Effect OF Ni Concentration on Structural and Optical Properties of ZnS Nanoparticles

Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

Effect OF Ni Concentration on Structural and Optical Properties of ZnS Nanoparticles9 B. Sreenivasulu1, S. Venkatramana Reddy1,a, P. Venkateswara Reddy1 1 – Department of Physics, Sri Venkateswara University, Tirupati-517502, A.P. India a – drsvreddy123@gmail.com DOI 10.2412/mmse.0.1.664 provided by Seo4U.link

Keywords. SEM, morphology, Raman studies, PL, DRS, absorption.

ABSTRACT. Ni (1, 5 mol %) doped ZnS nano particles are synthesized by chemical Co-Precipitation method. The prepared samples are characterized byXRD, Raman Spectroscopy, Photo luminescence (PL), Optical absorption, Diffused reflectance (DRS) and Scanning electron microscope (SEM). XRD Analysis confirms the Cubic blended Structure for Ni doped ZnS and no impurity peaks are presented in XRD pattern. The average particle sizes of Ni doped nanoparticles are in the range of 2-3 nm. Raman spectra show the Vibrational modes that represent the structure of ZnS. The PL spectra exhibit emission peaks in both UV and visible regions and these results are good agreement with the absorption spectra and DRS. The optical band gap of ZnS decrease with increase of Ni concentration. SEM micro graphs reveal that the surface morphology of Ni doped ZnS nanoparticles are spherical in shape.

Introduction. Research on nano sized semiconductors stimulated great interest in the recent past due to their unique properties and potential applications in diverse areas such as photo catalysis, solar cells, display panels, etc. [1–4]. These materials show unusual luminescence properties induced by the quantum size effect. Efforts have been made in realizing luminescence tuneable materials simply by changing the particle size and size distribution and great progress has been achieved [5–9]. They not only give luminescence in various regions but also can add to the excellent properties of ZnS. In doped ZnS nano crystals, impurity ions occupy the ZnS lattice site and behave as a trap site for electrons and holes. The electrons are excited from the ZnS valence band to conduction band by absorbing the energy equal to or greater than their band gap energy.

Subsequent relaxation of these photo excited electrons to some surface states or levels is followed by radiative decay, enabling luminescence in the visible region. However, two different kinds of ions simultaneously present in a host material produce fluorescence, which is completely different from the emission due to a single ion and this property is very beneficial for white light generation [10–12]. In this work, the PL properties of the ZnS doped with Ni were investigated in an air atmosphere.

Experimental details: All the chemicals used are of Analytical Reagent grade (Sigma Aldrich chemicals), such as Zinc acetate [Zn (CH3 COO)2.2H2O], sodium sulfide (Na2 S) and nickel chloride (NiCl2 .6H2 O) are used as source materials for Zn, S and Ni respectively, with double distilled water as solvent. Pure and Ni-doped ZnS nano particles have been synthesized using the soft chemical approach, known as chemical co-precipitation technique [13-15] by mixing nickel chloride of 1, 5 mol % with zinc acetate aqueous solution. Sodium sulfide aqueous solution was 9

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Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

also added to this solution drop-by-drop followed by PVP to prevent agglomeration of the particles. The precipitates so formed are washed and dried to remove the last adherent. The Pure and Ni (1, 5 mol %) doped ZnS nano particles obtained, are finally crushed to the powder form for further investigations.

Results and discussion: XRD analysis:

(111)

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Fig. 1. XRD patterns (a) pure (b) 1 mol% and (c) 5 mol% Ni doped ZnS nano particles.

The XRD spectra of pure ZnS and as prepared ZnS sample at different dopant concentrations (1, 5 mol %) are shown in Fig.1. The XRD spectra shows three major peaks at 2θ = 28.65°, 48.16° and 56.55° for the miller planes (111), (220) and (311) of Ni doped ZnS nano particles respectively. The indexed plane values of ZnS agree well with the JCPDS file No: 80-0020 for all samples. The diffraction peaks indicate the nano size of the synthesized particles. The full width at half maximum (FWHM) of the diffraction peaks are slightly increased by the addition of capping agent. This may be due to the reduction of particles size. The averaged crystallite sizes of D is calculated from the FWHM of the diffraction peaks using the Debye–Scherrer equation,

