Materials 2015, 8, 5336-5347; doi:10.3390/ma8085245
OPEN ACCESS
materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Metal-Organic Frameworks to Metal/Metal Oxide Embedded Carbon Matrix: Synthesis, Characterization and Gas Sorption Properties Jiun-Jen Chen 1 , Ya-Ting Chen 2 , Duraisamy Senthil Raja 2 , Yu-Hao Kang 1 , Pen-Chang Tseng 1 and Chia-Her Lin 2,3, * 1
Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan; E-Mails: [email protected] (J.-J.C.); [email protected] (Y.-H.K.); [email protected] (P.-C.T.) 2 Department of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan; E-Mails: [email protected] (Y.-T.C.); [email protected] (D.S.R.) 3 R&D Center for Membrane Technology & Research Center for Structure of Matter, Chung Yuan Christian University, Chung-Li 320, Taiwan * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-3-265-3315; Fax: +886-3-265-3399. Academic Editor: Rafael Luque Received: 5 July 2015 / Accepted: 11 August 2015 / Published: 19 August 2015
Abstract: Three isostructural metal-organic frameworks, (MOFs), [Fe(OH)(1,4-NDC)] (1), [Al(OH)(1,4-NDC)] (2), and [In(OH)(1,4-NDC)] (3) have been synthesized hydrothermally by using 1,4-naphthalene dicarboxylate (1,4-NDC) as a linker. The MOFs were characterized using various techniques and further used as precursor materials for the synthesis of metal/metal oxide nanoparticles inserted in a carbon matrix through a simple thermal conversion method. The newly synthesized carbon materials were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy analysis, powder X-ray diffraction and BET analysis. The results showed that the MOF-derived carbon composite materials maintained the morphology of the original MOF upon carbonization, and confirmed the insertion of metal/metal oxide particles in the carbon matrix.
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Keywords: metal-organic frameworks; 1,4-naphthalene dicarboxylate; nanoporous carbon; metal oxide nanoparticle; gas sorption
1. Introduction Metal/metal oxide nanoparticles have demonstrated distinct properties from their bulk materials and have shown promising applications in many fields such as catalysis [1], magnetism [2] and biochemistry [3]. Though extensive research has been done, the preparation of these nanomaterials still suffers from the aggregation problem. In order to resolve the aggregation issue, a new strategy has been developed in which these nanoparticles are dispersed into carbon matrices such as porous carbon, carbon nanotubes and graphenes [1–3]. The carbon matrices can not only block the nanoparticles from aggregation, but also modify their properties [1–4]. On the other hand, metal-organic frameworks (MOFs) are the new types of porous hybrid functional materials that are constructed by metal ions or metal clusters and organic linkers. Because of their high porosities and tunable structural properties, they have wide functional applications in terms of catalysis [5,6], gas storage [7,8], sensors [9], biomedicine [10], and so on. Apart from the use of MOFs as crystalline porous materials, the utilization of MOFs as a precursor for metal/metal oxide nanoparticle-embedded carbon frameworks has been researched actively in recent years [11–16]. The subject has become of particular interest for functional applications such as catalysis, gas storage materials, anode materials for lithium-ion batteries, etc. [13–16]. After the report by Q. Xu et al. on the carbonization of MOF-5 [11], various methods have been reported to synthesize materials that contain metal/metal oxide nanoparticle-embedded porous carbon matrix from MOF precursors [17–20]. Since carbon, metal and oxygen atoms are arranged periodically at the atomic level within MOF structures, the MOFs can be converted into metal/metal oxide nanoparticle-embedded carbon frameworks with a certain degree of permanent porosity, which is much useful for the aforementioned applications. Further, the synthesis of pure porous carbon materials has also been done using MOF as a precursor approach, which needs to remove the metal/metal oxide nanoparticles formed in situ. This can be done using many methods such as vaporization at high temperature, washing with an acid solution and so on. However, our aim is to synthesize the dispersed metal/metal oxide nanoparticles in porous carbon matrices without removing the metal particles formed in situ during calcination because of the fact that the MOFs are highly ordered materials, and the carbonized MOFs prepared from these MOFs are expected to induce homogeneously dispersed metal/metal oxide nanoparticles in the resulting porous carbon matrices. Furthermore, MOFs with numerous structural topologies and tunable pores will enable us to construct a variety of carbonized MOFs with different degrees of metal/metal oxide nanoparticle-embedded porous carbon matrices for various applications [21]. Though a few articles have been published on the synthesis of the dispersed metal/metal oxide nanoparticles in porous carbon matrices from MOFs, the synthesis of metal/metal oxide nanoparticle-embedded porous carbon hybrid functional materials using a simple, one-step thermal conversion is still a challenge. Hence, herein we report three new metal/metal oxide embedded porous carbon hybrid functional materials from three MOFs ([Fe(OH)(1,4-NDC)] (1), [Al(OH)(1,4-NDC)]
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(2), and [In(OH)(1,4-NDC)] (3)) of 1,4-naphthalene dicarboxylate (1,4-NDC) using a simple, one-step thermal cracking method. Further, the MOFs and their metal/metal oxide embedded carbon hybrid materials were characterized using various physico-chemical techniques, and their gas sorption properties have been studied. 2. Experimental Section All reagents were commercially available and used as received without further purification. In a typical synthesis of the MOFs, the mixture of FeCl3 (0.162 g, 1.0 mmol) or Al(NO3 )3 ¨ 9H2 O (0.375 g, 1.0 mmol) or In(NO3 )3 ¨ xH2 O (0.301 g, 1.0 mmol), H2 -1,4-NDC (0.108 g, 0.5 mmol), and H2 O (10 mL) was placed in a 23 mL Teflon autoclave and then heated at 180 ˝ C for 1 day. After filtering off and washing with distilled water, powder samples of MOFs were obtained. 1: color, white; yield, 0.104 g; elemental analysis, found/calcd.: C, 39.60/40.13; H, 4.16/4.21% for 1¨ 4H2 O. 2: color, white; yield, 0.117 g; elemental analysis, found/calcd.: C, 49.24/49.00; H, 3.67/3.77% for 2¨ 2H2 O. 3: color, white; yield, 0.113 g; elemental analysis, found/calcd.: C, 38.12/38.64; H, 2.65/2.70% for 3¨ 1.5H2 O. Conversely, the synthesis of metal/metal oxide nanoparticles within a porous carbon matrix involves a direct carbonization method using the above synthesized MOFs as precursors. The MOFs (0.200 g) were taken in a silica boat and then placed in a tube furnace and heated from room temperature to 800 ˝ C under N2 gas with a heating rate of 5 ˝ C¨ min´1 to carbonize the MOFs. After reaching the target temperature (800 ˝ C), the temperature was maintained at 800 ˝ C for 5 h, then cooled down to room temperature with a cooling rate of 1 ˝ C¨ min´1 . The final black-colored powder products were further characterized. 3. Results and Discussion The simple hydrothermal reactions of H2 -1,4-NDC (0.5 mmol) with corresponding metal salts (1.0 mmol) yielded the new MOFs. The new MOFs were initially characterized by elemental analysis, which agrees well with theoretical values. The as-synthesized MOFs may contain guest H2 O molecules which are removed by heating of these MOFs to 180 ˝ C for 12 h under vacuum („10´3 torr); the phase purity of the bulk materials were independently confirmed by powder X-ray diffraction (PXRD) measurements (Figure 1). Comparing the PXRD patterns of as-synthesized 1–3 with calculated PXRD patterns of reported aluminum naphthalenedicarboxylate (Al-1,4-NDC) MOF material [22] confirms that the these new MOFs (1–3) have the same type of structural architecture as Al-1,4-NDC (Figure 1a–c). Further comparison of these PXRD pattern of activated MOFs with one another showed that the three new MOFs are isostructural to each other (Figure 1d). It is to be noted that the diffraction peaks of compound 1 and 3 at 8.2˝ showed a slight shift from the calculated PXRD of Al-1,4-NDC, which may be due to the difference in the ionic radius of the metal ions (Al3+ < Fe3+ < In3+ ). The shape and microstructure of the resulting MOFs (1–3) were investigated via scanning electron microscopy (SEM) and images are presented in Figure 2. The SEM images of the compounds show that they are crystalline materials with an elongated block shape, with individual particle sizes of about 30 µm, 3 µm, and 15 µm (for 1, 2 and 3, respectively).
