Al System Due to Surface

Jan 31, 2014 - Our DFT simulations show that spin-polarized charge-transfer occurs from an adsorbed Al atom to O 2p state in surface and subsurface fo...
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Ferromagnetism in Nanostructured TiO2/Al System Due to Surface Charge Transfer Zhiqiang Jiang,† Shijian Chen,*,† and Dingke Zhang‡ †

School of Physics, Chongqing University, Shapingba, Chongqing 401331, China College of Physics and Electronic Engineering, Chongqing Normal University, Shapingba, Chongqing 401331, China

‡

ABSTRACT: Ferromagnetism has been induced in nanostructured TiO2/Al powders prepared by ball milling at room temperature. This study presents the investigation of the origin of the magnetism in a TiO2/Al system with a combination of experiments and density functional theory (DFT) simulations. Our results demonstrate that adsorption of Al on surfaces of TiO2 nanostructures yields spontaneous magnetization. Our DFT simulations show that spin-polarized charge-transfer occurs from an adsorbed Al atom to O 2p state in surface and subsurface for some unique configurations. X-ray absorption near edge structure (XANES) spectra of the magnetic nanostructured TiO2/Al provide the necessary experimental evidence of the charge transfer and confirm the origin of ferromagnetic behavior. On the basis of the experiments and DFT simulations, we believe the room temperature ferromagnetism in nanostructured TiO2/Al originates from charge-transfer from Al to O atoms. Our results illustrate a complex interplay between the atomic level interfacial structure and the resulting ferromagnetic ordering in metalcoated semiconductor oxide nanostructures.

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found experimentally and theoretically to be RT DMSs.25,26 These reports indicate that the semiconducting materials can display spontaneous magnetization even without any 3d transition metal element. Recently, a model based on the charge-transfer effect has been proposed for the ferromagnetism of doped oxide nanoparticles and related materials,27 which could be applicable to a wide range of nanoparticles and thin films of dilute magnetic oxides previously regarded as dilute magnetic semiconductors. We have showed nanostructured TiO2/Al powders prepared by ball milling possess ferromagnetism at room temperature.28 In this study, to better understand the origin of FM in TiO2, we perform first principles density functional theory (DFT) simulations to investigate the origins of magnetism in rutile TiO2/Al nanostructures. Our simulations of an Al doped rutile TiO2 system, in which one Ti atom is substituted by an Al atom, was found to be nonmagnetic. The results of ferromagnetism from Al atom adsorption on TiO2 surfaces suggest that the ferromagnetic behavior can be attributed to the spin-polarized charge-transfer.

INTRODUCTION Ferromagnetic semiconductors (FS) have been widely investigated in recent years because they not only combine the spin and charge of electronics1 but also have potential applications in spintronics.2 Although several systems with ferromagnetism at room temperature have been found, such as (Ga,Mn)As3 and ZnO/Al,4 it is necessary to synthesize a variety of materials. Titanium dioxide (TiO2), due to advantages of wide band gap,5 stability,6 applications of photocatalysis7 and dye-sensitized solar cell,8 becomes one of the major candidates. Traditionally, FS can be achieved by doping general semiconductors with a transition metal.9,10 Moreover, since room temperature ferromagnetism in HfO2 was reported by Coey et al.,11 experiments on d0-ferromagnetism have been carried on by other groups.12−14 As is well-known, TiO2 of the rutile phase, which is a stable form, exhibits a direct band structure. Many theoretical investigations of TiO2 have been carried out to study the band structure, electronic structure and optic properties.15 Since the discovery of room-temperature ferromagnetism in Co-doped TiO2,16 various 3d transition metal doped TiO2-based FS have been extensively studied, such as Co:TiO2,17 Cr:TiO218 and Fe:TiO2.19,20 However, some of the oxides doped with 3d elements often suffer from contamination of the magnetic precipitates or the secondary phase.21 These extrinsic magnetic behaviors are undesirable for practical applications. Hence, nonmagnetic dopants that do not have partially filled d or f bands have been sought. As a result of this, RT ferromagnetism has recently been found in Cu doped TiO2,22 Al doped ZnO films23 and nanostructured ZnO particles.24 In addition, both C- and N-doped ZnO were © 2014 American Chemical Society

