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Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle
Current Applied Physics 13 (2013) 1025e1031
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Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle Biswajit Choudhury, Amarjyoti Choudhury* Department of Physics, Tezpur University, Napaam 784028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2012 Received in revised form 11 February 2013 Accepted 12 February 2013 Available online 26 February 2013
Mn doped TiO2 nanoparticles are synthesized by solegel method. Incorporation of Mn shifts the diffraction peak of TiO2 to lower angle. The position and width of the Raman peak and photoluminescence intensity of the doped nanoparticles varies with oxygen vacancy and Mn doping level. The electron spin resonance spectra of the Mn doped TiO2 show peaks at g ¼ 1.99 and 4.39, characteristic of Mn2þ state. Reduction in the emission intensity, on Mn doping, is owing to the increase of nonradiative oxygen vacancy centers. Mn doped TiO2, with 2% Mn, shows ferromagnetic ordering at low applied field. Paramagnetic contribution increases as Mn loading increases to 4% and 6%. Temperature dependent magnetic measurement shows a small kink in the ZFC curve at about 40 K, characteristic of Mn3O4. The ferromagnetic ordering is possibly due to the interaction of the neighboring Mn2þ ions via oxygen vacancy (Fþ center). Increase in Mn concentration increases the fraction of Mn3O4 phase and thereby increases the paramagnetic ordering. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Ferromagnetism Oxygen defects F center Paramagnetism Exchange interaction
1. Introduction Diluted magnetic semiconductors (DMS) are class of materials where the materials observe ferromagnetism while preserving its semiconducting behavior. These classes of materials have found tremendous possibility in spintronic devices where both charge and spin degrees of freedom can be manipulated [1,2]. Initial trials to observe ferromagnetism was carried out in IIeVI and IIIeV semiconducting materials, such as Mn doped CdTe, ZnSe and Mn doped GaAs, etc [3,4]. Although Ohno achieved ferromagnetism in Mn doped GaAs, the Curie temperature (Tc) was very less (Tc w 110 K) [4]. Initial attempt on oxide based DMS was started by Matsumoto et al. in Co doped TiO2 thin film, prepared by combinatorial pulsed-laserdeposition (PLD) molecular-beam epitaxy (MBE), and obtained a Tc well above room temperature [5]. Although several reports have appeared on magnetism in oxide based diluted semiconductors, the results diverge from each other and therefore the intrinsic nature of magnetism have been questioned by the scientific community [6e11]. The search for room temperature ferromagnetism is carried out in Mn doped TiO2 thin film, nanoparticles, etc. Wang et al. [12] examined that coupling of Mn ions through large concentration of holes gave rise to the room temperature ferromagnetism in Mn doped TiO2 film, prepared by pulsed laser deposition (PLD). On the other hand, Hong
et al. [13] observed absence of magnetism in Mn doped TiO2 film prepared by PLD technique. Mie et al. [14] observed coexistence of ferromagnetism and antiferromagnetism in hydrothermally prepared Mn doped TiO2 nanoparticles. Bhattacharya et al. [15], in Mn doped TiO2 nanocrystals, observed ferromagnetism at low doping concentration and paramagnetism at higher Mn concentration. Kim et al. [16] reported the necessity of oxygen vacancies in Mn doped rutile TiO2 thin films. Li et al. [17] reported that oxygen vacancy alone could not induce ferromagnetism in TiO2 thin films and an optimum level of Mn would be necessary for observing ferromagnetism. In this report we have studied the role of oxygen vacancies and Mn concentration in regulating magnetic ordering in Mn doped TiO2 nanoparticles. The structural and morphological characterizations are done with X-ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy, etc. The valence state of Mn in host TiO2 matrix is analyzed with electron spin resonance (ESR) spectroscopy. We have observed that at low doping concentration the materials attain ferromagnetism with less paramagnetic contribution. But, as dopant concentration increases the paramagnetic contribution has dominance over ferromagnetism. 2. Experimental details 2.1. Preparation of Mn doped TiO2 nanoparticles
* Corresponding author. Tel.: þ91 3712 267120; fax: þ91 3712 267006. E-mail address: [email protected] (A. Choudhury). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.02.007
TiO2 nanoparticles doped with three different concentration of Mn, 2%, 4% and 6%, were prepared by solegel method. The precursors
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with intense CuKa (l ¼ 0.154 nm) radiation. The scanning step was 0.05 and the scanning speed was 1 /min. Transmission electron microscope images (TEM) were obtained with JEOL-JSM 3010 transmission electron microscope operating at a voltage of 300 K. Energy dispersive X-ray analysis (EDX) and elemental mapping were taken on FEI Nova Nano SEM 600. Raman spectroscopy was carried at room temperature with a Labram (Horiba Jobin-Yvon) high resolution Raman spectrometer at spectral resolution of 0.5 cm1. Electron spin resonance spectra (ESR) of the doped samples were carried out with a JEOL-JES-FA200 electron spin resonance spectrometer with an applied microwave frequency of 9.09 GHz. Magnetic measurements were carried out with a Lakeshore-7410 vibrating sample magnetometer (VSM). 3. Results and discussion Fig. 1. X-ray diffraction of pure and Mn doped TiO2 nanoparticles with Mn concentration of 2%, 4% and 6% respectively. Inset of the figure shows the enlarged view of the (101) peak displaying shifting of the peak to lower angle with Mn loading.
for dopant and host were manganese acetate tetra hydrate and titanium isopropoxide respectively. 5 ml of titanium isopropoxide and 15 ml of 2-propanol were added to a 100 ml conical flask. The reaction mixture was stirred for 15 min followed by addition of 1 ml of water to initiate the hydrolysis of the isopropoxide chain. This was followed by the addition of an appropriate amount of manganese acetate solution to the host solution and stirred for about 6 h. During the stirring process, the solution first became sol and at the end of the reaction it was converted to a gel. The gel was first sonicated and then centrifuged in water followed by ethanol for 3 times. The product was then dried in an oven at 80 C. The resulting amorphous Mn doped TiO2 was again annealed at 450 C for 3 h resulting in crystalline Mn doped anatase TiO2 nanoparticles. 2.2. Characterization details The crystalline phase of pure and doped TiO2 nanoparticles were studied with Rigaku Miniflex X-ray diffractometer (XRD) equipped
The X-ray diffraction (XRD) spectra of pure and Mn doped TiO2 nanoparticles correspond to the tetragonal anatase phase of TiO2 (JCPDS-782486). Out of the anatase, brookite and rutile phases of TiO2, anatase and brookite are the metastable phase while rutile is the stable phase. Solution phase synthesis and low temperature annealing preferably results in nanocrystalline anatase phase. On the other hand high temperature annealing (above 600 C) results in bulk rutile phase of TiO2. This metastable anatase phase of TiO2 contains many structural defects on the surface, grain boundary, etc. Although, we have not observed any distinguished peaks of Mn or its oxides, the presence of these phases cannot be completely ruled out. Since the added dopant concentration is very less, the fraction of impurity phases are probably too less to be detected by XRD. An enlarged view of the (101) peak shows shifting of the diffraction peak to lower angle with Mn concentration. Mn priA), Mn3þ (0.58 A) and Mn4þ (0.53 A) marily exists as Mn2þ (0.82 2þ respectively and the ionic radii of Mn is larger than that of Ti4þ (0.61 A) [18]. The shifting of the peak to lower angle indicates substitution of larger Mn2þ on the Ti4þ site. (Fig. 1) Fig. 2a and b shows the transmission electron microscope images of pure and 6% Mn doped TiO2 nanoparticles. The nanoparticles of pure TiO2 have spherical shape with agglomeration.
Fig. 2. Transmission electron microscope images of (a) pure and (b) 6% Mn doped TiO2 nanoparticles. (c) EDX pattern with the composition of the elements in the inset of the figure. Elemental mapping of (d) Ti (e) O and (f) Mn taken from the area of the SEM shown in the inset of Fig. 2d.
