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A 3,6-connected 3-D arsenotungstate framework based on unique sandwich-type metal-organic dimer chain
Inorganic Chemistry Communications 98 (2018) 87–91
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A 3,6-connected 3-D arsenotungstate framework based on unique sandwichtype metal-organic dimer chain
T
Liping Cuia,b, Jinghua Lva, Kai Yua,b, , Xinyue Maa,b, Wenting Daia,b, 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 Key Laboratory of Photochemical Biomaterials and Energy Storage Material, Heilongjiang Province, Harbin Normal University Harbin, 150025, People's Republic of China
b
GRAPHICAL ABSTRACT
A 3,6-connected 3-D arsenotungstate framework based on unique sandwich-type metal-organic dimer chain has been synthesized, which shows electrocatalytic performance for reduction of H2O2 and outstanding catalytic degradation activity for typical dyes MB, RhB and MO under UV light.
ARTICLE INFO
ABSTRACT
Keywords: Hydrothermal synthesis Keggin arsenotungstate Photocatalytic properties Electrocatalytic behavior
A 3-D inorganic-organic network based on kiggen-type arsenotungstat, [Ag(4,4′-bpy)]2[{Ag(4,4′V bpy)}2(AsWVI 11W O40)]·3H2O(1) (4,4′-bpy = 4,4′-bipyridine), has been synthesized by hydrothermal means. Xray single crystal analysis shows that compound 1 contains two crystallography independent silver ions, which adopt different coordination modes connecting bpy ligand alternately to form two kinds of metal-organic chains {Agbpy}n. Two parallel {Ag(1)bpy}n chains are linked together via {AsW12O40} linkers to result in a unique sandwich-type dimer chain along a axis. Each Keggin cluster on sandwich chain bonds with six cognate clusters on six adjacent sandwich chains via surrounding six Ag, forming a 2-D layer along c axis. The 2D layer and 1D sandwich chains are intersected to yield a 3D network with 1-D channel. The {Ag(2)bpy}n chains and lattice water reside in the channel and further stabilize the 3-D framework via H-bonding and π-π accumulation. From topological perspective, compound 1 can be viewed as a 3, 6-connected network with {43}2{46.66.83} topology. Compound 1 shows merit electrocatalytic activity for reduction of H2O2 and efficient catalytic degradation ability for three typical dyes under UV radiation.
With the development of modern science and technology, the concept of green chemistry has been paid more and more attention, especially in the field of material chemistry [1]. It is particularly important to develop photocatalytic materials with excellent catalytic effects to
control air and water pollution [2]. Polyoxometalates (POMs) have unique physical and chemical properties such as the tailorability of structure, the controllability of the property, high thermal stability, strong acidity, reversible multi-electron redox properties, and thus are
⁎ Corresponding authors at: Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, Harbin Normal University, Harbin 150025, People's Republic of China. E-mail addresses: [email protected] (K. Yu), [email protected] (B. Zhou).
https://doi.org/10.1016/j.inoche.2018.10.005 Received 4 September 2018; Received in revised form 25 September 2018; Accepted 8 October 2018 Available online 11 October 2018 1387-7003/ © 2018 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 98 (2018) 87–91
