Janus MXene Fiber Constructed via Flake Orientation Engineering

Abstract The orientation of MXene flakes has received increasing research attention as it plays a critical role in determining the performance of MXene‐based assemblies. Engineering MXene flakes into horizontal or vertical orientations can offer distinct advantages such as higher electrical conductivity, higher mechanical strength, and more efficient ion/molecule transport across the flakes. However, the benefits of horizontal and vertical orientations are mutually exclusive, and both of them possess structural symmetry that restricts their ability for stimuli‐responsive deformation. Herein, a Janus‐oriented MXene fiber is reported, consisting of vertically oriented flakes on one side and horizontally oriented ones on the other. The unique Janus orientation endows the fiber with good mechanical properties, high electrical conductivity, and asymmetric volume expansion capability. Interestingly, such asymmetric volume expansion between the two orientations induces a pronounced curling deformation under water stimuli, leading to a high length change of up to 2100%. Due to such a strong deformation response, the Janus MXene fiber is used to construct an event‐triggered logic gate, which enables the operation of a dual‐mode water immersion warning system featuring both light and Bluetooth signaling. This work is believed to provide inspiration for future orientation engineering of MXene flakes and the design of advanced MXene‐based information devices.