group 12 dithiocarbamate complexes: synthesis, spectral studies and their use as precursors for...

7/27/2019 Group 12 dithiocarbamate complexes: Synthesis, spectral studies and their use as precursors for metal sulfides na

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Group 12 dithiocarbamate complexes: Synthesis, spectral studies and

their use as precursors for metal sulfides nanoparticles and

nanocomposites

Peter A. Ajibade , Benjamin C. Ejelonu

Department of Chemistry, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa

h i g h l i g h t s

Three dithiocarbamate complexes of

group 12 metal ions were synthesized

and characterized.

Complexes characterized by

spectroscopic techniques and

elemental analyses.

Complexes used as precursors to

prepare metal sulfide nanoparticles/

PMMA nanocomposites.

Nanoparticles sizes varied between

3.03 and 23.45 nm.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 5 January 2013

Received in revised form 14 April 2013

Accepted 24 April 2013

Available online 7 May 2013

Keywords:

Dithiocarbamate complexes

Synthesis

Spectral studies

Precursors

Nanoparticles

Nanocomposites

a b s t r a c t

Zn(II), Cd(II) and Hg(II) dithiocarbamate complexes have been synthesized and characterized by elemen-

tal analysis, thermogravimetric analysis, UVVis, FTIR, 1H- and 13C NMR spectroscopy. The complexes

were thermolysed at 180 C and used as single molecule precursors for the synthesis of HDA capped

ZnS, CdS and HgS nanoparticles and polymethylmethacrylate (PMMA) nanocomposites. The optical and

structural properties of the nanoparticles and nanocomposites were studied by UVVis, PL, XRD and

SEM. The crystallites sizes of the nanoparticles varied between 3.03 and 23.45 nm. SEM and EDX analyses

of the nanocomposites confirmed the presence of the nanoparticles in the polymer matrix.

2013 Elsevier B.V. All rights reserved.

Introduction

Dithiocarbamate complexes are known to possess striking struc-

tural features, diversified industrial and biological applications [1].

The dithiocarbamate complexes of elements with d10 configura-

tions represent a large and interesting group of inorganic com-

pounds that have been extensively studied in recent years [25].

They have been found as useful precursors for the deposition of

II/VI compound semiconductor materials because of their reason-

able volatility and less carbon deposition as impurity [68]. Nano-

particles sizes range between 1 and 100 nm and they have wide

applications in catalysis, electronic, optical and magnetic because

of their unprecedented chemical and physical properties. They are

also used in light emitting devices, solar cells and bio-imaging [9].

The use of CdS nanoparticles in modern technology as light-emit-

ting diodes, solar cells, and optical devices based on its non-linear

optical properties has been reported. Also, ZnS semiconductor has

1386-1425/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.saa.2013.04.113

Corresponding author. Tel.: +27 40 602 2055; fax: +27 40 602 2094.

E-mail address: [email protected] (P.A. Ajibade).

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 408414

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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 . c o m / l o c a t e / s a a
http://dx.doi.org/10.1016/j.saa.2013.04.113mailto:[email protected]://dx.doi.org/10.1016/j.saa.2013.04.113http://www.sciencedirect.com/science/journal/13861425http://www.elsevier.com/locate/saahttp://www.elsevier.com/locate/saahttp://www.sciencedirect.com/science/journal/13861425http://dx.doi.org/10.1016/j.saa.2013.04.113mailto:[email protected]://dx.doi.org/10.1016/j.saa.2013.04.113http://crossmark.dyndns.org/dialog/?doi=10.1016/j.saa.2013.04.113&domain=pdf


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function of temperature or time [14]. Initial decomposition for the

metal complexes starts around 130 C, 125 C, and 89 C for the Zn,

Cd, and Hg complexes respectively (Fig. 1). At a temperature of

about 223334 C about 41% of the mass of the complexes lost.

Heating the metal complexes further results into much more grad-

ual loss of mass, with about 22.85% of the mass left for the Zn com-

plex; 30.85% for the Cd complex; and 0.5% for the Hg complex- allat 800 C.

The products of the thermal decomposition of the metal com-

plexes correspond to their respective oxides, but for Hg where vol-

atilization results as observed in the thermogram. The Hg complex

initial decomposition at 30 C (0.78% mass loss) suggests the es-

cape of entrapped solvent molecule within its matrix [15]. The cal-

culated and experimental masses could be said to agree favourably

(Table 2). The mercury complex is seen to have the least stability

[16,17], the stability trend of the complexes is in the order:

Hg < Cd < Zn.

