Metal oxide nanomaterial-based sensors for monitoring environmental NO₂ and its impact on the plant ecosystem: a review

Abstract

The substantial increase in the emission of air pollutants due to the rapid industrial growth, combustion of fossil fuels, pesticides, and insecticides used in the agriculture sector in the last few decades urgently requires sincere remedial efforts from the global scientific community. The elevated concentration of nitrogen dioxide (NO₂) makes it one of the most common and catastrophic air pollutants. NO₂ acts as a precursor for ozone (O₃) and particulate matter (PM) and participates in the phenomena of acidification and eutrophication in plant ecosystems. Therefore, worldwide research is ongoing towards designing an accurate, reliable, and environment-friendly NO₂ gas detection technology for the monitoring of NO₂ concentration in the earth's environment. Nanostructured metal oxide semiconductors (NMOSs) have been found to be very consistent and precise for NO₂ sensing due to their outstanding structural and morphological properties. The first part of this review presents an overview of the effects of NO₂ pollution on the plant ecosystem. The second part briefly describes the various other available techniques for NO₂ detection. Further, this article presents a comprehensive review on the progress of a wide variety of NMOS-based NO₂ gas sensors. NMOS materials with diverse morphologies such as nanorods, nanoparticles, nanowires, nano flowers, and nanosheets, fabricated into NO₂ gas sensors, are discussed extensively. Moreover, their fabrication methods and performance in terms of sensitivity/selectivity, operating temperature, response/recovery time, and detection limit, along with their gas sensing mechanism, are explained in the review in order to establish a reliable NO₂ gas detection technology.

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1. Introduction

Air pollution is an immense concern worldwide in recent times and can cause ruthless problems for human health as well as for the ecosystem. According to the World Health Organization (WHO), with atmospheric concentrations of NO₂ higher than 30 μg m⁻³ on a 24 hour yearly average and 95 μg m⁻³ on a 4 hour average, NO₂ is considered as the chief air pollutant.¹ The burning of fossil fuels and emissions from vehicles and industries are the main sources of excess NO₂ in ambient air.² NO₂ can now produce ground level or “bad” ozone multiple times, which in turn is a potent greenhouse gas and air pollutant.³ This ground-level ozone may penetrate the stomata of plants, causing them to close, thus preventing CO₂ transport, and ultimately ensuing the reduction of the photosynthesis process.⁴ Excess NO₂ not only poses a threat to living organisms but also accounts for serious impacts on the environment via acid rain, eutrophication, the greenhouse effect, global warming, and climate change. Fig. 1 depicts the various natural and anthropogenic sources of NO₂ emissions and the effects of high concentrations of NO₂ on foliar physiology, plant metabolism, leaf nutrition, and the mechanism of plant growth.

Fig. 1 Schematic diagram of the sources of NO₂ pollution and their effects on plant ecosystems.

In view of the above information, sincere and effective efforts are required for the accurate detection of NO₂ concentrations in the ecosystem. Several techniques are available for the monitoring of nitrogen dioxide; however, only a few can be applied to measure NO₂ at concentrations below parts per billion (ppb). These techniques of NO₂ detection are mainly based on the surface conversion techniques in which NO₂ is first converted to NO, which is subsequently detected by chemiluminescence technique. Other NO₂ detection techniques include optical,⁵ photoacoustic,⁶ gas chromatographic methods,⁷ and nanostructured metal oxide semiconductor (NMOS)-based gas sensors.⁸ NMOS gas sensors are one of the emerging technologies among the various NO₂ sensing techniques.⁹ The NMOS gas sensors have many advantages in comparison to other available gas sensors^10,11 in terms of their abundance, cost-effectiveness, ease of fabrication, small size, simple integration in electronics, easy maintenance, as well as moderate stability and safety.¹² The NMOS materials possess a wide band gap, which allows them to have a full spectrum of electronic properties.⁹ Gas sensors based on wide band gap MOS are suitable due to their reversible and big changes in the electrical resistance, along with the excellent thermal, chemical, environmental, and radiation stability and mechanical robustness.¹³ The properties of the NMOS sensors are dependent on the material's size¹⁴ and its microstructure.¹⁵ The small grain size of the material provides a more specific area, which allows more oxygen ions and NO₂ molecules to adsorb on the surface of the material. The various studies on the particle size and micro-structural dependent sensing property of the NMOS-based sensors are given in the following sections of the article. There are many fabrication processes to obtain NMOS materials with diversifying microstructures and morphologies. Sensing materials with specific morphologies can be synthesized using hydrothermal and sol–gel methods. Meanwhile, different techniques, including physical vapor deposition (PVD), epitaxial growth, chemical vapor deposition (CVD) and thermal spray, were used for the fabrication of thin films as sensing layers having controlled thickness.¹⁶ In addition, there are plenty of methods, including doping, organic sensitization, inorganic heterojunction sensitization, and oxygen vacancy modification, that can be used to improve the NO₂ sensing performance of NMOS sensors.¹⁷ These methods govern the type of reaction and rate of gases at the surface of the sensor, and demonstrate a significant effect on the sensitivity, response time, recovery time, and selectivity of the sensors.¹⁸ The NMOS gas sensors require high temperature to operate. Currently, various efforts have been made for NMOS sensors that can operate at room temperature for monitoring NO₂. Thus, NMOS gas sensors are nowadays progressing in the direction of low operating temperature, simple integration, and low energy consumption. In spite of some complications of the operating temperature, NMOS sensors are still an appealing technology for measuring NO₂.

The main objective of this review article is to comprehensively study the NMOS-based NO₂ gas sensors in order to establish a strong detection technique for the NO₂ concentrations. This review also elaborates on the effects of elevated levels of air pollutant ‘NO₂’ on the plant ecosystem. Specifically, the mechanism of plant growth in the excess NO₂ environment is discussed in the following section. A wide variety of NMOS (ZnO, SnO₂, In₂O₃, TiO₂, WO₃, CuO, NiO, and Co₃O₄)-based NO₂ gas sensors are studied, along with their gas sensing mechanism. The recent major developments in NMOS-based NO₂ gas sensors are also reviewed. Moreover, gas sensing parameters of primary interest, such as gas response time/sensitivity, recovery time, selectivity, and the detection limit of MOS nanomaterials for NO₂ gas sensing, are discussed in detail. Furthermore, this review article highlights the performance of NMOS-based NO₂ gas sensors over the various other available NO₂ sensing techniques.

2. Effects of NO₂ on the plant ecosystem

Plants play an important role in the construction of the ecological environment by decomposing and absorbing harmful gases. The amount of biologically active nitrogen compounds has risen dramatically as a result of human activities, both industrial and agricultural, disrupting the natural nitrogen cycle. Nitrogen in several forms, primarily NO_x (NO, NO₂) and NH₃ as dry deposition, and NO₃ and NH₄ as wet deposition, pollutes the air. NO₂ can also be converted to nitrates, which can cause fine particle pollution as a secondary pollutant.¹ It acts as a precursor for harmful secondary air pollutants, mainly ozone and particulate matter. Nitrogen dioxide contributes to ground-level ozone through a complex sequence of chemical reactions that take place in the sunlight. NO₂ levels above a certain threshold can harm vegetation, causing leaf damage and reduced growth. It increases the susceptibility of plants to disease and frost damage. In the presence of sunlight, NO₂ reacts with other air pollutants and forms ozone, which can damage plants at high concentrations. An excessive amount of nitrogen dioxide in the atmosphere can damage plants, harm the leaves, and reduce the growth and yield of many crops.⁴ High NO₂ concentration levels have potentially toxic effects on plants, such as acid rains, which lowers the pH of water and soil. This gives rise to harmful effects, and even death of plants and biological systems. Presently, researchers hold different opinions towards the effects of NO₂ on plants. NO₂ can form organic nitrogenous compounds in the plants after being metabolized and incorporated into the nitrate assimilation pathway.²¹ However, most plants have low NO₂–N penetration into total plant nitrogen, as well as resistance towards NO₂. Exposure of plants towards NO₂ causes a variety of physiological responses, including changes in the antioxidant enzyme activity,²² nitrogen metabolic enzyme activity, and dispersal of nitrogenous metabolic products in plant tissues. Low concentrations of NO₂ in the water lead to the formation of nitrate and nitrite, which are further used by plants during the normal phase of nitrate metabolism; therefore, NO₂ may be used as an airborne fertilizer.²³ High NO₂ concentrations, on the other hand, may cause excessive accumulations of nitrite and acidification of cells.^24,25 It shows negative responses in plants, such as the development of reactive oxygen species (ROS). In addition, the reduction of nitrogen assimilation and plant growth with severe leaf damage, chlorosis in the plant, or even plant death, can happen. However, different plant organisms have different physiological responses towards NO₂. Therefore, the effects of NO₂ on plant ecosystems are still passionately debated, and no consensus has been achieved.

