Synthesis, spectral and thermal studies of pyridyl adducts of Zn(ii) and Cd(ii) dithiocarbamates, and their use as single source precursors for ZnS and CdS nanoparticles

Article abstract of DOI:10.1039/c4dt00610k

The synthesis, spectroscopic characterisation, and thermal studies of pyridyl adducts of Zn(ii) and Cd(ii) complexes of N-ethyl-N-phenyl dithiocarbamate, represented as [ZnL₂py] and [CdL₂py ₂], are reported. Single-crystal X-ray structural analysis of the Zn compound showed that it is five-coordinate with four sulphurs from dithiocarbamate and one nitrogen from pyridine in a distorted square pyramidal geometry. The thermogravimetric studies indicate that the zinc and cadmium compounds undergo fast weight loss, and the temperature at maximum rate of decomposition is at 277 °C and 265 °C respectively, to give the metal (Zn or Cd) sulphide residues. These compounds were used as single molecule precursors to produce nanocrystalline MS (M = Zn, Cd) after thermolysis in hexadecylamine. The morphological and optical properties of the resulting MS nanocrystallites were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-Vis absorption and photoluminescence (PL) spectroscopy, and powdered X-ray diffraction (XRD). By varying the growth time, the temporal evolution of the optical properties and morphology of the nanocrystals were investigated.

Full text of DOI:10.1039/c4dt00610k

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Synthesis, spectral and thermal studies of pyridyl
adducts of Zn(II) and Cd(II) dithiocarbamates, and
their use as single source precursors for ZnS and
CdS nanoparticles†
Cite this: Dalton Trans., 2014, 43,
8703
Damian C. Onwudiwe,*a Christien A. Strydom,a Oluwatobi S. Oluwafemi,b
Eric Hostenc and Anine Jordaand
The synthesis, spectroscopic characterisation, and thermal studies of pyridyl adducts of Zn(II) and Cd(II)
complexes of N-ethyl-N-phenyl dithiocarbamate, represented as [ZnL2py] and [CdL2py2], are reported.
Single-crystal X-ray structural analysis of the Zn compound showed that it is five-coordinate with four sul-
phurs from dithiocarbamate and one nitrogen from pyridine in a distorted square pyramidal geometry.
The thermogravimetric studies indicate that the zinc and cadmium compounds undergo fast weight loss,
and the temperature at maximum rate of decomposition is at 277 °C and 265 °C respectively, to give the
metal (Zn or Cd) sulphide residues. These compounds were used as single molecule precursors to
produce nanocrystalline MS (M = Zn, Cd) after thermolysis in hexadecylamine. The morphological and
optical properties of the resulting MS nanocrystallites were investigated using transmission electron
microscopy (TEM), scanning electron microscopy (SEM), UV-Vis absorption and photoluminescence (PL)
spectroscopy, and powdered X-ray diffraction (XRD). By varying the growth time, the temporal evolution
of the optical properties and morphology of the nanocrystals were investigated.
Received 27th February 2014,
Accepted 15th April 2014
DOI: 10.1039/c4dt00610k
www.rsc.org/dalton
tors, catalysts, production of hydrogen, electro-luminescent
displays, anti-reflection coating for infrared devices and other
Introduction
The synthesis of II–VI semiconductor nanoparticles (NPs) has non-linear optical devices.8–11 Bulk ZnS crystallizes into two
become a niche focus area in materials syntheses. This group allotropic forms: a cubic form (C-ZnS) with sphalerite structure
of compounds continues to generate research interest leading having a band gap of 3.5–3.7 eV and a hexagonal form (H-ZnS)
to the development of many synthetic approaches. The major with wurtzite structure having a band gap of 3.7–3.8 eV.12 The
drive is to produce particles in the nanosize regime which are cubic form is the stable form at room temperature, while at
crystalline and regular in both size and shape. ZnS and CdS elevated temperatures the wurtzitic (hexagonal) phase
NPs have been extensively studied among the different II–VI becomes more dominant. However, in the nanoscale, the hexa-
metal chalcogenide semiconductor NPs.1–6 The reason is gonal phase has been reported at lower temperatures.13,14 The
mainly due to their nonlinear optical properties and the ability stable phase for bulk CdS is the wurtzite form, and this phase
of tuning their band gaps by controlling the NPs size.7 They undergoes structural transition to the cubic form in the temp-
have a wide range of applications including light-emitting erature range of 300–400 °C.15 The hexagonal–cubic phase
diodes (LEDs), solid state solar window layers, photoconduc- transition is also size-dependent with a critical transformation
size varying from 4 to 15 nm.16,17
Among the various synthetic methods adopted in the syn-
aChemical Resource Beneficiation (CRB) Research Focus Area, North-West University,
thesis of II–VI NPs such as wet chemical,18,19 micro emul-
Private Bag X6001, Potchefstroom 2520, South Africa.
