Synthesis of multi-podal CdS nanostructures using heterocyclic dithiocarbamato complexes as precursors

Article abstract of DOI:10.1016/j.poly.2013.03.027

Bis(dipiperidinyldithiocarbamato)cadmium(II) (1) and bis(ditetrahydroquinolinyldithio-carbamato)cadmium(II) (2) were used as precursors for the synthesis of oleylamine (OA), decylamine (DA) and dodecylamine (DDA) capped CdS nanoparticles. The optical properties of these particles have been studied. The absorption spectra for the amine capped CdS particles are blue shifted in relation to the bulk material. The corresponding photoluminescence spectra show a narrow band edge emission. High quality crystalline CdS particles of different shapes, ranging from short nanorods and elongated nanorods (rods, bipods, tripods and tetrapods) to nanocubes were obtained when the reaction temperature was varied between 180 and 270 C. A decrease in the length of the rods and bipodal nanoparticles was observed with an increase in the length of the chain of the amine (capping agent) used. The p-XRD patterns revealed the hexagonal phase of CdS to be dominant in all the samples. Infra-red studies suggest that the mode of bonding of the amines (oleylamine, decylamine and dodecylamine) on the CdS nanoparticle surfaces is through electron donation from the nitrogen atoms.

Full text of DOI:10.1016/j.poly.2013.03.027

Polyhedron 56 (2013) 62–70
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis of multi-podal CdS nanostructures using heterocyclic
dithiocarbamato complexes as precursors
a
a
Linda D. Nyamen a,b, Neerish Revaprasadu a, , Rajasekhar V.S.R. Pullabhotla , Adeola A. Nejo ,
⇑
Peter T. Ndifon b, Mohammad Azad Malik c, Paul O’Brien c
a Department of Chemistry, University of Zululand, Private Bag X1001, KwaDlangezwa 3880, South Africa
b Department of Inorganic Chemistry, University of Yaoundé I, P.O. Box 812, Yaoundé, Cameroon
c School of Chemistry and Materials Science Centre, The University of Manchester Oxford Road, Manchester M13 9PL, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(dipiperidinyldithiocarbamato)cadmium(II) (1) and bis(ditetrahydroquinolinyldithio-carbamato)cad-
mium(II) (2) were used as precursors for the synthesis of oleylamine (OA), decylamine (DA) and dodecyl-
amine (DDA) capped CdS nanoparticles. The optical properties of these particles have been studied. The
absorption spectra for the amine capped CdS particles are blue shifted in relation to the bulk material. The
corresponding photoluminescence spectra show a narrow band edge emission. High quality crystalline
CdS particles of different shapes, ranging from short nanorods and elongated nanorods (rods, bipods, tri-
pods and tetrapods) to nanocubes were obtained when the reaction temperature was varied between 180
and 270 °C. A decrease in the length of the rods and bipodal nanoparticles was observed with an increase
in the length of the chain of the amine (capping agent) used. The p-XRD patterns revealed the hexagonal
phase of CdS to be dominant in all the samples. Infra-red studies suggest that the mode of bonding of the
amines (oleylamine, decylamine and dodecylamine) on the CdS nanoparticle surfaces is through electron
donation from the nitrogen atoms.
Received 30 October 2012
Accepted 5 March 2013
Available online 28 March 2013
Keywords:
CdS nanostructures
Dithiocarbamates
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
[7], or trioctylphosphine sulfide [7,8], were the main sources of
cadmium and sulfur for the synthesis of CdS nanoparticles. Later,
The syntheses and applications of semiconductor nanomaterials
have been the subject of intense research during the past two dec-
ades. Synthetic routes to nanostructured materials are generally
divided into physical and chemical methods. The aim is to produce
particles in the nanosize regime which are crystalline and regular
in both size and shape. The ability to manipulate the size and shape
of the particles has become a niche focus area in the field. This con-
trol will assist workers to optimally harness the unique properties
of materials for applications. Cadmium sulfide, a II–VI type semi-
conductor, is a good example of a material whose size and shape
can be manipulated by variation of the reaction conditions in the
synthetic methodology [1–5].
