A catalytic CVD method was employed to synthesize carbon fibers in a quartz tube (8 cm in diameter and 100 cm in length). At first, cupric chloride (CuCl₂·2H₂O, ≥99.999%) or its aqueous solution (3 mol/L) as a catalyst precursor was put into a ceramic boat, which was placed at the center of the reaction tube. Then the ambient pressure inside the tube was maintained in a low vacuum environment of 0.133 Pa (10 mTorr) before the carbon source acetylene was introduced at 200 °C. Reaction temperature was set as 300, 350, 400 or 450 °C for 20 min. The conditions of catalyst precursor were investigated at 200 °C in a vacuum environment and at 300 °C after the introduction of acetylene for 10 min. To further verify effects of water, carbon fibers were fabricated with the introduction of water vapor at 450 °C.

The morphology and structures of as-synthesized carbon materials and catalyst precursors were examined by field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL) and transmission electron microscopy (TEM, JEM-2100, JEOL). Crystal structure of carbon materials was characterized by X-ray powder diffraction patterns (XRD, D-max-γA, Rigaku) with Cu Kα irradiation ( λ = 0.154178 nm).

Reaction temperature is one of the most important parameters in a CVD process^{[ 20]}. A higher temperature can provide more energy for the catalyst to decompose hydrocarbon gas, such as acetylene. In this study, cupric chloride was grinded prior to the CVD synthesis to enhance its catalysis activity. Fig. 1(a-d) displays FESEM images of carbon products using cupric chloride powder as a catalyst precursor at different reaction temperatures: 300, 350, 400, and 450 °C, respectively. Cupric chloride exhibits low catalytic activity at 300 °C since only small carbon pieces formed on the surface of catalyst particles (Fig. 1(a)). While the reaction temperature was increased to 350, 400 and 450 °C, carbon fibers were obtained (Fig. 1(b-d)). With the increase in reaction temperature, the morphology of carbon structures was changed from mixture of carbon pieces and fibers (Fig. 1(b)) to fibers (Fig. 1(d)). It is obvious that large catalyst particles produce pieces and small catalyst particles lead to fibers. This suggests that the conditions of catalysts can directly affect the growth of carbon structures.

Fig. 1.

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	Fig. 1. FESEM images of as-prepared carbon materials using cupric chloride powder as catalyst precursor at different reaction temperatures: (a) 300 °C; (b) 350 °C; (c) 400 °C; (d) 450 °C.

On the other hand, cupric chloride aqueous solution (3.0 mol/L) was used as catalyst precursor to synthesize carbon fibers, the aim of which is to alter initial reaction conditions for changing the growth mode of carbon materials^{[ 22]}. As shown in Fig. 2, similar to powder catalysts, no carbon materials with typical fiber morphology were obtained at the reaction temperature of 300 °C. When the reaction temperature was increased to 350 and 400 °C, carbon fibers and multi-branched carbon fibers were obtained, respectively. At the reaction temperature of 450 °C, the proportion of branched carbon fibers was circa 35%. Most of the fibers had three or four branches formed on the surface of catalyst particles, which are commonly called as Y-shaped and X-shaped carbon fibers.

Fig. 2.

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	Fig. 2. FESEM images of as-prepared carbon materials using cupric chloride aqueous solution as catalyst precursor at different reaction temperatures: (a) 300 °C; (b) 350 °C; (c) 400 °C; (d) 450 °C.

The most obvious difference appeared at 450 °C for the two kinds of carbon materials obtained using cupric chloride powder and aqueous solution. Only straight and coiled carbon fibers were synthesized using cupric chloride powder as precursor, and no multi-branched carbon fibers were observed. While cupric chloride aqueous solution was used, large amount of carbon fibers with multi-branched structures were obtained, and the branches formed on a catalyst particle had similar diameter, length, and surface smoothness.

TEM characterizations were further conducted to investigate the obvious distinction among carbon fibers synthesized at different temperatures. For example, at the reaction temperature of 350 °C, a faceted catalyst particle exists inside carbon material, consisting of several fibers (Fig. 3(a and b)). The fibers seem to form at the vertices instead of the planes of the particles, suggesting that the atoms at the vertices are more active at assisting the fiber growth than those at the plane. While at 450 °C, carbon fibers formed on the planes of a catalyst instead of the vertices (Fig. 3(c and d)). Both SEM and TEM characterizations above indicate that the diameter, length, and morphology of multi-branched carbon fibers can be tailored with the change of reaction temperature. A high reaction temperature affiliates the crystallization of catalyst particles with well-defined facets for the synthesis of multi-branched fibers with smooth surface. This means that reaction temperature has an obvious effect on the characteristics of catalyst particles, including particle size and particle regularity, and subsequently affects the property of carbon materials, such as morphology and surface smoothness.

Fig. 3.

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	Fig. 3. TEM images of multi-branched carbon fibers using cupric chloride aqueous solution as catalyst precursor at 350 °C (a and b) and 450 °C (c and d).

The XRD pattern of branched carbon fibers is shown in Fig. 4. The broad diffraction peak at around 20° results from carbon fibers, and three other distinctive diffraction peaks match well with standard copper diffraction patterns (PDF file No. 65-9026). No diffraction peaks of other metallic impurity or copper oxide were detected. Qin et al.^{[ 23]} reported that this kind of amorphous carbon materials was a mixture of solid polymers from acetylene coupling and carbon from acetylene decomposition, the products obtained at 300 °C had a C/H atomic ratio of 1.11, and their hydrogen content, weight loss, and weight loss rate decreased with increasing reaction temperatures.

Fig. 4.

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	Fig. 4. XRD pattern of multi-branched carbon fibers obtained at 450 °C.

Initial reaction conditions can decide the growth mode of carbon materials during a CVD process^{[ 22]}. For instance, He et al.^{[ 24]} reported that aqueous Ni-based mixtures can provide faster growth rate. Hansen et al.^{[ 25]} stated that Cu crystals were transformed into spherical morphology due to the introduction of water. The adsorption of water on different exposed facets of Cu crystals can induce surface reconstructions and result in the reshaping of the Cu crystals. In this study, in comparison to cupric chloride powder, the aqueous solution of cupric chloride contributes to the formation of multi-branched carbon fibers. In order to reveal the role of water during the CVD growth, some experiments were conducted to investigate the status of catalyst precursors at 200 °C prior to the introduction of acetylene and at 300 °C after the introduction of acetylene for 10 min.

Prior to the introduction of acetylene at 200 °C, catalyst precursors have a dimension of several tens of microns (Fig. 5(a)). While after the introduction of acetylene for 10 min, catalyst particles become smaller with a size of several microns (Fig. 5(c)). This indicates that acetylene molecules interact with catalyst particles and consequently break them into small particles at a higher temperature 300 °C.

Fig. 5.

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	Fig. 5. FESEM images of catalyst precursors prepared at 200 °C without the introduction of acetylene (a and b) and at 300 °C after the introduction of acetylene for 10 min (c and d).

On the other hand, a mixture of water vapor and acetylene was used for the growth of carbon fibers at 450 °C instead of pure acetylene in order to study effects of water vapor on the fiber growth. As shown in Fig. 1(d), Fig. 6(a and b), there are no obvious differences in the morphology of carbon fibers due to the introduction of water vapor during the fiber growth process. Experimental results above indicate that the use of cupric chloride solution is a key factor for the growth of multi-branched fibers instead of water vapor as reported in the literature.

Fig. 6.

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	Fig. 6. FESEM images with different magnification of as-prepared carbon fibers obtained at 450 °C with the introduction of water vapor during the CVD process.