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Advancements in Nanostructured Functional Constituent Materials for Gas Sensing Applications: A Comprehensive Review

Ivana Panžić

^1,*

Arijeta Bafti

Floren Radovanović-Perić

¹,

Davor Gašparić

¹,

Zhen Shi

Arie Borenstein

³ and

Vilko Mandić

^1,*

University of Zagreb Faculty of Chemical Engineering and Technology, Trg Marka Marulića 19, 10000 Zagreb, Croatia

Institute of Advanced Magnetic Materials, College of Materials and Environmental Engineering, Hangzhou Dianzi University, Xiasha Higher Education Zone, Hangzhou 310012, China

Chemistry Department, Ariel University, Ramat HaGolan St 65, Ariel 40700, Israel

Authors to whom correspondence should be addressed.

Appl. Sci. 2025, 15(5), 2522; https://doi.org/10.3390/app15052522

Submission received: 21 January 2025 / Revised: 13 February 2025 / Accepted: 19 February 2025 / Published: 26 February 2025

(This article belongs to the Special Issue Novel Nanomaterials in Gas Sensors)

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Abstract

The unique properties of nanostructures, such as their high surface-to-volume ratio, tunable physical and chemical characteristics, and enhanced sensitivity, are critical for advancing gas detection technologies. Therefore, this comprehensive review explores the recent advancements in nanostructured materials, emphasizing their pivotal role in enhancing gas sensing performance. A key focus of this review is metal oxide-based gas sensors, and the impact of synthesis methods and (micro)structural properties on sensor performance is thoroughly examined. By segmenting the discussion into 1D nanostructured materials, including different metal oxides, the review provides a broad yet detailed perspective on how different functional materials contribute to gas sensing efficiency. From a performance standpoint, this review highlights critical sensing parameters, including gas detection mechanisms, response times, selectivity, stability, and operating conditions. High-end detection values may reach around a few ppb for most gases. Beyond evaluating current advancements, this review also addresses existing challenges and future research directions, particularly in scalability, long-term sensor stability, low-temperature operation, and integration into real-world applications. By providing a comprehensive and up-to-date analysis, this review serves as a valuable resource for researchers and engineers, offering insights that can drive the next generation of high-performance, reliable, and selective gas sensors.

Keywords:

solid-state gas sensors; thin films; nanostructured nanomaterials; metal oxides; composites

1. Background of Nanostructured Materials for Gas Sensors

1.1. Nanostructured Metal Oxides

Metal oxide semiconductors are emerging as promising materials for gas sensing applications due to their high sensitivity to a variety of gases, ease of fabrication, low cost, and excellent compatibility with other components and processes [1,2,3,4]. Over the years, nanostructures of ZnO, SnO₂, TiO₂, In₂O₃, WO₃, CuO, CdO, Fe₂O₃, and MoO₃ have been engineered in various dimensions and sensor designs. The surface properties and morphology of these metal oxides are critical to their gas sensing efficiency [5]. Depending on the desired application and available fabrication techniques, different surface morphologies and configurations, such as single crystals, thin films, thick films, and one-dimensional (1D) nanostructures, have been realized [6]. Among these, 1D nanostructures have garnered significant attention for gas sensor development due to their high surface-to-volume ratio and excellent chemical and thermal stability across a range of operating conditions [7,8,9]. Due to their high sensitivity, fast response times, and durability, metal oxide-based gas sensors have found widespread applications in environmental monitoring, industrial safety, healthcare, and smart technology. In air quality monitoring, sensors for gases like NO₂, CO, and VOCs are integrated into urban air pollution monitoring networks, providing real-time data to regulate emissions and improve public health. In industrial safety, metal oxide sensors play a crucial role in detecting toxic or explosive gases such as H₂S, CO, and CH₄ in chemical plants, mines, and refineries, helping to prevent hazardous leaks. In the healthcare sector, breath analysis sensors detect biomarkers such as acetone (for diabetes monitoring) and ammonia (for liver function assessment), enabling non-invasive medical diagnostics. Additionally, automotive and smart home technologies leverage these sensors for CO detection in vehicles, air purification systems, and gas leak detection in residential and commercial settings. As advancements continue, the integration of metal oxide gas sensors with Internet of Things (IoT) platforms and wireless sensor networks is further expanding their real-world applicability, enabling continuous, remote, and energy-efficient gas monitoring solutions.

This review provides an in-depth overview of recent advances in the fabrication of metal oxide gas sensors based on nanostructured functional constituents. It covers various fabrication strategies, particularly those related to nanostructuring, and discusses their impact on sensing performance. This review delves into innovative techniques such as atomic layer deposition, sol–gel processes, and hydrothermal synthesis, highlighting their ability to create uniform and high-surface-area nanostructures that enhance sensor response times and sensitivity. This review will try to provide a comprehensive insight into the plethora of possible metal oxide and mixed metal oxide gas sensors, hopefully offering studies in the field a good starting point from which they can build their own research paths. The key goal is promoting better long-term stability, selectivity, and sensitivity of gas sensors.

The key findings from the current research are summarized, demonstrating how the incorporation of materials such as graphene, carbon nanotubes, and other conductive nanocomposites can improve the selectivity and stability of gas sensors. This review also discusses challenges related to material integration, scalability, and power consumption, emphasizing the need for environmentally friendly and cost-effective fabrication methods. Potential future developments in the field of nanostructured metal oxide gas sensors are proposed, focusing on hybrid material systems, the use of machine learning algorithms for enhanced signal processing, and the integration of sensors into flexible and wearable technologies. These insights provide a comprehensive outlook on the trajectory of next-generation gas sensing devices.

1.2. Fabrication Techniques for 1D Nanostructures

The development of techniques for fabricating one-dimensional (1D) nanostructures has been a central focus in nanotechnology and nanoscience [10]. Numerous methods have been explored to create 1D metal oxide nanostructures for gas sensing applications. These include conceptually different approaches, that is, bottom–up techniques and top–down techniques: hydrothermal synthesis [11], ultrasonic irradiation [12], electrospinning [13], anodization [14], sol–gel processing [15], molten-salt methods [16], carbothermal reduction [17], solid-state chemical reactions [18], thermal evaporation [19], vapor-phase transport [20], aerosol techniques [21], RF sputtering [22], molecular beam epitaxy [23], chemical vapor deposition (CVD) [24], nanocarving [25], UV lithography, and dry plasma etching [26].

Each of the mentioned fabrication routes and subsequent treatments results in distinct nanostructures with varying surface morphologies. Some examples include nanorods [5,7], nanotubes [14], nanowires [17], nanofibers [13], nanobelts [22], nanoribbons [27], nanowhiskers [28], nanoneedles [29], nanopushpins [30], fiber mats [21], urchin-like structures [31], and lamellar or dendritic architectures [32]. These diverse morphologies have exhibited different levels of success in detecting various reducing and oxidizing gases, such as H₂, H₂S, NH₃, CO, NO₂, O₂, liquefied petroleum gas (LPG), ethanol, methanol, xylene, propane, toluene, acetone, and triethylamine.

1.3. Configuration of Gas Sensors and Performance Indicators

When a gas comes into contact with a sensor, the molecules adsorb onto the metal oxide surface, either donating or withdrawing electrons depending on whether the gas is reducing or oxidizing. Reducing gases, like hydrogen (H₂) or carbon monoxide (CO), donate electrons, lowering the sensor’s resistance, while oxidizing gases, such as oxygen (O₂) or nitrogen dioxide (NO₂), withdraw electrons, increasing the resistance. This change in resistance, which can be measured and correlated with gas concentration, forms the basis of the sensor’s functionality [33].

The main performance parameters of metal oxide-based gas sensors are sensitivity, selectivity, response time, recovery time, operating temperature, stability, repeatability, detection limit, linearity, and power consumption. Sensitivity refers to the sensor’s ability to detect low concentrations of gas, while selectivity reflects its ability to distinguish between different gases. Response time measures how quickly the sensor reacts to gas exposure, and recovery time describes how fast it returns to baseline conditions once the gas is removed. Most sensors operate at elevated temperatures to accelerate reactions between the gas and the metal oxide surface, enhancing sensitivity and response time. Stability and repeatability are critical for consistent performance over time, ensuring the sensor provides reliable data without significant drift across multiple measurements. Detection limit refers to the lowest concentration of gas the sensor can detect, and linearity describes how the sensor’s output signal relates to gas concentration, with a more linear response being preferable for easier data interpretation. Power consumption is another key factor, especially for portable or battery-powered devices, as sensors that operate at higher temperatures often require more energy.

