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Electrolyte electroreflectance study of carbon monoxide adsorption on polycrystalline silver and gold electrodes
Electrochimica Acta 48 (2003) 2949 /2956 www.elsevier.com/locate/electacta
Electrolyte electroreflectance study of carbon monoxide adsorption on polycrystalline silver and gold electrodes A. Cuesta *, N. Lo´pez 1, C. Gutie´rrez Instituto de Quı´mica Fı´sica ‘‘Rocasolano’’, C.S.I.C., C. Serrano, 119, E-28006 Madrid, Spain Received 9 October 2002; received in revised form 5 December 2002; accepted 15 March 2003
Abstract We have carried out an electrolyte electroreflectance (EER) study of the adsorption of CO on polycrystalline Ag and Au electrodes. EER proved to be a very sensitive tool for detecting the presence of chemisorbed CO on the surface of the Ag electrode, clearly distinguishing between strong adsorption (chemisorption) and weak adsorption (physisorption). In the case of Au, onto which CO chemisorbs in both alkaline and acid media, EER also detects the presence of this chemisorbed CO. However, while in alkaline medium CO adsorption provokes major changes in the EER spectrum of Au, in acid medium only minor changes are observed, since the CO coverage is 10 times lower than that in the former case. An analysis of the EER spectra of CO-covered Au provided valuable information regarding the surface electronic levels involved in the formation of the chemisorption bond. # 2003 Elsevier Ltd. All rights reserved. Keywords: Electrolyte electroreflectance; Au; Ag; CO; Adsorption; Surface states
1. Introduction CO adsorption on metal electrodes constitutes a model system in electrocatalysis, since CO is an ubiquitous catalytic poison, and in interfacial electrochemistry, because it can be used as a probe molecule to obtain information on the surface structure and morphology using, for example, infrared spectroscopy. In 1990, our group reported the discovery, using potential-modulated reflectance spectroscopy (PMRS) (usually known as electrolyte electroreflectance (EER) when applied to systems in which the potential modulation produces only physical changes), of an electronic transition in the range 265 /331 nm of CO chemisorbed on several noble metal electrodes: Ru, Rh, Pd, Pt and Au [1] (see Ref. [2] for a review). This was completely unexpected, since the full panoply of surface spectroscopies had been, and continues to be, intensively applied to adsorbed carbon
* Corresponding author. Tel.: /34-91-5619400; fax: /34-915642431. E-mail address: [email protected] (A. Cuesta). 1 Present address: Instituto de Energı´a Solar, Universidad Polite´cnica de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain. 0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00360-8
monoxide, the molecule most studied in chemisorption. Therefore, a series of tests were carried out in order to exclude the possibility of the feature in the PMR spectra being an artifact. So, in the case of CO chemisorbed on Pt, we could confirm the existence in differential reflectance spectra of CO-covered vs. CO-free Pt of a maximum in the same wavelength region (240 /245 nm) where the PMRS signal of chemisorbed CO appears [3]. Furthermore, it was found [3] that the adsorption of one monolayer of carbon monoxide produced a decrease of 1% in the reflectance of the platinum electrode, this large reflectance decrease indicating that most probably the structure observed was due to a charge-transfer transition of the CO/Pt surface complex, either from a filled Pt level to an empty level of the chemisorbed CO, or from a filled CO level to the Pt Fermi level. The case of the PMR spectra of CO chemisorbed on Au and Ag is especially interesting, because the coinage metals (Cu, Ag and Au) are the only ones that show an appreciable electroreflectance signal that could interfere with, or be modified by, the signal due to chemisorbed CO. CO adsorbs strongly on group VIII metals, but weakly on the coinage metals. As an example, the initial heat of adsorption of CO gas on Ag(1 1 1) is 27 kJ mol 1 [4], which is only 50% higher than the heat of
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adsorption of Xe on Ag(1 1 1), determined in the same work to be 18 kJ mol1. However, as early as 1974, Gossner and Po¨lzl [5] showed that CO inhibits H2 evolution on Ag in 1 M H2SO4, and that therefore CO adsorbs on Ag. Spectroscopic detection of CO adsorbed on Ag was first reported in 1984, using surface-enhanced Raman spectroscopy (SERS), although a band was observed only after roughening the electrode with an oxidation /reduction cycle in the 0.1 M KCl electrolyte [6]. Vibrational spectra of CO adsorbed on silver have been reported for roughened electrodes in 0.1 M NaClO4 [7], for underpotential-deposited (UPD) Ag on Pt in 0.1 M KCl [8], and for Ag electrodeposited on Pt(1 1 0) [9]. Recently, it has been shown, using both cyclic voltammetry and infrared reflection /absorption spectroscopy (IRAS), that CO adsorbs on an unroughened Ag electrode over the pH range 0.3 /14 [10,11]. It has also been demonstrated, using cyclic voltammetry, that the CO /Ag bond is relatively strong at pH]/12, since only at pH5/11 was adsorbed CO displaced from the surface of Ag by N2 bubbling [11]. As far as we know, this is the only case in which a pH change of only one pH unit can dramatically change the strength of the CO /metal adsorption bond. In this work, we report for the first time EER spectra of CO adsorbed on Ag and show that EER is exceedingly sensitive to the strength of the CO/Ag adsorption bond. We have also carried out measurements of the CO / Au system, using an improved experimental setup, in order to check the validity of our previous results [12,13]. Our new results show that, also in the case of Au, EER can very sensitively distinguish between weak and strong CO adsorption. Furthermore, EER spectra provide very valuable information regarding the gold electronic levels involved in the chemisorption bond.
2. Experimental Silver and gold disks, 12 and 10 mm in diameter, respectively, were cut from 1 mm thick sheets of 99.97% pure Ag and 99.99% pure Au, supplied by Advent Research Materials. They were polished successively with 5.0, 0.3 and 0.05 mm alumina and then cleaned with Milli-Q water (18.2 MW cm, 3 ppb TOC) in an ultrasonic bath. Before each experiment the Ag electrode was etched with a mixture of 2 volumes of 25% NH3 and 1 volume of 30% H2O2 and thoroughly rinsed with Milli-Q water. The Au electrode was flameannealed in a hydrogen flame. Once prepared, the electrodes were inserted in a polypropylene holder with a Viton O-ring and immersed in the solution. The resulting cyclic voltammograms of the gold and silver
electrodes in the different solutions used coincide with the corresponding ones in the literature. A homemade Ag/AgCl (saturated KCl) electrode (0.197 V vs. SHE) was used as reference. The solutions were prepared using Milli-Q water and analytical grade NaOH and HClO4 from Merck. The EER spectra were measured with unpolarized light in a two-window (Spectrosil B) cell at an angle of incidence of 458. The light source was a 30 W deuterium lamp from Hamamatsu mounted in a convective lamp housing, Model 60000 from Oriel, with a condensing lens/collimating lens accessory. A photomultiplier tube, Model 1P28 from Hamamatsu, was used. A sine wave (DV /20 /200 mVrms, f/65 Hz) was superimposed on the electrode potential and the modulated reflectance signal was detected with a digital lock-in amplifier, Stanford SR 850 DSP. The same lock-in amplifier was used for the differential capacitance measurements, which were carried out using either 18 Hz/10 mVrms or 1 kHz/4 mVrms sine wave superimposed to the 10 mV s 1 triangular potential sweep. The capacity was calculated, assuming a series RC circuit for the double layer, as: 2 2 1 I0 I90 C 2pnDV I90 where n and DV are the sine wave frequency and amplitude, respectively, I0 the in-phase component of the current and I90 the out-of-phase component.
