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Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles
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Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles Sireenart Surinwong a , Apinpus Rujiwatra b,∗ a b
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
a r t i c l e
i n f o
Article history: Received 24 February 2012 Received in revised form 21 May 2012 Accepted 12 June 2012 Keywords: Zinc ferrite Nanoparticle Solvothermal Ultrasonic Superparamagnetism
a b s t r a c t ¯ ˚ were synthesized as a single phase by Nanoparticles of cubic zinc ferrites (Fd3m, a = 8.41(3)–8.44(1) A) an ultrasonic cavitation-assisted solvothermal technique using ethyl alcohol–water mixed solvents at temperatures of 150 ◦ C or higher for 18 h or more. The influences of the ultrasonic cavitation and the use of C2 H5 OH–H2 O mixed solvents in diminishing average particle size and in improving particle size uniformity were illustrated. The largest average size of nanoparticles obtained was 20 nm as measured from SEM photographs, with crystallite size of approximately 10 nm as estimated from XRD results. The room-temperature field-dependent magnetization of the nanoparticles obtained showed a characteristic S feature with magnetization of 24.32 emu/g at 1 T. © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Due to inherent superparamagnetism and a small energy band gap (Wu, Okuya, & Kaneko, 2001), spinel zinc ferrite has been found useful in various applications, e.g., electronic (Hakim, Manjurul Haque, Huq, & Nordblad, 2011), catalytic (Fan, Gu, Yang, & Li, 2009) and biomedical (Sharifi, Shokrollahi, & Amiri, 2012). Chemical and physical properties of zinc ferrite depend nonetheless on particle size, which varies drastically with synthesis routes and parameters. There are numerous techniques for preparing zinc ferrite nanoparticles, e.g., solid state combustion (Bardhan et al., 2010), sol–gel (Atif, Hasanain, & Nadeem, 2006), microemulsion (Zhihao & Lide, 1998) co-precipitation (Chen, Yang, Liu, & Huang, 2000), and solvothermal process (Chu, Jiang, & Zheng, 2006; Fan et al., 2009; Hu, Guan, & Yan, 2004; Yu, Fujino, & Yoshimura, 2003). Among these techniques, the solvothermal process appears supe¯ rior particularly for scaling up (Somiya & Roy, 2000; Zou, Xu, Hou, Wu, & Sun, 2007). Zinc ferrite nanoparticles can be prepared as a single phase, for example, under solvothermal conditions within a wide range of temperatures, 120–250 ◦ C, and over a wide range of reaction time, 1–48 h (Chu et al., 2006; Fan et al., 2009; Hu et al., 2004; Yu et al., 2003). Characteristics of the product nanoparticles however differ greatly, depending on synthesis parameters, e.g., reaction temperature and time, solvent, zinc and
∗ Corresponding author. Tel.: +66 5394 1906; fax: +66 5389 2277. E-mail addresses: [email protected], [email protected] (A. Rujiwatra).
iron precursors, and source of heating. Without using additives, a common drawback consists of poor particle size uniformity and hard agglomeration. To overcome such drawback and to improve the uniformity of particle sizes, the use of ultrasonic cavitation technique was recently reported (Reddy, Sivasankar, Sivakumar, & Moholkar, 2010; Sivakumar, Towata, Yasui, Tuziuti, & Iida, 2006). In this work, the ultrasonic cavitation technique was tried in the solvothermal synthesis of zinc ferrite nanoparticles, aiming at uniform particle size. Mixed solvents prepared with different ratios of ethyl alcohol and water were used as the liquid medium for the synthesis. Magnetic property of the nanoparticles prepared was studied at room temperature. 2. Materials and method Mixtures of ZnCl2 0.0083 mol (98.0%, Merck KGaA) and FeCl3 ·6H2 O 0.0166 mol (100.89%, Fluka) were first prepared using 10 cm3 of mixed solvents which were prepared from C2 H5 OH (99.5%, Merck) and deionized water in three different C2 H5 OH:H2 O volume ratios (v/v): 2:1, 1:1 and 1:2 (v/v). KOH pellets (85%, Merck) were then gradually added into each mixture with stirring to a concentration of 6.0 mol/L. The pHs of the mixtures were measured using universal pH indicator strips (Merck KGaA, Germany) to a common value of 14. Each mixture was ultrasonicated at 70(±5) ◦ C for 3 h in a laboratory ultrasonic bath (Bandelin Electronic RK255H, 160/320 W, 35 kHz), and transferred into a 25 mL solvothermal autoclave. The reactions were then conducted under an autogenous pressure generated at 130–180 ◦ C for 12–24 h. The synthesized