The thermolysis of metal and chalcogenide sources in
coordinating solvents is a common route to semiconductor nanom-
aterials. The so called ‘hot injection’ method allows for the manip-
ulation of the capping ligands, ligand–solvent pair, reactant
concentration and reaction temperature to control particle growth
and the final morphology. Initially, highly reactive and toxic pre-
cursors, such as dimethylcadmium [6], bis(trimethylsilyl)sulfur
less harmful cadmium sources such as CdCl2 [8,9], Cd-oleate [10],
or CdO [7,11], and sulfur precursors such as dodecanthiol [10], a
solution of elemental sulfur in oleylamine [9], or in octadecene
[11] were employed. A related method is the use of single source
precursors incorporating the elemental constituents as starting
materials. In these high temperature thermolyses or hot injection
routes, the precursor is introduced into the hot solvent. There is
a short burst nucleation, followed by slow growth and annealing.
The separation between nucleation and growth stages facilitates
shape control. The final shape, size, size-distribution and lattice
type are determined based on the cumulative effects of nucleation
and growth phases. Particles synthesized by this method exhibit
excellent crystallinity and monodispersity.
Various classes of inorganic compounds have been used as pre-
cursors to CdS nanoparticles. There is a substantial body of work on
the use of dithiocarbamato complexes of cadmium as single source
precursors to CdS [12–17]. Related compound classes such xan-
thates [18–20], thioureas [21–25] and thiosemicarbazides [26]
have also been extensively reported. Recently our group revisited
the use of dithiocarbamato complexes as starting materials for
CdS and PbS nanoparticles [3,5,14,27]. We found very interesting
results when heterocyclic dithiocarbamato complexes such as
cadmium and lead piperidinyl and tetrahydroquinolinyl
⇑
Corresponding author. Tel.: +27 35 902 6152; fax: +27 35 902 6568.
E-mail address: nrevapra@pan.uzulu.ac.za (N. Revaprasadu).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.03.027

L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
63
dithiocarbamates were used as precursors for CdS or PbS nanopar-
S
S
ticles. By changing the reaction temperature and reactant concen-
tration, particles in the shape of spheres, rods and cubes were
formed. In this work we report the synthesis of anisotropic CdS
nanoparticles by the thermolysis of heterocyclic dithiocarbamate
complexes in decylamine (DA), dodecylamine (DDA) and oleyl-
amine (OA) at various reaction temperatures.
N
Cd
N
S
S
complex 1
2. Material and methods
S
2.1. Chemicals
S
S
N
N
Decylamine (DA), dodecylamine (DDA), oleylamine (OA), tolu-
ene, tri-octylphosphine (TOP) 90%, 1,2,3,4-tetrahydroquinoline
98% (Aldrich), piperidine 99% (Sigma–Aldrich), petroleum ether,
methanol 99.5%, dichloromethane, carbon disulfide 99.5%, sodium
hydroxide 98%, cadmium chloride monohydrate 99% and acetone
(Merck) were used as purchased without any further purification.
Cd
S
complex 2
Fig. 1. Structures of the complexes.
2.2. Synthesis of the precursors
2.2.1. Preparation of the ligands
Carbon disulfide (6.0 mL, 0.1 mol) was added in small portions
to an equimolar mixture of sodium hydroxide (4.0 g, 0.1 mol) and
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(a)
the
corresponding
amine
(piperidine/tetrahydroquinoline,
(i)
0.1 mol) and cooled in an ice bath at 0 °C. After 15 min, the solidi-
fied mass was then dried in air and recrystallised in a mixture of
acetone/petroleum ether. The product was collected and washed
with chloroform and suction dried.
(ii)
Na(pip-dtc), yield: 90%. 1H NMR (400 MHz, DMSO-d6) d: 1.41
(m, 2H, 3-CH2), 1.53 (t, 2H, 4-CH2), 4.28 (t, 2H, 2-CH2). IR (cmÀ1
,
ATR): 3367 v(O–H); 967 v(C@S); 1468 v(C@N). Microanalysis, Calc.
for C6H14NS2O2Na: C, 32.87; H, 6.44; N, 6.39. Found: C, 32.57; H,
6.29; N, 5.96%.
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Na(thq-dtc), yield: 62%. 1H NMR (400 MHz, CD3OD) d: 2.07 (m,
2H, 3-CH2), 2.73 (t, 2H, 4-CH2), 4.58 (t, 2H, 2-CH2), 7.08–7.85 (m,
4H, Ar–H). IR (cmÀ1, ATR): 3324 v(O–H); 967 v(C@S); 1485
v(C@N). Microanalysis, Calc. for C10H18NS2O4Na: C, 39.59; H,
5.98; N, 4.62. Found: C, 39.15; H, 5.94; N, 4.27%.
