Humidity sensing application of CeO2 nanoparticles: mechanism interpretation with density functional theory

Humidity sensors play a crucial role in non-contact measurements, particularly in environmental monitoring and healthcare systems. Among various materials, semiconductor metal oxides have gained significant attention due to their favorable physicochemical and electrical properties, making them ideal candidates for humidity sensing applications. In this study, cerium oxide (CeO₂) nanoparticles (NPs) were synthesized via a green synthesis route using Morinda tinctoria leaf extract, inspired by biomimetic processes. Comprehensive characterization techniques-including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and UV–Vis spectroscopy-were employed to evaluate the structural, functional, morphological, elemental, and optical features of the synthesized NPs. XRD confirmed the formation of pure, crystalline CeO₂ in the cubic phase without any metallic Ce impurities. Microscopy revealed a porous morphology, contributing significantly to the enhanced humidity sensing performance. The sensor exhibited high sensitivity to relative humidity (RH) across the range of 5 %–98 % at room temperature, with a resistance variation of up to 2218 Ω—demonstrating a five-order magnitude response and excellent linearity. Moreover, the sensor showed rapid response and recovery times of 23 and 44 s, respectively, along with good long-term stability. These eco-friendly and cost-effective CeO₂-based humidity sensors are well-suited for agriculture and humidity monitoring applications. To further understand the sensing mechanism, density functional theory (DFT) calculations were performed. Topological analysis using electron localization function (ELF) maps elucidated the nature of bonding in CeO₂ and its interaction with water molecules.