Adsorption of pyridine on a polycrystalline gold electrode surface studied by infrared reflection absorption spectroscopy

The adsorption behavior of pyridine on a smooth polycrystalline gold electrode surface was investigated over a wide wavenumber region (2000–500 cm⁻¹) by in situ infrared reflection absorption spectroscopy (IRAS). The reversible adsorption/desorption of pyridine was observed upon the change in applied electrode potential, and the adsorption state at positive potentials was found to depend strongly on the kind of halide ion used as a supporting electrolyte. Symmetry analysis of absorption bands observed revealed that pyridine molecules adsorb with the molecular axis (C₂ axis) perpendicular to the electrode surface (vertical configuration) at positive potentials in 0.5 M KF, KCl and KBr solutions. A band due to the out-of-plane bending mode of the adsorbed pyridine molecule was observed at potentials more negative than ca. 0 V for 0.5 M KF solution containing 100 mM pyridine. We concluded that even in the 100 mM pyridine solution, adsorbed pyridine forms a monolayer and that the molecules reorient from a flat (parallel) to the vertical configuration as the potential becomes less negative. No bands due to adsorbed pyridine were detected for 0.5 M KI solution. The amount of adsorbed pyridine was found to depend strongly on the strength of specific adsorption of halide ions.     

Surface electrochemistry of CO as a probe molecule on carbon-supported Se-surface modified Ru nanoparticles via infrared reflection absorption spectroscopy

The carbon monoxide molecule is used to probe the electronic properties of Ru/C and Se surface-modified Ru/C nanocatalysts. Coordination of a Se to a Ru surface strongly affects the vibrational properties and reactivity of the CO adsorbed. Marked alteration of the CO stretching frequency; increase of the Stark tuning rate for C-O vibration; and a positive shift of the onset of CO oxidation are observed for Se-modified Ru particles and attributed to the electronic effect of Se.     

Oxidation of the cyanide ion at a platinum electrode studied by polarization modulation infrared reflection absorption spectroscopy

The anodic oxidation of the cyanide ion at a platinum electrode in aqueous solution was observed by polarization modulation infrared reflection absorption spectroscopy(PM IRRAS). The cyanide ion was adsorbed on the electrode surface in the potential region more negative than 0.4 V (versus Ag/AgCl). In the more positive region (> 0.4 V ), the adsorbed cyanide ion was oxidized to form the cyanate ion. Cyanogen was not detected during the oxidation reactions; this suggests direct electrochemical formation of the cyanate ion.     

Interaction of ester functional groups with aluminum oxide surfaces studied using infrared reflection absorption spectroscopy

The bonding of two types of ester group-containing molecules with a set of different oxide layers on aluminum has been investigated using infrared reflection absorption spectroscopy. The different oxide layers were made by giving typical surface treatments to the aluminum substrate. The purpose of the investigation was to find out what type of ester-oxide bond is formed and whether this is influenced by changes in the composition and chemistry of the oxide. The extent by which these bonded ester molecules resisted disbondment in water or substitution by molecules capable of chemisorption was also investigated. The ester groups were found to show hydrogen bonding with hydroxyls on the oxide surfaces through their carbonyl oxygens. For all oxides, the ester groups showed the same nu(C = O) carbonyl stretching vibration after adsorption, indicating very similar bonding occurs. However, the oxides showed differences in the amount of molecules bonded to the oxide surface, and a clear relation was observed with the hydroxyl concentration present on the oxide surface, which was determined from XPS measurements. The two compounds showed differences in the free to bonded nu(C = O) infrared peak shift, indicating differences in bonding strength with the oxide surface between the two types of molecules. The bonding of the ester groups with the oxide surfaces was found to be not stable in the presence of water and also not in the presence of a compound capable of chemisorption with the aluminum oxide surface.     

