- Email: [email protected]
Structure and magnetic properties of Ti1−xCoxO2 nanoparticles prepared by chemical route
Journal of Crystal Growth 321 (2011) 19–23
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Structure and magnetic properties of Ti1 xCoxO2 nanoparticles prepared by chemical route Sunil Sharma a,c, Nagesh Thakur a, R.K. Kotnala b, Kuldeep Chand Verma c,n a
Department of Physics, Himachal Pradesh University, Shimla 171005, India National Physical Laboratory, New Delhi 110012, India c Department of Physics, Eternal University, Baru Sahib, H.P., India b
a r t i c l e i n f o
abstract
Article history: Received 30 August 2010 Received in revised form 3 February 2011 Accepted 14 February 2011 Communicated by. K. Nakajima Available online 18 February 2011
The structural, microstructural, dielectric and ferromagnetic behavior of Co-doped TiO2 (TC) nanoparticles have been studied. TC nanoparticles were prepared by a chemical synthesis route using polyethylene glycol as a surfactant. The polymer serves as a surfactant to encapsulate the cationic species in divided groups during the reaction that confines the size and morphology of the specimen. X-ray diffraction pattern has been used to confirm the crystalline structure (rutile or anatase) and average particle size is measured using Debye–Scherer’s relation. The particle’s size is also measured by transmission electron microscopy. Fourier transform infrared spectroscopy has been used to confirm the formation of Ti–O bond. X-ray photoemission spectroscopy suggests that the Ti and Co ions have the oxidation states 4+ and 2 + , respectively, present in the 2p region and confirm the presence of Ti–O–Co bonds. The variation in magnetization with Co concentration is studied. Dielectric measurements were also carried at room temperature to confirm the storage charge capability of TC semiconductor, and the nano-size effect to control the dielectric constant up to higher frequency region has been observed. The enhancement in magnetization at room temperature is also observed. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A2. Growth from solutions B1. Dilute magnetic semiconductors B2. TiO2
1. Introduction Dilute magnetic semiconductors (DMSs) based on II–VI semiconductors such as CdTe and ZnSe exhibit ferromagnetic behavior only at low temperature (Tc o4 K) [1]. Also, the DMS based on III–V semiconductors such as InAs and Mn doped GaAs offers ferromagnetic order at about 100 K [2,3]. However, these materials show limitations in the potential applications of DMS due to their low Curie temperature [4]. From theoretical predictions, the occurrence of ferromagnetism in wide band gap semiconductors like ZnO, GaN, etc., by doping certain 3d elements, the DMSs have attracted widespread research attention due to their potential applications of spintronics and optical devices [5–8]. TiO2 is an attractive wide band gap semiconductor for the fabrication of DMSs [9]. Recently, research has been done on the magnetism of TiO2based DMSs doped with transition metals such as Co, Mn, Fe, Ni and Cr [9–11]. The crystalline structures of TiO2 are different, with rutile being the most stable one. At low temperatures there is only a slight difference between the stability of rutile and
n Corresponding author at: Department of Physics, Eternal University, Baru Sahib, H.P., India. Tel.: +91 9418941286; fax: + 91 0177 2830775. E-mail address: [email protected] (K.C. Verma).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.02.023
metastable anatase and brookite phases. TiO2 is also used in catalysis, photocatalysis, dye-sensitized solar cells, etc. In magnetic semiconductors the host material is lightly doped with magnetic impurity, without forming a second phase. The presence of magnetic impurities results in spin polarization in the majority carrier band. This polarization is induced by exchange interaction among the magnetic dopant spins and the carriers. Room temperature ferromagnetism in Co-doped TiO2 as the main focus of research in recent years was reported by Matsumoto et al. [12], who produced Ti1 xCoxO2 films by combinatorial laser molecular beam epitaxy. If the ferromagnetism arises from the surface of Co ions, only a few Co ions contribute to the ferromagnetism because most of Co ions reside in the core of nanoparticles. Ferromagnetism with high Curie temperature depends on many parameters such as the preparation condition, particles size and amount of dopant level [4,13]. Besides the ferromagnetic and optical behavior, its remarkable dielectric constant makes it a promising material for various memory applications such as tunable mixers, phase shifter, voltage controlled oscillators, delay lines and for ultra-large-scale integration dynamic random access memory (DRAM) capacitor [14]. Several attempts have been made by researchers to prepare TiO2 DMS using a variety of techniques, including sol–gel, solid state reaction, micro-emulsion, hydrothermal processing, sol–gel auto-combustion, etc. [8,11,15]. However, most of them have
20
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
been directed towards the preparation of particles with a large size distribution in the range of a few micrometers economically, despite complicated experimental steps and high reaction temperatures. In the present study, a chemical synthesis route similar to a sol–gel technique but modified has been applied to synthesize DMS Ti1 xCoxO2 (x ¼0.01 (TC1), 0.02 (TC2) and 0.04 (TC4)) nanoparticles. Polyethylene glycol (PEG) is used as a surfactant to reduce the grain size. As a result, size of grains below 100 nm is observed with annealing temperature of 700 oC for crystallization. The experimental characterizations were performed by Fourier transform infrared (FTIR) spectroscopy; bonding information was obtained using X-ray photoemission spectroscopy (XPS), magnetization measurements and dielectric properties. We also report an enhancement in magnetization at nano-level of DMS TiO2 with Co doping, which was not seen in the reported study on bulk and nanoparticles of doped or undoped TiO2 [15–17]. Measuring the value of dielectric constant is important in various memory based devices, which was not found in the previous literature.
