Understanding the Origin of Ferromagnetism in ZnO Porous

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Understanding the Origin of Ferromagnetism in ZnO Porous Microspheres by Systematic Investigations of the Thermal Decomposition of Zn5(OH)8Ac2 3 2H2O to ZnO Zhenbo Xia, Yewu Wang,* Yanjun Fang, Yuting Wan, Weiwei Xia, and Jian Sha* Department of Physics & State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, 310027, People's Republic of China ABSTRACT: In this paper, thermogravimetric analyses, Fourier transform infrared spectra, X-ray powder diffraction, photoluminescence, electron paramagnetic resonance, and superconducting quantum interference device magnetometer are applied to observe the thermal decomposition of Zn5(OH)8Ac2 3 2H2O to ZnO for understanding the origin of room temperature ferromagnetism in ZnO porous microspheres. The results show that both zinc vacancies and shallow donors induced by hydrogen may play an important role in triggering magnetic order in ZnO samples fabricated by annealing the precursor Zn5(OH)8Ac2 3 2H2O at temperature above 400 °C.

1. INTRODUCTION In 2000, Dietl et al. described a theoretical model and predicted that the Curie temperature (TC) could be raised upon room temperature for p-type semiconductors, especially for the wide band gap semiconductors, i.e., ZnO and GaN.1 Dilute magnetic semiconductors (DMSs) are therefore attracting intense interest due to the possibility to control both electron charge and spin degrees of freedom simultaneously for future spintronics devices.2 Recently, many groups have done profound investigations on room temperature ferromagnetism (RT-FM) in transition metal (TM) doped ZnO such as ZnO: Co,3 ZnO:Ni,4 ZnO:Cu5 and ZnO:Mn,6 which makes ZnO a promising candidate for spintronic applications. However, the origin of FM in ZnO:TM still remains a controversial topic. It is not unambiguously understood yet whether it is intrinsic or induced by extrinsic origins such as TM clusters and precipitates of impurity phase, and the indetermancy makes it the main obstacle to realize the stability and reproducibility of RT-FM in ZnO. More interestingly, unexpected RT-FM has also been observed in HfO2 without any doping element.7 Subsequently, a series of nonmagnetic oxides such as ZnO,8 In2O3,9 Al2O3,9 and TiO210 have been found to be ferromagnetic at room temperature, and it is widely accepted that the origin of RT-FM is due to intrinsic defects. Several types of defect, such as oxygen vacancy, zinc vacancy, zinc interstitial, hydrogen interstitial, and oxygen interstitial, have been thought to have some connection with the observed RT-FM of undoped nonmagnetic oxides. For example, Coey et al. reported RT-FM in HfO2 thin film and attributed it to the existence of oxygen vacancies,7a but the first-principles investigations carried out by Pemmaraju et al. showed that the observed RT-FM is most likely due to the presence of hafnium vacancy sites.7b Rao et al. assumed that the origin of RT-FM in nanoparticles of metal oxide such as CeO2, Al2O3, ZnO, and SnO2 may be the exchange interactions between unpaired elecr 2011 American Chemical Society

tron spins arising from oxygen vacancies at the surfaces of the nanoparticles;9 however Yi et al.,11 Khalid et al.,8a and Wang et al.8b demonstrated, both theoretically and experimentally, that zinc vacancy can be the origin of RT-FM in pure ZnO thin films, nanoparticles, and nanowires. Meanwhile Yan et al.12 and Li et al.13 reported that the interstitial zinc at the surface may contribute to the RT-FM in undoped ZnO nanorods and nanoparticles. Total energy calculations carried out by Gallegeo et al.14 predicted that the reversible switch of surface magnetism can be achieved by varying the hydrogen density on the ZnO (0001) surface. Therefore, although intense work has been carried out to explore the origin of RT-FM in undoped semiconductors including ZnO, the results are far from convincing and even some of them are against another. Usually, postannealing of ZnO samples carefully in different atmospheres such as nitrogen,8a oxygen,15 zinc vapor,16 and hydrogen17 is a common method to distinguish the defects inducing the observed RT-FM in earlier work. Here, the detailed investigations of the thermal decomposition of Zn5(OH)8Ac2 3 2H2O to ZnO porous spheres have been directly carried out to understand the origin of the observed RT-FM in pure ZnO. In addition, we have demonstrated that the shallow donors caused by hydrogen and zinc vacancy are probably responsible for the observed RT-FM in our undoped ZnO porous spheres. Layered basic zinc acetate (Zn5(OH)8Ac2 3 2H2O) spheres, the precursor of ZnO, are first grown by chemical bath deposition. The phase change, structure evolution, and optical and magnetic properties are then observed in detail during the prcoess of the thermal decomposition of Zn5(OH)8Ac2 3 2H2O precursor to ZnO porous spheres. Received: March 28, 2011 Revised: June 28, 2011 Published: June 28, 2011 14576

