Synthesis of anisotropic PbS nanoparticles using heterocyclic dithiocarbamate complexes

Article abstract of DOI:10.1039/c2dt30282a

We report the synthesis of lead piperidine and lead tetrahydroquinoline dithiocarbamate (DTC) complexes and their use as single source precursors for the preparation of anisotropic PbS nanoparticles. The complexes were thermolysed in coordinating solvents such hexadecylamime (HDA), tri-n-octylphosphine oxide (TOPO), oleylamine (OA) and decylamine (DA) at various reaction temperatures. The variation of the reaction conditions and precursors produced PbS particles with shapes ranging from spheres to cubes and rods. The size of the particles is generally larger than those synthesized by conventional precursor routes. The electron microscopy and X-ray diffraction data confirm the particles to be very crystalline with the dominant cubic rock salt phase present in all samples. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt30282a

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PAPER
Synthesis of anisotropic PbS nanoparticles using heterocyclic
dithiocarbamate complexes†‡
Linda D. Nyamen,a,b V. S. R. Rajasekhar Pullabhotla,a Adeola A. Nejo,a Peter T. Ndifon,b Jamie H. Warnerc
and Neerish Revaprasadu*a
Received 8th February 2012, Accepted 15th May 2012
DOI: 10.1039/c2dt30282a
We report the synthesis of lead piperidine and lead tetrahydroquinoline dithiocarbamate (DTC) complexes
and their use as single source precursors for the preparation of anisotropic PbS nanoparticles. The
complexes were thermolysed in coordinating solvents such hexadecylamime (HDA), tri-n-octylphosphine
oxide (TOPO), oleylamine (OA) and decylamine (DA) at various reaction temperatures. The variation of
the reaction conditions and precursors produced PbS particles with shapes ranging from spheres to cubes
and rods. The size of the particles is generally larger than those synthesized by conventional precursor
routes. The electron microscopy and X-ray diffraction data confirm the particles to be very crystalline with
the dominant cubic rock salt phase present in all samples.
oleic acid and trioctylphosphine.9 Each surfactant has a different
functional group that reacts with the crystal facets of the PbS
1. Introduction
The synthesis of semiconductor nanoparticles has been domi-
nated by CdSe and CdS due to their unique size dependent pro-
perties, which render them useful in diverse applications.
However, materials such as PbS have also been identified as
being important, as PbS displays a distinct quantum confinement
effect as a result of having a large excitation Bohr radius
(18 nm).1 PbS has a narrow band gap of ca. 0.41 eV with
reported applications in fields such as infrared (IR) detectors,2
solar absorbers,3 light emitting diodes (LEDs)4 and optical
switches.5
thereby controlling growth. Other diverse shapes such as star
shaped, dendrites, nanowires and hollow structures have also
been reported.10–13 The thermolysis of single molecule precur-
sors in alkylamines has also successfully yielded high quality
PbS nanoparticles.14 The amines have been reported to lower the
decomposition temperature of the precursor complexes. The use
of olive oil as coordinating solvent has also been identified as a
‘green’ alternative to amines at low temperatures.15
Recently, our group has reported the use of cadmium dithio-
carbamate complexes as precursors for CdS nanoparticles.16–18
The hot-injection method is a common route to produce
monodispersed PbS nanoparticles. In this method, the lead salt
and sulphur source are injected into coordinating solvents, which
also act as stabilizing ligands.6–9 The injection methods use long
chain phosphine or amine based ligands as capping groups.
Hyeon and co-workers reported a general synthetic route to
metal sulphide nanoparticles whereby the metal chloride is dis-
solved in oleylamine and reacted with the sulphur precursor
(sulphur mixed with oleylamine) under heating.7 Warner and
Cao prepared PbS particles, including cubes, stars, rods and
nanowires, by using a combination of surfactants i.e. oleylamine,
Cadmium
piperidine
dithiocarbamate
(DTC),
[Cd-
(S2CNC5H10)2], and cadmium tetrahydroquinoline dithio-
carbamate, [Cd(S2CNC9H10)2], complexes were thermolysed in
hexadecylamine (HDA) to give anisotropic CdS nanoparticles.
