C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8

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A convenient, general synthesis of carbide nanofibres viatemplated reactions on carbon nanotubes in molten saltmedia

Xuanke Lia,b,*, Aidan Westwoodb, Andy Brownb, Rik Brydsonb, Brian Randb

aHigh Temperature Ceramic and Refractory Key Laboratory of Hubei Province, Wuhan University of Science and Technology,

Wuhan, Hubei 430081, PR ChinabInstitute for Materials Research, University of Leeds, Leeds LS2 9JT, United Kingdom

A R T I C L E I N F O

Article history:

Received 9 May 2008

Accepted 27 September 2008

Available online 2 October 2008

0008-6223/$ - see front matter � 2008 Elsevidoi:10.1016/j.carbon.2008.09.050

* Corresponding author: Address: High TempScience and Technology, Wuhan, Hubei 4300

E-mail address: xkli8524@sina.com (X. Li)

A B S T R A C T

Carbide nanofibres were synthesized by reaction of various transition metals with carbon

nanotubes using a molten LiCl–KCl–KF salt system as a reaction medium. Metal sources

included titanium, zirconium, hafnium, vanadium, niobium and tantalum powders.

Multi-walled carbon nanotubes were used both as a carbon source and also as a template

for the preparation of titanium carbide, zirconium carbide, hafnium carbide, vanadium car-

bide, niobium carbide and tantalum carbide nanofibres. Generally, the carbide products

were characterized by scanning electron microscopy, transmission electron microscopy,

selected-area electron diffraction and electron energy loss spectroscopy. The polycrystal-

line carbide nanofibres produced in these reactions have a similar morphology to that of

their multi-walled carbon nanotube precursors. However, when using titanium mixed with

titanium dioxide as a titanium source, both polycrystalline and straight, single crystal tita-

nium carbide nanofibres are formed.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Transition metal carbides are key ceramics owing to their

excellent properties, which include high melting points, elas-

tic moduli, hardnesses, thermal and electrical conductivities,

high temperature strengths, resistance to creep/thermal

shock and low reactivities [1,2]. Owing to these attractive

properties, transition metal carbides have existing applica-

tions in coatings and structural reinforcement with potential

application in catalysis and microelectronics. The latter appli-

cations, in particular, would benefit from a capability to pro-

duce material with nanometer length scales. However, in

general, widespread application of carbides is currently hin-

dered by high manufacturing costs and poor control over

their purity, shape, size and structure. Existing carbide syn-

er Ltd. All rights reserved

erature Ceramic and Re81, PR China. Fax: +86 27.

theses generally yield low purity, non-stoichiometric prod-

ucts [1–9] or require high temperatures or rigorous handling

of volatile/highly-reactive [10–14] reagents. A possible way

to exercise control over the size and shape of a carbide prod-

uct would be to begin by manufacturing nanostructured

material. However, whilst there are reports of the production

of carbide nanorods [13,14], this synthesis, which involves

reaction of carbon nanotubes with unstable, volatile precur-

sors, is difficult, hard to control and not applicable to all tran-

sition metal carbides. Clearly it is of interest to develop a

simple, clean and controlled route for the synthesis of nano-

structured carbides.

The synthesis method reported in this work can produce

stoichiometric nanocarbides of controlled morphology and

structure by reaction of transition metals with carbon sources

.

fractory Key Laboratory of Hubei Province, Wuhan University of86551274.

202 C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8

using a convenient, general method in liquid phase molten

salt reaction media at relatively low temperatures (950 �Ccompared to the more typical synthesis temperature of

1400–1700 �C for carbothermal reduction).

Two types of multi-walled carbon nanotubes (MWCNTs)

were used as carbon sources for the production of carbide

nanofibres in this work: one type containing highly curved

and entangled MWCNTs [15], the other consisting of relatively

straight MWCNTs [16]. As will be shown, the molten salt route

enables controlled syntheses of carbide nanofibres to be

accomplished using easily-handled reactants at relatively

low temperatures and with simple and effective purification

steps. More generally, syntheses in which molten salt is used

as a reaction medium [17] offer convenient and cost-effective

routes to useful products at low temperatures in relatively

short reaction times. Carbide synthesis in molten salts thus

offers a potent, key enabling technology for overcoming some

of the existing synthesis problems, as outlined above, and for

producing a wide range of new materials, including nano-

structured carbides in particular.

