Abstract
Graphitic carbon nitrides (g-CN), composed of heptazines, show promise as photocatalysts for surface reactions and energy conversion. Despite the potential, their suboptimal visible light absorption and poor electrical conductivity limit their photocurrent density and catalytic activity. Carbon-rich g-CN, achieved by N-vacancies modification and Carbon (C) atom doping, has been proposed to enhance its photocatalytic performance in the photochemical NOX decomposition and water splitting, respectively. However, the underlying mechanisms of this enhancement require in-depth investigation. In this thesis, we utilized g-CN and heptazine models to explore photoinduced charge transfer and reaction pathways. The models have been demonstrated in recent theoretical studies to be suitable for revealing molecular-level mechanisms in the photocatalytic process of g-CN.In Chapter 1, the background, including structure and property, of g-CN has been summarized. The applications of g-CN as the photocatalysis in surface reactions and energy conversion are introduced. Followingly, Chapter 2 provides a brief introduction to the methods employed in this study, such as the density functional theory (DFT) and time-dependent DFT (TD-DFT), etc.
In Chapter 3, the principles of photocatalysis and surface reactions, introducing a graphitic carbon nitride (g-CN) film with N-vacancies (g-CNNV) for efficient NOX removal, have been studied. Utilizing DFT and TD-DFT calculations, we reveal the decomposition mechanism of NO/NO2 gas on g-CNNV. The process involves N-O bond cleavage, N occupancy in N-vacancy, oxygen formation, and subsequent illumination-driven oxygen desorption through photochemical processes. The illuminated g-CNNV film demonstrates effective NOX decomposition into harmless oxygen, suggesting its potential as a green and economical strategy for NOX removal when applied to building exteriors or chimneys. The synergistic effects of sunlight and surface defects make the g-CNNV film an eco-friendly and cost-effective photocatalyst for NOX removal. Additionally, considering the effects of temperature and entropy, the results of Gibbs free energy proved that the impact of temperature on the NO decomposition process is less significant for the g-CNNV film compared to NO2. This study contributes to understanding the strong attractions of N vacancies in g-CN film to NOX molecules and their fundamental photo-assisted decomposition processes.
In Chapter 4, this study presents a thorough exploration of electronic structure and excited-state nonadiabatic dynamics in water-splitting, focusing on its reaction mechanism in the ground and excited states on pure g-CN and C-doped g-CN (C/g-CN). It is well demonstrated that C atoms can be integrated into the g-CN structure, replacing either a three-coordinated N atom (N3C) (C(N3C)) or a two-coordinated N atom (N2C) (C(N2C)). The simulations reveal that the C dopants at N2C improve the activity for water-splitting in the ground state. Additional results highlight the impact of a π-electron conjugated system introduced by sp2 hybridized carbon atoms at N3C, leading to enhanced absorption and activity of C/g-CN under Vis-Blue lamps. The C dopants at N3C improve the distribution of photoinduced holes at active N atoms, resulting in H atom detachment from H2O via the electron-driven proton transfer (EDPT) process. Dynamic simulations reveal the nature of photoinduced charge carriers, showcasing transitions among local excitation (LE), hybrid LE−charge transfer (CT), and CT states. The synergistic effect of carbon dopants on enhanced catalytic activity of C/g-CN is elucidated.
To summarize, this thesis studied reaction mechanisms and photoinduced charge transfer in g-CN and carbon-rich g-CN films. The findings in this study are expected to provide novel insights into the process of photo-assisted reactions.
| Date of Award | 7 Aug 2024 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Ruiqin ZHANG (Supervisor) |
Cite this
- Standard