Ultrafast Spectroscopy And Dynamics Of Heterostructured Nanosystems

Based on the complex excited state dynamics background of the nano-heterostructure system,the work of this dissertation mainly uses ultrafast transient absorption spectroscopy technology(assisted by steady-state absorption,steady-state/transient fluorescence spectroscopy,electronic structure and performance characterization,etc.)to conduct in-depth systematic ultrafast dynamics research on some well-designed nano-heterostructure systems.Insight into the physical mechanisms that has not yet been discovered but are indeed closely related to the functionality of these heterogeneous structural systems,so as to provide key mechanism guidance for the regulation,design,synthesis,and application of nano heterogeneous structural systems.It mainly includes 5 work contents:(1)Study on photoexcited electron dynamics of ruthenium single-atom catalyst for photocatalytic nitrogen fixationIt is still a grand challenge to exploit efficient catalysts to achieve sustainable photocatalytic N2 reduction under ambient conditions.Here,we developed a ruthenium-based single atoms catalyst anchored on defect-rich TiO2 nanotubes(denoted Ru-SAs/Def-TNs)as a model system for N2 fixation.The constructed Ru-SAs/Def-TNs exhibited a catalytic efficiency of 125.2 ?mol g-1 h-1,roughly 6 and 13 times higher than that of the supported Ru nanoparticles and Def-TNs,respectively.Through ultrafast transient absorption and photoluminescence spectroscopy,we revealed the relationship between catalytic activity and photoexcited electron dynamics in such a model SAs catalytic system.The unique ligand-to-metal charge-transfer state formed in Ru-SAs/Def-TNs was found to be responsible for its high catalytic activity,as it can greatly promote the transfer of photoelectrons from Def-TNs to Ru-SAs center and the subsequent capture by Ru-SAs.This work sheds light on the origin of high performance of SAs catalysts from the perspective of photoexcited electron dynamics,and hence enriches the mechanistic understanding of SAs catalysis.(2)Efficient Photocatalytic N2 Fixation by Surface Plasmon on Molybdenum doped TiO2Photocatalytic N2 fixation has attracted substantial attention in recent years,as it represents a green and sustainable development route toward efficiently converting N2 to NH3 for industrial applications.How to rationally design catalysts in this regard remains a challenge.Here we propose a strategy that uses plasmonic hot electrons in the highly doped TiO2 to activate the inert N2 molecules.The synthesized semiconductor catalyst Mo-doped TiO2 shows a NH3 production efficiency as high as 134 ?mol g-1 h-1 under ambient conditions,which is comparable to that achieved by the conventional plasmonic gold metal.By means of ultrafast spectroscopy we reveal that the plasmonic hot electrons in the system are responsible for the activation of N2 molecules,enabling improvement of catalytic activity of TiO2.This work opens a new avenue toward semiconductor plasmon-based photocatalytic N2 fixation.(3)Triplet Photochemistry Enabled by Plasmon-induced Spin-exchanged Energy TransferPlasmon induced photochemical reactions from metal nanostructures are attracting heightened attention due to its potential for photocatalytic reactions.However,the nonmagnetic nature of plasmonic metals renders the plasmon-driven photochemical reactions limited to singlet state.Here we report an efficient plasmon-induced spin-exchanged energy transfer(PISEET)from plasmonic semiconductors to molecules,which enables the direct population of triplet electronic states of molecules.We demonstrated the PISEET pathway in plasmonic CuFeS2 nanocrystals,in which spin and plasmon interaction occurred as a result of spin-polarized character of conduction electron of plasmon.The PISEET process demonstrated to be dominated by the Dexter-type exchange interaction rather than the conventional plasmonic dipole-dipole coupling.Moreover,the PISEET mechanism has proved to be a generality,which undoubtedly shows great potential in the manipulation of emerging triplet photochemistry.(4)Semiconductor plasmons induced Molecular triplet population and triplet-triplet annihilationBased on the PISEET pathway observed in the CuFeS2-AP solid-state heterostructure,here we study the liquid-phase heterostructure of CuFeS2-AP molecules.When AP molecules are grafted onto the surface of CuFeS2 nanocrystals,the PISEET process from CuFeS2 nanocrystals to AP molecules can still be observed.Subsequently,we observed the delayed fluorescence of AP molecules on the surface of CuFeS2 nanocrystals,which is caused by the triplet-triplet annihilation process of AP molecules due to the local thermal field effect of plasmons.This can be confirmed by the pump fluorescent-dependent transient absorption experiment.This result indicates that while the spin exchanged energy transfer from semiconductor plasmon to molecule acceptor,it also has the local thermal field effect that enhances the triplet-triplet state annihilation process that is often observed in gold plasmon.This study shows that semiconductor plasmons have great potential in the regulation of triplet photochemistry.(5)Semiconductor plasmons induced Singlet and triplet energy transferBased on the above observation of CuFeS2 semiconductor plasmon-regulated molecular triplet state,we here constructed the CuFeS2-R6G molecular solid-state heterostructure system.Here we observe the simultaneous population of singlet and triplet electronic states of R6G molecules induced by CuFeS2 plasmons,that is,the singlet energy transfer of dipole-dipole interaction and the spin-exchanged energy transfer channel are simultaneously open.In addition,through the pump-flucent depended-transient absorption experiment,we found that the PISEET process observed in this system is not only dominated by the Dexter mechanism of the two-electron exchange in the traditional sense,but also involves the contribution of thermal effect of plasmon hot electrons.This work enriches the study of energy transfer between heterostructures and provides a key mechanism guide for the subsequent study of semiconductor plasmon-induced energy transfer.