Summary of the performance of sensors fabricated by using the nanocomposites of metal oxide nanoparticles (0-D) and graphene or graphene derivatives (GO, rGO).
Most of the available commercial solid-state gas/vapor sensors are based on metal oxide semiconductors. Metal oxides (MOs) change their conductivity while exposed to gas or vapors ambient can be utilized as gas or vapor sensing materials. In recent days, graphene has attracted tremendous attention owing to its two-dimensional structure with an extremely high surface to volume ratio, electron mobility, and thermal conductivity. However, intrinsic graphene is relatively inefficient for the adsorption of gas/vapor molecules. In this regard, graphene oxide (GO) and reduced graphene oxide (rGO), which are graphene species functionalized with different oxygen groups that offer a higher amount of adsorption sites improving the sensitivity of the film. Up to now, many research groups across the globe have reported the promising performance towards gas detection using various GO/rGO-metal oxide nanocomposites. This chapter reviews the composites of graphene oxide or reduced graphene oxide and metal oxides in nanoscale dimensions (0-D, 1-D, 2-D, and 3-D) for gas sensing applications considering two specific focus areas, that is, synthesis of nanocomposites and performance assessment for gas/vapor sensing.
- nanoscale metal oxide
- graphene derivatives
- efficient gas sensing
In today’s world, gas/vapor sensors have received significant attention because of their important applications in numerous areas such as environmental monitoring at industry and domestic area , disease diagnosis , agriculture , industrial wastes , food quality monitoring, etc. The detection of gases like NO, NO2, NH3, CO, CO2, SO2, H2S, etc. is essential in many fields especially in environmental monitoring due to their toxicity and the related risk to the ecosystem [1, 2, 3, 4]. Detection of volatile organic compounds (VOCs) is of great importance in environmental safety, supervision of human health, and food quality monitoring [1, 2, 3]. The detection of frequently used VOCs like acetone , formaldehyde , methanol , etc. is essential because they produce toxic effects, even in low concentrations, on human health. Detection of ethanol in human breath is important to restrict the drunken driving-related issue . Timely detection of released VOCs from stored vegetables and fruits is important to monitor their quality and freshness . So, simple and reliable detections of gases and VOCs are important in everyday life.
Most of the existing commercial gas/vapor sensors are based on metal oxide (MO) semiconductors and polymer materials. However, the limitations of these gas sensors can be one or more as follows: costly, low sensitivity in lower ppm or ppb level, poor selectivity, limited lifetime, poor repeatability, difficult to miniaturization high power consumption [4, 10, 11], etc. As an alternative, nanostructured material-based gas/vapor sensors have gained significant importance due to many promising electrical, thermal, and optical characteristics combined with very high effective surface area, high sensitivity, fast response and recovery, selectivity, repeatability and stability , etc. Different carbon nanomaterials, such as graphene, graphene oxide (GO), carbon nanotube (CNT), charcoal, etc. have been shown to be promising gas/vapor sensing behavior due to the simple modifying their sensitivity by easy chemical treatments [12, 13, 14].
The limitations of intrinsic graphene are: (i) difficult to synthesize in large scale, (ii) it has almost no functional groups that can use for the adsorption of gas/vapor molecules, and (iii) it has metallic behavior with almost zero band gap [4, 13]. The prime performance enhancement methods in graphene-based sensors are found to be suitable impurity doping, composite formation, functionalization, implementation in field-effect transistor (FET) structure, etc. In this situation, reduced graphene oxide (rGO), which is graphene functionalized with different oxygen groups that provide enhanced adsorption sites, is more favorable for improving sensitivity. Besides very high thermal stability, the rGO sample contains many dangling bonds which can act as adsorption sites for gas analytes [15, 16].
Although many literatures suggested that the gas sensing performance can be improved by the structural and morphological variations, this is an insufficient approach for the growing demands of the gas/vapor sensing device performance. Single component transition metal oxide and carbon-based materials still suffer from some limitations arising from their inadequate physical and chemical characteristics that may hinder their large scale applications for high-performance gas/vapor sensors. Owing to their variable chemical conformation, synergistic properties, heterostructured nano-hybrids components, and nanocomposites are expected to show more admirable gas/vapor sensing performance [15, 17].
Metal oxide nanostructures are frequently hybridized with (i) noble and transition metals like Pd, Pt, Au, Ag, Ni, Nb, and so on, (ii) other metal oxides, (iii) carbon-based nanomaterials like CNT, graphene, and graphene-derivatives like GO and rGO to improve the gas sensing performance. Among all these functionalized materials, graphene and its derivatives attract tremendous attention for hybridizing with nanostructured metal oxides for promising gas/vapor sensing applications. Improvement of gas sensing properties of graphene/metal oxides hybrids principally depends on the following four factors:
(i) graphene derivative like GO or rGO supplies more dangling bonds and active interaction sites for gas/vapor molecule adsorption/reaction; (ii) its large effective surface area also enhance the gas sensing performance [15, 16]; (iii) metal oxide nanostructures have been extensively discovered as gas/vapor sensors due to the relatively high sensitivity of their electrical conductance to the target adsorbents. Thus the presence of rGO layers on metal oxide surface, electrical properties exhibit large and fast changes in the occurrence of gases/vapors improving overall sensing performance of the sensor; (iv) while GO and rGO show ambipolar behavior in the electron and hole concentration, they show hole-dominant p-type conducting properties owing to the adsorbed water and oxygen molecular species. Also, a nanocomposite of p-type rGO with an n-type transition metal oxide form a p-n heterojunctions and the resulting complex nanostructure may exhibit better sensing performances than those of the individual materials. Numerous research has confirmed that the p-n heterojunction formed by p and n-type materials can play a positive role in the sensing mechanism [18, 19, 20].