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2. EXPERIMENTAL SECTION Details of the growth process of the Zn5(OH)8Ac2 3 2H2O porous spheres were described previously.18 To prepare Zn5(OH)8Ac2 3 2H2O microspheres, a sealed glass vial containing 20 mL of aqueous solution of Zn(CH3COO)2 3 2H2O (90 mM), hexamethylenetetramine (90 mM), and sodium citrate (9 mM) was kept at 95 °C for 4 h. The white precipitates in the solution were then collected, washed, and dried in an oven of 65 °C before characterization. The dried precipitates were then divided into five parts. One of them was untreated and others were annealed at 200, 300, 400, and 500 °C with a heating rate of 10 °C/min in the air for 30 min to analyze the thermal decomposition process of the Zn5(OH)8Ac2 3 2H2O precursor to ZnO porous spheres. The size and morphology of the raw materials and the products annealed at different temperatures were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The thermal decomposition process from Zn5(OH)8Ac2 3 2H2O to ZnO was studied by performing thermogravimetric analyses (TGA) on a Hi-Res TGA 2950 thermogravimetric analyzer in flowing air (50 sccm) from 50 to 550 °C at a heating rate of 10 °C/min. Fourier transform infrared spectra (FTIR) of the samples were recorded using a Nexus 670 spectrophotometer. The X-ray powder diffraction (XRD) spectra of the samples with variable annealing temperatures were collected from a Siemens D-500 diffractmeter with Cu KR as the incident radiation. The element states and composition of postannealed ZnO spheres were examined by X-ray photoemission spectroscopy (XPS) (ESCALAB).The photoluminescence (PL) measurements were performed on a luminescence spectrometer (Edinburgh Instruments FLS 920) with a Xe lamp emitting at 325 nm as excitation source. Electron paramagnetic resonance (EPR) data of the samples were taken using an ESRA-300 spectrometer. The magnetic characterization was carried out using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc.) at room temperature. 3. RESULTS AND DISCUSSION Figure 1a shows a typical SEM image of the collected white precipitates, which has been identified as Zn5(OH)8Ac2 3 2H2O by XRD shown in Figure 4. The products are flower-like, threedimensional (3D) microspheres with diameters about 5 μm. The microsphere is actually composed of a random growth of seemingly flexible nanoflakes with a thickness about 20 nm. The individual flakes are curved and interconnected with each other. In addition, this unique structure possesses larger surface area when compared to a similar-sized material, which makes it particularly promising for applications in biosensors and photodecomposition of organic compounds.19 The morphology of precipitates, or flower-like spheres, changed, when annealed at 300 °C in the air for 30 min, as shown in Figure 1b. On further increase of the annealing temperature to 400 °C, many micropores appear clearly on the surface of the thin flakes, as seen in Figure 1c. Finally the thin flakes changed to porous flakes composed of grainy crystals when annealed at 500 °C, as seen in Figure 1d. At the end of thermal treatment, the white precipitates, Zn5(OH)8Ac2 3 2H2O, have been decomposed to ZnO completely with wurtzite structure, which will be confirmed later by XRD patterns shown in Figure 4. In the following section, we analyzed the step-by-step thermal decomposition of Zn5(OH)8Ac2 3 2H2O in order to fully understand the defects and the origin of the observed RT-FM in

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Figure 1. SEM images of the intact precursor Zn5(OH)8Ac2 3 2H2O (a) and the samples obtained by annealing Zn5(OH)8Ac2 3 2H2O at (b) 300, (c) 400, and (c) 500 °C in air for 30 min, respectively.

