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A Surfaced-Enhanced Raman Spectroscopy and Density Functional Theory Study of [Au(CN)2]/[Au(CN)4] Adsorbed on Gold Nanoparticles Michael B. Jacobs,*,†,§ Paul W. Jagodzinski,† Travis E. Jones,‡ and Mark E. Eberhart‡ † ‡
Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401, United States Molecular Theory Group, Colorado School of Mines, Golden, Colorado 80401, United States ABSTRACT: Surface-enhanced Raman spectroscopy (SERS) can be used to study chemical reactions on metal surfaces. Its detection limit makes it possible to observe long-lived intermediates and conformational changes to reactants and products. However, the interaction between the surface and the adsorbate makes interpretation of SERS spectra difficult. A case in point is provided by our studies of gold nanoparticles exposed to aqueous cyanide solution. These systems are characterized by time-dependent SERS spectra, suggesting either conformational or chemical changes to gold adsorbed cyanide species. Though there has been conjecture as to the nature of the process responsible for the evolving spectra, there is no definitive answer. Herein, we report the results of first-principles studies that, when combined with the SERS data, suggest that the observed shift of the SERS signals is attributable to the transformation of dicyanoaurate ions ([Au(CN)2]) to tetracyanoaurate ions ([Au(CN)4]) adsorbed to the metal. In the former case, we argue that the C∞ axis of the ion is oriented parallel to the surface, and in the latter, the 4-fold axis is normal to the gold (111) surface. Further, the [Au(CN)4] complex is slightly distorted from its D4h symmetry to enhance surface bonding.
’ INTRODUCTION Surface-enhanced Raman spectroscopy (SERS) is a sensitive technique for analyte detection and identification in biological, catalytic, chemical, electrochemical, and organic systems. Its sensitivity derives from the enhancement to Raman-active modes when an analyte adsorbs to metal surfaces. In some instances, the enhanced signal is sufficient to allow for single molecule detection.1,2 This capability makes SERS a powerful tool in the effort to study, understand, and thereby control surface processes. Unfortunately, there are difficulties associated with the interpretation of SERS spectra. For many spectroscopic techniques, the principles of symmetry can be used to distinguish between closely related analytes. In SERS, this is seldom the case, for chemisorption of an analyte to a metal surface will break symmetry and mix Raman signals that would otherwise serve as molecular fingerprints. Furthermore, it is possible that the Raman signals from two closely related functionalities, simply through their differing proximities to the metal surface, can be enhanced to different extents and further mask symmetry effects. Thus, although SERS affords a limit of detection suitable for following chemical processes occurring on metal surfaces, it lacks the necessary specificity. The gold cyanide system has been studied quite extensively via SERS. Instances where lack of specificity has made it difficult to fully interpret the information SERS provides are common. An example is provided by our investigation of gold particles exposed to aqueous sodium cyanide. The SERS spectra for this system were recorded at roughly regular intervals for a period of approximately 35.5 h. The resulting spectra shown in Figure 1, r 2011 American Chemical Society
spectra ad, are a result of the sample aging and are characterized by a band at 2141 cm1 (Figure 1, spectrum a), a doublet at 2141 and 2189 cm1 (Figure 1, spectra bd), two bands at 298 and 400 cm1 (Figure 1, spectra a and b), and three bands at 298, 400, and 539 cm1 (Figure 1, spectra c and d). Most significant are the time-dependent features of the spectra. When the solution was aged over 8 h, a shoulder appears at 2189 cm1. This shoulder becomes more significant with continued aging until the band at 2141 cm1 is less intense than the band at 2189 cm1, as is evident in Figure 1. At the same time, there are changes in the SERS spectra in the low-frequency region between 200 and 600 cm1. Inspection of Figure 1 shows a slight decrease in the intensity of the bands at 298 and 400 cm1 beginning approximately 1 h following the initial mixing. After 12 h (Figure 1, spectrum c), a third band becomes apparent at 539 cm1. The intensity of this band increases throughout the experiment (∼35.5 h) while the band at 298 cm1 broadens (Figure 1, spectrum d). Uncertainties as to the enhancement mechanism compound the difficulties associated with the interpretation of SERS spectra. We have been studying gold nanoparticles in the presence of aqueous cyanide solution and have observed spectra that result in a daedal situation unresolved by current literature reports. Here, we show that electronic density functional theory (DFT) can be used in combination with SERS data to give remarkably detailed Received: April 6, 2011 Revised: October 17, 2011 Published: October 18, 2011 24115
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Figure 1. SER spectra of 4.0 104 M NaCN mixed with gold nanoparticles, with a collection time of 10 s: (a) aged ∼1.0 h, (b) aged ∼10.0 h, (c) aged ∼12.0 h, (d) aged ∼35.5 h.
