MXene has a clear layered structure with layered nanosheets(thickness<5 nm)laterally grown to several micrometers.The atomically layered structure significantly increases the available specific surface area,which maximizes the contact between the catalyst and the reaction intermediate,thereby improving the catalytic performance.This characteristic effectively shortens the charge transfer path and realizes fast charge transport,which can improve the conductivity of the electrocatalyst and the separation of electron-hole pairs in the photocatalyst,thus facilitating the catalytic reaction.Since the discovery of MXene in 2011,researchers have developed a variety of synthetic methods to prepare these materials,and the most commonly used synthetic method is chemical stripping by selective etching of the"A"layer from the parent MAX precursor through fluoride-based acidic solutions.After etching,the 2D structure usually loses its original surface and is filled with surface functional groups(usually composed of O,F,and OH,called terminal T_x）to obtain M_n+1X_nT_x.The adjustment of surface terminals is very important,and they will affect the interlayer spacing,adsorption and diffusion behavior of metal ions,etc^[19]。 In addition to the above top-down methods,the common strategies for MXene synthesis include bottom-up chemical vapor deposition and template methods.In 2011,Naguib et al.Synthesized Ti₃C₂MXene materials with clear layered structure,high specific surface area and abundant active sites from the corresponding Ti₃AlC₂MAX precursor by HF etching method using HF as an etchant^[20]。 Because of the danger of HF and its damage to the environment,other fluoride acids or mixtures of fluoride salts and acids are used instead to reduce HF use.In 2014,Ghidiu et Al.Used a mixture of 6 mol/L hydrochloric acid(HCl)and lithium fluoride(LiF)to selectively etch Al in MAX phase Ti₃AlC₂to synthesize Ti₃C₂MXene^[21]。 In 2017,Zuo et al.Used a mixture of HCl and iron fluoride（FeF₃）to synthesize Ti₃C₂MXene,which is the same as MXene synthesized by LiF/HCl etching,and its intercalation step is simpler than HF etching^[22]。 While HF etching and fluoride-based acid etching methods are commonly used to synthesize carbide-based MXene,they have not been successfully applied on nitride-based MAX to date.Urbankowski et al.Successfully synthesized monolayer Ti₄N₃T_xnitride MXene by heating a mixture of molten fluoride salts composed of sodium fluoride(NaF),potassium fluoride(KF),and lithium fluoride(LiF)at a high temperature of 550°C for 30 min^[23]。 In 2015,Xu et al.Reported the application of chemical vapor deposition(CVD)in the synthesis of MXene materials,and prepared ultrathin TMCα-Mo₂C superconducting crystals with large specific surface area and high stability^[24]。

Because MXene is usually not a semiconductor and can not be directly used as a photocatalyst,it is generally used as a co-catalyst to form a composite catalyst with other materials in the field of photocatalysis.Designing MXene-based composites is an effective way to improve the electrocatalytic and photocatalytic performance of MXene.The synthesis technologies of MXene-based composites mainly include mechanical mixing method,solvothermal method,calcination method,hydrothermal method and self-assembly method.In 2019,Wang et al.Successfully prepared a novel microporous MIL-100(Fe)/Ti₃C₂MXene composite by mechanical mixing method and used it as a non-noble metal-based Schottky junction photocatalyst under illumination.The results showed that the composite significantly improved the nitrogen fixation ability,and its NO conversion rate was 4 and 3 times higher than that of pure Ti₃C₂MXene and pure MIL-100(Fe)samples,respectively^[25]。 Chen et al.Synthesized the Bi₄O₅Br₂/Ti₃C₂composite by using a solvothermal method,and the ohmic heterojunction formed between the Bi₄O₅Br₂/Ti₃C₂interfaces can accelerate the separation of spatial carriers and the extraction of photoexcited carriers,while increasing the active edge sites for photocatalytic N₂reduction reaction(Fig.2 )^[26]。 Gao et al.Prepared MXene/TiO₂/Co composite photocatalyst by two-step calcination method,and Co as low-valent metal dopant modification improved the surface adsorption of N₂and increased the number of active sites^[27]。 Qin et al.Prepared AgInS₂/MXene by hydrothermal synthesis,and a Schottky junction was formed between AgInS₂and Ti₃C₂,which can be used as an electron trap to effectively capture and accumulate photogenerated electrons on Ti₃C₂,which is beneficial to the reduction reaction of multi-electron N₂,thereby improving the photocatalytic performance^[28]。 Zhang et al.Also designed a Ti₃C₂MXene/CdIn₂S₄Schottky heterojunction with sufficient interface contact by hydrothermal method,which greatly promoted the separation of photogenerated carriers and the efficiency of interface transmission,thus achieving efficient photocatalytic selective value-added conversion^[29]。 He et al.Synthesized black phosphorus quantum dots(BP QDs)immobilized on Ti₃C₂T_xMXene by self-assembly,and obtained 0D/2D composite colloidal suspension.The strong covalent P−O−Ti bond formed between BP QDs and MXene enhanced the interfacial mechanical stability and charge transfer of the heterostructure^[30]。

