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Insights Into Exfoliation Possibility of MAX Phases to Mxenes

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Insights Into Exfoliation Possibility of MAX Phases to Mxenes Insights into exfoliation possibility of MAX phases to MXenes Mohammad Khazaei,1, ∗ Ahmad Ranjbar,1 Keivan Esfarjani,2 Dimitri Bogdanovski,3 Richard Dronskowski,3 and Seiji Yunoki1, 4, 5 1Computational Materials Science Research Team, RIKEN Advanced Institute for Computational Science (AICS), Kobe, Hyogo 650-0047, Japan 2Department of Mechanical and Aerospace Engineering, University of Virginia, 122 Engineer’s Way, Charlottesville,VA 22904, USA 3Chair of Solid State and Quantum Chemistry, RWTH Aachen University, 52056 Aachen, Germany 4Computational Condensed Matter Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan 5Computational Quantum Matter Research Team, RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan (Dated: March 15, 2018) Chemical exfoliation of MAX phases into two-dimensional (2D) MXenes can be considered as a major breakthrough in the synthesis of novel 2D systems. To gain insight into the exfoliation possibility of MAX phases and to identify which MAX phases are promising candidates for successful exfoliation into 2D MXenes, we perform extensive electronic structure and phonon calculations, and determine the force constants, bond strengths, and static exfoliation energies of MAX phases to MXenes for 82 different experimentally synthesized crystalline MAX phases. Our results show a clear correlation between the force constants and the bond strengths. As the total force constant of an “A” atom contributed from the neighboring atoms is smaller, the exfoliation energy becomes smaller, thus making exfoliation easier. We propose 37 MAX phases for successful exfoliation into 2D Ti2C, Ti3C2, Ti4C3,Ti5C4, Ti2N, Zr2C, Hf2C, V2C, V3C2, V4C3, Nb2C, Nb5C4, Ta2C, Ta5C4, Cr2C, Cr2N, and Mo2C MXenes. In addition, we explore the effect of charge injection on MAX phases. We find that the injected charges, both electrons and holes, are mainly received by the transition metals. This is due to the electronic property of MAX phases that the states near the Fermi energy are mainly dominated by d orbitals of the transition metals. For negatively charged MAX phases, the electrons injected cause swelling of the structure and elongation of the bond distances along the c axis, which hence weakens the binding. For positively charged MAX phases, on the other hand, the bonds become shorter and stronger. Therefore, we predict that the electron injection by electrochemistry or gating techniques can significantly facilitate the exfoliation possibility of MAX phases to 2D MXenes. PACS numbers: I. INTRODUCTION MAX phases with different mixtures of transition metals and/or X elements have also been synthesized. Theoreti- cally, thousands of crystalline or alloy structures of MAX MAX phases are a large family of solid layered tran- phases have been predicted to be stable.[10–20] sition metal carbides or nitrides with a hexagonal lat- All MAX phases are metallic [13, 14, 21–25] because tice (space group P 63/mmc) and the chemical formula of the existence of partially occupied d orbitals of tran- of Mn+1AXn, where “M” stands for an early transition metal (Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, or Mo), “A” repre- sition metals near the Fermi energy.[13, 14] In other sents an element from main groups III VI of the periodic words, due to the metallic bonding between the tran- table (Al, Ga, In, Tl, Si, Ge, Sn, Pb, P,− As, Bi, S, or Te), sition metal atoms either on the same layer or across lay- “X” is carbon or nitrogen, and n = 1 4.[1–5] Recently, ers, MAX phases become electrically conductive. Since the family of crystalline MAX phases has− been expanded MAX phases are made of metal and nonmetal elements even further. For example, a set of ordered double transi- and are synthesized at high temperatures, they are usu- ′ ′ arXiv:1803.00692v3 [cond-mat.mtrl-sci] 14 Mar 2018 ally considered as ceramics. Owing to their high me- tion metals MAX phases of M2M AX2, M2M2AX3,[6, 7] ′ chanical and thermal stability,[10, 13–15, 26, 27] some and (M2/3M1/3)2AX2 has been synthesized,[8, 9] where ′ of the MAX phases could be used in harsh conditions, “M ” is another transition metal. The crystal structures i.e., in conditions where the materials should be resis- of MAX phases with varying stoichiometry are shown tant to thermal shock, fatigue, creep, oxidation, or cor- schematically in Fig. 1. To the best of our knowledge, rosive reactions.[1, 2] To enhance the technological appli- 82 different crystalline MAX phases have been experi- cations of MAX phases, the ductility of MAX phases can mentally fabricated [1, 3] as listed in Table 1S. In ad- be improved by lowering their ceramic brittleness with- dition to the crystalline MAX phases, various alloys of out harming their toughness when the number of valence electrons per unit cell is properly controlled by mixing different M, A, or X elements.