to (211) and (220) reflections of the cubic Ia3d type structure, and the SAXRD pattern is almost similar to that of the mesoporous silica template, KIT-6 (Figure S1 of Supporting Information).31 This reveals that both samples possess 3D cubic structure with an enatiomeric pair of independently interpenetrating 3D continuous networks of mesoporous channels that are mutually intertwined and separated by carbon walls. It should be noted that the intensity of (211) reflection of the SAXRD pattern of FePt@C-H-700 is smaller than that of FePt@C-L700, suggesting more defects in the carbon walls owning to the incorporation of more FePt nanoparticles in the carbon framework. When the samples were carbonized at different temperature (600-900 °C), the SAXRD patterns are similar each other for the same FePt loading (Figure S2 of Supporting Information).

6000 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Zhu et al.

Figure 2. WAXRD patterns of the samples of FePt@C-L-700 and FePt@C-H-700.

This can suggest that ordered mesoporous carbon with FePt nanoparticles can be obtained by carbonization in argon between 600 and 900 °C. Figure 2 shows the WAXRD patterns of the samples of FePt@C-L-700 and FePt@C-H-700. It can be seen that both patterns can be easily indexed to cubic FePt (JCPDS 29-0717) according to the reflection peak positions and relative intensities, suggesting FePt can be formed by using carbonization of polyfurfuryl alcohol/KIT-6 containing Fe(acac)3 and Pt(acac)2. Both patterns show weak and broad cubic FePt characteristic diffraction peaks, illustrating that the nanoscale FePt particles were formed in the carbon matrix. Compared to the sample of FePt@C-L-700, the diffraction peaks of the sample of FePt@CH-700 are much stronger, though they are still broad. This is due to the growth of FePt nanoaprticles into larger size and/or the formation of more FePt nanoparticles in the carbon matrix at increased FePt loading amount in the sample. On the other hand, the diffraction intensities of the samples with the same loading amount of FePt slightly increase with the increase of the carbonization temperatures (Figure S3 of Supporting Information). The representative TEM images of the samples of FePt@CL-700 and FePt@C-H-700 are shown in Figure 3. The ordered mesoporous structure can be observed for both samples, despite the ordered degree of FePt@C-H-700 decreased compared to that of FePt@C-L-700, which is consistent with the result of the SAXRD investigation. It also can be seen that a lot of small black points are uniformly distributed in the entire particles. By combination with the WAXRD results, it can be confirmed that these small black points are FePt nanoparticles. It is interesting that the size of most of FePt nanoparticles was similar to the diameter of carbon walls for the sample of FePt@C-L700. Moreover, these FePt nanoparticles are distributed in the carbon walls. This suggests that these FePt nanoparticles are embedded in carbon walls. It can be understood that mesoporous silica template provides a confined space to the growth of FePt crystal only in the channels of mesoporous silica, and the carbon also provides a block to confine the growth of FePt during the conversion of the cross-linked polymer web into compact carbon. Therefore, such a double confinement makes FePt particles embed in carbon walls. Compared to the sample of FePt@C-L-700, more FePt nanoparticles are observed in the sample of FePt@C-H-700, and some FePt nanoparticles are larger than those of carbon walls, which suggests a part of FePt nanoparticles are located on the surface of the carbon matrix. This may result from more Fe and Pt precursors that could not be absorbed into polyfurfuryl alcohol frameworks. Therefore, at lower FePt loading levels, almost all FePt nanoparticles can be embedded in the carbon walls. While at higher FePt loading

Figure 3. TEM images of the samples of (A) FePt@C-L-700 and (B) FePt@C-H-700.

level, some FePt nanoparticles are located on the surface of the carbon matrix. The nitrogen adsorption-desorption isotherms were recorded to investigate the effect of FePt loading amount on the pore properties of the samples. Figure 4 shows the nitrogen adsorption-desorption isotherms and the corrsponding pore size distributions of the samples of FePt@C-L-700 and FePt@CH-700, and the data of the surface area and pore volume are listed in Table 1. Both isotherm curves are found to be of type IV with a marked leap in the adsorption branch between relative pressures P/P0 of 0.4 and 0.6, which is typical for mesoporous solids. This suggests that the ordered mesoporous structure has been kept after the incorporation of FePt nanoparticles. The specific surface areas of FePt@C-L-700 and FePt@C-H-700 can reach 1300 and 1038 m2/g, respectively, obtained using the BET method. The pore volumes of both samples reach 1.29 and 1.04 cm3/g, respectively. Both samples exhibit large surface areas and high pore volumes, but the sample of FePt@C-H700 has some decrease for surface area and pore volume to some extent compared to the sample of FePt@C-L-700. This may be attributed to the presence of more FePt nanoparticles because of its much higher density than carbon. From the pore size distribution curves, both samples exhibit similar narrow pore size distributions, in the range of 3-5 nm. These pore structural data also can explain the fact of FePt nanoaprticles embedded in the carbon walls. According to previous studies, the presence of a large amount of metal oxides in the pore channels must have led to the remarkable decrease of the pore size, surface area, and pore volume. However, no such phenomenon occurred in this case. When the samples were carbonized at different temperatures, all samples with the same FePt loading show the similar N2 adsorption-desorption isotherm curves and pore size

Ordered Mesoporous Carbon with FePt Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6001

Figure 4. (A) Nitrogen adsorption-desorption isotherms and (B) the corrsponding pore size distributions of the samples of FePt@C-L-700 and FePt@C-H-700.

