Sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride photocatalyst: insights from first principles

We present here results of our first principles studies of the sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride (g-C3N4). Using the Ab initio thermodynamics approach combined with some kinetic analysis, we reveal the favorable S-doping configurations By analyzing the valence charge densities of the doped and un-doped systems, we find that sulfur partially donates its px- and py- electrons to the system with some back donation to the S pz-states. To obtain accurate description of the excited electronic states, we calculate the electronic structure of the systems using the GW method. The band gap width calculated for g-C3N4 is found to be equal to 2.7 eV that is in agreement with experiment. We find the S doping to cause a significant narrowing the gap. Furthermore, the electronic states just above the gap become occupied upon doping that makes the material a conductor. Analysis of the projected local densities of states provides insight into the mechanism underlying such dramatic changes in the electronic structure of g-C3N4 upon the S doping. Based on our results, we propose a possible explanation for the S doping effect on the photo-catalytic properties of g-C3N4 observed in the experiments

First principles studies of the size and shape effects on reactivity of the Se modified Ru nanoparticles

We present here the results of our density-functional-theory-based calculations of the electronic and geometric structures and energetics of Se and O adsorption on Ru 93- and 105-atom nanoparticles. These studies have been inspired by the fact that Se/Ru nanoparticles are considered promising electrocatalysts for the oxygen reduction reaction (ORR) on the direct methanol fuel cell cathodes and the oxygen binding energy is a descriptor for the catalyst activity towards this reaction. We find the character of chemical bonding of Se on a flat nanoparticle facet to be ionic, similar to that obtained earlier for the Se/Ru(0001) surface, while in the case of a low coordinated Ru configuration there is an indication of some covalent contribution to the bonding leading to an increase in Se binding energy. Se and O co-adsorbed on the flat facet, both accept electronic charge from Ru, whereas the adsorption on low-coordinated sites causes more complicated valence charge redistribution. The Se modification of the Ru particles leads to weakening of the oxygen bonding to the particle. However, overall, O binding energies are found to be higher for the particles than for Se/Ru(0001). High reactivity of the Se/Ru nanoparticles found in this work is not favorable for ORR. We thus expect that larger particles with well-developed flat facets are more efficient ORR catalysts than small nanoparticles with a large fraction of under-coordinated adsorption sites

Electronic Structure of the c(2x2)O/Cu(001) System

The locally self-consistent real space multiple scattering technique has been applied to calculate the electronic structure and chemical binding for the c(2x2)O/Cu(001) system, as a function of $d_{O-Cu1}$ -- the height of oxygen above the fourfold hollow sites. It is found that the chemical binding between oxygen and copper has a mixed ionic-covalent character for all plausible values of $d_{O-Cu1}$. Furthermore, the electron charge transfer from Cu to O depends strongly on $d_{O-Cu1}$ and is traced to the variation of the long-range electrostatic part of the potential. A competition between the hybridization of Cu1-$d_{xz}$ with O-$p_x,p_y$ and Cu1-$d_{x^2-y^2}$ with O-$p_z$ states controls modification of the electronic structure when oxygen atoms approach the Cu(001) surface. The anisotropy of the oxygen valence electron charge density is found to be strongly and non-monotonically dependent on $d_{O-Cu1}$.Comment: 14 pages, 7 figures, 1 tabl

Relationship between Electronic and Geometric Structures of the O/Cu(001) System

The electronic structure of the $(2\sqrt{2}\times\sqrt{2})R45^{\circ}$ O/Cu(001) system has been calculated using locally self-consistent, real space multiple scattering technique based on first principles. Oxygen atoms are found to perturb differentially the long-range Madelung potentials, and hence the local electronic subbands at neighboring Cu sites. As a result the hybridization of the oxygen electronic states with those of its neighbors leads to bonding of varying ionic and covalent mix. Comparison of results with those for the c(2x2) overlayer shows that the perturbation is much stronger and the Coulomb lattice energy much higher for it than for the $(2\sqrt{2}\times\sqrt{2})R45^{\circ}$ phase. The main driving force for the 0.5ML oxygen surface structure formation on Cu(001) is thus the long-range Coulomb interaction which also controls the charge transfer and chemical binding in the system.Comment: 17 pages, 8 figure

