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Alkanethiol Adsorption on Platinum: Chain Length Effects on the Quality of Self-Assembled Monolayers María Alejandra Floridia Addato,† Aldo A. Rubert,† Guillermo A. Benítez,† Mariano H. Fonticelli,*,† Javier Carrasco,‡ Pilar Carro,§ and Roberto C. Salvarezza† †
Instituto de Investigaciones Fisicoquímicas, Teoricas y Aplicadas (INIFTA), Universidad Nacional de La Plata—CONICET, Sucursal 4 Casilla de Correo 16 (1900) La Plata, Argentina ‡ Instituto de Catalisis y Petroleoquímica, CSIC, Marie Curie 2, E-28049 Madrid, Spain § Departamento de Química Física, Instituto de Materiales y Nanotecnología, Universidad de La Laguna, Tenerife, Spain
bS Supporting Information ABSTRACT: The adsorption of butanethiol (BT), hexanethiol (HT), and dodecanethiol (DT) on Pt from ethanolic solutions has been studied by electrochemical techniques, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. XPS data show two main S 2p components at 162.3 and 163.4 eV. The barrier properties estimated by doublelayer capacitance and the blockage of the electron transfer from a redox couple in solution are markedly improved for self-assembled monolayers (SAMs) of the longer DT molecule. While the behavior of DT monolayers on Pt is comparable to that found for those grown on Au, HT and BT SAMs on Pt are more defective, are less blocking, and have a slightly lower thiolate coverage than their Au counterparts. The chain length dependent quality of these SAMs is explained based on DFT and thermodynamics calculations. It is demonstrated that a lying down (LD) surface structure dominates the stability diagram for the shorter chain thiols, while the standing up (SU) phase is more important for DT. We propose that the poor quality observed for SAMs of short chain thiolates results from an easier CS bond scission of the thioalkyl radical in a LD configuration.
1. INTRODUCTION Self-assembled monolayers (SAMs) of thiolates on metal surfaces have attracted considerable attention due to their importance in fundamental research and also as far as technological applications are concerned. They have been used in sensing devices, in material protection, as resists and inks in lithography and as building elements in the development of supramolecular assemblies. SAMs on Au, Ag, and Cu have been extensively studied. For these systems, there is plenty of knowledge about the self-assembly in gas and liquid phases, the surface structures formed along this process, the different types of defects in the final state, and the chemical, thermal, and electrochemical stability of the SAMs. In contrast to the vast literature of thiolate SAMs on the coinage metals,1 the information regarding SAMs on Ni, Pd, and Pt is relatively scarce. The study of thiol selfassembly on these metals is difficult since they are excellent catalysts for many organic reactions and they also exhibit a great affinity for S atoms. Thus, different reaction pathways, involving molecule decomposition and S adsorption, could take place along with SAM formation. In the case of Pd it has been found that alkanethiol SAMs are formed on a diluted palladium sulfide adlayer, which is formed from the CS bond cleavage.2,3 The sulfide overlayer passivates the Pd surface avoiding further thiolate decomposition and allowing alkanethiol SAM formation.4 In the case of Pt the situation is also complex, and contradictory results have been reported. While there is agreement about r 2011 American Chemical Society
the presence of thiolate species on the Pt surface, other species such as alkyl disulfide, hydrocarbon chains and sulfides,5 and partially oxidized alkylthiolate species6 could also be present. The increasing interest in thiol SAMs on Pt711,5,12 arises not only from the point of view of basic surface science but also as a matter of direct technological relevance. In this sense, these monolayers are also investigated because of their possible applications in molecular electronics as contact materials,6 sensing systems,13,14 lithography,1518 catalysis,19 and electrochemical and molecular electronic devices.13,14 Furthermore, thiols are important as capping agents for the preparation of high-quality Pt nanoparticles. Regarding the last issue, it is well-known that both platinum metal and its alloys possess distinctive ability in catalyzing partial oxidation, hydrogenation and dehydrogenation of a variety of important molecules that are essential in many industrial processes.20 Therefore, it is a key issue to understand the chemistry, structure, and stability of the alkanethiols SAMs on Pt. In this work the self-assembly of butanethiol (BT), hexanethiol (HT) and dodecanethiol (DT) on polycrystalline Pt substrates from ethanolic solutions and the quality of their SAMs have been investigated. XPS data show two main S 2p components at 162.3 and 163.4 eV. The barrier properties estimated by Received: February 11, 2011 Revised: July 29, 2011 Published: August 05, 2011 17788
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The Journal of Physical Chemistry C double-layer capacitance and the blockage of the electron transfer from a redox couple in solution are markedly improved for SAMs of the longer DT molecule. Although the behavior of DT monolayers on Pt is comparable to that found for those grown on Au, HT and BT SAMs on Pt are more defective, less blocking, and have a slightly lower thiolate coverage than their Au counterparts. The qualitatively different behavior of long and short chain alkanethiols has been explained based on DFT and thermodynamics calculations. It is demonstrated that a lying down (LD) surface structure dominates the stability diagram for short chain thiols, while a standing up (SU) phase is more important for a long chain thiol. It is proposed here that the poor quality observed for SAMs of short chain thiolates results from an easier CS bond scission of the thioalkyl radical in a LD configuration.
