Quaternary Ammonium Bromide Surfactant Adsorption on Low-Index

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Quaternary Ammonium Bromide Surfactant Adsorption on LowIndex Surfaces of Gold. 1. Au(111) J. P. Vivek† and Ian J. Burgess* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5C9 Canada ABSTRACT: The coadsorption of the anionic and cationic components of a model quaternary ammonium bromide surfactant on Au(111) has been measured using the thermodynamics of an ideally polarized electrode. The results indicate that both bromide and trimethyloctylammonium (OTA+) ions are coadsorbed over a broad range of the electrical state of the gold surface. At negative polarizations, the Gibbs surface excess of the cationic surfactant is largely unperturbed by the presence of bromide ions in solution. However, when the Au(111) surface is weakly charged the existence of a low-coverage, gaslike phase of adsorbed halide induces an appreciable (∼25%) enhancement of the interfacial concentration of the cationic surfactant ion. At more positive polarizations, the coadsorbed OTA+/Br− layer undergoes at least one phase transition which appears to be concomitant with the lifting of the Au(111) reconstruction and the formation of a densely packed bromide adlayer. In the absence of coadsorbed halide, the OTA+ ions are completely desorbed from the Au(111) surface at the most positive electrode polarizations studied. However, with NaBr present in the electrolyte, a high surface excess of bromide species leads to the stabilization of adsorbed OTA+ at such positive potentials (or equivalent charge densities).

1. INTRODUCTION The halide salts of quaternary ammonium surfactant ions are extensively reported in the literature as being excellent stabilizing ligands for gold nanoparticles.1−4 A very popular member of this family is cetyltrimethylammonium bromide (CTAB), and the stability of CTAB stabilized gold nanoparticles can be attributed to the strong adsorption affinity of bromide on gold, which in turn is believed to help the cationic surfactant adhere to the bromide-covered metal surface.5 CTAB is also the most popular surfactant/stabilizer/ligand for the seed mediated synthesis of gold nanorods in aqueous solution.6−9 In a typical protocol for gold nanorod synthesis, the reaction mixture contains Au3+ ions (HAuCl4), quaternary ammonium bromide surfactant (CTAB), a mild reducing agent (e.g., ascorbic acid), and nanoparticle seeds (citrate or CTAB stabilized gold) dispersed in an aqueous medium. Over the course of the past several years, it has been demonstrated that gold nanorods can be formed in preference to quasi-spherical nanoparticles by varying parameters such as the source of gold ions,10 the size of the nanoparticle seed,11−13 the identity of the reducing agent,10 solution pH,14 and the chain length of the quaternary ammonium surfactant,15 but the presence of halide ions has been found to be crucial for the formation of nonspherical nanoparticles.16−19 At present, the role of quaternary ammonium bromide surfactants on the formation of gold nanorods is a subject of debate in the literature.20 Initially, the popular perception was that rodlike micelles of CTAB act as soft templates,8,11,21−24 but it was subsequently demonstrated that gold nanorods can be synthesized even at © 2012 American Chemical Society

concentrations below the CTAB critical micelle concentration (cmc).25 Currently, a popular perception is that nanorod growth is promoted by the preferential adsorption of the quaternary ammonium surfactant on different facets of a single crystal or twinned embryonic seed crystal.11,25 Using selective area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) studies, it has been demonstrated by various groups that gold nanorods synthesized by the reduction of gold chloride using ascorbic acid in the presence of CTAB and single crystal gold seed crystals have a pentagonal cross section with (100) side facets and (111) end facets.11 Crystallographic analysis of gold nanorods has led to speculation that the preferential adsorption of the cationic surfactant on (100) facets over (111) facets of the nanoparticle during the growth process promotes nanorod formation.21,26 This postulate is based upon the known differences in bromide adsorption on the low-index crystallographic surfaces of gold.27 In the case of CTAB adsorption, it is hypothesized that bromide acts as a bridging center between the nanorod’s metallic surface and the quaternary ammonium headgroup of the surfactant.5,23 Since the presence of bromide ions is crucial for the formation of nanorods,25,28 it has been envisaged that preferential adsorption of bromide ions on Au(100) surfaces brings more positively charged surfactant species to that facet, Received: January 3, 2012 Revised: February 25, 2012 Published: February 29, 2012 5031

