DFT and In-Situ Spectroelectrochemical Study of the Adsorption of

Dec 30, 2008 - Francisco J. Vidal-Iglesias , Jose Solla-Gullón , Jose M. Orts , Antonio Rodes , and Juan M. Pérez. The Journal of Physical Chemistry C...
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J. Phys. Chem. C 2009, 113, 989–1000

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DFT and In-Situ Spectroelectrochemical Study of the Adsorption of Fluoroacetate Anions at Gold Electrodes Jose´ Manuel Delgado, Raquel Blanco, Jose´ Manuel Orts, Juan Manuel Pe´rez, and Antonio Rodes* Departamento de Quı´mica Fı´sica e Instituto UniVersitario de Electroquı´mica., UniVersidad de Alicante, Apartado 99, E-03080 Alicante, Spain ReceiVed: August 6, 2008; ReVised Manuscript ReceiVed: October 24, 2008

The adsorption of mono-, di-, and trifluoroacetate anions at gold electrodes is studied in perchloric acid solutions by cyclic voltammetry and in situ infrared and Raman spectroscopies. Surface enhanced infrared reflection absorption spectroscopy experiments under attenuated total reflection conditions (ATR-SEIRAS) and surface enhanced Raman scattering (SERS) experiments were carried out with gold thin-film electrodes sputtered on silicon and on a polycrystalline gold substrate, respectively. Theoretical harmonic vibrational frequencies of trifluoroacetate species adsorbed with different geometries on Au clusters with (111), (100), and (110) orientations, as well as IR and Raman intensities, have been obtained from B3LYP/LANL2DZ, 6-31+G(d) calculations. This theoretical study was extended to di- and monofluoroacetate anions adsorbed on Au(111) clusters. The theoretical and experimental results confirm the bidentate bonding of fluoroacetate anions with the OCO plane perpendicular to the metal surface irrespective of the surface crystallographic orientation. No significant effect of either the surface crystallographic orientation or the total charge of the metal cluster-adsorbate on the vibrational frequencies of adsorbed fluoroacetate species can be deduced from the DFT calculations. The ATR-SEIRA spectra show the existence of nonhydrogen-bonded water molecules coexisting with adsorbed fluoroacetate anions, in an extent that depends on both the fluoroacetate coverage and the number of fluorine atoms in the fluoroacetate anion. Step-scan ATR-SEIRAS measurements carried out with the gold thin-film electrodes have allowed for the monitoring of the time-dependent behavior of trifluoroacetate adsorption in potential step experiments, showing a Langmuir-type adsorption kinetics. Introduction In addition to the macroscopic data derived from classical electrochemical experiments, which lack molecular specificity, the detailed characterization of adsorbed layers involves the determination of their chemical nature as well as relevant microscopic data such as preferred adsorption sites, bonding geometry, and molecular orientation with respect to the solid surface. In-situ vibrational spectroscopies, such as infrared and Raman, can provide valuable and complementary information on these points, as well as on the adsorbate-metal and adsorbate-adsorbate interactions.1-7 This information relies on a correct interpretation of the spectra of adsorbed species on metal electrodes, which was frequently based on the comparison with the spectra of the free species (either in solution or in the gas phase) or of coordination complexes with metals.2,3 The development of density functional theory (DFT) has facilitated a new approach for the interpretation of the spectra of adsorbed species by providing calculated vibrational frequencies of adsorbates with different adsorption geometries.8 As vibrational frequencies behave as local properties, their theoretical analysis in the case of small adsorbates on metal surfaces can be carried out by using the supermolecule or cluster model.9-11 The study of the specific adsorption of anions is a key step in order to reach a detailed knowledge of the adsorption and reaction processes taking place at the metal electrode/solution interface.12 Among others, the anions coming from carboxylic acids and their derivatives have deserved some attention in the * Corresponding author. E-mail: [email protected]., Fax: +349 65903537.

last years since they can be considered as model compounds for both adsorption and reactivity studies.13-24,24-29 Moreover, their weak acidic character makes them suitable to be used as probe species for testing the surface acid-base properties of the electrode surface.21,22,26,29,30 In this respect, estimated pKa values below the corresponding solution values have been reported for bioxalate,26 bimalonate,29 and bisuccinate29 anions adsorbed at gold electrodes. In previous papers, we reported on the adsorption of acetate anions on gold31 and silver28 single-crystal and thin-film electrodes by combining cyclic voltammetry with external reflection (gold single-crystals) and surface enhanced infrared reflection absorption spectroscopy experiments under attenuated total reflection conditions (ATR-SEIRAS, with gold and silver thin-films). The obtained spectra were analyzed on the basis of the theoretical harmonic vibrational frequencies, obtained from DFT-B3LYP calculations, of acetate adsorbed with different geometries on Au and Ag clusters with the basal orientations. The theoretical and experimental results confirmed bonding of the acetate to the metal surface in a bidentate configuration irrespective of the surface crystallographic orientation as previously suggested for acetate13,14,20,25 and other carboxylic acids adsorbed on metal electrodes.15-19,21-23,26,29 The ATR-SEIRAS experiments provided also information on the potential-dependent water-metal and water-adsorbate interactions. In this respect, the in situ infrared spectra showed the existence of weak acetate-water interactions related to bidentate bonding of adsorbed acetate and the hydrophobic character of the methyl group. The detection of the corresponding υ(Au-O) bands by

10.1021/jp807014f CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

990 J. Phys. Chem. C, Vol. 113, No. 3, 2009 in situ surface enhanced Raman scattering (SERS) spectroscopy32 is consistent with bidentate bonding, which involves the two oxygen atoms of the carboxylate group. In addition, no bands were detected which could be related to the formation of hydrogen bonds between neighboring acetate anions. These types of bands were clearly observed in the case of adsorbed bioxalate26 and bimalonate29 anions. The aim of this paper is to extend previous studies of the adsorption of the acetate anion on gold electrodes to its fluorinated derivatives (mono-, di-, and trifluoroacetate) in order to assess the effect on its adsorption properties due to the modification of the electronic molecular properties when replacing by fluorine the hydrogen atoms of the methyl group of acetate. Previous work on this topic is limited to an in situ external reflection infrared study of the adsorption of trifluoroacetate anions at platinum single-crystal electrodes,15 that reported main bands at approximately 1200 cm-1 that were assigned to C-F stretching vibrations on the basis of previous assignments made for bands observed in the case of trifluoroacetate coordination compounds. Based on the high intensity of the C-F stretching bands, bonding of trifluoroacetate through the two oxygen atoms of the carboxylate group was suggested irrespective of the orientation of the platinum electrode.15 The present work reports in situ infrared and Raman spectroelectrochemical experiments carried out with gold thin film electrodes obtained by argon sputtering. The nanostructure of these films deposited on silicon substrates is similar to that observed in the case of vapor deposited (either thermally33,34 or by e-beam35) or chemically deposited36,37 gold films, and allows the performance of ATR-SEIRAS measurements.26,29,31,32 It has been also shown, that these kinds of films also present a noticeable SERS activity.32 The ATR-SEIRAS6,35,38 and SERS4,5 techniques provide a significant enhancement of the infrared absorption and Raman scattering by adsorbed species, allowing the detection of weakly absorbing modes and/or low coverage intermediates. The theoretical vibrational frequencies of adsorbed species will be calculated in order to assign the infrared and Raman bands to vibrational modes and different adsorption geometries.8 We will also take advantage of the capability of the ATR-SEIRAS experiments to provide information about the vibrational properties of the water molecules in the vicinity of the metal surface6,35,38 in order to approach the study of the interactions between adsorbed fluoroacetate and interfacial water. Finally, the kinetics of trifluoroacetate adsorption will be addressed from the results of time-resolved step-scan ATRSEIRAS experiments in the submillisecond range.6,35,38 Computational Details Adsorption of the fluoroacetates was studied on the three basal crystallographic orientations of gold: Au(111), Au(100), and Au(110). Two kinds of clusters were studied for each of these surface planes, one for each type of bidentate bonding (bridge and chelate). These clusters were chosen to avoid bonding of acetate to the metal atoms in the cluster border. The clusters employed (Supporting Information, Figure S1) were composed of 10-15 gold atoms, arranged in two layers, having the symmetry and atomic arrangement characteristic of each surface crystallographic orientation.31 The geometry of all the metal clusters was kept fixed, with the nuclei located at their positions in the truncated crystal, and the same distances between neighboring gold atoms that in the bulk metal (0.28837229 nm).39 Because of the use of different clusters for obtaining the calculated frequencies of bridge- and chelate-bonded adsorbed fluoroacetates, a comparison of the relative energies of these two bidentate configurations has not been carried out.

