Hierarchically Flower-like N-Doped Porous Carbon Materials

Article pubs.acs.org/IC

Hierarchically Flower-like N‑Doped Porous Carbon Materials Derived from an Explosive 3‑Fold Interpenetrating Diamondoid Copper Metal−Organic Framework for a Supercapacitor Zuo-Xi Li,*,†,‡ Kang-Yu Zou,† Xue Zhang,† Tong Han,† and Ying Yang† †

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 ‡ Shannxi Key Laboratory of Phytochemistry, Baoji 721016, P. R. China S Supporting Information *

ABSTRACT: A peculiar copper metal−organic framework (Cu-MOF) was synthesized by a self-assembly method, which presents a 3-fold interpenetrating diamondoid net based on the square-planar CuII node. Although it exhibits a high degree of interpenetration, the Cu-MOF still exhibits a one-dimensional channel, which provides a template for constructing porous materials through the “precursor” strategy. Furthermore, the explosive ClO4− ion, which resided in the channel, could induce the quick decomposition of organic ingredients and release a huge amount of gas, which is beneficial for the porosity of postsynthetic materials. Significantly, we first utilize this explosive MOF to prepare a series of Cu@C composites through the calcination−thermolysis method at different temperatures, which contain copper particles exhibiting various shapes and combinations with the carbon substrate. Considering the hole-forming effect of copper particles, Cu@C composites were etched by HCl to afford a sequence of hierarchically flower-like N-doped porous carbon materials (NPCs), which retain the original morphology of the Cu-MOF. Interestingly, NPC-900, originating from the calcination of the Cu-MOF at 900 °C, exhibits a more regular flower-like morphology, the largest specific surface area, abundant porosities, and multiple nitrogen functionalities. The remarkable specific capacitances are 138 F g−1 at 5 mV s−1 and 149 F g−1 at 0.5 A g−1 for the NPC-900 electrode in a 6 M potassium hydroxide aqueous solution. Moreover, the retention of capacitance remains 86.8% (125 F g−1) at 1 A g−1 over 2000 cycles, which displays good chemical stability. These findings suggest that NPC-900 can be applied as a suitable electrode for a supercapacitor.

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of MOFs.10 Furthermore, the nitrogen element doped in carbon materials can improve the surface polarity and humidity, and finally the electric conductivity.11 The calcination of MOFs with N-containing ligands has proven to be a facile and efficient method for synthesizing N-doped carbon materials. Thereby, it is very important to develop regular synthesis protocols and ideas by employing MOFs as precursors to fabricate N-doped porous carbon materials (NPCs) with stable porous structures, large surface areas, and unique morphologies.12 As a long rodlike unit due to two sp2 N endings, rigid bis(imidazole) ligands have been widely explored in the construction of MOFs, and we also focus our continuing efforts on this area.13 In this work, 1,4-bis(imidazol-1-yl)benzene (bib) has been employed as a rodlike spacer to construct a new MOF {[Cu(bib)2(H2O)2](ClO4)2(H2O)}∞ (Cu-MOF), which exhibits a peculiar 3-fold interpenetrating diamondoid network based on the square-planar copper(II) node. A series of Cu@C composites were successfully fabricated by the calcination of explosive Cu-MOF at different temperatures. After the acid etching of Cu@C composites, a sequence of hierarchically flower-like NPCs were obtained, which have been characterized by X-ray

INTRODUCTION The continuing enthusiasm for metal−organic frameworks (MOFs) stems from not only the intriguing architecture1 but also the possible applications in gas adsorbents,2 luminescent labels,3 and molecular magnets.4 The extensive investigation during the past decade has demonstrated that interpenetration is often encountered in the self-assembly of porous MOFs.5 Definitely, porosity and interpenetration are two closely related aspects. In general, with a decrease in the size of the pore, interpenetration also enhances the thermal stability and enlarges the internal surface area of MOFs.6 Thus, intensive studies of interpenetration could provide crucial clues for the synthesis of porous MOFs. Besides intrinsic physicochemical properties, because of longterm ordered structures, large specific surfaces, adjustable pores, and various morphologies, MOFs have even been appropriately employed as sacrificial precursors to provide carbon and metal sources.7 Hence, many designed materials (such as carbon and metal/metal oxides) with large specific surface areas, high thermal stability, and good applications in a supercapacitor have been created by calcination and thermolysis of MOFs.8 Significantly, MOF-derived materials could retain their inherent morphologies of the original MOFs under proper conditions.9 Recently, plenty of porous carbon has been fabricated by calcination © XXXX American Chemical Society

