Highly Efficient Visible-Light-Driven H2 Production via an Eosin Y

Jun 12, 2018 - (3) Therefore, highly efficient photocatalysts should be developed for ... (20−26) However, most of these systems suffer from low act...
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Cite This: Inorg. Chem. 2018, 57, 7495−7498

Highly Efficient Visible-Light-Driven H2 Production via an Eosin Y‑Based Metal−Organic Framework Jian Wang,*,† Yanhong Liu,† Yang Li,† Lingling Xia,† Min Jiang,† and Pengyan Wu*,† †

School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou, Jiangsu 221116, China

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been widely used as photocatalysts for water splitting.20−26 However, most of these systems suffer from low activity, high cost, and poor visible-light utilization; photocatalytically active MOFs in the visible-light spectral region are still relatively rare. Therefore, a facile approach should be developed to fabricate visible-light-responsive MOFs. In this paper, an ecofriendly dye, Eosin Y, was first used as the linker and light harvester to construct a 3D Cd-MOF. The resultant Cd-MOF exhibited excellent visible-light-driven catalytic activity for H2 production. The solvothermal reaction of Cd(NO3)2·6H2O, Eosin Y, and 4,4′-bipyridine (bpy) in methanol and water afforded the compound {Cd(C20H6Br4O5)(C10H8N2)0.5(H2O)0.5}n (CdEY1). Single-crystal X-ray diffraction measurement displayed that Cd-EY1 belongs to the monoclinic system with the C2/c space group. The asymmetric unit of Cd-EY1 included one CdII atom, one Eosin Y anion, a half bpy ligand, and a half μ2-bridging water molecule. Each Cd2+ ion is bonded to two bimonodentate carboxylate groups from two Eosin Y molecules, one hydroxyl O atom from the other Eosin Y molecule, one N atom from a bpy ligand, and one water molecule, exhibiting trigonal-bipyramidal coordination geometry (Figures 1a and S1 and S2). Two adjacent Cd2+ ions are interconnected by one μ2-O bridge and two carboxylic groups to form a dimer unit [Cd2(COO)2(μ2H2O)] with a nonbonding Cd···Cd distance of 3.727 Å, further linking another dimer via Eosin Y ligands to produce a 1D chain along the c axis. These 1D chains are bridged together by bpy ligands to generate a 2D layer (Figure 1b), which is further faceto-face-interdigitated to a 3D framework via a π···π interaction (distance 3.54 Å) between two neighboring xanthene rings from different layers and C−H···π interactions (distance 2.64 Å) between the phenyl H atom of Eosin Y and the pyridine ring of the bpy ligand from neighboring layers. The structure of Cd-EY1 can be simplified as a sql topology with a Schläfli symbol of (44· 62). Notably, 1D zonal channels of 9.6 × 12.8 Å2 exist in Cd-EY1 (Figure 1c). The solvent-accessible volume is 1207.7 Å3, calculated using PLATON software, and possesses 20.8% of the unit cell. Thermogravimetric analysis data of Cd-EY1 displayed a weight loss of 1.21% with temperatures of 26−200 °C, which is in accordance with the fact that this structure loses a half water molecule (calculated as 1.02%) and remains stable until 350 °C (Figure S3). The Brunauer−Emmett−Teller surface area calculated from CO2 sorption data for Cd-EY1 turns out to be 114 m2 g−1 (Figure S5). The UV−visible spectrum measurement of solid Cd-EY1 samples displayed a broad absorption band ranging from 373 to

ABSTRACT: A simple and effective strategy was developed to immobilize ecofriendly dye inside a porous metal−organic framework (MOF) built from Eosin Y with [Cd2(COO)2(μ2-H2O)] secondary building units for the first time. The MOF exhibited efficient photocatalytic activity for H2 evolution under visible-light irradiation with a turnover number of 13920 and an initial turnover frequency of 7433 h−1, which was approximately 31 times the photocatalytic efficiency of Eosin Y.

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ydrogen is an alternative, sustainable, and clean energy source that can replace fossil fuels.1 Photocatalytic water splitting is considered to be a promising technique for H2 production, aimed at converting the abundant solar energy on earth into chemical fuel energy.2 However, most photocatalysts operate only under UV irradiation, which accounts for only 3− 5% of the solar spectrum.3 Therefore, highly efficient photocatalysts should be developed for maximizing visible-light utilization. Over the past 30 years, organometallic dyes such as ruthenium-, platinum-, and iridium-based complex-sensitized catalysts have been used to develop visible-light-responsive photocatalysts; nevertheless, their problematic recyclability, toxicity profile, and high cost restrict their more general application, particularly, on large scales.4,5 Recent studies have focused on using cheap ecofriendly dyes as alternatives to environmentally more benign photoactive catalysts. Among such dyes, Eosin Y has been the most widely employed.6 However, the homogeneous nature of Eosin Y drives low stability (dissolution and self-degradation) under a photocatalytic reaction in water and fast carrier recombination/deactivation. Therefore, we hypothesized that immobilization of Eosin Y fragments into a rigid skeleton would be a promising approach for building visiblelight-responsive photocatalysts for enhanced H2 production. Metal−organic frameworks (MOFs) are a new family of hybrid solids that are constructed from inorganic connecting points and organic bridging ligands.7−12 The highly crystalline nature of the MOFs ascertains charge transfer from the lightexcited organic ligands to metal clusters, whereas their tenability can facilitate mass transport in photocatalytic reactions and improve the efficiency of electron−hole separation,13−16 thereby making MOFs superior photocatalysts. In addition, their design flexibility, derived from the large variation of available organic ligands and metal centers, is beneficial for enlarging the light absorption regions, which improves their photoactivity; this phenomenon is unparalleled in the semiconductor-based photocatalysts.17−19 With all of these advantages, MOFs have © 2018 American Chemical Society

