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Cite this: Chem. Commun., 2014, 50, 13502
From metal–organic framework to carbon: toward controlled hierarchical pore structures via a double-template approach†
Received 8th August 2014, Accepted 8th September 2014
Jian-Ke Sun and Qiang Xu*
DOI: 10.1039/c4cc06212d www.rsc.org/chemcomm
For the first time, MOF-derived hierarchically porous carbons with controlled pore structures for on-demand applications have been achieved via a double-template approach.
Porous carbon materials have attracted much attention in a variety of energy and environmental applications including gas storage and separation,1 catalysis,2 sensing,3 and energy harvesting, storage and conversion.4 Recently, the fabrication of porous carbons with hierarchical pore structures has provided new insights into the advanced utilization of carbon materials.5 Compared with singlesized porous carbons, hierarchical carbons composed of mesopores in combination with macropores and/or micropores will favor the mass transport through the macropores/mesopores and increase the specific surface area through small mesopores/ micropores. To date, several methods such as chemical activation6 and nano-casting7 from colloidal silica, block-copolymer micelles, or zeolites have been employed for obtaining hierarchically porous carbon materials. However, the effective control of hierarchical carbons with tailored pore structures (size, surface area and pore volume) for on-demand applications is still a great challenge. Metal–organic frameworks (MOFs) as a new class of organic– inorganic hybrid materials have received much attention in the last decade due to their great potential for applications in gas adsorption and storage,8 catalysis,9 photoluminescence,10 drug delivery,11 and so on.12 Recently, the exploration of MOFs as a platform for clean energy applications has become a burgeoning field.13 Especially, the employment of MOFs as templates and/or precursors to prepare porous carbon materials shows great potential for applications in energy and environmental fields.14 In 2008, we firstly employed MOF-5 as a template and precursor with furfuryl alcohol (FA) as an additional carbon source to prepare the porous carbons.14a Recently, it has been shown that the direct carbonization of a MOF precursor without an additional carbon source is another way to prepare
National Institute of Advanced Industrial Science and Technology (AIST) Ikeda, Osaka 563-8577, Japan. E-mail: [email protected]; Fax: +81-72-751-9629 † Electronic supplementary information (ESI) available: Experimental details, characterization and additional structural data. See DOI: 10.1039/c4cc06212d
13502 | Chem. Commun., 2014, 50, 13502--13505
highly porous carbon materials.15 Over the past few years, although great progress has been achieved by using MOFs as templates and/or precursors to fabricate highly porous carbons,16 it is still quite difficult to effectively control the pore structures. Even if some hierarchical pore structures have been reported, the issue of design is often qualitative and phenomenological. The lack of an effective and general method to control the pore structures still hinders the development of carbons with desired pore textures and applications. Herein, we report a facile and effective method to prepare hierarchical pore structures of MOF-derived carbons with controllable pore metrics (pore size, surface area and pore volume). As a proof of concept, a double-template method is employed for the first time by using a MOF as a template and precursor and an additional metal ion (Cu2+) as the second template. Importantly, the preloaded Cu2+ ions in the MOF not only play a role of pore-forming agents through in situ transformation into Cu nanoparticles during the carbonization process, but also trigger the size and phase transformation of the metal oxides (Al2O3) generated from MOFs in the resulting carbon–metal hybrids due to the doping effect. Interestingly, the size of Al2O3 particles depends on the amount of preloaded Cu2+ ions prior to carbonization, which can generate porous carbons with controlled pore sizes after removal of the metal species from the raw carbons (Scheme S1, ESI†). Remarkably, the mesopore-connected micropores and/or macropores can be achieved on the basis of this method. Compared with MOF-derived carbons, such metal@MOF-derived on-demand hierarchically porous carbons exhibit enhanced rate performances as electrodes in EDLCs as well as significant enhancement in gas uptakes. In this context, Al-MIL-101-NH217 was employed as a prototypical MOF to prepare the carbons. Al-MIL-101-NH2 was synthesized from DMF solution at 110 1C in high yields. It is isostructural to MIL-101-Cr,18 which exhibits large pores near meso-sized pore spaces (Fig. S1a, ESI†). The phase purity of the as-synthesized Al-MIL-101-NH2 was demonstrated by powder X-ray diffraction (PXRD) (Fig. S2, ESI†). The direct carbonization of the MOF precursor at 800 1C gives raw carbon materials (carbon–metal oxide hybrids), which are purified by removal of Al2O3 with a HF solution.
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Fig. 2 HAADF-STEM images of (a) MIL-C, (b) MIL-C-0.5, (c) MIL-C-1, (d) MIL-C-2. (e) TEM and (f) HAADF-STEM images of the raw carbon derived from Al-MIL-101-NH2. (g) TEM and (h) HAADF-STEM images of the raw carbon derived from 2Cu@Al-MIL-101-NH2. Fig. 1 (A) The sorption isotherms and (B) pore size distributions for (a) MIL-C, (b) MIL-C-0.5, (c) MIL-C-1, and (d) MIL-C-2. (C) Correlation between the amount of Cu2+ in the MOF and the pore volume of the corresponding carbon. (D) Correlation between the amount of Cu2+ in the MOF and the surface area of the corresponding carbon.