However, a wide variety of nanostructured metal oxides and its composite with GO and rGO have been reported for efficient gas/vapor sensing applications in last one decade. In this chapter, we have categorized the graphene nanocomposites based on the morphology of metal oxides, that is, zero-dimensional (0-D like nanoparticles, quantum dots, etc.), one dimensional (1-D like nanorods, nanotubes, nanofibers, etc.), two dimensional (2-D like nanosheets, nanoplates, etc.), and three-dimensional (3-D like nanoflower, nanospheres, etc.). Synthesis, fabrication of graphene/nanoscale metal oxides nanocomposites and their performance assessment for gas/vapor sensing application are the main objective of the article.
2. Synthesis nanoscale metal oxides and graphene derivatives composite
In this section, the synthesis of graphene and its derivatives like graphene oxide (GO) and reduced graphene oxides (rGO) is described in the first sub-section. Then the synthesis of nanoscale metal oxides, as well as the nanohybrid formation, is described in the next sub-section.
2.1 Synthesis of graphene and graphene-derivatives
Graphene is considered as the parent of all graphitic forms . The purest form of graphene is named as pristine graphene (with no heteroatomic contamination) where ‘scotch tape method’ widely accepted for producing the highest quality of graphene . Graphene produced from micromechanical cleavage, that is, adhesive tape method can isolate only a small amount of graphene, hence this method is used to isolate graphene for research purposes. For large scale production of graphene, various methods have been reported in the literature which can be broadly classified into two categories: top-bottom approach and bottom-up approach .
Top-bottom methods mainly involve breaking of the van der Waals bonds which hold layers of graphene to form graphite . Top-bottom approach involves electrochemical exfoliation, exfoliation of graphite intercalation compounds (GIC), micromechanical cleavage, solvent-based exfoliation of graphite oxide, arc discharge, etc. . Among these methods, exfoliation of graphite oxide has received great attention as graphite oxide is easily produced by oxidation of graphite as reported in the Hummers method. Graphite oxide is exfoliated to obtain graphene oxide which is reduced to form reduced graphene oxide (rGO). Reduction process can be thermal, chemical, or UV-based method . Bottom-up approach involves forming of large-area graphene sheet via growth over the substrates and one of the most potential methods is chemical vapor deposition (CVD) .
Along with graphene, researchers have also worked on the synthesis of graphene oxide (GO) as well as reduced graphene oxide (rGO) in recent years. rGO nanoparticles was prepared by thermal reduction of GO which is again obtained from Hummer’s method . However, the required quality of graphene and graphene derivatives (rGO, GO) depends on its applications and based on that the methods of production are decided. Till date, CVD [26, 27] and modified Hummer’s method [28, 29, 30] are most suitable for the synthesis of graphene and GO, respectively, in context of the formation of metal oxide/graphene, metal oxide/rGO, metal oxide/GO nanocomposite.
2.2 Synthesis metal oxide nanostructures and graphene derivatives composites
Synthesis of hybrid graphene with different nanoscale metal oxides are classified in four categories, that is, graphene/0-D metal oxides, graphene/1-D metal oxides, graphene/2-D metal oxides, and graphene/3-D metal oxides.
2.2.1 Synthesis of graphene/0-D metal oxides composites
Synthesis of metal oxide nanoparticles (NPs) and GO/rGO composites which was used for efficient gas sensing applications is described in this section. Among all the metal oxides, SnO2 was reported mostly to synthesize nano composites with graphene and its derivatives (GO and rGO). At the same time, nanoparticles of metal oxides were preferred majorly to prepare the monohybrids with GO and rGO. Different chemical synthesis techniques were followed to develop the nanocomposites of metal oxide/rGO like hydrothermal, solvothermal, flame spray pyrolysis, etc. [31, 32].
Hydrothermal is one of the commonly reported techniques for preparing metal oxide nanoparticles-rGO composites. Among different metal oxides, SnO2 nanoparticles were reported extensively to prepare nano-hybrid with rGO for efficient gas sensing application [33, 34, 35, 36, 37, 38, 39, 40, 41, 42]. SnO2/rGO [33, 34, 35] nano-hybrid was prepared by facile hydrothermal treatment where precursor was prepared with mixture of SnCl4, HCl, H2O, and GO (or rGO). Heating temperatures were reported as 120°C  and 180°C [34, 35] whereas the heating time was 12 h, consistent for all the reports. Different weight% (0.5–5 wt.%) of Au was added in the SnO2/rGO nanocomposite by using HAuCl4 salt to study the effect of Au concentration on the sensitivity of SnO2/rGO gas sensors . Scanning electron micrograph (SEM) of SnO2/rGO films which was used for promising gas sensing application are represented in Figure 1(a and b). Mishra et al. reported rGO/SnO2 nanocomposite by surfactant-assisted hydrothermal method, in which hexamethyldisilazane (HDMS) was used as a surfactant . Ghosh et al.  reported SnO2 nanoparticle synthesis by hydrothermal method and SnO2/rGO film synthesis by mixing of SnO2 nanoparticles with GO. The GO-SnO2 mixture was then ultrasonicated to obtain uniform dispersion. Then the sample was drop cast on the platinum electrode and heated at 160°C to reduce GO and get SnO2/rGO hybrid sensing layer . The hydrothermal method was also used for the synthesis of SnO2/rGO hybrid with a high concentration of oxygen vacancy [40, 42] and Pt-activated SnO2 nanoparticles-rGO hybrid . Transmission electron micrograph (TEMs) of SnO2 quantum dot decorated on rGO surface is represented in Figure 2(a–c).
