Ultrathin Two-Dimensional Nanosheets:Electronic Structure Modulation And Applications In Photocatalysis

With the severe overuse of fossil fuels,the issue of energy shortage and serious environmental pollution are global problems.Thus,it is highly desirable to develop clean and efficient source of energy for the social sustainable development.In the field of energy conversion,photocatalytic reaction on semiconductors was regarded as an efficient route to directly convert inexhaustible solar energy into chemical energy,with environmental friendliness and low cost,which is considered to be a potential strategy to solve the energy and environment problems.In this regard,designing and synthesizing semiconductor photocatalysts with high activities has received great attention in the field of photocatalysis.Recently,ultrathin two-dimension?2D?nanosheets can provide great opportunities for the maximum of photocatalytic efficiency due to their unique structural features and significantly modified electronic structures.To this end,we used ultrathin 2D nanosheets as excellent catalyst models to investigate the crucial factor in the photocatalytic process and modulate the electronic structure of the semiconductors,which can not only significantly improve the photocatalytic performance,but also provide new possibilities for the design of promising catalysts in the field of photocatalysis.The main content of this dissertation are summarized briefly as follows:1.Elemental doping is one of the most effective approaches to regulating the electronic structures for improving the photocatalytic activity.In spite of a few researches on optimization of the photocatalytic activity by means of elemental doping in semiconductors,there still exists controversy on the role that the dopants play in photocatalysis.To this end,ultrathin 2D naosheets could be served as an ideal model with clear structure-property relationship to achieve in-depth atomic-level insights into the correlation between the doping atoms and the corresponding photocatalytic activity.Herein,we put forward a desired model of ZnIn2S₄?ZIS?nanosheets with oxygen doping to obtain in-depth understanding of the role that doping atoms play in photocatalysis,which shows enhanced photocatalytic activity compared with pristine ZIS.The electron dynamics analyzed by ultrafast transient absorption spectroscopy reveals that the average recovery lifetime of photoexcited electrons is increased by 1.53 times upon oxygen incorporation into the ZIS crystals,indicating enhanced separation of photoexcited carriers in oxygen-doped ZIS nanosheets.As expected,the oxygen-doped ZIS nanosheets show a remarkably improved photocatalytic activity with hydrogen evolution rate up to 2120?mol·h^-1·g^-1 under visible light irradiation,which is 4.5 times higher than that of the pristine ZIS nanosheets.2.'In this work,we put forward a common metal-silver-loading strategy for promoting the separation of photoexcited charges and thus enhancing the photocatalytic activity of metal-semiconductor hybrid system.On the other hand,we designed Ag-K₄Nb₆O₁₇ hybrid structure as a material model for getting insights into the key role of metal Ag during the photocatalytic process.The electron dynamics analyzed by ultrafast transient absorption spectroscopy reveals that metal Ag nanoparticles can extract the photoexcited electrons as a sink,significantly facilitating the transfer of photoexcited electrons from ultrathin K₄Nb₆O₁₇ nanosheets to metal.Photocatalytic hydrogen-evolution measurements clearly disclosed that the Ag-K₄Nb₆O₁₇ hybrid structure exhibit an obviously increased photocatalytic activity under the irradiation of solar light,which is approximately 20 times higher than that of the bare ultrathin K₄Nb₆O₁₇ nanosheets.Of note,no hydrogen generation was observed on the Ag-K₄Nb₆O₁₇ hybrid structure under the irradiation of visible light alone,which demonstrates that the injection of plasmonic hot electron from metal Ag to K₄Nb₆O₁₇ could not occur in this hybrid system.Furthermore,our photocatalytic experiment under both ultraviolet and additional visible light irradiation clearly showed that the introduction of metal Ag nanoparticles was responsible for the significantly increased photocatalytic activity,because the metal Ag nanoparticles not only played an important role in improving the electron-hole separation efficiency but also bring the intensified electric field induced by the surface plasmon resonance,which will provide valuable insight into the design of hybrid structures with improved photocatalytic activity.3.Structural distortion in semiconductors can significantly influence their electronic structures and endow them with enhanced photoabsorption and modified band structures,thus allowing for greatly improved photocatalytic performance.In this work,we successfully introduced structural distortion into the framework of g-C₃N₄ sample via a fast-heating strategy,which is a faster,lower cost and easier route with respect to the previously reported methods.X-ray absorption spectroscopy?XAS?shows that the fast-heating strategy can not only lead to the formation of structural distortion in g-C₃N₄ samples,but also regulate the degree of structural distortion with elevating temperature in the heating process.The electron spin resonance?ESR?spectra revealed that the presence of structural distortion is good for the redistribution of ? electrons,which helps to optimize the band structure for enhancing the visible light harvesting.The photocatalytic measurement demonstrated that the introduction of structural distortion can significantly increase the photocatalytic activity of g-C₃N₄ under both solar and visible light irradiation.Especially,the g-C₃N₄-700 sample exhibits the highest hydrogen evolution rate of up to 255.0 ?mol·h^-1 among the obtained samples,which is about 3 times higher than that of the pristine g-C₃N₄ sample.This work constructs the quantitative relationship between structural distortion and photocatalytic activity,offering a new direction on designing efficient photocatalysts in this field.