1,720,994 research outputs found
Van der Waals density functional studies of hydrogenated and lithiated bilayer graphene
Get PDFThesis (PhD)--University of Pretoria, 2014.In this thesis, we use rst principles density functional theory (DFT) to study the
energetics, structural and electronic properties of hydrogenated and lithiated bilayer
graphene material systems. The newly developed four variants of the non-local van der
Waals (vdW) exchange-correlation functionals (vdW-DF, vdW-DF2, vdW-DF C09x
and vdW-DF2 C09x) are employed to explore all the possible con gurations of hydrogen
adsorption at 50% and 100% coverage on a 1 1 unit cell. The results obtained are also
compared with the GGA PBE functional.
For 50% hydrogen coverage, 16 unique con gurations are identi ed in the unrelaxed
state. Formation energy analysis reveals six possible energetically favourable con gurations
with three low-energy competing con gurations. It is found that the properties
of hydrogenated bilayer graphene greatly depend on the hydrogen con guration. For
instance, the formation of a hydrogen dimer within the layers decouples the structure,
whereas the dimer formation outside surfaces does not have a signi cant in
uence on
the van der Waals forces; thus the bilayers remain coupled. In this coupled con guration,
the vdW-DF C09x functional predicts the lowest formation energy and shortest
interlayer separation, whereas the GGA PBE functional gives the highest formation
energy and largest interlayer distance. The reasons behind the variation of these functionals
are discussed. Two of the three low-energy competing con gurations exhibit
semimetallic behaviour, whereas the remaining con guration is a wide band gap material.
The wide band gap structure is found to undergo a hydrogen-induced spontaneous
phase transformation from hexagonal to tetrahedral (diamond-like) geometry. We conclude
that this wide band gap con guration represents a viable template for synthesizing
nanodiamonds from graphene by hydrogenation. At 100% coverage, ten unique
hydrogen con gurations are identi ed from a 1 1 unit cell. All exchange-correlation
functionals predict nine of the structures to have negative formation energies. From
these nine structures, three low-energy competing structures are noted and found to be wide band gap semiconductors, whereas the other con gurations exhibit either a
semimetallic or metallic character. Although a 1 1 unit-cell is able to present a clear
picture for the interaction between hydrogen and graphene, our results reveal that it
limits the occurrence of other interesting physics. The cell size was increased to 2 1, to
identify other low-energy con gurations that are not possible in a 1 1 cell. The identi
ed con gurations have shown physically interesting hydrogen arrangements such as
chair-like, zigzag-like and boat-like con gurations. Furthermore, our results reveal that
hydrogenation reduces the elastic properties of the pristine structures.
We further perform a systematic investigation of the e ects of lithium (Li) on AA
and AB stacking sequences of bilayer graphene. Two Li atoms are considered to examine
the e ects of the Li-Li interaction on bilayer graphene, and a total of 12 unique
con gurations for AB and 9 for AA stackings are identi ed. The vdW-DF consistently
predicts the highest formation energies, whereas vdW-DF2 C09x gives the lowest. Unlike
in the case of the pristine structures, it is noted that for lithiated bilayer graphene,
GGA PBE gives comparable results to the other functionals. One of the Li intercalated
con gurations undergoes a spontaneous translation from the AB to AA stacking,
and is found to be the most energetically stable con guration. We therefore conclude
that Li favours the AA stacking, and that con guration represents a feasible template
for experimentally synthesizing and characterizing a Li-based anode material. We noticed
that all identi ed Li con gurations exhibit metallic behaviour. Lastly, we found
that the intercalated Li dimer weakly interacts with the graphene layers, whereas the
intercalated isolated Li atom exhibits strong interaction.gm2014Physicsunrestricte
Theoretical studies of graphene and graphene-related materials involving carbon and silicon
