Article pubs.acs.org/Langmuir
Quaternary Ammonium Bromide Surfactant Adsorption on LowIndex Surfaces of Gold. 2. Au(100) and the Role of CrystallographicDependent Adsorption in the Formation of Anisotropic Nanoparticles J. P. Vivek† and Ian J. Burgess* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5C9 Canada ABSTRACT: A qualitative and quantitative description of the coadsorption of a quaternary ammonium bromide surfactant on Au(100) has been determined using electrochemical techniques. Cyclic voltammetry reveals that both the cationic surfactant ion and its halide counterion are adsorbed on the surface of unreconstructed Au(100) over a wide range of electrode potentials or charge densities. The relative Gibbs excesses of the cationic and anionic components of octyltrimethylammonium (OTA+) bromide have been determined using the thermodynamics of ideally polarized electrodes. Coadsorbed OTA+ does not strongly affect the behavior of bromide layers on Au(100) with low-coverage films being replaced by commensurate overlayers at positive electrode charge densities. The presence of surface bromide allows for the stabilization of adsorbed OTA+ at positive polarizations. Furthermore, charge-induced phase changes in the bromide layer lead to subtle but appreciable changes in the surface excesses of OTA+ ions which is consistent with a hierarchical model of surfactant adsorbed upon a halide-modified Au(100) surface. A comparison of the OTA+ adsorption isotherms on Au(100) and Au(111) reveals that the presence of coadsorbed bromide does not lead to preferential accumulation of cationic surfactant ions on a particular crystal facet. These results are inconsistent with explanations of anisotropic nanoparticle formation that invoke a thermodynamic argument of preferred surfactant adsorption on different crystal facets of an embryonic nanoparticle seed crystal.
1. INTRODUCTION The formation of anisotropic nanoparticles is a subject of intense research interest due to their unique and tunable optical properties. For example, assemblies of anisotropic metal nanoparticles have been shown to create massive localized electric fields, or hotspots, for surface enhanced Raman spectroscopy.1−4 The existence of a longitudinal plasmon excitation in metal nanorods provides opportunities for near-IR biosensing applications.5,6 Similarly, gold nanorods have been shown to be effective drug-delivery vectors,7 substrates for photothermal therapeutics,6 and contrast agents in medical imaging.8 Critical to these applications is the formation of a nonspherical, nanosized (5−100 nm) metallic core, and there are now numerous methodologies in the literature that outline synthetic strategies to produce such anisotropic nanoparticles. However, several critical notions associated with these synthesizes remain unproven including the role of surfactant/ stabilizer preferential adsorption on different crystallographic facets of an embryonic seed crystal. It has been established that the nature,9−12 concentration,10,13 and even the level of impurity halide ions plays a critical role in gold nanorod formation.14 This has led to the supposition that the presence of bromide in the synthesis leads to preferential adsorption of © 2012 American Chemical Society
surfactant molecules and subsequent directed growth along certain low-index crystallographic directions.9,15 However, studying the interface of a growing nanoparticle with crystallographic sensitivity in a milieu as complex as that used for nanorod synthesis is an Augean task, and rather unsurprisingly there is no direct experimental evidence in support of these hypotheses. In this work we test the basic premise that quaternary ammonium surfactants, such as the CTAB (cetyltrimethylammonium bromide) stabilizers used in Murphy’s synthetic protocol,16−19 show preferential equilibrium adsorption on the Au(111) surface compared to the Au(100) surface. We have previously demonstrated that there is little difference in the surface concentration of a model quaternary surfactant (the octyltrimethylammonium cation, OTA+) on reconstructed Au(100)-hex and unreconstructed Au(100)-1 × 1,20 the former of which has a near identical surface crystallographically as Au(111).21,22 Those studies were performed in the absence of bromide ions and various groups have shown that bromide Received: January 3, 2012 Revised: February 25, 2012 Published: February 29, 2012 5040
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adsorption is considerably different on Au(100) compared to Au(111).23 The measurement of significant differences in the coadsorption of OTA+ and Br− on these low index crystallographic surfaces would lend strong evidence supporting the theory that preferential adsorption leads to anisotropic nanoparticle growth. Therefore, the objective of this work is to assess the differences in surfactant coverage on Au(100) and Au(111) surfaces in the presence of bromide ions. Rather than working with polymorphic, nanosized particles dispersed in the complex and dynamic solution conditions used for nanoparticle synthesis, one can use macroscopic (surface area ∼0.2 cm2), cut, polished, and oriented single crystals exposed to simple solutions composed of well-defined supporting electrolytes. The approach employed herein relies on detailed electrochemical characterization and the thermodynamics of ideally polarized gold-solution interfaces. This methodology has been shown by our group24,25 and others26 to be an effective means of providing valuable insight into molecular adsorption pertinent to nanoparticle and colloidal systems. The adsorption behavior OTA+/Br− on Au(111) has been discussed in part 1,27 and herein, we characterize the adsorption behavior of OTA+/ Br− on Au(100). Quantification of both the halide and the surfactant Gibbs surface excess as a function of the metal surface’s electrical variable is achieved using the chronocoulometric method made popular by Lipkowski’s group.28 Finally, the isotherms for OTA+ and bromide on the two crystallographic surfaces will be compared to assess an underlying premise pertaining to the formation of anisotropic gold nanoparticles.
