Potential-Dependent Water Orientation on Pt(111), Pt(100), and Pt

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Potential-Dependent Water Orientation on Pt(111), Pt(100), and Pt(110), As Inferred from Laser-Pulsed Experiments. Electrostatic and Chemical Effects Nuria Garcia-Araez,* Victor Climent, and Juan Feliu Instituto UniVersitario de Electroquı´mica, UniVersidad de Alicante, Apdo. 99, E-03080 Alicante, Spain ReceiVed: January 27, 2009; ReVised Manuscript ReceiVed: April 1, 2009

The laser-induced temperature jump method is used to characterize the net orientation of interfacial water on well-defined platinum surfaces, Pt(111), Pt(100), and Pt(110), as a function of the applied potential. A clear effect of the surface structure on the potential of water reorientation is observed, being 0.37 for Pt(111), 0.33 for Pt(100), and 0.14 V vs RHE for Pt(110) in 0.1 M HClO4 solution. The potential of water reorientation also exhibits a different pH dependency for the three basal planes, shifting 0.060 for Pt(111), 0.030 for Pt(100), and 0.015 V/dec for Pt(110). Comparison with charge density data provides a deeper understanding of these results. A quantitative analysis of the electrostatic and chemical effects governing the potential-dependent reorientation of the interfacial water network is addressed. It is concluded that water on Pt(111) exhibits a small net orientation in the absence of electric field at the interphase. On the other hand, the agreement between the relative position of values of the potential of water reorientation and work functions, for the three basal planes, suggests that the same situation holds for Pt(100) and Pt(110). 1. Introduction Water has a paramount importance in a variety of fields such as chemistry, biology, and physics. Besides, water interaction with metal surfaces plays a central role in many disciplines such as catalysis, electrochemistry, corrosion, and biochemistry. Water can lower the activation energy of reactions through solvation of reactants and/or intermediates; new reaction paths can become accessible as a result of hydrogen bonding to water molecules, and water or the products of the autoprotolysis can also act as reagents. However, the role played by water molecules in electrified interphases is poorly understood. One of the main difficulties in the study of interfacial water is the interference from bulk water contributions. For this reason, most of the progress in the understanding of the interaction of water with metal surfaces has arisen from model experiments under ultrahigh-vacuum (UHV) conditions and density functional theory (DFT) calculations. Special mention is deserved of the work of Ogasawara et al. on water on Pt(111) by the combination of soft X-ray spectroscopy and DFT calculations, concluding that water molecules form a “flat-ice” structure on the platinum surface, with a preferential orientation with the hydrogen toward the metal.1 Also specially remarkable is the new structural model for water on Ru(001) proposed by Feibelman using DFT calculations, in which the water monolayer is halfdissociated with one OH broken.2 The characterization of the behavior of the interfacial water network in contact with platinum single-crystal surfaces is especially interesting. Platinum is the best base catalyst for fuelcell reactions, which are a very promising new source of energy for the near future. Since these reactions take place in aqueous environments, water will be ubiquitously present and therefore is expected to produce key effects in electrocatalysis. Recently, the behavior of interfacial water on polycrystalline platinum thin films was successfully characterized by in situ surface-enhanced infrared adsorption spectroscopy.3 However, the interpretation of these results at a molecular level is limited by the lack of control of surface structure, inherent to this technique. On the

other hand, when platinum single-crystal surfaces are employed, with other in situ spectroscopic techniques, the separation of the response from interfacial and bulk water becomes very difficult, and results obtained by different laboratories are discrepant.4,5 The laser-induced temperature jump method allows studying the behavior of interfacial water on metal surfaces, under “in situ” conditions, with high selectivity. This method is based on the use of short laser pulses to suddenly increase the temperature at the water-metal interphase. This technique was previously used by Benderskii et al.6 with mercury electrodes and by Smalley et al.7 with polycrystalline platinum. Recently, Climent et al. succeeded in the application of this method to the study of single-crystal surfaces.8,9 Previous work applied the laser-induced temperature jump method to the study of water reorientation on Pt(111).8 In the present work, this analysis will be extended to the other basal planes: Pt(100) and Pt(110). With this, a unique analysis of the effect of the surface structure on the fundamental properties of water adsorbed on platinum surfaces, under in situ conditions, will be performed. Furthermore, the results will be rationalized by means of a quantitative analysis on the electrostatic and chemical interactions governing the reorientation of the interfacial water network. Another big advantage of the laser-induced temperature jump method is the possibility of decoupling the charge-transfer process associated with adsorption phenomena from the purely double-layer response, by making the temperature jump sufficiently fast. This strategy was applied to study the kinetics of hydrogen adsorption on a Pt(111) electrode.8 It should be mentioned that, because of the very fast nature of this reaction, in an independent study by impedance spectroscopy, frequencies up to 1 MHz were required to achieve the separation of faradaic and double-layer processes.10 On the other hand, by decreasing the bulk proton concentration, the rate of hydrogen adsorption is slowed down, and the pure double-layer response can be obtained. Then, the response of the electrode potential to the change of the temperature under coulostatic conditions reflects

10.1021/jp900792q CCC: $40.75  2009 American Chemical Society Published on Web 05/01/2009

Potential-Dependent H2O Orientation on Pt Surfaces the temperature coefficient of the double-layer potential. Noteworthy, this coefficient is mainly determined by the change in the polarization of solvent molecules.11 At low enough potentials, water dipoles are oriented with the positive end (hydrogen) toward the metal, giving a positive contribution to the overall potential drop. The effect of the rise of the temperature is a decrease of this positive contribution, resulting in negative laserinduced transients. As the potential is increased, the oxygentoward-the-metal orientation is favored, thus resulting in positive laser-induced transients. One very important result that can be obtained from the laser-pulsed experiments is the value of potential where the transients change sign, which corresponds to the potential of turnover of water molecules and can be identified with the potential of maximum entropy (pme) of double-layer formation.6,11 As a result of the electrostatic interaction of the water dipoles with the electric field at the interphase, the pme is expected to be close to the potential of zero free charge (pzfc). It should be recalled here that for platinum electrodes, due to the existence of adsorption processes involving charge transfer, it is necessary to distinguish between two types of charge.12 While the free charge is the actual electronic charge on the metal, the total charge takes into account the charge involved in adsorption processes. Water dipoles will be strongly affected by the free charge density on the platinum surface, as a result of electrostatic interactions. However, the net orientation of the water network will be also sensitive to water-metal specific (chemical) interactions. Interestingly, the covering of platinum sites by specifically adsorbed species may also affect the water-metal specific (chemical) interactions. In this regard, the pme is also expected to show some sensitivity to the potential of zero total charge (pztc). Traditionally, in the absence of any electric field, water on metal surfaces was considered to form an hexagonal phase of ice, with a net orientation with the oxygen toward the surface.13,14 In agreement with this model, the pme is located at lightly negative charge densities on gold9,15 and mercury6,16 electrodes. Conversely, X-ray experiments indicate that water adsorption on Pt(111), under UHV conditions, leads to a flatice structure with a prevalent hydrogen-toward-the-metal orientation.1 However, the predominance of the hydrogen- or oxygen-toward-the-metal structures on Pt(111) is expected to be very sensitive to the experimental conditions, since a number of DFT calculations have shown that the adsorption energies of both adlayers are very similar.17-19 In addition, it should be pointed out that the extrapolation of results in UHV environment to the metal-liquid water interphase should be made with great caution, as a result of the very different experimental conditions of temperature and pressure. Consequently, the in situ structure of water on platinum single-crystal surfaces at the pzfc is still an open question. In the present work, we will address this issue from the careful comparison of pme and pzfc values, as obtained under in situ conditions. This paper is organized as follows. The fundamentals of the laser-induced temperature jump method and details of the experimental setup are briefly described in sections 2 and 3, respectively. The results of the application of the temperature jump method to Pt(111), Pt(100), and Pt(110) are discussed in section 4. First, the separation of the contributions from the double-layer response and charge-transfer processes to the laserinduced potential transients is addressed in section 4.1. The results of the temperature coefficient of the double-layer potential and pme values are presented in section 4.2. The comparison of this data with pztc and pzfc values is performed

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9291 in section 4.3. Finally, the main conclusions of the present work are summarized in section 5. 2. Preliminary Considerations In the present work, nanosecond laser pulses are used to suddenly increase the temperature of the interphase between a platinum single-crystal electrode and an aqueous solution. Although the exact temperature change induced by the laser pulse cannot be measured in our experiment, due to the short time scale of the perturbation, it can be calculated through a simple heat-transport model. This provides the following estimation of the temperature change at the interphase20

