Editing Alcohol Sensor Based on Gas-Sensitive Resistive Materials (section)

== Working Principle ==

The response of SnO<sub>2</sub>-based gas sensors originates from surface reactions between adsorbed oxygen species and reducing gases such as ethanol. This mechanism governs the relationship between gas concentration and electrical resistance.

In ambient air, oxygen molecules adsorb onto the SnO<sub>2</sub> surface and capture electrons from the conduction band, forming negatively charged oxygen species. This process creates an electron depletion layer and increases the potential barrier, resulting in high resistance.

When ethanol vapor is introduced, it reacts with the pre-adsorbed oxygen species, releasing electrons back into the conduction band. As a result, the depletion layer narrows, the potential barrier decreases, and the resistance drops.[1]

This mechanism explains the empirical observation in the MQ-3 datasheet: increasing ethanol concentration leads to a decrease in sensor resistance (R<sub>s</sub>).

The sensing process can be described in three sequential steps:

=== Oxygen adsorption ===

O<sub>2</sub>(g) + 2e<sup>-</sup> ⇄ 2O<sup>-</sup>(ads)

Oxygen molecules adsorb on the SnO<sub>2</sub> surface at elevated temperature and capture electrons, forming ionized oxygen species and creating an electron depletion layer.

=== Ethanol reaction ===

CH<sub>3</sub>CH<sub>2</sub>OH(g) + 6O<sup>-</sup>(ads) → 2CO<sub>2</sub>(g) + 3H<sub>2</sub>O(g) + 6e<sup>-</sup>

Ethanol reacts with the adsorbed oxygen species, producing CO<sub>2</sub> and H<sub>2</sub>O while releasing electrons.

=== Resistance change ===

The released electrons increase carrier density and conductivity, leading to a decrease in sensor resistance (R<sub>s</sub>). The magnitude of this change depends on ethanol concentration.

* electron density ↑  
* conductivity ↑  
* resistance (R<sub>s</sub>) ↓  

The resistance variation is converted into a voltage signal through a voltage-divider circuit and measured by an analog-to-digital converter (ADC), enabling real-time detection. The process is reversible: when ethanol is removed, oxygen re-adsorption restores the initial high-resistance state.[2]