LAUSR

2D MXenes are widely proved to be potential electrode materials, although the size effect on their electrochemistry is not fully understood. In this work, Ti3C2Tx nanoflakes are prepared through acidic etching of Ti3AlC2 powders, followed by the intercalation treatment with tetrapropylammonium hydroxide. Such a method produces large‐scale delaminated and oxygenated nanoflakes. With aid of centrifugation, the nanoflakes with varied lateral sizes and thicknesses are collected, where electrochemical response of charged redox probes and polar phenol molecules is varied. Density functional theory and energy dispersive spectroscopy confirm such electrochemical response is dependent on the size and thickness of used nanoflakes, more exactly the oxygen content on their surface. Taking the nanoflakes obtained using a centrifugal speed of 5000 rpm (MX‐TPA0.2) as an example, they feature good dispersibility, a high oxygen content, a small size, and a thin thickness. On these nanoflakes electrochemical response of polar p‐substituted phenols is pronounced, stemming from a strong electron‐withdrawing interaction of their oxygenated termination with the Ar‐OH. A sensitive electrochemical sensor is further constructed for the detection of p‐nitrophenol. This work thus provides an approach to synthesize MXenes with different sizes and thicknesses as well as further to reveal size‐dependent electrochemistry of MXenes.