Defects-Induced Room Temperature Ferromagnetism in ZnO

Article pubs.acs.org/JPCC

Defects-Induced Room Temperature Ferromagnetism in ZnO Nanorods Grown from ε‑Zn(OH)2 Jing Wang, Sichao Hou, Haoyuan Chen, and Lan Xiang* Department of Chemical Engineering, Tsinghua University, Beijing, 10084, China S Supporting Information *

ABSTRACT: This paper reports the zinc interstitial-induced room temperature ferromagnetism (RT-FM) in undoped ZnO nanorods synthesized by aging ε-Zn(OH)2 precursor in 0−2 mol·L−1 NaOH at 80 °C for 10.0 h. The variations of the defect states and ferromagnetism of the ZnO nanorods with NaOH concentration were investigated by X-ray diffraction, Raman scattering, photoluminescence, electron spin resonance, X-ray photoelectron spectroscopy, and superconducting quantum interference device so as to identify the origin of RTFM. The experimental results revealed that the increase of the NaOH concentration led to the increase of the oxygen-related defects but the decrease of the zinc interstitials in association with the magnetization value. A direct correlation between the ferromagnetism and the relative concentration of the zinc interstitials was established, which indicated that the zinc interstitials may play an important role in mediating the RT-FM in the undoped ZnO nanorods.

1. INTRODUCTION Diluted magnetic semiconductors with Curie temperatures (Tc) at room temperature have attracted much attention since their charge and spin can be tuned simultaneously.1−3 ZnO is a promising candidate for semiconductor-based spintronics owing to its wide band gap (3.37 eV) and large exciton binding energy (60 meV). Room temperature ferromagnetism (RT-FM) has been widely observed in ZnO doped with transition metals (TM) such as Mn, Ni, Co, etc. 4−7 Correspondingly, the models based on the exchanging coupling of localized spins8,9 or the segregation of the magnetic metal clusters/precipitates10,11 have been adapted to explain the RTFM origination. Recently, RT-FM has also been observed in nonmagnetic doped ZnO and undoped ZnO.12−15 Many works have been carried out to introduce intrinsic point defects (oxygen vacancy, oxygen interstitial, zinc vacancy, zinc interstitial, etc.) into ZnO and to reveal their correlations with RT-FM. However, up to now, the defect-induced RT-FM in ZnO still remains a controversial topic due to the sophisticated defect-chemistry. It was widely accepted that the oxygen-related defects, especially the oxygen vacancies, may be responsible for RT-FM in undoped ZnO.16−24 For example, Xing et al.17 tuned the content of oxygen vacancies from 4% to 20% in undoped ZnO nanowires prepared via a vapor transport method and found that the oxygen vacancies boosted the RT-FM. Xu et al.20 synthesized ZnO nanoparticles with tunable sizes and ferromagnetism via a wet chemical method and established a direct link between the ferromagnetism and the relative occupancy of the singly charged oxygen vacancies (VO+) located on the ZnO surface. You et al.24 prepared ZnO with rich oxygen interstitials by treating ZnO with H2O2 solution © XXXX American Chemical Society

and demonstrated that the oxygen interstitials could also boost the ferromagnetism in ZnO. Besides oxygen-related defects, some other reports have revealed that the RT-FM in ZnO could also be mediated by zinc-related defects. It was reported that the zinc vacancies generated in the thermal decomposition of Zn5(OH)8Ac2·2H2O to ZnO or in the mechanical milling of ZnO nanoparticles contributed to the ferromagnetism, and the zinc vacancy-induced RT-FM was also demonstrated by some first-principles calculations.25−28 Meanwhile, zinc interstitialinduced RT-FM has been commonly reported in ZnO doped with transition metals such as Mn, Cu, Co, Cr, etc.29−35 in Zn/ ZnO granular films,14 or in nanocrystallines Zn/ZnO core− shell structures,36 etc. The origin of zinc interstitials (Zni) was usually attributed to the diffusion of metal ions or the existence of heterostructure interface. However, up to now, to the best of our knowledge, only a few reports were concerned with the Zniinduced RT-FM in pure ZnO sample,37 the experimental evidence for the Zni-induced RT-FM in pure ZnO was still limited. Furthermore, the role of Zni in mediating the ferromagnetism in pure ZnO was still unclear. ε-Zn(OH)2 has been employed as a promising precursor for the fabrication of ZnO nanostructures since it can be converted to ZnO at temperature 20 MΩ·cm−1 was used in the experiments. Containers, lab spoons, and funnels made by Teflon were used throughout the experiments to eliminate the unintentional ferromagnetic contamination. Synthesis of ε-Zn(OH)2. : ε-Zn(OH)2 was prepared by dropwise addition of 2.0 mol·L−1 ZnSO4 into 4.0 mol·L−1 NaOH at 25 °C under stirring (300 r·min−1) until the molar ratio of Zn2+:OH− reached 1:2. The slurry was stirred for 10 min and then the precipitate was filtrated, washed with deionized water, and dried at 25 °C for 24.0 h. All the X-ray diffraction peaks can be indexed to the orthorhombic εZn(OH)2 (JCPDS 38−0385) (Figure S1, Supporting Information). Synthesis of ZnO. : A 1.50-g sample of the ε-Zn(OH)2 was dispersed in 40.0 mL of 0−2.0 mol·L−1 NaOH solutions, then the slurry was kept at 80 °C for 10.0 h. The final product was filtrated, washed, and dried at 60 °C for 12.0 h. 2.2. Analysis. The composition and structure of the samples were identified by an X-ray powder diffractometer (XRD, Bruker-AXS D8 Advance, Germany), using Cu Kα (λ = 0.154178 nm) radiation. The morphology and microstructure of the samples were examined with field emission scanning electron microscopy (FESEM, JSM 7401F, JEOL, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan). The Raman spectra were recorded with He−Ne laser excitation at 532 nm, using a Horiba Jobin Yvon LabRAM HR800 Raman spectrometer. The room temperature photoluminescence spectra were measured on a Hitachi F-7000 luminescence spectrometer, using a Xe lamp with an excitation wavelength of 325 nm. Electron-spinresonance (ESR) measurements were performed on a JEOLTE300 spectrometer operating at an X-band frequency of 9.4 GHz. The surface composition of the samples was characterized by an X-ray photoelectron spectrometer (XPS, PHI-5300, PHI, USA). The magnetic characterization was carried out at room temperature, using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc.). B

