Surface Plasmon Spectroscopy of Nanosized Metal

Surface Plasmon Spectroscopy of Nanosized MetalParticles

Paul Mulvaney

Advanced Mineral Products Research Centre, School of Chemistry, University of Melbourne,Parkville, Victoria, 3052, Australia

Received April 4, 1995. In Final Form: July 7, 1995X

The use of optical measurements to monitor electrochemical changes on the surface of nanosized metalparticles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shownto be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, andalloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarizationleads toaniondesorption. Underpotential deposition (upd) of electropositivemetal layers results indramaticblue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can bereadily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation canbe ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. Theunderpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silvershell, but the mean free path is less than predicted, due to lattice strain in the shell.

IntroductionInterest in theopticalpropertiesof colloidalmetalsdates

back toRoman times. Nanosizedgoldparticleswere oftenused as colorants in glasses, and quite complex opticaleffects were created using metal particles.1 In theseventeenth century, “Purple of Cassius”, a colloid ofheterocoagulated tin dioxide and gold particles, becameapopular colorant inglasses.2 These earlymanifestationsof the unusual colors displayed bymetal particles promptedFaraday’s investigations into the colors of colloidal goldin the middle of the last century. Today his studies aregenerally considered to mark the foundations of moderncolloid science.3 The formation of color centers and smallcolloidalmetal particles in ionicmatrices and glasses hasremained an area of very active research,4-6 driven, inpart, by the technical importance of the photographicprocess.7 However, colloid chemistshave tended toneglectthe study of metal particles in aqueous solution becauseof their complicated double layer structure,which ismoreamenable to direct electrochemical investigation. Themore recentdiscovery that the surfaceplasmonabsorptionband can also provide information on the development ofthe band structure in metals8-11 has led to a plethora ofstudies on the size dependent optical properties of metalparticles, particularly those of silver and gold,12-17 whileadvances in molecular beam techniques now enable

spectroscopic analysis of metal clusters to be carried outin vacuum.18,19

Although many of the optical effects associated withnanosized metal particles are now reasonably well un-derstood, thereare largediscrepanciesbetween theopticalproperties of metal sols prepared in water, particularlythose of silver, and sols prepared in other matrices.6,20-27

In a recent review Kreibig noted that while much workhas been done to isolate matrix effects and to determinethe roles of defects, grain boundaries, crystallinity, andpolydispersity on the optical properties of sols, little isknownabout thewayspecific surface chemical interactionsmay influence the absorption of light by small metalparticles.28 These differences are attributed to uniquedouble layer effects present at themetal-water interface.This review focuses on some of these surface chemicaleffects, and attempts to show how changes to the surfaceplasmon absorption band of aqueous metal colloids canbe related to electrochemical processes occurring atmetalparticle surfaces. Simple models are proposed to explainsome of these chemical changes within the Drude frame-work for surface plasmon absorption.

1. Light Absorption by Colloids

In the presence of a dilute colloidal solution containingN particles per unit volume, the measured attenuation oflight of intensity Io, over a pathlength d cm is given byX Abstract published in Advance ACS Abstracts, December 15,

1995.(1) See, for example: Savage, G. Glass and Glassware; Octopus

Books: London, 1975.One of themost famous examples is theLycurgusCupwhich is ruby red in transmitted light butappears green in reflectedlight. The color is due to colloidal gold. It was manufactured in the 4thcentury AD.

(2) See: Thiessen, P. A. Kolloid Z. 1942, 101, 241, for micrographsof this composite.

(3) Faraday, M. Philos. Trans. R. Soc. 1857, 147, 145.(4) Siedentopf, H. Z. Phys. 1905, 6, 855.(5) Mott, N. F.; Gurney, R. W. Electronic Processes in Ionic Crystals;

Oxford University Press: Oxford, 1948.(6) Hughes, A. E.; Jain, S. C. Adv Phys. 1979, 28, 717.(7) The Theory of the Photographic Process, 4th ed.; James, T. H.,

Ed.; MacMillan Press: New York, 1977.(8) Scott, A. B.; Smith,W. A.; Thompson,M. A. J. Phys. Chem. 1953,

57, 757.(9) Doremus, R. H. J. Chem. Phys. 1965, 42, 414.(10) Doyle, W. T. Phys. Rev. 1958, 111, 1067.(11) Romer, H.; von Fragstein, C. Z. Phys. 1961, 163, 27.(12) Perenboom, J. A. A.; Wyder, P.; Meier, F. Phys. Rep. 1981, 78,

173.(13) Papavassiliou, G. C. Prog. Solid State Chem. 1980, 12, 185.(14) Kreibig, U. J. Phys. F: Met. Phys. 1974, 4, 999.

(15) von Fragstein, C.; Schoenes, F. J. Z. Phys. 1967, 198, 477.(16) Kreibig, U. Z. Phys. B: Condens. Matter Quanta 1978, 31, 39;

J. Phys. (Paris) 1977, 38, C2-97.(17) Yanase, A.; Komiyama, H. Surf. Sci. 1991, 248, 11, 20.(18) Fallgren, H.; Martin T. P.; Chem. Phys. Lett. 1990, 168, 233.(19) (a) Tiggesbaumker, J.; Koller, L.;Meiwes-Broer, K.-H.; Liebsch,

A. Phys. Rev. A 1993, 48, R1749. (b) Huffman, D. R. Adv. Phys. 1977,26, 129.

(20) Frens, G.; Overbeek, J. Th. G. Kolloid Z. Z. Polym. 1969, 233,922.

(21) Berry, C. R.; Skillman, D. C. J. Appl. Phys. 1971, 42, 2818.(22) Miller, W. J.; Herz, A. H. In Colloid and Interface Science;

Academic Press: New York, 1976; Vol. 4.(23) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J.

Colloid Interface Sci. 1983, 93, 545.(24) Henglein, A. J. Phys. Chem. 1979, 83, 2209.(25) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.(26) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem.

Soc., Faraday Trans. 2 1979, 75, 790.(27) Linnert, T.;Mulvaney, P.;Henglein, A.J. Phys. Chem. 1993, 97,

679.(28) Kreibig, U.; Genzel, U. Surf. Sci. 1985, 156, 678.

788 Langmuir 1996, 12, 788-800

0743-7463/96/2412-0788$12.00/0 © 1996 American Chemical Society

whereCext is theextinctioncross sectionofa singleparticle.For spherical particles with a frequency dependentdielectric function ε ) ε′ + iε′′, embedded in a medium ofdielectric function εm, Cext is given by29-33

where k ) 2π xεm/λ and an and bn are the scatteringcoefficients, which are functions of the radius R and thewavelength λ in terms of Ricatti-Bessel functions. Theextinction cross section of a particle is often normalizedto give the extinction cross section per unit area:

Conventionally, chemists measure the extinction coef-ficient of a solution in units ofM-1 cm-1, where the colloidconcentration is themolarmetal atom concentration.Thisquantity is related to Qext by

where Vm (cm3 mol-1) is the molar volume of the metal.For very small particles where kR , 1, only the first,electric dipole term in eq 2 is significant, and

This equation can be also obtained by purely electrostaticarguments, and a clear derivation is given by Genzel andMartin.34 In many cases to be described here, it will benecessary to consider the perturbation introduced by athin surface layer. The extinction cross section of a small,concentric sphere is given by32

where εcore is the complex dielectric function of the corematerial, εshell is that of the shell, εm is the real dielectricfunction of the surrounding medium, g is the volumefraction of the shell layer, andR is the radius of the coatedparticle. When g) 0, eq 6 reduces to eq 5 for an uncoatedsphere, and for g ) 1, eq 6 yields the extinction crosssection for a sphere of the shell material.In the case of manymetals, the region of absorption up

to the bulk plasma frequency (in the UV) is dominated bythe free electron behavior, and the dielectric response iswell described by the simple Drude model. According tothis theory,35 the real and imaginaryparts of thedielectricfunction may be written

where ε∞ is the high frequency dielectric constant due tointerband and core transitions and ωp is the bulk plasmafrequency

in terms of N, the concentration of free electrons in themetal, andm, the effectivemass of the electron. ωd is therelaxation or damping frequency, which is related to themean free path of the conduction electrons,Rbulk, and thevelocity of electrons at the Fermi energy, vf, by