Enhancing the gas-sensing capabilities of these materials often involves modifying both surface properties and bulk characteristics. Such modifications can be achieved through techniques like nanoparticle deposition, surface coatings, or doping with foreign elements. These adjustments typically result in improved sensitivity compared to unmodified 1D nanostructures [34].

2. Mechanisms Suitable for Gas Sensing of Nanostructured Materials

Chemiresistive gas sensors operate based on the principle that exposure to target gas molecules induces changes in the electrical resistance of the sensing material, typically a semiconductor or metal oxide [35]. This process is influenced by the adsorption and desorption of gas molecules on the sensor surface, which modulates the concentration, mobility, and spatial distribution of charge carriers within the material. The mechanism can be broadly understood as a surface-mediated process, where interactions between the sensor surface and gas molecules directly affect the electron or hole density, causing measurable variations in resistivity (Figure 1 and Figure 2) [36,37].

When the sensing material is exposed to a gas, the molecules are adsorbed onto the surface, often in a manner governed by the Langmuir adsorption isotherm, which describes how the surface coverage of adsorbed molecules depends on the partial pressure of the gas and the affinity of the adsorbate to the surface. In this context, the Langmuir isotherm provides insight into the equilibrium adsorption state, which is particularly relevant for understanding sensor response at varying concentrationsThe adsorption process can lead to charge transfer between the adsorbed molecules and the sensing material, resulting in a depletion or accumulation of charge carriers in the sensor’s space-charge region. For instance, in n-type metal oxides like ZnO and SnO₂, reducing gases (such as CO) tend to donate electrons, increasing the charge carrier concentration, while oxidizing gases (such as NO₂) withdraw electrons, creating a depletion layer that reduces conductivity (Figure 3). This balance between adsorption and desorption is essential for establishing a steady-state response, as well as for sensor recovery.

Figure 2. The mechanism of free charge flow and changes in the resistance of MOS gas sensors depending on the type of sensor material and analyte (reproduced from [38] MDPI, 2023).

The space-charge layer model further explains the impact of gas adsorption on the electronic structure of the sensing material. In this model, adsorbed molecules influence the distribution of charge carriers by altering the surface potential, which extends into the bulk in the form of a space-charge region. This phenomenon is particularly pronounced in materials with high surface-to-volume ratios, such as nanostructured oxides, where surface effects strongly dominate over bulk properties. The width of the space-charge layer and the position of the Fermi level shift dynamically in response to surface interactions, modulated by factors such as gas concentration, temperature, and humidity. As a result, the sensor’s resistive behavior can vary significantly, influenced by the extent of band bending, and carrier modulation in the space-charge layer is another critical parameter that affects gas sensor performance. Elevated temperatures can enhance the adsorption and desorption kinetics, allowing sensors to achieve faster response and recovery times. However, excessive temperatures can also lead to an unwanted desorption of gas molecules, reducing sensitivity. For example, metal oxide-based chemiresistive sensors typically operate in the range of 200–400 °C, where sufficient thermal energy allows for optimal reaction kinetics without compromising stability [37]. Furthermore, temperature affects the energy barrier at the surface, altering the likelihood of charge transfer interactions with adsorbed molecules. The crystalline structure, morphology, and dopant types play an essential role in dictating sensor response and selectivity. Nanostructures, such as nanowires, nanosheets, and nanoparticles, with high surface-to-volume ratios provide more active sites for adsorption, potentially enhancing sensitivity. Additionally, doping or compositional modifications can tailor the band structure to favor interaction with specific gas molecules, improving selectivity. For example, introducing catalytic additives like Pd or Pt can enhance sensor response to particular gases by facilitating specific surface reactions [39]. By understanding how factors such as adsorption dynamics, surface potential, and material properties interact, researchers can develop sensors with improved sensitivity, selectivity, and response characteristics, thus advancing applications in environmental monitoring, industrial safety, and healthcare diagnostics.

Figure 3. Schematic representation of the surface of n-type MOS with ionosorbed oxygen species (b) and after its interaction with reducing gas CO (c) and oxidizing gas NO₂ (a) within the chemisorption model of sensor response (top). The corresponding modulation of band energy levels (bottom) (reproduced from [40] MDPI, 2021).

3. Segmentation of Nanostructured Materials for Gas Sensing

Gas sensors benefit significantly from nanostructuring due to the increased surface area and enhanced reactivity that nanomaterials provide. By reducing the size of the sensing materials to the nanoscale, a larger proportion of atoms are positioned at the surface, which improves the interaction between the gas molecules and the sensor material. This heightened surface-to-volume ratio facilitates more efficient gas adsorption and reaction kinetics, leading to faster response times and improved sensitivity (Figure 4).

For instance, nanostructured materials such as nanoparticles, nanowires, and nanotubes can capture gas molecules more effectively, resulting in a stronger sensor response. Moreover, nanostructured materials often exhibit unique electrical, optical, and catalytic properties that can be fine-tuned for specific gas detection applications. This tunability allows for the optimization of sensor performance, making it possible to selectively detect a wide range of gases at low concentrations. The incorporation of nanostructures can also contribute to enhanced stability and durability, which are critical for real-world applications. As a result, the integration of nanostructured materials into gas sensor designs not only improves their sensitivity and selectivity, but also paves the way for the development of more effective and reliable sensing devices across various industries, including environmental monitoring, industrial safety, and healthcare.

3.1. Zinc Oxide (ZnO)

Zinc oxide (ZnO) has emerged as a highly promising material for gas sensing applications, largely due to its intrinsic semiconducting properties, wide bandgap of 3.37 eV, and high exciton binding energy of approximately 60 meV, which contribute to its stability and versatility in various sensing environments [41]. ZnO is particularly sensitive to changes in ambient oxygen levels and responds well to a broad range of gases, including hydrogen, methane, ammonia, nitrogen oxides, and volatile organic compounds (VOCs). Its sensitivity and selectivity to specific gases can be tuned by manipulating factors such as dopant incorporation, morphology, and operating temperature, allowing ZnO-based sensors to be tailored to diverse applications in environmental monitoring, industrial safety, and medical diagnostics. The active sites in ZnO-based gas sensors are typically located at the surface of the material. These sites are responsible for the adsorption and interaction with gas molecules. Modifications to ZnO influence the nature and number of active sites. Surface defects, such as oxygen vacancies, act as active sites for gas adsorption. The increased concentration of oxygen vacancies leads to a greater number of active sites, enhancing sensitivity. The type of defect also influences the interaction with specific gas molecules, affecting selectivity. The introduction of specific dopants or surface functional groups can enhance the adsorption of the target gas, improving selectivity. For instance, Ni-doped ZnO nanowire arrays show high selectivity for H₂S [42], while Na-doped ZnO nanoneedles exhibit enhanced selectivity for specific VOCs [43]. The use of different transition metal dichalcogenides in heterojunctions with ZnO [44] can further alter selectivity. Doping ZnO with elements such as aluminum (Al), gallium (Ga), and indium (In) has proven effective in modifying the electronic structure and enhancing its gas sensing properties. Al-doped ZnO, for example, has shown increased conductivity and improved response times, particularly for reducing gases like hydrogen and hydrocarbons. Ga and In doping have similar effects, with In-doped ZnO exhibiting superior sensitivity to ammonia and nitrogen oxides by adjusting the electronic states at the surface, which enhances adsorption properties and can lead to a 62% response to 0.1% CH₄, which is 4.7 times higher than undoped ZnO [39,40]. The dopants act by creating additional oxygen vacancies or by modifying the carrier density, which directly influences the sensor’s ability to detect specific gas molecules. This process increases the material’s reactivity and helps stabilize the response in real-world conditions where gas concentrations may vary widely. Nanostructuring ZnO into forms like nanowires, nanoparticles, and thin films has been another critical strategy for improving gas sensing performance, since gas sensors based on ZnO nanostructures show a high sensitivity, reversible response, and good selectivity towards HCHO under UV light. Nanoscale ZnO structures, with their high surface-to-volume ratio, offer more active sites for gas molecule adsorption and exhibit unique facet-dependent properties. For instance, ZnO nanowires with exposed polar (001) facets have been shown to exhibit enhanced sensitivity to VOCs, largely due to the increased electron density and surface oxygen availability on these facets, which promote gas–surface interactions [45]. Nanoparticles provide similar advantages, as their size and shape can be engineered to maximize adsorption sites and facilitate faster response and recovery times. Thin films, particularly when synthesized with specific crystallographic orientations, also show improved gas sensing performance, as they provide a stable platform with enhanced carrier mobility, contributing to consistent sensor response across different operating conditions [46,47]. Thin-film sensing platforms based on T-ZnO exhibited the highest response at 200 °C (RM = 2.98; R = 66.4%) compared to ZnO-NR thin films at 230 °C (RM = 1.34; R = 25.5%), attributed to the interconnected network and effective bandgap and barrier height reduction in T-ZnO [45]. Operating temperature is another critical parameter that affects the gas sensing properties of ZnO. Higher temperatures can activate additional adsorption sites and accelerate the reaction kinetics on the ZnO surface, enhancing sensitivity and selectivity to certain gases. However, excessive temperatures may also lead to the desorption of target gas molecules, impacting sensor stability and limiting the temperature range suitable for specific applications. For instance, ZnO-based sensors typically perform optimally at temperatures between 200 and 300 °C, where gas molecule adsorption and desorption are balanced to maintain steady signal output [45]. A synergetic effect of decreasing grain size and increasing operating temperature was observed towards the improvement of sensitivity, reaching a value of 54 and a limit of detection as low as 0.61 ppm. The decrease in grain size resulted in a prolonged response time but faster recovery. In any case, both response time and recovery time were <400 s. The results demonstrate that room-temperature magnetron sputtering is a viable approach to enhance the performance of ZnO films in sensors for ethanol vapor [48].

