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Microstructure and magnetic properties in Sn1−xFexO2 (x = 0.01, 0.05, 0.10) nanoparticles synthesized by hydrothermal method
Journal of Alloys and Compounds 491 (2010) 679–683
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and magnetic properties in Sn1−x Fex O2 (x = 0.01, 0.05, 0.10) nanoparticles synthesized by hydrothermal method Limei Fang a,∗ , Xiaotao Zu a,∗ , Chunming Liu a , Zhijie Li a , Germanas Peleckis b , Sha Zhu c , Huakun Liu b , Lumin Wang c,d a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USA d Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104, USA b c
a r t i c l e
i n f o
Article history: Received 29 June 2009 Received in revised form 4 November 2009 Accepted 6 November 2009 Available online 11 November 2009 Keywords: Semiconductors Nanoparticles Hydrothermal Magnetic properties
a b s t r a c t Sn1−x Fex O2 (x = 0.00, 0.01, 0.05, 0.10) nanoparticles were synthesized by a simple hydrothermal method. X-ray diffraction (XRD) reveals that all samples are pure rutile-type tetragonal phase and the grain size decreases with the increase of Fe content. High-resolution transmission electron microscopy (HRTEM) images show that the samples are spherical in shape and most grains are uniform in size with diameters of 5–6 nm. Magnetic measurements by using magnetic property measurement system (MPMS) show that the Fe-doped SnO2 nanoparticles exhibit paramagnetic behavior and the calculated values of the effective Bohr magnetron number are 4.94 B (LT) and 3.92 B (HT) for x = 0.01, 5.04 B for x = 0.05, and 4.81 B for x = 0.1, respectively. The decrease of magnetic moment per iron atom with the increase of iron concentration could be attributed to the antiferromagnetically coupled spins. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, transition-metal doped semiconducting oxides have been attracting considerable attention due to their roomtemperature ferromagnetism. This kind of materials, named diluted magnetic semiconductors (DMS), is of great interest as potential materials for spintronic application. Nano-sized tin dioxide, as an n-type semiconductor with a wide band gap of 3.6 eV, has been widely used for applications such as optoelectronic devices [1,2], gas sensors [3–5], varistor [6,7], and photocatalysts [8,9] because of its excellent electronic and optical properties. As we all know, doping can improve the performance of SnO2 nanostructures. Many results have been reported on Fe [10–14], Co [15,16], Ni [17], Mn [18,19] -doped SnO2 oxides. In these papers, Coey et al. [10] found ferromagnetism in Sn0.95 Fe0.05 O2 thin film with moments ranging from 1.06 to 4.76 B /Fe expected for Fe3+ . It is reported that SnO2 powders doped with 10% Fe content showed ferromagnetism [11]. Ferromagnetism in Sn0.95 Fe0.05 O2 ceramic with magnetic moment of 0.95 B /Fe, about 85% of the iron being in a magnetically ordered
∗ Corresponding author at: Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China. Tel.: +86 13880258160. E-mail addresses: [email protected] (L. Fang), [email protected] (X. Zu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.044
high-spin Fe3+ , and Curie temperature of about 360 K were reported by Fitzgerald et al. [20]. On the other hand, Punnoose et al. [21] reported a paramagnetic behavior in chemically synthesized powder sample Sn0.95 Fe0.05 O2 prepared above 600 ◦ C and Adhikari et al. [22] observed an antiferromagnetic behavior of 5%Fe-doped SnO2 nanoparticles synthesized by a chemical coprecipitation method. Overall, the origin of ferromagnetism in transitional-metal-doped SnO2 system is controversial and more detailed investigations are required. In this paper, the microstructure and magnetic properties of Fe-doped SnO2 nanoparticles by a simple hydrothermal method are reported. Interestingly, Fe-doped SnO2 samples exhibit paramagnetic behavior and the magnetic moments decrease with the increase of Fe content. More details about these studies are as follows. 2. Experimental details Sn1−x Fex O2 (x = 0.00, 0.01, 0.05, 0.10) samples were synthesized by a simple sol–gel–hydrothermal method from SnCl4 ·5H2 O and FeCl3 ·6H2 O with different Fe nominal content. Firstly, citric acid was added to 96 ml of distilled water until the PH = 1–2 at 50 ◦ C with magnetic stirring; then 17.529 g of SnCl4 ·5H2 O were added and dissolved. Secondly, FeCl3 ·6H2 O (the Fe molar content in samples was 0, 1.0, 5.0, and 10.0 mol%) was added to the above solution. It was stirred for 10 min and a sol formed. Thirdly, an aqueous ammonia solution (15 mol/l, 30 ml) was added to the above sol dropwise (the controlled dropping rate is around 1 drop per 3–4 s) under magnetic stirring within 30 min. The dropping rate must be well controlled for the chemical homogeneity. Then, the hydrolysis product was stirred for 30 min to form a gel. The resultant gel was transferred into a Teflon-lined autoclave for
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L. Fang et al. / Journal of Alloys and Compounds 491 (2010) 679–683
Fig. 1. XRD patterns of Sn1−x Fex O2 samples.
