Synthesis of various magnetic nanostructures and the microwave characterizations 7

Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route 7.1 Introduction Chapter presented that the resonance frequency and the permeability of as-synthesized Zn-ferrite were higher than those of Fe3O4. Hence the enhanced saturation magnetization could extend the Snoek’s limitation. Another optional method to extend the Snoek’s limitation is to induce the shape anisotropic field into Fe3O4 particles. The shape effect on the Snoek’s law could be described as following: 1/2 H (μi − 1)fr = γ4πMs ( ) H ea (1.11 in Chapter 1) The value on the right side would be extended due to Hha > Hea . In this case, the permeability of Fe3O4 may be high at relative high frequency range (GHz range). In this chapter, we succeeded in obtaining uniform Fe3O4 particles with different shapes and studied the effect of various shapes on the resonance frequency. The synthesis process reported here is so called chemical reduction method, which is used to convert -Fe2O3 to Fe3O4 with the morphology being preserved. For several decades, shape control over iron oxide nanocrystals is one of the most interesting topics because their physical and chemical properties can be manipulated through variations on their morphology and size. Unique electron-transport behavior 113 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route was shown by Fe3O4 nanowires.[1] Very high specific capacity (~ 749 mA·h·g-1 at C/5 and ~ 600 mA·h·g-1 at C/2) was exhibited by carbon coated Fe3O4 nanospindles when used as an anode material for Li-Ion batteries.[2] Room temperature magnetoresistance as high as ~ 1.2% was observed in MgO/Fe3O4 core-shell nanowires.[3] Relative high luminescence and very strong magnetic resonance T2* effect was displayed by quantum dot capped Fe3O4 nanorings.[4] Extra high coercive fields of 76.5 ( 1.5) mT was detected in Fe3O4 tube arrays.[5] With these intriguing properties reported, the fabrication of various shapes of iron oxides, especially Fe3O4 particles, attracts more and more attentions. So far, many synthesis methods have been developed for synthesis of Fe3O4 particles, such as sol-gel in reverse micelles,[6] hydrothermal,[7] co-precipitation[8] and thermal decomposition.[9,10] However, these methods tend to form Fe3O4 nanoparticles with isotropic shapes. Recently, Fe3O4 with hollow structures (rings, tubes ad capsules, etc.) and 1-dimensional structures (wires, rods and spindles, etc.) were successfully synthesized by template method[11] - dehydration or reduction of premade isotropic -FeOOH or -Fe2O3 particles.[5,12] Compared with dehydration, reduction of premade -Fe2O3 seems to be a simple and effective method. Usually, the reduction of -Fe2O3 involves an annealing process in reductive atmosphere at elevated temperatures (typically in the range of 300-500 ℃). This annealing treatment of nanostructured materials may result in undesirable aggregation and sintering. In this work, we have developed a 114 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route chemical reduction method, which allows the reduction process to be carried out in organic solvent. The developed method aims to obtain pure Fe3O4 phase after the reduction with preserving the morphology of premade -Fe2O3 template. In this study, two kinds of reducing agents, i.e. oleic acid and H2-involved gas (5%H2-95%Ar gas mixture), are used, and their effects on the reduction process are investigated. The developed reduction method is suitable for massive production, which makes it possible to investigate the microwave absorption performance of as-reduced Fe3O4 particles with various structures. Since the shape of formed Fe3O4 nanoparticles is most related with that of premade -Fe2O3 template, it is important for us to prepare -Fe2O3 with various shapes prior to the reduction process. Unlike Fe3O4, the existing methods are more effective to form high-quality -Fe2O3 particles with varied shapes. -Fe2O3 nanobelts, nanowires and nanoflakes could be produced by thermal oxidation of iron in oxygen atmosphere[13,14] or by calcination of -FeOOH in air.[15] -Fe2O3 nanotubes and nanorods could be formed by template method[16,17] as well as hydrothermal method.[18] -Fe2O3 nanorings, nanodiscs and capsules could also be prepared by hydrothermal method.[19-21] Based on the references, the hydrothermal route is found to be versatile and able to synthesize -Fe2O3 nanoparticles of different shapes, such as rings, tubes, capsules, rods as well as discs. Hence, on the purpose to enrich the library of the shapes of Fe3O4 nanostructures, the hydrothermal method was 115 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route employed in the current study. As an extension work of our previous report,[22] ammonium phosphate (NH4H2PO4) was chose as additives to facilitate the hydrolysis of FeCl3 to produce Fe3+ ions and control the growth of -Fe2O3 nanoparticles. Besides the -Fe2O3 rings and tubes reported by the previous works, single-crystalline -Fe2O3 rods with tunable sizes were also developed by adjusting the ratio of [Fe3+]/[H2PO4-]. To the best of my knowledge, it is the first report on the synthesis of -Fe2O3 rings, tubes and rods by one single route. It is worthy to mention that the sizes of developed rods can be well controlled by adjusting the concentration of starting materials. The formation mechanism on the variety of as-prepared -Fe2O3 particles by hydrothermal method is also described in this chapter. 