Highly-enhanced gas-sensing performance of metal-doped In2O3 microtubes from acceptor doping and double surface adsorption

The different valent metal-doping is a feasible and convenient way to adjust the microstructures and electron concentration of In₂O₃ gas sensors. In this paper, the different valence metals (Zn²⁺, Sb³⁺, Zr⁴⁺ and Nb⁵⁺) are doped into MIL-68 (In) metal–organic frameworks (MOFs) by solvothermal method, and then In₂O₃ and metal-doped In₂O₃ microtubes are obtained by pyrolysis MIL-68 MOFs. All samples exhibit the similar microtubular structures, indicating oxygen adsorption on both inner and outer surface. The average grain size of metal-doped In₂O₃ microtubes decreases a little while the specific surface area increases greatly. Metal-doping greatly affects the formaldehyde gas-sensing performance, and Zn²⁺-doped In₂O₃ sensor presents the highest response value (188.56), shortest response/recovery times and excellent selectivity to formaldehyde gas at 210 ℃. Compared the microstructural and gas-sensing parameters of In₂O₃ sensor, the specific surface area and oxygen vacancies of metal-doped In₂O₃ sensors enhance the surface $O^{-}$. Moreover, acceptor Zn²⁺-doping directly extracts electrons from conduction band of Zn²⁺-doped In₂O₃ sensor, which greatly increases the resistance in air and the thickness of electron deletion layer for Zn²⁺-doped In₂O₃ sensor.