First-principles study of formation of Se submonolayer structures on Ru surfaces

The Ru nanoparticles with Se submonolayer coverage (Se/Ru) demonstrate high electrocatalytic activity toward oxygen reduction reaction (ORR) on cathodes of proton exchange membrane fuel cells. To understand the mechanisms of formation of Se structures on Ru surfaces, the geometric and electronic structures and energetics have been calculated in the present work for various distributions of Se atoms on the Ru (0001) surface and in the vicinity of the edge between the (0001) and (1101) facets. The calculations were performed within the density-functional theory with plane-wave expansion for wave functions and the projector augmented wave potentials. It has been found that due to electronic charge transfer from Ru to Se upon selenium adsorption, Se atoms become negatively charged and repel each other. This repulsion makes compact Se islands on Ru (0001) unstable. Se atoms prefer to separate from each other by the distance of similar to 5.47 angstrom or larger, which is possible for all Se adsorbates if coverage is not exceeding 1/3 ML. Further increase in Se coverage weakens Se-Ru bonding. Three-dimensional Se structure such as 4- and 11-atom pyramids are found to decompose spontaneously with scattering of Se atoms over the Ru (0001) surface. The Se adsorbates are also found to repel in the vicinity of the edge between the Ru facets, and a small increase in Se bonding to undercoordinated Ru atom does not change the trend of Se adsorbates to separate from each other. The obtained most stable configurations of Se on Ru with 1/3 ML coverage or less may also be optimal for ORR because they provide Ru sites available for O and OH adsorption

Gold-doped graphene: A highly stable and active electrocatalysts for the oxygen reduction reaction

In addressing the growing need of renewable and sustainable energy resources, hydrogen-fuel-cells stand as one of the most promising routes to transform the current energy paradigm into one that integrally fulfills environmental sustainability. Nevertheless, accomplishing this technology at a large scale demands to surpass the efficiency and enhance the cost-effectiveness of platinum-based cathodes, which catalyze the oxygen reduction reaction (ORR). In this work, our first-principles calculations show that Au atoms incorporated into graphene di-vacancies form a highly stable and cost-effective electrocatalyst that is, at the same time, as or more (dependently of the dopant concentration) active toward ORR than the best-known Pt-based electrocatalysts. We reveal that partial passivation of defected-graphene by gold atoms reduces the reactivity of C dangling bonds and increases that of Au, thus optimizing them for catalyzing the ORR and yielding a system of high thermodynamic and electrochemical stabilities. We also demonstrate that the linear relation among the binding energies of the reaction intermediates assumed in computational high-throughput material screening does not hold, at least for this non-purely transition-metal material. We expect Au-doped graphene to finally overcome the cathode-related challenge hindering the realization of hydrogen-fuel cells as the leading means of powering transportation and portable devices

The perturbation energy: A missing key to understand the nobleness of bulk gold

The nobleness of gold surfaces has been appreciated since long before the beginning of recorded history. Yet, the origin of this phenomenon remains open because the so far existing explanations either incorrectly imply that silver should be the noblest metal or would fail to predict the dissolution of Au in aqua regia. Here, based on our analyses of oxygen adsorption, we advance that bulk gold\u27s unique resistance to oxidation is traced to the large energy cost associated with the perturbation its surfaces undergo upon adsorption of highly electronegative species. This fact is related to the almost totally filled d-band of Au and relativistic effects, but does not imply that the strength of the adsorbate-Au bond is weak. The magnitude of the structural and charge-density perturbation energy upon adsorption of atomic oxygen-which is largest for Au-is assessed from first-principles calculations and confirmed via a multiple regression analysis of the binding energy of oxygen on metal surfaces

Factors controlling the energetics of the oxygen reduction reaction on the Pd-Co electro-catalysts: Insight from first principles

We report here results of our density functional theory based computational studies of the electronic structure of the Pd-Co alloy electrocatalysts and energetics of the oxygen reduction reaction (ORR) on their surfaces. The calculations have been performed for the (111) surfaces of pure Pd, Pd0.75Co0.25 and Pd0.5Co0.5 alloys, as well as of the surface segregated Pd/Pd0.75Co0.25 alloy. We find the hybridization of dPd and dCo electronic states to be the main factor controlling the electrocatalytic properties of Pd/Pd0.75Co0.25. Namely the dPd - dCo hybridization causes low energy shift of the surface Pd d-band with respect to that for Pd(111). This shift weakens chemical bonds between the ORR intermediates and the Pd/Pd0.75Co0.25 surface, which is favorable for the reaction. Non-segregated Pd0.75Co0.25 and Pd0.5Co0.5 surfaces are found to be too reactive for ORR due to bonding of the intermediates to the surface Co atoms. Analysis of the ORR free energy diagrams, built for the Pd and Pd/Pd0.75Co0.25, shows that the co-adsorption of the ORR intermediates and water changes the ORR energetics significantly and makes ORR more favorable. We find the onset ORR potential estimated for the configurations with the O - OH and OH - OH co-adsorption to be in very good agreement with experiment. The relevance of this finding to the real reaction environment is discussed