Effect of calcination temperature on the structural, optical and magnetic properties of pure and Fe-doped ZnO nanoparticles

X-ray diffraction patterns of all the powder samples were recorded by a rotating angle X-ray diffractometer (Bruker AXS D8 Advance, Germany) equipped with a graphite monochromator and a mirror, at a fixed incidence angle of 1° to 5° and CuKα (wavelength = 1.540 Å, 40 kV, 100 mA) radiation source. The angular accuracy of the setup was 0.001° and the angular resolution was about 0.01°. The samples were scanned in an angular direction in the range of 20° to 90° (2θ) with the step size of 0.01°. Surface morphology of undoped (pure) and doped synthesized samples was determined by using scanning electron microscope Carl Zeiss Supras 55 which operated at a high acceleration voltage of 15 kV. The UV absorption spectra of the samples were recorded on the Systronics-2205 double beam spectrophotometer having a bandwidth of 1 nm and wavelength range of 200 nm to 1100 nm. Elemental composition of various samples was determined by EDS spectroscopy at an acceleration voltage of 200 keV. Magnetic hysteresis loop at room temperature was measured by using a vibrating sample magnetometer (VSM, Microsense EZ9) with a maximum magnetic field strength of 22.5 kOe.

Fig. 1 shows X-ray diffraction patterns of the synthesized pure ZnO samples calcined at different temperatures. Each XRD sample exhibits a hexagonal wurtzite phase. In all the samples peak intensity is very high which confirms good crystalline formation. It is clear from the XRD patterns that all the characteristic diffraction peaks can be indexed to the diffractions of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) planes with a hexagonal wurtzite structure. These diffraction peaks agree well with the values of the standard JCPDS No. 36-1451. XRD patterns clearly and show that the intensity of the peaks increases with an increase in calcination temperature, indicating increase in crystallinity at higher temperature.

Fig. 1

The average crystallite size was calculated by using Scherrer’s equation 1 and was found to be in the range of 31 nm to 42 nm as shown in Table 1:

where τ is grain size, β is the full width at half maximum, λ is the wavelength of X-ray (1.548 Å) and θ is the diffraction angle. From Table 1, it can be observed that the increase in particle size is significant at 750 °C as compared with the size at 600 °C. Migration of grain boundaries occurs at higher temperature which results in coalescence of small grains and forming large grains.

From the d-spacing values, lattice constants ‘a’ and ‘c’ were calculated using equation 2. The values are given in Table 1:

The calculated lattice parameters were also in good agreement with the values reported in literature.

Fig. 2 shows X-ray diffraction patterns of Fe-doped ZnO nanoparticles calcined at 450 °C, 600 °C and 750 °C. Each XRD sample exhibits the hexagonal wurtzite phase without any impurities and the secondary phase which shows incorporation of Fe dopant into the ZnO wurtzite lattice as a substitute atom. The intensity of the peaks in the XRD pattern is similar to that of pure ZnO, i.e. the peak intensity increases as the calcination temperature is increased. But, if we compare the corresponding peaks of pure and doped ZnO samples, it is observed that the intensity of doped samples decreases as compared to the pure samples. The further reveals that crystallinity reduces with the incorporation of Fe atoms into the ZnO lattice. It has been reported in literature that when a foreign particle is incorporated in a crystal lattice, it produces a strain as well as defects in the lattice which may deteriorate the crystal quality [21]. Table 2 summarizes the calculated average particle size and unit cell parameters. Average particle size varies from 29 nm to 38 nm. Again the variation is similar to that in the pure ZnO nanoparticles but the only difference is that the particle size decreases for the doped samples as compared to the pure samples at corresponding temperatures. The reason for that is due to introduction of large amount of foreign particles into the crystal lattice causing strain within the lattice and leading to lattice distortion, which results in a decrease in crystallinity of the material.

