Time dependent density functional theory (TD-DFT) calculations have been carried out to study the electronic structure and the optical properties of five coumarin based dyes: C343, NKX-2311, NKX-2586, NKX-2753 and NKX-2593. We have found out that the position and width of the first band in the electronic absorption spectra, the absorption threshold and the LUMO energy with respect to the conduction band edge are key parameters in order to establish some criteria that allow evaluating the efficiency of coumarin derivatives as sensitizers in Dye Sensitized Solar Cells (DSSC). Those criteria predict the efficiency ordering for the coumarin series in good agreement with the experimental evidence. Presumably, they might be used in the design of new efficient organic based DSSC.

The structural and electronic properties of the alizarin dye supported on TiO2 nanoclusters have been examined by means of time-dependent density-functional (TD-DFT) calculations performed in the time-domain framework. The calculated electronic absorption spectrum of free alizarin shows a first band centered at 2.67 eV that upon adsorption features a red shift by 0.31 eV, in agreement with both experimental and previous theoretical work. This red shift arises from a relative stabilization of the dye LUMO when adsorbed. To analyze the dependence of the electronic properties of the dye-support couple on the size of metal-oxide nanoparticles, different models of (TiO2) n nanoclusters have been used (with n = 1, 2, 3, 6, 9, 15, and 38). As a conclusion, the minimal model is good enough to theoretically reproduce the main feature in the spectrum (i.e., the energy shift of the main band upon binding to TiO2). However, it fails in creating intermediate states which could play a significant role under real experimental conditions (dynamics of the electronic transfer). Indeed, as the size of the nanocluster grows, the dye LUMO moves from the edge to well inside the conduction band (Ti 3d band). On the other hand, to assess the consistency of the time-domain approach in the case of such systems, conventional (frequency-domain) TD-DFT calculations have been carried out. It is found that, as far as the functional and basis set are equivalent, both approaches lead to similar results. While for small systems the standard TD-DFT is better suited, for medium to large sized systems, the real-time TD-DFT becomes competitive and more efficient.

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Dissociation mechanisms for methanol and water on stoichiometric and defective TiO2 (110) surfaces have been clarified from periodic density functional calculations. When the molecules are adsorbed on an oxygen vacancy, the most favorable route to dissociation is an indirect mechanism involving, initially, the formation of an intermediate hydroxyl group on in-plane oxygen, followed by the hydrogen transfer to a neighbor bridging oxygen. Since the transient species lifetime is short, they are difficult to be detected. At variance, dissociation on stoichiometric surfaces proceeds readily through a direct hydrogen transfer process from the adsorbate to bridging oxygen. However, the energy barrier for the recombination is low and the species can return to the initial state.

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The electronic structure and the optical response of free catechol, [Ti(cat)(3)](2-) complex, and catechol bound to TiO(2) nanoclusters have been analysed using time dependent density functional theory (TD-DFT) performing calculations both in real time and frequency domains. Both approaches lead to similar results providing the basis sets and functionals are similar. For all cases, the simulated spectra agree well with the experimental ones. For the adsorption systems, the spectra show a band at 4.7 eV associated to intramolecular catechol π→π* transitions, and low energy bands corresponding to transitions from catechol to the cluster with a tail that is red-shifted when the coupling between the dye and the cluster is more effective. Thus, dissociative adsorption modes provide longer tails than the molecular mode. Although the bidentate complex is more stable than the monodentate, the energy difference between both is smaller when the cluster size increases. Small cluster models reproduce the main features of the optical response, however, the (TiO(2))(15) cluster constitutes the minimal size to provide a complete picture. In this case, the conventional TD-DFT (frequency domain) calculations are highly demanding computationally, while real time TD-DFT is more efficient and the calculations become affordable.

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.

In this work we report on theoretical calculations of methanol adsorption and dissociation on the stoichiometric and defective TiO2(110) surface. The periodic implementation of density functional theory (DFT) with plane waves and pseudopotentials was employed. A supercell made of 4 × 1 unit cells was used to represent the surface, which corresponds to methanol coverage of 0.25 ML. The defective surface was modeled by removing one bridging oxygen from the outermost layer. Several adsorption sites were explored through both static and molecular dynamics calculations. The most stable adsorption site on the defective surface is with the molecule directly adsorbed onto the vacancy, whereas adsorption on titania resembles the stoichiometric case. Our estimated adsorption energies are found to be in agreement with the features observed in previous experimental desorption data. One of the main aims of this study was to determine whether methanol could dissociate on the stoichiometric surface. From static calculations we find that both the molecular and the dissociated state are almost degenerate. In addition, molecular dynamics calculations show that the transition barrier between the two species is small. On the other hand, dissociation on defects is thermodynamically favorable by 0.5 eV. However, dynamic calculations show that in this case the conversion from the molecular to the dissociated state is not straightforward. Implications to these findings are discussed within the text.