| Sum-frequency generation(SFG)is a second order nonlinear spectroscopy with surface/interface selectivity and high time resolution.SFG has been widely used to investigate the structural and dynamic information,such as molecular adsorption,orientation,arrangement,electronic structure,charge transfer and vibrational relaxation.As an ?-? group semiconductor with a 2.42 eV direct band gap,cadmium sulfide(CdS)nanomaterials have received a lot of attentions as promising photochemical catalysts to degrade lignin due to their excellent photoelectric conversion efficiency and high electron mobility.2-phenoxy-l-phenylethanol(PP-ol)is the lignin model molecule with ?-O-4 linkage bond.Significant difference in catalytic activity of degradation of PP-ol with CdS nanoparticles prepared with different processes has been observed,and detailed structural information should be investigated to real the catalytic mechanism.Conventional spectral methods have been widely used in the investigation of catalytic activity and mechanism of NPs with certain limitations.The adsorption and desorption,especially the adsorption configuration of the PP-ol molecules,the intermediate products and the products on the surface of CdS NPs have impact on the reaction paths.The electronic structure and surface states of CdS NPs are key factors for the catalytic activity.The complexity of electronic structure,especially the surface states,causes the role of NPs on photocatalysis to be unclear,and sometimes even contradictory.For example,the surface states are the recombination centers of photo-generated free electrons and holes,which reduces the photocatalytic performance.On the other hand,the surface states are also the trapping centers of the free electrons and holes,which promotes electron-hole separation and may enhance the photocatalytic activity of NPs.Time resolved methods,such as transient absorption spectroscopy and time-resolved photoluminescence spectroscopy have been used to study the energy levels of surface states and its influence on the carriers'relaxations.However,the contributions of the surface are disguised by the dominating bulk signal.As the surface selective method,broadband sum frequency generation spectroscopy(BB-SFG)can be used to investigate the adsorption configuration of molecules and access the surface states of NPs.The CdS@Na2S NPs and CdS@SN NPs were the photocatalysts for the ?-O-4 bond cleavage of PP-ol model compounds.The CdS@Na2S NPs show much higher reaction activity.The NPs were characterized by XRD,ultraviolet-visible diffuse reflectance spectrum(Uv-Vis DRS)and steady-state fluorescence spectrum.The results show that they have different crystal types and similar bandgap.In this thesis,the adsorption configurations of PP-ol molecules at the surface of CdS NPs,the properties and energy levels of surface states of CdS NPs have been studied by BB-SFG to explain the differences in the PP-ol degradation by CdS NPs with different crystal types.In addition,surface state electronic relaxation and anisotropy have been investigated through time resolved sum frequency generation spectroscopy(TR-SFG)as well.The main contents and major results are as follows:(1).To further explore the reasons for the differences in photocatalytic activities of NPs with different crystal types,the crystal types,bandgaps and the energy levels of surface states of CdS@Na2S NPs and CdS@SN NPs,as well as the possible adsorption configurations of PP-ol molecules on the surface of the NPs have been investigated by the conventional techniques and BB-SFG.The main findings are as follows:(?)After photoexcitation,the photogenerated electrons of CdS@Na2S NPs are mainly relaxed by surface states emission.It may be that nonradiative relaxation such as internal transformation promotes the effective separation of photogenerated electrons and holes.However,the band edge emission of CdS@SN NPs is the main relaxation pathway,which means that photogenerated electrons are mainly recombined with holes.From this perspective,the CdS@Na2S NPs have higher photocatalytic activity.(?)The BB-SFG results sh ow that the PP-ol molecules were mainly adsorbed on the surface of CdS nanoparticles by O atoms rather than benzene rings.When adsorbed on the CdS@SN NPs,methylene and methyne of PP-ol molecules point to the surface of NPs.When adsorbed on the CdS@Na2S NPs,methylene and methyne of PP-ol molecules are far away from the surface of NPs.From the perspective of adsorption configuration and steric hindrance,the PP-ol molecules adsorbed on the CdS@Na2S NPs dehydrogenated on the ?-carbon(methyne)more easily and the C-O ether bond cleavage to yield acetophenone and phenol.(?)In terms of dephasing kinetics,the methyne of PP-ol adsorbed on the CdS@Na2S NPs is the most active functional group,while methylene of PP-ol adsorbed on the CdS@SN NPs is the most active functional group.The more active the methyne,the more easily the C-O ether bond cleavage.(2).The band gap,the properties and energy levcls of surface states are the important factors affecting the photocatalytic mechanism.the Uv-Vis absorption spectrum is not particularly sensitive to the information of surface.state.The excitation-wavelength-dependent photoluminescence phenomena caused by the surface states interference makes the determination of band gap and energy levels of the surface states very difficult.With tuning the incident wavelength,SFG intensity can be significantly enhanced when the incident or the signal light resonant with the electronic transitions of the medium.BB-SFG spectroscopy has the potential to access the "dark states" for the electronic structure characterization of NPs.The results show that the SFG,the two-photon luminescence(TPL)and SFG induced photoluminescence(SFG-PL)signals are sensitive to the crystal types,only the anisotropic hexagonal CdS NPs have the SFG,TPL and SFG-PL signals.The band gap and surface states of hexagonal CdS NPs have been investigated by the BB-SFG,TPL,SFG-PL combined with Uv-Vis DRS and fluorescence excitation emission matrix spectroscopy.The surface states are attributed to the S vacancy.(3).The preparation temperature has direct effect on the size,the band gap and the surface states of NPs.The crystal type and band gap of NPs at different preparation temperatures were investigated.The relationship between the particle size and band gap is as follows:under the current experimental conditions,the larger of the particle size,the band gap is smaller.The band gap of CdS NPs with different particle sizes obtained by SFG,TPL and SFG-PL are similar to the results obtained by Uv-Vis DRS.The particle size of CdS-180 NPs is the largest and the band gap is the smallest.(4).The TR-SFG spectroscopy has been used to investigate the electronic relaxation of the NPs.When electrons were excited to the surface states,the excited electrons are mainly relaxed through exciton-exciton annihilation and Auger recombination due to saturation of the surface states.With high excitation intensity,there are more excitons generated leading to stronger exciton-exciton annihilation and faster electron relaxation.When the excitation photon energy matches with the valance band to conduction band transitions,the excited electrons are mainly relaxed by SFG-PL,the remaining excited electrons are relaxed through exciton-exciton interaction or electron-electron interaction.The electrons at the conduction band relax faster than that at the surface states.(5)Compared with CdS NPs,CdS nanowires(CdS NWs)show advantage in photogenic charge transmission due to the anisotropy of CdS NWs.In particular,the NWs array increase the charge diffusion length along with the nanowire axis.The dependence between the intensity of SFG signal and polarization of excitation light has been discussed.The results show that when the excitation light is perpendicular to the axial direction of the nanowire,the electric field in the nanowire attenuates.The SFG intensity in perpendicular polarization is less than the parallel polarization,resulting in weaker exciton-exciton annihilation and longer relaxation time. |
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