Infrared Reflection Absorption Spectra Research Articles

Low Potential Electrochemical Synthesis of Polyfuran and Characterization of the Obtained Free-Standing Film

A free-standing polyfuran (PFu) film has been successfully synthesized by electrochemical polymerization of furan at low potential (1.2 V vs Ag/AgCl) in a binary solvent system consisting of boron trifluoride−ethyl ether (BFEE) and additional ethyl ether (EE). The obtained polymer contains much less ring-opened component and shows higher conductivity than that prepared by chemical method or by electropolymerization in other solvent systems. Its electric conductivity is as high as 10-2 S cm-1 both in its electrochemically doped state and chemically doped state with I2. Moreover, the obtained free-standing film shows good mechanical properties, which makes itself a powerful competitor to other conducting polymers. The orientation of PFu deposited on Au and stainless steel electrode was investigated by surface enhanced Raman spectra (SERS) and reflection−absorption infrared spectra (RA-IR) techniques. The furan ring in the polymer chain is inclined to be arranged parallel to the metal surface at the early stage of deposition.

Chemistry of 3-Hexyne on Ru(0001): A Reflection−Absorption Infrared Spectroscopy Study

3-Hexyne has been adsorbed on Ru(0001) at different temperatures under ultrahigh vacuum. At 100 K and very low coverage (0.05 L), the reflection−absorption infrared (RAIR) spectrum is consistent with the formation of a surface di-σ/π complex involving the rehybridization of the acetylenic carbon atoms. This complex is oriented with the plane containing the α and β carbon atoms tilted relative to the surface. For an exposure of 0.07 L, a more compact layer of the di-σ/π species or, alternatively, a second layer of a π complex was identified by the appearance in the RAIR spectrum of the methyl symmetric modes. For higher exposures, a surface multilayer of 3-hexyne is formed, which compares well with the transmission spectrum of solid-phase 3-hexyne. The thermal decomposition of 3-hexyne on Ru(0001) follows two different paths, depending on whether the molecule is chemically bonded to the surface at low temperature (below ∼200 K) and subsequently heated or adsorbed at higher temperatures, decomposing on adsorption. In the first case, the di-σ/π complex decomposes above 143 K into an adsorbed acetylenic species and other fragments. These are mostly pumped away, but some dehydrogenates are adsorbed at 223 K as ethylidyne, identified by the small band at 1344 cm-1 assigned to the methyl umbrella mode. In the second case (adsorption at the reaction temperature) the C−C triple bond breaks completely, leaving a mixture of surface species that depends on the temperature. Upon adsorption at 243 K, only adsorbed propylidyne and ethylidyne were identified. At 273 K, the RAIR spectrum is dominated by the intense methyl umbrella mode and, in the C−H stretching region, by the methyl symmetric stretching mode of ethylidyne bonded to the surface in a 3-fold hollow site in C3v symmetry. Only at 320 K ethylidyne is completely decomposed, leaving adsorbed methylidyne as the only species identified by RAIRS.

Optically Induced Band Shifts in Infrared Spectra of Mixed Self-assembled Monolayers of Biphenyl Thiols

Optically induced band shifts in reflection absorption infrared spectra (RAIR) of mixed self-assembled monolayers (SAMs) are reported. SAMs of 4-trifluoromethyl-4-mercaptobiphenyl dispersed in 4‘-methyl-4-mercaptobiphenyl on gold exhibit significant shifts of the spectral envelope of the symmetric CF3 stretch and the b1u skeletal mode. A linear relationship is observed between the band positions, the band intensity, and the surface CF3 group concentration. To rationalize the observed changes in band positions and shapes, a classical electromagnetic theory approach is used, providing an excellent agreement between theory and experiments. The simplified RAIR signature of 4‘-trifluoromethyl-4−mercaptobiphenyl on gold is a unique example of the surface selection rule.

