Nanostructured Metal Oxide Semiconductors towards Greenhouse Gas Detection

Abstract

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.

1. 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.

The main reasons for the increment in CO₂ concentration in the atmosphere are fossil fuel and land-use change by about two-thirds and one-third, respectively [2]. During the first three decades about half of released CO₂ is removed, whereas a further 30% is removed over a few centuries and the remainder will stay in the atmosphere for many thousands of years [2].

Methane is the main component of natural gas which has been extensively utilized in domestic and industrial applications as a source of energy. It is emitted by both natural and anthropogenic sources with 40% and 60% proportions, respectively [3]. In comparison with carbon dioxide, CH₄ has more potential for global warming, so that any rapid increase in its concentration in the atmosphere would be devastating. Thus, at the recent COP26 (Conference of the Parties in Glasgow), over 100 countries signed up to the Global Methane Pledge to decrease global CH₄ emissions by 30% by 2030.

Agriculture, such as soil and management of animal manure, is the main anthropogenic source of N₂O. Moreover, it is naturally emitted from a broad range of biological sources that exist in soils and oceans. Nevertheless, the major contributions of N₂O are due to fossil fuel combustion, sewage treatment and chemical industrial productions such as nylon [4]. Nitrous oxide has a noticeable effect on climate change, global warming and stratospheric ozone layer depletion. In comparison with carbon dioxide, N₂O causes about 300 times atmosphere warming per unit weight, and it can remain in the atmosphere for up to about 114 years [2,5].

Fluorinated gases are synthetic gases used in industrial applications (as substitutes for ozone-depleting substances such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and halons) since they are not harmful to the atmospheric ozone because they do not damage the atmospheric ozone layer. Hydrofluorocarbons (HFCs) are, for example, extensively applied in industrial and commercial refrigeration systems, heat pump equipment and air conditioners. In addition, HFC gases are used as blowing agents for fire extinguishers, solvents, foams and aerosol propellants. Perfluorocarbons (PFCs) find many applications such as cosmetics, as well as in the pharmaceutical, refrigeration and electronic industries. These gases are also by-products of the aluminum smelting process (like CF₄, carbon tetrafluoride, PFC-14, which is the most abundant perfluorocarbon in the atmosphere). They can be found in old fire protection systems too [6]. PFCs can also be used as chemical tracer molecules to monitor the movement of supercritical CO₂ for carbon capture and sequestration purposes. To this aim, usually, a sample of carbon-based sorbents is collected from either soil gas or the atmosphere and then analyzed in a laboratory [7]. C₂F₆ (perfluoroethane, PFC-116) is released predominantly during aluminum production and semiconductor manufacturing. Among the PFC group, C₃F₈ (perfluoropropane, PFC-218) is the least abundant and is released during the production of semiconductors and utilized as a refrigerant as well [8]. In comparison with CO₂ gas, F-gases such as SF₆ have a significant role in global warming (23,000 times more than CO₂). From a climate overview, hydrofluorocarbons are by far the most relevant fluorinated gases with a short atmospheric lifetime. In contrast, PFC gases, including CF₄, C₂F₆ and C₃F_8, are long lived and they can remain in the atmosphere for about 50,000, 10,000 and 2600 years, respectively.

One of the important GHGs is ozone that is formed and destroyed through the chemical reactions of other pollutants, including volatile organic compounds and NO_x [2,9]. Other chemically reactive gases such as sulfur dioxide and carbon monoxide with short atmospheric lifetime are commonly removed by raining or natural oxidation processes, so they have a variable concentration in the air

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

presents some GHGs with their emission percentage in 2019 [10]. As can be seen, the highest and lowest emission percentage of greenhouse gases are attributed to carbon dioxide and CFC11, respectively.

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.

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).

2. 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 inter-digitated 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 $O_2^{-}$ is physisorbed (Equation (1)), while above this temperature, the ionic species prevail: $O^{-}$ below 250 °C and $O^{2-}$ above 550 °C (Equations (2) and (3)) [21].

$$O_{2 \left(ads\right)}+ é \to O_{2 \left(ads\right)}^{-}$$

(1)

$$O_{2\left(ads\right) }^{-}+ é \to 2O^{-}$$

(2)

$$O_{2 \left(ads\right)}^{-}+ é \to O_{ \left(ads\right)}^{2-}$$

(3)

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).

