14
Nanosensor in Gas Monitoring: A Review

Nurhidayatullaili Muhd Julkapli and Samira Bagheri

University Malaya, Nanotechnology and Catalysis Research Centre (NANOCAT), IPS Building, 50603 Kuala Lumpur, Malaysia

14.1 Introduction

It has been demonstrated that exposure to toxic gases at high concentrations has profound health effects on the human respiratory system and could induce unconsciousness with attendant neurological sequelae and even death [1–3]. Even at relatively low concentrations, it can cause a malodor nuisance problem [2]. Although the chromatography-based systems have been currently and most frequently employed in petroleum industries due to their reliability of high detectability and precision, they are not convenient enough to track down short-term variations in behavior since their application involves a multistage protocol from the initial sampling to final determination [1–4]. In addition, offline analysis of toxic gases can be time-consuming and making accurate measurements is a big challenge.

The chemical sensitivity of nanosensors based on conductive nanomaterials and composites has been currently used for toxic gas detection due to their many benefits such as low cost, fast response, miniaturization, simple operation, and easy networking [3,5]. However, there are major obstacles to take this technology to an industrial scale including lack of high-quality, reproducible nanomaterials, nonuniform physical and chemical properties, relative inertness to the atmosphere due to lack of active groups, and sensitivity to humidity [6]. Depending on the level of humidity that can be obtained up to thousands of parts per million in the real-world environment in which the sensors are to be operated, their ability to detect gases can be completely screened. Hence, the success of a sensor largely depends on its sensitivity to humidity, ability to sense nonpolar and large molecules, and accessibility to be used under dry and vacuum conditions.

14.2 Sensing Technologies in Petroleum Industries

The development of gas sensors to monitor combustible gases is imperative due to concerns for safety, especially for liquefied petroleum gas [7,8]. Due to its high potential to produce hazardous gases, the detection is particularly important, because explosions may be caused when they leak out accidently or by mistake. Thus, demand for developing simple yet reliable gas sensors is huge for petroleum industries. The detection of gas molecules, including mononitrogen oxides (nitric oxide, NO), nitrogen dioxide (NO2), ammonia (NH3), carbon monoxide (CO), and others is important due to their toxicity and associated risks to ecosystem (Figure 14.1) (Table 14.1).

Figure 14.1 Optimized structures of the interaction of small gases (including highly toxic ones) with the open metal sites of [Cu3(btc)2]: (a) CO, (b) CO2, (c) O2, (d) OCS(O), (e) OCS(S), (f) PH3, (g) NH3, (h) N2, (i) NO, (j) N2O(O), (k) N2O(N), and (l) NO2 [1].

Table 14.1 Gaseous pollutants, sources, and effect on environment.

Gases Sources Effect on environment References
NO2 Combustion sources and automobile Harmful to plant and respiratory system of beings and animals [9]
CO2 Colorless, tasteless, toxic gas generated from the burning of fossil fuels and malfunctioning equipment
Used as fault characteristic gas in evaluating the insulation performance of power transformers		 Induces greenhouse gases and global warming [10]
NH3 Irritating and corrosive gases from industries Inhalation of even low concentrations causes coughing as well as nose and throat irritation
Contact with concentrated NH3 solutions may cause corrosive injury, including skin burns, permanent eye damage, or blindness		 [11]
H2S Produced in large quantity from industrial activities of petroleum/gas drilling and refining, wastewater treatment, coke ovens, tanneries, and landfills Involved in a wide range of physiological functions, and its abnormal level are associated with a series of disease, including diabetes, hypertension, stroke, and Alzheimer's disease [12]
CO Colorless, tasteless, toxic gas mainly generated from the burning of fossil fuels and malfunctioning equipment Effect on respiratory system [13]

Therefore, the detection of gases in domestic appliances must be no false alarm, which requires to be done before reaching the explosive limits. A lower explosive limit is the lowest concentration (percentage) of a gas or a vapor in the air capable of producing flash of fire in the presence of ignition sources including arc, flame, and heat [10–12].

