In Silico Study of Interactions between the Methylene Blue Molecule and the (TiO2)20 Cluster by Means of DFT Calculations.

In this work, the (TiO2)20 cluster is proposed to adsorb the methylene blue (BM) dye; thus, the quantum parameters to explain the adsorption process are calculated by means of density functional theory calculations. Eight possible configurations are obtained and labeled from M1 to M8. According to the adsorption energy values, they reveal physisorption for at least two cases, and for the rest of the systems, they exhibit chemisorption. The preferential positions that lead to good adsorption for the BM dye are parallel to the semiconductor cluster; however, when one end of the BM dye formed by hydrogen atoms is interacting with the cluster, a weak chemical interaction is reached. The chemical interactions for M4 and M5 systems generate considerable increases of their electronic gap values (E g) with respect to the rest, and this effect is explained based on iso-surfaces of frontier orbitals and electronic charge transference. The chemical interactions between these chemical species are stable under vibrational and thermal criteria. This semiconductor cluster arises as a good candidate to adsorb some dyes like BM.

Vickers hardness prediction from machine learning methods.

The search for new superhard materials is of great interest for extreme industrial applications. However, the theoretical prediction of hardness is still a challenge for the scientific community, given the difficulty of modeling plastic behavior of solids. Different hardness models have been proposed over the years. Still, they are either too complicated to use, inaccurate when extrapolating to a wide variety of solids or require coding knowledge. In this investigation, we built a successful machine learning model that implements Gradient Boosting Regressor (GBR) to predict hardness and uses the mechanical properties of a solid (bulk modulus, shear modulus, Young's modulus, and Poisson's ratio) as input variables. The model was trained with an experimental Vickers hardness database of 143 materials, assuring various kinds of compounds. The input properties were calculated from the theoretical elastic tensor. The Materials Project's database was explored to search for new superhard materials, and our results are in good agreement with the experimental data available. Other alternative models to compute hardness from mechanical properties are also discussed in this work. Our results are available in a free-access easy to use online application to be further used in future studies of new materials at www.hardnesscalculator.com .

Cu n Clusters (n = 13, 43, and 55) as Possible Degradant Agents of mSF6 Molecules (m = 1, 2): A DFT Study.

In order to obtain the structural and electronic properties of pristine copper clusters and Cu13-SF6, Cu43-SF6, Cu55-SF6, Cu13-2SF6, Cu43-2SF6, and Cu55-2SF6 systems, DFT calculations were carried out. For Cu13-mSF6, its surface suffers a drastic deformation, and Cu43-mSF6 at its outer surface reveals strong interaction for the first chemical molecule; when the second molecule is interacting, these outer surfaces are not severely affected. These two cases degraded fully the first SF6 molecule; however the second molecule is bonded to the latter systems and for Cu55-mSF6 (m = 1 and 2) a structural transformation from SF6 →SF4 appears as well as inner and outer shells that display slight deformations. The electronic gaps do not exhibit drastic changes after adsorption of mSF6 molecules, and the magnetic moment remains without alterations. The whole system shows thermal and vibrational stability. In addition, for Cu13-mSF6 the values of the optical gap and intensity of the optical exhibit changes with respect to the pristine case (Cu13), and the rest of the systems do not exhibit major oscillations. These icosahedral copper clusters emerge as a good option to degrade mSF6 molecules.

Effect of Chemical Order in the Structural Stability and Physicochemical Properties of B12N12 Fullerenes.

The effect of chemical order in the structural and physicochemical properties of B12N12 [4,6]-fullerene (BNF) isomers was evaluated using density functional theory and molecular dynamic calculations. The feasibility to find stable BNF isomers with atomic arrangement other than the well-known octahedral Th-symmetry was explored. In this study, the number of homonuclear bonds in the modeled nanostructures was used as categorical parameter to describe and quantify the degree of structural order. The BNF without homonuclear bonds was identified as the most energetically favorable isomer. However, a variety of BNF arrays departing from Th-symmetry was determined as stable structures also. The calculated vibrational spectra suggest that isomers with chemical disorder can be identified by infrared spectroscopy. In general, formation of homonuclear bonds is possible meanwhile the entropy of the system increases, but at expense of cohesive energy. It is proposed that formation of phase-segregated regions stablishes an apparent limit to the number of homonuclear bonds in stable B12N12 fullerenes. It was found that formation of homonuclear bonds decreases substantially the chemical hardness of BNF isomers and generates zones with large charge density, which might act as reactive sites. Moreover, chemical disorder endows BNF isomers with a permanent electric dipole moment as large as 3.28 D. The obtained results suggest that by manipulating their chemical order, the interaction of BNF's with other molecular entities can be controlled, making them potential candidates for drug delivery, catalysis and sensing.

