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Activation of metal oxides by light is a robust yet facile approach to manipulating their surface chemistry for favorable reactions with target molecules in heterogeneous catalysis and gas sensors. However, a limited understanding of interface chemistry and the involved mechanism impedes the development of a rational design of oxide interfaces for light‐activated gas sensing. Herein, the TiOx‐assisted photosensitization of In2O3 toward NO2 sensing is investigated as a case study to elucidate the detailed mechanism of light‐activated surface chemistry at the metal/gas interface. The resultant heterogeneous oxides exhibit outstanding NO2 sensing performance under light irradiation thanks to abundant photoexcited electrons and holes that serve as adsorption and desorption sites, respectively, to accelerate both surface reactions. Furthermore, the facile transfer of electrons and holes across the TiOx‐In2O3 interface contributes to improving the reversibility of sensing kinetics. Through this study, the mechanistic understanding is established of how the surface chemistry of metal oxide surfaces can be tuned by light activation providing an effective route to the design fabrication of high‐performance gas sensors. [ABSTRACT FROM AUTHOR]

The trace detection of biomarker molecules in exhaled breaths enables a facile disease diagnosis in early stages. However, the selective detection of each pathognomonic gas was difficult to achieve due to a poor understanding of the selectivity mechanism, which impeded the development of a rational design for practical diagnostics. Herein, we report a systematic study of the explicit roles of noble metal catalysts in modulating the gas selectivity of metal oxides and provide an in-depth mechanistic understanding of the reaction pathways specific to each element. Using a consistent synthetic platform to prepare highly dispersed catalysts on In2O3 nanofibers, we identified that Ru and Pd catalysts showed exceptional selectivity to CH3SH and H2S, respectively, while Pt selectively interacted with CH3COCH3 molecules. Furthermore, we revealed that the element-specific gas adsorption and the ensuing charge transfer form the basis for regulating selective surface reactions of target gases. Altogether, we propose a set of design principles for sensing layers based on the prominent, element-dependent roles of catalysts toward the development of a rational synthesis of sensing materials with optimal sensing performance. These principles can be summarized as follows: controlled chemisorption of oxygen species, selective gas adsorption, and subsequent activation of adsorbed gases. [ABSTRACT FROM AUTHOR]

It is well known that certain gas molecules can serve as specific markers for characterizing the physiological and environmental nature of a subject, such as acetylene gas which can be used for determining a person's smoking status as well as for monitoring air pollution levels. For their reliable detection, gas sensors should be designed with high sensing capabilities in terms of selectivity, sensitivity and low detection limit. In this work, we present a rational design approach for the synthesis of p-type LaFeO3/n-type SnO2 composite nanotubes (NTs) via galvanic replacement reaction (GRR) on electrospun perovskite LaFeO3 NTs for p–n type-converted sensing. The GRR process provides LaFeO3/SnO2 NTs with high surface area (146.6 m2 g−1) by generating SnO2 nanograins (<10 nm), as controlled by varying the reaction time. The abundant heterogeneous p–n junctions formed in LaFeO3/SnO2 NTs contribute to the dramatic improvements in their sensing characteristics. In fact, the LaFeO3/SnO2 NTs exhibited 31.2-fold increased response toward acetylene (5 ppm) with a far improved response speed (16 s) compared to that of pristine LaFeO3 NTs (64 s). Our results demonstrate that the GRR process can be used to engineer the morphology and composition of p-type perovskites to achieve exceptional chemical sensing performances. [ABSTRACT FROM AUTHOR]

Single‐atom catalysts (SACs) supported on inorganic materials have attracted much attention in numerous research fields for their high catalytic performance. However, such SACs have been limited by the low metal loading, especially on different types of inorganic supports. Herein, a general approach is presented for preparing SACs on metallic, metal oxide, and perovskite nanosheet (NS) supports to reach a high metal loading of up to 3.94 wt%, by utilizing N‐doped graphene as a sacrificial template that spatially confines the single atoms (SAs). Specifically, the target support material precursors are adsorbed onto the SAs‐stabilized sacrificial template, followed by subsequent heat treatment to transfer the SAs to the support material and to remove the graphene layer. Pt SAs on oxide support exhibits little to no aggregation throughout >10 000 min of annealing at 275 °C, demonstrating high thermal stability, as also supported by ex situ post‐anneal electron microscopy and X‐ray absorption fine structure studies. As a proof‐of‐concept, Pt SACs on SnO2 NSs exhibit high catalytic activity toward chemiresistive sensing of acetone gas (response = 95.4 at 10 ppm, 7.6‐fold enhancement compared with pristine SnO2 NSs) and unprecedented stability under highly humid conditions (27.4% response deterioration at 95% relative humidity). [ABSTRACT FROM AUTHOR]

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-CeO 2 ) catalysts embedded in a SnO 2 matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co-CeO 2 catalyst nanoparticles integrated with SnO 2 nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co-CeO 2 @SnO 2 NFs exhibited superior gas sensing characteristics toward isoprene (C 5 H 8 ) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co-CeO 2 catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO 2 sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C 5 H 8 (compared to pristine SnO 2 ), 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-CeO 2 catalysts in the SnO 2 matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co-CeO 2 , 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.

