Aerosol Jet Printing of Ti3C2 Mxene Aqueous Ink
Authors: Mansoor Naqsh E , Muramutsa Florent , Shuck Christopher E. , Subbaraman Harish , Pandhi Twinkle , Gogotsi Yury , Estrada David 
Published: February 2020

An emerging class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes have garnered significant interest in the scientific community due to their unique electrical, optical, and chemical properties [1]. Studies have reported high electrical conductivity and response, surface hydrophilicity, compositional control, and mechanical strength [1]. This unique combination of properties makes MXenes a contender for a wide variety of applications including batteries, super-capacitors, electronics, sensors, and others [2]. However, for MXenes to become industrially relevant, their compatibility with large scale synthesis and manufacturing techniques needs further investigation.
In this study, we report the first aerosol jet printing (AJP) of water based titanium carbide (Ti3C2) MXene ink on a flexible polyimide substrate (Fig. 1-a,b). AJP is a high resolution additive manufacturing technique expected to shorten production times, provide greater control over size, design and variability, and a demonstrated capability of conformal printing and flexible substrate processing [3]. Previous studies have reported nano-material inks composed of various 2D materials including molybdenum disulfide, black phosphorous and hexagonal boron nitride [4]. However, only graphene-based inks have been shown to be AJP compatible [5]. Compared to graphene, MXenes offer greater compositional control. About 30 different kinds of MXenes have been synthesized, while many more are theoretically studied [1]. 2D Ti3C2 was the first to be discovered and has been studied most extensively [6].
To develop our AJP MXene ink, multi-layer MXenes were obtained by in situ HF etching of the pre-cursor Ti3AlC2 MAX Phase [7]. The obtained colloidal solution, containing 2D MXene flakes was sonicated with deionized water for 3 hours at 55 W. The resultant ink had a concentration of 1 mg mL-1 and an average flake size of 100 nm +/- 10 nm, as confirmed by dynamic light scattering (DLS) measurements. Contact angle measurements of the ink on polyimide substrate revealed an angle of 58.13o (Fig 1-c), showing good wettability. Our aqueous ink was printed with Optomec AJ-200 printer using the ultrasonic atomizer and printer parameters such as nozzle diameter, sheath gas flow, atomizer gas flow, atomizer current and platen temperature were optimized to get even dispersion and uniform printing. The MXene solutions were then printed in 0.5 mm x 1 cm linear structures with varying pass numbers (N), N = 5 to N = 25 in 5 pass increments.
To investigate the quality of the printed structures, optical microscopy, height measurements, scanning electron microscopy (SEM) imaging and two-point probe resistance measurements were carried out. As revealed by optical images, the lines were even and didn’t show overspray, “coffee ring” effects, or runs typically associated with a solvent rich print (Fig. 1-d). SEM also showed even dispersion and consistent printing (Fig 1-e). However, some microcracks could be seen within the lines; which could be due to the absence of an annealing treatment. Stylus profilometer was used to determine the height of our printed MXene lines which varied from ~ 620 nm to 4.80 µm (Fig. 1-f). Our most uniform prints were N = 15 and N = 20 which had a height of 2.50 µm +/- 0.10 µm and 3.50 µm +/- 0.10 µm, respectively (Fig. 1-f). Electrical resistance (R) varied from R = 568 to R = 60 kΩ for N = 5 and N = 25 respectively (Fig. 1-g).
In conclusion, we report the first AJP of aqueous Ti3C2 MXene inks. We find that the dimensions and electrical resistance of AJP MXene devices scale with increasing pass number, as expected. Microscopy analysis suggests additional work remains to be done to optimize our ink rheology and post-processing. However, our work highlights the important potential of additive manufacturing of MXene based devices which could include microelectrodes, battery electrodes, thermoelectrics, micro-capacitors and flexible sensors.
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[5] T. S. Tran, N. K. Dutta, and N. R. Choudhury, Advances in Colloid and Interface Science, 261, 41–61 (2018).
[6] M. Naguib et al., ChemInform, 42 (2011).
[7] M. Alhabeb et al., Chemistry of Materials, 29, 7633–7644 (2017).

Figure 1