Figure 2. TG and DTG profiles of the thermolysis of Zn5(OH)8Ac2 3 2H2O (Δm: change in mass).

ZnO samples. The thermal decomposition process from Zn5(OH)8Ac2 3 2H2O to ZnO was studied by performing TGA in flowing air (50 sccm) from 50 to 550 °C at a heating rate of 10 °C/min. The TG and DTG profiles for Zn5(OH)8Ac2 3 2H2O are presented in Figure 2, showing that the mass of the sample reaches a constant value at ca. 400 °C. The previous three major stages of mass loss, I, II, and III are also observed. The first decomposition step ranges from 50 to 180 °C. Stage I involves losing bound moisture and releasing the crystal water as presented in eq 1. Subsequently, in stage II, Zn5(OH)8Ac2 groups begin chemically decomposing into ZnO, H2O, and Zn3(OH)4(CH3CO2)2 at temperatures from 180 to 310 °C as indicated in eq 2.20 Actually, the appearance of a ZnO phase in this stage has also been confirmed by FTIR and XRD. The final major stage of decomposition in the TGA profiles, stage III, is observed from 310 to 400 °C and corresponds to the decomposition of the Zn3(OH)4(CH3CO2)2 and zinc acetate as shown in eqs 3 and 4.20 And then the acetic anhydride may react with oxygen to 14577

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Figure 3. FT-IR absorption spectra of the intact precursor Zn5(OH)8Ac2 3 2H2O and the samples obtained by annealing Zn5(OH)8Ac2 3 2H2O at 200, 300, 400, and 500 °C in air for 30 min, respectively.

release CO2 and H2O, as outlined in eq 5. The mass losing of the sample ceases, as the precursor Zn5(OH)8Ac2 3 2H2O transformed completely into wurtzite ZnO since at ca. 400 °C. The expected mass loss of 34.1% agrees well with the observed 33.5% for the whole process. Zn5 ðOHÞ8 ðCH3 CO2 Þ2 3 2H2 O f Zn5 ðOHÞ8 ðCH3 CO2 Þ2 þ 2H2 O

ð1Þ

Zn5 ðOHÞ8 ðCH3 CO2 Þ2 f Zn3 ðOHÞ4 ðCH3 CO2 Þ2 þ 2H2 O þ 2ZnO

ð2Þ

Zn3 ðOHÞ4 ðCH3 CO2 Þ2 f ZnðCH3 CO2 Þ2 þ 2H2 O þ 2ZnO

ð3Þ

ZnðCH3 CO2 Þ2 f ðCH3 COÞ2 O þ ZnO

ð4Þ

ðCH3 COÞ2 O þ 4O2 f 4CO2 þ 3H2 O

ð5Þ

The stepwise transformation was also studied by FTIR and XRD, respectively. Figure 3 illustrates the FTIR spectra of the intact precursor (Zn5(OH)8Ac2 3 2H2O) and the samples by annealing the precursor at different temperatures (TA) for 30 min. From the intact precursor up to TA = 200 °C, carboxylate vibration bands broadening is observed, locating at 1582 and 1402 cm1.21 The apparent broadening of these bands is due to the decomposition of Zn5(OH)8Ac2 3 2H2O, to which the Ac signals are related.21 This temperature coincides with stage I of the TGA profiles (see Figure 2). As the annealing temperature further increases to 300 °C, two new bands appear at 1631 and 1373 cm1. The bands are probably due to the stretching vibrations of acetate anions in Zn3(OH)4Ac2. A broad absorption band appears at ca. 447 cm1, which can be identified as vibration modes of the ZnO band in bulk ZnO.21b,22 These investigations indicate that ZnO already forms after annealing at 300 °C (corresponding to stages I and II in the TGA curve shown in Figure 2). Further increasing the annealing temperature to 400 °C, the bands at 1582 and 1402 cm1 disappear and two new bands at 1631 and 1373 cm1 are also weaken. But the broad band at 447 cm1, corresponding to ZnO vibrations, is consistent with that observed in bulk ZnO. It indicates that ZnO with

