Arc synthesis of double-walled carbon nanotubes in low pressure air and their superior field emission properties

Abstract

Double-walled carbon nanotubes (DWCNTs) have been effectively synthesized by direct current (DC) arc discharge in low pressure air using a mixture of Fe catalyst and FeS promoter. Compared with conventional arc methods, this method is easier to implement without using expensive high purity gas sources. A tip structural DWCNT film has been successfully fabricated by a mixing process of electrophoresis, electroplating and electrocorrosion. The field emission properties of tip structural nanotube film are significantly increased compared with DWCNT film fabricated by electrophoresis. The turn-on electric field E_to decreases from 1.25 to 0.92 V/μm, the low threshold electric field E_th decreases from 1.45 to 1.13 V/μm, and the field enhancement factor β increases from about 2210 to 4450. Meanwhile, this tip structural CNT film shows remarkably stable within 2% fluctuations for several hours. The high-performance emitter material and preparation technologies are both easy to scale up to large areas.

Introduction

Carbon nanotubes (CNTs) have been attracted considerable scientific and technological attention for their unique structural, electronic, thermal and mechanical properties [1], [2], [3], [4]. Double-walled carbon nanotubes (DWCNTs) which consist of two concentric cylindrical graphene layers have both merits of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). DWCNTs with their outstanding properties are expected to be used in various attractive applications, such as field emission devices [5], solar cells [6], field effect transistors [7], and atomic force microscopy (AFM) tips [8].

Since the pioneering work on CNTs in 1991 [1], great attempts have been made and continue in optimizing CNT preparations. Till today, DWCNTs have been prepared by several methods such as the conventional arc discharge [9], [10], [11], the high-temperature pulsed arc discharge [12], the catalytic chemical vapor deposition (CVD) [13], the coalescence of C₆₀ inside SWCNTs at high temperature [14], and separating DWCNTs from SWCNTs by density gradient ultracentrifugation [6]. Among all these methods, arc discharge with its simplicity and reliability is widely used nowadays for preparing high quality DWCNTs because arc-synthetized nanotubes are known to be well graphitized and have less structural defects.

In this work, we demonstrate a promising and low-cost synthetic method for preparing DWCNTs using a mixture of Fe catalyst and FeS promoter by DC arc discharge in low pressure air. Compared with conventional arc methods, this method is easier to implement without using expensive high purity gas sources, such as hydrogen [9], [10], [11] and helium [15], which can reduce significantly the cost of preparing DWCNTs and minimize the danger of large-scale application of hydrogen gas. What is more, it also simplifies the preparation process because of the absence of pre-vacuum. Furthermore, a simple, scalable, and low-cost technique to fabricate a tip structural DWCNT film is developed by a mixing process of electrophoresis, electroplating and electrocorrosion, and emitters made of DWCNT films with and without this tip structure have been studied particularly.