Surface Enhanced Infrared Studies of 4-Methoxypyridine Adsorption

Feb 10, 2016 - Department of Chemistry, University of Saskatchewan, Saskatoon, ... being found on negatively charged surfaces in acidic media and the ...
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Surface Enhanced Infrared Studies of 4‑Methoxypyridine Adsorption on Gold Film Electrodes Amanda Quirk, Bipinlal Unni, and Ian J. Burgess* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5C9 Canada S Supporting Information *

ABSTRACT: This work uses electrochemical surface sensitive vibrational spectroscopy to characterize the adsorption of a known metal nanoparticle stabilizer and growth director, 4-methoxypyridine (MOP). Surface enhanced infrared absorption spectroscopy (SEIRAS) is employed to study the adsorption of 4-methoxypyridine on gold films. Experiments are performed under electrochemical control and in different electrolyte acidities to identify both the extent of protonation of the adsorbed species as well as its orientation with respect to the electrode surface. No evidence of adsorbed conjugated acid is found even when the electrolyte pH is considerably lower than the pKa. Through an analysis of the transition dipole moments, determined from DFT calculations, the SEIRA spectra support an adsorption configuration through the ring nitrogen which is particularly dominant in neutral pH conditions. Adsorption is dependent on both the electrical state of the Au film electrode as well as the presence of ions in the electrolyte that compete for adsorption sites at positive potentials. Combined differential capacitance measurements and spectroscopic data demonstrate that both a horizontal adsorption geometry and a vertical adsorption phase can be induced, with the former being found on negatively charged surfaces in acidic media and the latter over a wide range of polarizations in neutral solutions.

1. INTRODUCTION Studies of the orientation of pyridine adsorbed on metal surfaces have a long history starting with early ultrahigh vacuum (UHV) measurements1 and extending to subsequent work in aqueous solutions where electrochemical measurements have provided a rigorous and quantitative tool for characterizing the extent of adsorbed pyridine and its preferred adsorption orientation.2 Broadly speaking, pyridine adsorbs on coinage metals either through the nonbonding pair of electrons on its ring nitrogen (vertical configuration) or through its π-electron system (horizontal adsorption). However, other adsorption configurations have been reported including tilted vertical phases1,3,4 and the so-called α-pyridyl phase where coordination to the metal surface occurs via the ring nitrogen and an adjacent carbon atom after C−H bond cleavage.5,6 The orientation adopted by pyridine on metal surfaces depends on the identity of the metal,7−12 the packing density of its surface atoms, the electronic state of the metal, the vacuum/solution concentration of the adsorbate, and in the case of adsorption from solution, the pH of the electrolyte. Substitution effects also play a key role with reports of coordination through the substituent in both 4-cyanopyridine13 and pyridinecarboxlyates14 as well as other studies that indicate that the electron-donating ability of para-derivatized pyridines perturb the relative energetics associated with horizontal and vertical adsorption motifs.15,16 The strong affinity of pyridine and its derivatives for metal surfaces coupled with its rich, yet seemingly well understood © XXXX American Chemical Society

surface behavior, has made it popular in reports of nascent and developing surface characterization techniques based on vibrational spectroscopy. Examples include infrared reflection absorption spectroscopy (IRRAS)17−24 and its more quantitative analogue photoelastic modulation PM-IRRAS,25,26 tipenhanced Raman scattering (TERS),27 shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS),28−31 and surface enhanced infrared reflection absorption spectroscopy (SEIRAS).3,11,32 Renewed interest in the adsorption behavior of pyridine has been sparked in recent years due to the demonstration that substituted pyridines such as 4-dimethylaminopyridine (DMAP) can act as gold nanoparticle phase-transfer agents and stabilizers.33−35 Compared to the covalent Au−S bond found in thiol-based stabilizers, substituted pyridines can be desorbed from the metal surface under relatively mild conditions which opens new routes for postfabrication modification of metal nanoparticles.36−38 This research group has previously shown that DMAP adsorption on Au nanoparticles is highly dependent on surface crystallography39 which allows for strategies that produce highly anisotropic nanostructures.40 Key to this result was pre-existing fundamental knowledge of DMAP adsorption acquired through electroReceived: November 12, 2015 Revised: February 5, 2016

