Nanostructured Spinel ZnFe2O4 for the Detection of ... - ResearchGate

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-1195.2.4. Response and Recovery TimeThe response of ZnFe 2 O 4 sensor was found to be quick (~ 7 s) to 300 ppm Cl 2 , while the recovery wasfast (~ 25 s). The fast response may be due to faster oxidation of the gas. The negligible quantity of thesurface reaction product and its high volatility explains its fast response and quick recovery to itsinitial chemical status.6. DiscussionWhen reducing gases such as ammonia, ethanol, LPG, chlorine, hydrogen, acetone, H 2 S react withdifferent oxygen species, complex series of reactions take place, ultimately oxidizing the ammonia,ethanol, LPG, chlorine, hydrogen, acetone, and H 2 S as:C 4 H 10 + 13O 2- + 300 0 C → 5H 2 O + 4CO 2 + 26e - (3)Cl 2 + 4O 2- + 300 0 C → 2Cl - + 2O 2 + 6e - (4)C 2 H 5 OH + 6O - + 300 0 C → 3H 2 O + 2CO 2 +6e - (5)4NH 3 + 3O 2 - + 150 0 C → 6H 2 O + 2N 2 + 3e - (6)4H 2 S + 4O 2- + 300 0 C → SO 2 + 4H 2 O + 8e - (7)2H + + 2O 2 - → H 2 O 2 + O 2 (8)There are four adsorption behaviors of chlorine on the oxide surface [24- 25]Cl 2 + O 2(ad) 2− → 2Cl (ad) − +O 2 +2e − (9)Cl 2 +2O ox→ 2Cl o − +O 2 +2e − (10)Cl 2 +2e → 2Cl (ad) −Cl 2 +2V o +2e − → 2Cl o − ,where subscripts ad and o mean the species adsorbed on the surface and the species occupying thelattice oxygen site, respectively. Vo is the oxygen vacancy. Pure ZnFe 2 O 4 showed response to chlorineat high temperature.This shows n-type conduction mechanism. Thus on oxidation, a single molecule of gas liberatesplurality of electrons in the conduction band, resulting in an increase in conductivity of the sensor. Wefound that the gas responses increase with operating temperature, reach to their responses increasewith operating temperature, reach to their respective maxima at a particular temperature and thendecrease with a further increase in operating temperature. An increase in operating temperature causesoxidation of a large number of gas molecules, producing a very large number of electrons. Therefore,the conductivity increases this is the reason why the gas response increases with operatingtemperature. The temperature at which the gas response is maximum is the actual temperature neededto activate the material for progressing the reaction. However, the response decreases at higheroperating temperature, as the oxygen adsorbates are desorbed from the surface of the sensor. Also, athigher temperature, the carrier concentration increases due to intrinsic thermal excitation and the116