sion,20 hydrothermal technique,21 etc., the thermolysis of
E-mail: dcconwudiwe@gmail.com; Fax: +27 18 299 2350; Tel: +27 18 299 1068
chalcogenide sources in coordinating solvents also known as
single source precursor is a common and less complex
approach. This method allows the effective manipulation of
the synthetic conditions, control of the particle growth, and
the final morphology. Dithiocarbamate is one of the widely
used precursors. Dithiocarbamates exhibit a fast decompo-
sition in their thermal decomposition profile, which is an
bDepartment of Chemistry, Cape-Peninsula University of Technology, P.O. Box 652,
Capetown 8000, South Africa
cDepartment of Chemistry, Nelson Mandela Metropolitan University, P.O. Box 77000,
Port Elizabeth 6031, South Africa
dLaboratory for Electron Microscopy, CRB Research Focus Area, North-West
University, Private Bag X6001, Potchefstroom 2520, South Africa
†CCDC 935369. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4dt00610k
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important factor in single-source precursor method. Thus
dithiocarbamate of different groups ranging from hetero-
cyclic,22 alkyl,23 alkyl-phenyl dithiocarbamates,24 are among
such that have been used to prepare high quality II–VI nano-
crystalline semiconductors.
Herein, we report the syntheses and characterization of two
novel precursor complexes, [ZnL2py] and [CdL2py2]. These
single source complexes of dithiocarbamates were used in the
preparation of ZnS and CdS NPs by thermolysing the precursor
molecules in hexadecylamine. By varying the growth time, we
investigated the temporal evolution of the optical properties
and morphology of the as-synthesised nanocrystals.
Table 1 Crystallographic and refinement parameters
Compound
[ZnL2py]
Color
Colourless platelet
C23H25N3S4Zn
Formula
Formula mass
Crystal system
Space group
a (Å)
b (Å)
c (Å)
537.07
Trigonal
P3121
8.608 (5)
8.608 (5)
29.162 (5)
90
90
120
1871 (3)
3
1.430
α (°)
β (°)
γ (°)
V (Å3)
Z
Dcalc. (g cm−3
)
Reflections collected
Unique reflections (Rint
No. observations (I > 2.00σ(I))
No. variables
R, wR2
GOF
CCDC number
11 163
3082 (0.016)
2910
Experimental
Materials
)
164
0.0230, 0.0535
1.07
935369
All the chemical reagents used were of analytical grade and
were used as received without further purification. Zinc(II)
acetate, cadmium(II) chloride, N-ethyl aniline, carbon disul-
phide, sodium hydroxide were obtained from Sigma Aldrich.
Pyridine and chloroform were obtained from Merck.
Platon30 and Ortep-331 were used to prepare material and dia-
grams respectively for this publication.
Physical properties measurement
All non-hydrogen atoms were refined anisotropically.
C-bound H atoms were placed in calculated positions and
refined as riding atoms with C–H 0.95 (aromatic CH), 0.99
(CH2), 0.98 (CH3) Å and with U(H) = 1.2 (1.5 for methyl) Ueq(C).