Get PDFDissertation (MSc)--University of Pretoria, 2011.The structural and electronic properties of graphene and graphene-related materials have been intensively investigated using the plane wave based periodic density func- tional theory (DFT). The Vienna ab initio simulation package (VASP) code employing the generalized gradient approximation (GGA) for the exchange correlation potential was used. In all calculations, the geometry optimization option was employed in allow- ing the structure to fully relax. Hydrogen adatoms were adsorbed on C, Si and SiC in the graphene structure in-volving (1x1),(2x2),(3x3) and (4x4) two dimensional unit cells. The density of states reveals that the adsorption of 50% hydrogen makes the system metallic but 100% coverage at the on top sites generates a band gap. Our results show that SiC in the graphene structure is a plausible structure with a wide band gap. For adsoption of lithium adatoms, we considered various configurations involving the (1x1), (2x1) and (2x2) two-dimensional unit cells, and we consider the isolated Li dimer on graphene. We consider more detailed configurations than have been studied before, and our results compare favourably with previously calculated results where such results exist. For 100% coverage, we have new results for Li on the on-top site, which suggests a staggered configuration for the lowest energy structure for which the Li adatoms are alternately pushed into and pulled out of the graphene layer. For 50% coverage, Li favours the hollow site. We discovered that a careful relaxation of the system also shows a staggered configuration, a result that has not been investigated before.Physicsunrestricte
First-principles study of the enhancement of electrochemical performance of a SnS2 monolayer for lithium/sodium-ion batteries via vacancy defects
Get PDFDissertation (MSc (Physics))--University of Pretoria, 2023.Various transition metal dichalcogenides materials have been investigated from bulk to monolayer phases for different advanced technological applications. Tin disulfide monolayer offers advantages as an anode material for Li/Na-ion batteries, although it cannot be considered an ideal for direct exploitation. We systematically performed a comparative study of the adsorption and diffusion behaviour of Li/Na on a pristine SnS2 monolayer and on a SnS2 monolayer with a S-vacancy for enhancement of electrochemical performance, using the density functional theory approach. Although all the adsorption sites are exothermic, it was established that Li/Na adatoms mostly prefer to bind strongly on a SnS2 monolayer with a S-vacancy but avoiding the S-vacancy site. It was established that avoiding the S-vacancy site along the path, an excellent diffusion barriers of 0.19 eV for Li and 0.13 eV for Na were achieved, suggesting possible ultrafast charge/discharge rate. Due to reduced molar mass, the SnS2 monolayer with a S-vacancy has a slightly higher storage capacity than its pristine counterparts for both Li and Na adatoms. The obtained open circuit voltage values are within the range of 0.25–3.00 V assuring that the formation of dendrites can surely be averted for the envisaged battery operation. Understanding the effects of an S-vacancy on the electrochemical properties of Li/Na on the SnS2 monolayer allows us to consider possible improvements to energy storage devices that can be applied as a result of improved anode material.PhysicsMSc (Physics)Unrestricte
First-principles study of doped hematite surfaces for photoelectrochemical water splitting
No full textThesis (PhD (Physics))--Univesity of Pretoria, 2020.Photoelectrochemical (PEC) water splitting, using sunlight and appropriate semiconductors to produce hydrogen (H2) fuel, is a promising route to solve both the production of clean H2 fuel and storage for solar energy. Owing to its various advantages, hematite (α-Fe2O3) has emerged as a promising photoanode material for PEC water splitting. However, its poor electrical conductivity, low carrier mobility, short-hole diffusion length, and fast recombination rates of the electron-hole pairs have greatly limited its full potential for PEC performance. One way to improve the PEC activity of α-Fe2O3 is by doping with other elements. In particular, surface doping is proved to be more beneficial than bulk doping because it reduces the distance moved by the charge carriers from inside the bulk to the surface where they are required for interfacial transfer.
