Ultrafast Spectroscopy And Dynamics Of Photocatalytic Model Nanosystems

Nanomaterials have played a pivotal role in both fundamental research and in technological applications over the past few decades.Usually they can exhibit new electronic,optical,magnetic,and/or transport properties or effects not found in the bulk,mainly due to the quantum confinement effect at the nanoscale.To date,a vast amount of research has been devoted to their synthesis and the related aspects.Owing to the intrinsic complexity of nanomaterials,however,the mechanistic understanding remains rather limited,especially from the excited-state dynamics perspective.Currently,one of the major challenging tasks in the field lies in how to glean microscopic information and provide fundamental insight into a variety of working mechanisms involved in the designed model nanosystems,which are highly desirable for their realistic applications.In a typical photocatalytic process,the reaction mainly involves three consecutive steps:(1)light absorption by catalysts to generate electron-hole pairs,(2)charge separation and migration to the catalyst surface,and(3)reaction with molecules at the surface.Throughout the transfer process of the photogenerated electrons or holes,the charge loss will greatly affect the efficiency of the catalysts.Generally speaking,an optimal design of photocatalytic nanomaterials features one or more of the following five points:(1)bandgap engineering to achieve extended light absorption,(2)structure optimization to avoid charge loss,(3)potential regulation to obtain an appropriate redox potential,(4)increase of active sites to accumulate electrons or holes,and(5)usage of sacrificial agents to reduce electron-hole recombination.These points hold the key to the optimized design of photocatalytic nanomaterials.Notably,the past few decades have witnessed abundant research in this regard,with a focus mainly on the macroscopic functionality or performance;nevertheless,the microscopic mechanisms are still far from being explored,particularly in the photoexcited dynamics aspect.In view of aforementioned situation,during my PhD studies I have been committed to executing systematic scrutiny into the photoexcited electron/hole dynamics,exciton dark-state dynamics,and defect-state dynamics in a series of well-designed model nanosystems by means of ultrafast transient absorption(TA)spectroscopy in conjunction with steady-state absorption and photoluminescence(PL)as well as time-resolved PL spectroscopy.The major projects I have completed are summarized as follows.1.Plasmonic hot-electron dynamics in model nanosystemsIn metal nanosystems,surface plasmon relaxation mainly involves the electron-phonon scattering and charge recombination processes.Normally,the former leads to the heating of the lattice(i.e.,photothermal conversion)while the latter to the cooling of the hot electrons(i.e.,hot-electron effect).In photocatalysis,it is rather difficult to harness the two effects in a synergistic manner.We report here that in a well-designed model system of bimetallic Au@Pd core-shell nanostructure the two effects can be disentangled through tailoring the shell thickness at atomic-level precision.As demonstrated by our ultrafast absorption spectroscopy characterizations,the achieved tunability of the two effects in a model reaction of Pd-catalyzed organic hydrogenation offers a knob for enhancing energy coupling.The experiments also showed that the photothermal conversion effect makes a positive contribution while the hot-electron effect makes a negative one in this particular catalytic reaction.Through unraveling the competition and synergy between the two effects,this work opens a new avenue for rationally developing plasmonic-catalytic nanostructures toward efficient solar-to-chemical energy conversion.2.Hole-transfer dynamics in model nanosystemsIn photocatalysis with a focus on the participation of electrons,the effective transfer of holes may also play a key role.Effective hole transfer can efficiently reduce electron-hole recombination.After photoexcited,the electron transfer is typically sufficiently fast,whereas the transport of hole and subsequent reactions are generally slow,eventually leading to the high charge-recombination rate.Bearing this in mind,we have developed a water-soluble molecular co-catalyst strategy to promote the hole-transfer kinetics,i.e.,using small organic molecules to help accelerate the relatively sluggish hole-transfer process.We first designed and constructed a model nanosystem comprising the K4NbbO17 nanosheet and the molecular co-catalyst trifluoroacetic acid(TFA),and then gained a mechanistic understanding of the subtle mechanism associated with the photoexcited hole dynamics via ultrafast TA and PL characterizations.The molecular co-catalysis strategy developed in this work represents a facile yet highly effective approach to suppress recombination of photogenerated charges and may provide fertile ground for creating high-efficiency photosynthesis systems avoiding the use of costly noble-metal co-catalysts.3.Exciton dark-state dynamics in model nanosystemsThe transfer/transport of the photoexcited charge carriers and excitons in semiconductor nanomaterials may directly influence their photoelectrochemical performances.Although this is a well-known fact,various new subtle effects involved still await in-depth exploration.We have been devoted to this line of work with a focus on the photoexcited exciton dark-state dynamics in a series of CdS model nanosystems through joint observations from ultrafast TA and PL spectroscopy.With a set of deliberately designed control experiments(i.e.,regulating the defects via doping,introducing the electron or hole-acceptors,loading the noble metal,varying the environment of samples,and so on),we have discovered several new effects associated with the exciton dark-state dynamics in this "regular"nanosystem that has been subjected to extensive investigations.(1)A new exciton dark state(of non-radiative nature)within the bandgap caused by element doping was identified,and its related photoexcited electron reservoir effect(PEER)was proposed and further elucidated.(2)A new hole-related dark state(hitherto undocumented)was unveiled,and the related free hole carriers were found to induce different dynamical behavior from the bound excitons.Moreover,a careful examination of the semiconductor/metal composite system revealed that the metal component functions not only as a provider of active sites and an acceptor of photoexcited electrons,but also as a promoter for exciton dissociation favoring the free charge carrier-based photocatalysis.4.Defect-state dynamics in model nanosystemsDefect engineering,an extensively adopted strategy in semiconductor-based research and applications,usually brings about new phenomena or effects such as modifying the bandgap,optimizing the crystalline structure,and introducing the defect states(DS).We have been devoted to this line of work with a focus on the photoexcited defect-state dynamics in a diversity of prototypical model nanosystems through joint observations from ultrafast TA and PL spectroscopy.(1)Co-doped In2S3 system:The doping-induced localized DS(acting as the electron acceptor)was identified and the related relaxation dynamics was quantitatively characterized.The interplay between the defect-state dynamics and photocatalytic performances was also given.(2)Oxygen-doped ZnIn2S4 system:Oxygen doping was found to increase the energy separation between the conduction band and the H+/H2 potential,thereby improving the photocatalytic performance.Furthermore,the defect state-related charge carrier relaxation dynamics was quantitatively examined.(3)Single-atom Pt-doped g-C3N4 system:The key finding lies in that the cooperation of single-atom co-catalyst in g-C3N4 can effectively modulate the electronic structure,elongating the lifetime of photogenerated electrons due to the isolated single Pt atoms induced,intrinsic change of the surface trap states(or DS),as revealed by ultrafast TA spectroscopy.(4)CdS/CoP2 composite system:The ultrafast TA and PL spectroscopy characterizations enabled a mechanistic picture that clearly identifies the direction and rate of the DS-related electron transfer in this hybrid nanosystem as well as its intrinsic relation with the improved efficiency of charge separation for photocatalytic applications.