J. Phys. Chem. C 2008, 112, 11427–11432
11427
Dynamics of Double-Layer Restructuring on a Platinum Electrode covered by CO: Laser-Induced Potential Transient Measurement Akira Yamakata* and Masatoshi Osawa Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan ReceiVed: March 1, 2008; ReVised Manuscript ReceiVed: May 6, 2008
The dynamic behavior of water molecules on CO-covered Pt electrode has been studied by laser induced temperature jump method. A 532-nm laser pulse quickly heats up the interface (∆T ≈ 90 K) and perturbs the orientation of water, which results in a negative jump of the rest potential. The potential is relaxed with cooling the substrate (∼20 µs). This behavior is identical to that on the CO-free surface, but the amplitude of the negative jump is more than 30 times larger on the CO-covered surface. This large difference can be ascribed to the differences in the electric field at the interface and also in the interactions of water-Pt and water-CO; water is more flexible on CO. At high laser power densities exceed a threshold, the negative potential jump is followed by a slightly slower (∼70 µs) irreversible potential shift toward the positive direction. Infrared reflection absorption spectroscopy and picosecond two-pulse correlation experiments reveal that the subsequent positive shift arises from thermally activated desorption of CO. Adsorption of water and restructuring of the electric double layer after CO desorption is responsible to the positive shift. The rate of restructuring becomes slower as the initial potential becomes lower due to the increase in the electric field and the number of hydrated cations. The intense field and the hydrogen-bonding affect water structure in a long-range and enhance the collective motion of the water layers. 1. Introduction Water molecules play an important role in biological, chemical and physical processes. It is especially true in the electrochemical systems (liquid/solid interface). The static structure and dynamics of water control the electrochemical processes such as the rate of mass transport and electron transfer reactions.1–3 Water has a large dipole moment and the fluctuation of water structure changes the chemical potentials of the hydrated molecules and thus induces the chemical reactions. The behavior of water molecules depend on the electrostatic field at the interface, interactions with the surface or adsorbates, and hydrogen-bonding with the hydrated ions, etc. Thus the effects of these forces on the behavior of water molecules are important for full understanding of the electrochemical processes. The static structure of the water molecules on electrode surfaces has been studied by X-ray scattering,4,5 IR absorption,6–10 and sum-frequency generation (SFG)11,12 on Ag, Au and Pt surfaces. It has been reported that the orientation of water molecules changes from “hydrogen-down” to “hydrogen-up” as the potential is increased across the potential of zero charge (pzc); water is aligned directing its dipole moment opposite the electric field.4–12 Water molecules in the electric field produced by surface charge and ions in the solution (φcharge) supports huge (107 V cm-1) potential drop due to its large dipole moment (φdipole). The potential between the electrode and the solution (φM-S) is given by the potential drop by water as eq 1,1
φM-S ) φcharge + φdipole
(1)
The term of φdipole also contains the surface dipole created by the spillover electrons, but this contribution is much smaller than that by water molecules. Thus, water is the most important component to constitute the electric double layer. * Corresponding author. E-mail:
[email protected].
