synthesis, characterization and growing mechanism of monodisperse fe3o4 microspheres

Synthesis, characterization and growing mechanism of monodisperse Fe 3 O 4 microspheres Yingdi Lv a , Hui Wang a, Ã , Xiaofang Wang a , Jinbo Bai b a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), Department of Chemistry, Northwest University, Xi ’an 710069, PR China b Laboratory MSS/MAT, CNRS UMR 8579, Ecole Centrale Paris, 92295 Chatenay Malabry, France article info Article history: Received 14 January 2009 Received in revised form 8 March 2009 Accepted 31 March 2009 Communicated by S. Uda Available online 8 April 2009 PACS: 61.10.Eq 81.16.Dn 81.07-b 81.07.Lk 81.10.Dn Keywords: A1. Nanostructures A2. Growth from solutions A2. Hydrothermal crystal growth B1. Nanomaterials B2. Iron oxide abstract Monodisperse Fe 3 O 4 microspheres assembled by a number of nanosize tetrahedron subunits have been selectively synthesized through the hydrothermal process. The synthesized Fe 3 O 4 microspheres have good dispersibility. The subunits made up of microspheres were uniform in size and like-tetrahedron in shape. The average diameter of each Fe 3 O 4 microsphere is about 50–55 mm. The length of each edge of tetrahedron is about 100 nm. A series of experiments had been carried out to investigate the effect of reductant, precipitator and reaction time on the formation of Fe 3 O 4 microsphere and tetrahedron subunits. The results show that ascorbic acid as reductant and urea as precipitator supplied a relatively steady environment during the synthesis process and led to the formations of Fe 3 O 4 tetrahedron subunit and monodisperse Fe 3 O 4 microspheres. As the reaction time increased from 3 to 24 h, the Fe 3 O 4 microspheres tended towards dispersion and becoming large in size from 10–20 to 50–55 mm, and the subunits formed Fe 3 O 4 microspheres that varied from spheroid to tetrahedron and from a small nanoparticle (20–30 nm) to a large one (90–110 nm). A reasonable explanation for the formations of the Fe 3 O 4 microsphere and the tetrahedron subunit was proposed through Ostwald ripening and the attachment growth mechanism, respectively. & 2009 Elsevier B.V. All rights reserved. 1. Introduction It has been accepted that the special structure and character- istics of microspheres assembled by nanoparticles endow them with potential application such as catalysis, medicine, electronics and contrast agents in MIR [1]. Thus, the synthesis of magnetic particles with specific size and well-defined morphologies has become a hot topic in the related research field. Among magnetic particles, Fe 3 O 4 have been extensively investigated. Fe 3 O 4 as one of the most important transition magnetic metal oxides has received increasing attention due to its extensive applications. It has been considered as an ideal material for magnetic data storage [2], a candidate for biological application such as a tag for sensing and imaging [3], and a drug-delivery carrier for antitumor therapy [4]. The synthesis of Fe 3 O 4 nanocrystals with different sizes and shapes has attracted considerable interest in recent years. So a number of Fe 3 O 4 nanocrystals, such as nanoparticles, nanowires, nanorods, nano- belts, nanopyramids, and nanotubes via different synthesis methods have been reported by different research groups [5–11]. Although the fabrication of monodisperse magnetic particles have been pioneered by Mati jevic’s group in the early 1980s [1 2 ],no more attention has been paid to the synthesis of monodisperse- sphere microparticles assemb led by nanoparticles. I n this paper, we report a ne w synthesis method of Fe 3 O 4 microspheres assembled by a specific nanoparticles via hydr othermal method at a low temperatur e, in which FeCl 3 Á 6H 2 O, ascor bic acid, ure a and poly- ethy lene glycol 6000(PEG-6000) were u sed as precursor, r eductant, precipitator and surfactant, respectiv ely. The effects of the reductant and the precipitator on the formation of products were discussed. In addition, the function of the surfactants in the process of self- assembled F e 3 O 4 microspheres was a lso i nv estigated. 2. Experimental section 2.1. Materials Hexahydrated ferric chloride (FeCl 3 Á 6H 2 O), sodium hydroxide (NaOH) and urea were bought from Rgent Company in Tianjin. