Thin films containing Co, C, and N were co-sputtered in vacuum for kinetic studies of oxygen reduction in alkaline electrolytes. Stability tests show that the oxygen reduction activity of the Co-C-N catalysts deteriorates with time. Films were characterized with X-ray photon spectroscopy (XPS) and scanning electron microscopy (SEM) surface techniques before and after electrochemical studies. The analyses reveal a loss of cobalt and nitrogen from the surface region and the formation of cobalt oxide species through its reaction with oxygen. However, the activity of the optimal film, which decreased by less than 50% after two months of continuous operation, is considerably stable compared to metal chelates. The oxygen adsorption on the film surface appears to be consistent with the Griffith model, while the rate-determining step is the primary charge transfer. The mechanism for a four-electron oxygen reduction has been proposed.

Thin films of amorphous Co-C-N mixtures which were co-sputtered in vacuum showed good catalyst properties for oxygen reduction in alkaline solution. The optimal catalytic activity was Co0.68C0.16N0.16. The oxygen reduction for these films proceeds via a four-electron pathway and the Co2+/Co3+ redox couple is shown to be responsible for the oxygen reduction reaction. The performance is comparable to transition metal macrocycles in alkaline solutions. Evidently, intact structures of the macrocycles are not necessary for good catalyst activity.

The thermal behavior of Langmuir-Blodgett (LB) monolayers of DL-dipalmitoyl phosphatidylethanolamine (DPPE) deposited onto a ZnSe substrate was studied by polarized infrared attenuated total reflection (ATR), over the temperature range 25-140 degrees C. The results are discussed in terms of the imaginary part of the effective electric susceptibility tensor [gamma] of the monolayer, a function that approximately corrects for the effect of the local field. We show that the spectra of the imaginary parts of the principal components of [gamma], Im(gamma(t)) and Im(gamma(n)) (the subscripts t and n representing the directions tangent to and normal to the film, respectively), can be straightforwardly obtained from polarized reflection spectra. The results obtained using Im(gamma(t)) and Im(gamma(n)) are compared with those obtained from the extinction coefficients k(t) and k(n), which were calculated from adapted Kramers-Kronig relations. We show that Im(gamma(t)) and Im(gamma(n)) provide a better account of the molecular processes occurring in the DPPE LB films as a function of temperature. No abrupt phase transition was observed. This is attributed to positional disorder in the headgroups of the monolayer as it was initially transferred onto the ATR prism.

We compare two methods for obtaining structural information on Langmuir-Blodgett films from polarized infrared spectroscopy. For sufficiently uniform films, we have already shown (refs 11 and 12) that all polarized spectroscopic properties could be characterized by a single quantity that we call the "electrical surface susceptibility tensor", <(gamma)over tilde>. The imaginary parts of the susceptibility tensor could be readily obtained from reflectance measurements with the electric field parallel (Im(gamma(t))) and perpendicular (Im(gamma(n))) to the plane of the film, independently of any specific properties of the film. This, in turn, could be related to the characteristics of individual molecules comprising the film, and their geometric disposition if the molecules are assumed to be interacting dipoles, which corrects, to a large extent, for the local field effects. We report the results of an infrared study performed on a dipalmitoylphosphatidylethanolamime monolayer. Attenuated total reflection spectra for both s and p polarization were recorded, and the spectra of Im(gamma(t)) and Im(gamma n), as well as those of the imaginary part of the refractive index (k(t) and k(n)), ark presented. Finally, the information deduced from the Im(gamma(x)) and k(x) (x = s, p) are compared, and the molecular orientation is discussed both in terms of Im(gamma(x)) and k(x). We show that by using this approach one obtains more truthworthy measurements of the disposition of oriented molecules at surfaces than those obtained from absorption coefficients.

The model developed recently by us for calculating the optical properties of a small isotropic sphere, with a radially uniaxial coating of molecules, is extended here to include multicomponent coatings in an approximation in which the molecules, treated as point polarizable, are assumed to be randomly distributed over spherical shells of close to the same radius. Calculations are performed to determine the effect of coadsorption of Xe on the infrared absorption properties of CO2 physisorbed on silica particles with the molecular axes oriented radially. The calculations confirm an earlier suggestion by one of us that the effect of coadsorption of an ‘‘inert molecule,’’ such as Xe, on the infrared absorption spectrum of an adsorbed species can be used to provide information on the orientation of the species with respect to the local surface normal.

