Recent Research Progress of Metal Oxide Semiconductor-Based Gas Sensors for Benzene Series Compounds Detection: A Review

1. INTRODUCTION

Environmental air pollution stems primarily from the relentless rise in emissions of hazardous, even life-threatening, gases—most notably volatile organic compounds (VOCs), which refer to organic compounds with high saturated vapor pressure (greater than 13.33 Pa), low boiling point, low molecular weight, and high volatility at room temperature under standard conditions [1,2]. They can also be considered as any carbon compound that participates in atmospheric photochemical reactions, except for CO, CO₂, H₂CO₃, metal carbides, metal carbonates, and ammonium carbonate [3,4]. As the main atmospheric pollutants, these substances originate from industrial complexes, mining operations, household appliances and activities, and manufacturing facilities. Rapid advances in science and technology, aimed at improving living standards while accommodating a growing global population, have fueled an unprecedented expansion of industry, thereby intensifying air pollution worldwide [5-8].

Among VOCs, benzene-series compounds such as benzene, toluene (methylbenzene), ethylbenzene, xylene (dimethylbenzene), and aniline are among the most significant groups, and they are highly toxic and carcinogenic to humans. Exposure to benzene, toluene, ethylbenzene, xylene (collectively known as BTEX), and aniline vapors may cause acute health issues like myocardial infarction, neurological impairment, cancer, and bone marrow aplasia, depending on the duration of the exposure, the concentration of the gas, and how often one is exposed [9-12]. Therefore, international organizations (the World Health Organization, WHO; the International Agency for Research on Cancer, IARC; the Occupational Safety and Health Administration, OSHA) have strictly regulated the levels of BTEX and aniline in the atmosphere and the duration of human exposure (as shown in Table 1). Therefore, during the production, storage, or use of benzene, toluene, ethylbenzene, xylene, and aniline, their leakage must be strictly monitored.

Table 1.

Exposure levels to the Benzene series as indicated by OSHA and NIOSH.

Common methods for detecting BTEX and aniline indoors or outdoors include gas chromatography, Raman spectroscopy, mass spectrometry, optical spectroscopy, and ion mobility spectroscopy [10,13,14]. However, the aforementioned equipment, besides being expensive, is bulky, time-consuming, and difficult to use, and requires highly skilled workers to run, making real-time monitoring and analysis difficult. To address these challenges, a growing body of research has focused on low-cost, portable gas-sensing approaches with high sensing performance for practical applications in in-situ monitoring and quantifying BTEX and aniline vapor in the environment. Gas sensors based on metal oxide semiconductor (MOS) technology, as one of the low-cost, easy-to-integrate, and miniaturizable chemiresistive sensors with good sensitivity and stability, have been widely used to detect BTEX and aniline vapor.

As mentioned above, air pollutants such as BTEX and aniline are present in human living environments at varying concentrations, some of which remain very harmful even at low levels of one billionth (ppb) and one millionth (ppm) (as shown in Table 1). Therefore, developing high-performance gas sensors has been the goal of many researchers to prevent unintentional exposure to hazardous gases and mitigate environmental pollution.

Despite the significant potential demonstrated by MOS sensors for chemiresistive gas sensing over the past few decades, several challenges persist in their application for detecting BTEX and aniline [10,15-17]. These issues include: (1) low sensitivity, limiting their effectiveness in detecting trace concentrations; (2) poor selectivity, particularly in complex gas environments where interference from other VOCs can compromise accurate detection of benzene derivatives; (3) some MOS gas-sensing materials are highly sensitive to humidity, which can degrade their gas-sensing performance towards target gases; (4) high operating temperatures increase power consumption and limit portability. These issues are largely attributed to the high chemical stability of BTEX and aniline, which makes their detection inherently challenging for conventional MOS-based sensors. All these factors limit the practical use of the MOS sensor for monitoring BTEX and aniline vapor. This review aims to report on the latest developments in MOS gas sensors for detecting benzene-series compounds.