information about the structure and conformation of molecules adsorbed to a metal surface. Specifically, it is demonstrated that the long observed shift of the SERS signals associated with the adsorption of dicyanoaurate ions ([Au(CN)2]) to a gold surface results in the transformation of this ion to tetracyanoaurate ions ([Au(CN)4]) adsorbed to the metal so that its 4-fold axis is oriented normal to the gold (111) surface. This research provides a paradigm through which to combine DFT calculations with SERS data to give more detailed information about surface bonding and coordination than could be obtained by either method alone. Second, the results summarized here strongly support the speculation that, when adsorbed to gold nanoparticles, [Au(CN)2] transforms to [Au(CN)4]. Finally, we discuss the orientation of [Au(CN)2] and [Au(CN)4] complexes to the gold surface.
’ EXPERIMENTAL SECTION Sample Preparation and Instrumentation. The gold nanoparticle sample solution was prepared using a sodium citrate (Aldrich; 99%; Milawaukee, WI) reduction of hydrogen tetrachloroaurate(III) hydrate (HAuCl4 3 xH2O, Aldrich, 99.99%).35An aqueous solution of the gold salt was prepared by dissolving 240 mg of hydrogen tetrachloroaurate(III) hydrate in 500 mL of nanopure water. The solution was then heated to boiling in a conventional microwave (Sunbeam SBM 7500W) on the high setting (600 W). The solution was removed from the microwave, and 50 mL of a 1% w/w sodium citrate solution was added to the boiling solution. This changed the color of the solution from a bright yellow to a pale yellow color and then to a red wine color.
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The particle size is ∼15 nm in diameter, as shown by a UV visible absorption band at 520 nm.3,5 The solution was then returned to the microwave and heated for 15 min at the high setting (600 W). Preparation of the gold nanoparticle solution using microwave heating produces particles that are more uniform in size and shape compared to those prepared by standard boiling methods.3,5 The gold nanoparticle solution was stored at room temperature in the dark to prevent any degradation. A prepared stock gold nanoparticle solution has a lifetime of approximately 68 months. Aggregation occurs over time, indicated by a color change from red wine to maroon to purple, at which time the gold colloid solution is no longer suitable for experimental procedure. Raman samples were prepared in borosilicate glass test tubes after the gold nanoparticle solution was allowed cool to room temperature. Raman samples involving cyanide ions and the gold nanoparticle were prepared by adding 0.02 mL of a 1.0 103 M aqueous sodium cyanide (Baker, 98.3%) solution to 1.8 mL of the gold nanoparticle solution. The final concentration of the sodium cyanide was 1 104 M. Upon addition of the sodium cyanide salt, the solution changed from a red wine color to purple at the top of the test tube. This color change is an indication of aggregation due to the presence of the adsorbate. The sample was mixed immediately after the addition of the sodium cyanide, and the color of the entire solution remained a red wine color. SERS spectra were collected immediately after the samples were mixed. Raman spectra were collected using a spectrometer consisting of a Lexel (Fremont, CA) Ramanion krypton laser operating at 647.1 nm with approximately 40 mW at the sample and a Spex Industries (Edison, NJ) 1877 Triplemate System equipped with a 600 grooves/mm grating and an Oxford CCD detector operating at 140 K. A 90° geometry was used for collection of the scattered radiation. The filter stage slit width was set at 7.0 mm for a spectral bandpass of 42 nm. The instrument was calibrated with benzonitrile (Sigma-Aldrich, 99%), and the reported frequencies are accurate to (2 cm1. The typical collection time for each sample was 10 s. The samples for spectral collection were in the 10 75 mm (Pyrex or Fisherbrand) test tubes in which they were prepared. Each test tube was covered with Parafilm after the addition of aqueous cyanide. Each experiment (from stock solution and sample preparation through spectral collection) was repeated three times to ensure reproducibility. Computational