ABSTRACT
This chapter reviews the gas sensing properties of the carbon nanotubes (CNTs) at the current state of art. The physical properties of the CNTs are an outstanding mix for advanced nanosensors at high sensitivity and very low sub-ppm level of gas detection at room temperature. Fabrication techniques of the CNTs are surveyed as well. Pristine, modified, purified, and functionalized CNTs are considered as chemically interfaced materials for gas adsorption by using various transducers as innovative platforms. Here we review the key developments of the CNTs gas sensors by means of a comparative analysis of the chemical sensing performance and outline promising applications, challenges, and future perspectives. 9.1 IntroductionRecent advances in nanoscience and nanotechnology have led to the synthesis of novel functional materials — nanotubes, nanowires,
nanocrystals, nanobelts, nanoplatelets, nanofibers, nanorods, nanowalls, nanoparticles, such as ideal building blocks for emerging nanoscale sensing devices. The ability to tailor the size and structure and thus the properties of nanomaterials offers excellent prospects for designing advanced sensing devices and enhancing the performance of gas detection. The detection of chemical and biological species is critical to many sectors of environmental air pollution control, process monitoring, healthcare, medical diagnosis, life sciences, food technology, agriculture, homeland security, safety, automotive, and aerospace industries. Hence, the development of new sensing microdevices enabled by emerging nanotechnologies could significantly impact for practical applications. Chemical microsensors based on nanostructured materials have been fabricated with highperformance operations for gas detection [1-11] of hazardous and toxic analytes and biosensing [12-17]. Carbon nanotubes (CNTs) have exceptional physical properties that make them one of the most promising building blocks for future nanotechnologies. They play a key role in the development of innovative electronic devices in the fields of ultra-high-sensitivity chemical sensors, flexible electronics, field emitters, energy sources, nanoactuators, micro-electromechanical systems (MEMS), and nano-electromechanical systems (NEMS). CNTs are one of the hottest topics in modern physics. The interest in these objects has been sparked by the exceptional properties of those nano-sized objects combined with the ease of theoretical investigations due to the relatively limited number of surface atoms in CNTs, facilitating ab initio calculations [18-21] in the chemical interactions between adsorbed gas molecules and nanomaterial. While the different allotropic forms of carbon (diamond, graphite, graphene, C60 molecules, etc.) refer to well-defined structures, the name “carbon nanotube” encompasses a large variety of different nano-objects, which differ from each other in terms of diameter, length, chirality, electronic properties, and number of shells or walls in the case of multiwalled carbon nanotubes (MWCNTs) [22]. This can be a chance for future applications since different types of nanotubes can be better suited for different types of use. For example, semiconducting individual single-wall nanotubes can serve as chemical nanosensor at high sensitivity, self-heating and
ultra-low power consumption, or alternatively as a channel in highperformance field-effect transistors. The mechanical properties of the MWCNTs with intermediate diameter (5-20 nm) are of particular interest for fabricating NEMS or MEMS. The metallic single-walled carbon nanotubes (SWCNTs) can be important as nanoscale interconnections or nanoelectrodes. The discovery of CNTs in 1991 by Iijima [23] in the format of multiwalled nanotubules, and then in 1993 [24] his design and fabrication of CNTs in the format of single-walled nanotubes with a diameter of 1 nm, has generated a great interest among worldwide researchers to explore their unique mix of electrical, optical, thermal, mechanical, physical, and chemical properties to develop high-performance sensing devices. CNTs may be considered as graphene cylinders [25]. Graphene is a single planar sheet of sp2-bonded carbon atoms. Graphite consists entirely of individual graphene layers, which are stacked on each other. The properties of graphite already give a first hint toward the remarkable properties of CNTs, such as high mobility of the order of 104 cm2V-1s-1 at room temperature, in-plane electrical resistivity as low as ranging 1-50 µΩ.cm, thermal conductivity of graphite as high as 500 Wm-1K-1. The basic structure of CNTs in the geometry of SWCNTs can be thought of as a rolled-up graphene sheet in which the edges of the sheet are joined together to form a seamless tube. By changing the direction in the roll-up, different chiralities can be created. Furthermore, several tubes of different diameter can be fitted into each other to make MWCNTs. These consist of concentric cylindrical shells of graphene sheets coaxially arranged around a central hollow area. The diameter of the single nanotubules is at very narrow size distribution of 1-5 nm. The individual CNTs can be either metallic or semiconducting depending on their chirality and diameter, thus they electrically behave as a metal or semiconductor. For sensing applications, the semiconducting CNTs are useful for gas detection. In fact, exposure to target chemical species generally shifts the Fermi level in the individual semiconducting CNTs and determines also an intertube change modulation based on the hopping mechanism in the CNTs networked films with subsequent changes in the electrical conductance of the individual nanotubes and nanotube-based layers [26-28].