Infrared reflection–absorption spectroscopic study on the adsorption structures of acrylonitrile on Ag(111) and Ag(110) surfaces

Infrared reflection–absorption spectra in CN stretching, CH 2 out-of-plane wagging and CH 2 twisting vibration regions were measured for acrylonitrile (CH 2CHCN) exposed to Ag(111) and Ag(110) in increasing amounts at 77 K. The adsorbate on Ag(111) takes on a series of discrete adsorption states; i.e., an isolated state, associated states, and ordered and amorphous multilayer states. The adsorbate on Ag(110) at lower exposures is in a state with the CN group weakly coordinated to a silver atom (or silver atoms). The adsorbate on Ag(110) takes the associated state and the amorphous multilayer at larger exposures. On raising the temperature to 96 K, the amorphous states on both Ag(111) and Ag(110) are converted to the ordered multilayer. The desorption temperature of the ordered multilayer is below 99 K for Ag(110), while the temperature is above 107 K for Ag(111); the result indicates the effect of the surface morphology on the stability of the ordered state.

Infrared reflection absorption spectroscopic study on the adsorption structures of ethylene on Ag(110) and atomic oxygen pre-covered Ag(110) surfaces

Infrared reflection absorption spectra in the CH 2 out-of-plane wagging (ω(CH 2)) vibration region were measured for ethylene (C 2H 4) adsorbed on Ag(110) as well as on the oxygen-induced p( n×1) reconstructed surfaces of Ag(110) ( n=2, 3, 4 and 6) at 80 K. C 2H 4 on Ag(110) gives a main peak at 955 cm −1, while on p(2×1)O–Ag(110) it exhibits a broad features of at least four components (997, 984, 970 and 954 cm −1) at saturation coverage. C 2H 4 on p( n×1)O–Ag(110) ( n=6, 4, 3) gives rise to a 972–976 cm −1 band at low exposures, shifting to 966–970 cm −1 at saturation coverage. The spectral changes are interpreted by assuming a pair of adsorption sites on both sides of the added Ag–O rows of the reconstructed surfaces.

Infrared studies of mixed Langmuir–Blodgett monolayers of octadecyldimethylamine oxide and dioctadecyldimethylammonium chloride with arachidic acid and poly(acrylic acid)

Infrared reflection–absorption spectra (IRAS) have been measured for monolayers of octadecyldimethylamine oxide (C 18DAO), dioctadecyldimethylammonium chloride (2C 18DAC) and their mixtures with arachidic acid and poly(acrylic acid) prepared by Langmuir–Blodgett (LB) technique on gold-evaporated glass slides. The spectra show that arachidic acid and C 18DAO or 2C 18DAC molecules form well-ordered monolayers with trans zigzag conformations. The order of the alkyl chain in C 18DAO is greatly increased upon the formation of the mixed LB film. The relative intensities of infrared bands due to CH 2 and CH 3 stretching modes of the hydrocarbon chains indicate that they are tilted with respect to the surface normal in the mixed LB films of arachidic acid and C 18DAO or 2C 18DAC. The hydrocarbon chains of C 18DAO and 2C 18DAC are disordered in the mixed LB films of poly(acrylic acid) and C 18DAO or 2C 18DAC.

Coverage-Dependent Infrared Spectroscopy of Carbon Monoxide on Palladium(100) in Aqueous Solution: Adlayer Phase Transitions and Electrooxidation Pathways

Infrared reflection−absorption spectra (IRAS) measured for the C−O stretch (νCO) of carbon monoxide dosed on ordered Pd(100) in aqueous 0.1 M HClO4 as a function of the coverage (θCO) and the electrode potential (E) are examined in comparison with extant IRAS and adlayer structural data on Pd(100) in ultrahigh vacuum (UHV). A noteworthy feature of the electrochemical adlayer, as for the UHV system, is the presence of a single νCO band blue shifting markedly (by ca. 100 cm-1) with increasing coverage up to saturation (θCO ≈ 0.8), indicative of uniformly bridging CO coordination. Extrapolation of the θCO-dependent νCO frequencies to surface potentials corresponding to the UHV-based system, however, yielded reasonable concordance only at high coverages. The discrepancies at lower θCO are suggested to arise from solvation effects on the local CO-site surface potentials. Examination of the θCO-dependent νCO peak frequencies ( ), integrated absorbances (Ai), and bandwidth (Δν1/2) for the dosed electrochemical a...

