Superlattices and Microstructures 70 (2014) 98–108

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / s u p e r l a t t i c e s

Colloidal synthesis of monodispersed ZnS and CdSnanocrystals from novel zinc and cadmiumcomplexes

http://dx.doi.org/10.1016/j.spmi.2014.03.0110749-6036/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +27 18 299 1068; fax: +27 18 299 2350.E-mail address: dcconwudiwe@gmail.com (D.C. Onwudiwe).

Damian C. Onwudiwe a,⇑, Aliyu D. Mohammed a, Christien A. Strydom a,Desmond A. Young a, Anine Jordaan b

a Chemical Resource Beneficiation (CRB) Research Focus Area, North-West University, Private Bag X6001, Potchefstroom 2520,South Africab Laboratory for Electron Microscopy, CRB Research Focus Area, North-West University, Private Bag X6001, Potchefstroom 2520,South Africa

a r t i c l e i n f o

Article history:Received 25 February 2014Accepted 3 March 2014Available online 17 March 2014

Keywords:NanocrystalsZnSCdSXanthateDithiocarbamateMorphology

a b s t r a c t

Monodispersed spherical and hexagonal shaped ZnS and CdSnanocrystals respectively, have been synthesized using novelheteroleptic complexes of xanthate (S2CObu) and dithiocarbamate(S2CNMePh). The nanocrystals were prepared via colloidal routeand stabilized in hexadecylamine (HDA). The morphology of theas-prepared nanocrystals was characterized using transmissionelectron microscopy (TEM), high resolution transmission electronmicroscopy (HRTEM), and powdered X-ray diffraction (p-XRD)analysis. An average diameter of 7.2 nm and 8.6 nm were obtainedfor the ZnS and CdS respectively. The optical properties of thenanoparticles studied by UV–vis and photoluminescence (PL)spectroscopy showed a blue shift in the absorption spectra, andband edge emission respectively.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Semiconductor nanomaterials have unique optical and electronic properties for practical applica-tions, and have attracted a lot of attention. Among various semiconductor materials, much effort

D.C. Onwudiwe et al. / Superlattices and Microstructures 70 (2014) 98–108 99

has been made to fabricate group II–VI binary semiconductors which possess attractive optical prop-erties [1–3]. This class of nanomaterials has drawn considerable interest for use in photovoltaic (PV)technology, photocatalysis, electroluminescent displays, and tunable emission properties [4–6]. Thesize, morphology and dimensionality of nano-structured materials can strongly affect their properties.Thus, different low-dimensional ZnS and CdS nanostructures have been prepared. Examples includeZnS and CdS quantum dots [7,8], CdS nanorods and nanospheres [9,10], and ZnS nanotubes [11], etc.

While nanomaterials have been generated by several physical methods, chemical methods are stillthe most widely used, because they provide better control as well as enable different sizes, shapes andfunctionalization [12]. The synthetic procedures employed generally attempt to control the particlemorphology – size and shape. Thermal decomposition, in a coordinating medium, is one of the mostefficient pathways for synthesizing the II–VI nanoparticles. In the thermolysis process, the precursorpowder is heated so as to nucleate metal sulfide (MS) particles which are then grown to the desiredsize by controlled reaction of precursor molecules [13]. Suitable single-source precursors for MS(M = Zn or Cd) would contain direct metal–sulfur bonds. In our previous studies, we reported the suit-ability of N-alkyl-N-phenyl dithiocarbamato complexes of the group 12 for the production of MSnanoparticles [14,15]. However, relatively few reports exist on xanthates as precursors for nanoparti-cles synthesis [16]. One reason maybe because xanthates do not offer much options for derivatisationas found in the dithiocarbamates. We decided to investigate novel molecular precursor complexesbased on mixed ligands of xanthate and dithiocarbamate, and to study the structural and optical prop-erties of the ZnS and CdS nanoparticles obtained.