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.03.046 Ã Corresponding author. Tel.: +86 29 8836 3115; fax: +86 29 8830 3798. E-mail address: huiwang@nwu.edu.cn (H. Wang). Journal of Crystal Growth 311 (2009) 3445–3450 ARTICLE IN PRESS Ascorbic acid was bought from Northwest Geological Institute of Nonferrous Metals in Xi’an. Polyethylene glycol 6000 (PEG-6000) was bought from Kermel Company in Tianjin. All the reagents are AR. 2.2. Synthesis methods In a typical procedure, 0.003 mol FeCl 3 Á 6H 2 O, 0.003 mol ascorbic acid, 0.015 g PEG-6000 and 0.6 g urea were dissolved in a deionized water of 25ml to get an orange solution of 0.12 mol/L Fe 3+ ions. The precursor solution was sealed into a 50ml Teflon- lined autoclave, followed by the hydrothermal treatment at 120 1C for 3 or 8 or 24 h and then cooled to room temperature naturally. Three kinds of black solid products could be obtained at three various times, respectively. These hydrothermal products would be collected after washing with deionized water several times and subsequently dried in vacuum at 60 1C for 6 h. The above- described experiment conditions are denoted as the standard condition. The other conditions (under the standard condition) were invariable if changed the species of the precipitator or the amounts of the reductant. 2.3. Characterizations X-ray diffraction (XRD) patterns were recorded on a Rigaku (Japan) D/Max r-A X-ray diffractometer with Cu K a radiation (50 kV, 300 mA) at room temperature in air. Scanning electron microscopy (SEM) images were taken with Quanta 400 ESEM-FEG instrument operated at 25 kV, transmission electron microscope (TEM) images were taken with JEOL JEM-3010 instrument operated at 300 kV. They were used to characterize morphology, particle sizes, and compositions of the products. The specific surface areas of the samples were measured with a surface analyzer (JW-K) by the absorption of N 2 at liquid N 2 temperature (BET method). 3. Result and discussion Ascorbic acid had been reported as a relatively weak reductant in many studies [13,14]. Urea also had been reported as an effective precipitator in the previous research due to its alkaline reaction by slow hydrolysis at 70 1C [15,16]. This characteristic made the pH of the reaction system change in a narrow range. So we selected ascorbic acid as reductant and urea as precipitator. In addition, if ascorbic acid and urea are simultaneously utilized, a very soft reaction that favors the growth of crystal will take place. However, the changes in the number of the reductant and the species of precipitator would influence the synthesis of product. 3.1. Effect of reductant and precipitator Fig. 1 shows the SEM and TEM images of the product prepared at 120 1C for 24 h under the standard condition. Fig. 2 shows the XRD patterns of the various products synthesized by the above- mentioned methods in Section 2.2. It is apparent from Fig. 1a that the Fe 3 O 4 product synthesized in this work presents in the form of a large amount of microspheres, and all the microspheres are monodisperse with a relatively uniform size. XRD pattern shown in Fig. 2(a)-2 indicates the product to be a cubic-structured Fe 3 O 4 according to the standard pattern (JCPDS Card no. 85-1436, a =8.393 A). The average diameter of the microspheres shown in Fig. 1b is between 50 and 55 m m and each of them is very round. The existing small holes and chippings were due to outside force effect originating from SEM test on the surface of microspheres. Fig. 1c shows an image of individual microsphere with a clear surface and the surface is smooth. Fig. 1d reveals the specific surface condition of the microspheres shown in Fig. 1c. The surface of microspheres was made of a number of small subunits, each of which is like a tetrahedron crystal in shape and uniform in size and packed densely. Fig. 1e clearly exhibits that each of the subunits is a nanometer size of regular tetrahedron crystal with four symmetrical horns, and TEM imagine in Fig. 1f further supports this result. The TEM imagine also shows that the length of each edge of tetrahedron is about 100 nm. The selected area electron diffraction pattern indicates the nanosize tetrahedron to be a single-crystalline structure. To investigate the formation factors of Fe 3 O 4 microspheres, a series of experiments had been carried out in this work. The results show that ascorbic acid plays an important roll in the formation of Fe 3 O 4 microspheres. It is found that