Carbon nanotube (CNT) gas sensors for emissions

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Sensors and Actuators B 203 (2014) 349–362

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Review

Carbon nanotube (CNT) gas sensors for emissions from fossil fuel burning M. Mittal a,∗ , A. Kumar a,b a Sensors and Nanotechnology group, Council of Scientific and Industrial Research-Central Electronics Engineering Research Institute (CSIR-CEERI), Pilani (333031), Rajasthan, India b Academy of Scientific & Innovative Research (AcSIR), New Delhi, India

a r t i c l e

i n f o

Article history: Received 20 November 2013 Received in revised form 15 May 2014 Accepted 15 May 2014 Available online 27 May 2014 Keywords: Carbon nanotubes Gas sensors Fossil fuels Carbon monoxide Nitrogen dioxide Sulfur dioxide

a b s t r a c t Fossil fuels endow wide applications in industrial, transportation, and power generation sectors. However, smoke released by burning fossil fuels contains toxic gases, which pollutes the environment and severely affects human health. Carbon nanotubes (CNTs) are potential material for gas sensors due to their high structural porosity and high specific surface area. Defects present on the CNT sidewalls and end caps facilitate adsorption of gas molecules. The chemical procedures adopted to purify and disperse carbon nanotubes create various chemical groups on their surface, which further enhance the adsorption of gas molecules and thus improves the sensitivity of CNTs. Present review focuses on CNT chemiresistive gas sensing mechanisms, which make them suitable for the development of next generation sensor technology. The resistance of carbon nanotubes decreases when oxidizing gas molecules adsorb on their surface, whereas, adsorption of reducing gas molecules results in increasing the resistance of CNTs. Sensing ability of carbon nanotubes for the gases namely, NO, NO2 , CO, CO2 and SO2 , released on burning of fossil fuels is reviewed. This review provides basic understanding of sensing mechanisms, creation of adsorption sites by chemical processes and charge transfer between adsorbed gas molecules and surface of CNTs. In addition, useful current update on research and development of CNT gas sensors is provided. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Electronic properties of carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Chemical properties of carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Functionalization of carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon nanotube chemiresistive sensing mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon nanotubes for detection of gases released on burning of fossil fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Detection of carbon monoxide (CO) and carbon dioxide (CO2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Detection of nitric oxide (NO) and nitrogen dioxide (NO2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Sulfur dioxide (SO2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Challenges in fabricating carbon nanotube sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biographies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

350 351 351 351 351 353 355 355 356 358 358 359 359 359 359

∗ Corresponding author at: Present address: Pulse Power and Development of RFQ for Accelerator Division, Institute of Plasma Research (IPR), Bhat, Gandhinagar (382428), India. Tel.: +91 9464989975; +917923962140/41/42. E-mail addresses: [email protected] (M. Mittal), [email protected] (A. Kumar). http://dx.doi.org/10.1016/j.snb.2014.05.080 0925-4005/© 2014 Elsevier B.V. All rights reserved.

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1. Introduction Coal, petroleum, and natural gas are known as fossil fuels, which are formed over millions of years by anaerobic decomposition of dead and decaying organic matter. However, fossil fuels are limited in amount, but they harm our environment irreparably. On burning, the compounds of carbon, nitrogen, and sulfur present in fossil fuels, react with air to form gaseous oxides. The gases released are toxic in nature and are harmful for both human beings as well as the environment. Fast expanding industries, transportation, and power generation sectors are the biggest consumers of fossil fuels and hence are the major sources of pollution. Parts per million (ppm) or even parts per billion (ppb) concentrations of noxious gases are sufficient to pollute the environment. The gases released by burning fossil fuels, if inhaled, can cause dysfunction of various organs of living beings or may cause death. Thus, precise measurement of the gases emitted on burning of fossil fuels is very essential. Among the range of available gas sensors, solidstate gas sensors are widely preferred as they have robust design and are durable. Present solid-state gas sensors like metal oxide sensors are used since last few decades for the detection of toxic gases, however, their use have been limited due to higher operating temperature, power requirements, and selectivity issues. To overcome some of these limitations of metal oxide gas sensors, new materials with better sensing properties are being continuously investigated for the development of new generation sensor technology. Recent progress in nanomaterials research offers an opportunity to improve the response of new sensor materials upon exposure to the target gas. The improved response is mainly due to the large surface to volume ratio and the control over topography because the exposed surface area greatly affects the sensor performance. Carbon nanotubes (CNTs) [1] discovered by Sumio Iijima in 1991 have promising applications in the field of nanoscience and nanotechnology. Large surface to volume ratio, presence of defects, and porous structure of carbon nanotubes present effective binding sites for gas molecules. The gas molecules adsorbed on the surface of carbon nanotubes change their properties such as resistance and capacitance. Carbon nanotubes show great potential in making up next generation sensor technology because they are chemically, thermally, and mechanically very stable. Minimal sensing material requirement and high sensitivity at room temperature make carbon nanotubes suitable for developing highly sensitive, low power and low cost miniaturized gas sensors. Both the receptor and the transducer components of a sensor can be fabricated using carbon nanotubes [2]. Kong et al. [3] in year 2000 observed changes in the conductance of single walled carbon nanotube field effect transistors (SWCNT-FET) when exposed to nitrogen dioxide (NO2 ) and ammonia (NH3 ) gas environments. Their results sparked research activities in the field of CNT gas sensors. Valentini et al. [4] in 2000 observed adsorption of NO2 gas molecules on carbon nanotube mats. Li et al. [5], employed interdigitated electrodes (IDEs) and

