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Cite this: Chem. Commun., 2014, 50, 5476
One-pot synthesis of magnetic particle-embedded porous carbon composites from metal–organic frameworks and their sorption properties†
Received 14th March 2014, Accepted 4th April 2014
Hee Jung Lee, Won Cho, Eunji Lim and Moonhyun Oh*
DOI: 10.1039/c4cc01914h www.rsc.org/chemcomm
Nano- and micro-composites comprised of porous carbon and magnetic particles are prepared by one-step pyrolysis of metal– organic frameworks (MOFs). The porosity and composition of resulting magnetic porous carbons are facilely regulated by altering the pyrolysis temperature and changing the organic building blocks incorporated within the initial MOFs.
Porous coordination polymers (CPs) or metal–organic frameworks (MOFs) are of great interest due to their useful chemical and physical properties.1 Based upon their controllable chemical compositions and porosities, many practical applications, including gas storage, gas separation, optics and catalysis, have been intensively studied. In addition, the development of thin membranes2 from MOFs and nano- or micro-sized coordination polymer particles (CPPs)3 enhanced their original usefulness and also expanded their application range to bio-medicines.4 Recently, the utilization of CPPs as the starting materials for the production of diverse metal oxides has also been demonstrated.5 The simple calcination of welldefined CPPs provided a convenient way for the production of complicated and well-organized metal oxides. On the other hand, porous carbon materials have received great attention due to their many useful applications including in adsorbents, supercapacitors, gas storage and as catalyst supports.6–9 Moreover, the fusion of carbon materials with other functional materials to improve their usefulness is a central focus of interest.7,8 In particular, magnetic carbon materials are useful for lithium ion batteries, supercapacitors and separation of pollutants.8 Even though many methods have been developed to synthesize various carbon materials including electrical arc, chemical vapor deposition (CVD), nanocasting and ultrasonic spray pyrolysis,9 the development of more convenient synthetic methods for the functional carbon materials is still of great
Department of Chemistry, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea. E-mail: [email protected]; Fax: +82-2-364-7050; Tel: +82-2-2123-5637 † Electronic supplementary information (ESI) available: Experimental details, supplementary figures and tables. See DOI: 10.1039/c4cc01914h
5476 | Chem. Commun., 2014, 50, 5476--5479
interest. In recent years, the utilization of MOFs as templates or precursor materials for the production of porous carbon materials has been demonstrated by taking advantage of the fact that MOFs are typically porous materials and possess a significant amount of carbon source.10 Herein, we report a novel strategy for one-pot synthesis of nano- and micro-sized hybrid composites comprised of porous carbons and magnetic particles (g-Fe2O3, Fe3C and/or a-Fe) by a simple thermal treatment of iron-containing MOFs (Fe-MIL-88A or Fe-MIL-88B) as precursors. Furthermore, we report that both the porosity of the resulting carbon materials and the compositions (g-Fe2O3, Fe3C or a-Fe) of the resulting magnetic particles incorporated within porous carbons can be regulated by altering the pyrolysis temperature and changing the organic components present in the starting MOFs. Fe-MIL-88A micro-rods with a hexagonal shape were first prepared according to the reported solvothermal method using Fe(NO3)3 and fumaric acid (Scheme 1). Fe-MIL-88A structures ([Fe3O(O2CC2H2CO2)3(H2O)2(NO3)]n) have a three-dimensional hexagonal structure consisting of trimers of FeO6 octahedra linked to fumarate (Scheme 1 and also see Fig. S1, ESI†).11 Scanning electron microscopy (SEM) images of the resulting particles clearly revealed the formation of uniform micro-sized hexagonal rods (Fig. 1a). The comparison between the powder X-ray diffraction (PXRD) pattern of the resulting hexagonal-rods and the simulated PXRD pattern of the reported Fe-MIL-88A structure11 confirmed that the resulting hexagonal rods have a Fe-MIL-88A structure (Fig. S1, ESI†). Finally, A-series of hybrid composites (magnetic particle-embedded porous carbon materials) were prepared via a simple one-step thermal treatment (600–1000 1C) of Fe-MIL-88A hexagonal rods in the tube furnace under a nitrogen gas flow. The resulting products were designated based on the kind of starting MOFs and the pyrolysis temperature. For example, A-600 denotes the products obtained by thermal treatment of Fe-MIL-88A at 600 1C. The resulting magnetic particle-embedded porous carbon materials were fully characterized by SEM, transmission electron microscopy (TEM), PXRD, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) and Raman spectroscopy.
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Scheme 1 Preparation of magnetic particle-embedded porous carbon composites (magnetic particles@carbon, A-600–A-1000 and B-600–B-1000) via pyrolysis of micro- or nano-sized CPPs of Fe-MIL-88A or Fe-MIL-88B. Note that A-600 denotes the products obtained by the pyrolysis of Fe-MIL-88A at 600 1C and B-700 was obtained by the pyrolysis of Fe-MIL-88B at 700 1C.