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Although spherical in shape, agglomeration persists in the doped TiO2 nanoparticles. The average particle size of pure and doped TiO2 nanoparticles is 8 nm and 6 nm respectively. Fig. 2c depicts the EDX pattern of 6% Mn doped TiO2 nanoparticles. The elemental composition is shown in the inset of Fig. 2c. Fig. 2def exhibits the elemental mapping of Ti, Mn and O in the 6% Mn doped TiO2 nanoparticle. The EDX and mapping is carried out on the selected area of the sample shown in the inset of Fig. 2c. From the elemental composition it is found that O:Ti ratio is 3.75 which is higher than usual 2:1 ratio and also Ti/(Ti þ Mn) ratio is 1.5% which is again smaller than added 6%. EDX is mostly a surface characterizing technique and it can detect elements present upto few nm layer from surface and not of entire volume of the nanoparticle. The surface of TiO2 contains many active sites, such as Ti3þ and oxygen vacancies. Therefore, the atmospheric oxygen molecule, eOH moiety present in moisture can easily adsorb on these sites. Since, the initially prepared TiO2 is annealed in air to get crystalline anatase, the possibility of the adsorption of these O2 and eOH is very much feasible. Therefore, the value of 3.75 for O:Ti is not the same as that of 2:1 stoichiometric ratio of TiO2, since the surface of TiO2 contains entire oxygen that are adsorbed on the surface. Similarly, Mn concentration is 1.54%, which is less than nominal 6%. This is not the Mn concentration present in the entire volume of the nanoparticles. EDX cannot detect the composition of the elements in the core and since few regions of sample area are selected for EDX, the composition is not same everywhere and the dopants (present on surface) are in-homogeneously distributed on the surface. Apart from this, during doping the dopants are adsorbed on the surface of the nanoparticle and then it penetrate inside while annealing. Since, we have centrifuged the doped sample, loosely bound Mn2þ ions may have been detached from this. Therefore, some Mn ions might have been lost from surface. These may be reasons why the composition shown by EDX is different than that of stoichiometric of TiO2 or from added Mn concentration. The doping effect of Mn, on the TiO2 lattice, is further studied with Raman spectroscopy. Fig. 3 shows the room temperature Raman spectra of pure and doped TiO2 nanoparticles. Anatase TiO2 has six Raman active modes and three infra-red (IR) active modes [19e21]. The representation of optical phonon modes at the center of Brilliouin zone is given by:
Fig. 3. Raman spectra of pure and entire Mn doped TiO2 nanoparticles. The enlarged view of the intense Eg peak is shown in the inset of the figure.
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Gopt ¼ A1g ðRÞ þ 2B1g ðRÞ þ 3Eg ðRÞ þ B2u ðIRÞ þ A2u ðIRÞ þ 2Eu ðIRÞ:
(1)
In pure TiO2 the first intense Raman peak appears at 145 cm1 corresponding to Eg mode. The other Eg peak appears as two low intense peaks at 197 and 640 cm1 respectively. The B1g peak and the (A1g þ B1g) peak appear at 397 cm1 and 516 cm1 respectively [20,21]. The most intense Eg peak at 145 cm1 corresponds to Oe TieO type of bending vibration. This mode is very much sensitive to local oxygen coordination surrounding the metal ion [22]. The
Fig. 4. Room temperature ESR spectra of 2%, 4% and 6% Mn doped TiO2 nanoparticles.
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expanded view of this mode is shown in the inset of Fig. 3. The inset figure demonstrates that the Raman Eg peak of TiO2 becomes asymmetric and blue shifted as TiO2 is incorporated with 2%, 4% and 6% of Mn. This kind of changes in the Raman peak occurs owing to the disruption of the local structure and due to the generation of defects on the lattice site of TiO2 on Mn doping. The Raman peaks are very sensitive to any disorder occurring in the oxygen coordination in the TiO2 lattice. Substitution of Ti4þ ions by Mn2þ distort the TiO2 lattice and generate oxygen vacancies to maintain charge neutrality. Formation of oxygen vacancies, on substitution, decreases the average number of oxygen ions to bind with Ti4þ or Mn2þ ions. Decrease in the average number of TieO/MneO bonds will reduce the average number of bonds (due to entire TieO or MneO bonds) in TiO2 and thereby contract the lattice [19]. The strength of a bond is determined by its force constant and the force constant of a bond is associated with frequency by yNOk [19]. Therefore, decrease in the number of TieO/MneO bonds will contract the lattice and will shift the Eg position to higher wavenumber. Therefore, oxygen vacancies are mainly responsible for the blue shifting and widening of the Raman peak [23e25]. Fig. 4aec illustrates the room temperature ESR spectra of Mn doped TiO2 nanoparticles. The effective g-factor is calculated using the equation [26]:
geff ¼
hn ; mB H
(2)
where h, v, mB and H are the Plank constant, frequency of applied microwave field, Bohr magneton and applied magnetic field, respectively. The ESR curve contains two absorption peaks. The intense peak has a g-value of 1.99 and the second low intense peak has g at 4.39. These spectral positions of Mn appear due to the interaction of Mn (II) with the surrounding O2 ligands in octahedral symmetry [27e31]. In case of strong interaction of Mn (II) with the surrounding ligands, the expected signals comprise of a sextet at g ¼ 1.99 [27]. In our case, we have not observed any sextet lines instead an unresolved broad line appears at g ¼ 1.99 with a weak shoulder at g ¼ 4.39. This may be due to weak interaction of Mn2þ with the host lattice or due to clustering of the Mn2þ ions on the grain boundary [28e30]. Photoluminescence (PL) spectroscopy is an important characterizing tool to understand the presence of defects in a material. Fig. 5 depicts the room temperature photoluminescence spectra of pure and Mn doped TiO2 nanoparticles at an excitation wavelength
of 320 nm. We have fitted the emission peaks with Gaussian and observed that five different emission peaks are present in the spectra. The UV emission is the indirect band to band transition from conduction band to the valence band of TiO2 nanoparticles [32]. Although not shown here, the position of the different emission peaks are estimated by fitting the peaks with Gaussian (r2 ¼ 0.9956). The defect emission band spans from 420 to 540 nm. The 424 nm peak is associated with self trapped excitons (STE) [33]. The 491 nm peak is due to the charge transfer transition from Ti3þ to the TiO2octahedra associated with oxygen defects [34]. The 6 emission bands at 535 nm and 460 nm are associated with electrons bound (or trapped) to oxygen vacancy centers [35]. Depending on the number of trapped electrons, these oxygen vacancy centers are referred as F (two trapped electrons), Fþ (one electron) and Fþþ (two electrons) type color centers [36]. These color centers may act as luminescence enhancer or quencher. As it is seen in the spectra, there is neither any Mn emission peak nor there is any change in the position of the defect emission peaks. It is the intensity of the emission peaks that is quenched on Mn doping. In pure TiO2, the oxygen vacancies act as luminescence enhancer and increase the emission intensity. Doping of Mn disturbs the TiO2 lattice, breaks the TieO bond and generates many oxygen vacancies. With the increase of Mn doping, the number of nonradiative oxygen vacancy centers also increases, nearby Mn2þ. Since, the oxygen defect concentration increases, most of the photoexcited electrons are trapped and strongly localized in those oxygen vacancies, decreasing the availability for recombination with the holes. Since the nonradiative oxygen vacancy centers, with trapped electrons, increases with Mn, the emission intensity also decreases subsequently. Apart from the nonradiative oxygen vacancies, the other factor affecting PL intensity is the mobility of the carriers. The presence of dopants and defects on the interior, grain boundary and on the surface reduces the mobility of the free carriers. The mobile carriers are scattered when they approach charged dopants or oxygen defect (F, Fþ) states. Decrease in the mobility will increase the separation of carriers and hence reduces the PL intensity [37]. Room temperature magnetic measurements (MeH) of entire samples are carried out in the field range of 20 kOe. Fig. 6aed shows the MeH curves of pure and Mn doped TiO2 nanoparticles. Although pure TiO2 nanoparticles are showing diamagnetism, incorporation of Mn into TiO2 shows magnetism. Although total magnetization increases with dopant concentration, the actual saturation magnetization possibly occurs at low magnetic field. The linear magnetization
Fig. 5. Photoluminescence spectra of pure and Mn doped TiO2 nanoparticles excited at 320 nm.
