Benzoate Acid-Dependent Lattice Dimension of Co-MOFs and MOF

Article pubs.acs.org/IC

Benzoate Acid-Dependent Lattice Dimension of Co-MOFs and MOFDerived CoS2@CNTs with Tunable Pore Diameters for Supercapacitors Kang-Yu Zou, Yi-Chen Liu, Yi-Fan Jiang, Cheng-Yan Yu, Man-Li Yue, and Zuo-Xi Li* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry and Material Sciences, Northwest University, Xi’an 710069, P.R. China S Supporting Information *

ABSTRACT: Herein three novel cobalt metal−organic frameworks (Co-MOFs) with similar ingredients, [Co(bib)(o-bdc)]∞ (1), [Co2(bib)2(m-bdc)2]∞ (2), and {[Co(bib)(pbdc)(H2O)](H2O)0.5}∞ (3), have been synthesized from the reaction of cobalt nitrate with 1,4-bis(imidazol-1-yl)benzene (bib) and structure-related aromatic acids (1,2-benzenedicarboxylic acid = o-bdc, 1,3-benzenedicarboxylic acid = m-bdc, and 1,4-benzenedicarboxylic acid = p-bdc) by the solvothermal method. It is aimed to perform systematic research on the relationship among the conformation of benzoate acid, lattice dimension of CoMOF, and pore diameter of MOF-derived carbon composite. Through the precursor strategy, Co-MOFs 1−3 have been utilized to synthesize porous cobalt@carbon nanotube composites (Co@CNTs). After the in situ gas-sulfurization, secondary composites CoS2@ CNTs were successfully obtained, which kept similar morphologies of corresponding Co@ CNTs without destroying previous highly dispersed structures. Co-MOFs and two series of composites (Co@CNTs and CoS2@CNTs) have been well characterized. Topology and Brunauer−Emmett−Teller analyses elucidate that the bdc2− ion could control the pore diameters of MOF-derived carbon composites by adjusting the lattice dimension of Co-MOFs. The systematic studies on electrochemical properties demonstrate that (p)-CoS2@CNT possesses hierarchical morphology, moderate specific surface area, proper pore diameter distribution, and high graphitization, which lead to remarkable specific capacitances (839 F g−1 at 5 mV s−1 and 825 F g−1 at 0.5 A g−1) in 2 M potassium hydroxide solution. In addition, the (p)-CoS2@CNT electrode exhibits good electrochemical stability and still retains 82.9% of initial specific capacitance at the current density of 1 A g−1 after 5000 cycles.

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INTRODUCTION Supercapacitors, famous as electrochemical capacitors with high-energy storages, high-power transportations, and good cycle features, are regarded as the most favorable power for portable electronic devices and electric vehicles.1 Because of the increasing requirement of power and energy in human society, the development of new electrode materials for prominent energy storages are urgently desired.2 Carbon materials, such as activated carbon and carbon nanotubes (CNTs), which have a large specific surface area, high electric conductivity, and ecofriendly nature, are extensively investigated in electric double-layer capacitors (EDLCs), but they are limited in low specific capacitance and energy density.3 Cobalt sulfides, as a kind of typical pseudocapacitive material with various oxidation states for charge transfer, can achieve relatively high specific capacitance, but they suffer from intrinsic drawbacks of low specific surface area and electric conductivity.4 Thus, more and more research is focused on the cobalt sulfide/carbon composite to overcome the inherent deficiency of single material and explore novel promising materials for supercapacitors.5 Metal−organic frameworks (MOFs) with perfect selfassembly of metal centers and organic blocks, possess great advantages, such as a highly ordered architecture, ultrahigh © 2017 American Chemical Society

specific surface, adjustable pore diameter, and diverse topology.6 Nowadays, MOFs have ever been appropriately employed as sacrificial precursors or templates for synthesizing highly dispersed metal/metallic compound/carbon materials or composites with high specific surface areas and thermal stability by the calcination-thermolysis method.7 Besides, MOF-derived materials could retain the original morphologies of MOFs under proper conditions.8 Although the calcination-thermolysis strategy sheds new light on the fabrication of functional materials, and rapid progress has witnessed many MOF-derived high-performance materials, there are still many uncertain and troublesome problems to be resolved.9 Because of calcination process operated in a furnace, just like a black box, the thermolysis reaction cannot be directly monitored, and it is very difficult to control the characteristics of MOF-derived materials (such as the morphology, surface area, and porosity). Therefore, it is greatly necessary to explore regular synthesis concepts and strategies for artificially controllable syntheses of MOF-derived materials.10 Our group is focusing the continuing efforts on the calcination criterion of MOFs and MOF-derived materials for Received: January 23, 2017 Published: May 19, 2017 6184

DOI: 10.1021/acs.inorgchem.7b00200 Inorg. Chem. 2017, 56, 6184−6196

Article

Inorganic Chemistry supercapacitors.11 In this work, under the cooperation of ancillary ligand 1,4-bis(imidazol-1-yl)benzene (bib), we employed a series of benzoate acids to prepare three cobalt metal−organic frameworks (Co-MOFs) with similar ingredients, [Co(bib)(o-bdc)]∞ (1), [Co2(bib)2(m-bdc)2]∞ (2), and {[Co(bib)(p-bdc)(H2O)](H2O)0.5}∞ (3) (1,2-benzenedicarboxylic acid = o-bdc, 1,3-benzenedicarboxylic acid = m-bdc, and 1,4-benzenedicarboxylic acid = p-bdc). Then a sequence of Co@CNTs were well prepared via the calcination-thermolysis of Co-MOF precursors, which were followed by the in situ gassulfurization to afford CoS2@CNTs. The as-synthesized products have been well characterized, and the effects of benzoate acids on the lattice dimension of Co-MOFs and pore diameters of carbon composites have also been given a detail discussion. Significantly, the electrochemical performance of CoS2@CNT electrode in the supercapacitor has been intensively studied, and the (p)-CoS2@CNT electrode shows excellent electrochemical performance.