Fig. 1 SEM and high magnification TEM images of (a) Fe-MIL-88A, (b) A-600, (c) A-700, (d) A-800, (e) A-900 and (f) A-1000. The inset in (b) shows A-600 dispersed in water and magnetically separated.
The resulting black materials were strongly attracted by a magnet (Fig. 1b and Fig. S2, ESI†). The morphological details were identified by SEM and TEM images (Fig. 1). All A-series (A-600–A-1000) products moderately maintained the original rod-shape of the starting Fe-MIL-88A after pyrolysis. At the same time, a slight decrease in the overall particle size was observed after pyrolysis. The degree of contraction of particles increased with increasing pyrolysis temperature. Many nano-grains of iron-containing magnetic particles were discovered within products (Fig. 1 and 2). Note that these nano-grains were much obviously observed in the B-series of porous magnetic carbons. The magnetic materials incorporated within porous carbons were first characterized by PXRD spectroscopy (Fig. 2). The PXRD pattern of A-600 (Fig. 2a) revealed that maghemite (g-Fe2O3) was a major product with a minor portion of iron carbide (Fe3C)
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Fig. 2 PXRD patterns of (a) A-600, (b) A-700, (c) A-800, (d) A-900 and (e) A-1000. g-Fe2O3 (JCPDS No. 39-1346), Fe3C (JCPDS No. 35-0772) and a-Fe (JCPDS No. 06-0696). A magnified peak around 261 is shown in the inset of (e). HRTEM images of (f) A-600, (g, h) A-700, (i, j) A-800 and (k) A-900.
during low temperature pyrolysis. All of the diffraction peaks matched exactly with the reported values of the cubic phase of g-Fe2O3 (JCPDS No. 39-1346),12 even though the peaks were somewhat broad, and with the orthorhombic phase of Fe3C (JCPDS No. 35-0772).13 In fact, both g-Fe2O3 and magnetite (Fe3O4) had quite similar PXRD patterns; therefore, they could not be easily distinguished using only PXRD data. The formation of g-Fe2O3 rather than Fe3O4 was confirmed from the XPS spectrum (Fig. S3a, ESI†). Two broad peaks at 724.6 and 710.9 eV, assigned to Fe 2p1/2 and Fe 2p3/2, respectively, were observed in the XPS spectrum of A-600; additionally, the characteristic satellite peak14 at 719.8 eV for g-Fe2O3 was also observed. The intensity increase of PXRD peaks correlated to Fe3C and the intensity decrease of PXRD peaks for the initially dominant g-Fe2O3 were clearly detected in the PXRD pattern of A-700 (Fig. 2b). Most of the g-Fe2O3 was transformed into Fe3C during high temperature pyrolysis (T 4 700 1C). In addition, a small amount of metallic iron (a-Fe, JCPDS No. 06-0696)13 was detected in the PXRD pattern. The PXRD patterns of A-800 and A-900 (Fig. 2c and d) were similar to that of A-700. As confirmed by the PXRD pattern of A-1000 (Fig. 2e), most iron-containing magnetic materials (g-Fe2O3 and Fe3C) were transformed to a-Fe during the pyrolysis at 1000 1C. The characteristic peaks15 at 720.3 and 707.3 eV for a-Fe were observed in the XPS spectrum of A-1000 (Fig. S3e, ESI†). High-resolution TEM (HRTEM) images of the A-series also revealed the formation of magnetic particles. The HRTEM image of A-600 showed a calculated planar space of 0.25 nm in the (311) plane for g-Fe2O3 (Fig. 2f). The calculated planar spaces of 0.23 nm in the (002) plane for Fe3C (Fig. 2h), 0.21 nm in the (211) plane for Fe3C (Fig. 2k) and 0.20 nm in the (031) plane for Fe3C (or in the (110) plane for a-Fe, both have a quite similar value, Fig. 2g and i) were clearly observed in the HRTEM images of A-700–A-900. In addition, HRTEM images revealed the characteristic spaces of 0.34 nm in the (002) plane for the graphite (Fig. 2g–j). The Raman spectra of A-series porous
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Fig. 3 N2 sorption isotherms of (a) A-series and (b) B-series porous magnetic carbons. Solid symbols denote adsorption and open symbols denote desorption.