The corrosive and oxidative properties of NO₂ are the primary reason for plant injury that affects various biochemical and physiological processes after entering through the stomata of plant leaves. Several stomatal characteristics, such as dimensions and stomatal conductance, cuticular chemical compositions, and environmental factors, affect the foliar NO₂ uptake. The plant growth is reduced by NO₂ and the damage suffered by the plant is based on various factors, including the length of exposure, age of the plant, humidity conditions, light and edaphic factors.²⁶ Recently, Sheng et al.²⁸ explored the physiological and biochemical responses of two precious species, Carpinus betulus and Carpinus putoensis, for a high concentration of NO₂ stress and their recovery. The seedlings of the two species underwent fumigation with 12.0 mg m⁻³ NO₂ for 0, 1, 6, 12, 24, 48, and 72 h. In terms of peroxidase (POD) activity, the damage response of C. betulus under NO₂ stress appeared later than that of C. putoensis. C. putoensis exhibited more sensitive metabolisms towards NO₂ stress as compared to C. betulus. After stress removal, both Carpinus species recovered to normal growth through their metabolism after 30 days.²⁷ The leaf injury symptoms of both Carnipus species towards NO₂ exposure and after recovery, along with the fumigation test device setup, are shown in Fig. 2.

Fig. 2 Leaves of (A) Carpinus betulus (B) and Carpinus putoens showing injury symptoms under different NO₂ exposure times and after natural recovery. (c) The experimental set-up for timing regulation and recording of NO₂ concentration.²⁷

Sheng et al.²⁸ investigated the effects of NO₂ on biochemical responses in 41 garden plants by exposing all of the plants towards 6 μL L⁻¹ NO₂ in an open-top glass chamber. Measurements of the various physiological parameters, such as chlorophyll content (Chl), peroxidase (POD) activity, along with malondialdehyde (MDA) concentrations, and leaf mineral ion contents nitrogen, phosphorus, potassium, calcium, magnesium, and zinc of all 41 garden plants were carried out. The results reveal that NO₂ damages the plants and influences the Chl content in the leaf in most of the functional groups. Changes in mineral ion concentrations, as well as increases in POD activity and soluble protein and MDA concentrations, may serve as signals for inducing defense responses.²⁸ Chen et al. reported the effect of NO₂ fumigation oxidative stress and antioxidative response in Cinnamomum camphora seedlings at low-volume fractions for 60 days and demonstrated that 4.0 μL L⁻¹ of NO₂ exposure significantly reduced the Chl content in the leaves of Cinnamomum camphora seedlings. The change in the Chl content was responsible for changes in the leaf color that resulted in yellow spots or yellow leaves.²⁹ Hu et al. used a photosynthesis method and a scanning electron microscope to examine the effects of gaseous NO₂ on stomatal conductance, photosynthesis, dark- and photorespiration in Populus alba × Populus berolinensis hybrid leaves. The results indicated that after 14 and 48 hours of exposure towards 4 μL L⁻¹ of NO₂, the net photosynthesis of Populus alba × Populus berolinensis hybrid leaves decreased significantly, demonstrating that photosynthesis inhibition may be the reason for NO₂ growth suppression.³⁰ Sheng et al.³¹ reported leaf physicochemical responses, stomatal characteristics, and chloroplast structure of Carpinus putoensis W. C. Cheng after exposure to 6 μL L⁻¹ NO₂ for periods 0, 1, 6, 24, and 72 h and natural recovery of 30 days. The short-duration exposure towards a high concentration of NO₂ had major negative impacts (p < 0.05) on the Chl content, photosynthesis, and chloroplast-related physicochemical processes of C. putoensis leaves, except for one hour of NO₂ exposure, which was proven to be beneficial for the physiological responses of plant. Furthermore, exposure of NO₂ formed swelling of the thylakoids inside the chloroplasts and increased the thickness of the palisade/spongy tissue. This thylakoid swelling could be reduced by the removal of the pollutant from the air flow. Scanning electron microscopy (SEM) images, along with diurnal changes such as net photosynthesis (P_n), stomatal conductance (G_s), transpiration rate (T_r) and intercellular CO₂ concentration (C_i) of C. putoensis leaves in the form of bar graphs with exposure to 6 μL L⁻¹ NO₂ for 0, 1, 6, 24 and 72 h, are shown in Fig. 3.

Fig. 3 (A) (a) SEM images of a leaf cross-section, (b) leaf stomata in the control group, (c–e) NO₂-treated groups exposed to 6 μL L⁻¹ NO₂ for 1, 24 and 72 h, respectively, and (f) the post-recovery group. (B) The diurnal changes (bar graphs form) in photosynthesis in C. putoensis leaves grown with exposure to 6 μL L⁻¹ NO₂ for 0, 1, 6, 24, and 72 h after a recovery period; (a) net photosynthesis (P_n), (b) stomatal conductance (G_s), (c) transpiration rate (T_r) and (d) intercellular CO₂ concentration (C_i).³¹

It has been reported that the mature bean (Phaseolus vulgaris) tissue showed swelling of the thylakoids within the chloroplasts without any extrachloroplast damage after exposure towards 1 and 2 μL L⁻¹ of NO₂ for 1 and 2 h, and the swelling of the thylakoid could be inverted by removal of the air pollutant or by a change in the rate of gas flow within the tissue.³¹ Li et al. studied the NO₂ assimilation capacity of 70 woody plants. They demonstrated that deciduous broad-leaved trees possessed the maximum growth rate under high-concentration NO₂ stress, with Robinia pseudoacacia, Sophora japonica, Pterocarya stenoptera, and Cerasus serrulate showing the highest recovery among them.²⁷ The common symptoms experienced by several plants towards NO_x exposure, such as chlorosis of young needles and tip burn of older needles, were frequently detected in coniferous species.³³ Herringbone necrosis has been observed in the older leaves of beech, hazel, and apple trees. Various plant species demonstrate a water-soaked appearance on their leaves, which is followed by necrosis on exposure towards acute NO_x. Moreover, leaf glazing has been detected in the plants of the annual pea, cabbage, and spinach.³²

Air pollutants such as NO₂ can indirectly influence vegetation by chemical reactions in the atmosphere, and directly affect vegetation, soil, or water after deposition. The effect of pollutants on plants is considered by their adsorption and rate of uptake (flux) and the plant's reaction.⁴ High levels of nitrogen dioxide are detrimental for vegetation as it results in damage to foliage, a decrease in growth, and a reduction in crop yields.

In view of the above, it is necessary to determine the ecological and environment-friendly monitoring techniques to reduce atmospheric NO₂ concentrations. Therefore, researchers are trying to fabricate low cost, easy to process, fast response/recovery, selective, highly sensitive, and low operation temperature NO₂ sensors.

3. Different techniques for monitoring NO₂

The chemical sensor consists of a transducer and an active layer that converts chemical information into another form of an electronic signal, such as a frequency change, a voltage change, or a current change. Various gas sensor technologies, such as catalytic gas sensors, electrochemical gas sensors, semiconductor gas sensors, acoustic gas sensors, and optical gas sensors, have been used to detect NO₂ gas. The various NO₂ gas sensors, such as the MOS sensor, electrochemical sensor, catalytic sensor, optical sensor, acoustic wave sensor, chemiluminescence, and photoacoustic spectroscopy, are shown in Fig. 4. Table 1 displays the advantages/disadvantages associated with each technique.