2. Experimental technique Ti1 xCoxO2 (TC) nanoparticles were prepared from the precursor solutions of tetra-n-butyl orthotitanate and cobalt 2-ethylhexanoate (C7H15COO)2Co in xylene. The solution was prepared by mixing the precursor solutions in the desired molar ratio of Ti:Co. The solution was refluxed at 60 oC with constant stirring for 2 h for homogeneous mixing. PEG, at 3% (in volume) of the TC solution, was added dropwise. PEG can decompose the organic residual from metal and also encapsulate the TC constituents into the smallest limit during the heating process. The solution was dried at 200 1C and heated at 700 1C for 3 h for crystallization. The phase structure and nanobehavior of TC specimens were studied by X-ray diffraction (XRD) using a PAN analytical X-Pert PRO system and microstructure by the transmission electron microscopy (TEM) using Hitachi H-7500. The effect of light on TC nanoparticles has been studied in the infrared region by FTIR using a Nicolet Avtar 5700 system. The photoemission spectrum was recorded using a VSW make photoelectron spectrometer. Magnetization measurements were performed at room temperature using a vibrating sample magnetometer (VSM735). For electrical measurements, the powder of TC nanoparticles heated at 700 1C was pressed into pellets using a pressure of 5 bar for 10 min. The pellets were sintered at a high temperature of 1000 1C for 5 h. The relative electrical permittivity (e) and dissipation factor (tan d) were measured in the frequency range from 100 Hz to 20 MHz using a Precision Impedance analyzer (WayneKerr 6500B).
3. Results and discussion Fig. 1 shows a representative XRD pattern of the TC nanoparticles heated at 700 oC. It shows that the major Bragg’s peaks in all the samples can be indexed (peaks are indexed using ASTM card) to tetragonal rutile phase of TiO2 with Miller indices (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1) and (2 2 0) with diffraction angle at 2y ¼27.44o, 36.08o, 39.19o, 41.24o, 44.05o, 54.33o and 56.64o, respectively, observed in all TC specimens. However, a few minor peaks were identified as an impurity phase of CoTiO3 in TC4 specimen. Such an impurity phase generally existing at higher Co concentration in TiO2 was also observed in the reported literature [6,18]. Furthermore, the peak position shifts to smaller angle with increasing of Co content, revealing of changes in lattice parameters. The values of lattice parameters for rutile phase of, ˚ ˚ a(A)¼4.595, 4.596 and 4.603 and c(A)¼2.969, 2.968 and 2.970 and the average particle’s size of (using Debye–Scherer’s relation)
Fig. 1. XRD patterns of Ti1 xCoxO2 (x¼ 0.01, 0.02 and 0.04) nanoparticles heated at 700 oC/3 h.
Table 1 Values of rutile Co-doped TiO2 nanoparticles of the lattice parameters (a and c), average particles size (x) from XRD, (x’) from TEM, dielectric constant (e) and saturation magnetization (M). Specimen
˚ a (A)
˚ c (A)
x (nm)
x’ (nm)
e(1 MHz)
M (emu/g)
Ti0.99Co0.01O2 Ti0.98Co0.02O2 Ti0.96Co0.04O2
4.595 4.596 4.603
2.969 2.968 2.970
25 27 28
24 26 27
60.5 78.3 102.0
8.75 19.61 24.37
25, 27 and 28 nm, respectively, are calculated from XRD data for TC1, TC2 and TC4 nanoparticles. The values of a, c and average particle’s size are given in Table 1. The variations in the values of lattice parameters exist because the ionic radii of the substituted ˚ ion is larger than that of parent Ti4 + (0.64 A). ˚ The Co2 + (0.838 A) smaller crystalline size indicates that the preparation method is suitable for nano fabrication of TC specimens because in this case PEG is used as a surfactant. Fig. 2(a)–(c) presents typical TEM images of the Co-doped TiO2 nanoparticles. The particles show high monodispersity in size and the estimated average particle size is 24, 26 and 27 nm, respectively, for TC1, TC2 and TC4 specimens. Fig. 3 shows the FTIR spectra of Ti1 xCoxO2 nanoparticles acquired in the range 4000–200 cm–1 in KBr pellets. It shows several weak absorption bands in the region 4000–1500 cm–1 probably due to atmospheric moisture and CO2. However, as the spectra was recorded after heating the samples at 700 1C the presence of vibrations such as O–H, C–H, etc. characteristic of this region cannot be expected. The FTIR spectral bands in the frequency region of interest 450–432 cm 1 are assigned to the stretching vibrations of Ti–O bonds [19].
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
21
Fig. 2. TEM images of (a) TC1, (b) TC2 and (c) TC4 specimens.
Fig. 3. FTIR spectra of TC nanoparticles.
