High-Temperature Modification Of G-C₃N₄ And Their Photocatalytic Properties

As an organic semiconductor photocatalyst,graphitic carbon nitride?g-C₃N₄?has attracted much attention because of its high thermal and chemical stability,medium-bandgap?ca.2.7 eV?and wide-ranging photocatalytic applications.However,limited photocatalytic activity of bulk g-C₃N₄ hardly meets the practical requirements for application.Thus,a series of methods were used to modify g-C₃N₄,including morphological design,atomic-level doping,molecular-level copolymerization and construction of g-C₃N₄-based semiconductor composites,which provide a relatively deep understanding of the properties of g-C₃N₄.However,high thermal and chemical stability of g-C₃N₄ leads to the difficulty for surface modification of g-C₃N₄ by conventional chemical methods.Herein,we find g-C₃N₄ is instable under high temperature and can produce a series of derivatives with high molecular weights owning to the breaking of polymer molecular chains.Therefore,the properties based on breaking of g-C₃N₄ molecular chains were utilized to modify g-C₃N₄.The main contents are as following.?1?Excellent photocatalytic hydrogen evolution was achieved by high-temperature inducing breaking of g-C₃N₄ molecular chains.Bulk g-C₃N₄ was synthesized by the thermal condensation of dicyandiamide at 550°C.Then,bulk g-C₃N₄ was pyrolysed at 550°C with the evolution of the derivatives with high molecular weights under N₂ protection.Among the resultant products,CN-6h?heated for 6h?show 14 times higher photocatalytic activity for hydrogen production than g-C₃N₄ owning to the high surface area,expanded visible absorption and enhanced separation efficiencies of photogenerated electrons and holes.?2?The modified g-C₃N₄?CNS?was fabricated by CVD method during which gaseous derivatives derived from bulk g-C₃N₄ deposited on the inner wall of quartz tube.Bulk g-C₃N₄was synthesized by the thermal condensation of dicyandiamide at 550°C.Then,bulk g-C₃N₄was pyrolysed at 700°C and produced lots of derivatives with high molecular weights.With these derivatives deposited on the inner wall of quartz tube,the CNS was obtained.The breaking of tri-s-triazine rings leads to the apperance of C?N groups and the ineluctable introduction of H₂O makes CNS produce OH groups.As a result,CNS shows a small bandgap and expanded visible absorption.It provides a new method to study the chemical properties of g-C₃N₄.?3?The modified g-C₃N₄?B-CN?was fabricated by interfacial reaction of B-CN,halide salts?NaCl,KCl?and H₂O under high temperature,which produced C?N groups,OH groups and corresponding metal ions on its surface.B-CN was firstly synthesized by the thermal condensation of dicyandiamide at 550°C.Then,a series of modified B-CN were fabricated by chemical reaction of B-CN,halide salts?NaCl,KCl?and H₂O at 550°C for different time?Na-4h,K-3h,Na-K-1h,Na-K-2h and Na-K-4h?.Cl^-was released in the form of NH₄Cl;the remaining metal ions?Na⁺,K⁺?and C?N groups derived from the breaking of tri-s-triazine rings form the coordinate structure?N=C=N^-M⁺?.The ineluctable introduction of H₂O makes the modified B-CN produce OH groups.The electronic band structure can be regulated by changing heating time,and metal-ion loading increases with enhanced heating time.Because of the synergistic reaction of NaCl and KCl,the resulting Na-K-4h shows high surface area,expanded visible absorption and enhanced separation efficiencies of photogenerated electrons and holes compared to B-CN.As a result,Na-K-4h displays almost 26 times higher photocatalytic activity for hydrogen production than B-CN.