0.5
0.4
0.3
0.2
0.1
0
(ii)
(b)
(i)
2.3. Preparation of the complexes
Cadmium chloride (5.0 mmol) was dissolved in distilled water
(25.0 mL) and added dropwise to the corresponding solution of
the dithiocarbamate ligand (10.0 mmol) in water and cooled in
an ice bath at 0 °C. The reaction mixture was stirred for 1 h, and
the precipitate formed was filtered, washed with excess distilled
water and dried overnight in an oven at 70 °C.
300
350
400
450
500
550
600
650
700
Wavelength (nm)
0.6
0.5
0.4
0.3
0.2
0.1
0
(c)
(ii)
2.3.1. Complex 1
[Cd(pip-dtc)2] (pip = piperidinyl and dtc = dithiocarbamato),
yield: 82%. 1H NMR (400 MHz, CDCl3) d: 1.63 (m, 8H, 3-CH2),
1.73 (t, 4H, 3-CH2), 4.13 (t, 8H, 2-CH2). IR (cmÀ1, ATR): 3324 v(O–
H); 967 v(C@S); 1485 v(C@N); 388 v(Cd–S). Microanalysis, Calc.
for C12H20N2S4Cd: C, 45.40; H, 3.81; N, 5.29. Found: C, 45.57; H,
3.61; N, 5.73%.
(i)
2.3.2. Complex 2
375
425
475
525
575
625
675
[Cd(thq-dtc)2] (thq = tetrahydroquinoliny), yield: 71%. 1H NMR
(400 MHz, CDCl3) d: 1.99 (m, 4H, 3-CH2), 2.71 (t, 4H, 4-CH2), 4.24
(t, 4H, 2-CH2), 7.18–7.86 (m, 8H, Ar–H). IR (cmÀ1, ATR): 3324
v(O–H); 967 v(C@S); 1485 v(C@N); 365 v(Cd–S). Microanalysis,
Wavelength (nm)
Fig. 2. (i) UV–Vis and (ii) PL spectra of CdS nanoparticles synthesised using Cd(pip-
dtc) complex at 180 °C in (a) OA, (b) DA and (c) DDA.

64
L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
Fig. 3. TEM and corresponding HRTEM image of CdS nanoparticles synthesised at 180 °C using [Cd(pip-dtc)2] complex in (a) OA, (c) DDA and (e) DA.
Calc. for C20H20N2S4Cd: C, 39.51; H, 5.98; N, 4.62. Found: C, 39.15;
H, 5.94; N, 4.27%.
was repeated at a reaction temperature of 270 °C. The above reac-
tion procedure was then repeated using the [Cd(thq-dtc)2]
complex.
The structures of the complexes are shown in Fig. 1.
2.4. Synthesis of oleylamine (OA) capped CdS nanoparticles
2.5. Synthesis of decylamine (DA) capped CdS nanoparticles
A mass of 0.5 g (1.2 mmol) of the [Cd(pip-dtc)2] complex was
dissolved in 6.0 mL of tri-n-octylphosphine (TOP). The solution
was injected into 6.0 g (22.4 mmol) of hot oleylamine (OA) in a
three-necked flask at 180 °C. The solution turned to a yellowish or-
ange color and a drop in temperature of 32 °C was observed. The
reaction was allowed to stabilize at 180 °C. After a reaction time
of 1 h, aliquots of samples were taken and methanol was added,
resulting in the formation of a flocculent precipitate. The precipitate
was separated by centrifugation and then dispersed in toluene to
give yellowish orange OA-capped CdS nanoparticles. The reaction
A mass of 0.5 g (1.2 mmol) of the [Cd(pip-dtc)2] complex was
dissolved in 6.0 mL of TOP. The solution was injected into 6.0 g
(38.1 mmol) of the hot DA in a three-necked flask at 180 °C. The
solution turned a yellowish orange color and a drop in temperature
of 25 °C was observed. The reaction was allowed to stabilize at
180 °C. After the reaction time of 1 h, aliquots of samples were ta-
ken and methanol added, resulting in the formation of a flocculent
precipitate. The precipitate was separated by centrifugation, and
dispersed in toluene to give yellowish orange DA-capped CdS
nanoparticles. The reaction was repeated at a reaction temperature

L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
65
Table 1
Absorption band edge, photoluminescence emission maxima and particle dimensions for CdS nanoparticles synthesized from [Cd(pip-dtc)2] and [Cd(thq-dtc)2] under various
conditions.
