Structures and properties of Mg0.95Mn0.01TM0.04O (TM = Co, Ni, and Cu) nanoparticles synthesized by sol–gel auto combustion technique

The room temperature structural, optical and dielectric properties of Mg0.95Mn0.05O and Mg0.95Mn0.01TM0.04O (TM = Co, Ni, and Cu) nanoparticles are reported. All transition metal nanocrystalline samples were successfully prepared by sol–gel auto combustion method. X-ray powder diffraction patterns at room temperature confirmed the formation of single-phase cubic structure with an Fm3̄m space group for all prepared samples. Slight variation in the lattice parameter of TM doped Mg0.95Mn0.05O has been observed. Using Rietveld refinement of XRD data, the space group and lattice parameters are determined. Scanning electron microscopy (SEM) measurements were performed to understand the morphology and grain size of the Mg0.95Mn0.01TM0.04O (TM = Co, Ni, and Cu) nanocrystals. The estimated band gaps as calculated by using UV-Vis spectroscopy are found to be 3.59, 3.61, 5.63 and 3.55 eV for Mg0.95Mn0.05O and Mg0.95Mn0.01TM0.04O (TM = Co, Ni, and Cu) nanocrystals, respectively. Both dielectric constant and dielectric loss is found to decrease due to TM (transition metal) doping. The ac conductivity is found to increase with increase in frequency. Electric modulus spectra reflect the contributions from grain effects: the large resolved semicircle arc caused by the grain effect. The results obtained in this study were discussed comparatively with those cited in the literature.


Introduction
Nanostructures have potential applications in modern science and technology due to their intriguing structural and optical properties. 1,2 Recently, nanostructures based on oxides have received considerable attention from researchers of the elds of material science, physics and chemistry 3,4 due to the presence of oxygen, a highly electronegative element, which exhibits the tendency of pulling the bonding electrons towards itself and away from the other elements thereby inducing substantial electric eld at the interatomic scale. 5 Nanomaterials based on metal oxides with high surface to volume ratio have allured considerable interest from the research and scientic community due to their conceivable applications in the eld of optoelectronics, nanoelectronics and sensing devices. In particular, magnesium oxide (MgO) is a fascinating basic oxide that has potential applications in catalysis, adsorption, synthesis of refractory ceramics, 6,7 nano electronics, optoelectronics and sensing devices 8 and superconductor products. 9 Adsorption, catalyst supports, and optical sensors are the areas where metal oxides are especially used. Besides these, they are also used in biocompatibility, bioimaging 10 and many other elds by virtue of their exceptional nanosized structures, superior chemical, morphological and optical band characteristics. 11 The quantum size effect generated by an increase in the band gap due to a decrease in the quantum allowed state leads to the change in the electrical and optical characteristics of nanosized particles, which in other senses improves the surface and interface effects. 12 MgO as ceramic has been focused due to its applicability in several areas. MgO is an accepted photocatalyst with exceptional chemical, mechanical, optical and electrical properties. Besides, the inexpensiveness and non-toxicity were regarded as the main reason for the acceptability of MgO materials in modern age of materials. Keeping in view its photocatalytic properties, excellent dielectric properties, the multidimensional applications of MgO such as refractory, paint, translucent ceramics, plasma display panel, absorbent for many pollutants and superconductor products were explored. [13][14][15] Magnesium oxide (MgO) exhibits a large band gap of 7.7 eV, excitonic binding energy of $80 MeV and posses high transmission in the ultraviolet (Uv) region. 16 Therefore, MgO can be used to enhance the band gap of ZnO by forming its solid solution with MgO. Since the phase purity, homogeneity, particle size, morphology, as well as crystallinity are the tools to determine the optical properties of materials, the care, control and selection of the method to synthesize the material is of the utmost importance.
A large number of techniques were commonly used for the preparation of MgO powders, such as sol-gel method, 17 ame spray pyrolysis, 18 chemical vapour deposition, 19 co-precipitation method 20 etc. Manganese (Mn) enters the MgO crystal preferentially in the divalent charge state occupying cubic Mg sites. Depending on the thermal history of the crystal, higher valence states are possible by virtue of which charge compensation can be achieved by Mg vacancies. 21 Among the different methods, the sol-gel method is the most effective method to prepare the nanopowders of metal oxides as it is fast, economic and low temperature is required by this method to synthesize the nanosized samples.
The ability to obtain single-phase metal oxide magnetic nanoparticles with controllable particle size and size distribution improves its adequacy in a wide range of technological applications. The sol-gel auto combustion was utilized to synthesize the metal oxide nanoparticles by various researchers in this eld. NiFe 2 O 4 nanoparticles were prepared by a simple and cost-effective polyvinylpyrrolidone (PVP) assisted sol-gel auto-combustion method. 22 Recently this method also shows option to synthesize advanced spinel ferrite one-dimensional (1D) and two-dimensional (2D) nano-structures. 23,24 La 0.67 -Sr 0.33 MnO 3 nanoparticles were also successfully synthesized via the sol-gel auto-combustion technique. 25 Herein, the Mg 0.95 Mn 0.05 O and transition metal doped Mg 0.95 Mn 0.01 TM 0.04 O (TM ¼ Co, Ni, and Cu) nanoparticles were prepared by a sol-gel auto combustion method. The structural, optical and electric properties of as prepared powders have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultra-violet visible spectrum (UV-Vis), Fourier transformation infra red (FT-IR) spectroscopy and dielectric measurements. The main goal of this study is to investigate the effect of TM doping on the structural, vibrational, optical and dielectric properties of Mg 0.96 Mn 0.04 O nanoparticles. For optoelectronic device applications, the controlled band gap is of the utmost importance. In this regard, we made an effort to tune the band gap with different TM doping using UV-Vis spectroscopy.

