Surface Plasmon Spectroscopy of Nanosized Metal, Notas de estudo de Engenharia Elétrica

Baixe Surface Plasmon Spectroscopy of Nanosized Metal e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! Surface Plasmon Spectroscopy of Nanosized Metal Particles 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 metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leads toaniondesorption. Underpotential deposition (upd) of electropositivemetal layers results indramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in the shell. Introduction Interest in theopticalpropertiesof colloidalmetalsdates back toRoman times. Nanosizedgoldparticleswere often used as colorants in glasses, and quite complex optical effects were created using metal particles.1 In the seventeenth century, “Purple of Cassius”, a colloid of heterocoagulated tin dioxide and gold particles, became apopular colorant inglasses.2 These earlymanifestations of the unusual colors displayed bymetal particles prompted Faraday’s investigations into the colors of colloidal gold in the middle of the last century. Today his studies are generally considered to mark the foundations of modern colloid science.3 The formation of color centers and small colloidalmetal particles in ionicmatrices and glasses has remained an area of very active research,4-6 driven, in part, by the technical importance of the photographic process.7 However, colloid chemistshave tended toneglect the study of metal particles in aqueous solution because of their complicated double layer structure,which ismore amenable to direct electrochemical investigation. The more recentdiscovery that the surfaceplasmonabsorption band can also provide information on the development of the band structure in metals8-11 has led to a plethora of studies on the size dependent optical properties of metal particles, particularly those of silver and gold,12-17 while advances in molecular beam techniques now enable spectroscopic analysis of metal clusters to be carried out in vacuum.18,19 Although many of the optical effects associated with nanosized metal particles are now reasonably well un- derstood, thereare largediscrepanciesbetween theoptical properties of metal sols prepared in water, particularly those of silver, and sols prepared in other matrices.6,20-27 In a recent review Kreibig noted that while much work has been done to isolate matrix effects and to determine the roles of defects, grain boundaries, crystallinity, and polydispersity on the optical properties of sols, little is knownabout thewayspecific surface chemical interactions may influence the absorption of light by small metal particles.28 These differences are attributed to unique double layer effects present at themetal-water interface. This review focuses on some of these surface chemical effects, and attempts to show how changes to the surface plasmon absorption band of aqueous metal colloids can be related to electrochemical processes occurring atmetal particle surfaces. Simple models are proposed to explain some 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 containing N particles per unit volume, the measured attenuation of light of intensity Io, over a pathlength d cm is given by X 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 theLycurgus Cupwhich is ruby red in transmitted light butappears green in reflected light. The color is due to colloidal gold. It was manufactured in the 4th century AD. (2) See: Thiessen, P. A. Kolloid Z. 1942, 101, 241, for micrographs of 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) Römer, 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) Tiggesbaümker, J.; Köller, 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 dependent dielectric function  ) ′ + i′′, embedded in a medium of dielectric function m, Cext is given by29-33 where k ) 2π xm/λ and an and bn are the scattering coefficients, which are functions of the radius R and the wavelength λ in terms of Ricatti-Bessel functions. The extinction cross section of a particle is often normalized to 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 colloid concentration is themolarmetal atom concentration.This quantity 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 electrostatic arguments, and a clear derivation is given by Genzel and Martin.34 In many cases to be described here, it will be necessary to consider the perturbation introduced by a thin surface layer. The extinction cross section of a small, concentric sphere is given by32 where core is the complex dielectric function of the core material, shell is that of the shell, m is the real dielectric function of the surrounding medium, g is the volume fraction of the shell layer, andR is the radius of the coated particle. When g) 0, eq 6 reduces to eq 5 for an uncoated sphere, and for g ) 1, eq 6 yields the extinction cross section 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 by the free electron behavior, and the dielectric response is well described by the simple Drude model. According to this theory,35 the real and