| As one of the most important precursor materials for graphene nanosheet,Graphene oxide(GO)is mainly comprised of carbon atom,hydrogen atom and oxygen atom.With quasi-two-dimensional graphene-like sheet structure and non-stoichiometric property of GO,study on its molecular structure has always been a hotspot in the research field.The quasi-two-dimensional structural characteristic of GO endows it with large specific surface area(～2600 m2/g).Plenty of oxygen containing functional groups on the surface of GO facilitates surface modification and organic molecules physical adsorption One of the research directions is the surface modification and functionalization on GO surface,namely,chemically modified graphene(CMG).GO has favorable dispersibility in variety of organic solvents.It can be studied in various liquid phase environments.Despite plenty of unique energy-levels,the mechanisms of GO’s intrinsic photoluminescence(PL)properties are still uncertain.Thus,it makes sense to explore the PL mechanisms and the most critical influences on PL of GO,such as pH dependence and solvation effect.In this dissertation,with the help of steady state fluorescence spectra,time resolved spectra and femtosecond transient absorption spectra,the molecular structure and energy level of GO were studied and some new insights were put forward.The main contents of the dissertation are as follows:1.Many previous papers and experiments show that GO contains three PL emission centers,450 nm,500 nm and 600 nm,respectively.Although GO can be synthesized by different methods and GO is a complex non-stoichiometric material,GO contains the same kinds of oxygen-containing functional groups and chromophore.The chromophore belongs to the structures of polycyclic aromatic hydrocarbons(PAH)and linked with oxygen-containing functional groups,namely,quasi-molecular structures.By the methods of time resolved fluorescence,we confirm the size-independence PL of GO and this is the direct evidence that oxygen-containing functional groups are evenly distributed on the surface of GO instead of the edge.Generally,oxygen-containing functional groups are distributed on the edge of GO based on the Lerf-Klinowski and other models.In this section,we give the new insights of GO’s molecular structure and multi-peaks emission mechanisms.2.A new kind of functionalized graphene is synthesized;the synthesized functionalized GO was used to research the origin of the weak 500 nm emission from GO.It has large fluorescence quantum yield(18%).In this section,the steady-state fluorescence,time resolved fluorescence and femtosecond transient absorption spectra were used to give a comprehensive understanding of the GO-CONH-TPA molecular energy structure.For the first time,we found the phosphorescence emission from the large-scale GO.The intersystem crossing(ISC)process last 2 ns,and the phosphorescence emission located at 500 nm and has a lifetime of 7 μs.The ISC can induce lowering of the excited state energy with time and cause the excitation wavelength dependent fluorescence of GO.3.It has been proved that water molecules affect fluorescence emission of GO,and we found the water molecules inhibiting effect for the GO PL emission.In most of the organic polar solvent,GO has a quantum yield(QYs)about 3%.The time resolved fluorescence and femtosecond transient absorption spectra were used to investigate the relationship between water molecular and GO QYs and explore the mechanisms.There are three influence factors:Firstly,water molecular reduce the number of π-conjugated carbon radicals,these radicals have strong oxidizing property.When there are no π-conjugated carbon radicals,the carboxyl groups can be easily reduced and induce the PL intensity weak.Secondly,in the environment of water molecules the carboxyl groups are protonated and the protonated GO has larger non-radiative transition probability than deprotonated GO.Thirdly,water molecular can reduce the phonon scatter of the carriers.These can also suppress the GO’s QYs. |
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