Functional Nanomaterial-Based Gas Sensors and Humidity Sensors

Gas and humidity (water molecules) are important components of the environment and human respiration, which are closely related to human life and production. For example, the detection of toxic and flammable gases is of great significance for protecting the environment, ensuring safe production and human health. In addition, with the development of modern medical technology, the non-invasive early diagnosis of diseases can be achieved by analyzing the composition of exhaled gases [1,2,3], and respiratory behavior can be monitored by utilizing the high-humidity characteristics of exhaled gas [4,5,6]. It is widely agreed that the detection of gases and humidity relies on advanced gas and humidity sensors, most of which rely on gas molecules or water molecules that adsorb on the surface of sensitive functional materials, converting them into easy-to-process signals (such as resistance, capacitance, voltage, and frequency). Combining the advantages of the small size effects and rich functionality of nanomaterials, the design and development of sensitive functional nanomaterials are key to constructing high-performance gas and humidity sensors. As a result, in recent years various functional nanomaterials have blossomed and been used to develop high-performance gas and humidity sensors, such as Pd, Pd-Ni, and Pd-Mg nanofilms for H₂ sensors [7,8,9]; MoS₂/ZnO, GaN/reduced graphene oxide (rGO), polyaniline@MoS₂, and Co₃O₄/SnO₂ nanomaterials for NH₃ sensors [10,11,12,13]; Ag₂Te, GaSe_0.31O_0.69, In₂O₃/polypyrrole, CuO-ZnO, and MoSe₂-WS₂ nanomaterials for NO₂ sensors [14,15,16,17,18]; Pd@Ni/ZnO, Cd-doped In₂O₃, ZnO/g-C₃N₄, and Pt-Pd/SnO₂ for CH₄ sensors [19,20,21,22]; Au/SnO₂, Fe₂O₃/Ti₃C₂, Cu₂O/ZnO, and Ru@SnO₂ nanomaterials for H₂S sensors [23,24,25,26]; Pr-doped SnO₂, Fe₂O₃-MoS₂, Ti₃C₂T_x/LaFeO₃, and NiO/NdNiO₃ nanomaterials for SO₂ sensors [27,28,29,30]; ZnO/WSe₂, MoO₃/Ti₃C₂T_x, ZnO/Co₃O₄, and In₂O₃/ZnO/Ti₃C₂T_x nanomaterials for ethanol sensors [31,32,33,34]; WO₃/WS₂, MIL-88B@3-aminopropyltrimethoxysilane, Pt-modified BiVO₄, and SnO₂-ZnO nanomaterials for acetone sensors [35,36,37,38]; In₂O₃@TiO₂, Au/In₂O₃, SnO₂-Sn₃O₄, and Ag-modified ZnSnO₃ nanomaterials for formaldehyde sensors [39,40,41,42]; LaCo_0.1Fe_0.9O₃, NiO-In₂O₃, Au-ZnO, and p-Si/MoO₃ nanomaterials for CO₂ sensors [43,44,45,46]; and Bi₂O₂S, Ti₃C₂T_x/WS₂, Ti₃C₂T_x/single-walled carbon nanotubes, MoBT_x, V₂O₅, and KCl/carbon black/halloysite nanotubes for humidity sensors [47,48,49,50,51,52]. As mentioned above, precious metals and oxide materials have been used in the field of gas sensors for many years but still receive much attention due to the continuous development of nanomaterial technology. In addition, emerging two-dimensional materials, such as MXene, have received widespread attention in the field of gas and humidity sensors.

In this context, this Special Issue aims to showcase recent studies on gas and humidity sensors, including 12 research articles and 2 review articles. Among them, Sheng et al. fabricated a Sn-doped In₂O₃-based gas sensor using magnetron sputtering and rapid annealing processes. The results demonstrated that the Sn-doped In₂O₃ gas sensor had a good selective response to ethanol; the reason for this selectivity is explained in combination with density functional theory [53]. Usually, researchers mainly focus on the gas sensing characteristics of gas sensors at room temperature (~25 °C) and high temperatures. Petrushenko et al. reported a CuI gas sensor using a successive ionic layer adsorption and reaction method, achieving good NH₃ sensing responses at low temperatures (5 °C) [54]. The CuI gas sensor based on a polyethylene terephthalate substrate has demonstrated good flexibility and humidity resistance and is expected to be used for respiratory NH₃ detection. Guo et al. reported a In₂O₃/Ti₃C₂T_x gas sensor using an electrostatic self-assembly method, achieving high-selectivity NO₂ detection at room temperature [55]. In addition to resistive gas sensors based on gas sensing functional nanomaterials, Prof. Zhang’s group proposed a field effect transistor-type gas sensor using carbon nanotubes as a channel material and ZnO/WS₂ as gate sensing materials, achieving ppb-level benzene detection at room temperature [56]. In terms of humidity sensors, Laera et al. reported a flexible humidity sensor based on chemically rGO [57]. Zhang et al. reported a stable and fast-response humidity sensor based on a polyanionic liquid containing bromide ions, demonstrating multifunctional applications related to respiratory rates, speech recognition, and non-contact switches [58]. In addition, Prof. Shi’s group discusses the recent advances in metal oxide semiconductor heterojunctions for the detection of volatile organic compounds [59]. Regarding the gas sensing mechanism of chemiresistive gas sensors, Prof. Ma’s group provide a comprehensive overview of the modeling approaches for chemiresistive gas sensors [60].

I would like to express my sincere gratitude to all the authors for their significant contributions to this Special Issue, as well as to Ms. Polaris Zhang (Contact Editor) and all the reviewers for their hard work.