J. Cent. South Univ. Technol. (2010) 17: 239-243

DOI: 10.1007/s11771-010-0037-z                                                                                                                 

Coexistence of antiferromagnetic and ferromagnetic in Mn-doped anatase TiO₂ nanowires

LI Hong-mei(李红梅)^{1, 2}, LIU Min(刘敏)^{1, 2}, ZENG Yang-su(曾阳素)^{1, 2}, HUANG Tong-cheng(黄同成)^{1, 2}

1. Department of Information Engineering, Shaoyang University, Shaoyang 422000, China;

2. Institute of Laser and Information Technology, Shaoyang University, Shaoyang 422000, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: A series of Mn-doped TiO₂ nanowires (NWs) were prepared by hydrothermal method at the mole fraction of Mn changing from 0 to 12.0%. X-ray powder diffraction (XRD) analysis shows that all the samples have pure anatase structure. SEM and TEM studies show that the diameter and the length of the Mn-doped TiO₂ NWs are larger than those of the undoped TiO₂ NWs. Energy dispersive X-ray spectroscopy (EDX) reveals that the samples are composed of Ti, Mn and O. According to magnetization measurements, all samples show ferromagnetic behavior, but only the undoped TiO₂ NWs are completely ferromagnetic with a saturated magnetization about 1.0 mA?m²/kg. Mn-doped TiO₂ samples exhibit antiferromagnetic and ferromagnetic (AF-FM) behaviors simultaneously. Photoluminescence (PL) spectra demonstrate the existence of MnO₂ sublattice. These observations indicate that an AF-WF crossover is induced by the coexistence of TiO₂ sublattice and MnO₂ sublattice.

Key words: TiO₂; nanowires; Mn; antiferromagnetism; ferromagnetism

1 Introduction

A variety of doped-semiconducting materials possess two properties: semiconductor nature and ferromagnetic behavior, which are called diluted magnetic semiconductors (DMS). DMS materials have attracted much attention in recent years for their potential spintronics applications. Since Co-doped anatase TiO₂ thin film with room temperature (RT) ferromagnetism (FM) was found by MATSUMOTO et al [1], TiO₂-based DMS materials have been studied vigorously over recent years. However, the origin of magnetic moment, the coupling mechanism, and the precise state of the impurities remain unknown.

The fully stoichiometrical TiO₂ is a non-magnetic semiconductor, with a band gap of about 3.2 eV spanned by O 2p and Ti 3d states (for anatase structure) [2]. Theoretically, transition metal doped TiO₂ can be transformed into DMS. Many research groups have focused their work on doping TiO₂ with Co, Cu, and Fe [3-5]. However, there are a few reports on Mn-doped TiO₂. Great difference also exists in the results. Theoretical calculation revealed that Ti_1-xMn_xO₂ was antiferro-magnetic (AF) [6], while RT FM was found in Mn-doped TiO₂ thin films by WANG et al [7]. They thought that Mn strengthened RT FM of TiO₂. In contrast, HONG et al  [8] revealed that Mn did not play any key role in inducing the FM of TiO₂ and Mn, even decreased the FM of TiO₂ when its content reached some values.

In this work, the structure and the magnetic properties of Mn-doped TiO₂ NWs prepared by a simple and low temperature hydrothermal method were investigated, and a special phenomenon of AF-WF coexistence in Mn-doped TiO₂ NWs was analyzed.

TiO₂ NWs were synthesized by a typical hydrothermal method. All chemicals were used as received without further purification. Commercial TiO₂ powders (mixed with anatase structure and rutile structure), sodium hydroxide (NaOH), ethanol (C₂H₅OH) and potassium permanganate (KMnO₄) were all analytical reagents (AR). First, 10 mol/L NaOH solution was attained by solving NaOH in the mixed solution of deionized water and ethanol; the volume ratio of deionized water to ethanol was 1:1. 30 mL of this solution was mixed with 1 g TiO₂ powder and ultrasonic- vibrated for 5 min in order to make the solution uniform. Then, different contents of KMnO₄ were mixed with the solvent. The mole fraction of Mn was changed from 0 to 12%. The mixed solution was ultrasonic-vibrated for another 5 min and transferred into a teflon-lined stainless steel autoclave with 50 mL capacity. The autoclave was maintained at 150 ℃ under autogenous pressure for  15 h, and then cooled to room temperature.

The products were collected by centrifugation and thoroughly washed with 0.1 mol/L HNO₃ aqueous solution, and then washed with deionized water until the pH value of the washing solution was about 7.0, finally, dried at 60 ℃ for several hours in air.

The products were characterized by XRD using a Philips X’Pert MRD X-ray diffractometer with Cu K_α radiation (λ=0.154 056 nm). The images of SEM were recorded on an FEI QUANTA 200F analyzer. The images of TEM were recorded on a Hitachi H-9000 NAR microscope. Photoluminescence (PL) spectra were obtained using a Hitachi F-4500 fluorophotometer with the excited wavelength of 325 nm at room temperature. Magnetic properties were measured using an alternative gradient magnetometer.

