This thesis deals with the applications of the Kohn-Sham density functional calculation in electrochemical catalysts and the movement of molecules. The schematic configuration of Sb doped graphene, and the energies for dissociation and association steps for armchair and zigzag type Sb doped graphene. The molecular configuration of carbon dioxide and (c) the potential energy surface for the carbon dioxide bending mode.
The frequency scanning for a methane molecule. dmax and harmonic indicate the maximum of the C-H displacement and frequency respectively when dmax shows the highest value. a) The C-H trajectory for long-timescale molecular dynamics simulation with a single harmonic frequency field, and (b) the Fourier transform of a function of the C-H trajectory. dual-frequency trajectory from Fourier transform data. The maximum displacements according to the amplitude of the laser pulse; black and orange lines indicate the maximum displacements for the pulse with the single harmonic frequency and the combination of harmonic and anharmonic frequencies, respectively. A).
Introduction
These three studies present the application of DFT calculation to investigate the catalytic properties and show the advantages of the Ehrenfest dynamics method unable to be interpreted from the stationary electronic structure. It was realized that the time-dependent profile of physically observable properties becomes an important criterion to interpret the real-time dynamics in matter. The real-time dynamics simulated by the TDDFT method is applied to investigate the time propagation of the electron under external perturbations.
Theoretical background and computational details
Density functional theory
In the first step for the DFT simulation, the Kohn-Sham Hamiltonian for the simulation must be established. System configuration, initial atomic charge density, and basis set for Kohn-Sham conditions are needed to build the initial Kohn-Sham Hamiltonian. For example, the Kohn-Sham wave function based on the PW basis set can be represented as follows nKS( ) Ecut n( ) iG r.
Kohn-Sham equations are therefore computed iteratively by updating the Kohn-Sham Hamiltonian from the previous state. As a result, the Kohn-Sham wave functions and ground state eigenvalues can be calculated iteratively.
Time dependent density functional theory
In the RTP-TDDFT figure, the establishment of the time evolution operator derived from the time-dependent Kohn-Sham equation is the initial step for time propagation. In numerical calculation, there are different formulas for the time evolution operator based on the computer technique used. For example, the splitting of the Suzuki-Trotter (ST) type based time evolution operator has an exponential function.
Nevertheless, this ST-type operator requires a high computational performance for applying the time-evolution operator in exponential function to the Kohn-Sham wave function. To avoid the high-cost required exponential form, the time evolution operator of Crank-Nicholson (CN) type is used as the solver of linear equation. As a result, it brings the more effective mathematical algorithm compared to the ST-type time evolution operator in the exact exponential function.
To compare these two different time evolution operators, the ST and CN types, case studies in different systems were carried out. A smaller time step represents the more accurate time propagation and shows the lowest numerical error in the Kohn-Sham total energy for the ST and CN type time evolution operators. However, there is an insignificant accuracy difference between the ST and CN type time evolution operators.
The computational time cost of the ST-type time evolution operator increases logarithmically with respect to the size of Kohn-Sham states. For that reason, the CN-type time evolution operator is more useful for TDDFT simulation than the ST-type. a) Numerical error and (b) computation time cost for TDDFT calculation with different conditions. However, the TDDFT simulation can handle the non-equilibrium electronic dynamics by using the time-evolution operator.
Oxygen reduction reaction
- Introduction
- Carbon-coated Core-Shell Fe-Cu nanoparticles as Highly Active and Durable
- Antimony-doped graphene nanoplates
- Conclusion
In order to optimize the ORR process, the noble metal-based catalysts such as Pt metal have been thoroughly investigated both theoretically and experimentally. From the volcano plots, the highest activity for oxygen reduction among the transition metal is Cu, and the strong binding affinity with oxygen among the transition metal is Fe. The CuFe alloy nanoparticles can be synthesized experimentally, and the electrochemical test takes place in alkaline condition due to the corruption of Fe metal.
For that reason, the CuFe alloy configurations are proposed for the location of Fe ions and the electrochemical profile is calculated by the DFT. The bottom in Figure 2-1 describes the ORR processes on the CuFe alloy, and the Figure 2-2 represents the ORR profiles following the Figure 2-1 processes. However, for the CuFesurface case, the oxygen bond is strong, and the energy drop in 0→1 step is high.
For these reasons, CuFeinner shows better performance and the non-noble metal-based catalyst can replace the previous noble metal-based catalyst such as Pt. The net ORR reaction has a potential of 1.6 eV for a four-electron reaction, and the potential becomes zero when the applied potential is 0.4 eV (1.6 eV/4 electrons). The multiple oxidation states of Sb, Sb3+ and Sb5+, are responsible for ORR stability.