L. Cui et al.
widely used in various fields such as photocatalysis [3,4], electrochemistry [5,6], medicine [7,8] and materials science [9,10]. Among them, the Keggin-type POMs [XM12O40]n-(X = B, P, Si, Ge, As; M = W, Mo, V) as ideal inorganic building units are widely studied for their relatively wide synthesis conditions, high structural stability, and suitable size [11]. Many complicated Keggin derivatives can be synthesized by hydrothermal process using simple starting material or by routine solution method with precursor for their broad isolation environment [12]. Keggin compounds have very high structural stability for their highly symmetrical structure. Therefore, such materials are widely used in the field of catalysts due to their advantages of good catalytic stability, easy recovery, and long cycle life [13]. In addition, suitable size endow Keggin compounds with smaller steric hindrances and more flexible bonding modes. Therefore, they can not only be introduced into the metal-organic network in the form of guest molecules to form POMOF structure, but also connect organic ligands through secondary metal linking units to form organic-inorganic hybrids. Despite their many advantages, the arsenotungstat series is a relatively undeveloped field compared with other Keggin compounds. Only a handful of {AsW12O40}-based hybrids have been obtained so far, and most of them are zero-dimensional (0-D) structures. The 3-D highconnected Keggin arsenotungstats are still less common [14–16]. The application of these compounds is also very limited. In the contexts, further explore 3-D high-connected arsenotungstat architectures are necessary and meaningful in this branch. Transition metal silver ions are selected in our exploration for their soft d10 configuration and strong coordination ability with rigid ligands. They can take different bond numbers and show various bonding geometry such as linear, “square”, “tetragonal pyramid”, “trigonal-bipyramidal”, and so on [17–19]. Thus, Ag(I) and 4,4′-bpy ligand are introduced into the {AsW12O40} reaction systems in order to explore 3-D high-connected assemblies with outstanding properties. Fortunately a new 3,6-connected Keggin arsenotungstate based on unique organic-inorganic sandwich chain, [Ag(4,4′-bipy)]2[{Ag(4,4′V bipy)}2(AsWVI 11W O40)]·3H2O [20]. The electro- and photo-catalytic behaviors of compound 1 are studied in detail. X-ray diffraction analysis (Table S1 and S2) [21,22] reveals that compound 1 crystallizes in the monoclinic crystal system with the C2/ m space group. One basic unit of compound 1 consists of a single electron reduced {AsW12O40} (abbreviated as {AsW12}) polyanion cluster, four Ag+, four bpy ligands, and three lattice water molecules (Fig. S1). There are two crystallographically independent silver ions exhibit two kinds of bonding geometries coexisting in compound 1. Ag1 adopts five-coordinated mode defined by three oxygen atoms from three Keggin spheres with the distance of Ag1-O9, 2.661(2) Å and two nitrogen atoms from two bpy molecules with the distance of Ag1-N1, 2.202 Å (Fig. S2). Ag2 is linearly geometrically linked to two nitrogen atoms from two bpy ligands with Ag2-N2, 2.21(2) Å. In this way, each of Ag (Ag1 and Ag2) links bpy ligand alternately to form two kinds of parallel metal-organic chains {Agbpy}n (Fig. S3). Two Ag1 on adjacent
two {Ag(1)bpy}n chains bond to two opposite terminal O atoms of the same keggin sphere in a “T-type” geometry resulting in a unique sandwich-type dimer chain {Ag2(4,4′-bipy)2(AsW12 O40)}n (Fig. 1). Each Keggin cluster of sandwich chains links with six cognate clusters of six adjacent sandwich chains via surrounding six Ag(1), forming a 2D layer along c axis (Fig. 2). The 2-D layer and 1-D sandwich chains are intersected to yield a 3-D network with 1-D channel (Fig. S4 and S5). The size of the channel: 6.28 Å (O6-O6) × 7.70 Å (Ag1-Ag1). The other metal organic chain {Ag (2)bpy}n and lattice water molecules reside in the tunnel and further stabilize the 3-D framework via H-bonding and π-π accumulation. The {AsW12} clusters can be regarded as hexa-connected nodes, and Ag1 is tri-connected node, thus the overall structure of compound 1 can be simplified to a 3, 6-connected 3-D framework with {43}2{46.66.83} topology (Fig. 3). As shown in Fig. S6, IR spectra of compound 1 display the classical vibration peaks of the Keggin {AsW12} anions at 985, 873, 749, and 662 cm−1, which can be assigned to v(W-Ot), v(As-Oa), v(W-Ob), and v (W-Oc), respectively [23]. The bands at 1556 and 1742 cm−1are attribute to v(CeN) and v(CeC) for organic ligand 4,4′-bpy. The bands at 3604 cm−1 corresponded to v(NeH) and/or ν(OeH) vibration of of organic ligands and water molecules. The thermogravimetric (TG) analyses of compound 1 was carried out in flowing N2 in the temperature range of 25–800 °C, as shown in Fig. S7. The title compound shows two step weight losses: 1.41% at 160–250 °C and 16.12% at 300–650 °C, which corresponding to the loss of all the water molecule (calculated value 1.34% for 3 H2O) and organic ligands (calculated value 15.50% for 4 bpy), respectively. The total weight loss is about 17.53%, which is close to the calculated value of 17.46%. Cyclic voltammetric behaviors