Figure 5. (A) Magnetization curve measured at 300 K for the samples of FePt@C-L-700 and FePt@C-H-700. (B) Magnetic separation from phenol solution for FePt@C-L-700 under an external magnet.

TABLE 1: Structure Parameters of FePt@C-L-700 and FePt@C-H-700 samples

SBET (m2/g)

Vp (cm3/g)

Dp (nm)

FePt@C-L-700 FePt@C-H-700

1300 1038

1.29 1.04

3.58 3.64

distributions. However, the surface areas present a slight decrease with increasing the carbonization temperature, which may be due to the increased shrinkage with increasing the carbonization temperature (Figure S4 and Table S1 of Supporting Information). Figure 5a shows the magnetization curves of the samples of FePt@C-L-700 and FePt@C-H-700 measured at room temperature. The magnetization strengths (M) of FePt@C-L-700 and FePt@C-H-700 are 0.71 and 1.76 emu/g at 20K Oe, respectively. Almost no hysteresis loop can be observed in the magnetization curve for the sample of FePt@C-L-700, which suggests that the sample shows a superparamagnetic property. This may be due to almost all of FePt nanoparticles are distributed in the carbon walls. But for the sample of FePt@CH-700, the magnetization curve presents a very small hysteresis loop, which indicates that the sample has superparamagnetic small nanoparticles and ferromagnetic big nanoparticles. This is consistent with the result of TEM observation (small FePt nanoparticles are embedded in carbon walls and big FePt nanoparticles are located on the surface of the carbon matrix). Therefore, the superparamagnetic FePt@C sample can be obtained at the lower FePt loading level; while at higher FePt loading level, the FePt@C sample exhibits the ferromagnetic property. The magnetic separation of such magnetic mesoporous carbon was tested in phenol solution by placing a magnet beside the glass bottle. Figure 5b shows the example of FePt@C-L700. Without the magnet, the sample dispersed in phenol solution to form a black suspension. After placing a magnet beside the glass bottle, the black particles can be attracted to the wall of glass bottle. Therefore, this will provide an easy and effective way to separate the mesoporous carbon from the

Figure 6. Equilibrium adsorption curves of phenol adsorption on the samples of FePt@C-L-700 and FePt@C-H-700 at room temperature.

solutions, which facilitates the mesoporoua carbon to use in practical applications. To determine the adsorption capacities of the magnetic mesoporous carbons for phenol, the equilibrium adsorption experiments were introduced. Figure 6 shows the equilibrium adsorption curves of phenol adsorption on the samples of FePt@C-L-700 and FePt@C-H-700 at room temperature. The phenol adsorption capacities for FePt@C-L-700 and FePt@CH-700 increase with increasing equilibrium concentrations. Up to a certain equilibrium concentration, no more phenol can be adsorbed on the FePt@C-L-700 and FePt@C-H-700. Therefore, it can be estimated that the phenol adsorption capacities of FePt@C-L-700 and FePt@C-H-700 are about 139 and 114 mg/ g, respectively. This indicates that the samples of FePt@C-L700 and FePt@C-H-700 have good phenol adsorption capacity. Here FePt@C-L-700 has a higher adsorption capacity than FePt@C-H-700, which may be attributed to the larger surface area of FePt@C-L-700 compared to that of FePt@C-H-700. Figure 7 shows the recycling of the sample of FePt@C-L-700 for the adsorption of phenol in water at room temperature. The magnetic mesoporous carbon materials retained about 84% adsorption capacities for phenol after 5 consecutive operations. The result indicates that this kind of magnetic mesoporous carbon material has a good reusability.

6002 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Zhu et al. dispersive X-ray spectroscopy results for the sample of FePt@CL-700. These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. Relative recycling adsorption capacity of FePt@C-L-700 for phenol in water at room temperature after different cycles.