Xanthate and dithiocarbamato complexes are air-stable with reasonable volatility and are readilyobtained in good synthetic yields. Barreca et al. [17,18] have prepared metal sulfide thin films (ZnS,CdS, ZnxCd1�xS) from single-source O-alkylxanthate precursors via CVD. CdS nanocrystals includingnanorods and faceted nanoparticles have been synthesized via the thermolysis of cadmium ethylxant-hate [19]. Good quality organically capped ZnS and CdS nanocrystals prepared from different Zn andCd complexes of dithiocarbamates as precursors have been reported [20–22]. In addition to theirappreciable volatility and stability to air and moisture, the presence of pre-formed M–S bonds andthe absence of M–C bonds enable their clean conversion into the metal sulfide in an inert atmosphere[23]. In this study, we report the preparation of Zn(II) and Cd(II) complexes with xanthate and dithio-carbamate moiety, thermal decomposition to metal sulfide is described, and their use as single-sourceprecursor to synthesize monodispersed ZnS and CdS nanocrystals is presented.

2. Experimental

2.1. Materials

All the chemicals used were of analytical reagent grade and used as such. Zinc(II) chloride, hydratedcadmium(II) chloride, methyl aniline, and carbon disulfide were obtained from Sigma Aldrich.n-Butanol, sodium/potassium hydroxides were purchased from Merck, SA.

2.2. Physical measurements

The spectral analyses of the ligand and complexes were recorded on Bruker alpha-P FT-IRspectrometer in the 500–4000 cm�1 range, and 600 MHz Bruker Avance III NMR spectrometers.Microanalyses were carried out with Elementar, Vario EL Cube, set up for CHNS analysis. The thermaldecomposition study was performed on an SDTQ 600 Thermal instrument. 10–12 mg of samples werecontained within alumina crucibles and heated at a rate of 10 �C min�1 from room temperature to800 �C under flowing nitrogen. For the nanoparticles, the X-ray powder diffraction data were collectedon a Röntgen PW3040/60 X’Pert Pro X-ray diffractometer using Ni-filtered Cu Ka radiation(k = 1.5405 A) at room temperature. TEM measurements were performed on a TECNAI G2 (ACI) instru-ment operated at an accelerating voltage of 200 kV. The absorption measurements were carried outusing a PerkinElmer Lambda 20 UV–vis spectrophotometer at room temperature. Photoluminescencemeasurement was done on a PerkinElmer LS 55 luminescence spectrometer.

100 D.C. Onwudiwe et al. / Superlattices and Microstructures 70 (2014) 98–108

2.3. Synthesis procedure

The ligands, sodium salts of N-methyl-N-phenyl dithiocarbamate and butyl xanthate are wellknown and were prepared by methods based on the literature reports [24,25].

2.3.1. Synthesis of potassium butyl xanthate, KL1

1.12 g (0.02 mol) of KOH was suspended in a 50 mL portion of n-butyl alcohol (bp 117 �C) in anoven dried Schlenk flask, and stirred vigorously with nitrogen bubbling through the mixture. Carbondisulfide, 1.2 mL (0.02 mol), was added drop-wise to the KOH saturated solution over a period of30 min. After the addition of about half of the CS2, the solution becomes light yellow and finally turnscloudy after the addition of more than 0.65 mL of the CS2. The yellow precipitate was stirred for 2 h,washed with n-butanol and put into the desiccator containing KOH to dry overnight.

2.3.2. Synthesis of sodium N-methyl-N-phenyldithiocarbamate, NaL2

A solution of sodium hydroxide (8 g, 0.2 mol) in 10 mL of distilled water was prepared in a twonecked flask with a thermometer. To this solution, 21.80 mL of N-methyl aniline (density 0.985)was added and the mixture was stirred for approximately 2 h at a low temperature range of 2–4 �C.The yellowish-white solid product which separated out was filtered, washed with small portions ofether, and recrystallized in acetone.

2.3.3. Synthesis of the complexes, [ML1L2] (M = Zn, Cd)To a well stirred 50 mL aqueous solution of an equimolar (5 mmol) concentration of

ligands KL1 (0.94 g) and NaL2 (1.02 g), 20 mL equimolar aqueous solution of the metal chloride(ZnCl2, CdCl2�2½H2O) was added drop-wise with continuous stirring. An immediate formation ofprecipitate [ML1L2] (M = Zn, Cd) occurred . The mixture was stirred for 45 min, filtered, washed withwater and dried at room temperature in vacuo.

