Enhanced Formaldehyde Gas Sensing of Ag-Doped NiFe2O4 Spinel Ferrites

Abstract

In this study, NiFe_1.9 Ag_0.1 O₄ spinel ferrite nanoparticles were successfully prepared using the sol-gel method, followed by successive annealing steps at 600 °C and 700 °C for 24 h each. A key novelty of this work is the demonstration that silver incorporation significantly enhances gas-sensing performance toward low-concentration formaldehyde (HCHO) while strictly maintaining a single-phase cubic spinel structure (space group Fd$\bar{3}$m). Structural and morphological analyses via X-ray diffraction (XRD) confirmed high crystallinity, while TEM imaging revealed nearly spherical nanoparticles with an average diameter of 32 nm. Energy-dispersive X-ray (EDX) mapping verified a homogeneous elemental distribution of Ni, Fe, Ag, and O throughout the spinel lattice. To evaluate gas-sensing capabilities, the synthesized powder was spray-deposited onto alumina substrates to form thick-film sensors. The optimized sensor exhibited a peak response of 1.7 toward 5 ppm HCHO at an operating temperature of 275 °C, achieving a notable detection limit of 0.5 ppm. Furthermore, the device demonstrated rapid kinetics with response and recovery times of 58 s and 148 s, respectively alongside excellent repeatability and robust humidity tolerance. These findings position Ag-doped NiFe₂O₄ as a highly promising sensing material for precise environmental monitoring of formaldehyde.