2. Experimental

2.1. The preparation of transition metal carbidenanofibres from MWCNTs in molten salts

Curved and straight MWCNTs were obtained by a catalytic

chemical decomposition (CVD) method [15] and a floating cat-

alytic method [16], respectively. Both were purified by washing

in hydrofluoric acid and subsequent aqueous rinsing to pH 7.

Each was dispersed in trichloromethane by ultrasonic vibra-

tion for 15 min. The MWCNTs were then mixed with pure tita-

nium, zirconium, hafnium, vanadium, niobium or tantalum

powder along with the reaction medium of mixed salts com-

posed of ca. 98 wt% LiCl–KCl eutectic (LiCl:KCl = 58.8:41.2 -

mol%) and 2 wt% KF. The mixture was dried for several

hours in an oven at 120 �C and then placed in a covered alu-

mina crucible and heated at 950–960 �C for 5 h under a flowing

argon atmosphere. After cooling, the crucible was boiled in

water to remove the salts and the group IVand group V carbide

nanofibre products were recovered after rinsing and drying.

2.2. The characterization of MWCNTs and molten saltsynthesis products

The phases present in titanium, zirconium, vanadium,

niobium and tantalum carbides produced from the reaction

of MWCNTs with the corresponding metal in the molten salts

were identified by X-ray diffraction (XRD) using a Philips APD

1700 instrument with Cu Ka radiation.

The morphologies of all of the reaction products were ini-

tially characterized by imaging in a standard field emission

gun scanning electron microscope (FEG-SEM). In order to link

information on product microstructure and elemental com-

position, atomic lattice resolution transmission electron

microscopy (HRTEM), selected-area electron diffraction

(SAED), and electron energy loss spectroscopy (EELS) analyses

were conducted on representative areas/fibres. TEM samples

were prepared by ultrasonic dispersion of the powder prod-

ucts in methanol, which was dropped onto a standard TEM

holey carbon support film (Agar Scientific) and then dried in

air. The specimens were examined with a Philips CM200 field

emission gun transmission electron microscope (FEG-TEM)

operating at 197 kV and fitted with a Gatan imaging filter

(GIF 200) and Oxford Instruments UTW ISIS Energy Dispersive

X-ray detector. Bulk energy loss spectra were taken in diffrac-

tion mode (image coupled) with a �0.2 lm diameter selected-

area diffraction aperture inserted, a collection semi-angle of

6 mrad and convergence semi-angle of �1 mrad. HRTEM and

SAED were used to probe the microstructure and crystallinity

of the product. The d-spacings measured from electron dif-

fraction patterns were compared with the International Cen-

tre for Diffraction Data (ICDD) inorganic compound powder

diffraction file database to identify the crystalline phases

present.

3. Results and discussion

A SEM secondary electron image of the curved MWCNT start-

ing material is shown in Fig. 1a. The purity of this material

was confirmed by a combination of thermogravimetric analy-

sis and XRD (not shown). The diameter and length of the car-

bon nanotubes were typically around 15–40 nm and a few lm,

respectively. As an example of the purity of product of the

molten salt synthesis process, the X-ray diffraction pattern

of the bulk product of the synthesis carried out with titanium,

shown in Fig. 1e, can be indexed to cubic TiC and a small rem-

nant of the MWCNT source only. The derived cell edge dimen-

sion of the TiC product of 0.4330 nm is very close to the ICDD

database value of 0.4328 nm for cubic TiC and thus suggests a

highly stoichiometric product [18]. The very weak remnant

peak from the strongest MWCNT Bragg (0002) reflection, vis-

ible at about 25.4� also suggests that there is little contamina-

tion with the carbon starting material. However, EEL spectra

of this material (not shown) include an oxygen K-edge signal

(onset at 530 eV) that equates to an oxygen content of a few

at% (relative to carbon). This may indicate oxidation at grain

boundaries as well as on the surface. Some surface oxidation

would naturally be expected, just from exposure to air, but it

is possible that grain boundary oxidation results from rinsing

or drying of the product, particularly as development of peaks

corresponding to TiO2 was observed in XRD patterns of prod-

ucts obtained with increasing drying time, when the drying

temperature was raised from 50 �C to 150 �C. If necessary, this

oxidation could be decreased or removed by drying the sam-

ple at low temperature or by treatment in dilute hydrofluoric

acid prior to use of the product. As an example of the carbide

nanofibres produced from the MWCNT source, Fig. 1b illus-

trates the typical morphology apparent from SEM images of

the titanium-derived sample and shows that the sample is

mainly composed of fibrillar material. The fibres have a mor-

phology and length similar to their MWCNT template (and

carbon source), i.e. these are mostly curved and entangled.

