Polyoxometalate-Based Metal–Organic Frameworks as Visible-Light

Publication Date (Web): April 18, 2018 ... The basic building units of the two PMOFs are [CuI12(trz)8], but polyoxoanion (POA) template effect leads t...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Polyoxometalate-Based Metal−Organic Frameworks as Visible-LightInduced Photocatalysts Xiuxia Zhao,†,‡ Shaowei Zhang,†,‡,§ Junqing Yan,‡ Landong Li,‡ Guangjun Wu,‡ Wei Shi,*,‡ Guangming Yang,‡ Naijia Guan,‡ and Peng Cheng*,‡,⊥ ‡

College of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin, 300071, China Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of the Ministry of Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China ⊥ Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China §

S Supporting Information *

ABSTRACT: Two stable 3D polyoxometalate-based metal−organic frameworks (PMOFs), [CuI12(trz)8(H2O)2][α-SiW12O40]·2H2O (1) and [CuI12(trz)8Cl][αPW12O40] (2) (Htrz = 1-H-1,2,4-triazole) based on Keggin-type POMs were successfully obtained and fully characterized. The basic building units of the two PMOFs are [CuI12(trz)8], but polyoxoanion (POA) template effect leads to different structures and properties: 1 represents an interesting example that [α-SiW12O40]4− locate in the nine-membered Cu-trz rings through Cu···O weak interactions to form a 3D framework, whereas 2 shows a 3D structure constructed from 2D bilayer cationic network [CuI12(trz)8Cl]3+ and [α-PW12O40]3− lying in the adjacent layers via Cu···O weak interactions. PMOF 1 as unusual visible-light photocatalyst exhibit significantly enhanced photocatalytic activity under visible-light and excellent stability during the photocatalysis process for recovering and recycling, as well as photocatalytic hydrogen evolution activity.

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INTRODUCTION

Metal−organic frameworks (MOFs), as one of the most attractive porous organic−inorganic hybrid materials, have attracted great interest because of their versatile structures and potential applications.5 Especially, MOFs have widely served as porous supports for the heterogeneous catalysts.6 As a consequence, one promising approach to solve the disadvantages of significant solubility and poor recyclability is to incorporate homogeneous POMs with porous MOFs with the following advantages:7 (i) POM-based MOFs (PMOFs) usually possess high thermal stability and can be easily recycled as heterogeneous catalysts; (ii) the crystalline PMOFs with definite structures and chemical components are conducive for the study of catalytic mechanism; (iii) the porous MOFs with advantages including large surface area, high metal content, and flexibility in the design of the active sites in the framework facilitate the diffusion of substrates and products through the channels and make them as ideal platform to hierarchically organize light-harvesting antennae and catalytic centers to achieve solar energy conversion. Nevertheless, these two moieties (POMs and MOFs) have distinct solubility in either hydrophilic or hydrophobic solvents, which lead the preparation of PMOFs extremely difficult. Hydrothermal method has been proven to be an effective method in synthesis of organic−inorganic hybrid POMs,8 because the increased solubilities of materials and the reduced viscosity of water under

Because of a global energy shortage and environmental pollution, solar energy has been proposed as the most promising alternative energy source with abundant and environmentally friendly properties. Great efforts have been devoted for the usage of solar energy via various approaches such as photoelectrochemical cells and photovoltaics in the past two decades.1 Photocatalysis is also an effective approach to remedying environment and converting solar energy. Especially, photocatalytic water splitting for hydrogen production has attracted tremendous attention because it is not only a green route to convert solar energy into renewable hydrogen energy but also a promising technology to reduce fossil fuels consumption.2 As photocatalysts, polyoxometalates (POMs) acting as inorganic metal-oxide clusters have prominent photocatalytic activity under visible-light because they share very similar light absorption and electrochemical band-edge positions with TiO2.3 However, high solubility in solution makes them very difficult in recovering and recycling, which is highly critical to meet the industrial requirements. Furthermore, POMs as inorganic crystalline materials have their own shortages in compatibility and processability, hindering the further engineering of POMs into multifunctional materials. Therefore, the design and assembly of novel POM-based organic−inorganic hybrids with charming structures and optimizing properties has been an attractive and challenging topic.4 © XXXX American Chemical Society

