Photoactive Metal−Organic Framework and Its Film for Light-DrivenHydrogen Production and Carbon Dioxide ReductionPengyan Wu, Xiangyang Guo, Linjuan Cheng, Cheng He,* Jian Wang, and Chunying Duan

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024, China

*S Supporting Information

ABSTRACT: The design of a new photocatalytic system andintegrating the essential components in a structurally controlledmanner to create artificially photosynthetic systems is highdesirable. By incorporating a photoactive triphenylamine moietyto assemble a Gd-based metal−organic framework as aheterogeneous photosensitizer, new artificial systems wereconstructed for the proton and carbon dioxide reduction underirradiation. The assembled MOFs exhibited a one-dimensionalmetal-oxygen pillar that was connected together by thedepronated TCA3− ligands to form a three-dimensional non-interpenetrating porous framework. The combining of protonreduction and/or the carbon dioxide reduction catalysts, i.e., theFe-Fe hydrogenase active site models and the Ni(Cyclam)complexes, initiated a photoinduced single electron transfer fromits excited state to the substrate. The system exhibited an initial TOF of 320 h−1 of hydrogen per catalyst and an overall quantumyield of about 0.21% and is able to reduce carbon dioxide under irradiation. The deposit of the photoactive Gd-TCA into the filmof an α-Al2O3 plate provided a platform for the practical applications through prolonging the lifetime of the artifical system andallowed the easily operated devices being recyclable as a promising photocatalytic system.

■ INTRODUCTION

A renewable and clean energy source has been paid more andmore attention in recent years due to the exhausting fossil fueland the world’s growing demand for green energy. Hydrogenwas regarded as a clean fuel of the next generation to reduceconsumption of fossil fuels and emission of greenhouse gases.1,2

The promising strategy is hydrogen production from waterutilizing solar energy, since the harnessing of solar energywould contribute significantly to our electrical and chemicalneeds, reducing the carbon dioxide emission.3,4 To date, severaltypes of hydrogen evolution photocatalysts that operated inheterogeneous systems, including the heteroatom-doped,nanoxide type, dye-sensitized, and Z-scheme types, have beendeveloped.5,6 Inspirited by the structures and mechanism of thenatural photosynthetic systems, light-driven hydrogen evolutionhas been also realized by the employing of a multicomponentsystem consisting of a sacrificial electron donor, a photo-sensitizer, and a hydrogen evolution catalyst in a homogeneoussystems.7,8 These systems have shown beneficial features ineach of their fields; the design of new photoactive systems andintegrating the components in a structurally controlled mannerto create more efficient functional devices is still high desirable.9

Metal−organic frameworks (MOFs) are a new family ofhybrid solids with infinite structures built from organic bridgingligands and inorganic connecting points.10,11 The intrinsiccrystalline property provided precise knowledge about the porestructure and the nature and distribution of catalytically active

sites. In comparison to these heterogeneous catalytic systemsthat were examined earlier, the design flexibility and frameworktenability resulting from the huge variations of metal nodes andorganic linkers make MOFs interesting photocatalysts in thelight-driven hydrogen production from water.12 By incorporat-ing a triphenylamine moiety as the backbone of the organiclinker, we report herein the preparation of a new gadolinium-based lanthanide−organic framework and its film for theapplication as a heterogeneous photosensitizer in the light-driven hydrogen production and carbon dioxide reduction. Weenvisioned that the redox potential of the excited MOF-basedmaterial is negative enough to reduce these proton reductioncatalysts and/or the carbon dioxide reduction catalysts, i.e., theFe-Fe hydrogenase active site models13 and the Ni(Cyclam)complexes,14 as the triphenylamine moiety is a stable blueemitter and an efficient electron donor to initiate a photo-induced single electron transfer from its excited state to thesubstrate.15,16 (Scheme 1)The choice of gadolinium as the nodes of the framework

partly is due to that the Gd3+ has no energy level below 32 000cm−1 to accept any energy from the TCA3− moiety, whichpotentially prevents the unnecessary energy loss in the case ofother lanthanide ions.17−19 The pores of the photoactive MOFsprovided the possibility to adsorb redox catalysts with suitable