[10, 13] ∗Electronic address: [email protected] Up to a few years ago, the application of MAX 2 M M5AX 4 M’ A M M’ AX X M4AX 3 2 2 3 M3AX 2 M2M’AX 2 M1 M1 M2 M2 M M M1 M1 1 1 M2AX M M M2 M2 2 2 M M M1 M1 1 1 M2 M2 M2 M2 M1 M1 M2 M2 c a FIG. 1: Atomic structures of six different crystalline MAX phases. The a and c axes are common to all structures. Transition metals in different layers are indicated by “M1” and “M2” to denote different bonds between transition metals located in the same layer (M1−M1 or M2−M2 ) and in different layers (M1−M2). In Mn+1AXn (n = 1 − 4), M1 and M2 are the same type ′ ′ of transition metal, while they are different types in M2M AX2 and M2M2AX3. phases was based solely on their interesting mechanical these 2D MXenes are different from their corresponding properties, rather than their electronic, magnetic, or MAX phases. It is also shown theoretically that upon a optical ones. However, the possibility of exfoliation particular surface functionalization, some of the MXenes from MAX phases into two-dimensional (2D) transition turn into semiconductors.[38] In contrast to the limited metal carbides or nitrides, so called MXenes, has sig- application of MAX phases as structural materials, many nificantly broadened the range of potential applications interesting electronic,[38–44] optical,[45] mechanical,[46] of MAX phases.[28, 29] It has been repeatedly proven magnetic,[38, 47–51] and thermoelectric [38, 39], photo- experimentally that by using the combination of acid catalytic [52] and many other applications [53, 54] have treatment and sonication, the M A bonds in some been theoretically proposed for 2D MXenes. Experimen- of the MAX phases can be broken− while the M X tally, MXenes have attracted a lot of attention in the bonds are kept almost intact. After the exfoliation− materials science community for electronic transport,[55– process, the A atoms are washed out from the MAX 57] energy storage,[58, 59] energy conversion,[60–63] and phase and the remaining set of single or multilayers is development of novel hybrid nanocomposites.[64, 65] MXene. The crystal structures of derived 2D MXenes, There are comprehensive reviews on the synthesis and ′ ′ M2X, M3X2, M4X3, M5X4 M2M X2, and M2M2X3, are application,[66] and the current theoretical status [67] of shown schematically in Fig. 2. So far, the single or MXenes in the literature. multilayer 2D structures of Ti2C,[29] Ti2N,[30] V2C,[31] Nb2C,[31] Mo2C,[32] Ti3C2,[28] Zr3C2,[33] Nb4C3,[34] It is now well accepted that the MAX phases are a Ta4C3,[29] and Ti4N3[36] as well as TiNbC,[29] great source for producing novel 2D systems. Consider- (Ti0.5Nb0.5)2C,[29] (V0.5Cr0.5)3C2,[29] Ti3CN,[29] ing the compositional varieties of MAX phases, a large Mo2TiC2,[37] Mo2ScC2,[7] Cr2TiC2,[37] Mo2Ti2C3,[37] number of 2D MXenes with unprecedented properties are (Nb0.8Ti0.2)4C3,[35] and (Nb0.8Zr0.2)4C3 [35] have expected to become available. Therefore, an important already been synthesized. The electronic properties of question to be answered at this moment is which of the MAX phases are promising candidates for a successful 3 M M5X4 M M’ X M’ M4X3 2 2 3 X M2M’X2 M3X2 M2X c a FIG. 2: Atomic structures of six different 2D MXenes. The a and c axes are common to all structures. exfoliation into 2D materials. Theoretically, based on Burke-Ernzerhof (PBE) functional is used to compute the analysis of tensile and shear stresses, the mechan- the exchange-correlation energy.[70] The projected aug- ical exfoliation of various M2AlC (M = Ti, Zr, Hf, V, mented wave approach with a plane wave cutoff energy Nb, Ta, Cr, Mo, and W) MAX phases into 2D M2C of 520 eV is used to construct the wave functions. The MXenes have been investigated.[68] There, it was found atomic positions and lattice constants are fully opti- that by applying a large tensile strain, all the interlayer mized using the conjugate gradient method without im- M Al bonds can be broken, leading to the separation posing any symmetry. After structural optimization of − between M2C and Al layers.[68] However, the chemical atomic structures, the maximum residual force on each exfoliation process is a very complicated dynamical pro- atom is less than 0.0001 eV/A.˚ In the electronic self- cess, which is very difficult to model and simulate with consistency procedure, the total energies are converged to −8 all the details. Nevertheless, we can still gain great in- within 10 eV/cell. For the optimization of Mn+1AXn ′ sights through studying the bond strength and the exfo- (M2MnAXn+1), the Brillouin zone is sampled by taking liation energy because it is expected that MAX phases 18 18 9, 18 18 6, 18 18 3, and 18 18 1 (18 18 6 with weaker M A bonds are promising for the success- and× 18×18 3)× Monkhorst-Pack× × × k points× [71]× for n×= 1,× 2, ful exfoliation process.− In this paper, by using a set of 3, and× 4 (for× n = 1 and 2), respectively.
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