Vagner Rigo | UFSM - Universidade Federal de Santa Maria (original) (raw)

Papers by Vagner Rigo

Nanoalloys, 2020

Abstract Nanoparticles and nanoalloys are fascinating objects, also in relation to their energy a... more Abstract Nanoparticles and nanoalloys are fascinating objects, also in relation to their energy applications. This chapter focuses mainly on the use of metallic and bimetallic nanoparticles to this end. The state-of-the-art of electrochemical devices for energy conversion and storage is reported. Here, we mainly address the beneficial role of employing nanoparticles as building block of their electrodes. We highlight the implications that the rich variety of isomers, with the same size but different geometrical shapes, and homotops, with the same isomer but different chemical orderings, have. For example, there is the need for establishing robust structure–property relationships and then adapting available models. This is at the core of the so-called nanoparticles' design. We also address how isomerization processes could affect the catalytic performance of nanocatalysts. As a paradigmatic example, we focus on the design of Pt-based nanoparticles and nanoalloys for oxygen reduction and ethanol oxidation, two fundamental reactions in electrochemical devices.

Physical Review B, 2009

We have investigated the electronic and transport properties of zigzag Ni-adsorbed graphene nanor... more We have investigated the electronic and transport properties of zigzag Ni-adsorbed graphene nanoribbons ͑Ni/GNRs͒ using ab initio calculations. We find that the Ni adatoms lying along the edge of zigzag GNRs represent the energetically most stable configuration, with an energy difference of approximately 0.3 eV when compared to the adsorption in the middle of the ribbon. The carbon atoms at the ribbon edges still present nonzero magnetic moments as in the pristine GNR even though there is a quenching by a factor of almost five in the value of the local magnetic moments at the C atoms bonded to the Ni. This quenching decays relatively fast and at approximately 9 Å from the Ni adsorption site the magnetic moments have already values close to the pristine ribbon. At the opposite edge and at the central carbon atoms the changes in the magnetic moments are negligible. The energetic preference for the antiparallel alignment between the magnetization at the opposite edges of the ribbon is still maintained upon Ni adsorption. We find many Ni d-related states within an energy window of 1 eV above and below the Fermi energy, which gives rise to a spin-dependent charge transport. These results suggest the possibility of manufacturing spin devices based on GNRs doped with Ni atoms.

Journal of Composite Materials

Due to its exceptional biocompatibility, Polyurethane (PU) reinforced with calcium carbonate (CaC... more Due to its exceptional biocompatibility, Polyurethane (PU) reinforced with calcium carbonate (CaCO3) is a composite material with significant biomedical applications. However, much of the currently known mechanical and chemical information regarding composites has been obtained at low and moderate CaCO3 content levels. This study employs experimental and theoretical tools to evaluate the structural, morphological, and mechanical properties of pristine polyurethane, and when doped with CaCO3 at 25 and 50 wt.%. In the experiments the samples are characterized using X-ray diffraction (XRD), infrared spectrophotometry (FT-IR), scanning electron microscopy (SEM), and tensile and flexural mechanical tests, while theoretical calculations are performed to evaluate the carbonate-polymer interaction. The XRD and FT-IR results indicate that CaCO3 is at the calcite phase and that PU-CaCO3 materials exhibit a broadening of bands related to the NH2 group. This result is explained using theoretica...

ABSTRACT In this work, we study the adsorption of hydrocarbon molecules on carbonate surfaces by ... more ABSTRACT In this work, we study the adsorption of hydrocarbon molecules on carbonate surfaces by means of first-principles calculations based on Density Functional Theory (DFT) with and without van der Waals (vdW) corrections. Energetic, electronic, and structural properties have been determined for the adsorption of the representative hydrocarbons (hexane and benzene) on calcite (CaCO3) and dolomite [CaMg(CO3)2] (10–14) dry surfaces. Those hydrocarbons were selected to represent aromatics and alkanes on surfaces, and for each molecule the evaluated properties are similar for both surfaces. Due to the obtained similarities in both surfaces, we have evaluated the vdW corrections only for calcite. Our results suggest that Ca sites are the most energetically favorable for hydrocarbon adsorption on both minerals. This effect is mostly due to the electronic level ordering that leads to charge differences in the possible adsorbed sites (Ca, Mg, and CO3) in the carbonate surfaces. The vdW corrections strengthen the hydrocarbon–surface bond with a corresponding reduction in the bond distance between the benzene and the surface. However, this reduction is not even for all atoms in the molecule, and the angle between the benzene aromatic ring and the surface increases. The energy barrier, for the displacement of the hydrocarbons along the calcite surface, was determined for representative surface direction, using the Nudged Elastic Band method, and adsorption energies for the most stable sites show the same order of magnitude.

