A Clean and General Strategy To Decorate a Titanium Metal−OrganicFramework with Noble-Metal Nanoparticles for VersatilePhotocatalytic ApplicationsLijuan Shen,† Mingbu Luo,† Linjuan Huang,† Pingyun Feng,*,‡ and Ling Wu*,†

†State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, China‡Department of Chemistry, University of California, Riverside, California 92521, United States

*S Supporting Information

ABSTRACT: We demonstrate a facile and generalapproach for the fabrication of highly dispersed Au, Pd,and Pt nanoparticles (NPs) on MIL-125(Ti) without usingextra reducing and capping agents. Noble-metal NPformation is directed by an in situ redox reaction betweenthe reductive MIL-125(Ti) with Ti3+ and oxidative metalsalt precursors. The resulting composites function asefficient photocatalysts.

Metal−organic frameworks (MOFs), which are built frommetal cations/clusters with organic linkers, have emerged

as a class of very promising materials.1 Their distinguishableproperties, like ultrahigh surface areas and pore volume, and well-defined porosity together with tunable functionality have openedup new perspectives in gas storage,2 separation,3 and catalysis.4 Inview of catalytic applications, MOFs can acquire catalytic activityby using the open metal sites5 and catalytically active organiclinkers.6 Additionally, MOFs can serve as unique host matrixes,which allow implementation of the desired properties byintegration with different functional entities such as noble-metal nanoparticles (NPs).7 The introduction of noble-metalNPs can not only retain and enhance the functionalities of theMOFs but also generate new functionalities.8

Different strategies for the synthesis of metal NP/MOFcomposites have been reported in recent years, includingchemical vapor deposition,9 liquid/incipient-wetness impregna-tion,10 solid grinding,11 and microwave irradiation.12 Althoughthese methods have their own advantages, there are still severalinherent limitations. First, the above synthetic routes usuallyinvolve harsh treatment conditions and complicated proce-dures.13 Second, the crystalline texture of the starting MOFs canbe easily destroyed by the strong reducing agents used in thesynthesis process. Finally, to control the morphology and sizedistribution of the noble-metal NPs within the matrixes, variousorganic agents are necessary. The presence of organic layers maylead to the ineffective contact between the catalysts and reactants,thus lowering the catalytic efficiency of the obtained metal NP/MOF composites.14 To address these problems, it is highlydesirable to develop a complementary approach for preparinghigh-quality metal NPs/MOFs without using strong reducingagents and stabilizing molecules. Herein we report for the firsttime the synthesis of M/MIL-125(Ti) (M = Au, Pd, Pt)composites via a facile Ti3+-assisted method. MIL-125(Ti),

which contains cyclic octamers of TiO2 octahedra, possesses highstability.15Moreover, its photocatalytic and redox activity make itan intriguing candidate for our study.16 Through an in situ redoxreaction between reductive Ti3+-MIL-125(Ti) and oxidativemetal salt precursors, the M/MIL-125(Ti) composites weremade, without any extra reducing or stabilizing agents. Thismethod avoids the introduction of impurities and ensures theclean interface between metal NPs and MOFs.Scheme 1 illustrates the procedure for the fabrication of M/

MIL-125(Ti). The as-synthesized MIL-125(Ti) was dispersed in

methanol and purged by nitrogen in a sealed vessel. After lightirradiation (λ = 320−780 nm), the color of the suspensionchanged from white to blue. This phenomenon is due to theformation of Ti3+ by optically induced electron trapping at Ti4+

sites.16 The presence of Ti3+ species has been confirmed by in situelectron-spin-resonance (ESR) measurement. As shown inFigure 1, MIL-125(Ti) gives rise to a very strong ESR signalunder light irradiation, whereas no signal has been observed inthe dark. The observed g values (gx = 1.965, gy = 1.954, and gz =1.908) are characteristic of paramagnetic Ti3+ centers in adistorted rhombic oxygen ligand field.17 The obtained Ti3+-MIL-125(Ti) can be preserved in methanol steadily purged bynitrogen in the sealed vessel. However, once Au, Pd, and Ptprecursors were added into the Ti3+-MIL-125(Ti)/methanolsolution in the dark, because of the strong reducing ability of Ti3+

(−1.37 V vs SHE),18 these metal ions were immediately reducedand rapidly nucleated on the surface of MIL-125(Ti), growinginto clusters and eventually forming NPs. Meanwhile, the Ti3+

ions were oxidized and further converted to Ti4+ ions. A similarprocedure can be employed to load Ag and Ni NPs into MIL-