The H atoms of the methyl group were allowed to rotate with a
fixed angle around the C–C bond to best fit the experimental
electron density (HFIX 137 in the SHELX program suite28). The
structure was refined as a 2-component inversion twin and the
dithiocarbamate ethyl group is disordered in a 0.688(9) : 0.332(9)
ratio. Table 1 contains the crystallographic and refinement
parameters.
Infrared spectra were recorded on a Bruker alpha-P FT-IR
spectrometer in the 500–4000 cm−1 range. The NMR spectra
were recorded on a 600 MHz Bruker Avance III NMR spectro-
meter. Microanalyses were carried out using a Elementar,
Vario EL Cube, set up for CHNS analysis. The X-ray powder
diffraction data were collected on a Röntgen PW3040/60 X’Pert
Pro X-ray diffractometer using Ni-filtered Cu Kα radiation (λ =
1.5405 Å) at room temperature. Scanning electron microscopy
images were obtained on a Quanta FEG 250 Environmental
Scanning electron microscope (ESEM). TEM measurements
were performed on a TECNAI G2 (ACI) instrument operated at
an accelerating voltage of 200 kV. The thermal behaviour of
the adducts was studied using the simultaneous thermal ana-
lysis (SDTA) technique for parallel recording of TG (thermo-
gravimetry) and DSC (differential scanning calorimetry) curves.
The study was performed on a SDTQ 600 Thermal instrument.
Samples were contained within alumina crucibles and heated
at a rate of 10 °C min−1 from room temperature to 700 °C
under flowing nitrogen. The absorption measurements
were carried out using a PerkinElmer Lambda 20 UV-vis
spectrophotometer at room temperature. A PerkinElmer LS 55
luminescence spectrometer was used to measure the photo-
luminescence spectra of the nanoparticles.
The data was corrected for absorption effects using the
numerical method implemented in SADABS.27 CCDC 935369
contains the supplementary crystallographic data for this
paper.
General synthesis procedures
The parent complexes were prepared as reported earlier,25 and
the adducts were prepared by refluxing the respective precur-
sor complex in a pyridine solution and recrystallizing them in
a hot chloroform solution as reported earlier.26 In a typical
procedure, approximately 1.2 g of bis(N-ethyl-N-phenyldithio-
carbamate)M(II) [M = Zn and Cd] was added to 15 mL pyridine.
The mixture was refluxed at a temperature between 80 and
85 °C for 2 h. The pyridine solution was filtered off and the
X-ray crystallography
X-Ray diffraction studies were performed at 200 K using a solid precipitate was rinsed with water followed by ethanol and
Bruker Kappa Apex II diffractometer with graphite monochro- then recrystallized from hot chloroform.
mated Mo Kα radiation (λ = 0.71073 Å). APEXII27 was used for
(Pyridyl)bis(N-ethyl-N-phenyl dithiocarbamato)zinc(II): [ZnL2py].
data collection and SAINT27 for cell refinement and data The compound was obtained as a white solid. Yield: 1.28 g,
reduction. The structure was solved by direct methods (91%), M.p. 189–191 °C. 1H NMR (CDCl3) δ = 7.40–7.24 (m,
using SHELXS-9728 and refined by least-squares procedures –C6H5), 1.64 (t, –CH3), 4.20 (q, –CH2), 8.94 (d), 7.84 (t), 7.45 (q)
using SHELXL-9728 with SHELXLE29 as a graphical interface. (pyridine). 13C NMR (CDCl3) δ 144.94, 129.27, 128.05, 126.93
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(–C6H5), 53.78(–CH2), 12.41(–CH3), 149.39, 138.33, 124.61
(pyridine), 207.49 (NCS2). Selected IR, ν (cm−1): 1443 (CvN),
1260 (C2–N), 972 (CvS). Anal. Calc. for C23H25N3S4Zn (537.07):
C, 51.43; H, 4.69; N, 7.82; S, 23.88. Found: C, 51.04; H, 4.81; N,
7.80; S, 23.89. The product was recrystallized from a chloro-
form–ethanol solution to afford crystals suitable for X-ray
crystallography.
(Bis-pyridyl)bis(N-ethyl-N-phenyldithiocarbamato)cadmium(II):
[CdL2py2]. The compound was obtained as a white solid.
