The effect of KI on the formation of Tl2E (E = S, Se) nanorods via solvothermal route

Article abstract of DOI:10.1016/j.ssc.2004.02.022

Tl₂E (E=S or Se) nanorods were synthesized via solvothermal route with the addition of KI. The products were characterized with X-ray powder diffraction patterns and transmission electron microscope images. Their optical properties were studied by UV-vis transmittance and photoluminescence spectrum. The band gap of direct forbidden transitions was found larger than that of bulk materials, because of the blue shift caused by nanometer-scale crystalline particles due to quantum confinement effects. A possible growth mechanism is proposed.

Full text of DOI:10.1016/j.ssc.2004.02.022

Solid State Communications 130 (2004) 297–300
www.elsevier.com/locate/ssc
The effect of KI on the formation of Tl2E (E ¼ S, Se) nanorods
via solvothermal route
*
Youbao Ni, Mingwang Shao , Zhengcui Wu, Feng Gao, Xianwen Wei
College of Chemistry and Materials Science, Anhui Normal University, Anhui Province, Wuhu 241000, China
Received 21 July 2003; accepted 12 February 2004 by T.T.M. Palstra
Abstract
Tl2E (E ¼ S or Se) nanorods were synthesized via solvothermal route with the addition of KI. The products were
characterized with X-ray powder diffraction patterns and transmission electron microscope images. Their optical properties
were studied by UV–vis transmittance and photoluminescence spectrum. The band gap of direct forbidden transitions was
found larger than that of bulk materials, because of the blue shift caused by nanometer-scale crystalline particles due to quantum
confinement effects. A possible growth mechanism is proposed.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 78.40Fy; 81.07.Bc; 81.05.Hd
Keywords: A. Nanostructures; B. Crystal growth
1. Introduction
methods [6–8]. Because solvothermal route is one of the
most promising preparations [9], we employed it to prepare
thallium(I) sulfide and thallium(I) selenium nanorods.
In recent years, one-dimensional (1D) nanostructural
materials have become an exciting research focus because
of their great potential of addressing some basic issues about
dimensionality and space-confined transport phenomena
[1]. However, the fabrication or synthesis of these 1D
nanostructural materials is a challenge owing to their
extremely small size and their large anisotropy. Nowadays,
much attention has been paid to Tl2E (E ¼ S or Se) because
of their various interesting and potentially useful electrical
properties. These studies mainly stemmed from the detec-
tion of photoconductivity and photovoltaic effect in Tl2S
and semiconduction of Tl2Se [2,3]. Traditionally Tl2S was
synthesized using thallium ethoxide in ethanol and dry H2S
[4,5], while the compound of Tl2Se was prepared [4] by
heating the component elements in a sealed tube at about
400 8C.
2. Experimental procedure
All reagents were of analytical grade purity and were
purchased from Shanghai Chemical Co. Ltd. The typical
procedure for the preparations is as follows: 0.01 mol of
Tl2SO4, 0.01 mol sulfide powder and 0.005 mol KI were
added to a Teflon reactor of 50 ml capacity, then
ethylenediamine was filled into 90% of the total volume.
The reactor was maintained at the temperature of 150 8C for
48 h. Then the products were collected, washed with ethanol
and dried in vacuum at 60 8C for 6 h.
The phase and the crystallographic of the products were
characterized by X-ray diffraction (XRD) pattern, which
was recorded by using a Shimadzu XRD-6000 X-ray
diffractometer equipped with Cu Ka radiation ðl ¼
0:15406 nmÞ; the scanning rate of 0.058 s21 was applied to
record the pattern in the 2u range of 10–708.
Recently, the chemical solution routes are emerging as
effective, convenient and less energy-consuming synthesis
*
Corresponding author. Tel.: þ86-553-3869086; fax: þ86-553-
3869303.
E-mail address: mwshao@mail.ahnu.edu.cn (M. Shao).
The transmission electron microscope (TEM) image was
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.02.022

298
Y. Ni et al. / Solid State Communications 130 (2004) 297–300
Fig. 1. (a) The XRD pattern of as-prepared products, which
indicates a hexagonal phase of Tl2S. (b) The XRD pattern of as-
prepared products, which indicates a tetragonal phase of Tl2Se.
taken with a Hitachi Model H-600 TEM, using an
accelerating voltage of 75 kV to determine the morphological
features. Samples were prepared by placing a drop of dilute
alcohol supersonic disperser, onto a carbon-coated copper
grip and allowing the alcohol to evaporate in the dark.
UV–vis absorption spectra were measured on a Hitachi
U-4100 spectrophotometer with the samples dispersed in
alcohol at room temperature model. Photoluminescence
(PL) spectra were recorded using a Hitachi F-4500
fluorescence spectrophotometer at room temperature.
3. Results and discussions
The XRD pattern of as-prepared products (Fig. 1(a)),
showed the presence of reflections characteristic of the Tl2S
hexagonal phase of the products. The intense peaks indicate
that the as-prepared products have high degree of crystal-
linity. The lattice constants calculated for the sample are
a ¼ 1:221 ^ 0:008 nm; c ¼ 1:829 ^ 0:01 nm; which is in
agreement with the values of a ¼ 1:222 nm; c ¼ 1:821 nm
reported by JCPDS 06-378.
The typical morphology of Tl2S products is presented in
Fig. 2(a) via TEM observations. They all take the shape of
nanorods with the average size of 100 nm in diameter and
length up to 1.7 mm.
The optical property of the products was also investi-
gated. Fig. 3(a) shows the transmittance spectrum of Tl2S
Fig. 2. (a) TEM image of many Tl2S nanorods with the addition of
KI. (b) TEM image of Tl2S nanorods without the use of KI. (c) TEM
image of many Tl2Se nanorods with the addition of KI.

