Metal-Directed Assembly of Five 4-Connected ... - ACS Publications

Article Cite This: Cryst. Growth Des. 2018, 18, 979−992

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Metal-Directed Assembly of Five 4‑Connected MOFs: One-Pot Syntheses of MOF-Derived MxSy@C Composites for Photocatalytic Degradation and Supercapacitors Zuo-Xi Li,* Bo-Long Yang, Yi-Fan Jiang, Cheng-Yan Yu, and Lin Zhang 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: In this work, a series of divalent metal ions (M = CoII, ZnII, CdII, NiII, and CuII) have been used to react with phthalic acid (H2pa) and 1,4-bis(imidazol-1yl)benzene (bib) under proper solvethermal conditions, which afforded five metal− organic frameworks (MOFs) with the identical formula [M(pa)(bib)]∞, named Co-, Zn-, Cd-, Ni-, and Cu-MOF, respectively. Co-MOF and Zn-MOF are isoporphous, and present a diamondoid framework. Cd-MOF shows a distorted 2D (4,4) layer just like the herringbone. Ni-MOF and Cu-MOF are also isostructural, and illustrate a 3D CdSO4 net. These topologies are all based on the 4-connected node linked by the ligands pa2‑ and bib. It is the character of the metal ion that causes the structural diversity, which indicates that the metal ion plays an essential role in the self-assembly of MOFs. Furthermore, five well-dispersed MxSy@C composites have been successfully synthesized through the onepot sulfurization of MOF precursors, which prevents the sintered phenomenon in the stepwise carbonization−sulfurization process. Interestingly, the metal species originating from the MOF precursor exerts a crucial effect on the preparation of MOF-derived material, such as the pore-effect of Zn and Cd vapor, and catalytic graphitization of Ni species. Significantly, the pure ZnS and CdS samples, obtained by getting rid of carbon from corresponding composites, show moderate photocatalytic activities for degradation of MB dye under the visible light irradiation. Meanwhile, the other three MxSy@C composites have been intensively investigated on the supercapacitive properties. Especially, the NiS2@C electrode exhibits outstanding specific capacitances (806 F g−1 at 5 mV s−1 and 833 F g−1 at 0.5 A g−1) in 2 M KOH aqueous solution. Moreover, the NiS2@C composite displays excellent long-term cycle, and can be applied as an electrode material in the supercapacitor.

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INTRODUCTION

Besides intriguing topological architectures and intrinsic physicochemical properties, metal−organic frameworks (MOFs) display miscellaneous merits of long-range ordered structures, large specific surfaces, and adjustable pores, and MOFs have been widely employed as sacrificial precursors to fabricate porous carbon, metal/metal oxides, and the composites through the carbonization.11−15 Inevitably, if the sulfurization process is introduced, the metal sulfide@C composites will be successfully obtained. Furthermore, MOFderived materials can maintain the inherent features of original MOFs, such as the morphology, large specific surface area, and uniform pores.16−18 Thereby, the calcination−thermolysis strategy brings a new light on the synthesis of functional materials. However, there are still many uncertain and troublesome problems during the calcination of MOF precursors, such as the sintered phenomenon.19−21 In our continued efforts on the research of MOF-derived materials, herein, we delicately synthesized five 4-connected MOFs with the unique formula [M(pa)(bib)]∞ (M = CoII,

Metal sulfides, as fantastic materials, are attracting attention in many research fields, including solar cell, Li-ion battery, photocatalysis, and especially supercapacitor.1−3 However, the low specific surface area and electric conductivity reduce the specific capacitance, and greatly limit development to a higher level. Carbon materials are proven to be good candidates for the supercapacitive electrodes due to the abundant pores, which provide large accessible surface area for ion transportation and accommodation.4−6 Unfortunately, the electrochemical performance of carbon is relatively low, because the charge storage is solely dependent on the adsorption of electrolyte ion onto the electrode surface. Theoretically, the metal sulfide@carbon composite can make up the inherent single material deficiency, and integrate the advantages of different materials with enhanced performance for supercapacitors.7−10 Significantly, the introduction of carbon into the composite can tremendously improve the electric conductivity, which is beneficial to promoting the specific capacitance. Therefore, it is urgently necessary to develop regular concepts and strategies for the artificially controllable synthesis of metal sulfide@C composites. © 2018 American Chemical Society

Received: October 18, 2017 Revised: January 1, 2018 Published: January 16, 2018 979

DOI: 10.1021/acs.cgd.7b01463 Cryst. Growth Des. 2018, 18, 979−992

Crystal Growth & Design

Article

Table 1. Crystallographic Data and Structure Refinement Parameters for Five MOFs

a

MOFs

Co-MOF

Zn-MOF

Cd-MOF

Ni-MOF

Cu-MOF

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 col./uni. R(int) R1a [I > 2σ (I)] wR2b GOF Δρmax/Δρmin (e Ǻ −3)

C20H14CoN4O4 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 0.0410 0.0442 0.0882 1.214 0.378/-0.388

