Journal ArticleDOI

# Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo2ScAlC2, and its two-dimensional derivate Mo2ScC2 MXene

, Jun Lu1
15 Feb 2017-Acta Materialia (Pergamon)-Vol. 125, pp 476-480

AbstractWe present theoretical prediction and experimental evidence of a new MAX phase alloy, Mo2ScAlC2, with out-of-plane chemical order. Evaluation of phase stability was performed by ab initio calculations based on Density Functional Theory, suggesting that chemical order in the alloy promotes a stable phase, with a formation enthalpy of −24 meV/atom, as opposed to the predicted unstable Mo3AlC2 and Sc3AlC2. Bulk synthesis of Mo2ScAlC2 is achieved by mixing elemental powders of Mo, Sc, Al and graphite which are heated to 1700 °C. High resolution transmission electron microscopy reveals a chemically ordered structure consistent with theoretical predictions with one Sc layer sandwiched between two Mo C layers. The two-dimensional derivative, the MXene, is produced by selective etching of the Al-layers in hydrofluoric acid, resulting in the corresponding chemically ordered Mo2ScC2, i.e. the first Sc-containing MXene. The here presented results expands the attainable range of MXene compositions and widens the prospects for property tuning.

Topics: Phase (matter) (54%)

### 1. Introduction

• It has been about six decades since Nowotny et al. discovered a family of laminated material called H-phases [1].
• To date, more than 70 MAX phases have been synthesized in both bulk and thin film form.
• The first Mo2C MXene was reported in 2015 [18, 19], and has since been found to have high potential for e.g. energy storage, in particular for electrode material in e.g. Li-ion batteries [20].
• Structural and compositional characterization show separation of the elements into individual atomic layers.

### 2. Computational details

• First-principles calculations were performed by means of density functional theory (DFT) and the projector augmented wave method [23, 24] as implemented within the Vienna abinitio simulation package (VASP) [25-27].
• The authors adopted the non-spin polarized generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) [28] for treating electron exchange and correlation effects.
• These are modelled using the special quasi-random structure (SQS) method [30, 31] on supercells of 4×4×1 M3AX2 unit cells, with a total of 96 M-sites, respectively.
• Evaluation of phase stability was performed by identifying the set of most competing phases at a given composition, using a linear optimization procedure [31, 32] including all competing phases in the system.
• This gives an estimate above which temperature the disordered structure is energetically favorable as compared to the ordered structure.

### 3. Experimental details

• These powders were mixed in an agate mortar and placed in a covered Al2O3 crucible, which was inserted in a tube furnace.
• Both MAX and MXene samples were also characterized by using the Linköping double Cs corrected FEI Titan3 60– 300 operated at 300 kV, equipped with the Super-X EDX system to perform atomic structural analysis.
• The suspension was afterwards filtered and dispersed in water ~10 times in order to remove all the remaining acid and the reaction products.
• Subsequent intercalation of the MXene sheets were realized by adding ~0.1g of the powder in ~1ml of an organic base, tetrabutylammonium hydroxide , and shaking it manually for ~5 min.

### 4. Results and discussion

• For Mo2ScAlC2 and Sc2MoAlC2, six different layer sequences were considered, see Anasori et al. [37] for layer stacking definitions.
• The in-plane and out-of-plane lattice parameters, a and c, determined from Rietveld refinement, are 3.03 and 18.77 Å, respectively.
• Furthermore, if the M element closest to the Al layer has larger electronegativity than Al, this results in fewer electrons available for populating antibonding Al-Al orbitals, which is energetically expensive [6].
• X-ray diffractograms of the Mo2ScAlC2 MAX powder and its corresponding MXene, Mo2ScC2, after etching and intercalation, is shown in Fig.
• The etching is not fully completed as the scan for the MXene also contains residual Mo2ScAlC2.

### 5. Conclusions

• The authors have theoretically predicted the existence of a new quaternary MAX phase alloy with out-of-plane chemical order, Mo2ScAlC2, with a Sc atomic layer sandwiched between two Mo-C layers.
• The prediction has been experimentally verified through bulk synthesis and materials characterization, primarily from high resolution STEM with EDX elemental mapping.
• The a and c lattice parameters determined using Rietveld refinement are 3.03 and 18.77 Å, respectively.
• Furthermore, the MAX phase has been converted into twodimensional MXene by selective etching of Al.
• The resulting MXene, Mo2ScC2, is the first MXene to date comprising Sc. 12.

