**1. Introduction**

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 differ‐ ent 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 cou‐ pled 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 sens‐ ing material because of their attractive characteristics like chemical, thermal and mechanical stability, high surface area, metallic and semi-conductive properties and functionalization capability [1]. CNTs are graphene sheets rolled in a tubular fashion. Different types of CNTs can be synthesized: single walled carbon nanotubes (SWCNT), double wall carbon nano‐ tubes (DWCNTs) and multi walled carbon nanotubes (MWCNT).

The publication of the first CNT-based sensor for NH3 and NO2 detection using an individu‐ al semiconducting SWCNT [2] triggered the research activity in this area. Pristine CNTs have shown to be chemically inactive to gas molecules in general. However, their modifica‐

tion/functionalization capability has been exploited throughout the last years, especially for the development of devices with enhanced selectivity and sensitivity for the room tempera‐ ture detection of a wide variety of gases. Numerous articles and reviews focused on differ‐ ent 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 [35] 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.

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-sen‐ sor, 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

Latest Advances in Modified/Functionalized Carbon Nanotube-Based Gas Sensors

Cava, et al. proposed the use of a homogeneous film of MWCNTs prepared by the self-as‐ sembly technique and use it as an active layer for an oxygen gas sensor with increased sensi‐

resistance decreased and showed a better oxygen sensitivity when compared to sensors pre‐ pared 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 pro‐

The high van der walls attraction between CNTs causes them to remain in bundles or ag‐ glomerated. This can represent a problem for their application as gas sensors because it re‐ sults 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 [38]. 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

C. 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 heat‐

sensor. Both surfactant-dispersed and organic solvent-dispersed samples showed a decrease in resistance with increasing temperature, which demonstrate the semi-conductive behavior

A SWCNT-based gas sensor selective for NO2 and SO2 at room temperature and ambient pressure was developed by Yao *et al.* [39]. High sensitivity and selectivity for 2 ppm of each gas was achieved by controlling the humidity levels. For instance, at low humidity levels, the sensors showed to be selective for NO2and insensitive to SO2. At high humidity levels (92%), both gases were detected. However, NO2 showed a decrease in resistance and SO2

Continuous *in situ* UV illumination on SWCNTs during gas sensing experiments was used to enhance the sensor's overall performance in the detection of NO, NO2 and NH3 (Figure 1) [40]. Changes in conductance (∆G/G0) as function of time were recorded and used to prepare calibration curves in order to determine sensors sensitivity. It was found that the continuous exposure of the sensors to UV light under inert atmosphere (N2, Ar) regenerating the sur‐ face, therefore, enhances their sensitivity for the detection of NO and NO2. Linear responses

C, it does not have significant effect on the electronic behavior of the

C, the electrical

http://dx.doi.org/10.5772/52173

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tivity [37]. When the sensors were exposed to 10% O2 in Nitrogen at 160 o

moting a better gas adsorption between nanotubes (inter-tube contact).

one used to perform the gas sensing experiments.

of the SWCNTs and thus no effect of the solvent.

80 o

ing treatment at 150o

showed an increase in resistance.

This chapter presents a summary of selected original research articles that have been pub‐ lished 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 impor‐ tance 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.
