Photocatalysis is a promising technology that can effectively convert the solar energyinto sustainable green energy. However, theoretical studies on the molecular mechanisms of photocatalytic reactions are rare. This thesis is devoted to investigate several typical photocatalytic reactions on the surfaces of the most popular photocatalysis TiO2 with density functional theory. We start our study with the characterization of both the free and trapped hole on the surface generated by the light. The oxidation of physisorbed H2O molecule by the hole trapped at bridge oxygen on rutile TiO2(110) surface has been studied. The hole is found to transferto the molecule via the anti-bonding orbital as a result of the hybridization between the hole orbital and the HOMO of the molecule. The energy and symmetry mismatching between the trapped hole orbital and the HOMO of the molecule explains why the trapped hole cannot directly transfer to the chemisorbed H2O molecule. On the other hand, we have found that the chemisorbed H2O moleculecan be more efficiently oxidized by the free hole with a lower barrier and higher reaction energy compared to the oxidation by the trapped hole. In this reaction, the free hole is transferred to the chemisorbed H2O after the dissociation. This is different from the oxidation of chemisorbed H2O on anatase TiO2(101) surface by free hole, in which the hole is transferred concertedly with the dissociation of themolecule.     In order to understand the hole scavenger ability of organic molecules, the oxidation of three small organic molecules (CH3OH, HCOOH and HCOH) onanatase TiO2(101) surface has been systematically investigated. The concerted hole and proton transfer is found for all these molecules. The calculations suggestthat both kinetic and thermodynamic effects need to be considered to correctly describe the hole transfer process. The order of hole scavenging power is found tofollow: HCOH &gt; HCOOH &gt; CH3OH &gt; H2O, which agrees well with experiments.     Photo-selective catalytic reduction of the NO by NH3 and the photooxidationof CO by O2 are closely related to the environment application. Both reactionsinvolve the formation and/or breaking of non R–H bonds. The mechanism for the photoreduction of NO proposed by experiment has been verified by our calculations.The role of the hole is to oxidize the adsorbed NH3 into ·NH2 radical, which canform a NH2NO complex with a gaseous NO molecule easily. The photooxidation of CO by O2 is the first multi-step photoreaction we ever studied. By combining thepotential energy surfaces at the ground and excited state we have found that thehole and electron both take part in the reaction. A molecular mechanism which is in consistent with various experiments is proposed.     These studies show that density functional theory is a powerful tool for studying the photocatalytic reaction. Apparently, more work needs to be done in orderto improve the performance of the existing materials and to design new ones thatcan take advantage of the solar light more efficiently / <p>QC 20140522</p>

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16854号 / 工博第3575号 / 新制||工||1540(附属図書館) / 29529 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 邑瀬 邦明, 教授 中村 裕之 / 学位規則第4条第1項該当

New layered cathode materials, Li₀.₉Ni₀.₄₅Ti₀.₅₅O₂, were synthesized by means of ion-exchange from Na₀.₉Ni₀.₄₅Ti₀.₅₅O₂. The degree of cation disordering in the material depends critically on the synthesis conditions. Longer times and higher temperatures in the ion-exchange process induced more cation disordering. However, the partially disordered phase showed better capacity retention than the least disordered phase. First principles calculations indicated this could be attributed to the migration of Ti⁺⁴ into the Li layer during the electrochemical testing, which seems to depend sensitively on the Ni⁺² -Ti⁺⁴ configuration in the transition metal layer. The poor conductivity of this material could also be the reason for its low specific capacity according to the Density of States (DOS) obtained from first principles calculations indicating that only Ni participates in the electronic conductivity. / Singapore-MIT Alliance (SMA)

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Based on first principles Density Functional Theory calculations, we have investigated for possible paths for engineering electronic band structure of Transition Metal Dichalco- genides (TMDs). We have considered two approaches which have shown to be promising for engineering electronic bands of TMDs: substitutional chemical doping and heterostruc- turing. All the calculations are done using first principles Density Functional Theory as it is implemented in Quantum Espresso package. Two possible substitutional doping meth- ods for MoS2 are considered in our calculations; cation doping where Mo is substituted by metal atoms and anion doping where Nitrogen and halogen group atoms take the posi- tion of S-sites. We observe the n-type characteristics for halogen group doping and p-type characteristics for Nitrogen group doping at S site. Similarly, we observe these bipolar characteristics when substituted by the transition metal elements (4d elements in the peri- odic table) at Mo site. Our results on doping monolayer MoS2 are in agreement with those results obtained by Dolui et al. for similar systems. Our work is extended to explore the effect of substitutional doping in bilayer MoS2. We observe the promising bipolar char- acteristics on doping while the magnitude of the band gap decreases upon the controlled S-site doping with F and As. In the second part, we considered two types of heterostructuring; Van der Waals heterostructures, and lateral heterostructures. In Van der Waals heterostructures, a direct band gap is observed with a physical separation of charges into two layers from orbital isosurface plots. We present a brief overview of the folding of energy bands in supercell approach. Using heterogeneous supercell approach, we studied the electronic properties of a mixed system of MoS2 -WS2 . The separation of the charges into the two sections shows that our MoS2 -WS2 in-plane heterostructure shows a potential for a pn junction. These systematic studies of the doped and heterostructures of TMDs can be useful for device applications.

