fig9

Figure 9. (A) Schematic illustration of the band structure of a p-n heterojunction before and after contact. Figure 9A is quoted with permission from Mondal et al.^[46]; (B) Schematic illustration of the band structure (B₁) and surface oxygen vacancy sites (B₂) of the In₂O₃/NiO heterostructure. Figure 9B is quoted with permission from Xie et al.^[169]; (C) NO_x gas sensing mechanism on ordered mesoporous WO₃/ZnO n-n heterojunction. Figure 9C is quoted with permission from Han et al.^[174]; (D) NH₃ gas sensing mechanism on CuO/α-Fe₂O₃ p-p heterojunction. Figure 9D is quoted with permission from Bhuvaneshwari et al.^[175]; (E) Synthesis route diagram (E₁) and ethanol sensing performance (E₂) of multilevel dendritic mesoporous TiO₂-SnO₂ composites. Figure 9E is quoted with permission from Zhao et al.^[76]. PEO: Poly(ethylene oxide).