In order to check the presence of the oxidation state of titanium, oxygen and cobalt in these TC nanoparticles, high resolution XPS was performed. Fig. 4 shows the Ti 2p, O 1s and Co 2p regions for all TC specimens. Fig. 4(a) shows the Ti 2p XPS spectra for TC1, TC2 and TC4 specimens. The core levels of Ti 2p3/2
and Ti 2p1/2 in TC1 are observed at 458.81 and 464.36 eV, respectively. These peaks are assigned to Ti4 + [20] and the separation between these lines ( 5:55 eV) is slightly less than standard binding energy value (5.7 eV) for TiO2 because of 1% Co doping. After increasing Co doping in TiO2 lattice, the intensities of the Ti 2p peaks show depression, which was expected. In addition, the binding energies of Ti 2p peaks shift to lower energies for TC2 and TC4 than TC1, while the binding energy of O 1s peak shifts in the opposite direction (Fig. 4(b)) with a depression in intensity compared to that of TC1. The lower binding energy of Ti 2p with increasing Co doping resulted from the increased electron cloud density around Ti, which indicates that the atom possessing lower electronegativity was introduced into the TiO2 crystal structure [20]. The observed value of binding energy for TC2 and TC4 is 5.34 and 5.23, respectively. In Fig. 4(b), the O 1s XPS spectrum shows a prominent peak at 531.61 eV for TC1, which is slightly larger than that of pure TiO2 ( 530 eV) [21] ascribed to the Ti–O–Co bonds in TC1. A similar behavior is observed in TC2 ( 531.8 eV) and TC4 (532.06 eV) specimens with a slight increment in binding energy, which indicate that the Ti–O–Co bonds are quite stronger with increasing Co in TiO2. Fig. 4(c) shows Co 2p spectra of TC1, TC2 and TC4 specimens. These spectra show four peaks: the 2p3/2 and 2p1/2 spin doublet and their corresponding shake-up resonant transition peaks. The positions of Co 2p3/2 and Co 2p1/2 peaks are in agreement with those for Co in the oxidation state of 2+ in CoO [22]. If Co would have been present in the form of metallic clusters, the difference between Co 2p3/2 and 2p1/2 peaks would have been 15.05 [23]. In the present paper, the separation between 2p3/2 and 2p1/2 is
22
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
Fig. 4. X-ray photoelectron spectra of TC nanoparticles. (a) Ti 2p, (b) O 1s and (c) Co 2p.
increasing Co concentration because of increasing particle size [25]. The high dielectric constant measured at low frequency region might be attributed to the Maxwell-Wagner type interfacial polarization mechanism. The interfaces between the ferroelectric and ferromagnetic phases, which have significantly different conductivities, cause an additional polarization, which is termed as the interfacial polarization and boosts the dielectric constant [26]. Interfacial polarization responds very slowly to the external field; hence, it dominates in the lower frequency region and has no significant contribution in the higher frequency region. The values of e at 1 MHz are 60.5, 78.3 and 102, and resonance in e takes place after 11, 10 and 6 MHz, respectively, for TC1, TC2 and TC4 specimens. The discrimination in the value of resonant frequencies is due to their particles size effect [26]. A similar explanation is also applicable for loss tan d. Fig. 6 illustrates the variation of magnetization in Co-doped TiO2 nanoparticles at room temperature. From Fig. 6, it is clear that the magnetization (M–H curve) of doped specimens is greatly sensitive to Co dopant concentration. TC1 exhibits a weak ferromagnetic order even at room temperature. For TC2 and TC4, the curve assumes S-type shape. It suggests the emergence of ferromagnetic long range ordering with 2 and 4 mol% Co-doped TiO2 and the increase of magnetization with increasing Co concentrations. The values of saturation magnetization Ms are 8.75, 19.61 and 24.37 memu/g, respectively, observed for TC1, TC2 and TC4 specimens. This increase in magnetization could be due to the increase in the oxygen vacancies, which arises from oxygen deficient environment during the growth of the specimen. These observed values of magnetization show an enhancement over those reported in the literature [15–17]. In magnetic oxide semiconductors, the possible mechanisms responsible for FM exchange could be Zener’s double-exchange [27], RKKY interaction [28], superexchange [29] and the bound magnetic polaron model (BMP) [30]. In all these models, carrier density plays an important role in mediating the FM coupling among the magnetic ions. The carrier density, in turn, is influenced by the nature of the defects present in the materials, e.g. interstitial defects, cation vacancies and/or oxygen vacancies. Short range superexchange can be easily ruled out in this case because of the low concentration of Co in our specimens (r4%) and the particles size of TC, which is in the range of small nanoscale (r28 nm). This exchange is predominantly antiferromagnetic and appears for the doping concentration above the cation percolation limit (10%) [30].
Fig. 5. Variation of dielectric constant (e) and loss (tan d) with frequency.
measured to be 15.170.01, 15.770.03 and 15.870.01, respectively, for TC1, TC2 and TC4 specimens, which indicates that Co2 + is surrounded by oxygen in tetrahedral coordination. Moreover, the presence of the resonant shake up peaks along with the Co 2p doublets suggests that Co is in a high spin state [24]. This observation further confirms the substitution of Co on Ti sites. The variation of dielectric constants, e and loss (tan d) with frequency of all the TC specimens is shown in Fig.5. The dielectric constants were observed to decrease in the lower frequency region. But at higher frequency region, e remains constant. It is also clear from Fig. 5 that the dielectric constant increases with
Fig. 6. M–H hysteresis loops.
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
On the other hand, Zeners’ double-exchange mechanism cannot be invoked for carrying the ferromagnetic coupling in our samples because of the following two reasons: (i) it is also nearest neighbor interaction and (ii) it needs mix valence system, which is not the case with our specimens because in Fig. 4, Co is only in 2+ formal oxidation state. Another possibility could be the long range RKKY interaction, which can couple the magnetic moments ferromagnetically at larger distances. However, for this interaction to be brought into play one needs a ferromagnetic metal. Considering the semiconductor like behavior in our specimens, one can rule out the RKKY interactions to be operative. In the impurity band exchange model, ferromagnetism is formally explained by the formation of bound magnetic polaron [30]. An electron associated with the defects (e.g. interstitial defects due to nano-behavior, and oxygen vacancy in our case) will be confined in a hydrogenic orbital of radius r¼ e(m/mn)a0, with binding energy B.E.¼(mn/me2)RN. Here e is the dielectric constant of the host semiconductor, mn is the effective mass of the electron, a0 is the Bohr radius, m is the mass of electron and RN is the Rydberg constant. In this picture, the bound magnetic polaron formation takes place through the exchange interaction of the electron associated with the defect site and magnetic cation that resides within the localization radius r of the electron. Thus increasing carrier density leads to the formation of an impurity band in the band gap of the host material. When enough interstitial defects and oxygen vacancies and hence BMPs are present, they start to overlap with each other and eventually lead to the formation of shallow donor impurity bands. These impurity bands subsequently overlap with the Co 3d levels and ferromagnetically align the Co moments.
