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Nanocomposites Containing Keggin Anions Anchored on PyrazineBased Frameworks for Use as Supercapacitors and Photocatalysts Nana Du, Lige Gong, Lingyu Fan, Kai Yu, Huan Luo, Shengjie Pang, Jiaqian Gao, Zhuwu Zheng, Jinghua Lv, and Baibin Zhou ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00409 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Nano Materials

Nanocomposites Containing Keggin Anions Anchored on Pyrazine-Based Frameworks for Use as Supercapacitors and Photocatalysts Nana Dua,b, Lige Gonga,b, Lingyu Fana,b, Kai Yua,b,*,Huan Luoa,b, Shengjie Panga,b, Jiaqian Gaoa,b, Zhuwu Zhenga,b, Jinghua Lva, Baibin Zhoua,b,* a Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, Harbin Normal University, Harbin 150025, People's Republic of China. b Key Laboratory of Synthesis of Functional Materials and Green Catalysis, Colleges of Heilongjiang Province, Harbin Normal University, Harbin, 150025,People's Republic of China KEYWORDS: polyoxometalates, POMOFs, Keggin structures, supercapacitors, photocatalysis ABSTRACT Designing highly active Keggin-anion anchored on pzbased frameworks is desirable, but still a huge challenge. inorganic-organic

hybrid

[{Ag5(pz)7}(SiW12O40)](OH)·H2O(2),

Herein,

compounds:

three

new

POM-based

[{Ag5(pz)7}(BW12O40)](1),

(Hpyr)[{Ag(pz)}2(PMo12O40)](3)

(pz=pyrazine,

pyr=pyrrole) have been fabricated via a hydrothermal synthesis method. All compounds were characterized by elemental analyses, infrared spectroscopy (IR), powder X-ray diffraction

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(PXRD), thermogravimetric (TG), scanning electron micrographs (SEM) and transmission electron microscope (TEM). Single-crystal X-ray diffraction displays the configuration of compounds 1-2 are parallel. In compound 1, the [BW12O40]5-is as 8-connected node incorporated into the orifice of the Ag-pz framework forming the glamorous POM-based metal organic frameworks (POMOFs). And the 3D POMOFs possess a kind of novel topology {42·54}2{44·512·68·73·9}{44·53·63}{44·55·6·74·9}{73}. There is a metal-organic nanotube structure constructed by four Ag-pz chains in compound 3. Then, the opened POMOFs consist of [PMo12O40]3-

clusters

encircled

in

metal–organic

nanotubes

with

the

topology

{34·46·54·6}2{34·48·58·64·74}. When used as electrode materials, as-prepared compounds 1, 2 and 3 have high specific capacitance of 1058F g-1, 986F g-1 and 1611F g-1 at a current density of 2.16A g-1, and favorable cycling stability, after 1000 cycles, the retention rates of the capacitance are 90.3%, 94.5% and 84.8% at 15.12A g-1 in supercapacitors, respectively. Moreover, the ability of compounds 1-3 to degrade dyes are excellent under UV irradiation. The degradation rates of RhB are 94.89% for 1, 93.06% for 2 and 96.88% for 3 after 150min, and the degradation rates of MB are 96.48% for 1, 94.28% for 2 and 98.58% for 3 after 90min. These results illustrate that compounds 1-3 have potential applications in energy functional materials and organic pollutant degradation.

INTRODUCTION Electrochemical energy storage and treatment of environmental pollution are gradually becoming a hot new direction and attracting increasing interest in society.1-7As a string of molecule metal oxygen cluster compound, polyoxometalates(POMs) formed by the dehydration of different acid, are a sort of unique inorganic nanoscale metal oxygen clusters possessing diverse

configuration

and

desirable

properties

in

magnetism,6-11


2

catalysis,12-23proton

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conduction24-26and molecular electronics.27-28In particular, owing to their high redox ability, high intrinsic electron storage capacity, high chemical tunability and high acidic stability, POMs are ideal components for electrochemical energy storage. In 1998, Yamada29 reported a low-priced electrochemical capacitor and it was the first time the H3PMo12O40 was anchored in a Nafion. The reversible specific capacitance was 112F g-1. However, as electrode materials, the low conductivity of POMs limited their performance in supercapacitors. Thus, it was a promising method to design high capacity energy storage system by combining POMs with redox activity as electronic storage sites and conductive nanomaterials with high surface area. In 2007, Ana Karina Cuentas-Gallegos30 et al. employed film electrodes composed of late-model hybrid materials which consist of multiwalled carbon nanotubes and phosphomolybdate polyanion with polyvinyl acetate as the cement to fabricate symmetric solid-state supercapacitors. The capacitance value was 285F g-1 at 200mA g-1. Nevertheless, the cycle stability and specific capacitance of Nano hybrid material based on POMs still needed to be further improved. In 2017, Ma31 et al explored(H2bpe)(Hbpe)2{[Cu(pzta)(H2O)][P2W18O62]}·5H2O as electrode of supercapacitor, exhibiting specific capacitance of 168F g-1 at 5A g-1, which manifested POMbased coordination polymer modified by the nitrogen-containing organic molecules and transition metals can be used to improve the cycle stability and specific capacitance. However, the achieved specific capacitance was still not good enough due to the presence of weak forces of some POMs (hydrogen bonds, supramolecular interactions) in low-dimensional structures reduced the rate of ion or electron transport. Consequently, it is still essential to fabricate POMbased materials with mul-dimensional structures (strong force) to overcome the drawback. In 2018, Lan32 et al. researched the supercapacitive performances of NENU-5 and its nanocomposites modified by Ppy. Their specific capacitances were 432 and 5147mF cm−2.