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EXPERIMENTAL AND THEORETICAL METHODS Experiments. First, we prepared nanostructured (TiO2)1−x/Alx samples with different milling times varying from 1 to 24 h. More details on preparing, and the macroscopic structural and magnetic properties of, the samples can be consulted elsewhere.28 All prepared powders were characterized Received: November 13, 2013 Revised: January 21, 2014 Published: January 31, 2014 3789

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unit cells with 5 layers forming a slab. The periodic images of the slab were separated by 13 Å of vacuum in the direction normal to the surface. The value of k-points is set as 5 × 5 × 1.

by means of X-ray diffraction (XRD) using monochromatic Cu−Kα radiation, X-ray photoelectron spectroscopy (XPS), Xray absorption near edge structure spectroscopy (XANES) and transmission electron microscopy (TEM). Magnetic characterization was performed via a vibrating sample magnetometer (VSM) at room temperature. Structural characterization by Xray diffraction (XRD) and TEM show that the nanostructured TiO 2 /Al powders have a homogeneous rutile crystal structure.28 Calculation Model and Methods. We performed the DFT simulations to indentify optimized structures, electronic structures and magnetic properties of rutile TiO2/Al system, which were implemented in the simulation package VASP.29 In these simulations, electronic exchange-correlation functions are treated with the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) form.30 Core and valence electronics were used with the projector-augmented wave potentials supplied by VASP.31 In each case, we employed a 500 eV plane-wave kinetic energy cutoff, smearing was used with 0.5 eV, and a Γ-centered Monkhorst-Pack grid with kpoints was used to sample the irreducible Brillouin zone. In geometry optimization, the parameter of energetic convergence for self-consisted field is set as 10 × 10−5 eV/atom. Moreover, the structural optimization of all ionic positions would finish until the Hellmann−Feynman (HF) forces were less than 0.01 eV/Å. Finally, magnetic moments on each atom were obtained from calculation results of VASP by projecting atomic wave functions onto individual atomic orbitals. In this work, we specifically considered doping of Al in a bulk TiO2 crystal lattice and the adsorption of Al on the following low-energy bulk-terminated TiO2 surfaces: Ti-terminated and O-terminated nonpolar (110) surfaces. First, Al doped in rutile bulk TiO2 was calculated, as shown in Figure 1. The supercell bulk doping of crystalline TiO2

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DISCUSSION AND RESULTS Magnetic Properties. Figure 2 shows the room-temperature magnetization curves for nanostructured (TiO2)0.7/Al0.3.

Figure 2. Room-temperature magnetization curves for pure TiO2 and nanostructured (TiO2)0.7/Al0.3. (a) Ms dependence on the ratio of Al to TiO2; (b) Ms and 3/R (surface-to-volume ratio) dependence on the milling time.

A clear hysteresis behavior is evident on the M-H curve obtained from the nanostructured (TiO2)0.7/Al0.3 milled for 8 h, indicating that ferromagnetism is induced in the TiO2/Al powders simply by milling at room temperature. In contrast, pure TiO2 milled for 8 h with the same conditions shows weak paramagnetism. It is also seen in the insets (a and b) in Figure 2 that spontaneous magnetization (Ms) is highly dependent on both the ratio of Al to TiO2 and milling time. The Ms of the milled (TiO2)1−x/Alx powders depends on the Al content x and shows a tendency to increase with x when Al/TiO2 < 0.3, but it decreases slightly when Al/TiO2 > 0.3. It is worth noting in inset b that both Ms and 3/R (surface-to-volume ratio) exhibit similar dependence on the milling time, suggesting that the magnetic moment is localized on the surface and the spontaneous magnetization is governed by the surface-area-tovolume ratio of nanoparticles. In order to better understand the mechanism of ferromagnetism in nanostructured TiO2/Al system, we performed DFT simulations. DFT Simulations. First, we investigated the magnetism of pristine TiO2. The calculation results show that bulk TiO2 in the slab supercell with low-index TiO2 surface with dangling bonds saturated by H atoms revealed no ferromagnetism, which is consistent with our reported experimental results.28 Furthermore, our simulations for Al atoms doped in TiO2 crystal at a Ti site also reveal no magnetic moment. Experimental results (Figure 2) show that magnetism is highly dependent on the surface to volume ratio, which suggests that the origin of the ferromagnetic behavior can possibly be attributed to the interaction between Al and TiO2 at the surfaces of nanoparticles. Next, we examined magnetic properties for the adsorption of Al on the low-index surfaces as indicated above.

