Molecular engineering of CxNy: Topologies, electronic structures and multidisciplinary applications

As a class of metal-free two-dimensional (2D) semiconductor materials, polymeric carbon nitrides have attracted wide attention recently due to its facile regulation of the molecular and electronic structures, availability in abundance and high stability. According to the different ratios of C and N atoms in the framework, a series of CxNy materials have been successfully synthesized by virtue of various precursors, which further triggers extensive investigations of broad applications ranging from sustainable photocatalytic reactions and highly sensitive optoelectronic biosensing. In view of topological structures on their electronic structures and material properties, the as-reported CxNy could be generally classified into two main categories with three- or six-bond-extending frameworks. Owing to the effective n ->pi* transition in most CxNy materials, the relative energy level of the lone-pair electrons on N atoms is high, which thus endows the materials with the capability of visible light absorption. Meanwhile, the different repeating units, bridging groups and defect sites of these two kinds of CxNy allow them to effectively drive a diverse of promising applications that require specific electronic, interfacial and geometric properties. This review paper aims to summarize the recent progress in topological structure design and the relevant electronic band structures and striking properties of CxNy materials. In the final part, we also discuss the existing challenges of CxNy and outlook the prospect possibilities. (C) 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.