Construction and Photocatalytic Activities of a Series of Isostructural

Article pubs.acs.org/crystal

Construction and Photocatalytic Activities of a Series of Isostructural Co2+/Zn2+ Metal-Doped Metal−Organic Frameworks Jingjuan Liu, Jiannan Xiao, Dongbo Wang, Wei Sun, Xuechuan Gao, Haiyang Yu, Houting Liu, and Zhiliang Liu* College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010020, P. R. China S Supporting Information *

ABSTRACT: Photocatalytic behaviors of isostructural metaldoped metal−organic frameworks were rarely discussed in the field of metal−organic frameworks. Herein, a series of 2D isomorphous Co2+/Zn2+ metal-doped metal−organic frameworks, namely, [M(tpbpc)(bdc)0.5·H2O]·solvent (M = Co (1), Co0.545Zn0.455 (2), Co0.2165Zn0.7835 (3), Co0.0258Zn0.9742 (4), Zn (5), Htpbpc = 4′-[4,2′;6′,4″] terpyridin-4′-yl-biphenyl-4carboxylic acid, H2bdc = 1,4-benzenedicarboxylic acid), have been successfully synthesized under solvothermal conditions, and the crystal structures were testified by the single-crystal X-ray diffraction analysis and PXRD analysis. The photocatalysis activities of 1−5 were investigated by photodegradation of methyl orange (MO) experiments under visible light irradiation. Results indicated that 1 is nearly an inactive photocatalyst for degradation of MO, whereas 5 exhibits excellent photocatalytic activity under visible light irradiation. Interestingly, when Co2+ ions are gradually replaced by Zn2+ ions in the metal−organic frameworks, photocatalytic activities of which have improved gradually from 1 to 5; this offers a controllable regulation of photocatalytic properties by changing metal ions in isostructural metal-doped MOFs.

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INTRODUCTION Metal−organic frameworks (MOFs) have evoked considerable attention in the past two decades not only because of their intriguing topological structures1−4 but also due to their high surface areas, tunable pore sizes, and open metal sites,5,6 which endow them with potential applications in gas storage,7,8 separation,9,10 light emitting,11,12 chemical sensing,13,14 ion exchange,15,16 and heterogeneous catalysis.17−19 Thereinto, utilizing MOFs in heterogeneous catalysis to remove harmful pollutants in water became a hot topic for chemists.19−24 At present, many methods are being used for ecological elimination of organic compounds or harmful pollutants in water such as adsorption and separation, chemical treatment, and photocatalysis.25−28 By comparison, photocatalysis under the visible light irradiation is a more convenient and recyclable method to degrade organic dyes without further contamination.29 Some typical solid photocatalysts such as TiO2,30,31 semiconductor metal oxides and sulfide particles,32,33 layered titanates and niobates,34,35 and polyoxometalates36,37 have been extensively explored for oxidation or degradation of organic contaminants. Compared with traditional photocatalysts, MOFs materials not only can offer high surface areas and tunable band gaps but also can be recovered and reutilized easily.20,38−40 Therefore, remarkable attention has been paid to develop new MOFs photocatalytic materials with highly efficient photocatalytic activities and effective use of the solar energy sources.41,42 According to the pioneering works, some MOFs such as UIO-66, MOF-5, and MIL-100 (Fe) have been reported to © 2016 American Chemical Society