2. EXPERIMENTAL METHODS 2.1. Electrochemical Experiments. Standard three-electrode electrochemical cells were employed with an operational amplifier potentiostat (TEQ-Argentina). A saturated calomel electrode (SCE) and a large-area platinum foil were used as reference and counter electrode, respectively. All potentials in the text are referred to the SCE scale. The cyclic voltammetry in Figure1b and double-layer capacitance measurements have been carried out using 0.1 M NaOH aqueous solution as the base electrolyte, prepared from solid NaOH (analytical grade from Baker). The reagent employed for the study of the Faradaic response of a redox couple was K4Fe(CN)6 from Merck. In the latter case a 10 mM solution was prepared in 1 M KNO3 (Merck). In all cases the solutions were prepared with Milli-Q water and degassed with purified nitrogen prior to the experiments. All measurements were performed at room temperature. 2.2. Preparation of the Substrates. Alkanethiols were adsorbed on Pt foil substrates (Johnson Matthey Electronics, 99.99%, 0.025 mm thick). The substrates were washed by subsequent rinses with absolute ethanol, Milli-Q water, piranha solution, Milli-Q water, and ethanol. They were finally dried under a nitrogen atmosphere. The cleanness of the Pt substrates was tested electrochemically among experiments and in few occasions by XPS. The determination of the real surface area (A) of Pt electrodes was made by estimating the total charge involved in the H monolayer from cyclic voltammograms recorded in 0.1 M NaOH (peaks CII+CIII in Figure 1b) and 0.1 M H2SO4. All electrochemical data refer to the real surface area.21 2.3. Preparation of the Alkanethiolate- and Sulfur-Covered Surfaces. Alkanethiolate monolayers on Pt were prepared in liquid phase using alkanethiols with different chain lengths (n carbons): butanethiol (BT), hexanethiol (HT), and dodecanethiol (DT). BT was from Fluka and HT and DT were from Aldrich; all of them were used without further purification. The thiolate monolayers were prepared by immersing the Pt substrates in 50 μM alkanethiol ethanolic solutions overnight, considering that ethanol has been reported as the best solvent for alkanethiol SAMs grown on Pt.18 After the preparation, the samples were carefully cleaned in ethanol in order to remove physisorbed thiols, and then they were used for the electrochemical and XPS characterization. Sulfur-covered surfaces were prepared by immersion of the Pt substrates in 1 mM Na2S (Aldrich) + 0.1 M NaOH aqueous solution for the formation of approximately one monolayer.22
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2.4. XPS. The samples were characterized by XPS using a Mg KR source (XR50, Specs GmbH) and a hemispherical electron energy analyzer (PHOIBOS 100, Specs GmbH). A two-point calibration of the energy scale was performed using sputtered cleaned gold (Au 4f7/2, binding energy (BE) = 84.00 eV) and copper (Cu 2p3/2, BE = 932.67 eV) samples. For spectra deconvolution of the S 2p region, a Shirley type background was subtracted and a combination of Lorentianz and Gaussian functions was used. The full width at half-maximum (fwhm) was fixed at 1.1 eV and the spinorbit doublet separation of S 2p signal was set to 1.2 eV. The BEs and peak areas were optimized to achieve the best adjustment. Sulfur coverage was estimated by the measurement of the areas of Pt 4f and S 2p signals corrected by the relative sensitivity factor (RSF) of the elements. Pt 4f signal was corrected by the attenuation length for electrons in Pt in order to consider only the signal arising from the outermost Pt atomic monolayer. XPS spectra consisted of 260 scans, with each run taken with 0.025 eV steps, holding for 0.1 s. Special care was taken in order to ensure damage-free data collection. The spectra appearance remained unchanged as a function of measurement time, under the experimental conditions determined in the current study. It is well-known that long exposures of alkanethiol SAMs on Au to X-rays produce damages that alter the chemical nature of surface species.23 However, alkanethiol SAMs on Pt are not as affected by the irradiation, a fact that has already been reported.24 2.5. DFT Calculations. All calculations were performed using plane-wave pseudopotential periodic DFT. The exchange-correlation potential was described by means of the generalized gradient approach (GGA) with the PerdewWang (PW91)25 implementation. The one-electron wave functions have been expanded on a plane wave basis set with a cutoff of 450 eV for the kinetic energy. The Brillouin zone sampling was carried out according to the MonkhorstPack26 scheme with (9 9√ 1), (9 5 1) and (9 3 1) dense k-points meshes for ( 3 √ √ √ √ 3)R30, ( 3 2 3)R30, and 3 4 unit cells, respectively. The projector augmented wave (PAW) method,27,28 as implemented by Kresse and Joubert,29 has been employed to describe the effect of the inner cores of the atoms on the valence electrons. The tolerance used to define self-consistency was 105 eV for the single-point total energy and 104 eV for the geometry optimization. The energy minimization (electronic density relaxation) for a given nuclear configuration was carried out using a Davidson block iteration scheme. The dipole correction was applied to minimize polarization effects caused by asymmetry of the slabs. The calculations have been carried out using the VASP 4.6 package.29,30 The van der Waals density functional (vdWDF) calculations were performed with a modified version of the VASP code, which includes our own self-consistent implementation of the nonlocal van der Waals density functionals. The surface was modeled by a periodic slab composed of four metal layers and a vacuum of ∼12 Å. Adsorption was considered to occur only on one side of the slab. During the geometry optimization the two bottom layers were kept fixed at their optimized bulk truncated geometry for the Pt(111) surface. The two outermost atomic metal layers as well as the atomic coordinates of the adsorbed species were allowed to relax without further constraints. The atomic positions were relaxed until the force on the unconstrained atoms was less than 0.03 eV/ Å. The molecular calculations of BT and butane (B) have been done in a cubic supercell with side lengths of (20 Å 20 Å 20 Å). 17789
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Figure 2. Open circuit potential shifts of a Pt substrate in NaOH 0.1 M after addition of ethanol (final concentration 1 M) and the subsequent injection of hexanethiol (final concentration 0.5 mM).
Figure 1. (a) XPS data of a clean Pt foil. (b) Cyclic voltammograms of Pt in NaOH 0.1 M for three positive potential limits. Scan rate: 100 mV 3 s1.
