Probing defects and correlations in the hydrogen-bond network of ab initio water

The hydrogen-bond network of water is characterized by the presence of coordination defects relative to the ideal tetrahedral network of ice, whose fluctuations determine the static and time-dependent properties of the liquid. Because of topological constraints, such defects do not come alone, but are highly correlated coming in a plethora of different pairs. Here we discuss in detail such correlations in the case of ab initio water models and show that they have interesting similarities to regular and defective solid phases of water. Although defect correlations involve deviations from idealized tetrahedrality, they can still be regarded as weaker hydrogen bonds that retain a high degree of directionality. We also investigate how the structure and population of coordination defects is affected by approximations to the inter-atomic potential, finding that in most cases, the qualitative features of the hydrogen bond network are remarkably robust

XPS analysis of Fe2O3-TiO2-Au nanocomposites prepared by a plasma-assisted route

Fe2O3 nanodeposits have been grown on fluorine-doped tin oxide (FTO) substrates by plasma enhanced-chemical vapor deposition (PE-CVD). Subsequently, the obtained systems have been functionalized through the sequential introduction of TiO2 and Au nanoparticles (NPs) by means of radio frequency (RF)-sputtering. The target nanocomposites have been specifically optimized in view of their ultimate functional application in solar-driven H2 generation. In the present study, our attention is focused on a detailed X-ray photoelectron spectroscopy (XPS) characterization of the surface composition for a representative Fe2O3-TiO2-Au specimen. In particular, this report provides a detailed discussion of the analyzed C 1s, O 1s, Fe 2p, Ti 2p, and Au 4f regions. The obtained results point to the formation of pure Fe2O3-TiO2-Au composites, with gold present only in its metallic state and each of the constituents maintaining its chemical identity

A study of Pt-/alpha-Fe2O3 nanocomposites by XPS

alpha-Fe2O3 matrices were deposited on Fluorine-doped Tin Oxide (FTO) substrates by Plasma Enhanced- Chemical Vapor Deposition (PE-CVD) from Fe(hfa)_2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N’,N’-tetramethylethylenediamine). The obtained nanosystems were subsequently functionalized by platinum nanoparticles (NPs) via Radio Frequency (RF)-sputtering, exposing samples either to a pre- or post-sputtering thermal treatment at 650°C for one hour in air. Interestingly, Pt oxidation state in the final composite systems strongly depended on the adopted processing conditions. In this work, a detailed X-ray Photoelectron Spectroscopy (XPS) analysis was carried out in order to investigate the material chemical composition, with particular regard to the relative Pt(0)/Pt(II)/Pt(IV) content. The obtained results evidenced that, when annealing is performed prior to sputtering, only PtO and PtO2 are revealed in the final Pt/alpha-Fe2O3 nanocomposite. In a different way, annealing after sputtering results in the co-presence of Pt(0), Pt(II) and Pt(IV) species, the former arising from the thermal decomposition of PtO2 to metallic platinum

Pest and disease incidence in agroforestry systems of the Brazilian Central Amazon.

In Central Amazonia there are no examples of pests or diseases whose incidence is reduced in agroforestry systems

Desperfilhador por roto-compressão: novo equipamento para desperfilhar bananeiras.

O trabalho foi conduzido para desenvolver um novo equipamento para a eliminação de perfilhos, com redução de mão de obra e maior praticidade de uso

Heartwood formation and durability of the wood of plantation grown tree species of the central Amazon.

In this study the heartwood formation and the durability of seven years old plantation grown mogno, andiroba, cedro, cumaru, and jatoba was compared with wood from primary growth, in Manaus-AM (Brasil). The results are discussed with special regard to wood quality of plantation grown trees compared to primary growth

"Hot" Surface Activation of Molecular Complexes: Insight from Modeling Studies

Rock-and-roll over hot floors: Theoretical modeling of the first activation stages of a Cu complex on top of a heated surface (750 K) revealed two mobility regimes, a slow bump-and-rock diffusion over the surface and a fast roll-and-go motion accompanied by significant temperature-induced bond oscillations. This study enables a deeper insight into "hot" surface molecular activation processes.Tanz auf dem Vulkan: Das Modellieren der ersten Aktivierungsstufen eines Cu-Komplexes auf einer beheizten Oberfl\ue4che (750\u2005K) enth\ufcllte zwei Bewegungsarten: eine langsame Diffusion durch \u201eAnsto fen und Taumeln\u201c und eine schnelle Rollbewegung, die mit deutlichen temperaturinduzierten Bindungsoszillationen einhergeht. Diese Befunde geben einen Einblick in die Prozesse bei der Aktivierung durch \u201ehei fe\u201c Oberfl\ue4chen

An accurate and transferable machine learning potential for carbon

We present an accurate machine learning (ML) model for atomistic simulations of carbon, constructed using the Gaussian approximation potential (GAP) methodology. The potential, named GAP-20, describes the properties of the bulk crystalline and amorphous phases, crystal surfaces, and defect structures with an accuracy approaching that of direct ab initio simulation, but at a significantly reduced cost. We combine structural databases for amorphous carbon and graphene, which we extend substantially by adding suitable configurations, for example, for defects in graphene and other nanostructures. The final potential is fitted to reference data computed using the optB88-vdW density functional theory (DFT) functional. Dispersion interactions, which are crucial to describe multilayer carbonaceous materials, are therefore implicitly included. We additionally account for long-range dispersion interactions using a semianalytical two-body term and show that an improved model can be obtained through an optimization of the many-body smooth overlap of atomic positions descriptor. We rigorously test the potential on lattice parameters, bond lengths, formation energies, and phonon dispersions of numerous carbon allotropes. We compare the formation energies of an extensive set of defect structures, surfaces, and surface reconstructions to DFT reference calculations. The present work demonstrates the ability to combine, in the same ML model, the previously attained flexibility required for amorphous carbon [V. L. Deringer and G. Csányi, Phys. Rev. B 95, 094203 (2017)] with the high numerical accuracy necessary for crystalline graphene [Rowe et al., Phys. Rev. B 97, 054303 (2018)], thereby providing an interatomic potential that will be applicable to a wide range of applications concerning diverse forms of bulk and nanostructured carbon