of compound 1 were investigated with 1-modified carbon paste electrodes (1-CPE) in 1 mol·L−1 H2SO4 aqueous solution at different scan rates. As shown in Fig. 4a, three pairs of reversible redox peaks with the average peak potentials E1/ 2 = (Epa + Epc)/2, 0.203 V (I/I′), −0.140 V (II/II′), and −0.452 V (III/ III′) for 1-CPE in the potentials range of −0.8 to 0.7 V. Two pairs of redox peaks, II/II′ and III/III′, for 1-CPE are attributed to two successive bi-electron redox processes of the {AsW12} polyanions. The peaks I-I′ for 1-CPE should be assigned to the redox couple of AgI/Ag [24]. The cathodic peaks of 1-CPE shift to the lower potentials whereas the corresponding anodic peaks shift to the higher potentials with increasing of the scanning rates, as depicted in Fig. 4a. The plotting of the peak currents (II) of the anode and cathode against scanning rates are shown in insert of Fig. 4a. The peak currents were proportional to the sweep speed, which indicating that the redox process of 1-CPE is surfacecontrolled. The electrocatalytic properties of 1-CPEs were also explored. As can be seen from Fig. 4b, the peak currents vary gradually following the addition of H2O2: the reduction peak currents increase while the corresponding oxidation peak currents decrease dramatically, which indicate that H2O2 was reduced by {AsW12} polyoxoanion species. In contrast, the electroreduction of H2O2 at a naked electrode
Fig. 1. The 1D sandwich dimer chain of compound 1 along a axis. 88
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Fig. 2. The 2D layer based on {AsW12} cluster and Ag(bpy)2 linker along c axis.
Fig. 3. The polyhedral and ball-and-stick representation of 3D inorganic-organic network with 1D channel, and the simplified framework with {43}2{46.66.83} topology of compound 1.
generally needs a high overpotential and no obvious response could be detected at a bare CPE, which suggests that 1-CPEs have good electrocatalytic activities toward the reduction of H2O2.
Typical azo dyes methylene blue (MB), Rhodamine B (RhB), and Methyl Orange (MO) were selected as substrates to estimate the photocatalytic behavior of compound 1 under UV irradiation. The
Fig. 4. (a) Cyclic voltammograms of 1-CPE rates (from inner to outer: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, and 220 mV s−1) (Insert plots: The dependence of cathode and anode peak currents II on scan rates.); (b) Cyclic voltammograms of 1-CPE in 1 M H2SO4 solution containing H2O2 at different concentrations under the scan rate of 50 mV∙s−1 (Potentials vs. SCE). 89
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Fig. 5. The UV-vis absorption spectra of the MB, RhB, and MO solutions during the decomposition reaction under UV irradiation in the presence of compound 1.
degradation of dyes was monitored by UV-vis spectra and the decolorization rates of the dyes, which are shown in Fig. 5 and Fig. S8. With the increase of irradiation time, the characteristic absorption peaks at 665 and 610 nm for MB, 550 nm for RhB, and 460 nm for MO were decreases continuously under the same conditions. The degradation rate, defined as 1-C/C0 (C0 = initial concentration, C = concentration after irradiation) are 97.8% for MB, 92.9% for RhB, and 83.6% for MO after 90, 110, and 140 min, respectively. On the contrary, only small amounts of the substrates were degraded under the same experiment conditions without compound 1. The absorption wavelength of three dyes in the UV region did not shift during the degradation process, and the characteristic peaks of them had almost disappeared by the final stages of catalytic degradation, which indirectly suggests that the heterocycle groups had been destroyed. Furthermore, compound 1 shows different photocatalytic activities on three kinds of substrates, which can be due to their different structures and specific degradation pathways. Compared with reported other Keggin hybrids, compound 1 exhibit better degradation ability for above dyes under similar reaction conditions [25], which can be attributed to two reasons. First, {Ag (bpy)2} complexes chains promote electron transfer between {AsW12} cluster, which facilitates the rapid migration of electrons to the surface of POM. Second, the unique channel of compound 1 increases the contact area between catalyst and dyes, which promotes more active centers to participate in the degradation reaction. XRPD pattern and IR of compound 1 before and after photocatalytic reaction is shown in Fig. S9 and S6, respectively. The characteristic peaks positions of XRD before photocatalysis reaction are in accordance with that of simulated patterns. In addition, in contrast to the IR and XRD patterns of the compound 1 before and after degradation reaction, the positions of the main peaks have hardly changed, which not only reveals the phase purity of the compound, but also shows that it is possesses high catalytic stability during catalytic degradation [26]. In summary, A 3,6-connected 3-D arsenotungstate framework based on unique sandwich-type metal-organic dimer chain has been synthesized, which shows electrocatalytic activity for reduction of H2O2 and outstanding catalytic degradation activity for typical dyes MB, RhB and MO under UV light.
