Nanostructured Metal Oxide Semiconductors towards Greenhouse Gas Detection

: Climate change and global warming are two huge current threats due to continuous anthropogenic emissions of greenhouse gases (GHGs) in the Earth’s atmosphere. Accurate measurements and reliable quantifications of GHG emissions in air are thus of primary importance to the study of climate change and for taking mitigation actions. Therefore, the detection of GHGs should be the first step when trying to reduce their concentration in the environment. Throughout recent decades, nanostructured metal oxide semiconductors have been found to be reliable and accurate for the detection of many different toxic gases in air. Thus, the aim of this article is to present a comprehensive review of the development of various metal oxide semiconductors, as well as to discuss their strong and weak points for GHG detection.


Introduction
Greenhouse gases (GHG) are compounds that absorb infrared radiation and prohibit it from radiating out into space and subsequently trap the heat in the atmosphere, contributing to global warming. Common GHGs are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and fluorinated gases (F-gases: Hydrochlorofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3)). Figure  1 shows some activities responsible for greenhouse gas emissions into the atmosphere.  Table 1. Contributions of the most important long-lived greenhouse gases to the increase in global radiative force from the pre-industrial era to 2019 [3].

Greenhouse Gases
Sources/Emission (%) carbon dioxide deforestation and combustion of fossil fuel/66 methane wetlands, termites, ruminants, rice agriculture, fossil fuel exploitation, landfills and biomass burning/16 nitrous dioxide soil and animal manure management, sewage treatment, fossil fuel combustion, and chemical industrial processes/7 CFC12 (dichlorodifluoro methane) refrigeration systems/5 CFC11 (trichlorofluoro methane) refrigeration systems/2 other refrigeration systems, electrical insulator, semiconductors and LCD panels manufacturing, etc./4 As can be observed in Figure 2, in comparison to 2012, the global emissions of GHGs (excluding land-use change) increased to 52.4 gigatonnes of CO2 equivalent (GtCO2 eq) in 2019 with a rate of 1.1% per year. This was mainly due to the increase in CO2 and CH4 emissions to 0.9% and 1.3%, respectively. Methane and carbon dioxide have an approximate share of 19% and 73%, respectively, and are by far the most prominent greenhouse gases. In addition, in 2019, the emission of F-gases (3% share) and N2O (6% share) have grown by a respective 3.8% and 0.8%. As can be obviously seen in Figure 2, in 2019, the total emission of GHGs (except those from land-use change) had 59% and 44% growth compared to 1990 and 2000, respectively. However, the reported emission growth in 2019 was equal to 1.1%, like the average annual growth rate between 2012 and 2019. Emissions of global greenhouse gases based on the type and source of gas. Reprinted with permission from reference [11].
The most significant effects of increasing concentration of greenhouse gases in the Earth's environment are the following [12,13]: • Global warming, • Intense droughts, • Severe storms, • Glaciers melting, • Depletion of the ozone layer, which can occur by emission of CCl4, CFCs and HCFCs, • Changing the cycle of plant life and rain patterns, • Rising sea levels and warmer oceans, • Changing the lives of wildlife species.
For these reasons, already in 1997, the Kyoto Protocol tried to mitigate the emissions of the six main greenhouse gases, namely CO2, N2O, CH4 and F-gases (HFCs, PFCs and SF6). In 2015, the Paris Agreement was adopted because of the irreversible losses and damages caused to nature and society by climate change. Together with increasing pressure on natural resources and land area associated with the loss of species and ecosystems, climate change is a serious threat to the world's capacity to provide clean water, sufficient food and safe homes. Thus, the goal of the Paris Agreement was to limit the increase in the global average temperature to well below 2 °C (ideally 1.5 °C) above pre-industrial level. To achieve these long-term temperature targets, the parties agreed to reach their peaks of GHGs emissions as soon as possible and then to cut them drastically in the second half of the 21st century. Several countries, including the European Union (EU), have committed to be climate neutral by 2050 or later. These targets mean that a balance between anthropogenic releases and withdrawals by GHG sinks will have to be reached soon [14]. Despite the negative effect of GHGs, such as climate change, global warming, rise of ocean levels and environmental degradation, their presence in the atmosphere is vital to provide a habitable temperature. If the concentration of GHGs in the atmosphere becomes totally zero, the average surface temperature of the Earth would be about −18 °C [15].
