Preparation of PbS and PbSe nanocrystals by a new solvothermal route

Article abstract of DOI:10.1016/j.materresbull.2003.12.003

A new solvothermal route for the preparation of nanocrystals of PbS and PbSe, involving the reaction of lead stearate with sulfur or selenium and tetralin (tetrahydronaphthalene) in toluene solvent is described. Tetralin in the presence of S/Se gives H₂S/H₂Se and gets aromatized to naphthalene. The nanocrystals have been characterized by powder X-ray diffraction and electron microscopy. Use of surfactant Triton X-100 (polyoxyethylene(10)isooctylphenyl ether) resulted in both nanorods and nanoparticles of PbSe. Capping by citric acid and malonic acid reduce the particle sizes to less than 10nm.

Full text of DOI:10.1016/j.materresbull.2003.12.003

Materials Research Bulletin 39 (2004) 669–676
Preparation of PbS and PbSe nanocrystals by a
new solvothermal route
Ujjal K. Gautam, Ram Seshadri*
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
Received 28 March 2003; received in revised form 11 November 2003; accepted 3 December 2003
Abstract
A new solvothermal route for the preparation of nanocrystals of PbS and PbSe, involving the reaction of lead
stearate with sulfur or selenium and tetralin (tetrahydronaphthalene) in toluene solvent is described. Tetralin in
the presence of S/Se gives H2S/H2Se and gets aromatized to naphthalene. The nanocrystals have been
characterized by powder X-ray diffraction and electron microscopy. Use of surfactant Triton X-100
(polyoxyethylene(10)isooctylphenyl ether) resulted in both nanorods and nanoparticles of PbSe. Capping by
citric acid and malonic acid reduce the particle sizes to less than 10 nm.
# 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; A. Semiconductors; C. Electron microscopy; C. X-ray diffraction; D. Crystal structure
1. Introduction
Nanocrystals of semiconductors such as CdS/CdSe have been prepared by a variety of methods [1–4].
PbS and PbSe are important semiconducting materials with applications in electronic and optical
devices. Nanocrystalline PbS and PbSe have been reported by a few workers. Thus, the reaction of S
and Se with a lead salt in ethylenediamine solvent is reported to give the respective phase with particle
sizes in the range of 20–100 nm [5]. Alkaline aqueous synthesis of PbS and PbSe has been reported by
Qian and coworkers [6] where 20–35 nm PbS/PbSe particles are obtained by reacting lead acetate with
S/Se at a pH of 10 at room temperature. Elemental lead has been reacted with S/Se in liquid ammonia
to yield 50–80 nm particles [7]. There are also reports of microwave, sonochemical [8] and
photochemical [9] synthesis as well. Kerner et al. [10] have shown that spherical (8 nm) and rectangular
(25 nm) nanoparticles of PbSe are obtained with different capping agents using high intensity
*
Corresponding author. Present address: Materials Department, UC Santa Barbara, Santa Barbara, CA 93106,
USA. Tel.: þ1-805-893-6129; fax: þ1-805-893-8797.
E-mail addresses: gautam@sscu.iisc.ernet.in (U.K. Gautam), seshadri@mrl.ucsb.edu (R. Seshadri).
0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2003.12.003

