Effect of Nickel Vacancies on the Room-Temperature NO2 Sensing Properties of Mesoporous NiO Nanosheets

To improve the gas-sensing performance of metal-oxide-semiconductors, the effect of defects on gas-sensing properties has been widely investigated. Nevertheless, although the metal cation defect is the dominative acceptor defect in p-type semiconductors, its effect on the gas-sensing properties remains blank, which leads to a hindrance for further developing p-type semiconductor-based gas sensors. Accordingly, to eleborate the effect of metal cation defects on the sensing properties, mesoporous NiO nanosheets with different amounts of nickel vacancies were prepared by annealing at different temperatures. It was found that the amount of nickel vacancies increased with increasing the annealing temperature. Gas-sensing studies revealed that the NiO with a higher concentration of nickel vacancies exhibited higher sensitivity to NO2 at room temperature. With further increasing the annealing temperature to 600 °C, although the rapid decrease in the specific surface area of the NiO might limit the physisorption of NO2, the NiO could also present a better sensitivity to NO2 due to the abundant nickel vacancies with high activity. Furthermore, an in situ DRIFTS study demonstrated that the number of adsorbed nitrate and nitrite species on NiO surfaces increased with increasing the amount of nickel vacancies, indicating that the nickel vacancies acted as the dominative active sites participating in the gas–solid reaction and then determined the room-temperature sensing properties. According to the defect ionization equation, a hole conduction model was further proposed to decipher the dependency of sensing properties on the metal cation defects. We hope this work could make us better understand the roles of cation defect in the sensing properties, and it could also benefit the improvement of p-type semiconductor-based gas sensors.