Fig. 4 Different NO₂ gas sensors: (a) MOS sensor,⁴⁴ (b) electrochemical sensor,⁴⁵ (c) catalytic sensor,⁴⁶ (d) optical sensor, (e) acoustic wave sensor, (f) chemiluminescence and (g) photoacoustic spectroscopy.

Table 1 Different NO₂ detection techniques with their advantages and disadvantages

Name of the sensing technique	Measured quantities	Working principle	Advantages	Disadvantages
MOSs-based sensors	Conductivity	Conductometric	Low cost and long lifetime, wide range of target gases	Sensitivity to environmental components, high energy consumption, non-selective
Catalytic sensor	Temperature, resistance	Catalytic/gas oxidation	Simple and low-cost measures flammability of gases	Need of air or oxygen to work
Optical sensor	Polarization, light intensity, wavelength, etc.	Fluorescence, optical, etc.	Easy to operate in the absence of oxygen, unaffected from electromagnetic interference	High cost and miniaturization
Electrochemical sensor	Charge, current, resistance, voltage, inductance, etc.	Potentiometric, amperometric, resistive, etc.	Ability to measure toxic gases in low concentration	Easy contamination
Mass resistive sensor/acoustic wave sensor	Change in the characteristics such as velocity and amplitude	Acoustic	Long lifetime and no secondary pollution	Sensitive to environmental factors
Photoacoustic spectroscopy	Absorbed electromagnetic energy	Photoacoustic	High sensitivity	Miniaturization, stability
Chemiluminescence (fluorescence)	Dark current/photocurrent	Emission of radiation	Quick response high sensitivity	Non-linear behavior

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The sensing mechanism of each sensor depends on various properties, such as the operating temperature, selectivity, response time, detection limit, sensitivity, and recovery time, which are also well defined in a later section of this article.

3.1 Catalytic sensors

For almost a century, catalytic sensors have been used to detect combustible gases. Jonson discovered the first catalytic combustion type sensor in 1923.^33,34 Combustible gas mixtures do not burn until they attain a certain ignition temperature. However, in the presence of a particular chemical process, the gas will start to ignite even at lower temperatures.³⁵ This process is called catalytic combustion. A gas sensor made based on the catalytic principle is called a catalytic gas sensor. The catalytic gas sensor is classified into two types – thermoelectric type and Pallister type.³⁵

3.2 Electrochemical gas sensors

Electrochemical gas sensors allow gases to diffuse through a porous membrane to an electrode, where they are reduced or oxidized. These sensors work by reacting with a target gas and generating an electrical signal that is proportional to the gas concentration. These gas sensors are composed of a working electrode and a counter electrode, which are detached by a thin layer of electrolyte. Potentiometric, amperometric and conductometric are the categories of the electrochemical gas sensors. In these types of sensors, the resultant quantities, such as the potential, current, and frequent series of conductivities of the electrode, are measured.³⁶ Laref et al. evaluated the performance of a four-electrode electrochemical sensor for in-field nitrogen dioxide monitoring, and also proposed an empirical unsupervised recalibration method. This study revealed the effectiveness of the proposed drift correction approach.³⁷

3.3 Optical gas sensors

Optical gas sensors utilize optical absorption/emission scattering of gas molecules at determined optical wavelengths. Optical gas sensors comprise a photodetector component, a light-emitting element, a gas sensing element, and a filter for getting phosphorescence or fluorescence. Thin films of palladium or chemochromic oxides are generally used in most optical sensors. These thin films of oxides are coated along the length of an optical fiber and it is known as optodes. An infrared gas sensor is an example of this category.³⁸ Paliwal et al. developed a low-cost tabletop surface plasmon resonance system. They incorporated the RF magnetron sputtered WO₃ thin film with a gold-coated prism to excite the surface plasmon modes in the Kretschmann configuration (WO₃/Au/prim). The WO₃ thin film deposited at a sputtering pressure of 50 mT exhibited a very sharp SPR curve with a low value of reflectance (0.21) at the resonance angle of 46.6°. NO₂ gas was detected at room temperature by the optimized system (WO₃ (50 mT)/Au/prim). The WO₃/Au/prism SPR system was found to be suitable for the selective detection of low concentrations of NO₂ gas (0.5 ppm) with high sensitivity.³⁹

3.4 Acoustic wave sensors

The detection principle of acoustic wave sensors is based on a mechanical or acoustic wave. When the acoustic wave propagates through the material, a particular change is noticed in the characteristics of the wave (amplitude/velocity). The variation in velocity can be monitored by measuring the phase or frequency characteristics of the sensor. These sensors consist of a receptor and a transducer, which are sensitive to an analyte and convert the response into an electrical signal, respectively.^36,40 Li et al. used a cation-exchange method to synthesize PbS CQDs to enhance the performance of the SAW sensor. The Pb(NO3)₂-treated sensor showed potential for monitoring traces of NO₂ in the concentration range of 0.5–30 ppm with good sensitivity and selectivity at room temperature.⁴¹

3.5 Chemiluminescence

Emission of UV, IR, or Vis radiation by a chemical reaction or reactions defines a luminescent phenomenon that is widely known as chemiluminescence (CL). As an analytical technique, CL detection is widely known for its quick response, high sensitivity, and absence of undesirable luminescence in the background. This emission can be seen in the solid, liquid, and gas phases.⁴² In this manner, it can facilitate and widen its area of analytical applications. Among these gas phases, liquid-phase CL exhibits significant potential for analytical applications, while gas-phase CL plays a vital role in monitoring and measuring several gaseous and volatile elements in the environment.⁴³

3.6 Photoacoustic spectroscopy

Gas-phase spectroscopy is currently extremely famous in a wide scope of fields, for example, biology, atmospheric, chemistry, and medical sciences. This spectroscopy depends on the photoacoustic effect, which was first described by Alexander Graham Bell.⁴⁷ The spectroscopic gas sensors have proved to be worthwhile tools. For specific gas compounds, some applications require a sensor that can detect a wide range of gases, while others need high sensitivity. There is also a desirable selectivity, time resolution, and robustness. In this technique, there is no need for maintenance and sample preparation.⁴⁸ Peltola et al. demonstrated a simple and highly sensitive cantilever enhanced photo-acoustic sensor for the detection of nitrogen dioxide at 532 nm. It was observed that the sensitivity of the setup was enhanced by approximately a factor of eight for an averaging time of 120 s, and a CEPAS signal with a high laser power improved the NO₂ detection limit to as low as a few parts-per-trillion.⁴⁹