Figure 5 demonstrates the effective factors on the response of MOX gas sensors. 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 λ_D (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. Influence of bulk doping on gas-sensing characteristics of SnO₂ and In₂O₃ sensors [13].

Additive	Effect	Nature
Noble metals (less than 5 wt%) (Pd, Pt, Rh, Ag, Au)	increases response to reducing gases decreases operating temperature decreases response time	catalytic effect change of A/D parameters decrease of O2 dissociation temperature
Al2O3, SiO2	increases sensor response improves thermal stability	decrease of grain size decrease of area of intergrain contact increase of porosity
Ag (Ag2O), Cu (Cu2O)	increases response to H2S, SO2	two-phase system phase transformations during gas detection
Fe (Fe2O3)	increases response to alcohols	change of oxidation state
Ga (Ga2O3), Zn (ZnO)	increases sensor response	Decrease of grain size Increase of porosity
P, B	Improves selectivity	Creation of new phase
Ca, K, Rb, Mg	Increases sensor response Improves thermal stability	Decrease of grain size
La, Ba, Y, Ce	Improves thermal stability Increases sensor response	Stabilization of grain size (creation of new phase) Decrease of grain size
Transition MOXs (<0.5 wt%) (Co, Mn, Sr, Ni)	Increases sensor response Improves selectivity	Catalytic effect Change of electron concentration Change of A/D parameters Change of grain size

shows some of these additives. In addition, it should be noted that the main doping additives include noble and transition metals [30].

Humidity usually exhibits reducing features, even if the oxidizing effect of water was evidenced by Staerz et al. on the WO₃ surface at 300 °C [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:

$${\mathrm{S}}^{\mathrm{n}}=\frac{{\mathrm{R}}_{\mathrm{g}}}{{\mathrm{R}}_{\mathrm{a}}}$$

(4)

$${\mathrm{S}}^{\mathrm{p}}=\frac{{\mathrm{R}}_{\mathrm{a}}}{{\mathrm{R}}_{\mathrm{g}}}$$

(5)

In Equations (4) and (5), R_a shows the resistance of the gas sensor in the presence of reference gas that is usually air, and R_g is the gas sensor’s resistance when target gas exists [32].

Table 3. Change of resistance value for MOX-based sensor in the presence of oxidizing/reducing gases.

Type of Sensitive Material	Target Gas/Variation of Resistance	Response
p-type	reducing/increases	${\mathrm{S}}^{\mathrm{n}}$$= \frac{{\mathrm{R}}_{\mathrm{g}}}{{\mathrm{R}}_{\mathrm{a}}}$
	oxidizing/decreases	${\mathrm{S}}^{\mathrm{p}}$$= \frac{{\mathrm{R}}_{\mathrm{a}}}{{\mathrm{R}}_{\mathrm{g}}}$
n-type	reducing/decreases	${\mathrm{S}}^{\mathrm{p}}$$= \frac{{\mathrm{R}}_{\mathrm{a}}}{{\mathrm{R}}_{\mathrm{g}}}$
	oxidizing/increases	${\mathrm{S}}^{\mathrm{n}}$$= \frac{{\mathrm{R}}_{\mathrm{g}}}{{\mathrm{R}}_{\mathrm{a}}}$

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].

3. Nanostructured Metal Oxide Semiconductor (NMOS)-Based Greenhouse Gas Sensors

3.1. 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. Brief summary of metal oxide semiconductor-based CO₂ gas sensors.