In parts I and II of this series, the dominant role of ‘‘primary’’ Raman moments of degree greater than one, to Raman intensities for molecules forming a uniform coating on pairs of spherical particles in close proximity, is demonstrated. In this paper, the effect of chain length, coverage, particle size, and separation, and the location of the particle in the chain on the intensity are explored for linear chains of coated particles. Intensity enhancements calculated for visible excitation are up to 30 times those for two particles, being ∼4×106 for infinite chains of silver particles of radius 25–50 Å separated by the thickness of a layer of CO ∼105 for platinum and ∼104 for germanium chains. Furthermore, the enhancement for chains falls off appreciably more slowly with particle separation. Graphs of intensity vs coverage vary greatly in shape depending on particle size, molecular size, and polarizability.

In part I, a mathematical formalism was developed for calculating Raman scattering intensities from a collection of spherical particles uniformly coated with molecules, and applied to the special case of two CO-coated Ag particles in contact, with their centers aligned with the laser field and under parallel polarization conditions. In this paper, the additional relations required to perform the calculations for arbitrary orientation and polarization conditions are developed and used in calculations for CO and benzene physisorbed onto pairs of particles of Ag, Pt, Ge, and SiO. The large increase in Raman radiation intensity per molecule, reported in part I on going from a one-particle to a two-particle system, is shown here to apply only to the Raman radiation component polarized parallel to the interparticle axis and due to the incident radiation component similarly polarized. For all other components, the excitation spectra are similar for the two systems. The large increase for the parallel–parallel case is due to the process in which the primary Raman multipoles centered on one particle induce Raman dipoles on the pair of particles.

The theory of intense Raman scattering, generated by dipole active vibrational modes of molecules adsorbed on small metal particles, is reformulated here to make use of our recently developed approach to calculating the polarizability of small metal spheres and to include terms missed in our earlier development. The approach is based on the local-dielectric-response approximation and requires a knowledge of the conduction electron density profile at the metal/ambient interface. The modulation of the polarizability of a metal particle, due to the quadrupolar field generated by a phonon mode of the adlayer, is calculated using an integration procedure that incorporates both electron density and induced charge density profiles determined by Lang and Kohn for jellium. Scattering intensities for both Raman and dipole active modes of CO and CO2 on small silver particles are calculated and compared. The results show that the ratio of the scattering intensity for a Raman active molecular mode to that for a dipole active mode increases rapidly with the particle radius, being comparable for the two stretching modes of CO2 on Ag particles of radius 25–50 Å.

A differential equation for the polarizability of a small, continuum, spherically symmetric medium is developed in the local-dielectric-response approximation. The dynamic polarizability of a small metal sphere is then calculated through this equation using the Drude dielectric expression with the spatial dispersion of the free electron density given by the results of self-consistent density functional calculations on jellium. This approach is used to examine the size dependence of the plasmon resonance absorption of small metal particles. It is able to account for the observed broadening and peak frequency shifts, both "red," and "blue," as well as the additional absorption feature seen for small particles. To the extent that comparison with experimental data is possible, good agreement is found.

The methods developed in our earlier papers, dealing with Raman scattering from small isolated, spherical particles, uniformly coated with radially uniaxial molecules, are extended to include scattering driven by applied multipolar fields. Such fields are generated by the action of the incident electromagnetic radiation on extraneous material, such as other particles belonging to the same high-density dispersion. In this paper we treat inelastic scattering associated with the Raman dipole matrix elements of the adsorbed molecules ignoring other contributions. Analytical expressions are developed for calculating the primary Raman–Stokes moments given the amplitudes of the applied multipolar fields. As a test of the practicability of the procedure, a full calculation for parallel polarization has been performed for two identical contacting, CO-coated Ag particles oriented with their line of centers parallel to the direction of polarization of the incident radiation. For this case, the effect of interparticle coupling is to broaden and intensify the excitation spectrum, extending the region of intense Raman scattering to much lower frequencies. Intensity enhancements approaching 106 are calculated for frequencies as low as 80% of the single-particle surface-plasmon resonance frequency.

Our previous model dealing with the optical properties of small uniformly coated spheres is used iteratively to give the effective polarizability for homogeneous isotropic dielectric spheres and compared with results predicted by the Lorentz expression. Better agreement with the Lorentz expression is obtained using a modification of the model used to evaluate the self-polarizing field for the coating.

Cobalt oxide films, fabricated by reactive sputtering in a 100% pure O2 plasma are highly nonstoichiometric with a bulk oxygen to cobalt ratio of ∼ 1.15. Conversely, surface XPS studies indicate an oxygen to cobalt ratio ranging from 1.8 to 3.0 and show the presence of Co3+ only in the surface region of the as formed films. Infrared and Raman spectra however confirm the presence of both Co3+ and Co2+ in the bulk and indicate that these highly defective films possess the spinel structure. Photoelectrochemical and impedance studies show that the sputtered oxide is a highly doped p-type semiconductor with an indirect bandgap of Eg ∼ 1.50 eV. The electrocatalytic properties of these oxide films for oxygen reduction and evolution are reported.