C20H14ZnN4O4 439.74 Monoclinic P21/c 10.556(2) 11.849(2) 17.512(6) 90 117.96(2) 90 1934.7(8) 4 1.510 1.304 896 16218/3400 0.0783 0.0548 0.0973 1.155 0.349/-0.282

C20H14CdN4O4 486.75 Monoclinic P21/c 12.161(7) 11.342(6) 15.114(6) 90 115.75(3) 90 1877.7(2) 4 1.722 1.199 968 19109/4668 0.0285 0.0298 0.1109 1.158 1.149/-1.176

C20H14NiN4O4 433.06 Monoclinic C2/c 14.777(6) 9.776(5) 12.772(5) 90 107.703(17) 90 1757.7(2) 4 1.636 1.142 888 6206/2155 0.0259 0.0296 0.0916 1.068 0.379/-0.299

C20H14CuN4O4 437.89 Monoclinic C2/c 14.576(7) 9.808(5) 13.428(7) 90 113.456(8) 90 1761.2(2) 4 1.651 1.278 892 6253/2147 0.0336 0.0424 0.1369 1.147 0.787/-0.444

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

ZnII, CdII, NiII, and CuII). To explore the role of metal ion in the calcination, these MOFs were further used as precursors to generate metal sulfide@C composites through one-pot sulfurization, which effectively avoids the sintered phenomenon during the stepwise carbonization−sulfurization process. Comprehensive characterization was carried out, including powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) spectra, field emission scanning electronmicroscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and Brunauer−Emmett−Teller (BET) surface analysis. Significantly, the potential applications of these materials have been intensively investigated, such as the photocatalytic degradation and electrochemical capacitance.

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Syntheses of MOFs. [Co(pa)(bib)]∞ (Co-MOF). A mixture of Co(NO3)2 (0.12 mmol), H2pa (0.1 mmol), and bib (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 cooling to room temperature at 5 °C h−1, purple block single crystals were gained. The mother liquor was decanted, and crystals were rinsed three times with ethanol (8 mL × 3) and dried in air for 24 h (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. [Zn(pa)(bib)]∞ (Zn-MOF). A mixture of Zn(NO3)2 (0.5 mmol), NaOH (1 mmol), H2pa (0.5 mmol) and bib (0.5 mmol) in the mixed solvent (CH3OH:H2O = 1:2, 15 mL), was sealed in a Teflon-lined autoclave and heated to 170 °C for 4 days. After cooling to room temperature at 10 °C h−1, colorless block crystals were obtained. The mother liquor was filtered, and single crystals were washed with ethanol (8 mL × 3) and dried in air. Yield: ∼ 30% based on bib. Anal. Calcd for C20H14ZnN4O4: C, 54.63; H, 3.21; N, 12.74%. Found: C, 54.28; H, 3.36; N, 13.09%. IR (KBr, cm−1): 3124m, 2362w, 1567m, 1538m, 1445s, 1383s, 1307m, 1249s, 1143s, 1076s, 962s, 841s, 762s, 705m, 658s, 528m, 481m, 422w. [Cd(pa)(bib)]∞ (Cd-MOF). Single crystals of Cd-MOF suitable for X-ray analysis were also obtained by the solvethermal method. The suspension of Cd(NO3)2 (0.12 mmol), H2pa (0.10 mmol), and bib (0.1 mmol) in 12 mL component solvent (C2H5OH:H2O = 4:3) was sealed in a Teflon-lined autoclave and heated to 140 °C for 2 days. After the autoclave was cooled to room temperature at 5 °C h−1, colorless block crystals were obtained. The mother liquor was decanted, and crystals were rinsed three times with ethanol (8 mL × 3) and dried in air (yield: ∼ 45% based on bib). Anal. Calcd for C20H14CdN4O4: C, 49.35; H, 2.90; N, 11.51. Found: C, 49.11; H, 2.97; N, 11.68%. IR (KBr, cm−1): 3421s, 3132m, 2026w, 1628s, 1578m, 1529s, 1464m, 1390m, 1312s, 1134m, 1071m, 960w, 835m, 743m, 645m, 540m, 497m, 423w. Single crystals of Ni-MOF and Cu-MOF suitable for X-ray analysis were obtained by the similar method as described for Cd-MOF. [Ni(pa)(bib)]∞ (Ni-MOF). Yield: ∼ 40% (based on bib). Anal. Calcd for C20H14NiN4O4: C, 55.47; H, 3.26; N, 12.94. Found: C, 55.20; H, 3.34; N. 13.08%. IR (KBr, cm−1): 3447s, 3105s, 2993m, 2747w, 2361w, 2327w, 1775w, 1632m, 1532s, 1492s, 1446m, 1360m, 1298m,