Our group prepared ZnFe₂O₄@V₂CT_xhybrid materials by in situ chemical co-precipitation method,based on different lithium storage mechanisms and stable interfacial electron transfer,the ZnFe₂O₄@V₂CT_xhybrid materials showed an extraordinary reversible capacity of 1189 mAh·g⁻¹after 100 cycles,which was 7.62 times higher than that of pure ZnFe₂O₄^[31]。 We also synthesized the CoFe₂O₄&V₂CT_xcomposite by in-situ hydrothermal method as an anode material,which showed a good reversible capacity of 638.6 mAh·g⁻¹after 100 cycles at 1 C,and a coulombic efficiency of 97.8%^[32]。 The Fe₂O₃/LaFeO₃/g-C₃N₄/Ti₃C₂composite is synthesized in situ by a mechanical mixing method,and is used as an effective visible light photocatalyst for Cr(Ⅵ)reduction and organic pollutant removal by constructing a plurality of Z-type/Schottky heterojunctions,and the substitution of Ti₃C₂for noble metal not only improves the migration rate of photogenerated carriers,but also inhibits the rapid recombination of photogenerated carriers^[33]。

In general,there are many techniques to synthetically prepare MXene,MXene-based composites,and MXene-derived hybrids.In addition to the above common synthesis strategies,solid phase synthesis,electrodeposition,wet milling and sintering are also effective strategies for the preparation of MXene series materials。

MXene has attracted more and more attention in the field of photocatalytic Nitrogen reduction reaction(NRR)due to its abundant surface active sites and a large number of natural surface defects that are easy to activate the Nitrogen-nitrogen triple bond.Although MXene has N₂chemisorption ability,it still has the problem that the band gap energy is seriously lower than that required for photocatalytic NRR,so MXene usually needs to be coupled with other semiconductors regardless of its metal or semiconductor properties.As the most reported MXene material up to now,Ti₃C₂is widely used.In 2020,Hao et al.Prepared RuO₂loaded TiO₂-MXene as a high-performance photocatalyst for nitrogen fixation,and the yield of NH₃reached 425μmol·L^-1·g^-1after 450 min of xenon lamp irradiation^[34]。 Qin et al.Reported that in-situ grown AgInS₂/Ti₃C₂with a mass ratio of AgInS₂of 30 wt%was prepared by a hydrothermal method.By exploring the energy level alignment of the nanocomposite,it was concluded that the interfacial charge separation and migration ability were improved by constructing a Z-type heterostructure(Fig.3),and a high photocatalytic nitrogen fixation rate of 38.8μmol·g_cat^-1·h^-1was obtained,with an apparent quantum efficiency(AQE)of 0.07%at 420 nm^[28]。

