A solvothermal route to CdS nanocrystals

Article abstract of DOI:10.1016/S0009-2614(03)00912-6

A novel solvothermal route for the preparation of organic soluble CdS nanocrystals, involving the reaction of cadmium stearate with sulfur in the presence of tetralin has been described. Tetralin in the presence of S gives H₂S, getting aromatized to naphthalene. By using trioctylphosphineoxide as the capping agent, nanocrystals of 4 nm are obtained. Use of dodecanethiol cap results in 5 nm as well as 10 nm nanocrystals which can be separated readily. The nanocrystals have the cubic zinc blende structure and exhibit blue shifts of the absorption maximum.

Full text of DOI:10.1016/S0009-2614(03)00912-6

Chemical Physics Letters 375 (2003) 560–564
www.elsevier.com/locate/cplett
A solvothermal route to CdS nanocrystals
1
Ujjal K. Gautam, Ram Seshadri , C.N.R. Rao
*
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
Received 18 March 2003; in final form 19 May 2003
Published online
Abstract
A novel solvothermal route for the preparation of organic soluble CdS nanocrystals, involving the reaction of
cadmium stearate with sulfur in the presence of tetralin has been described. Tetralin in the presence of S gives H2S,
getting aromatized to naphthalene. By using trioctylphosphineoxide as the capping agent, nanocrystals of 4 nm are
obtained. Use of dodecanethiol cap results in 5 nm as well as 10 nm nanocrystals which can be separated readily. The
nanocrystals have the cubic zinc blende structure and exhibit blue shifts of the absorption maximum.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
workers [7] have reported the preparation of
various sizes of monodisperse CdE (E ¼ S, Se,
Te) starting from the corresponding organo-
metallic precursors. OÕBrien and coworkers [8,9]
have prepared 4 nm CdS nanocrystals using a
single source precursor. Other preparative tech-
niques include sol–gel processing [10], microwave
heating [11], photoetching [12] and reverse mi-
celle [13]. El-Shall and coworkers [14] have re-
cently prepared CdS nanocrystals at room
temperature by a reduction route, starting from
CdCl2, sulfur and NaBH4.
In this Letter, we report a novel solvothermal
route for the preparation of soluble CdS nano-
crystals. In this method, H2S generated in-situ by
the reduction of sulfur in presence of tetralin, re-
acts with cadmium stearate in toluene medium to
yield CdS, by a procedure analogous to that em-
ployed to synthesize CdSe nanocrystals [15]. By
employing a suitable capping agent such as tri-
octylphosphineoxide (TOPO) or dodecanethiol,
Semiconductor nanoparticles have attracted
widespread attention because of their special
optical and electronic properties arising from the
quantum confinement of electrons and large
surface area [1–3]. Nanocrystals of II–VI semi-
conductors and of CdS in particular, have been
extensively studied due to their potential appli-
cations such as field effect transistors, light
emitting diodes, photocatalysis and biological
sensors [3–6]. A variety of synthetic routes for
monodisperse CdS nanocrystals have been re-
ported in the literature. Thus, Bawendi and co-
* Corresponding author. Fax: +91-80-846-2766.
E-mail addresses: gautam@sscu.iisc.ernet.in (U.K. Gau-
tam), seshadri@mrl.ucsb.edu (R. Seshadri), cnrrao@jncasr.
ac.in (C.N.R. Rao).
1
Present address: Materials Department, UC Santa Barbara,
Santa Barbara, CA 93106, USA.
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00912-6

