Carbon Nanowalls: A Potential 2-Dimensional Material for Field Emission and Energy-Related Applications

Carbon, an abundant material in the earth crust, is also the most attracting, in particular, owing to variety of fascinating materials it forms. It can appear as a transparent crystal (such as diamond), but also as black amorphous soot. It is associated with a rich and diverse chemistry. Carbon materials, in general, can be classified into different dimensional categories such as three-dimensional (3-D), two-dimensional (2-D), one-dimensional (1-D), and zero-dimensional (0-D) depending on the relative sizes in different spatial directions. Fullerenes are 0-D whereas carbon nanotubes (CNTs), nanofibres, or nanorods are 1-D nanostructures. Graphite nanosheets or nanowalls are considered as 2-D structures. A great attention has been given to the 0-D and 1-D carbon nanostructures, but studies on the growth and the properties of 2-D carbon nanowalls (CNWs) are not so abundant as in the case of fullerene or CNTs. The CNWs are very promising class of 2-D nanomaterials since the CNWs are characterized by an open boundary structure. On the other hand, fullerene and CNTs are closed boundary structures. Each CNW is made of several graphene sheets stacked over each other. The CNWs have a large surface area, which makes them very attractive for various potential applications such as chemical and biosensors or energy storage devices. Moreover, the CNWs have sharp edges normal to the substrate which make them very useful for field emission applications. In this chapter, growth of CNWs by various methods is discussed with an emphasis on plasma-enhanced chemical vapor deposition (PECVD) method. This is followed by the morphological and structural characterization of the CNWs by different techniques. The formation mechanism of CNWs will be described. In addition, properties of CNWs in respect to their potential applications in field emission and energy related devices including lithium ion batteries, fuel cells and solar cells, is also reviewed in the light of their unique morphology and structure.