imaginaryparts of thedielectric function may be written where ∞ is the high frequency dielectric constant due to interband and core transitions and ωp is the bulk plasma frequency in terms of N, the concentration of free electrons in the metal, andm, the effectivemass of the electron. ωd is the relaxation or damping frequency, which is related to the mean free path of the conduction electrons,Rbulk, and the velocity of electrons at the Fermi energy, vf, by When the particle radius, R, is smaller than the mean free path in the bulk metal, conduction electrons are additionally scattered by the surface, and the mean free path, Reff, becomes size dependent with Equation 11 has been experimentally verified by the extensivework ofKreibig for both silver andgoldparticles right down to a size of 2 nm.14,16,28 The advantage of the Drude model is that it allows changes in the absorption spectrumtobe interpreteddirectly in termsof thematerial properties of the metal. The origin of the strong color changes displayed by small particles lies in the denomi- nator of eq 5,whichpredicts the existence of anabsorption peak when From eq 7 it can be seen that over the whole frequency regime below the bulk plasma frequency of a metal, ′ is negativewhich isdue to the fact that theelectronsoscillate out of phasewith the electric field vector of the lightwave. This is why metal particles display absorption spectra which are strong functions of the size parameter, kR. In a small metal particle the dipole created by the electric field of the lightwavesetsupasurfacepolarization charge, which effectively acts as a restoring force for the “free electrons”. The net result is that, when condition 12 is fulfilled, the longwavelengthabsorptionby thebulkmetal is condensed into a single, surface plasmon band. In the case of semiconductor crystallites, the free electron concentration is orders of magnitude smaller, even in degenerately doped materials (i.e., ωp is smaller), and as aresult surfaceplasmonabsorptionoccurs in the IR, rather than in the visible part of the spectrum. Semiconductor crystallites therefore do not change color significantly when the particle size is decreased below the wavelength of visible light, although the IR spectrummaybe affected. It should be noted that the strong color changes observed when semiconductor crystallites are in the quantum size regime (R < ∼50 Å), are due to the changing electronic band structure of the crystal, which causes the dielectric function of the material itself to change. In Figure 1, a “typical” surface plasmon band is shown calculated using eq 5with parameters typical of silver for several values of the damping parameter ωd. The most importantparameteraffectingωd is theparticle size.From eqs 10 and 11 it can be seen that decreases in the particle size lead to an increase inωd, causing the band to broaden and the maximum intensity to decrease. The position of the 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. ′ ) ∞ - ωp 2/(ω2 + ωd 2) (7) ′′ ) ωp 2ωd/ω(ω 2 + ωd 2) (8) ωp 2 ) Ne2/mo (9) ωd ) vf /Rbulk (10) 1/Reff ) 1/R + 1/Rbulk (11) ′ ) -2m (if ′′ small) (12) A ) log10 Io/Id ) NCextd/2.303 (1) Cext ) 2π/k 2∑(2n + 1) Re (an + bn) (2) Qext ) Cext/πR 2 (3) (M-1 cm-1) ) (3 × 10-3)VmQext/4(2.303)R (4) Cext ) 24π2R3m 3/2 λ ′′ (′ + 2m) 2 + ′′2 (5) Cext ) 4πR 2k* × Im{ (shell - m)(core - 2shell) + (1 - g)(core - shell)(m + 2shell)(shell + 2m)(core + 2shell) + (1 - g)(2shell - 2m)(core - shell)} (6) Optical Properties of Metal Particles Langmuir, Vol. 12, No. 3, 1996 789 difference spectrum is also shown. Thedielectric function ofbulksilverwas firstmodified to take intoaccount surface scattering. Then the plasma frequency was increased to mimic the polarization effects. As is evident, the general shape is similar, but the asymmetry in the experimental difference spectrum is not reproduced. Note that the citrate sols in Blatchford’s work also showed a very pronounced, asymmetric increase in absorbance in the presence of borohydride. The steady-state electron con- centration is determined by the redox potential of the reductant used to prepare the sol. The most blue-shifted spectra are found when the preparation utilizes boro- hydride as a reductant (λmax∼ 376 nm). Citrate is amuch weakerreductantandtheFermi level in thesilverparticles lies at much more positive values. In this case the maximum usually lies between 385 and 390 nm. These optical changes are analogous to the electroreflectance effects observed with silver electrodes upon cathodic polarization but are far more pronounced.67-72 The electroreflectance studies have also revealed that anion adsorption and desorption occur concomitantly.73-76 It will be shown below that the asymmetry in the difference spectrum is associated with changes in the degree of adsorption of the stabilizing anions with particle charge. Nanosized metal particles have an enormous double layer capacity and in changing the environment in the solution from oxidizing to reducing, a large double layer chargemust build up on the particles. If the double layer capacity is taken to be 20 µF cm-2, a crude estimate suggests that a 14% change in the electron density is needed if the redox potential in a 30 Å diameter particle (67) Feinleib, J. Phys. Rev. Lett. 1966, 16, 200. (68) Kolb, D. M.; Kötz, 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 dotted curves are the experimental spectra. Data sources for experimental spectra: Cd, ref 58; Pb, ref 59; Sn, ref 60; Cu, ref 61. Dielectric function data sources: Cd, ref 62; Pb, ref 63; Sn, ref 64; Cu, ref 65. 