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Please cite this article in press as: Surinwong, S., & Rujiwatra, A. Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles. Particuology (2012), http://dx.doi.org/10.1016/j.partic.2012.06.008
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particles were recovered by filtration, washed with deionized water until the pH of the filtrate was ca. 7, and dried in air. Powder X-ray diffractometer (XRD, Bruker D8 Advance, CuK␣, ˚ 40 kV, 30 mA) and transmission electron Ni filter, = 1.540598 A, microscope (TEM, JEOL JEM-2010) were used to characterize the crystalline phases. From the collected XRD patterns, crystallite sizes of the yielded particles were calculated based on the most intense (3 1 1) peak and using the Scherrer equation (Fan et al., 2009). Particle size, morphology, and elemental composition of the yielded particles were investigated using a field emission scanning electron microscope (FESEM, JEOL JSM-6335F) equipped with an energydispersive X-ray (EDX) microanalyzer. Room temperature magnetic measurement was performed on the zinc ferrite nanoparticles with an average particle size of 30 nm, obtained from reactions conducted at 150 ◦ C for 18 h, using a vibration sample magnetometer (VSM, Lake Shore Model 7404).
mobility during the solvothermal process (Byrappa & Yoshimura, 2001), lower solubility of the precursors in ethyl alcohol could reduce the contents of the dissolved precursors. According to the XRD results as exemplified in Fig. 1(a), the application of the ultrasonic wave on the reaction mixtures using the ultrasonic bath (160/320 W, 35 kHz, 30 min) without successive solvothermal reactions did not result in the desired zinc ferrite, but only amorphous powders. This was contrary to previous report on the use of ultrasonic probe (70 W, 20 kHz, 50 min) (Sivakumar et al., 2006), resulting in well crystallized zinc ferrite nanoparticles even at lower power and frequency. Among different synthesis variables, contact of the ultrasonic sources (probe and bath) with reaction mixtures and different solvents and metal precursors seemed to be important parameters. Attempt to carry out solvothermal reactions using non-ultrasonicated reaction mixtures, resulted in the formation of zinc ferrite as the major component, as illustrated in Fig. 1(b). The SEM images in Fig. 2(a) reveal that the size of the
3. Results and discussion 3.1. Ultrasonic wave assisted solvothermal preparation of zinc ferrite nanoparticles Fig. 1 shows the XRD patterns collected on particles synthesized at 150 ◦ C for 24 h using different volume ratios of the C2 H5 OH:H2 O mixed solvents. Among the three different volume ratios of the mixed solvents (Fig. 1(c)–(e)), nearly phase-pure and phase-pure zinc ferrite particles could be synthesized only when the 1:1 and 1:2 (v/v) C2 H5 OH:H2 O mixed solvents were used, respectively. A broad feature observed in the collected diffraction patterns should be attributed to the nano-sized nature of the particles obtained. The use of the 2:1 (v/v) C2 H5 OH:H2 O mixed solvent resulted in the presence of the oxide phases of zinc and iron. This may be due to a number of possibilities. Requirement of large amount of water in order to generate sufficient • H and • OH radicals which were reported to be important in successful synthesis of zinc ferrite (Reddy et al., 2010) could be a reason. Relatively less solubility of the chloride precursors in alcohol than in water (Resa, González, Fanega, Ortiz de Landaluce, & Lanz, 2002), may be another reason. When less polarity of ethyl alcohol might facilitate molecular
Fig. 1. XRD patterns of powders prepared from: (a) ultrasonic cavitation process only, (b) hydrothermal reaction (150 ◦ C for 24 h), and solvothermal reaction (150 ◦ C for 24 h) using different ethanol to water volume ratios: (c) 2:1, (d) 1:1 and (e) 1:2 of the mixed solvents.
Fig. 2. SEM photographs of particles resulting from (a) hydrothermal reaction, and (b) solvothermal reaction using 1:2 (v/v) C2 H5 OH:H2 O mixed solvent, both conducted at 150 ◦ C for 24 h.