Delgado et al. The optimization of the geometries of the fluoroacetate anions in contact with the gold clusters was carried out starting from bidentate configurations (both bridge and chelating). In the case of the chelate bonding, the OCO plane was aligned with the metal surface dense rows. After obtaining the optimized geometry for each fluoroacetate-metal adduct, the harmonic vibrational frequencies were calculated, as well as the IR and Raman theoretical intensities. For comparison, the harmonic frequencies and IR intensities were also calculated for the aqueous anions, within the SCRF-PCM solvation model.40,41 All calculations have been carried out with the B3LYP functional as implemented in the Gaussian 03 code.42 The B3LYP functional combines the three-parameter hybrid exchange functional of Becke43 with the Lee-Yang-Parr correlation functional.44 This functional, in combination with the 6-31+ G(d)45-47 basis for the C, O, and H atoms, and the LANL2DZ48 effective core potential and associated double-ζ basis set for describing the gold metal atoms, has been proven to yield theoretical harmonic frequencies that compare remarkably well with the experimental values measured for adsorbed acetate on Cu,49,50 Ag,28 and Au31 surfaces. Frequency values were calculated for overall charge values of -1 and 0, in order to check the effect of eventual discharge of the adsorbed fluoroacetate species on their vibrational behavior. In all cases, the calculations correspond to the lowest spin multiplicity compatible with the total charge. All frequency values are given without applying any scaling factor. Experimental Section Gold thin-film electrodes were deposited by argon sputtering on one of the sides of a silicon prism (Kristallhandel Kelpin, Germany). Deposition was carried out in the vacuum chamber of a MED020 coating system (BAL-TEC AG) equipped with a turbomolecular pump. After lowering the pressure down to 5 × 10-5 mbar, argon was admitted into the vacuum chamber to reach a pressure around 5 × 10-2 mbar. The deposition rate and thin film thickness were controlled with a quartz crystal microbalance. 0.1 M HClO4 test solutions, prepared from the concentrated acid (Merck Suprapur) and Purelab Ultra (Elga-Vivendi) water, were deaerated by bubbling argon (L’Air Liquid N50). Sodium trifluoroacetate (Fluka puriss. p.a.), difluoroacetic acid (Aldrich purum), or sodium fluoroacetate (Fluka techn.) were added to the perchloric acid solution to reach the desired concentration. Voltammetric and in situ infrared experiments were carried out at room temperature in a glass cell (described in ref 24), which was provided with a Si prismatic window bevelled at 60°. This prism was previously covered by a 20 nm thick gold thin-film sputtered at 0.006 ( 0.001 nm/s. Somewhat thicker gold films (35 nm) were used in the step-scan experiments. STM images obtained for films deposited under the same conditions on silicon plates showed grains with a mean size ranging from 30 to 35 nm and an aspect ratio of 1.1-1.2.32 The gold film sputtered on the silicon prism was electrochemically annealed in the spectroelectrochemical cell by cycling at 50 mV/s between 0.10 and 1.20 V for several hours in the fluoroacetate-containing solution. The ordering effect caused by this treatment is similar to that observed in sulfuric acid26,35 and in acetate-containing perchloric acid32 solutions as proved by the recording of typical voltammetric features of Au(111) electrodes in a sulfuric acid solution. A thin gold foil allows the electrical contact with the gold film electrodes. A reversible hydrogen electrode (RHE) and a gold foil were used as reference and counter electrodes, respectively.

Fluoroacetate Anions at Gold Electrodes

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TABLE 1: B3LYP/LANL2DZ, 6-31+G(d) Harmonic Frequencies (in cm-1) for Trifluoroacetate Adsorbed on Au(hkl) Clusters Vibrational Modes

Au(100)

Bridge (8 + 2) q ) -1 q ) 0 υAuO 129 136 υCC + δOCO 832 842 υasCF3 1124 1140 υsCF3 + υCC + υsOCO 1197 1198 υsOCO + υCC 1409 1420 υasOCO 1676 1655 b

a

Au(110)

Chelate (9 + 4) q ) -1a q ) 0 137 140 830 841 1123 1143 1192 1201 1386 1442 1734 1646 b

Bridge (8 + 2) q ) -1 q ) 0 117 127 827 833 1117 1133 1196 1197 1405 1417 1682 1660 b

Au(111)

Chelate (9 + 4) q ) -1 q ) 0 114 126 826 837 1123 1139 1199 1200 1424 1438 1683 1659

b

Bridge (10 + 5)b q ) -1 q)0 138 143 828 841 1122 1142 1195 1197 1405 1417 1674 1651

Chelate (7 + 3)b q ) -1a q ) 0a 127 130 815 830 1109 1131 1191 1196 1385 1399 1689 1703

Unidentate optimized geometries. b (h + k) refer to the number of atoms in the first, h, and second, k, atomic layer of the metal cluster.

