Current state of knowledge on the metal oxide based gas sensing mechanism

Direct in situ spectroscopic evidence of the crucial role played by surface oxygen vacancies in the O2-sensing mechanism of SnO2

“…Consequently, we conclude that the resistance change in SnO 2 sensors in response to O 2 does not result from extrinsic negatively charged surface oxygen species but rather from the dynamic exchange of lattice oxygen with the gas ambient via the formation and healing of surface oxygen vacancies. This challenges the current unproven conceptions in the field of gas sensing 8 but is fully consistent with the recent work in the field of solid-state physics on vacancy-induced surface conductivity layers in metal oxides. 16 …”

“…3 The most commonly described is the oxygen ionosorption model, in which monoatomic O À adsorbates are produced by dissociative adsorption of gaseous O 2 , trapping electrons from the conduction band (for an n-type semiconductor) at the material's surface. [4][5][6][7][8] The resulting accumulation of negative surface charge induces an 'electron depletion layer' in the material, a volume of decreased Fermi energy (relative to the bulk value in the absence of surface charge). This causes upward band bending (relative to the surface in the absence of adsorbatesthe 'at band' condition), which is witnessed as increased sensor resistance.…”

2D Amorphous/Crystalline a-In2O3/In2Se3 Nanosheet Heterostructures with Improved Capability for H2 and NO2 Sensing

“…All these mechanisms, producing electrons, are congruent with the decrease of the BLR in 40% RH as recorded in Figure g,h. Remarkably, as shown in Figure a,d, increasing the humidity content decreases the H 2 sensor’s signal amplitude, following the same response as to CO in SnO 2 and In 2 O 3 crystalline MOs. − Combining these observations, the decrease of the H 2 sensor signal with increasing humidity, accounts first for the synergistic action of reactions (, 5–7) which release electrons/vacancies (leading to a resistance decrease), followed by the recombination of air oxygen with the as-formed electrons/vacancies, as for reaction () (leading to a resistance increase). An equilibrium is established, where a resistance decrease is partially mitigated by a resistance increase.…”

Review on Porosity Control in Nanostructured Semiconducting Metal Oxides and Its Influence on Chemiresistive Gas Sensing

“…Among them, SMOs have been widely used as gas-sensing layers due to their superior gas sensitivity at elevated temperatures. − In particular, nanostructured materials have been actively investigated in the gas sensor . Thus far, different synthetic methods for porous nanostructured SMOs have been reported including electrospinning, ,− spray pyrolysis, hydrothermal, solvothermal, , and direct precipitation. , On the other hand, various techniques to overcome their poor intrinsic gas selectivity have been suggested including functionalization with catalysts, − defect creation, phase control, and coating with selectivity overlayer. ,, Many publications ,− summarizing the performance of state-of-the-art SMOs concerning sensitivity, selectivity, sensing mechanisms, and morphology engineering have been widely reported.…”