However, the diameters of these fibres are about 40–90 nm,

i.e. larger than that of their carbon nanotube source.

TEM images of curved fibrillar materials derived from tita-

nium are shown in Fig. 1c and d from which it can be clearly

seen that the curved fibres are polycrystalline. The inset in

Fig. 1 – (a) SEM images of carbon nanotubes, (b) fibrillar reaction products and (c and d) TEM images of the curved fibrillar

products derived from titanium after the reaction in molten salts; the inset in (c) is an SAED pattern (indexed to cubic TiC)

from the fibres shown in (c); the inset in (d) is the magnified image from the box in (d) showing that the lattice fringes have a

spacing consistent with the {111} plane of the cubic TiC; (e) X-ray diffraction pattern of the final bulk titanium-derived

product, indexed to stoichiometric TiC.

C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8 203

204 C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8

Fig. 1c shows their SAED pattern which displays diffraction

rings indexable to the {111}, {200}, {220}, {311} and {222}

planes of cubic TiC. The relative intensities of the rings in

the SAED pattern are similar to the standard XRD intensities

for bulk cubic TiC. An atomic resolution TEM image of the

TiC nanofibre is shown in Fig. 1d. Crystalline fringes of vari-

ous orientations can be identified in Fig. 1d, which confirms

that the nanofibre is composed of several nano-crystals. The

inset in Fig. 1d is a magnified HRTEM image of the lattice

fringes from within the box in Fig. 1d. These fringes have a

lattice spacing of 0.250 nm, consistent with {111} planes of

cubic TiC.

The dissolution of metals in molten salts is not well under-

stood, but it has been suggested that transport reactions oc-

cur because metals dissociate to mobile cations and

delocalized electrons, a state which is considered to be inter-

mediate between ionic and metallic [19–22]. The molten salt

mixture is thus believed to facilitate the dissolution and

Fig. 2 – (a) SEM image, (b) bright field TEM image of a straight Ti

from the nanofibre shown in Fig. 1b. The inset in (b) is a true or

fibre, indexed to the [�120] zone axis of TiC. The inset in (c) is the

fringes have a spacing consistent with the {200} plane of the cu

transport of the titanium, and hence the formation of the

TiC, through diffusion of titanium cations from the molten

salt to the surface of the carbon nanotubes with subsequent

reaction.

In some of the experiments using titanium, fibrillar mate-

rials (shown in Fig. 2a) which are straight and smooth in mor-

phology were also found. The diameter and length of these

straight fibres are about 10–35 nm and 10s of lm, respectively.

Fig. 2b shows a bright-field TEM image of a straight fibre and

the inset shows an indexed SAED pattern in true orientation

with respect to the long part of the nanofibre, from which it

was obtained. The SAED pattern can be indexed to the

{002}, {244} and {420} reflections in single crystal cubic TiC.

This is consistent with the crystal being viewed down the

[�120] axis and the orientation implies that the long axis of

the TiC nanofibre is parallel to [001]. The lattice spacing of

0.216 nm for the {002} plane of cubic TiC can be readily re-

solved in the HRTEM as shown inset in Fig. 2c. Taken with

C nanofibre, (c) HRTEM image and (d) EEL spectrum recorded

ientation SAED pattern from the long section of the straight

magnified image from the box in (c) showing that the lattice

bic TiC.

C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8 205

the SAED data, this confirms that the straight TiC nanofibre

has a single crystal structure. An EEL spectrum of this nano-

fibre taken in the TEM is shown in Fig. 2d and reveals the car-

bon K-edge and titanium L2,3-edge which are consistent in

shape, peak position and relative intensity with a fully stoi-

chiometric titanium carbide phase [23,24]. In this case, the

oxygen K-edge signal (onset at �530 eV) is barely detectable,

suggesting there is no more than a few at% oxygen relative

to carbon, and less than in the polycrystalline product.

