Structure Control Of Carbon Nitride For Photo Energy Conversion And Applications

Due to the severe energy and environmental issues,the development and utilization of solar energy and the development of catalytic systems based on light energy conversion and storage are very important.The chemical driving forces of oxidation and reduction can be obtained in situ by exciting the ground state electrons in the catalyst under light irradiation,and hence the initial light energy can be converted or stored.Light excitation can easily cross the reaction energy barrier,thereby obtaining reaction intermediates that are difficult to obtain in thermal reactions,expanding the application range of chemical synthesis.In photochemical process,the photocatalyst must absorb light,form an excited state,and induce the separation of the photoelectron-hole pair,which migrate to the reaction substrate to undergo a redox reaction with the substrate.The catalyst must first have a wide range of absorption in this process.Secondly,in the case of the photocatalysts with low dielectric constant,the electrons and holes formed by light excitation are difficult to separate due to the strong exciton effect.A long-lived separated charges are necessary to undergo a chemical reaction since the charges must be in contact with the reaction substrate after the charge migrates in the photocatalyst.In recent years,carbon nitride has become one of the promising photocatalysts due to its easy preparation and abundance,excellent physical and chemical stability,easily tunable chemical structure and suitable energy band structure.This thesis is based on low-dimensionai conjugated carbon nitride and its applications in the construction of long-lived separated charges and the photochemical reactions under dark-condition,exciton control and expanded light absorption range.It has the following four parts:1.Carbon nitride could generate separated photo-generated electrons and holes under light irradiation becuase the heptazine ring in the carbon nitride is capable of capturing photo-generated electrons by introducing a cyano group.After the holes are eliminated by the electron donor,the photogenerated electrons can be stored in situ in the carbon nitride,which exhibits a characteristic blue color and the stored photoelectrons have extremely strong reactivity.Also the stored electons could be quickly quenched by oxygen in the air and the blue color disappears.Further,small-sized carbon nitride nanosheets were prepared by the ultrasonic exfoliation method,which has a faster photoelectron storage rate and a larger photoelectron storage capacity.Electron paramagnetic resonance demonstrates that the stored electrons have an ultra-long lifetime,with little decay after storage for one month.2.The use of this long-lived charge and iodonium salt as a co-catalyst enables it to initiate photo-RAFT polymerization under dark conditions.Compared with the polymerization reaction driven by direct irradiation,the reaction under dark condition could effectively avoid the photobleaching and photodegradation,thereby improving the control over the polymerization reaction.This photo-polymerization under dark condition could improve the fidelity of the trithioester so that the chain extension reaction could be performed to synthesize block polymers.This research expands the use of long-lived charges store in nanomaterials to drive photochemical reactions under dark condition.3.Due to the weak screening effect and dielectric effect,the electron and hole pairs generated by light excitation in carbon nitride have a large Coulomb effect,which leads to a very strong exciton effect.By adjusting the charge distribution of the heptazine ring in carbon nitride,the influence of the charge distribution on the transfer of photogenerated electrons and the exciton binding energy in carbon nitride was investigated.We have found that carbon nitride with a positive charge distribution has a highly reduced exciton binding energy,reaching 24 meV.Since the reduced exciton binding energy promotes the separation and transfer of charges,compared with carbon nitride with negative charge distribution,carbon nitride with positive charged distribution exhibits enhanced photocatalytic efficiency in both photocatalytic hydrogen production and hydrogen peroxide production.4.Due to the easy modification of carbon nitride and its own conjugation characteristics,the absorption range of carbon nitride is highly increased up to 2000 nm by expanding the conjugate structure.The enhanced absorption in the infrared region makes carbon nitride have excellent photothermal conversion performance,with a photothermal conversion efficiency of 37%at 808 nm.The adsorption of organic conjugated small molecules is improved because of the enhanced conjugation.Combation of enhanced photothermal performance and excellent small molecule drug loading capacity makes it a good combined anti-cancer performance of chemotherapy and photothermal therapy.