Synthesis
The When all the reactants are completely dissolved, citric acid was added to make a metal complex maintaining pH value at 11. The best about the present study is the preparation with the maintenance of pH and citric acid assistance to control reaction and particle size. Citric acid acted as a chelating agent and helps the reaction to proceed. The addition of citric acid dissolved the insoluble residue leading to the formation of a cation-citric acid complex. Further the nitrate salts are favored as precursors, because they serve as water-soluble low temperature NO 3 À oxidant source for synthesis. Many other fuels including DLalanine, hydrazine, acrylic acid, carbo-hydrazide, ethylene glycol and polyacrylic acid have also shown great promises. The whole solution was stirred through magnetic beet using a magnetic stirrer for 5 hours at 80 C temperature until the gel was obtained. The gel obtained was dried at 400 C for 4 hours to remove water and solvent content. The synthesized MgO powder was white in colour. The powder was calcined in air at 600 C for 10 h and then pressed into pellets of 10 mm diameter with 2 mm thickness. Finally, the pellets were sintered at 600 C for 6 h. Similar procedure was adopted for the synthesis of all other transition metal doped samples.
The formation of Mg 0.95 Mn 0.05 O takes place according to procedure as follows:

Characterization
The crystal structure, type of phases and particle size of Mg 0.95 Mn 0.05 O and transition metal doped samples of Mg 0.95 -Mn 0.01 TM 0.04 O (TM ¼ Co, Ni, and Cu) nanoparticles were investigated by means of room temperature X-ray powder diffraction technique using Bruker D8-Advance X-ray diffractometer with CuKa 1 (1.5406 A) radiation. The XRD data were collected in the 2q range from 10 # 2q # 80 with a step size of 0.02 and a scanning rate of 2 /min. The X-ray generator was set at 40 kV and 40 mA power setting. Scanning electron microscope images were recorded with a Philips XL30 ESEM (environmental scanning electron microscope).
Diffuse reectance spectra were recorded in the wavelength range 200-850 nm using UV-Vis spectrometer (Perkin Elmer, Lambda 950 -USA to estimate energy band gap). Dielectric measurements were carried out as a function of frequency in the range of 1-10 MHz on Novocontrol alpha-A high performance frequency analyzer at room temperature. High purity silver conducting paste was used to coat on the pellets for better electrical contact for the dielectric measurements.