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2113w, 1536s, 1381s, 1307s, 1138m, 1063s, 1010m, 951m, 845s, 747s, 647s, 529s. Powder samples of 1−3 were obtained by a modified method as used in the syntheses of single crystals. 0.3 g of polyvinylpyrrolidone (PVP, K30) as surfactant was added into the solvothermal reaction before being heated. Then the autoclave was naturally cooled to room temperature. Syntheses of Co@CNTs and CoS2@CNTs. Co-MOF powder samples 1−3 are pure and easily yielded in large-scale, which are conveniently utilized as precursors to further prepare MOF-derived materials by the calcination process. First, 1−3 were put into a tube furnace and then calcinated under N2 flow at 900 °C for 3 h with the rate of 5 °C min−1. After the tube furnace naturally cooled down, black products were obtained and labeled as (o)-Co@CNT, (m)-Co@CNT, and (p)-Co@CNT, respectively. Two combustion boats were loaded with 0.5 g of sublimate sulfur powder and 0.5 g of Co@CNT, which were placed in the upstream and downstream terminals of tube furnace, respectively. Following, the tube furnace was heated under N2 gas at 400 °C for 3 h with a rate of 5 °C min−1. As a result, (o)-CoS2@CNT, (m)-CoS2@CNT, and (p)CoS2@CNT were obtained correspondingly after the tube furnace naturally cooled to room temperature. X-ray Data Collection and Structure Determinations. Singlecrystal X-ray diffraction data of 1−3 were recorded on a Rigaku MM007/Saturn 70 with graphite monochromatic Mo−Kα radiation (λ = 0.71073 Å). The software SAINT13 was utilized for integration of the diffraction patterns. All structures have been solved by the direct method with SHELXS and refined by the full-matrix least-squares method with SHELXL in the SHELXTL package.14 The cobalt atom was placed according to the E-map, and other heavy atoms were generated by the successive difference Fourier synthesis and refined with anisotropic thermal parameters based on F2. The hydrogen atom of organic compound was theoretically added onto the mother atom and refined isotropically. Nevertheless, the water hydrogen atom in 3 was generated by the difference Fourier map and refined by a riding model. Further structural analyses are concluded in Table 1, and coordinated bond lengths and angles are summarized in Table S1. Fabrication of Electrodes. A mixture of 8 mg of CoS2@CNT powder, 1 mg of acetylene black, and 1 mg of polyvinylidene fluoride binder were ground thoroughly with ethanol in an agate mortar until a homogeneous black slurry was acquired. The resulting slurry was subsequently brushed into a nickel foam (size: 1 cm2, thickness: 2 mm). After that, the coated nickel foam was dried at 120 °C for 12 h and pressed under 10 MPa pressure with a nickel wire. Then the nickel foam-coated electrode is already prepared for the following electrochemical measurements. Electrochemical Measurements. The electrochemical measurements were carried out by a CHI660E electrochemical workstation and 2 M standard KOH solution as an aqueous electrolyte with a three-electrode system. Saturated calomel electrode (SCE) and platinum wire were employed as the reference and counter electrodes, respectively. The as-prepared nickel foam-based electrode was applied as the working electrode, and the detail electrode fabrication was described above. Cyclic voltammograms (CV) were conducted within the potential range from −0.3 to 0.5 V at various scan rates. Galvanostatic charge−discharge (GCD) curves were obtained within a potential window from 0 to 0.4 V at different current densities. The long-period cycle performance of working electrode was assessed by the GCD measurement at the current density of 1 A g−1 in 2 M KOH aqueous solution. Electrochemical impedance spectrometry (EIS) was recorded at the open circuit voltage in the frequency range between 100 kHz and 10 mHz.