Y. Ni et al. / Solid State Communications 130 (2004) 297–300
299
Fig. 4. PL spectrum of as-prepared Tl2S nanorods.
speculate that the presentation of KI plays an important
role. During the formation of nanorods, Tl2S nucleus absorb
anion (I2) on crystal planes selectively and grow large along
the c-axis direction, which makes the nanoparticles long and
slim. To confirm our proposed mechanism, we did control
experiment without the addition of KI, keeping the other
experimental variables invariant, the nanoparticles could
develop in rod-like shape but irregular, as shown in Fig. 2(b).
The synthesis route may be used to other systems. When
selenium powder was used as starting material instead of
sulfide powder, Tl2Se was obtained.
The XRD pattern of Tl2Se products shows strong peaks
and can be indexed to tetragonal phase, as shown in
Fig. 1(b). The lattice constants calculated for the sample are
a ¼ 0:855 ^ 0 : 001 nm; c ¼ 1:267 ^ 0:003 nm; which is
good according to the values of a ¼ 0:854 nm; c ¼ 1:270
nm reported by JCDPS 47-1299.
2=3
Fig. 3. (a) The transmittance spectrum of Tl2S. (b) The ðhnaÞ
versus photon energy ðhnÞ of Tl2S.
Fig. 2(c) is the TEM image of Tl2Se, which show
inhomogeneous nanorods. The size of Tl2Se nanorods is
about 75 nm in diameter and 900 nm in length, which is
smaller and shorter than that of Tl2S. The morphology of
Tl2Se is not good because it is sensitive to electron beam
irradiation during the examination, which is expected due to
its low melting point (340 8C) while Tl2S melts at 448 8C.
We also did control experiment without the addition of KI,
the nanopaticles could develop in rod-like shape, too.
The transmittance spectrum of Tl2Se was also measured.
products. The clear maximum at 370 nm is assigned to the
optical transition of the first excitonic state.
According to Smith [10], the value of ðhnaÞ in
n
semiconductors is proportional to ðhn 2 EgÞ ; where h is
Planck’s constant and n; the frequency of the optical
radiation absorbed. Hence a plot ðhnaÞ versus ðhnÞ would
yield straight-line plots, with Eg as the intercept on the ðhnÞ-
axis. The optical transitions and hence the band gap are
classified as direct, direct forbidden, indirect, and indirect
forbidden for n ¼ 1=2; 3=2; 2 and 3, respectively. The band
gap of Tl2S is classified as direct forbidden and the value of
n is equal to 2/3 [11]. So we plot ðhnaÞ2=3 versus ðhnÞ; shown
in Fig. 3(b) and obtain the value of the band gap 4.55 eV.
The value is larger than that of bulk materials [11], which
would be the blue shift caused by nanometer-scale crystal-
line particles due to quantum confinement effects.
2=3
Like Tl2S, we plotted ðhnaÞ versus ðhnÞ and got the value
of the band gap 3.10 eV. The maximum in the PL emission
spectrum of Tl2Se is at 379 nm when 250 nm excitation
source was used.
4. Conclusion
PL emission spectrum of Tl2S is shown in Fig. 4. The
emission spectrum was measured with 280 nm excitation
source. The maximum is at 370 nm.
Under the solvothermal route, nanorods Tl2E (E ¼ S, Se)
The possible growth mechanism is proposed. We
were synthesized, with the additional KI, approving

300
Y. Ni et al. / Solid State Communications 130 (2004) 297–300
[3] M. Hansen, K. Anderko, Constitution of Binary Alloys,
second ed., McGraw-Hill, New York, 1958.
morphology was obtained. The solvothermal route is easy to
be maintained and controlled. This method may be useful in
the synthesis of the other composite materials.
[4] J.C. Bailar, H.J. Emeleus, A.F. Trotman-Dickenson, Compre-
hensive Inorganic Chemistry, Pergamon Press, New York,
1977, p. 1148.
[5] G. Brauer, Handbook of Preparative Inorganic Chemistry,
second ed., vol. 1, Academic Press, New York, 1963.
[6] F.F. Lange, Science 273 (1996) 903.
Acknowledgements
The project was supported by the National Natural
Science Foundation of China and Excellent Scholar
Foundation of Anhui Province Education Administration.
[7] W.S. Sheldrich, M. Wachhold, Angew Chem., Int. Ed. Engl.
36 (1997) 206.
[8] R. Komatsu, N. Wtanabe, E. Komai, A. Kitaaakaze, K. Ikeda,
J. Appl. Phys. Part 139 (9B) (2000) 5662.
[9] Y. Xie, Y. Qian, W.Z. Wang, S.Y. Zhang, Y.H. Zhang,
Science 272 (1996) 1926.
References
[10] R.A. Smith, Semiconductors, second ed., Cambridge Univer-
sity Press, Cambridge, 1978, p. 313.
[1] C. Dekker, Phys. Today 52 (1999) 22.
[11] V. Estrella, M.T.S. Nair, P.K. Nair, Thin Solid Films 414
(2002) 289–295.
[2] M. Wolf, in: C.E. Backus (Ed.), Proceedings of the 25th Power
Sources Symposium, May 23–25, 1972, pp. 120–124 and
reprinted in: Solar Cells, IEEE, New York, 1976, pp. 38–42.