Cr-Noble Metal Pair Engineering in Heteronuclear Phthalocyanine Monolayers for VOC Sensing: A First-Principles Study

Metal-pair regulation offers a practical strategy for tuning the adsorption affinity and signal response of phthalocyanine-based VOC sensing materials. In this study, heteronuclear CrMPc monolayers with M = Ru, Rh, and Pd were evaluated by density functional theory for the detection of five representative VOCs, including CH3Cl, CH3OH, C6H6, H2CO, and C2H4. The three CrMPc frameworks are thermodynamically stable, with binding energies of -15.67, -15.11, and -13.50 eV for CrRuPc, CrRhPc, and CrPdPc, respectively, and their pristine band gaps can be adjusted from 0.063 to 0.690 eV by changing the second metal center. Adsorption analysis indicates that the three substrates do not respond to VOCs in the same manner: CrRuPc exhibits stronger interaction with C2H4 and obvious electronic perturbation toward C6H6 and CH3OH; CrRhPc shows a more balanced adsorption and electronic response toward oxygen-containing VOCs and C2H4; and CrPdPc displays a clearer preference for C2H4 while maintaining distinguishable responses toward H2CO and CH3OH. Importantly, the preferred sensing channels are not identical for the three substrates: CrRuPc is more suitable for C6H6 detection at 298 K and CH3OH detection at 348 K, CrRhPc shows strong adsorption/enrichment ability toward C2H4 and CH3OH, and CrPdPc provides a balanced response and recovery behavior for C2H4 at about 348 K. These findings highlight the role of Cr-4d metal-pair engineering in constructing phthalocyanine monolayers with differentiated VOC recognition and multiple sensing modes.