Structural analysis
XRD analysis provides information about the structural characteristics of the material as the width and the intensity of the diffraction peaks depend on lattice strain, crystallite size and other imperfections such as stacking faults etc. The as prepared Mg 0.95 Mn 0.05 O based transition metal doped powders at the temperature of 600 C have been structurally characterized by room temperature X-ray powder diffraction (XRD). The XRD patterns of Mg 0.95 Mn 0.01 TM 0.04 O (TM ¼ Co, Ni, and Cu) ¼ (Co, Ni and Cu) samples are shown in Fig. 1 in which all the samples present similar peak positions. The diffraction peaks of samples are indexed as (111), (200), (220), (311) and (222). All the samples exhibit the reections corresponding to cubic MgO phase having space group Fm 3m. The powders obtained showed a crystallized structures, which is matched with JCPDS PDF (no. 45-946) and consistent with earlier reports. 26,27 All the samples are in single phase and no diffraction peaks from other species could be detected within measurement range. It means that the TM ion successfully occupies the lattice site rather than interstitial ones. One can observe a slight shi of the position in the diffraction peaks indicating a light variation in lattice parameters. The lattice parameters are calculated using the formula for cubic structure.
Here, d is the interplanar distance, h, k, l are the miller indices and 'a' is the lattice parameter. and Cu 2+ (0.72 A), respectively than ionic radii of Mn 2+ (80 A) ion. This is in good agreement with previous reports. 28 The width of the peak is inversely related to the crystallite size, which has been computed from the full width half maximum (FWHM) of the intense peak using the Debye-Scherer's formula: In eqn (2), symbols as 'l' is the wavelength of CuKa 1 radiation and 'b' is the full width half maximum (FWHM) of the highest intense peak of diffracting angle 2q. Table 1 shows the values of particle size (d) and lattice variables obtained from the diffraction patterns of the powdered samples of Mg 0.95 Mn 0.05 O and Mg 0.95 Mn 0.01 TM 0.04 O (TM ¼ Co, Ni, and Cu). It was found that the crystallite size of samples lies in the range of 32.3-47.6 nm.
No doubt, particle size is variable with temperature. On annealing the lattice defects and strains generally decreases, however, it can also cause coalescence of crystallites that result in increasing the average size of the nanoparticles. The nano particles of metal oxides in the range of 40-50 nm at around 600 C are also earlier reported. [29][30][31] In this work particle size is calculated using Debye-Scherer's formula: the width of the peak is inversely related to the crystallite size, which has been computed from the full width half maximum (FWHM) of the intense peak. Instrumental broadening is not considered in entire XRD measurement.
Rietveld analyses of the diffraction data collected at the room temperature were carried out using The typical values of structural parameters in cubic coordinates for all samples rened by a standard Rietveld technique using FullProf renement program are listed in Table 2 along with the values of the prole factor R p , weighted prole factor R wp , expected weighted prole factor R exp , Bragg factor R B , structure factor R F , goodness of t c 2 and goodness of t (GOF) index. Here, red symbols are the observed prole; the black solid line is the calculated prole, tick marks below indicate the position of the allowed Bragg reections, the blue line curve at the bottom gives the difference between the observed and calculated data. All these parameters were used as numerical criteria of the quality of the t of calculated to experimental diffraction data.  Agglomeration process can be suppressed to control the size of nano crystallites by the introduction of organic molecules during the synthesis process as a capping agent. From SEM images, the average grain size for all four samples under investigation is in the range of 31-48 nm. The crystallite size determined from the SEM measurement is in good agreement with the size as obtained from the XRD.
In order to conrm the exact composition of the prepared nanocrystals, the EDAX analysis were carried out for

UV-Vis analysis
UV-visible absorption study is a powerful probe for investigating the effects of impurity doping on optical properties of semiconductor nano structures. The doped nanostructures are expected to have different optical characteristics as compare to pristine nanostructures. The optical diffuse reectance spectra were recorded using diffuse reectance spectroscopy to determine the optical band gaps of the as synthesized samples. All spectra were recorded in the range of 200-800 nm.
In order to calculate the direct band gap, Tauc relation is used: In eqn (3), the notations a is the absorption coefficient, A is a constant, n ¼ 1/2 for direct band gap semiconductor. The E g values are determined by extrapolating the linear region of the (hyF(R)) 2 as functions of hy. In other words, the hy value of x-axis at (hyF(R)) 2 ¼ 0 gives the band gap (E g ). Fig. 5 shows the plot for the percentage of reection (hyF(R)) 2 as a function of band gap energy hy (eV) for all the studied samples. An extrapolation of the linear region of a plot of (ahn) 2 vs. hn gives the value of the optical band gap E g (Fig. 5) 32 E g values of MgMnO nanoparticles increased with Ni content. The incorporation of Ni is accompanied by a systematic high-energy shi of the band gap extending down to the blue spectral range. The increase in the band gap or blue shi can be explained on the basis of the Burstein-Moss effect about lling the bottom of the conduction band depending on the increase in the carrier concentration. Therefore, the interstitial of Ni 2+ in MgO lattice may cause an increase in the carrier concentration and the Fermi level moves closer to the conduction band with an increase in the carrier concentration. Consequently, the lling of the conduction band by electrons generally causes an increase in the optical band gap or blue shi. The same increase in E g was also earlier reported. 33 They reported a blue shi of the absorption edges from 3.22 eV (undoped ZnO) to 3.30 eV (5% Co-doped ZnO). Such an increase in the optical band gap is consistent with previous observations. 34-36