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finally dried under vacuum. The purity of the final product was tested by 1H NMR. The Kraft temperature of this surfactant is ∼16 °C, and the surfactant stock solution should be kept at a higher temperature because any minor amount of crystallization from the stock solution can act as nucleation sites leading to premicelle formation. Quaternary ammonium surfactants are known to make premicelles35−38 if nucleation sites are available, and hence, all these measurements were done after repeated recrystalliztion. Fresh surfactant solution was made before each set of measurements and kept incubated at 30 °C to minimize any possible pre-cmc aggregation. The critical micellar concentration of the surfactant was determined to be ∼8 mM on the basis of surface tension measurements. Single-Crystal Fabrication. The method used for fabricating and orienting single crystal electrodes is detailed elsewhere.34 The quality and cleanliness of the polished electrode was judged by recording a cyclic voltammogram in 0.1 M perchloric acid using the hanging

passivating the sides of the growing nanoparticle and making the Au(111) capping facets relatively more accessible for further growth. For completeness, we note that a significant variation of the original synthetic route, in which a small amount of silver ions is added during the growth of the nanorods, has been shown to significantly improve the yield of high aspect ratio nanorods. The crystallography of nanorods formed in the presence of Ag+ ions has been shown to be different from that formed in the absence of Ag+ ions,29 and the effect of silver on the crystallography of gold nanorods remains a contentious subject in the literature. However, both in the absence and presence of Ag+ ions, a preferential concentration of surfactant species on one crystallographic surface over another seems a viable means to direct anisotropic nanoparticle formation. While simple models that invoke preferential surfactant adsorption are often used to explain nanorod formation, there has been no direct experimental evidence in support of these hypotheses. This is largely due to the difficulty in studying the interface of single crystal gold seeds in situ. It has previously been demonstrated that electrochemical techniques and the thermodynamics of ideally polarized electrodes can be used to extract quantitative information pertinent to the adsorption of ligand protecting species as a function of the electrical state of the metal surface.30−32 Such adsorption information is highly germane to nanoparticles in aqueous solution which undoubtedly carry a net surface charge. Information obtained from electrochemical studies performed on electrodes several millimeters in dimension can be correlated to observations made of analogous nanoparticle systems provided that the size of the nanoparticles is not sufficiently small that their electronic structure differs significantly from that of bulk gold. Measurements of surfactant adsorption as a function of the electrical state of the metal electrode are particularly useful if the electrical state of the nanoparticles can be evaluated using, for example, zeta potential measurements. In part 1 of this two-part contribution, we employ an electrochemical approach to describe the coadsorption of octyltrimethylammonium (OTA+) and Br− ions, under equilibrium conditions, on the Au(111) surface. A shorter alkyl chain surfactant in comparison to CTAB was chosen because it provides a wider range of concentrations below the cmc and its shorter hydrocarbon chain renders it more amenable to application of the chronocoulometric method used to determine the Gibbs excesses. In part 2,33 we perform a similar analysis on the Au(100) surface and compare the results for these low-index surfaces. Combined, the thermodynamic data allow us to assess the feasibility of existing models that speculate that anisotropic nanoparticle growth is driven by preferential surfactant adsorption on certain facets of singlecrystal seed particles.

Figure 1. Cyclic voltammogram (20 mV/s) of Au(111) electrode in 0.10 M HClO4. meniscus configuration. Figure 1 reveals that the measured CV is in excellent agreement with published voltammograms for Au(111).39 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 value of Epzc was found to be 0.27 V vs SCE which is in close agreement with values reported in the literature.40 Electrochemical Measurements. A detailed description of the electrochemical setup and experimental methodologies for cyclic voltammetry, differential capacity, and chronocoulometry measurement has been provided previously.30 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.