In Situ Infrared Reflection Absorption Spectroscopy of Materials Formed on SiO2 in Inductively Coupled Plasma Etching Chamber

In situ infrared reflection absorption (IRA) spectroscopy was used to investigate materials formed on film coated on samples mounted on the inner surface of a process chamber for inductively coupled plasma etching. Aluminum wafers partially covered with photoresist film were etched using and plasma generated in a quartz chamber surrounded by a radio frequency (rf) coil. RF bias was applied to the aluminum wafers at the same time. This is the first time that, in order to predict the IRA spectra, the reflectivities of various inorganic multiple layers formed on metal substrates have been calculated using the optical constants of the materials. The experimental spectra for film and for the materials formed on it during a 2 μm deep etching process were well characterized by the calculated spectra of film and a layer on it, respectively. © 1999 The Electrochemical Society. All rights reserved.

In situ infrared reflection–absorption spectroscopic characterization of sustained kinetic oscillations in the Pt(100)/NO+CO system

We have excited kinetic oscillations in the Pt(100)/NO+CO system at 470 K and pressure region around 2×10 −4 Torr. These oscillations are sustained and they arise by simply adjusting the partial pressures of NO and CO while holding the surface temperature at 470 K. However, they can only be induced if the partial pressure of CO is greater than that of NO (i.e., 1.4±0.05< p CO/ p NO<1.7±0.04). Infrared reflection–absorption spectra (IRAS) collected during the oscillation showed that the periodic growth and decay of the CO band are counterphase to the reaction-rate oscillations. Infrared bands due to molecularly adsorbed NO were not found during the early stages of the temporal evolution of the system into the upper reaction-rate branch. However, during the later stages of the evolution, a single IR band due to chemisorbed NO was observed. This band eventually disappeared as the reaction rate finally reached its maximum value. With the use of the model by Fink et al., we were able to rationalize the occurrence of oscillations under the present experimental conditions. Oscillations are caused by the periodic change in the level of adsorbate coverages that controls the dissociation of NO.

Melting of Rodlike Molecules on Pt(111). Infrared Spectroscopic Studies of Isotopically Labeled n-Alkanes

The melting of monolayers of isotopically labeled n-alkanes on Pt(111) surfaces has been followed by reflection−absorption infrared (RAIR) spectroscopy, and the results are compared with those of an earlier study of the unlabeled molecules (J. Phys. Chem. 1995, 99, 15629−15278). Temperature-dependent studies show that monolayers of the n-octane isotopolog, CD3(CH2)6CD3, melt on Pt(111) from a two-dimensionally ordered phase to one having one-dimensional order (“hexatic”) near 220 K, a temperature essentially identical to that seen for unlabeled n-octane. The RAIR spectra of the labeled molecules generally confirm (but in one case clarify an overly simplistic interpretation of) the assignments of the bands made in the earlier study. Specifically, the assignment of a low-frequency “softened” mode near 2760 cm-1 to a νCH stretch for proximal (i.e., surface-contacting) methylene C−H bonds has been verified. A feature near 2900 cm-1 had previously been assigned to a distal methylene C−H stretch, and the line w...

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 HClO 4 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 ( ν NO) band at 1590–1625 cm −1, the frequency upshifting with increasing coverage, θ NO. The ν NO frequencies at a given θ NO value are sensitive to the electrode potential, d ν NO/d E decreasing towards higher coverage, from 90 cm −1 V −1 (at θ NO=0.2) to 40 cm −1 V −1 at saturation ( θ NO≈0.5). The frequencies as well as the form of the electrochemical ν NO 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 HClO 4 yields C–O stretching ( ν CO) 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 ν NO frequency, the ν NO– θ NO dependence being similar to that observed for NO adsorption alone. This coadsorbate behavior, along with related changes in the ν CO spectra, a strongly increased preference for atop CO binding, and intensity-transfer from the ν NO to the ν CO bands, is indicative of microscopic intermixing of the NO and CO components. Some aspects of the ν NO and ν CO spectral behavior, however, suggest that small NO aggregates may form at intermediate mixed NO/CO compositions. Unexpectedly small (<10 cm −1 V −1) ν NO– E and ν CO– 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.