Did you find this useful? Give us your feedback

Content maybe subject to copyright    Report

Theoretical stability and materials synthesis of a
chemically ordered MAX phase, Mo2ScAlC2,
and its two-dimensional derivate Mo2ScC2
MXene
Rahele Meshkian, Quanzheng Tao, Martin Dahlqvist, Jun Lu, Lars Hultman and Johanna
Rosén
Journal Article
N.B.: When citing this work, cite the original article.
Original Publication:
Rahele Meshkian, Quanzheng Tao, Martin Dahlqvist, Jun Lu, Lars Hultman and Johanna
Rosén, Theoretical stability and materials synthesis of a chemically ordered MAX phase,
Mo2ScAlC2, and its two-dimensional derivate Mo2ScC2 MXene, Acta Materialia, 2017. 125(),
pp.476-480.
http://dx.doi.org/10.1016/j.actamat.2016.12.008
http://www.elsevier.com/
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-136312

1
Theoretical stability and materials synthesis of a chemically ordered MAX phase,
Mo
2
ScAlC
2
, and its two-dimensional derivate Mo
2
ScC
2
MXene
Rahele Meshkian*, Quanzheng Tao, Martin Dahlqvist, Jun Lu, Lars Hultman, and Johanna
Rosen
Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden
Abstract
We present theoretical prediction and experimental evidence of a new MAX phase alloy,
Mo
2
ScAlC
2
, with out-of-plane chemical order. Evaluation of phase stability was performed
by ab initio calculations based on Density Functional Theory, suggesting that chemical order
in the alloy promotes a stable phase, with a formation enthalpy of -24 meV/atom, as opposed
to the predicted unstable Mo
3
AlC
2
and Sc
3
AlC
2
. Bulk synthesis of Mo
2
ScAlC
2
is achieved
by mixing elemental powders of Mo, Sc, Al and graphite which are heated to 1700 ºC. High
resolution transmission electron microscopy reveals a chemically ordered structure
consistent with theoretical predictions with one Sc layer sandwiched between two Mo-C
layers. The two-dimensional derivative, the MXene, is produced by selective etching of the
Al-layers in hydrofluoric acid, resulting in the corresponding chemically ordered Mo
2
ScC
2
,
i.e. the first Sc-containing MXene. The here presented results expands the attainable range
of MXene compositions and widens the prospects for property tuning.
Keywords: laminated structure, out-of-plane chemical order, MAX phase, 2D material;
MXene, DFT calculations
*Corresponding author; Tel. +4613286619, e-mail address: rahele.meshkian@liu.se

2
1. Introduction
It has been about six decades since Nowotny et al. discovered a family of laminated material
called H-phases [1]. After their revival by Barsoum et al. some decades later [2], the family
was expanded and given the nomenclature M
n+1
AX
n
(MAX) phases, n = 1-3, being composed
of an early transition metal (M), an A-group element primarily from group 13 and 14 (A),
and carbon and/or nitrogen (X). These compounds are inherently laminated, and exhibit a
combination of metallic and ceramic properties which stem from strong metallic-covalent M-
X bonds in combination with weaker bonding between M-A atoms. Consequently, MAX
phases display high electrical and thermal conductivity, good resistance to oxidation and
thermal shock, and are elastically stiff and easily machinable. To date, more than 70 MAX
phases have been synthesized in both bulk and thin film form.
Substitution of a fraction of M, A, or X atoms can be beneficial for property tuning, e.g., for
increasing the hardness [3], or for introducing magnetic properties [4, 5]. MAX phase alloys
to date are to a major extent solid solutions, and in particular alloys of 211 (n = 1)
stoichiometry have not shown any tendency to order in atomic layers composed of one
element only, possibly due to a high configurational entropy within these systems and only
one crystallographic site for each M, A, and X element [6]. This is opposed to quaternary
MAX phases of 312 (n = 2) or 413 (n = 3) stoichiometry and with M-site alloying, which can
display an out-of-plane chemical order. Such examples are the recently reported Mo
2
TiAlC
2
and Mo
2
Ti
2
AlC
3
, which were theoretically predicted and subsequently synthesized by
Anasori et al. [7]. This is in addition to previously discovered Cr
2
TiAlC
2
and V
1.5
Cr
1.5
AlC
2
,
reported by Liu et al. [8] and Caspi et al. [9], respectively. Note that for V
1.5
Cr
1.5
AlC
2
, a