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Acknowledgments This work was supported the National Natural Science Foundation of China (Grants Nos. 21771046 and 21571044) the Natural Science Foundation of Heilongjiang Province (JC2016001, ZD2015001, and B2017007). Appendix A. Supplementary material CCDC 1863997 for 1 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Supplementary data associated with this article can 90
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L. Cui et al. 30 min. The pH value was adjusted to about 3.5 by 6 M HCl. The above mixture was transferred into 25 ml Teflon reactor and heated at 140 °C for 6 days. After the autoclave was cooled to room temperature, black block crystals were isolated, and dried at room temperature to give a yield of 46% (based on W). Calcd for C40H38Ag4AsN8O43W12 (Mr = 4031.26) C, 11.91; H, 0.94;N, 2.78; As, 1.86; W, 54. 72; Ag, 10.70. Found: C, 11.89; H, 0.93; N, 2.79; As,1.83; W, 54.69; Ag,10.68. IR (KBr pellet, cm−1): 3604 (br), 1742(m), 1556(m), 985(s), 873(s), 749(s), and 662 (s). [21] The crystal data of 1 was collected on a Bruker SMART CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The structures were solved by the direct methods and refined by the full-matrix least-squares method on F2 with the SHELXTL crystallographic software package. Anisotropic thermal parameters were used to refine non-hydrogen atoms. Hydrogen atoms on C and N atoms of the organic ligands were included in their calculated positions. [22] (a) G.M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997;
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(b) G.M. Sheldrick, SHELXL97, Program for Crystal Structure Solution, University of Göttingen, Germany, 1997. S.B. Li, H.Y. Ma, H.J. Pang, Z.F. Zhang, Y. Yu, H. Liu, T.T. Yu, Tuning the dimension of POM-based inorganic-organic hybrids from 3D self-penetrating framework to 1D poly-pendant chain via changing POM clusters and introducing secondary spacers, CrystEngComm 16 (2014) 2045–2055. Z.Y. Shi, J. Peng, Y.G. Li, Z.Y. Zhang, X. Yu, K. Alimaje, X. Wang, Assembly of multinuclear Ag complexes and Keggin polyoxometalates adjusted by organic ligands: syntheses, structures and luminescence, CrystEngComm 15 (2013) 7583–7588. J.Q. Sha, M.T. Li, J.W. Sun, Y.N. Zhang, P.F. Yan, G.M. Li, An unprecedented 3D POM-Ag architecture with intertwined and homological helical structures, Dalton Trans. 42 (2013) 7803–7809. X.Y. Yang, G.J. Liu, P.P. Zhu, J.Q. Sha, X.M. Zhang, S.X. Li, Self-organization of unprecedented POMCPs with four-fold helixes based on the highest connected Keggin POMs, Inorg. Chem. Commun. 77 (2017) 23–26.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A 3,6-connected 3-D arsenotungstate framework based on unique sandwichtype metal-organic dimer chain
T
Liping Cuia,b, Jinghua Lva, Kai Yua,b, , Xinyue Maa,b, Wenting Daia,b, 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 Key Laboratory of Photochemical Biomaterials and Energy Storage Material, Heilongjiang Province, Harbin Normal University Harbin, 150025, People's Republic of China
b
GRAPHICAL ABSTRACT
A 3,6-connected 3-D arsenotungstate framework based on unique sandwich-type metal-organic dimer chain has been synthesized, which shows electrocatalytic performance for reduction of H2O2 and outstanding catalytic degradation activity for typical dyes MB, RhB and MO under UV light.
