An electrodeposition method for growing epitaxial Co(OH)2 films on single crystalline gold (111), (100), and (110) substrates is described. The films were grown by electrochemical reduction of [Co(en)3]3+ in an alkaline electrolyte. The Co(OH)2 grew with a [0001] out-of-plane orientation on all the gold crystal orientations. The in-plane orientation follows the symmetry of the gold (111), (100), and (110) substrates. The Co(OH)2 can be converted to CoOOH by electrochemical oxidation in 1 M KOH at 95 °C, and after conversion remains epitaxial with a [0001] out-of-plane orientation. The CoOOH film can be further converted to epitaxial Co3O4 with a [111] out-of-plane orientation by decomposition of the CoOOH film in air at 300 °C. This synthesis method allows for a simple fabrication of epitaxial catalysts and could be useful to probe the catalytic activity of specific crystal planes.

Electrodeposition (ED) and chemical bath deposition (CBD) are both solution processing methods that assemble solid materials from molecules, ions, or complexes in solution. The reactions occur on solid surfaces to produce polycrystalline and epitaxial films, porous networks, nanorods, superlattices, and composites. Both methods can be used to produce metals, semiconductors, magnetic materials, and ceramics in a wide array of architectures. Because of the low processing temperatures (often near room temperature), the techniques are ideal for producing nanostructured materials and interfaces. The methods are not only inexpensive and relatively simple, but they can often produce materials and nanostructures that cannot be accessed in ultrahigh vacuum. For example, the shape and orientation can be tuned by controlling the pH or through solution additives. Also, in ED, the departure from equilibrium can be precisely controlled through the applied potential. This issue attempts to provide understanding of the growth mechanisms—from the molecular to final structure—of interfacial electrochemical and CBD reactions. For example, the influence of solution additives and pH and the effect of external parameters such as the applied potential will be discussed. Practical applications of these films and nanostructures include photovoltaic/photoelectrochemical cells and chemical/magnetic sensors.

In this paper, we demonstrate that films of magnetite, Fe3O4, can be deposited by the electrochemical reduction of a Fe(III)-triethanolamine complex in aqueous alkaline solution. The films were deposited with a columnar microstructure and a [100] preferred orientation on stainless steel substrates. In-plane electrical transport and magnetoresistance measurements were performed on the films after they were stripped off onto glass substrates. The resistance of the films was dependent on the oxygen partial pressure. We attribute the increase in resistance in O2 and the decrease in resistance in Ar to the oxidation and reduction of grain boundaries. The decrease in resistance in an Ar atmosphere exhibited first-order kinetics, with an activation energy of 0.2 eV. The temperature dependence of the resistance showed a linear dependence of log(R) versus T−1/2, consistent with tunneling across resistive grain boundaries. A room-temperature magnetoresistance of −6.5% was observed at a magnetic field of 9 T.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Layered nanostructures of copper metal and cuprous oxide are electrodeposited from alkaline solutions of Cu(II) lactate at room temperature. No subsequent heat treatment is necessary to effect crystallization. The electrode potential spontaneously oscillates during constant-current deposition. At a fixed current density the oscillation period decreases as either the pH or temperature is increased. The oscillations are periodic in stirred solution, but show period doubling and evidence of quasi-periodic or chaotic behavior in unstirred solution. The phase composition and resistivity of the films can be controlled by varying the applied current density. The resistivity of the films can be varied over ten orders of magnitude. Scanning electron microscopy shows that the films are layered.

Electrodeposition is unique among thin film deposition techniques, since the current following a potential step from open circuit to a given overpotential (i.e., driving force) provides a real-time, in situ probe of the deposition process. We have used current-time transients to determine the kinetics and dimensionality of growth, and to calculatethe composition profiles of electrodeposited superlattices in the Pb-TI-O system. The transients are indicative of 2D growth, and the height of the 2D monolayer depends on the orientation of the film. A [100] oriented film has a step height of 0.13 nm and a [210] oriented film has a step height of 0.18 nm. These heights correspond to the (400) and (220) d-spacings, respectively. Using a Cottrell analysis, we were able to both calculate and tailor the composition profile in these superlattices. Superlattices grown by pulsing between 70 and 150 mV have square profiles, while those grown by pulsing between 70 and 230 mV have graded profiles.

