Oxygen Deficiency and Room Temperature ... - IEEE Xplore

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 10, NO. 3, MAY 2011

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Oxygen Deficiency and Room Temperature Ferromagnetism in Undoped and Cobalt-Doped TiO2 Nanoparticles N. Rajkumar and K. Ramachandran

Abstract—The compositional analysis done on the synthesized undoped and cobalt (Co)-doped titanium dioxide (TiO2 ) nanoparticles by X-ray photoelectron spectroscopy and energy dispersive X-ray analysis suggests oxygen deficiency in undoped and Co-doped TiO2 samples. Magnetic investigations by vibrating sample magnetometer (VSM) indicate that undoped TiO2 nanoparticles possess ferromagnetic ordering at room temperature due to the oxygen deficiency and in Co-doped TiO2 , the enhancement in magnetic moment is due to Co doping. Index Terms—Cobalt (Co), ferromagnetic materials, oxygen compounds.

I. INTRODUCTION XIDE-BASED ferromagnetic semiconductors with high Curie temperature have been under considerable attention recently for realizing spintronic devices [1], [2]. Among the oxide-based ferromagnetic semiconductor, undoped and Cobalt (Co)-doped titanium dioxide (TiO2 ) both in anatase and rutile forms are of considerable interests [3]–[6]. Although room temperature ferromagnetism (FM) in Co-doped TiO2 is very much demonstrated in literature, the origin of the FM is still controversial due to the sensitive dependence on the fabrication methods and process conditions [7]–[11], as some authors suggested that the FM originates from the Co-metal clusters [12]–[15] embedded in TiO2 , but now most researchers tend to believe that the FM is the intrinsic property of Co-doped TiO2 magnetic semiconductors [3], [16]–[19], even though the origin of the weak FM at room temperature is still an open question. Moreover, in these investigations, the Co-doping concentrations in TiO2 were usually limited to a few percent and the observed saturation magnetization was very small. Recently, FM in oxide-based diluted-magnetic semiconductors (DMS) has been observed due to the oxygen vacancy and structural defects [20]–[24]. Kikoin et al. [25] observed ferromagnetic order due to the super exchange between complexes (oxygen vacancies + magnetic impurities), which are stabilized by the electron transfer from vacancies to impurities. Kim et al. [26] observed FM in Mn-doped

O

Manuscript received May 18, 2009; revised January 9, 2010; accepted April 24, 2010. Date of publication May 6, 2010; date of current version May 11, 2011. This work was supported by the University Grants Commission-Departmental Research Support and University with Potential for Excellence (UGC-DRS and UPE). The review of this paper was arranged by Associate Editor D. Litvinov. The authors are with the School of Physics, Madurai Kamaraj University, Madurai 625021, India (e-mail: [email protected]; thirumalchandran@ gmail.com). Digital Object Identifier 10.1109/TNANO.2010.2049745

TiO2 , it is attributable to magnetic polaron formed by trapped electrons in oxygen vacancy and magnetic ions around it. The spintronic application requires that FM in semiconductor needs to be intrinsic and that should not be from the magnetic clusters of the doped-transition-metal impurity. Even in the absence of transition-metal doping, pure ZnO and TiO2 nanoparticles show FM above the room temperature [27]–[29] due to the oxygen deficiency. In this paper, undoped and Co-doped-anatase TiO2 nanoparticles have been synthesized by chemical route and characterized for structural, optical, and magnetic properties, with a specific focus on FM due to the oxygen deficiency in TiO2 nanoparticles. II. SYNTHESIS TiO2 nanoparticles were prepared by the reaction of titanium (IV) n-butoxide in an alcoholic (ethanol) medium [30]. First Ti(oBu)4 (0.1 M) was dissolved in alcoholic (ethanol) medium with constant magnetic stirring, then 0.05 M of nitric acid in deionized water was added drop wise to the solution. The mixed solution was stirred continuously at 65 ◦ C. TiO2 nanoparticles were obtained after the evaporation of solvents. Co doping (1, 3, and 5 at.%) was carried out by adding appropriate amount of cobalt (II) acetate tetrahydrate to the TiO2 stock solution. The Co concentration was monitored by energy dispersive X-ray analysis (EDAX) every time. The obtained nanoparticles were annealed in air for 2 h at 250 ◦ C. III. EXPERIMENT The structural analysis of undoped and Co-doped TiO2 samples were carried out by recording the X-ray diffraction (XRD) spectrum at room temperature using X-ray diffractometer (PANalytical X’Pert) and the spectrum was recorded in the 2θ range of 10◦ –90◦ with step size of 0.02◦ using Cu Kα radiation ˚ The surface morphology of the sam(wavelength: 1.54056 A). ples was investigated by scanning electron microscopy (Hitachi S-3400 N, Japan). The elemental compositions of the samples were carried out using EDAX spectrum (Nortan System Six, Thermo electron corporation Instrument Super DRY II, USA). In order to further check, if there exists Co clusters in the doped samples, X-ray photoelectron spectroscopy (XPS) experiments were carried out in a vacuum chamber. The samples were loaded into a PHOIBOS HSA3500 100 R4 MCD-5 XPS system. Al Kα at 1486.61 eV was used with the X-ray source operated at 11.65 kV, 194 W. An energy analyzer was operated at constant pass energy of 50 eV, which resulted in an energy resolution of