ARTICLE INFO
ABSTRACT
Keywords: Hydrothermal synthesis Keggin arsenotungstate Photocatalytic properties Electrocatalytic behavior
A 3-D inorganic-organic network based on kiggen-type arsenotungstat, [Ag(4,4′-bpy)]2[{Ag(4,4′V bpy)}2(AsWVI 11W O40)]·3H2O(1) (4,4′-bpy = 4,4′-bipyridine), has been synthesized by hydrothermal means. Xray single crystal analysis shows that compound 1 contains two crystallography independent silver ions, which adopt different coordination modes connecting bpy ligand alternately to form two kinds of metal-organic chains {Agbpy}n. Two parallel {Ag(1)bpy}n chains are linked together via {AsW12O40} linkers to result in a unique sandwich-type dimer chain along a axis. Each Keggin cluster on sandwich chain bonds with six cognate clusters on six adjacent sandwich chains via surrounding six Ag, forming a 2-D layer along c axis. The 2D layer and 1D sandwich chains are intersected to yield a 3D network with 1-D channel. The {Ag(2)bpy}n chains and lattice water reside in the channel and further stabilize the 3-D framework via H-bonding and π-π accumulation. From topological perspective, compound 1 can be viewed as a 3, 6-connected network with {43}2{46.66.83} topology. Compound 1 shows merit electrocatalytic activity for reduction of H2O2 and efficient catalytic degradation ability for three typical dyes under UV radiation.
With the development of modern science and technology, the concept of green chemistry has been paid more and more attention, especially in the field of material chemistry [1]. It is particularly important to develop photocatalytic materials with excellent catalytic effects to
control air and water pollution [2]. Polyoxometalates (POMs) have unique physical and chemical properties such as the tailorability of structure, the controllability of the property, high thermal stability, strong acidity, reversible multi-electron redox properties, and thus are
⁎ Corresponding authors at: Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, Harbin Normal University, Harbin 150025, People's Republic of China. E-mail addresses: [email protected] (K. Yu), [email protected] (B. Zhou).
https://doi.org/10.1016/j.inoche.2018.10.005 Received 4 September 2018; Received in revised form 25 September 2018; Accepted 8 October 2018 Available online 11 October 2018 1387-7003/ © 2018 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 98 (2018) 87–91
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widely used in various fields such as photocatalysis [3,4], electrochemistry [5,6], medicine [7,8] and materials science [9,10]. Among them, the Keggin-type POMs [XM12O40]n-(X = B, P, Si, Ge, As; M = W, Mo, V) as ideal inorganic building units are widely studied for their relatively wide synthesis conditions, high structural stability, and suitable size [11]. Many complicated Keggin derivatives can be synthesized by hydrothermal process using simple starting material or by routine solution method with precursor for their broad isolation environment [12]. Keggin compounds have very high structural stability for their highly symmetrical structure. Therefore, such materials are widely used in the field of catalysts due to their advantages of good catalytic stability, easy recovery, and long cycle life [13]. In addition, suitable size endow Keggin compounds with smaller steric hindrances and more flexible bonding modes. Therefore, they can not only be introduced into the metal-organic network in the form of guest molecules to form POMOF structure, but also connect organic ligands through secondary metal linking units to form organic-inorganic hybrids. Despite their many advantages, the arsenotungstat series is a relatively undeveloped field compared with other Keggin compounds. Only a handful of {AsW12O40}-based hybrids have been obtained so far, and most of them are zero-dimensional (0-D) structures. The 3-D highconnected Keggin arsenotungstats are still less common [14–16]. The application of these compounds is also very limited. In the contexts, further explore 3-D high-connected arsenotungstat architectures are necessary and meaningful in this branch. Transition metal silver ions are selected in our exploration for their soft d10 configuration and strong coordination ability with rigid ligands. They can take different bond numbers and show various bonding geometry such as linear, “square”, “tetragonal pyramid”, “trigonal-bipyramidal”, and so on [17–19]. Thus, Ag(I) and 4,4′-bpy ligand are introduced into the {AsW12O40} reaction systems in order to explore 3-D high-connected assemblies with outstanding properties. Fortunately a new 3,6-connected Keggin arsenotungstate based on unique organic-inorganic sandwich chain, [Ag(4,4′-bipy)]2[{Ag(4,4′V bipy)}2(AsWVI 11W O40)]·3H2O [20]. The electro- and photo-catalytic behaviors of compound 1 are studied in