Structural Regulation of D-A Conjugated Polymers and Study Its Structure-Property Relationship of Photocatalytic Hydrogen-Production Performance

Author:Wang Xue Peng

Supervisor:chen hao


Degree Year:2019





At present,the environmental pollution and energy shortage caused by the rapid consumption of fossil energy have seriously hindered the sustainable development of human society.Therefore,it is urgent to seek renewable and green energy.Hydrogen energy is considered to be an ideal clean energy because of its high combustion value and pollution-free.Compared with the traditional methods of hydrogen production,the use of semiconductor photocatalytic materials to achieve solar to hydrogen energy conversion has obvious advantages.However,insufficient utilization of visible light and low efficiency of solar-hydrogen conversion seriously restricts its practical application.Recently,D-A organic conjugated polymers,consisting of alternating electron-rich donors(D)and electron-deficient acceptors(A),have become an emerging photocatalytic material for water splitting because of its intrinsic advantages such as wide spectral response,high stability and tunable energy levels.In this dissertation,a series of D-A conjugated polymers were designed and synthesized from the three aspects:light absorption,photo-generated carrier separation,interfacial reaction,which have pivotal impact on the photocatalytic efficiency.The structure-property relationship between material structure and photocatalytic performance was revealed,which was characterized and researched by morphology,hydrogen-production performance,carrier migration behavior and theoretical calculation.Finally,the D-A conjugated polymer with excellent photocatalytic hydrogen production performance was obtained.The research content of this dissertation is as follows:1)A series of ethynyl-bridged D-A conjugated polymers were constructed based on the Sonogashira-Hagihara cross-coupling strategy with benzothiadiazole,pyrene,dibenzothiophene sulfone as electron acceptors and benzene as an electron donor(P7-E,B-BT-1,4-E and P17-E).Compared with initial polymers(P7,B-BT-1,4 and P17),the introduction of alkynyl groups greatly enhanced the visible light absorption capacity and increased LUMO energy level.The hydrogen evolution rate(HER)of P7-E,B-BT-1,4-E and P17-E were improved about 1.25,2.40 and 86.9 times,respectively.Further analysis showed that the introduction of alkyne groups could not only broaden the light response range,but also further accelerate the electron transport and promote the separation of photogenerated electrons-holes effectively during the photocatalytic hydrogen evolution reaction.2)Based on the D-A conjugated polymers B-BT-1,4-E and B-BT-1,3,5-E which composed of benzene(D)and benzothiadiazole(A).A series of typical weak D and strong A conjugated polymers were further constructed by introducing electron-withdrawing fluorine onto the benzothiadiazole(B-FBT-1,4-E,B-DFBT-1,4-E,B-FBT-1,3,5-E and B-DFBT-1,3,5-E).Theoretical calculation showed that the introduction of fluorine was beneficial to the regulation of active site and the electronic transfer process followed the proton-coupled electron transfer mechanism.It was further found that the simultaneous introduction of electron-withdrawing group(F)and electron-donating group(-OCH3)had the lowest free energy to produce hydrogen molecules(B-FOBT-1,4-E,B-FOBT-1,3,5-E).The series characterization confirmed that introduction of fluorine would accelerate the transfer of photogenerated carriers.Compared to the initial polymers B-FBT-1,4-E and B-BT-1,3,5-E,the HER of B-FOBT-1,4-E and B-FOBT-1,3,5-E was increased by 3.1 and28.8 times,respectively.This research provides a new idea for further improving the photocatalytic efficiency of D-A conjugated polymers.3)The conjugated porous polymers prepared based on Sonogashira-Hagihara coupling could be post-modified by thiol-yne reaction due to the rich alkynyl group.Many different functional groups were introduced into BBT by the thiol compounds such as2-aminoethanethiol,etc(BBT-SC2NH2,-SC2CH3,-SC3NH2,-SC4NH2,-SC5NH2).Photocatalytic hydrogen-production test results showed that the introduction of amino group could greatly enhance the photocatalytic performance,but as the carbon chain length between sulfur and nitrogen was increased gradually,the HER decreases sharply.Among them,BBT-SC2NH2 showed the highest HER,which was nearly 28 times higher than that of unmodified BBT.In-depth research found that the newly introduced amino group could prolong the lifetime of excited electrons,facilitate photogenerated electron-hole migration and improve the hydrophilicity of materials.At the same time,the introduced amino group could be used as the new reactive sites which played a key role in the improvement of photocatalytic hydrogen production.4)By introducing a strong electron-withdrawing cyano group(-CN)into the electron acceptor,the weak D and strong A conjugated polymers were further constructed based on benzene as an electron donor to accelerate the migration of photogenerated carriers(B-B,B-CNB,B-2,5-CNB and B-2,6-CNB).Compared with the initial B-B(11.3μmol/h),the HER of B-CNB(145μmol/h),B-2,5-CNB(377μmol/h)and B-2,6-CNB(343μmol/h)was increased 13,33 and 30 times with the increase of cyano groups.At the same time,the apparent quantum yield(AQY)of B-2,5-CNB reached 7.8%at 420 nm.In-depth research proved that the introduction of electron-withdrawing cyano group could reduce the dihedral angle between polymer monomers,and then significantly improve the migration ability of photogenerated electrons and increase the amount of active sites,which were conducive to the improvement of photocatalytic hydrogen production efficiency.