AU修饰的TIO2

Photocatalysis of Au 25-modi fied TiO 2under visible and near infrared lightAtsushi Kogo a ,Nobuyuki Sakai a ,Tetsu Tatsuma a ,b ,⁎a Institute of Industrial Science,The University of Tokyo,4-6-1Komaba,Meguro-ku,Tokyo 153-8505,Japan bPRESTO,Japan Science and Technology Agency (JST),4-1-8Honcho,Kawaguchi,Saitama 332-0012,Japana b s t r a c ta r t i c l e i n f o Article history:Received 28April 2010Received in revised form 13May 2010Accepted 17May 2010Available online 24May 2010Keywords:Gold cluster PhotocatalystVisible and near infrared light TiO 2Visible light-driven photocatalysis of Au 25-modi fied TiO 2was investigated.It induces oxidation of phenol derivates and ferrocyanide and reduction of Ag +,Cu 2+and dissolved oxygen.Thermodynamically uphill reactions such as oxidation of phenol accompanied by reduction of Cu 2+are also driven.The photocatalysis,which is based on the excitation of Au 25,is observed even under 860nm light.©2010Published by Elsevier B.V.1.IntroductionGold clusters,which are smaller than 2nm in diameter,have discrete electronic levels due to quantum size effects [1,2],whereas gold nanoparticles larger than 3nm and bulk gold have electronic band structure.Among the clusters,Au 25,which is thermodynami-cally the most stable,have been extensively studied and clari fied to have unique properties such as an optical absorption in visible and near infrared (NIR)region [1,3],paramagnetism [4],fluorescence [5]and two-photon absorption [6].Recently,we have found that the Au 25-modi fied TiO 2electrodes exhibit anodic photocurrents and negative photopotential shifts in response to visible and NIR light in the presence of appropriate electron donors [7].TiO 2modi fied with other Au n (n =15–39)clusters also gives photoresponses and each photocurrent action spectrum is consistent with the corresponding optical absorption spectrum.Of the clusters examined,Au 25shows the highest ef ficiency.Visible and NIR light excites electrons of the gold clusters from HOMO to LUMO level.The holes generated in HOMO receive electrons from donors,and the excited electrons in LUMO are injected into the TiO 2conduction band and eventually accepted by the oxidized donors at the counter electrode.The cluster-modi fied TiO 2is therefore expected to work as a photocatalyst for oxidation of donors and reduction of acceptors.TiO 2is the most widely used photocatalyst,which is applied to self-cleaning and self-sterilizing materials and removal of hazardous compounds [8].However,TiO 2works only under ultraviolet light.Recently,many approaches such as N-doping [9],dye-sensitization[10]and metal ion adsorption [11]are explored to utilize visible and NIR light for photocatalysis.In this study,we examined the photocatalysis of the Au 25-modi fied TiO 2under visible and NIR light.Although thermal catalysis of gold nanoparticles or clusters adsorbed on metal oxides [12]and photocatalysis sensitized by localized surface plasmon resonance of gold nanoparticles [13,14]have been studied,there are no reports on photocatalysis of TiO 2modi fied with gold clusters,which exhibits no plasmon resonance.In this study,we used Au 25,which gives the highest photovoltaic property among the gold clusters and studied the photocatalysis of the Au 25-modi fied TiO 2.2.Experimental 2.1.Sample preparationAnatase TiO 2films (600nm thick)were prepared by spin-coating (1500rpm,10s)on ITO-coated or Pyrex glass substrates from a TiO 2slurry (STS-21,Ishihara Sangyo)diluted to 50%with pure water,followed by annealing at 450°C for 1h.Glutathione-protected Au 25clusters (denoted as Au 25)were synthesized according to literatures [3,15].After the removal of excess glutathione by dialysis,Au 25solution was cast on the TiO 2films and left for 2h followed by washing with water and dried.The adsorbed densities on ITO and Pyrex were estimated to be 6.0±0.68and 5.7±0.48nm −2,respectively.2.2.Photocurrent measurementsPhotocurrents of a Au 25–TiO 2electrode were measured in a cell shown in Fig.1a (inset).The Au 25–TiO 2working electrode and an ITOElectrochemistry Communications 12(2010)996–999⁎Corresponding author.Institute of Industrial Science,The University of Tokyo,4-6-1Komaba,Meguro-ku,Tokyo 153-8505,Japan.E-mail address:tatsuma@iis.u-tokyo.ac.jp (T.Tatsuma).1388-2481/$–see front matter ©2010Published by Elsevier B.V.doi:10.1016/j.elecom.2010.05.021Contents lists available at ScienceDirectElectrochemistry Communicationsj o u r n a l h o m e p a g e :w ww.e l s ev i e r.c o m /l o c a t e /e l e c o mcounter electrode were soaked in 0.1M sodium acetate buffer containing 0.1M LiNO 3and a donor (pH 4.94±0.027,deaerated by N 