Room temperature ferromagnetism of pure ZnO ...

JOURNAL OF APPLIED PHYSICS 105, 113928 共2009兲

Room temperature ferromagnetism of pure ZnO nanoparticles Daqiang Gao,1 Zhaohui Zhang,1 Junli Fu,2 Yan Xu,1 Jing Qi,1 and Desheng Xue1,a兲 1

Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, People’s Republic of China 2 College of Science, Minzu University of China, Beijing 100081, People’s Republic of China

共Received 10 March 2009; accepted 4 May 2009; published online 11 June 2009兲 We report the room temperature ferromagnetism 共RTF兲 of pure ZnO nanoparticles, which were prepared by coprecipitation method. Magnetization measurement indicates that the ZnO nanoparticles annealed in air at 450, 550, 650, and 800 ° C exhibit the RTF and the decrease in the ferromagnetism is performed with the increase in annealed temperature. Selected area electron diffraction, x-ray diffraction, and x-ray photoelectron spectroscopy measurements show that all the samples possess a typical wurtzite structure and no other impurity phases are observed. The results of the Raman spectra indicate that there are lots of defects existing in the fabricated samples. It is also found that the ferromagnetism of ZnO nanoparticles increases after annealing in vacuum condition and decreases after annealing in a rich-oxygen atmosphere. These results confirm that the oxygen vacancies play an important role in introducing ferromagnetism for the ZnO nanoparticles in our case. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3143103兴 I. INTRODUCTION

Zinc oxide, one of the most promising multifunctional materials, has been demonstrated to be applicable in lightemitting diodes, solar cells, room temperature ultraviolet lasers, field-effect transistors, gas sensor, and piezoelectricgated diode.1–6 Recently, the ferromagnetic function by dilute doping of 3d transition-metal ions into its host also attracted wide attention due to its potential applications in spintronics.7,8 Although numbers of results have been reported, the origin of ferromagnetism in transition-metal doped ZnO has not been fully understood. When different groups try to give various explanations to the ferromagnetism in doped ZnO, an unexpected ferromagnetism was found in the undoped HfO2 thin films.9,10 This result challenges the understanding of magnetism for the researchers, because neither Hf4+ nor O2− are magnetic ions and the d and f shells of the Hf4+ ion are either empty or full. In succession, ferromagnetism was observed in undoped wide band gap semiconductor nanoparticles and thin films, such as TiO2, ZnO, SnO2, In2O3, Al2O3, CeO2, etc.11–14 These reports created great excitement and threw a wider debate onto the origin of magnetism in these pure semiconductors, as it is widely believed that a transition-metal doping plays an essential role in introducing ferromagnetism into nonmagnetic oxides. Furthermore, ZnO has its advantages as it is a transparent direct wide band gap semiconductor with a large exciton binding energy, which allows its application in optoelectronics. Therefore, it is important to know whether the undoped ZnO can be ferromagnetic or not. If it is, knowing its possible origins is more important. It seems that synthesizing pure magnetic ZnO and investigating its magnetic properties a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0021-8979/2009/105共11兲/113928/4/$25.00

are valuable. In this paper, ZnO nanoparticles were synthesized by a coprecipitation method, which can avoid the influences of the substrate or the interface between film and substrate. At the same time, the room temperature ferromagnetism 共RTF兲 of the nanoparticles was studied.

II. EXPERIMENTAL

ZnO nanoparticles were prepared by the coprecipitation technique with the postoxidation annealing in air atmosphere. Briefly, highly pure Zn共NO3兲2 • 6H2O 共99.999% from Aldrich兲 of 2.6 g was dissolved in the mixture of 50 ml de-ionized water and 20 ml alcohol. Then the NH4OH solution was added into this mix solution gradually until the pH level reached 8. The mixture was stirred for 4 h at room temperature and then roasted at 50 ° C for 6 h. In the end, the precursor was annealed at 450, 550, 650, and 850 ° C for 2 h in air atmosphere, respectively. The morphologies of the nanoparticles were obtained by using a scanning electron microscope 共SEM兲共Hitachi S-4800兲 and transmission electron microscope 共TEM兲共JEM2010兲. The selected area electron diffraction 共SAED兲 and x-ray diffraction 共XRD兲共X⬘ Pert PRO PHILIPS with Cu K␣ radiation兲 were employed to study the structure of the nanoparticles. The vibration properties were characterized by Raman scattering spectra measurement, which was performed on a Jobin-Yvon LabRam HR80 spectrometer 共with a 325 nm line of Torus 50 mW diode-pumped solid-state laser兲 under backscattering geometry. The doping levels and the bonding characteristics were determined by x-ray photoelectron spectroscopy 共XPS兲共VG ESCALAB 210兲. Peak positions were referenced to the adventitious C 1s peak taken to be 285.0 eV. The measurements of magnetic properties were made using the vibrating sample magnetometer 共VSM兲共Lakeshore 7304兲.