Fe 3 O 4 could be formed in the presence of ascorbic acid, and in contrast, not formed in the absence of ascorbic acid under the standard condition. The reason for it may be explained that an appropriate amount of ascorbic acid (0.003 mol), forming a gentle reductant, could slowly reduce Fe 3+ to Fe 2+ by the following reaction 2Fe 3+ +C 6 H 8 O 6 =2Fe 2+ +C 6 H 6 O 6 +2H 2+ , and presenting a gradual reduction effect at a moderate temperature of 120 1C; and subsequently, the reduced Fe 3+ (Fe 2+ ) and the unreduced Fe 3+ contributed to the formation of Fe 3 O 4 . The further experiment also found that the amount of ascorbic acid was an important impact factor in our system. As the content of ascorbic acid was increased to 0.005 mol with other condition as constant, FeCO 3 could be formed in the reaction system due to existing diffraction peak assigned to FeCO 3 (JCPDS Card no. 29-696, a =8.393 A) in the products, as described in Fig. 2(b). This may be due to the fact that the increase of ascorbic acid decreased the pH value in the system (about pH 6–5), and this relatively low pH value favored not only the reduction of all Fe 3+ to Fe 2+ but also the reaction of urea hydrolysis (NH 2 ) 2 CO+3H 2 O=2NH 3 Á H 2 O+CO 2 , led to producing much more CO 2 to form FeCO 3 . While the pH values were 8–9, the content of ascorbic acid was 0.003 mol under the standard condition. It is concluded that acidic solution was not favorable to the formation of Fe 3 O 4 . In addition, comparing the XRD patterns of FeCO 3 sample with FeCO 3 standard in Fig. 2(b), another crystal phase assigned to FeO can be observed in the sample (JCPDS Card no. 6-615, a =8.393 A). A reasonable explanation for it is that the excessive ascorbic acid could reduce almost all Fe 3+ to Fe 2+ , and formed Fe(OH) 2 due to urea hydrolysis and further generated FeO under hydrothermal condition. This change process could be also described as follows: Fe 2+ +2NH 3 Á H 2 O=Fe(OH) 2 +2NH 4 + and Fe(OH) 2 =FeO+H 2 O. As the quantity of ascorbic acid decreased to 0.001 mol in the reaction system, the XRD pattern in Fig. 2(a)-4 indicated the formation of Fe 2 O 3 . The reason may be that, on one hand, the amount of ascorbic acid added in the reaction system was relatively lower; on the other hand, a part of ascorbic acid in the system reacting with O 2 dissolved in water made the amount of ascorbic acid to be too low to reduce Fe 3+ to Fe 2+ , and finally led to the formation of Fe 2 O 3 . Urea as precipitator was indispensable to this reaction system, without urea Fe 3 O 4 would not produce. As the precipitator changed into sodium hydroxide (NaOH) the morphologies of precipitated Fe 3 O 4 occurred change. A number of microspheres of Fe 3 O 4 were formed in the (NaOH) system, but the surface of each microsphere shown in Fig. 3a is not tight and rather rough. Fig. 3b clearly shows sheet structures on the surface and there are many interspaces among sheets. The sheets have obvious edges. It indicates that different pH value affected the formation rate of Fe 3 O 4 and further changed the microsphere of Fe 3 O 4 , namely, the nucleation rate of Fe 3 O 4 could be controlled by pH [17]. One reason for it may be that, when urea was used as precipitator, pH Y. Lv et al. / Journal of Crystal Growth 311 (200 9) 3445–34503446 ARTICLE IN PRESS Fig. 1. The SEM and TEM imagines of Fe 3 O 4 products synthesized at 120 1C for 24 h under the standard condition: a solution of 25 ml with 0.003 mol FeCl 3 Á 6H 2 O, 0.003 mol ascorbic acid, 0.015 g PEG-6000 and 0.6 g urea. Intensity (a.u.) 20 (a)-5 (a)-4 (a)-3 (a)-2 (a)-1 Intensity (a.u.) (b)-3 (b)-1 (b)-2 2θ (degree) 30 40 50 60 70 20 30 40 50 60 70 2θ (degree) Fig. 2. XRD patterns. (a)-1 Fe 3 O 4 after reaction for 3h and (a)-2 after reaction for 24 h with 0.003 mol ascorbic acid, (a)-3 Fe 3 O 4 standard, (a)-4 Fe 2 O 3 after reaction for 24h with 0.001 mol ascorbic acid, (a)-5 Fe 2 O 3 standard, (b)-1 FeCO 3 standard, (b)-2 FeO standard and (b)-3 FeCO 3 product. Y. Lv et al. / Journal of Crystal Growth 311 (200 9) 3445–3450 3447 ARTICLE IN PRESS value in the (urea) system maintained a relative constant due to keeping a hydrolytic equilibrium of urea hydrolyzed step-by-step, so the nucleation rate of Fe 3 O 4 was relatively slower in urea system than in NaOH system. This relative steady environment during nucleation favored the formation of