possess semiconductor properties for photoelectronics and photocatalysis.43−45 Lots of efforts have been made focusing on the relationship between photocatalytic efficiency and the structure of MOFs46−49 or the size of dye molecules.50,51 However, little effort has been exerted to explore how do the center metal ions within MOFs impact their photocatalytic behaviors. Because different center metal ions in MOFs possess different electronic configurations as well as the excitation and transformation of electronics play an important role in the possible mechanism of photocatalytic,52,53 the different central metal ions would induce great impacts on the photocatalytic behaviors. In order to explore the above relationship, the isostructural metal-doped MOFs with different central metal ions (M1 and M2) and the intermediate M1-M2 MOFs should be studied. With this approach, the influence of ligands and geometries can be eliminated easily. Meanwhile, the relationship between central metal ions and the corresponding influence on photocatalytic properties can be found conveniently. In this work, [Co(tpbpc)(bdc)0.5·H2O]·solvent was synthesized first by solvothermal reaction of Htpbpc, H2bdc, and Co(NO3)2·6H2O in a mixing solvent of DMF, EtOH, and deionized water at 85 °C. The single-crystal X-ray diffraction analysis reveals that the structure of 1 is a 2D lattice further incorporating via hydrogen bonding to build a three-dimenReceived: October 10, 2016 Revised: November 24, 2016 Published: December 19, 2016 1096

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systems were collected every 15 min; then the supernatant liquid separated by centrifugation was subjected to UV/vis spectroscopic measurements. Crystal Data Collection and Refinement. Crystallographic data were collected at a temperature of 293(2) K on an Agilent Xcalibur E X-ray single crystal diffractometer equipment with graphite-monochromated Cu−Kα radiation (λ = 1.54184 Å). The empirical absorption corrections were carried out using the SADABS program.54 Using the SHELX-97 program, the structures of 1 and 5 were solved by direct methods and refined anisotropically by full-matrix leastsquares techniques on F2 values.55 All hydrogen atoms were added on appropriate positions in theory using the riding model. From the difference Fourier maps of 1 and 5, a number of diffuse scattered peaks with electron density were observed, which can be attributed to the disordered solvent molecules. Attempts to model these peaks were unsuccessful because the residual electron density peaks obtained were diffused. Therefore, PLATON/SQUEEZE56 was used to refine the structure further. Crystallographic data and refinement parameters for 1 and 5 are listed in Table S1. Selected bond lengths and angles are listed in Tables S2 and S3. Crystallographic data for 1 and 5 have been deposited at the Cambridge Crystallographic Date Center with the deposition numbers 1506252 and 1506269.

