Webinar Q&A: MXenes: a new family of two-dimensional materials

With an overwhelming response to Yury Gogotsi's webinar MXenes: a new family of two-dimensional materials on the 2nd October 2014, please see below the Q&A from the event.

Once again, thank you Yury for an amazing webinar, it really was a fantastic event. It is available to watch on demand here.
 

Victor Abrukov: Do you think the task of creating the knowledge base is relevant and realizable? Can you make a comment on the task of creating the knowledge base at the seminar?

Yury Gogotsi: Yes, I believe that it is possible to create a “knowledge base” – it is a relevant and realizable task. We have a so known “material genome” project in the US, which is close to your idea of “knowledge base”.

Zongyuan Lin: We use the Ti3C2(OH)x etched from Ti3AlC2 to get 2D Ti3C2 nanosheets. And we follow the method  mentioned in your papers to make it done. But, we haven't succeed! Would you please give us some  suggestions?

Yury Gogotsi: It is difficult to say why – several other groups in China have successfully reproduced the process. Maybe this is a different structure of the MAX phase. Effects of temperature and time were described in detail in our paper: O. Mashtalir, M. Naguib, B. Dyatkin, Y. Gogotsi, M.W. Barsoum, Kinetics of aluminum extraction from Ti3AlC2 in hydrofluoric acid, Materials Chemistry and Physics, 139, 147-152 (2013)
Also, a much simpler method of synthesis is described in our article that will appear in Nature on Nov. 26.

Chuanfang Zhang: Since MXenes, like Ti3CN is easy to be oxidized at RT, is there any efficient ways to protect them from the oxidation in air?

Yury Gogotsi: MXene without functional groups (M2C) is very reactive due to free electrons of the transition metal. Therefore, we always observe surface functional groups (O, OH). Yes, it is possible to protect single-layer terminated MXenes from further oxidation by placing them in an oxygen-free environment. Deaerating water will already drastically decrease oxidation rate. Keeping them in non-aqueous solvents will certainly do the job.  In the case of multilayer particles or films, only the outer layer and edges suffer from oxidation after lengthy exposure to environment.

Ioan Baldea: Are there estimates for the band gap of MX-enes based on post-DFT approaches?(ii) Genuine many-body calculations beyond DFT (e.g. GW) are computationally much more demanding than those based on DFT. Could you indicate simpler MX-ene structures to perform post-DFT calculations (model structures, for which such calculations are still feasible), which are nonetheless relevant, to convincingly demonstrate theoretically a behavior switching from metallic to semiconductor?

Yury Gogotsi: Agree about limitations of DFT. We have not done anything beyond DFT ourselves, but there are several groups around the world that published band gap calculations using different approaches, also predicted strain-induced transition from indirect to direct band gap in MXenes. We are planning experimental measurements of the band gap.

Youngbin Lee: Is it possible to fabricate bare MXenes and oxygen-functionalized MXene in real process?And if it is feasible, are they stable in room temperature?Finally, how ductile are MXene?

Yury Gogotsi: Bare MXenes must be made in vacuum. We have not produced them yet. Oxygen-terminated MXenes can be produced by high-temperature treatment of OH-terminated ones. They are stable.
MXenes are flexible and can form flexible films. We will have papers in Advanced Materials and PNAS on flexible MXene films appearing this month.

Deepak Dwivedi: Can we use this material in the form of composite which we can use for the Photo voltaic application and solar cell application? what is the band gap of this material?

Yury Gogotsi: The band gaps were only estimated theoretically for various MXenes. However, we can produce composite films. Watch for our paper on this subject to appear in PNAS soon.

John Zhang: Even after several weeks of storage in air at RT, the surface easily become metal oxides. My questions are: (1) how to alleviate this surface oxidation ? (2) can we make a layered metal oxides from the oxidized MXene?

