**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 cata‐ lytic activity, efficient charge transfer and adsorption capacity. However, it has been demon‐ strated that the improvements in gas detection at low temperature for CNTs/MO-based sensors is due to the introduction of CNTs in the nanocomposite.

**Figure 6.** Response of SnO2/MWCNTs to NO2 at (A) room temperature and (B) 150 oC. (From Leghrib *et al.* [64]. Copy‐

Wongchoosuk, *et al.* were the first to report the preparation of MWCNT-doped tungsten ox‐ ide (WO3) thin films for H2 sensing application [65]. The thin films of MWCNT-doped WO3 and undoped WO3 (for comparison purposes) were prepared using the electron beam (ebeam) evaporation technique and exposed to 1000 ppm of H2 at different temperatures

all, MWCNT-doped WO3 thin films showed higher responses for H2 at any operating tem‐ perature when compared to the undoped WO3 thin film. To demonstrate selectivity, the response of the MWCNT-doped WO3 thin films was measured in presence of H2, ethanol (C2H5OH), methane (CH4), acetylene (C2H2) and ethylene (C2H4) at ppm level concentrations

filmswere selective to H2 because they showed stronger response for H2, much weaker re‐

MWCNTs treated with nitric acid were used to fabricate MWCNTs-doped SnO2sensors for the detection of ethanol and liquid petroleum gases (LPG) [66]. Sensors were tested to 100-1000 ppm of ethanol and 1000-10, 000 ppm of LPG at different operation temperatures

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

site 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 pol‐ ymer assisted deposition (PAD) method [61]. For this, polyethyleneimine (PEI) was the pol‐ ymer 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

C. Considering the obtained results, the selectivity of the MCNTs-doped SnO2 compo‐

C was the optimum operation temperature. Over‐

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357

C. It was concluded that the MWCNT-doped WO3 thin

C. The detection of both chemicals was improved when the operating

C or lower and it was determined that the optimum operating tem‐

C. The MCNTs-doped SnO2 composite showed better selectivity for LGP

right © 2010, with permission from Elsevier. )

and operating temperature of 350 o

in the range of 10-360 o

temperature was 350 o

perature is 320 o


C). It was determined that 350 o

sponses for C2H5OH, CH4, C2H2 and insensitivity ethylene (C2H4).

(200-400 o

Tin oxide (SnO2)/MWCNTS were synthesized using different ratios of tin dioxide precursor and plasma treated MWCNTs (Figure 6) [64]. 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 o C. Pure tin oxide films and pure plasma treated MWCNT were also tested for comparison pur‐ poses. Pure tin oxide films were unresponsive to all of the tested concentrations at the two different temperatures because both room temperature and 150 o C are too low when com‐ pared 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 o C. As for the SnO2/MWCNTS composite, higher sensor response for both gases was ach‐ ieved 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 o C 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 o C)for testing the effect of temperature in their morphology, structure and gas sensing properties in the detection of NO*X* [63]. The synthesized composites were exposed to 60 ppm of NO*X* at 200 o C and it was found that the ones synthesized at 400 o C showed higher response. From here, composites synthesized at 400 o C were exposed to 30 ppm NO*<sup>X</sup>* at different operating temperatures and it was determined that the optimum operation temperature was 200 o C. Concluding that the optimum oxidizing and operating temperatures were 200 o C and 400 o C, respectively, the samples were then exposed to different concentrations of NO*X*. Under the aforementioned conditions, the SnO2/SWCNTs composite showed improved perform‐ ance for the detection of NO*X* when compared to thin films of SWCNTs or SnO2.

gap is due to charge transfer from the Pd and Pt atoms to the surface of the SWCNTs. Differ‐ ent 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

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 cata‐ lytic activity, efficient charge transfer and adsorption capacity. However, it has been demon‐ strated that the improvements in gas detection at low temperature for CNTs/MO-based

Tin oxide (SnO2)/MWCNTS were synthesized using different ratios of tin dioxide precursor and plasma treated MWCNTs (Figure 6) [64]. 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 o

Pure tin oxide films and pure plasma treated MWCNT were also tested for comparison pur‐ poses. Pure tin oxide films were unresponsive to all of the tested concentrations at the two

pared 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

C. As for the SnO2/MWCNTS composite, higher sensor response for both gases was ach‐ ieved 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 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

Different composite synthesis temperature can affect the sensor performance. SnO2/ SWCNTs composites were synthesized at different oxidizing temperatures (300-600 o

testing the effect of temperature in their morphology, structure and gas sensing properties in the detection of NO*X* [63]. The synthesized composites were exposed to 60 ppm of NO*X* at

temperatures and it was determined that the optimum operation temperature was 200 o

C, respectively, the samples were then exposed to different concentrations of NO*X*. Under the aforementioned conditions, the SnO2/SWCNTs composite showed improved perform‐

Concluding that the optimum oxidizing and operating temperatures were 200 o

ance for the detection of NO*X* when compared to thin films of SWCNTs or SnO2.

C.

C)for

C.