Fabrication and characterization of charge-transfer complex films by a bias-voltage LB technique

We have investigated the effect of bias voltage applied to an electrically conductive substrate on the formation of adsorption Langmuir–Blodgett (LB) films. In fabricating the adsorption LB films, we used tetracyanoquinodimethane (TCNQ) with a long alkyl chain as a molecule for spreading monolayers on the subphase and tetramethyl- p-phenylenediamine (TMPD) as a material for adsorbing from the subphase to the monolayers. It is well known that TCNQ and TMPD are acceptor-like and donor-like molecules, respectively. In the visible absorption spectra and Fourier transform infrared reflection–absorption spectra of the adsorption LB films fabricated under several bias conditions, the peaks assigned to TCNQ show a clear dependence on the bias voltage. The electrical conductivity corresponding to the quantity of charge-transfer complex was also obtained. These results demonstrate that the bias-voltage LB technique enables us to control the formation of the charge-transfer complex.

In situ UV-Vis and ex situ IR spectroelectrochemical investigations of amorphous and crystalline electrochromic Nb 2 O 5 films in charged/discharged states

Niobium oxide (Nb2O5) films and powders have been obtained via the sol-gel route from an NbCl5 precursor. XRD spectra revealed that films with pseudohexagonal (TT-phase) and orthorhombic (T-phase) structure were formed at 500 °C and 800 °C, respectively, while at 300 °C films were amorphous. Infrared (IR) and Raman spectra of powders and films of Nb2O5 in different polymorphic forms were detected, and vibrational band assignments were made. Electrochromic properties of amorphous films and films with the TT-phase were established from in situ ultraviolet visible (UV-Vis) spectroelectrochemical measurements and correlated with ex situ IR transmission spectra of charged films. Ex situ IR spectra revealed that charging of amorphous films is accompanied by variations of the Nb-O stretching mode intensity, while, for films with the TT- and T-phase, splitting of the Nb3-O stretching modes and the appearance of polaron absorption were noted with Li+ ion insertion. Ex situ X-ray diffraction (XRD) spectra of charged films with the TT-phase showed changes of the unit cell dimensions with charging. The influence of the polaron absorption on the ex situ near-grazing incidence angle (NGIA) IR reflection-absorption spectra of charged/discharged films is discussed in detail.

The effect of co-adsorption of on-top CO on the sum-frequency generation signal of bridge CO on the Ni(111) surface

Anomalous coverage dependence was observed on the sum-frequency generation signal of bridge CO on the Ni(111) surface when co-adsorbed by on-top CO. The bridge CO peak was distinct and normal from 0.04 to ∼0.40 monolayer (ML) but weakened after the appearance of the on-top CO peak at 0.47 ML and disappeared at 0.60 ML, although both peaks were observed in the infrared reflection absorption spectra up to 1.0 ML. The spectra of mixtures of C 16O and C 18O eliminate the possibility that the weakening of the bridge CO band is caused by its coupling with the on-top band.

Infra-red reflection absorption spectroscopic study on adsorption structures of acrolein on polycrystalline gold and Au(111) surfaces under ultra-high vacuum conditions

Infra-red reflection absorption (IRA) spectra were measured at 80 K under ultra-high vacuum conditions for acrolein adsorbed on two kinds of gold films; Au(111) and polycrystalline gold surfaces. Upon increasing the amount of exposure from 0.02 to 200 L (1 L=1×10 −6 Torr·s), the adsorbate at Au(111) gave rise to a series of sharp IRA bands due to a CH 2 out-of-plane wagging vibration [ ω(CH 2)] successively, indicating discrete adsorption states, i.e. 964 (type 1)→978(type 1′)→991(type 2)→1003 cm −1(type 3). All these states have the molecular plane parallel to the surface; type 1 is in an isolated state, and type 2 is in an associated state with a two-dimensional arrangement, whereas type 3 forms an ordered multilayered structure. Type 1′ was tentatively assigned either to a trapped state at step sites or to an associated state forming small oligomers at the surface. Only type 3 gives rise to IRA bands due to ν(CO), which appears at 1677 cm −1 as a singlet at relatively small exposure levels and splits into doublets, giving the 1686 and 1672 cm −1 components at 2.0 L. The doublets were explained as being due to a crystal field splitting, which conforms to the fact that the adsorbate forms an ordered three-dimensional arrangement. The IRA spectrum of type 3 is readily converted to that of a more stable polycrystalline state upon increasing the temperature from 80 to 100 K. Thus, type 3 is a thermodynamically metastable state. Acrolein adsorbed on a polycrystalline gold film assumes an amorphous state in the exposure level of 0.06–4.8 L, giving broad IRA bands due to ν(CO) and ω(CH 2) in the 1686–1699 and 974–991 cm −1 regions, respectively. The IRA spectra of acrolein adsorbed on Ag(111) were also measured, which indicated that the adsorbates exist in a less ordered state than those on Au(111), although a multilayered structure gives IRA features that are almost identical with those of type 3.