The Journal of Physical Chemistry C

The Journal of Physical Chemistry C, 2016

The Journal of Chemical Physics

The electronic and coordination environment of minerals surfaces, as calcite, are very difficult ... more The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca 2+. Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO 3 (1014)). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for 43 Ca, 13 C, and 17 O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated.

The European Physical Journal B, 2015

Applied Surface Science, 2014

The European Physical Journal B

By means of ab-initio calculations, we have investigated the chemisorption paroperties of ethanol... more By means of ab-initio calculations, we have investigated the chemisorption paroperties of ethanol onto segregating binary nanoalloys. We select nanostructures with icosahedral shape of 55 atoms with a Pt outermost layer over a M core with M=Ag,Pd,Ni. With respect to nanofilms with equivalent composition, there is an increse of the ethanol binding energy. This is not merely due to observed shortening of the Pt-O distance but depends on the nanoparticle distortion after ethanol adsorption. This geometrical distortion within the nanoparticle can be interpreted as a radial breathing, which is sensitive to the adsortion site, identified by the O-anchor point and the relative positions of the ethyl group. More interestingly, being core-dependent-larger in Pd@Pt and smaller in Ni@Pt-, it relates to an effective electron transfer from ethanol and the M-core towards the Pt-shell. On the view of this new analysis, Pd@Pt nanoalloys show the most promissing features for ethanol oxidation.

Physical Review B, 2009

Ab initio simulations of the nicotinamide (vitamin B3) and ascorbic acid (vitamin C) molecules ad... more Ab initio simulations of the nicotinamide (vitamin B3) and ascorbic acid (vitamin C) molecules adsorbed on single-walled carbon nanotubes are performed based on density functional theory. Using a combination with non-equilibrium Green's functions methods, the electronic transport properties of these molecules adsorbed onto semiconducting nanotubes are also studied. The adsorptions of these molecules on the nanotube surface are observed to depend strongly on the functionalization of the adsorbed species. It is demonstrated that when the functionalized vitamins are adsorbed on the nanotube via a strongly covalent bond, significant changes on the electronic transport properties of the nanotubes are verified. In all cases, significant reductions of the total transmissions, both at the valence and conduction bands, of the nanotubes are observed. In some cases, a sharp Fano-type resonance appears, indicating a weak coupling between the sharp states of the molecule and the block states belonging to the nanotube. Hence, it is remarkable to observe that carbon nanotubes adsorbing molecules could result in promising vitamin carriers in both, pristine or functionalized, forms.

Journal of Applied Physics, 2011

We show, using ab initio density functional theory calculations, that Mn dimers adsorbed on graph... more We show, using ab initio density functional theory calculations, that Mn dimers adsorbed on graphene nanoribbons (Mn 2 /GNRs) present a magnetic bistability, as does the isolated Mn dimer. Our total energy results indicate that Mn dimers lying along the edge sites of zigzag GNRs represent the most likely configuration. We find that similar to the isolated Mn 2 molecule, the antiferromagnetic coupling represents the ground state for Mn 2 /GNR, and the spin density configuration of the GNR does not play an important role on the net magnetic moment of Mn 2 , which makes GNRs an ideal substrate for adsorption of these molecules. The ground state and the excited state configuration of the Mn dimer, viz., low-spin (LS) and high-spin (HS), are maintained in the face of changes in the spin density configuration of the substrate. Here we find that the Mn 2 /GNR systems exhibit a LS $ HS binary behavior, which can be considered as a useful property in the development of nanomemories based upon metallic clusters.