The lattice parameter calculated for Pt bulk is 3.9860 Å in good agreement with the experimental value, 3.92 Å.31 van der Waals (vdW) intermolecular forces play a relevant role in the systems studied in this work. Nevertheless, one of the main shortcomings of GGAs functionals is the inability to describe nonlocal vdW dispersion forces. Several different approaches are available nowadays and have proven their suitability.3244 The development in the honing of accurate density functionals which are able to account for vdW forces is still an ongoing quest. One promising and efficient scheme is the nonlocal vdW-DF of Dion et al.36 Alternative underlying exchange functionals to the revPBE used in the original vdW-DF can actually improve significantly the accuracy of vdW-DF, reaching better than chemical accuracy (less than 43 meV) for the S22 data set.35 Specifically, we have used the optB88-vdW density functional, which considers an optimized Becke88 exchange functional to explore the role of vdW dispersion forces in the chainchain and substratechain interactions in lying down phases and the chainchain interactions for standing-up ones.42
3. RESULTS AND DISCUSSION 3.1. XPS and Electrochemistry of Clean Pt. Typical XPS spectra obtained for a clean Pt foil used in our experiments is shown in Figure 1a. The absence of contaminants (Cl, SO42, S) and significant amounts of other metals is evident from the spectra. We have only observed a small amount of C, O, and N
resulting from atmospheric contaminants as a consequence of the transfer process. However, it is well-known that thiol adsorption is strong enough to displace these weakly adsorbed species and oxides from metal surfaces.45 We have also tested the contamination level in our electrochemical system by performing voltammetric runs which are sensitive to traces of impurities. Figure 1b shows stationary voltammograms of Pt in 0.1 M NaOH for three different positive potential limits. The voltammetric profile covering the stability potential range of water (Figure 1b, deep red trace), shows the OH adsorption at ≈0.3 V (peak AI0 ). On the other hand, Pt oxidation begins at ≈0.1 V defining a broad anodic potential region related to the PtO oxide monolayer formation (AI). On the negative scan, the PtO layer is reduced at 0.3 V (CI). The voltammogram also shows the underpotential deposition (upd) of H atoms at peaks CII (strongly adsorbed H) and CIII (weakly adsorbed H), which are desorbed at peaks AII and AIII, respectively. There are extreme requirements for the purity of solutions and surface cleanliness in order to achieve both wellresolved structure and equal charges for the H anodic and cathodic sorption peaks. Namely, insufficient purity usually leads to a lack in both the fine structure of the H upd peaks and the symmetry of their cathodic versus anodic charges.46 The repetitive voltammetric scans recorded in this potential region (green line in Figure 1b) showing no changes in the shape of peaks CIICIII indicate the absence of significant amounts of contaminants in the Pt substrate. Hence, the XPS and the electrochemical data indicate that the Pt foil surfaces, which were used to adsorb the different alkanethiol molecules from ethanolic solutions, show low contamination levels making them suitable substrates for alkanethiol SAMs studies. 3.2. Alkanethiol Self-Assembly. The thiol self-assembly process has been studied by recording the changes in the open circuit potential (ocp) of the clean Pt (piranha treated - the last ethanol rinse was omitted in these samples) surface in 0.1 M NaOH produced by (1) ethanol addition (the solvent used for alkanethiol self-assembly) and (2) subsequent hexanethiol addition to the electrolyte (Figure 2). The ocp value for the Pt surface in contact with the 0.1 M NaOH is 0.07 V, a potential value consistent with the presence of PtOH/PtO species in agreement with the XPS (Figure 1a) and voltammetric data (Figure 1b). When ethanol is added to the 17790
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Figure 3. S 2p XPS spectra of (a) HT SAM and (b) DT SAM. The two main signals, at 162.3 and 163.4 eV, are indicated as orange and green lines, respectively. Deep red and gray lines represent the respective fit to the raw spectrum and the background, respectively.
electrolyte, the ocp suddenly decreases to 0.6 V. At this potential value the surface is mainly covered by residues of ethanol chemisorption.47 Hence, the water molecules present in the solution would not interfere with the SAM formation. Also, the ocp value indicates that no PtO is present on the substrate surface since it lies just at the beginning of the H-adatom sorption (Figure 1b). This result could explain why ethanol is a suitable solvent for SAM preparation on Pt, since it plays an important role in the PtO reduction. Finally, after HT addition, the ocp increases to 0.4/0.5 V indicating the replacement of the adsorbed ethanol species by HT species. The ocp value remains stable along the immersion time suggesting that this value mainly corresponds to a HT-covered Pt surface as discussed below. Our results are well in agreement with those reported in ref 11 showing that the initial presence of oxygen on the surface has only a slight influence on the organization of the thiol. In fact, it has been demonstrated by Auger electron spectroscopy that thiol displaces the oxygen initially present on the surface of platinum.11 Furthermore, it has been reported that the best SAMs on Pt were grown using ethanol as solvent, when compared with those prepared in dimethylformamide, dichloromethane, toluene, or hexadecane.18 In this regard, the ability that ethanol has to reduce platinum oxide might play a major role for the improvement of SAMs quality. Thus, from the ocp data described in this work and from previous reports, it can be claimed that PtO should not be considered an abundant surface species after alkanethiol adsorption. 3.3. XPS and Electrochemistry of Alkanethiolate Adlayers on Pt. 3.3.1. XPS Analysis. The XPS spectra analysis of alkanethiols
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on metals is useful to understand the chemical nature of the adsorbed species. The S 2p3/2 core level peak for a Pt surface covered by HT (Figure 3a) can be successfully fitted by a set of two doublets at 162.3 and 163.4 eV. The signal at 162.3 eV for SAMs on Pt has been previously ascribed to adsorbed thiolate species due to the similar behavior of alkanethiols on other face-centered cubic (fcc) metals.6,4850 However, this interpretation disregards the possible presence of atomic sulfur that might also appear in the 162 eV binding energy region. In fact, in contrast to Au, where atomic S appears at 161 eV,51 on Pt, at low coverage, this species originates a S 2p signal at 162.4 eV.52 Therefore, it is unambiguously not possible to assign this component to thiolate because if some amount of S, formed by CS bond cleavage, were present it could originate a signal at ≈162 eV. However, the presence of alkyl chain order after thiol adsorption on Pt, detected by reflection absorption infrared spectroscopy,6 supports the hypothesis that at least a part of the adsorbed species consists of intact thiolates. This idea is also supported by recent DFT calculations showing that the CS bond cleavage on Pt has an activation energy much larger than the SH bond cleavage,53 i.e. thiolate formation is kinetically favored. However, given the catalytic activity of the substrate, the presence of other species such as S and hydrocarbon chain fragments cannot be excluded. The component at 163.4 eV has been related to “unbound” alkanethiol derived species: alkyl disulfides,6 dialkyl sulfides formed by reaction of hydrocarbon radical and thiol radicals,5 and also to dimerized thiolate species.54,55 Alternatively, this component has been assigned to thiol molecules adsorbed on PtO species.6 Note that in the current study the open circuit potential values shown in Figure 2 are not compatible with the presence of PtO species on the Pt surface in contact with ethanol and thiols. Therefore, we can discard the adsorption of sulfur headgroups weakly bound to platinum oxide as the origin of the 163.4 eV component. Moreover, as reported for thiols on Au,48 the species which contributes to this component could be free thiols trapped by chainchain interactions in the SAM.48 However, the relative contribution of the 163.4 eV signal is slightly higher for HT (Figure 3a) than for DT SAMs (Figure 3b), suggesting that the “unbound” thiols are not simply free thiols trapped in the SAMs. If this were the case, the relative contribution of the 163.4 eV component should increase with the hydrocarbon chain length as was observed for alkanethiols on Au.48 Therefore, alkyl sulfide formation seems to be one possible explanation for the 163.4 eV component of the S 2p signal. For HT SAMs the total S coverage (162.3 + 163.4 eV components) results ≈0.34 with a 162.3 eV/Pt signal ratio indicating a surface coverage by these species of ≈0.26 rather than 0.33, the value √ expected √ for dense thiol SAMs of standing up molecules in a ( 3 3)R30 lattice.48 Similar results were found for BT with a total S coverage ≈0.32, and 162.3 eV/Pt signal ratio leading also to a surface coverage of ≈0.26. The same 162.3 and 163.4 eV components are observed in the XPS spectra of DT on the Pt surface although, in this case, the total coverage by S and the surface coverage by the 162.3 eV species increase (Figure 3b). Three electronvolt species increase (Figure 3b). In fact, these values go up to ≈0.38 and ≈0.30 for the total S and the 162.3 eV species, respectively. This latter value is closer to the 0.33 value reported for aliphatic thiols on Pt(111) by in situ STM imaging.9 In this regard, it should be noted that the XPS spectrum for Pt(111) modified by dodecanethiol was found to be similar to that measured with a polycrystalline substrate.5 Finally, blank experiments performed for HT and DT SAMs on Au yielded a coverage of 0.33, 17791
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Table 1. Interfacial Capacitances of Modified Pt and Au Electrodes capacitance/μF 3 cm2 Pt 18 ( 1
S covered
a
Figure 4. Comparison between the voltammetric response of Pt electrodes in 10 mM K4Fe(CN)6 solution in 1 M KNO3: orange and green, as prepared HT SAM and DT SAM, respectively; deep red, clean Pt. Scan rate: 100 mV 3 s1.