sional framework containing 1D nanotube channels. Photocatalytic studies indicate that 1 is nearly an inactive photocatalyst for degradation of MO under visible light irradiation. Then, Co(NO3)2·6H2O is gradually replaced by Zn(NO3)2· 6H2O; a series of isomorphous metal-doped MOFs, namely, [ZnxCo1−x(tpbpc)(bdc)0.5·H2O]·solvent (0 ≤ x ≤ 1), were successfully obtained. With the replacement of Co(II) ions by Zn(II) ions, the properties of the MOFs as a catalyst to degrade MO under visible light irradiation have been improved gradually, which can offer a controllable regulation of photocatalytic properties by changing metal ions in metaldoped MOFs. At last, 5 presents the best photocatalytic activity and can completely degrade MO aqueous solution in a short time.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All the chemicals and solvents were obtained from commercial sources and used without further purification. Fourier transform infrared (FT-IR) spectra were measured with a PerkinElmer Spectrum Two FT-IR spectrometer (4000−400 cm−1) using the KBr pellet method. The TG curves were carried out under a heating rate of 10 °C min−1 from 30 to 1000 °C in the nitrogen protection recorded with a NETZSCH STA409pc instrument. X-ray diffraction patterns (PXRD) data were measured on an EMPYREAN PANALYTICAL apparatus and recorded on crushed single crystals were measured in the range of 5−80° using Cu−Kα radiation. Inductively coupled plasma spectroscopy (ICP) was performed on an Ultima 2 spectrometer. ICP-MS was performed on a PerkinElmer NexION 300Q spectrometer. The morphologies of the synthesized samples were investigated using a field-emission scanning electron microscope (HITACHI S-4800) at an accelerating voltage of 200 kV. The UV−vis absorption spectra were measured with a U-3900 spectrophotometer. Synthesis. Synthesis of [Co(tpbpc)(bdc)0.5·H2O]·Solvent (1). Co(NO3)2·6H2O (0.05 mmol, 0.0146 g), Htpbpc (0.03 mmol, 0.0129 g), and H2bdc (0.03 mmol, 0.0050 g) were dissolved in a 8 mL mixing solution of N,N-dimethylformamide (DMF), alcohol, and deionized water (1:1:2 in volume). Then, the above mixture was placed in a sealed 23 mL Teflon-lined stainless vessel, which was heated at 85 °C for 72 h under autogenous pressure and cooled down to room temperature at 5 °C/h. The as-synthesized purple-bulk crystals are insoluble in water and common organic solvents. Yield: 0.0131 g (74.4%). IR (KBr pallet, cm−1): 1662.89(s), 1599.35(s), 1571.11(m), 1528.63(m), 1385.49(s), 1213.19(m), 1095.30(s), 1063.95(s), 1027.96(m), 1004.51(s), 827.96(s), 785.59(s), 753.56(m), 644.19(s), 502.98(m). Synthesis of [ZnxCo1−x(tpbpc)(bdc)0.5·H2O]·Solvent (0 ≤ x ≤ 1) (2−4). The synthesis method of metal-doped MOFs 2−4 was the same as the process above except that a 0.05 mmol mixture of Co(NO3)2·6H2O and Zn(NO3)2·6H2O with a ratio of 8:2, 5:5, and 2:8 was used to instead of Co(NO3)2·6H2O, respectively. The color of crystals becomes fade gradually. The actual content ratios of Co2+ and Zn2+ ions in MOFs 2−4 are determined by ICP, which are listed in Table S4. Synthesis of [Zn(tpbpc)(bdc)0.5·H2O]·Solvent (5). A procedure similar to 1 was employed to synthesize 5 except that Zn(NO3)2·6H2O (0.05 mmol, 0.0149 g) was used instead of Co(NO3)2·6H2O. As a result, yellow rhombus-like crystals were obtained. Yield: 0.0160 g (98.7%). IR (KBr pallet, cm−1): 1656.52(m), 1601.31(s), 1571.23(s), 1528.89(m), 1384.73(s), 1216.66(m), 1065.69(s), 1028.54(s), 1004.57(s), 829.03(s), 785.9(s), 752.05(m), 649.80(s), 505.54(m). Photocatalytic Experiments. Photocatalytic experiments were performed in aqueous solutions with a 500 W high-pressure Xe lamp. A 15 mg portion of catalysts of MOFs 1−5 was added into 25 mL of MO dyes (2 × 10−5 mol L−1) aqueous solution. The solution was stirred constantly in the photocatalytic process. At the same time, another mixture sample was placed in a dark environment with stirring constantly. A sample with a certain volume from these reaction

