Chalcogenide Letters Vol. 6, No. 12, December 2009, p. 713 – 722

ANALYSIS OF ZnS NANOPARTICLES PREPARED BY SURFACTANT MICELLE-TEMPLATE INDUCING REACTION

M. DHANAM*, B. KAVITHA, NEETHA JOSE, DHEERA P. DEVASIA Department of Physics, Kongunadu Arts and Science College, Coimbatore-641029, India

Semiconductor ZnS nanoparticles have been successfully synthesized from solutions containing a nonionic surfactant, Triton X-100 (t-octyl-(OCH2CH2)x OH, x = 9,10). The particle size was estimated as 7.7 nm. The optical spectra enabled to conclude that these nanoparticles exhibit quantum size effects. The prepared nanoparticles have been characterized by x-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) and the results are discussed in this paper in detail. (Received December 14, 2009; accepted December 20, 2009) Keywords: ZnS nanoparticles; XRD, SEM, TEM and optical analysis; Particle size; Quantum size effects

1. Introduction Synthesis of nanoparticles (zero, one, two and three-dimensional) has become a focal area

in nanostructured materials research owing to their low dimensions characteristics which differ from the bulk [1]. Nanomaterials are increasingly gaining the attention of not only the scientific community but also the public due to their unique properties, which lead to new and exciting applications. The physical and chemical properties of the nanomaterials tend to be exceptionally closely dependent on the size and shape or morphology. It is interesting to note that by changing the preparation parameters, nanomaterials having wide range of varying morphology such as spherical nanoparticles, nanowires, nanorods and nanotubes can be synthesized [2]. The particular area of nanostructure formation has tremendous scope and is a driving force for the electronic industry. Synthesis and self-assembly strategies of nanomaterials require precursors from liquid, solid or gas phase [3]. The method employed in the present study is surfactant-assisted technique. Particle size ranging between 1– 50 nm is strongly dependent on the surfactant employed. For the production of high surface area nanoparticles surfactant assisted method is used [4, 5]. Surfactant can also be used in order to prevent nanoparticle agglomeration in the solution [6-9].

Usually the micelle can act as template to induce the growth of one-dimensional nanoscale materials. In the present study the surfactant molecules are selected in such a way that the concentration of the surfactant is not ten times more than the critical micelle concentration and therefore the surfactant micelles are not used to produce one dimensional nanomaterials as usual instead to synthesis three dimensional nanoparticles as in the present study. Here ZnS nanoparticles are synthesized using Triton X-100 micelle as template, CS2 as sulfur source and oil phase [10].

*Corresponding author: monikagokul@yahoo.com

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2. Experimental All of the reactants and solvents used in our reaction system are analytical grade and used

without any further purification. In this method zinc acetate (Zn (AC) 2, 0.2772 gm, and 2.5 m mole) was added in to a 100 ml beaker containing 20 ml distilled water to form a transparent solution A. Next, surfactant Triton X-100 (1.05 ml), the aqueous ammonia solution (28 % wt, 0.6 ml) and CS2 (0.3 ml, 4.9 m mole) were added in to a 250 ml flask containing 20 ml distilled water. Subsequently solution A was introduced from the beaker in to the flask at room temperature. Then the flask was heated from 20-60oC at heating rate about 1oC/s and kept at 60oC for 24 hours. All the above steps were carried out with constant stirring. The flask was then cooled to room temperature naturally. The precipitate was filtered off, washed with distilled water and absolute ethanol for several times and then dried in vacuum desiccators for 24 hour. The as-obtained white powder was collected for characterizations.

XRD data were taken from XPERT PRO diffractometer with CuKα radiation (λ=1.54056Å).The UV-VIS spectra were recorded on a JASCO –UV/VIS/NIR double beam spectrophotometer. Scanning electron microscopy images were obtained using the instrument JEOL, JAPAN-JSM 6360. The morphology was also observed by TEM, HRTEM and SAED patterns by the instrument JEOL3010 with UHR pole piece. The samples used for TEM observations were prepared by dispersing the nanoparticles in absolute ethanol followed by ultrasonic vibration for 10 minutes, then placing a drop of dispersion on to a Cu grid coated with a layer of amorphous carbon.

CS2 - water is an interesting oil-water system in which CS2 is insoluble in water and it can exist as small oil droplets in water under stirring. The micelles (surfactant) provide a cage like effect around CS2 that can control nucleation growth and agglomeration. Since the concentration of the surfactant (Triton X-100) is less than ten times of the critical micelle concentration, instead of rod like micelles spherical micelles are formed and these enwrap the CS2 oil phase to give spheres. Due to the concentration difference between CS2 oil phase (in side) and water phase (out side) of the micelles, the Zn2+ and NH3 (acting as an attacking agent) transfer through the surfactant micelles to react with CS2 to produce ZnS.

