Shouzhong Zou

A concerted assessment of potential-dependent vibrational frequencies for nitric oxide and carbon monoxide adlayers on low-index platinum-group surfaces in electrochemical compared with ultrahigh vacuum environments: Structural and electrostatic implications

Electrode potential-dependent intramolecular stretching frequencies, νNO, for nitric oxide adlayers on ordered low-index Pt, Rh, Ir, and Pd electrodes in acidic aqueous solution measured by infrared reflection-absorption spectroscopy (IRAS) are compared with corresponding data obtained in ultrahigh vacuum (UHV) environments in order to assess the manner and degree to which the chemisorbate vibrational properties are controlled by electrostatic factors. For most of the seven surfaces for which corresponding UHV-based data are also available, the coverage-dependent νNO spectral fingerprints observed in the corresponding electrochemical case are closely comparable, suggesting the occurrence of the same (or similar) binding sites and adlayer structures. The νNO frequencies at a given coverage are typically 10–15-fold more sensitive to the electrostatic potential (or field) at the Pt-group electrodes than for isolated (gas-phase) NO, highlighting the importance of potential-dependent surface bonding. The νNO f...

Infrared spectroscopy of carbon monoxide and nitric oxide on palladium(111) in aqueous solution: unexpected adlayer structural differences between electrochemical and ultrahigh-vacuum interfaces

Abstract Coverage-dependent infrared reflection-absorption spectra (IRAS) measured for carbon monoxide and nitric oxide adlayers dosed onto Pd(111) in acidic aqueous solution are examined in comparison with IRAS and other adlayer structural data available on Pd(111) in ultrahigh vacuum (UHV). Unlike most other ordered Pt-group surfaces, the coverage-dependent spectral fingerprints for both electrochemical adlayers on Pd(111) are markedly different from their UHV-based counterparts. These disparities are magnified after allowance is made for the differences in surface potentials, and hence interfacial electrostatic fields, between the electrochemical and UHV-based systems. Specifically, at low dosed CO coverages ( θ CO ν CO) band is obtained at ca. 1830–1880 cm −1 , consistent with threefold-hollow coordination, which is replaced at higher coverages up to saturation ( θ CO =0.75) by a pair of bands at 1940–1960 and 1890–1920 cm −1 , suggestive of ‘bridging-like’ along with hollow CO binding. Unusually similar θ CO -dependent infrared spectra were obtained upon sequential partial electrooxidation of a saturated layer, indicating that any CO islands thereby forming are largely dissipated. While the infrared spectra are qualitatively similar to the behavior in UHV at low coverages, the electrochemical ν CO frequencies are significantly (30–40 cm −1 ) higher than expected, and the higher-frequency ν CO feature seen for θ CO >0.4 has no obvious counterpart at the Pd(111)/UHV interface. These behavioral differences can be partly, yet not entirely, rationalized in terms of water coadsorption. Dosed NO adlayers exhibit largely a single NO stretching ( ν NO ) band blueshifting by ca. 30 cm −1 up to ca. 1750 cm −1 at saturation, suggestive of largely atop NO coordination. The absence of the markedly lower-frequency ν NO bands seen at low dosages on Pd(111) in UHV is ascribed to partial segregation of NO and coadsorbed water, yielding higher local NO coverages. These unexpected ν CO and ν NO spectral properties at Pd(111) electrodes with respect to the corresponding UHV-based systems are also discussed in comparison with corresponding IRAS behavior for CO and NO adlayers on other low-index Pt-group surfaces. The coadsorption of CO and NO on Pd(111) is also briefly considered. The form of the ν CO / ν NO spectra indicate the presence of some CO segregation as well as CO/NO molecular intermixing.

Surface-Enhanced Raman Scattering from Substrates with Conducting or Insulator Overlayers: Electromagnetic Model Predictions and Comparisons with Experiment

