elvis shoko | University of the Witwatersrand (original) (raw)

Papers by elvis shoko

Research paper thumbnail of Designing coral-like Fe2O3-regulated Se-rich CoSe2 heterostructure as a highly active and stable oxygen evolution electrocatalyst for overall water splitting

Journal of Electroanalytical Chemistry, 2022

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Research paper thumbnail of 2020 Roadmap on Solid-State Batteries

Journal of Physics: Energy

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Research paper thumbnail of Polymorph of LiAlP2O7: Combined Computational, Synthetic, Crystallographic, and Ionic Conductivity Study

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Research paper thumbnail of Hierarchical Core‐Shell N‐Doped Carbon@FeP 4 ‐CoP Arrays as Robust Bifunctional Electrocatalysts for Overall Water Splitting at High Current Density

Advanced Materials Interfaces

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Research paper thumbnail of Metal–Organic Framework (MOF)‐Derived Electron‐Transfer Enhanced Homogeneous PdO‐Rich Co 3 O 4 as a Highly Efficient Bifunctional Catalyst for Sodium Borohydride Hydrolysis and 4‐Nitrophenol Reduction

Chemistry – A European Journal

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Research paper thumbnail of High selectivity of CO2 conversion to formate by porous copper hollow fiber: Microstructure and pressure effects

Electrochimica Acta

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Research paper thumbnail of Thermoelectric Materials Under Pressure

physica status solidi (RRL) - Rapid Research Letters

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Research paper thumbnail of Structural and thermoelectric properties of the type-I Sn clathrates Cs8Sn46−n(n=0,2) from Density Functional Theory (DFT)

MRS Advances

:Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon... more :Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon free paths are short and the electron free paths are long. We analysed the relaxed structure of Sn clathrates using four different Density Funtional Exchange-Correlation functionals. The phonon structures were investigated as a first step in order to determine the phonon contribution to the thermal conductivity. We determined the Seebeck coefficient and electrical conductivity of the clathrate compound and the thermoelectric figure of merit. A glimpse into the dynamics of the system for the evaluation of the thermoelectric and electronic properties is presented.

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Research paper thumbnail of Pressure-induced conduction band convergence in the thermoelectric ternary chalcogenide CuBiS2

Physical Chemistry Chemical Physics

The dependence of the electronic and thermoelectric properties of the ternary chalcogenides Cu(Sb... more The dependence of the electronic and thermoelectric properties of the ternary chalcogenides Cu(Sb,Bi)(S,Se)2 on hydrostatic pressure is determined.

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Research paper thumbnail of Phononic Structure Engineering: the Realization of Einstein Rattling in Calcium Cobaltate for the Suppression of Thermal Conductivity

Scientific reports, Jul 26, 2016

Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrati... more Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is funda...

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Research paper thumbnail of The effect of host relaxation and dynamics on guest molecule dynamics in H2/tetrahydrofuranhydrate

Faraday Discussions, 2011

We use ab initio molecular dynamics simulations to obtain classically the effects of H2O cage mot... more We use ab initio molecular dynamics simulations to obtain classically the effects of H2O cage motions on the potential-energy surface (PES) of encapsulated H2 in the H2/tetrahydrofuran-hydrate system. The significant differences between the PES for the H2 in rigid and flexible cages that we find will influence calculation of the quantum dynamics of the H2. Part of these differences arises from the relaxation of the H2O cage around the classical H2, with a second part arising from the coupling of both translational and rotational motions of H2 with the H20 cage. We find that isotopic substitution of 2H for 1H of the H2O cage affects the coupling, which has implications for experiments that require the use of 2H2O, including inelastic neutron scattering that uses 2H2O cages in order to focus on the H2 guest dynamics. Overall, this work emphasizes the importance of taking into account cage dynamics in any approach used to understand the dynamics of H2 guests in porous framework materials.