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Figure 1. 1. The The comparison comparison of of PXRD PXRD patterns patterns of of as-synthesized as-synthesized MOFs, MOFs, (a) 1, (b) 22 and (c) Figure (a) 1, 1, (b) (b) and (c) (c) 333 Figure 1. The comparison of PXRD patterns of as-synthesized MOFs, (a) 2 and with simulated simulated PXRD PXRD pattern pattern of of reported reported Al-1,4-NDC; Al-1,4-NDC; (d) (d) the the comparison comparison of of PXRD PXRD patterns patterns with with simulated PXRD pattern of reported Al-1,4-NDC; (d) the comparison of PXRD patterns of activated activated MOFs. of MOFs. of activated MOFs.
(a) (a)
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(c) (c) Figure 2. The SEM images of 1 (a), 2 (b) and 3 (c). Figure 2. 2. The The SEM SEM images images of of 11 (a), (a), 22 (b) (b) and and 33 (c). (c). Figure
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In order ordertotostudy studythe thethermal thermal properties of the MOFs, the TGA measurements forcompounds the compounds In properties of the MOFs, the TGA measurements for the were were performed N2 atmosphere, the are results areinshown Figure S1. The slightloss weight loss performed under under N2 atmosphere, and theand results shown FigureinS1. The slight weight observed observed(up initially (up˝ C) 30–150 all the compounds be due to theofpresence of a small initially 30–150 for all°C) the for compounds are may beare duemay to the presence a small amount of ˝ amountwater of lattice water TGA molecules. studies indicated 1 andstable 2 areup quite stable up to 300 °C lattice molecules. studiesTGA indicated that 1 and 2 that are quite to 300 C (compound 3 ˝ (compound stable to 250 °C).observations These TGAhave observations haveconfirmed further been confirmed is stable up 3to is250 C).up These TGA further been through variousthrough in situ various in situ temperature PXRD measurements for all the compounds (Figures S2–S4). The various temperature PXRD measurements for all the compounds (Figures S2–S4). The various temperature temperature PXRD data indicated of Fe-MOF indicated that 1 wastoconverted Fe2O3 on heating to temperatures PXRD data of Fe-MOF that 1 was converted Fe2 O3 ontoheating to temperatures of 350 ˝ C of 350 °C (Figure and above and 3 were converted to their respective metal and above S2),(Figure whereasS2), the whereas MOFs 2 the and MOFs 3 were 2converted to their respective metal oxides (Al 2 O3 ˝ oxides In2Oto to 400S3 °C and (Figures and S4). the compounds heated 3) on and In2(Al O3 )2O on3 and heating 400heating C (Figures S4). S3 Further, theFurther, compounds were heatedwere at various at various temperatures forand onethehour and of thethe images of the compounds 1–3 afteratcalcination at various temperatures for one hour images compounds 1–3 after calcination various temperature temperature are given in Figures which that thedid compounds didmuch not undergo much up color are given in Figures S5–S7, whichS5–S7, show that theshow compounds not undergo color change to ˝ ˝ 1–3 at 600 °C were change up toto300 duethermal to their stability. high thermal The calcined products 300 C due their°Chigh The stability. calcined products of 1–3 at 600 of C were characterized characterized using PXRD measurements and compared with calculated PXRD patterns of metal their using PXRD measurements and compared with calculated PXRD patterns of their corresponding corresponding metal indicated oxides. The indicated that3 compounds 1 and were converted tometal their oxides. The results thatresults compounds 1 and were converted to 3their corresponding ˝ corresponding metal oxides (Fe2O3 and In2O3, respectively) during calcination at 600 °C for one hour oxides (Fe2 O3 and In2 O 3 , respectively) during calcination at 600 C for one hour (Figure 3), whereas the (Figure 3),was whereas the Al-MOF was converted to an unidentified amorphous Al-MOF converted to an unidentified amorphous phase material (Figure phase S8). material (Figure S8). (a)
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Figure 3. Comparison of PXRD patterns of 1 (a) and 3 (b) calcined at 600 ˝ C with calculated Figurepattern 3. Comparison of PXRD patterns of 1 (a) and 3 (b) calcined at 600 °C with calculated PXRD of their respective metal oxides. PXRD pattern of their respective metal oxides. The porous properties of the MOFs have been analyzed using N2 gas sorption measurements at 77K, The porous properties of the MOFs have been analyzed using N2 gas sorption measurements at 77K, which are shown in Figure 4. The results showed that compound 2 has a type-I adsorption isotherm which are shown in Figure 4. The results showed that compound 2 has a type-I adsorption isotherm which which is is characteristic characteristic of of microporous microporous material, material, and and the the BET BET and and Langmuir Langmuir surface surface area area of of 22 were were 2 calculated to be 155 and 162 m 2/g, respectively. The N2 adsorption isotherms of 1 and 3 indicated the calculated to be 155 and 162 m /g, respectively. The N2 adsorption isotherms of 1 and 3 indicated the 3+ 3+ non-porous non-porous nature nature of of these these materials, materials, which which may may be be due due to to the the bigger bigger ionic ionic radius radius of of the the Fe Fe3+ and and In In3+ 3+ metal However, the the CO CO22 gas gas sorption sorption measurements measurements showed showed that metal ions ions than than that that of of Al Al3+.. However, that compounds compounds 11 and and 33 have have aa porous porous nature. nature.