The different ORR pathways are scanned and the ORR profiles for each pathway are calculated as shown in Figure 3-5. For the armchair edge box, step 3→4 shows little uphill even though the dissociation barrier is low. Free energy profile along the ORR reaction processes at the armchair and zigzag ends, and the atomic configurations in each process.
Hydrogen evolution reaction
- Introduction
- In situ Electrochemical Activation of Atomic Layer Deposition coated MoS2 Basal Planes
- Enhanced Electrocatalytic Activity by Chemical Nitridation of 2D Titanium Carbide MXene
- Conclusion
In situ electrochemical activation of atomic layer deposition coated with MoS2 basal planes for efficient hydrogen evolution reaction MoS2 basal planes for efficient hydrogen evolution reaction. Molybdenum disulfide, MoS2, composed of active edge sites and a catalytically inert basal plane, is a lifting catalyst that replaces noble metal-based catalysts such as Pt for electrochemical HER catalysis. Since MoS2 is a 2D material and the basal plane consists of the majority of MoS2 layers, the activation of the basal plane is an important challenge to improve the performance of HER.
Herein, the MoS2 basal plane is activated when the basal plane is distorted, such as convex or concave distortion. Improved electrocatalytic activity by chemical nitriding of 2D titanium carbide MXenes for hydrogen evolution Carbide MXenes for hydrogen evolution. The titanium carbide MXene is one of a promising material to replace the previous Pt catalyst.
For free space and the aluminum-treated surface, the Gibbs free energy for hydrogen bonding is strongly positive, 2.61 eV and 3.99 eV, respectively. On the other hand, the nitrogen-treated surface has not only low formation energy, 0.97 eV, but also low Gibbs free energy, -0.34 eV. MXene titanium carbide structure configuration and Gibbs free energies for hydrogen bonding at different reaction sites.
For the high-performance HER catalyst, the thermodynamically treated Gibbs free energy for hydrogen bonding ranges to 0.1eV to replace the Pt catalyst. The basic concept to improve HER performance is the same, which is to create a negatively charged surface. Both treatments cause the Gibbs free energy for TAJ to lie in the correct region.
Anharmonicity in molecular vibration
- Introduction
- Potential energy surface
- Classical model
- Harmonic and anharmonic frequency
- Amplitude effect
- Molecular dynamics
- Conclusion
In this thesis, the target molecules are methane for the stretching mode example and carbon dioxide for the bending mode example. For the case of carbon dioxide, the potential energy surface for the bending mode in Figure 5-1(c) shows that there is only a harmonic frequency for resonance. As shown in Figure 5-3(a), the bending angle is about 5 degrees at room temperature (300K) with no external field and shows that about 0.01 eV is accumulated in the bending mode of carbon dioxide according to Figure 5 -1 (c).
When the external field of bending mode frequency is applied, the shear of carbon dioxide intensifies. In Figure 5-3 (b), the carbon dioxide bending mode is amplified to 15 degrees, and this indicates that approx. 0.2 eV in the carbon dioxide bending state. As Figure 5-5 depicts, the responses to external field correspond to the carbon dioxide case.
-O degree trajectory in carbon dioxide in Figure 5-1(b) for (a) room temperature and (b) with external field at CO2 bending frequency (20.008 THz). Frequency scan for a methane molecule. dmax and harmonic indicate the maximum for C-H displacement and frequency when respectively dmax shows the highest value. a) The C–H trajectory for long-time scale molecular dynamics simulation with single harmonic frequency field, and (b) the Fourier transform of a function of the C–H trajectory. For the carbon dioxide case, there is only harmonic frequency that can affect the reaction.
The rate determining step for carbon dioxide capture or transformation is adsorption of carbon dioxide on catalytic surface step. 36] Usually, the carbon dioxide is non-polar gas and difficult to activate on a catalytic surface, and the energy barrier between physisorption and chemisorption state is the main obstacle for carbon dioxide adsorption. In Figure 5-8, the carbon dioxide leaves the Cu (110) surface without any external light pulse.
From the results of the molecular dynamics simulation, the first rate-determining step for the capture of carbon dioxide can be easily overcome by using external light pulses. The laser pulse with single harmonic frequency of O-C-O bending mode of carbon dioxide enhances the scissor motion of carbon dioxide, and this activation aims to adsorb the carbon dioxide molecule on a catalytic surface such as Cu (110).
Summary
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Rabitz, “Selective Bond Dissociation and Rearrangement by an Optimally Tailored High-Field Laser Pulse,” Science, 292, no. Ashihara, “Ground-state molecular dissociation in the condensed phase using plasmonic field enhancement of chirped mid-infrared pulses,” Nature Communications, 10, p. Lee, “Selective and sensitive detection of metal ions by energy transfer-based plasmonic resonance nanospectroscopy”, Nature Nanotech., 4, pp.
Acknowledgements