EXPERIMENTAL SECTION

Materials and General Methods. All commercially available chemicals and solvents are reagent grade and used without further purification. The ligand bib was synthesized by the method reported previously.12 Elemental analyses for C, H, and N were taken on a PerkinElmer 240C analyzer. IR spectra were recorded on a TENSOR 27 (Bruker) Fourier transform infrared spectrometer by using KBr pellets. Powder X-ray diffraction (XRD) was performed on a Rigaku D/Max-2500 diffractometer, at 40 kV, 100 mA with Cu Kα radiation (λ = 1.5418 Å). The accurate cobalt content was analyzed by an atomic absorption spectrophotometer (AAS, Thermo Scientific, SOLLAR M6). The morphologies and structures of as-prepared samples were characterized with field emission scanning electron microscopy (FESEM) (HITACHI, S-4800) with an energy dispersive X-ray detector (EDX) at an accelerating voltage of 20 kV, and high resolution transmission electron microscopy (HRTEM) (FEI TECNAI G2) with 200 kV. X-ray photoelectron spectroscopy (XPS) was performed by utilizing an apparatus (Thermo Scientific, K-Alpha) with an Al Ka X-ray source. A Brunauer−Emmett−Teller (BET) surface analyzer (Tri Star-3020, Micromeritics, USA) was employed to conduct the porous features of as-obtained materials. Raman spectra were carried out by using an inVia confocal Raman microscope (Renishaw Co., England) with an Ar ion laser (514.5 nm excitation wavelength). Syntheses of 1−3. [Co(bib)(o-bdc)]∞ (1). Suitable single crystals of 1 were prepared by the solvothermal reaction. The mixture of Co(NO3)2·6H2O (0.12 mmol), bib (0.1 mmol), and o-H2bdc (0.1 mmol) in the CH3OH/H2O solvent (4:3, 12 mL) was put into a Teflon-lined autoclave and heated to 145 °C for 2 days. After the autoclave was cooled to room temperature at 5 °C h−1, purple block single crystals for X-ray analysis were gained. The mother liquor was filtered, and single crystals were washed with ethanol (8 mL × 3) and dried in air (yield: ca. 40% based on bib). Anal. Calcd for C20H14CoN4O4: C, 55.44; H, 3.26; N, 12.93. Found: C, 55.23; H, 3.32; N. 13.05%. IR (KBr, cm−1): 3431w, 3123m, 2439w, 1542s, 1553s, 1440s, 1310m, 1251m, 1135w, 1073m, 956m, 838m, 760m, 659m, 538w, 470w. Suitable single crystals of 2 and 3 were fabricated by a similar procedure as stated above, except o-H2bdc was replaced by m-H2bdc and p-H2bdc, respectively. [Co2(bib)2(m-bdc)2]∞ (2). Yield: ∼50% (based on bib). Anal. Calcd for C40H28Co2N8O8: C, 55.44; H, 3.26; N, 12.93. Found: C, 55.28; H, 3.31; N, 12.78%. IR (KBr, cm−1): 3439s, 3256m, 2410w, 1604m, 1531s, 1392s, 1309m, 1251m, 1123w, 1065s, 951w, 846m, 727s, 652s, 538w, 441w. {[Co(bib)(p-bdc)(H2O)](H2O)0.5}∞ (3). Yield: ∼45% (based on bib). Anal. Calcd for C20H17CoN4O5.5: C, 52.19; H, 3.72; N, 12.17. Found: C, 51.43; H, 3.81; N, 12.29%. IR (KBr, cm−1): 3454s, 3132s, 2543m,

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RESULTS AND DISCUSSION Synthesis Consideration. Herein three structure-related benzenedicarboxylic acids, o-bdc, m-bdc, and p-bdc, were selected as the primary ligand to perform a systematic investigation on the structural variety of MOFs based on the cobalt-bib substrate. Interestingly, the conformational disparity 6185


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which is closely related to the pore diameter of MOF-derived materials. Furthermore, cobalt species in parent MOFs could highly catalyze the graphitization of carbon during the calcination process at high temperature.15 Thereby, a series of Co-MOFs were used as precursors to synthesize Co@CNTs. Significantly, the sublimed sulfur possesses a low sublimation temperature of about 250 °C, and easily sublimed sulfur is readily reacted with cobalt species to yield CoS2, which has a very high performance in supercapacitors.16 Therefore, it is a valuable synthetic strategy for the preparation of highly and evenly dispersed CoS2@CNTs with adjustable pore diameters by varying ligand backbones, which could be adapted to many other similar systems. Crystal Structure of 1. Co-MOF 1 crystallizes in the monoclinic space group P21/c with one crystallographically unique CoII ion in the asymmetric unit. Each CoII ion is tetrahedrally coordinated by two monodentate carboxylic O atoms of distinct o-bdc2− linkers and two imidazole sp2 N atoms from individual bib ligands (Figure 1a). The Co−O bond lengths are 2.037(2) and 2.080(2) Å, and Co−N bond lengths are 2.071(2) and 2.080(2) Å, which are all comparable to those typically observed.17 Furthermore, a slightly longer bond length of 2.287(2) Å indicates that a weak interaction exists between the CoII ion and one carboxylate O atom. However, this additional connector makes no difference on the topology analysis, which could be ignored. The fully deprotonated o-bdc2− ion behaves as a V-form ligand with two monodentate COO−, which bridges the CoII center to form a 1D W-type chain (Figure 1b). Each bib ligand, taking the trans-coordinated conformation, connects two CoII ions of two adjacent 1D chains, respectively. Finally, the 1D chain is linked by bib to generate a 3D framework (Figure 1c).

Table 1. Crystallographic Data and Structure Refinement Parameters for 1−3 Co-MOFs

1

2

3

chemical formula formula weight crystal system space group a (Å) b (Å) c (Å) α β (deg) γ V (Å3) Z Dcalcd. (g cm−3) μ (mm−1) F(000) reflns collected/ unique R(int) R1a [I > 2σ (I)] wR2b GOF Δρmax/Δρmin (e Ǻ −3)

C20H14CoN4O4

C40H28Co2N8O8

C40H34Co2N8O11

433.28 monoclinic P21/c 10.438(2) 11.775(2) 17.966(5) 90 119.20(2) 90 1927.5(7) 4 1.493 0.925 884 15916/3395

866.56 triclinic P1̅ 11.030(9) 12.028(9) 16.110(12) 102.985(12) 92.832(13) 111.390(13) 1919(3) 2 1.500 0.929 884 17750/6528

920.61 triclinic P1̅ 8.7625(18) 9.7232(19) 12.622(3) 71.18(3) 89.07(3) 79.75(3) 1000.7(4) 1 1.528 0.901 472 8753/3517

0.0410 0.0442

0.0384 0.1044

0.0304 0.0353

0.0882 1.214 0.378/−0.388

0.2953 1.051 2.077/−1.556

0.0816 1.072 0.261/−0.318

a

R = Σ||F0| − |Fc||/Σ|F0|. bRw = [Σ[w(F02 − Fc2)2]/Σw(F02)2]1/2.

of benzenedicarboxylic acid provides an opportunity to investigate how to control the lattice dimension of MOFs,

Figure 1. View of the (a) coordination environment of CoII ion (hydrogen omitted for clarity); (b) 1D CoII-carboxylic chain; (c) 3D framework; (d) diamondoid net in 1 (the blue and green lines demonstrate the o-bdc2− and bib linkers, respectively). 6186