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Figure 4. The N22 gas adsorption isotherms for 1–3 at 77 K. Figure gas adsorption adsorption isotherms isotherms for for 1–3 1–3 at at 77 77 K. K. Figure 4. 4. The The N N2 gas
The sorption isotherms for 1–3 298 S9–S11. The CO222gas gas sorption isotherms forcompounds the compounds 1–3K at 273 K are andgiven 298in are given in The CO CO gas sorption isotherms for the the compounds 1–3 at at 273 273 K and and 298 K K are given inKFigures Figures S9–S11. Compounds 1–3 CO of mmol/g, 2.80 2.29 respectively, Figures S9–S11. Compounds 1–3 exhibited CO of 1.30and mmol/g, 2.80 mmol/g, and 2 adsorption Compounds 1–3 exhibited exhibited CO22 adsorption adsorption of 1.30 1.30 mmol/g, 2.80 mmol/g, mmol/g, and 2.29 mmol/g, mmol/g, respectively, at K, whereas mmol/g, and 1.27 mmol/g CO 2 adsorption 2.29 respectively, at mmol/g, 273 K, 11.57 atm; whereas 1.57 andwere 1.27observed mmol/g at 273 273mmol/g, K, 11 atm; atm; whereas 0.76 0.76 mmol/g, 1.57 mmol/g, and0.76 1.27mmol/g, mmol/g of of COmmol/g, 2 adsorption were observed for compounds 1–3, respectively, at K atm. of were observed for compounds 2 adsorption forCO compounds 1–3, respectively, at 298 298 K and and 11 1–3, atm. respectively, at 298 K and 1 atm. In nanoparticle-embedded carbon materials, the were used In order to preparemetal/metal metal/metaloxide oxide nanoparticle-embedded carbon materials, the MOFs In order order totoprepare prepare metal/metal oxide nanoparticle-embedded carbon materials, the MOFs MOFs were were used ˝ as templates. In a typical carbonization procedure, MOFs (1–3) were calcined at 800 °C under nitrogen used as templates. In a typical carbonization procedure, MOFs (1–3) were calcined at 800 C under as templates. In a typical carbonization procedure, MOFs (1–3) were calcined at 800 °C under nitrogen gas for h. characterized with of The nitrogen forfinal 5 h. products The finalwere products were characterized with the aid techniques. of various techniques. The gas flow flow gas for 55flow h. The The final products were characterized with the the aid aid of various various techniques. The carbonized carbonized products 1, 33 are 1c, respectively. were carbonized products from 1, designated 2 and 3 areas 2c and 3c, Compounds respectively. Compounds 1c–3c products from from 1, 22 and and are designated asdesignated 1c, 2c 2c and andas3c, 3c,1c, respectively. Compounds 1c–3c 1c–3c were initially initially characterized PXRD PXRD matching 1c Fe (Figure 3C 1c were initially using characterized using PXRDThe measurements. PXRDof with 5a) Fe characterized using PXRD measurements. measurements. The PXRD pattern patternThe matching ofpattern 1c with withmatching Fe and and Fe Feof 5a) 3C (Figure clearly indicated of carbide particles inside the matrix of 1c. At and Fe3 C (Figure the 5a) presence clearly indicated the iron presence of iron and iron carbide particles inside clearly indicated the presence of iron iron and and iron carbide particles inside the carbon carbon matrix of the 1c. carbon At the the same time, the PXRD pattern of 2c revealed its amorphous nature (Figure S12). Interestingly, the PXRD matrix of 1c. the same time, therevealed PXRD its pattern of 2c revealed its amorphous nature (Figure S12). same time, theAt PXRD pattern of 2c amorphous nature (Figure S12). Interestingly, the PXRD O (Figure 5b) suggested pattern matching of 3c with the theoretical PXRD patterns of In and In 2 3 Interestingly, the PXRD pattern matching ofPXRD 3c with the theoretical patterns5b) of suggested In and In2the O3 the pattern matching of 3c with the theoretical patterns of In and PXRD In2O3 (Figure presence of indium oxide particles inside the matrix 3c. (Figure theindium presence of indium indium oxide particles inside presence5b) of suggested indium and and indium oxide particlesand inside the carbon carbon matrix of of 3c. the carbon matrix of 3c. (a) (a)
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Figure 5. (a) Comparison of PXRD pattern of 1c with calculated PXRD patterns of Fe and Figure 5. (a) of pattern of 1c with PXRD patterns of and Figure 5.Comparison (a) Comparison Comparison of PXRD PXRD pattern 1c calculated with calculated calculated patterns of Fe Fe and Fe of PXRD pattern of 3c of with PXRDPXRD patterns of In and In2 O 3 C. (b) 3. Fe 3C. (b) Comparison of PXRD pattern of 3c with calculated PXRD patterns of In and In2O3. Fe3C. (b) Comparison of PXRD pattern of 3c with calculated PXRD patterns of In and In2O3.
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Further investigation of the local structure of the synthesized carbon materials (1c–3c) was carried investigation of the Raman local structure carbon materials was carried outFurther via Raman spectroscopy. spectra of of the the synthesized obtained carbon samples are (1c–3c) shown in Figure 6, out via Raman spectroscopy. Raman spectra of the obtained carbon samples are shown in Figure 6, ´1 ´1 exhibiting D and G bands centered around 1320 cm and 1600 cm , respectively, which is due the exhibiting and Gstructures bands centered 1320vibrations cm−1 andin1600 cm−1,directions respectively, which is due the disordered Dcarbon and thearound stretching opposite of two carbon atoms disordered carbon structures and theratios stretching opposite directions of twofound carbon in in a graphene sheet. The relative of thevibrations G band toin the D band (IG /ID ) were to atoms be 1.11, a1.15 graphene sheet. ratios of the G band to the Dthat bandin(Iall G/ID) were found to be 1.11, 1.15 and and 1.18 forThe 1c, relative 2c and 3c, respectively, suggesting samples the graphene sheets were 1.18 for 1c, 2c and 3c, respectively, suggesting that in all samples the graphene sheets were not not well-developed and the local carbon structures contained both graphitic and disordered carbon well-developed and the local carbon structures contained both graphitic and disordered carbon atoms [23]. atoms [23].