792 Langmuir, Vol. 12, No. 3, 1996 Mulvaney is to be raised from the ambient level in air of about +0.4 VNHEto around-0.4VNHEwhereH2 evolution begins. This corresponds to a plasmon band shift from 390 to 375 nm. This is clearly consistent with the fact that citrate stabilized silver sols typically have a band around 385- 390nmwhereasextensivelyγ-irradiatedsilver sols (strong reducing conditions) possess absorption bands at λ < 380 nm and steadily evolve hydrogen. (For very small particles, the shift in the Fermi level is not due just to the double layer charging. The increase in the conduction band electron concentration also causes an additional increase 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 polycrystalline silver (-0.7 VNHE) chemically produced sols will always contain residual electrical chargeandwill possessplasmon absorption bands shifted from the position expected for anelectricallyneutralparticle. Wehavealready identified this wavelength by reference to peak positions in salt matrices where electrostatic charging can be neglected (Figure 2). An uncharged silver colloid should have a maximum at 382 ( 1 nm; wavelengths shorter than this are due to cathodic polarization, and longer wavelengths are due to incomplete reduction of silver ions. Blatchford et al. in fact reached similar conclusions, but their calculated spectra for the cathodically polarized silver particles had two peaks, because they assumed that the charge was confined to a very thin surface layer of thickness 2 Å, corresponding to the Fermi screening length.43 ωp was only increased in this thin shell layer, resulting in theappearanceof twoabsorptionpeaks,which is not observed experimentally. Smearing out the double layer charge by increasing ωp throughout the particles gives better agreement with experiment. These surface plasmon shifts could readily form the basis for very efficient electrochromic switching. Electrical polarization of a film of nanosized silver colloids can alter the position of the band by 20 nm quite reversibly. 5. Chemisorption When metal particles are prepared in solution, they must be stabilized against the vanderWaals forceswhich cause coagulation.77 This can be achieved in a number of ways: physisorbed surfactant and polymers create steric or electrosteric barriers,78 physisorption of ions produces purely electrostatic barriers,while depletion stabilization occurs in the presence of some polymers due to osmotic pressure.78 In many cases, the distinction between chemical adsorption (involving direct covalent bonding with thesurfacemetalatoms)andmoresubtleelectrostatic mechanisms (e.g. charge-induceddipolemechanismsand dispersion force mechanisms79 ) is largely a matter of degree. The problem distinguishing chemisorption and physisorption is further exacerbated in the case of metal clusters containing only a few tens of metal atoms, where it is not clear whether the stabilizingmolecules should be denoted “adsorbates” or “ligands”. Is the electrostatic charge on such a cluster containing, say, tenmetal atoms to be considered as a double layer charge or as an intrinsic molecular charge? What is clear is that for the coinage metals, silver and gold, sols prepared with different stabilizers often have quite different absorption spectra even though theparticle size distributionsappear similar. It is therefore pertinent to ask whether chemisorption of “ligands” can alter the optical properties of the sol. The gelatinused to stabilize photographic emulsions certainly affects theabsorption of silver sols, aswas shownbyBerry et al.21,45Changes to theabsorption spectra in thepresence of specifically adsorbed sulfides or thiolswere reported by Herz.22 They showed that chemical complexation and adsorption could be used to drive the aerial oxidation of the silver. In Figure 6a, the adsorption of iodide ion onto a silver sol is shown under nitrogen purging (<0.1 µM O2). A drastic dampingof theplasmonband is evident. Theband initially remains at 382nmbut at higher coverages slowly red-shifts. Solvated iodide ion possesses a CTTS band at 229 nm, but there is initially no increase at 229 nmwhen iodide ion is added. It is thereforedesolvatedas it adsorbs. Close to a monolayer of iodide ion can adsorb before the further addition of iodide leads to a clear increase in the 229 nm CTTS band. Even stronger damping is observed with mercaptans and sulfide ions. In all cases there is initially almost no shift in the position of the surface plasmon band with adsorption, but close