Communications tothe Editor Bull. Korean Chem. Soc. 2009, Vol. 30, No. 12 2875 DOI10.5012/bkcs.2009.30.12.2875
Pt Nano-Layer Formation by Redox Replacement of Cu Adlayer on Au(111) Surface
Deyu Qu, Chi-Woo J. Lee,* and Kohei UosakF
Department of Advanced MaterialsChemistry, College of Science and Technology, KoreaUniversity,Jo시hwon, Chungnam339-700,Korea. * E-mail:cwlee@korea.ac.kr
'Physical ChemistryLaboratory,Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received September 11, 2009, Accepted October 16, 2009 KeyWords: Pt,Cu, Au(111), Redox replacement
Platinum is one of themost important catalysts for many che mical and electrochemicalreactions.1Deposition of small amo untof platinum on metalsurfaces hasbeen shown toimprove the electrocatalytic properties ofthe metal electrodes.2 Platinum does notexhibit underpotential deposition(UPD)behaviorand studiesshowedthatelectrochemicaldeposition of platinumon Au(111)surface from platinum complex solutionsproceeded with a layer by layer growth mode to form a multilayerof Pt,3 orproduced3Dclusters and acauliflower-like Ptappearance4 or Pt nanoparticles.5,6 Recentlya newmethod, which is now re cognized as surfacelimited redoxreplacement reaction (SLR3), waspresented to prepare a Pt submonolayer/monolayeron gold surface.7,8This method has been widely used toform a Pt adlayer on golds fordifferent purposes.8,9
Since controversy exists onthe platinumfilm electrochemi callydepositedbyusing PtCl62- versus PtCl42-,3,4 anda single UPDCu replacement with Pt(II) ions can yield a full monolayer but the previous elaborationwaslimited to PtCl62-caseonly,7 weinvestigated theredox replacement of UPD Cuadlayer on Au(111)in PtCl42- solutionsas a function of time by using X-ray photoelectron spectroscopy(XPS),cyclic voltammetry (CV), and scanning tunneling microscopy (STM).
Allchemicalswere of the bestqualityavailable commercially and used asreceived. A Au (111)singlecrystalwaspreparedas before.10 XP spectrawere obtained using a Rigakudenki model
920 930 940 950 960 970 64 66 68 70 72 74 76
Binding Energy (eV)
Figure 1. XP Cu 2p and Pt 4f spectra of the UPD Cu/Au(111) elec
trodes immersed in PtCl42- solution at different replacing time; (A) 0 minute (before the replacement), (B) 10 minutes, (C) 20 minutes, and (D) 240 minutes.
XPS-7000X-ray photoelectron spectrometer with an Mg Ka radiation. STM measurementswerecarried out as before.11
Fig. 1 shows the XP spectraof Cu and Pt regions as a function of time. The Cu 2p3/2 and Cu 2p1/2 peaksrange 932 to934eV and951 to 954 eV, respectively, and each ofthem canbebest fitted with twopeaks.Each ofthese two peakscorrespondsto metallic Cu (dotted line)andcopper oxide (dashedline). The Cu 2p3/2 (932.2 eV) and Cu2p】/2 (952.0 eV) peaks inFig 1A totally disappeared after 10minutes incubation (Fig.1B). Simultane ously Pt 4f7/2 (71.2 eV)andPt4氐2 (74.5eV) peaks12 appeared, reflectingthat the Pt/Cu SLR3 rapidlyoccurred.The four other peaks in XP spectra ofCu 2p (Fig.1A) with the binding energies of 933.4,941, 953.2 and 962eV are identicaltothose in the XPspectra regionofCu oxides and similar to the XPspectra of UPD Cu onAu reported previously.13,14Theappearance of CuO is due toexposure of the sample electrode to theambient envi ronmentbefore the XPS measurements. Itwas reported that only a moderateamount ofcopper oxide was formed on UPD Cu/Au surfaceeven when theUPD Cusurface was deliberately oxidized byoxygenplasma.13,14 A Au 5p/ peak (75.8 eV) was found in thesimilarintensities through Fig. 1A to 1D. Another peak(66.5 eV)wasalsofound with thesamebehavior as Au 5p/ peakin Fig1.Thispeaktemporarily assignedto the uniden tifiedspecies existed in thechamber of XPS.Oneshould note thatthe Pt 4f signal does not remainthesame after replacement, Pt4f7/2 peak intensity wasfoundto be 5011, 5000 and15660 cps for thereplacementtime of 10, 20 and 240 min, respectively.