3. Results 3.1. EER of CO adsorbed on Ag The EER spectrum of silver is known since the first report of electroreflectance in metals by Feinleib [14], and Ag has been used as a model to investigate the validity of the different models proposed to explain the electroreflectance phenomenon in metals [15 /18]. Fig. 1 shows the EER spectra of a silver electrode in deoxygenated solutions (curve a) of pH values 14 (Fig. 1A), 13 (Fig. 1B), 12 (Fig. 1C), 11 (Fig. 1D) and 1 (Fig. 1E). The spectra were recorded at those potentials at which the signal reached its maximum intensity (0.9 V for 14]/ pH ]/11 and 0.4 V for pH 1). All the spectra are very similar, the most distinct feature in them being the sharp peak in the electroreflectance around 312 nm (3.97 eV), which coincides with a minimum in the absolute reflectance of silver [14]. This minimum has its origin in the onset of the d0/s interband transition [19]. Other features at lower energies, namely a shoulder around 330 nm (3.76 eV) and a sharp dip around 350 nm (3.54 eV), have been attributed to volume and surface plasma excitations, respectively [15]. It is to be noted that,
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Fig. 1. (A) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 1 M NaOH; (B) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M NaOH; (C) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M Na2SO4/10 2 M NaOH; (D) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M Na2SO4/10 3 M NaOH; (E) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M H2SO4. DV/50 mVrms; f /65 Hz.
although the plasma frequency for silver, as calculated using the free-electron model, is 9.0 eV, the onset of the interband transition at around 4 eV makes o ?, the real part of the dielectric constant, to pass through zero at 3.8 eV, thus fulfilling the necessary condition for plasma excitation. Very likely, electronic transitions from filled bulk bands to empty surface states also contribute to the features at lower energies, since all the Ag(1 1 1), Ag(1 0 0) and Ag(1 1 0) surfaces show in this wavelength region electronic transitions involving intrinsic surface states [20 /22]. Large changes occur in the EER spectra of silver at pH values 14 /12 when CO is bubbled through the solution, as shown in Figs. 1A /C (curve b): the intensity of the electroreflectance signal decreases dramatically by more than 10-fold at pH 13. Purging the CO-saturated solution with N2 did not bring about a recovery of the original signal. On the contrary, no change was observed when CO was bubbled through a solution of pH 5/11, as illustrated in Figs. 1D and E for pH values 11 and 1, respectively (intermediate pH values are not shown for the sake of clarity). The small differences observed at pH 11 (Fig. 1D) between curves a and b at wavelengths between 340 and 380 nm fall within the range of experimental reproducibility.
3.2. EER of CO adsorbed on Au Figs. 2A and B show the EER spectra of Au in COfree (solid line) and CO-saturated (dotted line) 0.1 M NaOH at /0.9 and /0.4 V, respectively. The spectra in Fig. 2 are plotted as /DR /R vs. energy in eV (instead of DR /R vs. wavelength in nm, as in Fig. 1) in order to make comparisons with spectra in references [22 /25] easier. At potentials more negative than /0.7 V the only remarkable feature in the spectra of CO-free Au is a small dip around 3.8 eV (see solid line in Fig. 2A). At E ]//0.6 V a small maximum appears around 5.4 eV and the feature at 3.8 eV becomes broader, covering the whole region between 3.8 and 4.5 eV. Above /0.4 V (solid line, Fig. 2B), new features around 3 and 3.5 eV, which become more defined as the potential is made more positive, appear in the spectra. Significant changes occur in the EER spectra of gold in 0.1 M NaOH after bubbling CO through the solution while keeping the electrode potential below /0.80 V, the most striking one being the appearance of a maximum around 5 eV (see dotted line in Fig. 2A). The dip around 3.5 eV in the dotted line in Fig. 2A corresponds to the maximum at 330 nm reported previously for the EER spectrum of a CO-covered
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Fig. 2. (A) EER spectra of CO-free (solid line) and CO-covered (dotted line) polycrystalline Au in CO-free 0.1 M NaOH at /0.9 V. (B) EER spectra of CO-free (solid line) and CO-covered (dotted line) polycrystalline Au in CO-free 0.1 M NaOH at /0.4 V. (C) EER spectra of polycrystalline Au in CO-free (solid line) and CO-saturated (dotted line) 0.1 M H2SO4 at /0.2 V. (D) EER spectra of polycrystalline Au in CO-free (solid line) and COsaturated (dotted line) 0.1 M H2SO4 at /0.2 V.
polycrystalline Au electrode in 1 M NaOH [13]. At this potential the spectrum does not change if CO is purged from the solution by N2 bubbling, which indicates that the adsorption of CO is fairly strong. Upon increasing the potential to /0.4 V the spectrum of a CO-covered surface in a CO-free solution nearly coincides with that recorded at the same potential before CO adsorption (see Fig. 2B), indicating that at /0.4 V CO is nearly completely oxidized at the positive limit of the potential modulation. At more positive potentials the coincidence with the spectra recorded before CO adsorption is complete within experimental reproducibility. The solid lines in Figs. 2C and D correspond to the spectra of polycrystalline gold in 0.1 M H2SO4. The same features as in 0.1 M NaOH can be observed: a maximum at energies /5 eV that becomes sharper as the potential is made more positive; a broad feature between 3.8 and 4.5 eV that becomes a sharp dip at approximately 4.5 eV with increasingly positive potentials; and, at potentials more positive than 0 V, features around 3 and 3.5 eV. Contrary to what was observed in 0.1 M NaOH, no marked changes occurred when the sulfuric acid solution was saturated with CO (see doted lines in Figs. 2C and D).
4. Discussion 4.1. EER as a tool for discriminating between weak and strong adsorption of CO on Ag and Au It was recently demonstrated, using cyclic voltammetry, that only at pH 12 or higher is the adsorption of CO on Ag strong enough for CO to remain on the surface once dissolved CO has been removed from the solution by N2 bubbling [11]. Therefore, the differences observed in the EER spectra of Ag recorded above and below pH 12 must reflect the transition from strong to weak CO adsorption. The behavior of CO on Au is somewhat similar to that reported on Ag; although it adsorbs irreversibly both in alkaline and acid media, in alkaline media (pH /11), provided that the electrode potential is kept below 0.25 V vs. RHE (in 1 M NaOH), the coverage reached is 10 times higher than in acid media [26 /28]. Again, EER proves to be a very sensitive tool for discriminating the extent of chemisorption; as discussed above and as can be seen in Fig. 2, clear changes are observed when CO is bubbled through the 0.1 M NaOH solution and only small changes occur in 0.1 M H2SO4. The small differences between CO-covered and CO-free
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Au in 0.1 M H2SO4, in contrast with the total coincidence (within experimental reproducibility) of the spectra in the case of Ag (Figs. 1D and E), must be due to a stronger adsorption of CO on Au, as indicated by the fact that the intensity of the IRAS band of CO chemisorbed on gold in 1 M HClO4 is the same with and without CO in the solution [28], whereas in the case of Ag, already at pH 5/11 the IR band disappears if CO is removed from the solution [10]. This proves again the sensitivity of EER towards the strength of the adsorption bond. As we discuss below, we believe that this sensitivity is due to the ability of EER spectroscopy to detect changes brought about in the electronic structure of the metal surface by the formation of the chemisorption bond. It should be emphasized that, in this particular case, while EER is very sensitive to the strength of the CO adsorption bond, IRAS shows no sensitivity at all. Based on the Blyholder [29,30] model for the adsorption bond of CO on a metal surface, a stronger CO adsorption should correspond to a higher degree of electron density back-donation from the metal to the 2p* (antibonding) orbital of the CO molecule, resulting in a weaker C /O bond and, hence, in a lower C /O stretching frequency. However, no significant differences have been observed in n¯/(CO) among the IRA spectra of CO chemisorbed on Ag at pH values 13, 9.2, 7 and 0.3 [10], nor between those of CO adsorbed on Au in 0.2 M NaOH and 1 M HClO4 [28]. This is in accordance with recent density functional theory calculations showing that there exists no correlation between n¯/(CO) and M /CO bonding energies, due to the differing influence of the individual interaction components (5s and 2p* orbital components, steric repulsions) on the potential energy surface well shape and depth [31]. 4.2. The physical origin of the spectral changes upon CO adsorption on Ag The most interesting question posed by the reported results is the physical origin of the dramatic decrease in the intensity of the Ag electroreflectance signal as a consequence of the strong adsorption of CO on silver at pH ]/12 (Figs. 1A /C). The simplest model of the EER spectra of metals is the ‘‘free-electron model’’ developed by Hansen and Prostak [32] and later refined by McIntyre [33,34]. According to this model, the observed electroreflectance effect is due to the change in the freeelectron concentration at the surface induced by the potential modulation. It is to be emphasized that electric fields penetrate into metals a very short distance only, ˚ , which the Thomas /Fermi screening length, about 0.5 A renders EER an extremely surface-sensitive technique in the case of metals. Although the contribution of the bound electrons (i.e. interband transitions) must also be taken into account in order to obtain a satisfactory
