WO2017068444A1 - Method of synthesizing ceramic oxide nanoparticles having tailored properties - Google Patents

Abstract

The present invention relates to a method of synthesizing soft magnetic Ni-Zn ferrite nanoparticles with different Ni-Zn contents. The method comprises preparing an aqueous solution of zinc salt, an aqueous solution of nickel salt and an aqueous solution of iron salt in stoichiometric amounts. The aqueous solutions of zinc salt, nickel salt and iron salt are dispersed into an aqueous solution of a polymer and a sugar at a predetermined temperature under constant stirring to obtain a homogeneous mixture. A solid mass of polymer precursor powder is obtained at a predetermined pH by drying the reaction mixture in a microwave oven. Recrystallized nanoparticles of Ni-Zn ferrites (NixZn1-xFe2O4 (x = 0.2-0.8)) are obtained by heating the polymer precursor powders at 400-600°C in ambient air.

Description

METHOD OF SYNTHESIZING CERAMIC OXIDE NANOPARTICLES HAVING

TAILORED PROPERTIES

BACKGROUND

Technical Field of Invention

[001] The present invention facilitates a novel and cost effective method of synthesizing nanomaterials with controlled shape and morphologies and more particularly having tailored properties. The present invention particularly relates to a chemical method of synthesizing (i) shape-controlled diluted magnetic semiconductor (DMS) materials comprising ZnO doped with transition metals (TMs), (ii) finely dispersed homogeneous and heterogeneous semiconductor-metal systems of ZnO:Ag and ZnO:Zn core-shell nanoparticles and (iii) magnetic nanoparticles of Ni-Zn ferrites having tailored properties.

Description of Related Art

[002] ZnO is a potentially attractive II- VI group semiconductor with a direct band gap of -3.37 eV at 300 K in bulk form and a large value of free exciton binding energy of 60 meV. It is recognized as an apposite candidate material for room temperature UV-visible optoelectronic applications including optical transducers, UV light emitters, transparent solar cells, acousto-optic media and ultraviolet photo detectors. Many attempts including heterostructures, alloying and doping have been engineered to tune its electronic and optical properties for pertinent applications. Theoretical studies of Dietl et al {Science 287, 1019- 1022, 2000}, and Sato and Katayama-Yoshida {Jpn. J. Appl. Phys. 40, 334-336, 2001 }have attracted intense searching for the ferromagnetic ordering in TM doped ZnO and similar DMS materials in which non-magnetic semiconducting host ions are replaced by magnetic TM ions. The DMS materials use a complementary degree of spin of electrons in addition to the charge of TM ions and quite often induce a room-temperature ferromagnetic (RTFM) ordering in the doped composites. It is observed that the ferromagnetic ordering in these DMS materials is very sensitive to the amount of TM ions doped in the matrix.

[003] Furthermore, the enhancement of the emission properties of oxide semiconductors by surface coating with metals has been reported. The surface-plasmon mediated emissions from the semiconductor-metal interfaces shows excellent enhancements of near-band edge emissions in semiconductors. The semiconductor-metal core-shell clusters, such as Ti0₂:Ag, Ti0₂:Au, ZnO:Au, ZnO:Pt, ZnO:Ag, etc., were developed to enhance the UV emissions in these wide band gap semiconductor materials. Another importance of ZnO:metal nanocomposites is that they display some unique features in photocatalytic degradation of wastewater treatment, chemical and biological sensing which are based on the surface- enhanced Raman scattering (SERS), localized surface plasmon resonance and metal enhanced fluorescence. The ZnO : metal system has two distinct features. First, it shows localized surface plasmon absorption of light which gives rise to wide absorption range in UV-visible region. Second, it forms Schottky junction at the interface, which creates an internal electric field at the junction causing the photo-generated electron and holes to move/separate in different directions which greatly reduces the recombination rate in these semiconductor systems. However, there are several engineering challenges in the formation of non- compatible ZnO:metal system that must be fully addressed to ensure the reliability of these novel structures for use in state of the art applications.

[004] The homogeneous ZnO:Zn semiconductor-metal composite system has potential applications in Zn rich paints to avoid any rust in scale-free steel surfaces, UV-visible optoelectronics, photocatalytic applications in decomposing wastewater, for hydrogen production using water-splitting thermo-chemical cycle, etc. The ZnO:Zn redox pair emerges as one of the most promising materials for solar-driven H₂0 and C0₂ thermo-splitting cycles. Wang et al [J. Phys. Chem. B 108, 570-574, 2004] synthesized Zn-ZnO core-shell nanobelts and nanotubes in a growth kinetics controlled solid-vapour decomposition process. Cai et al [J. Phys. Chem. B 109, 18260-18266, 2005] demonstrated the controlled synthesis of ZnO-Zn composite nanoparticles by laser ablation of Zn metal target in aqueous solution of sodium dodecyl sulphate (SDS). High SDS concentration yields higher relative amount of Zn nanoparticles existing as the core in the core-shell nanostructures, whereas low SDS concentration leads to higher ZnO amount. Ding et al [Materials for Renewable Energy and Environment (ICMREE), 2011 International Conference, 1350-1354] elaborated the successive steps for the synthesis of Zn-nanoparticles by vibration milling and hydrogen production by a water splitting reaction with Zn-nanoparticles. These approaches show much promise for scaled-up use in industrial processes. However, the production of ZnO:Zn core- shell system requires elaborate techniques and hence there are only limited reports available in the literature till date.

[005] The spinel ferrites, for example, Ni-Zn, Mn-Zn, etc., are another important class of high-tech materials. They are potentially attractive for pertinent applications in various fields including high frequency circuits, high-quality filters, ferrofluids, and read/write heads for high speed digital devices {J. Mater. Sci. 42, 779-783, 2007; J. Sol-Gel Sci. Technol. 58, 501- 506, 2011 }. Because of comparatively low losses at high frequencies, they are extensively used in the cores of RF transformers and inductors in applications such as switched-mode power supplies {Phys B: Cond. Mater. 363, 225-231, 2005}. These materials also have high resistivity, which prevents the eddy currents which is said to be another source of energy losses {J. Magn. Magn. Mater. 308, 177-182, 2007}.

[006] In general, spinel ferrites can be classified as normal and inverse spinels. ZnFe₂0₄ is a well-known normal spinel ferrite with Zn²⁺ ions at tetrahedral (A) sites and all Fe³⁺ ions at octahedral (B) sites. However, the NiFe₂0₄ has an inverse spinel structure where Ni²⁺ ions are at B sites and Fe³⁺ ions are equally distributed at A and B sites. Ni-Zn ferrites have mixed spinel structure with the chemical formula (Zni-_xFei-y)[NixFei_+y]0₄, in which the A sites are occupied by Zn²⁺ and Fe³⁺ ions and the B sites are occupied by Ni²⁺ and Fe³⁺ ions. The Ni-Zn ferrites are one of the most attractive ferrites due to their high magnetic permeability, electrical resistivity, and Curie temperature; and depending on their potential applications, these ferrites could have tailored magnetic parameters such as coercive field, threshold field and ferromagnetic resonance band {J. Magn. Magn. Mater. 215-216, 221-223, 2000}.

[007] Besides the cationic distribution, grain size and porosity play vital roles in determining the characteristics of these ferrites. The nanostructured ferrites have drawn recent attention due to their novel electrical and magnetic properties, which make them apposite for many technological applications, ranging from microelectronics, storage devices to electromagnetic wave absorbers {Adv. Mater. Lett. 3, 399-405, 2012}. The spinel ferrites form two types of ferromagnetically ordered sublattices, one by the ions at A sites and other by those at the B sites. The resultant inter sub-lattice coupling and intra sub-lattice coupling are responsible for the net magnetization in these mixed spinel ferrites {J. Phys. Chem. C 112, 15623-15630, 2008; J. Phys.: Condens. Mater. 21, 405303, 2009}. As a result, the magnetic properties of these ferrites are strongly influenced by the cationic distribution at A and B sites. The distribution of cations in the A and B sites is influenced by the synthesis technique followed to derive the ferrites. This distribution becomes more sensitive to the synthesis process especially when the particles are in nanometer scale. Various techniques were developed to synthesize nanoferrites such as co-precipitation, thermal decomposition, mechanical alloying, sol-gel method, hydrothermal method, etc. Among these, the chemical methods like the hydrothermal synthesis method have shown promising results in controlling the particle morphology and their size distribution, but have fallen short of providing reliable single phase nanoparticles where chemical composition can be controlled {J. Appl. Phys. 87, 4352-4357, 2000; J. Mater. Chem.ll, 1408-1416, 2001 }.

[008] Mixed Mn-Zn and Ni-Zn ferrites were developed by Verma and Chatterjee {J. Magn. Magn. Mater. 306, 313-320, 2006} using citrate precursor technique capable of giving atomic scale mixing of the constituent cations and formation of compositionally stoichiometric materials. Morrison et al {J. Appl. Phys. 95, 6392-6395, 2004} employed a surfactant system for room- temperature reverse micelle synthesis of Ni-Zn ferrite nanoparticles. Deka and Joy {Mater. Chem. Phys. 100, 98-101, 2006} synthesized nanocrystalline Nio.5Zno.sFe20₄ by an auto-combustion method using glycerine as a fuel. In another report, Carpenter et al {US patent application no. 2009/0184282 Al } reported a method of making monodispersed magnetic nanoparticles of Ni-Zn ferrite at room temperature using reverse micelle technique from an aqueous solution of metal (Ni, Zn and Fe) salts, surfactant (nonylphenol ethoxylate and sodium dioctyl sulfosuccinate) and hydrocarbon (cyclohexane and 2,2,4- trimethylpentane). Sarangi et al {Powder Technol. 203, 348-353, 2010} synthesized Ni-Zn ferrite nanoparticles following an oxalate based precursor method. Monodispersed Ni-Zn ferrite microspheres were synthesized via solvothermal method using glycol as a solvent by Yan et al {Mater. Sci. Engg. B 171,144-148, 2010}. Ghesari et al {J. Supercond. Nov. Magn. 26, 477-183, 2013 } performed comparative studies on structural and magnetic properties of Ni-Zn ferrite nanoparticles prepared by the techniques of glycine-nitrate auto-combustion process and solid state reaction followed by ball milling. Recently, Dzunuzovic et al {J. Magn. Magn. Mater 374, 245-251, 2015} derived Ni-Zn ferrites nanoparticles without any impurity phases by auto-combustion method using citric acid. Abbas et al {Mater. Chem. Phys.147, 443-451, 2014} reported shape and size controlled synthesis of Ni-Zn ferrite nanoparticles by sonochemical and polyol methods (polyethylene glycol as a solvent). [009] The above mentioned approaches for the development of novel high-tech nanoceramics show much promise for scaled-up use in industrial processes. However, there are still many challenges that must be fully addressed to ensure a precise control over crystal growth, particle size distribution and monodispersity of the derived nanoparticles. Hence there is a need to develop an improved and convenient preparation route for novel nanoceramics with tunable properties.

[0010] The above mentioned shortcomings, disadvantages and problems are addressed herein, as detailed below.

OBJECTS OF THE INVENTION

[0011] The primary object of the present invention is to provide a method of synthesizing shape-controlled DMS materials comprising ZnO doped with TMs for potential applications in several fields including spintronics and magneto-optical devices, biomedical applications, and magnetic sensors.

[0012] Another object of the present invention is to provide a convenient technique with efficient control over the doping level and microstructure in the DMS materials with excellent reproducibility.

[0013] Yet another object of the present invention is to provide a simple preparation route of ZnO-based novel semiconductor-metal (ZnO : metal, metal = Zn, Ag, etc.) core-shell nanoparticles with tailored properties.

[0014] Yet another object of the present invention is to provide a simple preparation route for a novel synthesis technique of Ni-Zn ferrite nanoparticles with tailored composition and controlled size distribution.

[0015] Yet another object of the present invention is to provide a novel synthesis technique for synthesizing Ni-Zn ferrite nanoparticles wherein the reactions can be easily tuned to induce tailored properties in the Ni-Zn ferrite nanoparticles for their pertinent applications.

[0016] These and other objects and advantages of the embodiments herein will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. SUMMARY OF INVENTION

[0017] The various embodiments of the present invention provide a method of synthesizing shape-controlled Diluted Magnetic Semiconductor (DMS) nanoparticles. The method comprises reacting a solution of a metal ion with an aqueous solution of a polymer and a sugar under continuous stirring at a predetermined reaction temperature to obtain a mixture. Plurality of polymer precursors are obtained at a predetermined pH from the mixture. The plurality of polymer precursors are heat treated at a predetermined range of temperature for a predetermined period of time in ambient air. The predetermined range of heat treatment temperature is 400-600°C and the predetermined period of time is 2 h.