Precursor
Growth temp. (°C) Capping group Absorption band edge (nm) PL emission max (nm) Particle shape/size (nm)
l = length, b = breadth, s = size
[Cd(pip-dtc)2] 180
[Cd(pip-dtc)2] 180
OA
DA
493
506
481
485
rods: l = (41.07 4.72), b = (3.66 0.45)
bipods: l = (29.76 4.91), b = (1.55 0.27)
rods: l = (38.01 5.33), b = (6.90 0.33)
bipods: l = (33.33 9.82), b = (6.90 0.33)
tripods: l = (29.28 3.50), b = (7.43 0.35)
rods: l = (28.42 4.21), b = (5.36 0.21)
bipods: l = (29.47 4.21), b = (6.84 1.29)
tetrapods: l = (11.67 6.50), b = (4.44 4.00)
rods: l = (35.78 3.94), b = (16.83 2.10)
bipods: l = (42.10 6.44), b = (12.49 2.18)
tripods: l = (28.41 4.20), b = (20.52 3.49)
Rods: l = (19.50 4.30), b = (6.00 1.22)
bipods: l = (17.50 3.53), b = (5.62 1.08)
tripods: l = (12.50 4.67), b = (6.25 1.25)
rods: l = (15.55 2.22), b = (6.11 1.41)
bipods: l = (17.22 2.72), b = (6.94 1.24)
tripods: l = (13.33 4.78), b = (7.50 2.08)
rods: l = (21.00 1.00), b = (2.00 0.90)
elongated tetrapods: l = (24.00 5.50), b = (4.00 1.25)
cubes: s = (13.25 1.95)
[Cd(pip-dtc)2] 180
[Cd(pip-dtc)2] 270
[Cd(pip-dtc)2] 230
[Cd(pip-dtc)2] 230
[Cd(thq-dtc)2] 180
DDA
OA
511
517
529
554
491
490
472
496
DA
DDA
OA
495
570
479
628
472
487
568
486
568
[Cd(thq-dtc)2] 270
[Cd(thq-dtc)2] 230
OA
DA
524
540
rods: l = (15.08 1.68), b = (9.37 1.80)
[Cd(thq-dtc)2] 230
DDA
575
rods: l = (17.39 1.66), b = (9.52 1.66)
of 230 °C. The same reaction procedure was repeated using the
[Cd(thq-dtc)2] complex.
measurements and the samples were placed in silica cuvettes
(1 cm path length), using toluene as a reference solvent. A Perkin
Elmer, LS 55 Luminescence spectrometer was used for recording
the photoluminescence spectra.
2.6. Synthesis of dodecylamine (DDA) capped CdS nanoparticles
A mass of 0.5 g (1.2 mmol) of the [Cd(pip-dtc)2] complex was
dissolved in 6.0 mL of TOP. The solution was injected into 6.0 g
(32.3 mmol) of hot DDA in a three-necked flask at 180 °C. The solu-
tion turned a yellowish orange color and a drop in temperature of
27 °C was observed. The solution was allowed to stabilize at
180 °C. After a reaction time of 1 h, aliquots of samples were taken
and methanol added, resulting in the formation of a flocculent
precipitate. The precipitate was separated by centrifugation, and
dispersed in toluene to give yellowish orange DDA-capped CdS
nanoparticles. The reaction was repeated at a reaction temperature
of 230 °C. The same reaction procedure was repeated with the
[Cd(thq-dtc)2] complex.
3. Results and discussion
3.1. Characterization of the precursors
Elemental analyses revealed that the ligands and the complexes
synthesized were pure and agreed with the proposed formulae.
The precursors were obtained in good yield with a simple reaction
procedure. The piperidine dithiocarbamate ligand and its corre-
sponding cadmium complex were obtained as white precipitates,
whilst the tetrahydroquinoline dithiocarbamate ligand was ob-
tained as yellow microcrystals and its cadmium complex as a pale
yellow powder. The compounds are air stable, easy to synthesize
and soluble in some organic solvents, such as chloroform.