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ultrasound radiation. Particles of 80 nm diameter have been obtained under solvothermal conditions in
ethylenediamine/pyridine using lead salts and S/Se [11]. Hydrothermal synthesis of particles of PbSe
smaller than 50 nm has also been reported [12].
We have explored the synthesis of PbS/PbSe nanocrystals by a novel solvothermal route wherein lead
stearate, Pb(C17H35COO)2 is reacted with S or Se in toluene in the presence of tetralin
(tetrahydronaphthalene). The novelty of the reaction is that tetralin, in the presence of S/Se, generates
H2S/H2Se in-situ transforming itself to naphthalene. This reaction has been carried out without any
capping agent and also in the presence of capping agents such as citric acid, malonic acid and Triton
X-100 (polyoxyethylene(10)isooctylphenyl ether) as surfactant. By this means, it has been possible to
prepare PbS/PbSe particles and rods of nanometric dimensions.
2. Experimental
Lead stearate, [Pb(St)2], prepared by adding a Pb(NO3)2 solution to an aqueous solution of sodium
stearate at 363 K, was used as the Pb source for PbS and PbSe. 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 any unreacted stearic acid.
In a typical reaction for the preparation of PbSe, 0.25 g (0.33 mmol) of Pb(St)2 was mixed with
0.0263 g (0.33 mmol) of selenium and 0.2 g of tetralin (1.5 mmol) in 45 ml toluene. The reactants were
sealed in a stainless steel 70 ml teflon gasketed autoclave and placed in an air oven which was
preheated to 493 K for 12 h. After the reaction was complete, the autoclave was taken out of the oven
and allowed to cool to room temperature. The solid black product obtained was collected by
centrifugation, washed several times with toluene, followed by isopropanol and air dried. The reaction
Fig. 1. (a) Powder XRD profile of PbS prepared solvothermally in toluene at 493 K for 12 h; (b) the Rietveld fit; and (c) the
˚
difference profile are displayed [RBragg ¼ 9%, a ¼ 5:943ð1Þ A]. Vertical lines at the top of the figure are expected peak
positions.

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time could be reduced considerably by increasing the reaction temperature (details in discussion). PbS
was synthesized using the same reaction conditions, by taking 0.0107 g of S in the reaction mixture.
The PbSe nanorods were achieved as follows: 0.2 g (0.33 mmol) of Triton X-100 was added to the
synthetic mixture described in the above paragraph. A black product was obtained while the solvent
Fig. 2. TEM image of (a) PbS nanoparticles prepared without any capping agent; (b) PbSe nanoparticles prepared without any
capping agent; (c) SEM image of the PbSe particles obtained as the solid residue.

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turned red. Solid product was washed with toluene, methanol and acetone. When methanol was added
to the centrifugate, reddish black particles were obtained. It was redissolved and reprecipitated using
toluene and methanol, respectively twice, before characterization. For citric acid capping, 0.05 g
(0.26 mmol) of citric acid was added to the reaction mixture. The black product obtained was collected
by centrifugation and washed thoroughly with methanol and acetone. For malonic acid capping, 0.05 g
(0.43 mmol) of malonic acid was used with similar reaction conditions.
All the samples were characterized by using Siemens D5005 powder X-ray diffractometer employing
˚
the reflection Bragg–Brentano geometry with Cu Ka radiation (l ¼ 1:5418 A). Pb(St)2 was
characterized by IR spectroscopy and PXRD technique. Rietveld refinements of the powder samples
were carried out using version 1.23 of the XND code [13]. The transmission electron microcopy (TEM)
images were obtained using a JEOL (JEM3010) transmission electron microscope operating with an
accelerating voltage of 300 kV. A solution of PbSe in toluene obtained by using Triton X-100 was
evaporated on a holey carbon grid for TEM imaging. Similarly a dispersion of citric acid and malonic
acid capped phases in methanol was used to prepare sample grids.
3. Results and discussion
Reaction of Pb(St)2 with sulfur and tetralin at 493 K resulted in PbS with the rocksalt structure.
Reduction on the reaction time leaves behind unreacted Pb(St)2. The powder XRD pattern of a sample
prepared at 493 K after a reaction time of 12 h is shown in Fig. 1. The profile could be Rietveld fitted to
Fig. 3. TEM image of PbSe nanorods.