In order to verify estimations of global emissions, it is essential to measure the concentration of GHGs in the air. To this aim, innovative, cheap, robust and accurate sensors are needed, and metal oxide (MOX)-based sensors have the potential to fulfil these requirements [13]. A gas sensor is a device for both qualitative and quantitative detection of the volatile substances in the vapor phase. In general, gas sensors are composed of a receptor and a transducer. A semiconductor gas sensor can be considered as a sensor including a semiconductor material which works as a receptor and/or transducer. Semiconductors are divided into two groups of oxides and non-oxides which are commonly based on silicon. Oxide semiconductors can be used as a receptor/transducer, while non-oxide ones cannot work as receptors because of the presence of an insulating layer [16].
Over the past few decades, most studies have focused on developing various types of sensors by applying different sensing materials and transduction stages [17]. In addition, an increasing number of papers investigated metal oxide semiconductor materials as gas sensors in environmental monitoring and in industrial applications due to their advantages such as low cost, easy synthesis methods, simplicity of use, fast recovery speed, non-toxicity, their abundance on earth, easy underlying mechanism, production of small devices, long lifetime and superior performance in detecting a wide range of target gases such as flammable and toxic gases. Of the many studied metal oxides, n-type oxides (like SnO2, In2O3, WO3, ZnO and γ-Fe2O3), as well as p-type oxides (like CuO and Co3O4) showed interesting gas sensing features. However, due to stability issues, when operating at high temperatures, only SnO2, In2O3 and WO3 have been adopted in commercial sensors [16].
The literature on MOX-based sensors has provided evidence that they have excellent sensitivity concerning many greenhouse gases (mainly, CO2, CH4, O3 and N2O) as well as a great response with the variation in the concentration of target gas and working temperature of the device. Nonetheless, MOX-based gas sensors need further improvement in their performances to accurately monitor greenhouse gases [18][19][20]. Hence, the aim of this paper is to overview the latest achievements (in the last decade) in the application of nanostructured metal oxide semiconductor gas sensors for the detection of the main GHGs such as CO2, CH4, N2O and fluorinated gases (F-gases).

Sensing Mechanism and Parameters of Gas Sensors
Chemoresistive sensors are generally made of a sensing film deposited on an electrically insulating layer (also called the substrate) provided with electrodes. This device can incorporate a heater, separated from the electrodes by the substrate [21]. The electrodes utilized for the readout of sensor resistance can be applied on one or two-electrode configurations. In a one-electrode configuration, a spiral heater based on noble metals (platinum, palladium and gold) is embedded inside the sintered ceramic bead, whereas in a two-electrode configuration, two electrodes are deposited on top or underneath the sensing material layer. As can be seen in Figure 3a, cylindrical type electrode is composed of a cylindrical alumina substrate with the electrode made of platinum deposited on it or a wire of Pt wound up around it. In this electrode, the sensing layer can be deposited on the platinum film or on the Al2O3 substrate. In disk type electrode ( Figure 3b) the sensing material is in the form of a pellet with electrodes deposited on the two sides of the disk. The third electrode type is known as a parallel plate (Figure 3c), which has a planar substrate with a sensing layer. Then, two metal electrodes are deposited on both edges of the active film. Planar-type gas sensors consist of two electrodes that comb parallel to each other. This type of sensor can also be made of a sensing film that is deposited over interdigitated electrodes (IDE s) as shown in Figure 3d [22]. In the case of metal oxide semiconductor sensors, the sensing mechanism is defined based on reduction-oxidation reactions between the target gas and the oxygen species, which are adsorbed on the surface of the sensing material. Details on sensing models can be found in several books and featured papers [21,[23][24][25][26][27]. MOX sensors detect target gases due to reduction-oxidation reactions between the target gas and adsorbed oxygen species on their surface. When n-type metal oxide semiconductor sensors are exposed to a reducing gas atmosphere, like NH3, they decrease the sensor's electric resistance. On the contrary, the adsorption process increases the surface resistance of the sensor in the presence of an oxidizing gas such as CO2 and ozone. Due to their high electron affinity, oxygen species adsorb on the surface of n-type metal oxide semiconductors and trap free electrons. Electron trapping creates a potential barrier at the grain boundaries which limits the flow of electrons (i.e., there is the creation of a depletion layer or a space charge region). This leads to an increase of the MOX electric resistance. Successively, when the sensor is exposed to a reducing atmosphere, with gases such as NH3, the gas molecules react with active oxygen species and liberate free electrons in the bulk. This reduces the potential barrier height, favors the flow of electrons and reduces the sensor's electric resistance. In contrast, with oxidizing gases like CO2 and ozone, the adsorption process raises the surface resistance [28]. In p-type metal oxide semiconductor sensors, electron exchanges in the presence of a gas cause a decrease or an increase in electron holes in the valence band when they are exposed to a reducing gas or an oxidizing gas, respectively [27].
