2006 NM CdS–Au–TiO2

LETTERSAll-solid-state Z-scheme in CdS–Au–TiO2 three-component nanojunction system HIROAKI TADA1*,TOMOHIRO MITSUI1,TOMOKAZU KIYONAGA1,TOMOKI AKITA2AND KOJI TANAKA21Department of Applied Chemistry,Faculty of Science and Engineering,Kinki University,3-4-1,Kowakae,Higashi-Osaka,Osaka577-8502,Japan2National Institute of Advanced Industrial Science and Technology,Midorigaoka1-8-31,Ikeda,Osaka563-8577,Japan*e-mail:h-tada@apch.kindai.ac.jpPublished online:10September2006;doi:10.1038/nmat1734N atural photosynthesis,which achieves efficient solar energy conversion through the combined actions of many types of molecules ingeniously arranged in a nanospace,highlights the importance of a technique for site-selective coupling of different materials to realize artificial high-efficiency devices1.In view of increasingly serious energy and environmental problems,semiconductor-based artificial photosynthetic systems consisting of isolated photochemical system1(PS1),PS2and the electron-transfer system have recently been developed2,3.However,the direct coupling of the components is crucial for retarding back reactions to increase the reaction efficiency.Here,we report a simple technique for forming an anisotropic CdS–Au–TiO2nanojunction,in which PS1(CdS),PS2(TiO2)and the electron-transfer system(Au)are spatiallyfixed.This three-component system exhibits a high photocatalytic activity,far exceeding those of the single-and two-component systems,as a result of vectorial electron transfer driven by the two-step excitation of TiO2and CdS.Semiconductors are a key material in modern optoelectronic and photoelectrochemical devices,and unique electronic states resulting from the quantum size effect and high dispersion with downsizing make them even more interesting from the perspective of both fundamentals and potential applications.Recently,the coupling of semiconductors with molecules and other solids on the nanoscale has been reported to improve the performance of various devices,including solar cells4,5,photoluminescence6 and electrochromic devices7and biosensors8.On the other hand,in artificial photosynthetic systems consisting of isolated semiconductor particles and redox mediators(Ox/Red),water splitting to hydrogen and oxygen has recently been shown to proceed via a Z-scheme2,3.However,the Ox and Red mobile in solutions can compete with the reduction in PS1and the oxidation in PS2,respectively,to reduce the reaction effiing a simple photochemical technique,we have been able to construct a site-selective CdS–Au–TiO2nanojunction achieving an all-solid-state Z-scheme.Au particles with a mean size of3.4nm werefirmly deposited on the anatase TiO2{101}surface with an orientation relationship of Au{111} TiO2{101}by the deposition–precipitation method (see Supplementary Information,Fig.S1)9.A high-resolution transmission electron microscopic(HRTEM)image of a sample prepared by irradiation(l ex>320nm)of a de-aerated S8ethanol solution containing Au/TiO2particles in the presence of Cd2+ ions shows that a hemispherical core–shell-type nanoparticle is formed on TiO2(Fig.1a).The lattice spacings of the core and shell, determined to be0.23and0.32nm,respectively,are in agreement with the values for the Au(111)plane(International Centre for Diffraction Data,No.04-0784)and for the hexagonal CdS(101) plane(International Centre for Diffraction Data,No.41-1049).The electron energy-loss spectra(EELS)were obtained by irradiating an electron beam focused on the support and the shell labelled 1and2,respectively,in Fig.1a(Fig.1b).The Ti and O signals are present in spectrum1,whereas the signals of Cd and S are