Received: May 25, 2016Published: August 1, 2016

Article

pubs.acs.org/IC

© 2016 American Chemical Society 8153 DOI: 10.1021/acs.inorgchem.6b01267Inorg. Chem. 2016, 55, 8153−8159

redox potential to form artificial systems, from which the closeproximity between the adsorbed redox catalyst and thephotosensitizer around the confined pores is expected tofacilitate the efficient photoinduced electron transfer, where theunwanted energy-transfer and electron-transfer process poten-tially was avoided.16 In the meantime, the deposition of MOF-based material onto the α-Al2O3 plate to create a membranewas expected to integrate the components in a structurallycontrolled manner to create more efficient functional devices. Itwill combine the large surface areas with pore characteristics ofMOFs to prolong the lifetime of the systems for light-drivenhydrogen evolution and carbon dioxide reduction, leading theMOFs to exhibit potentially practical applications.20

■ RESULTS AND DISCUSSIONSynthesis and Characterization of Gd-TCA. Solvother-

mal reaction of 4,4′,4″-tricarboxyltriphenylamine (H3TCA)with Gd(NO3)3·6H2O in a DMF/ethanol (1:1 in v:v) mixedsolvent gave a new compound Gd-TCA in a yield of 60%.Elemental analyses along with powder X-ray analysis indicatedthe pure phase of its bulky sample. Single-crystal structureanalysis reveals the formation of a three-dimensional non-interpenetrating porous framework.21 Each gadolinium ion iscoordinated by a bidentate carboxyl group, two neutral watermolecules, and four oxygen atoms from four different di-monodentate carboxyl groups (Figure 1a). The di-monodentatecarboxyl groups were divided into two parts, each part containsa pair of 2-fold bridging carboxyl groups to connect theneighboring Gd3+ ions into a one-dimensional metal-oxygenpillar along the a axis. The pillars were further connectedtogether by the depronated TCA3− ligands to form a three-dimensional framework. Such a metal-oxygen pillar-like net-work is stable in aqueous solution, allowing the MOF to beused in photocatalysts that operated in aqueous media. Thelargest running openings along the a axis are a tetragonconsolidated by two Gd3+ ions and two TCA3− ligands with theinteratomic Gd···Gd separation, and the separations betweenthe two N atoms were ca. 13.72 and 9.19 Å, respectively(Figure 1b). This opening enables the ingress and ingress of theproton reducing catalysts, i.e., the Fe-Fe hydrogenase active sitemodel compound 1 to interact with the photoactive catalyticsites within the pores.A dye-uptake study was displayed by soaking crystals of Gd-

TCA in a methanol solution containing 2′,7′-dichorofluorescein

for 24 h. The experimental results demonstrate a dye uptakeequivalent to as much as 15% of the framework weight.22,23

Confocal laser scanning microscopy of the guest-adsorbedcrystals gave a strong green fluorescence response22 that can beassigned to 2′,7′-dichorofluorescein. The uniform distributionof the dye molecules throughout the crystals suggests that thedyes penetrated deeply into the channels, rather than remainingon the external surface.23The results demonstrate the ability ofGd-TCA to adsorb organic substrates within its open channels.The solid-state UV−vis absorption spectrum of compound

Gd-TCA exhibited an absorption band centered at 350 nmtypically assignable to the π−π* transition of the triphenyl-amine group.24 Upon excitation at this band (350 nm), Gd-TCA showed an intense luminescence band at about 435 nm.Solid-state electrochemistry exhibited a redox potential at 0.82V (vs SCE), assignable to the redox potential of the Gd-TCA+/Gd-TCA couple (Figure 2). The redox potential of the excited-

state Gd-TCA+/Gd-TCA* couple was calculated as −2.30 V onthe basis of a free energy change (E0−0) between the groundstate and the vibrationally related excited state of 3.12 eV.24

