Surface-Enhanced Infrared Absorption (SEIRA) of Adsorbateson Copper Nanoparticles Synthesized by Galvanic Displacement

AYUBA FASASI, PETER R. GRIFFITHS,* and LOUIS SCUDIERODepartment of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 (A.F., P.R.G.); and Departments of Chemistry and Material Science

and Engineering, Washington State University, Pullman, Washington 99164-4630 (L.S.)

Copper nanoparticles (Cu NPs) were made by electroless deposition on Ge

disks as substrates for surface-enhanced infrared absorption (SEIRA).

Previous X-ray photoelectron spectra had shown that elemental copper is

deposited on the Ge substrate and that the nanoparticulate film remains

resistant to oxidation even after several days of air exposure at room

temperature. SEIRA spectra of p-nitrothiophenol (p-NTP) adsorbed on

the copper nanoparticles were measured. Freshly made substrates made

by electroless deposition gave higher enhancements than both the 12-day-

old oxidized substrates and substrates made by physical vapor deposition.

The intensity of the antisymmetric NO2 stretching band of p-NTP relative

to that of the symmetric stretch was significantly higher for p-NTP

adsorbed on copper than on silver nanofilms, indicating that the C2 axis of

the aromatic ring is tilted with respect to the copper surface.

Index Headings: Electroless deposition; Galvanic displacement; Copper

films; Nanoparticles; Surface-enhanced infrared absorption spectroscopy;

SEIRA; Germanium substrate; Surface selection rules.

INTRODUCTION

It is well known that the infrared absorption of molecules isenhanced when they are adsorbed on nanoparticle metal filmson dielectric substrates by an effect that has come to be knownas surface-enhanced infrared absorption (SEIRA).1–3 The firstreport of this phenomenon, in which p-nitrobenzoic acid (p-NBA) was deposited on vapor-deposited silver, was reportedby Hartstein et al. in 1980.4 (It was not until more than tenyears later that Osawa et al. recognized that the spectrum of p-NBA reported by Hartstein’s group was that of a fully saturatedhydrocarbon.1,2)

For several years after the report by Hartstein et al., manyspectroscopists believed that SEIRA was caused by aplasmonic mechanism analogous to that of surface-enhancedRaman scattering (SERS) and this misconception is stillapparent in relatively recent papers.5 Because SERS is usuallyobserved for adsorbates on silver or gold, most of the earlyreports of SEIRA involved the use of these substrates. The useof copper was less frequently studied, possibly because theSERS enhancement factor for copper is less than that of silveror gold, although Ishida and Griffiths6 reported what appearedto be the effect of SEIRA on a vapor-deposited copper surfacein 1994. Miyake and Osawa7 reported the spectrum of carbonmonoxide adsorbed on a thin Cu film on Ge. Merklin andGriffiths also described an investigation of the adsorption ofnitrophenols on vapor-deposited silver and copper, showingthat the SEIRA spectra on both metals were very similar.8

It was not until 1999 that infrared spectra of species on the

surface of metals other than Au, Ag, or Cu were reported. Inthat year, two papers indicated that SEIRA was not restricted tothe coinage metals.9,10 Krauth et al. reported that, underultrahigh vacuum conditions, the infrared transmission spectraof carbon monoxide adsorbed on epitaxial island films of ironon MgO could be measured.9 They showed that the intensities,positions, and shapes of a band due to adsorbed CO weredependent on the morphology and thickness of the iron film.The stretching band of CO exhibited an asymmetric shape thatrevealed non-adiabatic interaction of vibrations with acontinuum of other excitations, i.e., free electron transitions.The authors showed that the dynamic dipole moment wasenhanced by about two orders over the adiabatic values. In thesame year, Bjerke et al.10 reported the measurement of surface-enhanced IR spectra of CO on a platinized platinum electrodein an infrared spectroelectrochemical cell by external reflectionspectrometry. By varying the platinization conditions, surfaceswere prepared that yielded intensity enhancements for the banddue to adsorbed CO of up to 20 times that of CO adsorbed onsmooth Pt electrodes. When CO was adsorbed on a smooth Ptelectrode, the shape of the band due to the CO stretching modewas quite symmetrical. As the degree of platinization wasincreased, however, the band first became asymmetrical, thenbipolar, and finally appeared as a reflection maximum.

