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Formation of carbon fiber florets using copper tartrate catalyst precursors
Qian Zhang a, Fanglin Du a, Lifeng Dong a,b,⁎, Chuncheng Hao a
a College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR Chinab Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri 65897, USA
⁎ Corresponding author at: College of Materials ScieUniversity of Science and Technology, Qingdao 2660428402 2869.
A novel method was investigated to synthesize carbon fiber florets using copper tartrate catalyst precursorsby catalytic chemical vapor deposition at 300 °C. Samples were obtained for different growth periods. On thebasis of electron microscopy and X-ray diffraction characterizations, a growthmodel for the formation of fiberflorets was proposed as well as the branching of fibers nearby copper catalyst particles. These findings couldfacilitate the understanding of the catalytic growth process of different carbon materials including carbonnanotubes and graphene under different reaction parameters.
nce and Engineering, Qingdao, PR China. Tel./fax: +86 532
Carbon materials, including carbon fibers, fullerene, carbon nano-tubes, and graphene, have attracted great attention due to their uniqueproperties and broad applications [1,2]. As a result of persistent researchand improved understanding, a variety of carbon materialswith complicated structure and morphology, such as carbon tree [3],Y-branched and multi-branched carbon fibers/nanotubes [4–6], havebeen synthesized. Various carbon materials demonstrate diversechemical and physical properties, and, therefore, could be suitable forvarious applications [7,8]. In this study, we have synthesized a carbonfiber network array, termed the floret, using copper tartrate catalystprecursors andhave proposed a growthmechanism for the formation ofthe florets. The network array of fiber florets makes them attractive aspotential additive to enhance electrical conductivity and mechanicalproperties of carbon-fiber-based composite materials.
2. Experimental
2.1. The preparation of copper tartrate catalyst precursors
Similar preparation procedures for the copper tartrate catalystprecursors have been reported in the literature [9]. Briefly, potassiumsodium tartrate aqueous solution (0.0586 M) was added slowly into acopper dichloride solution (0.0586 M) with vigorous stirring, and theresultant copper tartrate was formed as a light blue precipitate.Subsequently, the precipitate was filtered and washed with distilledwater and ethanol and dried at 100 °C for 1 h.
2.2. The synthesis of carbon fiber florets
Copper tartrate, used as a catalyst precursor, was placed on a ceramicsubstrate and positioned in the central part of a reaction tube (9 cm indiameter and 90 cm in length). Using copper tartrate as a catalystprecursor, carbon fibers were obtained by the catalytic decomposition ofacetylene for two different periods of times: 15 min and 55 min. As thetemperature was increased from room temperature to 300 °C, theambient pressure inside the reaction tube was maintained at 0.5 MPa.When the temperature reached at 300 °C, hydrogen was introduced intothe reaction tube to replace theexistingatmosphereasapre-treatment forthe catalyst precursors. Subsequently, the synthesis reaction wasconducted at atmospheric pressure at 300 °C with acetylene used as thecarbon source gas. Finally, the reaction tube was cooled to roomtemperature.
2.3. Morphology and structural characterizations
Field emission scanning electron microscopy (FESEM) images wereobtained with a JEOL JSM-6700F microscope; transmission electronmicroscopy (TEM) was performed on a JEOL JEM-2000EX transmissionelectron microscope at an accelerating voltage of 160 kV, and X-raypowder diffraction (XRD) patterns were recorded on a Rigaku D-MAX2500/PC diffractometer with Cu Ka radiation (λ=1.5418 Å).
3. Results and discussion
3.1. Characterization of copper tartrate catalyst precursors
Asshown inFig. 1aandb, copper tartrate catalystprecursorspresent asmulti-layered hexagonal plates. During the reaction, acetylene andhydrogen gas molecules diffuse inside the copper tartrate through the
spaces between the layers. The cross sectional structure of copper tartrateis the salient feature for the formationof carbonfiberflorets,whichwill bediscussed in Section 3.3. The optimal concentrations of potassium sodiumtartrate and copper dichloride can result in the formation of the multi-layered hexagonal copper tartrate plates instead of other morphologies.
3.2. Characterization of carbon fiber florets
Fig. 2a–f display FESEM images of carbon fiber florets synthesizedusing copper tartrate catalyst precursors for two different growth times.
Fig. 2. FESEM images of carbon fibers for differen
Fig. 2a and b show SEM images of carbon fiber florets at differentmagnifications for a 15 min growth period. This carbon nanomorphol-ogy,whichwe term a floret, consists of an anastomosing network of finecarbon fibers displaying bright catalyst particles positioned mid-wayfrom their terminal ends. The termini attach to the ends of neighboringfibers to form apparent fusion centers in the network. Furthermore, asshown in Fig. 2c, numerous curled ultrafine fibers, having diameters inthe range of 20 to 50 nm, emanate from the carbon fiber terminal ends.These ultrafinefibersmight be formed during the initial reaction period.Fig. 2d displays bifurcation and trifurcation of carbon fibers flanking
Fig. 3. FESEM images of branching carbon fibers flanking copper catalyst foci. The fibers were prepared for a 55 min growth period.
2781Q. Zhang et al. / Materials Letters 65 (2011) 2779–2782
copper catalyst particles (bright foci) at a higher magnification. Theinsets in Fig. 2c and d demonstrate TEM images of ultrafine fibers andcatalyst particles within carbon fibers (bar, 100 nm), respectively. Withan increase of growth time period to 55 min (Fig. 2e and f), carbon fiberlength can attain several tens of μm; the fibers seem to be morerandomlyoriented compared to their putativeparallel alignment duringthe initial growth period (Fig. 2a and b).