This potential was more negative than that of the [FeFe]-H2ases model compound 1 (E0−0 = −1.71 V), suggesting thethermodynamically feasibility for the electron reductionprocesses of compound 1 by the photogenerated of Gd-TCA.Progressive addition of compound 1 into the Gd-TCAsuspension in CH3CN/H2O (7:3 in v/v) quenched theemission dramatically (Figure 3a) with the coefficient KSVcalculated as 1.80 ± 0.06 × 104 M−1.25 The lifetime for the

Scheme 1. Construction of Gd-TCA Consolidated by theDinuclear Units Exhibits the Opening of the Pores along anAxis and Its Application on the Light-Driven HydrogenProduction with Fe-Fe Hydrogenase Model Compound andCarbon Dioxide Reduction with Ni(Cyclam) Complex,Respectively

Figure 1. (a) The coordination configuration of metal centers in Gd-TCA. Symmetry codes: A. 1 − x, −y, −z; B. 1 − x, −y, 1 − z; C. x, 1 +y, z; D. 2 − x, 1 − y, 1 − z. (b) View of the crystal packing of Gd-TCAalong the b direction showing the cavities channels with metal ionshaving labile solvent molecules and photoactive moieties exposed.

Figure 2. (a) Normalized absorption (red line) and emission spectrum(black line) of Gd-TCA, excitation at 350 nm. (b) Solid-state CV ofGd-TCA in the scan range 0.5−1.1 V (scan rate: 50 mV s−1) inphosphate-buffered saline with a platinum-wire counter electrode andsaturated calomel reference electrode.

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emission band of 2.45 ns for Gd-TCA is reduced to 1.73 ns inthe presence of compound 1. The result indicated theoccurrence of a direct photoinduced electron-transfer processfrom Gd-TCA* to the redox catalyst 1,26 which suggests thepossibility of the excited state of Gd-TCA to reduce the modelcompound 1.Light-Driven Hydrogen Production Using [FeFe]-

H2ases Model Compound 1. Photolysis of a suspension ofGd-TCA (1.0 ppm) and compound 1 (8.0 mM) in a solventmixture of 7:3 CH3CN/H2O at 25 °C in the presence ofNiPr2EtH·OAc (0.80 M) as sacrificial electron donors results ina H2 generation, which was quantified at the end of thephotolysis by GC analysis of the headspace gases, which showsthe effect of varying the catalyst concentration on the overallyield of the hydrogen evolution. Increasing the catalystconcentration increases the overall rate of hydrogen evolutionand the total amount of hydrogen evolved (Figure 3b). Theinitial rates for H2 evolution are obtained from the linearportion of each curve during the first hour of illumination andindicate a first-order dependence on the catalyst concentration.At higher catalyst concentration (>10 mM), while morehydrogen is evolved, the rate of hydrogen production doesnot increase linearly with the catalyst concentration, due to thepoor solubility of compound 1 in the solution.After 6 h of irradiation, the rate of hydrogen evolution

decreased dramatically, indicating the decomposition of at leastone system component. The further addition of Gd-TCA (1mg) to a reaction flask that contained Gd-TCA (1 mg),compound 1 (8.0 mM), and NiPr2EtH·OAc (0.8 M) aftercessation of the 6 h irradiation for hydrogen evolution did notcause any additional hydrogen evolution, whereas the additionof NiPr2EtH·OAc (0.80 M) led to ca. 15% enhancing ofhydrogen evolution, and the further addition of compound 1 (8mM) led to continued hydrogen production up to 85% of theabove-mentioned system added. The addition of bothcompound 1 and NiPr2EtH·OAc at the same amount wasable to resume the production of hydrogen directly. This resultdemonstrated the fast decomposition of compound 1 duringthe light-driven H2 evolution, providing the possibility of theGd-TCA complex being reused. In fact, the time course ofphotocatalytic H2 evolution through the addition of a freshsolution containing compound 1 (8.0 mM) and NiPr2EtH·OAc(0.80 M) to the filtration of the used photosensitizer Gd-TCArevealed continuous H2 production from the beginning of theirradiation period, and the total amount of produced hydrogenreached to 15 mL after 40 h (4 rounds repeating) irradiation.Light-Driven Hydrogen Production Using [Co(bpy)3]-