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RESULTS AND DISCUSSION Crystal Structure of [Zn(tpbpc)(bdc)0.5·H2O]·Solvent (5). The single-crystal X-ray diffraction analysis reveals that MOFs 1 and 5 are isostructural as well as the powder XRD analysis and IR spectra confirm that heterometallic MOFs 2−4 are isomorphic with 1 and 5. Therefore, 5 is selected as a representative to describe the specific structure. The singlecrystal X-ray diffraction analysis reveals that 5 crystallizes in the triclinic system with the space group P1̅. There are one Zn2+ ion center, one tpbpc− ligand, 0.5 bdc2− ligand, and one coordinated water in its asymmetric unit (Figure 1a). The Zn(II) center is six-coordinated by two nitrogen atoms of two tpbpc− ligands [Zn−N = 2.065(6), 2.074(7) Å], two carboxyl oxygen atoms of one tpbpc− ligand [Zn−O = 2.354(6), 2.060(6) Å], one carboxyl oxygen atom of the bdc2− ligand [Zn−O = 2.010(10) Å], and one oxygen atom of the coordinated water molecule [Zn−O = 2.205(7) Å], resulting in a distorted octahedron geometry. The two carboxyl oxygen atoms and two nitrogen atoms of tpbpc− ligands formed the four vertices of the equatorial plane, and the oxygen atom of the bdc2− ligand and one oxygen atom of coordinated water formed the two vertices of axia in the distorted octahedron (Figure 1b). In particular, one Htpbpc ligand coordinates to three zinc centers via two nitrogen atoms as legs and one carboxylate as head. The deprotonated carboxylic acid group in Htpbpc adopts bidentate chelate model coordination with a metal ion, as well as the two nitrogen atoms on the pyridine ring are also available for coordination. Besides, the carboxylic acid groups in H2bdc are deprotonated and participate in coordination with two metal ions by monodentate modes (Figure 1c). After the H2bdc ligand is deleted from the structure of 5, Zn(II) ions centers are further connected by tpbpc− ligands to fabricate a highly ordered 2D sheet network structure with a rhombus channel of nearly 12.85 × 22.48 Å2. The sheet is bridged by a H2bdc ligand to exhibit a 2D latticed structure with a parallelogram channel of approximately 11.06 × 12.85 Å2 along the crystallographic c axis (Figure 1d). Furthermore, the 2D lattice structure is incorporated via hydrogen bonding to build a three-dimensional framework with nanochannels (Figure 2). The coordinated water molecules as donor are further bound to ligands acting as acceptors through O−H···O hydrogen bonds. As shown in Figure 3, the distances 1097

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Figure 3. Hydrogen bonding in 5 and the 3D H-bonded supramolecular network of 5. Symmetry codes for the atoms generated: B: −x + 1, −y + 2, −z + 1. C: x + 1, y + 1, z + 1. E: x + 1, y, z. F: −x + 1, −y, −z.

Figure 1. (a) Ball-and-stick representation of the asymmetric structural unit of 5. Hydrogen atoms are omitted for clarity. Symmetry codes for the atoms generated: A: x + 1, y, z. B: −x + 1, −y + 2, −z + 1. C: x + 1, y + 1, z + 1. D: x + 1, y + 1, z. (b) The coordination environment of central Zn2+ ion. (c) Different coordination environments of ligands Htpbpc and H2bdc. (d) The 2D structure of 5 from different directions.

shown in Figure 2c,d. PLATON analysis for 5 showed that the effective free volume is 29.5% of the crystal volume. To analyze the framework topology, the organic linkers of tpbpc− and bdc2− are treated as 3- and 2-connected nodes, respectively. The topological net of the 2D lattice structure and further 3D structure incorporated via hydrogen bonding for 5 are shown in Figure 4. PXRD and IR Analysis. The powder XRD analysis and IR spectra of H2bdc, Htpbpc, and MOFs 1−5 are shown in Figures S1−S3, respectively. It can be seen that the MOFs 1−5 possess similar XRD patterns and nearly all the diffraction peaks can be well matched with the simulated spectra. Compared with 1, the highest diffraction peak at 25.8° of 2− 4 has shifted a little, which can be owing to the replacement of central metal ions.57 For the samples of 1−5, the sharp peaks indicate the excellent crystallinity of as-synthesized samples. The IR spectra of 1−5 show similar characteristic bands; when compared with ligands, the absence of absorption bands at ∼1711 cm−1 from −COOH shows that the carboxylate groups of ligands are completely deprotonated. The bands around 3300 cm−1 are due to the water molecules. The powder XRD analysis and IR spectra further confirm the isomorphic structure of MOFs 1−5. Thermal Properties. In order to study the mobility of the solvent molecules within MOFs 1−5, the thermogravimetric analysis (TGA) was carried out, and the results are shown in Figures S4 and S5. The TGA curves of MOFs 1−5 are similar and show three distinct weight loss steps in the range of 30− 1000 °C. The first mass loss of 12−16% occurs between 30 and 232 °C, which could be attributed to the loss of solvent molecules. The second weight loss of 3.08% for 1 and 2.95% for 5 from 232 to 350 °C corresponds to the loss of one