Received: January 10, 2018

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DOI: 10.1021/acs.inorgchem.8b00098 Inorg. Chem. XXXX, XXX, XXX−XXX

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0.5 mmol), K4[α-SiW12O40]·17H2O (0.123 g, 0.05 mmol) and isonicotinic acid (0.0123 g, 0.1 mmol) was dissolved in 10 mL of distilled water at room temperature. The resulting mixture was transferred into a 25 mL Teflon-lined stainless steel autoclave and kept at 170 °C for 72 h. After slow cooling to room temperature, yellow block crystals were obtained. Yield: ca. 32% (based on W). Anal. Calcd (%) for C16H20Cu12N24O42Si W12: C 4.56, H 0.48, N 8.00. Found: C 4.50, H 0.52, N 8.09. IR (KBr pellet, cm−1): 3480 (w), 1520 (s), 1265 (s), 1166 (s), 1086 (s), 983 (s), 883 (s), 812 (m), 662 (s), 515 (s). Synthesis of [CuI12(trz)8Cl][α-PW12O40] (2). The synthetic method of 2 was similar to that of 1, except that K3[α-PW12O40] (0.105 g, 0.035 mmol) was used instead of K4[α-SiW12O40]·17H2O. Dark brown block crystals of 2 were obtained. Yield: ca. 46% (based on W). Anal. Calcd (%) for C16H16ClCu12N24O40PW12: C 4.55, H 0.38, N 7.97. Found: C 4.53, H 0.34, N 8.06. IR (KBr pellet, cm−1): 3482 (w), 1529 (s), 1258 (s), 1163 (s), 1004 (s), 961 (s), 910 (s), 785 (w), 716 (s), 526 (s). X-ray Crystallography. Crystallographic data of 1 and 2 were collected on an Oxford SuperNova diffractometer with a graphite monochromatic Mo Kα radiation (λ = 0.71073 Å) at 123 K. Routine Lorentz polarization and empirical absorption corrections were used. All the structures were solved by direct methods and refined by fullmatrix least-squares methods on F2 with the SHELXTL-97 program package.9 Anisotropic thermal parameters were assigned to all nonhydrogen atoms. Positions of H atoms attached to C and N atoms were geometrically added. The final formula was determined through single-crystal structures, element analyses and TGA. The crystallographic data and structural refinements for 1 and 2 are summarized in Table 1. CCDC nos. 1025222−1025223 are for 1 and 2, respectively.

hydrothermal conditions can enhance the opportunity of the reaction among different reactants. Various PMOFs crystalline materials have been reported in the literature, which were systematically classified and summaried in the review by Su et al.3c Although these hybrids overcome some difficulties in recovering, these available materials are largely limited by either poor photocatalytic efficiency in the visible light range or poor stability in the photocatalytic reaction process, even photochemical splitting of water. Therefore, the documented PMOFs, which exhibit both photocatalytic hydrogen production and photodegradation of RhB under visible-light and good recyclability, have rarely been reported.3 Herein, by using two different polyoxoanions (POAs) as template, two highly stable PMOFs, formulated as [CuI12(trz)8(H2O)2][α-SiW12O40]·2H2O (1) and [CuI12(trz)8Cl][α-PW12O40] (2), were successfully obtained. Although PMOFs 1 and 2 were prepared under the same reaction conditions except different POMs precursors, they displayed two distinct frameworks governed by different charge and bonding abilities of [α-SiW12O40]4− and [α-PW12O40]3− POAs. 1 represents the rare example that [α-SiW12O40]4− POAs locate in the nine-membered Cu-trz rings through Cu···O weak interactions to form the 3D framework, whereas 2 is another interesting 3D PMOF constructed by 2D bilayer cationic network [CuI12(trz)8Cl]3+ and [α-PW12O40]3− POAs lying in the adjacent layers via Cu···O weak interactions (Scheme 1). Both PMOFs 1 and 2 present high photocatalytic activities under visible-light with high reusability.