Material	Structure/Synthesis Method	Target Gas/ Concentration (ppm)	Operating T (°C)	Response (%)	Ref.
ZnO	thin film/chemical spray pyrolysis	CO2/400	350	65	[48]
ZnO	nanowires/sol-gel	CO2/15	200	1.04	[49]
ZnO	nanostructures films/sol-gel	CO2/50 (sccm)	RT	1.0	[41]
Na/ZnO	spin-coated			81.9
CeO2	nano pellets/co-precipitation	CO2/800	400	~33	[50]
Gd/CeO2	nano pellets/co-precipitation	CO2/800	250	45
W/ZnO	nanorods/mechanochemical combustion	CO2/1000	450	~98	[46]
La/ZnO	nanopowders/hydrothermal	CO2/5000	400	65	[51]
Ca/ZnO	nanopowders/sol-gel	CO2/5%	450	113	[52]
Ca/ZnO	thin film/wet chemical	CO2/25,000	400	32	[53]
CuO/BaTiO3	BaTiO3 spheroids decorated with CuO microleaves/co-precipitating	CO2/1000	140	52	[54]
CuO	porous film/pneumatic spray pyrolysis	CO2/100	RT	1.04	[55]
Zn/SnO2	thin films/spray pyrolysis	CO2/500	310	94.4	[56]
ZnO/SnO2	nanocomposites/screen printing	CO2/70	RT	~0.64	[57]
La2O3/SnO2	nanofibers/electrospinning	CO2/100	300	5.1	[58]
Au-La2O3/SnO2	nanofibers/electrospinning, sputtering	CO2/100	300	10.1
La/SnO2	nanofilm/hydrothermal, impregnation	CO2/500	250	29.8	[59]
		CO2/50		5.12
CdO	nanowires/microwave-assisted wet chemical	CO2/5000	250	~1.5	[60]
CdO	rod-like nanostructure/microwave radiation	CO2/5%	250	3	[61]
Sn/CdO	spherical shaped structures/microwave assisted wet chemical			15
CaO/In2O3	mesoporous/impregnation	CO2/2000	230	~1.8	[62]
YPO4	nanobelts/surfactant-assisted colloidal	CO2/200	400	-	[63]
La2O3	microrods thin films/chemical bath deposition	CO2/350	250	48	[64]
		CO2/100		4.8
LaFeO3/SnO2	nanocomposites porous film/sol-gel, hydrothermal	CO2/4000	250	2.72	[65]
LaFeO3	nanocrystalline/sol-gel	CO2/2000	300	2.19	[66]
Pd/La2O3	thin-film/spray pyrolysis, ionic layer adsorption and reaction	CO2/500	250	28	[67]
La2O3	thin-film/spray pyrolysis			13
Cr/TiO2	thin-film/RF magnetron sputtering	CO2/10%	55	~9	[68]
Al2O3/TiO2	heterostructure/ALD	CO2/5	RT	30.6	[69]

RT = room temperature; ALD = atomic layer deposition.

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 metal-doped 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 (1000 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 screen-printed 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 (Figure 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 (Mo_{1 − x}W_xO₃, 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, Gd-doped 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.

3.2. 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. Pt-doped 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 3000 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.

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-VO_x 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 1000 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. A brief summary of MOX-based CH₄ gas sensors.

Material	Structure/Synthesis Method	Target Gas/ Concentration (ppm)	Operating T (°C)	Response (%)	Ref.
TiO2	nanorods/hydrothermal	CH4/60	RT	6028	[87]
		CH4/5		987
Cd/TiO3	thin films/magnetron co-sputtering	CH4/500	250	3.4	[88]
VO2	nanorods/thermal evaporation	CH4/500	RT	35	[89]
V2O5	nano-flowers/magnetron sputtering	CH4/500	100	11.2	[90]
	nano-rods			8.9
	nano-urchins			9.1
Au/VO2	nanosheets/CVD, ion sputtering	CH4/500	RT	~70	[83]
SnO2	nanoparticles/sol-gel	CH4/20,000	80	~3.5	[91]
SnO2	quantum dots/sonication assisted precipitation	CH4/5000	>375	~100	[92]
SnO2/WO3	nanosheets/impregnation	CH4/500	90	~2	[80]
WO3	nanosheets/hydrothermal			~1.5
Pd/SnO2	hollow spheres/adsorption– calcination	CH4/250	300	4.88	[93]
SnO2	hollow spheres/progressive inward crystallization routine		400	1.31
SnO2/NiO	porousnanosheets/immersion-calcination	CH4/500	330	15.2	[94]
ZnO/rGO	nanorods, nanosheets/hydrothermal	CH4/100–4000	190	4.52	[95]
ZnO/NiO	porous nanosheets/hydrothermal, post-treatment	CH4/1000	340	34.2	[86]
Pd/SnO2	nanoporous/hydrothermal	CH4/3000	340	17.6	[76]
NiO/Al	thin films/RF sputtering	CH4/100	RT	58	[96]
ZnO	nanoporous/electrochemical deposition	CH4/100	220	~4.8	[97]
ZnO	nanowalls/thermal evaporation	CH4/100	300	8.1	[98]
Co/ZnO	nanoparticles/solvothermal	CH4/100	140	1.05	[99]
α-Fe2O3	nanoparticles/commercial	CH4/4000	RT	1.08	[100]
αFe1.92/Cu0.08O3	nanoparticles/homogenous co-precipitation			1.12