Research into ZnO composite materials, such as ZnO loaded with noble metal catalysts (e.g., Pt, Pd, or Au), has also yielded promising results. These catalytic metals can create localized hot spots on the ZnO surface, increasing the sensor’s sensitivity and reducing the response time. For example, Pd-loaded ZnO sensors exhibit an enhanced response to hydrogen due to the spillover effect, where hydrogen dissociates on Pd sites and migrates to the ZnO surface, allowing for more effective charge transfer [49]. These composite materials extend the capabilities of ZnO-based sensors by introducing additional mechanisms for gas interaction and detection, facilitating the selective and rapid detection of target gases, even at low concentrations. ZnO’s unique properties, combined with advances in doping, nanostructuring, and composite formation, make it a versatile platform for developing high-performance gas sensors. The continued exploration of these modifications will likely lead to ZnO-based sensors with even greater sensitivity, stability, and selectivity, suitable for a broad range of applications (Figure 5).

3.2. Titanium Oxide (TiO₂)

Titanium dioxide (TiO₂) has garnered significant attention as a gas sensing material due to its notable properties, including strong oxidizing power, high chemical stability, non-toxicity, and excellent optical properties—particularly its high refractive index and UV absorption capabilities [51]. TiO₂-based gas sensors are well suited for detecting volatile organic compounds (VOCs), nitrogen oxides (NO_x), hydrogen sulfide (H₂S), and other harmful gases. The semiconducting nature of TiO₂, with a bandgap of approximately 3.2 eV for anatase and 3.0 eV for rutile, allows for efficient charge transfer upon interaction with target gas molecules, especially under UV activation, enhancing sensitivity and response speed. The nanostructuring of TiO₂ into morphologies such as nanotubes, nanorods, and nanosheets has proven especially effective in enhancing gas sensing performance. These nanostructures provide a high surface-to-volume ratio, increasing the number of active sites for gas adsorption, which is crucial for detecting gases at low concentrations. For instance, TiO₂ nanotubes have shown increased sensitivity to VOCs and NO_x due to their hollow structure, which promotes gas diffusion and increases the interaction area. SnO₂-coated TiO₂ nanotube layers attained a higher sensing response than a reference SnO₂ sensor. Specifically, an 8 nm SnO₂-coated TiO₂ nanotube layer recorded up to ten-fold enhancement in response compared to blank nanotubes for the detection of 1 ppm NO₂ at an operating temperature of 300 °C with 0.5 V applied bias. This is attributed to the SnO₂/TiO₂ heterojunction effect [52].

Nanorods, on the other hand, facilitate faster response and recovery times due to their one-dimensional structure, which provides a direct pathway for charge carrier transport, minimizing grain boundary effects and enhancing conductivity [52,53]. Nanosheets, with their large surface area and exposed reactive facets, further improve TiO₂’s ability to interact with gas molecules, making them particularly effective for applications requiring rapid detection and high sensitivity. Doping TiO₂ with elements such as nitrogen (N) and platinum (Pt) has resulted in substantial improvements in its gas sensing capabilities, particularly in terms of sensitivity and response time. Nitrogen doping introduces mid-gap states that reduce the bandgap energy, allowing TiO₂ to be activated under visible light or low UV intensity. This enhances its gas sensing performance in ambient conditions, where UV light sources are either weak or absent. N-doped TiO₂ has shown increased sensitivity to NO_x gases and VOCs, as nitrogen atoms create additional adsorption sites and facilitate charge transfer upon interaction with target gases [54,55,56]. It was revealed that a s-TiO₂ sensor without additional N₂ treatment showed superior H₂-sensing performance than a N₂-treated s-TiO₂ sensor when operated in a N₂ atmosphere. In addition, the response to N₂-diluted H₂ was larger than that to air-diluted H₂, and the interference from water vapor was negligible in N₂ atmosphere. The inclusion of noble metals, such as Pt, further improves the response and recovery characteristics of TiO₂ gas sensors. Pt doping enhances the catalytic activity on the TiO₂ surface by providing active sites for gas dissociation, particularly under UV or visible light activation. For example, Pt-modified TiO₂ sensors exhibit a strong response to hydrogen sulfide (H₂S), where Pt acts as a catalyst, promoting H₂S oxidation on the TiO₂ surface and enhancing electron transfer, resulting in a pronounced change in conductivity. This catalytic effect not only improves sensitivity, but also accelerates the recovery process, making the sensor suitable for environments with fluctuating gas concentrations [57] (Figure 6).

UV activation is another essential aspect of TiO₂-based sensors, as it significantly boosts sensor performance by generating electron–hole pairs that participate in surface reactions. Upon UV illumination, TiO₂ produces reactive oxygen species (ROS), such as superoxide ions (O₂⁻) and hydroxyl radicals (•OH), which interact readily with gas molecules. For example, in the presence of VOCs, these ROS facilitate the oxidation of gas molecules, enhancing the depletion or accumulation of charge carriers in the TiO₂ surface layer, which translates to a detectable change in resistance. The efficiency of UV activation makes TiO₂-based sensors highly effective in low-temperature environments, extending their applicability to settings where thermal activation may not be feasible [58]. Moreover, composite materials incorporating TiO₂ with other oxides, such as SnO₂, ZnO, or WO₃, have shown enhanced performance due to synergistic effects. For instance, TiO₂-SnO₂ composites leverage the high surface area and fast electron mobility of SnO₂ while maintaining TiO₂’s stability and photoactivation capabilities. These composites have demonstrated excellent response to NO_x and H₂S gases, as the heterojunctions formed at the interface improve charge separation and enhance the overall sensing response [59,60,61]. TiO₂-nanostructure-based gas sensors demonstrate a remarkable sensitivity to acetone, up to 1.88 at a detection limit of 5 ppm at room temperature and 180.08 at a detection limit of 200 ppm at an optimal operating temperature of 65 °C. Meanwhile, the device exhibits long-term stability, good cycle-to-cycle repeatability, and selectivity to trace acetone at room temperature. Moreover, benefiting from the 3D penetrating porous structure, the average response time of the device is only about 1.8 s [62,63].

In summary, the intrinsic properties of TiO₂, combined with advances in nanostructuring, doping, and composite formation, make it a versatile and effective material for gas sensing. The continued development of TiO₂-based sensors is expected to yield devices with even higher sensitivity, faster response and recovery times, and broader selectivity, suitable for applications across environmental monitoring, industrial safety, and healthcare.