The utility of first-principles methods in the study and prediction of binary alloys is showcased by three detailed studies. In particular, the T = 0K cluster expansion methodology in conjunction with finite temperature statistical modeling by a Monte Carlo method is used to study two systems of practical interest, Mg-Li (magnesium-lithium) and Rh-W (rhodium-tungsten). Also, an empirically-informed, high-throughput approach to crystal structure prediction is shown by a study of the Pt$_8$Ti (the Pietrokowsky phase) phase and a broad and detailed analysis of binary Mg-X phases in 39 systems (X=Ag, Al, Au, Ca, Cd, Cu, Fe, Ga, Ge, Hf, Hg, In, Ir, K, La, Li, Pb, Pd, Pt, Mo, Na, Nb, Os, Rb, Re, Rh, Ru, Sc, Si, Sn, Sr, Ta, Tc, Ti, V, W, Y, Zn, Zr). These results are presented in the form of three publications (the first two are in print, and the third is nearing submission) co-authored with Gus Hart and Stefano Curtarolo.

Previous experimental studies have reported wave-like transport of heat in a small number of material systems, such as superfluids like helium II and crystal solids like bismuth. This phenomenon was henceforth referred to as 'second sound'. These rare observations of second sound are partly due to the challenge of obtaining accurate theoretical predictions. In this work, we use an ab-initio framework to study phonon hydrodynamics in 3D crystal fluorides and alkali hydrides, including sodium fluoride (NaF), lithium fluoride (LiF), lithium hydride (LiH), and sodium hydride (NaH). Moreover, we predict the existence of phonon hydrodynamics regime in bilayer graphene systems, including AA-bilayer graphene and AB-bilayer graphene. First, we obtain the second and third-order interatomic force constants using first-principles calculations, which are based on density functional theory (DFT). Secondly, we calculate the lattice thermal conductivity and phonon scattering rates by solving the Boltzmann transport equation (BTE). Thirdly, we apply Guyer's condition to show the phonon hydrodynamics regime based on the average Normal, Umklapp, and Boundary scattering rates. Finally, we examine the effect of different pseudopotentials on the thermal, electronic, and mechanical properties as well as the phonon hydrodynamics regime. In addition, we report the effect of isotopes on the lattice thermal conductivity and phonon hydrodynamics regime. Our calculations predict the existence of the second sound in NaF at 15 K and 8.3 mm characteristic length, consistent with previous experimental work. Based on Guyer's condition, the hydrodynamic window was determined in terms of characteristic lengths (~10² - ~10⁸ nm) and temperatures (up to ~80 K) for fluorides and alkali hydrides. On the other hand, second sound in 2D materials has been predicted to exist at much higher temperatures relative to 3D materials. We report the existence of a second sound for AA-bilayer graphene and AB-bilayer graphene above room temperature at a characteristic length of ~100 nm.

We developed and implemented a first-principles based theory of the Landauer ballistic conductance, to determine the transport properties of nanostructures and molecular-electronics devices. Our approach starts from a quantum-mechanical description of the electronic structure of the system under consideration, performed at the density-functional theory level and using finite-temperature molecular dynamics simulations to obtain an ensemble of the most likely microscopic configurations. The extended Bloch states are then converted into maximally-localized Wannier functions to allow us to construct the Green’s function of the conductor, from which we obtain the density of states (confirming the reliability of our microscopic calculations) and the Landauer conductance. A first application is presented to the case of carbon nanotubes. / Singapore-MIT Alliance (SMA)

First principles computation can be used to investigate an design materials in ways that can not be achieved with experimental means. We show how computations can be used to rapidly capture the essential physics that determines the useful properties in different applications. Some applications for predicting crystal structure, thermodynamic and kinetic properties, and phase stability are discussed. This first principles tool set will be demonstrated with applications from rechargeable batteries and hydrogen storage materials. / Singapore-MIT Alliance (SMA)

We determined the Landauer ballistic conductance of pristine nanotubes at finite temperature via a novel scheme that combines ab-initio molecular dynamics, maximally-localized Wannier functions, and a tight-binding formulation of electronic transport in nanostructures. Large-scale ab-initio molecular dynamics simulations are used to obtain efficiently accurate trajectories in phase space. The extended Bloch orbitals for states along these trajectories are converted into maximally-localized orbitals, providing an exact mapping of the ground-state electronic structure onto a short-ranged Hamiltonian. Green's functions, self-energies, and ballistic conductance can then be obtained for any given configuration, and averaged over the appropriate statistical ensemble. / Singapore-MIT Alliance (SMA)