A

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Langmuir chemical and surface vibrational studies. Experimental methods to evaluate nanoparticle capping agents that may selectively passivate different metal facets was recently noted in a perspective by Xia et al. as key to better shape-control in nanocrystal synthesis.41 It was suggested that screening new capping agents for their ability to direct anisotropic nanoparticle growth should be pursued in order to help design new synthetic strategies. We contend that fundamental (spectro)electrochemical adsorption studies provide an excellent means to achieve this goal. We recently reported that the reduction of gold salts in the presence of 4-methoxypyridine (MOP) leads to highly anisotropic nanocrystals and we have provided an indepth thermodynamic analyses of MOP adsorption on Au(111) single-crystal surfaces.42 Herein we provide complementary electrochemical SEIRAS studies on gold films in an effort to confirm previously reported orientation motifs and to investigate the influence solution pH plays on the adsorption behavior of MOP.

3. RESULTS AND DISCUSSION Transmission Spectra of MOP and MOP·HClO4. Infrared spectra of MOP and its conjugate acid were measured in an effort to help interpret the SEIRAS results. Figure 1 shows a

2. EXPERIMENTAL SECTION

Figure 1. Transmission spectra of (a) MOP (red line) and (b) MOP· HClO4 (black line).

Reagents, Solutions, and Electrode Materials. All chemicals including 4-methoxypyridine (97%), potassium perchlorate (+99%), KOH (semiconductor grade, 99.99%) and perchloric acid (70%) were purchased from Sigma-Aldrich. KClO4 was used after double recrystallization from Milli-Q (≥18.2 MΩ cm−1) water and all other chemicals were used as received. The conjugate acid, 4-methoxypyridinium perchlorate (MOP·HClO4), was obtained by the dropwise addition of neat MOP into concentrated HClO4. The white-colored, needle-like MOP·HClO4 crystals were dissolved in Milli-Q water and precipitated by addition of diethyl ether. The pH of the electrolyte solution was adjusted to the desired value using dilute KOH and HClO4 solutions. Transmission and ATR-SEIRAS Measurements. The transmission spectra of MOP were measured from drop casted thin films of neat MOP on a clean CaF2 window. The MOP·HClO4 transmission spectra were collected using KBr pellets. All ATR-SEIRAS measurements were performed using a glass spectroelectrochemical cell built in-house. A 25 mm diameter silicon hemisphere (Harrick) coated with gold was used as the working electrode. The silicon hemisphere was polished with 3 μm, followed by 0.5 μm diamond polishing suspensions and then cleaned ultrasonically with ethanol (95%) and Milli-Q water. Prior to the gold deposition, the silicon hemisphere basal plane was treated with NH4F (40 w/w%) to remove the oxide layer. The gold layer on the silicon hemisphere was achieved using a Denton Vacuum Desk IV cold sputter and a deposition rate of ∼0.02 nm s−1 for 25 min to achieve a final film thickness of ∼30 nm. The formal concentration of MOP in all experiments was 0.1 mM unless otherwise noted. A saturated KCl/Ag/AgCl was used as the reference electrode and externally connected to the cell. The supporting electrolyte solution (50 mM KClO4 for all experiments unless otherwise noted) was deoxygenated for 30 min prior to the measurements and an argon blanket was maintained inside the cell throughout the experiment. The potential of the Au-film serving as both the SEIRAS active substrate and the working electrode was controlled using a HEKA PG 590 potentiostat. Prior to the addition of MOP, the Au layer on the silicon hemisphere was electrochemically cleaned in 0.05 M KClO4 by cycling the potential between the open circuit potential (OCP) and 1.2 V. An in-house developed LabVIEW program was used to control both the Bruker Vertex 70 FT-IR spectrometer and the potentiostat while performing the potential dependent SEIRAS experiments. The incident angle of the IR beam was 70° and all spectra were collected with 4 cm−1 resolution between 4000 and 400 cm−1. Each spectrum was the result of the coaddition of 128 interferograms. Final spectra are reported as relative changes in absorbance, ΔAbs = −log(RE/REref) where RE and REref are the singlebeam signals recorded at the reference potential (−0.70 V vs Ag/ AgCl) and the sample potential, respectively.