N2 adsorption of purified carbon materials (MIL-C) shows a dramatical increase in uptakes at low relative pressure, indicating abundant micropores in the carbon materials. A little hysteresis loop is found in high relative pressure regions (P/P0 = 0.5–1), suggesting only a small portion of mesopores in this carbon material (Fig. 1A-a). The calculated BET surface area of the carbon material is 1328 m2 g 1 with a pore volume of 0.7 cm3 g 1. The pore size distribution on the basis of a non-local density function theory (NLDFT) analysis confirms that the carbon material is mainly composed of micropores with the pore size distributions of 1–2 nm (Fig. 1B-a). PXRD spectrum of the carbon materials shows two broad peaks centered at around 25 and 441, which could be assigned to the carbon (002) and (101) diffractions, respectively (Fig. S3b, ESI†). In order to tune the pore metrics of the carbon materials, preloading of metal ions as additional templates into the MOF prior to carbonization was carried out. Al-MIL-101-NH2 has hydrophilic pores similar to MIL-101-Cr and the decorated –NH2 groups on the pore wall further increase its hydrophilic characteristics. The doublesolvent approach is employed to effectively load the metal ions into the inner pore space.19,20 The Cu2+ is chosen as the second template in this work. The successful incorporation of different amounts of Cu2+ into the MOF (denoted as XCu@Al-MIL-101-NH2, X = 0.5, 1, 2, where X is the weight percent (wt%) of metal ions in the MOF) was demonstrated by gas sorptions (Fig. S1, ESI†). A decrease in the amount of N2 adsorption and the pore volume of these samples indicates that the cavities of the host framework are occupied by dispersed metal ions. Such a phenomenon has also been observed in the case of loading metal nanoparticles into MIL-101-Cr.20 It is noted that the main peaks of PXRD of XCu@Al-MIL-101-NH2 match well with those of Al-MIL-101-NH2 (Fig. S2, ESI†), which demonstrates that the original framework is maintained after loading the metal ions. XCu@Al-MIL-101-NH2 is directly carbonized under the same conditions as those used for Al-MIL-101-NH2. The obtained raw carbon materials were further purified by the acids to produce porous carbon materials (denoted as MIL-C-X, X = 0.5, 1, 2). PXRD measurements indicate no obvious change in the relative intensity between (002) and (101) peaks in the MIL-C-X series as compared with that of MIL-C (Fig. S3, ESI†), indicating the negligible
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graphitization effect of additional Cu2+ ions on the resultant carbons. The TEM micrographs clearly reveal the morphological evolution depending on the absence and presence of the second template after carbonization (Fig. 2a–d). The relatively uniform textures can be found in MIL-C, whereas the loading of the second template leads to distinct morphologies of the resultant carbons. The uniform textures are gradually destroyed in the MIL-C-X series upon increasing the amount of the second template, along with appearance of framework-like textures. This tendency becomes apparent in MIL-C-2, for which only aperiodic arrangement of frameworklike textures is observed (Fig. 2d). Such regular morphology transformations indicate the gradual formation of large-sized pores upon increasing the amount of the second template. Quantitative analysis of the changes in the texture of these carbon materials was carried out by N2 adsorption. As shown in Fig. 1A, the MIL-C-X series exhibit combined-characteristics of type I and IV isotherms, indicating that some large pores such as mesopores or macropores appeared besides the micropores. An obvious enhancement of the surface area from MIL-C (1328 m2 g 1) to MIL-C-1 (2116 m2 g 1) is observed. As for MIL-C-2, the specific surface area drops to 1397 m2 g 1, which is comparable to that of MIL-C (1328 m2 g 1). Unlike in MIL-C, the pore volume of MIL-C-2 reaches the highest value of 2.4 cm3 g 1. Considering that the surface areas are mainly determined by micropores and small mesopores, these observations suggest that the present doubletemplate approach can effectively introduce large-sized pores into the micropore-dominated carbons without obviously sacrificing the quality of original micropores derived from a MOF template/ precursor. The effect of an additional template on pore structures was further investigated by pore size distributions on the basis of NLDFT analysis (Fig. 1B). The sizes of micropores (1–2 nm) are little affected by the introduction of the second template, while great changes are observed for mesopores and macropores. Generally, the increase of the amount of the second template enlarges the pore size of resultant carbons. Finally, the highly hierarchical pore structures with micro- (B1 nm), meso- (B12 nm) and macropores (B62 nm) are formed in MIL-C-2. Therefore, systematic control of pore structures from micropore-dominated carbons (MIL-C) to micro– mesoporous carbons (MIL-C-0.5, 1), and ultimately hierarchically micro–meso–macroporous carbons (MIL-C-2) is achieved through this novel double-template method. Further quantitative studies of pore volume distributions are summarized in Table S1 (ESI†). The increase
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of the portion of meso and/or macropores upon enhancing the amount of the second template is observed, which is consistent with the adsorption isotherms as well as the pore size distribution analysis. Interestingly, there are some correlations between the pore metrics (pore volume and surface area) and the content of the second template. The pore volume of the resultant carbons enhances linearly upon increasing the amount of the second template (Fig. 1C). However, a nonlinear relation can be found between the surface area of resultant carbons and the amount of the second template (Fig. 1D). To shed light on the mechanism of formation of such hierarchical pore architectures, the PXRD patterns of raw carbons (before washing by acids) derived from Al-MIL-101-NH2 or XCu@Al-MIL-101-NH2 were investigated. As shown in Fig. S3a (ESI†), no obvious diffractions were observed in the Al-MIL-101-NH2-derived raw carbon except for the characteristic peaks from the carbon, indicating that the generated Al2O3 particles are in an amorphous state or have a small size. However, the loading of the second template makes a great impact on the phase of Al2O3 in the raw carbons. For the XCu@Al-MIL-101NH2-derived