Modeling investigation of alkali, alkaline-earth, and transition metal-decorated reduced graphene oxide for enhanced humidity sensing

Detecting humidity is considered as a critical issue for electronic devices, healthcare, industrial operations, and environmental monitoring, necessitating the development of high-performance sensors. Reduced graphene oxide (rGO) was functionalized with alkali metals (Li, Na, K), alkaline-earth metals (Mg and Ca), the transition metal Cu, and copper oxide (Cu 2 O) to improve its interaction with humidity (H 2 O). The density functional theory (DFT) at the B3LYP/LANL2DZ level was used to optimize structures and investigate electronic properties, such as HOMO-LUMO energy gaps (ΔE), density of states (DOS), molecular electrostatic potential (MESP), and global reactivity descriptors. Among the studied systems, it was found that, rGO/5K, rGO/2Mg, rGO/2Ca and rGO/2Cu exhibited decreased bandgaps, increased reactivity, and orbital redistribution. Interaction with H 2 O demonstrated hydrogen bonding via QTAIM analysis, which is an important interaction mechanism. Adsorption energy ( E a ) calculations identified rGO/2Ca and rGO/2Mg as the optimal candidates with high H 2 O affinity and significant ΔE modulation. The findings point to the possible application of metal-decorated rGO as humidity sensor, facilitating its applications in both environmental monitoring and smart sensing technology. These findings offer a critical contribution to the field by demonstrating how specific alkali and transition metal decorations, particularly Ca and Mg, uniquely lower resistance to charge transfer, thereby defining a new frontier for highly responsive and stable graphene-based humidity sensors.