Ultrafast Spectroscopy And Dynamics Of Several Nanostructures Constructed By Regulated Synthesis

In recent decades,functional nanomaterials have become one of the most promising materials in many fields such as materials,chemistry,and energy due to their intriguing chemical and physical properties.Researchers have spent much time on their syntheses and regulations to obtain better physicochemical properties;nevertheless,how to achieve effective design,controllable preparation,and successful application of nanomaterials is still a hot topic.More importantly,insights into the microscopic mechanisms behind the regulation of nanostructures are still lacking.Based on the above backgrounds,we designed and synthesized several nanostructures by regulation to optimize the photoelectric properties of materials and further investigated the involved mechanisms from the ultrafast dynamics perspective.The major tools we used were ultrafast transient absorption spectroscopy as well as UV-vis absorption spectroscopy and fluorescence spectroscopy.We revealed that the regulation of nanostructures can not only change their electronic and energy-band structures,but also affect the related dynamics of charge carriers and excitons in the systems.The investigation of the location effects in a copper-doped metal-organic framework(MOF)system of UiO-66-NH2 by ultrafast spectroscopy is the major part of this thesis.We constructed two types of copper-doped MOFS,i.e.,Cu@UiO-66-NH2 and Cu-UiO-66-NH2.In the former,Cu2+ions are impregnated in the pore space of the amine-functionalized,Zr-based UiO-66-NH2;while in the latter,Cu2+ions are incorporated to form a bimetal-center MOF with Zr4+ being partially replaced by Cu2+in the Zr-O oxo-clusters.Ultrafast spectroscopy revealed that the photoinduced relaxation kinetics associated with the ligand-to-cluster charge-transfer state are promoted for both Cu-doped MOFs relative to undoped one,but in a sequence of Cu-UiO-66-NH2>Cu@UiO-66-NH2>UiO-66-NH2.Such a sequence turned to be in line with the trend observed in the visible-light photocatalytic hydrogen evolution activity tests on the three MOFs.These findings highlighted the subtle effect of copper-doping location in this Zr-based MOF system,further suggesting that rational engineering of the specific metal-doping location in alike MOF systems to promote the photoinduced charge separation and hence suppress the detrimental charge recombination therein is beneficial for achieving improved performances in MOF-based photocatalysis.The investigation of the luminescence and its related exciton dynamics in an anion-regulated two-dimensional(2D)perovskite(PEA)2PbXnY4-n system is another work I carried out independently.With the help of UV-vis absorption,fluorescence,and transient absorption spectroscopy,we found that the substitution of Br with I not only leads to the red-shift of exciton absorption,emission,and exciton bleaching bands,but also results in acceleration of exciton relaxation,which can be attributed to the levitation of the valence band by such a halogen substitution.This was also observed for the mixed perovskite systems formed via physical mixing.These studies are expected to be of value to the design of photoelectric materials based on mixed halide 2D perovskites.In addition,the charge carrier dynamics involved in a photocatalytic oligomer system,i.e.,oligo phenylene butadiynylene(OPB)were investigated by ultrafast transient absorption spectroscopy.Our collaborators provided the samples of the self-assembled OPB,and their experimental results revealed that the highly ordered structure is very important for maintaining the desired photocatalytic activity of this system.Our ultrafast spectroscopy characterizations and analyses gave a consistent explanation of the involved mechanisms:The structural disorder after photocatalysis will bring on trap states with long lifetimes in the bandgap of OPB,not beneficial for the photocatalytic overall water splitting.