Ferromagnetism Induced by Oxygen Vacancies in Zinc Peroxide

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Ferromagnetism Induced by Oxygen Vacancies in Zinc Peroxide Nanoparticles Daqiang Gao, Jing Zhang, Guijin Yang, Jing Qi, Mingsu Si, and Desheng Xue* Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, P. R. China ABSTRACT: A correlation between oxygen vacancies and the magnetization for pure zinc peroxide nanoparticles (∼7 nm) synthesized by the hydrothermal method is demonstrated. Occurrence of room temperature ferromagnetism for zinc peroxide nanoparticles is established by the observed hysteresis loops and the ferromagnetic resonance signal. X-ray photoelectron spectroscopy and photoluminescence results reveal the oxygen insufficiency in the samples. The variation of oxygen vacancies concentration is consistent with the changes of the saturation magnetization for the samples. Combining with the results of electron spin resonance, it is suggested that the singly occupied oxygen vacancies forming the F+ centers play the key role in the exchange mechanism.

1. INTRODUCTION Diluted magnetic semiconductors, in which a small concentration of magnetic transition metal atoms are added to a host semiconductor lattice, possessing both ferromagnetic and semiconductor properties, have been widely studied mainly as the basis of future development of the spintronic technology.1
3 Even though room temperature ferromagnetism (FM) has been observed in a number of magnetic element doped semiconductors,4
9 the reproducible experimental search for FM in doped semiconductors is still absent in homogeneous magnetic materials up to now.10
12 Sometimes clustering was found to be responsible for the observed FM in ZnO and in TiO2.13,14 Therefore, the mechanism of the FM in these materials still remains unclear.15,16 Besides the magnetic transition metals, other elements, which display no magnetic properties in their elemental form, have also been able to induce the long-range ferromagnetic order in oxides and semiconductor hosts, such as Cu- and Er-doped ZnO,17,18 C-doped SrO, MgO,19 N-doped CaO,20 and Cu-, Cr-, or Mo-doped In2O3.21
23 Later on, the situation is further complicated by the presence of intrinsic point defects or extended defects which can also contribute to ferromagnetic behavior at room temperature in undoped nanomaterials such as TiO2, In2O3, ZnO, CeO2, SnO2, Al2O3, CaO, CuO, and MgO.24
27 At the same time, models were proposed to explain the weak ferromagnetic effects in these oxides as defect related.28,29 It was reported that the FM per unit volume decreases as the thickness of ZnO films increases, implying that the FM is due to surface defects.30 FM observed in TiO2 nanoclusters and SnO2 nanowires were also attributed to surface defects: It is considered that the FM is related to the surface-tovolume ratio of the samples.31,32 Zinc peroxide (ZnO2) can be use in many fields, such as in the rubber industry, photocatalysis, synthesis of ZnO, cosmetic and pharmaceutical industries, and for therapeutic applications.33,34 Structural stability of ZnO2 nanoparticles (NPs) was studied at r 2011 American Chemical Society

temperatures up to 800 °C and pressures up to 36 GPa. It is found no magnetic hysteresis were detected for ZnO2 NP down to 5 K, indicating a paramagnetic (PM) state for the sample.35 Recently, there were some observations of strong defect-related emission in ZnO2 NPs with the particle size of few nanometers,36 which motivates us to carry out a comparative study on their magnetic properties. Owing to have with a wide direct band gap of about 4.5 eV, ZnO2 shows excellent optical property. Meanwhile, its band gap energy as well as its optical properties can be controlled in a certain size range by the dimensions, the morphology, and surface treatment of the particles.37
39 If the ZnO2 system also behaves the FM, it may bring new opportunity to the field of spintronics. Besides the reports of FM in doped oxides are suspected in terms of cluster formation and evolution of secondary phases, the pure ZnO2 system is free from such fears to some extent, which is liable to reveal the exact mechanism of FM exchange. In this paper, a correlation between oxygen vacancies and the magnetization of pure ZnO2 NPs synthesized by the hydrothermal method is demonstrated. The results reveal that oxygen vacancies are the main reason in introducing the FM in our case. Combining with the results of electron spin resonance, the F-center exchange mechanism is used to explain the observed FM.

