Synthesis of tantalum carbide from multiwall carbon nanotubes in a molten salt medium

NEW CARBON MATERIALS Volume 32, Issue 3, Jun 2017 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2017, 32(3): 205-212

RESEARCH PAPER

Synthesis of tantalum carbide from multiwall carbon nanotubes in a molten salt medium Zheng-wei Cui1,2, Xuan-ke Li1,2,*, Ye Cong1, Zhi-jun Dong1, Guan-ming Yuan1, Jiang Zhang1 1

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China;

2

The Hubei Province Key Laboratory of Coal Conversion and New Carbon Materials, Wuhan University of Science and Technology, Wuhan

430081, China

Abstract:

Tantalum carbide (TaC) nanofibers and coatings were synthesized using multiwall carbon nanotubes (MWCNTs) with different

structures as templates and the carbon source in a KCl-LiCl molten salt mixture (41.2/58.8 mol/mol). The TaC and MWCNTs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and selected area electron diffraction. Results indicate that the microstructure of the MWCNTs has a distinct influence on the formation of a TaC coating on the MWCNTs. MWCNTs heat-treated at 2 900 o

C have a higher crystallinity and are harder to react with Ta to form TaC than those without the heat-treatment. The formation of TaC nanofibers

or TaC coatings on MWCNTs is dependent on the molar ratio of tantalum to carbon nanotubes. The morphology of the polycrystalline cubic TaC nanofibers and the TaC coating is similar to that of MWCNTs. The reaction time and temperature have a great influence on the conversion of carbon to TaC and its crystallite size. Key Words: Carbon nanotubes; Tantalum Carbide; Carbide coatings

1 Introduction Carbon nanotubes (CNTs) are attracting increasing scientific and technological interest owing to their novel properties and potential applications[1]. The morphology and size of carbon nanotubes suggest that they could be used as supports for heterogeneous catalysis, reinforcing agents for metallic and ceramic matrix materials or as templates for creating nano wires or tubular structures[2]. Despite their uniquely superior properties, the interface compatibility problems between carbon nanotubes and most matrices have limited their industrial applications in carbon nanotube-reinforced metallic and ceramic matrix composites[3]. It means that, if carbon nanotubes are used as reinforcing fibers for metal-matrix composites or as catalyst supports without any surface treatments, it will be difficult to achieve high strength interfacial adhesion or to anchor effectively catalyst particles on carbon nanotubes[4, 5]. Moreover, the applications of carbon nanotubes as composite reinforcement are also limited since carbon reacts with many metallic and ceramic matrix materials[6, 7]. Tantalum carbide (TaC) is a promising ceramic coating material owing to its very high melting point (>3 500 C), high hardness, high resistance to chemical attack and thermal shock[8-10], and excellent electronic conductivity[11, 12]. It has

been reported that TiC, SiC, NbC and TaC nanorods could be obtained in vapor-solid reactions using carbon nanotubes as a template[13, 14], or in chemical vapor infiltration in carbon fiber composites reinforced with layer-structured PyC and TaC phases[15, 16], or in spark plasma sintering by reaction of carbon materials such as CNTs with transition metals[17, 18]. All the carbon materials reinforced with carbides such as TaC show an improved mechanical strength and stability in high temperature. However, these syntheses are difficult to realize applications as they require high temperatures and / or rigorous handling of volatile / highly-reactive reagents, which results in a high cost. In our previous work, we have developed a simple process for coating carbon fibers with thickness-controllable TiC and TaC layers and for producing carbide nanofibers from carbon nanotubes in a liquid phase KCl-LiCl-KF molten salt medium[19-22]. This approach also provides the possibility for the controllable preparation of TaC products with various morphologies. However, the influence of crystal structure and morphology of carbon sources on the formation of tantalum carbide coating is still not very clear. In this work, we report that the various morphologies and crystal structures of multi-walled carbon nanotubes (MWCNTs) influenced the morphology of TaC in synthesis of TaC in molten salt reaction medium using MWCNTs as carbon sources and templates. The carbide products were

Received date: 20 Mar 2017; Revised date: 08 Jun 2017 *Corresponding author. E-mail: [email protected] Copyright©2017, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. Supplementary data associated with this article can be found in the online version. DOI: 10.1016/S1872-5805(17)60117-3

Zheng-wei Cui et al. / New Carbon Materials, 2017, 32(3): 205-212

characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and selected area electron diffraction (SAED).