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FIG. 3. XRD patterns of ZnO nanoparticles annealed at different temperatures in air. FIG. 1. The SEM images of ZnO nanoparticles annealed in air at 共a兲 450, 共b兲 550, 共c兲 650, and 共d兲 850 ° C.

III. RESULTS AND DISCUSSION

The morphologies of the ZnO nanoparticles annealed in air at 450, 550, 650, and 850 ° C were captured by SEM, as given in Fig. 1. It shows that the average diameter of the nanoparticles varies from 80 to 230 nm with the increase in annealing temperature. The TEM image of the as-prepared ZnO nanoparticles 共450 ° C兲 is shown in Fig. 2共a兲. It can be seen that the sample has particle morphology with moderate aggregation and the average diameter is about 80 nm, which corresponds to the result of SEM in Fig. 1共a兲. Figure 2共b兲 shows the high resolution TEM 共HRTEM兲 image of the corresponding ZnO nanoparticles. The two adjacent planes are 0.52 nm apart, which is equal to the lattice constant of the standard ZnO grown along the c-axis direction. The SAED pattern taken from the particle is shown in Fig. 2共c兲, consisting of seven obvious rings, which are corresponding to 共100兲, 共002兲, 共101兲, 共102兲, 共110兲, 共103兲, and 共112兲 planes of polycrystalline ZnO with a wurtzite structure. The typical XRD patterns of the nanoparticles annealed at different temperatures are shown in Fig. 3. For each sample, all observed peaks can be indexed with the wurtzite phase of ZnO 共JCPDS Card No. 36-1451兲. In addition, the intensity of the peaks increases and the full width at half maximum 共FWHM兲 decreases with the increase in annealed temperature, which indicates a possible change in the crystallite size. Using the Scherrer formula for the FWHM of the main peaks, the average crystalline size was estimated to be

FIG. 2. 共a兲 Low magnification TEM, 共b兲 HRTEM image, and 共c兲 corresponding SAED pattern of ZnO nanoparticles annealed at 450 ° C.

around 25, 28, 32, and 38 nm for the samples annealing at the temperature of 450, 550, 650, and 850 ° C, respectively. The additional information on the structure of ZnO nanoparticles were obtained by Raman spectroscopy. Figure 4 shows the micro-Raman spectra of the ZnO nanoparticles annealed at different temperatures in the range of 400– 1200 cm−1 measured at room temperature. The peak at about 579 cm−1 can be assigned to A1 longitudinal optical 共LO兲 mode 关A1 共LO兲兴 and the other peak at 1164 cm−1 is just a second order peak of the LO phonon, which is caused by defects such as O-vacancy, Zn-interstitial defect, or these complexes.15 The result indicates that the as prepared sample has some crystal vacancies in our case. The intensities of the Raman peaks located at 579 and 1164 cm−1 decrease as the annealed temperature increases, implying that the concentration of the defects decreases with the annealing temperature increase. Further evidence for the purity and composition of the products were obtained by XPS and the results showed that the indexed peaks correspond to C, O, and Zn for all the ZnO nanoparticles annealed at different temperatures. A representative XPS spectrum of ZnO nanoparticles annealed at 450 ° C is shown in Fig. 5. Figures 5共a兲–5共c兲 are the survey spectrum, the Zn 2p, and O 1s core-level spectrum, respec-

FIG. 4. Raman spectra of ZnO nanoparticles annealed at different temperatures in air.


