DFT-Based Functionalization of Graphene with Lithium-Modified Groups for Enhanced Hydrogen Detection: Thermodynamic, Electronic, and Spectroscopic Properties.

This study investigates the impact of oxygen-containing functional groups (COO-Li, CO-Li, and O-Li) on the electronic and optical properties of graphene, with a focus on hydrogen sensing applications. Using density functional theory (DFT) calculations, we evaluated the thermodynamic feasibility of the functionalization and hydrogen adsorption processes. The Gibbs free energy changes (ΔG) for the functionalization of pristine graphene were calculated as -1233, -1157, and -1119 atomic units (a.u.) for COO-Li, CO-Li, and O-Li, respectively. These negative values indicate that the functionalization processes are spontaneous (ΔG < 0), with COO-Li being the most thermodynamically favorable. Furthermore, hydrogen adsorption on the functionalized graphene surfaces also exhibited spontaneous behavior, with ΔG values of -1269, -1204, and -1175 a.u., respectively. These results confirm that both functionalization and subsequent hydrogen adsorption are energetically favorable, enhancing the potential of these materials for hydrogen sensing applications. Among the functional groups we simulated, COO-Li exhibited the largest surface area and volume, which were attributed to the high electronegativity and steric influence of the carboxylate moiety. Based on the previously described results, we analyzed the interaction of these functionalized graphene systems with molecular hydrogen. The adsorption of two H molecules per system demonstrated favorable thermodynamics, with lithium atoms serving as active sites for external adsorption. The presence of lithium atoms significantly enhanced hydrogen affinity, suggesting strong potential for sensing applications. Further, electronic structure analysis revealed that all functionalized systems exhibit semiconducting behavior, with band gap values modulated by the nature of the functional group. FTIR (Fourier-Transform Infrared Spectroscopy) and Raman spectroscopy confirmed the presence of characteristic vibrational modes associated with Li-H interactions, particularly in the 659-500 cm range. These findings underscore the promise of lithium-functionalized graphene, especially with COO-Li, as a tunable platform for hydrogen detection, combining favorable thermodynamics, tailored electronic properties, and spectroscopic detectability.