Applied Surface Science 257 (2011) 4505–4509
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Preparation, magnetic characterization, and optical band gap of EuTiO3 nanoparticles T. Wei a , H.P. Liu a , Y.F. Chen a , H.Y. Yan a , J.-M. Liu b,c,∗ a b c
College of Science, Civil Aviation University of China, Jinbei Road 2898, Tianjin 300300, China Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 6 November 2010 Received in revised form 21 December 2010 Accepted 21 December 2010 Available online 28 December 2010
a b s t r a c t Perovskite-type polycrystalline EuTiO3 (ETO) nanoparticles were synthesized using the simple sol–gel technique. We investigated the magnetic properties of the as-prepared ETO nanoparticles and revealed the G-AFM phase below the Néel temperature (TN ). Furthermore, the transition from the G-AFM order to the FM order, induced by magnetic ﬁeld, was also demonstrated. The optical band gap of 1.03 eV for the as-prepared ETO nanoparticles was determined from the UV–visible absorption spectra. © 2010 Elsevier B.V. All rights reserved.
PACS: 81.07.−b 78.67.−n Keywords: EuTiO3 Nanaoparticles Magnetic properties
1. Introduction Nanoscale materials with fundamental and technological interests have displayed a wide range of electrical, magnetic, and optical properties due to their low dimensionality and quantum conﬁnement effect [1–4]. Among them, oxide nanoparticles have exhibited a variety of signiﬁcant properties which make them useful for a wide range of applications including sensors, catalysts, magnetic devices, and so on. Referring to the fabrication of nanoparticles, even though a series of methods have been used, the sol–gel approach has been widely employed. Through this method, one can easily control the reaction pathways on a molecular level, enabling the synthesis of nanomaterials with high crystallinity and well deﬁned morphologies. Recently, considerably renewed interests have been focused on EuTiO3 (ETO) owing to its rich physical properties, such as magnetodielectric effect, quantum ﬂuctuations feature, low temperature magnetic properties, lattice dynamics, and so on [5–10]. ETO is a peculiar incipient ferroelectric which adopts a cubic perovskite
∗ Corresponding author at: Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China. E-mail address: [email protected]
(J.-M. Liu). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.112
structure (Pm3m, space group) with a lattice parameter of 3.905 A˚ at room temperature [5–9]. Physically, ETO displays many similarities with conventional incipient ferroelectric SrTiO3 (STO). However, except from the low temperature (T < 30 K) quantum ﬂuctuations, ETO also has an additional magnetic order parameter and exhibits the G-antiferromagnetic (G-AFM) order below ∼5 K (Néel temperature, TN ) due to the localized 4f moments on the Eu site (S = 7/2) [5–10]. At present, different structural and magnetic properties on ETO have been reported, however, conventional understanding of the magnetism of ETO is still in debate. For example, Fujita et al. reported the ferromagnetic (FM) behavior of pulsed laser deposition (PLD) prepared single crystalline ETO epitaxial thin ﬁlms on STO substrate [11,12], while AFM order in high quality ETO epitaxial thin ﬁlms on STO substrate prepared by molecular beam epitaxy (MBE) was reported by Lee et al. . Considering the preparation process of thin ﬁlms, extrinsic effects have important inﬂuence on their physical properties [11–15]. Thus, to explore the intrinsic magnetic behavior of ETO with low dimensionality, other nanostructures, such as nanoparticles, nanowires, and nanotubes, are constructive. It is believed that these substrate-free nanostructures will shed light on the current debate on the magnetic characterization of ETO. On the other hand, owing to the similarity between ETO and STO , it is predicted that ETO will contain interesting optical properties for photocatalytic applications. For the as-prepared ETO nanostructures, thus, it is important to study the optical absorption
T. Wei et al. / Applied Surface Science 257 (2011) 4505–4509
properties which are relevant to the energy band of semiconductor catalyst. In this work, we synthesized high quality ETO nanoparticles using the simple sol–gel method. The magnetic properties of as-prepared nanoparticles were investigated through magnetic measurement and conﬁrmed by speciﬁc heat characterization. Furthermore, the UV–vis absorption spectra were measured for the as-prepared ETO nanoparticles which reveal that ETO nanoparticles have a direct optical band gap of 1.03 eV.