Figure 4. XRD patterns of the intact precursor Zn5(OH)8Ac2 3 2H2O and the samples obtained by annealing Zn5(OH)8Ac2 3 2H2O at 200, 300, 400, and 500 °C in air for 30 min, respectively.

significant crystallographic long-range order is formed, which is also confirmed by XRD shown in Figure 4. Complete thermal decomposition of the Zn5(OH)8Ac2 3 2H2O to ZnO has finished at this temperature (corresponding to stages I, II, and III in the TGA curve). When the annealing temperature reaches 500 °C, only significant enhancement of the ZnO vibrations in IR spectra can be observed, indicating the improvement of the crystallinity of ZnO. However, after thermal decomposition at higher temperatures such as 400 and 500 °C, the obtained ZnO sample still does not have a free surface account for the broad band at 3437 cm1 observed in all samples, which is assigned to the vibration of OH groups.20 While IR spectroscopy delivers some information about the remnant organic groups and chemisorbed species, XRD provides perspective insights into the microstructure of the forming ZnO lattices. The XRD patterns shown in Figure 4 indicate the phase transition and the structure evolution of the collected white precipitates during the thermal decomposition process. Figure 4a is the XRD pattern of the collected white precipitate without any postannealing process. All diffraction peaks in this pattern agree with that of layered basic zinc acetate (Zn5(OH)8Ac2 3 2H2O), and thus, the component of the precipitate can be indentified .23 The diffraction peaks in the range 2θ < 25° are indexed as (001), (002), and (003) locating at 7.11°, 14.14°, and 21.36°, respectively. And the higher angles at 33.07°, 58.67°, and 69.24° are indexed as (100), (110), and (200). They are broad and not symmetrical due to the turbostratic effect.21b Moreover, no peak of any other ZnO phases is observed in the XRD pattern. When the white precipitates are annealed at 200 °C for 30 min, the peaks of Zn5(OH)8Ac2 3 2H2O weaken gradually and become broader, which indicates that Zn5(OH)8Ac2 3 2H2O starts to decompose. As the annealing temperature further increases to 300 °C, the peaks of Zn5(OH)8Ac2 3 2H2O almost disappear while a weak and broadened peak of ZnO appears, as indicated by FTIR. The extremely broadened and weak reflection peak of ZnO shows the absence of long-range ordering in ZnO due to the very small particles. On further increase of the annealing temperature to 400 °C, the peaks of Zn5(OH)8Ac2 3 2H2O totally disappear and peaks of ZnO observed in the bulk sample appear, which reveals that of Zn5(OH)8Ac2 3 2H2O has decomposed into 14578

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Figure 5. XPS spectra of the sample obtained by annealing Zn5(OH)8Ac2 3 2H2O at 400 °C in air for 30 min.

crystalline ZnO with wurtzite structure.21b,24 No peaks from any other phases are observed in the XRD pattern. With further increase of the annealing temperature to 500 °C, narrowing and enhancing characteristics of ZnO diffraction patterns can be observed. The average grain size of the annealed samples is calculated by Scherrer’s formula25 Dhkl ¼ Kλ=β cos θ where Dhkl is the average grain size perpendicular to the (hkl) plane, β is the fwhm of the XRD peak, K is the Scherrer contant and taken as 0.9 because of the spherical shape of ZnO particles, λ is the wavelength of X-ray which is 1.54058 nm, and θ is the Bragg angle. According to Scherrer’s formula, a further calculation can be carried out and the result shows that the average grain diameter of the ZnO crystallite increases from 16.2 to 23.2 nm as the annealing temperature increases from 400 to 500 °C, taking the three main peaks (100), (002), and (101) into consideration. The composition and chemical states of the as-prepared ZnO sample with TA = 400 °C are further investigated by X-ray photoelectron spectroscopy (XPS) analysis. Figure 5a shows a broad scan survey spectrum taken at hν = 1253.6 eV. Figure 5b presents the XPS spectra of Zn 2p, and the peak positions of Zn 2p1/2 and Zn 2p3/2 locate at 1045.4 and 1022.4 eV. Comparing the peak positions to those in ref 26, we can conclude that: (1) Zn element is in the state of Zn2+ and (2) the entire XPS spectrum has a left shift of 0.1 eV since the standard peak position of Zn 2p3/2 is at 1022.5 eV. Thus the Zn (L3M45M45) kinetic energy with AES signal located at 266.0 eV can be determined to be 987.7 eV, which coordinates to the kinetic energy of Zn (L3M45M45) in ZnO.26 This indicates that the AES signal presented in Figure 5c is considered to be associated with Zn LMM in ZnO. And no other AES signal is observed in the spectra, such as the signal relating to the metallic zinc.

