The Functional Control And Performance Prediction Of MXene Surface With Anion Group

Low-dimensional nano-scale materials have become one of the hot issues in the fields of material science in terms of their unique structures and distinctive physical/chemical properties.Two dimensional MXenes materials with hexagon skeleton structure were produced by selectively removing Al layer from the MAX phase.The MXenes materials show great potential applications in the fields of Li-ion batteries,supercapacitors and electrochemical catalysis,due to their stable structure and a large amount of activated surface groups.In present work,depending on the high specific surface and activated surface groups,the adsorption properties of MXenes have been investigated to remove heavy metal ions（Pb ions）for wastewater treatment by an ion-exchange method.The adsorption mechanisms and the relative affective factors were elucidated based on the first-principles calculations.In addition,the potential optical-adsorption properties of MXenes were also investigated by atomic doping and strain.（1）The Ti₃C₂（OH/ONa）₂（alk-MXenes）was produced by HF acid etching of Ti₃AlC₂ and alkalization intercalation.It exhibits preferential Pb ion sorption behavior when competing cations（Ca and Mg ions）coexisted at high levels.Attractively,the alk-MXenes presents efficient Pb ions uptake performance with the applied sorption capacities of 4500 kg water per alk-MXenes,and the effluent Pb ions contents are below the drinking water standard recommended by the World Health Organization（10 μg/L）.Experimental and computational studies suggest that the sorption behavior is related to the cations exchange between hydrogen and Pb ions to form the stable chemical bonds.（2）The adsorption kinetics of heavy-metal ions and the effect of intercalated sites on adsorption have been interpreted by first-principles calculations with density functional theory.The results show that Ti₃C₂（OH）₂ has very good adsorption effect on Pb ions and other common heavy metal Cu,Zn,Pd and Cd ions.Further investigation shows that the effect of H₂O molecules can be ignored in Pb ions removing of Ti₃C₂（OH）₂.The ion adsorption efficiency of alk-MXenes decreases due to the occupation of the F and O atomsbut accelerates by the intercalation of OLi,ONa,and OK groups.In addition,the surface adsorption of Pb ions on Ti₃C₂（OH）₂ has many possible structures.For example,the formation of Pb-Pb interaction stems from the anion ions exchanges and the adjoining restrain of OH ions.（3）Taking into account the complexity and diversity of MXenes,the Pb adsorption behavior of different MXenes with the highest valuable applied structure of M₂X（OH）₂（M =Sc,Ti,V,Cr,Zr,Nb,Mo,Hf,Ta,and X=C or N）has been systemically investigated by first-principles calculations,which paves a desirable trajectory for the applications of MXenes.The formation energies of M₂N(O₂H_2-2xPb_x)are lower than those of M₂C(O₂H_2-2xPb_x),suggesting the substitution of N is more effective for the Pb adsorption in contrast to C due to the different valence electron numbers between C and N atoms.In addition,the results show that the Ti₂C（OH）₂ is the most favorite one for the lowest atomic mass to Pb removal.Notably,both Sc₂C（OH）₂ and Zr₂C（OH）₂ structures do not bestow the ability of Pb removal.（4）The Ti₃C₂（OH）_0.8F_1.2 can be transferred to TiO₂-C by phase transformation under FeCl₃,where C provides the sketching structure to disperse TiO₂.The results show that the U-TiO₂（rutile structure）displays a high Cr（Ⅵ）adsorption capacity of ²25 mg g-1,which is more higher than that of the A-TiO₂（Anatase structure）.First principles calculations show that the Cr（Ⅵ）ability of U-TiO₂ with（110）surface and A-TiO₂（101）and（001）surface is related to the variation between their adsorption energies and H₂O molecules.The calculations also show that the MXenes is a good potential material for removing Cr（Ⅵ）ions.（5）We report that a new scandium-based carbide MXenes（ScNbCO₂）with direct band gap of ¹.4 eV has been predicted by Nb doping and tuning strain in terms of first principles calculations.The results show that the pristine scandium carbide MXenes（Sc₂CO₂）bestows an indirect band gap of 2.965 eV.This band gap disappears by replacing Ti,Zr and Hf,whereas it maintains by doping Nb.The Nb-doping changes indirect band gap into direct ones of 1.839 eV.Attractively,the direct band gap of ScNbCO₂ of ¹.4 eV is further attained by stain effect of 3% uniaxial strain.In addition,the visible light absorption efficiency is remarkably improved by Nb-doping and strain effect,comparedwith the pristine Sc₂CO₂.Especially,an increase by ²0 times is achieved in the case of 3% uniaxial ScNbCO₂.It reveals that these metal-doping MXenes materials are promising to develop new light-electron conversion components.