Registered charity number: 207890As featured in:See Liu, Zhang et al.,Chem. Commun., 2014, 50, 3786.Showcasing research from the laboratories of Bin Liu and Qichun Zhang, Nanyang Technological University, Singapore 639798, SingaporeA p-type Ti(IV)-based metalorganic framework with visible-light photo-responseA new Ti(IV)-based porous metalorganic framework (MOF) (NTU-9) with visible-light photo-response was reported. The p-type characteristics and ef cient dye photo-degradation results demonstrated that Ti(IV)-based MOFs could be promising visible-light photocatalysts for energy conversion and environmental remediation.www.rsc.org/chemcomm3786 | Chem. Commun.,2014, 50, 3786--3788 This journal is The Royal Society of Chemistry 2014Cite this: Chem. Commun., 2014,50, 3786A p-type Ti(IV)-based metalorganic frameworkwith visible-light photo-responseJunkuo Gao,aJianwei Miao,bPei-Zhou Li,cWen Yuan Teng,aLing Yang,aYanli Zhao,cBin Liu*band Qichun Zhang*aHere,wereportanewTi(IV)-basedporousmetalorganicframework(MOF) (NTU-9), which displayed strong absorption in the visible regionwith a bandgap of 1.72 eV. The electronic structure and bandgap werefurther investigated by DFT calculations. Photoelectrochemical studiesindicated that NTU-9 is photoactive under visible light illumination(k 4 400 nm) and acts as a p-type semiconductor. The results demon-strated that Ti(IV)-based MOFs could be promising visible-light photo-catalysts for energy conversion and environmental remediation.Exploringnewvisible-light-drivenphotocatalystshasbeenoneofthe most attractive topics to address energy problems and reduceenvironmental contamination.1Because of its good photostability,lowcost, high eciency, lowtoxicity and large abundance, titaniumdioxide (TiO2) has become one of the most studied semiconductorsfor photocatalysis.2However, its largebandgaphas limiteditsapplicationonlyintheUVlightrange, whichaccountsforonly5% of the solar spectrum. Therefore, it is essential to extend theoptical response of TiO2 into visible light through the modificationof TiO2 with inorganic or organic species.3However, most of suchcomposites suer from either poor stability or fast recombination.Toaddress these problems, metalorganic frameworks (MOFs)might be a possible solution because of their large surface areasanddownsizedcatalyticcenters(onemetalorsmallmetal-oxidecluster). Moreover, the surface areas, bandgaps and photoactivitiesof MOFs can be tuned by changing the organic ligands or metalcenters.4In fact, some MOFs such as MOF-5, UIO-66, MIL-100(Fe),and MIL-88B(Fe) have already been demonstrated to have semi-conductorpropertiesforphotoelectronicsandphotocatalysis.5Very recently, the successful demonstration of MIL-1256aand itsNH2-functionalizedisostructure(MIL-125-NH2) inlight-drivenwatersplitting andCO2reductionmakesTi-basedMOFs moreattractive asphotocatalysts.7However, it is worthy to note thatnoother Ti(IV)-basedporousMOFstructureexcept MIL-1256aand MIL-916bhave been reported.Here, wereport anewporousTi(IV)-basedMOFstructure(NTU-9), whose absorption reaches up to 750 nm, which showsp-typesemiconductorbehaviour. NTU-9wasobtainedasred,hexagonal prism crystals by the reaction of Ti(i-OPr)4 (titaniumisopropoxide)withH4DOBDC(2,5-dihydroxyterephthalicacid)inaceticacid. Singlecrystal XRDanalysisrevealsthatNTU-9crystallizesinthetrigonal spacegroupP-31c.AsshowninFig. 1a, the Ti atom is octahedrally coordinated with six oxygenatoms from hydroxide and carboxylate groups. The bond lengthbetweenTiandtheoxidooxygenatomis B1.858whiletheFig. 1 (a) The coordinationmode of the Ti atominNTU-9. (b) ThecoordinationoftheDOBDCligandin NTU-9.