Table 1. Crystallographic Data and Structure Refinements for 1 and 2

Scheme 1. Schematic Representation of the Formation Processes for PMOFs 1 and 2

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formula Mr (g mol−1) T (K) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (g cm−3) μ (mm−1) Rint GOF on F2 R1a, wR2b [I > 2σ(I)] Δρmax/Δρmin (e Å−3)

EXPERIMENTAL SECTION

General Methods and Materials. All the reagents are commercially available and used without further purification. Elemental analyses (C, H and N) were measured on a PerkinElmer 2400-II CHNS/O analyzer. Thermogravimetric analyses (TGA) were obtained under a N2 atmosphere on a Labsys NETZSCH TG 209 Setaram apparatus with the heating rate of 10 °C·min−1 from 25 to 800 °C. Field-emission scanning electron microscopy (FESEM) images were measured on SU3500 scanning electron microscope. Transmission electron microscope (TEM) images were taken on FEI Tecnai G2F20 electron microscope (200 kV). PXRD were measured on a Rigaku Ultima IV instrument with Cu Kα radiation (λ = 1.54056 Å), with a scan speed of 10° min−1 in the range 2θ = 3−60°. The solid diffuse reflectance UV−vis spectra were recorded on a Varian Cary 5000 UV−vis spectrometer. The UV−vis spectra for solution samples were obtained on a Jasco V-570 spectrophotometer. Synthesis of [CuI12(trz)8(H2O)2][α-SiW12O40]·2H2O (1). A mixture of Cu(NO3)2·3H2O (0.0483 g, 0.2 mmol), Htrz (0.035 g,

1

2

C16H20Cu12N24O42SiW12 4217.33 122.95(10) monoclinic C2/m 17.7303(9) 14.6054(7) 12.3057(7) 90.724(4) 3186.4(3) 2 4.396 25.590 0.0291 1.226 0.0692 0.1788

C16H16ClCu12N24O40PW12 4219.63 122.7(3) monoclinic I2/m 14.4650(8) 14.5500(7) 15.7323(10) 109.896(7) 3113.5(3) 2 4.501 26.235 0.0228 1.116 0.0560 0.1420

2.913/−1.739

2.953/−1.274

The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Photocatalytic Experiments. The photocatalytic hydrogen evolution experiments were carried out in a top-irradiation-type Pyrex reaction cell connected to a closed gas circulation and evacuation system under the irradiation of a 300 W Xe lamp (wavelength: 320−780 nm). In a typical experiment, a catalyst sample of 100 mg was suspended in 100 mL 20% CH3OH aqueous solution in the reaction cell. After being evacuated for 30 min, the reactor cell was irradiated under the Xe lamp with magnetic stirring at room temperature. The gaseous products were analyzed by an online gas chromatograph (Varian CP-3800) with thermal conductivity detector. B

DOI: 10.1021/acs.inorgchem.8b00098 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Structural unit of PMOF 1; (b) 2D bicyclic [CuI12(trz)8(H2O)2]4+ layers; (c, d) 3D framework of PMOF 1.

Figure 2. (a) Structural unit of PMOF 2; (b) 2D bilayer cationic network [CuI12(trz)8Cl]3+; (c, d) 3D framework of PMOF 2.

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RESULTS AND DISCUSSION Structural Descriptions. The experimental PXRD patterns for PMOFs 1 and 2 are consistent well with the simulated PXRD patterns from the single-crystal X-ray data, indicating the high phase purity for both 1 and 2. The bond-valence sum calculations10 suggest that all W and Cu atoms in 1 and 2 are +6 and +1 oxidation states, respectively. The SEM and TEM images (Figure S1) reveal that PMOFs 1 and 2 are rod and particle, respectively. Besides, the TGA curves (Figure S2) show that PMOFs 1 and 2 have high thermal stability.

