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Flexible and mechanically robust gas sensors are the key technologies for wearable and implantable electronics. Herein, the authors demonstrate the high-performance, flexible nitrogen dioxide (NO2) chemiresistors using a series of n-type conjugated polymers (CPs: PNDIT2/IM-x) and a polymer dopant (poly(ethyleneimine), PEI). Imine double bonds (C = N) are incorporated into the backbones of the CPs with different imine contents (x) to facilitate strong and selective interactions with NO2. The PEI provides doping stability, enhanced electrical conductivity, and flexibility. As a result, the NO2 sensors with PNDIT2/IM-0.1 and PEI (1:1 by weight ratio) exhibit outstanding sensing performances, such as excellent sensitivity (𝚫R/Rb = 240%@1 ppm), ultralow detection limit (0.1 ppm), high selectivity (𝚫R/Rb < 8%@1 ppm of interfering analytes), and high stability, thereby outperforming other state-of-the-art CP-based chemiresistors. Furthermore, the thin film of PNDIT2/IM-0.1 and PEI blend is stretchable and mechanically robust, providing excellent flexibility to the NO2 sensors. Our study contributes to the rational design of high-performance flexible gas sensors. [ABSTRACT FROM AUTHOR]

Perovskite oxides are promising candidates for chemiresistive-type gas sensors owing to their exceptional thermal and chemical stability during solid-gas reactions. However, perovskites suffer from critical issues such as low surface area and poor surface activity, which negatively influence the sensing characteristics. While metal nanoparticles can be incorporated in perovskites to improve their reactivity, the fundamental incompatibility between catalytic metals and perovskite oxides often leads to substantial structural degradation as well as phase instability. Herein, we overcome this challenge through the introduction of an intermediary phase that forms coherent interfaces with both the perovskite phase and catalyst metals. Specifically, we present the case study of p-type La 0.8 Ca 0.2 Fe 0.98 Pt 0.02 O 3 perovskite, whose hole accumulation layer was modulated by the incorporation of metal-organic framework (MOF)-derived n-type α-Fe 2 O 3 nanoparticles decorated with highly dispersed Pt catalysts. The resulting composite exhibited significantly improved surface activity over the nonmodified La 0.8 Ca 0.2 FeO 3 perovskite, leading to exceptional chemiresistive sensing performance toward acetone gas ( R g / R a = 39.8 toward 10 ppm of acetone at 250 °C) with high cross-sensitivity against interfering gases. Importantly, our findings reaffirm the critical influence of interfacial engineering in facilitating surface chemical reactions on perovskite oxides and, by doing so, effectively provide a general synthetic guideline to the design of perovskite-based chemiresistors.

Although p-type semiconductors exhibit highly selective and stable chemiresistive gas sensing performances compared to conventional n-type semiconductors, their low sensitivity had long impeded their practical development. In this work, we developed highly porous Co 3 O 4 /CoMoO 4 heterostructure nanosheets (NSs) with enhanced sensitivity and superior stability toward acetone gas through a facile solution-based approach with Mo-impregnated Co-based metal-organic frameworks as the starting material. The spontaneous formation of a large number of p-p heterojunctions at the Co 3 O 4 -CoMoO 4 interface would facilitate the adsorption of oxygen and acetone molecules, as verified by density functional theory calculations. Consequently, experimental results showed that the Co 3 O 4 /CoMoO 4 NSs have a greatly enhanced response of 8.5 toward 5 ppm acetone, which is 7.1 times higher than that of pure Co 3 O 4 NS, without involving any noble metal catalysts. Moreover, the limit of detection of the Co 3 O 4 /CoMoO 4 NSs was as low as 10 ppb. Altogether, we propose that our synthetic approach for the engineering of p-p heterojunctions is an effective strategy for the future development of highly practical and sensitive gas sensors based on p-type semiconductors.