Fang et al.Synthesized the BiOBr/MXene-Ti₃C₂composite,and the formation of the composite improved the efficiency of carrier migration,thereby improving the activity of catalytic nitrogen fixation^[35]。 The existence of O vacancy and Ti vacancy can further prolong the photogenerated carrier lifetime,which indicates that the photogenerated electron can be captured by the defect first and inhibit its recombination with the hole,thus prolonging the average carrier"effective working"lifetime.Its precipitation rate of NH₃under xenon lamp irradiation is up to 234.6μmol·g_cat^-1·h^-1,which is about 48.8 and 52.4 times higher than that of pure BiOBr and Ti₃C₂,respectively.Gao et al.Utilized in situ modification of Co on Ti₃C₂MXene for efficient nitrogen fixation,which exhibited outstanding NH₄⁺yield（110μmol·g_cat^-1·h^-1）under UV-vis conditions^[27]。 Chang et al.Designed a Ti₃C₂quantum dot/mesoporous C₃N₄hollow nanosphere Schottky junction rich in surface defects（Ti³⁺,Ovs)(Fig.4),which is formed at the interface between the C₃N₄sphere and the r-Ti₃C₂QD.The mesoporous hollow C₃N₄spheres serve as a supporting matrix with abundant interfacial sites,and the r-Ti₃C₂QDs act as electron acceptors for the extraction and capture of electrons from the C₃N₄hollow spheres^[36]。 This makes the photogenerated electrons and holes spatially separated,thus suppressing the recombination of carriers.Under white light irradiation,enhanced N₂fixation activity was shown with a NH₃yield of 328.9μmol·g_cat^-1·h^-1.Li et al.Utilized W atoms to dope Ti₃C₂T_xMXene and used it for full-spectrum photofixation of nitrogen without sacrificial agents at room temperature with NH₃yields up to 228μmol·g_cat^-1·h^-1[37]。 Sun et al.Prepared the Ti₃C₂/N defect g-C₃N₄heterostructure photocatalyst,and the construction of the heterointerface and the introduction of N defects contributed to the rapid interfacial charge transfer to the active site,showing enhanced photocatalytic nitrogen fixation activity with a NH₃yield of 5.792 mg·h^-1·g^-1[38]。 Chen et al.Reported that Bi₄O₅Br₂was anchored on Ti₃C₂MXene with an ohmic heterojunction,and the formed ohmic heterojunction could accelerate the separation of spatial carriers and the extraction of photoexcited carriers,which had a prominent contribution to the cleavage of N≡N bond,and the NH₃yield was 277.74μmol·g_cat^-1·h^-1,which was 5 times higher than that of Bi₄O₅Br₂^[26]。 Qin et al.Designed a Ti₃C₂-QD/Ni-MOF heterostructure and confirmed that the interaction of Ti₃C₂-QD and Ni-MOF could accelerate the electron transfer by X-ray near-edge absorption fine structure(XANES)technique,and the Ni center was the main region for electron density enrichment and adsorption activation of N₂,which showed a considerable ammonia yield of 88.79μmol·g_cat^-1·h^-1[39]。 Sun et al.Prepared a 1D/2D CdS@Ti₃C₂MXene composite material,whose relatively high specific surface area and pore volume are beneficial to the sufficient contact between the catalyst and the reactant and accelerate the electron transfer,which can achieve a 293.06μmol·g_cat^-1·h^-1photocatalytic nitrogen fixation efficiency and an apparent quantum efficiency of 7.88%^[40]。 Wang et al.Used MIL-100(Fe)and Ti₃C₂MXene as Schottky catalyst,and the NO conversion was 4 and 3 times higher than that of pure Ti₃C₂MXene and pure MIL-100(Fe)samples,respectively^[25]。 Liu et al.Reported that Ti₃C₂MXene modified g-C₃N₄nanosheets to improve the photocatalytic redox ability,and the ammonia formation yield was 601μmol·g_cat^-1·h^-1,which was 3.64 times higher than the yield of pristine g-C₃N₄,203μmol·g_cat^-1·h^-1[41]。 To achieve a high quantum yield,Shin et al.Synthesized NH₃under visible light by using nanoscale Ti₃C₂MXene and plasmonic gold nanoparticles(AuNPs),which increased the photocatalytic activity by 4.4 times compared with microscale Ti₃C₂and AuNPs nanocomposites^[42]。 The highest photocatalytic performance was achieved by the enhanced absorption of light by AuNPs,and the effective immobilization of N₂by the chemical state reduced nanoscale Ti₃C₂MXene,resulting in an ammonia yield of 5334μmol·g_cat^-1·h^-1 。