Nanoalloys, 2020

Abstract Nanoparticles and nanoalloys are fascinating objects, also in relation to their energy a... more Abstract Nanoparticles and nanoalloys are fascinating objects, also in relation to their energy applications. This chapter focuses mainly on the use of metallic and bimetallic nanoparticles to this end. The state-of-the-art of electrochemical devices for energy conversion and storage is reported. Here, we mainly address the beneficial role of employing nanoparticles as building block of their electrodes. We highlight the implications that the rich variety of isomers, with the same size but different geometrical shapes, and homotops, with the same isomer but different chemical orderings, have. For example, there is the need for establishing robust structure–property relationships and then adapting available models. This is at the core of the so-called nanoparticles' design. We also address how isomerization processes could affect the catalytic performance of nanocatalysts. As a paradigmatic example, we focus on the design of Pt-based nanoparticles and nanoalloys for oxygen reduction and ethanol oxidation, two fundamental reactions in electrochemical devices.

Physical Review B, 2009

We have investigated the electronic and transport properties of zigzag Ni-adsorbed graphene nanor... more We have investigated the electronic and transport properties of zigzag Ni-adsorbed graphene nanoribbons ͑Ni/GNRs͒ using ab initio calculations. We find that the Ni adatoms lying along the edge of zigzag GNRs represent the energetically most stable configuration, with an energy difference of approximately 0.3 eV when compared to the adsorption in the middle of the ribbon. The carbon atoms at the ribbon edges still present nonzero magnetic moments as in the pristine GNR even though there is a quenching by a factor of almost five in the value of the local magnetic moments at the C atoms bonded to the Ni. This quenching decays relatively fast and at approximately 9 Å from the Ni adsorption site the magnetic moments have already values close to the pristine ribbon. At the opposite edge and at the central carbon atoms the changes in the magnetic moments are negligible. The energetic preference for the antiparallel alignment between the magnetization at the opposite edges of the ribbon is still maintained upon Ni adsorption. We find many Ni d-related states within an energy window of 1 eV above and below the Fermi energy, which gives rise to a spin-dependent charge transport. These results suggest the possibility of manufacturing spin devices based on GNRs doped with Ni atoms.

Journal of Composite Materials

Due to its exceptional biocompatibility, Polyurethane (PU) reinforced with calcium carbonate (CaC... more Due to its exceptional biocompatibility, Polyurethane (PU) reinforced with calcium carbonate (CaCO3) is a composite material with significant biomedical applications. However, much of the currently known mechanical and chemical information regarding composites has been obtained at low and moderate CaCO3 content levels. This study employs experimental and theoretical tools to evaluate the structural, morphological, and mechanical properties of pristine polyurethane, and when doped with CaCO3 at 25 and 50 wt.%. In the experiments the samples are characterized using X-ray diffraction (XRD), infrared spectrophotometry (FT-IR), scanning electron microscopy (SEM), and tensile and flexural mechanical tests, while theoretical calculations are performed to evaluate the carbonate-polymer interaction. The XRD and FT-IR results indicate that CaCO3 is at the calcite phase and that PU-CaCO3 materials exhibit a broadening of bands related to the NH2 group. This result is explained using theoretica...

ABSTRACT In this work, we study the adsorption of hydrocarbon molecules on carbonate surfaces by ... more ABSTRACT In this work, we study the adsorption of hydrocarbon molecules on carbonate surfaces by means of first-principles calculations based on Density Functional Theory (DFT) with and without van der Waals (vdW) corrections. Energetic, electronic, and structural properties have been determined for the adsorption of the representative hydrocarbons (hexane and benzene) on calcite (CaCO3) and dolomite [CaMg(CO3)2] (10–14) dry surfaces. Those hydrocarbons were selected to represent aromatics and alkanes on surfaces, and for each molecule the evaluated properties are similar for both surfaces. Due to the obtained similarities in both surfaces, we have evaluated the vdW corrections only for calcite. Our results suggest that Ca sites are the most energetically favorable for hydrocarbon adsorption on both minerals. This effect is mostly due to the electronic level ordering that leads to charge differences in the possible adsorbed sites (Ca, Mg, and CO3) in the carbonate surfaces. The vdW corrections strengthen the hydrocarbon–surface bond with a corresponding reduction in the bond distance between the benzene and the surface. However, this reduction is not even for all atoms in the molecule, and the angle between the benzene aromatic ring and the surface increases. The energy barrier, for the displacement of the hydrocarbons along the calcite surface, was determined for representative surface direction, using the Nudged Elastic Band method, and adsorption energies for the most stable sites show the same order of magnitude.

