ABSTRACT

Carbon nanowalls (CNWs) are basically two-dimensional graphite sheets that are typically oriented vertically on a substrate. Each CNW originates from the stacking of several graphene sheets. A more detailed structure of each CNW exhibits domain structure that consists of nanographite domains that are several tens of nanometers in size. Such unique morphology and structure of CNWs have attracted much attention for various potential applications such as electronic devices and energy devices. In this chapter, the structural characterization of CNWs by Raman spectroscopy and transmission electron microscopy has been discussed. In addition, potential applications of CNWs as negative electrodes in lithium ion batteries and as catalytic supports in fuel cells have been reviewed in light of their unique structure.

6.1 IntroductionCarbon nanostructures such as fullerenes and carbon nanotubes possess unique structure and dimensionality, leading to new mechanical, chemical, and electronic properties. Recently two-dimensional carbon nanostructures such as graphene sheets and carbon nanowalls (CNWs) have attracted much attention for various potential applications. Graphene is a monatomic layer of carbon atoms arranged on a honeycomb lattice, and an ideal two-dimensional carbon material. Graphene possesses many extraordinary properties. Graphene was reported to exhibit many superior properties such as high ballistic electron mobility (>200,000 cm2V−1s−1 for particular samples) [1], high thermal conductivity (5000 W m−1K−1) [2], Young’s modulus (approximately 1150 GPa), fracture strength (125 GPa) [3], and a high specific surface area (approximately 2600 m2g−1) [4]. These properties are very attractive for various applications such as electronic and energy devices. CNWs were first reported by Wu et al. in 2002 [5]. They observed wall-like carbon nanostructures perpendicular to a substrate, and named them CNWs. Since this report, a lot of groups have carried out the synthesis of CNWs mainly by plasma-enhanced chemical vapor deposition (PECVD) [6−8] and their characterization mainly by scanning electron microscopy (SEM), transmission electron microscopy (TEM) [9,10], and Raman spectroscopy [8,11]. TEM studies revealed that CNWs are basically two-dimensional graphite sheets with an average thickness of several nanometers, which are typically oriented vertically on a substrate. Each CNW originates from the stacking of several graphene sheets. This suggests that CNWs might exhibit similar properties as graphene. A more detailed analysis by TEM showed that each CNW has a domain structure which consists of nanographite domains which are several tens of nanometers in size [10]. Such unique morphology and structure of CNWs have stimulated not only fundamental studies on transport properties [12−15] and hydrogen absorption [16−18] but also various applications to electron field emitters [19−23], negative electrodes in lithium ion batteries [24,25], and catalyst supports in fuel cells [26−28]. In this chapter, the structural characterization of CNWs by Raman spectroscopy and TEM has been discussed. In addition, the potential