1. Preparation process of Co₃O₄-E-450

2. Structure schematic (a) and photo (b) of sample gas sensor real object, and the measurement electric circuit for the gas sensor (c)

3. TG-DSC curves of the obtained precursors Co₃O₄-E (a) and Co₃O₄-N (b)

4. XRD patterns of the precursors (a) and products Co₃O₄ (b)

5. N₂ adsorption-desorpion isotherms (a) and pore size distributions (b) for Co₃O₄-E-450 and Co₃O₄-450

6. Typical SEM, TEM and HRTEM images of Co₃O₄-E-450 (a, c, e), Co₃O₄-450 (b, d, f) Yellow ovals indicate the defects

7. FT-IR spectra of the precursor Co₃O₄-E, Co₃O₄-N and the products of Co₃O₄-E-450, Co₃O₄-450

8. Co2p (a) and O1s (b) XPS spectra of Co₃O₄-E-450 and Co₃O₄-450 and O1s XPS spectra of Co₃O₄-E-450 after sensing experiments to 100×10^-6 toluene and 100×10^-6 acetone (c)

9. UV-Vis spectra of Co₃O₄-E-450 and Co₃O₄-450 (a1-a2); (αhν)²versus hν curves of as-prepared Co₃O₄-E-450 and Co₃O₄-450 (b1-b2)

10. Schematic of formation process of the as-prepared Co₃O₄ nanosheet

11. Response of the gas sensors of Co₃O₄-E-450 and Co₃O₄-450 to 100×10^-6 toluene (a) and 100×10^-6 acetone (b) at working temperatures; Responses of the Co₃O₄-E-450 to 100×10^-6 various gases at different operating temperatures (c); Dynamic response characterizations of Co₃O₄-E-450 towards 100×10^-6 toluene (205 ℃) and 100×10^-6 acetone (225 ℃) (d)

12. Dynamic response-recovery curve of Co₃O₄-E-450-based sensor to different concentrations of toluene at 205 ℃ (a) and to different concentrations of acetone (b) at 225 ℃ Insets are the corresponding linear relation fitting curves

13. Cyclic tests of the Co₃O₄-E-450 sensor to 100×10^-6 of toluene at 205 ℃ (a) and 100×10^-6 of acetone at 225℃ (b); Long term stability of the Co₃O₄-E-450 sensor to 100×10^-6 and 0.5×10^-6 of toluene at 205 ℃ (c) and 100×10^-6 and 1×10^-6 of acetone at 225 ℃ (d)

Table 1. Structural performance parameters of the samples

Table 2. Properties of various additive assisted synthesis Co₃O₄ semiconductor sensors