comparison of the transmission spectrum of MOP (spectrum a) and MOP·HClO4 (spectrum b). The results are in very good agreement with the reported experimental work of Spinner and White.43 The free base shows several dominant bands with the most intense appearing at 1591 cm−1. MOP can be assigned to the Cs point group if its methoxy group lies in the plane of the pyridine ring16 and the symmetry class of any normal mode is either A′ (transition dipole moment in the molecular plane) or A″ (transition dipole moment orthogonal to the molecular plane). The only A″ mode in the spectral region of interest arises from the CH3 scissoring vibration which is expected to be weak relative to the ring vibrations of the pyridine group. Quantum mechanical calculations were performed using Gaussian0944 and the B3LYP density functional with the 6311++G basis set in order to assist in the vibrational peak assignments (see Table 1). The calculated results for MOP largely support the interpretation of Spinner and White, but we note the following discrepancy regarding their assignment of the strong absorption at ∼1500 cm−1 solely to a single ring vibration (υ19a).45 Our calculations show this to be a composite of closely overlapping signals arising from skeletal ring vibrations as well as the aforementioned A″ CH3 scissoring vibration. It should be noted that the calculated magnitude of the transition dipole moment for the lower frequency A′ vibration, tentatively assigned as υ18a, in this region is four times larger than the other two other and is expected to dominate the observed spectrum. The cumulative interpretation of the experimental, literature, and calculated transmission spectra indicates that between 1250 and 1700 cm−1 MOP displays four, medium to strong A′ bands arising from ring skeletal vibrations and ring vibrations coupled to the atomic displacements of the methoxy group. The vibrational spectrum of MOP·HClO4 is also in very good agreement with previous reports of pyridinium salts.45 Protonation leads to both an increase in the number of bands and a tendency for similar features found in the spectrum of MOP to appear at higher wavenumbers. The conjugated acids of pyridine and its derivatives exhibit shifts in their ring vibration modes to higher frequencies due to delocalization of the positive charge,43,45,46 but the influence of protonation on the vibrational frequencies of the substituent is more complicated. For example, Spinner B

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Langmuir Table 1. Assignment of Transmission Infrared Vibrational Bands for MOP and Its Conjugate Acida description

symmetry class

measured

methoxypridine A′ 1591 A′ 1570 A′ 1501 A″ A′ A′ 1465 A′ 1454 A′ 1420 A′ 1285 methoxypridinium perchlorate ring vibration (8a) A′ 1633 ring vibration (8b) A′ 1600 ring vibration (19b) A′ 1528 ring vibration (18a) A′ 1511 CH3 bend A″ ring vibration (19a) A′ CH3 bend A′ 1457 CH3 bend #2 A′ 1432 unassigned A′ 1397 COC (asym. stretch) A′ 1322 COC (asym. stretch) A′ 1304 ring vibration (9b) A′ 1259 + COC a-st. ring vibration (8a) ring vibration (8b) Ring vibration (19a) CH3 bend ring vibration (18a) CH3 bend #1 CH3 bend #2 ring vibration (19b) COC (asym. stretch)

literatureb calculatedc 1597,s 1574,m 1507,m n/a n/a 1465,w 1445,w 1423,w 1289,s

1593 1562 1504 1500 1493 n/a 1455 1422 1269

1635,s 1600,m 1529,m 1512,s

1623 1600 1538 1528 1504 1500 n/a 1468 n/a n/a n/a 1251

1459,w 1433,w 1398,m 1322,s 1304,s 1259,w

Figure 2. SEIRAS spectra of MOP measured at the open circuit potential at pH 10 (blue), pH 7 (red), and pH 4 (black). The reference spectrum is the electrolyte in the absence of MOP.