Figure 2. (a, b) The 3D structures of 5 with a 1D chain represented by a yellow cylinder. (c, d) The space-filling models of 5.

between O5−H5B···O4E and O5−H5C···O1F are 2.608 and 2.741 Å, respectively. Besides, the space-filling models of 5 are 1098

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band around 525 nm probably can be attributed to the d−d spin-allowed transition of the Co2+ (d7) ions. Besides, the UV absorption band at 385 nm for MOF 5 can be ascribed to LMCT. Additionally, the relatively broader absorption band in the visible region of MOF 5 motivates us to explore its potential applications in heterogeneous photocatalysis. In addition, through changing the central metal ions in frameworks, the UV absorption bands for heterometallic MOFs 2−4 located in both the UV and the visible regions, which are around 258, 360, and 510 nm, respectively. The band gaps (Eg) were 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 Kubelka− Munk function F versus energy E.58,59 Based on the Kubelka− Munk equation,60 F = (1 − R)2/2R, where R is the reflectance of an infinitely thick layer at a given wavelength. After calculation, the band gaps of MOFs 1−5 are 2.96, 2.82, 2.79, 2.74, and 2.58 eV, respectively (Figure S9). We speculated that the difference in the band gaps may be probably influenced by the change of center metal ions which further impacts the photocatalytic activities. MO is a common organic dye which is extensively used in the textile industry and residual in wastewater; therefore, it usually is used for evaluating the activity of photocatalysts. In this work, MO was selected as a representative to investigate its degradation by MOFs 1−5. The photocatalytic performances of these MOFs were carried out under visible light irradiation. Remarkably, results indicate that the MO aqueous solution is hardly degraded efficiently with MOF 1 as a catalyst. In contrast, MO is completely degraded in a short time by MOF 5. Therefore, we doped Zn2+ ions into Co-MOF in order to investigate the photocatalytic activities of metal-doped MOFs. The photodegradation curves of MO for MOFs 1−5 are shown in Figure 5, and curves show that about 30% of MO had been decomposed by 1 upon visible light irradiation for 90 min. When the time increases to 300 min, MO had been decomposed nearly 90% (Figure S10). For comparison, the absorption behaviors of MO in the dark show that about 50% of MO had been absorbed after the same time. Experimental results show that MOF 1 has inefficient photodegradation performance. For heterometallic MOFs 2−4, the photodegradation curves show that the degradation efficiency of MO rose with the increase of the Zn2+ ions content in the frameworks. In addition, the photodegradation curve of MO assisted by MOF 5 shows that the maximum absorption band (464 nm) of the MO solution decreased obviously as a function of increasing reaction time under visible light irradiation. The color fading can be easily observed by the naked eyes in the process of the photocatalytic performance. After 90 min, the MO aqueous solution was completely degraded and became a colorless solution as well as the degradation efficiency can reach to 100%. In the dark environment, the absorption peak of the MO at 464 nm decreased 20% for 5 after the same time (Figure S11). This suggests MOF 5 is vigorous for the decomposition of MO under visible light. The changes in the C/C0 plot of the MO solution versus irradiation time for MOFs 1−5 are shown in Figure 5, wherein C0 is the initial concentration and C is the concentration after irradiation for a few minutes. The calculation results show that, after 90 min, the photocatalytic decomposition rates, defined as 1 − C/C0, are 0.34, 0.39, 0.60, 0.94, and 1, respectively. As listed in Table S7, compared with the catalytic performance among various MOFs in the

Figure 4. Crystal structures of 5. (a) Defined 2- and 3-connected nodes. (b) Schematic representation of topological net of 5 with different nodes discriminated by colors. (c) 3D structure representation of topological net of 5 incorporated via hydrogen bonding.