4. Metal oxide semiconductor (MOS) gas sensors

Metal oxides are recognized as suitable materials for gas sensor and biosensor applications. The surface of these materials possesses effective interaction with gas molecules. Using various tailoring techniques, the surface properties of metal oxide permit enhanced sensing properties. Reduction in the grain size at the nanoscale leads to a large active surface area, and induces novel effects like bandgap widening and room temperature photoluminescence due to quantum confinement. These innovative physical properties in the metal oxide nanostructures pave the way for the advancement in sensor technology with advanced parameters. Metal oxide nanomaterials with desired properties, such as composition, stoichiometry, phase control, and geometry, can be achieved by different synthesis methods. In order to achieve these properties, various synthesis methods, such as chemical vapor deposition, electro-deposition co-precipitation, sol–gel, thermal decomposition, microemulsion, solvothermal, and microwave-assisted methods, have been widely explored by researchers.⁵⁰ Various metal oxides, such as Fe₂O₃, ZnO, CuO, NiO, WO₃, TiO₂, MgO, and SiO₂, have been synthesized using these methods. Many efforts have been done by researchers in the last few years to improve the sensitivity of gas sensors. Metal oxides with different nanostructures, such as nanowires, nanofibers, nanotubes, nanosheets, and nanospheres, display excellent gas sensing properties as they possessed a large surface area and relatively mass reactive sites.⁵¹ In the vapor deposition process, metal oxides grow from atoms or molecules via the vapor–solid or vapor–liquid–solid process. Diverse morphologies, such as nanowires, nanorods, nanoribbons/belts, 2D nanosheets, and 3D hierarchically-ordered microstructures, have been achieved by controlling the parameters.⁵² The synthesis of metal oxide nanostructures by anodic oxidation or electrodepositions results in nanofibers, nanowires, nanorods, nanotubes, and 3D nanoflowers. Meanwhile, in the surfactant-assisted synthesis process, surfactants in the form of micelles and microemulsions are employed as templates, thus providing nanoparticles, 1D nanowires, nanorods, nanoribbons, nanotubes, and 3D nanospheres structures.⁵³ Metal oxide nanotubes and nanowires are mostly prepared through the hydrothermal method, anodizing processes, and sol–gel method. For example, SnO₂ nanowires prepared through glycolate precursors under mild conditions displayed good sensitivity towards certain gases, such as C₂H₅OH, CO, and H₂. Meanwhile, SnO₂ nanotubes using the multi-walled carbon nanotube (MWCNT) as a template prepared by the wet chemical method were more porous and showed excellent response towards ethanol and acetone.⁵⁴ In recent years, metal oxides have been widely studied as sensing materials for NO₂ gas sensing.⁵⁵ The metal oxide semiconductor gas sensors are composed of a sensitive metal oxide surface layer. The gas concentration of a target gas is measured by these metal oxides via measuring the electrical resistance. These sensors are based on the reversible gas adsorption process at the surface of the metal oxide.³⁸ Xin et al. synthesized a novel composite of PbS quantum-dots-modified MoS₂ by the combination of the hydrothermal method with the chemical precipitation method, and then fabricated into the gas sensor. It was found that the response of MoS₂/PbS is higher than that of the pure MoS₂ gas sensor, and the MoS₂/PbS gas sensor showed better selectivity compared with the pure MoS₂ gas sensor at room temperature. The response of the MoS₂/PbS gas sensor was about 50 times higher than that of the MoS₂ gas sensor at 100 ppm NO₂ concentration, and the recovery rate was also improved.⁵⁶

The gas sensing mechanism of MOS-based sensors

At first, when the MOS surface layer is heated in air, oxygen is adsorbed on the surface of the metal oxide. In this process, the adsorption of oxygen forms an ionic species, such as O₂⁻, O⁻ and O²⁻, and these species obtained electrons from the conduction band. The kinematics of this adsorption process can be explained via these eqn (1)–(4)^57,58

O₂(gas) ⇔ O₂(absorbed) (1)

O₂(absorbed) + e⁻ ⇔ O₂⁻ (2)

O₂⁻ + e⁻ ⇔ 2O⁻ (3)

O⁻ + e⁻ ⇔ O²⁻ (4)

These oxygen species are stable at different temperatures; for example, O₂⁻ shows stability at 100 °C, and O⁻ shows stability between 100 and 300 °C, while O²⁻ is stable above 300 °C.⁵⁹ When an electron transfers from the conduction band to chemisorbed oxygen, then there is a reduction in the electronic concentration. It is observed that the resistance increases for the n-type semiconducting metal oxide, while it decreases for the p-type metal oxide. Fig. 5(a) describes the mechanism of the decrease in resistance as the conduction increases in the case of the n-type MOS sensor. Sensing properties depend on the operating temperature, and this dependence could result in the change in adsorption and desorption rates of oxygen ions at the metal oxide surface.⁶⁰ Generally, environmentally unsafe gases can be classified into two categories depending on their oxidizing and reducing effects. H₂S, CO, NH₃, CH₄, and SO₂ gases fall in the group of reducing gases, while NO₂, NO, N₂O, and CO₂ gases fall into in the oxidizing group. When metal oxide surfaces are exposed to oxidizing gases, then these target gases react with the adsorbed O⁻ ions and adsorb directly on the surface of the metal oxide. NO₂ is a strongly oxidizing gas. When the molecules of this gas interact with the metal oxide surface through adsorbed oxygen ions, the potential barrier consequently increases at the grain boundaries.^61,62 As a result, the thickness and resistance of the depletion layer increase, as shown in Fig. 5(b).

Fig. 5 (a) Schematic diagram: sensing mechanism of n-type and p-type MOSs. (b) The interaction of oxygen and NO₂ with n-type semiconducting metal oxides. Charge distribution following the adsorption of oxygen (left) and NO₂ (right) on the surface of the metal oxide. The resulting potential distribution across the grain boundaries is also shown in the figure.

The oxidizing reactions for different oxidizing gases are given as ((5)–(12)):^63–69

NO₂(gas) + e⁻ → NO₂⁻(ads) (5)

NO²⁻(ads) + O⁻(ads) + 2e⁻ → NO(gas) + 2O²⁻(ads) (6)

NO(gas) + e⁻ → NO⁻(ads) (7)

2NO⁻(ads) → N₂(gas) + 2O⁻(ads) (8)

N₂O(gas) + e⁻ → N₂O⁻(ads) (9)

N₂O⁻(ads) → N₂(gas) + O⁻(ads) (10)

CO₂(gas) + e⁻ → CO₂⁻(ads) (11)

CO²⁻(ads) + O⁻(ads) + 2e⁻ → CO(gas) + 2O²⁻(ads) (12)

For the oxidizing gas, the gas sensing response for n-type (Sⁿ_ox) and p-type (S^p_ox) semiconducting oxide is generally explained by these eqn (13) and (14):

Sⁿ_ox = R_og/R_a[n-type] (13)

S^p_ox = R_a/R_og[p-type] (14)

where R_og and R_a are the electrical resistances of the sensors in the presence of the target oxidizing gas and pure air.

On the other hand, when metal oxide surfaces are exposed to reducing gases, the gas reacts with the chemisorbed oxygen by releasing an electron and goes back to the conduction band. The reducing reactions between the chemisorbed oxygen species and reducing gases are generally given as eqn (15)–(20).^70–82

2NH₃ + 3O⁻(ads) → N₂ + 3H₂O + 3e⁻ (15)

2NH₃ + 3O²⁻(ads) → N₂ + 3H₂O + 6e⁻ (16)

H₂S + 3O⁻(ads) → H₂O + SO₂ + 3e⁻ (17)

H₂S + 3O²⁻(ads) → H₂O + SO₂ + 6e⁻ (18)

CO + O⁻(ads) → CO₂ + e⁻ (19)

CO + O²⁻(ads) → CO₂ + 2e⁻ (20)

In the case of reducing gases, the gas sensing response for n and p-type semiconducting oxide is given by eqn (21) and (22):

Sⁿ_rd = R_a/R_rg[n-type] (21)

S^p_rd = R_rg/R_a[p-type] (22)

where R_rg and R_a are the electrical resistances of the sensor in the presence of the target oxidizing gas and pure air, respectively.

The gas sensing response of the MOS sensors relies on the ratio between the thickness of the depletion layer and the bulk semiconductor. Additionally, various other factors determine the gas-sensing mechanism of the specific nanostructured metal oxides semiconductors (NMOS). In these factors, the surface-to-volume ratio is an important factor, i.e., if the size of the metal oxide nanoparticles decreases, the number of adsorbed gas molecules increases, and then it results in a higher gas response.⁸³ The gas response strongly depends on the diameter, i.e., shape and size of the nanorods/nanowires of the metal oxides. If the metal oxide nanorods/nanowires have a small diameter, it directly suggests that more atoms are available for surface reactions.^84,85

For practical use, the gas sensors must meet various requirements based on the interesting field of application. These requirements are described by some fundamental properties of a gas sensor. The most commonly used gas sensing parameters with their definitions are summarized below:

Sensitivity

It is concerned with the changes in electrical resistance of the sensing layer due to the adsorbed or chemisorbed species of the oxidizing or reducing gas in its contact.⁸⁶

Selectivity

The selectivity of any gas sensor is determined by the sensor's ability to differentiate the desired gas from the mixture of gases at the same concentration level.