molecular C2 axis is parallel to the metal surface. SEIRAS surface selection rules would render adsorbed water IR inactive in this orientation50−52 which explains the absence of signature water bands in the difference spectra shown in Figure 2. On the basis of previous electrochemical studies of MOP on Au(111), it is expected that MOP, like pyridine and DMAP, would adsorb on the gold surface as the neutral molecule at pH ≫ pKa as the concentration of MOP is between 2 and 3 orders of magnitude larger than the concentration of its conjugate acid (the pKa of methoxypyridinium is 6.553) . Qualitatively, the IR bands observed in Figure 2 support this hypothesis as the spectrum certainly more closely resembles the transmission spectrum of MOP than the spectrum of MOP·HClO4. The appearance of discrete peaks rather than closely paired peaks at ∼1615, ∼1510, and ∼1290 cm−1 is most telling. However, the ocp SEIRAS spectrum at pH 10 does display some noticeable differences compared to the MOP transmission in Figure 1. In particular, the highest frequency band and the peak appearing at ∼1290 cm−1 are significantly shifted to higher frequencies compared to the analogous transmission bands. Additionally, weak absorption signals associated with the methyl bending modes are missing as is the strong ring vibration assigned as υ8b. The missing bands provide information regarding the orientation of the adsorbed molecules and will be discussed below. As the pH is lowered, the pyridinium ion becomes the principal species present in solution and, assuming that its free energy of adsorption is comparable to that of MOP, one would expect the ocp spectra to exhibit new bands as MOPH+ replaces MOP on the metal surface. The fact that all three spectra in Figure 2 are essentially identical indicates that the conjugate acid does not adsorb at the ocp even when it is the majority species in the electrolyte. Pyridine and its para substituted derivatives are well-known to feature two principal adsorption motifs on gold surfaces; horizontal adsorption where the heterocyclic π electron system interacts with the metal and vertical adsorption where the lone pair of electrons on the ring nitrogen form a σ-type bond with the Au surface. If MOP or its conjugate acid were to adopt the horizontal adsorption the surface selection rules for SEIRAS54 would render all the A′ modes inactive and only the A″ methyl bending mode would be observed. The fact that all the spectra in Figure 2 display three A′ bands is strong evidence that MOP is vertically oriented on the surface at the ocp. The more symmetric parent molecule, pyridine, has two types of in-plane ring vibrations. Those with A1 symmetry class have their transition dipole moment (TDM) collinear with the principal

a

s = strong; m = medium; w = weak; sh = shoulder; n/a = not observed. bAssignments are made on both the basis of reference.43 c Calculated frequencies have been multiplied by a factor of 0.985 to account for harmonic approximation.

and White showed that the frequencies of the symmetric and asymmetric COC bands in 4-MOP·HCl are lowered and raised, respectively, upon protonation of methoxypyridine43 making it difficult to unambiguously assign all the bands appearing in spectrum b of Figure 1. Assignment of the bands was further complicated by the poor agreement between the quantum mechanical calculations and the experimental data below 1400 cm−1 where the asymmetric COC stretches are expected. The assignments found in Table 1 are based largely on the work of Spinner and White although the DFT results were used to help interpret the poorly separated peaks between 1500 and 1550 cm−1. Like its free base analog, the most prominent vibrational peaks in the transmission spectrum of the conjugate acid are all assigned to A′ modes. SEIRA Spectra at the Open-Circuit Potential. The pyridine derivative was found to spontaneously adsorb on SEIRAS active gold films at the open circuit potential (ocp ∼ +0.1 V) and representative SEIRA spectra are provided in Figure 2 at three different pH values. The relative absorption spectra (the reference spectrum was the Au film in the corresponding, MOP-free electrolyte) are remarkably similar for all three electrolyte acidities and display three strong upward bands at ∼1300, ∼1510, and ∼1615 cm−1. We note that while the addition of MOP to the electrolyte provides positive absorbance changes characteristic of MOP adsorption, there is seemingly no evidence of water displacement (e.g., negative absorbance changes associated with the δ(H2O) band at ∼1640 cm−1). Molecular dynamic simulations47,48 and recent X-ray absorption studies49 have shown that water molecules in the first layer lie flat on uncharged gold electrodes such that the C