model. The dielectric measurements show e values up to 11, 10 and 6 MHz, respectively, for TC1, TC2 and TC4 specimens by the particles size effect, which indicates the applicability of Co-doped TiO2 semiconductor for the memory devices operated in the high frequency region.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16]
4. Conclusion In summary, we have fabricated magnetic semiconductors Ti1 xCoxO2 (TC) (x ¼0.01, 0.02 and 0.04) nanoparticles by a chemical route using PEG as surfactant. Analysis of the XRD and XPS data of TC nanoparticles indicates that cobalt in the form of Co2 + is incorporated into the TiO2 matrix. The crystalline structure is tetragonal rutile structure in all specimens; however, a few percentage of impurity phase is seen in TC4. The TEM measurements show average particle size of 24, 26 and 27 nm, respectively, for TC1, TC2 and TC4 specimens. FTIR measurement indicates the presence of Ti–O bonds in the frequency region of 450–432 cm 1. We also found an enhancement of ferromagnetism with small nanoparticles. The observed ferromagnetism in Co-doped TiO2 specimens can possibly be described on the basis of the spin split impurity exchange, which has its origin in BMP
23
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
R. Janisch, P. Gopal, N.A. Spaldin, J. Phys.: Condens. Matter 17 (2005) 657. H. Ohno, Science 281 (1998) 951. S. Sanvito, G. Theurich, N.A. Hill, J. Supercond. 15 (2002) 85. H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, Appl. Phys. Lett. 69 (1996) 363. C.W. Zou, H.J. Wang, M.L. Yin, M. Li, C.S. Liu, L.P. Guo, D.J. Fu, T.W. Kang, J. Cryst. Growth 312 (2010) 906. Z. Zuo, W. Huang, P. Han, Z. Li, Solid State Commun. 149 (2009) 2139. H. Sakuma, T. Nagamatsu, T. Kashiwakura, K. Ishii, J. Magn. Magn. Mater. 322 (2010) 786. A. Kubacka, B.B. Baeza, G. Colon, M.F. Garcia, Appl. Catal. B 93 (2010) 274. V. Shutthanandan, S. Thevuthasan, T. Droubay, S.M. Heald, M.H. Engelhard, D.E. McCready, S.A. Chambers, P. Nachimuthu, B.S. Mun, Nucl. Instrum., Methods Phys. Res. B 242 (2006) 198. Y.Z. Peng, W.D. Song, C.W. An, J.J. Qiu, J.F. chong, B.C. Lim, M.H. Hong, T. Liew, T.C. Chong, Appl. Phys. A 80 (2005) 565. X. Liu, S. Chen, M. Li, X. Wang, Thin Solid Films 515 (2007) 6744. Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.Y. Koshihara, H. Koinuma, Science 291 (2001) 854. M.D. Regulacio, S. Kar, E. Zuniga, G. Wang, N.R. Dollahon, G.T. Yee, S.L. Stoll, Chem. Mater. 20 (2008) 3368. K.C. Verma, R.K. Kotnala, M.C. Mathpal, N. Thakur, P. Gautam, N.S. Negi, Mater. Chem. Phys. 114 (2009) 576. K. Karthik, S.K. Pandian, K.S. Kumar, N.V. Jaya, Appl. Surf. Sci. 256 (2010) 4757. R.K. Singhal, A. Samariya, S. Kumar, Y.T. Xing, D.C. Jain, U.P. Deshpande, T. Shripathi, E. Saitovitch, C.T. Chen, Solid State Commun. 150 (2010) 1154. B. Ali, A.K. Rumaiz, A. Ozbay, E.R. Nawak, S.I. Shah, Solid State Commun. 149 (2009) 2210. M. Okutan, E. Basaran, H.I. Bakan, F. Yakuphanoglu, Physica B 364 (2005) 300. Y.Y. Wang, G.W. Zhou, T.D. Li, W.T. Qiao, Y.J. Li, Catal. Commun. 10 (2009) 412. A. Manivannan, M.S. Seehra, S.B. Majumder, R.S. Katiyar, Appl. Phys. Lett. 83 (2003) 111. T. Yu, X. Tan, L. Zhao, Y. Yin, P. Chen, J. Wei, J. Chem. Eng. 157 (2010) 86. H. Sakuma, T. Nagamatsu, T. Kashiwakura, K. Ishii, J. Magn. Magn. Mater. 322 (2010) 786. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, M.G.E. Muilenberg, HandBook of X-ray Photoelectron Spectroscopy (1979). J.W. Quilty, et al., Phys. Rev. Lett. 96 (2006) 027202. K.C. Verma, R.K. Kotnala, V. Verma, N.S. Negi, Thin Solid Films 518 (2010) 3320. C. Venkatesh, V. Srinivas, M. Panda, V.V. Rao, Solid State Commun. 150 (2010) 893. C. Zener, Phys. Rev 81 (1951) 440. D.J. Priour, E.H. Hwang, S.Das Sarma, Phys. Rev. Lett. 92 (2004) 117201. K. Kikoin, V. Fleurov, Phy. Rev. B 74 (2006) 174407. J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Structure and magnetic properties of Ti1 xCoxO2 nanoparticles prepared by chemical route Sunil Sharma a,c, Nagesh Thakur a, R.K. Kotnala b, Kuldeep Chand Verma c,n a
Department of Physics, Himachal Pradesh University, Shimla 171005, India National Physical Laboratory, New Delhi 110012, India c Department of Physics, Eternal University, Baru Sahib, H.P., India b
a r t i c l e i n f o
abstract
Article history: Received 30 August 2010 Received in revised form 3 February 2011 Accepted 14 February 2011 Communicated by. K. Nakajima Available online 18 February 2011