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Therefore, it is still a challenge to design and synthesize new compounds with excellent cycle stability and large specific capacitance to meet the needs of high performance energy devices. POMOFs have high-dimensional structures consisting of POMs and metal organic frameworks (MOFs). MOFs hold the crystalline ordered structure with well-defined adjustable pore size, large surface area, high porosity and good thermal stability.33-37 Consequently, it will be an efficacious way for origin POMs to improve its low surface areas, mechanical strength, highly water solubility and defective conductivity to combine MOFs. Based on the above researches, we intend to fabricate POMOFs by applying Keggin {XM12}n-, Ag ions, and pz ligands on the basic of the following considerations: (I) as one of the most researched POMs species, Keggintype anions have already become a sort of essential molecule's building blocks, since they own 36 oxygen atoms with coordinating ability and Td symmetry, that guarantee agile stereoscopic orientations and abundant covalent coordination. (II) Pyrazine with unique structure, showing straight-forward and manageable coordination patterns, make it easy to develop adjustable molecular designs on this basis. Furthermore, a porous framework composed of pyrazine is more conducive to large-sized POMs intercalation due to its small steric hindrance, which favors the construction of POMOFs. (III) The electronic configuration of silver ions are [Kr] 4d105s0, which belong to the electronic configuration of the closed shell. The coordination number of silver ions can vary from two-coordinate to eight-coordinate, which enhance chance for coordination with POMs. According to the above, we synthesized three novel POMOFs [{Ag5(pz)7}(BW12O40)](1), [{Ag5(pz)7}(SiW12O40)](OH)·H2O (2), (Hpyr)[{Ag(pz)}2(PMo12O40)](3) under different pH values, via a simple hydrothermal synthesis method. Moreover, compounds 1-3 were explored as electrode materials in supercapacitors. Moreover, we also investigated detailedly the photocatalytic properties of the compounds 1-3.


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EXPERIMENTAL Material synthesis Synthesis of compound 1.The K5BW12O40·15H2O was prepared in the light of the literature method.38 AgNO3 (0.15g, 0.88mmol), K5BW12O40·15H2O (0.40g, 0.13mmol), pz (0.11g, 1.37mmol) were put into H2O (18mL) and stirred for 30min. Adding 1 M HNO3 to regulate the pH=3.0. After that obtained solution was transferred to a 30mL Teflon-lined stainless steel reactor and maintained 180 ℃ for five days. The buff crystals were obtained by cooling to room temperature. (yield: 42% based on W). Analytical calculation for C28H28Ag5N14O40BW12 (Mr = 3956.89): Ag, 13.64; B, 0.27; W, 55.76; C, 8.50; H, 0.71; N, 4.95%. Found: Ag, 13.77; B, 0.26; W, 55.81; C, 8.41; H, 0.81; N, 4.81%. Synthesis of compound 2.The process for producing compound 2 was analogous to compound 1 except that substituted H4SiW12O40 for K5BW12O40·15H2O. Adding 1 M NaOH to regulate the pH=3.5. After five days, the yellow crystals were obtained by cooling to room temperature. (yield: 41% based on W). Analytical calculation for C28H28Ag5N14O42SiW12(Mr = 4006.17): Ag, 13.47; Si, 0.70; W, 55.07; C, 8.40; H, 0.70; N, 4.89%. Found: Ag, 13.45; Si, 0.71; W, 55.03; C, 8.39; H, 0.78; N, 4.91%. Synthesis of compound 3.The process for producing compound 3 was analogous to compound 2 except that substituted H3PMo12O40 for H4SiW12O40 and add pyr (1mL). After five days, the deep yellow crystals were obtained and the rate of production is 40% (based on W). Analytical calculation for C12H12Ag2N5O40PMo12 (Mr = 2264.26): Ag, 9.52; P, 1.36; Mo, 50.84; C, 6.36; H, 0.57; N, 3.09%. Found: Ag, 9.54; P, 1.37; Mo, 50.80; C, 6.39; H, 0.62; N, 3.05%. Electrochemical measurements.Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurements and electrochemical impedance spectroscopy (EIS) were carried out on a