dx.doi.org/10.1021/jp5058226 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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oxygen-related defects with the increase of the NaOH concentration. Figure 4a shows the room temperature photoluminescence (PL) spectra of S1−S3. All the samples exhibited the multipeak bands in the visible region of 400−610 nm, including the blue emissions centered at 431 nm and the green-orange emissions in the range of 500−610 nm. A blue emission located at 468 nm was also observed for S1. Panels b and c of Figure 4 show the deconvolutions of the peaks of the blue emissions and green-orange emissions by the Gaussian−Lorentz distribution method. The blue peak centered at 431 nm was ascribed to the recombination of the electron from the shallow level donor of zinc interstitial (Zni) and extended Zni state to a hole in the valence band,16 and the blue peak located at 468 nm was attributed to the transition electron from the conduction band to zinc vacancy (VZn).48 The broad-band green-orange emission was usually attributed to the oxygen-related defects, which could be divided into two peaks, including the green emission at ∼532 nm originating from the singly charged oxygen vacancies (VO+), and the orange one at ∼573 nm originating from the doubly charged oxygen vacancies (VO++).49,50 The order of the PL intensities of the blue emission located at 431 nm was S1 > S2 > S3, while the order of the green-yellow emission was S1 < S2 < S3, indicating that the increase of the NaOH concentration led to the decrease of the contents of Zni and the increase of the contents of VO+ and VO++ in the ZnO samples. Figure 5 shows the electron spin resonance (ESR) spectra of S1−S3. Two kinds of paramagnetic signals were observed for the samples. The low-field signal at g = 2.008 (inset in Figure 5) appearing only in S1 was assigned to VZn according to the PL analysis and previous work.51 The high-field signal at g = 1.954, which has sometimes been mistakenly attributed to unpaired electrons trapped on oxygen vacancies, has been widely accepted to result from shallow donor centers.25,44 The former research revealed that VO was the deep donor while Zni (g ≈ 1.96) and H interstitials (g ≈ 1.97) were the shallow donors.52,53 Therefore, the signal at g = 1.954 should be ascribed to the Zni, and no overlapped signal for H interstitials was observed. The order of the intensities of the signals at g = 1.954 was S1 > S2 > S3, demonstrating that the contents of Zni decreased with the increase of the NaOH concentration. The surface composite and defects of the ZnO samples were further characterized by XPS. The survey scans show no impurity above the detection limit (Figure S2, Supporting

Figure 2. SEM (a−c), TEM (d), and SAED (inset in Figure 1d) pattern of S1−S3: (a) S1; (b) S2; and (c, d) S3.

