Mechanistic Study for Facile Electrochemical Patterning of Surfaces with Metal Oxides

Reactive interface patterning promoted by lithographic electrochemistry serves as a method for generating submicrometer scale structures. We use a binary-potential step on a metallic overlayer on silicon to fabricate radial patterns of cobalt oxide on the nanoscale. The mechanism for pattern formation has heretofore been ill-defined. The binary potential step allows the electrochemical boundary conditions to be controlled such that initial conditions for a scaling analysis are afforded. With the use of the scaling analysis, a mechanism for producing the observed pattern geometry is correlated to the sequence of electrochemical steps involved in the formation of the submicrometer structures. The patterning method is facile and adds to electrochemical micromachining techniques employing a silicon substrate

Proton–Electron Transport and Transfer in Electrocatalytic Films. Application to a Cobalt-Based O₂‑Evolution Catalyst

Solar-driven electrochemical transformations of small molecules, such as water splitting and CO₂ reduction, pertinent to modern energy challenges, require the assistance of catalysts preferably deposited on conducting or semiconducting surfaces. Understanding mechanisms and identifying the factors that control the functioning of such systems are required for rational catalyst optimization and improved performance. A methodology is proposed, in the framework of rotating disk electrode voltammetry, to analyze the current responses expected in the case of a semigeneral reaction scheme involving a proton-coupled catalytic reaction associated with proton-coupled electron hopping through the film as rate controlling factors in the case where there is no limitation by substrate diffusion. The predictions concern the current density vs overpotential (Tafel) plots and their dependence on buffer concentration (including absence of buffer), film thickness and rotation rate. The Tafel plots may have a variety of slopes (e.g., <i>F</i>/<i>RT</i> ln 10, <i>F</i>/2<i>RT</i> ln 10, 0) that may even coexist within the overpotential range of a single plot. We show that an optimal film thickness exists beyond which the activity of the film plateaus. Application to water oxidation by films of a cobalt-based oxidic catalyst provides a successful test of the applicability of the proposed methodology, which also provides further insight into the mechanism by which these cobalt-based films catalyze the oxidation of water. The exact nature of the kinetic and thermodynamic characteristics that have been derived from the analysis is discussed as well as their use in catalyst benchmarking

Synthesis of Monoclinic and Tetragonal Chalcocite Nanoparticles by Iron-Induced Stabilization

Infrared absorbing monoclinic and tetragonal chalcocite nanoparticles were synthesized. These metastable copper sulfide phases were obtained by addition of varying amounts of iron to the reaction mixtures. Phases were identified by powder X-ray diffraction (PXRD), and the particles were characterized by UV–vis–NIR absorption spectroscopy (UV–vis–NIR), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). This synthesis affords monoclinic chalcocite, which is difficult to obtain in nanocrystalline form because of its ready transformation to copper-deficient djurleite. Nanoparticles of the little-studied, high-temperature-stable tetragonal chalcocite form were synthesized for the first time. These particles showed improved phase stability compared to monoclinic chalcocite, while maintaining the optical properties that made monoclinic chalcocite intensely investigated as a light absorber in photovoltaics. Together, these syntheses offer two routes toward managing an impediment to utilization of nanocrystalline chalcocite in photovoltaic applications, the transformation to djurleite, and uncover remarkable methods of nanocrystalline phase control

Facile, Rapid, and Large-Area Periodic Patterning of Semiconductor Substrates with Submicron Inorganic Structures

The development of high-throughput and scalable techniques for patterning inorganic structures is useful for the improved function and efficiency of photonic and energy conversion devices. Here we demonstrate a facile and rapid electrochemical method for patterning periodic metallic and nonmetallic submicron structures over large areas. Si substrates have been patterned with arrays of periodically spaced lines, rings, squares, and terraces of main-group and transition-metal oxides. In addition to planar substrates, three-dimensional surfaces and their vertical sidewalls have been patterned. The features are 20(±1) nm high and 360(±15) nm wide, and their period is finely tunable <i>in situ</i> from 500 nm to 7 μm. These features exhibit <3% variation in period and are rapidly patterned in <2 min. We demonstrate the versatility of the technique by rapidly patterning an efficient water splitting catalyst, Co phosphate oxide (CoP_i), and show that the integrated materials system performs water splitting with complete Faradaic efficiency. More generally, the ability to pattern submicron structures over large areas in a facile, reliable, and timely manner may be useful for the fabrication of devices for energy, meta-material, and sensing applications

Influence of Solvent Reducing Ability on Copper Sulfide Crystal Phase

Copper sulfide particles across a wide range of stoichiometries are obtained depending on the ratio of cosolvents from which they are grown. Copper sulfides are abundant, low-cost materials with phase-dependent properties relevant to solar energy conversion and (opto)­electronic devices. For this reaction, the reducing ability of dodecanethiol versus oleic acid affects the speciation of the precursors, as determined using UV–vis absorption spectroscopy. The ratio of dodecanethiol to oleic acid in the synthetic medium affects the solid-state structure and stoichiometry, as determined by powder X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and detailed investigation of the band edge positions and plasmon behavior using visible–NIR optical absorption spectroscopy and cyclic voltammetry. A range of phases was obtained, including monoclinic chalcocite, tetragonal chalcocite, digenite, and covellite. The thermodynamic relationships between these phases were elucidated using equilibration experiments, revealing methods for postsynthetic property alteration. Particle size and morphology were also affected by solvent ratio, as shown using scanning or transmission electron microscopy. Oleic acid accelerated particle growth and resulted in particles with an unusual faceted shape