One possible explanation for this behavior was reported in1985 by Langreth,11 who derived expressions for the bandshape for an isolated vibrational mode of a molecule adsorbedon a metallic surface in the presence of electron-hole damping,which lead to an asymmetrical band shape with a long tail.Osawa and Ikeda subsequently proposed that another cause forthe intensification of absorption bands on the surface ofisolated metal particles is the locally enhanced electric field,especially at the edges of small particles where the radius ofcurvature is small.2 This theory was expanded by Osawa et al.in a later study.12 Using a similar rationale, Bjerke et al.simulated the variations in both shape and intensity of the bandof CO adsorbed on platinized platinum using the Bergmanrepresentation of the effective dielectric function.10

These theories showed that the enhancement of theabsorption bands in the spectra of molecules adsorbed onmetal island films is due to the enhanced electric field betweenclosely spaced conducting particles, and hence infrared spectraof molecules adsorbed on any metal that had been deposited togive an island film should be measurable. The electric fieldbetween the particles depends on the dielectric constant of themedium between them. As the interparticle distance decreases,the effective dielectric constant becomes more dependent onthe dielectric constant of adsorbate molecules than on themedium between them. As the interparticle distance decreasesto the point where the particles touch (or percolate), theeffective dielectric constant of the medium between the metalparticles approaches that of the adsorbate layer and the band

Received 22 February 2011; accepted 12 April 2011.

* Author to whom correspondence should be sent. E-mail: pgriff@uidaho.edu.

DOI: 10.1366/11-06274

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� 2011 Society for Applied Spectroscopy

shape changes from absorptive to dispersive. Because intensebands in the refractive index spectrum of adsorbates are usuallybipolar and the dielectric constant is the square of the complexrefractive index in a non-magnetic medium, the dielectricconstant—and hence both the electric field between theparticles and the enhancement factor—is also bipolar.

Because a plasmonic mechanism does not have to beinvoked to explain the behavior of SEIRA spectra, it is to beexpected that the same effect would be found for all metals,and indeed several papers have been published in which similareffects have been reported for metals other than the coinagemetals. These metals include nickel,13 rhodium,14 iridium,15

palladium,16 and zinc,17 as well further papers reporting similareffects on iron,18 platinum,19–21 and palladium.21 It may benoted that the long-wavelength wing of the plasmon resonancebands of these metals does not extend into the mid-infrared;hence a plasmonic mechanism for SEIRA need not be invoked.

The strength of the enhancement depends on the metal andthe size2 and shape of the nanoparticle. In most of the earlywork, the substrates for SEIRA were usually prepared byphysical vapor deposition (PVD), which is time consuming andrequires high-vacuum equipment. Recently the far simplermethod of galvanic displacement (often called electrolessdeposition) has been used to prepare silver and goldnanoparticles that are suitable for SEIRA and/or SERS ondisks of semiconductors such as germanium or silicon.22–24 Inthis technique, a metal ion is spontaneously reduced on asurface of the semiconductor. Although the names are oftenused interchangeably, galvanic displacement and electrolessdeposition are not exactly the same: galvanic displacement willcease when the substrate surface is fully covered by the metal,while further deposition takes place in electroless deposition.

Here, we report the deposition of copper onto a germaniumsubstrate and the feasibility of measuring SEIRA and SERSspectra of an adsorbate on surfaces prepared in this way. Thedriving force to form metallic copper on the substrate isdetermined by the difference of the standard potentials andsubsequent free Gibbs energy of Cu2þ/Cu and Ge4þ/Ge halfcells.

Cu2þ þ 2e ¼ CuðsÞ E8 ¼ 0:339V

Ge4þ þ 4e ¼ GeðsÞ E8 ¼ 0:124V

The resulting cell potential for this reaction isþ0.215 V, wherethe positive cell potential means the reaction is spontaneous.

We have previously studied the growth mechanism,morphology/topography, and microstructure of copper filmsformed by electroless deposition on polished HF-treated Gesubstrates by atomic force microscopy (AFM).25 We showedby X-ray photoelectron spectroscopy (XPS) that oxide layersonly build up very slowly (over a period of several days) on thesurface of the copper.