Structural search for stable Mg-Ca alloys accelerated with a neural network interatomic model.

We have combined a neural network formalism with metaheuristic structural global search algorithms to systematically screen the Mg-Ca binary system for new (meta)stable alloys. The combination of these methods allows for an efficient exploration of the potential energy surface beyond the possibility of the traditional searches based on ab initio energy evaluations. The identified pool of low-enthalpy structures was complemented with special quasirandom structures (SQS) at different stoichiometries. In addition to the only Mg-Ca phase known to form under standard synthesis conditions, C14-Mg2Ca, the search has uncovered several candidate materials that could be synthesized under elevated temperatures or pressures. We show that the vibrational entropy lowers the relative free energy of several phases with magnesium kagome layers: C15 and C36 Laves structures at the 2 : 1 composition and an orthorhombic oS36 structure at the 7 : 2 composition. The estimated phase transition temperatures close to the melting point leave open the possibility of synthesizing the predicted materials at high temperatures. At high pressures up to 10 GPa, two new phases at the 1 : 1 and 3 : 1 Mg : Ca stoichiometries become thermodynamically stable and should form in multi-anvil experiments.

Elimination of textile dyes using activated carbons prepared from vegetable residues and their characterization.

In this study, three mesoporous activated carbons prepared from vegetable residues were used to remove acid, basic, and direct dyes from aqueous solutions, and reactive and vat dyes from textile wastewater. Granular carbons obtained by chemical activation at 673 K with phosphoric acid from prickly pear peels (CarTunaQ), broccoli stems (CarBrocQ), and white sapote seeds (CarZapQ) were highly efficient for the removal of dyes. Adsorption equilibrium studies were carried out in batch systems and treated with Langmuir and Freundlich isotherms. The maximum adsorption capacities calculated from the Langmuir isotherms ranged between 131.6 and 312.5 mg/g for acid dyes, and between 277.8 and 500.0 mg/g for basic dyes at 303 K. Our objective in this paper was to show that vegetable wastes can serve as precursors for activated carbons that can be used for the adsorption of dyes. Specifically CarBrocQ was the best carbon produced for the removal of textile dyes. The color removal of dyes present in textile wastewaters was compared with that of a commercial powdered carbon, and it was found that the carbons produced using waste material reached similar efficiency levels. Carbon samples were characterized by bulk density, point of zero charge, thermogravimetric analysis, elemental analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, methylene blue adsorption isotherms at 303 K, and nitrogen adsorption isotherms at 77 K (SBET). The results show that the activated carbons possess a large specific surface area (1025-1177 m(2)/g) and high total pore volume (1.06-2.16 cm(3)/g) with average pore size diameters between 4.1 and 8.4 nm. Desorption and regeneration tests were made to test the viability of reusing the activated carbons.

Investigating the electronic properties of silicon nanosheets by first-principles calculations.

Using first-principles total energy calculations within the density functional theory (DFT), we investigated the electronic and structural properties of graphene-like silicon sheets. Our studies were performed using the LSDA (PWC) and GGS (PBE) approaches. Two configurations were explored: one corresponding to a defect-free layer (h-Si), and the other to a layer with defects (d-Si), both of which were in the armchair geometry. These sheets contained clusters of the C(n)H(m) type. We also investigated the effects of doping with group IV-A elements. Structural stability was studied by only considering positive vibration frequencies. Results showed that both h-Si and d-Si present a corrugated structure with concavity. h-Si sheets were found to be ionic (D.M. = 0.33 Debye) with an energy gap (HOMO-LUMO) of 0.77 eV in the LSDA theory and 0.76 eV in the GGS approach, while d-Si sheets were observed to be covalent (D.M. = 2.78 D), and exhibited semimetallic electronic behavior (HOMO-LUMO gap = 0.32 eV within the LSDA theory and 0.33 eV within the GGS approach). d-Si sheets doped with one carbon or one germanium preserved the polarity of the undoped d-Si sheets, as well as their semimetallic electronic behavior. However, when the sheets were doped with two C or two Ge atoms, or with one of each atom (to give Si(52)CGeH(18)), they retained the semimetallic behavior, but they changed from having ionic character to covalent character.