Details. All calculations were performed using the Amsterdam Density Functional package version 2009.01 (ADF).6,7 A core double-ζ, valence triple-ζ, doubly polarized (TZ2P) basis set, and zero order regular approximation (ZORA) relativistic corrections were used for the calculations. The first step in modeling the surface/adsorbate interaction was determining the size of the gold cluster that is qualitatively representative of the gold nanoparticle surface. We chose the low-energy, gold (111) surface, ignoring the herringbone surface reconstruction. We then built three clusters: 11, 31, and 65 atoms. The 31-atom cluster was composed of two layers, whereas the 11- and 65-atom clusters had a single gold atom capping the second layer to avoid a singly occupied molecular orbital (SOMO), which would necessitate a spin-polarized calculation. A neutral adsorbate was placed on each of the surfaces and a constrained geometry optimization was performed, wherein the surface atoms were held fixed at their bulk positions, a = 4.08 Å, while the adsorbate was allowed to relax. This process was repeated on each surface for all three possible adsorbates: cyanide, dicyanoaurate, and tetracyanoaurate. The properties 24116
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Figure 2. Correlation diagrams for [Au(CN)2] on the different surfaces. No qualitative differences can be seen between the different cases.
of these different systems were then compared to check for convergence. It is known that the appropriate cluster size depends on the properties of interest. In our experience, the total energy is slow to converge, whereas the molecular orbitals (MOs) often do so in 12 coordination spheres. Thus, we chose to use the MOs in this study. A comparison of the MOs between the 11-, 31-, and 65-atom clusters agrees with this assertion. Although the binding energy of [Au(CN)2] on the 31-atom surface is 22.6% higher than that on the 11-atom surface and [Au(CN)2] on the 65-atom surface is a further 4.5% lower in energy, the orbitals change very little across the three surfaces, as can be seen in Figure 2. As such, the 11-atom cluster is the minimum-sized surface necessary to capture the chemistry involved in these adsorbate/surface interactions. However, because the binding energy of the 31-atom cluster is nearly converged, we chose to use the 31-atom surface throughout this study to allow the computation of reliable frequencies in addition to an investigation into the nature of the analyte surface interactions by way of the mixing of their MOs.
’ RESULTS AND DISCUSSION SERS Spectra. Our research group was engaged in the fundamental studies to elucidate the interactions between aqueous CN ions and gold nanoparticles when we observed a progressive evolution of the SERS spectra, suggestive of a chemical change to the adsorbed species. Although our gold surface is not a Au electrode, there is an extensive literature base on in situ SERS studies done on Au electrode surfaces complementary to the SERS work herein.817 This report shows a doublet (SERS signals at 2141 and 2189 cm1 shown in Figure 1, spectrum d) in
the 20002200 cm1 region, and the shifts of both the high- and low-frequency regions as a function of time (sample aging). Previous work provides some assistance in identifying the species and the modes responsible for the SERS signals. Similar bands in the high-frequency region between 2000 and 2200 cm1 were observed by others, shown in Table 1. Liao and Stern observe a similar shift of the SERS signal at 2144 cm1 to an equally intense signal at 2191 cm1.18 They attribute this to the formation of an unspecified complex containing cyanide. Two unindentified cyanide species were observed via SERS with gold particles in a solgel at ∼2150 cm1 and the other at ∼2200 cm1.19 Dorain et al.20 assigned a signal observed at 2140 cm1 to adsorbed [Au(CN)2] and tentatively assigned the signal at 2190 cm1 to adsorbed [Au(CN)4] or a hydrated ion on the nanoparticle upon the addition of an oxidant. However, they did not report any mode assignments for the 200600 cm1 region. We have added aqueous CN to our prepared gold nanoparticles via a citrate reduction. Aqueous CN reacts with gold to form [Au(CN)2] in