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agreement between theory and experiment [18], the model can reproduce the main features of the ER spectrum of polycrystalline surfaces, namely the peak at 3.9 eV for silver and at 2.5 eV for Au, although, as noted by Kolb [18], these peaks arise mainly from the 1/ R effect due to the rapid change of reflectance of both metals in these spectral regions. Within the free-electron model, the potential-induced reflectance change at normal incidence is given by:
pffiffiffiffi DR 8p o 1 CDL DE oˆ 1 Im 3f R lNe o 1 oˆ3
(1)
where o1 is the dielectric constant of the incident medium (the aqueous solution in this case), CDL the differential double-layer capacity, DE the amplitude of the potential modulation about a fixed d.c. bias potential, N the surface free-electron concentration, oˆ3 the bulk complex dielectric function of the metal and oˆ3f the free-electron contribution to oˆ3 :/ According to Eq. (1), the ER effect must be very sensitive to changes in CDL [33]. Hence, the simplest explanation of the changes observed in the EER spectra in Figs. 1A /C would be that strong adsorption of CO on Ag (at pH]/12) provokes a decrease of the differential double-layer capacity, while if CO adsorption on Ag is weak (at pH 5/11) no appreciable changes in CDL should occur. However, this seems not to be the case. Fig. 3 shows differential capacitance measurements for polycrystalline Ag at pH 12 (Figs. 3A and C) and pH 11 (Figs. 3B and D). The measurements were made at 18 Hz (Figs. 3A and B) and 1 kHz (Figs. 3C and D), since at 18 Hz the interfacial capacitance seems to be dominated by the OH adsorption capacitance. It can be seen that the capacity changes provoked by CO adsorption are very similar at pH 12 (strong adsorption) and pH 11 (weak adsorption). Furthermore, the measurements at 1 kHz show that, at both pH values, the double-layer capacitance increases upon CO adsorption. A not so simple, but much more interesting, explanation of the changes in the spectra observed when CO adsorbs strongly on a silver electrode at pH ]/12 is to assume that the chemisorption bond provokes changes in the electronic structure of the surface. These changes must correspond to variations in the populations of the states involved in the corresponding electronic transitions, since the same features at the same energies are still observed in the EER spectra of chemisorbed COcovered silver, albeit with a much smaller intensity (see Figs. 1A /C). At this point, we would like to recall the Blyholder [29,30] model for the chemisorption bond of CO on a metal surface, according to which CO bonding involves electron density donation from the 5s (non-bonding) orbital at the carbon atom into (partially) empty bands of the metal, and electron density back-donation from a
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4.3. The physical origin of the spectral changes upon CO adsorption on Au
Fig. 3. (A) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dashed line) 0.1 M Na2SO4/10 2 M NaOH. DV /10 mVrms; f /18 Hz. (B) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dotted line) 0.1 M Na2SO4/10 3 M NaOH. DV/10 mVrms; f /18 Hz. (C) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dotted line) 0.1 M Na2SO4/10 2 M NaOH. DV/4 mVrms; f /1 kHz. (D) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dotted line) 0.1 M Na2SO4/10 3 M NaOH. DV/4 mVrms; f/1 kHz.
(partially) filled band into the 2p* (antibonding) orbital of the CO molecule. With this model in mind, either injecting charge into the final state of the electronic transition responsible for the EER signal or withdrawing it from the initial state would provoke a decrease in the EER signal, as experimentally observed in the EER spectra in Figs. 1A /C. Accordingly, these spectra could contain valuable information regarding the surface electronic states involved in the chemisorption bond. We expect that further experiments with Ag(h k l ) electrodes, whose band structures are known [20,21,35] and whose EER spectra have been thoroughly studied and interpreted [15,17,21,22,35], will help to identify these electronic states and that knowing them will allow an explanation of the fact that CO chemisorbs on Ag only at pH ]/12. As shown below, the results obtained with Au support these ideas.
We will start the discussion by examining the EER spectra of CO-free polycrystalline gold in 0.1 M H2SO4 and 0.1 M NaOH. The spectrum at /0.2 V in 0.1 M H2SO4 (Fig. 2D) is dominated by two structures around 5 and 3.5 eV, marked A and B, respectively, in Fig. 2D. These features shift to higher energies as the potential is made more positive. Feature A is already observable at /0.3 V, although it becomes more intense at more positive potentials. On the other hand, feature B is clearly observed only at potentials more positive than 0 V. Very similar features at similar energies have been observed for unreconstructed Au(1 0 0) [24,25] and have been assigned to electronic transitions from filled d bands to empty surface states, according to previous theoretical calculations of the band structure of the Au(1 0 0) surface [23]. A shift of the transition energy with the electrode potential is typical of transitions between the metal and a surface state that is located within the double layer, and therefore is affected at least partly by changes in the applied field. We believe that the features observed by us for bare polycrystalline gold also correspond to electronic transitions from filled d bands into two empty surface states: surface state B, lying approximately 3.5 eV above the edge of the d band, and surface state A, lying approximately 5 eV above the edge of the d band. As the potential is made more positive (the Fermi level is pushed downwards), both surface bands A and B move upward relative to the edge of the d band. On the contrary, at very negative potentials the Fermi level lies above both surface states, which are then filled, and consequently no transition can be observed. Since surface state A lies higher in energy, it appears at more negative potentials, while more positive potentials must be reached to make surface state B visible. This is consistent with the observations in 0.1 M NaOH, in which solution measurements at very negative potentials can be carried out: surface state A is clearly observed only at potentials more positive than /0.7 V (although a weak feature at 3.75 eV at potentials as negative as / 0.9 V could be related to it), while very weak features corresponding to surface state B appear only at /0.4 V. The evolution of the peak position with the potential is shown in Fig. 4, in which we have plotted the energy at which the extremes of the features corresponding to surface states A and B in 0.1 M H2SO4 (filled symbols) and 0.1 M NaOH (open symbols) appear against the electrode potential. As mentioned above, in alkaline solution CO can adsorb strongly on polycrystalline gold and this is reflected by marked changes in the EER spectrum. As can be seen in Fig. 2A, the spectrum for CO-covered Au in 0.1 M NaOH is very similar to that of bare Au in 0.1
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Fig. 4. Plot of the energy of the different peaks observed in the EER spectra of CO-free and CO-covered Au in 0.1 M NaOH and 0.1 M H2SO4 vs. the electrode potential. The extremes of the features corresponding to surface states A and B (see text) are indicated in the figure. Filled symbols: CO-free Au in 0.1 M H2SO4; open symbols: CO-free Au in 0.1 M NaOH; half-filled symbols: CO-covered Au in 0.1 M NaOH.