The structural, microstructural, dielectric and ferromagnetic behavior of Co-doped TiO2 (TC) nanoparticles have been studied. TC nanoparticles were prepared by a chemical synthesis route using polyethylene glycol as a surfactant. The polymer serves as a surfactant to encapsulate the cationic species in divided groups during the reaction that confines the size and morphology of the specimen. X-ray diffraction pattern has been used to confirm the crystalline structure (rutile or anatase) and average particle size is measured using Debye–Scherer’s relation. The particle’s size is also measured by transmission electron microscopy. Fourier transform infrared spectroscopy has been used to confirm the formation of Ti–O bond. X-ray photoemission spectroscopy suggests that the Ti and Co ions have the oxidation states 4+ and 2 + , respectively, present in the 2p region and confirm the presence of Ti–O–Co bonds. The variation in magnetization with Co concentration is studied. Dielectric measurements were also carried at room temperature to confirm the storage charge capability of TC semiconductor, and the nano-size effect to control the dielectric constant up to higher frequency region has been observed. The enhancement in magnetization at room temperature is also observed. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A2. Growth from solutions B1. Dilute magnetic semiconductors B2. TiO2
1. Introduction Dilute magnetic semiconductors (DMSs) based on II–VI semiconductors such as CdTe and ZnSe exhibit ferromagnetic behavior only at low temperature (Tc o4 K) [1]. Also, the DMS based on III–V semiconductors such as InAs and Mn doped GaAs offers ferromagnetic order at about 100 K [2,3]. However, these materials show limitations in the potential applications of DMS due to their low Curie temperature [4]. From theoretical predictions, the occurrence of ferromagnetism in wide band gap semiconductors like ZnO, GaN, etc., by doping certain 3d elements, the DMSs have attracted widespread research attention due to their potential applications of spintronics and optical devices [5–8]. TiO2 is an attractive wide band gap semiconductor for the fabrication of DMSs [9]. Recently, research has been done on the magnetism of TiO2based DMSs doped with transition metals such as Co, Mn, Fe, Ni and Cr [9–11]. The crystalline structures of TiO2 are different, with rutile being the most stable one. At low temperatures there is only a slight difference between the stability of rutile and
n Corresponding author at: Department of Physics, Eternal University, Baru Sahib, H.P., India. Tel.: +91 9418941286; fax: + 91 0177 2830775. E-mail address: [email protected] (K.C. Verma).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.02.023
metastable anatase and brookite phases. TiO2 is also used in catalysis, photocatalysis, dye-sensitized solar cells, etc. In magnetic semiconductors the host material is lightly doped with magnetic impurity, without forming a second phase. The presence of magnetic impurities results in spin polarization in the majority carrier band. This polarization is induced by exchange interaction among the magnetic dopant spins and the carriers. Room temperature ferromagnetism in Co-doped TiO2 as the main focus of research in recent years was reported by Matsumoto et al. [12], who produced Ti1 xCoxO2 films by combinatorial laser molecular beam epitaxy. If the ferromagnetism arises from the surface of Co ions, only a few Co ions contribute to the ferromagnetism because most of Co ions reside in the core of nanoparticles. Ferromagnetism with high Curie temperature depends on many parameters such as the preparation condition, particles size and amount of dopant level [4,13]. Besides the ferromagnetic and optical behavior, its remarkable dielectric constant makes it a promising material for various memory applications such as tunable mixers, phase shifter, voltage controlled oscillators, delay lines and for ultra-large-scale integration dynamic random access memory (DRAM) capacitor [14]. Several attempts have been made by researchers to prepare TiO2 DMS using a variety of techniques, including sol–gel, solid state reaction, micro-emulsion, hydrothermal processing, sol–gel auto-combustion, etc. [8,11,15]. However, most of them have
20
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
been directed towards the preparation of particles with a large size distribution in the range of a few micrometers economically, despite complicated experimental steps and high reaction temperatures. In the present study, a chemical synthesis route similar to a sol–gel technique but modified has been applied to synthesize DMS Ti1 xCoxO2 (x ¼0.01 (TC1), 0.02 (TC2) and 0.04 (TC4)) nanoparticles. Polyethylene glycol (PEG) is used as a surfactant to reduce the grain size. As a result, size of grains below 100 nm is observed with annealing temperature of 700 oC for crystallization. The experimental characterizations were performed by Fourier transform infrared (FTIR) spectroscopy; bonding information was obtained using X-ray photoemission spectroscopy (XPS), magnetization measurements and dielectric properties. We also report an enhancement in magnetization at nano-level of DMS TiO2 with Co doping, which was not seen in the reported study on bulk and nanoparticles of doped or undoped TiO2 [15–17]. Measuring the value of dielectric constant is important in various memory based devices, which was not found in the previous literature.