Infrared spectra studies of the metal complexes

The infrared spectra of the ligands and complexes were com-pared and assigned on careful comparison. In dithiocarbamate

complexes, three regions of great importance are: 1580

1450 cm1 due to m(CAN) of NCS; 1060940 cm1 due to m(CAS)

of CSS and 420250 cm1 due to the MAS bond [16,18]. In the free

ligands, the m(CAN) bond that appears in the range 1509

1488 cm1 shifted to 15091488 cm1 in the metal complexes.

This suggests an increase in the CAN double bond characters as a

result of the delocalization of electrons towards the metal centre

after due to coordination to the dithiocarbamate ligands [16]. In

the ligands, the m(CAS) stretching vibrations were observed as

two bands in the region 1060940 cm1. These bands occurred

as single sharp bands in the complexes at about 990 cm

1

. Thisconfirms the coordination of the ligands to the metal ions as biden-

tate chelating ligands through the sulfur atoms. The presence of

the m(CN) and m(CSS) absorption bands in the metal complexes

confirm the presence of dithiocarbamate ligands in the complexes

[19]. It has been observed in previous studies that the coordination

modes of alkyl-aryl dithiocarbamate ligands with group 12 metals

are mostly through the sulfur atoms [2022]. This is further sup-

ported by the singular absorptions which appeared at about

1000 70 cm1 region (Zn complex at1003 cm1; Cd complex at

993 cm1; and Hg complex at 992 cm1) in the FTIR spectra of

the metal complexes [16]. The characteristic frequencies of the

aromatic groups in the complexes, resulting from the out-of-plane

bending vibrations of the aromatic CAH are said to appear in the

range 600900 cm1 [23]. The (@CAH) bending modes of the aro-

matic ring was seen to absorb at around 700 cm1 in the ethyl phe-

nyl dithiocarbamate free ligand; and at about 695, 695, and

696 cm1 respectively in the Zn, Cd, and Hg complexes. The char-

acteristic absorption peak of NAH of the butyl dithiocarbamate li-

gand is seen to appear at 3281 cm1 in the free ligand and between

3208 and 3263 cm1 in the metal complexes, which is slightly be-

low the literature value of 33003500 cm1 [24].

Electronic spectra of the metal complexes

The electronic spectra of the complexes are shown in Fig. 2.

Three characteristic absorption bands are known for dithiocarba-

mate complexes due to the chromospheres NS2 caused by p?p

transition of NCS and SCS moieties; andg?p

transition arisingfrom transition of an electron of the lone pair electrons on the sul-

fur atom to an antibondingp-orbital [1]. In the metal complexes an

intense band around 291294 nm was observed due to the p?p

transition of the phenyl ring [25]. The aniline group is also said to

show a secondary band at 285 nm [26]; this band was observed be-

tween 281 and 286 in the metal complexes. The d10 electronic con-

figuration of the pseudo-transition (d10) metals is suggestive why

no dd transition over the visible region was observed [1].

NMR spectra of the metal complexes

In the 1H NMR spectra of the metal complexes two signals were

observed as multiplets around 7.447.30 ppm, which corresponds

to the aromatic protons of the phenyl ring of the ethyl phenyl moi-ety [27]. The methyl protons of the butyl group experienced up

field signals due to more shielding effect of the nuclei of the carbon

chain, and it appeared as triplets about 0.99 ppm; while the CH 3protons of the ethyl group appeared around 1.30 ppm. The peaks

Table 1

Elemental analysis of the metal complexes.

Metal complexes Formula

weight

Analytical data calculated

(found) (%)

Carbon Hydrogen Nitrogen

[(C4H9)NHCS2ZnS2CN(C2H5)C6H5] 407.98 41.18

(40.88)

4.94

(5.20)

6.86

(7.10)

[(C4H9)NH

CS2CdS2CN(C2H5)C6H5]

456.98 36. 80

(35.39)

6.13

(6.26)

4.41

(4.06)

[(C4H9)NH CS2HgS2CN

(C2H5)C6H5]

546.02 30. 77

(30.99)

3.69

(3.89)

5.13

(5.45)

Fig. 1. TGA curves showing the degradation pattern of the metal complexes.

Table 2

Thermal decomposition data of the metal complexes.