The oxygen species on the surface of the MOX can be either physisorbed or chemisorbed in the function of the temperature. Below 150 °C, the molecular form dominates and is physisorbed (Equation (1)), while above this temperature, the ionic species prevail: below 250 °C and above 550 °C (Equations (2) and (3)) [21].
By adsorption of the oxygen molecules on the surface of MOX, the electrons transferred from the sensing material to these molecules. This movement leads to the production of negatively charged oxygen molecules on semiconductors in such a way that causes upward band bending at grain boundaries in the semiconductor ( Figure 4).  To improve the sensor's performance, such as selectivity (the ability of a sensor to identify the target gas when in the presence of different gases; if the selectivity is low, the sensor response will be due to the contribution of any molecules in contact with it), sensitivity (the capacity to discriminate small changes in the analyte's concentration over a wide range of gas concentration values) and durability of the sensor, the crystallite size of MOX should be as small as possible (in the nanometer range), as reported by Yamazoe [16]. Furthermore, sensitizers should be finely dispersed, and the thickness of sensing layers should also be optimized [29]. In fact, in large grains containing partly depleted layers, the conduction in the entire grain width is not affected by surface reactions. Thus, the process of conduction occurs in the bulk region more conductive compared to the outer space charge region. When the conduction is parallel to the surface, the sensor presents a limited sensitivity [21]. For thin layers, the thickness of the layer is comparable to λD (the Debye length, a measure of the electronic "crosstalk" between the surface phenomena and the bulk electronic properties [30]), the influence of surface processes is extended to the whole layer (there is no longer a conducting channel and a resistive one), and the conduction is then a function of the concentration of electrons influenced by the surface reactions. In the case of porous layers, there are three possibilities for the necks between grains, including surface/bulk, grain boundary and flat bands, which are for enough large necks, large grains not sintered together and small grains with small necks, respectively ( Figure  4). Moreover, as can be seen in Figure 4, the barrier height (qVS) at the grain boundaries is a function of the density of surface state NS modulated by the previously mentioned oxidation-reduction reactions.
The performance of the sensor depends on various properties, including response and recovery time (the time needed for a sensor to reach a variation of 90% of the total resistance, impedance or capacitance during gas adsorption or desorption, respectively), selectivity, sensitivity, operating temperature (the temperature at which the gas sensor shows the highest response with respect to the target gas) and limit of detection (LOD, the minimum concentration of target gas detected at a known confidence level), etc. [13]. Furthermore, reversibility (the capacity of a sensor to recover its initial resistance, impedance or capacitance value when the atmosphere with the target gas is switched to fresh air), long-term stability, consumption of energy and low dependence on humidity are other significant parameters for gas [32]. Generally, pure metal oxide gas sensors have some disadvantages, including poor selectivity and low sensitivity towards lower concentration of target gas, high consumption of energy and high operating temperature [33].
Any method able to modify on MOX surface, surface stoichiometry (oxygen vacancies are responsible for enhanced performances), work function, the binding energy of atoms, the concentration of carbon contaminants, chemical activity of surface and so on is effective at improving gas sensor performances. However, to this aim, surface modification of metal oxides and semiconductors by catalytically active atoms remains the most used method. An increase of sensitivity and fast surface reactions can occur in the presence of catalysts on the surface of metal oxides and semiconductors [31].