U.K. Gautam et al. / Chemical Physics Letters 375 (2003) 560–564
561
ꢀ
the growth of the resulting phase could be arrested
giving rise to toluene-soluble monodisperse CdS
nanocrystals. Besides the yield of the CdS nano-
crystals, this method has certain advantages over
other techniques. The reaction is carried out in a
closed vessel and hence inert conditions are not
required. Since the nanocrystals are toluene solu-
ble, they can be size selectively precipitated using a
polar solvent leading to monodispersity. Unlike in
the room temperature synthesis, one obtains
highly crystalline particles.
geometry with Cu-Ka radiation (k ¼ 1:5418 A).
Transmission electron microscope (TEM) images
were obtained using a JEOL (JEM3010) trans-
mission electron microscope operating with an
accelerating voltage of 300 kV. A solution of
capped CdS nanocrystals in toluene was evapo-
rated on a holey carbon grid for TEM imaging.
UV–vis absorption spectra of nanocrystals in tol-
uene solvent were recorded using a Hitachi U3400
spectrometer.
3. Results and discussion
2. Experimental
Reactions of Cd(St)2 with sulfur at 493 K in the
presence of tetralin and TOPO resulted in 4 nm
nanocrystals with nearly 100% yield. The presence
of tetralin facilitates the reaction because under
the reaction conditions, it reacts with sulfur and
produces H2S in-situ which reacts with Cd(St)2 to
give the desired phase. In the process, tetralin is
converted to naphthalene which is the more stable
compound. When the CdS particles are capped
with dodecanethiol, we obtain two different sizes
of nanocrystals. The smaller, 5 nm nanocrystals
remain in solution while 10 nanocrystals are
insoluble.
Cadmium stearate [Cd(St)2], prepared by the
addition of a Cd(CH3COO)2 solution to an
aqueous solution of sodium stearate at 363 K,
was used as the Cd source. The stearate was
washed thoroughly with hot water and dried. The
powder XRD pattern and the FT-IR spectrum
confirmed that the sample was free from unre-
acted stearic acid. In a typical reaction for the
preparation of dodecanethiol-capped CdS nano-
crystals, 0.46 g (0.68 mmol) of Cd(St)2 was mixed
with 0.022 g (0.68 mmol) of sulfur, 0.1 g of tetr-
alin (0.75 mmol) and 0.66 g (3.3 mmol) dodeca-
nethiol in 45 ml toluene. The reactants were
sealed in a stainless steel 70 ml teflon-lined auto-
clave and placed in an air oven which was pre-
heated to 493 K for 12h. After the reaction was
complete, the autoclave was taken out of the oven
and allowed to cool to room temperature. The
reaction mixture, upon centrifugation, gives a
yellow solid along with a yellow centrifugate. The
centrifugate contains the soluble smaller nano-
crystals which can be precipitated out using a
polar solvent, isopropyl alcohol. This was reso-
lubulized in toluene and reprecipitated twice be-
fore further characterization. The precipitate
contains the bigger nanocrystals which was wa-
shed thoroughly with toluene. A similar proce-
dure was followed in case of TOPO-capped CdS
nanocrystals, except instead of thiol, 0.10 g of
TOPO was used.
Powder XRD patterns of the various nano-
crystals are shown in Fig. 1. The peaks are con-
siderably broadened, indicating small size. Fig. 1a
represents the PXRD pattern of the TOPO-cap-
ped CdS nanocrystals. The pattern can be in-
ꢀ
dexed to a cubic (a ¼ 5:810 A) zinc blende
structure rather than the wurtzite structure. The
particle size estimated from X-ray broadening is
4 nm. To confirm the space group, a PXRD
pattern was simulated using the DIFFaX pro-
gram [16] wherein 4 ꢀ 4 nm2 slabs of hexagonal
CdS were stacked in an fcc arrangement. The
simulated pattern in Fig. 1d compares well with
the experimental pattern with comparable line
broadening. The PXRD patterns of the soluble
and the insoluble fractions of the CdS nanocrys-
tals obtained using dodecanethiol are shown in
Figs. 1b and c, respectively. From the peak
broadening the average size of the soluble nano-
crystals is estimated to be 5 nm and that of the
precipitated ones to be 10 nm.
All the samples were characterized by powder
X-ray diffraction using a Siemens5005 diffrac-
tometer employing the reflection Bragg–Brentano