Figure 1. rutile TiO2:Al 2 × 2 × 2 supercell, the Al dopant is placed at A (the side of the unit cell) and B (the center of the unit cell) sites, respectively.

consists of 2 × 2 × 2 rutile TiO2 unit cells in which one Ti atom is replaced by an Al atom. In rutile TiO2, two different types of Ti atoms (indicated as A and B shown in Figure 1) were considered. For the geometry optimization, we used the same k-points grids (9 × 9 × 13) for the supercells in both configurations. For Al adsorption on Ti-terminated and O-terminated (110) surfaces, the simulation supercell consists of 3 × 1 TiO2 surface 3790

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It can be seen from Table 1 that Al has negative adsorption energies for all the configurations of Al adsorption on Oterminated TiO2 (110) surface, because there is an attractive interaction between electropositive Al and electronegative O. On the contrary, the adsorption energies are thermodynamically unfavorable for Al adsorption on a Ti-terminated surface, owing to little difference of electronegativity between Al and Ti atoms. In addition, we can find the structure of Al adsorption at Bridge-O(I) site of O-terminated surface is most stable relatively with the adsorption energy of −4.998 eV. It is clear from Figure 3 that the Bridge-O site can allow Al bonds with the two nearest O, while Al can bond with the nearest O only in a Top-O configuration, which leads to the adsorption energy of Bridge-O configurations to be higher than those of Top-O for Al adsorption on an O-terminated (110) surface. The data summarized in Table 1 also shows the magnetic moment Ms for Al adsorption on different sites of TiO2 (110) surfaces. It is clear that for Al adsorption on an O-terminated (110) surface, the configurations that yield a nonzero magnetic moment are Bridge-O(I) and Bridge-O(II) site (with the magnetic moment of 1.00 μB), while there is no magnetic moment at Top-O(I) and Top-O(II) sites. In spite of a nonzero magnetic moment for Al adsorption on a Titerminated TiO2 (110) surface, the adsorption energy is positive except for Al adsorption at Bridge-Ti(I) sites with the adsorption energy of −5.467 eV. Figure 4a shows the spin density distribution of Al adsorption on the Bridge-O (I) site of the O-terminated

In our simulation of Al atoms adsorption on TiO2 surfaces, we first placed isolated Al atoms at the high symmetrical positions as much as possible. We found six and four sites for Al atoms adsorption at Ti-terminated and O-terminated (110) surfaces, respectively, as shown in Figure 3. These adsorption

Figure 3. Absorption sites for Al atom on O-terminated TiO2 (110) surfaces ((a) top view, (b) side view): a (top-O(I)), b (top-O(II)), c (Bridge-O(I) and d (bridge-O(II)) ; and on Ti-terminated TiO2 (110) surfaces (top view (c), side view (d)): A (top-Ti(I)), B (top-Ti(II)), C (top-O), D (bridge-Ti(I)), E (bridge-O), F (bridge-Ti(II)). Blue and red spheres indicate Ti and O atoms, respectively, and green spheres denote adsorption sites of the Al atom.

sites on Ti-terminated surfaces can be defined as Bridge-O (marked E, middle of two O atoms in the topmost surface layer), Bridge-Ti(I) (marked D), Bridge-Ti(II)(marked F, middle of two Ti atoms in the topmost surface layer), Top-O (marked C, above O in the topmost surface layer), Top-Ti(I) (marked A)and Top-Ti(II)(marked B, above Ti in the topmost surface), shown in Figure 3a. The sites for Al adsorbed on the O-terminated surface are defined as Bridge-O(I) (marked c), Bridge-O(II) (marked d), Top-O(I) (marked a) and TopO(II) (marked b), respectively, as shown in Figure 3c. Table 1 summaries the adsorption energies and net magnetic moments for Al adsorbed on different sites of TiO2 surfaces. We define the adsorption energy Ea as Ea = ET − ETiO2 − EAl, where ET is the total energy of the supercell containing Al adsorbed on the TiO2 surface, ETiO2 is the total energy of pure TiO2 in the same supercell, and EAl is the energy of one Al atom in fcc crystal lattice. With this definition, negative adsorption energy donates exothermic reaction, in other words, where it is energetically favorable for Al to adsorb on the surface.