the presence of oxygen. It has been reported that, under atmospheric conditions, the formation of [Au(CN)2] occurs upon the addition of aqueous CN to gold colloids prepared via excess reducing agent.21 Von Raben et al.21 observed a signal at 2138 cm1 assigned to adsorbed [Au(CN)2]. Murray et al.22 reported a signal in the high-frequency region at 2131 cm1 (CN stretching motion ν1) and attributed this, not to adsorbed [Au(CN)2], but instead to CN adsorbed in a linear orientation bound through the carbon atom. They also observed a SERS band at 2181 cm1 attributed to adsorbed CN interacting via a diagonal orientation through the carbon atom. In addition, Hesse et al.23 reported a SERS signal at 2139 cm1 due to adsorbed CN and not [Au(CN)2]. They also observed a similar SERS signal at 2190 cm1 and assigned it to adsorbed 24117
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Table 1. Mode Assignments in cm1 for Adsorbed Species on Gold Surfaces adsorbed species
Au surface
ν1(CtN)
δ(AuCtN)d
ν(AuCN)d
ref
[Au(CN2)]
colloid
2140
NA
NA
[Au(CN4)]
colloid
2190
NA
NA
20
CN
island film
2139
NA
NA
23
[Au(CN4)]
island film
2190
NA
NA
23
unidentified CN complex
particle
2144, 2191
NA
NA
18
unidentified CN complex
particle in a solgel
∼2150, ∼2200
NA
NA
19
[Au(CN2)]
colloid
2138
NA
NA
21
CN CN
island film island film
2131 2181
291 320
385 500
22 22
CN
electrode
∼2100
∼300
∼370
9, 25
[Au(CN2)]
nanoparticle
2141a
298a
400a
this work
[Au(CN4)]
nanoparticle
2141a, 2189b
∼298c
400a, 539b
this work
515a, 450b
this work
[Au(CN4)]
DFT
a
b
c
2152 , 2224
289
20
a
Indicates that cyanide groups are bent toward the surface. b Indicates that cyanide groups are parallel to the surface. c Indicates two AuCN bending modes, one for the cyanide groups bent and one for the cyanide groups parallel. d NA = no assignment.
[Au(CN)4 ] based on the reports of Dorain et al. and Jones.20,23,24 On a Au electrode surface, the CN stretching band is observed at ∼2100 cm1 due to linearly adsorbed CN.9,25 Few reports contain data for the low-frequency region between 200 and 600 cm1, as shown in Table 1. Murray et al.22 observed SERS signals at 291, 320, 385, and 500 cm1 and provided mode assignments assuming that CN is adsorbed to the gold surface with different orientations. Bozzini et al. reported a AuCN, δ(AuCN), bend at 298 cm1 and a AuC stretch, ν(AuCN), at 370 cm1.8 This is in agreement with two lowfrequency bands at ∼300 and ∼370 cm1 observed by Beltramo et al.25 A reverse assignment was given by Gao and Weaver: the band at 300 cm1 corresponds to the AuCN stretching mode, and the band at 370 cm1 corresponds to the AuCN bending mode.14,15 On the basis of these previous studies, we are comfortable making the following assignments. Starting with the high-frequency region, in Figure 1, spectrum a, we assigned the peak at 2141 cm1 to the CN ν1(∑g+) stretching motion due to the adsorbed complex ion [Au(CN)2]. This assignment is also given in comparison with the wavenumber (2164 cm1) of the CN ν1(∑g+) vibration of [Au(CN)2] ions in aqueous solution.20,24 Next, we assigned the SERS signals resulting from the progression shown in Figure 1, spectra bd. Similarities have been observed in SERS spectra under different oxidizing conditions.18,20,22,23 Samples in our experiment were exposed to atmospheric oxygen. We assigned the signal appearing at 2189 cm1 to the ν1 (a1g) CN stretching motion of the adsorbed complex ion [Au(CN)4]. This assignment is based on the wavenumber (2209 cm1) of the a1g ν1 CN vibration of [Au(CN)4] ions in aqueous solution.20,24 The SERS results in the low-frequency region between 200 and 600 cm1 are much more difficult to interpret and assign than those in the high-frequency region. Compared with previous studies, this is the first report to show three bands in the low-frequency region (Figure 1, spectrum d). We do not observe a shift in the wavenumbers of the SERS signals at 400 cm1 and at 298 cm1 (the later undergoes band broadening) (Figure 1, spectra ad). The SERS signal at 298 cm1 is tentatively assigned to the AuCN bending motion, δ(AuCN), the signal at 400 cm1 to the AuCN stretch, ν(AuCN), and the signal at 539 cm1 is assigned to a AuC stretch, ν(AuCN), based on similar assignments given in the literature.8,20,22,24,25