When the particle radius, R, is smaller than the meanfree path in the bulk metal, conduction electrons areadditionally scattered by the surface, and the mean freepath, Reff, becomes size dependent with

Equation 11 has been experimentally verified by theextensivework ofKreibig for both silver andgoldparticlesright down to a size of 2 nm.14,16,28 The advantage of theDrude model is that it allows changes in the absorptionspectrumtobe interpreteddirectly in termsof thematerialproperties of the metal. The origin of the strong colorchanges displayed by small particles lies in the denomi-nator of eq 5,whichpredicts the existence of anabsorptionpeak when

From eq 7 it can be seen that over the whole frequencyregime below the bulk plasma frequency of a metal, ε′ isnegativewhich isdue to the fact that theelectronsoscillateout of phasewith the electric field vector of the lightwave.This is why metal particles display absorption spectrawhich are strong functions of the size parameter, kR. Ina small metal particle the dipole created by the electricfield of the lightwavesetsupasurfacepolarization charge,which effectively acts as a restoring force for the “freeelectrons”. The net result is that, when condition 12 isfulfilled, the longwavelengthabsorptionby thebulkmetalis condensed into a single, surface plasmon band. In thecase of semiconductor crystallites, the free electronconcentration is orders of magnitude smaller, even indegenerately doped materials (i.e., ωp is smaller), and asaresult surfaceplasmonabsorptionoccurs in the IR, ratherthan in the visible part of the spectrum. Semiconductorcrystallites therefore do not change color significantlywhen the particle size is decreased below the wavelengthof visible light, although the IR spectrummaybe affected.It should be noted that the strong color changes observedwhen semiconductor crystallites are in the quantum sizeregime (R < ∼50 Å), are due to the changing electronicband structure of the crystal, which causes the dielectricfunction of the material itself to change.In Figure 1, a “typical” surface plasmon band is shown

calculated using eq 5with parameters typical of silver forseveral values of the damping parameter ωd. The mostimportantparameteraffectingωd is theparticle size.Fromeqs 10 and 11 it can be seen that decreases in the particlesize lead to an increase inωd, causing the band to broadenand the maximum intensity to decrease. The position ofthe peak is virtually unaffected by small changes to ωd

(29) Toon, O. B.; Ackerman, T. P. Appl. Opt. 1981, 20, 3657.(30) van der Hulst, H. C. Light Scattering by Small Particles; John

Wiley and Sons: New York, 1957.(31) Kurtz, V.; Salib, S. J. Imaging Sci. Technol. 1993, 37, 43.(32) Bohren,C.F.;Huffman,D.R.AbsorptionandScattering ofLight

by Small Particles; Wiley: New York, 1983.(33) Kerker, M. The Scattering of Light and Other Electromagnetic

Radiation; Academic Press: New York, 1969.(34) Genzel, L.; Martin, T. P. Phys. Status Solidi B 1972, 51, 91.(35) Kittel, C. Introduction to Solid State Physics, 2nd ed.; Wiley:

New York, 1956.

ε′ ) ε∞ - ωp

2/(ω2 + ωd2) (7)

ε′′ ) ωp2ωd/ω(ω

2 + ωd2) (8)

ωp2 ) Ne2/mεo (9)

ωd ) vf /Rbulk (10)

1/Reff ) 1/R + 1/Rbulk (11)

ε′ ) -2εm (if ε′′ small) (12)

A ) log10 Io/Id ) NCextd/2.303 (1)

Cext ) 2π/k2∑(2n + 1) Re (an + bn) (2)

Qext ) Cext/πR2 (3)

ε(M-1 cm-1) ) (3 × 10-3)VmQext/4(2.303)R (4)

Cext )24π2R3

εm3/2

λε′′

(ε′ + 2εm)2 + ε′′2

(5)

Cext ) 4πR2k* ×

Im{ (εshell - εm)(εcore - 2εshell) + (1 - g)(εcore - εshell)(εm + 2εshell)

(εshell + 2εm)(εcore + 2εshell) + (1 - g)(2εshell - 2εm)(εcore - εshell)}(6)

Optical Properties of Metal Particles Langmuir, Vol. 12, No. 3, 1996 789

but for largedampinga slowshift to lower energies occurs.Inan inertmatrix, the only cause of peak shifts is a changein the dielectric properties of the metal particles them-selves, due to this surface scattering or for exceedinglysmall particle sizes (<1-2 nm), to quantization of theenergy levels within the conduction band. In the case ofsilver particles, quantization results in a blue-shift of theplasmon band and a break-up into discrete excitationbands.36-41 Nevertheless, in water, the experimentallymeasured surface plasmonabsorption bands of silver solsvary enormously in position (ranging from375 to 405 nm)and the absorption coefficients vary by factors of 3 or420-27,42-45 These discrepancies cannot be explained onthe basis of eq 2 and provided the motivation for much ofthe work to be described below. We will considerexperimental spectroscopic data illustrating the effectsof anion adsorption, electronic charging, and under-potentialmetaldeposition, and try to interpret thespectralfeatures in terms of eqs 5-8. Becausemetal particles arestudied in a variety ofmatrices, it is worth reviewing firsthow the surface plasmon absorption is affected by thesolvent refractive index.

2. The Effect of the Solvent Refractive Index

For simplemetals obeying theDrudemodel, thepositionof the plasmon absorption peak does depend on therefractive index of the surrounding medium. Using eqs5and6,we find, for small ε′′, that the bandposition shouldobey46

where λp2 ) (2πc)2/ωp2 is the metal’s bulk plasma wave-

length. From a plot of the observed band position vs 2εm,both the high-frequency dielectric constant and the bulkplasma frequency (orwavelength) canbe extracted. Plotsof λ2 vs 2εm are shown for silver and lead colloids in Figure2. From the data for lead we find that the bulk plasmaenergy is 11.3 eV and ε∞ ) 1.1, in good agreement withelectron loss spectroscopy data of Ashton and Green andthat of Girault et al.47,48 In the case of silver, the bandposition in water is variable, but interpolating the valuesfrom the salt matrices, we predict that the “true” positionof the silver surface plasmon band inwater is 382( 1 nm.The high frequency dielectric constant is estimated to be5.9, not far from other estimates of 4.7-5.3. Adherenceto eq 13 demonstrates that the absorption of light by theparticles over the spectral region is due to the absorptionbyconductionelectrons, rather than interbandtransitions.It also suggests that the position of the surface plasmonbandmay be used tomeasure the local dielectric constantin microheterogeneous systems. Papavassiliou has useda similar procedure to analyze the optical properties ofparticulate metal films.49 He found that the color of thefilm was altered when it was immersed into solvents ofdiffering refractive index. More recently,Wilcoxonet al.50and Esumi et al.51,52 have prepared metal sols in non-aqueousmedia, andobserved spectral shifts inqualitativeaccord with eq 13. This optical approach to the measure-ment of the dielectric function of the environment isanalogous to the use of solvatochromic molecules such asET30.53Clearly, since the particles in these experiments are

prepared in different media, and may have variableshapes, sizes, and defect structures, good adherence to eq13 cannot always be assumed. Ideally, the sameparticles

(36) Mulvaney, P.; Henglein, A. Chem. Phys. Lett. 1990, 168, 391.(37) Mulvaney, P.; Henglein, A. J. Phys. Chem. 1990, 94, 4182.(38) Henglein, A.; Mulvaney, P.; Linnert, T. Ber. Bunsen-Ges. Phys.