detail. X-ray diffraction analysis (Table S1 and S2) [21,22] reveals that compound 1 crystallizes in the monoclinic crystal system with the C2/ m space group. One basic unit of compound 1 consists of a single electron reduced {AsW12O40} (abbreviated as {AsW12}) polyanion cluster, four Ag+, four bpy ligands, and three lattice water molecules (Fig. S1). There are two crystallographically independent silver ions exhibit two kinds of bonding geometries coexisting in compound 1. Ag1 adopts five-coordinated mode defined by three oxygen atoms from three Keggin spheres with the distance of Ag1-O9, 2.661(2) Å and two nitrogen atoms from two bpy molecules with the distance of Ag1-N1, 2.202 Å (Fig. S2). Ag2 is linearly geometrically linked to two nitrogen atoms from two bpy ligands with Ag2-N2, 2.21(2) Å. In this way, each of Ag (Ag1 and Ag2) links bpy ligand alternately to form two kinds of parallel metal-organic chains {Agbpy}n (Fig. S3). Two Ag1 on adjacent
two {Ag(1)bpy}n chains bond to two opposite terminal O atoms of the same keggin sphere in a “T-type” geometry resulting in a unique sandwich-type dimer chain {Ag2(4,4′-bipy)2(AsW12 O40)}n (Fig. 1). Each Keggin cluster of sandwich chains links with six cognate clusters of six adjacent sandwich chains via surrounding six Ag(1), forming a 2D layer along c axis (Fig. 2). The 2-D layer and 1-D sandwich chains are intersected to yield a 3-D network with 1-D channel (Fig. S4 and S5). The size of the channel: 6.28 Å (O6-O6) × 7.70 Å (Ag1-Ag1). The other metal organic chain {Ag (2)bpy}n and lattice water molecules reside in the tunnel and further stabilize the 3-D framework via H-bonding and π-π accumulation. The {AsW12} clusters can be regarded as hexa-connected nodes, and Ag1 is tri-connected node, thus the overall structure of compound 1 can be simplified to a 3, 6-connected 3-D framework with {43}2{46.66.83} topology (Fig. 3). As shown in Fig. S6, IR spectra of compound 1 display the classical vibration peaks of the Keggin {AsW12} anions at 985, 873, 749, and 662 cm−1, which can be assigned to v(W-Ot), v(As-Oa), v(W-Ob), and v (W-Oc), respectively [23]. The bands at 1556 and 1742 cm−1are attribute to v(CeN) and v(CeC) for organic ligand 4,4′-bpy. The bands at 3604 cm−1 corresponded to v(NeH) and/or ν(OeH) vibration of of organic ligands and water molecules. The thermogravimetric (TG) analyses of compound 1 was carried out in flowing N2 in the temperature range of 25–800 °C, as shown in Fig. S7. The title compound shows two step weight losses: 1.41% at 160–250 °C and 16.12% at 300–650 °C, which corresponding to the loss of all the water molecule (calculated value 1.34% for 3 H2O) and organic ligands (calculated value 15.50% for 4 bpy), respectively. The total weight loss is about 17.53%, which is close to the calculated value of 17.46%. Cyclic voltammetric behaviors of compound 1 were investigated with 1-modified carbon paste electrodes (1-CPE) in 1 mol·L−1 H2SO4 aqueous solution at different scan rates. As shown in Fig. 4a, three pairs of reversible redox peaks with the average peak potentials E1/ 2 = (Epa + Epc)/2, 0.203 V (I/I′), −0.140 V (II/II′), and −0.452 V (III/ III′) for 1-CPE in the potentials range of −0.8 to 0.7 V. Two pairs of redox peaks, II/II′ and III/III′, for 1-CPE are attributed to two successive bi-electron redox processes of the {AsW12} polyanions. The peaks I-I′ for 1-CPE should be assigned to the redox couple of AgI/Ag [24]. The cathodic peaks of 1-CPE shift to the lower potentials whereas the corresponding anodic peaks shift to the higher potentials with increasing of the scanning rates, as depicted in Fig. 4a. The plotting of the peak currents (II) of the anode and cathode against scanning rates are shown in insert of Fig. 4a. The peak currents were proportional to the sweep speed, which indicating that the redox process of 1-CPE is surfacecontrolled. The electrocatalytic properties of 1-CPEs were also explored. As can be seen from Fig. 4b, the peak currents vary gradually following the addition of H2O2: the reduction peak currents increase while the corresponding oxidation peak currents decrease dramatically, which indicate that H2O2 was reduced by {AsW12} polyoxoanion species. In contrast, the electroreduction of H2O2 at a naked electrode
Fig. 1. The 1D sandwich dimer chain of compound 1 along a axis. 88
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Fig. 2. The 2D layer based on {AsW12} cluster and Ag(bpy)2 linker along c axis.
Fig. 3. The polyhedral and ball-and-stick representation of 3D inorganic-organic network with 1D channel, and the simplified framework with {43}2{46.66.83} topology of compound 1.
generally needs a high overpotential and no obvious response could be detected at a bare CPE, which suggests that 1-CPEs have good electrocatalytic activities toward the reduction of H2O2.