2

Experimental

Three carbon nanotubes as both carbon sources and templates were employed: one set was the curved MWCNTs prepared by the catalytic decomposition of methane [23, 24], while the other two were the straight MWCNTs prepared by a floating catalytic method[25] and their heat treated products at 2 900 ºC. The carbon nanotubes were dispersed in trichloromethane by ultrasonication for 15 min and then mixed with the reaction medium of a salt mixture composed of LiCl-KCl eutectic (LiCl:KCl = 58.8:41.2, mol%) and Ta powder. The mass ratio of molten salt (Ms) and reactants (tantalum and MWCNTs) (Mr) is about 11:1. For three different sets, the molar ratio of the MWCNTs/Ta is 1:1, 2:1 and 3:1 in synthesis of TaC nanofibers and TaC-coated carbon nanofibers. This mixture was placed in a covered alumina crucible and reacted at the required temperature for various time under an argon flow. After cooling, the crucible was repeatedly boiled in water to remove the salts. The remaining product was dried at about 100 C for 5 h. In order to provide information on microstructure and elemental composition of the products, XRD, SEM, high resolution TEM (HRTEM) and SAED characterizations were conducted on representative sample areas. The powder products were ultrasonic dispersed in alcohol before dropped onto a standard TEM holey carbon support (Agar Scientific). The specimens were examined with a Philips CM200 FEG-TEM operating at 197 kV and fitted with a Gatan imaging filter (GIF 200) and an Oxford Instruments UTW ISIS X-ray detector (EDS). The d-spacings measured by electron diffraction patterns were compared with the International Centre for Diffraction Data (ICDD) of inorganic compound powder diffraction file (PDF) database in order to identify the crystalline structure and phases.

3

Results and discussion

Fig. 1a presents the X-ray diffraction (XRD) profiles of the curved, straight and heat-treated straight MWCNTs at 2 900 ºC. The pattern of the curved MWCNTs shows one broad and one weak peaks at about 2 = 25.9° and 42.9° corresponding to the (002) and (100) crystal planes of graphite, respectively. One broad peak at about 2 = 25.8°can also be seen in XRD profile of the straight MWCNTs. The (002) d-spacings of the curved and the straight MWCNTs are about 0.343 nm and 0.345 nm, corresponding to 2 = 25.9° and 25.8°, respectively. Two weak peaks in the XRD profiles of the curved MWCNTs at about 2 = 44.4° and the straight MWCNTs at about 2 = 44.6°correspond to (111) and (110) crystal planes of cubic nickel and iron from a few nickel and iron catalyst particles encapsulated in carbon nanotubes, respectively. In comparison with the straight MWCNTs, the

curved MWCNTs shows a stronger (002) crystal plane reflection at 2 = 25.9°. This indicates that the crystal structure of the curved MWCNTs is more ordered than that of the straight ones. The XRD profile of the heat-treated straight MWCNTs at 2 900 ºC shows a sharp diffraction peak at about 2θ = 26.4°and two weak and broad peaks at about 2θ = 54.4° and 77.6°, corresponding to (002), (004) and (110) crystal planes of a hexagonal graphite, respectively. The disappearance of iron catalyst diffraction peak at about 2 = 44.6°(shown in profile of the straight MWCNTs in Fig. 1a) suggests that the catalyst in the graphitized straight MWCNTs has been removed. In comparison to the curved MWCNTs and the straight MWCNTs, a broader (100) diffraction peak at 2 = 41.6°can be observed in the graphitized straight MWCNTs. In addition, the strong and sharp peak at about 2θ = 26.4° corresponding to (002) plane of graphite also indicates that the crystal structure of the graphitized straight MWCNTs become more perfect. Fig. 1b-i show the typical field emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images of the curved, straight and heat-treated straight MWCNTs at 2 900 ºC. Images from FESEM (Fig. 1b) and HRTEM (Fig. 1c) clearly present the abundant entangled morphology of the curved carbon nanotubes, which is composed of entangled MWCNTs with a fish-bone structure. The diameter and length of the curved carbon nanotubes are mainly around 15-40 nm and a few micrometers, respectively. The HRTEM image of the straight carbon nanotubes shown in Fig. 1f displays the misorientation of their graphene planes. The straight carbon nanotubes possess a turbostratic structure and are composed of some straight nanofibers and a few hollow spheres. The diameter and length of the straight carbon nanotubes are mainly about 10-70 nm and a few micrometers, respectively. It shows that the graphene orientation of the curved carbon nanotubes is better than that of the straight carbon nanotubes. The morphology of the straight MWCNTs does not show an obvious difference before (Fig. 1d) and after (Fig. 1g) heat-treatment at 2 900 ºC. In comparison to the turbostratic structure of the primary straight MWCNTs, the HRTEM image (Fig. 1i) of the straight MWCNTs treated at 2 900 ºC shows a distinct concentric nanotube morphology. The graphene planes of the graphitized MWCNTs in Fig. 1i have become straighter and more ordered, suggesting that the graphitization treatment effectively promotes the crystal growth of the graphitic walls of the MWCNTs and carbon spheres, and obviously improves their layer orientation degree. These results are consistent with the analysis of the XRD profiles shown in Fig. 1a. And the pristine straight MWCNT products with a turbostratic structure are converted to the highly oriented and concentric carbon nanotubes after heat-treatment at 2 900 ºC. As a result, the (100) plane diffraction peak of the latter becomes boarder and shifts to 2 = 41.6°owing to the decrease of the graphene curvature radius

