Size-Dependent Physical and Electrochemical Properties of Two-Dimensional MXene Flakes.

Two-dimensional (2D) particles, including transition metal carbides (MXenes), often exhibit large lateral-size polydispersity in delaminated colloidal solutions. This heterogeneity results in challenges when conducting fundamental studies, such as investigating correlations between properties and the 2D flake size. To resolve this challenge, we have developed solution-processable techniques to control and sort 2D titanium carbide (Ti3C2T x) MXene flakes after synthesis based on sonication and density gradient centrifugation, respectively. By tuning the sonication conditions, Ti3C2T x flakes with varied lateral sizes, ranging from 0.1 to ∼5 μm, can be obtained. Furthermore, density gradient centrifugation was used to sort Ti3C2T x flakes with different lateral sizes into more monodisperse fractions. These processing techniques allow for the characterization of size-dependent optical and electronic properties by measuring the absorption spectra and film conductivity, respectively. Additionally, by testing the material as electrochemical capacitor electrodes, we show the Ti3C2T x flake-size dependence of electrochemical performance. Ti3C2T x films made of flakes with lateral sizes of ∼1 μm showed the best capacitance of 290 F/g at 2 mV/s and rate performance with 200 F/g at 1000 mV/s. The work provides a general methodology which can be followed to control the size of MXenes  and other 2D materials for a variety of applications and fundamental size-dependent studies.