which √ in√this case can be unambiguously assigned to thiolates in ( 3 3)R30 lattice and c(4 2) arrangements.48 It is interesting to note that for alkanethiol adsorption on Pd, the total sulfur coverage is 0.8, much larger than the figure found for the DT SAMs on Pt. Consequently, the Ptthiol system seems to be different from that described for thiols adsorbed on Pd, where thiol SAMs are formed on top of a diluted palladium sulfide layer.2,3 No evidence of oxidized S species like sulfonates (167 eV) was detected in the XPS spectra. The absence of higher BE components and the presence of only two components as found here has already been reported for samples measured with Al KR5 and Mg KR24 sources. In conclusion, XPS data indicate differences in the properties of SAMs as the chain length is increased. The quality of BT and HT SAMs is poorer since even assuming the absence of adsorbed atomic S, i.e., all thiolate, the amount of this species is lower than the value expected for a dense SAM, when compared with alkanethiols on Au. In contrast, for DT SAMs the intensity of the 162.3 eV component is closer to that found for complete SAMs on Au in SU configuration. Unbound thiols and a small amount of S could be present, but the SAM quality is expected to be better than the one observed for short chain thiols. Note that these results contrast to those previously reported for the same system where no influence of the hydrocarbon chain length has been found based on XPS data.6 In the next section the conclusions about the chain length dependent quality of the alkanethiol/Pt system will be supported by electrochemical data. 3.3.2. Electrochemical Measurements. In order to test whether alkanethiolate SAMs on Pt are effective as blocking layers, the electrochemical response of a redox couple in solution, which does not penetrate alkanethiol SAMs,5658 has been studied. In Figure 4 the positive and negative faradaic currents due to the ferro/ferricyanide redox couple are shown. The voltammetric profiles have been acquired in a narrow potential domain in order to avoid considerable damage of the SAMs. Indeed, the possibility of thiolate or metal oxidation due to the use of ferricyanide as a redox probe has to be taking into account.59 In the same manner, electrodesorption, as a consequence of excursions into low potential regions, cannot be, in principle, ruled out. However, it was found that under the
Au 17a
BT covered
10 ( 2
7(1
HT covered DT covered
7(2 3(1
5(1 2.8 ( 0.5
Data taken from ref 65.
experimental conditions chosen in this report, stable profiles were taken along hundreds of cycles. From this result, it is inferred that no significant oxidation or reductive desorption takes place in these experiments. The marked inhibition of the electron transfer process as a consequence of the DT SAM on Pt can be clearly noted. This result supports the association of the 162.3 eV component with alkanethiolate rather than S. Certainly, if atomic S domains were present at the surface, a considerable redox current would be expected. On the other hand, in the case of HT, significant faradaic currents are observed at relatively low overpotentials (Figure 4, orange line). Considering that a major manifestation of the blocking SAM is the suppression of a simple faradaic process, the more suitable explanation for this behavior is the presence of a large amount of defects or S domains in the layer which control diffusional transport to the electrode.60 The barrier properties of the thiolate adlayers on Pt have also been studied carrying out interfacial capacitance measurements. The voltammetric capacitance (C) can be obtained by dividing the charging current density (j) by the scan rate (v) in a potential range where no faradaic processes occur (oxidation or reduction reactions). For the clean metal, the current density is almost constant within a narrow potential range, between 0.48 and 0.53 V (Figure 1b). Although the generally accepted definition of double layer region for platinum-group metals, i.e., a flat region appearing in cyclic voltammogram,61 is fulfilled in this potential domain, it must be taken into account that in alkaline media H and OH adsorption overlaps in this region.62 As a direct consequence of these phenomena, the C values obtained for clean Pt exceed the typical figure for an electrostatic double layer capacitance, which is about 30 μFcm2.63 On the other hand, the DT modified Pt showed capacitances significantly smaller than those of bare electrodes, as has been observed for other metals.64,3 For the sake of comparison, we have also performed capacitance measurements on alkanethiolate covered preferentially oriented Au(111) (Table 1). As already reported for Au66,67 and Pd3 it is evident that the adsorption of alkanethiols significantly reduces the Pt capacitance (Table 1). Since the decrease in the capacitance is related with a low dielectric constant layer on the electrode surface, it is clear that the low capacitances values obtained for alkanethiol modified Pt substrates should be ascribed to the presence of hydrocarbon chains, as the capacitance of S-covered Pt electrodes is considerable greater (Table 1). Also observed was the wellknown dependence of the capacitance value on the chain length. However, while the capacitance of the DT covered Pt is comparable to that measured in the analogous system with Au, the figures for HT and BT monolayers on Pt are significantly different to those found for Au. It has been reported that SAMs are less ordered on Pt than on Au, and a considerably high 17792
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Figure 5. Equilibrium structures of the different models studied for the BT adsorption on Pt(111). (I) BTH (LD) + H (θ = 1/6), (II) BTH (SU) + H (θ = 1/3), (III) S + BH + H (θ = 1/6), (IV) S + BTH (LD) + H (θ = 1/3), (V) BTH (SU) + S + H (θ = 1/3), and (VI) BTH (SU) + S + H (θ = 1/4).
