Precipitation derived ZnS:Ni nanocrystals: Study of Structural and Morphological Properties

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ScienceDirect Materials Today: Proceedings 3 (2016) 3892–3900

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ICMRA 2016

Precipitation derived ZnS:Ni nanocrystals: Study of Structural and Morphological Properties Umadevi Godavartia,c , Vishwanath Moteb, Madhava P Dasaric* a

Department of Physics, CMR Technical Campus, Hyderabad, Telangana, India Department of physics, Dayanand science college, Latur, Maharashtra, India c Department of physics, Gitam Institute of technology, Gitam University, Visakhapatnam, India

b

Abstract

Pure and Ni doped ZnS nanocrystals were synthesized using simple co-precipitation method. The obtained samples were characterized by different techniques such as X-ray diffraction pattern (XRD), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy. The X-ray diffraction (XRD) analysis confirms that the prepared nanocrystals have a cubic crystal structure. The lattice constant of Zn(1-x)NixS samples was calculated from XRD patterns, which were found to decrease with an increase of Ni content. The morphology of ZnS changed from cubic to spherical –cubic structure after Ni doping. The average crystallite size is in the range of 3-5 nm. The FTIR spectra confirmed the formation of pure and Ni doped ZnS nanocrystals. © 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International conference on materials research and applications-2016. Keywords: Nanoparticles; chemical synthesis; X-ray diffraction; TEM; FTIR

* Corresponding author. Tel.: +91-9348811777 E-mail address: [email protected] 2214-7853© 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International conference on materials research and applications-2016.

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1. Introduction In recent years, nanosized semi-conductor materials have drawn much attention due to their physical and chemical properties [1]. Their properties change drastically due to their size quantization effect which is improved compared with their bulk counterparts. At room temperature, the Sphalerite, cubic (Zinc blende) structure of ZnS is stable, while wurtzite possesses less dense hexagonal structure and is stable above 1020 0 C at atmospheric pressure. ZnS is an important II-VI semi-conducting material with wide band gap energy of 3.7 eV and a large exciton binding energy (40 meV) devices

[3,4]

. Due to its excellent properties, ZnS has versatile potential applications [1, 2] as optoelectronic

[5]

, e.g., photoconductors

[6]

, optical sensors

[7]

, solid state solar window layers[8], light – emitting materials

[9]

, field-effect transistors [10]. In the present paper, we try to get the bottom of the structural, morphological and optical properties of ZnS

particles doped with Ni2+. Divalent elements like Ni2+ ions doped ZnS nanocrystals can be obtained in many ways such as Gama irradiation method hydrothermal process

[16]

[11]

, chemical precipitation method

, sol-gel method

[17]

[12,13]

, Mechano-chemical method

and Reverse –Michelle method

[14,15]

, a

[18]

. We preferred co-precipitation

method for the synthesis of ZnS as this method is simple, low-cost and availability of the equipment. Crystal structure and average grain size were measured using XRD. TEM is used to study the particle size and morphology. The functional groups of ZnS and Ni are studied using FTIR spectroscopy. 2.

Experimental Samples with compositional formula Zn1-xNixS with x = 0.00, 0.05, 0.10 and 0.15 were prepared by co-

precipitation route. In this procedure, Zinc acetate dihydrate [ Zn(CH3COO)2 · 2H2O] of 1M is diluted in distilled water (50 ml) and sodium sulfide [ Na2S] 1M is diluted in distilled water (50 ml) were prepared and added by under vigorous stirring to obtain PH 13.5 for 2 h. A white precipitate was obtained which was separated by centrifugation. The precipitate which is separated is washed several times with distilled water and ethanol then dried under vacuum at 60oC to get the powder samples of ZnS nanoparticles. For the synthesis of 0.05% Ni doped ZnS nanoparticles were prepared at room temperature by mixing calculated amounts of zinc acetate solution and Nickel acetate solution [Ni(OCOCH3)2 · 4H2O] followed by dropwise addition of saturated solution of sodium sulfide up to PH 13.5. The mixture was vigorously stirred for 2 h. The precipitate was filtered from the reaction mixture and washed several times with ethanol to remove all sodium particles. The wet precipitate was then dried. Similarly prepared for samples of 0.10%, 0.15% Ni doped ZnS samples. XRD measurements were carried out to study the average crystallite size and structural properties of the Ni-doped ZnS nanocrystals (Model: PW-3710). The structural analysis of the synthesized samples was carried out using a powder X-ray diffractometer (XPERT-PRO) with a Cu-Kα radiation source of wavelength 1.5406 A0. The samples are named after their compositions, e.g., 00% Ni stands for Pure sample ZnS, Zn1-xNixS (x = 0.00), 05% Ni stands for Zn0.95Ni0.05S (x = 0.05),10% Ni stands for Zn0.90Ni0.10S(x = 0.10) and 15% Ni stands for Zn0.90Ni0.15S (x = 0.15).

