Ultrafast, One-Step, Salt-Solution-Based Acoustic Synthesis of Ti3C2 MXene

The current quest for two-dimensional transition metal carbides and nitrides (MXenes) has been to circumvent the slow, hazardous, and laborious multistep synthesis procedures associated with conventional chemical MAX phase exfoliation. Here, we demonstrate a one-step synthesis method with local Ti3AlC2 MAX to Ti3C2Tz MXene conversion on the order of milliseconds, facilitated by proton production through solution dissociation under megahertz frequency acoustic excitation. These protons combined with fluorine ions from LiF to selectively etch the MAX phase into MXene, whose delamination is aided by the acoustic forcing. These results have important implications for the future applicability of MXenes, which crucially depend on the development of more efficient synthesis procedures. For proof-of-concept, we show that flexible electrodes fabricated by this method exhibit comparable electrochemical performance to that previously reported.

: Safety data sheet for 0.1-0.25 % conc. HF. Figure S4A presents the cyclic voltammogram profile of Ti 3 C 2 T z collected at 10 mV s −1 in 1 M H 2 SO 4 electrolyte. Redox peaks can be identified in the CV profile, with both the oxidation peak and the reduction peak located at -0.55 V showing a highly reversible redox reaction. The open circuit voltage (OCV) was stable at 0.3 V. The electrodes used for current collection were glassy carbon, having the ability to suppress the hydrogen evolution reaction, with a wide voltage window of 0.9 V. The charge storage mechanisms were studied for the Ti 3 C 2 T z electrode through the CV currents collected at different scan rates, a method reported by Wang et al. , 23] that relates the sweep rates and collected current through the i= av b formula, in which the b value is the slope of the log I versus log V curve. The storage mechanism is diffusion limited if the b value is ∼0.5, while a capacitive storage mechanism is dominant for a b value of 1. As shown in Fig. S4A, the b value is close to1 at different scan rates. Figure S4B shows the Nyquist plot, and the observed straight vertical line in the low frequency region is an indication of pure capacitive behavior for the Ti 3 C 2 T z electrode. From the real axis, the equivalent series resistance (ESR) was 0.08 Ω.cm 2 . Both electrochemical stability and Coulombic efficiency are decisive factors in the evaluation of any energy storage system. By charging and discharging at a current density of 10 A g −1 for 10,000 cycles, the Ti 3 C 2 T z electrode (three-electrodes system) shows 92% stability and almost 100% Columbic efficiency (see Figure S4C), demonstrating outstanding long-life electrochemical stability. In the inset, the symmetric shape of the galvanostatic cycling profile is shown, being maintained after 10,000 cycles with only a very insignificant reduction in the capacitance. The CV profiles for a 2 μm film (see Fig. S4D) shows that the MXene charging mechanism is more or less maintained for different scan rates. Furthermore, a high-rate performance was observed. Figure   S2E represents the method to calculate the b-value fitting parameter. The symmetrical behavior in the charge-discharge curves at various current densities, shown in Fig. S4F, confirms the high electrochemical reversibility.

Electrochemical Characterization
For the sake of comparison between the produced MXene from the mild conventional etching (F 1 ) and the SAW technique (F 2 ), we prepare Ti 3 C 2 MXene by mild conventional etching (12 M HCl and 5M LiF) and filter a film from an unwashed colloidal solution. This allows a more direct comparison to the results from the film originating from the SAW technique.
The XRD patterns in Fig S6a show that both the F 1 and F 2 films contain residual LiF particles.
The electrochemical performance of a washed and unwashed film from conventional mild synthesis is 230 and 210 Fg -1 , respectively, i.e., about 8 % reduction when washing is omitted. This is close to within the experimental error range.
A comparison between the F 1 and F 2 films is shown in Figure S6b indicating that the film originating from the SAW technique has about 14% lower capacitance at low scan rates compared to the conventionally derived material, though this difference decreases with increasing scan rate. It should be stressed that the initial work on SAW-facilitated MXene is produced for proof of concept, and the slight difference may at least in part be due to changes in the electrode materials (density, thickness, sheet size, etc.)  Table S1.