EXPERIMENTAL SECTION

Materials and General Methods. All the solvents and reagents for syntheses were commercially available and used as received. The ligand 1,4-bis(imidazol-1-yl)benzene (bib) was synthesized according to the reported procedure.22 Elemental analyses for C, H, and N were taken on a PerkinElmer 240C analyzer. IR spectra were recorded on a TENSOR 27 (Bruker) FT-IR spectrometer by using KBr pellets. XRD was performed on a Rigaku D/Max-2500 diffractometer, at 40 kV, 100 mA with Cu Kα (λ = 1.5418 Å) radiation. TGA was performed using a STA 449 C instrument from room temperature to 800 °C with a heating rate of 10 °C min−1 under the N2 atmosphere. The morphologies and structures of as-prepared samples were characterized with FESEM (HITACHI, S-4800) with an EDX at an accelerating voltage of 20 kV, and HRTEM (FEI TECNAI G2) with 200 kV. XPS was performed by utilizing an apparatus (Thermo Scientific, K-Alpha) with an Al Ka X-ray source. The BET surface analyzer (Tri Star-3020, Micromeritics, USA) was employed to explore the porous features of as-obtained materials. Raman spectra were tested by using an inVia confocal Raman microscope (Renishaw Co., England) with an Ar ion laser (514.5 nm excitation wavelength). Ultraviolet visible (UV−vis) data were collected from a UV−vis spectrophotometer (Shimadzu UV-3600, Japan). 980



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measurement at the current density of 1 A g−1 in the 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.

1250m, 1124w, 1070s, 958w, 873w, 853m, 827m, 750s, 710m, 664m, 633m, 536m, 455w. [Cu(pa)(bib)]∞ (Cu-MOF). Yield: ∼ 25% (based on bib). Anal. Calcd for C20H14CuN4O4: C, 54.86; H, 3.22; N, 12.79. Found: C, 54.61; H, 3.29; N. 12.93%. IR (KBr, cm−1): 3445s, 3137m, 3109s, 2991w, 2747w, 2361s, 2340s, 1618w, 1603s, 1562s, 1490m, 1443m, 1399s, 1268m, 1123m, 1069s, 958w, 835m, 753s, 707m, 663s, 539m, 472w, 433w. One-Pot Syntheses of MxSy@C Composites. Five MOF samples are pure and easily collected in large-scale, which are conveniently utilized as precursors to prepare MOF-derived materials through the calcination process. The MOF samples and sublimate sulfur powder were completely mixed and ground at the mass ratio of 1:2. Then the mixture was transferred into a tube furnace and calcinated under N2 flow at 650 °C for 3 h with the rate of 5 °C min−1. After the tube furnace naturally cooled to room temperature, a sequence of MxSy@C composites (Co9S8@C, ZnS@C, CdS@C, NiS2@C, and Cu2S@C) were correspondingly obtained. Syntheses of ZnS and CdS Photocatalysts. ZnS@C and CdS@C composites were placed into an open muffle furnace at 450 °C with a heating rate of 5 °C min−1, which was aimed to remove the carbon substance. After naturally cooled to room temperature, pure ZnS and CdS materials were fabricated. X-ray Data Collection and Structure Determinations. X-ray single-crystal diffraction data of MOFs were collected on a Rigaku MM-007/Saturn 70 with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). The program SAINT23 was used for integration of the diffraction profiles. All the structures were solved by direct methods using the SHELXS program of SHELXTL package and refined by fullmatrix least-squares methods with SHELXL.24 Metal atoms in each MOF were located from the E-maps and other non-hydrogen atoms were projected according to successive difference Fourier syntheses, which are all refined with anisotropic thermal parameters on F2. The hydrogen atoms were generated theoretically onto the specific atoms and refined isotropically. Further details for structural analyses are summarized in Table 1, and selected bond lengths and angles are listed in Table S1. Photocatalytic Degradation. Photocatalytic activities of ZnS and CdS samples were evaluated by degradation of methylene blue (MB) dye in the liquid phase under visible light irradiation. The visible light source used in the experiment was a 300 W Xe lamp with a 400 nm cutoff light filter. In each entry, 40 mg photocatalyst was added into 50 mL MB solution (10 mg L−1). The aqueous suspensions were first ultrasonically treated for 5 min and then stirred in the dark for 60 min to ensure the adsorption−desorption equilibrium before the light irradiation. During the tracking reaction, 3 mL liquid was taken out at 10 min intervals and centrifuged to remove the catalyst. Moreover, the concentration of MB dye was measured at 663 nm absorbance by using a UV−vis spectrophotometer. Fabrication of Electrodes. A mixture of 8 mg active materials (e.g., Cu2S@C, Co9S8@C, and NiS2@C composites), 1 mg acetylene black and 1 mg polyvinylidene fluoride binder were ground thoroughly with ethanol in an agate mortar until homogeneous black slurry was acquired. The resulting slurry was subsequently brushed into 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 the 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. The 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 proper potential range at various scan rates. Galvanostatic charge− discharge (GCD) curves were obtained within a suitable potential window at different current densities. The long-period cycle performance of working electrode was assessed by the GCD

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RESULTS AND DISCUSSION Syntheses of Consideration. The calcination−thermolysis strategy, which employs proper MOFs as sacrificial

