Graphitic Carbon Nitride With Element Doping For Improved Photocatalytic Performance

The energy crisis and environmental deterioration caused by the rapid development of human society are becoming more and more serious.Photocatalysis technology,as a green technology,has attracted much attention for photocatalytic hydrogen evolution and pollutant degradation under sunlight irradiation.Graphitic carbon nitride?g-C₃N₄?is favored by many researchers because of its unique electronic structure,metal free materials and excellent responce to visible light.However,it is restricted by small specific surface area,easy recombination of electron-hole pairs and low light utilization.Therefore,the current research is focused on how to improve the light utilization and photocatalytic activity of g-C₃N₄.We intend to synthetize g-C₃N₄ microwires and nanowires to increase the specific surface area and tune the electronic band structure by doping Na,Fe and S elements to enhance photocatalytic activity for hydrogen production and pollutant degredation.In the first part,we fabricated porous fiber-like g-C₃N₄ photocatalysts by calcining melamine via a facile supramolecular hydrogel approach.The crystal structure,morphology,bonding state and the specific surface area of photocatalysts were characterized by XRD,SEM,TEM,FTIR,XPS and BET.The photoelectric properties were characterized by UV-Vis,PL and photoelectrochemical experiments.The photocatalytic performance was characterized by photocatalytic hydrogen evolution from water splitting under solar irradiation.The results show that the photocatalytic hydrogen evolution performance was enhanced via the larger specific surface area by providing more active sites on the surface.For the pure g-C₃N₄ synthesized by the same precursors,the greater crystallinity and the degree of polymerization,the better photocatalytic hydrogen evolution performance.This experiment may provide a new pathway to enhance photocatalytic hydrogen evolution of g-C₃N₄.In the second part,the Na atoms enter the g-C₃N₄ lattice to form a bridge for electron-hole pairs separation and transfer by a supramolecular hydrogels method.The crystal structure,morphology,bonding state and the photoelectric properties of Na doped g-C₃N₄ were characterized,and the photocatalytic performance was characterized by hydrogen evolution from water splitting under visible light at?>420 nm.The results show that Na/g-C₃N₄ nanoswires were successfully synthesized and the specific surface area was greatly increased.The weak covalent bond of Na-N was formed by Na⁺with the solitary electrons of N atoms at the edge of aromatic ring exist in the conjugate plane structure.Na/g-C₃N₄ nanoswires are about 100-150 nm in diameter and tens of microns in length.Compared with g-C₃N₄ nanowires and bulk g-C₃N₄,the absorptivity of Na/g-C₃N₄ in visible region is obviously enhanced,and more photogenerated carriers can be generated and effectively separated under light.The photocatalytic activity of10Na/CN synthesized with 10 mmol NaBH₄ precursors was the best among all photocatalysts under visible light of?>420 nm.The hydrogen evolution rate is up to17.4?mol/h and the average quantum efficiency is 4.67%,which is 12.4 times and 35.0times higher than g-C₃N₄ nanoswires and bulk g-C₃N₄,respectively.The calculated charge density difference map for?001?lattice confirmed that the weak covalent bond of Na⁺with N atoms exist in the vacancy of g-C₃N₄ aromatic ring plane.The calculated band structure,HOMO and LUMO results show that Na doping changes the electronic band structure of g-C₃N₄,and it effectively promotes the separation of photogenerated carriers to enhances photocatalytic activity via increasing the separation degree of HOMO and LUMO.The mechanism of Na doped g-C₃N₄ nanowires enhancing photocatalytic activity was deeply studied by experimental tests and theoretical calculations,which provided an effective way to synthesize high photocatalytic activity photocatalyst.In the third part,the visible light photocatalytic performance of graphitic g-C₃N₄can be enhanced by tuning its electronic structure and bandgap via metal and nonmetal elements doping.The Fe and S codoped g-C₃N₄ was synthesized by the polymerization of melamine,iron chloride and trithiocyanuric acid at elevated temperature and characterized as crimped nanosheets with mesoporous structures.The photocatalytic performance of Fe-S codoped g-C₃N₄ for RhB degradation increases seven times by enhancing visible light adsorption and increasing the mobility of photoinduced electron/hole pair by narrowing its bandgap compared to the pure g-C₃N₄ nanosheets.The synergetic effect of Fe ion coordinated in the pore centre among three triazine units and S dopant substituted the N in triazine skeleton causes much stronger delocalized HOMO and LUMO and increases the reactive sites,facilitating the migration of photogenerated charge carriers,thus enhances the visible-light driven photocatalytic performance.