| 1. |
- Yang, Yi, et al.
(författare)
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Tuning catalytic activity of Ni-Co nanoparticles synthesized by gamma-radiolytic reduction of acetate aqueous solutions
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Transition metal-based catalysts show great potential to replace Pt-based material in energy conversion devices thanks to their low cost, reasonable intrinsic activity, thermodynamic stability, and corrosion resistance. The electrochemical performance of such catalysts is sensitive to their composition and structure. Here we demonstrate that homogeneous alloy nanoparticles with a varying Ni-to-Co ratio and controlled structure can be synthesized from aqueous Ni (Co) acetate solutions using a facile γ-radiolytic reduction method. The obtained samples are found to possess stacking fault defects that are ordered to form polytype structures. The concentration of these defects depends on the Ni-to-Co ratio, as supported by the results of ab initio calculations. Structural defects may play a significant role in the enhanced activity of catalysts toward oxygen evolution and oxygen reduction reactions. At the same time, the activity of Ni-Co catalysts in the hydrogen evolution reaction is found to be affected by the formation of Ni-OH bonds on the surface rather than by the presence of structural defects. Our study demonstrates that the composition of Ni-Co nanoparticles is an essential factor having an impact on their structure and that both composition and structure can be tuned to optimize electrochemical performance with respect to various catalytic reactions.
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| 2. |
- Yang, Yi, et al.
(författare)
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Tuning Catalytic Activity of Ni-Co Nanoparticles Synthesized by Gamma-Radiolytic Reduction of Acetate Aqueous Solutions
- 2023
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Ingår i: Advanced Materials Interfaces. - : Wiley-Blackwell. - 2196-7350. ; 10:17
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal-based catalysts show great potential to replace Pt-based material in energy conversion devices thanks to their low cost, reasonable intrinsic activity, thermodynamic stability, and corrosion resistance. The electrochemical performance of such catalysts is sensitive to their composition and structure. Here, it is demonstrated that homogeneous alloy nanoparticles with varying Ni-to-Co ratio and controlled structure can be synthesized from aqueous Ni(Co) acetate solutions using a facile gamma-radiolytic reduction method. The obtained samples are found to possess defects that are ordered to form polytypes structures. The concentration of these defects depends on the Ni-to-Co ratio, as supported by the results of ab initio calculations. It is found that structural defects may influence the activity of catalysts toward the oxygen evolution reaction, while this effect is less pronounced with respect to the oxygen reduction reaction. At the same time, the activity of Ni-Co catalysts in the hydrogen evolution reaction is affected by formation of Ni-OH bonds on the surface rather than by the presence of structural defects. This study demonstrates that the composition of Ni-Co nanoparticles is an essential factor affecting their structure, and both composition and structure can be tuned to optimize electrochemical performance with respect to various catalytic reactions.
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| 3. |
- Ambrosch-Draxl, C., et al.
(författare)
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First-principles study of oxygen ordering in YBa2Cu3O7-x
- 2000
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 341, s. 1997-1998
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Tidskriftsartikel (refereegranskat)abstract
- We have performed first-principles calculations considering different patterns of oxygen/vacancy ordering in the Cu-O chains of YBa2Cu3O7-x. From the total-energy values we determine the effective interactions of the oxygen atoms within the basal plane by using the cluster expansion technique. Furthermore, we discuss the effect of ordering to the density of states at the Fermi level.
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| 4. |
- Andersson, David A., et al.
(författare)
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First-principles based calculation of binary and multicomponent phase diagrams for titanium carbonitride
- 2008
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Ingår i: Calphad. - : Elsevier BV. - 0364-5916 .- 1873-2984. ; 32:3, s. 543-565
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Forskningsöversikt (refereegranskat)abstract
- In this paper we have used a combined first principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on the vacancy-induced ordering of the substoichiometric carbonitride phase, TiCxNy (x + y <= 1). Results from earlier Monte Carlo simulations of the low-temperature binary phase diagrams are used in order to formulate sublattice models for TiCxNy within the compound energy formalism (CEF) that are capable of describing both the low temperature ordered and the high-temperature disordered state. We parameterize these models using first-principles calculations and then we demonstrate how they can be merged with thermodynamic descriptions of the remaining Ti-C-N phases that are derived within the Calphad method by fitting model parameters to experimental data. We also discuss structural and electronic properties of the ordered end-member compounds, as well as short range order effects in the TiCxNy phase.