Oxygen photoanodes formed by reactive sputtering of iron oxide onto conducting indium tin oxide (ITO) substrates held at 350 ° C have been investigated by conventional photoelectrochemical, impedance, XPS and auger spectroscopic methods. This fabrication procedure leads to films containing 8 to 20 atomic % indium in the front portion of the film, increasing to much higher values near the ITO interface (back portion of the film). Two interesting effects are observed with the thin-film iron oxide formed in this way. The first is that the as formed films must be vacuum annealed before an appreciable dc photoanodic response is observed. Secondly, films 250 nm thick display the property of giving about double the quantum yield for back face, than for front face, illumination over the spin and parity forbidden transition centered at 535 nm. On correcting for transmission and reflection losses, the resulting true quantum efficiencies satisfy the same inequality, a result that can only be accounted for by a higher ( > 2 × ) primary quantum efficiency for the back, versus the front portion of the film. As these films show substantially higher quantum efficiencies than do indium free films of the same thickness, it is concluded that the indium in the films is responsible for the increase in primary quantum efficiency. This result is discussed in terms of a localized states model for α-Fe2O3.

Atomic sputtering has been employed to fabricate a new, thin ( ⩽ 900 nm) n-type semiconducting phase of lead oxide of approximate stoichiometry PbO1.50. Films 120 to 880 nm in thickness deposited in a 100% O2 plasma and vacuum annealed for 8 h reproducibly exhibited indirect bandgaps of ~1.18 eV, while films of ~ 320 nm and vacuum annealed for ~1 h displayed higher bandgaps of ~1.30 eV. Films deposited in a 50:50 mixture of O2 and N2 resulted in an oxide displaying a bandgap of ~1.44 eV. All films thicker than 400 nm required vacuum annealing at ~ 250° C for periods of time > 2 h before any photoelectrochemical response was observed. The lower bandgap oxide phases are stable in aqueous alkaline media over a narrow potential range of ~ 0.20 V both in the dark as well as under chopped white light illumination (~ 200 mW/cm2). These films have been characterized using a variety of techniques including XPS, Auger, X-ray diffraction, impedance and photoelectrochemical spectroscopies.

Building on our recent paper on the optical properties of molecules forming a uniform, uniaxial layer on a small isotropic sphere, the present paper extends the treatment to include Raman scattering from both local modes and the coupled or surface phonon modes of such a system. As with the earlier paper, the treatment is restricted to the large wavelength approximation. While Raman inactive molecular modes can result in Raman active phonon modes, the present paper considers only those Raman active phonon modes that are connected with Raman active molecular modes. For such phonon modes, the field enhancement factors for the scattered wave are identical in form to those for the incident wave, derived in our recent paper, but differ significantly from those for local modes. Calculations are presented for CO and benzene on silver particles.

AbstractFilms of a new n‐type semiconducting lead oxide phase of approximate stoichiometry PbO1.50 are formed by reactive sputtering (lead metal target) in either a 100% O2 plasma or a 50:50 mixture of O2 and N2.

This project was inspired by the widespread use of the properties of coated spheres in modeling the dielectric and spectroscopic behavior, including surface enhanced Raman spectroscopy, of a variety of dispersed systems. In this paper the self-polarizing contributions to the absorption and elastic scattering properties of a coated sphere are obtained in the large wavelength limit, or dipole approximation, in which the coating is treated as a polarizable, uniaxially anisotropic, shell with the local optic axis directed radially. Numerical calculations for CO on Ag are provided to illustrate some of the phenomena predicted by the equations. Of particular interest are the dependence of the absorption band for CO stretch upon particle size, surface coverage, and molecular orientation, and the dependence of the surface plasmon resonance absorption on particle size and coverage. Some limitations of the model are examined.

This paper forms the third in a series on the optical properties of a system consisting of a uniform, radially uniaxial coating of molecules on a small isotropic sphere. In this paper, the induction of Raman activity in a coated metal sphere by the fields generated by the coupled surface phonon modes of the coating is considered. Thus, the surface phonon modes induce surface plasmons in the metal that result in its polarizability being modulated. The phenomenon, which requires dipole active rather than Raman active molecular modes to be operative, we have termed surface phonon induced Raman scattering (SPIRS). Scattering intensities predicted via SPIRS and surface enhanced Raman scattering (SERS) have been compared for CO on a spherical silver particle. That for SPIRS is found to be of the order of or greater than that for SERS, depending on exactly how the surface region of the metal sphere is modeled. Some predictions based on SPIRS are reminiscent of data previously assigned to SERS but not explained by current models.