Two-dimensional materials for gas sensors: from first discovery to future possibilities

Semiconductor gas sensors have been developed so far on empirical bases, but now recent innovative materials for advancing gas sensor technology have been made available for further developments. Two-dimensional (2D) materials have gained immense attention since the advent of graphene. This attention inspired researchers to explore a new family of potential 2D materials. The superior structural, mechanical, optical and electrical properties of 2D materials made them attractive for next-generation smart device applications. There are considerable improvements and research studies on graphene, molybdenum disulfide (MoS2), tungsten disulfide (WS2), tin sulfide (SnS2), black phosphorus and other 2D materials in the field of sensing devices. These materials have been reported to be used perfectly for sensing target gases at parts per million and parts per billion levels. A wide variety of mechanisms have been reported as main functions of 2D materials in sensing the target gas in gas sensors - that is, semiconductor, field-effect transistor, optical, electrochemical and piezoelectric sensors. This review addresses the current experimental and theoretical approaches to studying 2D materials, their mechanism of sensing and possible developments in fabrication of ultrasensitive gas sensors, along with the challenges and issues in the fabrication of the gas sensors and their selectivity and stability challenges.