Hydrogen (H 2 ) is one of the next-generation energy sources because it is abundant in nature and has a high combustion efficiency that produces environmentally benign products (H 2 O). However, H 2 /air mixtures are explosive at H 2 concentrations above 4%, thus any leakage of H 2 must be rapidly and reliably detected at much lower concentrations to ensure safety. Among the various types of H 2 sensors, chemiresistive sensors are one of the most promising sensing systems due to their simplicity and low cost. This review highlights the advances in H 2 chemiresistors, including metal-, semiconducting metal oxide-, carbon-based materials, and other materials. The underlying sensing mechanisms for different types of materials are discussed, and the correlation of sensing performances with nanostructures, surface chemistry, and electronic properties is presented. In addition, the discussion of each material emphasizes key advances and strategies to develop superior H 2 sensors. Furthermore, recent key advances in other types of H 2 sensors are briefly discussed. Finally, the review concludes with a brief outlook, perspective, and future directions.

Edges of 2D transition metal dichalcogenides (TMDs) are well known as highly reactive sites, thus researchers have attempted to maximize the edge site density of 2D TMDs. In this work, metal-organic framework (MOF) templates are introduced to synthesize few-layered WS 2 nanoplates (a lateral dimension of ≈10 nm) confined in Co, N-doped hollow carbon nanocages (WS 2 _Co-N-HCNCs), for highly sensitive NO 2 gas sensors. WS 2 precursors are assembled in the surface cavity of Co-based zeolite imidazole framework (ZIF-67) and subsequent pyrolysis produced WS 2 _Co-N-HCNCs. During the pyrolysis, the carbonized ZIF-67 are doped by Co and N elements, and the growth of WS 2 is effectively suppressed, creating few-layered WS 2 nanoplates functionalized Co-N-HCNCs. The WS 2 _Co-N-HCNCs exhibit outstanding NO 2 sensing characteristics at room temperature, in terms of response (48.2% to 5 ppm), selectivity, response and recovery speed, and detection limit (100 ppb). These results are attributed to the enhanced adsorption and desorption kinetics of NO 2 on abundant WS 2 edges, confined in the gas permeable HCNCs. This work opens up an efficient way for the facile synthesis of edge abundant few-layered TMDs combined with porous carbon matrix via MOF templating route, for applications relying on highly active sites.

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/SnO2 heterostructure. With the ultrahigh specific surface area (54.29 m2 g–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 SnO2 nanograins. 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 situ TEM analysis confirms excellent thermal stability and sinter resistance of the heterojunction-immobilized Pt single atoms.

Conductive porous materials having a high surface reactivity offer great promise for a broad range of applications. However, a general and scalable synthesis of such materials remains challenging. In this work, the facile synthesis of catalytic metal nanoparticles (NPs) embedded in 2D metal–organic frameworks (MOFs) is reported as highly active and conductive porous materials. After the assembly of 2D conductive MOFs (C‐MOFs), i.e., Cu3(hexahydroxytriphenylene)2 [Cu3(HHTP)2], Pd or Pt NPs are functionalized within the cavities of C‐MOFs by infiltration of metal ions and subsequent reduction. The unique structure of Cu3(HHTP)2 with a cavity size of 2 nm confines the bulk growth of metal NPs, resulting in ultra‐small (≈2 nm) and well‐dispersed metal NPs loaded in 2D C‐MOFs. The Pd or Pt NPs‐loaded Cu3(HHTP)2 exhibits remarkably improved NO2 sensing performance at room temperature due to the high reactivity of catalytic metal NPs and the high porosity of C‐MOFs. The catalytic effect of Pd and Pt NPs on NO2 sensing of Cu3(HHTP)2, in terms of reaction rate kinetics and activation energy, is demonstrated.

The colorimetric gas sensor offers an opportunity for the simple and rapid detection of toxic gaseous substances based on visually discernible changes in the color of the sensing material. In particular, the accurate detection of trace amounts of certain biomarkers in a patient's breath provides substantial clues regarding specific diseases, for example, hydrogen sulfide (H 2 S) for halitosis and ammonia (NH 3 ) for kidney disorder. However, conventional colorimetric sensors often lack the sensitivity, selectivity, detection limit, and mass-productivity, impeding their commercialization. Herein, we report an inexpensive route for the meter-scale synthesis of a colorimetric sensor based on a composite nanofiber yarn that is chemically functionalized with an ionic liquid as an effective H 2 S adsorbent and lead acetate as a colorimetric dye. As an eyereadable and weavable sensing platform, the single-strand yarn exhibits enhanced sensitivity supported by its high surface area and well-developed porosity to detect the breath biomarker (1 ppm of H 2 S). Alternatively, the yarn loaded with lead iodide dyes could reversibly detect NH 3 gas molecules in the ppm-level, demonstrating the facile extensibility. Finally, we demonstrated that the freestanding yarns could be sewn into patterned textiles for the fabrication of a wearable toxic gas alarm system with a visual output.