3.3. Copper Oxide (CuO)

Copper oxide (CuO) has gained significant attention as a gas sensing material due to its relatively narrow bandgap of approximately 1.2 eV, which allows for the sensitive detection of reducing gases, such as hydrogen (H₂), carbon monoxide (CO), and methane (CH₄). CuO’s p-type semiconducting behavior plays a critical role in its gas sensing properties, as exposure to reducing gases reduces the number of holes (the primary charge carriers) in the material, resulting in an increase in resistance that can be measured. This p-type behavior, combined with its narrow bandgap, enables CuO to operate effectively at lower temperatures than many n-type oxide sensors, making it especially valuable in applications where low-power operation and sensitivity to hazardous gases are required [64]. The nanostructuring of CuO into forms like nanoparticles, nanowires, and thin films has led to substantial improvements in sensitivity and response times. CuO nanoparticles, due to their high surface-to-volume ratio, provide a large number of active sites for gas adsorption, facilitating quicker and more effective interactions with target gases. Nanowires offer a unique advantage in charge transport, as their one-dimensional structure supports efficient electron transfer, leading to improved sensitivity and reduced response and recovery times.

Thin films, which can be precisely engineered to control thickness and crystallographic orientation, exhibit stable response characteristics and are ideal for applications requiring long-term stability and repeatability [65,66]. Doping CuO with elements such as zinc (Zn) or tin (Sn) has further enhanced its selectivity and sensitivity to specific hazardous gases. Zn doping, for example, has been shown to improve CuO’s sensitivity to carbon monoxide by increasing the density of active sites and modifying the electronic structure to favor CO adsorption. Tin-doped CuO has demonstrated improved selectivity and response to hydrogen and ammonia, which are critical in industrial monitoring and safety applications. These dopants function by altering the carrier density and oxygen vacancy concentration in the CuO lattice, which enhances the charge transfer process and creates preferential adsorption sites for target gases [64,67]. Additionally, the presence of dopants can adjust the response temperature range of CuO, allowing for greater flexibility in sensor operation under varying environmental conditions. CuO’s interaction with reducing gases involves efficient electron transfer processes, where the gas molecules donate electrons to the CuO surface, leading to a reduction in hole concentration and causing a measurable resistance increase. For instance, when exposed to hydrogen, CuO undergoes a reduction reaction where hydrogen molecules dissociate on the CuO surface, resulting in electron transfer that alters the material’s conductivity. Similarly, with methane and carbon monoxide, adsorption on the CuO surface leads to electron donation, affecting the concentration of charge carriers and producing a detectable change in resistance [66] (Figure 7).

Operating temperature is another important factor in CuO-based gas sensors, as temperature affects the kinetics of gas adsorption and desorption. CuO sensors often exhibit optimal performance in the temperature range of 100–300 °C, depending on the specific gas and nanostructure used. Lower temperatures can hinder gas adsorption, reducing sensitivity, while higher temperatures might lead to desorption of the gas molecules before they can significantly alter the material’s electrical properties. Conversely, excessively high operating temperatures can lead to the desorption of gas molecules from the CuO surface before they can significantly alter the material’s electrical properties [67]. At elevated temperatures, the adsorbed gas molecules gain sufficient kinetic energy to overcome the adsorption energy, leading to rapid desorption. This rapid desorption reduces the interaction time between the gas molecules and the CuO surface, diminishing the signal change and potentially leading to false negatives [67]. This effect is particularly pronounced for gases with low adsorption energies, which tend to desorb more readily at higher temperatures. Therefore, precise temperature control is often required to optimize CuO’s response to different gases, especially in industrial applications where rapid fluctuations in gas concentration occur [68].

CuO has also been investigated in composite form with other metal oxides, such as ZnO and SnO₂, to enhance gas sensing performance through synergistic effects. For example, CuO-ZnO composite sensors have shown high sensitivity and selectivity for hydrogen and carbon monoxide, as the heterojunctions formed at the interface between CuO and ZnO improve charge separation and facilitate electron–hole pair interactions with adsorbed gas molecules. Specifically, in one study, the response of the composite sensor to 100 ppm of H₂ was significantly higher than its response to 400 ppm of CO₂, 25 ppm of NH₃, 10 ppm of acetone, and 25 ppm of ethanol at an operating temperature of 350 °C [69]. CuO-SnO₂ composites similarly leverage the high surface area and reactivity of SnO₂ while utilizing CuO’s p-type properties to enhance the response to reducing gases, making these composites suitable for use in environmental monitoring and industrial safety devices [69].

The combination of CuO’s inherent p-type behavior, narrow bandgap, and versatile response to reducing gases makes it a highly effective material for gas sensing applications. Advances in nanostructuring, doping, and composite formation continue to expand the range and efficacy of CuO-based sensors, making them valuable tools in industrial safety, environmental monitoring, and other critical applications where sensitivity and selectivity are paramount.

3.4. Iron Oxides (Fe₂O₃ and Fe₃O₄)

Iron oxides, particularly hematite (Fe₂O₃) and magnetite (Fe₃O₄), are widely studied materials in gas sensing due to their affordability, chemical stability, and favorable electronic properties. Fe₂O₃, an n-type semiconductor with a bandgap of around 2.1 eV, and Fe₃O₄, known for its unique mixed valency and half-metallic behavior, have proven effective in detecting gases such as hydrogen (H₂), carbon monoxide (CO), nitrogen oxides (NO_x), and ammonia (NH₃). These iron oxides exhibit a strong affinity for oxygen, making them particularly sensitive to reducing gases that alter their charge carrier dynamics through redox reactions at the surface (Figure 8) [70,71,72].

The gas sensing mechanism of iron oxides is primarily governed by changes in resistance that occur when reducing or oxidizing gases interact with the sensor surface. In the presence of reducing gases like H₂ or CO, these gases donate electrons to iron oxide, leading to an increase in conductivity, especially in n-type Fe₂O₃. For oxidizing gases such as NO_x, the opposite effect occurs, as these gases extract electrons, decreasing the concentration of charge carriers and leading to an increase in resistance. Fe₃O₄, with its unique ability to exhibit both semiconducting and metallic-like behavior depending on conditions, has shown potential in detecting multiple gas types at varied temperatures, adding versatility to iron oxide-based sensors [71,72]. Since Fe oxide-based sensors respond to multiple gasses, selectivity can be improved quite successfully by functionalization with Pt, Pd, or Au, or temperature modulation [70,71,72,73,74].

Nanostructuring iron oxides into forms like nanospheres, nanotubes, nanorods, and thin films has greatly improved their sensitivity and response times. Nanospheres, for example, offer a large surface area that enhances gas adsorption, increasing the rate at which the sensor responds to target gases. Nanotubes and nanorods, due to their one-dimensional structure, support direct pathways for charge transport, enhancing conductivity and leading to faster response and recovery times. In particular, Fe₂O₃ nanorods have shown high sensitivity to nitrogen oxides due to their surface reactivity and stable resistance changes upon exposure to oxidizing gases [73,74]. These unique morphologies maximize active sites for gas interaction, making nanostructured Fe₂O₃ and Fe₃O₄ highly suitable for applications in environmental monitoring and industrial safety. Doping iron oxides with metals like tin (Sn) and nickel (Ni) has proven beneficial in enhancing their selectivity, response rates, and stability under varied environmental conditions. Sn-doped Fe₂O₃, for example, has shown improved sensitivity and selectivity towards reducing gases like CO and H₂ by increasing oxygen vacancy concentration and facilitating charge transfer between Fe₂O₃ and the adsorbed gas molecules. Similarly, Ni-doped Fe₂O₃ has been found to increase sensor response rates to ammonia and nitrogen oxides, largely due to nickel’s catalytic effect, which enhances adsorption kinetics and the stability of the surface reaction intermediates. The inclusion of these dopants modifies the electronic structure and creates additional defect sites, effectively tuning the sensor’s performance for specific applications [71,75]. Other examples of Fe oxide-based sensors include force and pressure sensors, which were produced using carrageenan films in one study [73]. The sensor’s output signal in analogue voltage was registered using an oscilloscope and transmitted to a PC for further analysis. The obtained results showed a very interesting outcome, where the sensor, which was intended to be piezoresistive, demonstrated a combination of behavior typical for galvanic cells and piezoelectric material. It provides a stable DC output that is sensitive to the applied static pressure, and in case of a sudden impact, like a hit, it demonstrates piezoelectric behavior with some particular effects [73].