Fig. 3a–d exemplify an interesting growth phenomenon for a55 min growth period: carbon fibers immediately adjacent to coppercatalyst foci demonstrate apparent trifurcation (Fig. 3a and b) or moreextensive splitting (Fig. 3c) or branching (Fig. 3d), and ultrafine fibersseem to emanate from different growth facets of the copper catalystparticles. This phenomenon will be discussed in the next section.
Fig. 4. XRD patterns of copper tartrate catalyst precursors at different growth periods: (a)tartrate annealed at an ambient atmosphere; and (d) copper tartrate.
XRD characterizations were conducted to examine the change ofcrystal structures of copper tartrate catalyst precursors at differentgrowth periods. As shown in Fig. 4, when copper tartrate with a crystalsize of 42 nm (Fig. 4d) was annealed at an ambient atmosphere fromroom temperature to 300 °C, the copper tartrate was oxidized to Cu2Oand CuO (Fig. 4c), the sizes of which are 34 nmand 21 nm, respectively.When hydrogenwas introduced into the reaction tube at 300 °C, copperoxides were reduced to Cu nanoparticles with a crystal size of 50 nm(Fig. 4b). During the growth period, copper acts as a catalyst for theformation of carbon fiber florets (Fig. 4a). In accordance with Scherrer'sequation, the crystal size of copper catalyst was calculated to be 44 nm.No copper carbide or other impurity was found among the carbon fiberflorets.
carbon fiber florets; (b) copper tartrate after the treatment with hydrogen; (c) copper
Fig. 5. Illustrations of the growth model for carbon fiber florets (a–c) and a hypothetical separation mechanism of carbon fibers adjacent to copper catalyst foci (d–f).
Based on electron microscopy observations and XRD characteriza-tions, the following growth model is proposed to explain the formationof carbon fiber florets (Fig. 5a–c). Three reaction steps constitute theformation process. First, oxygen in the ambient atmosphere enters thegaps among copper tartrate plates and reacts with copper tartrate nearthe gaps and surfaces. Small copper oxide particles (not shown inillustration) form on the surfaces, and larger particles appear internallydue to the Ostwald ripening process (Fig. 5a). During this step, coppertartrate is oxidized to Cu2O and CuO, and oxide particles remainconnected in the form of multi-layered plates instead of individualparticles. The oxidation temperature and duration time can affect thesize of catalyst particles. Second, hydrogen is introduced, and Cu2O andCuO particles are reduced to Cu. The oxygen and hydrogen introductionsteps result in floret formation, which are different from our previouswork [10]. Third, acetylene gas enters the reaction chamber, and C2H2
molecules polymerize, decompose, and diffuse through the coppercatalyst to formcarbonfibers. Thefiber diameter is controlledby the sizeof copper catalyst [10]. Large catalyst particles inside plates result inlarge carbon fibers; whereas, small catalyst particles on the surfacecontribute to the formation of ultrafine fibers on the terminal ends oflarge fibers (Fig. 5b). Copper catalyst particles (rendered as blackrectangles in Fig. 5) remain at the center of the fibers. Different growthrates result in the formation of fiber bundles having different lengths,thereby generating stress forces among the fibers. Carbon fiber floretsare formed because the stresses lead to the separation of fibers, but theends of fibers are connected (Fig. 5c). When the reaction is prolongedfrom 15 min to 55 min, the connected ends of the fibers would separateby the stress, and thus the fiber floret dissociates into individual fibers.
Another interesting growth phenomenon is that the carbon fibersimmediately adjacent to copper catalyst foci demonstrated separationor splitting followed by subsequent consolidation (Fig. 3). In accordancewith SEM and TEM observations, carbon fiber separation might beattributed to stresses generated because of thermal expansion andcontractionof copper catalyst particles (Fig. 5d–f). Initially, carbonfibersbind to each other due to van der Waals forces (Fig. 5d). When thereactor power was turned off after the reaction setting time wasattained, the temperature inside the reaction tube declined, and coppercatalyst particles inside the carbon fibers contracted due to thetemperature decrease. The contraction of copper particles could resultin stress being applied to the outboard surfaces of carbon fibers; thedirections of these stress forces are indicated by arrows in Fig. 5e.Meanwhile, carbon fibers would continue their growth due to residualacetylene molecules in the reaction tube. When the stresses among the
fibers become strong enough to exceed the van der Waals forcesconsolidating thefibers,fiber separation could occur (Fig. 5f). The lengthof the separated fibers is less than 1 μm.
4. Conclusions
In this paper, carbon fiber florets were synthesized using coppertartrate catalyst precursors by a catalytic CVDmethod. The pre-treatmentof catalyst precursors by oxidation and reduction reactions resulted in theformation of carbon fiber florets, and the thermal expansion andcontraction of metal catalyst particles inside the carbon fibers leadputatively to the carbon fiber separations immediately adjacent to thecopper catalyst particles.
Acknowledgment
We are grateful for the financial support of this work from theNational Natural Science Foundation of China (No. 50872059, no.510771075), theNatural Science Foundation of Shandong Province (No.Z2008F07) and the Taishan Scholar Overseas Distinguished Professor-ship program from the Shandong Province Government, P R China. Theauthors thank Dr. Michael M. Craig for helpful discussions.
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