number of gauche defects in alkyl thiol monolayers on Pt(111) has been found by vibrational sum frequency generation (SFG) spectroscopy, making the film permeable to ions.68 However, the fact that HT and BT SAMs on Pt exhibit C values ≈40% greater (Table 1) than their Au counterparts, can hardly be justified by conformational defects. The larger capacitances are indicative of partial permeation of electrolyte ions into the SAM, in agreement with the partial blocking in the electron transfer (Figure 4) and the lower coverage by the 162,3 eV component detected in the XPS data. Therefore, the electrochemical measurements and XPS data lead to the conclusion that DT forms dense SAMs on Pt, in agreement with previous results,5 while short chain thiols form SAMs of poorer quality. As a counterpart, while BT and HT SAMs on Au are more defective and disordered in comparison to those formed from longer thiols,69,70 the surface coverage is the same. Therefore, the defective nature of BT and HT on Pt should be related to incomplete SAMs, as already reported for Ni surfaces.71 In the following, DFT results are presented as an attempt to explain the difference in the quality of SAMs for short and long chain alkanethiols on Pt. 3.4. DFT Calculations. 3.4.1. Models for Thiol Adsoption on Pt. Before we begin this section, some important points related to the DFT calculations deserve to be stressed. First, a Pt(111) substrate has been selected for modeling thiol adsorption because it has been studied with different techniques and structural information about it can easily be found.9 Second, the calculations have been focused on the short BT molecule due to the computational time involved, although part of the approach has been extended to HT and DT. Finally, it should be noted that the hypothetical models under study are only examples with the purpose of explaining in a qualitative way the behavior of the adsorbed thiol. Namely, they represent different surface structures that could arise from the CS bond scission— formation of thiol+S and S+alkyl chain—setting up a competition against thiolate in the adsorption process. Certainly, other surface models are possible, but it is believed that the qualitative results from the present analysis are applicable to predict and understand thiol SAMs on Pt.
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It is generally accepted that the adsorption mechanism of thiols on Pt(111) occurs via the cleavage of the SH bond leading to the coadsorption of both hydrogen and radical thiol, thiobutyl radical (BTH), in this study. The SC bond in the adsorbed BTH can eventually be broken forming new adsorbed species, namely, butyl radical (BH) and sulfide, S. The BH species can react with coadsorbed H to form a butane molecule (B) that will be detached from the Pt(111) surface, allowing another BT molecule to undergo its adsorption process. The optimized structures for all adsorption models studied on the Pt substrate are presented in Figure 5(IVI). Models I (Figure 5-I) and II (Figure 5-II) represent the situation where no SC bond scission takes place, i.e., the 162.3 eV component in the XPS spectra is considered to be only thiolate species. Model I, corresponding to a diluted LD phase (Figure √ 5-I) √with surface coverage θ = 1/6, has been studied in a ( 3 2 3)R30 unit cell. This phase simulates the initial adsorption stage for the short chain thiol molecules where the intact thiolate species could interact with the Pt surface in LD configuration. On √the other √ hand, model II, consists of a dense SU phase in a ( 3 3) R30 unit cell with surface coverage θ = 1/3 and would correspond to an ideal close packed final state of the system. This surface structure has already been described for alkanethiolate SAMs on the Pt(111) substrate.9 For instance, thiols with long hydrocarbon chains such as DT should adopt the SU configuration in order to accommodate the 0.30 thiolate surface coverage experimentally observed. No matter what phase (LD or SU) is considered, after optimization the S atom of the alkanethiol molecule is bonded to the Pt(111) surface at hollow fccbridge positions. In the SU phase the tilt angle, the angle between the molecular backbone and the surface normal direction, is ≈4, close to the values reported for the Ptthiol system from IR measurements.6 The possible CS bond cleavage due to the catalytic activity of the Pt(111) √ surface√has also been considered in model III (θ = 1/6) using a ( 3 2 3)R30 unit cell (Figure 5-III). This model considers the possibility that the 162.3eV component measured for short thiols were only adsorbed sulfides. Therein, the fragments originating from the CS bond cleavage of the BTH species—S and butyl radicals—are represented. The optimal bonding site for S after their optimization was found to be fcc hollow, while the butyl chains were placed on top of a substrate atom in LD configuration. The chains were linked to the metal surface by the C1 atom, i.e., the carbon previously bonded to sulfur. Attempts were √ made to √ optimize these dissociated fragments (S + BH) in a ( 3 3)R30 unit cell where the initial geometry was built up with the butyl radical in SU configuration, the only possible configuration for this coverage. After optimization, this arrangement yielded again the BTH adsorbed species in SU configuration (Figure 5-II). It means that, for higher adsorbate coverage (θ = 1/3), the C1 atom of the butyl radical in SU configuration prefers to be bonded to the S atom rather than to a Pt surface atom. This fact indicates that the butyl species can only be adsorbed placed parallel to the Pt surface and denotes the difficulty of the CS bond scission when the thiolate adsorbate adopts the vertical position on the Pt surface as discussed further on. Note that the adsorption of H species at top positions has been included in the surface models, except in model III where this adsorbate has been located on fcc hollow site. Indeed, it is wellknown that the potential energy surface for H chemisorption on Pt is rather flat.72 In contrast to alkanethiol adsorption on 17793