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Fig. 1 (a) XRD profiles of the curved, straight and

heat-treated straight MWCNTs at 2 900 ºC; (b) SEM and (c) TEM images of the curved

MWCNTs; (d) SEM, (e) TEM and (f) HRTEM images of the straight MWCNTs; (g) SEM, (h) TEM and (i) HRTEM images of the straight MWCNTs treated at 2 900 ºC

of the latter. This suggests that the in-plane crystalline size Lc(100) of the MWCNTs may decrease with the graphene curvature radius of the concentric carbon nanotubes. The structure difference of these MWCNTs may have influence on the formation of tantalum carbide by reacting MWCNTs with Ta in the molten salt medium.

diffraction peaks of LiTaO3 decreases with the increase of reaction temperature. With increasing the reaction temperature to 950 ºC, the peak of LiTaO3 phase disappears. This indicates that the high reaction temperature is advantageous for a complete reaction of tantalum with carbon and decreasing of the impurity in TaC products.

In order to choose a proper reaction temperature, the TaC-coated nanofibers on the curved MWCNTs were synthesized at 800-950 ºC. Fig. 2 shows the XRD patterns of the products produced at various temperatures for 5 h in molten salts. The mass ratio of molten salt medium and reactant is 11:1 and the molar ratio of MWCNTs/Ta is 2:1. It can be seen from Fig. 2 that the TaC has been formed in the range of 800-950 ºC for 5 h. The intensity of the six diffraction peaks of cubic TaC phase (JCPDS: 00-035-0801) increases with the reaction temperature. It indicates that the high reaction temperature is beneficial to the growth of TaC crystal. However, a peak at about 2 = 23.7 corresponding to (012) crystal plane of rhombohedral phase of LiTaO3 (JCPDS: 00-029-0836) can be observed in Fig. 2a-c, which may be formed by reaction of Ta, molten salts and residual water in molten salts at 800-900 C. And the relative intensity of the

Fig. 3 shows the XRD patterns of the products produced at 950 ºC for different times (a) 1h, (b) 3h and (c) 6h in KCl-LiCl molten salts. The molar ratio of MWCNTs/Ta is 2:1. Six peaks of TaC and a weak peak at about 2  =26 of MWCNTs can be seen from Fig. 3. There is no other peak, which suggests that tantalum has completely reacted with carbon. The relative intensity of TaC diffraction peaks significantly increases with the reaction time, indicating that the crystal size of TaC grows with the reaction time. The disappearance of the MWCNTs peak in the product prepared at 950 ºC for 6h may result from the relative intensity increase of the diffraction peaks of TaC phase owing to the crystal growth of TaC. It seems that the TaC coating on carbon nanotubes can be formed at 950 ºC for 1h in KCl-LiCl molten salts.

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Fig. 2 XRD patterns of the products prepared at different reaction

Fig. 3 XRD patterns of the products prepared at 950ºC for different

temperatures (a) 800 ºC, (b) 850 ºC, (c) 900 ºC and (d) 950 ºC for 5 h

times (a) 1h, (b) 3h and (c) 6h in KCl-LiCl molten salt.

in KCl-LiCl molten salts.