3. RESULTS AND DISCUSSION Cyclic Voltammetry and Differential Capacity Measurements. It is necessary to be able to independently control the concentration of each adsorbing ion in the electrolyte in order to properly assess the coadsorption of bromide ions and quaternary ammonium surfactant ions. As triflate ions have previously been shown to only very weakly adsorb on Au(111)41 and Au(100),34 the synthesis of a triflate salt of OTA+ allows for the introduction of surfactant ions without the complication of simultaneously increasing the chemical potential of potentially coadsorbing counterions (e.g., Cl−, Br−, I−, OH−). In this study where a measurement of the effect

2. EXPERIMENTAL SECTION Reagents and Solutions. Sodium fluoride (99.99%) and sodium bromide (≥99.99%) salts were purchased from Sigma-Aldrich. NaF was bleached of organics by placing the powder in a UV-ozone chamber (Spectronics corporation, USA) for 30 min prior to being dissolved in water to make the electrolyte. All solutions were prepared with Milli-Q ultrapure water (>18.2 MΩ cm). Octyltrimethylammonium trifluoromethanesulfonate (triflate) was synthesized by methylation of N,N-dimethyloctylamine (purity 95%, Aldrich) using methyl trifluoromethanesulfonate (purity >98%, Aldrich). Details of the synthesis are published elsewhere.34 Crude OTATf was recrystallized first from a hexane-ethyl acetate mixture, then from water, and was 5032

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of bromide on OTA+ adsorption is desired, bromide is introduced by the independent addition of NaBr. Cyclic voltammograms (CVs) in Figure 2 present a qualitative picture

structure of the adsorbed surfactant aggregates. Figure 2b provides the CV of a 1:1 mixture of both OTA+ and Br− (1.0 mM) in 0.10 M NaF. The peaks corresponding to the adsorption/desorption of the surfactant have broadened somewhat in the presence of bromide. This is caused by the overlap of bromide adsorption and OTA+ adsorption. A small, but highly reproducible, current spike appears at ∼−0.41 V and indicates a possible phase transition within the adsorbed layer. The lifting of the reconstruction coincident with the formation of a condensed bromide layer is suppressed and an attenuated peak is seen to be shifted to more positive potentials in the presence of OTA+. Figure 2b qualitatively indicates that Br− and OTA+ coadsorption is highly likely on the Au(111) surface at potentials more positive than −0.60 V. Differential capacity (DC) curves in Figure 3 show the adsorption behavior of OTA+ in 0.10 M NaF electrolyte in the

Figure 2. (a) Cyclic voltammograms (20 mV/s) of Au(111) in 0.10 M NaF supporting electrolyte (black dotted line) in the presence of 1.0 mM NaBr (red solid line), 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.

Figure 3. (a) Differential capacity curves of Au(111) in 0.10 M NaF supporting electrolyte (black dotted line) plus 1.0 mM OTATf in the absence of bromide ions (blue dashed line) and in the presence of 1.0 mM NaBr (green solid line). All DC curves were recorded using a 25 Hz, 5 mV rms ac perturbation superimposed upon a 5 mV/s dc voltage sweep.

of bromide and OTA+ adsorption at the Au (111)/electrolyte interface. Figure 2a provides the CV for the 0.10 M NaF supporting electrolyte (black dotted line) as well as voltammograms for the electrolyte in the presence of 1.0 mM NaBr (red solid line) and in the presence of 1.0 mM OTATf (blue dashed line). The CV for 1.0 mM bromide shows a low coverage adsorption of bromide in the potential range between −0.45 and −0.15 V, followed by an increase in current leading to a sharp irreversible peak at −0.02 V which corresponds to the lifting of the (1 × 23) reconstruction of the Au(111) surface and consequent formation of a condensed layer of bromide.42 The experimental voltammogram is in excellent agreement with previous literature reports of the Au(111)/Br− interface.42 The voltammetric behavior of the 1.0 mM cationic surfactant in the absence of bromide is also shown in Figure 2a. As a detailed description of the adsorption behavior of OTA+ on Au(111) in the absence of adsorbing anions is the subject of parallel work,43 only a brief description of the most relevant features of this voltammogram are discussed herein. The voltammetric peaks at ∼−0.55 V represent the adsorption/desorption of OTA+ ions. At potentials positive of the adsorption peak, the capacitive current is less than that of the electrolyte-only curve indicating the formation of a surfactant layer at the interface. An elevated current plateau is observed at potentials positive of ∼0.10 V and the CV in this region is distinctly asymmetric about the zero current axis. This voltammetric behavior is associated with a kinetically slow phase transition in the