[0018] The step of obtaining the plurality of the polymer precursors further comprises cooling the mixture to room temperature. The mixture is aged at a temperature range of 20 to 25°C for a time period of 24 h to obtain a plurality of precipitates. The plurality of precipitates are washed with an alcohol. The alcohol is methanol. The plurality of precipitates are dried at a controlled temperature of 50-60°C to obtain the plurality of polymer precursors.

[0019] The solution of metal ion is selected from the group consisting of titanium tetrachloride, chromium trioxide, iron (III) nitrate nonahydrate, cobalt (II) nitrate hexahydrate, nickel (II) nitrate hexahydrate, copper (II) nitrate trihydrate, zirconyl nitrate hydrate, cerium (III) nitrate hexahydrate or hafnium (IV) chloride.

[0020] The polymer is poly-vinyl alcohol.

[0021] The sugar is sucrose.

[0022] The predetermined reaction temperature is in the range of 60-65°C in ambient air.

[0023] The shape-controlled DMS particles are ZnO nanoparticles doped with TMs, wherein the TM is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf.

[0024] The shape-controlled DMS particles are of nanoplatelet shape of TM-doped ZnO, wherein the TM is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf.

[0025] The predetermined pH is 9.

[0026] According to the embodiments of the present invention, the step of obtaining the plurality of the ZnO-poly-vinyl alcohol-sucrose polymer precursor comprises of a reaction between aqueous solution of zinc nitrate and aqueous poly-vinyl alcohol- sucrose at 60-65°C of reaction temperature and cooling the reaction mixture to room temperature. The mixture is aged at a temperature range of 20 to 25 °C for a time period of 24 h to obtain a plurality of precipitates. The plurality of precipitates is washed with methanol. The plurality of precipitates is dried at a controlled temperature of 50-60°C to obtain the plurality of ZnO- polymer precursor.

[0027] According to an embodiment of the present invention, the poly-vinyl alcohol is a weak reducing but an excellent capping agent thereby supports the controlled reductive oxidation of the metal cations inside the poly-vinyl alcohol-sucrose polymer micelles. The process allows controlled growth of the magnetic nanoparticles in the form of platelets in the reaction solution by encapsulating the nucleating sites inside the polymer micelles.

[0028] According to an embodiment of the present invention, the core-shell nanoparticles are ZnO : metal nanoparticles. The core is made up of ZnO and the shell is made up of a metal. The metal is selected from the group consisting of metals including zinc metal and silver metal.

[0029] According to an embodiment of the present invention, the step of obtaining the ZnO:Ag core-shell nanoparticles is performed by dispersing the obtained plurality of ZnO- polymer precursor in an aqueous solution of silver nitrate while continuous stirring for 20-30 minutes in dark at a temperature of 50-60°C and obtaining a dry powder by washing the solution and drying in a reduced pressure at 20-25°C. In the next step, the dried powders are heated at selected temperatures in the range 400-500°C for 2 h in ambient air to obtain ZnO : Ag core-shell nanoparticles.

[0030] According to another embodiment of the present invention, the step of obtaining the ZnO:Zn core-shell nanoparticles is performed by heating the dried ZnO precursor powders at 500°C for 2 h in ambient air to obtain recrystallized ZnO nanoparticles and further heating the obtained recrystallized ZnO nanoparticles at 1000°C in an autoclave at 2 bar pressure in an atmosphere of 5% H₂-95% Ar gas.

[0031] According to another embodiment of the present invention, the embodiments of the present invention can also be used for synthesizing soft magnetic Ni-Zn ferrite nanoparticles with different Ni-Zn contents. The method comprises preparing an aqueous solution of zinc salt, an aqueous solution of nickel salt and an aqueous solution of iron salt in stoichiometric amounts. The aqueous solutions of zinc salt, nickel salt and iron salt are dispersed into an aqueous solution of a polymer and a sugar at a predetermined temperature under constant stirring to obtain a mixture. A solid mass of polymer precursor powder is obtained at a predetermined pH by drying the mixture in a microwave oven. Recrystallized Ni-Zn ferrite nanoparticles are obtained by heating the polymer precursor powder at 400-600°C in ambient air.

[0032] According to one embodiment of the present invention, the zinc salt is zinc nitrate hexahydrate, the nickel salt is nickel nitrate hexahydrate, and the iron salt is iron nitrate.

[0033] According to one embodiment of the present invention, the predetermined temperature is 60-65°C.

[0034] According to one embodiment of the present invention, the predetermined pH is 9. The predetermined pH is maintained by adding 25% of ammonium hydroxide solution to the mixture.

[0035] According to one embodiment of the present invention, the aqueous solution of the polymer and the sugar is prepared by adding said polymer and the sugar in a ratio of 1: 10 by weight in water.

[0036] According to one embodiment of the present invention, the soft magnetic Ni-Zn ferrite nanoparticles are derived with a varied Ni and Zn contents as Ni_xZni-_xFe20₄, where in x = 0.2-0.8.

[0037] According to another embodiment of the present invention, a process of synthesizing magnetic nanoparticles of Ni_xZni-_xFe20₄ ferrites with tuneable composition and narrow particle size distribution is provided. The Ni_xZni__xFe20₄ (x=0.2-0.8) nanoparticles are synthesized by a chemical method which involves a reaction between aqueous solutions of salts of zinc, nickel, and iron with an aqueous solution of poly-vinyl alcohol and sucrose at 60-65°C. The poly-vinyl alcohol-sucrose polymer micelles are used as surfactants to control the growth of the nanoparticles. The poly-vinyl alcohol-sucrose polymer micellar system acts as a controlled reducing agent as well as a capping agent. In the reaction solution, the polymer micelles encapsulate the metal cations and thereby ensure a controlled growth of the nascent nucleating sites.

[0038] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF DRAWINGS

[0039] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:

[0040] FIG. 1 is a flow chart showing the various steps involved in the method of synthesizing shape-controlled DMS nanoparticles, according to an embodiment of the present invention.

[0041] FIG. 2 shows a flow chart showing the steps involved in the synthesis of Ni-doped ZnO DMS nanoparticles, according to an embodiment of the present invention.

[0042] FIG. 3a is showing the X-ray diffractograms of the pristine ZnO nanoplatelets after heating a Zn²⁺-poly-vinyl alcohol-sucrose polymer precursor powder at different temperatures, according to an embodiment of the present invention.

[0043] FIG. 3b is showing the X-ray diffractograms of various Ni-doped ZnO nanoplatelets, according to an embodiment of the present invention.

[0044] FIG. 4a and FIG. 4b are the High Resolution Transmission Electron Microscope (HRTEM) images of the pristine ZnO nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention.

[0045] FIG. 4c, FIG. 4d and FIG. 4e are the HRTEM images of the Zno.99Nio.01O nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention.

[0046] FIG. 4f and FIG. 4g are the HRTEM images of Zno.9sNio.02O nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention. [0047] FIG. 5a, FIG. 5b and FIG. 5c are the Field Emission Scanning Electron Microscope (FESEM) images of the Zno.9sNio.02O nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention.

[0048] FIG. 5d and FIG. 5e are the FESEM images of the pristine ZnO nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention.

[0049] FIG. 6a shows the room temperature magnetic hysteresis curves of the Ni-doped ZnO nanoplatelets obtained after heating the corresponding precursor powders at 400°C for 2 h in ambient air, according to an embodiment of the present invention.

[0050] FIG. 6b shows the room temperature magnetic hysteresis curves of the Ni-doped ZnO nanoplatelets obtained after heating the corresponding precursor powders at 500°C for 2 h in ambient air, according to an embodiment of the present invention.

[0051] FIG. 7 is a flow chart showing the method of synthesizing ZnO (core)-metal (shell) nanoparticles, according to an embodiment of the present invention.

[0052] FIG. 8 is a schematic illustration of the formation of Ag-coated ZnO nanoparticles, according to an embodiment of the present invention.

[0053] FIG. 9 shows the X-ray diffraction (XRD) patterns of the pristine ZnO nanoparticles derived after heating the ZnO-polymer precursor powders at 400°C (shown by (a)) and at 500°C (shown by (b)) for 2 h in ambient air, while (c) shows the XRD pattern of Ag-coated ZnO-polymer precursor powders, and (d) shows the XRD pattern of recrystallized Ag-coated ZnO nanoparticles after heat treating the precursor powders at 400°C and (e) shows the XRD pattern of recrystallized Ag-coated ZnO nanoparticles after heat treating the precursor powders at 500°C for 2 h in ambient air, according to the embodiments of the present invention.

[0054] FIG. 10A shows the Williamson-Hall plot of pristine ZnO nanoparticles and FIG. 10B shows the Williamson-Hall plot of Ag-coated ZnO nanoparticles.

[0055] FIG. 11A and FIG. 11B are the field emission scanning electron microscope

(FESEM) images of pristine ZnO processed at 500°C for 2 h in ambient air while FIG. 11C and FIG. 11D are the FESEM images of Ag-coated ZnO nanoparticles processed at 500°C for 2 h in ambient air and FIG. HE shows the energy dispersive X-ray spectroscopy (EDS) spectrum of Ag-coated ZnO nanoparticles, according to the embodiments of the present invention.

[0056] FIG. 12A and FIG. 12B are the high resolution transmission electron microscope (HRTEM) images of pristine ZnO nanoparticles calcined at 500°C for 2 h in ambient air while FIG. 12C and FIG. 12D are the HRTEM images of Ag-coated ZnO nanoparticles calcined at 500°C for 2 h in ambient air, according to the embodiments of the present invention.

[0057] FIG. 13A shows the UV-visible reflectance spectra and FIG. 13B shows the corresponding (ahv)² versus hv plots corresponding to the pristine ZnO and Ag-coated ZnO nanoparticles calcined at 500°C for 2 h in ambient air, according to the embodiments herein.

[0058] FIG. 14A and FIG. 14B show the room-temperature photoluminescence spectra of pristine ZnO and Ag-coated ZnO nanoparticles calcined at 400°C and 500°C for 2 h in ambient air, respectively, according to the embodiments of the present invention.

[0059] FIG. 15A and FIG. 15B represent the energy band diagram of pristine and Ag-coated ZnO nanoparticles, respectively, according to the embodiments of the present invention.

[0060] FIG. 16A-FIG.16 D show the X-ray photoelectron spectroscopy (XPS) results of Ag- coated ZnO core-shell nanoparticles (synthesized at 500°C), according to the embodiments of the present invention.

[0061] FIG. 17 shows the XRD patterns of ZnO nanoparticles and ZnO:Zn nanoparticles, wherein (a) shows XRD patterns of pristine ZnO nanoparticles synthesized at 500°C in ambient air while (b) shows the XRD patterns of the derived ZnO:Zn composite powders on Si substrates after heating the pristine ZnO samples at 1000°C in an atmosphere of 5% H₂- 95% Ar gas at 2 bar pressure, according to the embodiments of the present invention.

[0062] FIG. 18 shows the UV-visible absorption spectra of pristine ZnO and ZnO:Zn nanoparticles, where (a) corresponds to the pristine ZnO nanoparticles synthesized at 500°C in ambient air while (b) corresponds to the derived ZnO:Zn composite powders after heating the pristine ZnO samples at 1000°C in an atmosphere of 5% H₂-95% Ar gas at 2 bar pressure, according to the embodiments of the present invention. [0063] FIG. 19 is a flow chart showing the method of synthesizing soft magnetic Ni-Zn ferrite nanoparticles with different Ni-Zn contents, according to an embodiment of the present invention.

[0064] FIG. 20 shows the Raman spectra of the derived nickel-zinc ferrite nanoparticles after heating the corresponding precursor powders at 500°C for 2 h in ambient air, wherein (a) shows Raman spectrum of Nio.2Zno.₈Fe20₄, (b) shows Raman spectrum of Nio.4Zno.6Fe20₄, (c) Raman spectrum of Nio.5Zno.5Fe204, (d) Raman spectrum of Nio.6Zno.₄Fe20₄, and (e) Raman spectrum of Nio.sZno.2Fe20₄ nanoparticles, according to the embodiments of the present invention.

[0065] FIG. 21 shows the X-ray diffraction patterns of the derived nickel-zinc ferrite nanoparticles after heating the corresponding precursor powders at 500°C for 2 h in ambient air, wherein (a) shows X-ray diffraction pattern of Nio.2Zno.sFe20₄, (b) shows X-ray diffraction pattern of Nio.₄Zno.6Fe20₄, (c) shows X-ray diffraction pattern of Nio.5Zno.5Fe20₄, (d) shows X-ray diffraction pattern of Nio.6Zno.₄Fe20₄, and (e) shows X-ray diffraction pattern of Nio.sZno.2Fe20₄ nanoparticles, according to the embodiments of the present invention.

