Structural, vibrational and magnetic characterization of copper doped CoMn2O4 nano-particles synthesized by chemical route

The Co1-xCuxMn2O4 samples were effectively synthesized by means of chemical
 reagents and highly scalable co-precipitation technique. X-ray diffraction
 investigation affirmed the tetragonal structure possessed by Co1-xCuxMn2O4
 with crystal size ranging from 12 nm to 22 nm. The crystallite size and
 micro strain values were correlated using Williamson-Hall plot and
 size-strain plot method. SEM images showed highly porous, less dense,
 agglomerated grains with grain size from 3.93 ?m to 13.73 ?m. Vibrational
 characterization of the samples was completed using FTIR spectra and Raman
 spectra which affirm the tetragonal structure. The magnetic characterization
 demonstrates the ferromagnetic nature of the materials which varies with
 copper substitution


Introduction
The materials which are transition metal based originate from Mn 3 O 4 having spinel structure with general formula A 2+ B 3+ 2 O 4 [1], where Mn 3+ is often replaced by different transition metals such as Cu, Ni, Fe, Co, and Zn to induce semiconductor properties [2]. Super capacitors made with transition metal oxides undertake redox reaction for storage of charge which attracted a lot of attention in the past two decades due to their capability to produce high capacitance [3]. Super capacitors (SC's) widely used in energy storage applications due their low-cost, eco friendly nature and high energy density, which basically functions with the help of electrode. Among various electrode materials, transition metal oxides with larger surface-to-volume ratio and short path length for ion diffusion, which make them attractive candidates for applications in high-performance electrodes [4][5][6][7][8]. Manganese-based oxide electrodes are described by lower operating voltages (1.3-1.5 V for lithium extraction) and produce higher energy density. Cobalt manganite (CoMn 2 O 4 ) has attracted an interest in the field of catalyst, lithium ion battery and electrochemical super capacitor because of its different oxidation states during the redox reaction. Rechargeable lithium-ion batteries (LIBs) have a wide scope of uses in different fields as like telecommunication [9][10], entertainment industry [11][12], medicine [13][14][15], aeronautics [16], microelectronics [17] and electric automobiles [18]. The mechanical stability, electrical conductivity and recycling ability of the electrode material are appeared to develop by making binary composites of metal oxides as like Ni-Co, Co-Mo, Co-Ru oxides confirming fast redox reactions. Out of them, Mn-Co oxide was utilized hopefully as super capacitor electrode because of its high capacitance and better redox reactions over unitary Mn or Co oxide [19].
The cationic distribution of spinel is represented by 4 , where x is the degree of inversion, A type metal assumes tetrahedral positions and B type metal assumes octahedral positions. At the point when x=0, normal spinel will be formed and when x=1, inverse spinel will be framed. At the point when x gets value in between 0 and 1, a mixed spinel will be formed. As cobalt manganite is represented by CoMn 2 O 4 , where Co 2+ is situated in tetrahedral position (A-site) and Mn 3+ in octahedral position (B-site) producing tetragonal structure because of Jahn-Teller (J-T) effect experienced by Mn 3+ in octahedral position MnO 6 [20]. The cation distribution assumes an extensive job in resolving the properties of spinel oxides. Site preference of cations relies on: i) particle size, ii) electronic configuration of ion and iii) strength and symmetry of crystal filed at the site. The predominant exchange association between and inside the octahedral (B) and tetrahedral (A) sublattice direct the sort of magnetic ordering created in the material. As per Neel's two sublattice model, A-B interactions are stronger than A-A or B-B interactions. Attributable to the imbalance in the quantity of ions in the tetrahedral and octahedral positions and the strong exchange interaction between the sublattice prompts to ferromagnetic nature [20].
Numerous analysts have put forth different attempts in creating transition metal based oxide (essentially got from Mn 3 O 4 ) nanoparticles through various synthesis techniques, for example, ultrasonic technique [21],co-precipitation [22,23], auto-burning [24], solid state reaction [25,26], ball milling process [27,28], sol-gel [29] techniques and others. In light of the methods referenced above, we apply the co-precipitation route because of its simplest and adequacy in creating the nanomaterials. From the author's literature information, very few attempts were done in elaborate study regarding to structure, morphology, vibrational and magnetic properties of copper subbed Cobalt manganites [30][31][32][33]. In this work we are going to report the fabrication of copper substituted

