Ecology, Pollution and Environmental science: Open Access (EEO)

Greenhouse Gas Sensors Fabricated with New Materials for Climatic Usage: A Review



KSV Santhanam1*, NNN Ahamed1


1 School of Chemistry and Materials Science, Rochester Institute of Technology, Rochester, NY 14623, USA.


*Corresponding Author: KSV Santhanam, School of Chemistry and Materials Science, Rochester Institute of Technology, Rochester, NY 14623, USA, TEL: 5854752920 ; FAX: 5854752920;E-mail:ksssch@rit.edu


Citation: KSV Santhanam, NNN Ahamed (2018) Greenhouse Gas Sensors Fabricated with New Materials for Climatic Usage: A Review. SciEnvironm 1:106.


Copyright:© 2018 KSV Santhanam, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited


Received date: May 14, 2018; Accepted date: May 29, 2018; Published date: June 01, 2018.


Abstract

With increasing utilization of fossil fuels in today’s technological world, the atmosphere is having increased concentrations of greenhouse gases that need to be controlled in it. For achieving this goal, it is imperative to have sensors that could provide the data on greenhouse gases in the environment. The recent literature contains a few publications using new methods and materials for sensing these gases. The first part of this review is focused on the possible effects of greenhouse gases in the atmosphere and the second part surveys the developments of sensors for greenhouse gases with a coverage on carbon nano-materials and the composites directed towards sensing gases like CO2, CH4 and NOx. With carbon dioxide measurements, due consideration for the dissolved carbon dioxide gas in water (moisture) is focused. The density functional calculations projects Pd doped single walled carbon nanotubes as ideal for the development of NOx sensor. The current trend is to make sensors through 3D printing or inkjet printing to allow the reach of ppb levels of sensitivity that has not been realized before.


Keywords

: Greenhouse gases, carbon dioxide, methane, nitrogen oxides, fluorocarbons, optical methods, wireless sensors, resistive sensors, conducting polymers


Introduction

The entire world is eagerly looking forward to the atmosphere that provides clean air that is having reduced levels of carbon dioxide, nitrous oxide, methane, ozone and fluorocarbons that contribute to global disturbances. These gases are called the greenhouse effect gases. The atmosphere today is facing two situations that contribute to the increased greenhouse effect gases. The first one is rapid deforestation caused by forest fires and land requirement for developmental purposes. The other is the use of fossil fuels for variety of purposes. The fossil fuels generate gases that contribute to the greenhouse gas effect. The real concern is in the climatic status of the world for the generations ahead. The United Nations (UN) has proclaimed several steps in mitigating greenhouse gas emissions; by this effort it has reduced burning of 64,000 kilotons of wood and reduced the release of 118,000 tons of CO2 into the environment(http://www.undp.org/content/undp/en/home/ourwork/climate-and-disaster-resilience/climate-change/mitigating-greenhouse-gas-emissions.html). By encouraging the use of renewable sources of energy several wind farms and solar panels have been developed to reduce the greenhouse effect gases.


Among the greenhouse gases involved in environmental disturbances, carbon dioxide, nitrogen oxides and methane are given utmost importance as the concentrations of these gases are changing more than the others requiring immediate attention. Hence this review is devoted to these gases.


Status of Greenhouse gases


The concentration levels of greenhouse effect gases in the environment have been periodically monitored by several agencies and in this section, we review the concentrations of these gases in the environment for the purpose of developing sensors.


Status of carbon dioxide gas


The carbon dioxide level in the atmosphere is currently reported to be at about 400 ppm compared to a low value of about 200 ppm in 800,000 BC as shown in Figure 1. It reveals a rapid growth of carbon dioxide level in the atmosphere. This increase is attributed to the fuel combustion, forest fires, volcanic eruption and volatile organic compounds. An obvious choice to overcome this problem is to reduce the of use fossil fuels, nonpolluting fuels, forest conservation and stopping volcanoes [1,2]. The effectiveness of the measures taken requires sensors for monitoring the carbon dioxide levels in the atmosphere. This has resulted in the evolution of gravimetric sensors; they fall in the category of micro to nano electromechanical systems and are based on the properties such as chemical, electrical conductivity, optical, magnetic, acoustic, capacitive or ultrasonics. The active material response to the exposed gas is revealed by its physico-chemical interactions.