magnetic carbons showed the characteristic Raman-active E2g mode G-band for graphitic sheets at ca. 1586 cm 1 and a D-band attributed to the presence of defects within carbon materials at ca. 1331 cm 1 (Fig. S4, ESI†).16 Typically, the relative intensity ratio of the D-band to the G-band (ID/IG) provides the degree of graphitization of samples. Among the A-series porous magnetic carbons, A-1000 has the lowest ID/IG value and so it has the highest degree of graphitization. The EDX spectra of A-series porous magnetic carbons also provided information on their compositions (Table S1, ESI†). Oxygen atoms were abundant only in A-600, as can be expected from the dominant presence of g-Fe2O3 within A-600. A dramatic decrease in the oxygen content in A-700 was observed, while almost no oxygen was detected in A-1000. The porosity and surface area of the resulting A-series porous magnetic carbon were characterized by N2 sorption measurements. All sorption isotherms revealed type IV behavior (Fig. 3a). The steep increase in uptake amount in the low pressure region (P/P0 o 0.1) and the existence of the hysteresis curve indicated the presence of micropores and mesopores within the hybrid materials. The BET surface area and total pore volume calculated from the N2 sorption isotherm were 170 m2 g 1 and 0.15 cm3 g 1, respectively, for A-600. These values increased upon increasing the pyrolysis temperature up to 800 1C. The BET surface areas and total pore volumes were 204 m2 g 1 and 0.36 cm3 g 1 for A-700, and 265 m2 g 1 and 0.46 cm3 g 1 for A-800, respectively. However, these values decreased again in A-900 and A-1000, possibly due to the collapse of the pore within porous carbons and decrease in the overall size at high temperature. The pore size distributions of the resulting porous magnetic carbons, which were calculated using the NLDFT (non-local density function theory) method, are also shown in Fig. S5 (ESI†). Analogous magnetic particle-embedded porous carbons designated as B-600–B-1000 (B-series) were also prepared by a similar pyrolysis of Fe-MIL-88B ([Fe3O(BDC)3(H2O)2(NO3)]n) nano-rods at various temperatures. First of all, Fe-MIL-88B nano-rods (Fig. S6a, ESI†) were synthesized via a solvothermal reaction of Fe(NO3)3 with aromatic 1,4-benzenedicarboxylic acid (H2BDC) instead of aliphatic fumaric acid.17 The resulting B-series of porous magnetic carbons were characterized by SEM, TEM, PXRD, XPS, EDX and Raman spectroscopy. Generally, most data obtained from B-series products were quite similar to those of A-series products. The morphology of the initial Fe-MIL-88B was moderately maintained after pyrolysis (Fig. 4 and Fig. S6, ESI†).
5478 | Chem. Commun., 2014, 50, 5476--5479
Fig. 4 TEM and HRTEM images of (a) B-600, (b) B-700, (c) B-800, (d) B-900 and (e) B-1000. Elemental mapping data for (f) B-800 and (g) B-900. Scanning TEM (STEM) images (left), Fe mapping images (middle) and C mapping images (right).
The contraction phenomenon of the overall particle size observed in the B-series was similar to that observed in the A-series. Magnetic nano-grains incorporated within the porous carbons were clearly detected in TEM images and elemental mapping images (Fig. 4). In addition, the size increase of the magnetic nano-grains was observed with increasing pyrolysis temperature (Fig. 4). The average sizes of magnetic nano-grains within B-600–B-1000 determined from TEM data were 4.24 0.29, 7.04 0.50, 8.11 0.44, 19.13 2.69 and 36.67 3.29 nm, respectively (SD, n = 50). The elemental mapping images of B-800 and B-900 clearly showed the compositions of porous magnetic carbon nano-composites (Fig. 4f and g). Raman spectra of B-series porous magnetic carbons also showed the characteristic G-band at 1580 cm 1 and D-band at 1334 cm 1 (Fig. S7, ESI†) for carbon materials, which are similar to those observed in the A-series. Meanwhile, there was some difference between A-series and B-series products in the formation of magnetic materials. Even though a significant amount of Fe3C was formed during the pyrolysis process of Fe-MIL-88A, no considerable formation of Fe3C was observed during the pyrolysis of Fe-MIL-88B (Fig. S8, ESI†). Fe3C can be easily formed by the decomposition of aliphatic carbon (fumarate within Fe-MIL-88A). However, the formation of Fe3C by the decomposition of Fe-MIL-88B will be difficult because the decomposition of aromatic carbon cannot effectively supply the carbon sources for Fe3C formation. Specifically, g-Fe2O3 was the major product in B-600 (Fig. S8a, ESI†). In the PXRD pattern of B-700, the strong peaks assigned to a-Fe were detected and the intensity decrease of the peaks for the initially formed g-Fe2O3 was observed (Fig. S8b, ESI†). The amounts of a-Fe were increased with increasing pyrolysis temperature (Fig. S8b–e, ESI†). Finally, a-Fe was the only dominant product in B-1000, with a trace amount of g-Fe2O3 and Fe3C (Fig. S8e, ESI†). HRTEM images of B-series porous magnetic carbons also showed characteristic planar spaces for g-Fe2O3, a-Fe and graphite (Fig. 4). The porosity and
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materials, where the porosities and compositions can be regulated by altering the reaction conditions and varying the components of the initial MOFs. Furthermore, this method will be useful for the mass-production of porous magnetic carbon materials. This work was supported by the National Leading Research Lab Program (no. NRF-2012R1A2A1A03670409) through NRF grant funded by the MEST. Fig. 5 UV-vis absorption plot measured at various times after removing MB with (a) A-800 and (b) B-800. Insets in (a) and (b) show adsorption rate curves of MB (5 mg L 1, 20 mL) after the addition of A-800 and B-800, respectively. The photograph in (a) shows the MB solution before and after separation using A-800.