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Figure 6. Raman spectra of the obtained nanoporous carbon samples. Figure 6. Raman spectra of the obtained nanoporous carbon samples. Since compound 1c exhibited a strong interaction with a magnetic field (Figure S13), we were unable Since compound 1c exhibited a strong interaction with a magnetic field (Figure S13), we were unable to characterize it further using SEM, TEM or energy dispersive X-ray spectroscopy (EDS) analysis. The to characterize it further using SEM, TEM or energy dispersive X-ray spectroscopy (EDS) analysis. SEM and TEM images of 2c and 3c (Figure 7) suggested that the obtained nanoporous carbons retained a The SEM and TEM images of 2c and 3c (Figure 7) suggested that the obtained nanoporous carbons typical crystal morphology similar to that of the parent MOFs. Further, the SEM and TEM images of 3c retained a typical crystal morphology similar to that of the parent MOFs. Further, the SEM and TEM revealed that the surface of nanoporous carbon samples was embedded with indium oxide nanoparticles images of 3c revealed that the surface of nanoporous carbon samples was embedded with indium oxide (Figure 3b,d). nanoparticles (Figure 3b,d).
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Figure 7. The SEM images of 2c (a), and 3c (b) and the TEM images of 2c (c), and 3c (d). Figure 7. The SEM images of 2c (a), and 3c (b) and the TEM images of 2c (c), and 3c (d). The composition of the carbon matrix and the metal oxide nanoparticles were confirmed through EDS analysis, and the the results resultsare aregiven givenininFigures Figures8 8and andS14. S14. Surprisingly, EDS analysis of showed 2c showed analysis, and Surprisingly, thethe EDS analysis of 2c the the presence of aluminum oxide nanoparticles inside carbon matrix (Figure 8). the On other the other hand, presence of aluminum oxide nanoparticles inside the the carbon matrix (Figure 8). On hand, the the results of EDS analysis of 3c two differentSEM SEMimage imagelocations locationsclearly clearlyconfirms confirmsthe the presence presence of results of EDS analysis of 3c at at two different In222O of 3c 3c (Figure (Figure S14). S14). In O333 in in the the carbon carbon framework framework of
Figure 8. Cont. Figure 8. Cont.
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Element Weight Weight (%) (%) Atomic Atomic (%) (%) Element Element Weight (%) Atomic C 45.36 58.85(%) C 45.36 58.85 C 45.36 58.85 O 24.22 23.59 O 24.22 23.59 O 24.22 23.59 Al 30.42 17.57 Al 30.42 17.57 Al 30.42 17.57
Figure 8. The EDS analysis results for 2c. Figure Figure 8. 8. The The EDS EDS analysis analysis results results for for 2c. 2c. Nitrogen gas adsorption analysis was used to further characterize the nanoporous structure of 1c, 2c Nitrogen gas9). adsorption analysis was used usedindicated to further furtherthe characterize theboth nanoporous structure of 1c, 1c, 2c 2c gas adsorption was to characterize the nanoporous structure of andNitrogen 3c (Figure The shapeanalysis of the isotherms existence of micropores and mesopores. and 3c 3c (Figure 9). The of indicated of both micropores and and mesopores. and (Figure 9). The shape of the the isotherms isotherms indicated the the existence micropores The steep increase at shape low relative pressure indicates the existence presence of of both the micropores. Themesopores. isotherms The steep increase at low relative pressure indicates the presence of the micropores. The isotherms The steep increase at low relative pressure indicates the presence of the micropores. The isotherms showed a small hysteresis, which is typical for the presence of spherical mesopores randomly connected showed small hysteresis, The which is typical typical forcharacteristics the presence presence of ofofspherical spherical mesopores randomly connected showed aa small hysteresis, which is for the mesopores connected with weak microporosity. analyzed pore the carbon materialsrandomly are summarized in with weak microporosity. The analyzed pore characteristics of the carbon materials are summarized in with microporosity. pore characteristics carbon materials summarized in Tableweak 1. It is interesting toThe noteanalyzed that the porous properties of of thethe carbon materials areare better than that of Tableparent 1. ItIt isisMOF interesting totonote the ofofsome the areare better than that of Table 1. interesting note that the porous properties thecarbon carbon materials better than that their materials. Thisthat may beporous due toproperties the fact that of thematerials linker units of the MOFs have their parent MOF materials. This mayduring be due tocarbonization the that that some of the unitsthe ofresulting theofMOFs have of their parent MOF materials. may bethe due to fact the fact some oflinker themakes linker units the carbon MOFs gas process, which been removed in the form of CO 2This the carbonization process, which makes resulting carbon been been removed inporous thein form of COof 2 gas have removed the form COduring gas during the carbonization process, which the makes the resulting 2MOF materials more than the parent materials. materials more porous than thethan parent carbon materials more porous the MOF parentmaterials. MOF materials. 160
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Figure gas adsorption adsorption isotherms isotherms for Figure 9. 9. The The N N22 gas for 1c–3c 1c–3c at at 77 77 K. K. Figure 9. The N2 gas adsorption isotherms for 1c–3c at 77 K.