Experimental results on MOX gas sensors showed that the addition of dopants or impurities significantly improved metal oxide semiconductors' features ( Figure 6). The addition of various additives (based on catalytically active metals, non-metals, alkaline earth metals, metalloids, etc.) to the MOX is one of the main techniques to increase the performance of gas sensors and this can be done by bulk doping and surface doping [30]. Table 2 shows some of these additives. In addition, it should be noted that the main doping additives include noble and transition metals [30].   [35]. However, when doping with precious metals, water dissociation is not favored on either metal's close-packed face, as Rh and Pt are relatively unreactive [36]. Calculations showed that neither H2O bilayers, nor half-dissociated monolayers, are thermodynamically stable on clean Rh (111). On the contrary, impurities such as C atoms provide favorable binding sites for H atoms detached from H2O molecules [37].
Nevertheless, it is almost impossible to synthesize polycrystalline metal oxide films with suitable grain faceting. In 1D sensors, planes and faceting depend on the synthesis parameters that allow a tighter control of the sensor's performances [31].
Many studies reported that the power consumption and operating temperature of MOX gas sensors have decreased by using nanocomposites and make them suitable for remote monitoring systems [38,39]. Moreover, the utilization of nanocomposites as sensing materials can remarkably improve the selectivity of sensors [40]. By solving the 3 "S" problems of gas sensors (the improvement of sensitivity, selectivity and stability) with metal oxide nanocomposites, these could be successfully applied for the detection of GHGs [34].
Sensor's resistance changes when in presence of the test gas. The response of n-type and p-type MOSs-base gas sensor can be defined by Equations (4) and (5), respectively: In Equations (4) and (5), Ra shows the resistance of the gas sensor in the presence of reference gas that is usually air, and Rg is the gas sensor's resistance when target gas exists [32]. Table 3 shows the resistance changes in the presence or absence of oxidizing/reducing gases in the case of metal oxide semiconductor-based gas sensors. Generally, reducing gases like CO, CH4, H2S, H2, SO2, NH3 and HCHO leads to a decrease or an increase of conductivity in p-type and n-type semiconductors, respectively. Whereas the opposite behavior is reported in the case of oxidizing gases such as N2O, NO, NO2, O3, Cl2 and CO2 for both types of semiconductor sensors [41].

Sensing of Carbon Dioxide (CO2)
Several studies deal with the detection and control of CO2 emission owing to its significant effect on global warming. The global concentration of CO2 in the atmosphere was determined from historical ice core studies and recent air monitoring sites from 1950 to 2021 (Figure 7) [42]. As can be seen, the level of CO2 in the atmosphere was about 310 ppm in 1950 compared to about 416.7 ppm in December 2021 [43], measured at Mauna Loa Observatory in HI, USA. The mean annual absolute increase over the past 10 years was 2.4 ppm/y. This resulted in the evolution of CO2 sensors for monitoring and controlling the levels of CO2 in the atmosphere and indoor air quality [44,45].  Table 4 illustrates a summary of various metal oxide semiconductor-based CO2 gas sensors. The ZnO nanorods of length (1-3 µm) without and with the addition of Ge, Nd and W were synthesized through the mechanochemical combustion method in reference [46]. In this paper, CO2 sensors based on pure ZnO were less sensitive than the metaldoped ZnO ones under the air atmosphere. The authors justified these results based on the temperature that has a significant role in the adsorption of CO2 molecules onto the surface of the ZnO based sensor and its electrical response. They determined the working temperature by measuring the sensitivity of all the doped ZnO samples under a constant concentration of CO2 (1,000 ppm). As can be seen in Figure 8, all ZnO specimens show similar behavior with regard to the effect of working temperature. In fact, the response of sensors increased with temperature and concentration of doping. Shohany et al. reported similar results with respect to the temperature [47].  