Fig.1 shows XRD patterns of the raw TiO₂ powders and the TiO₂ NWs. It can be seen that anatase and rutile phases coexist in the raw TiO₂ powders. However, the obtained TiO₂ NWs show typical anatase structure (JCPDS No. 21-1272), after hydrothermal treatment. No metallic Mn precipitation was detected in XRD. As we know, KMnO₄ is a strong oxidizer, which can react with ethanol or water to form Mn⁴⁺ ions. The ionic radius and coordination number of Mn⁴⁺ are close to those of Ti⁴⁺. Therefore, the lattice of TiO₂ did not change significantly. This is consistent with the previous published data for Mn-doped TiO₂ thin films [8].

Fig.1 XRD patterns of raw TiO₂ powders (a), and 0% (b), 2.4% (c), 7.2% (d) and 12.0% (e) Mn-doped TiO₂ NWs

Fig.2 shows the SEM images of TiO₂ NWs. It can be seen that the undoped TiO₂ NWs are straight and uniform with smooth surface (Fig.2(a)). The diameter of the NWs is about 50 nm and the length of NWs is less than 1 μm. A few nanoparticles do not change to NWs because a low temperature, 150 ℃, is used. However, Mn-doped TiO₂ NWs exhibit different results. Figs.2(b)- (d) reveal that Mn-doped TiO₂ NWs are no longer straight and have rough surfaces. The curvature and the roughness of Mn-doped samples increase with the increase of the mole fraction of Mn. Both their diameter and length are larger than those of the undoped TiO₂ NWs, reaching several hundred nanometers and several micrometers or several tens micrometers, respectively. These indicate that Mn⁴⁺ can assist the growth of TiO₂ NWs. The possible speculation is that Mn⁴⁺ can help TiO₂ NWs to release the surface tension and reduce the free energy. These are beneficial to the growth of TiO₂ NWs. On the other hand, Mn-doped TiO₂ NWs cannot keep straight any more and have some rough materials on the surface. So, with the increase of KMnO₄ content, corresponding to the increase of Mn ions doped into TiO₂ NWs, the surfaces of TiO₂ NWs become rougher.

Fig.2 SEM images of TiO₂ NWs doped with different mole fractions of Mn: (a) 0%; (b) 2.4%; (c) 7.2%; (d) 12.0%

Fig.3 shows the EDX spectrum of 2.4% Mn-doped TiO₂ NWs. It reveals that the sample is composed of Mn, Ti and O, which demonstrates that Mn is doped in TiO₂ NWs. Fig.4 shows the TEM images of Mn-doped TiO₂ NWs, which present rough surfaces more clearly and coincide with the SEM images.

Fig.3 EDX result of 2.4% Mn-doped TiO₂ NWs

Fig.4 TEM images of 2.4% (a) and 12.0% (b) Mn-doped TiO₂ NWs

Fig.5 presents the magnetic hysteresis (M-H) curves of the samples measured at RT. As shown in  Fig.5, all the samples have clear magnetic hysteresis loops. However, only undoped TiO₂ sample shows clear RT FM. And its saturated magnetization is about 1.0 mA?m²/kg. Mn-doped samples exhibit AF-FM behaviors simultaneously. Their magnetizations are unsaturated even in 1.6 MA/m field (not shown). Similar behavior was observed in Cu-doped TiO₂ and Fe-doped TiO₂ [4-5], but never observed in Mn-doped TiO₂ samples so far. Noncollinear (canted) spin arrangements in two sublattices were used to understand this phenomenon [9]. In canted spin systems, the AF order is along one axis and the FM order is along another axis. It is proved that MnO₂ has AF [10]. Furthermore, KAMINSKI and DAS- SARMA [11] found that TiO₂ with oxygen defects (Vo) has RT FM. In our Mn-doped TiO₂ samples, the possible speculation is that FM arises from TiO₂ lattice with Vo and AF arises from MnO₂ lattice. Fig.6 shows that with the increase of mole fraction of Mn, the coercivities (H_c) of samples decrease. This indicates that FM decreases with the increase of mole fraction of Mn. As initial mole fraction of Mn increases from zero, remnant magnetization (RM) of the system gradually rises to a maximum at around 7% Mn, then decreases rapidly, and finally, this AF-WF coexistence phenomenon almost disappears at RM around at 12% Mn (Fig.7).