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Table 1. Pore characteristics of the synthesized nanoporous carbon materials. Materials
BET Surface Area (m2 /g)
Langmuir Surface Area (m2 /g)
Total Pore Volume (cm3 /g) P/Po „0.99
1c
155
261
0.28
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100
149
0.21
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236
342
0.16
4. Conclusions In summary, Fe-, Al-, and In-MOFs based on a 1,4-NDC linker have been prepared and characterized. Using these MOFs as precursor materials, metal/metal oxide nanoparticle-inserted nanoporous carbons were prepared via simple direct carbonization without using any additional carbon sources. The PXRD, SEM, TEM, EDS and nitrogen sorption measurements confirm the dispersion of metal/metal oxide nanoparticles in the resulting nanoporous carbon materials. It can be expected that these newly synthesized carbon composite materials have promising prospects for application in supercapacitors. Supplementary Materials Supplementary materials can be accessed at: http://www.mdpi.com/1996-1944/8/8/5336/s1. Acknowledgments Financial assistance received from the Bureau of Energy of the Ministry of Economic Affairs, Taiwan; Ministry of Science and Technology, Taiwan (MOST103-2632-M-033-001-MY3 and MOST104-2113-M-033-006); and Chung-Yuan Christian University are gratefully acknowledged. Author Contributions C.-H.L. conceived and designed the experiments; J.-J.C. and Y.-T.C. performed the experiments and analyzed the data; Y.-H.K. and P.-C.T. contributed reagents/materials/analysis tools; D.S.R. wrote the paper. Conflicts of Interest The authors declare no conflict of interest. References 1. Tong, X.; Qin, Y.; Guo, X.; Moutanabbir, O.; Ao, X.; Pippel, E.; Zhang, L.; Knez, M. Enhanced Catalytic activity for methanol electro-oxidation of uniformly dispersed nickel oxide nanoparticles-carbon nanotube hybrid materials. Small 2012, 8, 3390–3395. [CrossRef] [PubMed] 2. Sun, X. Microstructure and magnetic properties of carbon-coated nanoparticles. J. Dispersion Sci. Technol. 2003, 24, 557–567. [CrossRef]
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materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Metal-Organic Frameworks to Metal/Metal Oxide Embedded Carbon Matrix: Synthesis, Characterization and Gas Sorption Properties Jiun-Jen Chen 1 , Ya-Ting Chen 2 , Duraisamy Senthil Raja 2 , Yu-Hao Kang 1 , Pen-Chang Tseng 1 and Chia-Her Lin 2,3, * 1
Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan; E-Mails: [email protected] (J.-J.C.); [email protected] (Y.-H.K.); [email protected] (P.-C.T.) 2 Department of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan; E-Mails: [email protected] (Y.-T.C.); [email protected] (D.S.R.) 3 R&D Center for Membrane Technology & Research Center for Structure of Matter, Chung Yuan Christian University, Chung-Li 320, Taiwan * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-3-265-3315; Fax: +886-3-265-3399. Academic Editor: Rafael Luque Received: 5 July 2015 / Accepted: 11 August 2015 / Published: 19 August 2015
Abstract: Three isostructural metal-organic frameworks, (MOFs), [Fe(OH)(1,4-NDC)] (1), [Al(OH)(1,4-NDC)] (2), and [In(OH)(1,4-NDC)] (3) have been synthesized hydrothermally by using 1,4-naphthalene dicarboxylate (1,4-NDC) as a linker. The MOFs were characterized using various techniques and further used as precursor materials for the synthesis of metal/metal oxide nanoparticles inserted in a carbon matrix through a simple thermal conversion method. The newly synthesized carbon materials were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy analysis, powder X-ray diffraction and BET analysis. The results showed that the MOF-derived carbon composite materials maintained the morphology of the original MOF upon carbonization, and confirmed the insertion of metal/metal oxide particles in the carbon matrix.
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Keywords: metal-organic frameworks; 1,4-naphthalene dicarboxylate; nanoporous carbon; metal oxide nanoparticle; gas sorption
1. Introduction Metal/metal oxide nanoparticles have demonstrated distinct properties from their bulk materials and have shown promising applications in many fields such as catalysis [1], magnetism [2] and biochemistry [3]. Though extensive research has been done, the preparation of these nanomaterials still suffers from the aggregation problem. In order to resolve the aggregation issue, a new strategy has been developed in which these nanoparticles are dispersed into carbon matrices such as porous carbon, carbon nanotubes and graphenes [1–3]. The carbon matrices can not only block the nanoparticles from aggregation, but also modify their properties [1–4]. On the other hand, metal-organic frameworks (MOFs) are the new types of porous hybrid functional materials that are constructed by metal ions or metal clusters and organic linkers. Because of their high porosities and tunable structural properties, they have wide functional applications in terms of catalysis [5,6], gas storage [7,8], sensors [9], biomedicine [10], and so on. Apart from the use of MOFs as crystalline porous materials, the utilization of MOFs as a precursor for metal/metal oxide nanoparticle-embedded carbon frameworks has been researched actively in recent years [11–16]. The subject has become of particular interest for functional applications such as catalysis, gas storage materials, anode materials for lithium-ion batteries, etc. [13–16]. After the report by Q. Xu et al. on the carbonization of MOF-5 [11], various methods have been reported to synthesize materials that contain metal/metal oxide nanoparticle-embedded porous carbon matrix from MOF precursors [17–20]. Since carbon, metal and oxygen atoms are arranged periodically at the atomic level within MOF structures, the MOFs can be converted into metal/metal oxide nanoparticle-embedded carbon frameworks with a certain degree of permanent porosity, which is much useful for the aforementioned applications. Further, the synthesis of pure porous carbon materials has also been done using MOF as a precursor approach, which needs to remove the metal/metal oxide nanoparticles formed in situ. This can be done using many methods such as vaporization at high temperature, washing with an acid solution and so on. However, our aim is to synthesize the dispersed metal/metal oxide nanoparticles in porous carbon matrices without removing the metal particles formed in situ during calcination because of the fact that the MOFs are highly ordered materials, and the carbonized MOFs prepared from these MOFs are expected to induce homogeneously dispersed metal/metal oxide nanoparticles in the resulting porous carbon matrices. Furthermore, MOFs with numerous structural topologies and tunable pores will enable us to construct a variety of carbonized MOFs with different degrees of metal/metal oxide nanoparticle-embedded porous carbon matrices for various applications [21]. Though a few articles have been published on the synthesis of the dispersed metal/metal oxide nanoparticles in porous carbon matrices from MOFs, the synthesis of metal/metal oxide nanoparticle-embedded porous carbon hybrid functional materials using a simple, one-step thermal conversion is still a challenge. Hence, herein we report three new metal/metal oxide embedded porous carbon hybrid functional materials from three MOFs ([Fe(OH)(1,4-NDC)] (1), [Al(OH)(1,4-NDC)]