hydrothermal reaction at 150 ◦ C for 12 h. After that, the hydrothermal product was filtered and dried at 110 ◦ C for 10 h. Then, the Sn1−x Fex O2 samples were obtained. For all samples, phase analysis was structurally characterized by X-ray diffraction (XRD) at room temperature by using a Rigaku D/max-2400 diffractometer with Cu K␣ radiation ( = 0.15406 nm, 40 kV, 60 mA). The particle morphologies were observed by high-resolution transmission electron microscopy (HRTEM) using a JEOL 2010 F a field emission gun electron microscope with an accelerating voltage of 200 keV. Magnetic susceptibilities were measured using a magnetic properties measurement system (MPMS, Quantum Design MPMS - XL) over a wide temperature range from 7 up to 340 K and magnetic fields up to 10,000 Oe.
3. Results and discussion XRD patterns of Sn1−x Fex O2 with x-values ranging from 0 to 0.10 are shown in Fig. 1. The peaks pointed out at 26.65◦ (1 1 0), 33.83◦ (1 0 1), 37.90◦ (2 0 0), 51.83◦ (2 1 1), 54.71◦ (2 2 0), 57.76◦ (0 0 2), 61.82◦ (3 1 0), 65.88◦ (3 0 1), 71.23◦ (2 0 2), 78.81◦ (3 2 1) correspond to SnO2 cassiterite crystals (Powder Diffraction File No. 41-1445). For all samples, XRD data showed peaks corresponding to rutile structured SnO2 crystallite (P42 /mnm) only, and doping did not change the structure of SnO2 , which indicated that the obtained Sn1−x Fex O2 nanoparticles are single phase. No diffraction peaks from Fe or other impurities were detected. Furthermore, with the increase of Fe content, the intensity of diffraction peaks decreased accordingly, and the full width at half maximum (FWHM) of the diffraction peaks increased, which indicated possible changes in
Fig. 3. Temperature dependence of magnetization (M − T) with an applied magnetic field of 2000 Oe for as-prepared Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) samples.
the crystal size. Using the Scherrer equation, D = 0.9/ˇ cos (D is the grain size, is the wavelength of X-ray (0.15406 nm for Cu K␣ ), ˇ is the FWHM of the diffraction peaks and is the Bragg diffractive angle) [23], the crystallite size can be estimated. The crystal size determined by (1 1 0) diffraction peak decreased from 5.98 to 5.02 nm when the Fe content increased from 0 to 10 mol%. The particle size decreased with the increase of Fe content, which indicated that the introduction of Fe dopant prevented the growth of crystal grain of SnO2 . The high-resolution transmission electronic microscopy (HRTEM) images of the as-prepared Sn0.99 Fe0.01 O2 and Sn0.95 Fe0.05 O2 are shown in Fig. 2a and b, respectively. TEM images show that Fe-doped SnO2 samples are nearly spherical in shape. It can be seen that, most grains are uniform in size with diameters of 5–6 nm and narrow size distribution, and well dispersed. The selected area electronic diffraction (SAED) pattern (Fig. 2a) presents the polycrystalline rings that are all from the rutile SnO2 structure, which is in agreement with the XRD results. Fig. 3 shows the temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetizations measured in a field 2000 Oe from 7 to 340 K for the as-prepared Sn1−x Fex O2 samples with x = 0.01, 0.05, and 0.10. The zero field cooled (ZFC) curves, which are almost overlapped with the FC curves, were observed during warming in a field of 2000 Oe (Quantum Design, MPMS
Fig. 2. HRTEM of (a) Sn0.99 Fe0.01 O2 and (b) Sn0.95 Fe0.05 O2 samples.
L. Fang et al. / Journal of Alloys and Compounds 491 (2010) 679–683
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Fig. 5. (Left) The rutile structure model of SnO2 and (right) the octahedron structure model. Fig. 4. The plots of inverse magnetic susceptibility versus temperature for asprepared Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) samples; the insets show temperature dependence of inverse magnetic susceptibility for x = 0.01 in low and high temperature regions.