Figure 1. Diamondoid net in Co-MOF and Zn-MOF (the blue and green lines demonstrate the pa2‑ and bib linkers, respectively).

precursors, provides a promising approach for preparing highperformance MOF-derived materials. The metal sulfide behaves as a kind of typical pseudocapacitive materials to achieve high specific capacitance, and we previously adopted a stepwise synthesis, i.e., the carbonization−sulfurization procedure, to successfully prepare a series of CoS2@CNT composites. However, the carbonization of Co-MOF could result in carbon as a sintered pellet with awful morphology. Lately, more MOFs constructed from the H2pa ligand were also directly carbonized, and similar sintered carbon would be obtained. The sintered phenomenon will greatly reduce the BET surface area, and also the related properties. We have made much progress, and fortunately, when adopting the one-pot sulfurization method, i.e., calcinating the mixture of MOFs and elemental sulfur powder together, well-dispersed MxSy@C composites have been successfully pursued. The result shows that the one-pot sulfurization is a very convenient and fruitful technique for synthesizing MOF-derived sulfide materials. Crystal Structures of Co-MOF and Zn-MOF. These two MOFs are isomorphous, and their structures have been previously reported.25,26 Herein we give a brief description of the structures. They crystallize in a monoclinic space group P21/c, and the metal ions locate on a tetrahedral coordination environment with two carboxylic O atoms and two imidazole N atoms. The fully deprotonated pa2− ion acts as a V-shaped ligand, and bridges the metal centers to form 1D W-type chain. The bidentate ligand bib, adopting a trans-coordinated 981



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Figure 2. View of the (a) coordination environment of CdII ion (hydrogen omitted for clarity); (b) 1D CdII-carboxylate chain; (c) 2D herringbonepattern layer; (d) 2D (4,4) lattice in Cd-MOF.

Figure 3. View of the (a) octahedral coordination environment (hydrogen atoms omitted for clarity); (b) 1D linear chain in Ni-MOF.

2.251(2) and 2.306(2) Å. The coordination bond lengths are in accord with those observed values in related CdII coordination polymers.27,28 The fully deprotonated pa2− ion with two bidentate chelate carboxylic groups connects the CdII center into a W-type chain (Figure 2b). Each bib ligand, adopting a cis-coordinated conformation, links two CdII ions from two neighboring 1D chains. In detail, the 1D CdII-carboxylate chain is arranged in a parallel mode by the bib ligand, which finally results in a 2D herringbone-pattern layer (Figure 2c). If the pa2− and bib ligands are viewed as linear linkers, then the CdII ion could be

conformation, further connects the 1D chain into a 3D diamondoid framework with the Schäfli symbol of [66] (Figure 1). Crystal Structure of Cd-MOF. Single-crystal X-ray diffraction analysis reveals that Cd-MOF also crystallizes in P21/c. The asymmetric unit consists of one crystallographically independent CdII ion, one pa2− ion, and one bib ligand. As shown in Figure 2a, each CdII ion is octahedrally coordinated by two carboxylic groups of distinct pa2− ions with Cd−O bond distances from 2.294(2) to 2.448(2) Å, and two imidazole rings from individual bib ligands with Cd−N bond distances of 982



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Figure 4. View of the (a) criss-across bib ligand along the 1D NiII-carboxylate chain (the red line); (b) 4-connected CdSO4 net in Ni-MOF.

Crystal Structures of Ni-MOF and Cu-MOF. These two MOFs are also isostructural, and thus only the architecture of Ni-MOF is presented herein as an example. Ni-MOF crystallizes in a monoclinic space group C2/c with one crystallographically independent NiII ion located on the inversion center. As shown in Figure 3a, the NiII ion possesses an octahedral coordination environment, which is equatorially

treated as a 4-connected node. Based on this simplification, CdMOF presents a distorted nonplanar 2D (4,4) lattice (Figure 2d). Further analysis shows that there are no obvious hydrogenbonding or π−π stacking interaction between adjacent 2D layers, and they are packed tightly in a translational mode via the intermolecular or van der Waals interaction (Figure S1). 983



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Figure 5. Metal-directed assembly of five 4-connected MOFs.