Figure 1. Cyclic voltammogram (20 mV/s) of Au(100) electrode in 0.10 M HClO4. Au(100) surface.32−35 The electrode potential is then stepped and held at a variable potential, −1.00 < Evar. ≤ +0.45 V, for a sufficient duration (∼180 s) to achieve an equilibrated state of surfactant and/or halide adsorption. A current transient is measured upon a potential step to Edes = −1.00 V. The electrode is biased at the desorption potential for only 200 ms which minimizes the slow kinetics associated with the charge-assisted Au(100)-1 × 1 to Au(100)-hex reconstruc-
2. EXPERIMENTAL SECTION Reagents and Solutions. Details concerning the procurement and treatment of simple electrolytes (NaF and NaBr) are provided in the companion paper. 27 A description of the synthesis of octyltrimethylammonium triflate (OTATf) is provided in an earlier paper,20 and part 1 contains important information with regards to preparing OTATf solutions. Fresh surfactant solution was made before each set of measurements and kept incubated at 30 °C to minimize any possible precmc aggregation. All solutions were prepared with Milli-Q ultrapure water (>18.2 MΩ cm). Single-Crystal Fabrication. The method used for fabricating and orienting single-crystal electrodes is detailed elsewhere.3 The quality and cleanliness of the polished electrode was judged by recording a cyclic voltammogram in 0.1 M perchloric acid using the hanging meniscus configuration. Figure 1 reveals that the measured CV is in excellent agreement with published voltammograms for Au(100).21,29−31 The potential of zero charge, Epzc, for the thermally annealed Au(111) electrode was determined from the position of the double-layer minimum in capacitance measurements performed in 5 mM KClO4. The values of Epzc were found to be 260 mV vs SCE for the thermally reconstructed Au(100) crystal and 70 mV for the unreconstructed surface. These values are in close agreement with values reported in the literature.21,22,30 Electrochemical Measurements. All electrochemical measurements were done in 0.10 M sodium fluoride electrolyte (pH ∼ 8) under an argon atmosphere with the single crystal electrode in a hanging meniscus arrangement. A detailed description of the electrochemical setup and experimental methodologies have been provided previously.24 The chronocoulometric method used to determine the charge density, and subsequently, the relative Gibbs surface excess information, was modified from the sequence of potential steps described in part 1. An effort was made to minimize the time held at negative potentials where charge-induced reconstruction of the Au(100) crystal is known to occur. This can be achieved by applying a base potential (Ebase) of sufficiently positive potential where bromide adsorption (+0.15 V) results in a 1 × 1 (unreconstructed)
Figure 2. Potential step sequence used for chronocoulometry experiments with an unreconstructed Au(100) electrode. tion.21,22,33,36 To further minimize reconstruction effects, Figure 2 illustrates how the variable potential was descended from +0.45 to −1.00 V rather than the more common approach of stepping the variable potential away from the desorption potential. We have used a similar methodology in earlier chronocoluometry studies with Au(100)20 as have Prado et al.37
3. RESULTS AND DISCUSSION Cyclic Voltammmetry. The cyclic voltammograms in Figure 3a provide the basis for a general description of bromide and OTA+ adsorption on Au(100). The CVs of a Au(100) electrode in the presence of 0.10 M NaF supporting electrolyte, electrolyte plus 1.0 mM NaBr, and electrolyte plus 1.0 mM OTATf are overlaid to provide a qualitative description of the potential dependent behavior of the individual components. The CVs shown are those after repeated cycling to positive potentials where the thermal Au(100)-hex reconstruction is lifted and the surface is restored to Au(100)-1 × 1.22 However, 5041