Cl2. While the active sites of enzymes involved in the overallwater-splitting in natural systems, namely, hydrogenase and

photosystem I, use iron, nickel, and manganese ions, cobalt hasalso emerged in the past decade as the most versatile non-noblemetal for the development of synthetic hydrogen and oxygenevolving catalysts.25,27,28 Fluorescence titration upon additionof compound 2, [Co(bpy)3]Cl2, revealed the significantquenching of Gd-TCA emission with the Stern−Volmerconstant as KSV = 7.13 ± 0.4 × 103 M−1 (Figure 4a). Light-

driven hydrogen evolution was also observed after irradiatingthe system containing Gd-TCA (1 mg), and compound 2 (50μM) in 5 mL of a 1:1 CH3CN/H2O solution. The maximum ofhydrogen evolution efficiency was achieved at pH = 10.0 withthe concentration of the sacrificial electron donor triethylamine(Et3N) of 2.5% (v/v). The system exhibited an initial TOF of320 h−1 of hydrogen per cobalt compound for the first hour.This result is comparable to the best results for [Co(bpy)3]Cl2as catalyst with the ruthenium compound as photosensitizer.29

The overall quantum yield for the light-driven H2 evolution isabout 0.21%. The activity increases with the concentrationincreasing of 2 until 50 μM. Further adding 2 caused a littleincrease of hydrogen evolved, but the volume of hydrogenevoluted does not scale linearly with the catalyst concentration(Figure 4b). These results indicate the possible decompositionof 2 during the irradiation. The addition of a fresh solution of 2(50 μM) and Et3N (2.5% in v/v) resumed the hydrogenproduction directly. The reaction is able to be repeated, and thelifetime of the system was prolonged to more than 20 h (5rounds repeating) with hydrogen evolution reaches to 22 mL.Since the α-Al2O3 pattern with high affinity for carboxylic

groups is a commonly used support for the MOF-based thinfilms to be combined, our MOF-based film was preparedaccording to the classic method of MOFs film formation.30 TheXRD pattern of Gd-TCA film matches very well with thesimulated pattern, indicating that the formed film is a purephase of Gd-TCA (Figure 5c). The SEM images in Figure 5a,brevealed that the surface of the deposited support wascompletely covered with a continuous and dense Gd-TCAlayer. The amounts of the loading Gd-TCA material werequantified by ultraviolet−visible spectroscopy; an uptakeequivalent was demonstrated as much as 1 mg for a 1.5 ×0.5 cm2

film. The Gd-TCA deposited film exhibits high stabilityfor photocatalyzing hydrogen evolution with the initial rate ofabout 1.71 mL/h and about 3.8 mL for the first 5 h with 1.5 ×0.5 cm2

film in a fresh solution containing 2 (50 μM) and Et3N(2.5% v/v). While the rate of hydrogen decreased dramaticallyafter 5 h irradiation, the light-driven hydrogen evolution couldbe resumed by simply pulling out the film and reusing throughputting down in the above-mentioned solution. The prelimi-nary experiments suggested that a system lifetime was

Figure 3. (a) Family of emission spectra of the Gd-TCA suspension(0.02%) upon addition of compound 1 in 7:3 CH3CN/H2O up to 11μM. (b) H2 evolution of Gd-TCA (1 mg), in 5 mL of solutioncontaining NiPr2EtH·OAc (0.8 M) and 1 with various concentrations.