Figure 1
Figure 1: Global atmospheric concentrations of carbon dioxide over time. Taken From: https://www.epa.gov/climate-indicators/climate-change-indicators-atmospheric-concentrations-greenhouse-gases

Status of methane


Methane is another important gas contributing to greenhouse gas effect. Its increase is greatly changing the climatic condition. It absorbs the sun’s heat and warms up the atmosphere. It is 84 times more potent than carbon dioxide. It is produced by natural decomposition of rice paddies, marshes, guts of animals, rotting of rubbish and distribution of fossil fuels like coal, oil or gas. The use of natural gas for a variety of living conditions, if it can be replaced by other sources such as hydrogen [3], can certainly reduce the contribution of this gas towards greenhouse gas. Methane gas in the atmosphere has not been a serious concern as it was increasing by 0.5 parts per billion per year until year 2000. The current statistics shows that it has gone up by 12.5 parts per billion in 2014. Methane mole fraction (ppb) is expected to reach about 1850 by 2018 which was about 1780 in the year 2000 (https://www.washingtonpost.com/news/energyenvironment/wp/2016/12/11/atmospheric-levels-of-methane-a-powerful-greenhouse-gas-are-spiking-scientistsreport/?utm_term=.e490502182e6). A recently published review [4] demonstrated that a sensor developed for carbon dioxide should consider the response of it for methane in the atmosphere. Figure 2 shows the simulated effect of methane interference in the measurement of carbon dioxide gas. The error in measurement of carbon dioxide concentration has been indicated in the Fig. for different concentration ratios of carbon dioxide to methane.


Figure 2
Figure 2: Carbon dioxide sensor responses to different concentrations of methane. Y=Sensor response; Z= Concentration. Group 1 a: CO2 concentration, 390 ppm, b. Ratio of carbon dioxide/methane, c: methane concentration 1.8 ppm; Group 2 a: CO2 concentration, 390 ppm, b.

Ratio of carbon dioxide/methane, c: methane concentration 4.25 ppm; Group 3 a: CO2 concentration, 390 ppm, b. Ratio of carbon dioxide/methane, c: methane concentration 18 ppm; Group 4 a: CO2 concentration, 390 ppm, b. Ratio of carbon dioxide/methane, c: methane concentration 40 ppm; Group 5 a: CO2 concentration, 390 ppm, b. Ratio of carbon dioxide/methane, c: methane concentration 80 ppm; Group 6 a: CO2concentration, 390 ppm, b. Ratio of carbon dioxide/methane, c: methane concentration 100 ppm. Reproduced with permission from reference [4].


The effect of methane and carbon dioxide gases on the environment was recently analyzed by Charnay et al [4a] using 3D climate carbon model and it showed that the global albedo to change from 0.40 to 0.23 depending on the relative proportions of carbon dioxide and methane levels. The global temperature has been estimated by using the albedo value to increase from -11.5oC to 65oC. This model predicts that carbon dioxide level of 1 bar could produce hot climates at a low land fraction and cloud feed back. For the earth to reach the high temperature requires carbon dioxide in the atmosphere to be 1 Bar at 3.8 Ga.


Status of NOx


The nitrogen oxides have pronounced influence in the environment; they are responsible for producing yellow-brown smog. It is often called photochemical smog. The sources for nitrogen oxides are automobile exhaust and fuel burning (including bio-diesel). It is to be controlled at 0.03 ppm over a one-year period for the human health and environmental factors. It is currently estimated at 325 ppb in the environment (https://www.epa.gov/climate-indicators/climate-change-indicators-atmospheric-concentrations-greenhouse-gases).


Status of Fluorocarbons


The fluorocarbons are innumerable and they could be divided into two classes; one class acting on the ozone layer that reduces the ultraviolet rays reaching the earth and another class having environmental effects. The first-class compounds are methyl chloroform, halon-1211, CFC-12, HCFC-22 and HCFC-141b. In the other classes sulfur hexafluoride, HFC-23, HFC-134a, PFC-14, PFC-16 and nitrogen trifluoride. An examination of the ozone depleting fluorocarbons, except for methyl chloroform, the other ones are either increasing or reaching a constant value. They range from about 5 to 400 parts per thousand (ppt) in the atmosphere (https://www.epa.gov/climate-indicators/climate-change-indicators-atmospheric-concentrations-greenhouse-gases). Since fluorocarbons are not in the concentration levels of CO2 or CH4, it will not be considered here.


Sensors for Greenhouse gases


Since the time of realization of the importance of greenhouse gases influencing the environment, several different sensors have been developed and are well-reviewed in the literature [5-9]. Figure 3 describes the six different types of sensors used in the measurement of greenhouse gases.