(

∆T ) ∆T0

√t - √t - t0

√t0

)

t > t0

(1)

where t0 is the duration of the laser pulse (t0 ) 5 ns) and ∆T0 is the maximum temperature change (∆T0 ∼ 35 K20). The experimental response toward the laser pulse, under coulostatic conditions, is a displacement of the electrode potential from its initial value. This potential change is essentially given by the response of the interphase. Another contribution is due to the existence of a thermodiffusion potential, between the heated solution at the surface of the electrode and the cold solution near the reference electrode, but this contribution is small under the present experimental conditions, as will be shown in section 4.2. The response of the interphase toward the laser heating involves two main contributions. On the one hand, the response of the double-layer results from the temperature coefficient of the double-layer potential, and it is mainly due to reorganization phenomena of the interfacial water network. On the other hand, charge-transfer processes may also take place as a result of the modification of the equilibrium coverage, provided that their kinetics are fast enough. Consequently, the interpretation of the laser-induced transients first requires a careful separation of these contributions. Hydrogen adsorption kinetics can be simulated with ButlerVolmer equations for the forward and backward rate constants of the reaction, and including Frumkin-type interactions. Details of this derivation are given elsewhere8

Pt + H++e- a Pt-H 0.5 (1 - θH)cH+ exp - (FE + ∆G0 + ωθH) dθH RT 0 )k 0.5 dt -θH exp (FE + ∆G0 + ωθH) RT (hydrogen adsorption) (2)

{

[

]

[

]

}

where θ is the hydrogen coverage, k0 is the standard rate constant, ∆G0 is the standard Gibbs energy of adsorption, and ω is the interaction parameter. The temperature dependency of the standard rate constant is included in the activation energy, Ea:

(

0 k0 ) kT)298K exp

Ea Ea R 298 K RT

)

(3)

From the equilibrium condition, dθ/dt ) 0, the Frumkin adsorption isotherm is obtained. This isotherm is adequate for the description of charge-transfer processes studied here, within a certain potential window, as explained in Appendix 1. Analogous equations can be written for OH adsorption, which in acidic media will take place through

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{

Garcia-Araez et al.

Pt + H2O a Pt-OH + H++e-

}

0.5 (1 - θOH) exp (FE - ∆G0 - ωθOH) dθOH RT 0 )k 0.5 dt -θOHcH+ exp (-FE + ∆G0 + ωθOH) RT (OH adsorption) (4)

[

]

[

]

As above-mentioned, laser-induced potential transients contain a contribution due to the temperature coefficient of the doublelayer potential, ξdl, and also a contribution due to the chargetransfer reaction associated with the modification of the equilibrium coverage of specific adsorbing species

∆E ) ξdl∆T + z

qML∆θ Cdl

(5)

where Cdl is the double-layer capacity, z ) 1 for hydrogen adsorption and z ) -1 for OH adsorption, ∆E ) E - Ei, and ∆θ ) θ - θi, where Ei and θi are the initial potential and the initial coverage, respectively. The simulation of the laser-induced transients is performed by the numerical solution of the combination of the above equations solved with a fifth-order Runge-Kuta method with adaptative step size implemented in Fortran.21 3. Experimental Section The platinum electrodes were prepared from small platinum beads (2-3 mm diameter), oriented, cut, and polished to obtain the desired orientation, following Clavilier’s procedure.22 Prior to each experiment, the electrodes were annealed in a Bunsen flame (propane-air), cooled down in a flow of H2/Ar (N-50, Air Liquide in all gases used), and protected with a drop of ultrapure water in equilibrium with the H2/Ar gas mixture. All experiments were performed in a four-electrode electrochemical cell, with the working electrode under meniscus configuration. A coiled platinum wire was used as a counter electrode and a hydrogen-charged Pd wire was used as a reference electrode, which equals +50 mV vs RHE. However, all potentials in this work are quoted against the RHE scale, except otherwise stated, in order to make easier the comparison with other studies. Cyclic voltammograms were recorded using a computercontrolled µ-Autolab III potentiostat (Eco-Chemie, Utrecht, Netherlands) under the current integration mode. Solutions were prepared from concentrated perchloric acid (Merck, suprapur) and KClO4 (Merck, p.a.) diluted in ultrapure water (18.2 MΩ cm) obtained from an Elgastat water purification system. The KClO4 was purified by recrystallization, and the HClO4 was used as received. The electrolytes were purged with argon, and the solution was kept under an argon blanket throughout the duration of the experiment. The laser-induced potential transients were recorded as previously explained.20 Briefly, after recording of a voltammogram to ensure the surface order and cleanliness of the cell, the single-crystal working electrode is polarized at a given potential. The fourth platinum electrode is polarized exactly at the same potential as the working electrode. Around 200 µs before firing the laser on the single-crystal working electrode, both electrodes are disconnected from the potentiostat, and the potential difference between them (that appears as a consequence of the temperature jump at the working electrode) is measured with a home-built differential amplifier (bandwidth ca. 20 MHz). The experiment was repeated at a frequency of 10 Hz, which allows the relaxation of the temperature to its initial value between

consecutive pulses. The potentiostat is reconnected between successive laser pulses, ensuring that the potential is kept at the desired value. Potential transients (128 or 256) were averaged at each potential using a TDS 3054B Tektronix oscilloscope. After recording of the laser transients at different potentials, a new voltammogram was recorded and compared with the initial one, in order to test the stability of the surface during the whole experiment. The light source employed was a Brilliant Q-switched Nd:YAG laser (Quantel) operating in frequency-doubled mode at a wavelength of 532 nm and pulse duration of 5 ns. The beam diameter obtained directly at the laser output is ca. 6 mm, and this was reduced to ca. 4 mm by passing it through a conventional arrangement of lenses. Laser energy of ca. 1 mJ per pulse, i.e., 8 mJ cm-2, was used in all the experiments, well below the damage threshold of the electrode surface. The maximum temperature change induced by the laser pulse is estimated to be ∆T < 2 K at t > 0.5 µs.20 4. Results and Discussion 4.1. Separation of Charge-Transfer and Double-Layer Contributions to the Laser-Induced Transients on Pt(111), Pt(100), and Pt(110). Figures 1 and 2 show laser-induced potential transients for Pt(111), Pt(100), and Pt(110) electrodes in 0.1 M KClO4 + 1 mM HClO4 and 0.1 M HClO4 solutions, respectively. The corresponding cyclic voltammograms are shown in Figure 3. The characteristic profiles of the voltammograms reveal that the surfaces are highly well-ordered and virtually free of uncontrolled contaminants. Perchlorate solutions of different pH values are employed in order to minimize the extent of anion specific adsorption. In addition, the use of the perchlorate solution with a higher pH (pH ∼ 3) is also convenient for the interpretation of the laser-pulsed experiments.8,9 On the one hand, the higher pH diminishes the rate of hydrogen and OH specific adsorption, and hence the deconvolution of the pure double-layer response is facilitated. Indeed, it will be shown below that at pH ∼ 3, laser-induced transients are dominated by the double-layer response. On the other hand, the thermodiffusion potential produced by the temperature gradient in solution decreases to a negligible value. However, it should be noted that, even in 0.1 M HClO4 solutions, the thermodiffusion potential constitutes a minor contribution to the potential transients (∼-0.38 mV/K). We will first discuss the results obtained in 0.1 M KClO4 + 1 mM HClO4 solutions because the interpretation is more straightforward. In all cases, transients are negative at low enough potentials (see, for example, that transients at E ) 0.1 V are negative for the three basal planes), indicating that the temperature coefficient of the double-layer potential is negative at these potentials. These results, in turn, provide evidence that interfacial water molecules exhibit a net orientation with their positive end (hydrogen) toward the metal, within this potential region.11 Then, as the potential is increased, the absolute magnitude of the transients decreases. At high enough potentials, transients become positive, meaning that the oxygen-towardthe-metal orientation is favored. Noteworthy, the value of potential where the transients change sign can be identified with the pme of double-layer formation, and it is clear from this figure that it strongly depends on the surface structure. These values of pme for the three basal planes are plotted in Figure 9 and will be discussed in detail in sections 4.2 and 4.3. Finally, Figure 1 shows that if the potential is further increased, the magnitude of the transient decreases again, becoming negative at high enough potentials. Negative transients are observed within the

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Figure 1. Laser-induced potential transients on Pt(111), Pt(100), and Pt(110) electrodes, as indicated, in 0.1 M KClO4 + 1 mM HClO4 solution at different potentials, as labeled.