Light-activated chemiresistors offer a powerful approach to achieving lower-temperature gas sensing with unprecedented sensitivities. However, an incomplete understanding of how photoexcited charge carriers enhance sensitivity obstructs the rational design of high-performance sensors, impeding the practical utilization under commonly accessible light sources instead of ultraviolet or higher-energy sources. Here, a rational approach is presented to modulate the electronic properties of the parent metal oxide phase, exemplified by this model system of Bi-doped In 2 O 3 nanofibers decorated with Au nanoparticles (NPs) that exhibit superior NO 2 sensing performance. Bi doping introduces mid-gap energy levels into In 2 O 3 , promoting photoactivation even under visible blue light. Additionally, green-absorbing plasmonic Au NPs facilitate electron transfer across the heterojunction, extending the photoactive region toward the green light. It is revealed that the direct involvement of photogenerated charge carriers in gas adsorption and desorption processes is pivotal for enhancing gas sensing performance. Owing to the synergistic interplay between the Bi dopants and the Au NPs, the Au-Bi x In 2-x O 3 (x = 0.04) sensing layers attain impressive response values (R g /R a  = 104 at 0.6 ppm NO 2 ) under green light illumination and demonstrate practical viability through evaluation under simulated mixed-light conditions, all of which significantly outperforms previously reported visible light-activated NO 2 sensors., (© 2024 Wiley‐VCH GmbH.)

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 MoS 2 , followed by freeze-drying is utilized to successfully fabricate a 3D MoS 2 /MXene van der Waals heterostructure aerogel. The low-temperature synthetic approach effectively suppresses significant oxidation of the Ti 3 C 2 T x MXene and results in a hierarchical and freestanding 3D heterostructure composed of high-quality MoS 2 and MXene nanosheets. Functionalization of MXene with a MoS 2 catalytic layer substantially improves sensitivity and long-term stability toward detection of NO 2 gas, and computational studies are coupled with experimental results to elucidate that the mechanism behind enhancements in the gas-sensing properties is effective inhibition of HNO 2 formation on the MXene surface, due to the presence of MoS 2 . 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.

Metal-organic frameworks (MOFs) have emerged as attractive chemical sensing materials due to their exceptionally high porosity and chemical diversity. Nevertheless, the utilization of MOFs in chemiresistive type sensors has been hindered by their inherent limitation in electrical conductivity. The recent emergence of two-dimensional conductive MOFs (2D c-MOFs) has addressed this limitation by offering enhanced electrical conductivity, while still retaining the advantageous properties of MOFs. In particular, c-MOFs have shown promising advantages for the fabrication of sensors capable of operating at room temperature. Thus, active research on gas sensors utilizing c-MOFs is currently underway, focusing on enhancing sensitivity and selectivity. To comprehend the potential of MOFs as chemiresistive sensors for future applications, it is crucial to understand not only the fundamental properties of conductive MOFs but also the state-of-the-art works that contribute to improving their performance. This comprehensive review delves into the distinctive characteristics of 2D c-MOFs as a new class of chemiresistors, providing in-depth insights into their unique sensing properties. Furthermore, we discuss the proposed sensing mechanisms associated with 2D c-MOFs and provide a concise summary of the strategies employed to enhance the sensing performance of 2D c-MOFs. These strategies encompass a range of approaches, including the design of metal nodes and linkers, morphology control, and the synergistic use of composite materials. In addition, the review thoroughly explores the prospects of 2D c-MOFs as chemiresistors and elucidates their remarkable potential for further advancements. The insights presented in this review shed light on future directions and offer valuable opportunities in the chemical sensing research field., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

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/SnO 2 heterostructure. With the ultrahigh specific surface area (54.29 m 2 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 SnO 2 . X-ray absorption fine structure analysis and HAADF-STEM analysis reveal a homogeneous atomic dispersion of Pt species between carbon nitride and SnO 2 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.