2.3.4. Synthesis of hexadecylamine (HDA) capped MS nanoparticles (M = Zn, Cd)In a typical procedure, hexadecylamine (7.50 g) in a three-necked flask connected to a condenser

was degassed by slowly heating it to 120 �C and repeatedly flushing it with N2 under vigorous stirring.The temperature was thereafter increased to 280 �C. Then, 0.50 g of the precursor cadmium complex,[CdL1L2], was dispersed in 6 mL of tri-n-octylphosphine (TOP) and injected into the hot hexadecyl-amine. The temperature dropped by about 20 �C, and the solution turned to a bright yellow colour.The temperature was gradually increased to 280 �C, and allowed to stabilize. After 60 min, the solutionwas allowed to cool to about 70 �C, and addition of excess anhydrous methanol to the solution re-sulted in the reversible flocculation of the nanoparticles. The flocculent was separated by centrifuga-tion and was redispersed in toluene. The process was repeated three times to remove any excess HDA.The removal of the solvent yielded HDA-capped CdS nanoparticles. The reaction was repeated at260 �C, for the zinc complex, [ZnL1L2], following the same procedure.

3. Results and discussion

Preparation of the complexes, [ZnL1L2] and [CdL1L2], follow the general reaction (1). Xanthate anddithiocarbamato ligands are similar in their bonding mode. Both possess two sulfur atoms coordina-tion centres and may exist in different canonical forms.

In the xanthate, the stability of form (III) is considerably decreased. This is because four memberedrings are more strained in xanthate than in the dithiocarbamate, due to reduced mesomeric electronreleasing tendency of RAO group in the former compared to the later with R2N or RR0NA grouping[26]. The binding properties determine the structural organization of the metal complexes [27].During the synthesis, the aim was to prepare heteroleptic complexes with both xanthate anddithiocarbamate moiety following Eq. (1), and eliminating the possibility of Eqs. (2) and (3) bystoichiometric balance. Elemental analysis (Table 1) reveals that complexes are of good purity. The

Table 1Yield, melting point and micro analysis data of the complexes [ML1L2] where M = Zn and Cd; L1 = S2CObu, L2 = S2CNMePh.

Complex Found (calculated) (%)

Yield (%) M.Pt (�C) C H N S

[ZnL1L2] 63 238–239 40.02 (39.33) 4.35 (4.32) 3.65 (3.52) 32.62 (32.31)

[CdL1L2] 68 274–276 34.90 (35.17) 3.28 (3.86) 2.95 (3.16) 28.62 (28.89)

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complexes are soluble in dichloromethane, chloroform, acetone, DMSO, and insoluble in most of theorganic solvents. The zinc complex is off-white while the cadmium complex is faint yellow in colour.

Table 2Infra-re

Com

[ZnL

[CdL

MCl2 þ KL1 þ NaL2 !ML1L2 þ KClþ NaCl ð1Þ

MCl2 þ 2KL1 !MðL1Þ2 þ 2KCl ð2Þ

MCl2 þ 2NaL2 !MðL2Þ2 þ 2NaCl ð3Þ

M ¼ Zn; and Cd

3.1. IR and NMR spectral studies of the complexes

The infra-red frequencies of diagnostic values are given in Table 2. The spectra of both complexesshow three very strong bands in the region 1200–1035 cm�1 which are characteristic of xanthatecompounds [28]. The t(CS) stretching frequency of the xanthate is observed in the region 1063–1041 cm�1, while it occurred at relatively lower frequency range of 963–969 cm�1 for the dithiocar-bamate. The presence of a single t(CS) unsplitted peak in both complexes suggest the bidentate modeof co-ordination of the ligands [29], and the higher frequency values exhibited by the t(CS) of the xan-thate compared to the dithiocarbamate can be attributed to higher electronegativity of the adjacentoxygen atom in the former which strengthens the C@S bond order. The butyl group of the xanthateexhibit the vibrational frequency of t(ACH) in the range 2860–2953 cm�1, while the t(@CH) of thephenyl ring of dithiocarbamate occurred around 3055 cm�1; being the CH of an SP3 hybridised carbon.

