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 (Cu2O) to improve its interaction with humidity (H2O). 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 (Delta E), 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 H2O 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 H2O affinity and significant Delta E 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.