NiO/rGO nanohybrid  was prepared via two-step hydrothermal treatment. NiO nanoparticles powder was prepared by hydrothermal method using NiCl4·6H2O as the source of Ni and then calcined at 400°C. NiO nanoparticle powder was then mixed with rGO solution and treated by hydrothermal method with a various ratio of NiO/rGO as 2:1, 4:1, and 8:1 (Figure 3).
Undoped and Ni-doped SnO2 nanoparticle and graphene composites were developed by flame spray pyrolysis (FSP) method as reported in references [32, 43], respectively. About 0.1−2 wt.% Ni-doped SnO2 nanoparticles were synthesized by FSP technique and graphene was produced from graphite by the electrolytic exfoliation technique. Then, a paste was prepared by mixing Ni-doped SnO2 and graphene powder and finally spin coating method was used to deposit a film for gas sensing application. Bright field (BF) TEM images of 0.5 wt.% SnO2 NPs loaded graphene composites and 2 wt.% Ni doped SnO2 NPs loaded graphene composites are represented in Figure 4(a) and (b), respectively.
ZnO/rGO composite was prepared by the solvothermal method for low-temperature acetylene sensing as reported by Iftekhar Uddin et al. [44, 45]. ZnO powder was prepared through the solvothermal method by using Zn(NO3)2 and NaOH in ethanol at 120°C. Ag-loaded ZnO/GO hybrid was synthesized by chemical route. AgNO3 was added to the ZnO/GO solution with 2:1 ratio, then stirred continuously for 30 min. Hydrazine hydrate was then added to the mixer to reduce GO at 110°C for 8 h . Morphology of ZnO NPs and rGO nanocomposite is shown in Figure 5(a and b).
ZnO quantum dots (QDs) decorated on graphene nanosheets were synthesized by facile solution-processed method (Figure 6(a)). ZnO QDs were nucleated and grown on the surface of graphene by controlling the distribution density by reaction time and precursor concentration . ZnO-rGO hybrid was prepared by wet chemical method followed by deposition of Au using HAuCl4, which was added to the ZnO-rGO dispersion. Finally, the addition of NaBH4 through sonication process completed the formation of ZnO QD . To understand the impact of particle size on gas sensing performance, Tung et al.  prepared rGO-Fe3O4 nanoparticle hybrid with different particle sizes (5, 10, and 20 nm) via in situ chemical reduction of GO in presence of poly-ionic liquid (PIL) (Figure 6(b)). Kamal  prepared graphene-NiO nanoparticles composites by decomposition of nickel benzoate dihydrazinate complex used for hydrogen sensing application.
Graphene oxide was synthesized from natural graphite flakes by Hummers’ method which was further used to prepare rGO-CuFe2O4 nanocomposite by combustion method . In this process, sonicated GO was dissolved with 1:2 ratio of Cu2+ to Fe3+ salts and distilled water. The resulting mixture was stirred at 100°C to get a viscous solution which was further heated around 450°C in a muffle furnace. Finally, the mixture was frothed and it gave a foamy powder of nanocomposite (Figure 6(c)). The one-pot microwave-assisted non-aqueous sol-gel method was used to synthesize pure SnO2 nanoparticles and SnO2/rGO nanocomposite (Figure 6(d)) . Kim et al.  reported the microwave-assisted the formation of SnO2/graphene nanocomposite in which mixture of SnO2 nanopowder and graphene flakes dispersed in ethanol, the resulting solution was dried. The dried powder mixture was treated in the commercial microwave heater for heating process. Microwave-treated powder was again dispersed in ethanol and then the solution was spray-coated on SiO2 substrate placed on a hot plate. Along with SnO2/graphene nanocomposite, a small amount of secondary SnOx (x < 2) nanoparticles were also deposited on the surface. Secondary SnOx nanoparticles tend to increase as the microwave heating time is increased .
2.2.2 Synthesis of graphene/1-D metal oxides composites
One dimensional (1-D) nanostructures of the metal oxide like nanotubes, nanorods, nanofibers, nanowires, etc. are considered as most promising for the detection of analytes in gaseous phases [53, 54]. Owing to its large surface-to-volume ratio, large open porosity and most importantly one of its dimension is comparable to the Debye length which enhances the gas sensitivity significantly. Gas sensing performance is improved further in 1-D materials by using as a nanocomposite with graphene, GO and rGO. Here, we reviewed different methods used for the synthesis of 1-D metal oxide nanostructure and its composite with graphene and graphene derivatives.
ZnO nanowires (NW) and graphene hybrid architecture were reported by Yi et al.  where graphene sheets covered with thin metal layers were used as top electrodes for ZnO where graphene sheets coated with thin metal layers were employed as top electrodes for ZnO vertical-NW channels. The ZnO NWs-graphene/metal hybrid architectures maintained sufficient spaces between the NWs for easy and fast gas transport. However, ZnO nanorods (NRs) were synthesized by using hydrothermal reaction and graphene sheets were synthesized by the CVD method and transferred to the top of the ZnO NRs by PMMA treatment. The scanning electron micrograph of ZnO NRs-graphene/metal hybrid architectures are represented in Figure 7(a). Single crystalline WO3 nanorods on the surface of graphene were synthesized through a one-step hydrothermal method . WO3 nanorods with 3.5 wt.% graphene composites improved gas sensitivity of 25 times showing good selectivity towards NO2. SEM image of WO3 nanorods and graphene hybrids is shown in Figure 7(b).