3
partially ordered structure has been observed. In an explanatory and predictive theoretical
study by Dahlqvist et al. [6], the authors have investigated the stability of TiMAlC,
TiM
2
AlC
2
, MTi
2
AlC
2
, and Ti
2
M
2
AlC
3
where M is from group 4-6 in the Periodic table of
elements, trying to identify the origin behind the chemical ordering. Extending beyond that
study, exploring a combination of M elements that can neither be found in a pure 312 MAX
phase nor energetically promote a stacking in which M is surrounded by C in an face-centered
cubic (fcc) configuration, we have here investigated quaternary MAX phases in the Mo-Sc-
Al-C system.
Interest in Al-containing MAX phases increased after evidence of their resistance to
oxidation upon formation of protective oxide layers [10, 11], also used in studies focused
towards crack healing [12, 13]. Moreover, selective etching of Al has been shown to produce
MXenes, graphene analogous materials that are both electrically conducting and hydrophilic
[14]. The quest for Mo-containing MXenes in particular was elevated after a number of
theoretical studies, predicting these compounds as promising thermoelectric material [15], as
catalyst [16] and also as efficient electrodes for Li-ion batteries [17]. The first Mo
2
C MXene
was reported in 2015 [18, 19], and has since been found to have high potential for e.g. energy
storage, in particular for electrode material in e.g. Li-ion batteries [20].
There is only one previous report stating synthesis of a Sc-based MAX phase; Sc
2
InC [1,
21]. However, no information is presented on the specific synthesis conditions, and no
experimental evidence of the resulting material or its properties. There is a theoretical report
on the structural and elastic properties of a number of known and hypothetical M
2
InC phases

4
with M = Sc, Ti, V, Nb, Zr, Hf and Ta. Beside the calculated crystal parameters, the authors
have reported the theoretical Young’s, shear, and bulk moduli, which for Sc
2
InC are well
below the other phases investigated [22].
Consequently, exploring synthesis of a MAX phase based on Al, Mo and Sc is highly
motivated from a fundamental as well as a property perspective. In the present study, we have
theoretically predicted and experimentally verified the existence of Mo
2
ScAlC
2
as a new
chemically ordered MAX phase. Structural and compositional characterization show
separation of the elements into individual atomic layers. Furthermore, we present evidence
of the corresponding MXene; Mo
2
ScC
2
.
2. Computational details
First-principles calculations were performed by means of density functional theory (DFT)
and the projector augmented wave method [23, 24]
as implemented within the Vienna ab-
initio simulation package (VASP) [25-27]. We adopted the non-spin polarized generalized
gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) [28] for
treating electron exchange and correlation effects. A plane-wave energy cut-off of 400 eV
was used and for sampling of the Brillouin zone we used the Monkhorst-Pack scheme [29].
For each considered phase the calculated total energy is converged to within 0.5 meV/atom
with respect to k-point sampling and structurally optimized in terms of unit-cell volumes, c/a
ratios (when necessary), and internal parameters to minimize the total energy.

##### Citations
More filters

Journal ArticleDOI
Abstract: The recent chemical exfoliation of layered MAX phase compounds to novel two-dimensional transition metal carbides and nitrides, the so-called MXenes, has brought a new opportunity to materials science and technology. This review highlights the computational attempts that have been made to understand the physics and chemistry of this very promising family of advanced two-dimensional materials, and to exploit their novel and exceptional properties for electronic and energy harvesting applications.

399 citations

Journal ArticleDOI
Abstract: 2D transition metal carbides, carbonitrides, and nitrides, known as MXenes, are a rapidly growing family of 2D materials with close to 30 members experimentally synthesized, and dozens more studied theoretically. They exhibit outstanding electronic, optical, mechanical, and thermal properties with versatile transition metal and surface chemistries. They have shown promise in many applications, such as energy storage, electromagnetic interference shielding, transparent electrodes, sensors, catalysis, photothermal therapy, etc. The high electronic conductivity and wide range of optical absorption properties of MXenes are the key to their success in the aforementioned applications. However, relatively little is currently known about their fundamental electronic and optical properties, limiting their use to their full potential. Here, MXenes' electronic and optical properties from both theoretical and experimental perspectives, as well as applications related to those properties, are discussed, providing a guide for researchers who are exploring those properties of MXenes.

375 citations

Journal ArticleDOI
Abstract: Recent chemical exfoliation of layered MAX phase compounds to novel two-dimensional transition metal carbides and nitrides, so called MXenes, has brought new opportunity to materials science and technology. This review highlights the computational attempts that have been made to understand the physics and chemistry of this very promising family of advanced two-dimensional materials, and to exploit their novel and exceptional properties for electronic and energy harvesting applications.