CO adsorption and CO and O coadsorption on Rh(111) studied by reflection absorption infrared spectroscopy and density functional theory

The adsorption of carbon monoxide on Rh(111) and on oxygen modified Rh(111) was investigated using thermal desorption spectroscopy, reflection absorption infrared spectroscopy (RAIRS), and density functional theory. The results show that CO adsorbs on Rh(111) in on top sites at low coverages. With increasing coverage hollow sites and bridge sites get occupied according to the RAIRS results. A new vibrational feature at high wave numbers was found in the on top region of the CO stretching frequency. This feature can be explained by a local high density CO structure where two CO molecules are adsorbed in the ( radical3x radical3)R30 degrees structure. The coadsorption of oxygen and carbon monoxide leads to a shift of the CO stretching frequency to higher wave numbers with increasing O to CO ratio. CO adsorption on a (2x1) oxygen layer is possible and RAIRS shows that the CO adsorbs in on top and most likely in bridge sites in this case.     

Interaction of anhydride and carboxylic acid compounds with aluminum oxide surfaces studied using infrared reflection absorption spectroscopy

The chemical bonding of three different anhydride and carboxylic acid based compounds with a set of differently prepared aluminum substrates has been investigated using infrared reflection absorption spectroscopy. The compounds were selected to model typically used adhesives, coatings, and self-assembling monolayers. The purpose of the investigation was to study the interaction of these functional groups with the aluminum oxide surface and to determine whether this interaction is influenced by the changes in chemistry and composition of the oxide layer. The extent to which the compounds resisted disbondment in water was also investigated. The oxide layers on the differently prepared substrates were all found to be capable of hydrolysis of the anhydride group, resulting in the formation of two carboxylic acid groups. Subsequently, both of the carboxylic acid groups became deprotonated, to form a coordinatively bonded carboxylate species. The same behavior was also observed for monofunctional carboxylic acids. For all different oxides layers, the carboxylate was found to be coordinated in a bridging bidentate way to two aluminum cations in the oxide layer. The oxide layers showed however clear differences in the amount of molecules being chemisorbed. A relation was established with the amount of hydroxyls present on their surfaces, as determined from X-ray photoelectron spectroscopy measurements. The coordinative bonding of a monofunctional carboxylic acid group to the oxide surface was found to be not stable in the presence of water, while a bifunctional carboxylic acid group could resist displacement by water for a prolonged period of time.     

Surface dynamics of coadsorbed CO and D2O on Pt(100) in ultrahigh vacuum as studied by time-resolved infrared reflection absorption spectroscopy

Changes in the properties of CO adsorbed at saturation coverages on Pt(100) induced by subsequent coadsorption of fixed amounts of D2O at 105 K in ultrahigh vacuum (UHV) were monitored by time-resolved infrared reflection absorption spectroscopy (tr-IRAS). The linear- and bridge-bonded CO stretching features were found to change in intensity and shift toward lower energies as a function of time at fixed CO and D2O coverages. Also observed was the development of multiple features in both CO spectral regions depending on the amount of D2O on the surface. These findings indicate that, under the conditions of these experiments, the interfacial dynamics are relatively slow, on the order of minutes, involving a gradual rearrangement of adsorbed CO and D2O on the surface to yield surface solvated CO, as has been suggested in the literature (Kizhakevariam et al. J. Chem. Phys. 1994, 100, 6750). This factor should be considered when comparing, quantitatively, shifts induced by water coadsorption with CO on Pt single crystals in UHV with CO adsorption on the same surfaces in electrochemical environments.     

Adsorption kinetics of n-nonyl-beta-D-glucopyranoside at the air-water interface studied by infrared reflection absorption spectroscopy

The adsorption of the surfactant n-nonyl-beta-D-glucopyranoside at the air-water interface after injection of the surfactant into the subphase was studied by infrared reflection absorption spectroscopy. In the first part, we investigated the equilibrium adsorption of n-nonyl-beta-D-glucopyranoside and the Gibbs adsorption isotherm was measured by applying the film balance technique. In the second part, the adsorption kinetics was followed by changes in the surface pressure and in the intensities of the OH band, which is related to the layer thickness, and the CH(2) antisymmetric stretching vibrational band. During an induction period, when the molecules are still highly diluted and the surface pressure is low, they are oriented parallel to the air-water interface. IR band simulations for the CH(2) antisymmetric stretching vibrational band support the idea of horizontally oriented molecules at the air-water interface. Later on, when more molecules are adsorbed to the air-water interface, they suddenly rearrange to an upright orientation as indicated by changes of the OH and the CH(2) bands. The observations are discussed in comparison to results obtained for the adsorption kinetics of n-decyl-beta-D-maltopyranoside, n-dodecyl-beta-D-maltopyranoside, and sodium dodecyl sulfate.     