M H2SO4 in the energy region where surface state A appears. This is more clearly seen in Fig. 4, where the energies of the corresponding peaks have been plotted against the electrode potential, the half-filled symbols corresponding to the CO-covered gold electrode in 0.1 M NaOH. It can be clearly seen that the energy at which the maximum and the minimum in the spectra of COcovered polycrystalline Au in 0.1 M NaOH fit very well with the energy expected at the corresponding potential for an electronic transition into surface state A, CO adsorption apparently making this transition to appear at very negative potentials, at which otherwise it should not appear because surface state A would be filled. Kita et al. [26] reported that CO chemisorbs on polycrystalline Au in 1 M NaOH only if the potential is kept more negative than 0.25 V vs. RHE (/0.79 V vs. Ag/AgCl (saturated KCl)). This agrees very well with the most negative potential, /0.7 V, at which the features in the EER spectra of bare Au in 0.1 M NaOH corresponding to the electronic transition into surface state A start to be clearly observed. Our experimental results suggest that chemisorbed CO is withdrawing electronic density into its 2p* (antibonding) orbital from surface state A, strong adsorption being only possible at potentials more negative than / 0.7 V, where surface state A is filled. In acid medium, hydrogen evolution starts at around /0.35 V and, hence, surface state A is empty in the whole potential range available, strong adsorption being then impossible. Further work with gold single crystal electrodes should help confirm our hypothesis, since surface states similar to the one termed A here exist for unreconstructed Au(1 0 0) and for Au(1 1 0), but not for
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Au(1 1 1) or reconstructed Au(1 0 0). If our hypothesis is correct, CO should chemisorb in alkaline solutions at sufficiently negative potentials on Au(1 1 0) and Au(1 0 0)-(1 /1), but not on Au(1 1 1) or Au(1 0 0)(hex). Indeed, more than 10 years ago, Chang et al. [36] found, using IRAS, that in acid media a small amount of CO chemisorbs on Au(1 1 0), but that on Au(1 1 1) CO chemisorption is below the detection limit. These authors reported that the amount of CO chemisorbed on Au(1 0 0) is below the detection limit, too, although no information regarding the state of the surface (reconstructed or unreconstructed) was given. It should be indicated, however, that some years later, Edens et al. [37] could determine, using cyclic voltammetry, that CO adsorbs on the potential-induced reconstructed Au(1 0 0) in alkaline media, reaching a coverage of u /0.5 if Edose 5//0.3 V vs. SCE. They also reported that CO adsorption favors the formation of wellordered potential-induced reconstruction domains. Since it is well-known that the electronic structure of a Au(1 0 0)-(hex) surface is very similar to that of Au(1 1 1) [38], on which CO chemisorption occurs only up to a coverage 5/0.07, it is obvious that further experimental work is necessary to clarify this point. Our hypothesis is also in agreement with results from a cyclic voltammetry study: the charge corresponding to the oxidation of CO chemisorbed on Au in 1 M NaOH was approximately 300 mC cm2 for Au(poly) and Au(1 1 0), but it barely reached 30 mC cm 2 for Au(1 1 1) [39]. A final point which we would like to address is that, according to Fig. 4, the field felt at a given potential by the surface states is very similar in 0.1 M NaOH and 0.1 M H2SO4, indicating that the potential of the point of zero charge (pzc) of Au should not be very different in these two solutions. At the moment, we do not have experimental results available for the pzc of Au in 0.1 M NaOH and 0.1 M H2SO4 that could confirm this inference.
5. Conclusions Due to its extreme surface sensitivity, EER can detect the occurrence of changes in the electronic structure at the very surface of an electrode upon the formation of an adsorption bond, thus allowing to distinguish easily between CO strong adsorption and weak adsorption on Ag and Au. A close inspection of the EER spectra of CO-covered Au has allowed us to derive the following hypotheses regarding the CO /Au chemisorption bond: / A filled Au surface state at energies /5 eV above the Fermi level takes part in the chemisorption bond,
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injecting electron density into the 2p* orbital of the CO molecule. / Chemisorption can only occur if this surface state is filled. This provides an explanation for the experimental fact that CO chemisorbs on Au(poly) only in alkaline solutions and at potentials below a threshold value.
[13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements
[21]
Financial support from the Ministerio de Ciencia y Tecnologı´a through project BQU2000-1496 is gratefully acknowledged.
[22] [23] [24] [25] [26]
References [27] [1] J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 291 (1990) 289. [2] C. Gutie´rrez, in: B.E. Conway, J.O’M. Bockris, R.E. White (Eds.), Modern Aspects of Electrochemistry, vol. 28, Plenum Press, New York, 1995, pp. 61 /105. [3] A. Cuesta, C. Gutie´rrez, J. Electroanal. Chem. 383 (1995) 195. [4] G. McElhiney, H. Papp, J. Pritchard, Surf. Sci. 54 (1976) 617. [5] K. Gossner, H. Po¨lzl, Z. Phys. Chem. N. F. 90 (1974) 164. [6] M.R. Mahoney, M.W. Howard, R.P. Cooney, J. Electroanal. Chem. 161 (1984) 163. [7] Y. Ikezawa, H. Saito, H. Matsubayashi, G. Toda, J. Electroanal. Chem. 252 (1988) 395. [8] H. Furukawa, K. Ajito, M. Takahashi, M. Ito, J. Electroanal. Chem. 280 (1990) 415. [9] I. Oda, H. Ogasawara, M. Ito, Langmuir 12 (1996) 1094. [10] G. Orozco, M.C. Pe´rez, A. Rinco´n, C. Gutie´rrez, Langmuir 14 (1998) 6297. [11] A. Rinco´n, M.C. Pe´rez, G. Orozco, C. Gutie´rrez, Electrochem. Commun. 3 (2001) 357. [12] J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 306 (1991) 301.
[28] [29] [30] [31] [32] [33]
[34] [35] [36] [37] [38] [39]
J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 314 (1991) 259. J. Feinleib, Phys. Rev. Lett. 16 (1966) 1200. D.M. Kolb, R. Ko¨tz, Surf. Sci. 64 (1977) 96. D.M. Kolb, J. Phys. C5 167 (1977) 167. F. Forstmann, K. Kempa, D.M. Kolb, J. Electroanal. Chem. 150 (1983) 241. D.M. Kolb, in: R.J. Gale (Ed.), Spectroelectrochemistry: Theory and Practice, Plenum Press, New York, 1988, pp. 87 /188. H. Ehrenreich, H.R. Philipp, Phys. Rev. 128 (1962) 1622. K.-M. Ho, B.N. Harmon, S.H. Liu, Phys. Rev. Lett. 44 (1980) 1531. D.M. Kolb, W. Boeck, K.-M. Ho, S.H. Liu, Phys. Rev. Lett. 47 (1981) 1921. W. Boeck, D.M. Kolb, Surf. Sci. 118 (1982) 613. S.H. Liu, C. Hinnen, C. Nguyen van Huong, N.R. de Tacconi, K.M. Ho, J. Electroanal. Chem. 176 (1984) 325. D.M. Kolb, J. Schneider, Surf. Sci. 162 (1985) 764. D.M. Kolb, J. Schneider, Electrochim. Acta 31 (1986) 929. H. Kita, H. Nakajima, K. Hayashi, J. Electroanal. Chem. 190 (1985) 141. H. Nakajima, H. Kita, K. Kunimatsu, A. Aramata, J. Electroanal. Chem. 201 (1986) 175. K. Kunimatsu, A. Aramata, H. Nakajima, H. Kita, J. Electroanal. Chem. 207 (1986) 293. G. Blyholder, J. Phys. Chem. 68 (1964) 2772. G. Blyholder, M.C. Allen, J. Am. Chem. Soc. 91 (1969) 3158. S.A. Wasileski, M.J. Weaver, Faraday Discuss. 121 (2002) 285. W.N. Hansen, A. Prostak, Phys. Rev. 174 (1968) 500. J.D.E. McIntyre, in: P. Delahay, C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, vol. 9, Wiley, New York, 1973, pp. 61 /166. J.D.E. McIntyre, Surf. Sci. 37 (1973) 658. K.M. Ho, C.L. Fu, S.H. Liu, D.M. Kolb, G. Piazza, J. Electroanal. Chem. 150 (1983) 235. S.-C. Chang, A. Hamelin, M.J. Weaver, J. Phys. Chem. 95 (1991) 5560. G.J. Edens, X. Gao, M.J. Weaver, N.M. Markovic, P.N. Ross, Surf. Sci. 302 (1994) L275. D.M. Kolb, Prog. Surf. Sci. 51 (1996) 109. G. Scherb, C. Gutie´rrez, D.M. Kolb, Unpublished results.