Please cite this article in press as: Surinwong, S., & Rujiwatra, A. Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles. Particuology (2012), http://dx.doi.org/10.1016/j.partic.2012.06.008
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resulting particles varied over a wide range, as compared to the ultrasonicated case of Fig. 2(b), under similar synthesis conditions. Application of ultrasonic wave in Fig. 2(b) to the reaction mixtures prior to solvothermal synthesis resulted in both reduction of the resulting particle size and improved uniformity of particle sizes. This may be attributed to acoustic cavitation phenomenon and the formation of hot spots (Suslick & Price, 1999), from which the nuclei of zinc ferrite could be induced but with concentration low enough for the growth process to be outweighed in subsequent solvothermal reaction (Suslick & Price, 1999). The low concentration of the established zinc ferrite nuclei was attributed to the poor solubility of the metal precursors in ethyl alcohol (Resa et al., 2002). The influences of reaction temperature and time on phase purity, particle size and size distribution of the resulting zinc ferrites were further investigated using 1:1 and 1:2 (v/v) C2 H5 OH:H2 O mixed solvents. The results obtained for the two mixing ratios were noted to be closely similar. In comparison to the reactions conducted at 150 ◦ C, lowering of reaction temperature to 130 ◦ C resulted in other metal oxides as exemplified in Fig. 3(a). Zinc ferrite particles could not be synthesized as a pure phase even at 130 ◦ C when the reaction was extended to 24 h. This is similar to shortening the reaction time to 12 h at 150 ◦ C, which also resulted in the formation of undesired oxides (Fig. 3(b)). Reduction of reaction time to 18 h at 150 ◦ C, on the other hand, showed no significant effect on phase purity of the synthesized particles (Fig. 3(c)). Zinc ferrite nanoparticles could therefore be synthesized by the above ultrasonic cavitationassisted solvothermal reactions at temperatures of at least 150 ◦ C for at least 18 h. The XRD patterns of the particles obtained from the solvothermal reaction using either 1:1 and 1:2 (v/v) C2 H5 OH:H2 O mixed solvents, at 150 ◦ C for 18–24 h could be readily indexed as ¯ cubic zinc ferrite (JCPDS 82-1049); Fd3m, giving similar refined ˚ Fig. 4 shows a selected area electron diffraca = 8.41(3)–8.44(1) A. tion pattern taken on randomly chosen particles, to reveal the diffraction ring pattern characteristic of the nanoparticles. These rings correspond well to the characteristic interplanar spacing [2 2 0], [3 1 1], [4 0 0], [3 3 3] and [4 4 0] of the cubic spinel zinc
Fig. 3. XRD patterns of particles resulting from solvothermal reactions conducted at (a) 130 ◦ C for 24 h, and at 150 ◦ C for (b) 12 h, (c) 18 h and (d) 24 h, all using the 1:2 (v/v) C2 H5 OH:H2 O mixed solvent.
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Fig. 4. Selected area electron diffraction pattern of the synthesized zinc ferrites.
ferrite (JCPDS 82-1049). These results serve to confirm the well crystallized structure of the produced uniformly sized zinc ferrite nanoparticles. The EDX results as representatively shown in Fig. 5 confirmed the stoichiometric 1:2 Zn:Fe molar ratio. The crystallite sizes estimated from the (3 1 1) diffraction peaks of the collected XRD patterns were also almost identical, i.e., 10.1(4)–10.4(3) nm, and were comparable to those obtained from the colloid mill and hydrothermal technique (9–19 nm) (Fan et al., 2009) and the sputtering method (10.0 nm) (Nakashima et al., 2007). With the aid of appropriate reagents, the estimated crystallite sizes of the yielded particles could be significantly smaller or larger, depending on the types of the reagents added (Köseo˘glu et al., 2008; Wu et al., 2001; Xue, Li, Wang, & Fu, 2007; Zhihao & Lide, 1998). As shown in Fig. 2(b), the particles appeared mostly as agglomerates of more-or-less spherical particles. Fig. 6 shows particle size distributions measured from SEM photographs (di-axes measurements of over 500 particles for each sample), indicating an average size of smaller than 20 nm. In the case of 1:1 (v/v) C2 H5 OH:H2 O mixed solvent (Fig. 6(a) and (b)), extension of reaction time apparently resulted in the growth of the average particle size and in the widening of particle size distribution. Similar trend of particle growth with extended reaction time, particle size seemed to be distributed within smaller ranges for the 1:2 (v/v) C2 H5 OH:H2 O case (Fig. 6(c) and (d)). For both 1:1 and 1:2
Fig. 5. EDX result indicating the stoichiometric 1:2 Zn:Fe molar ratio.
