Reduced Graphene Oxide (rGO)-Based Nanohybrids as Gas Sensors: State of the Art

Abstract

The use of gas sensors in human and environmental safety needs remarkable sensitivity, selectivity, fast response/recovery, low detection limit and long-term stability. Accordingly, various organic as well as inorganic nanostructures have been developed and still remain an area of interest in the field of gas sensors. Among the various nanomaterials, graphene is two-dimensional (2-D) nanostructure that consists of sp2-hybridized carbon atoms arranged in hexagonal ring pattern with intra-atomic separation of 0.142 nm. The larger theoretical surface area (2630 m2 g−1), high carrier mobility at room temperature (2000 m2 V−1 s−1) makes graphene more attractive as a sensing material for gas sensors. Reduced graphene oxide (rGO) is a chemically derived form of graphene that consists of many oxygen functional groups and defects at the basal plane and the edges. The presence of oxygen-containing functional groups in rGO plays an important role in gas adsorption ability and hence enhancing the sensing properties with improved sensitivity. The easy, cost-effective and large-scale synthesis of rGO paved more attention in sensing application as compare to intrinsic graphene. The rGO-based gas sensors mainly include the nanohybrids of 0-D, 1-D, 2-D and 3-D nanostructures of metal/metal oxide with rGO. In gas sensors, the salient features of rGO nanohybrids are needed to study in detail along with the sensing mechanisms. The physical and chemical modifications of rGO surface with metal, metal oxides and polymers not only contribute to individual characteristics but also add or enhance the properties by synergistic effect between them. During the chemical synthesis of nanohybrids, rGO serves the reactive sites to control the particle size by the steric hindrance effect and contributes to improve the sensitivity towards the analytes. In the intrinsic graphene, the Fermi level located at the converging point changes due to adsorption of gas molecules, which is the key point for gas sensing application. Further, the chemical affinity and selectivity of the rGO can be achieved by surface functionalization. Being a most promising material in gas sensing, the systematic review is timely and essential to evaluate the outcomes and challenges in rGO nanohybrids for gas detection. Accordingly, the present chapter is mainly focused on the current state of the art of rGO nanohybrids for the gas sensing application. The classification of different configuration of rGO nanohybrids gas sensors along with its working principles is also described in detail. The chapter also includes the different sensing mechanisms and approaches used to engineer the surface as well as interface of the rGO-based nanohybrids in order to improve the overall sensor performance. The present chapter is mostly devoted to cover the rGO-based nanohybrids with simplified classifications along with application domains which is useful to academics, R&D and scientific communities.