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Rheological Characteristics of 2D Titanium Carbide (MXene) Dispersions: A Guide for Processing MXenes Bilen Akuzum, Kathleen Maleski, Babak Anasori, Pavel Lelyukh, Nicolas Javier Alvarez, E. Caglan Kumbur, and Yury Gogotsi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08889 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018
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Table of Contents Figure 81x38mm (300 x 300 DPI)
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Rheological Characteristics of 2D Titanium Carbide (MXene) Dispersions: A Guide for Processing MXenes
Bilen Akuzum1,2, Kathleen Maleski1, Babak Anasori1, Pavel Lelyukh1, Nicolas Javier Alvarez3,*, E. Caglan Kumbur2,*, and Yury Gogotsi1,*
1
A. J. Drexel Nanomaterials Institute and
Department of Materials Science and Engineering Drexel University, Philadelphia, PA 19104 USA 2
Electrochemical Energy Systems Laboratory
Department of Mechanical Engineering and Mechanics Drexel University, Philadelphia, PA 19104 USA 3
Department of Chemical and Biological Engineering Drexel University, Philadelphia, PA 19104 USA
Submitted as a Technical Manuscript to ACS Nano December 2017
________________________________ * Corresponding author: Yury Gogotsi, E-mail:
[email protected]; Phone: 1-215-895-6446; Fax: 1-215-895-1934 E. Caglan Kumbur, E-mail:
[email protected]; Phone: 1-215-895-5871 Nicolas Javier Alvarez, E-mail:
[email protected]; Phone: 1-215-571-4120
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Abstract Understanding the rheological properties of two-dimensional (2D) materials in suspension is critical for the development of various solution processing and manufacturing techniques. 2D carbides and nitrides (MXenes) constitute one of the largest families of 2D materials with >20 synthesized compositions and applications already ranging from energy storage to medicine to optoelectronics. However, in spite of a report on clay-like behavior, not much is known about their rheological response. In this study, rheological behavior of single- and multi-layer Ti3C2Tx in aqueous dispersions was investigated. Viscous and viscoelastic properties of MXene dispersions were studied over a variety of concentrations from colloidal dispersions to high loading slurries, showing that a multi-layer MXene suspension with up to 70 wt.% can exhibit flowability. Processing guidelines for the fabrication of MXene films, coatings, and fibers have been established based on the rheological properties. Surprisingly, high viscosity was observed at very low concentrations for solutions of single-layer MXene flakes. Single-layer colloidal solutions were found to exhibit partial elasticity even at the lowest tested concentrations (0.35 and ϕ >0.13, respectively). Conversely, the dispersions of multi-layer Ti3C2Tx particles (blue triangles, Fig. 2c) exhibit a similar power law increase in viscosity to the polystyrene36 and kaolin clay35 suspensions in a similar loading range. The fact that exfoliated single layer Ti3C2Tx flakes have a strong impact on the viscosity at low volume fractions can make processing limited to dilute concentration regimes and/or the use of high shear rates. On the other hand, the observed behavior could prove beneficial for the fabrication of transparent films of MXene, where higher relative viscosities allow for processability at low concentrations for fabricating ultra-transparent, conductive films.37 The effect of two-dimensionality is clearly expressed in the above differences in the rheological response of single- and multi-layer MXene flakes. Overall, results suggest that the dispersions of multi-layer and single-layer MXenes should be treated as two distinct colloidal systems as they show very different rheological properties at different concentration regimes (Fig. 2d). The single-layer Ti3C2Tx flakes exhibit versatile rheological properties, which are easily tunable by changing the concentration and the applied shear rate. However, one limitation seems to be the upper limit of concentration for forming a stable single-layer Ti3C2Tx flake solution as the viscosity increases rapidly at very dilute concentrations (0.18 - 3.6 mg/mL, ϕ=4.73x10-5 - 9.40x10-4). For processes that require higher viscosities in which consistency of the dispersion becomes important, this upper concentration limit of stability could be a potential issue for the single-layer Ti3C2Tx flake solutions. In such ACS Paragon Plus Environment
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instances, multi-layer Ti3C2Tx particles could be a better option, as they are capable of forming dispersions with much higher viscosities even at high shear rates. Viscoelastic Study of Single-Layer Ti3C2Tx Dispersions After assessing the viscous properties of MXene dispersions in water, a detailed viscoelastic study in the linear-viscoelastic regime has been conducted for single-layer Ti3C2Tx flake solutions to better understand the particle interactions in the dispersion. The elastic (G’) and viscous (G”) moduli of the MXene dispersions have been determined as a function of frequency at constant strain amplitude of 0.1%. The viscoelastic moduli provide detailed insights into the characteristics of the particle network that might form in the dispersion.38 Here, the elastic modulus (G’) provides information on the rigidity and nature of the particle networks that might form inside the studied system; meaning that a relatively high and frequency independent elastic modulus suggests a system that forms a gel-like state (volume spanning connectivity). The viscous modulus (G”) quantifies how readily an applied stress to the system is relaxed or dissipated. The relative magnitude of G’ to G’’ provides deeper insights into the fluid vs. solid-like tendencies of the suspension. Figure 3 shows the viscoelastic data collected for varying single-layer Ti3C2Tx flake concentrations. The schematics in Fig. 3 show possible representative microstructural evolutions of single-layer Ti3C2Tx dispersions at selected concentrations. At concentrations below 0.90 mg/mL (ϕ=2.36x10-4), the elastic modulus (G’) and the viscous modulus (G”) was found to increase similarly with increasing frequency. This has been reported in other colloidal solutions, suggesting that