The NMR spectral data is presented in Tables 3 and 4. The 1H NMR spectra of the zinc and cadmiumcomplexes are almost indistinguishable. The signals due to the xanthate appeared at relatively higherfield compared to the dithiocarbamate ligand. The signals due to the phenyl protons of the dithiocar-bamate resonate between 7.40 and 7.30 ppm. The methyl protons from the two dithiolate ligandsappear in two widely different environments, due to the difference in the electronegativity of the adja-cent atoms. While the CH3 of the xanthate appeared around 0.95 ppm, the CH3 of the dithiocarbamateresonated between 3.74 and 3.81 ppm in both complexes. Other peaks due to the methylene protonsof the xanthate were found between 4.47 and 1.46 ppm. The 13C NMR, Table 4, show a significantdifference in the resonant peaks of the CS2, which are indicative of its different environments in the

d spectroscopic data for the complexes [ZnL1L2], and [CdL1L2].

plexes IR (cm�1)

t(C@N) t(C2AN) t(C@S)dtc t(C@S)xan t(CAOAC) t(ACH) t(@CH)

1L2] 28681491 1379 969 1063 1200 2929 3057

29521L2] 2860

1490 1375 963 1041 1183 2930 30552953

Table 31H NMR spectral data of the complexes, [ML1L2], where M = Zn and Cd; L1 = S2CObun, L2 = S2CMePh.

Complex 1H NMR (ppm)

(C6H5) (NACH3) (OACH2) (OCH2CH2) (CH3CH2) (CH3)

[ZnL1L2] 7.40(t, 2H) 3.81 (s, 3H) 4.40(t, 2H) 1.77(m, 2H) 1.53(m, 2H) 0.95(t, 3H)7.33(t, 2H)

[CdL1L2] 7.43(t, 2H) 3.74 (s, 3H) 4.47(t, 2H) 1.81(m, 2H) 1.46(m, 2H) 0.93(t, 3H)7.36(t, 2H)7.31(d, H)

Table 413C NMR spectral data of the complexes, [ML1L2], where M = Zn and Cd; L1 = S2CObun, L2 = S2CMePh.

Complex 13C NMR (ppm)

(C6H5) (NACH3) (OACH2) (OCH2CH2) (CH3CH2) (CH3) OACS2 NACS2

[ZnL1L2] 147.50129.65 49.95 78.89 30.49 18.86 13.68 230.82 205.90128.53125.27

[CdL1L2] 145.98129.67 46.95 78.42 30.34 18.94 13.60 230.72 206.98128.69125.52

102 D.C. Onwudiwe et al. / Superlattices and Microstructures 70 (2014) 98–108

complexes. In both complexes, the peak appeared around 230 and 205 ppm for the xanthate anddithiocarbamate moiety respectively. The weak intensity of these peaks is characteristics of quater-nary carbon signals [30]. The reasons for the high CS2 resonant peak observed in the xanthate isascribed to the high electronegativity of the adjacent oxygen atom compared to the nitrogen, whichresults in higher deshielding effect. The complexes exhibits three signals for the methylene carbonsaround 78.0, 30.0 and 18.0 ppm, while the terminal carbons (ACH3) resonate around 13.60 ppm.The highest methylene carbon peak is assigned to the OACH2 of the butyl group, being more deshield-ed. The peaks due to the methyl groups are easily distinguishable. While the ACH3 peak of the

Fig. 1. Different resonant form of (a) dithiocarbamato, and (b) xanthate moiety.

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xanthate is a terminal CH3 and appeared around 13.60 ppm, the CH3 peak of the xanthate appeared at46.95 ppm due to the high electronegativity of the nitrogen atom (see Fig. 1).