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(2), and [In(OH)(1,4-NDC)] (3)) of 1,4-naphthalene dicarboxylate (1,4-NDC) using a simple, one-step thermal cracking method. Further, the MOFs and their metal/metal oxide embedded carbon hybrid materials were characterized using various physico-chemical techniques, and their gas sorption properties have been studied. 2. Experimental Section All reagents were commercially available and used as received without further purification. In a typical synthesis of the MOFs, the mixture of FeCl3 (0.162 g, 1.0 mmol) or Al(NO3 )3 ¨ 9H2 O (0.375 g, 1.0 mmol) or In(NO3 )3 ¨ xH2 O (0.301 g, 1.0 mmol), H2 -1,4-NDC (0.108 g, 0.5 mmol), and H2 O (10 mL) was placed in a 23 mL Teflon autoclave and then heated at 180 ˝ C for 1 day. After filtering off and washing with distilled water, powder samples of MOFs were obtained. 1: color, white; yield, 0.104 g; elemental analysis, found/calcd.: C, 39.60/40.13; H, 4.16/4.21% for 1¨ 4H2 O. 2: color, white; yield, 0.117 g; elemental analysis, found/calcd.: C, 49.24/49.00; H, 3.67/3.77% for 2¨ 2H2 O. 3: color, white; yield, 0.113 g; elemental analysis, found/calcd.: C, 38.12/38.64; H, 2.65/2.70% for 3¨ 1.5H2 O. Conversely, the synthesis of metal/metal oxide nanoparticles within a porous carbon matrix involves a direct carbonization method using the above synthesized MOFs as precursors. The MOFs (0.200 g) were taken in a silica boat and then placed in a tube furnace and heated from room temperature to 800 ˝ C under N2 gas with a heating rate of 5 ˝ C¨ min´1 to carbonize the MOFs. After reaching the target temperature (800 ˝ C), the temperature was maintained at 800 ˝ C for 5 h, then cooled down to room temperature with a cooling rate of 1 ˝ C¨ min´1 . The final black-colored powder products were further characterized. 3. Results and Discussion The simple hydrothermal reactions of H2 -1,4-NDC (0.5 mmol) with corresponding metal salts (1.0 mmol) yielded the new MOFs. The new MOFs were initially characterized by elemental analysis, which agrees well with theoretical values. The as-synthesized MOFs may contain guest H2 O molecules which are removed by heating of these MOFs to 180 ˝ C for 12 h under vacuum („10´3 torr); the phase purity of the bulk materials were independently confirmed by powder X-ray diffraction (PXRD) measurements (Figure 1). Comparing the PXRD patterns of as-synthesized 1–3 with calculated PXRD patterns of reported aluminum naphthalenedicarboxylate (Al-1,4-NDC) MOF material [22] confirms that the these new MOFs (1–3) have the same type of structural architecture as Al-1,4-NDC (Figure 1a–c). Further comparison of these PXRD pattern of activated MOFs with one another showed that the three new MOFs are isostructural to each other (Figure 1d). It is to be noted that the diffraction peaks of compound 1 and 3 at 8.2˝ showed a slight shift from the calculated PXRD of Al-1,4-NDC, which may be due to the difference in the ionic radius of the metal ions (Al3+ < Fe3+ < In3+ ). The shape and microstructure of the resulting MOFs (1–3) were investigated via scanning electron microscopy (SEM) and images are presented in Figure 2. The SEM images of the compounds show that they are crystalline materials with an elongated block shape, with individual particle sizes of about 30 µm, 3 µm, and 15 µm (for 1, 2 and 3, respectively).
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Figure 1. 1. The The comparison comparison of of PXRD PXRD patterns patterns of of as-synthesized as-synthesized MOFs, MOFs, (a) 1, (b) 22 and (c) Figure (a) 1, 1, (b) (b) and (c) (c) 333 Figure 1. The comparison of PXRD patterns of as-synthesized MOFs, (a) 2 and with simulated simulated PXRD PXRD pattern pattern of of reported reported Al-1,4-NDC; Al-1,4-NDC; (d) (d) the the comparison comparison of of PXRD PXRD patterns patterns with with simulated PXRD pattern of reported Al-1,4-NDC; (d) the comparison of PXRD patterns of activated activated MOFs. of MOFs. of activated MOFs.
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(c) (c) Figure 2. The SEM images of 1 (a), 2 (b) and 3 (c). Figure 2. 2. The The SEM SEM images images of of 11 (a), (a), 22 (b) (b) and and 33 (c). (c). Figure
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In order ordertotostudy studythe thethermal thermal properties of the MOFs, the TGA measurements forcompounds the compounds In properties of the MOFs, the TGA measurements for the were were performed N2 atmosphere, the are results areinshown Figure S1. The slightloss weight loss performed under under N2 atmosphere, and theand results shown FigureinS1. The slight weight observed observed(up initially (up˝ C) 30–150 all the compounds be due to theofpresence of a small initially 30–150 for all°C) the for compounds are may beare duemay to the presence a small amount of ˝ amountwater of lattice water TGA molecules. studies indicated 1 andstable 2 areup quite stable up to 300 °C lattice molecules. studiesTGA indicated that 1 and 2 that are quite to 300 C (compound 3 ˝ (compound stable to 250 °C).observations These TGAhave observations haveconfirmed further been confirmed is stable up 3to is250 C).up These TGA further been through variousthrough in situ various in situ temperature PXRD measurements for all the compounds (Figures S2–S4). The various temperature PXRD measurements for all the compounds (Figures S2–S4). The various temperature temperature PXRD data indicated of Fe-MOF indicated that 1 wastoconverted Fe2O3 on heating to temperatures PXRD data of Fe-MOF that 1 was converted Fe2 O3 ontoheating to temperatures of 350 ˝ C of 350 °C (Figure and above and 3 were converted to their respective metal and above S2),(Figure whereasS2), the whereas MOFs 2 the and MOFs 3 were 2converted to their respective metal oxides (Al 2 O3 ˝ oxides In2Oto to 400S3 °C and (Figures and S4). the compounds heated 3) on and In2(Al O3 )2O on3 and heating 400heating C (Figures S4). S3 Further, theFurther, compounds were heatedwere at various at various temperatures forand onethehour and of thethe images of the compounds 1–3 afteratcalcination at various temperatures for one hour images compounds 1–3 after calcination various temperature temperature are given in Figures which that thedid compounds didmuch not undergo much up color are given in Figures S5–S7, whichS5–S7, show that theshow compounds not undergo color change to ˝ ˝ 1–3 at 600 °C were change up toto300 duethermal to their stability. high thermal The calcined products 300 C due their°Chigh The stability. calcined products of 1–3 at 600 of C were characterized characterized using PXRD measurements and compared with calculated PXRD patterns of metal their using PXRD measurements and compared with calculated PXRD patterns of their corresponding corresponding metal indicated oxides. The indicated that3 compounds 1 and were converted tometal their oxides. The results thatresults compounds 1 and were converted to 3their corresponding ˝ corresponding metal oxides (Fe2O3 and In2O3, respectively) during calcination at 600 °C for one hour oxides (Fe2 O3 and In2 O 3 , respectively) during calcination at 600 C for one hour (Figure 3), whereas the (Figure 3),was whereas the Al-MOF was converted to an unidentified amorphous Al-MOF converted to an unidentified amorphous phase material (Figure phase S8). material (Figure S8). (a)
