Electrochemically Functionalized Graphene for Ultrafast NO₂ Detection at Room Temperature

Graphene is one of the most promising alternative sensing materials for next-generation applications such as flexible devices, and Internet of Things, since it exhibits high sensitivity at room temperature and stable sensing performance. Meanwhile, graphene has main drawbacks such as slow response and incomplete recovery reaction. To overcome the disadvantages, the functionalization of graphene has been widely explored as graphene have demonstrated a unique advantage in functionalization. Atomically thin carbon nanostructure, consisting entirely of surface, allows surface functionalization to impact the entire structure. As halogen atoms including bromine and iodine are excellent candidates for tuning the surface chemistry, but conventional halogenation methods of graphene typically involve the use of toxic halogen gas, requiring the advanced reactors or high vacuum systems. The development of efficient and safe halogenation methods has enormous potential for practical sensing application.
Herein, we fabricated halogenated graphene sensors via a facile electrochemical halogenation process of monolayer graphene using sodium chloride solution. Different voltages and sodium chloride concentration were used to control halogenation levels and investigate changes in subsequent sensing properties. The response and recovery times upon exposure to NO₂ were dramatically decreased after halogenation, from 157 to 38 s and from 1485 to 202 s, respectively. Also, the sensors exhibited high selectivity to NH₃ and NO₂ after halogenation. The selectivity was examined upon exposure to various gases including 5 ppm NO₂, 50 ppm CH₃COCH₃, C₂H₅OH, C₇H₈, H₂, and NH₃. For materials characterization, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and energy dispersive X-ray spectroscopy (EDS) have been exploited to identify the formation of C–Cl bonds and the presence of halogen atoms within the graphene structure. Density functional theory (DFT) calculation was performed to discuss the gas sensing mechanism of chlorinated graphene. The theoretical calculation was in agreement with the experimental results. The high sensitivity, selectivity, and enhanced reaction time play a key role in real-time sensing applications, as early-stage gas discrimination is important in various circumstances.