Copper-on-copper homoepitaxy studied by infrared spectroscopy of adsorbed CO

Infrared spectroscopy of adsorbed CO was used to characterize the dependence of surface structure on deposition temperature during homoepitaxial growth on Cu(100). Intensity borrowing due to dipole coupling greatly enhances the absorption signal due to defect-bonded CO, making it possible to detect and quantify defect concentrations at the level of a few percent. For deposition temperatures between 300 and 400 K, the defect density increases slightly with decreasing deposition temperature but remains below 2%. There is a sharp increase in defect density, up to 5%-6%, as the deposition temperature is decreased from 300 to 250 K. At lower deposition temperatures, there is some sign of a leveling off in defect density, but the IR absorption spectrum becomes so broad that meaningful analysis becomes impractical, while visible degradation of the low-energy electron diffraction pattern indicates worsening surface order. No indication of "re-entrant" ordering at low temperatures was observed for deposition temperatures down to 150 K.     

Surface enhanced infrared absorption spectroscopy (SEIRA) using external reflection on low-cost substrates

A novel approach for spectroscopic trace analysis is introduced by combining surface enhanced infrared absorption (SEIRA) spectroscopy with external reflection techniques on disposable inexpensive substrates. SEIRA-active surfaces produced by electrochemical deposition of silver on smooth metal surfaces and glass improve the sensitivity of IR reflection measurements significantly since the infrared absorption of organic substances such as p-nitrobenzoic acid is considerably increased in the vicinity of rough noble metal surfaces. The enhancement properties of thus prepared substrates are characterized and compared using IR-spectroscopy. These low-cost substrates used in single- and multiple external reflection arrangements, respectively, yield a significant increase of the detection level compared to conventional reflection absorption infrared spectroscopy (RAIRS) up to one order of magnitude. Hence, a notable step towards a wide-spread application of SEIRA in routine IR reflection analysis is presented. Received: 20 January 1998 / Revised: 3 June 1998 / Accepted: 5 June 1998     

Surface enhanced infrared absorption spectroscopy (SEIRA) using external reflection on low-cost substrates

A novel approach for spectroscopic trace analysis is introduced by combining surface enhanced infrared absorption (SEIRA) spectroscopy with external reflection techniques on disposable inexpensive substrates. SEIRA-active surfaces produced by electrochemical deposition of silver on smooth metal surfaces and glass improve the sensitivity of IR reflection measurements significantly since the infrared absorption of organic substances such as p-nitrobenzoic acid is considerably increased in the vicinity of rough noble metal surfaces. The enhancement properties of thus prepared substrates are characterized and compared using IR-spectroscopy. These low-cost substrates used in single- and multiple external reflection arrangements, respectively, yield a significant increase of the detection level compared to conventional reflection absorption infrared spectroscopy (RAIRS) up to one order of magnitude. Hence, a notable step towards a wide-spread application of SEIRA in routine IR reflection analysis is presented.     

Structures of 17,19-hexatriacontadiyne monolayers on Au(111) studied by infrared reflection absorption spectroscopy and scanning tunneling microscopy

The aggregation and reaction of 17,19-hexatriacontadiyne molecules are studied on a Au(111) surface. The molecular orientation and arrangement are elucidated by infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). A vapor-deposited monolayer and a multilayered film formed by adsorption from the solution provide IRA spectra with bands due to the antisymmetric and symmetric stretching of methylenes in the gauche conformation. After the adsorbed film is rinsed with the solvent, however, the spectrum loses the gauche bands and is characterized by the enhanced C-H(distal) and C-H(proximal) stretching bands, which means that all-trans molecules are laid flat. Only STM images for the rinsed film display columnar structures on the herringbones of the reconstructed Au(111) surface; the alkyl chain direction is found to be parallel to the Au atom row. The results indicate that an ordered monolayer is formed first at the liquid-solid interface, and then, disordered overlayers with the gauche conformation are grown but removed by a rinse. Upon exposure to UV light, thus obtained monomer columns are converted into oligomers with flexible backbones and an increased gauche population in the alkyl chains, which resemble red phase polydiacetylenes in LB films.     

