718
J. Phys. Chem. 1996, 100, 718-724
Ag-Clad Au Nanoparticles: Novel Aggregation, Optical, and Surface-Enhanced Raman Scattering Properties R. Griffith Freeman,† Michael B. Hommer, Katherine C. Grabar, Michael A. Jackson, and Michael J. Natan* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: May 18, 1995; In Final Form: October 4, 1995X
Ag-coated Au colloidal particles have been prepared by reduction of Ag+ in the presence of preformed Au colloids. The composition of the Au100-xAgx particles was varied from x ) 0 to 80. SERS spectra of pyridine, p-nitroso-N,N′-dimethylaniline (p-NDMA), and trans-1,2-bis(4-pyridyl)ethylene (BPE) have been obtained with these colloids. At monolayer Ag coverages (x < 10), the optical spectra of Ag-coated Au particles are indistinguishable from uncoated Au particles. However, the SERS behavior of aggregated colloids with 647.1 nm excitation is extremely dependent upon the Ag:Au ratio. Very small amounts of Ag (x e 5) lead to an increase in SERS intensity, but further increases lead to complete loss of signal. For p-NDMA and pyridine, these data can be explained by Ag inhibition of adsorbate-induced aggregation. The initial increase in SERS intensity results from production of smaller aggregates that exhibit a surface plasmon band in better alignment with the excitation wavelength; higher ratios of Ag eliminate aggregation and all SERS enhancement. For BPE, the same Ag-induced loss of SERS is observed, even though each of the Au100-xAgx colloidal solutions is clearly aggregated by BPE adsorption. This finding suggests that submonolayers of Ag modulate specific chemical interactions between the Au and BPE that are responsible for SERS.
Introduction The large number of experimental and theoretical approaches to understanding surface-enhanced Raman scattering (SERS) has led to an increased recognition of the factors responsible for the formation of SERS-active (and SERS-inactive) surfaces.1-6 For example, surface roughness is required to produce significantly enhanced Raman spectra of molecules adsorbed at noble metal surfaces. The two phenomena responsible for SERS are large increases in the electric field near the surface (electromagnetic effect) and specific adsorbate-surface interactions (chemical effect).1-6 Colloidal Au and Ag in solution can be rendered SERS-active by adsorbate-induced particle aggregation.1-9 Particle coagulation results in a new, lowenergy surface plasmon mode in the UV-vis spectrum, the magnitude of which correlates well with the wavelength dependence of SERS intensity.8 In principle, controlling the position and/or size of the surface plasmon band should lead to control of SERS intensity. In practice, modulating SERS activity by controlled aggregation is difficult, since aggregated Au colloids are both fractal10 (leading to a distribution of optical properties) and thermodynamically unstable with respect to precipitation.11 Our approach to tuning the optical properties of colloidal Au has centered on the preparation of Ag/Au composites.12 Au has lower enhancement factors in the visible than Ag1-6 but is easier to prepare as monodisperse sols in a variety of diameters.13 We reasoned that particles composed of one of more layers of Ag deposited on preformed colloidal Au might be both more monodisperse than typical colloidal Ag preparations14 and more strongly enhancing than pure Au. Moreover, it has been shown that the single-particle resonance for Ag-Au alloys can be continuously shifted, from 400 nm for pure Ag colloids to 520 nm for Au, by varying the composition.15 This suggests that † Current address: Division of Science, Northeast Missouri State University, Kirksville, MO 63501. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0718$12.00/0
the collective particle surface plasmon band, which is responsible for surface enhancement in aggregates, may be tunable. Au/Ag composites and alloys have received tremendous attention in the surface chemistry and electrochemical literature.16-25 SERS has been a useful probe of the resulting surfaces. For example, when monolayers of Ag were deposited on Au electrodes, a SERS signal was obtained using 514.5 nm excitation, a wavelength at which Au is SERS-inactive.22 These results indicated that Ag atoms provided SERS-active sites on the Au surface. Further work by the same group showed that Ag provided specific sites for chemical interaction with SERSactive adsorbates (CN- and pyridine).23 Examination of AgAu alloys24 indicated that