DOE OSTI2016
Since 1995 we have pursued a number of different studies that are quite diverse in nature but with the common theme of using novel laser based methods to study important processes at buried interfaces. Studies of Corrosion, Passivation on n-GaAs(100)Methanol Photoelectrochemical Cell In these studies we have used picosecond photoluminescence and electrochemical studies to understand the GaAs/methanol interface. In our most extensive set of studies we conducted photo-illumination and XPS experiments to understand the chemistry occurring in the GaAs/methanol photoelectrochemical during photoexcitation. An important distinction between photocorrosion and photoetching of GaAs is elucidated by these studies. The dependence of GaAs photocorrosion on light intensity has been explored to better understand intrinsic differences between the lamplight studies and the picosecond photoluminescence studies. The effect of coating the GaAs with a sulfide layer prior to immersion in the cell has also been explored. This last result has led us to examine n-GaAs as a function of crystallographic orientation after exposure to aqueous Na 2 S containing solutions has been studied as a function of crystallographic orientation of the GaAs surface. The (100) and (110) surfaces are relatively similar, with significant amounts of As-S species present at the interface. The (111)B surface lacks this constituent, but shows significant amounts of metallic As. The XPS results have been correlated with the results of previous photocorrosion and passivation studies conducted in a photoelectrochemical cell. The studies indicate that the metallic As present at (111)B surface contributes strongly to the large surface recombination velocity found there, and to the inability of Na 2 S to passivate the (111)B surface. SAMS Under Water: Water Molecular Structure and Bonding at Hydrophobic Surfaces In these DOE sponsored studies we have been interested in learning the similarities and differences in how water behaves at hydrophobic self-assembled monolayer (SAMS)/water interfaces relative to the organic liquid/water interfaces. Several monolayer films have been examined in these studies using a combination of vibrational sum frequency spectroscopy (VSFS), contact angle measurements and AFM. At the hydrocarbon monolayer/water interface we find that water has a weak bonding interaction with the monolayer film that results in an orientation of water at the terminus of these hydrocarbon chains. The water-film interaction is still present for fluorinated films but it is found to be considerably weaker. Hydration and Surfactant Adsorption at Salt/Water Interfaces This set of studies has examined the molecular characteristics of the CaF 2 /water interface using VSFS. Our first studies detailed the structure and orientation of water molecules adsorbed at this mineral surfaces including studies of the surface in the presence of aqueous solutions of salts. These studies have been followed by a series of static and time-resolved studies of the adsorption of carboxylic acid containing organics at this surface, specifically carboxylic acid surfactants and acetic acid. In the latter we have developed a new method for time resolved studies that involve sequential wavelength tuning and automated control of spatial beam overlap at the target can probe amplitude changes of sum-frequency resonances in widely spaced infrared regions. This offers great advantages for the study of the synchronism of molecular processes at interfaces. This approach is particularly suitable to investigate the synchronization of interfacial processes such as surfactant adsorption at charged mineral surfaces. Macromolecular Assembly at Liquid/Liquid Interfaces Macromolecular assembly at the interface between water and a hydrophobic surface underlies some of the most important biological and environmental processes on the planet. Our work has examined polymer adsorption and assembly of carboxylic acid-containing polyelectrolytes at the carbon tetrachloride–water interface, a model system for an oil–water interface. Using VSFS and interfacial tension techniques, these unique set of studies identify the factors that dictate whether or not polyelectrolytes will adsorb to the oil–water interface and also describe the specifics of the adsorption process that depend upon factors such as polymer size, charge density, hydrophobicity, conformation, and the effect of metal ion electrostatics and bonding. The systems studied include polyelectrolytes polyacrylic acid (PAA) and polymethylacrylic acid (PMA) of different polymer sizes and under different aqueous solution conditions. The studies are the first to show the highly ordered nature of the adsorption of the first monolayer with subsequent monolayers disordered. The second set of studies have examined how peptoid nanosheets assemble at the oil/water interface. Peptoid nanosheets are a recently discovered class of two-dimensional (2D) nanomaterial, which form from the self-assembly of a sequence-specific peptoid polymer at an air-water interface. Nanosheet formation occurs first through the assembly of a peptoid monolayer and subsequent compression into a bilayer structure. In these highly successful studies we have shown that the oil-water interface provides another opportunity for growth of these unique and highly ordered peptoid sheets. The monolayer formed at this interface are found through surface spectroscopic measurements to be highly ordered and electrostatic interactions between the charged moieties, namely carboxylate and ammonium residues, of the peptoid are essential in the ability of these peptoids to form ordered nanosheets at the oil-water interface.