2NH3 + CS2 NH4NHCSSH (1)

Zn2++NH4NHCSSH+2NH3 ZnS↓ + 2NH4

+ + NH4SCN (2)

Then the un-reacted CS2 is removed when it is heated to a temperature above the boiling point of CS2 (46oC-47oC). Surfactant is removed by water and absolute ethanol. ZnS will be left as spherical nanoparticles. The ability to vary the size of the micellar core by simply altering the water to surfactant ratio provides a very convenient handle to control particle size [11].

3. Results and discussion 3.1 Structural analysis Fig 1 shows the XRD pattern of as-synthesized white powder by surfactant micelle-

template inducing reaction in the presence of non-ionic surfactant, Triton X-100. The diffraction peaks in the x-ray diffraction pattern could be indexed to cubic phase ZnS with crystal constant of a = 5.437Å, which were in good agreement with those of bulk ZnS crystal (JCPDS file No.01-0792) and earlier report [12]. Zinc blend and wurtzite are the two most popular structural configuration of ZnS. Due to size effect the peaks broaden and then widths become larger as the crystal becomes smaller [13-18]. Ghosh et al [15] reported that the broadening of the peak may also occur due to micro strains of the crystal structure arising from defects like dislocation and twinning etc. They have considered these defects to be associated with the chemically synthesized spontaneously growing nanocrystals.

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But in the present work since the synthesize surfactant micelle-template inducing reaction in the presence of surfactant, the nanocrystal grow in controlled manner and there is no possibility of having considerable defects. The broadening of the peak may arise due to insufficient energy that is needed for atom to move to proper site in forming the crystallite as reported. Small crystallites have relatively few lattice planes that contribute to the broadening. The observed diffraction peaks correspond to the (111), (220) and (311) planes of the cubic crystalline ZnS and

are reported as identifying peaks of ZnS by earlier workers. Using the equation 2sin

ndhklλθ

= ,

the d-spacing has been calculated and the estimated values are in good agreement with JCPDS file No.01-0792.

The average sizes of the ZnS nanoparticles were calculated from the Debye–Scherrer

equation cosKD λ

β θ= and are found to be 4.8 – 7.7 nm. The mean crystal size of Mn2+ doped

ZnS nanoparticles is calculated to be 2.6nm [13]. The average size of ZnS:Tb nanocrystals were reported as 3nm [14]. The reported crystallite size from the major peak (111) centered at 2θ=28.52o was 2.2 nm [15]. It is interesting to note that surfactant assisted method helps to prepare large size nanoparticles than the doped ZnS nanoparticles prepared from other techniques. From

the particle size the dislocation density has been estimated using the expression 12D

δ = [19]. No

other crystalline impurities were detected within the detection limit indicating that the as-synthesized white powder was pure ZnS. The calculated structural parameters were presented in Tables 1 and 2. As particle synthesis time increases, the particle size increases and the dislocation density decreases considerably.

3.2 Optical analysis Fig. 2 shows the absorption spectra of ZnS nanoparticles prepared for different synthesis

time. Absorption peak will be comparatively bluer shifted and it will be strong for reduced particle size.

To examine the quantum confined effect of the synthesized nanoparticles, UV-VIS absorption spectra were employed. The peak in the UV absorption is indicative of the band gap of the semiconductor ZnS nanoparticles. The bulk ZnS absorption peak is expected at 337.6 nm (Eg=3.68 eV). As the particle size is reduced there is a significant blue shift in the optical absorption spectra and this is a tool to examine quantum confinement effect of nanoparticles [table 3]. From the Absorbance – Energy plot of ZnS nanoparticles, their band gap values have been determined [Fig.3]. Using the estimated band gap values of nanopartcles in the Brus equation the particle size has been calculated from the optical spectra. It has been found that as synthesis time increases from 5 to 24 hrs the particle size also increases. Intensity of absorption is also found comparatively high (0.9 AU).

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10 20 30 40 50 60 70

0100200300400500600

311220111

(d) t=24hrs

2theta(degrees)

inte

nsity

0100200300400500600

311220111 (c) t=9hrs

inte

nsity

0

100

200

300

400

311220

111 (b) t=7hrsin

tens

ity

050

100150200250300350400

311220

111

(a) t=5hrs

inte

nsity

Fig.1 XRD spectra of ZnS nanoparticles at different preparation times.