Figure 3. Raman spectra of S1−S3. Inset: Enlarged E1(LO) modes.

assigned to the oxygen deficiencies such as oxygen vacancies in ZnO.46,47 The order of the intensities of the E1(LO) mode was S3 > S2 > S1 (inset in Figure 3), indicating the increase of the

Figure 4. Room temperature photoluminescence spectra (a), Gaussian−Lorentz fitting curves of the blue emissions (b), and green-orange emissions (c) of S1−S3. C



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Table 1. XPS Data of O 1s Spectra and Zn L3M45M45 Auger Peaks S1

S2

S3

species

peak

%

peak

%

peak

%

O1 O2 O3 Zn2+ Zni

530.33 531.77 532.87 497.95 494.72

74.9 15.2 9.9 74.1 25.9

530.26 531.60 532.70 497.90 494.65

71.3 19.6 9.1 76.6 23.4

530.19 531.54 532.65 497.99 494.75

70.3 20.1 9.6 77.9 22.1

competing dissolution−precipitation and in situ crystallization routes:41−43 Zn(OH)2 + (x − 2)OH− dissolution

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn(OH)2x − x

Figure 5. Electron spin resonance spectra of S1−S3.

precipitation

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ZnO + H 2O + (x − 2)OH−

Information). Figure 6 shows the high-resolution XPS spectra, including the Gaussian−Lorentz fitting curves of O 1s spectra

(1)

solid state

Zn(OH)2 ⎯⎯⎯⎯⎯⎯⎯⎯→ ZnO + H 2O

(2)

If the conversion was performed in H2O, the in situ crystallization was dominant while the dissolution−reprecipitation pathway was suppressed due to the low solubility of εZn(OH)2 in H2O (Ksp = 3.5 × 10−17, 25 °C57). The isotope tracing experiments by Nicholas et al.43 demonstrated that 60− 73% of ZnO formed from ε-Zn(OH)2 via the in situ solid conversion in water, while the concentrated NaOH solutions facilitated the dissolution−precipitation process, which was favorable for the anisotropic growth of ZnO rods along the caxis,42,58 leading to the formation of ZnO rods with increased length and higher aspect ratio. The variation of the Zni content with NaOH concentration could also be explained by the evolution of the growth mechanisms. Here we proposed that the in situ crystallization mechanism favored the formation of Zni. To reveal the formation of Zni in the in situ crystallization process, the crystal structures of ε-Zn(OH)2 and ZnO were studied. As illustrated in Figure 7, ε-Zn(OH)2 is orthorhombic with space group of P212121,59 whereas ZnO is a hexagonal close-packed structure. Zn(II) ion is surrounded tetrahedrally by four oxygen atoms in both cases. Each oxygen is bridged with four zinc tetrahedral in the case of ZnO, while each oxygen

Figure 6. High-resolution of O 1s spectra (a) and Zn L3M45M45 Auger peaks (b) of S1−S3.

and Zn L3M45M45 Auger electron spectra (AES). For O 1s spectra, three species centered at the banding energy of ca. 530.2−530.3, 531.5−531.7, and 532.6−532.8 eV were indexed and denoted as O1, O2, and O3, respectively. The species of O1 originated from the lattice oxygen anions (O2−) in the würtzite structure, while the species of O2 belonged to the Ox− ions (O− and O2−) in the oxygen-deficient regions caused by VO, Oi, or the OH bonds on the ZnO surface, and the species of O3 were usually attributed to the O2 or H2O absorbed on the ZnO surface.54,55 For Zn L3M45M45 AES, the main peaks at 497.9−498.0 eV were assigned to the lattice Zn2+, whereas the shoulder peaks at ca. 494.7 eV were usually attributed to the Zni.56 The comparisons of the O 1s spectra and the Zn L3M45M45 AES data for S1−S3 are presented in Table 1. The increase of NaOH concentration led to the increase of oxygen defects and surface hydroxide-related species (O2), but the decrease of the Zni content, consistent with the analyses of PL and ESR. The variations of the aspect ratios and the defect states of S1−S3 may be connected with the phase transformation process of ε-Zn(OH)2 to ZnO. The former work showed that the conversion of ε-Zn(OH)2 to ZnO took place via the

Figure 7. Crystal structures of ε-Zn(OH)2 (left) and ZnO (right). The red sphere, the center of the blue tetrahedron, and the white sphere represent oxygen, zinc, and hydrogen, respectively. D



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is bridged with only two zinc tetrahedral in the case of εZn(OH)2. Therefore, compared with ZnO, ε-Zn(OH)2 shows a much lower density of Zn atoms (the packing densities of εZn(OH) 2 and ZnO are 44.4% and 53.3%42 ). Lattice contraction and volume shrinkage would occur during the in situ crystallization of ε-Zn(OH)2 to ZnO and the zinc atoms may occupy the lattice interstitial to form Zni during internal atomic rearrangements. Figure 8 shows the magnetic hysteresis (M−H) curves measured at room temperature for S1−S3. S1 exhibited the