Large-scale sandwich-like heterostructures of ZnO nanorod arrays with reduced graphene oxide sheets were reported by Zou et al.  as shown in Figure 7(c). Highly dense ZnO nanorods were grown by hydrothermal method and double sides coverage of reduced graphene sheets by ZnO NRs formed a sandwich like heterostructures of ZnO/graphene/ZnO for efficient ethanol detection.
Electrospinning is a potential and well-reported technique for the synthesis of nanofibers (NFs) network of metal oxides. N,N-dimethylformamide (DMF) and polyvinyl pyrrolidone (PVP) are mixed with target metal oxide precursor and the whole mixture is poured into a syringe having a suitable needle attached. A high voltage (a few kV, DC) is applied between the needle and the collector plate to get the NFs of the target metal oxides. In electrospinning method, composites of metal oxide NFs and graphene derivatives are synthesized by two different routes, that is, (i) GO or rGO solutions are added into the base mixture before electrospinning and (ii) synthesized NFs are decorated with GO or rGO solutions. rGO/Co3O4 NFs  and rGO/ZnONFs composites  were synthesized where rGO was added into the precursor before electrospinning and both the nanohybrids were tested towards different gases and vapors like NH3, ethanol, etc. rGO/Co3O4 NFs , rGO/SnO2 NFs , and rGO/WO3 NFs composites  were synthesized where Co3O4, SnO2, and WO3 nanofibers were synthesized by electrospinning method first and then functionalized with rGO solution. All three composites were used to detect acetone in the selective route. Figure 8(a) and (b) represents the TEM and SEM images of rGO/Co3O4 NFs  and rGO/ZnONFs composite , respectively. Here, rGO film was almost invisible due to the mixing of rGO in solution before electrospinning. rGO film was clearly visible in rGO/Co3O4 NFs  and rGO/SnO2 NFs  composites in SEM images shown in Figure 8(c) and (d) as the rGO functionalization was carried out after electrospinning.
Synthesis of TiO2 nanotubes array was done by electrochemical anodization of metallic titanium films  and rGO/TiO2 nanotubes composite was synthesized by electrodeposition of rGO on TiO2 nanotubes [61, 62]. Electrodeposition method was also used to synthesize ZnO nanorods and selective electrochemical etching of those nanorods to synthesized ZnO nanotubes. rGO/ZnO nanotubes hybrid structure was synthesized by dip-coating technique for efficient alcohol sensing application .
One step colloidal synthesis was employed for rGO/SnO2 quantum wire nanocomposite for room temperature H2S sensing . Single crystal SnO2 nanowire was directly grown on the platinum electrode by thermal evaporation and composites was formed by using CVD grown graphene layer for efficient NO2 sensing . Hydrolysis method was used in absence as well as in presence of GO to form ZnO nano-seed and GO supported ZnO nano-seed, respectively, and ultrathin ZnO nanorod/rGO mesoporous nanocomposites were synthesized for NO2 sensing .
2.2.3 Synthesis of graphene/2-D metal oxides composites
Metal oxides nano-sheets and nameplates were functionalized with graphene and its derivatives for efficient gas sensing behavior. Ni-doped ZnO nanosheets were deposited on a p-Si substrate by using radio frequency (RF) sputtering techniques. GO was synthesized by Hummer’s method and reduced thermally at high temperature (600°C). rGO flakes were then decorated on Ni-doped ZnO nanosheets by drop-casting method. rGO/Ni-doped ZnO nanosheets were used for low ppm hydrogen detection . Highly wrinkled SnO2/rGO composite was synthesized by one-time hydrothermal technique and used for the detection of ethanol at 250°C (Figure 9(a)) . Nanocomposites of ZnO nanosheets and GO were synthesized for highly efficient acetone sensing. The nanocomposites sensor was flexible, high effective surface area and enhanced functional groups due to GO which were in favor of gas adsorption (Figure 9(b)) . rGO/hexagonal WO3nanosheets hybrid materials were fabricated through the hydrothermal method and post-annealing treatment. 2-D porous WO3nanosheets were attached on rGO. The sensor based on 3.8 wt.% rGO/hexagonal-WO3composites offered promising sensing performance to H2S .
