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The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1) The potential of zero charge (pzc) of Pd(1 1 1) has been determined in dilute NaF solutions by measuring the Gouy?CChapman minimum of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of )0.12 V vs. SCE has been found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) have also been determined. While the pzc of the first, pseudomorphic Pd layer on Au(1 1 1) is )0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc??s and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/water interface is estimated. _ 2002 Elsevier Science B.V. All rights reserved. The potential of zero charge (pzc) is a fundamental property of the metal?Celectrolyte interface, and its knowledge is important for a detailed understanding of double-layer phenomena, electrochemical kinetics and the adsorption behaviour of charged and neutral species [1,2]. The pzc is commonly defined as the potential, at which the electrode surface has no excess charge; for solid surfaces its value can in many cases be obtained from double-layer capacity measurements in dilute solutions, where a pronounced minimum appears at the pzc (Gouy?CChapman minimum) [3]. This has been demonstrated very nicely by the Meudon group for the three low-index faces of gold and silver single crystals [4,5]. This technique, however, cannot be usually employed for the more reactive metals, such as transition metals, because of the formation of adlayers or surface oxides over a wide potential range. A typical example is Pt(1 1 1), where the Gouy?CChapman minimum is conspicuously absent [6], and the pzc still a matter of considerable controversy [7?C10]. Due to our continued interest in the electrocatalytic properties of Pd single crystal surfaces and epitaxially grown Pd overlayers on gold substrates [11,12], we performed capacitance measurements for a full interface characterisation. Unlike for Pt(1 1 1), a clear Gouy?C Chapman minimum was readily observed for Pd(1 1 1), which allowed the determination of the pzc in a rather direct and unambiguous manner. The corresponding values for a massive Pd(1 1 1) single crystal electrode and for thin Pd(1 1 1) overlayers of various thicknesses on Au(1 1 1) in NaF solutions are shown and discussed as follows. The Pd(1 1 1) electrode was a single-crystal cylinder (MaTecK, J?ulich), 4 mm in diameter and 4 mm long, with a Pd wire attached to its rear for better handling. Before each experiment, the Pd single crystal was annealed by inductive heating in an argon atmosphere, cooled down to room temperature for 1 min in a stream of argon and protected against contamination from air with a water droplet. The electrode was then brought in contact with the electrolyte under potential control, usually negative of the potential of zero charge. More details about the preparation of Pd single crystal electrodes are given elsewhere [13]. The Pd(1 1 1) overlayers on Au(1 1 1) were obtained by electrodeposition from 0.1 M H2SO4 t 0:1 mM PdSO4 at +0.4 V vs. SCE [14], the film thickness being controlled by the charge flow through the interface (1 ML ffi 440 lC=cm2T. The Pd-covered Au(1 1 1) electrode was removed from the electrochemical cell, carefully rinsed with Milli-Q water and transferred to a second cell which contained a deaerated x mM NaF solution (x 5; 10; 25; 50 and 100). The capacitance measurements were performed with a lock-in amplifier (Stanford Research System, SR 830 DSP) at 18 Hz and a 10 mV peak-to-peak sinusoidal perturbation using the same procedures as in [6]. The solutions were made of NaF (Merck, p.a) and Milli-Q water (18.2 MX cm and 2 ppb TOC). All potentials were measured and are quoted with respect to the saturated calomel electrode (SCE). In Fig. 1 is shown the cyclic voltammogram for an inductively heated Pd(1 1 1) electrode in 0.1 M H2SO4 which contains the distinct features that demonstrate high surface quality [15]. The curve reveals a rather wide double layer charging region between H adsorption at the negative end of the potential cycle and oxide formation at the positive end. Double-layer capacity measurements were then conducted in that potential region with freshly prepared Pd(1 1 1) electrodes in 5, 10, 25, 50 and 100 mM NaF, which clearly reveal a pronounced capacity minimum around )0.12 V vs. SCE for the lowest NaF concentrations (Fig. 2). From the C?CE curve for 5 mM NaF the pzc of Pd(1 1 1) in NaF is determined to be )0.124 V vs. SCE. Several features of the curves in Fig. 2 are worth mentioning: (i) The position of the minimum shifts slightly with the NaF concentration, indicating slight specific adsorption of fluoride on Pd(1 1 1). (ii) With increasing NaF concentration fluoride adsorption influences massively the double-layer capacity around the pzc, causing a CDL suppression for E > _0:15 V. (iii) At