Yield: 1.21 g, (88%), M.p. 249–252 °C. 1H NMR (DMSO) δ =
7.31–7.22 (m, –C6H5), 1.14 (t, –CH3), 4.11 (q, –CH2), 8.55(d),
7.78 (t), 7.40 (q) (pyridine).
13C NMR (DMSO) δ 146.27, 129.47, 127.89, 127.09 (–C6H5),
55.01(–CH2), 12.35(–CH3), 149.71, 138.42, 124.34 (pyridine),
207.74 (NCS2). Selected IR, ν (cm−1): 1443 (CvN), 1265 (C2–N),
985 (CvS). Anal. Calc. for C28H30N4S4Cd (663.24): C, 50.70;
H, 4.56; N, 8.45; S, 19.34. Found: C, 51.11; H, 4.52; N, 8.12;
S, 19.78.
Fig. 1 The molecular structure of pyridyl bis(N-ethyl-N-phenyl dithio-
carbamate) Zn(II), [ZnL2py], with displacement ellipsoids drawn at a 50%
probability level. Symmetry element i: y, x, −z.
Synthesis of hexadecylamine (HDA) capped ZnS nanoparti-
cles. In a typical procedure, 0.45 g of the precursor complex,
[ZnL2py], was dispersed in 6.0 mL of tri-n-octylphosphine
(TOP). The solution was injected into 6.0 g hot hexadecylamine
(HDA) in a three-necked flask at 280 °C. The temperature
dropped by approximately 20 °C, and the solution turned to a
whitish colour. The temperature was gradually increased to
280 °C, and the reaction was allowed to stabilize at this temp-
erature. Aliquots of the samples were taken at 30 min intervals.
Addition of excess anhydrous methanol to the solution
resulted in the reversible flocculation of the nanoparticles. The
flocculate was separated by centrifugation, and dispersed in
toluene to give a whitish solution of HDA-capped ZnS
nanoparticles.
Synthesis of hexadecylamine (HDA) capped CdS
nanoparticles. 0.45 g of the [CdL2py2] complex was dissolved
in 6.0 mL of TOP. The solution was injected into 6.0 g of hot
HDA in a three-necked flask at 268 °C. The solution turned to
a yellowish colour and a drop in temperature of 20 °C was
observed. The reaction was allowed to stabilize at 268 °C. Ali-
quots of samples were taken and precipitated with an excess
methanol. The precipitate was separated by centrifugation,
and dispersed in toluene to give yellowish HDA-capped CdS
nanoparticles.
Fig. 2 Crystal packing of pyridyl bis(N-ethyl-N-phenyl dithiocarbamate)
Zn(II) [ZnL2py] viewed normal to the a axis.
Table 2 Selected bond length (Å), and bond angles (°) for compound
[ZnL2py]
Bond length [ZnL2py] (Å)
Bond angles [ZnL2py] (°)
Zn(1)–N(2)
Zn(1)–S(1)
Zn(1)–S(2)
Zn(1)–S(1)i
Zn(1)–S(2)i
C(1)–N(1)
C(2)–N(1)
C(4)–N(1)
C(11)–N(1)
2.061(3)
S(1)–Zn(1)–S(2)
S(1)–Zn(1)–N(2)
S(1)–Zn(1)–S(1)i
S(1)–Zn(1)–S(2)i
S(2)–Zn(1)–N(2)
S(1)i–Zn(1)–S(2)
S(2)–Zn(1)–S(2)i
S(1)i–Zn(1)–N(2)
S(2)i–Zn(1)–N(2)
S(1)i–Zn(1)–S(2)i
72.55(2)
108.02(2)
143.96(2)
96.85(2)
106.97(8)
96.85 (2)
146.05(2)
108.02(2)
106.97(8)
72.55(2)
2.3583(15)
2.6047(17)
2.3583(15)
2.6047(17)
1.336(4)
1.559(8)
1.488(9)
Results and discussion
Characterization of precursor compounds
1.443(3)
The precursors [ZnL2py] and [CdL2py2] were prepared by reflux-
ing a solution of the parent complexes, Zn(II) bis(N-ethyl-N-
phenyl dithiocarbamates) and Cd(II) bis(N-ethyl-N-phenyl
dithiocarbamates) separately, in excess pyridine at 80–85 °C
for 2 hours. The chemical reactions proceed as shown in eqn
(1) and (2). The adducts are soluble in warm chloroform and
ZnL2 þ py
CdL2 þ py
! ZnL2py
ð1Þ
ð2Þ
ðexcessÞ
! CdL2py2
ðexcessÞ
tetrahydrofuran, and are stable in air under room temperature where L = bis(N-ethyl-N-phenyl dithiocarbamate), and py =
conditions.