addition of Au led to diminished enhancement of Raman scattering at 488.0 and 514.5 nm. Interestingly, when excited at 647.1 nm, the SERS response showed maxima at 100% Au and 100% Ag, with a broad minimum between the two, suggesting that Ag can act to either increase or decrease SERS activity. Murray showed that Au overlayers on Ag-island films reduced SERS intensity at a wavelength where Au does not typically cause surface enhancement.25 In related work, Ag has also been deposited on Pt electrodes, giving rise in some cases to moderately enhancing substrates.26 SERS properties of Au or Ag composites with nonenhancing metals have also been investigated. Small amounts of Pt added to colloidal Au greatly diminish the SERS signal.27 In contrast, SERS spectra could be acquired on Au and Ag electrodes modified with one or more monolayers of Rh or Pt.28 At low coverage, large signals were seen; as the amount of either Rh or Pt increased, the SERS intensity eventually dropped off. Finally, a series of studies showed that underpotential deposition of Pb on an enhancing surface led to severe diminution of SERS intensity.29 We report here the properties of Ag-clad Au particles with molar compositions Au100-xAgx, where x ) 0-80.30 Small amounts of Ag (x e 5) lead to substantial increases in SERS intensity for adsorbed pyridine, BPE, and p-NDMA with 647.1 © 1996 American Chemical Society
Ag-Clad Au Nanoparticles nm excitation. However, deposition of larger quantities of Ag leads to a dramatic decrease in signal, and for x ≈ 10, the signal disappears completely. The data presented herein show that modulation of aggregate size and chemical enhancement control the observed spectral intensities. Experimental Section Materials. HAuCl4‚3H2O, AgNO3, p-nitroso-N,N′-dimethylaniline [IUPAC name: N,N′-dimethyl-4-nitrosoaniline] (pNDMA), trans-1,2-bis(4-pyridyl)ethylene (BPE), trisodium citrate, and pyridine were purchased from Aldrich. BPE was recrystallized several times from a solution of H2O and CH3OH; the other materials were used as received. HNO3 and HCl were “Baker Analyzed” grade and used as received. HPLC grade CH3OH was obtained from EM Science. Solutions of pyridine and p-NDMA were prepared in doubly distilled H2O, and BPE solutions were prepared in CH3OH. Synthesis of Ag-Clad Au Colloids. All glassware was washed with freshly prepared aqua regia (3:1 HCl:HNO3) followed by extensive rinsing with doubly distilled H2O. Three solutions were prepared for these syntheses: 1.0 mM HAuCl4, 10 mM AgNO3, and 38.8 mM trisodium citrate. Preparation of colloidal Au followed literature procedures with slight modifications.13 Preparation of colloidal Ag followed the recipe of Lee and Meisel.31 Au particles with 12 ( 1 nm diameters were prepared by adding 50 mL of the citrate solution to 500 mL of boiling 1.0 mM HAuCl4 with vigorous stirring. After appearance of a deep red color, boiling and stirring were continued for 15 min. The solution was then allowed to cool to room temperature with continued stirring. To prepare solutions of Ag-coated Au particles, identical volumes corresponding to 20 µmol of Au were transferred into 50 mL Erlenmeyer flasks. After reheating these solutions to a boil, “y” mL of 10 mM AgNO3 was added with stirring to produce the desired final Au mole fraction (100 - x), where 100 - x ) 100{20/[10y + 20]}. Final volumes were adjusted to 25 mL. With Au seed particles having 12 nm diameter, we calculate a particle concentration of 9.0 × 1015 particles/L. In most cases the AgNO3 solution was added to the Au particles in a single aliquot. For those solutions containing larger amounts of Ag, e.g., less than 80% Au, addition of AgNO3 was done in 0.20 mL increments at 15 min intervals in order to prevent the formation of separate Ag particles. Stoichiometric equivalents of citrate were also added to ensure complete reduction of Ag+ in these solutions with higher [Ag+]. Solutions designed to produce particles with greater than 60% Ag yielded some precipitate during synthesis. To make 18 nm diameter Au particles, only 35 mL of citrate solution was used in the first step. The rest of the procedure was identical to that used for 12 nm Au particles. The elemental composition of composite particles was verified using a Leeman Labs PS3000UV inductively coupled plasma (ICP) spectrophotometer. Colloidal particles were separated from free metal ions in solution by filtering an aggregated colloid solution through a 0.1 µm Nucleopore filter. Isolated particles were then dissolved into dilute aqua regia (1:3:6 parts by volume HCl:HNO3:H2O). The ICP data for Ag agreed with the predicted Ag compositions within 1-1.5%. Control experiments on the filtrates, the dilute aqua regia, and the Nucleopore filter confirmed that Au and Ag resulting from these sources are negligible. Instrumentation. The Raman excitation source was a Coherent Kr+ laser with an Innova-200K gas tube. The laser passed through an Optometrics TGF-302 tunable grating filter to remove plasma lines. Light intensities were controlled with