In this study first-principles calculations based on density functional theory (DFT) were carried out to investigate the influence of Cu, Zn, Ti and Zr on the {0001} and {01 2} hematite surfaces for enhanced PEC water splitting. Various surfaces of hematite were constructed and their thermodynamic stabilities were determined by calculating surface and formation energies. The {0001} and {01 2} surfaces were found to be the most stable. Besides, all the doped systems were found thermodynamically stable. Furthermore, it was found that Cu doped surface systems does not only decrease the bandgap but also leads to the correct conduction band alignment for spontaneous water splitting. In all calculations, the charge density difference plots and the Bader charge analysis showed accumulation of charge at the top outmost surface, implying the photogenerated charge carriers can efficiently diffuse to the surface for enhanced interfacial charge transfer to the adsorbates.
Morever, it was found that even with mono doping of Zn on the topmost layer of the {0001} α-Fe2O3 surface, the bandgap can be decreased without impurity states in the band structure which normally acts as recombination centres. Furthermore, the energetic stability and electronic properties of bimetallic doped {0001} α-Fe2O3 surface with (Zn, Ti) and (Zn, Zr) pairs for enhanced PEC water splitting was also studied. Bimetallic doping is viewed as an important and executable way of not only increasing the conductivity of a semiconductor material but also reducing the quick recombination of the electron-hole pairs. The doped systems showed negative formation energies under both O-rich and Fe-rich conditions implying that they are thermodynamically stable and could be prepared experimentally. Additionally, bimetallic doping of (Zn, Ti) and (Zn, Zr) on the {0001} surface is expected to enhance the PEC performance of α-Fe2O3 because Ti or Zr is capable of increasing the conductivity of α-Fe2O3 due to the substitution of Fe3+ with Ti4+ or Zr4+, while Zn can foster the surface reaction and reduce quick recombination of the electron-hole pairs. We hope that our results provided here will be of great interest to both experimental and theoretical researchers.Ministry of Higher Education, Copperbelt University, ZambiaThe University of Pretoria, Department of PhysicsCentre for High-Performance Computer (CHPC), Cape TownPhysicsPhD (Physics)Restricte
Band gap engineering of a MoS2 monolayer through transition metal and chalcogen alloying : an ab initio study
Get PDFThesis (PhD)--University of Pretoria, 2019.In this thesis, density functional theory (DFT) calculations are performed to study
the transition metal and chalcogen alloying of a molybdenum disul de (MoS2) monolayer
for band gap engineering. The e ects of the foreign atoms on the thermodynamic
stability, structural and electronic properties of the MoS2 monolayer are investigated.
To study these e ects systematically at di erent alloying concentrations, the possible
line-ordered con gurations at each concentration are considered. Their energetics,
structural and electronic properties are compared with the well-known alloy shapes,
random and/or cluster con gurations.
For the case of the transition metal alloying, chromium (Cr) atoms are introduced
at the molybdenum (Mo) sites. Various unique line-ordered con gurations are considered
at each concentration. The most stable ones are identi ed by means of formation
energies. The energetics comparison of the line-ordered alloy and the random con gurations
generated using special quasirandom structure (SQS) shows that the line-ordered
alloy con gurations have relatively low formation energies compared to the random
con gurations. The formation energies of all considered con gurations are positive but
relatively small, revealing that both shapes of the Cr alloying can be synthesized and
co-exist at the same synthesis conditions. For the structural properties, the increase in
Cr concentration reduces the lattice constant of the MoS2 system following the Vegard's
law. The Cr atoms ne-tune the band gap of a MoS2 monolayer from 1.65 eV to 0.86
eV. Based on the partial density of states and the charge density analysis, the Cr 3d
and Mo 4d at the vicinity of the band edges are found to be the main responsible for
the reduction of the band gap.