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adsorption/desorption peaks centered at ∼−0.70 V, the CV merges with that of the electrolyte indicating complete desorption. The rising portion of this CV found at the most positive potentials has been discussed in our earlier work and arises from a phase transition within the adsorbed OTA+ layer on unreconstructed Au(100).20 Figure 3b shows the CV observed for 0.10 M NaF in the presence of both 1.0 mM NaBr and 1.0 mM OTATf. The sharp P1 peak corresponding to the lifting of reconstruction followed by the formation of a two-dimensional adlayer of bromide is greatly diminished and slightly shifted to more positive potentials (ΔE ∼ 25 mV in the presence of 1.0 mM OTA+). The P2 peak that corresponds to commensurate bromide layer formation in the absence of surfactant is also significantly shifted to more positive potentials (ΔE ∼ 75 mV in the presence of 1.0 mM OTA+). After the addition of the cationic surfactant, a broad plateau region is developed between peaks P1 and P2, suggesting the formation of a stable layer of adsorbate(s). The voltammetry is largely unaffected at negative potentials and the OTA+ ads/des peaks are slightly more symmetric about E ∼ −0.70 V compared to Figure 3a. Increasing the OTA+ concentration in the electrolyte while keeping the bromide concentration fixed leads to (1) a shift in the surfactant adsorption peaks to further negative potentials, (2) increased suppression (in magnitude) of peaks P1 and P2, and (3) increasing anodic shifts in peaks P1 and P2. Conversely, when the concentration of bromide in the electrolyte is varied under conditions of fixed surfactant concentration, the position of the ads/des peaks at negative potentials are unaffected but both peaks P1 and P2 shift cathodically and increase in magnitude. Cumulatively, these results indicate that the adsorption of OTA+ suppresses the kinetics associated with the formation of ordered bromide layers on the Au(100) surface. Although the voltammetry strongly indicates that the coexistence of adsorbed bromide and OTA+ over a wide range of electrode potentials, the qualitative data afforded from these measurements is insufficient to determine whether this is competitive or synergistic coadsorption. Chronocoulometry. The basic experimental strategy to extract quantitative information is the same as that adopted in part 1 for the study of the Au(111)−OTA+/Br− system. Using the modified chronocoulometric method described above, the charge density at the electrode−electrolyte interface was measured as a function of applied potential for two series of electrolytes. In series 1 experiments, a variable OTA+ concentration and a constant bromide (1.0 mM) concentration was used; whereas in series 2, the bromide concentration was varied while retaining a fixed OTA+ concentration (1.0 mM). In Figure 4a, the charge curves for series 1 chronocoulometry experiments are shown as well as the σm−E curve for the 0.10 M NaF supporting electrolyte (dotted line). Comparing the curve for the electrolyte plus 1.0 mM Br− (dashed line) to that of the pure electrolyte reveals that bromide only adsorbs on the surface at potentials more positive than −0.50 V. The steep increase in charge starting at −0.40 V is consistent with the onset of the broad feature observed in the voltammetry as are the inflections in the charge density curves at −0.10 V and +0.10 V. The charge curves for variable OTA+ concentrations (colored lines with symbols in Figure 4a) show that the surfactant starts adsorbing at potentials more negative to the onset of bromide adsorption on the Au(100) surface. The sigmoidal sections of the curves associated with OTA+
Figure 3. (a) Cyclic voltammograms (20 mV/s) of Au(100) in 0.10 M NaF supporting electrolyte (black, dotted line) in the presence of 1.0 mM NaBr (red, solid line) and in the presence of 1.0 mM OTATf (blue, dashed line). (b) Cyclic voltammograms (20 mV/s) of Au(111) in 0.10 M NaF supporting electrolyte in the presence of both 1.0 mM NaBr and 1.0 mM OTATf.
