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J. Phys. Chem. B 2001, 105, 7874-7877
In Situ Time-Resolved Second-Harmonic Generation from Pt(111) Microfacetted Single-Crystal Platinum Microspheres Boguslaw Pozniak, Yibo Mo, Ionel C. Stefan, Kevin Mantey, Mike Hartmann, and Daniel A. Scherson* Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7078 ReceiVed: April 24, 2001; In Final Form: June 25, 2001
In situ time-resolved second-harmonic generation (SHG) signals have been obtained from well-defined Pt(111) facets of micrometer dimensions formed spontaneously on the surface of Pt single-crystal microspheres grown by melting/cooling techniques in the ambient atmosphere. Plots of the intensity of the SHG signal, Ipp(2ω), versus the potential, E, where pp refers to p-input and p-output polarization, recorded while E was scanned linearly between two prescribed values at a rate of 50 mV/s in 0.1 M HClO4 aqueous solutions were found to be linear in the range 0.05-0.6 versus RHE and, thus, in excellent agreement with the behavior believed to be characteristic of larger Pt(111) surfaces prepared by more conventional techniques. Measurements were also performed in which the input voltage to the potentiostat was repeatedly stepped (>500 000 times) between 0.1 and 0.7 V versus RHE at frequencies of 250 Hz, while accumulating Ipp(2ω) signals from a single Pt(111) microfacet. In situ Ipp(2ω) transients acquired with submicrosecond resolution revealed a damped oscillatory behavior both in the actual potential of the Pt single-crystal microspherical electrode, as measured with a high-impedance oscilloscope, and also in Ipp(2ω). These oscillations lasted for ca. 100 µs before the potential and Ipp(2ω) settled at constant values. Extensions of this overall strategy to the acquisition of time-resolved SHG measurements of well-defined surfaces in even shorter time domains are discussed.
Introduction Recent advances in ultrafast laser pulse technology have opened new prospects for studying fundamental aspects of the kinetics of chemical reactions with unprecedented time resolution.1 Interest in our laboratory has been focused recently on the implementation of laser-based spectroscopic methods for monitoring dynamics of surface events at solid-liquid interfaces, with emphasis on surface reconstruction and oxidation-reduction of adsorbed species. One of such strategies involves application of a fast electrical perturbation, such as a potential or a current step, while following the time evolution of the interface using optical probes. Although conceptually simple, realization of such conditions requires, in practice, careful attention to cell design and instrumentation, as the response, even for ideally polarizable interfaces, is governed by the size of the electrode and the electrolyte resistance. As has been discussed in detail by McCreery and co-workers,2 a significantly faster time response can be achieved by reducing the electrode size. In fact, these authors succeeded in monitoring with submicrosecond resolution charge-transfer reactions involving electrogenerated solution-phase chromophores by absorption UV-visible spectroscopy via a combination of microelectrodes and special electronics, incorporating charge injection circuitry. Studies of surface dynamics at well-defined electrified interfaces requires development of techniques for producing high-quality single-crystal electrode surfaces of very small dimensions, as well as selection of a probe with optimized interfacial specificity. This paper presents results of in situ time-resolved secondharmonic generation (SHG) measurements on Pt(111) microfacets (10 µm) formed spontaneously on the surface of singlecrystal Pt microspheres (150 or 500 µm in diameter). The welldefined character of microfacets thus produced was verified