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Fig. 6. Room temperature MeH curve of (a) pure (b) 2% (c) 4% and (d) 6% Mn doped TiO2 nanoparticles (without subtracting high field linear part).
at the high magnetic field may be due to the presence of paramagnetic or some other magnetic phases which do not contribute to ferromagnetic ordering. Since the slope of the linear part increases with Mn concentration, we can attribute this variation to be dependent on Mn concentration. Therefore, the total magnetization of the sample is the sum total of a ferromagnetic (in the low field) and a paramagnetic (at the high field) part. The possible interaction/impurity phases that give rise to the paramagnetism or antiferromagnetism are Mn2þeMn2þ interaction, formation of antiferromagnetic manganese oxide phases [14,18,38,39]. The ferromagnetism may be due to the substitution of Mn into TiO2 lattice or due to the presence of secondary oxide phases of Mn [18]. Of all the oxides of Mn, only Mn3O4 is ferrimagnetic with a Curie temperature (Tc) of 42 K [18,38,39]. Since the Tc is too low, the observed room temperature ferromagnetic behavior in the samples cannot be entirely attributed to the Mn3O4 phase. For further understanding of the magnetic nature of the doped materials, temperature dependent magnetic (MeT) measurement is carried out. The MeT measurement is carried out at an applied field of 500 Oe and in the field range from 15 to 300 K. Fig. 7a and b shows the zero field cooling (ZFC) and field cooling (FC) MeT curves of 2% and 6% Mn doped TiO2 nanoparticles. It is seen that 2% Mn doped TiO2 exhibits a deviation in the ZFC and FC curve at 120 K with a small kink at 40 K in the ZFC curve. For 6% Mn doped TiO2, the ZFC and FC curve overlaps up to 50 K and then bifurcation occurs. The temperature at which the bifurcation between the ZFC and FC curve occurs is the thermomagnetic irreversible temperature (Tirr) [18]. Tirr depends on magnetocrystalline anisotropy, applied magnetic field, coercivity, etc. The peak appearing at w39e40 K in the two samples correspond to the Curie temperature of Mn3O4. This peak appears as a small kink in 2% Mn and becomes intense at 6% Mn doping, indicating increase in the fraction of Mn3O4 phase with the increase in Mn concentration. Another observation in the FC curve is the increase in the magnetization in the low temperature regime (below 39 K). The steady increase in magnetization on lowering the temperature indicates the presence of frozen isolated Mn2þ ions in
the host oxide matrix that contribute to paramagnetic ordering [18,40]. Antiferromagnetic oxide phases of Mn such as MnO, Mn2O3, MnO2 have Neel temperature, TN < 100 K [38,41]. Since we have not observed any characteristic peak corresponding to this TN in the MeT curves, we can neglect the presence of antiferromagnetic interaction due to these oxide phases. Therefore, the increase in the linearity of the magnetization, with the dopant concentration, at the high field region is possibly due to the Mn3O4 phase [18]. Besides, magnetic contribution coming from different magnetic phases, during magnetic measurement, may apparently contribute to this paramagnetic linearity at high field. Two theories are usually employed to discuss ferromagnetism in oxide based DMS, RudermaneKitteleKasuyaeYosida (RKKY) interaction and bound magnetic polaron (BMP) theory [42e45]. RKKY theory is applied in metallic system where electrons are delocalized and bound magnetic polaron theory is applied in case of insulating system where electrons are strongly localized [42e 45]. From PL analysis we have understood that the carriers are strongly localized and doping reduces mobility of the carriers. Since the mobility decreases on doping we can expect that the doped samples have high resistivity. Although we have proposed that Mn doped samples will have high resistivity, Sharma et al. [18], in fact, obtained high resistivity in Mn doped TiO2 films having resistivity (r) w107 U-m. Therefore, taking into consideration of the wide band gap of anatase TiO2 (3.2 eV), strong localization of carriers, and high resistivity (expected) of the doped samples, we can suggest that an F-center mediated bound magnetic polaron theory (FCBMP) or BMP will be useful to explain magnetism in these samples. This is only suggestive, we do not rule out any possible mechanism that can explain the magnetic origin. An electron trapped in an oxygen vacancy (F center) couple with the magnetic spins of the nearest Mn2þ ions within the radius of the hydrogen like orbit of the F center and forms a BMP. When Mn loading is 2%, most of the Mn2þ ions occupy substitutional position and a few Mn2þ ions are oxidized to Mn3O4. Mn2þ ions, on substitution, create
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vacancies (as inferred from the appearance of visible emission peaks), those oxygen vacancies alone cannot to contribute to ferromagnetism. Increase of Mn increases the number of oxygen vacancies and therefore an exchange interaction of the Mn ions via oxygen vacancies may induce ferromagnetism. Although oxygen vacancies can be expected to be increased at 4% and 6%, the interaction of Mn2þ via oxygen vacancies is less pronounced as most of the Mn2þ ions form oxide phases. Therefore, we can suggest that both oxygen vacancies and an optimum level of Mn (with entire Mn2þ taking part in BMP) are necessary for getting ferromagnetism in Mn doped TiO2 nanoparticles. Very high concentration of Mn does not contribute to ferromagnetism, instead forms secondary oxide phases and contribute to paramagnetism. 4. Conclusion In summary we have suggested that for getting ferromagnetic ordering both oxygen vacancies and an optimum level of Mn (on the TiO2 lattice) are necessary. Pure TiO2 nanoparticles do not exhibit any magnetic ordering. However, incorporation of Mn results in the ferromagnetic ordering at low applied file and paramagnetism at high magnetic field. We have proposed that Mn2þ ions undergo exchange interaction via oxygen vacancies and form bound magnetic polaron to induce ferromagnetism in TiO2. At low doping concentration Mn2þ ions occupy lattice site, interact with oxygen vacancies and increases ferromagnetic ordering. However, at high dopant concentration some of the Mn2þ forms Mn3O4 phase and thereby increases paramagnetic contribution. References
Fig. 7. Zero field cooling (ZFC) and field cooling (FC) MeT curves of (a) 2% and (b) 6% Mn doped TiO2 nanoparticles.
oxygen vacancies nearby Mn2þ in the lattice. The radius of the hydrogenic orbit of the F-center is given by rH ¼ εr ðm=m* Þa0, where 3 r is dielectric constant, m is electron mass and m* is effective mass of electrons and a0 is Bohr radius [42e45]. Those Mn2þ ions that fall within this F-center radius interacts with the spin of Fcenter, forms BMP and imparts ferromagnetism. The low value of magnetization indicates participation of less number of Mn2þ ions in the formation of BMP. However, as we increase the Mn concentration to 4% and 6%, the number of Mn atoms in the interior of TiO2 is comparatively less than that on its surface and on the grain boundary. In case of 4% and 6% Mn doping, only those Mn atoms are allowed to enter the lattice which are permissible by the host, the rest are expelled and are oxidized to Mn3O4 (during air annealing). Although 2% Mn doped TiO2 contains Mn3O4, the fraction of this oxide phase is less in the outside. However, in case of 4% and 6% of Mn, the fraction of Mn3O4 increases and thereby increasing paramagnetic contribution. Thus, we can suggest that 2% is the solubility limit of Mn inside TiO2. Sharma et al. [18] also observed that incorporation of high concentration of Mn forms Mn3O4 phase and increases paramagnetic ordering. Hong et al. [13] demonstrated that Mn doping just enhances the magnetism of a TiO2 film which is already ferromagnetic, it itself does not increase the magnetization. They observed that doping with high concentration of Mn destroys the ferromagnetism. On the other hand, Li et al. [17] observed that simple oxygen vacancy cannot impart magnetism and an optimum level of Mn is necessary for ferromagnetism. In our analysis, we have observed that although pure TiO2 nanoparticles contains oxygen
[1] I. Zuti c, J. Fabian, S.D. Sarma, Spintronics: fundamentals and applications, Rev. Mod. Phys. 76 (2004) 323e410. [2] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S.V. Molnár, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (1998) 1488e1495. [3] J.K. Furdyna, J. Kossut, Diluted Magnetic Semiconductors e Semiconductor and Semimetals, vol. 25, Academic Press, New York, 1988. [4] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, (Ga, Mn) As: a new diluted magnetic semiconductor based on GaAs, Appl. Phys. Lett. 69 (1996) 363e365. [5] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 291 (2001) 854e856. [6] S.R. Shinde, S.B. Ogale, J.S. Higgins, H. Zheng, A.J. Millis, V.N. Kulkarni, R. Ramesh, R.L. Greene, T. Venkatesan, Co-occurrence of superparamagnetism and anomalous Hall effect in highly reduced cobalt-doped rutile TiO2dfilms, Phys. Rev. Lett. 92 (2004) 166601. [7] G. Glaspell, A.B. Panda, M.S. El-Shall, Reversible paramagnetism to ferromagnetism in transition metal-doped TiO2nanocrystals prepared by microwave irradiation, J. Appl. Phys. 100 (2006) 124307. [8] C.E. Rodríguez-Torres, S.J. Stewart, C. Adán, A.F. Cabrera, Paramagnetism and clustering in Fe-doped TiO2 nanoparticles, J. Alloys Compd. 495 (2010) 485e487. [9] B. Choudhury, A. Choudhury, P. Alagarsamy, A. Islam, M. Mukherjee, Effect of oxygen vacancy and dopant concentration on the magnetic properties of high spin Co2þ doped TiO2 nanoparticles, J. Magn. Magn. Mater. 323 (2011) 440e446. [10] T. Dietl, Origin of ferromagnetic response in diluted magnetic semiconductors and oxides, J. Phys. Condens. Matter 19 (2007) 165204. [11] R. Janisch, P. Gopal, N.A. Spaldin, Transition metal-doped TiO2and ZnOdpresent status of the field, J. Phys. Condens. Matter 17 (2005) R657eR689. [12] Z. Wang, J. Tang, Y. Chen, L. Spinu, W. Zhou, L.D. Tung, Room-temperature ferromagnetism in manganese doped reduced rutile titanium dioxide thin films, J. Appl. Phys. 95 (2004) 7384. [13] N.H. Hong, J. Sakai, A. Ruyter, V. Brize, Does Mn doping play any role in magnetism in Mn doped TiO2 thin film? Appl. Phys. Lett. 89 (2006) 252504. [14] L.H. Mie, L. Min, Z.Y. Su, H.T. Cheng, Coexistence of antiferromagnetic and ferromagnetic in Mn doped TiO2 nanowires, J. Cent. South. Univ. Technol. 17 (2010) 239e243. [15] S. Bhattacharyya, A. Pucci, D. Zitoun, A. Gedanken, One-pot fabrication and magnetic studies of Mn-doped TiO2 nanocrystals with an encapsulating carbon layer, Nanotechnology 19 (2008) 495711. [16] K.J. Kim, Y.R. Park, J.H. Lee, S.L. Choia, H.J. Lee, C.S. Kim, J.Y. park, Roomtemperature ferromagnetic properties in Mn-doped rutile thin films, J. Magn. Magn. Mater. 316 (2007) e215ee218.