Table 1 XRD data of ZnS nanoparticles

Angle 2θ (degrees) Lattice constant (Å) Peak

observed Nanoparticle

synthesis time (hrs) Observed

ASTM Observed ASTM

111

5 7 9

24

28.723 28.787 28.787 28.656

28.216 5.39

220

5 7 9

24

47.774 47.770 47.707 47.974

47.043 5.38

311

5 7 9

24

56.597 56.664 56.865 56.460

56.025 5.39

5.43

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Table 2 Structural parameters of ZnS nanoparticles

Nanoparticle synthesis time(hrs)

Angle(2θ) (degrees)

d-spacing

(Ao)

Particle size

(nm)

Dislocation density(ρ)

Х1016 lines/m

5 28.723

3.104

4.24

5.56

7 28.787 3.097 4.57 4.80

9 28.787 3.097 4.86 4.24

24 28.656 3.111 7.78 1.65

Table 3 Optical parameters of ZnS nanoparticles.

Synthesis time (hrs)

Band gap of bulk

ZnS (eV)

Band gap of

nanoparticles(eV)

Particle size

(nm)

Absorption peak (nm)

Intensity of absorption

(au)

5

4.01

5.08

267.3

0.9

9 3.96 5.52 276.9 0.4

24

3.68

3.82 7.82 286.5 0.4

718

300 400 500 600

0.0

0.1

0.2

0.3

0.4

0.5 (c) t=24hrs

Wavelength(nm)

Abs

orba

nce(

AU

)

0.000.050.100.150.200.250.300.350.40

(b) t=9hrs

Abso

rban

ce(A

U)

0.0

0.2

0.4

0.6

0.8

1.0

(a) t=5hrs

Abso

rban

ce(A

U)

Fig.2 Absorption spectra of ZnS nanoparticles for various synthesis times.

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2 4

0.0

0.1

0.2

0.3

0.4

0.5

(c) t=24hrs

Energy(eV)

Abs

orba

nce

(AU

)

0.000.050.100.150.200.250.300.350.40

(b) t=9hrs

Abs

orba

nce(

AU

)

0.0

0.2

0.4

0.6

0.8

1.0

(a) t=5hrsA

bsor

banc

e(AU

)

Fig.3 Estimation of bandgap energy of ZnS nanoparticles [Eg = 4.01 eV (a), 3.96 eV (b) and 3.82 eV (c)]

3.3 Surface morphology analysis The effect of surfactants and the reactant concentration as well as the molar ratio of water

to surfactant determines the size and morphology of ZnS nanoparticles. Fig.4 and 5 shows the SEM photos of ZnS nanoparticles prepared at different synthesis time.

The surface of every particle is smooth and looks like spherical. Figures 6 and 7 shows TEM and HRTEM pictures which indicate that the sample is composed of a large quantity of nanoparticle with uniform size and shape. The average size of the nanoparticle estimated from the image is about 7.78 nm which is in good agreement with that calculated from that of XRD and optical studies. Fig 6 also confirms that the nanoparticles are spherical in nature. The lattice fringe is clearly exhibited from the individual nanocrystals, whose d spacing was evaluated as 3.11Å. This is in good agreement with lattice constant of cubic ZnS with d of 3.123Å for (111) plane. The Selected Area Energy Diffraction (SAED) pattern in Fig. 8 shows concentric rings instead of sharp spots. These diffuse rings indicate the polycrystalline nature of the material. The rings have been indexed to (111), (220) and (311) planes of the cubic ZnS phase (JCPDS No.03-0524) which corresponding to d (111) =3.1114Å, d(220)=1.904Å and d(311)=1.6277Å confirms the presence of cubic ZnS.

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Fig .4 SEM picture of ZnS nanoparticle at 9hrs

Fig. 5 SEM picture of ZnS nanoparticle at 24 hrs

Fig.6 TEM image of ZnS nanoparticles (x= 50 nm)

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Fig.7 HRTEM image of ZnS nanoparticle (x= 5nm)

Fig.8 SAED pattern of ZnS nanoparticle

4. Conclusions ZnS nanoparticles having different sizes (4.8–7.7 nm) have been successfully synthesized

through a simple surfactant micelle-template inducing reaction.. The formation of nanoparticle is strongly influenced by the quantity of surfactant and the aging time. From the x-ray diffraction spectra and the lattice constant, particle size and dislocation density were estimated and presented. From the absorption spectra the band gap and the particle size of the nanoparticle are calculated. The morphology of the particles has been identified from the SEM and TEM analysis.

Acknowledgements The authors are grateful to the Secretary, Principal, Dean and Head of the Department of

Physics, Kongunadu Arts and Science College, Coimbatore for their excellent encouragement and support. One of the author Mrs.B.Kavitha, was grateful to express her thanks to Jawaharlal Nehru Memorial Fund for financial support.

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