Janus Electrochemical Exfoliation of Two-dimensional Materials

Atomically thin two-dimensional (2D) materials have attracted increasing research interest due to their fascinating properties as well as multifarious potential applications. Despite tremendous achievements, the fundamental challenge in this rapidly growing field is fabricating these 2D materials with a well-controlled structure in an effective and scalable way. Electrochemical exfoliation, where bulk crystals can be electrochemically positively/negatively charged using intercalators, is an ingenious strategy to prepare high-quality 2D materials with Janus characteristics for a wide range of applications: on the one hand, the exfoliated 2D materials by cathodic exfoliation via cation intercalation under a reductive environment can possess advantages such as large size, high crystallinity, and a pure phase structure; on the other hand, by using anodic exfoliation via anion intercalation, surface modifications or oxidations could occur, which allows production of atomically thin 2D materials with diverse functionalities. This review article focuses mainly on the state-of-the-art developments in the smart and effective production of different categories of atomically thin 2D materials by cathodic and anodic electrochemical exfoliation, herein named as "Janus electrochemical exfoliation". The design strategies to realize Janus exfoliation of bulk crystals, the interfering factors such as intercalators, and the exfoliation mechanism are comprehensively summarized and discussed. More importantly, applications of these electrochemically exfoliated 2D materials in optoelectronic devices, energy storage, biosensing and catalysis are also detailed. Finally, the perspectives on the challenges and opportunities of this promising field are presented.