Structural and optical properties of CaTi'O IND.3' perovskite-based materials obtained by microwave-assisted hydrothermal synthesis: an experimental and theoretical insight

CaTiO3 powders were synthesized using both a polymeric precursor method (CTref) and a microwave-assisted hydrothermal (CTHTMW) method in order to compare the chemical and physical properties of the perovskite-based material as a function of the synthesis method. To this end, X-ray diffraction, Raman spectroscopy, inductively coupled plasma atomic emission spectroscopy and experimental Ti and Ca K-edge X-ray absorption near-edge structure spectroscopy, as well as measurements of photoluminescence (PL) emission, were used to characterize the typical bottom-up process of the CaTiO3 perovskite phase at different times. Detailed Rietveld refinements show a random polycrystalline distortion in the powder structure, which can be associated with the tilting (α angle < O–Ti–O) between adjacent TiO6 octahedra (intermediate range) for CTHTMW samples and an intrinsic TiO6 distortion (short range) in relation to the polymeric precursor CTref sample. These properties were further investigated by first-principles calculations based on the density functional theory at the B3LYP level. The relationship between this tilting on the PL profile is highlighted and discussed. Thus, a structural model derived from both experimental results and theoretical simulations reveals a close relationship between this tilting and the presence of intermediate energy states within the band gap which are mainly responsible for PL emissionsCAPESCNPQFAPESP/CEPID 98/14324-

Theoretical study of the stoichiometric and reduced Ce-Doped TiO2 anatase (001) surfaces

The effects of Ce doping (2.6%) on the oxygen vacancy (V-O) formation energy (Et) and the electronic structure of the anatase TiO2(001) surface were studied by ineans of periodic density functional calculations within the PBE and PBE + U approaches. Several situations were considered for V-o formation, differing in terms of the position in relation to the dopant site (at the surface and subsurface atomic layers). The vacancy energy of formation is almost always lower for the surface than for the bulk, but the difference is still larger with Ce dopant in the subsurface layers. Nevertheless, the Ce-for-Ti substitution is more stable at the outermost layers, indicating thermodynamically favorable dopant migration toward, the oxide surface. The PEE + U approach provides a physically meaningful description of localized d and f electrons in Ti3+ and Ce3+ species, respectively: Moreover, fully localized spin (simple and split) or partially localized spin solutions are found within similar to 0.5 eV range. Not unexpectedly, standard. PBE fails to describe electron-localized solutions, but interestingly, it predicts the same geometries and order of stability of different vacancy positions. The present work provides compelling evidence that oxygen vacancy formation is remarkably facilitated by Ce dopant in TiO2 anatase {001} facets, but only when Ce is in subsurface positions.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Graphenyldiene: A new sp2-graphene-like nanosheet

The race and the discovery of novel two-dimensional (2D) carbon-based materials have been intensified because many are suitable for energy storage systems, thermoelectric devices, and catalysis applications. Therefore, this study introduces to the scientific community a novel 2D nanosheet named graphenyldiene (GPD), which is formed by arranging cyclobutadiene and bi-phenyl groups to create a monolayer with octadecagonal, hexagonal and tetragonal rings. The cohesive energy of GPD is only 1.37 and 0.65 eV/atom higher than graphene and biphenylene, respectively. Molecular dynamics simulations confirmed its structural and thermal stability. The GPD monolayer remains stable, with no significant deformations at around 1000 K, and the disintegration of the geometry occurs only at a temperature of 1500 K, which is characterized by the formation of an amorphous graphdiyne. The GPD electronic structure shows a direct band gap transition, 1.26 eV, at the Γ point. GPD is a promising alternative to electronic devices due to its carrier mobility of around 103.cm2/V.s. Also, the GPD satisfies the Born-Huang criterion for mechanical stability with elastic constants C11 = 157.62 N/m, C12 = 53.66 N/m and C66 = 51.98 N/m. The Bader's topological analysis indicated that all bonds have strong shared shell characteristics. Finally, the vibrational analysis identified 54 modes, where 21 are Raman active, with A1g and E2g modes dominating the spectrum at 1347, 1685 and 1697 cm−1

DFT Study on Ce-Doped Anatase TiO2: Nature of Ce3+ and Ti3+ Centers Triggered by Oxygen Vacancy Formation

A systematic study of TiO2 anatase, Ce-doped TiO2 anatase with 2.8 and 5.6% dopant concentration and of the systems resulting from oxygen vacancy formation has been carried out by means of periodic density functional theory based calculations using PBE, PBE+U, and hybrid functionals. For each approach, several situations are considered for the oxygen vacancy formation, differing on the position of the removed oxygen or on the resulting electronic structure. The hybrid B3LYP functional and PBE+U approaches provide a physically meaningful description of localized d and f electrons in Ti3+ and Ce3+ species, respectively. Nevertheless, quasi-degenerate solutions were encountered featuring either fully localized spin (simple and split) or partially localized spin. Although standard PBE calculations result always in fully (unphysical) delocalized solutions, the most stable geometry thus predicted, in which Ce is six-coordinated and V-O folded by 3[TiO5], is in agreement with the B3LYP and PBE+U results. The present work provides compelling evidence that the remarkable catalytic properties of these systems partially arise from the facilitated oxygen vacancy (V-O) formation triggered by the Ce dopant, which is further enhanced when dopant concentration is increased.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Adsorption of NH₃ with Different Coverages on Single-Walled ZnO Nanotube: DFT and QTAIM Study

NH₃ adsorption with different coverages on single-walled armchair and zigzag ZnO nanotubes (ZnONT) has been studied via periodic computational simulations at the all-electron B3LYP level. In order to fully characterize the molecules-surface interaction, infrared (IR) spectra were calculated for the first time. A rigorous analysis of the electron density in the bonding region, according to the quantum theory of atoms in molecules, was performed. NH₃ molecules physisorb without dissociation via a self-catalyzed process. Although the nanotubes undergo sensitive lattice deformations with low coverages, its fundamental electronic properties were not modified. Owing to these analyses, the ZnONTs can be applicable as NH₃ gas sensor