XL). The magnetization (M) increased rapidly with the decrease of temperature (T) below 50 K, which is the typical behavior of a paramagnetic material. Magnetic characteristics at room temperature resulting from the slight increase of doping content was observed by Punnoose et al. [24], and those authors have found their chemically synthesized Sn1−x Cox O2 powders showed room-temperature ferromagnetism for x ≤ 0.01, but for x > 0.01, this ferromagnetism was completely destroyed and their samples demonstrated a paramagnetic behavior. As for our samples, the Fe content is higher than 0.01 and paramagnetism were observed. The magnetization increased more quickly with the higher Fe dopant, which indicated the magnetism of the samples depends on the Fe doping content. Thus, we assume that the ferromagnetism is either suppressed or disappears, only paramagnetism or antiferromagnetism is found in the fine doping samples. Inverse magnetic susceptibility versus temperature under magnetic field of 2000 Oe for these three samples is shown in Fig. 4. For x > 0.01, the 1/ − T curves are linear for the entire range of temperature and follow the Curie–Weiss behavior with a negative intercept on the temperature axis, which indicates the existence of a strong antiferromagnetic interaction between Fe ions in the two samples. Based on the experimental data of and T in Fig. 4, the estimated values of the effective Bohr magnetron number are obtained and decreased from peff = 5.04 for x = 0.05 to peff = 4.81 for x = 0.1 (see Table 1). The observed moments can be explained by the coexistence of high-spin and low-spin Fe3+ in different sites based on the following discussion. Sn1−x Fex O2 has a rutile tetragonal crystal structure with a space group P42 /mnm. The schematic structure is shown in Fig. 5(left). The Fe ion is located in the center of unit cell, coordinated with six nearest neighbouring oxygen atoms and surrounded by eight next nearest tin ions. The valence of Fe ion is supposed to be a mixed valence of 3+ and 2+ in our samples. Generally, Fe3+ or Fe2+ is located in the octahedron crystal field (see Fig. 5, right) and should have three different 3d5 or 3d6 electron configurations with S = 1/2, Table 1 Experimental value of Fe ion with different spin states. Sn1−x Fex O2 , x = 0.01
peff
LT
HT
4.94
3.92
Sn1−x Fex O2 , x = 0.05
5.04
Fig. 6. Schematic representation of the electron energy levels for the Fe3+ (d5 ) and Fe2+ (d6 ).
3/2, and 5/2 or S = 0, 1, and 2, respectively. In Fig. 6, the low spin (LS), intermediate spin (IS), and high spin (HS) states are schematically shown for the 3d5 (Fe3+ ) and 3d6 (Fe2+ ) configurations in a octahedron crystal field, from which and the formula of effective magnetron number p = 2 s(s + 1) (s is the spin quantum number) for transitional metal ions we can get the theoretical effective moment value of Fe ion with different spin states as shown in Table 2. The Fe spin states in our samples can be deduced from the comparison between the experimental and theoretical effective Bohr magnetron number value of Fe ion. The values of the effective Bohr magnetron number are 5.04 for Sn0.95 Fe0.05 O2 and 4.81 for Sn0.9 Fe0.1 O2 as against the expected spin only moment of Fe3+ (5.9). The experimental value is lower than the theoretical value, which can be explained by the coexistence of HS and LS states Fe3+ in different sites. As can be seen from Fig. 4, the inverse magnetic susceptibility for x = 0.01 does not obey the Curie–Weiss law and no direct proportion between the inverse magnetic susceptibility and the temperature can obtained in the 1/ − T curves. However, from the approximate linear extrapolation of the higher temperature (HT) part of the susceptibility curves (see the right inset of Fig. 4), the effective Bohr magnetron number peff = 3.92 is obtained, which coincides with the Table 2 Theoretical value of Fe ion with different spin states.