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absent. The enhancements of the observed A′ bands will depend on the values of both χ and φ, and, in principle, the precise average molecular orientation could be extracted from the calculated angles of the transition dipole moments and the observed spectra. However, this makes several assumptions, including the supposition that the magnitude of the TDMs calculated for the free molecule are not perturbed upon adsorption on gold and that the inherent SEIRAS enhancement factor is frequency independent. Rather than making these assumptions, and arguably overinterpreting the available data, we simply conclude that the SEIRAS results strongly support the vertical adsorption motif based on the observed/missing A′ bands and the absence of the υ8b band. Potential-Dependent Adsorption on Polycrystalline Gold. Metallic films formed by sputtering gold on Si wafers are usually polycrystalline. Delgado et al. reported that cycling asdeposited SEIRAS films in an acetic acid/sulfuric acid electrolyte facilitates surface reordering and greatly improves the fraction of {111} surface domains.56 As our previous electrochemical studies of MOP were performed on a Au(111) single-crystal, it would be ideal to collect complementary SEIRAS data on the same crystallographic orientation. However, our previous work with DMAP has shown that, on slightly miscut Au(111) crystals, N-bonded adsorption occurs on stepped surfaces and defects whereas π-bonded adsorption dominates on terraces. The presence of defects can dominate surface enhanced IR studies of molecular adsorption and lead to discrepancies between thermodynamic and spectroscopic studies on nominally {111} electrodes as previously reported by Hoon-Khoshla et al.55 In order to minimize the influence of these effects we chose to perform SEIRAS measurements on MOP adsorption on metallized Si without attempting to preferentially texture the surface. Differential capacitance (DC) measurements were performed on several different Au films deposited from identical conditions in order to ensure that the resulting gold layers provided a consistent electrochemical response. The DC curves indicate that the same average crystallography was consistently produced by the sputtering technique. SEIRA spectra were obtained as a function of potential using the single beam spectrum at E = −0.70 V as the reference. Electrochemical measurements revealed that in both acidic and neutral media, MOP was completely desorbed from the electrode surface at this potential (see below). In neutral pH, the three principal A′ peaks appear starting at E > −0.60 V and first grow in intensity before weakening at more positive potentials (Figure 4). On the basis of the transition dipole moment analysis provided above, it can be concluded that the spectra support an adsorption model whereby a significant component of the principal molecular axis is perpendicular to the metal surface at all potentials investigated in pH 7 electrolyte. The equivalent experiment in weakly acidic electrolyte is shown in Figure 5. Although the spectra are qualitatively very similar, we note three important differences. First, the onset of the IR absorption peaks in pH 4 electrolyte occurs at more positive potentials (−0.2 V). At more negative values the spectra contain no direct evidence of MOP adsorption but negative bands in the water bending region are seen at all potentials positive of the reference potential. Second, unlike the case for neutral pH, the A′ peak intensities do not decrease in magnitude at the most positive potentials. The two sets of data can be better interpreted by comparing the integrated peak areas with the corresponding differential capacitance curve as shown in Figure 6. The differential

molecular axis and those with B1 symmetry class have TDMs orthogonal to the C2 axis. The relatively high symmetry of pyridine has facilitated previous studies of its adsorption orientation using IRRAS and SEIRAS. Hoon-Khosla et al. were able to use measured A1 and B1 intensities and calculated electric field strengths to determine that (a) the plane of vertically oriented pyridine is tilted with respect to the electrode surface and (b) there exists some contribution from N-bonded pyridine on defect sites on Au(111) surfaces at potentials where previous thermodynamic studies had shown that the π-bonded configuration is favorable.55 Osawa described a similar strategy for determining the orientation of pyridine from SEIRAS measurements which does not require complicated calculations of the electric field but rather relies on the intrinsic intensities of the orthogonal A1 and B1 modes for the free molecule.54 In principle, the second approach can be applied to the current study of MOP adsorption even though all the observed vibrations in the SEIRA spectra have the same symmetry class. Figure 3 provides the DFT-calculated

Figure 3. DFT-calculated spectrum for MOP as well as a Cartesian coordinate system, showing (φ) the tilt angle between the gold surface’s perpendicular and the principal molecular axis, (χ) rotation around the molecular axis, and (θ) the angle between the TDM of any particular A′ vibration and the molecular axis.

