Nanostructured materials have attracted considerable research interest over the recent decades because of their potential applications in nanoengineering and nanotechnology. On the other hand, the developments in nanotechnology are strongly dependent on the availability of new materials with novel and engineered morphologies. Among the novel nanomaterials reported thus far, composite nanofibers (NFs) have attracted considerable attention in recent years. In particular, metal oxide NFs have great potential for the development of gas sensors. Highly sensitive and selective gas sensors can be developed by using composite NFs owing to their large surface area and abundance of grain boundaries. In composite NFs, gas sensing properties can be enhanced greatly by tailoring the conduction channel and surface properties by compositional modifications using the synergistic effects of different materials and forming heterointerfaces. This review focuses on the gas sensing properties of composite NFs synthesized by an electrospinning (ES) method. The synthesis of the composite NFs by the ES method and the sensing mechanisms involved in different types of composite NFs are presented along with the future perspectives of composite NFs.

We investigated the effect of grain size on the H2-sensing behavior of SnO2-ZnO composite nanofibers. The 0.9SnO2-0.1ZnO composite nanofibers were calcined at 700 °C for various times to control the size of nanograins. A bifunctional sensing mechanism, which is related not only to the SnO2-SnO2 nanograins, but also to the ZnO-SnO2 nanograins with surface metallization effect, is responsible for the grain-oriented H2-sensing properties and the selective improvement in sensing behavior to H2 gas compared to other gases. Smaller grains are much more favorable for superior H2 sensing in SnO2-ZnO composite nanofibers, which will be an important guideline for their use in H2 sensors. The one-dimensional nanofiber-based structures in the present study will be efficient in maximizing the sensing capabilities by providing a larger amount of junctions.

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We developed a hydrogen sensor of reduced graphene oxide-loaded ZnO nanofibers. An extremely high response of about 866 at a low concentration of 100 ppb was obtained. The combined effect of the presence of rGO nanosheets and hydrogen-induced metallization of ZnO played a crucial role in enhancing the detection behavior.