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Inorganic Chemistry The topological analysis was used to study the characteristics of this complicated architecture. The o-bdc2− and bib ligands are considered as linear linkers, and so the CoII center is viewed as a 4-connected tetrahedral node, which looks like the carbon atom in the diamond (Figure 1d). In other words, the topological simplification affords a diamondoid net with the Schäfli symbol of [66]. Crystal Structure of 2. Co-MOF 2 crystallizes in the triclinic space group P1,̅ which includes two crystallographically unique CoII ions in the asymmetric unit. However, two CoII ions exhibit similar tetrahedral coordination geometry (Figure 2a). They are surrounded by two monodentate COO− of

The fully deprotonated m-bdc2− ion acts as a V-form ligand with two monodentate COO−, which bridges the CoII center to form a 1D linear chain [Co(m-bdc)]∞. Meanwhile, the bidentate ligand bib, adopting a cis-coordinated conformation, connects the CoII node into another 1D linear chain [Co(cisbib)]∞. As shown in Figure 2b, these two types of 1D chains are aligned in a criss-cross mode to produce a 2D (4,4) layer. Moreover, two neighboring layers are entangled with each other by using the phenyl ring of the m-bdc2− ion to yield an interdigitated architecture. Interestingly, these bilayer architectures are stacked tightly in a parallel pattern via the van der Waals interaction to form a 3D supramolecular structure (Figure 2c). Crystal Structure of 3. Co-MOF 3 crystallizes in the triclinic space group P1̅. The asymmetric unit is composed of a crystallographically unique CoII ion, a p-bdc2− ion, a bib ligand, one and half a water molecules. The CoII ion holds the CoO4N2 deformed octahedral geometry, which is provided by three pbdc2− O atoms, two bib N atoms, and one water molecule (Figure 3a). The Co−O/N bond lengths range from 2.061(2) to 2.225(2) Å, comparable to the above-reported parameters.

Figure 2. View of the (a) coordination environment of the CoII ion; (b) 2D (4,4) layer; (c) 3D supramolecular framework in 2. Two neighboring entangled layers are separated by different colors.

Figure 3. View of the (a) coordination environment of the CoII ion; (b) 3D mixed-ligand framework in 3.

The completely deprotonated p-bdc2− ligands adopt two types of coordination patterns, one-half being the bis(monodentate) mode and the other half for the bis(chelating) mode. That is to say, each p-bdc2− ion bridges two CoII ions to afford a 1D chain, which is subsequently connected by the trans-bib ligand into a 3D framework (Figure 3b). The topological simplification on the 3D architecture results into a 4-connected framework with the Schäfli symbol of [66]. On the basis of the topology analysis, the 3D architecture represents a diamondoid framework (Figure 4a). The bulky

distinct m-bdc2− ions, and two imidazole groups of individual bib ligands. The coordination bond lengths around the CoII ion range from 2.039(5) to 2.095(7) Å, which are also in accord with those reported values of related CoII coordination polymers (Table S1). Similarly, the weak interactions between the CoII ion and carboxylic O atom with bond lengths of 2.296(7), 2.337(7), 2.408(8), and 2.436(7) Å are ignored, because they have no effect on the topology structure. 6187


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Figure 4. View of the (a) diamondoid framework (the blue and green lines demonstrate the p-bdc2− and bib linkers, respectively); (b) 5-fold interpenetrating network in 3.

Figure 5. XRD patterns of (a) Co@CNTs; (b) CoS2@CNTs.

reflections from (111), (200), (210), (211), (220), (311), (222), (230), and (321) crystal planes of CoS2 (JCPDS Card No. 41-1471), respectively.20 Meanwhile, the (002) diffraction of CNTs is still clearly observed in the XRD patterns of CoS2@ CNTs. Raman spectra illustrate two characteristic peaks approximately located at 1350 and 1585 cm−1 (Figure 6), which are indexed to D (the disordered structure of carbon materials) and G bands (the opposite vibration of two carbon atoms in a graphite sheet), respectively.21 The value of ID/IG is a significant parameter for assessing the graphitization degree of carbon materials. The ID/IG value of (p)-Co@CNT is 0.82, lower than those of other two Co@CNTs, which demonstrates that (p)-Co@CNT possesses the highest graphitization degree in Co@CNTs. After sulfurization, CoS2@CNTs keep the graphitization degrees of corresponding Co@CNTs, which suggests that (p)-CoS2@CNT owes the highest graphitization degree in CoS2@CNTs. The ID/IG values of CNTs in this work are considerable, which are larger than those of highly graphitized nanoporous carbon derived from ZIF-6722 (even down to 0.5), but lower than those of nanoporous carbon derived from Al-MOF23 (about 1.0). To further explore the element composition and complex state, XPS measurements of CoS 2 @CNTs have been implemented. As shown in Figure 7a, the XPS survey spectra for all samples show obvious peaks of O 1s at 530.9 eV, C 1s at 284.3 eV, S 2s at 230.8 eV, and S 2p at 162.7 eV.24 Moreover,

linkers p-bdc2− and trans-bib act as the lattice edges to generate a large adamantane cage with Co···Co separations of about 13.8, 13.7 Å, 11.5, and 10.9 Å (Figure S1). The potential void space in the adamantane cage is filled with another four independent equivalent frameworks. Thereby, Co-MOF 3 illustrates a 5-fold entangled diamondoid network (Figure 4b). Furthermore, the remnant space within the interpenetrating network is occupied by the free water molecule. Characterization and Morphology. The XRD experiments were first carried out, which were aimed to verify the phase purity of powder samples 1−3. The powder sample pattern is in good agreement with the corresponding simulated one, which reveals that the high purity of powder sample (Figure S2). The black products after the calcination were also investigated by XRD. In Figure 5a, three sharp peaks were located at 44.2°, 51.5°, and 75.8° in all XRD patterns, matching well with the (111), (200), and (220) crystal planes of the cubic species of cobalt (JCPDS Card No. 15-0806).18 In addition, an unobvious broad peak at around 26° was also observed, assigning to the (002) diffraction of CNTs.19 Therefore, a series of Co@CNTs were successfully prepared from the calcination-thermolysis of Co-MOFs 1−3. Significantly, CoS2@CNTs have been obtained by simple sulfuration treatments, and also characterized by XRD. Figure 5b displays nine sharp peaks located at about 28.0°, 32.4°, 36.5°, 40.0°, 46.5°, 55.1°, 57.6°, 60.3°, and 62.8°, which were assigned to the 6188


