Compositions comprising free-standing two-dimensional nanocrystals

1. A composition comprising at least one layer having first and second surfaces, each layer comprising:

a substantially two-dimensional array of crystal cells,

each crystal cell having an empirical formula of M n+1 X n , such that each X is positioned within an octahedral array of M,

wherein M is at least one Group IIIB, IVB, VB, or VIB metal,

wherein each X is C, N, or a combination thereof;

n=1, 2 or 3; and

wherein at least one of said surfaces of each layer has surface terminations comprising alkoxide, carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide, thiol, or a combination thereof.

2. The composition of claim 1 comprising a plurality of layers, wherein both surfaces of each layer have said surface terminations.

3. The composition of claim 1 , wherein M is at least one Group IVB, Group VB, or Group VIB metal.

4. The composition of claim 1 wherein M is Ti, and n is 1 or 2.

5. The composition of claim 1 wherein M n+1 X n comprises Sc 2 C, Sc 2 N, Ti 2 C, Ti 2 N, V 2 C, V 2 N, Cr 2 C, Cr 2 N, Zr 2 C, Zr 2 N, Nb 2 C, Nb 2 N, Hf 2 C, Hf 2 N, Ti 3 C 2 , Ti 3 N 2 , V 3 C 2 , Ta 3 C 2 , Ta 3 N 2 , Ti 4 C 3 , Ti 4 N 3 , V 4 C 3 , V 4 N 3 , Ta 4 C 3 , Ta 4 N 3 , or a combination thereof.

6. The composition of claim 1 wherein M n+1 X n comprises Ti 3 C 2 , Ti 3 CN, Ti 2 C, Ta 4 C 3 or (V 1/2 Cr 1/2 ) 3 C 2 .

7. The composition of claim 1 wherein M is Ta, and n is 2 or 3.

8. The composition of claim 1 , the crystal cells having an empirical formula Ti 3 C 2 or Ti 2 C and wherein at least one of said surfaces of each layer has surface terminations comprising hydroxide, oxide, sub-oxide, or a combination thereof.

9. The composition of claim 1 , wherein the composition comprises an electrically conductive or semiconductive surface.

10. The composition of claim 1 , wherein M is at least one of Sc, Y, Lu, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W.

11. A stacked assembly of at least two layers, each layer having first and second surfaces, each layer comprising:

a substantially two-dimensional array of crystal cells,

each crystal cell having the empirical formula of M n+1 X n , such that each X is positioned within an octahedral array of M;

wherein M is a Group IIIB, IVB, VB, or VIB metal;

each X is C, N, or a combination thereof; and

n=1, 2, or 3;

wherein the layers are characterized as having an average surface area and an average interlayer distance; and

wherein at least one of said surfaces of each layer has bound thereto surface terminations comprising alkoxide, carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide, thiol, or a combination thereof.

12. The stacked assembly of claim 11 , wherein M n+1 X n comprises Sc 2 C, Sc 2 N, Ti 2 C, Ti 2 N, V 2 C, V 2 N, Cr 2 C, Cr 2 N, Zr 2 C, Zr 2 N, Nb 2 C, Nb 2 N, Hf 2 C, Hf 2 N, Ti 3 C 2 , Ti 3 N 2 , V 3 C 2 , Ta 3 C 2 , Ta 3 N 2 , Ti 4 C 3 , Ti 4 N 3 , V 4 C 3 , V 4 N 3 , Ta 4 C 3 , Ta 4 N 3 , or a combination thereof.

13. The stacked assembly of claim 12 , wherein M n+1 X n comprises Ti 2 C, Ti 2 N, Ti 3 C 2 , Ti 3 N 2 , Ti 4 C 3 , Ti 4 N 3 , Ta 3 C 2 , Ta 3 N 2 , Ta 4 C 3 , or Ta 4 N 3 , or a combination thereof.

14. The stacked assembly of claim 13 , wherein M n+1 X n comprises Ti 3 C 2 , TiNbC, Nb 2 C, Ti 3 CN, Ti 2 C, Ta 4 C 3 , or (V 1/2 Cr 1/2 ) 3 C 2 .

15. The stacked assembly of claim 11 , wherein M n+1 X n is Ti 3 C 2 , TiNbC, Ti 3 CN, or Ti 2 C.

16. The stacked assembly of claim 15 , wherein M n+1 X n is Ti 3 C 2 .

17. The stacked assembly of claim 11 , wherein the number of layers is in the range of 2 to about 50.

18. The stacked assembly of claim 11 , wherein the average surface area of the layers is in the range of from about 100 nm 2 to about 10,000 nm 2 or from about 100 μm 2 to about 10,000 μm 2 .

19. The stacked assembly of claim 11 , wherein lithium atoms, lithium ions, or both lithium atoms and lithium ions are intercalated between at least some of the layers.

20. An energy-storing device or electrode comprising the stacked assembly of claim 11 .

21. A method of preparing a composition comprising:

removing substantially all of the A atoms from a MAX-phase composition having an empirical formula of M n+1 AX n ;

wherein M is at least one Group IIIB, IVB, VB, or VIB metal;

wherein A is an A-group element;

each X is C, N, or a combination thereof; and

n=1, 2, or 3,

thereby providing a composition comprising at least one layer having a first and second surface, each layer comprising a substantially two-dimensional array of crystal cells;

each crystal cell having an empirical formula of M n+1 X n , such that each X is positioned within an octahedral array of M; and

wherein at least one of said surfaces of each layer has surface terminations comprising alkoxide, carboxylate, halide, hydroxide, hydride, oxide, sub-oxide, nitride, sub-nitride, sulfide, thiol, or a combination thereof.

22. The method of claim 21 , wherein the A atoms are removed by a process comprising a treatment with a fluorine-containing acid.

23. The method of claim 22 , wherein the fluorine-containing acid is aqueous hydrofluoric acid.

24. The method of claim 22 , further comprising sonication.

25. The method of claim 24 , wherein sonication is done using ultrasonic or megasonic sources.

26. The method of claim 21 , wherein removing substantially all of the A atoms from a MAX-phase composition is done electrochemically.