electrochemical nitrogen fixation has attracted wide attention In scientific research due to its advantages such as abundant raw materials,mild reaction conditions,low equipment and operation costs,and low energy consumption.In recent years,MXene materials have been widely used in topology,catalysis,energy storage and other aspects because of their excellent conductivity,hydrophilicity,stability and other unique properties.In electrocatalysis,MXene,as an Electrochemical catalyst,has a strong function of adsorbing and activating reactants,thus improving the reaction rate and catalytic efficiency.In particular,when compounded with other semiconductor materials,due to the outstanding conductivity of MXene,it can be used as a conductive substrate to keep the heterojunction metallic and accelerate electron transport to improve electrocatalytic performance^[43]。 In addition,MXene can also regulate its electronic structure through surface functional group modification,and then adjust its catalytic performance^[10]。 Therefore,MXene materials have a wide range of applications in electrochemical nitrogen fixation,and have high research and application value。

However,due to the slow dissociation of N≡N,the electrochemical nitrogen reduction reaction(NRR)process usually results in low faradaic efficiency and ammonia yield.In order to improve the electrocatalytic activity and stability,researchers have adopted the method of doping metal,taking advantage of the high surface area and adjustable surface properties of MXene,to compound or dope it with metal materials such as metal nanoparticles,nanowires or nanosheets to improve their electrocatalytic activity and stability.Peng et al.Anchored single-atom Ru on MXene nanosheets as an important electron donor center for N₂activation,which not only enhanced the nitrogen adsorption and activation ability of the catalyst,but also helped to reduce the thermodynamic energy barrier of the first hydrogenation step^[44]。 This catalyst achieved an ammonia yield of 40.57μg·h^-1·mg^-1and a faradaic efficiency of 25.77%at−0.3 V.Du et al.Constructed a Ni nanoparticle/V₄C₃T_XMXene heterostructure with an ammonia yield of 21.29 mg·h^-1·mg_cat^-1,and the improved NRR activity attributed to the cooperative NRR pathway of Ni sites in the nanoparticles and surface O vacancies of V₄C₃T_xMXene was confirmed by density functional theory(DFT)calculations combined with various characterization methods^[45]。 Chen et al.Anchored single-atom ruthenium(Ru)on chemically activated Ti₃C₂with an O-terminated group（Ti₃C₂O）,DFT calculations combined with experiments showed that a single Ru site bound to 4 oxygens is the dominant reaction center,which allows the hydrogenation of*N-NH₂to*NHNH₂in a new distal/alternating hybrid path,At the same time,the energy barrier of the electricity limiting step was reduced from 0.96 eV in the distal path alone or 1.18 eV in the alternative path alone to 0.78 eV,thereby significantly promoting the NRR kinetics,achieving excellent activity and selectivity with an ammonia yield of 27.56μg·h^-1·mg^-1and a faradaic efficiency of 23.3%^[46]。 Luo et al.Adsorbed Fe on pure Ti₃C₂O₂and nonmetal(N,F,P,S,Cl)-doped Ti₃C₂O₂,and the charge transfer of the adsorbed Fe atom to the N₂molecule weakened the N≡N interaction,indicating that Fe/MXene is a potential catalytic material for NRR^[47]。 It is worth noting that doping with nonmetals F and S reduces the limiting potentials of the two potential-limited steps in the reduction reaction compared to the undoped pure structure.Chen et al.Reported that Ti₃C₂MXene loaded with semimetallic 1T'-MoS₂nanosheets could efficiently electrocatalytically convert N₂into NH₃,which was shown by DFT calculations.The 1T'-MoS₂/Ti₃C₂composite makes the activation and further reduction of*N₂thermodynamically more favorable than pure 1T'-MoS₂,with an ammonia yield of 31.96µg·h^-1·mg_cat^-1and a faradaic efficiency of 30.75%(Fig.5 )^[48]。

