Electrochemical production of low-melting metal nanowires

Article abstract of DOI:10.1016/S0009-2614(99)00003-2

The extent of nanotube formation arising from the electrolysis of molten LiCl, using graphite electrodes, is reduced by the addition of salts to the electrolyte. In certain cases (99% LiCl+<1% SnCl₂), considerable quantities of tin-containing n

Full text of DOI:10.1016/S0009-2614(99)00003-2

1
9 February 1999
Chemical Physics Letters 301 Ž1999. 159–166
Electrochemical production of low-melting metal nanowires
a
c
b
a
a
a
W.K. Hsu , J. Li , H. Terrones , M. Terrones , N. Grobert , Y.Q. Zhu ,
a
a
d
a
a,)
S. Trasobares , J.P. Hare , C.J. Pickett , H.W. Kroto , D.R.M. Walton
a
School of Chemistry, Physics and EnÕironmental Science, UniÕersity of Sussex, Brighton BN1 9QJ, UK
b
Instituto de Fisica, UNAM, Apartado Postal 20-364, Mexico D.F. 01000, Mexico
c
China UniÕersity of Geosciences, National Lab of Mineral and Rock Materials Exploitation and Application, Beijing 100083, China
d
John Innes Centre, Norwich Research Park, Colney, Norwich NR4 5UH, UK
Received 3 August 1998; in final form 12 November 1998
Abstract
The extent of nanotube formation arising from the electrolysis of molten LiCl, using graphite electrodes, is reduced by
the addition of salts to the electrolyte. In certain cases Ž99% LiClq-1% SnCl ., considerable quantities of tin-containing
2
nanotubes are formed in addition to Li C . However, the tube walls are poorly graphitised. Analogous results were obtained
2
2
by adding trace amounts of Bi, Pb or Sn powder to the LiCl. q 1999 Elsevier Science B.V. All rights reserved.
1
. Introduction
in amorphous carbon particles. However, when the
SnCl concentration was reduced to less than 1% by
2
In addition to the commonly employed arc dis-
charge technique, pyrolysis of hydrocarbons and
metal-catalyzed laser ablation of graphite w1–6x, a
weight, tin-containing nanotubes Žca. 30–40% of the
observed material. were produced w10,11x. The walls
of the tubes, however, appear to be fairly amor-
phous.
In this Letter, we describe a detailed and extended
study involving other mixed electrolytes, e.g. LiCl
containing ca. 1–50% by weight of KCl, CuCl2 ,
novel method for producing nanotubes and
fullerene-related materials, involving the passage of
a direct current through a graphite rod Žcathode.
immersed in a molten ionic salt Že.g., LiCl or LiBr.
contained in a graphite crucible Žanode. w7–9x, has
recently been developed. In our earlier studies w7–9x,
ZnCl2 or CaCl . Small amounts Žca. -1% by
2
weight. of Bi, Co, Cu, In, Pb, Sn or Zn powder were
2
0–40% of the carbon-based products consisted of
also added to the LiCl.
well-graphitised multi-walled nanotubes, the remain-
der being amorphous carbon and polyhedral parti-
cles. A proportion of the nanotubes and particles
contained encapsulated Li C w9x. Addition of 1–
2
. Experimental
2
2
5
0% Žby weight. of SnCl to the LiCl resulted only
2
The electrolysis apparatus consists of a heated
quartz glass tube Žlength, 40 cm; OD, 5 cm. with gas
inletroutlet flanges. A crucible anode was made by
drilling a 3 cm diameter hole to a depth of 7 cm in a
)
Ž
.
Corresponding author. Fax: q44 1273 677196; e-mail:
d.walton@sussex.ac.uk
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.00003-2

Table 1
Production of nanostructures using composite electrolytes
Electrolytes
Additive salts or metal powder in LiCl
Nanotubes
Nanowires
Metal-encapsulated particles
Amorphous and polyhedral carbon particles
Ž% by weight.
Ž%.
Ž%.
Ž%.
Ž%.
KClqLiCl
2
1–50
10–50
1–10
10–50
1–10
10–50
1–10
10–50
5–10
20–40
1–5
20–40
60–80
)95
60–80
100
)90
100
)90
100
95
CaCl qLiCl
CaCl qLiCl
2
CuCl qLiCl
2
CuCl qLiCl
-1
-1
5–10
5–10
2
ZnCl qLiCl
2
ZnCl qLiCl
2
SnCl qLiCl
2
SnCl qLiCl
5
1–5
5–10
10
5–10
5–10
30–40
5–10
10–20
30–40
2
SnCl qLiCl
1–5
-1
-1
-1
-1
-1
-1
1–2
-1
-1
1–5
90
2
SnCl qLiCl
0.8–1
0.8–1
0.8–1
0.8–1
0.8–1
0.8–1
0.8–1
0.8–1
30–40
30–40
30–40
20–30
50–60
50–60
50–60
50–60
60–70
)90
80–90
60–70
2
SnqLiCl
PbqLiCl
BiqLiCl
CuqLiCl
CoqLiCl
InqLiCl
ZnqLiCl
Column heading percentages based on TEM observations.

