Effect of doping concentration on absorbance, structural, and magnetic properties of cobalt-doped ZnO nano-crystallites

Background

ZnO is not new for researchers. Its fundamental structural properties were studied in 1935 for the first time by Bunn[ ¹ ], and then detailed optical studies were carried out by Damen et al.[ ² ] and Decremps et al.[ ³ ] using Raman spectroscopy.

Initially, ZnO got the attention of researchers because of its potential applications in laser due to large exciton binding energy (60 mev) and wide band gap about 3.3 ev[ ⁴ ]. Now, ZnO is being studied because of its promising application for spintronics and optoelectronics.

Spintronics requires controlled conduction and high degree of spin polarization. ZnO is a semiconductor in nature and has very favorable structure carrier-induced ferromagnetism. After the theoretical study of Dietl et al. in which they observed room temperature ferromagnetism in Mn-doped ZnO[ ⁵ ], the researchers started a comprehensive study of Mn and other transition metal-doped ZnO. The electronic structure of doped metal is strongly affected by the electronic structure of host lattice. There is a strong hybridization of 3D-host energy levels and 3D-3D Coulomb interactions[ ⁴ ]. Initially, some researchers reported absence of ferromagnetism at room temperature. Simultaneously, some of them reported room temperature ferromagnetism but in the materials fabricated at low temperatures[ ⁶ , ⁷ ]. In our work, Co-doped ZnO nano-particles (Zn _{1 ‏− x} Co _x O, x = 1, 3, and 7) were successfully fabricated using chemical co-precipitation method to study the effect of doping concentration on structural, absorbance, thermal, and magnetic properties.

Methods

Results and discussions

XRD graphs of doped ZnO nano-particles (shown in Figure ¹ ) representing the standard picture of pure ZnO even at higher doping concentrations show successful substitution of cobalt atoms with zinc atoms[ ⁸ – ¹⁰ ]. But the profiles of higher doping concentrations contain some additional peaks at 2 θ = 47° which is a diffraction from the cobalt-crystal system. Although these peaks are very feeble, they exist and represent agglomeration of cobalt atoms at higher doping ratios. Distortion or other type of degradation (like shift of peaks and decrease of crystallinity) was not observed in doped samples as claimed by researchers working with Fe and Mn as dopants[ ¹¹ , ¹² ]. The crystallite size calculated using Scherer's formula was found to be 14.61, 14.9, and 15.27nm for 1%, 3%, and5% (impurity vs. crystallite size graphically shown in Figure ² ) cobalt-doped ZnO, respectively. A simultaneous DSC/TGA study of precursor was carried out during the fabrication of ZnO. DSC/TGA SDT Q600 measures amount and rate of change of mass as a function of temperature or time. Our DSC/TGA instrument is capable of performing both DSC and TGA simultaneously. DSC/TGA analysis was performed to study the change of phases during crystallization. The mixture of Zn(OH) ₂ and Co(OH) ₂ was taken before annealing and was put into the furnace of DSC in an inert environment. The simultaneous DSC/TGA graph is shown in Figure ³ . The following results were deduced from the obtained data: Since the samples were fabricated in ethanol environment and then washed with distilled water and ethanol several times, the materials contain traces of ethanol. The first peak was observed at 57.45°C which represents the mass loss due to evaporation of ethanol. This behavior can be observed from both heat flow and mass loss graphs. The second downward peak in heat flow graph and further decrease of mass was observed at 178.54°C and 190.64°C in weight curve, which represent decompositions of Zn(OH) ₂ and Co(OH) ₂ , respectively. Both the materials liberate water molecules. Further, 10% mass loss was observed at approximately 500°C due to the evaporation of water of crystallization. These results were in good agreement with the literature[ ¹³ ]. The crystallization occurs during the removal of water molecules. So, heat loss during crystallization could not be observed from the heat graph because this heat is absorbed in the material to liberate water of crystallization. We can observe weight loss at 500°C and an endothermic peak of smaller intensity.

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Fourier transformed infrared spectroscopy (FTIR), presented in Figures ⁴ , ⁵ , and ⁶ , was performed to study the absorbance properties of fabricated samples and hence to deduce the nature of bonds present in the fabricated samples. In the given profile on wave number axis, the absorbance peaks from wave number 3,400 to 3,900 cm ⁻¹ are the peaks representing the hydroxyl (−OH) groups, which show incomplete removal of organic solvent. The next absorbance peak at wave number approximately 1,630 cm ⁻¹ is due to the absorbance of organic group carbon (carbonyl carbon -C=O). The absorbance at wave number approximately 466 cm ⁻¹ is due to the typical bond between zinc and oxygen (Zn-O)[ ¹⁴ – ¹⁶ ]. The peaks from 700 to 900 cm ⁻¹ are attributed to the bond between cobalt and oxygen (Co-O)[ ¹⁷ ]. From the obtained FTIR profiles, we can observe the typical ZnO peaks and presence of organic contents in 1% and 3% doped samples, but in 5% cobalt-doped ZnO sample, we can observe additional peaks in wave number region 600 to 1,200 cm ⁻¹ which is the region of ZnO and Co-O bonding which again represents the presence of additional phases at higher concentration of doping.

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We have done VSM analysis to observe magnetic characteristics of our fabricated samples to study room temperature ferromagnetism and the change in behavior of hysteresis curve by changing the magnetic doping concentration. The given Figures ⁶ , ⁷ , and ⁸ represent VSM profiles of Co-doped ZnO sketched at room temperature. Obtained profile confirms the presence of ferromagnetic behavior at room temperature and hence a successful doping where the impurity magnetic atom has successfully replaced the atoms of host crystal lattice, as XRD revealed pure ZnO at lower ratios. Presence of magnetic behavior depicts the presence of magnetic atoms. The smaller width of loop for 1% magnetic doping shows that it is a soft magnet, and 1% doped sample does not contain any cluster of atoms or segregation. So, we may deduce that presence of ferromagnetic behavior is due to intrinsic coupling between the atoms of doped materials but not due to the presence of secondary phase (segregation). We observed a proportional change in the shape and width of hysteresis loop, a stronger loop for 3% and 5%, as shown in Figures ⁸ and ⁹ , doped samples. These results were again contrasting, where some researchers observed ferromagnetic ordering at lower doping concentration but paramagnetic behavior for higher doping concentration. For iron doping, it was explained that paramagnetic behavior is due to anti-ferromagnetic coupling at higher doping concentration[ ¹¹ , ¹⁸ – ²⁰ ]. In our cobalt-doped samples, a cobalt proportional ferromagnetic strength was observed due to different electronic configurations of cobalt and agglomeration of a little fraction of cobalt atoms (as revealed by XRD) which prevents anti-ferromagnetic coupling.

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Conclusion

Different compositions of cobalt-doped ZnO nano-crystallites were successfully fabricated using co-precipitation method. For fabrication of Co-doped ZnO, decomposition temperate of precursor was determined by DSC/TGA analysis. XRD revealed standard ZnO profile for lower doping but additional phases were present for higher doping. Similarly FTIR and VSM studies also revealed doping proportional properties.

Authors’ information

All authors are experienced researchers of Materials Science and Nanotechnology. SAS and RA are PhD faculty members; the others are senior PhD students.