Electrical and hydrogen gas sensing properties of Co1-Zn Fe2O4 nanoparticles; effect of the sputtered palladium thin layer

Hydrogen gas sensing of Co1-xZnxFe2O4 (x = 0–0.45) nanoparticles synthesized by a simple hydrothermal process has been investigated. An n→p crossover in the electrical conductivity toward hydrogen gas was observed. However, no such charge carrier reversal is noticed at higher x values. In both cases, the related mechanisms are proposed. It has been found that reversal is temperature and doping ratio dependent. In this regard, the more compatible and realistic model is presented which explains the nature of our observations. By analyzing the adsorption kinetics of the surface, it is identified that at a higher percentage of Zn (x = 0.45) the sensor response deviates from the Freundlich isotherm and falls under the category of the Langmuir adsorption model toward H2 gas exposure. These strong correlations between the results of gas sensing measurements and those calculated based on the DC electric resistivity would pave the way for further investigation of the gas sensors from a fundamental point of view. Deposition of Palladium nano-structures (possibly island-like) on the surface of the CoFe2O4 sensor appeared to be effective in speeding up the response time and increasing the sensitivity. The remarkable response time, as low as 3 s, is obtained after modifying the sensor surface with the palladium deposition.