Received: March 25, 2016

A

DOI: 10.1021/acs.inorgchem.6b00746 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. Views of (a) the 3-fold interpenetrating diamondoid network and (b) the 1D irregular channel. TG-DTA analyzer under a N2 atmosphere with a heating rate of 10 °C min−1 in the range of 25−600 °C. The morphologies and structures of all synthesized products were observed by FESEM (Hitachi, S-4800) with an accelerating voltage of 20 kV and HRTEM (FEI TECNAI G2) at 200 kV. Energy-disperse spectroscopy (EDS) elemental mapping images were all recorded by FESEM (JEOL 7800F at 20 kV). XPS data were recorded by using an apparatus (Thermo Scientific, K-Alpha) with an Al Ka X-ray source. The porous features of as-prepared materials were evaluated by a BET surface analyzer (Tri Star-3020, Micromeritics). Raman spectra were recorded on an inVia confocal Raman microscope (Renishaw Co.) employing an argon ion laser with an excitation wavelength of 514.5 nm. Synthesis of Cu-MOF Crystals and Powder. The Cu-MOF crystals were obtained by the solvent-diffusing method. A 6 mL aqueous solution of Cu(ClO4)2·6H2O (0.02 mmol) was placed at the bottom of a glass tube. Then a component solvent of C2H5OH and H2O (1:1, 8 mL), as a buffer layer, was carefully added. At last, a solution of bib (0.06 mmol) in C2H5OH (6 mL) was layered upon the buffer layer, and the glass tube was fixed at an undisturbed place. After ∼3 weeks, blue block single crystals appeared at the buffer layer. Yield: ∼55% (based on bib). Anal. Calcd for C24H26Cl2CuN8O11: C, 39.11; H, 3.56; N, 15.20. Found: C, 38.92; H, 3.61; N, 15.11. IR (KBr, cm−1): 3556m, 3136s, 2968w, 1620w, 1529s, 1309m, 1250m, 1107s, 1070s, 960m, 835m, 737m, 625m, 544w, 496w. The Cu-MOF powder was obtained as follows. The ligand bib and Cu(ClO4)2·6H2O were dissolved in ethanol and deionized water to obtain a clear solution, respectively. Subsequently, the raw solution was mixed while being stirred at room temperature for 2 h. Then, blue powder precipitated. The powder was centrifuged and rinsed with deionized water and ethanol three times. Finally, the powder was evacuated at 80 °C under vacuum for 12 h.

Figure 1. (a) Square-planar coordination environment and (b) 3D diamondoid framework.

photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Brunauer−Emmett−Teller (BET) surface analysis, etc. Finally, MOF-derived NPCs have been developed as good electrodes of supercapacitors through intensive electrochemical measurements.

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EXPERIMENTAL SECTION

Materials and General Methods. All the commercially available solvents and reagents were analytical grade and used without any purification. The ligand bib was synthesized according to the reported method.14 Elemental analyses for C, H, and N were performed on a PerkinElmer 240C analyzer. Infrared (IR) spectra were recorded on a TENSOR 27 (Bruker) FT-IR spectrometer with KBr pellets. Powder X-ray diffraction (XRD) was performed on a Rigaku D/Max-2500 diffractometer with Cu Kα radiation (40 kV, 100 mA, λ = 0.15405 nm). Thermogravimetry analysis was performed on a Rigaku standard B