14.3 Nanosensor Technology

Nanostructured materials can improve the performance of gas sensors because of their much greater surface to volume ratio compared to micrograined materials. The large surface to volume ratio of nanomaterials can be used as an advantage as the number of defect sites for the reaction is increased [14–16]. The large surface area to volume ratio of the nanocrystalline structures increases the opportunity for this surface reaction to occur. This in turn will increase the sensitivity and response rate of the gas sensor [15]. There are several points that need to be considered to improve the nanosensor technology, including easy and low-cost nanomaterial synthesis, minimizing substrate effect in order to avoid undesired surface contamination produced during microfabrication processes, incorporating new functional molecules with specific interaction designed for improving the sensitivity and selectivity of the adsorbates, and the impact of humidity on the performance of sensors should be quantified and controlled [9,17].

14.3.1 Nanomaterials

Highly effective, stable, and sensitive gas sensors can be achieved by modifying materials at the nanoscale (Figure 14.2). First approach is by achieving a high nonequilibrium amount of O2 vacancies in a nanometal oxide sensor [18]. A high nanoequilibrium amount of O2 vacancies serves to contribute in more effective movement of charge across the sensor material. The nonequilibrium state is used because thermodynamics at room temperature does not predict the amount of vacancies required for a low operating temperature [19]. Second, by well-dispersed ion conductivity in polymeric composites that could increase the number of surface sites for gas interaction, even a low concentration of gas can be detected at room temperature. Both approaches are applied to changing the defect chemistry within the space charge layer and surface of materials making it possible to get maximum signal change in the presence of targeted gases (Table 14.2).

Figure depicting the structure and surface modification of nanomaterial for sensor properties improvement.

Figure 14.2 Structure and surface modification of nanomaterial for sensor properties improvement (reproduced with permission from Ref. [20]. Copyright 2014, Elsevier).

Table 14.2 Positive and negative aspects of material selection for sensor applications.

Nanomaterials Positive view Negative view References
Nanometal oxide Low response times and high sensitivity
Act as catalyst to improve speed of the detection process		 Require post-treatment such as annealing
High operation temperature
High power consumption
Limited for explosive gas detection		 [20,21]
Ion conductive polymeric composites Lower working temperature
Conductive nanoparticle consumption is less		 Advanced technique for synthesis to control the homogeneity and dispersion [22,23]
Nanocarbon High sensitivity with shorter response, recovery, and sensitivity
Mechanically flexible
Its two-dimensional structure is compatible with traditional planer process technology
Large surface area
Ballistic conductivity and high electron mobility at room temperature		 Required aging and thermal process to increase the sensitivity
Additional purification process required
Lack of selectivity and relatively high cost		 [24,25]

14.3.1.1 Metal Oxide

The synthesis of nanometal oxides has become a critical point of research activities due to their highly conductive properties. The charge carrier on the surface of metal oxides is sensitive to the composition of surrounding atmospheres and, therefore, considerable research has been carried out on novel solid-state gas sensors based on semiconductor metal oxides [20,26]. The selection of metal oxides is based on their excellent characteristics of higher mobility, high conductivity, and transparent and low free carrier absorbance. Furthermore, in the case of metal oxide gas sensing materials, it is generally a surface controlled process that is responsible for sensitivity [21,27]. The limitations of these gas sensors include high cost, sensitivity in parts per billion level is rare, selectivity is poor, lifetime is limited, repeatability is poor, miniaturization is difficult, and power consumption is high.