310 citations

Journal ArticleDOI
, Jun Lu1
TL;DR: This work designs a parent 3D atomic laminate, (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, shows evidence for 2D Mo1.33C sheets with ordered metal divacancies and high electrical conductivities.
Abstract: The exploration of two-dimensional solids is an active area of materials discovery. Research in this area has given us structures spanning graphene to dichalcogenides, and more recently 2D transition metal carbides (MXenes). One of the challenges now is to master ordering within the atomic sheets. Herein, we present a top-down, high-yield, facile route for the controlled introduction of ordered divacancies in MXenes. By designing a parent 3D atomic laminate, (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D Mo1.33C sheets with ordered metal divacancies and high electrical conductivities. At ∼1,100 F cm−3, this 2D material exhibits a 65% higher volumetric capacitance than its counterpart, Mo2C, with no vacancies, and one of the highest volumetric capacitance values ever reported, to the best of our knowledge. This structural design on the atomic scale may alter and expand the concept of property-tailoring of 2D materials. Vacancies in 2D materials can influence their properties, however controlling their formation remains a challenge. Here the authors show that selective etching of a 3D laminate with in-plane chemical ordering results in formation of MXenes with ordered divacancies, as well as elevated conductance and supercapacitance.

299 citations

Journal ArticleDOI
, Min Jang4, Yi Huang5
Abstract: Energy and environmental issues presently attract a great deal of scientific attention. Recently, two-dimensional MXenes and MXene-based nanomaterials have attracted increasing interest because of their unique properties (e.g., remarkable safety, a very large interlayer spacing, environmental flexibility, a large surface area, and thermal conductivity). In 2011, multilayered MXenes (Ti3C2Tx, a new family of two-dimensional (2D) materials) produced by etching an A layer from a MAX phase of Ti3AlC2, were first described by researchers at Drexel University. The term “MXene” was coined to distinguish this new family of 2D materials from graphene, and applies to both the original MAX phases and MXenes fabricated from them. We present a comprehensive review of recent studies on energy and environmental applications of MXene and MXene-based nanomaterials, including energy conversion and storage, adsorption, membrane, photocatalysis, and antimicrobial. Future research needs are discussed briefly with current challenges that must be overcome before we completely understand the extraordinary properties of MXene and MXene-based nanomaterials.

208 citations

##### References
More filters

Journal ArticleDOI
TL;DR: A simple derivation of a simple GGA is presented, in which all parameters (other than those in LSD) are fundamental constants, and only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked.
Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif

117,932 citations

### "Theoretical stability and materials..." refers methods in this paper

• ...We adopted the non-spin polarized generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) [28] for...

[...]

• ...We adopted the non-spin polarized generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) [28] for treating electron exchange and correlation effects....

[...]

Journal ArticleDOI
TL;DR: An efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set is presented and the application of Pulay's DIIS method to the iterative diagonalization of large matrices will be discussed.
Abstract: We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special metric'' and a special preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.

64,484 citations

### "Theoretical stability and materials..." refers methods in this paper

• ...First-principles calculations were performed by means of density functional theory (DFT) and the projector augmented wave method [23, 24] as implemented within the Vienna abinitio simulation package (VASP) [25-27]....

[...]

Journal ArticleDOI
TL;DR: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way and can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function.
Abstract: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.

48,474 citations

### "Theoretical stability and materials..." refers methods in this paper

• ...First-principles calculations were performed by means of density functional theory (DFT) and the projector augmented wave method [23, 24] as implemented within the Vienna abinitio simulation package (VASP) [25-27]....

[...]

Journal ArticleDOI
Abstract: The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Bl\"ochl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules $({\mathrm{H}}_{2}{,\mathrm{}\mathrm{H}}_{2}{\mathrm{O},\mathrm{}\mathrm{Li}}_{2}{,\mathrm{}\mathrm{N}}_{2}{,\mathrm{}\mathrm{F}}_{2}{,\mathrm{}\mathrm{BF}}_{3}{,\mathrm{}\mathrm{SiF}}_{4})$ and several bulk systems (diamond, Si, V, Li, Ca, ${\mathrm{CaF}}_{2},$ Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.

46,297 citations

### "Theoretical stability and materials..." refers methods in this paper

• ...First-principles calculations were performed by means of density functional theory (DFT) and the projector augmented wave method [23, 24] as implemented within the Vienna abinitio simulation package (VASP) [25-27]....

[...]

Journal ArticleDOI
Abstract: A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.

42,677 citations

### "Theoretical stability and materials..." refers methods in this paper

• ...was used and for sampling of the Brillouin zone we used the Monkhorst-Pack scheme [29]....

[...]