Article

Inorganic Chemistry

Figure 4. (a) FT-IR and (b) Raman spectra of NPCs. Figure 3. XRD patterns of (a) Cu@C composites and (b) NPCs. located from the E-maps, and other high-weight atoms were projected in successive difference Fourier syntheses, which are all anisotropically refined. H atoms in organic parts were produced theoretically on the parent atoms and refined with isotropic thermal parameters. However, water H atoms were added by difference Fourier maps and refined with a riding pattern. Cu-MOF exhibits a one-dimensional (1D) aperture with a porosity of 41.3%, which is resided by the ClO4− ion and H2O molecule. Guest molecules are isolated and dispersed without any obvious contacts. Without hydrogen bonds between the water molecule and other parts, there is no proper acceptor for the O−H donor, which causes two B level checkCIF alerts. Crystallographical data: C24H26Cl2CuN8O11, fw = 736.97, monoclinic C2/c, a = 16.159(3) Å, b = 14.035(3) Å, c = 15.438(3) Å, β = 109.09(3)°, V = 3308.7(11) Å3, Z = 4, Dc = 1.479 g/cm3, R1 = 0.0623, wR2 = 0.1570 (for I > 2σ), and GOF = 1.123. Fabrication of Electrodes. First, 1 mg of NPC powder (active material), 1 mg of acetylene black (conducting agent), and 1 mg of polyvinylidine fluoride (PVDF, binder) were mixed uniformly. Then a few drops of ethanol was added in an agate mortar and ground thoroughly until a homogeneous black slurry was obtained. Then, the as-prepared slurry was pasted on a piece of nickel foam (size of 1 cm2, thickness of 2 mm). After that, the coated nickel foam was dried at 120 °C for 12 h to remove the solvent and pressed under a 10 MPa pressure with a nickel wire to form an electric connector. Finally, the nickel foam-coated electrode was prepared for electrochemical performance measurements. Electrochemical Performance Measurements. All electrochemical characterizations were evaluated using a CHI660E electrochemical workstation (Chenhua Instrument Co. Ltd., Shanghai, China)

Synthesis of Cu@C Composites and NPCs. The Cu-MOF powder is pure and easily produced on a large scale, so it is conveniently used as a precursor to further synthesize materials by the calcination−thermolysis process. The Cu-MOF powder was placed in a furnace and heat-treated under a N2 flow at 600, 750, and 900 °C for 12 h with a heating rate of 0.6 °C min−1. After the sample had naturally cooled to room temperature, black powder was obtained. Cu@C composites prepared at the three carbonization temperatures were labeled as Cu@C-600, Cu@C-750, and Cu@C-900, respectively. For the preparation of NPCs, the as-synthesized Cu@C composites were stirred in concentrated hydrochloric acid for 24 h to remove the copper particles.15 Consequently, the samples were extensively washed with deionized water and ethanol. Finally, the resulting NPCs were dried under vacuum for 24 h at 80 °C (correspondingly, denoted as NPC-600, NPC-750, and NPC-900, respectively). Caution! The perchlorate compounds should be handled caref ully, because of the potential explosion. Thus, during the TG-DTA research, as little Cu-MOF as possible must be used, and only 2.5 mg of powder was supplied, which still caused a slight explosion. Simultaneously, only 60 mg of Cu-MOF powder was used as a precursor sample at a very low heating rate of 0.6 °C min−1 in this work, with the aim of preventing the explosion. X-ray Diffraction Measurements. Single-crystal diffraction of Cu-MOF was performed on a Rigaku MM-007/Saturn 70 instrument with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). SAINT16 was used to integrate the diffraction profiles. The initial structure was calculated by direct methods under the SHELXS module of the SHELXTL software and optimized by full-matrix least-squares methods under the SHELXL module.17 The CuII ion in Cu-MOF was C


Article

Inorganic Chemistry

Figure 5. XPS survey spectra of (a) Cu@C-900 (inset, high resolution of Cu 2p) and (b) NPCs. and a 6 M potassium hydroxide aqueous solution as an electrolyte with a three-electrode configuration at room temperature. The as-prepared nickel foam-based electrode was used as the working electrode, and the detailed electrode fabrication was described above. The platinum wire and Hg/HgO electrodes were employed as the counter and reference electrodes, respectively. Cyclic voltammograms (CV) were recorded within the range of −1 to 0 V at various scan rates. Galvanostatic charge−discharge (GCD) curves were obtained in the same potential range at different current densities. Moreover, the long-term cycle stability of the working electrode was determined by the GCD measurement at a current density of 1 A g−1.