From our experience on pure ZnO sensors for the detection of CO2, this sensing material is very sensitive not only to carbon dioxide but also to water vapor. On ZnO screenprinted sensors made from a commercial powder (Sigma Aldrich, Milan, Italy) and fired at 600 °C for 1 h, a sensor response was already observed from 100 ppm of CO2 under dry air at 200 °C. However, the response was significantly affected when humidity (75% relative humidity at room temperature) was introduced (Figure 9b; 250 ppm of CO2). A drift of the baseline was also evidenced during the measurements and in successive tests: The initial impedance value under dry air continuously increased (Figures 9a,b). To limit interferences with water vapor, an acetone gas sensor based on a core@shell structure ZnO nanowire-zeolite imidazole framework (ZIF) array was proposed by Yao et al. [70]. Both the selectivity and sensitivity to acetone were highly promoted due to the selectivity of zeolite imidazole frameworks which worked as sieve membranes. Subsequently, ZnO nanowires of various sizes were grown on a zinc oxide seeded sapphire substrate, followed by a hydrophobic and catalytic ZIF-CoZn coating. These authors found that thin ZnO (5 nm)-ZIF-CoZn nanowires had a better response toward 100 ppm acetone at 260 °C. In addition, the sensor response in the range from 10 to 90 RH% was almost constant. Such kinds of structures can also be investigated for the detection of GHGs in the air.  Molybdenum and tungsten oxide nanostructured thin film-based gas sensors (Mo1 − xWxO3, x = in the range of 0.6-1) were produced by Mohamed A. Basyooni et al. through reactive RF magnetron co-sputtering at 400 °C to detect carbon dioxide at room temperature [71]. Their results showed fast responses from 6.53 to 12.25 s and recovery times from 0.06 to 4.02 s for the detection of CO2 in the presence of UV light (365 nm) activation.
In recent years, there has been an increasing amount of literature on the synthesis of porous LaFeO3-SnO2 nanocomposites, Au-La2O3-doped SnO2 nanofibers and LaFeO3 microspheres by using various chemical methods in the presence of metals or metal oxides to increase the response of these sensors towards CO2. Zhang et al. described the synthesis of the LaFe2O3/SnO2 porous film-based sensor by mixing method and reported low response time (less than 20 s) at 250 °C [65]. The 5 wt% La2O3-SnO2 nanofibrous was obtained by using the electrospinning process. The results showed significant improvement in the response towards many types of gases, particularly carbon dioxide [58]. Copper oxide has also attracted more attention as a sensing material for CO2 detection with various structures such as nanocomposite, nanostructure, thick film and thin film [72][73][74]. Semiconducting pure and Gd doped CeO2 nanostructure were prepared by co-precipitation method for detection of CO2 in the range of 200-400 °C. According to the results, Gddoped CeO2 gas sensor showed maximum sensitivity at 250 °C, good stability and low operating temperature as well as a limit of detection of 800 ppm CO2, while maximum response for pure CeO2 was reported to be above 400 °C.

Sensing of Methane (CH4)
Methane is the simplest hydrocarbon, explosive, odorless and colorless gas, but it is known as a highly flammable GHG. Moreover, it can be considered as a significant constituent of natural gas such as compressed natural gas (CNG) and liquified natural gas (LNG), which are used as fuel for vehicles as well as in the generation of heat and electricity production. According to the report of the World Meteorological Organization [75], the increase of CH4 emissions from 2018 to 2019 (+11 ppb, part per billion) was negligibly lower than that reported between 2017 to 2018, although it was higher than the average during the last 10 years (+8 ppb/y). However, methane concentration in air is 262% higher with respect to the levels in 1750 when human activities started disrupting Earth's natural equilibrium. It reached 1889 ± 2 ppb in 2020. Thus, the detection of methane is essential due to the safety concern, although its detection is very difficult owing to the non-polarity and high enthalpy of the C-H bonds. Much of the current literature on MOX-based sensors pays particular attention to CH4 detection [76].