Response

When the sensor is exposed to an analyte gas, then the resistance of the sensor is changed. This change in resistance as a function of analyte gas concentration is defined as a response.
where R_a and R_g show the respective sensor resistance in air and analyte gas, respectively.⁸⁷

Response time and recovery time

The response time is the time required for achieving 90% of the final sensor signal,⁸⁸ while the recovery time is defined as the time required by the sensor to reach 10% of the baseline value.

Limit of detection

LOD is defined as the capability of a gas sensor to measure the least possible concentration of the analyte gas with reliability under the given conditions.

Working temperature

The temperature at which the gas sensor shows the highest response for a certain amount of the analyte gas is known as the working temperature of a gas sensor.⁵²

Repeatability

The repeatability of a sensor is defined as when it shows the same characteristics, such as response, sensitivity, and selectivity over the repeated tests.

Stability

Stability stands for the ability of a sensor to retain its properties when continuously operated over a longer time.

5. Nanostructured metal oxides semiconductors (NMOS)-based NO₂ gas sensors

Various n-type MOSs-based gas sensors, such as ZnO,^89–91 SnO₂,^92,93 In₂O₃,⁷⁷ TiO₂,⁹⁴ and WO₃,⁹⁵ have been studied for their excellent gas-sensing performance (response/recovery time (t_res/t_rec), selectivity, operating temperature (°C), and LOD) for NO₂, as listed in Table 2. One of the most outstanding features of the MOS-based NO₂ gas sensors is their fast response due to the strong oxidation of the NO₂ molecules.⁹⁶ The ZnO cross-linked nanowalls with a pore size of (200 nm to 500 nm) exhibited fast response/recovery times of 23/11 s and a high response value (6.4) towards 50 ppm NO₂ at RT with good repeatability.⁸⁹ The ZnO hierarchical nanostructure-based gas sensor prepared by drop-cast method showed a significant 28-fold improvement over the traditional widely adopted nanowire-based gas sensor with a fast response/recovery time of 72/69 s at RT for 20 ppm NO₂.⁹⁰ The sensor based on the SnO₂ material with a tube-like structure (5–10 nm) exhibited a very fast response time of 6 s and a low LOD of 9.7 ppb at RT. The sensing action was due to the nanocrystalline SnO₂ grains and the high density of defects (twin boundaries & dislocations).⁹⁷ The sensor using mesoporous In₂O₃ was reported with its high response value of 158.7 towards 97 ppm NO₂, with a fast response time of 96 s and a low LOD of 970 ppb at RT.¹⁹ The sensor made of TiO₂ nanoparticles (6.5 nm) prepared by hydrothermal method exhibited an ultra-high response of the value of 1093 to 40 ppm NO₂, fast response/recovery times (48/52 s), and a low LOD of 0.02 ppm at RT, but the selectivity among NH₃, H₂ and CH₄ was very poor.⁹⁴ The large surface area and pore volume of the TiO₂ nanoparticles provide a high concentration of oxygen vacancies and interstitial defect states on the surface. Thus, efficient adsorption and desorption of NO₂ gas molecules are feasible methods that can result in high sensitivity of the sensor.

Table 2 Summary of various NMOS-based NO₂ gas sensors with their sensing parameters