PMOF 1 belongs to monoclinic space group C2/m. Each asymmetric unit consists of a [CuI12(trz)8(H2O)2]4+, a [αSiW12O40]4− POA (Figure 1a and Figure S3a) and two lattice water molecules. Four crystallographically independent Cu ions all adopt three-coordinated ″T″-type geometry (Figure S4). Cu1 and Cu2 are linked by three trz− ligands through a “pyrazole-like” bridging mode to produce neutral trigonal {Cu3(trz)3} unit [Cu−N: 1.860(16)−1.885(19) Å]. Adjacent {Cu3(trz)3} units are further connected by two Cu ions (Cu3 and Cu4) and one trz− ligand (“imidazole-like” linking mode) C

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property allows 1 and 2 as the promising candidates applied for visible-light photocatalysts. Photocatalytic degradation of organic pollutants is one of the effective approaches to the purification of water. In order to evaluate the photocatalytic activities of PMOFs 1 and 2, the targeted substrate RhB, being stable in aqueous solution upon visible-light irradiation,14 was chosen as the model dye. In our experiments, 40 mg of PMOFs 1 or 2 were added into the jacketed beaker filled with 100 mL RhB aqueous solution (1.0 × 10−5 mol L−1), respectively. To reach the adsorption equilibrium, the sample was stirred in dark for 0.5 h before measurement. Afterward, the solution was under visible light irradiation (λ > 420 nm) from a 300 W Xe lamp in open air at room temperature. During the irradiation process, the suspension was kept continuously stirring during which 3 mL solution was withdrawn every 30 min and immediately centrifuged to separate photocatalysts before analysis. The influence of the irradiation time versus photodegradation reaction of RhB was studied by UV−vis spectra. The results show that the absorbance decreased as the irradiation time increased, indicating that RhB rapidly degraded. After irradiation for 240 min under visible-light, the conversions of RhB increased from 20% (without catalysts) to 91% for 1 and 81% for 2, respectively (Figure 4). It is worth noting that the

to give a 2D layer [Cu−N: 1.873(19)−1.896(23) Å] (Figure 1b). The whole 2D layer could be regarded as an alternative arrangement of three- and nine-membered {Cu-trz} rings. Neighboring 2D layers are bridged by [α-SiW12O40]4− POAs through Cu−O weak interactions to form a 3D framework [Cu−O: 2.667(18) Å] (Figure 1c, d). It is the first 3D framework that {Cu3(trz)3} units functionally combine with polyoxotungstates. When [α-SiW12O40]4− was replaced by [α-PW12O40]3−, PMOF 2 was harvested (Figure 2a and Figure S3b). The asymmetric unit of 2 contains four independent Cu ions, however, they exhibit unusual three different coordination geometries (Figure S5). The basic Cu-trz unit is a fourmembered {Cu4N8} heterocycle constructed from four Cu ions (two Cu1 and two Cu2) and eight N atoms from four trz− ligands [Cu−N: 1.884(13)−1.910(12) Å]. The remaining N atom at each trz− ligand is bridged to the two-coordinate exocyclic Cu ions (Cu3 or Cu4) to connect two {Cu4N8} heterocycles, giving an eight-membered {Cu8(trz)8} ring [Cu− N: 1.885(13)−1.919(16) Å]. Each {Cu4N8} heterocycle is surrounded with four {Cu8(trz)8} rings, and each {Cu8(trz)8} ring is connected by four {Cu4N8} heterocycles, which extend infinitely to build a square-grid pattern in the ab plane, being similar to the previous reported heterocycle {[Cu6(trz)4Br][Cu4Br4(OH)]}.11 Interestingly, the adjacent square-grid layers are linked by μ4-Cl ions to generate 2D bilayer cationic network [CuI12(trz)8Cl]3+ [Cu−Cl: 2.701(21) Å] (Figure 2b). The 2D networks are further connected by [α-PW12O40]3− POAs to produce an interesting 3D framework [Cu−O: 2.538(17) − 2.605(13) Å] (Figure 2c, d), representing the first example of PMOF consisted of the cationic network [CuI12(trz)8Cl]3+ and POM. Different to the [α-SiW12O40]4− POAs in 1, [αPW12O40]3− POAs in 2 locate in the adjacent layers instead of lying in the nine-membered rings. Photocatalysis Properties. The optical band gap (Eg) of the photocatalyst is the key factor affecting the efficiency of the photocatalysis process.12 The solid state diffuse reflectivity spectra of 1 and 2 were conducted for the Eg values which can be defined 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. The Kubelka−Munk function F = (1 − R)2/2R, was transformed from the recorded diffuse reflectance data, in which R is the reflectance of an infinitely thick layer at a given wavelength.13 Eg values are 2.11 eV for 1 and 2.35 eV for 2, respectively (Figure 3), which suggests that 1 and 2 are potentially semiconductive materials. The semiconductive