Laser induced temperature jump (T-jump) have been expected to be a promising method to study the dynamics at the electrochemical interfaces.13–20 Rapid T-jump by a pulsed laser quickly shifts the thermo-dynamical equilibrium and enables us to study the dynamics. For redox couples at the electrochemical interface, the change of temperature changes their equilibrium potentials as evident from the Nernst equation.1 This technique was also successfully used to study fast electron transfer dynamics of redox couples attached at the interface,21 which is not readily accessible by conventional potential control approaches due to the time-constant of the double layer charging (in the order of 10-4-10-3 s depending on the size of the electrode). The T-jump also changes the structure of the water molecules in the electric double layer, and changes the electrode potential (E-jump). Laser heating of the water layer randomizes its orientation, which results in the decrease of |φdipole| and changes φM-S as derived from equation 1. Climent et al. have studied the laser induced potential jump (E-jump) on clean Pt17 and Au15,18 electrodes, and showed that the potential is jumped to the negative direction at the negatively charged surface and to the positive direction at the positively charged surface. This effect was used to examine the pzc,18 entropy of double layer formation,15,17 double layer capacitance,19 and desorption/ adsorption of cations20 and hydrogen15 on clean surface, etc. Recently, we demonstrated that the use of picosecond timeresolved IR absorption spectroscopy coupled to the E-jump can overcome the time-resolution of the conventional electrochemical measurement (less than submicroseconds) and enables us to examine the picosecond dynamics.16 In the present study, we have examined the behavior of water molecules on CO-covered Pt electrode as a model system to study the behavior of water during reactions. The behavior of water molecules on the adsorbed molecular layer should be different from that adsorbate-free surface due to the different water-molecule and water-surface interactions; it depends on
10.1021/jp8018149 CCC: $40.75 2008 American Chemical Society Published on Web 07/03/2008
11428 J. Phys. Chem. C, Vol. 112, No. 30, 2008
Yamakata and Osawa
the nature of the adsorbed molecules (hydrophilic or hydrophobic). Adsorption and desorption of molecules should also drastically change the structure of the electric double layer. However, the behavior of water molecules has not been fully understood, yet. In this study, we found that the thermal orientational change of water molecules is more than 30 times larger on CO-covered surface than on the clean surface. Additionally CO desorption at high laser power densities gives further restructuring of the electric double layer in the time range of 70 µs. The rate of the latter restructuring depends on the initial potentials. The mechanisms of these phenomena are discussed. 2. Experimental Section The electrolyte solution, 0.1 M HClO4, was prepared from analytical grade HClO4 (Wako Chemicals, Tokyo) and Millipore water (>18 MΩ), and deaerated with Ar gas before use. The electrochemical cell used for laser-induced experiments was a three-electrode glass one with a flat window for introducing pulsed laser beam. The working electrode was a polycrystalline platinum plate (3 × 3 × 0.03 mm) mirror polished with 1 µm alumina slurry. One side of the electrode, which was not illuminated by the laser beam, was insulated by a resin to remove the electrochemical interference from this side. Potential was controlled and measured against an Ag/AgCl electrode with a homemade potentiostat that has a fast voltage follower (∼1 ns). The output signal from the potentiostat was amplified by a preamplifier (NF circuit, NF5307) at 1 MHz bandwidth to increase the S/N ratio. The risetime of the signal was limited to ∼300 ns. The cell constant was measured by a commercially available potentiostat (ALS Inc., Model 644). Prior to each measurement, the electrode surface was cleaned by repeating oxidation and reduction in the electrolyte solution. Then CO was adsorbed on the electrode at 0.1 V by bubbling the solution with CO gas. The laser-induced measurements were carried out in the COsaturated solution with the second harmonic output from a Spectra-Physics GCR-170 Nd:YAG pulse laser (532 nm, 6 ns duration, 10 Hz repetition rate). Temporal profiles of the potential jumps were recorded with a digital oscilloscope (Iwatsu, DS-4262). In the measurement, the counter electrode was mechanically disconnected 2 ms before the laser irradiation and reconnected 10 ms after. The disconnection is indispensable to prevent the charge flow and to observe the temporal profiles of the potential jump as has been suggested by Climent et al.17 In two-pulse correlation experiments22 for examining the mechanism of laser-induced CO desorption, a Continuum PY61C Nd:YAG laser (532 nm, 35 ps duration, 10 Hz repetition rate) was used. The time-resolution of this measurement is determined by the width of the pulse (35 ps). The two-pump pulses were irradiated on the surface at the normal incidence. The polarizations were perpendicular to each other to prevent a coherent artifact coming from interference effects.23 Maximum amplitude of the potential jumps were recorded by a gated integrator/boxcar averager (Stanford Research Systems, Inc., SR250) as a function of a time delay between the two pump pulses. Output signals from the boxcar averager were directly recorded as a voltage and not calibrated to the actual amplitude of the potential change. The beam sizes of these lasers were 9 mm, which was large enough to illuminate the full area of the electrode. To examine the desorption of CO by the laser pulse irradiation, infrared reflection absorption spectroscopy (IRAS) was also used. The spectra were recorded on a Bio-Rad FTS-
Figure 1. Potential transient induced by a 532 nm laser pulse (6 ns, 35 mJ cm-2, 10 Hz) on Pt electrode in 0.1 M HClO4 solution at -0.2 V vs Ag/AgCl. Blue and red curves are measured in the absence and presence of adsorbed CO, respectively.