Fe₃O₄, with its unique ability to exhibit both semiconducting and metallic-like behavior depending on conditions, has shown potential in detecting multiple gas types at varied temperatures, increasing the versatility of iron oxide-based sensors [76].

3.5. Tin Oxide (SnO₂)

Tin oxide (SnO₂) is one of the most widely researched n-type semiconductors for gas sensing applications, known for its excellent electronic properties, chemical stability, and versatility in detecting a wide range of gases. SnO₂ has a direct bandgap of around 3.6 eV, which allows it to respond quickly to reducing gases such as hydrogen (H₂), carbon monoxide (CO), volatile organic compounds (VOCs), and ammonia (NH₃). This wide bandgap also ensures that the sensor remains stable and highly resistive when not exposed to reducing gases, making it ideal for selective gas sensing [77,78]. Nanostructured SnO₂, such as in nanowires, nanoparticles, and thin films, offers an improved sensitivity due to its enhanced surface-to-volume ratio, which increases the active sites for gas adsorption. Nanowires, for instance, provide a direct pathway for electron transport, leading to faster response and recovery times for hydrogen and CO gases, as well as the sensitive monitoring of VOCs under various environmental conditions [79,80,81]. Thin films, on the other hand, allow for precise control over thickness and morphology, enabling stable sensor performance and long-term reliability.

Doping SnO₂ with noble metals like platinum (Pt) and palladium (Pd) further enhances its selectivity, response time, and overall performance. These dopants act as catalytic agents that lower the activation energy required for gas interactions at the sensor surface, leading to faster adsorption–desorption dynamics and improved sensitivity. Pd-doped SnO₂, for example, has demonstrated superior response to hydrogen due to Pd’s high catalytic activity in dissociating hydrogen molecules into atomic hydrogen, which enhances the electron transfer process. Pt doping, on the other hand, improves sensitivity to carbon monoxide and methane by promoting the adsorption of these gases and facilitating charge transfer to the SnO₂ surface, resulting in significant changes in conductivity [82] (Figure 9).

The gas sensing mechanism in SnO₂ relies on the changes in surface resistivity upon interaction with target gases. In the presence of a reducing gas, SnO₂ undergoes a reduction in surface resistance, as electrons are transferred from the adsorbed gas molecules to the SnO₂ lattice, increasing charge carrier concentration. For oxidizing gases like nitrogen dioxide (NO₂), the opposite effect occurs: the gas molecules capture electrons from the SnO₂ surface, reducing the conductivity. This reversible interaction with gases allows SnO₂-based sensors to operate in real time and makes them highly useful in detecting fluctuating concentrations of gases, especially in environmental and industrial settings [83]. Temperature plays a crucial role in SnO₂ gas sensing performance, as it influences the rate of the adsorption and desorption of gas molecules on the sensor surface. SnO₂ sensors typically operate within a temperature range of 150–400 °C, with the optimal temperature depending on the specific gas and nanostructure. Higher temperatures accelerate gas diffusion and improve sensitivity, but can also increase the desorption rate, potentially reducing sensitivity. Doped SnO₂, particularly with Pd or Pt, can achieve improved sensitivity at lower temperatures, which is advantageous for applications requiring low-power operation, such as wearable devices or portable gas monitors [83,84]. Light activation has also been explored to enhance the low-temperature performance of SnO₂ sensors, especially in applications requiring energy-efficient operation. By using UV light, for example, it is possible to activate the SnO₂ surface at lower temperatures, as UV photons generate electron–hole pairs that facilitate gas interactions without the need for thermal excitation. This method has shown promising results in enhancing sensitivity and extending the operating range of SnO₂ sensors, especially for gases like NO₂ and CO that require precise, low-level detection [85]. Composite materials combining SnO₂ with other metal oxides, such as ZnO, WO₃, or TiO₂, have demonstrated enhanced gas sensing properties through synergistic effects. SnO₂-ZnO composites, for instance, leverage the high surface area of ZnO and the electronic properties of SnO₂ to achieve superior sensitivity and selectivity for gases like ethanol and acetone. Similarly, SnO₂-TiO₂ composites improve stability and sensitivity to NO_x and CO by forming heterojunctions that enhance charge transfer and promote selective gas adsorption. These composite sensors are particularly useful in environmental monitoring, where a broad range of gases need to be detected accurately and reliably [86,87]. SnO₂’s high sensitivity, tunable selectivity, and adaptability to nanostructuring and doping make it a leading material in gas sensor technology. Its effectiveness in detecting a broad spectrum of gases has made it a key material for sensors in environmental monitoring, industrial safety, and healthcare.

Ongoing research into doping, nanostructuring, and composite formation continues to expand the capabilities of SnO₂-based sensors, supporting the development of high-performance, low-power devices for next-generation gas sensing applications.

3.6. Indium Oxide (In₂O₃)

Indium oxide (In₂O₃) is a promising material for gas sensing applications due to its wide bandgap of approximately 2.9 eV, high electrical conductivity, and exceptional chemical and thermal stability, which enable it to maintain performance under varied environmental conditions. These properties make In₂O₃ especially effective for detecting oxidizing gases, such as nitrogen dioxide (NO₂), ozone (O₃), and carbon dioxide (CO₂), as its n-type semiconducting nature readily facilitates charge transfer with these gas molecules. The high reactivity of In₂O₃ towards oxidizing gases is due to its propensity for electron donation and reduction upon gas exposure, which induces measurable changes in conductivity, allowing for fast, sensitive gas detection [88]. The nanostructuring of In₂O₃ into forms such as nanofibers, nanorods, and nanoparticles has been shown to significantly enhance its gas sensing performance. For instance, nanofibers provide a high surface area-to-volume ratio, improving the density of active sites for gas adsorption, which results in faster response and recovery times. Nanorods and nanoparticles, with their unique surface morphologies, offer additional active sites and pathways for efficient electron transport, leading to improved sensitivity, particularly for low-concentration gas detection. Nanostructured In₂O₃ has exhibited remarkable sensitivity to NO₂, which can be detected at parts-per-billion (ppb) levels, demonstrating its suitability for applications requiring precise gas measurements, such as environmental monitoring and industrial safety [89,90]. Doping In₂O₃ with metals like tin (Sn) and zinc (Zn) has further optimized its gas sensing performance by modifying its electronic structure, enhancing both sensitivity and selectivity. For example, Sn-doped In₂O₃ sensors have shown improved selectivity towards oxidizing gases such as NO₂ and O₃, as Sn atoms create additional oxygen vacancies that promote stronger interactions between the target gas molecules and the In₂O₃ surface. Similarly, Zn doping has been observed to shift the bandgap and enhance response to specific gases by introducing donor levels, which facilitate electron transfer and enhance response times at lower gas concentrations. This doping strategy allows In₂O₃-based sensors to achieve high performance even in complex and fluctuating environments, where multiple gases may be present [91,92]. In₂O₃ sensors also exhibit the advantage of operating at relatively low temperatures, which makes them more energy-efficient compared to other metal oxide sensors that require higher operating temperatures (typically above 200 °C). This low-temperature operability not only reduces energy consumption, but also broadens the potential applications of In₂O₃ sensors to include portable and wearable devices, where energy efficiency is essential. Additionally, low-temperature operation reduces the risk of interference from thermal noise and degradation, ensuring stable performance over extended periods [93]. The sensing mechanism of In₂O₃ is predominantly based on the modulation of resistance upon gas adsorption. In the presence of oxidizing gases like NO₂, electrons from the conduction band of In₂O₃ are transferred to gas molecules, leading to the formation of negatively charged adsorbates on the sensor surface. This electron withdrawal decreases the free electron concentration in the n-type In₂O₃, resulting in an increase in resistance. When the oxidizing gas desorbs, the electrons return to the In₂O₃, restoring its original conductivity. This reversible charge transfer underlies the real-time sensing capability of In₂O₃, making it suitable for applications where continuous monitoring is required [94]. Light activation has been studied as a method to enhance In₂O₃ sensor performance at even lower temperatures, as UV or visible light irradiation generates electron–hole pairs that facilitate the gas interaction process without the need for thermal activation. This approach has proven effective in enhancing sensitivity and enabling low-temperature operation for gases like NO₂ and CO₂. Photo-induced charge carriers not only improve the reaction kinetics, but also increase selectivity for certain gases by promoting specific surface reactions under light exposure, providing a pathway for developing low-power, light-activated gas sensors [94].