raw carbons (X = 0.5, 1), several obvious diffractions assignable to Al2O3 crystals (PDF#10-0425) and metallic Cu (PDF#040836) were also observed apart from the diffractions corresponding to the carbon. Notably, the phase of Al2O3 can be influenced by the amount of the second template. For example, the mixed phases of Al2O3 (PDF#10-0425 and PDF#26-0031) can be found in the raw carbon derived from 2Cu@Al-MIL-101-NH2 (Fig. S3g, ESI†). Moreover, the increase of the amount of the second template enhances the diffractions of Al2O3 and Cu nanoparticles, indicating that the sizes of Al2O3 and Cu nanoparticles increased as deduced from the Scherrer equation. This observation is further supported by TEM micrographs (Fig. 2). Before loading the Cu2+ ions, the Al2O3 nanoparticles with very small sizes of 1–2 nm are homogeneously distributed in the raw carbon matrix (Fig. 2e and f). However, loading small amounts of Cu2+ ions leads to large crystalline Al2O3 nanoparticles in the 2Cu@Al-MIL-101-NH2 derived raw carbon. The average size of Al2O3 nanoparticles is 60–70 nm (Fig. 2g and h), which is consistent with the macropore size distribution in MIL-2-C after removal of the metal species from the raw carbon. According to the above observations, a possible mechanism for gradual formation of hierarchically porous carbons is proposed. Before loading additional Cu2+ ions, the MOF serves as a template and precursor to generate the carbon mainly composed of micropores. The additional Cu2+ ions in MOFs play two roles in enhancing the porosity of carbons. Firstly, the in situ generated Cu nanoparticles from Cu2+ act as pore-forming agents, which increase the portions of fine mesopores after removal from the raw carbons. On the other hand, since the resulting Al2O3 crystal contains a small amount of elemental Cu (Fig. S9, ESI†), we propose that Cu2+ may be incorporated into the Al2O3 domain and occupy the Al lattice point at an initial nucleation stage (embryo formation) because of the comparable ion radii (Cu2+ = 72 pm, Al3+ = 50 pm). The initial embryo formation depends on the availability of nucleation sites in the lattice of the transforming phase. These can be point-defect-like vacancies or impurities. It has been reported that the oxygen vacancies are available nucleation centers for the growth of metal oxide.21 For example, Amores et al. have reported that the existence of Cu2+
13504 | Chem. Commun., 2014, 50, 13502--13505
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leads to generation of the defects (O2 ) at the surface of TiO2 particles, which accelerate the size growth and phase transformation of TiO2 by high temperature sintering.21a Nair and co-workers have reported that the doping of Cu2+ could affect the phase and crystal size of TiO2.21b The influence of cation dopants on the phase transformation behavior of TiO2 is based on the changes in the defect structure of the titania lattice. The introduction of low oxidation state metal ions (Cu2+) helps in occupying the Ti lattice points and leads to more available nucleation centers in the titania lattice, which is beneficial to the enhancement of the nucleation process. Moreover, the extent of the grain growth is found to depend on the amount of the dopant (Cu2+). A similar phenomenon has been observed in Fe-doped TiO2.21c Generally, the doping of small amounts of metal ions could effectively influence the size and phase of the host metal oxide, which changes the properties of the nanocrystals.22 In the present work, metal ions (Cu2+) of a low oxidation state occupy the Al lattice points, which leads to an increase in the oxygen vacancies in an embryo formation stage. The increase in oxygen vacancies will enhance the nucleation process, making the grains grow into large particles. Moreover, the crystal size of Al2O3 depends on the amount of the dopant (Cu2+), which leads to the particle size being systematically tuned in the raw carbons. Ultimately, the removal of Cu nanoparticles and large-sized crystalline Al2O3 not only generates the mesopores, but also produces the macropores as observed in MIL-C-2. It should be mentioned that the Al2O3 is still distributed over the entire area of raw carbons despite growing into large-sized particles (Fig. 2g, h and Fig. S10, ESI†), which leads to the resulting carbon containing highly connected hierarchical pores, as observed from TEM images. It is noteworthy that MOF-derived hierarchical carbons sometimes can be obtained by varying the calcination temperatures because the metal/metal oxide nanoparticles (pore-forming agents) generated from MOF precursors are generally sintered at high temperatures. However, the quantitative control of size and aggregation of metal–metal oxide nanoparticles is still a great challenge. The double-template method presented here provides a new insight into the area, which can be utilized to effectively control the pore metrics of hierarchical carbons. The hierarchically porous carbon-based electrode materials exhibit enhanced rate performances in EDLCs. The performance of these samples is evaluated by using different scanning rates from 10 to 50 mV s 1 (electrolyte: 1 M H2SO4). As shown in Fig. 3A and B, at a low scanning speed, the rectangular shape of all the carbon electrodes can be well maintained. However, increasing scanning speeds distort the rectangular CV shapes in MIL-C and MIL-C-X (X = 0.5, 1) series (Fig. 3A). The degree of the distortion depends on different kinds of pore structures. For MIL-C and MIL-C-0.5, such rectangular shapes (the specific capacitances for MIL-C and MIL-C-0.5 are 145 and 143 F g 1, respectively, at 10 mV s 1) are dramatically lost at a high scanning speed of 50 mV s 1, which makes these carbons fail to work as supercapacitors. For MIL-C-1, although the initial specific capacitance is 180 F g 1 at 10 mV s 1, a 32% decay in capacitance (123 F g 1) is observed when the scanning rate increases from 10 to 50 mV s 1. Moreover, the rectangular CV plot is accompanied with a serious distortion, suggesting that MIL-C-1 is also not suitable for high-speed
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Fig. 3 (A) Cyclic voltammograms of MIL-C, MIL-C-0.5 and MIL-C-1 based electrodes at different sweep rates (10 and 50 mV s 1). (B) Cyclic voltammograms of MIL-2 based electrodes at sweep rates from 10 to 400 mV s 1. (C) CO2 and (D) H2 adsorption isotherms for (a) MIL-C-1, (b) MIL-C-0.5, (c) MIL-C and (d) MIL-C-2.