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

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.

The thermal chemistry of adsorbed ethyl on the Pt(111) surface: infrared evidence for an ethylidene intermediate in the ethyl to ethylidyne conversion

The reflection absorption infrared (RAIR) spectrum of the adsorbed ethyl (C2H5) moiety produced by the dissociative adsorption of ethane from a supersonic molecular beam onto Pt(111) at 150 K is reported. The thermal chemistry of the C2H5 fragment so produced was then followed using a combination of RAIR spectroscopy and temperature programmed desorption (TPD). The RAIR spectra indicate the presence of ethylidene (CHCH3) and ethylidyne (CCH3) at 250 and 350 K respectively. This implies the stepwise loss of one and two hydrogen atoms from the ethyl moiety during the heating process. Temperature programmed desorption (TPD) measurements show that hydrogen desorption is not accompanied by desorption of either saturated or unsaturated C2 hydrocarbons or methane into the gas phase.

A study on adsorption structures of methacryloyloxyalkyl dihydrogen phosphates on silver substrates by infrared reflection absorption spectroscopy.

10-Methacryloyloxydecyl dihydrogen phosphate (M10P) for use in dentistry has recently been noted as an adhesive monomer contained in a metal primer. Although the treatment of a metal surface with primer before the application of resin is recognized to improve the adhesion between metal and resin, the role of M10P in the adhesion process has not been clarified. In this study, infrared reflection absorption (IRA) spectroscopy was employed to study the adsorption structures of M10P as well as 2-methacryloyloxyethyl dihydrogen phosphate (M2P) on evaporated silver substrates. The IRA spectra of the self-assembled films of those phosphates verified the adsorption of M10P or M2P on silver substrates from the methyl methacrylate solutions (5 x 10(-5) mol/L). The saturation coverages of M10P and M2P were completed after about 50 and 25 min, respectively. Two characteristic bands around 980 and 1080 cm-1 due to the PO(2-)3 stretching vibrations were observed. These results indicate that the phosphate groups of both monomers are adsorbed to silver surfaces in the dissociated form, -PO(2-)3, and form hydrophobic monolayers. The monolayer of M10P was found to be more durable against thermocycling in water than that of M2P by IRA measurements. The roles of M10P in the metal primer are presumably to form such a monolayer with appreciable durability and to promote polymerization with resin monomers.

Infrared Spectroscopy of Model Electrochemical Interfaces in Ultrahigh Vacuum: Roles of Solvation in the Vibrational Stark Effect

The effects of dosing various solvents along with potassium upon the infrared reflection−absorption spectra (IRAS) for saturated chemisorbed CO adlayers on Pt(111) at 90 K in ultrahigh vacuum (UHV)...

Construction of Semiconductor Nanoparticle Layers on Gold by Self-Assembly Technique

Cadmium sulfide (CdS) nanoparticles were prepared using the Aerosol-OT (AOT)/heptane inverse-micelle method. The particle size was controlled by changing the [H2O]/[AOT] ratio in the preparation process to be ca. 3 nm, which was estimated from their absorption maximum ( ca. 385 nm) and absorption edge ( ca. 450 nm) in heptane solutions. Self-assembled monolayers (SAMs) of α,ω-dithiols were prepared on (111)-oriented gold substrates. They were highly stable in aqueous solutions and showed a blocking effect toward electron exchange between the gold substrate and the electroactive species in the solution. Mono- and multi-layers of CdS nanoparticles were prepared by immersing the SAM-covered gold substrates in a heptane solution containing CdS particles. Infrared reflection-absorption spectra revealed that CdS particles on SAMs were surrounded by AOT and that a sequential immersion procedure led to the alternately layered structure of CdS particles and dithiol molecules.