surface area of the B-series were slightly larger than those of the A-series (Fig. 3b). The highest BET surface areas and total pore volumes were measured at 490 m2 g 1 and 0.52 cm3 g 1 for B-800, respectively. The application of the resulting porous magnetic carbons as adsorbents for the separation of waste from aqueous solutions was investigated using a typical dye (methylene blue, MB) as waste. UV-vis spectra measured at various times after mixing a MB aqueous solution and adsorbents revealed a fast adsorption capacity of A-800 and B-800 (Fig. 5). It took approximately 20 min to reach equilibrium, as shown in the adsorption curves in the insets of Fig. 5. Finally, MB was clearly and easily removed from the aqueous solution by using the magnetic properties of A- and B-series porous magnetic carbons, as shown in a photograph within Fig. 5a. The adsorption isotherms of MB on A-800 and B-800 were obtained with different initial concentrations of MB (Fig. S10, ESI†). The adsorption capacities of MB on A-800 and B-800 were calculated to be 60.2 and 83.9 mg g 1, respectively. In conclusion, we successfully fabricated magnetic particleembedded porous carbon materials via a simple one-step pyrolysis of nano- and micro-sized iron-containing MOFs. The porosity of the resulting magnetic carbon materials differed depending upon the pyrolysis temperature and the components of the starting MOFs. Porous magnetic carbon composites produced from the pyrolysis of Fe-MIL-88A and Fe-MIL-88B at 800 1C under a nitrogen gas flow possessed the highest surface area and total pore volume among the series. The compositions of the resulting magnetic materials were also found to be varied depending upon the pyrolysis temperature and the nature of the organic building blocks incorporated within the initial Fe-MIL-88 structures. The formation of a significant amount of Fe3C as an intermediate between the initial formation of g-Fe2O3 and the final transformation to a-Fe resulted from the pyrolysis of Fe-MIL-88A containing aliphatic fumarate; however, the direct conversion of the initially formed g-Fe2O3 to a-Fe was discovered in the case of the pyrolysis of Fe-MIL-88B enclosing aromatic 1,4-benzenecarboxylate. This methodology is quite convenient for the production of hybrid porous magnetic carbon
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Notes and references 1 (a) H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, ¨ . Yazaydin, R. Q. Snurr, M. O’Keeffe, J. Kim and O. M. Yaghi, Science, A. O 2010, 329, 424–428; (b) E. D. Bloch, W. L. Queen, R. Krishna, J. M. Zadrozny, C. M. Brown and J. R. Long, Science, 2012, 335, 1606–1610; (c) J. An and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 5578–5579; (d) M. Kim, J. F. Cahill, H. Fei, K. A. Prather and S. M. Cohen, J. Am. Chem. Soc., 2012, 134, 18082–18088; (e) S. Shimomura, M. Higuchi, R. Matsuda, K. Yoneda, Y. Hijikata, Y. Kubota, Y. Mita, J. Kim, M. Takata and S. Kitagawa, Nat. Chem., 2010, 2, 633–637. 2 (a) B. Liu, M. Tu and R. A. Fischer, Angew. Chem., Int. Ed., 2013, 52, 3402–3405; (b) Q. Liu, N. Wang, J. Caro and A. Huang, J. Am. Chem. Soc., 2013, 135, 17679–17682. 