capacitor performance. For MIL-C-2, such a rectangular shape is well-preserved without remarkable distortion as the scanning rate is increased. The specific capacitance of 185 F g 1 observed at 10 mV s 1 is only slightly decreased to 172 F g 1 at 50 mV s 1, corresponding to a decay as small as 7% (Fig. 3B). In fact, the MIL-C-2-based electrode can even retain the rectangular shape at a scanning speed as high as 400 mV s 1, 82% of initial capacitance is retained. This enhanced rate performance is attributed to the hierarchical micro–meso–macropore architecture in MIL-C-2, and the interconnected large pores within the networks are favorable for buffering ions to shorten the diffusion distances from the external electrolyte to interior surfaces, which is essential for rapid transport of electrolyte ions.23 These hierarchically porous carbons also exhibit enhanced performances in gas storage (Fig. 3C and D). Compared with MIL-C, the hierarchically porous carbons (MIL-C-0.5 and MIL-C-1) exhibit enhanced uptakes of H2 (77 K, 1 bar) and CO2 (273 K, 1 bar). In particular, the MIL-C-1 displays the highest uptakes of H2 and CO2 with amounts of 220 and 124 cm3 g 1, respectively. Different from the requirement for ion transport, the micropores and small mesopores in MIL-C-1 can increase the interactions between the pore wall and small gas molecules at low relative pressure (P/P0 o 0.1), which is essential for highperformance gas storage. Accordingly, it can explain the dramatical decrease of H2 and CO2 uptakes from MIL-C-1 to MIL-C-2. This results from the weak interactions between the pore wall and gas molecules in porous carbon MIL-C-2 containing a large portion of large mesopores and macropores. In conclusion, a double-template approach has been developed for the first time to control the pore structures of MOF-derived carbons. The preloaded metal ions (Cu2+) in the MOF not only play a role of pore-forming agents by in situ transforming into
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Cu nanoparticles during the carbonization process, but also trigger the size and phase transformation of the metal oxides (Al2O3) generated from MOFs in the resulting carbon-metal hybrids due to the doping effect. The size of the Al2O3 depends on the amount of preloaded Cu2+ ions, which can be further utilized to systematically control the porous carbons with hierarchical micro-/meso-/macropores. The present work may provide a general approach for preparing MOF-derived carbons with controlled hierarchical pore structures. The authors thank the reviewers for valuable suggestions, Dr Takeyuki Uchida for TEM and FE-SEM measurements and AIST for financial support. J. K. S. thanks JSPS for a postdoctoral fellowship.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
16 17 18 19 20 21
22 23
C. Z. Wu and H. M. Cheng, J. Mater. Chem., 2010, 20, 5390. X. H. Li and M. Antonietti, Chem. Soc. Rev., 2013, 42, 6593. D. S. Su and G. Centi, J. Energy Chem., 2013, 22, 151. Y. P. Zhai, D. Y. Zhao, P. F. Fulvio, R. T. Mayes and S. Dai, Adv. Mater., 2011, 23, 4828. D. C. Guo, J. Mi, G. P. Hao, W. Dong, G. Xiong, W. C. Li and A. H. Lu, Energy Environ. Sci., 2013, 6, 652. J. Wang and S. Kaskel, J. Mater. Chem., 2012, 22, 23710. H. Nishihara and T. Kyotani, Adv. Mater., 2012, 24, 4473. M. Eddaoudi, J. Kim, N. L. Rosi, D. T. Vodak, J. Wachter, M. O’Keeffe and O. M. Yaghi, Science, 2002, 295, 469. L. Q. Ma, C. Abney and W. B. Lin, Chem. Soc. Rev., 2009, 38, 1248. Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112, 1126. P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, ´rey, R. E. Morris and C. Serre, Chem. Rev., 2012, 112, 1232. G. Fe ¨ll and R. A. Fischer, Chem. Soc. Rev., D. Zacher, O. Shekhah, C. Wo 2009, 38, 1418. S. L. Li and Q. Xu, Energy Environ. Sci., 2013, 6, 1656. (a) B. Liu, H. Shioyama, T. Akita and Q. Xu, J. Am. Chem. Soc., 2008, 130, 5390; (b) B. Liu, H. Shioyama, H. L. Jiang, X. B. Zhang and Q. Xu, Carbon, 2010, 48, 456. (a) J. Hu, H. Wang, Q. Gao and H. Guo, Carbon, 2010, 48, 3599; (b) L. Radhakrishnan, J. Reboul, S. Furukawa, P. Srinivasu, S. Kitagawa and Y. Yamauchi, Chem. Mater., 2011, 23, 1225; (c) H. L. Jiang, B. Liu, Y. Q. Lan, K. Kuratani, T. Akita, H. Shioyama, F. Q. Zong and Q. Xu, J. Am. Chem. Soc., 2011, 133, 11854; (d) A. J. Amali, J. K. Sun and Q. Xu, Chem. Commun., 2014, 50, 1519. J. K. Sun and Q. Xu, Energy Environ. Sci., 2014, 7, 2071, and references there in. M. Hartmann and M. Fischer, Microporous Mesoporous Mater., 2012, 164, 38. ´rey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, G. Fe ´ and I. Margiolaki, Science, 2005, 309, 2040. S. Surble M. Imperor-Clerc, D. Bazin, M. D. Appay, P. Beaunier and A. Davidson, Chem. Mater., 2004, 16, 1813. ¨nnebro, T. Autrey, A. Aijaz, A. Karkamkar, Y. J. Choi, N. Tsumori, E. Ro H. Shioyama and Q. Xu, J. Am. Chem. Soc., 2012, 134, 13926. (a) J. M. G. Amores, V. S. Escribano and G. Busca, J. Mater. Chem., 1995, 5, 1245; (b) J. Nair, P. Nair, F. Mizukami, Y. Oosawa and T. Okubo, Mater. Res. Bull., 1999, 34, 1275; (c) X. H. Wang, J. G. Li, H. Kamiyama and T. Ishigaki, Thin Solid Films, 2006, 506–507, 278. D. J. Norris, A. L. Efros and S. C. Erwin, Science, 2008, 319, 1776. D. W. Wang, F. Li, M. Liu, G. Q. Lu and H. M. Cheng, Angew. Chem., Int. Ed., 2008, 47, 373.