Figure 6. PL spectra of the intact precursor Zn5(OH)8Ac2 3 2H2O and the samples obtained by annealing Zn5(OH)8Ac2 3 2H2O at 200, 300, 400, and 500 °C in air for 30 min, respectively.

In order to identify the defects in porous ZnO microspheres, photoluminescence (PL) and electron paramagnetic resonance (EPR) are applied to investigate the thermal decomposition from Zn5(OH)8Ac2 3 2H2O to ZnO. Figure 6 illustrates the PL spectra of the intact precursor (Zn5(OH)8Ac2 3 2H2O) and the samples by annealing the intact precursor at different temperatures (TA) for 30 min. Actually, for the intact precursor, Zn5(OH)8Ac2 3 2H2O, no obvious defect related emission can be observed. But when the intact precursor is annealed at 200 °C for 30 min, an obvious and broad emission peak located at ca. 520 nm appears. As the annealing temperature increases to 300 °C, a broad emission peak located at ca. 550 nm is observed. Indeed the broad emission peak located at ca. 550 nm can be easily divided into three peaks at 466, 520, and 550 nm, respectively. Although the origins of the emissions are still an open and controversial question, these different peaks in the visible spectral region have been attributed 14579

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Figure 7. EPR spectra of the intact precursor Zn5(OH)8Ac2 3 2H2O and the samples obtained by annealing Zn5(OH)8Ac2 3 2H2O at 200, 300, 400, and 500 °C in air for 30 min, respectively.

Figure 8. Room-temperature magnetic hysteresis loop of the samples obtained by annealing Zn5(OH)8Ac2 3 2H2O at 300, 400, and 500 °C in air for 30 min, respectively.

to the defect emission of ZnO. For example, oxygen and zinc vacancy or interstitials and their complexes are assigned to the origin of the emission at 466 nm, respectively. Green emission is usually attributed to singly ionized oxygen vacancies, although this assignment is highly controversial.27 Yellow defect emission is also commonly reported in ZnO nanostructures prepared from aqueous solution. This emission is typically attributed to interstitial oxygen.28 The observations shown in Figure 6 indicate that tiny ZnO nanoparticles have already formed at 300 °C and even at 200 °C, which is also confirmed by TGA, FTIR and XRD spectra. It is very interesting to observe that emission peaks in the visible region disappear and an obvious UV emission peak appears when the annealing temperature is increased to above 400 °C. The quenching of the visible emission illustrates that the concentration of the corresponding defects in ZnO are significantly decreased eventually. At the same time, one broad and main UV emission band located at 397 nm is observable. The peak located at 397 nm is usually observed in the ZnO nanorods.3,27 It is believable that the UV emission peak at 397 nm in our samples is the near band-edge emission of ZnO, which comes from the excitonic transitions. These results also demonstrate that Zn5(OH)8Ac2 3 2H2O has decomposed into crystalline ZnO completely when the annealing temperature is increased to above 400 °C. Electron paramagnetic resonance (EPR) spectroscopy, a useful technique for indentifying paramagnetic defects, is used to identify the defects in ZnO combining with photoluminescence. Figure 7 shows the EPR spectra of the intact precursor (Zn5(OH)8Ac2 3 2H2O) and the samples after annealing at different temperatures (TA) for 30 min, respectively. From the intact precursor up to TA = 200 °C, a very weak paramagnetic signal near 3600 G on the high-field side of the spectra is observed and its g factor is approximately 1.9629. It is well-known that unpaired electrons are responsible for the generation of the EPR signal. Usually many face-bound radicals such as •OH and •CH3 (containing unpaired electrons) may also be the paramagnetic centers,29 which are responsible for the EPR signal in the initial Zn5(OH)8Ac2 3 2H2O and the sample annealed at 200 °C. FTIR (Figure 3) clearly shows these radicals. When the annealing temperature increases to 300 °C, ZnO phase forms and the number of the face-bound radicals would decrease according to the TGA, FTIR, and XRD results. On further increase of the annealing temperature to 400 °C, a new low-field signal with g factor close to the free-electron value (g = 2.0058) appears while the relative intensity of the high-field signal (g = 1.96) increases significantly. At this stage, Zn5(OH)8Ac2 3 2H2O has