so small that the size effects were significant due to the high surface-to-volume ratio. However, in the current case, the average diameters of the three samples were similar and were all in the submicron scale. Furthermore, since the Ms values decreased with the decrease of the diameters, it seemed that the size effect could not be the key factor in the ferromagnetism. In the previously reported TM-doped ZnO or Zn/ZnO heterostructures,14,29,32−36,60 the Zni-induced RT-FM was usually described by the bound magnetic polaron (BMP) model proposed by Coey et al.,9 in which the ferromagnetic exchange can be mediated by shallow donor electrons that form bound magnetic polarons which overlap to create a spin-split impurity band. In the BMP model, the presence of dopants or heterostructure interface may be essential for the sufficient charge transfer that would induce RT-FM.14 On the other hand, the ab initio study by Sato et al.61 showed that the magnetic properties of ZnO could be controlled by changing the carrier density, suggesting the possible carrier-induced ferromagnetism. It was also reported that the shallow donor Zni may act as the charge carrier.62−64 Moreover, the carrierinduced ferromagnetism mediated by shallow donor Zni has also been demonstrated in some TM-doped ZnO such as ZnO:Co, ZnO:Mn, ZnO:Cu, etc.62,65,66 Therefore, in the current case, we tentatively proposed the carrier-induced ferromagnetism to explain the origin of the observed RT-FM in pure ZnO samples.67 In this mechanism, the free carrier concentration is vital to determine the FM order.68 Figure 9

Figure 8. Room temperature magnetic hysteresis loop of S1−S3.

largest saturation moments (MS ≈ 2 × 10−4 emu/g) and the magnetism order of the samples was S1 > S2 > S3. The RT-FM in S1−S3 should be related to the intrinsic point defects. Considering the coexistence of several defects in ZnO samples, including the oxygen-related defects (VO+ and VO++) and the zinc-related defects (VZn and Zni), it was essential to identify which type of defects were responsible for RT-FM in S1−S3. Some previous studies have investigated the correlation between the defect-related luminescence and the ferromagnetism of ZnO and found that the oxygen-related defects were responsible for RT-FM since the increase of oxygen-related defects (including VO+, VO++, and Oi) boosted the defectsrelated emission and the ferromagnetism simultaneously.16,17,20,22 However, the PL results in the present work revealed that the sample exhibiting intensive oxygen defectrelated green-orange emission showed comparatively weak ferromagnetism. The XPS results also indicated that the content of the oxygen defects or the surface hydroxide-related species increased with the increase of NaOH concentration, in contrast to the variation of RT-FM. Hence, the RT-FM in S1− S3 may not be related to the oxygen-related defects or the surface hydrogenation. On the other hand, the PL, ESR, and XPS analyses revealed that the Zni contents decreased with the increase of NaOH concentration, consistent with the variation of Ms, indicating that the observed RT-FM in S1−S3 may be connected to the Zni. The ferromagnetism of solution-grown ZnO nanorods was suggested to originate from the surface defects in some previous reports, since strong dependence of Ms values on size was observed. Panigrahy et al.16 demonstrated the decrease of ferromagnetism of the ZnO nanorods with the increase of the diameters from 85−110, to 250−300, to 500−550 nm. Yan et al.37 demonstrated that the nanorods with diameters of ∼10 nm showed a higher saturated magnetization value compared to those with diameters of ∼20 nm. In these works, the difference in diameters was comparatively notable, or the diameters were

Figure 9. Comparison of Zni/Zn2+, ESR intensities, Zni PL peak areas (inset), and saturation magnetizations of S1−S3.

summarizes the comparison of Zni/Zn2+, intensities of Zni ESR signal and Zni PL peak areas (inset in Figure 9) versus the saturation magnetizations of S1−S3. The approximated linear variation trends of the three curves indicated a direct correlation between Ms and the shallow donor Zni concentration, indicating that the RT-FM in S1−S3 may be carrierdependent30 and the shallow donor Zni as the charge carriers may play a key role in mediating the RT-FM in ZnO grown from ε-Zn(OH)2. In addition, since the XPS results gave out the information about surface defect states, while PL and ESR results were connected with both surface and bulk defects, the similar variation trends of the three curves shown in Figure 9 implied that there was no significant difference between the relative concentration of Zni on the surface and in the bulk of the ZnO rods, thus we suggested that the observed ferromagnetism in the present work may originate from the E