According to the theory of topology, crystal structures of MOFs have been described in terms of nets in which metal atoms are the nodes and organic ligands are the links. So the uninodal nets could be discriminated from each other based on the link numbers (n) of metal nodes, and corresponding MOFs are assigned to the n-connected nets. Referring to the wellknown free-charge database of Reticular Chemistry Structure Resource, there are mainly three sorts of 4-connected nets, namely, the 3D diamondoid, 2D (4,4) lattice, and 3D CdSO4, which are frequently reported in the area of crystal engineering.31 Generally, the 3D diamondoid net is caused by the tetrahedral node, which is exemplified in Co-MOF and ZnMOF once more (Figure 5). The 2D (4,4) lattice and 3D CdSO4 net both happen in the planar-square node, and the extension of square node highly determines the topology. If viewed along the pricipal axis, the square node is arranged in a criss-cross mode, and so it is oriented in three orthogonal directions, finally leading to the 3D CdSO4 net, just as shown in Ni-MOF and Cu-MOF. Compared to the CdSO4 net, the square node of regular 2D (4,4) lattice is arranged in the same plane, and the node is oriented in two orthogonal directions. However, in Cd-MOF, four linkers of the 4-connected node are not orthogonal with each other, and so a distorted 2D (4,4) lattice is formed, looking like the herringbone. Summarily, the differences in the extension of metal nodes cause diverse topologies for [M(pa)(bib)]∞, and the results indicate that the metal ion exerts vital effect on the construction of MOFs. Characterization and Morphology. The XRD experiments of MOF samples were first carried out, which were aimed at verifying the phase purity. The experimental patterns are in good agreement with the corresponding simulated, which reveals that the powder sample is identical to the crystal phase (Figure S2). Then TGA curves have been measured to investigate the thermal stability of MOFs (Figure S3), and the results show that all MOFs maintain the original phases before 230 °C. Upon further heating, MOF samples begin to lose weight rapidly, which indicates the collapse of frameworks.

surrounded by four O atoms from two carboxylic groups, and axially coordinated by two bib N atoms. The coordination bond lengths are 2.1078(15) and 2.1680(13) Å for the Ni−O bond, whereas 2.0791(15) Å for the Ni−N bond, which are all comparable to the typical value of nickel complexes.29,30 The fully deprotonated pa2− ion with two bidentate chelate carboxylic groups, connects the NiII center into a 1D linear chain (Figure 3b). Two axial coordination sites of NiII ion in the 1D chain are occupied by the imidazole ring. Therefore, the 1D NiII-carboxylate chain is extended by bib in a criss-cross mode to generate a 3D framework (Figure 4a). If viewed along the 1D NiII-carboxylate chain, one-half of bib ligand lies in the horizontal plane and the other half is in the polar axis (the inset of Figure 4a). The topological analysis was applied to get more insight into this elegant structure. Ligands pa2− and bib are both regarded as a linear rod, and thus the NiII ion could be simplified as a 4-connected node. In such a way, Ni-MOF illustrates a uninodal 4-connected framework with the Schäfli symbol of [65·8], which is topologically identical to the augmented square CdSO4 net (Figure 4b). For Cu-MOF, it is worth mentioning that the CuII ion lies in a square-planar coordination environment surrounded by two monodentate carboxylic groups and two imidazole rings. Similar to Ni-MOF, the CuII ion is still extended by pa2− and bib ligands in a criss-cross planar-square mode, undoubtedly leading to the CdSO4 net. Structural Correlation Analysis. Five MOFs have been synthesized from the solvethermal reaction of M(NO3)2 (M = CoII, ZnII, CdII, NiII, and CuII) with ligands H2pa and bib under proper conditions. There exist some characteristics in common, and the metal−ligand ratio is apparently the same according to the identical formula [M(pa)(bib)]∞. Furthermore, similar 1D MII-carboxylate chains are illustrated in these MOFs. The metal ions in these MOFs show three kinds of coordination environments, including the tetrahedral, octahedral, and planar square; however, all of them could be simplified as a 4connected node. 984



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Figure 6. XRD patterns of all metal sulfide@C composites.

Specifically, the TGA trace of Cu-MOF shows a sharp decline from 230 to 280 °C, and then descends slowly. This is attributed to the decomposition of Cu-MOF skeleton. The thermal stability of Zn-MOF and Cd-MOF are slightly superior to Cu-MOF, with the onset degradation temperature of 280 and 300 °C, respectively. Compared to three previous MOFs, Co-MOF and Ni-MOF show even better thermal stability, and the decomposition temperatures are 330 and 360 °C, respectively. The calcinated products of MOFs have also been investigated by XRD experiments. As shown in Figure 6, corresponding metal sulfides were successfully synthesized by studying the characteristic XRD diffractions, which were indexed to the hexagonal phases of wurtzite-type ZnS (JCPDS NO. 36−1450) and CdS (JCPDS NO. 65−3414), and cubic phases of Cu2S (JCPDS NO. 65−2980), Co9S8 (JCPDS NO. 65−6801), and NiS2 (JCPDS NO. 65−3325), respectively.32−36 On the other hand, a broad peak with low intensity at around 26° was captured in each XRD pattern,

Figure 7. Raman spectra of all metal sulfide@C composites.