7.2 Experimental results 7.2.1 Synthesis of -Fe2O3 with various shapes by hydrothermal treatment 7.2.1.1 Mechanism on the formation of -Fe2O3 nanoparticles with different morphology In this work, large-scale -Fe2O3 nanoparticles with various shapes and sizes can be prepared via a facile hydrothermal treatment. The formation of -Fe2O3 nanocrystals starts from the -Fe2O3 monomers generated by the hydrolysis of Fe3+ ions at 220 ℃ in the presence of phosphate ions. The formed monomers possess very high surface energy and tend to aggregate rapidly. The aggregation process is affected greatly by 116 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route the concentration of Fe3+ ions as well as the phosphate ions, resulting in -Fe2O3 of different shapes. Similar mechanism has been already revealed by some previous reports regarding to the hollow nanocrystals, such as nanorings and nanotubes.[19,22,23] For the formation of nanorings, -Fe2O3 monomers aggregate to form a disk first, followed by a subsequent ‘etching’ of the -Fe2O3 nanodisks by phosphate ions at the central part, leading to ring-structure particles formed. In our experimental results, when the concentrations of Fe3+ and H2PO4- ions are mM and 0.72 mM, 150 nm disks would be formed if the heating period at 220 ℃ is around 10 h; while 154 nm rings would be obtained if the heating period at 220 ℃ is prolonged to 48 h, as shown in Fig. 7.1. The SEM image in Fig. 7.1b shows a mixture of disks and freshly formed rings, which is a kind of intermediate product before the final Fig. 7.1 SEM images of (a) -Fe2O3 disks (10 h at 220 ℃); (b) mixed product of disk and rings (20 h at 220 ℃) and (c) -Fe2O3 rings (48h at 220 ℃). The scale bars for all the images stand for 200 nm. formation of nanorings. The morphology evolution of formed particles from disk to ring is consistent with the previous reports.[19] However, the mechanism (why the aggregation of -Fe2O3 monomers forms disk at the middle stage) is still not well understood. Although we know that molar ratio of Fe3+ to H2PO4- ions is crucial to this problem, no specific answer could be given so far. Nevertheless, researchers have 117 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route tried to explain the dissolution of the hematite particles by the following reactions: Fe2O3 + H+ Fe3+ + H2O (7.1) Fe3+ + xH2PO4- [Fe(H2PO4)x]3-x (7.2) According to Eq. (7.2), the formation of [Fe(H2PO4)x]3-x will consume Fe3+ ions and lower the concentration of Fe3+ in the aqueous solution. Then the decomposition of Fe2O3 to Fe 3+ ions is forced by the lack of Fe3+ ions to reach a thermodynamic equilibrium state, as indicated by Eq. (7.1). The central part of as formed disk is rich of H2PO4- because of the aggregation of -Fe2O3 monomers starts from there, leading to a final ring-structure after the dissolution process. The above mechanism is also valid for the formation of -Fe2O3 tubes. With using different concentrations of FeCl3 and NH4H2PO4, as listed in Table 2.3, the aggregation of -Fe2O3 monomers forms spindle-like crystals first, as revealed by our previous study,[22] and followed by a dissolution along the central axis. As reported, for -Fe2O3 tubes fabricated by this hydrothermal route, the central axis is always along the c axis of trigonal Fe2O3, corresponding to [0 1] crystal orientation. The preferential growth along [0 1] direction may be dominated by the selective adsorption of phosphate ions on different crystal facets of Fe2O3 other than (0 1) facets. As investigated by Jia et al.,[23] the adsorption capacity and affinity of (0 1) plane to phosphate ions are much lower than that of other planes, such as (1 0), (0 2) and (1 4) planes, originating from the absence of singly coordinated hydroxyl 118 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route groups on (0 1) plane. In other words, the attachment of phosphate ions on the facet will hinder the growth in the direction normal to the facet. And the density of attached phosphate ions, depending on the molar ratio of phosphate ions to iron precursor, is very important to the morphology of formed -Fe2O3 particles. The function of phosphate ions in the employed hydrothermal route is similar with that of oleic acid and oleylamine used in the thermal decomposition method for synthesis of shaped metal oxides.