3 (a) M. Oh and C. A. Mirkin, Nature, 2005, 438, 651–654; (b) H. J. Lee, Y. J. Cho, W. Cho and M. Oh, ACS Nano, 2013, 7, 491–499. ´rey, R. Gref, 4 (a) A. C. McKinlay, R. E. Morris, P. Horcajada, G. Fe P. Couvreur and C. Serre, Angew. Chem., Int. Ed., 2010, 49, 6260–6266; (b) J. D. Rocca, D. Liu and W. Lin, Acc. Chem. Res., 2011, 44, 957–968. 5 (a) W. Cho, Y. H. Lee, H. J. Lee and M. Oh, Chem. Commun., 2009, 4756–4758; (b) W. Cho, Y. H. Lee, H. J. Lee and M. Oh, Adv. Mater., 2011, 23, 1720–1723. 6 (a) S. Feng, W. Li, Q. Shi, Y. Li, J. Chen, Y. Ling, A. M. Asiri and D. Zhao, Chem. Commun., 2014, 50, 329–331; (b) S. Zhang, L. Chen, S. Zhou, D. Zhao and L. Wu, Chem. Mater., 2010, 22, 3433–3440; (c) L. Zhou, H. Li, C. Yu, X. Zhou, J. Tang, Y. Meng, Y. Xia and D. Zhao, Carbon, 2006, 44, 1601–1604. 7 W. Li, F. Zhang, Y. Dou, Z. Wu, H. Liu, X. Qian, D. Gu, Y. Xia, B. Tu and D. Zhao, Adv. Energy Mater., 2011, 1, 382–386. 8 (a) T. Muraliganth, A. V. Murugan and A. Manthiram, Chem. Commun., 2009, 7360–7362; (b) X. Du, C. Wang, M. Chen, Y. Jiao and J. Wang, J. Phys. Chem. C, 2009, 113, 2643–2646; (c) Z. Sun, L. Wang, P. Liu, S. Wang, B. Sun, D. Jiang and F.-S. Xiao, Adv. Mater., 2006, 18, 1968–1971. 9 (a) A. Yu, E. Bekyarova, M. E. Itkis, D. Fakhrutdinov, R. Webster and R. C. Haddon, J. Am. Chem. Soc., 2006, 128, 9902–9908; (b) J.-S. Lee, S.-I. Kim, J.-C. Yoon and J.-H. Jang, ACS Nano, 2013, 7, 6047–6055; (c) B.-H. Han, W. Zhou and A. Sayari, J. Am. Chem. Soc., 2003, 125, 3444–3445; (d) S. E. Skrabalak and K. S. Suslick, J. Am. Chem. Soc., 2006, 128, 12642–12643. 10 (a) B. Liu, H. Shioyama, T. Akita and Q. Xu, J. Am. Chem. Soc., 2008, 130, 5390–5391; (b) W. Chaikittisilp, K. Ariga and Y. Yamauchi, J. Mater. Chem. A, 2013, 1, 14–19; (c) A. Banerjee, R. Gokhale, S. Bhatnagar, J. Jog, M. Bhardwaj, B. Lefez, B. Hannoyer and S. Ogale, J. Mater. Chem., 2012, 22, 19694–19699; (d) A. J. Amali, J.-K. Sun and Q. Xu, Chem. Commun., 2014, 50, 1519–1522. ´ and G. Fe ´rey, Angew. Chem., Int. Ed., 11 C. Serre, F. Millange, S. Surble 2004, 43, 6286–6289. 12 B. H. Kim, N. Lee, H. Kim, K. An, Y. I. Park, Y. Choi, K. Shin, Y. Lee, S. G. Kwon, H. B. Na, J.-G. Park, T.-Y. Ahn, Y.-W. Kim, W. K. Moon, S. H. Choi and T. Hyeon, J. Am. Chem. Soc., 2011, 133, 12624–12631. 13 D.-H. Liu, Y. Guo, L.-H. Zhang, W.-C. Li, T. Sun and A.-H. Lu, Small, 2013, 9, 3852–3857. 14 Y. Ma, G. Ji and J. Y. Lee, J. Mater. Chem., 2011, 21, 13009–13014. 15 M. Descostes, F. Mercier, N. Thromat, C. Beaucaire and M. GautierSoyer, Appl. Surf. Sci., 2000, 165, 288–302. 16 F. Tuinstra and J. L. Koenic, J. Chem. Phys., 1970, 53, 1126–1130. 17 W. Cho, S. Park and M. Oh, Chem. Commun., 2011, 47, 4138–4140.
Chem. Commun., 2014, 50, 5476--5479 | 5479
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Cite this: Chem. Commun., 2014, 50, 5476
One-pot synthesis of magnetic particle-embedded porous carbon composites from metal–organic frameworks and their sorption properties†
Received 14th March 2014, Accepted 4th April 2014
Hee Jung Lee, Won Cho, Eunji Lim and Moonhyun Oh*
DOI: 10.1039/c4cc01914h www.rsc.org/chemcomm
Nano- and micro-composites comprised of porous carbon and magnetic particles are prepared by one-step pyrolysis of metal– organic frameworks (MOFs). The porosity and composition of resulting magnetic porous carbons are facilely regulated by altering the pyrolysis temperature and changing the organic building blocks incorporated within the initial MOFs.