Received: October 29, 2014Published: January 16, 2015

Scheme 1. Schematic Presentation for the Synthesis of M/MIL-125(Ti) (M = Au, Pd, Pt) Composites

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© 2015 American Chemical Society 1191 DOI: 10.1021/ic502609aInorg. Chem. 2015, 54, 1191−1193

125(Ti). More significantly, the Ti3+ ions are reproducible(Figure S1 in the Supporting Information, SI), which can be usedto deposit multimetallic NPs on MIL-125(Ti) in sequence. Infact, the unique property of MOFs found in this study is quitesimilar to metal oxides like TiO2 orWO3; their low valence statesoften show considerable reducing ability, allowing them to reactwith the oxidative noble-metal salt directly.19 As a proof-of-concept, we have extended this synthetic protocol to otherMOFs. Taking UiO-66(Zr) as an example, we have alsosuccessfully designed the Au/UiO-66(Zr) composite via asimilar procedure (Figures S2 and S3 in the SI). On the basisof these results, we believe that this approach is a general strategyfor developing metal NP/MOF composite systems.Figure 2 shows the typical transmission electron microscopy

(TEM) images of the as-prepared M/MIL-125(Ti) composites.

The MIL-125(Ti) substrate displays some oblate nanocrystalswith a diameter of ca. 300 nm (Figure S4 in the SI), and noobvious change of the characteristic structure of MIL-125(Ti) isobserved upon noble-metal NP loading. The small Au, Pd, and PtNPs are homogeneously distributed throughout the MIL-125(Ti) substrate with an intimate interfacial contact. Inaddition, the average diameters of Au, Pd, and Pt are estimatedto be about 6, 3, and 3 nm, respectively (Figure S5 in the SI).High-resolution TEM (HRTEM) images in Figure 2b,e,h clearlyshow the characteristic lattice fringes of the NPs. The lattice

spacing is measured to be 0.231, 0.224, and 0.226 nm, which canbe indexed to the (111) plane of face-centered-cubic structures ofAu, Pd, and Pt NPs, respectively.20 Furthermore, energy-dispersive X-ray (EDX) analysis on individual M/MIL-125(Ti)composites confirms the existence of Ti, O, and Au/Pd/Ptelements. Detailed X-ray photoelectron spectroscopy (XPS)analysis clearly shows that Au, Pd, and Pt elements in M/MIL-125(Ti) exist in the metallic state (Figure S6 in the SI).The crystal structure of the M/MIL-125(Ti) composites has

been determined by powder X-ray diffraction (XRD) measure-ment. As shown in Figure S7 in the SI, the XRD pattern of MIL-125(Ti) is in good agreement with that in the previous report.21

After the loading of noble metals, there is no apparent loss ofcrystallinity in the XRD patterns, indicating that MIL-125(Ti) isstable and the structure of the substrate is preserved throughoutthe treatments, which is consistent with the result of the aboveTEM analysis. Notably, no typical diffraction peaks of Au, Pd, orPt are observed. This may be due to the even distribution and lowloading amount (2% by weight) of these noble metals.22

The surface area and porosity of the samples have also beenstudied, as displayed in Figure S8 in the SI. The surface area of thesample decreases slightly after the loading of noble-metal NPs;this can be ascribed to the introduction of noble-metal NPs,which causes the slight pore blocking of MIL-125(Ti).11 Theoptical properties of the M/MIL-125(Ti) composites have beeninvestigated by UV−vis diffuse-reflectance spectroscopy. Asshown in Figure S9 in the SI, the light absorption spectrum of theMIL-125(Ti) sample shows an absorption edge at 350 nm. Thenoble-metal loading obviously enhances the light absorption ofMIL-125(Ti), which is also in agreement with the color changeof the MIL-125(Ti) composites (as shown in the inset).The photocatalytic activities of the MIL-125(Ti) and M/MIL-

125(Ti) composites are first evaluated using selective oxidationof benzyl alcohol to benzaldehyde. A set of control experimentshave been carried out to demonstrate the photocatalytic natureof the reaction (Figure S10 in the SI). As can be seen from Table1, all of these samples show activities for the photocatalytic

selective oxidation of benzyl alcohol to benzaldehyde. Theincorporation of noble-metal NPs with MIL-125(Ti) effectivelyimproves the conversion while simultaneously maintaining thehigh selectivity. Photocatalytic degradation of RhB over M/MIL-125(Ti) has also been studied. As shown in Figure S11 in the SI,the M/MIL-125(Ti) composites display obviously improvedphotoactivity compared to blank MIL-125(Ti). The enhancedactivity could be ascribed to the fact that the introduction ofnoble metals into MIL-125(Ti) might form a Schottky barrierand the photoelectrons can easily transfer from MIL-125(Ti)with low work function to Au, Pd, and Pt NPs with high workfunction because of their intimate interfacial contact, whichsignificantly enhances the separation and lifetime of photo-

Figure 1. ESR spectra of MIL-125(Ti) before (a) and after lightirradiation (b) and after a Pd precursor (H2PdCl4) is added (c).