Chem. Commun., 2014, 50, 13502--13505 | 13505
Published on 08 September 2014. Downloaded by University of West Florida on 11/10/2014 04:28:26.
COMMUNICATION
View Article Online View Journal | View Issue
Cite this: Chem. Commun., 2014, 50, 13502
From metal–organic framework to carbon: toward controlled hierarchical pore structures via a double-template approach†
Received 8th August 2014, Accepted 8th September 2014
Jian-Ke Sun and Qiang Xu*
DOI: 10.1039/c4cc06212d www.rsc.org/chemcomm
For the first time, MOF-derived hierarchically porous carbons with controlled pore structures for on-demand applications have been achieved via a double-template approach.
Porous carbon materials have attracted much attention in a variety of energy and environmental applications including gas storage and separation,1 catalysis,2 sensing,3 and energy harvesting, storage and conversion.4 Recently, the fabrication of porous carbons with hierarchical pore structures has provided new insights into the advanced utilization of carbon materials.5 Compared with singlesized porous carbons, hierarchical carbons composed of mesopores in combination with macropores and/or micropores will favor the mass transport through the macropores/mesopores and increase the specific surface area through small mesopores/ micropores. To date, several methods such as chemical activation6 and nano-casting7 from colloidal silica, block-copolymer micelles, or zeolites have been employed for obtaining hierarchically porous carbon materials. However, the effective control of hierarchical carbons with tailored pore structures (size, surface area and pore volume) for on-demand applications is still a great challenge. Metal–organic frameworks (MOFs) as a new class of organic– inorganic hybrid materials have received much attention in the last decade due to their great potential for applications in gas adsorption and storage,8 catalysis,9 photoluminescence,10 drug delivery,11 and so on.12 Recently, the exploration of MOFs as a platform for clean energy applications has become a burgeoning field.13 Especially, the employment of MOFs as templates and/or precursors to prepare porous carbon materials shows great potential for applications in energy and environmental fields.14 In 2008, we firstly employed MOF-5 as a template and precursor with furfuryl alcohol (FA) as an additional carbon source to prepare the porous carbons.14a Recently, it has been shown that the direct carbonization of a MOF precursor without an additional carbon source is another way to prepare
National Institute of Advanced Industrial Science and Technology (AIST) Ikeda, Osaka 563-8577, Japan. E-mail: [email protected]; Fax: +81-72-751-9629 † Electronic supplementary information (ESI) available: Experimental details, characterization and additional structural data. See DOI: 10.1039/c4cc06212d
13502 | Chem. Commun., 2014, 50, 13502--13505
highly porous carbon materials.15 Over the past few years, although great progress has been achieved by using MOFs as templates and/or precursors to fabricate highly porous carbons,16 it is still quite difficult to effectively control the pore structures. Even if some hierarchical pore structures have been reported, the issue of design is often qualitative and phenomenological. The lack of an effective and general method to control the pore structures still hinders the development of carbons with desired pore textures and applications. Herein, we report a facile and effective method to prepare hierarchical pore structures of MOF-derived carbons with controllable pore metrics (pore size, surface area and pore volume). As a proof of concept, a double-template method is employed for the first time by using a MOF as a template and precursor and an additional metal ion (Cu2+) as the second template. Importantly, the preloaded Cu2+ ions in the MOF not only play a role of pore-forming agents through in situ transformation into Cu nanoparticles during the carbonization process, but also trigger the size and phase transformation of the metal oxides (Al2O3) generated from MOFs in the resulting carbon–metal hybrids due to the doping effect. Interestingly, the size of Al2O3 particles depends on the amount of preloaded Cu2+ ions prior to carbonization, which can generate porous carbons with controlled pore sizes after removal of the metal species from the raw carbons (Scheme S1, ESI†). Remarkably, the mesopore-connected micropores and/or macropores can be achieved on the basis of this method. Compared with MOF-derived carbons, such metal@MOF-derived on-demand hierarchically porous carbons exhibit enhanced rate performances as electrodes in EDLCs as well as significant enhancement in gas uptakes. In this context, Al-MIL-101-NH217 was employed as a prototypical MOF to prepare the carbons. Al-MIL-101-NH2 was synthesized from DMF solution at 110 1C in high yields. It is isostructural to MIL-101-Cr,18 which exhibits large pores near meso-sized pore spaces (Fig. S1a, ESI†). The phase purity of the as-synthesized Al-MIL-101-NH2 was demonstrated by powder X-ray diffraction (PXRD) (Fig. S2, ESI†). The direct carbonization of the MOF precursor at 800 1C gives raw carbon materials (carbon–metal oxide hybrids), which are purified by removal of Al2O3 with a HF solution.