Figure 4. (a) Family of emission spectra of the Gd-TCA suspension(0.02%) upon addition of complex 2 [Co(bpy)3]Cl2 up to 1.1 × 10−4

M in 1:1 CH3CN/H2O. (b) H2 evolution of Gd-TCA (1 mg), in 5 mLof a solution containing Et3N (2.5%) and 2 ([Co(bpy)3]Cl2) withvarious concentrations (right), respectively.

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prolonged to more than 40 h with a total of 33.5 mL (about1000 TON per TCA moiety, Figure 5d) H2 evolved. The indexof XRD patterns of the Gd-TCA film pulled out from thecatalytic reaction evidences the maintenance of the frameworkand the stability of the materials (Figure S6). To our bestknowledge, this is the first example of MOF film forphotochemical reduction of water into H2 molecules.Light-Driven Carbon Dioxide Reduction by Using

Ni(Cyclam)2+. Most importantly, the excited state potential ofGd-TCA is negative enough to reduce the well-known carbondioxide electrochemical reduction catalyst Ni(Cyclam)Cl23;14,31 we try to create the artificial system for the light-drivenreduction of carbon dioxide. As shown in Figure 6, progressive

addition of compound 3 into the Gd-TCA suspension inCH3CN/H2O (1:1) quenched the emission with the coefficientKSV calculated as 2.65 ± 0.2 × 103 M−1. The lifetime for theemission band of 2.38 ns for Gd-TCA is reduced to 1.58 ns inthe presence of compound 3. These results confirmed theoccurrence of a photoinduced electron-transfer process from

the excited state of Gd-TCA to the model compound 3. For thesystem containing Gd-TCA (1 mg), and compound 3 (50 μM)in 5 mL of CH3CN/H2O (1:1) solution, the maximum ofhydrogen evolution efficiency was achieved at pH = 11.0 withthe concentration of the sacrificial electron donor triethylamineof 10.0% (in v/v). After irradiation of 12 h, about 0.11 mL ofhydrogen was produced. Further addition of compound 3 didnot enhance the volume of the product hydrogen of the system.To optimize the reaction conditions of the carbon dioxide

reduction under irradiation, we used a cyclic carbonate, 4-methyl-1,3-dioxolan-2-one, 4 as the model compound. Asshown in Figure 4, after irradiation of the system containing aGd-TCA suspension (1.0 ppm), redox catalyst 3 (50 μM),Et3N (10.0% v/v) as sacrificial electron donor, and the modelcompound 4 (5 μL, 11 mM), in a solvent mixture of 7:3CH3CN/H2O (5 mL) at 25 °C for 12 h, a total of 17.1% of thecyclic carbonate was reduced, by measuring the quantum of theglycol using GC. Of course, at these reaction conditions, withthe absence of light, only negligible (lower than 1.8%)hydrolysis of the model compound 4 was found.The reduction procedure of carbon dioxide displayed by

bubbling the carbon dioxide into the solution up to solutionsaturation and carefully adjusting the pH value of the system to11.0 of a CH3CN/H2O (7:3 in volume 5.0 mL) mixture solventat 25 °C. Upon irradiation of the aforementioned solutioncontaining a Gd-TCA suspension (1.0 ppm), redox catalyst 3(50 μM), and Et3N (10.0% v/v) for 12 h, the system gives theconcentration of the HCOO− of about 22.7 μM. Furthermore,the isotopic 13CO2 was employed in the photocatalytic reaction,and the product was identified by 13C NMR spectroscopy toconfirm the origin of HCOO−. The 13C NMR spectrum clearlygave a peak at 164.6 ppm, corresponding to HCOO−, when13CO2 was introduced, while that signal was not detected with12CO2 only or with 13CO2 in dark. The results unambiguouslydemonstrate that the produced HCOO− anion indeed comesfrom CO2.

