3. PREAMBLE TO THE DESCRIPTION

COMPLETE

Field of Invention

FACILE SYNTHESIS OF NiSe.Zn NANOPARTICLES BY METHOD VARIATION AND THEIR MORPHOLOGICAL STUDIES

Introduction

Recently there is an extensive attention given to the preparation and characterization of metal chalcogenides due to their properties and potential applications [1-2]. There are different methods of synthesis used for preparing NiSe like chemical method, hydrothermal process, solvothermal, CVD, ultrasonic synthesis [3-8], using poly vinyl alcohol (PVA) and/or cetyl trimethyl ammonium bromide (CTAB) as surfactants.

The doped metal chalcogenides have attracted more attention and many such chalcogenides are successfully synthesized [9-12]. Zinc being a leading metal used in corrosion resistance, Zn plating of Fe, in batteries and other fields, it is chosen as a doping element. As it was reported that the size and morphology of NiSe depends on nature of capping agent, reductant, temperature and time of reaction [6], in this work, we have taken an attempt to synthesize Zn doped NiSe using two different modes - one using the reducing agent hydrazine hydrate at 100°C with PVP as capping agent and the other using PVP only at room temperature. Wehave also taken an attempt to study the effect of the modes on the size, morphology and fluorescence properties. The selenides were identified and compared using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform Infrared spectroscopy (FT-IR), ultra violet-visible spectroscopy (UV) and photoluminescence (PL) techniques.

Summary of Invention

The zinc doped nickel selenide was successfully synthesized via two methods -hydrothermal and co-precipitation process. EDX analysis confirms the presence of these elements. SEM and XRD revealed that the particle size and shape is greatly influenced by the mode of synthesis, which in turn attributed to the effect of temperature and use of reducing agent hydrazine hydrate. Honey comb like structure of NiSe:Zn was obtained by hydrothermal process, whereas, perfect cubic and mono clinic crystals were obtained by co-precipitation process. Possible mechanism for the formation of different shape crystals is also explained. PL spectra revealed that conductivity of NiSe is enhanced by doping with zinc. There is no observable difference noticed in conductivity by adopting the different modes of synthesis.

4. DESCRIPTION

Experimental

Synthesis of zinc doped NiSe - Hydrothermal method using PVP (method-1)
NiCI2.6H20 and Na2Se03/ with a stoichiometric ratio of 1:1 were dissolved in 50 ml of deionized water. With this solution, certain amount of poly-N-vinyl pyrrolidone (PVP) is added with constant stirring. 20 ml of hydrazine hydrate and 25 ml of anhydrous ZnCI2 are added to the above solution and kept for heating in a mantle at 100°C for 5 hours. On adding hydrazine hydrate, greenish yellow color changed to black. The black precipitate thus obtained was washed with deionized water and acetone several times, to remove PVP and impurities. Finally, it was dried at 100°C for 2 hours.
Synthesis of zinc doped NiSe - Chemical Co precipitation route using PVP (method-2)
NiCI2.6H20 and Na2Se03/ with a stoichiometric ratio of 1:1 were dissolved in 25 ml of deionized water. The addition was carried out drop by drop with constant stirring. 10 ml of anhydrous ZnCI2 and a certain amount of PVP were added to the above solution. This solution was kept in a magnetic stirrer at room temperature for 4 hours. The yellowish green precipitate obtained was washed with deionized water and acetone several times to remove PVP and other impurities. Finally, it was dried at 100°C for 1.30 hours.

Mechanism

Method-1

This method is carried out by using hydrazine hydrate as reducing agent and PVP as surfactant at 100°C. The honeycomb like structure is due to the presence of hydrazine hydrate and temperature. Probable mechanism is given below.

Method-2

This method is carried out by using PVP as surfactant as well as reducing agent [13]. The cubic and mono clinic structure is due to the process being carried out at room temperature in the absence of hydrazine hydrate. Probable mechanism is given below.

Results and discussion:

Figures 1 and 2 show XRD patterns of as-prepared NiSe:Zn based on method-1 and method-2 respectively. XRD is a powerful non-destructive technique for characterizing crystalline materials. It provides information about the structures, phases, preferred crystal orientations and other structural parameters such as average grain size, crystallinity, strain and crystal defects. XRD peaks are produced by constructive interference of a monochromatic beam of X-rays scattered at specific angles from each set of lattice planes in a sample. The peak intensities are determined by the distribution of atoms within the lattice. When NiSe:Zn synthesized using hydrazine hydrate and PVP as surfactant, sharp peaks are obtained indicating crystallinity.

The peak positions of fig 1 and 2 are similar, indicating same 2-6 values, the difference lies in the intensity of the peaks. More intensity peaks are obtained by method-2, indicating more crystalline. The highest peak position corresponds to rhombohedral structure corresponding to cell constant values a=b=0.806 nm and c=2.215 nm in case of noise: Zn obtained by the method-1. Whereas the highest peak position corresponds to monoclinic-primitive structure corresponding to cell constant values a=1.214nm, b=0.575nm and c=1.53nm in case of NiSe.Zn obtained by the method-2 (obtained by JCPDS). This indicates more orderly arrangement of plane in particles obtained by method-2 at room temperature. The axial angle increases from 90° and the geometry changes to rhombohedral. This change is attributed to reaction temperature, which is carried out at 100°C.