2. Experimental technique Ti1 xCoxO2 (TC) nanoparticles were prepared from the precursor solutions of tetra-n-butyl orthotitanate and cobalt 2-ethylhexanoate (C7H15COO)2Co in xylene. The solution was prepared by mixing the precursor solutions in the desired molar ratio of Ti:Co. The solution was refluxed at 60 oC with constant stirring for 2 h for homogeneous mixing. PEG, at 3% (in volume) of the TC solution, was added dropwise. PEG can decompose the organic residual from metal and also encapsulate the TC constituents into the smallest limit during the heating process. The solution was dried at 200 1C and heated at 700 1C for 3 h for crystallization. The phase structure and nanobehavior of TC specimens were studied by X-ray diffraction (XRD) using a PAN analytical X-Pert PRO system and microstructure by the transmission electron microscopy (TEM) using Hitachi H-7500. The effect of light on TC nanoparticles has been studied in the infrared region by FTIR using a Nicolet Avtar 5700 system. The photoemission spectrum was recorded using a VSW make photoelectron spectrometer. Magnetization measurements were performed at room temperature using a vibrating sample magnetometer (VSM735). For electrical measurements, the powder of TC nanoparticles heated at 700 1C was pressed into pellets using a pressure of 5 bar for 10 min. The pellets were sintered at a high temperature of 1000 1C for 5 h. The relative electrical permittivity (e) and dissipation factor (tan d) were measured in the frequency range from 100 Hz to 20 MHz using a Precision Impedance analyzer (WayneKerr 6500B).
3. Results and discussion Fig. 1 shows a representative XRD pattern of the TC nanoparticles heated at 700 oC. It shows that the major Bragg’s peaks in all the samples can be indexed (peaks are indexed using ASTM card) to tetragonal rutile phase of TiO2 with Miller indices (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1) and (2 2 0) with diffraction angle at 2y ¼27.44o, 36.08o, 39.19o, 41.24o, 44.05o, 54.33o and 56.64o, respectively, observed in all TC specimens. However, a few minor peaks were identified as an impurity phase of CoTiO3 in TC4 specimen. Such an impurity phase generally existing at higher Co concentration in TiO2 was also observed in the reported literature [6,18]. Furthermore, the peak position shifts to smaller angle with increasing of Co content, revealing of changes in lattice parameters. The values of lattice parameters for rutile phase of, ˚ ˚ a(A)¼4.595, 4.596 and 4.603 and c(A)¼2.969, 2.968 and 2.970 and the average particle’s size of (using Debye–Scherer’s relation)
Fig. 1. XRD patterns of Ti1 xCoxO2 (x¼ 0.01, 0.02 and 0.04) nanoparticles heated at 700 oC/3 h.
Table 1 Values of rutile Co-doped TiO2 nanoparticles of the lattice parameters (a and c), average particles size (x) from XRD, (x’) from TEM, dielectric constant (e) and saturation magnetization (M). Specimen
˚ a (A)
˚ c (A)
x (nm)
x’ (nm)
e(1 MHz)
M (emu/g)
Ti0.99Co0.01O2 Ti0.98Co0.02O2 Ti0.96Co0.04O2
4.595 4.596 4.603
2.969 2.968 2.970
25 27 28
24 26 27
60.5 78.3 102.0
8.75 19.61 24.37
25, 27 and 28 nm, respectively, are calculated from XRD data for TC1, TC2 and TC4 nanoparticles. The values of a, c and average particle’s size are given in Table 1. The variations in the values of lattice parameters exist because the ionic radii of the substituted ˚ ion is larger than that of parent Ti4 + (0.64 A). ˚ The Co2 + (0.838 A) smaller crystalline size indicates that the preparation method is suitable for nano fabrication of TC specimens because in this case PEG is used as a surfactant. Fig. 2(a)–(c) presents typical TEM images of the Co-doped TiO2 nanoparticles. The particles show high monodispersity in size and the estimated average particle size is 24, 26 and 27 nm, respectively, for TC1, TC2 and TC4 specimens. Fig. 3 shows the FTIR spectra of Ti1 xCoxO2 nanoparticles acquired in the range 4000–200 cm–1 in KBr pellets. It shows several weak absorption bands in the region 4000–1500 cm–1 probably due to atmospheric moisture and CO2. However, as the spectra was recorded after heating the samples at 700 1C the presence of vibrations such as O–H, C–H, etc. characteristic of this region cannot be expected. The FTIR spectral bands in the frequency region of interest 450–432 cm 1 are assigned to the stretching vibrations of Ti–O bonds [19].
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
21
Fig. 2. TEM images of (a) TC1, (b) TC2 and (c) TC4 specimens.
Fig. 3. FTIR spectra of TC nanoparticles.