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CHI660E electrochemical workstation in 1 M H2SO4 solution under traditional three-electrode system. The working electrodes were fabricated by the mixture of title compound and acetylene black in a certain ratio with ethyl alcohol. Then, the mixture (5μL) was dripped on to the surface of glassy carbon electrode (GCE). The as-prepared GCE was working electrode, the Ag/AgCl (3 M KCl) electrode was used as a reference electrode and a Pt wire was acted as the counter electrode. The measurement of photocatalytic.50mg of the sample was mixed in 10mg·L-1 aqueous solution of dye 100mL and stirred in the dark for 20min to assure adsorption equilibrium. Then, stirred and exposed to UV irradiation at the same time. At given time intervals, 4mL of samples were extracted and separated by centrifugation to gain clarification solution for analysis by UVvisible spectroscopy. RESULTS AND DISCUSSION Crystal structures The similar polyoxoanion of compounds 1-3 are the classic Keggin polyoxoanion building blocks, [BW12O40]5-({BW12}), [SiW12O40]4-({SiW12}) and [PMo12O40]3-({PMo12}). In the {BW12}, there is a W3O13 tripolymer which comprised three WO6 octahedra sharing edges combined in a triangular arrangement. Then, a centric {BO4} tetrahedron is embossed with four W3O13 tripolymer making up of a {BW12}. The {SiW12} and {PMo12} are similar to {BW12} except that the {BO4} is substituted with {SiO4} and {PO4}, as well as the {WO6} is substituted with {MoO6}. The bond distances of B-O, Si-O and P-O are 1.46-1.55Å, 1.55-1.65Å and 1.511.55 Å for compounds 1-3, respectively. While the distances of W-O and Mo-O can be be split into three types: W-Od 1.63-1.69Å, W-Ob/c 1.80(2)-1.94 Å and W-Oa 2.31-2.50Å, W-Od 1.64-


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1.69 Å, W-Ob/c 1.84-1.92 Å and W-Oa 2.32-2.49Å, Mo-Od 1.63-1.67 Å, Mo-Ob/c 1.86-1.96 Å and Mo-Oa 2.43-2.48 Å for compounds 1-3, respectively. Structural description of compound 1.Single-crystal X-ray diffraction analysis indicates that compound1 crystallizes in the triclinic and space group is P-1. The constitutional unit of compound 1 comprises one {BW12} anion, five Ag(I) ions, seven pz ligands (Figure S1). In compound 1, the Ag1 ion is six-coordinated in a slightly twisted octahedron achieved by two O atoms from two {BW12} anions and four N atoms of four pz ligands, with bond distances of AgO for 2.77-2.80Å, Ag-N for 2.33(3)-2.46(5) Å and the angle of N-Ag-N for 83.1(16)-178.3(16)°. The Ag2 ion exhibits a “Y”-shaped geometry, linked by three N atoms from three pz ligands, with the bond distances of Ag-N for 2.19(3)-2.26(3)Å and angles of N-Ag-N for 119.1(10)120.5(9)°. Ag3 ion is connected by two O atoms from two {BW12} anions and three N atoms from three pz ligands forming a trigonalbiyramid, with bond distances of Ag-O for 2.64-2.68Å, Ag-N for 2.16(3) and 2.27(5) Å and angle of N-Ag-N for 107.9(16)-144.1(15)°. Ag4 and Ag5 ions adopt the distorted tetrahedron, connected by two N atoms from two pz ligands, two O atoms from{BW12} polyanion, with bond distances of Ag-O for 2.44-2.61Å, Ag-N for 2.19(3)2.26(4) Å and angle of N-Ag-N 86.4(10)-124.2(10)°. In the structure of compound 1, the adjacent Ag5, Ag1, Ag4, Ag3 and Ag5 ions connected by bridging pz ligands constructed a Ag-pz chain (Figure S2a). As shown in Figure S2b, each {BW12} anions are linked with Ag1 and Ag3 ions of the Ag-pz chain to form a wave-like 1D chain by terminal oxygen. In the 1D chain, there are two kinds of {BW12} anions (Abbreviated as B1, B2) which arranged in the form of ABAB. The adjacent 1D chains are farther connected by organic pz linkages to shape a 2D layer (Figure 1a). Significantly, there are symmetrical 10-


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Figure 1. Structure of compound 1. (a) the 2D layer; (b) the neighboring layers combined by the Ag2-pz chain and Ag-O bonds; (c) Keggin ions embedded in the orifices of the 3D Ag-pz framework.

Figure 2. Topology structure of compound 1.