observed linear response for nitrotoulene and NO2 gas concentration. They also employed UV irradiation for complete desorption of gases from the surface of CNTs which improved the recovery time of the sensor from a few hours to few minutes. Suehiro et al. [6] introduced dielectrophoresis (DEP) process to align carbon nanotubes between metal electrodes. Aligned CNTs provide larger surface area for the adsorption of gas molecules thus increased the sensitivity of CNTs. In addition, aligned CNTs provide improved carrier transport between the two metal contacts. Sin et al. [7] in 2007 observed ∼10 folds increase in the sensitivity of carboxyl functionalized carbon nanotubes (f-CNTs) for alcohol vapors. Many groups have reported conjugation of polymers with carbon nanotubes for gas sensing applications [8–12]. Abraham et al. [9] observed fast response of CNT/PMMA composites towards dichloromethane, chloroform, and acetone with 102 –103 orders of change in the resistance of synthesized conjugates. Shirsat et al. [11] conjugated free base and metal (like iron and ruthenium) substituted porphyrins with SWCNTs for enhancing the selectivity and conductivity of chemiresistor sensor arrays. For different central metal atoms, difference in sensitivity of the devices was recorded towards volatile organic compounds present in air like acetone, methanol, ethanol, and water. Jiana et al. [12] aligned PEDOT/PSS-CNT composites using AC-DEP and observed that aligned CNT/polymer composite films show better response to 10 ppm NH3 , trimethylamine, and methanol vapors at room temperature in comparison to the films formed by normal drop casting method. Strong binding between polymer-conjugated CNTs and gas molecules results because of the porous structure of polymers [13]. Hence, the rate of charge transfer between the carbon nanotubes and the adsorbed gas molecules increases when some polymer molecules are used as intermediate. The increased charge transfer enhances the sensitivity of the sensor. However, irreversibility and poor selectivity of gaseous molecules are the major issues for polymer hybridized CNT sensors [14]. Currently, carbon nanotube-nanoparticle conjugates are under intensive studies for gas sensor applications [15–21]. Metallic nanoparticles, namely Pd, Pt, Au, Ag, Rh, Pb, Sn show catalytic properties and allow specific binding of gas molecules. Gas sensor arrays using carbon nanotubes with different binding sites are promising and are much researched sensor systems now a days as they allow screening of multiple analytes. During the past decade, several reviews summarizing CNT potential for the fabrication of gas sensors [22–27] have been published. The need of the hour is to control the amount of pollutants liberated by burning fossil fuels due to their hazardous effects on human beings as well as on the environment. Gaseous byproducts released on combustion of fossil fuels are responsible for situations like acid rain, global warming and winter-smog. Human health, environment protection and new scientific developments in CNT based gas sensors were the motivation for writing this review article. Permissible, short-term and immediate danger to health and life exposure limits for these gases are given in Table 1. This review is organized as follows. Section 2 details the electronic and chemical

Table 1 Immediately dangerous to life or health (IDLH) concentration, permissible and short-term exposure limits (PEL and STEL) of the various gases released by burning of fossil fuels. Gaseous product

Immediately dangerous to life or health (IDLH)a concentration

Permissible exposure limit (PEL)b

Short term exposure limit (STEL)c

References

Nitrogen oxide (NO) Nitrogen dioxide (NO2 ) Carbon monoxide (CO) Carbon dioxide (CO2 ) Sulfur dioxide (SO2 )

100 ppm 20 ppm 1200 ppm 40,000 ppm 100 ppm

25 ppm 5 ppm 50 ppm 5000 ppm 5 ppm

– 25 ppm 1500 ppm at 10 min exposure 30,000 ppm 30 ppm for 10 min exposure

[28,29] [28,30] [28–31] [28–30] [28,29,32]

a b c

IDLH (immediately dangerous to life or health): situation that can cause an immediate danger to life. PEL (permissible exposure limit): acceptable average exposure over the continuous working period of 8 h. STEL (short term exposure limit): acceptable exposure over period of 15 min.

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Fig. 1. 3-D plots for the dispersion relation of (a) graphene (b) metallic CNT (c) semiconducting CNTs [48].

properties of carbon nanotubes and functionalization techniques. Section 3 discusses the sensing mechanisms of CNT based resistive gas sensors. Section 4 summarizes the work done by various researchers to detect the presence of toxic gases (NO2 , NO, CO, CO2 and SO2 ) released on burning of fossil fuels. Challenges in commercializing CNT sensors are addressed in Section 5 followed by conclusions. 2. Carbon nanotubes Carbon nanotubes are seamless cylinders formed by wrapping of graphene sheets along the axial direction. The ends of CNTs are sealed with fullerene like hemispherical molecules. Depending upon the number of graphene sheets rolled, carbon nanotubes are classified as single-walled [33,34] or multi-walled carbon nanotubes [1]. CNTs show great potential in every field of science and technology, as they own both metallic and semiconducting properties. Physical and chemical properties of the carbon nanotubes can be controlled through the synthesis processes used for their production [35]. Small size, high surface to volume ratio, mechanical strength [36–38], electrical and [39–42] thermal conductivity [43–45], and porous structure make carbon nanotubes potential candidate for energy storage and sensing applications. 2.1. Electronic properties of carbon nanotubes Electronic properties of carbon nanotubes can be understood from the band structure of graphene [46]. The dispersion relation for graphene (as shown in Fig. 1(a)) represents the bonding (␲) and anti-bonding (␲*) orbitals degenerated at K-points (Fermi points) in the Brillouin zone. At K-points, valence and conduction bands are connected with each other due to which band gap in graphene is zero at that points and hence, graphene acts as a zero band gap semiconductor. However, for carbon nanotubes, each graphitic band opens up to form a number of sub-bands due to confinement of electrons in the radial direction. If the sub-bands pass through Fermi points, CNTs are metallic as shown in Fig. 1(b) otherwise semiconducting as shown in Fig. 1(c) [47]. The direction of axial wrap decides whether the carbon nanotubes formed are armchair, zigzag, or chiral. Rolling of graphene sheet along the symmetry axis results in the formation of armchair or zigzag carbon nanotubes otherwise chiral CNTs are formed. For armchair type CNTs, the circumferential vector (C) lies along the direction exactly between the two basis vectors i.e. the chiral angle,  is 30◦ and the chiral indices, n = m. The nanotubes so formed are purely metallic. Whereas, for zigzag CNTs the circumferential vector lies purely along one of the two basis vectors, i.e. the chiral angle  is 0◦ and out of n or m, one of the chiral indices is zero. The chiral nanotubes are formed for m = / n and chiral angle, 0◦ ≤  ≥ 30◦ [48]. CNTs with n − m = 3i (where i is an integer) have curvature induced energy band-gap typically of the order of few meV. However, carbon nanotubes for / 3i, have larger energy band-gap ∼ 1 eV [49,50]. It is which n − m =

