Summary of metal oxide NPs used to modify CNTs for gas sensor applications.
A gas sensor is a device that when exposed to gaseous species, is able to alter one or more of its physical properties, so that can be measured and quantified, directly or indirectly. These devices are used for applications in homeland security, medical diagnosis, environmental pollution, food processing, industrial emission, public security, agriculture, aerospace and aeronautics, among others. Desirable characteristics of a gas sensor are selectivity for different gases, sensitivity at low concentrations, fast response, room temperature operation (some applications may require high temperature), low power consumption, low-cost, low maintenance and portability. Traditional techniques like gas chromatography (GC), GC coupled to mass spectrometry (GC-MS), Fourier transmission infrared spectroscopy (FTIR) and atomic emission detection (AED) provide high sensitivity, reliability and precision, but they are also bulky, time consuming, power consuming, operate at high temperature, and the high maintenance and requirement of trained technicians translate in high costs. In an effort to overcome those disadvantages, research in the area has been focused on the search for functional sensing materials.
Carbon nanotubes (CNTs) have been have been focus of intense research as alternative sensing material because of their attractive characteristics like chemical, thermal and mechanical stability, high surface area, metallic and semi-conductive properties and functionalization capability . CNTs are graphene sheets rolled in a tubular fashion. Different types of CNTs can be synthesized: single walled carbon nanotubes (SWCNT), double wall carbon nanotubes (DWCNTs) and multi walled carbon nanotubes (MWCNT).
The publication of the first CNT-based sensor for NH3 and NO2 detection using an individual semiconducting SWCNT  triggered the research activity in this area. Pristine CNTs have shown to be chemically inactive to gas molecules in general. However, their modification/functionalization capability has been exploited throughout the last years, especially for the development of devices with enhanced selectivity and sensitivity for the room temperature detection of a wide variety of gases. Numerous articles and reviews focused on different aspect of CNTs-based sensors and summarizing their progress and potential have been published throughout the years. Some of them are listed in references [3-34]. A most recent review  addressing the technological and commercial aspects of CNTs sensors presents evidence of the continuous active research in the area and that they have real potential to complement or substitute current technologies.
This chapter presents a summary of selected original research articles that have been published between 2010 and present in which the main subject are modified/functionalized SWCNTs, DWCNTs and MWCNTs and their use as gas sensing material. The majority of the references included in this chapter content are based on experimental results. However, theoretical studies based on computational science are also included because of their importance in the study of CNT-based sensors. The use of different methods of calculations and simulation has been useful to design new structures and materials and to study, evaluate and predict the interactions and adsorption energies between those materials and gaseous molecules. First, we present current research activities on pristine CNTs-based sensors and the different approaches used to improve their sensitivity and selectivity without modifying the CNTs structure, followed by the review of CNTs modified with conducting polymers, metallic nanoparticles (NPs), nanostructured oxides and sidewall modification, doping and others. Different modification/functionalization techniques like chemical deposition, plasma, sputtering and electrodeposition are discussed. Gas sensors based on changes of electrical conductivity caused by adsorption of gas molecules (resistors) are the most common sensor type discussed in this review. Other sensing platforms like surface acoustic wave (SAW) and quartz microbalance (QMB)are also included.
2. Unmodified carbon nanotubes
Pristine CNTs are known for their high stability because of their strong sp2 carbon-carbon bonds and thus insensitive when used as sensing material for certain gases. However, the detection of NO, NO2 and NH3 has been previously reported. In order to improve their sensitivity and recovery time, different approaches like dispersion techniques to debundling the CNTs ropes, humidity assisted detection, application of an electric field, continuous use of ultraviolet (UV) light and even separation of semi-conductive types from conductive have been reported. The detection of NO, NO2 and NH3, as well as other gases like formaldehyde and dimethyl methylphosphonate (DMMP) with pristine SWCNTs are discussed in this section.