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a standard deviation of 0.4 nm (10% of the
mean). In the inset, we show the histogram of
particle size distribution. The electron diffraction
(ED) pattern of the nanocrystals is shown in
panel b. The calculated d values from this ED
pattern closely matches with that of the reported
for space group F-43m. Thus, the d-values cor-
responding to the (1 1 1), (220) and (3 1 1) ap-
ꢀ
pears at 3.358, 2.038 and 1.729 A, respectively
while the corresponding literature values are
ꢀ
3.385, 2.040 and 1.754 A. Lattice fringes are ob-
served in the high resolution electron microscope
(HREM) images of nanocrystals as shown in
panel c, indicating the high degree of crystallinity
and spherical nature of the particles. It is inter-
esting that the CdS nanocrystals posses the cubic
space group, since the hexagonal wurtzite struc-
ture is thermodynamically more stable. This is
probably because of the high pressure generated
inside the autoclave which directs the Cd–S
planes to be stacked in an ABC ABC pattern
rather than the AB AB pattern, the former being
entropically more favourable.
Fig. 3 gives a representative TEM image of the
soluble dodecanethiol-capped CdS nanocrystals.
The average size of the particles is found to be
5 nm although a few bigger particles are also seen.
The panels b and c show, respectively, the ED
pattern and the HRTEM image of one of the
Fig. 1. Powder XRD patterns of (a) TOPO-capped CdS par-
ticles, (b) soluble dodecanethiol-capped 5 nm particles, (c) do-
decanethiol-capped 10 nm particles, (d) shows the simulated
XRD pattern for the 4 nm particles. Vertical lines at the top of
the figure are expected peak positions.
Fig. 2shows a TEM image of the TOPO-
capped CdS nanocrystals. The particles are fairly
monodisperse with a mean diameter of 4 nm and
Fig. 2. (a) TEM image of TOPO-capped CdS particles. Inset
shows the size distribution of the nanocrystals. The electron
diffraction pattern of the nanocrystals is shown in (b) and
HREM image of a nanocrystal is shown in (c).
Fig. 3. (a) TEM image of soluble dodecanethiol-capped CdS
nanocrystals. The electron diffraction pattern and the HREM
image of a nanocrystal are shown in (b) and (c), respectively.

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Fig. 5. UV–vis absorption spectra of (a) TOPO-capped CdS
nanocrystals (4 nm), (b) dodecanethiol-capped soluble (5 nm)
nanocrystals and (c) insoluble (10 nm) CdS nanocrystals.
In summary, we have carried out a new one-pot
synthesis of organic solvent soluble CdS nano-
crystals. The method is safe, cheap and convenient
for large scale production, and yields nanocrystals
of relatively narrow size distribution. CdS nano-
crystals obtained by this method are in the meta-
stable cubic structure.
Fig. 4. TEM image of insoluble dodecanethiol-capped CdS
nanocrystals.
nanocrystals. In Fig. 4 we show the dodecanethiol-
capped 10 nm particles obtained as insoluble
fraction. It is observed that there is a larger par-
ticle size distribution in case of the bigger particles
compared to the smaller ones.
Acknowledgements
UKG thanks Council of Scientific and In-
dustrial Research, India, for Senior Research
Fellowship.
Fig. 5 displays the UV–vis absorption spectra
of the various CdS nanocrystals. The TOPO-
capped (4 nm) nanocrystals exhibit a well-defined
absorption feature at 455 nm (Fig. 5a). This is
considerably blue-shifted relative to the bulk CdS
(515 nm) due to quantum confinement effects [17].
Narrow size distribution can be inferred from the
small difference between the peak position and
the onset of the spectrum. The absorption bands
of the dodecanethiol-capped nanocrystals are
broader compared to the TOPO-capped due to a
greater size distribution. However, absorption
band of the smaller thiol-capped particles (5 nm)
at 470 nm are considerably blue-shifted compared
to that of the 10 nm nanocrystals (490 nm).
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