[0066] FIG. 22 shows the Fe K-edge X-ray absorption near-edge spectra (XANES) of the derived Nii-_xZn_xFe20₄ (x=0.2-0.8) nanoparticles, according to an embodiment of the present invention. The inset shows the pre-edge features in the derived samples (*'x'NZF = Nio.xZni- o._xFe20₄; e.g., 2NZF = Nio.2Zno.sFe20₄).

[0067] FIG. 23A shows the Fe K-edge normalized extended X-ray absorption fine structure (EXAFS) spectra and FIG. 23B shows the Fourier transformed EXAFS spectra along with the best fit theoretical curves of Ni_xZni-_xFe20₄ (x=0.2-0.8) nanoparticles heat treated at 500°C, wherein 'x'NZF = Nio.xZni-o._xFe20₄; e.g., 2NZF = Nio.2Zno.sFe20₄, according to an embodiment of the present invention.

[0068] FIG. 24A and FIG. 24B show SEM images of Nio.2Zno.sFe20₄ nanoparticles calcined at 500°C and FIG. 24C shows the EDX spectrum of Nio.2Zno.sFe20₄ nanoparticles, according to the embodiments of the present invention.

[0069] FIG. 25 shows the room-temperature magnetic hysteresis (M verses H) curves of the Ni-Zn ferrite nanoparticles derived after heating the corresponding precursors at 500°C for 2 h in ambient air, wherein (a) shows the magnetic hysteresis curve of Nio.2Zno.₈Fe20₄, (b) shows magnetic hysteresis curve of Nio.5Zno.sFe20₄, and (c) shows magnetic hysteresis curve of Nio.sZno.2Fe20₄ nanoparticles, according to the embodiments of the present invention.

[0070] FIG. 26A, FIG. 26B and FIG. 26C show the variation of dielectric constant (ε'), dielectric loss (ε") and ac conductivity (c_ac) of Nio.2Zno.₈Fe20₄, Nio.5Zno.sFe20₄ and Nio.sZno.2Fe20₄ nanoparticles in the frequency range, 1 Hz- 10 MHz, respectively, according to the embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0071] In the following detailed description, a reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. The embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that any logical change may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.

[0072] The various embodiments of the present invention provide a simple and cost effective method of synthesizing shape-controlled DMS nanoparticles. The shape-controlled DMS nanoparticles are highly pure. The shape-controlled DMS nanoparticles are of TM-doped ZnO. The method is a convenient route for the mass scale production of the pristine and TM- doped ZnO nanoparticles. The method also provides an easy way for controlled doping of TM ions in wide band gap semiconductor oxides. In addition, the method according to the present invention allows a precise control over the shape and size of the particles in these DMS materials for pertinent applications. The pristine as well as TM-doped ZnO nanoparticles synthesized in the present invention have application in the fields of spintronics, magneto- optics, biosensors, medical imaging, drug delivery, etc.

[0073] FIG. 1 is a flow chart showing the various steps involved in the method of synthesizing shape-controlled DMS nanoparticles, according to an embodiment of the present invention. With respect to FIG. 1, the method comprises of the step of reacting a solution of a metal ion with an aqueous solution of a polymer and a sugar (101). The reaction is done under continuous stirring at a predetermined temperature to obtain a mixture. Plurality of polymer precursors are obtained (102). The plurality of polymer precursors are obtained at a predetermined pH from the mixture. The step of obtaining the plurality of the polymer precursors further comprises cooling the mixture to room temperature. The mixture is aged at a temperature range of 20 to 25 °C for a time period of 24 h to obtain a plurality of precipitates. The plurality of precipitates are washed with an alcohol. The alcohol is preferably methanol. The plurality of precipitates are dried at a controlled temperature of 50- 60°C to obtain the plurality of polymer precursors. The plurality of polymer precursors are heat treated in ambient air (103). The heat treatment is done at a predetermined range of temperature for a predetermined period of time. The predetermined range of temperature is 400-600°C and the predetermined period of time is 2 h.

[0074] According to an embodiment of the present invention, the solution of metal ion is selected from the group consisting of titanium tetrachloride, chromium trioxide, iron(III) nitrate nonahydrate, cobalt(II) nitrate hexahydrate, nickel(II) nitrate hexahydrate, copper(II) nitrate trihydrate, zirconyl nitrate hydrate, cerium(III) nitrate hexahydrate or hafnium(IV) chloride.

[0075] According to another embodiment of the present invention, the polymer is poly-vinyl alcohol.

[0076] According to another embodiment of the present invention, the sugar is sucrose.

[0077] According to another embodiment of the present invention, the predetermined temperature is in the range of 60-65 °C in ambient air.

[0078] According to another embodiment of the present invention, the shape-controlled DMS particles are ZnO nanoparticles doped with TMs, wherein the TM is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf.

[0079] According to another embodiment of the present invention, the shape-controlled DMS particles are nanoplatelets of pristine and TM-doped ZnO, wherein the TM is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf.

[0080] According to another embodiment of the present invention, the shape-controlled DMS particles are hexagonal nanoplatelets of pristine and TM-doped ZnO, wherein the TM is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf.

[0081] According to another embodiment of the present invention, the predetermined pH is 9.

[0082] According to an embodiment of the present invention, the method comprises a reaction between aqueous solutions of metal ions and a freshly prepared aqueous solution of poly-vinyl alcohol and sucrose under continuous magnetic stirring at 60-65°C in ambient air. The process allows controlled growth of the magnetic nanoparticles in the form of platelets in the reaction solution by encapsulating the nucleating sites inside the polymer micelles. The stable re-crystallized nanoplatelets of pristine and TM-doped ZnO in the form of a finely divided loose powder are obtained by heat treating the derived polymer precursors at 400- 500°C for 2 h in ambient air.

[0083] According to one embodiment of the present invention, the shape-controlled DMS particles are in the form of hexagonal nanoplatelets comprising pristine and TM-doped ZnO. The method according to the present invention uses micellar microemulsion of poly-vinyl alcohol and sucrose as a reactor medium and controls the shape and size of the particles by encapsulating the nucleating sites inside the polymer micelles. After aging, a controlled drying and scaffolding at 60-70°C in air preserves the nascent particles to be capped in the polymer micelles.

[0084] The poly-vinyl alcohol is a weak reducing but an excellent capping agent thereby supports the controlled reductive oxidation of the metal cations inside the poly-vinyl alcohol- sucrose polymer micelles. The possible mechanism behind the formation of hexagonal platelet type morphology is the texturing effect of the poly- vinyl alcohol lamellae in the form of micelles. The poly- vinyl alcohol-sucrose capping ligands (micelle wall) can change the surface energy of different crystal facets in the nascent ZnO lattice at the nucleating stage, the side facets may possess higher energy than the top and down surfaces, thus leading to a preferential lateral growth in the form of platelets. The recrystallized particles appear with a reconstructive decomposition of the polymer encapsulated precursor structure after a heat treatment at 400-600°C for 2 h in ambient air.

[0085] The materials used in the synthesis process were zinc nitrate hexahydrate [mol. wt. = 297.47, 99.98% pure], poly-vinyl alcohol [mol. wt. -96800, degree of polymerization -2000], 25% ammonia solution, sucrose (99% pure) and a suitable salt of the desired dopants.

[0086] Table 1 shows the specific salts used in this work to incorporate the TM dopants in the pristine ZnO lattice. Table 1: Salts used to incorporate the TM dopants in the pristine ZnO lattice

[0087] In most of the cases, the easily available nitrate salts were used, e.g., nickel nitrate hexahydrate [mol. wt. = 290.81, 99.95% pure] was used to dope ZnO with Ni.

[0088] FIG. 2 shows a flow chart showing the steps involved in the synthesis of Ni-doped ZnO DMS nanoparticles, according to an embodiment of the present invention. With respect to FIG. 2, the synthesis method of the present invention includes the steps of preparing a homogenous mixture of aqueous solution of poly-vinyl alcohol and aqueous solution of sucrose (201). The aqueous solutions of zinc nitrate hexahydrate and nickel nitrate hexahydrate are added in the homogenous mixture of poly-vinyl alcohol and sucrose (202) in presence of ammonia solution (203). The Zn²⁺ and Ni²⁺ ion solutions are dispersed in small amounts in the poly-vinyl alcohol-sucrose polymer molecules. The Zn²⁺ and Ni²⁺ ions are enclosed/capped inside the poly-vinyl alcohol-sucrose polymer micelles in a rather stable form (204). The reaction solution is aged in the air at room temperature or lower for 24 h (205). The derived precipitate is dried at 50-60°C (206) to obtain the amorphous precursor (207). The as-obtained polymer precursor is heat treated at 400-600°C in air (208). A product of recrystallized Ni-doped ZnO nanoparticles in the form of finely divided loose powders are obtained in the end (209). The Ni-doped ZnO nanoparticles are in the form of nanoplatelets.

[0089] In typical synthesis, a drop wise addition of Zn(N0₃)₃.6H₂0 (in 0.2 M solution) and Ni(N0₃)₃.6H₂0 (in 0 to 21.77 mM solution as per the required doping level) to a mixed polyvinyl alcohol-sucrose solution (in 1 : 10 ratio by mass) in water at 60-65 °C with continuous magnetic stirring results in a homogeneous dispersion of the salts in the polymer micelles. The sucrose additive, used in this method, improves average viscosity of the reaction solution such that the poly-vinyl alcohol- sucrose polymer micellar wall behaves to be rather stable and does not pile off so easily during the reaction. It is observed that in the solution the salts have an endothermic reaction with the poly-vinyl alcohol-sucrose molecules. During the reaction, pH of the mixed solution is maintained at ~9 by adding ammonium hydroxide solution to support the hydrogenation of Zn²⁺-Ni²⁺ ions in the reaction solution. Just after the addition of the metal salts in the poly-vinyl alcohol-sucrose solution, the mixed reaction solution appears to be light paris green in colour, however the turbid precipitates that start forming almost instantaneously is whitish in colour (irrespective of Ni concentration). The changes in the reaction solution during the process are studied further with in-situ measurement of the UV- visible spectrum, which confirms the nucleation of ZnO crystallites in the reaction solution.

[0090] The reactions involved are,

(Zn²⁺ + 2N0 ^2") + 2(NH₄ ⁺ + OH^")→ Zn(OH)₂ + 2NH₃† + 2HN0 (la)

(Ni²⁺ + 2N0 ^2") + 2(NH₄ ⁺ + OH^")→ Ni(OH)₂ + 2NH₃† + 2HN0 ( lb)

[0091] The metal hydroxide precipitates are soluble in basic medium (at pH ~ 9), producing the metal oxide anions (zincates-nickelates), which are trapped inside the polymer micelles and after aging, nucleate to produce the stable, shape and size controlled metal oxide nanoparticles,

Zn(OH)₂ + 2(NH₄ ⁺ + OH^")→ Zn0₂ ^2" + 2NH₃† + 2H₂0 + H₂† (2a)

I

ZnO

Zn(OH)₂ + Ni(OH)₂ + 4(NH₄ ⁺ + OH^")→ Ni_xZni-_x0₂ ²- + 4NH₃† + 6H₂0 (2b)

I

[0092] Effect of pH: It has been observed that the pH ~ 9 is the optimum value for the controlled growth of the nanoparticles inside the polymer micelles and is highly decisive in determining the size and morphology of the final products. Any further increase in the pH level will lead to a competition between the growth and erosion of the metal oxide nanoparticles inside the polymer micelles, which will affect the resultant growth of the particles.

[0093] After the reaction, the solution is cooled to room temperature and aged at 20 to 25 °C for 24 h before decanting out the transparent top layer of excess poly-vinyl alcohol-sucrose. Finally, the as-obtained precipitate was washed with methanol to remove the remaining polyvinyl alcohol-sucrose and dried at a controlled temperature of 50-60°C to obtain the fluffy voluminous whitish mass of polymer capped precursor powders. Refined ZnO or Ni-doped ZnO nanoplatelets are obtained by heat treating the polymer precursor powders, after pulverizing by grinding in a mortar with a pestle, at 400-600°C in ambient air for 2 h. The heat treatment temperature is decisive in controlling the final size of the particles. The same procedure is followed to obtain a series of pristine and Ni-doped ZnO samples (Zni-_xNi_xO, x = 0-0.1).

[0094] The structural and morphological properties of the derived samples were studied with X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and high resolution transmission electron microscope (HRTEM).