to monolayer coverage, a shift to longerwavelengths is observed. These changes can be reversed by exposing the coated sol to γ-radiation in the presence of 2-propanol. Organic radicals transfer electrons to the sol (eq 14), cathodically polarizing them (Figure 6b). In the case of iodide adsorption, the peak immediately recovers to its initial intensity and position, and simultaneously the CTTS absorption band of the solvated ion increases in intensity until it too attains its initial intensity. In the case of sulfide ion, complete reversibility is not achieved. Sulfide ion remains partly adsorbed even during vigorous hy- drogen evolution from silver sols.81 In order to model this chemical damping, a number of simple models can be suggested, based on a core-shell structure. We treat the adsorbed iodide ion as a non- absorbing layerwithan ionicdiameter of 2.5Åandassume a 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 has been 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 colloidal silver in water. (b) The calculated difference spectrum due to an increase of 0.5% in the particle electron density. Mean free path effects included in calculation. (c) The observed difference spectra obtained after electron injection into the particles by hydroxyalkyl radicals generated by pulse radiolysis.66 Optical Properties of Metal Particles Langmuir, Vol. 12, No. 3, 1996 793 the value for KI, but the polarizability due to the iodide ion dominates.) The results of such a calculation are shown in Figure 7 where the spectra for several coating thicknesses are shown. They do not in any way explain the observed damping. The nonabsorbing coat cannot introduce any damping, and themodel predicts an almost linear shift in the resonance frequency with coating thickness. However the increased absorption and shift to longer wavelengths do account for the effects of some stabilizers such as gelatin (n ) 1.5), PVP,21,45 and hydrophobically bound surfactant stabilizers.82 The 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 damping but also introduces absorption peaks due to exciton formation 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 be ruledout. Chemisorptionofanionsmustalter themetallic properties of the silver core. A number of authors have, in the past, invoked demetalization to explain phenomenaassociatedwith the surface of metals.84-86 The adsorption of gases onto thin metal filmshas longbeenknownto increase the resistivity of the film, and this has been variously attributed to changes in the free electron concentration, to changes in specularity, or to the formation of unusual surface phases. In fact, comparing the spectra in Figure 6a with those in Figure 1, it is evident that these model spectra mimick quite well the effects of chemisorption. Chemisorption of anions appears to result in an increase in the damping frequency of the core conduction electrons. The major difficulty is that if we interpret the data purely in terms of a reduction in electron mobility or electron mean free path via eqs 10 and 11, then for SH- adsorption onto Ag sols 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 plasmon band 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 spectrum of iodide damped silver colloids.80 Spectrum a is that of iodide- coated silver particles. Spectrum b is obtained after electron transfer to the colloidusing radiolytically generated reductants. The iodide ion desorbs which causes a strong decrease in the surface plasmon damping, and the blue shift is due to the increase in the conduction band electron density. Figure 7. Effect of a dielectric layer of KI on the absorption spectrum of colloidal silver in water. Ag radius ) 3 nm, m ) 1.77, shell ) 2.76. Numbers refer to the shell layer thickness. Inset: Position of the surface plasmon band as a function of the coating thickness. Figure 8. Effect of an AgI shell on the spectrum of colloidal silver inwater.Thicknessof shell layersare indicated.Agradius ) 3 nm, m ) 1.77. Dielectric data for AgI taken from ref 83. 794 Langmuir, Vol. 12, No. 3, 1996 Mulvaney hydrogen atoms. No conductivity changes are observed due to particle charging, and hydrogen is evolved stoi- chiometrically, as seen from Figure 11.104,105 When the same experiment is carried out in the presence of 0.1 mM Pb2+, conductivity changes are seen immediately, due to the deposition of lead adatoms: The deposition of Pb poisons the hydrogen evolution reaction, and the particles become coated with lead. In Figure 12, electron micrographs of the Pt sol particles after Pb deposition are shown.106 The lead mantle is clearly visible. Thus, underpotential deposition is also capable of interfering with very facile catalytic processes on nanosized metal particles. 