Material	Structure	Synthesis method	Target gas	C (ppm)	Operating temp. (°C)	Response	t res/trec	LOD	Ref.
ZnO	Nanowalls	Solution	NO2	50	RT	∼6.2	23/11 s	∼5 ppm	89
NiO/WO3	Plates	Annealing	NO2	30	RT	4.8	2.5/1.1 s	5 ppm	124
WO3/S/rGO	Nanorods	Hydrothermal	NO2	20	RT	149.5	6/56 s	0.25 ppm	125
NiO/rGO	Nanosheets	Hydrothermal	NO2	0.25	RT	0.04	576/121 s	∼0.25 ppm	126
ZnO	Nanowires	Drop-cast	NO2	20	RT	32	72/69 s	∼5 ppm	90
NiO/SnO2	Nanosheets	Annealing	NO2	60	RT	∼7.5	—	∼5 ppm	102
ZnO/rGO	Nanowalls	Solution	NO2	50	RT	9.61	25/15 s	∼5 ppm	117
CuO/rGO	Nanosheets	Chemical solution	NO2	1	RT	14	66/34 s	60 ppb	114
ZnO			NOx	1	RT		240/— s	10 ppb	91
SnO2/S/rGO	Nanoparticles	Hydrothermal	NO2	5	RT	20.31	40/357 s	1 ppm	127
ZnO	Nanorods	Wet chemical route	NO2	1	RT	100	∼5/∼20 min	∼1 ppm	128
α-Fe2O3/rGO	Nanospheres	Hydrothermal	NO2	90	RT	150.63	—/1648 s	0.18 ppm	129
ZnO	Nanowires	CVD	NO	10	RT	46	—	1.5 ppm	130
NiO/CuO	Flower-like	Hydrothermal	NO2	100	RT	77.16	2 s/—	1 ppm	131
SnO2	Nanotubes	Electrospinning	NOx	9.7	RT	89.2	6/218 s	9.7 ppb	97
In2O3/rGO	Layers	Reflux	NOx	97	RT	1.45	25/— s	970 ppb	132
SnO2	Nanocrystals	Chemical precipitation	NO2	11	RT	33	100/250 s	∼3 ppm	133
ZnO			NOx	1	200		60/— s	0.1 ppm	134
In2O3/SnO2	Nanorods	Electrospinning	NOx	100	RT	8.98	4.67/— s	0.1 ppm	118
SnO2	Thin films	Pulsed laser deposition	NO2	4	RT	7730	3/176 s	∼4 ppm	92
CeO2/graphene	Nanosheets	Solvothermal	NOx	300	RT	12.76	1.3/— s	5 ppm	135
In2O3	Octahedra	Sol–gel	NO2	200	RT	∼70	∼500/∼500 s	0.1 ppm	136
SnO2/rGO	Nanoparticles	Hydrothermal	NO2	1	RT	3.8	14/190 s	50 ppb	121
SnO2	Nanowires	—	NO2	10	RT, UV assisted	∼85	10/15 min	∼0.1 ppm	93
In2O3	Mesoporousnanocrystals	Hydrothermal	NOx	97	RT	158.7	96/— s	970 ppb	19
TiO2	Thin films	Rf-Sputter	NO2	250	RT, UV assisted	∼1.8	100/210 s	100 ppm	137
TiO2	Nanodots	Nano-oxidation	NO	10	RT	31	91/184 s	∼5 ppm	138
In2O3/TiO2	Nanofibers	Electrospinning	NOx	97	RT	41.1	3/— s	97 ppb	119
TiO2	Nanoparticles	Hydrothermal	NO2	40	RT	1093	48/52 s	0.02 ppm	94
In2O3	Nanoparticles	Ultrasonic spray pyrolysis	NOx	100	250		600/— s	0.1 ppm	20
CuxO/graphene	Nanoflowers	Reflux	NOx	97	RT	95.1	9.6/— s	97 ppb	139
ZnO/SnO2	Nanorods	Wet chemical	NO2	0.5	RT, UV assisted	1266	7/8 min	∼200 ppb	120
MoO3	Thin films	Magnetron sputtering	NO	200	RT	92	30/1500 s	5 ppm	140
Al2O3/TiO2	Nanotubes	Induction	NOx	97	RT	88.04	∼8/∼8 s	0.97 ppm	141
WO3	Nanoplates	Chemical	NOx		300		80/— s	0.5 ppm	95
ZnO1−x	Sheet-like	Suspension flame spraying	NO2	1	RT	∼2.568	60/230 min	0.25 ppm	142
MWCNTs/WO3	Nanoparticles	Hydrothermal	NO2	5	RT	14	10/27 min	0.1 ppm	143
PdO/Co3O4	Nanocubes	Chemical precipitation	NO2	20	RT	27.33	—	1 ppm	144
CuO	Microspheres	Reflux method	NOx	97	RT	64.93	5.33/— s	0.97 ppm	98
WO3	Hollow microspheres	Hydrothermal	NOx	2.5	300		—	0.5 ppm	145
K2O/In2O3	Nanowires	Template	NOx	97	RT	151.78	12/— s	48.5 ppb	146
α-Fe2O3/rGO	Nanoparticles	Hydrothermal	NO2	5	RT	3.86	76/946 s	0.1 ppm	147
Cu2O/rGO	Nanoparticles	Chemical solution	NO2	1	RT	5.2	29.2/76.8 s	100 ppb	148
ZnO	Microwires	Surface etching	NO2	20	RT, UV assisted	411	221/118 s	∼10 ppm	149
ZnO/graphene	Spheres	Solvothermal	NO2	50	RT	8	132/164 s	∼1 ppm	150
Co3O4/rGO	Thin sheets	Hydrothermal	NO2	800	RT	∼8	1.5/1 min	60 ppm	151
ZnO	Nanoparticles	Hydrothermal	NO2	20	RT, UV assisted	85	26/16 s	∼1 ppm	152
In2O3/rGO	Nanosheets	Hydrothermal	NO2	30	RT	8.25	4/24 min	∼5 ppm	153
SnO2/graphene	Nanoparticles	Sol–gel	NO2	20	RT	∼9.5	1/5 min	5 ppm	154
CuO	Virus-like	Chemical solution	NO2	4	RT	28.1	22/42 s	1 ppm	99
SnO2	Nanoparticles		NOx		300			0.5 ppm	155
Pd/ZnO	Nanoparticles	Sol–gel	NO2	50	RT	45.2	67/250 s	10 ppm	106
WO3/MWCNT/rGO	Nanoparticles	Hydrothermal	NO2	5	RT	17	7/15 min	1 ppm	156
In2O3	Nanostructures	Arc-discharge	NO	50	RT, UV assisted	41.7	∼10/— min	∼2 ppm	157
Au/SnO2	Nanoparticles	Sputtering	NO2	50	RT	90	70/— s	600 ppb	113
CuO	Nanoplatelets	Sonochemical method	NO2	40	RT	53 737	—	—	100
WO3	Nanoparticles	Chemical	NOx		350		180/— s	1 ppm	158
Ag/SnO2/rGO	Nanoparticles	Hydrothermal	NO2	5	RT	2.17	49/339 s	1 ppm	110
NiO	Nanosheets	Microwave synthesis	NO2	10	RT	0.56	—	—	101
Au/VO2	Nanowires	CVD/ion sputtering	NO2	5	RT	3.22	∼50/∼600 s	∼0.5 ppm	109
Pd/Ga2O3	Nanowires	Thermal evaporation	NO2	100	RT	41.44	200/70 s	~10 ppm	107
SnO2	Nanofibers	Chemical	NO2	5	200		85/110 s	1 ppm	159
CeO2/rGO	Bilayer	Spray	NO2	10	RT	20.5	92/— s	∼1 ppm	160
SnO2/rGO	Nanoparticles	Hydrothermal	NO2	1000	RT	22.87	100/— s	1 ppm	122
Fe3O4/graphene	Nanoparticles	Hydrothermal	NO2	400	RT	24.2	275/738 s	∼30 ppm	161
ZnO/rGO	Nanorods	Oriented growth	NO2	1	RT	119	75/132 s	50 ppb	162
NiO	Nanosheets	Hydrothermal	NO2	60	RT	3.05	∼200/∼300 s	∼5 ppm	102
Al/NiO	Nanosheets	Solvent-thermal	NO2	10	RT	2.77	50/200 s	250 ppb	101
NiO	Nanosheets	Hydrothermal	NO2	60	RT	1.8	∼250/∼250 s	∼7 ppm	103
Cu/Cu2O	Hollow spheres	Hydrothermal	NO2	10	RT	6.27	34/— s	—	111
Co3O4	Nanoparticles	Thermal treatment	NOx	100	RT	52.1	—	100 ppb	104
Sb/WO3	Nanoparticles	Chemical solution	NO2	10	RT	51	∼150/∼200 s	∼1 ppm	96
WO3/MWCNTs	Nanoparticles	Metal organic decomposition	NO2	0.1	RT	0.25	10.5/20 min	100 ppb	163
Cellulose/Fe2O3	Nanoparticles	Hydrothermal	NO2	200	RT	∼1100	50/30 s	1 ppm	164
WO3-δ	Films	Granule spray	NO2	10	RT	18 500	17/25 s	1.88 ppm	165
CNT	Thin films		NO2	100	165		∼30 min	10 ppm	112
Co3O4	Flower-like	Hydrothermal	NO2	60	RT		∼25/70 s	—	105
SnO2-CuO/rGO	Composite	Hydrothermal	NO2	50	RT	∼250%		150 ppb	166
α-Fe2O3	Thin film	Hydrothermal	NO2	100	RT	∼86%	∼31/32 s	—	167
V2O5	Microbelts	Mold-casting	NO2	100	RT		∼35/47 s	—	168
N-GQDs/ZnO composite	Nanosheet to spherical	Hydrothermal	NO2	05	100 °C	∼22%	—	0.1 ppm	169
CuO/rGO	Nanoflakes	Hydrothermal	NO2	05	RT	∼400.8%	6.8/55.1 s	50 ppb	115
SnO2/RGO	Spherical	Hydrothermal	NO2	100	250 °C	∼88.9	12/34	—	116
SnO2	Nanowires	Lithography and thermal evaporation	NO2	05	RT	∼50	7 min/100 s	—	170
CuO/ZnO	Nanowires	Thermal oxidation and sol–gel	NO2	100	250 °C	∼4.1	25/150 s	—	171
Cd-Doped Co3O4	Nanosheet	Microwave-assisted solvothermal	NO2	10	RT	∼3.38	—	154 ppb	172

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P-Type nanostructures, such as CuO,^98–100 NiO,^101–103 and Co₃O₄,¹⁰⁴ have also been reported as good NO₂ sensing materials. Hierarchical CuO microspheres prepared by using the facile reflux method exhibited high sensitivity of 64.9 and response time of 5.33 s to 97 ppm at RT. Good selectivity, stability, and low LOD of 0.97 ppm NO_x gas are the other advantages of these sensors. This was attributed to the hierarchical structure of the CuO microspheres that allows for effective and rapid gas diffusion towards the sensing surfaces.⁹⁸ The sensor based on CuO virus-like microspheres synthesized by the chemical solution method exhibited a high response of 28.4 and response/recovery times of 22/42 s towards 4 ppm NO₂ at RT.⁹⁹ CuO nanoplatelets-based NO₂ gas sensors prepared by sonochemical method revealed an ultra-high response to 40 ppm NO₂ operated at RT. This sensor exhibited good sensitivity, selectivity, and low power consumption due to the small crystallite size, higher surface area, and point defects.¹⁰⁰ The NO₂ gas sensors were also prepared using self-assembled polycrystalline hexagonal mesoporous NiO nanosheets,^102,103 and the sensors showed much better responses to NO₂ operated at room temperature. The micro flowers-like Co₃O₄ structures have also been synthesized using a low-cost single-step hydrothermal method.¹⁰⁴ The hierarchical 3D flower-like Co₃O₄-based sensor has a fast response/recovery time of the order ∼25/70 s and good selectivity towards 60 ppm NO₂ at RT. In a recent investigation, Kumar et al.¹⁰⁵ synthesized Co₃O₄ flower-like structures using the hydrothermal method. It was also noticed that the 3D flower-like Co₃O₄ (Fig. 7d) sensor at RT attained the maximum response to 60 ppm of NO₂ gas at ∼22%, and these hierarchical 3D micro flowers might become the reason for the improved sensor response. In this way, the Co₃O₄ sensor could be a promising candidate for NO₂ detection at RT. The resistance response during cyclic exposure at different and fixed concentrations, sensor response versus different chemical inputs and stability of the sensor at RT are shown in Fig. 7a, b, c, e and f, respectively.¹⁰⁵

5.1 Metal-doped NMOS-based NO₂ gas sensors

The gas-sensing performance of NO₂ gas sensors has also been improved by the doping of noble metals, such as Pd,^106,107 Au (ref. 108 and 109) and Ag,¹¹⁰ and metal ions, such as Al³⁺,¹⁰¹ Cu²⁺,¹¹¹ Sb⁵⁺ (ref. 96) and Zn²⁺,¹¹² in MOS. The noble metals are highly active catalysts that produce more oxygen species by adsorbing oxygen molecules, and more oxygen ions are formed on the surface of the MOS. Thus, the gas sensing performance is enhanced. On the other hand, doping with metal ions in MOS can increase the number of active sites and defects on the surface of MOS. Thus, increase in the number of oxygen species and enhancement of the adsorbed gas molecules on the sensor's surface increase the gas sensing performance.