[24,25] 7.2.1.2 Shape controllable synthesis of -Fe2O3 nanoparticles As a key factor that influences the morphology of as-synthesized -Fe2O3 nanocrystals, the molar ratio of iron precursor to phosphate ions, i.e. [Fe3+]/[H2PO4-], was adjusted in this work. Besides the repeated work on the synthesis of -Fe2O3 rings and tubes by following the previous works, we further developed -Fe2O3 balls and rods, as shown in Fig. 7.2. The concentrations of used materials are listed in Table 2.3. From Table 2.3, we further observed that 74 nm -Fe2O3 rings and 70 nm -Fe2O3 tubes were produced with using the same ratio of [Fe3+]/[H2PO4-] but different concentrations of iron precursors. Due to their similar outer diameters, tubes with longer sizes could be seen as elongated rings. This observation may reveal that (a) the morphology of -Fe2O3 nanoparticles is mainly controlled by the ratio of [Fe3+]/[H2PO4-]; while (b) the size of -Fe2O3 nanoparticles is mainly dependent on the concentration of iron precursor when the ratio of [Fe3+]/[H2PO4-] is fixed to keep 119 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route the morphology. This finding is further supported by the experimental work on the synthesis of -Fe2O3 rods with different sizes. Fig. 7.2 SEM images of -Fe2O3 with different shapes: (a) 117 nm -Fe2O3 balls; (b) 74 nm -Fe2O3 rings; (c) 70 nm -Fe2O3 tubes and (d) 98 nm -Fe2O3 rod. The scale bars for all the images stand for 200 nm. 7.2.1.3 Size controllable synthesis of -Fe2O3 rods For the synthesis of -Fe2O3 rods, the ratio of [Fe3+]/[H2PO4-] was adjusted to be 20:0.36. The concentration of iron precursor was tuned by adding different amount of distilled water, as demonstrated by Table 2.3. The results indicated that the size of as-synthesized -Fe2O3 rods could be successfully controlled by only adjusting the concentration of precursor. A trend that higher concentrations of the iron precursor lead to larger sizes of -Fe2O3 rods was observed. Referring to the size control of as-obtained -Fe2O3 rods, a maximum length of 120 nm was reached. However, the uniformity and quality is poor as shown by the SEM images in Fig. 7.3a. Some fragments of -Fe2O3 rods, as pointed out by the arrowheads, could be found. The 120 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route Fig. 7.3 (a) SEM images of 120 nm -Fe2O3 rods and (b) TEM images of capsules and broken ones involved in as-prepared 120 nm -Fe2O3 rods. TEM images in Fig. 7.3b indicate that some hollow capsules and broken ones are involved in the sample of 120 nm -Fe2O3 rods. Hence, there exists a maximal size limitation for as-prepared -Fe2O3 rods by the hydrothermal method with employing FeCl3-NH4H2PO4 system. When the concentration of iron precursor decreased, we can obtain shorter but high-quality rods, as shown in Fig. 7.4a&b. The size and size distribution of as-synthesized -Fe2O3 particles was obtained by counting 80 to 100 particles in the SEM images. The statistical results were also listed in Table 2.3. The average value of outer diameter is adopted to name the sample. Take 98-rod as an example, the outer diameter is 98 nm with a deviation less than nm. As far as I know, it is the first time that -Fe2O3 rods with tunable sizes via hydrothermal method are reported. To learn more about the structure of as-prepared -Fe2O3 rods, HRTEM images and SAED patterns are acquired. The results reveal a perfect single crystal structure for all -Fe2O3 rods with different sizes. The lattice spacing in Fig. 7.4c is measured at about 0.253 nm, which is close to the standard d 121 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route spacing of {1 0} at 0.252 nm for the hematite. The SAED patterns and HRTEM analyses reveal that the nanorods grow along [0 1] (c axis), as labeled in the image. The oriented growth of -Fe2O3 rods along [0 1] direction further illustrates the weak adsorption affinity of (0 1) phosphate ions onto (0 1) plane of trigonal hematite. Different from the previously reported hollow structure, 98 nm-rods, 61 nm-rod as well as 55 nm-rods are solid. This may be due to the high ratio of [Fe3+]/[H2PO4-] and the low concentration of phosphate ions in the aqueous solution, resulting in insufficient phosphate ions for the dissolution process, as displayed by Eq. (7.2). Fig. 7.4 SEM images of (a) 61 nm Fe3O4 rods and (b) 55 nm Fe3O4 rods; (c) the HRTEM image for as-synthesized -Fe2O3 rod and the corresponding SAED pattern (inset). 122 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route Chemical reduction of -Fe2O3 to Fe3O4 nanoparticles 7.2.2 7.2.2.1 Effect of reducing agent (oleic acid) on the reduction process For the reduction process, trioctylamine (TOA) with a boiling point above 365℃ was chosen as the solvent, and oleic acid was used as reducing agent. To investigate the effect of oleic acid on the phase transformation from -Fe2O3 to Fe3O4 in the current work, a set of experiments was applied to 74 nm -Fe2O3 rings firstly. The experimental conditions were listed in Table 7.1. With using 100 mg Fe2O3 rings as starting material, the amount of oleic acid was adjusted from 0.5 g to g, then to 3.5 g, corresponding to as-obtained sample B, C and D. To be noted that the reduction process for these three samples was under pure Ar gas flow, indicating that no other reducing agent but only oleic acid was used. After phase identification by XRD, Table 7.1 Reduction conditions for phase transformation from -Fe2O3 to Fe3O4 nanorings. Sample Oleic acid Flow rate Gas (sccm) Phase Ms No. (g) B 0.5 Ar 80 α-Fe2O3 + Fe3O4 50.9 C Ar 80 Fe3O4 59.7 D 3.5 Ar 80 Fe3O4 67.3 E 3.5 5% H2 + 95% Ar 80 Fe3O4 69.1 F ----- 5% H2 + 95% Ar 80 α-Fe2O3 + Fe3O4 12.9 G ----- 5% H2 + 95% Ar 120 α-Fe2O3 + Fe3O4 48.2 (emu/g) Note: For each batch of experiments, 100 mg of 74 nm -Fe2O3 rings dispersed in 35 mL TOA was used as starting materials (so called sample A). 