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Figure 6. Raman spectra of (a) Co@CNTs; (b) CoS2@CNTs.

the deconvoluted high resolution spectrum of Co 2p for (p)CoS2@CNT shows two characteristic peaks at 778.8 and 794.1 eV, which are attributed to the Co 2p3/2 and Co 2p1/2 states, respectively (Figure 7b). The shakeup feature at higher binding energies confirms the Co2+ state.25 The peaks at 780.8 and 801.2 eV are satellite peaks. Following, the high resolution S 2p spectrum of (p)-CoS2@CNT has been analyzed (Figure 7c), and the binding energies at 162.5 and 163.8 eV are identical to S 2p3/2 and S 2p1/2, respectively. The peak of 163.8 eV in the S 2p spectrum reveals the valence is −1, which demonstrates the existence of S22− species in the sulfurized composite.26 There is also a peak of sulfur and oxygen bonding at 168.6 eV, which might derive from the surface absorption of O species. In addition, the deconvolution of Co 2p and S 2p spectra for (o)CoS2@CNT and (m)-CoS2@CNT are shown in Figure S3. The XPS research illustrates that CoS2@CNTs are successfully synthesized, which are in good accordance with XRD results. The morphologies of Co-MOFs and carbon-based composites were initially studied by FESEM. Co-MOF powder clearly appears as tiny quadrangular blocks (Figure S4), and the synthesis experiments elucidate that the particles will be smaller and more uniform by the addition of PVP. After calcinations of Co-MOFs at 900 °C, (o)-Co@CNT presents as agminated and scorch mud, while (m)-Co@CNT and (p)-Co@CNT retain the origin appearances of corresponding Co-MOFs, but with a smaller size and rough surface. We speculate that Co-MOF 1

Figure 7. View of (a) XPS survey spectra of CoS2@CNTs; (b) Co 2p spectrum of (p)-CoS2@CNT; (c) S 2p spectrum of (p)-CoS2@CNT.

may be melted before decomposition, which loses the origin shape of tiny crystal. Interestingly, various CNTs grow on the rough surface of Co@CNTs, and highly dispersed cobalt particles are embedded in the carbon matrix (Figure S5). The cobalt species in parent MOFs can catalyze the graphitization during the calcination process and plays a very crucial role during the formation of CNTs.15 After the in situ sulfurization of Co@CNTs, the Co species is transformed into CoS2 and 6189


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Inorganic Chemistry leads to the generation of CoS2@CNTs, which keep the morphologies of corresponding Co@CNTs without destroying the highly dispersed structure, especially CNTs almost the same as before (Figure 8). The EDX analyses illustrate that the

Figure 9. TEM and HRTEM images of (a, b) (m)-CoS2@CNT; (c, d) (p)-CoS2@CNT.

distribution plot of (o)-Co@CNT only shows a predominant peak centered at 4 nm, revealing the scarcity of meso-/macroporosity (Figure 10b). After gas-sulfuration, the N2 adsorption−desorption isothermal of CoS2@CNTs shows some differences (Figure 10c). The BET specific surface areas of CoS2@CNTs have been obviously decreased and are lower than those of corresponding Co@CNTs, which may be caused by the larger size of CoS2 particle and so lower porosities in CoS2@CNTs. The values of BET specific surface area are 28, 8, and 18 m2 g−1 for (o)-CoS2@CNT, (m)-CoS2@CNT, and (p)CoS2@CNT, respectively. Moreover, pore volumes of (o)CoS2@CNT, (m)-CoS2@CNT, and (p)-CoS2@CNT are 0.33, 0.25, and 0.38 cm3 g−1, respectively. In addition, pore diameter distribution plots show that (p)-CoS2@CNT possesses abundant porosity and wide range pore diameter (Figure 10d), which contributes to intercalation and deintercalation of electrolyte ions, quickly reaching the maximum accessible surface.28 Effects of Benzoate Acids on the Pore Diameters of Composites. Co-MOFs 1−3 illustrate a series of mixed-ligand frameworks based on bib and three kinds of benzoate acids. The CoII ion keeps a 4-connected center, and the bib ligand maintains a bidentate connecting mode, which promotes us to perform systematic research on the influence of benzoate acid. Interestingly, all bdc2− ions own two carboxylic groups with different positions on the identical benzene ring, and thus our study could be focused on the orientation of −COOH. The bdc2− ion connects two CoII ions by using two −COO− groups bonded with one CoII ion for each, and the linkages Cobdc-Co associated with Co-bib-Co serve as the lattice edges of Co-MOFs. The separation of Co···Co bridged by bib is almost invariable, but the span of Co···Co connected by the bdc2− ion could be greatly different. Therefore, the lattice dimension is mainly determined by the bdc2− ion. In Co-MOF 1, the separation of Co−o-bdc−Co is about 6.3 Å. However, the spans of Co−m-bdc−Co and Co−p-bdc−Co in 2 and 3 are ca. 8.9 and 11.2 Å, respectively. 1 and 3 both exhibit the

Figure 8. FESEM images of (a, b) (o)-CoS2@CNT; (c, d) (m)CoS2@CNT; (e, f) (p)-CoS2@CNT.

