Reduced Graphene Oxide (rGO)-Loaded Metal-Oxide Nanofiber Gas Sensors: An Overview

Reduced graphene oxide (rGO) is a reduced form of graphene oxide used extensively in gas sensing applications. On the other hand, in its pristine form, graphene has shortages and is generally utilized in combination with other metal oxides to improve gas sensing capabilities. There are different ways of adding rGO to different metal oxides with various morphologies. This study focuses on rGO-loaded metal oxide nanofiber (NF) synthesized using an electrospinning method. Different amounts of rGO were added to the metal oxide precursors, and after electrospinning, the gas response is enhanced through different sensing mechanisms. This review paper discusses rGO-loaded metal oxide NFs gas sensors.


Introduction
Metal oxide gas sensors are used to sense various toxic gases and vapors [1,2] in many areas [3]. These sensors are quite popular owing to their low preparation costs, high sensitivity, fast dynamics, high stability, simple operation, and small size [4]. The general gas sensing mechanism of metal oxide gas sensors stems from the modulation of electrical resistance in the presence of target gases [5]. In these sensors, the sensing layer is exposed directly to the gas. The interaction between the target gas and sensing layer modulates the electrical resistance of the gas sensor, resulting in the generation of a sensing signal. This conductivity change is due to variations in the width of the electron depletion layer across the exposed area of the sensing layer [6]. Various strategies can be used to enhance the gas sensing characteristics of metal oxide-based gas sensors, such as UV light activation [7], fabrication of p-n, p-p and n-n heterojunctions [8], noble metal decoration [9],surface engineering, and the generation of structural defects [10]. Thus far, different morphologies of metal oxides, such as nanowires [11], nanotubes [12], nanorods [13], nanobelts [14], nanofibers (NFs), nanosheets [15], and hierarchical structures [16], have been used in gas sensing studies. This is because gas adsorption on the surface of gas sensing relates directly to the surface area of gas sensors and higher adsorption of gas means higher sensing signal. Thus, several studies have attempted to synthesize morphologies with a high surface area to increase the sensing performance of gas sensors. In addition, a porous morphology [17] and hollow structures [18,19] can be other useful techniques to increase the gas sensing properties. Therefore, metal oxide NFs are highly popular for sensing studies because of a very high surface area results from the one-dimensional morphology and the presence of nanograins on their surfaces, as shown in Figure 1. The generation of depletion layers on nanograins causes the development of potential barriers, which will be modulated during exposure to the target gases, contributing to the sensing signal generation.  [20] (b) TEM image of 0.5 SnO2-0.5 Co3O4composite NFs [21].
Another advantage of NFs is their ease of preparation using a facile electrospinning technique. In general, the electrospinning technique can be used to fabricate continuous fibers with the possibility of control over the fiber diameter [22]. In addition, different NFs, such as porous NFs [23], core-shell NFs [24,25], and complex NFs [26], can be easily synthesized by electrospinning. Furthermore, NFs with controllable alignment can be produced by modifying the electrospinning [27]. Electrospinning also can be used for the mass production of NFs because of the easy handling, possibility of control of the diameter, low cost, simple operation, and high reproducibility [28]. Briefly, the features of electrospun fibers can be controlled by electrospinning parameters, including the solution variables (e.g., surface tension, viscosity, and conductivity), operating variables (e.g., applied voltage, spinning distance, and solution flow rate), and ambient variables (e.g., humidity and temperature) [29].Li et al. [30] (Figure 2) and Xue et al. [31] reviewed these factors, so this review paper does not discuss them further.  [30]. Figure 3 shows the major components of electrospinning include a syringe pump, spinneret, conductive collector, and power supply. The electrospinning technique can be described as follows: (1) charging of the liquid droplet and the formation of a Taylor Another advantage of NFs is their ease of preparation using a facile electrospinning technique. In general, the electrospinning technique can be used to fabricate continuous fibers with the possibility of control over the fiber diameter [22]. In addition, different NFs, such as porous NFs [23], core-shell NFs [24,25], and complex NFs [26], can be easily synthesized by electrospinning. Furthermore, NFs with controllable alignment can be produced by modifying the electrospinning [27]. Electrospinning also can be used for the mass production of NFs because of the easy handling, possibility of control of the diameter, low cost, simple operation, and high reproducibility [28]. Briefly, the features of electrospun fibers can be controlled by electrospinning parameters, including the solution variables (e.g., surface tension, viscosity, and conductivity), operating variables (e.g., applied voltage, spinning distance, and solution flow rate), and ambient variables (e.g., humidity and temperature) [29].Li et al. [30] (Figure 2) and Xue et al. [31] reviewed these factors, so this review paper does not discuss them further.
in Figure 1. The generation of depletion layers on nanograins causes the development of potential barriers, which will be modulated during exposure to the target gases, contributing to the sensing signal generation. Another advantage of NFs is their ease of preparation using a facile electrospinning technique. In general, the electrospinning technique can be used to fabricate continuous fibers with the possibility of control over the fiber diameter [22]. In addition, different NFs, such as porous NFs [23], core-shell NFs [24,25], and complex NFs [26], can be easily synthesized by electrospinning. Furthermore, NFs with controllable alignment can be produced by modifying the electrospinning [27]. Electrospinning also can be used for the mass production of NFs because of the easy handling, possibility of control of the diameter, low cost, simple operation, and high reproducibility [28]. Briefly, the features of electrospun fibers can be controlled by electrospinning parameters, including the solution variables (e.g., surface tension, viscosity, and conductivity), operating variables (e.g., applied voltage, spinning distance, and solution flow rate), and ambient variables (e.g., humidity and temperature) [29].Li et al. [30] (Figure 2) and Xue et al. [31] reviewed these factors, so this review paper does not discuss them further.    [30]. Figure 3 shows the major components of electrospinning include a syringe pump, spinneret, conductive collector, and power supply. The electrospinning technique can be described as follows: (1) charging of the liquid droplet and the formation of a Taylor cone, (2) formation of the charged jet, (3) thinning of the jet by the applied voltage, and (4) collection and solidification of the jet on a grounded collector. A Taylor cone is formed owing to surface tension and the applied electric field, from which a charged jet is ejected. The ejected jet was solidified quickly, resulting in the collection of solid fibers on the collector [32,33]. cone, (2) formation of the charged jet, (3) thinning of the jet by the applied voltage, and (4) collection and solidification of the jet on a grounded collector. A Taylor cone is formed owing to surface tension and the applied electric field, from which a charged jet is ejected. The ejected jet was solidified quickly, resulting in the collection of solid fibers on the collector [32,33].

