Two-Dimensional Polymers for Photocatalytic Overall Water Splitting:Design,Synthesis and Mechanism Studies

Author:Wang Lei

Supervisor:xu hang xun


Degree Year:2019





Energy and environmental crisis has attracted global attention and the development of clean conversion route to produce renewable energy has become the focus of current scientific research.Given that solar energy and water are the most abundant resources on the earth,converting solar energy into storable chemical fuels via photocatalytic overall water splitting through semiconductors is a potentially viable route to address the global energy crisis and environmental issues.Compared to the traditional inorganic semiconductor materials,metal-free semiconducting polymers representing an emerging class of photocatalyts for photocatalytic overall water splitting as their optical and electronic structures can be conveniently designed and well controlled in molecular level.In particular,polymer semiconductors with two-dimensional(2D)planar conjugated structures offers distinct features in catalytic applications and can be flexibly designed and modified,which stands out as the most intriguing polymeric materials for photocatalytic energy conversion.This thesis mainly focuses on the facile fabrication of high-performance 2D polymer-based photocatalysts for photocatalytic overall water splitting.Meanwhile,the mechanism of catalytic reaction is discussed in depth based on the structure-activity relationship.The main results can be summarized as follows:1.Developed unique 1,3-diyne-linked 2D conjugated polymer photocatalyst(PTEPB and PTEB)for efficient visible-light-driven overall water splitting.Our results indicate that PTEPB and PTEB exhibit suitable band gaps for simultaneously generating stoichiometric amounts of H2 and O2 under visible light irradiation.More importantly,the ultrathin sheet-like structure enables the photogenerated excitons to immediately reach the polymer surface to drive redox reactions,suppressing undesirable electron-hole recombination.Furthermore,the experimental results and first-principles calculations also indicate that the 1,3-diyne covalent linkage could serve as the active sites for water splitting.2.The water dissociation pathways on 2D conjugated polymer photocatalysts(PTEPB and PTEB)under visible light irradiation are unveiled using in situ spectroscopic techniques.By correlating to the computational results,the possible active sites that are responsible for H2 or O2 evolution reactions can be accurately identified.Meanwhile,it is shown that the water dissociation pathways are different for PTEPB and PTEB,exemplifying the significance of structural control in developing polymer photocatalysts.3.Based on the design of semiconductor electronic structure,we successfully demonstrate that 2D aza-CMP is a unique low-bandgap metal-free photocatalyst with suitable band edge positions for photocatalytic O2 evolution.Our results indicate that aza-CMP nanosheet is very efficient in photocatalytic O2 generation,exhibiting impressive O2 evolution efficiency compared to other metal-free polymer photocatalysts.This is also the first time that a metal-free polymer photocatalyst is shown to be able to catalyze O2 evolution reaction in the NIR region.Furthermore,first-principles calculations were carried out to reveal the possible active sites for the O2 evolution process,providing more insights into the physicochemical behaviors of the 2D aza-CMP photocatalyst.4.Based on Principles photocatalytic reduction and oxidation processes at different catalytically active centers in natural plants,we developed aza-CMP/C2N van der Waals heterostructures as Z-scheme systems for efficient visible-light-driven overall water splitting.Our results indicate that efficient charge transfer between the interfaces of aza-CMP/C2N Van der Waals heterostructures is crucial to achieve high photocatalytic activity,the solar-to-hydrogen(STH)conversion efficiency was measured to be 0.23%.By introducing RGO as the solid electron mediator,the STH value could further be increased to 0.40%.Furthermore,we show that the STH value could increase to 0.73%by depositing cocatalysts on aza-CMP and C2N nanosheets,respectively.5.Inspired by the mechanism of molecular assemblies in natural photosynthesis,we demonstrate that 2D crystalline self-assembled organic(OPB)nanosheets can split pure water into H2 and O2 under visible light irradiation.Both computational and experimental results show that these self-assembled 2D crystalline nanosheets exhibit a bandgap and an electronic structure suitable for overall water splitting under visible light irradiation.Meanwhile,we reveal that the photocatalytic activity is closely related to the crystalline structure of the self-assembled OPB nanosheets.