[0095] FIG. 3a is showing the X-ray diffractograms of the pristine ZnO nanoplatelets after heating a Zn²⁺-poly-vinyl alcohol-sucrose polymer precursor powder at selected temperatures, according to an embodiment of the present invention. With respect to FIG. 3a, (i) is for precursor powder, (ii) is for the pristine ZnO nanoplatelets obtained after heating the Zn²⁺- poly-vinyl alcohol- sucrose polymer precursor powders at 400°C and (iii) is for the pristine ZnO nanoplatelets obtained after heating the Zn²⁺-poly-vinyl alcohol-sucrose polymer precursor powders at 500°C for 2 h in ambient air.

[0096] FIG. 3b is showing the X-ray diffractograms of various Ni-doped ZnO nanoplatelets, according to an embodiment of the present invention. With respect to FIG. 3b, (iv) is for Zno.99Nio.01O, (v) is for Zno.9sNio.02O, (vi) is for Zno.95Nio.05O, (vii) is for Zno.93Nio.07O, and (viii) is for Zno.90Nio.10O nanoplatelets after heating the respective Zn²⁺-Ni²⁺-polymer precursors at 500°C for 2 h in ambient air (* denotes Ni peak).

[0097] The XRD analysis revealed the formation of ZnO crystallites with hexagonal wurtzite type (JCPDS card no. 036-1451) crystal structure without any detectable impurity phases (Ni concentration, x < 0.1). The crystallite size calculated from XRD peak widths and Debye Scherrer s formula comes out to be around 10-15 nm in the Ni-doped ZnO samples heat treated at 500°C.

[0098] FIG. 4a and FIG. 4b are the HRTEM images of the pristine ZnO nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention. With respect to FIG. 4a, the scale is at 50 nm. With respect to FIG. 4b, the scale is 2 nm. The monodispersed nanoplatelets can be seen in these HRTEM images. The interplanar distance is found to be approximately 0.25 nm which corresponds to the (002) plane of ZnO.

[0099] FIG. 4c, FIG. 4d and FIG. 4e are the HRTEM images of the Zno.99Nio.01O nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention. With respect to FIG. 4c, the scale is 20 nm. With respect to FIG. 4d, the scale is 5 nm. With respect to FIG. 4e, the scale is 5 nm. The nanoplatelets can be clearly seen. The interplanar distance is found to be approximately 0.26 nm which corresponds to the (002) plane of ZnO.

[00100] FIG. 4f and FIG. 4g are the HRTEM images of Zno.9sNio.02O nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention. With respect to FIG. 4f, the scale is 50 nm. With respect to FIG. 4g, the scale is 20 nm. The monodispersed nanoplatelets can be clearly seen in these HRTEM images.

[00101] The HRTEM images confirm the uniformity in the shape and size of the finely dispersed particles in the form of hexagonal nanoplatelets.

[00102] FIG. 5a, FIG. 5b and FIG. 5c are the FESEM images of the Zno.9sNio.02O nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention. With respect to FIG. 5a and FIG. 5b, the scale is 200 nm while with respect to FIG. 5c, the scale is 20 nm. The nanoplatelets can be clearly seen.

[00103] FIG. 5d and FIG. 5e are the FESEM images of the pristine ZnO nanoplatelets obtained after heating the corresponding precursor at 500°C for 2 h in ambient air, according to an embodiment of the present invention. With respect to FIG. 5d, the scale is 200 nm while with respect to FIG. 5e, the scale is 20 nm. The nanoplatelets can be clearly seen. [00104] The effects of Ni doping on the optical, magnetic, and electrical properties of the derived TM-doped ZnO nanoplatelets are analyzed with UV-vis reflectance and photoluminescence spectroscopy, vibrating sample magnetometer (VSM), and a physical quantities measurement system (PQMS).

[00105] FIG. 6a shows the room temperature magnetic hysteresis curves of the nanoplatelets obtained after heating the corresponding precursor powders at 400°C for 2 h in ambient air, according to an embodiment of the present invention. With respect to FIG. 6a, (a) is for Zno.9sNio.02O, (b) is for Zno.95Nio.05O, (c) is for Zno.93Nio.07O and (d) is for Zno.90Nio.10O.

[00106] FIG. 6b shows the room temperature magnetic hysteresis curves of the nanoplatelets obtained after heating the corresponding precursor powders at 500°C for 2 h in ambient air, according to an embodiment of the present invention. With respect to FIG. 6b, (a) is for Zno.99Nio.01O, (b) is for Zno.9sNio.02O, (c) is for Zno.95Nio.05O, (d) is for Zno.93Nio.07O, and (e) is for Zno.90Nio.10O.

[00107] All the TM-doped samples show hysteretic behaviour in their magnetization

(M) versus magnetic field (H) plots, confirming the induced RTFM in the samples. The structural, optical, electrical, and magnetic properties of the Ni-doped ZnO samples derived after heat treating the corresponding precursors at 500°C are precisely given in Table 2. All the values are reproducible and highly attractive for pertinent applications.

Table 2: Structural, magnetic, optical, and electrical properties of Ni-doped ZnO nanoplatelets

Ni0.₀₁zn₀.99o 0.3258 0.5226 1.6040 48.04 11.24 11 11.2 1.48 73.59 3.20 11.76

Nio.₀₂zno.9so 0.3258 0.5225 1.6037 48.03 11.23 12 11.6 1.14 43.72 3.19 10.16

Nio.₀₅zno.950 0.3256 0.5226 1.6050 47.98 11.21 12 12.4 1.74 95.69 3.22 9.62

Nio.₀₇zno.930 0.3258 0.5229 1.6049 48.06 11.18 10 11.9 1.37 73.83 3.22 9.47

Ni₀.ioZn₀.9oO 0.3260 0.5228 1.6036 48.11 11.14 10 24.9 2.60 79.20 3.23 8.86

[00108] As mentioned before, this synthesis technique has been followed to develop a series of TM-doped ZnO (viz, Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf) samples with controlled morphology. In all the samples, the structural, optical, electrical and magnetic properties are highly reproducible in nature.

[00109] In general, the method developed in this invention has many advantages and benefits that can make it a success in production of TM-doped ZnO of controlled morphology. This cost effective method deals with a low-temperature reaction in water with commercially available economic raw materials. The final finish products appearing in the form of finely divided loose powders with striking RTFM properties offer great opportunities for the processing of TM-doped ZnO based devices and components in desired shapes.

[00110] The present invention provides a novel method towards the mass scale synthesis of highly pure shape-controlled pristine as well as TM-doped ZnO nanoparticles. The method described in the present invention provides an excellent and comparatively easy way of controlled doping of TM ions in such wide band gap semiconductor oxides. The method is cost effective which deals with a low -temperature reaction in water with commercially available economic raw materials. Further the final finish products appear in the form of finely divided loose powders with striking RTFM properties, which offer great opportunities for pertinent applications in spintronics, magneto -optics, biosensors, medical imaging, drug delivery, etc.

[00111] The embodiments of the present invention also provide a method of synthesizing ZnO-based core-shell nanoparticles. The core-shell nanoparticles are made of metal oxides and metals. The core-shell nanoparticles are synthesized using a sol-gel type chemical reaction technique.

[00112] According to an embodiment of the present invention, a method is provided for the synthesis of core-shell nanoparticles. The core shell nanoparticles comprise of ZnO core and metallic shell wherein the metal is selected from the group consisting of zinc metal and silver metal.

[00113] After carrying out the sol-gel type chemical reaction using aqueous solutions of poly-vinyl alcohol, sucrose and zinc salt, the Zn²⁺- poly-vinyl alcohol- sucrose polymer precursor powders are obtained. These Zn²⁺- poly-vinyl alcohol-sucrose polymer precursor powders are further explored for the synthesis of ZnO : Zn or ZnO : Ag nanoparticles.

[00114] According to one embodiment of the present invention, the key part of the process of synthesizing the core-shell nanoparticles is the use of the ZnO-polymer precursor powders as templates to obtain the ZnO:metal core-shell nanostructures.

[00115] The surface modified ZnO nanoparticles synthesized in the present invention show several striking properties including colossal enhancement of UV emission primarily due to the surface plasmon resonance (SPR) effect, giving rise to excellent photocatalytic/photo-degradation efficiency. Furthermore, a convenient synthesis technique of ZnO : Zn core-shell nanoparticles, a highly attractive redox pair for solar-driven H₂0 and C0₂ thermo-splitting cycles, is also provided in the present invention.

[00116] This application is an extension or improvement or advancement of the previously filed Indian patents having application no. 1275/Del/2014, dated May 12, 2014 and application no. 3409/DEL/2015, dated Oct. 21, 2015 by the applicant. The reported synthesis process of pristine ZnO nanoparticles is further explored in a systematic way to develop homogeneous and heterogeneous semiconductor-metal core-shell nanoparticles of ZnO:Zn and ZnO:Ag.

[00117] The present invention also discloses the effects of metal coatings on the structural, morphological and optical properties of the ZnO-based core-shell nanomaterials. The significantly enhanced UV emission properties in the derived ZnO:Ag nanoparticles has been achieved, while the convenient synthesis process of ZnO:Zn redox pair promises its applications in water- splitting thermo-chemical cycles. [00118] The present invention involves a convenient synthesis technique of ZnO-based core-shell nanoparticles for potential applications in diverse fields including chemical sensors, catalysts for synthesis of various gases, photo catalytic applications and highly efficient light emitting diodes (LEDs). Following a chemical synthesis method, finely dispersed homogeneous and heterogeneous semiconductor-metal systems of ZnO:Zn and ZnO:Ag core- shell nanoparticles are developed. These nanostructures show surface plasmon resonance (SPR) effects arising due to the thin uniform surface coating of metals on the ZnO nanoparticles. The enhanced UV emission from these tailored semiconductors opens up the possibility of developing highly efficient optoelectronic devices. Apart from tuneable luminescence, these semiconductor-metal nanoparticles are highly attractive for catalytic and sensor applications.

[00119] FIG. 7 is a flow chart showing the method of synthesizing the core-shell nanoparticles, according to an embodiment of the present invention. With respect to FIG. 7, the method comprises of the step of reacting a solution of a metal salt with an aqueous solution of a polymer and a sugar under continuous stirring at a predetermined temperature to obtain a mixture (301). A plurality of polymer precursors at a predetermined pH from the mixture are obtained (302) and further treated to obtain the core-shell nanoparticles (303). The core-shell nanoparticles are ZnO : metal nanoparticles. The core is made up of zinc oxide and the shell is made up of a metal. The metal is selected from the group consisting of zinc metal and silver metal.

[00120] According to an embodiment of the present invention, the step of obtaining the

ZnO:Ag core-shell nanoparticles (303) is performed by dispersing the obtained plurality of ZnO-polymer precursors in an aqueous solution of silver nitrate while continuous stirring for 20-30 minutes in dark at a temperature of 50-60°C and obtaining a dry powder by washing the solution and drying in a reduced pressure at 20-25 °C. Then, heating the dry powder at 400- 500°C for 2 h in ambient air to obtain ZnO : silver core-shell nanoparticles.

[00121] According to another embodiment of the present invention, the step of obtaining the ZnO:Zn core-shell nanoparticles (303) is performed by heating the dried ZnO- polymer precursor powders at 500°C for 2 h in ambient air to obtain recrystallized ZnO nanoparticles and further heating the obtained recrystallized ZnO nanoparticles at 1000°C in an autoclave at 2 bar pressure in an atmosphere of 5% H₂-95% Ar gas to obtain ZnO : Zn core-shell nanoparticles.

[00122] According to the embodiments of the present invention, the plurality of polymer precursors is ZnO-poly-vinyl alcohol-sucrose polymer precursor powder.

[00123] According to the embodiments of the present invention, the solution of metal ion is a zinc salt solution.

[00124] According to the embodiments of the present invention, the polymer is polyvinyl alcohol.

[00125] According to the embodiments of the present invention, the sugar is sucrose.

[00126] Synthesis procedure: The synthesis method of ZnO : Ag and ZnO : Zn nanoparticles consist of two broad stages. The first stage is the step of synthesizing ZnO precursor powders. In the next stage, the precursor powders were further processed to form the ZnO : metal (metal = Zn, Ag) core-shell nanoparticles.

[00127] Materials: The materials used in the synthesis process were zinc nitrate hexahydrate [mol. wt. = 297.47, 99.98% pure], silver nitrate [mol. wt. = 169.9, 99.99% pure], poly-vinyl alcohol [mol. wt. -96800, degree of polymerization -2000], 25% ammonia solution, and sucrose (99% pure).