123 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route sample B was examined to be a mixture of hematite and magnetite, as shown in Fig. 7.5. Most of as-reduced particles in sample B were broken into pieces. The saturation magnetization of sample B (only 50.9 emu/g) was relatively low due to the existence of hematite phase, as recorded by the magnetic hysteresis loops in Fig. 7.6. With enhancing the amount of oleic acid to g, an improvement could be observed based on the XRD and SEM results. No hematite phase could be found for sample C and fewer broken particles were shown compared with sample B, and the magnetization was enhanced as well. These results allow us to speculate that oleic acid acts not only as reducing agent but also as capping agent. The function of oleic acid as reducing agent is trying to break down the particles to finish the reduction from Fe3+ to Fe2+; Fig. 7.5 SEM images of (a) 74 nm -Fe2O3 rings, i.e. sample A; and as reduced samples: (b) sample B; (c) sample C; (d) sample D. The scale bars on these images stand for 200 nm. (e) The photo image of two samples, the one labeled with letter ‘A’ is for sample A dispersed in TOA, the other one with ‘T’ is for transparent solution obtained after reduction process when the ratio of oleic acid to -Fe2O3 rings is adjusted to be 29:1. The color of sample B, C and D seems the same, as shown by the inset photo in figure (c). (f) The XRD patterns for different samples. 124 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route the other function as capping agent is responsible for protecting the particles from being broken during the redox reaction. The multifunction of oleic acid has been investigated in other chemical reactions, such as the synthesis of Fe3O4 nanoparticles via thermal decomposition method.[26,27] Compared with these reference works, the refluxing temperature in the current study is much higher (350 ℃ versus 280 ℃ or 290 ℃). As revealed by Dieste et al.,[28] the higher the temperature, the less C=O double bonds of oleic acid could be detected due to the decomposition of carboxyl Fig. 7.6 The M-H loops of as-reduced samples B, C and D. group. When the temperature reaches 430 ℃ or above, no C=O bonds could be detected because of the complete decomposition of carboxyl group. This also means stronger reducibility of oleic acid but weaker stability as capping agent at higher temperatures, because more CO, H2 and C will be produced as a result of the decomposition of carboxyl group.[29] Our first concern is the phase conversion from -Fe2O3 to Fe3O4. Hence, we set the refluxing temperature of the reduction process at a relative high value, i.e. 350 ℃. At this temperature, the oleic acid still performs multifunction but with a relative strong reducibility. Temperature higher than 350 ℃ will damage the magnetic stirrer severely. 125 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route As such, when we increased the amount of oleic acid to 3.5 g (sample D), the morphology of as-reduced particles was further improved, as shown by the SEM image Fig. 7.5d. This indicates that the increase of oleic acid results in a stronger stability. But, a further increase of oleic acid (5 g - corresponding to a molar ratio of 29:1) is not helpful to the reduction because the TOA solvent involving -Fe2O3 particles would become transparent and show a red color due to the presence of Fe3+ ions after high temperature reaction, as displayed by the photo labeled with letter ‘T’ in Fig. 7.5e. This indicated that all the Fe3+ ions were released from particles instead of being reduced to Fe2+ ions due to excessive oleic acid. No Fe3O4 particles could be obtained. The reason is unknown so far. What we know is that the molar ratio of oleic acid to -Fe2O3 particles should be well controlled to make sure the complete reduction from hematite to magnetite as well as the unchanged morphology of particles. For 74 nm -Fe2O3 rings, a molar ratio around 20:1 (oleic acid to -Fe2O3 particles) was found to be effective to accomplish the reduction process. Nevertheless, there were still some defects on the particles surface, which could be seen directly from the SEM image in Fig. 7.5d. 7.2.2.2 Effect of 5%H2/95%Ar protection gas on the reduction process In order to improve the quality of Fe3O4 nanoparticles, a gas mixture including 5% H2 and 95% Ar instead of pure Ar was adopted as protection atmosphere to facilitate the reduction process. The gas flow rate was controlled by a gas mass flow meter. Sample 126 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route E, F and G were obtained under the protection of 5%H2/95%Ar gas, as listed in Table 7.1. By comparing the SEM images of sample E (Fig. 7.7a) and D (Fig. 7.5d), we could find that the usage of 5%H2/95%Ar gas made a great improvement on the surface morphology of as-reduced particles. Fig. 7.7 SEM images of as-reduced samples: (a) sample E; (b) sample F; (c) sample G. The scale bars on these