homogeneous and tight Fe 3 O 4 . In contrast, pH values in NaOH system changed all through and decreased with the nucleation time of forming Fe 3 O 4 , and further resulted in the formation of Fe 3 O 4 microsphere with the loose and sheet structure. The other reason may be that the crystal seed already existed in NaOH system before Fe 3 O 4 generation due to existing OH À and the solid seed would influence the morphology of final crystal; while there was no crystal seed before Fe 3 O 4 formation in the system used urea as precipitator, so the morphology of Fe 3 O 4 , formed by gradual hydrolysis of urea, was more uniform in urea system than in NaOH system. 3.2. Effect of reaction time To have further insight into the formation process of the Fe 3 O 4 microspheres, the products formed at different reaction times of 3 and 8h were characterized by SEM observation and BET method. The SEM images are shown in Fig. 4. Fig. 4a reveals that the microspheres of Fe 3 O 4 formed at 120 1C for 3 h are the aggregates of a lot of spheroidal particles (subunits) and have small opening 2μ μ m 50 μ m Fig. 3. SEM imagines of Fe 3 O 4 products synthesized at 120 1C for 24 h with a solution of 25 ml with 0.003 mol FeCl 3 Á 6H 2 O, 0.003 mol ascorbic acid, 0.015 g PEG-6000 and 0.4 g NaOH as precipitator. 200nm dc 50μ μ m a 200nm b Fig. 4. SEM images of Fe 3 O 4 products synthesized at 120 1C for 3 (a and b) and for 8 h (c and d) with a solution of 25 ml with 0.003 mol FeCl 3 Á 6H 2 O, 0.003 mol ascorbic acid, 0.015 g PEG-6000 and 0.6 g urea as precipitator. Y. Lv et al. / Journal of Crystal Growth 311 (200 9) 3445–34503448 ARTICLE IN PRESS on the surface. The size of microspheres agglomerates is about 10–20 m m(Fig. 4a). The spheroidal subunit shown in Fig. 4bisa uniform nanospheroid and its average diameter is around 20–30 nm. As the reaction time was prolonged to 8 h, it is obvious from Fig. 4c that some monodisperse microspheres with a size of 15–25 m m can be observed and their dispersion became better than those formed at 120 1C for 3 h. The surface of the microspheres is incompact compared to the surface shown in Fig. 4a and the microspheres were also the aggregates of many of small subunits with a nanosize of 40–60nm in Fig. 4d. But the subunit in shape was somewhat close to tetrahedron between spheroid and tetrahedron, as the image inserted in Fig. 4d. As the reaction time was further prolonged to 24 h a completely tetrahedron subunit with a nanosize of 90–110 nm that made up the monodisperse microspheres was formed, as shown in Fig. 2e and f. Accordingly, it concludes that as the reaction time prolonged from 3 to 24 h the aggregated microspheres made up of subunits tended towards dispersion and becoming large in size from 10–20 to 15–25 to 50–55 m m again, and the subunits tended to vary from spheroid to tetrahedron and from a small nanoparticle (20–30 nm) to a large one (90–110 nm). But they were still the same crystal as Fe 3 O 4 due to no difference between the products of 3 and 24 h supported by the XRD patterns in Fig. 2(a)-1 and (a)-2 which agreement with standard card (JCPDS Card no. 85-1436, a =8.393 A). The attachment growth mechanism may be introduced in our system. Because any nanoparticle has an active face with high surface energy during crystal growth process, the crystal growth preferentially occurs on this active face [18]. So in our reaction system, one spheroidal was attached with subsequent formed Fe 3 O 4 crystal from the four-equipotent directions to decrease its surface area to decrease the surface energy at maximum degree. This assumption is further confirmed by BET results listed in Table 1. As the reaction time was 3, 8 and 24 h, the BET specific surface areas of three Fe 3 O 4 products distinctly tended to decrease, and being 23.9174, 19.8599 and 13.6639 m 2 g À1 , respectively. The experimental result indicates that the surface energy was indeed the driven force of this process. All this could be explained in Fig. 5. The spherical subunit generated firstly at the early stage of the fast nucleation in the first 3 h. As the reaction time prolonged, a dissolution-recrystallized process happened in this system. The formed fresh Fe 3 O 4 particle preferentially grew on the surface of the spherical subunit from the four-equipotent direction (step 1). As the reaction time further prolonged, the dissolution-recrystallized