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Research paper thumbnail of Temperature Dependence of Alkali-Metal Rattling Dynamics in the { beta}-Pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD Simulation

We investigate the temperature response of the alkali-metal rattling modes in {\beta}-pyrochlores... more We investigate the temperature response of the alkali-metal rattling modes in {\beta}-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K, and 300 K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potentials reveals that, for K, the potential at a higher temperature is not a simple extrapolation to higher energy of that at a lower temperature. Instead, we find significant reconstruction of the potential at different temperatures. Finally, we explore the temperature response of the coupling between the alkali metals and find a complex temperature dependence which suggests that the origin of the coupling may be more complex than a pure Coulomb interaction. We also find an unexpected increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores.

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Research paper thumbnail of Electronic Properties of Cerium Oxides:Towards an Effective Valence Model Hamiltonian

The primary aim of this thesis is to develop a minimal model Hamiltonian to describe the electron... more The primary aim of this thesis is to develop a minimal model Hamiltonian to describe the electronic properties of ceria (CeO$_{2}$) and its reduced phases. In order to do this, several energy scales of the problem were explored to determine their relative significance to the problem. These included the crystal electric field ($\Delta_{CEF}$), the spin-orbit coupling ($\lambda_{so}$), electron hopping ($t$), on-site Coulomb repulsion for the Ce 4f4f4f states ($U$), the reorganization energy ($\lambda_{0}$), the direct exchange ($J$) and the energy gap between the oxygen valence band level and the cerium 4f4f4f states ($\Gamma\epsilon$). Once the relative magnitudes of the various energy scales were determined, it was then possible to define a minimal set of degrees of freedom required to obtain a meaningful description of the system in the minimal model. The first task was to obtain baseline data for both CeO$_{2}$ and Ce$_{2}$O$_{3}$ as well as the metallic phases of Ce. Density Functio...

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Research paper thumbnail of MSS membrane reactors for the dehydrogenation of cyclohexane

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Research paper thumbnail of Temperature dependence of alkali-metal rattling dynamics in the β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD simulation

Journal of physics. Condensed matter : an Institute of Physics journal, Jan 11, 2014

We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2... more We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K and 300 K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potenti...

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Research paper thumbnail of Novel rattling of K atoms in aluminium-doped defect pyrochlore tungstate

Journal of Physics: Condensed Matter, 2014

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Research paper thumbnail of Molecular dynamics evidence for alkali-metal rattling in the β-pyrochlores, AOs 2 O 6 (A = K, Rb, Cs)

Journal of Physics: Condensed Matter, 2013

We have used ab initio molecular dynamics simulations validated against inelastic neutron scatter... more We have used ab initio molecular dynamics simulations validated against inelastic neutron scattering data to study alkali-metal dynamics in the β-pyrochlore osmates AOs2O6 (A=K, Rb, Cs) at 300 K to gain insight into the microscopic nature of rattling dynamics in these materials. Our results provide new evidence at the microscopic level for rattling dynamics: (1) the elemental magnitude spectra calculated from the MD show a striking dominance by the alkali metals at low energies indicating weak coupling to the cage, (2) the atomic root-mean-square displacements for the alkali metals are significantly larger than for the other atoms, e.g., 25% and 150% larger than O and Os, respectively, in KOs2O6, and (3) motions of the alkali metals are weakly correlated to the dynamics in their immediate environment, e.g. K in KOs2O6 is 6 times less sensitive to its local environment than Os, indicating weak bonding of the K. There is broadening of the elemental spectra of the alkali metals from Cs to K corresponding to a similar broadening of the local potential around these atoms as determined from potential of mean-force calculations. This feature of the spectra is partly explained by the well-known increase in the relative cage volume with decreasing atomic size of the alkali metal. We find that for the smallest rattler in this series (K) the larger relative cage volume allows this atom freedom to explore a large space inside the cage leading to vibration at a broader range of frequencies, hence a broader spectrum. Thus, since K is considered the best rattler in this series, these findings suggest that a significant feature of a good rattler is the ability to vibrate at several different but closely spaced frequencies.

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Research paper thumbnail of Charge distribution and transport properties in reduced ceria phases: A review

Journal of Physics and Chemistry of Solids, 2011

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Research paper thumbnail of Toward a mechanism of rattler coupling in the β-pyrochlores AOs2O6 (A = K, Rb, Cs)

Journal of Materials Science, 2014

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Research paper thumbnail of Novel K rattling: A new route to thermoelectric materials?