2. Experimental procedures In this experiment europia (Eu2 O3 ) and tetrabutyl titanate (Ti(OC4 H9 )4 ) are chosen as the europium (Eu) and titanium (Ti) sources, respectively. First, 0.03 molar Eu2 O3 was carefully measured and located in a beaker. Then, 0.08 molar nitric acid (HNO3 ) was added in the beaker. The solution was continuously stirred until the Eu2 O3 was absolutely dissolved. Second, 0.06 molar tetrabutyl titanate (Ti(OC4 H9 )4 ) was carefully measured and 2methoxyethanol (C3 H8 O2 ) (volume is about 100 mL) was added to dilute it. The obtained solution was stirred at 50 ◦ C about 0.5 h. Third, mixing the above two solutions and continuously stirring about half an hour. Then, glycol (C2 H6 O2 ) in 1:1 molar ratio with respect to the metal nitrates was added to the solution as a dispersant. The ﬁnal solution was stirred at 50 ◦ C for sufﬁcient time to obtain the sol, which was kept at 90 ◦ C for 3 days to form the dried gel powder. The obtained powder was ﬁrstly annealed at 350 ◦ C at air about 10 h to remove the organic material. Then, part of the ﬁnal powder was annealed at 800 ◦ C in pure H2 for 20 min and the obtained ETO nanoparticles labeled as ETO1. Furthermore, to conﬁrm the effects of oxygen vacancies (OVs) on magnetic properties, other parts of the ﬁnal powder were annealed at 800 ◦ C and 700 ◦ C in pure H2 for 10 min and 20 min and the obtained ETO nanoparticles labeled as ETO2 and ETO3, respectively. Note that we would focus our attention on the physical properties of ETO1 which can reveal the intrinsic properties of ETO nanoparticles owing to avoid the disturbance of defects (for example, OVs) as shown in the following part. The structure and morphology of the ETO nanoparticles were investigated by X-ray diffraction (XRD) with Cu K˛ radiation and transmission electron microscopy (TEM). The superconducting quantum interference device (SQUID) magnetometer was employed to characterize the magnetization (M) as a function of temperature (T) and magnetic ﬁeld (H). The speciﬁc heat measurement was carried out by means of the relaxation method and the data were collected during the cooling process from 15 to 2 K
Fig. 1. XRD pattern of as-prepared ETO1 nanoparticles at room temperature.
under different magnetic ﬁelds, using the speciﬁc heat option of the Quantum Design physical property measurement system (PPMS). Electron paramagnetic resonance (EPR) measurements are performed to detect the OVs concentration. UV–vis absorption spectra were measured on a UV–vis spectrophotometer at room temperature. 3. Results and discussion Fig. 1 presents the XRD pattern of as-prepared ETO1 nanoparticles at room temperature. It is revealed that the ETO1 nanoparticles are highly crystallized and exhibit a single cubic perovskite structure [5,6,13]. No other non-perovskite phases such as Eu2 Ti2 O7 , Eu2 O3 , and TiO2 , are detected in current XRD spectrum . The diffraction peaks shown in Fig. 1 are consistent with the standard JPCD database of ETO ceramics. Furthermore, Fig. 2(a) displays the representative transmission electron microscopy (TEM) images of ETO1 nanoparticles. The averaged grain size is measured directly from these images and is ∼100 nm. Moreover, Fig. 2(b) shows the high-resolution TEM (HRTEM) picture of the ETO1 nanoparticles and the selected area electron diffraction pattern (SAED) shown in the inset of Fig. 2(b). Both of them conﬁrm that the ETO1 nanoparticles are well crystallized with a single cubic perovskite structure. In addition, X-ray energy dispersive spectroscopy (EDS) attached to the TEM was used to measure the Eu:Ti ratio. The measured points (crossing point 1, 2, 3, 4, and 5) selected from Fig. 2(a). Fig. 3 representatively presents the typical EDS plot of point 1. According to the characteristic X-
Fig. 2. Transmission electron microscopy (TEM) image of the ETO1 nanoparticles (a). High-resolution TEM (HRTEM) picture of the ETO1 nanoparticles (b) and the selected area electron diffraction pattern (SAED) (inset of (b)). The crossing points in (a) correspond with the EDS measurement points and the sideward numbers are their labels.