CDV = chemical vapor transport deposition; RT = room temperature; rGO = reduced graphene oxide; RF = radio frequency.

.

3.3. 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. Summary of MOX-based nitrous oxide gas sensors.

Material	Structure/Synthesis Method	Target Gas/ Concentration (ppm)	Operating T (°C)	Sensitivity	Ref.
BaO/SnO2	co-precipitation	N2O/300	500	3	[38]
Sm2O3/SnO2				3
PbO/SnO2				2.5
Gd2O3/SnO2				2.7
SrO/SnO2	co-precipitation	N2O/300	450	1.66
			500	4.5
In2O3	nanowires/carbothermal	N2O/10	250	~5	[103]
WO3	nanowires/solvothermal			25
ZnO	nanorods/self-assembly of ZnO nanodots			~5
Au/SnOx	films/L-MOCVD	N2O/100	210	11.5	[104]
SnO2		N2O/300	450	1.66

L-MOCVD = laser-induced metal–organic chemical vapor deposition; RT = room temperature.

summarizes the reported results in the literature on MOX-based N₂O gas sensors.

3.4. 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. Summary of MOX-based sensor towards SF₆ decomposed products.

Material	Structure/Synthesis Method	Target Gas/ Concentration (ppm)	Operating T (°C)	Response (%)	Ref.
NiO/ZnO	nanoflowers/hydrothermal	SO2, SOF2, SO2F2/100	260	SO2 (33.35) SOF2 (22.25) SO2F2 (36.67)	[108]
Au/ZnO	nanowires/hydrothermal	H2S/1	RT	38	[109]
In2O3	nanowires/chemical vapor deposition	H2S/1	RT	30.4	[110]
In2O3	single crystal whisker/carbothermal method	H2S/200 ppb	RT	47.9	[111]
CuO	nanowires/resistive heating of Cu wires	H2S/10 ppb	325	40.9	[112]
ZnO	flower-like nanorods/ hydrothermal	SO2, SOF2, SO2F2/10 (μL/L)	(SO2)250 (SOF2 and SO2F2) 300	SO2 (−33.44) SOF2 (−12.47) SO2F2 (−18.06)	[113]
TiO2	nanotube array/anodic oxidation	SO2, SOF2, SO2F2/50	200	SO2 (−76) SOF2 (−7.8) SO2F2 (−5.5)	[114]
CuO/WO3	nanowires/thermal evaporation followed sputter-deposition, thermal annealing	H2S/100	300	6.72	[115]
TiO2	nanotube array/anodic oxidation	SO2, SOF2, SO2F2/50	110	SO2 (−74.6) SOF2 (−7.82) SO2F2 (−5.52)	[116]
Au/TiO2	nanotube array/ deposition-precipitation	SO2, SOF2, SO2F2/50	110	SO2F2 (−19.95) SOF2 (−9.97) SO2 (−8.73)
TiO2/NiSO4	nanofibers/electrospun, hydrothermal	SO2F2/100	RT	189	[117]
SnO2	microspheres/hydrothermal	SO2/30	300	43.15	[118]
Cu/SnO2	microspheres/conventionalindirect heating		275	23.37
Fe2O3/NiO	nanoplates/solvothermal	H2S/50	200	26.48	[119]
Pt/TiO2	nanotube arrays/pulse electrodeposition	SO2, SOF2, SO2F2/50	150	SO2 (8.38) SOF2 (6.11) SO2F2 (17.91)	[120]
ZnO/SnO2	nanofibers/electrospinning	H2S/50	250	66.23	[121]
WO3/NiO	nanoflowers/hydrothermal	H2S/20	160	33.34	[122]

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.

4. 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 (Table 4, Table 5, Table 6 and Table 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.