Effect of grain size and grain boundary defects on electrical and magnetic properties of Cr doped ZnO nanoparticles

https://doi.org/10.1016/j.molstruc.2014.02.056Get rights and content

Highlights

  • Single phase nanoparticles of pure ZnO and Cr doped ZnO have been synthesized through sol–gel route.

  • Defects like Hsingle bondO, H, and oxygen vacancy have been estimated via FTIR and XPS.

  • Dielectric and impedance spectroscopy have been discussed on the light of grain size and grain boundary defects.

  • There surface defects and size of nanoparticles effect on the FM properties of the samples.

  • The appearances of FM in pure ZnO indicate to the importance of grain sizes and grain boundary defect magnetic properties.

Abstract

Nanostructure of Zn1−xCrxO (x = 0.0, 0.05 and 0.1) were synthesized successfully through sol–gel route. The effects of grain size and grain boundary defects on the electrical and magnetic properties have been investigated. X-ray diffraction (XRD) and selected area electron diffraction (SAED) results reveal the single-phase character. Crystallite sizes were obtained from the XRD patterns whose values are decreasing from ∼27 to ∼16 nm with increase in Cr content from 0% to 10% respectively. X-ray photoelectron spectroscopy (XPS) confirms the incorporation of Cr3+/Cr2+ ions in the lattice structure of ZnO, which causes decreasing in the valence electron density of Zn. Dielectric constant (ε) has been explained in the light of Maxwell–Wagner interfacial model, which differentiates between the structure of grain-core and grain-boundary. Complex impedance spectroscopy has been used to separate the grain and grain boundary contributions, the high resistivity values (107 Ω) can be attributed to the dominance of grain boundary resistance. The samples exhibit room temperature ferromagnetic (RT-FM) behavior, which has been discussed based on BMP model and effect of grain/grain boundary structure.

Introduction

Transition metal (TM) doped Zinc oxide have attracted considerable interest as promising diluted magnetic semiconductor (DMS) owing to the possibility of inducing room temperature (RT) ferromagnetism for advanced spintronic applications [1], [2], [3]. The operations of spintronic devices can be controlled by electrical signal as well as by magnetic field. The origin of FM in DMS is not understood completely. Dietl model requires spin-polarized free carriers to mediate indirect exchange interaction between the Mn impurities in the Ruderman–Kittel–Kasuya–Yosida–Zener fashion [4], while the defect like oxygen vacancy is required in bound magnetic polaron (BMP) model to mediate the exchange mechanism of TM dopant [5]. The appearance of FM in pure ZnO NPs without magnetic dopant elements turns the attention to the importance of grain size and grain boundary defects and their effect on many characters [6]. The grain boundary of ZnO nanoparticles (NPs) contains many defects like dangling bonds, vacancies and micropores that can play an important role in the electrical and magnetic properties of NPs. Furthermore, the nanomaterials are highly sensitive and functionally efficient because of smaller grain size and high surface to volume ratio, where in case of nano-scale materials most atoms, ions and defects would reside at the surface of NPs rather than the core. The oxygen vacancy (VO) and Zn interstitial atoms (Zni) as defect in grain boundary are responsible for the conduction process in the host material ZnO [7]. Ionic radius of Cr3+ is less than Zn2+ which makes easy to incorporate into ZnO crystal structure and induce the ferromagnetism at room temperature and also enhances the green emission [8]. Duan et al. have prepared Cr-doped ZnO nanoparticles through soft chemistry route and studied ferromagnetic properties at room temperature [9]. In the present work, we report the effect of grain size and the layers of grain boundary structure on the electrical and magnetic properties of ZnO and Cr doped ZnO NPs were prepared through sol gel method, annealed at 450 °C. The structure and grain size have been investigated by using XRD and HRTEM. The organic defect has been detected by using FTIR and the Vo defect has been estimated via XPS. The dielectric properties and ac complex impedance spectroscopy have been studied. Vibrating sample magnetometer (VSM) technique has been used for studying the magnetic properties, the field dependence of magnetization (M–H curve) exhibits RT-FM, the origin of FM has been discussed based on BMP model and different effect from grain and from grain boundary structure. We aim to show the contributions of grain-core and the layer of grain-boundary on electrical and magnetic properties.

Section snippets

Experimental details

NPs of Zn1−xCrxO (x = 0.0, 0.05 and 0.1) have been synthesized through simple sol–gel route, using analytical reagents grade zinc nitrate [Zn (NO3)·6H2O] and chromium nitrate [Cr (NO3)2·9H2O] as the raw materials. Nitrates were dissolved in distillated water at 70 °C by using magnetic stirrer. Citric acid (C6H8O7) dissolved separately in distilled water for 30 min and then added to the nitrate solution slowly. The molarity ratio of nitrate to citric acid was 1:1 and the pH was about ∼3. Few drops

XRD and TEM analysis

Fig. 1 shows the XRD patterns of the un-doped ZnO and Cr doped ZnO NPs, the patterns are matched well with the standard peak positions of ZnO (JCPDS card no. 36-1451), revealing the single-phase nature with hexagonal wurtzite structure and possess p63mc (186) space group. The index of the diffraction peaks of all the samples have been done by using the PowderX diffraction refinement program. The crystallite size of NPs has been calculated using Debye–Scherer’s formula [10]:D=0.9λßcosθwhere D is

Conclusion

Nanoparticles of un-doped ZnO and Cr (5% and 10%) doped ZnO have been successfully prepared through sol–gel route. The XRD and SAED confirm the single phase nature. FTIR spectroscopy shows some organic defects in the NPs due to polarity character of ZnO. XPS show the oxidation state of Zn, Cr and the presence of oxygen vacancy at 530 eV, which may mediate the exchange interactions between the electrons in 3d state of Cr ions according to BMP model. The dielectric study shows normal behavior and

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