W.K. Hsu et al.rChemical Physics Letters 301 (1999) 159–166
161
Fig. 1. Carbon nanotubes produced using: Ža. LiCl as electrolyte; Žb. LiClqKCl Ž1–50% by weight. as electrolyte.

1
62
W.K. Hsu et al.rChemical Physics Letters 301 (1999) 159–166
Fig. 2. Ža. Encapsulated particles produced from LiClqCuCl Ž1;5% by weight.. X-ray diffraction measurements confirm the presence of
2
pure Cu within the carbon cage. Žb. Sn nanowires produced from LiClqSnCl Ž0.8–1% by weight.. EDX and X-ray diffraction analyses
2
confirm the presence of b-Sn containing nanowires.

W.K. Hsu et al.rChemical Physics Letters 301 (1999) 159–166
163
cylindrical block of high-purity graphite Žlength, 10
cm; OD, 4 cm; Le Carbone, Portslade, UK.. The
salts, were added to the LiCl. After electrolysis, the
apparatus was set aside to cool. Distilled water was
added to the crucible and the mixture was worked up
in order to obtain the carbonaceous products Žca.
20–30 mg. Žsee Refs. w7–9x for a detailed description
of this procedure.. The results of these experiments
cathode consisted of a 3 mm diameter carbon rod
positioned at various depths within the crucible by
means of a screw mechanism. The crucible was
filled with the ionic saltsrpowder Žca. 50 g. and the
apparatus was evacuated and purged with argon
Ž100–150 Torr.. The oven temperature was then
raised Ž208Crmin. until the ionic salts melted Žca.
are given in Table 1.
6
008C.. The current was varied by carefully adjust-
3
. Results
ing the voltage Ž0–20 V at 3–5 A.. For the sake of
uniformity, the current was maintained for 2 min in
each electrolysis experiment and the cathode immer-
sion depth was set at 2 cm.
Electrolysis of LiClrKCl mixtures, containing up
to 50% Žby weight. KCl, gave results identical to
Experiments were conducted using various salt
ratios, e.g., KClrLiCl, CaCl rLiCl, CuCl rLiCl,
those obtained using pure LiCl, i.e., 20–40% nan-
otubes ŽFig. 1.. Addition of CaCl2 Ž10–50% by
weight. to LiCl, however, led to a decrease in nan-
otube formation Žto ca. 1–5%.. Reduction of the
2
2
and ZnCl rLiCl. In addition, experiments were car-
2
ried out in which trace amounts Ž-1% by weight. of
metal powders, e.g., Bi, Co, Cu, In, Pb, Sn and Zn
CaCl content to below 10% by weight lead to the
2
Ž325 mesh, 99.8%., Aldrich Chem.., instead of metal
reestablishment of the nanotube yield. Addition of
Fig. 3. Ža. Pb nanowires produced from LiClqPb Ž0.8–1% by weight.. Žb. Bi nanowires produced from LiClqBi Ž0.8–1% by weight.
mixtures.

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64
W.K. Hsu et al.rChemical Physics Letters 301 (1999) 159–166
CuCl , or ZnCl Ž10–50% by weight. to LiCl pro-
otubes ŽFig. 3a,b.. By contrast, the addition of Co,
2
2
duced only amorphous carbon, whereas the presence
Cu, In or Zn powder to LiCl yielded predominantly
amorphous carbon, accompanied by encapsulated
Žmetal. particles but negligible amounts of nan-
of less than 10% of CuCl or ZnCl resulted in the
2
2
formation of some encapsulated particles and amor-
phous carbon ŽFig. 2a. to the exclusion of nanotubes.
otubes.
In none of these experiments were metal-containing
nanotubes observed.
The nanotube length varies from metal to metal,
e.g., 3–5 mm for Sn and Pb and 1–3 mm for Bi.
HRTEM images ŽFig. 4a,b. ŽED-X analyses. of Bi-
In previous papers we demonstrated that tin-con-
taining nanotubes were formed by the electrolysis of
LiCl-containing SnCl2 Ž-1% by weight. w10,11x.
and Pb-containing nanotubes, respectively, confirm
that only the metals, and not their halides, were
present. A previous report described the movement
of encapsulated tin within nanotubes under the influ-
ence of electron irradiation w10,11x. Similar behavior
We have now shown that replacement of SnCl by
2
tin powder Ž-1% by weight. gives the same result
ŽFig. 2b.. Experiments using powdered Bi or Pb also
generated the corresponding metal-containing nan-
was observed for Pb but not for Bi.
Fig. 4. HRTEM images: Ža. a single Pb-containing nanowire; Žb. a single Bi-containing nanowire. The EDX spectrum of the individual
wires is shown in the insert. The Cu profiles were built up from a TEM copper grid.