EXPERIMENTAL

Substrates. Germanium disks (99.999% purity) 1.2 cm indiameter and 4 mm thick (Lattice Materials Corp., Bozeman,MT) were polished to a mirror surface with Metadi II 1 lmdiamond polishing compound (Buehler, Lake Bluff, IL) andwashed with ethanol and deionized (DI) water.

Electroless Deposition. Copper was deposited on Gesubstrates by galvanic displacement at room temperature. We

found that the most reproducible way to deposit copper on Gewas to immerse a well-polished Ge disk into a low-concentration (low millimolar) aqueous solution of coppersulfate. The copper film was allowed to grow on the Ge diskfor several minutes. The Ge disk was removed and immersed inDI water for 5 minutes to discontinue the reaction and toremove all unreacted copper sulfate solution. The samples werethen left to dry in air. Even though electroless deposition ofseveral other metals (Au, Pd, Pt, and Ag) on Ge or Si has beenreported previously, as noted above, this procedure was foundto produce more reproducible copper films than these or anyother procedures we have investigated. Silver films wereproduced in an analogous manner.

Physical Vapor Deposition. For CuNPs prepared by PVD,copper shot was vapor deposited on a Ge substrate that hadbeen previously polished with a diamond polishing compoundand washed with ethanol and DI water and ethanol byresistively heating the copper shot in a tungsten basket.

Adsorption of p-NTP. To investigate the SEIRA effect oncopper deposited by either electroless deposition or PVD, themetal-coated disks were immersed in a 50 ppm solution of p-NTP in ethanol for several hours before being removed andwashed with ethanol to remove any excess p-NTP.

Spectra. The SEIRA spectra were measured by transmissionspectroscopy at 4 cm�1 resolution using a standard NicoletMagna 760 Fourier transform infrared (FT-IR) spectrometerequipped with a deuterated triglycine sulfate detector byaveraging 128 scans. The spectrum of a chloroform solution ofp-NTP was measured using a three-reflection attenuated totalreflection (ATR) accessory with a ZnSe internal reflectionelement (Harrick Scientific Products, Inc., Pleasantville, NY).

RESULTS

Effect of Concentration of Copper Sulfate and Immer-sion Time. The size and morphology of the coppernanoparticles vary with the concentration of copper sulfateand the time of deposition. Knowledge of the size andmorphology of the copper nanoparticles is important becausethe microstructure of the deposited Cu film affects both theSEIRA effect6 and the mechanical and electrical properties ofthe film as discussed in detail in the literature.26,27 Bycontrolling the particle size and film thickness, the physicalproperties of the copper film can be tailored to satisfy therequirements for a specific application.

To assess the effect of the concentration of copper sulfate onthe morphology of the copper that was deposited and theSEIRA spectrum of the adsorbed p-NTP, the concentration ofCuSO4 was varied from 1 mM to 10 mM and the depositiontime was varied from 10 min to as much as 2 h. Ten copper-coated Ge disks were soaked in the 50 ppm p-NTP solution andthe spectrum was measured at intervals of 15 minutes.

The morphology of the deposited copper varied significantlydepending on the deposition conditions. For example, scanningelectron microscopy (SEM) images of copper deposited from 1mM CuSO4 for 100 min and from 5 mM CuSO4 for 30 minutesare shown in Figs. 1A and 1B, respectively. Initially, copper isdeposited as small islands on the surface of the germaniumdisk. Growth of these particles results in the formation ofnanowires. When the deposition time is increased further,additional copper is deposited at the junctions of the nanowires.

When a monolayer of p-NTP is bound to the surface of thecopper, the intensity of the bands in the p-NTP spectrum was

APPLIED SPECTROSCOPY 751

found to increase with the time for which the Ge disk wasimmersed in the CuSO4 solution until a plateau was attainedafter about 1.5 h (see Fig. 2A). Shorter times were required toreach equilibrium if the concentration of the CuSO4 solutionwas increased, as shown in Fig. 2B. The time for which the Gedisk was immersed in the ethanolic p-NTP solution was heldconstant at 100 minutes and was used for all thesemeasurements.