Figure 3
Figure 3: Types of sensors developed for greenhouse gas measurements. M=Digital multimeter, C=Computer having software for monitoring the measuring multimeter. P=Potentiostat. Field effect transistor parameters are monitored by the potentiostat. D=detector such as photomultiplier or Bolometer. GG=Greenhouse gas. IDT= Inter digitized electrodes.

Type 1 sensor generally has a nonmetallic substrate on which the active material is deposited and kept in either ambient or selected experimental conditions such as inert atmosphere in the measuring chamber. The change in resistance of the active material is measured as a function of concentration of the greenhouse gas. The successive measurement requires flushing the measuring chamber. Type 2 sensor is a field effect transistor having active material placed between the source and drain. The source, drain and gate electrodes are connected to a potentisotat. The drain current is measured as a function of greenhouse gas concentration. Type 3 sensor is a modification of type 2 sensor in that the voltage is plotted as a function of concentration of greenhouse gas. Type 4 sensor depicts the general scheme for electromagnetic interaction with the greenhouse gas. The photons used in these measurements are generally in the energy range of 104-10-2 kcal. Type 5 sensor utilizes a piezo electric crystal carrying interdigitized electrodes. The active material is placed between digitized electrodes. The radiofrequency(Rf) shift is measured with different concentrations of greenhouse gas. This type of sensor is also called a SAW (surface acoustic wave) sensor. Type 6 sensor is a micromechanical sensor using a cantilever beam which has a coating of an active material. The resonant frequency of the cantilever beam is measured as a function of the greenhouse gas. This review contains examples of all the six different types of sensors.


The conventional instrumental analysis such as optical spectroscopy, Fourier transform infrared spectroscopy (FTIR), semiconducting devices, mass spectroscopy and Raman spectroscopy have been used for analyzing greenhouse gases. These techniques will be continued to be used as they provide the advantage of selectivity and sensitivity. With the discovery of new nanomaterials, several less expensive methods such as using resistance measurement (Type 1 sensor), field effect transistor (Types 2 and 3 sensors), optoacoustic (Type 4 sensor), wireless measurements (Type 5 sensor) (surface acoustic wave (SAW) and micromechanical measurements (Type 6) have been developed in recent times. These new methods or new material-oriented research have provided the advantage of speed and accuracy in the measurement of greenhouse gases.


Carbon dioxide sensors

Carbon dioxide gas is safe for humans up to 5000 ppm and dangerous when it reaches a level of 40,000 ppm. A short-term exposure of 30,000 ppm is bearable (http://www.cdc.gov/niosh/idlh/intridl4.html) [9,10]. In the atmosphere, the carbon dioxide level is changing year by year. The present level of carbon dioxide in the atmosphere is over 400 ppm. The presence of it in the atmosphere affects the Albedo value of the earth in reflecting the solar radiation and is currently estimated at 0.39. While the planet Mercury has an Albedo value of 0.1 (receives maximum amount of sunlight), the planet Venus has a value 0.84 [3]. The temperature of the Venus is estimated at 462oC with carbon dioxide level in the atmosphere amounting to 96.5%. Based on these facts, the need for limiting carbon dioxide in the atmosphere of the earth is of utmost importance. Hence a good reliable sensor for carbon dioxide is required to monitor the atmospheric carbon dioxide. A variety of sensors have been developed based on optical absorption, change in semi conducting property, electrical resistance, amperometry and field effect transistor. The semiconducting materials used in these sensors operate at high temperatures (>200oC); the optical detectors use either fiber optics methods with sophisticated instrumentation or conventional infrared detectors. The electrical methods have been successfully used in biomedical applications in the detection of carbon dioxide.


The main thrust in this decade has been to find new materials that would enable easy detection and determination. The gravimetric sensors have been of great interest in this category. The infra-red sensors are developed using a combination of a wavelength filter and detector. This technology can reach a detection limit of 10 ppm of CO2. The upper limit of this sensor is 10,000 ppm [11,12]. Mayrwögera et al. [11] proposed a Fabry–Perot based bolometer using a glass plate as simple infrared filter for carbon dioxide determination (Type 4 sensor). Figure 4 shows the analyte that is mixed with nitrogen for analysis.


Figure 4
Figure 4: The dielectric layer used for the measurement and the methodology. Ge is used as dielectric layer. Taken from: Reproduced with permission from reference [11].

The interference coming from water vapor in the measurement of carbon dioxide concentration was removed by using glass filter as shown in Figure 5.


Figure 5
Figure 5: Carbon dioxide determination using glass filters. Reproduced with permission from reference [11].