A MWCNTs based sensor was used for the detection of 50 ppm of nitrogen monoxide (NO) . With the purpose to increase the sensitivity, an electric field was applied between two copper plates as electrodes, one of them containing MWCNTs-silicon wafer. It was found that when a positive potential was applied to the copper plate and the negative potential applied to the copper plate containing the MWCNTs-sensor, NO, being an electron acceptor, moves to the electron enriched zone, which is the one containing the MWCNTs-sensor and thus enhancement in the sensitivity is observed. The stronger the applied electric field, the better the sensitivity. However, applying a negative electric field was applied to the copper plate and a positive potential was applied to the copper plate containing the MWCNTs-sensor, the NO molecules moved away from the MWCNTs sensor and thus a decrease in the sensitivity is observed. The more negative the applied electric field, the lower the sensitivity of the sensor. Recovery of the sensors was achieved by applying reverse potential from the one used to perform the gas sensing experiments.
Cava, et al. proposed the use of a homogeneous film of MWCNTs prepared by the self-assembly technique and use it as an active layer for an oxygen gas sensor with increased sensitivity . When the sensors were exposed to 10% O2 in Nitrogen at 160 oC, the electrical resistance decreased and showed a better oxygen sensitivity when compared to sensors prepared under the same condition but using the drop-cast method. The reason for this is that the self-assembly technique provides a better distribution of the nanotubes and thus promoting a better gas adsorption between nanotubes (inter-tube contact).
The high van der walls attraction between CNTs causes them to remain in bundles or agglomerated. This can represent a problem for their application as gas sensors because it results in less adsorption/interaction (binding) sites, which translates in less sensitivity. Considering this, different dispersion techniques were used by Ndiaye and coworkers for the preparation of CNTs based sensors for NO2 detection . SWCNTs were dispersed in a surfactant, sodium dodecylbenzene sulfonate (NaDDBs) and an organic solvent, chloroform (CHCl3), drop-casted in IDEs and tested for the detection of 50, 100, 120, 200 ppb of NO2 at 80 oC. Sensors prepared with SWCNTs dispersed in NaDDBs showed better sensitivity than those with SWCNTs dispersed in chloroform. The explanation to this is that the surfactant was more effective in debundle the SWCNTs than the organic solvent. It was stated that even though the surfactant was not completely removed after several rinsing steps and heating treatment at 150oC, it does not have significant effect on the electronic behavior of the sensor. Both surfactant-dispersed and organic solvent-dispersed samples showed a decrease in resistance with increasing temperature, which demonstrate the semi-conductive behavior of the SWCNTs and thus no effect of the solvent.
A SWCNT-based gas sensor selective for NO2 and SO2 at room temperature and ambient pressure was developed by Yao
Battie and coworkers used sorted semi-conducting SWCNTs as sensing film for the detection of NO2 and NH3 . The density gradient ultracentrifugation (DGU) technique was used to separate semi-conducting from as produced SWCNTs. Films of as produced and sorted semi-conducting SWCNTs were exposed to NO2 and NH3 in air. Both films showed decrease in resistance and increase in resistance as exposed to NO2 and NH3, respectively. Full recovery was achieved by applying heat after NH3 exposure and vacuum after NO2 exposure. However, semi-conducting SWCNTs films were more sensitive for NH3 than to NO2 at different concentrations at ppm level.
3. Surface modified carbon nanotubes
CNTS modified with different functional groups have been used for the development of sensors for detection of volatile organic compounds (VOCs) in the environment as well as in exhaled breath. For the detection of VOCs in air, Wang
Two different CNT-based sensor arrays have been reported for the detection and pattern recognition of VOCs present in exhaled breath samples for medical diagnosis, Tisch
SWCNTs were functionalized with tetrafluorohydroquinone (TFQ) at the room temperature for detection of dimethyl methylphosponate (DMMP) at parts per trillion (ppt) levels (Figure 2) . The conductance of the TFQ-SWCNTs samples increased as function of concentration when exposed to DMMP in N2 in a concentration range from 20ppt to 5.4ppb. Sensors showed fast response and ultra sensitivity down to 20ppt when compared to unmodified SWCNTs sensor, which had a DL of nearly 1 ppm. The presence of TFQ clearly showed to improve the sensitivity and this is because it provides additional binding sites thru hydrogen bonds between hydroxyl groups in TQF and DMMP. In addition, TQF tailors the electronic properties of SWCNTs via hole doping.