Chem. 1990, 94, 1449.(39) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. Radiat.

Phys. Chem. 1989, 34, 605.(40) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678.(41) Ershov, B.G.; Janata, E.;Henglein, A.; Fojtik, A.J. Phys. Chem.

1993, 97, 4589.(42) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci.

1982, 120, 435.(43) Blatchford, C. G.; Siiman, O.; Kerker, M. J. Phys. Chem. 1983,

87, 2503.(44) Richard, J.; Donnadieu J. Opt. Soc. Am. 1969, 59, 662.(45) Skillman, D. C.; Berry, C. R., J. Chem. Phys. 1968, 48, 3297.(46) Shklyarevskii, I. N.; Anachkova, E.; Blyashenko, G. S. Opt.

Spectrosc. (USSR) 1977, 43, 427.

(47) Ashton, A. M.; Green, G.W. J. Phys. F: Met. Phys. 1973, 3, 179.(48) Girault, P.; Seignac,A.; Priol,M.; Robin, S.C.R.Acad. Sci.1968,

266B, 688.(49) Papavassiliou, G. C. Z. Phys. Chem. (Leipzig) 1976, 257, 241.(50) Wilcoxon, J. P.;Williamson,R. L.; Baughman,R.J.Chem.Phys.

1993, 98, 9933.(51) Torigoe, K.; Nakajima, Y.; Esumi, K. J. Phys. Chem. 1993, 97,

8304.(52) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tano, T.; Torigoe, K.;

Meguro, K. Langmuir 1991, 7, 457.(53) Drummond,C.J.;Grieser,F.;Healy,T.W.J.Chem.Soc.,Faraday

Discuss. 1986, 81, 95.

Figure 1. Calculated surface plasmon absorption band of atypical free electron metal particle for several values of thedamping parameter, ωd, within the dipole approximation, eq6. Parameters, ε∞ ) 5.0, ωp ) 10 eV, R ) 5.0 nm. εm ) 1.77.Damping frequencies in eV: (1) 0.4, (2) 0.6, (3) 0.8, (4) 1.2, (5),2.4, (6) 3.6.

Figure 2. Plots of the square of the observed position of thesurface plasmon bands of colloidal lead and silver as a functionof twice themediumdielectric function. Data fromHughes andJain.6

λ2 ) λp2(ε∞ + 2εm) (13)

790 Langmuir, Vol. 12, No. 3, 1996 Mulvaney

should be transferred from one medium to the next toobviate all the matrix effects. This has recently beencarried out with gold sols using a comb polymer asstabilizer.54 The sols ranged in color from red to purple,dependingon thesolutionrefractive index, andwerestablefor many months, particularly in butyl acetate. Theresults are shown in Figure 3. Rather than assume freeelectron behavior, the predicted position of the surfaceplasmonbandwas calculatedusing the fullMie equationswith dielectric data for gold from the literature. Thedielectric data of JohnstonandChristywere found to yieldexcellent agreement. This calibration curve allows goldsols to be used as “dielectric indicators” in a variety ofmedia.

3. The Extinction Spectra of Base Metal SolsThe unique sensitivity of the absorption spectrum of

silver sols to double layer perturbations makes inter-pretation of spectral changes difficult, and we haveexpended considerable effort synthesizing sols of otherfree-electron metals in order to extend the number ofsystemswheresurfaceplasmoneffects canbe investigated.Mie calculations by Creighton and Eadon have indicatedthat for themajority ofmetals, surfaceplasmonabsorptioncontributes significantly to the UV-visible spectrum.57Very little experimental work has been done on the moreelectropositive metal sols, because they are so readilyoxidized by air, and the work is almost entirely confinedto salt matrices.6 However, more recently it has becomepossible to prepare well-characterized samples usingradiolytically generated reductants to ensure rapid nucle-ation. A primary experimental difficulty is the determi-nation of the particle size, since the sols dissolve instan-taneously in air and must be transferred into electronmicroscopes with the rigorous exclusion of oxygen. InFigure 4, the extinction spectra of several metal sols are

shown togetherwith the calculated spectra using eq 2. Ascan be seen, the agreement is generally very good, withthe intensity and position of the surface plasmon bandsof colloidal cadmiumand tin beingwell reproduced by theavailable dielectric data. The observed position of theband for colloidal lead at 215 nm is clearly red-shiftedfromthepositionpredictedusing thedataofLemmonnier63but the observed position is in excellent agreement withthe values in salt matrices using eq 13 (Figure 2), andtheir values of ε′ for lead must be a little too negative.Their data also predict the existence of a small peak at280 nm, which is not observed experimentally. The factthat the intensity and peak widths can be reproducedwith bulk dielectric data implies immediately that thereis littledampingdue to surface scattering in theseaqueoussols, i.e. the mean free path for conduction electrons inthesemetal particles is substantially smaller than 100Å.

4. Electronic EquilibriumThe fundamental reason for the variable absorption

spectra of metal sols prepared in water is the existenceof specific ion adsorption and electronic couplingwith thesolvent. The first attempts to quantify the effects ofelectronic polarizationon theoptical properties of colloidalmetals were those of Blatchford et al. as part of theirstudies into the origin of the SERS effect.29,42,43 Theypointed out that the spectrum of colloidal silver preparedwith citrate could be drastically altered by addition ofborohydride ion. Thebandwasblue-shiftedand increasedin intensity by a factor of 50%. They attributed this to achange in the electron density in the particles. Similarresults have often been obtained in other studies butwerenot correlated directly with redox potential. Directevidence for a blue-shift following electron injection wassubsequently found using pulse radiolysis.66 Exposure ofa silver sol to amicrosecond pulse of electrons from aVande Graaff generator produces (CH3)2COH radicals (Eo )-1.5 V NHE). The radicals react with the sol particlesaccording to

The transfer of electrons results in the release of protonsinto solution, and the resultant transient conductivitysignal can be used tomonitor the rate of electron transfer.Absorption changes to the sol take place at the same time,with bleaching at longer wavelengths and increasedabsorptionat shorterwavelengths thanthepeak,as shownin Figure 5. Further bleaching occurs at 320 nm, close tothe onset of d-band to Fermi level transitions. Thedischarge of the sols by solvent reduction or other oxidantslimits the attainable negative charge on the particles.

The transfer of electrons alters the free electron concen-trationand therefore thebulkplasma frequency,ωp. Fromeqs 7 and 9, it is clear that this will result in a blue-shiftof the surface plasmon absorption band. The simulated

(54) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427.(55) Optical Properties of Metals vol. 2; Weaver, J. H., Krafka, C.,

Lynch, D. W., Koch, E. E., Eds.; Physics Data Series, No. 18-2;Fachinformationszentrum: Karlsruhe, 1981.

(56) Johnston, P. B.; Christy, R. W. Phys. Rev. B 1972, 8, 4370.(57) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans.

1991, 87, 3881.(58) Henglein, A.; Gutierrez, M.; Janata, E.; Ershov, B. G. J. Phys.

Chem. 1992, 96, 4598.(59) Henglein,A.;Mulvaney,P.;Holzwarth,A.; Sosebee,T.E.; Fojtik,

A. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 754.

(60) Henglein, A.; Giersig, M. J. Phys. Chem. 1994, 98, 6931.(61) Sosebee, T.; Giersig, M.; Holzwarth, A.; Mulvaney, P. Ber.