The subsequent discovery of an air leak in the furnace dur-

ing some of the titanium carbide syntheses suggested that

the oxidation of some titanium powder particles may create

conditions which encourage the formation of single crystal

TiC nanofibres. In order to understand the formation mecha-

nism of the single crystal TiC nanofibre, mixed TiO2 and Ti

powders with a molar ratio in 1:1 were reacted with curved

carbon nanotubes in molten salts at 950 �C for 5 h. Fig. 3

shows the SEM and TEM images of the product derived from

the mixed TiO2 and Ti powders. This figure clearly shows

two different and distinct morphologies. Compared to the

product shown in Fig. 2, a significantly greater number of

Fig. 3 – (a) SEM image and (b–d) TEM images of the fibrillar produ

MWCNTs in molten salts. The inset in (d) is the SAED pattern f

straight nanofibres can now be seen in the SEM and TEM

bright-field images in Fig. 3a and b. Fig. 3c illustrates a higher

magnification TEM image of two straight nanofibres, which

shows that the nanofibres are smooth. An HRTEM image of

a straight nanofibre, is shown in Fig. 3d and crystalline fringes

can be identified which have a lattice spacing of 0.216 nm,

consistent with (200) planes of cubic TiC. Inset in Fig. 3d is

the SAED pattern which shows that the straight nanofibre

consists of single crystal TiC. The pattern can be indexed to

the 111, 200 and 1 �1 �1 reflections in cubic TiC consistent

with the crystal viewed down the [01 �1] axis. The EEL spec-

trum of the single crystal nanofibres (not shown) is also con-

sistent with a fully stoichiometric titanium carbide phase.

The latter result indicates that the addition of TiO2 has a

critical influence on the formation and yield of single crystal

TiC nanofibres. The enhancement of single crystal TiC growth

when oxygen atoms are introduced via the oxide may be be-

cause the oxide represents a less active source of titanium

than the metal, allowing a more gradual, single crystal growth.

Alternatively, a derivative sub-oxide or possibly defective sub-

oxide such as cubic TiO or an oxycarbide may act as a prefer-

cts derived from the reaction of a mixture of TiO2 and Ti with

rom the fibre shown in (d).

Fig. 4 – (a) SEM, (b) bright-field TEM and (c) HRTEM images of zirconium-derived curved nanofibres prepared by reaction of

(template) curved carbon nanotubes in molten salts. Inset in (b) is the corresponding SAED pattern, showing that the product

consists of polycrystalline cubic zirconium carbide. Corresponding (d) SEM, (e) bright-field TEM and (f) HRTEM images of the

tantalum-derived straight fibres prepared from a straight carbon nanotube template in the molten salts. Inset in (e) is the

corresponding SAED pattern which shows that the product is composed of polycrystalline cubic tantalum carbide.

206 C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8

Table 1 – Morphological and crystalline structure data for the transition metal carbide nanofibres as identified by analyticalelectron microscopy and XRD.

Co-reactant with MWCNTs Morphology, length and diameter Crystalline structure

Titanium Templated, few lm long and 40–90 nm diameter Templated fibre fits polycrystalline cubic TiC

Titanium

(with oxide or oxygen)

Templated, few lm long and 40–90 nm diameter;

Straight, 10s of lm long, 10–35 nm diameter

Templated fibre fits polycrystalline cubic TiC;

Straight fibre SAED fits single crystal cubic TiC

Zirconium Templated, few lm long and 40–90 nm diameter Polycrystalline cubic ZrC

Hafnium Templated, few lm long, 40–90 nm diameter Polycrystalline cubic HfC

Vanadium Templated, 200–1000 nm long (950 �C product),

few lm long (900 �C product), 40–90 nm diameter

Polycrystalline cubic VC

Niobium Templated, few lm long, 40–90 nm diameter Polycrystalline cubic NbC

Tantalum Templated, few lm long, 40–90 nm diameter Polycrystalline cubic TaC

C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8 207

ential nucleation site for carbide formation. In addition, the

close correspondence between the graphite 100 d-spacing

(0.213 nm) and the cubic TiC 200 d-spacing (0.216 nm) may

suggest that the single crystal growth may occur epitaxially

on the MWCNTwith little strain. Based on simple geometrical

arguments, this would be expected to form a coherent inter-

face provided the interfacial length in one dimension is no

greater than approximately 15 nm. However, the exact growth

mechanism of single crystal TiC nanofibres is still under study

and remains to be confirmed unequivocally.