2. EXPERIMENTAL SECTION Preparation of ZnO2 NPs. All the chemical reagents used in our experiments are analytical grade. In a typical synthesis, Zn(Ac)2 3 2H2O powders were dissolved into 80 mL of H2O2 (3%, v/v) aqueous solution. Then the pH value of the solution was adjusted to 10 with the diluted ammonia. The resultant Received: February 22, 2011 Revised: July 23, 2011 Published: July 24, 2011 16405

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Figure 1. (a) XRD patterns of the samples Z10, Z20, and Z30. (b) The estimated average crystalline size by Scherrer’s relation and the particle size by TEM results of each sample. (c) The representative TEM image and (d) the HRTEM image of sample Z20.

starting solution was transferred into a Teflon-lined stainless steel autoclave of 100 mL capacity, sealed, maintained at 90 °C for 12 h, and then air-cooled to room temperature. The as-formed products were filtered, washed with water, and then dried in air at 60 °C. Three different samples were obtained by changing the quantity of Zn(Ac)2 3 2H2O to 1, 2, and 3 g, respectively, which were denoted as sample Z10, Z20, and Z30, respectively. Characterization Techniques. The morphologies of the samples were characterized by transmission electron microscope (TEM, JEM-2010). The X-ray diffraction (XRD, X’ Pert PRO PHILIPS with Cu KR radiation) and selected area electron diffraction (SAED) were employed to study the structure of the samples. X-ray photoelectron spectroscopy (XPS, VG ESCALAB 210) and inductively coupled plasma
atomic emission spectrometer (ICP, IRIS, ER/S) were utilized to determine the bonding characteristics and the composition of the samples. Micro-photoluminescence (PL, Shi-Madzu, RF-540) measurement was carried out at room temperature using a He
Cd laser with the wavelength of 325 nm and output power of 15 mW as the excitation source. The measurements of magnetic properties were made using the Quantum Design MPMS magnetometer based on superconducting quantum interference device (SQUID) and vibrating sample magnetometer (VSM, Lakeshore 7304). The spectrometer at microwave frequency of 8.984 GHz was used for electron spin resonance (ESR JEOL, JES-FA300) measurements.

3. RESULTS AND DISCUSSION XRD patterns of ZnO2 NPs are shown in Figure 1a. All the diffraction peaks can be readily indexed to a pure cubic phase of ZnO2 (JCPDS card no. 13-311, space group: Pa3), and no other additional reflections are observed. The average crystalline size of each sample is estimated by Scherrer’s relation, and the results are shown in Figure 1b. The crystalline sizes for the three samples are nearly the same (∼6.8 nm). The particle sizes for the samples were also obtained by analyzing the TEM images, which reveals the nearly same sizes as estimated by XRD results. A

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Figure 2. PL spectra of samples (a) Z10, (b) Z20, and (c) Z30 fitted by two Gaussian peaks marked as P1 and P2. (d) The relative area of P1 for the samples of Z10, Z20, and Z30 versus their corresponding levels of oxygen vacancies (δ).

representative TEM image of sample Z20 is shown in Figure 1c, which shows the formation of uniform nanocrystals of NPs with the average diameter of 7.2 nm. An SAED pattern of sample Z20 is shown in inset of Figure 1c, revealing the polycrystalline structure of ZnO2 NPs. From high-resolution electron microscopy (HRTEM) image observed in sample Z20 shown in Figure 1d, one can see that the well-defined two-dimensional lattice fringes go straight throughout the whole structure, which indicates that the sample is well crystallized. The distance between adjacent lattice fringes in the HRTEM image is 0.28 nm, corresponding to (111) planes of cubic-structured ZnO2 nanostructures.33 PL measurement was preformed for all the samples, and the results are shown in Figure 2. It can be seen that the spectrum of each sample is characterized by a broad green emission band. The band-to-band emission peak (∼330 nm) of ZnO2 is not observed here because of PL detection limitations. The PL spectra of each sample can be well fitted by two Gaussian peaks marked as P1 and P2, which are centered at 452 and 520 nm. It is suggested that the green-blue wavelength centered at 452 nm originates from the oxygen vacancy, and the other peak at 520 nm may be attributed to zinc and zinc interstitial defects.40 Here, the O/Zn atomic ratios for the three samples were also assessed by XPS measurement. The survey scans show no impurity above the detection limit. Calculation on relative chemical composition shows that the nonstoichiometric formula for the sample Z10, Z20, and Z30 is ZnO1.95, ZnO1.88, and ZnO1.93, respectively. The relative area of P1 for sample Z10, Z20, and Z30 are shown in Figure 2d, which demonstrates that high-density oxygen vacancies exist in these samples and their concentrations are systematically adjusted. Room temperature FM has been observed previously in undoped wide band gap oxide thin films and NPs such as TiO2, HfO2, In2O3, CeO2, and SnO2, and its origin is usually linked to the oxygen deficiency.24
29 The contrasts in oxygen concentrations and optical characteristics in samples Z10, Z20, and Z30 motivated us to carry out a comparative study on their magnetic properties. The magnetization versus magnetic field (M
H) curves of samples Z10, Z20, and Z30 measured at room 16406

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Figure 5. XPS survey spectrum of sample Z20.