2.7. Instrumentation
The FT-IR spectra of the hydrated sodium salts of the dithiocar-
bamate ligands showed very broad bands in the 3500–3000 cmÀ1
region, typical of OH of water molecules. The C–N stretching vibra-
tion usually appears in dithiocarbamates as a strong band around
1500 cmÀ1. The position of this band is indicative of the degree of
the double bond character in the C–N bond [28] (v(C@N) = 1690–
1640; v(C–N) = 1350–1250 cmÀ1). The IR spectra of the bis(dite-
trahydroquinolinyldithiocarbamato)cadmium(II) and bis(dipi-
peridinyldithiocarbamato)cadmium(II) complexes showed v(C@N)
bands at 1446 and 1483 cmÀ1, respectively, indicating partial dou-
ble bond character. The v(C–N) band is higher for bis(dipi-
peridinyldithiocarbamato)cadmium (II) than for bis(ditetrahy
droquinolinyldithiocarbamato)cadmium(II). This may be due to
the electron withdrawing resonance effects of the phenyl ring in
tetrahydroquinolinedithiocarbamate, resulting in a decrease in the
double bond character of the C–N bond [29]. The v(C@S) band is
observed in the 999–975 cmÀ1 range in the spectra of the sodium
salts of the ligands. The upward shift of the band for the dithiocar-
bamate from that of the free ligand, together with a strong band
(or two very close bands) attributed to v(C–S), is indicative of a
Microanalysis was performed on a Perkin–Elmer automated
model 2400 series II CHNS/O analyzer. Infrared spectra were
recorded on a Bruker FT-IR tensor 27 spectrophotometer directly
on small samples (10 mg) of the compounds in the range
200–4000 cmÀ1. The 1H NMR spectra of both the ligands and the
metal complexes were obtained using
400 MHz spectrophotometer. The crystalline phase was identified
by X-ray diffraction (XRD), employing scanning rate of
0.05° minÀ1 in a 2h range from 20° to 80°, using a Bruker AXS D8
diffractometer equipped with nickel filtered Co K radiation
(k = 1.5418 Å) at 40 kV, 40 mA and at room temperature. The mor-
phology and particle sizes of the samples were characterized by a
JEOL 1010 TEM with an accelerating voltage of 100 kV, Megaview
III camera and Soft Imaging Systems iTEM software. The detailed
morphological and structural features were investigated using
HRTEM images with a JEOL 2010 transmission electron microscope
operated at an accelerating voltage of 200 kV. A Varian Cary 50
Conc UV–Vis spectrophotometer was used to carry out the optical
a Bruker advance III
a
a

66
L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
bidentate or slightly anisobidentate dithiocarbamate [30]. In the far
IR region, a new additional band, absent in the spectra of the ligands,
is observed in the 365–388 cmÀ1 region, which possibly corre-
sponds to v(Cd–S).
3.3. Synthesis of CdS from the cadmium piperidine dithiocarbamato
complex
In our previous work [5,15], we reported the synthesis of HDA
and TOPO-capped CdS nanoparticles using bis(dipiperidinyldithio-
carbamato)cadmium(II) and bis(ditetrahydroquinolinyldithiocarb-
amato)cadmium(II) complexes. The thermolysis of the precursors
at moderate reaction temperatures (140 and 180 °C) using HDA
as the capping group resulted in elongated and spherically shaped
particles [5,15]. At a higher reaction temperature (240 °C), large
faceted CdS particles in the shape of hexagons and cubes were
formed [15]. In this work we have used the same precursors, but
changed the length of the chain of the linear shaped amine, using
OA, DA and DDA as the passivating group, at reaction temperatures
of 180, 230 and 270 °C.
3.2. Synthesis of CdS nanoparticles
Long chain amines are found to be suitable surfactants for II–VI
semiconductor nanomaterials. The as-prepared unshelled CdSe
nanoparticles with long chain primary amines were found to have
emission quantum yields of 60%, without the need for an inorganic
shell [31]. In this work we decided to explore the use of long chain
amines, such as hexadecylamine (HDA), oleylamine (OA), decyl-
amine (DA) and dodecylamine (DDA), as capping groups for CdS
nanoparticles.
Fig. 4. TEM and corresponding HRTEM image of CdS nanoparticles synthesised using [Cd(pip-dtc)2] with 3 g of (a) OA at 270 °C and with 6 g of (c) DA and (e) DDA.

