Development and Application of First-Principles Techniques for the Study of Electron Transfer Properties in Energy Materials

Abstract

Electron transfer reactions are fundamental in energy and environment applications. Understanding such processes allow us to better design and optimize the performance of the materials used for energy and environment applications such as solar energy conversion and catalysis. However, the theoretical modelling of electron transfer reactions had proved difficult especially at the mean-field level of theory of the density functional theory (DFT) as electron transfer reactions involve the calculation of excited state properties.

In this thesis, I develop and implement a new simulation technique based on the constrained DFT (CDFT) to directly control the oxidation state of a transition metal ion. This approach called the oxidation-state constrained DFT (OS-CDFT), is then used to study electron transfer processes in systems containing transition metal ions. The calculation of forces due to the constraint is implemented to determine the reorganization energies and driving forces of electron transfer reactions. Additionally, the calculation of electronic coupling is also implemented by explicitly constructing the initial and final diabatic states. After the introduction and a discussion on methodologies in Chapter 1 and 2, respectively, the implementation and verifications of the OS-CDFT method for electron transfers in several prototypical systems including the aqueous ferrous-ferric self-exchange, and in polaron hopping in anatase and bismuth vanadate are discussed in Chapter 3. Additionally, OS-CDFT is also used to study the origins of the color of blue sapphires. The result from the OS-CDFT calculations suggests that photoexcited electron transfer between Fe–Ti pair impurities absorbs red light, which explains the characteristic blue color of the crystal.

I then apply this technique to study numerous electron transfer reactions. In Chapter 4 of this thesis, I study the effect of S and Mo vacancies on the rate of electron polaron hopping in monolayer and bulk molybdenum disulphide (MoS₂) to explain the drop in electron mobility when MoS₂ is thinned to a monolayer. I find that electron polaron hopping toward an S vacancy is highly exothermic which puts the reaction in the Marcus inverted regime. This explains the drop in electron mobility in monolayer MoS₂ as standard fabrication techniques tend to introduce high densities of S vacancies. In contrast, bulk MoS₂ is not affected as most S vacancies in it are found on the first few layers near the surface. Furthermore, I find that electron transfers near the S and Mo vacancy have asymmetric reorganization energies that prompt the development of a generalization of electron transfer theories for systems with asymmetric reorganization energies.

In Chapter 5, I explore the nature of electron and hole polaron hopping in mixed valence cobalt(II,III) oxide (Co₃O₄) in pristine condition and with an oxygen vacancy. Cobalt ions can be found in tetrahedral and octahedral configurations in Co₃O₄. I find that the tetrahedral site repels electron polarons because hopping from a neighboring octahedral site towards the tetrahedral site is highly endothermic while hopping away to a neighboring octahedral site is nearly instantaneous. Hence, electron polarons travel through bulk Co₃O₄ using a pathway through only octahedral Co. In contrast, while the octahedral pathway remains the dominant pathway for hole polaron transport, the pathways involving both the octahedral and the tetrahedral sites are still viable with about two thirds the rate of the octahedral pathways. Next, I study the oxygen evolution reaction (OER) in a Co₃O₄ slab. Since the OER process releases electrons to the anode, a Co₃O₄ anode would have excess electrons in the form of polarons. Therefore, I investigated the effects of electron polarons on the OER reaction in a Co₃O₄ anode. The electron polaron is localized at or near the active site by changing the oxidation state of the target Co atom using OS-CDFT.

Aside from electron transfer problems, I also explore the usage of OS-CDFT for studying the low-lying excited states of transition metal systems. This is discussed in Chapter 6 of this thesis. I first use OS-CDFT to calculate the optical band gap of transition metal oxides and dopant state energies of doped anatase. Here, OS-CDFT is used to constraint an electron from the valence band maximum (VBM) to either the conduction band minimum (CBM) or the dopant state. This approach is more computationally accessible than higher order approaches such as the hybrid functional or GW calculations. Hence, this approach is potentially useful for screening semiconductors for materials design.

In the final chapter, I present conclusions regarding the development and applications of the OS-CDFT method along with a discussion on future outlooks. In summary, I develop and implement the OS-CDFT technique for the study of electron transfer processes and low-lying excited states in various transition-metal-containing systems which contributes an important understanding of these properties.

Date of Award	31 Aug 2022
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Patrick SIT (Supervisor)