Ultrafast Spectroscopic Study Of Graphene Oxides

Graphene oxide?GO?is a two-dimensional carbon nanomaterial,in which a large number of oxygen-containing functional groups are distributed on its basal plane and edges.At first,it is only regarded as a‘precursor'of graphene in a large-scale production way.Recently,due to the deeper understanding of GO,researchers have realized that oxygen-containing functional groups play a significance role on the optophysical properties of GO.Those functional groups can link GO with other nanomaterial by non-covalent or covalent chemical bonds.As a result,it creates a series of nanocomposites with excellent optical and electrical properties.How to further improve the opto-electron conversion performance of these GO-based nanocomposites has been a hot issue in this field,which is also deeply affected by the electronic structures of GO and GO-based nanocomposites.However,due to the complex electronic structure of GO,it is difficult for common steady-state characterization methods to give a comprehensive information of the electronic structure of GO.This leads to the key questions related to the electronic structure of GO,which are still not well understood.For example,there are the questions on the origin of red-side fluorescence in GO,and the relationship between the ensemble electronic structures of GO and its oxygen contents.Hence,studying on these fundamental problems could be of great significance for improving the optoelectronic properties of GO-based nanocomposites.In this thesis,the electronic structures in GO with different oxygen contents and the excited state processes in GO-titanium dioxide?TiO2?nanocomposites are studied by femtosecond transient absorption spectroscopy and time-resolved fluorescence spectroscopy.The main contents are as follows:1.GO samples are prepared by a modified Hummers method,and GO-TiO2nanocomposite materials are prepared by physical adsorption and chemical synthesis,respectively.2.Ultrafast transient depolarization spectroscopic experiments are performed to explore the excitation energy migration/redistribution processes among different electronic states in GO.It gives a more clear picture for the interaction among various electronic states in GO.The experimental results show that in the sp³ region of GO,a charge transfer state will be formed between the sp³ hybridized carbon atoms and the oxygen-containing functional groups,and the energy redistribution and charge migration in the sp³ region occur on the time scale from sub-picoseconds to tens of picoseconds.In the sp² regions of GO,the energy redistribution does not occur,which indicates that the electronic states of sp² clusters are much localized.Furthermore,accompanied with the time-resolved fluorescence depolarization experimental results,it indicates that the red-side fluorescence?590-650 nm?of GO is dominantly contributed by the electronic states of sp² clusters.3.Femtosecond transient absorption spectroscopy combined with X-ray photoelectron spectroscopy experiments are performed to quantitatively analyze the relationship between the electronic structure and oxygen contents of GO.Those experimental results show that when GO has a relatively high oxygen content?such as?35%?,due to the high ratio of sp³/sp² region around the sp² regions,the photogenerated electrons/holes are localized in the excited sp² regions,leading to a insulator-like GO.When the oxygen content of GO is reduced to?34%?a critical oxygen concentration?,GO presents semiconductor-like properties.When the oxygen content is further reduced to?29%,GO begins to appear semi-metallic properties,which is in coexistence with the semiconductor-like states?so-called two-phase coexistence state?.4.At first,femtosecond transient absorption spectroscopic experiments are performed in various kinds of TiO2 nanoparticles in water and ethanol solutions,respectively,in order to explore the generation and relaxation processes of photogenerated electron-hole pairs in TiO₂ nanoparticles.It is found that the dynamics of trapped holes and the deep trapping processes of electrons in different kinds of TiO2 nanoparticles are almost identical.Secondly,for the same crystal type TiO2 in water and ethanol solutions,which represents the hole trapping processes,we find that the trapped holes also show the same dynamics behavior.Finally,the GO-TiO₂nanocomposites are investigated by femtosecond transient absorption spectroscopy.It demonstrates that the photogenerated electrons in GO will not transfer to TiO2.This could explain the reason why GO-TiO2 nanocomposites have higher photocatalytic efficiency.Combined with previous results,it implies that the photogenerated electrons of TiO₂ could effectively transfer to the GO and reduced GO,inhibiting the recombination of electron-hole pairs,thereby improving the photocatalytic efficiency in GO-TiO2 nanocomposites.