Sn1−x Fex O2 , x = 0.1
4.81
3d6 3d5
peff
LS
IS
HS
Fe2+ Fe3+
0 1.7
2.8 3.9
4.9 5.9
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expected spin only moment of the IS state Fe3+ (3.9). On the other hand, in the lower temperature (LT) region (see the left inset of Fig. 4), the linear fitting for the part of data gives peff = 4.94 and the inverse magnetic susceptibility goes to the zero, which indicates that the Fe spins remain uncoupled with an effective Bohr magnetron number closing to that of the HS state in Fe2+ (4.9). Although the two peff values for x = 0.01 are very rough estimation, it is possible to obtain a mixed spin state of the Fe2+ (HS) and the Fe3+ (IS) in Sn0.99 Fe0.01 O2 . As discussed above, the LS, IS, and HS states of Fe3+ or Fe2+ seem to be mixed depending on x-values and temperature. Coey and Fitzgerald et al. [10,20] reported that the high-spin Fe3+ ions were found in Sn0.95 Fe0.05 O2 films prepared by pulsedlaser deposition. A stabilized structure was obtained because of the charge balance due to the substitution of two Sn4+ by one Nb5+ and one Fe3+ in Fe–Nb co-doped SnO2 [12], where the values of effective Bohr magnetron number are 4.9 and 5.17 for Fe content being 33% and 15%, respectively. Cabrera et al. [13] revealed the iron atoms are approximately equally distributed in the 3+ and 2+ oxidation states in the Fe-doped SnO2 nanoparticles prepared by mechanosynthesis. As for our samples, we found Fe2+ (HS) and the Fe3+ (IS) are co-existent in lower dopant samples, and Fe3+ (LS and HS) was observed in higher dopant ones, which suggests that Fe3+ is more steady-going with the increase of Fe content. In Fig. 7, magnetization M of the Sn1−x Fex O2 samples with x = 0.01, 0.05, and 0.10 measured at 10 K is shown as a function of magnetic field H. Magnetization measured up to 10,000 Oe is linear and no hysteresis and saturation is observed in any samples in applied maximal field, which indicates that these samples are not ferromagnetic. It is only possible to be either paramagnetic or antiferromagnetic. Based on the XRD and TEM results, the particles are perfect fine and dispersed well. In this situation, these Fe spins are distributed as uncoupled spins or mediated through oxygen ions, and the magnetic interaction is paramagnetic or antiferromagnetic coupling. Thus a strong paramagnetic or antiferromagnetic behavior is observed, as shown in Figs. 3 and 7. Assuming Fe is 3+ or 2+, it contributes to 1, 3, and 5 or 0, 2, and 4 B /Fe for three different spin states (LS, MS, and HS). Fitzgerald et al. reported that a Sn0.95 Fe0.05 O2 [20] sample exhibited an Ms value of 0.95 B per Fe atom, which is higher than the one (0.31 B /Fe) reported here when the field was 10,000 Oe. This value is not of the order of those reported by Coey et al. [10] for 5 at.% Fe-doped SnO2 films grown on sapphire substrates (1.06–4.76 B /Fe). However, for other Fe-doped SnO2 film with a FM-like behavior, a magnetic moment of 2.0 B /SC was obtained and increased up to 4.0 B /SC
by introducing the oxygen vacancies near Fe atom [25]. The measured magnetization of Sn1−x Fex O2 corresponded to a moment of 0.2–0.6 B per iron atom when magnetic field was 10,000 Oe. Moreover, magnetization saturation (Ms ) could not be obtained even at a higher magnetic field due to the paramagnetic or antiferromagnetic. In addition, the magnetic moments per iron atom decreased with the increase of x, which represents antiferromagnetical nature of Fe–O–Fe interaction. The particle size grows smaller for samples with higher Fe dopant based on the fore-mentioned discussion. As a consequence, the concentration of surface defects and the number of Fe ions residing at surface will reasonably increase compared with the samples with lower Fe dopant. Therefore, there should be several antiferromagnetic regions with some reversed spin and many isolated paramagnetic iron sites. However, the antiferromagnetic pair can act within small groups of nearest-neighbour cations supplying clusters of antiferromagnetically coupled spins such as ↓↑, ↑↓↑, . . ., which reduces the average moment per Fe, as seen in Fig. 7. Therefore, the magnetization values are lower than saturation in the samples even in applied maximal field, which can be explained by the stronger antiferromagnetic interactions among the magnetic ions in the matrix. 4. Conclusions The microstructure and the magnetic properties in Sn1−x Fex O2 synthesized by a simple hydrothermal method are reported in this paper. It is found that Fe dopant in the SnO2 nanoparticles with a tetrahedral rutile structure dramatically influences the grain size and the magnetic properties. Room temperature paramagnetism is observed in Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) nanoparticles with magnetic moments of 4.94 (LT) and 3.92 (HT), 5.04, and 4.81 B per iron atom, respectively. The magnetic moments per iron atom decrease with the increase of Fe content due to the antiferromagnetically coupled spins. All in all, further investigations of the electronic structure and magnetism of transition-metal doped SnO2 are required. Acknowledgments This work was supported by the CSC, NSAF Joint Foundation of China (10376006), Program for New Century Excellent Talents in University (NCET-04-0899), Ph.D. Founding Support Program of Education Ministry of China (20050614013), Program for Innovative Research Team in UESTC. References
