U.P.B. Sci. Bull., Series B, Vol. 82, Iss. 1, 2020 ISSN 1454-2331 THE EFFECT OF Mn +2 DOPING RATIO ON THE SENSING PROPERTIES OF CoFe2O4 NANOPARTICLES FOR CO2 AND NH3 GASES Rihab JABBAR 1* , Sabah H. SABEEH 2 , Awham M. HAMEED 3 In this work, undoped and manganese (Mn +2 ) doped spinel cobalt ferrites nanoparticles (NPs) having a composition (MnxCo1-xFe2O4 where x = 0.2 and 0.8) were synthesis by sol-gel precipitation method. The results showed that the average crystallite size (D) was found to decrease from 20.68 nm to 9.95 nm with increasing the Mn +2 doping ratio. The samples have flat surface morphology as indicated by SEM and the grain sizes decrease with increasing the Mn-doping ratio as indicated by AFM. The sensing property was carried out at room temperature for CO2 and NH3 gases. It observed that all sample with different compositions sensitive to both gases, where the Mn0.8Co0.2Fe2O4 samples have the highest sensitivity for CO2 and NH3 were 32.12 % and 98 % respectively. Keywords: CoFe2O4, Nanoparticles, Gas Sensor, Sol-Gel method. Mn-doped CoFe2O4, CO2, NH3 1. Introduction Nowadays, the great effect of poisonous gases like CO, CO2, NO, and NO2 on the environmental damage resulting from automobile exhausts has become progressively clear as well as Cl2, CH4, and NH3 gases resulting from the industrial operations [1]. Therefore, the metal oxide gas sensor, especially the spinel ferrites in the general formula (MB2O4) (M = bivalent metal ions, such as Mn, Fe, Mg, Zn, Ni, Co, Cu, etc.) in which the metal cations M and B are positioned at A-site (tetrahedral) and B-sites (octahedral) sublattice and oxygen has an (FCC) close packing structure, it had attracted more interest in recent years due to their properties such as low cost, good chemical properties, good thermal properties, rapid response and recovery time and modest electronic design. Moreover, it can be provided substantial sensitivity, selectivity, and stability required by these systems [2, 3]. Among these spinel ferrites, cobalt ferrite (CoFe2O4) essentially interesting due to its properties that includes chemical stability, mechanical hardness, strong anisotropy, high forced field and moderate saturation magnetization [4]. 1,2,3 Department of Applied Sciences, University of Technology, Baghdad, Iraq, * Corresponding author’s e-mail: rihabjabbar@yahoo.com