This paper presents the synthesis of the organic-inorganic hybrid perovskite, CH3NH3PbI3, doped in the Pb(2+) position with Sn(2+), Sr(2+), Cd(2+) and Ca(2+). The incorporation of the dopants into the crystalline structure was analysed, observing how the characteristics of the dopant affected properties such as the crystalline phase, emission and optical properties. XRD showed how doping with Sn(2+), Sr(2+) and Cd(2+) did not modify the normal tetragonal phase. When doping with Ca(2+), the cubic phase was obtained. Moreover, DR-UV-Vis spectroscopy showed how the band gap decreased with the dopants, the values following the trend Sr(2+) < Cd(2+) < Ca(2+) < CH3NH3PbI3 ≈ Sn(2+). The biggest decrease was generated by Sr(2+), which reduced the CH3NH3PbI3 value by 4.5%. In turn, cathodoluminescence (CL) measurements confirmed the band gap obtained. Periodic-DFT calculations were performed to understand the experimental structures. The DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy, with the values calculated following the trend Sn(2+) ≈ Pb(2+) > Cd(2+) > Sr(2+) for the tetragonal structure and Pb(2+) > Ca(2+) for the cubic phase. The electron localization function (ELF) analysis showed similar electron localizations for undoped and Sn(2+)-doped tetragonal structures, which were different from those doped with Sr(2+) and Cd(2+). Furthermore, when Cd(2+) was incorporated, the Cd-I interaction was strengthened. For Ca(2+) doping, the Ca-I interaction had a greater ionic nature than Cd-I. Finally, an analysis based on the non-covalent interaction (NCI) index is presented to determine the weak-type interactions of the CH3NH3 groups with the dopant and I atoms. To our knowledge, this kind of analysis with these hybrid systems has not been performed previously.

The outstanding catalytic properties of cerium oxides rely on the easy Ce(3+) ↔ Ce(4+) redox conversion, which however constitutes a challenge in density functional based theoretical chemistry due to the strongly correlated nature of the 4f electrons present in the reduced materials. In this work, we report an analysis of the performance of five exchange-correlation functionals (HH, HHLYP, PBE0, B3LYP, and B1-WC) implemented in the CRYSTAL06 code to describe three properties of ceria: crystal structure, band gaps, and reaction energies of the CeO2 → Ce2O3 process. All five functionals give values for cell parameters that are in fairly good agreement with experiment, although the PBE0 hybrid functional is found to be the most accurate. Band gaps, 2p-4f-5d in the case of CeO2 and 4f-5d in the case of Ce2O3, are found to be, in general, overestimated and drop off when the amount of Hartree-Fock exchange in the exchange-correlation functional decreases. In contrast, the reaction energies are found to be underestimated, and increase when the amount of HF exchange lowers. Overall, at its standard formulation, the B1-WC functional seems to be the best choice as it provides good band gaps and reaction energies, and very reasonable crystal parameters.

The interaction of Cu, Ag, and Au atoms with the regular terrace sites of the CeO 2 (111) surface has been investigated within the LDA+U and GGA+U density functional theory approaches using different U values and periodic slab surface models. For the interaction of Cu and Ag with this surface the different methods consistently predict the same qualitative description of stable active sites, the same order of stability and the oxidized character of adsorbed Cu and Ag. For the case of Au the description is more method dependent due to the nearly degeneracy between the solutions between cationic and neutral Au, in agreement with a recent study. The present results are indicative of the strength and limitations of the present density functional theory approaches.

We use density functional theory calculations with Hubbard corrections (DFT+U) to investigate electronic aspects of the interaction between ceria surfaces and gold atoms. Our results show that Au adatoms at the (111) surface of ceria can adopt Au(0), Au(+) or Au(-) electronic configurations depending on the adsorption site. The strongest adsorption sites are on top of the surface oxygen and in a bridge position between two surface oxygen atoms, and in both cases charge transfer from the gold atom to one of the Ce cations at the surface is involved. Adsorption at other sites, including the hollow sites of the surface, and an O-Ce bridging site, is weaker and does not involve charge transfer. Adsorption at an oxygen vacancy site is very strong and involves the formation of an Au(-) anion. We argue that the ability of gold atoms to stabilise oxygen vacancies at the ceria surface by moving into the vacancy site and attracting the excess electrons of the defect could be responsible for the enhanced reducibility of ceria surfaces in the presence of gold. Finally, we rationalise the differences in charge transfer behaviour from site to site in terms of the electrostatic potential at the surface and the coordination of the species.

