Teresa D. Golden

X-ray photoelectron spectroscopy study of electrodeposited nanostructured CeO2 films

Nanostructured CeO2 films are produced by anodic electrodeposition onto a variety of substrates. The crystal structure and oxidation state of the films are studied by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS), respectively. Calculations from XRD show that crystallite sizes of the electrodeposited films range from 6–10 nm. Sintering of these films to 700 °C increases the grain size to approximately 25 nm. A study of Ce 3d, Ce 4d, O 1s, and the valence-band region indicates that the Ce(IV)/Ce(III) ratio increases with sintering temperature, with features of both Ce4+ and Ce3+ identified by XPS. Ce 3d and O 1s characteristics show that high-temperature sintering of the films facilitates Ce(IV) oxide formation.

Amperometric detection of cationic neurotransmitters at nafion-coated glassy carbon electrodes in flow streams

Abstract Substantial improvements in amperometric monitoring of flowing streams are obtained by using Nafion-coated working electrodes. The charged coating tends to exclude anionic and neutral interferences, thus adding a new dimension of selectivity to electrochemical detection for flow-injection and liquid-chromatographic systems. A highly selective response is observed for cationic neurotransmitters in the presence of otherwise interfering substances (e.g., ascorbic acid, uric acid, bilirubin or chlorpromazine). The permselectivity and transport characteristics are evaluated with respect to solution pH, solvent, flow rate, film thickness, and other variables. The reduced flow-rate dependence results in low noise levels and detection limits of 0.04 and 0.10 ng of norepinephrine and epinephrine, respectively. A bilayer electrode coating, with a cellulose acetate film over the Nafion layer, offers a bifunctional (selectivity, protection) capability. Applicability to urine samples is demonstrated.

Metalloporphyrin chemically modified glassy carbon electrodes as catalytic voltammetric sensors

Abstract Metalloporphyrin-coated glassy carbon electrodes are used as electrocatalytic voltammetric sensors for numerous clinically important solutes. For such compounds, heterogeneous charge- transfer rates are often very slow at carbon electrodes, leading to poorly defined voltammetric responses. The metalloporphyrin-modified electrodes are shown to decrease by several hundred millivolts the potential required for the oxidation of ascorbic acid, penicillamine, acetaminophen, dihydronicotinamide adenine dinucleotide, hydralazine, epinephrine, cysteine and oxalic acid. The faster rates of electron transfer result in a well defined voltammetric response and increased sensitivity. The differential pulse peaks for caffeic acid, ascorbic acid, acetaminophen and dopamine are enhanced by 18, 10.5, 9.4 and 8.4, respectively. When used for amperometric monitoring of flowing streams, the coated electrode permitted detection at lower potentials than at the naked surface and greatly facilitated assays of urine samples.

Electrodeposited defect chemistry superlattices

Nanometer-scale layered structures based on thallium(III) oxide were electrodeposited in a beaker at room temperature by pulsing the applied potential during deposition. The conducting metal oxide samples were superlattices, with layers as thin as 6.7 nanometers. The defect chemistry was a function of the applied overpotential: High overpotentials favored oxygen vacancies, whereas low overpotentials favored cation interstitials. The transition from one defect chemistry to another in this nonequilibrium process occurred in the same potential range (100 to 120 millivolts) in which the rate of the back electron transfer reaction became significant. The epitaxial structures have the high carrier density and low electronic dimensionality of high transition temperature superconductors.

Anodic Electrodeposition of Cerium Oxide Thin Films I. Formation of Crystalline Thin Films

Cerium oxide thin films have been produced by anodic electrodeposition with a preferred orientation onto a variety of substrates. The Ce(III) ions are complexed with suitable organic ligands in solution to form stable complexes in basic pH solutions. Ligands with mild complexing power are suitable for the formation of cerium oxide films by anodic electrodeposition. The deposition conditions investigated include deposition modes, substrates, complexing agents, pH, temperature, and current density. X-ray diffraction (XRD) and scanning electron microscopy characterizations of the cerium oxide films show that the texture of the films can be controlled by temperature, pH, and current density. XRD analysis reveals that CeO 2 thin films deposited with crystallite sizes as small as 6 nm are possible with this method. A preferred (111) orientation of polycrystalline CeO 2 is formed when the plating bath temperature is higher than 50°C and the current density lower than 0.30 mA/cm 2 . XPS results reveal that the cerium oxide films deposited at room temperature are nonstoichiometric.