decomposed into crystalline ZnO completely and the face-bound radicals are almost removed. Therefore the signal of EPR is closely related to the ZnO phase. However the relative intensity of both of these two paramagnetic signals decreases when the annealing temperature increases to 500 °C. The line in the EPR spectrum with g factor close to g = 2.0058 is commonly attributed to Zn vacancy.30 Many EPR studies attribute the commonly observed signal with g = 1.96 to the singly ionized oxygen vacancy.31 However, Driess et al.22b have demonstrated that the signal at g = 1.96 is not related to oxygen vacancy, which is probably due to one electron being weakly bound to ionized impurity atoms. There is evidence proving that this kind of signal is due to neutral shallow donors (e.g., Al, Ga, In).22b,31 Although the origin of the signal with g = 1.96 is still a controversial issue, it is widely accepted that g = 1.96 is assigned to the shallow donors. Some calculations reveal that oxygen vacancy is actually the deep donor, while zinc interstitial is the native shallow donor.30,32 Look et al. also have assigned the zinc interstitial or the Znirelated complex to the native shallow donors in the electronirradiated ZnO samples.33 However, in our case, zinc interstitial has not been detected according to the XPS measurements as shown in Figure 5. Therefore zinc interstitial inducing the signal with g = 1.96 in the EPR spectrum can be excluded. It is wellknown that hydrogen is one of the most abundant impurities in ZnO, and it is undetectable for most experimental techniques. More recently, Selim et al. show the experimental evidence to support the recent calculations that predicted hydrogen in an oxygen vacancy forms multicenter bonds and acts as a shallow donor.34 This kind of shallow donor is probably responsible for the signal with g = 1.96 in EPR spectrum in our ZnO samples according to the aforementioned analysis. Room temperature ferromagnetism (RT-FM) has been observed in our pure ZnO samples fabricated by annealing the precursor Zn5(OH)8Ac2 3 2H2O at 300, 400, and 500 °C, respectively. Figure 8 shows the magnetic hysteresis (M-H) curves measured at room temperature for the three samples. The saturation magnetic moments Ms of sample 300 (TA = 300 °C) is ∼5  103 emu/g. The Ms increases by 4-fold (∼2  102 emu/g) when TA increases to 400 °C. However, on further increase of TA to 500 °C, the Ms decreases to ∼1.5  102 emu/g. Indeed, the origin of RT-FM in undoped ZnO is not clear and is still a controversial issue. Many hypotheses have been presented to give a reasonable interpretation for the origin of the RT-FM in pure ZnO. But most researchers agree that the defects of ZnO induce the observed RT-FM.8,35 The main point 14580

