Computational Investigations of the Exciton Dynamics in Low-Dimensional Graphitic Carbon Nitrides

低維石墨氮化碳中激子動力學的計算研究

Student thesis: Doctoral Thesis

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Award date30 Aug 2019

Abstract

Graphitic carbon nitrides (g-C3N4) are promising materials for energy applications and catalysis in general. Since the discovery of their capability for photocatalytic activity towards hydrogen evolution from water, many attempts have been employed for improvement; still, the mechanistic features of the water-splitting reaction catalyzed by g-C3N4 are not fully understood. This thesis focusses on the mechanism of exciton generation in g-C3N4 and carbon nano-dot sensitized g-C3N4 for enhanced charge separation and the dynamics of the water-splitting with g-C3N4 based on calculations using the density functional theory (DFT) and time-dependent DFT (TDDFT). Present work provides the fundamental knowledge of the efficient sensitization of g-C3N4, clarifies the water-splitting mechanism via excited-state non-adiabatic dynamics, and offers guidance for further computational and experimental investigations to enhance the material’s photocatalytic performance.

In chapter 3, it is shown that carbon nitride quantum dots (CNQDs) possess effective localization of frontier orbitals and charge separation observed on the first excited-state energy surface compared to the graphene quantum dots (GQDs). Upon optical excitation of the electron-hole pairs in CNQDs, the hole is spatially confined to the N-atom lone-pair electron site and the electron at the C-N nonbonding π site, while there is no such electron-hole separation in GQDs, rationalizing the great potential of CNQDs for photo-catalytic applications and the promise of GQDs for luminescent applications. The fundamental mechanism of n→π* (carbon nitride) and π→π* (sp2-bonded carbon) transitions distinguish them for photocatalytic and luminescent applications, respectively.

In Chapter 4, the influence of sp2-bonded carbon nano-dots on g-C3N4 nanosheets is studied in order to understand the mechanism of enhanced photocatalytic efficiencies. GQDs-based sensitization of g-C3N4 nanosheets significantly influences the electronic structure of g-C3N4 and induces enhanced excited-state charge separation, presenting a suitable podium for improving photocatalytic activity. The fundamental transition is of π→π* nature and takes place from GQD (π-state) to g-C3N4 nanosheet (π*), inducing the electron-hole spatial separation. Concentration varied GQD sensitization of g-C3N4 nanosheets through tuning the nitrogen to carbon (N:C) ratio regulates the electron-hole separation, providing the guidance to design appropriate sensitization for controlled synthesis for enhanced photocatalytic performance.

In Chapter 5, using the excited-state non-adiabatic dynamics simulations, the molecular level picture of the decomposition of heptazine hydrogen-bonded to water molecule(s) (heptazine-water complex) into heptazinyl and hydroxyl biradical product is revealed. Hydrogen detachment from the water molecule to the heptazine occurs within tens of femtosecond. Extending the heptazine structure to an oligomer comprising of three heptazines with larger water cluster shown the same phenomena of water-splitting within the same timescale. The observation of the photorelaxation-induced water-splitting by heptazine is a proof of the principle of water-splitting reaction, which is less challenging rather than the careful deactivation of biradical product, tempting for further computational and experimental investigations for deactivation of biradical product for efficient hydrogen evolution. With findings in the above chapters considered together, it is concluded that the photocatalytic capability of g-C3N4 is determined by the exciton generation and, the water-splitting into biradical product followed by the efficient deactivation for hydrogen evolution.

The results unveil the significance of the electronic structure modification, the role of electron-hole separation on excited-state energy surface and water-splitting mechanism for energy harvesting applications with low-dimensional graphitic carbon nitrides.