Compounds Decomposition ranges

(C)

DTG max value

(C)

Weight loss

(%)

Decomposition reaction Product

expected

Mass

changes (mg)

Cal. Found

Zn 130752 181 76.98 [(C4H9)NHCS2HgS2CN(C2H5) C6H5]? ZnS? ZnO ZnO 3.63 3.63

Cd 125340 177 68.24 [(C4H9)NHCS2CdS2CN(C2H5) C6H5]? CdS? CdO CdO 4.20 4.20

Hg 89469 144 98.89 [(C4H9)NHCS2HgS2CN(C2H5) C6H5]? HgS

volatilization

HgS 0.61 0.61

410 P.A. Ajibade, B.C. Ejelonu / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 408414


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at about 4.20 (quartets) and 3.50 ppm (triplets) are assigned to the

NACH2 protons of the ethyl and butyl groups respectively. The

methylene protons NA(CH2)2 peaks were seen at around 1.70

and 1.40 ppm in the complexes. The NH proton peak appeared

around 1.50 ppm.

In the 13C NMR spectra, two different signals were observed for

the phenyl ring attached to the N-ethyl group. The first signal ap-

peared around 145.91 ppm; while the second signal with higher

intensity was observed as three peaks between 129.14 and

126.86 ppm. This is assigned to the N-butyl group. The methylene

carbon peaks of the complexes were observed as four signals be-

tween 54.69 and 19.50 ppm. The peak observed at 78.92 ppm is

attributed to the signal of the NA(CH2) of the ethyl group which

is more deshielded than the NA(CH2) of the butyl ring. The CS2for the ZnL1L2, CdL1L2 complexes were observed at 207.87,

205.19 and 207.55, 205.44 ppm respectively.

Optical properties of the metal sulfides nanoparticles

The absorption and emission of ZnS, CdS and HgS nanoparticlesare given in Fig. 3. The peak of the spectra of the nanoparticles is

288 nm (ZnS); 291 nm (CdS) and 285 nm (HgS), which are at short-

er wavelengths than that of bulk ZnS, CdS and HgS (334, 516 and

620 nm for ZnS, CdS and HgS bulks respectively) as a result of

quantum confinement effects. This observation suggests a decrease

in the particle size of the respective nanoparticles compared to the

bulk materials. The band gap in a nanomaterial has been obtained

from the absorption maxima of their spectra. Using the fundamen-

tal absorption edges in the samples (absorption maxima), their

band gaps are calculated thus; ZnS = 4.31 eV (288 nm),

CdS = 4.26 eV (291 nm) and HgS = 4.35 eV (285 nm). The band

gap energies of the bulk samples are also calculated to be:

ZnS = 3.71 eV (334 nm), CdS = 2.40 eV (516 nm) and HgS = 2.0 eV

(620 nm) [28,29].Two emission peaks due to the exciton and the trapped lumi-

nescence are said to exist for semiconductor nanocrystals. The

exciton emission is usually observed as sharp band while the

trapped emission appears as broad band [29]. The ZnS, CdS and

HgS nanoparticles were found to exhibit emission maxima at

Fig. 2. Electronic spectra of the metal complexes.

Fig. 3. Absorption and emission spectra of the nanoparticles: (A) absorption (a) and emission (b) spectra of ZnS nanoparticle; (B) absorption (a) and emission (b) spectra ofCdS nanoparticle and (C) absorption (a) and emission (b) spectra of HgS nanoparticle.

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Fig. 4. SEM/TEM images of ZnS (A), CdS (B) and HgS (C) nanoparticles.

Fig. 5. Powder XRD pattern of ZnS nanoparticle (A); powder XRD pattern of CdS nanoparticle (B) and powder XRD pattern of HgS nanoparticle (C).



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393, 391 and 389 nm respectively. As shown in Fig. 3, a shift was

observed for all the nanoparticles with respect to absorption edge.

Fig. 4 shows the SEM/TEM pictures of samples ZnS (A); CdS (B)

and HgS (C) nanoparticles. They all gave a somewhat spherical

morphology, which agglomerated to form larger particles; thus

giving them a dense surface morphology [2931]. The nanoparticle

gave different shapes and sizes under the TEM analysis. Dots-, rice-

like- and spherical- shapes were observed for the ZnS, CdS and HgS

nanoparticles respectively. Their sizes ranged between 2.78 and

14.54 nm [29].

The XRD patterns of the metal sulfide nanoparticles are

presented in Fig. 5. The XRD patterns reveal their nanocrystalline

Table 3

Values of crystallite sizes for ZnS, CdS and HgS nanoparticles.

