Article pubs.acs.org/JPCC
Electric Field Effect on Condensed-Phase Molecular Systems. III. The Origin of the Field-Induced Change in the Vibrational Frequency of Adsorbed CO on Pt(111) Hani Kang, Sunghwan Shin, Youngwook Park, and Heon Kang* Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 08826, Republic of Korea ABSTRACT: The effect of an external electric field on the C−O stretch frequency, ν(C−O), of carbon monoxide was studied for CO in different environments of condensed molecular films: (I) chemisorbed CO on Pt(111) covered with amorphous solid water (ASW), (II) CO trapped in an ASW matrix, (III) chemisorbed CO on Pt(111) covered with solid Ar, and (IV) CO trapped in a solid Ar matrix. Changes in ν(C−O) of these samples under an electric field were measured to investigate the Stark frequency shift and the effect of metal−adsorbate charge transfer on the frequency change. The electric field was applied up to 4.3 × 108 V·m−1 using the ice film capacitor method. Reflection absorption infrared spectroscopy was used to monitor the spectral changes of the ν(C−O) band. The Stark shift was measured from the ν(C−O) change of isolated CO in the ASW matrix under the field. The effect of metal−adsorbate charge transfer was estimated for chemisorbed CO by measuring the ν(C−O) shift under the field and subtracting the electrostatic Stark effect. The electrostatic Stark effect appeared with a Stark tuning rate of Δμ = 0.64 ± 0.04 cm−1/(108 V·m−1) for CO in the ASW matrix. −1 −2 The charge transfer effect on the frequency change had a sensitivity factor of Δν/σ ̅ ≈ 200 cm /C·m for chemisorbed CO on Pt(111), where σ is the excess charge density of the Pt surface. From these observations, we suggest that in electrochemical experiments, where ν(C−O) of CO adsorbates on the electrode surface changes with the electrode bias potential, the frequency shift may result predominantly from the metal−adsorbate charge transfer rather than the electrostatic Stark shift.
1. INTRODUCTION The absorption frequencies of molecular vibrations can vary with an applied electric field in the so-called vibrational Stark effect (VSE).1 Because the observation of the VSE requires strong electric fields, this phenomenon has been widely studied in electrochemical cells,2−20 where the electrical double layer (EDL) near the electrode surface provides a strong-field environment for adsorbed molecules. Carbon monoxide is a prototypical simple molecule for studying the VSE of adsorbed species. CO is readily detectable using infrared spectroscopy, and its stretching frequency ν(C−O) is sensitive to the adsorption geometry and the local environment of the surface. In addition, CO is a commonly present adsorbate on metal surfaces under catalytic and electrochemical reaction environments.2 Vibrational spectroscopic studies have shown that ν(C−O) of adsorbed CO molecules on the electrode surface changes with the electrode bias potential, even when the binding site of CO remains invariant during the potential change.2−11,17 The observation indicates that the frequency change is due to the VSE of CO inside the EDL field. However, the observed frequency shifts are often greater than what is expected for the EDL field strengths of the generally accepted range.3 The discrepancy suggests a possibility that additional effects also contribute to the frequency shifts, such as charge transfer between the adsorbate and metal surface, which depends on the bias potential as well as the presence of specifically adsorbed ions on the surface.3 To better understand the origin of the ν(C−O) shift of the adsorbed CO on metal surfaces, it is desirable to isolate © 2016 American Chemical Society
different effects that may contribute to the frequency shift and study them separately under a well-defined experimental condition. One approach is to study a model system of the electrode surface in the ultrahigh vacuum (UHV) environment.21 In the early stage of the UHV model experiment, the effect of an electrode potential change was simulated by changing the work function of the metal surface,21 which was probably too simplified an approach. Subsequently, Lambert and co-workers7,22 studied the field effect on ν(C−O) of chemisorbed CO on a Pt(111) surface in UHV by applying an AC electric field (∼3 × 106 V·m−1) between the Pt substrate and a metal sphere separated by a known distance. In the present work, we study the field effect on ν(C−O) under a stronger field condition (≤4.3 × 108 V·m−1) provided by the ice film capacitor method,23 which can generate an electric field comparable to the EDL field strength. The investigated model systems include chemisorbed CO on a Pt(111) surface in the presence or absence of an overlaying ASW film and CO trapped in an amorphous solid water (ASW) or solid Ar matrix.
2. EXPERIMENTAL METHODS The experiment was performed in an UHV surface analysis chamber with a base pressure of 6 × 10−10 Torr. The chamber was equipped with a low-energy Cs+ ion gun (Kimball Physics), Kelvin work-function probe (McAllister), Fourier transform Received: June 1, 2016 Revised: July 7, 2016 Published: July 20, 2016 17579
DOI: 10.1021/acs.jpcc.6b05494 J. Phys. Chem. C 2016, 120, 17579−17587
Article
The Journal of Physical Chemistry C
path outside the UHV chamber was purged with dry nitrogen gas. The difference in the RAIR spectra measured before and after charging the sample with Cs+ ions represented the absorbance change (ΔA) of the sample induced by the electric field. RAIR spectra were recorded at a spectral resolution of 4 cm−1 except for the high-resolution measurements specified in the text.
infrared (FTIR) spectrometer (Bruker, Vertex 70), and quadrupole mass spectrometer (QMS; Extrel). The apparatus was an improved version of the previous one described elsewhere,23 with an optimized instrumental configuration for the study of the electric field effect on frozen molecular films. A Pt(111) single-crystal sample was installed onto a customdesigned holder stage of an UHV sample manipulator (ColdEdge Co.). The sample holder was cooled using a closed-cycle liquid He cryostat (Sumitomo, CH-204SN) to a temperature beyond the readability of N-type thermocouple (1300 K, while the sample holder was in thermal contact with the cryostat. The sample was electrically isolated from the ground for applying the bias voltage or reading the current during ion or electron beam exposure. Frozen molecular films were grown on a Pt(111) substrate surface cooled to a desired temperature. H2O and Ar films were grown by backfilling the chamber with the corresponding gases. For CO, a tube doser was used to guide the gas close to the sample surface and thus to minimize the contamination of the chamber wall due to CO adsorption. An electric field was applied across a frozen molecular film using the ice film capacitor method, which was described in detail previously for its principle23 and applications.24,25 The method is based on the charging of a molecular film by Cs+ ion deposition on the surface. Cs+ ions were sprayed onto the sample surface at low incidence energy (≤30 eV). The sample surface was precovered with an ASW film, such that Cs+ ions remained afloat on the surface of ASW without penetrating the sample interior due to its thermodynamic propensity for the water surface. The Cs+ beam current was several nanoamperes. The Cs+ deposition generated the voltage (V) across the molecular film. This voltage was monitored using a Kelvin work-function probe, which measured the contact potential difference (CPD) between the sample surface and the probe. The CPD was measured before and after Cs+ deposition, and the difference between the two measurements, ΔCPD, represented the charging voltage of the sample.26 The electric field strength (F) through the sample was estimated by F = V/ d, where d is the thickness of the sample. The thickness of the molecular film was estimated based on the temperature-programmed desorption (TPD) measurements. For an ASW sample, the film thickness was calculated by dividing the TPD intensity of water by that of the corresponding water monolayer (ML) formed on Pt(111), which gave the film thickness in units of ML (1 ML = 1.1 × 1015 water molecules cm−2). This quantity was converted to d by using the density of the ASW film, ρ(H2O). For the present ASW film that was prepared using water vapor deposition at