absence (blue dashed line) and in the presence of bromide ions (green solid line). The DC curve for OTATf shows typical onestate adsorption behavior, with a nearly constant capacitance of ∼10 μF cm−2 measured at all potentials positive of the adsorption peak. In the presence of bromide ions the DC curve shows an increase in the adsorption peak intensity (E ∼ −0.50 V) and also reveals a capacitive spike at −0.41 V, consistent with the corresponding features in the cyclic voltammograms. Due to the presence of bromide ions at the interface, the capacity is consistently larger than the curve for 1.0 mM OTA+ in the bromide-free electrolyte. After the pronounced capacitive feature at ∼0 V corresponding to the lifting of reconstruction due to the presence of strong bromide adsorption, several broad capacitive features are observed. These may indicate the formation of stable surface aggregates or phase changes within the mixed halide−surfactant adlayer. However, overinterpretation of such a DC curve is unwarranted primarily for the reason that these measurements do not necessarily correspond to a state of adsorption equilibrium. It is important to note that at potentials negative of −0.70 V, the DC curve of the Au(111) electrode in the presence of OTA+ and Br− ions merges with that of the 0.10 M NaF electrolyte (black dotted line). This indicates that negative potentials result in the complete desorption of specifically adsorbed ions which is a critical 5033

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where cOTA+ and cBr− are the bulk electrolyte concentrations of OTA+ and Br−, respectively. Partial differentiation of eq 3 with respect to ln cOTA+ at constant electrode potential provides

criterion for the chronocoulometry methodology described below. Coadsorption and the Thermodynamics of Ideally Polarized Electrodes. Quantitative information concerning the Br− and OTA+ Gibbs excesses can be obtained using a chronocoulometric method whereby the charge density (σm) of an equilibrated, ideally polarized electrode interface in the presence of adsorbed species is determined. In a chronocoulometry experiment, the electrode’s charge density is obtained over a range of potentials of interest by integrating the current transient which arises upon stepping the electrode potential from a value where ions of interest are adsorbed, E, to a potential where all adsorbed ions are completely desorbed, Edes. Such measurements are then repeated over the same range of potentials for a series of electrolyte compositions. Details concerning the collection and analysis of such current transients have been thoroughly described by Lipkowski’s group.44 Interpretation of chroncoulometry data is based on the application of the electrocapillary equation for an electrified, ideally polarized interface. For the Au(111) electrode in the presence of OTATf surfactant, NaBr, and NaF supporting electrolyte, the electrocapillary can be written as follows for conditions of constant temperature and pressure

⎛ ⎞ ∂γ ⎜⎜ ⎟⎟ = −RT Γ + OTA ⎝ ∂ ln cOTA+ ⎠ E

Integration of eq 3 from Edes to E under conditions of constant surfactant concentration provides γ(E) = γ(Edes) −

E

∫E

des

(6)

σm dE)/∂ ln cOTA+]E (7)

As long as the desorption potential corresponds to conditions where there is no surfactant adsorption at the electrode interface, irrespective of the concentration in the bulk of solution, the first term on the right-hand side of eq 7 vanishes. Comparing the result with eq 5 reveals

(1)

ΓOTA+ = −

E 1 [∂( σm dE)/∂ ln cOTA+]E , μ − Br RT Edes

∫

(8)

Thus, the Gibbs surface excess of surfactant cations in the presence of a fixed concentration of bromide ions can be calculated by integrating σm−E curves measured for a series of bulk surfactant concentrations (series 1). Identical thermodynamic arguments can be made for analogous experiments where σm−E curves are measured for a series of bulk bromide concentrations and a constant OTA+ electrolyte concentration (series 2). ΓBr− = −

E 1 [∂( σm dE)/∂ ln cBr−]E , μ + OTA RT Edes

∫

(9)