Electrolyte electroreflectance study of carbon monoxide adsorption on polycrystalline silver and gold electrodes A. Cuesta *, N. Lo´pez 1, C. Gutie´rrez Instituto de Quı´mica Fı´sica ‘‘Rocasolano’’, C.S.I.C., C. Serrano, 119, E-28006 Madrid, Spain Received 9 October 2002; received in revised form 5 December 2002; accepted 15 March 2003
Abstract We have carried out an electrolyte electroreflectance (EER) study of the adsorption of CO on polycrystalline Ag and Au electrodes. EER proved to be a very sensitive tool for detecting the presence of chemisorbed CO on the surface of the Ag electrode, clearly distinguishing between strong adsorption (chemisorption) and weak adsorption (physisorption). In the case of Au, onto which CO chemisorbs in both alkaline and acid media, EER also detects the presence of this chemisorbed CO. However, while in alkaline medium CO adsorption provokes major changes in the EER spectrum of Au, in acid medium only minor changes are observed, since the CO coverage is 10 times lower than that in the former case. An analysis of the EER spectra of CO-covered Au provided valuable information regarding the surface electronic levels involved in the formation of the chemisorption bond. # 2003 Elsevier Ltd. All rights reserved. Keywords: Electrolyte electroreflectance; Au; Ag; CO; Adsorption; Surface states
1. Introduction CO adsorption on metal electrodes constitutes a model system in electrocatalysis, since CO is an ubiquitous catalytic poison, and in interfacial electrochemistry, because it can be used as a probe molecule to obtain information on the surface structure and morphology using, for example, infrared spectroscopy. In 1990, our group reported the discovery, using potential-modulated reflectance spectroscopy (PMRS) (usually known as electrolyte electroreflectance (EER) when applied to systems in which the potential modulation produces only physical changes), of an electronic transition in the range 265 /331 nm of CO chemisorbed on several noble metal electrodes: Ru, Rh, Pd, Pt and Au [1] (see Ref. [2] for a review). This was completely unexpected, since the full panoply of surface spectroscopies had been, and continues to be, intensively applied to adsorbed carbon
* Corresponding author. Tel.: /34-91-5619400; fax: /34-915642431. E-mail address: [email protected] (A. Cuesta). 1 Present address: Instituto de Energı´a Solar, Universidad Polite´cnica de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain. 0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00360-8
monoxide, the molecule most studied in chemisorption. Therefore, a series of tests were carried out in order to exclude the possibility of the feature in the PMR spectra being an artifact. So, in the case of CO chemisorbed on Pt, we could confirm the existence in differential reflectance spectra of CO-covered vs. CO-free Pt of a maximum in the same wavelength region (240 /245 nm) where the PMRS signal of chemisorbed CO appears [3]. Furthermore, it was found [3] that the adsorption of one monolayer of carbon monoxide produced a decrease of 1% in the reflectance of the platinum electrode, this large reflectance decrease indicating that most probably the structure observed was due to a charge-transfer transition of the CO/Pt surface complex, either from a filled Pt level to an empty level of the chemisorbed CO, or from a filled CO level to the Pt Fermi level. The case of the PMR spectra of CO chemisorbed on Au and Ag is especially interesting, because the coinage metals (Cu, Ag and Au) are the only ones that show an appreciable electroreflectance signal that could interfere with, or be modified by, the signal due to chemisorbed CO. CO adsorbs strongly on group VIII metals, but weakly on the coinage metals. As an example, the initial heat of adsorption of CO gas on Ag(1 1 1) is 27 kJ mol 1 [4], which is only 50% higher than the heat of
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adsorption of Xe on Ag(1 1 1), determined in the same work to be 18 kJ mol1. However, as early as 1974, Gossner and Po¨lzl [5] showed that CO inhibits H2 evolution on Ag in 1 M H2SO4, and that therefore CO adsorbs on Ag. Spectroscopic detection of CO adsorbed on Ag was first reported in 1984, using surface-enhanced Raman spectroscopy (SERS), although a band was observed only after roughening the electrode with an oxidation /reduction cycle in the 0.1 M KCl electrolyte [6]. Vibrational spectra of CO adsorbed on silver have been reported for roughened electrodes in 0.1 M NaClO4 [7], for underpotential-deposited (UPD) Ag on Pt in 0.1 M KCl [8], and for Ag electrodeposited on Pt(1 1 0) [9]. Recently, it has been shown, using both cyclic voltammetry and infrared reflection /absorption spectroscopy (IRAS), that CO adsorbs on an unroughened Ag electrode over the pH range 0.3 /14 [10,11]. It has also been demonstrated, using cyclic voltammetry, that the CO /Ag bond is relatively strong at pH]/12, since only at pH5/11 was adsorbed CO displaced from the surface of Ag by N2 bubbling [11]. As far as we know, this is the only case in which a pH change of only one pH unit can dramatically change the strength of the CO /metal adsorption bond. In this work, we report for the first time EER spectra of CO adsorbed on Ag and show that EER is exceedingly sensitive to the strength of the CO/Ag adsorption bond. We have also carried out measurements of the CO / Au system, using an improved experimental setup, in order to check the validity of our previous results [12,13]. Our new results show that, also in the case of Au, EER can very sensitively distinguish between weak and strong CO adsorption. Furthermore, EER spectra provide very valuable information regarding the gold electronic levels involved in the chemisorption bond.
2. Experimental Silver and gold disks, 12 and 10 mm in diameter, respectively, were cut from 1 mm thick sheets of 99.97% pure Ag and 99.99% pure Au, supplied by Advent Research Materials. They were polished successively with 5.0, 0.3 and 0.05 mm alumina and then cleaned with Milli-Q water (18.2 MW cm, 3 ppb TOC) in an ultrasonic bath. Before each experiment the Ag electrode was etched with a mixture of 2 volumes of 25% NH3 and 1 volume of 30% H2O2 and thoroughly rinsed with Milli-Q water. The Au electrode was flameannealed in a hydrogen flame. Once prepared, the electrodes were inserted in a polypropylene holder with a Viton O-ring and immersed in the solution. The resulting cyclic voltammograms of the gold and silver
electrodes in the different solutions used coincide with the corresponding ones in the literature. A homemade Ag/AgCl (saturated KCl) electrode (0.197 V vs. SHE) was used as reference. The solutions were prepared using Milli-Q water and analytical grade NaOH and HClO4 from Merck. The EER spectra were measured with unpolarized light in a two-window (Spectrosil B) cell at an angle of incidence of 458. The light source was a 30 W deuterium lamp from Hamamatsu mounted in a convective lamp housing, Model 60000 from Oriel, with a condensing lens/collimating lens accessory. A photomultiplier tube, Model 1P28 from Hamamatsu, was used. A sine wave (DV /20 /200 mVrms, f/65 Hz) was superimposed on the electrode potential and the modulated reflectance signal was detected with a digital lock-in amplifier, Stanford SR 850 DSP. The same lock-in amplifier was used for the differential capacitance measurements, which were carried out using either 18 Hz/10 mVrms or 1 kHz/4 mVrms sine wave superimposed to the 10 mV s 1 triangular potential sweep. The capacity was calculated, assuming a series RC circuit for the double layer, as: 2 2 1 I0 I90 C 2pnDV I90 where n and DV are the sine wave frequency and amplitude, respectively, I0 the in-phase component of the current and I90 the out-of-phase component.