W.K. Hsu et al.rChemical Physics Letters 301 (1999) 159–166
165
4
. Discussion
dissolution of graphite and re-formation of carbon
nanostructures? The two obvious cathodic reactions
of Li are reduction to the metal at the surface and
The fascinating discovery that during electrolysis
q
of molten lithium chloride in a single compartment
cell with a graphite cathode Žrod. and graphite anode
Žcrucible., well-graphitised nanotubes are formed is
not an obvious or the expected result, nor one that is
easy to explain.
intercalation of Li between graphitic layers. At tem-
peratures above 6008C, graphite dissolves in molten
Li to give a thermodynamically very stable Li C
2
2
phase. The material, identified by electron diffraction
w9x, was found to be encapsulated within well-graphi-
q
The prerequisite for Li reduction, the observa-
tised carbon nanotubes, suggesting that the formation
of Li C may be a key step in cathode dissolution
tion of Li C formation, and the inhibition of nan-
otube formation when ions with a more positive
discharge potential ŽCa , Co , Zn , Cu , Sn
2
2
2
2
and subsequent nanotube formation ŽReaction Ž1..:
2
q
2q
2q
2q
2q
,
2
q
Pb , etc.. are present at high concentration in the
molten salt provides an insight into the electrochem-
istry of nanotube formation which is now considered.
The experimental data in Table 1 suggests that re-
duction of lithium cations at the graphite cathode
plays a crucial role in the formation of nanostruc-
tures. Thus when easily reducible metal cations
q
y
2
Li q2e q2C
™Li C .
1
Ž .
graphite
2
2
Slugs of Li C are often found encapsulated
2
2
within the well-graphitised carbon nanotubes and we
estimate that ca. 10% of available nanotube inner
core volume is filled with this material. That the
material is within the nanotubes prompts the sugges-
tion that C atoms extrude or segregate as an ordered
hexagonal array, resulting in graphitic nanotubes. A
somewhat analogous process has been suggested in
order to explain the metal-catalysed gas-phase for-
mation of boron nitride ŽBN. nanotubes w12x. In this
ŽCu2 , Zn , Sn . are present at high concentra-
tion, they are discharged as the free metals in prefer-
ence to Li and the formation of nanostructures is
prevented. The presence of KCl does not interfere in
q
2q
2q
q
q
the process because Li is preferentially reduced.
2
q
Intermediate behaviour is observed with Ca . At
high concentration this cation is preferentially dis-
charged and the formation of nanotubes is thus inhib-
ited, lowering the loading of Ca and re-establish-
ing Li as the major charge carrier. The yield of
experiment, amorphous BN material reacts with a
metal ŽTa. to form a crystalline TarBN particle.
Subsequently, segregation or extrusion of ordered
BN from the alloy facilitates the formation of hexag-
onally infrastructured nanotubes.
2
q
q
nanotubes consequently increases.
The question now arises: what is the nature of the
cathodic reaction of lithium cations which leads to
While the decomposition of Li C to graphite
nanotubes and Li metal might occur under the condi-
2
2
Fig. 5. Plausible scheme for electrolytic formation of amorphous carbon-coated metal nanowires.

1
66
W.K. Hsu et al.rChemical Physics Letters 301 (1999) 159–166
tions of our experiments, an alternative possibility is
oxidation of Li C leading to graphitisation, i.e.,
reversing Reaction Ž1.. The most likely oxidant in
the system is chlorine which is generated at the
Žlower. anode and sparges through the melt ŽReac-
tion Ž2...
melting points Ž200–3008C. so that the molten state
of metal may facilitate nanowire generation. This
behaviour would account for the presence of metal-
encapsulated particles, instead of nanowires in exper-
iments involving high-melting-point Co and Cu. The
preferential formation of metal-encapsulated parti-
cles in the presence of low-melting-point metals,
such as Zn and In, remains to be explained.
2
2
y
Li C qCl ™C q2Liq2Cl .
2
Ž .
2
2
2
2
The formation of a conducting sheath around
Li C suggests a growth mechanism whereby chlo-
2
2
rine is reduced on the graphite surface by oxidation
of the carbide interior, i.e., a short-circuited electro-
chemical cell. As graphite is deposited on the surface
Li C is extruded forming a fresh surface for graphi-
tisation, thus propagating tubule growth.
The presence of easily reducible metal halides of
Sn, Pb or Bi provides alternative oxidants to chlorine
ŽReaction Ž3...
Acknowledgements
We thank the Royal Society ŽUK. for financial
assistance Žto WKH, MT and YQZ., Conacyt-
Mexico, DGAPA-UNAM IN-107-296 ŽHT. and the
DERA ŽUK. for supporting NG. We also thank the
EPSRC for financial support, Dr. J. Thorpe and D.P.
Randall for help with electronmicroscopy.
2
2
Li C qM2q™2C
q2Li qM .
q
Ž .
3
2
2
graphite
In such cases the encapsulated material is the
metal itself, not the carbide. However, the same
concept can be applied to the self-assembly of
nanowires. Sn or Pb cations are discharged at the
interior metal surface by electrons furnished by oxi-
dation of adsorbed Li C . Thus coupling of oxida-
tion of the carbide and deposition of metal would
produce a graphite sheet around a molten metal core
by further adsorption of the carbide, oxidative
graphitisation and metal deposition ŽFig. 5..
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