2.Graphene, Graphene Oxide, and Reduced Graphene Oxide
Graphene is a single-layer comprised of sp 2 carbon atoms that can be used as a gas sensor owing to its high charge carrier mobility (200,000 cm 2 V −1 s −1 ), high mechanical stiffness, high environmental compatibility, and huge surface area (2630 m 2 g −1 ) [35,36]. Schedin et al. [37] introduced the first graphene gas sensor in 2007. Because atoms in a single layer of graphene can be considered surface atoms, graphene can interact with even a single molecule [37]. Even though nowadays pristine graphene can be synthesized on a large scale [38] with good water solubility [39,40], it can be easily agglomerated in solution due to surface interactions [41]. Moreover, graphene has no bandgap or functional groups, limiting its gas sensors applications, particularly in the pristine form [42]. Therefore, reduced graphene oxide (rGO), which is synthesized by the reduction of graphene oxide (GO), is a better choice for gas sensing applications because it has many functional groups and defects [43]. GO has also been used for gas sensing studies [44]. On the other hand, GO has very high resistance due to the presence of alkoxy (C-O-C), hydroxyl (-OH), carboxylic acid (-COOH), carbonyl (C=O), and other oxygen-based functional groups [45,46]. rGO has more defects and dangling bonds than graphite, resulting in better sensing properties [47]. GO is widely prepared using either Hummers [48] or Brodie [49] methods. In these methods, differences are in both the acid used (nitric or sulfuric acid), and the type of salt used (potassium chlorate or potassium permanganate). By subsequent reduction of GO, rGO can be obtained. In fact, rGO can be prepared easily from GO by chemical reduction, thermal reduction, and UV light reduction [50]. Figure 4 shows the structure and preparation of GO and rGO [44].

Graphene, Graphene Oxide, and Reduced Graphene Oxide
Graphene is a single-layer comprised of sp 2 carbon atoms that can be used as a gas sensor owing to its high charge carrier mobility (200,000 cm 2 V −1 s −1 ), high mechanical stiffness, high environmental compatibility, and huge surface area (2630 m 2 g −1 ) [35,36]. Schedin et al. [37] introduced the first graphene gas sensor in 2007. Because atoms in a single layer of graphene can be considered surface atoms, graphene can interact with even a single molecule [37]. Even though nowadays pristine graphene can be synthesized on a large scale [38] with good water solubility [39,40], it can be easily agglomerated in solution due to surface interactions [41]. Moreover, graphene has no bandgap or functional groups, limiting its gas sensors applications, particularly in the pristine form [42]. Therefore, reduced graphene oxide (rGO), which is synthesized by the reduction of graphene oxide (GO), is a better choice for gas sensing applications because it has many functional groups and defects [43]. GO has also been used for gas sensing studies [44]. On the other hand, GO has very high resistance due to the presence of alkoxy (C-O-C), hydroxyl (-OH), carboxylic acid (-COOH), carbonyl (C=O), and other oxygen-based functional groups [45,46]. rGO has more defects and dangling bonds than graphite, resulting in better sensing properties [47]. GO is widely prepared using either Hummers [48] or Brodie [49] methods. In these methods, differences are in both the acid used (nitric or sulfuric acid), and the type of salt used (potassium chlorate or potassium permanganate). By subsequent reduction of GO, rGO can be obtained. In fact, rGO can be prepared easily from GO by chemical reduction, thermal reduction, and UV light reduction [50]. Figure 4 shows the structure and preparation of GO and rGO [44].  Furthermore, rGO has high thermal stability, and the total weight loss of rGO was reported to be only 11% up to 800 °C, which was attributed to the absence of most oxygen functional groups. [51]. Pristine rGO gas sensors have a long response and recovery times, and incorporating rGO with metal oxides can be a good strategy to increase the Furthermore, rGO has high thermal stability, and the total weight loss of rGO was reported to be only 11% up to 800 • C, which was attributed to the absence of most oxygen functional groups. [51]. Pristine rGO gas sensors have a long response and recovery times, and incorporating rGO with metal oxides can be a good strategy to increase the sensing capabilities of rGO-based gas sensors [52]. The synthesis and properties of rGO have been reviewed comprehensively [41]. The following section explains the gas sensing capability of rGO-loaded metal oxide NFs. This paper does not discuss the combination of rGO with other materials, such as mesh fabric [53] or polymers [54,55], for gas sensing studies. Furthermore, composites of metal oxides-rGO in morphologies other than NFs are not discussed.