3. Results 3.1. EER of CO adsorbed on Ag The EER spectrum of silver is known since the first report of electroreflectance in metals by Feinleib [14], and Ag has been used as a model to investigate the validity of the different models proposed to explain the electroreflectance phenomenon in metals [15 /18]. Fig. 1 shows the EER spectra of a silver electrode in deoxygenated solutions (curve a) of pH values 14 (Fig. 1A), 13 (Fig. 1B), 12 (Fig. 1C), 11 (Fig. 1D) and 1 (Fig. 1E). The spectra were recorded at those potentials at which the signal reached its maximum intensity (0.9 V for 14]/ pH ]/11 and 0.4 V for pH 1). All the spectra are very similar, the most distinct feature in them being the sharp peak in the electroreflectance around 312 nm (3.97 eV), which coincides with a minimum in the absolute reflectance of silver [14]. This minimum has its origin in the onset of the d0/s interband transition [19]. Other features at lower energies, namely a shoulder around 330 nm (3.76 eV) and a sharp dip around 350 nm (3.54 eV), have been attributed to volume and surface plasma excitations, respectively [15]. It is to be noted that,
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Fig. 1. (A) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 1 M NaOH; (B) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M NaOH; (C) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M Na2SO4/10 2 M NaOH; (D) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M Na2SO4/10 3 M NaOH; (E) EER spectra of polycrystalline Ag in CO-free (curve a) and CO-saturated (curve b) 0.1 M H2SO4. DV/50 mVrms; f /65 Hz.
although the plasma frequency for silver, as calculated using the free-electron model, is 9.0 eV, the onset of the interband transition at around 4 eV makes o ?, the real part of the dielectric constant, to pass through zero at 3.8 eV, thus fulfilling the necessary condition for plasma excitation. Very likely, electronic transitions from filled bulk bands to empty surface states also contribute to the features at lower energies, since all the Ag(1 1 1), Ag(1 0 0) and Ag(1 1 0) surfaces show in this wavelength region electronic transitions involving intrinsic surface states [20 /22]. Large changes occur in the EER spectra of silver at pH values 14 /12 when CO is bubbled through the solution, as shown in Figs. 1A /C (curve b): the intensity of the electroreflectance signal decreases dramatically by more than 10-fold at pH 13. Purging the CO-saturated solution with N2 did not bring about a recovery of the original signal. On the contrary, no change was observed when CO was bubbled through a solution of pH 5/11, as illustrated in Figs. 1D and E for pH values 11 and 1, respectively (intermediate pH values are not shown for the sake of clarity). The small differences observed at pH 11 (Fig. 1D) between curves a and b at wavelengths between 340 and 380 nm fall within the range of experimental reproducibility.
3.2. EER of CO adsorbed on Au Figs. 2A and B show the EER spectra of Au in COfree (solid line) and CO-saturated (dotted line) 0.1 M NaOH at /0.9 and /0.4 V, respectively. The spectra in Fig. 2 are plotted as /DR /R vs. energy in eV (instead of DR /R vs. wavelength in nm, as in Fig. 1) in order to make comparisons with spectra in references [22 /25] easier. At potentials more negative than /0.7 V the only remarkable feature in the spectra of CO-free Au is a small dip around 3.8 eV (see solid line in Fig. 2A). At E ]//0.6 V a small maximum appears around 5.4 eV and the feature at 3.8 eV becomes broader, covering the whole region between 3.8 and 4.5 eV. Above /0.4 V (solid line, Fig. 2B), new features around 3 and 3.5 eV, which become more defined as the potential is made more positive, appear in the spectra. Significant changes occur in the EER spectra of gold in 0.1 M NaOH after bubbling CO through the solution while keeping the electrode potential below /0.80 V, the most striking one being the appearance of a maximum around 5 eV (see dotted line in Fig. 2A). The dip around 3.5 eV in the dotted line in Fig. 2A corresponds to the maximum at 330 nm reported previously for the EER spectrum of a CO-covered
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Fig. 2. (A) EER spectra of CO-free (solid line) and CO-covered (dotted line) polycrystalline Au in CO-free 0.1 M NaOH at /0.9 V. (B) EER spectra of CO-free (solid line) and CO-covered (dotted line) polycrystalline Au in CO-free 0.1 M NaOH at /0.4 V. (C) EER spectra of polycrystalline Au in CO-free (solid line) and CO-saturated (dotted line) 0.1 M H2SO4 at /0.2 V. (D) EER spectra of polycrystalline Au in CO-free (solid line) and COsaturated (dotted line) 0.1 M H2SO4 at /0.2 V.
polycrystalline Au electrode in 1 M NaOH [13]. At this potential the spectrum does not change if CO is purged from the solution by N2 bubbling, which indicates that the adsorption of CO is fairly strong. Upon increasing the potential to /0.4 V the spectrum of a CO-covered surface in a CO-free solution nearly coincides with that recorded at the same potential before CO adsorption (see Fig. 2B), indicating that at /0.4 V CO is nearly completely oxidized at the positive limit of the potential modulation. At more positive potentials the coincidence with the spectra recorded before CO adsorption is complete within experimental reproducibility. The solid lines in Figs. 2C and D correspond to the spectra of polycrystalline gold in 0.1 M H2SO4. The same features as in 0.1 M NaOH can be observed: a maximum at energies /5 eV that becomes sharper as the potential is made more positive; a broad feature between 3.8 and 4.5 eV that becomes a sharp dip at approximately 4.5 eV with increasingly positive potentials; and, at potentials more positive than 0 V, features around 3 and 3.5 eV. Contrary to what was observed in 0.1 M NaOH, no marked changes occurred when the sulfuric acid solution was saturated with CO (see doted lines in Figs. 2C and D).
4. Discussion 4.1. EER as a tool for discriminating between weak and strong adsorption of CO on Ag and Au It was recently demonstrated, using cyclic voltammetry, that only at pH 12 or higher is the adsorption of CO on Ag strong enough for CO to remain on the surface once dissolved CO has been removed from the solution by N2 bubbling [11]. Therefore, the differences observed in the EER spectra of Ag recorded above and below pH 12 must reflect the transition from strong to weak CO adsorption. The behavior of CO on Au is somewhat similar to that reported on Ag; although it adsorbs irreversibly both in alkaline and acid media, in alkaline media (pH /11), provided that the electrode potential is kept below 0.25 V vs. RHE (in 1 M NaOH), the coverage reached is 10 times higher than in acid media [26 /28]. Again, EER proves to be a very sensitive tool for discriminating the extent of chemisorption; as discussed above and as can be seen in Fig. 2, clear changes are observed when CO is bubbled through the 0.1 M NaOH solution and only small changes occur in 0.1 M H2SO4. The small differences between CO-covered and CO-free
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Au in 0.1 M H2SO4, in contrast with the total coincidence (within experimental reproducibility) of the spectra in the case of Ag (Figs. 1D and E), must be due to a stronger adsorption of CO on Au, as indicated by the fact that the intensity of the IRAS band of CO chemisorbed on gold in 1 M HClO4 is the same with and without CO in the solution [28], whereas in the case of Ag, already at pH 5/11 the IR band disappears if CO is removed from the solution [10]. This proves again the sensitivity of EER towards the strength of the adsorption bond. As we discuss below, we believe that this sensitivity is due to the ability of EER spectroscopy to detect changes brought about in the electronic structure of the metal surface by the formation of the chemisorption bond. It should be emphasized that, in this particular case, while EER is very sensitive to the strength of the CO adsorption bond, IRAS shows no sensitivity at all. Based on the Blyholder [29,30] model for the adsorption bond of CO on a metal surface, a stronger CO adsorption should correspond to a higher degree of electron density back-donation from the metal to the 2p* (antibonding) orbital of the CO molecule, resulting in a weaker C /O bond and, hence, in a lower C /O stretching frequency. However, no significant differences have been observed in n¯/(CO) among the IRA spectra of CO chemisorbed on Ag at pH values 13, 9.2, 7 and 0.3 [10], nor between those of CO adsorbed on Au in 0.2 M NaOH and 1 M HClO4 [28]. This is in accordance with recent density functional theory calculations showing that there exists no correlation between n¯/(CO) and M /CO bonding energies, due to the differing influence of the individual interaction components (5s and 2p* orbital components, steric repulsions) on the potential energy surface well shape and depth [31]. 4.2. The physical origin of the spectral changes upon CO adsorption on Ag The most interesting question posed by the reported results is the physical origin of the dramatic decrease in the intensity of the Ag electroreflectance signal as a consequence of the strong adsorption of CO on silver at pH ]/12 (Figs. 1A /C). The simplest model of the EER spectra of metals is the ‘‘free-electron model’’ developed by Hansen and Prostak [32] and later refined by McIntyre [33,34]. According to this model, the observed electroreflectance effect is due to the change in the freeelectron concentration at the surface induced by the potential modulation. It is to be emphasized that electric fields penetrate into metals a very short distance only, ˚ , which the Thomas /Fermi screening length, about 0.5 A renders EER an extremely surface-sensitive technique in the case of metals. Although the contribution of the bound electrons (i.e. interband transitions) must also be taken into account in order to obtain a satisfactory
