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

With the development of economy and society,the traditional fossil fuels will not be able to meet the needs of human beings in the foreseeable future.Over the past decades,a variety of renewable energy technologies have emerged,among which photovoltaic and photocatalytic technologies represent two important branches and hot areas simply because they only require sunlight to generate electricity and fuel.Of course,the most important issue is how to improve the efficiencies of photoelectric conversion and photocatalytic reactions.Tremendous efforts have been made from the perspectives of material design,structural characterization,and performance testing.However,the traditional characterization techniques are mostly of steady-state nature or have low time resolution,making them difficult to achieve the real-time tracking of excited-state dynamics,such as photogenerated carriers/excitons/phonons,in material systems.Femtosecond time-resolved ultrafast spectroscopy,thanks to its ultra-high time resolution,makes it possible to directly capture the excited-state dynamics of interest.Nowadays,ultrafast spectroscopy has been widely used to reveal various ultrafast physical processes in material systems,such as hot-carrier cooling(typically on the femtosecond timescale),coherent phonon relaxation(typically on the picosecond timescale),and electron-hole recombination and processes related to defect states(typically on the picosecond-nanosecond timescale).The in-depth understanding of these processes is crucial for the theoretical development of excited-state dynamics and the design and development of optoelectronic devices.As is well known,the excitedstate dynamics in condensed-phase systems are rather sophisticated,and the existing insights into the pertinent microscopic mechanisms still remain quite limited.Starting from the structural regulation of model nanomaterial systems,one may interrogate the mechanistic aspects of photophysical and photochemical processes of close relevance to excited states would provide crucial clues and valuable guidance for the practical research and development of material systems.In this dissertation,we systematically investigate a class of model twodimensional hybrid perovskite materials,with a focus on the subtle correlations between organic-cation modulation and electron-phonon/phonon-phonon interactions,hot phonon bottleneck effects,and exciton/phonon dynamics,mainly by means of ultrafast transient absorption spectroscopy and steady-state/transient fluorescence spectroscopy,along with structure/property characterizations as well as analyses based on rational simulations.Additionally,we have also examined the photogenerated carrier/exciton relaxation dynamics in several typical photocatalytic material systems and elucidated their intrinsic connection to photocatalytic efficiency.The major research progress achieved is summarized as follows:1、The emerging two-dimensional(2D)lead-halide perovskite materials hold great promise for next-generation photovoltaic and optoelectronic applications,in which phonon engineering plays a crucial role.However,detailed mechanistic exploration related to phonon effects,especially from a dynamics perspective,remains rather limited.Herein,we present a systematic demonstration of phononic fine-tuning in a prototype 2D hybrid organic-inorganic perovskite(HOIP)system,i.e.,phenethylammonium lead iodide[(PEA)2PbI4]with each hydrogen atom at positions 2(ortho),3(meta),and 4(para)on the PEA’s phenyl group being replaced by a fluorine atom.Through a set of joint observations via ultrafast spectroscopy and temperaturedependent photoluminescence spectroscopy,we reveal that such a fluorination can subtly exert profound impacts on its structural distortion-induced phononic properties,including coherent phonon modes,phonon-phonon/electron-phonon interactions,and the hot-phonon bottleneck effect.This work highlights the significant importance of the atomic level tailoring of organic cations in low-dimensional HOIP systems,which is usually ignored in conventional notion and practice.2、Organic-cation engineering has recently proven effective in flexibly regulating two-dimensional hybrid organic-inorganic perovskites(2D HOIPs)to achieve a diversity of newly emerging applications.There have been many mechanistic studies based on structural tunability of organic cations;nevertheless,those with emphasis on the effect solely caused by the organic cations remain lacking.To this end,here we deliberately design a set of 2D HOIPs in which the inorganic layers are kept nearly intact upon cation modification,i.e.,the precursor phenethylammonium lead iodide and its four derivatives with the phenyl group’s para-position H being replaced by CH3,F,Cl,and Br.By means of femtosecond time-resolved transient absorption spectroscopy and temperature-dependent/time-resolved photoluminescence spectroscopy,we interrogate the subtle impact of cation modification on phonon dynamics,coherent phonon modes,phonon-dressed exciton dynamics,and excitonic emissions.A concerted trend for phonon lifetimes and exciton relaxation lifetimes regulated by cation modification is revealed,evidencing the existence of strong exciton-phonon coupling in this 2D HOIP system.The observed mass effect can be ascribed to the change in moment of inertia of organic cations.Also,we observe an interesting interplay of exciton kinetics pertinent to population transfers between two emissive states,likely linked to the subtle variation in crystal symmetry induced by cation modification.The mechanistic insights gained from this work would be of value for the 2D HOIPs-based applications.3、By virtue of the drawbacks of carrier-recombination dynamics and insufficient catalytic active sites in non-metallic catalyst C3N4 material,the regulations based on single-atom Fe doping and introduction of carbon defects have been conducted to enhance its photocatalytic performance.The major findings are as follows:(1)The single-atom Fe doping aids in opening a new relaxation pathway for photogenerated carriers,which effectively promotes the separation of electrons and holes.Additionally,the single-atom Fe itself provides sites for adsorption of oxygen molecules,further increasing the efficiency of transferring photogenerated carriers to oxygen molecules and hence enhancing the photocatalytic performance.(2)Through the design of multilevel micro-nano structures and the modification based on carbon defects,the light absorption capacity is improved,and meanwhile the desirable exciton dissociation is promoted,thereby increasing the concentration of carriers and leading to improved photocatalytic efficiency.4、The effect of single-atom Ce doping on the photogenerated carrier dynamics has also been investigated in two photocatalytic nitrogen-fixation materials,i.e.,W18O49 nanowires and BiOCl nanoplates.As revealed by ultrafast spectroscopy,the introduction of single-atom Ce enables promotion of carrier transfer in both systems and suppression of charge recombination,thereby bringing forth the desirable improvement in photocatalytic efficiency.