8. Optical Detection of Alloy Formation Although core-shell structures have generally been clearly identified by HRTEM following metal deposition on nanosized metal particles, alloy formation has some- times been postulated to account for differences between experimental and observed spectra or to explain sluggish reoxidation of shell layers. The melting point of metal particles decreases rapidly once the diameter is less than 100Å,107 whichmeans that alloying and surface diffusion of adatomswill bemore facile on nanosized particles than on bulk electrodes. Belloni et al. reported that simulta- neous reduction of Cu and Pd resulted in colloidal alloy formation,108 and recent work on coreduction of Ag, Au, and Pt salts by Liz-Marzan et al.109 suggested that alloy formation also occurred in these sytems. Alloy formation was also reported recently for Sn reduction on colloidal Au.60 Berry and Skillman suggested that reaction of lead and silver sols resulted in mixed metal particles,110 although itwas subsequently found that the equilibration of lead and silver sols takes place via underpotential deposition of lead onto the more noble metal.111 Duff et al. carried out extensive work on the nucleation of mixed noble metal sols, and presented TEM results and optical spectra on a number of systems of catalytic interest.112 Bradleyandco-workersdemonstrated inaseriesof elegant papers onCOadsorption ontometal sols that the bonding to the different surface metals could be readily distin- guished.113 This may prove a useful chemical technique for distinguishing alloys and shell structures. The question arises whether the optical spectra alone can reveal whether alloying has taken place during deposition of a second metal onto a seed metal particle. In the case of mixed gold-silver colloid particles, this can bedoneby comparing theexperimental absorption spectra of the “coated” particles with calculated spectra for the gold-coated silver sol and the calculated gold-silver alloy sol based on the alloy dielectric data.114,115 The calculated spectra are shown in Figure 13, and it is obvious that whereas gold deposition should result only in damping of the underlying silver surface plasmon band, alloy forma- tion is accompanied by a continuous red-shift in the band with increasing gold content. In Figure 14 the position of the surface plasmon bands and the experimentally observed positions101 are plotted as a function of themole fraction of Au in the bimetallic particles. It is apparent that the strong red-shifting observed experimentally is best explained by spontaneous alloy formation during the electrodeposition process. Papavassiliou has prepared alloy colloids of gold and silver by evaporation and condensation of the alloys,116 and the observed position of theplasmonbandsare indeedconsistentwith thepositions predicted from the dielectric data of Ripken.115 The origin of the red-shift in the case of the AuAg alloy spectra is quite interesting. Silver andgoldhave identical bulk plasma frequencies, so a peak shift due to a changing electron density is not expected. However, the high- frequency dielectric constants are quite different, prima- rilybecause the interbandtransitions ingoldextendacross most of the visible spectrum. The absorption band shift in this case is due to the perturbation of the d-band energy levelsandnot to changes in the freeelectronconcentration. This results in a steady increase in the effective value of ∞ for thealloyand, consequently, a red-shift in theposition of the absorption band. The fact that a linear shift is found, as observed experimentally by Papavassiliou and as predicted using Ripken’s data, can be explained if the alloy dielectric function takes the form, (R) ) (1 - R) Ag + RAu, 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 homogeneous deposition once the particle size has significantly in- creased, it is difficult to assess how homogeneously the first fewmonolayers are deposited because the small size changes are dwarfed by the core particle size distribution. We can in principle characterize the quality of the shell layer in termsof theelectronmean freepath. Forasphere, (106) The author is indebted toM. Giersig for providing the electron micrographs. (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. Nature 1985, 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) Schlüter, 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 evacuated solutions of 0.3 mM colloidal Pt at pH 4.0 as a function of the irradiation time at 8.5 × 104 rad h-1 in the presence of 0.1 M 2-propanol and 0.01 M acetone in the absence of lead and in the presence of 0.1 mM Pb2+ and 0.2 mM Pb2+. The plateau values correspond to complete reduction of lead ions and the slopes yield G(Pb0) ) 2.04. Sodium poly(vinylsulfonate) (0.1 mM) was used to stabilize the sols. 2(CH3)2COH + Pb 2+ + Ptn f PtnPb + 2H + + 2(CH3)2CO (23) Optical Properties of Metal Particles Langmuir, Vol. 12, No. 3, 1996 797 the mean free path was shown by Euler to be equal to the radius of the sphere,R.117 Granqvist et al. proposed that the mean free path in the shell layer of a layered particle should be given by where dcoat is the diameter of the coated particle and dcore the diameter of the core.118 In Figure 15, the absorption spectra obtained from the chemical reduction of silver