Preparation Of GC₃N₄-based Photocatalytic Nanomaterials And Their Hydrogen Production Performance

In the face of increasingly urgent energy and environmental issues,the development of clean,renewable energy to maintain human sustainable development is imminent,and the conversion of solar energy into hydrogen energy through semiconductor photocatalysis technology is considered one of the most promising solutions.Graphitic carbon nitride（g-C₃N₄）has proven to be one of the most promising semiconductor catalysts for photocatalytic hydrogen（H2）evolution.However,as a photocatalytic material,pristine g-C₃N₄ still shows unsatisfactory properties due to the deficient specific surface area,inadequate light absorption ability and high recombination rate of photoinduced carriers,which limits its practical application.This work focuses on the controllable preparation methods of P-doped g-C₃N₄nanosheets（PCNNSs）,Pd/PCNNSs composites,and MoS₂/PCNNSs composites.The structure and morphology of all samples were characterized by XRD,SEM,TEM,XPS,etc.and the photoelectrochemical properties were analyzed by UV-Vis absorption spectra,photoluminescence and electrochemistry,etc.Combined with the photocatalytic activity measurements,the relationship between the photocatalytic H₂ evolution performance and the structure was further explored.The main contents and conclusions of this study are as follows:（1）Porous P doped g-C₃N₄ nanosheets（PCNNSs）were prepared from urea and adenine phosphate by thermal condensation and multiple thermal oxidation exfoliation.The porous structure of PCNNSs increases its specific surface area and provides more active sites for the reaction.P doping introduces impurity energy levels,optimizes the transfer path of photogenerated electrons,which makes the visible light absorption of the material obviously enhance in the range of 450⁷00 nm.Therefore,the photocatalytic H₂ evolution activity of PCNNSs is significantly enhanced.When the amount of P source added is 2 wt%of urea,the H₂ evolution rate is the highest,which is about 20 times that of the original g-C₃N₄.（2）Based on the porous P doped g-C₃N₄ nanosheets（PCNNSs）,Pd/PCNNSs composites were prepared by in situ reduction under ice bath by using NaBH₄ as reducing agent and PdCl₂as the Pd source.The introduction of Pd NPs effectively transfers electrons,optimizes the separation efficiency of photogenerated electron-hole pairs,and enhances the n→π*electron transition of PCNNSs,which increases the absorption of visible light in the range of 450⁷00nm.Therefore,the photocatalytic H₂ evolution activity of the Pd/PCNNSs composites is significantly improved under visible light.The highest H₂ evolution rate can reach 1523μmol?g^-1?h^-1 when the loading of Pd is 2 wt%,which is about 8 times that of the PCNNSs,and it shows excellent photocatalytic stability.（3）MoS₂/PCNNSs composites were prepared by impregnation-pyrolysis by using porous P doped g-C₃N₄ nanosheets（PCNNSs）as matrix and ammonium molybdate and thiourea as raw materials.The different migration directions of photogenerated electrons and holes reduce the probability of their recombination.The type II heterojunction is formed between the MoS₂and PCNNSs,and the photogenerated electrons that transition to the conduction band（CB）of PCNNSs migrate to the conduction band of MoS₂ under the action of energy difference,meanwhile,the holes on the valence band（VB）of MoS₂ migrate to the valence band of PCNNSs.The different migration directions of photogenerated electrons and holes reduce the probability of their recombination.In addition,the narrow band gap of MoS₂ extends the light absorption range from⁷00 nm to⁸00 nm.Therefore,the photocatalytic H₂ evolution activity of the MoS₂/PCNNSs composites is significantly improved under visible light.The highest H₂ evolution rate can reach 1741μmol?g^-1?h^-1 when the addition amount of ammonium molybdate is 0.1 mmol,which is about 9 times that of the PCNNSs.