Electron Induced Synthesis

Organic compounds containing linear sp-bonded carbon chains have been known to be useful reactants for synthesis of carbonaceous materials [69]. It has also long been known that organic compounds and their polymeric materials decompose easily under the irradiation of electron beam during the measurement with electron microscopy. In the nanocarbon investigations, energetic electron beams in TEM have been well known to cause reconstruction (or transformation) of carbon nanoparticles to more compact carbon onions [70]. Very recently, a new process for solid-state synthesis of nanocarbon materials under TEM electron irradiation has been developed, in which polyyne-containing carbons are involved as precursor compound for CNT formation [71, 72].

The polyyne-containing carbon is prepared with an electrochemical reduction between a poly(tetrafluoroethylene) (PTFE) film and a sacrificial magnesium anode [72, 73]:

The above reduction was carried out by a two-electrode method (anode: magnesium; cathode: stainless steel) in tetrahydrofuran, containing lithium chloride and iron (II) chloride as supporting electrolytes [73]. The polyyne-containing carbon was formed on the reduced surfaces (about 1 ¡m in depth) of PTFE films (10 mm x 10 mm x 60 ¡m), and the carbonized layer was confirmed to contain the polyyne structure by Raman spectroscopy (e.g., C=C stretching vibration at 2100 cm-1). The in-situ synthesis of CNTs using the carbonized PTFE films can be described in the sequence shown in Figure 4. First, the cross-section specimen was heated towards 800 °C (at the rate of 15 °C/min) without the electron irradiation, during which the remaining PTFE part in the thin film was melted and disappeared at about 350 °C. After an electron irradiation at 800 °C (electron acceleration voltage 100 kV, irradiated charge density about 103 C/cm2), copious amounts of CNTs were formed on the surface of the layer. The length of these CNTs is typically at 1 ¡m while the outer diameter of these tubules is in the range of 10-50 nm and the inner diameter is of about 8-30 nm. It is noted that graphitization of the shell of the tubules was low although the tubules resembled "CNTs." Interestingly, the carbon tubules were found only on the substrate side of the polyyne-containing carbon layer, as illustrated in Figure 4. Based on these observations, it is believed that the PTFE layer can protect the polyyne structure in the carbonized layer from the oxidation in moist air. The undamaged polyyne structure and free space are two important factors responsible for the observed CNT growth. When the heating temperature is below ca. 400 °C, the same electron irradiation does not cause any CNT growth. Instead, damages such as bond rupture and loss of mass and crys-tallinity take place due to the electronic irradiation on the organic materials [71].

Very recently, the same material system has been investigated in a greater depth [72]. The in-situ TEM observation indicates that graphene layers are first formed around the magnesium particles (from electrochemical reduction process) with low crystallinity. The metal particles evaporate gradually while a further graphitization takes place upon the heating (600 °C) and irradiation, leading to formations of carbon nanocapsules (CNCs) and nanoparticles (CNPs), as depicted in Figure 5. One important finding in this work is

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