Preparation Of One-dimensional GC₃N₄ Nanostructures And Its Photocatalytic Hydrogen Production Performance

Photocatalytic water splitting for hydrogen generation using solar is an important way to solve the current problems of environmental pollution and energy shortage.However,general wide bandgap semiconductors have limited conversion efficiency of solar energy due to their response only to ultraviolet light.Therefore,photocatalysts with visible light response have received wide attention from researchers.Polymeric semiconductor graphitic carbon nitride?g-C₃N₄?has suitable band gap,non-toxicity,low cost,high thermodynamics and chemical stability.It is widely used in photocatalytic water splitting for hydrogen generation,photocatalytic carbon dioxide reduction,photocatalytic pollutant decomposition and photocatalytic bacteria disinfection.However,g-C₃N₄ has poor photocatalytic activity due to defects such as low surface area,rapid recombination of charge carriers and insufficient solar-light absorption.In view of the shortcomings of g-C₃N₄,this paper proposes the preparation of holey g-C₃N₄ nanotubes and triazine-heptazine based g-C₃N₄ nanorods with high crystallinity by supramolecular precursor strategy and molten salt post-treatment method,which increased the specific surface of g-C₃N₄ and reduced the rate of photogenerated electron hole recombination,thereby significantly improving photocatalytic production hydrogen activity.The main research contents and results of this paper are as follows:?1?A simple one-step thermal polymerization method was developed for synthesis of holey g-C₃N₄ nanotubes with intermediate band gap,involving direct heating of mixtures of cyanuric acid and melamine in mass ratios of 5:1.Melamine-cyanuric acid supramolecular polymer nanorods were formed by hydrogen bonding between cyanuric acid and melamine during calcination.With the increasing of temperature,the supermolecular polymer nanorods were transformed into holey g-C₃N₄ nanotubes with a length of 200-800 nm and a diameter of 150 nm.The walls of the nanotubes had thicknesses of 15 nm.The existence of intermediate band gap extended the light absorption of porous g-C₃N₄ nanotubes to the whole visible light region.The porous and nanotubular structure of the nanotubes facilitated efficient charge carrier migration and separation?photocurrent density of 0.18?A/cm²?,provided active sites and promoted the mass-transfer process for the surface reaction.Therefore,the holey g-C₃N₄ nanotubes exhibited the highest photocatalytic hydrogen evolution rate of687.4?mol h^-1 g^-1,which was 3 times higher than that of g-C₃N₄ samples prepared by direct thermal polymerization of melamine.?2?Urea can be converted to cyanuric acid in situ,and further to form melamine-cyanuric acid supramolecular precursor by hydrogen bonding between cyanuric acid and melamine.A simple one-step thermal polymerization method was developed for synthesis of holey g-C₃N₄ nanotubes with a length of 2-3?m and a diameter of 250 nm,involving direct heating of mixtures of melamine and urea in mass ratios of 10:1.The walls of the nanotubes had thicknesses of 40 nm.The formation process of holey g-C₃N₄ nanotubes was studied in detail.The holey g-C₃N₄nanotubes facilitated efficient charge carrier migration and separation?photocurrent density of 0.3?A/cm²?,provided more active sites and promoted the mass-transfer process for the surface reaction.Therefore,the holey g-C₃N₄ nanotubes exhibited the highest photocatalytic hydrogen evolution rate of 1073.6?mol h^-1 g^-1,which was 4.7times higher than that of g-C₃N₄ samples prepared by direct thermal polymerization of melamine.?3?The triazine-heptazine based g-C₃N₄ nanorods with high crystallinity were synthesized by simple calcination of heptazine based g-C₃N₄ in molten salt.The high crystallinity and the internal triazine-heptazine donor-acceptor?D-A?heterostructure significantly accelerated the interfacial charge transfer?photocurrent density of 1.6?A/cm²?,and the large specific surface can provide more catalytic active sites,thereby increasing the photocatalytic hydrogen production activity.Therefore,the triazine-heptazine based g-C₃N₄ nanorods with high crystallinity exhibited an excellent hydrogen production rate of 2980.8?mol h^-1 g^-1,which was 6.1 times than that of g-C₃N₄ obtained without molten salt treatment.