| We have faced with the severe energy and environmental crisis since the 21st century.So a new type of energy source is urgently needed to be developed in oder to solve the growing problem of energy demand.Hydrogen energy is a new kind of pollution-free renewable energy source which can replace the traditional coal and oil fuels.So hydrogen energy has great development potential.Scientists have been looking for a simple and cheap way to produce hydrogen.In recent years,with the in-depth study and utilization of solar energy,researchers began to produce hydrogen and oxygen through the solar photocatalytic water splitting reaction.This has become an important and promising research direction for the production of hydrogen energy.The key to realize photocatalytic water splitting is searching an excellent photocatalyst.Due to the visible-light-responsive ability and non-metallic nature,as a highly stable,cheap,non-toxic two-dimensional(2D)semiconductor photocatalyst,graphitic carbon nitride(g-C3N4)has attracted wide attention.In order to utilize g-C3N4 more efficiently,we first need to have a clear understanding of its photocatalytic mechanism.Excited-state reactions are involved during the photocatalytic water splitting process.So the adsorption configurations,energy barriers and energy level alignments in the ground state and excited state are analyzed comprehensively when we studied the mechanism of photocatalytic water splitting reaction on g-C3N4.These ground-state and excited-state problems were mainly studied by GW method and Bethe-Salpeter equation(BSE)within many-body Green's function theory(MBGFT).We use the GW method to calculate the accurate energy level alignments of the systems.The state-of-the-art BSE we used is capable of calculating the excited-state properties of periodic systems.The optical absorption spectra,energy values and electron/hole spatial distributions etc in excited state were calculated by BSE under the condition of considering the electron-hole interaction.As most other photocatalysts,the photocatalytic efficiency of water splitting reaction on g-C3N4 is not satisfactory.So seeking an improved structure of g-C3N4 is the other emphasis for research.On the one hand,we studied the improved structures of g-C3N4 with excellent performance and special configurations that have already been experimentally synthesized,in order to explain some outstanding phenomena in experiments using high-accuracy first-principles methods at molecular level.On the other hand,we also tried to design new types of improved structures of g-C3N4 according to the ultimate requirements of an ideal photocatalyst proposed by scientists.And the electronic and optical properties of the improved photocatalyst were studied systematically in theory.We hope to provide more choices for the experimental synthesis of the improved structures of g-C3N4.Therefore,we mainly studied about the mechanism of photocatalytic water splitting reaction on g-C3N4(Chapter 3&4)and the improved structures of g-C3N4(Chapter 5&6)in this dissertation.The main contents and innovative contributions are listed as follows:1.During photocatalytic water splitting process,the behavior of photogenerated holes on metal-free semiconductor is still an open and controversial issue.The photogenerated holes will migrate to an oxygen-containing species when photocatalytic water splitting reaction occurs.Hydroxyl anion(OH-)is always present whether in water solution with self-ionization phenomenon or in alkali solution.So we also included the contribution of OH-except for water molecules in order to expore the effect of the existence of different kinds of oxygen-containing species on the behavior of photogenerated holes.We think that OH-can capture the free photogenerated holes according to our MBGFT study on the free photogenerated holes and coupled electron-hole pairs at the interfaces of composite systems.But the holes will finally relax onto substrate g-C3N4 with lower excited-state energy via no-adiabatic transition if the coupling between electron and hole remains.In addition,the energy levels of OH-are always higher than the energy levels of g-C3N4 when OH-and g-C3N4 touch to each other directly.But the probability of the energy levels of OH-locating at the valence band maximum reduced if there are water molecular layers between OH-and g-C3N4.This indicates that the photogenerated holes tend to gather at the interface of the system in order to promote the photocatalytic water splitting reaction.2.In recent years,with the rapid increase of experimental researches related to water splitting reaction on g-C3N4,it is urgently to reveal the entire water splitting reaction process in depth from the ground-state and excited-state aspects.We found that the main active site of the whole water splitting reaction process is the nitrogen atom of g-C3N4.The species adsorbed on nitrogen sites are much more stable at excited state,but some species adsorbed on carbon sites may much more stable at ground state.So we speculate that some strategies that can eliminate the interference of carbon atom in experiment will promote the reaction at some extent.More importantly,we found that there are two non-adiabatic potential energy surface crossings during the production of hydrogen at excited state.The first potential energy surface crossing appears during the process of the excited-state proton transfer reaction.The second potential energy surface crossing appears during the process of the adsorption of OH radical at excited state.The system will return to ground-state after the second potential energy surface crossing.Our calculations indicate that the energy barriers of some follow-up ground-state reactions are not high at all.So we think that not all photocatalytic water splitting reaction steps on g-C3N4 would occur at excited state and involve the transfer of electrons and holes.3.Scientists have been seeking an ideal photocatalyst to realize the broad adsorption of solar spectrum,overall water splitting and the obvious separation of electron-hole.In recent years,an emerging covalent organic framework(COF)material provide a probability for the structure controlling and functionality designing.So we tried to construct a COF based on g-C3N4 in order to precisely adjust the electronic and optical properties of g-C3N4,and to fulfill the three ultimate requirements of an ideal photocatalyst.The adjust ways include changing the quantity of C?C triple bonds,functionalizing the C?C triple bonds and increasing the layers of structures.Our results show that these COFs can produce charge transfer excitons which are dipole allowed and loosely bound.The exciton binding energies of these excitons are comparable to the thermal energies of room-temperature.And the separation distance of electron and hole in these excitons reaches to about 10 nm.Compared to the pristine g-C3N4,the exciton binding energies in these COFs are two orders of magnitude lower,and at the same magnitude with the 3D inorganic semiconductor.This is a significant feature of our 2D COFs different from traditional 2D materials.4.Experimental studies found that embedding graphitic carbon rings(Cring)into g-C3N4 can show high quantum efficiency.Cring-C3N4 is a novel ?-conjugated in-plane heterostructure which is different from the common doping/defect structures and van der Waals heterostructures.So the electronic and optical properties of different kinds of Cring-C3N4 structures were calculated by GW+BSE method in Chapter 6.We found that the band gap reduces greatly compared to the pristine g-C3N4 due to the enhanced?-conjugated structure in Cring-C3N4.A lot of charge transfer excited states appear in Cring-C3N4 as result of the formation of in-plane heterostructure.This may account for the good electron-hole separation efficiency and high quantum efficiency as found in this material experimentally.Our calculations show that the electronic levels and gaps can be adjusted by tuning the ratio between Cring and g-C3N4.and the driving force for oxygen evolution reaction can also be adjusted by functionalizing Cring-C3N4. |
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