MWCNTs were oxidized with KMnO4 to add oxygenated functional groups, mainly (-COOH) for the detection of organic vapors . Oxidized MWCNTs in form of a bucky paper were exposed to different concentrations of acetone. Variations in electrical resistance were recorded for both unmodified and oxidized MWCNTs. Oxidized MWCNTs showed higher sensitivity to acetone (2.3 vol. %) than unmodified ones, and good selectivity when sensing other oxygen containing vapors such as diethyl ether and methanol. The sensors also showed complete reversibility and high reproducibility for all tested vapors.
MWCNTs were chemically treated with acid to obtain hydroxyl groups (OH) and used as sensing material for humidity sensors . Changes in resistance were measured as the RH was varied from 11% to 98%. It was observed that the resistance increased as sensors were exposed to the different humidity levels. It was found that acid treated SWCNTS were more sensitive to humidity than pristine MWCNTs. The higher sensitivity of Acid treated SWCNTs is attributed to their higher surface and thus more adsorption sites that result from the acid treatment. Sensors showed fast response and to be stable. As with most humidity sensors, the recovery time was longer than response time due to slow desorption of water molecules.
Purified MWCNTs were treated with oxygen plasma or fluorine plasma and used for the detection of ethanol (Figure 3) . Changes in resistance as function of time were recorded for the sensors when exposed of 50-500 ppm of ethanol vapor in air. Samples treated with oxygen plasma for 30 sec and with fluorine plasma for 60 sec showed the highest sensitivity to 100 ppm of ethanol, compared to pristine MWCNTS and other oxygen and fluorine plasma treated for different duration time. However, fluorine plasma treated samples showed the better sensitivity and reduced response and recovery time. The improvement of the fluorine plasma treated samples is explained by the difference in electronegativity between oxygen and fluorine.
Thermally fluorinated MWCNTs (TFC) were used for NO gas detection at room temperature . The effect of thermal fluorination process was performed at various temperatures (100 -1000 oC) and 200 oC was found to be the optimum fluorination temperature. TFC samples prepared at temperatures higher than 200 oC showed a decrease of the fluorine functional groups, and even fluorine-assisted pyrolysis and fluorine-induced reorientation of the MWCNTs structure occurred at 1000 oC. TFC prepared at 200 oC showed high sensitivity, stability, reproducibility and full recovery, when their gas sensing properties were evaluated towards the detection of 50 ppm NO in dry air. Interestingly, the presence of fluorine reverses the electron transfer process, when compared to pristine MWCNTs, allowing them to go from NO to MWCNTs and thus causing an increase in resistance. The fluorination not only helped to enhance the sensitivity but also made the sensors insensitive to humidity changes.
MWCNTs were modified with amino groups for the detection of formaldehyde at ppb level . Changes in resistance as function of time were measured as the sensors were exposed to formaldehyde in a concentration range between 20 and 200 ppb. Sensors containing MWCNTs with higher amino group content (18%) were 2.4 and 13 times more sensitive to formaldehyde than samples containing 5% amino groups and pristine CNTs, respectively. Short response times are due to a chemical reaction between the aldehyde and amino group. For the same reason, the recovery times are longer, since chemical desorption is a slow and irreversible process. SWCNTs with 18% amino groups showed to be selective to formaldehyde when tested against interferences like acetone, CO2, ammonia, methanol and ethanol.