32

■ CONCLUSIONIn a summary, a new strategy to create MOFs as photo-sensitizer for light-driven H2 evolution and carbon dioxidereduction was achieved through incorporating a triphenylaminemoiety as the photoactive organic linkers. Gd-TCA exhibitedexcellent photocatalytic efficiency comparable to the commonlyused photosensitizer Ru(bipy)3

2+ in aqueous media with redoxcatalysts including Fe-Fe hydrogenase models and the tris-(bipyridine)cobalt complexes for the H2 evolution. Theprolonged lifetime of MOFs through simply repeating thecatalytic reactions and the easy preparation of the film ensuredthem as promising candidates for the photocatalytic H2evolution and carbon dioxide reduction. The deposition ofMOF-based material into the α-Al2O3 plate to create amembrane combined with the large surface areas with porecharacteristics of a MOF allows the MOF-based materialsexhibiting potentially practical application.

■ EXPERIMENTAL SECTIONMaterial and Methods. Unless otherwise stated, all chemical

materials were purchased from commercial sources and used withoutfurther purification. 4,4′,4″-Tricarboxytriphenylamine (H3TCA) wassynthesized according to the published procedure.331H NMR spectrawere measured on a Varian INOVA 400 M spectrometer. Elementalanalyses were obtained on an Elemental Analyzer Vario EL III. PowderXRD diffractograms were obtained on a Riguku D/Max-2400 X-ray

Figure 5. (a) SEM image of Gd-TCA film on α-Al2O3 support (topview). (b) SEM image of Gd-TCA film on α-Al2O3 support (sideview). (c) XRD patterns for the Gd-TCA film, α-Al2O3, and thesimulation pattern of the single crystal of Gd-TCA. (d) Time course ofphotocatalytic hydrogen production over a 1.5 × 0.5 cm2

film(containing 1 mg of Gd-TCA) for a total of 40 h with a fresh solutioncontaining catalyst 2 (50 μM) and Et3N (2.5%) was added every 5 h.

Figure 6. Left: Family of emission spectra of the Gd-TCA suspension(0.02%) upon addition of complex 3 up to 1.1 × 10−4 M in 1:1CH3CN/H2O. Right:

13C NMR spectra for the product obtained fromreaction with (a) 13CO2 under irradiation; (b)

13CO2 in the dark, and(c) 12CO2 under irradiation.

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diffractometer with a Cu sealed tube (λ = 1.54178 Å). Thermogravi-metric analysis (TGA) was carried out at a ramp rate of 5 °C/min in anitrogen flow with a Mettler-Toledo TGA/SDTA851 instrument. FT-IR spectra were recorded as KBr pellets on a JASCO FT/IR-430. SolidUV−vis spectra were recorded on an HP 8453 spectrometer. The solidfluorescent spectra were measured on a JASCO FP-6500. Bothexcitation and emission slits were 3 nm wide. The HCOO− in theliquid phase was analyzed using an IC (DIONEX ICS-5000, (ThermoFisher Scientific Inc.) with an IonPac AS11-HC column and an IonPacAG11-HC column.The solution of Gd-TCA was prepared in a CH3CN/H2O mixture

solution with the concentration of 0.02%. Stock solutions of the[FeFe]-H2ases model compound 1 (1.0 × 10−2 M) and [Co(bpy)3]Cl2(4.0 × 10−2 M) were prepared in 7:3 CH3CN/H2O and 1:1 CH3CN/H2O solvents, respectively.Solid-state voltammograms were measured by using a carbon-paste

working electrode, and a well-ground mixture of each bulk sample andcarbon paste (graphite and mineral oil) was set in the channel of aglass tube and connected to a copper wire. A platinum wire with a 0.5mm diameter counter electrode and saturated calomel referenceelectrode were used. Measurements were performed by using a three-electrode system in phosphate-buffered saline [PBS] at a scan rate of50 mV s−1, in the range of 0.5−1.1 V. Dye-uptaking experiments weredisplayed by soaking Gd-TCA (1.2 mg, 2 μmol) in a methanolsolution of 2′,7′-dichorofluorescein dye (24 mM, 2 mL) overnight.The resulting crystalline powders were washed thoroughly withmethanol until the solution became colorless. The washed sampleswere digested by Na2EDTA (0.01 M, 2 mL) and NaOH (1.5 M, 0.1mL), and the resultant clear solution with light olivine color wasdiluted to 25 mL and adjusted to a pH of 1.5. Absorption experimentswere performed on a TU-1900 UV−vis spectrophotometer. Theconcentration of 2′,7′-dichorofluorescein was determined by compar-ing the UV−vis absorption with a standard curve.Syntheses and Characterizations. Synthesis of Gd-TCA. A

