Structural, optical, and magnetic properties of Mn and Fe-doped Co 3 O 4 nanoparticles

Mn and Fe-doped Co 3 O 4 nanoparticles were prepared by a simple precipitation method. The synthesized particles were characterized by X-ray di ff raction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), UV-Vis absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and vibrating sample magnetometer (VSM) techniques. XRD analysis showed the cubic structure of Co 3 O 4 . SEM and TEM images confirmed the formation of interconnected nanoparticles. Mn and Fe-doped Co 3 O 4 showed broad absorption in the visible region compared to undoped sample and the band gap values are red shifted. Five Raman active modes were observed from the Raman spectra. FTIR spectra confirmed the spinel structure of Co 3 O 4 and the doping of Mn and Fe shifts the vibrational modes to lower wave number region. The magnetic measurements confirmed that Fe-doped Co 3 O 4 shows a little ferromagnetic behavior compared to undoped and Mn-doped Co 3 O 4 , which could be related to the uncompensated surface spins and the finite size e ff ects. C 2015 All where noted,

of ferromagnetism under external magnetic field due to unpaired atom spin orientation near the surface of the nanocrytals and the interaction of atoms. It is well known that the magnetic properties of nanomaterials strongly dependent on the shape and size of their particles, crystallinity, magnetization direction, and so on. 13 Recently, several strategies are employed to improve the behavior of metal oxides by means of introducing various dopants 15 or forming nanocomposites with p-n junction, 16 or by tuning the morphology. 17 Here, we concentrate on doping, since doping is utilized to modify the electronic structure of nanoparticles to achieve improved catalytic, electro-optical, magnetic, chemical, and physical properties. Anandhababu et al. 18 worked on size and surface effects of Ce-doped NiO and Co 3 O 4 nanostructures. The increase of Ce doping concentration induces ferromagnetic phase in antiferromagnetic NiO and Co 3 O 4 due to finite size and oxygen vacancy created by the Ce ion, where the existence of Ce 3+ state is responsible for the weak RTFM. Grewe et al. 9 observed enhanced electrochemical performance in Fe-doped Co 3 O 4 , where the doping of Fe changes the electronic structure, which in turn affects the conductivity and transferability of the material. Mariammal et al. 15 studied the RTFM in Mn-doped CuO nanoflakes induced by oxygen vacancies and surface related defects. Ju et al. 19 studied the effect of dopants in Co 3 O 4 and reported that doping of Mg in Co 3 O 4 reduces the particle size. Hence, the present work is devoted to study the effect of doping in the structural, optical, and magnetic properties (at RT and low temperature) of Co 3 O 4 .

II. EXPERIMENTAL
Undoped, Mn, and Fe-doped (1, 3 at.%) Co 3 O 4 nanoparticles were prepared by precipitation method. 20 Cobalt nitrate was used as a precursor solution and the aqueous solution of oxalic acid was used as a precipitation agent. In a typical synthesis process, 0.2 M cobalt nitrate and 0.4 M oxalic acid were dissolved in distilled water separately. The aqueous solution of precipitating agent was added to the cobalt precursor solution. The mixture was stirred for 2 h at RT. The pink coloured precipitates of

III. CHARACTERIZATION
X-ray diffraction (XRD) measurements of undoped, Mn and Fe-doped Co 3 O 4 nanoparticles were performed at RT using PANalytical X'Pert X-ray diffractometer with Cu-Kα radiation (wavelength: 1.54056 Å) at a step size of 0.02 • over the 2θ range of 10 to 90 • . The morphology of the samples were analyzed by scanning electron microscope (SEM, VEGA 3 TESCAN) operating at an accelerating voltage of 30 kV and high resolution transmission electron microscope (HRTEM) (JEOL 3010) with an accelerating voltage of 300 kV. The RT UV-Vis absorption spectra were recorded by using UV-Vis absorption spectrometer (Shimadzu). Fourier transform infrared (FT-IR) spectra were measured using the KBr method on a Fourier transform infrared spectrometer (Shiraz) at RT in the range of 4000 − 400 cm −1 with a resolution of 1 cm −1 . Micro-Raman spectra were  measured at RT using LABRAM HR visible (400-1100 nm) model with a 632.8 nm excitation source of He-Ne laser. The magnetic measurements were carried out using 14T-PPMS vibrating sample magnetometer.

A. XRD analysis
The XRD spectra of undoped, Mn and Fe-doped Co 3 O 4 nanoparticles are shown in Fig. 1, from which the phase and crystal structure of the samples are analyzed. It can be seen that all the samples are well crystallized with all diffraction peaks, which can be well indexed to the cubic structure of Co 3 O 4 , without any traces of other phases or impurities. The peaks are in good agreement with the JCPDS data (JCPDS Card No. 78-1970). The doping doesn't affect the cubic structure, whereas it affects the crystallinity of the samples, which can be seen from the changes in the intensity of the diffraction peaks. The competition between the tetrahedral (A) and octahedral (B) coordinate cations in the spinel structure (AB 2 O 4 ) can also be responsible for the different intensity ratios observed for the doped samples. 21 Mn-doping decreases the crystallinity but Fe-doping increases the crystallinity. Slight shift in the peak position of doped samples is observed, which is due to the variation in the lattice constants. The calculated lattice parameters are presented in Table I Table I.