Dielectric measurement
The dielectric constant and dielectric loss of a material are two basic criteria that a material must match for the better applicability and efficiency as they affect many optoelectronic and transport properties. Dielectric studies have been done on Mg 0.95 Mn 0.05 O and Mg 0.95 Mn 0.01 TM 0.04 O (TM ¼ Co, Ni, and Cu) nanocrystals to investigate any variation of dielectric constant and dielectric loss with frequency and different transition metal ion doping.
The dielectric properties of materials are characterized by the complex dielectric constant (3) which is represented by 3 ¼ 3 0 À j3 00 . The real part (3 0 ) of dielectric constant is the measure of the amount of energy stored in a dielectric due to the applied eld and the imaginary part (3 00 ) of dielectric constant describes the dissipated energy in dielectric. The value of real part of dielectric constant (3 0 ) is calculated by using 3 ¼ Ct/(A3 0 ) where 3 0 is the permittivity of free space, t is the thickness of pellet, A is the cross sectional area and C is the capacitance of pellet. Fig. 6 shows the variation of dielectric constant (3 0 ) with frequency for Mg 0.95 Mn 0.05 O and Mg 0.95 Mn 0.01 TM 0.04 O (TM ¼ Co, Ni, and Cu) at room temperature. At lower frequency the dispersion of dielectric constant was observed. The large value of dielectric constant at lower frequency observed is attributed to the grain boundary defects or the presence of oxygen vacancies. 37 In addition to that, the large value of the dielectric constant is also due to the fact that the nanoparticles of Mg 0.95 Mn 0.05 O under the application of electric eld act as nano dipoles. With the decrease in the size of nano particle, the particles per unit volume increases and thereby increases dipole moment per unit volume and hence the high dielectric constant. 38   However, in the higher frequency regime i.e. frequency above 0.5 MHz, 3 0 is independent of frequency. As beyond a certain frequency limit, the hopping between different metal ions cannot follow the changing eld.  39 We note that the imaginary part of dielectric constant (3 00 ) shows a decreasing trend with increase in frequency (see Fig. 7), almost similar to real part of dielectric constant and loss tangent. This variation in imaginary part of dielectric constant (3 00 ) with respect to frequency may be due to several factors; such as conduction mechanism (hopping of electron between Mn 3+ and Mn 2+ ), materials composition of sample, annealing temperature, grown technique and particle size. 40 The ratio of energy dissipated and energy stored in the material determines the dielectric loss factor (tan d) and variation of dielectric loss with frequency at room temperature is shown in Fig. 8 41 In this work, we have observed that dielectric loss decreases at higher frequency which is due to suppression of domain wall motion. The dielectric loss is maximum at lower frequencies and is due to nearly equal hopping frequency between different ionic sites and the frequency of the applied eld.  Fig. 9. It is found that the ac conductivity progressively increases with the increase in the frequency of the applied ac eld. This is because rising in frequency would improve the electron hopping frequency. The ac conductivity is initially high for pure Mg 0.95 Mn 0.05 O and found to be less in transition metal doped Mg 0.95 Mn 0.01 TM 0.04 O (TM ¼ Co, Ni, and Cu) nanoparticles. The substitution of transition metal doping may initiate the defect ions, oxygen vacancies in the Mg 0.95 Mn 0.05 O nanoparticles and tends to segregate at the grain boundaries due to the diffusion at the time of sintering and cooling processes. These defects block the ow of charge carriers at the grain boundaries and cause decreases in the conductivity initially thereaer the conductivity of doped nanoparticles increases. Mg 0.95 Mn 0.01 -Co 0.04 O has highest value of ac conductivity both at low and high frequencies as compared to other transition metal doped samples.

Conclusions
The O nanostructured materials decreases with increase in frequency having wide peaks in certain range of frequency, which is due to the resonance among the hopping frequency of charge carriers and applied frequency. Electric modulus spectra reect the contributions from grain effects: the large resolved semicircle arc caused by the grain effect.

Conflicts of interest
There are no conicts to declare.