Structural and magnetic properties of Fe and Ni co-doped SnO2 nanoparticles prepared by co-precipitation method

In this research work Fe and Ni co-doped Tin oxide (SnO2) nanoparticles have been prepared by co-precipitation method. The samples were prepared at various combination of Fe and Ni from 0% up to 10%. The produced nanoparticles were studied by x-ray diffraction (XRD), Scanning Electron Microscopy (SEM), UV–vis Spectrophotometer, Fourier Transformation Infrared Spectroscopy (FTIR) and Vibrating Sample Magnetometer (VSM). The XRD study reveals the formation of rutile structure of the undoped and doped SnO2 nanoparticles with the average crystallite size of 1.5–10.8 nm. Metal oxide bonding is confirmed through FTIR measurement. Optical band gap redshift (3.9 to 3.64 eV) with doping of Fe and Ni atom is observed. SEM image confirms the formation of spheroidal nanoparticles and size of the nanoparticle varies from 36 to 15 nm. The VSM study shows the ferromagnetic phase transition at 7% Ni, Fe doped SnO2 nanoparticles. This ferromagnetism arises for the oxygen vacancies and defect states. Further, increase of doping concentration of 10%, nanoparticles show the phase transition from ferromagnetic to paramagnetic. Such transition can be applicable in hyperthermia treatment and memory devices.


