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Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo2ScAlC2, and its two-dimensional derivate Mo2ScC2 MXene
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0001-5036-2833
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
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2017 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 125, p. 476-480Article in journal (Refereed) Published
Abstract [en]

We 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 degrees 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. (C)2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD , 2017. Vol. 125, p. 476-480
Keywords [en]
Laminated structure; Out-of-plane chemical order; MAX phase; 2D material; MXene; DFT calculations
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-136312DOI: 10.1016/j.actamat.2016.12.008ISI: 000394201500047OAI: oai:DiVA.org:liu-136312DiVA, id: diva2:1087958
Note

Funding Agencies|Swedish Research Council (VR) [621-2012-4425, 642-2013-8020]; Knut and Alice Wallenberg (KAW) Foundation; Swedish Foundation for Strategic Research (SSF) through the synergy grant FUNCASE; KAW Foundation

Available from: 2017-04-10 Created: 2017-04-10 Last updated: 2020-02-21
In thesis
1. Synthesis and characterization of Mo- and W-based atomic laminates
Open this publication in new window or tab >>Synthesis and characterization of Mo- and W-based atomic laminates
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mn+1AXn (MAX) phases are inherently nanolaminated compounds based on transition metals (M), A group elements (A), and carbon or/and nitrogen (X), which exhibit a unique combination of ceramic and metallic properties. My thesis work has focused on exploring novel MAX phase chemistries, including elemental combinations beyond those traditionally used for MAX phases, and their graphene-analogous 2D counterpart, MXenes.  

The first part of the thesis investigates Mo-based MAX phases, which are among the least studied, despite indication of superconducting properties and potential for derivation of Mo-based MXenes. Initially, I performed theoretical calculations focused on evaluation of phase stability of the Mon+1GaCn MAX phases, and synthesized the predicted stable Mo2GaC in thin film form using DC magnetron sputtering. Close to phase pure epitaxial films were grown at ~590 °C, and electrical resistivity measurements using a four-point probe technique suggest a superconducting behavior with a critical temperature of ~7 K. The follow-up of this work, was synthesis of a new MAX related material, Mo2Ga2C, also by means of DC magnetron sputtering. The theoretical predictions as well as the materials characterization by X-ray diffraction and neutron powder diffraction, suggested a Ga bilayer interleaved between a set of Mo2C blocks, arranged in a simple hexagonal structure.   

It is known that selectively etching of the A-layer in a MAX phase, shown for A=Al, can lead to realization of a MXene. Hence, the next step in my research was to explore the possibility of etching of A=Ga in Mo2GaC as well as in Mo2Ga2C, targeting a Mo2C MXene, as motivated by theoretically proposed superior thermoelectric properties of this 2D material. While Mo2GaC did not allow removal of the A-layer, I showed that Mo2C MXene could be realized from etching Mo2Ga2C thin films, removing the Ga bilayer, in 50% hydrofluoric acid at a temperature of ~50 °C for a duration of ~3 h. Hence, the results did not only produce the first Mo-based MXene, it also showed that MXenes can be obtained for etching A-elements other than Al. This, in turn, increase the pathways for expanding the family of MXenes.    

I thereafter set out to explore the magnetic properties resulting from Mn-alloying of the non-magnetic Mo2GaC MAX phase. For that purpose, (Mo,Mn)2GaC was synthesized using a  DC magnetron sputtering system with Ga and C as elemental targets and a 1:1 atomic ratio  Mo:Mn compound target. Heteroepitaxial films on MgO(111) substrates were grown at  ~530 °C, as confirmed by X-ray diffraction. Compositional analysis using energy dispersive X-ray spectroscopy showed a 2:1 ratio of the M- and A-elements and a 1:1 ratio for the Mo and Mn atoms in the film. Vibrating sample magnetometry was utilized to measure the magnetic behavior of the films, showing a magnetic response up to at least 300 K, and with a coercive field of 0.06 T, which is the highest reported for any MAX phase to date.  

The second part of my research has been dedicated to realizing new MAX phase related, chemically ordered compounds and their MXene derivatives, and to initiate exploration of their properties. Materials synthesis was performed by pressureless bulk sintering, and inspired by theoretical calculations we showed evidence for a new so called o-MAX phase, Mo2ScAlC2, with an out-of-plane chemically ordered structure. It is the first experimentally verified Sc-containing MAX phase, which makes its corresponding MXene, Mo2ScC2, also presented in this work, the first MXene including Sc. Moreover, I discovered two so called i-MAX phases including W, (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC, which display in-plane chemical ordering in the M-layer. Furthermore, both was shown to allow synthesis of their corresponding 2D counterpart; W1.33C MXene, with ordered vacancies.  Initial test on these novel MXenes showed a high potential for hydrogen evolution reaction.  