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Au(111), the H atoms resulting from the SH bond scission during thiolate formation can remain adsorbed on the Pt surface. Finally, S+BTH lattices were studied considering the possibility that the 162.3 eV components in the XPS spectra were a mixture of thiolate and sulfide √ adsorbed√species. Models IV and V have also been studied in a ( 3 2 3)R30 unit cell with a total S coverage (sulfide+thiolate) θ = 1/3. They differ in the configuration of the adsorbed thiobutyl radical, LD in model IV (Figure 5-IV) and SU in model V (Figure 5-V). The last model (model VI) also represents a S+BTH lattice where the adsorbed BT √H is in SU configuration but in this case it has been employed a 3 4 unit cell with a total S coverage (sulfide+thiolate) of θ = 1/4, close to that found in BT and HT SAMs for the 162 eV component (θ = 0.26) (Figure 5-VI). These three lattices explore the possibility that after CS bond cleavage the butyl radical desorbs from the surface as a butane molecule, thus allowing the adsorption of a new BT molecule. As a result of this process, a S+BTH lattice should be expected. In these cases, after incorporation of some S the Pt surface is considered to be “passivated” allowing the adsorption of intact thiolates resembling the mechanism proposed for thiol adsorption on Pd.2,4 The adsorption energy (Eads) on the Pt(111) surface is defined as Eads
1 ¼ ½E@Pt EPt NBT EBT þ NB EB NBT
1 @Pt ½G NPt μPt A
∑N@ μ@ γclean
model I model II
ð2Þ
where A is the surface area, G@Pt is the Gibbs free energy of the adsorbed system, and μPt and μ@ are the chemical potentials of the bulk metal surface and the adsorbed species, respectively. NPt and N@ are the number of platinum atoms and the adsorbed species in the slab unit cell. On the other hand, γclean represents the surface free energy of the clean surface.76 In eq 2 the chemical potential of the Pt surface (μPt) is equated to the total energy of a bulk Pt atom (EPt Bulk). On the other hand the Gibbs free energy (G@Pt) is estimated by the total energy of the adsorbatesubstrate system at T = 0 K (E@Pt). The chemical potentials of the BTH, H, B, and S can be written as a function of the chemical potential of BT as follows μBT ¼ μBTH þ μH
ð3Þ
μBT ¼ μB þ μS
ð4Þ
model III
model IV model V model VI
Eads/eV 1.34
0.75
1.91
1.54
1.58
1.63
dsPt/Å 2.32
2.32
2.21 (C1Pt)
2.33
2.32
2.32
2.28 (SPt)
2.28
2.28
2.28
139
140
140
R/deg
133
139
Likewise, the chemical potential of BT can be written in relation to its DFT total energy (EBT), as follows μBT ¼ EBT þ ΔμBT
ð5Þ
where ΔμBT includes the realistic thermodynamic conditions, like the temperature and BT concentration. The most stable surface structure is the one which minimizes the surface free energy, which can be expressed as a function of the adsorption energy (eq 1) γLD ðΔμBT Þ ¼
1 ½NBT Eads þ ELD vdW A þ γclean
ð1Þ
where NBT is the number of BT molecules implied in the adsorption process and NB is the number of butane molecules produced and further detached from the surface in models IV, V, and VI. E@Pt, EPt, EBT, and EB stand for the total energy of the adsorbatesubstrate system, the clean surface, and the BT and B molecules, respectively. Negative numbers indicate an exothermic adsorption process with respect to the clean surface and the adsorbates. Following previous works,73,74 in order to compare the stability of these phases containing different surface arrangements, different numbers of SPt bonds, and different numbers of adsorbed species (Figure 5(IVI)), ab initio atomistic thermodynamics has been used taking into account that it has been successfully applied on several SAM systems. In this way, the stability of each model has been determined through the surface free energy,75 defined by γ¼
Table 2. Adsorption Energy (Eads), Averaged Distance SPt (dSPt) and C1SPt Angle (R) Calculated for the Different Models Shown in Figure 5
γSU ðΔμBT Þ ¼
NBT Δ μBT A
ð6Þ
1 ½NBT Eads þ ESU vdW A þ γclean
NBT Δ μBT A
ð7Þ
In eqs 6 and 7 the energetic contributions arising from dispersion forces for these systems have been estimated by using the optB88-vdW functional as an additional energetic term (see Supporting Information). Using this method we have obtained 0.147 eV/CH2 for LD and 0.063 eV/CH2 for SU configurations. Thus, the Eads values derived from DFT calculations using the PW91 functional were corrected by adding ELD vdW = nC 0.147 eV and ESU vdW = nC 0.063 eV for the LD and SU phases, respectively, being nC the number of methylene units. It is worth to mention that these values are in a reasonable agreement with the vdW forces estimated from experimental data: ELD vdW = nC 78 0.087 eV,77 ESU The smaller vdW experivdW = nC 0.044 eV. mental values can be explained considering the chain disorder present in the real SAMs that avoid a complete optimization of the interactions. Moreover, the optB88-vdW functional is expected to be including some small amount of residual semilocal correlation. This additional attractive energy to the pure vdW dispersion could also contribute to the apparent overestimation of optB88-vdW correction terms. In addition to the comments made above, it results necessary to clarify some points related to the inclusion of vdW interactions in this treatment. The dispersive interaction energy for each CH2 group has been considered constant independently of its position along the chain and the molecule conformation. These factors clearly exert an influence on chainchain and substrate chain interactions. However, it is a fair approximation to neglect them given that the current treatment deals with vdW interactions in a phenomenological way. Note that in models V and VI ESU vdW is divided by three because each alkanethiol is surrounded by only two neighbors instead of six. The analysis of surface free energy vs chemical potential of the adsorbate plots 17794
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Figure 7. Schematic representation of the LD and SU arrangements for DT, HT, and BT on Pt(111). The coverage for each model is noted.
Figure 6. γLD and γSU vs Δμ for BT (green), HT (red), and DT (orange) adsorbed on Pt(111). γLD and γSU values are represented by solid and dashed lines, respectively. Black trace: clean surface. The transition points are indicated by vertical lines. van der Waals interactions calculated using (a) vdW-DF and (b) experimental data.