spectrum for MOP as well as a Cartesian coordinate system that defines (1) the tilt angle φ between the surface normal (the Z-axis) and the principal molecular axis, (2) rotation, χ, around the molecular axis, and (3) the angle θ between the TDM of any particular A′ vibration and the molecular axis. It must be emphasized that the DFT calculations did not include gold clusters to mimic the metal surface and therefore no in silico tilt angles were actually determined. In the calculated spectrum the wavenumber axis has been scaled by an empirical factor of 0.985 to account for well-known errors associated with the assumption of a harmonic oscillator. Analysis of the calculated vibrations allows for the evaluation of θ for the dominant A′ bands and the values are shown in parentheses in the main part of Figure 3. For the bands at ∼1225, ∼1500, and ∼1600 cm−1, the TDM is less than 20° from the principal axis implying that their magnitudes in the SEIRA spectra provide a measure of the orientation of the molecular axis. On the other hand, the TDM of the band at ∼1560 cm−1 is considerably offaxis. If the tilt angle approaches 0°, vibrational modes with TDMs nearly collinear with the molecular axis should be enhanced relative to those with larger absolute θ values. This is clearly the case when analyzing the higher frequency end of the SEIRA spectra in Figure 2 where υ8a is observed but υ8b is D

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Figure 6. (a) Integrated peak areas (left ordinate) for the MOP vibrational peaks at 1300 cm−1 (black square), 1500 cm−1 (red circle), and 1615 cm−1 (blue triangle) and the differential capacitance curves (right ordinate) for the supporting electrolyte in the absence (purple line) and presence (dark red line) of 0.1 mM MOP (formal concentration) in pH 4 electrolyte. (b) Equivalent set of data in pH 7 electrolyte.

Figure 4. Relative change in absorbance in SEIRAS spectra of 0.1 mM MOP, at pH 7, as a function of potential. The reference spectrum was collected at −0.70 V.

reference spectra in Figure 5 are taken at −0.70 V, the potential induced surface replacement of water with infrared inactive MOP (i.e., horizontally adsorbed) is consistent with the relative absorbance spectra for −0.60 V < E < −0.20 V. For completeness, we note that the lack of direct IR signals prevents unambiguous determination of whether the horizontally adsorbed species is MOP or its conjugate acid. Concurrent with the capacitive drop at −0.20 V, all three A′ bands increase in intensity and reach a plateau at potentials matching the limiting capacitance observed for E ≥ +0.10 V. The combined data in Figure 6a strongly support a horizontal to vertical phase transition in mildly acidic solutions, and no spectroscopic evidence of adsorbed conjugate acid is evident at larger positive potentials. In neutral electrolyte (Figure 6b), MOP appears to adsorb in a single state between −0.40 V < E < 0.10 V as evidenced by the capacitive minimum in this region. Application of potentials greater than 0.10 V shows an increasing capacitance. However, the DC curve in the presence of 0.1 mM MOP does not remerge with the capacitance of the supporting electrolyte. The three overlaid spectroscopic signals register a maximum at the same potential as the minimum in the differential capacitance curve (ca. −0.10 V). At more positive potentials the relative integrated peak areas decrease at roughly the same rate but never completely disappear. Cumulatively, Figure 6b indicates that at positive potentials MOP is either partially desorbed from the electrode or reorients to form a less compact inner Helmholtz layer. An edge-on motif has been reported for pyridine on various single crystal surfaces in ultrahigh vacuum whereby the C(2)−H bond is broken and η2 α-pyridyl coordination results.5 Such an adsorption orientation has also been observed for MOP on the oxophilic Ag(100) surface.57 A transition from vertically oriented MOP at 0.0 V to edge-on adsorption at higher potentials requires realignment of the molecular axis with respect to the surface normal. Analysis of the transition dipole moments predicts that this would cause different intensity changes in the three observed A′ modes which is not observed and thus we can eliminate the possibility of edge-on adsorption. A complete transition from vertical to horizontal adsorption can also be discounted as the adoption of the latter orientation would provide no A′ modes along the

Figure 5. Relative change in absorbance in SEIRAS spectra of 0.1 mM MOP, at pH 4, as a function of potential. The reference spectrum was collected at −0.70 V.