To investigate the repeatability of this process, eight Gedisks were simultaneously coated with copper by soaking in 5mM copper sulfate for 30 minutes and p-NTP was adsorbed onthe surface. When the spectra of all eight disks were measuredat five locations, the relative standard deviation (rsd) of peakabsorbance of the symmetric NO2 stretching band, ms(NO2),was less than 3% for measurements of a given batch. The rsd ofthe ms(NO2) band from batch to batch over a period of twoweeks was less than 12%.

Comparison with Vapor Deposition. The morphology ofcopper films prepared by electroless deposition appears to leadto twice the enhancement of SEIRA spectra in comparison tovapor deposition, as estimated from the intensity of thems(NO2) band of p-NTP. Furthermore, the bands of p-NTPon copper produced by PVD are observed to be more distortedthan the ones produced by electroless deposition, indicatingthat the copper films formed by PVD are more percolated.However, bearing in mind the morphology of the particles seenin Fig. 1, the asymmetry of the bands in the SEIRA spectrum ofp-NTP is less than would be expected from previous work. The

spectra of adsorbed p-NTP on copper films prepared by PVDand electroless deposition under conditions that gave themaximum enhancement with each technique are compared inFig. 3.

SEIRA of p-NTP on Freshly Prepared and OxidizedCopper. Our previous work showed that copper nanofilmsformed by electroless deposition oxidized very slowly25 andthis behavior is reflected in the SEIRA spectra measured afteradsorption of p-NTP. Spectra of p-NTP on freshly made coppernanoparticles and on copper nanoparticles after 12 days ofexposure to ambient air are remarkably similar in intensity,although a small increase in the asymmetry of bands can beobserved, as shown in Fig. 4. The decrease in absorbance of thebands by about 30% when the substrate is exposed to air isbecause the oxide layer that is formed on the surface of thecopper nanoparticles, which is composed of CuO andCu(OH)2,22 makes part of the surface unavailable to the p-NTP adsorbate.

Comparison with p-NTP Adsorbed on Silver. The SEIRAspectra of p-NTP adsorbed on silver and copper are shown inFig. 5. It can be seen that the antisymmetric NO2 stretchingband, mas(NO2), at approximately 1510 cm�1 is three timesstronger than ms(NO2) at approximately 1340 cm�1 in thespectrum of p-NTP adsorbed on CuNPs than for thecorresponding spectrum of p-NTP on AgNPs. The surfaceselection rule for infrared spectra of molecules adsorbed on thesurface of flat metals gives that absorption bands for which acomponent of the dipole moment derivative is perpendicular to

FIG. 1. SEM images of copper deposited from (A) 1 mM CuSO4 for 100 min and (B) 5 mM CuSO4 for 30 minutes.

FIG. 2. Variation of the absorbance of the ms(NO2) band in the SEIRA spectra of p-NTP at approximately 1340 cm�1 with the length of time each germaniumsubstrate was immersed in copper sulfate solution. The concentrations of CuSO4 and immersion times were (A) 1 mM and 100 min and (B) 5 mM and 30 min. Theerror bars represent the standard deviation for triplicate measurements.

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the metal surface are allowed whereas any band for which thedipole moment is parallel to the metal surface is forbidden.28

The same rule has been shown to apply to SEIRA spectra ofadsorbates on metal nanoparticles.1 Thus, if p-NTP wereadsorbed on either copper or silver so that the metal–S–Cangle, h, is 1808, i.e., the C2v axis of the –C6H4NO2 moiety isperpendicular to the metal surface, mas(NO2) would beforbidden whereas ms(NO2) would be strongly allowed.Because the ms(NO2) band in the spectrum of p-NTP adsorbedon AgNPs is so much stronger than when p-NTP is adsorbedon CuNPs, it is clear that the –C6H4NO2 moiety is more nearlyperpendicular to the silver surface than to the copper surface, asshown schematically in Fig. 6. A simple calculation of theangle by which the aromatic ring is tilted relative to the planeof the metal requires the relative intensities of mas(NO2) andms(NO2) in the spectrum of unoriented p-NTP to be known.These bands have approximately the same intensity in thespectrum of a KBr disk29 but the peak absorbance of ms(NO2) isonly 55% that of mas(NO2) in the ATR spectrum of a solution ofp-NTP in CHCl3 after the ATR correction has been applied(see Fig. 7). If it is assumed that the mas(NO2) and ms(NO2)bands have the same intensity, h is calculated to be 1608 for p-NTP on CuNPs and 1738 for p-NTP on AgNPs, whereas if it isassumed that the intensity of ms(NO2) is one-half that ofmas(NO2), the corresponding angles are 1378 and 1678,