Figure 4. (a) Spin density and (b) electronic charge difference profiles for Al adsorption on O-terminated TiO2(110) surface at Bridge-O(I) site. The yellow color donates isodensity of magnitude of 0.0019 Å−3 for (a) and 0.0036 Å−3 for (b). Blue and red and green spheres indicate Ti, O and Al atoms, respectively.

TiO2 (110) surface. It is clear that the magnetic moment mainly distributes on an Al atom, while the nearest neighboring and the next nearest neighboring O atoms in topmost layers, the rest of O anions and Ti cations remain unpolarized, which clearly shows that only the surfaces become ferromagnetic in

Table 1. Adsorption Energy (Ea) and Magnetic Moment (Ms) for Al Adsorption on TiO2 (110) Surfaces surfaces

position

Top-Ti(I)

Ti-terminated

Ea (eV) Ms (μB)

0.081 0.12

surfaces

position

O-terminated

Ea (eV) Ms (μB)

Top-Ti(II) 3.625 1.57 Top-O(I)

Top-O 0.779 3.46 Top-O(II)

−2.053 0

−1.450 0 3791

Bridge-Ti(I)

Bridge-Ti(II)

−0.546 2.87

7.46 3.59

Bridge-O

Bridge-O(I)

5.363 0.73 Bridge-O(II)

−4.998 1.00

−2.977 1.00

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the nanostructured TiO2/Al system, while the rest remain paramagnetic. The corresponding electronic charge density difference that distributes over adsorbed Al and the nearest O atoms is shown in Figure 4b, which indicates that there is a charge transfer between the Al atom and the nearest O atoms. By performing the Bader analysis, we estimated the magnitude of the charge transfer of 2.0e− from adsorbed Al to the nearest neighboring O and the next nearest neighboring O atoms. It is evident that the alteration of the electronic structure and the spin structure due to Al adsorption on the surface extend to the second nearest-neighbor oxygen atoms in the TiO2 surface and subsurface layer. In order to demonstrate the origin of net magnetic moments, we analyzed partial density of state (PDOS, shown in Figure 5)

Figure 6. Normalized O K-edge XANES for ball-milled (TiO2)0.7Al0.3x and ball-milled pure TiO2.

range order.34,35 It can be observed that for the (TiO2)0.7Al0.3 sample, the intensity of low-energy features t2g and eg is reduced compared with the pure TiO2. This can be understood as being caused by the hybridization of 3sp orbitals of adsorbed Al and 2p orbitals of neighboring O, promoting a charge transfer from Al to O. As a consequence of this charge transfer, there is an increase in the occupation of O 2p states leading to a reduction in t2g and eg absorption intensities. This decrease of the absorption intensity coincides with the enhancement of spontaneous magnetization, thus confirming that the charge transfer from adsorbed Al to TiO2 surfaces as the origin of the ferromagnetic behavior in the nanostructured TiO2−Al system. We then examined the long-rang ferromagnetic coupling in nanostructure of TiO2/Al. Here we also take the configuration of Al adsorption at the Bridge-O(I) site as an example. In order to investigate the magnetic coupling between adsorbed Al atoms, we adopted a supercell twice as large as that of the case discussed thus far. For the new structure, two Al atoms were placed at Bridge-O(I) site initially, separated by 3.3 Å (nearest neighbor Bridge-O(I) sites) and 6.0 Å (next-nearest neighbor Bridge-O(I) sites), respectively. After relaxation, we found that the configuration with two Al adsorbents separated by 6.0 Å favors ferromagnetic couplingthe total energy of the ferromagnetic state is lower by 21 meV than the configuration without spin polarization. Therefore, the nanostructured TiO2/ Al system is weak ferromagnetism at low Al adsorption concentration. Figure 7 shows the magnetic coupling between the two Al ions separated by 6.0 Å. As can be clearly seen, the coupling between O 2p states in the surface and subsurface

