Design,synthesis And Charge Transfer Performance Tuning Of Non-phase-separated H-BCN

Metal-free semiconductor photocatalytic reduction of CO₂ has been considered as one of the best sustainable solutions to the crises of energy and environmental problems.As an emerging type of promising metal-free photocatalyst,graphene-like hexagonal boron carbon nitride（h-BCN）has attracted tremendous attention due to its flexible element proportion and adjustable band gap.Due to the various arrangements of B,C,and N atoms in the plane,h-BCN has two types of structures:one is phase-separated structure with graphene and h-BN domains joint together,while the other is non-phase-separated structure with chemically mixed ternary B-C-N phase.The former is easy to synthesize but has limited charge transfer,which greatly restricts its application in photocatalysis.The latter has uniformly distributed delocalized electrons,which are beneficial to the continuous transport of photogenerated charges,however suffering from the difficulty of synthesis,it has not been practically applied and studied in depth.In this thesis,non-phase-separated h-BCN is taken as the research object.The relationship between the structures and photocatalytic performance of h-BCN is first established from the molecular,atomic and electronic levels through first-principle calculations,and the proper h-BCN structures for CO₂ photoreduction are proposed.Then,on basis of the calculation results,by well designing the ratio of boron-containing inorganics and nitrogen-containing organics as well as the thermodynamic and kinetic parameters in the reaction process,non-phase-separated h-BCN is controllably synthesized by organic-inorganic hybrid precursor pyrolysis method.The formation mechanism and photocatalytic CO₂ reduction performance are explored.And the enhanced mechanism of charge separation and transfer are further studied in order to achieve efficient photocatalytic reduction of CO₂.The main research work and results are as follows:（1）The structures and optoelectronic properties of homogeneous and phase-separated BCN monolayers under gradient carbon concentrations（8.33%～58.33%）were systematically studied using first-principles calculations based on density functional theory（DFT）.And the intrinsic relationship between the photocatalytic performance and the energy band structure,density of states,band-edge potential,effective carrier mass as well as light absorption properties of h-BCN with different compositions and structures were investigated.The results show that with increasing C concentration,the band gap of h-BCN gradually decreases from 4.69 e V to 1.50 e V,showing excellent tunability.Compared with the locally phase-separated h-BCN,the B,N,and C atoms in the non-phase-separated h-BCN show obvious bonding characteristics.The uniformly distributed delocalized electrons provide continuous carrier transport channels,making it more suitable for photocatalytic CO₂ reduction.When the concentration of C atoms is≥33.33%,carbon-containing products such as CH₄,C₃H₇OH,C₂H₅OH,C₂H₄,CH₃OH,CO,and HCOOH can be obtained by photocatalytic reduction of CO₂ on the non-phase-separated h-BCN.（2）The system of boric acid and ethylenediamine in aqueous solution was calculated by all-atom molecular dynamics simulation method,and results showed that the boric acid and ethylenediamine would undergo coordination reaction to form B-N-C bonds.Based on the theoretical calculations,organic-inorganic hybrid precursor pyrolysis method was conducted to elaborate non-phase-separated h-BCN at ammonia atmosphere.The results show that the pyrolysis atmosphere and reaction time have significant effects on the formation of non-phase-separated h-BCN.The redistribution ofπelectrons forms continuous carrier transport channels in the B-C-N atomic plane,which promotes the separation and transport of photogenerated carriers.The resultant BCN-1 exhibits excellent photocatalytic activity for CO₂ reduction with no co-catalysts or sacrificial agents,giving the CO generation rate of 13.94μmol g^-1 h^-1 under 350-780 nm illumination,which is 9.4 times that of g-C₃N₄ under the same conditions.（3）Taking advantage of the high electronegativity of halogen atoms,the chemical states and charge distributions of non-phase-separated BCN with F,Cl or Br modification were studied by DFT calculations.The results show that F atoms can be stably adsorbed on B and C atoms,generating a polarization electric field on the surface of BCN.Based on this,F-modified non-phase-separated boron carbonitride（BCN-F-x）was synthesized by co-heating BCN-1 prepared in the previous chapter with fluoride salt.Although the amount of unpaired electrons and CO₂ adsorption capacity was significantly reduced after F modification,the CO yield was 1.8 times higher than that of pristine BCN-1,which is resulted from the fact that F-induced polarization electric field enhances the separation and transfer efficiency of photocarriers,thus promotes the photocatalytic activity.（4）Using boric acid,ethylenediamine and glucose as raw materials,the organic-inorganic hybrid precursor was synthesized,and the Graphite_sheet-BCN in-plane heterostructures（G_x-BCN）were obtained by organic-inorganic hybrid precursor pyrolysis method.The results revealed that the graphitized carbon sheets were embedded in the flakes of non-phase-separated h-BCN.The prepared G₁₅₀-BCN exhibits 10 times longer carrier decay lifetime and 3.7 times higher CO yield(51.14μmol g^-1 h^-1)than that of BCN-1.DFT calculations reveal that the graphite sheet creates new energy levels in the in-plane heterostructure,which promotes the efficient separation and local directional transfer of photoinduced carriers,accelerates the surface reaction kinetics,and therefore enhances the photocatalytic CO₂ reduction performance.