2.2.4 Synthesis of graphene/3-D metal oxides composites
Hybrids with flower-like hierarchical ZnO and rGO were synthesized by the facile and mild solution-processed method. Compared with the pristine flower-like ZnO, NO2 sensing was increased significantly in case of hierarchical rGO/ZnO hybrids . A facile one-pot hydrothermal method was used to synthesize rGO/In2O3 composites. The flower-like hierarchical structure of In2O3 showed high effective surface area enhancing the active interaction sites. In the composite, rGO formed local p-n heterojunctions enhancing the gas sensing performance significantly. The rGO/In2O3 composite exhibited an excellent selectivity towards NO2 in the wide concentration range from 10 ppb to 1 ppm . α-Fe2O3/rGO nanocomposites with nanosphere-like α-Fe2O3 structure were synthesized by a hydrothermal route at 120°C. α-Fe2O3 nanosphere was 40–50 nm in diameter and constructed by a few nanometer-sized nanoparticles where rGO was intercalated single sheets. These nanocomposites showed excellent response and selectivity towards NO2 at room temperature . Graphene-WO3 nanostructure with gauze-like graphene nanosheets wrapping up spherical WO3 nanoparticles was synthesized by a facile sol-gel method. Graphene-wrapped WO3 nanocomposites offered uniform nanospheres with 200–400 nm diameter. Graphene/WO3 nanocomposites showed good sensitivity and selectivity to low concentrations of NO2 gas at room temperature when pure WO3 and graphene-based sensors did not show any response towards NO2 at room temperature . Scanning electron micrograph of 3-D metal oxides and graphene nanocomposites is shown in Figure 10(a–d).
3. Assessments of gas/vapor sensing performances
3.1 Sensing performance of graphene/0-D metal oxides composites
Gas/vapor sensing performance of metal oxide nanoparticles (0-D) functionalized with graphene or graphene derivatives (GO, rGO) is represented in Table 1 where 37 references are considered for performance assessment of graphene/0-D metal oxide oxides composites sensors.
|Composite material||Target gas/vapor||Operating temperature (range) (°C)||Concentration and range (ppm)||Response magnitude||Response/recovery time (s)||Ref.|
|Graphene-SnO2 nanoparticles||NO2||150 (30–190)||5 (1–5)||Rgas/Rair = 72.6||129/107|||
|rGO/SnO2 nanoparticles||Acetone||Room temperature||2000 (10–2000)||Rair − Rgas/Rair = 9.72%||95/141|||
|Graphene aerogel-SnO2 nanoparticle||NO2||Room temperature||200 (10–200)||Rgas − Rair/Rair = −12%||190/224|||
|Graphene-SnO2 nanoparticles||H2||50||100 (1–100)||Igas/Iair = 6||1.1/1.1|||
|rGO-SnO2 quantum dots||H2||200||500||Rair − Rgas/Rair = 89.3%||∼50/∼155|||
|LPG||250||500||Rair − Rgas/Rair = 92.4%||∼80/∼155|
|rGO/SnO2 nanoparticles||NH3||200 (100–200)||1000 (25–2800)||—||—|||
|rGO/SnO2 nanoparticles||C2H2||180 (100–300)||50 (0.5–500)||Rair/Rgas = 12.4||54/23|||
|rGO-SnO2 nanoparticles||NO2||30 (30–100)||1 (0.05–2)||Rair/Rgas = 3.8||14/190|||
|rGO-SnO2 nanoparticles||NO2||Room temperature||5 (1–20)||Iair − Igas/Iair = 65.5%||12/17|||
|Graphene/SnO2 nanoparticles||NO2||150 (25–350)||5||Rgas/Rair = 26342||13/—|||
|rGO/SnO2 nanoparticles||NO2||50 (30–60)||5 (0.5–500)||Rair/Rgas = 3.31 (25% RH)||135/200|||
|rGO/SnO2 nanoparticles||SO2||60 (22–220)||500 (10–500)||Rair − Rgas/Rgas = ∼22||144/210|||
|Graphene/SnO2 nanoparticles||NO2||Room temperature||100 (0.3–100)||Gg − Go/Go = ∼11||—|||
|Graphene/SnO2 nanoparticles||Ethanol||350 (150–350)||1000 (50–1000)||965||1.8/∼120|||
|Graphene/SnO2 nanoparticles||NO2||60 (25–120)||4 (1–4)||Rg – Ra/Ra = ∼22||—|||
|rGO/SnO2 nanoparticles||CO||Room temperature||1600 (50–1600)||Rg – Ra/Ra = 9.5%||∼60/∼60|||
|Sulfonated graphene/SnO2 nanoparticles||NO2||Room temperature||5 (1–50)||Rair − Rgas/Rgas = 1.203||40/357|||
|Graphene-SnO2 nanoparticle with doped Ni||Acetone||350 (150–350)||200 (1–50)||Rair/Rgas = 169.7||5.4/150|||
|Ro − Rgas/Ro = 11%(H2), 14.8% (ethanol)||34/27|||
|rGO/SnO2 nanoparticles decorated Au NPs||NO2||50 (30–60)||50 (5–100)||Rgas/Rair = 2.68||19/20|||
|rGO-SnO2 nanoparticles (activated by Pt)||Methanol||110 (20–180)||500 (10–500)||Rair/Rgas = 203||6/21|||
|rGO/SnO2 nanoparticles (with Ag NPs)||NO2||Room temperature||5 (0.5–500)||Rair/Rgas = 2.17 (25% RH)||49/339|||
|Graphene-ZnO quantum dots||HCHO||Room temperature||100 (25–100)||Gg − Go/Go = 1.1||30/40|||
|Graphene/ZnO nanoparticles||C2H2||250 (25–350)||100 (30–1000)||Rair/Rgas = 143||100/24|||
|rGO/ZnO nanoparticles||NO2||50 (25–140)||50 (5–275)||Rg − Ra/Ra = 32%||96/1552|||
|rGO/ZnO nanoparticles||NO2||Room temperature||5 (1–25)||Rg – Ra/Ra = 25.6%||165/499|||
|3-D rGO/ZnO nanoparticle||CO||200||1000 (1–1000)||Rg – Ra/Ra = 85.2%||7/9|||
|rGO-ZnO/Ag nanoparticles||C2H2||150 (25–250)||100 (1–1000)||Rair/Rgas =21.2||25/80|||
|rGO/ZnO-Au nanoparticles||NO2||80 (60–90)||100 (20–100)||Rair − Rgas/Rair = 32.55||27/86|||
|Graphene-NiO nanoparticles||H2||250 (100–350)||2000 (400–2000)||Rg – Ra/Ra = 52.4%||NA|||
