Preparation Of Graphitic Carbon Nitride-based Photocatalysts And Study Their Photocatalytic Hydrogen Production

With the development of modern industrialization and the increasing human demand for energy,the consumption of fossil energy accompanied by the production of harmful gases and greenhouse gases(such as NO_x,SO₂,CO,CO₂,etc.),which brings a series of environmental pollution problems.Semiconductor-based photocatalysis technology can effectively convert solar energy into clean hydrogen energy,which can provide power without generating pollutants,and has great potential in mitigating energy crisis and environmental pollution.The photocatalyst is a very critical component in the solar-based photocatalysis system,which determines the efficiency of solar-to-fuel conversion.In recent years,due to the advantages of low cost,simple synthesis,and controllable electronic structure,graphite-like carbon nitride(g-C₃N₄)materials have become a research hotspot in the field of photocatalytic energy conversion.However,the g-C₃N₄prepared by the traditional thermal polycondensation method has the disadvantages of small specific surface area,active site coverage,high photogenerated charge recombination rate and weak catalytic activity.This paper designs and synthesizes a series of g-C₃N₄-based photocatalysts,and studies their catalytic performance and reaction mechanism for the decomposition of water to hydrogen under visible light.The main research contents are summarized as follows:(1)A three-dimensional network-like g-C₃N₄nanobelt(g-C₃N₄NB)prepared by the hydrothermal etching bulk g-C₃N₄.Compared to bulk g-C₃N₄,the characterizations show that the three-dimensional structure of g-C₃N₄NB can enhance the absorption of visible light,provide more reactive sites,reduce the recombination of photogenerated electron-hole pairs,and prolong the lifetime of photogenerated charges.3 wt%Pt was deposited on g-C₃N₄NB as a hydrogen evolution promoter by an in-situ photodeposition technique.Under visible light illumination(??420 nm),3 wt%Pt/g-C₃N₄NB reached a hydrogen evolution rate(HER)of 1.36 mmol·g^-1·h^-1.The photocatalytic HER of 3 wt%Pt/g-C₃N₄NB is 10.9 times higher than that of the Pt/bulk g-C₃N₄.The apparent quantum yield(AQE)of 3 wt%Pt/g-C₃N₄NB reached12.0%at?=420 nm.(Chapter 2).(2)One-dimensional porous g-C₃N₄nanorods(O-g-C₃N₄NR)have been facile prepared by direct calcination of hydrous melamine nanofibers precipitated from an aqueous solution of melamine.First,the melamine nanofiber precursor is prepared by pretreating the melamine monomer used in thermal polymerization,and then the O-g-C3N4 NR with a porous structure is synthesized by thermal polymerization.The porous structure can enhance the absorption to visible light,mass transfer efficiency of O-g-C₃N₄NR.Oxygen atom doping can significantly promote the separation of photogenerated electron-hole pairs,and can also enhance the oxidation potential of the valence band of O-g-C₃N₄NR,thereby improving the visible-light-driven hydrogen evolution reaction rate.The O-g-C₃N₄NR loaded with Pt as a cocatalyst presented an excellent visible-light-driven photocatalysis HER(732?mol·g^-1·h^-1),5.7 times higher than that of Pt/bulk g-C₃N₄.The AQE of 3 wt%Pt/O-g-C₃N₄NR reached 7.1%under?=420 nm.The proposed synthesis strategy can overcome the shortcomings of uneven heat and mass transfer,stacking of materials and embedding of catalytically active sites during the preparation of carbon nitride by traditional thermal polymerization,and improve g-C₃N₄hydrogen production performance under visible light.(Chapter2).(3)The sea urchin-like g-C₃N₄(CNSC)with nitrogen vacancies was prepared by an in situ thermal polymerization.The energy bandgap of CNSC decreased to 2.0 e V due to nitrogen defects,endowing it more efficient to utilize solar;the three-dimensional sea urchin-like nanostructure can suppress the recombination of photoinduced electron-hole pair and the charge transfer resistance.In situ photodeposition 3 wt%Pt onto the CNSC,3 wt%Pt/CNSC reaches 3.36 mmol·g^-1·h^-1HER under visible light,26.4 times higher than that of Pt/bulk g-C₃N₄.At 420 nm light,the AQE of 3 wt%Pt/CNSC reaches 32%.In comparison with g-C3N4 from traditional polymerization(energy bandgap:2.7 e V),The photocatalytic HER performance of 3wt%Pt/CNSC is mainly due to the narrow energy bandgap,high visible light absorption capacity and unique nanostructure of CNSC.(Chapter 4).(4)The Co P QDs/g-C₃N₄fiber(Co P/CNF)composite was prepared via in situ electrostatic adsorption deposition followed by low-temperature phosphatization treatment.Carbon nitride itself has weak photocatalytic hydrogen evolution activity and often requires the loading of co-catalyst Pt to improve the surface hydrogen evolution reaction.Pt is an excellent hydrogen evolution co-catalyst but has the disadvantages of being expensive and low in abundance.In recent years,researchers have discovered that Co P has excellent hydrogen evolution performance and is expected to replace Pt for hydrogen evolution reaction.Electron microscopy results show that Co P is evenly distributed on the CNF surface in the form of quantum dots,exposing a large number of highly active crystal planes.Under visible light excitation,the Co P/CNF photocatalytic hydrogen evolution rate is as high as 121 mmolg-1·h-1,which is 34.9 times higher than that of Pt/CNF.At a wavelength of 420 nm monochromatic light,the AQE of Co P/CNF is as high as 59.9%.The experimental results show that the turnover number and the turnover frequency of Co P quantum dots reach 519 and 172.9 h^-1,respectively.The photocatalytic HER activity of Co P/CNF is due to the monodisperse Co P quantum dots providing abundant active sites and the synergistic effect between Co P quantum dots and CNF.(Chapter 5).