observed in spectrum2.In addition,the molar ratio of Cd/S in the deposits was determined to be about1,irrespective of the irradiation time(t p),by inductively coupled plasma spectroscopy and ion chromatography.Evidently,CdS deposits on the Au surface to yield CdS-coated Au nanocrystals on the TiO2surface (Au@CdS/TiO2).We have recently specified the reduction sites in Au/TiO2-photocatalysed reduction of S8to S2−ions as being the Au surface atoms having a great affinity to sulphur10.Thus,in the presence of Cd2+ions,the S2−ions produced selectively on the Au surface are considered to bond to Cd2+ions to form Au@CdS on TiO2.TEM images of the samples prepared by changing the t p show that all the Au particles are covered with CdS(Fig.1c).The CdS growing rate of4.8nm h−1at t p<0.5h decreases to0.27nm h−1 at0.5<t p<10h(Fig.1d).Electronic absorption spectra of TiO2, Au/TiO2and Au@CdS/TiO2with varying CdS thickness(l)show that Au/TiO2has an absorption peak(l max)at530nm due to the Au surface plasmon resonance with the TiO2interband transition band at l<385nm.The formation of a1.3-nm-thick CdS layer causes significant broadening and a redshift of the Au surface plasmon band of about50nm,which suggest a strong electronic interaction between Au and CdS11.The bandgap of CdS(E g)estimated from the absorption edge is shown as a function of l in Fig.2c:theLETTERSAu{111}= 0.232 nmTiO 2CoreShellCdS{101}= 0.318 nm2l (n m )t p = 0 ht p = 0.5 ht p = 10 ht p = 4 h 20 nm1STi×202CdO5 nm123456100200300400500600684210Energy loss (eV)I n t e n s i t y (a .u .)I n t e n s i t y (a .u .)t p (h)30040050060001abc dFigure 1Geometr i cal structure and compos i t i on of an Au–CdS compos i te nanopart i cle formed on T i O 2,and t i me evolut i on of the CdS shell layer.a ,HRTEM image of Au–CdS/TiO 2.b ,EELS of the support (1,blue line)and shell layer (2,red line)in a .c ,TEM images of Au@CdS/TiO 2prepared by changing t p .d ,Plots of l versus t p .The l values were determined by TEM observation carried out at an acceleration voltage of 300kV.The error bars represent one standard deviation.HRTEM observation and EELS analysis were carried out using a JEOL JEM 3000F electron microscope with a Gatan imaging filter (applied voltage:300kV)and TEM observations were carried out using a JEOL JEM 3010electron microscope (acceleration voltage:300kV).theoretical curve is calculated using the Brus equation for spherical CdS particles with a radius of l (ref.12).At l <4nm,the E g increases relative to the bulk value 13(2.4eV)which is ascribable to the quantum size e ffect of the CdS shell layer because the theoretical curve fits well with the experimental data.To study the e ffects of the CdS–Au–TiO 2nanojunction on the photocatalytic activity,methylviologen (MV 2+)reduction was used as a test reaction,in which sol–gel TiO 2films 14(TiO 2-TF)were used as a support in place of TiO 2particles (Fig.3a).Both Au/TiO 2-TF and CdS (about 5nm)/TiO 2-TF show higher photocatalytic activities than TiO 2-TF;these probably arise from the increase in the charge-separation e fficiency due to the electron transfer from TiO 2to Au with a large work function 15and from CdS to TiO 2with a conduction band (cb)edgelying lower than that of CdS 16,respectively.The photocatalytic activity of Au@CdS/TiO 2-TF far exceeds that of either the single-component (1C)or 2C systems.In addition,the MV +yield in the photostationary state for Au@CdS/TiO 2-TF reaches 52.2%,which is larger than that achieved in the 1C and 2C systems:the di fference in the CdS morphology between CdS/TiO 2and Au@CdS/TiO 2might make the activity comparison di fficult;however,CdS (about 