Reduced Graphene Oxide-Loaded Metal-Oxide Nanofiber Gas Sensors

Reduced Graphene Oxide-Loaded Metal-Oxide Nanofiber Gas Sensors: History

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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.

gas sensor
reduced graphene oxide
rGO-loading
metal oxide
sensing mechanism

1. 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. 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 ^[20]. In addition, different NFs, such as porous NFs ^[21], core-shell NFs ^[22]^[23], and complex NFs ^[24], can be easily synthesized by electrospinning. Furthermore, NFs with controllable alignment can be produced by modifying the electrospinning ^[25]. 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 ^[26]. 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) ^[27].Li et al. ^[28] ) and Xue et al. ^[29] reviewed these factors, so this entry does not discuss them further.

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 ^[30]^[31].

2. Graphene, Graphene Oxide, and Reduced Graphene Oxide

Graphene is a single-layer comprised of sp²carbon atoms that can be used as a gas sensor owing to its high charge carrier mobility (200,000 cm² V⁻¹ s⁻¹), high mechanical stiffness, high environmental compatibility, and huge surface area (2630 m²g⁻¹) ^[32]^[33]. Schedin et al. ^[34] 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 ^[34]. Even though nowadays pristine graphene can be synthesized on a large scale ^[35] with good water solubility ^[36]^[37], it can be easily agglomerated in solution due to surface interactions ^[38]. Moreover, graphene has no bandgap or functional groups, limiting its gas sensors applications, particularly in the pristine form ^[39]. 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 ^[40]. GO has also been used for gas sensing studies ^[41]. 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 ^[42]^[43]. rGO has more defects and dangling bonds than graphite, resulting in better sensing properties ^[44]. GO is widely prepared using either Hummers ^[45] or Brodie ^[46] 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 ^[47].

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. ^[48]. 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 ^[49]. The synthesis and properties of rGO have been reviewed comprehensively ^[38].

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. ^[50], 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. Gas sensing characteristics of rGO-loaded metal oxide NFs.

Sensing Material	Target Gas	Conc. (ppm)	Response (R_a/R_g)	T (°C)	Ref.
rGO-loaded ZnO NFs	H₂	10	2542	400	^[50]
rGO-loaded ZnO NFs	NO₂	5	123	400	^[51]
Au/rGO-loaded ZnO NFs	CO	5	35.8	400	^[52]
Pd/rGO-loaded ZnO NFs	C₆H₆	5	22.8	400	^[52]
0.44 wt.% rGO-loaded SnO₂ NFs	NO₂	5	100	200	^[53]
rGO-loaded-SnO₂ NFs under UV light	NO₂	5	100%(ΔR/R_a × 100)	25	^[54]
0.01 wt.% rGO-loaded-SnO₂ NFs	H₂S	5	34	200	^[55]
5 wt.% rGO-loaded-SnO₂ NFs	C₃H₆O	5	15	350	^[55]
rGO/Pd co-loaded SnO₂ NFs	C₆H₆	5	12.3	200	^[56]
rGO/Pt co-loaded SnO₂ NFs	C₇H₈	5	16	200	^[56]
1 wt.% rGO-loaded Fe₂O₃ NFs	C₃H₆O	100	8.9	375	^[57]
1 wt.% rGO-loaded Fe₂O₃ NFs	H₂S	1	9.2	350	^[58]
rGO-loaded In₂O₃ NFs	NH₃	15	23.37	25	^[59]
2.2 wt.% rGO-loaded In₂O₃ NFs	NO₂	5	43	50	^[60]
1wt.% rGO–Co₃O₄NFs	NH₃	50	53.6%(ΔR/R_a) × 100	25	^[61]
0.5 wt.% rGO-loaded CuO NFs	H₂S	10	1.95	300	^[62]
rGO-loaded ZnFe₂O₄ NFs	H₂S	1	147	350	^[63]

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 m²/g. In addition, in most cases the NFs with different diameters and mesoporous nature can be obtained by electrospinning.

Table 2. Precursors, NF diameter, surface area and porosity nature of rGO-loaded NF gas sensors reported in literature.

Sensing Material	Precursors	NF Diameter (nm)	Surface Area (m²/g)	Porosity Type	Ref.
rGO-loaded ZnO NFs	Zinc acetate, PVA	190	NA	NA	^[50]
rGO-loaded ZnO NFs	Zinc acetate, polyvinyl alcohol (PVA)	~150	NA	NA	^[51]
Au and Pd/rGO-loaded ZnO NFs	Zinc acetate, PVA, HAuCl₄⋅nH₂O, PdCl₂	~200	NA	Mesoporous	^[52]
0.44 wt.% rGO-loaded SnO₂ NFs	SnCl₂.2H₂O, polyvinyl acetate (PVAc)	~180	7.0574	Mesoporous	^[53]
rGO-loaded-SnO₂ NFs under UV light	SnCl₂.2H₂O, PVP, dimethyl formamide (DMF)	80–250	NA	NA	^[54]
0.01 wt.% rGO-loaded-SnO₂ NFs	PVP, polymethylmethacrylate (PMMA) + tin(IV) acetate, acetic acid	370	NA	NA	^[55]
rGO/Pt and Pd co-loaded SnO₂ NFs	SnCl₂.2H₂O, PVAc, PdCl₂, H₂PtCl₆.nH₂O, DMF	NA	NA	Mesoporous	^[56]
1 wt.% rGO-loaded Fe₂O₃ NFs	Ferric acetylacetonte PVP	100	NA	NA	^[57]
1 wt.% rGO-loaded Fe₂O₃ NFs	PVA, Fe(NO₃)₃.9H₂O	50–100	NA	NA	^[58]
rGO-loaded In₂O₃ NFs	InCl₃, PVP, Monomethylamine (MMA), Trimethylamine (TMA), Triethylamine (TEA), and N,N-Dimethylformamide (DMF)	50	NA	Mesoporous	^[59]
2.2 wt.% rGO-loaded In₂O₃ NFs	PVP, DMF and (In(NO₃)₃·4.5H₂O	76	39.82	Mesoporous	^[60]
1wt.% rGO–Co₃O₄NFs	Co(NO₃)₂·6H₂O, PVP, Ethanol	200–300	78.57	Mesoporous	^[61]
0.5 wt.% rGO-loaded CuO NFs	PVA, (Cu (CH₃CO₂)₂)	∼50	NA	NA	^[62]
rGO-loaded ZnFe₂O₄ NFs	Zn(CH₃COO)₂·2H₂O, Fe(NO₃)₃·9H₂O and PVA	50–100	NA	Mesoporous	^[63]

NA: not available.

This entry is adapted from the peer-reviewed paper 10.3390/s21041352

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