for the different models shown in Figure 5 allows comparison of the relative stability of the BT/Pt(111) surface structures. 3.4.2. DFT Results. In Table 2 we report the obtained structural parameters for the models studied here. The larger adsorption energy is expected for model III where one BT molecule in gas phase gives rise to three adsorbed fragments: H, S, and butyl. We have also estimated the binding energy, Eb, for BTH from eq 8 Eb ¼ ½E@Pt EPt NBT-H EBT-H
ð8Þ
The Eb values for models I and II result in 2.77 and 2.57 eV, respectively. These values are comparable with those reported for a thiomethyl radical on Pt(111) in refs 79 (2.78 eV) and 53 (2.68 eV). Although there is difference in chain length, the comparison is plausible considering that for the CH3S radical the C1SPt angle53,79 is similar to those found for models I and II (Table 2). It is possible to compare the DFT parameters for √ obtained √ model II with those estimated for BT in a ( 3 3)R30 lattice (SU) on an unreconstructed Au(111). In this case the , R = 145, and Eads = 1.8 eV. It is values found are dSAu = 2.42 Å evident that the dSAu is larger than dSPt, although smaller dSAu distances have been reported for RSAu adatom models (2.46 Å < dSAu < 2.33 Å).74 Finally, the Eb values for BT on Pt (models I and II) are considerably larger than those reported on Au irrespective of the model used.48 This fact could explain the increased difficulty to desorb thiolates from the Pt surfaces; i.e., they do not exhibit a desorption peak before the hydrogen evolution reaction8 as in the case of thiolates on Au(111). The Eads values shown in Table 2 were used to build a BT stability diagram for LD (model I) and SU (model II) surface structures. In Figure 6 the surface free energy (γ) for these
surface structures on the Pt(111) substrate, calculated with eqs 6 and 7, as a function of the chemical potential of the BT molecule (ΔμBT) are plotted using van der Waals interactions from vdWDF (Figure 6a) and from experimental data (Figure 6b). The surface free energy of the clean Pt(111) substrate (γclean) is also included. As expected, the surface free energy of the clean substrate is independent of the chemical potential of the BT molecule; thus it appears as a parallel line to the x axis (Figure 6a). On the other hand, the BT phases yield γ vs Δμ straight lines whose slopes are determined by the NBT/A ratio. At low chemical potentials (Δμ f ∞) the surface free energy is more positive than the γclean, reflecting that adsorbed BTH is unstable with respect to the clean surface. However, when Δμ = 1.93 eV, the diluted BTH LD phase becomes more stable than the clean surface (Figure 6a). The chemical potential range for the thermodynamic stability of the LD phase extends up to Δμ = 0.08 eV where the γLD equals γSU and a surface structure transition should take place. Now the effect of increasing the hydrocarbon chain length on the thiol stability will be discussed. The analysis is started with the BTH in LD phase with the molecular axis parallel to the surface and the hydrocarbon backbone extended along the Æ112 æ direction (Figure 7). When the BTH adsorbed in LD configuration is replaced by thiols with longer hydrocarbon chains, namely, HT√(nC = 6) √ or DT (nC = 12), the y component of the unit cell ( 3 2 3) increases in √ √ a proportional way, and the new unit cell should be ( 3 2 3λ) where λ = (nC)/4 is a factor that relates the number of C atoms between the HT or DT and the BT molecules. Thus, the area of the unit cell for longer thiols can be related to the area of BT unit cell as Aλ (Figure 7). In contrast, for the SU phase the area of the unit cell remains independent of nC (Figure 7). Taking into account these considerations for longer thiols, eq 6 can be modified as γLD ðΔμBT Þ ¼
1 ½NBT Eads þ ELD vdW Aλ þ γclean
17795
NBT Δ μBT Aλ
ð9Þ
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Figure 9. γLD and γSU vs Δμ for the clean Pt surface and the different BT related models (IVI) shown in Figure 5.
Figure 8. Projected density of states, PDOS, of the C1 atom (s and p states) and the Pt surface atom (dz2 states) in model I, model II, and model III. The zero of energy corresponds to the Fermi level (EF).
On the other hand, the eq 7 is also valid for the HT and DT adsorption. When the hydrocarbon chain length is increased (Figure 6a) the stability range of the clean Pt and the LD surface structure is dramatically reduced. In fact, the clean Pt is covered by thiododecyl radical (DTH) at Δμ = 3.10 eV and by thiohexyl radical (HTH) at Δμ = 2.2 eV. On the other hand, the LD to SU transitions occur at Δμ = 1.2 eV for DTH and Δμ = 0.6 eV for HTH (Figure 6). Note that similar results and conclusions on the LD to SU phase transition are reached by using experimental van der Waals interaction data (Figure 6b). In fact, the main driving force for the LD to SU transition is the number of thiolate bonds that can be accommodated per surface area unit. In the case of BT, this number is similar in LD or SU configuration but decreases dramatically for DT in LD so that this system prefers to be arranged in the SU configuration. Certainly, the van der Waals energetic term, from either DFT calculation or experimental data, also plays some role in the stabilization of both phases, taking into account the dispersion forces in the chainchain and substratechain in LD and SU phases, although
its value has a minor influence in the stability diagram as shown by comparing panels a and b of Figure 6. The main conclusion from these calculations is that the SU phase dominates the DT adsorption in almost the entire range of chemical potential values with experimental significance. As discussed below, it has important implications on the SAM chemistry and quality. Next the possibility of CS bond scission for both the LD and SU adsorbed species is analyzed. The analysis is focused on the BTH LD (model I), BTH SU (model II), and S+BH (model III) surface structures, in particular the interaction between the C1 atom and the Pt atom underneath. The analysis of the projected density of states (PDOS) of the C1 atom (s and p states) and the Pt atom (dz2 states) provides a possible explanation for the CS bond scission in the LD phase. In fact, the PDOS of C1 for model I (Figure 8a) and model III (Figure 8c) surface structures shows a peak located at ≈3 eV. This peak coincides with the (dz2) state of the Pt atom that lies just below the C1 atom. Obviously, in the S+BH system the Pt (dz2) state is strongly affected by the CPt bond formation, a feature that is less evident in the BTH LD system. However, the presence in both systems of the 3 eV peak suggests that the Pt atom strongly interacts with the C1 atom in model III and also exerts a clear influence on the C1 atom of model I. It is also noted in Figure 8b that this peak is absent in the BTH SU phase (model II). This feature indicates that negligible influence of the Pt surface atom is exerted on the states of the C1 atom when BTH is in SU configuration. This analysis is supported by the adsorption of alkyl radicals at top sites due to the strong overlap between the p-lobe of the C1 atom and the Pt dz2 orbital.72 From these results we can conclude that the LD phase can be decomposed in alkyl chain and S species while the SU phase should remain intact. This is an important result that could explain why the short alkanethiols on Pt originate poor quality SAMs while long alkanethiols produce better quality SAMs approaching those shown on Au substrates. In fact, while for short chain thiols the LD phase dominates the phase diagram, for long chain thiols the stability range of the LD phase is only observed at extremely low Δμ values (Figure 6). Figure 9 exhibits the complexity of the phase diagram for the short chain thiols, for which the LD phase dominates over a wide range of Δμ (Figure 6). Thus, a larger probability of CS bond scission is expected (Figure 8). In this diagram the surface structure models shown in Figure 5 are analyzed, including the products resulting from the Pt induced SC bond cleavage. 17796
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The Journal of Physical Chemistry C The diagram shows that at low Δμ the BTH LD lattice is more unstable than the corresponding S+BH, i.e., the adsorbed BTH species in LD configuration should decompose into the hydrocarbon chain radical BH and S. As expected, similar calculations for BTH on Au(111) yield completely different results: BTH adsorbed in LD configuration is more stable than its adsorbed fragments, S+BH. It means that the butyl radical could be desorbed from the Pt surface as a butane molecule after hydrogenation and substituted by a new BTH radical. Increasing Δμ results in the formation of S+BTH LD lattices. However the stability of this lattice is only slightly higher than that corresponding to the S+BTH SU surface structure. Therefore, for short chain thiols it is possible that mixtures of BTH LD, BTH SU, and S were present, justifying our experimental results for HT and BT where the SAMs exhibit higher capacitance values than those expected and lower thiolate coverage. Finally, the effect of H2 formation instead of H adatom upon thiol adsorption on the stability of the different phases has been also tested. In these calculations a H2 pressure of 5 107 atm, considering the H2 content in equilibrium with the ethanolic solutions, has been used. This value is reasonable, taking into account the H2 levels in a recipient containing ethanol in contact with the atmosphere, typical for self-assembly from solutions.4 Results from these calculations are in agreement with our previous analysis, indicating that both processes, the H2 molecule formation and the H adsorption on Pt(111) are energetically comparable.
