Synthesis and magnetic properties of Zr doped ZnO Nanoparticles
© Zhang et al; licensee Springer. 2011
Received: 1 June 2011
Accepted: 10 November 2011
Published: 10 November 2011
Zr doped ZnO nanoparticles are prepared by the sol-gel method with post-annealing. X-ray diffraction results show that all samples are the typical hexagonal wurtzite structure without any other new phase, as well as the Zr atoms have successfully entered into the ZnO lattices instead of forming other lattices. Magnetic measurements indicate that all the doping samples show room temperature ferromagnetism and the pure ZnO is paramagneism. The results of Raman and X-ray photoelectron spectroscopy indicate that there are a lot of oxygen vacancies in the samples by doping element of Zr. It is considered that the observed ferromagnetism is related to the doping induced oxygen vacancies.
Diluted magnetic semiconductors (DMSs) have attracted intense interest due to their potential applications in spintronic devices [1–3]. DMSs are usually produced by doping semiconductors with transition metals (TMs). Through theoretically predicting, GaN and ZnO as typical n-type semiconductors would be ideal candidates for room-temperature (RT) DMSs . The room temperature ferromagnetism (RTFM) in TM-doped GaN has been reported in experiment and theroy, such as, Mn [5, 6], Gd , and Cr [8, 9]. Compared with GaN, ZnO has a lot of outstanding superiorities, as is known to all, which has a wide band-gap (3.37 eV at RT) and a high excitation binding energy (60 meV at RT), so ZnO has been got more and more attention. Otherwise, since Dietl et al. predicted that Mn-doped ZnO can show the clear RTFM and also has a higher Curie temperature (T C ) than RT , which triggered worldwide interest in research of the doping ZnO materials. At first, RTFM has been demonstrated for various kinds of TM-doped ZnO, for example, Mn , Co , Ni , and Fe . However, the origin of their magnetism remains controversy, because it is not yet clear whether the observed RTFM is truly intrinsic or related to secondary phases such as clusters . To avoid the impact from ferromagnetic (FM) elements, in recent years, RTFM in ZnO doping with other non-ferromagnetic elements has been discovered in experiment and theory, for instance, Cu [15, 16], V , Cr [18, 19], Li [20, 21], C , Er -doped ZnO. However, until now there is no consensus on the origin of FM in doping ZnO, so we researched the origin of RTFM in the doping ZnO materials, it was hoped that we could get a better explanation about this intractable issue.
Paul et al. prepared the Zr doped ZnO films using a sol-gel technique with post-annealing successfully and found the films of extremely great properties, such as in the structural, optical, and electrical aspects, otherwise, at higher Zr concentrations, increasing dopant atom forms some kinds of defects . Defects may cause FM to appear reported before [15, 23], so in this paper, we prepared Zr doped ZnO nanoparticles (NPs) by the same method and studied the structure and their magnetic property with the different Zr doping contents.
Zn1-xZrxO NPs were prepared by the sol-gel method with post-annealing. All the chemical reagents used as starting materials are analytic grade reagents and purchased without any further treatment. Firstly, 0.1 M Zn(NO3)2·6H2O and y M (y = 0.0005, 0.001, 0.0015, and 0.002) Zr(NO3)4·5H2O were dissolved into the ethylene glycol monomethylether (C3H8O2). Then, the dissolved solution was stirred for 4 h at 80°C and dried at 80°C in the oven to form the precursor. Finally, the precursor was annealed at 500°C for 1.5 h in the air and the series of Zn1-xZrxO NPs were obtained. At the same time, Zr contents of Zn1-xZrxO samples are consistent with the mole percentage (x = 0.005, 0.01, 0.015, and 0.02).
The morphologies of samples were characterized by scanning electron microscope (SEM, Hitachi S-4800, Hitachi High Technologies America, Inc., Schaumburg, IL, USA) and transmission electron microscope (TEM, JEM-2010, JEOL Ltd., Tokyo, Japan). Selected area electron diffraction (SAED) and x-ray diffraction (XRD, X' Pert PRO PHILIPS with Cu Kα radiation, PANalytical, Shanghai, People's Republic of China) were employed to study the structure of the samples. The vibration properties were characterized by the Raman scattering spectra measurement, which was performed on a Jobin-Yvon LabRam HR80 spectrometer (Horiba Jobin Yvon Inc., Edison, NJ, USA) with a 325 nm line of Torus 50 mW diode-pumped solid-state laser under backscattering geometry. X-ray photoelectron spectroscopy (XPS, VG ESCALAB 210, VG Scientific Ltd., East Grinstead, UK) was utilized to determine the bonding characteristics and the composition of the particles. The measurements of magnetic properties were made using vibrating sample magnetometer (VSM, Lakeshore 7304, Lakeshore Cryotronics, Inc., Westerville, OH, USA) and Quantum Design MPMS magnetometer based on superconducting quantum interference device (SQUID).
Results and discussion
In other element-doping systems, different mechanisms of FM have been reported. Hou et al. reported that the carrier-induced FM (RKKY or double exchange mechanism) might be applied to explain the FM in Cu-doped ZnO films, in which the free carrier concentration is vital to determine whether the material is PM or FM . Meanwhile, Hu et al. found that Cr ion substitution is necessary for establishing FM in Cr-doped ZnO films containing VZn. However, Ran et al. suggested that defects of Cu-doped ZnO films, such as oxygen vacancies and/or zinc interstitials, might contribute to the RTFM, thus, the observed RTFM was explained in terms of defect-related models . Otherwise, Qi et al. concluded that an exchange mechanism associated with oxygen vacancies was responsible for the FM in the Zn1-xErxO thin films . At the same time, the RTFM was clearly observed in In-doped ZnO nanowires, which may be associated with oxygen vacancies induced by In doping . In our system, the pure ZnO NPs show the PM behavior, but all of the other doping samples exhibit the clear RTFM, so it's sure that the RTFM is induced by doping of Zr. In the XRD patterns, all the intense peaks from Zn1-xZrxO (x = 0.005, 0.01, 0.015, 0.02) could be indexed the same hexagonal wurtzite structure as pure ZnO NPs, the increase in a and c parameter as a function of Zr concentration is consistent with the substitution of Zn2+ ions (0.74 Å) by Zr4+ ions (0.84 Å) [25, 26]. The more Zn2+ were substituted by Zr4+, the greater lattice distortion of ZnO would be generated, the more vacancies and/or interstitials should be got. After measured the Raman and XPS, our supposition has been affirmed that there are lots of oxygen vacancies in our samples. As a result, oxygen vacancies should be considered as the origin of FM in our samples, which seems to be similar to the series of Er , In -doped ZnO, where the oxygen vacancies also play a crucial role in the RTFM.
We successfully prepared Zn1-xZrxO NPs with the typical pure ZnO hexagonal wurtzite structure by the sol-gel method with post-annealing. All the samples have the clear RTFM, and M s per Zr atom of samples is sensitive to the content of Zr, and decreases continuously as the increase of the doping Zr content through the magnetic measurement at RT. Combining with the results of Raman and XPS, we suppose that the FM of the Zn1-xZrxO NPs is owing to the oxygen vacancies inducing by doping of the nonmagnetic element of Zr.
This work is supported by National Science Fund for Distinguished Young Scholars (Grant No. 50925103 and 11034004), the Keygrant Project of Chinese Minisity of Education (Grant No. 309027), and NSFC (Grant No.50902065).
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