possible to tune the band gap of semiconducting carbon nanotubes during synthesis. The energy band gap is related to the diameter, d (in nm) of single walled carbon nanotubes as, Egap = 0.7/d (in eV) [51]. The length independent conductance of carbon nanotubes of length upto few microns is an added advantage for the fabrication of ballistic conductors. The one-dimensional structure and strong covalent bonding in carbon nanotubes reduces small angle scattering events, which may arise due to lattice vibrations and defects. Only back and forth motion of electrons is allowed in carbon nanotubes. Since carbon nanotubes have symmetrical band structure; backward scattering events of the electrons in 1-D carbon nanotubes are reduced [52,53]. Poncharal et al. [54] reported ballistic transport regime in carbon nanotubes upto a length of ∼200 ␮m. The current density for ballistic carbon nanotubes has been reported to be as high as ∼ 109 A/cm2 [55]. However, for longer carbon nanotubes many scattering events are possible and thus, diffusive transport takes place. 2.2. Chemical properties of carbon nanotubes Three nearest neighbors for each carbon atom in carbon nanotubes bind according to sp2 hybridization. Therefore, the pyramidalization angle ( p ) should be 0◦ . Hamon et al. [56] and Niyogi et al. [57] observed that the angle of pyramidalization is not 0◦ but it is 11.6◦ at the end caps. This value of the angle of pyramidalization is close to that of a tetrahedral structure i.e. 19.5◦ . In addition, the angle of pyramidalization for the nanotube sidewalls is 6.0◦ . The finite value of  p at the sidewalls introduces local strain in the tubular nanostructures [56,57].  orbital mismatch is another reason for the presence of strain in CNTs [58]. The end caps in CNTs exhibit larger reactivity due to large angle of pyramidalization because of two-dimensional curvature.  orbital misalignment is negligible at the end caps [59]. Thess et al. [60] observed that the value of angle of pyramidalization is low for sidewalls as compared to end-caps of carbon nanotubes. Even though high strain is present, the strong carbon–carbon bond interactions in carbon nanotubes suppress their reactivity. This limits the effective sites on carbon nanotubes and hence their sensing potential. Defects may occur in carbon nanotubes during synthesis process. The presence of structural defects like pentagon–heptagon pairs, Stone–Wales, and vacancies [61–63] builds sp3 character on the hexagonal network of tubular nanostructures [64–66]. Presence of defects enhances the chemical reactivity of carbon nanotubes [67,68]. Defect sites and end-caps of carbon nanotubes being highly reactive adsorb gaseous molecules easily and undergo chemical functionalization at faster rate [69–71]. 2.3. Functionalization of carbon nanotubes Functionalized carbon nanotubes exhibit tremendous potential in developing gas nanosensors as functional groups present on the CNT surface make them very reactive and adsorb gas molecules

• Storage of liquid fuels e.g. H2 • Drug delivery

[99,100]

• CNT–polymer composites in aerospace science • CNT–Biomolecule composites in biosensors

[97,98,100]

• Outer surface of CNTs remain unused

References

Area of application

Disadvantages

• Easy dispersibility in polar solvents • Control over the density of generated groups • Length of CNTs is rarely affected • Production of stable f-CNTs • Destruction of the carbon framework • Deterioration of intrinsic electronic and mechanical properties • Small surface area of CNTs is addressed • Chemical sensors • Light Emitting Diodes • Solar cells • Transistors [95,96,100] Advantages

• Complete CNT surface can be addressed • High degree of functionalization • Easy dispersion in chemical solvents • Production of stable f-CNTs • Destruction of the carbon network • Shortening of CNTs • Degradation of intrinsic electronic and mechanical properties • Chemical sensors • Light Emitting Diodes • Solar cells • Transistors [93,94,100]

• -stacking • van der Waals interaction • e.g. wrapping of CNTs with polymers, surfactants and peptides • Preservation of structural integrity • Length of CNTs is not affected • Electronic and chemical properties of the CNTs are not affected • Produced f-CNTs are unstable • Dispersibility of non-covalent f-CNTs is comparatively low • Oxidation of carbon nanotubes e.g. carboxyl functionalization • Cyclo addition • Electro and nucleophilic addition • Ozonolysis and radical reactions Schematic Reaction mechanisms

Non-covalent functionalization End or defect functionalization Covalent sidewall functionalization

Exohedral functionalization

Hirsch [100] summarized the surface activation routes for carbon nanotubes through covalent, non-covalent, defect, ␲stacking, sidewall and endohedral functionalization. Covalent and non-covalent addition of chemical groups to the ␲-conjugated framework of carbon nanotubes are the two most extensively studied methodologies used for CNT functionalization [98,101–105]. Covalent functionalization routes include cyclo additions, electrophilic and nucleophilic addition, ozonolysis, radical (oxidative or reductive) reactions [101,102,105]. Anchoring carboxyl or carbonyl group onto CNTs follows covalent routes, which uses nitric

Table 2 Summary of techniques for functionalization of carbon nanotubes.

a. Covalent defect and end-cap functionalization: The fullerene like end-caps of CNTs are highly strained resulting in preferential oxidation at end-caps. Various topological disorders, like, incomplete bonding and Stone–Wales defects are the locations of higher reactivity that oxidize during purification. Oxidation results in opening up of sp2 bonding sites of CNTs and anchoring carboxyl, hydroxyl, and carbonyl groups at these sites. CNTs are oxidized using nitric acid [88], mixture of nitric and sulfuric acid [89], mixture of hydrogen peroxide and sulfuric acid [90], potassium permanganate [91] or ozone oxidation [92]. b. Covalent sidewall functionalization: Functionalization of complete CNT surface can be achieved with this method. Covalent functionalization routes, though provides high solubility, stability, processibility and formation of strong bonds but not preferred much as it destroys the unique properties of CNTs and introduces additional defects on their surface. Moreover, covalent functionalization may require harsh reaction conditions to take place. c. Non-covalent functionalization: An alternative method to preserve the structure of CNTs is to functionalize CNTs through non-covalent approach. With non-covalent functionalization, their electronic and optical characteristics remain unchanged. Uniform wrapping of functional groups over the CNT surface is an added advantage. Interactions involved in this approach are ␲ stacking and van der Waals. However, because of poor interaction among ␲ orbitals, stability is quite low. Table 2 summarizes the techniques used for the functionalization of carbon nanotubes.