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Fig. 2.

SEM image of undoped TiO2 nanoparticles. TABLE I CHEMICAL COMPOSITIONS OF UNDOPED AND CO-DOPED TIO2 NANOPARTICLES (EDAX)

Fig. 1.

XRD Spectra of undoped and Co-doped TiO2 nanoparticles.

about 0.9 eV. The binding energy (BE) shifts due to the surface charging were corrected using C 1s level at 285 eV, as an internal standard. UV-Vis absorption measurements were carried out at room temperature, using Shimadzu UV-Vis absorption spectrometer. Raman modes were analyzed by using Renishaw InVia laser Raman microscope with He–Ne laser (633 nm). The magnetic hysteresis (M-H) loops were measured using vibratingsample magnetometer (Lakeshore 7400) between ±15 kOe at room temperature. IV. RESULTS AND DISCUSSION A. XRD Analysis Fig. 1 shows the XRD pattern for undoped and Co-doped TiO2 samples. The Bragg line at (3 1 0) with less intensity corresponds to rutile phase and all other diffraction peaks in undoped and Co- doped (1, 3, and 5 at.%)TiO2 samples can be indexed as TiO2 with anatase structure (JCPDS Card no: 54–1846 and 78– 2486). No diffraction peaks related to the metallic Co and Co in oxide forms were detected. This indicates that the anatase structure is not affected by Co doping. The calculated lattice ˚ parameters for undoped TiO2 nanoparticles are a = 3.795 A ˚ and c = 9.502 A, and the lattice constants for doped samples differ from undoped sample by a very small amount because of the difference in the ionic radius of host (Ti) and doped (Co) ions and similar results were also observed by Maensiri et al. [31]. The crystallinity of the doped samples decreases with increasing the doping percentage. Further, by looking at

the full-width at half-maximum of Bragg peaks, the average crystallite size of undoped and doped samples are calculated from Scherrer’s equation as around 5 nm. B. SEM with EDAX Analysis The morphology of the undoped TiO2 sample is shown in Fig. 2 and it reveals that the agglomeration of nanocrystals to form particles or grains, which are expected to contribute more grain boundary effects. The chemical compositions of undoped and doped samples are very essential to know the exact concentration of the dopant (Co here) and the defects. The EDAX spectrum of undoped TiO2 shows the presence of Ti and O elements alone in the sample, confirming the absence of any other impurity. The atomic percentage of Ti and O elements in undoped TiO2 sample is 54.65 and 45.35, respectively, but actual stoichiometric atomic percentage of Ti is 33.33 and O is 66.67, which shows oxygen deficiency in the undoped TiO2 sample. Likewise, all the doped samples show oxygen deficiency. Table I gives the atomic percentage of Ti, O, and Co. Due to the limitations of EDAX measurements, the percentage of oxygen vacancy is taken only as a guideline value and the exact percentage is found here by XPS. Even though when doping is done for a particular percentage, it should be known how much the dopant has actually entered into the lattice sites. Some of the less energetic dopant atoms may not substitute the host site. Here, the question is what will happen to the remaining percentage of dopant atoms that could not find place in the host lattice? This should be washed

RAJKUMAR AND RAMACHANDRAN: OXYGEN DEFICIENCY AND ROOM TEMPERATURE FERROMAGNETISM

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Fig. 3. UV-Vis absorption spectra of undoped and Co-doped TiO2 nanoparticles.

