Understanding the Increasing Trend of Sensor Signal with Decreasing Oxygen Partial Pressure by a Sensing-Reaction Model Based on O^2– Species

Abstract

Although the increasing trend of sensor signal with decreasing oxygen partial pressure was observed quite early, the underlying mechanism is still elusive, which is a hindrance to accurate gas detection under varying oxygen partial pressure. In this work, a sensing model based on previous experimental and theoretical results is proposed, in which the O2– species is determined to be the main oxygen species because O– species has not been observed by direct spectroscopic studies. On this basis, combined with the band bending of SnO2 at different oxygen partial pressures, the functional relationship between the surface electron concentration, oxygen partial pressure, and reducing gas concentration is established, which includes three forms corresponding to the depletion layer, accumulation layer, and flat band. In the depletion layer case, the variation of the sensor resistance to different concentrations of CO and oxygen can be well fitted with our function model. Besides, this model predicts that the response of sensors will no longer maintain the increasing trend in an extremely hypoxic atmosphere but will decrease and approach 1 with the background oxygen content further going down to 0.

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

“…A conductivity model based on vacancy healing and formation was recently proposed for SnO 2 -based CGS by Sun and Lu, which allowed them to model the conduction electron concentration and sensor response in both the ‘electron depletion’ and ‘electron accumulation’ regime depending on the density of near-surface vacancies relative to the bulk value. 29 Our direct in situ spectroscopic evidence of the correlation between near-surface vacancy density and sensor's resistance and band bending fully supports such a mechanism.…”

Comment on “Understanding the Increasing Trend of Sensor Signal with Decreasing Oxygen Partial Pressure by a Sensing-Reaction Model Based on O2–Species”

“…So, applying the mass action law, the rate for this reaction is proportional to the CO pressure and the oxygen density at the surface. Thus, the rate equation for O o is different than that found in ref . Indeed, by using eqs and , we can write normald [ O o ] normald t = k normala normald normals false[ V o 2+ false] 2 n normals 4 p normalO 2 − k normald normale normals false[ normalO normalo false] 2 − k normalr normale normala normalc normalt p normalC normalO [ O o ] …”

“…Relevant surface reactions to sensing have been summarized in ref for single charged vacancies. Accordingly, for the oxygen-vacancy model and the notation of ref , the oxygen adsorption–desorption process for doubly charge vacancies, can be written as normalO 2 ( g ) + 4 normale − + 2 V o 2 + ⇌ 2 normalO normalo where V o 2 + accounts for doubly ionized vacancies and O o for lattice oxygen. Equation indicates that as oxygen pressure decreases the density of vacancies increases.…”

“…Indeed, the law of mass action proposes that the rate of the chemical reaction is directly proportional to the product of the activities or concentrations of the reactants. On the other hand, in ref , the oxygen adsorption–desorption process is described as 1 2 normalO 2 ( g ) + 2 normale − + V o 2 + ⇌ normalO normalo …”

“…At first sight, the above considerations seem not to be crucial, but they lead to inconsistencies when a reducing gas is incorporated . According to ref , carbon monoxide reacts with surface lattice oxygen to generate vacancies as described as follows: normalC normalO ( g ) + normalO normalo → normalC normalO 2 ( g ) + normalV normalo …”