B. Choudhury, A. Choudhury / Current Applied Physics 13 (2013) 1025e1031 [17] X.Y. Li, S.X. Wu, L.M. Xu, C.T. Li, Y.J. Liu, X.J. Xing, S.W. Li, Effects of depositing rate on structure and magnetic properties of Mn:TiO2 films grown by plasmaassisted molecular beam epitaxy, Mater. Sci. Eng. B 156 (2009) 90e93. [18] S. Sharma, S. Chaudhary, S.C. Kashyap, V.K. Malik, DC magnetization investigations in Ti1xMnxO2 nanocrystalline powder, J. Alloys Compd. 509 (2011) 7434e7438. [19] M.G. Broj cin, M.J. S cepanovi c, Z.D. Doh cevi c-Mitrovi c, I. Hini c, B. Matovi c, G. Stanisi c, Z.V. Popovi c, Infrared study of laser synthesized anatase TiO2 nanopowders, J. Phys. D Appl. Phys. 38 (2005) 1415e1420. [20] B. Choudhury, A. Choudhury, Dopant induced changes in structural and optical properties of Cr doped TiO2 nanoparticles, Mater. Chem. Phys. 132 (2012) 1112e1118. [21] J. Zhang, M. Li, Z. Feng, J. Chen, C. Li, UV Raman spectroscopic study on TiO2. I. Phase transformation at the surface and in the bulk, J. Phys. Chem. B 110 (2006) 927e935. [22] H.Y. Lin, Y.Y. Chou, C.L. Cheng, Y.F. Chen, Giant enhancement of band edge emission based on ZnO/TiO2 nanocomposites, Opt. Express 15 (2007) 13832e 13837. [23] J.C. Parker, R.W. Siegel, Calibration of the Raman spectrum to the oxygen stoichiometry of nanophase TiO2, Appl. Phys. Lett. 57 (1990) 943e945. [24] J.G. Li, X. Wang, K. Watanabe, T. Ishigaki, Phase structure and luminescence properties of Eu3þ-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists, J. Phys. Chem. B 110 (2006) 1121e1127. [25] Q. Wu, Q. Zheng, R. Krol, Creating oxygen vacancies as a novel strategy to form tetrahedrally coordinated Ti4þ in Fe/TiO2 nanoparticles, J. Phys. Chem. C 116 (2012) 7219e7226. [26] J.A. Weil, J.R. Bolton, Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, second ed., John Wiley & Sons, Inc., New Jersey, 2007. [27] D. Dvoranová, V. Brezová, M. Mazúra, M.A. Malati, Investigations of metaldoped titanium dioxide photocatalysts, Appl. Catal. B 37 (2002) 91e105. [28] R. Arroyo, G. Cordoba, J. Padilla, V.H. Lara, Influence of manganese ions on the anataseerutile phase transition of TiO2 prepared by the solegel process, Mater. Lett. 54 (2002) 397e402. [29] K.C. Heo, C.I. Ok, J.W. Kim, Influence of manganese ions on the anatase-rutile phase transition of TiO2 prepared by solegel process, J. Korean Phys. Soc. 47 (2005) 861e865. [30] R.V. Reinaa, S. Chaob, V. Petrovskyb, F. Doganb, S. Greenbaum, Electrical and electron paramagnetic resonance spectroscopy characteristics of Mn doped nanostructured TiO2 for capacitor applications, J. Power Sources 210 (2012) 21e25. [31] A. Venkatasubbaiah, J.L. Rao, S.V.J. Lakshman, B. Sreedhar, Electron paramagnetic resonance and optical absorption spectra of manganese (II) ions in lead acetate glasses, Polyhedron 12 (1993) 1539e1543.
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[32] N. Serpone, D. Lawless, R. Khairutdinov, Size effects on the photophysical properties of colloidal anatase TiO2 particles: size quantization versus direct transitions in this indirect semiconductor? J. Phys. Chem. 99 (1995) 16646e 16654. [33] J. Preclikova, P. Galar, F. Trojanek, S. Danis, B. Rezek, I. Gregora, Y. Nemcova, P. Maly, Nanocrystalline titanium dioxide films: influence of ambient conditions on surface- and volume-related photoluminescence, J. Appl. Phys. 108 (2010) 113502. [34] J. Liu, J. Li, A. Sedhain, J. Lin, H. Jiang, Structure and photoluminescence study of TiO2 nanoneedle texture along vertically aligned carbon nanofiber arrays, J. Phys. Chem. C 112 (2008) 17127e17132. [35] Y. Lei, L. Zhang, G. Meng, G. Li, X. Zhang, C. Liang, W. Chen, S. Wang, Preparation and photoluminescence of highly ordered TiO2 nanowire arrays, Appl. Phys. Lett. 78 (2001) 1125e1127. [36] N. Serpone, Is the band gap of pristine TiO2 narrowed by anion- and cationdoping of titanium dioxide in second-generation photocatalysts? J. Phys. Chem. B 110 (2006) 24287e24293. [37] B. Choudhury, A. Choudhury, Tailoring luminescence properties of TiO2 nanoparticles by Mn doping, J. Lumin. 136 (2013) 339e346. [38] Z.M. Tian, S.L. Yuan, Y.Q. Wang, J.H. He, S.Y. Yin, K.L. Liu, S.J. Yuan, L. Liu, Magnetic properties in Mn doped TiO2 bulk samples, J. Phys. D Appl. Phys. 41 (2008) 055006. [39] S. Sharma, S. Chaudhary, S.C. Kashyap, S.K. Sharma, Room temperature ferromagnetism in Mn doped TiO2 thin films: electronic structure and Raman investigations, J. Appl. Phys. 109 (2011) 083905. [40] O.D. Jayakumar, H.G. Salunke, R.M. Kadam, M. Mohapatra, G. Yaswant, S.K. Kulshreshtha, Magnetism in Mn doped ZnO nanoparticles prepared by a co precipitation method, Nanotechnology 17 (2006) 1278e1285. [41] L.M. Xu, Y.P. Yu, X.J. Xing, X.Y. Wu, S.W. Li, Enhancement of ferromagnetic upon thermal annealing in plasma assisted MBE grown mixed-phase Mndoped insulating TiO2 thin films, Appl. Phys. A Mater. Sci. Process. 92 (2008) 361e365. [42] Z.L. Lu, H.S. Hsu, Y.H. Tzeng, F.M. Zhang, Y.W. Du, J.C.A. Huang, The origins of ferromagnetism in Co doped ZnO single crystalline films: from bound magnetic polaron to free carrier mediated exchange interaction, Appl. Phys. Lett. 95 (2009) 102501. [43] L.R. Shah, B. Ali, H. Zhu, W.G. Wang, Y.Q. Song, H.W. Zhang, S.I. Shah, J.Q. Xiao, Detailed study on the role of oxygen vacancies in structural, magnetic and transport behaviour of magnetic insulator: co CeO2, J. Phys. Condens. Matter 21 (2009) 486004. [44] J.M. Coey, M. Venkatesan, C.B. Fitzgerald, Donor impurity band exchange in dilute ferromagnetic oxides, Nat. Mater. 4 (2005) 173e179. [45] J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, M. Venkatesan, Ferromagnetism in Fe- doped SnO2 thin films, Appl. Phys. Lett. 84 (2004) 1332e1334.
