Maicon P. Lourenço

Imogolite-like nanotubes: structure, stability, electronic and mechanical properties of the phosphorous and arsenic derivatives

Imogolite is a single-walled aluminosilicate nanotube (NT) found in nature that can be easily synthesized, as well as its analogue aluminogermanate NT. Based on geometrical assumptions and pKa values, species such as H3PO4, H3PO3, H3AsO3, H3AsO4 could also be candidates to form imogolite-like structures. In the present work, we provide insights about the stability, electronic, structural and mechanical properties of possible imogolite like NTs by means of self-consistent charge density-functional tight-binding method (SCC-DFTB). Similarly to aluminogermanate, where the tetrahedral silicate groups are replaced by germanate, in this work tetrahedral silicate groups are substituted by phosphate, phosphite, arsenate and arsenite units in the imogolite structure. Detailed analysis is focused on structural properties, strain energy, band gap and Mulliken charges distribution. The calculated strain energy curves for all studied zigzag imogolite-like NTs present well-defined minima, which change as a consequence of composition variation. Moreover, the strain energy curves of armchair imogolite-like NTs also present minima, although in all cases less stable than zigzags by at least 2.2 meV per atom. The insulating NT behaviour changes after internal modification from silicate to phosphate, phosphite, arsenate and arsenite, as well as the charge distribution inside and outside the nanotubes.

Clay Mineral Nanotubes: Stability, Structure and Properties

The emerging field of nanotechnology is mostly focused on carbon and inorganic based nanomaterials, such as carbon nanotubes, graphene, transition metal nanotubes and nanowires (Iijima, 1991; Tenne et al., 1992; Endo et al., 1996; Dresselhaus et al., 2001). Systems containing aluminosilicates have been investigated as mesoporous materials in the form of zeolite and alumina. Although they have not yet received as much attention, clay minerals can also form nanostructured layered materials and nanotubes with remarkable geometric properties. Imogolite is the most representative species of this case, since it has been studied in a pre-nano (1970) decade (Cradwick et al., 1972) and has been nearly forgotten until recently. Since 2000 (Bursill et al., 2000; Tamura & Kawamura, 2002; Mukherjee et al., 2005; Nakagaki & Wypych, 2007), these structures gained again prominence in the literature and appear as an emerging field of research. They can be used as nanoreactors for selective catalysts, adsorbent, nanocable, support for the immobilization of metalloporphyrins, encapsulation and ionic conductor (Nakagaki & Wypych, 2007; Kuc & Heine, 2009).

Benzocaine Complexation with p-Sulfonic Acid Calix(n) arene: Experimental ( 1 H-NMR) and Theoretical Approaches

The aim of this work was to study the interaction between the local anesthetic benzocaine and p‐sulfonic acid calix[n]arenes using NMR and theoretical calculations and to assess the effects of complexation on cytotoxicity of benzocaine. The architectures of the complexes were proposed according to 1H NMR data (Job plot, binding constants, and ROESY) indicating details on the insertion of benzocaine in the cavity of the calix[n]arenes. The proposed inclusion compounds were optimized using the PM3 semiempirical method, and the electronic plus nuclear repulsion energy contributions were performed at the DFT level using the PBE exchange/correlation functional and the 6‐311G(d) basis set. The remarkable agreement between experimental and theoretical approaches adds support to their use in the structural characterization of the inclusion complexes. In vitro cytotoxic tests showed that complexation intensifies the intrinsic toxicity of benzocaine, possibly by increasing the water solubility of the anesthetic and favoring its partitioning inside of biomembranes.

Structural, Electronic, and Mechanical Properties of Inner Surface Modified Imogolite Nanotubes

The electronic, structural and mechanical properties of the modified imogolites have been investigated using self consistent charge-density functional-tight binding method with “a posteriori” treatment of the dispersion interaction (SCC-DFTB-D). The zigzag (12,0) imogolite has been used as the initial structure for the calculations. The functionalization of the interior (12,0) imogolite nanotubes by organosilanes and by heat treatment leading to the dehydroxylation of the silanols were investigated. The reaction of the silanols with the trimethylmethoxysilanes is favored and the arrangement of the different substitutions that leads to the most symmetrical structures are preferred. The Young moduli and band gaps are slightly decreased. However, the dehydroxylation of the silanol groups in the inner surface of the imogolite leads to the increase of the Young moduli and a drastic decrease of the band gap of about 4.4 eV. It has been shown that the degree of the dehydroxylation can be controlled by heat treatment and tune the band gap, eventually, leading to a semiconductor material with well defined nanotube structure.

Stability, Structure, and Electronic Properties of the Pyrite/Arsenopyrite Solid–Solid Interface–A DFT Study

Pyrite is the most common sulfide in the Earth. In the presence of arsenopyrite its oxidation is delayed, and instead, the arsenopyrite increases its oxidation rate, releasing As(III) and As(V) species in the medium. DFT/plane waves calculations were performed on pyrite/arsenopyrite interface models to understand the stability, structure, and electronic properties of the interface. This is the first step to understand the influence of the inlaid arsenopyrite in the pyrite oxidation mechanism. The interface is slightly stressed with minor changes in the bond lengths and lattice parameters with respect to the pure phases. The work of adhesion and the formation energy indicate that the miscibility of the two phases is not favorable, explaining the presence of large domains of either pyrite or arsenopyrite forming bulk granular regions. The valence band of the pyrite/arsenopyrite interface has large contributions from the pyrite phase, while the conduction band has large contributions from the arsenopyrite. T...

FASP: a framework for automation of Slater–Koster file parameterization

The self-consistent-charge density-functional tight-binding method (SCC-DFTB) is largely used for investigating systems of increasing complexity. However, the SCC-DFTB parameters are not available for the whole periodic table and some issues related to its transferability can limit its usefulness. The framework for automating the repulsion energy (Erep) parameterization uses reference chemical models to parameterize the repulsion term of the SCC-DFTB. The aim is to provide a straightforward and accessible manner to obtain accurate SCC-DFTB Erep parameters automatically. Large number of reference chemical models can be simultaneously used to increase the accuracy and transferability of the parameters with respect to the total energies and derived properties. SCC-DFTB parameters for the C, O, N, H set were obtained. The results are in good agreement with the well-established SCC-DFTB parameters available for this test case. Extensions to include other properties such as reaction energies in the Erep parameterization are briefly discussed.

Nanotubes with well-defined structure: imogolites

Imogolites (aluminosilicate and aluminogermanate) are nanotubes (NTs) with well-defined diameter and chirality that can be synthesized in aqueous solution in mild conditions. It has been shown that the diameter and the single- or double-walled NT formation can be controlled depending on the synthesis parameters such as anionic ligands, temperature, pH and reactant concentration. In this chapter, the advances for understanding at a molecular level the remarkable properties of imogolites, their stability and formation mechanism are reviewed highlighting the computational chemistry contribution. Different theoretical methods have been used, in special the Self-Consistent-Charge Density-Functional Tight-Binding method (SCC-DFTB), the most used for investigating imogolites, is here briefly described.