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1、BULLETIN Investigation on composite Au/T O nanoparticles(1)-Synthesis and characterization WANG Chuanyi,LIU Chunyan and SHEN Tao Institute of Photographc Chemistry,Chinese Academy of Sciences,Beijing 100101,China Abstdct Naionieter sizcd Au/Ti()z particles were synthesized by irradiation of a HAuCh

2、solution containing colloidal Ti%with light of wavelength(A)330 nm.Ilie absorption maximum attributed to the surface plasmon band of gold was observed at 540 nm,a red shift of about 20 nrn from thr position in aqueous solution.The Au clusters are situated on the surface of TiQ in twns of microcrysta

3、llite,which was confirmed by HRTEM,EDS m d XRI).The electronic interfaction between the metal and the suprt was discussed.Keywords:photocatalytic reduction,composite Au/TiOz nanoparticles,surface plasmon band.IN the past decades,much more Interest has been shown in the research field of nanometer-si

4、zed metal and semiconductor particles.These ultrafine particles exhibit novel properties very different from the bulk properties,such as unique catalytic behavior,size quantization effects,nonlinear optical properties and unusual luminescence.Recently,coupled particles are extensively investigated i

5、n view of improving the properties of the nanostructure materials.However,less attention has been given to the coupled system of metal-semiconductor compared with that of metal-metal and semiconductor-semiconductor21.Some work has been carried out on composite metal-semiconductor particles since the

6、 phenomenon of the so-called strong metal-support interaction(SMSI)was first found by Tauster et aL.13.The system of M/Ti02,such as v/To,P /T O,and i/T i 0 is by far the most extensively investigated catalytic sys-tem,in which the support of Ti&is bulk material.In the present work,the synthesis of A

7、u/Ti02 in nanometer-sized scale was reported for the first time,and the prepared particles were characterized by a transmission electron microscope(TEM)1 Experimental(1)Materials.letrachloroauric(111)acid(HAuC14-4H20)and titanium-tetraisopropoxide(Ti(O-i-C3H,)4)were purchased from Aldrich.All the ot

8、her chemicals used in the experiments were analytical grade reagents,and the doubly distilled deionized water was used for solution preparation.Colloidal TO2 particles were prepared by hydrolysis of titanium-tetraisopropoxide:(i)1 mL of Ti(O-i-C3H7)4 WilS dissolved in 10 mL of ethanol;(ii)the soluti

9、on was then slowly dropped into 100 mI,of diluted hydroperchloric acid(0.1 mol/I)under vigorous stirring at 0C.The resulted solution was peptized by further stirring for 3-5 h to give a transparent TiOz sol which contained 3.2 x lo-mol/I,Tic&;(iii)the Ti02 sol was dialyzed for purification as well a

10、s the adjustment of pH.In this work,the pH value of colloidal TiOz was adjusted to 3.The average diameter of the prepared TiOZ parti-cles determined by a transmission electron microscope(TEM)was about 5 nm.(2)Photoreduction.Au/Ti02 coupled particles were prepared by the photoreduction of AuC1;in the

11、 presence of Ti02 colloid.The photoreduction was carried out in a home-made photochemical reactor.A 300 W high-pressure mercury lamp(X330 nm)was used as a light source which was cooled by cold water in the jacket.The samples were deoxygenated as well as stirred up by bubbling N2 before and dur-ing i

12、rradiation.(3)Measurements.Optical absorption was measured on a Hewlett-Packard 8451A Diode Array UV-vis spectrophotometer.TEM was performed with a Hitachi H-9000 high-resolution transmission electron microscope(HRTEM)equipped with an EDAX DX-4 X-ray spectrometer analyzer for the recording of the el

13、ectron digital spectrum(EIIS).All experiments were carried out at room temperature unless specified in the text.2 Results and discussion 2.1 Spectral evolution upon illumination Figure 1 shows thc optical evolution of 8 x 10 mol/L HAuC14 solutions with the presence of 1.2 x mol/I,TiOz during irradia

14、tion.As can be seen,the solution of AuCI,exhibits an absorption 2 10 Chinese Science Bulletin Vol.43 No.3 February 1998 BULLETIN maximum at 330 nm,which is attributed to the ligand-to-metal charge transfer(IMCT)band of AuCW ions.This peak decreases with irradiation proceeding.After a certain irradia

15、tion time,a broad 5-7 absorption band centered at around 540 nm attribut-ed to the surface plasmon band of colloidal gold starts to develop.This band becomes more intense 1.0-with increasing irradiation time.The appearance of the peak at around 540 nm indicates the formation of colloidal gold.During