Endohedral functionalization

fast [72–77]. Chemical routes adopted to functionalize CNTs largely help in exploiting their reactive properties. Formation of rope like bundles and contamination (due to amorphous carbon and catalytic impurities) in carbon nanotubes during synthesis render them insoluble in chemical solvents [78–84]. In addition, being hydrophobic in nature due to the presence of van der Waal forces [85] CNTs are not dispersible in chemical solvents. New chemical techniques for dispersing the CNTs involve modification of their surface [57,86,87]. Functional groups bound to the backbone of carbon nanotubes determine their characteristic properties and chemical reactivity. These are less stable than the carbon nanotubes and accelerate chemical reactions between the CNTs and the adsorbing molecules. Common groups used to functionalize CNTs are; hydroxyl (–OH), carboxyl (–COOH), carbonyl (–C O) and amino (–NH2 ). Carbon nanotubes are functionalized using either endohedral or exohedral approach. In endohedral functionalization, CNTs are filled with nanoparticles or bio-functionalities. Functionalities penetrate inside the CNTs by placing the CNTs inside the suspension containing desired functionalities. This approach depends on the surface energy of the liquid. Another route for endohedral functionalization involves filling of the CNTs with metal salts followed by heating at high temperature, is known as the wet chemistry. The metal salts melt inside the CNTs forming small nanoparticles. Outer surface of CNTs is functionalized using exohedral approach, which is further categorized as:

• Spontaneous penetration by capillary action • Wet chemistry involves filling of CNTs with metal salts followed by heating at high temperature • Structure of the CNTs is preserved • Length of CNTs is not affected

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Method

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(b) Sensitivity: According to IUPAC (International Union of Pure and Applied Chemistry) definition, sensitivity (S) is the slope of calibration curve, that is, sensor response versus concentration. (c) Response time: Response time is the time taken by the sensor to reach its stable output value when subjected to a step change in input. (d) Selectivity: Ratio of sensitivity for all the interfering gases to that of the desired gas of a sensor represents selectivity. (e) Recovery time: Recovery time is the time required by the sensor to return to initial value when the concentration of the adsorbing gas changes to zero from a certain values.

Fig. 2. Schematic diagram representing working of a gas sensor.

acid (HNO3 ) or mixture of sulfuric acid (H2 SO4 ) and nitric acid (HNO3 ) in ratio 3:1 [106]. Carboxyl functionalization of CNTs is widely studied as it improves the reactivity of CNTs and allows the conjugation of other nanoparticles and biological elements with the carbon nanotubes. CNTs end-caps and defect sites are selectively functionalized using concentrated nitric acid (HNO3 ). Amine functionalized carbon nanotubes are obtained by first acylating the carboxylated CNTs with the help of thionyl chloride (SOCl2 ) followed by reaction with an amine compound such as ethanediamine (EDA) or octadecylamine (ODA) or didecylamine (DDA) [107–109]. Pillai et al. [110] proposed a single step procedure to obtain amine-functionalized nanotubes using hexadecylamine (HDA). Acid treatment improves the reactive chemistry of carbon nanotubes largely. In addition, functionalization purifies carbon nanotubes by removing the surface impurities. However, functionalization also leads to the degradation of electrical properties of CNTs as it introduces defects and impurities in CNTs.

3. Carbon nanotube chemiresistive sensing mechanism Chemiresistive sensors [5,111] are the sensors in which output resistance of the device changes when a particular gas is adsorbed on the sensing element from the surroundings [112–115]. The adsorbed gas molecules desorb from the sensor surface when the gas disappears and the sensor regains its original resistance. Ease of fabrication and simple structure of chemiresistive sensor ensures valuable sensing of gases with low power consumption. Thus, chemiresistive sensors are preferred over other type of sensors, like chemicapacitive [116] and field-effect transistor (FET) sensor [117,118]. A chemiresistive sensor first recognizes the molecules adsorbed on the surface of the sensing element and then, the transduction unit provides the output resistance of the sensor [119]. Schematic shown in Fig. 2 represents the working of chemiresistive gas sensor. A sensor is characterized by following parameters: (a) Response: Sensor response represents the change in resistance of the gas sensor with respect to the initial resistance of the device when exposed to various gases.

 R =

(Rgas − Rair ) Rair

 × 100

(1)

where, Rair is initial resistance of the sensor measured in ambient air and Rgas is resistance of the sensor in the presence of test gas.

Sensing parameters depend on the structure of the sensing element and the temperature. The nature of the sensing element decides whether molecular adsorption is physical or chemical. Physical adsorption occurs due to the presence of the van der Waals forces of attraction having low binding energy (∼meV for small gas molecules and ∼ few eV for large gas molecules). Absence of chemical bonding retains the electronic and chemical structure of gas molecules. Further, low binding energies (B.E.) results in complete and easy desorption of gas molecules, thus, full and fast recovery of the sensor. As physisorption is not sight selective, it results in low sensitivity and selectivity of the sensors. Chemical adsorption is favored at highly reactive sites and formation of chemical bonds (orbital overlap) between the sensing element and gaseous molecules results in high binding energies ∼several eV [120] and is suitable for fabrication of gas sensors with high selectivity. However, molecular desorption is quite low for chemisorbed molecules resulting in slow recovery. Carbon nanotubes can be used as an efficient sensing material due to the presence of defect sites (pentagon, heptagon, impurities, Stone–Wales), porous structure, and high surface to volume ratio offering a large number of binding sites to the gas molecules [5,3,121–123]. Large thermal stability of CNTs makes them robust under different reaction conditions preserving their intrinsic structure. Molecular detection at room temperature with CNTs serves numerous advantages like elimination of heating element, no power loss, simplicity of sensor design, miniaturization etc. However, the major drawback of room-temperature operation is the potential interference from humidity. Water molecules present on CNTs may donate electrons to CNTs and/or form hydrogen bonding with oxygen atoms on the CNT surface and/or may introduce charge trapping sites, which change the electrical response of CNT sensors. In CNT sensors water molecules may adsorb either on CNT surface or on supporting silicon substrate. The molecules adsorbed on CNT surface can be easily removed by vacuum pumping or by thermal annealing at high temperature (∼200 ◦ C). Water molecules bound to the substrate are difficult to remove and are the main cause of the hysteresis in the electrical response of the sensors [124,125]. When the target gas from the surrounding adsorbs on the surface of carbon nanotubes, charge transfer takes place between the CNTs and gas molecules. As adsorption is a surface phenomenon, atoms from the topmost layer in CNTs are of prime interest. For SWCNTs all the atoms behave as surface atoms [126,127], whereas in the case of MWCNTs the atoms present in the outermost layer are responsible for the sensor response [128]. As prepared CNTs behave as p-type semiconductors due to the generation of defects and oxidation states during synthesis and purification. The transfer of electrons to and from the CNTs depends on the nature of the adsorbing gas [129]. As soon as the CNT surface comes in contact with reducing gases [130,131] (for example NH3 ), the electrical resistance of the carbon nanotubes increases due to the recombination of free electrons (donated by the gas molecules) and holes (present in CNTs). However, oxidizing gases (like NO2 ) withdraw electrons from CNTs when adsorb on their surface. Withdrawal of electrons from carbon nanotubes increases hole population in CNTs