Fig. 4 (a) SEM and TEM images of the curved TaC fibers prepared at 950C for 5h in the molten salt, (b) EDX spectrum from the fibers shown in (c), (c) the low magnification bright-field image, (d) the corresponding SAED pattern, (e) high magnification images and (f) HRTEM image. The inset in (f) is a power spectrum from the box in (f).

Fig. 4a illustrates the typical morphology of products prepared at 950 ºC for 5h by reacting curved MWCNTs with Ta in the molten salt medium, and shows that the sample is mainly composed of fibers. The molar ratio of the curved MWCNTs and Ta is 1:1. Generally, the products are curved and entangled, and they have a morphology and length similar to their MWCNTs. However, the diameter of these fibers is about 40-90 nm, i.e. larger than their carbon nanotubes. A bright-field TEM image of the curved fibers is shown in Fig. 4c. Contrast can be distinguished along the fibers, which suggests that the fibers are not of single crystal phase. In these images, regions of similar crystal orientation show at similar

contrast levels. From Fig. 4c, it can be seen clearly that the curved fibers are composed of many grains. Fig. 4d shows the SAED pattern obtained from a 200-400 nm diameter region of the curved fibers in Fig. 4c and this displays diffraction rings indexable to the {111}, {200}, {220}, {311}, {222}, {400}, {331}, {420}, {422} and {511} planes of cubic TaC. This diffraction rings of tantalum carbide confirm that the curved fibers are of polycrystalline structure. No carbon nanotube reflections are evident. The relative intensities of the rings in the SAED pattern are similar to the standard XRD intensities for bulk TaC. EDX analysis (Fig. 4b) from the same area (Fig. 4c) confirms the SAED result. The carbon and tantalum

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Fig. 5 (a) FESEM, (b) TEM and (c) HRTEM images of the straight nanofibers prepared by reaction of the straight MWCNTs at 950C for 5h in the molten salts. The inset in (c) is the corresponding SAED pattern of the straight nanofibers in Fig. 5b.

Fig. 6 (a) Low magnification and (b) high magnification TEM images of partially formed and curved TaC nanofibers.

compositions of these nanofibers were measured by semi-quantitative TEM-EDX analysis on ultrathin sample areas indicate that the atomic ratio of C/Ta is about 1:1. Fig. 4e presents a high magnification TEM image of a curved TaC nanofiber, which shows that the nanofiber is composed of abundant grains. Most of the grains are smaller than the fiber diameter. An atomic resolution TEM image of the TaC nanofiber is shown in Fig. 4f. Several groups of clear crystalline fringes of one grain can be identified in Fig. 4f, which can be measured that the d-spacing are 0.257 nm and 0.223 nm, consistent with the lattice spacings of (111) and (200) planes of cubic TaC, respectively. Inset in Fig. 4f is the power spectrum produced via a Fast Fourier Transform (FFT) of the lattice fringes within the box in Fig. 4f, which can be - indexed to the 111, 200 and 111 reflections of cubic TaC, consistent with the crystal viewed down the [011] axis. The SEM micrograph in Fig. 5a illustrates the typical morphology of the nanofibers obtained at 950ºC for 5h by the molten salt reaction with the molar ratio of the straight MWCNTs to Ta of 1:1, which reveals similar morphology and length characteristics to the straight MWCNTs (Fig. 1d). The processed nanofibers possess a rougher surface and larger diameter (some 20-90 nm) than the straight MWCNTs do. Fig. 5b shows a bright-field and low magnification TEM image of the straight TaC nanofibers. A close inspection reveals that the TaC nanofibers are actually composed of many small domains.

