Spectroscopic link between adsorption site occupation and local surface chemical reactivity.
In this Letter we show that sequences of adsorbate-induced shifts of surface core level (SCL) x-ray photoelectron spectra contain profound information on surface changes of electronic structure and reactivity. Energy shifts and intensity changes of time-lapsed spectral components follow simple rules, from which adsorption sites are directly determined. Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the d-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity.
Stability, structure, and electronic properties of chemisorbed oxygen and thin surface oxides on Ir(111)
We present ab initio calculations for atomic oxygen adsorption on Ir111 for a wide range of oxygen coverages, , namely from 0.11 to 2.0 monolayers ML, including subsurface adsorption and thin surfaceoxide-like structures. For on-surface adsorption, oxygen prefers the fcc-hollow site for all coverages considered. Similarly to oxygen adsorption on other transition metal surfaces, as increases from 0.25 ML to 1.0 ML, the binding energy decreases, indicating a repulsive interaction between the adsorbates. For the coverage range of 0.11 to 0.25 ML, there is an attractive interaction, suggesting the possible formation of a local 22 periodicity with a local coverage of =0.25 ML. Pure subsurface oxygen adsorption is found to be metastable and endothermic with respect to the free O2 molecule. For structures with coverage beyond one full ML, we find the incorporation of oxygen under the first Ir layer to be exothermic. As the subsurface O coverage increases in these structures from 0.5 to 1.0 ML, the energy becomes slightly more favorable, indicating an attractive interaction between the O atoms. The structure with the strongest average O binding energy is however a reconstructed trilayer-like structure that can be described as a 33R30° oxide-like layer in p22 surface unit cell, with coverage 1.5 ML. Through calculation of the surface Gibbs free energy of adsorption, taking into account the pressure and temperature dependence through the oxygen atom chemical potential, the calculations predict only three thermodynamically stable regions, namely, the clean surface, the p22-O phase, and bulk IrO2. Thin trilayer surface oxide structures are predicted only to form when kinetic hindering occurs, in agreement with recent experimental work.
Energy
Energy is often defined as the ability to do work. There are many forms of energy. A few examples include; Gravitational energy, Kinetic energy (energy of motion), Thermal energy (heat energy), Potential energy. Potential energy is the energy something has based on its position in the universe. A stretched elastic band has potential energy. A rock held up in the air has potential energy based on its position.