ZnFe2O4 Nanoparticles for Gas-Sensing Applications: Monitoring of Structural Parameters while Exposing the Ferrite in Gas Atmospheres

Abstract

Abstract ZnFe2O4 materials are promising for several applications, including catalysis, sensors, and supercapacitors. This paper presents a hydrothermal-based facile method for synthesizing ZnFe2O4, whose size can be controlled with the concentration of sodium acetate used as a fuel. The characterization of the morphology, structure, composition, and electronic properties of the synthesized samples is also presented in this paper. The crystal structure of the synthesized samples was determined using an X-ray Diffractometer (XRD). The results revealed fluctuations in the size, lattice parameter, and strain in the nanoparticles with increasing the concentration of sodium acetate fuel. Field-Emission Scanning Electron Microscopy (FESEM) was used to determine the morphology and elemental composition of synthesized materials, and it revealed that the particles in synthesized samples possessed approximately spherical morphology whose size decreased significantly with the increasing amount of sodium acetate. Transmission Electron Microscopy (TEM) was utilized to determine the structure, morphology, and elemental distributions in particles at the nanoscale, and it confirmed the findings of XRD and FESEM analyses. The high-resolution TEM (HRTEM) imaging analysis of the nanoparticles in samples revealed that the particles predominantly possessed (001) type facets. X-ray photoelectron spectroscopy (XPS) and core-loss electron energy loss spectroscopy (EELS) showed an increasing fraction of Fe2+ with the decreasing size of the particles in samples. The Brunauer, Emmett, and Tellers (BET) analysis of samples revealed a higher surface area as the particle size decreases. In addition, the determined surface area and pore size values are compared with the literature, and it was found that the synthesized materials are promising for gas-sensing and supercapacitor applications. The ab initio calculations of the Density of States (DOS) and Band structure of (001) surface terminating ZnFe2O4 particles were carried out using Quantum Espresso software to determine the bandgap of the synthesized samples. They were compared to the experimentally determined bandgap values for the corresponding samples. Finally, in-situ TEM measurement was carried out on one sample and revealed that the d-spacing of ZnFe2O4 NPs showed a noticeable fluctuation reaching more than 5% upon exposure to CO2 and Ar gases. It is concluded from the presented study that the reduction in the size of the nanoparticles provides more active sites due to a higher concentration of oxygen vacancies and tunes the bandgap.