Figure 5. Partial density of state (PDOS) for Al adsorption at BridgeO(I) site of O-terminated TiO2 (110) surface. The PDOS of adsorbed Al atom, the nearest neighboring O atom (marked 1) and the all the Ti atoms are shown. The positions of atoms marked 1 are indicated in Figure 4. The Fermi energy is set to zero.

for Al and neighboring O and Ti atoms for the configuration of Al adsorption at Bridge-O (I) site on the O-terminated (110) surface. It can be seen from Figure 5 that Al 3s and 3p electronic state overlaps largely with O 2p electronic state near the Fermi level. This result indicates that there is a strong attractive interaction, which causes splitting of Al 3s, Al 3p and O 2p states near Fermi level, between Al and the nearest O atoms. We can find that spin-up states are fully occupied while spin-down states are unfilled for Al 3s, Al 3p and O 2p orbitals near the Fermi level, which yields a magnetic moment of 1.0 μB. It is a fact that the electronic structure and the spin structure have been changed, since Al atom adsorbed onto the TiO2 surface. Moreover, this is evident that charge transfer from Al to the nearest O atoms gives rise to the spin-polarized Al atom and O atoms. XANES Results. In order to verify this charge-transfer ferromagnetism for the TiO2−Al system, we acquired XANES for O K-edge from nanostructured (TiO2)0.7Al0.3. Figure 6 shows the O K-edge NEXAFS spectra of the pure TiO2 sample, and the nanostructured (TiO2)0.7Al0.3. The O K-edge NEXAFS spectra correspond to the electron transitions from the oxygen 1s core level into empty or partially filled O 2p states. The prominent doublets centered at 531.4 and 534.2 eV can be assigned respectively to transitions into the t2g and eg bands of Ti.32,33 The features above 536 eV are due to the covalent mixing of O 2p and Ti 4sp orbitals and are sensitive to the long-

Figure 7. Ferromagnetic coupling between Al atoms adsorbed at Bridge-O(I) sites on O-terminated of TiO2(110) surface and separated by 5.97 Å. The yellow denotes spin density of magnitude of 0.0019 Å−3. 3792

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layers mediates the long-range ferromagnetic coupling. It should be noted that magnetic moment is zero when two Al atoms are placed in neighboring Birdge-O(I) sites. After relaxation, Al atoms move closer from an initial distance of 3.3 to 2.8 Å, leading to antiferromagnetic coupling and zero net magnetization. Therefore, one should note that the magnetic moments of nanostructured TiO2/Al are highly dependent on the concentration of Al adsorbents. Finally, the simulation results of spin-polarized charge transfer as the mechanism of origin of ferromagnetism observed in TiO2/Al nanoparticles may explain the experimental observations. The experimental results show that the Ms strongly depends on the Al dopants concentration, initially increasing before x > 30% then decreasing. It is agreeable with the simulation results that as the Al concentration increases, more Al atoms become available for adsorption, increasing the degree of spin polar charge transfer to TiO2 surface, and magnetic moment would increase accordingly. On the other hand, our calculations of ferromagnetic coupling also demonstrate that magnetization depends on surface density of Al, the coupling is ferromagnetism when the Al atoms are placed at the next-nearest neighboring sites (6.0 Å), while the coupling is antiferromagnetic with the distance of Al atoms decreasing to the nearest neighboring site. Thus, there must exist a critical surface density of adsorbed Al beyond which there is a loss of magnetization. Moreover, our experiments show that both Ms and 3/R (surface-to-volume ratio) exhibit similar dependence on the milling time, suggesting that the magnetic moment is localized on the surface and the spontaneous magnetization is governed by the surface-area-tovolume ratio of nanoparticles. This argument is fully supported by our simulation results. The spin density distribution and corresponding electronic charge density difference, as shown in Figure 4, clearly show that the spin distribution and the alteration of the electronic structure due to Al adsorption on the surface only extend to the second nearest-neighbor oxygen atoms in the TiO2 surface and subsurface layer. In other words, only the surfaces of nanostructured TiO2/Al system become ferromagnetic.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 23 65678362. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC) (grants 11304406 and 61307035), Fundamental Research Funds for the Central Universities (Project No. CQDXWL-2013-Z001), and Natural Science Foundation of Chongqing (grant cstc2013jcyjA0732).