Figure 2. As in Figure 1 but in 0.1 M HClO4 solution.

oxide formation region for Pt(111) and Pt(110), while for Pt(100), negative transients are observed at potentials before surface oxidation takes place. When the proton concentration is increased, the hydrogen and OH adsorption/desorption processes become faster and can contribute to the potential transients. Still, Figure 2 shows that the pme values can be directly obtained from the experimental transients, from the value of the potential where the laserinduced response becomes negligible. A marked effect of the

surface structure on pme values is observed. These values are also included in Figure 9 and will be discussed in sections 4.2 and 4.3. On the other hand, in order to determine the temperature coefficient of the double-layer potential within the whole available potential window, it is necessary to characterize in detail the contribution from these charge-transfer processes to the laser-induced transients. In this way, the contribution from the double-layer response can be separated from the contribution from charge-transfer processes (eq 5). The method for the

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Figure 3. Cyclic voltammogram of Pt(111), Pt(100), and Pt(110) electrodes, as indicated, in (a) 0.1 M HClO4 and (b) 0.1 M KClO4 + 1 mM HClO4. Scan rate: 50 mV/s. Dashed line corresponds to the current density calculated from the thermodynamic parameters in Table 1. See text for details.

TABLE 1: Thermodynamic Data Employed for the Simulation of Laser-Induced Transients for Pt(111), Pt(100), and Pt(110) Electrodes, as Indicated

H on Pt(111) OH on Pt(111) H on Pt(100) H on Pt(110)

(∆G0)/ (kJ mol-1)

0 )/ (∆Snonisothermal (J mol-1 K-1)

-28.1 65.9 -36.7 -12.5

10.1 ∼-50 ( 5a 12.4 ∼15 ( 5a

(ω)/(kJ mol-1) 28.5 13.3 5.58 -2.7

a As determined from the fit of laser-induced transients. See text for details.

separation of these contributions will be described below, with the help of Figure 4. In this figure, laser-induced transients for Pt(111), Pt(100), and Pt(110) electrodes in 0.1 M HClO4 are compared in the low, middle, and high potential region. 4.1.1. Kinetics of Charge-Transfer Processes in the Low Potential Region. Figure 4A compares transients measured at a potential E ) 0.1 V, within the hydrogen adsorption region. It is observed that, for the three basal planes, transients exhibit a bipolar shape, with a sharp negative peak at very short times, t < 0.5 µs, and a broader positive contribution at longer times, t > 1 µs. Interestingly, the time scale where the positive contribution appears strongly depends on the surface structure. As will be shown below, this behavior reflects the differences in the rate of hydrogen adsoption/desorption on the three basal planes. The nature of the two contributions of opposite sign of transients in Figure 4A was previously identified by means of simulations of the transients on the bases of Butler-Volmer kinetics for hydrogen adsorption on Pt(111).8 It was concluded that the negative fast contribution can be ascribed to the doublelayer response of the electrochemical interphase toward the laser heating. The negative sign of this contribution is a consequence of the negative thermal coefficient of the double-layer potential, which in turn indicates that the net orientation of the interfacial water network is with the hydrogen toward the metal. This initial change of the potential induces a subsequent increase of the hydrogen coverage, the process of hydrogen adsorption withdrawing electrons from the surface and thus producing a positive displacement of the potential at longer times. As a consequence

of these opposing effects, double-layer and charge-transfer processes, the transients exhibit a bipolar shape. When the pH is increased, the rate of hydrogen adsorption is slowed down.8 As a result, simulation of the transients at different pH values allows characterizing the hydrogen adsorption kinetics. The standard rate constant of hydrogen adsorption on Pt(111) can be thus estimated from the fit of the laser-induced potential transients measured at various pH values (pH ) 1-4), and employing the thermodynamic data for hydrogen adsorption on Pt(111) summarized in Table 1. This provides a standard rate constant of k0 ≈ 104.5 s-1, corresponding to a charge-transfer resistance of Rct ≈ 25-21 mΩ cm2. The kinetics can be also described by an effective relaxation time of τ ≈ 0.2-0.4 µs in 0.1 M HClO4. Noteworthy, these results are in very close agreement with the charge-transfer resistance of Rct ≈ 26 -30 mΩ cm2 reported from impedance measurements of Pt(111) in 0.5 M H2SO4 and 0.5 M HClO4 solutions within the hydrogen adsorption region.10 Laser-induced transients for Pt(110) in 0.1 M HClO4 at E ) 0.1 V also exhibit a bipolar shape (Figure 4A). As for Pt(111), the negative fast contribution at t < 0.1 µs is due to the doublelayer response, while the positive contribution at longer times can be ascribed to the kinetics of hydrogen adsorption. It is reasonable to assume that the slower contribution is due to hydrogen adsorption on the (110) sites, associated with the voltammetric peak centered at E ≈ 0.14 V. Consequently, the kinetics of this process can be studied from the simulation of the transients on the bases of Butler-Volmer equations, as for the study of Pt(111).8 As above-mentioned, this analysis also requires the thermodynamic parameters of hydrogen adsorption on Pt(110), summarized in Table 1. Details of this calculation are given in Appendix 1. It was found that the best fit of the measured transients at E ) 0.10 V corresponds to an entropy of hydrogen adsorption of ≈15 ( 5 J/mol K (nonisothermal conditions) and a standard rate constant of k0 ≈ 104.8 s-1, corresponding to a characteristic response time of τ ≈ 0.5 µs and a charge-transfer resistance of Rct ≈ 15 mΩ cm2. Figure 5 compares the results of the simulations with the laser-induced transients measured in (0.1 - x) M KClO4 + x M HClO4 solutions, where x varies between 10-4 and 0.1. The four curves in Figure 5A were obtained with the same values of the thermal



Figure 4. Comparison of transients obtained for Pt(111), Pt(100), and Pt(110) electrodes, as indicated, in 0.1 M HClO4, at (A) E ) 0.1 V, (B) E ) 0.35 V, and (C) E ) 0.75 V.

at pH ) 3. At pH ) 4, the contribution of hydrogen adsorption in the potential transient is almost negligible, and at lower pH values, the kinetics of hydrogen adsorption are not diffusion limited. Figure 4A reveals that for the laser-induced transients on Pt(100), the contribution from hydrogen adsorption kinetics takes place at much longer times than on Pt(111) and Pt(100). This is due to the fact that hydrogen adsorption/desorption on (100) sites is much slower than on (111) or (110) sites. Indeed, impedance measurements on Pt(100) in 0.5 M H2SO4 solutions24 have reported a charge-transfer resistance of Rct ≈ 0.3-0.4 Ω cm2 for hydrogen adsorption at potentials within 0.06-0.35 V. Impedance measurements also provided an estimation for the double-layer capacity, Cdl ≈ 20-35 µF cm-2, and for the activation energy for hydrogen adsorption, Ea ≈ 30-40 kJ mol-1. The combination of this data with the thermodynamic parameters for hydrogen adsorption on Pt(100) can be used to estimate the contribution from hydrogen adsorption kinetics to the laser-induced transients.

Figure 5. Comparison of the simulated (A) and experimental (B) potential transients after laser illumination on Pt(110) at E ) 0.10 V. (A) Simulated curves with ξdl ) -1.5 mV K-1, Cdl ) 20 µF cm-2, k0 ) 104.8 s-1, and Ea ) 40 kJ mol-1. (B) Experimental curves for (0.1 x) M KClO4 + x M HClO4, where x equals (a) 0.1, (b) 10-2, (c) 10-3, and (d) 10-4.

coefficient of the double-layer potential (ξdl ) -1.5 mV/K), the activation energy for hydrogen adsorption (Ea ≈ 40 kJ mol-1), and the double-layer capacity (Cdl ≈ 20 µF cm-2 from ref 23). The only parameter that was varied is the concentration of protons in solution. Mass transport effects were not considered, but the introduction of the depletion of proton concentration at the interphase is expected to produce only small effect

In order to obtain the thermodynamic parameters for hydrogen adsorption on Pt(100), it should be taken into account that two different hydrogen adsorption processes can be distinguished in the voltammograms of Pt(100) in perchlorate solutions (Figure 3). The main hydrogen adsorption on (100) sites takes place between 0.25 and 0.45 V vs RHE. Another hydrogen adsorption process takes place at E < 0.2 V, which was attributed to hydrogen adsorption on defect sites, in view of voltammetric and charge density data on Pt(100) stepped surfaces.25 Unfortunately, adsorption of hydrogen on defect sites strongly overlaps with hydrogen evolution. As a result, the thermodynamic parameters characterizing this process cannot be determined with enough precision from the analysis of the voltammetric profiles. Consequently, the modeling of the kinetics of this process by means of Butler-Volmer equations, as was performed in the case of Pt(111) and Pt(110), is not attainable with the presently availabledata.Alternatively,forsmallperturbations,Butler-Volmer equations can be reduced to the following simplified equation, which allows estimating the characteristic relaxation time of this reaction (see Appendix 2 for details)

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∆E ) ξ1∆T(t) +

ξ2 τ

∫0t ∆T(t′) exp( t -τ t′ ) dt′

Garcia-Araez et al.