Model electromagnetic (EM) calculations are presented of the surface-enhanced Raman scattering (SERS) intensities expected for gold and silver particles coated with conducting as well as insulating dielectric films, with the objective of assessing their thickness-dependent properties and hence the anticipated scope of such “overlayer SERS” tactics to chemically diverse interfacial materials. Most calculations refer to Raman enhancements at the film outer edge, relevant to species adsorbed on the overlayer. Spheroidal and spherical metal particles are treated by using the electrostatic approximation, with a first-order electrodynamic correction for “radiation damping”, in vacuum and aqueous environments. The presence of simple insulating dielectric (such as organic) overlayers yields progressive decays in the calculated Raman enhancement factor, G, at the film edge with increasing thickness, d, throughout the visible optical region of experimental significance, even though the largest G values are necessarily obtained close to the plasmon resonance energy. These G–d dependencies are markedly milder than predicted for bare metal particles in water or vacuum, suggesting the potential broad-based analytical utility of nanoscale (∼ 1–10 nm thick) molecular dielectric overlayers in SERS. Furthermore, Raman enhancements increasing with film thicknesses are typically obtained at locations close to the inner film edge, relevant to moieties imbedded in dielectric overlayers. Sharper G–d decays are predicted at the film edge of transition-metal overlayers, especially near the plasmon resonance; unlike dielectric insulators, the metal overlayers progressively “quench” the optical frequency-dependent enhancement at the film edge arising from substrate plasmon resonance. Reasonable agreement is obtained in comparison with the Raman intensity-thickness dependence measured for chemisorbed carbon monoxide on rhodium films electrodeposited onto gold. The model calculations can also account qualitatively for the nonmonotonic Raman intensity-thickness dependencies observed for phonon bands from semiconducting (cadmium chalcogenide) overlayers on gold, attributed to film-induced redshifts in the substrate plasmon resonance. More general implications for the analytical utility of “overlayer SERS” tactics are also pointed out.

Surface-enhanced Raman spectroscopy of cadmium sulfide/cadmium selenide superlattices formed on gold by electrochemical atomic-layer epitaxy

Abstract The phonon properties of ultrathin CdS/CdSe superlattice films formed on gold by electrochemical atomic-layer epitaxy are characterized by means of surface-enhanced Raman spectroscopy (SERS). Substantial (15–25 cm −1 ) red-shifts in the CdS phonon frequencies are observed, whereas the CdSe frequencies are essentially unaltered, indicating that substantial crystallographic strain occurs in the former, but not the latter, superlattice component. The findings demonstrate the virtues of SERS for exploring the structure of such solid–solid interfaces with unique monolayer-level sensitivity.

Infrared spectroscopy of carbon monoxide at the ordered palladium (110)-aqueous interface: evidence for adsorbate-induced surface reconstruction

Abstract In situ infrared reflection-absorption spectra in the CO stretching ( ν CO ) region for carbon monoxide adsorbed on an ordered Pd(110) electrode in 0.1 M HClO 4 as a function of coverage ( θ CO ) and electrode potential ( E ) are compared with corresponding θ CO -dependent spectra reported in previous studies for the same surface in ultrahigh vacuum (UHV). The latter system has been shown from electron diffraction to involve an unusual case of CO-induced surface reconstruction which is reflected also in marked changes in the θ CO -dependent ν CO spectra. Detailed comparison of the infrared data obtained in the electrochemical and UHV environments therefore enables the possible occurrence of Pd(110) reconstruction in the former system to be assessed. Indeed, rich θ CO -dependent ν CO spectra were obtained which provide strong evidence that similar surface structural changes also occur at the Pd(110)-aqueous interface. Specifically, dosing CO from dilute (ca. 0.01 mM) solution yields a single ν CO band (at 1810–1840 cm −1 ) at low coverages. These spectra transform for θ CO values above ca. 0.4 into a complex multiple-band pattern which contains an additional band near 1930–1940 cm −1 and weaker higher-frequency ν CO features along with a peak at ca. 1850–1880 cm −1 . Significantly, this spectral pattern, obtained for coverages between ca. 0.4 and 0.75, is similar to that observed in the UHV system over a comparable θ CO range, the latter being diagnostic of the CO-induced formation of a (1 × 2) “missing-row” reconstruction. Increasing the coverage further, from 0.75 up to saturation, θ CO = 1.0, triggers another spectral transition to a single dominant ν CO band at 1960–1965 cm −1 , consistent with the formation of a (2 × 1) atop CO adlayer on a (1 × 1) (i.e. unreconstructed) substrate. Further evidence that the freshly prepared surface is unreconstructed prior to CO dosing, as well as in the presence of a saturated adlayer, was obtained from ν CO spectra following CO removal and redosing, and also from energetic considerations along with voltammetric data. Intermediate-coverage CO adlayers formed by solution dosing followed by partial electrooxidative removal exhibit much smaller θ CO -dependent spectral changes, indicative of adsorbate island formation. However, the ν CO spectra for such adlayers transformed within 10–20 min into θ CO -dependent patterns comparable to those obtained upon initial solution CO dosing. This “island dissipation” may be driven by the resultant formation of reconstructed nanoscale domains.