images stand for 200 nm. Photo images of sample B and sample C dispersed in TOA solution are also displayed by insets. (d) The XRD patterns for different samples. Such as samples F and G, were prepared with using only H2 as the reducing agent in the experiments. Without the presence of oleic acid, the as-reduced particles of samples F and G possess perfect ring-structure as well as smooth surface, as shown by the SEM images in Fig. 7.7. When the gas flow rate of H2 gas was 80 sccm (sample F), the reduction effect from H2 gas was so weak that a very strong hematite peak (1 4) was observed in the XRD pattern (Fig. 7.7d). The brown color shown by the as-reduced precipitate also demonstrated the large proportion hematite phase in Sample F, as seen from the photo in Fig. 7.7b (inset). With the gas flow rate 127 Chapter increasing to 120 sccm Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route (sample G), the precipitates became black and the hematite peak in XRD pattern got less strong, indicating more magnetite phase transformed after reduction process compared with sample F. The magnetic properties could also illustrate the effect of H2 on the transformation from hematite to magnetite phase. The Ms values read from the hysteresis loops (Fig. 7.8) were 12.9 emu/g and 48.2 emu/g, corresponding to Fig. 7.8 The M-H loops of as-reduced samples E, F and G. sample F and G. The increment of magnetization could prove that Sample G had less hematite phase, resulting from the higher gas flow of 5%H2/95%Ar gas employed for reduction process. Even the gas flow rate was adjusted to 120 sccm, the phase transformation was still uncompleted. This phenomenon further indicates that oleic acid is necessary for the reduction process in the current method, while the H2 gas facilitates the phase transformation from hematite to magnetite and benefits to the maintenance of morphology. 7.2.2.3 Characterizations on the as-reduced Fe3O4 nanoparticles Based on above analysis, we have learnt that both the ratio of oleic acid to -Fe2O3 128 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route particles and the gas flow of H2 are key factors for a successful reduction work. A usage of 3.5 g oleic acid combined with a gas flow around 80 sccm of H2 is sufficient to accomplish the reduction of 100 mg of 74 nm -Fe2O3 rings. This optimized condition was further applied to other kinds of as-synthesized -Fe2O3 particles, such as 164 nm rings, 70 nm tubes as well as the rods with three different sizes. The SEM images of as-obtained Fe3O4 particles were shown in Fig. 7.9 and Fig. 7.10. Comparing with unreduced -Fe2O3 particles, the perfect ring-, tube- and rod-structure were maintained after reduction process. Before the reduction process, Fig. 7.9 SEM images of as-reduced samples: (a) 154 nm rings; (b) 70 nm tubes; (c) 98 nm rods. The scale bars on these images stand for 500 nm. All the scale bars stand for 500 nm. (d) The M-H loops for as reduced samples. the phase of as-prepared -Fe2O3 particles was confirmed as the rhombohedral -Fe2O3 (JCPDS 88-2359) and no impurities were observed, as shown by the X-ray diffraction patterns in Fig. 7.11a. After reduction, the XRD result showed pure Fe3O4 phase, which match well with the cubic structure magnetite (JCPDS 82-1533). The 129 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route apparent difference between the XRD patterns of the as-synthesized -Fe2O3 particles and the as-reduced Fe3O4 particles clearly shows the structure conversion from corundum to spinel. To further confirm that the magnetite phase rather than maghemite phase was obtained after the reduction process, XPS spectrum was recorded and shown in Fig. 7.11b. Both of the two curves show two main peaks at the binding energies of 711 eV and 724 eV, corresponding to Fe2p1/2 and Fe2p3/2 peaks. However, a feature on the Fe2p spectra line shapes for -Fe2O3 is the small satellite peak,[30] which is used to differentiate Fe2O3 and Fe3O4. Obviously, in our Fe3O4 samples, no satellite peak was detected. This further proves that the phase conversion from hematite to magnetic was complete. Fig. 7.10 SEM images of as reduced sample: (a) 61 nm Fe3O4 rods; (b) 55 nm Fe3O4 rods. The scale bars on these images stand for 200 nm. (c) The HRTEM image for as-synthesized Fe3O4 rod and the corresponding SAED pattern (inset). (d) The M-H loops of as-reduced Fe3O4 rods with different sizes. Fig. 7.10c shows HRTEM images and SAED patterns of as-reduced Fe3O4 rods. A 130 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route perfect single crystal structure was clearly observed and the measured d spacing from HRTEM image was around 0.47 nm, which is close to the standard d spacing of {111} at 0.48 nm for the magnetite. The crystallographic direction is along the longitudinal direction of the magnetite rods. The results show a preserved single crystal structure after phase transformation from -Fe2O3 to Fe3O4. Meanwhile, the crystal orientations change from [0 1] hematite  [1 1] magnetite and [1 0] hematite  [3 1] magnetite, as indexed on SAED patterns. Fig. 7.11 (a) The X-ray patterns and (b) XPS spectra of -Fe2O3 and Fe3O4 samples. The particles are successfully reduced from -Fe2O3 to Fe3O4 phase, which is further proved by the magnetic hysteresis loops in Fig. 7.9d. All of as-reduced Fe3O4 particles show ferromagnetic property. The saturation magnetization ( Ms ) value of 154 nm-rings is 89.3 emu/g, comparable with bulk Fe3O4 (85-90 emu/g).