T. Wei et al. / Applied Surface Science 257 (2011) 4505–4509
Fig. 3. EDS plot of ETO1 nanoparticles from point 1 as shown in Fig. 2(a).
ray positions of different elements, we can determine the content of the test sample. Comparing with the standard energy database of TEM, it is revealed that the sample is constituted by O, Ti, and Eu elements. Furthermore, the relative concentration ratio can be calculated through the X-ray intensity of each element. The quantitative analysis of the EDS data of these points (1, 2, 3, 4, and 5) reveals that the averaged atomic ratio of Eu, Ti, and O is about 1:1:3 which clearly shown in Table 1. In order to shed light on the currently conﬂicting magnetic behavior of ETO and explore the intrinsic magnetic properties of low dimensional samples, the magnetic measurements on the ETO1 nanoparticles were performed. Fig. 4(a) gives the magnetization (M)–T curves at zero-ﬁeld cooled (ZFC) and ﬁeld cooled (FC) cases, respectively. Under the ZFC case, the samples were cooled from 50 K to 2 K under zero ﬁeld, and then the ZFC M–T measurements were performed in the warming process with a ﬁeld H = 100 Oe. For the FC case, the samples were ﬁrst cooled down to 2 K in the presence of H = 100 Oe, and then the M–T data were probed under the same ﬁeld during the heating cycle. Clearly, as shown in Fig. 4(a), one can observe that the ZFC and FC plots give the same variation of M with T. These M–T curves have the same cusp feature at ∼5 K, indicating the G-AFM transition induced by Eu2+ spins ordering below TN ∼ 5 K [5,11–13]. This transition point is identical with the AFM transition point of bulk ETO. Furthermore, the inset of Fig. 4(a) gives the dependence of 1/M on T and the solid line is the Curie–Weiss law ﬁtting, giving Tc ∼ 3.3 K. The positive Curie point Tc is attributed to the competition between the next nearest-neighbor FM exchange interaction and the nearest-neighbor AFM exchange interaction. It is believed that the G-AFM order for ETO is not very stable [5,17,18]. Considering the Eu2+ spins are of Heisenberg character and weak magnetic anisotropy, upon an external H, these AFM ordered Heisenberg spins are immediately ﬂipped into an alignment perpendicular to H and then gradually transited into the FM arrangement with increasing H. To conﬁrm this point, Fig. 4(b) gives the M–H curve measured at 2 K, showing a linearly increasing M with T at low H and saturated above ∼1.5 T (saturated M ∼ 7 B /Eu). This conﬁrms the transition from the AFM state to the FM state, induced by H. As we know, speciﬁc heat is a parameter sensitive to magnetic transitions. The H-induced magnetic transition can be also reﬂected in the speciﬁc heat (Cp ) data, as shown in Fig. 5. Under H = 0 in Fig. 5(a), the Cp of ETO1 exhibits obviously increase with T decreas-
Fig. 4. M–T curves at zero-ﬁeld cooled (ZFC) and ﬁeld cooled (FC) cases, respectively (a). The plot of 1/M with T and the solid line is ﬁtting results (the inset of Fig. 3(a)). M–H curve measured at 2 K (b).
ing below ∼6 K, and subsequently a clear anomaly peak appears at T = TN ∼5 K which indicates the G-AFM transition. To explore H inﬂuence, the Cp data under different H (H = 0.1 T, 0.5 T, 1 T, 3 T, and 5 T) are also given in Fig. 5(a) and (b), respectively. As given in Fig. 5(a), one can see that the height of AFM speciﬁc heat peak is suppressed and the TN shifts to the low T aside with the increasing of H. The phenomena can be attributed to the result of the suppression of AFM state under the inﬂuence of magnetic ﬁeld. In contrast, Fig. 5(b) displays the Cp –T curves under H = 3 T and 5 T which is larger than the saturated H (∼1.5 T). One can see that current Cp –T plots display obvious difference with that of plots H ≤ 1 T. On the one hand, the T value of speciﬁc heat peak (Tp ) is greatly enhanced under the inﬂuence of H and the Tp is about 5.5 K and 10 K at H = 3 T and 5 T, respectively. This case contradicts with the variation trend of AFM transition under H, thus, the current speciﬁc heat peaks should not ascribe to the AFM transition. Furthermore, considering the aforesaid unstable G-AFM state, it is believed that the current speciﬁc heat anomaly peak and enhanced transition T under H (3 T and 5 T) is attributed to the FM transition induced by H. On the other hand, we can see the speciﬁc heat peak in Fig. 5(b) gives evident diffusion behavior compared with that AFM transition plots. The broad diffusion peaks indicate the com-
Table 1 The quantitative element analysis of ETO1 sample from the EDS data and the averaged values of the obtained parameters. Elements
Eu Ti O
60 17 23
18 17 65
1 1 1
0.795 0.993 0.495
4.471 1.290 2.059
0.997 0.996 0.921
T. Wei et al. / Applied Surface Science 257 (2011) 4505–4509
Fig. 7. M–T curves at zero-ﬁeld cooled (ZFC) for ETO1, ETO2, and ETO3, respectively.