2. BODY

2.1 Benzene gas sensors

Benzene (C₆H₆) is the simplest aromatic hydrocarbon, a nonpolar molecule, and a basic raw material in the petrochemical industry. It is a well-known carcinogen, and exposure to high concentrations can affect the blood, possibly leading to the development of leukemia [18,19]. Benzene has a hexagonal ring structure consisting of six carbon atoms. The C-C bonds in the ring are of a special type, known as delocalized π bonds, which give benzene its high chemical stability. Owing to the remarkably high chemical stability conferred by its delocalized π-electron system, benzene is notoriously resistant to oxidation or reduction, making its detection by conventional MOS gas sensors exceedingly challenging. Therefore, it is difficult to detect benzene vapor using traditional MOS such as SnO₂, ZnO, WO₃, and In₂O₃. However, binary metal oxides with a spinel structure (AB₂O₄) exhibit good benzene gas-sensing performance. Krishna et al. reported that nanoflake-structured CuFe₂O₄ and cotton-like porous nano-heterostructured CuCeO₂ exhibit a large specific surface area and uniform porosity, resulting in enhanced gas-sensing performance for benzene detection at room temperature [20]. However, its response to benzene vapor is very low ((R_g-R_a)/R_a≈13%). Tohidi et al. reported that nanostructured thin films of Zn₂SnO₄ deposited by CVD showed good sensitivity to 100 ppb benzene at 290°C [21]. The gas-sensing performance of Zn₂SnO₄ thin films was modeled and optimized using the surface response method. However, the gas-sensing performance of this thin film toward benzene vapor, particularly its response and selectivity, still does not meet practical requirements. Lee and Fu et al. reported a MEMS-based benzene gas sensor with a self-heating WO₃ thin film. In this work, a WO₃ sensing layer with micro-sized grains was deposited on a quartz chip with Pt interdigitated electrodes and Pt micro-heaters by sputtering. The study found that the sensor exhibited excellent sensitivity to benzene gas at 300°C, with a limit of detection (LOD) of 0.2 ppm and a very fast response speed (35 s). However, this study did not investigate sensor selectivity, and there remain doubts about their practical applications. Zhang and Zhou et al. reported a benzene sensor based on a novel concave Cu₂O octahedron, which was evaluated as an excellent sensing material for benzene vapor detection at 230°C (a response of about 9.7 at 50 ppm) [22]. Guo and Li et al. successfully synthesized a Bi-doped SnO₂/rGO nanocomposite, which significantly enhanced gas-sensing performance at low temperature for benzene vapor [23]. Compared with that of the pure SnO₂ and SnO₂/rGO, the Bi-doped SnO₂/rGO sensor exhibited high response (48.6 to 5 ppm), fast response-recovery speed (9/13 s), good stability, and selectivity to detect benzene vapor at low temperature (150°C). Beyond its impressive sensitivity, a notable advantage of this sensor is its response/recovery speed, which presented a response time of 3 s and a recovery time of 4 s for 50 ppm benzene vapor. However, the reported minimum concentration is only 5 ppm, which obviously does not meet the relevant regulations on benzene gas content in the environment.

Fig. 1.

Schematic illustration of (a) the synthetic process of bimetallic Au-Pt nanoparticle-supported ZnO porous nanobelts and (b) the fabrication of the corresponding gas-sensing device. Adapted from Ref. [29].