We have imaged fractured cross-sections of electrodeposited ceramic oxides based on the TI-Pb-O system using a scanning tunneling microscope. The goal of this work is to measure both the modulation wavelength and compositional profile of the superlattices by mapping out the electronic properties in real space on a nanometer scale. Fourier analysis was done on STM images of all superlattices to yield the modulation wavelength. The modulation wavelength from STM was then compared with those obtained, by Faraday calculation and x-ray diffraction. The STM can be used to design “better” superlattices. We have found that the composition profile in superlattices deposited by modulating the potential was more square than in superlattices deposited by modulating the current.

Potential step transients were investigated as an in-situ probe of epitaxial growth for electrodeposited conducting metal oxides of the Tl2O3 and PbaTlbOc systems. Changes in the induction time and growth type were observed for Tl2O3 deposition as a function of the substrate. The substrates studied were glassy carbon and [210]-textured Pb0.8Tl0.2O1.9· T12O3 deposited onto the glassy carbon electrode showed a distinct induction time, the magnitude of which was potential dependent. Also the type of growth was characterized as three dimensional with instantaneous nucleation. X-ray diffraction of this film shows a nearly random pattern. The potential step transient for Tl2O3 deposited onto the [210]-textured Pb0.8Tl0.2O1.9, showed the absence of an induction time and elimination of the growth segment. The type of growth was characterized as two dimensional. X-ray diffraction indicated epitaxy was obtained for the Tl2O3 films which grew two dimensionally onto the [210]-textured mixed oxide.

Electrochemistry can be used for the atomic-level architecture of ceramic materials. In this report, ceramic superlattices based on the TlaPbbOc/TldPbeOf system were electrodeposited with individual layer thicknesses as thin as 3nm. The superlattices were deposited from a single aqueous solution at room temperature, and the layer thicknesses were galvanostatically controlled. Substitution of Tl2O3 into PbO2 appears to stabilize a face-centered cubic structure with an average lattice parameter of 0.536nm. The lattice parameters for the T1aPbbOc. mixed oxides vary by less than 0.3% when the Pb/Tl ratio is varied from 0.84 to 7.3. Because the modulation wavelengths are of electron mean free path dimensions, this new class of degenerate semiconductor metal-oxide superlattices may show thickness-dependent quantum optical, electronic, or optoelectronic effects.

Thallium (III) oxide is a degenerate n-type semiconductor with high optical transparency and electrical conductivity. Films of thallium(III) oxide can be electrochemically deposited onto conducting and p-type semiconducting substrates, and photoelectrochemically deposited onto n-type semiconducting substrates. Films deposited at currents below the mass transport limit onto platinum or stainless steel were columnar, and the current efficiency on stainless steel was 103 ±2%. Dendritic films were deposited at mass-transport-limited currents. Films were deposited with thicknesses ranging from 0.1 μm on n-silicon, to 170 μm on stainless steel. The photoelectrochemically deposited films were “direct-written” onto n-silicon, since the material was deposited only at irradiated portions of the electrode. Thin films were grown by irradiating the n-silicon with 450 nm monochromatic light, since the light was strongly absorbed by the thallium(III) oxide. The most uniform thin films were deposited when the n-silicon was initially irradiated with a short pulse of high intensity light. The pulse apparently promoted instantaneous nucleation of a high density of thallium(III) oxide nuclei.

Zirconia powder was produced in aqueous solution from zirconyl nitrate using electrogenerated base. Both divided and undivided electrochemical cells were used. In the divided cell, hydroxide ion was discharged in the cathode compartment, and hydrated zirconia was produced. The as-produced material was weakly agglomerated, amorphous, and had a surface area of up to 316 m2/g. The surface area of the powder did not vary systematically with the electrosynthesis current density, but did depend on subsequent processing. Crystalline zirconia was produced by calcining in air at temperatures above 400°C. Tetragonal zirconia was the only phase observed until about 800°C. After calcining at 800°C the crystallite size increased to about 20nm and about 34% monoclinic zirconia was produced. When an undivided cell was used, the pH remained constant (1–1.5) throughout the electrosynthesis, and amorphous zirconia deposited on the cathode.