Fig. 5. Speciﬁc heat (Cp (T)) data under different H (H = 0 T, 0.1 T, 0.5 T, 1 T, 3 T, and 5 T).
Fig. 8. Absorption spectra of the as-prepared ETO1 nanoparticles at room T (a). Plot of (˛h)2 against (h) and the solid line is the linear extrapolation to obtain the optical band gap (b).
Fig. 6. Electron paramagnetic resonance (EPR) spectra at room T for ETO1, ETO2, and ETO3, respectively.
petition interactions between FM long range order stabilized by H and thermal perturbation. At this stage, it is time for us to conﬁrm the intrinsic magnetic properties of the as-prepared ETO1 nanoparticles. Thus, we measured the EPR of ETO1 at room-T. For comparison, the EPR spectra of ETO2 and ETO3 were also measured. Fig. 6 presents the plots of EPR spectra for ETO1, ETO2, and ETO3, respectively. It is clearly shown that no EPR signal was detected for ETO1 sample, however,
the remarkable EPR signal appears for ETO2 and ETO3 samples as shown in Fig. 6. Moreover, we further calculated the Lande factor (g) from the EPR spectra and found that the g value for ETO2 and ETO3 is 2.0036. This indicates that the EPR signal originates from the OVs [19,20]. Now, according to the EPR and EDS results, we can conclude that the as-prepared ETO1 nanoparticles have normal stoichiometric composition which indicates the magnetic properties of ETO1 are the intrinsic behavior. Furthermore, Fig. 7 shows the ZFC curves for ETO1, ETO2, and ETO3, respectively. One can see that G-AFM transition happens for ETO1, however, there are no G-AFM transition for the oxygen deﬁcient ETO2 and ETO3 which indicates the inﬂuence of OVs on magnetic properties. To proceeding, owing to the possible photocatalytic property of ETO, referring to similar effect for STO, we measured the optical absorption spectrum of the as-prepared ETO1 nanoparticles. Fig. 8(a) displays the data at room temperature. It is shown that the absorption cut-off wavelength is ∼1400 nm, suggesting that the ETO1 nanoparticles can absorb photons over the whole visible light spectrum. Furthermore, the energy band estimated from a linear extrapolation of (˛h)2 against photon energy (h) plot, as shown in Fig. 8(b), gives the band gap of ∼1.03 eV [13,21,22]. It is
T. Wei et al. / Applied Surface Science 257 (2011) 4505–4509
believed that the as-prepared ETO1 nanoparticles are direct band gap materials and the measured band gap value is consistent with that of ETO thin ﬁlms . 4. Conclusions In conclusion, we have synthesized high quality ETO nanoparticles by the simple sol–gel approach and the grain size is ∼100 nm. The magnetic ground state of the nanoparticles is the G-AFM order, while magnetic ﬁeld can induce a transition into the FM state. The optical band gap is ∼1.03 eV. Acknowledgements This work was supported by the National Natural Science Foundation of China (11074113, 50832002, and 10874075), the Scientiﬁc Research Foundation of Civil Aviation University of China (2010QD01X and 05yk25s). References  R. Skomski, J. Phys.: Condens. Matter 15 (2003) R841.  C.R. Martin, Science 266 (1994) 1961.  C.C. Chen, A.B. Herhold, C.S. Johnson, A.P. Alivisatos, Science 276 (1997) 398.  I. Naumov, H. Fu, Phys. Rev. Lett. 98 (2007) 077603.
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