using in situ second harmonic generation (SHG) in 0.1 M HClO4,3 yielding results believed to be characteristic of Pt(111) specimens produced by more conventional means. Experimental Section Preparation of Pt(111) Microfacets. The method employed for growing single crystal microspheres followed closely that described by Clavilier.4 For our experiments, the tip of Pt wires of either 50 or 250 µm in diameter were heated beyond the melting point using a hydrogen-oxygen minitorch (Smith Equipment), until spheres of a diameter 1.5-3.5 times larger than that of the wire were formed. After some time, the flame was slowly retracted, and once the temperature had reached a sufficiently low value, the sphere crystallized, yielding typically 1 to 11 facets, as could be visually discerned using a telescope (10 × 30, Specwell) (see insert Figure 1). Once the specimen had cooled to room temperature, soft glass was used to seal the wire, exposing the sphere at the end and a short segment of the wire to enable fine position adjustments. The relative orientation of the facets was determined by illuminating a section of the sphere with an unfocused laser beam and measuring the angles between the incident and reflected beams from the facets (see below), yielding a spatial arrangement consistent, within 0.6°, with their (111) (or (110)) orientation (see below). This procedure also allows measurements of the azimuthal angle, i.e., between the plane of reflection and a selected crystallographic direction of a facet surface; however, for this series of experiments, the angle was regarded as unimportant and therefore was not determined. For some of the microspheres, all possible Pt(111) facets could be found; in contrast, Pt(110) facets, if present at all, were always of much smaller dimensions than their Pt(111) counterparts. The size of a specific (111) facet on a microsphere was determined either ex situ by electron
10.1021/jp0115502 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001
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J. Phys. Chem. B, Vol. 105, No. 33, 2001 7875
Figure 1. Second-harmonic generation response, Ipp(2ω) as a function of potential, from a Pt (111) microfacet in 0.1 M HClO4 during a cyclic voltammetric scan at 50 mV/s before (upper panel) and after several potential excursions into the oxide region (lower panel). Points represent averages of three scans. Arrows indicate the direction of the potential scan. Solid lines are best polynomial fits to the experimental data. Inserts: upper panel. Scanning electron microscope image of a Pt(111) facetted microsphere. Lower panel, Ipp(2ω) vs potential for a Pt(111) surface (curve a), a disordered Pt(111) surface (curve b), and a Pt(100) surface (curve c) obtained in 0.1 M HClO4 at a scan rate of 20 mV/s (adapted from Lynch et al.5). Please note that potentials in this insert are given with respect to a SCE as a reference electrode.
microscopy (see insert, upper panel, Figure 1), or in situ, via the analysis of ring-disk Airy diffraction patterns obtained by reflecting collimated laser light from the facet. For a microsphere formed on the 50 µm wire, the only specimen examined in detail, both techniques yielded virtually identical results. Optics. Picosecond light pulses were generated from a dye laser (Coherent, 702-2) coupled to a cavity dumper (Coherent, 7220) pumped by a frequency-doubled mode-locked Nd:YLF laser (Coherent, Antares 76-YLF). The temporal behavior of the system was measured with an autocorrelator (Coherent, FR 103) and monitored on a digital oscilloscope (Tektronix TDS 744A), yielding 3 ps (HWFH) pulses with a repetition rate of ca. 8 MHz. The dye laser wavelength was set 592 nm, as determined by a monochromator (Jobin-Yvon, H-10). The output power was measured with a power meter (Coherent Fieldmaster), yielding typical values of about 60 mW; however, owing to losses due to reflections from various optical elements in the beam path, the power incident on the electrode surface was on the order of 30 mW. The overall response of the laser system was found to be stable for prolonged periods of time; hence, no reference line was used to normalize against power fluctuations.