Received: March 17, 2018 Published: June 12, 2018 7495

DOI: 10.1021/acs.inorgchem.8b00718 Inorg. Chem. 2018, 57, 7495−7498

Communication

Inorganic Chemistry

Figure 2. (a) Fluorescence response spectra of 0.17 mM Cd-EY1 suspension upon the gradual addition of the compound [Co(bpy)3]Cl2. (b) Transient emission spectra of solid Cd-EY1 and Cd-EY1 incorporated with [Co(bpy)3]Cl2.

that the excited state of Cd-EY1 could reduce the proton reduction catalyst [Co(bpy)3]Cl2. Irradiation of an ethanol/water mixture (1/1, v/v) suspension including Cd-EY1 and [Co(bpy)3]Cl2 with a 500 W xenon lamp at 25 °C resulted in H2 generation using triethylamine (TEA) as a sacrificial electron donor. The reaction was optimized by regulating the pH value of the solution and changing the amount of TEA (Figures S10 and S11). The highest H2 production efficiency was obtained at pH = 9 and a 5% (v/v) concentration of TEA; a decrease in the activity at both lower and higher TEA concentrations and/or pH was observed. When the photocatalysis conditions were fixed (Cd-EY1 and [Co(bpy)3]Cl2 with concentrations of 26 μM and 0.34 mM, respectively; 5% TEA in an ethanol/water mixture solution; pH = 9), the initial turnover frequency (TOF) was 7433 h−1, and the highest H2 evolution turnover number (TON) based on Cd-EY1 was 13920 (Figure 3). These values are the largest reported to date for related systems with [Co(bpy)3]Cl2 as the proton reduction catalyst.27,23,26 Notably, the index of PXRD patterns of the Cd-EY1 materials collected from the solution via centrifugation wellmatched that of the original one, confirming the stability of the MOF material during catalysis (Figure S7). Also, inductively coupled plasma mass spectrometry analysis of the supernatant solution after H2 evolution showed trace amounts of Cd leaching into the solution; therefore, its original structural framework was retained during the photocatalytic process. The high H2 production efficiency showed that Cd-EY1 is an excellent photolysis hydrogenation system. This led to a further investigation of the potential factors influencing the photocatalytic efficiency during the H2 evolution process. Several

Figure 1. (a) Construction of Cd-EY1 consolidated by Eosin Y ligands and the coordinated environment of Cd2+ ions in Cd-EY1. (b) 2D layer structure in Cd-EY1 composed of 1D chains interconnected by bpy units. (c) Connolly surface of Cd-EY1 with a radius of 1.4 Å along the [001] direction.

650 nm, demonstrating excellent absorption for visible light (Figure S4). Upon excitation at 452 nm, the Cd-EY1 suspension in water displayed an emission band centered at 558 nm. Even after adjustment of the pH value of the Cd-EY1 system with HCl or NaOH, the Cd-EY1 suspension showed steady fluorescence in the pH range of 5.7−10.5 (Figure S6). Furthermore, the chemical resistance of Cd-EY1 was examined via powder X-ray diffraction (PXRD) in harsh conditions: soaking in an aqueous solution with pH = 10.5 and 5.6 for 1 day, demonstrating the maintenance of the framework under these conditions (Figure S7). These results indicate that Cd-EY1 exhibited excellent luminescence stability and high chemical resistance, providing a basis for photodriven H2 evolution in water. Gradually adding 0.24 mM [Co(bpy)3]Cl2 to a 0.17 mM Cd-EY1 suspension in water quenched approximately 99% of its emission intensity with KSV (Stern−Volmer quenching constant) of (1.23 ± 0.37) × 105 M−1 (Figures 2a and S8). Also, transient emission spectra of solid Cd-EY1 showed that their fluorescence lifetime decreased from 2.07 to 1.61 ns when [Co(bpy)3]Cl2 existed (Figure 2b). The result indicated that a direct photoinduced electron-transfer process occurred from the excited state of Cd-EY1 to the proton reducing agent [Co(bpy)3]Cl2, which suggested that it is possible 7496

