Large Scale Synthesis of Carbon Nanofibers Using Co-P Catalyst Films

Tien-Yu Wu, Shinn-Shyong Tzeng Department of Materials Engineering, Tatung University, Taiwan

Email: sstzeng@ttu.edu.tw (S.-S. Tzeng)

Introduction

Carbon nanotubes (CNTs), which is one of the most famous one-dimensional nano-materials in 21th century [1], has been wildly investigated because of their unique structure and remarkable properties [2~4]. Carbon nanofibers (CNFs), as well as carbon nanotubes, also draw much attention due to their similar processing methods and their remarkable and special properties. Based on those unique properties and structures, CNTs and CNFs all have a great potential for the fundamental applications, such as in nano-electronic devices, filed emission displays, energy storage media, etc. However, finding an effective catalyst for synthesizing the CNFs has been one of the most important research issues. Many preparation methods of catalyst have been reported, for examples, impregnation [5], spray [6], catalyst film [7] and sol-gel method [8]. In this study, a simple electroless coating technique was used to prepare the catalyst film for the catalytic synthesis of CNFs through chemical vapor deposition (CVD). It is demonstrated that large scale production of CNFs with a quite uniform diameter distribution could be achieved using a simple electroless Co-P film as catalyst.

Experimental

The Co-P film was prepared by electroless plating on the surface of Al2O3

substrate, which was activated in a PdCl2 solution in advance. The electroless Co-P film was synthesized at 90 for 90~630 seconds using the following plating solution, which contained CoSO4 (0.1M), NaH2PO2, (0.2M), Na3C6H5O7

(0.2M) and H3BO3 (0.5M). NaOH was added to adjust pH value. Synthesis of CNFs was carried out in a horizontal tube furnace at 600 with a typical reaction time of 2 hours. Acetylene (C2H2, 99.93%) was used as the carbon source and hydrogen (H2, 99.999%) as carrier gas. The flow rate of the acetylene/hydrogen mixture was 10/90 sccm. Some of the as-grown CNFs will also be heat treated at higher temperatures. The yield of the carbon deposited (%) was calculated as follows:

Carbon yield (%) = 100catalyst

CNF

m

m ,

where mCNF is the weight of as-deposited CNFs and mcatalyst is the weight of catalyst. The morphology of CNFs was observed by field emission scanning electron microscopy (FE-SEM), and the microstructure was characterized by x-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM).

Results and discussion

The SEM morphology of Co-P film after H2 pretreatment is presented in Fig. 1, which shows that small nano-sized particles, ranged form several nanometers to several hundred nanometers, were distributed on the substrate. These nanoparticles are responsible for the growth of CNFs. It has been reported that the Co-P film prepared by electroless plating has a unique feature of the microstructure, the existence of the so called “channels”, which is resulted from the phosphorous segregation in the grain boundaries of cobalt because of the essentially no solubility of phosphorous in cobalt gains [9~11]. This particular phenomenon makes the grain size of Co-P film to be in the range of a few tens of nanometers. The formation of nano-sized particles for the growth of CNFs is attributed to the above-mentioned unique feature.

Fig.1 The SEM image of Co-P film after H2 pretreatment.

Fig. 2 The SEM image of as-grown CNFs.

Fig.2 presents the SEM image of the as-deposited CNFs grown at 600 for 2 hours. It shows that the CNFs have a curved and entangled morphology. However, they have a quite uniform diameter distribution ranged from 20 nm to 60 nm.

The structure of as grown CNFs was characterized by x-ray diffraction (Fig. 3) and Raman spectroscopy (Fig. 4). As shown in Fig.3, the x-ray diffraction pattern shows a broad (002) reflection and a much weaker (10 ) band, which corresponds to a turbostratic carbon structure [12]. The d002 and Lc values are 3.465 Å and 25 Å, respectively. Raman spectrum as presented in Fig. 4 shows two overlapped D and G bands, located at 1326 cm-1 and 1595 cm-1,respectively. Results of curve fitting indicate that in addition to the D and G bands, a broad band appeared at ~1545 cm-1, which was assigned to an amorphous graphitic phase [13,14]. Fig. 5 shows the TEM images of CNFs heat treated at 2400 . Two different types of CNFs were observed with graphite layers arranged perpendicular and at an angle with respect to the fiber axis, which are designated as “platelet” and “herringbone structures, respectively [15].

2( (degree)

Fig. 3 X-ray diffraction pattern of as-prepared CNFs grown at 600

Fig. 4 Raman spectrum of as-prepared CNFs grown at 600

800 1000 1200 1400 1600 1800 2000

Inte

nsity

13261595

wave numbers(cm-1)

5 10 15 20 25 30 35 40 45 50 55 60 65

Si

(10 )

(002)

Inte

nsity

(a)

(b)

Fig. 5 The TEM images of 2400 heat treated CNFs.

In the synthesis of CNTs using metal films as catalyst, it is usually required that the thickness of the film is smaller than a critical value on the order of several tens of nanometers. It is quite interesting, however, to find that the yield of CNFs produced using electroless Co-P films as catalyst increases with increasing thickness of the film (Fig. 6). As indicated in Fig. 6, the weight of carbon deposited increases as the weight of the catalyst Co-P film, controlled by increasing the plating time, is raised. 200 mg of CNFs can be grown in two hours at 600 when using 4 mg of Co-P catalyst film on an alumina substrate with an area of 10*20 mm2, which corresponds to a yield of 5000 . Diameter measurements (Fig. 7) indicated that regardless of the thickness of catalyst film, the CNFs have a quite uniform diameter distribution in the range of 20~60 nm.

Fig. 6 The weight of carbon deposited as a function of weight of Co-P catalyst film.

Fig. 7 The diameter distribution of CNFs grown at 600 for different plating times of Co-P catalyst film.

1.5 2.0 2.5 3.0 3.5 4.00

30

60

90

120

150

180

210

Plating times90 sec

180 sec 270 sec 360 sec 450 sec630 sec

Wei

gh

t o

f ca

rbo

n d

epo

site

d (m

g)

Weight of catalyst film (mg)

10 20 30 40 50 60 70 800

25

50

630 sec

Diameter (nm)

Per

cen

tag

e (%

)

0

25

50

450 sec

0

25

50

360 sec

0

25

50

270 sec

0

25

50

180 sec

0

25

50

90 sec

Conclusions

CNFs with a quite uniform diameter distribution in the range of 20~60 nm were synthesized by catalytic chemical vapor deposition using the electroless Co-P film as catalyst. It was found that the yield of CNFs increases with increasing thickness of the catalyst film, and a carbon yield of 5000 can be achieved in two hours at 600 when using an alumina substrate with an area of 10*20 mm2.Structural and morphology characterizations by XRD, Raman spectroscopy, and SEM indicated that the CNFs have a turbostratic graphite structure and a curved and entangled morphology. Two different arrangements of graphene layers in CNFs, “platelet” and “herringbone”, were found by TEM observation.

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