Construction Of Defective MXenes-Ti₃C₂O_x Based Composites And Their Photocatalytic Nitrogen Fixation Performanc

Since the 20th century,ammonia（NH₃）has been used extensively in agriculture,high-tech industries and traditional manufacturing,and is an important carbon-free energy storage carrier due to its high hydrogen capacity and weak liquefaction pressure.Until now,large-scale production of NH₃has relied on the energy-intensive and carbon-intensive Haber-Bosch process.In recent years,efforts have been made to explore new ways of green NH₃synthesis in order to overcome these drawbacks.In contrast to the Haber-Bosch process,photocatalytic nitrogen reduction reactions（PNRR）at ambient temperature and pressure as a replacement strategy have attracted a great deal of scholarly attention.Despite pioneering advances in this most attractive area of research,catalytically efficient ammonia conversion efficiency remains a great challenge.The key to improving PNRR efficiency lies in the rational design and construction of efficient catalysts.Most photocatalytic materials suffer from narrow photoresponse regions,low carrier separation efficiency and lack of abundant N₂adsorption activation centres.Therefore,it is particularly important to construct catalytic materials with strong interaction with N₂.MXenes-Ti₃C₂Ox（terminal O functional group）has excellent charge low transport resistance,adjustable band gap structure and high affinity with N₂,which makes MXenes-Ti₃C₂Ox with its own intrinsically defective Ti³⁺structure an ideal material for nitrogen fixation.To address the limitations of the PNRR process,this paper uses MXenes-Ti₃C₂Ox as a co-catalyst to achieve complementary catalytic nitrogen fixation performance far beyond that of a single component by constructing different2D heterostructures:single-atom defect structures,double-defect types and a series of2D heterojunctions that modulate the"electron"transfer of the reduced layer Fe^Ⅱare designed and constructed."transfer,and a series of 2D heterojunctions were designed and constructed,respectively,and a discussion including the pathway and mechanism of the nitrogen fixation reaction,between nanostructure and catalytic performance was developed.To provide a reference for promoting the application of defective MXenes-Ti₃C₂Tx in catalysis,the main findings are as follows:1.Cu single-atom（Cu-SACs）anchored defect states Ti³⁺-MXenes-Ti₃C₂Ox:A simple electrostatic adsorption coupling enrichment theory was used.2D layered Ti³⁺-MXenes-Ti₃C₂Ox materials rich in defect sites were first etched and prepared,and Cu²⁺was adsorbed or embedded in the defective host structure under aniline modification to reduce the Cu-SACs/Ti₃C₂Ox composites.This unique"accordion-like"structure facilitates the modulation of the charge distribution of Cu species between the carriers,and more effectively ensures the dispersion of Cu-SACs to enhance their contact area with the defective carriers and facilitate the response of carriers to inert N₂.The precise coordination information between this single atom and the carrier has been resolved by combining X-ray photoelectron spectroscopy（XPS）and synchrotron radiation techniques（XAS）.Meanwhile,the low-valent electron-rich Cu(3d¹⁰4s¹)configuration of d-orbital electrons can effectively participate in the feedback ofπ-architecture electrons on N₂,and the heterogeneous configuration enhances the catalytic reaction（thermodynamics）(?G（*NN）-?G（*H）=-0.39_Cu-N3eV).Next,using theoretical calculations（DFT）to analyse the effect of different structures on nitrogen fixation performance and competing reactions（HER）,we learned that the Cu-SACs/Ti₃C₂Ox structure has a higher thermodynamic stability of Cu-N₃.Also,we found that the Cu-N-Ti ligands assisted in producing versatility in photocatalytic nitrogen fixation by（1）inhibiting the evolution of the HER competition process;（2）asymmetric(Cu-Nx-Ti³⁺)charge sites induced an increase in N₂adsorption density;and（3）weakening the catalytically determined*NN/*NNH step energy barrier,allowing the optimised samples to show a 29μmol·g^-1·h^-1photocatalytic N₂conversion.At the same time,the existence of an asymmetric electronic structure between the Ti³⁺defect and the derived electron-rich Cu-Nx facilitates the polarization of the adsorbed N₂molecules for better activation.2.Introduction