5nm)/TiO 2coupled using mercaptoacetic acid (MAA)showed a photocatalytic activity higher than those of CdS and TiO 2(ref.17).These findings are indicative of the progress of MV 2+reduction by the electrons excited to the cb(CdS)with a high potential in Au@CdS/TiO 2,whereas the cb(TiO 2)electrons reduce MV 2+in the 2C system 16.The MV +yield for Au@CdS/TiO 2-TF as a function of l indicates that the photocatalytic activity of Au@CdS/TiO 2reachesLETTERS23456E g (e V )350400450500550600650A b s o r b a n c eA b s o r b a n c e0.1Au@CdS (l = 1.3 nm)/TiO 2l = 3.8 nml = 4.1 nm l = 5.2 nm l = 2.6 nm l = 2.4 nmex > 300 nm0.05Beforeirradiationex > 400 nmd A / d λl (nm)λ(nm)λλ(nm)λλ(nm)λAu/TiO 2TiO 2–0.05–0.03–0.010.010.030.05300400500600700800abcd2.32.42.52.62.72.82.93.0650550λλFigure 2Optical properties of Au@CdS/TiO 2.a ,Electronic absorption spectra of Au@CdS/TiO 2with varying l .b ,Change in electronic absorption spectra of Au@CdS/TiO 2with irradiation.c ,Size dependence of the E g (CdS):the curve is calculated by the Brus equation of E g =E g (bulk)+(¯h 2π2/2l 2)(1/m ∗2e+1/m ∗2h )−1.8e 2/εl using the values of m ∗e =0.21m 0,m ∗h =0.8m 0and ε(CdS)=8.45.The error bars represent one standard deviation.d ,Derivative spectra of b .a maximum at l ∼3nm (Fig.3b).This probably results from the balance of the increase in the cb-electrons(CdS)potential due to the quantum size e ffect (Fig.2c)and the decrease in light absorption.The l max of metal nanoparticles is related to electron density (n )by l max =4πc (2ε0m /e 2n )1/2(ref.18),where ε0is the vacuum permittivity,e and m are the charge and mass of the electron,respectively,and c is the speed of light.Thus,the Au nanoparticles can be regarded as a sensor for electron transfer in the 3C system.The electronic absorption spectra of Au@CdS/TiO 2before and after ultraviolet (l ex >300nm)and visible light (l ex >400nm)irradiation in de-aerated ethanol show that the l max redshifts by 24nm after visible light irradiation,whereas it blueshifts by 20nm after ultraviolet light irradiation (Fig.2b,d).A similar trend was also observed in water,although the amounts of the shift decreased (see Supplementary Information,Table S1).The influence of semiconductor charging during the reaction on the Au surface plasmon resonance might be excluded in these ex situ optical measurements.The essential reaction scheme can be discussed on the basis of the energy band diagram of the 3C system (Fig.4).For l ex >400nm irradiation,the cb-electrons(CdS)would be used for MV 2+reduction rather than transferred to Au becauseof the increase in their surface population due to the CdS size quantization 19;the injection of the holes left in the CdS valence band (vb)into Au reduces n ,explaining the redshift of l max (electron transfer I ,Au →CdS).For l ex >300nm,the vb-holes(TiO 2)with a strong oxidation power oxidize the solvent,and the electrons left in the cb(TiO 2)flow into Au,which increases n to cause the blueshift of l max (electron transfer II,TiO 2→Au).Thus,simultaneous electron transfer I and II (that is,vectorial electron transfer of TiO 2→Au →CdS)should occur as a result of excitation of both TiO 2and CdS under the conditions in MV 2+reduction.Furthermore,to specify the reduction sites of Au@CdS/TiO 2,Pt photodeposition was carried out by irradiating visible and ultraviolet light in the 3C system.If the electron transfer from CdS to TiO 2rapidly takes place through their contact with Au@CdS/TiO 2,Pt should be photodeposited on TiO 2.However,annular dark-field scanning TEM images and EELS of the Pt-photodeposited Au@CdS/TiO 2(Pt/Au@CdS/TiO 2)have demonstrated that Pt is deposited