The laser beam polarization direction was rotated to the desired angle using a broadband polarization rotator (Newport model PR 550) and focused on one of the facets of the Pt microsphere (see below) at an angle of 45° (as imposed by the cell geometry, see below) by a single lens. The beam reflected from the facet was passed first through a polarizer, to control the output polarization, then through a UV filter (Schott UG11), to remove the fundamental frequency, and finally focused by a fused silica lens onto the entrance slit of a monochromator (Jobin-Yvon H-10) set at a wavelength of 291 nm, i.e., half that of the incident laser. The intensity of the SHG signal, I(2ω), was measured with a photomultiplier tube (Hamamatsu R 928) supplied with a nominal voltage of 1.25 kV (Bertran Associates Inc. model 205B-05R) attached to the monochromator output, which was then fed to either of two photon counters (Stanford Research Systems SR 400 or Stanford Research Systems SR 430). The SR 400 was used in cyclic voltametry experiments, since it allowed the input gate to be opened for a long time and, hence, for the signal to accumulate. The SR 430 is a multichannel counter with 5 ns temporal resolution and accumulation of signals in independent bins when triggered synchronously with other events. The photon counters and other
7876 J. Phys. Chem. B, Vol. 105, No. 33, 2001 relevant devices were controlled remotely by a PC through a GPIB board with custom written software working under LabVIEW visual programming language. The SHG signal showed, as expected, a quadratic dependence on the incoming light intensity before reaching saturation level for high values. No surface damage due to the laser light was discerned after the SHG measurements. All experiments were performed with p-input and p-output polarizations. Spectroelectrochemistry. A 1 × 1 cm quartz fluorimetric cuvette with five transparent windows was used for the in situ SHG experiments. The single-crystal Pt microsphere working electrode was mounted on the Teflon cap of the cuvette and a 1 × 1 cm Pt gauze counter electrode placed at the bottom of the cuvette. A reversible hydrogen electrode in the same solution (RHE) connected to the cuvette through a thin Teflon tubing was used as a reference electrode. An EG&G Model 175 Universal Programmer and Model 173 Potentiostat/Galvanostat was used to scan or step the input potential to the potentiostat and to trigger initialization of the data acquisition system. All experiments were performed at room temperature in 0.10 M HClO4 (Ultrex) solutions (Barnstead ultrapure water) purged using UHP Ar (Praxair).
Letters
Figure 2. Plot of Ipp(2ω) vs potential for a Pt(111) microfacet in a CO saturated 0.1 M HClO4 solution during a cyclic voltammetric scan at a rate of 20 mV/s following polarization of the electrode at +0.10 V vs RHE for ca. 1 h. The increase in Ipp(2ω) in the region above 0.75 V is due to the reconstruction of the adsorbed CO superlattice on Pt(111), from a (2 × 2) at low potentials to a (x19 × x19)R23.4° at high potentials (see text for details).
Results and Discussion Figure 1 shows plots of the intensity of the SHG signal, Ipp(2ω), where the subscript pp refers to p-input and p-output polarizations, for a Pt(111) microfacet formed on a single-crystal microsphere, ca. 0.75 mm diameter, recorded in situ in aqueous 0.10 M HClO4 during a linear potential scan in the region 0.0 < E < 0.8 V versus RHE at a rate of 50 mV/s. It is evident from these data that Ipp(2ω) is directly proportional to the applied potential over the range 0.1-0.6 V, with a slope virtually independent of the direction of the scan. As reported by Corn and co-workers,5,6 the potential region over which the SHG response is linear is much wider than that found for other Pt low index faces (see insert, lower panel in this figure), providing strong evidence that the Pt(111) facets are indeed well-defined. Also in agreement with data reported by these workers,5 was the much narrower Ipp(2ω) versus E linear region observed after extending the scan into the oxide formation region (see lower panel in this figure and also insert), a procedure known to promote irreversible surface disorder.7,8 The well-defined character of the Pt(111) surface was further evidenced by the SHG signals obtained in experiments performed in CO-saturated 0.10 M HClO4 solutions. As indicated in Figure 2, a sizable increase in Ipp(2ω) was observed in the region above 0.75 in the scan in the positive direction following polarization of the electrode at +0.10 V versus RHE for ca. 1 h. This phenomenon has been attributed by Akemann et al.9,10 to the reconstruction of the adsorbed CO superlattice on Pt(111), from a (2 × 2) at low potentials to a (x19 × x19)R23.4° at high potentials, where CO occupies on-top and 2-fold bridge sites.11 It may be noted that in our measurements the potential was scanned at 20 mV/s, as opposed to stepped in sequence as in the latter work. This factor may explain the much higher potential at which the onset of this reversible transition was observed. Nevertheless, the relative increase in Ipp(2ω), ca. 50%, agrees very well with the data of Akemann et al. cited above. As noted these and other authors,12 electrooxidation of CO is accompanied by a drastic drop in Ipp(2ω), which is partially recovered during the scan in the negative direction at a potential of about 0.9 V versus RHE. Further aspects of this oxidation process and the overall dynamics of the adlayer reconstruction will be discussed in a forthcoming publication.
