Electron Excitation and Transfer Mechanism in Au-TiO2 Nanoparticle System

Author:Li Bo Chao

Supervisor:fu jiang min shang toshihisa tomie


Degree Year:2019





Au-TiO2 nanoparticle system has profound influence on high efficiency solar cell and photocatalysis.In these applications,the key physical problem involved is the high efficiency electron transfer from Au nanoparticle to TiO2 nanoparticle and the corresponding underlying mechanism.The previous experimental results have proved the presence of electron transfer from noble metal nanoparticle to the attached semiconductor.The transfer process is extremely complicated,which is related to metal and nanophysics,semiconductor physics,interface physics,photoexcitation process and electron transport etc.To clarify the electron transfer process and its underlying mechanism,as well as to improve the transfer efficiency are challenging tasks.Hence,a lot of researches have been carried out by adopting femtosecond pump-probe technique to explore the transfer process in Au-TiO2 nanoparticle system.Although the currently available technique can provide useful information on the electron excitation and thermionic electron dynamics,the disadvantage of having no spatial resolution limits the experimental results to only obtaining the total electron transfer behavior over all nanoparticle systems in time domain,not offering any information on the contribution of individual metal and semiconductor nanoparticles or individual clusters composed with several metal and semiconductor nanoparticles.Moreover,the difference in electron transfer mechanism of various nanoparticle systems can not be obtained as well.Above all,with ultrafast pump-probe technique only,it is impossible to precisely clarify the electron transfer process in Au-TiO2 nanoparticle system and the corresponding underlying mechanism.In this thesis,femtosecond pump-probe technique was combined with photoemission electron microscope of high spatial resolution to explore the electron excitation process and electron lifetime of Au and TiO2 nanoparticles,as well as the electron transfer process in Au-TiO2 nanoparticle system and its underlying mechanism,the main achievements are summarized as follows:1.In the respect of exploring the electronic structures and photoelectron emission mechanisms of TiO2 and Au nanoparticle.In the research on photoelectron emission process of TiO2 nanoparticles,all particles can be categorized into two types,3-photon nanoparticles and 4-photon nanoparticles,according to the order of nonlinear process at all wavelengths employed in the experiment.And the total photon energy of 3 or 4 photons is smaller than the expected photon numbers needed for photoelectron emission from the TiO2 nanoparticles,in which 5-photon at 700nm and 6-photon is required at 790nm.Hence,femtosecond laser can not provide the electrons in the valence band of TiO2 with enough energy to overpass the work function of 7.5eV according to the multiphoton electron emission mechanism.Furthermore,the electron yield for 4-photon nanoparticles is much larger than that of 3-photon nanoparticles,which is against our expectation.Meanwhile,the electron yield shows abnormal dependent on the laser power,and a difference electron yield of as large as 4 orders of magnitude between700nm and 790nm is observed.This large difference in electron yield can not be explained by the available photoelectron emission theory.Therefore,we proposed the multiphoton excitation thermionic emission theory,and accordingly a new model is built.In the new model,the valence band electrons of TiO2 are excited to an excited state in the conduction band close to the vacuum level,and thermalized into Boltzmann distribution.Subsequently,the electrons in the high energy tail of Boltzmann distribution can emit into the vacuum.The theoretical equation is deduced on the basis of the model,and fits well with the experimental results.The new theory successfully interpreted the abnormal electron yield on the dependent of laser wavelength.Moreover,by combining with the existing research result on electronic structure of TiO2,eg state and t2g state are revealed.Based on the newly built model,we interpret the larger electron yield difference between 4-photon particles and that 3-photon particles by the appearance of high density of defect levels.In the research on photoelectron emission process of Au,nanoparticles,it is found that when the laser power is lower than 200mW,multiphoton excitation process domains in photoelectron emission;and thermionic emission domains over multiphoton excitation process in photoelectron emission when the laser power is increased further.2.In the respect of measuring the lifetime of hot electrons in TiO2 nanoparticle and Au nanoparticle.The wavelength dependence of lifetime of hot electrons is found for Au and TiO2nanoparticles.For TiO2 nanoparticles,all wavelengths employed in the experiment can be sorted into three groups according to the dependence of electron yield on delay time:In the first wavelength group of 740nm,780nm and 790nm as fundamental pump laser,the hot electron lifetime are 0.25ps,0.28ps,0.23ps,respectively;second group of 770nm,the corresponding lifetime is 0.47ps;and third group of 750nm,760nm and 800nm,the lifetime are 0.4ps,4ps,0.45ps,respectively.Similarly,for Au nanoparticles,the first wavelength group of 750nm,760nm and 790nm,the hot electron lifetime are 1.71ps,1.4ps and 1.53ps,respectively;the second group of 780nm and 800nm,the lifetimes are 0.27ps;and third group of 740nm and 770nm,the hot electron lifetime are 0.45ps,0.43ps,respectively.And the maximum value is 1.71ps which is acquired at 750nm.Compared with the several tens of femtosecond electron lifetime of metals estimated by other researchers,the measured lifetime of hot electrons in nanoparticles is increased by 10-fold.We attributed this long lifetime of hot electrons to the defect levels with high density of states as they are often existed in the nanometer size of particles.These defect levels can act as excited states to prolong the lifetime of hot electrons.This property of Au nanoparticles facilitates the electron transfer process and improves the transfer efficiency greatly.In addition,the delay time dependence of electron yield of an individual TiO2,differs from another one,which illustrates the varied characteristics of the individual TiO2 nanoparticle.This property is applied to the Au nanoparticle’s case as well.3.In the respect of studying the electron transfer mechanism in Au-TiO2 nanoparticle system.Particles in Au-TiO2 region are classified into two groups according to the electron yield on fundamental and second harmonic illumination:one group having more TiO2 component and the other group having more Au component,and the wavelength dependence of each group is given.The results show that the electron yield enhancement of the particles having more Au component decreased rapidly at 750nm,where as pure Au nanoparticles having the longest lifetime at 750nm.This phenomenon justifies the occurrence of electron transfer from Au nanoparticle into the conduction band of attached TiO2 nanoparticle.A new electron transfer mechanism in Au-TiO2 nanoparticle system is proposed.In the new mechanism,electrons in Au nanoparticles will make upward transition to the excited state by absorbing photons.The lower excited electrons having long lifetimes are then thermalized to Boltzmann distribution,served as fundamental state to redistribute the electrons,and those in the high energy tail of Boltzmann distribution have enough energy to overpass the Schottky barrier at the interface of Au and TiO2 nanoparticle,and transfer into the attached TiO2 nanoparticles.Comparing with the previous mechanism in which the excited electrons are thermalized on the basis of Fermi level,the energy needed to conquer the barrier height is reduced in the new electron transfer mechanism.And it is believed that the newly generated excited states are responsible for the high catalytic activity(high electron transfer efficiency)of nanoparticles.This thesis could lay a solid foundation to the optimization of Au-TiO2 nanoparticle system in the field of plasmonic solar cell and photocatalysis.