almost selectively on CdS under both conditions (see Supplementary Information,Fig.S2).This finding that CdS acts as the reduction sites of Au@CdS/TiO 2indicates that the back electron transfer from CdS to TiO 2is a minor path,strongly supporting the vectorial electron transferLETTERSAu@CdS(l = 2.96 nm)/TiO 2Au/TiO 2CdS/TiO 2TiO 2M V + (10–4 M )t p (min)l (nm)0204060801000.51.01.52.02.5021345630405060Y i e l d o f M V +(%)2070abFigure 3Photocatalytic activity of Au@CdS/TiO 2.a ,Time courses forphotocatalytic reduction of MV 2+.b ,Dependence of the MV +yield after 100-min irradiation on l .The error bars represent one standard deviation.in the 3C system:although the TiO 2–CdS contact area increases with the growth of CdS,the ratio of the contact area to the CdS surface area never exceeds 50%for hemispherical CdS and Au particles.This Z-scheme simultaneously generates vb-holes(TiO 2)with a strong oxidation power and cb-electrons(CdS)with a strong reduction power,which explains the high photocatalytic activity and high yield for MV 2+reduction.The photocatalytic activity of Pt/Au@CdS/TiO 2for H 2generation from H 2O was studied further (see Supplementary Information,Fig.S3).Although no H 2was detected in the Pt/CdS system,Pt/Au@CdS/TiO 2yielded H 2with an almost constant rate.These results can also be interpreted within the framework of the Z-scheme 20(that is,the electron supply from TiO 2to CdS via Au restricts the self-decomposition of CdS due to the oxidation of surface S 2−ions by the vb-holes(CdS)).The photoinduced reductive desorption of sulphur can occur for semiconductors with a cb edge greater than −4.6eV versus vacuum 21.In addition,anion-doped TiO 2,of which the vb edge is raised with the cb edge almost maintained,was shown to exhibit photocatalytic activities under visible light irradiation 22.Owing to the versatility,rational coupling of the components and their dimensional control will enable the development of our prototypeCdSAucbvb ex > 400 nmVacuum level (eV)TiO 2cbvbex > 300 nmvb–3–4–5–6–7Red 1OxidationReduction h +e –e –Ox 1D Ox2D Red 2E 0(R/O)h +λλλλFigure 4Energy band diagram scheme of the CdS–Au–TiO 2system.E 0(R/O)is the standard electrode potential of MV +/MV 2+.D Red 2and D Ox 2represent thedistribution function for occupied and unoccupied states,respectively,and l is the reorganization energy.of an all-solid-state Z-scheme for visible-light-induced e fficient artificial photosynthetic systems.METHODSSAMPLE PREPARATIONTiO 2particles (anatase,surface area =8.1m 2g −1)were used as a support.The pH of a 4.86×10−3M aqueous solution (100ml)of HAuCl 4·4H 2O was adjusted to 6.0with a 1mol dm −3NaOH aqueous solution.The solution turned from yellow to lighter yellow,accompanied by ligand exchange from [AuCl 4]−to [Au (OH )4−x Cl x ]−.To this solution,10g of TiO 2particles were added and magnetically stirred at 343K for 1h.The particles were washed with distilled water three times,and then heated at 673K for 4h in air(Au (0.33mass%)/TiO 2).After an Au/TiO 2(1g)ethanol suspension (250ml)containing S 8(0.344mmol)and Cd (ClO 4)2·6H 2O (3.46mmol)had been bubbled with argon for 30min in the dark,irradiation was carried out for a given period with a high-pressure mercury lamp at 298K;the light intensity integrated from 320to 400nm (I 320−400)was 3.7mW cm −2.CdS/TiO 2-TF was prepared using MAA as a bifunctional coupling agent.A Cd (ClO 4)2·6H 2O aqueous solution (1.0mmol dm −3,50ml)and a Na 2S ·9H 2O aqueous solution (1.0mmol dm −3,50ml)containing MAA (1.0mmol dm −3)were mixed,and stirred for 10min.After TiO 2film-coated glass substrates had been immersed in the suspension of