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Fig. 2 HAADF-STEM images of (a) MIL-C, (b) MIL-C-0.5, (c) MIL-C-1, (d) MIL-C-2. (e) TEM and (f) HAADF-STEM images of the raw carbon derived from Al-MIL-101-NH2. (g) TEM and (h) HAADF-STEM images of the raw carbon derived from 2Cu@Al-MIL-101-NH2. Fig. 1 (A) The sorption isotherms and (B) pore size distributions for (a) MIL-C, (b) MIL-C-0.5, (c) MIL-C-1, and (d) MIL-C-2. (C) Correlation between the amount of Cu2+ in the MOF and the pore volume of the corresponding carbon. (D) Correlation between the amount of Cu2+ in the MOF and the surface area of the corresponding carbon.
N2 adsorption of purified carbon materials (MIL-C) shows a dramatical increase in uptakes at low relative pressure, indicating abundant micropores in the carbon materials. A little hysteresis loop is found in high relative pressure regions (P/P0 = 0.5–1), suggesting only a small portion of mesopores in this carbon material (Fig. 1A-a). The calculated BET surface area of the carbon material is 1328 m2 g 1 with a pore volume of 0.7 cm3 g 1. The pore size distribution on the basis of a non-local density function theory (NLDFT) analysis confirms that the carbon material is mainly composed of micropores with the pore size distributions of 1–2 nm (Fig. 1B-a). PXRD spectrum of the carbon materials shows two broad peaks centered at around 25 and 441, which could be assigned to the carbon (002) and (101) diffractions, respectively (Fig. S3b, ESI†). In order to tune the pore metrics of the carbon materials, preloading of metal ions as additional templates into the MOF prior to carbonization was carried out. Al-MIL-101-NH2 has hydrophilic pores similar to MIL-101-Cr and the decorated –NH2 groups on the pore wall further increase its hydrophilic characteristics. The doublesolvent approach is employed to effectively load the metal ions into the inner pore space.19,20 The Cu2+ is chosen as the second template in this work. The successful incorporation of different amounts of Cu2+ into the MOF (denoted as XCu@Al-MIL-101-NH2, X = 0.5, 1, 2, where X is the weight percent (wt%) of metal ions in the MOF) was demonstrated by gas sorptions (Fig. S1, ESI†). A decrease in the amount of N2 adsorption and the pore volume of these samples indicates that the cavities of the host framework are occupied by dispersed metal ions. Such a phenomenon has also been observed in the case of loading metal nanoparticles into MIL-101-Cr.20 It is noted that the main peaks of PXRD of XCu@Al-MIL-101-NH2 match well with those of Al-MIL-101-NH2 (Fig. S2, ESI†), which demonstrates that the original framework is maintained after loading the metal ions. XCu@Al-MIL-101-NH2 is directly carbonized under the same conditions as those used for Al-MIL-101-NH2. The obtained raw carbon materials were further purified by the acids to produce porous carbon materials (denoted as MIL-C-X, X = 0.5, 1, 2). PXRD measurements indicate no obvious change in the relative intensity between (002) and (101) peaks in the MIL-C-X series as compared with that of MIL-C (Fig. S3, ESI†), indicating the negligible
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graphitization effect of additional Cu2+ ions on the resultant carbons. The TEM micrographs clearly reveal the morphological evolution depending on the absence and presence of the second template after carbonization (Fig. 2a–d). The relatively uniform textures can be found in MIL-C, whereas the loading of the second template leads to distinct morphologies of the resultant carbons. The uniform textures are gradually destroyed in the MIL-C-X series upon increasing the amount of the second template, along with appearance of framework-like textures. This tendency becomes apparent in MIL-C-2, for which only aperiodic arrangement of frameworklike textures is observed (Fig. 2d). Such regular morphology transformations indicate the gradual formation of large-sized pores upon increasing the amount of the second template. Quantitative analysis of the changes in the texture of these carbon materials was carried out by N2 adsorption. As shown in Fig. 1A, the MIL-C-X series exhibit combined-characteristics of type I and IV isotherms, indicating that some large pores such as mesopores or macropores appeared besides the micropores. An obvious enhancement of the surface area from MIL-C (1328 m2 g 1) to MIL-C-1 (2116 m2 g 1) is observed. As for MIL-C-2, the specific surface area drops to 1397 m2 g 1, which is comparable to that of MIL-C (1328 m2 g 1). Unlike in MIL-C, the pore volume of MIL-C-2 reaches the highest value of 2.4 cm3 g 1. Considering that the surface areas are mainly determined by micropores and small mesopores, these observations suggest that the present doubletemplate approach can effectively introduce large-sized pores into the micropore-dominated carbons without obviously sacrificing the quality of original micropores derived from a MOF template/ precursor. The effect of an additional template on pore structures was further investigated by pore size distributions on the basis of NLDFT analysis (Fig. 1B). The sizes of micropores (1–2 nm) are little affected by the introduction of the second template, while great changes are observed for mesopores and macropores. Generally, the increase of the amount of the second template enlarges the pore size of resultant carbons. Finally, the highly hierarchical pore structures with micro- (B1 nm), meso- (B12 nm) and macropores (B62 nm) are formed in MIL-C-2. Therefore, systematic control of pore structures from micropore-dominated carbons (MIL-C) to micro– mesoporous carbons (MIL-C-0.5, 1), and ultimately hierarchically micro–meso–macroporous carbons (MIL-C-2) is achieved through this novel double-template method. Further quantitative studies of pore volume distributions are summarized in Table S1 (ESI†). The increase