In this work, we report for the first time the synthesis of the lead
analogues and their use as precursors for PbS nanoparticles.
Lead piperidine dithiocarbamate (DTC), [Pb(S2CNC5H10)2], and
Lead tetrahydroquinoline dithiocarbamate, [Pb(S2CNC9H10)2],
were thermolysed in hexadecylamine, dodecylamine, decylamine
and oleylamine to give PbS particles with varying shapes.
aDepartment of Chemistry, University of Zululand, Private Bag X1001,
KwaDlangezwa, 3880, South Africa. E-mail: nrevapra@pan.uzulu.ac.za;
Fax: +27 35 9026568; Tel: +27 35 9026152
2. Experimental
bDepartment of Inorganic Chemistry, University of Yaoundé I, PO Box
812, Yaoundé, Cameroon
2.1 Chemicals
cDepartment of Materials, University of Oxford, 16 Parks Road, Oxford,
OX1 3PH, UK
†Dedicated to Professor David Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
Hexadecylamine (HDA) 98%, tri-n-octylphosphine (TOP) 90%,
tri-n-octylphosphine oxide (TOPO) 99% and 1,2,3,4-tetrahydro-
quinoline 98% were purchased from Aldrich and piperidine 99%
from Sigma-Aldrich. Petroleum ether, methanol 99.5%, carbon
disulphide 99.5%, sodium hydroxide 98%, lead chloride 99%
and acetone were purchased from Saarchem, UnivAR and Merck
‡Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt30282a
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chemicals. The chemicals were used as purchased without any
purification.
2.2 Instrumentation
Microanalysis. Microanalysis was performed on a Perkin-
Elmer automated model 2400 series II CHNS/O analyzer.
1H NMR. The 1H NMR spectra of the both ligands and metal
complexes were obtained using a Bruker advance III 400 MHz
spectrophotometer.
Scheme 1 Reaction scheme for the synthesis of (a) Pb(II) piperidine
dithiocarbamate and (b) Pb(II) tetrahydroquinoline dithiocarbamate
complexes.
Infra-red analysis. Infrared spectra were recorded on a Bruker
FT-IR tensor 27 spectrophotometer directly on small samples of
the compounds in the range 200–4000 cm−1
.
Na(thq-dtc), yield: 82.7%. 1H NMR (400 MHz, CD3OD):
δ 2.07 (m, 2H, 3-CH2), 2.73 (t, 2H, 4-CH2), 4.58 (t, 2H, 2-CH2),
Thermal analysis. Thermogravimetric analysis was carried
out at 20 °C min−1 heating rate using a Perkin Elmer Pyris
6 TGA up to 900 °C in a closed perforated aluminium pan. The
DSC analysis was carried out at 20 °C min−1 heating rate up to
450 °C using a Perkin Elmer DSC 4000 series, which was cali-
brated with indium metal. Both TGA and DSC were run under
N2 gas flow.
7.08–7.85 (m, 4H, Ar–H). Significant IR bands: ν(O–H)
:
3324 cm−1, ν(CvS): 967 cm−1, ν(CvN): 1485 cm−1. Anal. calcd
for C10H18NS2O4Na: C, 39.59; H, 5.98; N, 4.62. Found: C,
39.15; H, 5.94; N, 4.27.
2.3.2 Preparation of the lead complexes. Lead acetate
(5 mmol) dissolved in distilled water (25.0 mL) and added drop-
wise to the solution of the dithiocarbamate ligand (10 mmol)
cooled in an ice-bath at 0 °C. The reaction mixture was stirred
for 1 h, and the precipitate formed was filtered, washed with
warm distilled water and dried in an oven overnight at 70 °C.
The chemical reaction for the preparation of the precursors is
detailed in Scheme 1 shown above.
Transmission electron microscopy (TEM) and high resolution
TEM. Samples were prepared by placing a drop of a dilute sol-
ution of nanoparticles on Formvar-coated grids (150 mesh) for
TEM and holy carbon grids for HRTEM. The samples were
allowed to dry completely at room temperature and viewed using
a JEOL 1010 TEM and JEOL 2100 HRTEM. Viewing was done
at an accelerating voltage of 100 kV (TEM) and 200 kV
(HRTEM), and images captured digitally using a Megaview III
camera; stored and measured using Soft Imaging Systems
iTEM software (TEM) and Gatan camera and Gatan software
(HRTEM).