Figure 4. UV−visible absorption spectral changes observed for the RhB solution under visible light irradiation in the presence of (a) 1 and (b) 2 as photocatalysts, respectively. Inset: the conversion of RhB (K) with reaction time (t). The conversion of RhB (K) can be expressed as K = It/I0, where I0 represents the UV−vis intensity of RhB at the initial time (t = 0) and It is the UV−vis absorption intensity at a given time (t).

absorption band of the dye at around 554 nm blue-shifts gradually as the irradiation time increases, which can be explained by either the destruction of its conjugated structure or the N-deethylation of RhB during photocatalytic process.15 In addition, the long-term stability of 1 was tested for the practical applications. After each cycle when the photo-

Figure 3. Diffuse reflectance UV−vis−NIR spectra of K−M functions versus energy (eV) of (a) 1 and (b) 2. D

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properties. In order to compare with PMOFs 1 and 2, the photodegradation of RhB in the presence of (NBu4)4[SiW12O40] or (NBu4)3[PW12O40] was examined under the same conditions (Figure S8). The conversion rates were 82% and 71%, respectively. The above results suggest that the photocatalytic activities of PMOFs 1 and 2 are mainly derived from the coefficient effect of POMs and MOFs. Additionally, the reasons why the degradation efficiency of 1 is significantly higher than that of 2 are as follows: (i) The band gap of 1 is narrower than that of 2, which is more conducive to the separation of the charges. (ii) The differences of components and structures between 1 and 2. As described in the crystal structures, the structural unit of 1 consists of a [CuI12(trz)8(H2O)2]4+ and a [α-SiW12O40]4−, whereas 2 contains a [CuI12(trz)8Cl]3+ and a [α-PW12O40]3−. Importantly, [α-SiW12O40]4− POAs locate in the nine-membered Cu-trz rings, and [α-PW12O40]3− POAs lie in the adjacent layers (built by [CuI12(trz)8Cl]3+ units) via Cu···O weak interactions. The skeleton of PMOF 2 is not beneficial to the adsorption and desorption of hydroxyl radicals and superoxides on its surface. Besides, the skeleton of PMOF 2 is not suitable to the transport of excited electrons to its surface, because it retards the generation of hydroxyl radicals and superoxides and further hinders the occurrence of the photocatalytic process. Impressively, the performance of PMOF 1 is among the best reported PMOFs with high photocatalytic activity under visible light and photostability (Table S1). In consideration of the excellent performances of PMOFs 1 and 2 in photodegradating RhB under visible light, moreover, previous studies have claimed that the use of organic dyes as a test compound for photocatalytic activity is inadequate.19 Hence, the photocatalytic hydrogen evolution activities of PMOFs 1 or 2 (100 mg) were preliminarily examined using 1.0% Pt (wt %) as a cocatalyst in 20% methanol aqueous solution (100 mL) under a 300 W Xe lamp irradiation. During this catalysis process, methanol serves as the sacrificial electron donor as well as the partial source of the protons required in the reduction semireaction of water. As shown in Figure 6 and

degradation process was completed, the catalyst was centrifuged and filtered, then immersed into another identical amount of RhB aqueous solution for another run in the same condition. Moreover, the PXRD patterns after five cycles of photodegradation process match well with the simulated one (Figure S6), indicating that 1 is very stable and reusable for at least five cycles (Figure 5).