Porous coordination polymers (CPs) or metal–organic frameworks (MOFs) are of great interest due to their useful chemical and physical properties.1 Based upon their controllable chemical compositions and porosities, many practical applications, including gas storage, gas separation, optics and catalysis, have been intensively studied. In addition, the development of thin membranes2 from MOFs and nano- or micro-sized coordination polymer particles (CPPs)3 enhanced their original usefulness and also expanded their application range to bio-medicines.4 Recently, the utilization of CPPs as the starting materials for the production of diverse metal oxides has also been demonstrated.5 The simple calcination of welldefined CPPs provided a convenient way for the production of complicated and well-organized metal oxides. On the other hand, porous carbon materials have received great attention due to their many useful applications including in adsorbents, supercapacitors, gas storage and as catalyst supports.6–9 Moreover, the fusion of carbon materials with other functional materials to improve their usefulness is a central focus of interest.7,8 In particular, magnetic carbon materials are useful for lithium ion batteries, supercapacitors and separation of pollutants.8 Even though many methods have been developed to synthesize various carbon materials including electrical arc, chemical vapor deposition (CVD), nanocasting and ultrasonic spray pyrolysis,9 the development of more convenient synthetic methods for the functional carbon materials is still of great
Department of Chemistry, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea. E-mail: [email protected]; Fax: +82-2-364-7050; Tel: +82-2-2123-5637 † Electronic supplementary information (ESI) available: Experimental details, supplementary figures and tables. See DOI: 10.1039/c4cc01914h
5476 | Chem. Commun., 2014, 50, 5476--5479
interest. In recent years, the utilization of MOFs as templates or precursor materials for the production of porous carbon materials has been demonstrated by taking advantage of the fact that MOFs are typically porous materials and possess a significant amount of carbon source.10 Herein, we report a novel strategy for one-pot synthesis of nano- and micro-sized hybrid composites comprised of porous carbons and magnetic particles (g-Fe2O3, Fe3C and/or a-Fe) by a simple thermal treatment of iron-containing MOFs (Fe-MIL-88A or Fe-MIL-88B) as precursors. Furthermore, we report that both the porosity of the resulting carbon materials and the compositions (g-Fe2O3, Fe3C or a-Fe) of the resulting magnetic particles incorporated within porous carbons can be regulated by altering the pyrolysis temperature and changing the organic components present in the starting MOFs. Fe-MIL-88A micro-rods with a hexagonal shape were first prepared according to the reported solvothermal method using Fe(NO3)3 and fumaric acid (Scheme 1). Fe-MIL-88A structures ([Fe3O(O2CC2H2CO2)3(H2O)2(NO3)]n) have a three-dimensional hexagonal structure consisting of trimers of FeO6 octahedra linked to fumarate (Scheme 1 and also see Fig. S1, ESI†).11 Scanning electron microscopy (SEM) images of the resulting particles clearly revealed the formation of uniform micro-sized hexagonal rods (Fig. 1a). The comparison between the powder X-ray diffraction (PXRD) pattern of the resulting hexagonal-rods and the simulated PXRD pattern of the reported Fe-MIL-88A structure11 confirmed that the resulting hexagonal rods have a Fe-MIL-88A structure (Fig. S1, ESI†). Finally, A-series of hybrid composites (magnetic particle-embedded porous carbon materials) were prepared via a simple one-step thermal treatment (600–1000 1C) of Fe-MIL-88A hexagonal rods in the tube furnace under a nitrogen gas flow. The resulting products were designated based on the kind of starting MOFs and the pyrolysis temperature. For example, A-600 denotes the products obtained by thermal treatment of Fe-MIL-88A at 600 1C. The resulting magnetic particle-embedded porous carbon materials were fully characterized by SEM, transmission electron microscopy (TEM), PXRD, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) and Raman spectroscopy.
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Scheme 1 Preparation of magnetic particle-embedded porous carbon composites (magnetic particles@carbon, A-600–A-1000 and B-600–B-1000) via pyrolysis of micro- or nano-sized CPPs of Fe-MIL-88A or Fe-MIL-88B. Note that A-600 denotes the products obtained by the pyrolysis of Fe-MIL-88A at 600 1C and B-700 was obtained by the pyrolysis of Fe-MIL-88B at 700 1C.
Fig. 1 SEM and high magnification TEM images of (a) Fe-MIL-88A, (b) A-600, (c) A-700, (d) A-800, (e) A-900 and (f) A-1000. The inset in (b) shows A-600 dispersed in water and magnetically separated.
The resulting black materials were strongly attracted by a magnet (Fig. 1b and Fig. S2, ESI†). The morphological details were identified by SEM and TEM images (Fig. 1). All A-series (A-600–A-1000) products moderately maintained the original rod-shape of the starting Fe-MIL-88A after pyrolysis. At the same time, a slight decrease in the overall particle size was observed after pyrolysis. The degree of contraction of particles increased with increasing pyrolysis temperature. Many nano-grains of iron-containing magnetic particles were discovered within products (Fig. 1 and 2). Note that these nano-grains were much obviously observed in the B-series of porous magnetic carbons. The magnetic materials incorporated within porous carbons were first characterized by PXRD spectroscopy (Fig. 2). The PXRD pattern of A-600 (Fig. 2a) revealed that maghemite (g-Fe2O3) was a major product with a minor portion of iron carbide (Fe3C)
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Fig. 2 PXRD patterns of (a) A-600, (b) A-700, (c) A-800, (d) A-900 and (e) A-1000. g-Fe2O3 (JCPDS No. 39-1346), Fe3C (JCPDS No. 35-0772) and a-Fe (JCPDS No. 06-0696). A magnified peak around 261 is shown in the inset of (e). HRTEM images of (f) A-600, (g, h) A-700, (i, j) A-800 and (k) A-900.