As mentioned above, benzene is a non-polar molecule with excellent chemical stability due to its large delocalized bonds. It is precisely this excellent chemical stability that makes it difficult to apply single-metal-oxide gas sensors for benzene detection [24-27]. To improve sensor performance, extensive research has been conducted on the use of precious-metal modifications and various metal oxide composites to construct heterojunctions that enhance benzene gas sensing. Zhang et al. investigated a benzene sensor based on a Pd-doped CaTiO₃/TiO₂ nanocomposite, which showed remarkable benzene sensing performance at room temperature (high response of 33.46-50 ppm benzene and a low detection limit of 100 ppb) [28]. Kim et al. reported an excellent selective-sensing sensor for benzene using SnO₂-ZnO core-shell nanowires functionalized with Pd nanoparticles. It presented a high response of 71 for 100 ppb benzene vapor at 300°C. Chen and Guo developed a chemiresistive gas sensor based on bimetallic Au-Pt nanoparticle-supported ZnO porous nanobelts that displayed a high response of approximately 39-50 ppm to benzene vapor at 300°C, with satisfactory response time (~8 s) and recovery time (~30 s) [29]. This gas sensor also displays excellent selectivity, stability, repeatability, and resistance to humidity interference. Motaung et al. reported low-concentration detection of benzene using a metal-organic framework (MOF)- derived Co₃O₄/TiO₂. heterostructures loaded with 0.5~2 wt.%Fe synthesized using the coprecipitation method [30]. The 1.0 wt.% Fe/Co₃O₄/TiO₂ heterostructure-based sensor disclosed higher responses of ~3 toward 2 ppm benzene vapor at 175°C and was able to experimentally detect benzene vapor as low as 0.35 ppm at 175°C. The good gas-sensing performance of this sensor is mainly attributed to the loading of Fe³⁺ on the surface of Co₃O₄/TiO₂, which increased the specific surface area, narrowed the band gap, and provided numerous oxygen vacancies. In addition, they also prepared a Pt-loaded NiO-CeO₂ nanosheet-assembled hierarchical structure, and this composite disclosed a superior response of 2.7 to 2 ppm benzene vapor and a very low detection limit of 70 ppb at 100°C [31]. Lee et al. investigated a bi-layer sensor constructed with Ru-TiO₂ as the catalytic layer and SnO₂ as the sensing layer for detecting benzene vapor [32]. The sensor based on Ru-TiO₂/SnO₂ bi-layer exhibited a high response value of 81.4 to 5 ppm benzene vapor. Taken together, these results indicate that the synergistic effect of constructing heterojunctions and noble-metal sensitization effectively enhances the gas-sensing performance of metal oxide gas sensors towards benzene vapor. In addition, some previous works reported that the application of metal-ion-doped metal oxides effectively enhances the gas-sensing performance of sensors towards benzene vapor. However, the enhancement effect still lags behind that of the noble-metal modification method [33-35].

Table 2.

Sensing performance of the MOS-based sensor on Benzene vapor.

2.2 Toluene gas sensors

As an important aromatic organic compound and raw material, toluene (C₇H₈) is extensively used in pharmaceuticals, laboratory solvents, fuels, paint thinners, and inks. Toluene vapor rapidly targets the central nervous system, triggering acute respiratory distress, while its widespread emissions make it a global contributor to air-quality degradation. Under normal conditions, toluene is highly stable, but it can react violently with acids or oxidants. Due to the presence of a methyl group, toluene is prone to free radical or oxidation reactions. This makes it relatively easy to detect toluene vapor using MOS [47].

Given the presence of active methyl groups in toluene, some metal oxides with strong catalytic activity (e.g., Co₃O₄ and NiO) can be used to enhance the gas-sensing performance of MOS. Jin et al. reported a toluene sensor based on mesoporous SnO₂@Co₃O₄ core-shell nanospheres, which exhibited an extremely high response (Rg/Ra = 10.2) to 100 ppm toluene at 300°C. The LOD reached 831 ppb [48]. The sensor's improvement in toluene performance is primarily attributed to the stable heterojunction formed between SnO₂ and Co₃O₄. Volanti et al. investigated a sensor based on Co₃O₄ coating with ZIF-67 (Co₃O₄/ZIF-67) for detecting toluene vapor [49]. The Co₃O₄/ZIF-67 exhibited a high response (61.22) to 100 ppm toluene at 250°C, which is 3 times higher than that of pure Co₃O₄. The ZIF-67 layer also enhanced the selectivity to toluene vapor. However, the sensor's slow response and recovery times hinder its practical application. Lee et al. prepared Nb-doped NiO hollow spheres by one-pot ultrasonic spray pyrolysis. The sensor based on Nb-doped NiO hollow spheres demonstrated dual functionality for ultrasensitive, highly selective detection of p-xylene and toluene by simply modulating the sensing temperature [50]. The maximum response of the Nb-doped NiO hollow spheres to 5 ppm xylene is about 1752 at 350°C, and to 5 ppm toluene is about 103 at 400°C. The Nb-doped NiO hollow spheres can be employed to design a single-gas sensor with dual selectivity for xylene and toluene for reliable monitoring of indoor air quality. Zhang et al. prepared a rationally designed Co₃O₄ hierarchical structure via a hydrothermal route [51]. The sheet-shaped Co₃O₄ exhibited higher sensitivity (8.5 to 200 ppm), faster response-recovery speed (10/30 s), and better selectivity to toluene at 180°C. It is evident from the above introduction that, whether using cobalt oxide and nickel oxide alone or forming heterojunctions with other metal oxides, they all exhibit good gas-sensing performance for toluene. However, the high working temperature and detection concentration limits also limit its practical application. In addition, some researchers have encountered similar problems when using sulfides as sensing materials for toluene detection [52,53]. Lowering the working temperature and detecting the minimum concentration are important issues that need to be addressed at present.