Electrochemical adsorbate-induced substrate restructuring: gold(110) in aqueous bromide electrolytes

The electrode potential-dependent atomic structure of ordered Au(110) in bromide electrolytes is examined by in situ scanning tunneling microscopy, focusing on the ordered adlayer arrangements formed on the (1◊1) substrate terraces, and especially the adsorbate-induced nanoscale restructuring which is observed to occur at higher potentials. Comparison is made with the previously examined behavior of Au(110) in iodide electrolytes. Several ordered bromine structures were identified, all featuring ‘close-packed’ Br rows along the (001) direction, i.e. perpendicular to the (110) ‘rails’. At lower coverages, h Br <0.75, rectangular packing was commonly observed, featuring also Br rows parallel to the (110) direction. The Br binding sites were determined unambiguously from ‘potentiodynamic’ STM data, featuring juxtaposed substrate and adlayer domains created by potential-step perturbations during image acquisition. The rectangular unit cells contain Br atoms lying within the (110) troughs, and exhibit periodic variations from ‘long twofold’ to fourfold-hollow binding sites. These include (3◊1) and (4◊1) unit cells, corresponding to h Br =0.66 and 0.75, respectively. At similar potentials to the latter, an alternative (4◊1) structure can be formed having the same coverage, yet with quasi-hexagonal Br packing. Increasing the potential further yields erosion of terrace edges, especially along the (001) direction, and the appearance of arrays of single atomic-layer gold ‘nanorods’, 1.8 nm wide, also oriented along (001). These strings eventually self-assemble into ordered arrays, with rod separations of 0.9‐1.2 nm. The restructuring can be reversed rapidly by returning the potential to lower values. The ordered restructuring is markedly diVerent from that observed previously in the Au(110)‐I system, which features ‘miniterrace stripes’ largely paralleling the (11:0) direction. These structural diVerences can be understood in terms of the known dissimilarities in the close-packed Br and I adlattices; the strict (001) orientation of the nanorods in the Au(110)‐Br system is apparently triggered by the similarly oriented Br strings. The widths of the restructured gold miniterraces in both electrolytes are surmised to be determined by adsorbate-induced dipole repulsion forces.

Coadsorbate vibrational interactions within mixed carbon monoxide-nitric oxide adlayers on ordered low-index platinum-group electrodes

Abstract In-situ infrared reflection-absorption spectra are described for saturated mixed carbon monoxide-nitric oxide adlayers in comparison with CO and NO adsorbed separately on selected ordered Pt-group electrodes in aqueous solution in order to assess the nature of the coadsorbate vibrational interactions and hence the adlayer structure and bonding. Both chemisorbates exhibit pronounced intramolecular vibrational fingerprints ( ν CO , ν NO bands) that are sensitive to the local vibrational environment as well as the surface bonding geometry, enabling a microscopic-level assessment of the coadsorbate interactions in relation to CO and NO coadsorbed with water. The surfaces selected—Ir(110), Ir(111), Pt(100), Pt(111), Rh(100), Rh(111), and Pd(111)—yield near-exclusive molecular (rather than dissociative) NO adsorption, yet exhibit differing coverage-dependent NO, and especially CO, spectral and coordinative properties. Progressive displacement of NO by exposing NO-saturated electrodes to dilute CO solutions yielded chiefly molecularly intermixed CO/NO adlayers. This deduction is most straightforward (and quantitative) on Ir(110) and Ir(111), facilitated by the exclusively atop-like adsorption of CO and NO, as gleaned from the single ν CO and ν NO band frequencies. Both these features on Ir(110) redshift markedly (by 40–60 cm −1 ) upon dilution with either the other chemisorbate or with coadsorbed water. Such redshifts, along with the observed suppression of the ν NO band intensity in the CO/NO mixtures, arise chiefly from composition-dependent dynamic-dipole coupling within the intermixed dipolar adlayers, and are diagnostic of microscopic coadsorbate structure. Similar ν NO redshifts induced by dilution with coadsorbed CO are observed on each of the other surfaces. The composition-dependent analysis is facilitated by the observation of a lone coverage-dependent ν NO band, associated probably with multifold NO coordination, in both the absence and presence of coadsorbed CO in most cases. While corresponding effects upon the ν CO band frequencies are also observed, the spectra on Pt and Rh surfaces include a pair of ν CO bands with frequencies, ca. 1800–1850 and 2000–2070 cm −1 , suggestive of bridging and atop-like CO coordination, respectively. On Rh(100), Rh(111), Pt(111) and especially Pt(100), NO coadsorption induces CO ‘bridging’ to ‘atop’ site switching. This is largely consistent with the formation of molecularly intermixed CO/NO adlayers where the strong preference of NO for multifold sites shifts CO molecules into neighboring atop-like configurations. On Pt(111), Rh(111), and Pd(111), however, the ν CO spectral fingerprints for the CO/NO adlayers suggest the additional presence of segregated compressed ‘CO-rich’ domains. Comparisons are made also with the behaviour of analogous CO/NO adlayers in ultrahigh vacuum.