Bunsen-Ges. Phys. Chem. 1995, 99, 40.(62) (a) Schwarz, H. Phys. Status Solidi B 1971, 43, 755. (b) Olsen,

C. G.; Lynch, D. W. Phys. Rev. B 1974, 9, 3159.(63) Lemmonnier, J. C.; Priol, M.; Robin, S. Phys. Rev. B 1973, 8,

5452.(64) Vina, L.; Hochst, H.; Cardona, M. Phys. Rev. B 1985, 31, 958.(65) Johnston, P. B.; Christy, R. W. Phys. Rev. B 1974, 11, 1315.(66) Henglein, A.; Mulvaney, P.; Linnert, T. J. Chem. Soc., Faraday

Discuss. 1991, 92, 31.

Figure 3. Position of the plasmon band of colloidal gold innumerous solvents of different refractive index.54 Fits to eq 2using dielectric data from Weaver et al.55 and Johnston andChristy.56 The solvents used are indicated. The full circles referto binary mixtures of butyl acetate and CS2.

(CH3)2COH + Agn f Agn- + H+ + (CH3)2CO (14)

e-coll + H+ f 1/2H2 (15)


difference spectrum is also shown. Thedielectric functionofbulksilverwas firstmodified to take intoaccount surfacescattering. Then the plasma frequency was increased tomimic the polarization effects. As is evident, the generalshape is similar, but the asymmetry in the experimentaldifference spectrum is not reproduced. Note that thecitrate sols in Blatchford’s work also showed a verypronounced, asymmetric increase in absorbance in thepresence of borohydride. The steady-state electron con-centration is determined by the redox potential of thereductant used to prepare the sol. The most blue-shiftedspectra are found when the preparation utilizes boro-hydride as a reductant (λmax∼ 376 nm). Citrate is amuchweakerreductantandtheFermi level in thesilverparticleslies at much more positive values. In this case themaximum usually lies between 385 and 390 nm. Theseoptical changes are analogous to the electroreflectanceeffects observed with silver electrodes upon cathodicpolarization but are far more pronounced.67-72 The

electroreflectance studies have also revealed that anionadsorption and desorption occur concomitantly.73-76 Itwill be shown below that the asymmetry in the differencespectrum is associated with changes in the degree ofadsorption of the stabilizing anions with particle charge.Nanosized metal particles have an enormous double

layer capacity and in changing the environment in thesolution from oxidizing to reducing, a large double layerchargemust build up on the particles. If the double layercapacity is taken to be 20 µF cm-2, a crude estimatesuggests that a 14% change in the electron density isneeded if the redox potential in a 30 Å diameter particle

(67) Feinleib, J. Phys. Rev. Lett. 1966, 16, 200.(68) Kolb, D. M.; Kotz, R. Surf. Sci. 1977, 64, 96.(69) Hansen,W.N.; Prostak,A.Phys.Rev.1967, 160, 600;1968, 174,

500.

(70) Takamura, T.; Takamura, K.; Yeager, E. J. Electroanal. Chem.1971, 29, 279.

(71) McIntyre, J. D. E. Surf. Sci. 1973, 37, 658.(72) McIntyre, J. D. E. In Advances in Electrochemistry and

Electrochemical Engineering, Vol. 9; Muller, R. H., Ed.; Wiley-Inter-science: New York, 1973; p 61.

(73) Anderson,W. J.; Hansen,W. N. J. Electroanal. Chem. 1973, 47,229.

(74) Cahan, B.D.; Horkans, J.; Yeager, E.FaradaySoc. Symp. 1970,4, 36.

(75) McIntyre, J. D. E.; Aspnes, D. E. Surf. Sci. 1971, 24, 417.(76) Kolb, D. M. In Trends in Interfacial Electrochemistry; Silva, A.

F., Ed.; Reidel Publishing Co.: Dordrecht, 1986; p 301.

Figure 4. Experimental and calculated absorption spectra of a number of colloids of electropositive metals in water. The dottedcurves are the experimental spectra. Data sources for experimental spectra: Cd, ref 58; Pb, ref 59; Sn, ref 60; Cu, ref 61. Dielectricfunction data sources: Cd, ref 62; Pb, ref 63; Sn, ref 64; Cu, ref 65.


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is to be raised from the ambient level in air of about +0.4VNHEto around-0.4VNHEwhereH2 evolution begins.This corresponds to a plasmon band shift from 390 to 375nm. This is clearly consistent with the fact that citratestabilized silver sols typically have a band around 385-390nmwhereasextensivelyγ-irradiatedsilver sols (strongreducing conditions) possess absorption bands at λ < 380nm and steadily evolve hydrogen. (For very smallparticles, the shift in the Fermi level is not due just to thedouble layer charging. The increase in the conductionband electron concentration also causes an additionalincrease in theFermienergy, but this varies onlyas∆N2/3.)Unless a reductant is used which has a redox potential

identical to the potential of zero charge of polycrystallinesilver (-0.7 VNHE) chemically produced sols will alwayscontain residual electrical chargeandwill possessplasmonabsorption bands shifted from the position expected foranelectricallyneutralparticle. Wehavealready identifiedthis wavelength by reference to peak positions in saltmatrices where electrostatic charging can be neglected(Figure 2). An uncharged silver colloid should have amaximum at 382 ( 1 nm; wavelengths shorter than thisare due to cathodic polarization, and longer wavelengthsare due to incomplete reduction of silver ions. Blatchfordet al. in fact reached similar conclusions, but theircalculated spectra for the cathodically polarized silverparticles had two peaks, because they assumed that thecharge was confined to a very thin surface layer ofthickness 2 Å, corresponding to the Fermi screeninglength.43 ωp was only increased in this thin shell layer,resulting in theappearanceof twoabsorptionpeaks,whichis not observed experimentally. Smearing out the doublelayer charge by increasing ωp throughout the particlesgives better agreement with experiment. These surfaceplasmon shifts could readily form the basis for veryefficient electrochromic switching. Electrical polarizationof a film of nanosized silver colloids can alter the positionof the band by 20 nm quite reversibly.

5. Chemisorption

When metal particles are prepared in solution, theymust be stabilized against the vanderWaals forceswhich

cause coagulation.77 This can be achieved in a number ofways: physisorbed surfactant and polymers create stericor electrosteric barriers,78 physisorption of ions producespurely electrostatic barriers,while depletion stabilizationoccurs in the presence of some polymers due to osmoticpressure.78 In many cases, the distinction betweenchemical adsorption (involving direct covalent bondingwith thesurfacemetalatoms)andmoresubtleelectrostaticmechanisms (e.g. charge-induceddipolemechanismsanddispersion force mechanisms79 ) is largely a matter ofdegree. The problem distinguishing chemisorption andphysisorption is further exacerbated in the case of metalclusters containing only a few tens of metal atoms, whereit is not clear whether the stabilizingmolecules should bedenoted “adsorbates” or “ligands”. Is the electrostaticcharge on such a cluster containing, say, tenmetal atomsto be considered as a double layer charge or as an intrinsicmolecular charge? What is clear is that for the coinagemetals, silver and gold, sols prepared with differentstabilizers often have quite different absorption spectraeven though theparticle size distributionsappear similar.It is therefore pertinent to ask whether chemisorption of“ligands” can alter the optical properties of the sol. Thegelatinused to stabilize photographic emulsions certainlyaffects theabsorption of silver sols, aswas shownbyBerryet al.21,45Changes to theabsorption spectra in thepresenceof specifically adsorbed sulfides or thiolswere reported byHerz.22 They showed that chemical complexation andadsorption could be used to drive the aerial oxidation ofthe silver.In Figure 6a, the adsorption of iodide ion onto a silver

sol is shown under nitrogen purging (<0.1 µM O2). Adrastic dampingof theplasmonband is evident. Thebandinitially remains at 382nmbut at higher coverages slowlyred-shifts. Solvated iodide ion possesses a CTTS band at229 nm, but there is initially no increase at 229 nmwheniodide ion is added. It is thereforedesolvatedas it adsorbs.Close to a monolayer of iodide ion can adsorb before thefurther addition of iodide leads to a clear increase in the229 nm CTTS band. Even stronger damping is observedwith mercaptans and sulfide ions. In all cases there isinitially almost no shift in the position of the surfaceplasmon band with adsorption, but close to monolayercoverage, a shift to longerwavelengths is observed. Thesechanges can be reversed by exposing the coated sol toγ-radiation in the presence of 2-propanol. Organicradicals transfer electrons to the sol (eq 14), cathodicallypolarizing them (Figure 6b). In the case of iodideadsorption, the peak immediately recovers to its initialintensity and position, and simultaneously the CTTSabsorption band of the solvated ion increases in intensityuntil it too attains its initial intensity. In the case ofsulfide ion, complete reversibility is not achieved. Sulfideion remains partly adsorbed even during vigorous hy-drogen evolution from silver sols.81