Liu et al.Prepared two-dimensional Ti₃C₂MXene loaded with copper nanoparticles,and the Cu/Ti₃C₂could achieve a high Faradaic efficiency of 7.31%and a NH₃yield of 3.04μmol·h^-1·cm^-2,which was attributed to the wider conduction and valence bands and larger Fermi level of the Cu/Ti₃C₂^[49]。 Li et al.Designed to couple Sb with Nb₂CT_xto construct an electron-rich interface,which makes the p-band center of the interfacial Sb atom move down and become the main active center to promote the activation and hydrogenation of N₂,while limiting the competitive hydrogen evolution,with a NH₃yield of 49.8 mg·h^-1·mg^-1and a high Faradaic efficiency of 27.3%^[50]。 Liu et al.Synthesized the Bi@Ti₃C₂nanocomposite by depositing metallic Bi nanoparticles on two-dimensional Ti₃C₂MXene nanosheets via a liquid-phase reduction method,and the unique N-philic and H-phobic characteristics of Bi atoms and the excellent electronic conductivity of MXene made it have excellent NRR activity,with NH₃yield and faradaic efficiency as high as 28.3μg·h^-1·cm^-2and 27.2%^[51]。

In addition,the surface characteristics of MXene can be changed by surface modification of monoatomic or polyatomic adsorbents using co-doping strategy,which can improve its catalytic activity,selectivity and stability.The carrier mobility,the photoelectric conversion efficiency and the catalytic activity are improved by constructing a heterostructure,oxygen vacancies and the like.Zeng et al.Used a co-doping strategy to report the synergistic performance of N and S co-doped Ti₃C₂T_xfor the electrocatalytic hydrogenation of N₂to NH₃.The synergistic effect of N and S dopants significantly improved the electron/ion transport ability and increased the catalytic active sites,with a NH₃yield of 34.23μg·h^-1·mg_cat^-1and a Faradaic efficiency of 6.6%^[52]。 The MXene catalyst can be tuned by optimizing the size and functional groups of QDs,and Jin et al.Found that the surface functional groups play a key role in the electrocatalytic performance,and confirmed the excellent NRR activity of hydroxyl groups on Ti₃C₂T_xby DFT calculation,with NH₃yield and faradaic efficiency of 62.94 g·h^-1·mg_cat^-1and 13.30%,respectively^[53]。 Xia et al.Reported that surface hydroxyl modification could effectively promote electron transfer,surface adsorption,and activation of N₂.With the increase of the number of surface hydroxyl groups,Ti₃C₂showed an increase in the yield of NH₃with a yield of 1.71 g·h^-1·cm^-2and a faradaic efficiency of 7.01%^[54]。 Chu et al.Reported the synergistic effect of O vacancy and heterostructure on MoO_3-x/MXene,demonstrating that the O vacancy on MoO_3-xis the active site for N₂chemisorption,and the MXene substrate can further adjust the O vacancy site to break the proportional relationship.The*N₂/*N₂H is effectively stabilized while the*NH₂/*NH₃is destabilized,so that the binding affinity of the NRR intermediate is stronger to reduce the energy barrier and enhance the NRR activity of the MoO_3−x/MXene,the NH₃yield is 95.8µg·h^-1·mg^-1,and the faradaic efficiency is 22.3%^[55]。 Shi et al.Synthesized an N-doped Ti_V-Ti_3−xC₂T_y-1.2 MXene.The Ti³⁺species is the intrinsic active site of ENRR,and the electronic state of the active Ti³⁺species can be adjusted by surface atom engineering.The introduction of Ti vacancy can capture the electrons injected into the antibonding orbital of adsorbed N₂,which is beneficial to the activation of N₂^[56]。 In addition,the introduction of N doping into MXene can not only effectively act as a stable active site for ENRR,but also through N doping,additional electrons can be introduced to change the electronic structure of the material,making the active site of Ti³⁺as a catalyst more stable.In addition,N doping can also adjust the chemical properties and electron affinity of MXene surface,and narrow the orbital overlap between Ti³⁺and N₂to promote the desorption process of NH₃molecules on MXene surface 。

