Computational Simulation And Design Of Low Dimensional Covalent Organic Polymers For Photocatalytic Water Splitting

Sustainable development of human beings in the future mainly depends on continuous energy resource and good environment.Reliance on fossil fuels such as coal,oil and natural gas is at the heart of the environmental pollution and energy shortages that we face today and in the future.Fossil energy is not renewable,and the carbon dioxide,sulfur and nitrogen oxides emitted by its combustion will lead to greenhouse effect,air pollution and other environmental problems.Therefore,the search for environmentally friendly,renewable energy is the key to achieve sustainable development of human beings.In a broad sense,almost all forms of energy on earth eventually come from solar energy.If the conversion and storage of solar energy can be realized efficiently,the energy problem will be solved.Hydrogen is an efficient clean energy with high energy density,and the combustion product is water,which can be recycled and is very environmentally friendly.Therefore,producing hydrogen from water splitting by means of photocatalysts is an ideal way to convert and store solar energy and obtain efficient and clean energy.During the past nearly 60 years,inspied by the photodissociation of water on the surface of titanium dioxide by Fujishima et al.in 1972,a large number of catalytic systems were used for the study of water splitting.Howerve,the practical application of water splitting is still limited by the low solar-to-hydrogen conversion effiency and expensive farbrication of photocatalyst.Compared with inorganic catalysts,organic polymer catalysts have been extensively studied in recent ten years,such as g-C3Na.Polymer nanomaterials,especially organic polymeric two dimensional materials with good periodic,such as Covalent Organic Frameworks(COF),Conjugate Microporous Polymers(CMP),these kinds of materials can be controllablely synthesized and prepared by experiment from bottom to top method,composed of abundant non-metallic elements,with large specific surface area and active sites and high chemical stability,are potential catalytic materials.However,the application of this kind of materials in photocatalytic water splitting has attracted attention until recent years.Compared with a large number of low-dimensional polymers synthesized experimentally,only a small proportion of them have been used in the study of water splitting.At the same time,compared with the abundant and mature synthetic methods in the experiment,it is relatively difficult to predict and design the polymer materials for photocatalytic water splitting and study the reaction mechanism in the experiment,which requires a lot of financial and material resources.Thanks to the rapid development of computer technology,first principle calculations based on Density Functional Theory(DFT)play an increasingly important role in material simulation and design.For photocatalysis,DFT can accurately predict the basic electrical,optical,mechanical and thermal properties of materials.Especially for organic polymers containing only non-metallic elements such as carbon,hydrogen,oxygen and nitrogen,DFT theoretical calculations can quickly and accurately predict their properties.At the same time,the complex electrochemical reactions in photocatalytic process at atomic level can also be studied by theoretical calculation.In this paper,the application of low dimensional covalent organic polymer materials in photocatalytic water splitting was investigated by using DFT calculations.This paper is divided into five chapters.The first chapter briefly introduces the the basic principle and approximate method of DFT,the basic principle of photocatalytic water splitting and the computational method and theoretical model for water splitting used in this paper,as well as the design criteria and naming of low dimensional covalent organic polymers.In chapter 2,a low band gap conjugated microporous polymer(aza-CMP)was investigated for photocatalytic oxygen evolution reaction under visible and near-infrared light and to the best of our knowledge,it is the first work of photocatalytic oxygen evolution using metal-free catalysts in infrared light.In chapter 3,a class of polymers containing diacetylene groups is reported,including two conjugated microporous polymers(PTEPB and PTEB)and a one dimensional wire.Among them,PTEPB and PTEB can utilize visible light to achieve overall water splitting via a four-electron pathway without using sacrificing agent and cocatalyst.Using DFT theory,we studied the reaction mechanism of water splitting on the surface of PTEB and PTEPB,and the predicted reaction intermediates were confirmed by experiments.In this chapter,we also theoretically predicted that a one dimensional polymer containing dialkynyl can also be used for overall water splitting.In addition,we studied the hydrogen storage behavior of PTEB,and it is of great significance for the practical utilization of hydrogen energy to simultaneously realize hydrogen storage and hydrogen generation on a single material.Finally,two other polymers containing alkynyl groups,CTF-EDDBN and CTF-BDDBN,were synthesized by introducing the alkynyl group into CTF-2,where CTF-BDDBN can be regarded as the analogues of PTEPB.Photocatalytic oxygen reduction and water oxidation via a two-electron pathway were simultaneously realized on these two materials,namely producing H2O2,which can also be used as energy.In chapter 4,a class of two dimensional(2D)covalent organic frameworks,L-TST,based on molecular design is proposed.They have beeter optical absorption than g-C3N4,PTEPB and PTEB,and can also achieve overall water splitting.At the same time,this chapter also designed a z-type system for photocatalytic water splitting,namely Ao-TST@aza-CMP,whose theoretical maximum light conversion efficiency exceeds 20%.In the last chapter,based on the existing 2D COF materials have been synthesized by experiment,we used DFT calculations to screen out about two hundred COFs,and found five COFs with the potential to achieve photocatalytic overall water splitting,and also obtained a large number of photocatalytic hydrogen evolution and oxygen evolution half reaction catalysts.