60A/896 FT-IR spectrometer equipped with an MCT detector and operated at a 4-cm-1 resolution. The electrochemical cell for IRAS was a glass one with a 60° beveled CaF2 window, the basic design of which was similar to that reported in the literature.24,25 A Pt disk (10 mm in diameter and 1 mm thickness) served as a working electrode in this experiment. The measurements were carried out with p-polarized IR beam after removing dissolved CO by Ar gas pursing. 3. Results and Discussion 3.1. Laser Induced Potential Jump at Pt Electrode with and without Adsorbed CO. Figure 1 shows typical potential transients induced by a 532 nm laser pulse at 35 mJ cm-2 at an initial potential of -0.2 V vs Ag/AgCl. In the absence of adsorbed CO (blue curve), the potential jumps to the negative direction and recovers within ∼20 µs consistent with the report by Climent et al.15 The E-jump has been ascribed to the orientation change of water molecules on the surface. On the other hand, when CO was adsorbed on the surface, the behavior is different; the potential first jumps negatively and then shifts positively. It is also worth noting that the amplitude of the initial negative E-jump on the CO-covered Pt is over 30 times larger than that on the clean Pt. 3.2. Laser Power and Electrode Potential Dependence of the Potential Jump. To investigate the E-jumps on the COcovered Pt in more detail, laser power and initial potential dependences were examined. The results of the power dependence are shown in Figure 2A. At low power densities up to 20 mJ cm-2, the potential changes only to the negative side. The following positive shift is observed only for laser power densities higher than 25 mJ cm-2. The positive shift reaches a maximum at about 70 µs and decays very slowly. The maximum and minimum of the potential shift for each laser power density are summarized in Figure 2B. This figure shows that amplitude of the negative jump linearly increases up to the power density of 30 mJ cm-2, while the following positive shift increases nonlinearly. The decrease of the negative jump for laser power densities lager than 30 mJ cm-2 is due to the superposition of the positive shift. The power dependence suggests that at least two independent processes (or mechanisms) contribute to the potential transient. Figure 3A shows the initial potential dependence on the jump for 50 mJ cm-2. The positive shift is a great function of the initial potential and becomes larger as the initial potential is lowered, while the initial potential dependence for the initial negative jump is much less significant up to 0.4 V at which adsorbed CO is electrochemically oxidized. The different
Dynamics of Double-Layer Restructuring
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11429 in Scheme 1 (left and middle panels). Since the pzc of the COcovered Pt surface locates at 1.1 V vs RHE (reversible hydrogen electrode, which is ∼0.9 V vs Ag/AgCl)26 and the surface is charged negatively at the potentials examined in the present study, water molecules on the CO adlayer are oriented by their hydrogen atoms pointing toward the negatively charged surface due to the electrostatic interaction with the field (that is, φdipole is positive). If the interface is heated and the orientation of interfacial water molecules is randomized, φdipole is reduced and hence the rest potential is negatively shifted as schematically shown in the Scheme 1. The averaged angle of the orientation change can be estimated by the potential drop across the interfacial water layer given by
φdipole ) -
Figure 2. Laser power dependence of the potential transient. (A) Potential transient measured by a 532 nm laser pulse (6 ns duration, 10 Hz) at -0.2 V vs Ag/AgCl. The laser power is indicated in the figure. Gray straight lines and arrows show the potential and the delay time at which the maximum is reached. (B) Power dependence of the potential transient. Peak top potential of the negative jump (9) and the positive shift at ∼80 µs (b).