14.3.1.1.1 ln2O3

Lanthanum oxide (ln2O3) has gained potential application in nanosensors due to its high sensing ability for different chemical compounds including H2S, NH3, CO, O3, and Cl2; formaldehyde, acetone, and toluene; and H2, CO, and NO2 [27–29]. On account of different surface areas and potential properties, the nanostructure morphology of ln2O3 plays a significant role in determining their electrochemical and optical properties. ln2O3-based sensors' performance toward toxic gases produced from petroleum industries has been much studied and received much attention due to its accurate and selective sensing at high temperature (>268 °C) among the normal gas molecules [28,30].

Different fabrication techniques will produce different morphologies and nanophysical structures, which consequently affect the final sensor prophesies of ln2O3 (Table 14.3). These synthesis techniques produced several morphologies of ln2O3, converted from indium hydroxide, including nanodots, nano- and microspheres, nanocubes, multipods, lotus roots-like nanocrystals, and one-dimensional nanocrystals [27,31]. However, in these studies on the sensing performance at room temperature, the responses of the ln2O3-based sensors to H2S could not reach a stable and maxima stage that is not restored to the baseline unless heated subsequently. Thus, the ultrahigh sensing of ln2O3 fabricated mainly originated from the ambient humidity induced by hydrolyzation of toxic gases [32]. Such hydroxylation will lead to desorption of the chemisorbed O2 and formation of H2O thin film on the surface of ln2O3 significantly increasing the conductance of the sensor and hence the sensitivity.

Table 14.3 Fabrication technique of ln2O3 and its variation on surface and morphological properties.

Fabrication technique Structure of ln2O3 Sensor properties References
Hydrothermal Nanoparticles Strong and selective sensing to H2S at 268 °C among the normal gas molecules [33]
Spray dry hydrolysis Nanostructured thin film Good sensitivity toward H2S at low temperature of 50 °C [34]
Chemical vapor deposition Nanowires High sensitivity to H2S with low detection limit of 1 ppm at room temperature [35]
Electrospinning Highly porous nanowires Good response to the dilute H2S at room temperature [36]
Chemical solution method Self-assembled nanoparticles High sensitivity to H2S down to 20 ppb in the low detection limit at room temperature [37,38]
Electrospinning and calcining technique Fabrication of NiO-ln2O3 nanofibers Excellent sensing properties at room temperature [39,40]
Solution dipping monolayer organic colloids templates Micro/nanostructured orderly porous ln2O3 films Possess ultrahigh sensitivity to H2S at room temperature and the ambient humidity [41]
Low-temperature wet chemical method Cracked cube and maize-corn nanostructured H2S detection at room temperature [42]

Investigations of gas sensor performance with ln2O3 morphologies concluded that pore size and specific surface area of a particular morphology can be correlated with sensing properties to establish basic morphology sensing relation.

14.3.1.1.2 TiO2

Titanium oxide (TiO2) has proven itself as one of the competitive and promising compounds in gas sensor applications due to its optical and electronic properties. The anatase phase of TiO2 is found to exhibit interesting stable and low-temperature sensing properties [43–45]. The relatively high mobility of anatase in comparison to rutile and relatively few trapping centers and compensating levels are considered as reasons for the sensing response [46]. The development of TiO2 thin-film sensors will contribute to the higher sensing properties related to the material surface [44]. Synthesis of TiO2 films has been reported by many researchers using different chemical methods including chemical deposition, electrodeposition, spray pyrolysis deposition, sol–gel, successive ionic layer adsorption, and reaction (Figure 14.3).

Figure depicting (a) SEM image of TiO2 nanowire gas sensor. (b) XRD patterns of TiO2 thin film with 450 °C annealing for 1 h and without the annealing process.

Figure 14.3 (a) SEM image of TiO2 nanowire gas sensor. (b) XRD patterns of TiO2 thin film with 450 °C annealing for 1 h and without the annealing process [44]. Source: http://www.mdpi.com/1424-8220/13/1/865. Licensed under CC BY 3.0.