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Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle Biswajit Choudhury, Amarjyoti Choudhury* Department of Physics, Tezpur University, Napaam 784028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2012 Received in revised form 11 February 2013 Accepted 12 February 2013 Available online 26 February 2013
Mn doped TiO2 nanoparticles are synthesized by solegel method. Incorporation of Mn shifts the diffraction peak of TiO2 to lower angle. The position and width of the Raman peak and photoluminescence intensity of the doped nanoparticles varies with oxygen vacancy and Mn doping level. The electron spin resonance spectra of the Mn doped TiO2 show peaks at g ¼ 1.99 and 4.39, characteristic of Mn2þ state. Reduction in the emission intensity, on Mn doping, is owing to the increase of nonradiative oxygen vacancy centers. Mn doped TiO2, with 2% Mn, shows ferromagnetic ordering at low applied field. Paramagnetic contribution increases as Mn loading increases to 4% and 6%. Temperature dependent magnetic measurement shows a small kink in the ZFC curve at about 40 K, characteristic of Mn3O4. The ferromagnetic ordering is possibly due to the interaction of the neighboring Mn2þ ions via oxygen vacancy (Fþ center). Increase in Mn concentration increases the fraction of Mn3O4 phase and thereby increases the paramagnetic ordering. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Ferromagnetism Oxygen defects F center Paramagnetism Exchange interaction
1. Introduction Diluted magnetic semiconductors (DMS) are class of materials where the materials observe ferromagnetism while preserving its semiconducting behavior. These classes of materials have found tremendous possibility in spintronic devices where both charge and spin degrees of freedom can be manipulated [1,2]. Initial trials to observe ferromagnetism was carried out in IIeVI and IIIeV semiconducting materials, such as Mn doped CdTe, ZnSe and Mn doped GaAs, etc [3,4]. Although Ohno achieved ferromagnetism in Mn doped GaAs, the Curie temperature (Tc) was very less (Tc w 110 K) [4]. Initial attempt on oxide based DMS was started by Matsumoto et al. in Co doped TiO2 thin film, prepared by combinatorial pulsed-laserdeposition (PLD) molecular-beam epitaxy (MBE), and obtained a Tc well above room temperature [5]. Although several reports have appeared on magnetism in oxide based diluted semiconductors, the results diverge from each other and therefore the intrinsic nature of magnetism have been questioned by the scientific community [6e11]. The search for room temperature ferromagnetism is carried out in Mn doped TiO2 thin film, nanoparticles, etc. Wang et al. [12] examined that coupling of Mn ions through large concentration of holes gave rise to the room temperature ferromagnetism in Mn doped TiO2 film, prepared by pulsed laser deposition (PLD). On the other hand, Hong
et al. [13] observed absence of magnetism in Mn doped TiO2 film prepared by PLD technique. Mie et al. [14] observed coexistence of ferromagnetism and antiferromagnetism in hydrothermally prepared Mn doped TiO2 nanoparticles. Bhattacharya et al. [15], in Mn doped TiO2 nanocrystals, observed ferromagnetism at low doping concentration and paramagnetism at higher Mn concentration. Kim et al. [16] reported the necessity of oxygen vacancies in Mn doped rutile TiO2 thin films. Li et al. [17] reported that oxygen vacancy alone could not induce ferromagnetism in TiO2 thin films and an optimum level of Mn would be necessary for observing ferromagnetism. In this report we have studied the role of oxygen vacancies and Mn concentration in regulating magnetic ordering in Mn doped TiO2 nanoparticles. The structural and morphological characterizations are done with X-ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy, etc. The valence state of Mn in host TiO2 matrix is analyzed with electron spin resonance (ESR) spectroscopy. We have observed that at low doping concentration the materials attain ferromagnetism with less paramagnetic contribution. But, as dopant concentration increases the paramagnetic contribution has dominance over ferromagnetism. 2. Experimental details 2.1. Preparation of Mn doped TiO2 nanoparticles
* Corresponding author. Tel.: þ91 3712 267120; fax: þ91 3712 267006. E-mail address: [email protected] (A. Choudhury). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.02.007
TiO2 nanoparticles doped with three different concentration of Mn, 2%, 4% and 6%, were prepared by solegel method. The precursors
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with intense CuKa (l ¼ 0.154 nm) radiation. The scanning step was 0.05 and the scanning speed was 1 /min. Transmission electron microscope images (TEM) were obtained with JEOL-JSM 3010 transmission electron microscope operating at a voltage of 300 K. Energy dispersive X-ray analysis (EDX) and elemental mapping were taken on FEI Nova Nano SEM 600. Raman spectroscopy was carried at room temperature with a Labram (Horiba Jobin-Yvon) high resolution Raman spectrometer at spectral resolution of 0.5 cm1. Electron spin resonance spectra (ESR) of the doped samples were carried out with a JEOL-JES-FA200 electron spin resonance spectrometer with an applied microwave frequency of 9.09 GHz. Magnetic measurements were carried out with a Lakeshore-7410 vibrating sample magnetometer (VSM). 3. Results and discussion Fig. 1. X-ray diffraction of pure and Mn doped TiO2 nanoparticles with Mn concentration of 2%, 4% and 6% respectively. Inset of the figure shows the enlarged view of the (101) peak displaying shifting of the peak to lower angle with Mn loading.
for dopant and host were manganese acetate tetra hydrate and titanium isopropoxide respectively. 5 ml of titanium isopropoxide and 15 ml of 2-propanol were added to a 100 ml conical flask. The reaction mixture was stirred for 15 min followed by addition of 1 ml of water to initiate the hydrolysis of the isopropoxide chain. This was followed by the addition of an appropriate amount of manganese acetate solution to the host solution and stirred for about 6 h. During the stirring process, the solution first became sol and at the end of the reaction it was converted to a gel. The gel was first sonicated and then centrifuged in water followed by ethanol for 3 times. The product was then dried in an oven at 80 C. The resulting amorphous Mn doped TiO2 was again annealed at 450 C for 3 h resulting in crystalline Mn doped anatase TiO2 nanoparticles. 2.2. Characterization details The crystalline phase of pure and doped TiO2 nanoparticles were studied with Rigaku Miniflex X-ray diffractometer (XRD) equipped
The X-ray diffraction (XRD) spectra of pure and Mn doped TiO2 nanoparticles correspond to the tetragonal anatase phase of TiO2 (JCPDS-782486). Out of the anatase, brookite and rutile phases of TiO2, anatase and brookite are the metastable phase while rutile is the stable phase. Solution phase synthesis and low temperature annealing preferably results in nanocrystalline anatase phase. On the other hand high temperature annealing (above 600 C) results in bulk rutile phase of TiO2. This metastable anatase phase of TiO2 contains many structural defects on the surface, grain boundary, etc. Although, we have not observed any distinguished peaks of Mn or its oxides, the presence of these phases cannot be completely ruled out. Since the added dopant concentration is very less, the fraction of impurity phases are probably too less to be detected by XRD. An enlarged view of the (101) peak shows shifting of the diffraction peak to lower angle with Mn concentration. Mn priA), Mn3þ (0.58 A) and Mn4þ (0.53 A) marily exists as Mn2þ (0.82 2þ respectively and the ionic radii of Mn is larger than that of Ti4þ (0.61 A) [18]. The shifting of the peak to lower angle indicates substitution of larger Mn2þ on the Ti4þ site. (Fig. 1) Fig. 2a and b shows the transmission electron microscope images of pure and 6% Mn doped TiO2 nanoparticles. The nanoparticles of pure TiO2 have spherical shape with agglomeration.
Fig. 2. Transmission electron microscope images of (a) pure and (b) 6% Mn doped TiO2 nanoparticles. (c) EDX pattern with the composition of the elements in the inset of the figure. Elemental mapping of (d) Ti (e) O and (f) Mn taken from the area of the SEM shown in the inset of Fig. 2d.
