Effect of Mn Dopant on Structural and Optical Properties of Nife 2 o 4 Nanoparticles Synthesized by Autocombustion Method

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Introduction
2] Due to different properties of ferrites it has been prepared to meet the remarkable uses in microwave fields, engineering, permanent magnet, transformer core and memory chips etc.The spinel ferrites are semiconducting in nature.The electrical property depends on several factors including the method of preparation, sintering temperature, sintering atmosphere, chemical composition and microstructure.as well as, by addition of impurities. 3errites containing Mn 2+ ions tend to form α-Fe 2 O 3 phase when heat treated above 200 o C in air atmosphere. 4It is very important to understand the mechanism involve in the change of properties.6][7][8] Manganese ferrite (MnFe 2 O 4 ) nanoparticles (NPs) with other ferrites are considered as a crucial tool for enhancing efficiency of magnetic resonance imaging, hyperthermia, and drug delivery. 9Also, it is a mixed ferrite, where Mn 2+ and Fe 3+ occupies both tetrahedral and octahedral bonding sites. 10he chemical changes in ferrites depend on their structural parameters of particle size and shape, which can be modified in the synthesis processes.In spinel ferrites, the physical and magnetic properties are powerfully reliant on cation distribution and process of research. 11,12 the present work, the studies on Mn x Ni 1-x Fe 2 O 4 (x = 0,0.2,0.4,0.6,0.8,1.0)ferrites annealed at 600 o C are reported.Powder X-ray diffraction (XRD), FT-IR spectroscopy techniques and spectrophotometer stayed active for structural and optical characterizations.

Sample Preparation
Manganese doped Nickel ferrite nanoparticles were prepared by self-propagating auto combustion route from a mixture of stoichiometric amount of (Mn (NO 3 ) 2 .6H 2 O, Fe(NO 3 ) 3 .9H 2 O and Nickel Nitrates (Ni(NO 3 ).6H 2 O).10% of PVA and sucrose solutions were added it, where the sucrose is utilized for the combustion purpose and PVA forms the polymer resin with metal ions trapped in it.The whole mixture was heated to about 80 o C for the evolution of NO 2 , CO 2 , and H 2 O.The mixture in solution transforms and converted to black fluffy gel.The PVA (the matrix) and Sucrose (the fuel) are dissolved together and at elevated temperature, and the brown fumed of solution gets evaporated.Sucrose provides the wrapping throughout the coordination for the cations in solution and circumvents their selective precipitation during the evaporation process.Black fluffy, and voluminous gel gets burnt in the selfpropagating manner at about 90 o C. for 6 h to remove In order of residual water, the as-prepared Nickel ferrite nanoparticles was dried overnight and calcined at 600 o C. The prepared samples were used to study the structural and optical characterisation.

Result and Discussion Structural Analysis
The XRD pattern of the prepared samples is shown in The intensity of these peaks gently increases with an increase in the concentration of Mn 2+ ion.
An enlarged view of the high-intensity characteristic peak (311) as shown in Fig. 2. Manganese ions may have different ionic radius compared to nickel ions, leading to change in the lattice parameter of crystal structure, effect on spacing between lattice planes influencing the shift in position of (311) peak.
It can induce strain in the crystal lattice, altering the diffraction pattern and casing peak shifts. 13e average crystallite size of the resulting powder materials, calculated using the Debye-Scherrer formula (equation 1), increased monotonically with increasing Mn concentration (0.0-1.0) from 20.0 to 26.38 nm, respectively 14 as shown in Fig. 3. .

..(1)
X-ray wavelength, FWHM in radians and Bragg's angle, respectively, refer to λ, β, and θ. and dislocation density (δ) as Also, the larger ionic radius of Mn 2+ compared to N 2+ leads to stronger ionic interactions between them. 15An increase in crystallite size typically results sharper and narrower peaks in an XRD pattern.This is because larger crystallites lead to a decrease in width of diffraction peaks, which enhances resolution of XRD pattern.So, for Mn doped nickel ferrites, sharp and crystalline peaks are observed after doping.The dislocation density decreases with increase in Mn concentration reveals that the less distortion in crystal lattice.

Scanning Electron Microscopy Study
The SEM micrographs for ferrite phase i.e.Mn x Ni 1- x Fe 2 O 4 ferrite (x = 0.2, 0.4, 0.6,0.8,1.0) composition is shown in Fig. 4. The grains in the ferrite phase are smaller in size.The uniform nature of the particles is revealed with some agglomeration.The average grain size is about 97 nm.There is progress of grain size with calcination temperature.Increasing the Mn 2+ ion concentration, the powder show irregular microstructures with small spherical particles and size of the particle is varied. 16Generally, the grain size increases with an increase in Mn 2+ ion concentration.

Optical analysis
UV-Visible Spectroscopy is used for the examination of the optical properties of Mn doped Ni ferrite nanoparticles.Solutions of Mn-Ni nano ferrites prepared by sonication in double distilled water were used to record the spectra.The absorption spectra were observed from 200nm to 800 nm as shown in figure Fig. 5.   From absorption spectra, the variation in Mn composition in Ni ferrites causes the absorption change in the UV section about 300 nm (for Mn 0.8 Ni0. 2 Fe 2 O 4 ) moves to longer wavelength visible region.This shifting to longer wavelength can be attributed to changes in electronic structure and composition of the material.Manganese doping alters the energy level and band structure, affecting absorption characteristics and again increase in absorption wavelength.To examine influence of Manganese effect on optical band gap, the Tauc equation is used for fitting absorption data. 17 where C is proportionality constant, h is Planck constant, Eg is band gap, n is an integer which is '2' for direct band gap transition in present study and α is the absorption coefficient.
The plot of (ahν) 2 with respect to energy of photon (hν) for all the Mn doped nickel nano ferrites are shown in Fig. 6.The extrapolation of the curve to the energy axis gives the band energy value for particular composition.
The values of band gap vary from 3.80 eV -3.57eV for Mn doped nickel ferrites.Thus, up to Mn=0.6 the projected value of band gap decreases is due to the quantum confinement effect.Further for Mn = 0.8 and 1.0 composition, the band gap rises may due to the little disorder in crystallinity of compositions.The increase in Mn produces additional energy levels in band structure.These levels can hybridize with existing energy bands of material, altering the band after Mn =0.8.Thus addition of Manganese for nickel ferrite leads to changes in electronic properties of materials in terms of tuned band gap, conductivity and absorption spectra.

Conclusions
Manganese doped nickel nanoparticles was synthesized by using self-propagating auto combustion method.The cubic spinel structure of Mn-Ni nanoparticles was identified by XRD.The agglomerated nature of ferrite particles nature is confirmed by Scanning electron Microscopy.The result indicate that the structural properties of nickel ferrite can be improved by Mn addition.The optical band gap for Mn-Ni nanoparticles decreases from 3.80 to 3.57 eV up to Mn = 0.6.The further rise of band gap only because of preparative crystallinity defects in composition revealed the variation of Mn in Ni ferrites for tuning its band gap.Thus variable narrow band gap can increase the efficiency of photo catalysis by enabling the absorption of a wide range of photons from solar spectrum and enhancing the generation of electron-hole pairs and facilitating more photocatalytic reactions.