coordination water molecule (calcd: 3.06% for 1 and 3.03% for 5). The overall framework begins to collapse from 350 °C, owning to the decomposition of organic ligands, and the final residue can be ascribed to the formation of ZnO and CoO. Furthermore, the TGA curves were measured after heating the samples of 1−5 at 230 °C for several hours, and the results are shown in Figure S5. Before the overall framework begins to collapse, the little weight loss of nearly 3% corresponds to the loss of one coordination water molecule. Compared with the weight loss of the as-synthesized sample (Figure S4), we speculate that there are approximately five guest water molecules in the structure. Morphology Analysis. In order to investigate the size and morphologies of the Co2+/Zn2+ metal-doped samples, SEM images were taken out and are shown in Figure S6. The images confirmed that the samples 1−5 possess a uniform bulk structure and smooth surface, which illustrates that the Co2+/ Zn2+ metal-doping does not cause significant changes in MOF size and morphology. Photocatalytic Property. The solid state diffuse-reflectance UV−vis spectra for ligands Htpbpc and H2bdc are recorded (Figure S7), which show that the UV absorption bands for ligand Htpbpc are around 360 nm and the absorption bands for H2bdc are found around 320 nm. In order to elucidate the photoresponse wavelength region, the solid state diffuse-reflectance spectra for the MOFs 1−5 were recorded and are shown in Figure S8. MOF 1 consists of absorption components in both the UV (around 300 nm) and the visible regions (around 525 nm). The UV absorption band from 265 to 360 nm can be owing to ligand-to-ligand charge transfer and ligand-to-metal charge transfer (LMCT). The broad absorption 1099

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Figure 5. (a) Absorption spectra of the MO solution and the degradation curve during the decomposition reaction under visible light irradiation with the presence of MOFs 1−5. (b) Photographs showing the photocatalytic degradation under visible light for 1−5 (left) and for 5 in different times (right).

literatures, MOF 5 exhibits excellent photocatalytic degradation of MO under visible light. The kinetic data for MOFs 1−5 to degrade MO can be approximately fitted with the apparent first-order rate equation. The relationship between the percentage of Co(II)/Zn(II) and the corresponding band gaps and the photocatalytic activities of metal-doped MOFs are shown in Figure 6. With the increasing of Zn2+ content in the metal-doped MOFs, the value of the band gap reduces gradually and the photocatalytic performance for degradation of MO improves significantly. In addition, PXRD patterns of the samples after the photocatalytic reactions have been investigated, which have no obvious changes (Figure S12). The results demonstrate that 1−5 maintain their structural integrity and possess good stability as catalysts in the photocatalytic reaction system. The photographs of 1−5 before and after photocatalytic experiments are shown in Figure S13. Furthermore, after three times of photocatalytic tests on MOF 5, the photocatalytic efficiency shows little loss when MO is added to the system again (Figure S14). The slight decrease of the photocatalytic efficiency might be caused by the weight loss of catalyst during the process of

Figure 6. Corresponding band gaps and photocatalytic decomposition rates in metal-doped MOFs with different Zn/Co molar ratios.

recovery, which is hard to avoid. In order to confirm that if the partial samples would be degraded, the samples 3 and 5 were selected as representatives to test ICP-MS. As shown in the Table S6, there are little Zn2+/Co2+ ions that can be detected in 1100