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Fig. 3. Pictorial representation of electron transfer into/out of pristine carbon nanotubes, (a) reducing gas molecules are adsorbed on CNT surface and transfer electrons to CNT through defects and oxygen atoms present on surface due to chemical processes, (b) oxidizing gas molecules are adsorbed on CNT surface and withdraw electrons from CNT.

which decreases the output resistance of the sensor [127,3,132]. Fig. 3 represents the electron transfer mechanism between CNTs and adsorbed oxidizing and reducing gaseous analytes. Khojin et al. in 2011 [133] studied change in the output resistance of CNTs on adsorption of gases. Their studies showed that the output resistance might vary due to changes in potential barrier (i) between the carbon nanotubes and the electrodes (ii) between interconnected carbon nanotubes, or (iii) due to variation in the number of charge carriers within the nanotube itself on adsorption of gases. They performed a comparative study between the pristine, slightly defected, and highly defected carbon nanotubes and found that the resistance of pristine CNTs changed because of variation in the potential either at the junction of two adjacent CNTs or at the metal–CNT interconnect [133]. Further, they observed that with increase in the number of defect sites, number of charge carriers in the carbon nanotubes change. The changes within the carbon nanotubes are responsible for variation in its resistance. They concluded that the gaseous molecules adsorb preferentially on defected CNTs compared to pristine nanotubes [134]. Theoretical observations of Khojin et al. [133] agreed well with the experimental observations of Bradley et al. [135], Peng et al. [136], and Liu et al. [137]. The most favored locations for the adsorption of gas molecules on CNTs are the top surface, hollow sites, zigzag, and the axial as shown in Fig. 4(A) [138,139]. Battie et al. [140] in 2012 observed that percolation in metallic CNTs is another feature that decides the dominating mechanism for changes in the electrical response in CNT nanosensors. Below the charge percolation threshold, sensing is dominated by modulation of the Schottky barrier (SB), while, above the percolation threshold response of CNT sensors is attributed to charge transfer between the SWCNTs and the gas molecules. There exist three types of junctions within the CNT films: (a) between two semiconducting SWCNTs (b) between two metallic SWCNTs and (c) between one metallic and one semiconducting SWCNT. Fuhrer et al. [141] reported that the contact resistance between a metallic and a semiconducting SWCNT is large as compared to the contact resistance between two semiconducting or between two metallic SWCNTs. Charge between the two SWCNTs is transferred through the Schottky barrier. Therefore, more current flows in CNT films comprising entirely of either semiconducting or metallic SWCNTs

compared to the CNT films formed from mixture of semiconducting and metallic CNTs [142,143]. During the synthesis process, bundled carbon nanotubes are obtained due to the presence of the strong van der Waals force of attraction at the graphitic surface of CNTs [144–146]. Bundled carbon nanotubes present different adsorbing locations for gaseous molecules as compared with a single carbon nanotube. The adsorbing sites for the bundled CNTs are: (i) outermost surface (ii) grooves formed at the periphery of bundle due to contact between different nanotubes (iii) internal hollow sites or pores, and (iv) gap created at the interconnect of three nanotubes, known as interstitial channel [129,24] as shown in Fig. 4(B). The unique structure of CNT bundles provides novel adsorption and transport properties [147]. The binding energies of the gas molecules play an important role in allocating preferred binding sites for their adsorption on CNTs. However, sometimes, it may happen that the energy favored sites on carbon nanotubes are already occupied and thus, are no more available for the adsorption of gas molecules [148]. In addition, the presence of fullerene caps at the ends of carbon nanotubes ceases the possibility for the adsorption of molecules inside the nanotubes. The formation of bundles during synthesis and the variations in the processes for purification of carbon nanotubes result in the varying number and type of sites available for adsorption. The purification of nanotubes with acid reagents is well known procedure to debundle the entangled CNTs. The separated nanotubes offers more adsorption sites compared with the bundled ones. In addition to the removal of impurities, functionalization process also results in anchoring of the different functional groups on the highly strained end caps or the defect sites in the carbon nanotubes. This way, CNTs can undergo further conjugation reactions with nanoparticles and bio-molecules. Functional groups with some specific atoms within molecules are responsible for the characteristic chemical reactions of those molecules. New defects generate under vigorous conditions during chemical functionalization of CNTs. Carboxyl group is the most versatile group anchored on the carbon nanotubes. Acidic nature of carboxyl group forces it to drop its proton to form COO− ion (electrophile). The electrophile thus formed reacts with the nucleophilic reagents (bases) readily, to form ionic salts. Some molecules attach to the hydroxyl group while making

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Fig. 4. Diagrammatic illustration of adsorption sites on (A) isolated carbon nanotubes (B) carbon nanotube bundles.

hydrogen bond with the H atom of the –OH group. The f-CNTs thus, show variations in their output resistance due to the transfer of charge carriers inside and/or outside the carbon nanotubes on the adsorption of gaseous molecules. Moreover, functionalization provides selectivity in the binding of gaseous molecules. Doping of CNTs with heteroatoms is another phenomenon that influences the response of nanosensors. Chemical doping replaces carbon atoms in carbon nanotubes by heteroatoms like boron and nitrogen. Doping alters the physical and chemical properties around the dopant atom in CNTs that increases binding energy of interaction of gas molecules. However, binding energies of the carbon atoms located far away from the heteroatoms are not affected significantly. Boron or nitrogen doped CNTs show enormous change in the electrical response on exposure to certain gases like NO2 , NH3 , and O2 [149,150]. Nevidomskyy et al. [151] performed ab initio studies for nitrogen doped carbon nanotubes. Nitrogen being an electron donor forms an energy level lying 0.2 eV below the bottom of conduction band. Doped nitrogen atoms hybridize with the ␲ orbitals of CNTs to create spatially localized states that are singly occupied and are chemically active, which increases the interaction of CNTs with gas molecules [152]. Battie et al. [153] in 2011 studied the sensitivity of nitrogen doped CNT sensors towards NO2 , H2 O and NH3 . They observed that sensitivity of pristine and nitrogen doped CNTs towards NO2 and H2 O remains almost similar. But for NH3 , sensitivity was 2.3 times higher in case of nitrogen doped CNTs compared with that of pristine CNTs. It is due to charge transfer from NH3 molecules to N-doped SWCNTs [154]. 4. Carbon nanotubes for detection of gases released on burning of fossil fuels Fossil fuels are storehouse of energy and are used abundantly in industries, thermal power plants, and transportation. However, these fuels cause irreparable damage to the environment. Smoke released by burning fossil fuels contain carbon, nitrogen and sulfur which react with atmospheric oxygen to form their monoxides and dioxides (CO, CO2 , NO, NO2 and SO2 ) and when these gases come in contact with water vapors produce acids. Carbonic gases form an insulating blanket in the atmosphere, which restricts infrared radiations to escape from the surface of earth. These gases are responsible for greenhouse effect and hence global warming. Other gases, NO, NO2 and SO2 result in acid rain and winter smog. Further, ozone erosion is a major hazard caused by these gases. To monitor the release of these gases, researchers are continuously looking for efficient sensor materials. Chemical interactions of gases, released by burning fossil fuels, with carbon nanotubes and their effects on the resistance of nanotubes are discussed in the following subsections.