respectively. We note that a caveat was given by Merklinand Griffiths in their investigation of the adsorption of p-nitrobenzoic acid on vapor-deposited copper and silverwarning that the degree to which the stretching modes of the–CO2

� group of p-nitrobenzoate ions adhered to the surfaceselection rule was dependent on the local environment andadvising caution in ascribing intensity differences of theantisymmetric stretching mode of the –CO2

� group to surfaceselection rules.8

Measurement of the tilt angle of aromatic thiols adsorbed onmetal surfaces has been a subject of some controversy andestimates vary with both the experimental technique used forthe measurement and the theory used to interpret theexperimental data. Several reports on the SERS spectra ofbenzenethiol (BT) adsorbed on Ag and Au have beenpublished. From SERS spectra, Sandroff and Herschbach30

and Joo et al.31 deduced that the phenyl ring lies nearly flat onAu and Ag surfaces (h ’ 908) because of the red shift of one ofthe BT modes on adsorption. Using similar arguments, Carronand Hurley calculated h¼ 858 and 768 for BT on Ag and Au,respectively.32 Conversely, Takahashi et al. concluded that thephenyl ring is nearly perpendicular to the surface (h ’ 1808)because out-of-plane ring modes were enhanced less than thein-plane modes.33 In a SERS spectroelectrochemical investi-gation of aromatic thiols bonded to gold electrodes, Szafranskiet al. found that the aromatic ring in BT is tilted slightly relativeto the mean surface plane of the gold at all applied potentials.34

Similarly, from SEIRA measurements, Wan et al. reported that

FIG. 3. Comparison of SEIRA spectra of p-NTP on copper prepared (A) byelectroless deposition of 1 mM CuSO4 solution on a Ge substrate for 100minutes at room temperature and (B) by PVD to yield a 6 nm thick film.

FIG. 4. SEIRA spectra of p-NTP on (A) freshly prepared copper and (B)copper exposed to air for 12 days. Copper was deposited by immersing Ge in 5mM copper sulfate for 30 minutes.

FIG. 5. SEIRA spectra of p-NTP adsorbed on (A) copper and (B) silvernanofilms formed on a Ge disk by electroless deposition. The mas(NO2) band ismarked with an arrow on both spectra.

FIG. 6. Possible orientation of p-NTP on (A) silver and (B) coppernanoparticles.

APPLIED SPECTROSCOPY 753

for BT adsorbed on Au(111) the phenyl ring is tiltedapproximately 308 from the surface normal (h ’ 1508).35

Tao and co-workers investigated the conformation ofadsorbed BT with substituents para to the thiol group bycyclic voltammetry, reflection–absorption IR spectroscopy,scanning tunneling microscopy (STM), contact angle measure-ments, and ellipsometry.36 They found that inserting a CH2

group between the sulfur atom and the aromatic ring improvedthe order of the thiol monolayer. Their results indicated that theC2 axis of the –SC6H5 group is tilted with respect to the goldsurface. Conversely, using high-resolution electron energy lossspectroscopy, Whelan et al. found that an adsorption geometrywith the plane of the aromatic ring largely parallel to theAu(111) surface is favored.37

The metal–S–C angle, h, is determined by the hybridizationof the sulfur in the metal thiolate bond. Shaporenko et al.reported that the sulfur in 4,40-terphenyl-substituted alkane-thiols (C6H5(C6H4)2(CH2)nSH on polycrystalline (largely 111)gold and silver substrates is sp3 hybridized (h ’ 1048) for Auand sp hybridized (h ’ 1808) for Ag.38 If it is assumed that p-NTP behaves in the same way, we would tentatively concludethat h is also approximately 1808 for p-NTP adsorbed on silver,which our results indicate to be the case. Previous experimentaldeterminations of the orientation of aromatic thiols adsorbed onAu, Ag, or Cu do not give much insight as to whether a tilted orperpendicular alignment should be expected for p-NTPadsorbed on noble metals because the conclusions reached bydifferent groups appear to depend on the technique that wasused. Using contact angle measurements and ellipsometry,Sabatani et al. reported that BT forms poorly defined layers onAu(111) but that p-biphenyl mercaptan (BPM) and p-terphenylmercaptan (TPM) are both approximately perpendicular to thesurface (h ’ 1808).39

