Future Perspective for Emerging Applications Using Carbon Nanowalls

Abstract

Self-organized graphite nanostructures composed of graphene have been studied intensively. Carbon nanowalls and related sheet nanostructures are layered graphenes with open boundaries. The sheets form a self-supported network of wall structures with thicknesses in the range from a few nanometers to a few tens of nanometers, and with a high aspect ratio. The large surface area and sharp edges of carbon nanowalls could prove useful for a number of different applications. Electron field emitters and electrodes for fuel cell would be promising applications. To date, carbon nanowalls and related materials have been grown using various CVD methods. The morphology and structure of carbon nanowalls depend on the source gases, pressure, process temperature as well as the type of plasma used for the growth of carbon nanowalls. Isolated nanosheets, vertically standing nanowalls with a maze-like structure, highly branched type, and a kind of porous film have been fabricated. We can expect a wide variety of applications based on their structure or morphology, as illustrated in Fig. 8.1. Dense, highly branched carbon nanowall films with extremely large surface area are used for gas storage application. For this purpose, vertical alignment and crystallinity of carbon nanowalls can be less crucial, while low-temperature growth with relatively high growth rate may be important for the practical use. In the case of the application for membrane filter, honeycomb structure with controlled spacing is required. For this application, some methods for exfoliation of carbon nanowalls from the substrate to obtain free-standing membrane, or attachment of carbon nanowall membrane to the different materials should be developed. For the electron emitters, carbon nanowalls with atomically thin edges, moderate spacing, and excellent height uniformity are required. For the practical use of carbon nanowalls in the electron emitter application, evaluations of stability and lifetime of emitters are also important.