Composite materials that combine In₂O₃ with other metal oxides, such as SnO₂, TiO₂, or WO₃, further enhance the selectivity, stability, and sensitivity of In₂O₃-based sensors. In₂O₃-SnO₂ composites, for example, leverage the high sensitivity of SnO₂ to reducing gases with In₂O₃’s selective response to oxidizing gases, creating a sensor with balanced performance characteristics across multiple gas types. These composites form heterojunctions that enhance charge transfer and carrier separation, allowing for improved sensing accuracy in multi-gas environments. Similarly, In₂O₃-TiO₂ composites exhibit increased stability under humid conditions, extending the sensor’s applicability to varied environments [91]. In₂O₃’s high sensitivity, low-temperature operation, and adaptability to nanostructuring and doping make it a valuable material for gas sensors in applications ranging from environmental monitoring to portable air quality measurement devices. Continued research into doping, light activation, and composite engineering is likely to further expand the capabilities and applications of In₂O₃-based gas sensors (Figure 10).

3.7. Molybdenum Oxide (MoO₃)

Molybdenum oxide (MoO₃) is gaining attention in gas sensing applications due to its high sensitivity to both oxidizing and reducing gases, including ammonia (NH₃), hydrogen (H₂), and nitrogen oxides (NO_x). Its layered structure and bandgap, typically around 2.9–3.1 eV, provide unique electronic properties that facilitate strong gas adsorption and high catalytic activity, making MoO₃ suitable for sensitive and selective gas detection. The layered arrangement of MoO₃ contributes to its exceptional adsorption capacity, as it allows gases to intercalate between layers, which maximizes the interaction area and promotes effective electron transfer upon gas exposure [95].

Nanostructuring MoO₃ into forms such as nanorods, nanosheets, and nanoflakes greatly enhances its surface area and reactivity. These nanostructures allow more active sites for gas adsorption and create efficient electron pathways, resulting in faster response and recovery times. For example, nanorods provide a high surface-to-volume ratio with directional electron transport, which is particularly beneficial for detecting gases like H₂ and NO₂ at low concentrations. Nanosheets and nanoflakes, with their high surface area and open edges, enable rapid adsorption and desorption cycles, leading to more reliable, repeatable sensing [96,97]. The performance of MoO₃-based sensors is further optimized through doping with elements such as vanadium (V) and chromium (Cr). These dopants modify the band structure of MoO₃, introducing defect sites and oxygen vacancies that enhance the sensor’s interaction with target gases. V-doped MoO₃, for instance, shows improved sensitivity to NH₃ and NO_x gases by promoting stronger chemical interactions at the sensor surface, while Cr doping enhances MoO₃’s response to reducing gases like H₂ by facilitating electron transfer processes.

Doping with transition metals creates localized electronic states that improve charge carrier mobility and reactivity, which leads to faster response and recovery times [98,99]. In addition to doping, combining MoO₃ with other metal oxides, such as titanium dioxide (TiO₂), has proven effective for improving sensitivity and selectivity. MoO₃-TiO₂ composites, for instance, benefit from the formation of heterojunctions that facilitate charge separation and increase selectivity for specific gases. TiO₂ enhances the catalytic activity of MoO₃, particularly under UV light, allowing the composite to perform well in detecting volatile organic compounds (VOCs) and NO_x gases. These composites take advantage of the synergistic effects between MoO₃ and TiO₂, leading to enhanced sensitivity and stability, even under fluctuating environmental conditions [100]. The gas sensing mechanism in MoO₃ relies on the modulation of electrical conductivity upon gas adsorption. For oxidizing gases like NO₂, MoO₃ interacts by trapping electrons from the conduction band, leading to an increase in resistance. For reducing gases like H₂ and NH₃, the interaction results in the donation of electrons to the MoO₃ surface, decreasing its resistance. This resistance modulation is reversible, enabling MoO₃ sensors to operate in real-time applications. MoO₃-based sensors can detect even trace levels of gases due to this effective charge transfer mechanism, making them ideal for applications in environmental monitoring and industrial safety [101].

Another key advantage of MoO₃-based sensors is their stability across a wide range of operating temperatures, making them robust enough for industrial and environmental applications where temperature fluctuations are common. MoO₃ can operate effectively at temperatures from room temperature up to several hundred degrees Celsius, which is useful in high-temperature industrial environments. Additionally, MoO₃’s high chemical stability prevents degradation under extended exposure to reactive gases, ensuring long-term operational reliability and reducing maintenance needs [102,103,104,105]. The potential of MoO₃-based sensors is further expanded by the use of light activation, which can reduce the need for high operating temperatures.

UV or visible light activation generates electron–hole pairs that assist in gas adsorption and desorption processes, enabling sensitive detection at room temperature. This photoactivation is particularly advantageous in wearable and portable gas sensors, as it allows low-power operation without sacrificing performance. Studies have demonstrated that light-activated MoO₃ sensors can detect gases like NH₃ and NO₂ with high sensitivity and selectivity, making them ideal for applications requiring minimal energy consumption [102,103]. MoO₃’s high sensitivity to a broad spectrum of gases, coupled with its stability, adaptability to nanostructuring, and compatibility with doping and composite strategies, makes it a versatile material for gas sensing.

Recent research has mainly focused on using MoO₃ for detecting ammonia, VOCs, and hydrogen sulfide in industrial surroundings. Research is mostly focused on enhancing sensitivity, selectivity, and response time. And even though MoO₃ is not as common as TiO₂ in this field, the ongoing research into light-activated and doped MoO₃ sensors continues to enhance its performance, making it a valuable material in the development of next-generation sensors for environmental and industrial applications, and further increasing its use (Figure 11).

3.8. Mixed Metal Oxides and Other Systems

Mixed metal oxides (MMOs) represent an advanced class of materials in gas sensing, combining two or more metal oxides to leverage synergistic effects that enhance their sensitivity, stability, and selectivity. By forming heterojunctions between the different metal oxides, MMOs can achieve tailored electronic and surface properties, resulting in superior gas sensing performance compared to single metal oxides. For instance, ZnO-CuO, SnO₂-TiO₂, and Fe₂O₃-SnO₂ composites are popular MMO combinations, each demonstrating a marked improvement in response to specific target gases due to optimized band alignment, electron transfer efficiency, and selective adsorption capabilities [106,107,108,109,110,111,112,113,114,115,116,117,118].

One key advantage of MMOs is their ability to support heterojunctions, which form at the interface of two metal oxides with different energy band structures. These heterojunctions facilitate electron transfer by creating built-in electric fields that enhance the sensor’s response to target gases. For example, in ZnO-CuO heterojunctions, the p-n junction created between p-type CuO and n-type ZnO leads to a modulation in resistance upon gas adsorption. When exposed to reducing gases like hydrogen (H₂), electrons from the ZnO layer flow into CuO, increasing resistance and enabling the detection of low-concentration gases. It is observed that H₂ sensitivity of the heterojunction is increased with increases in temperature, as well as the thickness of CuO film. A sensitivity value as high as 266.5 is observed at 300 °C when biased at 3 V in the presence of approximately 3000 ppm of H₂. Similarly, SnO₂-TiO₂ heterojunctions improve the sensor’s response to oxidizing gases such as nitrogen dioxide (NO₂) by enhancing charge carrier mobility and creating additional adsorption sites [106]. Response (recovery) times to 400 ppb NO₂ were determined as a function of the operating temperature and indicated a significant decrease from 62 (42) s at 123 °C to 12 (19) s at 385 °C. A much smaller sensitivity to H₂ was observed, which might be advantageous for the selective detection of nitrogen oxides. The influence of humidity on NO₂ response was demonstrated to be significant below 150 °C, and systematically decreased upon increasing the operating temperature up to 400 °C [107].