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the liquid supernatant of samples 3 and 5 after the photocatalytic reaction. We speculate that there are very little Zn2+ or Co2+ released from the samples to water. In general, the photocatalytic degradation of MO may be attributed to the generation of hydroxyl radicals (·OH) in the photocatalytic systems. Therefore, the ·OH quenching experiment was carried out to investigate the degradation mechanism of MO on MOF 5. As the result, the photodegradation efficiency of MO on MOF 5 was sharply decreased in the presence of t-BuOH (IPA, a widely used ·OH scavenger), and the relevant rate constant for MOF 5 decreased from 100% to 58% under visible light irradiation (Figure S15). The result suggests that the ·OH radicals are predominate factors to degrade the organic dyes effectively. Moreover, other active species such as positively charged holes (h+), and O2 trapping experiments were further carried out, and the results are displayed in Figures S16 and S17. When molecular nitrogen is used instead of molecular oxygen in the reaction, the degradation efficiency of MO has a slight influence. While ammonium oxalate ((NH4)2C2O4) as holes scavenger is added, the degradation efficiency decreases significantly. Capture tests suggest that O2 is insignificant but h+ is a special vital factor in the photocatalytic reaction. In other words, the generation of · OH radicals from water molecules consumes lots of the photogenerated holes, which make a great contribution in the photocatalytic process. In addition, Scheme 1 summarizes a

Figure 7. Transient photocurrent response of MOF 5.

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CONCLUSIONS In summary, a series of 2D isostructural metal-doped MOFs have been successfully synthesized and exhibited different photocatalytic activities, respectively. Remarkably, when Co(II) ions are gradually replaced by Zn(II) ions in the isostructural MOFs as a catalyst, the ability of degradation for MO has improved gradually. In addition, MOF 5 can completely degrade MO aqueous solution under visible light irradiation in a short time. Then, the influence of center metal ions on the photodegradation activities for MO was explained through diffuse-reflectance UV/vis spectra analysis combined with the calculated MOFs’ band gaps. This work can offer a controllable regulation of photocatalytic properties by changing metal ions within isostructural metal-doped MOFs.

Scheme 1. Possible Mechanistic Proposal of the Photocatalytic Reaction

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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.cgd.6b01488. X-ray crystallographic data, selected bond lengths (Å) and bond angles (deg) for 1 and 5, ICP, FT-IR spectra, SEM, powder X-ray patterns, thermogravimetric curves, UV−vis absorption spectra, the calculation of band gaps for 1−5, and some additional figures (PDF)

possible mechanistic proposal of the photocatalytic reaction. The oxygen and nitrogen 2p bonding orbitals (valence band, VB) constitute the HOMO, and the empty transition metal orbitals (conduction band, CB) make up the LUMO. Under visible light irradiation, when absorption energy is equal to or greater than the band gap of the catalyst, the electrons (e−) were excited from the HOMO (VB) of MOFs to their LUMO (CB); meanwhile, the same amount of h+ can be formed in VB. The e− and h+ migrate to the surface of MOFs. In order to resume its stable state, the positively charged h+ strongly needs one e− from water molecules, which was oxidized to ·OH active constituent. In the meantime, the e− combined with the O2 on the surfaces of MOFs to produce superoxide radical (·O2−); then they might be translated into the ·OH radicals. Eventually, the formed ·OH radicals have the ability to decompose MO effectively to complete the photocatalytic process23,29. In order to confirm the efficient separation of photogenerated charge carriers, the photoelectron chemical properties of MOF 5 were measured and are shown in Figure 7, and the result shows that the photocurrent can be produced with a repeatable photocurrent of ∼0.12 μA cm−2 response to on/off cycle, suggesting that the MOF 5 is active to produce the photoinduced charge carriers under visible light irradiation and the charge carriers can transfer effectively.

Accession Codes

CCDC 1506252 and 1506269 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-471-4994375. Tel: +86471-4994375. ORCID

Zhiliang Liu: 0000-0003-3917-6014 Notes

The authors declare no competing financial interest. 1101

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ACKNOWLEDGMENTS We are grateful for the financial support provided by the NSFC of China (21361016) and the Inner Mongolia Autonomous Region Fund for Natural Science (2013ZD09).

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DOI: 10.1021/acs.cgd.6b01488 Cryst. Growth Des. 2017, 17, 1096−1102