Several authors have studied the use of various SnO2 sensors for detecting CH4. Ptdoped SnO2 nanofibers (100-150 nm) were obtained by using the electrospinning method [77]. The results showed a response time of 4.48 s at 350 °C towards CH4 in the presence of 20 mol% of Pt. Bunpang et al. [78] measured a response time of 3.9 s at 350 °C towards 1 vol% CH4 for 0.5 wt% Cr-doped SnO2 nanoparticles. In recent years, Lijia Yao et al. synthesized Pd-SnO2 composite nanoporous structure by applying a low-power hydrothermal technique. They found that, in the presence of Pd, the gas sensing performance towards methane significantly increased compared to pure SnO2. As shown in Figure 10, by addition of 2.5 mol% Pd, the sensor showed fast recovery time (5 s), high repeatability, long-term stability and excellent response (17.60) toward 3,000 ppm of CH4 at the operating temperature 340 °C [76]. A similar study was also reported for the Pd-SnO2 gas sensor, which was fabricated by the sol-gel method, for the detection of CH4 in the air [79]. Their experimental results exhibited that the addition of palladium causes a remarkable increase in the sensor response to CH4 of about 6-7 times when compared with a pure tin dioxide sensor.
(a) (b) Figure 10. (a) The corresponding exponential correlation between the gas response and gas concentration; (b) stability state towards 3,000 ppm of methane at 340 °C based on 2.5 mol% Pd-SnO2 nanoporous composite. Reprinted with permission from reference [76]. Dongping Xue et al. [80] fabricated SnO2-loaded WO3 nanosheet composites and were able to detect CH4 with a response time of about 1.4 times higher than pure tungsten trioxide at the optimum working temperature of 90 °C. Figure 11 shows the optimal operating temperature towards 500 ppm of CH4. They observed that by raising the operating temperature up to 90 °C, the gas response was enhanced, while by a further increase in temperature, the sensor response decreased. γ-Fe2O3 nanoparticles were prepared using a green method for the detection of CH4 with a concentration of 100 ppm at the operating temperature of 150 °C [81]. The results showed a higher response and selectivity, as well as a short response and recovery time in comparison with euphorbia extracted γ-Fe2O3 nanoparticles. Pt-VOx thin film obtained by the magnetron sputtering technique also showed a high sensing response towards 500 ppm of methane at room temperature [82]. Au-decorated vanadium dioxide nanosheets exhibited a good sensing response in presence of methane concentration in the range from 100 to 500 ppm at room temperature [83]. A gas sensor based on VO2 nanoparticles was prepared by vapor transport method with a sensor response equal to 652 towards 500 ppm CH4 concentration at 150 °C [84]. The cobalt-doped zirconium oxide exhibited a superior recovery time and response time of 27 s and 19 s, respectively, under 100 ppm methane with respect to the pure ZnO sensor [85]. The NiO decorated ZnO porous nanosheets showed high response and long-term stability too at 340 °C towards 1,000 ppm of CH4. These results were higher than that of pure ZnO porous nanosheets, due to the larger surface area and interfacial interaction of NiO and ZnO by p-n heterojunction formation [86]. A summary of metal oxide semiconductor-based CH4 gas sensors is reported in Table 5.

Sensing of Nitrous Oxide Gas
Another important greenhouse gas is nitrous oxide (N2O), which is a colorless gas with a sweet odor. The World Meteorological Organization reported that N2O concentration in air was 333.2 ± 0.1 ppb in 2020 which was 123% more with respect to pre-industrial levels. The increase of N2O from 2018 to 2019 was comparable with the average growth rate over the last decade (+0.99 ppb/y), although the increase of nitrous gases in these years was lower than reported results between 2017 and 2018 [75]. For these reasons, it is very important to develop gas sensor for N2O for the protection of humans' health and the environment. Nevertheless, there is a relatively small body of literature on the use of metal oxide semiconductor-based sensors towards N2O detection.