potentials clearly negative of the pzc (E < _0:2 V), all CDL?CE curves merge as it is expected for non-specifically adsorbing ions, and they reach a value of 20 lF=cm2 at very negative potentials. Such a capacity value is commonly observed for singlecrystal metal electrodes (i.e., with a roughness factor of unity) like Au(hk l) or Ag(hk l) [4,5]. The above-mentioned fluoride adsorption and the beginning oxide formation (hydroxide adsorption) accompanied by slight pH changes at potentials positive of the pzc cause a marked irreversibility in the cyclic voltammogram (see inset of Fig. 2) and in the CDL?CE curves (not shown). For the latter, the capacitance minimum at the pzc in dilute solutions is no longer visible in the negative scan, but it reappears after a short waiting period at )0.45 V. Consequently, the curves in Fig. 2 refer to the first positive scan for freshly annealed Pd(1 1 1) electrodes, which were immersed at )0.45 V vs. SCE. This procedure ensures that the CDL?CE curves in Fig. 2 reflect the electrochemical behaviour of a genuine Pd(1 1 1) surface, at least for the lowest electrolyte concentration. The curve for the latter calls for an evaluation of the inner-layer capacity Ci by calculating the diffuse-layer capacity according to the Gouy?CChapman theory [3]. Unfortunately, the classic model that does not take specific adsorption into account, did not yield meaningful results, suggesting that specific adsorption of fluoride contributes significantly to the surface charge near the pzc. A more sophisticated evaluation of Ci will be attempted at a later stage. In previous publications we have demonstrated that thin Pd overlayers on Au(1 1 1) single crystal electrodes represent ideal, because easy to prepare substitutes for massive Pd(1 1 1) crystals [11,12]. While up to now the single-crystallinity of the Pd overlayers was tested by structure-sensitive reactions like the underpotential deposition of Cu, the hydrogen adsorption, or the oxidation of formic acid, the double-layer capacity of such films were not yet investigated. For this purpose, we have deposited Pd films of various thicknesses onto Au(1 1 1) from a 0.1 mM PdSO4 solution, where in STM studies a layer-by-layer growth was observed for the first two monolayers [14]. The result of a double-layer capacity measurement for epitaxially grown Pd(1 1 1) films on Au(1 1 1) in 10 mM NaF is shown in Fig. 3 for film thicknesses of 1, 2 and 5 monolayers (ML), the latter number representing an average thickness. The corresponding curve for massive Pd(1 1 1) in the same solution has been included for comparison. There is a remarkable shift in the Gouy?CChapman minimum with increasing film thickness, indicating a distinct thickness dependence of the pzc. The potential values of the capacity minimum for 1, 2 and 5 ML Pd on Au(1 1 1) are )90, )100, and )122 mV vs. SCE, respectively, as compared to )124 mV for massive Pd(1 1 1). This shift in pzc with thickness and its possible relation to the overlayer structure will be discussed in the following section. We will first discuss the relation between pzc and work function for Pd(1 1 1), in order to get an estimate of the surface dipole contribution to the pzc. According to Trasatti [16,17], we may write??????????????????????. where Er 0 is the pzc, U is the work function as determined under UHV conditions, dvM 0 is the perturbation of the metal surface dipole by the neighbouring solvent, gH2OedipT0 is the surface potential of the solvent (in our case water) at the pzc, and const(Ref) is a constant that depends solely on the reference electrode (the so-called absolute electrode potential in the case of SHE). Although work functions of metals are often a matter of debate, a value of 5.60 eV for Pd(1 1 1) [18] appears to be very reasonable, also in comparison to U 5:8?C5.9 eV reported for Pt(1 1 1) [19,20]. Likewise, for reconstructed Au(1 1 1) a work function value of 5.3?C5.4 eV is found in the recent literature [21,22]. The absolute electrode potential is still an issue of controversy, the value for const(SHE) ranging from 4.5 eV [23] to 4.8 eV [24?C26]. However, since we are aiming at a crude estimate only of the water dipole contribution at the palladium?Cwater interface, we will not worry too much about the 0.3 eV discrepancy for the value of the absolute electrode potential. We start our considerations by fixing the energy scales for a metal in vacuum and in contact with the electrolyte with the help of Au(1 1 1) ?C e ffiffiffi 3 p 22T, the work function of which is taken as 5.4 eV [22] and the pzc as +0.56 V vs. SHE [27]. Adopting Trasatti??s value of )0.3 V for the term in the square brackets in Eq. (1) [16] would lead us to an absolute electrode potential of consteSHET 5:4 _ 0:3 _ 0:56 _ 4:5 eV. Consequently, for Pd(1 1 1) with U 5:6 eV and the pzc t0:12 V vs. SHE ()0.12 V vs. SCE), a surface dipole contribution