Gas sensing of metaloxide sensors is based on the resistance change of the material due to the chemical and electronic interaction between the gas and sensing matrix. The chemical interaction involves the adsorption of the target gas molecules on the surface of metal oxide thin film, where subsequent reaction with

oxygen species adsorbed on the metal oxide surface, results in the detection for the target gas. The adsorption and reaction process can be described as follows:

When metal oxide sensor is exposed to air, oxygen molecules adsorb on the surface of the materials to form chemisorbed oxygen anions (O2(ads)) by capturing electrons from the conductance band, resulting in the formation of a depletion layer, known as the space charge layer, on the surface of the sensor material which results in an increase in resistance of the material.

The sensitivity of sensing film is defined as the slope of the resistance-time curve and is given as below;

S = change in conductivity/time

https://pubs.rsc.org/en/content/articlelanding/2016/ra/c6ra07103a#!divAbstract

(DOI: 10.1039/C6RA07103A )

Article Development of Fe 2 O 3 –PANI nanocomposite thin film based ...

Decrease in conductivity occurs in the gas of oxidizing gases, in metal oxide gas sensor. Sensitivity depends on surface area, microstructure of sensing layers, surface additives, temperature, humidity etc.S = (Ra-Rg)/Rg, where, Ra, Rg are the stable values of the resistance of the material before and after exposure to gas.

Suppose when the sensor is exposed to reducing gases, the electrons trapped by the oxygen adsorbate will return to the surface of the sample, leading to a decrease in the potential barrier height and thus an increase in conductivity. I think the conductivity depends on the properties and pore size of the materials.

dear Ausama I. Khudiar,

the sensitivity of the metal oxide gas sensors, is the variation of conductivity during the time period it changed with the factors influencing the surface reactions, such as chemical components, surface-modification and micro structures of sensing layers, temperature and humidity.

Dear Mr Ausama

As I understand the question, you seek to know "if there is any dependence of gas sensitivity/ response on the DC conductivity value of the oxide"

Here, for oxide semiconductors, the DC conductivity could be dependent on several factors like dimensions, defects etc and hence can vary among samples of same compositions.

In addition to microstructure, morphological properties, the sensitivity would certainly depend on the "order of conductivity" of the material. Since, the sensitivity is defined as normalized change in conductivity which arises because of exchange of charge carriers between the gas and metal oxide.

So here, if the conductivity is too high, the change in carrier concentration with gas exposure would be too low as compared to the actual value and hence the change will be nominal or insignificant.

On the other hand, when the conductivity too low, it gets difficult for measurement and the corresponding change is carrier concentration is also too low to measure significantly.

Thus, the intermediate conductivities are desirable (few kilo Ohm to a few mega ohms) for the optimized sensor device.

Thank you Sir