pyridine.
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with the Zn1, S1, C1, S2 plane. There is one long intramolecu-
lar
H bond C2⋯H2A⋯S2 of 2.67 Å. There are two
C23⋯H23⋯π ring interactions of 2.69 Å with the two Zn1, S1,
C1, S2 planes. The crystal packing is shown in Fig. 2. Selected
crystal data, collection and refinement parameters are sum-
marized in Table 1, and selected bond distances and angles
are presented in Table 2.
IR studies
The IR spectra of compounds [ZnL2py] and [CdL2py2] showed
evidence for the formation of the dithiocarbamate groups. The
thioureide bands appeared at 1433 and 1443 cm−1 for the
[ZnL2py] and [CdL2py2] complexes respectively. The band is
intermediate between the stretching vibrations of the C–N
single bonds, at 1250–1350 cm−1 and CvN double bonds at
1640–1690 cm−1, suggesting partial double bond character
and, therefore, partial delocalization of π-electron density
within the dithiocarbamate group.32 The band also showed
a
lower value compared to the parent compound
(1454–1456 cm−1),25 and it is attributed to the change in
coordination number from 4 (in the parent complex) to a
higher coordination number: 5 and 6 (after adduct formation),
which affect the degree of interaction between the dithiocarba-
mate ligands and the metal ion leading to a reduction in the
vibrational frequency. It is also due to the transfer of electrons
from the nitrogen of the pyridine ligand to the metal ion,
thus, enriching the metal electron density upon the adduct
formation.34 The ν(C–S) bands appear around 980 cm−1 in all
the complexes, without any splitting, supporting the bidentate
coordination of the dithiocarbamate to the metal centre.35
Bands due to the pyridine molecule are masked by those due
to dithiocarbamate ligand.
Fig. 3 TG/DTG and DSC curves of compounds (a) [ZnL2py], and (b)
[CdL2py2] obtained under a nitrogen atmosphere (75 mL min−1) and
heating rate of 10 °C min−1
.
Description of crystal structures
Pyridyl bis(N-ethyl-N-phenyl dithiocarbamate) Zn(II) [ZnL2py]
(Fig. 1) is five coordinate with a distorted square pyramidal
geometry. There are two bidentate dithiocarbamate ligands
and one monodentate pyridine ligand. The two dithiocarba-
mate ligands are symmetry arranged around a two-fold
rotational axis. The Zn atom lies towards the pyridine ligand
0.7450 (2) Å above the plane formed by the four sulphur atoms
of the two dithiocarbamate ligands. The Zn–S bond lengths
are 2.3583(15) Å and 2.6047(17) Å and the Zn–N bond length is
2.061(3) Å. The two dithiocarbamate ligands form two Zn1, S1,
S2, C1 planes with an acute angle of 47.16(6)° with each other
and 72.07(7) Å with the pyridine plane. The phenyl group on
the dithiocarbamate ligand makes an acute angle of 73.11(16)°
NMR studies
The chemical shifts of the pyridine group are between 7.40
and 8.94 ppm, and overlapped with the signals due to the
phenyl protons observed between 7.40 and 7.22 ppm. The
signals due to the methylene protons and the protons of the
terminal methyl group of the ethyl substituent appeared as
quartet and triplet around 4.20 and 1.15 ppm respectively. The
observed deshielding of the methylene and methyl protons on
the nitrogen atom is attributed to the release of electrons on
the nitrogen of the NRR′ (R = ethyl, R′ = phenyl) group, forcing
high electron density toward the sulphur (or) the metal via the
Table 3 Result from TG/DTG and DSC curves for the compounds
Compounds
[ZnL2py]
[CdL2py2]
Decomposition stages
First stage
Second stage
First stage
Second stage
Temperature range of decomposition (°C)
DTG peak maximum T (°C)
DSC peak: T at maximum peak height (°C)
Melting point
150–185
176
176
201–380
277
280
66–142
105, 123
107
212–320
265
266
201
252
Mass loss (%)
Found
Calculated
14.80
14.70
79.60
78.70
23.10
23.80
70.80
71.40
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Fig. 5 Photoluminescence spectra of (a) ZnS and (b) CdS NPs after
different growth times.
