Mark G. Shumsky

Hydrothermal precipitation and characterization of nanocrystalline BaTiO3 particles

Crystalline BaTiO3 powders were precipitated by reacting fine TiO2 particles with a strongly alkaline solution of Ba(OH)2 under hydrothermal conditions at 80°C to 240°C. The characteristics of the powders were investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and atomic emission spectroscopy. For a fixed reaction time of 24 hours, the average particle size of BaTiO3 increased from ∼50 nm at 90°C to ∼100 nm at 240°C. At synthesis temperatures below ∼150°C, the BaTiO3 particles had a narrow size distribution and were predominantly cubic in structure. Higher synthesis temperatures produced a mixture of the cubic and tetragonal phases in which the concentration of the tetragonal phase increased with increasing temperature. A bimodal distribution of sizes developed for long reaction times (96 h) at the highest synthesis temperature (240°C). Thermal analysis revealed little weight loss on heating the powders to temperatures up to 700°C. The influence of particle size and processing-related hydroxyl defects on the crystal structure of the BaTiO3 powder is discussed.

POTENTIAL OSCILLATIONS DURING THE ELECTROCHEMICAL SELF-ASSEMBLY OF COPPER/CUPROUS OXIDE LAYERED NANOSTRUCTURES

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.

Low-temperature electrodeposition of the high-temperature cubic polymorph of bismuth(III) oxide

Abstract Nanocrystalline films of δ-Bi 2 O 3 were electrodeposited at 65°C directly from alkaline solutions of tartrate-complexed Bi(III). This face-centered-cubic polymorph of Bi 2 O 3 is normally only stable at high temperatures (729–825°C). The material has the highest known oxide ion mobility. We propose that the high temperature form of the oxide is stabilized due to the nanocrystalline (70 nm) size of the particles in the film. The oxide also deposits epitaxially onto a single-crystal Au(110) substrate with strong in-plane and out-of-plane orientation. The large lattice mismatch (35.4%) is accommodated by forming a coincidence lattice, in which the δ-Bi 2 O 3 is rotated 90° relative to the Au (110) substrate. The epitaxial relationship between film and substrate may also serve to stabilize the high-temperature structure.

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.

Structure and phase transformation of lanthanum chromate

Lanthanum chromate (LaCrO4) was synthesized as a low temperature ( < 700 °C) intermediate to perovskite-type, lanthanum chromite (LaCrO3). The lattice parameters, atom positions and bond lengths determined from X-ray powder diffraction show that LaCrO4 forms a monazite-type crystal structure. Lanthanum chromate forms a solid solution with Ca, having a solubility between 10 and 20 at%. Thermal analysis shows that at ambient oxygen pressure, LaCrO4 transforms to LaCrO3 at a temperature near 700 °C. It also indicates that (La,Ca)CrO4 initially transforms to LaCrO3 and CaCrO4 with the subsequent formation of a (La,Ca)CrO3 solid solution. Evidence of Ca segregation to the surface of (La,Ca) CrO3 particles is given by Auger electron spectroscopy and scanning electron microscopy.[/p]

Epitaxial Electrodeposition of Orthorhombic α ­ PbO2 on (100)-Oriented Single Crystal Au

Epitaxial films of orthorhombic lead dioxide α-PbO 2 (Pbcn, a = 0.4971, h = 0.5956, and c = 0.5438 nm) were electrodeposited at room temperature on a [100]-oriented single crystal Au substrate (cubic, Fm3m, a = 0.4079 nm). The values of lattice mismatch are 21.9, 46.0, and 33.3% in the a, b, and c directions of α-PbO 2 , respectively. The films are [100]-oriented, with the a axis perpendicular to the substrate. Depending on the preparation conditions, the film was found to form two different in-plane orientations, with the unit cell of the PbO 2 structure rotated either 45 or 11.5° relative to the substrate. For the films deposited at low current density, only the first type of the in-plane texture is observed, while at high current densities both types are present in comparable amounts.

Epitaxial Electrodeposition of Lead Sulfide on (100)‐Oriented Single‐Crystal Gold

Small lattice mismatches and gas-phase deposition are typically used for growing epitaxial films on single-crystal substrates. A 1-µm thick film of PbS can be epitaxially electrodeposited onto a Au (100) single crystal. The large lattice mismatch (45.5 %) between Au and PbS is accommodated by the formation of a coincidence lattice, in which the epitaxial film is rotated by 45 degrees relative to the substrate. The coincidence lattice reduces the mismatch to +2.9 %.