A Pd/ZnO nanoparticles-based Schottky diode gas sensor fabricated by thermal evaporation method showed a high response with a value of ∼45.2 and good response/recovery times of 67/250 s for 50 ppm NO₂ at RT.¹⁰⁶

The Au-loaded SnO₂ thin film was prepared using DC/RF sputtering and heat-treated at 600 °C in argon (Ar) atmosphere for the duration of 1 h. Fig. 6(a) shows the SEM images, while the response-recovery curves of the sensor in various concentrations of NO₂ are shown in Fig. 6(b). The graph between the response and response time at different operating temperatures is shown in Fig. 6(c). The sensor shows good selectivity compared to the other gases, including H₂, CO₂, NH₃, and C₄H₁₀, as shown in Fig. 6(d).¹¹³ The sensor revealed a good response with a value of 90 and a fast response time (70 s) towards 50 ppm NO₂ at RT compare to pure Au and SnO₂ film-based sensors. The high performance of the sensor can be ascribed to the efficient dispersion of Au-nanoparticles over SnO₂ films. In this study, it was observed that the gold nanoparticles acted as the catalyst leading to oxygen dissociation, which facilitated the adsorption of oxygen ions, as shown in Fig. 6(e), while the extraction of more electrons from the conduction band is shown in Fig. 6(f), due to which the sensing response property improved.¹¹³

Fig. 6 (a) SEM image of the Au/SnO₂ thin film, (b) response-recovery curves of the Au/SnO₂ sensor in various concentrations of NO₂, (c) response and response time at different operating temperatures, (d) selectivity of the fabricated sensor for various target gases, and (e and f) the schematics of the sensing mechanism.¹¹³

Fig. 7 (a) Log scale I–V curves. Resistance response during cyclic exposure at (b) different and (c) fixed concentrations. Sensor response versus (d) Co₃O₄ FESEM and (e) different chemical inputs, and (f) stability of the sensor at RT.¹⁰⁵

The Al/NiO-based gas sensor prepared by solvent-thermal technique exhibited fast response/recovery time (50/200 s) and very low LOD of the value of 250 ppb towards NO₂.¹⁰¹ The response value of the Al/NiO sensor is 19.66 to 10 ppm of NO₂ at RT, which is higher than that of pure NiO (0.56). This enhancement of the response value can be attributed to the increased oxygen vacancies after Al³⁺ doping.¹⁰¹

5.2 Carbon NMOS-based NO₂ gas sensors

The NO₂ gas sensing performance of MOS can be improved by combining them with other metal oxides or carbon-based nanomaterials. The sensor-based on CuO/rGO nanohybrid nanosheets fabricated by the hydrothermal method can detect a very low concentration of NO₂ (60 ppb) with a good sensitive response (18 to 1 ppm NO₂).¹¹⁴ The sensor presented a fast response/recovery time (66/34 s) and high stability for sensing down to 5 ppm NO₂. This CuO/rGO heterojunction-based sensor was also found to be very selective towards NO₂ against various interfering gases, such as CO₂, SO₂, and HCHO.¹¹⁴ CuO nanoflakes modified with rGO nanosheets (CuO/rGO) were synthesized by using a cost-effective hydrothermal method with thermal treatment. The synthesized composite CuO/rGO-based sensor showed a high response behavior of ∼400.8% towards 5 ppm NO₂ gas with good stability (30 days) and reliable repeatability.¹¹⁵ SnO₂/rGO-based sensors showed good sensing performance towards NO₂ gas with a fast response (12 s) and recovery time (34 s), as well as good selectivity and repeatability. The enhancement in the sensing performance was observed due to the synergistic effect of SnO₂ and rGO.¹¹⁶

5.3 Composites and MOS heterostructures-based NO₂ gas sensors

Various composite nanomaterials, such as, NiO/SnO₂,¹⁰² ZnO/rGO,¹¹⁷ CuO/rGO,¹¹⁴ In₂O₃/SnO₂,¹¹⁸ In₂O₃/TiO₂,¹¹⁹ ZnO/SnO₂,¹²⁰ and SnO₂/rGO,^121,122 and their sensing parameters are also listed in Table 2. The NO₂ sensor-based In₂O₃-composited SnO₂ nanorod heterostructures were prepared using the electrospinning method.¹¹⁸ The sensor exhibits remarkable response times of 4.67 s and LOD as low as 0.1 ppm at RT, and showed a response value of (8.98) for 100 ppm NO_x, which was 11 times higher than that of pure SnO₂ nanorods.

The ZnO/SnO₂ composites (with a Zn and Sn molar ratio of 1 : 1) were prepared by hydrolyzing SnCl₂ on ZnO nanorods using the wet chemical method. The ZnO/SnO₂ composite nanomaterials-based sensors showed an excellent response value of 1266 to 0.5 ppm NO₂ at RT and a low LOD of ∼200 ppb.¹²⁰

Recent investigations demonstrated that heterojunction structures of various materials could be a good candidate for gas sensors as they improved the gas sensing performances by charge transfer effects. Especially, graphene-based metal oxide heterojunction structures are emerging as the best candidates for gas sensing operations because they have high mobility carriers to enhance the charge transfer effects.¹²³

Liu et al. reported the enhancement in NO₂ gas sensing performance by ZnO/rGO nano composite heterojunction structures. The ZnO nanowalls were vertically grown on the rGO thin film using a soft solution process to obtain a heterojunction structure. The ZnO/rGO nanocomposite-based sensor showed a higher response of 9.61 and fast response/recovery speeds of 25/15 s to 50 ppm NO₂ at RT. The improvement in these sensing properties was observed due to the 3D network of planar rGO sheets and porous structure of ZnO nanowalls, and a strong connection between the ZnO nanowalls and rGO.¹¹⁷ A similar study on the SnO₂/rGO hybrid composite-based NO₂ sensing film was carried out by Zhang et al.¹²² using the hydrothermal method, in which SnO₂ nanocrystals were attached on the surface of the rGO sheets. The SnO₂/rGO hybrid composite sensor exhibits a higher response value than the pure rGO film sensor towards NO₂. This may be due to the formation of a p–n heterojunction at the interface between rGO and SnO₂. The Fermi energy of rGO is higher than that of SnO₂, so electrons can be transported to SnO₂ from rGO, which can improve the adsorption of NO₂ molecules, resulting in enhanced gas-sensing performance.¹²²

A chemically prepared SnO₂–CuO/rGO ternary composite-based sensor showed 8–15 times higher sensing response than those of CuO/rGO towards 50 ppm NO₂. It was observed that the SnO₂–CuO/rGO ternary composite showed a low limit of detection (150 ppb), good selectivity, and long-term stability. These results can be attributed to the synergistic effect and nanostructure nature of the composite.¹⁶⁶ The SnO₂–boron nitride nanotubes-based gas sensor was studied for NO₂ detection.¹⁷³ A design of a MOS-based (SnO₂–BNNTs) sensing device is shown in Fig. 8. A thin film of BNNTs was coated onto the sensor platform by a spin coating process. Furthermore, SnO₂ was deposited on the surface of BNNT by the RF-sputtering technique. Pt wire electrodes were connected with the help of a silver paste for making the NO₂ sensor device (shown in Fig. 8).