Fig. 2.

Schematic illustration of (a) the synthesis of Co3O4, (b) the synthesis procedure of Co3O4/ZIF-67, and (c) the role of ZIF-67 in the composite. Adapted from Ref. [49].

Noble metals exhibit higher catalytic activity, and their application to modify the surface of MOS can enhance MOS gas-sensing performance for toluene detection. Tatsuya Joutsuka, Yoshiteru Itagaki, et al. investigated the synergistic effects of integrating gold (Au) nanoparticles into SmFeO₃ for the detection of toluene [54]. Liang et al. demonstrated a Pd nanoparticle-functionalized In₂O₃ nanosphere-based gas sensor for detecting benzene, toluene, and xylene gases [55]. The sensor based on 0.75 wt% Pd-In₂O₃ nanospheres exhibited a high response (R_a/R_g = 21) to 100 ppm toluene vapor, approximately four times better than pure In₂O₃ at its respective optimum operating temperature. Moreover, this sensor showed enhanced sensing performance towards toluene, including low operating temperature, exceptional selectivity, and good stability. Nehru and Kalaiselvan et al. investigated a room-temperature-operable Ag/Bi₂O₃ nanocomposite-based chemiresistive toluene gas sensor that exhibits a high, rapid response, good repeatability, and excellent stability to toluene vapor [56]. After Ag modification, the gas sensitivity of Ag/Bi₂O₃ to toluene is significantly improved at room temperature. Yuan and Lin et al. reported an aluminum-doped ZnO-supported Au nanoparticle (Au/ZnO-Al) sensor with good gas-sensing performance for toluene vapor [57]. The Au/ZnO-Al sensor showed a high response value of 18.3 towards 5 ppm toluene and can detect ppb-level toluene with a low detection limit of 1.1 ppb at 275°C. Moreover, the Au/ZnO-Al sensor enhanced toluene selectivity. Kim et al. reported a selective toluene and benzene gas sensor based on Pt- and Pd-functionalized ZnO nanowires (NW) in self-heating mode. By adjusting the sputtering process, the Pt-functionalized ZnO NW prepared showed a response of 2.86 to 50 ppm toluene at room temperature. The response of ZnO NW functionalized with Pd to 50 ppm benzene at room temperature is 2.20 [58]. This study opens a pathway to the fabrication of selective toluene and benzene gas sensors with low power consumption operating in the self-heating mode.

To improve the gas-sensing performance of MOS sensors towards toluene, bimetallic-modified MOS toluene sensors have received attention. The use of alloy nanoparticles composed of two precious metals to modify the surface of MOS materials not only enhances sensitivity but also effectively reduces operating temperature. Song et al. investigated a sensor based on Pd/Au-nanocluster-functionalized 3D SnO₂ nanotube arrays that exhibited high sensitivity to formaldehyde (response of 2.55 to 1 ppm), toluene (response of 5.33 to 1 ppm), and acetone (response of 2.20 to 1 ppm) at room temperature [59]. Hahn and Kim et al. employed Au and Pd bimetallic clusters to modify the surface of a TiO₂ nanohelix array, which significantly enhanced the gas-sensing performance of TiO₂ towards toluene [60]. By adjusting the Au:Pd ratio, the sensor can achieve a response of up to 130000 to 100 ppm toluene with a fast recovery time (4 s) at 200°C. By incorporating single- or bimetallic metal-nanocluster-decorated sensors into a sensor array and employing a pattern recognition algorithm, they achieve successful discrimination of the target gases in real time. From the above introduction, it can be seen that metal oxide surfaces modified with precious metals exhibit better gas-sensing performance towards toluene, especially by significantly reducing the operating temperature of MOS. MOS modified with precious metals can detect toluene at room temperature, and the detection limit is also reduced. This is mainly due to the excellent catalytic effect of precious metal clusters, which allow toluene to react on the surface of metal oxides at low temperatures, altering the depletion layer and generating response signals. In addition to the above-mentioned heating-based MOS, some researchers have used light irradiation to excite MOS for toluene detection, achieving good results at room temperature [61]. Park et al. developed a porous In₂O₃-ZnO nanofiber-based sensor for detecting toluene vapor under ultraviolet light (UV, 365 nm) irradiation. The sensor exhibited good gas-sensing performance for toluene vapor at room temperature. Kim et al. prepared Pt and Pd-functionalized ZnO nanowires by the vapor-liquid-solid growth method [58]. The sensor based on Pt- and Pd-functionalized ZnO nanowires showed good gas-sensing performance for toluene and benzene under UV irradiation.