The SERS bands in our observed results cannot be assigned definitively. It appears that these bands may be due to CN, [Au(CN)2], or [Au(CN)4] based on previous literature reports.9,14,15,1823 First-principles methods have been shown to be helpful when interpreting SERS data.8,2533 Using our SERS data in combination with DFT, we provide new information as to the structure and orientation of the species giving rise to our observed signals on a gold nanoparticle surface. DFT Studies of Adsorbed Gold/Cyanide Complexes. The first step in modeling the complexes on a gold surface was to determine the orientation of the adsorbed ions. Two models were chosen to represent the extreme orientations of the CN and [Au(CN)2] ions. These are designated end-on or flat-on depending on the angle between the adsorbate internuclear axis and the surface. Both linear ions have a C∞ internuclear axis. If the angle the C∞ axis makes with the gold surface is greater than 45°, then it is considered to be in an end-on orientation, if less than 45°, it is considered to be in a flat-on orientation. Most often, a molecule with a lone pair, such as nitrogen, interacts with the metal surface through the lone pair, in a perpendicular or end-on fashion. In the case of cyanide, there is an alternate end-on orientation where the ion interacts with the surface through the carbon atom. Despite the typical assumption that the ions will adopt the end-on orientation, we have also considered the possibility that the linear ions may adopt the flaton orientation. In cyanide, this offers no obvious benefit, whereas in the case of the dicyanoaurate, it allows the lone pairs from both terminal cyanide groups to interact equally, albeit to a lesser extent, with the surface. Insight into the orientation can be garnered through a comparison of the one-electron orbitals associated with each orientation. This was done for both orientations of the linear molecules by inspecting each MO that has character on the adsorbate and the gold surface to determine if it was bonding or antibonding between the surface and the adsorbate. In the case of cyanide, we found strong AuC bonding interactions when the ion was oriented end-on with carbon on the surface, but a mix of bonding and antibonding interactions in the other orientations. However, the MOs of dicyanoaurate in the end-on configuration appear to have a large amount of antibonding character, whereas in the flat-on orientation, they are more bonding in character. These observations 24118
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Figure 3. Simplified correlation diagrams for the [Au(CN)2] molecule in the (a) “end-on” and (b) “flat-on” conformations. The lower-lying adsorbate orbitals are in the pink region. The higher-lying adsorbate orbitals are in the yellow region.
allowed us to determine that the cyanide ion will chemisorb to the surface end-on, interacting through its carbon atom, whereas, unexpectedly, the dicyanoaurate favors the flat-on orientation. The origin of this anomalous behavior can be seen by way of the orbital correlation diagrams shown in Figure 3, which illustrate the strength of mixing between the [Au(CN)2] complex ion and the surface. The portion of the diagram is composed of three columns. MOs of the surface are shown on the left, those of the complex ion on the right, and those of the combined surfaceion lie in the center. The lines connecting the central MOs to the complex ion and gold surface orbitals provide a visual representation as to the extent to which one set of molecular orbitals interacts with another set so as to lower the combined orbital energies. For clarity, these diagrams show only two representative orbitals drawn from the entire molecular orbital (MO) manifold. The coloring reflects the density of nodes in the gold surface orbitals. Surface MOs falling within the yellow-colored region are characterized by high nodal density, whereas those in the pink-colored region are typified by low nodal density. The factors that will influence the extent of interaction can be inferred from the first-order correction to the wave function from RayleighSchr€odinger perturbation theory Ψn ð1Þ ≈Ψn ð0Þ þ
^ n ð0Þ >
