Surface-enhanced Raman and electron spectroscopic investigations of lead-modified silver surfaces.
AuthorKellogg, Diane Schneider
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PublisherThe University of Arizona.
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AbstractSurface enhanced Raman scattering (SERS) is a powerful means for obtaining vibrational data from the metal/electrolyte or metal/gas interfacial environment. However, SERS is only observed for a limited number of metal surfaces under certain experimental conditions. Before this method can become a universal tool, the enhancement mechanism(s) must be understood. The results reported in this dissertation assess both electronic and chemical contributions to the SERS mechanism. The electronic properties of the metal are altered by systematic deposition of Pb or Cu onto a substrate that supports intense SERS, Ag. The chemical nature of the interface is altered with different probe molecules. The effect of Pb deposition on the SERS enhancing ability of Ag electrodes has previously been investigated with strongly adsorbed probe molecules. The behavior of cyanide species in the presence of Pb⁺² is complicated by the necessity of maintaining low solution pH to prevent Pb(OH)₂ precipitation; thus, the predominant solution species is HCN. Although previous reports state that no SERS can be detected from cyanide-containing solutions below pH 6, intense SERS signals can be obtained at pH 2 if sufficiently positive electrode potentials are maintained. The two unresolved SERS bands observed in acidic solutions are attributed to HCN which interacts with the Ag surface in end-on and side-on configurations. The predominant effect of Pb deposition on HCN SERS is HCN displacement. Enhancement due to charge transfer processes is not significant, while electromagnetic effects dictate the residual SERS intensity remaining after the initial HCN displacement. The supporting electrolyte anion affects the rate of change of the potential dependent C≡N stretch in basic CN⁻ media. A correlation between the rate of frequency change and anion charge/radius ratio was observed at potentials near and slightly negative of the Ag potential of zero charge in basic CN⁻ media. These results demonstrate the extraordinary sensitivity of SERS to interfacial conditions. The contributions from chemical and electromagnetic enhancement are further assessed by following excitation wavelength dependence of the SERS intensity of pyridine and Cl⁻ as a function of Cu coverage. Contributions from both are observed, but chemical enhancement is less evident for Cu than for Pb deposition. This is related to the smaller change in work function that occurs as a consequence of Cu versus Pb deposition on Ag surfaces.
Degree GrantorUniversity of Arizona
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SURFACE REACTIONS AND SURFACE ANALYSIS OF LITHIUM METAL AND ITS COMPOUNDS STUDIED BY AUGER ELECTRON SPECTROSCOPY, X-RAY PHOTOELECTRON SPECTROSCOPY AND RUTHERFORD BACKSCATTERING SPECTROMETRY (THERMAL BATTERIES).Burrow, Bradley James (The University of Arizona., 1984)The development of analysis techniques necessary for the quantitative, chemical surface analysis of lithium-containing solids important in the construction of high energy density batteries is presented. Electron beam damage is discovered to be the source of apparent lithium metal formation in Li(ls) XPS spectra of lithium salts. Beam Damage thresholds of Li₂O, Li₂CO₃ and Li₂SO₄ are calculated using time-dependent Auger spectra, and possible mechanisms are discussed. The variables which affect Auger quantitation are reviewed with particular emphasis on low energy transitions. Two experimental attempts at measuring the instrument response function for the cylindrical mirror analyzer and electron multiplier are discussed. Background correction techniques proposed in the literature are compared using synthesized Auger data. Auger lineshapes are synthesized by a series of calculations which mimic each step of the Auger electron's path from the atomic core level to the detector. The results indicate that the SIBS (Sequential Inelastic Background Subtraction) method is more applicable to Auger analysis because of its analytical accuracy, speed and ability to handle spectra with poor signal to noise. The special problem of low energy background subtraction is resolved through the use of a new five-parameter function which adequately accounts for the analyzer distortions and secondary cascade in one calculation. Using the above correction techniques, Auger spectra, peak energies, relative intensities and FWHM's of Li₂O, LiOHNH₂O, LiH, Li₃N, Li₂CO₃ and Li₂SO₄NH₂O are presented. Despite special handling techniques, the hydroxide, hydride and nitride reveal extensive oxidation. The oxyanion salts reveal little Li Auger intensity until substantial anion desorption had occurred. The reaction products of lithium with oxygen, water and carbon dioxide are studied by AES. Results indicate the formation of Li₂O, LiOHNH₂O and Li₂O with hydrocarbons, respectively. These results are used to construct a plausible surface structure of the Li-SO₂ interface which explains its stability to self-discharge corrosion and yet maintain electronic conductivity for external discharge. RBS and AES depth profiling are used to analyze potassium-implanted glasses. The results indicate a great deal of ionic migration for glasses which leads to a speculative mechanism for alkali corrosion of glasses.
