Chapter-3

Synthesis of ZnS nanoparticles by

chemical route

Chapter -3 Page | 88

3.1 INTRODUCTION

II-VI binary compound semiconductor nanomaterials such as ZnS possess many

interesting physical properties and have potential applications in electronic and

optical devices [1,2]. There are various preparation techniques by which this

semiconductor nanoparticles can be synthesized viz. solid-phase reactions [3] gas

phase reaction with H2S or sulphur vapour [4] , sol gel process [5], spray pyrolysis

methods [6] hydro and solvo thermal routes [7,8], electro and sono chemistry

method [9,10] gamma irradiation technique [11,12], Inverse micellar method[13]

direct chemical reaction synthesis[14] and green reaction synthesis [15].

Wide band gap II-VI semiconductors are ideal materials for the study of discrete

states in the energy gap. One of the member of the said family ZnS exists in two

forms: cubic and hexagonal with a transition temperature of 1028 C [16]. There

are various irradiation methods for preparing nanoparticles one of them is

microwave irradiation. Microwave irradiation has attracted chemists’ attention,

although the exact nature of the interaction between reactants and microwaves

during the preparation of materials is not entirely understood. Different from

normal heating, where the heat energy is transferred from outside to inside,

microwave irradiation induces interaction of the dipole moment or molecular ionic

aggregates with alternating electronic and magnetic fields causing molecular-level

heating, which leads to a homogenous and quick thermal reaction. Compared with

conventional methods, microwave irradiation synthesis has many advantages such

as very short reaction time, production of small inorganic particles with narrow

particle size distribution, and high purity owing to short reaction time. It is found

that this method is non toxic route to produce ZnS nanoparticles which could be

attributed to fast homogeneous nucleation and easy dissolution of the gel [17, 18,

19].

The inverse miceller route is an established technique to prepare nanoparticles

with a narrow size distribution. The nanoparticles are generated inside the water

pools of the inverse micellar system and thus size-selective nanoparticles with

narrow size distribution can be obtained. Additionally, it is quite easy to modify

the surface of the nanoparticles during reverse micellar preparation with a shell

Chapter -3 Page | 89

material. ‘‘Water-in-oil’’ microemulsion contain dynamic structures of nano-sized

water droplets in a continuous oil medium that are stabilized by surfactant

molecules at the water/oil interface. The stable microenvironment offers a suitable

reaction media for the synthesis of nanoparticles and restricts the excess particles

growth when the sizes of the particles approach that of water nanodroplets. As a

result, the microemulsion-mediated processing method has been utilized as an

effective pathway to synthesize several nanocrystalline compounds with narrow

size distribution and good monodispersity. In addition, the inverse micelle method

is a soft technique and does not require special instruments or extreme conditions

[13].

Looking towards the synthesis of nanoparticles by chemical route, molarity of the

solution used makes great impact on the properties of prepared nanostructures.

Keeping this point in mind, the molarity variation of the precursors used for the

synthesis of ZnS nanoparticles is carried out. Further, synthesis of ZnS

nanoparticles by three different methods is adopted viz by

1. Microwave irradiation

2. Chemical reaction in alkaline medium and

3. Inverse micellar method.

3.2 Chemical approach for synthesis of ZnS nanoparticles.

3.2.1 Preparation method for of ZnS nanoparticles using

microwaves

Method –I Zinc acetate as Zinc source and Sodium Sulphide as

Sulphur source

Sample Name given: ZnS_MW_1:1

All the reagents used were analytically pure and used without further

purification. In a typical synthesis procedure Zn(CH3COO)2.2H2O (5 mmol) and

Na2S.9H2O (5 mmol) were put into a round-bottom glass flask of 50 ml containing

20 ml ethylene glycol (EG) as shown in Figure 3.1 . The flask was immediately

placed in a microwave oven for 10 minutes with continuous power of 180 W and

Chapter -3 Page | 90

then the system was allowed to cool naturally to room temperature which resulted

in white precipitate at the bottom of the flask.

Sample Name given: ZnS_MW_1:1.5

In 2nd case , Zn(CH3COO)2.2H2O (5 mmol) and Na2S.9H2O (7.5 mmol) were put

into a round-bottom glass flask of 50 ml containing 20 ml ethylene glycol (EG).