J. Phys. Chem., Vol. 100, No. 2, 1996 719 a polarization rotator/beam splitter combination (Meadowlark Optics), neutral density filters, and/or a ThorLabs iris. The beam was brought to a 3 mm line focus using a 10 cm focal length cylindrical lens. The beam struck the sample at an angle of 30° below the horizontal plane in which the backscattering was observed. The samples were held in 5 mm outer diameter quartz or glass NMR tubes placed in a NMR sample holder. The sample was placed in a rigid, fixed-position NMR tube spinner. The NMR tube/sample holder could be removed and replaced repeatedly with less than 5% signal variation. Thus, entire experiments were carried out without any adjustment of optics. We have found that highly aggregated colloids have a tendency to coat the NMR tube wall when spun rapidly. To avoid this complication, samples were spun slowly or not at all. Scattered light was collected and collimated with a f#/1.2 Minolta 50 mm camera lens and finally focused via a 25 cm focal length lens through a polarization scrambler onto the entrance slit of a Spex 1404 0.85 m double monochromator equipped with 1800 lines/ mm gratings. The detector was a thermoelectrically cooled Hamamatsu R928P photomultiplier tube in photon counting mode. The monochromator was operated and data were collected by an 80286-PC clone running DM3000 software from Spex. For the SERS spectra shown here, the 647.1 nm Kr+ line was used with 75 mW power at the sample. The entrance and exit slits of the monochromator were set at 700 µm and the interior slits at 1400 µm, resulting in a nominal 5 cm-1 bandpass. TEM data were acquired on a JEOL-JEM 1200 II EX electron microscope operated at 80 kV accelerating voltage. Colloid samples were prepared by placing 10 µL of solution on formvarcoated Cu grids purchased from Ted Pella Inc. The samples were allowed to dry at room temperature overnight. Optical spectra were collected on a Hewlett-Packard 8452 diode array spectrometer using a 1 cm quartz cell. Solutions were removed from the SERS apparatus and diluted 4-fold to keep absorbances in the Beer’s law regime. Calculations. Calculations were performed using an adaptation of COATME with the subroutine BHCOAT.32 Refractive indices for Au and Ag12d were entered into the adapted program as functions of the wavelength fitted to the data. Results It is known from work in histochemistry that Au particles act as seeds for Ag growth.13b In our work, citrate reduction of Ag+ in the presence of preformed colloidal Au causes deposition of Ag onto the particles. Size analysis of TEM micrographs depicting Au100 samples indicates a 12 ( 1 nm diameter, based on a sampling of approximately 100 particles (Figure 1, top panel). For small Ag/Au ratios, addition of Ag causes no discernible change in particle diameter or morphology, as evidenced by TEM data on Au90Ag10 particles (Figure 1, middle panel). Simple geometric arguments using a Ag atomic radius of 1.4 Å predict the radius for Au90Ag10 particles exceeds that for the initial Au100 particles by only 3%, a change too small to observe by TEM. In other words, the Ag cladding consists at most of one or two atomic layers. Thus, all particles having between 100 and 90 mol % Au consist of spherical 12 nm diameter particles. Solutions with larger amounts of Ag, especially Au70Ag30 through Au20Ag80, show some signs of aggregation and somewhat increased size irregularity. However, it should be noted that these Ag-clad Au preparations are still more spherical than Ag sols that we have prepared using literature protocols.14 Importantly, there is no evidence for the formation of separate Ag colloids in any TEM micrographs. Moreover, if separate colloidal Ag particles were formed, their
720 J. Phys. Chem., Vol. 100, No. 2, 1996
Freeman et al.
Figure 2. SERS spectra at Au100-Au90Ag10 colloids made 20 mM in pyridine. Conditions: 75 mW, 647.1 nm excitation, 1 cm-1/step, 2 s integration, 5 cm-1 band-pass.
