Graphitic Carbon Nitride Based Photocatalysts: Design,Synthesis And Properties Investigation

Over the past few years,the development of society and increased population not only lead to a series of environmental problems,but also increase the human demand for energy greatly.Environmental pollution and energy shortage have become two main challenges in the21^st century.It is urgent to develop new type of clean,environment-friendly and sustainable energy sources.The design and synthesis of efficient,simple and sustainable materials as solar-driven photocatalysts has been considered as one of promising strategies toward addressing the future environmental and energy issues,because photocatalysis can convert low density solar energy into high density electric and chemical energy.Among the numerous types of photocatalysts,graphitic carbon nitride?g-C₃N₄?,as a?-conjugated metal-free polymer,has attracted increasing attention due to its reasonable cost,suitable band structure,excellent structural and chemical stability.However,high recombination rate of photo-induced charges,low specific surface area and limited active sites remain the major obstacles to its development and practical applications.In order to improve the photocatalytic activity of g-C₃N₄,some strategies,including heterostructure design,microstructure control and molecular composition adjustment have been developed to overcome these drawbacks.The main results are listed as follow:?1?Ultrathin g-C₃N₄ nanosheets obtained from thermal oxidative exploitation bulk g-C₃N₄ in air atmosphere were used as an excellent substrate,and Ag₂WO₄ or Ag₂CO₃nanoparticles with high dispersion have been loaded on their surface by a facile deposition-precipitation method.The as-prepared Ag₂WO₄/g-C₃N₄ and Ag₂CO₃/g-C₃N₄heterostructure photocatalysts exhibit an enhanced electron-hole separation rate.The large surface area of g-C₃N₄ nanosheets not only increases the adsorption of pollutants,but also makes the heterojunction more effective,which greatly improves the synergetic effect between Ag₂WO₄?or Ag₂CO₃?and g-C₃N₄ nanosheets.In addition,the N-H groups or conjugated?structures in g-C₃N₄ can bond with Ag⁺tightly due to the chemical adsorption to inhibit the light corrosion of silver containing compounds.Under visible light irradiation,the as-prepared heterostructure photocatalysts show excellent photo-degradation activities for both RhB and MO pollutants.?2?Macroscopic foam-like holey ultrathin g-C₃N₄ nanosheets?CNHS?with abundant micro-,meso-and macropores have been fabricated through long-time thermal treating of bulk g-C₃N₄ under an air atmosphere.The as-prepared CNHS nanosheets possess a high specific surface area of 277.98 m² g^-1,more exposed new active edges and catalytic active sites,and the enhanced electron transport ability.The rich in-plane holes in CNHS are also beneficial to the rapid cross-plane diffusion of photo-generated carriers.Moreover,the macroscopic loose,porous,foam-like structure contributes to a high dispersity of CNHS in water,which can greatly improve the catalytic performances and applications in aqueous solution.Due to their unique physicochemical properties,CNHS exhibit a superior photocatalytic hydrogen evolution rate of 57.20?mol h^-1 under visible light irradiation,an apparent quantum efficiency of 4.03%at?=420±15 nm and a turnover number of 18.04 in1 h.?3?Exfoliation of layered bulk g-C₃N₄?CNB?to thin g-C₃N₄ sheets in nanodomains has attracted much attention in photocatalysis because of the intriguing properties of nano-scaled g-C₃N₄.In our work,carbon-rich g-C₃N₄ nanosheets?CNSC?were easily prepared by self-modification of polymeric melon units through successively thermally treating bulk g-C₃N₄ in an air and N₂ atmosphere.The prepared CNSC not only retain the outstanding properties of nanosheets,such as large surface area,high aspect ratios and short charges diffusion distance,but also overcome the drawback of enlarged bandgap caused by the quantum size effect,resulting in an enhanced utilization of visible light and photo-induced electron delocalization ability.As such,the as-prepared CNSC show a high hydrogen evolution rate of 39.6?mol h^-1 with a turnover number of 24.98 in 1 h at?>400 nm.Under irradiation by longer wavelength of light??>420 nm?,CNSC still exhibit a superior hydrogen evolution rate,which is 72.9 and 5.4 times higher than that of bulk g-C₃N₄ and g-C₃N₄ nanosheets,respectively.The apparent quantum efficiency of CNSC sample was tested to be 4.52,1.41,1.03 and 0.25%at wavelength of 420±15 nm,450±15 nm,475±15nm and 520±15 nm,respectively.?4?Compared to the intensive research on doping or constructing heterostructure,developing co-polymerization strategy for improving the photocatalytic performance of g-C₃N₄ has received much less attention,owing to the lack of appropriate polymerization route being exploited to introduce of organic motifs into carbon nitride networks.In the present work,benzamide is employed as a new,low-cost co-monomer of urea to prepare the phenyl group functionalized g-C₃N₄?CNPF?through a convenient one-pot thermal induction process.Introducing of phenyl group into melon frame can essentially extend the original?-conjugation system of g-C₃N₄,leading to the improved visible light utilization and increased separation rate of electron-hole pairs.Furthermore,adding some benzamide into urea during the polymerization process enriches the pore structure of the as-obtained CNPF samples,resulting in a larger specific surface area.Under visible light irradiation,the as-prepared CNPF exhibits a superior catalytic activity in a variety of photocatalytic systems,such as hydrogen generation from water,photo-degradation of dye and photo-reduction of Cr?VI?.