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| 5. |
- Andersson, David A., et al.
(författare)
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Thermodynamics of structural vacancies in titanium monoxide from first principles calculations
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:14, s. 144101-
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Tidskriftsartikel (refereegranskat)abstract
- The structure, stability and electronic properties of the low oxygen oxides of titanium, TiOx with 1/3 <= x <= 3/2, have been studied by means of accurate first-principles calculations. In both stoichiometric and nonstoichiometric TiO there are large fractions of vacant lattice sites. These so-called structural vacancies are essential for understanding the properties and phase stability of titanium oxides. Structures with an ordered arrangement of vacancies were treated with a plane wave pseudo-potential method, while calculations for structures with disordered vacancies were performed within the framework of the Korringa-Kohn-Rostoker Green's function technique. The relaxed structural parameters in general compare well with experimental data, though some discrepancies exist for stoichiometric TiO in the ideal B1 structure, i.e., without any vacancies. The equation of state as well as the elastic properties are also derived. A monoclinic, vacancy-containing, structure of stoichiometric TiO is confirmed to be stable at low temperature and pressure. Experimentally a transition from a stoichiometric cubic structure with disordered vacancies to the ideal B1 structure without any vacancies has been observed at high pressure. It is discussed how this experimental observation relates to the present theoretical results for defect-containing and defect-free TiO.
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| 6. |
- Andersson, David, 1978-
(författare)
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From the Electronic Structure of Point Defects to Functional Properties of Metals and Ceramics
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Point defects are an inherent part of crystalline materials and they influence important physical and chemical properties, such as diffusion, hardness, catalytic activity and phase stability. Increased understanding of point defects enables us to tailor the defect-related properties to the application at hand. Modeling and simulation have a prominent role in acquiring this knowledge. In this thesis thermodynamic and kinetic properties of point defects in metals and ceramics are studied using first-principles calculations based on density functional theory. Phenomenological models are used to translate the atomic level properties, obtained from the first-principles calculations, into functional materials properties. The next paragraph presents the particular problems under study. The formation and migration of vacancies and simple vacancy clusters in copper are investigated by calculating the energies associated with these processes. The structure, stability and electronic properties of the low-oxygen oxides of titanium, TiOx with 1/3 < x < 3/2, are studied and the importance of structural vacancies is demonstrated. We develop an integrated first-principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on vacancy-induced ordering of the substoichiometric carbonitride phase, TiCxNy (x+y < 1). The possibility of forming higher oxides of plutonium than plutonium dioxide is explored by calculating the enthalpies for nonstoichiometric defect-containing compounds and the analysis shows that such oxidation is only produced by strong oxidants. For ceria (CeO2) doped with trivalent ions from the lanthanide series we probe the connection between the choice of a dopant and the improvement of ionic conductivity by studying the oxygen-vacancy formation and migration properties. The significance of minimizing the dopant-vacancy interactions is highlighted. We investigate the redox thermodynamics of CeO2-MO2 solid solutions with M being Ti, Zr, Hf, Th, Si, Ge, Sn or Pb and show that reduction is facilitated by small solutes. The results in this thesis are relevant for the performance of solid electrolytes, which are an integral part of solid oxide fuel cells, oxygen storage materials in automotive three-way catalysts, nuclear waste materials and cutting tool materials.
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| 7. |
- Baykov, Vitaly, et al.