it is important to note that the voltammograms presented are not true representations of adsorption on an unreconstructed Au(100) surface as minor amounts of reconstruction may happen as the electrode is cycled through negative potentials. In the first cycle of a thermally annealed Au(100) electrode in 0.10 M NaF supporting electrolyte, the potential was scanned to 0.50 V vs SCE and a strong voltammetric peak (j ∼ 8 μA cm−2) was observed at 0.40 V (not shown) corresponding to the potential induced lifting of the reconstruction.22 Subsequent cycles show an increasingly attenuated and cathodically shifting peak that eventually becomes the small feature observed at 0.10 V in the steady-state CV shown as the black dotted line in Figure 3a. When 1.0 mM NaBr was added to the electrolyte, the voltammetry is dominated by two sharp peaks in the positive-going potential scan (solid red line in Figure 3a). The first peak, P1, occurs at ∼−0.10 V and corresponds to the lifting of the reconstruction38,39 followed by the formation of a low-coverage adlayer of bromide. The presence of this peak indicates that excursions to negative potentials provide sufficient time and driving force to at least partially reconstruct the Au(100) surface. A second sharp peak, P2, corresponding to the formation of the commensurate c(√2 × 2√2)R45° bromide layer is found at 0.125 V.38 The positive potential limit was restricted to E < 0.35 V and hence the peak associated with the transition between the c(√2 × 2√2)R45° and the incommensurate c(√2p)R45° adlayer phase was not observed. The CV for 1.0 mM OTATf in the absence of bromide ions is shown as the dashed line in Figure 3a. Between −0.50 and 0.20 V, the double layer charging currents in the presence of the cationic surfactant are smaller in magnitude than those of the supporting electrolyte indicating that OTA+ is adsorbed on the electrode surface. At potentials more negative than the 5042
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steep increase in the charge at ca. −0.20 V. The slopes of the curves are consistent with the large inner layer capacitance expected for the formation of a layer of specifically adsorbed small inorganic anions such as Cl−, Br−, I−, or SO42−.40 Increasing the amount of bromide in the electrolyte shifts the steep portion of the charge plots to more negative potentials but has no effect on the OTA+ adsorption found at ca. −0.60 V which is consistent with the voltammetry experiments. The chronocoulometry method relies on the premise that any adsorbed molecules are completely displaced from the electrode surface upon the step to the desorption potential, Edes, thus negating any “memory” of the path taken to reach adsorption equilibrium (i.e., the order in which components of the electrolyte are added should be entirely irrelevant). The data in Figures 4a and b contain a common data set (namely an electrolyte composed of 1.0 mM NaBr plus 1.0 mM OTATf) and differ only in the sequence taken to reach their final composition. To test the reproducibility of the data, Figure 4c overlays the common charge curves for series 1 and 2. The two curves are almost entirely superimposable and demonstrate the self-consistency of the experimental data. Gibbs Surface Excesses. The two series of charge density data were analyzed using the thermodynamics of ideally polarized electrodes and the same arguments constructed in part 1. The Gibbs surface excess of OTA+ in the presence of 1.0 mM Br− on Au(100) determined from the charge curves in Figure 4a is plotted in Figure 5a. The maximum in the Gibbs excess is obtained at ∼−0.35 V and slightly shifts to negative potentials with increasing OTATf concentration. A quasiplateau region with slightly lower Gibbs excesses can be observed over the range −0.30 < E < 0.0 V. For the highest concentration of OTATf studied (1.0 mM), the maximum
Figure 4. Charge density as a function of applied potential curves for 0.10 M NaF supporting electrolyte (dotted lines) with series 1 and 2 electrolytes (colored lines with data points). (a) Series 1 electrolytes contain 1.0 mM NaBr (black, dashed line) plus 0.01, 0.025, 0.050, 0.075, 0.10, 0.15, 0.25, 0.50, 0.75, and 1.0 mM OTATf. (b) Series 2 electrolytes contain 1.0 mM OTATf (black, dashed line) plus 0.01, 0.025, 0.050, 0.075, 0.10, 0.15, 0.25, 0.50, 0.75, 1.0, and 1.5 mM NaBr. (c) Charge data for 1.0 mM OTATf and 1.0 mM NaBr electrolytes taken from series 1 measurements (red circle) and series 2 measurements (black square).