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nuclear group {Ag(pz)}10 in the layer, which the size of each void is ca. 12.94Å× 23.065Å(Figure S3).Another interesting feature for the 2D layer is the two kinds of {BW12} anions located on either side of the void from the view of b axis (Figure S4). The two types of {BW12} anions coordinate in the same way:{BW12} anions serve as a tridentate ligand coordinating with Ag ions through Ag-O bonds, except for the oxygen atoms connected to the Ag atoms [Ag1-O20, Ag3-O14, Ag5-O3 for B1 and Ag1-O5, Ag3-O13, Ag4-O9 for B2]. The parallel 2D layers are bridged by the Ag-pz zigzag chain which constructed by the Ag2 ions connecting pz ligands and Ag-O bonds (Ag5-O11 is yellow and Ag4-O18 is green) (Figure 1b), generating a 3D structure with several unique features. In the pilotaxitic POMOFs 3D network, there is a large parallelogram-like void in which a symmetrical {BW12} polyanions are occupied and connected to eight Ag atoms(2Ag5, 2Ag1, 2Ag4, 2Ag3) of 3D Ag-pz MOFs (a metal organic frameworks constructed by Ag ions and pz ligand) as inorganic ligand (Figure 1c).According to the topological image, assume each Ag(1) as a 6-connected node, Ag(2) as 3connected node, Ag(3) as 5-connected node, Ag(4) and Ag(5) as 4-connected node, and {BW12} anions as a 8-connected node (B1 is yellow, B2 is green and Ag ions are purple ), then the structure of compound 1 can be condense into regular network with a original {42·54}2{44·512·68·73·9}{44·53·63}{44·55·6·74·9}{73} topology (Figure 2). Structural description of compound 2.Single-crystal X-ray diffraction analysis indicates that compound 2 crystallizes in the monoclinic and space group is C 2/c. The constitutional unit of compound 2 comprises one {SiW12} anion, five Ag(I) ions, seven pz ligands, one hydroxyl and one water molecule (Figure S5). The structure of compound 2 is parallel with compound 1 aside from substituting Ag1, Ag3, Ag5 and {BW12} anions with Ag3, Ag1, Ag4 and {SiW12} anions.


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In addition, there is only one type of {SiW12} anions. The topology of Compounds 1 and 2 are identical(Figure S6). Structural description of compound 3.Single-crystal X-ray diffraction analysis indicates that compound 3 crystallizes in the orthorhombic and space group is Cmc21. The constitutional unit of compound 3 comprises one [PMo12O40]3- anion, two Ag ions, two pz ligands and one protonated pyrrole ligand, as shown in Figure S7. In compound 3, the {PMo12} cluster plays the part of an octodonate inorganic node to connect eight Ag ions (Figure S8). Each independent Ag ion is linked by four O atoms from four {PMo12} anions, and two N atoms of two pz ligands exhibiting a slightly twisted octahedron, with bond distances of Ag-O for 2.65-2.77Å, Ag-N for 2.156(10) and 2.179(10)Å and angle of N-Ag-N for 169.5(4)°. In the structure of compound 3, the adjacent Ag1 ions connected by bridging pz ligands construct a Ag1-pz chain (Figure S9a). A metal-organic nanotube is built by the Ag-pz chains quadrangularly arranging. And the symmetrical eight-connected {PMo12} polyanions play the part of objects locating the channels of metal-organic nanotube to form a 1D structure. Further, pyrrole molecule encircles in the hole between the adjacent {PMo12}anion(Figure S9b). Each {PMo12} cluster forms a 2D layers by linking {Ag2(pz)}, with an aperture of 12.62×12.62Å2, which pyrrole molecule encircles in the viod (Figure 3a). In the 2D layers, each {PMo12} cluster links four adjacent {Ag2(pz)} (Figure S10a) surrounded by four peripheral {PMo12} clusters to make up a square structure. Interestingly, each {Ag2(pz)} is shared by six staggered {PMo12}(Figure S10b). Eventually, the 2D layers are linked the 1D metal-organic nanotubes to construct highly opened POMOFs (Figure 3b). According to the topological image, assume each Ag(1) ion as a 6-connected node and {PMo12} anions as a 8-connected node, then the structure


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of compound 3 can be condense into regular network with a {34·46·54·6}2{34·48·58·64·74} topology (Figure 3c).

