9 results on '"Chaves, Anderson"'
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2. Density functional investigation of the adsorption effects of PH3 and SH2 on the structure stability of the Au55 and Pt55 nanoclusters.
- Author
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Guedes-Sobrinho, Diego, Chaves, Anderson S., Piotrowski, Maurício J., and Da Silva, Juarez L. F.
- Subjects
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METAL clusters , *DENSITY functional theory , *ADSORBATES , *ADSORPTION (Chemistry) , *ICOSAHEDRA - Abstract
Although several studies have been reported for Pt55 and Au55 nanoclusters, our atomistic understanding of the interplay between the adsorbate-surface interactions and the mechanisms that lead to the formation of the distorted reduced core (DRC) structures, instead of the icosahedron (ICO) structure in gas phase, is still far from satisfactory. Here, we report a density functional theory (DFT) investigation of the role of the adsorption effects of PH3 (one lone pair of electrons) and SH2 (two lone pairs) on the relative stability of the Pt55 and Au55 nanoclusters. In gas phase, we found that the DRC structures with 7 and 9 atoms in the core region are about 5.34 eV (Pt55) and 2.20 eV (Au55) lower in energy than the ICO model with Ih symmetry and 13 atoms in the core region. However, the stability of the ICO structure increases by increasing the number of adsorbed molecules from 1 to 18, in which both DRC and ICO structures are nearly degenerate in energy at the limit of 18 ligands, which can be explained as follows. In gas phase, there is a strong compression of the cationic core region by the anionic surface atoms induced by the attractive Coulomb interactions (core+-surface-), and hence, the strain release is obtained by reducing the number of atoms in the cationic core region, which leads to the 55 atoms distorted reduced core structures. Thus, the Coulomb interactions between the core+ and surface- contribute to break the symmetry in the ICO55 structure. On the other hand, the addition of ligands on the anionic surface reduces the charge transfer between the core and surface, which contributes to decrease the Coulomb interactions and the strain on the core region of the ICO structure, and hence, it stabilizes a compact ICO structure. The same conclusion is obtained by adding van der Waals corrections to the plain DFT calculations. Similar results are obtained by the addition of steric effects, which are considered through the adsorption of triphenylphosphine (PPh3) molecules on Au55, in which the relative stability between ICO and DRC is the same as for PH3 and SH2. However, for Pt55, we found an inversion of stability due to the PPh3 ligand effects, where ICO has higher stability than DRC by 2.40 eV. Our insights are supported by several structural, electronic, and energetic analyses. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
3. A theoretical investigation of the structural and electronic properties of 55-atom nanoclusters: The examples of Y-Tc and Pt.
- Author
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Batista, Krys E. A., Piotrowski, Maurício J., Chaves, Anderson S., and Da Silva, Juarez L. F.
- Subjects
ATOMIC clusters ,ELECTRIC properties ,SURFACE chemistry ,TRANSITION metals ,DENSITY functional theory ,POLAR effects (Chemistry) - Abstract
Several studies have found that the Pt55 nanocluster adopts a distorted reduced core structure, DRC
55 , in which there are 8-11 atoms in the core and 47-44 atoms in the surface, instead of the compact and high-symmetry icosahedron structure, ICO55 , with 13 and 42 atoms in the core and surface, respectively. The DRC structure has also been obtained as the putative global minimum configuration (GMC) for the Zn55 (3d), Cd55 (4d), and Au55 (5d) systems. Thus, the DRC55 structure has been reported only for systems with a large occupation of the d-states, where the effects of the occupation of the valence anti-bonding d-states might play an important role. Can we observe the DRC structure for 55-atom transition-metal systems with non-occupation of the anti-bonding d-states? To address this question, we performed a theoretical investigation of the Y55 , Zr55 , Nb55 , Mo55 , Tc55 , and Pt55 nanoclusters, employing density functional theory calculations. For the putative GMCs, we found that the Y55 adopts the ICO55 structure, while Nb55 and Mo55 adopt a bulk-like fragment based on the hexagonal close-packed structure and Tc55 adopts a face-centered cubic fragment; however, Zr55 adopts a DRC55 structure, like Zn55 , Cd55 , Pt55 , and Au55 . Thus we can conclude that the preference for DRC55 structure is not related to the occupation of the anti-bonding d-states, but to a different effect, in fact, a combination of structural and electronic effects. Furthermore, we obtained that the binding energy per atom follows the occupation of the bonding and anti-bonding model, i.e., the stability of the studied systems increases from Y to Tc with a small oscillation for Mo, which also explains the equilibrium bond lengths. We obtained a larger magnetic moment for Y55 (31 μB) which can be explained by the localization of the d-states in Y at nanoscale, which is not observed for the remaining systems (0-1 μB). [ABSTRACT FROM AUTHOR]- Published
- 2016
- Full Text
- View/download PDF
4. Ab Initio investigation of the role of CO adsorption on the physical properties of 55-atom PtCo nanoalloys
- Author
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Chaves, Anderson Silva, 1986 and UNIVERSIDADE ESTADUAL DE CAMPINAS
- Subjects
Adsorção ,Cluster ,Density functional theory ,Artigo original ,Adsorption ,Teoria do funcional de densidade - Abstract