Figure 1. (top) TEM image of 12 nm diameter colloidal Au. (middle) TEM image of 12 nm diameter Au90Ag10. The scale is the same for both images. (bottom) Normalized UV-vis spectra of 12 nm Au90Ag10 and of a 90:10 molar mixture of 12 nm diameter Au:12-15 nm diameter colloidal Ag.
surface plasmon band at ≈400 nm would be easily observed. The bottom panel of Figure 1 shows the normalized optical spectra of a solution of Au90Ag10 and a 90:10 molar (by atom) mixture of colloidal Au:colloidal Ag. The absence of separate Ag resonances argues strongly against the formation of separate particles. The lack of size variation in composites with a low molar concentration of Ag should lead to negligible changes in absorption cross section; in fact, the optical spectra for Au100 through Au92Ag8 are virtually superimposable (data not shown). The optical density changes by only a few percent over this sample range, and λmax ) 522 nm for all particles. Even at much higher Ag concentrations (e.g., Au80Ag20), where the characteristic absorption band for colloidal Ag is present at ≈400 nm, the 522 nm feature persists. This finding accords with expectations based on a computer algorithm for modeling the optical properties of concentric, coated colloidal particles32 and
strongly suggests alloys are not formed. Further evidence against alloying is that Au/Ag composites prepared by citrate reduction of aqueous Au3+/Ag+ mixtures or by melting of AuAg mixturessspecies both described as alloyssexhibit continuous changes in λmax between 520 and 400 nm as the composition is varied.15 It is possible that the [AuCl2]- layer present on the surface of Au colloids13b prepared by reduction of [AuCl4]-scolloidal Au is negatively chargedsserves as a barrier to alloying. In summary, optical properties of isolated Au particles coated with e1-2 layers of Ag atoms are unchanged relative to uncoated colloidal Au. However, differences between coated and uncoated particles are immediately apparent upon adsorbate-mediated aggregation. Figure 2 shows a portion of a pyridine SERS spectrum obtained after addition of a small quantity of pyridine to solutions containing 11 different particle compositions (Au100, Au99Ag1, Au98Ag2, Au97Ag3, ..., Au90Ag10). The intensity of both ring stretching bands for pyridine adsorbed to Au99Ag1, Au98Ag2, Au97Ag3, Au96Ag4, and Au95Ag5 is increased relative to Au100. For Au94Ag6, the signal abruptly drops into the background and remains there for all particles with 6 e x e 20. Colloids containing much larger amounts of Ag (x g 25) have Ag-like optical properties and are SERS-active upon addition of pyridine (data not shown). The return in SERS activity is marked by a shift in the larger of the two pyridine ring breathing modes from 1014 to 1010 cm-1, coinciding with values for pyridine adsorbed to pure Au and Ag, respectively. This change in SERS intensity in the x ) 0-10 range may be understood upon examination of the respective [colloid solution + pyridine] optical spectra (Figure 3). For Au99Ag1 to Au96Ag4, aggregation leads to a broad collective-particle surface plasmon band centered around 690 nm. Comparison to the aggregated Au100 optical spectrum reveals two differences, a slight blue shift and a decrease in scattering, manifested in a reduced background. Both of these changes are consistent with reduced aggregate size. This trend is continued for the spectrum resulting from addition of pyridine to colloidal Au95Ag5, for which there is a significant shift in λmax to 670 nm and a large increase in absorbance. For Au93Ag7 through Au90Ag10, there is no appreciable aggregation: Au94Ag6 has a small shoulder at 600 nm, while the remainder resemble isolated, uncoated colloidal Au. These spectra show that deposition of small quantities of Ag on colloidal Au clearly inhibits adsorbateinduced aggregation. Since colloid aggregation is the major contributor to SERS enhancement, the absence of this absorption feature from particles with x ) 6-10 provides an explanation for the corresponding lack of SERS signal. It should be noted that the decrease in SERS intensity is not specific to the bands in Figure 3: when the Ag content of the particles increases above the required threshold, the entire Raman spectrum disappears.
Ag-Clad Au Nanoparticles
Figure 3. Optical absorption spectra (after 4-fold dilution) of Au100Au90Ag10 colloids made 20 mM in pyridine.
J. Phys. Chem., Vol. 100, No. 2, 1996 721
Figure 5. (top) SERS spectra at Au100-Au90Ag10 colloids made 10 µM in BPE. See Figure 2 for conditions. (bottom) Plot of the peak intensity at 1610 cm-1 vs mol % Ag (x) from top-panel data.
Figure 4. Optical absorption spectra (after 4-fold dilution) of Au100Au90Ag10 colloids made 10 µM in BPE.