4.1. Detection of carbon monoxide (CO) and carbon dioxide (CO2 ) Oxides of carbon, i.e. carbon monoxide and carbon dioxide are the major environmental pollutants. Burning of fossil fuels in motor vehicles, industries and power plants and volcanic eruptions are the major sources of release of CO and CO2 in the atmosphere. Combustion of hydrocarbons in the absence of sufficient oxygen results in emission of CO. Presence of relatively large concentration of carbon monoxide in atmosphere results in the formation of photochemical smog and ground level ozone on interaction with moisture, nitrogen oxide and aldehydes like acetaldehyde. Ever increasing levels of carbon dioxide in the natural environment has raised the earth’s temperature. The increased earth’s temperature results in global warming which is further responsible for melting of glaciers and rising sea level. Though, non-toxic, CO2 causes suffocation if present in excessive concentration in the work area. Thus, precise measurement of CO and CO2 is essential. Adsorption of carbon monoxide on CNTs was investigated theoretically by Peng and Cho [155] in year 2000. They calculated exchange of electronic charge between SWCNTs and CO molecules with the help of density functional theory (DFT). The results showed zero charge exchange between the CNTs and CO molecules, so carbon nanotubes are unable to sense the CO gas. Similar results were observed experimentally by Santucci et al. in 2003 [156]. Insensitivity of CNTs towards carbon monoxide forced researchers to investigate new ways for the detection of CO gas using carbon nanotubes. In 2005, Matranga and Bockrath [157] reported the interaction between carbon monoxide and hydroxyl modified CNTs. They demonstrated that interaction occurs due to the formation of hydrogen bond between the hydroxyl groups anchored onto CNTs and CO molecules. The bonding behavior and charge transfer between carbon nanotubes and C–O molecule is illustrated in Fig. 5. The H atom of hydroxyl modified CNTs binds

Fig. 5. Adsorption of CO gas molecule on the CNTs functionalized with hydroxyl group. Hydrogen bond is formed between the hydrogen atom of the hydroxyl group and the oxygen atom from carbon monoxide.

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4.2. Detection of nitric oxide (NO) and nitrogen dioxide (NO2 )

Fig. 6. Response of the CNT chemiresistor sensor as a function of time for 10 ppm CO gas at 150 ◦ C [158].

with the electronegative oxygen of carbon monoxide. Hydroxyl groups are attached onto CNTs during the purification procedures for the removal of impurities. Dong et al. [158] in 2013 observed a decrease in the resistance of carboxyl modified CNT gas sensor in the presence of CO gas molecules as shown in Fig. 6. Carboxyl groups were anchored onto the carbon nanotubes by sonicating them in nitric acid for 6 h followed by dispersion in sodium dodecyl sulphate (SDS). The CO molecules form hydrogen bond with the H atom of COOH group. Carbon monoxide molecules draw electrons from the nanotube using carboxyl group as an intermediate thus increasing the holes concentration and the current through carbon nanotubes in CO atmosphere. Peng and Cho [149] in 2003 observed that the boron doped CNTs serve as potential nanosensors for CO gas molecules as there exists strong binding between boron and CO molecules leading to large charge transfer from CNTs to CO. The binding energies of CO molecules with the boron-doped CNTs (B.E. = −0.85 eV) and the binding distance of 1.52 A˚ indicated the chemical adsorption of CO molecules, while physical adsorption was observed for nitrogen-doped CNTs having B.E. of −0.22 eV and ˚ The strong binding energy and short bindbinding distance 2.99 A. ing distances are consistent with the larger charge transfer between gas molecules and doped CNT systems. CO2 being reducive in nature, when adsorb on CNTs transfer its electrons to the CNTs leading to recombination of electrons and holes inside CNTs [159] and decrease the hole current. CO2 molecules adsorb on the surface of CNTs due to van der Waals force of attraction, which is exerted by carbon atoms of CNTs on the CO2 molecules. The desorption of CO2 from CNT surface was studied by Firouzi et al. [160]. Incomplete desorption from CNT sensors was observed after each exposure to CO2 gas as shown in Fig. 7. This is because of higher adsorption energies of CO2 molecules on CNT surface [5]. The changes in resistance of CNTs with time as observed by Firouzi et al. were 0.33%, 0.34% and 0.37% at room temperature for the three exposure cycles of CO2 . Carbon nanotubes were exposed to CO2 for 30 s during each cycle. The increase in sensitivity during each succeeding cycle was observed because CO2 molecules were not desorbed completely from the CNT surface. The undesorbed molecules during the preceding cycles also contribute in the sensitivity of CNTs for the next exposure cycles. To conclude, pristine carbon nanotubes are insensitive to CO gas but are sensitive to CO2 gas. CNTs can be made sensitive to CO gas by functionalizing them with carboxyl group (–COOH), as the hydrogen of anchored group will bond with the oxygen of CO molecule, thus helps in binding CO on CNT surface.