Figure 3. Upper panel: Plot of Ipp(2ω) vs potential for a Pt(111) microfacet in 0.10 M HClO4 during a potential step from Einp ) + 0.7 to Einp ) + 0.1 V vs RHE. Data represent the average of over half a million sequential acquisitions. The lower panel shows a single Egr transient as measured with the oscilloscope, while the SHG data were acquired (see text for details).
The rates of adsorption/desorption of atomic hydrogen on Pt in aqueous electrolytes are very high and in all likelihood faster than the time constant of the cell used for these experiments. Under these conditions, the SHG signal may be regarded as tracking the instantaneous coverage of hydrogen on the Pt(111) facet and, thus, the actual potential of the electrode, Eact, as opposed to the square wave signal applied to the input of the potentiostat, Einp. Correlations between Einp and Eact could be obtained applying to the potentiostat input a square wave between +0.1 V < Einp < +0.7 V versus RHE, at a rate of 250 Hz, while accumulating Ipp(2ω) data from a single Pt(111) facet via the photon counter, and also monitoring the potential of the working electrode with respect to ground, Egr, using the Textronix oscilloscope. Panel A of Figure 3 shows the average of over half a million sequential SHG acquisitions for the section of the duty cycle in which Einp was stepped in the negative
Letters direction, i.e., + 0.7 f + 0.1 V versus RHE, whereas panel B in this same figure shows a single Egr transient as monitored with the oscilloscope. As evidenced from these data, both Egr and Ipp(2ω) transients, which are caused by electronic instabilities, displayed a damped oscillation of rather large amplitude with a common period of about 21 µs, reaching a constant value only for times longer than 90 µs. Although the ringing character of the Egr remained unaffected upon blocking the light, the oscillations in Ipp(2ω) totally disappeared. This observation provides ample evidence that the effects observed are not due to electronic artifacts external to the potentiostat/cell/optics arrangement but represent an accurate measure of the actual working electrode potential. Access to faster time domains without complications due to spurious responses from the potentiostat can be readily realized by change injection techniques of the type reported by McCreery et al.2 and employing electrodes of much smaller diameters compared to those used in this work. Implementation of both
J. Phys. Chem. B, Vol. 105, No. 33, 2001 7877 these approaches are currently under way and will be reported in due course. References and Notes (1) Zewail, A. H. J. Phys. Chem. A 2000, 104, 5660-5694. (2) Robinson, R. S.; McCreery, R. L. J. Electroanal. Chem. 1985, 182, 61-72. (3) Corn, R. M.; Higgins, D. A. Chem. ReV. 1994, 94, 107-125. (4) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211. (5) Lynch, M. L.; Barner, B. J.; Corn, R. M. J. Electroanal. Chem. 1991, 300, 447-465. (6) Lynch, M. L.; Corn, R. M. J. Phys. Chem. 1990, 94, 4382-4385. (7) Sashikata, K.; Furuya, N.; Itaya, K. J. Vac. Sci. Technol, B 1991, 9, 457-464. (8) Furuya, N.; Shibata, M. J. Electroanal. Chem. 1999, 467, 85-91. (9) Akemann, W.; Friedrich, K. A.; Linke, U.; Stimming, U. Surf. Sci. 1998, 404, 571-575. (10) Akemann, W.; Friedrich, K. A.; Stimming, U. J. Chem. Phys. 2000, 113, 6864-6874. (11) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648-1660. (12) Bae, I. T. J. Phys. Chem. 1996, 100, 14081-14086.
