Synthesis, characterization and humidity sensing performance of chromium-substituted lithium–copper ferrites

In this study, chromium-substituted lithium–copper ferrites with the nominal composition LixCu1-xCryFe2-yO4 (x = 0.2; y = 0, 0.2, 0.4) were prepared through the traditional ceramic route. X-ray diffraction (XRD) confirmed the formation of a single-phase cubic spinel structure (space group Fd3m), with crystallite sizes in the 37.1–37.2 nm range. The microstructural features—including particle shape, grain connectivity and porosity—were examined by scanning electron microscopy (SEM), while Fourier-transform infrared (FTIR) spectroscopy revealed characteristic ferrite absorption bands, with the most prominent features appearing between 420–497 cm^-1 and 547–600 cm^-1. The optical response of the materials, evaluated by ultraviolet–visible (UV–Vis) spectroscopy, showed an increase in band gap upon chromium incorporation up to y = 0.2, followed by a reduction at y = 0.4. X-ray photoelectron spectroscopy (XPS) confirmed the presence of chromium in the lithium–copper ferrite system, while the specific surface area was evaluated using nitrogen adsorption–desorption measurements based on the Brunauer–Emmett–Teller (BET) method. Humidity-sensing behavior was assessed by monitoring the electrical resistance of the samples over a broad relative humidity interval (8.9–97%) at room temperature. All compositions exhibited the expected decrease in resistance with rising humidity and maintained stable performance over a 90-day period. Among the investigated ferrites, the y = 0.2 sample displayed the most favorable characteristics, offering response and recovery times of 77.8 s and 88.5 s, respectively. The experimental data over the entire humidity range were well fitted by the Freundlich adsorption isotherm, providing additional insight into the humidity sensing mechanism.