Doping in MMO systems adds further versatility by fine-tuning properties such as carrier density, selectivity, and response time. Dopants can modify the electronic structure, induce oxygen vacancies, and increase active sites for gas adsorption, significantly improving sensitivity and selectivity. For instance, doping ZnO-CuO composites with palladium (Pd) increases sensitivity to gases like methane (CH₄) and carbon monoxide (CO) due to the enhanced catalytic properties Pd provides. Sensitivity improved by 17.1 times and 327.8 times compared with the pristine CuO- and ZnO-based gas sensors, respectively. Moreover, the detection limit to H₂S of such sensors could be reduced as low as 300 ppb [105]. Similarly, adding nickel (Ni) to SnO₂-TiO₂ systems has been shown to improve sensitivity to VOCs by promoting rapid electron transfer and enabling selective adsorption under lower operating temperatures, which is critical for applications that require energy-efficient, low-temperature operation [108]. Nanostructuring plays a crucial role in further enhancing the performance of MMO-based gas sensors.

Structuring MMOs into nanoscale morphologies, such as nanorods, nanosheets, and nanocomposites, significantly increases their surface area and active sites, promoting higher gas adsorption rates and faster electron transport. Nanorod arrays, for example, provide a high surface-to-volume ratio and directional pathways for electron flow, improving sensitivity and response time. In NiO₂-CeO₂ nanostructures, this structure facilitates the rapid adsorption and desorption cycles necessary for detecting NO₂ and H₂ gases at low concentrations. The decoration of NiO on the CeO₂ surface develops a built-in potential at the interface of NiO and CeO₂, which plays a vital role in the superior sensing performance of the NiO/CeO₂ sensor. Sharp response and recovery times (15 s/19 s) were observed for the NiO/CeO₂ sensor towards 100 ppm isopropanol at room temperature [110]. Nanosheets and nanoflakes, with large surface areas and open edges, allow for efficient charge transfer and enhanced interaction with target gases, which is particularly useful for detecting low-concentration CO and NO₂ [109]. In addition to structural and doping modifications, photoactivation has emerged as a method for enhancing MMO sensor performance.

Under UV or visible light, electron–hole pairs are generated, which improve the adsorption and desorption kinetics at lower operating temperatures. This activation method has shown particular promise in ZnO-CuO- and TiO₂-based MMOs, where light exposure can reduce the sensor’s response time, lower power consumption, and extend its applicability to portable and wearable devices. In light-activated SnO₂-TiO₂ sensors, for example, UV-induced electron–hole pairs enhance gas sensitivity by increasing the number of reactive oxygen species on the surface, allowing for the efficient detection of NO₂ and VOCs without the need for elevated temperatures [110]. MMOs are also highly customizable to complex and fluctuating environmental conditions, which is crucial for real-world applications in air quality monitoring, industrial safety, and environmental control. The robustness of these materials is enhanced by their ability to maintain stability over long periods, even under varied humidity and temperature conditions. For instance, Fe₂O₃-SnO₂ composites demonstrate exceptional stability in humid environments while retaining high sensitivity to target gases. This robustness is largely attributed to the synergistic interaction between Fe₂O₃ and SnO₂, which strengthens the material’s resistance to degradation and maintains consistent performance across a wide range of conditions [111,112].

The gas sensing mechanisms in MMOs are typically based on the modulation of electrical resistance or conductance when exposed to target gases. Upon the adsorption of reducing gases, such as H₂ or CO, the electrons from these gases transfer to the conduction band of the n-type metal oxide component (e.g., SnO₂ in SnO₂-TiO₂), decreasing resistance. Conversely, when exposed to oxidizing gases like NO₂, electrons are withdrawn from the conduction band, increasing resistance. This electron modulation is highly reversible, which is crucial for applications that require continuous, real-time monitoring [111,112,113,114,115,116,117,118,119,120]. In summary, MMOs are valuable for their adaptability, improved sensitivity, and selectivity across a range of gases, making them highly suitable for complex environmental applications. Advances in nanostructuring, doping, and photoactivation continue to expand their potential, promising enhanced performance for the next generation of gas sensors (Table 1).

Table 1. Comparison of metal oxide-based sensors, target gases, sensitivity, limits of detection for the selected gases, and operating conditions.

Metal Oxides	Sensitive Gases	Sensitivity Characteristics	Limit of Detection (LOD)	Operating Temperature Range	Light Use During Test
ZnO	CO, H₂, NO₂, NH₃	High sensitivity to NO₂ and NH₃, enhanced by nanostructures (nanowires, nanoparticles) and doping with Al, Ga, and In.	~7 ppb for NO₂ [43] Specific LOD varies	200–400 °C	Not typically used
TiO₂	VOCs, NO_x, H₂S	Strong sensitivity to VOCs and NO₂, improved by UV activation; doping with Pt or N enhances recovery time.	~15 ppb for NO₂ [53] Specific LOD varies	RT-300 °C; UV activation can enable lower T	UV light enhances sensitivity
CuO	CO, CH₄, H₂, NH₃	Effective for reducing gases like H₂ and CO; p-type behavior enables selective detection with Zn or Sn doping.	~8 ppb for CO [61] Specific LOD varies	200–400 °C	Not typically used
Fe₂O₃, Fe₃O₄	H₂, CO, NO_x	Chemically stable; high sensitivity to CO and NO₂ when Sn- or Ni-doped; nanostructuring enhances response rate.	~1 ppm for H₂S [69] Specific LOD varies	200–400 °C	Not typically used
SnO₂	H₂, CO, VOCs	Highly sensitive to reducing gases like CO and H₂; doping with Pt or Pd improves selectivity and response time.	~10 ppb for H₂ [74] Specific LOD varies	200–400 °C	Not typically used
In₂O₃	NO₂, O₃, CO₂	Sensitive to oxidizing gases like NO₂ and O₃; Sn and Zn doping improves response to low-concentration gases.	~10 ppb for NO₂ [83] Specific LOD varies	145 °C	Not typically used
MoO₃	NH₃, H₂, NO_x	Versatile for oxidizing and reducing gases; V and Cr doping enhances selectivity; nanostructuring yields faster response.	~20 ppb for H₂ [90] Specific LOD varies	200–400 °C	Not typically used
ZnO-CuO	H₂, CO, NH₃	High sensitivity to reducing gases; the p-n junction formed enhances charge separation and improves response time.	15 ppb for H₂ [109] Specific LOD varies	200–400 °C	Not typically used
SnO₂-TiO₂	NO₂, VOCs	Enhanced sensitivity due to improved charge transfer; effective for detecting low concentrations of NO₂.	~50 ppm for VOCs [110] Specific LOD varies	200–400 °C	Not typically used
Fe₂O₃-SnO₂	CO, H₂, NO_x	High stability and sensitivity to CO; doping with Sn enhances selectivity and reduces response times in fluctuating environments.	~20 ppm for acetone [107] Specific LOD varies	200–400 °C	Not typically used
ZnO-SnO₂	H₂, CO, VOCs	Synergistic effects improve sensitivity to various gases; effective under low-temperature conditions.	~1 ppm for CO [111] Specific LOD varies	200–400 °C	Not typically used
In₂O₃-TiO₂	NO₂, CO, O₃	High sensitivity to oxidizing gases; doping with Sn improves selectivity and lowers detection limits for environmental monitoring.	~10 ppm for NO₂ [112] Specific LOD varies	200–400 °C	Not typically used
CuO-Fe₂O₃	H₂, CO, CH₄	Good response to reducing gases; the combination offers improved selectivity/stability due to the electronic properties of both oxides.	~2 ppm for H₂ [106] Specific LOD varies	200–400 °C	Not typically used
TiO₂-MoO₃	NH₃, NO_x, VOCs	Enhanced sensitivity and response times, particularly in detecting NH₃; doping further optimizes selectivity.	~100 ppm for NH₃ [113] Specific LOD varies	200–400 °C	Not typically used
SnO₂-CuO	CO, H₂, CH₄	High sensitivity to reducing gases; synergistic effect improves response time and stability under varying conditions.	~70 ppb for H₂ [114] Specific LOD varies	200–400 °C	Not typically used

4. Future Directions and Challenges

Despite significant advancements, challenges remain in optimizing the selectivity, reducing response times, and extending the operational lifespan of gas sensors for real-world applications. While single metal oxides like ZnO, TiO₂, CuO, and Fe₂O₃ have demonstrated substantial sensitivity to various gases, mixed metal oxides (MMOs) and doped systems (e.g., Sn-doped In₂O₃ or Ni-doped Fe₂O₃) have shown promising improvements in selectivity and stability. The improved sensitivities of some gases are highlighted in Table 1, showing just a small portion of the research into metal and mixed metal oxides. This should serve as a guide for future research, showcasing the immense possibilities these systems offer in terms of gas sensing.