mixture of 4,4′,4″-tricarboxytriphenylamine (H3TCA) (94 mg, 0.25mmol) and Gd(NO3)3·6H2O (455 mg, 1 mmol) was dissolved in 15mL of mixed solvents of DMF and ethanol in a screw-capped vial. Theresulting mixture was kept in an oven at 100 °C for 3 days. Yellowblock-shaped crystals were obtained after filtration and dried invacuum. Yield: 60%. Anal. Calcd For C21H17NO6Gd·2H2O (%): C,44.44; H, 2.47; N, 2.84; Found: C, 44.40; H 2.43; N, 2.87. FTIR (KBrpellet) (cm−1): 3428 (w), 1659 (m), 1591 (m), 1532 (m), 1510 (m),1311 (m), 1174 (m), 1105 (s), 1015 (s) cm−1.Synthesis of Gd-TCA Film.20b α-Al2O3 particles (1.5 cm × 0.5 cm)

with a large average pore size of 100 nm and porosity of 30−40% weretreated with 4 mL of APTES in 150 mL of toluene at 120 °C for 24 hunder N2 conditions, leading to an APTES monolayer deposited onthe α-Al2O3 particle surface. Then, the crystal seedings were depositedonto the surface of the APTES functionalized α-Al2O3 patterns bydispersion in mother solution with a microwave method. Thesolvothermal reaction of H3TCA and Gd(NO3)3·6H2O in thepresence of the functionalized α-Al2O3 pattern in a mixed solvent ofDMF and ethanol gave Gd-TCA film in a yield of 10%, and the filmswere washed with pure ethanol and then dried in a vacuum oven(Scheme 2).

Single-Crystal X-ray Crystallography. Crystal data of Gd-TCA:C22H20NO9Gd, Mr = 599.64, Triclinic, space group P1 , a = 9.541 (2)Å, b = 12.842(2) Å, c = 14.455(3) Å, α = 116.32(1)°, β = 109.20(1)°,γ = 90.15(1)°, V = 1476.1(5) Å3, Z = 2, Dc = 1.349 g cm−3, μ(Mo−Kα) = 2.287 mm−1, 5606 unique reflections [Rint = 0.0402]. Final R1[with I > 2σ(I)] = 0.0587, wR2 (all data) = 0.1650, GOF = 1.036.CCDC No. 933349. Intensities were collected on a Bruker SMART

APEX CCD diffractometer with graphite monochromated Mo−Kα (λ= 0.71073 Å) using the SMART and SAINT programs.34 Thestructure was solved by direct methods and refined on F2 by full-matrixleast-squares methods with SHELXTL version 5.1.35 Of two benzoicacid moieties in the ligand, a total of 10 carbon atoms on the twobenzene rings and 2 carboxylic acid groups were disordered into twoparts and their site occupied factor (s. o. f.) fixed as 0.75 and 0.25,respectively. The oxygen atoms in other benzoic acid moieties weredisordered into two parts and their (s. o. f.) were fixed as 0.75 and0.25, respectively. Except for these disordered carbon and oxygenatoms with minor site occupied factors (0.25), non-hydrogen atoms ofthe ligand backbones were refined anisotropically. Except for thesolvent methanol and coordinated water molecules, hydrogen atomswere fixed geometrically at calculated positions and allowed to ride onthe parent non-hydrogen atoms. Bond distances constraintscorresponding to the disordered phenyl rings and the isolated solventmolecules were used to help the refinement on the disordered parts.The A alert in the checkCIF file is due to the close distance betweenthe atoms in different parts of the disordered phenyl rings. The first Balert in the checkCIF file is due to that the hydrogen atoms were notfixed on the solvent molecules, and the second B alert in the checkCIFfile is due to the possible presence of high disordered solventmolecules in the lattice.