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(4) Ando, K.; Saito, H.; Jin, Z.; Fukumura, T.; Kawasaki, M.; Matsumoto, Y.; Koinuma, H. Magneto-Optical Properties of ZnOBased Diluted Magnetic Semiconductors. J. Appl. Phys. 2001, 89, 7284−7286. (5) Sharma, P.; Gupta, A.; Rao, K.; Owens, F. J.; Sharma, R.; Ahuja, R.; Guillen, J. O.; Johansson, B.; Gehring, G. Ferromagnetism above Room Temperature in Bulk and Transparent Thin Films of MnDoped ZnO. Nat. Mater. 2003, 2, 673−677. (6) Cui, J.; Gibson, U. Electrodeposition and Room Temperature Ferromagnetic Anisotropy of Co and Ni-Doped ZnO Nanowire Arrays. Appl. Phys. Lett. 2005, 87, 133108. (7) Walsh, A.; Da Silva, J. L.; Wei, S. H. Theoretical Description of Carrier Mediated Magnetism in Cobalt Doped ZnO. Phys. Rev. Lett. 2008, 100, 256401. (8) Dietl, T.; Ohno, H.; Matsukura, F. Hole-Mediated Ferromagnetism in Tetrahedrally Coordinated Semiconductors. Phys. Rev. B 2001, 63, 195205. (9) Coey, J.; Venkatesan, M.; Fitzgerald, C. Donor Impurity Band Exchange in Dilute Ferromagnetic Oxides. Nat. Mater. 2005, 4, 173− 179. (10) Park, J. H.; Kim, M. G.; Jang, H. M.; Ryu, S.; Kim, Y. M. CoMetal Clustering as the Origin of Ferromagnetism in Co-Doped ZnO Thin Films. Appl. Phys. Lett. 2004, 84, 1338−1340. (11) Deka, S.; Pasricha, R.; Joy, P. Experimental Comparison of the Structural, Magnetic, Electronic, and Optical Properties of Ferromagnetic and Paramagnetic Polycrystalline Zn1−XCoXO (X = 0, 0.05, 0.1). Phys. Rev. B 2006, 74, 033201. (12) Garcia, M.; Merino, J.; Fernández Pinel, E.; Quesada, A.; De la Venta, J.; Ruíz González, M.; Castro, G.; Crespo, P.; Llopis, J.; Gonzalez-Calbet, J. Magnetic Properties of ZnO Nanoparticles. Nano Lett. 2007, 7, 1489−1494. (13) Khalid, M.; Ziese, M.; Setzer, A.; Esquinazi, P.; Lorenz, M.; Hochmuth, H.; Grundmann, M.; Spemann, D.; Butz, T.; Brauer, G. Defect-Induced Magnetic Order in Pure ZnO Films. Phys. Rev. B 2009, 80, 035331. (14) Zhang, X.; Cheng, Y.; Li, L.; Liu, H.; Zuo, X.; Wen, G.; Li, L.; Zheng, R.; Ringer, S. Evidence for High-T C Ferromagnetism in ZnX(ZnO)1−X Granular Films Mediated by Native Point Defects. Phys. Rev. B 2009, 80, 174427. (15) Herng, T.; Qi, D. C.; Berlijn, T.; Yi, J.; Yang, K.; Dai, Y.; Feng, Y.; Santoso, I.; Sanchez-Hanke, C.; Gao, X. Room-Temperature Ferromagnetism of Cu-Doped ZnO Films Probed by Soft X-Ray Magnetic Circular Dichroism. Phys. Rev. Lett. 2010, 105, 207201. (16) Panigrahy, B.; Aslam, M.; Misra, D. S.; Ghosh, M.; Bahadur, D. Defect-Related Emissions and Magnetization Properties of ZnO Nanorods. Adv. Funct. Mater. 2010, 20, 1161−1165. (17) Xing, G.; Wang, D.; Yi, J.; Yang, L.; Gao, M.; He, M.; Yang, J.; Ding, J.; Sum, T. C.; Wu, T. Correlated d0 Ferromagnetism and Photoluminescence in Undoped ZnO Nanowires. Appl. Phys. Lett. 2010, 96, 112511. (18) Liu, W.; Li, W.; Hu, Z.; Tang, Z.; Tang, X. Effect of Oxygen Defects on Ferromagnetic of Undoped ZnO. J. Appl. Phys. 2011, 110, 013901−013901−5. (19) Venkatesh, P. S.; Purushothaman, V.; Muthu, S. E.; Arumugam, S.; Ramakrishnan, V.; Jeganathan, K.; Ramamurthi, K. Role of Point Defects on the Enhancement of Room Temperature Ferromagnetism in ZnO Nanorods. CrystEngComm 2012, 14, 4713−4718. (20) Xu, X.; Xu, C.; Dai, J.; Hu, J.; Li, F.; Zhang, S. Size Dependence of Defect-Induced Room Temperature Ferromagnetism in Undoped ZnO Nanoparticles. J. Phys. Chem. C 2012, 116, 8813−8818. (21) Zhan, P.; Wang, W.; Liu, C.; Hu, Y.; Li, Z.; Zhang, Z.; Zhang, P.; Wang, B.; Cao, X. Oxygen Vacancy-Induced Ferromagnetism in UnDoped ZnO Thin Films. J. Appl. Phys. 2012, 111, 033501. (22) Xu, X.; Xu, C.; Lin, Y.; Li, J.; Hu, J. Comparison on Photoluminescence and Magnetism between Two Kinds of Undoped ZnO Nanorods. J. Phys. Chem. C 2013, 117, 24549−24553. (23) Zhan, P.; Xie, Z.; Li, Z.; Wang, W.; Zhang, Z.; Li, Z.; Cheng, G.; Zhang, P.; Wang, B.; Cao, X. Origin of the Defects-Induced

Zni on both the surface and in the bulk. It is still an experimental challenge to identify in detail the types of defects responsible for RT-FM since the observed RT-FM may originate from the combined effects of several defects including VO, VZn, and Zni. Nevertheless, the present work not only provides some evidence for the Zni-induced RT-FM in undoped ZnO, but also provides an alternative strategy for controlling the RT-FM of ZnO by varying the Zn i concentrations.