=

0.91 cos

Raman Analysis. The Raman spectra of ZnS poly types have been described by Schneider and Kirby [16]. Nilsen et al. [17] reported the one- and two-phonon Raman spectra of bulk cubic ZnS. The Raman Spectra for pure and Ni doped ZnS nano particles are recorded in the frequency range 200-500 nm as shown in Fig. 2. The Raman spectra of undoped nano particles exhibit strong peaks at 270 and 350 cm-1 . Ni doped ZnS nanoparticles exhibits the transverse optical (TO) and longitudinal optical (LO) zone center phonons near 262 cm-1 , 342 cm-1 , 273 cm-1 and 351cm1 .The TO and LO mode positions are slightly changed for Ni doped ZnS.

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Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954 24000

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Fig. 2. Raman Spectra of (a) pure (b) 1 mol% and (c) 5 mol% Ni doped ZnS nano particles.

Morphological studies.

a)

b)

c)

Fig. 3. SEM and EDX of (a) pure (b) 1 mol% and (c) 5 mol% Ni doped ZnS nano particles.

SEM and EDAX analysis.

Fig.3 shows SEM images of different doping concentrations (1, 5 mol %) PVP capped ZnS nano particles and the corresponding EDX spectrum. The spectra demonstrate the presence of various elements in the prepared ZnS nano samles. The peaks corresponding to the elements Zn, Ni and S confirm the presence of the nanoparticles in the polymer matrix. Thus chemical precipitation method is very effective as no loss of elements occurrs during the synthesis.

Optical Properties.

PL Spectra.

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120000 100000 80000 60000

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Fig. 4. PL Spectra of (a) pure (b) 1 mol% and (c) 5 mol% Ni doped ZnS nano particles.

Pure and Ni doped ZnS nano particles are shown in Fig.4. It can be observed that PL emission spectra of doped ZnS have sharp intensity and symmetric with four dominant peaks at 419 nm, 438 nm and 467nm corresponding to blue emission and the peak at 539 nm identified as green emission. The emission peak at 467 nm is attributed to sulfur vacancies. Borse et al. [18] and Lu et al. [19] have reported the peak in the range 450 – 460 nm and have been assigned to sulfur vacancies i.e., to the recombination of electrons at sulfur vacancy with holes in the valence band.

The origin of emission peak observed at 539 nm does not result from impurity states related with dopant, but originated from native defect state.

Optical absorption and UV-Visible spectra. Absorption shoulder for pure ZnS and 1, 5 mol % Ni doped ZnS nano particles peaks are centered at 320 nm. The absorption peak positions are compared, the capped particles are significantly shifted to blue region.The relation between reflectance R and absorption coefficient α as given by Kubelka–Munk method [20] is

( ) =

(

)

=

where F (R) is the Kubelka–Munk function, S is the scattering coefficient.

The band gap for undoped ZnS came out to be 3.7 eV which is quite higher as compared to its bulk counterpart (3.54 eV). The band gap of 1 mol % Ni doped ZnS has decreased whereas for 5 mol % Ni, it has increased slightly. According to the Burstein–Moss shift [21-22], at high doping content, the Fermi level shifts into the conduction band.

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-1000 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

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Fig. 5. UV-Visible and Band gap Spectra of (a) pure, (b) 1 mol% and (c) 5 mol% Ni doped ZnS nano particles.

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Summary. In the present work, Ni doped ZnS nanoparticles have been synthesized successfully through chemical co-precipitation method using PVP as the capping agent. Structural analysis indicates that the Ni doped ZnS nanoparticles crystallize in a cubic structure without forming other secondary phases. The substitution of Ni ions at Zn sites is also confirmed by XRD and Raman studies. Raman studies show the quantum confinement effects in ZnS nanoparticles.. It is found that the undoped sample exhibits PL emission peaks at 466 nm but Ni2+ doped ZnS sample exhibits PL emission covering the whole visible region with multiple peaks at 438, 450, 467 and 539 nm.This research is supposed to have positive impact on nano technology in fetures.

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Cite the paper B. Sreenivasulu, S. Venkatramana Reddy, P. Venkateswara Reddy (2017). Effect OF Ni Concentration on Structural and Optical Properties of ZnS Nanoparticles. Mechanics, Materials Science & Engineering, Vol 9. doi:10.2412/mmse.0.1.664

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