The flask was immediately placed in a microwave oven for 10 minutes with

continuous power of 180W and then the system was allowed to cool naturally to

room temperature which resulted in white precipitate at the bottom of the flask.

Sample Name given: ZnS_MW_1:2

In 3rd case, Zn(CH3COO)2 .2H2O (5 mmol) and Na2S.9H2O (10 mmol) were put

into a round-bottom glass flask of 50 ml containing 20 ml ethylene glycol (EG).

The flask was immediately placed in a microwave oven for 10 minutes with

continuous power of 180W and then the system was allowed to cool naturally to

room temperature which resulted in white precipitate at the bottom of the flask.

Figure 3.1 Schematic of ZnS nanoparticles preparation by method-I

Chapter -3 Page | 91

Method –II Zinc Nitrate as Zinc source and Thiourea as Sulphur source

Figure 3.2 Schematic of ZnS nanoparticles preparation by method-II

Sample Name Given: ZnS_MW_1:1_n

All the reagents used were analytically pure and used without further purification.

In a typical synthesis procedure Zinc nitrate Zn(NO3)2.(1 mmol) and thiourea

SC(NH2)2 (1 mmol) were put into a round-bottom glass flask of 50 ml containing

20 ml ethylene glycol (EG) as shown in Figure 3.2. The flask was immediately

placed in a microwave oven for 8 minutes with continuous power of 180W and

then the system was allowed to cool naturally to room temperature which resulted

in white precipitate at the bottom of the flask.

Sample Name Given: ZnS_MW_1:1.5_n

In the 2nd case Zinc nitrate Zn(NO3)2.(1 mmol) and Thiourea SC(NH2)2 (1.5

mmol) were put into a round-bottom glass flask of 50 ml containing 20 ml

ethylene glycol (EG). The flask was immediately placed in a microwave oven for

6 minutes with continuous power of 720W and then the system was allowed to

cool naturally to room temperature which resulted in white precipitate at the

bottom of the flask.

Chapter -3 Page | 92

Sample Name Given: ZnS_MW_1:2_n

In the 3rd case Zinc nitrate Zn(NO3)2.(1 mmol) and thiourea SC(NH2)2 (2 mmol)

were put into a round-bottom glass flask of 50 ml containing 20 ml ethylene

glycol (EG). The flask was immediately placed in a microwave oven for 6 minutes

with continuous power of 720W and then the system was allowed to cool

naturally to room temperature which resulted in white precipitate at the bottom of

the flask.

The proposed mechanism of microwave assisted Ethylene glycol EG synthesis of

Zinc sulphide can be explained as the strong complexion between Zn2+ and

thiourea lead to the formation of Zinc-thiourea complexes [20-23] in the

microwave synthesis ,which prevent the production of a large number of free S-2

in the solution and will be favourable for the formation of the products.

Our experiments confirm that Zinc salts and thiourea easily dissolve in EG

indicating the formation of Zn-thiourea complexes, which have been reported in

the literature [24-26]. Secondly the Zinc-thiourea complexes undergo thermal

decomposition under the microwave irradiation to produce ZnS.

Zn+2+SC(NH2)2

Ethylene glycol

stirring [Zn(SCN2H4)2]

2+

[Zn(SCN2H4)2]2+ Decomposition under

Microwaveirradiation ZnS Nanoparticles

In this microwave synthesis EG and microwave irradiation play vital roles for the

preparation of ZnS. EG acting as both reaction media and dispersion media can

efficiently adsorb and stabilize on the surface of the particle [27] helping the

producing monodispersed ZnS with good dispersivity. Meanwhile EG with the

high permanent dipole is an excellent susceptor of the microwave irradiation,

which can take up the energy from microwave field and get the polar reaction

solution heated up to high temperature instantaneously [28,29]. All these help to

the decomposition of Zinc-thiourea complexes and the formation of the products.

Apart from that microwave irradiation as a heating method affords a uniform

growth environment for the formation of nanosized ZnS. With microwave

irradiation of reactants in polar solution, temperature and concentration gradient

can be avoided, providing a uniform environment for the nucleation and the

Chapter -3 Page | 93

growth of the products [30]. So it can be assumed that microwave irradiation not

only provides the energy for the decomposition of the Zinc-thiourea complexes

but also enhances the formation of ZnS as a result of the fast and homogeneous

heating effects of microwaves [31].