adsorption progressively become more pronounced and shift to increasingly negative potentials with increasing surfactant concentration in the electrolyte until, at the highest surfactant concentration studied (1.0 mM), OTA+ is seen to start to adsorb at −0.80 V. Figure 4a also shows that the influence of varying the surfactant’s concentration in the presence of a fixed amount of bromide in the electrolyte is negligible at potentials positive of −0.30 V. In general, the qualitative trend of the charge curves for series A is very similar to that observed for the same set of measurements on Au(111). Charge curves for series B electrolytes (variable bromide concentration and 1.0 mM OTATf) are shown in Figure 4b. In the absence of bromide ions, the charge curve for OTA+ merges with that of the pure electrolyte for E < −0.80 V and otherwise shows one sigmoidal feature indicative of a single state of surfactant adsorption within the range of potentials studied. The addition of bromide ions to the electrolyte (colored lines with symbols) does not perturb the σm−E plots at negative potentials but does lead to a
Figure 5. Gibbs surface excesses on Au(100) as a function of potential for (a) OTA+ in the presence of 1.0 mM NaBr plus the variable electrolyte concentrations of OTATf listed in Figure 4 and (b) Br− in the presence of 1.0 mM OTATf plus the variable electrolyte concentrations of NaBr listed in Figure 4. 5043
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value of ΓOTA+ in the presence of 1.0 mM NaBr is ∼3 × 10−10 moles cm−2 which is essentially the same as the corresponding experiment performed using Au(111). At potentials positive of 0.05 V, the surface excess drops to another quasi-plateau region which is also qualitatively similar to the data shown in part 1. The surface excess of bromide in the presence of OTA+ is determined from the charge curves provided in Figure 4b, and the results are plotted in Figure 5b. From this latter plot, it is clear that there is predominant adsorption of bromide on the surface at potentials more positive than −0.40 V. The bromide adsorption isotherms are characterized by two steep slopes that shift monotonically to more negative potentials with increasing levels of bromide present in the electrolyte. The steepest rise is observed over the range 0 < ΓBr− < 4 × 10−10 moles cm−2, and its upper limit corresponds to a bromide coverage of approximately θ = 0.2. This coverage is significantly smaller than that expected for ordered bromide adlayers reported in previous electrochemical studies.23 However, as the potential is increased toward the most positive potentials studied, the surface excesses of bromide begin to exceed 9.0 × 10−10 moles cm−2 which is equivalent to θ = 0.45. This coverage is consistent with the ordered c(√2 × 2√2)R45° bromide adlayer in which each anion is located in a 2-fold bridge site on the Au(100) surface.23 Admittedly, in the absence of scanning probe32,41,42 or surface X-ray scattering verification,34,35,41,43,44 the assignment of a commensurate Br adlayer in the presence of the OTA+ is speculative. Nevertheless, an implication that can be drawn from these data is that the bromide is likely contact adsorbed to the Au(100) surface and the organic cations are adsorbed upon the bromide modified surface. Electrostatic screening of the bromide ions by the quaternary ammonium headgroup would also reduce repulsive interactions and facilitate the induction of the higher coverage c(√2p)R45° usually found at more positive potentials. Such a “sandwich” model has been previously proposed by Jaschke et al. on the basis of atomic force microscopy imaging of CTAB adsorption on Au(111).45 As discussed in part 1, plotting the Gibbs excess isotherm data versus surface charge density rather than applied potential is more beneficial when attempting to relate measurements made on macroscopic electrodes to nanoparticle systems. The surface charge of nanoparticles can be obtained by zeta potential measurements, and in an effort to facilitate possible future comparisons, we have transformed the data in Figure 5 using the Parsons function, ξ = σmE + γ.46 The Gibbs excesses of OTA+ and Br− as a function of the Au(100) surface charge density are plotted in Figures 6a and b, respectively. It can be seen that the maximum adsorption of OTA+ is achieved when the surface is weakly charged and there is a minimal amount of bromide adsorbed on the surface. Bromide is shown to adsorb in a superequivalent fashion at weakly negative charge densities, but its surface excess increases nearly linearly as the electrode’s surface charge is made increasingly positive. The influence of the bromide adsorption on the Gibbs surface excess of OTA+ is more clearly illustrated in Figure 7. Here the Gibbs excess results for both 1.0 mM OTA+ in the presence of 1.0 mM Br− and 1.0 mM Br− in the presence of 1.0 mM OTA+ which are plotted as a function of electrode charge density. At negative potentials, the adsorption of bromide is negligible and the surface excess of OTA+ reaches a limiting value of 2.8 × 10−10 moles cm−2 which is within 10% of the maximum OTA+ coverage found in our previous studies using Au(100) in the absence of coadsorbing halide.20 As the electrode becomes
Figure 6. Gibbs surface excesses on Au(100) as a function of electrode charge density for (a) OTA+ in the presence of 1.0 mM NaBr plus the variable electrolyte concentrations of OTATf listed in Figure 4 and (b) Br− in the presence of 1.0 mM OTATf plus the variable electrolyte concentrations of NaBr listed in Figure 4.