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The Journal of Physical Chemistry C defects in ZnO are Zni (interstitial zinc) and VO (oxygen vacancies), but VZn (zinc vacancy) and Oi (interstitial oxygen) may also be thermodynamically stabilized in the ZnO crystal lattice only if there is higher oxygen partial pressure.22b In our case, the samples are fabricated by annealing the intact precursor at high temperature in air, so all four kinds of defects might exist in ZnO. PL spectra (Figure 6) shows that the emission peaks in visible region disappear when the annealing temperature increases from 300 to 400 °C. The quenching of the visible emission reveals that the concentration of the corresponding defects in ZnO is significantly decreased and even eliminated. Meanwhile, it is notable that the relative intensity of the high-field signal (g = 1.96) increases significantly when the annealing temperature increases from 300 to 400 °C. As mentioned earlier, the signal with g = 1.96 is due to shallow donors caused by hydrogen. Recently, the first-principles calculations made by Sanchez et al.14 suggest that the hydrogen acts as an oxygen vacancy passivating center, forming strong ZnH bonds and leading to a metallic, hole-doped surface with a net magnetic moment. As displayed in Figure 8, the Ms increases 4-fold (∼ 2  102 emu/g) when TA increases from 300 to 400 °C. Hence we believe that the presence of the shallow donors caused by hydrogen is probably responsible for the observed weak RTFM in sample 300 (TA = 300 °C) and the significant increase of Ms in sample 400 (TA = 400 °C). EPR spectra in Figure 7 also indicates that a new low-field signal with g factor close to the freeelectron value (g = 2.0058) appears when the annealing temperature increases from 300 to 400 °C. The appearance of the new signal (g = 2.0058) is usually attributed to zinc vacancy,30 which may also contribute to the observed RT-FM of ZnO samples. In summary, the shallow donors caused by hydrogen may make major contribution to the RT-FM in undoped ZnO samples fabricated by annealing at the temperature above 300 °C. As to the observed decrease of Ms in sample (TA = 500 °C), it is attributed to the decrease of defect concentration caused by the growing of the particle size when the sample is annealing at higher temperature as illustrated by other groups.9 In fact, grain size growth has been proved by the XRD spectra in Figure 4. The spin numbers decrease by 28% as the annealing temperature increases from 400 to 500 °C according to the calculation from the EPR patterns, which is very close to the decreasing value of Ms.

4. CONCLUSIONS In summary, we have used several techniques (TGA, FTIR, XRD, PL, EPR, and SQUID) to systematically investigate the thermal decomposition of Zn5(OH)8Ac2 3 2H2O to ZnO and concluded that the shallow donors caused by hydrogen and zinc vacancy are major defects in our undoped ZnO samples, fabricated by annealing the precursor Zn5(OH)8Ac2 3 2H2O at temperatures above 400 °C. We propose that both the shallow donors caused by hydrogen and zinc vacancy perhaps play an important role in triggering magnetic order in our pure ZnO samples. The detailed investigations introduced in this paper are an effective way to understand the origin of the observed RT-FM in ZnO samples. It is therefore favorable to figure out the real origin of the RT-FM in ZnO. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Y.W.), [email protected] (J.S.).