rGO-Loaded ZnO NFs
Zinc oxide (ZnO) is one of the most common metal oxides in the gas sensing area because of its unique properties, such as n-type conductivity, low toxicity, ease of synthesis, high availability, good thermal stability, and high mobility of electrons [56,57]. Abideen et al. [58] prepared rGO-loaded ZnO NFs for H 2 gas sensing investigations. At 400 • C, the sensor exhibited an exceptionally strong response (R a /R g ) of 2542 to 10 ppm H 2 gas. The presence of rGO and a semiconductor (ZnO)-to metal (Zn) transition in a H 2 atmosphere were the main reasons behind the enhanced response to H 2 gas. They suggested that ZnO could be converted to metallic Zn in the presence of H 2 gas because of the high-sensing temperature. Owing to the differences in the work functions of rGO, ZnO, and Zn, at equilibrium, potential barriers were generated at both rGO/Zn and Zn/ZnO interfaces. ZnO was more n-type because of the flow of electrons from the Zn layer to ZnO. Accordingly, high resistance modulation occurred, contributing to the sensing signal. In addition, the potential barriers of rGO/Zn prevented the flow of electrons to rGO, which acted as a source of resistance modulation in the presence of H 2 gas.
In another similar investigation, Abideen et al. reported the effects of the rGO-loading (0.04, 0.11, 0.17, 0.44, 0.77, and 1.04 wt.%) on the NO 2 sensing response of ZnO NFs [59]. Among rGO-loaded ZnO NF gas sensors, the sensor with a 0.44 wt.% rGO-loading showed an enhanced response to NO2 gas. The sensor showed a response (Ra/Rg) of ~123 to 5 ppm NO2 gas at 400 °C.The boundaries between the nanograins acted as a source of resistance modulation caused by the generation of potential barriers resulting from oxygen adsorption. After introducing NO2 gas, the height of the potential barriers increased across the grain boundaries, leading to the sensor signal ( Figure 6a). Among rGO-loaded ZnO NF gas sensors, the sensor with a 0.44 wt.% rGO-loading showed an enhanced response to NO 2 gas. The sensor showed a response (R a /R g ) of~123 to 5 ppm NO 2 gas at 400 • C.The boundaries between the nanograins acted as a source of resistance modulation caused by the generation of potential barriers resulting from oxygen adsorption. After introducing NO 2 gas, the height of the potential barriers increased across the grain boundaries, leading to the sensor signal ( Figure 6a).
Among rGO-loaded ZnO NF gas sensors, the sensor with a 0.44 wt.% rGO-loading showed an enhanced response to NO2 gas. The sensor showed a response (Ra/Rg) of ~123 to 5 ppm NO2 gas at 400 °C.The boundaries between the nanograins acted as a source of resistance modulation caused by the generation of potential barriers resulting from oxygen adsorption. After introducing NO2 gas, the height of the potential barriers increased across the grain boundaries, leading to the sensor signal ( Figure 6a).  In the rGO/ZnO heterojunctions, Ohmic contact was generated (Figure 6b), acting as a non-rectifying barrier to charge transfer, which affected the sensing behavior. In a NO 2 atmosphere, the resistance of the gas sensor was increased because of the abstraction of more electrons. In addition, discretely distributed rGO nanosheets, which had a high surface area, played a catalytic role on NO 2 gas molecules. Other contributions were the presence of defects in rGO, which were favorable sites for the adsorption of gases.
Abideen et al. synthesized rGO-loaded ZnO NFs functionalized by Au and Pd NPs, as shown in Figure 7 [60]. Figure 8a-c shows typical TEM images of rGO nanosheets, ZnO NFs and Au-decorated ZnO NFs, respectively. At 400 • C, the sensors functionalized with Au and Pd showed an enhanced gas response towards CO and C 6 H 6 gases, respectively. Owing to the formation of ZnO/rGO heterojunctions, the electron depletion layer was wider in the ZnO NFs. Upon exposure to reducing gases, they interacted with oxygen species and liberated electrons, resulting in increased sensor conductivity, contributing to the sensing signal.
as shown in Figure 7 [60]. Figure 8a-c shows typical TEM images of rGO nanosheets, ZnO NFs and Au-decorated ZnO NFs, respectively. At 400 °C, the sensors functionalized with Au and Pd showed an enhanced gas response towards CO and C6H6 gases, respectively. Owing to the formation of ZnO/rGO heterojunctions, the electron depletion layer was wider in the ZnO NFs. Upon exposure to reducing gases, they interacted with oxygen species and liberated electrons, resulting in increased sensor conductivity, contributing to the sensing signal.  rGO has a high surface area, many defects and functional groups, and a different work function than ZnO. These factors contributed to the sensing enhancement. The effects of noble metals were considered. In the case of electronic sensitization, oxidized Pd (PdO), which initially took electrons from ZnO, was reduced in the presence of C6H6. Accordingly, the space charge layer was relaxed by the return of electrons to the ZnO. In chemical sensitization, Au and Pd NPs acted as catalytic metals. The incoming oxygen species were adsorbed on the surface of Au or Pd, leading to the dissociation of oxygen and subsequent spillover to neighboring ZnO. This increased the initial width of the electron depletion layers, which acted as a source of resistance modulation in the presence of the target gases. Furthermore, Schottky barriers were formed in the interfaces Abideen et al. synthesized rGO-loaded ZnO NFs functionalized by Au and Pd NPs, as shown in Figure 7 [60]. Figure 8a-c shows typical TEM images of rGO nanosheets, ZnO NFs and Au-decorated ZnO NFs, respectively. At 400 °C, the sensors functionalized with Au and Pd showed an enhanced gas response towards CO and C6H6 gases, respectively. Owing to the formation of ZnO/rGO heterojunctions, the electron depletion layer was wider in the ZnO NFs. Upon exposure to reducing gases, they interacted with oxygen species and liberated electrons, resulting in increased sensor conductivity, contributing to the sensing signal.  rGO has a high surface area, many defects and functional groups, and a different work function than ZnO. These factors contributed to the sensing enhancement. The effects of noble metals were considered. In the case of electronic sensitization, oxidized Pd (PdO), which initially took electrons from ZnO, was reduced in the presence of C6H6. Accordingly, the space charge layer was relaxed by the return of electrons to the ZnO. In chemical sensitization, Au and Pd NPs acted as catalytic metals. The incoming oxygen species were adsorbed on the surface of Au or Pd, leading to the dissociation of oxygen and subsequent spillover to neighboring ZnO. This increased the initial width of the electron depletion layers, which acted as a source of resistance modulation in the presence of the target gases. Furthermore, Schottky barriers were formed in the interfaces rGO has a high surface area, many defects and functional groups, and a different work function than ZnO. These factors contributed to the sensing enhancement. The effects of noble metals were considered. In the case of electronic sensitization, oxidized Pd (PdO), which initially took electrons from ZnO, was reduced in the presence of C 6 H 6 . Accordingly, the space charge layer was relaxed by the return of electrons to the ZnO. In chemical sensitization, Au and Pd NPs acted as catalytic metals. The incoming oxygen species were adsorbed on the surface of Au or Pd, leading to the dissociation of oxygen and subsequent spillover to neighboring ZnO. This increased the initial width of the electron depletion layers, which acted as a source of resistance modulation in the presence of the target gases. Furthermore, Schottky barriers were formed in the interfaces between Au/ZnO and Pd/ZnO (Figure 9). The width of the electron depletion layer decreased because of the flow of electrons from ZnO to metal. Moreover, the width of the electron depletion layer decreased by subsequent interaction of reducing gases and the liberation of electrons, causing a large change in resistance of the gas sensor. between Au/ZnO and Pd/ZnO (Figure 9). The width of the electron depletion layer decreased because of the flow of electrons from ZnO to metal. Moreover, the width of the electron depletion layer decreased by subsequent interaction of reducing gases and the liberation of electrons, causing a large change in resistance of the gas sensor. The good selectivity of the Au-functionalized gas sensor to CO was attributed to the low (1.20eV) oxidation barrier of CO on Au and the strong interaction of Au with CO gas. In addition, strong adsorption of C6H6 on Pd resulted in the strong response of the Pd-functionalized gas sensor to benzene.