Fig.5 M-H curves of Mn-doped TiO₂ NWs at 300 K

Fig.6 Coercivities of samples at RT vs mole fraction of Mn

Fig.7 RM of samples vs mole fraction of Mn at RT

In order to investigate the defects in the samples, PL measurements were employed. As shown in Fig.8, the maximum peaks at 3.39, 3.38 and 3.36 eV samples with Mn mole fractions of 2.4%, 7.2% and 12.0%, respectively. These peaks can be attributed to band-to- band recombination whose values should be equal to the values of E_g [12]. However, the values of these peaks are larger than 3.2 eV, the regular E_g of anatase TiO₂. This may be related to band gap broaden caused by quantum confinement effect when the diameter is reduced to nano-scale. However, band gap broaden is not just caused by quantum confinement effect. Someone thought that lattice distortions are also contributed to the change of band gap. WUNDERLICH et al [13] believed that distortion of the lattice made E_g higher, and with the increase of distortion, E_g decreases again. In our samples, a possible explanation of the higher E_g can be given if we consider a distortion of TiO₂ lattice by doping Mn. Because of doping Mn, many MnO₂ lattices are formed, which bring distortions to TiO₂ lattice. At the beginning, these distortions lead to an increase of E_g. With the increase of mole fraction of Mn, the degree of disorder in TiO₂ lattices increases and E_g decreases finally. The peaks at 3.13, 2.74 and 2.65 eV appear in all Mn-doped TiO₂ NWs. The emission peak at 3.13 eV results from electronic transitions between MnO₂ and TiO₂ nanointerfaces. This is similar to the PL spectra observed by PALOMINO-MERINO et al [14]. It can be seen that the intensity ratio of 3.13 eV to the maximum peak increases with the increase of the mole fraction of Mn. So we can infer that there are more electrons transferring to MnO₂ with the increase of mole fraction of Mn, which indicates that the number of MnO₂ lattice increases with the increase of mole fraction of Mn. Two peaks at 2.74 and 2.65 eV existing in all samples are attributed to oxygen defects of TiO₂ [15]. The interval of the two peaks is 0.09 eV. So, in this case, we attribute 2.74 eV to Ti⁴⁺ adjacent to oxygen vacancies (Vos) and 2.65 eV to Vos [14].

Fig.8 PL spectra of 2.4%, 7.2%, 12.0% Mn-doped TiO₂ NWs

Theoretical calculation reveals that Ti_1-xMn_xO₂ is AF [6]. However, FM was found in Mn-doped TiO₂ thin films [7-8]. Different from thin films usually prepared at high temperature or with a high temperature post treatment, in this work, TiO₂ NWs were prepared by a low temperature hydrothermal method. So many defects exist in TiO₂ NWs, such as undercoordinated surface Ti sites and Vos. Vos are considered to be the origin of RT FM of TiO₂. The PL peaks at 2.74 and 2.65 eV confirm the existence of Vos. The PL peak at 3.13 eV, which results from electronic transitions between intraband shallow levels at the nanointerfaces between MnO₂ and TiO₂, proves the existence of MnO₂ sublattices [14]. As demonstrated before, TiO₂ with Vos has FM and MnO₂ has AF [10-11]. So, the reason for AF-FM in Mn-doped TiO₂ NWs is the coexistence of TiO₂ lattice and MnO₂ lattice. With the increase of mole fraction of Mn, E_g of Mn-doped TiO₂ NWs increases firstly and decreases finally, and the intensity ratio of 3.13 eV to the maximum peak increases. These indicate that the number of MnO₂ lattice increases with increasing mole fraction of Mn. Furthermore, H_c and RM of the Mn-doped samples decrease with the increase of mole fraction of Mn and completely show AF at around 12% Mn. The possible reason is that Vos of TiO₂ lattice decrease because MnO₂ lattice increases or more Mn substitutes Ti in TiO₂ with the increase of mole fraction of Mn. So, with the increase of mole fraction of Mn, both H_c and RM decrease and show AF at RT finally. It is also found that Mn does not have any contribution to FM of TiO₂ NWs. In contrast, Mn ions have destructive effect on FM of TiO₂.

4 Conclusions

(1) Mn-doped TiO₂ nanorods are prepared by a simple and low temperature hydrothermal method. All of the samples are of anatase structure. The diameter and the length of the Mn-doped NWs are larger than those of the undoped TiO₂ NWs.

(2) The magnetic property of Mn-doped TiO₂ NWs is different from that of undoped TiO₂ NWs with FM and has AF-WF at RT.

(3) Further studies show that, FM arises from TiO₂ lattices with Vos and AF arises from MnO₂ lattices. Due to canted spin arrangements in these two sublattices, AF and FM coexist in Mn-doped TiO₂ NWs simultaneously. FM at RT decreases with the increase of mole fraction of Mn, and Mn-doped samples show complete AF finally.

(4) Mn ions do not contribute to the FM of TiO₂, instead, they have destructive effect on FM of TiO₂.

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Foundation item: Project(06JJ2031) supported by Natrual Science Foundation of Hunan Province, China; Project(06A065) supported by Education Department of Hunan Province, China

Received date: 2009-03-10; Accepted date: 2009-06-02

Corresponding author: ZENG Yang-su, PhD, Professor; Tel: +86-739-5305947; E-mail: zengyangsu@126.com

(Edited by CHEN Wei-ping)