DOI: 10.1021/acs.inorgchem.8b00718 Inorg. Chem. 2018, 57, 7495−7498

Communication

Inorganic Chemistry

Figure 4. Structure of Cd-EY2 showing the coordination geometry of the Cd ions. Color code: Cd, cyan; O, red; Br, green; C, gray. Free solvent molecules have been omitted for clarity. Figure 3. H2 generation from Cd-EY1, a Cd-EY2 suspension, or a Eosin Y solution containing [Co(bpy)3]Cl2 with a concentration of 0.34 mM. All samples were placed in 5% TEA in 1/1 ethanol/water, pH = 9, and irradiated with 500 W xenon lamps.

systems exhibited almost identical photoactive units, the superiority of Cd-EY1 over Cd-EY2 was attributed to the close proximity of [Co(bpy)3]Cl2 and the photoactive units within the confined spaces, encouraging an efficient photoinduced electrontransfer process, which further increased the H2 evolution efficiency. In summary, this study established a novel and simple strategy to immobilize ecofriendly Eosin Y into the MOF material. When Eosin Y was introduced as the organic linker of the MOF generated, visible-light-driven H2 production was enhanced; its highest activity was a TON reaching 13920 per mole of MOF. Detailed studies demonstrated that the improved catalytic performance could be ascribed to the efficient photoinduced electron-transfer process originating from the close proximity of the photosensitizer and the redox sites within confined channels of the framework. The approach reported in this study could open a new avenue to the development of novel porous functional materials for a broad scope of visible-light-driven applications.

control experiments were conducted; e.g., the artificial system cannot perform well without light; the absence of any of the individual components, containing TEA, Cd-EY1, and [Co(bpy)3]Cl2, led to no H2 production (Table S1, entries 2−5). The subcompounts of Cd-EY1 were then investigated independently. Eosin Y instead of Cd-EY1 was applied to photodriven H2 production under the same conditions as mentioned above, leading to ca. a 96.8% decrease in H2 evolution, and the TON was only 445 per mole of photosensitizer. Moreover, the reactions catalyzed by a simple mixture of Eosin Y and bpy, Eosin Y and Cd2+, or Eosin Y, bpy, and Cd2+ produced hydrogen in low yields, as shown in entries 7−9 of Table S1. Most likely, this is due to an effecient interaction between the cobalt complex and the host photosensitizer Eosin Y immobilized in the confined space of the MOF. To confirm this hypothesis, Fourier transform infrared spectroscopy of the photosensitizer Cd-EY1 impregnated with an aqueous solution of [Co(bpy)3]2+ displayed a C−H stretching vibration at 3067 cm−1, a CN stretching vibration at 1241 cm−1, and a C−H bending vibration at 976 cm−1 for [Co(bpy)3]2+. The apparent red shift from 3097, 1315, and 1013 cm−1 (free [Co(bpy)3]2+), respectively, suggested that [Co(bpy)3]2+ was adsorbed through a weak interaction between Cd-EY1 and the compound [Co(bpy)3]2+ (Figure S9). To verify the importance of the confined space in the MOF, a new homodinuclear complex, Cd-EY2 [Cd(C 20 H 7 Br 4 O 5 )(CH3OH)3(H2O)]2(OH), containing the same photosensitizer Eosin Y and Cd ions but no bpy moieties was designed and prepared as a reference. The crystal structure showed one Cd atom, one Eosin Y anion, three coordinated methanol molecules, one coordinated water molecule, and a hydroxyl group in an asymmetric unit (Figure S12). The CdII atom links its symmetric part through a hydroxyl−O bridge to construct a C2-symmetric dinuclear complex with a nonbonding Cd···Cd distance of 4.157 Å (Figure 4). A fluorescence titration of Cd-EY2 (0.17 mM) also exhibited approximately 99% quenching in the emission intensity upon the addition of 0.24 mM [Co(bpy)3]Cl2 (Figure S13). Thus, in the same photocatalytic conditions as those of Cd-EY1, the TOF was approximately 714 h−1, the calculated TON was approximately 2830 based on Cd-EY2, and its stability was confirmed by electrospray ioniozation mass spectrometry (Figure S14). Although this value was 6 times higher than that of the simple blending of Eosin Y and Cd2+, it exhibited an approximately 78.6% decrease compared with that produced using Cd-EY1. Because the two compounds in the 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.inorgchem.8b00718. Experimental details, crystal data, and related spectra (PDF) Accession Codes

CCDC 1822271−1822272 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] (J.W.). *E-mail: [email protected] (P.W.). ORCID

Jian Wang: 0000-0002-0282-3911 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (Grants 21401087 and 21401086), the NSF of Jiangsu Province (Grant BK20140234), and TAPP and PAPD of Jiangsu Higher Education Institutions. 7497

DOI: 10.1021/acs.inorgchem.8b00718 Inorg. Chem. 2018, 57, 7495−7498

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00718 Inorg. Chem. 2018, 57, 7495−7498