Co/S ratio of CoS2@CNTs is about 0.5, and elemental mapping images demonstrate that the C, O, Co, and S species are uniformly dispersed in CoS2@CNTs (Figures S6−S8), which are well matched with XRD and XPS results. Moreover, the accurate Co contents of all MOF-derived materials are concluded in Table S2. Owing to the 1-derived composite being bulky and thick scorch sample, it is improper for TEM measurements, so only 2- and 3-derived composites have been further investigated by TEM analyses. As shown in Figures S9 and 9, the Co and CoS2 particles are embedded in the carbon matrix, and surrounded by plenty of CNTs with different diameters. Moreover, the size of CoS2 particle becomes larger than that of Co particle after sulfuration. Significantly, characteristic lattice fringes 0.2 and 0.34 nm are detected in the HRTEM images of Co@CNTs, which are assigned to the (111) crystal plane of cobalt and (002) plane of graphite carbon, respectively (Figure S9b). Interestingly, long highquality CNTs were generated, which are shown in Figure S10. For CoS2@CNTs, besides the lattice fringe 0.34 nm of CNTs, the HRTEM images clearly display a lattice fringe 0.31 nm for the (111) spacing of the highly crystalline CoS2 phase, which are still wrapped in the graphitic carbon layer (Figure 9b). The porous features of Co@CNTs and CoS2@CNTs were assessed by N2 adsorption−desorption isothermal analyses at 77 K (Table 2). As seem from Figure 10a, Co@CNTs exhibit the classical type-IV curve with an obvious type-H3 hysteresis loop from 0.45 to 1.0 of P/P0 values, which reveals the presence of meso-/macro-porosity.27 Moreover, an uptake at very low pressure shows the existence of micropores. (o)-Co@CNT has a larger BET specific surface area (186 m2 g−1) than those of other Co@CNTs; however, the simple pore diameter 6190


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Inorganic Chemistry Table 2. Summaries of the Porosity Parameters for As-Prepared Materials sample

SBETa (m2 g−1)

Vtotalb (cm3 g−1)

Vmicroc (cm3 g−1)

Vmeso+macrod (cm3 g−1)

pore size (nm)

(o)-Co@CNT (m)-Co@CNT (p)-Co@CNT (o)-CoS2@CNT (m)-CoS2@CNT (p)-CoS2@CNT

186 33 99 28 8 18

0.118 0.050 0.20 0.032 0.025 0.038

0.046 0.006 0.017 0.003 0.002 0.003

0.072 0.044 0.183 0.029 0.023 0.035

2.57 6.11 8.16 4.58 12.8 13.6

a Calculated from the BET surface area analysis. bThe total pore volume is calculated at relative pressure of 0.97. cVmirco referring to micropore volume calculated by using t-plot (FHH) method. dVmeso+macro referring to meso and macro pore volumes determined by subtracting micropore volume from the total pore volume.

Figure 10. View of the (a, c) N2 adsorption−desorption isotherms; (b, d) pore diameter distribution curves.

lattice dimension of MOFs could exert a profound effect on the pore diameter of calcinated products. Generally, MOFs with bigger lattice dimension could result in larger pore diameters after calcination. As seen from the pore diameter distribution, all Co@CNTs show a common peak at ca. 4 nm (Figure 10b). (o)-Co@CNT exhibits poor porosity in the region of above 10 nm, and (p)-Co@CNT shows more abundant porosity than (m)-Co@CNT in the meso-/macropore region (10−140 nm). The order of pore diameters is consistent with the lattice dimension of Co-MOFs, also with the separation of Co−bdc− Co. Thus, the title research demonstrates a feasible strategy for tuning the pore diameter of MOF-derived material by varying the backbone of H2bdc.

diamondoid topology and are suitable to elucidate the influence of the bdc2− ion on the size of adamantane cage. The length of Co−p-bdc−Co in 3 is much larger than that of 1, which makes the cage much bigger and capable of being penetrated by the other four equivalent frameworks. However, 1 represents a nonpenetrated diamondoid framework due to a small cage caused by the short length of Co−o-bdc−Co. With a 2D (4,4) layer, different from the diamondoid net, we can also deduce that 2 shows middle lattice dimension between 1 and 3 (Figure 11). During the syntheses of carbon composites by calcinationthemolysis of MOFs, the metal-free organic component could be directly turned into carbon. Thereby, as a basic unit, the 6191


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Figure 11. Correlation among the backbone of bdc2− ion, lattice dimension of Co-MOFs, and pore diameters of Co@CNTs.

Electrochemical Properties. Because of the hierarchical morphology and CoS2 nanoparticles highly dispersing in the carbon matrix, CoS2@CNTs may have good supercapacitive performance.29 Hence, the electrochemical properties of CoS2@CNT electrode were intensively investigated by CV, GCD, and cycling life tests in a three-electrode configuration in 2 M potassium hydroxide electrolyte. To confirm the potential application of CoS2@CNTs in the supercapacitor, CV and GCD profiles of CoS2@CNT electrodes have been studied within a proper potential window at a scan rate of 10 mV s−1 and a density current of 1 A g−1, respectively. As shown in Figure 12a, CV curves exhibit these distinct redox peaks, different from the ideal rectangular CV figure in ELDCs,30 which illustrates that the capacitance merits are primarily determined by faradaic redox reactions. The faradic redox reactions result from the reversible conversion among the multiple cobalt states at the CoS2 surface in the alkaline KOH electrolyte.31 The mechanism for charge storage of CoS2@CNT electrodes may be explained by the following redox reactions:

