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Self-Assembly and Visible-Light Photocatalytic Properties of W/Nb Mixed-Addendum Polyoxometalate and Transition-Metal Cations [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Keywords: niobium . photocatalysis . polyoxometalates . synthetic methods . tungsten Nowadays, energy issues are one of the most pressing problems faced by human beings. The conversion of solar energy into hydrogen as a clean and renewable fuel is a promising and attractive approach. Since Fujishima and Honda first reported photoinduced water splitting on titanium dioxide electrodes under UV irradiation in 1972,[1] large numbers of metal oxides and sulfides have been explored as heterogeneous photocatalysts for hydrogen production from splitting of water.[2] In this field, although significant progress has been made as hundreds of photocatalysts have been prepared and tested, great effort is needed to search for photocatalysts with high enough efficiencies and visible-light activity for practical applications.[ 3] Polyoxometalates (POMs) are an outstanding class of inorganic metal oxide clusters with abundant structural diversity and versatile physical and chemical properties that have potential applications in catalysis, electrical conductivity, photochemistry, medicine, and magnetism.[4] Similar to metal oxide semiconductors (such as TiO2, titanates, Ta2O5, and tantalates), POMs are mostly constructed from corner- and edge-shared MO6 (M=Mo6+, W6+, V5+, Nb5+, Ta5+) octahedra. The similarities in composition and structure endow POMs and metal oxide semiconductors with similar electronic and light-absorption characteristics in the photocatalytic system. Moreover, these clusters are soluble in water and could be viewed as the smallest nanoparticles with ordered structures at the atomic level. Therefore, besides the advantage of extra-large surface area allowing for full contact of photocatalysts with water, POMs provide an ideal opportunity to study the relationship between photocatalytic activity and inorganic cluster structures.[ 5] During the past few years, the photocatalytic H2 evolution activity of polyoxotungstates has been studied.[6] However, the development of polyoxoniobate photocatalysts is still in its infancy,[ 7] and the reported polyoxoniobates are mostly active under UV irradiation.[8] Therefore, it is of great interest to develop novel polyoxoniobate photocatalysts with visible-light activity for more efficient use of solar energy. To achieve visiblelight H2 evolution activity, the only solution is to increase the valence-band position of the photocatalysts. One effective strategy to achieve this is to dope transition-metal cations (Ni2+, Cr3+) or to introduce main-group cations into the metal oxide photocatalysts to increase the valence-band position by hybridization of O2p with d or s orbitals of metal cations.[5a, 9] The crystals of Cs6H[Si(NbO2)3W9O37].8H2O are colorless and have no visible-light absorption (Figure 1).[10] We propose that by sandwiching of transition-metal cations, visible-light-active photocatalysts may be achieved. Herein, by utilizing the self-assembly of W/Nb mixed-addendum POM and transition-metal cations (Cr3+, Fe3+) in aqueous solution, we isolated three sandwichlike compounds with unprecedented architectures: Cs5KH[(Si2W18Nb6O78)Cr- (H2O)4].11H2O (1), Cs4[(Si2W18Nb6O78)Cr2(H2O)8].14H2O (2), and Cs4K4H[(Si2W18Nb6O78)FeCl2(H2O)2].13.5H2O (3). To the best of our knowledge, these are the first W/Nb mixed-addendum POM photocatalysts reported with visible-light activity for the H2 evolution reaction. Compounds 1 and 2 were synthesized by heating at reflux a 0.5m HCl solution containing different ratios of Cr- (NO3)3.9H2O and Cs6H[Si(NbO2)3W9O37].8H2O. Compound 3 was prepared by following the procedure described for 1, except for the use of FeCl3.6H2O instead of Cr(NO3)3.9H2O. The maintenance of {SiNb3W9O40} units in 1-3 under such highly acidic conditions (pH 0.3) indicates the stability of the products in acidic medium, which is higher than that of lacunary polyoxotungstates that are usually sensitive to pH and sometimes undergo disassembly and reassembly in the reaction process.