Synthesis and characterization 2.1 Preparation of Co 1-x Cu x Mn 2 O 4
Co 1-x Cu x Mn 2 O 4 nanoparticles were prepared by co-precipitation technique. Analytical grade Cobalt chloride (CoCl 2 ‧6H 2 O, 97%), manganese chloride (MnCl 2 ‧4H 2 O, 99%) and copper chloride (CuCl 2 ‧2H 2 O, 99%) were taken as starting materials for the synthesis. The precursors were bought from Burgoyne and Burbridges Company, Mumbai, India. An ideal stoichiometric proportion of precursors were weighed and dissolved in 200 ml of ionized water individually and further, all the solutions mixed each other. At room temperature the mixture was stirred using a magnetic stirrer to get liquid solution. At that point liquid ammonia (NH 3 ) was added to the arrangement drop wise to get a pH~7 and mixed for an additional 1 hour for precipitation development. The arrangement was washed over and over with refined water and filtered to get mud colored wet precipitate. Following 24 hours the dry precipitate was finely and consistently powdered using agate mortar and pestle. Utilizing alumina crucible the dry powder was sintered at 600 o C for 4 hours in a muffle furnace to get last dark powdered cobalt copper manganite sample.

Characterization Details
The structural characterization of the fabricated samples was completed using Bruker AXS D8 Advance XRD instrument (Detector Si(Li) PSD) using Copper Kα radiation (λ=1.5405Å) at room temperature in the range 0 o to 90 o with a step of 0.02 o per sec to explore the phase composition and crystallite size. The morphology of the samples was examined on SEM (JEOL Model JSM). The FTIR spectral examination was done utilizing spectra got on Thermo Nicolet, Avatar370 and the magnetic investigation was done with the help of VSM spectra got on Lakeshore 7410 arrangement. The Raman examination was done with the assistance of Raman spectra acquired on Horiba Jobin Vyon, Model Lab Ram HR. The entire procedure of synthesis and characterization of samples is as appeared in Fig. 1.    The lattice parameters a and c of the tetragonal crystal structure was evaluated by using:

XRD structural analysis
where d is interpalnar distance while (h k l) are the Miller indiex values of the crystal planes. Crystal size D was found using the Debye-Scherer equation [35]: where λ is the wavelength of radiation (λ=1.5406 Å), β is the full width half maximum (FWHM) of diffraction peak, and θ is the Bragg's angle.The mean crystal size (D) was determined from Debye-Scherer condition which varies from 12 nm to 22 nm. According to Debey-Scherer, the peak is wider for small grains and narrower for larger grains. All the samples showed sharp and highly intense peaks as shown in Fig. 2, which were produced by larger grains as evident from Table I. It was clear that, particles with higher grain size will have a lesser grain boundary resistance thusly prompting to enhance the material electrical conductivity [35]. The dislocation is the morphological irregularity within a crystal structure, which strongly determines the material properties. Dislocation density ( ) is the ratio of number of abnormalities present per unit volume of a crystalline substance, which can be determined using [36,37] The micro strain is expressed by: The X-ray density (∆ ) was determined by the formula: where M is the molecular weight of composition, N is the Avogadro's number and V is unit cell volume. Quantitative data concerning the particular preferential crystal orientation can be acquired from the texture coefficient, Tc (h k l).The reflection peak from each XRD graph contain data related to preferential growth of phases in polycrystalline material. The degree of orientation in crystal planes can be found from the articulation: where I and I 0 stand for the observed and standard intensities and Nis number of peaks. The spectra from Fig. 2 showed high intensity peaks for (022), (020) and (011) [39]. Kshirasagar et al reported that Co 1-x Cu x Mn 2 O 4 samples show tetragonal structure for x = 0.05 to x = 0.55 because of J-T effect which could be evacuated once 0.55<x<1. So decisively we report that there was a considerable enhancement in crystal volume, crystal size and decrease in X-ray density, dislocation density, micro strain because of addition of copper ions instead of cobalt ions at tetrahedral sites. All the announced values show some sort of reduction in their values may due to distortions of J-T effect shown by Mn 3+ ions at octahedral sites, as introduced in Table I.