For the chalcogen alloying, the in
uence of the oxygen (O) and tellurium (Te) atoms
are considered. We start with the study of the O alloying in a MoS2 monolayer appearing
in di erent shapes: line-ordered, cluster and random. The small calculated
formation energy values of the various O alloy con gurations show that this alloying
are stable and should be synthesizable under favorable conditions. At high concentration,
the O line-ordered alloys seem to be constantly most stable compared to the
considered random and cluster alloy con gurations, while the formation energies of all
the con gurations are nearly the same at low concentration. Although the O atom has
small atomic radii compared to the S atom, their alloying preserve the 2D hexagonal
structure of the MoS2 monolayer at each concentration. However, the lattice constant
decreases linearly with the increase in O concentration, consistent with Vegard's law.
The introduction of O atom in the MoS2 monolayer also ne-tunes the band gap of the
MoS2 monolayer with a range of 1.65 eV to 0.98 eV. The band gap reduction is mainly
contributed by the Mo 4d and O 2p orbitals at the band edges.
We further carried out a thorough systematic study of Te line-ordered alloys in a
MoS2 monolayer. The low formation energies of the Te line-ordered alloy con gurations
indicate that they are also thermodynamically stable at low concentration. The
obtained formation energies for line-ordered alloy con gurations at each concentration
compete very well with the random con gurations that are already achieved experimentally.
The structural characterization indicates that the lowest energy con guration at
each concentration corresponds to the con guration where the Te atom rows are far
apart from each other within the supercell. Similar to that of O alloying, the variation
of the lattice constant at di erent concentrations obeys Vegard's law, but its values
increase with the concentration since Te atom has larger atomic radius than S and O
atoms. The Te alloying ne-tunes the band gap ranging between the MoS2 (1.65 eV)
and the MoTe2 (1.04 eV) band gap values .
In brief, the Cr, O and Te successfully engineer the band of a MoS2 monolayer, and
their study should be bene cial for nanotechnological applications.PhysicsPhDUnrestricte
Computational insights of hydrogen vacancies mediating interaction of lithium and sodium adatoms with an electrode graphane monolayer
Get PDFDissertation (MSc (Physics))--University of Pretoria, 2023.Density functional theory calculations were performed to enhance interaction of Li/Na atoms with a graphane monolayer for LIBs and NIBs. The energetic stability, electronic and electrochemical properties of Li/Na atoms on a pristine graphane monolayer as well as on a graphane monolayer with H vacancies (VH) following a Line pathway and zigzag pathway were evaluated. Firstly, different Li adsorption sites and Na adsorption sites on a large supercell of 5×5×1 pristine graphane monolayer were considered, to establish the most energetically favourable site, based on binding energy calculations. All the adsorption sites of Li/Na atom on pristine graphane are energetically unfavourable (endothermic reaction). This is an indication that Li/Na atoms interacts weakly with graphane monolayer and could result with formation of unwanted Li/Na clusters. As a strategy to improve Li/Na atom interaction with graphane monolayer, a H vacancy (VH) was created. Different Li/Na adsorption sites at the vicinity of H vacancy were considered to establish the most energetically favourable site. All the Li/Na configurations relaxes towards the VH site. The Li/Na binding energies are higher than the cohesive energies of metallic bulk counterparts suggesting that clustering will not be reached easily. An increase in H vacancies along the line pathway from one VH1(L)to five VH5(L) leaves behind localized electrons ready to interact with the Li/Na atom resulting in high binding energies. The creation of H vacancies along the zigzag pathway from one VH1(Z) to ten VH10(Z) leavesbehind the dangling electrons on the nearest neighbour C atoms that pair and repel Li/Na atom away, yielding undesired low binding energies which become a setback for LIBs and NIBs. On the density of states analysis, adsorption of Li/Na atoms introduced new electronic states crossing the Fermi level. An observed transition from insulator to metallic behaviour will enhance the electron transmission in the graphane monolayer during battery operations. Li/Na storage capacities plots indicate that an increase of the Li/Na content result in an increase in the storage capacities. The average voltage obtained for the adsorption of five Li/Na atoms is 1.55 V/1.04 V. At this concentration, undesired clustering is unexpected until the voltages approaches zero. This implies that more adsorption of Li/Na atoms can still be considered to achieve a maximum concentration.NITheCSPhysicsMSc (Physics)Unrestricte