The selection of synthesis methods is based on the simplicity of the apparatus and good productivity on a large scale (Table 14.4). Surface functionalization of TiO2 with gold nanoparticles has been successfully used as a sensor to identify hydrogen peroxide and glucose [47]. The sensing mechanism is generally based on either an influence of gold nanoparticles on electrical properties of TiO2 or a change in optical features of the localized surface plasmon resonance upon interaction with target analyst. A combination of TiO2 and doped alumina increases the sensing properties up to 400 ppm of liquefied petroleum gas at an operational temperature of 448 K.

Table 14.4 Synthesis methods of TiO2 and its effect on sensor properties.

Synthesis methods Features of TiO2 Sensor properties References
Anodization process TiO2 nanotubes H2 sensor at room temperature [48]
Spray pyrolysis Porous TiO2 UV sensor [49]
Sputtering and screen printing TiO2 thin and thick films NO2 and CO gas sensor [50]
Screen printing process Nanostructured Nb-doped TiO2 thick film CO gas sensor [51]
14.3.1.1.3 ZnO

Znic oxide (ZnO) is a typical n-type semiconductor with a direct wide bandgap of 3.37 eV and large excitation binding energy of 60 meV [52,53]. The use of ZnO-based gas sensor showed improved gas sensing properties including higher gas response to a variety of reducing or oxidizing gases, low cost, being environment-friendly, and low operating temperature (350 °C). The most attractive features of ZnO is its ability to make the crystal structure O2 deficient, giving it the chemistry possible to change the resistance based on adsorbed surface species (Figure 14.4) [54]. The sensor properties of ZnO can be further improved by addition of small amounts of noble metal catalyst such as palladium or platinum that not only promote gas response but also decrease the operating temperature up to 250 °C. Heterojunction of ZnO with p-type quaternary kesterite makes this sensor applicable in room temperature with 19.3% achievement in gas sensing less than 1200 ppm of liquefied petroleum gas. The compact morphology of p-type quaternary kesterite effectively reduces the surface area that severely hampers the gas response [55–57]. Meanwhile, their nanoheterojunction effectively improves the surface area and results in improved gas response.

A bar graphical representation where response (%) is plotted on the y-axis on a scale of 0–40 and different gases are represented on the x-axis.

Figure 14.4 Sensor properties of ZnO nanowires toward different types of toxic gases (reproduced with permission from Ref. [56]. Copyright 2012, Elsevier).

14.3.1.1.4 SnO2

Tin dioxide (SnO2) is an n-type semiconductor oxide widely used as a gas sensor due to its high level of sensitivity, simple design, low weight, and low cost. With a bandgap of 3.62 eV at 298 K, SnO2 has promising gas sensing properties for various types of toxic gases and organic vapor [58–61]. The sensing mechanism of SnO2 sensor is based on the fact that adsorbed oxygen (O2 or O) on the SnO2 surface interacts with target gas molecules and results in changing SnO2 conductivity [59]. The sensitivity and selectivity have been tuned by using different catalysts and promoters and by varying the operating temperature. The role of different additives including Al, Pt, Pd, Sb, Fe, Cu, Zn, and others on the gas sensing behavior of SnO2 has been studied to develop high-performance sensors [60–62]. Meanwhile, the conductivity and sensor of SnO2 could be improved by doping with other metal elements. Doping creates more of the desired O2 vacancies by substitution of the dopant on Sn lattice site. Doping increases the number of intermediate energy levels created in the forbidden energy region [60,63]. Among the doping elements that have been found to impart sensitivity and selectivity to SnO2 for gas sensing, CuO is found to have an outstanding promoter action. The p–n heterojunction with grain size of 20 nm from this combination has an outstanding sensitivity and selectivity to H2S. Indium and ruthenium dopants in SnO2 not only improve sensing properties toward NO2 but also lower the operating temperature [64]. The high sensitivity is also attributed to the tiny size of SnO2 due to its high surface to volume ratio, which leads to ample adsorption site in sensing process.