SURFACE CHARACTERIZATION OF TITANIUM AND TITANIUM DEUTERIDE GAS-PHASE AND SOLUTION-PHASE OXIDATION PROCESSES (SURFACE ANALYSIS, ANGER ELECTRON SPECTROSCOPY).Burrell, Michael Craig (The University of Arizona., 1984)The reactions of atomically clean, titanium film surfaces with oxygen, deuterium, and water have been investigated. Auger Electron Spectroscopy was utilized to monitor the formation 9f a surface oxide in the case of oxygen exposure, and to characterize the deuteride which formed upon deuterium absorption, and its subsequent oxidation. Quantification of surface oxide stoichiometries was facilitated by novel data acquisition and treatment schemes. The quartz crystal microbalance was used to measure the mass of adsorbed oxygen or deuterium with submonolayer sensitivity. Electron energy loss spectroscopy was sensitive to the presence of Ti⁺³ in the surface oxide. The initial oxidation of the titanium surface was characterized by the dissociative adsorption of three mono1ayers of oxygen atoms at a constant rate. The oxide formed during this reaction stage was a Ti₂0₃/Ti0₂ mixture with a total thickness of 13 A. The rate of oxygen adsorption then decreased such that oxide growth was logarithmic with time. When the oxide had attained a total thickness of 20 A, the initial suboxide was converted to Ti0₂, and subsequent oxide formed was purely Ti0₂. Oxide growth occurred by oxygen anion migration under the influence of an electrostatic field, set up across the oxide layer by electron transfer from the metal to adsorbed oxygen species. The pressure dependence of the oxide growth rate and terminal thickness suggested a constant field growth mechanism. Clean titanium films reacted with deuterium to form a bulk deuteride TiDₓ (x<2). The oxide layer which resulted from oxygen exposure was characterized by the above techniques. Oxide layers greater than 20 A completely inhibited deuterium absorption by prohibiting 02 dissociation, but did not act as a diffusional barrier when additional dissociation sites were provided. Iron adlayers were found to accelerate the D₂ absorption reaction. Removal of the titanium films from the vacuum chamber to an isolable electrochemical reaction chamber, without exposure to the atmosphere, allowed a determination of the effect of the various gas/solid reactions on the subsequent electrochemical oxidation processes.
Surface-enhanced Raman scattering and electron spectroscopic studies of copper and silver surfaces.Ingram, Jani Cheri (The University of Arizona., 1990)The overall goal of this research is to investigate the enhancement mechanisms associated with the surface enhanced Raman scattering (SERS) phenomenon. The approach taken in these investigations is to directly compare the SERS abilities of Cu and Ag surfaces using both SERS and electron spectroscopy. One set of experiments has been directed toward studying the contribution of the charge transfer (CT) mechanism to the SERS of pyridine adsorbed on Cu and Ag electrodes. These studies involve modifying the electrodes by depositing submonolayer amounts of Pb which serves to quench the SERS. By using a variety of excitation energies, the resulting quenching profiles track the CT process. Additionally, other electrochemical/SERS experiments have been pursued in order to probe the contribution of the CT mechanism. The second set of experiments involved measuring the optical properties of the Pb-modified Cu and Ag surfaces using electron energy loss spectroscopy in the reflection mode (REELS). Based on electromagnetic (EM) theory, the enhancement is, to a rough approximation, indirectly proportional to ε₂, the imaginary part of the dielectric constant. Thus, the goal of these studies is to determine the changes in ε₂ as a function of Pb coverage in order to determine the contribution of the EM mechanism to the SERS quenching profile. It was necessary to develop a method to determine optical constants from REELS data. A number of pure metals (Al, Cu, Ag, Au, Ti, V, Fe, Co, and Ni) were chosen to test the method. In all cases, our results compare well to the literature values with a relative standard deviation of 20% or less. Having established the method, the next step was to apply it to the Pb-modified Cu and Ag surfaces. Due to instrumental limitations, only semi-quantitative optical constants could be determined. From these values, it was found that the relative changes in the ε₂ values were larger for submonolayer coverages of Pb on Ag compared to Cu. The EM enhancements determined from these results did not predict the SERS-quenching behavior suggesting that other mechanisms must be considered.