Fig. 7. Magnetic field dependence of magnetization (M − T) at 10 K for as-prepared Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) samples.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and magnetic properties in Sn1−x Fex O2 (x = 0.01, 0.05, 0.10) nanoparticles synthesized by hydrothermal method Limei Fang a,∗ , Xiaotao Zu a,∗ , Chunming Liu a , Zhijie Li a , Germanas Peleckis b , Sha Zhu c , Huakun Liu b , Lumin Wang c,d a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USA d Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104, USA b c
a r t i c l e
i n f o
Article history: Received 29 June 2009 Received in revised form 4 November 2009 Accepted 6 November 2009 Available online 11 November 2009 Keywords: Semiconductors Nanoparticles Hydrothermal Magnetic properties
a b s t r a c t Sn1−x Fex O2 (x = 0.00, 0.01, 0.05, 0.10) nanoparticles were synthesized by a simple hydrothermal method. X-ray diffraction (XRD) reveals that all samples are pure rutile-type tetragonal phase and the grain size decreases with the increase of Fe content. High-resolution transmission electron microscopy (HRTEM) images show that the samples are spherical in shape and most grains are uniform in size with diameters of 5–6 nm. Magnetic measurements by using magnetic property measurement system (MPMS) show that the Fe-doped SnO2 nanoparticles exhibit paramagnetic behavior and the calculated values of the effective Bohr magnetron number are 4.94 B (LT) and 3.92 B (HT) for x = 0.01, 5.04 B for x = 0.05, and 4.81 B for x = 0.1, respectively. The decrease of magnetic moment per iron atom with the increase of iron concentration could be attributed to the antiferromagnetically coupled spins. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, transition-metal doped semiconducting oxides have been attracting considerable attention due to their roomtemperature ferromagnetism. This kind of materials, named diluted magnetic semiconductors (DMS), is of great interest as potential materials for spintronic application. Nano-sized tin dioxide, as an n-type semiconductor with a wide band gap of 3.6 eV, has been widely used for applications such as optoelectronic devices [1,2], gas sensors [3–5], varistor [6,7], and photocatalysts [8,9] because of its excellent electronic and optical properties. As we all know, doping can improve the performance of SnO2 nanostructures. Many results have been reported on Fe [10–14], Co [15,16], Ni [17], Mn [18,19] -doped SnO2 oxides. In these papers, Coey et al. [10] found ferromagnetism in Sn0.95 Fe0.05 O2 thin film with moments ranging from 1.06 to 4.76 B /Fe expected for Fe3+ . It is reported that SnO2 powders doped with 10% Fe content showed ferromagnetism [11]. Ferromagnetism in Sn0.95 Fe0.05 O2 ceramic with magnetic moment of 0.95 B /Fe, about 85% of the iron being in a magnetically ordered
∗ Corresponding author at: Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China. Tel.: +86 13880258160. E-mail addresses: [email protected] (L. Fang), [email protected] (X. Zu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.044
high-spin Fe3+ , and Curie temperature of about 360 K were reported by Fitzgerald et al. [20]. On the other hand, Punnoose et al. [21] reported a paramagnetic behavior in chemically synthesized powder sample Sn0.95 Fe0.05 O2 prepared above 600 ◦ C and Adhikari et al. [22] observed an antiferromagnetic behavior of 5%Fe-doped SnO2 nanoparticles synthesized by a chemical coprecipitation method. Overall, the origin of ferromagnetism in transitional-metal-doped SnO2 system is controversial and more detailed investigations are required. In this paper, the microstructure and magnetic properties of Fe-doped SnO2 nanoparticles by a simple hydrothermal method are reported. Interestingly, Fe-doped SnO2 samples exhibit paramagnetic behavior and the magnetic moments decrease with the increase of Fe content. More details about these studies are as follows. 2. Experimental details Sn1−x Fex O2 (x = 0.00, 0.01, 0.05, 0.10) samples were synthesized by a simple sol–gel–hydrothermal method from SnCl4 ·5H2 O and FeCl3 ·6H2 O with different Fe nominal content. Firstly, citric acid was added to 96 ml of distilled water until the PH = 1–2 at 50 ◦ C with magnetic stirring; then 17.529 g of SnCl4 ·5H2 O were added and dissolved. Secondly, FeCl3 ·6H2 O (the Fe molar content in samples was 0, 1.0, 5.0, and 10.0 mol%) was added to the above solution. It was stirred for 10 min and a sol formed. Thirdly, an aqueous ammonia solution (15 mol/l, 30 ml) was added to the above sol dropwise (the controlled dropping rate is around 1 drop per 3–4 s) under magnetic stirring within 30 min. The dropping rate must be well controlled for the chemical homogeneity. Then, the hydrolysis product was stirred for 30 min to form a gel. The resultant gel was transferred into a Teflon-lined autoclave for
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L. Fang et al. / Journal of Alloys and Compounds 491 (2010) 679–683
Fig. 1. XRD patterns of Sn1−x Fex O2 samples.