Yury Gogotsi: Yes, layered metal oxides can be produced from MXenes (see H. Ghassemi, W. Harlow, O. Mashtalir, M. Beidaghi, M. R. Lukatskaya, Y. Gogotsi, M. L. Taheri, In Situ Environmental Transmission Electron Microscopy Study of Oxidation of Two-Dimensional Ti3C2 and Formation of Carbon-Supported TiO2, Journal of Materials Chemistry A, 2, 14339-14343, 2014)

Youngbin Lee: How do C and N atoms arrange in Ti3(C0.5N0.5)2?

Yury Gogotsi: We didn’t study carbon and nitrogen location. Should be the same way as in the MAX phase precursor – I’d assume a random mix.

Wang Hao: How can we determine the volume of MXenes as they are particles?

Yury Gogotsi: From their structure models (see, e.g., C. Shi, M. Beidaghi, M. Naguib, O. Mashtalir, Y. Gogotsi, S. J. L. Billinge, Structure of nanocrystalline Ti3C2 MXene using atomic pair distribution function, Phys. Rev. Letters, 112, 125501, 2014)

Partho Pratim Chatterjee: How does the Electrical and Magnetic behaviour of MXenes change at near absolute zero?

Yury Gogotsi: Only Ti3C2Tx was studied and showed a transition from metallic to semiconductor behavior around 100K - see J. Halim, M. R. Lukatskaya, K. M. Cook, J. Lu, C. Smith, L.-Å. Näslund, S. May, L. Hultman, Y. Gogotsi, P. Eklund, M. W. Barsoum, Transparent Conductive Two-Dimensional Titanium Carbide Epitaxial Thin Films, Chemistry of Materials, 26 (7), 2374–2381 (2014)

Deepak Dwivedi: What are the methods of production and have any one used this for solar cell?

Yury Gogotsi: We have not seen reports on use in solar cells.
Synthesis by HF etching was described in detail in our paper: O. Mashtalir, M. Naguib, B. Dyatkin, Y. Gogotsi, M.W. Barsoum, Kinetics of aluminum extraction from Ti3AlC2 in hydrofluoric acid, Materials Chemistry and Physics, 139, 147-152 (2013)
Also, a much simpler method of synthesis is described in our article that will appear in Nature on Nov. 26.

Julia Bykova: Has the exfoliation of magnesium diboride (MgB2) have been ever approached?

Yury Gogotsi: No.

Roberto Matassa: Is it possible to determine the layers distance by PDF analysis of a X-ray diffraction measurements?

Yury Gogotsi:  Yes, see C. Shi, M. Beidaghi, M. Naguib, O. Mashtalir, Y. Gogotsi, S. J. L. Billinge, Structure of nanocrystalline Ti3C2 MXene using atomic pair distribution function, Phys. Rev. Letters, 112, 125501, 2014

Roberto Matassa: Charge storage depends on the porosity of the used materials. Is it possible to determine the porosity in dimension without BET analysis?

Yury Gogotsi: We can only estimate the available surface area theoretically from the structure of MXene. Unlike ions, gas molecules don’t penetrate between MXene layers, so measuring the porosity experimentally is difficult. SAXS or SANS may be used.

Roberto Matassa: How long time do you need for exfoliate your 2D materials? and How is the amount of the produced exfoliate materials?

Yury Gogotsi: From minutes to hours, depends on the MAX phase, particle size, temperature and other parameters. We produce grams of material in a single batch, but potentially you can make kilograms if you have a large container – this is a wet chemistry process at room or slightly elevated temperature.

Deepak Dwivedi: Thin film making and powder nano particles making methods are simple in terms of synthesis or not? What would be the possible methods?

Yury Gogotsi: Yes. See our publications, also forthcoming papers in Nature and PNAS this month.

Bilal Ahed: Is there any other delaminated material other than Ti3C2?