Journal of Applied Physics, 2014

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Research paper thumbnail of Designing coral-like Fe2O3-regulated Se-rich CoSe2 heterostructure as a highly active and stable oxygen evolution electrocatalyst for overall water splitting

Journal of Electroanalytical Chemistry, 2022

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Research paper thumbnail of 2020 Roadmap on Solid-State Batteries

Journal of Physics: Energy

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Polymorph of LiAlP2O7: Combined Computational, Synthetic, Crystallographic, and Ionic Conductivity Study

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Hierarchical Core‐Shell N‐Doped Carbon@FeP 4 ‐CoP Arrays as Robust Bifunctional Electrocatalysts for Overall Water Splitting at High Current Density

Advanced Materials Interfaces

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Metal–Organic Framework (MOF)‐Derived Electron‐Transfer Enhanced Homogeneous PdO‐Rich Co 3 O 4 as a Highly Efficient Bifunctional Catalyst for Sodium Borohydride Hydrolysis and 4‐Nitrophenol Reduction

Chemistry – A European Journal

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Research paper thumbnail of High selectivity of CO2 conversion to formate by porous copper hollow fiber: Microstructure and pressure effects

Electrochimica Acta

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Thermoelectric Materials Under Pressure

physica status solidi (RRL) - Rapid Research Letters

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Structural and thermoelectric properties of the type-I Sn clathrates Cs8Sn46−n(n=0,2) from Density Functional Theory (DFT)

MRS Advances

:Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon... more :Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon free paths are short and the electron free paths are long. We analysed the relaxed structure of Sn clathrates using four different Density Funtional Exchange-Correlation functionals. The phonon structures were investigated as a first step in order to determine the phonon contribution to the thermal conductivity. We determined the Seebeck coefficient and electrical conductivity of the clathrate compound and the thermoelectric figure of merit. A glimpse into the dynamics of the system for the evaluation of the thermoelectric and electronic properties is presented.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Pressure-induced conduction band convergence in the thermoelectric ternary chalcogenide CuBiS2

Physical Chemistry Chemical Physics

The dependence of the electronic and thermoelectric properties of the ternary chalcogenides Cu(Sb... more The dependence of the electronic and thermoelectric properties of the ternary chalcogenides Cu(Sb,Bi)(S,Se)2 on hydrostatic pressure is determined.

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Research paper thumbnail of Phononic Structure Engineering: the Realization of Einstein Rattling in Calcium Cobaltate for the Suppression of Thermal Conductivity

Scientific reports, Jul 26, 2016

Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrati... more Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is funda...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of The effect of host relaxation and dynamics on guest molecule dynamics in H2/tetrahydrofuranhydrate

Faraday Discussions, 2011

We use ab initio molecular dynamics simulations to obtain classically the effects of H2O cage mot... more We use ab initio molecular dynamics simulations to obtain classically the effects of H2O cage motions on the potential-energy surface (PES) of encapsulated H2 in the H2/tetrahydrofuran-hydrate system. The significant differences between the PES for the H2 in rigid and flexible cages that we find will influence calculation of the quantum dynamics of the H2. Part of these differences arises from the relaxation of the H2O cage around the classical H2, with a second part arising from the coupling of both translational and rotational motions of H2 with the H20 cage. We find that isotopic substitution of 2H for 1H of the H2O cage affects the coupling, which has implications for experiments that require the use of 2H2O, including inelastic neutron scattering that uses 2H2O cages in order to focus on the H2 guest dynamics. Overall, this work emphasizes the importance of taking into account cage dynamics in any approach used to understand the dynamics of H2 guests in porous framework materials.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Temperature Dependence of Alkali-Metal Rattling Dynamics in the { beta}-Pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD Simulation