3.2. Thermal decomposition studies

The thermogravimetric analysis graphs presented in Fig. 2(a) and (b) shows clear 2-step decompo-sition. In the zinc complex, the first decomposition has its maximum decomposition temperature at

129.22°C

255.94°C

182.49°C

-2

0

2

4

6

8

10

12

Der

iv. W

eigh

t (%

/min

)

20

40

60

80

100

Wei

ght (

%)

8006004002000Temperature (°C)

145.37°C

276.11°C

181.68°C

-2

0

2

4

6

8D

eriv

. Wei

ght (

%/m

in)

20

40

60

80

100

120

Wei

ght (

%)

8006004002000Temperature (°C)

(a)

(b)

Fig. 2. TG/DTG and DSC curves of complexes (a) [ZnL1L2], and (b) [CdL1L2] obtained in nitrogen atmosphere (75 mL/min),heating rate 10 �C/min.

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129 �C; and 145 �C for the cadmium complex. This step terminates at about 182 �C in both complexes,and it is due to the decomposition of the xanthate group. The amount of samples left after this stepagrees favourably with the estimated weight, zinc complex: 6.35 mg (found), 5.72 mg (calc.); cad-mium complex: 6.23 mg (found), 5.94 mg (calc). In a similar thermal decomposition reaction involvingcadmium ethyl xanthate [31], decomposition occurred around 160 �C. However, the increase in thedecomposition temperature of the xanthate group, of the cadmium complex, observed here couldbe attributed to the presence of the dithiocarbamato group which increases the bond strength. Thesecond decomposition step has its maximum temperature at 256 �C and 276 �C for the zinc and cad-mium complexes respectively. This stage corresponds to the decomposition of the dithiocarbamategroup. The residue obtained at the end of the decomposition indicates the formation of metal sulfidesas the only constituents. The experimental and calculated values are in good agreement, zinc complex;2.80 mg (found), 2.33 mg (calc); and cadmium complex: 3.32 mg (found), 2.58 mg (calc.).

3.3. Optical properties of the HDA-capped ZnS and CdS NPs

The UV–visible absorption spectra of the colloidal ZnS and CdS stabilized by hexadecylamine areshown in Figs. 3(a) and 4(a) respectively. The spectra of the nanoparticles showed characteristicabsorption band with a maximum located at 290 nm for the ZnS nanoparticles, and at 492 nm forthe CdS nanoparticles. These absorbance bands appear at shorter wavelengths in semiconductor nano-crystals when compared to the bulk materials. For example, ZnS has a band gap of 3.6 eV at 300 K. Thiscorresponds to ultraviolet (UV) radiation for optical interband transition, with a wavelength of 340 nm

(a)

(b)

Fig. 3. Absorption (a), and photoluminescence (b) spectra of ZnS synthesized from [ZnL1L2] at 260 �C.

(a)

(b)

Fig. 4. Absorption (a), and photoluminescence (b) spectra of CdS synthesized from [CdL1L2] at 280 �C.

D.C. Onwudiwe et al. / Superlattices and Microstructures 70 (2014) 98–108 105

[32], and between (515–520 nm) for CdS nanoparticles [33,34]. The considerable blue-shift relative tobulk samples indicates that the particles are formed in the quantum confinement size regime.

The fluorescence spectra of the ZnS and CdS samples are shown in Figs. 3b and 4b respectively. Thespectra showed emission peaks at 330 and 495 nm for the ZnS and CdS nanoparticles respectively. Thefeature of the fluorescence band and the position of the emission maxima suggest that the fluores-cence spectra exhibit band edge emission. Band-edge emission originates from the recombinationof free excitons of nanocrystalline size regime. Usually, two emissions are observed from semiconduc-tor nanoparticles: an excitonic and a trapped emission. The excitonic emission is usually sharp andlocated near absorption edge of the particles, while the trapped emission is broad and stokes shifted[35]. Band edge photoluminescence at room temperature is observed in high quality crystal.

