Synthesis of two-dimensional molybdenum
carbide, Mo2C, from the gallium based atomic
laminate Mo2Ga2C
Rahele Meshkian, Lars-Åke Näslund, Joseph Halim, Jun Lu, Michel W. Barsoum and Johanna Rosén
Linköping University Post Print
N.B.: When citing this work, cite the original article.
Original Publication:
Rahele Meshkian, Lars-Åke Näslund, Joseph Halim, Jun Lu, Michel W. Barsoum and Johanna Rosén, Synthesis of two-dimensional molybdenum carbide, Mo2C, from the gallium based atomic laminate Mo2Ga2C, 2015, Scripta Materialia, (108), 147-150.
http://dx.doi.org/10.1016/j.scriptamat.2015.07.003
Copyright: Elsevier
http://www.elsevier.com/
Postprint available at: Linköping University Electronic Press
Synthesis of two-dimensional molybdenum carbide, Mo
2C, from the gallium
based atomic laminate Mo
2Ga
2C
Rahele Meshkiana,*, Lars-Åke Näslunda, Joseph Halima,b, Jun Lua, Michel W. Barsouma,b,
Johanna Rosena
aThin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University,
SE-581 83, Linköping, Sweden
bDepartment of Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United
States
We report on the synthesis of a two-dimensional transition metal carbide, Mo2C, (MXene)
obtained by immersing Mo2Ga2C thin films in hydrofluoric acid. Experimental evidences for
neither synthesis of a Mo-based MXene nor selective etching of Ga from an atomic nanolaminate have previously been presented. MXene formation is verified through X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. This discovery unlocks new potential applications for Mo-based MXenes in a host of applications, from thermoelectrics to catalysis and energy storage.
Keywords: 2D materials; Layered structures; MXene; Transmission electron microscopy
(TEM); Energy dispersive X-ray spectroscopy (EDS) *Corresponding author; e-mail: rahele.meshkian@liu.se
The interest in two-dimensional (2D) materials has elevated after the discovery of graphene. Recently a new family of graphene-analogous material, known as MXenes, was discovered [1,2]. As for graphene, the properties of the MXene compounds, e.g. wetting, electrical, and electrochemical characteristics, can be tunable through selections of surface functional groups, which makes them interesting in multiple fields and technologies such as reinforcements in polymers, energy storage, and catalysis. MXenes are, for example, promising candidates as high-performance anode materials in lithium ion as well as non-lithium ion batteries [1,2]. Recently a new study on Ti3C2 showed that the volumetric capacitance of these
materials can be as large as 900 F/cm3 [3], which is three times higher than previously reported
carbon-based electrodes [4].
Examples of experimentally realized MXene compounds reported to date, are: Ti3C2,
Ti2C, Ta4C3, TiNbC, (V0.5, Cr0.5)3C2, Ti3CNx (x < 1), V2C, and Nb2C [5–7]. These compounds
can be generated by exfoliation of the corresponded laminated MAX phases upon selective etching of the A element using a suitable etchant. The MAX phases are a family of materials composed of a transition metals (M), an A group element (A), and carbon and/or nitrogen (X)
with a combination of metallic and ceramic properties [8–10]. So far, all synthesized MXene compounds have been limited to be generated from MAX phases with aluminum (Al) as the A element. Potential MXene compounds, such as Hf2C, Mo2C, and Zr2C, are yet to be produced,
although corresponding MAX phase materials with Al as the A element cannot be formed. Also among the still not yet synthesized MXene compounds, Mo2C stands out as a promising
candidate for a high-performance thermoelectric material [11,12]. Band structure calculations on functionalized Mo2C indicate metallic character when the functional groups are, e.g., O and
OH, and semiconducting properties when the functional groups are, e.g., F and Cl. A large Seebeck coefficient and a decent electrical conductivity have also been suggested, with optimal combination of properties for thermoelectric applications indicated for Mo2C functionalized
with F [11]. In addition, as a low cost material, functionalized 2D layered Mo2C could find
application within, e.g., power generation, energy storage devices, and catalysis.