of double-defective doping to modulate the electronic structure of BiOBr/MXenes-Ti₃C₂Ox to achieve efficient photocatalytic nitrogen fixation:Using the conventional solvothermal technique,a series of double-defective BiOBr/MXenes-Ti₃C₂Ox composites with different Vo contents were constructed on the surface of MXenes using ethylene glycol as a structural guide to facilitate the electron transfer between N₂and the catalytic material.The complexes were constructed on the surface of MXenes with different Vo contents of double-defective BiOBr/MXenes-Ti₃C₂Ox composites to facilitate the electron transfer between N₂and catalytic materials,while improving the effective separation of photogenerated electron pairs and inhibiting carrier complexation.The presence of Ti³⁺defects and oxygen vacancies was well demonstrated using advanced characterisation techniques including high angle dark field scanning electron microscopy（HAADF-STEM）,XPS and low temperature electron paramagnetic resonance（EPR）.Meanwhile,the double defects introduced a large number of discontinuous lattices and disordered edge structures to enrich the BiOBr/MXenes-Ti₃C₂Ox local electronic structure and increase the catalytic active sites,exhibiting a catalytic ammonia yield of 234.6μmol·g^-1·h^-1and maintaining a relatively stable nitrogen fixation performance after six cycles.In addition,time-resolved spectroscopy（TRPL）reveals that the formation of weak interaction forces at the interface of the two-defective heterojunction can inhibit the formation of electronic states and Fermi energy level pegging between the two-dimensional materials and improve the response time of the carriers with the reaction solution.At the same time,the unsaturated electronic structure causes excitation of the indirect subband resulting in a broad response to visible light（absorption edge red shift:431 nm→628 nm）.COHP further revealed the interfacial charge transfer process,while ¹⁵N₂isotope labelling experiments and N₂programmed temperature desorption（N₂-TPD）spectroscopy demonstrated that the composites could provide adsorption and excitation sites for N₂molecules,thus further improving the nitrogen fixation efficiency.3.Construction of NM-101（Fe^Ⅱ/Fe^Ⅲ）/MXenes-Ti₃C₂Ox high-efficiency photocatalytic nitrogen fixation materials on defective carriers:similar to the active centre of the Fecluster of biological nitrogen fixation enzymes,this link uses the defective carrier MXenes-Ti₃C₂Ox with a solvent（ethylene glycol）to modulate the Fe-MOF reduced layer"Fe^Ⅱ"charge ratios.In-depth study of the interfacial reconfiguration behaviour of the defective carriers revealed that the heterojunction material exhibited a 75.1%Fe^Ⅱ（Fe^Ⅱ:Fe^Ⅲ）rich reduced layer structure and the defective Ti³⁺-Ti₃C₂Ox effectively impeded the fouling process between the reduced layer and the oxide layer（Fe^Ⅱ/Fe^Ⅲ）.At the same time,various characterizations of the catalyst structure,apparent chemical state and optoelectronic properties were carried out to deeply analyse the connection between the reconstructed charge clusters and PNRR:NM-101（Fe^Ⅱ/Fe^Ⅲ）overlaying the defective carrier Ti₃C₂Ox and presenting an amorphous"phase"exposing more nitrogen activation sites.Simultaneously coupled heterojunction interfaces are in close contact and XPS,XAS,EIS and TRPL characterisation together reveal that the constructed"bridge（Fe-O₂-Ti）"is able to efficiently transport and store electrons.In addition,the presence of high spin Fe^Ⅲand low/medium spin Fe^Ⅱstructures in the heterojunction is revealed by using Musburger spectroscopy,and the reduced layer charge is formed to redistribute the Fe^Ⅱ/Fe^Ⅲstructure;furthermore,the defective Ti³⁺-Ti₃C₂Ox enhances the reduced nitrogen fixation capacity of the heterojunction（CB=-0.84 eV）,which leads to the formation of surface/internal homogeneous junctions in the composite and facilitates This facilitates charge migration to the"defective layer"for interfacial reactions to achieve effective carrier separation,and exhibits a yield of 450μmol·g^-1·h^-1.Finally,the XPS characterisation has identified the charge shedding from the reduced layer during cyclic nitrogen fixation as the cause of the weakened cyclic activity.The photocatalytic nitrogen fixation mechanism of the reduction layer-rich structure NM-101（Fe^Ⅱ/Fe^Ⅲ）-1.5 was further revealed.