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Catalysis with single-atom catalysts (SACs) exhibits outstanding reactivity and selectivity. However, fabrication of supports for the single atoms with structural versatility remains a challenge to be overcome, for further steps toward catalytic activity augmentation. Here, we demonstrate an effective synthetic approach for a Pt SAC stabilized on a controllable one-dimensional (1D) metal oxide nano-heterostructure support, by trapping the single atoms at heterojunctions of a carbon nitride/SnO2heterostructure. With the ultrahigh specific surface area (54.29 m2g–1) of the nanostructure, we obtained maximized catalytic active sites, as well as further catalytic enhancement achieved with the heterojunction between carbon nitride and SnO2. X-ray absorption fine structure analysis and HAADF-STEM analysis reveal a homogeneous atomic dispersion of Pt species between carbon nitride and SnO2nanograins. This Pt SAC system with the 1D nano-heterostructure support exhibits high sensitivity and selectivity toward detection of formaldehyde gas among state-of-the-art gas sensors. Further ex situTEM analysis confirms excellent thermal stability and sinter resistance of the heterojunction-immobilized Pt single atoms.

Conductive metal–organic frameworks (cMOFs) offer high porosity and electrical conductivity simultaneously, making them ideal for application in chemiresistive sensors. Recently, incorporating foreign elements such as catalytic nanoparticles into cMOFs has become a typical strategy to enhance their sensing properties. However, this approach has led to critical challenges, such as pore blockage that impedes gas diffusion, as well as limited improvement in reversibility. Herein, single-atom catalyst (SAC)-functionalized cMOF is presented as a robust solution to the current limitations. Facile functionalization of SACs in a cMOF can be achieved through electrochemical deposition of metal precursors. As a proof of concept, a Pd SAC-functionalized cMOF is synthesized. The Pd SACs are stabilized at the interplanar sites of cMOF with Pd–N4coordination while preserving the porosity of the MOF matrix. Notably, the microenvironment created by Pd SACs prevents irreversible structural distortion of cMOFs and facilitates a reversible charge transfer with NO2. Consequently, the cMOF exhibits a fully recoverable NO2response, which was not previously attainable with the nanoparticle functionalization. Additionally, with the combination of preserved porosity for gas diffusion, it demonstrates the fastest level of response and recovery speed compared to other 2D-cMOFs of this class.

Chemiresistive gas sensors based on semiconducting metal oxides typically rely on noble metal catalysts to enhance their sensitivity and selectivity. However, noble metal catalysts have several drawbacks for practical utilization, including their high cost, their propensity for spontaneous agglomeration, and poisoning effects with certain types of gases. As such, in the interest of commercializing the chemiresistive gas sensor technology, we propose an alternative design for a noble-metal-free sensing material through the case study of Co-doped ceria (Co–CeO2) catalysts embedded in a SnO2matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co–CeO2catalyst nanoparticles integrated with SnO2nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co–CeO2@SnO2NFs exhibited superior gas sensing characteristics toward isoprene (C5H8) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co–CeO2catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO2sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C5H8(compared to pristine SnO2), exceptionally high selectivity, and a low detection limit of 100 ppb. The sensor also exhibited high stability for prolonged response–recovery cycles, attesting to the strong anchoring of Co–CeO2catalysts in the SnO2matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co–CeO2, demonstrate strong potential to completely replace noble metal catalysts, thereby advancing the development of the commercially viable chemiresistive gas sensors free from noble metals, capable of detecting target gases at sub-ppm levels.

The concept of integrating diverse functional 2D materials into a heterostructure provides platforms for exploring physics that cannot be accessed in a single 2D material. Here, physically mixing two 2D materials, MXene and MoS2, followed by freeze-drying is utilized to successfully fabricate a 3D MoS2/MXene van der Waals heterostructure aerogel. The low-temperature synthetic approach effectively suppresses significant oxidation of the Ti3C2TxMXene and results in a hierarchical and freestanding 3D heterostructure composed of high-quality MoS2and MXene nanosheets. Functionalization of MXene with a MoS2catalytic layer substantially improves sensitivity and long-term stability toward detection of NO2gas, and computational studies are coupled with experimental results to elucidate that the mechanism behind enhancements in the gas-sensing properties is effective inhibition of HNO2formation on the MXene surface, due to the presence of MoS2. Overall, this study has a great potential for expansion of applicability to other classes of two-dimensional materials as a general synthesis method, to be applied in future fields of catalysis and electronics.