1
[Pb(pip-dtc)2], yield: 89.5%. H NMR (400 MHz, CDCl3): δ
2.07 (m, 8H, 3-CH2), 2.71 (t, 4H, 4-CH2), 4.50 (t, 8H, 2-CH2).
Significant IR bands: ν(O–H): 2937 cm−1, ν(CvS): 974 cm−1
,
ν(CvN): 1465 cm−1, ν(Pb–S): 408 cm−1. Anal. calcd for
C12H22N2S4OPb: C, 26.41; H, 4.06; N, 5.13. Found: C, 26.19;
H, 3.52; N, 4.77.
X-ray diffraction. The crystallinity of the dried colloids was
studied by using powder X-ray diffractometer (XRD). Powder
diffraction patterns were recorded in the high angle 2θ range of
20–90° using a Bruker AXS D8 Advance X-ray diffractometer,
equipped with nickel filtered Co Kα radiation (λ = 1.5418 Å) at
40 kV, 40 mA and at room temperature. The scan speed and step
sizes were 0.05° min−1 and 0.00657, respectively.
1
[Pb(thq-dtc)2], yield: 93.2%. H NMR (400 MHz, CDCl3): δ
2.10 (m, 4H, 3-CH2), 2.80 (t, 4H, 4-CH2), 4.30 (t, 4H, 2-CH2),
7.26–7.92 (m, 8H, Ar–H). Significant IR bands: ν(O–H)
:
:
3038 cm−1, ν(CvS): 964 cm−1, ν(CvN): 1485 cm−1, ν(Pb–S)
367 cm−1. Anal. calcd for C20H26N2S4O3Pb: C, 35.44; H, 3.87;
N, 4.13. Found: C, 35.39; H, 3.06; N, 3.85.
2.3 Synthesis of the precursors
2.4 Preparation of nanoparticles
2.3.1 Preparation of the ligands. Carbon disulfide (0.1 mol,
6.0 mL) was added in small portions to an equimolar solution
mixture of sodium hydroxide (4.0 g, 0.1 mol) and the corres-
ponding amine (piperidine, tetrahydroquinoline, 0.1 mol),
cooled in an ice bath at 0 °C. After 15 min, a precipitate formed,
which was then dried in air and re-crystallised in a mixture of
acetone–petroleum ether. The final product was collected,
washed with chloroform and suction dried.
2.4.1 Synthesis of HDA capped PbS nanoparticles. The [Pb-
(pip-dtc)2] complex (0.5 g) was dissolved in tri-n-octylphos-
phine (6.0 mL). The resultant solution was then injected into hot
HDA (6.0 g) in a three neck flask at 180 °C. The solution was
turned to a yellow colour and a decrease in temperature of
20–30 °C was observed. The solution was allowed to stabilize at
180 °C. After a 1 h reaction time, heating was stopped. Excess
methanol was then added to the cooled reaction mixture resulting
in the formation of a flocculent precipitate, which was separated
by centrifugation, and redispersed in toluene to give HDA
capped PbS nanoparticles. The reaction was repeated at an
elevated temperature of 250 °C.
1
Na(pip-dtc), Yield: 94.5%. H NMR (400 MHz, DMSO-d6):
δ 1.41 (m, 2H, 3-CH2), 1.53 (t, 2H, 4-CH2), 4.28 (t, 2H, 2-CH2).
Significant IR bands: ν(O–H): 3367 cm−1, ν(CvS): 967 cm−1
,
ν(CvN): 1468 cm−1. Anal. calcd for C6H14NS2O2Na: C, 32.87;
H, 6.44; N, 6.39. Found: C, 32.57; H, 6.29; N, 5.96.