4. CONCLUSION In summary, the present work presents a simple aqueous solution route for the fabrication of single-crystalline ZnO nanorods exhibiting tunable RT-FM by treating ε-Zn(OH)2 in 0−2 mol·L−1 NaOH solutions. The correlation between the ferromagnetism and the relative concentration of Zni was established based on the results obtained by XRD, Raman, PL, ESR, XPS, and SQUID analyses, which suggested that the shallow donor Zni may play a crucial role in mediating the RTFM in the undoped ZnO nanorods grown from ε-Zn(OH)2.

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ASSOCIATED CONTENT

* Supporting Information S

XRD pattern of the precursor formed at room temperature; enlarged (002) diffraction peaks of S1−S3; diameter and length distributions of S1−S3; and XPS survey scan spectrum of representative sample S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-1062772051. Tel: +86-10-62788984. Author Contributions

J.W. and L.X. conceived and designed the study. J.W., S.C.H., and H.Y.C/ performed the experiments. J.W. and L.X. wrote the paper. All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Nos. 51174125, 51234003, and 51374138), National Hi-Tech Research and Development Program of China (863 Program, 2012AA061602), and National Key Technology Research and Development Program of China (2013BAC14B02). The authors thank those who helped with the data collection: Dr. M. Jiang (ESR), Dr. Y. Zhou (PL), Mrs. X.Y. Ye (XPS), and Dr. J.L. Xu (SQUID).

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REFERENCES

(1) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors. Science 2000, 287, 1019−1022. (2) Wolf, S.; Awschalom, D.; Buhrman, R.; Daughton, J.; Von Molnar, S.; Roukes, M.; Chtchelkanova, A. Y.; Treger, D. Spintronics: A Spin-Based Electronics Vision for the Future. Science 2001, 294, 1488−1495. (3) Pearton, S. J.; Abernathy, C.; Norton, D.; Hebard, A.; Park, Y.; Boatner, L.; Budai, J. Advances in Wide Bandgap Materials for Semiconductor Spintronics. Mater. Sci. Eng., R 2003, 40, 137−168. F