Silicon (Si) nitrogen (N), and phosphorous-nitrogen (P-N) were used to modify MWCNTs and study their gas sensing properties for hydrogen peroxide, sodium hypochlorite and1, 2-dichloromethane, nitrogen, and ammonia . Samples of Si-MWCNTs, N-SWCNTs, and P-N-SWCNTs were prepared by aerosol chemical vapor deposition. It is known that the incorporation of heteroatoms in the CNT structure changes its morphology and thus the reactivity. To evaluate the gas sensing properties of the prepared materials, changes in resistance as function of time were recorded when they were exposed to the different gases. Exposure to N2 caused the removal of physisorbed water molecules and thus a decrease in the resistance values. Sodium hypochlorite and dichloroethane caused decrease in resistance of pristine MWCNTs and Si-MWCNTs due to charge transfer (electrons) from CNTs to Chlorine atoms and increase in resistance of N-P MWCNTs. Ammonia showed the opposite effect in resistance. These results demonstrate the p-type semiconductor behavior for pristine MWCNTs and Si-MWCNTs and n-type of N-MWCNTs and N-P-MWCNTs. All sensors recovered in 10 min for all gases with the exception of ammonia that exceeded 1 hour.
In an effort to enhance the selectivity of SWCNTs-based vapor sensors, Battie
Computational studies based on SWCNTs doped with heteroatoms have been also reported.
|SnO2||MWNTs||RT, 150||NO2||1ppm||3min (150C)|
4. Conducting polymers and CNT composites
Conducting polymers have been widely used to enhance the sensing properties of CNTs-based sensors. CNTs unique characteristics combined with polymer’s delocalized bonds, high permeability and low density have demonstrated that it is possible to detect many different gases with high sensitivity, fast response and good reproducibility. Previous reports on polymer/CNTs-based sensor have been summarized and reviewed [7-9, 12]. However, some challenges to overcome are aggregation or agglomeration of CNTs, thermal stability and selectivity, among others. Polymer/CNTs composites used in resistors, SAW and QMB type of sensors are discussed.
CNTs were used to improve Polyaniline (PANi) poor thermal stability (Figure 4) . The proposed solution to this was the uniform incorporation of CNTs in the polymer network. Considering that one of the problems of MWCNTs is their aggregation and agglomeration, they were oxyfluorinated under different conditions in order to obtain a better dispersion in aqueous solution and it was found that the better dispersion was obtained with the MWCNTs with the highest oxygen content. The oxyfluorinated MWCNTs were then mixed with aniline and ammonium persulfate (APS) and other chemicals, for in-situ polymerization. Changes in resistance as function of time were used to characterize and evaluate the resulting PANi/MWCNTs composite was for the detection of ammonia (NH3) in a concentration range of 1-50 ppm. PANi/MWCNTs composite with the highest oxygen content had a uniform composition, improved thermal stability and highest and faster response for 50 ppm of NH3. The composite was able to detect 1 ppm and showed excellent repeatability for cycling exposures to 50 ppm and it needed heat treatment to accelerate the NH3 desorption and thus the recovery of the sensor. A possible drawback is the selectivity to NH3 among gases that can extract protons (H+) from PANi.
Another SAW gas sensor was reported by Viespe
Biopolymer/CNTs composites for chemical vapor sensors were produced by using two different biopolymers, cellulose, the most naturally abundant one and poly (lactic acid) (PLA). Considering that previous studies have shown that a homogeneous distribution of MCNTs in the cellulose matrix can improve the polymer’s mechanical and electrical characteristics, MWCNTs were functionalized with imidazolide groups and covalently attached it to cellulose chains . The resulting material, a paper-like film, was then used as sensing element for the detection of methanol, ethanol, 1-propanol and 1-butanol at ppm levels. Responses were measured by changes in resistance and were found to be reversible and consistent for all the tested vapors. However, the sensor only showed linear responses as function of concentration for 1-propanol in the range of 400-3600 ppm. The other composite, PLA/MWCNTs was prepared by doping the biopolymer with2 and 5% of MWCNTs and annealing, in order to understand the effect of MWCNTs in the crystallinity of the polymer and its performance in the detection of toluene, water, methanol and chloroform . PLA/2%-MWCNTs showed highest responses for all gases, when compared to PLA/3%-MWCNTs. However, it was found that all samples were selective to chloroform. Moreover, annealing the samples showed a decrease in the responses that were significantly lower than the untreated ones. Annealing did not affect the selectivity to chloroform but it considerably affected its sensitivity.