B. SEM and HRTEM analyses
The morphology of the as synthesized samples were characterized by SEM. Fig. 2 reveals the presence of rod like morphology with the length and diameter ranging from 3 -9 µm and 0.5 -1.5 µm, respectively. The rod like morphology was formed from the interconnected nanoparicles with average particle size of 150 nm. The doping slightly changes the particle size. Additional structural characterization was done through TEM equipped with the SAED, as shown in Fig. 3. Fig. 3(a) and 3(b) shows the TEM images of Co 3 O 4 nanoparticles at different magnifications. The particles almost possess spherical shaped morphology, where the size ranges from 82 -244 nm. It can be seen that the nanoparticles are arranged in such a way to form rod like structure. The HRTEM image of the nanoparticles (Fig. 3(c)) reveals the interlayer spacing of about 2.439Å, which corresponds to (311) plane of cubic Co 3 O 4 . The SAED pattern shown in Fig. 3(d) consists of concentric rings, which correspond to (220), (400), and (331) planes of polycrystalline Co 3 O 4 with cubic structure.

C. UV-Vis analysis
The UV-Vis absorption spectrum of Co 3 O 4 is shown in Fig. 4(a). The peak at 275 nm corresponds to the bonding-antibonding (π-π * ) electronic transition between cobalt and oxygen (Co= = O). 22  while the second band with the O 2− → Co 3+ charge transfer (Co 3+ level located below the conduction band). 24,25 With Mn and Fe doping, the spectra ( Fig. 4(b)-4(e)) show broad absorption, which is due to the creation of surface related defects in the nanoparicles. 15 The optical band gap of undoped and doped Co 3 O 4 is obtained from Tauc's plot and the values are tabulated in Table II. The first band gap shows red shift and the second band gap shows blue shift compared to that of bulk (E g = 1.48 and 2.19 eV) due to the non-uniform size of the particles.   Fig. 6 shows the FTIR spectra of undoped, Mn and Fe-doped Co 3 O 4 nanoparticles. Two absorption bands at 663 and 571 cm −1 are originated from the stretching vibrations of the metal-oxygen bond, which confirms the spinel structure of Co 3 O 4 . The first band is associated with OB 3 vibration in the spinel lattice and the second band with the ABO 3 vibration, where A and B denotes the Co 2+ (3d 7 ) in tetrahedral site and Co 3+ (3d 6 ) in an octahedral site, respectively. 27 With the addition of Mn and Fe, these modes are shifted slightly to lower wave number region. The shift in the characteristic peaks can be seen from the dotted lines presented in the inset of Fig. 5, which is associated with changes in the surface area and surface defects due to doping. 15

F. VSM analysis
The magnetic properties of the undoped and doped samples were accessed by recording both temperature dependent magnetization and magnetic hysteresis curves. The M(T) and M(H) were carried out by vibrating sample magnetometer in the temperature range of 10 -300 K and the field range of −5 ≤ H ≥ +5 KOe, respectively. The notations M, H and T represent magnetic moment, magnetic field and temperature, respectively.  signature of hysteresis is observed in both the samples, as seen in Fig. 7 Fig. 7, 8 and 9. The broad peak is observed around 35 K, which is the freezing temperature of residual spin moments, and this peak suggests that undoped and doped samples of Co 3 O 4 nanoparticles have weak ferromagnetic behavior at low temperature. The ZFC (Zero field cooled) and FC (Field cooled) points overlap above 35 K and magnetization curve decreases with increasing temperature, which indicates the antiferromagentic behavior at high temperature, and weak ferromagnetic behavior at low temperature. 17

V. CONCLUSIONS
Undoped, Mn and Fe-doped Co 3 O 4 were synthesized by simple precipitation method. The structural characterization of the samples explored by XRD showed cubic structure with reduced crystallite size of 24-27 nm upon Fe-doping compared to undoped and Mn-doped Co 3 O 4 samples. The size and morphology of undoped, Mn, and Fe-doped samples were investigated by TEM and the plane of reflections corresponding to Co 3 O 4 was identified from SAED pattern. The incorporation of Mn and Fe into Co 3 O 4 broadened the absorption band due to the creation of surface related defects in the nanoparticles. Raman spectra confirmed the presence of Co 3 O 4 nanoparticles and the peaks were shifted to lower wave number region due to changes in the grain size of the particles after doping. FTIR spectra confirmed the spinel structure of Co 3 O 4 nanoparticles. The magnetic properties were analyzed from VSM, which confirmed that Co 3 O 4 possess weak ferromagnetic behavior after Fe-doping due to the smaller particle size and uncompensated surface spins.