Several conducting polymer based resistive sensors have been developed for the detection of carbon dioxide. The role of doping polyaniline (PANI) has been shown to be playing a role in the detection of CO2 [12,13]. The working range of the sensor for CO2 has been reported as 102 to 104 ppm. Figure 6 shows the conductivity change with the concentration of CO2; the conductivity decreases when carbon dioxide is absorbed on the sensor material (PANI) (Type 1 sensor). The sensor performance has been shown to depend on whether it is having emeraldine base or sulfonated polymer as active material. Both of them respond to carbon dioxide; the emeraldine base response to lower ppm levels is reported to be negligible.


Figure 6
Figure 6: Conductivity of sensor with concentration of carbon dioxide. Reproduced with permission from reference [13a]. EB-PANI:Emaraldine base PANI. NaSPANI:Sodium sulfonated PANI.

The carbon dioxide concentrations in the atmosphere and the corresponding pH are controlled by the humidity in the atmosphere. The dissolved carbon dioxide is in equilibrium with other species [13] and Table 1 shows the calculated concentrations of the equilibrating species.


Table 1: The CO2 concentrations in the atmosphere, pH value and equilibrating species concentrations.


CO2 (gas)(ppm) pH Concentration [CO2] Concentration [H2CO3] Concentration [HCO3] Concentration [CO32-]
1.0 x 10-1 6.94 3.36 nM 5.71 pM 5.90 nM 1.90 pM
1.0 x100 6.81 33.6 nM 57.1 pM 91.6 nM 33.0 pM
1.0 x101 6.42 0.33 μM 5.71 nM 0.378 μM 55.7 pM
1.0 x102 5.92 3.36 μM 5.71 nM 0.378 μM 56.0 pM
1.0 x103 5.42 33.6 μM 57.1 nM 3.78 μM 56.1 pM
1.0 x104 4.92 0.336 mM 0.571 μM 0.119 μM 56.1 pM
1.0 x105 4.42 3.36 mM 5.71 μM 0.378μM 56.1 pM
1.0 x106 3.92 33.6 mM 57.1 M 12.0 mM 56.1 pM

The true concentration of CO2 in the atmosphere can be evaluated by taking the concentrations in Table 1 into consideration as there is equilibrium between CO2 and the protonated species as


CO2 + H2O = H2CO3 ->HCO3- + H+ (1)


An infrared fiber optic optical spark plug sensor has been used for measuring CO2 and water. As both the molecules are infrared active with strong overlap, the spark plug was kept close to the electrodes for the in-cylinder measurement of CO2 and gaseous water (Type 4 sensor). A tungsten halide lamp with two infra-red detectors having different optical band pass filter was used. The test was carried out using spark ignited engine [14]. Air quality monitoring sensors using a cluster of metal oxide or electrochemical sensors have been used for analysis of a mixture of nitrogen monoxide and carbon dioxide. The measurements were used to assess the practicality of carbon dioxide sensor for Data Quality objective with 25% uncertainty [15]. Table 2 shows the number of sensors and the manufacturers used for data acquisition. Figure 7 shows the carbon dioxide levels obtained from the sensor with linear regression analysis [NDIR=Non-dispersive infrared technology].


Table 2: Commercialized carbon dioxide sensors for field experiments.


Marketed By Model Concentration range Sensitivity Response time Resolution Comments
ELT Sensors S-100H 0-5 mmol/mol 1V/mmol/mol 60 s 70 nmol/mol Uses 16 bits ADC with DAQ range of 5V
Edinburgh sensors Gas card NG 0-1mmol/mol 1V/0.1mmol/mol 10s 60 nmol/mol Uses 14 bits ADC with DAQ range of 10 V
Figure 7
Figure 7: Red data measured. Green: simulated. Reproduced with permission from reference [15].

1.Resistive sensor. (Type 1 sensor) Rs resistance of the sensor in ambient air and Ro is the resistance when not exposed to air. ADC:anolog to digital converter and DAQ:data acquisition.


It is shown that the predicted values are lower compared to calibration R2=0.71 as against 0.93. When an electrochemical sensor is used for the measurement of CO2, the interference of ozone has been encountered and has been removed by linear/multi linear regression [15].


Carbon nanotube (CNT) based sensors have been developed for carbon dioxide gas utilizing the principle of physico-chemical adsorption of the gas by the carbon nanotubes. Two types of sensors have been developed based on this principle. One type is based on resistance change upon adsorption of the gas on the active material and the other dependent on the adsorption of the gas on active material causing the effects on transistor properties such as the voltage or current in the Field effect transistor [9] (Types 2 and 3 sensors). The adsorption of carbon dioxide gas produces an increase in resistance that is attributed to increase in the energy barrier for electron movement as shown in Figure 8.