|rGO/NiO nanoparticles||CH4||260 (20–400)||1000 (100–1000)||Rair − Rgas/Rgas = 15.2||16/20|||
|rGO/NiO NP with SnO2 nanoplates||NO2||Room temperature||60 (5–60)||Gg − Go/Go = 62.28||220/835|||
|rGO/CuO nanohybrid||NO2||135 (25–225)||75 (1–75)||Igas − Iair/Iair = 51.7||50/105|||
|rGO/CuFe2O4 nanoparticles||NH3||110||200 (5–200)||Rg – Ra/Ra = 24.41||3/6|||
|rGO/Fe3O4 nanoparticles||Ethanol||Room temperature||1||Rair − Rgas/Rgas = 1.86||—|||
|NO2||200 (250–450)||2.5 (1–5)||Rair − Rgas/Rgas = 4.68||—|
|Graphene/WO3 nanoparticle||NO2||250 (200–300)||5 (1–20)||Rgas/Rair = 133||∼25/—|||
|Graphene-CeO2 nanoparticles||NO2||—||(10–200 ppm)||—||—|||
Graphene/metal oxide NPs (0-D) composites and its gas sensing performance were explored extensively in case of SnO2 where hydrothermal method was commonly used as the synthesis technique. The gas sensing performance of rGO/SnO2 NPs was further improved by functionalization with Pd, Au, Pt, and Ag nanoparticles [31, 34, 41, 78]. ZnO nanoparticles occupied the second position to use as a nanocomposite with graphene and its derivatives other metal oxide nanoparticles like NiO, CuO, WO3, Fe3O4, CeO2, etc. were reported as promising gas/vapor sensing composite materials with graphene and its derivatives. However, among all the target gases and vapors, NO2 was the mostly explored gas and detected successfully by graphene/metal oxide nanocomposite sensors . Other gases like H2, NH3, CO, C2H2, CH4, SO2, and organic vapors like acetone, ethanol, methanol, and formaldehyde were detected successfully by using graphene/NP metal oxide hybrids. Sensors were tested at different temperature range varying from room temperature ∼25°C to 400°C. The average operating temperature of graphene/NP metal oxide nanocomposites was recorded below 150°C. Among the 37 research articles, room temperature sensing was reported by 13 groups of researchers mentioned in the literature. The detailed transient behavior of graphene/metal oxide NPs sensors is shown in Figure 11(a–f) where all the sensing was reported at room temperature. So, overall study confirms the lowering of gas sensing the temperature of metal oxide nanoparticles due to the formation of composites with graphene, GO, and rGO. Most of the article showed a lower detection limit of gases and vapors (<100 ppm). Depending on the surface morphology, sensing temperature and device structures the detection range varied from ppb to ppm level as shown in Table 1. Finally, the response magnitude and response/recovery time were fully dependent on the operating temperature and concentration range of the analyte. However, all the sensors showed adequate response magnitude towards the target gas/vapors. Response/recovery time increased and decreased with decrease and increase of operating temperature, respectively. Response time and recovery time varied from 1 s to 220 s and 1 s to 1552 s, respectively, as shown in Table 1. Wang et al. confirmed that the uniform distribution of SnO2 nanoparticles on rGO sheets is an effective factor for enhanced NO2 sensing performances . The p-n junction existed in the interface of nanoparticle and rGO contributed to good room temperature NO2 sensing properties which is associated with the valid electron flow from SnO2 nanoparticle to rGO.
3.2 Sensing performance of graphene/1-D metal oxides composites
One dimensional (1-D) metal oxide like nanorods, nanotubes, nanowires, nanofibers, quantum nanowires, etc. functionalized by graphene, GO, and rGO were reported as the promising gas sensing materials. A total of 21 reports on graphene/1-D metal oxide nanocomposites have been summarized in Table 2 and compared in terms of target gas/vapors and its concentration, operating temperature, response magnitude, and response/recovery time. In the case of 1-D nanomaterials, ZnO was most explored metal oxide used to synthesize nanocomposites with rGO in the form of nanorods, nanotubes, and nanofibers. Other metal oxides are SnO2 nanofibers/nanorods, TiO2 nanotubes, Co3O4 nanofibers, WO3nanorods, αFe2O3 nanofibers, etc. reported for nanocomposites with graphene for gas sensing application. NO2 and ethanol were the mostly explored target gas and vapor in case of graphene/1-D metal oxide nanocomposites. Other selective gases and vapors are CO, H2S, CH4, H2, NH3, acetone, benzene, methanol, etc. Though the operating temperature range was in-between room temperature ∼25°C and 450°C, average sensing temperature was slightly high (∼200°C) in case of 1-D compared with 0-D metal oxide nanocomposites with graphene. However, the detection range of gases/vapors was quite small in case of graphene/1-D metal oxide nanocomposites where 1 ppm and below 1 ppm detection were reported frequently. Quite a high response magnitude and moderate response/recovery time were recorded in case of graphene/1-D metal oxide nanocomposites.