Fig. 4 Absorption spectra of (a) ZnS and (b) CdS NPs at different
growth time.
[ZnL2py] but with the loss of two molecules of pyridine (calc.:
23.8%, found: 23.1%), and the expulsion of the pyridine mole-
cules occurred at lower temperatures (66–142 °C). The second
stage of decomposition shows a weight loss of 78.7% and
71.4% for [ZnL2py] and [CdL2py2] respectively, and sub-
sequently leaving behind residues which corresponds to their
respective metal sulphides. It is observed that the temperature
at maximum rate of decomposition of the adducts are lower
compared to that of the parent complexes: [ZnL2], 321 °C;
[CdL2], 333 °C.38 The reason could be the dimeric nature of
the parent complexes.33 The introduction of a Lewis base (pyri-
dine), destroys the dimeric nature resulting in monomeric
adducts, thereby lowering the temperatures at maximum rate
of decomposition. The DSC curves of both complexes exhibit
three endotherms. The first, low-temperature peak corres-
ponds to the release of the pyridine molecule at approximately
176 °C and 107 °C for [ZnL2py] and [CdL2py2] respectively. The
second endotherm is the melting temperature, and the third
endotherms are attributed to the thermal decomposition of
complexes resulting in the metal sulphides.
thioureide π-system.36 All the signals due to the protons in
[ZnL2py] were observed at a lower field compared to [CdL2py2].
The 13C NMR spectra of the complexes show one sharp singlet
signal at 207 ppm which is ascribed to the carbon of the N–
CS2 moiety. The down field shift of the NCS2 moiety may be
attributed to an increase in π-bond character or delocalization
of an electron along the C–N bond contributed by the
unshared electron pair in the nitrogen atom,37 and an electron
donating effect by the ethyl substituents. In addition, the
signals for the carbons of methyl and methylene groups con-
nected directly to the N atoms appear at 55 and 12 ppm
respectively. The compounds also exhibit the expected signals
due to the carbons of aryl groups at 126–146 ppm.
Thermal studies
The thermal analyses (TGA and DSC) were conducted to evalu-
ate the thermal behaviour of the complexes, and ascertain the
temperature of decomposition. The TGA/DTG and DSC curves
of the complexes are presented in Fig. 3 and the results are
tabulated in Table 3. The TGA/DTG curve of [ZnL2py] (Fig. 3a)
showed that the decomposition started at 150 °C, and contin-
ued till 185 °C. 14.80% of the sample was lost, which corres-
Characterization of the nanoparticles
Optical properties. The UV-Vis spectra of the ZnS and the
ponds to the pyridine molecule (calc.: 14.70%). The TGA/DTG CdS nanoparticles grown at 280 and 268 °C respectively are
curve of [CdL2py2] shows almost the same behaviour with shown in Fig. 4. All the spectra show a blue shift in their band
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Fig. 6 TEM images of ZnS nanoparticles synthesized at 280 °C after (a) 30 min, (b) 60 min and (c) 90 min growth time.