(c)1-Dchannelsof NTU-9viewedalongthec-axis. Colorrepresentation: red, O; gray, C; blue, Ti.H atoms are removed for clarity.aSchool of Materials Science and Engineering, Nanyang Technological University,Singapore 639798, Singapore. E-mail: qczhang@ntu.edu.sgbSchool of Chemical and Biomedical Engineering, Nanyang TechnologicalUniversity, Singapore 637459, Singapore. E-mail: liubin@ntu.edu.sgcSchool of Physical and Mathematical Sciences, Nanyang Technological University,Singapore 637371, SingaporeElectronic supplementary information(ESI) available: Experimental details,summary of crystal structure and refinement details of NTU-9, IRspectra,powder-XRDandTGAspectra of NTU-9. CCDC974959. For ESI andcrystal-lographic data in CIF or other electronic format see DOI: 10.1039/c3cc49440cReceived 12th December 2013,Accepted 4th February 2014DOI: 10.1039/c3cc49440cwww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 04 February 2014. Downloaded by Nanyang Technological University on 24/06/2015 09:47:13. View Article OnlineView Journal| View IssueThis journal is The Royal Society of Chemistry 2014 Chem. Commun.,2014, 50, 3786--3788 | 3787distance between Ti and the oxygen atom from the carboxylategroup is B2.037 . The coordination geometries of the Ti centerresemble the structure of [Ti(salicylate)3]2 ions.8Note that theTiO bonds in NTU-9 are much shorter than the metalO bondsin MOF-749which is constituted of divalent metal ions (such asCo2+, Fe2+, Mg2+)andDOBDCligands, indicatingthestrongerbondingbetweenTi(IV)andOatoms.TheconnectionbetweenTi(IV) atoms and DOBDC ligands forms a two dimensional (2-D)honeycomb-like layer. These layers are stacked along the c-axis.There exist one dimensional (1-D) channels about 11 11 along the c-axis, which are similar to the 1-D channels in MOF-74(Fig. 1c).9bAll uncoordinated O atoms from the carboxylate groupspoint to the 1-D channels. The framework {Ti2(HDODBC)2-(H2DOBDC)}nisneutral, andtheemptyspacesinchannelsareoccupiedby2-propanolmolecules.Afterremovingguestsinthechannels, the solvent-accessible void in the structure of NTU-9 wascalculated to be about 50.7% using PLATON. Thermogravimetricanalysis (TGA) of NTU-9 revealed that most of the guest moleculesinthechannelswereremovedbelow1001C(Fig. S1inESI).CrystallineNTU-9isstableinair, water andcommonorganicsolvents. After removal of the guest molecules (treated at 120 1Cfor 12 h under vacuum), the crystalline structure of NTU-9 wasretainedwhichwas confirmedviathe powder XRDpattern(Fig. S2 in ESI). Details of the crystal structure and refinementdataareprovidedintheESI(TableS1inESI). Theexperi-mental powder XRD pattern for NTU-9 matches very well withthesimulatedone(generatedonthebasis of singlecrystalstructureanalysis), whichconfirmedthephasepurityof thebulk materials (Fig. S2 in ESI).The diuse reflectance spectra were recorded on a PerkinEl-mer Lambda 750s UV-Vis spectrometer. As shown in Fig. 2, thespectrumofNTU-9showsabroadrangeofabsorptioninthevisible region from 400 nm to 750 nm with an absorption peakcentered at around 520 nm. The calculated bandgap of NTU-9from the absorption onset is about 1.72 eV. The absorption ofNTU-9 was dramatically red-shifted when compared with TiO2(3.2 eV for anatase and 3.0 for rutile), and the reported Ti(IV)-basedMOF MIL-125 (3.6 eV) and its isostructure MIL-125-NH2 (2.6 eV).7cTo understand the electronic structure of NTU-9, the densityof states (DOS) and the electronic band structure of NTU-9 werecalculatedusingthe density functional theory (DFT) intheCASTEP program.10As depicted in Fig. S4S6 (ESI), the valenceband (VB) of NTU-9 between the energy level (4.0 eV) and theFermi level (0.0 eV) is mainly contributed from the O 2p statesmixedwithasmall amount of C2pstates. Theconductionbands(CB)aboveFermienergyaremostlymadeupofTi3dstates, with minor contributions from the C 2p and O 2p states.The DOS results indicate the existence of charge transfer fromthe organic ligand to Ti(IV) atoms in NTU-9. The dominations ofVB and CB by O and Ti atoms in NTU-9 are similar to that ofTiO2, whoseVBismainlyformedbytheoverlappingofO2porbitals and CB is mainly constituted of the 3d states of Ti. Thesmall contributions of p atomic orbitals of carbon to the CB inNTU-9 may alter and further reduce the band gap by changingthe degree of conjugationor functional groupmodificationin the ligands. This designable character of MOFs could leadto the synthesis of more Ti(IV)-based MOF semiconductorswithvisiblelightactivity.TheCBminimum(1.74eV)andVBmaximum (0 eV) are located at the same k-point in the Brillouinzone(Fig. S4, ESI). Thebandstructureillustrates that theinterband transitions of NTU-9 are direct, which may exhibit ahigherphotonicefficiencycomparedwithindirect bandgapsemiconductors such as TiO2. The calculated energy band gapfor NTU-9is1.74eV, whichmatches theexperimental datavery well.The photoelectrochemical properties of NTU-9 were studiedina three-electrode set-up. Fig. 3a shows the photocurrentFig. 