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Figure 4. The N22 gas adsorption isotherms for 1–3 at 77 K. Figure gas adsorption adsorption isotherms isotherms for for 1–3 1–3 at at 77 77 K. K. Figure 4. 4. The The N N2 gas
The sorption isotherms for 1–3 298 S9–S11. The CO222gas gas sorption isotherms forcompounds the compounds 1–3K at 273 K are andgiven 298in are given in The CO CO gas sorption isotherms for the the compounds 1–3 at at 273 273 K and and 298 K K are given inKFigures Figures S9–S11. Compounds 1–3 CO of mmol/g, 2.80 2.29 respectively, Figures S9–S11. Compounds 1–3 exhibited CO of 1.30and mmol/g, 2.80 mmol/g, and 2 adsorption Compounds 1–3 exhibited exhibited CO22 adsorption adsorption of 1.30 1.30 mmol/g, 2.80 mmol/g, mmol/g, and 2.29 mmol/g, mmol/g, respectively, at K, whereas mmol/g, and 1.27 mmol/g CO 2 adsorption 2.29 respectively, at mmol/g, 273 K, 11.57 atm; whereas 1.57 andwere 1.27observed mmol/g at 273 273mmol/g, K, 11 atm; atm; whereas 0.76 0.76 mmol/g, 1.57 mmol/g, and0.76 1.27mmol/g, mmol/g of of COmmol/g, 2 adsorption were observed for compounds 1–3, respectively, at K atm. of were observed for compounds 2 adsorption forCO compounds 1–3, respectively, at 298 298 K and and 11 1–3, atm. respectively, at 298 K and 1 atm. In nanoparticle-embedded carbon materials, the were used In order to preparemetal/metal metal/metaloxide oxide nanoparticle-embedded carbon materials, the MOFs In order order totoprepare prepare metal/metal oxide nanoparticle-embedded carbon materials, the MOFs MOFs were were used ˝ as templates. In a typical carbonization procedure, MOFs (1–3) were calcined at 800 °C under nitrogen used as templates. In a typical carbonization procedure, MOFs (1–3) were calcined at 800 C under as templates. In a typical carbonization procedure, MOFs (1–3) were calcined at 800 °C under nitrogen gas for h. characterized with of The nitrogen forfinal 5 h. products The finalwere products were characterized with the aid techniques. of various techniques. The gas flow flow gas for 55flow h. The The final products were characterized with the the aid aid of various various techniques. The carbonized carbonized products 1, 33 are 1c, respectively. were carbonized products from 1, designated 2 and 3 areas 2c and 3c, Compounds respectively. Compounds 1c–3c products from from 1, 22 and and are designated asdesignated 1c, 2c 2c and andas3c, 3c,1c, respectively. Compounds 1c–3c 1c–3c were initially initially characterized PXRD PXRD matching 1c Fe (Figure 3C 1c were initially using characterized using PXRDThe measurements. PXRDof with 5a) Fe characterized using PXRD measurements. measurements. The PXRD pattern patternThe matching ofpattern 1c with withmatching Fe and and Fe Feof 5a) 3C (Figure clearly indicated of carbide particles inside the matrix of 1c. At and Fe3 C (Figure the 5a) presence clearly indicated the iron presence of iron and iron carbide particles inside clearly indicated the presence of iron iron and and iron carbide particles inside the carbon carbon matrix of the 1c. carbon At the the same time, the PXRD pattern of 2c revealed its amorphous nature (Figure S12). Interestingly, the PXRD matrix of 1c. the same time, therevealed PXRD its pattern of 2c revealed its amorphous nature (Figure S12). same time, theAt PXRD pattern of 2c amorphous nature (Figure S12). Interestingly, the PXRD O (Figure 5b) suggested pattern matching of 3c with the theoretical PXRD patterns of In and In 2 3 Interestingly, the PXRD pattern matching ofPXRD 3c with the theoretical patterns5b) of suggested In and In2the O3 the pattern matching of 3c with the theoretical patterns of In and PXRD In2O3 (Figure presence of indium oxide particles inside the matrix 3c. (Figure theindium presence of indium indium oxide particles inside presence5b) of suggested indium and and indium oxide particlesand inside the carbon carbon matrix of of 3c. the carbon matrix of 3c. (a) (a)
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Figure 5. (a) Comparison of PXRD pattern of 1c with calculated PXRD patterns of Fe and Figure 5. (a) of pattern of 1c with PXRD patterns of and Figure 5.Comparison (a) Comparison Comparison of PXRD PXRD pattern 1c calculated with calculated calculated patterns of Fe Fe and Fe of PXRD pattern of 3c of with PXRDPXRD patterns of In and In2 O 3 C. (b) 3. Fe 3C. (b) Comparison of PXRD pattern of 3c with calculated PXRD patterns of In and In2O3. Fe3C. (b) Comparison of PXRD pattern of 3c with calculated PXRD patterns of In and In2O3.
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Further investigation of the local structure of the synthesized carbon materials (1c–3c) was carried investigation of the Raman local structure carbon materials was carried outFurther via Raman spectroscopy. spectra of of the the synthesized obtained carbon samples are (1c–3c) shown in Figure 6, out via Raman spectroscopy. Raman spectra of the obtained carbon samples are shown in Figure 6, ´1 ´1 exhibiting D and G bands centered around 1320 cm and 1600 cm , respectively, which is due the exhibiting and Gstructures bands centered 1320vibrations cm−1 andin1600 cm−1,directions respectively, which is due the disordered Dcarbon and thearound stretching opposite of two carbon atoms disordered carbon structures and theratios stretching opposite directions of twofound carbon in in a graphene sheet. The relative of thevibrations G band toin the D band (IG /ID ) were to atoms be 1.11, a1.15 graphene sheet. ratios of the G band to the Dthat bandin(Iall G/ID) were found to be 1.11, 1.15 and and 1.18 forThe 1c, relative 2c and 3c, respectively, suggesting samples the graphene sheets were 1.18 for 1c, 2c and 3c, respectively, suggesting that in all samples the graphene sheets were not not well-developed and the local carbon structures contained both graphitic and disordered carbon well-developed and the local carbon structures contained both graphitic and disordered carbon atoms [23]. atoms [23].