L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
67
The bis(dipiperidinyldithiocarbamato)cadmium(II) complex
1000
800
600
400
200
0
was thermolysed in OA at 180 °C. The absorption and photolumi-
nescence spectra of the OA capped CdS nanoparticles are shown
in Fig. 2a. The absorption band edge is sharp, with a band gap posi-
tioned at 493 nm. The photoluminescence spectrum shows two
peaks (Fig. 2a). The first narrow emission peak has a maximum
at 481 nm, whereas a broad weak emission peak is observed in
the 550–700 nm region. The latter emission is due to trap emission,
observed in poorly passivated surfaces and/or defects in the crystal
lattice.
A similar reaction procedure was adopted for the particles
synthesized in DA and DDA. The absorption and photolumines-
cence spectra of the DA and DDA capped CdS nanoparticles at
180 °C are shown in Fig. 2b and c. The absorption spectra show
distinct band edges at 506 and 511 nm for the DA and DDA capped
CdS nanoparticles, respectively. The positions of the band edges
show a slight blue shift from bulk CdS (515 nm). The corresponding
photoluminescence spectra (Fig. 2b and c) show narrow emission
peaks with the emission maxima at 485 (DA) and 490 nm (DDA).
The TEM and HRTEM images of the OA, DA and DDA capped CdS
nanoparticles synthesized at 180 °C are shown in Fig. 3. All three
capping groups led to elongated particles in the form of rods,
bipods, tripods and tetrapods, the dimensions of which are shown
in Table 1. There is a decrease in the length of the rods and bipods
as we increase the length of the carbon chain from DA
(l = 38.01 5.33 nm) to DDA (l = 28.42 4.21 nm).
(a)
(b)
(c)
20
30
40
50
60
70
80
90
2 Theta (deg)
Fig. 5. XRD patterns of CdS nanoparticles synthesised using [Cd(pip-dtc)2] in(a) OA
at 270 °C, (b) DDA at 230 °C and (c) DA at 230 °C.
0.6
(a)
0.5
(b)
0.4
0.3
0.2
0.1
0
In our previous work with HDA (which is longer than DDA) at
180 °C, we obtained shorter rods of length (l = 19.11 7.29 nm)
[5]. An aspect ratio of 11.22, 5.50 and 5.30 is obtained for OA, DA
and DDA respectively. The HRTEM image for the OA capped CdS
shows an elongated particle with distinct lattice fringes (Fig. 3b)
The d-spacing of 3.3 Å corresponds to the [001] plane of hexagonal
CdS. A CdS particle in the form a tetrapod is observed in the HRTEM
image of the DDA capped CdS (Fig. 3d).
300
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
When the reaction temperature is increased to 230 °C for DA
and DDA, and 270 °C for the OA capped CdS nanoparticles, a change
in the optical properties of the CdS nanoparticles is observed. The
absorption spectra are broader and there is a general decrease in
the intensity of the emission peaks and a shift to longer wave-
lengths (Table 1). This change is due to the change in particle size
and size distribution due to the higher reaction temperatures. The
higher temperature also favors a reduction of surface defects
through annealing. The TEM and HRTEM images (Fig. 4) show that
the breadth of the nanorods increased while the length decreased.
Short rods, bipods and tripods of CdS are clearly visible, whose
dimensions are presented in Table 1. The crystallinity of the parti-
cles is confirmed, with distinct lattice fringes observed in the
HRTEM images. The HRTEM images of the OA and DA capped
CdS show the presence of tetrapod shaped particles (Fig. 4b and
d). CdS nanoparticles with the tetrapod morphology are achieved
through the formation of a zinc blende seed at a relatively low
temperature, allowing growth along the four [111] faces, forming
wurtzite arms. There is a separation angle of 113 °C between the
arms. In Fig. 4f, a bipodal structure is visible with a zinc blende
core, having a lattice spacing of 0.29 nm, corresponding to the
[200] lattice plane. The two side arms have a lattice spacing of
0.34 nm, assigned to the [002] lattice planes of wurtzite CdS.
The X-ray diffraction (XRD) spectra for the amine capped CdS
are shown in Fig. 5. The (110), (103) and (112) planes, indexed
to wurtzite CdS, are present in the diffraction patterns. The sharp
[002] planes, observed for all samples, are consistent with the
preferential [001] growth direction in anisotropic CdS nanoparti-
cles. There is evidence of diffraction from the zinc blende seed.
