Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

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wo-Dimensional Nanocrystals Produced by Exfoliation Tof Ti 3AlC 2

Michael Naguib ,Murat Kurtoglu ,Volker Presser ,Jun Lu ,Junjie Niu ,Min Heon ,Lars Hultman ,Yury Gogotsi ,*and Michel W. Barsoum *T ypically two-dimensional (2D) free-standing crystals exhibit

[1properties that differ from those of their 3D counterparts. ]

Currently, however, there are relatively few such atomically lay-[2–5ered solids. ] Here, we report on 2D nanosheets, composed

of a few Ti layers and conical scrolls, produced by the room 3C2

temperature exfoliation of Ti uoric acid. The 3AlC2 in hydro

large elastic moduli predicted by ab initio simulation, and the possibility of varying their surface chemistries (herein they are terminated by hydroxyl and/or uorine groups) render these nanosheets attractive as polymer composite llers. Theory also predicts that their bandgap can be tuned by varying their surface terminations. The good conductivity and ductility of the treated powders suggest uses in Li-ion batteries, pseudo-capacitors, and other electronic applications. Since Ti 3AlC 2 is a member of a 60 + group of layered ternary carbides and nitrides known as the MAX phases, this discovery opens a door to the synthesis of a large number of other 2D crystals.

Arguably the most studied freestanding 2D material is

graphene, which was produced by mechanical exfoliation into

[1]single-layers in 2004. Some other layered materials, such as [2][4hexagonal BN, transition metal oxides, and hydroxides, ] as [3well as clays, ] have also been exfoliated into 2D sheets. Inter-estingly, exfoliated MoS 2 single layers were reported as early as [5]in 1986. Graphene is nding its way to applications ranging

[6from supercapacitor electrodes ] to reinforcement in compos-[7ites. ] Although graphene has attracted more attention than

all other 2D materials combined, its simple chemistry and the weak van der Waals bonding between layers in multilayer struc-tures limit its use. Complex, layered structures that contain more than one element may offer new properties because they

provide a larger number of compositional variables that can be tuned for achieving speci c properties. Currently, the number of non-oxide materials that have been exfoliated is limited to two fairly small groups, hexagonal van der Waals bonded struc-tures (e.g., graphene and BN) and layered metal chalcogenides

[8](e.g., MoS 2,WS 2,etc.).

It is well established that the ternary carbides and nitrides

with a M formula, where n = 1, 2, or 3, M is an early n+1AX n

transition metal, A is an A-group (mostly groups 13 and 14) element, and X is C and/or N, form laminated structures with

[9anisotropic properties. , 10 ] These, so-called MAX, phases are

layered hexagonal (space group P63/mmc ), with two formula units per unit cell ( Figure 1 a). Near-close-packed M-layers are interleaved with pure A-group element layers, with the X-atoms lling the octahedral sites between the former. One of the most widely studied and a promising member of this family

[11,12]is Ti 1 a). Over 60 MAX phases are currently 3AlC 2. (Figure

[9]known to exist.

The M n+1X layers are chemically stable. By comparison, n

because the A-group atoms are relatively weakly bound, they are the most reactive species. For example, heating Ti 3SiC 2in a C-rich atmosphere results in the loss of Si and the formation of TiC .[13 ] When the same compound is placed in molten cryo-x [14[15lite ] or molten Al, ] essentially the same reaction occurs: the Si escapes and a TiC forms. In the case of cryolite, the vacan-x

cies that form lead to the formation of a partially ordered cubic TiC 0.67 phase. In both cases, the high temperatures led to a structural transformation from a hexagonal to a cubic lattice and a partial loss of layering. In some cases, such as Ti 2InC, simply heating in vacuum at ≈800 ° C, results in loss of the A-group

[16]element and TiC x formation. Removing both the M and A

elements from the MAX structure by high-temperature chlo-rination results in a porous carbon known as carbide-derived

[17carbon with useful and unique properties. , 18 ]

Mechanical deformation of the MAX phases, which is medi-

ated by basal dislocations and is quite anisotropic, can lead to partial delamination and formation of lamellas with thicknesses

[19]that range from tens to hundreds of nanometers. However,

none of the MAX phases have ever been exfoliated into few-nanometer-thick crystalline layers reminiscent of graphene. Furthermore, as far as we are aware, there are no reports on the selective room temperature or moderate-temperature liquid or gas-phase extraction of the A-group layers from the MAX phases and/or their exfoliation. Here, we report the extraction of the Al from Ti 3AlC 2 and formation of a new of 2D material (Figure 1 b,c) that we propose to call “MXene” to emphasize its graphene-like morphology.

M. Naguib ,Dr. M. Kurtoglu ,Dr. V. Presser ,Dr. J. Niu ,M. Heon ,Prof. Y. Gogotsi ,Prof. M. W. Barsoum Department of Materials Science and EngineeringDrexel University

Philadelphia, PA 19104, USA

E-mail: gogotsi@drexel.edu; barsoumw@drexel.edu M. Naguib ,Dr. M. Kurtoglu ,Dr. V. Presser ,Dr. J. Niu ,M. Heon ,Prof. Y. Gogotsi A.J. Drexel Nanotechnology InstituteDrexel University

Philadelphia, PA 19104, USA Dr. J. Lu ,Prof. L. Hultman Department of PhysicsIFM, Linkoping UniversityLinkoping 58183, Sweden

DOI: 10.1002/adma.201102306 4248

http://doc.xuehai.net© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Mater. 2011, 23, 4248–4253

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