3.4. Structural properties

The TEM images of the ZnS nanoparticles show that the particles are spherical with an averagediameter 7.2 nm, Fig. 5. A hexagonal shaped particle with average diameter of 8.6 nm was obtainedfor the CdS nanoparticles, Fig. 6. The high-resolution transmission electron microscopy (HRTEM)images confirm the presence of free standing crystalline particles, with good degree of monodisper-sity, which is an indication of the good stabilizing property of hexadecylamine. The distinct latticefringes in the HRTEM images (Figs. 5c and 6c) confirm the highly crystalline nature of the particles.The lattice planes of ZnS spherical particles, 3:1 Å is unique to zinc blende and corresponds to the(111) reflection of the cubic structure. The lattice planes of the CdS (3:5 Å) are constituents of(100) planes of hexagonal structure. The corresponding fast Fourier transform (FFT) patterns(Figs. 5d and 6d) show several well-defined sharp spots which further confirm the high crystallinity

(a) (b) (c)

(d)

Fig. 5. (a) TEM image, (b) HR-TEM image (c) expanded individual quantum dots showing distinct lattice fringes, and (c) thecorresponding Fourier transforms for the ZnS nanoparticle.

(a) (b) (c)

(d)

Fig. 6. (a) TEM image, (b) HR-TEM image (c) expanded individual quantum dots showing distinct lattice fringes, and (c) thecorresponding Fourier transforms for the CdS nanoparticle.

106 D.C. Onwudiwe et al. / Superlattices and Microstructures 70 (2014) 98–108

of the nanoparticles. The crystallographic phase and growth of the nanoparticles are controlled by theconditions at the nucleation stage. In this case, hexadecylamine as surfactant molecule can be selec-tively adsorbed onto the surface thereby modulating the surface energy. In the ZnS NPs, uniformadsorption of amine in all the sites lead to the uniform growth in all the sites [36]. In the CdS NPs,the surface energy of the crystallographic faces of the initially formed seeds has a dominant effecton the anisotropic growth pattern of the nanoparticles [37]. These two different phenomena have sig-nificant effect on the morphology of the obtained nanoparticles, spherical shape for the ZnS NPs andhexagonal shape for the CdS NPs.

The crystalline phases of the prepared samples were investigated by X-ray diffraction (XRD), asshown in Fig. 7. The patterns of the ZnS sample (Fig. 7a) are indexed to cubic sphalerite phase withcharacteristics (111), (220), (311) peaks. The CdS sample exhibit patterns which can be indexed aswurtzite-phase with characteristic (100), (002), (101) (110), (103), and (112) peaks. The sharp(002) plane is consistent with the preferential (001) growth direction in anisotropic CdS nanoparti-cles. In a study involving the shape control of monodispersed hexagonally shaped CdS by Cheng et al.[19], it was reported that the top and bottom faces of the hexagonal plates are bounded by the (001)planes, while the six lateral faces are bounded by the (100) planes.

30 40 50 60

30 40 50 60

(a)

(b)

Fig. 7. XRD pattern of (a) ZnS and (b) CdS nanoparticles.

D.C. Onwudiwe et al. / Superlattices and Microstructures 70 (2014) 98–108 107

4. Conclusion

We synthesized new heteroleptic complexes by the reaction of the chloride salts of zinc and cad-mium with equimolar concentrations of xanthate and dithiocarbamate, with a view of using them assingle source precursors for the preparation of ZnS and CdS nanoparticles. Monodispersed ZnS and CdSnanocrystals were synthesized via the colloidal synthesis method. Hexadecylamine (HDA) was used asa surface stabilizing agent to prevent surface dangling bonds and agglomeration of the nanocrystals.Spherically shaped ZnS and hexagonally shaped CdS nanocrystals were obtained, as confirmed by theHRTEM. The XRD analysis showed patterns which are indexed for cubic sphalerite and wurtzite-phasefor the ZnS and CdS nanoparticles respectively. UV–visible optical spectroscopy indicated a blue shiftin the absorption spectra due to the quantum size effect. The fluorescence spectra of the nanocrystalsdisplay emission which is attributed to the recombination of free excitons of nanocrystalline sizeregime.

Acknowledgements

This work was supported by National Research Foundation (NRF), South Africa, and North–WestUniversity, Potchefstroom, South Africa. Any opinion, findings and conclusions or recommendations

108 D.C. Onwudiwe et al. / Superlattices and Microstructures 70 (2014) 98–108

expressed in this material are those of the author(s) and therefore NRF does not accept any liability inregard thereto.

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