16、 irradiation,the color of the reaction solution changes from light yellow to color-less at first,and then to violet-red.The violet-red colloid solution is transparent and stable for a few 300 400 500 600 700.800 A/nm weeks.As we know,the surface plasmon band of a Fig.1.Changes in the optical absorpt

17、ion during illumination.metal is greatly affected by the particle size and its The irradiation time for curves from 0 to 7 are 0,5,10,15,surface environment,ln fig,1 two points be 3%45,60 and 90 min.1.2x lo-mol/L Ti02 was taken as stressed.First,at the initial stage of the particle reference formati

18、on,the absorption band is relatively broad.This is related to the particles formed at this time which are very snlall and unevenly distributed.or ern us has found that particles with average diameters oi 2.9-6 nm exhibit a broader plasmon band.Second,as shown in fig.1,the peak position for gold de-p

19、osited on Ti&is at around 540 nm,which is a red-shift of several ten nanometers compared with that for aqueous gold sols(514-521 nmr8).This is most probably due to the high refractive index of the supporting Ti02.Details about the influence of the medium refractive index on the position of the surfa

20、ce plasmon bands for gold sols has been demonstrated by Mulvaney et a1.83.It was confirmed that the peak of gold plasmon shifted toward a longer wavelength when the medium refractive index increased.Consid-ering the fact of LOz as the supporting medium,the position of the plasmon band for gold shoul

21、d be strongly affected by the high refractive index of Ti02.The refractive index of Ti02 is much higher than that of both water and ethanol,and naturally from that of gold.2.2 Mechanism for the formation of gold on Ti02 nanoparticles Ti02 is an n-type semiconductor.When it is illuminated with ultrab

22、and-gap(23.2 eV)irradiation,electron(e-)and hole(h)pairs are generated.The photogenerated charge that carries(e-,h+)would migrate to the particle surface in a very short time scale(about 10 ps)I,and react with surface adsorbates or recombine with each other.The pH value of the solution in the experi

23、ment(around 3.0)is lower than that for the point of zero charge(pH,=6.3).Therefore,the surface of the Ti02 particles is positively charged,which facilitates the adsorption of the AuCL,-anions to the surface of Ti02 particles,and the reaction between the AuCL anions and the photogenerated electrons.T

24、i02+hv-,hf+e-(1)AuC1;+e-+AuCli+C1-(2)2AuC1;-t AuClT+AuC1;(3)AuC1,+e-+AO+2C1-(4)n u +(AU),(5)h+Scavenger 4 Scavenger+(6)h+e-,hu(7)AUO formed on the Ti02 surface can act as an electron relay for the photogenerated e-to react with AU*.The AUO nuclei can aggregate and deposit on the surface of colloidal

25、 TiQ particles.These processes are il-lustrated in figure 2.Further experiment shows that no stable violet-red gold colloid can be formed in the absence of Ti under the same conditions.Bas.ed on the above analysis,it can be concluded that TiQ particles have the Chinese Science Bulletin Vol.43 No.3 F

26、ebruary 1998 21 1 following effects during the formation of Au particles on the surface of the metal oxide:(i)a photocatalyst for the photoreduction of AuCL complexes;(ii)crystal seeds of heteronucleus for the formation of Au colloid;(iii)stabilizer for Au deposited on its surface because of its pol

27、arity and:high specific surface.I 2.3 Characterization by TEM and EDS Figure 3(a)shows the TEM micrograph of i the finally formed nanoparticles.Clearly,the de-posited Au clusters are indeed localized on the sur-face of the Ti02 particles after the photoreduction.The coupled particles are nearly mono

28、dispersed,and the average diameter is about 25 nm.In addi-0-0 tion,from fig.3 one can also find that the size of the substrate part of Au/Ti02 is much larger than that of an individual Ti02 particle before the depo-sition of Au clusters(5 nm).That is to say,ag-O H,+O A U C 6 :-(A&.gregation to some

29、extent occurs for Ti02 particles during the illumination.The surface of T i Q is Fig.2.Schematic drawings of the formation of metal gold on the positively charged in acidic solution,which makes surface of Ti02 particles.the colloidal particles stable.When AuCli ions are adsorbed,however,the positive

30、 charge on the particle surface will be partially neutralized;more-over,with the photoirradiation proceeding,C1-ions are liberated from the coordinate group of Au(III),and Hf on the surface perhaps reacts with photogenerated electrons.All these factors make the Ti&particles turn unstable,and result