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Although spherical in shape, agglomeration persists in the doped TiO2 nanoparticles. The average particle size of pure and doped TiO2 nanoparticles is 8 nm and 6 nm respectively. Fig. 2c depicts the EDX pattern of 6% Mn doped TiO2 nanoparticles. The elemental composition is shown in the inset of Fig. 2c. Fig. 2def exhibits the elemental mapping of Ti, Mn and O in the 6% Mn doped TiO2 nanoparticle. The EDX and mapping is carried out on the selected area of the sample shown in the inset of Fig. 2c. From the elemental composition it is found that O:Ti ratio is 3.75 which is higher than usual 2:1 ratio and also Ti/(Ti þ Mn) ratio is 1.5% which is again smaller than added 6%. EDX is mostly a surface characterizing technique and it can detect elements present upto few nm layer from surface and not of entire volume of the nanoparticle. The surface of TiO2 contains many active sites, such as Ti3þ and oxygen vacancies. Therefore, the atmospheric oxygen molecule, eOH moiety present in moisture can easily adsorb on these sites. Since, the initially prepared TiO2 is annealed in air to get crystalline anatase, the possibility of the adsorption of these O2 and eOH is very much feasible. Therefore, the value of 3.75 for O:Ti is not the same as that of 2:1 stoichiometric ratio of TiO2, since the surface of TiO2 contains entire oxygen that are adsorbed on the surface. Similarly, Mn concentration is 1.54%, which is less than nominal 6%. This is not the Mn concentration present in the entire volume of the nanoparticles. EDX cannot detect the composition of the elements in the core and since few regions of sample area are selected for EDX, the composition is not same everywhere and the dopants (present on surface) are in-homogeneously distributed on the surface. Apart from this, during doping the dopants are adsorbed on the surface of the nanoparticle and then it penetrate inside while annealing. Since, we have centrifuged the doped sample, loosely bound Mn2þ ions may have been detached from this. Therefore, some Mn ions might have been lost from surface. These may be reasons why the composition shown by EDX is different than that of stoichiometric of TiO2 or from added Mn concentration. The doping effect of Mn, on the TiO2 lattice, is further studied with Raman spectroscopy. Fig. 3 shows the room temperature Raman spectra of pure and doped TiO2 nanoparticles. Anatase TiO2 has six Raman active modes and three infra-red (IR) active modes [19e21]. The representation of optical phonon modes at the center of Brilliouin zone is given by:
Fig. 3. Raman spectra of pure and entire Mn doped TiO2 nanoparticles. The enlarged view of the intense Eg peak is shown in the inset of the figure.
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Gopt ¼ A1g ðRÞ þ 2B1g ðRÞ þ 3Eg ðRÞ þ B2u ðIRÞ þ A2u ðIRÞ þ 2Eu ðIRÞ:
(1)
In pure TiO2 the first intense Raman peak appears at 145 cm1 corresponding to Eg mode. The other Eg peak appears as two low intense peaks at 197 and 640 cm1 respectively. The B1g peak and the (A1g þ B1g) peak appear at 397 cm1 and 516 cm1 respectively [20,21]. The most intense Eg peak at 145 cm1 corresponds to Oe TieO type of bending vibration. This mode is very much sensitive to local oxygen coordination surrounding the metal ion [22]. The
Fig. 4. Room temperature ESR spectra of 2%, 4% and 6% Mn doped TiO2 nanoparticles.
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expanded view of this mode is shown in the inset of Fig. 3. The inset figure demonstrates that the Raman Eg peak of TiO2 becomes asymmetric and blue shifted as TiO2 is incorporated with 2%, 4% and 6% of Mn. This kind of changes in the Raman peak occurs owing to the disruption of the local structure and due to the generation of defects on the lattice site of TiO2 on Mn doping. The Raman peaks are very sensitive to any disorder occurring in the oxygen coordination in the TiO2 lattice. Substitution of Ti4þ ions by Mn2þ distort the TiO2 lattice and generate oxygen vacancies to maintain charge neutrality. Formation of oxygen vacancies, on substitution, decreases the average number of oxygen ions to bind with Ti4þ or Mn2þ ions. Decrease in the average number of TieO/MneO bonds will reduce the average number of bonds (due to entire TieO or MneO bonds) in TiO2 and thereby contract the lattice [19]. The strength of a bond is determined by its force constant and the force constant of a bond is associated with frequency by yNOk [19]. Therefore, decrease in the number of TieO/MneO bonds will contract the lattice and will shift the Eg position to higher wavenumber. Therefore, oxygen vacancies are mainly responsible for the blue shifting and widening of the Raman peak [23e25]. Fig. 4aec illustrates the room temperature ESR spectra of Mn doped TiO2 nanoparticles. The effective g-factor is calculated using the equation [26]:
geff ¼
hn ; mB H
(2)
where h, v, mB and H are the Plank constant, frequency of applied microwave field, Bohr magneton and applied magnetic field, respectively. The ESR curve contains two absorption peaks. The intense peak has a g-value of 1.99 and the second low intense peak has g at 4.39. These spectral positions of Mn appear due to the interaction of Mn (II) with the surrounding O2 ligands in octahedral symmetry [27e31]. In case of strong interaction of Mn (II) with the surrounding ligands, the expected signals comprise of a sextet at g ¼ 1.99 [27]. In our case, we have not observed any sextet lines instead an unresolved broad line appears at g ¼ 1.99 with a weak shoulder at g ¼ 4.39. This may be due to weak interaction of Mn2þ with the host lattice or due to clustering of the Mn2þ ions on the grain boundary [28e30]. Photoluminescence (PL) spectroscopy is an important characterizing tool to understand the presence of defects in a material. Fig. 5 depicts the room temperature photoluminescence spectra of pure and Mn doped TiO2 nanoparticles at an excitation wavelength
of 320 nm. We have fitted the emission peaks with Gaussian and observed that five different emission peaks are present in the spectra. The UV emission is the indirect band to band transition from conduction band to the valence band of TiO2 nanoparticles [32]. Although not shown here, the position of the different emission peaks are estimated by fitting the peaks with Gaussian (r2 ¼ 0.9956). The defect emission band spans from 420 to 540 nm. The 424 nm peak is associated with self trapped excitons (STE) [33]. The 491 nm peak is due to the charge transfer transition from Ti3þ to the TiO2octahedra associated with oxygen defects [34]. The 6 emission bands at 535 nm and 460 nm are associated with electrons bound (or trapped) to oxygen vacancy centers [35]. Depending on the number of trapped electrons, these oxygen vacancy centers are referred as F (two trapped electrons), Fþ (one electron) and Fþþ (two electrons) type color centers [36]. These color centers may act as luminescence enhancer or quencher. As it is seen in the spectra, there is neither any Mn emission peak nor there is any change in the position of the defect emission peaks. It is the intensity of the emission peaks that is quenched on Mn doping. In pure TiO2, the oxygen vacancies act as luminescence enhancer and increase the emission intensity. Doping of Mn disturbs the TiO2 lattice, breaks the TieO bond and generates many oxygen vacancies. With the increase of Mn doping, the number of nonradiative oxygen vacancy centers also increases, nearby Mn2þ. Since, the oxygen defect concentration increases, most of the photoexcited electrons are trapped and strongly localized in those oxygen vacancies, decreasing the availability for recombination with the holes. Since the nonradiative oxygen vacancy centers, with trapped electrons, increases with Mn, the emission intensity also decreases subsequently. Apart from the nonradiative oxygen vacancies, the other factor affecting PL intensity is the mobility of the carriers. The presence of dopants and defects on the interior, grain boundary and on the surface reduces the mobility of the free carriers. The mobile carriers are scattered when they approach charged dopants or oxygen defect (F, Fþ) states. Decrease in the mobility will increase the separation of carriers and hence reduces the PL intensity [37]. Room temperature magnetic measurements (MeH) of entire samples are carried out in the field range of 20 kOe. Fig. 6aed shows the MeH curves of pure and Mn doped TiO2 nanoparticles. Although pure TiO2 nanoparticles are showing diamagnetism, incorporation of Mn into TiO2 shows magnetism. Although total magnetization increases with dopant concentration, the actual saturation magnetization possibly occurs at low magnetic field. The linear magnetization
Fig. 5. Photoluminescence spectra of pure and Mn doped TiO2 nanoparticles excited at 320 nm.