(6)

where ξ1 is the temperature coefficient of the double-layer potential, with virtually infinite response time, ξ2 is the temperature coefficient of hydrogen adsorption, with response time τ, and ∆T is the temperature change at the interphase, which can be estimated from eq 1. This analysis provides a characteristic relaxation time of τ ≈ 5-10 µs, which approximately corresponds to a charge-transfer resistance of Rct ≈ 0.15-0.5 Ω cm2, according to the following approximation (see Appendix 2 for the derivation)

τ)

(

Rct ≈ CdlRct ∂Ee ∂θ Ti 1 Cdl qML

( )

)

(7)

Noteworthy, these values of Rct are in good agreement with the results of the above-cited impedance study of Pt(100) in 0.5 M H2SO4.24 4.1.2. Kinetics of Charge-Transfer Processes in the Middle Potential Region. Figure 4B compares laser-induced transients measured for Pt(111), Pt(100), and Pt(110) in 0.1 M HClO4 at a potential E ) 0.35 V. Transients for Pt(111) exhibit a monotonous decay, as a result of the fact that E ) 0.35 V is very close to the double-layer region, and the contribution from kinetics of hydrogen adsorption is negligible.8 On the other hand, transients for Pt(100) exhibit also a nearly monotonous decay. This is a consequence of the slower kinetics of hydrogen adsorption on Pt(100) sites. The contribution from hydrogen adsorption to the laser-induced transients can be calculated from the combination of kinetic data from impedance measurements24 and the thermodynamic parameters in Table 1. Following this approximation, a good fit of the laser-induced transients is obtained, with an average value of the standard rate constant of k0 ≈ 103.4 s-1. With this analysis, the thermal coefficient of double-layer potential on Pt(100) was determined. Although this analysis has been performed by employing Butler-Volmer equations without any further approximation, it is worth mentioning that this reaction kinetics can be also well-described with the simplified eq 6. The corresponding effective parameters, for transients measured at E ) 0.35 V in 0.1 M HClO4, are ξ1 ) 0.5 mV/K, ξ2 ) - 0.6 mV/K, and τ ) 9 µs. Furthermore, this value of the response time is in excellent agreement with the product CdlRct ≈ 9.5 µs, in agreement with eq 7. Finally, laser-induced transients for Pt(110) at E ) 0.35 V (Figure 4B) exhibit two marked contributions from opposite signs: Transients present a very fast, positive contribution at t < ∼0.1 µs, followed by a slower, negative contribution at longer times. Interestingly, this very marked bipolar shape prevails in a very broad potential window (0.35-0.65 V), where no contribution from specific adsorption processes is observed in the voltammograms of Figure 3. Therefore, the finite response time of the laser-induced transients cannot be ascribed to the contributions from kinetics of a charge-transfer process. Conversely, possible kinetic contributions from OH adsorption, corresponding to the voltammetric peak at E ∼ 0.23 V, are hidden by the presence of these two contributions of opposite sign in transients at E > 0.2 V. The origin of these two contributions of opposite sign is unclear. Interestingly, it is observed that the characteristic response time of the slower contributions increases as the proton concentration in solution is decreased. This behavior is similar to that observed for Pt(111) stepped surfaces.26 Laser-induced

transients for these surfaces exhibit a relatively slow contribution in 0.1 M KClO4 + 1 mM HClO4 solutions at E > 0.35 V, whose relaxation time increases with the step density. Conversely, the double-layer response toward the laser heating was essentially instantaneous at these potentials in 0.1 M HClO4, except for the surfaces with higher step density, like Pt(331) or Pt(S)[2(111) × (110)]. An acid catalysis mechanism was tentatively proposed to explain the faster response in more acidic solutions. The present results show that for Pt(110), which can also be understood as a Pt(S)[(111) × (110)] surface, the response time is relatively slow even in 0.1 M HClO4. As the proton concentration is decreased, the response time gets even higher, thus producing a small contribution to the laser-induced transients in 0.1 M KClO4 + 1 mM HClO4. It is well-known that the structure of water on metal surfaces is given by a delicate balance between the interaction with the metal surface and hydrogen bonding between neighboring water molecules. Accordingly, the relatively slow reorganization of the water adlayer was tentatively proposed to explain the finite response time of laser-induced transients for Pt(111) stepped surfaces. The disruption of the bidimensional order on these surfaces is expected to weaken the bonding energy between adsorbed water molecules, thus resulting in longer relaxation times. The present results on Pt(110) indicate that stabilizing interactions between neighboring water molecules are even weaker on this surface. It is worth mentioning that recent DFT calculations have shown that lateral interactions between neighboring water molecules, as estimated from the difference in adsorption energy of a monolayer and a monomer, are much smaller for Pt(110) than for Pt(111): 0.128 and 0.218 eV, respectively.17,27 As a result of the relatively slow response of water on Pt(110) toward the thermal perturbation at E > 0.25 V, the determination of the value of the temperature coefficient, within this potential region, is uncertain, since this value will be given by the sum of two contribution of opposite sign. The characteristic response time of the slower contribution can be estimated from the fit of the experimental transients to eq 6. According to the above explained interpretation, the coefficients ξ1 and ξ2 would correspond to different components of the double-layer potential temperature coefficient. Figure 6 shows that good agreement between the experimental and simulated curves is obtained by employing constant values for the temperature coefficient of the virtually instantaneous response (ξ1 ) 1 mV/K) and for the slower process (ξ2 ) -1.2 mV/K), and values for the response time, τ, of the slower process that are inversely proportional to the proton concentration: τ ) (0.004/cH+) µs. These results corroborate the hypothesis of an acid-catalysis mechanism, where the reaction rate would be proportional to the proton concentration. 4.1.3. Kinetics of Charge-Transfer Processes in the High Potential Region. Figure 4C compares laser-induced transients measured for Pt(111), Pt(100), and Pt(110) in 0.1 M HClO4 at a potential E ) 0.75 V. Transients for Pt(100) exhibit an essentially monotonous decay, suggesting that kinetics of OH adsorption on Pt(100) sites are not fast enough to significantly contribute to the transients. In the case of Pt(110), transients exhibit a contribution with a finite response time. The origin of this relatively slow contribution is possibly the same as that observed at potentials between 0.35 and 0.65 V (see above). The quantitative characterization of this process at E > 0.65 is complicated by the early oxidation of the surface, which also induces the disordering of the surface structure. Finally, transients for Pt(111) exhibit a nonmonotonous decay, indicating



Figure 6. Comparison of the simulated (A) and experimental (B) potential transients after laser illumination on Pt(110) at E ) 0.35 V. (A) Simulated curves with ξdl ) 1 mV/K, ξads ) -1.2 mV/K, and τ ) (0.004/cH+) µs. (B) Experimental curves for (0.1 - x) M KClO4 + x M HClO4, where x equals (a) 0.1, (b) 10-2, (c) 10-3, and (d) 10-4.

that kinetics of OH adsorption contribute to the transients. Figure 7 compares transients for Pt(111) at E ) 0.75 V in (0.1 - x) M KClO4 + x M HClO4 solutions, where x varies between 10-4 and 0.1. The standard rate constant of OH adsorption on Pt(111) can be thus determined from the fit of these transients by means of Butler-Volmer equations for OH adsorption/desorption (see section 2 for details of this analysis). The simulated curves are included in Figure 7 for the sake of comparison. These curves have been obtained with the same values of the thermal coefficient of the double-layer potential (ξdl ) 1.5 mV/K), the activation energy for OH adsorption (Ea ) 40 kJ mol-1), and the double-layer capacity (Cdl ) 20 µF cm-2 23). The only parameter that was varied was the concentration of proton in solution. It is observed that the rate of OH adsorption is increased as the proton concentration is increased. This result agrees with previous observations from impedance measurements,23,28 where a Warburg resistance was identified within the OH adsorption region, as the proton concentration in solution was decreased. This resistance was assigned to the diffusion of protons in solution. Therefore, the present results support the generally accepted interpretation of OH adsorption through water dissociation in acidic media:

