NiFe2O4 and ZnFe2O4 nanoparticles synthesis by sol-gel auto-combustion for humidity sensor applications

The sol-gel auto-combustion approach was used to prepare nickel and zinc spinel ferrites. X-ray diffraction (XRD) studies were used to assess phase purity and crystal lattice parameters. An XRD analysis has revealed that the synthesized nanocrystalline has a spinel structure with crystallite sizes of 32.3 nm and 38.9 nm for NiFe2O4 (NFO) and ZnFe2O4 (ZFO), respectively. Transmission electron microscopy and field emission scanning electron microscopy were used for morphological analysis to determine the size and shape of the samples' nanoparticles. Energy dispersive spectroscopy verified good agreement of the experimental composition with the nominal composition of ferrite powders. The optical direct and indirect energy gaps were obtained in the range of 2.0-2.25 eV. The magnetic studies revealed the ferromagnetic nature of the NFO, whereas the ZFO was observed to be paramagnetic. The frequency, temperature, and humidity dependence of dielectric properties and AC conductivity were studied. The dielectric permittivity, dielectric loss tangent, and AC conductivity of NFO are greater than those of ZFO at different frequencies, temperatures, and relative humidity. The humidity sensitivity of ZFO is greater than that of NFO nanoparticles, and both of them have good stability.