rGO-Loaded SnO2 NFs
SnO2 is among the most popular metal oxides for sensing studies because of its high stability, favorable bandgap, low price, and good intrinsic sensing properties [61,62]. Lee et al. [63] examined the effects of rGO-loadings (0.04-1.04 wt.%) on the NO2 gas sensing properties of SnO2 NFs produced by electrospinning. The sensor with 0.44 wt.% rGO exhibited the highest resistance and the strongest response (Ra/Rg) of ~100 to 5 ppm NO2 gas at 200 °C. The pristine sensors showed a change in resistance due to the grain boundary mechanism and radial modulation, as shown in Figure 10a. For rGO-loaded sensors (Figure 10b), p-n heterojunctions were formed, generating potential barriers to the flow of electrons. The heights of these barriers were changed in the target gas atmosphere, resulting in sensing signal generation. The good selectivity of the Au-functionalized gas sensor to CO was attributed to the low (1.20 eV) oxidation barrier of CO on Au and the strong interaction of Au with CO gas. In addition, strong adsorption of C 6 H 6 on Pd resulted in the strong response of the Pd-functionalized gas sensor to benzene.

rGO-Loaded SnO 2 NFs
SnO 2 is among the most popular metal oxides for sensing studies because of its high stability, favorable bandgap, low price, and good intrinsic sensing properties [61,62]. Lee et al. [63] examined the effects of rGO-loadings (0.04-1.04 wt.%) on the NO 2 gas sensing properties of SnO 2 NFs produced by electrospinning. The sensor with 0.44 wt.% rGO exhibited the highest resistance and the strongest response (R a /R g ) of~100 to 5 ppm NO 2 gas at 200 • C. The pristine sensors showed a change in resistance due to the grain boundary mechanism and radial modulation, as shown in Figure 10a. For rGO-loaded sensors (Figure 10b), p-n heterojunctions were formed, generating potential barriers to the flow of electrons. The heights of these barriers were changed in the target gas atmosphere, resulting in sensing signal generation.
Hollow NFs are also interesting morphologies for gas sensing studies because of their higher surface areas that can interact with gas species [64]. For example, a BET surface area of hollow SnO 2 NFs composited with GO was reported to be 33.4m 2 /g. In this regard, Li et al. [65] investigated rGO-loaded SnO 2 NFs for gas sensing studies. Figure 11 shows typical SEM and TEM images of rGO-loaded SnO 2 NFs. Hollow NFs are also interesting morphologies for gas sensing studies because of their higher surface areas that can interact with gas species [64].For example, a BET surface area of hollow SnO2NFs composited with GO was reported to be 33.4m 2 /g. In this regard, Li et al. [65] investigated rGO-loaded SnO2 NFs for gas sensing studies. Figure 11 shows typical SEM and TEM images of rGO-loaded SnO2 NFs.  Hollow NFs are also interesting morphologies for gas sensing studies because of their higher surface areas that can interact with gas species [64].For example, a BET surface area of hollow SnO2NFs composited with GO was reported to be 33.4m 2 /g. In this regard, Li et al. [65] investigated rGO-loaded SnO2 NFs for gas sensing studies. Figure 11 shows typical SEM and TEM images of rGO-loaded SnO2 NFs.  p-n heterojunctions were formed between the connections owing to the intimate contact between rGO and SnO 2 . (Figure 12a). Under UV illumination (Figure 12b), SnO 2 acted as a UV absorber and a collector of generated electrons, whereas rGO acted as a photoelectron acceptor and provided many electron transport pathways. Generated electrons, as a result of UV illumination, can be adsorbed by oxygen species. The holes generated can be combined with chemisorbed oxygen, resulting in the desorption of oxygen. Thus, equilibrium is achieved under UV light. The best response could be achieved by tuning the UV light intensity. Oxidizing gases, such as NO 2 , can abstract electrons from the surface of the gas sensor. Compared to the dark condition, many more photoelectrons are captured by NO 2 gas, resulting in a stronger response to NO 2 gas. The strongest response was obtained under a 97 mW/cm 2 UV light intensity, which indicated a response of~100% (∆R/R a ×100) to 5 ppm NO 2 at room temperature. p-n heterojunctions were formed between the connections owing to the intimate contact between rGO and SnO2. (Figure 12a). Under UV illumination (Figure 12b), SnO2acted as a UV absorber and a collector of generated electrons, whereas rGO acted as a photoelectron acceptor and provided many electron transport pathways. Generated electrons, as a result of UV illumination, can be adsorbed by oxygen species. The holes generated can be combined with chemisorbed oxygen, resulting in the desorption of oxygen. Thus, equilibrium is achieved under UV light. The best response could be achieved by tuning the UV light intensity. Oxidizing gases, such as NO2, can abstract electrons from the surface of the gas sensor. Compared to the dark condition, many more photoelectrons are captured by NO2 gas, resulting in a stronger response to NO2 gas. The strongest response was obtained under a 97 mW/cm 2 UV light intensity, which indicated a response of ~100% (ΔR/Ra ×100) to 5 ppm NO2 at room temperature. Choi et al. [66] prepared 0.01 and 5 wt.% rGO-loaded SnO2 NFs for sensing studies. In the 0.01 wt.% rGO-loaded sensor, the electrical transport and sensing characteristics of the sensor were dominated by the SnO2 NFs, which showed an enhanced gas response to H2S. In contrast, in the sensor with 5 wt.% rGO, the rGO NSs showed an enhanced response to acetone. The 0.01 wt.% rGO NS-loaded SnO2 NFs showed a response (Ra/Rg) of 33.7 to 5 ppm H2S at 200 °C. On the other hand, the 5 wt.% rGO-loaded sensor showed a response of 10.4 to 5 ppm acetone at 350 °C. Accordingly, the selectivity of the gas sensor towards H2S or acetone could be tuned by changing the amount of rGO.