Introduction
Recently nanoparticle research have got much attention due to its potential application in various field of science and technology, like energy harvesting ( e.g.nanoparticle solar cell) [1], gas sensing [2], data storage [3] catalyst agent [4], biomedical etc [5,6] due to its improved chemical or physical properties relative to those of large-sized particles.The physical and chemical properties of nanoparticles can be tuned by adjusting the particle size, surface structure, chemical composition, particle surface etc [7,8].
The progress of nanotechnology with high performance in different areas requires materials of low cost, good chemical and thermal stability, desired magnetic, electrical, and optical properties, nontoxicity etc Oxide nanoparticles have a number of benefits, including high thermal and chemical stability, straight forward preparation processes, ease of engineering to the desired size with controllable electrical, optical and magnetic property.Recently research on room temperature ferromagnetism of oxide semiconductor called diluted magnetic semiconductor (DMS) is greatly enhanced due to its immediate applications in spintronics, nanoelectronics, nanophononics, magnetoelectronic, microwave devices, memory devices and medical treatment [9][10][11][12][13][14].
Different oxide nanoparticles are synthesized in various fields of research depending on their physical and chemical properties.Among them SnO 2 receives much attention due to their various mechanical, optical electrical and magnetic properties.Tin oxide is an n-type, IV-VI group semiconductor of significant band gap (E g = 3.6 eV), has thermal and chemical stability, high optical transparency, low electrical resistance, and low toxicity and room temperature magnetic property when doped with transition metal ions.It has potential applications in solar cells and optoelectronic devices as transparent conducting layer and electrode, gas sensing, photo catalytic activities, lithium batteries and various field of biomedical.It has potential applications in Si solar cells as window layer, conducting electrodes, perovskite solar cell as electron transport layer and buried layer, photodetector as buried layer, optoelectronic devices as transparent conducting layer and electrode, gas sensing devices, photo catalytic activities, flat panel displays, lithium batteries and various field of biomedical [15][16][17][18][19][20].In biomedicine there are some difficulties resolving the significant therapeutic benefits of metal oxide nanoparticles with their harmful side effect.As SnO 2 is nontoxic it can be a potential candidate in this area.Transition metal (TM) doping in SnO 2 alters its bulk, thin film, and nanocrystalline structural, optical, and magnetic properties.Moreover, because of the localized oxygen vacancies and high carrier density, it may be considered as a host semiconductor for the construction of DMS.The study of transition metal doped SnO 2 reported the stable ferromagnetic phase rather than antiferromagnetic phase.For example, Aditya Sharma et al reported the ferromagnetic properties of Fe and Ni doped SnO 2 nanoparticles [21].They found the saturation magnetization and magnetic moment decrease with the increase of Fe or Ni concentration due to the super exchange interaction of metal ions.Besides, Khan et al reported the increase of ferromagnetism up to 2% Ni doping of Co and Ni doped SnO 2 nanoparticles [22].They found the maximum saturation magnetization 0.201 emu g −1 , coercivity 50 Oe and a remanant magnetization of 0.1 emu g −1 .Furthermore, Sharma et al reported the weak ferromagnetic properties of 1 wt% and 2 wt% Fe doped SnO 2 nanoparticles [23].On the other hand, Supin et al reported the zero coercivity and negligible remanence for Fe, Ni and Co doped SnO 2 nanoparticles [24].They explain the coexistence of ferromagnetic and paramagnetic phase through bound polaron model.M. Duhan et al also reported the Cr and Fe co-doped SnO 2 nanoparticles and found the increase of saturation magnetization from 4 × 10 -3 emu g −1 to 6 × 10 -2 emu g −1 due to creation of oxygen vacancies [25].However, it is difficult to produce stable nanostructured materials with the appropriate properties because they are very reactive and have a huge surface area, which leads to the production of secondary phases.To achieve the desired properties, such as homogeneity, shape, crystallite size, etc, the selection of the synthesis technique is also crucial.Pure and TM-doped SnO 2 nanostructured materials can be prepared using a variety of synthesis techniques, including sol-gel [26], sputtering [27], solvothermal processes [28], hydrothermal methods [29], microwave synthesis [30], co-precipitation [31], flame spray pyrolysis method [32], surfactantmediated methods [33] etc Here, we have selected a chemical co-precipitation procedure because it has many advantages over alternative synthesis methods.Particularly, it is a straightforward, cost-effective, and simple method for manufacturing fine, crystalline, and homogenous magnetic nano particles at comparably lower calcination temperature, yielding higher product yields and greater surface areas.
Although several researcher reported Fe, Ni, Co and Cr doped SnO 2 nanoparticles but the co-doping of Fe and Ni with oxide matrix is rarely reported.Besides, the magnetic behavior of simultaneous doping of Fe and Ni is not well understood.Hence in this research, we have synthesized Fe and Ni co-doped SnO 2 nanoparticles through co-precipitation method.The influence of simultaneous incorporation of Fe and Ni on the microstructural, optical, and magnetic properties of SnO 2 have been studied.The enhanced ferromagnetic is observed for 7% Ni and Fe co-doped SnO 2 with coercivity 205 Oe and saturation magnetization 0.21 emu gm −1 , which is crucial for diluted magnetic semiconductor.On the other hand, the coercivity is decreased (101 Oe) but the saturation magnetization is increased (0.37 emu/gm) for 10% Fe and Ni doped SnO 2 .The phase transition of the as synthesized Fe and Ni doped SnO 2 nanoparticles can be used in spintronic application.The solution is then placed on an electric heater containing magnetic stirrer.The solution was stirred at 500 rpm for about 15 min at a temperature of 75 °C.During stirring a 25% of ammonia water was added drop by drop to the solution for 1 min to obtain a milk white precipitation.After stopping the addition of ammonia water, the solution continued to stir another 15 min.It was kept 24 h in equilibrium for precipitation.Then it was filtered and washed with deionized water several times.The precipitate was then dried for 24 h at 45 °C.Thereafter, it was calcinated for 1 h at 600 °C and crushed using mortar and pastel to obtain SnO 2 nanopowder.The following figure 1 shows color of the nanopowder prepared with various Ni and Fe concentrations.