In situ infrared experiments were carried out under attenuated total reflection conditions with a Nicolet Magna 850 spectrometer equipped with a narrow-band DC-coupled MCT-A detector. Spectral resolution in the linear-scan experiments was 8 cm-1. The spectra were collected with p-polarized light and are presented as -log(R2/R1), where R2 and R1 are the reflectance values corresponding to the single-beam spectra recorded at the sample and reference potentials, respectively. Each one of these single-beam spectra is calculated from 100 interferograms. Time-dependent spectra collected in the step-scan experiments were calculated from a single interferogram obtained from the average of 50 data points around each sampling time. Time and spectral resolution were respectively 0.5 ms and 16 cm-1. In-situ Raman spectra were obtained using a Teflon cell as described in ref 51. A 20 nm thick gold film was deposited by argon sputtering on the flat surface of a polished polycrystalline gold cylinder. The use of the gold substrate is necessary in our experimental setup for SERS experiments in order to keep the electric contact with the gold thin-film electrode. As shown in a previous paper,32 the nature of the substrate (either gold or silicon) does not affect significantly the nanostructure of the gold film sputtered at a given deposition rate. In addition, the enhancement of the Raman scattering in the case of the film sputtered on the gold substrate is specifically related to the presence of the gold film.32 Since the intensities of the adsorbate bands do not vary significantly with the gold deposition rate, a higher value for this parameter was chosen with respect to that used for the ATR-SEIRAS experiments (0.080 vs 0.006 nm s-1). The grain sizes observed under these conditions range from 20 to 30 nm, whereas the aspect ratio was around 1.1-1.2. Electrochemical annealing of the sample was avoided since it causes a significant decrease of the SERS enhancement.32 A Ag/AgCl electrode and a Pt wire were used as reference and counter electrodes, respectively. However, all potentials are referred to the RHE scale. A fused silica window separated the microscope objective from the solution. Raman spectra were collected with a LabRam spectrometer (from Jovin-Yvon Horiba). The slit and pinhole used were 200 or 300 µm and 500 or 600 µm, respectively. The excitation line was provided by a 17 mW He-Ne laser at 632.8 nm. The laser beam was focused through a 50× long working distance objective (0.5 NA) into a 2 µm spot at the electrode surface. A Peltier cooled charge-couple device (CCD) (1064 × 256 pixels) was used as detector. Signal averaging of two spectra with a spectrometer resolution better than 3 cm-1, 20 s acquisition time each, has been performed. Results and Discussion Computational Results. Tables 1 to 3 summarize the theoretical (B3LYP/LANL2DZ,6-31+G(d)) harmonic vibrational frequencies obtained for the mono-, di-, and trifluoroacetate anions adsorbed on the Au clusters. Figure 1 shows

TABLE 2: B3LYP/LANL2DZ, 6-31+G(d) Harmonic Frequencies (in cm-1) for Difluoroacetate Adsorbed on Au(111) Clusters Vibrational Modes

υAuO υCC + δOCO υasCF2 υsCF2 δi.p.CH υsOCO + υCC υasOCO υCH a

Au(111) Bridge (10 + 5) q ) -1 q)0 142 156 938 945 1046 1068 1116 1122 1327 1333 1419 1420 1646 1628 3141 3151

Chelate (7 + 3) q ) -1a q ) 0a 135 137 934 941 1032 1052 1104 1113 1329 1337 1415 1404 780 1673 3132 3149

Unidentate optimized geometries.

TABLE 3: B3LYP/LANL2DZ, 6-31+G(d) Harmonic Frequencies (in cm-1) for Monofluoroacetate Adsorbed on Au(111) Clusters Vibrational Modes

υAuO υCC + δOCO υCF δsCH2 υsOCO + υCC υasOCO υsCH2 a

Au(111) Bridge (10 + 5) q ) -1 q)0 158 177 921 928 1057 1079 1361 1367 1408 1409 1620 1600 3073 3078

Chelate (7 + 3) q ) -1a q ) 0a 135 142 912 923 1031 1059 1366 1374 1392 1406 1663 1641 3068 3077

Unidentate optimized geometries.

theoretically calculated spectra at the same theory level, for the three fluoroacetates studied. Spectra a correspond to the IR of the aqueous fluoroacetate anions, calculated within the SCRFPCM solvation model,40,41 while spectra b and c correspond to the theoretical IR and Raman response of the anions adsorbed in a bridge bidentate configuration on the Au(111)(10 + 5) cluster. In the case of trifluoroacetate, the theoretical frequencies are presented for clusters modeling the (111), (100), and (110) unreconstructed surfaces of gold (Table 1). The geometry optimizations that started in the bridge configuration maintained this type of bonding geometry on the three surfaces, both for charges 0 and -1. However, some of the calculations starting from a chelate geometry ended in unidentate geometry (namely, for q ) -1 on Au(100)(9 + 4) and for both charges on Au(111)(7 + 3) clusters). The calculated frequencies for the symmetrical OCO stretching of bridge-bonded adsorbed trifluoroacetate (combined with C-C stretching) are very similar on the three orientations for q ) -1 (around 1405-1410 cm-1) and also for q ) 0 (around 1417-1420 cm-1). They are slightly higher, however, for the chelate configuration (between 1424 and 1442 cm-1), depending

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Figure 1. Theoretical infrared (a,b) and Raman (c) spectra calculated at the B3LYP/LANL2DZ,6-31+G(d) theory level for fluoroacetate anions (A ) trifluoroacetate; B ) difluoroacetate; C ) monofluoroacetate). Spectra a correspond to the solvated anions in water and were calculated within the SCRF-PCM model. Spectra b and c correspond to the anions adsorbed on a Au(111)(10 + 5) cluster, in a bridge configuration.

on surface orientation and charge. Conversely, for unidentate adsorption values below 1400 cm-1 were obtained. The calculated frequency for the asymmetric OCO stretching of the carboxylate group appears in most cases between 1645 and 1690 cm-1, although a value as high as 1734 cm-1 was obtained in the case of unidentate adsorption on Au(100), with negative total charge. Note that the red- and blue-shifts observed, respectively, for the calculated υs(OCO) and υas(OCO) frequencies when going from bidentate to unidentate fit well with those measured for acetate52-54 and trifluoroacetate52,54 coordination compounds. For the bidentate adsorption configuration, the frequencies were slightly lower for the neutral case than for the negative charge. This mode gives one of the main absorption bands in the experimental spectra obtained for the dissolved trifluoroacetate anion55 (see also Supporting Information, Figure S2), in agreement with the calculated spectra within the SCRF-PCM model (Figure 1, spectra a). However, in the case of the trifluoroacetate adsorbed in bidentate configurations (either bridge or chelate), this mode would have a zero component of the transition dipole moment (and of the transition polarizability) in the direction normal to the surface. This means that, if the trifluoroacetate anions were in a bidentate bonding mode, no contribution from this mode would be expected to the experimental IR (and Raman) spectra, because of the surface selection rule.56 Other characteristic modes of the adsorbed trifluoroacetate have calculated frequency values around 1200 cm-1 (υs(CF3) + υ(CC) + υs(OCO)), 1130 cm-1(υas(CF3)), and 830 cm-1 (υ(CC) + δ(OCO)), with very small effects of total charge and surface crystallographic orientation. As in the case of the υas(OCO) mode discussed above, no band for the υas(CF3) mode would be expected from the surface selection rule in the spectra for bidentate trifluoroacetate. Regarding the 1200 cm-1 feature, its main contribution comes from the C-F stretching. For the sake of simplicity, the υs(CF3) + υ(CC) + υs(OCO) mode will be noted in the following as υs(CF3). The theoretical frequencies obtained for the Au-O stretching mode lie between 114 and 143 cm-1. Slightly higher values are obtained for each surface and bonding configuration when the charge is 0 (as compared with the charge -1). The absolute frequency values corresponding to the molecule-surface vibration, however, are not very reliable, as in the calculations the metal cluster atoms have been kept frozen in fixed positions. However, it can be noted here that the calculated frequencies are significantly lower than those calculated for adsorbed acetate, which are between 165 and 207 cm-1.57 These observations fit with the analysis by Holze58 of the frequency of the Me-O

