Study On The Mechanism Of Water Oxidation Reaction Of Graphene Oxide And Nitrogen Doped Carbon Quantum Dots

As one of the most important step of artificial photosynthesis,water oxidation reaction has been considered as the rate-determining-step to restrict the reaction efficiency because of the multi-step electron and proton transfer in the reaction process.In recent years,people pay more attention to new carbon materials because they can react with water to produce oxygen.However,the mechanism of water oxidation of most carbon materials is not clear.It is of great significance to explore the mechanism of oxygen production in water oxidation of carbon materials for further understanding of water oxidation and improving the efficiency of light energy conversion in artificial photosynthesis.In this dissertation,the mechanism of water oxidation of graphene oxide and nitrogen doped carbon quantum dots is studied by density functional theory(DFT)and isotope labeling experiments.The mechanism of water oxidation is discussed from the perspective of photophysics and photochemistry,and rate-determining-step of the reaction is found.The research contents are as follows:(1)In this paper,the heavy oxygen water(H218O)labeling experiment was used to determine that water oxidation occurred in the photoreduction process of graphene oxide.Half of the oxygen generated in the reaction came from water,and the other half came from graphene oxide.Combining with density functional theory(DFT)calculation and various characterization analyses before and after photoreaction,we have defined that epoxy group is the reaction center of water oxidation reaction,and proposed the mechanism of oxygen generation:two epoxies react with one water molecule to generate one hydroxyl,one C-H bond and one molecule of oxygen.At the same time,we also found that there is no water oxidation reaction in the formation of carbon dioxide,which is a simple decarboxylation reaction.In the process of illumination,the carboxyl group is continuously formed by the reaction of carbonyl pairs,epoxy pairs and water,which reaches a dynamic balance with decarboxylated.(2)In this paper,DFT and TD-DFT are used to calculate the rate coefficient of the photophysical process of hydrogen bond complex.It is found that the hydrogen bond prolongs the lifetime of graphene oxide in T1 state in the photophysical process of water oxidation of graphene oxide.The reason is that the enhancement of hydrogen bond in the excited state stabilizes the excited state structure of graphene oxide.At the same time,the enhancement of hydrogen bond breaks the C-O bond and activates the water molecule,which creates opportunity for the photochemical reaction.In the process of photochemistry,hydrogen bond enhances and induces proton transfer and electron transfer,which promotes the water oxidation reaction.By calculating the rate coefficients of photophysics and photochemistry,it is found that the rate-determining-step of the whole reaction is the formation of O-O bond,which is because of the limitation of electron transfer.In order to improve the reaction rate,we added Fe3+ions into the system to introduce coordination bonds instead of hydrogen bonds to induce electron transfer,and achieved the increase the yield of oxygen.(3)In this paper,the ability of nitrogen doped carbon quantum dots to react with water by water oxidation has been proved by experiments.The electron transfer and proton transfer induced by excited state hydrogen bond in the photophysical and photochemical processes of the water oxidation reaction over nitrogen doped carbon quantum dots are investigated by theoretical calculation.The results show that the hydrogen bond formed by pyridine nitrogen and water is enhanced in the excited state,which is conducive to the occurrence of photophysical process;in the photochemical process,the enhancement of hydrogen bond induces proton transfer and electron transfer,which makes water lose electrons to generate hydroxyl radicals,and then forms O-O bond to generate hydrogen peroxide.Different from the water oxidation of graphene oxide takes place on the epoxy group,the water oxidation of nitrogen doped carbon quantum dots takes place on the carbon atom activated by nitrogen atom.The increase of reactive sites is one of the important reasons that nitrogen doping is beneficial to water oxidation.By calculating the rate coeficient,we find that the rate control step of the reaction is the intersystem crossing process from S1 to T1 in the photophysical stage.