Williamson -Hall plot and size-strain plot method
Lattice strain η and mean crystal size D were discovered by Williamson-Hall equation [40]: The equation (7) can be composed as y = mx + c where m = η and c = 1/D, with the goal that the linear plot of βcosθ versus sinθ gives the slope as lattice strain η and the intercept as 1/D as appeared in Fig. 3. Mean crystallite size D and lattice strain η got from the graph were classified in Table II which are in concurrence with the respective values tabulated in Table I.  The "size-strain plot" (SSP) is advantageous tool to recognize the isotropic nature and micro-strain contribution. The determination of the size-strain boundaries can be acquired by thinking about an average SSP, which has the advantages that less weight is given to information from reflections at high edges, where the accuracy is generally lower which is appeared in Fig. 4. We assumed that the "crystallite size" profile is represented by a Lorentzian function and the "strain profile" by a Gaussian function [41,42]. Accordingly, we have: where K is a constant that relies upon the shape of the particles (for spherical particles K = 3/4). The correlation between determined values of crystallite size D and micro strain ɛ from W-H plot and SSP were accounted for in Table II

Surface Morphology
The surface investigation of the fabricated samples was completed utilizing SEM (Scanning Electron Microscope) and the respective surface micrographs are appeared in Fig  5. The pictures reveal an exceptionally porous, less dense and agglomerated grains. The grain sizes of the samples extending from 1.67 μm to 6.80 μm, estimated using ImageJ programming as reported in Table III. The pictures delights that the samples are less thick and the grain distribution was fundamentally moderate, because of an unpredictable development of the ions. The morphological interpretation indicated that the homogeneity and grains size consistently increases with copper enhancement because of difference in ionic radii of Cu 2+ (0.73 Å), Mn 3+ (r = 0.645 Å) and Co 2+ (r = 0.72 Å). The porosity of samples continuously decreases as x increments and at x = 0.45 the concentration of both Cu 2+ and Co 2+ become approximately equal and the material denser.

FTIR analysis
The affirmation of crystal structure and infrared spectral investigation was carried with the assistance of FTIR spectra of the prepared samples. The spectra were taken in the range 700-400 cm -1 as appeared in Fig. 6. The position of transmission groups with frequency are organized in Table IV. There exist two principle absorption groups ν 1 and ν 2 relating to the stretching vibration of tetrahedral (A-site) and octahedral (B-destinations) positions around 667 cm -1 and 505 cm -1 [43,44]. The band frequencies (ν 2 ) around 505 cm -1 are related with an internal bending mode of Mn-O 6 octahedral and the band frequencies (ν 1 ) around 667cm -1 are credited to phase stretching mode of Mn-O, differs nonlinearly because of Jahn-Teller distortions [45,46]. Additionally the curve turns out to be increasingly pronounced for higher x values demonstrating the impact of copper at both the locations. The increase in Mn-O vibration frequency from 500 cm -1 to 517cm -1 demonstrates strong coupling constant which tends to shorter band length and reduced lattice constant. The values of ν 1 are higher than ν 2 , shows that the normal vibrational mode of the tetrahedral complexes is higher than that of the relating octahedral locations. The band ν 1 moved to higher values and band ν 2 moved to bring down values after enhancement in concentration of copper ions [47]. This might be because of a shorter bond length in the tetrahedral site compared with octahedral one. Due to nano sized grains, there is a shift in IR frequencies when compared with FTIR groups of bulk materials additionally there is a degradation of crystal symmetry. Nano sized grains are giving some disorder, as their atomic arrangements on the boundaries differ greatly from that of bulk crystals both in co-ordination number and bond lengths [48].