Inset in Fig. 5c is the corresponding SAED pattern of the straight fibers. The d-spacings and relative intensities of the diffraction rings in the SAED pattern are in accordance with the standard XRD diffraction data for bulk cubic TaC. This shows that the straight nanofibers consist of a polycrystalline cubic TaC. A low magnification TEM image of the curved nanofibers is shown in Fig. 6a. Interestingly, it is possible to identify two ends of carbon nanotube that remain intact, as indicated by the arrows in Fig. 6a. The high magnification TEM image of one nanofiber end is shown in Fig. 6b. A clear contrast variation can be distinguished within the nanofiber, indicating that it is composed of carbon nanotubes and tantalum carbide on the outer layers around a carbon nanotube core. At this stage, the nanofibers retain a size and morphology similar to their carbon nanotubes. This suggests that the reaction occurs via diffusion of Ta in the molten salts to the surface of the carbon nanotubes, and the reaction proceeds in the out surface of MWCTs to form the carbide. However, it is worth noting that the graphene planes of the carbon nanotube ends that remain intact seem to be more ordered than that of other regions of the curved MWCNTs. It implies that there may be relevance between the layer orientation of MWCNTs and the TaC layer grown thereon in molten salts.

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Fig. 7 TEM images of products produced by reaction of (a-b) the pristine straight carbon nanotubes with Ta and (c-d) their

graphitized products

at 2 900 ºC with Ta in the molten salt with a C/Ta molar ratio of 3:1. The inset in (a) corresponds to SAED pattern from the product in Fig. 7a. The inset in Fig.7d is the magnified image from the box in (d).

In order to explore the possible influence of the layer orientation of MWCNTs on the formation and growth of TaC coatings, TaC-coated carbon nanotubes were produced using a substantial excess of pristine straight carbon nanotubes produced by a floating catalytic method and their graphitized products at 2 900 ºC as the carbon sources. In this case, the molar ratio of carbon and tantalum is 3:1 in molten salts. Fig. 7a and b show that a TaC coating is formed on the pristine straight carbon nanotubes. The SAED pattern (inset in Fig. 7a) also reveals that the nanotubes consist of both polycrystalline cubic tantalum carbide and graphite-like carbon. This suggests that it is possible to control the thickness of tantalum carbide coating on carbon nanotubes by adjusting the processing conditions. Fig. 7c and d illustrate a high magnification TEM and a HRTEM image of the product prepared from the straight carbon nanotubes graphitized at 2900ºC, which shows that the product is composed mostly of highly oriented carbon nanotubes. The morphology and the orientation nature of graphene planes of the product are also well consistent with the TEM images of its carbon source shown in Fig. 1(h-i). Contrast variation in products shown in Fig. 7c and d indicates that TaC layer is formed at few regions. The inset in Fig.7d is the magnified image from the box in Fig.7d, which shows that the lattice fringes have a spacing consistence with the {111} planes of the cubic TaC. The latter result indicates that the crystal structure or the graphene plane orientation degree has a critical influence on the formation and

yield of TaC layer on carbon nanotubes. With the increase of heat-treatment temperature, the curvature radius of carbon nanotubes increase obviously and the curved graphenes show a highly orientation degree along the axis of carbon nanotube to form a concentric nanotube, thus leading to the stacked size of planar graphenes decreases remarkably. It suggests that the stacked size of planar graphenes is a key factor for the formation of TaC on MWCNTs. The increase of the stacked size of planar graphenes is beneficial for the formation of TaC on carbon source. In addition, the close coincidence between the graphite 100 d-spacing (0.213 nm) and the cubic TaC 200 d-spacing (0.223 nm) may suggest that the TaC crystal growth probably occurs epitaxially on the 100 planes of the stacked planar graphenes. However, the exact formation mechanism of TaC on carbon surface in a molten salt medium will be the subject of further investigation.

4

Conclusions

A simple template method for producing tantalum carbide nanofibers from carbon nanotubes using molten salts as reaction medium has been presented. Both TaC nanofibers and TaC coated carbon nanotubes were produced from carbon nanotube templates with a turbostratic structure. The morphologies of the tantalum carbide nanofibers have been shown to be similar to that of carbon nanotubes. Microstructural analysis shows that the TaC nanofibers have a

Zheng-wei Cui et al. / New Carbon Materials, 2017, 32(3): 205-212

cubic, polycrystalline structure. However，the graphene layer orientation of MWCNTs has a significant influence on the formation and growth of TaC coating on carbon nanotubes. It suggests that the formation of transition carbides in the molten salts appears to have a close relation with the crystal structure and graphene layer orientation of carbon sources.

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Acknowledgements

Wiley Online Library, 2001.

The authors acknowledge the financial support of the National Natural Science Foundation of China (Project 50972110 and 51472186), Hubei Provincial Natural Science Foundation of China (Project 2009CDA036) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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