modes detected in SERS for adsorbed oxoanions, which suggests a good correlation between the frequency and the values of reduced mass equal to the total anion mass. We will come back later to this point when discussing the experimental υ(Au-O) data obtained from the SERS spectra collected for the adsorbed fluoroacetate anions. As a conclusion, the B3LYP calculations suggest that no significant effect of surface crystallographic orientation is expected on the values of harmonic frequencies of adsorbed trifluoroacetate. The differences are usually below 10 cm-1, being comparable to the spectral resolution in the infrared experiments (8 cm-1). Similar conclusions were drawn from B3LYP studies of acetate adsorption on Ag(hkl)28 and Au(hkl)31 clusters. On this basis, only the harmonic frequencies corresponding to adsorption of di- and monofluoroacetate on Au(111) were calculated (given in Tables 2 and 3). Besides the vibrational modes described above for trifluoroacetate, di-, and monofluoroacetate also present a vibrational mode for the in-plane bending of the C-H bond at approximately 1330 and 1365 cm-1, respectively. As in the case of adsorbed trifluoroacetate, the effect of total charge on the corresponding frequency values is also very small. For a given fluoroacetate, and the same type of adsorption bonding geometry, the values obtained for the neutral system are slightly higher (in a few cm-1) for all the modes, except for the asymmetric OCO stretch, that shows frequencies for the neutral system that are slightly lower than those for the negative system. The differences between the frequencies obtained for the two charges studied are comparable to the experimental error and do not allow us to draw any conclusion regarding the charging state of adsorbed fluoroacetates. Besides the harmonic frequency values, the infrared and Raman intensities were also calculated. These data are better analyzed in spectral form. Figure 1 provides representative simulated vibrational spectra for the three fluoroacetate anions, both in solution and adsorbed on model clusters. In the case of the aqueous solvated fluoroacetate anions, the main IR band is expected to be observed between 1600 and 1700 cm-1, due to absorption by the asymmetric stretch mode of the carboxylate group. For the calculated IR spectra of the adsorbed trifluoroacetate, the most intense bands correspond to the combination of modes (υs(CF3) + υ(CC) + υs(OCO)) appearing at 1195 cm-1, while for the di- and monofluoroacetate, these are the in-plane deformation of CH in difluoroacetate (at 1327 cm-1), and the CH2 symmetric bending of monofluoroacetate (at 1361 cm-1), together with the signals due to C-F stretching, at 1057 cm-1

Fluoroacetate Anions at Gold Electrodes

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Figure 2. Cyclic voltammograms recorded for an electrochemically annealed gold thin film electrode in 0.1 M HClO4 (a, dotted line) and 10 mM CF3COONa + 0.1 M HClO4 (b, solid line). Sweep rate: 50 mV s-1. Dashed line (c) represents the charge density vs electrode potential curve calculated by integrating the difference between voltammetric curves b and a.

for monofluoroacetate and at 1046 and 1116 cm-1 for the asymmetric and symmetric stretch modes of CF2 in difluoroacetate. The most intense signals in the calculated Raman spectra of the adsorbed fluoroacetates on the Au(111)(10 + 5) cluster (spectra c in Figure 1A-C) correspond to the symmetric OCO stretch (between 1405-1420 cm-1). This feature, which has a low intensity in the calculated infrared spectra, is for the three fluoroacetate anions more intense than the corresponding CFn stretching band. It has to be noted that, both in the calculated Raman and infrared spectra for the adsorbed fluoroacetate anions (spectra b and c in Figure 1A-C), the bands due to the corresponding υas(OCO) modes are much less intense than those calculated for the anions within the SCRF-PCM solvation model (spectra a). In situ Spectroelectrochemical Behavior. Stationary Trifluoroacetate Adsorption. Figure 2 shows cyclic voltammograms obtained in a perchloric acid solution for a thin-film gold electrode sputtered on a silicon substrate. Curves a and b were recorded in the absence and in the presence of trifluoroacetate anions, respectively. As a difference with acetic acid (pKa ) 4.7639), the significantly lower pKa value for trifluoroacetic acid (pKa ) 0.5239) makes the dissociated anions to be the prevailing species at the solution pH (around 1.0). As reflected by the charge density curve reported in Figure 2, curve c, the addition of sodium trifluoroacetate to the test solution gives rise to an increase of the voltammetric charge between 0.30 and 1.20 V that can be related to the specific adsorption of trifluoroacetate anions. The nature of the adsorbed species giving rise to the excess of voltammetric charge in Figure 2 can be derived from in situ spectroscopic data. Figure 3 shows a set of potential-dependent spectra collected for the gold thin-film electrodes in a 10 mM CF3COONa + 0.1 M HClO4 solution. All of the spectra have been referred to the single-beam spectrum collected at 0.10 V. Positive- and negative-going bands in these spectra correspond to adsorbed species being formed/consumed in the potentialdependent adsorption processes. On the other hand, Figure 4 shows SERS spectra obtained in the same working solution with a gold thin-film sputtered on a polycrystalline gold substrate. A first point to be remarked in the discussion of the spectra in Figures 3 and 4 is the absence of carbonyl bands that could be related to adsorbed trifluoroacetic acid molecules. The spectra in Figure 3 show a strong positive-going band at approximately 1200 cm-1 together with a much smaller band appearing at approximately 1425 cm-1. The former band is similar to that observed in external reflection spectra obtained for platinum

Figure 3. Potential-difference ATR-SEIRA spectra collected with an electrochemically annealed gold thin-film electrode in contact with a 10 mM CF3COONa + 0.1 M HClO4 solution. Reference potential: 0.10 V; 100 interferograms were collected at each potential.

Figure 4. (A) SERS spectra collected at different electrode potentials with a gold thin-film electrode sputtered on a polished gold substrate. Test solution: 10 mM CF3COONa + 0.1 M HClO4 solution. Acquisition time: 20 s. (B) Raman spectra for solid sodium trifluoroacetate. Two spectra collected with an acquisition time of 20 s were averaged.

single crystal electrodes.15 On the other hand, main bands in the SERS spectra appear at approximately 1425, 852, and 180 cm-1. The frequencies of all these features in the ATR-SEIRA

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Figure 5. Plots against electrode potential of (A,C) the integrated band intensities and (B,D) the band frequencies for adsorbed trifluoroacetate anions in the (A,B) ATR-SEIRA and (C,D) SERS spectra collected with a gold thin-film electrode in contact with a 10 mM CF3COONa + 0.1 M HClO4 solution. The solid line in panel A represents the charge density values calculated as described in Figure 2.