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Figure 8. FESEM images of (a) Zn-MOF, (a′) ZnS@C composite, (a″) ZnS particle; (b) Cd-MOF, (b′) CdS@C composite, (b″) CdS particle; (c) Cu-MOF, (c′) Cu2S@C composite; (d) Co-MOF, (d′) Co9S8@C composites; (e) Ni-MOF, (e′) NiS2@C composite.

which is assigned to the (002) diffraction of carbon.37,38 Therefore, five MxSy@C composites have been obtained by the one-pot sulfurization of corresponding MOF precursors. Significantly, an obvious sharp peak of carbon (002) diffraction is detected in the XRD pattern of NiS2@C composite, which shows high graphitizaion degree for the carbon substrate and also excellent catalytic graphitization effect of nickel species. Furthermore, pure ZnS and CdS samples were successfully prepared after the combustion of corresponding MxSy@C composites to get rid of carbon (Figure S4). As seen from the Raman spectra of MxSy@C composites (Figure 7), two obvious characteristic peaks were observed, indexing to the D and G bands, which were approximately located at 1350 and 1585 cm−1, respectively.39,40 The ratio of IG/ID is a crucial parameter for evaluating the graphitization of carbon materials. Apparently, five MxSy@C composites possess different IG/ID values, and the NiS2@C composite has the highest graphitization degree due to the largest IG/ID value of

0.90. The Raman analysis exhibits that the nickel species owns the best catalytic graphitization effect during the carbonization, which is coincident with the XRD result. XPS measurements have been implemented for the investment on the element composition and complex state. Based on the fitting, the high resolution S 2p spectra of MxSy@C composites present three types of states, including the S 2p3/2 (161.6 ± 0.6 eV), S 2p1/2 (162.7 ± 0.6 eV), and S−O bond (168.5 ± 0.5 eV) (Figure S5 and S6).41−43 For the XPS studies of metal species, the deconvoluted high resolution Zn 2p spectrum exhibits two characteristic peaks at 1024.1 and 1047.1 eV, which are attributed to the Zn 2p3/2 and Zn 2p1/2 states, respectively.44 The deconvolution of Cd 3d spectrum also shows two characteristic peaks ascribed to the Cd 3d5/2 and Cd 3d3/2 states.45 Meanwhile, the high resolution Cu, Co, and Ni 2p spectra are fitted into two kinds of triplets, which at low and high binding energies could be mainly attributed to 2p3/2 and 2p1/2, respectively.46−48 Besides the 2p3/2 and 2p1/2 states, there 986



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

Table 2. Summaries of the Porosity Parameters for AsPrepared Materials sample ZnS@C composite CdS@C composite Cu2S@C composite Co9S8@C composite NiS2@C composite

Figure 9. TEM and HRTEM images of (a, a′) ZnS particle; (b, b′) CdS particle; (c, c′) Cu2S@C composite; (d, d′) Co9S8@C composites; (e, e′) NiS2@C composite.

SBETa (m2 g−1)

Vtotalb (cm3 g−1)

Vmicroc (cm3 g−1)

Vmeso+macrod (cm3 g−1)

pore size (nm)

251

0.27

0.01

0.26

4.45

611

0.54

0.03

0.51

3.52

62

0.04

0.004

0.036

2.64

202

0.15

0.038

0.112

2.93

349

0.20

0.05

0.15

2.30

a

Calculated from the BET surface area analysis. bTotal pore volume is calculated at relative pressure of 0.97. cVmicro 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.

exists a satellite peak at the relatively high binding energy of each triplet. Taking the Ni 2p spectrum as an example, the binding energy located at 856.6 and 858.4 eV can be attributed to Ni 2p3/2, and the peaks of 874.1 and 876.2 eV are assigned to Ni 2p1/2.49 The binding energies of 879.3 and 861.1 eV are the satellite peaks. Therefore, the XPS results clearly demonstrate that five MxSy@C composites are successfully prepared, which also match well with the XRD analyses. The morphologies of MOFs were initially studied by FESEM, and the MOF samples behave as regular polyhedrons with smooth facets (Figure 8). After the one-pot sulfurization, the products MxSy@C almost maintain the morphologies of

corresponding parent MOFs, but the surface looks much rougher. The ZnS and CdS particles, obtained by getting rid of carbon from ZnS@C and CdS@C composites, turn smaller and the surface becomes even rougher. The EDX results reveal that the relevant elements are uniformly dispersed in the bulky samples (Figures S7−S11). Thereby, the one-pot sulfurization could effectively suppress the sintered phenomenon when using the stepwise carbonization−sulfurization.25,26 Following, the 987



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Figure 11. View of the (a, b) time-dependent absorption of MB; (c) photocatalytic performances; (d) first-order kinetics plots for ZnS and CdS, respectively.