Figure 8
Figure 8: Sensing mechanism for carbon dioxide molecule. Reproduced with permission from reference [9].

A number of other reports on measuring CO2 using chemo resistive method with CNT have been reported in the literature [9,11]. Trans et al. [17] have fabricated a field effect transistor using carbon nanotubes (NTFET) whose sensitivity was examined by Star et al. [18-20]. A prototype sensor chip was packaged for measuring the carbon dioxide level by using a computer. Figure 9 shows the response of the sensor for carbon dioxide in breath analysis. The reproducibility in the response to successive injections of the gas is established by these measurements. The NTFET board containing the field effect transistor is shown in Figure 10. The reproducibility of the pulses shows the feasibility of its usage for successive measurements.


Figure 9
Figure 9: NTFET sensor fabricated for Carbon dioxide. Reproduced with permission from reference [19].
Figure 10
Figure 10: A. Functional Test Board (FTB) loaded with eight packaged die containing functionalized NTFET devices. B. Mockup capnography device. Reproduced with permission from reference [19].

The sensor monitors the %CO2 that can be detected by the NTFET which can be used conveniently in several locations such as hospitals and paramedics. Being a noninvasive and disposable device, it can be used in monitoring greenhouse gas for about 6 hours. The performance of the NTFET in measuring the concentration of CO2 in the presence of moisture has also been carried out; it has tolerance up to 80% RH (relative humidity). The sensors made with CNT for gases generally have the following advantages a) room temperature operation b) facile property adjustment c) high sensitivity and response time d) easy device fabrication e) low selectivity and f) long time instability [8,21-26].


The low selectivity and longtime instability has been active area of investigations in the last decade and several new composites of carbon nanotubes have been recommended for gas detection. Chen et al. [13] have reported that by cleaning the carbon nanotubes with ultraviolet light resulted in a dramatically enhanced performance. The mechanism by which ultra violet light enhances the performance has not been explored in this work. Presumably it is making the surface active by oxidation of amorphous carbon or defect centers in the nanotubes. A quantum mechanical investigation of carbon dioxide adsorption on S functionalized boron nitride and aluminum nitride (AlN) nanotubes has shown it to be an exothermic, opening the prospects of developing thermal sensor for carbon dioxide [27]. The adsorption of carbon dioxide on AlN nanocage and nanosheet has also been investigated using density functional calculations [28] with emphasis on chirality. An amine functionalized TiO2 has been used to capture the carbon dioxide from flue gases that is amenable for developing a gas sensor. The presence of moisture has been demonstrated to have significant effect on adsorption/desorption processes [29].


A nanothin film functionalized chemiresistor sensor operating at room temperature (Type 1 sensor) for the detection of carbon dioxide in the range of 50-500 ppm has been reported [30]. The sensor has negligible interference from ammonia, carbon monoxide, methane and nitrogen dioxide. The sensing of carbon dioxide using wireless network [9,31-34] (Type 5 sensor) uses the analyte induced changes such as in mass or elasticity or complex permittivity [35]. The diverse temperature requirements in this class of carbon network are shown in Figure 11.


Figure 11
Figure 11: The temperature of operation of sensors in relative scale. Taken from: Reproduced with permission from reference [35].

A SAW (surface acoustic wave) sensor using random copolymer Teflon AF2400 (Figure 12) prepared from tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro- 1,3-dioxole shows variations in carbon dioxide detection depending on the amount of water present. The phase shift changes with carbon dioxide level are shown in Figure 13; it shows a phase shift of about 1.140/ppm of CO2.


Figure 12
Figure 12: The structure of Teflon AF2400.
Figure 13
Figure 13: Phase shift observed as a function of carbon dioxide concentration at different water levels (RH=relative humidity). Reproduced with permission from reference [35].

The permittivity and conductivity of multiwalled carbon nanotubes has been used for the detection of carbon dioxide [35]; incorporation of SiO2 matrix along with the incorporation of wireless inductor-capacitor resonator in the sensor showed a decrease in effective permittivity as shown in Figure 14. The analyte induced changes in complex permittivity (r’-j r’’) where r’ and r’’ are the real and imaginary parts of the complex permittivity are measured in this approach. The imaginary permittivity of the sensing material is directly proportional to its conductivity. A hysteresis free operation of the sensor is remarkable and advantageous for fast measurements.