Pt + H2O a Pt-OH + H+ + e-

(8)

The simulated curves are in reasonable agreement with the experimental results, except for data in 0.1 M HClO4. The reason for this deviation is likely related to the fact that the present mathematical treatment, based on Butler-Volmer equations and Frumkin-type interactions, is too simplistic to describe OH

Figure 7. Comparison of the simulated (A) and experimental (B) potential transients after laser illumination on Pt(111) at E ) 0.75 V. (A) Simulated curves with ξdl ) 1.5 mV K-1, Cdl ) 20 µF cm-2, k0 ) 104.7 s-1, and Ea ) 40 kJ mol-1. (B) Experimental curves for (0.1 - x) M KClO4 + x M HClO4, where x equals (a) 0.1, (b) 10-2, (c) 10-3, and (d) 10-4.

adsorption on Pt(111). Indeed, previous impedance measurements showed that OH adsorption on Pt(111) involves at least two phenomena, with strikingly different dynamics.10,28 One of these phenomena involves an effective charge-transfer resistance of Rct ≈ 72 mΩ cm2 and exhibits a diffusion-limited behavior when the proton concentration in solution is decreased to 10-3 M, suggesting that this phenomenon can be attributed to OH adsorption from water dissociation (eq 8). Conversely, the other phenomenon remains virtually instantaneous up to frequencies of 1 MHz. From the simulation of the laser-induced transients, an averaged, approximated value of the standard rate constant is obtained, k0 ≈ 104.7 s-1, corresponding to Rct ≈ 13 mΩ cm2. 4.2. Temperature Coefficient of the Double-Layer Potential and pme of Double-Layer Formation. In the absence of kinetic complications, the temperature coefficient of the doublelayer potential can be determined directly from the laser-induced transients, by considering that eq 1 provides a reasonable estimation of the temperature change induced by the laser pulse. Under these conditions, the effect of the thermodiffusion potential, produced by the temperature gradient in solution, can be easily corrected for, as explained elsewhere.9 On the other hand, when hydrogen or OH adsorption is fast enough to contribute to the transients, the temperature coefficient of the double-layer potential can be still evaluated by means of the above-explained analysis employing Butler-Volmer kinetics. Due to the limitations of this analysis, a pH-independent value for the thermal coefficient of the double-layer potential has been employed, and hence other refinements, such as the correction for the thermodiffusion potential, were inapplicable. Figure 8 shows selected results of the temperature coefficient of the double-layer potential for the three platinum basal planes.

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Figure 8. Temperature coefficient of the double-layer potential (A, B, C) and total charge density data (D, E, F) for Pt(111), Pt(100), and Pt(110) electrodes, as indicated, in (a) 0.1 M HClO4 and (b) 0.1 M KClO4 + 1 mM HClO4.

Figure 9. Values of (A) pme’s, uncorrected (open symbols) and corrected (filled symbols) from the thermodiffusion potential, and (B) pztc’s, for Pt(111), Pt(100), and Pt(110) electrodes in (0.1 - x) M KClO4 + x M HClO4 solutions. Lines are drawn to indicate the tendencies of pztc values, and they are reproduced in the left figure in order to facilitate the comparison with pme values.

The pme of double-layer formation corresponds to the value of potential of zero temperature coefficient of the double-layer potential. Consequently, values of pme can be easily extracted from the results of the temperature coefficient of the doublelayer potential in Figure 8, since they will correspond to the intersection with the y ) 0 axis. Alternatively, values of pme can be also evaluated directly from the laser-pulsed measurements. As a first approximation, the pme can be identified with the value of potential where the laser-induced transients change sign. This is equivalent to assuming that the contribution of the thermodiffusion potential is negligible. Conversely, when pme values are obtained from the condition of zero tempera-

ture coefficient of the double-layer potential, the effect of the thermodiffusion potential can be corrected. This is done by calculating the value of the thermodiffusion potential from the tabulated values of the entropy of transport and transport numbers of the ions composing the solutions.9 This gives values between -0.38 and -0.04 mV/K, when the pH of the solution is varied from pH ) 1 to pH ) 4. Therefore, this procedure provides more refined values of the pme. Figure 9A shows values of pme for Pt(111), Pt(100), and Pt(110) electrodes in (0.1 - x) M KClO4 + x M HClO4 solutions, as determined with and without the correction from the contribution of the thermodiffusion potential. It is observed that the correction from



the thermodiffusion potential produces a small decrease on the pme in the more acidic solutions, while the effect of this correction is essentially negligible for pH > 3. Noteworthy, the pme of double-layer formation essentially corresponds to the potential of water reorientation. Another contribution to the pme comes from the entropy of metal electrons, which is related to the temperature coefficient of the work function of the metal.11 Unfortunately, values of the temperature coefficient of the work function have only been reported for Pt(111), -0.15 mV/K.29 Introduction of a correction for this contribution on Pt(111) would decrease the value of the potential of water reorientation by less than 10 mV. It is reasonable to assume that for Pt(100) and Pt(110) the effect of this correction will be also small. 4.3. Electrostatic and Chemical Effects on the Net Orientation of Interfacial Water on Pt(111), Pt(100), and Pt(110). As a result of the electrostatic interaction of water dipoles with the electric field at the interphase, the net orientation of the interfacial water network is expected to be strongly influenced by the electronic charge density on the metal. Then, if the surface is negatively charged, we can expect the water molecules to be polarized with the positive end closer to the metal, while the opposite is true when the surface is positively charged. The picture is complicated by the existence of a chemical interaction between water molecules and the metal surface and hydrogen bonding within the water adlayer. As explained in the Introduction, the natural net orientation of water on platinum surfaces, in the absence of any electric field, is still under debate. Consequently, the behavior of interfacial water is determined by the balance between electrostatic and chemical interactions. A quantitative characterization into these effects can be obtained from the comparison of the net orientation of interfacial water, as estimated from the temperature coefficient of the doublelayer potential, with charge density data. It should be taken into account, though, that the occurrence of charge-transfer processes on platinum electrodes complicates the very notion of charge density. It becomes necessary to distinguish between the total charge (Q), which includes the charge involved in the adsorption processes, and the free charge (σ), which is the actual charge residing on the metallic side of the electrochemical double-layer. For platinum electrodes in the absence of specific adsorption other than hydrogen and OH, these two quantities are related by30

Q ) σ - FΓH + FΓOH

(9)

where ΓH and ΓOH are the thermodynamic excesses of the adsorbed hydrogen and OH. Consequently, two different kinds of potentials of zero charge (pzc’s) should be distinguished: the pztc and the pzfc. Accordingly, the pme is expected to be closely related to the pzfc, since it is expected that the orientation of water dipoles at the interphase will be very sensitive to the actual excess of electrons at the metal. In addition, the displacement of the pme with respect to the pzfc provides fundamental information on the specific interaction of water molecules with the metal surface. If, in the absence of any electric field, water adsorption is favored with the oxygen toward the metal, then the pme would be lower than the pzfc, since negative charge is needed to compensate the chemical interaction and to achieve the degree of maximum disorder. The opposite would be true for a preferential orientation with the hydrogen toward the metal. On the other hand, the comparison of the pztc with the pme may provide important information on the effect of adsorption