Plasma-treated MWCNTs (p-MWCNTs) polyimide (PI) composite films (p-MWCNTs-PI) were developed by Yoo
Yuana and coworkers prepared a sensor array based on polymer/MWCNTs composites for the selective detection of chloroform (CHCl3), tetrahydrofuran (THF) and methanol (MeOH) . Ethyl cellulose (EC), poly [methyl vinyl ether-alt-maleic acid] (PMVEMA), hydroxypropyl methyl cellulose (HPMC), poly (alpha-methylstyrene) (PMS), poly (vinyl benzyl chloride) (PVBC) and poly (ethyleneadipate) (PEA) were the polymers used to prepare the polymer/MWCNTs composites and to provide uniqueness to each sensor in terms of their physical and chemical characteristics like molecular structure, polymer length, polarity and intermolecular forces. Changes in resistance as function of time were recorded for the sensor array when exposed to the different gases at different temperatures (30 40, 50, 60 oC) and 50-60% R. H. The sensors showed to be selective to the three gases at the different temperatures when in presence of vapor molecules of chloride, cyclic oxide and hydroxide groups. The decreasing order of sensitivity concurred with the order of decreasing conductivity: PEA/MWCNTs >EC/MWCNTs >PMVEMA/MWCNTs >PVBC/MWCNTs >HPMC/MWCNTs >PMS/MWCNTs.
SWCNTs modified with Poly- (D) glucosamine (Chitosan) (SWCNTs-CHIT) were as high performance hydrogen sensor . Three types of sensors were prepared: SWCNTs deposited on glass substrate (type 1), SWCNTs deposited over a glass substrate modified with CHIT-film (type 2), Chit-film deposited over SWCNTs deposited over a glass substrate. Each type of sensor showed a different changes in resistance when exposed to 4% H2 in air. Increase in resistance was observed for three types of sensors and good recovery but for type (3) sensor. Sensors modified with CHIT showed better sensitivity. Moreover, the authors explained that the improved sensitivity was far higher than that reported for Pd-SWCNTs based sensors, which are commonly used for H2 detection. The enhanced performance of SWCNTs-CHIT can be explained by the strong interaction/ binding of the H2 molecules with/to the –OH and –NH3 groups contained in CHIT, which also explains the poor recovery of type 3 sensor.
SWCNTs were modified with Polypyrrole (PPy) and 5, 10, 15, 20-tetraphenylporphyrine (TPP) to prepare composites for the detection of 1-butanol in nitrogen using a quartz microbalance (QMB) . The QMB gold electrodes were coated with PPy/SWCNTs-COOH, PPy/SWCNTs-COOH/SWCNTs-TPP via electropolymerization. Frequency shifts of the quartz crystal resonator as function of time were measured for the sensors when exposed to 1-butanol in ppb concentration range. Even though both composites showed good and higher response magnitudes than QMB prepared with other composites that did not contain CNTs, PPy/SWCNTs-COOH/SWCNTs-TPP showed better performance than PPy/SWCNTs-COOH. The results demonstrate that the incorporation of CNTs enhanced the sensitivity towards the detection of 1-butanol.
|PECH, PEUT, PIB||MWCNTs||SAW||Toluene||1.7-12.2ppm||[70 72]|
|EC||MWCNTs||Resistor||THF, CH3Cl2, MeOH||NS|||
5. Metal nanoparticlesdecorated CNTs
Electronic, physical and chemical properties of metallic nanoclusters are usually sensitive to the changes in environment . CNTs decorated with metallic nanoparticles (NPs) have been widely used to achieve selectivity and improve the sensitivity, response time and DLs for a variety of gas detections. Layer by layer, electrodeposition, chemical deposition, electrochemical deposition and sputtering are the methods used to prepare the metallic NP-CNTs composites discussed in this section.