Enhancement of electrochemical performance of monolayer SnS2 for Li/Na-ion batteries through a sulphur vacancy : a DFT study
Get PDFDATA AVAILABILITY : The datasets generated during and/or analysed during
the current study are available from the corresponding author on
reasonable request.Various transition metal dichalcogenides materials have been investigated from bulk to monolayer phases for different
advanced technological applications. Tin disulfide monolayer offers advantages as an anode material for Li/Na-ion batteries,
although it cannot be considered ideal for direct exploitation. We systematically performed a comparative study of the
adsorption and diffusion behaviour of Li/Na on a pristine SnS2 monolayer and on a SnS2 monolayer with S-vacancy for
enhancement of electrochemical performance, using density functional theory approach. Although all the adsorption sites
are exothermic, it was established that Li/Na adatoms mostly prefer to bind strongly on SnS2 monolayer with S-vacancy but
avoiding the S-vacancy site. It was established that avoiding the S-vacancy site along the path, excellent diffusion barriers of
0.19 eV for Li and 0.13 eV for Na were achieved, suggesting possible ultrafast charge/discharge rate. Due to reduced molar
mass, the SnS2 monolayer with S-vacancy has a slightly higher storage capacity than its pristine counterparts for both Li
and Na adatoms. The obtained open circuit voltage values are within the range of 0.25–3.00 V assuring that the formation
of dendrites can surely be averted for the envisaged battery operation. Understanding the effects of an S-vacancy on the
electrochemical properties of Li/Na on the SnS2 monolayer allows us to consider possible improvements to energy storage
devices that can be applied as a result of improved anode material.The University of Pretoria and the National Institute for Theoretical and Computational Sciences (NITheCS). Open access funding provided by University of Pretoria.https://link.springer.com/journal/10008am2024PhysicsSDG-09: Industry, innovation and infrastructur
Investigating electrocatalytic properties of β12-borophene as a cathode material for an efficient lithium-oxygen battery : a first-principles study
Get PDFDATA AVAILABITY STATEMENT: Data will be made available on request.Please read abstract in article.The University of Pretoria.http://link.springer.com/journal/13204PhysicsSDG-07:Affordable and clean energySDG-13:Climate actio
Electronic properties of vacancies in bilayer graphane
Get PDFPlease read abstract in the article.The University of Pretoria and the National Institute for Theoretical Physics (NiThep).http://www.elsevier.com/locate/physbhj2019Physic
Band gap engineering of a MoS2 monolayer through oxygen alloying : an ab initio study
Get PDFOxygen (O) alloying in a MoS2 monolayer appearing in different shapes: line-ordered, cluster and random have been theoretically designed, for band gap engineering in order to extend its nanotechnological applications. The thermodynamic stability, structural and electronic properties of these alloy configurations at each concentration have been comparatively studied using the density functional theory methods. Based on the formation energy analysis, the O line-ordered alloys are most stable compared to the well known random and cluster alloys at high concentration, while at low concentration they compete. The lattice constants of all the alloyed systems decrease linearly with the increase in O concentration, consistent with Vegard's law. The Mo–O bond lengths are shorter than Mo–S leading to a reduction in the band gap, based on density of state analysis. The partial charge density reconciling with the partial density of states analysis reveals that the band gap reduction is mainly contributed by the Mo 4d and O 2p orbitals as shown at the band edges of the density of states plots. Creation of stacking of MoS2 with MoO2 gives metallic character, with Mo 4d orbital crossing the Fermi level. The O alloys in a MoS2 monolayer should be considered to be an effective way to engineer the band gap for designing new nanoelectronic devices with novel performance.The University of Pretoria for computational resources and financial support. NC would also like to thank the National Institute of Theoretical Physics for financial support.https://iopscience.iop.org/journal/0957-44842019-10-09hj2019Physic
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