away from the system by some means, so that they will not form clusters to give unusual properties. This shows that, even though we have intended for 1, 3, and 5 at.% doping, only 0.88, 2.29, and 3.36 at.% of Co could enter into the TiO2 systems, i.e., this is the approximate concentration that enters into doped systems, as given in Table I. C. UV-Vis Analysis We measured the optical absorption of undoped and Co-doped TiO2 nanoparticles as a function of wavelength, as shown in Fig. 3. All the samples were highly transparent in the visible region. Fig. 3(Inset) shows the α-absorption plot to determine the optical bandgap of the undoped and Co-doped samples. The optical bandgap is found to be around 3.48 eV for undoped TiO2 sample and for Co-doped, it shifts very slightly towards lower energy side up to 3.46 eV from undoped TiO2 and this negligibly small shift is due to less crystallinity of the doped nanoparticles, which was confirmed by the XRD. The optical absorption edges remain almost same after doping with Co. Similar result was also observed by Park et al. [32] for sputtered Co-doped TiO2 rutile thin films. The optical absorption studies of spin-coated Co-doped TiO2 thin film by Suryanarayanan et al. [33] show the absorption edge remains same after doping with Co. The bandgap of all the samples were blue shifted by ∼0.28 eV from bulk TiO2 and with this observation of blue shift, the quantum confinement in the synthesized samples were tested and the calculated particle size from the Brus equation [34] is ∼10 nm. D. Raman Analysis Fig. 4 shows Raman spectra of undoped and Co-doped (3 at.%) samples. The characteristic Raman modes of strong anatase phase were observed at 399, 517, and 641 cm−1 [35] along with weak brookite and rutile modes [36], [37] for undoped TiO2 sample. The phases determined in XRD agree with the modes corresponding to crystalline TiO2 observed in Raman spectrum. But Raman modes observed in Co-doped

Fig. 4. Raman spectra of undoped and Co-doped TiO2 nanoparticles (A: anatase; B: brookite; and R: rutile).

(3 at.%) TiO2 were slightly broadened and shifted to higher frequency. The peak broadening derives from the deterioration of the lattice periodicity, which indicates the incorporation of Co in the TiO2 host lattice. Also, the peak shift to higher frequency suggests that the lattice was expanded by Co incorporation and Ti ions were substituted by heavier Co ions. The XRD analysis also indicated that the lattice constant slightly deviates on Co doping. From these results, it was suggested that the unique features of the phonon signals are predominantly due to the Co substitution in Ti sites and the relaxation of lattice strain by the defects caused by Co doping. Nakano et al. [38] also observed similar result for Co-doped TiO2 nanocrystals. In addition to quantum confinement of electronic energy levels, lattice vibrations, i.e., optical as well as acoustic phonons also confined in nanomaterials. Phonons can be Raman active and phonon from whole brillouin zone will contribute to Raman scattering. In general, a quantum-confined system means that the electronic energy levels are discrete and, hence, the confined electrons. But phonon confinement in nanomaterials is an interesting issue, when a nanoparticle is assumed as a collection of atoms, then the atomic vibrations will be confined as the particle size is decreased. So, phonon confinement is possible in nanomaterials. The theoretical model for phonon confinement had been proposed by Richter et al. [39] and Campbell et al. [40]. When phonons propagate in nanocrystalline materials, the fundamental q = 0 Raman selection rule is relaxed for a finite-size domain, allowing the participation of phonons away brillouin Zone. Raman intensity I(ω) is a superposition of weighted Lorentzian contributions over the first brillouin zone [23] I(ω) = BZ

|c(q0 = 0, q)|2 dq. (ω − ω0 (q))2 + (Γ/2)2

(1)

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With phonon confinement function within an isolated nanoscale sphere |c(0, q)|2 = e(−q

2

L 2 /4)

.