Typical Procedure for Hydrogen Production and CarbonDioxide Reduction. For [FeFe]-H2ase model compound 1 ascatalysts, each sample was made in a 23 mL flask with a volume of 5mL in MeCN/water (7:3 v/v). Typically, the sample contained 0.02%Gd-TCA, 8 mM model compound 1, and 0.8 M NiPr2EtH·OAc as thesacrificial electron donor. The flask was sealed with a septum andprotected from light, then degassed by bubbling nitrogen for 15 minunder atmospheric pressure at room temperature. After that, thesamples were irradiated by a 500 W xenon lamp, and the reactiontemperature was 293 K by using a water filter to absorb heat. Thegenerated photoproduct of H2 was characterized by GC 7890Tinstrument analysis using a 5 Å molecular sieve column (0.6 m × 3mm) and a thermal conductivity detector, and nitrogen was used ascarrier gas. The amount of hydrogen generated was determined by theexternal standard method. Hydrogen in the resulting solution was notmeasured, and the slight effect of the hydrogen gas generated on thepressure of the flask was neglected for calculation of the volume ofhydrogen gas.27a,36 Quantum yield for light-driven H2 evolution wasdetermined according to the literature method.29 The differencebetween the power of light passing through the blank and that throughthe sample was considered to be absorbed by Gd-TCA. Each value ofquantum yield was tested three times to reduce errors. The powers ofthe light passing through the blank and the sample were measuredwith an L30A-BB-13 thermal sensor and a Nova II power meter usinga 500 W Xe lamp equipped with a 365 nm band-pass filter.

For [Co(bpy)3]2+ as catalysts, each sample was made in a 23 mL

flask with a volume of 5 mL in MeCN/water (1:1 v/v) at pH = 10.Typically, the sample contained 0.02% Gd-TCA, 50 μM [Co(bpy)3]

2+,and 2.5% v/v of Et3N as the sacrificial electron donor. The flask wassealed with a septum and protected from light, then degassed bybubbling nitrogen for 15 min under atmospheric pressure at roomtemperature. After that, the samples were irradiated by a 500 W xenonlamp, and the reaction temperature was 293 K by using a water filter toabsorb heat.

The photon flux was determined by eq 1

λ=FP

S hc2Photon

Beam (1)

where P (in W) is determined by the difference in the power of lightpassing through the blank and the sample, respectively; λ is theirradiation wavelength number (365 nm); SBeam is the beam area; h isPlanck’s constant; and c is the speed of light.

The quantum yields (φ) were determined by eq 2

φλ

=×

=×

⎜ ⎟⎛⎝

⎞⎠

n NF S t

n NhcP t

12

H(H )

2(H )

22

Photon Beam

2

(2)

Scheme 2. Synthetic Process of Gd-TCA Film

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where the amount of hydrogen produced (n(H2)) was estimated formGC analysis; N is the Avogadro constant; and t is the irradiationperiod.The reduction procedure of carbon dioxide displayed by bubbling

the carbon dioxide into the solution up to solution saturation andcarefully adjusting the pH value of the system to 11.0 of a CH3CN/H2O (7:3 in volume 5.0 mL) mixture solvent at 25 °C. Theaforementioned solution containing a Gd-TCA suspension (1.0 ppm),redox catalyst 3 (50 μM), and Et3N (10.0% v/v) was irradiated for 12h.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.inorg-chem.6b01267.

Experimental details and related spectra (PDF)Crystal data of Gd-TCA (CIF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: hecheng@dlut.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge financial support from the NaturalScience Foundation of China (21421005 and 21531001), theNatural Science Foundation of Jiangsu Province(BK20140234).

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