during low temperature pyrolysis. All of the diffraction peaks matched exactly with the reported values of the cubic phase of g-Fe2O3 (JCPDS No. 39-1346),12 even though the peaks were somewhat broad, and with the orthorhombic phase of Fe3C (JCPDS No. 35-0772).13 In fact, both g-Fe2O3 and magnetite (Fe3O4) had quite similar PXRD patterns; therefore, they could not be easily distinguished using only PXRD data. The formation of g-Fe2O3 rather than Fe3O4 was confirmed from the XPS spectrum (Fig. S3a, ESI†). Two broad peaks at 724.6 and 710.9 eV, assigned to Fe 2p1/2 and Fe 2p3/2, respectively, were observed in the XPS spectrum of A-600; additionally, the characteristic satellite peak14 at 719.8 eV for g-Fe2O3 was also observed. The intensity increase of PXRD peaks correlated to Fe3C and the intensity decrease of PXRD peaks for the initially dominant g-Fe2O3 were clearly detected in the PXRD pattern of A-700 (Fig. 2b). Most of the g-Fe2O3 was transformed into Fe3C during high temperature pyrolysis (T 4 700 1C). In addition, a small amount of metallic iron (a-Fe, JCPDS No. 06-0696)13 was detected in the PXRD pattern. The PXRD patterns of A-800 and A-900 (Fig. 2c and d) were similar to that of A-700. As confirmed by the PXRD pattern of A-1000 (Fig. 2e), most iron-containing magnetic materials (g-Fe2O3 and Fe3C) were transformed to a-Fe during the pyrolysis at 1000 1C. The characteristic peaks15 at 720.3 and 707.3 eV for a-Fe were observed in the XPS spectrum of A-1000 (Fig. S3e, ESI†). High-resolution TEM (HRTEM) images of the A-series also revealed the formation of magnetic particles. The HRTEM image of A-600 showed a calculated planar space of 0.25 nm in the (311) plane for g-Fe2O3 (Fig. 2f). The calculated planar spaces of 0.23 nm in the (002) plane for Fe3C (Fig. 2h), 0.21 nm in the (211) plane for Fe3C (Fig. 2k) and 0.20 nm in the (031) plane for Fe3C (or in the (110) plane for a-Fe, both have a quite similar value, Fig. 2g and i) were clearly observed in the HRTEM images of A-700–A-900. In addition, HRTEM images revealed the characteristic spaces of 0.34 nm in the (002) plane for the graphite (Fig. 2g–j). The Raman spectra of A-series porous
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Fig. 3 N2 sorption isotherms of (a) A-series and (b) B-series porous magnetic carbons. Solid symbols denote adsorption and open symbols denote desorption.
magnetic carbons showed the characteristic Raman-active E2g mode G-band for graphitic sheets at ca. 1586 cm 1 and a D-band attributed to the presence of defects within carbon materials at ca. 1331 cm 1 (Fig. S4, ESI†).16 Typically, the relative intensity ratio of the D-band to the G-band (ID/IG) provides the degree of graphitization of samples. Among the A-series porous magnetic carbons, A-1000 has the lowest ID/IG value and so it has the highest degree of graphitization. The EDX spectra of A-series porous magnetic carbons also provided information on their compositions (Table S1, ESI†). Oxygen atoms were abundant only in A-600, as can be expected from the dominant presence of g-Fe2O3 within A-600. A dramatic decrease in the oxygen content in A-700 was observed, while almost no oxygen was detected in A-1000. The porosity and surface area of the resulting A-series porous magnetic carbon were characterized by N2 sorption measurements. All sorption isotherms revealed type IV behavior (Fig. 3a). The steep increase in uptake amount in the low pressure region (P/P0 o 0.1) and the existence of the hysteresis curve indicated the presence of micropores and mesopores within the hybrid materials. The BET surface area and total pore volume calculated from the N2 sorption isotherm were 170 m2 g 1 and 0.15 cm3 g 1, respectively, for A-600. These values increased upon increasing the pyrolysis temperature up to 800 1C. The BET surface areas and total pore volumes were 204 m2 g 1 and 0.36 cm3 g 1 for A-700, and 265 m2 g 1 and 0.46 cm3 g 1 for A-800, respectively. However, these values decreased again in A-900 and A-1000, possibly due to the collapse of the pore within porous carbons and decrease in the overall size at high temperature. The pore size distributions of the resulting porous magnetic carbons, which were calculated using the NLDFT (non-local density function theory) method, are also shown in Fig. S5 (ESI†). Analogous magnetic particle-embedded porous carbons designated as B-600–B-1000 (B-series) were also prepared by a similar pyrolysis of Fe-MIL-88B ([Fe3O(BDC)3(H2O)2(NO3)]n) nano-rods at various temperatures. First of all, Fe-MIL-88B nano-rods (Fig. S6a, ESI†) were synthesized via a solvothermal reaction of Fe(NO3)3 with aromatic 1,4-benzenedicarboxylic acid (H2BDC) instead of aliphatic fumaric acid.17 The resulting B-series of porous magnetic carbons were characterized by SEM, TEM, PXRD, XPS, EDX and Raman spectroscopy. Generally, most data obtained from B-series products were quite similar to those of A-series products. The morphology of the initial Fe-MIL-88B was moderately maintained after pyrolysis (Fig. 4 and Fig. S6, ESI†).
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Fig. 4 TEM and HRTEM images of (a) B-600, (b) B-700, (c) B-800, (d) B-900 and (e) B-1000. Elemental mapping data for (f) B-800 and (g) B-900. Scanning TEM (STEM) images (left), Fe mapping images (middle) and C mapping images (right).