2)and 0.1M aqueous LiNO 3containing an acceptor,respectively.The solutions were bridged with a KNO 3salt bridge.The working electrode was irradiated with 500–750nm light (10mW cm −2)using a xenon lamp (LA-251Xe,Hayashi Tokei)through a sharp-cut filter (SCF-25C-52Y,Sigma Koki),and short-circuit photocurrents were measured with an ammeter (1280C,Solartron).2.3.Spectroscopic measurementsThe Au 25–TiO 2film-coated glass substrate was soaked in acetoni-trile containing a donor and an acceptor,and absorbance changes of the film were measured in situ with a spectrophotometer (HR4000CG-UV-NIR,Ocean Photonics)under 500–750nm light (10mW cm −2).A sharp-cut filter (SCF-25C-48Y,Sigma Koki)was employed to prevent excitation of TiO 2by the probe light.A monochromatic light (6×1015photons cm −2s −1)was irradiated by using a xenon lamp (LAX-102,Asahi Spectra)with a band-pass filter (FWHM 10nm,Asahi Spectra)to record the action spectrum for the photocatalysis.3.Results and discussion 3.1.Photocatalytic oxidationThe Au 25–TiO 2electrode was irradiated with 500–750nm light in the two-compartment cell.The working and counter electrode compartments contain a donor (1mM)and Ag +as an acceptor (0.1M),respectively.Observed anodic short-circuit photocurrent densities after a 1-h irradiation are plotted against standard electrode potential E °of the donors in Fig.1a.In the presence of hydroquinone as a donor,the Au 25–TiO 2electrode exhibited a much higher photocurrent density (11.5μA cm −2)than a TiO 2electrode without clusters (0.18μA cm −2)or the Au 25–TiO 2electrode without donors (0.16μA cm −2).The photocurrent,which reverts to zero immediately after turning the light off,is therefore generated as a result of the electron transfer from the photoexcited Au 25to TiO 2and that from hydroquinone to Au 25.The electrons transferred to TiO 2are eventually accepted by Ag +,via the external circuit and the counter electrode.In the presence of a phenol derivative (hydroquinone,catechol,phenol or p -cresol)or ferrocyanide as a donor,the anodic photocurrent was larger than that in a control experiment without donors.In the presence of methanol,ethanol,formaldehyde or formic acid,however,the photocurrent was as low as that in the absence of donors.In the control experiment,in which a small current was observed,water may work as a donor.In a practical system,photocatalytic oxidation and reduction proceed on the same film (Fig.1b,inset).We therefore immersed the Au 25-modi fied TiO 2film on a glass substrate in acetonitrile containing AgNO 3(0.1M)and an electron donor (0.1M),and irradiated it with 500–750nm light.The absorbance of the film was monitored in situ at 500nm.In the presence of hydroquinone as a donor,the absorbance increased under irradiation as shown in Fig.2(inset).The increase is ascribable to the plasmon resonance of the silver nanoparticles deposited by the photocatalytic reduction of Ag +.Absorbance increases during the initial 3-min irradiation are plotted in Fig.1b as indices for the oxidation rates of donors.In the presence of hydroquinone,catechol,bisphenol A,p -cresol,phenol or formic acid as a donor,the increase rate of absorbance was signi ficantly larger than that in a control experiment without donors.On the other hand,in the presence of methanol,ethanol or formaldehyde,the increase rate was as low as that in the absence of donors.Again,in thecontrolFig.1.Photocatalytic oxidation ability of the Au 25-modi fied TiO 2under visible light irradiation (500–750nm,10mW cm −2)accompanied by reduction of Ag +:(a)short-circuit photocurrent densities measured in the two-compartment cell (inset)and (b)absorbance increase rates plotted against E °of the donors.The E °values (pH 5)of methanol,ethanol,formaldehyde and formic acid are calculated from the standard Gibbs energy of formation and those of the other donors are obtained from literatures [16–19].Fig.2.Action spectrum for the photocatalytic reduction of 0.1M Ag +accompanied by oxidation of 0.1M hydroquinone by the Au 25-modi fied TiO 2(6×1015photons cm −2s −1)(circles)and absorbance spectrum of 5.0×10−6M Au 25aqueous solution (curve).Inset shows the absorbance change during 500–750nm light irradiation (180–360s,10mW cm −2).997A.Kogo et al./Electrochemistry Communications 12(2010)996–999experiment,water dissolved in acetonitrile may play the role of a donor.Irradiation of Au 25with 500–750nm light should excite electrons from HOMO to LUMO,LUMO+1and LUMO+2[1,2].Since the potential of HOMO is +1.52V vs.NHE (pH 5),donors that have more negative E °could be oxidized in the photocatalysis.Although all the donors examined fall into this