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Fig. 6. Histograms showing particle size distributions measured from the SEM photographs for the particles produced from solvothermal reaction using 1:1 (v/v) C2 H5 OH:H2 O solvent at (a) 150 ◦ C for 18 h and (b) 150 ◦ C for 24 h, compared to those using 1:2 (v/v) C2 H5 OH:H2 O solvent at (c) 150 ◦ C for 18 h and (d) 150 ◦ C for 24 h.
(v/v) C2 H5 OH:H2 O cases, average particle sizes approach approximately 20 nm when the reactions were extended to 24 h. Compared to previous reports on the solvothermal synthesis of zinc ferrite nanoparticles, sizes of the nanoparticles reported in this work were comparable to those synthesized using similar ranges of reaction
temperature and time; 150–180 ◦ C and 18–24 h (Chu et al., 2006; Fan et al., 2009; Hu et al., 2004; Yu et al., 2003). It may be noted that there was a report on the use of ammonia for similar synthesis of zinc ferrite nanoparticles, though the sizes of the yielded particles were significantly larger, i.e. 300 nm (Yu et al., 2003).
Please cite this article in press as: Surinwong, S., & Rujiwatra, A. Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles. Particuology (2012), http://dx.doi.org/10.1016/j.partic.2012.06.008
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XRD results. The room-temperature field-dependent magnetization of the yielded nanoparticles showed characteristic S feature with magnetization of 24.32 emu/g at 1 T, which was relatively high as compared to reported values of nanoparticles of similar sizes, and relatively large degree of inversion in the synthesized spinel structure could be assumed.
Acknowledgments This work is financially supported by the Thailand Research Fund. Sireenart Surinwong thanks the Development and Promotion for Science and Technology Talents Project and the Graduate School, Chiang Mai University for the graduate scholarship. Dr. Santi Maensiri is thanked for the VSM measurements. Fig. 7. Field dependent magnetization at room temperature for synthesized zinc ferrite nanoparticles of 20 nm in average size.
3.2. Room temperature magnetic coupling of the zinc ferrite nanoparticles Regarding the crystal structure of zinc ferrite, the normal spinel structure should be expected to be antiferromagnetic in nature (Tanaka, Makita, Shimizugawa, Hirao, & Soga, 1998). At room temperature, paramagnetic behavior is thus normal. The structure of zinc ferrite nanoparticles can however be partially inverted: a fraction of FeIII is distributed out of the preferred octahedral A sites to the tetrahedral B sites (Upadhyay, Verma, Sathe, & Pimpale, 2007). This then leads to the coupling between the cations at both A and B sites, and therefore the occurrence of special features such as enhanced magnetic susceptibility, hyperfine magnetic field and superparamagnetic coupling (Yao et al., 2007). These features are nonetheless well acknowledged to be size dependent (Liu et al., 2003; Zhou, Xue, Chan, & Wang, 2001). The room-temperature field dependent magnetization, M(H) of zinc ferrite nanoparticles synthesized at 150 ◦ C for 18 h exhibited the characteristic S curve for superparamagnetic ordering, as shown in Fig. 7. The magnetization at the highest applied field of 1 T was 24.32 emu/g. There was no sign of saturation in magnetization in the recorded M(H) curve, suggesting the highly disordered surface-spin structure of the nanoparticles (Upadhyay et al., 2007). Neither is coercivity found near the origin, implying ideal superparamagnetism. Compared to previously reported room temperature magnetization for zinc ferrite nanoparticles of similar sizes (Atif et al., 2006; Köseo˘glu et al., 2008; Xue et al., 2007; Yao et al., 2007), the observed value of 24.32 emu/g at 1 T is notably high. This implies relatively large degree of inversion in the synthesized spinel structure, which then results in the strong ferrimagnetic coupling between the FeIII ions located in the octahedral and tetrahedral sites (Köseo˘glu et al., 2008). 4. Conclusions ¯ In summary, nanoparticles of cubic zinc ferrites (Fd3m) could be synthesized as a single phase using the ultrasonic cavitation assisted solvothermal technique at reaction temperatures of at least 150–180 ◦ C for at least 18 h. The influences of the ultrasonic cavitation process and the use of C2 H5 OH:H2 O mixed solvents in diminishing average particle size and in improving particle size uniformity were illustrated. Size of the yielded nanoparticles is distributed in narrow ranges with the largest average size of approximately 20 nm as measured from SEM photographs, and the crystallite sizes of approximately 10 nm as estimated from