Chronocoulometry and Gibbs Excesses. Equilibrium charge density versus electrode potential data for series 1 chronocoulometry measurements are shown in Figure 4a. The charge density data for the supporting electrolyte (0.10 M NaF) in the absence of bromide and OTA+ is shown as the dotted line. The charge density curve for the supporting electrolyte in the presence of 1.0 mM bromide is shown as the dashed line and is consistent with that reported in the literature.42,47,48 This latter curve is composed of sections having different slopes, each related to different states of bromide adsorption. Though a detailed description of the charge curve for the adsorption of bromide on Au(111) is already available in the literature,42 a brief description is provided here as it is helpful for the ensuing discussion. Between ∼−0.60 and ∼−0.10 V, the charge density in the presence of bromide ions is only slightly larger than the charge density for the base electrolyte. This so-called “foot” region corresponds to a highly disorganized and low-coverage layer of adsorbed bromide ions. A steep increase in the measured charge density is observed at potentials positive of the limit of the foot region and correlates with the sharp peak

(2)

(3)

+

For series 2 experiments (constant OTA concentration), eq 1 becomes −dγ = σm dE + RT ΓBr− d ln cBr−

σm dE

− [∂(

For series 1 experiments (constant bromide concentration) performed in the presence of a large excess of supporting electrolyte, such that changes in activity coefficients are negligible, eq 1 simplifies to −dγ = σm dE + RT ΓOTA+ d ln cOTA+

des

⎛ ⎞ ⎛ ∂γ(E ) ⎞ ∂γ des ⎟ ⎟⎟ = ⎜⎜ −⎜⎜ ⎟ + ∂ ∂ ln c ln c ⎝ ⎝ OTA ⎠ E OTA+ ⎠ E

where γ is the interfacial tension, σm is the metal charge density, E is the electrode potential, Γj is the Gibbs excess of the adsorbed species j, and μj is the chemical potential of species j in the bulk of the electrolyte. In order to obtain insight into the adsorption behavior of OTA+ on the Au(111) surface in the presence of Br− ions, we employed an approach similar to that used by Shi and Lipkowski to quantify the coadsorption of copper and counterions on Au(111).45−50 In series 1 measurements, the concentration of the quaternary ammonium surfactant was varied while maintaining a constant bromide concentration. In series 2, the bromide concentration was varied and the OTA+ surfactant concentration was kept constant. In both sets of experiments, the concentration of fluoride ions is fixed and defined by the 0.10 M NaF electrolyte such that dμF− = 0. Additionally, the concentration of sodium ions was either unchanged by the addition of OTATf (series 1) or varied by less than 2% by the addition of NaBr (series 2). Thus, the last two terms in eq 1 are either equal to zero or negligibly small. Furthermore, on the basis of previous electrochemical measurements, the Gibbs excess of triflate ions in the interfacial region approaches zero when the ratio of fluoride to triflate ions in the bulk of solution is large.41 Thus, for both series of experiments, eq 1 can be simplified to the following −dγ = σm dE + ΓOTA+ dμOTA+ + ΓBr− dμBr−

E

∫E

which can be partially differentiated with respect to ln cOTA+ at constant electrode potential

−dγ = σm dE + ΓOTA+ dμOTA+ + ΓBr− dμBr− + ΓTf − dμ Tf − + ΓF− dμ F− + ΓNa+ dμNa+

(5)