SWCNTs films were modified with Pd NPs using sputtering method . After apply different deposition times (40 – 160 s), it was found that 120 s was the optimum deposition time to obtain enhanced sensor response for 1% H2 in dry air at 50 oC. A typical response curve (igas/iair vs. time) showed differences in response and recovery between the first and following H2 sensing cycles. FTIR studies were used to support and explain those differences and mechanisms of detection. The first cycle showed an overall larger electrical current in the presence of H2and then it reached a new steady state. When the atmosphere was change to dry air, the current did not go to its original value but remained in the steady state, which is considered as an irreversible response. The explanation to this is that is atomized by the Pd NPs and spilled to the surface of the MWCNTs, occurring the chemical and irreversible reaction of hydrogenation of the carbonyl groups of the MWCNTs at the first cycle. The second cycle and following ones started at the steady state where the first cycle finished and the electrical current showed a decrease in the presence of H2 and when the atmosphere was changed to dry air, the electrical current recovered back to where the cycle started. This reversible behavior is explained as physisorption of H2 molecules onto Pd/SWCNTs.
Pd/MWCNTs and Pt-Pd/MWCNTs composites were tested for the detection of H2 in a concentration range of 20 ppm– 2% in N2 and 200 ppm – 2% in air . Composites were prepared by growing CNTs yarns and then covered them with a layer of Pd NPs or sequentially deposited layers of Pd and Pt NPs, using a recently developed technique called self-fuelled electrodeposition (SFED). Exposure to 1% H2using N2 with 1% air as carrier gas. As with other Pd/CNTs-based sensors , an initial irreversible drop in resistance was observed, and after that, the sensor reached a steady state. A stable baseline was established just after a couple of exposure/recovery cycles. Pd-MWCNTs was not able to detect H2 concentrations below 20 ppm but with the Pt-Pd/MWCNTs composites it was possible to detect concentrations as small as 5 ppm (0.0005%). The sensor saturated at 100 ppm and higher concentrations. When the same experiments were performed using air as carrier gas, it was found that the detection limit for both composites decreased.
A Pd/MWCNTs flexible substrate H2 sensor was fabricated using the layer-by-layer technique . For this, an Au IDE was sputtered over a polyester (PET) film, followed by the fabrication of a poly (4-styrenesulfonic acid-
Pd/SWCNT composites were prepared by using a poly (amido amine) (PAMAM) dendrimer assisted synthesis, followed by a pyrolysis step to remove the dendrimers . For H2 sensing experiments, the samples were deposited over Ti/Au electrodes and changes in resistance as function of time were measured. The Pd/SWCNTs samples showed to be more sensitive to 10, 000 ppm of H2 at room temperature, when compared to samples of chemically reduced Pd on SWCNTs (without the presence of dendrimer) and Pd/PAMAM-SWCNTs. Moreover, these samples were able to detect all of the concentrations in a 10-100 ppm range. It was concluded that not only the dendrimers provided more nucleation sites for the Pd NPs and thus higher NPs density, but also that the removal of the dendrimers thru the pyrolysis step reduces the distance between the NPs and the SWCNTs and consequently reduces the delay in electron transfer, allowing faster response times.
Double wall carbon nanotubes (DWCNTs) have shown to have longer length, compared to SWNTs, provide a better percolation behavior, the possibility of modifying the outer layer without modifying the inner one and flexibility are some of their attractive characteristics. Considering those characteristics, Rumiche
Another H2 sensor based on N-doped MWCNTs electrochemically decorated with Au NPs (Au-NMWCNTs) was presented by Sadek and collaborators . N-doped MWCNTs were chosen because they have enhanced surface reactivity and chemically active sites for the nucleation of Au NPs during the electrodeposition process. NPs of different sizes were obtained with variation of electrodeposition potential. Changes in resistance as function of time were used to evaluate the performance of the Au-NMWCNTs sensors when exposed to different H2 concentration between 0.06% and 1%. Sensitivity, response time and recovery time highly depended on the size of the AuNPs: the smallest the size the better the sensitivity and the shorter the response and recovery times.
