Atomic-scale study of TiO2-GR nanohybrid formation by ALD: the effect of the gas phase precursor.

In the present work, we report on a theoretical-computational study of the growth mechanism of the TiO2-Graphene nanohybrid by atomic layer deposition. Hydroxyl groups (OH) are anchoring sites for interacting with the main ALD titanium precursors (Tetrakis (dimethylamino) Titanium, Titanium Tetrachloride, and Titanium Isopropoxide). Results demonstrate that the chemical nature of the precursor directly affects the reaction mechanism in each ALD growth step. Tetrakis(dimethylamino)titanium is the precursor that presents a higher affinity (lower energy barriers for the reaction) to hydroxylated graphene in the growth process. A complete reaction mechanism for each precursor was proposed. The differences between precursors were discussed through the non-covalent interactions index. Finally, the water molecules help reduce the energy barriers and consequently favor the formation of the TiO2-graphene nanohybrid.

Strain Effects on the Two-Dimensional Cr2N MXene: An Ab Initio Study.

Structural, electronic, and magnetic properties of two-dimensional Cr2N MXene under strain were studied. The uniaxial and biaxial strain was considered from -5 to 5%. Phonon dispersion was calculated; imaginary frequency was not found for both kinds of strain. Phonon density of states displays an interesting relation between strain and optical phonon gaps (OPGs), that it implies tunable thermal conductivity. When we apply biaxial tensile strain, the OPG increases; however, this is not appreciable under uniaxial strain. The electronic properties of the dynamically stable systems were investigated by calculating the band structure and electron localization function (ELF) along the (110) plane. The band structure showed a metallic behavior under compressive strain; nevertheless, under tensile strain, the system has a little indirect band gap of 0.16 eV. By analyzing, the ELF interactions between Cr-N are determined to be a weaker covalent bonding Cr2N under tensile strain. On the other hand, if the Cr atoms reduce or increase their self-distance, the magnetization alignment changes, also the magnetic anisotropy energy displays out-of-plane spin alignment. These properties extend the potential applications of Cr2N in the spintronic area as long as they can be grown on substrates with high lattice mismatch, conserving their magnetic properties.

Predicting the stability and electronic structure of alkali metal aurides.

Density functional theory calculations of phonon modes predict that some compounds of the alkali metal aurides family, general formulaA2MAu6(A= K, Rb or Cs;M= Ti, Zr, Hf, Sn or Pb), have stable three-dimensional phase with a double perovskite-type structure and cubicFm3¯mspace group (K2PtCl6-type). Bader's charge analysis shows that most electron density is located within the six atoms at the octahedra vertices like double perovskite halides. However, the short spacing between Au anions enables d-orbital interactions between them. Compounds of this family, with group 4 metals only, carry conduction states around the Γ point (k= 0). On the other hand, compounds with group 14 metals possess more conduction states around all the Brillouin zone and have electron pockets in their band structures. These compounds provide further insights into the unusual anionic behavior of gold and present other alternatives for the construction of divergent nanodevices.

Adsorption of crotonaldehyde on metal surfaces: Cu vs Pt.

The thermal chemistry of crotonaldehyde on the surface of a polished polycrystalline copper disk was characterized by temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) and contrasted with previous data obtained on a Pt(111) single crystal substrate. A clear difference in the adsorption mode was identified between the two surfaces, highlighted by the prevalence of RAIRS peaks for the C=C bond on Cu vs for C=O on Pt. Adsorption was also determined to be much weaker on Cu vs Pt, with an adsorption energy on the former ranging from -50 kJ/mol to -65 kJ/mol depending on the surface coverage. The experimental data were complemented by extensive quantum mechanics calculations using density functional theory (DFT) to determine the most stable adsorption configurations on both metals. It was established that crotonaldehyde adsorption on Cu occurs via the oxygen atom in the carbonyl group, in a mono-coordinated fashion, whereas on Pt multi-coordination is preferred, centered around the C=C bond. The contrasting surface adsorption modes seen on these two metals are discussed in terms of the possible relevance to selectivity in single-atom alloy hydrogenation catalysis.

Determining Surface Terminations and Chirality of Noncentrosymmetric FeGe Thin Films via Scanning Tunneling Microscopy.