Nanofibres of the carbides of zirconium, hafnium, vana-

dium, niobium and tantalum were all obtained at 950 �Cusing a method similar to that used to produce the TiC

nanofibres, presumably via the same cation dissolution

and diffusion mechanism as outlined above for titanium.

Their morphology and crystalline structure were also char-

acterized by SEM and TEM as exemplified for the cases of

zirconium carbide and tantalum carbide in Fig. 4. Both

curved ZrC nanofibres and straight TaC nanofibres are

formed, with a polycrystalline cubic structure in each case,

and with a morphology similar to that of the original

curved or straight carbon nanotube reactive templates,

respectively. The corresponding EEL spectra for these two

nanofibres show carbon K-edges which are consistent in

shape, peak position and relative intensity with the rele-

vant stoichiometric carbide phase [23,24]. Polycrystalline

HfC, VC and NbC were also produced from reaction of

MWCNTs with the relevant element under these conditions

as summarised in Table 1. Under the oxygen-free condi-

tions employed for the reactions of these five metals, no

evidence of single crystal nanofibre production was ob-

served in any of their carbide products. The fact that the

vanadium carbide nanofibres are shorter than those of

the other carbides suggested that the reactivity of vana-

dium is higher than that of the other metals under the

standard reaction conditions (950 �C). This idea was sup-

ported by the fact that a later supplementary reaction with

vanadium under lower temperature conditions (900 �C)

yielded templated vanadium carbide nanofibres of similar

length to that of the MWCNT carbon source.

4. Conclusions

Carbide nanofibres were produced by reactions of transition

metals on MWCNTs in molten salts. Titanium, zirconium,

hafnium, vanadium, niobium and tantalum powders were

each employed as a metal source for the synthesis of nano-

structured carbide fibres and this method produced carbide

nanofibres from all of the six early transition metals that

have been trialed so far. Significantly, it has been shown that

the carbon source employed in this process can act as a

reactive template which dictates the resulting carbide mor-

phology. In this case, since MWCNTs were used as the car-

bon source, their unique morphology was exploited as a

template in order to yield carbide nanofibres. In all cases,

transition metal carbide nanofibres were produced from

their metal sources in polycrystalline (cubic) form and with

morphologies that were reflective of the MWCNT source.

However, when a mixture of titanium and titanium dioxide

was used as the titanium source, cubic titanium carbide

was produced in both straight, single crystal and templated,

polycrystalline forms.

This work has demonstrated that molten salt synthesis

provides a convenient and general route to the production

of carbide materials with MWCNTs acting not only as a

source of carbon but also as a template to produce carbide

nanofibres of controlled morphology and structure. However,

it can be envisaged that, in general, other carbide morpholo-

gies will be produced depending upon the morphology of the

original carbon source, e.g. carbide coatings from reaction on

carbon fibres [22] or carbide particles from reaction with car-

bon black. The method therefore offers possibilities for con-

trollable preparation of transition metal carbide materials

using a variety of carbon sources so as to obtain materials

with differing morphologies, sizes and microstructures. In fu-

ture studies, following elucidation of the nucleation and

growth mechanisms of the carbides, variation of the synthe-

sis conditions can be expected to alter the yield, and possibly

the crystallinity, of the carbide products and to offer control

over the size and shape of the carbide product on the nano-

meter scale. Potential applications for carbide nanomaterials

include their use as catalysts [25,26] and cold-field electron

emitters [27].

Acknowledgements

The authors acknowledge the financial support of the

National Natural Science Foundation of China (Grant No.

50672070) and Royal Society K.C. Wong Education

Foundation.

208 C A R B O N 4 7 ( 2 0 0 9 ) 2 0 1 – 2 0 8

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