and SERS spectra are potential-dependent (see Figure 5), shifting to higher values when the electrode potential increases. This behavior, together with the surface specificity of the SEIRA and SERS effects, strongly suggests their adscription to adsorbed species. On the basis of the results from DFT calculations discussed above, the bands appearing at 1200 cm-1 in the ATR-SEIRA and at 1425 cm-1 in the ATR-SEIRA and SERS spectra can be assigned respectively to the υs(CF3) and υs(OCO) modes of adsorbed trifluoroacetate. The much lower intensity of the υs(OCO) band, which is predicted by the calculated spectrum shown in Figure 1A (spectrum b), can be related to a decrease of the corresponding dynamic dipole due to the electron withdrawing effect associated to the presence of the electronegative fluorine atoms in the -CF3 group. On the other hand, the absence of the υs(CF3) in the SERS spectra can be understood on the basis of the low polarizability of the C-F bond.59,60 A key point in the spectra shown in Figure 3 is the lack of a band for the υas(OCO) mode. According to the DFT calculations, this band should appear at approximately 1670 cm-1. As in the case of adsorbed acetate,31 its absence in the experimental spectra can be rationalized by assuming bidentate (either bridge or chelate) coordination to the gold surface. With such adsorption geometry, the surface selection rule56 precludes the observation of the band since the corresponding dynamic dipole would be parallel to the gold surface. The same argument holds to explain the absence of the υas(OCO) band in the SERS spectra (Figure 4). The bands at approximately 850 and 170 cm-1 in the SERS spectra can be associated respectively to the υ(CC) + δ(OCO) and υ(Au-O) modes of adsorbed trifluoroacetate (see Table 1). A small feature at approximately 930 cm-1 could be related to perchlorate anions in solution. This latter assignment is supported by the potential-independent behavior of this band, which can also be observed in the spectra collected at an electrode potential of 1.45 V, for which the electrode surface is oxidized. It has to be noted that, as it happens in the case of the corresponding calculated values (see above), the experimental frequency of the υ(Au-O) band is significantly shifted to lower values when compared with the band observed for adsorbed acetate, which appears around 260 cm-1.32 On the other hand, it has to be noted that the experimental value for the υ(Au-O) band for adsorbed trifluoracetate is well-above the corresponding

calculated value (see Table 1). These results will be discussed below together with frequency data obtained for adsorbed monoand difluoroacetate anions. A small potential-independent feature observed at 242 cm-1 in the SERS spectra, which is also observed at 250 cm-1 in the spectrum obtained for solid sodium trifluoroacetate (Figure 4B), can be assigned to the gear-wag mode61 of adsorbed trifluoroacetate. Finally, it has to be noted that all of the bands for adsorbed trifluoracetate disappear in the SERS spectra recorded at 1.45 V. Instead, a broad feature is observed at approximately 575 cm-1 that can be assigned to the Au-OH stretching mode of the hydroxyl layer,62,63 which displaces adsorbed trifluoracetate from the electrode surface. The band intensities and band frequencies of the surface bands related to trifluoroacetate adsorbed at the gold thin-film electrode are plotted as a function of the electrode potential in Figure 5A,B (ATR-SEIRA spectra) and C,D (SERS spectra). The intensity of the υs(CF3) band in the ATR-SEIRA spectra steadily increases with the electrode potential for potentials above 0.40 V. The shape of the intensity versus potential curve fits with that of the charge density curve obtained for the same gold thinfilm electrode (solid line in Figure 5A), thus suggesting that the changes in the band intensity are mostly related to changes in the adsorbate coverage. Note that the spectra in Figure 3 indicate that bidentate adsorption is prevalent irrespective of the adsorbate coverage whereas optimized geometries obtained from DFT calculations suggest that the OCO plane is perpendicular to the electrode surface. The plot in Figure 5B shows that the υs(CF3) band frequencies are also shifted toward higher wavenumbers with increasing electrode potential paralleling the changes in the band intensity. This shift can be originated from the increase of adsorbate coverage (with increasing dipole coupling) with the applied potential. Some contribution to the observed frequency shifts that could be associated to an effect of the electrode potential at constant coverage (Stark effect) can not be discarded. Figure 5C shows that the intensities of the υs(OCO), υ(CC) + δ(OCO), and υ(Au-O) bands in the SERS spectra also have a potential-dependent behavior, which is paralleled by a blue-shift of the corresponding band frequency (Figure 5D). The latter is smaller for the υ(CC) + δ(OCO) band (not shown), which remains almost constant with the electrode potential. No negative-going bands related to the consumption of either trifluoroacetic acid or trifluoroacetate solution species can be

Fluoroacetate Anions at Gold Electrodes observed in the ATR-SEIRA spectra. This behavior is again related to the high surface specificity of the SEIRA effect.35,38,64,65 Instead, the negative-going bands observed in Figure 3 correspond to interfacial water molecules adsorbed at the reference potential. The frequency values for the δ(OH) and υ(OH) bands, appearing, respectively, at approximately 1616 cm-1 and 3500 cm-1, are characteristic of weakly hydrogen bonded water molecules, which prevail at the electrified metal/solution interface at potentials below the potential of zero charge.33,35,66,67 The sign of these bands indicates that interfacial water molecules adsorbed at the reference potential are being displaced and/or reoriented at the sample potential. The spectra in Figure 3 show for potentials above 0.80 V a sharp positive-going band at approximately 3680 cm-1. This feature lies in the spectral region typical for hydrogen bond-free water molecules.33 Similar features have been observed in ATR-SEIRA spectra obtained in the presence of carbon monoxide adsorbed on platinum,68-70 palladium,71,72 platinum-ruthenium alloy,73,74 ruthenium-modified platinum,75 and nickel76 films. The observation of such a feature has been justified by the existence of interfacial water molecules isolated within the hydrophobic adsorbate layer. In the present case, the band at 3680 cm-1 can be related to water molecules with an -OH group exposed to the -CF3 group of vicinal adsorbed trifluoracetate anions. This assignment is consistent with the observation by Falk77,78 and co-workers of similar features in the infrared spectra of hydrated Nafion membranes, which were assigned by these authors to water molecules exposed to the fluorcarbon environment. Note that the spectra in Figure 3 indicate that a trifluoroacetate coverage above some threshold is needed for the observation of this water band. Its appearance is paralleled by the sudden blue-shift of the υs(CF3) band showed in Figure 5B. The absence of other positive-going water bands at potentials at which trifluoroacetate anions are adsorbed, similar to those observed for sulfate35,66 and perchlorate33 anions, indicates the existence of weak interactions between adsorbed trifluoroacetate anions and interfacial water molecules. It has to be also noted here that no bands have been observed between 2500 and 2700 cm-1 and around 2000 cm-1. These bands were observed in the ATRSEIRA spectra of bioxalate26 and bimalonate29 anions adsorbed on gold thin-film electrodes and related to the formation of hydrogen bonds between neighboring adsorbed bicarboxylate anions or between adsorbed bicarboxylate and water molecules.26 As in the case of adsorbed acetate,31 these hydrogen bonds could not be formed in the case of adsorbed trifluoroacetate if, as suggested by the DFT calculations (see above), the hydrophobic CF3 group is pointing to the solution side of the interface in a bidentate adsorption configuration. In this respect, bonding through the oxygen atoms is confirmed by the presence of the corresponding υ(Au-O) band in the SERS spectra. The high signal-to-noise ratio in the ATR-SEIRA spectra facilitates the study of the effect of trifluoroacetate concentration on the intensity and frequency of the adsorbed trifluoroacetate band. Spectroscopic experiments similar to that reported in Figure 3 have been performed in perchloric acid solutions containing sodium trifluoroacetate with concentrations ranging from 0.1 to 10 mM. Figure 6 shows the spectra collected in these solutions at 1.10 V, together with the spectrum recorded at this electrode potential in a trifluoroacetate-free 0.1 M HClO4 solution. As previously reported by Ataka et al.,33 this latter spectrum shows a strong band at approximately 1100 cm-1 that can be related to the υas(Cl-O) mode of adsorbed perchlorate anions. The spectra in Figure 6 show clearly how the increase

J. Phys. Chem. C, Vol. 113, No. 3, 2009 995

Figure 6. Potential-difference ATR-SEIRA spectra collected at 1.10 V with an electrochemically annealed gold thin-film electrode in x M CF3COONa + 0.1 M HClO4 solutions. Reference potential: 0.10 V; 100 interferograms collected at each potential.