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of the portion of meso and/or macropores upon enhancing the amount of the second template is observed, which is consistent with the adsorption isotherms as well as the pore size distribution analysis. Interestingly, there are some correlations between the pore metrics (pore volume and surface area) and the content of the second template. The pore volume of the resultant carbons enhances linearly upon increasing the amount of the second template (Fig. 1C). However, a nonlinear relation can be found between the surface area of resultant carbons and the amount of the second template (Fig. 1D). To shed light on the mechanism of formation of such hierarchical pore architectures, the PXRD patterns of raw carbons (before washing by acids) derived from Al-MIL-101-NH2 or XCu@Al-MIL-101-NH2 were investigated. As shown in Fig. S3a (ESI†), no obvious diffractions were observed in the Al-MIL-101-NH2-derived raw carbon except for the characteristic peaks from the carbon, indicating that the generated Al2O3 particles are in an amorphous state or have a small size. However, the loading of the second template makes a great impact on the phase of Al2O3 in the raw carbons. For the XCu@Al-MIL-101NH2-derived raw carbons (X = 0.5, 1), several obvious diffractions assignable to Al2O3 crystals (PDF#10-0425) and metallic Cu (PDF#040836) were also observed apart from the diffractions corresponding to the carbon. Notably, the phase of Al2O3 can be influenced by the amount of the second template. For example, the mixed phases of Al2O3 (PDF#10-0425 and PDF#26-0031) can be found in the raw carbon derived from 2Cu@Al-MIL-101-NH2 (Fig. S3g, ESI†). Moreover, the increase of the amount of the second template enhances the diffractions of Al2O3 and Cu nanoparticles, indicating that the sizes of Al2O3 and Cu nanoparticles increased as deduced from the Scherrer equation. This observation is further supported by TEM micrographs (Fig. 2). Before loading the Cu2+ ions, the Al2O3 nanoparticles with very small sizes of 1–2 nm are homogeneously distributed in the raw carbon matrix (Fig. 2e and f). However, loading small amounts of Cu2+ ions leads to large crystalline Al2O3 nanoparticles in the 2Cu@Al-MIL-101-NH2 derived raw carbon. The average size of Al2O3 nanoparticles is 60–70 nm (Fig. 2g and h), which is consistent with the macropore size distribution in MIL-2-C after removal of the metal species from the raw carbon. According to the above observations, a possible mechanism for gradual formation of hierarchically porous carbons is proposed. Before loading additional Cu2+ ions, the MOF serves as a template and precursor to generate the carbon mainly composed of micropores. The additional Cu2+ ions in MOFs play two roles in enhancing the porosity of carbons. Firstly, the in situ generated Cu nanoparticles from Cu2+ act as pore-forming agents, which increase the portions of fine mesopores after removal from the raw carbons. On the other hand, since the resulting Al2O3 crystal contains a small amount of elemental Cu (Fig. S9, ESI†), we propose that Cu2+ may be incorporated into the Al2O3 domain and occupy the Al lattice point at an initial nucleation stage (embryo formation) because of the comparable ion radii (Cu2+ = 72 pm, Al3+ = 50 pm). The initial embryo formation depends on the availability of nucleation sites in the lattice of the transforming phase. These can be point-defect-like vacancies or impurities. It has been reported that the oxygen vacancies are available nucleation centers for the growth of metal oxide.21 For example, Amores et al. have reported that the existence of Cu2+
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leads to generation of the defects (O2 ) at the surface of TiO2 particles, which accelerate the size growth and phase transformation of TiO2 by high temperature sintering.21a Nair and co-workers have reported that the doping of Cu2+ could affect the phase and crystal size of TiO2.21b The influence of cation dopants on the phase transformation behavior of TiO2 is based on the changes in the defect structure of the titania lattice. The introduction of low oxidation state metal ions (Cu2+) helps in occupying the Ti lattice points and leads to more available nucleation centers in the titania lattice, which is beneficial to the enhancement of the nucleation process. Moreover, the extent of the grain growth is found to depend on the amount of the dopant (Cu2+). A similar phenomenon has been observed in Fe-doped TiO2.21c Generally, the doping of small amounts of metal ions could effectively influence the size and phase of the host metal oxide, which changes the properties of the nanocrystals.22 In the present work, metal ions (Cu2+) of a low oxidation state occupy the Al lattice points, which leads to an increase in the oxygen vacancies in an embryo formation stage. The increase in oxygen vacancies will enhance the nucleation process, making the grains grow into large particles. Moreover, the crystal