Study On Surface Modification Of Carbon Nitride With Non-noble Metals And Its Photocatalytic Performance

With the rapid development of industry,the society’s demand for energy has risen sharply.Looking for renewable energy to replace traditional non-renewable energy is considered an effective way to solve environmental problems and energy crises.Solar energy is the most abundant renewable energy on earth,and converting solar energy into storable chemical energy through photocatalytic technology is one of the effective ways to solve energy and environmental problems.The development of high-efficiency,stable,and inexpensive catalysts for hydrogen production by photo-splitting water is the focus of research in this field.Graphite phase carbon nitride（g-C₃N₄）is composed of C and N elements rich in earth.It has many advantages,such as simple synthesis process,suitable energy band structure and good chemical stability,so it is used in photocatalytic water decomposition.It has broad application prospects in hydrogen production.This dissertation first summarizes the recent research progress of g-C₃N₄surface modification system in photocatalytic hydrogen production,and then prepares a series of g-C₃N₄-based photocatalytic materials by means of phosphorus element structure control and cocatalyst modification.It solves the shortcomings of the pure phase g-C₃N₄,such as poor visible light absorption,high photo-generated carrier recombination rate,and low conductivity.And we are conducting research on the mechanism of photocatalysis.The specific research content is as follows:1)The adjustment of the band structure and the improvement of carrier separation efficiency can be achieved through the design of dual-site phosphorus-modified photocatalysts with a regulated band structure and surface state.We designed a two-site phosphorous modified g-C₃N₄photocatalyst based on the special redox properties of ammonium hypophosphite.Compared with pure g-C₃N₄and traditional single-site phosphorus-modified g-C₃N₄,the photocatalytic activity of dual-site phosphorus-modified g-C₃N₄is significantly improved.The results confirm that two existence forms of P–N coordination were induced by the addition of NH₄H₂PO₂,one involving the replacement of C by P atoms in the frameworks,and the other involving P surface states in the form of the P–O bond.In-depth studies have shown that this two-site modification can not only adjust the energy band structure of g-C₃N₄,but also improve the separation efficiency of its photogenerated carriers,thereby speeding up the photocatalytic hydrogen production reaction.2)The surface of g-C₃N₄was modified with Mo P as a cocatalyst,and the non-noble metal modification on the surface of g-C₃N₄was realized,and the mechanism of Mo P was studied.The Mo P/g-C₃N₄composite material was prepared by a simple physical compounding method.Without the noble metal platinum modification,the composite material has good hydrogen production performance under visible light irradiation.This is due to the fact that Mo P modification improves the carrier separation efficiency and transport efficiency of g-C₃N₄,and reduces the overpotential of the hydrogen production reaction on its surface.3)Prepared phosphorylated Ni Al layered double hydroxide（P-LDH）nanosheets.Using it as a cocatalyst for the photocatalytic hydrogen evolution reaction of g-C₃N₄,the structure and formation process of P-LDH as well as the influence mechanism on the photocatalytic hydrogen evolution reaction were systematically studied.The P-LDH and g-C₃N₄composite photocatalyst was prepared by in-situ method,which showed excellent photocatalytic hydrogen production activity.The characterization results show that P-LDH has a lower hydrogen evolution overpotential and faster hydrogen evolution reaction kinetics in a neutral electrolyte.In addition,the surface work function of P-LDH is higher than that of g-C₃N₄,and the interfacial electric field directed from g-C₃N₄to P-LDH can be formed after recombination.Therefore,the modification of P-LDH can effectively improve the interface charge transfer efficiency of g-C₃N₄,inhibit the recombination of photogenerated carriers,and reduce the surface overpotential of g-C₃N₄.