The Journal of Physical Chemistry C

The Journal of Physical Chemistry C, 2016

The Journal of Chemical Physics

The electronic and coordination environment of minerals surfaces, as calcite, are very difficult ... more The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca 2+. Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO 3 (1014)). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for 43 Ca, 13 C, and 17 O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated.

The European Physical Journal B, 2015

Applied Surface Science, 2014

The European Physical Journal B

By means of ab-initio calculations, we have investigated the chemisorption paroperties of ethanol... more By means of ab-initio calculations, we have investigated the chemisorption paroperties of ethanol onto segregating binary nanoalloys. We select nanostructures with icosahedral shape of 55 atoms with a Pt outermost layer over a M core with M=Ag,Pd,Ni. With respect to nanofilms with equivalent composition, there is an increse of the ethanol binding energy. This is not merely due to observed shortening of the Pt-O distance but depends on the nanoparticle distortion after ethanol adsorption. This geometrical distortion within the nanoparticle can be interpreted as a radial breathing, which is sensitive to the adsortion site, identified by the O-anchor point and the relative positions of the ethyl group. More interestingly, being core-dependent-larger in Pd@Pt and smaller in Ni@Pt-, it relates to an effective electron transfer from ethanol and the M-core towards the Pt-shell. On the view of this new analysis, Pd@Pt nanoalloys show the most promissing features for ethanol oxidation.

Physical Review B, 2009

Ab initio simulations of the nicotinamide (vitamin B3) and ascorbic acid (vitamin C) molecules ad... more Ab initio simulations of the nicotinamide (vitamin B3) and ascorbic acid (vitamin C) molecules adsorbed on single-walled carbon nanotubes are performed based on density functional theory. Using a combination with non-equilibrium Green's functions methods, the electronic transport properties of these molecules adsorbed onto semiconducting nanotubes are also studied. The adsorptions of these molecules on the nanotube surface are observed to depend strongly on the functionalization of the adsorbed species. It is demonstrated that when the functionalized vitamins are adsorbed on the nanotube via a strongly covalent bond, significant changes on the electronic transport properties of the nanotubes are verified. In all cases, significant reductions of the total transmissions, both at the valence and conduction bands, of the nanotubes are observed. In some cases, a sharp Fano-type resonance appears, indicating a weak coupling between the sharp states of the molecule and the block states belonging to the nanotube. Hence, it is remarkable to observe that carbon nanotubes adsorbing molecules could result in promising vitamin carriers in both, pristine or functionalized, forms.

Journal of Applied Physics, 2011

We show, using ab initio density functional theory calculations, that Mn dimers adsorbed on graph... more We show, using ab initio density functional theory calculations, that Mn dimers adsorbed on graphene nanoribbons (Mn 2 /GNRs) present a magnetic bistability, as does the isolated Mn dimer. Our total energy results indicate that Mn dimers lying along the edge sites of zigzag GNRs represent the most likely configuration. We find that similar to the isolated Mn 2 molecule, the antiferromagnetic coupling represents the ground state for Mn 2 /GNR, and the spin density configuration of the GNR does not play an important role on the net magnetic moment of Mn 2 , which makes GNRs an ideal substrate for adsorption of these molecules. The ground state and the excited state configuration of the Mn dimer, viz., low-spin (LS) and high-spin (HS), are maintained in the face of changes in the spin density configuration of the substrate. Here we find that the Mn 2 /GNR systems exhibit a LS $ HS binary behavior, which can be considered as a useful property in the development of nanomemories based upon metallic clusters.