We present a study concerning the effect of the on site d-d Coulomb interaction energy U on the band-gap states of nonstoichiometric rutile ͑110͒ TiO 2 surface. As well known, the excess electrons resulting from the formation of oxygen vacancies localize on the Ti 3d orbitals forming band-gap states. Local density approximation ͑LDA͒ does not give a correct description of these band-gap states, either with or without gradient corrections. The failure of LDA is often attributed to an inadequate treatment of electron correlation in systems with localized orbitals and is commonly corrected with an empirical local Coulomb repulsion term, i.e., the LDA+ U method. This study provides a completely general strategy to estimate the U value in this kind of systems, illustrated here for reduced ͑110͒ TiO 2 surface, well characterized from experiments. From ab initio embedded cluster configuration interaction calculations, combined with the effective Hamiltonian theory, a value of U of 5.5± 0.5 eV is obtained, in good agreement with those reported for this system from x-ray photoemission spectroscopy experiments ͑U = 4.5± 0.5 eV͒. It is observed that when the ab initio estimate of U is injected into the periodic LDA+ U calculations, a correct description of the gap states is obtained from the periodic LDA+ U calculations. Additionally, the results indicate that the position of these states on the band gap strongly depends on the level at which lattice relaxation is taken into account, with significant differences between the density of states curves at the LDA+ U level obtained using the optimal generalized gradient approximation or LDA+ U geometries. These results suggest that this combined strategy could be a useful tool for those systems where electron correlation plays a key role, and no experimental data are available for the on-site Coulomb repulsion.

A good correlation was obtained between the electronic properties of Cu-doped anatase TiO2 by virtue of both physical chemistry characterization and theoretical calculations. Pure and Cu-doped TiO2 were synthesized. The composition, structural and electronic properties, and the band gap energy were obtained using several techniques. The method of synthesis used produces Cu-doped anatase TiO2, and XRD, XPS and Raman spectroscopy indicate that Cu atoms are incorporated in the structure by substitution of Ti atoms, generating a distortion of the structure and oxygen vacancies. In turn, the band gap energy of the synthesized samples decrease drastically with the Cu doping. Moreover, periodic density functional theory (DFT-periodic) calculations were carried out both to model the experimentally observed doped structures and to understand theoretically the experimental structures obtained, the formation of oxygen vacancies and the values of the band gap energy. From the analysis of density of states (DOS), projected density of states (PDOS) and the electron localization function (ELF) a decrease in the band gap is predicted upon increasing the Cu doping. Thus, the inclusion of Cu in the anatase structure implies a covalent character in the Cu-O interaction, which involves the appearance of new states in the valence band maximum with a narrowing in the band gap.

The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO(2)(111) surface have been investigated in detail by means of periodic density functional theory-based calculations, using the LDA+U and GGA+U potentials for a broad range of U values, complemented with calculations employing the HSE06 hybrid functional. In addition, the effects of the lattice parameter a(0) and of the starting point for the geometry optimization have also been analyzed. From the present results we suggest that the oxidation state of single Au atoms on CeO(2)(111) predicted by LDA+U, GGA+U, and HSE06 density functional calculations is not conclusive and that the final picture strongly depends on the method chosen and on the construction of the surface model. In some cases we have been able to locate two well-defined states which are close in energy but with very different electronic structure and local geometries, one with Au fully oxidized and one with neutral Au. The energy difference between the two states is typically within the limits of the accuracy of the present exchange-correlation potentials, and therefore, a clear lowest-energy state cannot be identified. These results suggest the possibility of a dynamic distribution of Au(0) and Au(+) atomic species at the regular sites of the CeO(2)(111) surface.

We have used computer modeling techniques to investigate the interaction of Cu, Ag, and Au atoms (at 0.25 ML coverage) with the (111) surface of zirconia, ZrO2. The surface was simulated by means of periodic slabs, and the calculations were performed using spin-polarized density functional theory (DFT) within the generalized gradient approach. We show that, for the three metals, the most stable adsorption sites are not on top of the outmost oxygen atoms, as previously suggested for the Cu/ZrO2 case, but at less-symmetric bridge sites between oxygen and zirconium. Furthermore, the examination of the charge density and the electronic density of states shows that no full charge transfer takes place between any of the metals and the zirconia surface, as, in all cases, the metal's unpaired electron remains largely localized on the metal adatom upon adsorption. The origin of the interaction appears to be the polarization of the electronic density of the adsorbed metal atom, together with a modest chemical contribution arising from the mixing of the orbitals of the metal atom and the surface oxygen. The adsorption energies follow the order |E ads(Ag)| < |E ads(Au)| < |E ads(Cu)|.