8298 | Dalton Trans., 2012, 41, 8297–8302
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The above reaction procedure was repeated using the [Pb(thq-
dtc)2] complex at reaction temperatures of 180 and 250 °C.
the spectra of the complexes is assignable to ν(Pb–S), which is
absent in the spectra of the dithioligands.
The 1H NMR data of Na(thq-dtc) showed three signals at
chemical shifts 2.07, 2.73 and 4.54 ppm for the aliphatic
methylene protons while the peaks observed at the range of
7.08–7.85 ppm are assignable to the proton of the aromatic ring
as multiplet peaks. The methylene protons signals of Na(pip-
dtc) were observed as three different chemical shift values of
1.41, 1.53 and 4.28 ppm. The minor changes in chemical shift
for N–CH2 protons of the lead complexes of the dithioligands
may be due to the coordination of the ligands to the metal atom.
The TGA curve of Na(pip-dtc) showed that decomposition
occurred in three steps. The first step corresponds to the loss of
two moles of water molecules while the last two steps are attri-
buted to the decomposition of the compound leaving sodium sul-
phide as the residue. The DSC thermogram of Na(pip-dtc)
showed three endothermic peaks. The first endothermic peak at
156.5 °C corresponds to the loss of water of crystallization and
the second to the melting point at 303.5 °C. The third endother-
mic peak at 345.7 °C corresponds to the decomposition of the
compound. The TGA thermogram of Na(thq-dtc) also showed a
three step decomposition pattern. The first decomposition step is
attributed to the loss of water molecules while the last two
decomposition steps correspond to the loss of the organic moiety
leaving sodium sulphide as the residue. The loss of water mol-
ecules from this ligand was confirmed by the appearance of an
endothermic peak at 108.0 °C in the DSC thermogram.
2.4.2 Synthesis of TOPO capped PbS nanoparticles. The
[Pb(pip-dtc)2] complex (0.5 g) was dissolved in TOP (6.0 mL)
and the resultant solution was injected into TOPO (6.0 g) in a
three neck flask at 240 °C. The brownish solution was allowed
to stabilize at 240 °C for 1 h. Excess methanol was then added
to the cooled solution to flocculate the particles, which were then
separated by centrifugation. The supernatant was discarded and
the precipitate of TOPO capped PbS nanoparticles was dissolved
in toluene for further analysis.
2.4.3 Synthesis of oleylamine (OA) capped PbS nano-
particles. The [Pb(thq-dtc)2] complex (0.5 g) was dissolved in
TOP (6.0 mL). The resultant solution was injected into 6.0 g of
hot oleylamine (OA) in a three neck flask at 230 °C. The sol-
ution was turned into a blackish-brown colour and was allowed
to stabilize at 230 °C. After 2 h reaction time methanol was
added, resulting in the formation of a flocculent precipitate,
which was separated by centrifugation. The OA capped PbS
nanoparticles were then dispersed in toluene. The reaction was
repeated at 180 and 270 °C.
The above reaction conditions were repeated for the [Pb(pip-
dtc)2] complex.
2.4.4 Synthesis of decylamine (DA) capped PbS nano-
particles. The [Pb(pip-dtc)2] complex (0.5 g) was dissolved in
TOP (6.0 mL). This solution was injected into 6.0 g of hot de-
cylamine (DA) in a three neck flask at 230 °C. The solution
turned to a blackish-brown colour and was allowed to stabilize at
230 °C. After 2 h reaction time methanol was added and the
formed precipitate was separated by centrifugation. The obtained
particles were dissolved in toluene to give a brownish black
solution.
Both Pb(pip-dtc) and Pb(thq-dtc) showed similar two-step
thermal decomposition behaviour. In the first step, rapid mass
loss corresponded to the dehydration of the complexes while
the second stage weight loss is due to the formation of the lead
thiocyanate, which did not decompose any further up to 900 °C
as shown in the thermograms. The Pb(pip-dtc) complex de-
composes in a single step after dehydration. The second step
involves the decomposition of the hydrated complex, which
resulted in the formation of Pb(SCN)2 as residue. The DSC
thermogram of Pb(pip-dtc) showed two endothermic peaks
attributed to a dehydration of water molecules and melting/
decomposition process. The first stage decomposition in the
TGA thermogram of Pb(thq-dtc) resulted in the formation of the
hydrated complex with a weight loss of 8.41% (calcd = 7.97%).