4. CONCLUSIONS The adsorption of short and long thiols on Pt from ethanolic solutions has been studied by electrochemical techniques, XPS and DFT calculations. In contrast to previous results on these systems, we found that the SAM quality is chain length dependent. Butanethiol and hexanethiol SAMs exhibit low thiolate coverage and poor barrier properties suggesting that thiolates could coexist with adsorbed sulfides and broken hydrocarbon chains. On the other hand, dodecanethiol SAMs exhibit higher thiolate coverage and good barrier properties similar to those found for the dense phases of thiolates on Au. The observed chain length dependent quality of SAMs on Pt can be explained by DFT and thermodynamic calculations. They show that the LD phases strongly interact with the Pt atoms so that the thiol adsorbate can be broken. Therefore, for short chain thiols, where the LD phase dominates the stability diagram, one should expect mixtures of adsorbed sulfides and hydrocarbon chains at low Δμ values, and sulfides and adsorbed alkanethiolates at higher Δμ values. On the other hand, DT SAMs exhibit a narrow stability range for the LD surface structure. Even at low Δμ values the adsorbed dodecanethiolates prefer to adopt the SU configuration where the Pt atomadsorbate interaction is negligible, and therefore, the possibility of molecule decomposition into sulfide and hydrocarbon fragments is expected to be smaller than for shorter thiols. This fact explains the better SAM quality found for these SAMs. ’ ASSOCIATED CONTENT
bS
Supporting Information. A more detailed description about the calculation of the vdW interaction energy per methylene group of the alkyl chain in SU and LD configurations.
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This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support has been provided by the Argentine “Agencia Nacional de Promocion Científica y Tecnologica” (PICT 062111111), CONICET (PIP 11220090100139), the National University of La Plata (Argentina), MCI (Spain, CTQ200806017/BQU), and ACIISI (ID20100152, Gobierno de Canarias, Spain). M.A.F.A. is a doctoral fellow of “Agencia Nacional de Promocion Científica y Tecnologica”. ’ REFERENCES (1) Alexiadis, O.; Harmandaris, V. A.; Mavrantzas, V. G.; Site, L. D. J. Phys. Chem. C 2007, 111, 6380–6391. (2) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 2003, 125, 2597–2609. (3) Corthey, G.; Rubert, A. A.; Benitez, G. A.; Fonticelli, M. H.; Salvarezza, R. C. J. Phys. Chem. C 2009, 113, 6735–6742. (4) Carro, P.; Corthey, G.; Rubert, A. A.; Benitez, G. A.; Fonticelli, M. H.; Salvarezza, R. C. Langmuir 2010, 26, 14655–14662. (5) Laiho, T.; Lukkari, J.; Meretoja, M.; Laajalehto, K.; Kankare, J.; Leiro, J. A. Surf. Sci. 2005, 584, 83–89. (6) Petrovykh, D. Y.; Kimura-Suda, H.; Opdahl, A.; Richter, L. J.; Tarlov, M. J.; Whitman, L. J. Langmuir 2006, 22, 2578–2587. (7) Sartenaer, Y.; Dreesen, L.; Humbert, C.; Volcke, C.; Tourillon, G.; Louette, P.; Thiry, P. A.; Peremans, A. Surf. Sci. 2007, 601, 1259–1264. (8) Williams, J. A.; Gorman, C. B. J. Phys. Chem. C 2007, 111, 12804–12810. (9) Yang, Y.; Yen, Y.; Ou Yang, L.; Yau, S.; Itaya, K. Langmuir 2004, 20, 10030–10037. (10) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1995, 99, 11472–11480. (11) Lang, P.; Mekhalif, Z.; Rat, B.; Gamier, F. J. Electroanal. Chem. 1998, 441, 83–93. (12) Karhanek, D.; Bucko, T.; Hafner, J. J. Phys.: Condens. Matter 2010, 22, 265005. (13) Mallon, C. T.; McNally, A.; Keyes, T. E.; Forster, R. J. J. Am. Chem. Soc. 2008, 130, 10002–10007. (14) Mallon, C. T.; Forster, R. J.; McNally, A.; Campagnoli, E.; Pikramenou, Z.; Keyes, T. E. Langmuir 2007, 23, 6997–7002. (15) Lis, D.; Peremans, A.; Sartenaer, Y.; Caudano, Y.; Mani, A. A.; Dreesen, L.; Thiry, P. A.; Guthmuller, J.; Champagne, B.; Cecchet, F. J. Phys. Chem. C 2009, 113, 9857–9864. (16) Beebe, J. M.; Kim, B.; Frisbie, C. D.; Kushmerick, J. G. ACS Nano 2008, 2, 827–832. (17) Williams, J. A.; Gorman, C. B. Langmuir 2007, 23, 3103–3105. (18) Geissler, M.; Chen, J.; Xia, Y. Langmuir 2004, 20, 6993–6997. (19) Collman, J. P.; Decreau, R. A.; Lin, H.; Hosseini, A.; Yang, Y.; Dey, A.; Eberspacher, T. A. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 7320–7323. (20) Peng, Z.; Yang, H. Nano Today 2009, 4, 143–164. (21) Marinkovic, N.; Markovic, N.; Adzic, R. J. Electroanal. Chem. 1992, 330, 433–452. (22) Sung, Y.; Chrzanowski, W.; Wieckowski, A.; Zolfaghari, A.; Blais, S.; Jerkiewicz, G. Electrochim. Acta 1998, 44, 1019–1030. (23) Duwez, A. J. Electron Spectrosc. Relat. Phenom. 2004, 134, 97–138. 17797
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