Multi-wall carbon nanotubes: Purification, morphology and field emission performance
Introduction
Multi-wall carbon nanotubes (MWNTs) have unique electrical and structural properties, including high aspect ratio, small tip radii of curvature, chemical inertness, high mechanical strength and high electrical conductivity. They were discovered by Iijima [1]. They have a wide range of applications and have been investigated extensively. In particular, electronic and electrochemical applications are some of the most promising, and include field emission displays (FEDs) [2], [3], nanodevices [4], [5], [6], fuel cells [7] and electroactive composites [8], [9]. Various synthetic methods for producing nanotubes have been developed [10], [11], [12]. In addition to laser evaporation and electric arc discharge approaches, the catalytic process is very efficient for producing MWNTs. However, as-prepared carbon nanotubes (CNTs) generally are not pure MWNTs, but contain other impurities of carbonaceous particles and metal catalysts. Carbonaceous particles include amorphous carbon particles, fullerenes and nanocrystalline polyaromatic shells. The presence of impurities may significantly affect the characteristics of MWNTs and the behavior of any device that is based on them. Therefore, the purification of MWNTs is very important for applications that exploit their intrinsic properties. The use of physical and chemical oxidation to purify the MWNTs has been extensively investigated. Purification processes that involve physical oxidation are based on the fact that the oxidation temperature of carbonaceous particles is different from that of MWNTs in air or oxygen [13], [14]. However, a persistent problem, associated with physical oxidation, is that materials such as transition metal catalysts that remain encapsulated in the wall structure can affect performance in many practical applications. Accordingly, numerous research groups have attempted to eliminate the need for a metal catalyst. Bandow et al. [15] developed a method of purification by microfiltration, which separates carbon nanospheres, metal nanoparticles, polyaromatic carbons and fullerenes from the fraction of single-walled carbon nanotubes (SWNTs). Many investigations have utilized chemical oxidation to purify MWNTs [16], [17], [18]. Among the materials that are used for field emission, CNTs and graphite fibers have attracted much attention because of their unique one-dimensional structures and environmental stability. Although their field emission capacity must still be improved, CNTs have been accepted as field emitters because of their unique characteristics for field emission, including high enhancement factor (β) and low turn-on voltage [19]. Therefore, purified MWNTs are useful for field emission [20], [21], [22].
This study proposes the purification of MWNTs using acids. The concentration of sulphuric acid (H2SO4)/nitric acid (HNO3) solution and the time required to purify the MWNTs are discussed and optimized. Finally, the field emission characteristics are demonstrated.
Section snippets
Experiment
As-prepared MWNTs (10 nm in diameter) were dispersed in 100 ml deionized water by ball-milling for 24 h. After ball-milling treatment, MWNTs were dried. The MWNTs were suspended in H2SO4/HNO3 solution (20 ml in volume) at various concentrations (1:1 and 1:3) and purification times, 5, 20, and 48 h, to eliminate metal catalysts and impurities from MWNTs.
After treatment with acids, profuse water is added to dilute the heavy acidic suspension that is caustic, and oxidized until the pH reaches 7. Then,
Results and discussion
Fig. 1 shows the morphologies of MWNTs after treatment with various concentrations of H2SO4/HNO3 acidic solutions and purification times. In Figs. 1(a)–(c), the purification time varies from 5 to 48 h when the concentration ratio of the H2SO4:HNO3 solution is fixed at 1:1. The purified MWNTs retain numerous carbonaceous particles, as shown in Figs. 1(a) and (b). In Fig. 1(c), CNTs are cut into short pieces as the purification time extends to 48 h. Restated, the MWNTs will be destroyed by
Conclusion
Highly pure MWNTs were successfully prepared by acid purification. H2SO4/HNO3 purification solution not only oxidizes carbonaceous particles, but also dissolves the metal catalysts. A 1:3 concentrated H2SO4:HNO3 solution for 20 h yields highly pure MWNTs. The ID/IG ratio of MWNTs is as low as 0.27. Highly purified MWNTs provide emission tips with a high efficiency. At an electric field of 1.9 V/μm, the field emission current density is as high as 170 μA/cm2. Therefore, enhanced electrical
Acknowledgments
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 96-2221-E-214-015. The authors would also like to thank the MANALAB at ISU, Taiwan.
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