Fig. 2

The ionic radii of Fe²⁺ and Fe³⁺ are different from that of Zn²⁺. The ionic radii of Fe²⁺, Fe³⁺ and Zn²⁺ are 0.078 nm, 0.068 nm and 0.074 nm, respectively. If Fe ions exist in ZnO mainly in the form of Fe²⁺, due to larger ionic radius of Fe²⁺ ion than that of Zn²⁺, it will lead to compression strain and, as a result, particle size increases [22]. On the other hand, if Fe ions exist in ZnO mainly in the form of Fe³⁺, due to smaller ionic radius of Fe³⁺ ion than that of Zn²⁺, it will lead to tensile strain and, as a result, particle size decreases [23]. Therefore, it is reported that Fe²⁺ and Fe³⁺ ions may coexist which requires further investigation. For maintaining charge neutrality and to substitute Zn²⁺ ionic sites, the valence of Fe ion should be +2. The ionic radius of Fe²⁺ is larger than that of Zn²⁺ whereas that of Fe³⁺ is smaller. Therefore, it can be inferred that Fe²⁺ and Fe³⁺ ions coexist and are expected to distort the lattice structure for holding charge neutrality. So, in order to incorporate the Fe ions, crystal lattice of ZnO contracts or expands depending on the nature of the ion. So, all these factors lead to distortion or strain in the material and also produce defects which in turn affect the properties of the material.

The morphology of pure and Fe-doped samples was observed by using a scanning electron microscope (SEM). Fig. 3 shows SEM images of pure ZnO nanoparticles calcined at 450 °C, 600 °C and 750 °C, respectively. The SEM images confirm that all the samples are in nano range which is in good agreement with the particle size calculation using the XRD technique. It is clearly seen that the particles are spherical in shape. Further, it is clear that the particle size goes on increasing with an increase in calcination temperature.

Fig. 3

Similarly, Fig. 4 shows SEM images of Fe-doped ZnO nanoparticles calcined at 450 °C, 600 °C and 750 °C, respectively. All the samples are composed of particles in nano range which are spherical in shape as clearly seen from the SEM images. Further, it is clear that the particle size goes on increasing with an increase in calcination temperature. Comparing Fig. 3 and Fig. 4, the difference in particle size is clearly visible and again in good agreement with the XRD calculations.

Fig. 4

Fig. 5 shows energy dispersive X-ray spectra (EDS) of pure and Fe-doped ZnO nanoparticles which confirm the elemental composition of the synthesized nanoparticles (undoped and doped). The EDS analysis confirms the presence of Zn, O and Fe in the doped sample with no other impurity element.

Fig. 5

Fig. 5a to Fig. 5c show the EDS spectra of pure ZnO samples calcined at 450 °C, 600 °C and 750 °C, respectively. The EDS spectra indicate that the synthesized pure ZnO sample mainly contains Zn and O elements. The EDS spectral peak of O appears at 0.5 keV whereas Zn appears at 1 keV, 8.6 keV and 9.5 keV in all three samples. Fig. 5d shows the EDS spectrum of Fe-doped ZnO sample calcined at 600 °C. The EDS spectrum indicates that the synthesized Fe-doped sample mainly contains Fe, Zn, and O elements. The EDS spectral peak of O appears at 0.5 keV, Zn appears at 1 keV, 8.6 keV and 9.5 keV, while Fe signal appears at 0.7 keV, 6.4 keV and 7.2 keV.

The Zn ions concentration (wt.%) was found to be approximately 92.01 %, 91.69 % and 91.51 % for samples calcined at 450 °C, 600 °C and 750 °C, respectively, in pure ZnO samples as highlighted in Table 3. Fe ions concentration was found to be 2.18 % for Fe-doped samples calcined at 600 °C. The EDS results further verify the XRD results, which reveal that Fe-doped ZnO diluted magnetic semiconductor was successfully synthesized by the chemical precipitation method.

Results for pure and doped ZnO nanoparticles were compared by using the UV-Vis absorption spectroscopy. The band gap of semiconductor nanoparticles changes with a change in particle size. The UV-Vis absorption study of pure and Fe-doped ZnO nanoparticles samples has been carried out over the wavelength range of 300 nm to 800 nm. Fig. 6a shows the absorption spectra of the pure ZnO nanoparticles with the absorption peaks at 388 nm, 392 nm and 398 nm for calcination temperatures of 450 °C, 600 °C and 750 °C, respectively. Similarly, Fig. 6b shows the absorption spectra of the Fe-doped ZnO nanoparticles with the absorption peaks at 384 nm, 389 nm, and 395 nm for calcination temperatures of 450 °C, 600 °C and 750 °C, respectively.