The [103] plane for pure wurtzite is normally more intense than
the [110] and [112] plane. These [110] and [112] planes could
also correspond to the [220] and [331] planes of the zinc blende
Fig. 6. (a) UV–Vis and (b) PL spectra of CdS nanoparticles synthesised at 180 °C
using [Cd(thq-dtc)2] in OA.
structure. However the weaker [102] plane confirms the wurtzite
phase is dominant.
3.4. Synthesis of CdS nanoparticles from the cadmium
tetrahydroquinolinedithiocarbamate complex
The CdS nanoparticles were synthesized in OA at 180 °C using
the bis(ditetrahydroquinolinyldithiocarbamato)cadmium(II) com-
plex. The optical spectrum of the OA capped CdS using this precur-
sor is similar to that of the OA capped particles synthesized from
the bis(dipiperidinyldithiocarbamato)cadmium(II) complex. The
UV–Vis spectrum shows a sharp absorption band edge of 491 nm
(Fig. 6a). Two peaks are observed in the photoluminescence spec-
trum, a narrow peak with an emission maximum at 479 nm and
a second deep trap peak with a maximum at 628 nm (Fig. 6b).
The TEM image of the OA capped CdS synthesized at 180 °C shows
a mixture of tripods and tetrapods, similar to those obtained using
the piperidinyldithiocarbamato complex at the corresponding
temperature. However when the reaction temperature is increased
to 270 °C, the morphology of the particles changes drastically. The
TEM image of the OA capped CdS (Fig. 7c) shows a mixture of
cubic- and hexagonally shaped particles. The average size of the
cubes is 13.25 1.95 nm. A well-defined cube shaped CdS particle
with distinct lattice fringes is observed in the HRTEM image
(Fig. 7d). In our previous work on CdS using bis(dite-
trahydroquinolinyldithiocarbamato)cadmium(II) as a precursor,
thermolysed in HDA at 240 °C, we obtained large faceted CdS
particles having hexagonal, cubic and other irregular shapes [5].

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L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
Fig. 7. TEM and corresponding HRTEM images of OA-capped CdS nanoparticles synthesised using [Cd(thq-dtc)2] at (a) 180 and (c) 270 °C.
It is evident that the higher temperature favors the formation of
3.5. Branched growth
large close to cubic particles when using this class of precursor. We
have also synthesized CdS nanoparticles using the bis(dite-
trahydroquinolinyldithiocarbamato)cadmium(II) pre-cursor therm
olysed in DA and DDA at a reaction temperature of 230 °C. The TEM
images of the CdS particles synthesized in DA (Fig. 8a) show short
nanorods with an average length of 15.08 1.68 nm and breadth of
9.37 1.80 nm. The DDA capped particles have a similar morphol-
ogy with a length of 17.39 1.66 nm and breadth of 9.52 1.66 nm
(Fig. 8c). A single crystalline particle in the form of a rectangle is
observed in the HRTEM image of the DA capped CdS particles
(Fig. 8d).
Ab initio calculations suggested that the mode of bonding of
amines to the nanoparticle surfaces is through the lone pair of elec-
trons on the nitrogen atom [32]. There is an argument that the
acidic proton in the NH2 group on a primary amine can coordinate
to a surface metal site [33]. The use of tertiary amines in nanopar-
ticle synthesis does appear to contradict this [34,35], and it is
possible that the coordination of amines to nanoparticle surfaces
may actually occur through both the lone pair of electrons and
the protons. FTIR spectra of oleylamine-capped FePt particles
confirm the presence of N–H and suggested that oleylamine
appeared to bind through electron donation from the nitrogen
atom [36]. The capping agents used in our work (oleylamine, decyl-
amine and dodecylamine) are all primary amines. The IR spectra of
the OA-, DA- and DDA-capped CdS nanoparticles are shown in
Fig. 9. All three show the same peaks, with some differences in
the level of their intensity at some wavenumbers. The peaks ob-
served below 300 cmÀ1 are characteristic of the Cd–S bond. The
C–H stretching is observed in the range 2911–2914 cmÀ1. The peak
at 3229 cmÀ1 is attributed to the N–H stretching, thus confirming
the presence of N–H and indicating that oleylamine, decylamine
and dodecylamine appear to bind through electron donation from
the nitrogen atom, as reported previously [36].