[00128] Method of synthesis of ZnO : Ag core-shell nanoparticles: As mentioned before, the ZnO precursor powders were explored to synthesize silver coated ZnO nanoparticles. A batch of 5.0 g powder of the dried ZnO precursor powders were dispersed in an aqueous solution of AgN0₃ (0.25 M, 100 ml) with continuous magnetic stirring at 50-60°C in dark. After 20-30 min of reaction at this temperature, the powders were recovered by decanting out the AgN0₃ solution followed by washing in hot water at least 2-3 times and drying in a reduced pressure (10-100 mbar) at 20-25 °C. Finally, the dried powders were heat treated at 400-500°C for 2 h in ambient air. The recrystallized ZnO nanoparticles so obtained are now capped with a stable thin shell layer of Ag.

[00129] A solution-processed two step chemical synthesis method is explored for the synthesis of ZnO : Ag core-shell nanoparticles. The crystalline structure, morphology and other physical properties of the derived Ag-coated ZnO nanoparticles were analyzed with standard characterization techniques. FIG. 8 is a schematic illustration of the formation of Ag-coated ZnO nanoparticles, according to an embodiment of the present invention. With respect to FIG. 8, the schematic model of the growth mechanism of ZnO:Ag core-shell nanoparticles can be seen. In the sol-gel type chemical reaction, the Zn²⁺ ions get trapped in the poly-vinyl alcohol polymer as shown by (401) and forms a Zn²⁺-poly-vinyl alcohol complex structure. After the addition of silver nitrate, the Ag²⁺ cations align as shown in (402). The Zn²⁺ cations remain inside the poly- vinyl alcohol micelles while the Ag²⁺ ions align on the surface of the poly-vinyl alcohol micelles. On heating, the ZnO:Ag core-shell nanoparticles are obtained wherein the core is made up of ZnO while the shell is made up of Ag as shown by (403).

[00130] The chemical reactions involved in the synthesis process are given below. As described previously, the poly-vinyl alcohol is used to precisely control the reductive oxidation of the metal salt. With respect to FIG. 8 and the given reactions, when the Zn salt solution is added to the poly-vinyl alcohol-sucrose solution, zinc hydroxide anions (zincates) are formed and get trapped inside the polymer micelles (401).

(Zn²⁺ + NO3^2") + 4(NH₄ ⁺ + OH^")→ Zn(OH)₄ ² + 4NH₃† + HN0₃

Zn(OH)₄ ^2" + poly- vinyl alcohol→ Zn(OH)₄ ^2~- poly-vinyl alcohol (3)

[00131] In the second step, the zinc -poly-vinyl alcohol complexes acted as the reaction centre for the reduction and dehydrolysis of silver hydroxide anions followed by nucleation of Ag atoms on ZnO surface.

(Ag⁺ + NO3^") + 20H→ Ag(OH)₂ ^2"

Ag(OH)₂ ^2" + Zn(OH)₄ ^2"- poly-vinyl alcohol → Zn(OH)₄ ² - poly-vinyl alcohol - Ag(OH)₂ ²-

Zn(OH)₄ ^2"- poly-vinyl alcohol - Ag(OH)₂ ^2"→ heat treatment→ ZnO:Ag core-shell nanoparticles (4)

[00132] The as-derived ZnO:Ag core-shell nanoparticles were characterized with standard instrumental techniques. The crystal structure of the derived nanoparticles was analysed with an X-ray diffractometer of Panalytical s X Pert Pro using CuKa radiation of 0.1545 nm. FIG. 9 shows the XRD patterns of the pristine ZnO nanoparticles derived after heating at 400°C (shown by (a)) and at 500°C (shown by (b)) for 2 h in ambient air (this is already described), while (c) shows the XRD pattern of Ag-coated ZnO precursor powders, and (d) shows the XRD pattern of recrystallized Ag-coated ZnO nanoparticles after heat treating the precursor powders at 400°C and (e) shows the XRD patterns of recrystallized Ag- coated ZnO nanoparticles after heat treating the precursor powders at 500°C for 2 h in ambient air, according to the embodiments of the present invention. With respect to FIG. 9, the formation of ZnO crystallites with hexagonal wurtzite type (JCPDS card no. 036-1451) crystal structure along with the intense peaks of cubic Ag (JCPDS card no. 04-0783) can be observed in the X-ray diffracto grams. The average crystallite size is estimated (from XRD peak widths) to be 12 nm and 21 nm in pristine ZnO and ZnO:Ag core- shell nanoparticles processed at 400°C, which increase to 13 nm and 22 nm in pristine ZnO and ZnO:Ag nanoparticles, respectively when processed at 500°C. The lattice parameters were calculated from the Rietveld refinement of the XRD data. The peak broadening analysis was also extended to calculate the lattice strain in the pristine ZnO and Ag-coated ZnO. FIG. 10A and FIG. 10B show the Williamson-Hall plots of pristine and Ag-coated ZnO nanoparticles drawn to estimate lattice strain values in the derived nanoparticles where FIG. 10A shows the Williamson-Hall plot of pristine ZnO and FIG. 10B shows the Williamson-Hall plot of Ag- coated ZnO nanoparticles. Both samples were synthesized at 500°C. The XRD results are precisely shown in Table 3.

Table 3 : Comparison of the structural properties of pristine and Ag-coated ZnO nanoparticles

[00133] With respect to Table 3, the average crystallite size, lattice constants and lattice strain in the pristine and Ag-coated ZnO nanoparticles can be seen and compared.

[00134] The morphology of the derived nanoparticles was analyzed with a Carl Zeiss

SUPRA 55 FESEM. FIG. 11A and FIG. 11B are the FESEM images of pristine ZnO processed at 500°C in ambient air while FIG. 11C and FIG. 11D are the FESEM images of Ag-coated ZnO nanoparticles processed at 500°C in ambient air and FIG. HE shows the EDS spectrum of Ag-coated ZnO nanoparticles, according to the embodiments of the present invention. Loose agglomerates of typical size 40-60 nm can be seen in the pristine ZnO samples (FIG. HA and FIG. 11B). A close up view reveals that the agglomerates are essentially consisting of several tiny particles of sizes 15-20 nm. Few of the agglomerates have grown bigger by forming clusters. The Ag-coated ZnO also show similar type of loose agglomerates, but unlike pristine ZnO, they are much bigger in size, typically 500 nm-ΐμιη (FIG. 11C). A magnified view reveals flower like morphologies in the particle clusters (FIG. 11D). With respect to FIG. HE, the EDS spectrum displays the peaks corresponding to the elements Zn, O and Ag and no other impurity peaks were detected. The analysis of data yields 34.81 at.% of zinc, 61.05 at.% of oxygen and 4.14 at.% of silver composition in the ZnO:Ag core-shell nanoparticles. With respect to FIG.llE, the high purity of the ZnO:Ag core-shell nanoparticles was confirmed.

[00135] The microstructure in the derived nanoparticles was further analyzed with a

JEOL-2100 HRTEM. FIG. 12A and FIG. 12B are the HRTEM images of pristine ZnO nanoparticles processed at 500°C in ambient air while FIG. 12C and FIG. 12D are the HRTEM images of Ag-coated ZnO nanoparticles processed at 500°C, according to the embodiments of the present invention. With respect to FIG. 12A-FIG.12D, the micrographs clearly show the existence of a uniform thin layer of Ag on the ZnO nanoparticles. In the pristine samples, finely dispersed ZnO nanoparticles with an average size of 20 nm can be observed. The Ag-coated ZnO nanoparticles are comparatively bigger in size with an average size of 25 nm. Also, the difference in the contrast at the edges of the particles due to the surface coating of the thin silver layer can be clearly observed, which was further confirmed by measuring the interplanar spacings in the highly magnified HRTEM images as seen in FIG. 12B and FIG. 12D. In FIG. 12B of ZnO nanoparticles, an interplanar spacing of 0.26 nm was observed, which corresponds to the (002) plane of ZnO crystal. In FIG. 12D of Ag- coated ZnO nanoparticles interplanar spacing of 0.24 nm was observed, which corresponds to the (111) planes of metallic silver.

[00136] The effects of Ag coating on the optical properties of the derived Ag-coated

ZnO nanoparticles were analyzed with UV-visible diffuse reflectance spectroscopy and photoluminescence spectroscopy. UV-visible reflection spectra of the samples were recorded with a Shimadzu UV-2450 spectrometer. FIG. 13A shows the UV-visible reflectance spectra and FIG. 13B shows the corresponding (ahv)² versus hv plots in pristine and Ag-coated ZnO nanoparticles processed at 500°C in ambient air, according to the embodiments herein. The band gap values calculated to be 3.2 eV and 3.1 eV in pristine ZnO and Ag-coated ZnO nanoparticles, respectively. The observed red shift in the band gap value of ZnO:Ag is directly indicating towards a strong interfacial coupling between Ag and ZnO in the hetero structure.

[00137] Room-temperature photoluminescence spectra of the samples were obtained with a Perkin Elmer LS 55 spectrophotometer at an excitation wavelength of 300 nm. FIG. 14A and FIG. 14B show the room-temperature photoluminescence spectra of pristine and Ag- coated ZnO nanoparticles processed at 400°C and 500°C, respectively, according to the embodiments of the present invention. With respect to FIG. 14A and FIG. 14B, it was observed that the Ag-coated ZnO nanoparticles show enhanced (in comparison to pristine ZnO nanoparticles of similar size) UV emission at 389 nm wavelength with considerable suppression of the visible emission at 468 nm. The observed changes in the photoluminescence of the ZnO:Ag nanoparticles are attributed to the increased electron density in the conduction band of ZnO through the localized surface plasmons in the ZnO:Ag nanoparticles.

[00138] The mechanism of charge transfer between ZnO and Ag is schematically shown in the band energy diagrams in FIG. 15. FIG. 15A and FIG. 15B represent the energy band diagrams in pristine and Ag-coated ZnO nanoparticles, respectively, according to the embodiments of the present invention. The electron affinity and the work function of ZnO are 4.3 eV and 5.2 eV, respectively, whereas the work function of Ag is about 4.1 eV. Therefore, the Fermi energy level of ZnO is formed at lower energy level than that of Ag. As a result, electron transfer from Ag to ZnO takes place at the core-shell interface until the two systems attain equilibrium. The increased electron density at the conduction band of the ZnO leads to the increase in radiative transition rate leading to the enhancement in UV emission from the ZnO:Ag core-shell nanoparticles.

[00139] The chemical states of the elements in the ZnO-Ag core-shell nanoparticles were analysed with X-ray photoelectron spectroscopy (XPS). FIG. 16A-FIG.16D show the XPS spectra of ZnO-Ag core-shell nanoparticles, according to the embodiments of the present invention. FIG. 16A shows the XPS core level full spectrum of Ag-coated ZnO nanoparticles processed at 500°C. The bands of Ag3d, Zn2p and Ols were further deconvoluted to analyse the chemical states of the elements present in the ZnO:Ag composite structure. The details of the XPS bands are given in Table 4.

Table 4 : XPS bands observed in the Ag-coated ZnO nanoparticles processed at 500°C

[00140] With respect to Table 4, the binding energy (Eb) and full width at half peak maximum (FWHM) values of the observed XPS bands in the ZnO:Ag nanoparticles derived at 500°C can be studied. With respect to FIG. 16B, the Zn 2p₃/2 and 2pm bands were observed at 1022.31 eV and 1045.40 eV, respectively. With respect to FIG. 16C, the Ag 3d₅/2 and 3d _/2 bands were observed at 368.76 eV and 374.76 eV, respectively. The Ag 3d bands confirm the presence of metallic silver (Ag°). No peaks corresponding to Ag₂0 and AgO were observed. With respect to FIG. 16D, the deconvoluted Ols band shows two symmetrical peaks, indicating two different kinds of oxygen species in the samples. The peak centered at 530.64 eV and the other peak at 532.82 eV were assigned to the core level oxygen of ZnO and the chemisorbed hydroxyl groups (OH ), respectively.

[00141] Synthesis method of ZnO:Zn core-shell nanoparticles: The process for the synthesis of ZnO:Zn core-shell nanoparticles is described here in detail. A batch of dried ZnO precursor powders were heated at 500°C for 2 h in ambient air to obtain recrystallized ZnO nanoparticles. The derived ZnO nanoparticles were subjected to a heat treatment at 1000°C in an autoclave at 2 bar pressure in an atmosphere of 5% H₂-95% Ar gas.