Kim et al. [67] fabricated Pt or Pd rGO-co-loaded SnO2 NFs for sensing purposes. The sensors sensitized with Pt showed an enhanced gas response to toluene, while the sensor with Pd showed an improved response to benzene. Figure 13 shows the sensing mechanism. rGO plays a role in the formation of heterojunctions with SnO2, where electrons from SnO2 migrate to rGO, leading to an expansion of the electron depletion layer of SnO2 and a change in the width of the electron depletion layer in the presence of target gases, which greatly affect the response to gases. The sensing enhancement of the Pt-functionalized sensor was attributed partially to the H2 dissociation of gases. Upon dissociation, C7H8 can generate more H2 molecules. Thus, Pt dissociated C7H8 more effectively than the remaining gases. Furthermore, the methyl group (-CH3) on toluene made it easier to adsorb on the Pt surface. Regarding the Pd-functionalized gas sensor, the adsorption energy of C6H6/Pd (1.35 eV) was lower than that of C6H6/Pt (1.49 eV); thus, Choi et al. [66] prepared 0.01 and 5 wt.% rGO-loaded SnO 2 NFs for sensing studies. In the 0.01 wt.% rGO-loaded sensor, the electrical transport and sensing characteristics of the sensor were dominated by the SnO 2 NFs, which showed an enhanced gas response to H 2 S. In contrast, in the sensor with 5 wt.% rGO, the rGO NSs showed an enhanced response to acetone. The 0.01 wt.% rGO NS-loaded SnO 2 NFs showed a response (R a /R g ) of 33.7 to 5 ppm H 2 S at 200 • C. On the other hand, the 5 wt.% rGO-loaded sensor showed a response of 10.4 to 5 ppm acetone at 350 • C. Accordingly, the selectivity of the gas sensor towards H 2 S or acetone could be tuned by changing the amount of rGO.
Kim et al. [67] fabricated Pt or Pd rGO-co-loaded SnO 2 NFs for sensing purposes. The sensors sensitized with Pt showed an enhanced gas response to toluene, while the sensor with Pd showed an improved response to benzene. Figure 13 shows the sensing mechanism. rGO plays a role in the formation of heterojunctions with SnO 2 , where electrons from SnO 2 migrate to rGO, leading to an expansion of the electron depletion layer of SnO 2 and a change in the width of the electron depletion layer in the presence of target gases, which greatly affect the response to gases. The sensing enhancement of the Pt-functionalized sensor was attributed partially to the H 2 dissociation of gases. Upon dissociation, C 7 H 8 can generate more H 2 molecules. Thus, Pt dissociated C 7 H 8 more effectively than the remaining gases. Furthermore, the methyl group (-CH 3 ) on toluene made it easier to adsorb on the Pt surface. Regarding the Pd-functionalized gas sensor, the adsorption energy of C 6 H 6 /Pd (1.35 eV) was lower than that of C 6 H 6 /Pt (1.49 eV); thus, the Pd-functionalized gas sensor showed an enhanced response to C 6 H 6 compared to C 7 H 8 gas. the Pd-functionalized gas sensor showed an enhanced response to C6H6 compared to C7H8 gas.

rGO-Loaded α-Fe2O3
NFs n-Type α-Fe2O3 is one of the most popular oxides, with many excellent features, such as low cost and facile preparation [68]. In that study, 0.5, 1.0, and 3.0wt.% rGO-loaded α-Fe2O3 NFs with approximate diameters of 100 nm were prepared using an electrospinning technique. The response of the optimal gas sensor, namely 1wt.% rGO/α-Fe2O3NFs at 375 °C to 100ppm acetone was approximately 8.9, which was approximately 4.5 times higher than the pure α-Fe2O3gas sensor [69]. The resistance of the composite sensors was lower than that of the pristine sensor, and the sensors with larger amounts of rGO showed lower conductivity. Ohmic contacts were generated because of the formation of rGO-Fe2O3 heterojunctions, which contributed to the sensing signal. Furthermore, defects and functional groups in rGO provided strong adsorption sites for gas molecules. In addition, the spaces between the layers of rGO nanosheets acted as an effective gas diffusion channel, which also provided more adsorption sites for gas molecules. When the amount of rGO was 3 wt.%, the surface of Fe2O3 was covered further with rGO, resulting in a significant decrease in the number of gas adsorption sites of α-Fe2O3. Thus, acetone gas reacted mainly with rGO rather than α-Fe2O3NFs, leading to a low response.
Hoang et al. [70] prepared rGO-loaded (0-1.5wt.%) α-Fe2O3 NFs for sensing studies. The 1 wt.% rGO loaded sensor showed a strong response (Ra/Rg) of ~9.2 to 1ppm H2S gas at 350 °C. On the other hand, when rGO was increased, rGO nanosheets were dominant for the electron pathways, which decreased the overall sensor resistance, causing a weaker sensor response. The formation of rGO/Fe2O3 heterojunctions (Figure 14), high surface area resulting from the NFs, and strong adsorption sites, such as oxygen functional groups and structural defects, also enhanced the sensor response.