Frey et al.40 studied the adsorption of BT, BPM, and TPMon gold and silver by X-ray photoelectron spectroscopy (XPS)and angle-resolved near-edge X-ray absorption fine structurespectroscopy (NEXAFS) and, like Carron and Hurley32 andTao et al.,36 found that BT gave poorly defined layers. Theyshowed that BPM and TPM are slightly inclined with respect tothe surface normal and exhibit smaller tilt angles on silver thangold.35 Wong et al. investigated the effect of halogen

substitution on the geometry of arenethiol films on Cu(111)by STM.41 They showed that the thiol molecules adsorb inpatterns that depend on their quadrupole moment but they didnot report on the axial angle of the adsorbates. (This isunfortunate; as we have previously reported,25 copper inelectrolessly deposited films exists largely in the (111) state.)Beccari et al.42 characterized BT on Cu(100) by XPS andNEXAFS and showed that h ’ 1608, which is in line with ourresult for p-NTP on copper.

The conclusion that h is closer to 1808 for p-NTP adsorbedon AgNPs than when it is adsorbed to CuNPs is further borneout when the quadrant vibrations of the aromatic ring thatabsorbs near 1600 cm�1 are examined. The form of these twomodes is shown schematically in Fig. 8. In the spectrum ofbenzene, these two modes, which were designated as m8 byWilson,43 are degenerate. On substitution, the degeneracy isremoved and two bands are seen, with the m8a mode usuallyabsorbing near 1600 cm�1 and the m8b mode near 1580cm�1.44,45 Calculation of the vibrational frequencies of nitro-aromatic molecules by density functional theory (DFT) showsthat the m8b mode of these molecules absorbs at higherfrequency than the m8a mode, presumably because of second-order coupling (Fermi resonance) between the mas(NO2) modeand the m8b ring mode since they have the same symmetry andare separated by less than 100 cm�1. The m8a band is usuallymore intense than m8b, which is the case for unoriented p-NTP,as can be seen from the ATR spectrum of a chloroform solutionof p-NTP shown in Fig. 7. The peak of the m8b band is at 1598cm�1 while that of the m8a band is at 1579 cm�1.

From Fig. 8 it can be seen that the direction of the dipolemoment derivative for m8a is along the C2 axis of the molecule,whereas the direction for m8b is at an angle to the C2 axis. Usingan argument analogous to the one made for the NO2 stretchingvibrations, the m8a mode of p-NTP should be intensified to agreater degree relative to the m8b mode when the molecule isadsorbed on silver (where the C2 axis is approximatelyperpendicular to the metal surface, i.e., h ’ 1808), with theintensification being less for adsorption on copper, where h ,1808, which is what is found in practice.

CONCLUSION

The infrared spectra of monomolecular layers of p-NTPadsorbed on thin electrolessly deposited copper films are easilymeasured by transmission spectrometry. A comparison of the

FIG. 7. ATR spectrum of a saturated solution of p-NTP in chloroform, withthe chloroform bands subtracted.

FIG. 8. Atomic displacements for the degenerate m8 mode of benzene and them8a (left) and m8b (right) modes of substituted aromatic molecules.

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intensity of the ms(NO2) and mas(NO2) bands and of the m8a andm8b bands in spectra of p-NTP adsorbed on copper and silverfilms prepared by electroless deposition suggests that the C2

axis of the aromatic ring is tilted with respect to the coppersurface and close to perpendicular with respect to the silversurface.

ACKNOWLEDGMENT

We would like to thank one of the reviewers of the original manuscript forpointing out the possibility that the frequency at which the m8a and m8b bands ofaromatic rings absorb could be different for nitro-substituted aromaticmolecules than for other substituents.

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