Figure 5. Reusability experiments of PMOF 1.

The photocatalytic performance of PMOF 1 is comparable to most of reported photocatalysts for the degradation of RhB. These reported photocatalysts include many UV range catalysts, such as the POM-based hybrid with [ring + helix] channels,16a the semiconducting microporous framework built by Cd6Ag4(SPh)16 clusters and bipyridines,16b and the AgPMOFs with helix/loop subunits,16c as well as several rare visible-light catalysts (e.g., the nanosized inorganic catalyst Bi2WO6,17a the 3D inorganic heteropoly blue,17b and the metalazole framework containing only CuI ions and trz− ligands17c). More importantly, the amazing stability and reusability of PMOF 1 recommend it as a promising candidate for visiblelight photocatalyst. The photocatalytic mechanisms can be derived as follows: POMs easily generate the excited *POMs under irradiation, and then *POMs ([*SiW12O40]4− or [*PW12O40]3−) could extract electrons from H2O molecules ̇ to form the hydroxyl radicals (OH) during the photocatalytic process. The reduced POMs (POMs−) are quite stable when encapsulated in 3D framework and can be quickly reoxidized in the presence of O2. The reoxidation accompanies the ̇ −). These cycles take generation of superoxide radicals (O 2 place continuously while the system is exposed to the visible light. Simultaneously, the RhB is also excited by light to generate *RhB molecules. As a result, the photodegradation process of RhB can be effectively accomplished by the hydroxyl and superoxide radicals.18 In order to confirm the above mechanism, we introduced isopropyl alcohol (IPA) and benzoquinone (BQ) into the photocatalytic RhB degradation ̇ reaction system as the quencher of OH and ̇O2−, respectively. As shown in Figure S7, the photocatalytic degradation rates of RhB for PMOF 1 were dramatically depressed by the addition ̇ H and ̇O2− of IPA and BQ, respectively, which indicates that O do emerge and play a crucial role in the process of RhB oxidation. For PMOFs 1 and 2, the modification of POMs by {CuI-trz} units provides the possibility to get enhanced photocatalytic activity. {CuI-trz} units as linkers connect POMs and promote the transfer of the electrons from POM to POM. This promotion can prevent the deactivation and conglomeration of POMs, resulting in the enhancement of photocatalytic

Figure 6. Time course of H2 evolution from 1 (red), 2 (blue), and without catalyst (black) with the 1.0% Pt cocatalyst under 300 W Xe lamp (without filter) irradiation in 100 mL of 20% methanol aqueous solution.

Figure S9, the total amount of H2 was 44.4 μmol in 5 h for PMOF 1, and the H2 continuously evolved at a rate of 192.2 μmol g−1 h−1, which could be comparable to the well-known catalysts, such as the phosphoniobate-based 3D frameworks,20a the amino-functionalized TiIV-MOF,20b the dye-sensitized Pt@ UiO-66(Zr) MOF,20c the postsynthesis modification of MOF253-Pt,20d as well as the biomimetic {[FeFe]@ZrPF}.20e E

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introduction of POMs into MOFs gives rise not only to the increase of visible-light absorption, but also to more-efficient separation of photogenerated electron/hole pairs. The PMOFs as heterogeneous catalysis exhibit significantly enhanced photocatalytic activity under visible light and excellent stability during the catalysis process. Especially, PMOF 1 is not only among the best reported PMOFs with high photodegradation activity and photostability but also presents high photocatalytic hydrogen evolution activity. This work presents a rational way to convert homogeneous POMs to heterogeneous catalysts with accessorily photocatalytic property.