|Composite material||Target gas/vapor||Operating temperature (range) (°C)||Concentration and range (ppm)||Response magnitude||Response/recovery time (s)||Ref.|
|rGO/ZnO nanorods||NO2||Room temperature||1 (0.120–1)||Rgas − Rair/Rair = 50%||120/320|||
|Functionalized graphene/ZnO nanorods||Ethanol||340 (200–370)||100||Rair − Rgas/Rair = 93.5%||5/20|||
|rGO/ZnO nanofibers||NO2||400 (300–400)||5 (1–5)||Rgas/Rair = 119||143/259|||
|CO||400 (300–400)||5 (1–5)||Rair/Rgas = 22.6||—|
|Non-oxidized graphene/ZnO nanofibers||Acetone||350 (250–450)||5 (1–5)||Rair/Rgas = 18.5||12.8/—|||
|rGO/ZnO nanotubes||Ethanol||125 (27–150)||100 (1–800)||79.14%||41.1/98.32|||
|rGO/ZnO nanorods||Ethanol||260||50 (5–50)||Rair/Rgas = ∼27||<10/<10|||
|rGO/ZnO nanorods||NO2||Room temperature||1 (1–10)||Rair − Rgas/Rgas = 1.19||75/132|||
|Graphene/ZnO nanorod doped by Au/Ti||Ethanol||300||50 (10–50)||Rair/Rgas = ∼90||—|||
|Au/Pd functionalized rGO/ZnO nanofiber||CO||400 (300–450)||5 (1–5)||Rair/Rgas = 35.8||191.3/82.1|||
|C6H6||400 (300–450)||5 (1–5)||Rair/Rgas = 22.8||110.3/318.2|
|rGO/SnO2 nanofibers||H2S||200||5 (1–5)||Rair/Rgas = 33.7||<198/<114|||
|Acetone||350 (150–400)||5 (1–5)||Rair/Rgas = 10.4||<198/<114|
|Graphene/SnO2 nanowires||NO2||150 (100–250)||0.1 (0.01–0.1)||Rair/Rgas = 11||43/37|||
|Graphene/SnO2 nanorods||H2S||260||50 (1–50)||Rair/Rgas = 130||5/10|||
|rGO/SnO2 quantum wires||H2S||Room temperature||50 (10–100)||Rair/Rgas = 33||2/292|||
|Nanoporous graphene hybrid-SnO2 nanorods||CH4||150 (100–200)||1000|||Rgas – Rair|/Rair = 24.9 %||369/519.8217.9/242|||
|rGO/TiO2 nanotubes||H2||200 (100–300)||480 (120–480)|||Ggas − Go|/Go = 37.6||1110/<300|||
|rGO/TiO2 nanotubes||Methanol||Room temperature||800 (10–800)||Rair − Rgas/Rair = 96.93%||18/61|||
|GO/Co3O4 nanofibers||Acetone||300 (200–350)||5 (1–5)||Rgas/Rair = 2.29||—|||
|rGO/Co3O4 nanowires||NH3||Room temperature||50 (5–100)||ΔR/Rair = 53.6%||4/300|||
|rGO/Cu2O nanowires||NO2||Room temperature||2 (0.4–2)||Igas − Iair/Iair = 67.8%||—|||
|Graphene/WO3 nanorods||NO2||300||20 (0.025–20)||Rgas/Rair = 202||—|||
|rGO/αFe2O3 nanofibers||Acetone||375 (250–400)||100 (0.05–2)||Rair/Rgas = 8.9||3/9|||
Figure 12(a) represents very efficient H2S sensing for rGO/SnO2 quantum wires sensor for the concentration range of 10–100 ppm at room temperature. Being a room temperature sensing, the sensor showed a very fast response of 2 s only . Highly selective NH3 sensing was reported for rGO/Co3O4 nanowires at room temperature as shown in Figure 12(b) where response time was only 4 s . Improvised NO2 sensing was observed for rGO/Cu2O nanowires compared with the pure Cu2O nanowires and pure rGO in the concentration range of 0.4–2 ppm at room temperature (Figure 12(c)) . However, the overall study envisages the potential gas sensing of 1-D metal oxides functionalized by graphene, GO, and rGO. The 1-dimensional structure of TiO2 nanotubes (NTs) in its hybrid with rGO provided large amount of gas interaction sites which lead to high response magnitude of the sensor .
3.3 Sensing performance of graphene/2-D metal oxides composites
Table 3 shows gas sensing performance of 2-D metal oxides and GO (or rGO) nanocomposites where ZnO, SnO2, and WO3 nanosheets were used as 2-D materials. Relatively high operating temperature was reported for graphene/2-D metal oxide nano composites. Average sensing temperature was more than 200°C. Moderate response magnitude, response/recovery time were recorded for graphene/2-D metal oxide nano composites. Transient behavior of GO/ZnO nanosheets in the exposure of 100 ppm acetone at 240°C and rGO/hexagonal WO3nanosheets in the exposure of 40 ppm H2S at 350°C are shown in Figure 13(a) and (b) [66, 67].