Fig. 3.

(a, b) SEM images, (c) cross-sectional SEM image, (d, e) TEM images, (f) high-resolution lattice fringe image, and (g) elemental mapping images of 10Nb-NiO. Gas-sensing characteristics of (h) pure NiO, (i) 1Nb-NiO, (j) 5Nb-NiO, (k) 10Nb-NiO, and (l) 20Nb-NiO sensors toward 5 ppm of various gases at 350–450°C; polar plots of gas responses of the 10Nb-NiO sensor to 5 ppm analyte gases at (m) 350°C and (n) 400°C (E: ethanol, X: p-xylene, T: toluene, B: benzene, C: carbon monoxide, F: formaldehyde). Adapted from Ref. [50].

Fig. 4.

Schematic illustration of fabrication procedure for a homogeneous Au@Pd NCs decorated TiO2 NHs gas sensor. Adapted from Ref. [60].

Table 3.

Sensing performance of the MOS-based sensor on Toluene vapor.

2.3 Ethylbenzene gas sensors

Ethylbenzene, also known as phenylethane, is an aromatic hydrocarbon with the molecular formula C₆H₅C₂H₅. Ethylbenzene can be used as a major raw material in the pharmaceutical industry. Ethylbenzene is highly irritating to the skin and mucous membranes, and at high concentrations it can cause anesthetic effects. Direct inhalation can cause chemical pneumonia and pulmonary edema. Long-term exposure can cause eye and upper respiratory irritation, as well as neurasthenia. In addition, its vapor can form explosive mixtures with air, posing a risk of combustion and explosion when exposed to open flames, high heat, or contact with oxidants. The presence of an ethyl group on the benzene ring of ethylbenzene activates the ring, making it more prone to chemical reactions than benzene. It is precisely because of its higher activity than benzene and toluene that it is difficult to accurately distinguish benzene, toluene, and ethylbenzene using a MOS sensor. Zhang and Wei et al. obtained MOS sensors with good gas-sensing performance for benzene series vapors by regulating the surface oxygen vacancy concentration on WO₃ nanosheets [100]. The oxygen-vacancy-rich WO₃ nanosheet material exhibited a relatively low working temperature of 180°C and a high sensing response (R_a/R_g = 64.15, 50.54, 58.79, and 56.26 to 50 ppm ethylbenzene, benzene, toluene, and xylene, respectively). Although the gas-sensing performance of the material is good, it also makes it difficult to distinguish ethylbenzene. Xu et al. prepared hexagonal WO₃ (h-WO₃) nanosheets, nanoparticles, and nanorods by a facile, low-cost, and environmentally friendly hydrothermal and sol-hydrothermal method. The sensor based on h-WO₃ nanosheets showed good gas-sensing performance for benzene-series gases at 320°C [37]. At the optimal working temperature, the response values of the h-WO₃ nanosheets sensor to 50 ppm BTEX vapors were 12.33, 27.73, 43.17, and 36.27, respectively. The same problem is that its response to benzene-series gases is similar, making it difficult to distinguish ethylbenzene effectively. Xu and Wang et al. used CeO₂, which has strong catalytic performance, as the catalytic layer and formed a bi-layer sensor with ZnO, which has good gas-sensing performance, effectively improving the sensor's response to benzene-series gases [97]. As the CeO₂ catalytic layer is located at the top of the bi-layer structure, the sensor not only significantly improves its gas-sensing performance for benzene-series gases but also effectively suppresses its responses to ethanol, acetone, and formaldehyde, thereby enhancing its selectivity to benzene-series gases. This method of leveraging catalytic properties to enhance gas-sensing performance is highly effective.