rGO-Loaded α-Fe 2 O 3 NFs
n-Type α-Fe 2 O 3 is one of the most popular oxides, with many excellent features, such as low cost and facile preparation [68]. In that study, 0.5, 1.0, and 3.0wt.% rGO-loaded α-Fe 2 O 3 NFs with approximate diameters of 100 nm were prepared using an electrospinning technique. The response of the optimal gas sensor, namely 1wt.% rGO/α-Fe 2 O 3 NFs at 375 • C to 100ppm acetone was approximately 8.9, which was approximately 4.5 times higher than the pure α-Fe 2 O 3 gas sensor [69]. The resistance of the composite sensors was lower than that of the pristine sensor, and the sensors with larger amounts of rGO showed lower conductivity. Ohmic contacts were generated because of the formation of rGO-Fe 2 O 3 heterojunctions, which contributed to the sensing signal. Furthermore, defects and functional groups in rGO provided strong adsorption sites for gas molecules. In addition, the spaces between the layers of rGO nanosheets acted as an effective gas diffusion channel, which also provided more adsorption sites for gas molecules. When the amount of rGO was 3 wt.%, the surface of Fe 2 O 3 was covered further with rGO, resulting in a significant decrease in the number of gas adsorption sites of α-Fe 2 O 3 . Thus, acetone gas reacted mainly with rGO rather than α-Fe 2 O 3 NFs, leading to a low response.
Hoang et al. [70] prepared rGO-loaded (0-1.5wt.%) α-Fe 2 O 3 NFs for sensing studies. The 1 wt.% rGO loaded sensor showed a strong response (R a /R g ) of~9.2 to 1ppm H 2 S gas at 350 • C. On the other hand, when rGO was increased, rGO nanosheets were dominant for the electron pathways, which decreased the overall sensor resistance, causing a weaker sensor response. The formation of rGO/Fe 2 O 3 heterojunctions (Figure 14), high surface area resulting from the NFs, and strong adsorption sites, such as oxygen functional groups and structural defects, also enhanced the sensor response.

rGO-Loaded In2O3 NFs
As a semiconducting metal oxide, In2O3 (Eg= 3.6 eV) is used for sensing studies [71]. On the other hand, pristine In2O3 NFs show high resistance at room temperature because of the low mobility of electrons. A good strategy to overcome this problem is to form p-n heterojunctions and increase the overall sensing performance [72,73]. NH3is a highly reactive gas that harms the global greenhouse balance. Furthermore, it can be regarded as a biomarker of the human breath to diagnose lung diseases. Hence, the precise sensing of NH3gas is necessary [74]. Andre et al. synthesized rGO-loaded In2O3 NFs for gas sensing applications [75]. Figure 15 shows TEM analyses of rGO-loaded In2O3 NFs. At room temperature, the sensor showed a response of 23.36 to 15 ppm NH3 gas. p-n heterojunctions of rGO-In2O3 were formed in the air upon intimate contact and exposure to NH3 gas; the electrons released greatly modulated the height of these potential barriers, resulting in the appearance of the sensing signal. Other factors that contributed to the sensing signal were the increase in surface area due to the presence of rGO nanosheets and the synergistic effect between the In2O3 structure and rGO nanosheets, which formed a three-dimensional interconnected morphology, facilitating the gas accessibility depth parts of the sensing layer.

rGO-Loaded In 2 O 3 NFs
As a semiconducting metal oxide, In 2 O 3 (E g = 3.6 eV) is used for sensing studies [71]. On the other hand, pristine In 2 O 3 NFs show high resistance at room temperature because of the low mobility of electrons. A good strategy to overcome this problem is to form p-n heterojunctions and increase the overall sensing performance [72,73]. NH 3 is a highly reactive gas that harms the global greenhouse balance. Furthermore, it can be regarded as a biomarker of the human breath to diagnose lung diseases. Hence, the precise sensing of NH 3 gas is necessary [74]. Andre et al. synthesized rGO-loaded In 2 O 3 NFs for gas sensing applications [75]. Figure 15 shows TEM analyses of rGO-loaded In 2 O 3 NFs.

