Fig. 1. Schematic diagram of gas sensing mechanism. (a) In air; (b) in H₂; (c)(d) energy band diagram of depletion region change after reaction of ZnO with gas

Fig. 2. Schematic diagram of fiber optic sensing. (a) Optical path transmission diagram; (b) equivalent diagram of optical path; (c) numerical relationship simulation plot

Fig. 3. Schematic diagram of sensor gas sensitivity test

Fig. 4. Morphology and elemental composition of Ni-doped ZnO nanorod arrays. (a) Optical fiber with etched light intensity reduced to 30%; (b) optical fiber grown with ZnO nano arrays; (c) scanning electron microscopy (SEM) image of pure ZnO arrays; (d) SEM image of Ni∶ZnO-4 sample; (e) EDS mapping of Ni∶ZnO-4 sample

Fig. 5. XRD pattern of Ni-doped ZnO

Fig. 6. Response characteristics of Ni-doped ZnO nanorod arrays fiber optic sensors to H₂ gas. (a) Recovery curves of ZnO nanorod arrays sensors with different Ni doping concentrations in response to H₂ volume fraction of 1×10^－3; (b) response recovery time of Ni∶ZnO-4 sensors; (c) response spectra of Ni∶ZnO-4 sensors; (d) Ni∶ZnO-4 sensor response to different H₂ volume fractions

Fig. 7. Performance of Ni∶ZnO-4 sensor at H₂ volume fraction of 1×10^－3. (a) Repeatability; (b) stability; (c) selectivity