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RESULTS AND DISCUSSION Consideration of Synthesis as Part of the Precursor Strategy. Cu-MOF is easily collected in massive amounts and exhibits a 1D primitive aperture, which could be appropriately employed as a sacrificial precursor to further fabricate porous carbon and metal/metal oxide materials through the calcination− thermolysis process.18 Moreover, the ClO4− ion in the 1D aperture, as an initiator, accelerates the decomposition of the organic ingredient and releases a huge number of gases, resulting in greater porosity. Fortunately, the Cu@C composite was afforded after calcination, and the copper particle could behave as a hole-forming agent.15 In other words, the removal of the copper particle may further yield carbon materials with a larger specific surface area and more plentiful pores. These statements

Figure 6. Deconvoluted N 1s spectra of (a) NPC-600, (b) NPC-750, and (c) NPC-900.

demonstrate how a variety of porosity-forming effects were considered for the generation of porous materials and could be adapted to many similar systems. Crystal Structure of Cu-MOF. The title compound crystallizes in the monoclinic space group C2/c with one crystallographically independent CuII ion in the asymmetric unit. The CuII ion lies in a square-planar environment and is coordinated by four nitrogen atoms from individual bib ligands D


Article

Inorganic Chemistry Table 1. Elemental Compositions and Surface Functional Groups of NPCs content of N 1s groups

content of O 1s groups

sample

C

N

O

pyridinic-N

pyrrolic-N

graphitic-N

CO

O−C−O

adsorbed O2/H2O

NPC-600 NPC-750 NPC-900

85.55 92.57 92.87

8.65 2.55 1.90

5.78 4.89 5.23

40.4 22.3 27.6

59.6 62.6 61.8

− 15.1 10.6

96.0 87.6 84.7

4.0 8.3 9.7

− 4.1 5.6

(Figure 1a). The coordination bond lengths (dCu−N) are 2.005(3) and 2.020(3) Å, while the angles around the CuII node are 85.8(2)° and 90.03(14)° (Table S1), which demonstrates that the CuII ion has a distorted square coordination geometry.19 Furthermore, there are weak interactions between the CuII ion and two water molecules, and the Cu−O bond length of 2.549(3) Å is slightly greater than the classic value. This additional coordination has no effect on the topology analysis, and thus, the CuII ion is still considered as a squareplanar node if the weak Cu−O bond is ignored. The bidentate ligand bib, taking a trans-coordinated conformation, links two adjacent CuII ions. In such a way, the squareplanar CuII centers are connected by the rodlike spacer into a three-dimensional (3D) framework (Figure 1b). The topological method was used to study the nature of 3D structure, and surprisingly, a diamondoid architecture with a Schäfli symbol of 66 is recognized if the bib ligand is viewed as a linker and the CuII ion as a four-connected node.20 It is well-known that the square-planar node often leads to a two-dimensional (2D) (4,4)-lattice, and the diamondoid framework is usually based on the tetrahedral node, if the linear ligand is being used.21 Because of the nonlinear character of rigid ligand bib (two terminal N atoms diverge from the spacer direction), the extended direction of the metal node is significantly influenced by the coplanarity of four imidazole groups around the CuII ion. In the title complex, the four imidazole rings are significantly apart from the coordination plane, and two rings are even perpendicular. It means that all bib ligands are not located on the coordination plane, and also the CuII nodes are not extended along this plane, which breaks the square D4h symmetry of the CuII node. After careful analysis, each CuII ion is connected to four other equivalent CuII ions in a distorted tetrahedral mode by the bib ligand (Figure S1a), finally resulting in a diamondoid framework. Because of the long edges with dimensions of approximately 13.5 Å × 13.6 Å × 13.5 Å provided by the rodlike bib block, the diamondoid framework presents a hexagon-shaped channel along the c axis (Figure S1b). Unfortunately, the porosity is greatly reduced by interpenetration, and the void space of the 1D channel is large enough to allow three equivalent frameworks to be entangled with each other. Thus, Cu-MOF represents a 3-fold interpenetrating diamondoid network (Figure 2a). Interestingly, the overall void volume of the interpenetrating network is ∼1366.6 Å3, i.e., 41.3% of the cell volume based on the PLATON program,22 and the interpenetrating network still exhibits a 1D irregular aperture (Figure 2b). Furthermore, the void space of the 1D channel is filled up with the ClO4− ion and water molecule (Figure S2). Characterization and Morphology. To certify the phase purity of the bulky sample, the Cu-MOF powder was first characterized by XRD. The experimental XRD picture is identical to the corresponding computer-simulated pattern (Figure S3), which indicates that the powder sample and block single crystals are homogeneous. As seen from the FESEM image, Cu-MOF presents the petals of a flower-like morphology (Figure S4).