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FIG. 5. 共a兲 XPS survey spectrum, high resolution scan of 共b兲 Zn 2p, and 共c兲 O 1s for ZnO nanoparticles annealed at 450 ° C.

tively. The double spectral lines of Zn 2p are seen at the binding energy of 1022 eV 共Zn 2p3 / 2兲 and 1045 eV 共Zn 2p1 / 2兲 with a spin-orbit splitting of 23 eV, which coincides with the findings for the Zn2+ bound to oxygen in the ZnO matrix.16 Asymmetric O 1s peak is observed for the sample, which has a shoulder at the higher binding energy side fitting with Gaussian distribution. The buildup of two peaks at 531 and 533 eV is found. The dominant one is located at 531 eV, which is assigned to the O2− ion in the wurtzite structure surrounded by the Zn ions with their full complement of nearest-neighbor O2− ions.17 The peak at 533 eV is ascribed to loosely bound oxygen, such as absorbed O2 or adsorbed H2O on the ZnO surface.18,19

FIG. 6. M-H curves for the ZnO nanoparticles annealed at different temperatures.

Figure 6 depicts the magnetization versus magnetic field 共M-H兲 curves of the ZnO nanoparticles annealed at different temperatures, which were measured at room temperature using VSM. The hysteresis loops indicate that all the nanoparticles clearly have RTF. The annealing temperature dependence of magnetization is shown in the inset. It can be seen that the magnetization decreases from 0.0054 to 0.0015 emu/g while the annealing temperature increases. Some other groups reported the ferromagnetism in undoped semiconducting and insulating oxides. Some explained that the origin of ferromagnetism in these samples is due to defects.20,21 The Raman results showed that there were large numbers of defects in ZnO nanoparticles, in order to check if the magnetism in our case is due to defects such

FIG. 7. M-H curves of the as-prepared particles, as well as that annealed in vacuum, and in the mix atmosphereture of nitrogen and oxygen at 400 ° C for 1 h.


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TABLE I. The magnetization of the as-prepared sample and that of annealing at different conditions.

Annealed conditions Magnetization 共emu/g兲

As prepared

Vacuum 共1 ⫻ 10−3 Pa兲

N2 : O2 = 4 : 5

N2 : O2 = 2 : 5

N2 : O2 = 1 : 5

0.0054

0.0185

0.0045

0.0015

0.0003

as oxygen vacancies, vacuum-annealing and oxygenannealing tests were done, respectively. The as-prepared particles 共450 ° C兲 were divided into four parts. Then they were annealed in vacuum 共1 ⫻ 10−3 Pa兲 or in the mix atmosphere at different rations of nitrogen and oxygen at 400 ° C for 1 h, respectively. Figure 7 shows the M-H curves of them. The data of the as-prepared particles 共450 ° C兲 is also shown here in order to compare directly. It is easy to observe that the sample annealed in vacuum has a larger saturation magnetization of 0.018 emu/g, which is about trinary larger than that of as-prepared particles 共450 ° C兲. At the same time, we can clearly see that annealing in rich-oxygen atmosphere can gradually reduce the magnetization from 0.0045 to 0.0003 emu/g. The varieties of the ferromagnetism at different annealing conditions are described in Table I. As we know, heat treatment could provide thermal energy and then the crystal lattice reconformation happens to form high quality crystal and the larger size of the particles, but this decreases surface defects, which can explain the magnetization decreasing with the increase in annealed temperature. Meanwhile, it is found that the ferromagnetism of ZnO nanoparticles increases after annealing in vacuum condition and decreases after annealing in rich-oxygen atmosphere. These evidences clearly proved that the RTF of undoped ZnO nanoparticles in our case really originates from oxygen vacancies locating at the surface of nanoparticles, which are due to the losing of oxygen. According to the principle, the oxygen atoms at the surface escape from the bondage of the chemical bond because of heating, then the unpaired electrons show the abnormal spin phenomenon, which causes the magnetism. We suggest that the unpaired electron spins on the surfaces of the particles are responsible for the ferromagnetism in ZnO nanoparticles. Further theoretic investigations on the defect introducing ferromagnetism are expected and our work is on the way. IV. CONCLUSION

In summary, ZnO nanoparticles were prepared by coprecipitation method with postoxidation annealing in air. Clear hysteresis loops are observed at room temperature in ZnO nanoparticles annealed at different temperatures and the extrinsic impurity origin is excluded. The decrease in the ferromagnetism is observed with the increase in volume ration of oxygen when the as-prepared sample is annealed in the

mix atmosphere of nitrogen and oxygen. The ferromagnetism decreases with the increase in the ZnO nanoparticle size. These results indicate that oxygen vacancies at the surface of the nanoparticles are likely to be responsible for the ferromagnetism. ACKNOWLEDGMENTS

This work is supported by NSFC 共Grant No. 50671046兲 and MOE 共Grant No. J0630313兲. 1

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