capacitance curves with and without the addition of MOP were found to merge at negative potentials indicating that any adsorbed species is completely removed at sufficiently negative potentials. In slightly acidic electrolyte (Figure 6a) a weak capacitive minimum is observed at ∼ −0.45 V and a region of low capacitance is observed at E > 0.0 V. A pseudocapacitance peak separates these two regions and represents a phasetransition between two adsorption states. In the narrow region of potential between the adsorption onset peak at −0.60 V and the pseudocapacitive peak at −0.25 V no MOP bands are found in the SEIRA spectra. However, Figure 5 shows a loss of surface water in this region of potentials as manifested by the presence of negative δ(HOH) bands at ∼1640 cm−1. Previous SEIRAS studies have shown that water is hydrogen bonded and infrared active at negatively charged metal surfaces.50−52 As the E

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ClO4− anion has sufficient adsorption strength to displace MOP. Hydroxide ions are well-known to have very high affinity for positively charged gold and adsorbed hydroxide species are precursors in the oxidation of gold surfaces. It is conceivable that hydroxide partially displaces MOP even though in neutral solutions its concentration is nearly 3 orders of magnitude smaller than that of the pyridine derivative. A clear trend is present in Figure S-3 of the Supporting Information which shows that even over a rather narrow range of electrolyte acidities (pH 6−8) an increasing concentration of hydroxide ions leads to more pronounced decreases in the MOP IR bands at positive potentials. The positions of all three observed bands in neutral electrolyte vary linearly with increasing potential and plateau at the same polarizations where the peak areas begin to decrease (∼ +0.10 V). The potential dependence of the MOP bands (see Figure S-4 in the Supporting Information) shows that the tuning rates of the bands at ∼1600 and ∼1500 cm−1 are both ∼6.5 cm−1 V−1. This dependency is consistent with previously reported ring vibrations for DMAP32 and pyridine.21 Potential dependent shifts in the vibrational band positions of molecules adsorbed on electrified interfaces can be attributed to changes in the electric field, charge-redistribution between the adsorbate and the metal, or dipole-coupling caused by the lateral interactions of adsorbed species.59 The similar potential dependence for the ring mode positions in MOP, DMAP, and pyridine can be explained by a common vertical adsorption motif whereby the potential tunes the extent of σ donation between the ring nitrogen and the Au surface. The plateaus at positive potentials in the current study are clearly correlated with the partial desorption of MOP and indicate that dipolecoupling between the adsorbed species is a significant contributor to the potential dependence of the bands. It is not immediately clear why the ∼14 cm−1 V−1 potential dependence of the asymmetric COC stretch at ∼1300 cm−1 is more than double that of the higher frequency ring vibrations. It is much stronger compared to the DMAP system where the C−N stretch of the dimethylamino substituent was found to vary weakly with potential. As the electric field drops rapidly across the inner Helmholtz layer, the much greater Stark effect for the COC asymmetric stretch could imply that MOP is adsorbed to the gold through its oxygen atom. However, this contradicts our previous electrochemical studies where MOP was shown to have the negative end of its permanent dipole (i.e., the ring nitrogen) directed toward the metal surface. An alternative explanation invoking dipole coupling effects between the methoxy groups is more plausible. The higher degree of steric bulk afforded by the dimethylamino group would mitigate this affect in adsorbed DMAP. Monotonic tuning rates over the potential range −0.20 V ≤ E ≤ + 0.50 V are seen for all the bands in the acidic electrolyte. The absence of a plateau and the similar Stark shift values provide additional evidence that there is common adsorption state in pH 7 and pH 4 electrolytes, that is, the vertical Nbonded orientation.