We investigate the temperature response of the alkali-metal rattling modes in {\beta}-pyrochlores... more We investigate the temperature response of the alkali-metal rattling modes in {\beta}-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K, and 300 K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potentials reveals that, for K, the potential at a higher temperature is not a simple extrapolation to higher energy of that at a lower temperature. Instead, we find significant reconstruction of the potential at different temperatures. Finally, we explore the temperature response of the coupling between the alkali metals and find a complex temperature dependence which suggests that the origin of the coupling may be more complex than a pure Coulomb interaction. We also find an unexpected increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Electronic Properties of Cerium Oxides:Towards an Effective Valence Model Hamiltonian

The primary aim of this thesis is to develop a minimal model Hamiltonian to describe the electron... more The primary aim of this thesis is to develop a minimal model Hamiltonian to describe the electronic properties of ceria (CeO$_{2}$) and its reduced phases. In order to do this, several energy scales of the problem were explored to determine their relative significance to the problem. These included the crystal electric field ($\Delta_{CEF}$), the spin-orbit coupling ($\lambda_{so}$), electron hopping ($t$), on-site Coulomb repulsion for the Ce 4f4f4f states ($U$), the reorganization energy ($\lambda_{0}$), the direct exchange ($J$) and the energy gap between the oxygen valence band level and the cerium 4f4f4f states ($\Gamma\epsilon$). Once the relative magnitudes of the various energy scales were determined, it was then possible to define a minimal set of degrees of freedom required to obtain a meaningful description of the system in the minimal model. The first task was to obtain baseline data for both CeO$_{2}$ and Ce$_{2}$O$_{3}$ as well as the metallic phases of Ce. Density Functio...

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Research paper thumbnail of MSS membrane reactors for the dehydrogenation of cyclohexane

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Temperature dependence of alkali-metal rattling dynamics in the β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD simulation

Journal of physics. Condensed matter : an Institute of Physics journal, Jan 11, 2014

We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2... more We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20 K, 100 K and 300 K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potenti...

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Research paper thumbnail of Novel rattling of K atoms in aluminium-doped defect pyrochlore tungstate

Journal of Physics: Condensed Matter, 2014

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Research paper thumbnail of Molecular dynamics evidence for alkali-metal rattling in the β-pyrochlores, AOs 2 O 6 (A = K, Rb, Cs)

Journal of Physics: Condensed Matter, 2013

We have used ab initio molecular dynamics simulations validated against inelastic neutron scatter... more We have used ab initio molecular dynamics simulations validated against inelastic neutron scattering data to study alkali-metal dynamics in the β-pyrochlore osmates AOs2O6 (A=K, Rb, Cs) at 300 K to gain insight into the microscopic nature of rattling dynamics in these materials. Our results provide new evidence at the microscopic level for rattling dynamics: (1) the elemental magnitude spectra calculated from the MD show a striking dominance by the alkali metals at low energies indicating weak coupling to the cage, (2) the atomic root-mean-square displacements for the alkali metals are significantly larger than for the other atoms, e.g., 25% and 150% larger than O and Os, respectively, in KOs2O6, and (3) motions of the alkali metals are weakly correlated to the dynamics in their immediate environment, e.g. K in KOs2O6 is 6 times less sensitive to its local environment than Os, indicating weak bonding of the K. There is broadening of the elemental spectra of the alkali metals from Cs to K corresponding to a similar broadening of the local potential around these atoms as determined from potential of mean-force calculations. This feature of the spectra is partly explained by the well-known increase in the relative cage volume with decreasing atomic size of the alkali metal. We find that for the smallest rattler in this series (K) the larger relative cage volume allows this atom freedom to explore a large space inside the cage leading to vibration at a broader range of frequencies, hence a broader spectrum. Thus, since K is considered the best rattler in this series, these findings suggest that a significant feature of a good rattler is the ability to vibrate at several different but closely spaced frequencies.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Charge distribution and transport properties in reduced ceria phases: A review

Journal of Physics and Chemistry of Solids, 2011

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Toward a mechanism of rattler coupling in the β-pyrochlores AOs2O6 (A = K, Rb, Cs)

Journal of Materials Science, 2014

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Research paper thumbnail of Novel K rattling: A new route to thermoelectric materials?

Journal of Applied Physics, 2014

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