Agradecimentos: The authors thank the São Paulo Research Foundation (FAPESP), National Council for Scientific and Technological Development (CNPq), and Coordination for Improvement of Higher Level Education (CAPES) for the financial support. The authors also thank the Laboratory of Advanced Scientific Computing (University of São Paulo) and the Department of Information Technology - Campus São Carlos for hosting our cluster Abstract: The knowledge of the physical and chemical properties of PtCo nanoparticles as a function of the Pt/Co composition and atomic distribution is crucial for several potential applications, which includes catalysis, anticorrosion, data storage, etc. However, our current atom-level understanding is far from satisfactory, in particular due to the challenges to take into account chemical environment effects. In this work, we report a density functional theory investigation of the structural, energetic, and electronic properties of binary 55-atom PtCo particles at a saturated CO atmosphere (31 molecules), (CO)(31)/Pt(n)Coss(55-n). For PtCo in the gas phase, which adopts an icosahedron-like (ICO-like) structure in the lowest-energy configurations for all studied compositions, we found a rough correlation between stability and the number of bonds among the Pt and Co species; i.e., the stability (excess energy) increases (decreases) by increasing the number of Pt and Co bonds and with a minimum at about n = 28-42 (Pt-rich). However, at a saturated CO atmosphere, we found a stability displacement toward higher Co concentration (n = 6-20, Co-rich), which can be explained by the structural expansion of the nanocluster surface driven by the CO ligands. That is, the CO adsorption contributes to release the strain, which is induced by the attractive Coulomb interactions between the anionic surface and cationic core regions. Furthermore, for particular compositions (n = 42), we found a displacement of the Co atoms toward the surface upon the CO adsorption, which can be explained also by strain release as the adsorption energy of CO is larger on the Pt surfaces, which could favor Pt-rich surfaces FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado
- Published
- 2017
5. Density functional investigation of the adsorption effects of PH3 and SH2 on the structure stability of the Au-55 and Pt-55 nanoclusters
- Author
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Chaves, Anderson Silva, 1986 and UNIVERSIDADE ESTADUAL DE CAMPINAS
- Subjects
Adsorção ,Density functional theory ,Artigo original ,Adsorption ,Gold compounds ,Teoria do funcional de densidade ,Compostos de ouro - Abstract
Agradecimentos: Authors thank the São Paulo Research Foundation (FAPESP, Grant Nos. 2013/21045-2 and 2013/15112-9), Rio Grande do Sul Research Foundation (FAPERGS), National Council for Scientific and Technological Development (CNPq, Grant Nos. 2013/15112-9, 301190/2015-1, and 305161/2015-6), and Coordination for Improvement of Higher Level Education (CAPES) for financial support. Authors also thank the Laboratory of Advanced Scientific Computing (University of Sao Paulo) and the Department of Information TechnologyCampus São Carlos, for hosting our cluster Abstract: Although several studies have been reported for Pt-55 and Au-55 nanoclusters, our atomistic understanding of the interplay between the adsorbate-surface interactions and the mechanisms that lead to the formation of the distorted reduced core (DRC) structures, instead of the icosahedron (ICO) structure in gas phase, is still far from satisfactory. Here, we report a density functional theory (DFT) investigation of the role of the adsorption effects of PH3 (one lone pair of electrons) and SH2 (two lone pairs) on the relative stability of the Pt-55 and Au-55 nanoclusters. In gas phase, we found that the DRC structures with 7 and 9 atoms in the core region are about 5.34 eV (Pt-55) and 2.20 eV (Au-55) lower in energy than the ICO model with I-h symmetry and 13 atoms in the core region. However, the stability of the ICO structure increases by increasing the number of adsorbed molecules from 1 to 18, in which both DRC and ICO structures are nearly degenerate in energy at the limit of 18 ligands, which can be explained as follows. In gas phase, there is a strong compression of the cationic core region by the anionic surface atoms induced by the attractive Coulomb interactions (core(+)-surface(-)), and hence, the strain release is obtained by reducing the number of atoms in the cationic core region, which leads to the 55 atoms distorted reduced core structures. Thus, the Coulomb interactions between the core(+) and surface(-) contribute to break the symmetry in the ICO55 structure. On the other hand, the addition of ligands on the anionic surface reduces the charge transfer between the core and surface, which contributes to decrease the Coulomb interactions and the strain on the core region of the ICO structure, and hence, it stabilizes a compact ICO structure. The same conclusion is obtained by adding van der Waals corrections to the plain DFT calculations. Similar results are obtained by the addition of steric effects, which are considered through the adsorption of triphenylphosphine (PPh3) molecules on Au-55, in which the relative stability between ICO and DRC is the same as for PH3 and SH2. However, for Pt-55, we found an inversion of stability due to the PPh3 ligand effects, where ICO has higher stability than DRC by 2.40 eV. Our insights are supported by several structural, electronic, and energetic analyses FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DO RIO GRANDE DO SUL - FAPERGS CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Fechado
- Published
- 2017
6. Theoretical Investigation of the Adsorption Properties of CO, NO, and OH on Monometallic and Bimetallic 13-Atom Clusters: The Example of Cu13, Pt7Cu6, and Pt13.