The second weight loss is due to partial decomposition of the
organic moiety resulting in Pb(SCN)2 as residue, which corres-
ponds to an endothermic peak at 293.5 °C on the DSC thermo-
gram of the complex.
3. Results and discussion
The pure ligands and the lead complexes were isolated from
water and in very good yields. The lead complexes showed
different degrees of hydration depending on the ligands.
The purity of the dithioligands and their metal complexes were
established by microanalyses as formulated. The piperidine
dithiocarbamate and its complex were white while the tetra-
hydroquinoline dithiocarbamate and its complex were yellow in
colour. The compounds were microcrystalline, stable in air and
soluble in some organic solvents.
The 1H NMR data of Na(pip-dtc) showed the methylene
protons signals as three different chemical shift values of 1.41,
1.53 and 4.28 ppm. Information about the mode of bonding and
the effect of the lead ion on the ligand was obtained by compar-
ing the infrared spectra of the metal complexes with that of the
dithioligands. The hydrated sodium salts of the dithioligands
exhibit very broad bands centred at 3324–3367 cm−1 assigned
to water of crystallization. The bands present in the 1465–
3.1 Synthesis of PbS nanoparticles
The use of long chain amines as capping groups in nano-
materials has proven to be very successful. In the hot injection
method the high reaction temperature, typically between 250 and
350 °C facilitates the dynamic bonding of the surface ligands to
the nanocrystals.21 The high temperatures are also needed to
improve the crystallinity of the particles. At high temperatures
the influence of the surface ligands are minimized through the
dynamic bonding nature of the ligands and as a result particles
revert to thermodynamically stable shapes such as spheres. The
low temperature routes offer more flexibility in shape control
1485 cm−1 range in both the ligands and metal complexes are
19
assigned to ν(C–N), which lie between ν(CvN) and ν(C–N).
A
single sharp band in the 964–974 cm−1 region in the complexes,
which is attributed to the ν(CvS), indicates that the dithioligands
act as bidentate.20 A new band in the 367–405 cm−1 region in
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Fig. 2 TEM images of HDA capped PbS nanoparticles synthesised
from the Pb(thq-dtc) complex at (a) 180 °C and (b) 250 °C.
Fig. 1 TEM and corresponding HRTEM images of HDA capped PbS
nanoparticles synthesised from the Pb(pip-dtc) complex at (a), (b)
180 °C and (c), (d) 250 °C.
with anisotropic shaped particles such as rods formed through
judicious use of surfactants. In this work we aim to study the
effect of using various capping groups, mainly amines and
TOPO, on the shape of PbS nanoparticles at different reaction
temperatures.
Fig. 3 (a) TEM and (b) HRTEM images of TOPO capped PbS nano-
particles synthesised from the Pb(pip-dtc) complex at 240 °C.
3.3 Variation of capping group, TOPO, OA and DA
In contrast to HDA, the bulky nature of TOPO provides
increased steric hindrance. Also, the stability of the TOPO
capped particles is due to the high affinity of TOPO for the Cd2+
ions.22 The Pb(pip-dtc) complex was thermolysed in TOPO at
250 °C. The TEM image shows clustered quasi-cubic particles
(Fig. 3a). The particles have an average size of 30.70 3.52 nm.
The HRTEM image shows an oblate particle with distinct lattice
fringes showing no discontinuities or stacking faults (Fig. 3b).