surface normal and a complete loss of MOP signals. A more plausible interpretation is a potential dependent increased tilt in the ring system toward the gold surface. This “flattening” of the adsorbed layer diminishes the component of all three A′ modes in the Z-direction but in such a way that they retain their relative magnitudes which is in agreement with the data shown in Figure 6b. A tilted adsorption state, intermediate between the vertical and horizontal states, has been reported for pyridine on Au on the basis of STM and IR studies.3 In the case of MOP adsorption it would allow the para substituent to potentially bind to the gold surface via nonbonding electrons on the oxygen atom. Alternatively, the SEIRAS data in Figure 6b can be explained by the partial replacement of the pyridine derivative adsorbed on the gold surface by another species present in the solution. In Figure S-1 of the Supporting Information, this is clearly shown to be the case. The SEIRAS experiment was repeated at pH 7, 50 mM KClO4 electrolyte, but with a 10-fold higher concentration of MOP. Increasing the activity of MOP (but not that of any electrolyte species) exclusively enhances its free energy of adsorption, and as a result no decrease in the MOP SEIRAS peak intensities is evident. This provides a compelling argument that the behavior observed in Figure 6b is likely not caused by a reorientation of MOP but rather by partial desorption from a competing species in the electrolyte. Recent electrochemical and infrared studies by Zhumaev et al. revealed that the specific adsorption of perchlorate on positively charged Au(111) is significantly larger than previous estimates.58 The frequency of the Cl−O stretching mode of adsorbed perchlorate, υ(ClO), is ∼1150 cm−1, and Figure S-2 in the Supporting Information reveals a complete lack of discernible signal in this region of the SEIRAS spectra. Conclusively eliminating the possibility of competitive adsorption by ClO4− due to the absence of υ(ClO) is somewhat problematic due to the poor IR transparency of the Si ATR hemisphere at these wavelengths. SEIRAS studies on MOP adsorption were performed using 10, 50, and 100 mM KClO4 electrolytes to strengthen the argument that the perchlorate anion is inert. Figure 7 provides the relative peak areas for the three MOP bands as a function of potential in these solutions. As before, the MOP peaks display a maximum intensity around 0.0 V but there is clearly no dependence on the concentration of perchlorate ions in the solution, making it unlikely that the

4. CONCLUSIONS The present study has provided infrared spectroscopic characterization of the adsorbed state of 4-methoxypyridine on polycrystalline gold. The only spectroscopically visible form of the pyridine derivative detected on the Au surface comes from the free base (i.e., MOP and not MOPH+) even when the electrolyte pH is considerably more acidic than the pKa of the

Figure 7. Relative peaks areas for the 1500 cm−1 band of MOP as a function of applied potential in 0.1 mM MOP, pH 7 solutions with (blue squares) 100 mM KClO4, (black circles) 50 mM KClO4, and (red triangles) 10 mM KClO4. F

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conjugate acid. In neutral electrolytes, MOP only adopts a vertical, N-bonded configuration when it adsorbs on polycrystalline gold. At positive potentials, competitive adsorption of hydroxide ions leads to a partial displacement of the pyridine derivative. Adsorption from lower pH conditions provides horizontally (π-bonded) adsorbed species at negative potentials, but surface selection rules prevent an unambiguous assignment of the extent of protonation. MOP is adsorbed in a vertical configuration positive of the pzc for bare polycrystalline gold (∼0 V) in pH 4 solutions. This work can assist in developing new strategies for nanoparticle design. For example, by adjusting the pH and the effective surface charge density of gold seed particles, one can, in principle, manipulate subsequent nanoparticle growth rates by controlling the degree of MOP coverage on the seed particle surfaces. As seed particles tend to have well-defined crystallographic facets, the polycrystalline nature of the current study likely needs to be extended to single crystal surfaces. Nevertheless, we are currently utilizing the results of the present study to control the growth of gold nanoparticles deposited on conductive supports as will be described in a subsequent publication.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04178. Influence of increasing MOP concentration in pH 7 electrolyte, SEIRAS spectra in the perchlorate stretching region, influence of electrolyte pH on MOP SEIRAS signal intensities, and Stark shift plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by a grant from the Natural Science and Engineering Research Council (NSERC) of Canada. The computational aspect of this research was enabled in part by support provided by WestGrid (www.westgrid.ca) and Compute Canada Calcul Canada (www.computecanada.ca). The authors would like to thank one of the reviewers of the original manuscript who suggested competitive adsorption as an alternative to potential induced reorientation of MOP at positive potentials.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b04178 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b04178 Langmuir XXXX, XXX, XXX−XXX