Both the initial increase in signal and the transition from increase to decrease at Au94Ag6 are quite reproducible. Similar data sets have been generated several times by different individuals working independently in our lab. For a given size Au particle, there is no more than 1% variation in the particle composition that yields the maximum peak intensity. Moreover, the increases and decreases are occasionally even more pronounced than the data set shown. In this respect, the goal of tuning the SERS behavior of particle aggregates has been met: using red excitation, aggregated Au97Ag3-Au95Ag5 colloids are clearly more enhancing than pure Au and equally monodisperse. In contrast, the steep loss in signal with further addition of Ag is an unforeseen consequence of this strategy and must be reconciled with existing SERS models (i.e., electromagnetic and/ or chemical enhancement).1-6 Three experiments show that the phenomenon occurs only with zerovalent Ag (Ag0) on Au, as opposed to Ag+ or any other ionic species. First, we and others have shown that the presence of Ag+ on a SERS-active surface can be detected by the appearance of a new band in the SERS spectrum of adsorbed pyridine.33,34 We observe no such band in these experiments. Second, experiments were repeated with NaNO3 and KNO3 added (rather than AgNO3), followed by a heating procedure, identical to that which was used to deposit Ag on the Au core particle. This procedure resulted in UV-vis and SERS behavior indistinguishable from that of the control (Au100). Finally, the aggregation and SERS changes could not be reproduced by addition of Ag+ without heating (a requirement for citrate reduction to Ag0). These experiments indicate that all Ag is present as Ag0. For pyridine, it appears that Ag0 modulation of SERS intensity on colloidal Au is indirect, mediated by control of aggregation. When the same set of experiments is carried out with BPE as the adsorbate, direct inhibition of SERS activity is demonstrated. Figure 4 shows the optical spectra of the previously described set of 11 colloidal particle solutions (Au100-Au90Ag10) in the presence of 10 µM BPE. Aggregation occurs in all of them, as evidenced by the large new surface plasmon bands centered
Figure 6. SERS spectra at Au100-Au90Ag10 colloids made 0.5 nM in BPE. See Figure 2 for conditions. The unenhanced spectra are not labeled. (inset) Optical absorption difference spectra (after 4-fold dilution) of Au100, Au98Ag2, Au96Ag4, Au94Ag6, and Au94Ag6 colloids made 0.5 nM in BPE.
between 685 and 715 nm. This contrasts the pyridine data, for which colloids with x g 5 do not aggregate. As the concentration of Ag increases from x ) 0 to x ) 10, there is a roughly one-third decrease in extinction of the 710 nm band, along with a slight blue-shifting of the absorbance maximum, e.g., for Au95Ag5, λmax ) 695 nm. Figure 5 shows the corresponding SERS spectra. The data reveal a slight increase followed by a gradual loss of SERS intensity that is similar to but not as abrupt as that seen with pyridine. Nevertheless, for Au90Ag10, the spectral intensity is over 50 times weaker than for the peak intensities found for Au97Ag3. The lack of SERS activity from a collection of aggregated Au particles that exhibits large extinction in the 650-800 nm range is an uncommon occurrence in the SERS literature. Since adsorption of BPE initiated aggregation in the first place, the molecule must be adsorbed. Furthermore, it is extremely unlikely that the loss of signal intensity is due to reorientation of the adsorbate on the surface. Such a process should produce some new vibrational features and no new bands are observed. Even with BPE concentrations too low to initiate significant aggregation, the presence of Ag0 inhibits SERS. The inset to Figure 6 shows five of the 11 optical difference spectra obtained when solutions containing Au99Ag1, Au98Ag2, Au97Ag3, Au96Ag4, ..., Au100 are made 500 nM in BPE (difference ) spectrum after BPE addition-spectrum before BPE addition). At this adsorbate concentration, the only spectral change is the appearance of extremely small features (0.01-0.03 absorbance unit) centered between 620 and 650 nm. Although these data are noisier than those obtained at higher concentrations, it appears that the degree
722 J. Phys. Chem., Vol. 100, No. 2, 1996
Figure 7. SERS spectra at Au100-Au90Ag10 colloids made 20 mM in p-NDMA. See Figure 2 for conditions. Only six of the 11 spectra are labeled.
Figure 8. Optical absorption spectra (after 4-fold dilution) of Au100Au90Ag10 colloids made 20 mM in p-NDMA.
of aggregation is again roughly the same for all BPE solutions. Considering theoretical investigations of closely spaced colloidal particles,1-8,35 it is likely that these small absorbance changes derive from the creation of dimers, trimers, or similar lownuclearity structures. In accord with these findings, we have shown that, in aggregates derived from 20 nm diameter Au, removal of all particle clusters with effective diameters greater than 100 nm eliminates only 90-95% of the SERS activity.34 The optical spectrum of the residual clusters (i.e., those with effective diameters
