Title : Structural and gas-sensing properties of nano-structured mixed ferrites synthesized by citrate-gel auto-combustion technique
Abstract:
The nano-structured ferrites are known as gas sensing materials due to their reduced particle size, porosity, high surface-to-volume ratio, and conductivity. Spinel ferrites are metal oxide semiconductor (MOS) based sensors that exhibit a significant variation in the electrical resistance because of the reversible interaction of the gas with the preadsorbed ambient oxygen. This work presents synthesis of nano-structured nickel ferrite and its composition with manganese ferrite by using citrate-gel auto-combustion technique to study the structural properties and gas sensing application. The citrate-gel auto-combustion route carries the advantage of quick preparation of nanoferrite powders at a much lower temperature (180°C-200°C) compared to the conventional ceramic method. The stoichiometry of ferrite powders is confirmed by energy dispersive x-ray analysis (EDAX). X-ray diffraction (XRD) analysis confirmed the formation of single-phased ‘fcc’ cubic spinel structure of prepared nanoferrite powder. The crystallite size, porosity, and other structural parameters were obtained by XRD analysis. The morphology of prepared nanoparticles was observed by transmission electron microscopy (TEM) measurement. The average particle size of the ferrite samples was deduced by employing the statistical histogram of grain size and was compared with crystallite size. The optical bandgap energy value of samples was found by employing the Tauc plot of ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis. DRS) analysis. The metal oxide semiconductor-based gas sensing devices were successfully fabricated in the form of pellets to study their response toward the target gases. The gaseous state of ammonia and ethanol solution was achieved in the laboratory. The gas sensing setup was arranged in the laboratory using apparatus like thermocouple, LCR meter, gas chamber, temperature indicator, etc. The gas sensing experiment was performed to study the gas response (%) at different gas concentrations (ppm) and temperatures (℃). In metal oxide semiconductor-based gas sensors, an electron-depleted surface layer is formed in the presence of atmospheric oxygen that is adsorbed or chemisorbed on the surface, under normal atmospheric conditions and typical operating temperatures. The sensitivity of a material was determined by the change of its resistance in the presence of gas and air atmosphere. The best response was observed by the nickel ferrite sample against ammonia gas while another sample showed the maximum gas response for ethanol gas. The mixed type of semiconducting trend (n and p-type) was observed by the gas sensing devices for both the gases at different temperatures. The sensitivity increased with the temperature and peaked at its operating temperature for the prepared ferrite devices. It showed an excellent response in terms of sensitivity (%) for both gases. The maximum gas sensitivity (Smax) of around 99% was exhibited by Mn0.8Ni0.2Fe2O4 at different temperatures. This study confirmed that all the prepared nano-structured ferrites are efficient to detect the gases and can be used as the gas sensors.
