Konstanze R. Hahn

First principles analysis of H2O adsorption on the (110) surfaces of SnO2, TiO2 and their solid solutions.

Both associative and dissociative H(2)O adsorption on SnO(2)(110), TiO(2)(110), and Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces have been investigated at low ((1)/(12) monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO(2)(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H(2)O configurations. Increasing the coverage from (1)/(12) to 1 ML stabilized the (associatively or dissociatively) adsorbed H(2)O on SnO(2)(110) because of the formation of intermolecular H bonds. In contrast, on TiO(2)(110), the adsorption of isolated H(2)O groups ((1)/(12) ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H(2)O adsorption on Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti(6c) and bridging O atoms that had to be broken upon H(2)O adsorption. As a result, the H(2)O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for (1)/(12) ML. This behavior was even more accentuated at high H(2)O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn(1-x)Ti(x)O(2) gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H(2)O is facilitated at a high fractional coverage.

Free energy and electronic properties of water adsorption on the SnO2(110) surface.

A molecular understanding of the adsorption of water on SnO2 surfaces is crucial for several applications of this metal oxide, including catalysis and gas sensing. We have investigated water adsorption on the SnO2(110) surface using a combination of dynamic and static calculations to gain fundamental insight into the reaction mechanism at room temperature. The reaction dynamics are studied by following water adsorption and dissociation on the SnO2 surface with metadynamics calculations at low and high coverage. The electronic structure in the relevant isolated minima is investigated through Mulliken charge analysis and projected density of states analysis. Surface bridging oxygen (Obr) is found to play a decisive role in water adsorption forming rooted hydroxyl groups with the water H atoms. Bond formation with H significantly changes the electronic configuration of Obr and presumably leads to reduced band bending at the SnO2 surface. The free-energy estimation indicates that on a clean SnO2(110) surface at room temperature both associative and dissociative adsorption occur, with the latter being thermodynamically favored. Oxygen coverage strongly affects the ratio between associatively and dissociatively adsorbed H2O, favoring associative adsorption at high oxygen coverage (oxidized surface) and dissociative adsorption at low oxygen coverage (reduced surface). Electronic analyses of isolated surface minima show the existence of two different electron-transfer phenomena occurring at the surface, depending on the water adsorption mechanism. The relevance of these findings in explaining the changes in electric conductivity occurring in SnO2-based gas sensors upon water adsorption is discussed. Whereas associative adsorption leads to electron enrichment of the metal oxide surface, dissociative adsorption induces surface electron depletion. Both mechanisms are consistent with the electrical conductivity changes occurring upon interaction of SnO2 with water, causing cross sensitivity to the latter. The theoretical results form the basis for correlating the existing atomistic models with the experimental data and offer a coherent description of the reaction events on the surface at room temperature.

Functionalization of CeO2(1 1 1) by Deposition of Small Ni Clusters: Effects on CO2 Adsorption and O Vacancy Formation

In this study we have used density functional theory calculations to investigate the stability, structure and catalytic properties of Ni clusters supported on CeO2. We have found Ni clusters to be stabilized with increasing cluster size up to ten atoms both as isolated clusters and adsorbed on CeO2(1 1 1). Analysis of the structural properties showed an opening of the Ni particles when deposited on CeO2(1 1 1) indicating facilitated accessibility for reacting molecules. The reactivity of the functionalized surface has been examined on the example of CO2 adsorption and activation and compared to the one on pristine CeO2(1 1 1) and isolated Ni clusters. Furthermore, the effect of Ni cluster deposition on the formation and characteristics of surface and subsurface oxygen vacancies in CeO2 has been investigated. The oxygen vacancy position is found to significantly affect the stability and electronic structure of the Ni/CeO2 system.

Effect of asymmetric concentration profile on thermal conductivity in Ge/SiGe superlattices

The effect of the chemical composition in Si/Ge-based superlattices on their thermal conductivity has been investigated using molecular dynamics simulations. Simulation cells of Ge/SiGe superlattices have been generated with different concentration profiles such that the Si concentration follows a step-like, a tooth-saw, a Gaussian, and a gamma-type function in direction of the heat flux. The step-like and tooth-saw profiles mimic ideally sharp interfaces, whereas Gaussian and gamma-type profiles are smooth functions imitating atomic diffusion at the interface as obtained experimentally. Symmetry effects have been investigated comparing the symmetric profiles of the step-like and the Gaussian function to the asymmetric profiles of the tooth-saw and the gamma-type function. At longer sample length and similar degree of interdiffusion, the thermal conductivity is found to be lower in asymmetric profiles. Furthermore, it is found that with smooth concentration profiles where atomic diffusion at the interface...

Thermal Transport in Nanocrystalline Graphene: The Role of Grain Boundaries

Single grain boundaries of crystalline graphene with varying mismatch angles from 3° to 16° have been investigated using molecular dynamics simulations. Four- to eight-atomic rings are found to be the most abundant non-hexagonal polygons in the grain boundary for all mismatch angles. Tetra- and octagons are predominant for mismatch angles of 4.1° and 6.6° in contrast to nanocrystalline samples where penta- and heptagons are dominating. Out-of-plane buckling at the grain boundary is most pronounced for a mismatch angle of 3.0° and it tends to decrease with increasing mismatch angle. At 16.1°, the out-of-plane buckling vanishes. Analysis of the vibrational density of states of boundary atoms revealed a significant decrease of the main peak of optical vibrations and the evolution of secondary peaks below and above the major frequency attributed to vibrations of non-hexagonal rings. The thermal boundary resistance in single graphene interfaces has been approximated. It tends to increase with increasing mismatch angle, indicating reduced thermal conductivity when such interfaces are present in crystalline graphene. In nanocrystalline graphene samples, the thermal conductivity is significantly reduced with respect to crystalline graphene and it decreases with decreasing grain size according to an increasing number of single boundaries.