Density functional approaches for the interaction of metal oxides with small molecules

Under ambient conditions, most metals are covered with an oxide layer. The interaction of these oxide surfaces with different molecules has been widely investigated as the formation of molecular-metal oxide complexes has found a broad range of applications in technology. Theoretical insights into the physics and chemistry of such hybrid interfaces are crucial for the growth of technological advancement. Approaches based on Density Functional Theory (DFT) constitute an efficient alternative to wavefunction-based methods and have gained a prominent role in the study of systems and processes of substantial complexity. The density functional tight binding (DFTB) approach and its time-dependent extension (TD-DFTB) combine the accuracy typical of first principles DFT with the efficiency representative of semiempirical methods. Despite their success, serious shortcomings of DFTB/TD-DFTB have been identified. Those include the inaccurate description of hydrogen bond interactions and σ → π∗ electron transitions. The DFTB and TD-DFTB methods are here extended in order to overcome the aforementioned deficiencies. The incorporation of one-center exchange-like terms in the expansion of multicenter integrals leads to a DFTB scheme in which the fluctuation of the dual density matrix is treated self-consistently. This formalism improves upon hydrogen bond energies of neutral, protonated and hydroxide water clusters as well as of methylimidazole-water complexes. An analogous correction for TD-DFTB leads to marked qualitative and quantitative improvements over the original method. Especially, the failure for the description of σ → π∗ and n→ π∗ excitations is surmounted. Benchmark calculations on a large set of organic molecules also indicate a better description of triplet states. Overall, the accuracy of the revised TD-DFTB approach is found to be similar to that of ab initio TD-DFT calculations, at a highly reduced computational cost. Furthermore, TD-DFTB is generalized to account for fractional occupation and spin polarization. DFTB and TD-DFTB are employed to investigate the structural and optical properties of titanium dioxide (TiO2) complexed with two important environmental pollutants, namely, nitric oxide (NO) and acetaldehyde (CH3CHO). The TiO2-NO systems are shown to absorb visible light by formation of charge-transfer (CT) complexes. In contrast, complexation with acetaldehyde does not lead to solar activation of TiO2. The CT complexes may act as visible light sensitizers which prompt the self-degradation of the contaminant. Our predictions are confirmed by experiments conducted in collaboration within the present work. We also explain the unexpected photo-decomposition of NO in the presence of aluminum oxide (Al2O3) under ultraviolet illumination. Finally, the structural and electronic properties of the interfaces between zinc oxide (ZnO) and several organic molecules are investigated using DFT and DFTB. The influence of the ZnO surface coverage on the adsorption energies and geometries of the ligands is analyzed. Stable interfaces in the gas phase are found for carboxylic acids, thiols and phosphonates. We find that all these compounds prefer to bind dissociatively on the nonpolar surfaces of ZnO. Furthermore, glycine is found to bind to the (1010) ZnO surface through either the carboxyl or the amino group with similar adsorption energies at full coverage. However, the electronic properties of the adduct depend on the orientation of the aminoacid. For lower coverages, glycine strongly adsorbs on the surface through both functional groups. We find that the presence of surface oxygen vacancies slightly strengthens the ligand-substrate interaction.