Figure 3. Structure of compound 3.(a) view of the 2D layer; (b) view of the 3D structure; (c) view of the topology structure. Structural and morphology characteristics of compounds 1-3 IR Spectrum. The IR spectrum of compounds 1-3 are presented in Figure S11. In Figure S11a, 961, 918,823 and 750cm-1 are owing to ν(W-Od),ν(B-Oa), ν (W-Ob) and ν(W-Oc), the bands are typical for the reported {BW12} anion. In Figure S11b, 1010, 952, 902 and 785cm-1 are due to ν(W-Od), ν(Si-O), ν(W-Ob) and ν(W-Oc), the bands are typical for the reported {SiW12}


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anion. In Figure S11c, 961, 1065, 884 and 797cm-1 is thanks to ν(Mo-Od), ν(P-O), ν(Mo-Ob) and ν(Mo-Oc), the bands are typical for the reported {PMo12} anion. Moreover, vibration bands ranging from 1400 to 1640cm-1 in compounds 1-3 are ascribed to the pz and pyrrole ligands, while the vibration bands ranging from 3450 to 3470cm-1 in compounds 1-3 are ascribed to ν(OH). TG Analysis. To study the thermal stabilization of compounds 1-3, the TG analysis were explored under O2 atmosphere from 20 to 800℃.Showed in Figure S12, compound 1 displays two-step weight losses. The first weight loss of 9.26% (Calc. 8.1%) range from 110 to 318℃, ascribed to four pz molecules. The second step loss of 5.44% (Calc. 6%) occurs within the scope of 342-630℃, attributed to remaining pz ligands molecules. Only one-step weight loss presents in compound 2. The progress of decomposition range from 340 to 440 ℃ with weight loss of 14.1%, can be assigned to seven pz molecules, one hydroxyl and one H2O molecule (Calc. 14.85%). Compound 3 is the same as compound 2, presents one-step weight loss. The loss stage of 10.15% (Calc. 10.03%) arises within the scope of 330 to 485 ℃ , attributed to the loss of a pyrrole ligand and two pz ligands. The results of TG analysis are in step with the crystal structure analysis. XRPD Analysis. The XRPD analysis of compounds 1-3 are presented in Figure S13.The simulated diffraction peaks are in accord with experimental patterns which illustrate the degree of purity of the compounds 1-3. There is only a little different in the intensity of diffraction peaks for compounds 1-3, may be as a result of the preferred orientation of the crystal samples. SEM and TEM. SEM and TEM were also conducted to further confirm the morphology and microstructure of compounds 1-3. The representative SEM images of compounds 1-3 display in Figure 4a and Figure S14a-15a. As observed from the amplified SEM images, it can be seen that


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an ordinary polyhedron with a length and breadth of approximately 300 and 220μm for 1, 320 and 200μm for 2 and 336 and 220μm for 3, respectively. The corresponding TEM analysis (Figure 4b, S14b-15b) further confirms the existence of Keggin cluster. In the TEM images of compounds 1-3, it is evident for POM modication that numerous dark spots distributed. These spots are approximately 4-7nm in diameter, which are the monodispersed approximate diameter of the Keggin cluster. The average sizes of Keggin particles are about 6.4nm for 1, 5.8nm for 2 and 4.1nm for 3. EDS micro-analysis of compounds 1-3 are shown in Figure S16. EDS mapping of a single crystal of compounds 1-3 were measured, finding out more information on compound, that they were made up of C, N,O, B, Ag and W for compound 1, C, N, O, Si, Ag and W for compound 2, and C, N, O, P, Ag and Mo for compound 3, respectively. Furthermore, it is obvious that the elements are uniformly distributed in Figure 4c-h and Figure S14-15c-h. The results of SEM and TEM are in keeping with single-crystal X-ray diffraction analysis.


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Figure 4. The morphology images of compound 1. (a) SEM image; (b) TEM image; (c-h) EDS elemental mappings of C, N, O, B, Ag, and W. Properties of compounds 1-3 Cyclic voltammetry studies.With 1 M H2SO4 aqueous as the electrolyte, electrochemical performances of compounds 1-3 were implemented in a three-electrode system. The CV curves of K5BW12O40-based electrode ({BW12}-based electrode), H4SiW12O40-based electrode ({SiW12}-based electrode), and H3PMo12O40-based electrode ({PMo12}-based electrode) in the voltage window range from -0.2 to 0.7V, -0.1 to 0.7V and 0 to 0.7V at scan rates from 20 to 500mV s-1 are exhibited in Figure S17. And 1-based, 2-based, 3-based electrodes within the same voltage range as their precursor-based electrodes are displayed in Figure 5a-c. The comparison of CV curves of 1-basedand {BW12}-based electrode, 2-based and {SiW12}-based electrode, 3-based and {PMo12}-based electrode at 20mV s-1 scan rate are presented in Figure S18(a-c). Obviously, the CV curves of 1-based, 2-based and 3-based electrodes exhibit reversible redox peaks, which all show pseudocapacitor. In addition, the CV curves of 1-based, 2-based and 3-based electrodes display larger enclosed area than their precursor-based electrodes at the same scan rate, and it is possible that the highly porous Ag-MOF materials (compounds 1-3) supply more passageways for the charge transfer improving their capacitances. Furthermore, the redox peak still maintains original shape at 100mV s-1, indicating the excellent electrochemical behavior of the compounds 1-3. The extensive charge storage kinetics in the 1-based, 2-based and 3-based electrodes were obtained from CV.6 Popularly, the mathematical relationship (1) of the log of current density in the CV curve to the corresponding log of scan rate can be used to illustrate the charge storage kinetics,


