Jeanne E. Pemberton

Colloidal Polymerization of Polymer- Coated Ferromagnetic Nanoparticles into Cobalt Oxide Nanowires

The preparation of polystyrene-coated cobalt oxide nanowires is reported via the colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs). Using a combination of dipolar nanoparticle assembly and a solution oxidation of preorganized metallic colloids, interconnected nanoparticles of cobalt oxide spanning micrometers in length were prepared. The colloidal polymerization of PS-CoNPs into cobalt oxide (CoO and Co(3)O(4)) nanowires was achieved by bubbling O(2) into PS-CoNP dispersions in 1,2-dichlorobenzene at 175 degrees C. Calcination of thin films of PS-coated cobalt oxide nanowires afforded Co(3)O(4) metal oxide materials. Transmission electron microscopy (TEM) revealed the formation of interconnected nanoparticles of cobalt oxide with hollow inclusions, arising from a combination of dipolar assembly of PS-CoNPs and the nanoscale Kirkendall effect in the oxidation reaction. Using a wide range of spectroscopic and electrochemical characterization techniques, we demonstrate that cobalt oxide nanowires prepared via this novel methodology were electroactive with potential applications as nanostructured electrodes for energy storage.

Surface Enhancement Factors for Ag and Au Surfaces Relative to Pt Surfaces for Monolayers of Thiophenol

Raman signal intensities from the 997 cm−1 ring breathing mode of thiophenol monolayers adsorbed at Ag, Au, and Pt surfaces were employed for determination of absolute surface enhancement factors (SEFs). Unlike previous estimations of SEFs, these SEFs are determined by referencing the surface-enhanced Raman scattering (SERS) intensities to the unenhanced Raman scattering at Pt surfaces. The surfaces studied include those commonly prepared in a laboratory ambient and those prepared in vacuum. Surfaces prepared in ambient include polycrystalline Ag electrochemically roughened by an oxidation-reduction cycle (ORC), mechanically polished (MP) polycrystalline Ag, chemically polished (CP) polycrystalline Ag, Ag (111), MP polycrystalline Au, and MP polycrystalline Pt. Vacuum environment surfaces include coldly deposited Ag films (cold Ag) and room temperature-deposited (RT) “thick” Ag films. Each of these thiophenol/metal systems was sampled with an excitation wavelength (λex) of 514.5 nm; MP Au surfaces were also studied with λex of 720 nm. SEFs of 2.0 × 104 for ORC Ag, 5.3 × 103 for MP Ag, 160 for cold Ag, 64 for MP Au720 (λex = 720 nm), 69 for Ag (111), 39 for CP Ag, 7.9 for RT “thick” Ag, and 2.2 for MP Au514.5 (λex 514.5 nm) are observed relative to the SEF of MP Pt, which is assigned as 1. For practical purposes, the significance of the magnitudes of these SEFs is discussed in terms of estimated surface Raman limits of detection (LODs).

Surface enhanced Raman scattering in electrochemical systems: The complex roles of surface roughness

Abstract A series of experiments designed to elucidate the presence and properties of large-scale and atomic-scale roughness produced on Ag electrodes with electrochemical oxidation-reduction cycle (ORC) pretreatments are presented. This report reviews surface enhanced Raman scattering (SERS) and scanning electron microscopic (SEM) characterization of Ag electrodes roughened with controlled-rate ORCs, and presents new results for the laser-induced thermal decay of SERS as a probe of Ag surface active sites and differential reflectance spectroscopy of electrochemically roughened Ag electrodes. These results are interpreted in terms of the presence and properties of both large-scale and atomic-scale roughness on these surfaces.

Efficient purification of the biosurfactant viscosin from Pseudomonas libanensis strain M9-3 and its physicochemical and biological properties.

Viscosin (1), an effective surface-active cyclic lipopeptide, was efficiently recovered from Pseudomonas libanensis M9-3 with a simple purification protocol. A major pigment also obtained during this process was identified as phenazine-1-carboxylic acid. The critical micelle concentration (cmc) of viscosin was determined to be 54 mg L (-1), and the minimum surface tension between air and water at the cmc was 28 mN m (-1). Viscosin forms stable emulsions even at low concentrations (7.5 mg L (-1)), and the conditional stability constant for a cadmium-viscosin complex was determined to be 5.87. The physicochemical properties measured for viscosin are similar to other well-studied biosurfactants such as rhamnolipid and surfactin. Viscosin inhibited migration of the metastatic prostate cancer cell line, PC-3M, without visible toxicity. These properties suggest the potential of viscosin in environmental and biomedical applications.

Phosphonic Acids for Interfacial Engineering of Transparent Conductive Oxides

Transparent conducting oxides (TCOs), such as indium tin oxide and zinc oxide, play an important role as electrode materials in organic-semiconductor devices. The properties of the inorganic-organic interface-the offset between the TCO Fermi level and the relevant transport level, the extent to which the organic semiconductor can wet the oxide surface, and the influence of the surface on semiconductor morphology-significantly affect device performance. This review surveys the literature on TCO modification with phosphonic acids (PAs), which has increasingly been used to engineer these interfacial properties. The first part outlines the relevance of TCO surface modification to organic electronics, surveys methods for the synthesis of PAs, discusses the modes by which they can bind to TCO surfaces, and compares PAs to alternative organic surface modifiers. The next section discusses methods of PA monolayer deposition, the kinetics of monolayer formation, and structural evidence regarding molecular orientation on TCOs. The next sections discuss TCO work-function modification using PAs, tuning of TCO surface energy using PAs, and initiation of polymerizations from TCO-tethered PAs. Finally, studies that examine the use of PA-modified TCOs in organic light-emitting diodes and organic photovoltaics are compared.