CuO has been applied as an outstanding promoter to increase the surface conductance to gas adsorption behavior of SnO2. Subsequently, various materials based on SnO2-CuO have been synthesized and extensively studied for sensing including CuO-SnO2 heterostructures, CuO-doped SnO2 nanorods, and CuO-doped SnO2 nanoribons [65,66]. Their gas sensing mechanism has been attributed to the formation of p–n junctions between n-type SnO2 and p-type CuO, which makes it applicable for high-performance gas sensor [65–68].

14.3.1.1.5 Fe3O4

Magnetic particles with smaller size become single domain in contrast with the usual multidomain structures for bulk magnetic material exhibiting superparamagnetization. Iron oxide (Fe3O4) exhibiting superparamagnetic behavior displays greater saturation magnetization and low coercivity having potential application as a sensor [69–71]. The magnetic properties and gas sensing efficiency of the material depends on its microstructural properties related to its method of synthesis including coprecipitation, sol–gel, standard double-sintering method, microwave-induced combustion, microwave sintering method, sol–gel autocombustion method, reverse micelle as reaction process, ceramic method, and oxalate method [70,72]. An advanced sputtering technique has been used to increase the purity and directly assemble onto the substrate with which no further experimental step is required to control the size and yield of nanoferrite.

Ferrite alloys including magnesium ferrite, zinc ferrite, copper ferrite, and nickel ferrite has been used to increase its variations in electrical resistance that in turn improves the selectivity and sensitive toward a variety of toxic gases such as CH4, H2S, and C2H5OH [73,74]. Addition of zinc element to ferrite in nanorod composites resulted in highly selective and sensitive sensors toward liquefied petroleum gas than CO2 gas [75]. This also contributed to the mesoporous structure of the composites in enhancing the sorption capability of the sensor.

14.3.1.1.6 CdO

Cadmium oxide (CdO) has been synthesized by physical and chemical methods including sputter deposition, metal organic chemical vapor deposition, chemical bath deposition, and pulsed laser deposition [76–78]. These techniques apply a small degree of supersaturation of the solution (such as cadmium hydroxide) to cause the heterogeneous nucleation of the metal oxide on the substrate. The solubility of the solutes changes as a result of chemical reaction in the solution. Once the solution reaches supersaturation points, the solid particles are formed through the nucleation and crystal growth process [77,79] (Figure 14.5).

Figure 14.5 (a) Typical N2 adsorption–desorption isotherm and (b) pore-size distribution curve of the 3D hierarchical CdO nanostructures (reproduced with permission from Ref. [77]. Elsevier).

14.3.1.1.7 WO3

Tungsten oxide (WO3) as an important n-type semiconductor has attracted considerable interest due to its promising physical and chemical properties [80]. Taking into account its small bandgap (2.59 eV) and stable physicochemical properties, nanostructured WO3 is generally considered a feasible candidate for chemical and gas sensors [81]. A deeper understanding of the effect of temperature on Raman spectral characteristics of a metal oxide such as WO3 has helped to extend our knowledge regarding the behavior of metal oxide–gas interactions for sensing applications [80,82,83] (Figure 14.6).

Figure 14.6 Determination of the existence of a correlation between the thermal effects and the changes in Raman spectra for multiple WO3 structures. We have obtained results utilizing Raman spectroscopy for three different structures of WO3 (monoclinic WO3 on Si substrate, nano powder, and nanowires) that were subjected to temperatures in the range of 30–160 °C (reproduced with permission from Ref. [80]. Copyright 2013, American Chemical Society).

14.3.1.1.8 Co3O4

Spinel cobalt (II) oxide (Co3O4) is rich in O2 content and thus exhibits p-type semiconducting properties [84,85]. Its gas sensing properties have been explored in which sensing is usually based on catalytic properties of the surface of the oxide and the sensors are usually operated at elevated temperatures above 200 °C [85,86].

