Enhanced Photocatalytic Hydrogen Evolution From Organic Semiconductor Donor-Acceptor Heterojunctions

In the context of the energy crisis and global climate deterioration,the sustainable development of clean energy will become a new direction for future energy development.The current global energy crisis has highlighted the importance of ensuring that the energy needs are in line with the climate goals,so many governments regard the hydrogen production as a pillar industry for the development of energy strategies.Hydrogen will become the superior green and low-carbon energy in the future because it can enhance the energy security by reducing the dependence on the fossil fuels.Hydrogen energy can be derived from water during the process of the photocatalytic hydrolysis of water with the great advantage that solar energy is an inexhaustible energy.Photocatalytic hydrogen evolution is a promising approach to supplying clean fuels from abundant solar energy.The organic semiconductors have many advantages,such as the design of their material structures to change their photoelectric properties,the modification of the side chains to change their physical properties and their structural variability.Therefore,they are widely used and have great application potential in the photoelectric field including the light-emitting diodes,the solar cells and the field effect transistors.However,there are few studies on the application of the organic semiconductors in the photocatalytic hydrogen evolution.Due to the hydrophobic structures and the exciton recombination of the organic semiconductor materials,the efficiency of the photocatalytic hydrogen evolution is low when they are used as the photocatalysts.The immediate problems are how to improve the contact between the materials of the organic semiconductors and the water,and how to increase the efficiency of the exciton dissociation,so as to improve the performance of the photocatalytic hydrogen evolution.This dissertation mainly focuses on the organic materials for the photocatalytic hydrogen evolution.A series of highly efficient materials were designed and synthesized for the photocatalytic hydrogen evolution.The synthesized materials by using the surfactants and the hydrophilic groups improved the contact between the polymers and the water.The materials which were used as the donor-acceptor heterojunctions improved the efficiency of the photocatalytic hydrogen evolution.And the influence of the material structures on the efficiency of the photocatalytic hydrogen evolution is also discussed.In the second chapter,we prepared the blending conjugated polymeric micelles encapsulated in the biomass by using the amphiphilic xylan derivatives X-g-NMe as the surfactants,and the effective photocatalytic hydrogen evolution was achieved.We realized the efficient photoinduced charge transfer in the different polymeric heterojunctions by selecting the energy level-matched conjugated polymers.The results show that,in these polymeric heterojunction micelles,the more effective photoinduced charge transfer the conjugated polymers have,the stronger charge separation their heterojunction has,which leads to the higher rate of the photocatalytic hydrogen evolution.In the third chapter,we designed and prepared the blending conjugated polymeric micelles with a novel amphoteric non-ionic surfactant F127,achieving the efficient photocatalytic hydrogen evolution.The results show that the surfactant F127 has the stronger dispersivity and the smaller micelle volume than the surfactant X-g-NMe,which is beneficial for improving the efficiency of the photocatalytic hydrogen evolution.By matching the conjugated polymers with the different energy levels,the blending conjugated polymeric micelles achieved the effective photoinduced charge transfer（PCT）.In these complex micelles,the blending conjugated polymers exhibited the stronger charge separation during the PCT process,thereby improving the rate of the photocatalytic hydrogen evolution.In the fourth chapter,we designed and synthesized a kind of new donor material P4EOBDT-TTE with hydrophilic side chains.And then we matched them with the classic acceptor small molecule PC₆₁BM and EH-IDTBR,and built the polymer and small-molecule heterojunctions without the surfactants.By adjusting the ratio of the polymer to the small molecules,the donor material P4EOBDT-TTE showed the highest rate of the photocatalytic hydrogen evolution at about 164.32μmol·h^-1,which is 6 times as the rate of the single component photocatalytic hydrogen evolution.In the fifth chapter,we synthesized a new kind of bipolar organic polymer PF6NDPP and studied the performance of the photocatalytic hydrogen evolution of the polymer PF6NDPP as an acceptor or a donor.That is to say,we explored the performance of the photocatalytic hydrogen evolution of the combinations of P4EOBDT-TTE/PF6NDPP and PF6NDPP/PC₆₁BM.It was found that there is a certain synergistic effect between the donor P4EOBDT-TTE and the receptor PF6NDPP,leading to the improvement of the photocatalytic efficiency.And there is not a good synergistic effect between the donor PF6NDPP and the acceptor PC₆₁BM,without improving the photocatalytic efficiency.This result shows that a suitable donor-acceptor material system is the key to obtaining the efficient heterojunction photocatalysts.This dissertation focuses on the photocatalytic hydrogen evolution of the organic semiconductors,and makes a series of innovations to solve the problem of the low efficiency of the photocatalytic hydrogen evolution.1.The dispersion of surfactant-assisted hydrophobic polymers and the design of materials imitating the surfactant made the polymer form the good morphology in water.The regulation of different surfactants（X-g-NMe and F127）changed the dispersion of the hydrophobically conjugated polymers in water,which made them disperse well in water and improved the efficiency of photocatalytic hydrogen evolution.A kind of hydrophilic donor material（P4EOBDT-TTE）was designed and synthesized,which can form the good phase morphology in water with the hydrophobic small molecules(PC₆₁BM and EH-IDTBR)and realize the efficient photocatalytic hydrogen evolution.2.The efficiency of the photocatalytic hydrogen evolution of the organic semiconductors was greatly improved by constructing a donor-acceptor heterojunction.For the first time,a donor heterojunction was introduced into the organic photocatalytic hydrogen evolution system.The selection of the level-matched polymers and small molecules made the highly efficient photoinduced charge transfer between the organic semiconductors,which increased the efficiency of the exciton dissociation,thus improved the efficiency of photocatalytic hydrogen evolution.3.A kind of new bipolar semiconductor（PF6NDPP）was designed and used as both donor and acceptor for the photocatalytic hydrogen evolution.