M et al sulfide nanop articles Angle (d eg re e) observe d maxima Full we ig ht half maxima (FWH M) Gra in siz e from XRD (nm)

ZnS 28.86 1.41 6.45

47.94 1.57 6.15

56.94 1.32 3.03

CdS 26.86 0.72 12.65

44.17 1.76 5.42

52.29 1.84 5.35

HgS 20.18 0.38 23.45

23.40 0.59 15.20

33.93 0.45 20.41

39.90 0.49 19.27

Fig. 6. SEM/TEM micrographs of the respective nanocomposites: PMMA (A); ZnS/PMMA (B/B); CdS/PMMA (C/C) and HgS/PMMA (D/D).

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natures, which indicate the broadening of the diffraction peaks due

to the higher surface to volume ratio [28]. ZnS nanoparticle

showed broad peaks at 2h = 28.86, 47.95 and 56.94, which can

be indexed to (111), (220) and (311) planes of cubic zinc blende

structure (JCPDS 05-0566) Fig. 3 [28,29]. The three peaks ob-

served in the XRD patterns of CdS nanoparticle correspond to

(111), (220) and (311) of the cubic crystal phase of CdS (JCPDS

5-0566) [32]. The XRD patterns (111), (200), (220) and (311) ob-tained for HgS nanoparticle are in agreement with the pattern

JCPDS 00-006-0261, which correspond to HgS metacinnabar, syn

[29]. The ZnS and CdS samples appeared to be of much more smal-

ler particle sizes than that of HgS as indicated by the observed rel-

atively broader diffraction peaks for the ZnS and CdS nanoparticle

XRD patterns compared to that of HgS nanoparticle (Fig. 5AC). Ta-

ble 3 contains the values of crystallite sizes for ZnS, CdS and HgS

nanoparticles.

Structural properties of the metal sulfide/PMMA nanocomposites

SEM was used to study the morphology and the extent of inter-

action of the ZnS, CdS and HgS nanoparticles with PMMA. Fig. 6A

shows the SEM image of PMMA, while Fig. 6BD shows the SEM

images of ZnS/PMMA, CdS/PMMA and HgS/PMMA respectively.

The SEM monograph obtained for the polymer (PMMA) shows a

well-defined shape of spherical aggregates [33,34]. The pro-

nounced spherical aggregates observed in PMMA micrograph were

somehow lost (dissolved) in the PMMA interaction with different

nanoparticles; thus suggesting an interfacial interaction between

the polymer and the metal sulfide nanoparticles. The TEM images

revealed that some interaction occurred between the inorganic

nanoparticle materials and the polymer, which has been observed,

could help to increase the stability of the pure PMMA polymer and

hence, enhance its usability [33]. The chemical composition of the

metal sulfide/polymer nanocomposites were studied by EDX,

which confirmed the presence of Zn, S; Cd, S and Hg, S from ZnS,

CdS and HgS nanoparticles respectively.

Some broadening of the diffraction peaks and the decrease ofthe intensity of the spectra were observed in the XRD patterns of

the nanocomposites. This may be attributed to the pore filling ef-

fects, which is said to reduce the scattering contrast between the

pores and the frame work PMMA materials [3537].

Infrared spectral studies of the nanocomposites

The FTIR data revealed that the absorption band patterns of the

ZnS/PMMA, CdS/PMMA and HgS/PMMA nanocomposites are simi-

lar to those of the pure PMMA. However, a slight shift was ob-

served in their stretching frequencies suggesting the possibility

of an interaction between the metal sulfide nanoparticles and the

polymer. This possible interaction between the inorganic and or-

ganic phases could influence the thermal stability of the purePMMA [32,33].

Conclusion

Zn(II), Cd(II) and Hg(II) complexes of some alkyl-phenyl dithio-

carbamates were synthesized and characterized by elemental anal-

ysis, TGA, UVVis, FTIR, 1H- and 13C NMR spectroscopy.

Spectroscopic analyses confirmed the coordination of the metal

ions to the dithiocarbamate anions acting as bidentate chelating li-

gands. The complexes were used as single molecular precursors to

synthesize ZnS, CdS and HgS nanoparticles and PMMA nanocom-

posites. The crystallites sizes of the nanocomposites varied be-

tween 3.03 nm and 23.45 nm. EDX analysis from the SEM of the

nanocomposites confirmed the presence of the nanoparticles in

the polymer matrix and the broadening observed in the XRD of

the nanocomposites of the nanocomposites also confirmed the for-

mation of the nanocomposites.

Acknowledgements

The author acknowledged with thanks financial support from

Govan Mbeki Research and Development Centre, University of Fort

Hare. BCE acknowledged Adekunle Ajasin University, Akungba

Akoko, Nigeria for study leave to undertake this study.

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