# Zinc Sulfide Tubes Reinforced with Carbon Nanofibers

Authors: N. N. Kolesnikov; D. N. Borisenko; E. B. Borisenko; A. V. Timonina; V. V. Kveder
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

Zinc sulfide submicron and nanotubes with outer diameters in the range from 100 to 1000 nm were produced through chemical deposition from vapor under argon pressure. The novel process provides formation of ZnS tubes reinforced with carbon nanofibers. This is the first time that the ZnS tubes are grown with fibers during deposition.


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## Body

## 1. Introduction

Interest  in nanocomposites increases
due to their special properties and potential applications. Semiconductor II-VI
composites are interesting because of their size-dependent optical properties,
size-dependent band gap energies [1–3]. Core/shell
products, for example, ZnS/HgS, CdSe/ZnS fabricated by electro-deposition, or
chemical solution transport [1, 2] are of special interest due to varying
electronic properties dependent on core/shell weight ratio. In [3] ZnS/Si
core/shell tubes were fabricated by epitaxial growth. ZnS was a substrate used
to produce Si tubes for semiconductor industry. ZnS templates were afterwards
chemically removed. The core/shell structures are similar to the structures we obtain, but not
the same.We have developed a method
of fabrication of ZnS/C nanocomposite tubes by direct ZnS evaporation in a
graphite crucible. This technique is a kind of CVD method: carbon from graphite
crucible reacts with ZnS. Then at lower temperature carbon deposits along with
ZnS according to (1), so that the ZnS forms tubes and the tubes’ walls are
reinforced with the graphite nanofibers. This material is new and may be
interesting as a reinforced semiconductor nanomaterial.


## 2. Experimental

Zinc sulfide tubes of the outer diameter 100–500 nm were grown
in a graphite crucible of 40 mm in diameter and 100 mm in height. The
crucible was loaded with 100 g of zinc sulfide. The crucible was placed into the autoclave which had a
graphite resistance heater. The setup is shown in Figure1.Figure 1
Scheme of the setup for growth of ZnS+C nanocomposite: (1) heater, (2) graphite
crucible with ZnS in the working position, (3) current
leads, (4) heat
screen, (5) case
of high-pressure vessel (with water cooling jacket), (6) flange, and (7) soot
with ZnS and graphite nanocomposite.First, the pressure in the autoclave reached1.3⋅10−2 mbar, then the chamber was filled with argon. The ZnS powder was preliminary
annealed for 1 h at the temperature 600°C in argon atmosphere under pressure
52 atm. After cooling to room temperature, the furnace was three times cleaned
with high-purity argon and then heated to 1800°C. The synthesis was performed for
1.5 h by the first reaction in (1) in the temperature range 1800–1850°C under 58 atm argon pressure, then the furnace was cooled and ZnS submicro and nanotubes
deposited simultaneously  with graphite nanofibers by the second reaction in (1):(1)C(S)+ZnS(V)→≥1800°CZn(V)+CS2→T↓ZnS↓nano+CGNV, where C(S) is graphite, ZnS(V) is
gaseous ZnS, CGNV
is
graphite nanofibers.The ZnS submicro and nanotubes with carbon fibers were deposited on
cool parts of the furnace (no. 7 in Figure1).
Macroscopically, the product of the reaction looked like gray soot. The deposited tubes were studied by transmission electron microscopy
(TEM) on a JEM 100 CX TEM microscope. X-ray diffraction (XRD) patterns were
taken on a Siemens 500D X-ray diffractometer, whereas electron probe microanalysis
(EPMA) was performed on a VegaTescan setup.The tubes were then etched in hot HCl and rinsed in distillated water.
Some of these samples were annealed after etching at 870–890°C during 7
days, then cooled to room temperature. Microstructure, phase content and
composition of the etched, and annealed samples were also studied by the
abovementioned TEM, XRD, and EPMA techniques.


## 3. Results and Discussion

The X-ray diffraction pattern of the product of the reaction is shown in
Figure2. The product consists mostly  of wurtzite ZnS. Some  carbon is sometimes
present in the diffractograms.Figure 2
X-ray diffraction pattern of the
deposited ZnS tubes.Figure3 shows (a) ZnS tubes, (b) the tube
wall (3) containing channels (2) filled with graphite fibers (1), (c) the
schematic view of a ZnS tube (3) containing channels (2) filled with graphite
fibers (1). TEM image and the electron diffraction pattern of the grown ZnS
crystals (2) containing C fibers (1) are shown in Figure 4 on the left. The
electron diffraction pattern in the inset contains reflections from ZnS crystal
and rings from carbon. The electron diffraction patterns were taken so that the
incident beam was normal to the amorphous substrate. The ZnS tubes are randomly
oriented on the surface. However, one zone 〈0001〉 can be distinguished.ZnS tubes deposited from vapor: (a) ZnS
tubes; (b) ZnS tube walls with channels containing graphite nanofibers; (c)
schematic diagram of ZnS tubes reinforced with C nanofibers: (1) graphite
fibers, (2) channels, (3) ZnS tubes.(1) ZnS tubes; on the right: graphite
nanofibers, (2) ZnS tube; on the left: after etching in HCl a network of C
fibers is revealed.In the stable state ZnS has a cubic lattice with
sphalerite structure. However, it undergoes a polymorphic transition at 1020°C when sphalerite
transforms in wutrzite. In our case ZnS is quenched from the high-temperature
state (see the reaction) and exists in hexagonal phase at room temperature. Intensities
of the diffraction lines in the XRD patterns (Figure2) differ from the ratio of
intensities of a standard sample (JCPDS card 36-1450). These TEM diffraction and XRD
data confirm that some grown directions prevail in the tubes, in particular, c
direction of the wurtzite structure.The image in Figure4 on the right is taken from the samples etched in
hydrochloric acid. It is seen that etching in HCl causes dissolution of zinc
sulfide and reveals network of carbon fibers.The following annealing ZnS removes completely and only carbon fibers
remain. This is seen from Figure5 which shows the TEM images of the etched and
annealed samples (left) and the annealed sample (right). In the insets on the
left and on the right, electron diffraction patterns show rings from carbon.Graphite fibers from ZnS tubes remained after etching and
annealing (on the left), annealing (on the right).It should be noted that the effect of annealing can be detected visually:
the initially gray powder of ZnS and carbon mixture becomes absolutely black
after annealing, which confirms indirectly that only carbon remains after quite
long heating. White pure ZnS crystallites deposit on the cold end of the sealed
tube. Semiquantitative EPMA microanalysis performed by energy-dispersion (ED)
detection system confirms that only about 4% of ZnS remain after annealing, while
2% remain after combination of etching in hot HCl and the following annealing.The experimental data show that during growth of ZnS submicro and
nanotubes in graphite crucible the nanocomposite forms ZnS tubes reinforced
with graphite nanofibers deposit as the furnace temperature is decreased. The
annealing or etching of the tubes removes ZnS, so that only graphite nanofibers
from the tube walls remain.Thus, we have developed a chemical vapor deposition technique to produce
ZnS submicro and nanotubes reinforced with graphite nanofibers.



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*Source: 126354-2009-04-16.xml*