Nµ sinθ ε0
(2)
In eq 2, N is the number of water molecules per unit area (1.1 × 1019 m-2), µ is the dipole moment of a water molecule (6.24 × 10-30 C m), 0 is the permittivity of the vacuum (8.85 × 10-12 J-1 C2 m-1), and θ is the averaged angle between the dipole moment and the direction parallel to the surface.26 According to this equation, the observed potential jump on the order of 10 mV can be achieved by a very slight orientation change less than 0.1°. Note that the rate of the E-jump is governed by the reorientation of water and should be much faster than the double layer charging (time constant of which RsolCdl was ∼170 µs for the clean surface, where Rsol and Cdl are the solution resistance and double-layer capacitance, respectively). We have reported that the E-jump occurs within ∼200 ps by means of picosecond time-resolved IR absorption spectroscopy.16 The thermal contribution to the E-jump is estimated by a simple estimation of temperature at the interface. The maximum temperature change (∆Tm) achieved at the end of the laser pulse is given by13
∆Tm )
2(1 - R)I
√πκcd + √πκsolcsoldsol
√t0
(3)
where κ, c, d, and κsol, csol, dsol are the thermal conductivity, thermal capacity, and density of platinum and the aqueous electrolyte solution, respectively, R is the reflectivity of the surface (0.56 for Pt in water), I is the power density of the laser, and t0 is the laser pulse width (6 ns in the present case). By putting standard values into eq 3, a maximum temperature change of ∆Tm ≈ 90 K is calculated for the laser power of 20 mJ cm-2. The temperature starts to decrease soon after the pulse irradiation, which is represented by,14
1 ∆T(t) ) ∆Tm 2 Figure 3. Initial potential dependence of the potential jump induced by 532 nm laser pulse (6 ns pulse duration, 50 mJ cm-2) on the CO covered Pt electrode. (A) Potential transient measured at a power density of 50 mJ cm-2. The initial potential is indicated in the figure. Gray straight lines and arrows show the potential and the delay time at which the maximum is reached. (B) Peak top of the negative jump (b) and positive shift (9). In the case of negative jump, the curves were measured at a power density of 15 mJ cm-2 to avoid the superposition of the positive shift.
potential dependences also suggest different mechanisms for the initial negative and subsequent positive potential changes. 3.3. Mechanism of the Initial Negative E-Jump. Orientation change of water molecules induces an E-jump as described
t0 t
(4)
This equation indicates that the ∆T after 300 ns (time-resolution of this potential measurements) becomes ∼6.5 K, and the system is cooled down to the ambient temperature within 20 µs. The transient profile and the linear laser power dependence of the negative jump shown in Figures 1 and 2, respectively, are well correlated with the change in temperature. The negative potential jump can be caused by the T-jump on both CO-free and CO-covered surfaces, but the amplitude of the negative jump on the CO-covered Pt surface was more than 30 times larger than on the clean Pt electrode. This large difference of the potential jump can be ascribed to the differences in the electric field at the interface and also in water-Pt and water-CO interactions. The potential differences
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SCHEME 1: Schematic Representation of the Laser-Induced Potential Changes Caused by the Structural Change of the Electric Double Layer (OM-S, Which Is Negative in This Case)a
a φM-S is determined by the electric field created by the surface charge and ions (φcharge; negative in this case) and the potential drop at the water layers (φdipole; positive in this case). A laser induced T-jump randomizes the orientation of water to decrease φdipole, which brings a negative E-jump under the constant surface charge and ion distribution (i.e., φcharge is constant). More intense laser irradiation causes thermal desorption of CO, which induces adsorption of water, and results in the increase of φdipole.