C=

∫ I dV /2υΔVm

(1)

where C (F g−1) is the specific capacitance, I (A) is the instant current, ΔV (V) is the potential change, υ (mV s−1) is the scan rate, and m (g) is the mass of active material. After calculation, the specific capacitance of the (p)-CoS2@CNT electrode is 839 F g−1 at 5 mV s−1, which obviously decreases to 447 F g−1 when the scan rate increases to 100 mV s−1. Moreover, the CV curve of 100 mV s−1 shows some deformation, which may result from the limitation of mass transfer or ionic transport at high scan rates. The specific capacitance of GCD curves from 0.5 to 10 A g−1 (Figure 13b) was deduced from the discharge curve by using eq 2:32 C = I Δt /ΔVm

(2)

where C (F g−1) represents the mass specific capacitance, I (A) the discharge current, ΔV (V) the potential change during the discharge time Δt (s), and m (g) the mass of active material. The maximum specific capacitance of the (p)-CoS2@CNT electrode is up to 825 F g−1 at 0.5 A g−1. The specific capacitances of (p)-CoS2@CNT electrode decrease to 782, 550, 375, and 268 F g−1, as the current densities increase to 1, 2, 5, and 10 A g−1, respectively. It is well-known that the decrease of capacitance with the increase of current density is mainly caused by the low usage of active materials at large charge−discharge current densities.33 A negligible dissymmetry is detected in the GCD profiles, which may be attributed to the incompletely reversible redox reaction during the electrochemical measurements.34 Significantly, the (p)-CoS2@CNT electrode still retains 82.9% of initial specific capacitance (687 F g−1) at a current density of 1 A g−1 after 5000 cycles (Figure 13c), demonstrating the (p)-CoS2@CNT electrode displays moderate long-term cycle period and reversibility.

CoS2 + OH− ↔ CoS2OH + e− CoS2 OH + OH− ↔ CoS2O + H 2O + e−

Moreover, as shown in Figure 12, (p)-CoS2@CNT exhibits the largest area from the CV image and longest discharging time from the GCD curve, which indicate that (p)-CoS2@CNT possesses the highest specific capacitance. Hence, the supercapacitive properties of (p)-CoS2@CNT electrode were further investigated by a series of electrochemical measurements in the 2 M potassium hydroxide solution. Figure 13a illustrates CV profiles of (p)-CoS2@CNT at varied scan rates from 5 to 100 mV s−1. The specific capacitance originated from CV profiles at different scan rates can be calculated according to eq 1:32 6192


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Figure 12. View of (a) CV curves of CoS2@CNTs at 10 mV s−1; (b) GCD curves of CoS2@CNTs at 1 A g−1.

For the (o)-CoS2@CNT and (m)-CoS2@CNT electrodes, their CV and GCD curves are shown in Figure S11. All specific capacitance values of CoS2@CNT electrodes are summarized in Table S3 and Figure 14. Obviously, the specific capacitance of (p)-CoS2@CNT electrode is larger than those of (o)-CoS2@ CNT and (m)-CoS2@CNT electrodes. To further explore the electrochemical behaviors, EIS measurements of Co-MOFs and MOF-derived materials were conducted at the open circuit potential between 100 kHz and 10 mHz (Figure S12 and Figure S13). The Nyquist plots Co-MOFs 1−3 almost show a curve with tremendous impedance values, which demonstrates that Co-MOFs exhibit poor electrical conductivity.35 In addition, the Nyquist plots of all MOF-derived materials mainly consist of a semicircle and straight line in the highfrequency and low-frequency regions, respectively, which illustrates relatively low impedance values for the enhanced electroconductibility. The intersection point of above curves with the axis of real impedance exhibits a combined resistance of the intrinsic resistance of active material, ionic resistance of electrolyte, and contact resistance at the active material/current collector interface (Rs).36 The Rs values are 1.41 Ω, 1.53 Ω, and 1.38 Ω for (o)-CoS2@CNT, (m)-CoS2@CNT, and (p)-CoS2@ CNT electrodes, respectively. Obviously, (p)-CoS2@CNT shows a lower equivalent-series resistance, more vertical slope, and smaller semicircle diameter than other two CoS2@ CNTs, which indicate better conductivity, quicker ion diffusion, and lower charge-transfer resistance for improving the

Figure 13. (a) CV curves at different scan rates; (b) GCD curves at various current densities; (c) cyclic stability at 1 A g−1 for the (p)CoS2@CNT electrode.

capacitance. The above results are associated with moderate surface area and abundant inherent porosity of (p)-CoS2@ CNT electrode, which could provide enough interfaces, rapid mass transfer, and smooth electrolyte diffusion. On the other hand, graphitic crystallization CNTs could enhance the electron transfer during the electrochemical process.37 It is worth mentioning that (o)-CoS2@CNT exhibits the highest BET surface area, but the specific capacitance is the lowest, which may be caused by the singlet pore-size distribution, poor 6193


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zuo-Xi Li: 0000-0002-9683-862X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21673177) and Natural Science Foundation of Shannxi (2014JQ2057 and 2016KJXX-67).

