Fig. 1. SEM/TEM images of the materials in Table 1. (a) Co₃O₄ porous microspheres in Ref. [39]; Reproduced with permission from Ref. [39]. (b) SnO₂ nanospheres in Ref. [46]; Reproduced with permission from Ref. [46]. (c) SnO₂ elephantidens-like nanospheres in Ref. [47]; Reproduced with permission from Ref. [47]. (d) In₂O₃ microspheres in Ref. [48]; Reproduced with permission from Ref. [48]. (e) ZnO hollow microspheres in Ref. [30]; Reproduced with permission from Ref. [30]. (f) ZnO double-shelled microspheres in Ref. [43]. Reproduced with permission from Ref. [43].

Fig. 2. SEM images of (a) pristine In₂O₃ NWs, (b) Au–In₂O₃ NWs, (c) Ag–In₂O₃ NWs, and (d) Pt–In₂O₃ NWs. Dynamic response curves of (e) Au–In₂O₃ and pristine In₂O₃ NWs, (f) Ag–In₂O₃ and pristine In₂O₃ NWs, (g) Pt–In₂O₃ and pristine In₂O₃ NWs, (h) schematic diagram of the electrospinning process for In₂O₃ NWs and the preparation of the sensor array, (i) selective tests of three gas sensors, (j) pattern recognition based on PCA using three sensor arrays. Reproduced with permission from Ref. [51].

Fig. 3. (a) FESEM image of PdAu/SnO₂; (b) six periods of response curve of three sensors to 20-ppm acetone at working time of 250 °C; (c) the response of the PdAu/SnO₂ sensor to different concentrations of acetone in 94% RH environment at working temperature of 250 °C. Reproduced with permission from Ref. [54].

Fig. 4. Gas responses of the (a) pure SnO₂ and (b) 5Tb–SnO₂ sensors to acetone at 450 °C under both humid and dry conditions; (c) 30 repetitive sensing transients to 20-ppm acetone and (d) long-term stability of 5Tb–SnO₂ sensor at 450 °C in RH 80%. Reproduced with permission from Ref. [59]. (e) Gas responses of the 12Pr–In₂O₃ macroporous spheres to 20 ppm of acetone. (f) Dynamic sensing transients, (g) responses and (h) 15 repetitive sensing transients of the 12Pr–In₂O₃ macroporous spheres to 20-ppm acetone at 450 °C. Reproduced with permission from Ref. [60].

Fig. 5. (a) TEM images of 3D OP SnO₂–ZnO HM; (b) the sensor-based SnO₂–ZnO HM to acetone in the range 0.25 ppm–100 ppm at 275 °C; (c) the identification of human exhaled breath (healthy subjects and simulated diabetics) based on SnO₂–ZnO sensor; (d) dynamic resistance change transients of the SnO₂–ZnO sensor to human exhaled breath. Reproduced with permission from Ref. [30].

Fig. 6. (a) The SEM and (b) TEM images of the GQD-modified 3DOM ZnO sample; (c) XRD patterns of 3DOM ZnO and GQD-modified 3DOM ZnO samples; (d) the dynamic response curves in the acetone concentration range of 0.3 ppm–2 ppm; (e) the linear relationship response of various acetone concentrations; (f) the response/recovery time to acetone; (g) the selectivity tests for the 3DOM ZnO sensor and GQD-modified 3DOM ZnO sensor; (h) the responses of the GQD-modified 3DOM ZnO sensor toward healthy and simulated diabetes exhaled breaths and a schematic diagram of the breath collecting process; (i) the band diagram structure. Reproduced with permission from Ref. [8].