Figure 14
Figure 14: Carbon dioxide sensor using wireless transducer constructed with MWCNT-SiO2 film. Reproduced with permission from reference [8].

The sensor operates through dipole-dipole interaction mechanism as shown below. The functionalized MWCNT has partial negative charge to which the


Figure 14

analyte is bridged through its partial positive charge end as shown in the above illustration [8]. Optical sensing of carbon dioxide has been reported using different types of membranes [25]. As an offshoot of carbon nanotubes, the discovery of graphene by Novoselov and Geim in 2004 has opened up new carbon-based sensors [26]. In one approach, 3 μL of 1 mg/mL of graphene oxide solution was spin coated on silicon fingers 3 μm width. The graphene resistive sensor response in the concentration range of 0-1500 ppm is measured [36,37]. Figure 15 provides the resistance response with carbon dioxide concentration at the ppm levels.


Figure 15
Figure 15: Graphene resistive sensor. Reproduced with permission from reference [36].

The initial resistance measurement of the sensor was carried out in nitrogen atmosphere (RN2). The change in resistance [33] of the sensor when carbon dioxide gas was injected (Rx) is used in the construction of the graph in Figure 15. Recently a miniature resistive carbon dioxide sensor has been reported that operates in the concentration range of 50-50000 ppm. A photoacoustic spectroscopic method has been developed for the simultaneous determination of carbon dioxide and methane gases [38] with high precision and a large dynamic range.


A new silicon substrate micro sensor has been developed [4] using a composite made of carbon nanotube and Baytron-P that senses the greenhouse gas carbon dioxide at 22oC. The sensor was constructed with a Si chip by depositing the composite between two gold electrodes. Two identical Si chips were connected in a parallel configuration (Figure 16) to reduce the initial resistance of the sensor. The resistance of the sensor decreases upon exposure to the greenhouse gas that is proportional to the concentration of carbon dioxide.


Figure 16
Figure 16: Twin sensors in parallel configuration. (A) The arrangement of the sensors and the measurement details. (B) The equivalent circuit of the arrangement of the sensors. S1 and S2 are the sensors. C. Sensor dimensions. Reproduced with permission from reference [4].

The sensor showed a semiconducting behavior with a negative temperature coefficient (Type 1 sensor). The response time of the sensor is about 40 s. The Fourier transform infrared spectroscopy showed peaks for the nanocomposite at 1056 cm-1, 1195 cm-1, 1296 cm-1, 1635 cm-1, 2083 cm-1, 2345 cm-1 and 3278 cm-1. The carbon dioxide adsorption on the composite results in the polystyrene sulfonate absorption band shifting from 1195 cm-1 to 1176 cm-1 suggesting a phase separation occurring in the nanocomposite that result in the increased conductivity.


Metal oxide semiconductors (MOS) films, nanowires, nanocage, powder and microspheres have been investigated [39-46] for carbon dioxide sensing. The sensors developed using this approach operate at a temperature range of 200-700oC and can detect concentrations in the range of 100-10,000 ppm with a response time falling in the range of 3 s to 9.5 h. The recovery times are also in the range of 4 s to 700s. The performance of an MOS sensor depends on its morphology and composition. The results obtained here have shown that the grain size influences significantly the performance sensitivity (Type 2 and Type 3 sensors).


Poly (ionic liquid)-wrapped single-walled carbon nanotubes has been found to be sensitive for carbon dioxide detection in low concentrations of 500 ppt of CO2 [47]. The chemiresistive dynamic response of the carbon nanotubes is shown in Figure 17 (Type 1 sensor) where the mechanism for sensing is based on the interaction between BF4 anion and CO2. The charge transfer interaction between BF4- and CO2 is depicted in Figure 17E.


Figure 17
Figure 17: [A] shows the response of pristine single walled carbon nanotubes [B] Change in resistance with respect to CO2 concentrations. [C] Successive injections of 1 ppm CO2 to the sensor. [D] Response of the sensor to a) CO2 (10 ppm) b) CO2 (10 ppm) +relative humidity (42%) c) 100 ppm H2 d) 100 ppm CH4 e) 50 ppm ethanol f) 10 ppm O2 and relative humidity (42%). [E] Sensor mechanism. Reproduced with permission from [47].

