Study On G-C₃N₄-Based Photocatalysts:Design,Synthesis And Photocatalytic Performance For Hydrogen Production

Hydrogen(H2)production via photocatalytic water splitting driven by solar energy is highly considered to be a promising alternative energy source in solving the global energy crisis and environmental pollution.Recently,graphitic carbon nitride(g-C3N4)has emerged as a prominent metal-free organic semiconductor for visible-light photocatalytic H2 production from water splitting.However,the photocatalytic activity of pure g-C3N4 is unsatisfactory due to the fast recombination rate of photogenerated electron-hole pairs.In order to solve the above challenge,in this dissertation we have strategically designed,synthesized and studied economically favorable and highly efficient g-C3N4-based heterostructured photocatalysts to enhance charge migration and separation and finally to improve the photocatalytic H2 production.In addition,the proposed mechanisms of electron-hole transport in g-C3N4-based heterostructured photocatalysts are studied.This study includes three parts which are summarized as follows:In the first part,we have synthesized highly efficient visible light driven C3N4/CoTPP type-? heterostructured photocatalysts for H2 production from water splitting by decorating cobalt(?)meso-tetraphenylporphine(CoTPP)molecule on the surface of g-C3N4 via ?-?stacking interaction.The introduction of CoTPP molecules not only enhances the visible light absorption but also accelerates the charge migration and transfer through the formation of C3N4/CoTPP type-II heterojunctions.When 4 wt%CoTPP is introduced onto g-C3N4,the H2 evolution rate of C3N4/CoTPP hybrid is significantly increased up to 46.93 ?mol h-1 under visible-light irradiation(??420 nm),which is 2.73 times higher than that of g-C3N4 alone(17.21 ?mol h-1).Furthermore,the C3N4/4 wt%CoTPP composite shows good stability for H2 production over continuous irradiation of up to 20 h.Conclusively,the g-C3N4 photocatalyst merged with CoTPP conjugated molecules significantly improves the photocatalytic efficiency and stability for H2 evolution from water splitting under visible light irradiation.This study provides an opportunity on developing other highly efficient g-C3N4-based photocatalytic systems for solar-to-fuel conversion.In the second part,a high efficiency C3N4/NPBIm type-? heterostructured photocatalysts are prepared by decorating nickel(?)-3-pyridyl benzimidazole(NPBIm)molecule on the surface of g-C3N4 nanosheets through ?-? interaction,and the interfacial charge transfer through g-C3N4/NPBIm interface is thus improved.The maximum H2 evolution rate of 46.97 ?mol h-1 is achieved when loading an optimal 6 wt%of NPBIm molecule under visible light irradiation(??420 nm),which is 3.5 times higher than pure g-C3N4(13.47 ?mol h-1)with corresponding quantum efficiency of 5.13%at 420 nm.Moreover,the C3N4/NPBIm composite exhibits excellent stability toward H2 evolution after recycling experiments,which demonstrates its potential in the practical application of this composite photocatalysts.The separation and migration of electron-hole pairs are greatly enhanced between g-C3N4 and NPBIm molecule,as clearly revealed by the photoluminescence spectra,time-resolved photoluminescence spectra and photoelectrochemical results.Taken together,this study could provide opportunity to design and develop other highly efficient type-?heterostructured photocatalyst for the improvement of H2 production from water splitting under visible light irradiation.In the last and third part,we develop a novel C3N4/CoBP composite photocatalysts by decorating cobalt-boron-phosphide(CoBP)molecule on the surface of g-C3N4 via ultrasonic process for efficient visible light H2 production from water splitting.The obtained C3N4/CoBP hybrid photocatalyst exhibits remarkable photocatalytic performance in H2 evolution reaction,and reached an optimum H2 evolution rate of 51.68 pmol h-1 under visible-light irradiation(??420 nm),which is about 3.8 times higher than that of pristine g-C3N4(13.4 ?mol h-1).Meanwhile,the C3N4/CoBP composite photocatalyst shows good photostability and recyclability for H2 evolution over a long-term visible light illumination.Based on the detailed analyses,the remarkably photocatalytic performance of C3N4/CoBP composite photocatalyst is mainly attributed to the formation of heterojunction between g-C3N4 and an amorphous CoBP molecule,which accelerates the migration and separation of photogenerated charge carriers and effectively suppresses the charge recombination.Overall,this study provides a new opportunity for the design and synthesis of highly efficiency composite photocatalysts for energy conversion and utilization.