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共价有机框架（COFs）因其独特的周期性多孔结构和结构易调控的优点，在光催化等领域具有广阔的应用前景。本研究利用基于第一性原理的多体格林函数理论方法计算了三种三嗪基COFs的电光学性质，探究供体构筑单元、多层堆叠在调控体系电子结构、激发能与激子特性的作用。研究发现，供体构筑单元的改变会导致体系VBM和CBM能级发生不同程度的变化。由TPA更替为TFPB或TFPT时，带隙、激发能和激子束缚能均变大，而TFPB和TFPT的更替则对带隙、激发能和激子束缚能影响不大。所有COFs的能级与激发能随层数的变化规律相同。单层中不同构筑单元对COFs电子结构、激发能与激子特性的影响足以反映多层COFs及体相由构筑单元不同造成的影响，这些研究结果对COFs设计和改性有着至关重要的意义。Covalent organic frameworks (COFs) have been a potential candidate for applications in photocatalysis due to its periodically porous structures and tunable structure. The COF skeletons consisted of different building blocks may result in different performance. Investigating the effects of different building blocks on energy levels and excitons for COF can provide some insight for designing excellent COF catalysts. Based on the first-principles many-body Green’s function theory, the electronic structures and optical properties of the three donor-acceptor COFs by employing the monomer 2,4,6-trimethyl-1,3,5-triazine (TMT) as the key acceptor subunit and the trigonal aldehyde monomers including the tris(4-formylphenyl) amine (TPA), 1,3,5-tris(4-formylphenyl) benzene (TFPB) and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) as the donor subunit are calculated in this work. Regulation of the donor unit and interlayer interactions on the electronic structures and excitonic properties are analyzed. The results show that the valence band maximum (VBM) and conduction band minimum (CBM) energies of the system are varied by the alteration of donor subunit. From TPA to the TFPB or TFPT, the bandgaps of the system increase, the light absorption blue shift, and the exciton binding energies gradually increase. There is little effect on the band gap and excitation energy by replacing the TFPB with the TFPT. Among the three COFs, the positions of both CBM and VBM of the TFPT-TMT COF only match well with the chemical reaction potential of H2/H+ and O2/H2O, which is capable of photocatalytic overall water splitting. But the photocatalytic performance for the TFPT-TMT COF might be inhibited by the higher exciton binding energy. The exciton for the TPA-TMT COF is easier to separate according to the exciton distributions and the exciton binding energy. The effect of different building units on the electronic structure, excitation energy, and excitonic properties of COFs in monolayer COFs are in line with that in multilayer and bulk COFs. The variation of the energy levels and excitation energies of all the three COFs as the number of layers are consistent. With the increasing number of layers, the VBM and CBM shift up and down with respect to the vacuum level, respectively. The band gap gradually decreases. The energy tend to decrease slower with the more layer. The exciton energy for multilayer COFs is close to the bulk state. These results are significant to design and modify COFs.
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Keywords:
- Covalent organic frameworks /
- Electronic structure /
- exciton /
- Many-body Green’s function theory
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