Meanwhile, the blank experiment (without PMOF 1 as the catalyst) was also performed, and the amount of H2 produced was no more than 0.5 μmol in 5 h. In addition, the photocatalytic hydrogen evolution activity of PMOF 2 was also measured at the same conditions (Figure 6), which was less active than that of PMOF 1. The reason can be ascribed to the different band gaps, as well as the differences of components and structures between PMOFs 1 and 2. In fact, the prolonged experiment in which the photocatalytic process was performed for 5 h showed that the hydrogen evolution decreased gradually over time toward a plateau, indicating the efficient photocatalytic abilities of PMOFs. Besides, three cycles of photocatalytic hydrogen evolution for PMOF 1 (Figure S10) have been tested, and the recovered PMOF 1 shows no obvious alterations in the XRD patterns (Figure S11), which suggests that PMOF 1 is recyclable in the photocatalytic hydrogen production reaction. The possible mechanism for H2 production of this system is illustrated in Figure 7. When the full spectrum light excites

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00098. TGA spectra, additional figures, and crystallographic data of 1 and 2 (PDF) Accession Codes

CCDC 1025222−1025223 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 Authors

*E-mail: [email protected]. Fax: (+86) 22-23502458. *E-mail: [email protected]. ORCID

Landong Li: 0000-0003-0998-4061 Wei Shi: 0000-0001-6130-1227 Peng Cheng: 0000-0003-0396-1846

Figure 7. Possible mechanism for photocatalytic hydrogen production over PMOF 1 under full spectrum light irradiation.

Author Contributions †

X.Z. and S.Z. contributed equally to this work.

POMs in PMOFs, the ligand to metal charges transfer (LMCT) will be induced. LMCT facilitates an electron transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) to produce the photoelectrons, which is similar to the process in semiconductor photocatalysts.16a The photogenerated holes will be trapped on the surface defects and then react with methanol to produce CO2 and protons through the intermediates, e.g. formaldehyde and formic acid.21 Subsequently, the protons generated from water as well as methanol22 (Figure S12) and gathered on the surface of PMOFs are transferred to the Pt centers, and react with the photoelectrons to produce H2. In this system, the PMOF acts as a photoelectron generator to enhance the activity of H2 production, and the Pt center, a good electron trap, can effectively inhibit the recombination of photogenerated electrons and holes and provide redox reaction sites for hydrogen evolution. In PMOFs 1 and 2, the HOMO and LUMO are composed of oxygen 2p and metal (CuI) d orbitals, respectively. The diffuse reflectance UV−vis spectra of PMOFs 1 and 2 were measured to achieve the HOMO− LUMO gap (Eg).13

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 (21622105 and 21421001) and the Ministry of Education of China (B12015).

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REFERENCES

(1) (a) Gust, D.; Moore, T. A.; Moore, A. L. Solar fuels via artificial photosynthesis. Acc. Chem. Res. 2009, 42, 1890−1898. (b) Chen, H.; Nanayakkara, C. E.; Grassian, V. H. Titanium dioxide photocatalysis in atmospheric chemistry. Chem. Rev. 2012, 112, 5919−5948. (c) Montoya, J. H.; Seitz, L. C.; Chakthranont, P.; Vojvodic, A.; Jaramillo, T. F.; Nørskov, J. K. Materials for solar fuels and chemicals. Nat. Mater. 2017, 16, 70−81. (2) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 2010, 110, 6503− 6570. (b) Gao, C.; Wang, J.; Xu, H.; Xiong, Y. Coordination chemistry in the design of heterogeneous photocatalysts. Chem. Soc. Rev. 2017, 46, 2799−2823. (c) Maeda, K.; Domen, K. Photocatalytic water splitting: recent progress and future challenges. J. Phys. Chem. Lett. 2010, 1, 2655−2661. (d) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy. Nat. Mater. 2011, 10, 911−921.

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CONCLUSIONS In conclusion, two 3D hybrid MOFs based on Keggin-type POMs have been successfully prepared, in which the F

DOI: 10.1021/acs.inorgchem.8b00098 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00098 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00098 Inorg. Chem. XXXX, XXX, XXX−XXX