in trifluoroacetate concentration gives rise to a decrease of the intensity of the perchlorate band paralleling the expected increase of the intensity of the υs(CF3) band for adsorbed trifluoroacetate. The perchlorate band is absent in the spectrum collected in the solution with a 10 mM trifluroacetate concentration. Note that the same happens in the SERS spectra reported in Figure 4, with no significant bands for adsorbed perchlorate. Figure 7 allows the comparison of the changes of the band intensities and band frequencies of the perchlorate and trifluoroacetate bands as a function of the trifluoroacetate concentration. As previously observed in the case of acetate anions,31 increasing surface coverage of adsorbed trifluoroacetate at a constant electrode potential gives rise to a decrease of the intensity of the perchlorate band together with a red-shift of the corresponding band frequency. This behavior clearly suggests that adsorbed trifluoroacetate and perchlorate anions coexist at the electrode surface for solution trifluoroacetate concentrations up to 1 mM. Figure 6 also allows the observation of the effect of trifluoroacetate concentration on the water features characteristic of the gold thin-film electrode in the 0.1 M HClO4 solution, which appear at approximately 1650, 3400, and 3615 cm-1.33 The coadsorption of trifluoroacetate anions at low concentration, and thus low coverages, gives rise to a decrease of the aforementioned bands. For trifluoroacetate concentrations equal or higher than 1 mM, the ATR-SEIRA spectra obtained at 1.10 V show clearly the appearance of the water band at 3680 cm-1. In agreement with the effect of the potential-dependent coverage changes described for the spectra reported in Figure 3 for a 10 mM trifluoroacetate solution, the spectra in Figure 6 show how the intensity of the water band increases paralleling the increase of the υs(CF3) band for trifluoroacetate concentrations up to 10 mM. Again, this intensity change takes place together with a shift of the υs(CF3) band frequency.

996 J. Phys. Chem. C, Vol. 113, No. 3, 2009

Figure 7. Plots against trifluoroacetate concentration of (A,B) the integrated band intensities and (C,D) the band frequencies for (A,C) asymmetric Cl-O stretching band for adsorbed perchlorate and (B,D) symmetric CF3 stretching band for adsorbed trifluoroacetate anions as measured in the potential-difference spectra collected for a gold thinfilm electrode in contact with x mM CF3COONa + 0.1 M HClO4 solutions.

Delgado et al.

Figure 9. Potential-difference ATR-SEIRA spectra collected for an electrochemically annealed gold thin-film electrode in (A) 10 mM CHF2COOH + 0.1 M HClO4 and (B) 10 mM CH2FCOONa + 0.1 M HClO4 solutions. Reference potential: 0.10 V; 100 interferograms collected at each potential.

Figure 10. SERS spectra collected at different electrode potentials with a gold thin-film electrode sputtered on a polished gold substrate. Test solution: (A) CHF2COOH + 0.1 M HClO4 solution; (B) 10 mM CH2FCOONa + 0.1 M HClO4. Acquisition time: 20 s. Figure 8. Cyclic voltammograms for electrochemically annealed gold thin film electrodes in (a) 0.1 M HClO4, and (b,c,e) 10 mM CH(3 - x)FxCOONa + 0.1 M HClO4 solutions. x ) 0, 1, and 3 for curves b, c, and e, respectively. Curve d was recorded in a 10 mM CHF2COOH + 0.1 M HClO4 solution. Sweep rate: 50 mV s-1.

Mono- and Difluoroacetate Adsorption. The experimental spectroelectrochemical study presented above for adsorbed trifluoroacetate anions has been extended to mono- and difluoroacetate species. Figure 8, curves b, c, and e, shows cyclic voltammograms obtained for the gold thin-film electrode in 10 mM CFxH3-xCOONa + 0.1 M HClO4 solutions (x ) 0, 1, and 3). Curve d was recorded in a 10 mM CHF2COOH + 0.1 M HClO4 solution. Curve a in the same figure corresponds again to the cyclic voltammogram recorded in a fluoroacetate-free 0.1 M HClO4 solution. From the comparison of the voltammograms in Figure 8, it can be concluded that the presence of fluorine atoms shifts the onset of anion adsorption toward less positive potentials when compared with the behavior observed after the addition of sodium acetate (curve b). This trend can be related to the low solution pKavalues for the corresponding (fluoro)acetic acids, which decrease as the number of fluorine atoms increases from 0 to 3 (solution pKa values for mono- and difluoroacetic acids are respectively 2.59 and 1.3539). Thus, the solution concentration of the dissociated anion increases from acetate

to trifluoroacetate. This facilitates the adsorption process, which does not necessarily involve the dissociation of the acid as it happens in the case of acetic acid, for which the solution concentration of the anion is almost negligible. Another point to consider is the diminution of the voltammetric charge with the increase in the number of fluorine atoms, which suggests a decrease of the adsorbate saturation coverage. Increasing repulsions and steric effects associated to the replacement of hydrogen by fluorine atoms in the methyl group could explain this observation. Figure 9A,B shows the ATR-SEIRA spectra obtained at different electrode potentials with a gold thin-film electrode in contact with a 0.1 M HClO4 solution containing di- and monofluoroacetate anions, respectively. The corresponding SERS spectra are reported in Figure 10A,B. As in the case of trifluoroacetate, there are no bands that could be related to the adsorbed undissociated acids. Conversely, the spectra show bands at approximately 1425 and 1414 cm-1 for di- and monofluoroacetate, respectively, that can be assigned to the corresponding υs(OCO) mode of the adsorbed dissociated anion as suggested by the B3LYP calculated frequencies reported in Tables 2 and 3. Based on the DFT results, the bands appearing at approximately 1120 cm-1 (difluoroacetate) and 1065 cm-1

Fluoroacetate Anions at Gold Electrodes

Figure 11. Plots as a function of the number of fluorine atoms of (A) the integrated band intensities and (B) the band frequencies measured at 1.10 V for adsorbed fluoroacetate anions in the potential-difference ATR-SEIRA spectra collected for a gold thin-film electrode. Working solutions: 10 mM CH(3-x)FxCOONa + 0.1 M HClO4 (except for adsorbed difluoracetate: 10 mM CHF2COOH + 0.1 M HClO4).

