Interfacial Modulation Of Carbon Nitrides For Photochemical Applications

As a new type of metal-free semiconductor material,polymeric carbon nitride?CN,often denoted as g-C₃N₄?has shown great economic and social benefits in the fields of molecule conversion,solar-chemical energy conversion and photoelectric biosensing due to its unique molecular/electronic structure and surface properties,thus has aroused researchers'extensive interests.However,like others semiconductors,the limited utilization of sunlight,photogenerated electron-hole recombination and slow semiconductor surface/interface photocatalytic reactions also restrict the wide applications of carbon nitride.This dissertation mainly involves the"Top-down" dissolution of carbon nitride,"Bottom-up" synthesis of carbon nitride,and alternative assembling of CN-based photoelectrodes with improve activity.The main contents are as follows:1.The Top-Down strategy to dissolve carbon nitride and homogeneous photocatalysis:Carbon nitride is insoluble in most solvents,due to the interlayer van der Waals forces and hydrogen bond,which greatly limits the potential active sites accessible at the molecular level,understanding of synthesis theory and subsequent potential applications.Taking these into consideration,concentrated H₂SO₄ was reported as the first solvent,but the harsh conditions hindered the further application.We proposed the dissolution of carbon nitride in environmentally friendly methane sulfonic acid at room temperature.The Density Functional Theory results verified that the sulfonic acid group significantly reduced the adsorption energies.Taking the dehydrogenation of 3,3,5,5-tetramethylbenzidine and the N-demethylation of Azure B as examples,the homogeneous CN photocatalyst exhibited a dramatic enhancement in activity by a factor of up to 10 relative to the heterogeneous one due to the increase of potential accessible catalytic sites.Moreover,the reversible dissolution of CN using good/poor solvents allowed the homogeneous CN to be effectively recycled and reused,which united the advantages of both homo/heterogeneous catalysts.This work may extend the application of CN as a homogeneous?photo?catalyst and highlights the utility of polymeric catalysts with intrinsic catalytic properties in bridging the gap between homo/heterogeneous catalysis.2.The Bottom-Up strategy to synthesis carbon nitride and photocatalytic oxidation of tetracycline:Comparing with the traditional thermal condensation,the microwave-assisted approach synthesized the carbon nitride of low molecular weight in 90 second.Taking photocatalytic oxidation of tetracycline as an example,the as-obtained CN catalyst exhibited an excellent enhancement in activity by a factor of up to 11-fold over the thermal condensation CN due to the activation of O₂ by incompletely polymerized heptazine-unit according to DFT calculation,and the phase junction enhancing the electron-hole separation and immigration.The revealing of the more detailed polymerization kinetics would guide the construction and modification of heptazine-and triazine-based carbon nitrides in the future.3.Alternative assembling of carbon nitride-based semiconductor for enhancing photoelectrochemical activity:New insight into junction-based designs for efficient charge separation is vitally important for photoelectrochemical research.The assembling of BiOI-CN phase junction by alternative electrochemical and electrophoresis deposition showed excellent performance in charge separation and immigration.The maximum current density enhanced 2 and 6-folds,compared to BiOI and CN films,respectively.The alternate assembling of CN showed uniform film formation,exact composition and adjustable sequence.Therefore,this work would pave the related research on improving activity of carbon nitride-based photoelectrodes.