**Acknowledgements**

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

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

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)

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

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

Modification and functionalization of CNTs have shown to greatly improve the sensitivity and selectivity of CNTs-based sensors. For instance, great improvements for room tempera‐ ture detection of different gases have been reported, especially when using metal oxide/ SWCNTs composites. Another subject of high interest is the development of deposition

when compared to pure MWNT film, ZnO/MWCNTs and SnO2/MWCNTs.

C. Higher responses of the Co3O4/SWCNTs composite when com‐

C.

good recovery at 250 o

358 Syntheses and Applications of Carbon Nanotubes and Their Composites

level and consequently, its conductivity.

prepared under the same conditions.

**7. Conclusion**

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