In order to model this chemical damping, a number ofsimple models can be suggested, based on a core-shellstructure. We treat the adsorbed iodide ion as a non-absorbing layerwithan ionicdiameter of 2.5Åandassumea real refractive index in this layer of n ) 1.66. (This is

(77) (a) Biggs, S.; Mulvaney, P. J. Chem. Phys. 1995, 100, 8501. (b)Mahanty, J.;Ninham,B.N.DispersionForces;AcademicPress: London,1976.

(78) (a) Heller,W.; Pugh, T. L. J. Polym. Sci. 1960, 47, 203. (b) Pugh,T. L.; Heller, W. J. Polym. Sci. 1960, 47, 219.

(79) The polarization of neutral molecules at a metal surface hasbeen considered in Antoniewicz, P. R. Surf. Sci. 1975, 52, 709.

(80) Mulvaney, P. In Electrochemistry of Colloids and Dispersions;Mackay, R. A., Texter, J. Eds.; VCH: New York, 1991.

(81) Linnert, T.;Mulvaney, P.;Henglein, A.J. Phys. Chem. 1993, 97,679.

Figure 5. (a) Absorption spectrum of 30 Å radius colloidalsilver in water. (b) The calculated difference spectrum due toan increase of 0.5% in the particle electron density. Mean freepath effects included in calculation. (c) The observed differencespectra obtained after electron injection into the particles byhydroxyalkyl radicals generated by pulse radiolysis.66


the value for KI, but the polarizability due to the iodideion dominates.) The results of such a calculation areshown in Figure 7 where the spectra for several coatingthicknesses are shown. They do not in any way explainthe observed damping. The nonabsorbing coat cannotintroduce any damping, and themodel predicts an almostlinear shift in the resonance frequency with coatingthickness. However the increased absorption and shiftto longer wavelengths do account for the effects of somestabilizers such as gelatin (n ) 1.5), PVP,21,45 andhydrophobically bound surfactant stabilizers.82The damping can also be modeled by treating the shell

layer as bulk silver iodide.Typical spectra are shown in Figure 8. It is clear that

the presence of silver iodide will induce some dampingbut also introduces absorption peaks due to excitonformation in the shell layer, as well as predicting pro-

nounced red-shifts in the band position. Thus the forma-tion of a bulk silver iodide surface phase appears to beruledout. Chemisorptionofanionsmustalter themetallicproperties of the silver core.A number of authors have, in the past, invoked

demetalization to explain phenomenaassociatedwith thesurface of metals.84-86 The adsorption of gases onto thinmetal filmshas longbeenknownto increase the resistivityof the film, and this has been variously attributed tochanges in the free electron concentration, to changes inspecularity, or to the formation of unusual surface phases.In fact, comparing the spectra in Figure 6a with those inFigure 1, it is evident that these model spectra mimickquite well the effects of chemisorption. Chemisorption ofanions appears to result in an increase in the dampingfrequency of the core conduction electrons. The majordifficulty is that if we interpret the data purely in termsof a reduction in electron mobility or electron mean freepath via eqs 10 and 11, then for SH- adsorption onto Agsols at pH 10.5, where the surface plasmon intensity

(82) Huy, T.; Mulvaney, P. To be submitted for publication.(83) Bedikyan, L. D.;Miloslavskii, V. K.; Ageev, L. A.Opt. Spectrosc.

(USSR) 1979, 47, 225.

(84) Gordon, J. G., II; Ernst, S. Surf. Sci. 1980, 101, 499.(85) Sonderheimer, E. H. Adv. Phys. 1952, 1, 1. Watanabe, M. Surf.

Sci. 1973, 34, 759.(86) Ehrlich, G. J. Chem. Phys. 1961, 35, 2165.

Figure 6. (a) Effect of added iodide ion on the surface plasmonband of colloidal silver. Experiment carried out undernitrogen,pH 5. Figures refer to added [KI]/µM (1) 0; (2) 2; (3) 6; (4) 20;(5) 30; (6) 40. (b) Effect of cathodic polarization on the spectrumof iodide damped silver colloids.80 Spectrum a is that of iodide-coated silver particles. Spectrum b is obtained after electrontransfer to the colloidusing radiolytically generated reductants.The iodide ion desorbs which causes a strong decrease in thesurface plasmon damping, and the blue shift is due to theincrease in the conduction band electron density.

Figure 7. Effect of a dielectric layer of KI on the absorptionspectrum of colloidal silver in water. Ag radius ) 3 nm, εm )1.77, εshell ) 2.76. Numbers refer to the shell layer thickness.Inset: Position of the surface plasmon band as a function of thecoating thickness.

Figure 8. Effect of an AgI shell on the spectrum of colloidalsilver inwater.Thicknessof shell layersare indicated.Agradius) 3 nm, εm ) 1.77. Dielectric data for AgI taken from ref 83.




decreases by about a factor of 5, themean free pathwouldhave to be decreased to just 6 Å.81 This implies almostcompletedemetalizationof theentire silvermetal particle!One might argue that the sulfide ion can diffuse into theparticle, but we consistently find that damping ceases ataround one monolayer coverage (in the absence of air).There is no doubt that the surface atoms themselves

are demetalized because it has beenwell established fromstudies of the underpotential deposition of anions ontosilver that the anion is oxidized as it adsorbs.87,88 Underopencircuit conditions, theoxidationof theanion is coupledtooxygenorprotonreduction.89 Underpotentialdepositionof iodide occurs at -0.6 V NHE,87 which is sufficient todrivehydrogenevolution inacid solution, untilmonolayercoverage is attained. The net effect is the formation of amonolayer of AgI via reaction 16.

A similar reaction takes place during the formation ofself-assembled alkanethiol monolayers on gold.90 Thereis no simple way to account for the intensity of thischemical damping at present. However, the fact that thedamping strength is correlated with the strength of theAg-anion bond suggests that the effect is not simply dueto a reduction in the electron mean free path but ratherto the channeling of the surface plasmon energy intoexcitationmodes of the surfacemetal-adsorbate complex.Perssonhas recentlypresentedamodelalong these lines.91These damping effects are not confined to silver particles,although theyaremostpronounced for silver sols. Similardamping experiments have now been carried out oncolloidal Cd and Pb.92 It is worth noting that the UV-visible spectrum of the water soluble metal cluster[Au55{Ph2P(m-C6H4SO3Na)}12Cl6] shows no surface plas-mon resonance. This has been attributed to size depend-ent single electron 5d-6s interband transitions,93 but itis equally likely that the covalently bonded ligands areresponsible for the damping, which is about twice thatpredicted by surface scattering (eq 11).Comparing the silver colloid spectra in Figure 6b with

those of Blatchford et al.,43 we see that in both casescathodic polarization produces an asymmetric increasein absorbance, and we conclude that when borohydride isadded to the citrate stabilized sol, the particles becomenegatively charged, but in addition, citrate ions desorb,reducing the plasmondamping. Conversely, the additionof silver ions to a silver sol will drive the redox potentialof the particles to very positive potentials, and a red-shiftis expecteddue toadecrease in electrondensity. Howeverthis anodic shift also induces adsorption of anionicstabilizers from solution causing plasmon damping, sothe band will broaden considerably, as indeed is ob-served.24,94 Finally, it is worth noting that in the case ofdye adsorption onto silver sols, strong coupling of theelectronic states of the dye with the metal is observed.95Not only is the colloid surface plasmon band red-shifted

due to the increased refractive index of the dye layer butthepositionof thedyeabsorptionband isalso red-shifted.95

In summary, the adsorption of complexing anions ontocolloidal silver results in strong damping of the surfaceplasmon absorption band. Demetalization of severalatomic layers would be necessary to account for thisprocess. Thedamping is stronger for anions that complexmore strongly with silver ions, suggesting that plasmonenergy is channeled into the electronic absorption bandsof the adsorbate complex. The degree of damping iscontrolled by the redox potential of the sol particles sincethis controls the degree of adsorption from solution.Electrostatic chargingof theparticlesbyreductants causesthe specifically adsorbed anions to desorb, which drasti-cally reduces the amount of surface plasmon damping.The intensity, position, and width of the surface plasmonbandare then similar to the values found for sols preparedin salt matrices.