We decided to change the capping group to oleylamine. Pre-
vious reports have shown that oleylamine used as a surfactant
in combination with trioctylphosphine (TOP) resulted in shape
tunable PbS particles.9 The Pb(thq-dtc) complex was thermo-
lysed in oleylamine at 180 °C. The TEM images show very well
defined PbS particles with a cubic geometry (Fig. 4a). The
3.2 Synthesis of HDA capped PbS nanoparticles
The synthesis of PbS nanoparticles in HDA using the Pb(pip-
dtc) complex at 180 °C resulted in the formation of cubic shaped
particles (Fig. 1a). The cubes have an average size of 41.10
8.45 nm. The HRTEM image (Fig. 1b) of a single particle shows
lattice fringes with the spacing of 0.295 nm matching the
spacing between the [200] plane of rock-salt PbS. Previous
reports suggest that primary amines such as oleylamine preferen-
tially coordinate to the [111] facets of PbS thereby preventing
the formation of [100] terminated cubes. However Warner and
Cao did report the formation of PbS nanocubes synthesized in
oleylamine citing the presence of lead acetate in the synthetic
procedure, which inhibits the formation of spherical PbS nano-
particles.9 When the temperature is increased to 250 °C close to
spherical particles (smaller particles with average particle size of
13.50 1.05 nm) are observed (Fig. 1c). The HRTEM image
shows the presence of an oblate particle with the lattice spacing
of 0.21 nm corresponding to the [220] plane of cubic PbS
(Fig. 1d). The change in the shape of oleylamine capped PbSe
from spheres to cubes and a resultant increase in size has been
previously reported.21 By increasing the oleylamine concen-
tration, an increase in size occurs, which results in the formation
of cubic particles dominated by the [200] facets. We have
observed that an increase in the temperature results in the change
of shape from cubes to spheres.
average size of the nanocubes was 24.77
5.04 nm. The
HRTEM image shows a single cubic shaped particle (Fig. 4b).
These results are very similar to those reported by Warner and
Cao.9 When the reaction temperature was increased to 270 °C,
nanorods of PbS were obtained as shown in Fig. 4c. The rods
were 99.95
21.84 nm in length and 16.73
5.20 nm in
breadth. When the reaction time was increased to 4 h, the length
of the rods increased to 153.59 42.70 nm and the breadth to
23.19 5.52 nm (Fig. 4e). The increase in length of the rods
with reaction time is accompanied by a subsequent increase in
standard deviation of the length and decrease in standard devi-
ation of the breadth. The degree of anisotropy or aspect ratio
(L/W) increases from 5.93 to 6.62 as the reaction proceeds. The
HRTEM image shows clearly defined lattice fringes with a
lattice spacing of 0.34 nm corresponding to the [111] plane of
cubic PbS (Fig. 4d). This evolution in shape from cubes to rods
can be attributed to the increase in reaction temperature.
The reaction was repeated with the Pb(thq-dtc) complex at
similar reaction temperatures. At the lower reaction temperature
of 180 °C, close to spherical particles with an average size of
6.79 1.60 nm are observed (Fig. 2a). However, when the temp-
erature is increased to 250 °C, rod shaped particles are formed
(Fig. 2b).
When the Pb(pip-dtc) complex was thermolysed in oleyl-
amine under similar reaction conditions, the highest quality
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Fig. 5 (a) TEM and (b), (c) and (d) HRTEM images of DA capped
PbS nanoparticles synthesised from the Pb(pip-dtc) complex at 230 °C.
Fig. 4 TEM and corresponding HRTEM images of OA capped PbS
nanoparticles synthesised from the Pb(thq-dtc) complex at (a), (b)
180 °C, (c), (d) 2 h sample at 270 °C, (e) 4 h sample at 270 °C and (f)
from Pb(pip-dtc) at 230 °C.
particles were obtained at 230 °C in the form of nanocubes,
which had an average size of 24.53 4.39 nm (Fig. 4f). There
was no variation of shape in the form of rods at higher tempera-
tures. Therefore it can be inferred that the variation in the pre-
cursor does also contribute to shape evolution.
We decided to decrease the chain length of the amine by using
decylamine as the capping group. The Pb(pip-dtc) complex was
thermolysed in decylamine at 230 °C. The TEM image shows
particles that are predominantly cubic or close to cubic in shape
(Fig. 5a). The HRTEM image (Fig. 5b) shows a particle with a
length of 11.64 nm and breadth of ∼10 nm. A rectangular par-
ticle with a length of 12.96 nm and breadth of 7.62 nm is also
observed (Fig. 5c). There is also evidence of twinning, as seen
in Fig. 5d. The twin boundaries are shown by the arrows
(Fig. 5d). Primary amines such as hexadecylamine and oleyl-
amine selectively adsorb onto different facets and allow growth
along a certain facet while inhibiting growth along another one.