Nitrogen monoxide (NO) and nitrogen dioxide (NO2 ) gases are of concern as these are toxic in nature and have nasty smell. Burning of coal, oil, and gas at high temperature in explosive industries and motor vehicles are the sources producing NO and NO2 . Presence of NO and NO2 in the environment results in acid rain, smog formation and depletion of ozone layer. NO and NO2 severely affect human beings as inhalation of these gases causes nausea, irritation in eyes, throat, and nose and may damage lungs and may cause other respiratory problems. Safe limits of exposure to these gases are given in Table 1. Kong et al. [3] reported studies on the adsorption of NO2 on the surface of carbon nanotubes. NO2 being an oxidizing gas, withdraws electrons from the CNTs. The rate at which charge is removed from the nanotube was estimated to be about 0.1 electron per adsorbed NO2 molecule [3,161]. Peng et al. [136] suggested that electrons from the valence band edge of CNTs move towards the lowest unoccupied molecular orbital (LUMO) of NO2 located below the CNT valence band. Chang et al. [162], Yim et al. [163], Zhang et al. [164], Ricca and Baushlicher [165] and Peng et al. [166] also obtained similar results as by Kong et al. through the theoretical calculations. The non-zero charge transfer calculations between CNTs and NO2 molecules make researchers to look for the ways to employ CNTs as nitrogen dioxide sensor. The dominant adsorption phenomenon for NO2 molecules is the chemisorption. Defect sites play an important role in the adsoption of NO2 molecules [167,168]. After adsorption, NO2 molecules may decompose to form NO3 and NO as 2NO2 → NO3 + NO. NO3 thus formed, interacts strongly with CNTs resulting in delayed desorption of these molecules from CNTs as compared to NO. The adsorption of NO2 molecules led to the formation of new electronic states near the fermi-level in carbon nanotubes that changed their ouput resistance [169–171]. Recently, Muangrat et al. [172] in 2012 studied the the response of CNTs synthesized at three different temperatures towards NO2 gas. For CNTs produced at 850 ◦ C, 600 ppm concentration of NO2 gas showed small and unstable response with time whereas, for 1800 ppm and 3000 ppm NO2 gas change in response with respect to time was 0.97% and 1.76%, respectively as shown in Fig. 8. Further, CNTs synthesized at 950 ◦ C showed the highest change in response for each of the three cases, i.e. 2.46% for 600 ppm, 3.51% for 1800 ppm and 6.35% for 3000 ppm. CNTs synthesized at 900 ◦ C exhibited the response of 1.17%, 2.92%, 3.66% for the three concentrations of NO2 gas. Cantalini et al. [173] in 2003 observed an increased resistance of NO2 CNT sensors in the presence of humidity. They exposed CNTs to 80% relative humidity and 100 ppb concentration of NO2 . In the presence of humidity, an increase of 12.5% in resistance is observed compared with a decrease of 52.8% without humidity. The polarized water molecules, which are reducing in nature, interact physically with gas molecules and donate electrons to CNTs. The decrease in the number of holes in CNTs enhances the separation between the Fermi level and valence band in CNTs. To overcome the problem of humidity in CNT sensors at room temperature, Battie et al. [174] in 2012 proposed silica nanoparticles coated CNT sensors. They observed that highly polarized molecules like water (1.8546 ± 0.0040 D) and ammonia (1.4718 ± 0.0002 D) are captured by the mesoporous silica layer while NO2 molecules with very low dipole moment (0.316 ± 0.010 D) diffuse through silica layer and reach the SWCNTs without affecting the response time. Presence of oxygen molecules on the surface of carbon nanotubes plays an important role in the detection of NO gas [171,175]. As NO molecules react with the oxygen molecules and form NO2 , following the chemical reaction; NO + 1/2O2 → NO2 , which adsorbs on the active sites of CNTs. Ueda et al. [176] observed decrease in the output resistance of carbon nanotubes with corresponding

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Fig. 7. Variation of electrical resistance of CNTs for various CO2 gas exposure cycles. The curves indicate the incomplete desorption of CO2 gas molecules after each exposure cycle [160].

increase in the concentration of NO gas. As the NO molecules adsorb on CNTs, the number of holes in the nanotubes increase as electrons are withdrawn by NO molecules from CNTs. Sensitivity of CNTs at room temperature as observed by them is 44.0% for 50 ppm and

Fig. 9. Change in sensitivity of CNTs with respect to temperature for NO gas at 50 ppm and 100 ppm concentration [176]. (© Copyright (2006) Japanese Journal of Applied Physics).

42.5% for 100 ppm of NO gas [176] (as shown in Fig. 9). Moreover, with increasing temperature the diference in sensitivities at different gas concentrations reduced. In the year 2007, Mäklin et al. [177] observed change in the sensitivity of carboxylated CNTs due to the adsorption of NO gas. The NO molecules bind with the O atoms of COOH group as illustrated in Fig. 10. The reponse of carboxylated SWCNTs and MWCNTs to 100 ppm concentration of NO molecules was ∼40% and ∼12% respectively. Moreover, the sensitivity of the carbon nanotube gas sensor increases with the increase in concentration of the gas [173]. There exists strong binding between NO molecules and the sp2 hybridized strained CNT surface [176] as NO is a strong ␲-acceptor ligand [178]. Due to the strong ␲–␲ interaction between CNTs and the NO molecules, slower recovery of the CNT sensors was observed [177].

Fig. 8. Change in response of CNTs prepared at three different temperatures, that is, 850 ◦ C, 900 ◦ C and 950 ◦ C with respect to time for three different concentrations of NO2 gas (a) 600 ppm, (b) 1800 ppm and (c) 3000 ppm exposure [172].

Fig. 10. Illustration representing the interaction between the oxygen of carbonyl group generated on carbon nanotubes during Carboxylic functionalization and the N atom of nitrogen monoxide gas molecules.

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Fig. 11. Change in response of SWCNTs chemiresistor sensors with time towards 50 ␮L/L H2 S (ppm) and SO2 (ppm) at 25 ◦ C for (A) hydroxyl modified SWCNTs (B) carboxyl modified SWCNTs [179].

4.3. Sulfur dioxide (SO2 )

5. Challenges in fabricating carbon nanotube sensors

Hydrogen sulfide reacts with oxygen to form sulfur dioxide (SO2 ). Combustion of coal, ore refining processes, processing of fossil fuels and natural gas, and manufacturing of chemicals are the major sources of production of sulfur dioxide. When SO2 comes in contact with water in the presence of oxygen, it forms sulfuric acid leading to acid rain. Rainwater being acidic in nature corrodes metals, buildings, historical monuments, and textiles. SO2 also responsible for the formation of smog or haze that reduces the visibility. If inhaled, SO2 affects human beings as it causes irritation in lungs and throat and can even damage the respiratory system. Zhang et al. [179] in 2012 measured the resistance of SWCNT gas sensor in the presence of SO2 . They compared the response of hydroxyl modified and carboxyl modified CNTs for SO2 and H2 S gases. They observed that carboxylated single walled carbon nanotubes show better response compared to that of hydroxyl modified SWCNTs for both H2 S and SO2 gases having concentration of 50 ␮L/L each at 25 ◦ C. The observed response of hydroxyl-modified and carboxylated CNTs with time for SO2 was 0.6% and 2% respectively as shown in Fig. 11. This is because the COOH modified carbon nanotubes have stronger oxidizability compared to that of OH modified CNTs. Therefore, the amount of charge transferred from the gas molecules to carboxylated CNTs is much greater than that for hydroxyl modified CNTs. In addition, H2 S gas molecules present higher response compared to SO2 because H2 S molecules have high reducing capability than SO2 . Yao et al. in 2011 [180] observed that resistance of CNTs increase in SO2 environment in the presence of humidity. This is because as-prepared CNTs being p-type have higher electron affinity in the presence of H2 O, thus H2 O and SO2 donate electrons to CNTs. Additionally, chemical reaction between H2 O and SO2 produce sulfurous acid (SO2 + H2 O → H2 SO3 ), which is a reducing agent, and donate electrons to CNTs as shown in Fig. 12. SO2 adsorbs physically on pure activated carbon but it adsorbs chemically in the presence of water and oxygen [181]. In the presence of oxygen, sulfur dioxide is oxidized to form SO3 (SO2 + 1/2O2 → SO3 ) [182]. Further reaction with H2 O results in the formation of sulfuric acid, SO3 + H2 O → H2 SO4 . From the above discussion, it can be concluded that carboxyl and hydroxyl modified CNT sensors show remarkable change in the output resistance as compared with pristine CNT sensors. In addition, anchoring of specific functional groups onto the CNT surface helps in screening multiple analytes.