(monofluoroacetate) can be assigned to the υs(CF) mode. Note that these bands may have some contribution from coadsorbed perchlorate anions. This is clearly observed in the case of monofluoroacetate, for which the spectra in this region show a broad feature which splits at potentials above 1.0 V. The spectra in Figure 9 also show additional bands at approximately 1330 and 1345 cm-1 for the in-plane δ(CH) modes. Positive-going bands for isolated water molecules, similar to those described above from the spectra obtained in the trifluoroacetate-containing solutions, are clearly observed in the presence of adsorbed difluoroacetate (Figure 9A). Much less intense features can be seen in the spectra obtained for adsorbed monofluoroacetate (Figure 9B). The analysis of the band intensities and frequencies for the adsorbed fluoroacetate species observed in Figure 3 and Figure 9A,B shows, for the same fluoroacetate concentration and electrode potential, a clear dependence on the number of fluorine atoms. The corresponding data, including those for adsorbed acetate, are reported in Figure 11. The intensities of the υs(OCO) modes decrease when the number of fluorine atoms increases, while the intensities of the υs(CFn) modes increase. As commented above, increasing the number of fluorine atoms could give rise to an increase of the corresponding dynamic dipole of the C-F modes (as compared with the C-H case) and a decrease of the dynamic dipole of the υs(OCO) modes, due to the electron withdrawing effect associated to the presence of the fluorine atoms in the -CF3 group. This behavior could explain the observed intensity changes with the number of fluorine atoms, since the IR band intensities are proportional to the variation of the dynamic dipole moment.65 On the other hand, the increase of the frequencies of υs(CFn) modes when increasing the number of F atoms (Figure 11B) could be related to the formation of stronger C-F bonds. Moreover, the expected weakening of the C-H bonds when increasing the fluorine atoms could explain the decrease of the frequencies of δs(CHn) modes of adsorbed fluoroacetates when increasing the number of fluorine atoms. The SERS spectra in Figure 10A,B for adsorbed di- and monofluoroacetate anions show typical bands for the υs(OCO) mode at 1425 and 1414 cm-1, respectively. As in the case of

J. Phys. Chem. C, Vol. 113, No. 3, 2009 997 trifluoroacetate anions, no bands can be observed for the corresponding υs(CF) mode. Instead, the SERS spectra for adsorbed difluoroacetate (Figure 10A) shows bands at 944 and 180-220 cm-1 that can be assigned respectively to the υ(CC) and υ(Au-O) modes. In the case of adsorbed monofluoroacetate (Figure 10B), these features appear at 932 and 226-240 cm-1. For both adsorbed anions, the υ(CC) appear in the same frequency range expected for the band for adsorbed perchlorate. Thus, no information on the eventual coadsorption of these fluoroacetate anions and perchlorate anions can be derived from the SERS spectra. Figure 12 shows plots of the intensities and frequencies for the bands observed in the SERS spectra in Figure 10 for diand monofluoroacetate, respectively. Data for adsorbed trifluoroacetate are also plotted for comparison. A decrease of the intensities (Figure 12A) and frequencies (Figure 12B) of the υ(Au-O) bands for adsorbed fluoroacetates is clearly observed when increasing the number of fluorine atoms. This trend is correctly predicted from the calculated DFT frequencies (see Table 4). However, it has to be noted that, as commented above for adsorbed trifluoracetate, the calculated frequencies are significantly lower than those observed experimentally in SERS spectra. The underestimation of the υ(Au-O) B3LYP frequency is probably due to the cluster model employed, which keeps metal atoms in fixed positions. This means that the vibrational modes involving movement of the metal atoms are not properly described. In agreement the observations by Holze58 of the frequency of the Me-O modes detected in SERS for adsorbed oxoanions, decreasing υ(Au-O) values (both experimental and calculated) with the number of fluorine atoms can be explained by an increase of the reduced mass of fluoroacetate anions. An eventual weakening of the Au-O bond (due to the electron withdrawing effect associated with the presence of the fluorine atoms in the CF3 group) can not be discarded. In the case of υs(OCO) modes of fluoroacetates, the intensities observed in SERS spectra (Figure 12C) decrease slightly when increasing the number of fluorine atoms. However, this decrease is less important than that in the ATR-SEIRA spectra, suggesting that the presence of fluorine atoms does not modify significantly the polarizability of the carboxylate group. Trifluoroacetate Adsorption Kinetics: Step-Scan Experiments. Time-dependent step-scan spectra were obtained during a potential step from 0.20 to 1.10 V (for 35 ms) and back to 0.20 V in the 10 mM CF3COONa + 0.1 M HClO4 solution. The reference spectrum was collected at 0.20 V just before stepping the electrode potential. Figure 13 reports a threedimensional (3D) plot of these spectra showing the rising of the bands of trifluoroacetate and water at approximately 1200 and 3680 cm-1, respectively. As a difference with the results reported for a 10 mM CH3COONa + 0.1 M HClO4 solution,31 at short times after the potential step, no clear band attributable to adsorbed perchlorate (at ca. 1100 cm-1) can be observed. Under these latter conditions, the perchlorate bands appeared first and then decreased in parallel to the increase of the acetate band.31 In the trifluoroacetate-containing solution and in the present potential range, adsorbed trifluoroacetate prevails at the electrode surface in the whole time domain. This result is consistent with the stationary measurements reported above. The integrated intensities of the trifluoroacetate and water bands recorded in the step-scan spectra can be compared in Figure 14 with the corresponding current density transients. The latter are mainly dominated by the double-layer charging currents and do not provide direct information about the changes undergone by the different adsorbed species and by interfacial

998 J. Phys. Chem. C, Vol. 113, No. 3, 2009

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Figure 12. Plots against electrode potential of (A,C) the integrated band intensities and (B,D) the band frequencies of (A,B) υ(Au-O) and (C,D) υs(OCO) bands for adsorbed fluoroacetate anions in the SERS spectra collected with a gold thin-film electrode in contact with (a,c) 10 mM CH(3 - x)FxCOONa + 0.1 M HClO4 and (b) CHF2COOH + 0.1 M HClO4 solutions.

TABLE 4: Experimental and B3LYP/LANL2DZ, 6-31+G(d) Harmonic Frequencies for Au-O Stretching Modes of Acetate and Its Fluoro-Derivatives Functional group

Stretching Au-O/cm-1

-CF3 -CF2H -CFH2 -CH3

Experimentala 180 196 236 270

DFT/B3LYPb 143 156 177 207

a Taken from SERS spectra collected at 1.20 V. b B3LYP/ LANL2DZ, 6-31+G(d) frequencies on Au(111)(10 + 5) clusters (q ) 0).

Figure 14. Plots of (A) the current density transient and (B) the timedependent integrated band intensities for (a) trifluoroacetate and (b) water bands measured from the spectra obtained in the potential-step experiment reported in Figure 13. Curve (c, dotted line) corresponds to the fit of the time-dependent intensity of the symmetric CF3 stretching band for adsorbed trifluoroacetate to a Langmuir-type adsorption kinetics.

dependent behavior of the trifluoroacetate band during the adsorption process fits well to a Langmuir kinetics (curve c, dotted line) according to the following equation19,79 Figure 13. 3D plot of the step-scan spectra obtained in a potential step experiment from 0.20 to 1.10 V and back to 0.20 V in a 0.01 M CF3COONa + 0.1 M HClO4 solution. Reference spectrum was collected at 0.20 V just before the potential step. Temporal and spectral resolution were 0.5 ms and 16 cm-1, respectively. The inset shows the potential program used.

water. This information can be derived from the time-dependent spectra. In this way, the plots of the integrated band intensities measured in the trifluoroacetate-containing solution for adsorbed trifluroacetate and water (Figure 14B, curves a and b, respectively) show the parallel adsorption of these species. Note that at 1.10 V the adsorption of trifluoroacetate starts first, and then isolated water adsorption is detected. This result is consistent with the coverage- and potential-dependent data reported in Figure 3 and Figure 6, showing that some trifluoroacetate coverage threshold has to be reached in order to detect the existence of isolated adsorbed water molecules. The time-

I(t) ) I∞[1 - exp(-ktn)]

