Ultrafast Spectroscopy Investigations On 2D Hybrid Perovskite And Photocatalytic Nanocomposite Systems

Energy shortage and environmental pollution have become serious problems with the development of modern society.To solve these problems,people are making efforts in various scientific fields,e.g.,by synthesizing energy-saving LED devices to replace traditional energy-consuming light sources,exploiting photocatalytic H2 to provide renewable energy,and converting CO2 to reduce environmental pollution.The current situation is that researches concerning the synthesis and design of nanomaterials dominate in the fields,while the those concerning the mechanistic understanding of these nanomaterials are still lacking.Ultrafast spectroscopy(e.g.,femtosecond time-resolved transient absorption measurements)enables us to look into the microscopic processes,behaviors,and effects involved in nanomaterial systems,such as exciton dynamics and charge separation mechanisms in photocatalytic systems,thereby providing useful guidance for the rational design of nanomaterials in relevant fieldsIn the first part of this dissertation,two thin-film 2D organic-inorganic hybrid perovskites,i.e.,2-phenylethylammonium lead iodide(PEPI)and 4-phenyl-1-butylammonium lead iodide(PBPI)were synthesized and investigated by steady-state absorption,temperature-dependent photoluminescence,and temperature-dependent ultrafast transient absorption spectroscopy.PBPI has a longer organic chain(via introducing extra ethyl groups)than PEPI,thus its inorganic skeleton can be distorted bringing on structural disorder.The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics.In addition,the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy,suggesting its correlation with inorganic skeleton rather than organic chain.Moreover,the photoexcited coherent phonons were observed in both PEPI and PBPI,pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton.This work provides valuable insights into the optical properties,excitonic behaviors and dynamics,as well as coherent phonon effects in 2D hybrid perovskitesIn the second part of this dissertation,we employed ultrafast TA spectroscopy to examine two types of inorganic semiconductor heterojunction photocatalytic nanocomposite systems.The one is the CdS/a-TiO2 system.We anchored a-TiO2 interlayers onto CdS NRs(denoted CT)in order to build interfacial chemical connections with the carboxylic-acid-group-functionalized cobaloxime,which turned out to effectively accelerate charge transfer and hence promote charge separation in the system leading to a more efficient photocatalytic efficiency.The other system is the heterojunction formed between CdS quantum dots and three-dimensional ZnIn2S4 nanoflowers.Our ultrafast TA characterizations revealed that the unique structure of such a semiconductor heterojunction enables the opening of new channels for charge transfer,thereby promoting the desired charge separation and improving the photocatalytic efficiency in the system.