[11] Therefore, the saturated mixed-addendum POMs are not only active but also more stable in retaining their structure type in a broad pH range. In the synthesis of 1-3, we also tried to use other metal ions (ZnII, CoII, MnII, or NiII) to replace CrIII and FeIII, but no crystal sample was obtained in our experiments. Notably, the pH value is crucial for the successful preparation of 1- 3. The pH should be strictly controlled in the range 0.3-0.5. Compounds 1-3 are stable in air and soluble in acidic and alkaline solutions. In 1-3, the oxidation states of all W, Nb, Cr, and Fe centers are +6, +5, +3, and +3, respectively, based on the charge balance consideration and bond valence sum calculations.[12] The phase purity of 1-3 is confirmed by the agreement between the experimental powder X-ray diffraction (PXRD) pattern and the simulated pattern based on structural analysis (Supporting Information, Figures S16-S18). X-ray structural analysis indicates that 1-3 all contain two identical {SiNb3W9O40} clusters, which are tris(niobium)-substituted A-a- SiW9 derivatives. Each of the three niobium atoms is ligated by six oxygen atoms of the A-a-SiW9 unit. Compound 1 is composed of two {SiNb3W9O40} fragments that are linked through two Nb-O-Nb bridges and one (Nb)Ot-Cr-Ot(Nb) bridge (Ot= terminal oxygen atom; Figure 2a). However, in compound 2 two {SiNb3W9O40} units are linked by two CrIII ions through (Nb)Ot-Cr-Ot(Nb) bridges and one Nb-O-Nb bridge (Figure 2 b). Similar to the polyanion of 1, compound 3 is made up of two {SiNb3W9O40} clusters that are linked through two Nb- O-Nb bridges and one (Nb)Ot-Fe-Ot(Nb) bridge (Figure 2 c). To our knowledge, the transition-metal cations are preferentially coordinated with terminal oxygen atoms rather than bridging oxygen atoms. Moreover, Ot(Nb) is more nucleophilic than Ot(W), because the formal valence of Nb is +5 and that of W is +6. Therefore, all the transition-metal cations bind to the {NbO6} octahedra through Cr/Fe-Ot(Nb) bonds. Each Cr and Fe atom is six-coordinate and links to two {SiNb3W9O40} fragments with an average Cr/Fe-Ot(Nb/W) bond length of 1.86 - (Figure S1). The Nb and W of 1-3 all display octahedral coordinated environments. Compounds 1-3 have three types of O atoms: terminal oxygen atoms (Ot), bridging m2-oxygen atoms, and m4-oxygen atoms. The W-Ot, Nb/W-m2-O, and Nb/ W-m4-O bond lengths in 1-3 are in the range 1.616-1.765, 1.777-2.030, and 2.265-2.430 -, respectively. To study the conductivity of 1-3, the UV/Vis diffuse reflectance spectra of their powder samples were measured to achieve their band gaps (Eg), which were determined as the intersection point between the energy axis and the line extrapolated from the linear portion of the adsorption edge in a plot of the Kubelka-Munk function F against E.[13] As shown in the Supporting Information (Figures S8-S10), the corresponding well-defined optical absorption associated with Eg can be assessed at 2.49, 2.56, and 2.16 eV for compounds 1-3, respectively, which reveals the presence of an optical band gap and the nature of the semiconductivity. In the photocatalytic system, we used compounds 2 and 3 as the visible-light photosensitizers and catalysts, Pt as H2 evolution cocatalyst, and triethylamine (TEA) as a sacrificial electron donor. It is well known that some factors, such as the concentrations of TEA, sample, and H2PtCl6, the amount of the solvent, and the content of water in the solvent, will greatly influence the performance of the catalyst system. Studies on the effects of solvents were made at a concentration of 100 mg of 2 and 3, 100 mg of H2PtCl6, and 10 mL of TEA. The optimum conditions for H2 evolution were based on TEA (0.72 mm) in 90mL of water (9:1 v/v). More or less TEA content in the water solution may result in a decrease of H2 evolution. To investigate the importance of 2 and 3 in the photocatalytic reactions, more experiments with different combinations were performed. Lines a (2) and b (3) in Figure 3 are derived from the first run of data. As shown in Figure 4, no