In this work, three-dimensional (3D) hierarchical In₂O₃@SnO₂ core-shell nanofiber (In₂O₃@SnO₂) was designed and successfully prepared via a facile electrospinning and further hydrothermal methods. Vertically aligned SnO₂ nanosheets uniformly grown on the outside surface of In₂O₃ nanofibers were clearly observed by field emission scanning electron microscopy. Besides, hierarchical core-shell nanostructure of In₂O₃@SnO₂ was characterized by elemental maps using scanning transmission electron microscopy. The formaldehyde (HCHO) sensing performances of pure In₂O₃ nanofibers, SnO₂ nanosheets, and In₂O₃@SnO₂ core-shell nanocomposite were compared, and the In₂O₃@SnO₂ nanocomposite possessed highest response value, fast response/recovery speed, best selectivity, and lowest HCHO detection limit. Specifically, the response value (R_a/R_g) of the In₂O₃@SnO₂ nanocomposite reached 180.1 toward 100 ppm of HCHO gas, which was near 9 and 6 times higher than that of the pure In₂O₃ nanofibers (R_a/R_g = 19.7) and pure SnO₂ nanosheets (R_a/R_g = 33.2), respectively. In addition, the gas sensor showed instantaneous response/recovery time (3/3.6 s) toward 100 ppm of HCHO at the optimal operation temperature of 120 °C. More importantly, the detection limit toward HCHO gas was as low as 10 ppb (R_a/R_g = 1.9), which could be used for trace HCHO gas detection. The excellent sensing properties of the In₂O₃@SnO₂ were attributed to the synergistic effect of large specific surface areas of SnO₂ nanosheet arrays, abundant adsorbed oxygen species on the surface, unique electron transformation between core-shell heterogeneous materials, and long electronic transmission channel of SnO₂ transition layer. This work provides an efficient route for the preparation of novel hierarchical sensitive materials.

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