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Fig. 4. (a) The powder XRD pattern of the PbSe nanorods prepared with Triton X-100; (b) Rietveld simulated PXRD pattern
for PbSe without preferred orientation; (c) the difference profile of plots a and b; (d) Rietveld fit with strong suppression of the
(h 0 0) peaks; (e) Rietveld fit assuming partial suppression of the (h h h) peaks; (f) The difference profile of the plots a–e.
Vertical lines at the top of the figure are expected peak positions.
˚
the rock salt structure with cubic cell parameter of a ¼ 5:943ð1Þ A. Particle size calculated using the line
broadening of the powder XRD peaks obtained from Rietveld refinement is 200 nm. Varying the
concentrations of the reactants yielded impurity phases. TEM images of the sample show particles in two
different size domains. Fig. 2a shows the particles of 3 nm present in the solution phase with a fair
amount of monodispersity. The electron diffraction (ED) pattern of the particles show three rings which
can be indexed with d values of 2.89, 2.04 and 1.14. These values closely matches the d values of 2.97
(1 1 1), 2.09 (2 2 0) and 1.14 (5 1 1) reported for the PbS phase crystallizing in the space group Fm3m
(JCPDF 050592). It is possible that the stearic acid released in the reaction caps the PbS particles
inhibiting further growth leading to 3 nm particles. This can be speculated because we have observed
that other organic acids such as malonic and citric acid can also act as capping agents, as reported in this
publication. The solid residue has considerably bigger particles with sizes ranging from 100 to 300 nm.
PbSe was obtained when the reaction of Pb(St)2 was carried out with Se and tetralin at 493 K for 12 h
or at 513 K for 3 h. A refinement of the powder XRD pattern of the phase prepared at 493 K confirmed
˚
the rocksalt structure (a ¼ 6:128ð1Þ A). Based on the X-ray line width obtained from the Rietveld
refinement of the powder XRD pattern, the particle size is estimated to be ꢀ150 nm. TEM images were
similar to those of PbS, revealing different sizes of particles. Fig. 2b shows the TEM image of the 3 nm
particles. The ED pattern obtained from these particles shows three faint rings with d values of 2.90,
2.19 and 1.14. These values corresponds to the reported d values of 3.06 (2 0 0), 2.17 (2 2 0) and 1.4
(3 3 1), respectively (JCPDF 060354). We believe, as in case of PbS, that the stearic acid released in the
reaction caps the particles. Bigger particles (200 nm–1 m) were present in the solid residue as revealed
by the SEM images (Fig. 2c).
The toluene soluble fraction of the product obtained in the presence of surfactant Triton X-100
contained nanorods of the dimensions 50 nm ꢁ 300 nm as revealed by the TEM image in Fig. 3. The

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Fig. 5. TEM image of (a) nanoparticles and (b) submicron particles of PbSe obtained by using Triton X-100.

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Fig. 6. Transmission electron micrograph of malonic acid capped PbSe showing cauliflower aggregation of tiny particles.
powder XRD pattern gives evidence for preferred orientation of the particles in terms of different
intensities of reflections as shown in Fig. 4. The (h h h) peaks have grown in intensity while those of
(h 0 0) peaks have decreased. Ideally, an extended growth of the particles along the (h 0 0) direction
should expose more number of (h h h) lattice planes to the incident X-ray leading to intense (h h h)
peaks. Rietveld simulated powder XRD patterns presuming strong suppression of (h 0 0) peaks and
partial suppression of the (h h h) peaks are shown in the plots d and e respectively. The proposed growth
along the (h 0 0) is evident from the satisfactory fit of the experimental data to these two plots. In
addition to nanorods, we also observe the presence of nanoparticles of 20 nm diameter as shown in the
image (Fig. 5a). The toluene insoluble product consisted of rectangular submicron particles (Fig. 5b).
Effort to reduce PbS particle size with Triton X-100 was unsuccessful.
The line widths in the powder XRD pattern of citric-acid capped PbS nanocrystals showed the
average particle size to be ꢀ26 nm. In case of PbSe, 30–38 nm particles could be obtained using citric
acid capping. TEM images, however, show that along with the large nanocrystals, there were large
amounts of agglomerates of 3 nm nanoparticles. Accordingly, the citric acid-capped PbSe particles
show a 10 nm blue shift of the UV–Vis absorption edge. Using malonic acid as capping agent, we
obtained similar agglomerates of PbSe nanoparticles. In Fig. 6, we show a typical TEM image of a
cauliflower type agglomerate formed by malonic acid capped PbSe nanoparticles of 3–4 nm diameter.
Acknowledgements
The authors thank Prof. C.N.R. Rao for helpful discussions. UKG thanks Council of Scientific and
Industrial Research, India, for a Senior Research Fellowship.

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