Article

Ferromagnetism in Un-Doped ZnO Single Crystals. Appl. Phys. Lett. 2013, 102, 071914. (24) You, H.; Yang, J.; Zhu, J.; Xu, W.; Tang, X. Oxygen Interstitials Enhanced Room Temperature Ferromagnetism in Undoped Zinc Oxide. Appl. Surf. Sci. 2012, 258, 4455−4459. (25) Xia, Z.; Wang, Y.; Fang, Y.; Wan, Y.; Xia, W.; Sha, J. Understanding the Origin of Ferromagnetism in ZnO Porous Microspheres by Systematic Investigations of the Thermal Decomposition of Zn5(OH)8(Ac)2·2H2O to ZnO. J. Phys. Chem. C 2011, 115, 14576−14582. (26) Xing, G.; Lu, Y.; Tian, Y.; Yi, J.; Lim, C.; Li, Y.; Li, G.; Wang, D.; Yao, B.; Ding, J. Defect-Induced Magnetism in Undoped Wide Band Gap Oxides: Zinc Vacancies in ZnO as an Example. AIP Adv. 2011, 1, 022152. (27) Peng, C.; Liang, Y.; Wang, K.; Zhang, Y.; Zhao, G.; Wang, Y. Possible Origin of Ferromagnetism in an Undoped ZnO d 0 Semiconductor. J. Phys. Chem. C 2012, 116, 9709−9715. (28) Zhang, Y.; Yang, D.; Nghia, N.; Thanh, T.; Yu, S. DefectInduced Ferromagnetism in ZnO Nanoparticles Prepared by Mechanical Milling. Appl. Phys. Lett. 2013, 102, 072408. (29) Baik, J. M.; Kang, T. W.; Lee, J. L. Effects of N2O Plasma Treatment on Magnetic Properties of (Zn, Mn)O Nanorods. Nanotechnology 2007, 18, 095703. (30) Kittilstved, K. R.; Schwartz, D. A.; Tuan, A. C.; Heald, S. M.; Chambers, S. A.; Gamelin, D. R. Direct Kinetic Correlation of Carriers and Ferromagnetism in Co2+:ZnO. Phys. Rev. Lett. 2006, 97, 037203. (31) Herng, T.; Lau, S.; Yu, S.; Chen, J.; Teng, K. Zn-InterstitialEnhanced Ferromagnetism in Cu-Doped ZnO Films. J. Magn. Magn. Mater. 2007, 315, 107−110. (32) Liu, H.; Zhang, X.; Li, L.; Wang, Y.; Gao, K.; Li, Z.; Zheng, R.; Ringer, S.; Zhang, B.; Zhang, X. Role of Point Defects in RoomTemperature Ferromagnetism of Cr-Doped ZnO. Appl. Phys. Lett. 2007, 91, 072511. (33) Shi, T.; Xiao, Z.; Yin, Z.; Li, X.; Wang, Y.; He, H.; Wang, J.; Yan, W.; Wei, S. The Role of Zn Interstitials in Cobalt-Doped ZnO Diluted Magnetic Semiconductors. Appl. Phys. Lett. 2010, 96, 211905. (34) Wan, W.; Huang, J.; Zhu, L.; Hu, L.; Wen, Z.; Sun, L.; Ye, Z. Defects Induced Ferromagnetism in ZnO Nanowire Arrays Doped with Copper. CrystEngComm 2013, 15, 7887−7894. (35) Schwartz, D. A.; Gamelin, D. R. Reversible 300 K Ferromagnetic Ordering in a Diluted Magnetic Semiconductor. Adv. Mater. 2004, 16, 2115−2119. (36) Li, L.; Cheng, Y.; Luo, X.; Liu, H.; Wen, G.; Zheng, R.; Ringer, S. Room-Temperature Ferromagnetism and the Scaling Relation between Magnetization and Average Granule Size in Nanocrystalline Zn/ZnO Core−Shell Structures Prepared by Sputtering. Nanotechnology 2010, 21, 145705. (37) Yan, Z.; Ma, Y.; Wang, D.; Wang, J.; Gao, Z.; Wang, L.; Yu, P.; Song, T. Impact of Annealing on Morphology and Ferromagnetism of ZnO Nanorods. Appl. Phys. Lett. 2008, 92, 081911. (38) McBride, R. A.; Kelly, J. M.; McCormack, D. E. Growth of WellDefined ZnO Microparticles by Hydroxide Ion Hydrolysis of Zinc Salts. J. Mater. Chem. 2003, 13, 1196−1201. (39) Li, P.; Liu, H.; Xu, F. X.; Wei, Y. Controllable Growth of ZnO Nanowhiskers by a Simple Solution Route. Mater. Chem. Phys. 2008, 112, 393−397. (40) Xie, J.; Li, P.; Li, Y.; Wang, Y.; Wei, Y. Morphology Control of ZnO Particles Via Aqueous Solution Route at Low Temperature. Mater. Chem. Phys. 2009, 114, 943−947. (41) Li, P.; Liu, H.; Lu, B.; Wei, Y. Formation Mechanism of 1D ZnO Nanowhiskers in Aqueous Solution. J. Phys. Chem. C 2010, 114, 21132−21137. (42) Wang, M.; Zhou, Y.; Zhang, Y.; Hahn, S. H.; Kim, E. J. From Zn(OH)2 to ZnO: A Study on the Mechanism of Phase Transformation. CrystEngComm 2011, 13, 6024−6026. (43) Nicholas, N. J.; Franks, G. V.; Ducker, W. A. The Mechanism for Hydrothermal Growth of Zinc Oxide. CrystEngComm 2012, 14, 1232−1240.