ontoPdcolloidsare shown,and inFigure16, the calculated spectra are shown. Pd was chosen as a core because it is noble and has a fairly featureless absorption spectrum. Silverwas chosenas the shellmaterial because it displays strongly size-dependent, surfaceplasmonbroadening.For each coating thickness ofAg (found byHRTEM) themean free path was calculated according to eq 24, and the dielectric function of the silver layer was corrected using eqs 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 core shell particles of PtPb are formed. The spectrumon the left shows the coating to be fairly homogeneous, the higher resolution picture on 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 frequency in 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 plasmon resonance 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)(dcoat 2 - dcore 2)}1/3/2 (24) 798 Langmuir, Vol. 12, No. 3, 1996 Mulvaney colloidal palladium results in the appearance of a surface plasmon resonance at about 340 nm which red-shifts toward the bulk value as the coating thickness increases. The Mie calculations reproduce the shift in the peak position very well as seen in Figure 17a, which suggests the shell thicknesses determined by HRTEM were quite accurate. However, the observed peaks are very much broader than those predicted. The plasmon oscillations should have been evident at 340 nm and just 2 ML coverage. The broader peaks imply larger damping and therefore that the electron mean free path (MFP) in the shell layer is smaller than that predicted by eq 24. (Note the decrease in the MFP causes only a very small shift in the peak position of (2 nm, as shown in Figure 17a.) In Figure 17b, the mean free path, as calculated from the peak intensity, is showntogetherwith thevaluespredicted by eq 24. It is clear that in all cases the MFP in the shell layer is at least a factor of two smaller than theGranqvist modelpredicts. Underpotential deposition leads toa large number of nucleation sites and, as these patches merge to form the apparently homogeneous shell layer, consid- erable lattice strain must exist. TheMFP is governed by thepatch size. Interestingly,when the coated sol particles were subsequently aged at room temperature after complete reduction of the silver in solution, the plasmon band increased in intensity. Aging must allow some annealing of the patches, and this leads to a slow increase in the crystallinity of the shell layer and a corresponding increase in the electron mobility, which results in an increase in the intensity of the surfaceplasmonabsorption band. Conclusions In this review, a number of chemical effects that alter the optical behavior of small metal particles have been identified and discussed. It is obvious that optical measurements can provide important insights into the processes of redox catalysis on nanosizedmetal particles. Turkevich long ago mused that it may be simpler and perhaps also more accurate to determine the dielectric function of a metal from the colloid extinction spectrum, rather than from reflectivity measurements made in vacuo. It is clear fromFigure 4 that whereas this is quite reasonable in some cases, for metals where double layer effects are important, this procedure can only be adopted once the chemical perturbations to the extinction spectra discussed in this review have been accounted for. For this reason, it is stillworthwhile comparing themeasured spectrawithones calculatedusingdielectric data obtained independently, and this is theprocedurewehaveadopted. Particularly interesting is the fact that in the cluster regimewhere covalent bonding is so important to prevent coalescence, plasmondamping is very pronounced.93 The results of the experiments on anion adsorption onto metallic silver particles substantiate the important hy- pothesis that the ligands may largely determine the conduction electron mobility in metal clusters. It is also worthwhile reflecting on the implications that anydielectricmodulationhasondouble layer interactions. Metals have large Hamaker constants precisely because the free electrons contribute enormously to the polariz- Figure 14. (a) Experimentally observed position for gold electrodeposited onto silver sols.101 (b) Position of the surface plasmon band in nanosized colloids of AuAg alloys for various mole fractions of Au in the particles. (c) Positions predicted for Au-coated Ag sols. The coated particle shows two resonances due to the silver core and for highermole fractions of Au a band due to the shell. Figure 15. Experimental absorption spectra of Ag-coated Pd colloids in water.119 Numbers refer to monolayers of Ag calculated fromparticle sizes foundbyHRTEM.ThePd colloids had a radius of 4.6 nm. [Pd] ) 2.9 × 10-5 M. Figure 16. Calculated extinction spectra of Ag-coated Pd particles assuminga core radius of 4.6 nm. 1monolayer)0.257 nm. Dielectric data for Pd from ref 55. Mean free path in shell layer calculated from eq 24. Figures refer to monolayers of Ag. Optical Properties of Metal Particles Langmuir, Vol. 12, No. 3, 1996 799