In order to check the presence of the oxidation state of titanium, oxygen and cobalt in these TC nanoparticles, high resolution XPS was performed. Fig. 4 shows the Ti 2p, O 1s and Co 2p regions for all TC specimens. Fig. 4(a) shows the Ti 2p XPS spectra for TC1, TC2 and TC4 specimens. The core levels of Ti 2p3/2
and Ti 2p1/2 in TC1 are observed at 458.81 and 464.36 eV, respectively. These peaks are assigned to Ti4 + [20] and the separation between these lines ( 5:55 eV) is slightly less than standard binding energy value (5.7 eV) for TiO2 because of 1% Co doping. After increasing Co doping in TiO2 lattice, the intensities of the Ti 2p peaks show depression, which was expected. In addition, the binding energies of Ti 2p peaks shift to lower energies for TC2 and TC4 than TC1, while the binding energy of O 1s peak shifts in the opposite direction (Fig. 4(b)) with a depression in intensity compared to that of TC1. The lower binding energy of Ti 2p with increasing Co doping resulted from the increased electron cloud density around Ti, which indicates that the atom possessing lower electronegativity was introduced into the TiO2 crystal structure [20]. The observed value of binding energy for TC2 and TC4 is 5.34 and 5.23, respectively. In Fig. 4(b), the O 1s XPS spectrum shows a prominent peak at 531.61 eV for TC1, which is slightly larger than that of pure TiO2 ( 530 eV) [21] ascribed to the Ti–O–Co bonds in TC1. A similar behavior is observed in TC2 ( 531.8 eV) and TC4 (532.06 eV) specimens with a slight increment in binding energy, which indicate that the Ti–O–Co bonds are quite stronger with increasing Co in TiO2. Fig. 4(c) shows Co 2p spectra of TC1, TC2 and TC4 specimens. These spectra show four peaks: the 2p3/2 and 2p1/2 spin doublet and their corresponding shake-up resonant transition peaks. The positions of Co 2p3/2 and Co 2p1/2 peaks are in agreement with those for Co in the oxidation state of 2+ in CoO [22]. If Co would have been present in the form of metallic clusters, the difference between Co 2p3/2 and 2p1/2 peaks would have been 15.05 [23]. In the present paper, the separation between 2p3/2 and 2p1/2 is
22
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
Fig. 4. X-ray photoelectron spectra of TC nanoparticles. (a) Ti 2p, (b) O 1s and (c) Co 2p.
increasing Co concentration because of increasing particle size [25]. The high dielectric constant measured at low frequency region might be attributed to the Maxwell-Wagner type interfacial polarization mechanism. The interfaces between the ferroelectric and ferromagnetic phases, which have significantly different conductivities, cause an additional polarization, which is termed as the interfacial polarization and boosts the dielectric constant [26]. Interfacial polarization responds very slowly to the external field; hence, it dominates in the lower frequency region and has no significant contribution in the higher frequency region. The values of e at 1 MHz are 60.5, 78.3 and 102, and resonance in e takes place after 11, 10 and 6 MHz, respectively, for TC1, TC2 and TC4 specimens. The discrimination in the value of resonant frequencies is due to their particles size effect [26]. A similar explanation is also applicable for loss tan d. Fig. 6 illustrates the variation of magnetization in Co-doped TiO2 nanoparticles at room temperature. From Fig. 6, it is clear that the magnetization (M–H curve) of doped specimens is greatly sensitive to Co dopant concentration. TC1 exhibits a weak ferromagnetic order even at room temperature. For TC2 and TC4, the curve assumes S-type shape. It suggests the emergence of ferromagnetic long range ordering with 2 and 4 mol% Co-doped TiO2 and the increase of magnetization with increasing Co concentrations. The values of saturation magnetization Ms are 8.75, 19.61 and 24.37 memu/g, respectively, observed for TC1, TC2 and TC4 specimens. This increase in magnetization could be due to the increase in the oxygen vacancies, which arises from oxygen deficient environment during the growth of the specimen. These observed values of magnetization show an enhancement over those reported in the literature [15–17]. In magnetic oxide semiconductors, the possible mechanisms responsible for FM exchange could be Zener’s double-exchange [27], RKKY interaction [28], superexchange [29] and the bound magnetic polaron model (BMP) [30]. In all these models, carrier density plays an important role in mediating the FM coupling among the magnetic ions. The carrier density, in turn, is influenced by the nature of the defects present in the materials, e.g. interstitial defects, cation vacancies and/or oxygen vacancies. Short range superexchange can be easily ruled out in this case because of the low concentration of Co in our specimens (r4%) and the particles size of TC, which is in the range of small nanoscale (r28 nm). This exchange is predominantly antiferromagnetic and appears for the doping concentration above the cation percolation limit (10%) [30].
Fig. 5. Variation of dielectric constant (e) and loss (tan d) with frequency.
measured to be 15.170.01, 15.770.03 and 15.870.01, respectively, for TC1, TC2 and TC4 specimens, which indicates that Co2 + is surrounded by oxygen in tetrahedral coordination. Moreover, the presence of the resonant shake up peaks along with the Co 2p doublets suggests that Co is in a high spin state [24]. This observation further confirms the substitution of Co on Ti sites. The variation of dielectric constants, e and loss (tan d) with frequency of all the TC specimens is shown in Fig.5. The dielectric constants were observed to decrease in the lower frequency region. But at higher frequency region, e remains constant. It is also clear from Fig. 5 that the dielectric constant increases with
Fig. 6. M–H hysteresis loops.
S. Sharma et al. / Journal of Crystal Growth 321 (2011) 19–23
On the other hand, Zeners’ double-exchange mechanism cannot be invoked for carrying the ferromagnetic coupling in our samples because of the following two reasons: (i) it is also nearest neighbor interaction and (ii) it needs mix valence system, which is not the case with our specimens because in Fig. 4, Co is only in 2+ formal oxidation state. Another possibility could be the long range RKKY interaction, which can couple the magnetic moments ferromagnetically at larger distances. However, for this interaction to be brought into play one needs a ferromagnetic metal. Considering the semiconductor like behavior in our specimens, one can rule out the RKKY interactions to be operative. In the impurity band exchange model, ferromagnetism is formally explained by the formation of bound magnetic polaron [30]. An electron associated with the defects (e.g. interstitial defects due to nano-behavior, and oxygen vacancy in our case) will be confined in a hydrogenic orbital of radius r¼ e(m/mn)a0, with binding energy B.E.¼(mn/me2)RN. Here e is the dielectric constant of the host semiconductor, mn is the effective mass of the electron, a0 is the Bohr radius, m is the mass of electron and RN is the Rydberg constant. In this picture, the bound magnetic polaron formation takes place through the exchange interaction of the electron associated with the defect site and magnetic cation that resides within the localization radius r of the electron. Thus increasing carrier density leads to the formation of an impurity band in the band gap of the host material. When enough interstitial defects and oxygen vacancies and hence BMPs are present, they start to overlap with each other and eventually lead to the formation of shallow donor impurity bands. These impurity bands subsequently overlap with the Co 3d levels and ferromagnetically align the Co moments.