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Please cite this article in press as: Surinwong, S., & Rujiwatra, A. Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles. Particuology (2012), http://dx.doi.org/10.1016/j.partic.2012.06.008
ARTICLE IN PRESS Particuology xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles Sireenart Surinwong a , Apinpus Rujiwatra b,∗ a b
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
a r t i c l e
i n f o
Article history: Received 24 February 2012 Received in revised form 21 May 2012 Accepted 12 June 2012 Keywords: Zinc ferrite Nanoparticle Solvothermal Ultrasonic Superparamagnetism
a b s t r a c t ¯ ˚ were synthesized as a single phase by Nanoparticles of cubic zinc ferrites (Fd3m, a = 8.41(3)–8.44(1) A) an ultrasonic cavitation-assisted solvothermal technique using ethyl alcohol–water mixed solvents at temperatures of 150 ◦ C or higher for 18 h or more. The influences of the ultrasonic cavitation and the use of C2 H5 OH–H2 O mixed solvents in diminishing average particle size and in improving particle size uniformity were illustrated. The largest average size of nanoparticles obtained was 20 nm as measured from SEM photographs, with crystallite size of approximately 10 nm as estimated from XRD results. The room-temperature field-dependent magnetization of the nanoparticles obtained showed a characteristic S feature with magnetization of 24.32 emu/g at 1 T. © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Due to inherent superparamagnetism and a small energy band gap (Wu, Okuya, & Kaneko, 2001), spinel zinc ferrite has been found useful in various applications, e.g., electronic (Hakim, Manjurul Haque, Huq, & Nordblad, 2011), catalytic (Fan, Gu, Yang, & Li, 2009) and biomedical (Sharifi, Shokrollahi, & Amiri, 2012). Chemical and physical properties of zinc ferrite depend nonetheless on particle size, which varies drastically with synthesis routes and parameters. There are numerous techniques for preparing zinc ferrite nanoparticles, e.g., solid state combustion (Bardhan et al., 2010), sol–gel (Atif, Hasanain, & Nadeem, 2006), microemulsion (Zhihao & Lide, 1998) co-precipitation (Chen, Yang, Liu, & Huang, 2000), and solvothermal process (Chu, Jiang, & Zheng, 2006; Fan et al., 2009; Hu, Guan, & Yan, 2004; Yu, Fujino, & Yoshimura, 2003). Among these techniques, the solvothermal process appears supe¯ rior particularly for scaling up (Somiya & Roy, 2000; Zou, Xu, Hou, Wu, & Sun, 2007). Zinc ferrite nanoparticles can be prepared as a single phase, for example, under solvothermal conditions within a wide range of temperatures, 120–250 ◦ C, and over a wide range of reaction time, 1–48 h (Chu et al., 2006; Fan et al., 2009; Hu et al., 2004; Yu et al., 2003). Characteristics of the product nanoparticles however differ greatly, depending on synthesis parameters, e.g., reaction temperature and time, solvent, zinc and
∗ Corresponding author. Tel.: +66 5394 1906; fax: +66 5389 2277. E-mail addresses: [email protected], [email protected] (A. Rujiwatra).
iron precursors, and source of heating. Without using additives, a common drawback consists of poor particle size uniformity and hard agglomeration. To overcome such drawback and to improve the uniformity of particle sizes, the use of ultrasonic cavitation technique was recently reported (Reddy, Sivasankar, Sivakumar, & Moholkar, 2010; Sivakumar, Towata, Yasui, Tuziuti, & Iida, 2006). In this work, the ultrasonic cavitation technique was tried in the solvothermal synthesis of zinc ferrite nanoparticles, aiming at uniform particle size. Mixed solvents prepared with different ratios of ethyl alcohol and water were used as the liquid medium for the synthesis. Magnetic property of the nanoparticles prepared was studied at room temperature. 2. Materials and method Mixtures of ZnCl2 0.0083 mol (98.0%, Merck KGaA) and FeCl3 ·6H2 O 0.0166 mol (100.89%, Fluka) were first prepared using 10 cm3 of mixed solvents which were prepared from C2 H5 OH (99.5%, Merck) and deionized water in three different C2 H5 OH:H2 O volume ratios (v/v): 2:1, 1:1 and 1:2 (v/v). KOH pellets (85%, Merck) were then gradually added into each mixture with stirring to a concentration of 6.0 mol/L. The pHs of the mixtures were measured using universal pH indicator strips (Merck KGaA, Germany) to a common value of 14. Each mixture was ultrasonicated at 70(±5) ◦ C for 3 h in a laboratory ultrasonic bath (Bandelin Electronic RK255H, 160/320 W, 35 kHz), and transferred into a 25 mL solvothermal autoclave. The reactions were then conducted under an autogenous pressure generated at 130–180 ◦ C for 12–24 h. The synthesized