In situ monitoring of protein adsorption on a nanoparticulated gold film by attenuated total reflection surface-enhanced infrared absorption spectroscopy

In situ surface enhanced infrared absorption spectroscopy (SEIRAS) with an attenuated total reflection (ATR) configuration has been used to monitor the adsorption kinetics of bovine hemoglobin (BHb) on a Au nanoparticle (NP) film. The IR absorbance for BHb molecules on a gold nanoparticle film deposited on a Si hemispherical optical window is about 58 times higher than that on a bare Si optical window and the detection sensitivity has been improved by 3 orders of magnitude. From the IR signal as a function of adsorption time, the adsorption kinetics and thermodynamics can be explored in situ. It is found that both the electrostatic interaction and the coordination bonds between BHb residues and Au NP film surface affect the adsorption kinetics. The maximum adsorption can be obtained in solution pH 7.0 (close to the isoelectric point of the protein) due to the electrostatic interaction among proteins. In addition, the isotherm of BHb adsorption follows well the Freundlich adsorption model.     

Formation and bonding of alane clusters on Al(111) surfaces studied by infrared absorption spectroscopy and theoretical modeling

Alanes are believed to be the mass transport intermediate in many hydrogen storage reactions and thus important for understanding rehydrogenation kinetics for alanates and AlH3. Combining density functional theory (DFT) and surface infrared (IR) spectroscopy, we provide atomistic details about the formation of alanes on the Al(111) surface, a model environment for the rehydrogenation reactions. At low coverage, DFT predicts a 2-fold bridge site adsorption for atomic hydrogen at 1150 cm(-1), which is too weak to be detected by IR but was previously observed in electron energy loss spectroscopy. At higher coverage, steps are the most favorable adsorption sites for atomic H adsorption, and it is likely that the AlH3 molecules form (initially strongly bound to steps) at saturation. With increasing exposures AlH3 is extracted from the step edge and becomes highly mobile on the terraces in a weakly bound state, accounting for step etching observed in previous STM studies. The mobility of these weakly bound AlH3 molecules is the key factor leading to the growth of larger alanes through AlH3 oligomerization. The subsequent decomposition and desorption of alanes is also investigated and compared to previous temperature programmed desorption studies.     

Surface orientation of a liquid-crystalline polymer studied by polarized reflection spectroscopy

The polarized reflection spectrum was used to evaluate the surface orientation of extruded sheets of liquid-crystalline copolyester consisting of 73 mol % 4-hydroxybenzoic acid units and 27 mol % 2-hydroxy-6-naphthoic acid units. The anisotropy of absorption index was evaluated from the specular reflectance using the Fresnel equation and the Kramers-Kronig relation. An intense absorption and observed in the ultraviolet region was assigned to the π^* ←π transitions of the constituent monomeric units. The observed spectra of the liquidcrystalline polymer were successfully interpreted by use of the spectroscopic data on the monomers. The transition moment directions of the monomeric units were calculated by molecular orbital calculations within the framework of the CNDO/S-Cl approximation. The Surface orientation functions obtained from polarized reflection spectra were compared with the crystal orientation functions in the bulk, which were obtained from the azimuthal intensity distribution of wide-angle x-ray diffraction. The orientation behaviour at the surface of the liquid-crystalline polymer was shown to be equivalent to the orientation behaviour in the bulk at higher draw-down ratio.     

