Surface Tailoring of MoS₂ Nanosheets with Substituted Aromatic Diazonium Salts for Gas Sensing: A DFT Study

Two-dimensional (2D) nanomaterials are useful for building gas sensors owing to their desirable electronic and optical properties. However, they usually suffer from selectivity, because they cannot discriminate between gas molecules. Functionalization with organic molecules can be used to tailor their surfaces to recognize a specific family of compounds. In this study, solid-state density functional theory (DFT) was used to elucidate the functionalization of MoS₂ with substituted aromatic diazonium salts (<i>R</i> = -H, - CH₃, -CO₂H, -CHO, -OCH₃, and -NO₂). Results showed that chemical reaction with diazonium salts is favored to their physical adsorption (<i>E</i> _ads = -0.04 to -0.38 eV vs <i>E</i> _rxn = -1.47 to -2.20 eV), where organic cations have a preference to attach atop of sulfur atoms. Chemical functionalization induced a small variation in the bandgap energy not exceeding 0.04 eV; thus, the optical properties were well preserved. In the presence of ammonia, the substituted <b>MoS</b> _<b>2</b> <b>/2(a-f)</b> responded to the target analyte through a change in the interaction energy, varying from -0.08 to -0.83 eV, where the best interaction energy was obtained for <b>MoS</b> _<b>2</b> <b>/2c</b>, bearing the carboxylic acid group. In the presence of other gases such as CO₂, SO₂, and H₂S, the interaction energy is lower (-0.14 to -0.35 eV), indicating good selectivity of the nanomaterials. Furthermore, the interaction increased in the presence of humidity, which was more realistic than that in the presence of neat NH₃. This interaction was confirmed by computing the partial charges. Recovery times estimated from the interaction energies ranged from 0.31 s to several minutes, depending on the interacting molecules. Phenylcarboxyl-modified MoS₂ nanosheets show great potential as candidates for the development of chemoresistive gas sensors that are specifically designed for detecting ammonia.