processes on water reorientation. It is expected that the blockage of platinum sites by hydrogen or OH adsorption can notably alter the water-metal chemical interactions. In addition, these specific adsorption processes may also affect the magnitude of the free charge density, and/or screen it, thus also altering the electrostatic interactions. Unfortunately, there is at present no direct experimental method for the determination of the pzfc of platinum singlecrystal electrodes. An estimation of pzfc values for Pt(111) is given below, based on results from CO charge displacement experiments. This approach constitutes the most reliable and precise method for the determination of pztc’s for platinum electrodes at present. It is based on the effective, in situ quenching of the interphase by the adsorption of CO at saturation under potentiostatic conditions. Since CO adsorption takes place without change in its oxidation state, the charge involved in CO displacement experiments is equal to the difference in the total electrode charges after (Qf) and before (Qi) the CO adsorption. As a first approximation, the charge on the COcovered electrode can be neglected, and hence the CO displaced charge provides the total charge on the CO-free electrode. The pztc can be then identified with the potential at which the CO displaced charge is negligible. However, a better estimation of pztc values can be obtained if the value of the charge density on the CO-covered electrode is evaluated. For Pt(111), this can be done from the combination of differential capacity and pzc values of the CO-covered electrode, this last quantity being estimated from work function measurements.31,32 This gives values of Qf between -10 and -15 µC/cm2, resulting in a small increase of ≈30 mV of the corrected pztc values. Unfortunately, for Pt(100) and Pt(110), there is no analogous estimation of the pzc of the CO-covered electrode. However, it is expected that the displacement of the corrected pztc with respect to the uncorrected values would be less significant, because the capacity of these electrodes near the pztc is markedly higher. Total charge density curves for Pt(111), Pt(100), and Pt(110) electrodes in 0.1 M HClO4 and 0.1 M KClO4 + 1 mM HClO4 solutions are included in Figure 8. Total charge density data for Pt(111) in 0.1 M HClO4 was determined by using the value of pztc from CO displacement experiments, further corrected from the remaining charge on the CO-covered electrode, as explained above. Then, total charge density curves in (0.1 - x) M KClO4 + x M HClO4 solutions, where x varies between 10-3 and 0.05, were evaluated from successive integration of voltammetric profiles in the presence and absence of 2.5 × 10-3 M NaCl.30 In this way, the uncertainty in the relative position of the curves is greatly diminished.30 For Pt(100) and Pt(110) electrodes, total charge density curves in 0.1 M HClO4 were obtained by considering that pztc values can be identified with the values of potential where the CO displaced charge is negligible.25,33 Then, total charge density curves for solutions with other proton concentration were determined by considering that at low enough potentials, within the hydrogen adsorption region, charge density values can be considered essentially independent of pH, in agreement with the fact that CO displaced charges at 0.1 V are independent of pH.33 From the total charge density data of the three basal planes in (0.1 - x) M KClO4 + x M HClO4 solutions, values of pztc as a function of the proton concentration can be easily determined from intersection on the Q ) 0 axis. Results are shown in Figure 9B, in order to facilitate the comparison with pme data. Error bars for Pt(111) data correspond to the uncertainty in the estimation of the remaining charge on the CO-covered electrode (Qf) in CO displacement experiments.

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Error bars for Pt(100) and Pt(110) show the effect of the introduction of a reasonable upper-limit value of Qf ) -15 µC/ cm2. The comparison between pztc and pme values will be discussed first for the case of Pt(111). Since this is the most extensively studied basal plane, the present analysis can take advantage of a broader literature. A second advantage is that hydrogen and OH adsorption take place in separated potential regions on Pt(111), while these processes overlap for other crystallographic orientations. This fact greatly simplifies the present analysis, as will be explained below. Finally, the conclusions obtained for Pt(111) will shed light on the understanding of Pt(100) and Pt(110) results. Interestingly, it is observed that values of pztc for Pt(111) in Figure 9B are located within a potential region where the total capacity (as measured from cyclic voltammetry) equals the double-layer capacity.30 This indicates that this region can be considered a double-layer region, and hence values of pztc should correspond to values of pzfc. On the other hand, comparison to pme values for Pt(111) in Figure 9A reveals that pzfc and pme values are very close. This result indicates that water on Pt(111) exhibits a small net orientation in the absence of electric field at the interphase. In this regard, DFT calculations showed that the adsorption energies of water monolayers on Pt(111) in the oxygen-toward-the-metal or hydrogen-towardthe-metal orientations are very similar,17-19 indicating that the actual structure of the water adlayer on the Pt(111) can be a mixture of these configurations. The formation of these more complex, mixed structures was further studied by ab initio molecular dynamic simulations of water on Rh(111), which exhibits very similar characteristics to water on Pt(111), concluding that the orientation of the average dipole moment of the whole water adlayer is approximately in the plane of the surface,19 in agreement with the present results. It should be noted that the above conclusion is in apparent contradiction with our previous explanation for the increase in pme values for Pt(111) by adatom deposition at low-medium coverages, and by the introduction of step sites for Pt(111) stepped surfaces.11,26 Those results were tentatively explained by the disruption of an oxygen-toward-the-metal water preferential orientation on Pt(111) at the pzfc. Accordingly, water reorientation on Pt(111) would take place at potentials lower than the pzfc. The introduction of adatom species or step sites produces a decrease of the size of the Pt(111) domains. This effect would likely disrupt the formation of this preferential orientation, and as a result, pme values would be shifted toward higher potentials, closer to the pzfc. For adatom modification, this shift is around ca. 50 mV, while after the introduction of the steps, a larger shift is observed (up to ca. 250 mV for the surfaces with higher step density). Close inspection of Figure 9 shows that, after correction for the thermodiffusion potential, pme values are slightly lower than pzfc values. Furthermore, if the effects of the temperature coefficient of the work function, and of the temperature coefficient of potential drop across the diffuse-layer, are corrected, it is concluded that the potential of water reorientation is located around 40 mV negative with respect to the pzfc.34 Unfortunately, this shift is smaller than the uncertainty in the determination of pme and pzfc values, and therefore, the existence of an oxygen-toward-the-metal preferential orientation on Pt(111), at the pzfc, cannot be corroborated with the present experimental data. On the other hand, this shift is similar to that observed with adatom deposition at low-medium coverages, but it is clearly smaller than the effect of the introduction

Garcia-Araez et al. of step sites. Consequently, additional effects are probably involved in the behavior of Pt(111) stepped surfaces, but the explanation of these effects is beyond the scope of the present work. In conclusion, it should be emphasized that the present work only demonstrates that the net orientation of the interfacial water network in the absence of electric field at the interface (i.e., at the pzfc) is clearly smaller than that achieved at negative or positive charge densities (i.e., at negative or positive rational potentials). The observed variation of pzfc and pme with the proton concentration in solution also deserves special attention. The lineal fit of corrected pme and pzfc values against the logarithm of the proton concentration gives a slope of -0.060 V/dec. This dependency essentially corresponds to the variation in the values of the RHE reference electrode and indicates that pzfc and pme values on Pt(111) are essentially unaffected by the proton concentration in solution.35 Noteworthy, this is the behavior to be expected in the absence of hydrogen and OH specific adsorption, and indeed, previous studies on fully polarizable electrodes have shown that pzc values are essentially independent of the proton concentration in solution.36 The above conclusion gives support to the identification of pztc as pzfc values for Pt(111). Accordingly, free charge density curves can be determined from integration of double-layer capacities. For this purpose, previously reported double-layer capacities, as determined from the effect of pH on charge density data in (0.1 - x) M KClO4 + x M HClO4 solutions, will be employed.30 With this procedure, the analysis of the waterPt(111) interaction will be extended to the whole available potential window. Results of free charge density curves are shown in Figure 10, plotted against a RHE and a SHE scale. It is observed that free charge density values, plotted against a SHE scale, are essentially independent of the proton concentration within the double-layer region, which includes the pzfc, as above commented. Conversely, within the hydrogen and OH adsorption region, free charge density values strongly depend on pH. This is a consequence of the fact that hydrogen and OH adsorption produce a marked effect on free charge density values, and the potential region where these processes take place (in a SHE scale) shifts with pH ca. 0.06 V/dec.37 Interestingly, it is observed that free charge density values, plotted against a RHE scale, are essentially independent of the proton concentration within the hydrogen and OH adsorption regions. Apparently, the overall free charge density on the platinum surface is dominated by the extent of hydrogen or OH adsorption, and the absolute value of the applied potential (measured vs an external reference electrode) produces a smaller effect. Remarkably, the behavior of the temperature coefficient of the double-layer potential of Pt(111), as a function of potential (Figure 8A), closely parallels that of free charge densities (Figure 10B). This correspondence reinforces the idea that the net orientation of water on Pt(111) is indeed dominated by the free charge density. In conclusion, the net orientation of interfacial water for Pt(111) is essentially driven by electrostatic interactions. Chemical interaction between water molecules and the Pt(111) surface produce minor effects, as evidenced by the fact that pzfc and pme values are essentially equal, and the fact that the net orientation of interfacial water within the whole available potential window faithfully parallels the variations on the free charge density.38 At this point, the comparison of pztc and pme values for Pt(100) and Pt(110) will be addressed (Figure 9). It is observed that pztc values for Pt(100) and Pt(110) remain essentially