rGO-Loaded In2O3 NFs
As a semiconducting metal oxide, In2O3 (Eg= 3.6 eV) is used for sensing studies [71]. On the other hand, pristine In2O3 NFs show high resistance at room temperature because of the low mobility of electrons. A good strategy to overcome this problem is to form p-n heterojunctions and increase the overall sensing performance [72,73]. NH3is a highly reactive gas that harms the global greenhouse balance. Furthermore, it can be regarded as a biomarker of the human breath to diagnose lung diseases. Hence, the precise sensing of NH3gas is necessary [74]. Andre et al. synthesized rGO-loaded In2O3 NFs for gas sensing applications [75]. Figure 15 shows TEM analyses of rGO-loaded In2O3 NFs. At room temperature, the sensor showed a response of 23.36 to 15 ppm NH3 gas. p-n heterojunctions of rGO-In2O3 were formed in the air upon intimate contact and exposure to NH3 gas; the electrons released greatly modulated the height of these potential barriers, resulting in the appearance of the sensing signal. Other factors that contributed to the sensing signal were the increase in surface area due to the presence of rGO nanosheets and the synergistic effect between the In2O3 structure and rGO nanosheets, which formed a three-dimensional interconnected morphology, facilitating the gas accessibility depth parts of the sensing layer. At room temperature, the sensor showed a response of 23.36 to 15 ppm NH 3 gas. p-n heterojunctions of rGO-In 2 O 3 were formed in the air upon intimate contact and exposure to NH 3 gas; the electrons released greatly modulated the height of these potential barriers, resulting in the appearance of the sensing signal. Other factors that contributed to the sensing signal were the increase in surface area due to the presence of rGO nanosheets and the synergistic effect between the In 2 O 3 structure and rGO nanosheets, which formed a three-dimensional interconnected morphology, facilitating the gas accessibility depth parts of the sensing layer.
Yan et al. [76] fabricated rGO-loaded In 2 O 3 NFs by an electrospinning technique. The sensor with 2.2 wt.% rGO showed an enhanced gas response of 43 to 5 ppm NO 2 gas at 50 • C. Figure 16a,b show the sensing mechanism for the pristine and rGO-loaded gas sensors, respectively. Several contributions were mentioned to explain the sensing mechanism in rGO-loaded In 2 O 3 NFs. First, rGO with a high surface area and defects and functional groups provided many adsorption sites for incoming NO 2 gas molecules. Furthermore, potential barriers were formed due to the formation of rGO-In 2 O 3 heterojunctions, and the width and height of these barriers were modified upon exposure to NO 2 gas, which changed the resistance of the gas sensor. On the other hand, the amount of rGO was not optimized in the fabricated sensor. Yan et al. [76] fabricated rGO-loaded In2O3 NFs by an electrospinning technique. The sensor with 2.2 wt.% rGO showed an enhanced gas response of 43 to 5 ppm NO2 gas at 50 °C. Figure 16a,b show the sensing mechanism for the pristine and rGO-loaded gas sensors, respectively. Several contributions were mentioned to explain the sensing mechanism in rGO-loaded In2O3 NFs. First, rGO with a high surface area and defects and functional groups provided many adsorption sites for incoming NO2 gas molecules. Furthermore, potential barriers were formed due to the formation of rGO-In2O3 heterojunctions, and the width and height of these barriers were modified upon exposure to NO2 gas, which changed the resistance of the gas sensor. On the other hand, the amount of rGO was not optimized in the fabricated sensor.

rGO-Loaded Co3O4 NFs
Co3O4 is a p-type (Eg=1.6-2.2eV) semiconductor and is used for sensing different gases [77][78][79]. Feng et al. [80] synthesized the rGO-loaded Co3O4NFs using the electrospinning technique. At room temperature, the rGO-loaded sensor showed a 10-fold stronger response to NH3 than the pristine gas sensor. Ammonia is an electron donor gas that donates electrons to rGO upon adsorption. Therefore, the resistance of the gas sensor was increased. The rGO nanosheets interacted with the Co3O4to form Co-C bonds and therefore were polarized. Thus, the interaction with NH3, which had one lone pair of electrons, was stronger. The good selectivity was attributed to the different adsorption ability of different gases. Furthermore, NH3 is more polar than other tested gases, leading to better interactions with the sensing layer.

rGO-Loaded Co 3 O 4 NFs
Co 3 O 4 is a p-type (E g = 1.6-2.2 eV) semiconductor and is used for sensing different gases [77][78][79]. Feng et al. [80] synthesized the rGO-loaded Co 3 O 4 NFs using the electrospinning technique. At room temperature, the rGO-loaded sensor showed a 10-fold stronger response to NH 3 than the pristine gas sensor. Ammonia is an electron donor gas that donates electrons to rGO upon adsorption. Therefore, the resistance of the gas sensor was increased. The rGO nanosheets interacted with the Co 3 O 4 to form Co-C bonds and therefore were polarized. Thus, the interaction with NH 3 , which had one lone pair of electrons, was stronger. The good selectivity was attributed to the different adsorption ability of different gases. Furthermore, NH 3 is more polar than other tested gases, leading to better interactions with the sensing layer.

rGO-Loaded CuO NFs
CuO is a p-type metal oxide for H 2 S sensing, even for sub-ppb detection [81]. This is due mainly to the conversion of semiconducting CuO to CuS with metallic-like conductivity [82,83]. Kim et al. [84] used rGO (0.05-1.5 wt.%)-loaded CuO NFs for H 2 S gas sensing. The sensor with 0.5 wt.% rGO exhibited the maximum response of 1.95 to 10 ppm H 2 S at 300 • C. Figure 17 presents the sensing mechanism. Upon intimate contact between rGO and CuO, heterojunctions were formed, increasing the resistance of the gas sensor. In the H 2 S gas atmosphere, CuO was transformed into the CuS phase according to the following reaction: CuO(s) + H 2 S(g) → CuS (s) + H 2 O(g) (1) ppm H2S at 300 °C. Figure 17 presents the sensing mechanism. Upon intimate contact between rGO and CuO, heterojunctions were formed, increasing the resistance of the gas sensor. In the H2S gas atmosphere, CuO was transformed into the CuS phase according to the following reaction: CuO(s)+H2S(g)→CuS (s)+H2O(g) (1) Accordingly, the CuS with metallic conductivity was formed, which led to a significant change in the potential barrier height and decreased the hole accumulation layer, contributing to the sensing signal. rGO nanosheets were separated by CuO crystals, and there was no direct contact among the rGO nanosheets. For small amounts of rGO, the current flowing through CuO was blocked by the presence of discrete rGOs, which acted as a source of modulation. On the other hand, for a high content (>0.5 wt.%) of rGO, the rGO nanosheets provided additional paths for current flow, leading to a higher conduction path along rGO, which reduced the initial resistance and the response of the sensor. The promising roles of rGO nanosheets were also attributed to defects and oxygen functional groups on rGO, which provided many adsorption sites for H2S gas molecules. Furthermore, with the high surface area of rGO and spillover effect of rGO, H2S molecules were dissociated on rGO and spilled over to CuO crystals, leading to an increase in the gas response.