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Fig. 6. Room temperature MeH curve of (a) pure (b) 2% (c) 4% and (d) 6% Mn doped TiO2 nanoparticles (without subtracting high field linear part).
at the high magnetic field may be due to the presence of paramagnetic or some other magnetic phases which do not contribute to ferromagnetic ordering. Since the slope of the linear part increases with Mn concentration, we can attribute this variation to be dependent on Mn concentration. Therefore, the total magnetization of the sample is the sum total of a ferromagnetic (in the low field) and a paramagnetic (at the high field) part. The possible interaction/impurity phases that give rise to the paramagnetism or antiferromagnetism are Mn2þeMn2þ interaction, formation of antiferromagnetic manganese oxide phases [14,18,38,39]. The ferromagnetism may be due to the substitution of Mn into TiO2 lattice or due to the presence of secondary oxide phases of Mn [18]. Of all the oxides of Mn, only Mn3O4 is ferrimagnetic with a Curie temperature (Tc) of 42 K [18,38,39]. Since the Tc is too low, the observed room temperature ferromagnetic behavior in the samples cannot be entirely attributed to the Mn3O4 phase. For further understanding of the magnetic nature of the doped materials, temperature dependent magnetic (MeT) measurement is carried out. The MeT measurement is carried out at an applied field of 500 Oe and in the field range from 15 to 300 K. Fig. 7a and b shows the zero field cooling (ZFC) and field cooling (FC) MeT curves of 2% and 6% Mn doped TiO2 nanoparticles. It is seen that 2% Mn doped TiO2 exhibits a deviation in the ZFC and FC curve at 120 K with a small kink at 40 K in the ZFC curve. For 6% Mn doped TiO2, the ZFC and FC curve overlaps up to 50 K and then bifurcation occurs. The temperature at which the bifurcation between the ZFC and FC curve occurs is the thermomagnetic irreversible temperature (Tirr) [18]. Tirr depends on magnetocrystalline anisotropy, applied magnetic field, coercivity, etc. The peak appearing at w39e40 K in the two samples correspond to the Curie temperature of Mn3O4. This peak appears as a small kink in 2% Mn and becomes intense at 6% Mn doping, indicating increase in the fraction of Mn3O4 phase with the increase in Mn concentration. Another observation in the FC curve is the increase in the magnetization in the low temperature regime (below 39 K). The steady increase in magnetization on lowering the temperature indicates the presence of frozen isolated Mn2þ ions in
the host oxide matrix that contribute to paramagnetic ordering [18,40]. Antiferromagnetic oxide phases of Mn such as MnO, Mn2O3, MnO2 have Neel temperature, TN < 100 K [38,41]. Since we have not observed any characteristic peak corresponding to this TN in the MeT curves, we can neglect the presence of antiferromagnetic interaction due to these oxide phases. Therefore, the increase in the linearity of the magnetization, with the dopant concentration, at the high field region is possibly due to the Mn3O4 phase [18]. Besides, magnetic contribution coming from different magnetic phases, during magnetic measurement, may apparently contribute to this paramagnetic linearity at high field. Two theories are usually employed to discuss ferromagnetism in oxide based DMS, RudermaneKitteleKasuyaeYosida (RKKY) interaction and bound magnetic polaron (BMP) theory [42e45]. RKKY theory is applied in metallic system where electrons are delocalized and bound magnetic polaron theory is applied in case of insulating system where electrons are strongly localized [42e 45]. From PL analysis we have understood that the carriers are strongly localized and doping reduces mobility of the carriers. Since the mobility decreases on doping we can expect that the doped samples have high resistivity. Although we have proposed that Mn doped samples will have high resistivity, Sharma et al. [18], in fact, obtained high resistivity in Mn doped TiO2 films having resistivity (r) w107 U-m. Therefore, taking into consideration of the wide band gap of anatase TiO2 (3.2 eV), strong localization of carriers, and high resistivity (expected) of the doped samples, we can suggest that an F-center mediated bound magnetic polaron theory (FCBMP) or BMP will be useful to explain magnetism in these samples. This is only suggestive, we do not rule out any possible mechanism that can explain the magnetic origin. An electron trapped in an oxygen vacancy (F center) couple with the magnetic spins of the nearest Mn2þ ions within the radius of the hydrogen like orbit of the F center and forms a BMP. When Mn loading is 2%, most of the Mn2þ ions occupy substitutional position and a few Mn2þ ions are oxidized to Mn3O4. Mn2þ ions, on substitution, create
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vacancies (as inferred from the appearance of visible emission peaks), those oxygen vacancies alone cannot to contribute to ferromagnetism. Increase of Mn increases the number of oxygen vacancies and therefore an exchange interaction of the Mn ions via oxygen vacancies may induce ferromagnetism. Although oxygen vacancies can be expected to be increased at 4% and 6%, the interaction of Mn2þ via oxygen vacancies is less pronounced as most of the Mn2þ ions form oxide phases. Therefore, we can suggest that both oxygen vacancies and an optimum level of Mn (with entire Mn2þ taking part in BMP) are necessary for getting ferromagnetism in Mn doped TiO2 nanoparticles. Very high concentration of Mn does not contribute to ferromagnetism, instead forms secondary oxide phases and contribute to paramagnetism. 4. Conclusion In summary we have suggested that for getting ferromagnetic ordering both oxygen vacancies and an optimum level of Mn (on the TiO2 lattice) are necessary. Pure TiO2 nanoparticles do not exhibit any magnetic ordering. However, incorporation of Mn results in the ferromagnetic ordering at low applied file and paramagnetism at high magnetic field. We have proposed that Mn2þ ions undergo exchange interaction via oxygen vacancies and form bound magnetic polaron to induce ferromagnetism in TiO2. At low doping concentration Mn2þ ions occupy lattice site, interact with oxygen vacancies and increases ferromagnetic ordering. However, at high dopant concentration some of the Mn2þ forms Mn3O4 phase and thereby increases paramagnetic contribution. References
Fig. 7. Zero field cooling (ZFC) and field cooling (FC) MeT curves of (a) 2% and (b) 6% Mn doped TiO2 nanoparticles.
oxygen vacancies nearby Mn2þ in the lattice. The radius of the hydrogenic orbit of the F-center is given by rH ¼ εr ðm=m* Þa0, where 3 r is dielectric constant, m is electron mass and m* is effective mass of electrons and a0 is Bohr radius [42e45]. Those Mn2þ ions that fall within this F-center radius interacts with the spin of Fcenter, forms BMP and imparts ferromagnetism. The low value of magnetization indicates participation of less number of Mn2þ ions in the formation of BMP. However, as we increase the Mn concentration to 4% and 6%, the number of Mn atoms in the interior of TiO2 is comparatively less than that on its surface and on the grain boundary. In case of 4% and 6% Mn doping, only those Mn atoms are allowed to enter the lattice which are permissible by the host, the rest are expelled and are oxidized to Mn3O4 (during air annealing). Although 2% Mn doped TiO2 contains Mn3O4, the fraction of this oxide phase is less in the outside. However, in case of 4% and 6% of Mn, the fraction of Mn3O4 increases and thereby increasing paramagnetic contribution. Thus, we can suggest that 2% is the solubility limit of Mn inside TiO2. Sharma et al. [18] also observed that incorporation of high concentration of Mn forms Mn3O4 phase and increases paramagnetic ordering. Hong et al. [13] demonstrated that Mn doping just enhances the magnetism of a TiO2 film which is already ferromagnetic, it itself does not increase the magnetization. They observed that doping with high concentration of Mn destroys the ferromagnetism. On the other hand, Li et al. [17] observed that simple oxygen vacancy cannot impart magnetism and an optimum level of Mn is necessary for ferromagnetism. In our analysis, we have observed that although pure TiO2 nanoparticles contains oxygen
[1] I. Zuti c, J. Fabian, S.D. Sarma, Spintronics: fundamentals and applications, Rev. Mod. Phys. 76 (2004) 323e410. [2] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S.V. Molnár, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (1998) 1488e1495. [3] J.K. Furdyna, J. Kossut, Diluted Magnetic Semiconductors e Semiconductor and Semimetals, vol. 25, Academic Press, New York, 1988. [4] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, (Ga, Mn) As: a new diluted magnetic semiconductor based on GaAs, Appl. Phys. Lett. 69 (1996) 363e365. [5] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 291 (2001) 854e856. [6] S.R. Shinde, S.B. Ogale, J.S. Higgins, H. Zheng, A.J. Millis, V.N. Kulkarni, R. Ramesh, R.L. Greene, T. Venkatesan, Co-occurrence of superparamagnetism and anomalous Hall effect in highly reduced cobalt-doped rutile TiO2dfilms, Phys. Rev. Lett. 92 (2004) 166601. [7] G. Glaspell, A.B. Panda, M.S. El-Shall, Reversible paramagnetism to ferromagnetism in transition metal-doped TiO2nanocrystals prepared by microwave irradiation, J. Appl. Phys. 100 (2006) 124307. [8] C.E. Rodríguez-Torres, S.J. Stewart, C. Adán, A.F. Cabrera, Paramagnetism and clustering in Fe-doped TiO2 nanoparticles, J. Alloys Compd. 495 (2010) 485e487. [9] B. Choudhury, A. Choudhury, P. Alagarsamy, A. Islam, M. Mukherjee, Effect of oxygen vacancy and dopant concentration on the magnetic properties of high spin Co2þ doped TiO2 nanoparticles, J. Magn. Magn. Mater. 323 (2011) 440e446. [10] T. Dietl, Origin of ferromagnetic response in diluted magnetic semiconductors and oxides, J. Phys. Condens. Matter 19 (2007) 165204. [11] R. Janisch, P. Gopal, N.A. Spaldin, Transition metal-doped TiO2and ZnOdpresent status of the field, J. Phys. Condens. Matter 17 (2005) R657eR689. [12] Z. Wang, J. Tang, Y. Chen, L. Spinu, W. Zhou, L.D. Tung, Room-temperature ferromagnetism in manganese doped reduced rutile titanium dioxide thin films, J. Appl. Phys. 95 (2004) 7384. [13] N.H. Hong, J. Sakai, A. Ruyter, V. Brize, Does Mn doping play any role in magnetism in Mn doped TiO2 thin film? Appl. Phys. Lett. 89 (2006) 252504. [14] L.H. Mie, L. Min, Z.Y. Su, H.T. Cheng, Coexistence of antiferromagnetic and ferromagnetic in Mn doped TiO2 nanowires, J. Cent. South. Univ. Technol. 17 (2010) 239e243. [15] S. Bhattacharyya, A. Pucci, D. Zitoun, A. Gedanken, One-pot fabrication and magnetic studies of Mn-doped TiO2 nanocrystals with an encapsulating carbon layer, Nanotechnology 19 (2008) 495711. [16] K.J. Kim, Y.R. Park, J.H. Lee, S.L. Choia, H.J. Lee, C.S. Kim, J.Y. park, Roomtemperature ferromagnetic properties in Mn-doped rutile thin films, J. Magn. Magn. Mater. 316 (2007) e215ee218.