H2 is detected if TEA is absent, which suggests that if the oxidation half-reaction does not work, the reduction half-reaction also does not work (line e of Figure 3). If 2 or 3 is absent (line f of Figure 3), H2PtCl6 alone shows no photocatalytic activity, thus indicating that 2 and 3 play a crucial role in light harvesting for photoca- talysis. If H2PtCl6 is absent, the H2 evolution rates of samples 2 and 3 are 3.05 and 1.85 mmolh-1g-1, respectively (lines c and d of Figure 3). To explore the photocatalytic H2 evolution activities of 2 and 3, samples 2 and 3 (100 mg) and H2PtCl6 (100 mg) were dissolved in 10% TEA solution (100 mL) in a quartz cell. The catalyst solution was irradiated under a 500 W Xe lamp with a 400 nm cutofffilter. As shown in Figure 4a, for compound 2, the H2 evolution rates in three runs were 19.5, 20, and 20.2 mmolh-1g-1, respectively. The total evolved H2 during 12 h was 24 mmol. However, for compound 3, the H2 evolution rates in three runs were 7.8, 7.6, and 7.5 mmolh-1g-1 and the total H2 evolved over 12 h was 9.2 mmol (Figure 4b). The electrochemical behavior of compounds 1-3 was studied in 0.5m (CH3COOK+CH3COOH) pH 4.7 buffer solution. The cyclic voltammograms of 1 in the potential region of -1.3 to +0.6 V at different scan rates are presented in Figure 5. There are three reversible redox peaks, III-III', IV-IV', and V-V', with midpoint potentials E1=2 of -0.833, -0.977, and -1.154 V, respectively (E1=2=(Epa+Epc)/2; Epa and Epc are the anodic and cathodic peak potentials), corresponding to the redox processes of W centers.[14] In addition, there are two reversible redox couples with the E1=2=(Epa+Epc)/2 located at +0.293 and +0.126 V, which are attributed to the Cr3+ redox processes of the center (Figure 5). Above 100 mVs-1, the peak currents were proportional to the scan rate, which suggests the redox process is surface-controlled. For compound 2, the redox processes of W and Cr centers are similar to those of 1 (Figure S14). The cyclic voltammetric behavior for compound 3 also exhibits three reversible redox peaks in the potential range of -1.3 to +0.6 V and the mean peak potentials are -0.853, -0.982, and -1.13 V (vs. Ag/AgCl; Figure S15). In addition, the irreversible anodic peak at 0.412 V is assigned to the oxidation process of Fe3+.[15] In summary, we have synthesized and structurally characterized the novel compounds 1-3, which all contain the unique {SiNb3W9O40} cluster. The successful synthesis of 1-3 adds three new Nb-based POMs and unveils the potential for further development of polyoxoniobate chemistry. To the best of our knowledge, these are the first W/Nb mixed-addendum POM photocatalysts with visible-light activity for the H2 evolution reaction. Our results reveal the promising application of W/Nb mixed-addendum POMs in visible-light photocatalysis, which may stimulate further development in this area. Experimental Section Materials Cs6H[Si(NbO2)3W9O37].8H2O was synthesized according to the procedure described in the literature,[10] and the powder product was used without further purification. The material was characterized by IR spectroscopy and elemental analysis. All other reagents and solvents for the syntheses were purchased from commercial sources and used as received. Instruments Fourier transform infrared spectroscopy data were recorded from KBr pellets in the range 4000-300 cm-1 on a Mattson Alpha-Centauri spectrometer. PXRD patterns were recorded on a Siemens D5005 diffractometer with CuKa (l=1.5418 -) radiation in the range 3-508. Elemental analyses (W, Nb, Cr, Fe, K, and Na) were performed on a Plasma-SPEC(I) inductively coupled plasma atomic emission spectrometer. UV/Vis absorption spectroscopy was achieved on a U-3010 spectrophotometer (Hitachi, Japan). Thermogravimetric analysis was performed on a PerkinElmer TGA 7 analyzer heated from room temperature to 9008C under a nitrogen gas atmosphere with a heating rate of 108Cmin-1 (Figures S2-S4). Synthesis and characterization of 1 Solid Cr(NO3)3.9H2O (0.04 g, 0.1 mmol) was added to a solution of Cs6H[Si(NbO2)3W9O37].8H2O (0.359 g, 0.1 mmol) in HCl (0.5m, 30 mL). The