Temperature-dependent chemical state of the nickel catalyst for the growth of carbon nanofibers
Introduction
Catalytic chemical vapor deposition (CCVD) has been a widely used approach for industrial production of carbon nanofibers/nanotubes (CNFs/CNTs) due to its low cost, high yield and ease of scale-up [1], [2], [3], [4]. However, some longstanding and fundamental problems about this method have not yet be clarified, such as the state of the catalysts [1], [2]. In the early 1970s, Baker and co-workers proposed that the formation of CCVD-grown CNFs was induced by the liquid catalyst [5], [6], and thus triggered the important issue about the state of catalysts, including both physical and chemical states [1], [7]. To date, with the aid of the advanced in situ characterization techniques, some general agreements on the physical state, i.e. liquid state or solid state, of the catalyst have been achieved. For example, in situ transmission electron microscopy (TEM) confirmed that the Ni particles in a crystalline state could also induce the growth process of CNTs [8]. The recent in-situ Raman analyses also revealed that CNTs could grow from both solid and liquid catalyst Fe, depending on the growth temperature [9].
The debate on the chemical state of catalysts was originated from the argument in 1980s that the deviation of C equilibrium during the growth process of CNFs could be attributed to the formation of the metastable carbides, Ni3C or Ni3C1-x [7], [10], [11], [12]. However, the validity of such a carbide-assisted mechanism has been strongly questioned in the past. Because some early experimental results and analyses [13], [14] suggested that carbides should not be active catalysts for the formation of CNFs, and moreover, the convincing evidence for carbide-assisted mechanism was absent until very recent years. The direct evidence for carbide-assisted growth was firstly given by in situ TEM observation that Fe3C was able to induce the growth of CNFs at the temperatures of 600–700 °C [15], [16]. After that, comparative ex situ TEM analyses also demonstrated that both Fe and Fe3C could induce the catalytic growth of CNFs [17].
Ni is one of the most common catalysts used for the growth of CNFs/CNFs. The chemical state of Ni catalyst exists as metallic Ni during the catalytic growth of CNFs/CNTs at the temperatures of above 500 °C, which has been well demonstrated by both in situ TEM and XPS characterization [8], [18], [19]. However, the possibility of Ni3C-assisted growth of CNFs/CNTs at temperatures of below 500 °C still cannot be ruled out. In fact, some earlier theoretical research has predicted that the surface carbide, Ni3C, could induce the growth of CNFs at temperatures of below 400 °C [20], which was significantly lower than the common Ni-catalytic growth temperatures of above 500 °C.
In our recent work, we successfully synthesized CNFs at 300 °C by metal-organic CVD, and demonstrated that the Ni catalyst appeared as Ni3C, instead of metallic Ni during the catalytic growth process [21]. It is known that metastable Ni3C is sensitive to the environmental temperature and decomposes totally at temperatures of above 500 °C [22]. It is therefore quite reasonable to expect that the chemical state of Ni catalyst will be dependent on the growth temperature. In this work, we systematically investigate the chemical state of Ni catalyst at the catalytic growth temperatures ranged from 300 to 600 °C, and find that the chemical state of Ni catalyst might shift from Ni3C to Ni–Ni3C1-x, and finally to metallic Ni as the increase of the growth temperature.
Section snippets
Experimental details
CNFs were synthesized by metal-organic CVD in a horizontal tube furnace. Nickel(II) acetylacetonate (Ni(acac)2, Aldrich Chemical Co., 95%), as the precursor, was loaded in the quartz tube that was located at the upstream of the furnace with a temperature of 150 °C. The SiO2 wafer (4 cm × 15 cm) was mounted at the downstream of the furnace with a temperature of 300–600 °C, as the substrate for collecting the synthesized products. During the deposition process, the pressure and flow rate of the
Morphological and structural characteristics of CNFs
The phase compositions of the products synthesized at different temperatures were examined by XRD, as shown in Fig. 1. It is seen that the main diffraction peaks of the product obtained at 300 °C are located at 39.2°, 41.6°, 44.6° and 58.5°, which match well with the indexes of the (110), (006), (113) and (116) crystal planes of Ni3C with a rhombohedral structure (JCPDS 06–0697; a = 0.458 nm, c = 1.299 nm), respectively. There exists a broadened peak ranged from 20 to 30°, which should come
Conclusion
The CNFs were synthesized by use of catalytic CVD at temperatures ranged from 300 to 600 °C. It was found that the chemical state of Ni catalyst was dependent on the growth temperature, i.e., Ni3C at 300 °C, Ni3C1-x-Ni at 400 and 500 °C, and metallic Ni at 600 °C. This suggested that the growth of CNFs at different temperatures should be controlled by different growth models, Ni3C-assisted model, composite-assisted model and Ni-assisted model. This work not only provided the fundamental
Acknowledgments
This study is financially supported by the National Natural Science Foundation of China (51074188, 11502080), Research Funding of Central South University (2014JSJJ024) and the Australia Research Council (DP130101828).
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