Figure 7. FESEM images of (a) Cu@C-600, (b) Cu@C-750, and (c) Cu@C-900.

When the Cu-MOF powder was subjected to a TG-DSC experiment, it lost all crystalline water (7.7% observed, 7.3% calcd) in the first region of 82−125 °C. When the sample was heated further, a rapid mass loss with a slight explosion was observed at ∼240 °C due to the ClO4− ion, and further quantitative analysis was meaningless (Figure S5). The black products after calcination were also studied by XRD. As seen from Figure 3a, three characteristic peaks were located at 44°, 51°, and 74° in all XRD pictures, which match well the (111), (200), and (220) crystal planes, respectively, of the cubic copper phase (PDF Card No. 04-0836). Furthermore, an obvious broad peak at ∼25° has been recorded, which is assigned to the (002) diffraction of carbon species.23 Therefore, a series of Cu@C composites have been successfully fabricated from the thermolysis of Cu-MOF. The CuII ion is a strong oxidant with a reduction potential above −0.27 V, so the metal copper, not the oxides, is generated after the thermolysis of E


Article

Inorganic Chemistry

Figure 8. FESEM images of (a and b) NPC-600, (c and d) NPC-750, and (e and f) NPC-900.

Cu-MOF under a N2 atmosphere.24 Besides, these three peaks of the copper phase gradually become narrow as the temperature increases, which suggests that the average grain size of the copper particle is enlarged according to the Scherrer formula.25 Significantly, NPCs have been obtained by simple acid treatments and also characterized by XRD (Figure 3b), which displays two broad peaks of ∼25° and ∼44° appointed to the carbon (002) and (100)/(101) crystal planes,26 respectively. The broadness of XRD peaks reflects the defective nature of carbon. The absence of any Cu phase peaks obviously shows that the copper particle is successfully removed by acid etching. The FT-IR spectrum is used to confirm the functional groups of NPCs. As shown in Figure 4a, NPCs exhibit strong and abroad absorption peaks at ∼3450 cm−1, which could be assigned to the -OH group or adsorbed water. Moreover, the FT-IR spectrum shows peaks at 2085, 1635, 1347, and 650 cm−1, which could be attributed to the CN stretching vibration, the CC stretching vibration, and C−O and N−H deformation vibrations, respectively.27 Therefore, FT-IR analysis denotes the existence of nitrogen and oxygen elements in the NPCs. The Raman spectrum of NPCs illustrates that two peaks located at approximately 1350 and 1585 cm−1 (Figure 4b) correspond to D (the disordered structure of carbon materials) and G (the bond vibration within the sp2 carbon atom) bands, respectively.28 The relative IG/ID ratios are 0.72 for NPC-600, 0.78 for NPC-750, and 0.87 for NPC-900. The IG/ID value is an