(44) Zeng, H.; Duan, G.; Li, Y.; Yang, S.; Xu, X.; Cai, W. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561−572. (45) Chen, L. Y.; Yin, Y.-T.; Chen, C. H.; Chiou, J. W. Influence of Polyethyleneimine and Ammonium on the Growth of ZnO Nanowires by Hydrothermal Method. J. Phys. Chem. C 2011, 115, 20913−20919. (46) Ö zgür, Ü .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M.; Doğan, S.; Avrutin, V.; Cho, S.-J.; Morkoc, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301. (47) Ashkenov, N.; Mbenkum, B.; Bundesmann, C.; Riede, V.; Lorenz, M.; Spemann, D.; Kaidashev, E.; Kasic, A.; Schubert, M.; Grundmann, M. Infrared Dielectric Functions and Phonon Modes of High-Quality ZnO Films. J. Appl. Phys. 2002, 93, 126−133. (48) Tam, K.; Cheung, C.; Leung, Y.; Djurišic, A.; Ling, C.; Beling, C.; Fung, S.; Kwok, W.; Chan, W.; Phillips, D. Defects in ZnO Nanorods Prepared by a Hydrothermal Method. J. Phys. Chem. B 2006, 110, 20865−20871. (49) Vanheusden, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Voigt, J. A. Correlation between Photoluminescence and Oxygen Vacancies in ZnO Phosphors. Appl. Phys. Lett. 1996, 68, 403−405. (50) Song, J.; Baek, S.; Lim, S. Effect of Hydrothermal Reaction Conditions on the Optical Properties of ZnO Nanorods. Phys. B: Condens. Matter 2008, 403, 1960−1963. (51) Kakazey, N.; Sreckovic, T.; Ristic, M. Electronic Paramagnetic Resonance Investigation of the Evolution of Defects in Zinc Oxide During Tribophysical Activation. J. Mater. Sci. 1997, 32, 4619−4622. (52) Garces, N.; Giles, N.; Halliburton, L.; Cantwell, G.; Eason, D.; Reynolds, D.; Look, D. C. Production of Nitrogen Acceptors in ZnO by Thermal Annealing. Appl. Phys. Lett. 2002, 80, 1334−1336. (53) Hofmann, D. M.; Hofstaetter, A.; Leiter, F.; Zhou, H.; Henecker, F.; Meyer, B. K.; Orlinskii, S. B.; Schmidt, J.; Baranov, P. G. Hydrogen: A Relevant Shallow Donor in Zinc Oxide. Phys. Rev. Lett. 2002, 88, 045504. (54) Kotsis, K.; Staemmler, V. Ab Initio Calculations of the O1s XPS Spectra of ZnO and Zn oxo Compounds. Phys. Chem. Chem. Phys. 2006, 8, 1490−1498. (55) Chen, M.; Wang, Z.; Han, D.; Gu, F.; Guo, G. Porous ZnO Polygonal Nanoflakes: Synthesis, Use in High-Sensitivity NO2 Gas Sensor, and Proposed Mechanism of Gas Sensing. J. Phys. Chem. C 2011, 115, 12763−12773. (56) Chen, W.; Wang, J.; Wang, M. R. Influence of Doping Concentration on the Properties of ZnO: Mn Thin Films by Sol-Gel Method. Vacuum 2007, 81, 894−898. (57) Jacobs, K.; Balitsky, D.; Armand, P.; Papet, P. Low-Temperature Chemical Bath Deposition of Crystalline ZnO. Solid State Sci. 2010, 12, 333−338. (58) Wang, J.; Xiang, L. Formation of ZnO Rods with Varying Diameters from ε-Zn(OH)2. J. Cryst. Growth 2014, 401, 279−284. (59) Schnering, H. Zur Konstitution Des ε-Zn(OH)2. Anorg. Allg. Chem. 1964, 330, 170−178. (60) MacManus-Driscoll, J. L.; Khare, N.; Liu, Y.; Vickers, M. E. Structural Evidence for Zn Intersititials in Ferromagnetic Zn1−XCoxO Films. Adv. Mater. 2007, 19, 2925−2929. (61) Sato, K.; Katayama-Yoshida, H. Ab Initio Study on the Magnetism in ZnO-, ZnS-, ZnSe- and ZnTe-Based Diluted Magnetic Semiconductors. Phys. Status Solidi B 2002, 229, 673−680. (62) Zhang, Y.; Liu, Q.; Sritharan, T.; Gan, C.; Li, S. Pulsed Laser Ablation of Preferentially Orientated ZnO:Co Diluted Magnetic Semiconducting Thin Films on Si Substrates. Appl. Phys. Lett. 2006, 89, 042510. (63) Zhu, T.; Zhan, W.; Wang, W.; Xiao, J. Q. Room Temperature Ferromagnetism in Two-Step-Prepared Co-Doped ZnO Bulks. Appl. Phys. Lett. 2006, 89, 022508. (64) Liu, Q.; Yuan, C.; Gan, C.; Han, G. Effect of Substrate Temperature on Pulsed Laser Ablated Zn0. 95Co0. 05O Diluted Magnetic Semiconducting Thin Films. J. Appl. Phys. 2007, 101, 073902. G



Article

(65) Hou, D.; Ye, X.; Zhao, X.; Meng, H.; Zhou, H.; Li, X.; Zhen, C. Room-Temperature Ferromagnetism in n-Type Cu-Doped ZnO Thin Films. J. Appl. Phys. 2007, 102, 033905. (66) Mukherjee, D.; Dhakal, T.; Srikanth, H.; Mukherjee, P.; Witanachchi, S. Evidence for Carrier-Mediated Magnetism in MnDoped ZnO Thin Films. Phys. Rev. B 2010, 81, 205202. (67) Ueda, K.; Tabata, H.; Kawai, T. Magnetic and Electric Properties of Transition-Metal-Doped ZnO Films. Appl. Phys. Lett. 2001, 79, 988−990. (68) Liu, X.; Shi, E.; Chen, Z.; Zhang, H.; Xiao, B.; Song, L. HighTemperature Ferromagnetism in (Co, Al)-Codoped ZnO Powders. Appl. Phys. Lett. 2006, 88, 252503.

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