Altogether, I have in my thesis work realized 6 novel MAX phases and related materials, and have shown evidence for 4 new MXenes. These materials inspire a wide range of future studies, with respect to fundamental properties as well as potential for future applications.   

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. p. 59
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1933
National Category
Condensed Matter Physics Nano Technology
Identifiers
urn:nbn:se:liu:diva-148012 (URN)10.3384/diss.diva-148012 (DOI)9789176853122 (ISBN)
Public defence
2018-06-11, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2018-05-24 Created: 2018-05-24 Last updated: 2019-09-30Bibliographically approved
2. Synthesis and characterization of two- and three-dimensional nanolaminated carbides
Open this publication in new window or tab >>Synthesis and characterization of two- and three-dimensional nanolaminated carbides
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused towards the synthesis and characterization of novel nanolaminated materials in primarily bulk (powder) form. Of particular interest is magnetic materials, or laminates that can be used as precursor for two-dimensional (2D) materials. 2D materials typically display a large surface-to-volume ratio, and as such they are very promising for applications within energy storage and catalysis. A more recently discovered family of 2D transition metal carbides/nitrides, called MXenes, are currently attracting a lot of attention. MXenes are produced by selective etching of parent 3D nanolaminates, so called MAX phases, facilitating removal of selected atomic layers, and formation of 2D sheets.

In my work on new nanolaminates as precursors for 2D materials, I have synthesized (Mo2/3Sc1/3)2AlC and have studied its crystal structure. It was found that Mo and Sc are chemically ordered in the metal layers, with the in-plane ordering motivating the notation i-MAX for this new type of MAX phase alloy. By selective etching of Sc and Al, we thereafter produced a 2D materials with ordered vacancies, Mo1.33C, and studied the electrochemical properties. It was found that the material displayed a high capacitance, ~1200 F cm-3, which is 65% higher that the counterpart without vacancies, Mo2C.

I also synthesized a previously not known out-of-plane ordered Mo2ScAlC2 MAX phase. By selective etching of Al, we produced a 2D material, Mo2ScC2, which is correspondingly ordered in the out-of-plane direction. Another related laminated material was also discovered and synthesized, Sc2Al2C3, and its crystal structure was determined. The material is potentially useful for conversion into a 2D material. I have also shown that Sc2Al2C3 is an example of a series of materials with the same crystal structure, with Sc replaced by other metals.

Magnetic materials are used in many applications, such as for data storage devices. In particular, layered magnetic materials are of interest due to their anisotropic structure and potential formation of interesting magnetic characteristics. I have been synthesizing and characterizing magnetic nanolaminates, starting with the (V,Mn)3GaC2 MAX phase in the form of an epitaxial thin film. Analysis of the magnetic behavior showed a ferromagnetic response above room temperature I thereafter showed that our previously discovered family of i-MAX phases could be expanded with a subclass of ordered nanolaminates based on rare earth (RE) elements, of the general formula (Mo2/3RE1/3)2AlC , where RE=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. I studied their crystal structure by scanning transmission microscopy (STEM), X-ray diffraction (XRD), and neutron diffraction. We found that these phases can crystalize in three different structures, of space group C2/m, C2/c, and Cmcm, respectively. The magnetic behavior was studied and the magnetic structure of two materials could be determined. We suggest that the complex behavior identified is due to competing magnetic interaction and frustration.

I also synthesized another rare earth-based nanolaminate, Mo4Ce4Al7C3. The crystal structure was investigated by single crystal X-ray diffraction and STEM. Magnetization analysis reveal a ferromagnetic ground state below 10.5 K. X-ray absorption near-edge structure provide evidence that Ce is in a mixed-valence state. X-ray magnetic circular dichroism shows that only one of the two Ce sites are magnetic. 

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2020. p. 44
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2058
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-163847 (URN)10.3384/diss.diva-163847 (DOI)9789179298791 (ISBN)
Public defence
2020-03-26, F-building, Campus Valla, Linköping, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2020-03-02 Created: 2020-02-21 Last updated: 2020-03-11Bibliographically approved

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