In this letter we presents the first detailed micrographs of a synthesized 2D Mo2C
compound, for which no experimental evidence has previously been presented. The Mo2C
compound was generated through selective etching of gallium (Ga) from a thin film of the new ternary nanolaminated MAX phase related material Mo2Ga2C [13]. The produced MXene
material was characterized using X-ray diffraction (XRD), (scanning) transmission electron microscopy ((S)TEM), and energy dispersive X-ray spectroscopy (EDX).
Mo2Ga2C thin films were deposited using DC magnetron sputtering from three elemental
targets: Mo, Ga and C. Since Ga has a melting point of about 30 ºC the depositions were performed using a previously established procedure [14], which included a liquid Ga target. The depositions were performed at a temperature of 560 ºC. The base pressure in the chamber was 1.5 x 10-7 Torr, with Ar gas introduced to a pressure of 4.1 mTorr. The Mo
2Ga2C thin films
were grown on MgO[111] substrates, which were ultrasonically cleaned in acetone, ethanol, and isopropanol for 10 min each. Prior to deposition the substrate was annealed in the chamber for 15 min. Optimization of the Mo2Ga2C thin film deposition was performed in line with
previous work [13,15]. The overall composition of resulting samples, determined through elastic recoil detection analysis (ERDA), was 39, 43, and 18 at% of Mo, Ga, and C, respectively [13], with a structural analysis showing primarily Mo2Ga2C, although with some intergrown
layers of Mo2GaC [13]. Selective etching of the Ga layers was carried out by immersing the
thin films in 50% concentrated hydrofluoric acid (HF(aq)) for 3 h at a temperature of 50 ºC. The sample was subsequently rinsed in deionized water and ethanol.
Selective etching of the Ga layers was carried out by immersing the thin films in 50% concentrated hydrofluoric acid (HF(aq)) for 3 h at a temperature of 50 ºC. The sample was subsequently rinsed in deionized water and ethanol.
Structural analysis of the Mo2Ga2C and Mo2C was made through XRD, using a
Panalytical Empyrean MRD with a line focus Cu Kα source equipped with a hybrid mirror and
a 0.27º parallel plate collimator in the incident and diffracted beam side, respectively. Further structural characterization was performed through TEM, with cross-sectional TEM samples being prepared using conventional mechanical methods with additional low angle ion milling, followed by a fine-polishing step with a low acceleration voltage. In order to investigate the elemental distribution in the film with atomic resolution, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) characterization and EDX mapping was carried out using Linköping double Cs corrected FEI Titan3 60–300 operated at 300 kV and
equipped with the Super-X EDX system.
Figure 1. Symmetric 2θ XRD scan of a Mo2Ga2C thin film before and after 3 h etching in 50% HF acid
heated to 50 ºC.
The θ-2θ XRD diffractogram of a representative sample, before and after the etching process, is shown in Fig. 1. All peaks displayed in the diffractogram of the Mo2Ga2C material
are present in the sample after the etching process, i.e. the basal plane peaks of the Mo2Ga2C
important, however, is the additional peak at ~7.1 º in the diffractogram of the etched sample, which indicates the presence of a component with a larger c-lattice parameter determined to be ~24.7 Å. This observation suggests the formation of the Mo2C MXene upon HF(aq) etching
[5,6,16]. Further, and evident from the figure comparing the basal plane peaks that stems from the Mo2Ga2C, e.g. from the (0002) plane, before and after the etching process, there is a
reduction of peak intensities.
In addition, there is a feature at the left side of the MgO(222) peak that belongs to Mo3Ga(400) with a cubic structure within the space group Pm–3n(223), which is one of the
most competing phases in the Mo-Ga-C system [15]. The peak of this impurity phase (Mo3Ga)
in the diffractogram of the etched sample shows a substantial reduction in intensity, which suggests that this phase to a large extent is dissolved during etching.