important parameter for assessing the degree of crystallization of graphite, which usually increases with calcination temperature. The results imply that NPC-900 exhibits the highest degree of graphitization. For the sake of further investigation of the element composition and complex state, XPS measurements of Cu@C-900 and NPCs were taken. The XPS survey spectra for the as-prepared samples all show a predominant peak of C 1s at 284.3 eV, N 1s at 400.1 eV, and O 1s at 530.9 eV (Figure 5). Moreover, the absence of Cu phase peaks of NPCs indicates that the copper particle is removed, in good accordance with the XRD results described above. Then the high-resolution XPS spectrum of Cu 2p for Cu@C-900 shows two characteristic peaks located at 932.8 and 951.7 eV, which are assigned to the Cu 2p3/2 and Cu 2p1/2 states, respectively. Then, the high-resolution N 1s spectra of NPCs were carefully analyzed (Figure 6). The spectra of NPC-750 and NPC-900 are deconvoluted to three peaks, including the pyridinic-N (398.3 ± 0.2 eV), pyrrolic-N (400.5 ± 0.1 eV), and graphitic-N (401.1 ± 0.1 eV) forms. However, there is no visible peak in the graphitic-N form for NPC-600, which may be due to the low calcination temperature. Furthermore, the high-resolution O 1s spectra of NPC-750 and NPC900 are fitted to three peaks (Figure S6), including the CO bond (531.5 ± 0.3 eV), the O−C−O ether-like bond (532.7 ± 0.2 eV), and the physically adsorbed oxygen/water molecule (533.9 ± 0.4 eV).29 However, the high-resolution O 1s spectrum F


Article

Inorganic Chemistry

Figure 9. TEM and HRTEM images of (a and b) NPC-600, (c and d) NPC-750, and (e and f) NPC-900.

captured through the HRTEM image (Figure S7). The particle presents a lattice fringe (0.209 nm), which corresponds to the (111) crystal plane of the face-cubic copper phase. At 750 °C, the copper particle presents as polyhedra, such as the truncated pyramid. When calcinated at 900 °C, the copper particle changes into a ball and almost floats on the carbon substrate. Thereby, the copper particle holds the tendency to grow larger as the calcination temperature increases. It is well-known that different surface free energies make the crystal seeds grow at disparate velocities along distinct crystal planes. Therefore, the bulky copper phase usually emerges as polyhedral particles. However, at high calcination temperatures (for example, 900 °C), the thermodynamic growth of crystal seeds is greatly accelerated in every direction, which makes the disparity of surface free energy negligible.31 Therefore, the growth velocity at the high calcination temperature is isotropic, finally resulting in bulky spherical copper particles in Cu@C-900.

of NPC-600 shows only the two peaks mentioned above, without the peak of the adsorbed oxygen/water molecule. The presence of a distinct peak of the adsorbed oxygen/water molecule indicates that NPC-750 and NPC-900 possess larger surface areas. The relative percentage contents of multiple nitrogen and oxygen functionalities are listed in Table 1. In general, N- and O-containing functional groups could improve the wettability and electronic conductivity of NPCs, which is beneficial for enhancing the specific capacitance.30 The morphology of the Cu@C composite was studied by FESEM, which demonstrates that the carbon substrate almost retains the original morphology of Cu-MOF with evident agglomeration after the calcination treatment (Figure 7). However, the copper particle shows different combinations with the carbon substrate. For Cu@C-600, the copper phase is sparsely embedded in the carbon surface with irregular particles, seeming like the stamen of the flower. Interestingly, the copper nanoparticle is G


Article

Inorganic Chemistry The morphologies and porous structures of NPCs were also investigated by FESEM and HRTEM. As seen in Figures 8 and 9, besides the original meso/marco porous textures, plenty of additional irregular gullies appear on the surface of the flower-like structure, which are formed by the removal of copper particles and provide a much higher connecting porosity to enhance mass transfer. Thus, the copper particle on the carbon substrate has a hole-forming effect on the NPCs. Moreover, there are abundant disordered mircopores clearly observed on the carbon surface from HRTEM images. Moreover, the elemental mapping images demonstrate that the C, N, and O elements are uniformly dispersed in the NPC samples (Figures S8−S10) The porous features of Cu@C composites and NPCs were further explored by N2 adsorption−desorption isothermal analyses at 77 K, which are summarized in Table 2. As shown Table 2. Summary of the Porosity Parameters for AsPrepared Samples sample Cu@C-600 Cu@C-750 Cu@C-900 NPC-600 NPC-750 NPC-900

SBETa Vtotalb Vmircoc (m2 g−1) (cm3 g−1) (cm3 g−1) 151 320 323 286 440 553

0.20 0.29 0.31 0.23 0.54 0.60

0.05 0.11 0.08 0.09 0.10 0.09

Vmeso+macrod (cm3 g−1)

pore size (nm)