Figure 3. (a) M
H curves measured on the samples Z10, Z20, and Z30 at 300 K, where the PM signal contribution due to the holder and the samples has been subtracted. (b) M
H curves for sample Z20 measured at different temperatures. The insets in Figure 3a,b provide a magnified view of the low-field data.

exhibit the hysteresis curve with different coercivity and saturation magnetization, which indicate the clearly room temperature FM and the sample Z20 has the maximum saturation magnetization of 0.021 emu/g. The M
H curves for sample Z20 measured at different temperatures are shown in Figure 3b; one can see that the coercivity and saturation magnetization for sample Z20 decrease respectively from 270 to 50 Oe and from 0.0276 to 0.021 emu/g monotonically with increasing of the measured temperature. The insets in both of the two figures provide a magnified view of the low-field data to demonstrate the nature of coercive field and their drastically change at different measurement temperature. Figure 4a shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves at the dc field of 100 Oe on sample Z20. The ZFC result shows no blocking temperature in the range of 5
300 K, indicating that there is no ferromagnetic contamination in the sample. The FC curve exhibits an obvious deviation from the ZFC curve until the temperature above 300 K, which can be clearly seen from the temperature dependence of the ratio between the magnetization obtained after FC and ZFC (shown in inset). These results further reveal that the sample is ferromagnetic at room temperature. The temperature dependence of magnetization (M
T) can also give some magnetic information on the samples.41,42 Figure 4b shows the M
T for the sample Z20 measured in the temperature range of 5
300 K at 5000 Oe. It can be seen that the curve shows a rapid decay of magnetization at low temperature region as the increasing of the measured temperature. Then the magnetization declines gradually and does not go to zero until 300 K, indicating a PM phase mixed with a FM one for the sample. The M
T curve is fitted by the T3/2 law for FM phase and the Curie
Weiss law for PM phase, as shown by the equation MðTÞ ¼ Ms0 ð1
AT 3=2 Þ þ CH=T

Figure 4. (a) ZFC and FC curves at the dc field of 100 Oe on sample Z20, and the inset shows the temperature dependence of the ratio between the magnetization obtained after FC and ZFC, respectively. (b) M
T curve of the sample Z20 in the range of 5
300 K at 5000 Oe.

temperature are shown in Figure 3a, where the PM signal contribution due to the holder and the samples has been subtracted. It can be seen from the M
H curves that all the samples

ð1Þ

where Ms0 is the saturation magnetization at T = 0 K, A is a coefficient correlated to the structure and properties of materials, and C is the Curie constant. As shown in Figure 4b, the PM and FM part as well as the total magnetization are plotted separately; the total simulation from eq 1 gives a good fitting of the experimental results. To explain the origin of the FM in ZnO2 NPs in our case, a careful consideration that whether the contamination is responsible for the FM has to be undertaken. In our experiments all the scoops were plastic and the capsules used to hold the samples during the magnetic measurements were checked and showed no ferromagnetic signal. In addition, the variation of the saturation 16407

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Figure 7. (a) Normalized EPR spectra of the samples Z10, Z20, and Z30. (b) Magnified view of the EPR single centered at 321 mT. (c) Integration curves for A1 component for samples Z10, Z20, and Z30. Figure 6. (a) Temperature dependence ESR spectrum of the sample Z20 ranging from 100 to 300 K. (b) Estimated geff for the two components of “A1” and “A2” at different temperatures. (c) Variation in ΔH(321 mT
Hcenter) as a function of temperature for sample Z20.

magnetization values for the three samples is not proportional to the amount of the Zn(Ac)2 3 2H2O in the fabrication progress, so the magnetic impurity in the precursor materials also can be ruled out. The further evidence for the purity and composition of the products was obtained by XPS, and the results show that the indexed peaks are corresponding to O and Zn for all the samples (shown in Figure 5 is the XPS spectrum of sample Z20). So far, the role of trace impurities in producing room temperature FM in ZnO2 NPs can be ruled out decisively. Therefore, we suggest that the observed room temperature FM is intrinsic. The temperature dependence ESR spectrum of Z20 ranging from 100 to 300 K is also performed, and the results are shown in Figure 6a. It can be seen that the spectrum for each curve consists of two quite different components: a narrow component centered at 321 mT with a line width of few mT (denoted as “A1”) and a strong broaden resonance signal appeared at low-field region with the line width more than 100 mT (denoted as “A2”). According to the theory of electron spin resonance, the effective g factor (geff) is given by equation43 geff ¼ hγ=μB Hcenter