Figure 10. Free charge density data for Pt(111) in (0.1 - x) M KClO4 + x M HClO4 solutions, where x varies between 10-3 and 0.1. Arrows indicate the direction of increasing proton concentration.

unaffected by the proton concentration (the variation against the logarithm of the proton concentration is -0.008 and -0.009 V/dec, respectively). This contrasts with the behavior exhibited for Pt(111). The behavior of pztc values for Pt(100) and Pt(110) is a consequence of the fact that these pztc’s are located in a potential region where hydrogen and OH coadsorption takes place. Under these conditions, total charge density data is largely dominated by the extent of hydrogen and OH adsorption (see eq 9). Since on a RHE potential scale these adsorption processes are expected to be little affected by pH variations, in the absence of other coadsorption phenomena, it is logical that pztc values also remain essentially constant. Interestingly, pme values for Pt(100) and Pt(110) shift with pH less than the 0.06 V/dec observed for Pt(111) (the variation of corrected pme values against the logarithm of the proton concentration is -0.030 V/dec for Pt(100) and -0.015 V/dec for Pt(110)). It is worth recalling, once again, that pme values are expected to be close to pzfc values, and their difference is related to water-metal specific interactions. Therefore, the observed behavior suggests that pzfc values for Pt(100) and Pt(110) should follow a similar dependence with the proton concentration. Accordingly, the behavior of pzfc values for Pt(100) and Pt(110) would be similar to the behavior of charge density values on Pt(111) within the hydrogen and OH adsorption region (Figure 10B). In that situation, the extent of hydrogen and OH adsorption dominates the magnitude of the free charge density, and as a result, free charge density values remain little affected by the proton concentration on a RHE scale. While pztc, pzfc, and pme values were found to be essentially equal for Pt(111); for Pt(100) and Pt(110), it is observed that pztc values are somewhat higher than pme values. The interpretation of this result without the knowledge of pzfc values for Pt(100) and Pt(110) is unclear. The determination of pzfc values for Pt(100) and Pt(110) requires an independent method which could selectively probe the free charge density, in contrast to the CO displacement method and laser-pulsed measurements, which are also sensitive to charge-transfer processes and water-metal chemical interactions, respectively. The relative position of the pme values can be compared with the values of work function to obtain more information about the possible position of the pzfc’s. In view of Figure 9, the difference between pme values for Pt(111) and the other two

basal planes is ca. 0.1 V for Pt(100) and ca. 0.3 V for Pt(110). These numbers are close to the difference of work function values, with respect to the work function of Pt(111): 0.09 eV for Pt(100) and 0.26 eV for Pt(110).39,40 This suggest that pme’s for Pt(100) and Pt(110) are close to pzfc’s, despite the presence of coadsorbed hydrogen and OH at these potentials. Accordingly, the net orientation of the water network, in the absence of electric field at the interphase, would be small. This agrees with DFT calculations of water monomers on the three platinum basal planes, concluding that water adsorbs with its molecular plane nearly parallel to the surface.41 Finally, the fact that pztc values are somewhat higher than both pzfc and pme values would imply that, at these potentials, the surface excess of hydrogen is somewhat higher than that of OH (see eq 9). In addition, this conclusion explains that pztc values for Pt(100) are higher than those for Pt(111) at pH < 2, despite the fact that work function values for Pt(100) are lower than those for Pt(111). 5. Conclusions Laser-induced potential transients for Pt(111), Pt(100), and Pt(110) in perchloric acid solutions have been analyzed in detail. First, the contribution from kinetics of charge-transfer processes [namely, hydrogen adsorption on the three basal planes and OH adsorption on Pt(111)] has been characterized with the help of Butler-Volmer equations, thus providing values of the standard rate constant of these processes [k0 ≈ 104.5, 103.4, and 104.8 s-1 for hydrogen adsorption on Pt(111), Pt(100), and Pt(110), respectively, and k0 ≈ 104.7 s-1 for OH adsorption on Pt(111)]. In addition, this analysis has allowed the separation of the double-layer response from the contribution from charge-transfer processes. In this way, values of the temperature coefficient of the double-layer potential for the three basal planes have been estimated. From these results, it is concluded that in all cases, water exhibits a net orientation with the hydrogen toward the metal at low enough potentials. Then, when the applied potential is increased, the oxygen toward the metal orientation is favored. A clear effect of the surface structure on the potential of water orientation is observed, being 0.37 for Pt(111), 0.33 for Pt(100), and 0.14 V for Pt(110) in 0.1 M HClO4 solution. The potential of water reorientation also exhibit a different pH dependency

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for the three basal planes, shifting 0.060 for Pt(111), 0.030 for Pt(100), and 0.015 V/dec for Pt(110). Comparison of these results with charge density data sheds light into the chemical and electrostatic effects governing the reorientation of the interfacial water network. It is concluded that water on Pt(111) exhibits a small net orientation in the absence of electric field at the interphase. On the other hand, the lack of free charge density data for Pt(100) and Pt(110) limits the interpretation of the experimental results. However, the agreement between the relative position of values of the potential of water reorientation and work functions, for the three basal planes, suggests that the same situation holds for Pt(100) and Pt(110). Acknowledgment. Financial support from the MEC (Spain) through project CTQ 2006-04071/BQU is gratefully acknowledged. N.G. thanks the MEC (Spain) for the award of a postdoctoral fellowship. Appendix 1 The thermodynamic characterization of charge-transfer processes on Pt(111), Pt(100), and Pt(110) is based on a generalized adsorption isotherm.42-44 This method of analysis employs voltammetric data as a function of the temperature, as recorded in an isothermal electrochemical cell. For Pt(111), hydrogen and OH adsorption take place in separated potential regions. Therefore, hydrogen and OH coverages can be easily calculated from integration of the voltammograms, after correction for the double-layer charging as a constant baseline.43,44 For this calculation, a saturation charge density of 240 µC cm-2 for hydrogen adsorption and 110 µC cm-2 for OH adsorption is considered. Conversely, for the analysis of hydrogen adsorption on Pt(100) and Pt(110), the experimental data has first to be corrected from the OH adsorption contribution. For Pt(100), results from CO displacement experiments were employed to estimate the charge corresponding to OH adsorption. Then, the contribution from OH adsorption was subtracted by using a Frumkin isotherm.45 The resulting voltammetric profiles exhibit a main feature between 0.25 and 0.45 V, corresponding to the main hydrogen adsorption on (100) sites.25 The charge under this peak is 180 µC cm-2, corresponding to a saturation hydrogen coverage of 0.85. As discussed elsewhere,25 these results of the saturation coverage, smaller than unity, are probably due to the intrinsic presence of defects on this type of electrode surface, probably produced during the preparation of the electrode, which includes the cleaning step by flame annealing and quenching in water in equilibrium with H2 + Ar. The adsorption of hydrogen on the defects takes place at E < 0.2 V.25 As a result, the situation is equivalent to considering an ideal Pt(100) bidimensional domain which has a lower effective area than the geometrical electrode surface. Consequently, the study of hydrogen adsorption on (100) sites was performed employing results of hydrogen coverages scaled to the unity saturation coverage. On the other hand, the characterization of hydrogen adsorption on Pt(100) defects is not attainable, as a result of the strong overlapping of this process with hydrogen evolution. For Pt(110), the correction of the voltammetric profile from the contribution of OH adsorption is not trivial.42,43 In the present calculation, the voltammetric peak centered at E ≈ 0.14 V will be ascribed to only hydrogen adsorption, and competition with OH adsorption within this potential region will be neglected. The charge under this peak is 120 µC/cm2, and hence hydrogen coverages were calculated by using a scaling factor of 120/150 ) 0.8.