This allows the possibility of anisotropic nanostructures. The
decomposition pathway of the precursor may also play a role in
the growth mechanism and final morphology. There have been a
few studies that attempt to correlate the nature of the precursor to
the growth dynamics of the particles, however, there has been no
conclusive evidence as to how the type of precursor influences
the size or shape of the nanoparticles.23,24
Fig. 6 Powder X-ray diffraction pattern for PbS nanoparticles syn-
thesised from the Pb(pip-dtc) complex (a) at 180 °C in HDA (b) at
230 °C in DA, (c) from the Pb(thq-dtc) complex at 230 °C in DA, (d) at
270 °C in OA and (e) from the Pb(pip-dtc) complex at 240 °C in TOPO
and (f) at 230 °C in OA.
(220), (311), (222) and (420) planes of cubic rock salt PbS are
observed. The diffraction pattern shows no evidence of peaks
belonging to impurities or by-products. The Fast Fourier Trans-
form (FFT) images (inset Fig. 4d and 5b) show plane spacings
of 0.34 nm, 0.20 nm and 0.28 nm corresponding to the (111),
(220) and (200) planes of PbS.
The nature of the lead precursor has some influence on the
formation of large, faceted crystalline PbS nanoparticles under
high reaction temperatures in various coordinating solvents. The
particles synthesized in this work are generally larger than the
particles synthesized by conventional ‘hot injection’ precursor
routes. The only plausible explanation is the effect of the nature
of the precursor on the growth mechanism resulting in larger
The powder X-ray diffraction patterns of the PbS nano-
particles synthesized using the two precursors in various capping
groups is shown in Fig. 6. All peaks are sharp confirming the
crystalline nature of the material. The characteristic (111), (200),
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faceted particles. The formation of regular, large faceted particles
has been previously observed for CdS, which were grown using
the cadmium analogues as precursors.16–18 We intend to further
study the role of the precursors in the growth mechanism and
morphology of the particles.
for Nano-structured Materials (NCNSM), CSIR, Pretoria for
HRTEM measurements.
References
1 F. W. Wise, Acc. Chem. Res., 2000, 33, 773.
2 D. Qi, M. Fischbein, M. Drndic and S. Selmic, Appl. Phys. Lett., 2005,
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3 T. K. Chaudhuri and S. Chatteries, Proc. Int. Conf. Thermoelectr., 1992,
11, 40.
4. Conclusions
We have reported the synthesis of heterogeneous lead piperidine
and lead tetrahydroquinoline dithiocarbamate (DTC) complexes.
The complexes were characterized by 1H NMR, infrared and
thermogravimetric techniques. The thermolysis of the complexes
in capping groups such as HDA, TOPO, OA and DA produced
PbS nanoparticles with varying shapes and sizes. HDA gave
PbS particles with shapes ranging from close to spheres, cubes
and rods. In TOPO, the PbS appeared spherical and agglomer-
ated. The OA capped PbS were regular cubic shaped at lower
temperatures but changed to rods at higher (270 °C) tempera-
tures. The DA capped particles were very crystalline, forming
perfect cubes and rectangular shaped particles at 230 °C. The
X-ray diffraction and Fast Fourier Transform studies confirm the
cubic rock salt phase of PbS. The nature of the precursor plays a
key role in the formation of the larger than usual particles and
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Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF) of
South Africa through DST/NRF South African Research Chairs
Initiative (SARCHi) program. Neerish Revaprasadu would also
like to acknowledge the Royal Society of Chemistry (RSC) for
an international author travel grant to visit the University of
Oxford, UK. Ms LD Nyamen also acknowledges the Organis-
ation of Women in Science for Developing World (OWSDW)
formerly TWOWS for a sandwich Postgraduate Fellowship. The
authors also acknowledge the Centre for Electron Microscopy,
University of Kwa-Zulu Natal for the TEM and National Centre
8302 | Dalton Trans., 2012, 41, 8297–8302
This journal is © The Royal Society of Chemistry 2012