edge compared to the value of their respective bulk, ZnS ing of the emission to higher wavelengths as the reaction time
(345 nm) and CdS (512 nm).39 This blue shift is associated increased. The enhancement of the maximum emission wave-
with an increase in the band gap energy and the manifestation length position with increase in the nanoparticle size is a well-
of quantum confinement. The spectra of the ZnS nanoparticles known phenomenon.40,41 The PL spectra for the ZnS nanopar-
indicate that the absorption edge shift to a higher wavelength ticles (Fig. 2a) shows two types of emission – a stronger peak at
as the growth time increased. Similarly, the spectra of the CdS around 320 nm and a weaker and broader peak at around
nanoparticles show a shift of the characteristic shoulder to 406 nm. In a colloidal ZnS semiconductor, the photo-excited
higher wavelength. The typical CdS shoulder was evidenced carriers (electrons and holes) typically recombine with each
between 474 and 500 nm, as the growth time increased from other through various recombination paths such as direct
30 to 90 min.
band-to-band recombination, shallowly trapped recombination
Fig. 5a and b show the photoluminescence spectra of the via lattice vacancies, and deeply trapped recombination via
ZnS and CdS nanoparticles respectively, after different growth surface vacancies.42 The stronger peak at 320 nm (3.87 eV)
times. The spectra also confirm the general trend, i.e. the shift- could be related to the UV excitonic emission,43 while the blue
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Fig. 7 TEM images of CdS nanoparticles synthesised at 268 °C after (a) 30 min, (b) 60 min and (c) 90 min growth time.
emission at 406 nm (3.05 eV) can be ascribed to vacancies or size. A small blue emission of CdS in Fig. 5b at around 530 nm
surface states (SS). A broad blue emission (centered at has been attributed to surface defect states.
430–450 nm) has been reported for the ZnS nanoparticles gen-
Structural properties. The TEM images of the as-synthesized
erated in situ within the meso-porous material.44 Peaks in the ZnS and CdS nanoparticles and corresponding size distri-
emission spectrum between 400–600 nm are generally butions at different reaction times are shown in Fig. 6 and 7
suggested to originate from the surface defect states such as respectively. Fig. 6a indicates that the ZnS particles are small
sulphur vacancies located at the surface of ZnS nanoparti- and spherical at the beginning of the reaction. This grows into
cles.45,46 The emission spectra for CdS NPs show band edge larger, well dispersed particles as the reaction time increased.
luminescence. The emission maxima increased from 375 nm The size distribution has been fit using a Gaussian function,
after 30 min growth time, to 387 nm after 60 min, and 404 nm and the average particle diameters were estimated from the
after 90 min growth time indicating an increase in the particle peak position. The average particle diameter, as determined
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Fig. 8 SEM images of ZnS nanoparticles synthesised at 280 °C after (a) 30 min, and (b) after 90 min growth time (insets are higher magnification
images).
Fig. 9 SEM images of CdS nanoparticles synthesised at 265 °C after (a) 30 min, and (b) after 90 min growth time (insets are higher mag. images).
from the TEM images are 4.17 0.95 nm (60 min) and 4.83
uted to the phenomenon of oriented attachment or self-
1.16 nm (90 min) for ZnS NPs. The TEM results indicate an assembly. The presence of these rods is proposed to be respon-
increase in particle size as the growth time increased. The sible for the large shift of the CdS NPs absorption and emis-
increase in the average particle diameter as the reaction time sion spectra to higher wavelength as the reaction time
increased is in agreement with Ostwald ripening. The larger increased. This shift in the absorption and emission spectra of
particles would grow by mass transferred due to dissolution of the elongated particles to higher wavelengths, as compared to
smaller particles. The process corresponds to a decrease of the the spherical particles has been previously reported by Wang
surface free energy when large particles developed at the et al.49 The authors attributed this increase to the fact that the
expense of smaller particles.47 Similar observation was band gap of the elongated particles depends on both their
reported in our earlier study.48 The results also confirmed that width and length even though it is more sensitive to their
larger particles are produced at higher reaction time. The TEM width. The result obtained for CdS NPs is contrary to the one
images of the CdS NPs (Fig. 7) show that larger particles, pro- obtained for the ZnS NPs, where smaller particles are pro-
duced at the beginning of the reaction, breakdown slightly duced at the beginning of the reaction. The reason for this
into smaller particles with narrow size distribution as the reac- difference may be the different precursors used however, more
tion time increased. The average particle diameter as deter- detailed work is ongoing to ascertain this.