Theoretical studies have been used to study the interaction between Pd and Pt NPs decoratedCNTs and a wide variety of gases for different applications. Zhou
|Pt, Ru, Ag||SWCNTs||CO2,CH4, NH3,NO2||Resistor||100ppb NO2||[89 , 90]|
Li and co workers investigated the adsorption of CO and NO on SWCNTs-decorated with Pd and Pt using first-principle calculations . It was found that the electronic properties of SWCNTs change upon modification with Pd or Pd atoms. The semi-conductive band gap is decreased compared to pristine SWCNTs. The reason for the observed decrease in the band gap is due to charge transfer from the Pd and Pt atoms to the surface of the SWCNTs. Different from pristine SWCNT that show poor adsorption, Pd-SWCNTs and Pt-SWCNTs showed to chemisorb CO molecules as well as NO. However, Pt-SWCNTs showed bigger binding energy and charge transfer than Pd-SWCNTs. The formation of C-Pd, N-Pd, C-Pt, and N-Pt bonds demonstrate that the metal atoms provide additional adsorptions sites for gases and open the possibility to use both materials as sensors for the detection of CO and NO.
6. Nanostructured oxides mixed with CNTs
Sensors made of metal oxides films have been used for a long time because of they provide high sensitivity for the detection of a wide variety of gases. However, their major drawback is their elevated operating temperatures. The development of metal-oxide NPs based films and nanocomposites has shown advantages like higher surface area and porosity, high catalytic activity, efficient charge transfer and adsorption capacity. However, it has been demonstrated that the improvements in gas detection at low temperature for CNTs/MO-based sensors is due to the introduction of CNTs in the nanocomposite.
Tin oxide (SnO2)/MWCNTS were synthesized using different ratios of tin dioxide precursor and plasma treated MWCNTs (Figure 6) . The composites was tested for 2, 10, 20 ppm for CO and 50 100, 500, 1000 ppb of NO2 in dry air at both room temperature and 150 oC. Pure tin oxide films and pure plasma treated MWCNT were also tested for comparison purposes. Pure tin oxide films were unresponsive to all of the tested concentrations at the two different temperatures because both room temperature and 150 oC are too low when compared to the operation temperature for pure tin oxide-based sensors. On the other hand, pure plasma treated MWCNT responded to both gases at room temperature but not at 150 oC. As for the SnO2/MWCNTS composite, higher sensor response for both gases was achieved from samples prepared with an intermediate ratio of tin dioxide precursor and plasma treated MWCNTs (i. e. 20mL and 12mg, respectively), especially when operated at room temperature. Response time for 1 ppm of NO2 was 3minutesat 150 oC and 4 minutes at room temperature. Response time for 2 ppm of CO is stated to be 5 minutes but the temperature was not specified. SnO2/MWCNTS showed higher sensor response to NO2 than to CO, and was also sensitive to humidity changes.
Different composite synthesis temperature can affect the sensor performance. SnO2/SWCNTs composites were synthesized at different oxidizing temperatures (300-600 oC)for testing the effect of temperature in their morphology, structure and gas sensing properties in the detection of NO
MWCNTs treated with nitric acid were used to fabricate MWCNTs-doped SnO2sensors for the detection of ethanol and liquid petroleum gases (LPG) . Sensors were tested to 100-1000 ppm of ethanol and 1000-10, 000 ppm of LPG at different operation temperatures in the range of 10-360 oC. The detection of both chemicals was improved when the operating temperature was 350 oC or lower and it was determined that the optimum operating temperature is 320 oC. The MCNTs-doped SnO2 composite showed better selectivity for LGP than for ethanol and the calibration curve showed the sensors saturated at concentrations higher than 5000 ppm. The90% response and recovery time were 21s and 36s, respectively. When undoped SnO2 sensors were exposed to 250 ppm of ethanol and 2500 ppm of LPG, they showed higher sensitivity to ethanol than to LPG at operation temperature range of 190 - 360 oC. Considering the obtained results, the selectivity of the MCNTs-doped SnO2 composite for LGP can be attributed to the presence of MWCNTs but further studies are required.