[00142] The pristine ZnO nanoparticles were loaded in alumina boats and placed inside a horizontal cylindrical autoclave furnace. Few glass substrates were placed near the outlet end of the chamber at an angle of 45° with respect to the tube axis to get the final products deposited on them. The chamber was filled with 5% H₂-95% Ar gas up to a pressure of 2 bar and the temperature was raised to 1000°C. The temperature and pressure inside the chamber was maintained for 2 h. The gas was flushed out at a regular interval of 20 min followed by reintroduction of fresh gas. After 2 h, the tube furnace was cooled down to room temperature keeping the gas pressure at 2 bar as it is. The deposited core-shell type ZnO:Zn nanoparticles were collected from the substrates with a sharp metallic blade and were characterized with XRD, UV- visible spectroscopy, HRTEM, and XPS. The XRD and UV-visible spectroscopic analyses are incorporated herewith.

[00143] FIG. 17 shows the XRD patterns of ZnO nanoparticles and ZnO:Zn nanoparticles, wherein (a) shows XRD patterns of pristine ZnO nanoparticles synthesized at 500°C in ambient air while (b) shows the XRD patterns of ZnO:Zn composite nanoparticles deposited on Si substrates after heating the pristine ZnO samples at 1000°C in an atmosphere of 5% H₂-95% Ar gas at 2 bar pressure, according to the embodiments of the present invention. With respect to FIG. 17, the XRD patterns of pristine ZnO nanoparticles (processed at 500°C in ambient air) with that of the ZnO:Zn nanocomposites derived (on Si substrates) after heating the pristine ZnO samples at 1000°C in an atmosphere of 5% H₂-95% Ar gas at 2 bar pressure are compared. In the ZnO:Zn samples, diffraction peaks corresponding to metallic Zn (JCPDS card # 04-0831) can be seen along with the ZnO (JCPDS card # 036-1451) peaks.

[00144] FIG. 18 shows the UV-visible absorption spectra of ZnO nanoparticles and

ZnO:Zn nanoparticles, where (a) shows the pristine ZnO nanoparticles synthesized at 500°C in ambient air while (b) shows the derived ZnO:Zn composite powders after heating the pristine ZnO samples at 1000°C in an atmosphere of 5% H₂-95% Ar gas at 2 bar pressure, according to the embodiments of the present invention. With respect to FIG. 18, the pristine ZnO shows two characteristics absorption peaks at 359 nm and 380 nm. The absorption peak at 359 nm corresponds to the near-band edge absorption and the peak at 380 nm corresponds to defects or electron trap level transitions in ZnO. In contrast, the spectrum of ZnO:Zn nanocomposite has five absorption maxima at 206 nm, 290 nm, 335 nm, 348 nm and 380 nm, as shown in (b). The peak at 206 nm is due to the transition of the inner shell electrons of Zn to the conduction band, which is commonly observed in the UV-visible absorption of metal- metal oxide nanoparticles. The absorption peaks at 290 nm and 335 nm are due to the surface plasmon absorptions of Zn, while that of the peaks at 348 nm and 380 nm correspond to the near-band edge and defect level transitions in ZnO, respectively in the ZnO : Zn composite system. It can be clearly observed that the surface plasmon transitions of metallic Zn are absent in the pristine ZnO nanoparticles. Also the near-band edge absorption of ZnO in ZnO:Zn composites shows blue shift compared to pristine ZnO. This shift in the near-band edge absorption of ZnO is attributed to the size confinement effect of ZnO in the ZnO:Zn core-shell nanoparticles.

[00145] The present invention provides a novel, highly efficient and easier method of mass scale synthesis of ZnO:metal core-shell nanoparticles. This approach facilitates a simple method of synthesizing ZnO:Ag core-shell nanoparticles and similar type of semiconductor- metal heterostructures. The core-shell structures possess an excellent interfacing between semiconductor (ZnO) and metals (Zn, Ag) for pertinent applications in wide domains including photo-catalysis, plasmonics, optoelectronics, chemical sensors, and waste water management.

[00146] The embodiments of the present invention further extended for the synthesis of monodispersed nanoparticles of NiZnFe₂0₄ soft ferrites with tailored properties. The controlled growth of the ferrite nanoparticles is achieved by encapsulation of the nucleating sites in the poly-vinyl alcohol- sucrose polymer micelles. The said ferrite nanoparticles show the desired structural, magnetic and electrical properties for applications in broad areas comprising sensors and antennas, power applications, high-performance broad-band radar absorbing materials (RAMs), as well as in other fields where reduction of undesired electromagnetic interference (EMI) in electrical equipments operated at high frequencies and solutions to electromagnetic compatibility problems are necessary.

[00147] The present invention explores a facile approach towards the synthesis of the magnetic nanoparticles of Ni-Zn ferrites (Ni_xZni-_xFe20₄, x = 0.2-0.8) with tailored properties following a modified version of the novel chemical synthesis technique described in detail in the Indian patents (application no. 1275/Del/2014, filed on May 12, 2014 and application no. 3416/Del/2015, filed on Oct. 21, 2015).

[00148] According to an embodiment of the present invention, the method of synthesizing Ni-Zn nanoparticles comprises the steps of (i) reacting aqueous solutions of metal salts of zinc, nickel, and iron with an aqueous solution of poly-vinyl alcohol and sucrose at 60-65°C to obtain precipitates. The reaction precipitates are separated and dried in microwave oven to derive the amorphous precursor powders. The precursor powders are heat treated at selected temperatures in the range 400-600°C in ambient air to obtain the recrystallized NiZnFe₂0₄ nanoparticles.

[00149] According to the embodiments of the present invention, the novelty lies in the use of the controlled reducing medium of poly-vinyl alcohol and sucrose to produce the NiZnFe₂0₄ nanoparticles with tailored composition, morphology and size distribution. The synthesized soft ferrite nanoparticles have important applications in broad areas associated with filtering, transforming, absorbing or concentrating electromagnetic signals.

[00150] FIG. 19 is a flow chart showing the method of synthesizing soft magnetic Ni-

Zn ferrite nanoparticles with different Ni-Zn contents, according to an embodiment of the present invention. With respect to FIG. 19, the method of synthesizing soft magnetic Ni-Zn ferrite nanoparticles with different Ni-Zn contents comprises preparing an aqueous solution of zinc salt, an aqueous solution of nickel salt and an aqueous solution of iron salt in stoichiometric amounts (501). The aqueous solutions of zinc salt, nickel salt and iron salt are dispersed into an aqueous solution of a polymer and a sugar at a predetermined temperature under a constant stirring to obtain a mixture (502). A solid mass of polymer precursor powder is obtained at a predetermined pH by drying the mixture in a microwave oven (503). Recrystallized Ni-Zn ferrite nanoparticles are obtained by heating the polymer precursor powder at 400-600°C in ambient air (504).The zinc salt is zinc nitrate hexahydrate. The nickel salt is nickel nitrate hexahydrate. The iron salt is iron nitrate nonahydrate. The predetermined temperature is 60-65°C. The predetermined pH is 9. The predetermined pH is maintained by adding 25% of ammonium hydroxide solution to the mixture. The polymer is poly-vinyl alcohol and the sugar is sucrose. The aqueous solution of the polymer and the sugar is prepared by adding said polymer and the sugar in a ratio of 1: 10 by weight in water. The soft magnetic Ni-Zn ferrite nanoparticles are derived with a varied Ni and Zn contents as Ni_xZni_ xFe₂0₄, wherein x = 0.2-0.8.

[00151] According to another embodiment of the present invention, the Ni-Zn ferrite magnetic nanoparticles are obtained in various chemical concentrations. The Ni-Zn ferrite nanoparticles have a molecular formula of Ni_xZni-_xFe₂0₄, wherein x varies from 0.2 to 0.8.

[00152] According to one embodiment of the present invention, high purity in the Ni_xZni-xFe₂0₄ (x=0.2-0.8) nanoparticles synthesized by a novel chemical method via a polymer precursor is achieved. The synthesis route followed provides a precise control over the crystal growth and the final size distribution in the derived nanoparticles.

[00153] Materials Required : The materials used in the synthesis process were zinc nitrate hexahydrate [mol. wt. = 297.47, 99.98% pure], nickel nitrate hexahydrate [mol. wt. = 290.81, 99.95% pure], and iron nitrate nonahydrate [mol. wt. = 404, 99.99% pure]. All the salts were procured from Sigma Aldrich and used without further purifications. The polyvinyl alcohol [mol. wt. -96800, degree of polymerization -2000], 25% ammonia solution, and sucrose (99% pure) were purchased from Thermo Fisher Scientific.

[00154] Synthesis Method : The key step in the synthesis process is to control the growth of the nanoparticles in the reaction solution by encapsulation in the polymer micelles { Chem. Phys. 306, 163-169, 2004}. To synthesize Ni_xZni-_xFe₂0₄ ferrite nanoparticles, aqueous solutions of Ni²⁺, Zn²⁺ and Fe³⁺ ions in stoichiometric amounts were dispersed in the poly-vinyl alcohol- sucrose solution at 60-65°C with continuous magnetic-stirring. The polyvinyl alcohol is a good capping agent and it serves as a stabilizing medium for the nascent nanoparticles inside the short polymer micelles {AIP Conf. Proc. 1536, 283-284, 2013 }. It was observed that a basic medium supports the reductive-oxidation of the metal cations in the solution. After the reaction, the solution was aged at room temperature for 12 h. The obtained precipitate was dried in ambient air and the solid mass was heat treated in a microwave oven to derive the dried amorphous precursor powders. Refined ferrite nanoparticles were obtained after heat treating the precursor powders in covered silica crucibles in a muffle furnace at selected temperatures in the range 400-600°C in ambient air for selected time periods up to 6 h.

[00155] Details of the processing technique are described below for a specific composition, viz., Nio.2Zno.₈Fe20₄.

[00156] A transparent aqueous solution of poly-vinyl alcohol (100 ml, 4%) was prepared by magnetically stirring for 24 h. After continuous stirring, all the three phases of poly-vinyl alcohol [viz., isotactic, syndiotactic and atactic configurations] are still present in the solution but the syndiotactic phase appears as the most prominent due to its stable stereochemical configuration. The syndiotactic phase forms stable micelles, required for the controlled growth of the nanoparticles {Chem. Phys. 306, 163-169, 2004}. In the next step, the sucrose solution (50 ml, 40%) was added to the poly-vinyl alcohol solution. The mixed solution was stirred for another 3-4 h and its temperature was raised to 60-65°C.

[00157] The Ni²⁺, Zn²⁺ and Fe³⁺ solutions were prepared by dissolving the metal salts in stoichiometric amounts in water. In this specific example of Nio.2Zno.₈Fe20₄, the molarity ratio between Fe and Ni-Zn salts is maintained to be 2 : 1. Aqueous solutions of 0.2 M Fe-salt, 0.02 M Ni-salt and 0.08 M Zn-salt were prepared. The three solutions were slowly dispersed in the poly-vinyl alcohol-sucrose solution drop wise with continuous stirring. The pH during the reaction was maintained at ~9 by adding ammonium hydroxide solution in required amount. An optimized value of pH~9 effectively forms stable metal hydroxide anions in the solution. The as-formed hydroxide anions trapped in the polymer micelles, act as the coordinating sites for a stable growth of the oxide nanoparticles. After the reaction, the solution was aged at room temperature for 24 h. The obtained precipitate was dried at around 60°C and finally the solid mass was heat-treated in a microwave oven to derive the amorphous precursor powders. Recrystallized nanoparticles of Nio.2Zno.₈Fe20₄ were obtained by heat treating the precursor powders at selected temperatures in the range 400-600°C in ambient air for 2 h. The derived nanoparticles were washed with distilled water to remove any residual impurities.

[00158] A series of Ni_xZni-_xFe20₄, x = 0.2-0.8 nanoparticles were prepared following the same synthesis procedure with the metal salt solutions in the required stoichiometric amount. [00159] Sample Characterization : The derived Ni-Zn ferrite (Ni_xZni-_xFe20₄, x=0.2-

0.8) nanoparticles were studied with different characterization techniques.

[00160] The crystalline structure of the derived nanoparticles was studied with X-ray diffraction (XRD). The micro structural details in the derived samples were evaluated using scanning electron microscope (SEM) and transmission electron microscope (TEM). The magnetic properties of the derived samples were studied with vibrating sample magnetometer (VSM). The X-ray absorption spectroscopic (XAS) studies were performed to investigate the cationic distribution and the degree of inversion at lattice sites in the Ni_xZni-_xFe20₄ (x=0.2- 0.8) ferrite structure. The XAS is the most suitable technique for structural characterization of the local atomic environment of individual atomic species in such type of complex crystal lattice. The structural investigations were further extended to explain the effects of cationic distribution on the magnetic properties of the derived nanoparticles having mixed spinel structure.