rGO-Loaded ZnFe2O4 NFs
Spinel ZnFe2O4 (ZFO) is an n-type metal oxide with a normal spinel structure [85] that has gained considerable attention for the detection of gases [86]. Hoang et al. [87] synthesized rGO-loaded ZFO NFs (50-100 nm) for H2S gas sensing applications. The sensor showed a response of 147 to 1 ppm of H2S gas at 350 °C. Figure 18 shows the mechanism of gas detection. Electrons from rGO were transferred to ZFO owing to dif- Accordingly, the CuS with metallic conductivity was formed, which led to a significant change in the potential barrier height and decreased the hole accumulation layer, contributing to the sensing signal. rGO nanosheets were separated by CuO crystals, and there was no direct contact among the rGO nanosheets. For small amounts of rGO, the current flowing through CuO was blocked by the presence of discrete rGOs, which acted as a source of modulation. On the other hand, for a high content (>0.5 wt.%) of rGO, the rGO nanosheets provided additional paths for current flow, leading to a higher conduction path along rGO, which reduced the initial resistance and the response of the sensor. The promising roles of rGO nanosheets were also attributed to defects and oxygen functional groups on rGO, which provided many adsorption sites for H 2 S gas molecules. Furthermore, with the high surface area of rGO and spillover effect of rGO, H 2 S molecules were dissociated on rGO and spilled over to CuO crystals, leading to an increase in the gas response.

rGO-Loaded ZnFe 2 O 4 NFs
Spinel ZnFe 2 O 4 (ZFO) is an n-type metal oxide with a normal spinel structure [85] that has gained considerable attention for the detection of gases [86]. Hoang et al. [87] synthesized rGO-loaded ZFO NFs (50-100 nm) for H 2 S gas sensing applications. The sensor showed a response of 147 to 1 ppm of H 2 S gas at 350 • C. Figure 18 shows the mechanism of gas detection. Electrons from rGO were transferred to ZFO owing to different work functions, forming heterojunctions on the rGO-ZFO interfaces. Furthermore, potential barriers were formed along the grain boundaries of ZFO due to the adsorption of oxygen molecules. Upon the introduction of H 2 S gas, the electrons generated from the interaction of H 2 S with oxygen molecules, returned to the surface of the gas sensor, decreasing the barrier height, leading to the sensing signal. ferent work functions, forming heterojunctions on the rGO-ZFO interfaces. Furthermore, potential barriers were formed along the grain boundaries of ZFO due to the adsorption of oxygen molecules. Upon the introduction of H2S gas, the electrons generated from the interaction of H2S with oxygen molecules, returned to the surface of the gas sensor, decreasing the barrier height, leading to the sensing signal.  Table 1 lists the gas sensing characteristics of rGO-loaded metal oxide NFs reported in literature. Different oxidizing (NO2) and reducing (C6H6, H2, CO, H2S, C3H6O, and C7H8) gases, have been successfully detected by these gas sensors. The sensing temperatures ranging from room temperature up to 400 °C have been reported. This demonstrates the possibility of a high sensing temperature of rGO-loaded gas sensors. In some cases, such as Ref. [58], a very high response to target gas can be obtained. Finally, in some cases, high responses to low concentrations of gases have been reported. Overall, the data in Table 1 shows the promising effects of rGO to be used along with metal oxide NFs for gas sensing studies.  Table 1 lists the gas sensing characteristics of rGO-loaded metal oxide NFs reported in literature. Different oxidizing (NO 2 ) and reducing (C 6 H 6 , H 2 , CO, H 2 S, C 3 H 6 O, and C 7 H 8 ) gases, have been successfully detected by these gas sensors. The sensing temperatures ranging from room temperature up to 400 • C have been reported. This demonstrates the possibility of a high sensing temperature of rGO-loaded gas sensors. In some cases, such as Ref. [58], a very high response to target gas can be obtained. Finally, in some cases, high responses to low concentrations of gases have been reported. Overall, the data in Table 1 shows the promising effects of rGO to be used along with metal oxide NFs for gas sensing studies. Also, Table 2 summarizes precursors, NF diameter, surface area and porosity of different rGO-loaded NF gas sensors reported in the literature. In all cases, initially, a viscous solution of precursor materials was prepared and then it was loaded into a syringe and subsequently NFs were produced by electrospinning process. It should be noted that in most cases, rGO was reduced from the synthesized GO (via Hummers method). In most cases, the surface area is not mentioned, but it could reach to 78.57 m 2 /g. In addition, in most cases the NFs with different diameters and mesoporous nature can be obtained by electrospinning.

Conclusions and Outlook
This review discussed various aspects of the sensing mechanisms of rGO-loaded metal oxide NFs gas sensors. Different metal oxides, such as ZnO, SnO 2 , Co 3 O 4 , CuO, In 2 O 3 , and other metal oxides, were used in combination with rGO for gas sensing applications. Metal oxide NFs with a high surface area, resulting from long and continuous morphology of NFs, produced by electrospinning, are among the most popular choices for gas sensors. Furthermore, as shown in Figure 1, nanograins on the surface of NFs, can increase the surface area of NFs and act as a high resistance source for the gas sensors. Pristine rGO gas sensors generally show weaker sensing properties than metal oxides because of the long response time and recovery time, as well as the lower response. When loaded on metal oxides, rGO nanosheets can increase the overall sensing surface and adsorption sites, which leads to the generation of p-n or p-p heterojunctions with metal oxides and modulate the resistances of the gas sensors. Furthermore, defects in rGO, such as oxygen vacancies and functional groups, can provide strong adsorption sites for target gases. Generally, there is an optimal amount of rGO that can give the strongest gas response. When the amount of rGO is higher than the optimal value, the main current can pass from rGO, which leads to a weaker response. In addition, for larger amounts of rGO nanosheets, some agglomeration may occur, resulting in a weaker response of the gas sensor. There are some strategies, such as functionalization with noble metals, to increase the overall sensing performance of rGO-loaded metal oxide NFs. Therefore, rGO-noble metal co-loaded metal oxides can be quite effective for sensing studies. Further advances can help load rGO nanosheets on metal oxide composite NFs. For example, p-n heterojunctions between different metal oxides can be fabricated, and the rGO loading can increase the sensing performance further.