Therelative amount of UPDCu andformedPt atdifferent re
placement timeontheAusurfacewasdeterminedby CuUPD : Pt10 min : Pt20 min : Pt240 min = (Acu/Rcu) :(Api,10 min/R?t) :(Ap&20 min/ RPt) : (APt,240 min/RPt),where Aisthe peakareaandRis the atomic sensitivityfactor.12,15,16 Using Rcu and Rpt of 4.4 and 6.3, res
pectively,16 the ratio wasfoundtobe 1 : 0.17 : 0.23 : 0.93.
The Pt/Au electrodes withdifferent incubation time were also tested by CVmeasurements in 0.05 M H2SO4 solution (Figure2a). Thepotentialwas initiallyscanned from0.1 Vto negative direction.The platinum oxide reductionpeakaround 0.4 V as well asa well-shapedhydrogenadsorption/desorption featurein the potential regionof +0.1 V to-0.20 V were observ ed, similarto that ofa polycrystalline Ptreported previously.17 Thevoltammogram is differentfrom that ofPt film formedvia replacement of UPDCufrom PtCk2-solution but similartothe onethrough PtCl42-.8The charges associatedwith the desorption of hydrogenonthe platinumwere found to be 62, 107, 103
2876 Bull. KoreanChem. Soc. 2009,Vol.30,No. 12 Communications to the Editor (A)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Potential (V vs Ag/AgCl)
Figure 2. (A) Cyclic voltammogram of Pt/Au electrode in 0.05 M H2SO4 solution with potential scan rate of 20 mV s-1. Incubation time in 0.5 mM PtCl42- solution; (solide) 5 minutes, (dash) 10 minutes, (dot) 20 minutes, and (dash dot) 4 hours. (B) & (C) STM images (Ut = 100 mV, It = 100 pA) of Pt film on Au(111) electrode obtained after 240 (B) and 10 (C) minutes replacement of Cu adlayer in 0.5 mM K2PtCl4 solu
tion. The electrodes were rinsed by Pt ions free sulfuric acid solution before CV and STM measurements.
and155 pC cm-2 at theincubation time of 5, 10,20 and 240 min, respectively. These valuesare equal to about 30%, 51%, 49%
and74% of thechargeassociated with theadsorption/desorp- tion ofone hydrogenfullmonolayer on a Pt(111) surface (210 卩C cm-2),18suggestingthateven after 4 hoursimmersion, a full monolayer ofPtwas not formed andbare Au surfacestill existed aswas foundinXPS experiments.The voltammogram little changed withfurtherincubationtime.
The STMimages ofthe electrodestakenafter240 and10 minutes incubation are shownin Figure 2B and C,respectively.
Pt nano-clusters and bare Au(111) surface are seen in both cases.Thedatashowthat larger and more uncoveredAu(111) surface wasfoundwith the smaller incubation time. The STM images are different fromthe one in the previous report,7where apparently the entire Au(111)surface was coveredbyauniform Ptfilmandno largegoldsurfacewasobserved. However, Pt clusters with 0.3 to 0.5 nm high were found in the closeup STM image. Inaddition, ultrafine and narrow regionsof bare goldsurface were seen between thePt nano-clusters in the STM image of the originalwork.Thedifference betweenthis study and the previous work may come fromthe use of different pla
tinum ions. In the present case PtCl42-ions were hired, onePt atom replacedoneCu atom on the Au surfaceandthe significant congregation of deposited Pt atoms could occur at firstand then the atomicrearrangements proceed as time passed out.
When the sizesof the atomicnuclei (Cu: 0.256 nm, Pt:
0.278 nm,Au: 0.288nm) are considered, the commensurate Pt(111)surfacemay be expectedifallthe Cu atoms were re placedbyPt atoms at the samelocation. The aboveresults of XPS,CV, andSTMdemonstrate that the UPD copperwascom pletelyremoved upon electrontransfer,that the adsorption of Pt atoms waslessthan100%withaggregation insome preferred locations sothatsignificant portion of Ausubstrate was unoccu
pied, andthatthe deposited platinum atoms were mobilefor hours afterreplacingcopper atoms. SLR3 iscomposed of charge transfer,adsorption of the reduced particles,and the rearrange ment of theadsorbed atoms.Except the electrontransfer,the otherprocessesareslowand the detailsofSLR3 remain to be explored. This isthefirst example toshow the long-term atomic rearrangement or relaxation in SLR3.
In summary, kineticaspects of Pt monolayer formation by a spontaneous redox replacement of UPDCu adlayeronAu(111) surface werestudied. UPD Cuadlayer was totally removed immediately after it wasexposed to the solutioncontaining PtCl42- ions. The deposited platinum atoms were mobilefor hours thatatomicrearrangementstookplace. Pt clusters and uncovered Au surfaceweredetected.
Acknowledgments. This work was financially supportedby Korea University.Wearegrateful to Professor K. Shimazu and Dr. T. Matsuda, HokkaidoUniversity for use of the XPSinstru ment.
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