1674-2001/$ – see front matter © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.partic.2012.06.008
Please cite this article in press as: Surinwong, S., & Rujiwatra, A. Ultrasonic cavitation assisted solvothermal synthesis of superparamagnetic zinc ferrite nanoparticles. Particuology (2012), http://dx.doi.org/10.1016/j.partic.2012.06.008
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particles were recovered by filtration, washed with deionized water until the pH of the filtrate was ca. 7, and dried in air. Powder X-ray diffractometer (XRD, Bruker D8 Advance, CuK␣, ˚ 40 kV, 30 mA) and transmission electron Ni filter, = 1.540598 A, microscope (TEM, JEOL JEM-2010) were used to characterize the crystalline phases. From the collected XRD patterns, crystallite sizes of the yielded particles were calculated based on the most intense (3 1 1) peak and using the Scherrer equation (Fan et al., 2009). Particle size, morphology, and elemental composition of the yielded particles were investigated using a field emission scanning electron microscope (FESEM, JEOL JSM-6335F) equipped with an energydispersive X-ray (EDX) microanalyzer. Room temperature magnetic measurement was performed on the zinc ferrite nanoparticles with an average particle size of 30 nm, obtained from reactions conducted at 150 ◦ C for 18 h, using a vibration sample magnetometer (VSM, Lake Shore Model 7404).
mobility during the solvothermal process (Byrappa & Yoshimura, 2001), lower solubility of the precursors in ethyl alcohol could reduce the contents of the dissolved precursors. According to the XRD results as exemplified in Fig. 1(a), the application of the ultrasonic wave on the reaction mixtures using the ultrasonic bath (160/320 W, 35 kHz, 30 min) without successive solvothermal reactions did not result in the desired zinc ferrite, but only amorphous powders. This was contrary to previous report on the use of ultrasonic probe (70 W, 20 kHz, 50 min) (Sivakumar et al., 2006), resulting in well crystallized zinc ferrite nanoparticles even at lower power and frequency. Among different synthesis variables, contact of the ultrasonic sources (probe and bath) with reaction mixtures and different solvents and metal precursors seemed to be important parameters. Attempt to carry out solvothermal reactions using non-ultrasonicated reaction mixtures, resulted in the formation of zinc ferrite as the major component, as illustrated in Fig. 1(b). The SEM images in Fig. 2(a) reveal that the size of the
3. Results and discussion 3.1. Ultrasonic wave assisted solvothermal preparation of zinc ferrite nanoparticles Fig. 1 shows the XRD patterns collected on particles synthesized at 150 ◦ C for 24 h using different volume ratios of the C2 H5 OH:H2 O mixed solvents. Among the three different volume ratios of the mixed solvents (Fig. 1(c)–(e)), nearly phase-pure and phase-pure zinc ferrite particles could be synthesized only when the 1:1 and 1:2 (v/v) C2 H5 OH:H2 O mixed solvents were used, respectively. A broad feature observed in the collected diffraction patterns should be attributed to the nano-sized nature of the particles obtained. The use of the 2:1 (v/v) C2 H5 OH:H2 O mixed solvent resulted in the presence of the oxide phases of zinc and iron. This may be due to a number of possibilities. Requirement of large amount of water in order to generate sufficient • H and • OH radicals which were reported to be important in successful synthesis of zinc ferrite (Reddy et al., 2010) could be a reason. Relatively less solubility of the chloride precursors in alcohol than in water (Resa, González, Fanega, Ortiz de Landaluce, & Lanz, 2002), may be another reason. When less polarity of ethyl alcohol might facilitate molecular
Fig. 1. XRD patterns of powders prepared from: (a) ultrasonic cavitation process only, (b) hydrothermal reaction (150 ◦ C for 24 h), and solvothermal reaction (150 ◦ C for 24 h) using different ethanol to water volume ratios: (c) 2:1, (d) 1:1 and (e) 1:2 of the mixed solvents.
Fig. 2. SEM photographs of particles resulting from (a) hydrothermal reaction, and (b) solvothermal reaction using 1:2 (v/v) C2 H5 OH:H2 O mixed solvent, both conducted at 150 ◦ C for 24 h.