hydrothermal reaction at 150 ◦ C for 12 h. After that, the hydrothermal product was filtered and dried at 110 ◦ C for 10 h. Then, the Sn1−x Fex O2 samples were obtained. For all samples, phase analysis was structurally characterized by X-ray diffraction (XRD) at room temperature by using a Rigaku D/max-2400 diffractometer with Cu K␣ radiation ( = 0.15406 nm, 40 kV, 60 mA). The particle morphologies were observed by high-resolution transmission electron microscopy (HRTEM) using a JEOL 2010 F a field emission gun electron microscope with an accelerating voltage of 200 keV. Magnetic susceptibilities were measured using a magnetic properties measurement system (MPMS, Quantum Design MPMS - XL) over a wide temperature range from 7 up to 340 K and magnetic fields up to 10,000 Oe.
3. Results and discussion XRD patterns of Sn1−x Fex O2 with x-values ranging from 0 to 0.10 are shown in Fig. 1. The peaks pointed out at 26.65◦ (1 1 0), 33.83◦ (1 0 1), 37.90◦ (2 0 0), 51.83◦ (2 1 1), 54.71◦ (2 2 0), 57.76◦ (0 0 2), 61.82◦ (3 1 0), 65.88◦ (3 0 1), 71.23◦ (2 0 2), 78.81◦ (3 2 1) correspond to SnO2 cassiterite crystals (Powder Diffraction File No. 41-1445). For all samples, XRD data showed peaks corresponding to rutile structured SnO2 crystallite (P42 /mnm) only, and doping did not change the structure of SnO2 , which indicated that the obtained Sn1−x Fex O2 nanoparticles are single phase. No diffraction peaks from Fe or other impurities were detected. Furthermore, with the increase of Fe content, the intensity of diffraction peaks decreased accordingly, and the full width at half maximum (FWHM) of the diffraction peaks increased, which indicated possible changes in
Fig. 3. Temperature dependence of magnetization (M − T) with an applied magnetic field of 2000 Oe for as-prepared Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) samples.
the crystal size. Using the Scherrer equation, D = 0.9/ˇ cos (D is the grain size, is the wavelength of X-ray (0.15406 nm for Cu K␣ ), ˇ is the FWHM of the diffraction peaks and is the Bragg diffractive angle) [23], the crystallite size can be estimated. The crystal size determined by (1 1 0) diffraction peak decreased from 5.98 to 5.02 nm when the Fe content increased from 0 to 10 mol%. The particle size decreased with the increase of Fe content, which indicated that the introduction of Fe dopant prevented the growth of crystal grain of SnO2 . The high-resolution transmission electronic microscopy (HRTEM) images of the as-prepared Sn0.99 Fe0.01 O2 and Sn0.95 Fe0.05 O2 are shown in Fig. 2a and b, respectively. TEM images show that Fe-doped SnO2 samples are nearly spherical in shape. It can be seen that, most grains are uniform in size with diameters of 5–6 nm and narrow size distribution, and well dispersed. The selected area electronic diffraction (SAED) pattern (Fig. 2a) presents the polycrystalline rings that are all from the rutile SnO2 structure, which is in agreement with the XRD results. Fig. 3 shows the temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetizations measured in a field 2000 Oe from 7 to 340 K for the as-prepared Sn1−x Fex O2 samples with x = 0.01, 0.05, and 0.10. The zero field cooled (ZFC) curves, which are almost overlapped with the FC curves, were observed during warming in a field of 2000 Oe (Quantum Design, MPMS
Fig. 2. HRTEM of (a) Sn0.99 Fe0.01 O2 and (b) Sn0.95 Fe0.05 O2 samples.
L. Fang et al. / Journal of Alloys and Compounds 491 (2010) 679–683
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Fig. 5. (Left) The rutile structure model of SnO2 and (right) the octahedron structure model. Fig. 4. The plots of inverse magnetic susceptibility versus temperature for asprepared Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) samples; the insets show temperature dependence of inverse magnetic susceptibility for x = 0.01 in low and high temperature regions.
