Natural tannic acid (green tea) mediated synthesis of ethanol sensor based Fe3O4 nanoparticles: Investigation of structural, morphological, optical properties and colloidal stability for gas sensor application

A green tea mediated combustion synthesis route is implemented to prepare iron oxide nanoparticles. Chemical ferric nitrate and natural tannic acid extracted from green tea are used to prepare iron oxide nanoparticles, and the prepared sample is annealed at 350 degrees C. The crystal structure and phase of iron oxide nanoparticles have been analyzed using X-ray Diffraction (XRD). The crystallite size (D) and lattice constant (a) of the prepared and annealed samples are calculated. The mean crystallite size is found to be 23.4 nm for the prepared sample and 30.1 nm for the annealed sample. The morphological and compositional analysis are characterized through scanning electron microscopy (SEM) and EDX. The particle shape and size are recorded through tunneling electron microscopy (TEM) and the average grain size is 25 nm for as-prepared and 32 nm for annealed samples. The stability of colloidal systems of iron oxide nanoparticles has been assessed using the zeta potential. The asprepared sample has a decent stability value of - 58.3 mV, whereas the annealed sample has an exceptional value of - 60.1 mV. The optical band gap of the samples is calculated from ultra-violet (UV-vis) spectra, and the energy band gap of the as-prepared samples is 2.24 eV, and the annealed sample is 2.78 eV. The gas sensing behavior of as-prepared and annealed iron oxide samples is analyzed for different aspects related to operating temperature, gas concentration, response-recovery time, and different gases.