mined from the TEM images for CdS NPs are 6.92 1.52 nm
The SEM images of the synthesized ZnS and CdS nanoparti-
(30 min), 6.08 0.24 nm (60 min) and 6.04 1.28 nm cles at lower and higher growth times are presented in Fig. 8
(90 min). The TEM images for CdS NPs also shows the pres- and 9 respectively. The SEM results show a stack of platelets at
ence of elongated particles (short rods), which could be attrib- shorter reaction time (30 min) which graduates to clusters of
8710 | Dalton Trans., 2014, 43, 8703–8712
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Paper
Conclusion
(Pyridyl)bis(N-ethyl-N-phenyl dithiocarbamato)zinc(II): [ZnL2py],
and (bis-pyridyl)bis(N-ethyl-N-phenyldithiocarbamato)cadmium(II):
[CdL2py2] compounds were synthesized. The single crystal
X-ray structure of the [ZnL2py] compound shows a zinc atom
bonded to four sulphur atoms from each dithiocarbamato
ligand, and one nitrogen atom from the pyridine ligand.
Thermal decomposition studies gave a weight loss resulting in
the respective metal chalcogenides. The compounds were
effective as single molecule precursors for hexadecyl amine
capped ZnS, and CdS nanoparticles. The optical absorption of
the as-synthesized particles showed blue shifted band edges
with hexagonal wurtzite structure. The TEM analysis showed
that small spherical nanoparticles with an average particle dia-
meter that is <7 nm are produced with the presence of short
nanorods for CdS NPs.
Acknowledgements
This work was supported by National Research Foundation
(NRF), South Africa, and North-West University, Potchefst-
room, South Africa. Any opinion, findings and conclusions or
recommendations expressed in this material are those of the
author(s) and therefore NRF does not accept any liability in
regard thereto.
Fig. 10 Typical p-XRD patterns of (a) ZnS NPs, and (b) CdS NPs syn-
thesised from [ZnL2py] and [CdL2py], at 280 and 268 °C respectively.
particles with amorphous shape at longer reaction time
(90 min).
References
Fig. 10 shows the typical powdered XRD (p-XRD) pattern of
the ZnS and CdS nanoparticles. ZnS nanoparticles mostly syn-
thesized by colloid chemistry usually have a cubic-zinc blende
(sphalerite) structure, C-ZnS, which is a stable phase at low
temperatures for ZnS. On the other hand, hexagonal (wurtzite)
phase, H-ZnS, is the higher temperature polymorph of sphaler-
ite, and can be formed at temperatures higher than 1023 °C.50
There are only a few reports, where pure H-ZnS nanocrystals
were obtained with either special solvo-thermal reactions or
modified colloid chemistry methods or single-source mole-
cular precursor routes at temperatures below 500 °C.50–54 The
broad nature of the XRD diffraction peaks in Fig. 10 indicates
the high nanocrystalline nature of the particles. The XRD
pattern of the ZnS NPs (Fig. 10a) show diffraction peaks at
2θ values of 25.7, 28.5, 31.2, 40.5, 47.5, 53.0 and 56.5°
corresponding respectively to the (100), (002), (101), (102),
(110), (103), and (112) crystalline planes of hexagonal
ZnS. Similar to ZnS NPs, the CdS NPs (Fig. 10b) show peaks at
2θ values of 25.2, 26.5, 28.0, 43.6, 48.6, and 51.7° corres-
ponding to the (100), (002), (101), (110), (103), and (112) crys-
talline planes of hexagonal CdS. In our earlier study, using
Zn(II) bis(N-ethyl-N-phenyl dithiocarbamate) as precursor
molecules, we obtained H-ZnS with sizes less than 3 nm at
200 °C.48 This shows that these types of precursors are
excellent material to produce hexagonal ZnS NPs at low
temperature.
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