Alkali Metal Salts Assisted Modification Of Carbon Nitride And Their Photocatalytic Application

Graphite phase carbon nitride（g-C₃N₄）,as an organic semiconductor catalyst,has been favored by researchers because of its excellent physical and chemical stability,moderate band gap（about 2.7 e V）and extensive photocatalytic applications.However,it is the excellent physicochemical stability of g-C₃N₄ that makes it difficult to enhance its photocatalytic performance by surface modification.The alkali metal halide salt has the potential to modify the surface of the carbon nitride because of its halide anion which easily removes the amino group on the surface of the carbon nitride.Here,we promote the transfer of electrons by modifying the carbon nitride with alkali metal halide salts,and adjusting the lattice structure of carbon nitride with high crystallization.The alkali metal salts in the unmelted condition can be used as a template to increase the surface area of the modified carbon nitride.In addition,part of the alkali metal ions could coordinate with the carbon nitride,leading to the electron cloud distribution change of the carbon nitride and the optimization of carriers’directional migration.Based on the modification mentioned above,the photocatalytic performance of g-C₃N₄ is improved.Unlike previous report,we developed a feasible method to improve the crystallinity of carbon nitride using unmelted Cs Cl as mild chemical scissors to trim pristine carbon nitride instead of its precursor or the intermediates with incomplete structure.As a result,the regular poly（heptazine imides）structures with higherπconjugation were exposed to the surface,wherein the bonded Cs⁺ions on the surface of carbon nitride changed the charge distribution.The high regular structure can wipe out the defect sites reducing recombined sites of electron and hole,and poly（heptazine imides）structure can greatly reserve the visible light absorption ability from pristine carbon nitride.Benefiting from these excellent features,the resultant product exhibits excellent photocatalytic hydrogen evolution activity,which is about 23 times higher than that of bulk carbon nitride.We use a series of alkali metal salts（Na I,KI,RbI,CsI,CsBr,CsCl）,CN and a small amounts of deionized water as raw materials,and calcined at 550°C for a certain time to modify carbon nitride.The lattice structure can be observed in the modified carbon nitride,and the introduction of water causes more hydroxyl groups on the surface to improve the hydrophilic property of the material.During the modification process,the alkali metal cations can coordinate to the carbon nitride,which further influences the charge distribution on the surface of the carbon nitride,thereby realizing the separation of photogenerated electrons and holes.Finally,the photocatalytic seawater is applied to hydrogen evolution.Since the surface of the modified carbon nitride contains a large amount of negative charges,it is easier to adsorb the cations and Na-TEOA in the seawater aqueous solution,forming an electrical layer structure model,and the electric layer structure.The model can further optimize the optical activity of the material,improve the separation of charges,and make the holes more easily react with the sacrificial agent,and finally the hydrogen evolution effect is obviously improved.Compared with the effect of bulk carbon nitride in deionized water,the hydrogen evolution effect of modified carbon nitride in seawater is increased by more than two hundred times.At the same time,the stability of hydrogen evolution has also improved.The carbon nitride was modified by NaCl,KI,CN and a small amount of deionized water,and calcined at 550°C for two hours under a nitrogen atmosphere.The modified carbon nitride can be seen the lattice structure,and the introduction of water bings the surface more hydroxyl groups,so that it can be dispersed into a colloid in water.During the modification process,the Na⁺and K⁺cations are coordinated to the carbon nitride,which further affects the charge distribution on the surface of the carbon nitride,thereby separating the photogenerated electrons and holes,and generating a large amount of negative charges on the surface of the carbon nitride.Finally,because of the synergistic effect of Na and K,photocatalytic degradation of Rhodamine B by 88%in 10 min.In addition,its degradation mechanism was further explored.