Graphitization and microstructure transformation of nanodiamond to onion-like carbon
Introduction
The discovery of onion-like carbons has been of great interest from the viewpoints of science and technology [1]. Onion-like carbon consisting of concentric multilayer graphite sheets is one of the fullerenic forms of the new carbon allotropes, and can be produced by many techniques [2], [3], [4], [5], [6], [7], such as arc-discharge, irradiation and nanodiamond annealing. The preparation and growth mechanism of onion-like carbons by nanodiamond annealing have been studied extensively because of their wide potential applications in electromagnetic devices, field emission and solid lubricants [8], [9], [10], [11].
The quasi-spherical onion is formed by the closure of curved graphite sheets. The transformation is a process of diamond graphitization [12] and starts at a much lower temperature than that of bulk diamond. Bulk diamond graphitizes into planar graphite, whereas nanodiamond transforms into onion-like carbon. The presence of onion-like carbon after the graphitization of nanodiamond makes it both scientifically interesting and technologically important to study the transformation from diamond nanoparticles to onion-like carbons [13], [14], [15], [16].
Despite the intense interest in the creation of onion-like carbons from diamond nanoparticles, the formation mechanism is not very clear, e.g. how the multishell fullerenes come into being and why the shape of onion-like carbons is diverse. The purpose of this work was to study the graphitization and microstructure transformation of nanodiamond, including graphitization temperature and the resulting shapes of onion-like carbons. We have obtained onion-like carbons by annealing diamond nanoparticles at low temperature and studied the formation mechanism of onion-like structures by X-ray diffraction (XRD) and high-resolution electron microscopy (HREM) analysis. We also propose a model to explain the experimental phenomena in the structure rearrangement of nanodiamond.
Section snippets
Experimental procedures
Annealing of diamond nanoparticles (2–10 nm) was performed in a tube furnace in argon atmosphere at temperature from 900 to 1400 °C for 1 h. The nanodiamond samples were placed in a quartz boat and then heated to the annealing temperature at a rate of 10 °C/min. Afterwards the annealed samples were furnace cooled to room temperature under argon atmosphere.
The samples were characterized using XRD to determine crystalline structure and HREM for microstructure observation. The XRD measurements were
XRD analysis
Fig. 1 shows the XRD patterns of nanodiamond annealed at different temperature from 900 °C to 1400 °C for 1 h. The diffraction pattern of nanodiamond shows broader peaks at 2θ = 43.7° and 75.1° corresponding to (1 1 1) and (2 2 0) diamond planes and no graphite peak can be observed in Fig. 1(a). The diffraction peaks are obviously broadened owing to the very small crystallite size, strains and defects [17].
The XRD patterns of sample (b) and (c) are similar, indicating no obvious change in diamond
Conclusion
Diamond nanoparticles graphitize into onion-like carbons by annealing at low temperature. The onion-like carbons begin to form in the range of 1100–1200 °C and the graphitization temperature of diamond nanoparticles changes with the degree of crystallinity corresponding to particle size. All the nanodiamond can transform into onion-like carbons by annealing at 1400 °C for 1 h. The outer shells restrict the inner transformation in a spherical space and thus the interlayer spacing decreases with the
Acknowledgment
The present research is financially supported by Tianjin Natural Science Foundation: 05YFJZJC01900.
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