14.3.1.2 Glassy Carbon Electrode

Different nanocarbon materials including carbon nanotubes, charcoal, graphene, and graphene oxide have been demonstrated to be useful as gas sensors due to ease in tailoring their sensitivity by simple chemical treatments. In CNTs, the point defect plays a dominant role as a sensing active site. Line defects and edges seem to be the main contribution to the sensing properties of graphene. Adsorption from the gaseous phase resulted in ion exchanges between the adsorbate layer and the materials, meaning a variation of free electron concentration.

14.3.1.2.1 CNT Electrode

Sensing materials based on CNT network have shown high resistance, fast response time, rapid recovery, and good reproducibility [87–89]. CNTs have shown great potential for detection of volatile organic compounds and nonpolar molecules. The sensitivity of CNT sensors is contributed by their Π-Π stacking and can reach 1 ppm in case of dimethyl methylphosphate [88,90]. Besides, it has been demonstrated that cross-reactive arrays of synthetically designed polycyclic aromatic hydrocarbon and single-walled CNT bilayers can discriminate between polar and nonpolar volatile organic compounds as well as between the different volatile organic compounds from each subgroup [91]. The incorporation of gold nanoparticles onto the surface of CNTs has reported high sensitivity and low response/recovery time for detection of methane at room temperature.

14.3.1.2.2 Graphene/graphene Oxide Electrode

In recent years, graphene and its derivatives such as pristine graphene, graphene oxide, and reduced graphene oxide have been reported to show sensing applications due to their exceptional properties including low cost, high surface to volume ratio, ease of processing, high mechanical strength, good thermal stability, ballistic conductivity, high carrier mobility at room temperature, low electrical noise due to unique two-dimensional honeycomb lattice, and large surface area (92 630 m2 g−1) [92–95]. Graphene's two-dimensional structure can screen charge fluctuations better than one-dimensional materials. The planar structure of graphene eases Hall pattern fabrication and four probe measurements, limiting the contact resistance impact and helping to focus only on the active area compared to its one-dimensional counterpart [93]. With that, micrometer-size sensors made of graphene are capable of detecting individual gas molecules that attach to or detach from graphene surfaces. It demonstrated that the adsorbed molecules change with the local carrier concentration in grapheme, one electron by one electron, which leads to step-like changes in resistance [94,96]. Graphene nanoplanar could be synthesized by different methods including micromechanical (Scotch tape), chemical vapor deposition, epitaxial growth on epitaxial matched, chemical exfoliation, and thermal exfoliation unzipping CNTs (Table 14.5).

Table 14.5 Series on graphene synthesis method.

Synthesis method Parameters References
Mechanical cleaving of graphite A repeated stripping of graphite with adhesive tape is involved to obtain single layer
These isolated layers can be resolved using optical microscope, atomic force microscopy, or Raman spectroscopy		 [98,99]
Chemical exfoliation of graphite A strong acid solution has been employed to introduce O2 into graphene interlayers and form graphene oxide
Graphene oxide can be easily separated into single layers after dispersion into aqueous solution
Heating of hexagonal silicon carbide crystal to 1200 °C to produce epitaxial graphene		 [100,101]
Chemical vapor deposition Allow graphene growth on metal substrate like copper and nickel from hydrocarbon gases at 700–1000 °C [102,103]
Combination of chemical treatment and thermal exfoliation Synthesis process above 800 °C by incorporation of nickel nanoparticles that substantially eliminates the O2-containing impurities [104,105]

The gas-induced changes in resistivity had different magnitudes corresponding to different types of gases and the sign of the change indicated whether the gas was an electron acceptor like NO2, H2O, and iodine or an electron donor including NH3, CO, and ethanol (Figure 14.7). The interaction between graphene sheets and adsorbents could vary from weak van de Waals to strong covalent bonding. The above-mentioned interactions tune the electronic structure of graphene, which can be readily monitored by convenient electronic methods [106–108]. The level of interactions between target gas and vapor molecules may reach the lower limit of even single molecules. With that high sensitivity of sensor could be obtained even at low gas concentrations.