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i = avb(1) where i is the current (A), a and b are constants, and v is the scan rate (mV s-1). The calculated b value can be applied to estimate the capacitive contribution and the diffusion controlled charge contributions. In the event of the value of b is 0.5, it is resulted from the diffusion-controlled charges (Qd), and if b is 1, it is due to the surface capacitive (Qs). Figure 5d and Figure S19 exhibit b (slope of the line drawn from dots) is 0.90 for the 1-based electrode, 0.91 for the 2based electrode and 0.89 for the 3-based electrode, respectively. Therefore, the entire electrical charge (Q) contribution is divided into two parts with the following formula, Q = Qs + Qd (2). In addition, the entire electrical charge stored can be analyzed by the relationship (3)39 Q = Qc + kv ―1/2(3) that k is an adjustable parameters. Figure 5e and Figure S20 show the plot of stored charge vs. the scan rate to the power of -0.5 (v-1/2) for compounds 1-3. The Qs and Qd are estimated and displayed in Figure 5f and Figure S21, illustrated the contribution rate of surface capacitive and the diffusion-limited capacities to the total value when scan rates range from 30 to 80mV s-1. Obviously, the capacitance resulted from the capacitive course is 75.83% for 1-based electrode, 77.84% for 2-based electrode and 73.76% for 3-based electrode with a scan rate at 30mV s-1. At the same scan rate, capacitance from the diffusion-controlled process is 24.17% for 1-based electrode, 22.16% for 2-based electrode and 26.24% for 3-based electrode. Qs increasing corresponds to Qd droping when the scan rates change from 30 to 80mV s-1. From the graph, it is obvious that Qs is more important in the 1-3 based electrodes and it demonstrates the pseudocapacitive nature of the 1-3 based electrodes.


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Figure 5. (a-c) CV curves of the 1-based, 2-based and 3-based electrodes at scan rates of 20, 30, 40, 50, 60, 80, 100, 120, 150, 200, 250, 300, 350, 400, 450 and 500mV s-1; (d) the image of log of current density vs. the log of scan rate with the scan rate change from 30 to 80mV s-1 for 1based electrode; (e) the image of the Q vs. the scan rate to the power of -0.5 (v

-1/2);

(f)

contribution ratio of the Qs and Qd; (g-i) GCD curves for the 1-based, 2-based and 3-based electrodes at the current density of 2.16A g-1, 4.32A g-1, 6.48A g-1, 8.64A g-1, 10.8A g-1, 12.96A g-1, 15.12A g-1. Charge/discharge studies.To investigate the capacitive performance of all three electrodes (1based, 2-based and 3-based), GCD were implemented within the same voltage range and


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electrolyte as CV at different current densities. The discharge curves are consistent well with the CV curves, indicating the capacitance is a pseudocapacitive behavior caused by the faradaic redox reactions of electrode (Figure 5g-i). In addition, the specific capacitances (Cs) are calculated on the basis of discharge curves with the formulas below: I × ∆t

𝐶𝑠 = m × ∆V (5) That m is the mass of the active electrode material (g), ∆t is the discharge time (s), ∆V is the operating potential (V) and I is the discharge current (A). At current densities of 2.16A g-1, the Cs were 1058, 986, 1611 F g-1for 1-, 2- and 3-based electrode, respectively.Figure S22 further display the good rate capabilities of these electrodes at the different current density. Both of them are higher than POMOFs electrode materials reported in the literatures.31,40 In addition, we compared with the reported other POMs-based(Table S5) and MOF-based(Table S6) electrode materials, and the results showed that compounds 1-3 have high specific capacitance. The obverse of rising current density is continued the specific capacitance decreasing. The decreasing reason may be that when the current density is too large, here is some sacrifice of efficiency of the active material. Accordingly, the effective reaction between the electrode and the electrolyte is also reduced. Further, the energy density (E) and power density (P) are calculated on the basis of discharge curves with the expressions below:41,42 E=

Cs × ∆V2 2 × 3.6

(6)

P=

3600 × E ∆t

(7)

That ∆t is the discharge time and ∆V(V) is the discharge voltage window. Ragone plot of E versus P for 1-based, 2-based and 3-basedelectrodes are shown in Figure S23. The largest E can run up to 119W h kg-1 with a P of 971W kg-1 for 1-based electrode, 87.7W h kg-1 with a P of 865W kg-1 for 2-based electrode and 109.6W h kg-1 with a P of 755W kg-1 for 3-based electrode.