Scanning tunneling microscopy was used to study the surfaces of 20-100 nm thick FeGe films grown by molecular beam epitaxy. An average surface lattice constant of ∼6.8 Å, in agreement with the bulk value, was observed via scanning tunneling microscopy, low energy electron diffraction, and reflection high energy electron diffraction. Each of the four possible chemical terminations in the FeGe films were identified by comparing atomic-resolution images, showing distinct contrast with simulations from density functional theory calculations. A detailed study of the atomic layering order and registry across step edges allows us to uniquely determine the grain orientation and chirality in these noncentrosymmetric films.

Functionalization of silicene and silicane with benzaldehyde.

Organic functionalization of nanomaterials offers exceptional flexibility in materials design, and applications in molecular sensors and molecular electronics are expected. However, more studies should be conducted to understand the interaction between nanomaterials and organic molecules. In this work, we studied the functionalization of silicene and silicane with benzaldehyde, performing nudged elastic band calculations within density functional theory. We calculated the structural changes of the adsorption process, electronic properties of the main states, and the energetics. In silicene, the adsorption of benzaldehyde on the top site was found to be the most stable, with an adsorption energy of -0.55 eV. For silicane, the functionalization proceeds through a self-propagating reaction on a highly reactive dangling bond generated by a hydrogen atom vacancy. Benzaldehyde adsorbed on this site depicts an adsorption energy of -1.39 eV, which is larger than in bare silicene. Upon attaching, the double C=O bond breaks down turning the molecule into a highly reactive radical, which in this case, abstracts a neighboring H atom of the sheet. This process is highly achievable since the energy barrier to abstract the H atoms is 0.81 eV, whereas the one needed to desorb the molecule is 1.39 eV. After H abstraction, a new dangling bond is generated at the substrate, making a chain reaction possible to potentially form benzaldehyde monolayers. Organic functionalization is an excellent tool to engineer properties of 2D systems, and having a deeper understanding of the adsorption processes is the first step toward the development of new generation devices. Graphical abstract Benzaldehyde adsorbed on silicene and silicane.

N-Doped carbon nanotubes enriched with graphitic nitrogen in a buckypaper configuration as efficient 3D electrodes for oxygen reduction to H2O2.

Herein, a series of N-doped carbon nanotube (CNx) samples were obtained by modifying the synthesis temperature. Consequently, the proportion of graphitic nitrogen (Ngraph) in the samples was systematically increased as a function of temperature. This allowed evaluation of the role of the CNx graphitic nitrogen in the oxygen reduction reaction (ORR). A correlation between the Ngraph content and the ORR onset potential was observed, which shifted to more positive potentials with an increase in kinetic current density (jk); this showed that Ngraph played a significant catalytic role in the ORR. The samples with high Ngraph content favored the two-electron pathway for the ORR not only in basic media (pH = 13) but also in neutral media (pH = 7), representing an attractive alternative for wastewater remediation through the on-site generation of H2O2. The energetic calculations showed that the formation of H2O2 must be favorable in the presence of graphitic nitrogen sites. Finally, the performance of the buckypaper arrangement was evaluated, and the CNx buckypaper showed a higher cathodic current peak as compared to CNx traditional ink dispersions. Overall, this study not only sheds light on the role of Ngraph in the ORR, but also demonstrates that CNx buckypaper is an efficient 3D electrode for electrocatalytic applications.

Density functional theory studies of the adsorption of hydrogen sulfide on aluminum doped silicane.

First principles total energy calculations have been performed to study the hydrogen sulfide (H2S) adsorption on silicane, an unusual one monolayer of Si(111) surface hydrogenated on both sides. The H2S adsorption may take place in dissociative or non-dissociative forms. Silicane has been considered as: (A) non-doped with a hydrogen vacancy, and doped in two main configurations; (B) with an aluminum replacing a hydrogen atom and (C-n; n = 1, 2, 3) with an aluminum replacing a silicon atom at a lattice site. In addition, three supercells; 4x4, 3x3 and 2x2 have been explored for both non-doped and doped silicane. The non-dissociative adsorption takes place in geometries (A), (C-1), (C-2) and (C-3) while the dissociative in (B). Adsorption energies of the dissociative case are larger than those corresponding to the non-dissociated cases. In the dissociative adsorption, the molecule is fragmented in a HS structure and a H atom which are bonded to the aluminum to form a H-S-Al-H structure. The presence of the doping produces some electronic changes as the periodicity varies. Calculations of the total density of states (DOS) indicate that in most cases the energy gap decreases as the periodicity changes from 4x4 to 2x2. The features of the total DOS are explained in terms of the partial DOS. The reported charge density plots explain quite well the chemisorptions and physisorptions of the molecule on silicane in agreement with adsorption energies.