Therefore, improving the selectivity of the MOS sensor towards ethylbenzene is the most important issue to be addressed for its practical applications. Tshabalala and Motaung et al. synthesized sea-urchin-like TiO₂ hierarchical spheres by the hydrothermal method [101]. After different high-temperature calcination treatments, the TiO₂ phase transformed and exhibited temperature-dependent selectivity towards ethylbenzene at 75°C, with a detection limit of ~0.1 ppm, and toluene at 150°C. Research has found that the main reason for TiO₂'s performance is the production of sufficient, highly active oxygen on the high-energy crystal facets of TiO₂ after high-temperature treatment, which plays a crucial role in dissociating gas during interaction with the sensing layer. Swart and Motaung et al. doped Co₃O₄-In₂O₃ nanorods with Sm³⁺ and Yb³⁺, and prepared composite materials for ethylbenzene, xylene, and toluene gases at different temperatures by adjusting the doping amount [102]. The sensor based on Co-In 0.25 mol.% Sm³⁺ displayed superior gas sensing toward ethylbenzene gas at 75°C. Precise selectivity toward xylene vapor was observed for Co-In:0.25 mol.% YbO at 100°C.

This method of regulating the selectivity of sensors for xylene and ethylbenzene at different temperatures by controlling surface defects in metal oxides is a very good solution to the problem of gas-sensing selectivity.

With the continuous development of machine learning technology, using deep learning and neural network methods to distinguish benzene-series gases based on differences in gas-response features of MOS devices to benzene, toluene, ethylbenzene, and xylene has become an efficient and accurate approach. Dong et al. achieved chemical discrimination of benzene series (toluene, xylene isomers, and ethylbenzene) using a Ti-doped Co₃O₄ sensor [72]. Benzene series gases exhibited distinct gas-response features due to differences in redox rates at the surface of the Ti-doped Co₃O₄ sensor, providing an opportunity to discriminate benzene series via algorithmic analysis. This method can achieve 100% recognition of the benzene series. Pt-decorated WO₃ nanosheets with different electronic metal–support interactions are successfully prepared by finely tuning the oxygen vacancy structure of WO₃ nanosheets [103]. A rich-oxygen-vacancy Pt-WO₃ nanosheet-based sensor was prepared and exhibited a high response of 377.33 to 50 ppm ethylbenzene vapor at 140°C. Chen and Guo et al. successfully prepared an ultrafine Au nanoparticle-loaded porous ZnO nanobelts (Au NP-loaded porous ZnO nanobelts) [39]. The sensor based on Au NP-loaded porous ZnO nanobelts exhibited different temperature-modulated responses to BTEX gases. By employing linear discrimination and convolutional neural network analyses, highly effective BTEX identification was achieved across all investigated volatile organic compounds, a feat difficult to achieve with single chemiresistive sensors at constant working temperatures.

Fig. 5.

(a) Schematic illustration of the bi-layer structure of the BTEX sensor, in which an insulating CeO2 catalyst is coated onto the top-surface of the ZnO sensing layer. (b) Selectivity of ZnO-CeO2 and bare ZnO sensors towards different gases (all with identical concentrations of 50 ppm). (c) Response values of the ZnO-CeO2 sensor to a mixture of vapors (the concentrations of these gases were 20 ppm) at 200°C. (d) Response curves of the ZnO-CeO2 and bare ZnO sensors for toluene vapors with concentrations in the range of 10 ppb to 100 ppm. (e) The linear relationship between the sensing response and toluene concentration. (f) Reproducibility of the ZnO-CeO2 bi-layer sensor for 20 ppm toluene at 200°C. (g) Long-term stability testing results of the ZnO-CeO2 bi-layer sensor for 50 ppm toluene at 200°C; the time interval between two detection points is 7 days. All sensing measurements were conducted at 20°C and 60% RH. Adapted from Ref. [97].

