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Surface characterization of 2D transition metal carbides (MXenes)
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Research on two-dimensional (2D) materials is a rapidly growing field owing to the wide range of new interesting properties found in 2D structures that are vastly different from their three-dimensional (3D) analogues. In addition, 2D materials embodies a significant surface area that facilitates a high degree of surface reactions per unit volume or mass, that is imperative in many applications such as catalysis, energy storage, energy conversion, filtration, and single molecule sensing. MXenes constitute a family of 2D materials consisting of transition metal carbides and/or nitrides, which are typically formed after selective etching of their 3D parent MAX phases. The latter, are a family of nanolaminated compounds that typically follow the formula Mn+1AXn (n=1-3), where M is a transition metal, A is a group 13 or 14 element, and X is C and or N. Selective etching by aqueous F- containing acids removes the A layer leaving 2D Mn+1Xn slabs instantly terminated by a mix of O-, OH- and F-groups. The first and most investigated MXene is Ti3C2TX, where TX stands for surface termination, which has shown record properties in a range of applications (eg. electrode in Li-batteries, supercapacitors, sieving membrane, electromagnetic interference shielding, and carbon capture). Adding to that, over 30 different MXenes have been discovered since 2011, exhibiting alternative or superior properties. Most importantly, elegant routes for property design in the MXene family has been demonstrated, by means of either varying the chemistry in the Mn+1Xn compound, by alloying two M elements, or by changing the structure of the MXene by introducing vacancies.

The present work has a led to an additional route for post synthesis property tuning in MXenes by manipulation of surface termination elements. This enables a unique toolbox for property tuning which is not available to other 2D materials and is highly beneficial for applications that is dependent on surface reactions. Furthermore, chemical and structural characterization of terminations on single sheets is essential to rule out the influence of intercalants or contamination that is typically present in multilayer MXene samples or thin films. For that purpose, a method for preparing isolated contamination free single sheets of MXene samples for transmission electron microscopy (TEM) characterization was established. In order to determine vacancy and termination sites, atomically resolved scanning (S)TEM imaging and image simulations was carried out. Two main processes were employed to substitute the termination elements.

1) An initial thermal treatment in vacuum facilitates F desorption and it was shown that O-terminations rearranges on the evacuated sites. H2 gas exposure in a controlled environment demonstrated a removal of the remaining O-terminations. As a result, termination-free MXene is possible to realize under vacuum conditions.

2) CO2 was introduced as a first non-inherent termination on MXene by in situ CO2 gas exposure at low temperatures. That was a first demonstration of Ti3C2TX as promising material for carbon capture. Additionally, O-saturated surfaces were demonstrated after introduction of O2 gas on the F-depleted Ti3C2TX MXene, which is highly relevant for hydrogen evolution reactions where fully O-terminated Ti3C2TX are predicted to improve efficiency.

A Lewis acid melt synthesis method was used to realize the first MXene exclusively terminated with Cl. Moreover, this was the first report of a MXene directly synthesised with terminations other than O, OH, and F.

Furthermore, we have expanded the space of property tuning by introduction of chemical ordering, by selective etching of Y in an alloyed (Mo2/3Y1/3)2CTX MXene. This either produced chemical ordering with one M (Mo) element and vacancies, or ordering between two M (Mo and Y) elements. This was further reported to significantly increase volumetric capacitance because of the increased number of active sites around vacancies, leading to an increasing charge density. As a final note, the stability of Nb2CTX MXene under ambient conditions was investigated. It was found that the surface Nb adatoms, present after etching, got oxidized over time which resulted in local clustering and effectively degraded the MXene.

This work has demonstrated reproducible surface characterization methods for determining termination elements and sites in 2D MXenes, that is ultimately governing MXene properties. Most importantly, we report on a new approach for MXene property tuning as well as contributing to several existing property tuning approaches. 

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. , p. 68
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1986
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-156935DOI: 10.3384/diss.diva-156935ISBN: 9789176850855 (print)OAI: oai:DiVA.org:liu-156935DiVA, id: diva2:1316001
Public defence
2019-06-07, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2019-05-15 Created: 2019-05-15 Last updated: 2019-05-20Bibliographically approved
List of papers
1. On the organization and thermal behavior of functional groups on Ti3C2 MXene surfaces in vacuum
Open this publication in new window or tab >>On the organization and thermal behavior of functional groups on Ti3C2 MXene surfaces in vacuum
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2018 (English)In: 2D MATERIALS, ISSN 2053-1583, Vol. 5, no 1, article id 015002Article in journal (Refereed) Published
Abstract [en]

The two-dimensional (2D) MXene Ti(3)C(2)Tx is functionalized by surface groups (T-x) that determine its surface properties for, e.g. electrochemical applications. The coordination and thermal properties of these surface groups has, to date, not been investigated at the atomic level, despite strong variations in the MXene properties that are predicted from different coordinations and from the identity of the functional groups. To alleviate this deficiency, and to characterize the functionalized surfaces of single MXene sheets, the present investigation combines atomically resolved in situ heating in a scanning transmission electron microscope (STEM) and STEM simulations with temperature-programmed x-ray photoelectron spectroscopy (TP-XPS) in the room temperature to 750 degrees C range. Using these techniques, we follow the surface group coordination at the atomic level. It is concluded that the F and O atoms compete for the DFT-predicted thermodynamically preferred site and that at room temperature that site is mostly occupied by F. At higher temperatures, F desorbs and is replaced by O. Depending on the O/F ratio, the surface bare MXene is exposed as F desorbs, which enables a route for tailored surface functionalization.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2018
Keywords
MXene; Ti3C2Tx in situ heating; STEM; temperature-programmed XPS; surface functionalization
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-142131 (URN)10.1088/2053-1583/aa89cd (DOI)000412302100002 ()
Note