0.15 0.18 0.23 0.14 0.44 0.51

5.38 3.69 3.86 3.21 5.38 5.38

a

Calculated from the BET surface area analysis. bThe total pore volume is calculated at a relative pressure of 0.30. cVmirco refers to the micropore volume and is calculated using the t-plot (FHH) method. d Vmeso+macro refers to meso/marco pore volumes and is determined by subtracting the micropore volume from the total pore volume.

in Figure 10a, all samples exhibit the classical type IV curve with a typical type H3 hysteresis loop within the P/P0 range of 0.45−1.0, which reveals the presence of meso/macro porosity.32 Moreover, a high rate of uptake at a very low pressure implies the presence of a large amount of micropores. Cu@C-600 has a BET specific surface area of 151 m2 g−1 and a pore volume of 0.20 cm3 g−1, while Cu@C-750 has a BET specific surface area of 320 m2 g−1 and a pore volume of 0.29 cm3 g−1, which obviously demonstrates that the BET specific surface area and pore volume of the Cu@C composite increase with calcination temperature. However, Cu@C-900 has a BET specific surface area of 323 m2 g−1 and a pore volume of 0.31 cm3 g−1, almost the same as those of Cu@C-750. It reveals that the BET specific surface area and pore volume of Cu@C composites vary little at calcination temperatures of >750 °C. Compared to Cu@C composites, it can be seen that NPCs have larger specific surface areas and larger pore volumes after acid etching. The BET specific surface areas are 286, 440, and 553 m2 g−1 for NPC-600, NPC-750, and NPC-900, respectively. Moreover, pore volumes are 0.23, 0.54, and 0.60 cm3 g−1 for NPC-600, NPC-750, and NPC-900, respectively. Therefore, BET specific surface areas and pore volumes of NPCs always obviously increase with calcination temperature. In addition, pore size distribution plots show that sharp peaks of all samples are almost centered at 4 and 30 nm (Figure 10b), which shows the major mesoporosity and contributes to the enhancement of mass transfer.33 The discussions presented above reveal that the carbonization temperature is a very important factor for the preparation

Figure 10. (a) N2 adsorption−desorption isotherms and (b) pore size distribution curves.

of porous carbon materials. Significantly, removal of the holeforming agent (the copper particle on the carbon substrate) is beneficial for the enlargement of the meso/macro porosity and surface area. Considering their specific surface areas and pore volumes are larger than those of Cu@C composites, NPCs with multiple-pore structures could serve as more suitable candidates for the supercapacitor.34 Electrochemical Properties. Because of large specific surface areas and large pore volumes, NPCs may exhibit considerable potential application in high-performance electric double-layer capacitors (EDLCs).35 To examine the electrochemical performance in the supercapacitor, the NPC electrode was intensively investigated by CV, GCD, and cycling life tests in a three-electrode system by using a 6 M KOH electrolyte. First, to confirm the applicability of NPCs in the supercapacitor, the CV and GCD curves of NPC electrodes have been studied with a scan rate of 10 mV s−1 and a density current of 0.5 A g−1, respectively. As seen from Figure 11a, the CV curves exhibit a slightly distorted rectangular shape based on the EDLC behavior and NPC-900 possesses the largest CV area. Figure 11b shows the nearly triangular GCD curves, and NPC-900 also exhibits the longest discharging time. These results demonstrate NPC-900 possesses the highest specific capacitance.36 Hence, the capacitive properties of the NPC-900 electrode were further investigated by a series of electrochemical measurements. Figure 12a illustrates the CV curves of NPC-900 with scan H


Article

Inorganic Chemistry

Figure 11. (a) CV curves at 10 mV s−1 and (b) GCD curves at 1 A g−1 of NPC electrodes.

rates that varied from 5 to 100 mV s−1. The specific capacitance C (farads per gram) originated from CV curves can be estimated according to eq 1:37