Fig. 6.

(a) Schematic illustration of the Ti-doped Co3O4 sensor. (b) Response-recovery curves of a single 3-Ti-Co3O4 sensor toward 50 ppm toluene, o-xylene, m-xylene, p-xylene, and ethylbenzene gases at 280°C. (c) PCA based on features extracted from the response-recovery curves. Adsorption configurations and charge density differences of (d) O2 and (e) Ti-doped Co3O4. Adapted from Ref. [72].

Table 4.

Sensing performance of the MOS-based sensor on Ethylbenzene vapor.

The main challenge in ethylbenzene vapor detection is distinguishing it from other VOCs efficiently and accurately. It is very difficult to achieve this goal through material design alone. With the continuous development of machine learning technology, based on deep learning and neural network methods, by establishing an array of sensing systems, in-depth analysis of the response characteristics of different materials and different VOCs can more efficiently and accurately achieve the goal of identifying ethylbenzene gas. And this method is also becoming a hot topic in the development of MOS sensors [13, 104-106].

2.4 Xylene gas sensors

Xylene (C₈H₁₀, dimethylbenzene) is a typical aromatic hydrocarbon with two methyl groups on its benzene ring. It is divided into three isomers: ortho-, meta-, and para-, based on the relative position of the methyl groups, but their core chemical properties are almost identical [110]. Xylene has a pungent odor and is flammable. Although it belongs to the low-toxicity class of chemicals, inhaling high concentrations over a short period can cause damage to the heart, kidneys, liver, and lungs, leading to symptoms such as headache, dizziness, nausea, vomiting, difficulty breathing, and numbness in the limbs. In severe cases, it can cause convulsions, coma, and respiratory arrest. In addition, its chronic effects on the human body mainly manifest as headaches, dizziness, fatigue, sleep disorders, decreased appetite, nosebleeds, gum bleeding, hair loss, and skin bruising. Long-term exposure may cause keratitis and chronic dermatitis, and women may experience menstrual abnormalities [110].

Motaung et al. have conducted extensive research in this field. They prepared various CuO-ZnO (0.1-1.0 wt%) heterostructures by using the hydrothermal method [111]. CuO-ZnO (1.0 wt%) disclosed an excellent selectivity towards 100 ppm of xylene and ultra-low LOD of 9.5 ppb at 100°C. The superior gas-sensing characteristics could be ascribed to the high surface area, the formation of a p-n heterojunction, the strong chemical affinity, and the catalytic performance of p-type CuO toward xylene vapor. In addition, they designed and fabricated TiO₂ nanoparticles decorated with CuAg alloy nanoparticles, and the sensor based on 0.5% AgCu-loaded TiO₂ displayed a remarkable response (R_g/R_a = 33.2) to 100 ppm xylene and superior selectivity at 150°C [112]. Research on spinel-structured binary metal oxide gas sensors for xylene detection has also been reported [113]. Lu and Sun et al. applied spinel-type MOS ZnCr₂O₄ with yolk-shell microspheres for detecting xylene vapor, which exhibited good gas-sensing performance towards xylene at 225°C [114]. The sensor based on ZnCr₂O₄ microspheres showed good selectivity for xylene, with a high response of 200.7 at 100 ppm. Zhang et al. synthesized a hierarchical hollow NiCo₂O₄ microtubule for enhanced xylene vapor sensing [115]. The NiCo₂O₄-based sensors not only exhibited rapid response/recovery (20/9 s) to 100 ppm xylene at a relatively low working temperature of 220°C, but also showed a low detection limit (1 ppm), high response, excellent gas selectivity, and superior long-term stability.

Fig. 7.

(a) Online mass spectra of mixed gases without passing through the 0.9% Ru@CeO2 layer. (b) Online mass spectra of mixed gases passing through the 0.9% Ru@CeO2 layer. (c) Gas-sensing mechanism of the WO3/Ru@CeO2 bilayer sensor. Adapted from Ref. [116].