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Moreover, comparing with their simple POM (Figure S24), GCD measurements demonstrate the capacitance of the 1-based, 2-based and 3-based electrodes are preferable. The metal organic frameworks of compounds 1-3 supply more channels to accelerate transition rates of electron as well as supply more sites to increase the storage of electrochemical energy, which consumedly enhances the electroactive of the materials and lessens the internal resistance. The contrast images of charging-discharging curves of 1-based and {BW12}-based, 2-based and {SiW12}-based and 3-based and {PMo12}-based electrodes are shown in Figure S25. Electrochemical cycle stability and electrochemical impedance studies. Cycling performance of electrode materials is another significant capability for the actual application of supercapacitors. Cycling performance of 1-based, 2-based and 3-based electrodes were measured at 15.12A g-1 for 1000 cycles. As exhibited in Figure 6a-c, after 1000 cycles, 1-based, 2-based and 3-based electrodes can still retain 90.3%, 94.5% and 84.8% of the primary specific capacitance, respectively. From the inset, the curve shape has no significant change at the beginning and the end of the seven cycles. Each curve has analogous time-potential response behavior, illustrate the outstanding electrochemical stability of compounds and good application prospect in the field of supercapacitors. As one of the primary means to make out the electrochemical performances, the EIS was measured in the frequency region of 10-2-105Hz at 1st and 1000th cycle with 1M H2SO4 solution as the electrolyte. The Nyquist plots are made up of two parts: the diagonal lines in the low frequency and a hemicycle in the high frequency which corresponded to the Warburg resistance and the charge-transfer resistance, respectively (Figure 6d-f).An equivalent electric circuit is established according to the experimental impedance spectra, as displayed in Figure S26. The intercept of the plots at the axis of reals close to zero, indicates solution resistance (Rs) are 5.037 for 1-based electrode, 4.932 for 2-based


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electrode and 4.073 for 3-based electrode, respectively. The large slope of straight line at the low frequency scope, demonstrates little Zw of the H2SO4 electrolyte ion diffusion, namely, the slight diffusion impedance of the 1M H2SO4 electrolyte. Low solution resistance and Warburg resistance make charge transfer faster and easier in electrolyte solution, illustrating the excellent electrochemical performance of 1-based, 2-based and 3-based electrodes. Furthermore, there is only slight change for the Warburg resistance and the solution resistance after 1000 cycles.

Figure 6. (a-c)Cycling stability of compounds 1-3 after 1000 cycles (the inset are the first and the last seven cycles of compounds 1-3); (d-f) Impedance analysis of the 1-based, 2-based and 3based electrodes. The superior supercapacitor performance of the 1, 2 and 3-based electrodes might be attributed to the several mechanisms: (I)Keggin-type POMs inlaid in the metal organic framework with excellent redox properties, participate in the redox reaction to supply pseudocapacitance, so Keggin-type POMs are favorable candidate to serve as the supercapacitor electrode electrodes with high-performance.(II) MOFs are crystal-ordered structures, possessing high surface area and abundant channels and holes, which can accommodate the Keggin-type POMs embedded. In


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addition, they also serve as Faraday pseudo-capacitors, which are conducive to accelerate the rates of charge storage/transport and ion diffusion. (III) Compounds 1-3 have three-dimensional structure, which will effectively overcome the shortcomings of low ion or electron transport rate due to the existence of weak forces (hydrogen bond, supramolecular interactions) and insufficient performance of POMs-based low-dimensional structures in electrochemical energy storage. (IV) Compounds 1-3 are fabricated via a hydrothermal synthesis method and insoluble in water, which prevents polyacid from dissolving in electrolyte solution, significantly improving the performance in supercapacitor. Photocatalytic. It is well known that worked as photocatalysts to degrade organic dyestuff, POM-based compounds are good candidate. In addition, the degradation efficiency of dyes varies greatly with the structure and constituent of POMs.43-48 Therefore, the degradation of RhB and MB solution as model organic pollutants were investigated. In order to compare, the degradation of organic dyestuff without photocatalyst in the same situation was investigated. As shown in Figure S27, obviously, in the presence of photocatalyst, when reaction time raises, the intensity of the RhB absorption peaks lessen. Moreover, in the light of Figure 7a, it is clear that approximately 94.89% of RhB for 1, 93.06% of RhB for 2 and 96.88% of RhB for 3 has been decomposed during 150min, while dissociation of RhB is no more than 10% without any catalyst. Similarly to RhB, as the reaction time increases, the absorption peaks of MB obviously decrease (Figure S28). As exhibited in Figure 7b, the photocatalytic decomposition rates increase from 8.77% (without catalysts) to 96.48% for 1, 94.28% for 2 and 98.58% for 3 after 90min of irradiation. The results are higher than reported in the literature (Table S7) and indicate that compounds 1, 2 and 3 possess prominent photocatalytic activity to degrade RhB and MB. In the surface of the POM, some effective reactions occurred, resulting in photocatalytic activity.