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resulting particles varied over a wide range, as compared to the ultrasonicated case of Fig. 2(b), under similar synthesis conditions. Application of ultrasonic wave in Fig. 2(b) to the reaction mixtures prior to solvothermal synthesis resulted in both reduction of the resulting particle size and improved uniformity of particle sizes. This may be attributed to acoustic cavitation phenomenon and the formation of hot spots (Suslick & Price, 1999), from which the nuclei of zinc ferrite could be induced but with concentration low enough for the growth process to be outweighed in subsequent solvothermal reaction (Suslick & Price, 1999). The low concentration of the established zinc ferrite nuclei was attributed to the poor solubility of the metal precursors in ethyl alcohol (Resa et al., 2002). The influences of reaction temperature and time on phase purity, particle size and size distribution of the resulting zinc ferrites were further investigated using 1:1 and 1:2 (v/v) C2 H5 OH:H2 O mixed solvents. The results obtained for the two mixing ratios were noted to be closely similar. In comparison to the reactions conducted at 150 ◦ C, lowering of reaction temperature to 130 ◦ C resulted in other metal oxides as exemplified in Fig. 3(a). Zinc ferrite particles could not be synthesized as a pure phase even at 130 ◦ C when the reaction was extended to 24 h. This is similar to shortening the reaction time to 12 h at 150 ◦ C, which also resulted in the formation of undesired oxides (Fig. 3(b)). Reduction of reaction time to 18 h at 150 ◦ C, on the other hand, showed no significant effect on phase purity of the synthesized particles (Fig. 3(c)). Zinc ferrite nanoparticles could therefore be synthesized by the above ultrasonic cavitationassisted solvothermal reactions at temperatures of at least 150 ◦ C for at least 18 h. The XRD patterns of the particles obtained from the solvothermal reaction using either 1:1 and 1:2 (v/v) C2 H5 OH:H2 O mixed solvents, at 150 ◦ C for 18–24 h could be readily indexed as ¯ cubic zinc ferrite (JCPDS 82-1049); Fd3m, giving similar refined ˚ Fig. 4 shows a selected area electron diffraca = 8.41(3)–8.44(1) A. tion pattern taken on randomly chosen particles, to reveal the diffraction ring pattern characteristic of the nanoparticles. These rings correspond well to the characteristic interplanar spacing [2 2 0], [3 1 1], [4 0 0], [3 3 3] and [4 4 0] of the cubic spinel zinc
Fig. 3. XRD patterns of particles resulting from solvothermal reactions conducted at (a) 130 ◦ C for 24 h, and at 150 ◦ C for (b) 12 h, (c) 18 h and (d) 24 h, all using the 1:2 (v/v) C2 H5 OH:H2 O mixed solvent.
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Fig. 4. Selected area electron diffraction pattern of the synthesized zinc ferrites.
ferrite (JCPDS 82-1049). These results serve to confirm the well crystallized structure of the produced uniformly sized zinc ferrite nanoparticles. The EDX results as representatively shown in Fig. 5 confirmed the stoichiometric 1:2 Zn:Fe molar ratio. The crystallite sizes estimated from the (3 1 1) diffraction peaks of the collected XRD patterns were also almost identical, i.e., 10.1(4)–10.4(3) nm, and were comparable to those obtained from the colloid mill and hydrothermal technique (9–19 nm) (Fan et al., 2009) and the sputtering method (10.0 nm) (Nakashima et al., 2007). With the aid of appropriate reagents, the estimated crystallite sizes of the yielded particles could be significantly smaller or larger, depending on the types of the reagents added (Köseo˘glu et al., 2008; Wu et al., 2001; Xue, Li, Wang, & Fu, 2007; Zhihao & Lide, 1998). As shown in Fig. 2(b), the particles appeared mostly as agglomerates of more-or-less spherical particles. Fig. 6 shows particle size distributions measured from SEM photographs (di-axes measurements of over 500 particles for each sample), indicating an average size of smaller than 20 nm. In the case of 1:1 (v/v) C2 H5 OH:H2 O mixed solvent (Fig. 6(a) and (b)), extension of reaction time apparently resulted in the growth of the average particle size and in the widening of particle size distribution. Similar trend of particle growth with extended reaction time, particle size seemed to be distributed within smaller ranges for the 1:2 (v/v) C2 H5 OH:H2 O case (Fig. 6(c) and (d)). For both 1:1 and 1:2
Fig. 5. EDX result indicating the stoichiometric 1:2 Zn:Fe molar ratio.