Figures 3 and 4 show SEM pictures of the samples prepared by two routes. SEM provides information about the surface morphology. It is observed that the size and shape of the molecules are greatly affected due to synthesis of NiSe:Zn at 100°C compared to the same carried out at room temperature as well as the use of hydrazine hydrate which greatly affects the shape [14]. It was noticed that NiSe: Zn prepared by method-1 has a honeycomb like structure while the same prepared by method-2 in the absence of hydrazine hydrate has perfect cubic and monoclinic structure though the size of the particles is increased. The presence of the elements Ni, Se and Zn by both the methods are also confirmed from respective EDX spectra as given in Fig. 5 and 6.
One of the impressive features of semiconductors is their ability to emit light. Upon excitations with wavelength shorter or equivalent to the absorption onset, an electron is promoted from the valence band to the conduction band and upon relaxation; a photon of light is emitted. The minimum photon energy that is required to excite an electron into the conduction band is associated with the band gap of a material. PL describes the phenomenon of light emission from any form of matter after the absorption of photons. It is one of many forms of luminescence and is initiated by photo excitation. The most common use of PL is band gap or band to band transitions determination and detection of impurity and defect. Fig. 7 and 8 show PL spectra of NiSe:Zn obtained by methods 1 and 2 respectively.

On comparison to literature, NiSe is a p-type semiconductor with a band gap of 620 nm = 2.0 eV. On doping with zinc, the gap increases from 620 nm to around 720 nm, thereby decreasing the band gap energy to 1.722eV. This shows that the conductivity of NiSe has increased on doping with zinc. In comparing the graphs 7 and 8, it is observed that there is no change in the band-gap value by changing the mode of synthesis. This is attributed to the composition of the elements remains unaffected by the mode of synthesis.
Figures 9 and 10 show absorption spectra of NiSerZn obtained by methods 1 and 2 in the wavelength range of 190-800 nm. Literature survey has revealed that the absorption curve of nickel selenide is obtained at about 350 nm. As seen in Fig. 9 and 10, the absorption peaks are obtained at 350 nm and 355nm in method-1 and method-2 respectively. This confirms the presence of NiSe. It also indicates that doping does not affect the absorption spectra of NiSe.

IR spectrum of NiSe:Zn is shown in figures 11 & 12. In figure 11, the most intense and broadest band appears at 3458 cm'1. This can be associated with the vibrational modes of -H hydroxyl groups bonded with external hydrogen and / or atoms of the chalcogenides structural network. A similar absorption frequency appears at 3450 cm"1 in figure 12. The other, much weaker band, associated with the adsorbed molecule H20 which appear at 1658 cm"1. The reason for the appearance of these absorption bands is mostly due to the moisture absorbed by the sample from the environment during the sample preparation and due to IR measurement. The oxides are among the impurities that are strongly dependent on the chemical composition of chalcogenides. In all IR spectra, absorption bands are present related to vibrations of oxygen atoms bonded to basic elements or to carbon impurity atoms. A band appearing at 800-70 cm"1

can be due to the presence of Ni-0 bonds in the powders. The broad and asymmetric band centered at 578 and 499 cm"1 can be dependent on the composition, attributed to the vibrational mode of Se-0 bonds.

This table shows the different vibrational frequencies corresponding to the various modes of vibrations confirming the presence of NiSe:Zn.

Conclusion:

Zinc doped nickel selenide was successfully synthesized via two methods i) hydrothermal and ii) co-precipitation processes. EDX analysis confirms the presence of these elements. SEM and XRD revealed that the particle size and shape is greatly influenced by the mode of synthesis, which in turn attributed to the effect of temperature and use of reducing agent hydrazine hydrate. Honey comb like structure of NiSe:Zn was obtained by hydrothermal process, whereas perfect cubic and monoclinic crystals were obtained by co-precipitation process. A possible mechanism for the formation of different shaped crystals is also explained. PL spectra revealed that conductivity of NiSe is enhanced by doping with zinc. There is no observable difference noticed in conductivity by adopting the different modes of synthesis.

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5. CLAIMS

We claim that

1. Zinc doped nickel selenide (NiSe:Zn) nanoparticles have been synthesized.

2. Poly-N-vinyl pyrrolidone (PVP) has been used during the synthesis of NiSe:Zn as surfactant as well as reductant.

3. NiSe doped with Zn has been synthesized by hydrothermal process.

4. Also, NiSe doped with Zn has been synthesized by co-precipitation process.

5. Beautiful honeycomb like structure has been observed for the NiSe:Zn which was synthesized by hydrothermal route.

6. But special feature is that perfect crystalline monoclinic structure has been obtained for the NiSe:Zn nanoparticles, which was synthesized by co-precipitation route.

7. Photoluminescence property of NiSe gets enhanced by doping with Zn.

8. Photoluminescence property of NiSe:Zn is not affected by the modes of synthesis.

9. In a nut-shell, new nanoparticle, namely, NiSe:Zn has been synthesized and reported for the first time with suitable evidences for confirming the structure, with very good photoluminescence property, that this material shall open a new path for studies in a new indigenous material.