Figure 14.7 Transient gas-sensing characteristics of graphene device. (a) Responses to NO2 gas from 1.2 to 5 ppm. (b) Responses to NH3 gas from 5 to 100 ppm. All gas-sensing tests were performed at the operating temperature of 150 °C (reproduced with permission from Ref. [97]. Copyright 2014, Royal Society of Chemistry).

However, there are some limitations regarding the graphene sensor including not producible in large scale, do not have functional groups required for adsorption of gas and vapor and have no bandgap. Therefore, researchers are looking for the surface modification of graphene by doping, hybridization, functionalization, nanomesh formation, FET modulation, and others [109–111]. The reduced graphene oxide is produced by reduction that contains many functional groups and defects (Table 14.6). The decoration of graphene sheets with n-type metal oxides can lead to the formation of n–p junction and the resulting novel nanostructure may exhibit performance far better than that of individual materials. The n–p junction also resulted in a depletion zone by introducing more electrons to be attached from graphene oxide and shifting the Fermi level of graphene oxide toward the valence band. This consequently improves the conductivity of graphene oxide. The metal oxide-loaded graphene hybrid architecture has produced sensors that are highly sensitive, selective, and cost effective applicable to be used at room temperature. In general, graphene sensors combined with SnO2, ZnO2, WO3, Cu2O, Co3O4, ln2O3, and NiO. Nanocrystal decoration on the graphene oxide surface increases the overall active sensing surface area and subsequently increases the gas molecule adsorption during sensing properties. The attachment of nanocrystal onto surface of graphene/graphene oxide leads to more active sites including vacancies, defects, O2 functional groups, and sp2-bonded carbon for adsorption of gas molecules. Meanwhile, there are some report claims that nanocrystal served both as a nanospacer between dried graphene sheets and as a primary sensing transducer for gas sensing applications. This provides a net gas sensing mechanism and not the individual components, and hence the improvement of room temperature response with good selectivity to electron donor gases such as CO and NH3. Indeed, some of the nanocrystals such as Co3O4 behave like the nanopillars resulting in extra macroporous structures between graphene oxide layers that further improve the gas diffusion to the surface of graphene oxide driven by the capillary force leading to the improvement in gas responses. The improvement in O2 reduction ability could be achieved by a strong coupling between O2 ions in the graphene oxide and nanocrystals making them ionic in nature. This consequently serves as extra adsorption sites for NO2 gases and electrons would be extracted indirectly from p-type of graphene oxide through the bridging of O2, leading to an extra decrease in resistance in the presence of NO2.

Table 14.6 Modification of graphene for improving sensing properties.