[31,32] For Fe3O4 rods, the magnetization seems to be size/volume dependent. With their outer diameters decreasing from 98 nm to 55 nm, the Ms values reduced from 89 emu/g to 71 emu/g (Fig. 7.10d). This trend allows us to synthesize different size of rod structure Fe3O4 particles according to the required magnetic property. 131 Chapter 7.2.3 Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route Microwave characterizations on as-reduced Fe3O4 particles As mentioned above, Fe3O4 nanoparticles with various structures, including rings, tubes and rods are synthesized by employing chemical reduction method. The microwave absorption performance of these magnetic nanostructures is further investigated. The Fe3O4 particles (74 nm rings, 160 nm rings, 70 nm tubes and 98 nm rods) are dispersed into paraffin wax at a volume concentration of 20% for the electromagnetic parameters (μr = μ′ + jμ′′; εr = ε′ + jε′′ )measurements. Based on the measured parameters, the reflection loss curves are further obtained through calculation work using Eq. (2.5) and Eq. (2.6). The acquired electromagnetic spectra (the left side of each figure) and reflection loss curves are showed in Fig. 7.12 to Fig. 7.15 for different samples. When compared with as-synthesized Fe3O4 nanocrystals with octahedron-structures, the enhancement in the resonance frequency has been brought by as-reduced Fe3O4 particles with different shapes. Some characterized parameters are summarized in Table 7.2. As we can see from Table 7.2, the resonance frequency is enhanced from 1.57 GHz for octahedral Fe3O4 to 3.53 GHz for 74 nm rings and to 4.01 GHz for 154 nm rings. The smaller size of 74 nm rings results in a relative lower saturation magnetization Ms than 154 nm rings. As we know from the Snoek’s law, for the materials with similar structure, the saturation magnetization Ms is decisive to the product of frequency multiplies permeability. The higher Ms of 154 nm rings may account for 132 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route Fig. 7.12 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 74 nm-Fe3O4 rings. Fig. 7.13 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 154 nm-Fe3O4 rings. Fig. 7.14 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 70 nm-Fe3O4 tubes. Fig. 7.15 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 98 nm-Fe3O4 rods. 133 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route its larger permeability at a higher resonance frequency relative to that of 74 nm rings, resulting in a more effective microwave absorption. The reflection loss of 154 nm rings can reach -28 dB at the relative small thickness t = 4.2 mm. There is no optimal thickness observed in the reflection loss curves of 74 nm rings, which is similar to that of Fe3O4 nanocrystals (Fig. 5.15b in Chapter 5). Their resonance frequency shifts to lower band with lower the reflection loss when the thickness increases. Table 7.2 Characterized parameters for various magnetic structures summarized from the measured electromagnetic spectra and the calculated reflection loss curves. Samples 𝐌𝐬 (emu/g) ƒ𝐫 (GHz) 𝛍′′ Optimal thickness 𝐭 (mm) RL (dB) 114nm octahedron_Fe3O4 90 1.57 0.8 6.0 -17 104nm octahedron_ZnFe2O4 104 3.45 1.4 4.2 -38 74nm ring_Fe3O4 69 3.53 0.35 6.0 -12 154nm ring_Fe3O4 81 4.01 0.53 4.2 -28 70nm tube_Fe3O4 73 4.46 0.35 4.2 -20 98nm rod_Fe3O4 85 4.82 0.34 3.6 -15 The resonance peaks of 70 nm Fe3O4 tubes and 98 nm Fe3O4 rods shift to even higher frequency, i.e. 4.46 GHz and 4.82 GHz, respectively. We may speculate that the shape anisotropy contributes to the resonance frequency somehow. For 70 nm Fe3O4 tubes, the optimal thickness is at 4.2 mm corresponding to a reflection loss of -20 dB. There 134 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route is also no optimal thickness observed in the reflection loss for 98 nm Fe3O4 rods. Unlike the trend shown by Fe3O4 nanooctahedron and 74 nm rings, the resonance peak of 98 nm rods shifts to higher frequency band along with lower reflection loss when the thickness decreases. This means that the resonance peak could shift to higher frequency band along with lower reflection loss values if the thickness could be further decreased. Although the mechanism on the structure effect to electromagnetic performance is still unclear, we could find that the magnetic structures impact the microwave performance. Based on our work, the 98 nm Fe3O4 rods display the highest resonance frequency at 4.82 GHz; while Zn-ferrite octahedra display the lowest reflection loss of -38 dB. Hence we could conclude that the anisotropic structure is promising to enhance the resonance frequency to higher band and the high saturation magnetization is necessary for an effective microwave absorber. 