Typical azo dyes methylene blue (MB), Rhodamine B (RhB), and Methyl Orange (MO) were selected as substrates to estimate the photocatalytic behavior of compound 1 under UV irradiation. The
Fig. 4. (a) Cyclic voltammograms of 1-CPE rates (from inner to outer: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, and 220 mV s−1) (Insert plots: The dependence of cathode and anode peak currents II on scan rates.); (b) Cyclic voltammograms of 1-CPE in 1 M H2SO4 solution containing H2O2 at different concentrations under the scan rate of 50 mV∙s−1 (Potentials vs. SCE). 89
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Fig. 5. The UV-vis absorption spectra of the MB, RhB, and MO solutions during the decomposition reaction under UV irradiation in the presence of compound 1.
degradation of dyes was monitored by UV-vis spectra and the decolorization rates of the dyes, which are shown in Fig. 5 and Fig. S8. With the increase of irradiation time, the characteristic absorption peaks at 665 and 610 nm for MB, 550 nm for RhB, and 460 nm for MO were decreases continuously under the same conditions. The degradation rate, defined as 1-C/C0 (C0 = initial concentration, C = concentration after irradiation) are 97.8% for MB, 92.9% for RhB, and 83.6% for MO after 90, 110, and 140 min, respectively. On the contrary, only small amounts of the substrates were degraded under the same experiment conditions without compound 1. The absorption wavelength of three dyes in the UV region did not shift during the degradation process, and the characteristic peaks of them had almost disappeared by the final stages of catalytic degradation, which indirectly suggests that the heterocycle groups had been destroyed. Furthermore, compound 1 shows different photocatalytic activities on three kinds of substrates, which can be due to their different structures and specific degradation pathways. Compared with reported other Keggin hybrids, compound 1 exhibit better degradation ability for above dyes under similar reaction conditions [25], which can be attributed to two reasons. First, {Ag (bpy)2} complexes chains promote electron transfer between {AsW12} cluster, which facilitates the rapid migration of electrons to the surface of POM. Second, the unique channel of compound 1 increases the contact area between catalyst and dyes, which promotes more active centers to participate in the degradation reaction. XRPD pattern and IR of compound 1 before and after photocatalytic reaction is shown in Fig. S9 and S6, respectively. The characteristic peaks positions of XRD before photocatalysis reaction are in accordance with that of simulated patterns. In addition, in contrast to the IR and XRD patterns of the compound 1 before and after degradation reaction, the positions of the main peaks have hardly changed, which not only reveals the phase purity of the compound, but also shows that it is possesses high catalytic stability during catalytic degradation [26]. In summary, A 3,6-connected 3-D arsenotungstate framework based on unique sandwich-type metal-organic dimer chain has been synthesized, which shows electrocatalytic activity for reduction of H2O2 and outstanding catalytic degradation activity for typical dyes MB, RhB and MO under UV light.
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Acknowledgments This work was supported the National Natural Science Foundation of China (Grants Nos. 21771046 and 21571044) the Natural Science Foundation of Heilongjiang Province (JC2016001, ZD2015001, and B2017007). Appendix A. Supplementary material CCDC 1863997 for 1 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Supplementary data associated with this article can 90
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L. Cui et al. 30 min. The pH value was adjusted to about 3.5 by 6 M HCl. The above mixture was transferred into 25 ml Teflon reactor and heated at 140 °C for 6 days. After the autoclave was cooled to room temperature, black block crystals were isolated, and dried at room temperature to give a yield of 46% (based on W). Calcd for C40H38Ag4AsN8O43W12 (Mr = 4031.26) C, 11.91; H, 0.94;N, 2.78; As, 1.86; W, 54. 72; Ag, 10.70. Found: C, 11.89; H, 0.93; N, 2.79; As,1.83; W, 54.69; Ag,10.68. IR (KBr pellet, cm−1): 3604 (br), 1742(m), 1556(m), 985(s), 873(s), 749(s), and 662 (s). [21] The crystal data of 1 was collected on a Bruker SMART CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The structures were solved by the direct methods and refined by the full-matrix least-squares method on F2 with the SHELXTL crystallographic software package. Anisotropic thermal parameters were used to refine non-hydrogen atoms. Hydrogen atoms on C and N atoms of the organic ligands were included in their calculated positions. [22] (a) G.M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997;
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