Modification technique Sensing properties References
Introduction of positive gate potential (n-type conductance) Instantaneous response and fast recovery for NH3 sensing
Far superior to the performance in p-mode at zero or negative gate potential		 [112]
Hybrid of graphene oxide, tin oxide, and platinum Room temperature H2 sensor
Fast response and recovery time		 [113,114]
Nanomesh patterned from a CVD-grown large area graphene Exhibit sensitivity at about of about 4.32% per ppm in NO2 and 0.71% per ppm in NH3 with a detection limit of 15 and 160 ppb [115,116]
Lateral confinement of graphene into sub-10 nm wide nanoribbon Induce a bandgap by the quantum confinement and edge effect [117,118]
Nanocrystal SnO2-graphene-gold interdigital electrodes Excellent response to target gases at room temperature with detection limit of 1 ppm for NO2
However, weakened the signal for NH3
Sensor sensitivity to NO2 increased from 2.16 to 2.87
Sensor sensitivity to NH3 decreased from 1.46 to 1.1.2		 [119,120]
Alumina substrates supplied with comb-like Pt on surface of graphene oxide Good sensing characteristics to NO2 at low temperature (50 °C) [121,122]
SnO2-graphene on ceramic plate coated with gold electrodes and ruthenium oxides as heater The target NO2 gases directly adsorbed on the surface of SnO2 and modify the depth of the first depletion layer, which in turn alters the depletion at SnO2-graphene layers [123,124]
Indium-doped SnO2-graphene nanohydrid NO2 sensing properties at room temperature with detection limits as low as 0.3 ppm.
Excellent selectivity to several gases
Recovery process slow and takes overnight to completely recover to the initial state		 [125]
SnO2-graphene gold Good gas-sensing properties toward NH3 of 10–50 ppm at room temperature, high response magnitude, fast response/recovery, and good reversibility and repeatability [126]
ZnO-graphene oxide Detection of common industrial toxins such as CO, NH3, and NO for concentrations as low as 1 ppm at room temperature
Shorter response and recovery time compared to neat graphene oxide for NO2 detection		 [127,128]
ZnO-graphene oxide-supported ceramic substrate Response and recovery of sensing to 5–25 ppm NO2, respectively [129]
WO3-graphene oxide 2.5 times higher NO2 sensing response than pure WO3 film [130,131]
Pd-doped WO3-graphene oxide Good sensitivity to NO2 with recovery time less than 1 min [132]
CuO2 nanowire mesocrystal on graphene oxide sheet Response was 67.8% for 2 ppm NO2, much higher than graphene oxide (22.5%) at room temperature [133,134]
CuO-functionalized graphene oxide H2S gas detection at room temperature with sensitivity of 11% even at the exposed concentration of 5 ppb [135]
Intercalation of Co3O4-graphene oxide nanocrystal Co3O4 effectively attaches to the surface of graphene oxide to result in longer response than pure graphene oxide
Co3O4 acts as nanopillar to form extra macroporous structures between graphene oxide layers leading to enhanced responses		 [136]
Ir functionalized Co3O4 nanofibers/graphene oxide sheet Highly sensitive and selective acetone sensor [137]

14.3.2 Properties of Nanosensor Technology

The sensor performance strongly depends on the microstructural features including crystallite size and size grain boundaries' characteristics. Sensor properties can be controlled by controlling the grain size and surface morphology. The effect of the different surface treatments on active sensor surface causes the change in morphological properties of materials and hence on the sensor performance. Mesoporosity plays a crucial role in improving both the rate of response and sorption capability of nano sensors.

14.4 Conclusions

In recent year, gas detection and monitoring in petroleum industries has created an increasing concern regarding the awareness of environmental protection and human health. Toxic gases including CO2, NO2, and H2S are flammable and hazardous to both humans and environment. During the petroleum synthesis process, combustion accidents might be caused due to leakage or by mistakes. Thus, the requirement for reliable and sensitive gas detecting instruments for safety has increased. Therefore, nanosensor has become the subject of intense research due to its quick response and recovery periods and extremely small detection level capacity and it is reliable, efficient, simple, and cost-effective for good sensitivity. Many effort have being devoted for synthesizing novel sensing nanomaterials of various conducting nanomaterials including metal/metal oxide, ionic liquids, and nanocarbon for improving the gas detection level. Metal oxide nanomaterials have been fabricated because of their abundant nature, low synthesis cost, and availability in various appearances. Ion conductive polymer metal composites have been envisaged in nanosensors for petroleum industries due to their remarkable electronic and optical properties, high electrical conductivity, highly sensitive to external environment, wide bandgap energy, and high free carrier mobility. However, it has been noted here that these achievements are still in the stage of laboratory investigation and much research is needed to take these developments to an industrial scale. For real-life applications, it is noted that the performance of these sensors may strongly depend on the relative humidity of the environment.

Acknowledgment

The authors would like to thank Grand Challenge (GC001B-14SBS) of University Malaya, Malaysia, for their cordial support in completing this work.

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