Interactions within mixed NO/CO adlayers at the Pt(100)–aqueous electrochemical interface as probed by infrared spectroscopy

In situ infrared reflection–absorption spectra are reported for NO/CO adlayers on Pt(100) in aqueous 0.1 M HClO4 as a function of the coadsorbate composition and electrode potential, E, and compared with corresponding coverage-dependent spectra for pure NO and CO layers with the objective of elucidating the microscopic mixed adlayer structure. The spectra for irreversibly adsorbed NO at 0.3 V feature a single N–O stretching (νNO) band at 1590–1625 cm−1, the frequency upshifting with increasing coverage, θNO. The νNO frequencies at a given θNO value are sensitive to the electrode potential, dνNO/dE decreasing towards higher coverage, from 90 cm−1 V−1 (at θNO=0.2) to 40 cm−1 V−1 at saturation (θNO≈0.5). The frequencies as well as the form of the electrochemical νNO spectra are closely similar to corresponding published infrared data on Pt(100) in ultrahigh vacuum at 300 K when the comparison is made at equivalent surface potentials. As reported previously, CO adsorption on Pt(100) in 0.1 M HClO4 yields C–O stretching (νCO) bands located at 2015–2060 and 1800–1870 cm−1, attributed to atop and bridging coordination, respectively, the relative band intensities being sensitive to both the CO coverage and electrode potential. The mixed adlayers were formed by exposing a saturated NO layer to solution CO at 0.2–0.3 V vs. Ag/AgCl. They are stable over the potential range ca. 0.1–0.35 V, higher and lower potentials yielding CO electrooxidation and NO electroreduction, respectively. Replacement of NO by CO at a given potential yields progressive downshifts in the νNO frequency, the νNO–θNO dependence being similar to that observed for NO adsorption alone. This coadsorbate behavior, along with related changes in the νCO spectra, a strongly increased preference for atop CO binding, and intensity-transfer from the νNO to the νCO bands, is indicative of microscopic intermixing of the NO and CO components. Some aspects of the νNO and νCO spectral behavior, however, suggest that small NO aggregates may form at intermediate mixed NO/CO compositions. Unexpectedly small (<10 cm−1 V−1) νNO–E and νCO–E “Stark-tuning” slopes are observed under these conditions. The broader significance of such coadsorption effects to the elucidation of chemisorbate properties at electrochemical interfaces is also noted.

DIRECT OBSERVATION OF INFRARED BAND INTENSITY TRANSFER BETWEEN COADSORBATES HAVING WIDELY SEPARATED OSCILLATOR FREQUENCIES : INTERMIXED NO/CO ADLAYERS ON ORDERED IRIDIUM ELECTRODES

The occurrence of substantial (two–threefold) transfer of infrared band intensity between juxtaposed coadsorbates having widely separated (200–250 cm−1) oscillator frequencies is demonstrated directly for intermixed NO/CO adlayers on ordered Ir(111) and (110) electrodes by selectively removing CO or NO by electrochemical oxidation and reduction, respectively. The surprisingly large effect is nevertheless semiquantitatively consistent with the predictions of theoretical dipole-coupling models.

Surface potentials of metal–gas interfaces compared with analogous electrochemical systems as probed by adsorbate vibrational frequencies

The dependence of adsorbate vibrational frequencies on the surface potential, φ (“Stark-tuning” effect), observed in electrochemical systems is exploited for the same metal surfaces in contact with ambient-pressure gases so to estimate φ values in the latter environment. Saturated CO adlayers on palladium and platinum films are examined along these lines by using surface-enhanced Raman spectroscopy (SERS) to obtain frequencies for both the C–O (νCO) and metal–carbon stretching (νM--CO) vibrations in CO-containing aqueous electrochemical and gaseous environments. The effective gas-phase surface potentials extracted by matching the vibrational frequencies with the corresponding potential-dependent electrochemical spectra are substantially (ca. 1–1.5 V) lower than the work functions for such interfaces under “clean” (ultrahigh vacuum) conditions. These disparities are ascribed to the occurrence of electrochemical-like redox half-reactions in the ambient-pressure gas-phase environment, leading to surface charging, and, hence, marked alterations in the surface potential as controlled by potential-dependent redox kinetics. The possible implications of these and related findings to ambient-pressure adsorption and catalysis are discussed.