Graphene-based two-dimensional Janus materials

Two-dimensional (2D) Janus materials with opposing components and properties on two sides have recently attracted fevered attention from various research fields for use as, for example, oil/water separating membranes, interfacial layers for mass transfer, 2D sensors and actuators. The Janus structure allows for a unidirectional transportation system and programmed response to certain stimuli to be achieved. Graphene, the 2D honeycomb network formed from one atomic layer of carbon atoms, has also received substantial research interest because of its intriguing structure and fascinating properties. The high mechanical strength, flexibility and optical transparency make graphene a unique candidate as a building block of 2D Janus materials through asymmetric modification with different functional groups on the graphene surfaces. This article reviews graphene-based 2D Janus materials, starting with a theoretical understanding of the behavior of Janus graphene. Then, different strategies for fabricating Janus graphene and its derivatives are reviewed in detail according to the chemical strategies of the modification methods. The applications of graphene-based Janus materials are discussed with a specific focus on the Janus structures that lead to bandgap engineering, as well as the construction of a responsive system on graphene. The two-dimensional material graphene could find use as a one-way permeable membrane that improves the performance of batteries. Asymmetry has practical advantages in that it supports an action or motion in one direction while suppressing it in the other. Zijian Zheng and co-workers from The Hong Kong Polytechnic University review the different techniques developed to produce such asymmetric materials using graphene. These so-called Janus materials, named after the two-faced Roman god, can be fabricated by adding another substance to one side of the graphene. The authors summarize techniques for adding hydrogen or halogen atoms, small molecules, metal nanoparticles, or layers of a metal or polymer. These Janus materials could improve the efficiency of batteries, transistors and solar cells. Graphene has received enormous research interest in recent years owing to its intriguing structure and fascinating properties. Its high mechanical strength, flexibility and optical transparency make it a desire building block of 2D Janus materials. Through asymmetric surface modifications on target graphene derivatives, including hydrogenation and halogenation, grafting of organic molecules and polymers, and deposition of metal/metal oxides, different graphene-based Janus materials have been achieved with various shapes, sizes, and compositions. This review presents and discusses the development, fabricating strategies and applications of these 2D Janus materials, starting with the theoretical understanding of the behavior of Janus graphene.