Phosphorus-doped T-graphene nanocapsule toward O₃ and SO₂ gas sensing: a DFT and QTAIM analysis

Tetragonal graphene nano-capsule (TGC), a novel stable carbon allotrope of sp² hybridization is designed and doped with phosphorus (P) to study the O₃ and SO₂ gas sensitivity via density functional theory calculation. Real frequencies verified the natural existence of both TGC and P-doped TGC (PTGC). Both TGC and PTGC suffer structural deformations due to interaction with O₃ and SO₂ gases. The amount of charge transfer from the adsorbent to the gas molecule is significantly greater for O₃ adsorption than SO₂ adsorption. The adsorption energies for TGC + O₃ and PTGC + O₃ complexes are - 3.46 and - 4.34 eV respectively, whereas for TGC + SO₂ and PTGC + SO₂ complexes the value decreased to - 0.29 and - 0.30 eV respectively. The dissociation of O₃ is observed via interaction with PTGC. A significant variation in electronic energy gap and conductivity results from gas adsorption which can provide efficient electrical responses via gas adsorption. The blue/red shift in the optical response proved to be a way of detecting the types of adsorbed gases. The adsorption of O₃ is exothermic and spontaneous whereas the adsorption of SO₂ is endothermic and non-spontaneous. The negative change in entropy verifies the thermodynamic stability of all the complexes. QTAIM analysis reveals strong covalent or partial covalent interactions between absorbent and adsorbate. The significant variation in electrical and optical response with optimal adsorbent-gas interaction strength makes both TGC and PTGC promising candidates for O₃ and SO₂ sensing.