NOx Gas sensors

Faraday rotation spectroscopy has been proposed for NO2 determination [47,48] (Type 4 sensor). It uses a widely tunable external cavity quantum cascade laser (EC-QCL) and operates mode-hop free between 1600 cm-1 to 1650 cm-1 and allows Q-branch transition of NO2 at 1613.2 cm-1. A detection limit of 95 ppt has been reported. Meyyappan et al. [49] used a simple casting of single walled carbon nanotubes on an interdigitated electrode for detection of NO2 ranging from sub ppm to 44 ppb. The response time of the sensor is in the order of seconds.


Table 3: Several MOS materials have been examined for NOx detection.


MOS Operating Temperature (oC) Response time, s Detection concentration ppm Reference
WO3 200 150-280 0.005-1 78
WO3 300 80-300 0.5 53
WO3 300 0.5-2.5 54.
WO3 350 180 1 79
WO3 174 468 0.6 80
ZnO RT 240 0.01 50

Among the sensors reported in the literature, there are a few sensors reaching lowest detection limits of 0.01 to 0.5 ppb. The ZnO [50,51], In2O3 [52] and WO3 [53,54] sensors showing variable response times fall into this category of low detection limits. A sensor developed with reduced graphene oxide-Cu2O nanowire interestingly reaches the lowest detection concentration of 0.064 ppm [55].


Nitrous oxide (N2O) is released into the atmosphere from the chemical plants producing nitric acid and polymers(https://www.eia.gov/environment/emissions/ghg_report/ghg_nitrous.cfm). N2O is a colorless toxic pollutant gas with a slightly sweetish odor. It is widely used as an anesthetic and analgesic agent in clinical field and also as a propellant for pressurized containers in food industry. It is neither flammable nor explosive. One molecule of N2O has the same greenhouse warming power of 300 molecules of carbon dioxide. Two-thirds of anthropogenic N2O emissions arise from agricultural soils [58] where N2O is formed as part of the bacterial denitrification pathway, in which soil and marine bacteria use oxidized nitrogen compounds as terminal electron acceptors for anaerobic respiration [59]. Once that N2O molecule gets into the upper atmosphere, it can stay there for more than 100 years before getting destroyed naturally. Even though nitrous oxide is a moderately undisruptive substance unlike pollutants known as NOx, it has recently been reported to participate in the depletion of the ozone layer in the stratosphere (https://www.eia.gov/environment/emissions/ghg_report/ghg_nitrous.cfm). So, it is crucial to control and convert N2O to a harmless gas by catalytic surface reactions.


Yoosefian [60] performed Density functional studies on the adsorption behavior of nitrous oxide (N2O) onto intrinsic carbon nanotube (CNT) and Pd-doped (5,5) single-walled carbon nanotube (Pd-CNT). Pd dopant facilitates in adsorption of N2O on the otherwise inert nanotube as observed from the adsorption energies and global reactivity descriptor values. The adsorption energy of N2O on CNT was investigated in three orientations; vertical (VC) and horizontal (HC) to the nanotube axis and the nitrogen atom toward the C-C bond (NC). The full optimized structures are indicated in Figure 18.


Figure 18
Figure 18: The optimized structures of adsorbed N2O from three orientations on intrinsic CNT; A vertical (VC) to the nanotube axis, B horizontal (HC) to the nanotube axis and C the nitrogen atom toward the C-C bond (NC). Reproduced with permission from reference [60].

The adsorption of N2O changes the electronic conductivity of Pd-CNT which is attractive for developing the sensor.


Methane sensors

A large number of detectors [61-68] have been developed in the last couple of decades for the detection of methane mainly to detect the leakage of natural gas as methane and air combination constitutes explosive mixture. These detectors are based on optical fiber sensing [62], resistive change [62a-66a] using graphene-polyaniline composite, cataluminescence [67] and refractive index [68,69]. The lower detection limit that can be reached with these methods is about 10 ppm [62a,67]. A theoretical calculation of methane adsorption on graphene has been investigated and it shows opening of band structure of graphene upon adsorption [69]. It predicts that adsorption energy of defected graphene is increased with the number of layers. A low-cost sensor has been developed [70] using a metal oxide that has suitability for detecting methane leaks. The detection of the sensor is in the range of 0.8-2.7 ppm. It is ideally suited for environmental analysis of methane.


Humayun et al. [71,72] fabricated a highly sensitive, low-cost and energy efficient distributed methane (CH4) sensor system (DMSS) comprising metal oxide nanocrystals (MONC) functionalized MWCNT-based CH4 chemiresistor sensor (Type 1 sensor). The sensor showed the ability of sensing below 10 ppm of CH4 in dry air at room temperature. They developed a Gaussian plume triangulation algorithm for the DMSS as per which if a geometric model of the surrounding environment is given, then the algorithm can precisely detect and localize a CH4 leak as well as estimate its mass emission rate. To facilitate faster leak detection, a control algorithm based on the UV-accelerated recovery was developed.