Figure 2.TEM andHRTEM images of Au/MIL-125(Ti) (a and b), Pd/MIL-125(Ti) (d and e), Pt/MIL-125(Ti) (g and h). EDX images of Au/MIL-125(Ti) (c), Pd/MIL-125(Ti) (f), and Pt/MIL-125(Ti) (i).

Table 1. Photocatalytic Selective Oxidation of Benzyl Alcoholover MIL-125(Ti) and M/MIL-125(Ti)a

entry catalyst conversion (%) selectivity (%)

1 MIL-125(Ti) 18.9 >992 Au/MIL125(Ti) 36.0 >993 Pd/MIL-125(Ti) 32.7 >994 Pt/MIL-125(Ti) 26.4 >99

aReaction conditions: 16 mg of catalysts; 1.5 mL of BTF; 0.1 mmol ofbenzyl alcohol; reaction for 4 h at 25 °C; λ = 320−780 nm.

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generated carriers (Figure S12 in the SI).23 Thus, thephotocatalytic activity of M/MIL-125(Ti) can be improved.To illustrate the benefits of such a method in the synthesis of

metal NPs/MOFs and further explore the potential photo-catalytic application of Pt/MIL-125(Ti), H2 production from anaqueous solution containing triethanolamine as a sacrificial agenthas been performed. Figure 3 displays a photoactivity

comparison of the as-prepared samples. It can be seen thatMIL-125(Ti), without Pt deposition, exhibits very low photo-activity. The presence of Pt NPs as cocatalysts plays an importantrole in enhancing the efficiency of H2 production. Additionally, itis worth noting that Pt/MIL-125(Ti) obtained through this Ti3+-assisted method exhibits superior activity compared to thesample [Pt/MIL-125(Ti)-PD] prepared via a direct photo-deposition (PD) method (for details, see the SI). After 5 h ofirradiation, the total production of H2 over Pt/MIL-125(Ti) is38.68 μmol. The corresponding turnover number calculatedbased on the Pt on the catalyst is 30.2, which is about an 80%increase in the activity compared to Pt/MIL-125(Ti)-PD.In summary, we have introduced a novel in situ synthetic

method for the preparation of M/MIL-125(Ti) composites andevaluated the photocatalytic properties of these compositematerials. Through a redox reaction between the reductive MIL-125 (Ti) with Ti3+ and metal salt precursors, noble metals Au, Pt,and Pd with small sizes can be uniformly deposited onto MIL-125(Ti) in the absence of any extra reducing or stabilizing agents.The synthetic strategy used in this work represents a generalapproach for the introduction of uniform metal particles intoporous framework materials. Considering the excellent proper-ties and broad applications of the obtained composite materials,we anticipate that this synthetic strategy can be extended to otherporous and nonporous materials for different applications.

■ ASSOCIATED CONTENT*S Supporting InformationFull experimental and characterization data. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: pingyun.feng@ucr.edu.*E-mail: wuling@fzu.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe work was supported by the National Natural ScienceFoundation of China (Grants 21273036 and 21177024) and theNational Science Foundation (Grant CHE-1213795 to P.F.).