Luo et al.Designed sulfur-deficient Bi₂S₃nanoparticle decorative Ti₃C₂T_x-MXene by combining vacancy and interface engineering.The S vacancy and interfacial Bi sites can synergistically promote N₂adsorption and*N₂H formation with a NH₃yield of 68.3μg·h⁻¹·mg⁻¹and a Faradaic efficiency of 22.5%^[57]。 Zhang et al.Demonstrated that Ti₃C₂T_xMXene nanosheets can be used as precursors and conductive substrates for in situ hydrothermal growth of TiO₂nanoparticles^[58]。 The combination of TiO₂and Ti₃C₂T_xresulted in a synergistically active Ti-based nanohybrid catalyst for enhanced N₂reduction electrocatalysis with a NH₃yield of 26.32μg·h^-1·mg_cat^-1and a Faradaic efficiency of 8.42%.Chu et al.Designed a Nb₂CT_x-MXene modified by BN QDs,and DFT calculations showed that the synergistic effect of BN QDs and Nb₂CT_xcould produce unique interface B sites.As the NRR catalytic center,it can enhance the activation of N₂,reduce the reaction energy barrier and prevent the release of H₂,and the NH₃yield and Faradaic efficiency are 66.3µg·h^-1·mg^-1and 16.7%,respectively^[59]。

Ba et al.Designed multiple sets of heterostructure catalysts based on partially etched Ti₃AlC₂MAX/Ti₃C₂MXene(Fig.6 )^[60]。 The results show that the surface potential difference between MAX and MXene is about 40 mV,which means that electrons can be easily transported from MAX to MXene through the interface,thus facilitating the immobilization of N₂and achieving a Faradaic efficiency of 36.9%.DFT calculations further revealed a billiard-like catalytic mechanism,with intermediates alternately adsorbed on the surface of MAX or MXene,and*NH→*NH₂,the rate-determining step,with an energy barrier of 0.96 eV at the heterointerface,following a correlation-distal mechanism.Xu et al.Developed 1T-MoS₂nanodots assembled on conductive Ti₃C₂MXene,due to the synergistic effect between 1T-MoS₂and Ti₃C₂MXene.It has excellent NRR catalytic activity with a faradaic efficiency of 10.94%and a NH₃yield of 30.33μg·h^-1·mg_cat^-1[61]。

Most NRR catalysts operate by using associative or dissociative mechanisms,during which NRR competes with HER,resulting in low selectivity.MvK mechanism reduces this competition by eliminating the adsorption and dissociation processes of NH₃synthesis sites.Johnson et al.Reported that the Ti₂N nitride MXene initiates the Mars-van Krevelen mechanism to achieve high selectivity of nitrogen reduction reaction with a NH₃yield of 11.33μg·cm^-2·h^-1and a Faradaic efficiency of 19.85%^[62]。 Cao et al.Studied the electrocatalytic nitrogen fixation of transition metal atoms(Fe,Co,Mo,Ru)anchored on defective V₂C based on first-principles calculations,and the overpotential of NRR on Mo-doped V₂C was only 0.49 eV along the mixed enzyme pathway,which was much lower than that of undoped V₂C(0.61 eV )^[63]。 The low overpotential is associated with the synergistic effect of the diatoms of Mo and V,promoting the activation of the nitrogen molecule.In addition,the adsorption of nitrogen by Mo-doped V₂C is significantly greater than that of H⁺,and thus,the HER can be effectively suppressed 。