To enhance the gas-sensing performance of MOS for xylene vapor, a special bi-layer-structured sensor that leverages the synergistic effects of catalysis and sensitivity has been developed in recent years. Wang and Ma et al. constructed catalytically sensitive synergistic bilayer sensors by using Ru@CeO₂ nanosheets as the catalytic layer and WO₃ nanowires as the sensitive layer [116].

The bi-layer sensor exhibited a high response of 37.04 to 5 ppm xylene, reacting significantly to low concentrations as low as 1 ppb at 160°C. This method, which leverages the synergy of catalysis and sensitivity, is highly effective at enhancing the sensor's response to xylene gas. However, the catalytic properties of Ru and CeO₂ towards xylene, as well as their effectiveness towards other aromatic compounds with methyl groups, result in poor cross-selectivity. Zhang and Li et al. fabricated a bi-layer sensor consisting of an Ag@CeO₂ nanosheet layer and a WO₃ nanowire layer [117]. The sensor's response to 10 ppm xylene was 32.13, and it showed an exceptional response to even trace amounts, down to parts per billion (ppb). The idea of leveraging the synergistic effects of catalysis and sensitivity to enhance the gas-sensing performance of MOS gas sensors is not limited to benzene-series gases. It can be widely applied to improve the gas-sensing performance of gas sensors for other gases [118,119].

Due to xylene's relatively high chemical reactivity, numerous related studies have been conducted on MOS sensors [120-124]. As mentioned above, ethylbenzene and xylene exhibit higher activity than benzene and toluene, so the MOS sensor shows poorer selective response to ethylbenzene and xylene. Improving selectivity is a pressing challenge for xylene gas sensors. Motaung and Swart et al. developed a TiO₂ nanowire with the highest concentration of oxygen vacancies and Ti³⁺, which could influence the dual-selectivity functionality of TiO₂ nanowire toward C₇H₈ and C₈H₁₀ at 25 and 125°C, respectively. The sensor based on defect-rich TiO₂ nanowires displayed admirable, temperature-dependent dual-functionality, selectively detecting C₇H₈ and C₈H₁₀ vapors with robust performance at 25 and 150°C, respectively.

Table 5.

Sensing performance of the MOS-based sensor on Xylene vapor.

Table 6.

Sensing performance of the MOS-based sensor on Aniline vapor.

3. CONCLUSIONS

This review summarizes recent research on the MOS gas sensors for detecting BTEX and aniline vapors. The following conclusions and future perspectives have been drawn from a comprehensive review of MOS-based sensors for benzene-series gas detection.

Due to the presence of benzene rings, the benzene series exhibits excellent chemical stability, making them difficult to detect using MOS-based chemoresistance sensors. The main problems of MOS-based benzene-series gas sensors currently include: low sensitivity, mainly due to poor detection ability for low concentration (≤20 ppm) benzene-series gases; high working temperature, which poses safety hazards and is not conducive to the development of low-power and flexible devices; poor selectivity, which is also due to the similar chemical properties between benzene series. To address these issues, extensive research has focused on developing novel sensing materials. Precious-metal modification is an effective method for significantly enhancing the performance of MOS sensors for detecting benzene-series vapors. By modifying MOS or heterojunction nanocomposites with noble metal nanoclusters, the sensitivity of MOS can be significantly improved, and the operating temperature can be reduced. However, this method cannot address the poor selectivity of MOS sensors for detecting benzene-series gases. By leveraging the less gas-sensitive reaction characteristics of MOS to benzene-series gases and combining them with machine learning and a sensor array, the gas-sensing selectivity performance of MOS sensors to benzene-series gases can be effectively improved. We anticipate that this way of improving the selectivity and detection accuracy of MOS sensors will be an important development direction in the future.

This review summarizes recent progress and discusses future prospects for MOS gas sensors to detect benzene-series gases over the past few years. As an important chemical raw material and carcinogen, the strict monitoring and environmental testing requirements for the use of benzene derivatives will continue to increase. The detection system based on MOS sensors and deep learning will greatly advance this field.

Acknowledgments

This work was supported by the Science, Technology, and Innovation Commission of Shenzhen Municipality (No. JCYJ20240813153442055), Henan Provincial Natural Science Foundation General Project (No. 252300420391).

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