2 Diuse reflectance spectra of NTU-9.Fig. 3 (a) Zero-bias photocurrent response of the NTU-9/FTO electrodeuponchoppedvisiblelight illumination, inset shows thephotovoltage(illuminatedopen-circuitpotential)responsesof the NTU-9/FTOelectrode.(b) MottSchottky plot of the NTU-9/FTO electrode measured at a frequencyof 1000 Hz. The flat-band potential of the NTU-9 is indicated by the interceptof the dashed lines.Communication ChemCommPublished on 04 February 2014. Downloaded by Nanyang Technological University on 24/06/2015 09:47:13. View Article Online3788 | Chem. Commun.,2014, 50, 3786--3788 This journal is The Royal Society of Chemistry 2014profileof theNTU-9/FTOelectroderecordedunderzero-bias(two-electrode, short-circuit) conditions, indicating that theNTU-9 is active under visible light (l 4 400 nm) illumination.The repeatable cathodic (negative) photocurrent of B60 nA cm2suggests that NTU-9 is a p-type semiconductor. As shown in theinset ofFig.3a, theNTU-9/FTO cellsalso showed highlyrepea-table photovoltage (illuminated open-circuit potential) responsesof B7mVduringtheonoff cycles ofillumination throughoutthemeasurement.Tofurtherrevealtheconductivityandflat-band potential of NTU-9, the MottSchottky measurement wasperformedin0.5MNa2SO4aqueous solution. As showninFig. 3b, the negative slope indicates p-type behaviour of NTU-9,consistent with our previous analysis. Besides the p-typebehavior,theMottSchottkymeasurementgivestheflat-bandpotential of NTU-9 at around 0.76 V vs. the reversible hydrogenelectrode(RHE). Thecarrier concentrationof NTU-9canbeestimated from the slope of the MottSchottky plot using Na =(2/eoeeo)[d(1/C2)/dV]1, where eo is the electron charge, e is theNTU-9 dielectric constant, eo is the vacuum permittivity, and Naistheacceptor density. If wetakethedielectricconstant ofNTU-9 to be B101(a typical value for most of the MOFs),11theestimated carrier density of NTU-9 is B1015cm3.The visible light photocatalytic activity of NTU-9 was studiedthrough the degradation of organic dyes. NTU-9showed goodphotocatalytic activity in the degradation of rhodamine B (RB) andmethylene blue (MB) in aqueous solution under visible light irradia-tion (l 4 420 nm). Photodegradation of RB and MB were completeafter 80 min and 20 min, respectively (Fig. 4 and Fig. S8S10, ESI).During the 6 h testing period, NTU-9 exhibited persistent highphotoactivity and good photostability.Inconclusion, wehavereportedanewTi(IV)-basedMOF,which absorbs visible photons up to 750 nm and shows a p-typesemiconductor behavior. ThisnewTi(IV)-basedMOFexhibitsgood photocatalytic activity and stability in the degradation oforganic contaminants. Our results suggest that Ti(IV)-basedMOFscouldbepromisingcandidatesforthedevelopmentofecient visible light photocatalysts.Notes and references DatacollectionofcrystalswascarriedoutonaBrukerAPEXIICCDdiractometer equipped with a graphite-monochromatized Mo-Ka radia-tion source (l = 0.71073 ). Empirical absorption was performed, and thestructurewassolvedbydirectmethodsandrefinedwiththeaidofaSHELXTLprogrampackage. All hydrogenatomswerecalculatedandrefined using a riding model. The CCDC number for NTU-9 is 974959.1(a) S. Chu and A. Majumdar, Nature, 2012, 488, 294; (b) G. D. Scholes,G. R. Fleming, A. Olaya-CastroandR. vanGrondelle, Nat. Chem.,2011, 3, 763; (c) Y. Tachibana, L. Vayssieres andJ. R. Durrant,Nat. Photonics, 2012, 6, 511; (d) A. Kubacka, M. Ferna ndez-Garc aand G. Colo n, Chem. Rev., 2012, 112, 1555.2(a) A. Fujishima and K. Honda, Nature, 1972, 238, 37; (b) X. B. Chen,L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331, 746; (c) M. DArienzo,J. Carbajo, A. Bahamonde, M. 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