■ REFERENCES(1) (a) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999,402, 276−279. (b) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.;Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105−1125.(c) Gao, J.; Ye, K.; Yang, L.; Xiong, W.-W.; Ye, L.; Wang, Y.; Zhang, Q.Inorg. Chem. 2013, 53, 691−693.(2) Collins, D. J.; Zhou, H.-C. J. Mater. Chem. 2007, 17, 3154−3160.(3) Li, J. R.; Sculley, J.; Zhou, H. C. Chem. Rev. 2012, 112, 869−932.(4) Lu, H.-S.; Bai, L.; Xiong, W.-W.; Li, P.; Ding, J.; Zhang, G.; Wu, T.;Zhao, Y.; Lee, J.-M.; et al. Inorg. Chem. 2014, 53, 8529−8537.(5) (a) Muller, M.; Turner, S.; Lebedev, O. I.; Wang, Y.; van Tendeloo,G. R.; Fischer, A. Eur. J. Inorg. Chem. 2011, 2011, 1876−1887. (b) Liu,Y.; Mo, K.; Cui, Y. Inorg. Chem. 2013, 52, 10286−10291.(6) Hasegawa, S.; Horike, S.; Matsuda, R.; Furukawa, S.; Mochizuki, K.;Kinoshita, Y.; Kitagawa, S. J. Am. Chem. Soc. 2007, 129, 2607−2614.(7) (a) Zhou, T.; Du, Y.; Borgna, A.; Hong, J.; Wang, Y.; Han, J.;Zhang, W.; Xu, R. Energy Environ. Sci. 2013, 6, 3229−3234. (b) Shen, L.;Wu, W.; Liang, R.; Lin, R.; Wu, L. Nanoscale 2013, 5, 9374−9382.(c) Zhang, H. X.; Liu, M.; Bu, X.; Zhang, J. Sci. Rep. 2014, 4, 3923.(8) Wang, C.; deKrafft, K. E.; Lin, W. J. Am. Chem. Soc. 2012, 134,7211−7214.(9) Hermes, S.; Schroter, M.-K.; Schmid, R.; Khodeir, L.; Muhler, M.;Tissler, A.; Fischer, R. A. Angew. Chem., Int. Ed. 2005, 44, 6237−6241.(10) Gu, X. J.; Lu, Z. H.; Jiang, H. L.; Akita, T.; Xu, Q. J. Am. Chem. Soc.2011, 133, 11822−11825.(11) Ishida, T.; Nagaoka, M.; Akita, T.; Haruta, M. Chem.Eur. J.2008, 14, 8456−8460.(12) El-Shall, M. S.; Abdelsayed, V.; Khder, A. E. R. S.; Hassan, H. M.A.; El-Kaderi, H. M.; Reich, T. E. J. Mater. Chem. 2009, 19, 7625−7631.(13) (a) Wang, X.; Yang, J.; Yin, H.; Song, R.; Tang, Z. Adv. Mater.2013, 25, 2728−2732. (b) Liu, Y.; Zhang, W.; Li, S.; Cui, C.; Wu, J.;Chen, H.; Huo, F. Chem. Mater. 2014, 26, 1119−1125.(14) Pan, X.; Xu, Y.-J. Appl. Catal., A 2013, 459, 34−40.(15) (a) Hendon, C. H.; Tiana, D.; Fontecave, M.; Sanchez, C.;D’Arras, L.; Sassoye, C.; Rozes, L.; Mellot-Draznieks, C.; Walsh, A. J.Am. Chem. Soc. 2013, 135, 10942−10945. (b) Gao, J.; Miao, J.; Li, P.-Z.;Teng, W. Y.; Yang, L.; Zhao, Y.; Liu, B.; Zhang, Q. Chem. Commun.2014, 50, 3786−3788.(16) Dan-Hardi, M.; Serre, C.; Frot, T.; Rozes, L.; Maurin, G.; Sanchez,C.; Ferey, G. J. Am. Chem. Soc. 2009, 131, 10857−10859.(17) Zuo, F.; Wang, L.; Wu, T.; Zhang, Z.; Borchardt, D.; Feng, P. J.Am. Chem. Soc. 2010, 132, 11856−11857.(18) Xiang, G.; He, J.; Li, T.; Zhuang, J.; Wang, X. Nanoscale 2011, 3,3737−3742.(19) (a) Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo,M.-H. J. Mater. Chem. 2011, 21, 9079−9087. (b) Xi, G.; Ye, J.; Ma, Q.;Su, N.; Bai, H.; Wang, C. J. Am. Chem. Soc. 2012, 134, 6508−6511.(20) (a) Pan, X.; Xu, Y.-J. J. Phys. Chem. C 2013, 117, 17996−18005.(b) Jin, T.; Guo, S.; Zuo, J. L.; Sun, S. Nanoscale 2013, 5, 160−163.(c) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594−600.(21) Fu, Y.; Sun, D.; Chen, Y.; Huang, R.; Ding, Z.; Fu, X.; Li, Z.Angew.Chem., Int. Ed. 2012, 124, 1−9.(22) Yang, M.-Q.; Pan, X.; Zhang, N.; Xu, Y.-J. CrystEngComm 2013,15, 6819−6828.(23) Yang, M.-Q.; Weng, B.; Xu, Y.-J. J. Mater. Chem. A 2014, 2, 1710−1720.

Figure 3. Photocatalytic H2 production over different samples. Reactionconditions: 50 mg of catalyst; 80 mL of aqueous solution; 10 vol %triethanolamine; λ = 320−780 nm.

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