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3. 3.1

Umadevi Godavarti/ Materials Today: Proceedings 3 (2016) 3892–3900

Results and discussion Structural studies

(311)

(220)

(111)

XRD diffraction patterns of Zn1-xNixS (x=0.00, 0.05, 0.10 and 0.15) nanocrystals are shown in figure (1).

15% Ni

Intensity (a.u.)

10% Ni

5% Ni

Pure ZnS

20

30

40

50

60

70

80

2 (Degree) Figure(1) X-ray diffraction pattern of undoped and Ni doped ZnS nanocrystals.

It can be seen from the figure (1) that XRD peaks are broadened with three main peaks corresponding to (111), (220) and (311) planes. Ni doped ZnS samples showing Zinc blende structure of ZnS which confirms the formation of Ni doped ZnS solid solution with no secondary phases. It is also observed that the diffraction peaks intensities were decreased for Ni doped ZnS, which indicates that the Ni2+ ions are substituted in the inner lattice of Zn2+ ions of ZnS lattice. The lattice constant 'a' was calculated using the following relation

1  h2  k 2  l 2  d 2  a2

  

(1)

Where d is the interspacing distance, h, k & l are the Miller indices and ‘a’ is lattice constant. The calculated lattice parameter of the samples is given in table (1). From table (1), it is observed that lattice parameter ‘a’ decreases for Ni doped nanocrystals. This may be due to the substitution of Ni2+ into Zn2+ ions. Since the ionic radius of Ni2+ ions (0.72 nm) is smaller than that of Zn2+ ions (0.80 nm). The lattice parameter ‘a’ can be used to determine the volume of the unit cell and it is found that the unit cell volume decreases with increasing Ni concentration as shown in figure (2). This indicates that Ni2+ ions go to the Zn site in the structure due to the larger ionic radius. The values of the volume of unit cell are tabulated in the table (1).

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Table (1). Lattice constant (a), volume (V), average crystallite size (D) and Microstrain (ε) of pure and Ni doped ZnS nanocrystals. .

Lattice constant ’a’ in (Å)

Volume(V)

D (nm)

Strain (ε)

00%

5.3930

156.8537

7.2677

0.00471

05%

5.3765

155.4149

6.5975

0.00525

10%

5.3362

151.9474

5.3043

0.00653

15%

5.3322

151.6103

2.6919

0.01289

Ni Content-(X)

The average crystallite size of Ni doped ZnS samples was determined by extra broadening of the X-ray diffraction peaks of the samples using Debeye Scherrer’s formula

D

0.9  cos 

(2)

Where D is the average crystallite size, λ is wavelength, θ is diffraction angle and β is the full-width half maxima (FWHM). The average crystallite size (D) is decreasing with increasing Ni doping concentration in ZnS samples which is in the range 2-7nm which means that the synthesized nanoparticles are in the quantum confinement regime as shown in the table (1) [13]. This could be attributed to the small grain growth of Ni doped ZnS as compared to ZnS sample. This shows that the Ni2+ substitution in an interstitial position would affect the concentration of interstitial Zinc, sulfur and Zn vacancies. The average crystallite size and microstrain are tabulated in table (1). The observation of peak broadening is due to size and microstrain of nanocrystals. The structural change can be acquired from the diffraction peaks illustrates the incorporation of Ni 2+ in ZnS lattice which indicates that the crystal lattice has no change by Ni doping. The microstrain was determined using the equation:



 4 tan 

(3)

Figure 2 shows the comparison of crystallite size and strain with increasing of Ni doping concentration. From figure (2), it is observed that the average crystallite size decreases and strain increases with increasing Ni content. It means that Ni has been entering into the ZnS lattice.

Umadevi Godavarti/ Materials Today: Proceedings 3 (2016) 3892–3900

8

0.014

D (nm) 

Average Crystallite Size (nm)

7

0.012

6 0.010 5 0.008 4

Microstrain

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0.006 3

2

0.004 0.00

0.05

0.10

0.15

Ni Content

Figure (2) Average crystallite size and microstrain versus Ni Content.