(1)

with n ) 1. The fitting to the Langmuir model, which is better than that in the case of the instantaneous (n ) 2) and progressive (n ) 3) nucleation models, suggests the random adsorption of trifluoroacetate anions. A similar behavior has been reported for acetate,31 fumarate,17 and p-nitrobenzoate19 on gold electrodes in step-scan experiments. A value of 0.213 ms-1 is obtained for the kinetic constant for trifluoroacetate adsorption, k, from the slope of the plot of ln(I∞ - I(t)) versus t. This value is significantly higher than that obtained in the case of acetate (k ) 0.091 ms-1). This result can be connected with the different pKa values for trifluoroacetic and acetic acids, which give rise to a nearly negligible acetate anion concentration in the acetic acid-containing solution. Curve a in Figure 14B also shows that the kinetics for trifluoroacetate adsorption is much slower than that for trifluoroacetate desorption. The time scale for trifluoroacetate desorption is in the range of both the time resolution

Fluoroacetate Anions at Gold Electrodes (0.5 ms) and the time constant of the spectroelectrochemical cell (around 0.3 ms80), and hence, the kinetics of this process can not be properly studied under the present experimental conditions. Conclusions This work deals with the spectroelectrochemical study of the adsorption processes taking place at gold electrodes in contact with acidic solutions containing fluorinated derivatives of acetate anion. The voltammetric results obtained for the gold thin-film electrodes indicate in all cases the existence of specific adsorption of anions. Increasing the number of fluorine atoms in the methyl group shifts the adsorption process toward less positive potentials and leads to a decrease of the associated voltammetric charge (in the saturation limits) compared with that measured for acetic acid-containing solutions. These observations can be related, respectively, to the increasing anion concentration (associated with increasingly lower pKa values) and to an increase of repulsions and steric effects associated with the replacement of hydrogen by fluorine atoms in the methyl group of the adsorbate. In-situ infrared and Raman spectra have been obtained for gold thin-film electrodes, taking advantage of the high surface sensitivity of the ATR-SEIRA and SERS effects. The interpretation of these spectra on the basis of B3LYP/LANL2DZ,6-31+G(d) calculated optimized geometries and band frequencies has allowed the identification of the adsorbates formed at the gold electrode surface in the presence of the corresponding fluoroacetate anion. The main adsorbate band in the infrared spectra is characteristic of the υs(CFn) mode of adsorbed fluoroacetate, irrespective of the electrode potential and fluoroacetate concentration. Conversely, the SERS spectra show a main band due to the υs(OCO) mode. The much lower intensities of the υs(OCO) bands in the infrared spectra can be related to a decrease of the dynamic dipole moment when increasing the number of fluorine atoms, whereas that of the υs(CFn) mode in the SERS spectra can be associated to the low polarizability of the C-F bonds. Additional bands for the C-H bending modes of the adsorbed mono- and difluoroacetate anions are observed in the corresponding ATR-SEIRA spectra, whereas the SERS spectra show also bands for the υ(CC) and υ(Au-O) modes. The absence in both the ATR-SEIRA and the SERS spectra of any adsorbate band at approximately 1600-1700 cm-1, where the signal for the asymmetric O-C-O stretching mode of the carboxylate group is expected to appear on the basis of the B3LYP results, together with the observation of the corresponding υ(Au-O) band in the SERS spectra, suggests that fluoroacetate anions are bonded to the metal surface in a bidentate configuration, that is, through the two oxygen atoms of the carboxylate group. The spectroscopic data reported here do not allow for the discrimination between the bridge and the chelating configurations for the fluoroacetate anions since differences in the B3LYP calculated vibrational frequencies for these two configurations are within the experimental error. However, it has to be recalled that, in the calculations, chelate bonding tends to evolve toward unidentate adsorption, which can be definitively discarded from the absence of the υas(OCO) band in the infrared spectra. It should also be noted that no bands are observed between 3000 and 2000 cm-1 in the ATR-SEIRA spectra. Signals in this spectral range, that were reported for adsorbed bioxalate and bimalonate anions, would be related to the formation of hydrogen bonds between the carboxylate group of fluoroacetate anions and the interfacial water molecules. This observation is in agreement with the proposed geometry in bidentate configuration of the adsorbed fluoroacetates, with bonding to the metal surface through the two atoms of the carboxylate group.

J. Phys. Chem. C, Vol. 113, No. 3, 2009 999 The in situ infrared spectra provide also complementary information on the coadsorption of fluoroacetate and perchlorate anions. No coadsorption takes place in trifluoroacetate solutions with a 10 mM concentration, whereas adlayers containing intermixed trifluoroacetate and perchlorate anions are formed for trifluoroacetate concentrations equal to or below 1 mM. In the case of mono- and difluroacetate anions, intermixed layers are detected also for the 10 mM fluoroacetate concentration. These conclusions are derived from the band frequency for adsorbed fluoroacetate and perchlorate bands in the stationary infrared spectra collected at different fluoroacetate concentrations in the perchloric acid solution. The ATR-SEIRA spectra obtained for adsorbed trifluoroacetate have shown the appearance of a water band at approximately 3680 cm-1 that can be related to the presence of isolated nonhydrogen-bonded water molecules associated with the trifluoroacetate adlayer. This band, which is observed with decreasing intensities in the case of di- and monofluoroacetate anions, is similar to that previously reported in the case of carbon monoxide adlayers formed either after CO dosing or methanol/ formic acid dissociative adsorption. A time-resolved (step scan) ATR-SEIRA study of the adsorption/desorption processes of trifluoroacetate anions at the gold thin-film electrode in potential step experiments has been carried out, and results have been compared with those previously reported in the case of acetate anions. As in this latter case, the quantitative analysis of the time-dependent intensity of the trifluoroacetate band in the adsorption process fits with a Langmuir adsorption kinetics, thus suggesting the occurrence of random adsorption in both cases. The higher value for the kinetic constant obtained in the case of trifluoroacetate can be related to its lower pKa value, which gives rise to a higher trifluoroacetate anion concentration in solution when compared with the acetate concentration in the acetic acid-containing acidic solution. As a difference with adsorbed acetate, no bands for coadsorbed perchlorate and associated water molecules are observed during trifluoroacetate adsorption at short times after the potential step. Conversely, it has been observed that the band for the isolated water molecules associated to trifluoroacetate adsorption appears once the coverage for this latter species has reached a threshold value. Acknowledgment. Financial support from Ministerio de Educacio´n y Ciencia (Spain; Project No. CTQ2006-09868/ BQU, Fondos FEDER) and the University of Alicante is greatly acknowledged. R.B. is grateful to Ministerio de Educacio´n y Ciencia for the award of a FPU grant. The authors also thank the SS.TT.I. of the University of Alicante for allowing the use of the sputtering and Raman facilities. Supporting Information Available: Top view of the clusters used to model the Au(100), Au(110), and Au(111) surfaces for bridge and chelate adsorption of fluoroacetate anions; ATR spectra for 0.5 M CF3COONa solutions adjusted with HCl at different pH; ATR spectra collected for 0.5 M CFxH(3-x)COONa solutions (x ) 1 and 3) and a 0.5 M CF2HCOOH + 0.5 M NaOH solution. This information is free of charge via the Internet at http://pubs.acs.org References and Notes (1) Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N. Eds.;VCH: Weinheim, 1992; p 347. (2) Iwasita, T.; Nart, F. C. In AdVances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W. Eds.;VCH: Weinheim, 1995; Vol. 4, p 123.

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