3.2.2 Preparation method for ZnS nanoparticles by chemical

reaction in alkaline medium

Preparation method for ZnS nanoparticle by direct Sulphur reaction for synthesis with molar ratio (1:1)

Sample Name Given: ZnS_1:1_direct sulphur

Dissolve 1.3628 gm of ZnCl2 powder in 100 ml of deionized water in a

beaker (0.1 M).

Prepare another beaker containing 0.3264 gm of sulphur powder in 100 ml

of deionized water (0.1M).

For alkaline solution dissolve 2 gm of NaOH in 100 ml of deionized water

in a separate beaker.(5 M)

Mix all three beakers and refluxed it by constant stirring for 7 hrs which

will result into white precipitation in solution.

Centrifuge the solution for 1.5 hrs with 2000 rpm yields white precipitates

at the bottom of the container for further wash.

Yielded precipitates were washed five times with deionized water. Further

wash was given to the precipitates using methanol in order to remove

chlorine.

The final product then was dried at 60C in normal atmosphere which

results into white brownish dry powder as shown in Figure 3.3.

Preparation method for ZnS nanoparticle by direct sulphur reaction

for synthesis with molar ratio (1:2)

Sample Name Given: ZnS_1:2_direct sulphur

Dissolve 1.3628 gm of ZnCl2 powder in 100 ml of deionized water in a

beaker. (0.1 M).

Prepare another beaker containing 0.6528 gm of sulphur powder in 100 ml

of deionized water.(0.2 M) then same process was repeated to synthesize

ZnS nanoparticles as explained earlier.

Chapter -3 Page | 94

Proposed chemical reaction for samples prepared in alkaline medium can be

where ZnCl2 dissociates in to deionized water to form Zn+2 and two Cl-2 and then

it reacts with S in a basic medium provided by NaOH to form of whitish

precipititates of ZnS nanopartciles.

2 2 22( ) 2in deionized waterZnCl S NaOH ZnCl Na S ZnS NaCl

Figure 3.3 Schematic of ZnS nanoparticles preparation by direct sulphur

reaction(1:1)

3.2.3 Room temperature synthesis of ZnS nanoparticles by

Inverse Micellar method.

Sample Name Given ZnS_AOT

Typical synthesis using water-in-oil microemulsions (w = 5) system.

111.53 g of surfactant (AOT) was dissolved in 477.5 ml of n-heptane.

The resulting solution was divided in half and 11.25 ml of 0.15 M Zn(Ac)2

was added to one half and 11.25 ml of 0.15 m Na2S(aq) was added to the

other.

The microemulsion was vigorously stirred for 45 min. Until they became

optically clear.

Chapter -3 Page | 95

The solution containing AOT/n-heptane/ S-2 was cannulated into the

solution of AOT/n-heptane/Zn+2(aq) to synthesis the ZnS nanoparticles.

To cap the nanoparticles, 1.8 ml of 4-fluorobenzenethiol and 2.2 ml of

Triethylamine were added, resulting in a white precipitate.

After 1 h of stirring, the precipitates was isolated by centrifugation and

washed three times with n-heptane to remove the AOT surfactant and

excess capping agent (4-flurobenzenethiol).

The resulting precipitates were subsequently dispersed in 20 ml of acetone

to form the nanoparticle solution which is shown in Figure 3.4.

Figure 3.4 Schematic of ZnS nanoparticles synthesis by Inverse micellar method

Figure 3.5 Pictorial representation of Water in oil microemulsion and

proposed chemical reaction

Chapter -3 Page | 96

In inverse micellar method the narrow size distribution of particles is possible by

controlling the size of the particle using surfactant which will provide hindrance to

the agglomeration of the particle and uniformity of the particle size can be

achieved the pictorial representation of particle size formation in water in oil

medium and reactions between [32] the chemical used is shown in Figure 3.5.

3.3 Conclusion

ZnS nanoparticles are successfully synthesized using three different

techniques viz microwave irradiation method, chemical reaction in

alkaline medium and inverse micellar method.

ZnS nanoparticles with different precursors of Zn and S are successfully

prepared using microwave irradiation technique.

Apart from precursor variations ZnS nanoparticles are successfully

synthesized varying the molar ratio of Zn and S precursors in microwave

irradiation and chemical reaction in alkaline medium method.