(4) 5034

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with the electrolyte curve at the negative end of the potential domain, confirming that both the surfactant ions and the bromide ions are completely desorbed from the surface at very negative electrode polarizations. Between −0.60 < E < −0.05 V, the charge curves are consistently above the bromide-only curve suggesting the existence of coadsorption of Br− and OTA+ in this potential range. Surfactant adsorption on metal electrodes has been extensively studied in the past, and combined electrochemical, spectroscopic, and scanning probe measurements have demonstrated that charged surfactants can form a rich array of aggregated structures whose shape depends on the surfactant surface concentration and the electrical state of the electrode.5,53−58 Details of the OTA+ aggregate structure on Au(111) in the absence of coadsorbed bromide will be reported elsewhere. Even though the data in Figure 4a do not allow for immediate determination of the coadsorbed OTA+/ Br− aggregate structure, it is apparent that a significant amount of OTA+ is adsorbed at the interface even in the presence of bromide ions, in this region of potentials. For E > −0.05 V, the metal charge density becomes less positive with increasing additions of OTA+, which is an expected change for the adsorption of cationic species based on simple electrostatic considerations. The OTA+ curves are almost indistinguishable from the bromide-only curve at the most positive potentials studied. This suggests that the adlayer of bromide is not strongly affected by the presence of the cationic surfactant in the electrolyte. Figure 4b shows charge curves corresponding to variable bromide concentration in the presence of 1.0 mM OTA+ (series 2 chronocoulometry measurements). At negative potentials, all the curves are once again indistinguishable from the electrolyte curve, indicating complete desorption of OTA+/ Br− from the surface. Series 1 and 2 contain a common data set, namely σm−E measurements performed in the presence of both 1.0 mM OTA+ and 1.0 mM Br−. As the chronocoulometry method provides information pertinent to an equilibrated interface, these two data sets should be identical and independent of the experimental sequence preceding their measurement. Thus, as a means to illustrate the precision of our data, Figure 4c overlays the common charge curves for series 1 and 2. The two curves are almost superimposable everywhere other than at the most positive of potentials and demonstrate the self-consistency of the experimental data. Using eqs 8 and 9, the Gibbs surface excesses of OTA+, at constant Br− concentration in the bulk solution, and the Gibbs surface excesses for Br−, at constant OTA+ concentration in the bulk of solution, were calculated using the appropriate series of charge data. Figure 5a shows Gibbs excess data for OTA+ in the presence of 1.0 mM bromide. The cationic surfactant begins to adsorb at ∼−0.60 V, and the maximum Gibbs excess is attained at the onset of the potential corresponding to the lifting of the reconstruction due to bromide adsorption (∼−0.05 V). The Gibbs excess decreases at more positive potentials and plateaus at approximately +0.30 V at higher bulk concentrations of OTA+. The Gibbs excess versus potential plot of bromide in the presence of 1.0 mM OTA+ is shown in Figure 5b. In the presence of the cationic surfactant, the foot region of the isotherms (−0.50 < E < −0.05 V) reveals a very low coverage of adsorbed bromide. Assuming that these bromide species are all contact (specifically) adsorbed on the Au(111) surface, the maximum coverage in the foot region corresponds to about 4% occupancy of an unreconstructed gold surface. Such low surface coverages equate to the gas phase low coverage state of

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, 0.05, 0.075, 0.10, 0.15, 0.25, 0.50, 0.75, 1.0, and 1.5 mM OTATf. (b) Series 2 electrolytes contain 1.0 mM OTATf (black dashed line) plus 0.01, 0.025, 0.05, 0.10, 0.20, 0.35, 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).

observed in the cyclic voltammetry. Previous chronocoulometry42 and surface X-ray scattering27,51,52 studies have shown this to be a consequence of the formation of a much denser layer of bromide and a concomitant lifting of the gold surface reconstruction. After 0 V, the slope becomes smaller and is consistent with the quasi-plateau region observed in the voltammogram. Previous studies have demonstrated that at potentials larger than those of the inflection there is a significant amount of bromide adsorbed on the surface but it retains a largely disordered adlayer structure.27,42 The slope of the σm−E plot significantly decreases after 0.30 V where an ordered layer of bromide is formed. The colored lines in Figure 4a provide the measured charge densities for varying OTA+ concentrations in the presence of 1.0 mM bromide and the supporting electrolyte. All charge curves in Figure 4a merge 5035

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Figure 6. Gibbs surface excesses versus potential at the Au(111)/0.1 M NaF interface for 1.0 mM OTA+ in the absence of NaBr (red triangle) and 1.0 mM OTA+ in the presence of 1.0 mM Br− (red circle). The adsorption isotherm for 1.0 mM bromide coadsorption in the presence of 1.0 mM OTATf is also plotted using the right-hand ordinate (open circle).