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’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 10874148 and 60976012), the Zhejiang Provincial Natural Science Foundation of China (No. Y4090251), the “Qianjiang Talent Project” of Zhejiang Province (No. J20091163), the Fundamental Research Funds for the Central Universities (No. 2010QNA3023), the Program for New Century Excellent Talents in University, the Special Program for the Science and Technology Plan of Zhejiang Province (2009C14005), the Science and Technology Innovative Research Team of Zhejiang Province (2009R50010), and the National Basic Research Program of China (2007CB613403). ’ REFERENCES (1) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferr, D. Science 2000, 287, 1019–1022. (2) (a) Yu, U.; Nili, A. M.; Mikelsons, K.; Moritz, B.; Moreno, J.; Jarrell, M. Phys. Rev. Lett. 2010, 104, 037201. (b) Xu, Q. Y.; Hartmann, L.; Zhou, S. Q.; Mcklich, A.; Helm, M.; Biehne, G.; Hochmuth, H.; Lorenz, M.; Grundmann, M.; Schmidt, H. Phys. Rev. Lett. 2008, 101, 076601. (c) Neal, J. R.; Behan, A. J.; Ibrahim, R. M.; Blythe, H. J.; Ziese, M.; Fox, A. M.; Gehring, G. A. Phys. Rev. Lett. 2006, 96, 197208. (3) Kittilstved, K. R.; Schwartz, D. A.; Tuan, A. C.; Heald, S. M.; Chambers, S. A.; Gamelin, D. R. Phys. Rev. Lett. 2006, 97, 037203. (4) Jung, S. W.; Park, W. I.; Yi, G. C.; Kim, M. Adv. Mater. 2003, 15, 1358–1361. (5) (a) Ferhat, M.; Zaoui, A.; Ahuja, R. Appl. Phys. Lett. 2009, 94, 142502. (b) Herng, T. S.; Lau, S. P.; Yu, S. F.; Chen, J. S.; Teng, K. S. J. Magn. Magn. Mater. 2007, 315, 107–110. (6) Lee, S.; Shon, Y.; Kang, T. W.; Yoon, C. S.; Kim, E. K.; Kim, D. Y. Appl. Phys. Lett. 2008, 93, 022113. (7) (a) Venkatesan, M.; Fitzgerald, C. B.; Coey, J. Nature 2004, 430, 630–630. (b) Pemmaraju, C. D.; Sanvito, S. Phys. Rev. Lett. 2005, 94, 217205. (8) (a) Khalid, M.; Ziese, M.; Setzer, A.; Esquinazi, P.; Lorenz, M.; Hochmuth, H.; Grundmann, M.; Spemann, D.; Butz, T.; Brauer, G.; Anw, W.; Fischer, G.; Adeagbo, W. A.; Hergert, W.; Ernst, A. Phys. Rev. B 2009, 80, 35331. (b) Wang, Q.; Sun, Q.; Chen, G.; Kawazoe, Y.; Jena, P. Phys. Rev. B 2008, 77, 205411. (9) Sundaresan, A.; Bhargavi, R.; Rangarajan, N.; Siddesh, U.; Rao, C. Phys. Rev. B 2006, 74, 161306. (10) (a) Hong, N. H.; Sakai, J.; Poirot, N.; Brize, V. Phys. Rev. B 2006, 73, 132404. (b) Hong, N. H.; Sakai, J.; Gervais, F. J. Magn. Magn. Mater. 2007, 316, 214–217. (11) Yi, J. B.; Lim, C. C.; Xing, G. Z.; Fan, H. M.; Van, L. H.; Huang, S. L.; Yang, K. S.; Huang, X. L.; Qin, X. B.; Wang, B. Y.; Wu, T.; Wang, L.; Zhang, H. T.; Gao, X. Y.; Liu, T.; Wee, A.; Feng, Y. P.; Ding, J. Phys. Rev. Lett. 2010, 104, 137201. (12) Yan, Z.; Ma, Y.; Wang, D.; Wang, J.; Gao, Z.; Wang, L.; Yu, P.; Song, T. Appl. Phys. Lett. 2008, 92, 081911. (13) Li, L. Y.; Cheng, Y. H.; Luo, X. G.; Liu, H.; Wen, G. H.; Zheng, R. K.; Ringer, S. P. Nanotechnology 2010, 21, 145705. (14) Sanchez, N.; Gallegeo, S.; Cerda, J.; Munoz, M. C. Phys. Rev. B 2010, 81, 115301. (15) Zhang, X.; Cheng, Y. H.; Li, L. Y.; Liu, H.; Zuo, X.; Wen, G. H.; Li, L.; Zheng, R. K.; Ringer, S. P. Phys. Rev. B 2009, 80, 174427. (16) MacManus-Driscoll, J. L.; Khare, N.; Liu, Y. L.; Vickers, M. E. Adv. Mater. 2007, 19, 2925–2929. (17) (a) Lee, H. J.; Park, C. H.; Jeong, S. Y.; Yee, K. J.; Cho, C. R.; Jung, M. H.; Chadi, D. J. Appl. Phys. Lett. 2006, 88, 062504. (b) Wang, Z. H.; Geng, D. Y.; Guo, S.; Hu, W. J.; Zhang, Z. D. Appl. Phys. Lett. 2008, 92, 242505. (18) Xia, Z. B.; Sha, J.; Fang, Y. J.; Wan, Y. T.; Wang, Z. L.; Wang, Y. W. Cryst. Growth Des. 2010, 10, 2759–2765. 14581

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