Chapter -3 Page | 97

3.4 References

[1] A.G. Stanley, in: R. Wolfe (Ed.), Applied Solid State Science, 15.

Academic Press, New York, (1975).

[2] A.P. Alivisatos, Science 271, 933,(1996).

[3] C. Kaito, Y. Saito, K. Fujita, Jpn. J. Appl. Phys. 26,1973, (1987)

[4] M. Abboudi, A. Mosset, J. Solid State Chem. 109, 70, (1994).

[5] V. Stanic, T.H. Etsell, A.C. Pierre, Mater. Lett. 31, 35, (1997).

[6] K. Okuyama, I.W. Lenggoro, N. Tagami, J. Mater. Sci. 32, 1229, (1997).

[7] S.H. Yu, J. Yang, Y.S. Wu, Z.H. Han, Y. Xie, Y.T. Qian, Mater. Res. Bull.

33,1661 (1998) .

[8] S.H. Yu, Y.T. Qian, L. Shu, Y. Xie, J. Yang, C.S. Wang, Mater. Lett. 35,

116,(1998).

[9] J.J. Zhu, S.W. Liu, O. Palchik, Y. Koltypin, A. Gedanken, J. Solid State

Chem. 153, 342, (2000).

[10] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. 100,

14037, (1975).

[11] Y.H. Ni, X.W. Ge, H.R. Liu, X.L. Xu, Z.C. Zhang, Rad. Phys. Chem. 61,

61, (1975).

[12] Y.D. Yin, X.L. Xu, X.W. Ge, Y. Lu, Z.C. Zhang, Rad. Phys. Chem. 55

,353, (1975).

[13] G.Murugadoss, J. of Lum. 131, 2216, (2011).

[14] N Sarvanan, G B The, S Y Peen Yap, K M Cheong, J Mater Sci: Mater

Electron , 19(12),1206, (2008).

[15] U.S .Senapati, D.K .Jha, D.Sarkar , Res. J. of Phy. Sciences, 1(7), 1,

(2013).

[16] N. Kumbjhojkar, V.V. Nikesh, A.Kshirsagar, S.Mahamuni, Appl. Phys.

88, 6260, (2003).

[17] X.H. Liao, H. Wang, J.J. Zhu, H.Y. Chen, Mater. Res. Bull. 36, 2339,

(2011).

[18] J.C. Jansen, A. Arafat, A.K. Barakat, H. Van Bekkum, in: M.L. Occelli, H.

Robson (Eds.), Syn. of Micro. Mat., 1, 507, (1992).

[19] B.H.Van, P.V.Ben, H.N.Nhat, N.T.Uyen, T.M.Thi Comm. In Phy. 22(2),

167 (2012).

Chapter -3 Page | 98

[20] M. Krunks, E Mellikov, E. Sork, Thin solid Films, 145, 105, (1986).

[21] V N Semyonov, E M Averbach, L A Michalyeva, Zh. Neorg Khim, 24 (4),

911, (1979).

[22] K V Kerm, M O Tilling, J A Varvas, Proc. Tallinn Tech Univ. 479,101,

(1980).

[23] F. Dutault, J. Lahaye, BullSoc. Chim., France, 5, (1979).

[24] Inorganic Chemistry, 3rd ed., Wuhan University and Jilin University and

other Universities, High Education press, Beijing, China (1993).

[25] V N Popov, A B Kolodezev, V P Safonov, Tezisy Dokl Vses Soveshch.

Technol. , Portsessy, appa. Kach Prom. Lyuminoforov 98,(1977).

[26] S Yu., L Shu. J Yang, Z Han, Y Qian, Y Zhang, J. Mater. Res. 14, 4157,

(1999).

[27] C Feldman, C Metzmatcher, J. Mater. Chem. 11, 2603,(2001).

[28] W. X Tu, H F Liu, Chem. Mater. 12, 564, (2011).

[29] K J Rao, B Vaidyanathan, M. Ganguli, , P A Ramkrishnan, Chem. Mater.

11, 882, (1999).

[30] W.Tu, H Liu, J. Mater. Chem. 10, 2207, (2000).

[31] H Wang, J R Zhang, J J Zhu, J. Cryst. Growth, 233,829,(2001)

[32] N Yonghong, Gui Yin, J Hong, Z Xu, Mat. Res. Bul., 39, 1967, (2004).