Fig. 6

The value of the absorbance is dependent on the various factors, such as size of particles, flaws or defects in grain structure and oxygen deficiency. Lower band gap value (Eg) of pure and doped ZnO sample compared to bulk ZnO (3.39 eV) might be due to the presence of oxygen vacancy defects, which is in good agreement with the Eg value determined in the literature [24, 25]. From Table 4 and Table 5, it is seen that the band gap energy decreases with the increase in calcination temperature due to the increase in the particle size as has also been evident from the XRD results.

In contrast to pure ZnO, an increase in the value of band gap energy in doped ZnO nanoparticles is due to the sp-d spin exchange interaction between ocalized d electron and band electrons of doped transition metal ions substituting the vacancy of cation. The observed increase in value of Eg with transition metal ion doping could be mainly due to Fe incorporation at substitutional or/and interstitial sites of wurtzite structure of ZnO.

The room temperature M-H curves of the Zn_0.97Fe_0.03O nanoparticles calcined at different temperatures in the magnetic field ranging from 0 kOe to ±20 kOe, obtained from VSM measurements are shown in Fig. 7. Magnetic parameters of all samples are summarized in Table 6. Fig. 7a and Fig. 7b show M-H curves for Fe-doped sample calcined at 450 °C, 600 °C and 750 °C. From Fig. 7a, it can be observed that the sample calcined at 450 °C has very low coercivity and remanence whereas its magnetization saturation is high. On the other hand, it is observed from Fig. 7b that with an increase in calcination temperature, magnetic saturation decreases whereas both coercivity and remanence of Fe-doped sample increase. The magnitude of thermal vibrations of atoms increases with the increase in temperature. The increase in thermal atomic motions neutralizes the coupling forces between the adjacent atomic dipole moments and it causes misalignment of some dipoles, irrespectively of external field. This results in a decrease in magnetic saturation.

Fig. 7

So far, several reasons have been discussed to explain the origin of ferromagnetism and still this subject is controversial. One of the theories proposed that room temperature ferromagnetism is exhibited due to various intrinsic and extrinsic phenomena. Intrinsic phenomenon results from exchange interaction, and extrinsic phenomenon occurs due to formation of clusters or secondary phases of dopant semiconductor atoms [26].

In our study, the XRD and EDS results showed that dopant ions are successfully incorporated in the Zn wurtzite lattice without formation of any secondary phases and metallic clusters. However, there is a possibility of existence of secondary phases in the samples although they were not identified in our case. Therefore, let us consider all impurity phases that might be present in the samples and discuss about their ferromagnetic properties. Possible Fe-based oxides that might be present in Fe-doped samples are FeO and Fe₂O₃, Fe₃O₄ and ZnFe₂O₄. Secondary phases, such as FeO and Fe₂O₃ are anti-ferromagnetic at Neel temperature of 198 K and 963 K, respectively [27, 28]. Similarly, ZnFe₂O₄ phase is antiferromagnetic and can be excluded as the origin of ferromagnetism in our samples. In contrast to this, Fe₃O₄ phase is ferromagnetic but at Curie temperature of approximately 858 K [29]. Therefore, it is reasonable to conclude that the presence of ferromagnetic behavior in our samples is due to intrinsic coupling between the atoms of doped materials but not due to the presence of metallic clusters or secondary phases.

However, there are certain studies which claim that ferromagnetic behavior in transition metal-doped ZnO is correlated with defects such as oxygen or zinc vacancies [30–32]. Further, in some studies, the correlation between intrinsic defects such as oxygen or Zn vacancies and magnetism is also discussed [33, 34]. Karmakar et al. [35] investigated the origin of ferromagnetism in Fe-doped ZnO using ESR and M¨ossbauer spectroscopy. The analysis revealed that the Fe ions are present in both Fe²⁺ and Fe³⁺ valence states.

The presence of uncoupled Fe³⁺ ions is possibly due to hole doping in the system, which is caused by cation (i.e. Zn) vacancies. Therefore, once again it is concluded that the ferromagnetism observed in our Zn_0.97Fe_0.03O sample was due to the presence of Fe atoms in the form of Fe²⁺ and Fe³⁺ ions.