- Author
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Chaves, Anderson S., Piotrowski, Maurício J., Guedes-Sobrinho, Diego, and Da Silva, Juarez L. F.
- Subjects
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PHYSICAL & theoretical chemistry , *METAL clusters , *ADSORPTION (Chemistry) , *DENSITY functional theory , *BINDING energy - Abstract
We report a density functional theory investigation of the adsorption properties of CO, NO, and OH on the Cu13, Pt7Cu6, and Pt13 clusters in the cationic, neutral, and anionic states with the aim to improve our atomistic understanding of the adsorption properties on bimetallic clusters compared with monometallic clusters. The adsorption energy of CO and NO are substantially stronger on Pt13 than on Cu13, and hence, CO and NO bind preferentially on Pt sites on Pt7Cu6. Thus, it can contribute to drive the migration of the Pt atoms from the core to the surface region in large PtCu nanoalloys. The CO and NO adsorption energies on the bimetallic cluster are enhanced by a few percent compared with the energies of the monometallic clusters, which shows that the Pt-Cu interaction can contribute to an increase in the adsorption energy. In contrast with CO and NO trends, the OH adsorption energies on Cu13, Pt7Cu6, and Pt13 deviates only up to 0.31 eV, and hence, there is no clear preference for Cu or Pt sites on Pt7Cu6 or an enhancement of the adsorption energy on the bimetallic systems. We found a reduction of the CO and NO vibrational frequencies upon adsorption, which indicates a weakening of the CO and NO binding energies, and it is supported by a slight increase in the bond lengths. However, the OH vibrational frequency increases upon adsorption, which indicates an enhancement of the OH binding energy, which is supported by a slight decrease in the bond length by about 0.01 Å. It can be explained by the large charge transfer from the clusters to the O atom, which enhances the electrostatic interaction in the O-H bonding. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
7. Structural formation of binary PtCu clusters: A density functional theory investigation.
- Author
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Chaves, Anderson S., Rondina, Gustavo G., Piotrowski, Maurício J., and Da Silva, Juarez L.F.
- Subjects
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MOLECULAR structure , *BINARY metallic systems , *METAL clusters , *DENSITY functional theory , *TRANSITION metals , *CENTER of mass - Abstract
Binary transition-metal clusters have attracted great attention and a large number of studies have been reported, however, our atomistic understanding of the structural formation mechanisms of those clusters is still far from satisfactory. In this paper, we report a systematic study of the Pt n Cu m − n clusters ( m = 2 , 3 , … , 14 , n = 0 , 1 , … , m ) employing Ab-Initio density functional theory calculations within the Perdew–Burke–Ernzerhof functional. Using a set of structural design principles, we obtained a hierarchical set of atomic configurations from which the structural formation mechanisms are discussed in details. We found a negative excess energy for m > 2 , providing strong evidence favouring the formation of binary PtCu clusters. In general, the Cu atoms tend to form agglomerates located near the center of gravity of the clusters while the Pt atoms tend to lie further away from the center of gravity and separated from each other. Therefore, this behaviour tends to reduce the number of Pt–Pt bonds whereas the number of Cu–Cu and Pt–Cu bonds tends to be maximized. These mechanisms help to release strain energy by means of relaxation of the gas-phase clusters, and the locations of the Cu and Pt atoms suggest that the formation of core–shell like structures starts already in this small size regime. Furthermore, the formation of the PtCu alloy does not lead to changes in the magnetic properties of the clusters in comparison with the parent Pt and Cu clusters. The systematic work presented here provides a basis to understand and tailor the properties of binary cluster. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
8. Ethanol and Water Adsorption on Transition-Metal 13-Atom Clusters: A Density Functional Theory Investigation within van der Waals Corrections.
- Author
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Zibordi-Besse, Larissa, Tereshchuk, Polina, Chaves, Anderson S., and Da Silva, Juarez L. F.