6. Underpotential Deposition (upd) of Metals

In the previous sections we have examined the effectsof electron transfer and anion adsorption on the opticalspectrum of nanosized particles. Dramatic optical shiftsare also associated with the electrodeposition of metalatoms ontometal particles to formbilayer colloids. Whenelectrical charge is accumulated on metal particles, theyare cathodically polarized, and the surface plasmon bandis blue-shifted. In the presence of metal ions, elec-trodeposition of metal atoms can compete with protonreduction for the stored electrons.96 A number of papershave demonstrated that the formation of well-definedbilayer or multilayer metal particles is possible by thistechnique.97-101 The first case was Zsigmondy’s classicwork on the use of gold nuclei to formmonodisperse silverparticles.102 Bradley has recently presented a thoroughreview of much of the work on the preparation of mixedmetal particles in water and organic solvents.103 Most ofthe catalytically importantmetalsdonot showpronouncedsurface plasmon absorption because of damping by thed-d interband transitions. This leads to large values ofε′′ in the visible part of the spectrum which, through eq5, causesdampingof theplasmonoscillations. Effectively,the plasmon energy is lost by excitation of the singleelectron interband transitions.In Figure 9, the calculated spectra of lead-coated silver

sols is shownusing dielectric data fromLemonnier et al.63The spectra reproduce the experimental results exceed-ingly well.59 Initial reduction leads to a blue-shift andincrease in the band intensity as the particles arecathodically polarized. When the redox potential issufficiently negative, reduction of the metal ions begins,possibly in competition with H2 formation. The metaldepositioncausesstrongblue-shiftinganddamping,whichdistinguishes it from simple double-layer charging. Theshifts are extremely dramaticsa monolayer (ML) of leadatoms is capable of shifting the surface plasmon band by

(87) Wierse, D. G.; Lohrengel, M. M.; Schultze, J. W. J. Electroanal.Chem. 1978, 92, 121.

(88) Koppitz, F. D.; Schultze, J. W. Electrochim. Acta 1976, 21, 337.(89) Nazmutdinov, R. R.; Spohr, E. J. Phys. Chem. 1994, 98, 5956.(90) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem.

1991, 310, 335.(91) Persson, B. N. J. Surf. Sci. 1993, 281, 153.(92) Henglein, A.; Holzwarth, A. Submitted to J. Phys. Chem.(93) Schmid, G. In Clusters and Colloids; Schmid, G., Ed.; VCH

Publishers: Weinheim, 1994.(94) Henglein, A.; Mulvaney, P.; Linnert, T. Discuss. Faraday Soc.

1991, 92, 31.

(95) Kerker, M. J. Colloid Interface Sci. 1985, 105, 297.(96) Gerischer, H.; Kolb, D. M.; Sass, J. K. Adv. Phys. 1978, 27, 437.(97) Morris, R. H.; Collins, L. F. J. Chem. Phys. 1964, 41, 3357.(98) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1992,

96, 10419.(99) Henglein, A.; Mulvaney, P.; Linnert, A.; Holzwarth, A. J. Phys.

Chem. 1992, 96, 2411.(100) Henglein,F.;Mulvaney,P.;Henglein,A.Ber.Bunsen-Ges.Phys.

Chem. 1994, 98, 180.(101) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993,

97, 7061.(102) Zsigmondy, R.; Thiessen, P. A. Das Kolloidale Gold; Akadem.

Verlag: Leipzig, 1925.(103) Bradley, J. S. In Clusters and Colloids; Schmid, G., Ed.; VCH

Publishers: Weinheim, 1994.

Agn + mH+ + mI- f Agn-mAgIm + (m/2)H2 (16)


40 nm! This changes the color of the sol from yellow tobrown, or in the case of deposition onto gold sols fromruby red to brown. Thedeposition of just 0.1ML is readilydiscernible to thenaked eye. We cangain amore concreteunderstanding of the origin of these dramatic shifts byappealing to eq 6, the extinction cross section of a smallcoatedparticle. The condition for excitation of the surfacemode is that the denominator equal zero, or

Around monolayer coverage, g , 1 and we can simplifythis to

Assuming the real part of the dielectric functions for bothmetals is given by eq 7, we find on substitution into eq 18that

where λo is the position of the surface plasmon band priorto deposition,ωpc is the coremetal bulk plasma frequency,and∆ωpsc is the difference in the bulk plasma frequenciesof the shell and coremetals. (Wehaveneglected the smallshift due to the imaginary part of the metal dielectricfunctions.) ForCdandPb,whichhavesimilarbulkplasmaenergies of about 12 eV48,62 the slope of a plot of λ2 vs gis predicted to be around 0.8, which is fairly close to theexperimental value. In Figure 10, the experimentallyobserved position of the plasmon band is shown for Pbdeposition onto Ag together with the calculated positionusing bulk dielectric data and the simpler analyticexpression, eq19. It is clear fromeq18 that strong surfaceplasmon shifts are associated with large values of (εshell- εm). For dielectric coatings such as gelatin, surfactantmolecules, or citrate ions, (εshell - εm) is at most about 0.5,whereas for Pb, (ε′shell - εm) ∼ -16 at 380 nm.

The use of the bulk dielectric constants to model theshift due to deposition of less than a monolayer of metalatoms is clearly questionable, although it yields almostthe correct behavior. A simpler “chemical” explanationof the surface plasmon shifts during the deposition of thefirstmonolayer is as follows. Both lead and cadmiumaredivalent, and each adatom occupies a single lattice site,but contributes two electrons to the particle conductionband. The overall electron density increases (though theelectrical double layer is not being charged significantlybecause the extra charge is neutralized by the divalentlattice cation). The rate of plasmon band shift is thenpredicted to be just

where z is thedifference in valency of thePbandAg latticeions and Vm values are the molar volumes. This modelpredicts that all divalent metals will cause a blue shift ofthe surface plasmon band, but that monovalent adatomswill cause amuchweaker shift. This simplemodel is onlyvalid for submonolayer coverage, because it treats theadatoms as dopants, not as a discrete layer.

7. Catalyst PoisoningThe deposition of electropositive metals increases the

overpotential forhydrogen formationbyblocking theactivesites for H atom formation. It is therefore of interest toask whether deposition of metal atoms can compete withhydrogenevolutionon colloidalPtn,whichhasavery smalloverpotential for hydrogen formation. It has been shownthat radiolysis of Ptn sols in the presence of 2-propanolleads to the following reactions:104,105

The transfer of electrons to the sol is accompanied byproton uptake, converting the electrons to adsorbed

(104) Westerhausen, J.; Lindig,B.;Henglein,A.J.Phys.Chem.1981,85, 1627.

