Efficient Construction Of Polymer Semiconductor Heterojunction For Photocatalytic Hydrogen Production

Semiconductor photocatalytic technology directly uses clean and renewable solar energy to address the two major challenges of energy and environment,which has attracted widespread attention.Conjugated polymers（CPs）photocatalysts with broadly adjustable band gap,abundant structures,excellent electronic and optical properties have important application value in the field of photocatalytic water splitting,carbon dioxide（CO₂）reduction and pollutant degradation,etc.Simultaneously,they also have the drawbacks of poor thermal stability and short exciton diffusion length,like many organic semiconductor photocatalysts,which provide unfavorable factors for photocatalytic reactions.In recent years,many researchers have focused on the modification of photocatalytic properties of organic semiconductors.At present,the methods to improve the photocatalytic performance of organic semiconductors include the construction of donor-acceptor（D-A）structure,the regulation of band gap through molecular design engineering,the improvement of photon utilization,the improvement of carrier separation and the shortening of dipole moment and the construction of polymer-inorganic heterojunction.In this paper,the semiconductor modification of conjugated polymers and the performance of photocatalytic hydrogen production（PHP）are studied.The details are as follows:（1）The conjugated polymer framework based on tetra-bromo-pyrene-thiophene was synthesized by C–H direct arylation reaction method,and three kinds of CPs,Py DF0.0,Py DF0.1 and Py DF0.2 CPs were synthesized by adding 2,7-Dibromo-9H-fluoren-9-one in different proportions through bandgap engineering.Thiophene as electron donor unit（D）,2,7-Dibromo-9H-fluoren-9-one as electron acceptor unit（A）,the formation of intramolecular donor-acceptor（D-A）.The results show that the polymer Py DF0.1 sample with an optimum band gap（1.99 e V）has outstanding photocatalytic performance for hydrogen production.In addition,all the CPs show excellent dispersibility in aprotic cosolvent1-Methyl-2-pyrrolidinone（NMP）,which greatly improves photocatalytic hydrogen evolution rate（HER）.（2）Heterojunctions based on polymeric and inorganic semiconductors,namely Py OT@Ti O₂,have been successfully constructed via a C–H activation in-situ polycondensation based pyrene unit（Py-）and 3-（2-（2-ethoxyethoxy）ethoxy）thiophene unit（-OT）in the presence of titanium dioxide（Ti O₂）.The combination of polymeric semiconductor Py OT with Ti O₂ not only broaden light response of Ti O₂,but also dramatically promote the photo-generation exciton separation of Py OT.As a result,the optimized 50%Py OT@Ti O₂ composite,exhibited extensive visible light absorption（400～520 nm）and the highest photocatalytic HER using ascorbic acid（AA）as sacrificial agent,which is approximately 135 and 21 times higher than those of single components Ti O₂ and Py OT,respectively.This is mainly attributed to the construction of p-n heterojunction at the interface between Py OT and Ti O₂.In addition,the wettability of Py OT also plays an important role in improving the activity of hydrogen production.（3）The heteroatomic engineering strategy and heterojunction method were innovatively proposed to prepare organic-inorganic heterojunction for PHP.Three functionalized conjugated polymers（TFl,TSO and TSO₂）with different heteroatoms（C,S and O=S=O）were synthesized through C–H activation using heteroatomic engineering strategy.Additionally,CPs@Ti O₂ heterojunction was synthesized by a simple in situ polymerization method combining heteroatom engineering and heterojunction,which was applied to efficient PHP and the reasons of enhanced photocatalytic hydrogen evolution performance was studied.Because soft-hard polymer-inorganic heterojunction has the advantages of large interface contact,high efficiency of photogenerated carrier separation,wide light absorption and stronger redox ability.The photocatalytic properties of the three CPs and CPs@Ti O₂heterojunctions were compared and HERs of CPs@Ti O₂ were much higher than that of the single CPs material and Ti O₂ alone.And the HER of TSO₂@Ti O₂ is the highest,which can reach 11.2 mmol g^-1h^-1,5.5 time higher than those of pure TSO₂.