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METAL nanoparticles , *TRANSITION metals , *METAL clusters , *ADSORPTION (Chemistry) , *VAN der Waals forces , *CARBON-black , *DENSITY functional theory , *HYDROGEN production - Abstract
Transition-metal (TM) nanoparticles supported on oxides or carbon black have attracted much attention as potential catalysts for ethanol steam reforming reactions for hydrogen production. To improve the performance of nanocatalysts, a fundamental understanding of the interaction mechanism between water and ethanol with finite TM particles is required. In this article, we employed first-principles density functional theory with van der Waals (vdW) corrections to investigate the interaction of ethanol and water with TM13 clusters, where TM = Ni, Cu, Pd, Ag, Pt, and Au. We found that both water and ethanol bind via the anionic O atom to onefold TM sites, while at higher-energy structures, ethanol binds also via the H atom from the CH2 group to the TM sites, which can play an important role at real catalysts. The putative global minimum TM13 configurations are only slightly affected upon the adsorption of water or ethanol; however, for few systems, the compact higher-energy icosahedron structure changes its configuration upon ethanol or water adsorption. That is, those configurations are only shallow local minimums in the phase space. Except few deviations, we found similar trends for the magnitude of the adsorption energies of water and ethanol, that is, Ni13 > Pt13 > Pd13 and Cu13 > Au13 > Ag13, which is enhanced by the addition of the vdW correction (i.e., from 4% to 62%); however, the trend is the same. We found that the magnitude of the adsorption energy increases by shifting the center of gravity of the d-states toward the highest occupied molecular orbital. On the basis of the Mulliken and Hirshfeld charge analysis, as well as electron density differences, we identified the location of the charge redistribution and a tiny charge transfer (from 0.01 e to 0.19 e) from the molecules to the TM13 clusters. Our vibrational analysis indicates the red shifts in the OH modes upon binding of both water and ethanol molecules to the TM13 clusters, suggesting a weakening of the O-H bonding. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
9. Theoretical Study of the Structural, Energetic, and Electronic Properties of 55-Atom Metal Nanoclusters: A DFT Investigation within van der Waals Corrections, Spin-Orbit Coupling, and PBE+U of 42 Metal Systems.
- Author
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Piotrowski, Maurício J., Ungureanu, Crina G., Tereshchuk, Polina, Batista, Krys E. A., Chaves, Anderson S., Guedes-Sobrinho, Diego, and Da Silva, Juarez L. F.
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ELECTRONIC structure , *METAL clusters , *DENSITY functional theory , *VAN der Waals forces , *SPIN-orbit interactions , *CHEMICAL systems , *GROUND state energy - Abstract
An atom-level ab initio understanding of the structural, energetic, and electronic properties of nanoclusters with diameter size from 1 to 2 nm figures as a prerequisite to foster their potential technological applications. However, because of several challenges such as the identification of ground-state structures by experimental and theoretical techniques, our understanding is still far from satisfactory, and further studies are required. We report a systematic ab initio investigation of the 55-atom metal nanoclusters, (M55), including alkaline, transitional, and post-transitional metals, that is, a total of 42 systems. Our calculations are based on all-electron density functional theory within the Perdew-Burke-Ernzerhof (PBE) functional combined with van der Waals (vdW) correction, spin-orbit coupling (SOC) for the valence states. Furthermore, we also investigated the role of the localization of the d states by using the PBE+U functional. We found a strong preference for the putative PBE global-minimum configurations for the compact Mackay icosahedron structure, namely, 16 systems (Na, Mg, K, Sc, Ti, Co, Ni, Cu, Rb, Y, Ag, Cs, Lu, Hf, Re, Hg), while several systems adopt alternative compact structures such as 6 polytetrahedron (Ca, Mn, Fe, Sr, Ba, Tl) and 10 structures derived from crystalline face-centered cubic and hexagonal close-packed (HCP) fragments (Cr, Nb, Mo, Tc, Ru, Rh, Pd, Ta, W, Os). However, the 10 remaining systems adopt less compact structures based on the distorted reduced-core structure (V, Zn, Zr, Cd, In, Pt, Au), tetrahedral-like (Al, Ga), and one HCP wheel-type (Ir) structure. The binding energy shows a quasi-parabolic behavior as a function of the atomic number, and hence the occupation of the bonding and antibonding states defines the main trends (binding energy, equilibrium bond lengths, etc.). On average, the binding energy of the M55 systems represents 79% of the cohesive energy of the respective bulk systems. The addition of the vdW correction changes the putative global-minimum configurations (pGMCs) for selected cases, in particular, for post-transitional metal systems. As expected, the PBE+U functional increases the total magnetic moment, which can be explained by the increased localization of the d states, which also contributed to increase the number of atoms in the core region (increase coordination) of the pGMCs. In contrast with the effects induced by the vdW correction and localization of the d states, the addition of the SOC coupling cannot change the lowest energy configurations, but it affects the electronic properties, as expected from previous calculations for 13-atom clusters. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
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