Investigating the Effect of Active Site Density in Transition Metal-Doped Graphene on CO Gas Sensing Performance: A DFT Study

Developing sensitive and reversible CO sensors requires precise control of material–analyte interactions. Using DFT, we investigate CO sensing on bimetallic (Fe, Pt) anchored on N-doped graphene (TM2–N4–C), focusing on active-site density effects. Three densities are considered: low (12.7 Å), medium (8.5 Å), and high (4.2 Å). FePt–N4–C band gaps exhibit non-monotonic tuning, approaching metallicity at high density. CO chemisorbs on Fe sites, but physisorbs on Pt sites. FePt exhibits stronger synergistic adsorption than homonuclear counterparts. While adsorption generally strengthens with density, spin-polarized calculations qualitatively reorder this trend via spin delocalization. High temperatures drastically improve recovery; low-density FePt–N4–C reaches 65 s at 498 K. Three design principles emerge: low-density heteronuclear systems for reversible sensing, medium-density high-spin states for ultra-sensitive capture, and high-density configurations for work function sensors. This work establishes active site density as a key electronic and kinetic knob for graphene-based CO sensors.