(författare)
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Ab Initio Studies of the Energy Characteristics and Magnetic Properties of Point Defects in GaAs
- 2005
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Ingår i: Physics of the solid state. - : Pleiades Publishing Ltd. - 1063-7834 .- 1090-6460. ; 47:10, s. 1831-1836
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Tidskriftsartikel (refereegranskat)abstract
- The formation energies of intrinsic point defects and solution energies of transition metal impurities in gallium arsenide are determined on the basis of ab initio calculations using the method of a locally self-consistent Green's function, which is a generalization of the coherent potential approximation. Based on the calculated energies, the conclusion is made that the As-Ga antisite defect is the most common intrinsic defect in GaAs. Calculations showed that transition metal impurities, except for Ni, preferentially occupy gallium sites substitutionally. The magnetic moments of impurity atoms are calculated as a function of the chemical environment. It is shown that, in compensated GaAs, Mn atoms tend to form clusters.
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| 8. |
- Baykov, Vitaly, et al.
(författare)
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Diffusion of Interstitial Mn in the Dilute Magnetic Semiconductor (Ga,Mn)As : The Effect of a Charge State
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 101:17, s. 177204-
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Tidskriftsartikel (refereegranskat)abstract
- Migration barriers for diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As are studied using first-principles calculations. The diffusion pathway goes through two types of interstitial sites: As coordinated and Ga coordinated. The energy profile along the path is found to depend on the ratio of concentrations between substitutional and interstitial Mn in GaAs. Two regions of distinctly different behavior, corresponding to n-type and p-type (Ga,Mn)As, are identified. The difference in mobility is a reflection of the change in the charge state of Mn interstitials (double donors) that occurs in the presence of substitutional Mn impurities (acceptors). In addition, substitutional Mn impurities are shown to act as traps for interstitial Mn. The effective migration barrier for the positively doubly charged Mn interstitials in p-type (Ga,Mn)As is estimated to vary from 0.55 to about 0.95 eV.
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| 9. |
- Baykov, Vitaly, et al.
(författare)
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Magnetic properties of 3d impurities in GaAs
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 310:2, s. 2120-2122
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Tidskriftsartikel (refereegranskat)abstract
- Electronic structure, thermodynamic, and magnetic properties of 3d-transition metal (TM) impurities in GaAs have been studied from first principles using Green's function approach. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The possibility of raising the Curie temperature TC in (GaMn) As by co-doping it with Cr impurities was examined on the basis of total energy difference between the disordered local moment (DLM) and the ferromagnetically ordered (FM) spin configurations. The calculated Curie temperature and magnetic moment have maxima for GaAs doped with Cr and Mn. The magnetic properties of Mn-doped GaAs are shown to be more sensitive to antisite As defects than those of Cr-doped GaAs. However, the Cr impurities are sensitive to the presence of acceptor defects, such as vacancies on the Ga sublattice. The investigation of the electronic structure of pseudo-ternary alloys (Ga(1-x-y)MnxCry) As has shown a mutual compensation of Mn and Cr impurities. Therefore, in order to reach the highest critical temperature, GaAs has to be separately doped with Cr or Mn impurities. The GaAs doped with Fe is found to be non-ferromagnetic.
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| 10. |
- Baykov, Vitaly, et al.
(författare)
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Structural stability of intermetallic phases in the Zr-Sn system
- 2006
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 55:5, s. 485-488
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Tidskriftsartikel (refereegranskat)abstract
- A thermodynamic description of the intermetallic compounds in the Zr-Sn binary system has been obtained using total energy calculations by means of the Vienna ab initio simulation package. Our calculations show that hexagonal compounds Zr5Sn4 and Zr5Sn3 are the most stable phases in the Zr-Sn binary system. Their high stability is found to be due to hybridization of the Sn 5p with Zr 4d electronic states. Based on the calculated energies, the conclusion is made that Zr substitution on the Sri sites takes place in the Zr4Sn phase, which accounts for the unusual stoichiometry of this Cr3Si structure type compound.
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