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REFERENCES

(1) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Spintronics: A Spin-Based Electronics Vision for the Future. Science 2001, 294, 1488−1495. (2) Ohno, H. Making Nonmagnetic Semiconductors Ferromagnetic. Science 1998, 281, 951−956. (3) Ohno, H.; Shen, A.; Matsukura, F.; Oiwa, A.; Endo, A. (Ga,Mn)As: A New Diluted Magnetic Semiconductor Based on GaAs. Appl. Phys. Lett. 1996, 69, 363−365. (4) Park, J. H.; Kim, M. G.; Jang, H. K. Co-metal Clustering As the Origin of Ferromagnetism in Co-doped ZnO Thin Films. Appl. Phys. Lett. 2004, 84, 1338−1340. (5) Burdett, J. K. Structural-Electronic Relationships in Inorganic Solids: Powder Neutron Diffraction Studies of the Rutile and Anatase Polymorphs of Titanium Dioxide at 15 and 295 K. J. Am. Chem. Soc. 1987, 109, 3639−3646. (6) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical Processes for Water Treatment. Chem. Rev. 1993, 93, 671−698. (7) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269−271. (8) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (9) Baik, J. M.; Jang, H. K.; Kim, J. K. Effect of Microstructural Change on Magnetic Property of Mn-Implanted p-Type GaN. Appl. Phys. Lett. 2003, 82, 583−585. (10) Chang, T. Q.; Wang, D. B.; Luo, X. H. Synthesis, Optical, And Magnetic Propeerties of Diluted Magnetic Semeconductor Zn1‑xMnxO Nanowires via Vaporphase Growth. Appl. Phys. Lett. 2003, 83, 4020− 4022. (11) Venkatesan, M.; Fitzgerald, C. B.; Coey, J. M. D. Thin Films: Unexpected Magnetism in a Dielectric Oxide. Nature 2004, 430, 630− 633. (12) Xing, G. Z.; Wang, D. D.; Yi, J. B.; Yang, L. L.; Gao, M.; He, M.; Yang, J. H.; Ding, J.; Sum, T. C.; Wu, T. Correlated d 0 Ferromagnetism and Photoluminescence in Undoped ZnO Nanowires. Appl. Phys. Lett. 2010, 96, 112511. (13) Hong, N. H.; Poirot, N.; Sakai, J. Evidence for Magnetism Due to Oxygen Vacancies in Fe-Doped HfO2 Thin Films. Appl. Phys. Lett. 2006, 89, 042503. (14) Kang, H. J.; Park, K. H.; Yeal, Y. Y.; Lee, E. K.; Lee, S. S.; Oh, S. K.; Yu, S. C.; Yang, D. S. Magnetic and Structural Properties of Mn:ZnO Thin Film Grown on Sapphire(0001) Substrates by using Pulsed Laser Deposition. J. Korean. Phys. Soc. 2009, 55, 2685−2688. (15) Valentin, C. D.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Paganini, M. C.; Giamello, E. N-doped TiO2: Theory and Experiment. Chem. Phys. 2004, 339, 44−56. (16) Yuji, M.; Ryota, T.; Makoto, M. Ferromagnetism in Co-Doped TiO$_{2}$ Rutile Thin Films Grown by Laser Molecular Beam Epitaxy. Jpn. J. Appl. Phys. 2001, 40, L1204−L1206.

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CONCLUSIONS In summary, we have investigated the origins of the ferromagnetism in a nanostructured TiO2/Al system with the first principle DFT calculation method. Ferromagnetism at room temperature has been observed in nanostructured TiO2/ Al prepared by ball milling, the magnetization is highly dependent on Al concentration and governed by the surfacearea-to-volume ratio. Our calculated results demonstrate that the system of Al adsorption on rutile TiO2(110) surfaces can yield a nonzero net magnetic moment. Adsorption of Al on TiO2(110) surface gives rise to charge transfer from adsorbed Al to the nearest O atoms in the surface layer. The strong attractive interactions between Al 3s and 3p electronic states and O 2p states result in spin-polarized Al 3s, Al 3p and O 2p electrons in the surface layer. Furthermore, our calculations also show that such a system favors long-range ferromagnetic coupling at low Al adsorption concentration, which makes nanostructured TiO2/Al weakly ferromagnetic. As there are no magnetic elements in our system, room temperature ferromagnetism of TiO2/Al system is believed to origin from spinpolarized charge transfer. 3793