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agreement between theory and experiment [18], the model can reproduce the main features of the ER spectrum of polycrystalline surfaces, namely the peak at 3.9 eV for silver and at 2.5 eV for Au, although, as noted by Kolb [18], these peaks arise mainly from the 1/ R effect due to the rapid change of reflectance of both metals in these spectral regions. Within the free-electron model, the potential-induced reflectance change at normal incidence is given by:
pffiffiffiffi DR 8p o 1 CDL DE oˆ 1 Im 3f R lNe o 1 oˆ3
(1)
where o1 is the dielectric constant of the incident medium (the aqueous solution in this case), CDL the differential double-layer capacity, DE the amplitude of the potential modulation about a fixed d.c. bias potential, N the surface free-electron concentration, oˆ3 the bulk complex dielectric function of the metal and oˆ3f the free-electron contribution to oˆ3 :/ According to Eq. (1), the ER effect must be very sensitive to changes in CDL [33]. Hence, the simplest explanation of the changes observed in the EER spectra in Figs. 1A /C would be that strong adsorption of CO on Ag (at pH]/12) provokes a decrease of the differential double-layer capacity, while if CO adsorption on Ag is weak (at pH 5/11) no appreciable changes in CDL should occur. However, this seems not to be the case. Fig. 3 shows differential capacitance measurements for polycrystalline Ag at pH 12 (Figs. 3A and C) and pH 11 (Figs. 3B and D). The measurements were made at 18 Hz (Figs. 3A and B) and 1 kHz (Figs. 3C and D), since at 18 Hz the interfacial capacitance seems to be dominated by the OH adsorption capacitance. It can be seen that the capacity changes provoked by CO adsorption are very similar at pH 12 (strong adsorption) and pH 11 (weak adsorption). Furthermore, the measurements at 1 kHz show that, at both pH values, the double-layer capacitance increases upon CO adsorption. A not so simple, but much more interesting, explanation of the changes in the spectra observed when CO adsorbs strongly on a silver electrode at pH ]/12 is to assume that the chemisorption bond provokes changes in the electronic structure of the surface. These changes must correspond to variations in the populations of the states involved in the corresponding electronic transitions, since the same features at the same energies are still observed in the EER spectra of chemisorbed COcovered silver, albeit with a much smaller intensity (see Figs. 1A /C). At this point, we would like to recall the Blyholder [29,30] model for the chemisorption bond of CO on a metal surface, according to which CO bonding involves electron density donation from the 5s (non-bonding) orbital at the carbon atom into (partially) empty bands of the metal, and electron density back-donation from a
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4.3. The physical origin of the spectral changes upon CO adsorption on Au
Fig. 3. (A) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dashed line) 0.1 M Na2SO4/10 2 M NaOH. DV /10 mVrms; f /18 Hz. (B) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dotted line) 0.1 M Na2SO4/10 3 M NaOH. DV/10 mVrms; f /18 Hz. (C) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dotted line) 0.1 M Na2SO4/10 2 M NaOH. DV/4 mVrms; f /1 kHz. (D) Differential capacitance curves of a polycrystalline Ag electrode in CO-free (solid line) and CO-saturated (dotted line) 0.1 M Na2SO4/10 3 M NaOH. DV/4 mVrms; f/1 kHz.
(partially) filled band into the 2p* (antibonding) orbital of the CO molecule. With this model in mind, either injecting charge into the final state of the electronic transition responsible for the EER signal or withdrawing it from the initial state would provoke a decrease in the EER signal, as experimentally observed in the EER spectra in Figs. 1A /C. Accordingly, these spectra could contain valuable information regarding the surface electronic states involved in the chemisorption bond. We expect that further experiments with Ag(h k l ) electrodes, whose band structures are known [20,21,35] and whose EER spectra have been thoroughly studied and interpreted [15,17,21,22,35], will help to identify these electronic states and that knowing them will allow an explanation of the fact that CO chemisorbs on Ag only at pH ]/12. As shown below, the results obtained with Au support these ideas.
We will start the discussion by examining the EER spectra of CO-free polycrystalline gold in 0.1 M H2SO4 and 0.1 M NaOH. The spectrum at /0.2 V in 0.1 M H2SO4 (Fig. 2D) is dominated by two structures around 5 and 3.5 eV, marked A and B, respectively, in Fig. 2D. These features shift to higher energies as the potential is made more positive. Feature A is already observable at /0.3 V, although it becomes more intense at more positive potentials. On the other hand, feature B is clearly observed only at potentials more positive than 0 V. Very similar features at similar energies have been observed for unreconstructed Au(1 0 0) [24,25] and have been assigned to electronic transitions from filled d bands to empty surface states, according to previous theoretical calculations of the band structure of the Au(1 0 0) surface [23]. A shift of the transition energy with the electrode potential is typical of transitions between the metal and a surface state that is located within the double layer, and therefore is affected at least partly by changes in the applied field. We believe that the features observed by us for bare polycrystalline gold also correspond to electronic transitions from filled d bands into two empty surface states: surface state B, lying approximately 3.5 eV above the edge of the d band, and surface state A, lying approximately 5 eV above the edge of the d band. As the potential is made more positive (the Fermi level is pushed downwards), both surface bands A and B move upward relative to the edge of the d band. On the contrary, at very negative potentials the Fermi level lies above both surface states, which are then filled, and consequently no transition can be observed. Since surface state A lies higher in energy, it appears at more negative potentials, while more positive potentials must be reached to make surface state B visible. This is consistent with the observations in 0.1 M NaOH, in which solution measurements at very negative potentials can be carried out: surface state A is clearly observed only at potentials more positive than /0.7 V (although a weak feature at 3.75 eV at potentials as negative as / 0.9 V could be related to it), while very weak features corresponding to surface state B appear only at /0.4 V. The evolution of the peak position with the potential is shown in Fig. 4, in which we have plotted the energy at which the extremes of the features corresponding to surface states A and B in 0.1 M H2SO4 (filled symbols) and 0.1 M NaOH (open symbols) appear against the electrode potential. As mentioned above, in alkaline solution CO can adsorb strongly on polycrystalline gold and this is reflected by marked changes in the EER spectrum. As can be seen in Fig. 2A, the spectrum for CO-covered Au in 0.1 M NaOH is very similar to that of bare Au in 0.1
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Fig. 4. Plot of the energy of the different peaks observed in the EER spectra of CO-free and CO-covered Au in 0.1 M NaOH and 0.1 M H2SO4 vs. the electrode potential. The extremes of the features corresponding to surface states A and B (see text) are indicated in the figure. Filled symbols: CO-free Au in 0.1 M H2SO4; open symbols: CO-free Au in 0.1 M NaOH; half-filled symbols: CO-covered Au in 0.1 M NaOH.