[00161] The chemical structure of the derived Ni_xZni-_xFe20₄ (x=0.2-0.8) nanoparticles were analysed with micro Raman Horiba Jobin-Yvon HR800 spectrometer. The crystalline structure of the derived samples was evaluated with PanalyticaFs X^{^}Pert Pro X-ray diffractometer using CuK_a radiation of 0.1545 nm. The morphological analysis of the derived nanoparticles was performed with scanning electron microscope (SEM) and transmission electron microscope (TEM). X-ray absorption spectroscopic (XAS) measurements were carried out to probe the local atomic environment of individual atomic species in the ferrite samples. These studies were conducted with the scanning EXAFS beam line (BL-09) at the INDUS-2 synchrotron source (2.5 GeV, 125 mA) at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India. Room temperature magnetic characterizations of the samples were performed using a Lakeshore 7410 vibrating sample magnetometer (VSM) with a maximum applied field of 18 kOe. Dielectric measurements were performed at room- temperature using an impedance analyzer of Novocontrol Technology in the frequency range 1 Hz to 10 MHz.

[00162] FIG. 20 shows the Raman spectra of the derived nickel-zinc ferrite nanoparticles after heating the corresponding precursor powders at 500°C for 2 h in ambient air, wherein (a) shows Raman spectrum of Nio.2Zno.₈Fe20₄, (b) shows Raman spectrum of Nio.₄Zno.6Fe20₄, (c) Raman spectrum of Nio.5Zno.sFe20₄, (d) Raman spectrum of Nio.6Zno.₄Fe20₄, and (e) Raman spectrum of Nio.sZno.2Fe20₄ nanoparticles, according to the embodiments of the present invention. With respect to FIG. 20, the Raman analysis confirmed the cubic spinel structure of the Ni-Zn ferrite samples of space group Oh⁷ (Fd3m). The Raman spectra were recorded at room-temperature using He-Ne (632 nm) laser as the excitation source. The different Raman vibration modes observed in the derived nanoparticles are listed in Table 5.

Table 5 : Average crystallite size, lattice parameter (aReitveu), and the observed Raman peaks in the derived Ni_xZni-_xFe204 (x = 0.2-0.8) nanoparticles

*A11 samples were synthesized at 500°C.

[00163] With respect to Table 5, the intense Raman bands observed at -323 cm^"1 (E_g mode), -480 cm^"1 (F2_g mode), and in the range 670-690 cm^"1 (A_lg mode) belong to the cubic spinel system {J. Magn. Magn. Mater. 374, 245-251, 2015}. Other weak vibrations (F2_g mode) of the mixed spinel system were also observed in the range 200-230 cm^"1 and 550-560 cm^"1.

[00164] The crystal structure analysis of the derived Ni-Zn ferrite nanoparticles were performed with XRD technique. FIG. 21 shows the XRD patterns of the derived nickel-zinc ferrite nanoparticles after heating the corresponding precursor powders at 500°C for 2 h in ambient air, wherein (a) shows XRD pattern of Nio.2Zno.₈Fe20₄, (b) shows XRD pattern of Nio.₄Zno.6Fe20₄, (c) shows XRD pattern of Nio.5Zno.sFe20₄, (d) shows XRD pattern of Nio.6Zno.₄Fe20₄, and (e) shows XRD pattern of Nio.sZno.2Fe20₄ nanoparticles, according to the embodiments of the present invention. With respect to FIG. 21, the XRD patterns confirm the formation of highly crystalline single phase cubic spinel ferrite structure of space group Oh⁷ (Fd3m). The indexing of the observed XRD peaks was performed with JCPDS card no. 08- 0234. The average crystallite size was calculated using the Debye Scherrer s formula and the full width at half maxima (FWHM) values of the diffraction peaks. The values are given in Table 5. The lattice constant, a was calculated using the relationship for cubic crystal structure, sin² 6» = — [— ^ J (5) where, λ is the wavelength of the X-ray radiation source (0.15452 nm), 2Θ is the diffraction angle, and [h, k, 1] are the miller indices of the XRD peaks. The lattice constant values in the derived samples after Rietveld refinement {a etveid) are also listed in Table 5. It can be observed that ametveid monotonically decreases with the increase in Ni concentration. This can be attributed to the smaller ionic radius of Ni²⁺ (0.069 nm) compared to the ionic radius of Zn²⁺ (0.074 nm). As the Ni concentration increases, the lattice seems to be contracted as smaller Ni ions get accommodated in the pool of Zn ions of relatively larger radius.

[00165] To estimate the distribution of cations in the tetrahedral and octahedral sites,

X-ray intensity analyses were carried out using the formula suggested by Burger et al { Crystal Structure Analysis, John Wiley, New York, I960} .

where, hki is the relative integrated intensity, Fhki is the structural factor, P_m is the multiplicity factor and L_p is the Lorentz polarization factor. The structural factor Fm is a function of oxygen position parameter u, as well as cation distribution factor x. The formulas for structural factor Fm of cubic ferrite system were reported by Furuhashi et al {J. Inorg. Nucl. Chem 35, 3009-3014, 1973 } . The multiplicity factor is taken from the literature of B. D. Cullity {Elements of X-ray Diffraction, Addison- Wesley Publishing Company, 1967 } . The ionic scattering factors reported in the international tables for X-ray crystallography { Volume 3, 182, 1983, Kynoch Press, Birmingham} were used for the calculation of structural factor Fhki- The distribution of cations at A and the B sites were determined from the best fit intensity rations of I (220)/I (440) and I (422)/I (440) calculated from Equation 6 with that in the observed XRD intensity. The estimated best fit distribution of cations at lattice sites are given in Table 6. Table 6 : Distribution of cations (from XRD analysis) at lattice sites, the calculated lattice parameter (dcai) values (from cationic distribution), and % distribution of Fe cations (from EXAFS analysis) in the derived Ni_xZni-_xFe204 (x=0.2-0.8) nanoparticles

[00166] The obtained results indicate a chemically homogeneous molecular composition in the derived ferrites with a general formula (Νϊ_χΖηι-_χ)δ(Ρε₂-δ0₄-δ) (x=0.2-0.8) with δ < ± 0.04.

[00167] Furthermore, to elucidate the local environment of atomic species in the ferrites, EXAFS studies were undertaken. FIG. 22 shows the Fe K-edge XANES spectra of the derived Nii-_xZn_xFe20₄ (x=0.2-0.8) nanoparticles, according to an embodiment of the present invention. With respect to FIG. 22, the normalized XANES spectra show the plot between absorption coefficient μ(Ε) versus photon energy (E). The near-edge features of the ferrite samples show close similarity with that of a-Fe₂03, which confirms that the Fe cations are in the same oxidation state, i.e., Fe³⁺, in all the samples. The inset figure shows the enlarged portion of the pre-edge features. The pre-edge feature of XANES spectra is mainly due to the Is to 3d (electric dipole forbidden) quadrupole transition inside the crystal lattice. The quadrupole transitions of the spinel ferrites are attributed to the local mixing of Ap and 3d atomic orbitals, which is allowed in the tetrahedral symmetry but forbidden in the octahedral symmetry {Solid State Sci. 14, 1536-1542, 2012}. Therefore, the observed pre-edge features can be attributed to the Fe³⁺ ions occupying the tetrahedral sites. Furthermore, in mixed spinel structure, the Ni²⁺ ions show a favourable preference for octahedral sites due to their best fit charge distribution in the octahedral crystal field, while the Zn²⁺ ions prefer the tetrahedral sites due to their stable tetrahedral sp³ hybrid configuration {J. Mater. Sci. 42, 779-783, 2007}. It has been found that as Ni concentration at octahedral sites increases, Fe occupancy in the tetrahedral sites gets enhanced, which is further confirmed from the cationic distribution derived from X-ray intensity calculations (see Table 6).

[00168] To further investigate the cationic distribution, EXAFS analysis is undertaken at Fe K-edge. The set of EXAFS data analysis codes available within the IFEFFIT software package have been used. FIG. 23A shows the Fe K-edge normalized EXAFS spectra and FIG. 23B shows the Fourier transformed EXAFS spectra along with the best fit theoretical curves of the ferrite nanoparticles heat treated at 500°C, wherein 'y'NZF = Nio._yZni-o._yFe20₄; e.g., 2NZF = Nio.2Zno.₈Fe20₄, according to an embodiment of the present invention. With respect to FIG. 23A, the typical EXAFS spectra consist of a plot between absorption coefficient μ(β) as a function of photon energy (E). The energy dependent absorption coefficient μ(β) has been converted to the wave number dependent absorption coefficient X(k) using the relationship,

where m is the mass of the electron. x(k) is weighted by k² to amplify the oscillation at high k and the functions are Fourier transformed in R space to generate the x(R) versus R spectra in terms of the real distances from the centre of the absorbing atom (i.e., Fe). With respect to FIG. 23B, the Fourier transformed data along with theoretical fitting using IFEFFIT package of Ni_xZni-xFe20₄ (x=0.2-0.8) nanoparticles can be seen. The fitting approach for spinel structure as described by Henderson et al {J. Phys.: Condens. Mater 19, 076214, 2007} has been followed in this case, in which two cluster of atoms were generated using Zn(Ni)Fe20₄ spinel structure. In the first atomic cluster, Fe is at the octahedral site and for the second atomic cluster Fe is at the tetrahedral site. For each sample the proportion of Fe in each site was refined as a single parameter viz., if the proportion of metal in tetrahedral site is y, then the proportion of metal in octahedral site will be (1-y). The two layer fitting model yields the % of Fe cations occupied in the tetrahedral and octahedral sites. The contribution of Fe cations at tetrahedral and octahedral sites is in the ratio of 13:87 for Nio.2Zno.₈Fe20₄, 21 :79 for Nio.₄Zno.6Fe20₄, 27:73 for Nio.5Zno.sFe20₄, 31:69 for Nio.6Zno.₄Fe20₄, and 39:61 for Nio.sZno.2Fe20₄ nanoparticles, respectively. The distribution of Fe cations at the lattice sites derived from EXAFS analysis matched reasonably well with that from XRD analysis (see Table 6).

[00169] The morphology of the derived nanoparticles was studied with a scanning electron microscope (SEM). FIG. 24A and FIG. 24B show the SEM images of Nio.2Zno.₈Fe20₄ nanoparticles calcined at 500°C and FIG. 24C shows the corresponding EDX spectrum of Nio.2Zno.₈Fe20₄ nanoparticles, according to the embodiments of the present invention. With respect to FIG. 24A and FIG. 24B, the micrographs show well dispersed particles of nanorod shaped. The particles have a typical dimension of 100- 150 nm along length and an average cross section of 50 nm. With respect to FIG. 24C, the characteristic peaks correspond to Ni, Zn, Fe and O elements yield the atomic percentage of elemental composition : Ni(2.62) : Zn(10.15) : Fe(29.14) : 0(58.09), which yields the molecular formula of the sample as Nio.isZno.73Fe2.o30₄.o6.

[00170] The room-temperature magnetic properties of the derived Ni_xZni-_xFe20₄

(x=0.2-0.8) nanoparticles was studied with applied fields upto 18 kOe. FIG. 25 shows the room-temperature magnetic hysteresis (M verses H) curves of nickel-zinc ferrite nanoparticles derived after heating the corresponding precursors at 500°C for 2 h in ambient air, wherein (a) shows magnetic hysteresis curve of Nio.2Zno.₈Fe20₄, (b) shows magnetic hysteresis curve of Nio.5Zno.5Fe20₄, and (c) shows magnetic hysteresis curve of Nio.sZno.2Fe20₄ nanoparticles, according to the embodiments of the present invention. With respect to FIG. 25, the saturation magnetization (Ms), remanent magnetization (MR) and coercivity (He) values of the samples are given in Table 7. Table 7 : Measured values of saturation magnetization (Ms), remanent magnetization (Ms), and coercivity (He) in the derived Ni_xZni-_xFe204 (x = 0.2-0.8) nanoparticles

^'Temperature at which the samples were synthesized is given in parenthesis.

