Understanding the mechanisms which determine the band offsets and Schottky barriers at semiconductor contacts and engineering them for specific device applications are important theoretical and technological challenges. In this review, we present a theoretical approach to the band-line-up problem and discuss its application to prototypical systems. The emphasis is on ab initio computations and on theoretical models derived from first-principles numerical experiments. An approach based on linear-response-theory concepts allows a general description of the band alignment for various classes of semiconductor contacts and predicts the effects of various bulk and interfacial perturbations on the band discontinuities.
Oxygen-containing functional groups can be present in considerable amount intentionally or unintentionally on graphene, and a complete reduction of graphene oxide is difficult to achieve. To address the origin of this behavior, we have performed pseudopotential density functional theory calculations to investigate in particular the adsorption of hydroxyl (OH) on perfect and defected graphene, individually and in the presence of other coadsorbed functional groups. We found that hydroxyl groups weakly adsorb on perfect graphene, easily aggregate, also with coadsorbed epoxy groups, and can react with each other with a barrier of about 0.5 eV forming water. Defect sites are more reactive for OH adsorption but play different roles. At variance with single vacancy defects where the OH adsorption is highly dissociative, Stone−Wales defects could stabilize the hydroxyl groups on the graphene basal plane, with a much stronger binding and higher barriers for recombination and water formation than pristine
The nanoscale description of the reaction pathways and of the role of the intermediate species involved in a chemical process is a crucial milestone for tailoring more active, stable, and cheaper catalysts, thus providing “reaction engineering” capabilities. This level of insight has not been achieved yet for the catalytic hydrogenation of CO2 on Ni catalysts, a reaction of enormous environmental relevance. We present a thorough atomic-scale description of the mechanisms of this reaction, studied under controlled conditions on a model Ni catalyst, thus clarifying the long-standing debate on the actual reaction path followed by the reactants. Remarkably, formate, which is always observed under standard conditions, is found to be just a “dead-end” spectator molecule, formed via a Langmuir−Hinshelwood process, whereas the reaction proceeds through parallel Eley−Rideal channels, where hydrogen-assisted C−O bond cleavage in CO2 yields CO already at liquid nitrogen temperature.
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