A methane chemiresistive sensor has been fabricated using multiwalled carbon nanotubes/metal oxide nano particles that shows a detection limit of 10 ppm. The sensor response to methane and water vapor has been measured in this work which shows the sensor is inactive to water.


Modern Technology using 3D and Ink Jet Printing of Gas sensors

With rapid developments in finding active materials suitable for greenhouse gas sensors, there is need for consideration of the cost of making these devices. In this context, the 3D-printing of the active material would be of interest. High quality 3D printed desk top devices have been produced at a low cost without conventional micromechanical systems. Taylor and Velásquez-García [73] have reported a novel electrospray printed nanostructured graphene oxide for gas detection. A number of gases have been examined in this work with a variety of detection limits. For the greenhouse gas, CO2 the detection limit has been set at 1000 ppm. The preliminary reports open up opportunities to modify the spray printing for the lower detection limits. Rieu et al. [74] have developed an ink jet printing of SnO2 gas sensor on plastic substrate. Both the gas sensitive layer and heating transducer have been inkjet printed. This method of making the sensor compares well with sol-gel tin oxide film. The following detection limits have been reported for CO and NO2; the limit of detection falls far above the other methods discussed in the earlier sections. Furthermore, the operating temperature of the sensor is in the range of 200-300oC (Table 4). Sulfonated graphene has been used for detecting NO2 at room temperature by Liua et al. [79]. The sulfonated graphene and SnO2 particles are combined to form the active material of the sensor. The process involves direct deposition of SnO2 nanoparticles on reduced graphene oxide. A high performance and a good sensitivity have been achieved by this active material. A detection limit of 1 ppm of NO2 has been reached with this sensor. The room temperature detection using sulfonated graphene opens up opportunities for developing other greenhouse gas sensors. A screen printed piezoelectric microcantilever (Type 6 sensor) has been used to detect and determine NO2 and CO gases using oxygen plasma treated multiwalled carbon nanotubes as sensitive layer [76]. Figure 19 shows the response of the cantilever for the two gases with both positive and negative shifts in resonant frequency. It is attributed to stress and mass effects at different levels of concentrations of the analytes.


Table 4: Limits of detection of CO and NO2 using the SnO2 inkjet printed sensors.


CO (ppm) 0.5 (300o, dry air); 24 (300oC; wet air); 0.4 (200oC, dry air); 46 (200oC, wet air)
NO2(ppm) 6 (300oC, dry air); 9 (300oC, wet air); 1 (200oC, dry air); 6 (200oC, wet air)
Figure 19
Figure 19: Screen printed Cantilever with multiwalled carbon nanotube functionalized by oxygen –argon plasma as sensitive material. Reproduced with permission [76].

The sensor’s response to NO2 has been remarkable at ppb levels. A new generation of sensors for the greenhouse gases is to be expected in future utilizing 3D printing and inkjet printing that are being researched in the literature [76,79-83].


Outlook and Conclusions

The recent reports of monitoring greenhouse effect gases have conclusively shown that CO2 level in the atmosphere has been steadily increasing; the UN reports have consistently been promoting the use of renewable energies for controlling the atmospheric conditions. The atmospheric conditions need to be steadily monitored and for this purpose we need to have reliable measurements of the greenhouse effect gases. The acquisition of the data requires the development of cheap, robost and accurate sensors. Although the area of sensors has been growing rapidly, the sensors directed towards the greenhouse effect gases by using nano materials have been recent. This survey of the greenhouse effect gas sensors suggests that there has been active interest in developing gas sensing technologies. The first part of the paper projects the progressive increase in the concentrations of greenhouse gases in the atmosphere and its consequences. The second part discusses the developments in the areas of carbon dioxide, nitrogen oxide and methane sensors using carbon nanotubes and polymer composites. A few reports on using graphene which is a material having large surface area relative to carbon nanotubes, high electrical and thermal conductivities, opens opportunities for developing novel greenhouse gas sensors. The current trend is to use 3D printing or ink jet printing in making the sensor devices. This survey of greenhouse gas sensors shows that so far, the developments have been based on discovering active new materials and examining their characteristics limited to either pure gases with limited interferences. There are very few sensors that would respond to soley to only one greenhouse gas.


Acknowledgments

One of the authors (KSVS) thanks the National Science Foundation for financial support.


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