resulting clear solution was heated at reflux for 8 h and then cooled to room temperature. After a small amount of white insoluble solid had been removed by filtration, the filtrate was allowed to evaporate at room temperature. Over a period of 1 week, green block single crystals for X-ray crystallography were obtained (yield: 58%). IR (KBr disks): ñ=522, 667, 775, 918 cm-1 (Figure S11); elemental analysis calcd (%) for H31KCs5Nb6W18CrSi2O93 : Si 0.91, Nb 8.99, W 53.39, Cr 0.84, Cs 10.72, K 0.63; found: Si 0.89, Nb 9.06, W 53.17, Cr 0.85, Cs 10.58, K 0.66. Synthesis and characterization of 2 Compound 2 was prepared by following the procedure described for 1, but using a larger quantity of Cr(NO3)3.9H2O (0.08 g, 0.2 mmol). The resulting products were green block crystals (yield: 62%). IR (KBr disks): ñ=520, 645, 774, 905 cm-1 (Figure S12); elemental analysis calcd (%) for H44Cs4Nb6W18Cr2Si2O100 : Si 0.91, Nb 8.99, W 53.35, Cr 1.68, Cs 8.57; found: Si 0.93, Nb 8.86, W 52.21, Cr 1.63, Cs 8.8. Synthesis and characterization of 3 The same procedure was followed as that for 1, except for using FeCl3.6H2O instead of Cr(NO3)3.9H2O (yield: 70% based on W). IR (KBr disks): ñ=520, 645, 774, 905 cm-1 (Figure S13); elemental analysis calcd (%) for H32K4Cs4Nb6W18FeSi2O93.5Cl2 : Si 0.89, Nb 8.9, W 52.6, Fe 0.89, Cs 8.48, K 2.49, Cl 1.13; found: Si 0.85, Nb 8.97, W 52.21, Fe 0.91, Cs 6.21, K 2.6, Cl 1.02. Single-crystal studies Suitable single crystals of 1-3 were selected and mounted onto the end of a thin glass fiber using Fomblin oil. Single-crystal X-ray diffraction data for 1-3 were recorded on a Bruker APEXII CCD diffractometer with graphite-monochromated MoKa radiation (l= 0.71069 -) at 293 K. Absorption corrections were applied using the multiscan technique. The structure was solved by the Direct Method of SHELXS-97[16] and refined by full-matrix least-squares techniques using the SHELXL-97 program[17] within WINGX.[18] Those hydrogen atoms attached to lattice water molecules were not located. 1: H31KCs5Nb6W18CrSi2O93 ; Mr=6198.08; triclinic; space group P1- ; a=13.045(5), b=19.286(5), c=20.857(5) -; a=97.596(5), b=102.011(5), g=99.08(5)8; V=4994(3) -3; Z=2; 1calcd= 4.121 gcm-3 ; final R1=0.0593 and wR2=0.1287 (Rint=0.0566) for 17207 independent reflections (I>2s(I)). 2: H44Cs4Nb6W18Cr2Si2O100 ; Mr=6202.6; monoclinic; space group c 2/m; a=19.827(5), b= 18.945(5), c=32.518(5) -; a=90(5), b=90.047(5), g=90(5)8; V= 12214(3) -3 ; Z=4; 1calcd=3.375 gcm-3 ; final R1=0.0605 and wR2=0.176 (Rint=0.094) for 11 098 independent reflections (I> 2s(I)). 3: H32K4Cs4Nb6W18FeSi2O93.5Cl2 ; Mr=6265.7; monoclinic; space group c 2/m; a=34.34(5), b=16.271(5), c=21.588(5) -; a= 90(5), b=114.783(5), g=90(5)8; V=10951(3) -3 ; Z=4; 1calcd= 3.791 gcm-3 ; final R1=0.0494 and wR2=0.1277 (Rint=0.0587) for 9979 independent reflections (I>2s(I)). CCDC 929357 (1), 929358 (2), and 929359 (3) contain 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. Photocatalytic measurements Photocatalytic reactions were performed in a Pyrex inner-irradiation- type reaction vessel with a magnetic stirrer at room temperature. The reactant solution was evacuated using N2 several times to ensure complete air removal and then irradiated by using a 500 W Xe lamp with a 400 nm cutofffilter. The produced H2 was analyzed by a GC9800 instrument with a thermal conductivity detector and a 5 - molecular sieve column (2 mm-2 m) with N2 as carrier gas. Electrochemistry A CHI 440 electrochemical workstation connected to a Digital-586 personal computer was used for control of the electrochemical measurements and for data collection. 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Sun, Y.-Q. Jiao, Y. Xing, Z.-M. Su, K.-Z. Shao Department of Chemistry, Northeast Normal University Institute of Functional Materials Chemistry Key Laboratory of Polyoxometalate Science of Ministry of Education National & Local United Engineering Lab for Power Battery Changchun, Jilin 130024 (P. R. China) Fax: (+86) 431-5684009 E-mail: [email protected] [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201300175. (c) 2013 Blackwell Publishing Ltd. |