process continually occurred on the active face by overlapping layer by layer (step 2) until the subunits of tetrahedron shape were completely formed (step 3) in order to lower-specific surface energy. There are many reports concerning the formation mechanism of inorganic microspheres [19,20]. The formation of microspheres after fast nucleation in solution is related to two primary mechanisms: random aggregation and Ostwald ripening. In the present work, Ostwald ripening may be involved in the formation of Fe 3 O 4 microspheres. Firstly, some Fe 3+ was reduced by ascorbic acid to Fe 2+ in our reaction system. When the reaction tempera- ture reached 70 1C urea hydrolysis produced OH À to increase pH values in the system and subsequently the nanospheroid crystal generated no sooner than the precipitation reaction happened. Driven by the minimization of interfacial energy, the spheroidal nanocrystals would act as primary building units to produce larger self-assembled aggregates (Fig. 4a). This was a kinetically fast process because our system was aqueous with many surface Table 1 The results of BET surface area of Fe 3 O 4 synthesized in 3, 8 and 24 h. Sample Reaction time (h) BET surface area/m 2 g À1 Fe 3 O 4 3 23.9174 Fe 3 O 4 8 19.8599 Fe 3 O 4 4 13.6639 Step 1 Step 2 Step 3 Overlapping layer by layer Continuing layer by layer Fig. 5. The schematic sketch of formation process of Fe 3 O 4 tetrahedron crystal. FeCl 3 ·6H 2 O VC Urea + Form Aggregation Microspheres Shape change Subunits Monodisperse Microspheres Generate Ostwald Ripening Microspheres Fast Nucleation Subunits 3h-24h 3h 24h 8h 24h Fig. 6. The schematic sketch of formation process of microspheres. Y. Lv et al. / Journal of Crystal Growth 311 (200 9) 3445–3450 3449 ARTICLE IN PRESS hydroxyls and did not allow spheroidal subunit to rotate adequately to find the low-energy configuration interface and form perfectly single crystal [21]. These aggregates formed microspheres and continued to grow through Ostwald ripening. As stated early, it is obviously observed from Figs. 4 and 1 that the size of the agglomerates changed from 10–25 to 50–55 m m with increasing time from 3 to 24 h (Figs. 4a and b and 1c) and the size of the subunits varied from 20–30 to 90–110 nm (Figs. 4b, d and 1f), which was in agreement with the result of Ostwald ripening. In addition, as the time prolonged the microsphere surface became more and more compact in structure due to the change of subunit in shape from spheroid to tetrahedron at the cost of newly formed smaller Fe 3 O 4 crystal, and the microspheres became more and more dispersed (see Fig. 2a) because of the continuous hydrothermal process and the presence of PEG-6000. We found that polyethylene glycol 6000 (PEG-6000) kept microspheres monodisperse. When PEG-6000 was removed from our system with the other condition as constant, the microspheres still existed and most of them aggregated together. When we used 0.015 g of poly-vinylpyrrolidone (PVP) instead of PEG-6000, the laminated structure was formed. The results show that PEG-6000 was not only acting as a surfactant but also as a molding agent. Fig. 6 illustrates the schematic sketch of formation process of the microsphere. 4. Conclusion We developed a convenient method to synthesize Fe 3 O 4 microspheres assembled by nanosize subunits with a tetrahedron structure through hydrothermal process. Ascorbic acid as reduc- tant and urea as precipitator played an important role in the process of Fe 3 O 4 crystal formation. Both of them supplied a relatively steady environment, which was a crucial factor that determined the morphologies of subunits and Fe 3 O 4 microspheres formed by the subunits. On the other hand, the reaction time was also an important factor that determined the dispersion of Fe 3 O 4 microspheres and the generation of tetrahedron subunits. We also proposed a reasonable exploration for the growth mechanism of tetrahedron subunit crystal and the aggregations mechanism of Fe 3 O 4 microsphere in the present work. Ostwald ripening and the attachment growth mechanism were responsible for the forma- tion of Fe 3 O 4 microspheres and the growth of tetrahedron subunit, respectively. 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Effect of reductant and precipitator Fig. 1 shows the SEM and TEM images of the product prepared at 120 1C for 24 h under the standard