[00171] With respect to Table 7, it can be observed that the Ms increases with the increase in Ni concentration and a maximum value of 45.15 emu/g is observed in the Nio.5Zno.5Fe20₄ samples. Further increase in Ni concentration decreases the magnetization as can be observed in Nio.sZno.2Fe20₄ showing an Ms of 35.54 emu/g. This concentration dependence of magnetization in the Ni-Zn ferrite samples is attributed to the inter sub-lattice and intra sub-lattice exchange coupling interactions. In mixed spinel ferrites, there are two ferromagnetically ordered sub-lattices formed, one by the ions at the A-sites and the other by those at the B-sites. The inter sub-lattice coupling is antiferromagnetic in nature and relatively strong due to super-exchange interaction between these ions. The intra sub-lattice coupling between the different ions on B-sites and A-sites is also an antiferromagnetic super-exchange coupling but is weaker than the inter sub-lattice coupling due to larger distance between the different (Ni and Zn at B-sites and Zn and Fe at A-sites) ions, and therefore the intra sub- lattice ordering is forced to be ferromagnetic { J. Super cond. Nov. Magn. 25, 1907- 1911, 2012} . Thus, the net magnetization in the lattice is expressed as M = MB-MA, where MA and MB are the magnetic moments at the A and B sites, respectively. As Zn is a nonmagnetic element and (Fe(Zn))A(Fe(Ni))B0₄, a predominantly inverse ferrite, Zn²⁺ ions occupy tetrahedral A sites while Ni²⁺ ions occupy octahedral B-sites. At low Ni concentration (high Zn²⁺ concentration), maximum number of Fe³⁺ ions are at B-sites and the net magnetization is primarily due to the weak antiferromagnetic coupling between intra sub-lattice Fe³⁺ ions, which is in the range of 18-22 emu/g in Nio.2Zno.₈Fe20₄ (Table 7). As the Ni concentration at octahedral sites increases, the Fe ions are forced to migrate to tetrahedral sites (confirmed by XRD and EXAFS analysis) (see Table 6). Initially, with the increase in Ni concentration, the magnetic moments of both A- and B-sites increase, however, the net inter sub-lattice antiferromagnetic coupling interaction remains weak due to the large distance between Fe³⁺ cations at A and B sites. The net magnetization increased up to an optimal value in Nio.5Zno.5Fe20₄. Further increase in Ni concentration (decreasing Zn²⁺ concentration), the concentration of Fe³⁺ ions at the A-sites increases and hence the magnetization of the A-sites. As a result of the anti-parallel spin coupling between A and B sites, the net resultant magnetization starts reducing as observed in Nio.sZno.2Fe20₄.

[00172] The dielectric properties of the ferrites were evaluated by measuring the complex permittivity, as given by the equation, ε^* = ε' + ίε^" (8) where, ε ' is the dielectric constant representing the stored part of energy in the dielectric materials and corresponds to the real part of permittivity and ε", the imaginary part of permittivity represents the dielectric loss or the dissipated energy in the dielectric materials. The ac conductivity of the material is given by a_ac = 2Uf ε₀ε = ωε₀ε tanS (9) where, /is the frequency of the applied field, ω is the angular frequency, ε₀ is permittivity of free space and tanS is electric loss tangent = ε"/ε '.

[00173] FIG. 26A, FIG. 26B and FIG. 26C show the variation of dielectric constant, dielectric loss and ac conductivity of Nio.2Zno.₈Fe20₄, Nio.5Zno.sFe20₄ and Nio.sZno.2Fe20₄ nanoparticles, respectively in the frequency range, 1 Hz- 10 MHz. With respect to FIG. 26A, FIG. 26B and FIG. 26C, it is observed that ε ' and ε" decrease monotonically with the increase in frequency in all the samples. However, ε" decreases faster than ε ' in the selected frequency range. The values of ε ' and ε" are higher in Nio.5Zno.sFe20₄ compared to Nio.2Zno.₈Fe20₄ and Nio.sZno.2Fe20₄ samples over the selected frequency range. The a_ac increases slowly with frequency at lower frequencies but above 10⁵ Hz, starts showing a steep rise with frequency. At 10⁷ Hz frequency, a bit larger value of viz, 1-35 x 10^~3 Ω^"1 m^"1 lies in Nio.5Zno.5Fe20₄, in comparison to that of 1.05 x 10^~3 Ω^"1 m^"1 in Nio.2Zno.₈Fe20₄ or 1.75 x 10^"5 Ω^"1 m^"1 in Nio.sZno.2Fe20₄.

[00174] The observed dielectric behaviour can be explained qualitatively by exploring the mechanism of polarization processes in the ferrite nanoparticles. Iwauchi et al {Jap. J. Appl. Phys. 10, 1520-1528, 1971 } and Mazen et al {ISRN Cond. Mater. Phys. 2012, 907257, 2012} observed a strong correlation between conduction mechanism and dielectric behaviour of ferrite nanomaterials. The electrical exchange mechanism proposed by Heikes and Johnston { . Chem. Phys. 26, 582, 1957} explained that the electronic hoping (Fe²⁺ → Fe³⁺ and/or Ni²⁺ → Ni³⁺) occurring by electron transfer between adjacent octahedral sites remains decisive in giving large dielectric constant at lower frequencies. Beyond a certain frequency of applied electric field (10⁵ Hz), the electronic exchange between Fe²⁺ → Fe³⁺ or the hole hopping between Ni³⁺ → Ni²⁺ cannot follow the applied alternating electric field, which in turn decreases the values of dielectric constant. It can be noted that the ferrite compositions with high concentration of Fe (i.e., Nio.sZno.2Fe20₄) or Zn (i.e., Nio.2Zno.₈Fe20₄) at A-sites (tetrahedral sites) brings about a significant decrease in dielectric constants compared to the Nio.5Zno.5Fe20₄ sample. This happens due to the lower concentration of charge carriers with less concentration of Fe ions at octahedral sites (since considerable amount of Fe cations moved to tetrahedral sites) in Nio.sZno.2Fe20₄ and the formation of more homogeneous electronic structure that decreases polarization in Nio.2Zno.₈Fe20₄, respectively. Furthermore, the high value of a_ac observed in Nio.5Zno.sFe20₄ is because of the prominent number of available electrons (due to Fe³⁺→ Fe²⁺) and holes (due to Ni²⁺→ Ni³⁺) in this sample.

[00175] The above descriptions are part of the detail examination on the effects of cationic distributions at lattice sites in the derived Ni_xZni-_xFe20₄ samples. The derived Ni-Zn ferrite nanoparticles show attractive magnetic and dielectric properties along with the compositional homogeneity, which make them potentially attractive for pertinent applications. It has been observed that, several modifications are possible in terms of the distribution of cations at lattice sites. Therefore, it is to be understood that the invention may be practiced avoiding conflicts with the scope of appended claims.

[00176] The present invention provides a novel, highly effective and facile method of mass scale synthesis of nickel-zinc soft ferrite nanoparticles. The derived Ni-Zn ferrite nanoparticles show excellent magnetic and dielectric properties which are governed by the cationic distributions at lattice sites. The derived Ni-Zn ferrite nanoparticles are potentially attractive for applications in microelectronics, ferrofluids, RF transformers and inductors, radar absorbing composite materials, etc.

[00177] It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the claims.

Claims

[00178] CLAIMS: We claim:

1. A method of synthesizing shape-controlled Diluted Magnetic Semiconductor (DMS) nanoparticles comprises:

reacting a solution of a metal ion with an aqueous solution of a polymer and a sugar under continuous stirring at a predetermined temperature to obtain a mixture;

obtaining a plurality of polymer precursors at a predetermined pH from the mixture; and

heat treating the plurality of polymer precursors at a predetermined range of temperature for a predetermined period of time in ambient air, wherein the predetermined range of temperature is 400-600°C and wherein the predetermined period of time is 2 h.

2. The method as claimed in claim 1, wherein the step of obtaining the plurality of the polymer precursors further comprises:

cooling the mixture to room temperature;

ageing the mixture at a temperature range of 20 to 25 °C for a time period of 24 h to obtain a plurality of precipitates;

washing the plurality of precipitates with an alcohol, wherein the alcohol is methanol; drying the plurality of precipitates at a controlled temperature of 50-60°C to obtain the plurality of polymer precursors.

3. The method as claimed in claim 1, wherein the solution of metal ion is selected from the group consisting of titanium tetrachloride, chromium trioxide, iron(III) nitrate nonahydrate, cobalt(II) nitrate hexahydrate, nickel(II) nitrate hexahydrate, copper(II) nitrate trihydrate, zirconyl nitrate hydrate, cerium(III) nitrate hexahydrate or hafnium(IV) chloride.

4. The method as claimed in claim 1, wherein the polymer is poly-vinyl alcohol.

5. The method as claimed in claim 1, wherein the sugar is sucrose.

6. The method as claimed in claim 1, wherein the predetermined temperature is in the range of 60-65 °C in ambient air.

7. The method as claimed in claim 1, wherein the shape-controlled DMS nanoparticles are ZnO nanoparticles doped with transition metals, wherein the transition metal is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf.

8. The method as claimed in claim 1, wherein the shape-controlled DMS nanoparticles are nanoplatelets of pristine and transition metal doped ZnO, wherein the transition metal is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf.

9. The method as claimed in claim 1, wherein the shape-controlled DMS nanoparticles are hexagonal nanoplatelets of pristine and transition metal doped ZnO, wherein the transition metal is selected from the group consisting of Ti, Cr, Fe, Co, Ni, Cu, Zr, Ce, and Hf .

10. The method as claimed in claim 1, wherein the predetermined pH is 9.

11. A method of synthesizing ZnO based core-shell nanoparticles, comprises:

(a) reacting a solution of a zinc metal salt with an aqueous solution of a polymer and a sugar under continuous stirring at a predetermined temperature to obtain a mixture;

(b) obtaining a plurality of ZnO-polymer precursor at a predetermined pH from the mixture; and

(c) obtaining a series of core-shell nanoparticles, wherein the core-shell nanoparticles are ZnO : metal nanoparticles, wherein the core is made up of zinc oxide and the shell is made up of a metal, wherein the metal is selected from the group consisting of zinc metal and silver metal.

12. The method as claimed in claim 11, wherein the ZnO:Ag core-shell nanoparticles are obtained by dispersing the plurality of ZnO-polymer precursor in an aqueous solution of silver nitrate while continuous stirring for 20-30 minutes in dark at a temperature of 50-60°C and obtaining a dry powder by washing the solution and drying in a reduced pressure at 20-25°C;

heating the said dry powder at 400-500°C for 2 h in ambient air to obtain ZnO : silver core-shell nanoparticles.

13. The method as claimed in claim 11, wherein the ZnO:Zn core-shell nanoparticles are obtained by heating dried ZnO precursor powders at 500°C for 2 h in ambient air to obtain recrystallized ZnO nanoparticles and further heating the said obtained recrystallized ZnO nanoparticles at 1000°C in an autoclave at 2 bar pressure in an atmosphere of 5% H₂-95% Ar gas to obtain ZnO : Zn core-shell nanoparticles.

14. The method as claimed in claim 11, wherein the plurality of polymer precursor is ZnO -polymer precursor powders.

15. The method as claimed in claim 11, wherein the step of obtaining the plurality of the ZnO-polymer precursors further comprises:

cooling the reaction mixture to room temperature;

ageing the mixture at a temperature range of 20 to 25 °C for a time period of 24 h to obtain a plurality of precipitates;

washing the plurality of precipitates with an alcohol, wherein the alcohol is methanol; drying the plurality of precipitates at a controlled temperature of 50-60°C to obtain the plurality of polymer precursors.

16. The method further modified to synthesize soft magnetic Ni-Zn ferrite nanoparticles with different Ni-Zn contents, comprises:

(a) preparing an aqueous solution of zinc salt, an aqueous solution of nickel salt and an aqueous solution of iron salt in stoichiometric amounts;

(b) dispersing the said aqueous solution of zinc salt, the said aqueous solution of nickel salt and the said aqueous solution of iron salt into an aqueous solution of a polymer and a sugar at a predetermined temperature under constant stirring to obtain a mixture;

(c) obtaining a solid mass of polymer precursor powder at a predetermined pH by drying the mixture in a microwave oven; and

(d) obtaining recrystallized Ni-Zn ferrite nanoparticles by heating the polymer precursor powder at 400-600°C in ambient air.

17. The method as claimed in claim 16, wherein the said zinc salt is zinc nitrate hexahydrate.

18. The method as claimed in claim 16, wherein the said nickel salt is nickel nitrate hexahydrate.

19. The method as claimed in claim 16, wherein the said iron salt is iron nitrate nonahydrate.

20. The method as claimed in claim 16, wherein the predetermined temperature is 60- 65°C.

21. The method as claimed in claim 16, wherein the said predetermined pH is 9, and wherein the said predetermined pH is maintained by adding ammonium hydroxide solution (25% concentration) to the mixture.

22. The method as claimed in claim 16, wherein the polymer is poly- vinyl alcohol and wherein the sugar is sucrose.

23. The method as claimed in claim 16, wherein the said aqueous solution of the polymer and the sugar is prepared by adding said polymer and the said sugar in a ratio of 1: 10 by weight in water.

24. The method as claimed in claim 16, wherein the soft magnetic Ni-Zn ferrite nanoparticles are derived with a varied Ni and Zn contents as Ni_xZni-_xFe20₄, wherein x = 0.2-0.8.