Hui Yang,Liang Chu

a Tianjin Key Laboratory for Optoelectronic Materials and Devices,Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education),School of Materials Science and Engineering,Tianjin University of Technology,Tianjin 300384,China

b New Energy Technology Engineering Laboratory of Jiangsu Province & School of Science,Nanjing University of Posts and Telecommunications,Nanjing 210023,China

Keywords:Transition-metal borides i-MAB phases MBenes Monolayer Ordered metal vacancies

ABSTRACT 2D MBenes have been theoretically predicted to possess unique electronic structures and physicochemical properties,and thus shown great promise in various applications.However,the synthesis of individual single-layer MBene remains a grand challenge due to its orthorhombic structure of MAB phases.Recently,scientists from Link?ping University have fabricated 2D monolayer Mo4/3B2-xTz with ordered metal vacancies.Their results demonstrated the feasibility of top-down approach by chemical exfoliation of laminated compounds and provided the principle for further preparation of a wealth of MBenes.

Ternary layered transition metal nitrides/carbides/carbonitrides(MAX phases,M is a transition metal,A is an A-group element,and X is either carbon or nitrogen) and the derived two-dimensional(2D) MXenes have attracted extensive attention since 2011,due to the abundance of their forms,unique electronic structures and physicochemical properties,and thus shown great promise in applications.Recently,Ade and Hillebrecht successfully fabricated a series of single-crystalline ternary transition metal borides (MAB phases) as analogs of MAX by introducing B as the X element [1].Therefore,boron-based 2D materials,referred to as MBenes,have correspondingly emerged and theoretically predicted to possess diverse applications in electronic,optoelectronic,and energy devices.

Motivated by chemical exfoliation of MAX to obtain 2D MXene,the Al-containing MAB phases are as promising candidates for obtaining new 2D MBenes.Experimentally,there are mainly three methods for synthesizing 2D MBenes.2D MoB [2] was first reported to obtain by partially etching of Mo2AlB2phases through the deintercalation of Al layers from ordered stacking faults region.Selectively HCl-etching Al layers from Cr2AlB2yielded the bulk layered CrB nanosheets [3].Removal of the indium layer through a high-temperature dealloying of Ti2InB2also obtained a bulk layered TiB structure [4].However,the synthesis of individual singlelayer MBene,to date,has not been realized.Because single-layer MBene was quite difficult to synthesize due to its orthorhombic structure of MAB phases which has much higher static exfoliation energy of Al removal than that of MAX phase [5].To enhance the exfoliation possibility for MAB phases,ternary laminated MxM′1-xAlB phases (called i-MAB phases) formed by alloying MAB with other transition metals were naturally developed.In this way,obtaining single-layer 2D MBenes with complete Al removal from the i-MAB phases which possess lower static exfoliation energy can become easier.

Recently,single-layer Mo4/3B2-xTzderived by selective etching of laminated i-MAB phases was successfully fabricated in aqueous hydrofluoric (HF) acid (Fig.1A) [6].First,the target material (Mo2/3Y1/3)2AlB i-MAB phase was with close-packed hexagonal structure along [0001] direction (Fig.1B) with Mo and Y partially overlapped along [1ˉ100] direction (Fig.1C).Furthermore,Mo and Y atoms in the M layer are laminated ordering with Y atoms extending toward the Al layer along the [11ˉ20] axis,which can form a Kagomé lattice and was confirmed to be i-MAB structure(Fig.1D).Thus,after the removal of Al layers,ordered metal vacancies can appear in the 2D Mo4/3B2-xlayers due to the additional removal of Y etching.Theoretically,the Y-B bond strength is only 1.3 times as strong as that of Y-Al interaction,so chemical exfoliation to remove Al can also induce the lack of Y atoms.The structure of Mo4/3B2-xTzlayers with XRD (000l) peak at a low angle of 9.35° corresponded to the d-spacing value of 9.23 ?A (Fig.1E)and can increase to 20.37 ?A after the intercalation by tetrabutylammoniumhydroxide (TBAOH) with multilayer sheets delaminating in water.The composition in the delaminated layers revealed the presence of Mo,B,O and F,and cross-sectional SEM images and corresponding XPS analysis confirmed the complete conversion from 3D (Mo2/3Y1/3)2AlB to 2D Mo4/3B2-xduring the etching process.This work indicates that single-layer 2D MBenes can be prepared from i-MAB phases and also provide a powerful strategy for a wealth of MBenes preparation.

Fig.1.Synthesis and characterization of 3D (Mo2/3Y1/3)2AlB2 and its 2D derivative Mo4/3B2-xTz (boridene).(A) Schematic of the transformation process from 3D i-MAB to 2D boridene,(B to D) In-plane chemical ordering of the (Mo2/3Y1/3)2AlB2 i-MAB phase evident from STEM images along the (B) [0001],(C) [1ˉ100],(D) [11ˉ20] with corresponding SAED patterns.(E) XRD pattern of (Mo2/3Y1/3)2AlB2 before (black) and after (red) HF etching and after TBAOH intercalation (blue) and delamination (green).(Inset) SEM image showing the cross-section of a Mo4/3B2-xTz film (scale bar,2 mm).High-resolution XPS spectra of a filtered film of boridene with peak fittings for the (F) Mo 3d and(G) B 1s regions.Reprinted with permission [6].Copyright 2021,Science.

2D MBenes have been theoretically predicted to possess many potential applications in various fields such as Li- and Na-ion batteries,electrocatalysis,and magnetic devices.2D MnB MBene[7] exhibits robust metallic ferromagnetism (3.2μBper Mn atom)and high Curie temperature (345 K),which can increase to 405 K and 600 K after the functionalization with -F and -OH groups,respectively.W2B2monolayer [8] has an excellent catalytic activity to reduce N2to NH3under ambient conditions with a low limiting potential of -0.24 eV,as well as Nb3B4(0.50 eV),Ta3B4(0.39 eV),and Ti2B2(0.37 eV).The improved catalytic activity of nitrogen reduction reactions was attributed from strong backdonation between the hybridized d orbital of metal atoms and the 2p orbital in N2.The theoretical results also confirm that monolayer MBenes are promising electrode materials for hydrogen evolution [9],lithium/sodium ion batteries (LIBs/NIBs) [10] with low diffusion energy barriers (0.22/0.13,0.28/0.17 and 0.29/0.17 eV for V2B2,Cr2B2,and Mn2B2,respectively) and high Li/Na atomic storage capacities (969/614,696/492 and 679/483 mAh/g).In addition,2D MBenes was also attempted and considered to apply in biotechnological fields [11] such as biological recognition,response,and potential impact to living organisms by connecting their biological properties with structural and chemical features.

In summary,boron-based 2D materials as MBenes have been emerged and theoretically predicted to possess diverse applications in Li- and Na-ion batteries,electrocatalysis,and magnetic devices.The synthesis of individual single-layer Mo4/3B2-xTzderived by selective etching of laminated i-MAB phases have been successfully realized.The top-down approach provides principles and guidance for further preparation of a wealth of MBenes for the common electronic,optoelectronic,and energy devices applications.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos.52172205,11804166) and the China Postdoctoral Science Foundation (No.2018M630587) and Tianjin Natural Science Foundation (No.18JCQNJC77100).