Characterization
Crystal structure and phase composition of the prepared nanoparticle samples were studied by Rigaku DMAX-1000 x-ray diffractometer (XRD) with Cu-K α radiation (λ = 1.54056Å).The data acquisition was performed over the diffraction angle ranging from 20°to 80°.FTIR analysis was performed by a Perkin Elmer  SPECTRUM-GX spectrometer scanning over a range of between 3000 and 4000 cm −1 to identify the bonding vibration.The FTIR samples were prepared by mixing about 2% of synthesized powder with KBr.The particle size and morphology were studied by a scanning electron microscope (SEM) model JSM-7610F, Japan.EDX analysis was performed to study the composition of the doped nanoparticle.Using a vibrating sample magnetometer (VSM), Lakeshore 8604, the produced nanoparticles room temperature magnetic properties were studied.
The XRD patterns demonstrate the formation of a single-phase rutile tetragonal crystal structure [34].There were no impurity peaks in the region of x-ray detection in the powder sample of Fe, Ni co-doped SnO 2 .The broad peaks in the x-ray diffraction patterns show that the produced samples' particles are in the nanometer range.The samples average crystallite size was calculated using Debye-Scherrer's formula [35], where λ is the x-ray wavelength (Cu K α radiation and equals to 0.154 nm), θ is the Bragg diffraction angle, and β is the full width at half maximum (FWHM) of the XRD peak appearing at the Bragg diffraction angle θ.The following relation can be used to estimate the macrostrain (ε): Table 2 shows the summary of the possible phase and crystallite size and lattice constant of the samples determined from the dominant peaks.It is observed that the crystallite size varies from 1.5 nm to 10.8 nm with increase in doping concentration.Lattice parameter is influenced by a number of variables, including defects, external strain, the dopant concentration, and the difference in the ionic radii of host and doping ions [36].As the Fe and Ni content increases the lattice parameter a and c decreases.The decrease in lattice parameters confirmed the substitution of Fe 3+ ions and Ni 2+ ions into Sn 2+ lattice sites.This is due to the fact that the smaller ionic radius of Ni (70 pm) and Fe (60 pm) substituted to the higher radius of Sn (118 pm).The positive values of microstrain (table 2) of the samples reveals existence of tensile stress on the surface of nanoparticle.The introduction of tensile stress is originated from the doping of Fe and Ni atoms in SnO 2 matrix that inhibits the crystal growth as a consequences the crystallite size reduces.

SEM and EDX analysis
The SEM (Scanning Electron Microscopy) images of the powder samples in figures 3(a), (b), (c), (d) depicted that particle are in the nanometer range with spheroidal shape.Besides the SEM micrograph reveals that the samples contain aggregates of microscopic particles.The morphology and size distribution of the particles were analyzed from the images using Image J software.The average particle size of the undoped sample is around 36 nm, which is larger than crystal size obtained from XRD.The average particle size of 4%, 7% and 10% Fe and Ni co-doped SnO 2 sample are around 20 nm, 18 nm and 15 nm, respectively.The reduction of the size of the nanoparticles with doping is due to the smaller ionic radii of Fe and Ni compared to the host material.impurities were confirmed by EDX spectra.Due to the formation of oxide, the atomic percentage of Nickel (Ni) is less than that of Iron (Fe).The EDX pattern demonstrated that the nanoparticles are effectively synthesized.

Optical properties
The optical bandgap of the synthesized nanoparticles have been calculated by using Tauc's relation [37].
where, α is the absorption coefficient, A indicates a constant, hν is the photon energy and E g is the band gap. Figure 5 displays the variation of (αhν) 2 with photon energy for undoped and Fe, Ni doped SnO 2 .The extrapolating the linear portion of the graphs' energy axis indicated the optical bands position.The band gap was found to be 3.9 eV, 3.79 eV 3.74 eV and 3.64 eV for undoped , 4% doped, 7% doped and 10% doped samples, respectively.Similar red shift in the band gap of SnO 2 was also reported by other researchers [38][39][40].The possible causes to decrease the band gap are Ni and Fe atoms occupy the Sn atom sites.Besides, the formation of new recombination centers with lower emission energies responsibe for the energy band gap reduction in the presence of metallic dopant [23].Furthermore, introdution of foreign atoms in the oxide matrix may create defect states in the forbidden region, therefore the reduction of band gap is due to the transition of electrons thorugout those defects [41].The sp-d spin exchange interaction between band electrons and localised d electron of Fe and Ni which susbtitute Sn ions causes also to decrease the band gap [42][43][44][45].

FTIR analysis
Figure 6 illustrates the FTIR spectra of the prepared Fe, Ni doped SnO 2 nanoparticles.In the FTIR spectra the absorption band near 3400 cm −1 , 1600 cm −1 and 1400 cm −1 are due to vibration of the hydroxyl group (O-H) stretching of water molecule absorbed on the surface of SnO 2 [46].Absorption peaks below 670 indicate metaloxide (Sn-O) and metal-metal bonding.A major peak appeared at 620 cm −1 is responsible for Sn-O-Sn bonding.Peaks appear at around 1640 cm −1 are associated with the surface absorbed water molecules.Besides, there is a shift in the band position for all doped sample with respect to the undoped SnO 2 , which indicates the presence of extra metal bonding other than Sn after doping.It is also observed that the intensity of the absorption band decreases for 4% and 10% doped sample but it is increases for the 7% doped sample.Which can be correlated with the magnetic phase transition of the sample as described later in magnetic hysteresis measurement.