Raman Analysis
The Raman spectra of Co 1-x Cu x Mn 2 O 4 samples (x = 0.15 and x = 0.35) with wave numbers from 300 cm -1 to 900 cm -1 , is introduced in Fig. 7. As the spinel sample has tetragonal system belonging to the symmetry group I41/amd which produces 10 permitted Raman modes with Γ= 2A1g+3B1g+B2g+4Eg [49][50][51]. The Fig. 7 comprises of broad bands at 437, 442,520 and a solid band at 670 cm -1 . The wider peak at 670 cm -1 having broad shoulder is credited to A 1g sensitive states, which evolved because of symmetric stretching vibrations of Mn-O in MnO 6 octahedral position. The peaks at 630 cm -1 -680 cm -1 affirm the tetragonal structure formation which is a characteristic one for manganese oxide materials. As per Malavasi et al, wave numbers of A 1g modes are conversely corresponding to lattice parameter of the spinel. The peaks 437, 442 might be ascribed to B g states [52] additionally these are assigned to A-O bending, especially with Cu-O bending and 520 cm -1 is credited to axial motion of O 4 molecules at the tetrahedron positions [53]. We get more pronounced peaks at 520 cm -1 and 670 cm -1 when copper content increments to 0.35 from 0.15 which may be because of distinction in ionic radii of Co and Cu. At the point when the grain size builds, these Raman peaks become stronger and sharper, and move to higher wave number as obvious from Fig. 5 and Fig. 7.

Magnetic Study
Vibrating sample magnetometer was used to investigate the magnetic properties of synthesized Co 1-x Cu x Mn 2 O 4 with x = 0.15, 0.35, 0.55 at room temperature. Fig. 8 shows VSM plots. The values of saturation magnetization (Ms), coercivity (Oe), retentivity (Mr) and magnetic moment (η B ) of the samples are listed in Table V. The magnetic moment per formula unit in Bohr magneton (μ B ) was calculated by using the following relation [54]: Magnetic moment η B = M * Ms 5585 (9) where M is the molecular weight of a particular composition and Ms is saturation magnetization (emu/gm). All the samples demonstrated depict moderate hysteresis loops, showing low ferromagnetic nature as shown in Fig. 8. The ferromagnetic properties of considered samples were ascribed to the concurrent presence of Mn 4+ , Mn 3+ and Mn 2+ ions. They offer ferromagnetism by means of double-exchange mechanism [34]. The Cu 2+ and Mn 3+ ions in the spinel put added Jahn-Teller stabilization on interaction, which results in ions pair formation as like Mn 3+ -Mn 4+ in association with Cu 1+ or Cu 2+ . With increment in copper content in Co 1-x Cu x Mn 2 O 4 , the B-B interaction improved with increment in Mn 3+ -Mn 4+ ion pairs which thus brings about increment of magnetization [31].
As indicated by the Anuradha et al [55], maximum magnetization was shown for the sample with the smallest grain size, as obvious from the Fig. 8 and Table III. However, at x=0.55, concentration of both Cu and Co turns out to be similar when their ionic radii are like one another, the saturation magnetization suddenly decreases because of phase transmission of spinel from tetragonal to cubic as clarified by Kshirsagar et al [33]. Additionally, from same author, it was presumed that not just J-T ions Mn 2+ , Co 2+ and non J-T Cu 2+ ions, both at A site settles the B site J-T distortion by Mn 3+ particles, yet it relies upon electronic design, charge dispersion and cationic sizes moreover [33]. The magnetic investigation of all the samples shows that, the variation in saturation magnetization, retentivity, coercivity and magnetic moment is because of the substitution of copper in to the CoMn 2 O 4 spinel. Tab. V Calculated values of crystallite size (D), coercivity (Oe), saturation magnetization (Ms), retentivity (Mr) and magnetic moment (η B ) for Co 1-x

Conclusion
Based on spectral investigation it was affirmed that Co 1-x Cu x Mn 2 O 4 samples were effectively integrated by means of simplest and profoundly versatile co-precipitation technique. X-ray diffraction examination confirmed the tetragonal structure possessed by Co 1- x Cu x Mn 2 O 4 with crystal size increasing from 12 nm to 22 nm. So indisputably we report that there is a significant increase in crystal volume, crystal size and reduction in X-ray density, dislocation density and micro-strain because of addition of copper in place of cobalt at octahedral positions. The XRD patterns demonstrated strong preferential orientation about (020), (022) and (011) planes for all the samples. Williamson-Hall plot and size-strain plot methods were utilized to correlate crystallite size and micro strain values of the samples. SEM micrographs demonstrated highly porous, less dense, agglomerated grains which go on enhance their size from 3.93 μm to 13.73 μm, as copper content increases. Vibrational characterization of the samples was completed utilizing FTIR spectra and Raman spectra which affirm the tetragonal structure formation. The magnetic characterization of the materials demonstrated increase in ferromagnetism as copper substitution increases, which relies upon ionic size and cation distribution.