Nanocomposite structures of cobalt oxide (Co3O4) and SWCNTs were prepared using a polymer assisted deposition (PAD) method . For this, polyethyleneimine (PEI) was the polymer used to bind the cobalt ions from and adjust the viscosity of the solution during the deposition process in order to get a homogeneous distribution of the particles on the SWCNTs thin film. The Co3O4/SWCNTs composite sensor was tested for the detection of NOx in a concentration range of 20-100 ppm at room temperature. It showed proportional increases in response as function of concentration, poor recovery at room temperature and good recovery at 250 oC. Higher responses of the Co3O4/SWCNTs composite when compared to pristine CNTs are attributed to the high adsorption power of Co3O4 particles. The composite was also exposed to 4% of H2 in air, and showed enhanced responses than pure SWCNTs at room temperature and than Co3O4 films at both room temperature and 250 oC.
MO NPs (ZnO, SnO2, TiO2) and MWCNTs composites were simultaneously grown on silicon and silica on silicon substrates by catalytic pyrolysis method and used for gas sensing . Current differential-voltage (∆I-V) curves were recorded for all the prepared composites, while to 100 ppm ethanol. TiO2/MWCNTs showed better sensitivity (defined as ∆I/I-V) when compared to pure MWNT film, ZnO/MWCNTs and SnO2/MWCNTs.
N-doped, B-doped and O-doped CNTs were used to prepare doped-CNTs/SnO2hybrids . All doped-CNTs and doped-CNTs/SnO2hybrids were used to study the effect of functional groups on their gas sensing properties for 100, 200, 500, 1000ppb of NO2 at room temperature. The responses were as follows: B-doped hybrid> N-doped hybrid>O-doped hybrid. All doped-CNTs/SnO2hybrids responded better than N-doped and B-doped and O-doped CNTs. B-doped-CNTs/SnO2 hybrids showed an improvement in the response time when compared to bare CNTs and recovered its baseline, which was not achieved with B-doped CNTs. The high sensitivity and improved performance achieved with the B-doped and N-doped-CNTs/ SnO2 hybrids for low concentrations of NO2 at room temperature are attributed to two main factors: the interaction of the N2 gas with the n-SnO2/p-CNTs hetero-structure that affects the conduction of the CNTs and the addition of new functionalities (i. e. B and N atoms) to the CNTs surface that affects the electronic density of states and Fermi level and consequently, its conductivity.
A combination of ZnO layer with functionalized MWCNTs for the room temperature detection of NH3 has been reported by Tulliani and coworkers . Samples of Pd-doped/COOH-MWCNTs, N-MWCNTs, and F-MWCNTs were deposited over a screen-printed ZnO layer. The materials were evaluated by measuring changes in resistance as the sensors were exposed to NH3 at room temperature, in a concentration range 0-75 ppm and different relative humidity levels. The sensor based on ZnO with Pd-doped/COOH-MWCNTs was the only one that showed sensitivity to humidity. When exposed to NH3, all sensors showed a decrease in electrical resistance but did not show better DL than other graphite-based sensors prepared under the same conditions.
Modification and functionalization of CNTs have shown to greatly improve the sensitivity and selectivity of CNTs-based sensors. For instance, great improvements for room temperature detection of different gases have been reported, especially when using metal oxide/SWCNTs composites. Another subject of high interest is the development of deposition methods and synthesis of Pd/CNTs for H2 detection at room temperature. Interestingly, there is an increase in the tendency of combining other materials with modified CNTs. For example, CNTs decorated with metal NPs embedded in a polymer matrix or CNTs doped or CNTs doped with heteroatoms and decorated with NPs or metal oxides are some composites that have been successfully used as gas sensing materials. But not only the characteristics of the CNTs have contributed to these improvements. In fact, the reported improvements are attributed to the combination of materials and the intrinsic characteristics of the composites. This trend of combining materials demonstrates that the there is broad range of possibilities for the design of new materials to meet the requirements of an ideal sensor by showing selectivity for different gases, sensitivity at low concentrations, fast response, and room temperature operation among others.
The authors would like to acknowledge the NASA-URC Center for Advanced Nanoscale Materials (CANM) Grant # NNX08BA48A and NNX10AQ17A, NASA GSRP fellowship under Grant # NNX09AM23H and NASA Ames Research Center-Nanosensor Group