ð2Þ

where h, γ, μB, and Hcenter are the Planck constant, frequency of the applied microwave magnetic field, Bohr magnetron, and resonance magnetic field, respectively. Thus, the geff for the two components can be easily determined: The narrow component with a geff = 2.0027, which nearly equals to the free-electron value of 2.0023, indicating the existence of unpaired electrons in the sample. The broad component with a geff ∼ 2.21 at room temperature, which is far from 2.0023 and suggests considerable spin
orbit coupling. It can be seen from Figure 6b that geff for component A1 is temperature independent, while geff for component A2 increases gradually with the temperature decreasing owing to the enhanced spin
orbit coupling at low temperature. Figure 6c shows the variation in ΔH(321 mT
Hcenter) as a function of temperature for sample Z20. It can be seen that the ΔH increases gradually as the temperature decreasing, indicating

Figure 8. Dependence of the area of the integration curves for A1 component on Ms for samples Z10, Z20, and Z30.

the increased spin
orbit coupling. This is a typical behavior of a ferromagnetic material. These results indicate that the resonance of “A1” is the PM resonance induced by the unpaired electrons and “A2” is the ferromagnetic resonance of the samples. Recently, the subcategory of the bound magnetic polaron (BMP) model, known as the F-center exchange mechanism, has been used to explain the FM in CeO2, TiO2, and ZnO.44
46 In this framework, three possible states of the oxygen vacancies can be (a) the F2+ center with no trapped electrons, (b) the F+ center with one trapped electron, and (c) the F0 center with two trapped electrons. Generally, the F0 centers in the singlet state (S = 0) form the shallow donor levels above the conduction band, which can mediate only weak antiferromagnetic exchanges. In contrary to this, singly occupied vacancies (F+ centers) lie deep in the gap and favor a FM ground state. As discussed earlier, electrons in these singly occupied oxygen vacancies (F+) are strongly localized.46,47 The localization radius is ε(m/m*)a0, where ε is the dielectric constant and m and m* are the mass and effective mass of an electron with a0 as the Bohr radius. Once the F+ center density reaches the critical value (nvo) for magnetic percolation, these F+ centers overlap, which results in a long-range ferromagnetic ordering even in the absence of itinerant carriers.48 The temperature dependence ESR spectrum of Z20 reveals existence of the unpaired electrons (singly occupied oxygen vacancies) in the sample. To account for the origin of the observed room 16408

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The Journal of Physical Chemistry C temperature FM in ZnO2 NPs, room temperature ESR measurement was applied to all the samples, and the results are shown in Figure 7a. It can be seen that all samples show the ferromagnetic resonance signal of “A2” and the PM resonance of “A1”. Figures 7b and 7c provide the magnified view of A1 component and the integration curves of A1 component for the three samples, respectively. The number of spins (Ns) participating in this resonance are considered to be related with the intensity (I) and the half-width (ΔH) of the integration curves, the area of which represents the relative number of unpaired electrons in the samples.49,50 The dependence on the area of the integration curves and Ms for sample Z10, Z20, and Z30 are summarized in Figure 8. The trend is clear: The changes of saturation magnetization are corresponding to that of the relative number of unpaired electrons in the samples. It seems that singly charged oxygen vacancies can be able to boost the magnetism of the samples. However, although both PL and magnetic properties are closely linked with the oxygen deficiency, the associated physical mechanisms is very different.51,52 In addition, besides the FM phase, the singly charged oxygen vacancies also contribute to the PM part where the ration is difficult to be determined. These adverse factors result in the unquantitative variation of the value for the P1, area of the integration curves for A1 component for sample Z10, Z20, and Z30, and Ms, which is a challenge for future work.

4. CONCLUSIONS In conclusion, the structure and magnetic properties of ZnO2 NPs synthesized by the hydrothermal method were studied. Magnetic measurement indicates that the synthesized samples show intrinsic room temperature FM behavior, which are also confirmed by the observation of ferromagnetic resonance signal. PL results reveal the intensity broad defect-related emission, and the relative area of oxygen vacancy emission is different for the three samples. Combining with the ESR analysis and the variation of saturation magnetization for the samples, it is suggested that oxygen vacancies (F+ center) may be related to the FM in our case. In addition, the tunable room temperature FM indicates that oxygen vacancies play main role in establishing and regulating the ferromagnetic ordering in ZnO2 NPs. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by National Science Fund for Distinguished Young Scholars (Grant 50925103), the Keygrant Project of Chinese Ministry of Education (Grant 309027), the National Nature Science Foundation of China (Grants 10804038, 10804038, and 50902065), and the Fund for Academic Newcomer of PhD of Lanzhou University and Ministry of Education. ’ REFERENCES (1) Ohno, H. Science 1998, 281, 951. (2) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (3) Sharma, P.; Gupta, A.; Rao, K. V.; Owens, F. J.; Sharma, R.; Ahuja, R.; Guillen, J. M. O.; Johansson, B.; Gehring, G. A. Nature Mater. 2003, 2, 673.

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