Methodology for electronic excitations

Kolesov G, Vinichenko D, Tritsaris GA, Friend CM, Kaxiras E. Anatomy of the Photochemical Reaction: Excited-State Dynamics Reveals the C−H Acidity Mechanism of Methoxy Photo-oxidation on Titania. Journal of Physical Chemistry Letters. 2015;6 :1624-1627.Abstract

Light-driven chemical reactions on semiconductor surfaces have potential for
addressing energy and pollution needs through efficient chemical synthesis; however, little is known about the time evolution of excited states that determine reaction pathways. Here, we study the photo-oxidation of methoxy (CH3O) derived from methanol on the rutile TiO2(110) surface using ab initio simulations to create a molecular movie of the process. The movie sequence reveals a wealth of information on the reaction intermediates, time scales, and energetics. The reaction is broken in three stages, described by Lewis structures directly derived from the “hole” wave functions that lead to the concept of “photoinduced C−H acidity”. The insights gained from this work can be generalized to a set of simple rules that can predict the efficiency of photo-oxidation reactions in reactant−catalyst pairs.

Tritsaris GA, Vinichenko D, Kolesov G, Friend CM, Kaxiras E. Dynamics of the Photogenerated Hole at the Rutile TiO2(110)/Water Interface: A Nonadiabatic Simulation Study. Journal of Physical Chemistry C . 2014;118 :27393-27401.Abstract

Hydrogen production in photoelectrochemical cells constitutes an important avenue toward carbon-free fuel. The most convenient process for hydrogen production is the splitting of water molecules, which necessitates a catalytic
reaction involving a semiconductor. Here, we introduce a framework for the study of photocatalyzed reactions on semiconductor surfaces based on time-dependent density functional theory that explicitly accounts for the evolution of electronically excited states. Within this framework, we investigate the possibility of hole-mediated splitting of molecularly adsorbed water on a representative metal oxide surface the rutile TiO2(110). We find that oxidative dehydrogenation of water is possible in synergy with thermal effects at temperatures between 60 and 100 K only when defects like Ti interstitials are present in the subsurface region. This study presents a general computational strategy for describing photoexcited semiconductor/adsorbate interfaces and also demonstrates that the occurrence of water dissociation on the rutile TiO2(110) surface depends sensitively on the local atomic environment and external parameters such as temperature.

Tritsaris GA, Malone BD, Kaxiras E. Structural stability and electronic properties of low-index surfaces of SnS. Journal of Applied Physics. 2014;115 :173702.Abstract

Thin film photovoltaic cells are increasingly important for cost-effective solar energy harvesting. Layered SnS is a promising absorber material due to its high optical absorption in the visible and good doping characteristics. We use first-principles calculations based on density functional theory to study structures of low-index surfaces of SnS using stoichiometric and oxygen-containing structural models, in order to elucidate their possible effect on the efficiency of the photovoltaic device. We find that the surface energy is minimized for the surface with orientation parallel to the layer stacking direction. Compared to stoichiometric surfaces, the oxygen-containing surfaces exhibit fewer electronic states near the band gap. This reduction of near-gap surface states by oxygen should reduce recombination losses at grain boundaries and interfaces of the SnS absorber, and should be beneficial to the efficiency of the solar cell.

Tritsaris GA, Malone BD, Kaxiras E. Optoelectronic properties of single-layer, double-layer, and bulk tin sulfide: A theoretical study. Journal of Applied Physics. 2013;113 :233507.Abstract

SnS is a metal monochalcogenide suitable for use as absorber material in thin film photovoltaic cells. Its structure is an orthorhombic crystal of weakly coupled layers, each layer consisting of strongly bonded Sn-S units. We use first-principles calculations to study model single-layer, double-layer, and bulk structures of SnS in order to elucidate its electronic structure. We find that the optoelectronic properties of the material can vary significantly with respect to the number of layers and the separation between them: the calculated band gap is wider for fewer layers (2.72 eV, 1.57 eV, and 1.07 eV for single-layer, double-layer, and bulk SnS, respectively) and increases with tensile strain along the layer stacking direction (by ∼55 meV/1% strain).

Tritsaris GA, Mathioudakis C, Kelires PC, Kaxiras E. Optical and elastic properties of diamond-like carbon with metallic inclusions: A theoretical study. Journal of Applied Physics. 2012;112 :103503.Abstract

A tough material commonly used in coatings is diamond-like carbon (DLC), that is, amorphous carbon with content in four-fold coordinated C higher than ∼70%, and its composites with metal inclusions. This study aims to offer useful guidelines for the design and development of metal-containing DLC coatings for solar collectors, where the efficiency of the collector depends critically on the performance of the absorber coating. We use first-principles calculations based on density functional theory to study the structural, electronic, optical, and elastic properties of DLC and its composites with Ag and Cu inclusions at 1.5% and 3.0% atomic concentration, to evaluate their suitability for solar thermal energy harvesting. We find that with increasing metal concentration optical absorption is significantly enhanced while at the same time, the composite retains good mechanical strength: DLC with 70–80% content in four-fold coordinated C and small metal concentrations (<3 at. %) will show high absorption in the visible (absorption coefficients higher than 105 cm−1) and good mechanical strength (bulk and Young's modulus higher than 300 and 500 GPa, respectively).