Fig. 8 The schematic of the flow process for the SnO₂–BNNTs sensor.¹⁷³

The response/recovery times were studied by exposure of varied NO₂ concentrations from 250 ppb to 5 ppm at different operating temperatures (25–300 °C). The SnO₂–BNNTs sensor showed good selectivity and a maximum response of ∼2610 towards 5 ppm NO₂ at low operating temperature.

Fig. 9(a) shows the morphology of the as-developed sensor, and the gas response towards NO₂ can be seen in Fig. 9(b). Fig. 9(c) shows the change in resistance of the sensor to 5 ppm of various target gases (hydrogen, benzene, ethanol, carbon monoxide, and nitrogen dioxide) at optimal operating temperature. The sensor showed the highest sensor response towards NO₂ gas and lower response to other interfering gases. This was due to the modulation of the space charge depletion layer at the p–n heterojunction, which encouraged the tunneling effect by the trapping of NO₂ molecules and offered conducted channels through BNNTs to charge carriers at low operating temperatures (shown in Fig. 9(d)).¹⁷³

Fig. 9 (a) FESEM image of SnO₂–BNNTs; inset shows the enlarged view. (b) Gas responses of the sensor for 5 cycles to 250 ppb NO₂ at 100 °C. (c) Change in resistance of the sensor towards 5 ppm of various target gases (hydrogen, benzene, ethanol, carbon monoxide, and nitrogen dioxide) at 100 °C, (d). Variation of the space charge region at the interface of the n-type SnO₂ and p-type BNNTs–SnO₂ in the absence and presence of the NO₂ target gas.¹⁷³

In addition to the MOS sensing technique, several other detection techniques are available for NO₂ detection, such as optical, photoacoustic, gas chromatographic methods, and chemiluminescence. Although these sensing techniques provide good sensitivity, selectivity, and limit of detection, the large size and high cost of these conventional gas detectors, such as gas chromatography–mass spectrometry (GC–MS), Fourier transform infrared (FT-IR) and photoacoustic spectrometry, restrict their usage for portable applications. On the other hand, NMOS-based gas sensors are compact, portable, and cost-effective. Therefore, NMOS-based sensors can be a suitable replacement for these technologies.

Thus, the monitoring of NO₂ gas from the environment is necessary. Therefore, the development of an efficient sensing technology is extremely important. The above review and Table 2 summarize the outcome of various research studies for the detection of NO₂ gas using a wide range of materials and a variety of synthesis methods. The composite materials consisting of MOS as one of the components are highly promising candidates to be used as practical NO₂ gas sensors because of their high response, fast response/recovery time, and high repeatability and durability.

6. Fabrication of NMOS-based NO₂ gas sensors

The NMOS-based NO₂ gas sensors are fabricated by direct growth of MOS nanostructures on the substrate material through ultrasonic spray pyrolysis,²⁰ thermal evaporation,^106,174 CVD,¹³⁰ electrospinning,¹¹⁸ PLD,⁹² magnetron sputtering,¹³⁷ microwave synthesis,¹⁰¹ lithography and thermal evaporation.¹⁷⁴ A different process for the manufacturing of MOS-nanostructured NO₂ gas sensors is pre-synthesis, followed by deposition (using spin coating, paste printing, and PLD methods) of the as-prepared MOS-based nanomaterials synthesized by hydrothermal,^{19,102,103,125,126} solvothermal,¹³⁵ sol–gel,^106,136 and others.^175,176

An easy process for the fabrication of a MOS-based nanomaterials sensor for the detection of NO₂ gas is pelletization. In this approach, MOS nanomaterials are pressed into a pellet with the desired diameters and thicknesses under a high pressure of ∼10 MPa. An appropriate adhesive agent such as ethanol is mixed and ground prior to pressing the MOS material to make a pellet. Soldering wires (Au and Pt) are soldered on both sides of the prepared pellet using Au and Ag paste to shape a sensor constituent.¹⁷⁵

In addition to these methods, micro-electromechanical systems (MEMS) NO₂ gas sensors are mostly designed on metal oxides. MEMS-based gas sensors are trendy and have several advantages, such as their compact size, good repeatability, good mechanical stability, and low power expenditure, and they are well-suited for wearable applications.¹⁷⁷ In MEMS, the design of the sensing device is started on a wafer. The wafer is then decorated using the photo-lithography process for heater configuration, followed by electrode deposition using physical vapor deposition (thermal and e-beam evaporation methods). In the next step, the micro-heaters are isolated using a passivation layer like Si₃N₄ by lithography and reactive ion etching. Furthermore, inter-digitated-electrodes (IDE) are patterned on wafers by lithography, followed by Pt coating and lift-off process. The sensing area is also defined using a layer of MOS-based nanostructures on the device by lithography, followed by deposition and lift-off processes.¹⁷⁸

The excess concentration of NO₂ gas is a great challenge for agriculture practices. The concentration of NO₂ gas in the atmosphere/ecosystem should be monitored smartly for intelligent agricultural applications for high crop yield, and this can only be possible with the design of smart metal oxide-based NO₂ sensors.

The MOS-based NO₂ gas sensors applications mainly include the refinement of air quality equally in indoor and outdoor environments. MOS-based NO₂ sensors have significant down-streaming applications in various areas such as commercial areas, pesticides, and insecticides in the agricultural sector.

7. Conclusions

With its high concentration, nitrogen dioxide (NO₂) is one of the harmful and widespread air pollutants for human health, as well as to the plant ecosystem. The excess NO₂ in the ambient air can tarnish the plants in several ways by deteriorating their leaves, limiting their overall growth, and capitulating several crops. To prevent the ecosystem from this harm, the precise monitoring of NO₂ gas levels in the atmosphere/ecosystem is needed for high crop yield. The NMOS-based sensors are found to be more appropriate in terms of their low cost, robustness, LOD, accuracy, long lifetime and wide range of target gases compared to other sensing techniques. MOS and its composites consisting of the rGO and catalyst (Pd, Pt, Au) with nanorods/nanowires morphology have been found to have excellent sensing characteristics for NO₂ gas.

Until now, NMOS-based sensors detect NO₂ gas in a few seconds. Therefore, designing NO₂ gas sensors with a response time of a few milliseconds or microseconds is still challenging. The approach to pick up the ultrafast sensors depends mostly on the interconnection between the NO₂ gas molecules and the surface of the sensing layer, and the transportation of charge in MOS. The fast transfer of charge in sensing devices based on MOS heterostructures can provide a notable improvement in the response and recovery time. In addition, light-assisted MOS-based NO₂ sensors have opened a new way to design fast, reversible and room temperature-operated NO₂ sensors. Thermal isolation in integrated circuits is essential to overcome the high operating temperature challenge in MOS-based sensors. Environmental conditions, such as temperature, humidity, corrosion, residual charges, and poisonous vapor, significantly decrease the strength, consistency, and repeatability of the NO₂ gas sensors. The excellent selectivity and repeatability with no drift fault owing to age and environment are still big challenges for researchers. Efforts are necessary to conquer the challenges of MOS-based NO₂ gas sensors, and to manufacture capable and trustworthy sensor devices for their use in the agriculture field and real-life applications. A more efficient and suitable NMOS-based sensor can be designed by doping, organic sensitization, inorganic heterojunction, and oxygen vacancy modification.

CRediT authorship contribution statement

Shrestha Tyagi: writing – original draft, methodology, data curation. Manika Chaudhary: investigation, formal analysis, software. Anit K. Ambedkar: investigation, preparation. Kavita Sharma: writing – review & editing, conceptualization, visualization, validation. Yogendra K. Gautam: writing – conceptualization, review & editing, supervision. Beer Pal Singh: supervision, resources, conceptualization, funding acquisition.

Conflicts of interest

The authors have no conflicts of interest to declare.

Acknowledgements

Author Ms. Shrestha Tyagi would like to acknowledge the Council of Scientific & Industrial Research (CSIR), Government of India, for providing financial assistance [File no. 09/113(0031)/2020-EMR-I].

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