from pzc (electric field intensity) are -0.2 and -1.1 V for COfree (pzc ≈ 0 V27) and CO-covered (pzc ≈ 0.9 V26) surfaces at the examined potential (-0.2 V, in Figure 1); the electric field for CO-covered surface is ∼5 times larger. Thus the potential jump can be expected ∼5 times larger for CO-covered surface when we assume the jump linearly depends on the electric fields. In spite of this potential difference, the potential jump is still ∼5 times larger on CO-covered surface. This difference is responsible to the difference in the water-Pt and water-CO interactions. The motion of water is restricted by the interaction with the electrostatic field and the surface (or adsorbates), and also water-water interaction (hydrogen bonding) that are largely different on the CO-free and CO-covered surfaces. Water can directly interact with the clean surface via oxygen lone pair, which has been exemplified by the remarkable red-shift of the bending mode of water.7,9,10 On the other hand, strongly adsorbed CO molecules prevent the direct interaction of water molecules with the surface and also affects hydrogen bonding among water molecules. Recent IR studies showed that water molecules on the CO adlayer are free from hydrogen bonding.10 Thus water molecules are more flexible on the CO adlayer and can be reoriented more easily by the T-jump. The T-jump also may change the ratio of on-top and bridged CO slightly. This phase transition also may change the structure of the water layers. The larger amplitude of the E-jump on the CO-covered surface is explained in this way. This tendency is also suggested by the temperature dependent potential drop measurements across the water layer.26 3.4. Desorption of Adsorbed CO by Laser Pulse Irradiation. Desorption of CO from the surface by the laser pulse irradiation has been reported in similar experiments under UHV.22 We examined this possibility at the Pt/solution interface by IRAS (Figure 4). Spectra were measured after pursing dissolved CO in the solution by Ar to minimize readsorption of CO from the solution. Figure 4A is the spectrum of CO measured at -0.2 V before laser irradiation and referenced to the CO-free surface, while Figure 4B shows the spectra recorded at the same potential after laser irradiation. To make clear the spectral changes, the latter spectra were referenced to the spectrum before the irradiation (Figure 4A). The band centered at 2074 cm-1 is assigned to CO adsorbed on-top site. After 20 mJ cm-2 pump pulse irradiation for 40 min, for which only negative jump occurs, no change occurs in the adsorbed CO. However, at the pump power of 40 mJ cm-2 for which the positive potential shift occurs after the negative jump, the difference spectra show the decrease of adsorbed CO; the absorption intensity of CO decreased by 35% after 40 min irradiation. Since desorbed CO can be readsorbed on the surface,28 this measurement cannot be used for quantitative analysis of the CO desorption.
Figure 4. (A) IRA spectrum of adsorbed CO on Pt electrode at -0.2 V vs Ag/AgCl. (B) Difference IRA spectra before and after the 532 nm light at the laser power density of 20 or 40 mJ cm-2. Irradiated time is indicated in the figure.
Nevertheless, the result clearly shows that the desorption of CO is responsible for the subsequent positive potential shift and not to the initial negative jump. 3.5. Mechanism of the CO Desorption-Induced Positive Potential Shift. Adsorption and desorption of molecules should bring a drastic change in the structure of the electric double layers. The observed positive shift is due to the desorption of CO as evidenced by the IRAS measurements. The desorption can drastically change the rest potential, as schematically illustrated in the Scheme 1 (right panel). When adsorbed CO desorbes from the surface, vacant sites are produced on which water molecules can adsorb. Water molecules adsorb with the hydrogen atoms pointing toward the negatively charged surface. The potential drop at the interface (φdipole; positive in this case) is largely increased by replacement of CO with water due to the large dipole moment of the latter (16 times larger, 6.24 × 10-30 C m/0.39 × 10-30 C m). The increase of φdipole decreases |φM-S| under the constant φcharge. The decrease of |φM-S| brings a potential shift to the positive direction. Adsorption of ions also can change the potentials via the charge transfer with the surface. In the case of anion, its adsorption induces the charge transfer to the surface and brings an opposite negative potential shift. The size of ClO4- is much larger than CO, and hence the effect of adsorption of anion can be neglected. Adsorption of cation (proton) induces an electron transfer from the surface to give adsorbed hydrogen atom. This process causes a positive potential shift.15 Proton can adsorb below 0.1 V; thus, part of the positive shift below 0.1 V may be due to the adsorption of proton. However, the temporal profiles of the positive shift in Figure 3 cannot be explained by only the adsorption of proton. The number of protons increases
Dynamics of Double-Layer Restructuring at lower potentials. Thus, the rate of the positive potential shift should become faster at lower potentials. But the expriemental result in Figure 3 is opposite from this assumption. Adsorption of proton proceeds with an accompanying structural change of water layers, thus these two effects should be considered cooperatively. Temporal profile of the positive jump contains information about the time for the restructuring of the electric double layer after CO desorption. Desorption of CO occurs while the surface temprature is higher (