Magnetic properties
Using a vibrating sample magnetometer room temperature magnetic property of the obtained nanoparticles has been studied.Figure 7 shows the magnetic hysteresis (M-H) curve for all doped samples.The value of saturation magnetization, coercivity and remanent magnetization are summarized in table 3. It is noticeable that sample with 4% doping shows diamagnetic behavior.However, the sample with 7% doping shows ferromagnetic behavior with coercive field (H c ) 205 Oe.The observed ferromagnetism is due to oxygen vacancies or defect in the crystal [47].As SnO 2 surface comprising of Sn 4+ ions and O 2-ions, therefore doping of Fe and Ni in the oxide matrix initiates negative charge in the lattice and system becomes charge imbalance.To maintain the charge balance, O 2-ions need to left from the lattice and oxygen vacancies formed.Thus, doping might increase the concentration of oxygen vacancies in very small nanoparticle, and both cations and oxygen vacancies act as electron acceptors to promote electrons/holes transfer to the nanoparticle surface.This electron transfer creates magnetic bipolaron which is associated with spin split impurity band and ferromagnetic phase appears.Furthermore, it is observed that saturation magnetization is increased with increase in doping concentration but  the coercivity decreases.The maximum saturation magnetization is 0.37 emu/g for 10% sample, while the maximum coercivity (205 Oe) for 7% doping sample.As the concentration of dopant ion increases there is a greater probability for Fe or Ni atom to occupy Sn atom or O atom site, which causes to increase unpaired electrons i.e. magnetic moment and finally the saturation magnetization.Moreover, the XRD data confirm the rutile phase of undoped and doped SnO 2 nanoparticles and excludes the existence of secondary phases.Therefore, the increase of saturation magnetization is intrinsic property of Fe and Ni doped SnO 2 sample.Besides from the SEM image it is observed that the size of the nanoparticle decreases with increase in doping concentration.As the size of the particle is reduced, the coercivity increases due to the formation of single domain.Before application an external field the magnetization of a single domain particle lies along easy direction.Which is determined by the shape and magneto crystalline anisotropy.When an external field is applied in the opposite direction the particle is unable to respond by domain wall motion.The anisotropy forces which holds the magnetization in an easy direction are strong and so the coercivity large.Below some critical radius however the coercivity decreases.The drops in coercivity at very small particle size is the result of a corresponding reduction in anisotropy energy with size.The anisotropy energy, which hold the magnetization along an easy direction is given by the product of the anisotropy constant and the volume of the particle [48].
Further increase of doping concentration (10%), H c decreases to 101 Oe but saturation magnetization increases.Which indicates that it again switches towards paramagnetic phase.The antiferromagnetic interaction seems to dominate for Ni and Fe cluster and paramagnetic phase is observed.The transition occurs from relatively hard ferromagnetic to soft magnetic behavior is associated with residual strain as obtained from XRD data which reduces magnetic anisotropy of the doped sample.

Conclusion
Fe, Ni co-doped SnO 2 nanoparticles have been successfully prepared by co-precipitation method.XRD study confirms the crystalline nature of the samples with average crystallite size in the range of 1.5 to 10 nm.The SEM study shows the nanoparticle size of the sample below 80 nm.EDX analysis confirms Ni and Fe atoms substitute Sn atoms.Band gap of the nanoparticles is observed to decrease with doping concentration which is due to the sp-d spin exchange interaction among Fe and Ni ions that substitute Sn ions.VSM study confirms the ferromagnetic nature for 7% doping.The magnetic properties are highly dependent on the doping concentration.At first the ferromagnetic properties improved with the increase of doping concentration.Then phase transition occurs and the samples show paramagnetic behavior with higher Ni and Fe doping concentration.This phase transition property suggests the nanoparticles may use in the application of hyperthermia treatment and memory devices.

Figure 2 .
Figure 2. X-ray diffraction pattern of as deposited powder samples.

Figure 5 .
Figure 5. Variation of (αhν) 2 with photon energy for undoped and Ni and Fe codoped SnO 2 films.

Table 1 .
Sample preparation condition summary.