Hence, applying this concept to (1), we get 1 2 2 f (q)e(−q L /4) I(ω) = dq 2 2 0 (ω − ω0 (q)) + (Γ/2)

(2)

where f (q) = 1 for thin slab, while f (q) = 4πq 2 for an isolated sphere, q is magnitude of wave vector in units of π/a, where “a” is lattice constant, ω0 (q) denotes the phonon frequency at wave vector q, Γ is Raman line width, and L represents phononcorrelation length. Here, we use this concept to find the size of the particles. In order to evaluate the crystallite size, the dispersion curve of rutile TiO2 [41] and the present Raman spectra are used. The calculation was then performed for the crystallite size, namely, L and found as 5.82 nm for undoped TiO2 sample. E. XPS Analysis Whenever magnetic studies are planned for any system (DMS), then photoelectron spectroscopy measurements are very much essential to understand the valency of the magnetic ion. Even though EDAX could identify the concentration, XPS can give not only the valency of the magnetic ion but also the clustering. The XPS survey spectra of undoped and 3 at.% Co-doped TiO2 samples are shown in Fig. 5(a) and (b), respectively. The carbon peak is from surface contamination of samples due to the exposure in air. The magnified Ti 2p and O 1s regions of undoped and Co (3 at.%)-doped TiO2 samples are presented in Fig. 6(a) and (b), respectively. In Fig. 6(a), two peaks at 458.7 and 464.4 eV were identified as Ti 2p3/2 and Ti 2p1/2 [42], for both undoped and doped samples, respectively. For metallic Ti0 , these two peaks are expected at 455 and 459 eV [43]. The shifts in Ti 2p3/2 and Ti 2p1/2 peak positions and the change in the separation between these two peaks 5.7 eV, as well as the intensity ratio of these components 0.5, is caused by the presence of tetravalent Ti4+ , which is consistent with TiO2 formation [42]. No BE components associated with Ti2+ and Ti3+ was identified in our samples [44]. From Fig. 6(b), the BE value at 530.1 eV attributed to TiO4+ -O [44] and the higher BE component at 532.3 eV is attributed to chemisorbed oxygen [45]. The metallic Co 2p3/2 peak occurs at 778 eV [42] but in Fig. 7, no such peak was observed and the energy difference between Co 2p3/2 and Co 2p1/2 is 15.5 eV, which excludes the possibility of the formation of Co metal clusters, because the energy difference for Co metal clusters is 15.05 eV [46]. On the other hand, if Co is surrounded by oxygen, these differences should be 15.5 eV [42]. This is in agreement with the XRD result, which did not show the existence of any pure Co phase. The BE peak for Co 2p3/2 was observed at 781 eV, which corresponds to the Co(II) in CoO. The reported peak position for satellite peak in paramagnetic CoO is 786.3 eV [47] and the measured peak position in our sample is 786.3 eV. These confirm the presence of Co in Co (II) form. The peaks, due to the presence of Co2 O3 or mixed valent Co3 O4 reportedly occur at

Fig. 5. XPS survey spectra of (a) undoped and (b) 3 at.% Co-doped TiO2 nanoparticles.

779.9 and 779.3 eV [42], respectively, are not observed in our samples. So, it is concluded that Co is present in divalent form only in our samples. According to the results of XPS, the relative quantitative analysis of each element is completed using the XPS-peak-area data (after the peak fitting) of different elements and shown in Table II. The elemental composition obtained by XPS also suggests the oxygen deficiency in undoped and 3 at.% Co-doped TiO2 nanoparticles and given in the same Table II. Here, it is seen that 3 at.% Co doping would give the exact percentage as 2.08 at.% and since there is no other Ti peak, the remaining is oxygen deficiency. F. VSM Analysis The specific magnetization curve obtained from VSM measurement at room temperature is shown in Fig. 8 and the clear hysteresis shows that FM exists in all undoped and Co-doped TiO2 samples. The values of magnetic moment, coercivity, and retentivity for all the samples are tabulated in Table III. The magnetic moment of undoped TiO2 nanoparticles was calculated as 0.204 emu/g and the magnetic moment increases with


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TABLE II CHEMICAL COMPOSITIONS OF UNDOPED AND CO-DOPED TIO2 NANOPARTICLES (XPS)

Fig. 8.

VSM spectra of undoped and Co-doped TiO2 nanoparticles.

TABLE III VSM DATA FOR UNDOPED AND CO-DOPED TIO2 NANOPARTICLES

Fig. 6. Magnified XPS spectra of (a) Ti 2p region and (b) O 1s region for undoped, and 3 at.% Co-doped TiO2 nanoparticles.