Specific crystal structure of poly(3-hydroxybutyrate) thin films studied by infrared reflection–absorption spectroscopy

Infrared reflection–absorption (IR-RAS) and transmission spectra were measured for poly(3-hydroxybutyrate) (PHB) thin films to explore its specific crystal structure in the surface region. As IR-RAS is sensitive to the vibration mode of perpendicular orientation of the surface, differences between IR-RAS and transmission spectra indicate an orientation of the lamella structure in the surface of PHB thin films. The relative intensity of the crystalline CO stretching band in the IR-RAS spectrum is significantly weaker than that in the transmission spectrum. It may be concluded that the transient dipole moment of the CO stretching mode of the crystalline state is not oriented perpendicular but nearly parallel to the substrate surface. On the other hand, the relative intensity of the band at 3009 cm⁻¹ due to the C–H stretching mode of the C–HOC hydrogen bonding is similar between the IR-RAS and transmission spectra, suggesting that the C–H bond is oriented neither perpendicular nor parallel to the substrate surface but in an intermediate direction. Since the CO group of the C–HOC hydrogen bonding is oriented nearly parallel to the surface and its C–H group is in the intermediate direction, it is very likely that the C–HOC hydrogen bonding has a somewhat bent structure. These results are in good agreement with our previous conclusion that the C–HOC hydrogen bonding of PHB exists along the a-axis (not the b-axis) between the CH₃ group of one helix and the CO group of another helix.     

A reflection absorption infrared spectroscopy and density-functional theory investigation of methanol dehydrogenation on Rh(111)V alloy surfaces

The dehydrogenation reaction of methanol on a Rh(111) surface, a Rh(111)V subsurface alloy, and on a Rh(111)V islands surface has been studied by thermal-desorption spectroscopy, reflection absorption infrared spectroscopy, and density-functional theory calculations. The full monolayer of methanol forms a structure with a special geometry with methanol rows, where two neighboring molecules have different oxygen-rhodium distances. They are close enough to form a H-bonded bilayer structure, with such a configuration, where every second methanol C-O bond is perpendicular to the surface on both Rh(111) and on the Rh(111)V subsurface alloy. The Rh(111)V subsurface alloy is slightly more reactive than the Rh(111) surface which is due to the changes in the electronic structure of the surface leading to slightly different methanol species on the surface. The Rh(111)V islands surface is the most reactive surface which is due to a new reaction mechanism that involves a methanol species stabilized up to about 245 K, partial opening of the methanol C-O bond, and dissociation of the product carbon monoxide. The latter two reactions also lead to a deactivation of the Rh(111)V islands surface.     

Formic acid electrooxidation on Pd in acidic solutions studied by surface-enhanced infrared absorption spectroscopy

A mechanistic study of electrocatalytic oxidation of formic acid on Pd in sulfuric and perchloric acids is reported. Surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) shows the adsorption of CO, bridge-bonded formate, bicarbonate, and supporting anions on the electrode surface. Poisoning of the Pd surface by CO, formed by dehydration of formic acid, is very slow and scarcely affects formic acid oxidation. The anions are adsorbed more strongly in the order of (bi)sulfate > bicarbonate > perchlorate, among which the most strongly adsorbed (bi)sulfate considerably suppresses formic acid oxidation in the double layer region. The oxidation is suppressed also at higher potentials in both acids by the oxidation of the Pd surface. Adsorbed formate is detected only when formic acid oxidation is suppressed. The results show that formate is a short-lived reactive intermediate in formic acid oxidation and is hence detected when its decomposition yielding CO(2) is suppressed. The high electrocatalytic activity of Pd can be ascribed to the high tolerance to CO contamination and also high catalytic activity toward formate decomposition.     

Adsorption of ethyl ether on polycrystal and the (110) single crystal face of gold by admittance measurements and modulated reflection spectroscopy

The adsorption of ethyl ether on polycrystalline and on (110) gold electrodes has been studied by simultaneous measurements of differential capacity and modulated electroreflectance (ER) in aqueous solutions of 0.02 M NaF containing various concentrations of ethyl ether. The results indicate that the adsorption can be studied only at negative charges, the behaviour at positive charges being masked by the oxidation of ether. A comparison of our capacity curves with the data obtained for Hg would indicate that the ether molecules are less strongly adsorbed on Au than on Hg. The ER measurements show that there is a proportionality between the amount adsorbed and the reflectivity change, and confirms that a simple interfacial model may be used to evaluate the amount adsorbed. Qualitative analysis of the ER spectra reveal that the adsorbed layer exhibits transparent optical properties and displays anisotropic characteristics in the adsorption—desorption region, probably due to the sudden change in the layer structure. Furthermore, the ER spectra provide evidence for the absence of any chemical interaction between gold and ether molecules.