Water dynamics in pristine and porous ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{\mathrm{T}}_{x}$ MXene as probed by quasielastic neutron scattering

Water dynamics in pristine and porous ${Ti}_{3} C_{2} T_{x}$ MXene as probed by quasielastic neutron scattering

Murillo L. Martins, Tyler S. Mathis, Xuehang Wang, Luke L. Daemen, Yury Gogotsi, and Eugene Mamontov

Phys. Rev. Materials 6, 034001 – Published 1 March 2022

Abstract

MXenes are a novel class of two-dimensional (2D) materials whose applications in energy-storage systems have attracted substantial attention. Still, the rate performance of these materials is often diminished by sheet restacking, which is attenuated via controlled etching in $H_{2} {SO}_{4}$ solution. With this process, micropores are formed in MXene sheets that allow transport across 2D layers. As a result, the intercalation and diffusivity of ions are facilitated resulting in improved capacitance retention at high charge-discharge rates. In the present work, we used quasielastic neutron scattering to evaluate the potential changes in water dynamics as a consequence of this mechanism by assessing the behavior of weakly and strongly confined water populations in pristine and porous MXenes. First, we have found that the porous sample accommodates a noticeably higher content of both water populations. Additionally, the fraction of mobile molecules is higher either under strong or weak confinement. Interestingly, regardless of the abundance of weakly confined water in the porous sample, no considerable changes in the dynamical behavior were detected in comparison with the dynamics measured in the pristine material. For the strongly confined populations of water, our results show that water is able to permeate the micropores introduced by etching and perform unlocalized motions.

Received 19 October 2021
Accepted 9 February 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.034001

Physics Subject Headings (PhySH)

Surface & interfacial phenomena

2-dimensional systems

Neutron techniques Quasi elastic neutron scattering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Murillo L. Martins^1,*, Tyler S. Mathis ², Xuehang Wang ², Luke L. Daemen¹, Yury Gogotsi², and Eugene Mamontov ^1,†

¹Neutron Scattering Division, Oak Ridge National Laboratory, P.O. Box 2008, MS6455, Oak Ridge, Tennessee 37831, USA
²A. J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA

^*longomartiml@ornl.gov
^†mamontove@ornl.gov

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Vol. 6, Iss. 3 — March 2022

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Figure 1
(a) XRD data for pristine and etched MXene. (b) Results from $N_{2}$ sorption experiments for pristine and etched porous MXene. The linear relative pressure ranges for each sample are highlighted by the dotted lines.
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Figure 2
Temperature-dependent neutron elastic scattering intensity scans for pristine and porous MXene powders before (air-dried) and after annealing. The results are presented as the sums of the data collected between $Q = 0.3$ and $1.1 Å^{– 1}$ . In (a), the data were normalized by the samples’ masses. In (b,c) the data collected at 20 K were normalized to unit. In (b,c) the red lines are guides for the eye.
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Figure 3
Representation of the changes caused in MXene by treatment in $H_{2} {SO}_{4}$ solution and distribution of water populations in interlayer and interstack spaces. After etching, micropores are formed in the MXene sheets and the interlayer space subtly increases due to the presence of sulfur- and carbon-based compounds as discussed in Ref. [17]. After etching, micropores are formed in the sheets of ${Ti}_{3} C_{2} T_{x}$ MXene. Overall, the porous material retains a larger content of weakly and strongly confined water as compared with the pristine material.
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Figure 4
Representative QENS spectra collected at 300 K and $Q = 1 . 1 Å^{– 1}$ for the air-dried (a) pristine and (b) etched porous samples. The resolution and the Lorentzian $S$ ( $Q$ , $E$ ) functions are also shown (red and green lines) together with a fit of the data obtained with Eq. (1) (orange lines). (c) Normalized $S$ ( $Q$ , $E$ ) for the air-dried pristine and porous samples as determined by fitting the data with Eq. (1). In (d), (e), the QENS spectra collected at 300 K and $Q = 1.1 Å^{– 1}$ for the annealed pristine and porous samples are shown, respectivelly. (f) Normalized $S$ ( $Q$ , $E$ ) for the annealed pristine and porous samples as also determined by fitting the data with Eq. (1). The intensities in (c), (f) are in linear scale while the others are in log scale.
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Figure 5
(a) $Q$ dependence of the elastic fraction $x$ ( $Q$ ) as obtained by fitting the QENS data with Eqs. (1) and (2). As explained in the text, $x$ ( $Q$ ) has been fixed during the fits for the annealed pristine sample and no error bars are shown. (b) $Q$ dependence of the broadening, Γ( $Q$ ), of the QENS signals from annealed samples of pristine and porous MXenes. The lines represent the fits with either Eq. (3) (jump diffusion), or the $ℏ {D Q}^{2}$ law, and the diffusivities and relaxation times (if obtained) of each sample are also shown. (c) Relationship between Γ( $Q$ ) and $x$ ( $Q$ ) at $Q = 1.1 Å^{– 1}$ . The dotted line is a guide for the eye.
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Physical Review Materials

Water dynamics in pristine and porous Ti3C2Tx MXene as probed by quasielastic neutron scattering

Murillo L. Martins, Tyler S. Mathis, Xuehang Wang, Luke L. Daemen, Yury Gogotsi, and Eugene Mamontov

Phys. Rev. Materials 6, 034001 – Published 1 March 2022

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Water dynamics in pristine and porous ${Ti}_{3} C_{2} T_{x}$ MXene as probed by quasielastic neutron scattering