Preparation Of Graphitic Carbon Nitride Nanosheets And Researching Its Photocatalytic Performance

As a green renewable energy,solar energy is deemed to replace fossil fuels as an energy pillar for future social sustainable development.Semidconductor photocatalysis can make efficiently use of solar energy to produce hydrogen and degrade organic pollutants,which is regarde as an effective strategy to solve environmental and energy issues.Semidconductor photocatalysis has progress for more than 40 years since titanium dioxide was used in photocatalytic hydrogen production in 1972,and the semiconductor materials have gradually taken the stage from theoretical research to practical application.Graphite carbon nitride?g-C₃N₄?,one of carbon material family,is an environment-friendly metal-free material that exhibits potential excellent properties in the field of photocatalysis and has been extensively explored since it was proposed.Bulk g-C₃N₄ has intrinsic shortcomings due to its bulky morphology,such as low specific surfaces area and the high recombination of photogenerated e--h+ pairs,which limit the application of this material.Therefore,numerous efforts are focused on the research for preparation of g-C₃N₄ nanosheets with excellent photocatalytic activity.In recent years,scientists have prepared g-C₃N₄ nanosheets mainly by top-down strategies and bottom-up routes.However,these approaches have some shortcomings,such as low efficiency,tedious process and destruction of the conjugated CN structure.Hence,we still need to develop novel and thorough synthetic approaches.In this dissertation,we respectively employed chemical blowing method,pure water exfoliation route,blowing-template approach and graphene quantum dot?GQDs?assisted ultrasonic exfoliation strategy to prepare g-C₃N₄ nanosheets,and studied the degradation of organic dyestuff and hydrogen production performance under visible light illumination.Moreover,we characterized the structure and performance of materials employing many techniques,such as XRD,SEM,TEM,FTIR,AFM,Raman,UV-vis,PL,XPS and so on.Their photocatalytic performances and universal application were also investigated by photodegradation of Rh B and other dyes.The correlative works were summarized as follows.1.Ammonium chloride is decomposed into HCl and NH3 at high temperature.The generated gases,NH3 and HCl,did not only separate the CN sheets,but also prohibited the polymerization of g-C₃N₄ precursor of melamine,consequently forming nitrogen-richgraphitic carbon nitride nanosheets?NRCNNS?.The influence of dosage of chemical blowing agent on the morphology,structure and physicochemical property of NRCNNS was systematically studied.The optimal NRCNNS material exhibited a promoted activity,stability and reusability for degradation of different organic dyes under visible light irradiation.Moreover,a possible mechanism was proposed based on the band structure of NRCNNS.2.Porous g-C₃N₄ flake?PCNF?was synthesized via thermally polymerizing melamine with the presence of Mg CO3 that was used as a blow agent and template agent.CO2 gas released from the decomposition of Mg CO3 prohibited the further polymerization of g-C₃N₄ framework and separated the CN layers to produce a flake-like morphology,and meanwhile the removal of Mg O nanoparticles resulted in a porous structure.The resultant PCNF exhibited nice photocatalytic performance due to its good optical absorption and effective photoelectric-hole pair separation.This synthesis method is not only facile and efficient,but also able to control the morphology of materials,which provide a potential method to prepare other porous two-dimensional materials.3.We reported a new and environment-friendly strategy to exfoliate g-C₃N₄ into nanosheets?CNNS?with the intercalation of water molecules and the hydrolysis of bridge-linked N units.The physicochemical characterization indicated that the as-prepared CNNS had a typical 2D morphology with a ¹.2 nm thickness and numerous-OH groups on surface.In addition,the high charge separation and transport ability was achieved in CNNS because of the retaining of conjugated CN system.CNNS exhibited an enhanced photocatalytic performance,superior reusability and the excellent generality for decomposing various pollutants.Finally,we proposed a possible mechanism of CNNS based on the band structure characterized by XPS,UV-vis and other characterization techniques.The main active species were determined by quenching of various active species.4.We one-step prepared well dispersed GQDs modified CNNSs?GQDs/CNNSs?colloids via a facile and sufficient GQDs assisted exfoliation approach in a normal ultrasonic water bath.The exfoliation procedure was optimized by tuning the dopant in GQDs,ultrasonic time and GQDs dosage,and the obtained colloidal GQDs/CNNSs showed a typical 2D morphology with lateral size of several hundred nanometers and ultrathin thickness of 1.5-1.8nanometers.What was more,we could tailor the semiconductive behavior of GQDs by heteroatom doping and achieved a p-n type P-doped GQDs?P-GQDs?modified CNNSs colloids.This p-n GQDs/CNNSs material presented the enhanced separation efficiency of photo-excited carriers and photocatalytic activity in comparison with bulky g-C₃N₄ and other CNNSs materials from acid or alkali exfoliation.