XL). The magnetization (M) increased rapidly with the decrease of temperature (T) below 50 K, which is the typical behavior of a paramagnetic material. Magnetic characteristics at room temperature resulting from the slight increase of doping content was observed by Punnoose et al. [24], and those authors have found their chemically synthesized Sn1−x Cox O2 powders showed room-temperature ferromagnetism for x ≤ 0.01, but for x > 0.01, this ferromagnetism was completely destroyed and their samples demonstrated a paramagnetic behavior. As for our samples, the Fe content is higher than 0.01 and paramagnetism were observed. The magnetization increased more quickly with the higher Fe dopant, which indicated the magnetism of the samples depends on the Fe doping content. Thus, we assume that the ferromagnetism is either suppressed or disappears, only paramagnetism or antiferromagnetism is found in the fine doping samples. Inverse magnetic susceptibility versus temperature under magnetic field of 2000 Oe for these three samples is shown in Fig. 4. For x > 0.01, the 1/ − T curves are linear for the entire range of temperature and follow the Curie–Weiss behavior with a negative intercept on the temperature axis, which indicates the existence of a strong antiferromagnetic interaction between Fe ions in the two samples. Based on the experimental data of and T in Fig. 4, the estimated values of the effective Bohr magnetron number are obtained and decreased from peff = 5.04 for x = 0.05 to peff = 4.81 for x = 0.1 (see Table 1). The observed moments can be explained by the coexistence of high-spin and low-spin Fe3+ in different sites based on the following discussion. Sn1−x Fex O2 has a rutile tetragonal crystal structure with a space group P42 /mnm. The schematic structure is shown in Fig. 5(left). The Fe ion is located in the center of unit cell, coordinated with six nearest neighbouring oxygen atoms and surrounded by eight next nearest tin ions. The valence of Fe ion is supposed to be a mixed valence of 3+ and 2+ in our samples. Generally, Fe3+ or Fe2+ is located in the octahedron crystal field (see Fig. 5, right) and should have three different 3d5 or 3d6 electron configurations with S = 1/2, Table 1 Experimental value of Fe ion with different spin states. Sn1−x Fex O2 , x = 0.01
peff
LT
HT
4.94
3.92
Sn1−x Fex O2 , x = 0.05
5.04
Fig. 6. Schematic representation of the electron energy levels for the Fe3+ (d5 ) and Fe2+ (d6 ).
3/2, and 5/2 or S = 0, 1, and 2, respectively. In Fig. 6, the low spin (LS), intermediate spin (IS), and high spin (HS) states are schematically shown for the 3d5 (Fe3+ ) and 3d6 (Fe2+ ) configurations in a octahedron crystal field, from which and the formula of effective magnetron number p = 2 s(s + 1) (s is the spin quantum number) for transitional metal ions we can get the theoretical effective moment value of Fe ion with different spin states as shown in Table 2. The Fe spin states in our samples can be deduced from the comparison between the experimental and theoretical effective Bohr magnetron number value of Fe ion. The values of the effective Bohr magnetron number are 5.04 for Sn0.95 Fe0.05 O2 and 4.81 for Sn0.9 Fe0.1 O2 as against the expected spin only moment of Fe3+ (5.9). The experimental value is lower than the theoretical value, which can be explained by the coexistence of HS and LS states Fe3+ in different sites. As can be seen from Fig. 4, the inverse magnetic susceptibility for x = 0.01 does not obey the Curie–Weiss law and no direct proportion between the inverse magnetic susceptibility and the temperature can obtained in the 1/ − T curves. However, from the approximate linear extrapolation of the higher temperature (HT) part of the susceptibility curves (see the right inset of Fig. 4), the effective Bohr magnetron number peff = 3.92 is obtained, which coincides with the Table 2 Theoretical value of Fe ion with different spin states.
