Study On Surface Modification Of Graphite Carbon Nitride Photocatalytic Materials By Atomic Layer Deposition

The rapid development of modern science and technology brings much convenience to people's life,but environmental pollution and energy shortage have emerged as an urgent issue.How to realize sustainable development of human society has become a concern.Sunlight is an inexhaustible energy source.Photocatalytic degradation of organic compounds under the sunlight illumination is a promising approach to solve water pollution.Graphite carbon nitride(g-C3N4),as a typical non-metallic semiconductor material,has been proven for the first time in 2009 to decompose water for hydrogen under visible light.It has been extensively and intensively studied due to its unique electronic structure,physical and chemical stability,easy synthesis,and abundant resource.However,its disadvantages,such as high recombination rate of photo-generated electron-hole pairs,low quantum efficiency,small specific area,and slightly larger bandgap,impede its practical applications.To improve the photocatalytical activity and make full use of visible-light energy,the modification of g-C3N4 materials has become a hotspot.Atomic layer deposition(ALD)is a novel thin film deposition technique that has been rapidly developed in the last two decades.Based on sequential self-limited and self-saturated chemisorption reaction mechanism,ALD is very suitable for surface modification of materials with broad application prospects in the fields of energy and photocatalysis.In this thesis,g-C3N4 powders,synthesized by thermal polymerization of melamine,were used as the substrates and modified by novel ALD technique.Different cycles of TiO2 or Fe2O3 were deposited on g-C3N4 powders in order to explore the effect of ALD cycles and post-anneal on the visible light photocatalytic activity and reaction kinetics.The crystal structure,elemental composition,chemical bond,morphology,microstructure,specific surface area,band gap and fluorescence spectra of various samples have been characterized systematically to study the possible photocatalytic mechanism of TiO2/g-C3N4 and Fe2O3/g-C3N4 composite systems.Main achievements can be summarized as follows1.TiO2 with various cycles of 20,50 and 100 were deposited on g-C3N4 powders by thermal ALD and post-annealed at 500? in air.The effect of post-annealing and ALD cycle number on the photocatalytic properties of TiO2/g-C3N4 samples was investigated deeply.50-cycle annealed TiO2/g-C3N4 sample exhibits the highest photocatalytic activity with the 98.2%degradation of methyl orange in 90 min under visible light irradiation.The first-order reaction rate constant is 0.0406 min-1 with the half-life period of only 17 min,which is 5.8times higher than that of the pure g-C3N4 powders.The corresponding photocatalytic reaction mechanism is proposed based on a series of characterization results.The enhanced photocatalytic activity of annealed TiO2/g-C3N4 sample can be ascribed to two reasons.First,as-deposited TiO2 is amorphous,and 500? postanneal produces anatase TiO2 nanocrystals on g-C3N4 powders with the formation of heterojunctions between TiO2 and g-C3N4.The heterojunctions of TiO2/g-C3N4 promote the separation of photo-generated electrons and holes with improved photocatalytic efficiency.Second,thermal stripping of g-C3N4 occurs during the annealing process,leading to the significant increase the specific surface areas from 3.3m2/g to 31.2m2/g and the greatly enhanced adsorption capacity of organic pollutant molecules.2.Fe2O3 with various cycles of 50,100,200,400 and 600 were deposited on g-C3N4powders by ozone-assisted thermal ALD technique.Among them,100-cycle Fe2O3/g-C3N4 sample shows the optimal photocatalytic activity.Compared to pure g-C3N4,the degradation rate of methyl orange in 100 min increases from?-44.6%to-60.4%.The first-order reaction rate constant is 0.00896 min-1 and the half-life period is 77 min.According to a series of characterizations,it can be deduced that a part of Fe elements enter into the macrocyclic unit of pyridinium nitrogen in 3-s-triazine structure of g-C3N4 in the form of Fe doping ions.One hand,the formation of Fe-N ligand bonds act as catalytic active sites to promote the separation of photo-generated electrons and holes.Meanwhile,the doped Fe ions change the energy band structure of g-C3N4 with the increased visible light adsorption and the reduced optical bandgap by?0.16 eV.On the other hand,a part of Fe elements form amorphous Fe2O3 on the surface of g-C3N4 as recombination centers of photo-generated carriers,thereby causing a decrease in degradation efficiency.The comprehensive role of the two factors contributes to the enhanced photocatalytic performance of 100 cycles Fe2O3/g-C3N4,however it is not as remarkable as the 50 cycles annealed TiO2/g-C3N4 composite sample.