degradation of MB dye under the visible light irradiation. Based on the tentative experiment, the color of MB dye is gradually faded when added the ZnS and CdS powder (Figure S12). Furthermore, the decolorization is a little quicker in the presence of CdS powder, which infers that CdS has better catalytic degradation of MB dye. The UV−vis spectrum is used to quantitatively monitor the photocatalytic reaction. As shown in Figure 11a,b, after the irradiation, the maximum absorbance of MB dye gradually decrease with time, which illustrates that the ZnS and CdS samples possess remarkable photocatalytic degradation activities.56−60 The ZnS and CdS samples behave a little different degradation properties, and approximately 90.2% and 91.6% of MB dye were degraded after 100 min, respectively (Figure 11c). According to the Beer−Lambert law, the degradation rate of MB dye was also investigated, which was assessed by using the following equation:61,62

morphologies of related samples have been investigated by TEM measurements. As shown in Figure 9, the obvious lattice fringes of 0.159 and 0.206 nm are assigned to the (201) crystal plane of ZnS and (110) crystal plane of CdS, respectively. For the Cu2S@C, Co9S8@C, and NiS2@C composites, the metal sulfides are evenly dispersed in the carbon matrix, and their characteristic facets of (220), (331), and (311) are captured, which origin from the Cu2S, Co9S8, and NiS2 phases, respectively. Significantly, the NiS2 particles are obviously surrounded by the graphitic carbon, which illustrates that the nickel species exerts effectively catalytic graphitization during the carbonization of Ni-MOF.50,51 The porous features of all MxSy@C composites were characterized by N2 adsorption−desorption isothermal curves at 77 K. As shown in Figure 10a, all samples exhibit the classical type-IV curve with an obvious type-H3 hysteresis loop from 0.4 to 1.0 of P/P0 value, which reveals the existence of meso/ macropores.52 Moreover, the gradually increasing tendency at low relative pressures demonstrates the features of the micropore. Obviously, the ZnS@C composite, especially CdS@C, shows relatively large BET specific surface and pore volume (Table 2), which indicates the pore effect of zinc and cadmium species during the one-pot sulfurization.53−55 The pore diameter distribution of all MxSy@C composites is mainly located in a narrow range from 3 to 7 nm, with poor meso/ macropores (above 10 nm) (Figure 10b). The small pore diameters are caused by the H2pa ligand with two neighbor carboxylic groups, which provide relatively short lattice dimension Photocatalytic Degradation Activity. The photocatalytic activities of pure ZnS and CdS samples were explored by the

Degradation rate(%) = (C0 − C)/C0 × 100%

where C0 is the initial concentration of MB dye, and C represents the solution concentration of MB dye at different reaction intervals. The photodegradation of MB dye obeys the pseudo-firstorder kinetics, which can be expressed in the equation below:63,64 ln(C0/C) = kt

where k (min−1) is the degradation rate constant. The rate constants (k) were calculated to be ca. 2.23 × 10−2 and 2.38 × 10−2 min−1 for ZnS and CdS, respectively (Figure 11d). The larger k value for CdS manifests the superior catalytic degradation over ZnS. Compared to the reported liter988



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Figure 12. View of the CV and GCD curves of Cu2S@C, Co9S8@C, and NiS2@C composites.

ature,65−67 the photocatalytic degradation activities of ZnS and CdS samples in this work are considerable. For example, Wang et al. have prepared a raspberry-ZnS material with the photodegradation rate constant of 2.4 × 10−2 min−1 to MB.68 Cui et al. have successfully synthesized a copper complex to be applied as the photocatalyst, and 90.8% of MB dye was degraded after 120 min, comparing to 90.2% and 91.6% for ZnS and CdS samples after 100 min in our work, respectively.69 These results demonstrate that the one-pot sulfurization is an appreciable approach to prepare the sulfide photocatalyst. Supercapacitive Performance. Motivated by the hierarchical morphology and metal sulfide nanoparticles highly dispersing in the carbon matrix, the electrochemical properties of Cu2S@C, Co9S8@C, and NiS2@C composites were initially studied by CV and GCD measurements in a standard threeelectrode system. Figure 12a shows the CV curves of above three composites at a scan rate of 5 mV s−1, which exhibit obvious redox peaks of metal sulfides, corresponding to the individual reversible conversion among multiple metal states in the KOH electrolyte.70−72 Therefore, the capacitive behavior are primarily determined by the faradic redox characteristics. Obviously, the CV curve of NiS2@C electrode has the largest enclosed area, indicating the best capacitive performance among three kinds of electrode materials. The GCD curves of different electrodes were recorded at a current density of 0.5 A g−1 (Figure 12b). The asymmetrical configurations with apparent curvatures show a dominant faradaic capacitive

Figure 13. View of the (a) CV curves at different scan rates; (b) GCD curves at various current densities; (c) cycling period at 0.5 A g−1 for the NiS2@C electrode.

behavior, which are in good coincidence with the CV results. Moreover, NiS2@C exhibits the longest discharging time from the GCD curve, which also indicates that NiS2@C possesses the highest specific capacitance. To better evaluate the supercapacitive performance of NiS2@ C, CV curves were recorded at different scan rates and GCD measurements were carried out at different current densities (Figure 13a,b). The CV curves from 5 to 100 mV s−1 show more and more serious deformation, which may result from the limitation of mass transfer or ion transportation as the scan rates increase.73,74 It is well-known that the CV deformation 989