Sn1−x Fex O2 , x = 0.1
4.81
3d6 3d5
peff
LS
IS
HS
Fe2+ Fe3+
0 1.7
2.8 3.9
4.9 5.9
682
L. Fang et al. / Journal of Alloys and Compounds 491 (2010) 679–683
expected spin only moment of the IS state Fe3+ (3.9). On the other hand, in the lower temperature (LT) region (see the left inset of Fig. 4), the linear fitting for the part of data gives peff = 4.94 and the inverse magnetic susceptibility goes to the zero, which indicates that the Fe spins remain uncoupled with an effective Bohr magnetron number closing to that of the HS state in Fe2+ (4.9). Although the two peff values for x = 0.01 are very rough estimation, it is possible to obtain a mixed spin state of the Fe2+ (HS) and the Fe3+ (IS) in Sn0.99 Fe0.01 O2 . As discussed above, the LS, IS, and HS states of Fe3+ or Fe2+ seem to be mixed depending on x-values and temperature. Coey and Fitzgerald et al. [10,20] reported that the high-spin Fe3+ ions were found in Sn0.95 Fe0.05 O2 films prepared by pulsedlaser deposition. A stabilized structure was obtained because of the charge balance due to the substitution of two Sn4+ by one Nb5+ and one Fe3+ in Fe–Nb co-doped SnO2 [12], where the values of effective Bohr magnetron number are 4.9 and 5.17 for Fe content being 33% and 15%, respectively. Cabrera et al. [13] revealed the iron atoms are approximately equally distributed in the 3+ and 2+ oxidation states in the Fe-doped SnO2 nanoparticles prepared by mechanosynthesis. As for our samples, we found Fe2+ (HS) and the Fe3+ (IS) are co-existent in lower dopant samples, and Fe3+ (LS and HS) was observed in higher dopant ones, which suggests that Fe3+ is more steady-going with the increase of Fe content. In Fig. 7, magnetization M of the Sn1−x Fex O2 samples with x = 0.01, 0.05, and 0.10 measured at 10 K is shown as a function of magnetic field H. Magnetization measured up to 10,000 Oe is linear and no hysteresis and saturation is observed in any samples in applied maximal field, which indicates that these samples are not ferromagnetic. It is only possible to be either paramagnetic or antiferromagnetic. Based on the XRD and TEM results, the particles are perfect fine and dispersed well. In this situation, these Fe spins are distributed as uncoupled spins or mediated through oxygen ions, and the magnetic interaction is paramagnetic or antiferromagnetic coupling. Thus a strong paramagnetic or antiferromagnetic behavior is observed, as shown in Figs. 3 and 7. Assuming Fe is 3+ or 2+, it contributes to 1, 3, and 5 or 0, 2, and 4 B /Fe for three different spin states (LS, MS, and HS). Fitzgerald et al. reported that a Sn0.95 Fe0.05 O2 [20] sample exhibited an Ms value of 0.95 B per Fe atom, which is higher than the one (0.31 B /Fe) reported here when the field was 10,000 Oe. This value is not of the order of those reported by Coey et al. [10] for 5 at.% Fe-doped SnO2 films grown on sapphire substrates (1.06–4.76 B /Fe). However, for other Fe-doped SnO2 film with a FM-like behavior, a magnetic moment of 2.0 B /SC was obtained and increased up to 4.0 B /SC
by introducing the oxygen vacancies near Fe atom [25]. The measured magnetization of Sn1−x Fex O2 corresponded to a moment of 0.2–0.6 B per iron atom when magnetic field was 10,000 Oe. Moreover, magnetization saturation (Ms ) could not be obtained even at a higher magnetic field due to the paramagnetic or antiferromagnetic. In addition, the magnetic moments per iron atom decreased with the increase of x, which represents antiferromagnetical nature of Fe–O–Fe interaction. The particle size grows smaller for samples with higher Fe dopant based on the fore-mentioned discussion. As a consequence, the concentration of surface defects and the number of Fe ions residing at surface will reasonably increase compared with the samples with lower Fe dopant. Therefore, there should be several antiferromagnetic regions with some reversed spin and many isolated paramagnetic iron sites. However, the antiferromagnetic pair can act within small groups of nearest-neighbour cations supplying clusters of antiferromagnetically coupled spins such as ↓↑, ↑↓↑, . . ., which reduces the average moment per Fe, as seen in Fig. 7. Therefore, the magnetization values are lower than saturation in the samples even in applied maximal field, which can be explained by the stronger antiferromagnetic interactions among the magnetic ions in the matrix. 4. Conclusions The microstructure and the magnetic properties in Sn1−x Fex O2 synthesized by a simple hydrothermal method are reported in this paper. It is found that Fe dopant in the SnO2 nanoparticles with a tetrahedral rutile structure dramatically influences the grain size and the magnetic properties. Room temperature paramagnetism is observed in Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) nanoparticles with magnetic moments of 4.94 (LT) and 3.92 (HT), 5.04, and 4.81 B per iron atom, respectively. The magnetic moments per iron atom decrease with the increase of Fe content due to the antiferromagnetically coupled spins. All in all, further investigations of the electronic structure and magnetism of transition-metal doped SnO2 are required. Acknowledgments This work was supported by the CSC, NSAF Joint Foundation of China (10376006), Program for New Century Excellent Talents in University (NCET-04-0899), Ph.D. Founding Support Program of Education Ministry of China (20050614013), Program for Innovative Research Team in UESTC. References
Fig. 7. Magnetic field dependence of magnetization (M − T) at 10 K for as-prepared Sn1−x Fex O2 (x = 0.01, 0.05, and 0.10) samples.
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