a gelation transition should be close to this concentration regime.39,40 Noticeable presence of the elastic component (G’) could be observed even at the lowest tested concentrations (7.11x10-4), the dispersions seem to have two distinct curves for the elastic G’ and the viscous moduli G”. This indicates that the extent of the dominant structural length scale has become much larger, producing a frequency independent G’ response in the system.35 With increasing concentration, it is apparent that the crossover point for the G’-G” transition shifts to higher frequencies as well. This rheological behavior resembles that of a soft gel.43 At the highest studied concentration (3.60 mg/mL, ϕ=9.47x10-4), G’ increases significantly and becomes more independent of frequency (Fig. 3c). A slight deviation from the overall trend in G’ was observed in Fig. 3c at the highest studied frequency (100 rad/s), which indicates a shift to non-linear viscoelastic regime of the dispersion, where instrumental errors can be expected. Though not included in this study, further increase in the concentration is expected to cause an increase in the number of ordered domains and a subsequent increase in the elastic modulus due to increased density of defects similar to that of aqueous graphene dispersions.37,44 Due to inherent rheological similarities found between weak gels and Ti3C2Tx dispersions at high concentrations, well established processing techniques specific to gel-like materials could be utilized in fabricating MXenes such as wet spinning. Overall, dispersions of single-layer Ti3C2Tx display a wide range of rheological properties that seem to suit many well-established industrial fabrication techniques. As
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discussed before, one challenge remains for single-layer Ti3C2Tx is the upper concentration limit of stability. Although single-layer MXene dispersions offer rheological versatility for many of the proposed applications, some commonly used fabrication processes such as extrusion printing or dry spinning require much higher absolute elasticity values (>100 Pa) to enable processing, which was not observed for the studied concentration range.37,45 Although the viscoelastic data indicate the formation of a gel-like state in the studied dispersions, a simple gravity-test of the tested solutions shows that the formed gel structures are relatively weak and not able to withstand the gravitational forces. The absolute value of the elastic modulus for the highest studied concentration (3.60 mg/mL, ϕ=9.47x10-4) is approximately 2 Pa, which could be insufficient for fabrication methods that require much higher elasticity for processing. Viscoelastic Study of Multi-Layer MXene Dispersions Along with the viscosity data, a detailed viscoelastic study has also been performed for the dispersions of multi-layer MXenes. As mentioned before, the only drawback for single-layer MXene dispersions is their relatively low elastic modulus (~2 Pa) even at the highest tested concentration for the study (3.60 mg/mL, ϕ=9.47x10-4), which does not offer processability for some fabrication techniques that require higher elastic moduli such as extrusion printing45 (>100 Pa). Here, the geometry of the suspended particles has a significant impact on the gelation transition. For instance, the high aspect ratio of the single-layer flakes results in a much lower packing density, which limits the amount of multi-particle interactions that can offer high elastic modulus.46 On the other hand, dispersions of multi-layer MXenes are expected to form strong gel-like systems much easier as the random radial morphology of the particles enables reaching higher packing densities at ease. “Swelling” of the multi-layer particles, resulting multi-
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particle interactions, and the high amount of friction between the closely packed particles are expected to promote drastic improvements in the elastic modulus once a critical concentration is achieved.47 Additionally, ζ-potential measurements conducted on the MXenes showed that single-layer flakes (-45.9 mV) exhibit almost double the surface charge measured for multi-layer particles (-28/5 mV) in solution. This also suggests that multi-layer dispersions will necessitate higher solid loadings for gelation due to lack of strong electrostatic forces and relatively higher mass of the particles. Hence, understanding the viscoelastic properties of multi-layer Ti3C2Tx particles is of critical importance for describing a complete processability map for Ti3C2Tx in aqueous environments. At low concentrations (10-30 wt.%, ϕ=0.028-0.107), the dispersions of multilayer MXene behave similar to the low-concentration regime of single-layer MXene with viscous component (G”) slightly dominate at all tested frequencies (Fig. 4a). At 30 wt.% (Fig. 4b, ϕ=0.107) the elastic modulus G’ overlaps with the viscous component and follows a similar increase in value with increasing frequency. This suggests a gelation transition state near this concentration regime similar to that of traditional colloidal gels.39 Overall, similar to the dilute regime of single-layer Ti3C2Tx, dispersions of multilayer Ti3C2Tx particles would be suitable for processing methods such as spray or spin coating, which require ease of flowability (high viscous modulus, G”) with a certain level of elasticity (G’) to hold the system intact upon application.
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Figure 4: Plots showing viscoelastic measurements conducted on multi-layer MXene dispersions at various solid loadings: a) 10 wt.%, b) 30 wt.%, c) 40 wt.%, d) 60 wt.%, e) 65 wt.%, and f) 70 wt.%.
At moderate concentrations (40-60 wt.%, ϕ=0.166-0.375), G’ begins to dominate at lower frequencies (100 Pa) for preserving the printed form compared to singlelayer dispersions. At high concentrations (>60 wt.%), G’ dominates over the entire frequency range. In addition, a rapid increase of both moduli with increasing concentration is also apparent (Fig. 4e-f), indicating that a rheological percolation threshold is most likely somewhere in this concentration range49. Another interesting rheological feature for this concentration appears to be the almost overlapping values of G’ and G” at very low frequencies (