model. The dielectric measurements show e values up to 11, 10 and 6 MHz, respectively, for TC1, TC2 and TC4 specimens by the particles size effect, which indicates the applicability of Co-doped TiO2 semiconductor for the memory devices operated in the high frequency region.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16]
4. Conclusion In summary, we have fabricated magnetic semiconductors Ti1 xCoxO2 (TC) (x ¼0.01, 0.02 and 0.04) nanoparticles by a chemical route using PEG as surfactant. Analysis of the XRD and XPS data of TC nanoparticles indicates that cobalt in the form of Co2 + is incorporated into the TiO2 matrix. The crystalline structure is tetragonal rutile structure in all specimens; however, a few percentage of impurity phase is seen in TC4. The TEM measurements show average particle size of 24, 26 and 27 nm, respectively, for TC1, TC2 and TC4 specimens. FTIR measurement indicates the presence of Ti–O bonds in the frequency region of 450–432 cm 1. We also found an enhancement of ferromagnetism with small nanoparticles. The observed ferromagnetism in Co-doped TiO2 specimens can possibly be described on the basis of the spin split impurity exchange, which has its origin in BMP
23
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
R. Janisch, P. Gopal, N.A. Spaldin, J. Phys.: Condens. Matter 17 (2005) 657. H. Ohno, Science 281 (1998) 951. S. Sanvito, G. Theurich, N.A. Hill, J. Supercond. 15 (2002) 85. H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, Appl. Phys. Lett. 69 (1996) 363. C.W. Zou, H.J. Wang, M.L. Yin, M. Li, C.S. Liu, L.P. Guo, D.J. Fu, T.W. Kang, J. Cryst. Growth 312 (2010) 906. Z. Zuo, W. Huang, P. Han, Z. Li, Solid State Commun. 149 (2009) 2139. H. Sakuma, T. Nagamatsu, T. Kashiwakura, K. Ishii, J. Magn. Magn. Mater. 322 (2010) 786. A. Kubacka, B.B. Baeza, G. Colon, M.F. Garcia, Appl. Catal. B 93 (2010) 274. V. Shutthanandan, S. Thevuthasan, T. Droubay, S.M. Heald, M.H. Engelhard, D.E. McCready, S.A. Chambers, P. Nachimuthu, B.S. Mun, Nucl. Instrum., Methods Phys. Res. B 242 (2006) 198. Y.Z. Peng, W.D. Song, C.W. An, J.J. Qiu, J.F. chong, B.C. Lim, M.H. Hong, T. Liew, T.C. Chong, Appl. Phys. A 80 (2005) 565. X. Liu, S. Chen, M. Li, X. Wang, Thin Solid Films 515 (2007) 6744. Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.Y. Koshihara, H. Koinuma, Science 291 (2001) 854. M.D. Regulacio, S. Kar, E. Zuniga, G. Wang, N.R. Dollahon, G.T. Yee, S.L. Stoll, Chem. Mater. 20 (2008) 3368. K.C. Verma, R.K. Kotnala, M.C. Mathpal, N. Thakur, P. Gautam, N.S. Negi, Mater. Chem. Phys. 114 (2009) 576. K. Karthik, S.K. Pandian, K.S. Kumar, N.V. Jaya, Appl. Surf. Sci. 256 (2010) 4757. R.K. Singhal, A. Samariya, S. Kumar, Y.T. Xing, D.C. Jain, U.P. Deshpande, T. Shripathi, E. Saitovitch, C.T. Chen, Solid State Commun. 150 (2010) 1154. B. Ali, A.K. Rumaiz, A. Ozbay, E.R. Nawak, S.I. Shah, Solid State Commun. 149 (2009) 2210. M. Okutan, E. Basaran, H.I. Bakan, F. Yakuphanoglu, Physica B 364 (2005) 300. Y.Y. Wang, G.W. Zhou, T.D. Li, W.T. Qiao, Y.J. Li, Catal. Commun. 10 (2009) 412. A. Manivannan, M.S. Seehra, S.B. Majumder, R.S. Katiyar, Appl. Phys. Lett. 83 (2003) 111. T. Yu, X. Tan, L. Zhao, Y. Yin, P. Chen, J. Wei, J. Chem. Eng. 157 (2010) 86. H. Sakuma, T. Nagamatsu, T. Kashiwakura, K. Ishii, J. Magn. Magn. Mater. 322 (2010) 786. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, M.G.E. Muilenberg, HandBook of X-ray Photoelectron Spectroscopy (1979). J.W. Quilty, et al., Phys. Rev. Lett. 96 (2006) 027202. K.C. Verma, R.K. Kotnala, V. Verma, N.S. Negi, Thin Solid Films 518 (2010) 3320. C. Venkatesh, V. Srinivas, M. Panda, V.V. Rao, Solid State Commun. 150 (2010) 893. C. Zener, Phys. Rev 81 (1951) 440. D.J. Priour, E.H. Hwang, S.Das Sarma, Phys. Rev. Lett. 92 (2004) 117201. K. Kikoin, V. Fleurov, Phy. Rev. B 74 (2006) 174407. J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173.