Garcia-Araez et al. Once the coverages of charge-transfer species are determined, then formal Gibbs energies for hydrogen or OH adsorption are easily calculated from

( 1 -θ θ ) + zRT ln c

∆Gformal ) -zFE - RT ln

H+

) ∆G0 + g(θ)

(a1.1) where z ) 1 for hydrogen and z ) -1 for OH adsorption; E are potential values measured vs SHE. (In previous works based on the application of the generalized isotherm, the term corresponding to the proton concentration is absent, and as a result the potential values are given vs RHE.) The term g(θ) equals g(θ) ) ωθ for a Frumkin isotherm. Since the plot of ∆Gformal vs θ is lineal for all charge-transfer processes studied here (within an appropriate coverage range), the applicability of Frumkin type equations for the simulation of the laser-induced transients is corroborated. The slope of the ∆Gformal vs θ plot provides the value for the interaction parameter, ω, and the value of the intercept provides the standard Gibbs energy of adsorption, ∆G0. The reliability of these results can be tested by comparing the corresponding adsorption pseudocapacity with the experimental voltammograms. For this purpose, the adsorption pseudocapacity is calculated as a parametric function of the potential

θ ∆G0 RT ωθ RT + ln ln cH+ zF zF zF 1-θ F qML -1 1 ω RT C) ) qML ∂E zF zF θ(1 - θ) ∂θ T

(

E)-

( )

)

[

]

(a1.2) (a1.3)

where qML is the charge density corresponding to the saturation coverages. The results are compared to the experimental voltammograms in Figure 3. Reasonable good agreement is obtained between the experimental and calculated curves in 0.1 M HClO4, giving support to the use of this thermodynamic data for the simulation of the laser-induced transients. Some deviations are observed, though, for Pt(100) and Pt(110) at pH ) 3, but under these conditions, hydrogen adsorption kinetics are too slow to contribute to the laser-induced transients, and therefore these deviations will not affect the results of the simulation of the laser-induced transients. Finally, the formal entropy for hydrogen or OH adsorption is obtained from

∆Sformal ) -

(

∂∆Gformal ∂T

)

θ

(a1.4)

and extrapolation to zero coverage provides the value of the standard entropy for hydrogen or OH adsorption under isother0 . Essentially the same results are mal conditions, ∆S isothermal obtained from the temperature dependency of the standard Gibbs 0 energy of adsorption: ∆S isothermal ) -((d∆G0)/(dT )). It should be noted that isothermal thermodynamic parameters are obtained as a result of the fact that the present analysis employs experimental voltammograms recorded in an isothermal cell (i.e., where the working and reference electrodes were maintained at the same temperature). However, laser-induced transients are recorded in a nonisothermal cell, since the temperature of the working electrode is changed by laser pulses, but the temperature of the reference electrode remains unchanged. Therefore, for the simulation of the laser-induced transients, the thermodynamic parameters corresponding to a nonisothermal cell are required. The conversion is performed with the use of the temperature coefficient of a 0.1 M HCl RHE (+0.640 mV/K)46

Potential-Dependent H2O Orientation on Pt Surfaces 0 0 ∆Snonisothermal ) ∆Sisothermal + z 61.75 J/mol K


(a1.5)

It should be noted that the above explained analysis gives 0 ≈ -40 J/mol K for hydrogen adsorption values of ∆Snonisothermal on Pt(110), but this results does not produce a reasonable fit of the laser-induced transients at E ) 0.1 V. Instead, the simulated curves are in good agreement with the experimental transients 0 ≈ 15 ( 5 J mol-1 K-1. Similarly, for for values of ∆Snonisothermal OH adsorption on Pt(111), the thermodynamic analysis gives 0 ≈ -97 J/mol K. However, in order to obtain a ∆Snonisothermal reasonable fit of the experimental transients measured at E ) 0 ≈ -50 ( 5 J mol-1 K-1 has to 0.75 V, a value of ∆Snonisothermal be employed. These discrepancies are related to the fact that interaction parameter, ω, exhibits a noticeable dependency on temperature for hydrogen adsorption on Pt(110) and OH adsorption on Pt(111) [dω/dT equals -10, -18, and -102 J mol-1 K-1 for hydrogen adsorption on Pt(111), Pt(100), and Pt(110), respectively, and dω/dT ) -117 J mol-1 K-1 for OH adsorption on Pt(111)]. As a result, the effective entropy of the adlayer, used for the simulation, includes also a contribution due to the temperature dependence of the interaction parameter. The influence of this contribution on the shape of the transient is better understood after the mathematical treatment explained in Appendix 2. Appendix 2

dθ 0.5F 0.5F ) V0 exp -z η - exp z η dt RT RT

[ (

)

(

)]

(a2.1)

where η is the displacement of the electrode potential from its equilibrium value, η ) E - Eeq. The exchange rate is related to the standard rate constant by

V0 ) k0√(1 - θ)θcH+

(a2.2)

For small thermal perturbations, η is small, and the exponential function can be approximated by a Taylor series, giving

dθ F ≈ qMLV0 η dt RT

(a2.3)

The value of ∆E is given by eq 5. The value of∆Eeq can be approximated by

∆Ee ≈

( )

( )

∂Ee ∂Ee ∆T + ∆θ ∂T θi ∂θ Ti

(a2.4)

where

( ) ( ) )

θi

∂E0 ∂T

( ) ∂Ee ∂θ

dθ ≈ -zC1∆T - C2∆θ dt

(a2.7)

where

C1 ) C2 )

Fk0√(1 - θ)θcH+ (bbl - gT) RT

(

(a2.8)

)

Fk0√(1 - θ)θcH+ qML - zgθ RT Cdl

(a2.9)

If the temperature dependence of k0 is neglected, eq a2.7 can be solved analytically, giving eq 6 as the solution to the variation of potential

∆E ) ξ1∆T(t) +

ξ2 τ

∫0t ∆T(t′) exp( t -τ t′ ) dt′

(a2.10)

where ξ1 ) ξdl, ξ2 ) -(qML/Cdl)(C1/C2), and τ ) C2-1. References and Notes

For small perturbations, Butler-Volmer equations for the laserinduced transients (eqs 1-5) can be reduced to the simplified eq 6. In the following, the approximations involved in this derivation will be explained. For this purpose, the rate eqs 2-4, expressed as a function of the standard rate constant, k0, will be first rewritten as a function of the exchange rate, V0

∂Ee ∂T

included this term, the ((∂E0)/(∂T))θi should be understood as an effective parameter. This explains the discrepancy between the ∆S0 derived from the thermodynamic analysis and the values used in the simulation of the laser-induced transients. Combination of the above equations gives the following differential equation

+

θi

)Ti

(

( )

)

(1 - θi) cH+ R R R ln + ln 0 - gθi ) zF θi F zF c gT (a2.5)

(

)

RTi gRTi 1 ) gθ zF θi(1 - θi) zF

(a2.6)

where g ) ω/(RT) and ((∂E )/(∂T))θi ) (∆S )/(zF). In eq a2.5, the parameter g has been considered independent of temperature. If the temperature dependence of g needs to be considered, an additional term should be added. Since we have not explicitly 0

0

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(28) Sibert, E.; Faure, R.; Durand, R. J. Electroanal. Chem. 2002, 528, 39–45. (29) Kaack, M.; Fick, D. Surf. Sci. 1995, 342, 111–118. (30) Garcia-Araez, N.; Climent, V.; Herrero, E.; Feliu, J.; Lipkowski, J. Electrochim. Acta 2006, 51, 3787–3793. (31) Weaver, M. J. Langmuir 1998, 14, 3932–3936. (32) Cuesta, A. Surf. Sci. 2004, 572, 11–22. (33) Climent, V.; Go´mez, R.; Orts, J. M.; Aldaz, A.; Feliu, J. M. In The Electrochemical Society Proceedings (Electrochemical Double Layer); Korzeniewski, C., Conway, B. E., Eds.; The Electrochemical Society, Inc.: Pennington, NJ, 1997; Vol. 97-17, pp 222-237. (34) Garcia-Araez, N.; Climent, V.; Feliu, J. In preparation, 2009. (35) Previous studies on the effect of pH on pztc values for Pt(111), as obtained from CO displacement experiments in phosphate buffer solutions, showed that pztc values remained essentially pH independent of a RHE scale.33 This result is in contrast with the behavior observed here, with pztc values for Pt(111) shifting ca. -0.06 V/dec with the logarithm on the proton concentration. The discrepancy has two origins. First, an improved evaluation of the effect of the proton concentration on values of potential at constant charge has been employed here, based on a careful comparison of charge density data of Pt(111) in (0.1 - x) M KClO4 + x M HClO4 solutions with and without the addition of 2.5 · 10-3M NaCl.30 Second, more refined values of pztc are considered here, since values of potential of zero CO-displaced charge are corrected for the remaining charge on the COcovered electrode.31 This correction displaces pztc values to the doublelayer region, as evidenced by the fact that total capacities are equal to doublelayer capacities at these potentials.30 Previous studies have shown that, within this region, values of potential at constant charge, measured vs SHE, are essentially independent of the proton concentration in solution.30

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JP900792Q

Potential-Dependent Water Orientation on Pt(111), Pt(100), and Pt

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