versus potential curves for 1.0 mM OTA+-only, 1.0 mM OTA+ in the presence of 1.0 mM NaBr, and 1.0 mM NaBr in the presence of 1.0 mM OTA+. The data indicate that the presence of a low density state of bromide adsorption provides a cooperative effect for the accumulation of cationic surfactant ions at the interface. This can be rationalized by the known superequivalent adsorption of bromide anions on Au(111)61 whereby the absolute value of the negative charge accumulated by adsorbed halide is greater than the positive electronic charge density of the metal.61 The excess negative charge provided by the superequivalent adsorption of bromide would provide an electrostatic basis for the observed increase in the interfacial concentration of OTA+ relative to the halide free adsorption isotherm. At 0 V, the surface excess of the cationic surfactant continues to increase more or less monotonically with potential in the absence of coabsorbed bromide but decreases when NaBr is present in the electrolyte. The differential capacitance curves in Figure 3 indicate that the reconstructed Au(111) surface is lifted at this potential, and Figure 6 reveals that this is commensurate with the steep increase in the surface concentration of adsorbed halide species. The strong adsorption of Br− serves to both lift the reconstruction but also apparently disrupts the surfactant adlayer and induces a phase change that leads to a slightly lower Gibbs excess of OTA+. Interpreting the thermodynamic data in terms of a detailed molecular level picture is rather speculative without excess to high resolution scanning probe microscopies. However, Jaschke et al.’s previous atomic force microscopy (AFM) study has shown that cetyltrimethylammonium surfactant ions in the presence of bromide form elongated, cylindrical micelles on a Au−Br surface.5 It is possible that the inflection seen in Figure 6 at ∼0 V for the OTA+ plus 1.0 mM NaBr isotherm results from the formation of such a hierarchical structure, i.e. a bromide underlayer supporting surface aggregates of the cationic surfactant. A second decrease in the OTA+ isotherm in the presence of halide is observed at E ∼ 0.20 V which is consistent with the potentials at which an ordered layer of bromide is found to form on Au(111).62−64 It is important to note that, at potentials positive of this second inflection, the OTA+ surface excess reaches a limiting value of ∼2 × 10−10 moles cm−2. This is in stark contrast to the isotherm in the absence of NaBr which reveals a dramatic

Figure 5. Gibbs surface excesses on Au(111) 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.

bromide adsorption on Au(111).42 The Gibbs excesses reported in Figure 5b are very similar to those reported by Lipkowski et al. for bromide in the absence of potential coadsorbates.42 After the lifting of the reconstructed surface, a condensed layer of bromide is adsorbed on the surface and hence large values of Gibbs excess are seen at positive potentials. At the most positive polarizations, the Gibbs excess values obtained herein for 1.0 mM bromide on Au(111) in the presence of 1.0 mM OTA+ is roughly 25% higher than the reported Gibbs excess of bromide on bare Au(111) in the absence of any other specifically adsorbing species.42 This moderate increase in the Gibbs excess of bromide in the presence of OTA+ could be caused by additional bromide anions carried by adsorbed cationic OTA+ ions. More importantly, the high values of ΓBr imply that the OTA+ ions do not competitively replace bromide species on the Au(111) surface. Quantification of OTA+ adsorption in the absence of halide coadsorption on Au(111) has been performed and will be described in detail elsewhere. For completeness, we note that our measured surfactant Gibbs excesses are not consistent with the formation of a compact bilayer of OTA+. Recent TEM results have shown that CTAB bilayers can form on the (100) and other open facets of gold nanorods.59,60 As the anion and the quaternary ammonium headgroup of our system match those of CTAB, it is possible that the relatively short chain length employed in the current study may have insufficient intermolecular forces to drive bilayer formation. A clearer picture of the influence of bromide on the adsorption behavior of the cationic surfactant on Au(111) can be formed by comparing the adsorption isotherm for OTA+ in the presence of bromide (this study) to the equivalent isotherm in the absence of coadsorbing halide. Figure 6 shows the Gibbs excess 5036

