Study On Surface/interfacial Modulation Over G-C₃N₄ And Relevant Photocatalytic Properties

Graphitic carbon nitride?g-C₃N₄?is a new metal-free and visible-light-sensitive organic semiconductor.It has attracted extensive attention in recent years due to its low cost,ease of synthesis,and appropriate band structure.However,the low photocatalytic efficiency is still one of the most important factors to restrict the practical application of g-C₃N₄,which can be ascribed to its poor crystallization,excessive defect and therefore high recombination rate of photocarriers.Surface/interface modulation is one of the intensive methods to enhance the photocatalytic activity.Current study status of surface/interface modulation over g-C₃N₄ can be summarized as following.Firstly,alkali treatment of g-C₃N₄ is restricted to be a post-treatment process,such as soaking samples in alkaline solution and tuning the pH value of the photocatalytic reaction solution.Secondly,few studies are focused on the simultaneous adoption and corresponding synergistic effect of multiple surface/interface modulation methods.Thereby,two topics were carried out in this thesis and the main results and conclusions are as below.We developed an in situ method to graft hydroxyl groups over the g-C₃N₄photocatalyst by K ions modification during the polymerization of melamine.In situ surface alkalization achieving a 15-fold enhancement compared with the pristine g-C₃N₄ sample in photocatalytic H₂ evolution,which significantly higher than those of ex situ methods ranging from 2⁶ times.The enhanced H₂ evolution is attributed to the more negative conduction-band level of the sample compared to the pristine g-C₃N₄sample via K ion modification,which offer a stronger potential for water reduction.Moreover,surface hydroxyl groups can effectively trap the holes to reduce the recombination of photo-generated carriers.Our study demonstrates that a solid-gas interfacial Fenton reaction can be constructed via modifying hydroxyl and Fe³⁺species over g-C₃N₄-based photocatalyst.This photocatalytic system exhibits highly efficient and universal activity for photodegradation of volatile organic compounds?VOCs?including isopropanol,acetaldehyde,acetone,acetic acid and Toluene.Taking the photooxidation of isopropanol as model reaction,this system achieves a photoactivity of 2-3 orders of magnitude higher than that of pristine g-C₃N₄,which corresponds to a high apparent quantum yield of 49%at around 420 nm.So far,it's the record among the reported g-C₃N₄-based photocatalysts for degradation of pollutions.The significant photoactivity enhancement could be ascribed to the efficient and synergetic utilization of photoelectrons and photoholes to produce abundant oxygen-related radicals.Specific mechanism can be summarized to a three-step cascading reaction to generate reactive radicals?such as·OH and·O₂^-?as following:?1?the surface hydroxyl groups are activated by photoholes to generate hydroxyl radicals which oxidize VOCs to produce protons;?2?the O₂ and protons evolve into H₂O₂ via a two-photoelectron reduction process;?3?the Fe²⁺/Fe³⁺pair reacts with the H₂O₂ to provide oxygen-related radicals.This study provides the most sufficient and convincing evidence for solid-gas interfacial Fenton reaction for the first time.The proposed material surface modulation coupled with solid-gas interfacial Fenton reaction may open a new route for the efficient production of active radicals at photocatalyst surface,which therefore significantly promotes photocatalytic oxidation.In summary,this thesis indicate that the surface/interface modulation is one of the most effective strategies to promote the photocatalytic activity of C₃N₄.