Humidity-activated inorganic Co-Ni layered double hydroxides-organic polyethyleneimine hybrids for room-temperature, sensitive and stable CO2 sensing.

Carbon dioxide (CO2) detection plays a vital role in the fields of indoor ventilation, agriculture production, and global climate monitoring. Commercial CO2 detection primarily relies on non-dispersive infrared technology (NDIR), and still suffers from high cost, large size, and susceptibility to the fluctuation of ambient temperature and humidity. To address these bottlenecks, we propose cobalt-nickel layered double hydroxides (Co-Ni LDHs)-polyethyleneimine (PEI) composites-based chemiresistive CO2 sensors. After subtle componential adjustment, the optimal LDHs-PEI-3 sensor exhibited a larger response (209 % vs. 3 %) to 100 ppm CO2 and better selectivity with respect to pure LDHs counterpart at room temperature (23 ± 2 °C), accompanied with a short response/recovery time of 23/137 s. In addition, a sensitivity of 6.6 × 10-3 and 3.3 × 10-3/ppm for 15-40 ppm CO2 and 200-900 ppm CO2 were separately achieved, much larger than those for pure LDHs one (1.6 × 10-4 and 1.2 × 10-5/ppm). The response of the LDHs-PEI-3 sensor showed a 98 % retention after 41 days, confirming an excellent stability. Interestingly, further Ag doping downshifted the lowest operational RH from 50 % RH to 30 % RH and effectively suppressed the baseline drift, thus extending the application scenarios. The Co-Ni LDHs as inorganic scaffold provided robust conduction pathways while mitigating the aggregation of PEI. Besides, PEI offered abundant basic amine groups to react with acidic CO2 molecules via reversible acid-base reaction under humid atmosphere. All these collectively brought about a high and stable response. This work provides an alternative paradigm for room-temperature CO2 detection via inorganic-organic hybrids, and showcases vast application potential in the future.