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Fig. 6. Histograms showing particle size distributions measured from the SEM photographs for the particles produced from solvothermal reaction using 1:1 (v/v) C2 H5 OH:H2 O solvent at (a) 150 ◦ C for 18 h and (b) 150 ◦ C for 24 h, compared to those using 1:2 (v/v) C2 H5 OH:H2 O solvent at (c) 150 ◦ C for 18 h and (d) 150 ◦ C for 24 h.
(v/v) C2 H5 OH:H2 O cases, average particle sizes approach approximately 20 nm when the reactions were extended to 24 h. Compared to previous reports on the solvothermal synthesis of zinc ferrite nanoparticles, sizes of the nanoparticles reported in this work were comparable to those synthesized using similar ranges of reaction
temperature and time; 150–180 ◦ C and 18–24 h (Chu et al., 2006; Fan et al., 2009; Hu et al., 2004; Yu et al., 2003). It may be noted that there was a report on the use of ammonia for similar synthesis of zinc ferrite nanoparticles, though the sizes of the yielded particles were significantly larger, i.e. 300 nm (Yu et al., 2003).
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XRD results. The room-temperature field-dependent magnetization of the yielded nanoparticles showed characteristic S feature with magnetization of 24.32 emu/g at 1 T, which was relatively high as compared to reported values of nanoparticles of similar sizes, and relatively large degree of inversion in the synthesized spinel structure could be assumed.
Acknowledgments This work is financially supported by the Thailand Research Fund. Sireenart Surinwong thanks the Development and Promotion for Science and Technology Talents Project and the Graduate School, Chiang Mai University for the graduate scholarship. Dr. Santi Maensiri is thanked for the VSM measurements. Fig. 7. Field dependent magnetization at room temperature for synthesized zinc ferrite nanoparticles of 20 nm in average size.
3.2. Room temperature magnetic coupling of the zinc ferrite nanoparticles Regarding the crystal structure of zinc ferrite, the normal spinel structure should be expected to be antiferromagnetic in nature (Tanaka, Makita, Shimizugawa, Hirao, & Soga, 1998). At room temperature, paramagnetic behavior is thus normal. The structure of zinc ferrite nanoparticles can however be partially inverted: a fraction of FeIII is distributed out of the preferred octahedral A sites to the tetrahedral B sites (Upadhyay, Verma, Sathe, & Pimpale, 2007). This then leads to the coupling between the cations at both A and B sites, and therefore the occurrence of special features such as enhanced magnetic susceptibility, hyperfine magnetic field and superparamagnetic coupling (Yao et al., 2007). These features are nonetheless well acknowledged to be size dependent (Liu et al., 2003; Zhou, Xue, Chan, & Wang, 2001). The room-temperature field dependent magnetization, M(H) of zinc ferrite nanoparticles synthesized at 150 ◦ C for 18 h exhibited the characteristic S curve for superparamagnetic ordering, as shown in Fig. 7. The magnetization at the highest applied field of 1 T was 24.32 emu/g. There was no sign of saturation in magnetization in the recorded M(H) curve, suggesting the highly disordered surface-spin structure of the nanoparticles (Upadhyay et al., 2007). Neither is coercivity found near the origin, implying ideal superparamagnetism. Compared to previously reported room temperature magnetization for zinc ferrite nanoparticles of similar sizes (Atif et al., 2006; Köseo˘glu et al., 2008; Xue et al., 2007; Yao et al., 2007), the observed value of 24.32 emu/g at 1 T is notably high. This implies relatively large degree of inversion in the synthesized spinel structure, which then results in the strong ferrimagnetic coupling between the FeIII ions located in the octahedral and tetrahedral sites (Köseo˘glu et al., 2008). 4. Conclusions ¯ In summary, nanoparticles of cubic zinc ferrites (Fd3m) could be synthesized as a single phase using the ultrasonic cavitation assisted solvothermal technique at reaction temperatures of at least 150–180 ◦ C for at least 18 h. The influences of the ultrasonic cavitation process and the use of C2 H5 OH:H2 O mixed solvents in diminishing average particle size and in improving particle size uniformity were illustrated. Size of the yielded nanoparticles is distributed in narrow ranges with the largest average size of approximately 20 nm as measured from SEM photographs, and the crystallite sizes of approximately 10 nm as estimated from
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