d _5:6 t 4:5 t 0:12 ffi _1:0 V is obtained. This dipole contribution (square brackets in Eq. (1)) consists of two parts that in general cannot be easily separated. However, such a separation appears possible for the Hg ?C aqueous solution interface, for which Trasatti in his classic studies arrives at dvM0 _0:31 V [16]. He also presents strong arguments for dvM0 being rather insensitive to the chemical nature of the metal. Assuming dvM0 )0.3 V would yield a surface potential due to interfacial water of gH2OedipT0 t0:7 V. This value has to be compared with the only one available theoretical value of d 0:7 _ 0:1 V for Pt(1 0 0) in contact with water [28]. Although the very good agreement must be considered fortuitous, it certainly confirms the correct order of magnitude and it supports the notion of a strong interaction of water with Pd(1 1 1). (We stress the point that the above derived values rest on the assumption of the absolute electrode potential, const( SHE), being 4.5 eV. A value of 4.8 eV for the latter would bring down all dipole contributions by 0.3 V, i.e.,d _0:7 V and gH2OedipT0 t0:4 V.) The large contribution of water to the surface dipole of Pd(1 1 1) calls for a renewed discussion of the pzc of Pt(1 1 1), which is still a matter of controversy. Taking 5.9 eV as work function of Pt(1 1 1) and a dipole contribution similar to that for Pd(1 1 1), i.e., )1 V, a pzc of Er 0 5:9 _ 1:0 _ 4:5 V _ t0:4 V vs. SHE (t0:16 V vs. SCE) appears now more realistic than the very positive value which had been derived from immersion experiments [7], and which had invoked a vanishingly small dipole contribution from the solvent. The pronounced peak at t0:12 V vs. SCE of the double-layer capacity for Pt(1 1 1) in perchlorate solutions [6] and the conspicuous absence of a Gouy?CChapman minimum in dilute solutions, however, remains to be explained. We now turn to the pzc??s of thin Pd(1 1 1) overlayers on Au(1 1 1), which reveal a marked thickness dependence (Fig. 3). In previous STM studies of Pd deposition onto Au(1 1 1) it was concluded that the first Pd layers grow pseudomorphic, while a layer-by-layer type growth continues beyond the transition to bulk Pd [12,14]. Hence, the first two monolayers of Pd on Au(1 1 1) are expected to have a more open structure than massive Pd(1 1 1) (next-neighbour distance of 0.288 nm instead of 0.274 nm) and therefore, should acquire a work function value lower than that of Pd(1 1 1) with bulk structure. The higher value of the pzc for the 1st and the 2nd (presumably pseudomorphic) monolayer could either reflect a higher work function or be due to a somewhat different water structure, resulting in a slightly smaller surface dipole contribution. The higher work function of the 1st Pd layer on Au(1 1 1) may be explained by a direct chemical influence of the substrate (similar to a ligand effect), but for the two monolayer thick Pd(1 1 1) overlayer such an argument should no longer hold. On the other hand, a slight change in the palladium-water interaction due to the increase in the next-neighbour Pd distance from 2.74 to 2.88 _AA for the pseudomorphic film, could possibly reduce the 1 V dipole contribution by some 30?C40 mV. We recall that the difference in pzc??s of massive Pd(1 1 1) and 1 ML Pd on Au(1 1 1) is 34 mV only (Fig. 3). In this connection we may cite very preliminary results with Pd(1 0 0) in NaF solutions, which reveal a capacity dip (Gouy?C Chapman minimum) around )0.20 V vs. SCE. From this observation we conclude that the pzc of Pd(1 0 0) is about 80 mV more negative than that of Pd(1 1 1), whereas the work function of Pd(1 0 0) has been reported in the literature to be 5.3 eV [29], i.e., 200 mV lower than that of Pd(1 1 1). This leaves an 0.2 V lower dipole contribution of )0.8 V for Pd(1 0 0), suggesting a noticeable dependence of that quantity on the surface structure. In agreement with structure information from STM measurements, we find the pzc of a 5 ML thick Pd overlayer on Au(1 1 1) practically identical to that of massive Pd(1 1 1), despite the higher surface defect density of the Pd film. The pzc of Pd(1 1 1) in a dilute NaF solution is )0.12 V vs. SCE. From a comparison of pzc and work function, and assuming 4.5 eV for the absolute electrode potential of SHE, a water dipole contribution of about 0.7 V to the potential drop across the interface was estimated. A similarly large contribution from the interfacial water is also expected for Pt(1 1 1), which raises serious doubts about the previously published value of 0.85 V vs. SCE for the pzc of Pt(1 1 1) [7]. The pzc of a thin pseudomorphic Pd(1 1 1) overlayer on Au(1 1 1) is about 30 mV more positive than that for massive Pd(1 1 1). It is assumed that a slightly different water structure is induced by the pseudomorphy, which is responsible for this shift in pzc. |
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