C=

∫ I dV /2υΔVm

(1)

where I (amperes) denotes the instant current, ΔV (volts) represents the potential change, υ (millivolts per second) refers to the scan rate, and m (grams) is the mass of NPC samples. After calculation, the specific capacitance of the NPC-900 electrode is 138 F g−1 at 5 mV s−1, which obviously decreases to 94 F g−1 when the scan rate increases to 100 mV s−1. Moreover, the CV curve at 100 mV s−1 shows some deformation, which may be caused by the restriction of mass transfer or ionic transportation at high scan rates. The observation of linear and symmetrical GCD curves from 0.5 to 10 A g−1 (Figure 12b) suggests that the NPC-900 electrode presents a good capacitance and reversibility. The specific capacitance C (farads per gram) from the discharge curve was evaluated on the basis of eq 2:37 C = I Δt /ΔVm

Figure 12. (a) CV curves at different scan rates, (b) GCD curves at various current densities, and (c) cyclic stability at 1 A g−1 for the NPC-900 electrode.

2, 5, and 10 A g−1, the specific capacitance is reduced to 139, 132, 120, and 105 F g−1, respectively. The phenomenon is mainly attributed to the poor availability of active materials at high current densities.38 Significantly, the NPC-900 electrode exhibits moderate cycling performance and reversibility at a current density of 1 A g−1 (Figure 12c), and the specific capacitance decreases to 125 F g−1 (13.2% decay) after 2000 cycles. Besides, with respect to the NPC-600 and NPC-750 electrodes, the CV and GCD curves are shown in Figure S11. All specific capacitance values of NPC electrodes are summarized in Table S2. Obviously, the specific capacitance of the NPC-900

(2)

where I (amperes) represents the discharge current, ΔV (volts) is the potential difference within the discharge time Δt (seconds), and m (grams) denotes the mass of active material. The maximal specific capacitance of the NPC-900 electrode is calculated to be 149 F g−1 at 0.5 A g−1. When the current density adds up to 1, I


Article

Inorganic Chemistry

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of Cu@C-600, EDS elemental mapping images of NPCs, specific capacitances of NPC-600 and NPC-750, and comparison data (PDF) X-ray crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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 (21103138) and the Natural Science Foundation of Shannxi (2014JQ2057, 14JS007, and 2016KJXX-67).

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Figure 13. Comparison of specific capacitances of NPC electrodes at different current densities.

electrode is larger than those of NPC-600 and NPC-750 electrodes (Figure 13). The best electrochemical performance of the NPC-900 electrode can be strongly attributed to the following cooperative features: (1) a larger surface area, which provides larger interfaces for the development of the electrostatic charge separation layer; (2) the abundant inherent porosity (mesopores/macropores permit quick mass mobility and good smooth diffusion of electrolytes, and micropores are responsible for charge accommodation); (3) the multiple nitrogen and oxygen functionalities, which could improve surface properties and cause pesudocapacitance;39 and (4) graphitic crystallization, which enhances electron transfer at high charge−discharge rates.

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CONCLUSION In summary, we herein report a peculiar 3-fold interpenetrating diamondoid network (Cu-MOF) based on the square-planar Cu(II) node, which illustrates a 1D perchlorate-contained aperture. Cu-MOF, as an explosive MOF, was first employed as a precursor to synthesize a sequence of porous Cu@C composites through a facile and efficient calcination method. The results show that the 1D primitive aperture and explosive ClO4− ion promote the porosity of Cu@C composites. Interestingly, NPCs prepared by acid etching of Cu@C composites present larger specific surface areas and more plentiful pores, which are attributed to the hole-forming effect of the copper particle. It is deduced that NPCs may be more suitable candidates for the supercapacitor. Significantly, NPC-900 demonstrates electrochemical properties superior to those of other NPCs, possessing remarkable specific capacitances of 138 F g−1 at 5 mV s−1 and 149 F g−1 at 0.5 A g−1. Furthermore, the NPC-900 electrode maintains 86.8% of the original capacitance after 2000 cycles at 1 A g−1, which exhibits good cycling stability. Therefore, NPC-900 may have a potential application in the electrode material for the supercapacitor.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00746. Selected bond lengths and angles and additional plots, including XRD figures, FESEM images, and the TG-DSC curve of Cu-MOF, XPS data of NPCs, HRTEM images J


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