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(1) (a) Forse, A. C.; Merlet, C.; Griffin, J. M.; Grey, C. P. New perspectives on the charging mechanisms of superpcacitors. J. Am. Chem. Soc. 2016, 138, 5731−5744. (b) Zhi, M.; Xiang, C.; Li, J.; Li, M.; Wu, N. Nanostructured carbon-metal oxide composite electrodes for supercapacitors: a review. Nanoscale 2013, 5, 72−88. (c) Wang, Y.; Song, Y.; Xia, Y. Electrochemical capacitors: mechanism, materials, systems, characterization and applications. Chem. Soc. Rev. 2016, 45, 5925−5950. (2) (a) Huang, J.; Wang, J.; Wang, C.; Zhang, H.; Lu, C.; Wang, J. Hierarchical porous graphene carbon-based supercapacitors. Chem. Mater. 2015, 27, 2107−2113. (b) Liu, Y.; Zhou, J. Y.; Chen, L. L.; Zhang, P.; Fu, W. B.; Zhao, H.; Ma, Y. F.; Pan, X. J.; Zhang, Z. X.; Han, W. H.; Xie, E. Q. Highly flexible freestanding porous carbon nanofibers for electrodes materials of high-performance all-carbon supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 23515−23520. (3) (a) Yan, Y.; Miao, J.; Yang, Z.; Xiao, F. X.; Yang, H. B.; Liu, B.; Yang, Y. Carbon nanotube catalysts: recent advances in synthesis, characterization and applications. Chem. Soc. Rev. 2015, 44, 3295− 3346. (b) Yang, Z.; Ren, J.; Zhang, Z.; Chen, X.; Guan, G.; Qiu, L.; Zhang, Y.; Peng, H. Recent advancement of nanostructured carbon for energy applications. Chem. Rev. 2015, 115, 5159−5223. (c) Chen, T.; Dai, L. Flexible supercapacitors based on carbon nanomaterials. J. Mater. Chem. A 2014, 2, 10756−10775. (4) (a) Rakhi, R. B.; Alhebshi, N. A.; Anjum, D. H.; Alshareef, H. N. Nanostructured cobalt sulfide-on-fiber with tunable morphology as electrodes for asymmetric hybrid supercapacitors. J. Mater. Chem. A 2014, 2, 16190−16198. (b) Liu, S.; Mao, C.; Niu, Y.; Yi, F.; Hou, J.; Lu, S.; Jiang, J.; Xu, M.; Li, C. Facile synthesis of novel networked ultralong cobalt sulfide nanotubes and its application in supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 25568−25573. (c) Li, H.; Gao, Y.; Shao, Y.; Su, Y.; Wang, X. Vapor-phase atomic layer deposition of Co9S8 and its application for supercapacitors. Nano Lett. 2015, 15, 6689−6695. (d) Liu, Y.; Li, Y.; Kang, H.; Jin, T.; Jiao, L. Design, synthesis, and energy-related applications of metal sulfides. Mater. Horiz. 2016, 3, 402−421. (5) (a) Higgins, D. C.; Hassan, F. M.; Seo, M. H.; Choi, J. Y.; Hoque, M. A.; Lee, D. U.; Chen, Z. Shape-controlled octahedral cobalt disulfide nanoparticles supported on nitrogen and sulfur-doped graphene/carbon nanotube composites for oxygen reduction in acidic electrolyte. J. Mater. Chem. A 2015, 3, 6340−6350. (b) Peng, S.; Li, L.; Mhaisalkar, S. G.; Srinivasan, M.; Ramakrishna, S.; Yan, Q. Hollow nanospheres constructed by CoS2 nanosheets with a nitrogen-dopedcarbon coating for energy-storage and photocatalysis. ChemSusChem 2014, 7, 2212−2220. (c) Han, F.; Tan, C. Y. J.; Gao, Z. Template-free formation of carbon nanotube-supported cobalt sulfide@carbon hollow nanoparticles for stable and fast sodium ion storage. J. Power Sources 2017, 339, 41−50. (6) (a) Li, J. R.; Yakovenko, A. A.; Lu, W.; Timmons, D. J.; Zhuang, W.; Yuan, D.; Zhou, H. C. Ligand bridging-angle-driven assembly of molecular architectures based on quadruply bonded Mo-Mo dimers. J. Am. Chem. Soc. 2010, 132, 17599−17610. (b) Tian, D.; Chen, Q.; Li, Y.; Zhang, Y. H.; Chang, Z.; Bu, X. H. A mixed molecular building block strategy for the design of nested polyhedron metal-organic

Figure 14. Comparison of specific capacitances of CoS2@CNT electrodes at different current densities.

meso-/macro-porosity for mass transfer and largest intrinsic resistance.

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CONCLUSION In this work, three new Co-MOFs with similar ingredients were synthesized to perform systematic research on the role of the bdc2− ion in the self-assembly of MOFs and preparation of MOF-derived materials. The results reveal that the backbone of the bdc2− ion exerts an important effect on the lattice dimension of Co-MOFs and finally controls the pore diameters of MOF-derived Co@CNTs. After the in situ gas-sulfurization, the Co species is transformed into CoS2, and corresponding products CoS2@CNTs keep the morphologies of Co@CNTs without destroying the highly dispersed structures. The extensive characterizations show that (p)-CoS2@CNT possesses hierarchical morphology, moderate specific surface area, proper pore diameter distribution, and high graphitization, which are all beneficial to the supercapacitive performance. (p)CoS2@CNT demonstrates remarkable specific capacitances of 839 F g−1 at 5 mV s−1 and 825 F g−1 at 0.5 A g−1 in 2 M potassium hydroxide electrolyte. In addition, (p)-CoS2@CNT electrode still maintains a specific capacitance of 687 F g−1 (17.1% decay) at the current density of 1 A g−1 after 5000 cycles, which displays good electrochemical stability.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00200. Coordinated bond lengths and angles, and additional structural profiles; XRD and FESEM images of CoMOFs; FESEM and HRTEM images of Co@CNTs; the accurate Co contents of all MOF-derived materials; XPS data and EDX results of CoS2@CNTs; the Nyquist plots of all materials; supercapacitor behaviors of (o)-CoS2@ CNT and (m)-CoS2@CNT, and further comparison (PDF) X-ray crystallographic information files (CIF) 6194


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