31、in the aggregation.Fig.3(b)shows HRTEM micro-graph of an Au/Ti02 particle corresponding to the same sample in fig.3(a).As shown in fig.3(b),the lattice planes of Au are clearly visible in a local area of the images.This indicates that the Au cluster deposited on the Ti02 particles exhibits the prope

32、rties of microcrystallite.Figure 4 is the EDS spectrum of a particle Fig.3.TEM micrograph o f the prepared Au/Tiq nanoparticles corresponding to that in fig.3(a).As shown in(a)and high-resolution TEM micrograph of a prepared composite fig.4,the particle indeed consists of Au and Ti el-Au/TiG nanopar

33、ticle(b).ements.The EDS peaks attributed to Cu result from copper grid.The weight ratio of Au and Ti,determined from the EDS,is 49.18:50.82.Consid-ering the results of TEM and the density of T i Q and gold(hiO1=4 g/cm3,PA,=19.3 g/cm3),the di-ameter of Ti02 substrate in Au/Ti02 can be approximately e

34、stimated.It is about 15.7 nm,almost three times the primitive size of Ti02 particles.However,the size of Au clusters cannot be calculated,because the accurate number of Au clusters on an Au/TiQ particle is still uncertain.3 Conclusions Composite Au/Ti&nanoparticles were synthesized by a photochemica

35、l method in the present work.The particle size is about 25 nm.The gold clusters deposited on the surface of TiQ show the property of microcrystallite,and exhibit a typical surface plasmon band.A red shift of about several ten nanometers in the surface plasmon band occurs compared with that in aqueou

36、s solution.The composite Au/Tiq 212 Chinese Science Bulletin Vol.43 No.3 February 1998 BULLETIN FS:213 CPS:90 Cnts:23 KeV:0.53 Fig.4.EDS of a composite Au/Tiq nanoparticle.nanoparticles are expected to provide a new class of potentially interesting heterogeneous catalysts,and the preparation conditi

37、ons and catalytic properties of which should be further investigated.Acknowledgement This project w a supported by the National Natural Science Foundation of China(Grant No.295731411,the Photochemical Laboratory,Institute of Photographic Chemistry,Chinese Academy of Sciences and American Eastman Kod

38、ak Com-pany,and partially supported by Zhongguancun Association Measurement Foundation.References 1 Joselevich,E.,Willner,I.,Photosensitization of quantum-sized Ti02 particles in water-in-oil microemulsions,J.Phys.Chem.,1994,98:7628.2 Honma,I.,Sano,T.,Komoyama,H.,Surface-enhanced Raman scattering(SE

39、RS)for semiconductor mirocrstallites ob-served in Ag-CdS hybrid particles,J.Phys.Chem.,1993,97:6692.3 Tauster,S.J.,Fung,S.C.,Garten,R.I,.,Strong metal-support interactions.Group 8 noble metals supported on TiOz,J.Am.Chem.Soc.,1978,100:170.4 Sambi,M.,Pin,E.,Sangiovanni,G.et al.,Photoelectron diffract

40、ion study on the structure of a vanadium ultrathin film deposited at the Ti02(110)surface,Surf.Sci.,1996,349:L169.5 Poirier,G.E.,Hance,B.K.,White,J.M.,Tunneling probe manipulation of individual rhodium nanoparticles supported on Ti02,J.Phys.Chem.,1993,97:6500.6 Sault,A.G.,Peden,C.H.F.,Boespflug,E.P.

41、,Metal-support interactions in hydrous titanium oxide-supported nickel catalysts,J.Phys.Chem.,1994,98:1652.7 Doremus,R.H.,Optical properties of small gold particles,J.Chem.Phys.,1964,40:2389 8 Liz-Manan,I-.M.,Giersig,M.,Mulvaney,P.,Synthesis of nanosized gold-silica core-shell particles,Langmuir,199

42、6,12:4329.9 Griitzel,M.,Frank,A.J.,Interfacial electron-transfer reactions in colloidal semiconductor dispersions.Kinetic analysis,I.Phys.Chem.,1982,86:2964.10 Kamat,P.V.,Chauvet,J.P.,Fessenden,R.W.,Photoelectmhernistry in particulate systems 4.Photosensitization of a Ti02 semiconductor with a chlorophyll analogue,J.Phys.Chem.,1986,90:1389.(Received July 22,1997)Chinese Science Bulletin Vol.43 No.3 February 1998