The contraction phenomenon of the overall particle size observed in the B-series was similar to that observed in the A-series. Magnetic nano-grains incorporated within the porous carbons were clearly detected in TEM images and elemental mapping images (Fig. 4). In addition, the size increase of the magnetic nano-grains was observed with increasing pyrolysis temperature (Fig. 4). The average sizes of magnetic nano-grains within B-600–B-1000 determined from TEM data were 4.24 0.29, 7.04 0.50, 8.11 0.44, 19.13 2.69 and 36.67 3.29 nm, respectively (SD, n = 50). The elemental mapping images of B-800 and B-900 clearly showed the compositions of porous magnetic carbon nano-composites (Fig. 4f and g). Raman spectra of B-series porous magnetic carbons also showed the characteristic G-band at 1580 cm 1 and D-band at 1334 cm 1 (Fig. S7, ESI†) for carbon materials, which are similar to those observed in the A-series. Meanwhile, there was some difference between A-series and B-series products in the formation of magnetic materials. Even though a significant amount of Fe3C was formed during the pyrolysis process of Fe-MIL-88A, no considerable formation of Fe3C was observed during the pyrolysis of Fe-MIL-88B (Fig. S8, ESI†). Fe3C can be easily formed by the decomposition of aliphatic carbon (fumarate within Fe-MIL-88A). However, the formation of Fe3C by the decomposition of Fe-MIL-88B will be difficult because the decomposition of aromatic carbon cannot effectively supply the carbon sources for Fe3C formation. Specifically, g-Fe2O3 was the major product in B-600 (Fig. S8a, ESI†). In the PXRD pattern of B-700, the strong peaks assigned to a-Fe were detected and the intensity decrease of the peaks for the initially formed g-Fe2O3 was observed (Fig. S8b, ESI†). The amounts of a-Fe were increased with increasing pyrolysis temperature (Fig. S8b–e, ESI†). Finally, a-Fe was the only dominant product in B-1000, with a trace amount of g-Fe2O3 and Fe3C (Fig. S8e, ESI†). HRTEM images of B-series porous magnetic carbons also showed characteristic planar spaces for g-Fe2O3, a-Fe and graphite (Fig. 4). The porosity and
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materials, where the porosities and compositions can be regulated by altering the reaction conditions and varying the components of the initial MOFs. Furthermore, this method will be useful for the mass-production of porous magnetic carbon materials. This work was supported by the National Leading Research Lab Program (no. NRF-2012R1A2A1A03670409) through NRF grant funded by the MEST. Fig. 5 UV-vis absorption plot measured at various times after removing MB with (a) A-800 and (b) B-800. Insets in (a) and (b) show adsorption rate curves of MB (5 mg L 1, 20 mL) after the addition of A-800 and B-800, respectively. The photograph in (a) shows the MB solution before and after separation using A-800.
surface area of the B-series were slightly larger than those of the A-series (Fig. 3b). The highest BET surface areas and total pore volumes were measured at 490 m2 g 1 and 0.52 cm3 g 1 for B-800, respectively. The application of the resulting porous magnetic carbons as adsorbents for the separation of waste from aqueous solutions was investigated using a typical dye (methylene blue, MB) as waste. UV-vis spectra measured at various times after mixing a MB aqueous solution and adsorbents revealed a fast adsorption capacity of A-800 and B-800 (Fig. 5). It took approximately 20 min to reach equilibrium, as shown in the adsorption curves in the insets of Fig. 5. Finally, MB was clearly and easily removed from the aqueous solution by using the magnetic properties of A- and B-series porous magnetic carbons, as shown in a photograph within Fig. 5a. The adsorption isotherms of MB on A-800 and B-800 were obtained with different initial concentrations of MB (Fig. S10, ESI†). The adsorption capacities of MB on A-800 and B-800 were calculated to be 60.2 and 83.9 mg g 1, respectively. In conclusion, we successfully fabricated magnetic particleembedded porous carbon materials via a simple one-step pyrolysis of nano- and micro-sized iron-containing MOFs. The porosity of the resulting magnetic carbon materials differed depending upon the pyrolysis temperature and the components of the starting MOFs. Porous magnetic carbon composites produced from the pyrolysis of Fe-MIL-88A and Fe-MIL-88B at 800 1C under a nitrogen gas flow possessed the highest surface area and total pore volume among the series. The compositions of the resulting magnetic materials were also found to be varied depending upon the pyrolysis temperature and the nature of the organic building blocks incorporated within the initial Fe-MIL-88 structures. The formation of a significant amount of Fe3C as an intermediate between the initial formation of g-Fe2O3 and the final transformation to a-Fe resulted from the pyrolysis of Fe-MIL-88A containing aliphatic fumarate; however, the direct conversion of the initially formed g-Fe2O3 to a-Fe was discovered in the case of the pyrolysis of Fe-MIL-88B enclosing aromatic 1,4-benzenecarboxylate. This methodology is quite convenient for the production of hybrid porous magnetic carbon
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