Funding Agencies|Swedish Research Council [621-20124359, 622-2008-405, 2013-5580, 2016-04412, 642-2013-8020]; Knut and Alice Wallenbergs Foundation [KAW 2015.0043]; Swedish Foundation for Strategic Research (SSF) [RIF14-0074]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]

Available from: 2017-10-24 Created: 2017-10-24 Last updated: 2019-06-28Bibliographically approved
2. 2D Transition Metal Carbides (MXenes) for Carbon Capture
Open this publication in new window or tab >>2D Transition Metal Carbides (MXenes) for Carbon Capture
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2019 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 31, no 2, article id 1805472Article in journal (Refereed) Published
Abstract [en]

Global warming caused by burning of fossil fuels is indisputably one of mankinds greatest challenges in the 21st century. To reduce the ever-increasing CO2 emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO2. However, challenges remain because of limited CO2/N-2 selectivity and long-term stability. The effective adsorption of CO2 gas (approximate to 12 mol kg(-1)) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N-2 gas results in no adsorption, consistent with first-principles calculations. The adsorption efficiency combined with the CO2/N-2 selectivity, together with a chemical and thermal stability, identifies the archetype Ti3C2 MXene as a new material for carbon capture (CC) applications.

Place, publisher, year, edition, pages
WILEY-V C H VERLAG GMBH, 2019
Keywords
carbon capture; environmental TEM; MXene; surface terminations
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-154119 (URN)10.1002/adma.201805472 (DOI)000455111100003 ()30393920 (PubMedID)
Note

Funding Agencies|Swedish Research Council [2016-04412, 2016-00889, 642-2013-8020]; Knut and Alice Wallenbergs Foundation [KAW 2015.0043]; Swedish Foundation for Strategic Research (SSF) [EM16-0004, RIF 14-0074, FL12-0181]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

Available from: 2019-01-29 Created: 2019-01-29 Last updated: 2019-06-28Bibliographically approved
3. Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases
Open this publication in new window or tab >>Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases
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2018 (English)In: Small, ISSN 1613-6810, E-ISSN 1613-6829, Vol. 14, no 17, article id 1703676Article in journal (Refereed) Published
Abstract [en]

The exploration of 2D solids is one of our times generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo2/3Y1/3)(2)AlC, the resulting MXene can be either: i) (Mo2/3Y1/3)(2)C with in-plane elemental order through selective removal of Al atoms or ii) Mo1.33C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo2/3Y1/3)(2)C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolytea volumetric capacitance exceeding 1500 F cm(-3) is obtained, which is 40% higher than that of its Mo1.33C counterpart. With H2SO4, the trend is reversed, with the latter exhibiting the higher capacitance (approximate to 1200 F cm(-3)). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.

Place, publisher, year, edition, pages
WILEY-V C H VERLAG GMBH, 2018
Keywords
2D materials; capacitance; in-plane order; MXene; vacancies
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-147926 (URN)10.1002/smll.201703676 (DOI)000430922100010 ()29611285 (PubMedID)
Note

Funding Agencies|Swedish Research Council [2016-04412, 621-2012-4359, 622-2008-405, 2013-5580, 642-2013-8020, 2016-00889]; Knut and Alice Wallenbergs Foundation [KAW 2015.0043]; Swedish Foundation for Strategic Research (SSF) through the Synergy Grant FUNCASE; Swedish Foundation for Strategic Research (SSF) [RIF 14-0074]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

Available from: 2018-05-23 Created: 2018-05-23 Last updated: 2019-06-28
4. On the Structural Stability of MXene and the Role of Transition Metal Adatoms
Open this publication in new window or tab >>On the Structural Stability of MXene and the Role of Transition Metal Adatoms
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2018 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 10, no 23, p. 10850-10855Article in journal (Refereed) Published
Abstract [en]

In the present communication, the atomic structure and coordination of surface adsorbed species on Nb2C MXene is investigated over time. In particular, the influence of the Nb adatoms on the structural stability and oxidation behavior of the MXene is addressed. This investigation is based on plan-view geometry observations of single Nb2C MXene sheets by a combination of atomic-resolution scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and STEM image simulations.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2018
Keywords
2D material; MXene; Scanning Transmission Electron Microscopy; Structural Stability; Adatoms
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-148143 (URN)10.1039/C8NR01986J (DOI)000435358600004 ()29870038 (PubMedID)
Note

Funding agencies:The authors acknowledge the Swedish Research Council for funding under grants no. 2016- 04412 and 642-2013-8020, the Knut and Alice Wallenberg’s Foundation for support of the electron microscopy laboratory in Linköping, a Fellowship grant and a project grant (KAW 2015.0043). The authors also acknowledge Swedish Foundation for Strategic Research (SSF) through the Research Infrastructure Fellow program no. RIF 14-0074. The authors finally acknowledge support from the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No 2009 00971

Available from: 2018-05-31 Created: 2018-05-31 Last updated: 2019-06-28Bibliographically approved

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