Fig. 7. Magnified XPS spectrum of Co 2p region for 3 at.% Co-doped TiO2 nanoparticles.

increasing doping concentration. Here, it is interesting to note that the undoped TiO2 nanoparticles also show FM above room temperature with the coercivity of 135 G. Nguyen et al. [5]

reported similar result for TiO2 films deposited on LaAlO3 substrates and investigated various possible reasons for FM, finally concluded that the FM was due to the defects and/or oxygen vacancies. Coey et al. [48] found a strong anisotropy is the reason for FM. Here, it is necessary to compare the work by Sundaresan et al. [29] who observed FM for nanoparticles (7– 30 nm) of nonmagnetic oxides also such as CeO2 , Al2 O3 , ZnO, In2 O3 , and SnO2 . This shows that the origin of FM may be due to the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of the nanoparticles. In this study, the observed FM for undoped TiO2 nanoparticles is due to oxygen vacancies, because EDAX and XPS analyses show the deficiency of oxygen in the synthesized samples. The oxygen vacancies at the surface of the nanoparticles introduce exchange interactions between localized electronspin moments and these exchange interactions induce FM in undoped TiO2 . Table III shows that the magnetic moment of the Co-doped TiO2 nanoparticles is higher than the undoped TiO2 nanoparticle and also the magnetic moment slightly increases with increasing Co-doping concentration. Park et al. [32] show

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similar results that the magnetic moment increases with increase of Co concentration above 3 at.% for sputtered Co-doped TiO2 thin films. The magnetism is mainly due to Co doping and not because of defects or oxygen vacancies. However, in this study, oxygen vacancies play an important role in magnetism in doped samples. The enhancement of magnetic moment in the doped samples is due to the Co doping. From the XPS and XRD analyses, the absence of metallic Co or Co-related magnetic oxides was confirmed, hence, the enhancement in magnetic moment for doped samples is not because of magnetic Co clusters. This enhancement in the magnetic moment is, maybe, due to the exchange interaction between the sp-bond electrons or holes and the d-electron spins localized at the magnetic ions [49], super exchange between complexes (oxygen vacancies + magnetic impurities), which are stabilized by the electron transfer from vacancies to impurities [26] and magnetic polaron formed by the trapped electrons in oxygen vacancy and magnetic ions around it [27]. So, it is concluded that the oxygen vacancies play an important role for FM ordering but the doping enhances the ferromagnetic ordering. The observed magnetism is purely intrinsic property in undoped and Co-doped TiO2 nanoparticles. We can control the FM in nonmagnetic oxides by controlling the defects or vacancies so that the magnetism will depend on the preparation conditions alone and not other defects.

V. CONCLUSION The undoped and Co -doped (1, 3, and 5 at.%) TiO2 nanoparticles of 5–10 nm were synthesized by chemical route. Structural analyses showed that the synthesized samples were in anatase phase with slight deviation in lattice parameters. These deviations were due to the Co doping, because doping affects the crystallinity of the samples due to the difference in ionic radius of host (Ti) and dopant (Co) ions. The Raman peaks were slightly broadened and shifted to the higher frequency in Co-doped sample from undoped TiO2 sample and confirms the XRD result. The bandgap of undoped TiO2 nanoparticles was blue shifted by 0.28 eV from bulk and the particle size was calculated as 10 nm, hence compared with XRD particles size. Also, the bandgap of doped samples showed negligible red shift from undoped TiO2 nanoparticles, this infers that doping could not affect the samples. EDAX and XPS analyses indicate the presence of oxygen vacancies in all the samples. The XPS result for doped samples confirms that there is no trace of metallic Co and it shows the presence of Co in divalent state. Room temperature FM was observed in all doped and undoped samples and its magnetic moment gets enhanced with increase of doping concentration. Thus, the oxygen vacancies played an important role for FM and the observed enhancement in magnetic moment is due to the Co doping.

ACKNOWLEDGMENT The authors would like to thank Dr. Barman, Scientist, UGCDAE Consortium for Scientific Research, Indore, for the XPS measurements.

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N. Rajkumar is a research student working under Dr. K. Ramachandran, Senior Professor and Head in the Department of Theoretical Physics, School of Physics, Madurai Kamaraj University, Madurai, India. His research interests include diluted magnetic nano semiconductors.

K. Ramachandran received the Ph.D. degree in lattice dynamical studies in semiconductors from Madurai Kamaraj University, Madurai, India. He is a Senior Professor and the Head in the Department of Theoretical Physics, School of Physics, Madurai Kamaraj University. His research interests include nanoscience, simulation, and lattice dynamics. He has published hundreds of research works in international journals such as the Journal of Crystal Growth, the Journal of Luminescence, etc.