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Figure 6. Raman spectra of the obtained nanoporous carbon samples. Figure 6. Raman spectra of the obtained nanoporous carbon samples. Since compound 1c exhibited a strong interaction with a magnetic field (Figure S13), we were unable Since compound 1c exhibited a strong interaction with a magnetic field (Figure S13), we were unable to characterize it further using SEM, TEM or energy dispersive X-ray spectroscopy (EDS) analysis. The to characterize it further using SEM, TEM or energy dispersive X-ray spectroscopy (EDS) analysis. SEM and TEM images of 2c and 3c (Figure 7) suggested that the obtained nanoporous carbons retained a The SEM and TEM images of 2c and 3c (Figure 7) suggested that the obtained nanoporous carbons typical crystal morphology similar to that of the parent MOFs. Further, the SEM and TEM images of 3c retained a typical crystal morphology similar to that of the parent MOFs. Further, the SEM and TEM revealed that the surface of nanoporous carbon samples was embedded with indium oxide nanoparticles images of 3c revealed that the surface of nanoporous carbon samples was embedded with indium oxide (Figure 3b,d). nanoparticles (Figure 3b,d).
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Figure 8. Cont. Figure 8. Cont.
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Element Weight Weight (%) (%) Atomic Atomic (%) (%) Element Element Weight (%) Atomic C 45.36 58.85(%) C 45.36 58.85 C 45.36 58.85 O 24.22 23.59 O 24.22 23.59 O 24.22 23.59 Al 30.42 17.57 Al 30.42 17.57 Al 30.42 17.57
Figure 8. The EDS analysis results for 2c. Figure Figure 8. 8. The The EDS EDS analysis analysis results results for for 2c. 2c. Nitrogen gas adsorption analysis was used to further characterize the nanoporous structure of 1c, 2c Nitrogen gas9). adsorption analysis was used usedindicated to further furtherthe characterize theboth nanoporous structure of 1c, 1c, 2c 2c gas adsorption was to characterize the nanoporous structure of andNitrogen 3c (Figure The shapeanalysis of the isotherms existence of micropores and mesopores. and 3c 3c (Figure 9). The of indicated of both micropores and and mesopores. and (Figure 9). The shape of the the isotherms isotherms indicated the the existence micropores The steep increase at shape low relative pressure indicates the existence presence of of both the micropores. Themesopores. isotherms The steep increase at low relative pressure indicates the presence of the micropores. The isotherms The steep increase at low relative pressure indicates the presence of the micropores. The isotherms showed a small hysteresis, which is typical for the presence of spherical mesopores randomly connected showed small hysteresis, The which is typical typical forcharacteristics the presence presence of ofofspherical spherical mesopores randomly connected showed aa small hysteresis, which is for the mesopores connected with weak microporosity. analyzed pore the carbon materialsrandomly are summarized in with weak microporosity. The analyzed pore characteristics of the carbon materials are summarized in with microporosity. pore characteristics carbon materials summarized in Tableweak 1. It is interesting toThe noteanalyzed that the porous properties of of thethe carbon materials areare better than that of Tableparent 1. ItIt isisMOF interesting totonote the ofofsome the areare better than that of Table 1. interesting note that the porous properties thecarbon carbon materials better than that their materials. Thisthat may beporous due toproperties the fact that of thematerials linker units of the MOFs have their parent MOF materials. This mayduring be due tocarbonization the that that some of the unitsthe ofresulting theofMOFs have of their parent MOF materials. may bethe due to fact the fact some oflinker themakes linker units the carbon MOFs gas process, which been removed in the form of CO 2This the carbonization process, which makes resulting carbon been been removed inporous thein form of COof 2 gas have removed the form COduring gas during the carbonization process, which the makes the resulting 2MOF materials more than the parent materials. materials more porous than thethan parent carbon materials more porous the MOF parentmaterials. MOF materials. 160
1c 2c 1c 3c 2c 3c
160 140 140 120
3
3
(cm Vabs V (cm /g) /g) abs
120 100 100 80 80 60 60 40 40 20 20 0 0
0.0 0.0
0.2 0.2
0.4
0.6
Relative pressure0.6P/P0 0.4
0.8
1.0
0.8
1.0
Relative pressure P/P0
Figure gas adsorption adsorption isotherms isotherms for Figure 9. 9. The The N N22 gas for 1c–3c 1c–3c at at 77 77 K. K. Figure 9. The N2 gas adsorption isotherms for 1c–3c at 77 K.
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Table 1. Pore characteristics of the synthesized nanoporous carbon materials. Materials
BET Surface Area (m2 /g)
Langmuir Surface Area (m2 /g)
Total Pore Volume (cm3 /g) P/Po „0.99
1c
155
261
0.28
2c
100
149
0.21
3c
236
342
0.16
4. Conclusions In summary, Fe-, Al-, and In-MOFs based on a 1,4-NDC linker have been prepared and characterized. Using these MOFs as precursor materials, metal/metal oxide nanoparticle-inserted nanoporous carbons were prepared via simple direct carbonization without using any additional carbon sources. The PXRD, SEM, TEM, EDS and nitrogen sorption measurements confirm the dispersion of metal/metal oxide nanoparticles in the resulting nanoporous carbon materials. It can be expected that these newly synthesized carbon composite materials have promising prospects for application in supercapacitors. Supplementary Materials Supplementary materials can be accessed at: http://www.mdpi.com/1996-1944/8/8/5336/s1. Acknowledgments Financial assistance received from the Bureau of Energy of the Ministry of Economic Affairs, Taiwan; Ministry of Science and Technology, Taiwan (MOST103-2632-M-033-001-MY3 and MOST104-2113-M-033-006); and Chung-Yuan Christian University are gratefully acknowledged. Author Contributions C.-H.L. conceived and designed the experiments; J.-J.C. and Y.-T.C. performed the experiments and analyzed the data; Y.-H.K. and P.-C.T. contributed reagents/materials/analysis tools; D.S.R. wrote the paper. Conflicts of Interest The authors declare no conflict of interest. References 1. Tong, X.; Qin, Y.; Guo, X.; Moutanabbir, O.; Ao, X.; Pippel, E.; Zhang, L.; Knez, M. Enhanced Catalytic activity for methanol electro-oxidation of uniformly dispersed nickel oxide nanoparticles-carbon nanotube hybrid materials. Small 2012, 8, 3390–3395. [CrossRef] [PubMed] 2. Sun, X. Microstructure and magnetic properties of carbon-coated nanoparticles. J. Dispersion Sci. Technol. 2003, 24, 557–567. [CrossRef]
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