Ideal, Defective, and Gold--Promoted Rutile TiO2(110) Surfaces: Structures, Energies, Dynamics, and Thermodynamics from PBE+U

Extensive first principles calculations are carried out to investigate gold-promoted TiO2(110) surfaces in terms of structure optimizations, electronic structure analyses, ab initio thermodynamics calculations of surface phase diagrams, and ab initio molecular dynamics simulations. All computations rely on density functional theory in the generalized gradient approximation (PBE) and account for on-site Coulomb interactions via inclusion of a Hubbard correction, PBE+U, where U is computed from linear response theory. This approach is validated by investigating the interaction between TiO2(110) surfaces and typical probe species (H, H2O, CO). Relaxed structures and binding energies are compared to both data from the literature and plain PBE results. The main focus of the study is on the properties of gold-promoted titania surfaces and their interactions with CO. Both PBE+U and PBE optimized structures of Au adatoms adsorbed on stoichiometric and reduced TiO2 surfaces are computed, along with their electronic structure. The charge rearrangement induced by the adsorbates at the metal/oxide contact are also analyzed and discussed. By performing PBE+U ab initio molecular dynamics simulations, it is demonstrated that the diffusion of Au adatoms on the stoichiometric surface is highly anisotropic. The metal atoms migrate either along the top of the bridging oxygen rows, or around the area between these rows, from one bridging position to the next along the [001] direction. Approximate ab initio thermodynamics predicts that under O-rich conditions, structures obtained by substituting a Ti5c atom with an Au atom are thermodynamically stable over a wide range of temperatures and pressures.Comment: 20 pages, 12 figures, accepted for publication in Phys. Rev.

La sovranit\ue0 scomposta. Sull'attualit\ue0 del Leviatano

Una lunga tradizione interpretativa considera il Leviatano di Thomas Hobbes il testo in cui ha preso forma il dispositivo logico-concettuale con cui la modernit\ue0 ha pensato la politica. I tre studi qui contenuti \u201cscompongono\u201d la teoria hobbesiana per metterne in evidenza alcune aporie che manifestano oggi, nell\u2019apparente crisi della sovranit\ue0 statale, una cogente attualit\ue0. Nella dottrina di Hobbes l\u2019individuo, presentato come presupposto della sovranit\ue0, \ue8 anche prodotto di una biopolitica che della sovranit\ue0 \ue8 correlato essenziale; la natura umana \ue8 segnata dalla scissione tra bisogno di riconoscimento e affermazione di quella individualit\ue0 irrelata che \ue8 premessa necessaria della rappresentanza politica; la condizione umana viene sospesa in un presente astorico pensato come intrascendibile, per eliminare ogni pulsione al mutamento. Senza voler risolvere le tensioni tra sovranit\ue0 e biopolitica, riconoscimento e rappresentanza, eternit\ue0 e mutamento, questo libro mostra la centralit\ue0 di queste aporie tanto nel momento di affermazione della scienza politica moderna, quanto negli odierni processi di soggettivazione

Gallium Nitride-based Materials as Promising Catalysts for CO2 Reduction: A DFT Study on the Effect of CO2 Coverage and the Incorporation of Mg Doping or Substitutional In

Catalytic CO2 conversion to fuels and chemicals is important for mitigating the climate change and reducing the dependence on fossil resources. In order to achieve this goal on a large industrial level, effective catalysts need to be developed. Among them, gallium nitride (GaN) and related Mg-doped and In-alloyed systems have been proven as efficient materials for the reduction of highly stable CO2 molecules. This work presents a density functional theory (DFT) investigation, performing periodic boundary condition (PBC) calculations which allow to employ a more extended surface for a detailed analysis of the CO2 coverage, and the effect of Mg doping and In alloying on the CO2 adsorption and its conversion to CO. The results show the great potential of GaN(100) surfaces to simultaneously bind and strongly activate multiple CO2 molecules, which is a crucial aspect for an efficient CO2 conversion process. Moreover, the presence of Mg-dopant on the top layer is found to be more beneficial for the CO2 adsorption and activation with respect to both the pristine and In-alloyed system, and this effect is further improved by the inclusion of a second impurity on the top layer. In line with the previous experimental findings, these calculations support the potential of pristine GaN(100) to catalyze the CO2-to-CO reduction. The results presented here offer crucial information for the development of more efficient and selective catalysts for the CO2 reduction

Density Functional Theory Study and Photocatalytic Activity of ZnO/N-Doped TiO2Heterojunctions

The beneficial effects on ZnO and TiO2photocatalytic activity resulting from a suitable combination of doping and heterojunctions were proposed and investigated by a synergistic experimental and theoretical study. In detail, the ZnO/N-doped TiO2heterojunction was synthesized and tested in the photocatalytic degradation of atrazine under UV and visible light. Wide-angle X-ray diffraction (WAXD) analysis reveals the presence of both ZnO and TiO2crystalline phases in the heterojunction. UV-vis DRS analysis shows that the simultaneous presence of ZnO and N-doped TiO2in the heterojunction results in a slight increase in the band gap value in the UV region, while the shoulder in the visible region typically of bare N-doped TiO2is preserved. On the basis of these experimental observations, density functional theory (DFT) calculations were carried out to model both the N-doped TiO2structure and to rationalize the experimental decrease in the band gap energy. The DFT modeling of band alignment of the ZnO/N-doped TiO2heterojunction allowed us to define the "minimal band gap" (MBG), corresponding to the interface gap, which resulted in smaller band gap energy than the two separate semiconductor band gaps. Furthermore, the DFT modeling of the electronic structure of N-doped TiO2predicted the existence of additional energy levels between the TiO2valence and conduction bands, which allowed us to justify the activity of the heterojunction under visible light irradiation. Photocatalytic tests showed that the ZnO/N-doped TiO2heterojunction performance was better than that of both N-doped TiO2and ZnO alone, allowing us to achieve almost complete atrazine degradation under UV light irradiation, whereas 24% of atrazine degradation was achieved after 180 min of visible light irradiation. Finally, the photocatalytic results achieved by using scavenger molecules for reactive oxygen species showed that ·OH and ·O2-are both reactive species in atrazine photocatalytic degradation under UV irradiation, while ·OH is responsible for the photocatalytic processes under visible irradiation. DFT modeling, validated by these results, finally allowed us to define a model of the band alignment and photogenerated charge-transfer mechanism for the ZnO/N-doped TiO2heterojunction