3.2

Morphological studies It is necessary to know the exact particle size and structures of nanomaterials by direct measurements, such as transmission electron microscope (TEM) which can reveal the particle size, shape and orientation of the pure and Ni doped ZnS nanoparticles. Figure (3) shows TEM images of pure ZnS sample and Ni doped samples. The electron diffraction patterns at different regions of the TEM grid were recorded which shows uniform size distribution. We did not find any other diffraction rings that cannot be indexed by sphalerite structure [18]. The images show that particles are nearly in cubic shape and having an average particle size of 3-6 nm. This value is consistent with the XRD results of the samples. These figures confirm the high crystallinity with uniform sizes and shapes.

Figure 3(a).

Figure 3(b).

Umadevi Godavarti/ / Materials Today: Proceedings 3 (2016) 3892–3900

Figure 3(c).

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Figure 3(d).

Figure (3) TEM images figure (3a) undoped , figure (3b) 0.05 % Ni doped , figure (3c) 0.10 % Ni doped and figure (3d) 0.15 % Ni doped.

3.3. FTIR Study

FTIR gives the qualitative information about the adsorbed surfactant molecules which are bound to the surface of ZnS:Ni2+ nanoparticles. Figure (4) shows FTIR spectra of pure and Ni doped ZnS samples were recorded at room temperature. The peak values of FTIR spectra for all obtained samples are assigned at room temperature and are listed in the table (2). It is clearly shown a broad peak has been observed at 3400-3600 cm-1 due to O-H stretching in all samples because of some absorbed moisture. The samples at room temperature show characteristic peaks at 600-700 cm-1 are assigned to the ZnS band (i.e. corresponding to sulfides) and (Zn,Ni)-S band for all samples. FTIR spectra of our samples yield the bands which are in good agreement with the reported values [20]. The increment in Ni concentration of ZnS samples changes the observed values it means that the formation of nanophase in the prepared samples. IR absorption peaks at 900-1500 cm-1 are due to the oxygen stretching and bending frequency. The additional weak bands and shoulders at 1554 and 1409 cm-1 are observed. It may be due to the microstructure formation of the samples. Bands around 1117-1121 cm-1 are due to the characteristics frequency of inorganic ions. Weak additional bands are observed at 928, 931, 930 and 874s cm-1 due to low temperature. These modes indicate the presence of resonance interaction between vibrational modes of sulfide ions in the crystals [21].

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Table(2). Assignment of frequencies of FTIR spectra of Ni doped ZnS nanocrystals.

x=0.00 (cm-1) 487.22 671.32 1019.69

x=0.05 (cm-1) 494.75 674.15 1020.38

x=0.10 (cm-1) 480.6 671 1018.64

x=0.15 (cm-1) 488.61 617.91 1017.94

1399.71

1400.36

1401.37

1399.22

Weak additional stretching

1554.59

1566.96

1559.12

1570.12

C=O Symmetric stretching

3381.56

3388.07

3380.64

3381.03

O-H stretching

Assignment of frequencies Zn-S Asymmetric bending Zn-S stretching Shoulder with asymmetric stretching

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Figure (4) FTIR images of pure , 0.05 % Ni doped , 0.10 % Ni doped and 0.15 % Ni doped

4.

Conclusion

Ni2+ doped ZnS is synthesized using co-precipitation method. The undoped and Ni doped ZnS nanocrystals were synthesized by a chemical method at room temperature. The XRD characterization reveals that Ni ions go to the Zn sites in the crystals without changing the cubic structure. The lattice parameter decreases with increase in doping concentration. We also observe that the average crystallite size is reduced with increasing Ni concentration; it may be due to smaller grain growth in comparison with undoped ZnS nanocrystals. No secondary phases are observed in the Ni doped samples. It indicates the homogeneous substitution of the Ni ions in the ZnS lattice structure. The TEM images of undoped and doped ZnS nanoparticles exhibits an average particle size of about 3-4 nm . The TEM result is in good agreement with the XRD results. The FTIR spectra confirmed the formation of pure and Ni doped ZnS nanocrystals. Acknowledgements The authors thankful to Punjab University, Chandigarh for providing XRD and TEM results. We are also thankful to IICT, Hyderabad for providing FTIR spectra. References 1. 2. 3. 4. 5.

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