Pentagon-based 2D materials: Classification, properties and applications

Since penta-graphene (PG), a two-dimensional (2D) carbon allotrope exclusively composed of five-membered rings, was proposed in 2015, a great deal of effort has been devoted to the rational design and synthesis of pentagon-based 2D materials with novel properties and promising applications. As a result, over one hundred new pentagonal 2D materials have been theoretically predicated, and some of them have been experimentally synthesized. Due to the unique geometries of these materials, they exhibit many interesting properties, including negative Poisson's ratio, intrinsic piezoelectricity, giant out-of-plane second harmonic generation susceptibility, ferromagnetism with high Curie temperature and topological quantum state, and have broad applications. In this paper, we review the recent progress made in this emerging field, and classify all the pentagon-based 2D structures reported so far based on their geometric characteristics and space groups. Especially, we highlight the major breakthroughs that have occurred over the past several years, discuss the unexpected physical properties that have enabled these materials to bridge many disciplines in science and technology, and analyze the potential applications in metal-ion batteries, thermoelectricity, photocatalysis, electrocatalysis, and heterojunction. The challenges and opportunities in this field are also discussed for future study.