(105) Matheson, M. S.; Lee, P. C.; Meisel, D.; Pelizzetti, E. J. Phys.Chem. 1983, 87, 394.

Figure9. Calculated spectra ofPb-coatedAgparticles inwaterfor the first four monolayers of deposition assuming bulkdielectric behavior in the lead shell. Letters refer to spectra of3 nm radius silver with (a) no Pb, (b) 0.1 nm Pb, (c) 0.2 nm Pb,(d) 0.3 nmPb, (e) 0.7 nmPb, and (f) 1.5 nmPb shell. Note thesedielectric data reproduce the spectra of the puremetal solswell(Figure 4), but the small peak at 280 nm is an artifact of thedielectric data.63

εcore ) -2εshell [εshellg + εm(3 - g)]/[εshell(3 - 2g) +2εmg] (17)

εcore ∼ -2εm - 2g/3 (εshell - εm) (18)

λo2/λ2 ) 1 + 2g/3 ∆ωpsc

2/ωpc2 (19)

Figure 10. Plot of λ2 vs the mole fraction of lead in the coatedparticles for first fourmonolayers: (a) experimentally observedposition of the band;59 (b) using eq 19; (c) using bulk dielectricdata.Thesimpleadatommodel (eq20)predictsa slopeof around-1. Note no account is taken of the double layer charge whichwill cause a slight, additional blue-shift.

λo2/λ2 ) 1 + zVm(Pb)[Pb]/(Vm(Pb)[Pb] + Vm(Ag)[Ag])

(20)

2(CH3)2COH + Ptn f (CH3)2CO + Ptn(H)2 (21)

(H)2-Ptn f Ptn + H2(aq) (22)



hydrogen atoms. No conductivity changes are observeddue to particle charging, and hydrogen is evolved stoi-chiometrically, as seen from Figure 11.104,105 When thesame experiment is carried out in the presence of 0.1 mMPb2+, conductivity changes are seen immediately, due tothe deposition of lead adatoms:

The deposition of Pb poisons the hydrogen evolutionreaction, and the particles become coated with lead. InFigure 12, electron micrographs of the Pt sol particlesafter Pb deposition are shown.106 The lead mantle isclearly visible. Thus, underpotential deposition is alsocapable of interfering with very facile catalytic processeson nanosized metal particles.

8. Optical Detection of Alloy FormationAlthough core-shell structures have generally been

clearly identified by HRTEM following metal depositionon nanosized metal particles, alloy formation has some-times been postulated to account for differences betweenexperimental and observed spectra or to explain sluggishreoxidation of shell layers. The melting point of metalparticles decreases rapidly once the diameter is less than100Å,107 whichmeans that alloying and surface diffusionof adatomswill bemore facile on nanosized particles thanon bulk electrodes. Belloni et al. reported that simulta-neous reduction of Cu and Pd resulted in colloidal alloyformation,108 and recent work on coreduction of Ag, Au,and Pt salts by Liz-Marzan et al.109 suggested that alloyformation also occurred in these sytems. Alloy formationwas also reported recently for Sn reduction on colloidalAu.60 Berry and Skillman suggested that reaction of lead

and silver sols resulted in mixed metal particles,110although itwas subsequently found that the equilibrationof lead and silver sols takes place via underpotentialdeposition of lead onto the more noble metal.111 Duff etal. carried out extensive work on the nucleation of mixednoble metal sols, and presented TEM results and opticalspectra on a number of systems of catalytic interest.112Bradleyandco-workersdemonstrated inaseriesof elegantpapers onCOadsorption ontometal sols that the bondingto the different surface metals could be readily distin-guished.113 This may prove a useful chemical techniquefor distinguishing alloys and shell structures.The question arises whether the optical spectra alone

can reveal whether alloying has taken place duringdeposition of a second metal onto a seed metal particle.In the case of mixed gold-silver colloid particles, this canbedoneby comparing theexperimental absorption spectraof the “coated” particles with calculated spectra for thegold-coated silver sol and the calculated gold-silver alloysol based on the alloy dielectric data.114,115 The calculatedspectra are shown in Figure 13, and it is obvious thatwhereas gold deposition should result only in damping ofthe underlying silver surface plasmon band, alloy forma-tion is accompanied by a continuous red-shift in the bandwith increasing gold content. In Figure 14 the positionof the surface plasmon bands and the experimentallyobserved positions101 are plotted as a function of themolefraction of Au in the bimetallic particles. It is apparentthat the strong red-shifting observed experimentally isbest explained by spontaneous alloy formation during theelectrodeposition process. Papavassiliou has preparedalloy colloids of gold and silver by evaporation andcondensation of the alloys,116 and the observed position oftheplasmonbandsare indeedconsistentwith thepositionspredicted from the dielectric data of Ripken.115The origin of the red-shift in the case of the AuAg alloy

spectra is quite interesting. Silver andgoldhave identicalbulk plasma frequencies, so a peak shift due to a changingelectron density is not expected. However, the high-frequency dielectric constants are quite different, prima-rilybecause the interbandtransitions ingoldextendacrossmost of the visible spectrum. The absorption band shiftin this case is due to the perturbation of the d-band energylevelsandnot to changes in the freeelectronconcentration.This results in a steady increase in the effective value ofε∞ for thealloyand, consequently, a red-shift in thepositionof the absorption band. The fact that a linear shift isfound, as observed experimentally by Papavassiliou andas predicted using Ripken’s data, can be explained if thealloy dielectric function takes the form, ε(R) ) (1 - R) εAg+ RεAu, where R is the mole fraction of Au in the particle.

9. The Mean Free Path in the Shell Layer

While electron microscopy reveals quite homogeneousdeposition once the particle size has significantly in-creased, it is difficult to assess how homogeneously thefirst fewmonolayers are deposited because the small sizechanges are dwarfed by the core particle size distribution.We can in principle characterize the quality of the shelllayer in termsof theelectronmean freepath. Forasphere,

(106) The author is indebted toM. Giersig for providing the electronmicrographs.

(107) (a) Buffat, P. A.; Borel, J.-P. Phys. Rev. A 1976, 13, 2287. (b)Sambles J. R. Proc. R. Soc. London, A 1971, 324, 339.

(108) Marignier, J.; Belloni, J.; Delcourt, M.; Chevalier, J. Nature1985, 317, 344.

(109) Liz-Marzan,Luis;Philipse,A.P.J.Phys.Chem.1995,41, 15120.

(110) Berry, C. R.; Skillman, D. C. J. Photogr. Sci. 1969, 17, 145.(111) Henglein,A.;Holzwarth,A.;Mulvaney,P.J.Phys.Chem.1992,

96, 8700.(112) Duff, D. Ph.D. Thesis, Cambridge University, 1989.(113) (a) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.; Chaudret,

B.; Duteil, A. Chem. Mater. 1992, 4, 1234. (b) Bradley, J. S.; Millar, J.M.;Hill, E.W.;Melchior,M.;J. Chem. Soc., Chem.Commun. 1990, 705.(c) Mucalo, M. R.; Cooney, R. P. J. Colloid Interface Sci. 1992, 150, 486.

(114) Schluter, M. Z. Phys. 1972, 250, 87.(115) Ripken, K. Z. Phys. 1972, 250, 228.(116) Papavassiliou, G. C. J. Phys. F: Met. Phys. 1976, 6, L103.

Figure 11. Conductivity of ion-exchanged and evacuatedsolutions of 0.3 mM colloidal Pt at pH 4.0 as a function of theirradiation time at 8.5 × 104 rad h-1 in the presence of 0.1 M2-propanol and 0.01 M acetone in the absence of lead and inthe presence of 0.1 mM Pb2+ and 0.2 mM Pb2+. The plateauvalues correspond to complete reduction of lead ions and theslopes yield G(Pb0) ) 2.04. Sodium poly(vinylsulfonate) (0.1mM) was used to stabilize the sols.