Additionally, nanostructuring techniques, such as the formation of nanorods, nanosheets, and nanotubes, have enhanced the surface area and facilitated faster electron transport, improving response times and enabling low-temperature operation. When metal oxides are structured into nanorods, nanosheets, or nanotubes, their surface-to-volume ratio increases significantly compared to bulk materials. This allows more active sites for gas adsorption, leading to higher sensitivity and lower detection limits. Traditional metal oxide gas sensors require high operating temperatures (200–400 °C) to achieve adequate gas sensing performance. Nanostructures, especially when combined with catalyst doping (e.g., Pt, Pd) or UV/visible light activation, allow gas detection at lower temperatures or even room temperature. However, issues such as environmental interference, long-term stability, and high-power consumption remain significant barriers to practical deployment in complex, real-world environments.

Emerging fabrication techniques, including 3D printing and atomic layer deposition, enable precise control over sensor architecture and material composition, potentially addressing issues related to response time and selectivity. Furthermore, integrating metal oxides with nanocomposites like graphene and carbon nanotubes offers additional pathways to enhance conductivity, sensitivity, and selectivity by creating high-surface-area interfaces and enhancing electron mobility. These advancements hold potential for developing low-power, highly responsive, and selective gas sensors suitable for environmental monitoring, industrial safety, and wearable devices in diverse conditions.

However, implementing these advancements will face challenges such as scalability and cost, as high-precision methods like atomic layer deposition and 3D printing are difficult and expensive to scale for mass production. Material stability and durability remain concerns, as hybrid materials and composites may degrade under varying environmental conditions. Achieving reproducibility and consistent performance across production batches is also a hurdle, as minor variations can affect sensor responses. Complex fabrication protocols must be simplified for broader industrial use, and maintaining ultra-low power consumption without compromising sensitivity is challenging. Additionally, environmental and safety concerns related to nanomaterial production and disposal need to be addressed, along with navigating regulatory standards to ensure compliance for market readiness. Overcoming these issues is essential for realizing the potential of advanced gas sensors.

Author Contributions

Conceptualization, V.M., I.P., A.B. (Arijeta Bafti), F.R.-P. and D.G.; methodology, V.M., I.P., A.B. (Arijeta Bafti), F.R.-P. and D.G.; software, V.M., I.P., A.B. (Arijeta Bafti), F.R.-P. and D.G.; validation, V.M., I.P., A.B. (Arijeta Bafti), F.R.-P. and D.G.; formal analysis, I.P., A.B. (Arijeta Bafti) and F.R.-P.; investigation, V.M., I.P., A.B. (Arijeta Bafti), F.R.-P. and D.G.; resources, V.M., Z.S. and A.B. (Arie Borenstein); data curation, V.M., I.P., A.B. (Arijeta Bafti), F.R.-P. and D.G.; writing—original draft preparation, V.M. and I.P.; writing—review and editing, V.M., I.P., A.B. (Arijeta Bafti) and F.R.-P.; visualization, V.M., I.P., A.B. (Arijeta Bafti), F.R.-P. and D.G.; supervision, V.M.; project administration, V.M.; funding acquisition, V.M., Z.S. and A.B. (Arie Borenstein). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Croatian Science Foundation under the project SLIPPERY SLOPE, UIP-2019-04-2367, and EU-cofounded under the project C3.2.R3-I1.05.0091.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of the sensing mechanism of n-type semiconducting metal oxide nanostructures for reducing gas (reproduced from [36], MDPI, 2020).

Figure 4. The effect of different (nano) structuring approaches on the active surface of a thin film solid-state gas sensor layer.

Figure 5. Synthesis of hierarchical nanostructured ZnO nanorods decorated with Pd, and the proposed ethanol sensing mechanism (reproduced from [50], MDPI, 2023).

Figure 6. (A) FE-SEM images of the gas sensors are shown. (a,b) are the results of the hydrothermal synthesis. (c,d) are TiO₂ NRs and Pt NPs of Sensor A. (e,f) are TiO₂ NRs and Pt NPs of Sensor B. (g,h) are TiO₂ NRs and Pt NPs of Sensor C. The inset in (f) shows an enlarged image to clearly display the Pt NPs on the TiO₂ NRs. The red arrows indicate Pt NPs. The green arrows in (c) indicate chunks of Pt agglomerated together on the top surface of TiO₂ NRs. (B) Proposed sensing mechanisms are depicted. Photoelectrochemical activities for sensing NO₂ with respect to time of annealing TiO₂ are shown. (a) is the schematic of Sensor A, which did not have an annealing process to TiO₂. (b,c) suggest the working mechanisms of Sensor B and Sensor C, of which TiO₂ NRs were annealed for 1 h and 2 h, respectively. e⁻ and h⁺ indicate electron and hole, respectively (reproduced from [57], MDPI, 2021).

Figure 7. (a) SEM and (b) HR-SEM images of the as-obtained samples. HR-SEM images of the samples annealed at (c) 205 °C and (d) 450 °C (reproduced from [66], MDPI, 2022).

Figure 8. Visual representation of charge transfers in carrageenan film generated by applied force. (a) Visual representation of carrageenan film with iron (III) oxide particles; (b) charge generation in prepared film and the release of charge-carrying particles due to force; (c) visual representation of experiment scheme of film under short-term mechanical load (reproduced from [73], MDPI, 2024).

Figure 9. (top): SEM images of tin oxide nanofibers and nanoribbons doped with PG after calcination at 450 °C. (a) Low and (b) high magnifications. (bottom): Sensor responses to 2 ppm ethanol, 4 ppm acetone, 5 ppm CO, and 100 ppb NO at different operating temperatures (25, 100, 200, and 300 °C) (reproduced from [82], MDPI, 2020).

Figure 10. (top): (a) TEM images of the In₂O₃ thin film deposited at 500 °C. (b) SAED of the In₂O₃ thin film deposited at 500 °C. (bottom): Repeatability of indium oxide thin films sprayed at 500 °C toward 50 ppm toluene (reproduced from [92], ACS, 2021).

Figure 11. SEM images of (a) as-grown α-MoO₃ nanoribbon network on SiO₂/Si interdigitated substrate and (b) sulfurized α-MoO₃ nanoribbons. Insets in (a,b) show corresponding optical micrographs. High-resolution SEM images of (c) as-grown α-MoO₃ and (d) sulfurized α-MoO₃. The inset in (d) is a magnified view of an individual sulfurized MoO₃ nanoribbon (reproduced from [102], MDPI, 2022).

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Panžić, I.; Bafti, A.; Radovanović-Perić, F.; Gašparić, D.; Shi, Z.; Borenstein, A.; Mandić, V. Advancements in Nanostructured Functional Constituent Materials for Gas Sensing Applications: A Comprehensive Review. Appl. Sci. 2025, 15, 2522. https://doi.org/10.3390/app15052522

AMA Style

Panžić I, Bafti A, Radovanović-Perić F, Gašparić D, Shi Z, Borenstein A, Mandić V. Advancements in Nanostructured Functional Constituent Materials for Gas Sensing Applications: A Comprehensive Review. Applied Sciences. 2025; 15(5):2522. https://doi.org/10.3390/app15052522

Chicago/Turabian Style

Panžić, Ivana, Arijeta Bafti, Floren Radovanović-Perić, Davor Gašparić, Zhen Shi, Arie Borenstein, and Vilko Mandić. 2025. "Advancements in Nanostructured Functional Constituent Materials for Gas Sensing Applications: A Comprehensive Review" Applied Sciences 15, no. 5: 2522. https://doi.org/10.3390/app15052522

APA Style

Panžić, I., Bafti, A., Radovanović-Perić, F., Gašparić, D., Shi, Z., Borenstein, A., & Mandić, V. (2025). Advancements in Nanostructured Functional Constituent Materials for Gas Sensing Applications: A Comprehensive Review. Applied Sciences, 15(5), 2522. https://doi.org/10.3390/app15052522

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Advancements in Nanostructured Functional Constituent Materials for Gas Sensing Applications: A Comprehensive Review

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