7.3 Summary In this chapter, we synthesized -Fe2O3 nanoparticles of different shapes and sizes via hydrothermal method. The dependence of the shape and size of as-synthesized -Fe2O3 on the reactant concentration was studied. -Fe2O3 rods with controllable sizes were developed. The length of as-produced rods was found to increase with the reactant concentration. In a subsequent step, Fe3O4 nanoparticles of various shapes were obtained after a phase conversion from as-prepared -Fe2O3 by a chemical 135 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route reduction process. The phase transformation from hematite to magnetite mostly relies on the reducing agent, such as oleic acid and 5%H2-95%Ar gas. As investigated in this work, oleic acid is effective to reduce the Fe3+ to Fe2+ even at the inner part of particles. But this usually leads to broken particles after the reduction process. Instead, H2 gas is conducive to maintain the morphology of as-reduced particles but its reduction effect is not strong enough to realize the complete reduction of -Fe2O3 particles. Based on the investigation, an optimal molar ratio of 20:1 (oleic acid to -Fe2O3 particles) was chosen in this work, as well as 5%H2 plus 95% Argon gas was used to facilitate the reduction process. The results indicate that pure hematite phase could be obtained after reduction process, and the morphology of unreduced hematite particles, including rings, tubes as well as rods, is well kept. In other words, the chemical reduction method makes it possible to obtain Fe3O4 particles with various structures as long as the template of -Fe2O3 particles with well-shaped surface are provided. Furthermore, this method is favorable to large-scale synthesis of Fe3O4 particles, which is required in many applications. Through the comparison of the microwave performance of as-prepared Fe3O4 with various structures (114 nm octahedra, 74 nm rings, 154 nm rings, 79 nm tubes and 98 nm rods), we have found that the anisotropic structures could contribute to the enhancement of resonance frequency of materials. The 98 nm Fe3O4 rods show a highest resonance peak at 4.82 GHz. The balance between the resonance frequency and the permeability of a 136 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route magnetic material is important to its microwave absorption property. As such, the 154 nm Fe3O4 rings show relative low reflection loss of -28 dB corresponding to an optimal thickness of 4.2 mm. This point could be further proved by the 104 nm Zn-ferrite particles, which show the lowest reflection loss of -38 dB. 7.4 References [1] M. T. Chang, L. J. Chou, C. H. Hsieh, Y. L. Chueh, Z. L. Wang, Y. Murakami and D. Shindo, Adv. Mater., 19, 2290 (2007) [2] W. M. Zhang, X. L. Wu, J. S. Hu, Y. G. Guo and L. J. Wan, Adv. Funct. Mater., 18, 3941 (2008) [3] D. H. Zhang, Z. Q. Liu, S. Han, C. Li, B. Lei, M. O. Stewart, H. M. Tour and C. W. zhou, Nano Lett., 11, 2151 (2004) [4] H. M. Fan, M. Olivo, B. Shuter, J. B. Yi, R. Bhuvaneswari, H. R. Tan, G. C. Xing, C. T. Ng, L. Liu, S. S. Lucky, B. H. Bay, J. Ding, J. Am. Chem. Soc., 132, 14803 (2010) [5] J. Bachmann, J. Jing, M. Knez, S. Barth, H. Shen, S. Mathur, U. Gösele and K. Nielsch, J. Am. Chem. Soc., 129, 9554 (2007) [6] O. M. Lemine, K. Omri, B. Zhang, L. El Mir, M. Sajieddine, A. Alyamani and M. Bououdina, Supperlattices Microstruct., 52, 793 (2012) [7] T. J. Daou, G. Pourroy, S. Bégin-Colin, J. M. Grenèche, C. Ulhaq-Bouillet, P. Legaré, P. Bernhardt, C. Leuvrey and G. Rogez, Chem. Mater., 18, 4399 (2006) [8] Z. Li, B. Tan, M. Allix, A. I. Cooper and M. J. Rosseinsky, Small, 4, 231 (2008) [9] C. Yang, J. J. Wu and Y. L. Hou, Chem. Commun., 47, 5130 (2011) [10] S. H. Sun, H. Zeng, D. B. Robinson, S. Raoux, P. M. Rice, S. X. Wang and G. X. Li, J. Am. Chem. Soc., 126, 273 (2004) [11] Y. Piao, J. Kim, H. B. Na, D. Kim, J. S. Baek, M. K. Ko, J. H. Lee, M. Shokouhimehr and T. Hyeon, Nat. Mater., 7, 242 (2008) [12] Y. L. Chueh, M. W. Lai, J. Q. Liang, L. J. Chou and Z. L. Wang, Adv. Funct. Mater., 16, 2243 (2006) [13] X. H. Su, C. S. Yu and C. W. Qiang, Appl. Surf. Sci., 257, 9014 (2011) [14] X. G. Wen, S. H. Wang, Y. Ding, Z. L. Wang and S. H. Yang, J. Phys. Chem. B, 109, 215 (2005) 137 Chapter Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route [15] X. Wang, X. Y. Chen, L. S. Gao, H. G. Zheng, M. R. Ji, C. M. Tang, T. Shen and Z. D. Zhang, J. Mater. Chem., 14, 905 (2004) [16] S. Mandal and A. H. E. Müller, Mater. Chem. Phys., 111, 438 (2008) [17] Z. Y. Sun, H. Q. Yuan, Z. M. Liu, B. X. Han and X. R. Zhang, Adv. Mater., 17, 2993 (2005) [18] L. Liu, H. Z. Kou, W. L. Mo, H. J. Liu and Y. Q. Wang, J. Phys. Chem. B, 110, 15218 (2006) [19] X. L. Hu, J. C. Yu, J. M. Gong, Q. Li, G. S. Li, Adv. Mater., 19, 2324 (2007) [20] C. H. Su and H. Wang, Cryst. Res. Technol., 47, 896 (2012) [21] B. Tao, Q. Zhang, Z. Z. Liu, B. Y. Geng, Mater. Chem. Phys., 136, 604 (2012). [22] H. M. Fan, G. J. You, Y. Li, Z. Zheng, H. R. Tan, Z. X. Shen, S. H. Tang, Y. P. Feng, J. Phys. Chem. C, 113, 9928 (2009) [23] C. J. Jia, L. D. Sun, Z. G. Yan, L. P. You, F. Luo, X. D. Han, Y. C. Pang, Z. Zhang, C. H. Yan, Angew. Chem. Int. Ed., 44, 4328 ( 2005) [24] L. H. Zhang, J. J. Wu, H. B. Liao, Y. L. Hou and S. Gao, Chem. Commum., 4378 (2009) [25] Z. C. Xu, C. M. Shen, Y. L. Hou, H. J. Gao and S. H. Sun, Chem. Mater., 21, 1778 (2009) [26] D. Kim, N. Lee, M. Park, B. H. Kim, K. An, T. Hyeon, J. Am. Chem. Soc., 131, 454 (2009) [27] L. Li, Y. Yang, J. Ding, J. M. Xue, Chem. Mater., 22, 3183 (2010) [28] V. P. Dieste, O. M. Castellini, J. N. Crain, M. A. Eriksson, A. Kirakosian, J. L. Lin, J. L. McChesney, F. J. Himpsel, Appl. Phys. Lett., 83, 5053 (2003) [29] S. G. Kwon, Y. Piao, J. Park, S. Angappane, Y. Jo, N. M. Hwang, J. G. Park, T. Hyeon, J. Am. Chem. Soc., 129, 12571 (2007) [30] T. Yamashita and P. Hayes, Appl. Surf. Sci., 254, 2441 (2008) [31] S. H. Xuan, L. Y. Hao, W. Q. Jiang, L. Song, Y. Hu, Z. Y. Chen, L. F. Fei, T. W. Li, Cryst. Growth Des., 7, 430 (2007) [32] J. H. Zhang, Q. H. Kong, J. Du, D. K. Ma, G. C. Xi, Y. T. Qian, J. Cryst. Growth, 308,159 (2007) 138 [...]... in the XRD pattern (Fig 7. 7d) The brown color shown by the as-reduced precipitate also demonstrated the large proportion hematite phase in Sample F, as seen from the photo in Fig 7. 7b (inset) With the gas flow rate 1 27 Chapter 7 increasing to 120 sccm Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route (sample G), the precipitates became black and. .. images of sample E (Fig 7. 7a) and D (Fig 7. 5d), we could find that the usage of 5%H2/95%Ar gas made a great improvement on the surface morphology of as-reduced particles Fig 7. 7 SEM images of as-reduced samples: (a) sample E; (b) sample F; (c) sample G The scale bars on these images stand for 200 nm Photo images of sample B and sample C dispersed in TOA solution are also displayed by insets (d) The XRD... the H2 gas facilitates the phase transformation from hematite to magnetite and benefits to the maintenance of morphology 7. 2.2.3 Characterizations on the as-reduced Fe3O4 nanoparticles Based on above analysis, we have learnt that both the ratio of oleic acid to -Fe2O3 128 Chapter 7 Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route particles and. .. the product of frequency multiplies permeability The higher Ms of 154 nm rings may account for 132 Chapter 7 Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route Fig 7. 12 (a) The permittivity (ɛ', ɛ") and permeability (μ', μ") spectra and (b) the calculated frequency dependent reflection loss plots for 74 nm-Fe3O4 rings Fig 7. 13 (a) The permittivity... to magnetic was complete Fig 7. 10 SEM images of as reduced sample: (a) 61 nm Fe3O4 rods; (b) 55 nm Fe3O4 rods The scale bars on these images stand for 200 nm (c) The HRTEM image for as-synthesized Fe3O4 rod and the corresponding SAED pattern (inset) (d) The M-H loops of as-reduced Fe3O4 rods with different sizes Fig 7. 10c shows HRTEM images and SAED patterns of as-reduced Fe3O4 rods A 130 Chapter 7 Synthesis. .. Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route perfect single crystal structure was clearly observed and the measured d spacing from HRTEM image was around 0. 47 nm, which is close to the standard d spacing of {111} at 0.48 nm for the magnetite The crystallographic direction is along the longitudinal direction of the magnetite rods The. .. particles according to the required magnetic property 131 Chapter 7 7.2.3 Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route Microwave characterizations on as-reduced Fe3O4 particles As mentioned above, Fe3O4 nanoparticles with various structures, including rings, tubes and rods are synthesized by employing chemical reduction method The microwave absorption... Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route the other function as capping agent is responsible for protecting the particles from being broken during the redox reaction The multifunction of oleic acid has been investigated in other chemical reactions, such as the synthesis of Fe3O4 nanoparticles via thermal decomposition method.[26, 27] Compared... somehow For 70 nm Fe3O4 tubes, the optimal thickness is at 4.2 mm corresponding to a reflection loss of -20 dB There 134 Chapter 7 Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route is also no optimal thickness observed in the reflection loss for 98 nm Fe3O4 rods Unlike the trend shown by Fe3O4 nanooctahedron and 74 nm rings, the resonance peak of 98... favorable to large-scale synthesis of Fe3O4 particles, which is required in many applications Through the comparison of the microwave performance of as-prepared Fe3O4 with various structures (114 nm octahedra, 74 nm rings, 154 nm rings, 79 nm tubes and 98 nm rods), we have found that the anisotropic structures could contribute to the enhancement of resonance frequency of materials The 98 nm Fe3O4 rods . Chapter 7 Synthesis and microwave absorption of Fe3O4 particles with various structures by chemical reduction route 113 Chapter 7 Synthesis and microwave absorption of Fe 3 O 4 particles. learnt that both the ratio of oleic acid to -Fe 2 O 3 Fig. 7. 8 The M-H loops of as-reduced samples E, F and G. Chapter 7 Synthesis and microwave absorption of Fe3O4 particles with various structures. 1 27 E, F and G were obtained under the protection of 5%H 2 /95%Ar gas, as listed in Table 7. 1. By comparing the SEM images of sample E (Fig. 7. 7a) and D (Fig. 7. 5d), we could find that the