Anodic Electrodeposition of Cerium Oxide Thin Films II. Mechanism Studies

Nanocrystalline cerium oxide films with grain sizes as small as 6 nm are deposited by anodic deposition. Cyclic voltammetry studies of a series of complexing ligands indicate that formation constants below I x 10 3 and a solution pH between 7 and 10 are needed for CeO 2 film formation. A mechanism is proposed for the deposition of polycrystalline CeO 2 films by anodic deposition.

Scanning tunneling microscopy of electrodeposited ceramic superlattices

Cleaved cross sections of nanometer-scale ceramic superlattices fabricated from materials of the lead-thallium-oxygen system were imaged in the scanning tunneling microscope (STM). The apparent height differences between the layers were attributed to composition-dependent variations in local electrical properties. For a typical superlattice, the measured modulation wavelength was 10.6 nanometers by STM and 10.8 nanometers by x-ray diffraction. The apparent height profile for potentiostatically deposited superlattices was more square than that for galvanostatically deposited samples. These results suggest that the composition follows the applied potential more closely than it follows the applied current. The x-ray diffraction pattern of a superlattice produced under potential control had satellites out to the fourth order around the (420) Bragg reflection.

Electrosynthesis of nanocrystalline cerium oxide/layered silicate powders

Cerium oxide/montmorillonite layered silicate powder nanocomposites were synthesized electrochemically. The produced nanocomposites were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. X-Ray diffraction shows the nanocomposites retain the face-centered cubic structure of cerium oxide while incorporating delaminated platelets of the layered silicate into the matrix. By a negative shift in the OH stretching band position, infrared spectroscopy measurements indicate the layered silicates are delaminated. Photoluminescence background in the Raman spectra also shows the incorporation of the layered silicates within the cerium oxide matrix. The thermal properties of the nanocomposites were investigated with sintering and differential scanning calorimetry. The results indicate that low concentration of layered silicate mixed into the cerium oxide increases crystal growth rate whereas high concentration retards the increase of crystallite size during the densification process.

Synthesis of Nickel and Nickel Hydroxide Nanopowders by Simplified Chemical Reduction

Nickel nanopowders were synthesized by a chemical reduction of nickel ions with hydrazine hydrate at pH ~12.5. Sonication of the solutions created a temperature of 54–65°C to activate the reduction reaction of nickel nanoparticles. The solution pH affected the composition of the resulting nanoparticles. Nickel hydroxide nanoparticles were formed from an alkaline solution (pH~10) of nickel-hydrazine complexed by dropwise titration. X-ray diffraction of the powder and the analysis of the resulting Williamson-Hall plots revealed that the particle size of the powders ranged from 12 to 14 nm. Addition of polyvinylpyrrolidone into the synthesis decreased the nickel nanoparticle size to approximately 7 nm. Dynamic light scattering and scanning electron microscopy confirmed that the particles were in the nanometer range. The structure of the synthesized nickel and nickel hydroxide nanoparticles was identified by X-ray diffraction and Fourier transform infrared spectroscopy.

Cross‐sectional scanning tunneling microscopy of electrodeposited metal oxide superlattices

We have used scanning tunneling microscopy to characterize cleaved cross sections of Pb‐Tl‐O superlattices. The metal oxide ceramic superlattices were electrodeposited from a single solution, with layer thicknesses as small as 1.5 nm. The lattice parameter of the fcc fluorite‐type oxides is approximately 0.536 nm. Modulation wavelengths were determined using Fourier analysis of the STM images and found to be in good agreement with Faraday calculations and x‐ray diffraction measurements. The STM is especially well suited for the measurement of modulation wavelengths that are too large to measure by x‐ray diffraction, but too small to measure by scanning electron microscopy.