Figure 7. (a) Gibbs surface excesses versus electrode charge density at the Au(100)/0.1 M NaF interface for 1.0 mM OTA+ in the presence of 1.0 mM NaBr (black square) and 1.0 mM Br− in the presence of 1.0 mM OTA+ (red circle).
positively charged, the bromide surface concentration increases but the value of ΓOTA remains largely invariant at 2.5 × 10−10 moles cm−2. The slight decrease in the Gibbs excess of OTA+ after the onset of bromide coadsorption is consistent with an expansion of the effective area of the surfactant’s headgroup.47 The nature of neither the inorganic nor the organic adsorbate layer is readily evident in this region of the isotherms but the low coverage of bromide implies a disordered, gaslike state. Avranas and co-workers have previously described the adsorption of quaternary ammonium bromide surfactants on mercury electrodes48−50 and reported that short-chained species do not form condensed layers50 which is consistent 5044
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with the low-coverage OTA+ isotherms reported in this study. Interestingly, when the surface excess of bromide exceeds 7 × 10−10 moles cm−2 (corresponding to approximately 1/3 of a monolayer), the amount of cationic surfactant at the interface begins to decrease but does not fall to zero as might be expected for a cation at a highly positive charged surface. This can be explained by the fact that the charge from the adsorbed bromide, when summed with the positive electronic charge, results in a net negative surface charge. Thus, at a minimum, the coadsorption of bromide on the Au(100) surface serves to mitigate repulsive electrostatic forces which would otherwise make OTA+ adsorption unfavorable at positive electrode polarizations. The decrease in ΓOTA as θBr approaches 0.5 may be concomitant with the formation of an ordered inner layer of bromide upon which the surfactant aggregates. Gibbs Surface Excesses of OTA+/Br− on Au(111) and Au(100). Having the surface concentration data for OTA+/Br− on both Au(111) and Au(100) surfaces, it is possible to evaluate the influence of crystallography on the magnitude of surfactant adsorption. Since the potential of zero charge of the two crystals is significantly different, the Gibbs surface excess for 1.0 mM OTA+/Br− bulk concentration is plotted on a rational potential scale (E−Epzc) in Figure 8. The data suggests that within the experimental error (the estimated error in surface excess using the chronocoulometry back integration technique is 10%28), there is no significant difference in the adsorption behavior of OTA+/Br− on the two crystals. At positively charged surfaces there is significantly more bromide
adsorbed on the Au(111) surface compared to the Au(100) surface in the presence of OTA+ (Figure 8a, inset). However, this is not unexpected as Wandlowski and co-workers have shown that even in the absence of any other specifically adsorbing species, the surface concentration of bromide is higher on Au(111) compared to Au(100).35 The Gibbs surface excess of OTA + /Br − corresponding to 1.0 mM bulk concentration is plotted against metal charge density in Figure 8b, which also shows that the surfactant has essential the same isotherm on both Au(111) and Au(100). It is only perhaps at the most positive polarization that there is an indication of preferred surfactant adsorption on Au(111). On the basis of these results, it appears that even in the presence of bromide ions, quaternary ammonium surfactants show no preferential adsorption on Au(100) surfaces. This challenges the popular postulate that nanorod growth is based on differences in surfactant coverages on the Au(100) and Au(111) facets of embryonic nanoparticle seeds. It is important to note that the measurements described in these reports are made under equilibrium conditions. It is quite possible that the nonequilibrium conditions associated with nanorod formation and growth could lead to different coverages. However, this work illustrates that there is no thermodynamic basis for invoking a model of preferential R−N(R′)3+ adsorption on different low-index gold facets in the presence of bromide ions. A recent report from Mirkin’s group suggests that it is actually trace levels of iodide impurities in commercial CTAB that lead to the formation of gold nanorods.51 The strong preferential adsorption of iodide on the (111) end-cap facets could provide relatively unencumbered surfaces for the reduction of gold ions compared to the (110) and (100) faces if the higher energy facets are modified by strongly adsorbed CTAB