B. Choudhury, A. Choudhury / Current Applied Physics 13 (2013) 1025e1031 [17] X.Y. Li, S.X. Wu, L.M. Xu, C.T. Li, Y.J. Liu, X.J. Xing, S.W. Li, Effects of depositing rate on structure and magnetic properties of Mn:TiO2 films grown by plasmaassisted molecular beam epitaxy, Mater. Sci. Eng. B 156 (2009) 90e93. [18] S. Sharma, S. Chaudhary, S.C. Kashyap, V.K. Malik, DC magnetization investigations in Ti1xMnxO2 nanocrystalline powder, J. Alloys Compd. 509 (2011) 7434e7438. [19] M.G. Broj cin, M.J. S cepanovi c, Z.D. Doh cevi c-Mitrovi c, I. Hini c, B. Matovi c, G. Stanisi c, Z.V. Popovi c, Infrared study of laser synthesized anatase TiO2 nanopowders, J. Phys. D Appl. Phys. 38 (2005) 1415e1420. [20] B. Choudhury, A. Choudhury, Dopant induced changes in structural and optical properties of Cr doped TiO2 nanoparticles, Mater. Chem. Phys. 132 (2012) 1112e1118. [21] J. Zhang, M. Li, Z. Feng, J. Chen, C. Li, UV Raman spectroscopic study on TiO2. I. Phase transformation at the surface and in the bulk, J. Phys. Chem. B 110 (2006) 927e935. [22] H.Y. Lin, Y.Y. Chou, C.L. Cheng, Y.F. Chen, Giant enhancement of band edge emission based on ZnO/TiO2 nanocomposites, Opt. Express 15 (2007) 13832e 13837. [23] J.C. Parker, R.W. Siegel, Calibration of the Raman spectrum to the oxygen stoichiometry of nanophase TiO2, Appl. Phys. Lett. 57 (1990) 943e945. [24] J.G. Li, X. Wang, K. Watanabe, T. Ishigaki, Phase structure and luminescence properties of Eu3þ-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists, J. Phys. Chem. B 110 (2006) 1121e1127. [25] Q. Wu, Q. Zheng, R. Krol, Creating oxygen vacancies as a novel strategy to form tetrahedrally coordinated Ti4þ in Fe/TiO2 nanoparticles, J. Phys. Chem. C 116 (2012) 7219e7226. [26] J.A. Weil, J.R. Bolton, Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, second ed., John Wiley & Sons, Inc., New Jersey, 2007. [27] D. Dvoranová, V. Brezová, M. Mazúra, M.A. Malati, Investigations of metaldoped titanium dioxide photocatalysts, Appl. Catal. B 37 (2002) 91e105. [28] R. Arroyo, G. Cordoba, J. Padilla, V.H. Lara, Influence of manganese ions on the anataseerutile phase transition of TiO2 prepared by the solegel process, Mater. Lett. 54 (2002) 397e402. [29] K.C. Heo, C.I. Ok, J.W. Kim, Influence of manganese ions on the anatase-rutile phase transition of TiO2 prepared by solegel process, J. Korean Phys. Soc. 47 (2005) 861e865. [30] R.V. Reinaa, S. Chaob, V. Petrovskyb, F. Doganb, S. Greenbaum, Electrical and electron paramagnetic resonance spectroscopy characteristics of Mn doped nanostructured TiO2 for capacitor applications, J. Power Sources 210 (2012) 21e25. [31] A. Venkatasubbaiah, J.L. Rao, S.V.J. Lakshman, B. Sreedhar, Electron paramagnetic resonance and optical absorption spectra of manganese (II) ions in lead acetate glasses, Polyhedron 12 (1993) 1539e1543.
1031
[32] N. Serpone, D. Lawless, R. Khairutdinov, Size effects on the photophysical properties of colloidal anatase TiO2 particles: size quantization versus direct transitions in this indirect semiconductor? J. Phys. Chem. 99 (1995) 16646e 16654. [33] J. Preclikova, P. Galar, F. Trojanek, S. Danis, B. Rezek, I. Gregora, Y. Nemcova, P. Maly, Nanocrystalline titanium dioxide films: influence of ambient conditions on surface- and volume-related photoluminescence, J. Appl. Phys. 108 (2010) 113502. [34] J. Liu, J. Li, A. Sedhain, J. Lin, H. Jiang, Structure and photoluminescence study of TiO2 nanoneedle texture along vertically aligned carbon nanofiber arrays, J. Phys. Chem. C 112 (2008) 17127e17132. [35] Y. Lei, L. Zhang, G. Meng, G. Li, X. Zhang, C. Liang, W. Chen, S. Wang, Preparation and photoluminescence of highly ordered TiO2 nanowire arrays, Appl. Phys. Lett. 78 (2001) 1125e1127. [36] N. Serpone, Is the band gap of pristine TiO2 narrowed by anion- and cationdoping of titanium dioxide in second-generation photocatalysts? J. Phys. Chem. B 110 (2006) 24287e24293. [37] B. Choudhury, A. Choudhury, Tailoring luminescence properties of TiO2 nanoparticles by Mn doping, J. Lumin. 136 (2013) 339e346. [38] Z.M. Tian, S.L. Yuan, Y.Q. Wang, J.H. He, S.Y. Yin, K.L. Liu, S.J. Yuan, L. Liu, Magnetic properties in Mn doped TiO2 bulk samples, J. Phys. D Appl. Phys. 41 (2008) 055006. [39] S. Sharma, S. Chaudhary, S.C. Kashyap, S.K. Sharma, Room temperature ferromagnetism in Mn doped TiO2 thin films: electronic structure and Raman investigations, J. Appl. Phys. 109 (2011) 083905. [40] O.D. Jayakumar, H.G. Salunke, R.M. Kadam, M. Mohapatra, G. Yaswant, S.K. Kulshreshtha, Magnetism in Mn doped ZnO nanoparticles prepared by a co precipitation method, Nanotechnology 17 (2006) 1278e1285. [41] L.M. Xu, Y.P. Yu, X.J. Xing, X.Y. Wu, S.W. Li, Enhancement of ferromagnetic upon thermal annealing in plasma assisted MBE grown mixed-phase Mndoped insulating TiO2 thin films, Appl. Phys. A Mater. Sci. Process. 92 (2008) 361e365. [42] Z.L. Lu, H.S. Hsu, Y.H. Tzeng, F.M. Zhang, Y.W. Du, J.C.A. Huang, The origins of ferromagnetism in Co doped ZnO single crystalline films: from bound magnetic polaron to free carrier mediated exchange interaction, Appl. Phys. Lett. 95 (2009) 102501. [43] L.R. Shah, B. Ali, H. Zhu, W.G. Wang, Y.Q. Song, H.W. Zhang, S.I. Shah, J.Q. Xiao, Detailed study on the role of oxygen vacancies in structural, magnetic and transport behaviour of magnetic insulator: co CeO2, J. Phys. Condens. Matter 21 (2009) 486004. [44] J.M. Coey, M. Venkatesan, C.B. Fitzgerald, Donor impurity band exchange in dilute ferromagnetic oxides, Nat. Mater. 4 (2005) 173e179. [45] J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, M. Venkatesan, Ferromagnetism in Fe- doped SnO2 thin films, Appl. Phys. Lett. 84 (2004) 1332e1334.