Yury Gogotsi: At least a dozen. We have published 9 so far. See M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi, M.W. Barsoum, Two-Dimensional Transition Metal Carbides, ACS Nano 6 (2) 1322–1331, 2012; M. Ghidiu, M. Naguib, C. Shi, O. Mashtalir, L. M. Pan, B. Zhang, J. Yang, Y. Gogotsi, S. J. L. Billinge, M. W. Barsoum, Synthesis and Characterization of Two-Dimensional Nb4C3 (MXene), Chem. Comm. 50, 9517-9520, 2014; M. Naguib, J. Halim, J. Lu, K. Cook, L. Hultman, Y. Gogotsi, M. W. Barsoum, New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-ion Batteries, J. Am. Chem. Society, 135 (43) 15966–15969 (2013)

Tao Hui: Are Mxenes are equlibrium-state materials? Can they exist stably if they are used as electronic materials?

Yury Gogotsi: Yes, they are thermodynamically stable.

Paradisanos Ioannis: Is there any DFT calculations showing the electronic structure of these materials for various number of layers?

Yury Gogotsi: Yes, see a paper by Yu Xie and Paul Kent in Phys. Rev. B. (2013)

Fernando Costa Oliveira: How were E values determined? Can theoretical predictions of Li uptake be confirmed experimentally? How? Would it be reasonable to envisage the possibility of using concentrated solar power for producing 2D transition metal carbides and nitrides?

Yury Gogotsi: E values are from as initio calculations. The theoretical predictions of Li intercalation and capacity have been confirmed experimentally on free-standing MXene films and powder electrodes tested against a Li foil.  
Concentrated solar power can potentially be used for producing 2D transition metal carbides and nitrides, just like any other source of energy.

Nikolaos Papadopoulos: What is the best method to estimate the number of the layers? Can Raman spectroscopy be used for that purpose?

Yury Gogotsi: We estimate the number of layers by TEM or by AFM. We need to achieve a much better understanding or Raman spectra of MXenes before we can use Raman spectroscopy to the same extent as for carbons.

Erie Morales: In terms of the electronic structure of the MXenes, are the properties coming from the metal 3d states, or is something else presumably happening?

Yury Gogotsi: Certainly, transition metal atoms are largely responsible for the electronic properties of MXenes. I suggest you to have a look at papers on electronic structure of MXenes. You may wish to check our review article for appropriate references: M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, MXenes: A New Family of Two-Dimensional Materials, Advanced Materials, 26, 992-1005 (2014)

Chunlei Wang: Have you tried to use dry etching (RIE or DRIE) to fabricate MXenes? 2. During wet etching for Ti2AlC, is it possible to etch Ti away?

Yury Gogotsi: We have not been successful in producing MXene by dry etching of MAX phases. However, it does not mean that dry etching cannot produce MXenes, it only shows that our few attempts did not lead to success.

Giovanni Bruno: Is there any chance for the ALD (atomic layer deposition) for this Mxene growth?

Yury Gogotsi: I’m sure it is possible. It just has not been done yet.

Pavel Istomin: Can you say what tipically is lateral size of produced MXene sheets?

Yury Gogotsi: Typically, MXene flakes are a few microns in size. The largest flakes we observed were 10-20 microns. Intensive sonication can break MXene sheets into smaller pieces.

Nikolaos Papadopoulos: When MXenes are with terminals that transform them in semiconductors. Do they have direct or indirect bandgap?

Yury Gogotsi: There are several groups around the world that published band gap calculations using different approaches. Indirect to direct band gap transition has been predicted to be induced by strain or applied field in Sc2CO2 MXene by Yong-Chae Chung from Korea.

Anna Castaldo: It is possible to obtain MXenes from reactive sputtering starting from metallic targets (e.g. Al) and nitrogen, depositing very thin films onto apposite templates?

Yury Gogotsi: This should be possible, because MAX phases are grown by this method. However, no one has reported directly grown MXene films yet.

Chunlei Wang: Can you comment the effort using MXenes for gas sensing?

Yury Gogotsi: We only know that MXenes are capable of adsorbing some gases and that their properties change as a result of intercalation/adsorption. However, I’m not aware of any published MXene-based sensors.