Raman spectroscopy and vibrational assignments of 1- and 2-methylimidazole

Vibrational modes are assigned to the Raman bands of 1- and 2-methylimidazole. Assignments are based on a review of the literature assignments of imidazole, 1-methylimidazole and 4(5)-methylimidazole supplemented by Raman depolarization studies of 1.0 M solutions of 1- and 2-methylimidazole. Raman shifts observed for N-deuteration of 2-methylimidazole are also considered.

Orientation of phenylphosphonic acid self-assembled monolayers on a transparent conductive oxide: a combined NEXAFS, PM-IRRAS, and DFT study.

Self-assembled monolayers (SAMs) of dipolar phosphonic acids can tailor the interface between organic semiconductors and transparent conductive oxides. When used in optoelectronic devices such as organic light emitting diodes and solar cells, these SAMs can increase current density and photovoltaic performance. The molecular ordering and conformation adopted by the SAMs determine properties such as work function and wettability at these critical interfaces. We combine angle-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to determine the molecular orientations of a model phenylphosphonic acid on indium zinc oxide, and correlate the resulting values with density functional theory (DFT). We find that the SAMs are surprisingly well-oriented, with the phenyl ring adopting a well-defined tilt angle of 12-16° from the surface normal. We find quantitative agreement between the two experimental techniques and density functional theory calculations. These results not only provide a detailed picture of the molecular structure of a technologically important class of SAMs, but also resolve a long-standing ambiguity regarding the vibrational-mode assignments for phosphonic acids on oxide surfaces, thus improving the utility of PM-IRRAS for future studies.

Characterization of Octadecylsilane Stationary Phases on Commercially Available Silica-Based Packing Materials by Raman Spectroscopy

Two commercially available solid phase extraction packing materials containing octadecylsilane stationary phases, Isolute C18MF and Isolute C18, were characterized using Raman spectroscopy. Raman spectra of excellent quality can be obtained from such systems and provide direct information about the alkyl chain conformation in the stationary phase. Data from the ν(C-C) and ν(C-H) spectral regions suggest that the alkyl chains of these stationary phases exist in a highly disordered state which is very similar to that of the neat liquid.

Modification of BaTiO3 thin films: adjustment of the effective surface work function

Sputter-deposited BaTiO3 thin films have been modified with an alkylphosphonic acid and a partially-fluorinated alkylphosphonic acid in order to model the surface composition of similarly modified BaTiO3 nanoparticles. We present here the surface characterization of these modified films by a combination of X-ray photoelectron spectroscopy (XPS) and UV-photoelectron spectroscopy (UPS). BaTiO3 layers of average thicknesses ca. 2 nm were prepared by radio frequency (rf) magnetron sputter deposition on Ag films, to avoid charging effects during XPS/UPS characterization. Octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (perfluorohexyloctyl phosphonic acid, FHOPA), molecules with quite different molecular dipole moments, were chemisorbed from solution to the BaTiO3 surface. Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) of the modified BaTiO3 films indicated bidentate bonding of the alkylphosphonic acid to the oxide. Modification of the BaTiO3 surface with the partially-fluorinated alkylphosphonic acid (versus the normal alkylphosphonic acid) significantly changes the BaTiO3 interface dipole as revealed by UPS/XPS measurements, which, in turn, changes the frontier orbital offsets between the oxide and the organic modifier.

XPS Characterization of a Commercial Cu/ZnO/Al2O3 Catalyst: Effects of Oxidation, Reduction, and the Steam Reformation of Methanol

X-ray photoelectron spectroscopy (XPS) is used to characterize the surface region of a commercial Cu/ZnO/Al2O3 (33/66/1 wt %) catalyst. A systematic study of the effects of oxidation, reduction, and the steam reformation of methanol on the oxidative state of the Cu component is presented. The Zn XPS features show no changes due to the various treatments. Peak fitting procedures were developed to quantitate the Cu oxidation states on the basis of the XPS Cu 2P3/2 main and satellite features. After oxidation in pure O2 at 300°C, all Cu exists as Cu+2. The Cu/Zn ratio changes from 0.28 to 0.37 as a result of this oxidation, in comparison to the ratio in the catalyst as-received. The reduction studies involved different H2/N2 mixtures (15 to 100% H2) and temperatures (250 to 300°C). The catalyst always contains Cu+1 (7.0 ± 5.0%) and Cu° (93.0 ± 5.0%) sifter reduction. The Cu/Zn ratio decreases from approximately 0.37 in the oxidized catalyst to 0.13 after reduction. After methanol-steam reformation with a 50/50 vol % mixture, the Cu 2P3/2 and Auger features are indicative of complete reduction of all Cu in the catalyst to a reduced Cu° state not seen previously. Changes in the Cu/ Zn ratio of the surface are interpreted in terms of changes in surface morphology of the Cu species.