C and 400

C are too low when com‐

C and 4 minutes at room

C showed higher response. From

C were exposed to 30 ppm NO*<sup>X</sup>* at different operating

open the possibility to use both materials as sensors for the detection of CO and NO.

**6. Nanostructured oxides mixed with CNTs**

356 Syntheses and Applications of Carbon Nanotubes and Their Composites

sensors is due to the introduction of CNTs in the nanocomposite.

different temperatures because both room temperature and 150 o

temperature. Response time for 1 ppm of NO2 was 3minutesat 150 o

C and it was found that the ones synthesized at 400 o

was also sensitive to humidity changes.

here, composites synthesized at 400 o

o

200 o

o

**Figure 6.** Response of SnO2/MWCNTs to NO2 at (A) room temperature and (B) 150 oC. (From Leghrib *et al.* [64]. Copy‐ right © 2010, with permission from Elsevier. )

Wongchoosuk, *et al.* were the first to report the preparation of MWCNT-doped tungsten ox‐ ide (WO3) thin films for H2 sensing application [65]. The thin films of MWCNT-doped WO3 and undoped WO3 (for comparison purposes) were prepared using the electron beam (ebeam) evaporation technique and exposed to 1000 ppm of H2 at different temperatures (200-400 o C). It was determined that 350 o C was the optimum operation temperature. Over‐ all, MWCNT-doped WO3 thin films showed higher responses for H2 at any operating tem‐ perature when compared to the undoped WO3 thin film. To demonstrate selectivity, the response of the MWCNT-doped WO3 thin films was measured in presence of H2, ethanol (C2H5OH), methane (CH4), acetylene (C2H2) and ethylene (C2H4) at ppm level concentrations and operating temperature of 350 o C. It was concluded that the MWCNT-doped WO3 thin filmswere selective to H2 because they showed stronger response for H2, much weaker re‐ sponses for C2H5OH, CH4, C2H2 and insensitivity ethylene (C2H4).

MWCNTs treated with nitric acid were used to fabricate MWCNTs-doped SnO2sensors for the detection of ethanol and liquid petroleum gases (LPG) [66]. 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 o C. The detection of both chemicals was improved when the operating temperature was 350 o C or lower and it was determined that the optimum operating tem‐ perature is 320 o C. 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 o C. Considering the obtained results, the selectivity of the MCNTs-doped SnO2 compo‐ site 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 pol‐ ymer assisted deposition (PAD) method [61]. For this, polyethyleneimine (PEI) was the pol‐ ymer 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 o C. Higher responses of the Co3O4/SWCNTs composite when com‐ pared 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 o C.

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 compo‐ sites that have been successfully used as gas sensing materials. But not only the characteris‐ tics 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 re‐

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359

The authors would like to acknowledge the NASA-URC Center for Advanced Nanoscale Materials (CANM) Grant # NNX08BA48A and NNX10AQ17A, NASA GSRP fellowship un‐

der Grant # NNX09AM23H and NASA Ames Research Center-Nanosensor Group

and Carlos R. Cabrera1\*

1 Department of Chemistry and NASA-URC Center for Advanced Nanoscale Materials,

[1] Niyogi S, Hamon MA, Hu H, Zhao B, Bhowmik P, Sen R, et al. Chemistry of single-

[2] Kong J, Franklin NR, Zhou CW, Chapline MG, Peng S, Cho KJ, et al. Nanotube mo‐

[3] Ciraci S, Dag S, Yildirim T, Gülseren O, Senger RT. Functionalized Carbon Nano‐ tubes And Device Applications. J Phys: Condens Matter. 2004;16(29):R901-R960.

[4] Chu H, Wei L, Cui R, Wang J, Li Y. Carbon nanotubes combined with inorganic nanomaterials: Preparations and applications. Coord Chem Rev. 2010;254(9–10):

walled carbon nanotubes. Acc Chem Res. 2002 Dec;35(12):1105-1113.

lecular wires as chemical sensors. Science. 2000 Jan;287(5453):622-625.

sponse, and room temperature operation among others.

, Jing Li2

2 NASA-Ames Research Center, Moffett Field, California, USA

University of Puerto Rico, Puerto Rico

**Acknowledgements**

**Author details**

**References**

1117-1134.

Enid Contés-de Jesús1

MO NPs (ZnO, SnO2, TiO2) and MWCNTs composites were simultaneously grown on sili‐ con and silica on silicon substrates by catalytic pyrolysis method and used for gas sensing [60]. Current differential-voltage (∆I-V) curves were recorded for all the prepared compo‐ sites, 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 [67]. All doped-CNTs and doped-CNTs/SnO2hybrids were used to study the effect of func‐ tional groups on their gas sensing properties for 100, 200, 500, 1000ppb of NO2 at room tem‐ perature. 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 Odoped 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 Bdoped 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 heterostructure 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 detec‐ tion of NH3 has been reported by Tulliani and coworkers [62]. 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 ex‐ posed 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 de‐ crease in electrical resistance but did not show better DL than other graphite-based sensors prepared under the same conditions.