2(CH3)2COH + Pb2+ + Ptn f

PtnPb + 2H+ + 2(CH3)2CO (23)


the mean free path was shown by Euler to be equal to theradius of the sphere,R.117 Granqvist et al. proposed thatthe mean free path in the shell layer of a layered particleshould be given by

where dcoat is the diameter of the coated particle and dcorethe diameter of the core.118 In Figure 15, the absorptionspectra obtained from the chemical reduction of silver

ontoPdcolloidsare shown,and inFigure16, the calculatedspectra are shown. Pd was chosen as a core because it isnoble and has a fairly featureless absorption spectrum.Silverwas chosenas the shellmaterial because it displaysstrongly size-dependent, surfaceplasmonbroadening.Foreach coating thickness ofAg (found byHRTEM) themeanfree path was calculated according to eq 24, and thedielectric function of the silver layer was corrected usingeqs 7, 8, and 11. The chemical deposition of silver onto

(117) Euler, J. Z. Phys. 1954, 137, 318.(118) Granqvist, C. G.; Hunderi, O. Z. Phys. 1978, B30, 47.

(119) Michaelis,M.;Henglein,A.;Mulvaney, P.J.Phys.Chem.1994,98, 6212.

Figure 12. High-resolution electronmicrographs of colloidal Pt particles after radiolysis in the presence of Pb2+. Well-defined coreshell particles of PtPb are formed. The spectrumon the left shows the coating to be fairly homogeneous, the higher resolution pictureon the right clearly shows the lattice planes of bulk lead. The PtPb particles were prepared as outlined in Figure 11.

Figure 13. (left) Calculated spectra of 6 nm sized particles of AuAg alloys of various composition in water using full Mie equations,and the dielectric data for the alloys from refs 114 and 115. No mean free path effects were used, since the damping frequencyin the alloys is unknown. The numbers refer to themole fraction of gold. (right) Calculated spectra of Au-coated Ag colloids inwater.Core radius ) 3.0 nm. Note the silver plasmon band is strongly damped but does not shift. Thick coatings show a surface plasmonresonance in the gold layer close to 500 nm. Surface scattering in the core is included. Gold layer thicknesses used were 0, 0.32,0.6, 1.0, 1.5, 2.0, and 3.0 nm.

R ) {(dcoat - dcore)(dcoat2 - dcore

2)}1/3/2 (24)


http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/la9502711&iName=master.img-004.jpg&w=459&h=226


colloidal palladium results in the appearance of a surfaceplasmon resonance at about 340 nm which red-shiftstoward the bulk value as the coating thickness increases.The Mie calculations reproduce the shift in the peakposition very well as seen in Figure 17a, which suggeststhe shell thicknesses determined by HRTEM were quiteaccurate. However, the observed peaks are very muchbroader than those predicted. The plasmon oscillationsshould have been evident at 340 nm and just 2 MLcoverage. The broader peaks imply larger damping andtherefore that the electron mean free path (MFP) in theshell layer is smaller than that predicted by eq 24. (Notethe decrease in the MFP causes only a very small shift inthe peak position of (2 nm, as shown in Figure 17a.) In

Figure 17b, the mean free path, as calculated from thepeak intensity, is showntogetherwith thevaluespredictedby eq 24. It is clear that in all cases the MFP in the shelllayer is at least a factor of two smaller than theGranqvistmodelpredicts. Underpotential deposition leads toa largenumber of nucleation sites and, as these patches mergeto form the apparently homogeneous shell layer, consid-erable lattice strain must exist. TheMFP is governed bythepatch size. Interestingly,when the coated sol particleswere subsequently aged at room temperature aftercomplete reduction of the silver in solution, the plasmonband increased in intensity. Aging must allow someannealing of the patches, and this leads to a slow increasein the crystallinity of the shell layer and a correspondingincrease in the electron mobility, which results in anincrease in the intensity of the surfaceplasmonabsorptionband.

ConclusionsIn this review, a number of chemical effects that alter

the optical behavior of small metal particles have beenidentified and discussed. It is obvious that opticalmeasurements can provide important insights into theprocesses of redox catalysis on nanosizedmetal particles.Turkevich long ago mused that it may be simpler and

perhaps also more accurate to determine the dielectricfunction of a metal from the colloid extinction spectrum,rather than from reflectivity measurements made invacuo. It is clear fromFigure 4 that whereas this is quitereasonable in some cases, for metals where double layereffects are important, this procedure can only be adoptedonce the chemical perturbations to the extinction spectradiscussed in this review have been accounted for. Forthis reason, it is stillworthwhile comparing themeasuredspectrawithones calculatedusingdielectric data obtainedindependently, and this is theprocedurewehaveadopted.Particularly interesting is the fact that in the cluster

regimewhere covalent bonding is so important to preventcoalescence, plasmondamping is very pronounced.93 Theresults of the experiments on anion adsorption ontometallic silver particles substantiate the important hy-pothesis that the ligands may largely determine theconduction electron mobility in metal clusters.It is also worthwhile reflecting on the implications that

anydielectricmodulationhasondouble layer interactions.Metals have large Hamaker constants precisely becausethe free electrons contribute enormously to the polariz-

Figure 14. (a) Experimentally observed position for goldelectrodeposited onto silver sols.101 (b) Position of the surfaceplasmon band in nanosized colloids of AuAg alloys for variousmole fractions of Au in the particles. (c) Positions predicted forAu-coated Ag sols. The coated particle shows two resonancesdue to the silver core and for highermole fractions of Au a banddue to the shell.

Figure 15. Experimental absorption spectra of Ag-coated Pdcolloids in water.119 Numbers refer to monolayers of Agcalculated fromparticle sizes foundbyHRTEM.ThePd colloidshad a radius of 4.6 nm. [Pd] ) 2.9 × 10-5 M.

Figure 16. Calculated extinction spectra of Ag-coated Pdparticles assuminga core radius of 4.6 nm. 1monolayer)0.257nm. Dielectric data for Pd from ref 55. Mean free path in shelllayer calculated from eq 24. Figures refer to monolayers of Ag.


ability of the particles. Shifts in ωp induced by electrontransfer will alter the value of the Hamaker constant ofthe colloidal metal particle and, thus, the strength of thevan der Waals interaction becomes a function of theelectrostatic charge on the particle.120 The fundamentaltenet of DLVO theorysthe additivity of van der Waalsand electrostatic contributions to the double layer

forcessdoesnot strictlyapply tometalparticles. Theeffectis, of course, most pronounced in the case of very smallparticles, where the ratio of the double layer to volumecharge is most significant.

Acknowledgment. The author thanks his colleaguesat the Hahn-Meitner Institute in Berlin for many stimu-lating discussions. The receipt of an ARCQEII ResearchFellowship and the financial support of the AdvancedMineral Products Research Centre are gratefullyacknowledged.

LA9502711

(120) Within the dipole approximation, it can be shown that for twosmall metal particles, the van der Waals interaction energy takes theformU ) 3R6pωm/4d6, where R is the particle radius, d the separation,and ωm

2 ) ωp2/3. An increase in the stored electron concentration will

help stabilize the particles electrostatically but is partially offset by anincrease in the van derWaals interaction energy between the particles.Conversely however, sols stabilized by excess silver ions shouldexperience smaller dispersion forces for similar (positive) surface chargedensities.

Figure 17. (a) Comparison of experimental and calculated positions of silver shell surface plasmon absorption band. Curve aassumes damping is given by eq 24. Curve b is with increased damping to give correct intensity of the plasmon band. (b) Predictedand observed values of the electron mean free path in the silver shell layer, based on eq 24. The experimental values indicate theelectron mobility in the shell is a factor of 2 lower than that predicted by eq 24.


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Surface Plasmon Spectroscopy of Nanosized Metal