MAA-capped CdS nanoparticles (about 5nm)with stirring for 18h,the sample was washed with distilled water and dried.The loading with Pt on Au@CdS/TiO 2and CdS was carried out byphotoplatinization.After Au@CdS/TiO 2(or CdS)particles (0.2g)had been dispersed into a 1.93mmol dm −3aqueous H 2PtCl 6solution and de-aerated with Ar for 60min,the dispersion was illuminated by a 500W Xe lamp through a 430nm cuto fffilter (l ex >400nm,I 420−485=3.7mW cm −2)or a 300–400nm bandpath filter (300<l ex <400nm,I 320−400=4.0mW cm −2)for 6h at 298K.The solids (Pt/Au@CdS/TiO 2or Pt/CdS)were repeatedly washed with ethanol and dried under vacuum.OPTICAL MEASUREMENTSAfter suspensions of Au@CdS/TiO 2particles (0.2g)in ethanol or water (50ml)were irradiated (l ex >400nm or l ex >300nm)for 4h,the solvent was removed by evaporation,and the solids were further dried in a vacuum desiccator for 0.5h.The di ffuse reflectance spectra of the Au@CdS/TiO 2particles were recorded on a Hitachi U-4000spectrophotometer equipped with an integrating sphere to be transformed into the absorption spectra.PHOTOCATALYTIC ACTIVITY TESTMV +is easily oxidized by oxygen in air,and thus the reactions were carried out in a closed reaction vessel with an optical cell using a sol–gel TiO 2film as aLETTERSsupport in place of TiO2particles,which enabled optical measurements to determine the MV+concentrations(εat605nm=1.1×104mol−1dm3cm−1) without opening the system.As the l value for the thin-film system could not be determined by electron microscopy,the corresponding value for the particulate system under the same conditions is shown.After Au@CdS/TiO2-TF samples (8mm×40mm)had been immersed in a0.4mM MV2+ethanol solution followed by de-aeration with argon bubbling for1h,irradiation was carried out using a500W Xe lamp under I320−400=8.0mW cm−2at305K.As another test reaction,photocatalytic H2generation from water was examined forPt/Au@CdS/TiO2or Pt/CdS.After the photocatalyst(0.02g)had been dispersed in water(20ml)and de-aerated with Ar for15min,the suspension was irradiated by a500W Xe lamp through a300–400nm bandpathfilter (300<l ex<400nm,I320−400=4.0mW cm−2).The reaction temperature was kept at298or323K by circulating thermostatted water through an outer jacket around the reaction cell.The H2that evolved under illumination was analysed by gas chromatography(Shimadzu GC-8A);tcd column SHINCARBON ST, carrier gas was Ar,both the injection and column temperatures were323K.Received26April2006;accepted2August2006;published10September2006. 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Competingfinancial interestsThe authors declare that they have no competingfinancial interests.Reprints and permission information is available online at /reprintsandpermissions/Figure S1 HRTEM image of an Au nanoparticle loaded on TiO2.Fig. 1S H. Tada et al.Figure S2 HRTEM and ADF-STEM images, and EELS of Pt/Au@CdS/TiO 2 particles prepared by Vis-light ( ex > 400 nm) and UV-light (300 < ex < 400 nm) irradiation. a , HRTEM image, b , ADF-STEM image, c , EELS for the sample prepared by Vis-light irradiation. d , HRTEM image, e , ADF-STEM image, f , EELS for the sample prepared by UV-light irradiation.a d 051015Energy [ keV ]e51015Energy [ keV ]cb fFig. 2S H. Tada et al.Figure S3048121620p H 2 / n m o LFig. 3S H. Tada et al.Table S1 Au surface plasmon resonance of Au@CdS/TiO2 in H2O and C2H5OHsolvent conditions t p / hmax/ nmafter irradiationmax/ nmex> 300 nmI320-400 = 8.0 mW cm-24578 (± 2)- 14 (± 2) H2Oex> 400 nmI420-485 = 8.0 mW cm-24601 (± 1)+ 8 (± 1) ex> 300 nmI320-400 = 8.0 mW cm-24572 (± 3)- 20 (± 3) C2H5OHex> 400 nmI420-485 = 8.0 mW cm-24616 (± 4)+ 24(± 4)。