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(17) Kim, J. Y.; Park, J. H.; Park, B. G. Ferromagnetism Induced by Clustered Co in Co-Doped Anatase TiO2 Thin Films. Phys. Rev. Lett. 2003, 90, 17401. (18) Kaspar, T. C.; Droubay, T.; Shutthanandan, V.; Heald, S. M. Ferromagnetism and Structure of Epitaxial Cr-doped Anatase TiO2 Thin Films. Phys. Rev. B 2006, 73, 155327. (19) Errico, L. A.; Renteria, M.; Weissmann, M. Theoretical Study of Magnetism in Transition-metal-doped TiO2 and TiO2−δ. Phys. Rev. B 2005, 72, 184425. (20) Kim, Y. J.; Thevuthasan, S.; Droubay, T.; Lea, A. S.; Wang, C. M.; Shutthanandan, V.; Chambers, S. A.; Sears, R. P.; Taylor, B.; Sinkovic, B. Growth and Properties of Molecular Beam Epitaxially Grown Ferromagnetic Fe-doped TiO2 Rutile Films on TiO2(110). Appl. Phys. Lett. 2004, 84, 3531−3533. (21) Coey, J. M. D. d0 Ferromagnetism. Solid State Sci. 2005, 7, 660− 667. (22) Duhalde, S.; Vignolo, M. F.; Golmar, F.; Chiliotte, C. Appearance of Room-Temperature Ferromagnetism in Cu-doped TiO2−δ Films. Phys. Rev. B 2005, 72, 161313. (23) Chen, S. J.; Medhekar, N. V.; Garitaonandia, J. S.; Suzuki, K. Surface Charge Transfer Induced Ferromagnetism in Nanostructured ZnO/Al. J. Phys. Chem. C 2012, 116, 8541−8547. (24) Chen, S. J.; Suzuki, K.; Garitaonandia, J. S. Room Temperature Ferromagnetism in Nanostructured ZnO−Al System. Appl. Phys. Lett. 2009, 95, 172507. (25) Pan, H.; Yi, J. B.; Shen, L. Room-Temperature Ferromagnetism in Carbon-Doped ZnO. Phys. Rev. Lett. 2007, 99, 127201. (26) Xu, H. Y.; Liu, T. C.; Xu, C. S.; Liu, Y. X. Room-temperature Ferromagnetism in (Mn, N)-Codoped ZnO Thin Films Prepared by Reactive Magnetron Cosputtering. Appl. Phys. Lett. 2006, 88, 242502. (27) Coey, J. M. D.; Wongsaprom, K.; Alaria, J.; Venkatesan, M. Charge-transfer Ferromagnetism in Oxide Nanopaticles. J. Phys. D: Appl. Phys. 2008, 41, 134012. (28) Chen, S. J.; Garitaonandia, J. S.; Ortega, D.; Suzuki, K. Roomtemperature Spontaneous Magnetization in a Nano Structure TiO2-Al System Prepared by Ball-Milling. J. Alloys Compd. 2012, 536, S287− S290. (29) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (30) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, And Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671− 6687. (31) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (32) Khan, M. A.; Kotani, A.; Parlebas, J. C. Electronic Structure and Core Level Photoemission Spectra in TiO Compounds. J. Phys.: Condens. Matter 1991, 3, 1763−1772. (33) Jacobs, K.; Wickham, J.; Alivisatos, A. P. Threshold Size for Ambient Metastability of Rocksalt CdSe Nanocrystals. J. Phys. Chem. B 2002, 106, 3759−3762. (34) Chen, J. G. NEXAFS Investigations of Transition Metal Oxides, Nitrides, Carbides, Sulfides and Other Interstitial Compounds. Surf. Sci. Rep. 1997, 30, 1−152. (35) Ruus, R.; Kikas, A.; Saar, A.; Ausmees, A.; Nommiste, E.; Aarik, J.; Aidla, A.; Uustare, T.; Martinson, I. Ti 2p and O 1s X-Ray Absorption of TiO2 Plymorphs. Solid State Commun. 1997, 104, 199− 203.

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