After the formation of a crystalline seed, the growth process
occurs through a delicate balance between kinetic and thermody-
namic preferences [37]. Factors such as reaction temperature and
precursor temperature will determine the growth path. It has been
shown that high temperatures and low precursor concentrations
favor isotropic structures [38]. These structures are in the form
of perfect or quasi-spheres. At low reaction temperatures and high
precursor concentrations the growth path is kinetically driven,
favoring the formation of anisotropic structures in the form of rods
or branched structures. In our work we observed that there is a
difference in the dimensions of the amine capped CdS obtained
at different reaction temperatures using the [Cd(pip-dtc)2] com-
plex. At reaction temperatures of 180 °C the average length of
the rods in the form of bipods, tripods and tetrapods for the OA,
DA and DDA capped CdS particles is 26.91 3.64 nm. When the
temperature is increased to 230 °C (DA and DDA) and 270 °C for
the OA capped CdS, shorter and broader branched rods (av. length
22.43 4.09 nm and breadth 9.81 1.78 nm) are observed.
A
similar trend is observed when [Cd(thq-dtc)2] is thermolysed in
OA. At 180 °C narrow branched CdS particles in the form of bipods
and tetrapods are observed, however when the temperature is in-
creased to 270 °C cube shaped particles are observed. The DA and
DDA capped particles obtained at 230 °C are also large faceted par-
ticles. This trend has been previously observed for oleylamine
capped CdS nanoparticles synthesized at 175 and 100 °C. At
175 °C ellipsoidal particles were observed, but at the lower tem-
perature of 100 °C rods, bipods, tripods and tetrapods were
obtained. It is interesting to note that we have observed branched
structures of CdS at relatively high reaction temperatures (230 and
270 °C) when the bis(dipiperidinyldithiocarbamato)cadmium(II)
complex was thermolysed in all three amines. There have been
no other reports of branched CdS at these high temperatures. The

L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
69
Fig. 8. TEM and corresponding HRTEM images of CdS nanoparticles synthesised at 230 °C using [Cd(thq-dtc)2] in (a and b) DA and (c and d) DDA.
4. Conclusions
1.6
1 4
1.2
1
Two heterocyclic dithiocarbamatocadmium(II) complexes have
been thermolysed in oleylamine, decylamine and dodecylamine at
various reaction temperatures (180, 230 and 270 °C) to give
branched shaped CdS nanorods. The optical properties of the mate-
rials under all reaction conditions confirmed the quantum confined
nature of the particles. We found that at relatively high reaction
temperatures branched particles in the form of tetrapods were
formed when the bis(dipiperidinyldithiocarbamato)cadmium(II)
complex was thermolysed in all three amines, whereas the bis(ditet-
rahydro- quinolinyldithiocarbamato)cadmium (II) complex gave
cubic shaped particles at high reaction temperatures. A decrease
in the length of the rods, and the bipodal nanoparticles was ob-
served with an increase in the length of the chain of the alkylamine.
The dominant hexagonal phase in both is confirmed by X-ray dif-
fraction measurements. Infra-red studies indicate that the amines
used in our work as capping agents bind on the surface of the CdS
nanoparticles through electron donation from the nitrogen atoms.
(a)
(b)
(c)
0.8
0.6
0.4
0.2
0
200
700
1200
1700
2200
2700
3200
3700
4200
Wavenumber (cm-1)
Fig. 9. IR spectra of CdS nanoparticles synthesised using [Cd(pip-dtc)2] in (a) OA, (b)
DA and (c) DDA.
reaction temperature of 180 °C is also relatively high, producing
branched shaped particles without the addition of a shape direct-
ing agent. There is also no discernable difference in the morphol-
ogy of the particles with respect to the capping group.
Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF) of

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L.D. Nyamen et al. / Polyhedron 56 (2013) 62–70
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South Africa through the DST/NRF South African Research Chairs
Initiative (SARCHi) program. Ms. L.D. Nyamen also acknowledges
the Organization for Women Scientists for the Developing World
(OWSDW) formerly TWOWS for a Sandwich Postgraduate Fellow-
ship. The authors also acknowledge the Centre for Electron Micros-
copy, University of Kwa-Zulu Natal for the TEM and National
Centre for Nano-structured Materials (NCNSM), CSIR, Pretoria for
HRTEM measurements.
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