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Exciting POM with light energy to obtain catalytic activity, form the excited-state species (POMs) and bring about an intermolecular O-M charge transfer.49,50Shining an ultraviolet light on compounds 1-3, generates motivated state Keggin-based species and O-W/O-Mo charge transfer. And then, produce considerable electrons and holes to oxidize the RhB and MB dye. Besides, MOFs, possessing highly porous architecture in numerous varieties, consisted of inorganic units and organic ligands, with high surface area and hollows which are sufficient for diffusing molecules, even though the POMs clusters are encircled. Hence, it is feasible for POMOFs with porous to perform diversiform chemical reactions, for instance, catalysts.

Figure 7. Plot of reaction time vs. the concentration of dyestuff with compounds 1-3. (a) RhB (b) MB; Changes of concentration for RhB (c) and MB (d) under the same condition with repeating compound 3 in the reactions.


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In addition, we recycled compound 3 which was isolated and collected. At same reaction conditions, we reused the required compound 3 to degrade MB and RhB for four times. The photocatalytic decomposition rates of compound 3 after recycling four times are displaying Figure S29. As presented in (Figure 7c-d), compared with the first time, after four recycles of the catalysts, the detection indicated that there was only little increasing of residual concentration. Figure S30 displays the IR spectra of the compound 3 and after recycling. Experimental results indicate that the structure and photocatalytic activities of compound 3 are unchanged after four cycles.

CONCLUSIONS Briefly summarizing the results above, we have managed to synthesize three new Keggin-anion anchored on pz-based frameworks by altering the different Keggin-type POMs. Compounds 1 and 2 are 3D Ag-pz framework encapsulating {BW12} or {SiW12} templates, while compound 3 is a 3D nanotube incorporating {PMo12} clusters. Compounds 1-3 display high Cs of 1058F g-1, 986F g-1and 1611F g-1 at 2.16A g-1, respectively, when served as the electrode materials in supercapacitors. The Cs retention rates are retained at 90.3% for 1, 94.5 % for 2 and 84.8 % for 3 after 1000 cycles. In addition, the degradation rates of RhB are 94.89% for 1, 93.06% for 2 and 96.88% for 3 after 150min, and the degradation rates of MB are 96.48% for 1, 94.28% for 2 and 98.58% for 3 after 90min, showing good application in photocataly. This work reveals the novel and effective design by the construction of Keggin-type POMs based Ag-pz metal organic framework is promising for bifunctional materials. ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXXX. Details of crystallographic data; the structures of compounds 1-3; characterization of compounds 1-3; electrochemical and photocatalytic data of compounds 1-3 (PDF) Crystallographic data of compounds 1-3(CIF) Accession Codes CCDC1883018−1883020 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. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Bai-Bin Zhou: 0000-0001-8304-5092 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources Baibin Zhou received funding from the National Natural Science Foundation of China Grants 21571044, 21771046 and the Natural Science Foundation of Heilongjiang Province Grants JJ2016JQ0037, B2017007. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported the National Natural Science Foundation of China (Grants Nos. 21571044 and 21771046) the Natural Science Foundation of Heilongjiang Province (JJ2016JQ0037 and B2017007). REFERENCES 1. Li, M. T.; Cong, L. T.; Zhao, J.; Zheng, T. T.; Tian, R.; Sha, J. Q.; Sua, Z. M.; Wang, X. L. Self-organization towards complex multi-fold meso-helices in the structures of WellsDawson polyoxometalate-based hybrid materials for lithium-ion batteries. J. Mater. Chem. A, 2017, 5, 3371-3376 2. Zhang, M.; Zhang, A. M.; Wang, X. X.; Huang, Q.; Zhu, X. S.; Wang, X. L.; Dong, L. Z.; Li, S. L.; Lan, Y. Q. Encapsulating ionic liquids into POM-based MOFs to improve their conductivity for superior lithium storage. J. Mater. Chem. A, 2018, 6, 8735-8741


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Nanocomposites Containing Keggin Anions Anchored on Pyrazine-Based Frameworks for Use as Supercapacitors and Photocatalysts Nana Dua,b, Lige Gonga,b, Lingyu Fana,b, Kai Yua,b,*, Huan Luoa,b, Shengjie Panga,b, Jiaqian Gaoa,b, ZhuwuZhenga,b, Jinghua Lva, Baibin Zhoua,b,*

Graphical Abstract

Three novel Keggin-type POMOFs exhibit high-performance for supercapacitors and photocatalysts.


Nanocomposites Containing Keggin Anions ... - ACS Publications

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