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approximately equal to their surface concentrations and that the adsorbed halide species has an electrosorption valency close to unity,42,47 Figure 7b reveals that there are sufficient quantities of interfacial bromide at σm = 0 to render the surface net negatively charged and promote OTA + adsorption. A comparison of the Gibbs excess of OTA+ as a function of metal charge density with and without bromide in the electrolyte is presented in Figure 8. Relative to the halide-free

desorption of the cationic surfactant at positive electrode polarizations. Clearly, the presence of a high concentration of adsorbed halide species increases the driving force for surfactant adsorption on the Au(111) surface. Describing surfactant adsorption with respect to the applied potential is often inconvenient as it is, of course, determined by the choice of reference electrode. Parsons has advocated the use of electrode charge as the preferred electrical variable.64,65 The chronocoulometric data shown in Figure 4 can be readily transformed using the Parsons function, ξ = σmE + γ, to provide Gibbs excesses as a function of the metal surface charge density for both series 1 and 2 solutions. Such a data representation is potentially very useful as it has previously been shown that the metal charge density is a more accessible evaluation of the electrical state of colloids and nanoparticles compared to potential.30−32 Gibbs excess plots for (a) OTA+ in the presence of 1.0 mM bromide and (b) bromide in the presence of 1.0 mM OTA+ are plotted against the metal charge density in Figure 7.

Figure 8. Gibbs surface excesses versus electrode charge density at the Au(111)/0.1 M NaF interface for 1.0 mM OTA+ in the absence of NaBr (red triangle) and 1.0 mM OTA+ in the presence of 1.0 mM Br− (red circle). The adsorption isotherm for 1.0 mM bromide coadsorption in the presence of 1.0 mM OTATf is also plotted using the right-hand ordinate (open circle).

isotherm, in the presence of bromide there is a noticeable increase in the Gibbs excesses of OTA+ beginning at charge densities where bromide starts to adsorb on the surface. This supports the argument from above that it is the superequivalent adsorption of bromide that overcompensates positive surface electronic charge densities and mitigates unfavorable electrostatic interactions between positively charged surfactant ions. This effect is most greatly exhibited at highly positively charged electrode surfaces. In the absence of bromide, OTA + completely desorbs from the surface at the most positive polarizations while in the presence of bromide there is still a significant amount of OTA+ (>2 × 10−10 mol cm−2) on the surface even though the metal’s electronic surface charge exceeds +60 μC cm−2. Figure 7. Gibbs surface excesses on Au(111) 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.

4. SUMMARY AND CONCLUSIONS This work has provided a thermodynamic evaluation of the adsorption of a cationic surfactant on Au(111) in the presence of coadsorbing bromide ions. A thermodynamic treatment of an ideally polarized electrode surface has led to the quantification of the amounts of both surfactant and halide coadsorbed on the Au(111) electrode surface. The experimental data indicates that both quaternary ammonium ions and bromide ions are present over a wide range of the electrode’s electrical state. Furthermore, at most potentials (and corresponding surface charge densities), the presence of bromide ions in the electrolyte solution leads to an enhancement in the surface excess of OTA+ ions. This synergistic effect is most greatly observed at very positive electrode charge densities which can be explained by the ability of the adsorbed bromide species to mitigate electrostatic repulsions between the electrode and the cationic surfactant ions. The information presented in this work will be used in a subsequent

Gibbs excess plots for OTA+ in the presence of bromide show maximum surfactant adsorption at zero surface charge density (Figure 7a), but there is a significant amount of OTA+ adsorbed even at positive surface charge densities where there is also a significant amount of adsorbed bromide (Figure 7b). The observation of maximum cationic surfactant adsorption at zero charge densities is superficially surprising given that the adsorption of a neutral molecule is most favorable at uncharged electrodes and a cationic surfactant would be expected to have maximal affinity to a negatively charged electrode. It has previously been shown that the electrosorption of small cationic organic ions on mercury is strongly influenced by counterion coadsorption.66−68 Assuming that the halide Gibbs excess are 5037

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contribution (part 2)33 to compare quaternary ammonium surfactant and bromide coadsorption on low index facets of gold. Therein, we will evaluate the merits of existing models that propose preferential surfactant adsorption as an explanation of the formation of anisotropic gold nanoparticles.

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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.

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