Figure 2. TEM micrographs of a Mo2Ga2C thin film after ~3 h of etching in 50% HF at 50 ºC. a) An
overview TEM image reveals the formation of MXene, i.e. 2D layered Mo2C, in the tilted grains. b) A
higher magnification micrograph of one of the tilted grains shows exfoliation of the layers. c) A higher resolution image from the top part in (b) confirms the existence of Mo2C 2D layers. The left side of the
image also reveals the difficulty of etching the Ga layers of Mo2GaC that co-exists with the Mo2Ga2C
Fig. 2(a) shows an overview TEM micrograph revealing epitaxially grown layers (on a MgO(111) substrate) and the formation of tilted grains in the topmost part of the film. The tilted grains appear exfoliated and are 2D Mo2C layers. The effect of the HF(aq) etching is more
apparent in Fig. 2(b) where the higher magnification micrograph of one of the exfoliated tilted grains clearly shows delamination indicating that the A layer is removed. The fact that the titled grains are the first to be etched is consistent with basal plane etching. Said otherwise, the titled grains allow easier access of the etchant in, and reaction products out, along the basal planes. When the etchant reaches the epitaxially grown layers the A element is protected by the MX layer and the rate of the etching process decreases significantly and thus prevents the complete transformation of the film into MXene. Still a substantial fraction of the epitaxially grown layers are, however, converted into MXene as evidenced by the XRD diffractogram in Fig. 1. The fact that the epitaxially grown layers are converted into MXene is also observed in the TEM micrograph shown in Fig. 2(d).
Fig. 2(c) shows a higher magnification image from part of Fig. 2(b), where the exfoliation of the layers is even more apparent. Interestingly, this image also shows regions – on the left-hand side of the micrograph – where the Ga layers are intact. Since this region displays three Mo2C-Ga-Mo2C sheets it is, thus, obvious that removing the Ga layers in the Mo2GaC MAX
Figure 3. (a) High resolution STEM image from an etched Mo2Ga2C film. (b) The elemental mapping
of the Mo, Ga and O from the acquired STEM image shown in (a). The elemental mapping shows formation of the Mo2C phase after the etching procedure. (c) An elemental profile across the planes
shown in (a) confirms the removal of Ga upon etching.
Fig. 3(a) shows a high-resolution STEM image of an etched thin film initially composed of Mo2Ga2C (middle section) interspersed with sheets of Mo2GaC (thicker bright region to the
right and left). Corresponding elemental maps of Mo, Ga, and O in (b) shows a rather homogeneous distribution of in particular Mo, but also O. Most important, though, is that the Ga content is low in the middle section. This is confirmed by the elemental profile shown in (c), where the compositional analysis from the (S)TEM/EDX data reveals a ratio of approximately 50:5:40:5 for Mo, Ga, O and F, respectively. Hence, the absence of Ga in the middle section of the HRSTEM image verifies the removal of the A layer, and the formation of Mo2C MXene.
In summary, 2D layers of exfoliated Mo2C were formed from Mo2Ga2C thin films etched
etched to create MXenes and the first time a Mo-based MXene has been produced. These results unlock new potential applications for MXene materials involving Mo, will initiate research on 2D layered Mo2C as a high-performance thermoelectric material, and potentially opens up a
new set of 2D materials obtained from Ga-based MAX phases and related materials.
The research was funded by the European Research Council under the European Community Seventh Framework Program (FP7/2007-2013)/ERC Grant agreement no [258509]. J. Lu acknowledges the KAW Foundation for the Ultra Electron Microscopy Laboratory in Linköping. J. Rosen acknowledges funding from the Swedish Research Council (VR) grant no. 642-2013-8020 and 621-2012-4425, from the KAW Fellowship program, and from the SSF synergy grant FUNCASE.
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