|Composite material||Target gas/vapor||Operating temperature (range) (°C)||Concentration and range (ppm)||Response magnitude||Response/recovery time (s)||Ref.|
|rGO/Ni-doped ZnO nanoplates||H2||150||100 (1–100)|||Rair – Rgas|/Rair = 63.8%||28/—|||
|GO/ZnO nanosheets||Acetone||240||100 (50–500)||Rair/Rgas = 35.8%||13/7|||
|rGO nanosheets/wrinkled SnO2||Ethanol||250 (150–300)||100 (5–5000)||Rair/Rgas = 80%||9/457|||
|rGO/hexagonal WO3 nanosheets||H2S||350 (50–400)||40 (0.01–40)|||Rgas – Rair|/Rair = 168.58%||7/55|||
3.4 Sensing performance of graphene/2-D metal oxides composites
The gas sensing performance of nanocomposites developed by 3-D metal oxide and graphene derivatives are shown in Table 4. Nanoflowers and nanosphere structures were reported here. Interestingly, all the nanocomposites showed their selective behavior towards NO2. Operating temperature of the sensor was quite low and most of the cases room temperature sensing was reported. The detection range was quite high where lower and higher detection limit varied from a few ppb to 1000 ppm. Very high response magnitude was reported in case of graphene/3-D metal oxide nanocomposites. Reported response time and recovery time were quite high in the case of 3-D metal oxide composites compared with 0-D and 1-D metal oxide nanocomposites. Highly selective NO2 sensing was reported for rGO/ZnO nanoflower (1.7% rGO in ZnO) as shown in Figure 14(a). Promising NO2 sensing was observed for rGO/In2O3 nanoflower where the response was poor for pure In2O3nanoflower as shown in Figure 14(b). The nanoflower-shaped CuO nanostructure in its nanocomposite with rGO is effective to prevent the aggregation of rGO sheets and form porous structure with rGO, which greatly facilitate the adsorption and diffusion of gas molecules .
|Composite material||Target gas/vapor||Operating temperature (range) (°C)||Concentration and range (ppm)||Response magnitude||Response/recovery time (s)||Ref.|
|rGO/ZnO nanoflower||NO2||100 (50–150)||0.5 (0.005–0.5)||Rgas/Rair = 12||258/288|||
|rGO/In2O3 nanoflower||NO2||74 (25–110)||1 (0.01–1)||Rgas/Rair = 1337||208/39|||
|rGO/Fe2O3 nanosphere||NO2||Room temperature||90 (0.18–90)||Rair − Rgas/Rgas = 150.63%||80/1648|||
|rGO/CuO nanoflower||NO2||Room temperature||1000 (0.25–1000)|||Rgas – Rair|/Rair = 6.61 %||76/232|||
|Graphene/WO3 nanosphere||NO2||Room temperature||56 (7–56)||Igas – Iair/Iair = 40.8%||—|||
|rGO/WO3 porous nanoflakes||NO2||90 (20–200)||10 (5–200)||Rgas/Rair = 5%||4.1/5.8|||
Hybrids of graphene/nanoscale metal oxides have been extensively discussed in this chapter where the major focused area was synthesis/fabrication of monohybrid and its performance assessment for gas/vapor sensing applications. Detailed literature survey confirmed that metal oxide nanoparticle (0-D) are the most reported nanostructure used for the synthesis of nanocomposites with graphene (and GO and rGO) for potential gas sensing application while 1-D metal oxides like nanorods, nanotubes, nanowires, etc. were in the second place. Use of 2-D and 3-D metal oxides were relatively less to form composites with graphene. In chemical synthesis, GO/rGO functionalization was carried out in two routes, that is, (i) mixing of GO/rGO in precursor before synthesizing a nanostructure and (ii) functionalization by GO/rGO after synthesis of nanostructures. Hydrothermal was the most popular method followed by solvothermal, sol-gel, spray pyrolysis, etc. reported to synthesize a composite of 0-D metal oxides. Hydrothermal, electrospinning, electrochemical anodization, etc. were used for the synthesis of graphene/1-D metal oxide composites. Most of the 2-D and 3-D nanocomposites were grown by low-cost chemical methods. Therefore, the graphene/nanoscale metal oxide composites can synthesize via a cost-effective way.
Among all the metal oxides, SnO2 was mostly reported materials in 0-D structure used in composites with graphene. Other popular metal oxides are ZnO, WO3 and TiO2 mostly used for 1-D metal oxide hybrid. A large number of the report showed NO2 selective behavior of rGO/metal oxide nanocomposite gas sensors especially for 3-D and 0-D metal oxide hybrids. Other reported gases/vapors are H2, NH3, CO, H2S, C2H2, ethanol, methanol, acetone, benzene, etc. A significant variation was observed in case of operating temperature of the sensors in case of different nanoscale metal oxides. The average sensing temperature was highest in case of 2-D nanocomposites and decreased from 1-D, 0-D and 3-D. However, the overall study confirmed the relatively low-temperature detection of gases and vapors after the formation of composites of graphene and nanoscale metal oxides. The detection range was varying from lower ppb to higher ppm level but most of the report was confined near a low ppm range (1–100 ppm). A significant improvement was also observed in case of response magnitude and response time/recovery time.
Finally, we would like to conclude with the comment that gas/vapor sensing performance was improved significantly due to the formation of nanohybrid of nanoscale metal oxides and graphene derivatives like GO and rGO. Further study may be necessary with these nano thin-film sensors to encourage the performance in terms of high selectivity and long-term stability. Then these hybrid sensors to make these nanocomposite sensors more suitable for practical applications.
This work was supported in part by Department of Biotechnology grant (Letter No. BT/PR28727/NNT/28/1569/2018) and SPARC grant (SPARC/2018-2019/P1394/SL), Govt. of India.
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