Preparation And Photocatalytic Properities Of Nanodiamond/g-C₃N₄ Composite

With the development of human society and the progress of science and technology,energy crisis and environmental pollution are also increasing,which have been becoming urgent problems to be solved on the road of social sustainable development at present.Based on semiconductor materials,photocatalytic technology,on the one hand,can convert the energy density lower solar energy to chemical energy with higher energy density;on the other hand,it can degrade organic pollutants without secondary pollution.However,traditional semiconductor photocatalysts?TiO₂ and ZnO?have some shortages,such as low photocatalytic activity,easy deactivation and high cost,etc,which seriously hinder the practical application of photocatalytic technology.Therefore,researchers are trying to develop a high-efficiency and practical photocatalyst.Graphiticcarbon nitride?g-C₃N₄?has attracted extensive attention in the field of photocatalys due to their simple preparation,abundant raw materials,environment friendly and special electronic structure.However,low surface area,high combination rate and lower light utilization rate lead to low photocatalytic efficiency.Carbon nano-materials,small biotoxicity,simple preparation and unique photoelectric properties,are commonly used to modify g-C₃N₄ to improve its photocatalytic efficiency.Nanodiamond?ND?,as one of carbon nanomaterials,has good chemical stability,thermal conductivity,and high carrier mobility,which may promote transfer of the photogenerated carriers.In addition,the surface of ND rich in oxygen-containing functional groups,can provide more active sites for photocatalytic reaction.All of these properties have made ND to be a potential good photocatalyst.In this paper,we prepared ND and g-C₃N₄ composite,forming a semiconductor heterojunctions,which should promote the separation of photogenerated carriers,reducing recombination rate.On this basis,boron doped nanodiamond?BDND?with relative low band gap was coupled withg-C₃N₄.The BDND@g-C₃N₄ heterostructure exhibits a higher photocatalytic activity.The detailed contents of this thesis are as following:1.Research on morphology structure and photoelectric properties of ND.The morphology and structure of ND after pickling purification were studied.The results suggest that the average size of ND grains was 50 nm,which are composed of individual diamond nanocrystals with a size of 3⁵ nm.ND is a nuclear–shell structure,in which sp³ hybridized carbon was surrounded by a layer of sp²hybridized carbon,and outermost layer of ND is rich in oxygen-containing functional groups,such as hydroxyl,carboxyl and carbonyl,etc.Its special structural characteristics make the band gap of ND?3.25 eV?is lower than block diamond?5.47eV?.ND has a strong light scattering effect in aqueous solution.The excitation-dependent fluorescence from NDs has a large-scale red shift from 400 to620 nm under different excitation.The fluorescence from ND can be attributed to the combined effects of the fraction of sp² hybridized carbon and the defect energy trapping states on the surface of diamond.The ND with excitation-dependence fluorescence have been applied in plant cell imaging for the first time,which can avoid the interference of spontaneous fluorescence of plant cells by adjusting the excitation wavelength of ND.The report provides a pathway for multicolor imaging for biological cells using ND.2.Study on the preparation and photocatalytic properties of ND@g-C₃N₄.ND@g-C₃N₄ heterostructure was prepared by a one-step pyrolysis method.We have found that with the increasing of the amount of ND in ND@g-C₃N₄,the yield hydrogen evolution rate first increases,and then decreases.The highest yield of the hydrogen evolution rate was obtained when the ND loading was 10 wt%.ND@g-C₃N₄ shows a hydrogen evolution rate of 6490 mol·g^-1·h^-11 and 1182mol·g^-1·h^-1 under AM 1.5 irradiation and visible light irradiation,respectively,which is 3.22 times and 5.62 times of g-C₃N₄ under the same conditions.At 420 nm,the apparent quantum efficiency?AQE?for hydrogen evolution was increased from3.64%for g-C₃N₄ to 5.98%for ND@g-C₃N₄.The results of the cyclic experiments indicate that the ND@g-C₃N₄ has a good stability.The superior photocatalytic performance of ND@g-C₃N₄ can be attributed to the following two factors:first,the strong light scattering effect of ND should increase the light-trapping of the g-C₃N₄,which is favorable to the generation of the localized charge carriers;second,the ND and g-C₃N₄ can form a type-II heterostructure,which helps the carrier-separation at the heterostructure interface.As a result,the synergistic effect of the two factors leads the heterostructure showing superior photocatalytic activity.In addition,there is no O₂ generated in the experimental process.Therefore,the ND@g-C₃N₄ photocatalyst is inactive for water oxidation.3.Study on preparation and photocatalytic properties of BDND@g-C₃N₄.The BDND@g-C₃N₄ heterostructures were prepared by a one-step pyrolysis method.A small amount of BDND would not change the morphology and structure of g-C₃N₄.The two components are closely combined to form a type II heterostructure,which help the transfer and separation of the photogenerated carriers.The BDND with a relative small gap?2.43 eV?should broaden spectral response range of g-C₃N₄?2.75 eV?.The BDND@g-C₃N₄ photocatalystwith BDND loading as 3 wt%showed excellent photocatalytic activity.The hydrogen evolution rate of BDND@g-C₃N₄ was96.3 and 5.3 times of g-C₃N₄ under visible light irradiation.At 420 nm,the AQE for hydrogen evolution of BDND@g-C₃N₄ was 6.91%and 1.72 times that of g-C₃N₄.Cyclic experimental results indicate that BDND@g-C₃N₄ has good stability.Compared with g-C₃N₄,the photocatalytic activity of BDND@g-C₃N₄ was also optimized in the experiment of degradation of organic pollutant rhodamine B?RhB?.Under visible light irradiation,the reaction rate constant of BDND@g-C₃N₄ for degradation Rh B is 3.1 times of g-C₃N₄.The enhanced photocatalytic activity of BDND@g-C₃N₄ should be attributed to two factors:first,the enhanced spectral response of BDND@g-C₃N₄ after loading BDND can produce more photogenerated carriers to participate the photocatalytic reaction;second,the type II heterojunction formed by BDND and g-C₃N₄ would help transfer and separation of the photogenerated electrons and holes.The synergistic effect of the two factors leads the BDND@g-C₃N₄ showing superior photocatalytic activity for water splitting and degradation of organic pollutant.