Among the MOX-based gas sensors, several researchers reported SnO2 and WO3 for N2O detection. In2O3 based gas sensors provide noticeably sensing properties for the N2O detection too. WO3 nanowire mats and nanoparticle films were obtained by HF-CVD technique (hot filament chemical vapor deposition) and showed a good sensitivity towards N2O in the operating temperature range 100-500 °C [101]. In comparison with nanoparticle films, by increasing the temperature, the mat-like nanowire network provided higher sensitivity with response time and recovery time of 75 s and 360 s, respectively, at about 450 °C; 0.5 wt% Sm2O3-loaded SnO2 nanopowders were produced by the electrochemical route and showed high sensitivity towards 35 ppm of N2O in air. The experimental results highlighted that, by introducing Sm2O3, sensitivity increased up to approximately 1.5 times higher than pure SnO2 [102]. The sensing characteristics of nanowire indium oxide and tungsten trioxide were investigated by Rout et al. [103]. The In2O3 nanowire (~20 nm in diameter) sensor showed a response and recovery time of about 20 s along with a sensor response of 60 in the presence of 10 ppm N2O at 150 °C, whereas WO3 nanowire (5-15 nm in diameter)-based sensors exhibited a sensor response of 20-25 with a response and recovery times of 10 s and 60 s, respectively, under 10 ppm N2O at 250 °C. They reported that the sensitivity of indium oxide and tungsten trioxide nanowires is not affected in the presence of water vapor, up to 90% relative humidity. SrO-doped SnO2 demonstrated higher sensitivity and response time (three times) under 10-300 ppm N2O at 500 °C than pure SnO2 [38]. Table 6 summarizes the reported results in the literature on MOX-based N2O gas sensors.

Fluorinated Gases (HFCs, PFCs and SF6)
In the 19th century, the levels of methane were stable at 34.1 ± 0.3 ppt (part per trillion), while for both C2F6 and C3F8 gasses, they were below detection limits of 0.002 and 0.01 ppt, respectively. Around 1940, due to the high production of aluminum because of World War II, peaks corresponding to the emission of CF4 and C2F6 were detected. Furthermore, the emissions of PFCs were drastically decreased for CF4 (in 1980) and C2F6 and C3F8 (early to mid-2000s), but they then slowed down and probably stopped recently [8]. Due to their low concentration, these gases can be detected with good accuracy by gas chromatography with electron capture detector (GC/ECD). Figure 12 shows the baseline of fluorinated gas emission from 2005 to 2050. As can be seen, the emissions of F-gases are estimated to increase five times in this period. This increase can be described by a sixfold enhancement in the demand for air conditioners and refrigeration services, especially in developing countries, along with the effect of the replacement of CFCs and HCFCs with HFCs. A growth of 25% is expected for PFC emissions over the same period because of industry development and emission contractions in primary aluminum production. It is expected that SF6 emissions will enhance too (by almost 50%), owing to the increase in emissions from high and medium voltage switches as a result of the increase in electricity consumption and the development of magnesium production [6].
SF6 has been extensively applied in gas-insulated high-voltage switchgear (GIS, a compact metal encapsulated switchgear made of high-voltage components which can be safely operated in confined spaces) [105]. Due to partial discharge fault, sulfur hexafluoride decomposes, resulting in the generation of different low-fluoride sulfides. The reaction of low-fluoride sulfides with H2O and O2 in gas-insulating equipment leads to the production of different decomposition products such as SO2, H2S, SOF2 and SO2F2. In order to assess the operating state and insulation level of GIS, the partial discharge detection and analysis of content and composition of SF6 gas components have significant importance [106]. To this aim, the development of gas sensing equipment able to detect SF6 decomposition products can be considered as a proper way for online monitoring of partial discharge fault in GIS. Hongcheng Liu et al. [108] produced pure and NiO-decorated ZnO nanoflowers by the hydrothermal process for the detection of SF6 decomposition products (SO2, SO2F2, and SOF2). They reported that, by the addition of NiO, the properties of the ZnO-based sensor clearly increased, such as higher gas response, lower optimal operating temperature and shorter response and recovery time. Table 7 presents a brief summary of reported results on MOX-based sensors for the detection of decomposition products of sulfur hexafluoride. Au modified (1.2 at%) ZnO nanowires prepared by hydrothermal synthesis showed a high response (79.4) to 5 ppm H2S at room temperature in reference [109] due to the formation of nano-Schottky type barrier junctions at the interface between 5 nm Au particles and ZnO nanorods.