category,methanol,ethanol,formal-dehyde,formic acid and water are poorly oxidized.In the present system,species that generally show fast outer-sphere electrochemical reactions seem to be good donors.Inner-sphere electrochemical reactions,which are frequently seen on bare metal surfaces,are not expected for the clusters densely protected by glutathione.3.2.Photocatalytic reductionOpen-circuit potential of the Au 25–TiO 2electrode measured in a three-electrode system shifted from +0.40to +0.15V vs.NHE by 500–750nm light irradiation (10mW cm −2)in 0.1M aqueous LiNO 3without donors and acceptors.It is suggested from the thermody-namic viewpoint that acceptors whose E °is more positive than the observed photopotential should be reduced by the Au 25–TiO 2photocatalysis.The potential is more positive than the lower edge of conduction band of TiO 2(−0.5V at pH 5),probably because the rate of back electron transfer from TiO 2conduction band to Au 25HOMO level is not negligible in comparison with the rate of electron injection from Au 25LUMO to TiO 2conduction band in the absence of donors.We measured the photocurrents generated under visible light using the two-compartment cell.The working and counter electrode compartments contain hydroquinone as a donor (0.1M)and an acceptor (1mM,or 1.3mM for oxygen [20]),respectively.Ag +and Cu 2+(Fig.3)as well as oxygen (3.7μA cm −2)as acceptors gave much higher photocurrents than that observed in a control experiment without acceptors (0.06μA cm −2).In contrast,the photocurrents in the presence of Pb 2+or Ni 2+were as small as that in the control experiment.These results were consistent with the experimentally evaluated photopotential shown above.The absorbance of the Au 25–TiO 2film on a glass substrate was measured in situ during visible light irradiation in acetonitrile contain-ing hydroquinone (0.1M)and an acceptor (0.1M).When Ag +is employed as an acceptor,the absorbance signi ficantly increased (63×10−5s −1),suggesting that Ag +is reduced to silver nanoparticles.In contrast,the absorbance scarcely changed in the presence of the other metal ions examined (i.e.Cu 2+and Ni 2+).Although the photocurrentwas observed in the presence of Cu 2+,the absorbance did not increase possibly due to oxidative dissolution of the deposited copper by dissolved oxygen.3.3.Uphill reactionsSome of the reactions examined are thermodynamically uphill ones.For instance,whereas phenol (E °=+0.87V vs.NHE at pH 5)cannot reduce Ag +(E °=+0.80V)spontaneously,the reaction is driven by the Au 25–TiO 2photocatalyst.One of the advantages of photocatalyses over conventional catalyses is driving such uphill reactions,resulting in conversion of light energy to chemical one.Here we examined an even steeper uphill reaction,phenol oxidation coupled with Cu 2+reduction (E °=+0.34V).In the cell that contains 1mM phenol and 1mM Cu(NO 3)2in the working and counter electrode compartments,respectively,0.38μA cm −2of photocurrent was observed under 500–750nm light.The current was much larger than that in the absence of any donors and acceptors (0.03μA cm −2).The Au 25–TiO 2can therefore photocatalytically lift electrons by +0.53V by the aid of visible light energy.3.4.Action spectrum for the photocatalysisIn order to clarify the active wavelength range of the photo-catalysis and to con firm that Au 25is essential for the photocatalysis,an action spectrum was measured for photocatalytic reduction of Ag +with oxidation of hydroquinone,on the basis of the absorbance increase of the Au 25–TiO 2film under monochromatic light due to deposition of silver nanoparticles.As shown in Fig.2,the action spectrum for the absorbance increase rate showed good agreement with the optical absorption spectrum of a Au 25aqueous solution.This result strongly indicates that the observed photocatalysis is based on the excitation of Au 25and that the internal quantum ef ficiency is virtually constant over the range examined.The photocatalysis can be driven even at 860nm.4.ConclusionsThe Au 25-modi fied TiO 2has been proved to work as a photo-catalyst under visible and NIR light (≤860nm),which drives oxidation of phenol derivatives and ferrocyanide and reduction of Ag +,Cu 2+and oxygen.The photocatalysis is based on electron transfer from photoexcited Au 25to TiO 2and that from an electron donor to the Au 25.Uphill reactions (e.g.ΔE °=0.53V)can also be driven photocatalytically.AcknowledgmentsThis work was financially supported in part by KAKENHI (No.19049008for TT)on Priority Area “Strong Photon-Molecule Coupling Fields (No.470)”and KAKENHI (No.22710100for NS)from MEXT 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