Designing and Synthesis of Graphitic Carbon Nitride Based Photocatalysts with High Activity

Author:Liu Qiong

Supervisor:fang xiao ming


Degree Year:2018





Semiconductor photocatalytic technology is one of the most promising strategies to solve the energy crisis and environmental contamination issues under the current economic and social development.The application of photocatalysts would transfer the solar energy into the chemical energy such as hydrogen or methaol which is facile to transport and storage,or leading the solar iradiation as the energy source to initiate photocatalytic oxidation reaction to degraded the refractory organic pollutants.The development the high efficient,stable and cheap photocatalysts is the crucial issue of photocatalytic technology.However,various kinds of photocatalysts material have been developed,few of them could meet the large-scale practical application.Graphitic carbon nitride(g-C3N4)has be considered as a promising photocatalytic material because of its wide range of raw materials,simple preparation process and good chemical stability.However,as the high combination possibility of photogenerated electron-hole pairs,the relatively small visible light absorption range which render g-C3N4 exhibited the low photocatalytic activity.This thesis is devoted to develop the high photocatalytic activity of g-C3N4 based materials,which mean to focus the strategies of wide spectrum absorption,high efficiency charge separation and nanostructured engineering.The mainly research contents are as follow:1.Constructed an kind of 2D/0D type photocatalyst of g-C3N4 nanosheets/carbon nanodots(CQDs)which based on the type-II van der Waals heterojunction,the CQDs well dispersed and uniformly anchored into the CN network viaπ-πstacking interactions.As CQDs facilitate electron transfer and serve as the surface activity sites,which could suppress the recombination of photo-induced carriers.Additionally,The incorporation of CQDs can effectively expand visible light absorption regions via photosensitization and upper coversion effect,the more charge can be generated which leading the obtained samples exhibit high photocatalytic activity of hydrogen release under the visible light irradiation.2.Developed two kinds of Fe(III)/graphene/g-C3N4 and Fe(III)/CQDs/Fe-CN ternary composites with the wide optical spectrum absorption property.The interfacial charge transfer effect(IFCT)was introduced to g-C3N4 for largely enhanced the visible absorption region by grafting Fe(III)species on the surfaces of g-C3N4.And the energy gap is obviously reduced,the optical absorption region is extended to the near infrared region among 800-1200 nm.The incorporation of graphene and carbon quantity dots in the interfacial of Fe(III)and g-C3N4,which optimize the grafted of Fe(III)and facilitate the charge carriers transportation.When using the samples for photodegradation of MO,the degraded rate by using the former ternary sample reaching 82%and the other is 93%.After using the TOC and LC-MS to analysis the decompose process and mineralization,verifying the photocatalysts have the relatively strong oxidation capability.From the ESR probe technology and radical scavenger experiments,it is found that·OH active species could be generated from the ternary Fe(III)/CQDs/Fe-doped g-C3N4 photocatalyst under visible light irritation originating from the cooperation between the IFCT effect and the photo-Fenton reaction.And the·OH and hole adducts erve as the main active species.This work demonstrated a simple way to to modify the optical absorption and enhance the oxidation capability of pure CN by the introduction of the IFCT effect.3.A facile process for synthesizing the supramolecular precursor is presented.The precursor obtained from the hydrothermal treatment of dicyandiamide exhibits a similar structure to that of the melamine-cyanaurate(MCA)complex.After the hydrothermal process,the holey g-C3N4 nanosheets could be synthesized,the surface specific area and pore volme are enhanced.And by modification of the thermal polymerization process,the holey carbon nitride(HCN)nanosheets with high crystallization network and extended optical absorption region(600 nm)can be obtained,which the hydrogen evolution rate is enhanced 11.4 folds.And based on it,we also synthesize an other kinds of ultrathin g-C3N4 nanosheets with the uniform mesoporous,the hydrogen evolution rate is enhanced 15.4 folds.4.Based on the former hydrothermal pretreatment of dicyandiamide route,we further synthesized the well-defined 1D pillar crystals over the supermolecular precursor by introducing the liquid-liquid interface via supermolecular self-assembly.After the high thermal polymerization,the polymerized carbon nitride which possesseses the one dimension nanotube morphology with hierarchy porous network.It is found that the internal of nanotube consisted of dentritic leaves-like nanosheets connected with each other and also possessed the abundant uniform mesoporous channels.This unique network facilitate the charge carriers and reactant molecules transportation and provide more surface active sites in favor of the surface reaction,also enhance the visible absorption capability as the multiple reflection and scattering when the photon induced.The hydrogen evolution rate of the obtained sample reached 277.12μmol/h,is32.4 folds of the pristine g-C3N4.This synthesis method would apply a new way to gain the 1D g-C3N4 nanostructured.