M H2SO4 in the energy region where surface state A appears. This is more clearly seen in Fig. 4, where the energies of the corresponding peaks have been plotted against the electrode potential, the half-filled symbols corresponding to the CO-covered gold electrode in 0.1 M NaOH. It can be clearly seen that the energy at which the maximum and the minimum in the spectra of COcovered polycrystalline Au in 0.1 M NaOH fit very well with the energy expected at the corresponding potential for an electronic transition into surface state A, CO adsorption apparently making this transition to appear at very negative potentials, at which otherwise it should not appear because surface state A would be filled. Kita et al. [26] reported that CO chemisorbs on polycrystalline Au in 1 M NaOH only if the potential is kept more negative than 0.25 V vs. RHE (/0.79 V vs. Ag/AgCl (saturated KCl)). This agrees very well with the most negative potential, /0.7 V, at which the features in the EER spectra of bare Au in 0.1 M NaOH corresponding to the electronic transition into surface state A start to be clearly observed. Our experimental results suggest that chemisorbed CO is withdrawing electronic density into its 2p* (antibonding) orbital from surface state A, strong adsorption being only possible at potentials more negative than / 0.7 V, where surface state A is filled. In acid medium, hydrogen evolution starts at around /0.35 V and, hence, surface state A is empty in the whole potential range available, strong adsorption being then impossible. Further work with gold single crystal electrodes should help confirm our hypothesis, since surface states similar to the one termed A here exist for unreconstructed Au(1 0 0) and for Au(1 1 0), but not for
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Au(1 1 1) or reconstructed Au(1 0 0). If our hypothesis is correct, CO should chemisorb in alkaline solutions at sufficiently negative potentials on Au(1 1 0) and Au(1 0 0)-(1 /1), but not on Au(1 1 1) or Au(1 0 0)(hex). Indeed, more than 10 years ago, Chang et al. [36] found, using IRAS, that in acid media a small amount of CO chemisorbs on Au(1 1 0), but that on Au(1 1 1) CO chemisorption is below the detection limit. These authors reported that the amount of CO chemisorbed on Au(1 0 0) is below the detection limit, too, although no information regarding the state of the surface (reconstructed or unreconstructed) was given. It should be indicated, however, that some years later, Edens et al. [37] could determine, using cyclic voltammetry, that CO adsorbs on the potential-induced reconstructed Au(1 0 0) in alkaline media, reaching a coverage of u /0.5 if Edose 5//0.3 V vs. SCE. They also reported that CO adsorption favors the formation of wellordered potential-induced reconstruction domains. Since it is well-known that the electronic structure of a Au(1 0 0)-(hex) surface is very similar to that of Au(1 1 1) [38], on which CO chemisorption occurs only up to a coverage 5/0.07, it is obvious that further experimental work is necessary to clarify this point. Our hypothesis is also in agreement with results from a cyclic voltammetry study: the charge corresponding to the oxidation of CO chemisorbed on Au in 1 M NaOH was approximately 300 mC cm2 for Au(poly) and Au(1 1 0), but it barely reached 30 mC cm 2 for Au(1 1 1) [39]. A final point which we would like to address is that, according to Fig. 4, the field felt at a given potential by the surface states is very similar in 0.1 M NaOH and 0.1 M H2SO4, indicating that the potential of the point of zero charge (pzc) of Au should not be very different in these two solutions. At the moment, we do not have experimental results available for the pzc of Au in 0.1 M NaOH and 0.1 M H2SO4 that could confirm this inference.
5. Conclusions Due to its extreme surface sensitivity, EER can detect the occurrence of changes in the electronic structure at the very surface of an electrode upon the formation of an adsorption bond, thus allowing to distinguish easily between CO strong adsorption and weak adsorption on Ag and Au. A close inspection of the EER spectra of CO-covered Au has allowed us to derive the following hypotheses regarding the CO /Au chemisorption bond: / A filled Au surface state at energies /5 eV above the Fermi level takes part in the chemisorption bond,
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injecting electron density into the 2p* orbital of the CO molecule. / Chemisorption can only occur if this surface state is filled. This provides an explanation for the experimental fact that CO chemisorbs on Au(poly) only in alkaline solutions and at potentials below a threshold value.
[13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements
[21]
Financial support from the Ministerio de Ciencia y Tecnologı´a through project BQU2000-1496 is gratefully acknowledged.
[22] [23] [24] [25] [26]
References [27] [1] J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 291 (1990) 289. [2] C. Gutie´rrez, in: B.E. Conway, J.O’M. Bockris, R.E. White (Eds.), Modern Aspects of Electrochemistry, vol. 28, Plenum Press, New York, 1995, pp. 61 /105. [3] A. Cuesta, C. Gutie´rrez, J. Electroanal. Chem. 383 (1995) 195. [4] G. McElhiney, H. Papp, J. Pritchard, Surf. Sci. 54 (1976) 617. [5] K. Gossner, H. Po¨lzl, Z. Phys. Chem. N. F. 90 (1974) 164. [6] M.R. Mahoney, M.W. Howard, R.P. Cooney, J. Electroanal. Chem. 161 (1984) 163. [7] Y. Ikezawa, H. Saito, H. Matsubayashi, G. Toda, J. Electroanal. Chem. 252 (1988) 395. [8] H. Furukawa, K. Ajito, M. Takahashi, M. Ito, J. Electroanal. Chem. 280 (1990) 415. [9] I. Oda, H. Ogasawara, M. Ito, Langmuir 12 (1996) 1094. [10] G. Orozco, M.C. Pe´rez, A. Rinco´n, C. Gutie´rrez, Langmuir 14 (1998) 6297. [11] A. Rinco´n, M.C. Pe´rez, G. Orozco, C. Gutie´rrez, Electrochem. Commun. 3 (2001) 357. [12] J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 306 (1991) 301.
[28] [29] [30] [31] [32] [33]
[34] [35] [36] [37] [38] [39]
J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 314 (1991) 259. J. Feinleib, Phys. Rev. Lett. 16 (1966) 1200. D.M. Kolb, R. Ko¨tz, Surf. Sci. 64 (1977) 96. D.M. Kolb, J. Phys. C5 167 (1977) 167. F. Forstmann, K. Kempa, D.M. Kolb, J. Electroanal. Chem. 150 (1983) 241. D.M. Kolb, in: R.J. Gale (Ed.), Spectroelectrochemistry: Theory and Practice, Plenum Press, New York, 1988, pp. 87 /188. H. Ehrenreich, H.R. Philipp, Phys. Rev. 128 (1962) 1622. K.-M. Ho, B.N. Harmon, S.H. Liu, Phys. Rev. Lett. 44 (1980) 1531. D.M. Kolb, W. Boeck, K.-M. Ho, S.H. Liu, Phys. Rev. Lett. 47 (1981) 1921. W. Boeck, D.M. Kolb, Surf. Sci. 118 (1982) 613. S.H. Liu, C. Hinnen, C. Nguyen van Huong, N.R. de Tacconi, K.M. Ho, J. Electroanal. Chem. 176 (1984) 325. D.M. Kolb, J. Schneider, Surf. Sci. 162 (1985) 764. D.M. Kolb, J. Schneider, Electrochim. Acta 31 (1986) 929. H. Kita, H. Nakajima, K. Hayashi, J. Electroanal. Chem. 190 (1985) 141. H. Nakajima, H. Kita, K. Kunimatsu, A. Aramata, J. Electroanal. Chem. 201 (1986) 175. K. Kunimatsu, A. Aramata, H. Nakajima, H. Kita, J. Electroanal. Chem. 207 (1986) 293. G. Blyholder, J. Phys. Chem. 68 (1964) 2772. G. Blyholder, M.C. Allen, J. Am. Chem. Soc. 91 (1969) 3158. S.A. Wasileski, M.J. Weaver, Faraday Discuss. 121 (2002) 285. W.N. Hansen, A. Prostak, Phys. Rev. 174 (1968) 500. J.D.E. McIntyre, in: P. Delahay, C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, vol. 9, Wiley, New York, 1973, pp. 61 /166. J.D.E. McIntyre, Surf. Sci. 37 (1973) 658. K.M. Ho, C.L. Fu, S.H. Liu, D.M. Kolb, G. Piazza, J. Electroanal. Chem. 150 (1983) 235. S.-C. Chang, A. Hamelin, M.J. Weaver, J. Phys. Chem. 95 (1991) 5560. G.J. Edens, X. Gao, M.J. Weaver, N.M. Markovic, P.N. Ross, Surf. Sci. 302 (1994) L275. D.M. Kolb, Prog. Surf. Sci. 51 (1996) 109. G. Scherb, C. Gutie´rrez, D.M. Kolb, Unpublished results.