In past two decades, after their discovery, carbon nanotubes have shown blooming progress in the electronic device technology. Exciting features and structure of carbon nanotubes evidence for their enormous potential in developing the wide variety of gas sensors including chemiresistive. Although, with the present technology carbon nanotube gas sensors showed great achievements, but a great deal of research is still required to attain commercialization. The first and foremost challenge to the researchers is the device reproducability [183]. Controlled synthesis of carbon nanotubes is a prerequisite for large-scale manufacturing of sensors. Currently, chemical and physical routes are employed for sorting the nanotubes. Moreover, production of defect-free carbon nanotubes with length upto few micrometers is challanging task. So, optimatization and command over CNT growth parameters is necessary. Further, production yield of pure and defect free nanotubes is very low, making these CNTs quite expensive. Reproducible electronic properties for CNT network can be obtained using percolation as individual variations in nanotube chirality are statistically averaged. End to end alignment of carbon nanotubes between two metal electrodes imposes another challenge to device fabrication. Techniques like dielectrophoresis, friction alignment, fiber drawing, magnetic field, acoustic waves etc. provide a way to align nanotubes between metal electrodes in a controlled manner [184]. Toxicity of carbon nanotubes is another major topic of concern. Unprocessed nanotubes if inhaled can agglomerate in the lungs blocking air passage and causes asphyxiation [185–188]. Study of nanotube toxicity is crucial for evaluation of safe exposure of human immune system to the CNT based products. Apart from the work done to improve the nanotube morphology, a great deal of effort is required to improve the adsorption and desorption rates of target molecules. Chen et al. [189] recently showed that continuous in-situ cleaning of carbon nanotubes with ultraviolet light during gas sensing, dramatically enhanced their sensing performance. The foremost problem is to check the cross-sensitivity towards different molecules present in a gaseous mixture. Non-specific binding on CNTs can be reduced using surface passivation of CNTs. Selectivity in CNT sensors can be further improved by hybridizing CNTs with nanoparticles, polymers and heteroatoms. It can be further enhanced using gas sensor arrays and pattern recognition [8,11]. Effect of humidity on senstivity is minimized by vacuum pumping, annealing at high temperature (∼200 ◦ C) or by coating

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Fig. 12. (A) Highest occupied molecular orbital (HOMO) and Lowest unoccupied molecular orbital (LUMO) of as-prepared CNTs (p-type) representing the electron transfer from water, sulfur dioxide and sulfurous acid to HOMO of CNTs. (B) Schematic representing the adsorption of sulfurous acid molecule on as-prepared CNTs.

with silica nanoparticles [173]. Response and recovery time can be improved by using microheaters under CNT layer, U.V. light exposure or increasing carrier gas flux [190,191]. Furthermore, carbon nanotubes are not stable at high temperature in open atmosphere that limits the operating temperature of CNTs to ∼200 ◦ C. Stability of the sensor response needs to be addressed to bring the CNT sensors in practical use. With scientific and technological developments carbon nanotubes will bring revolution in the gas sensors field.

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Toxic gases emitted by burning of fossil fuels are of primary concern. To save the environment and human population from the hazards caused by the gases emitted by burning of fossil fuels, detection of these gases is foremost essential. Physical and chemical properties of carbon nanotubes have been discussed with a view of gas sensor applications. In the defect free carbon nanotubes, changes in the barrier potential of CNT–metal contact or CNT–CNT junctions produce change in the resistance of CNTs. Mechanisms underlying the adsorption phenomenon for oxidizing and reducing gases are described. The gases liberated on combustion of fossil fuels, i.e. SO2 , NO2 , NO, CO2 and CO adsorb on the CNT surface either via physical or chemical adsorption. Chemical functionalization improves sensitivity and selectivity of CNT gas sensor when compared with pristine CNTs due to increased number of defect sites. Charge transfer mechanisms between adsorbed gas molecules and CNTs have been elucidated with illustrations. Better understanding of adsorption mechanisms for gas molecules on CNT surface and their manipulation by attaching selective chemical groups may be considered as a foundation for the prospective research. Issues like reproducibility, durability, response and recovery time still need to be addressed. Acknowledgments Authors would like to acknowledge Dr. Chandrashekhar, Director CSIR-CEERI, for his support and encouragement. We are also greatful to Council of Scientific and Industrial Research (CSIR), India for providing grant under the Project No. PSC0102. References

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Biographies

M. Mittal received her M.Sc. degree in Physics and Electronics and M. Tech degree in Microelectronics from Panjab University, Chandigarh, India in 2010 and 2012, respectively. Presently, she is working as senior project fellow in Sensors and Nanotechnology group, Council of Scientific and Industrial Research-Central Electronics and Engineering Research Institute, Pilani, Rajasthan, India. Her current research interests include fabrication of carbon nanotube and quantum dots based electronic devices.

A. Kumar obtained M. Tech. degree in solid-statematerials from IIT Delhi in 1982. Since December 1982, he is working as scientist at Central Electronics Engineering Research Institute, Pilani, India. He is also a faculty (Professor) at academy AcSIR. He is well experienced in semiconductor devices and fabrication technology. His current research interests are nanoelectronic devices, novel silicon PV, CNT based nanosensors, and nanofabrication. He has authored more than 50 research papers in journals and conferences, and supervised several UG/PG students’ dissertations. He has visited Canada and Germany. He is a fellow of IETE, life member of IPA and SSI (India).