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To further explore the electrochemical behaviors, the EIS measurements of these composites were conducted at the open circuit potential between 100 kHz and 10 mHz (Figure S14). All Nyquist plots mainly consist of a semicircle and straight line in the high-frequency and low-frequency regions, respectively. The intersection point of Nyquist curve with the axis of real impedance exhibits a combined resistance (Rs) of the intrinsic resistance of active material, ionic resistance of electrolyte, and contact resistance at the active material/current collector interface.81 The Rs values are 0.76, 0.93, and 1.10 Ω for the Cu2S@C, Co9S8@C, and NiS2@C electrodes, respectively. However, with a relatively large Rs value, the NiS2@C electrode shows a more vertical slope and smaller semicircle diameter than those of other two electrodes, which indicate better conductivity, quicker ion diffusion, and lower charge-transfer resistance for enhancing the capacitance. Overall, the best electrochemical performance of NiS2@C electrode can be attributed to the following synergistic features: (1) the relatively high theory capacitance of NiS2, which is mainly caused by quick redox reaction between the NiS2 surface and OH− ion;82 (2) a larger surface area, which provides larger interfaces for improving the mass transfer and electrolyte diffusion; (3) the highest graphitic crystallization, which enhances the electron transfer at high scan rate; (4) the relatively low resistance, which indicates better intrinsic conductivity for promoting the pesudocapacitance.

Figure 14. Comparison of specific capacitances for all MxSy@C electrodes.

causes the decrease of capacitance as the increase of scan rate.75,76 The specific capacitance originated from the CV curve can be calculated according to eq 1:77 C=

∫ I dV /2νΔVm

(1)

−1

where C (F g ) is the specific capacitance, I (A) the instant current, ΔV (V) the potential change, υ (mV s−1) the scan rate, and m (g) the mass of active material. The specific capacitances of NiS2@C electrode are 806, 732, 608, 551, and 422 F g−1 at 5, 10, 20, 50, and 100 mV s−1, respectively. The GCD profiles present a wider hunch peak as the current densities increase, implying the pseudocapacitance and quick faradaic redox reaction:78,79

CONCLUSION

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ASSOCIATED CONTENT

In summary, a series of MOFs with the identical formula [M(pa)(bib)]∞ (M = CoII, ZnII, CdII, NiII, and CuII) have been successfully synthesized, which exhibit three kinds of typical 4connected net. The structural diversity indicates that the metal ion plays an important role in the assembly of MOFs. Interestingly, the one-pot sulfurization of these MOF precursors yields five MxSy@C composites, which suppresses the sintered phenomenon during the stepwise carbonization− sulfurization process. Moreover, the metal ion also exerts crucial effect on the synthesis of MOF-derived material, such as the pore-effect of Zn and Cd vapor, and catalytic graphitization of Ni species. Significantly, pure ZnS and CdS samples exhibit moderate photocatalytic degradation of MB dye under the visible light irradiation. On the other side, the Cu2S@C, Co9S8@C, and NiS2@C composites own the supercapacitive properties, and especially the NiS2@C electrode shows remarkable specific capacitances of 806 F g−1 at 5 mV s−1 and 833 F g−1 at 0.5 A g−1 in 2 M KOH electrolyte. Moreover, the NiS2@C electrode displays good long-term cycle, which has potential application in the supercapacitor.

NiS2 + OH− ↔ NiS2OH + e− NiS2 OH + OH− ↔ NiS2O + H 2O + e−

The specific capacitance deduced from the discharge curve is counted by eq 2:77 C = I Δt /ΔVm

■

(2)

−1

where C (F g ) 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 specific capacitance of NiS2@C electrode is up to 833 F g−1 at 0.5 A g−1, and decreases to 750, 720, 589, and 465 F g−1 as the current density increases to 1, 2, 5, and 10 A g−1, respectively. The decreasing capacitance is mainly caused by the low usage of active materials at large charge−discharge current density.80 Significantly, the NiS2@C electrode still retains 81.6% of initial specific capacitance (679 F g−1) at the current density of 0.5 A g−1 after 5000 cycles (Figure 13c), demonstrating that the NiS2@C electrode owns good longterm cycle and reversibility. For the Cu2S@C and Co9S8@C electrodes, their CV curves at different scan rates and GCD profiles at different current densities have drawn in Figure S13. All specific capacitance values of MxSy@C electrodes are summarized in Table S2. Obviously, their specific capacitances follow the order of NiS2@ C > Co9S8@C > Cu2S@C at any scan rate or current density (Figure 14).

S Supporting Information *

These materials are available and free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01463. Selected bond lengths and angles, and additional plots and figures of Cd-MOF; XRD patterns of MOFs; TGA curves of MOFs; XRD and TEM images of ZnS and CdS samples; high resolution XPS spectra of MxSy@C composites; EDX results of ZnS, CdS, Cu2S@C, Co9S8@C, and NiS2@C; digital photographs of MB solution; supercapacitive properties of Cu2S@C, Co9S8@ 990



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C, and NiS2@C composites, and the further comparison (PDF) Accession Codes

CCDC 1580027−1580029 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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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), Natural Science Foundation of Shannxi (2016KJXX-67), and Top-rated Discipline Construction Scheme of Shaanxi Higher Education.

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