size of Al2O3 depends on the amount of the dopant (Cu2+), which leads to the particle size being systematically tuned in the raw carbons. Ultimately, the removal of Cu nanoparticles and large-sized crystalline Al2O3 not only generates the mesopores, but also produces the macropores as observed in MIL-C-2. It should be mentioned that the Al2O3 is still distributed over the entire area of raw carbons despite growing into large-sized particles (Fig. 2g, h and Fig. S10, ESI†), which leads to the resulting carbon containing highly connected hierarchical pores, as observed from TEM images. It is noteworthy that MOF-derived hierarchical carbons sometimes can be obtained by varying the calcination temperatures because the metal/metal oxide nanoparticles (pore-forming agents) generated from MOF precursors are generally sintered at high temperatures. However, the quantitative control of size and aggregation of metal–metal oxide nanoparticles is still a great challenge. The double-template method presented here provides a new insight into the area, which can be utilized to effectively control the pore metrics of hierarchical carbons. The hierarchically porous carbon-based electrode materials exhibit enhanced rate performances in EDLCs. The performance of these samples is evaluated by using different scanning rates from 10 to 50 mV s 1 (electrolyte: 1 M H2SO4). As shown in Fig. 3A and B, at a low scanning speed, the rectangular shape of all the carbon electrodes can be well maintained. However, increasing scanning speeds distort the rectangular CV shapes in MIL-C and MIL-C-X (X = 0.5, 1) series (Fig. 3A). The degree of the distortion depends on different kinds of pore structures. For MIL-C and MIL-C-0.5, such rectangular shapes (the specific capacitances for MIL-C and MIL-C-0.5 are 145 and 143 F g 1, respectively, at 10 mV s 1) are dramatically lost at a high scanning speed of 50 mV s 1, which makes these carbons fail to work as supercapacitors. For MIL-C-1, although the initial specific capacitance is 180 F g 1 at 10 mV s 1, a 32% decay in capacitance (123 F g 1) is observed when the scanning rate increases from 10 to 50 mV s 1. Moreover, the rectangular CV plot is accompanied with a serious distortion, suggesting that MIL-C-1 is also not suitable for high-speed
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Fig. 3 (A) Cyclic voltammograms of MIL-C, MIL-C-0.5 and MIL-C-1 based electrodes at different sweep rates (10 and 50 mV s 1). (B) Cyclic voltammograms of MIL-2 based electrodes at sweep rates from 10 to 400 mV s 1. (C) CO2 and (D) H2 adsorption isotherms for (a) MIL-C-1, (b) MIL-C-0.5, (c) MIL-C and (d) MIL-C-2.
capacitor performance. For MIL-C-2, such a rectangular shape is well-preserved without remarkable distortion as the scanning rate is increased. The specific capacitance of 185 F g 1 observed at 10 mV s 1 is only slightly decreased to 172 F g 1 at 50 mV s 1, corresponding to a decay as small as 7% (Fig. 3B). In fact, the MIL-C-2-based electrode can even retain the rectangular shape at a scanning speed as high as 400 mV s 1, 82% of initial capacitance is retained. This enhanced rate performance is attributed to the hierarchical micro–meso–macropore architecture in MIL-C-2, and the interconnected large pores within the networks are favorable for buffering ions to shorten the diffusion distances from the external electrolyte to interior surfaces, which is essential for rapid transport of electrolyte ions.23 These hierarchically porous carbons also exhibit enhanced performances in gas storage (Fig. 3C and D). Compared with MIL-C, the hierarchically porous carbons (MIL-C-0.5 and MIL-C-1) exhibit enhanced uptakes of H2 (77 K, 1 bar) and CO2 (273 K, 1 bar). In particular, the MIL-C-1 displays the highest uptakes of H2 and CO2 with amounts of 220 and 124 cm3 g 1, respectively. Different from the requirement for ion transport, the micropores and small mesopores in MIL-C-1 can increase the interactions between the pore wall and small gas molecules at low relative pressure (P/P0 o 0.1), which is essential for highperformance gas storage. Accordingly, it can explain the dramatical decrease of H2 and CO2 uptakes from MIL-C-1 to MIL-C-2. This results from the weak interactions between the pore wall and gas molecules in porous carbon MIL-C-2 containing a large portion of large mesopores and macropores. In conclusion, a double-template approach has been developed for the first time to control the pore structures of MOF-derived carbons. The preloaded metal ions (Cu2+) in the MOF not only play a role of pore-forming agents by in situ transforming into
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Cu nanoparticles during the carbonization process, but also trigger the size and phase transformation of the metal oxides (Al2O3) generated from MOFs in the resulting carbon-metal hybrids due to the doping effect. The size of the Al2O3 depends on the amount of preloaded Cu2+ ions, which can be further utilized to systematically control the porous carbons with hierarchical micro-/meso-/macropores. The present work may provide a general approach for preparing MOF-derived carbons with controlled hierarchical pore structures. The authors thank the reviewers for valuable suggestions, Dr Takeyuki Uchida for TEM and FE-SEM measurements and AIST for financial support. J. K. S. thanks JSPS for a postdoctoral fellowship.
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