H2S has strong electron-accepting capacities and can be detected by In2O3 nanowire (80 nm in diameter) transistors down to 1 ppm at room temperature with reasonably fast response and recovery times (48 s and 56 s, respectively, when switching the atmosphere from under air to 1 ppm of H2S and vice versa) [110]. Single crystal whiskers and thick films of bipyramids of cubic In2O3 were grown by carbothermal method in reference [111] and showed that the single crystal whiskers were selective and could be used to detect 200 ppb of H2S while bipyramids films could be used in a 10-80 ppm range of hydrogen sulphide at room temperature. The lower detection limit of single crystal whiskers with respect to bipyramid crystals was attributed to a higher presence of singly charged oxygen vacancy defects.
Conductimetric sensors based on a suspended single CuO nanowire were able to sense H2S in dry and humid atmosphere (65 RH%) down to 10 ppb at 325 °C in reference [112]. Suspended nanowires are in a highly favorable configuration because they can be fully immersed in the gas atmosphere. Humidity-induced Debye length modulation was probably small compared to the large nanowire diameter (170 nm) thus, no evident increase or decrease of the resistance was evidenced when testing the sensors either in a dry or humid environment. These results are promising in view of the integration of CuO nanowires on CMOS-fabricated micro-hotplates to achieve miniaturized, low power consumption portable devices.

Conclusions and Perspectives
Accurate measurements and reliable quantifications of anthropogenic GHG emissions in air are of primary importance to study climate change and for taking mitigation actions. In this review, the general properties and gas sensing mechanisms of semiconductor metal oxides were first presented. Then, the factors that influence their sensitivity, selectivity and stability were also illustrated. Because of GHG concentration in air, MOX gas sensors have the potential to detect mainly CO2, CH4, N2O and PFC decomposition products in GIS in remote monitoring systems.
In the case of CO2 detection in air, MOX gas sensors already show the sensor response and the LOD compatible with the targeted application from room temperature and up to 400 °C operating temperatures. Their cross-sensitivity needs, however, to be fully evaluated (for example, ZnO sensors can detect almost all the above-cited GHGs (Tables 4-7)). Humidity strongly interferes when doing measurements in air. The core@shell structure based on ZnO nanowire-zeolite imidazole framework (ZIF) array could be investigated for GHG sensors with an improved selectivity with respect to water vapor.
Direct growth techniques of vertically oriented ZnO nanomaterials on electrode surfaces appear to be interesting manufacturing processes and allow the miniaturization of sensors and their incorporation in remote monitoring systems. The surface functionalization of vertically aligned ZnO nanomaterials with specific metal catalysts, dopants, and other metal oxides is a promising approach for improving sensing response, selectivity and response/recovery time too [123]. However, their ageing behavior has also to be studied: according to our experience in CO2 detection with ZnO screen-printed sensors, the sensor's response significantly decreases after one year of intermittent operation at 200 °C. Thus, other 1D nanomaterials have to be investigated for the detection of GHGs.
Methane monitoring is also of utmost importance since the COP26 in Glasgow and the Global Methane Pledge signed up to by more than 100 countries. The MOX gas sensors can detect CH4 in the concentration range from 60 to 20,000 ppm in the working temperature range from room temperature to 500 °C (Table 5). However, much effort will be needed to detect methane in air (targeting the 2 ppm current atmospheric concentration). The development of new nanocomposites could be the successful key to target this aim.
Nitrous oxide detection by means of MOX gas sensors is effective in the range 10-300 ppm at 200-500 °C (Table 6). For N2O detection in air, the sensor response and the LOD must be significantly improved too (the target is about 0.3 ppm). Moreover, for this application, significant improvements can be expected from new nanocomposite developments.
Finally, sulfur hexafluoride decomposition products such as SO2, H2S, SOF2 and SO2F2 can be evidenced by means of MOX gas sensors (Table 7). Even if concentrations of 20-100 ppm of these gases can be already detected, these performances could be considered as satisfying because GIS are compact metal encapsulated systems and the measurements are done in a closed environment and not in air. However, conductimetric sensors showed a lower limit of detection with respect to resistive semiconducting metal oxides and allowed the detection of hydrogen sulfide concentrations as low as 10 ppb at 325 °C.