Interface-engineered novel PPy-TiO₂-SnO₂ ternary nanocomposite for high-sensitivity low-concentration CO₂ gas sensing

Conducting polymer polypyrrole (PPy) and its metal oxide composites, namely PPy-TiO₂ and PPy-SnO₂, were synthesized via in-situ chemical oxidative polymerization with 10% (w/v) metal oxide loading. To advance beyond conventional bi-composites, a new synergistic tri-composite sensor PPy-TiO₂-SnO₂ was developed through ex-situ mixing of the two binary composites. UV–Visible spectroscopy was employed to determine the optical band gaps, while SEM revealed granular, densely connected morphologies, further supported by elemental confirmation from EDX. FTIR analysis verified the interaction between PPy and the incorporated metal oxides, and TGA established the thermal sustainability of the composites for room-temperature sensing applications. Impedimetric analysis showed a linear 45° complex impedance behaviour and confirmed that AC conductivity decreases with increasing bandgap. All composite films were fabricated on glass substrates and evaluated for CO₂ sensing using a laboratory-developed sensing unit. Linear, dynamic, and static responses were recorded across varying CO₂ concentrations. The bi-composites displayed sensitivities of 3.43 (PPy-SnO₂) and 14.15 (PPy-TiO₂), whereas the newly synthesized PPy-TiO₂-SnO₂ exhibited an intermediate yet enhanced sensitivity of 8.09. The tri-composite also demonstrated rapid gas-sensing performance, with a short recovery time of 76 s and response time of 184 s. Energy-level alignment and Fermi-level pinning effects correlated well with the bandgap values, supporting the proposed sensing mechanism involving interfacial depletion and accumulation layers. Overall, the PPy-TiO₂-SnO₂ tri-composite emerges as a robust, sustainable, and efficient room-temperature CO₂ sensing material. The sensor also exhibits good selectivity towards CO₂ over interfering gases such as NH₃, ethanol, and CO. Furthermore, it demonstrates excellent long-term stability with a sensitivity drift of 6.10% over 35 days and minimal baseline variation. The sensor performance remains largely unaffected under varying humidity conditions (30–70% RH), indicating its suitability for practical environmental applications. Systematic CO 2 gas sensing assembly along with the proposed schematics for the CO 2 gas sensing in Novel PPy-TiO 2 -SnO 2 tri-composite sensor. • Synthesis and characterization of a novel PPy–SnO₂–TiO₂ tri-composite via in-situ polymerization and ex-situ mixing. • Comparison of CO₂ sensing (linear, dynamic and static ) of PPy, PPy–SnO₂, PPy–TiO₂ and PPy–SnO₂–TiO₂ sensors at RT. • Sensing mechanism via Tauc bandgap and energy diagrams highlighting Fermi-level pinning and heterojunction effects. • Room-temperature CO₂ sensor with good sensitivity and fast response, using PPy–SnO₂–TiO₂ for low-power CO₂ monitoring. • PPy-TiO₂-SnO₂ sensor shows CO 2 selectivity,6.10% drift over 35 days, and stability at 30-70 % RH.