Conclusion Magnetically separable ordered mesoporous carbon with FePt nanoaprticles was successfully prepared via a nanocasting route. Polyfurfuryl alcohol with Fe(acac)3 and Pt(acac)2 in the channels of mesoporous silica KIT-6 was converted to mesoporous carbon containing FePt nanoparticles through carbonization under argon atmosphere. The results indicated that almost all FePt nanoparticles were confined within the carbon framework at lower FePt loading level because of the confinement of the channels of mesoporous silica KIT-6 template; while at higher FePt loading level, some FePt nanoaprticles are located on the surface of the carbon matrix. Moreover, the materials still have large surface area higher than 1000 m2/g and keep the mesopores open without any blocking effect. By use of phenol as a model pollutant, the adsorption capacities of FePt@C-L-700 and FePt@C-H-700 can reach 139 and 114 mg/g, respectively. Because FePt nanoparticles in the sample of FePt@C-L-700 exhibit superparamagnetic property, the magnetic separation can be realized using an external magnetic field. Therefore, the magnetic ordered mesoporous carbon has potential application to catalyst support and adsorbent. Acknowledgment. The authors gratefully acknowledge the support provided by the International Center for Young Scientists Interdisciplinary Materials Research (No. 205108), Key Laboratory of Inorganic and Composite New Materials of Jiangsu Province (No. Wjjqfhxc1200804), and the Alexander von Humboldt Foundation. Supporting Information Available: SAXRD pattern and N2 adsorption isotherm of KIT-6 template, SAXRD patterns, WAXRD patterns, and N2 adsorption isotherms and the corresponding structural parameters of the samples of FePt@C-L and FePt@C-H at different carbonization temperature. Energy-

(1) Corma, A. Chem. ReV. 1997, 97, 2373–2420. (2) Davis, M. E. Nature 2002, 417, 813–821. (3) Li, L.; Shi, J. L.; Yan, J. N. Chem. Commun. 2004, 17, 1990– 1991. (4) Shi, J. L.; Hua, Z. L.; Zhang, L. X. J. Mater. Chem. 2004, 14, 795–806. (5) Vallet-Regı´, M. Chem.-Eur. J. 2006, 12, 5934–5943. (6) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56–77. (7) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743– 7746. (8) Lee, J.; Yoon, S.; Hyeon, T.; Oh, S. M.; Kim, K. B. Chem. Commun. 1999, 2177–2178. (9) Wen, Z. H.; Liu, J.; Li, J. H. AdV. Mater. 2008, 20, 743–747. (10) Wang, H.; Wang, A. Q.; Wang, X. D.; Zhang, T. Chem. Commun. 2008, 22, 2565–2567. (11) Vinu, A.; Miyahara, M.; Mori, T.; Ariga, K. J. Porous Mater. 2006, 13, 379–383. (12) Liang, C. D.; Hong, K. L.; Guiochon, G. A.; Mays, J. W.; Dai, S. Angew. Chem., Int. Ed. 2004, 43, 5785–5789. (13) Tanaka, S.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Chem. Commun. 2005, 2125–2126. (14) Meng, Y.; Gu, D.; Zhang, F.; Shi, Y.; Yang, H.; Li, Z.; Yu, C.; Tu, B.; Zhao, D. Angew Chem. Int. Ed. 2005, 44, 7053–7059. (15) Zhang, F.; Meng, Y.; Gu, D.; Yan, Y.; Yu, C.; Tu, B.; Zhao, D. J. Am. Chem. Soc. 2005, 127, 13508–13509. (16) Lu, A.-H.; Schu¨th, F. AdV. Mater. 2006, 18, 1793–1805. (17) Ryoo, R.; Joo, S. H.; Jun, S.; Tsubakiyama, T.; Terasaki, O. Stud. Surf. Sci. Catal. 2001, 135, 150–156. (18) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Hsuna, T. O.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712–10713. (19) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169–172. (20) Lu, A.-H.; Schmidt, W.; Matoussevitch, N.; Bo¨nnemann, H.; Spliethoff, B.; Tesche, B.; Bill, E.; Kiefer, W.; Schu¨th, F. Angew. Chem., Int. Ed. 2004, 43, 4303–4306. (21) Dong, X.; Chen, H.; Zhao, W.; Li, Xia.; Shi, J. Chem. Mater. 2007, 19, 3484–3490. (22) Cao, Y.; Cao, J.; Zheng, M.; Liu, J.; Ji, G.; Ji, H. J. Nanosci. Nanotechnol. 2007, 7, 504–509. (23) Tian, Y.; Li, G.-D.; Gao, Q.; Yang, X.; Li, X.-H.; Chen, J.-S. Chem. Lett. 2007, 36, 422–423. (24) Service, R. F. Science 2000, 287, 1902–1903. (25) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D 2003, 36, R167-R181. (26) Gu, H. W.; Xu, K. M.; Xu, C. J.; Xu, B. Chem Commun. 2006, 941–949. (27) Sun, S. AdV. Mater. 2006, 18, 393–403. (28) Kockrick, E.; Krawiec, P.; Schnelle, W.; Geiger, D.; Schappacher, F. M.; Po¨ttgen, R.; Kaskel, S. AdV. Mater. 2007, 19, 3021–3026. (29) Jung, J.-S.; Lim, J.-H.; Malkinski, L.; Vovk, A.; Choi, K.-H.; Oh, S.-L.; Kim, Y.-R.; Jun, J. H. J. Mag. Mag. Mater. 2007, 310, 2361–2366. (30) Fuller, R. O.; Hondow, N. S.; Koutsantonis, G. A.; Saunders, M.; Stamps, R. L. J. Phys. Chem. C 2008, 112, 5271–5274. (31) Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136–2137.

JP8105309