layers. This model would also be highly contentious as contradictory results (i.e., no anisotropic particle formation) concerning the role of iodide have been reported.10,12,52 The methodologies reported in these contributions could be readily applied to iodidecontaining electrolytes. However, it is likely that the formation of anisotropic gold nanorods follows a much more complex mechanism. Other species present in the medium (gold salt, ascorbic acid, etc.) may have a significant role in determining the anisotropy. For instance, the cationic surfactant might form complexes with ascorbic acid and/or gold salt, and it is these species that may have dramatic preferential adsorption on different faces of the seed crystal. The results of this work suggest that a mechanism based on simple quaternary ammonium bromide adsorption on different facets of gold seed crystal is highly unlikely. Further investigation with more sophisticated techniques is necessary to determine the actual growth mechanism of gold nanorods under nonequilibrium conditions. Electrochemical scanning probe microscopy would be an ideal tool to follow the rate of Au(Cl)4− discharge on Au(111) and Au(100) in the presence of quaternary ammonium bromide surfactants such as CTAB.
4. SUMMARY AND CONCLUSIONS Combined, parts 1 and 2 of this study have determined the effect of bromide on the surface concentration of a quaternary ammonium surfactant on two, low-index gold surfaces. Coadsorption of the two ions occurs over a broad range of electrode polarizations and is largely cooperative rather than competitive in nature. The surface concentrations of OTA+ and Br− ions on Au(111) and Au(100) were independently obtained from chronocoulometry measurements over a series
Figure 8. Adsorption isotherms for 1.0 mM OTA+ adsorption in the presence of 1.0 mM NaBr on the Au(111) (black square) and Au(100) (red circle) electrode surfaces as a function of rational potential (a) and surface charge density (b). (inset) Corresponding adsorption isotherms for 1.0 mM Br− adsorption in the presence of 1.0 mM OTATf. 5045
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of concentrations ranging from 0.01 to 1.0 mM. Gibbs excess values have been provided as a function of both metal charge density and a rational potential scale. These results can be used for comparison purposes if the electrical state of the metallic centers in nanoparticle systems can be determined in future studies. On both crystal facets, the onset of bromide adsorption serves to promote the adsorption of the cationic surfactant on a positively charged electrode surface. Even in the presence of coadsorbed quaternary ammonium surfactant ions, the measured bromide surface coverages at positive polarizations are consistent with the formation of ordered bromide layers. The electrochemical data indicate that the formation of ordered bromide adlayers on Au(111) and Au(100) leads to quantitative changes in the layer of adsorbed surfactant ions. This is interpreted as being consistent with a hierarchical structure whereby OTA+ ions are at least partially commensurate with the halide adlayer. Comparison of the data for the two lowest energy FCC crystal surfaces shows that in the presence of OTA+ there is moderately higher bromide coverage on Au(111) compared to Au(100). However, the OTA+ isotherms indicates that the presence of adsorbed bromide does not lead to significant differences in the adsorption of OTA+ on Au(111) and Au(100). These results show that any explanation of gold nanorod growth based on a thermodynamic argument derived from a presumed preferential adsorption of quaternary ammonium bromide on different facets of gold is unlikely. However, as a cautionary note, it is important to consider that a gold nanoparticle would have a single electrochemical potential, and by virtue of their different potentials of zero charge, each different crystallographic facet would have a different surface charge density. The isotherms in this study show that it is conceivable that the electronic state of each interface could be sufficiently different enough to promote preferential surfactant adsorption. This issue could only be conclusively reconciled if direct measurements of the electronic state of the particle were available.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Department of Physics E19, Technical University of Munich, James-Franck-Strasse 1, 85748 Garching, Germany. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada. REFERENCES
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