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Large-Scale and Controlled Synthesis of Iron Oxide Magnetic Short Nanotubes: Shape Evolution, Growth Mechanism, and Magnetic Properties Wei Wu,†,‡ Xiangheng Xiao,‡ Shaofeng Zhang,‡ Juan Zhou,‡ Lixia Fan,§ Feng Ren,‡ and Changzhong Jiang*,†,‡ Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education and Center for Electronic Microscopy and Department of Physics, Wuhan UniVersity, Wuhan 430072, PR China, and School of Materials and Metallurgy, Wuhan UniVersity of Science and Technology, Wuhan 430081, PR China ReceiVed: February 2, 2010; ReVised Manuscript ReceiVed: July 27, 2010
We present a facile approach to the production of magnetic iron oxide short nanotubes (SNTs) employing an anion-assisted hydrothermal route by simultaneously using phosphate and sulfate ions. The size, morphology, shape, and surface architecture control of the iron oxide SNTs are achieved by simple adjustments of ferric ions concentration without any surfactant assistance. The result of a formation mechanism investigation reveals that the ferric ions concentrations, the amount of anion additive, and the reaction time make significant contributions to SNT growth. The shape of the SNTs is mainly regulated by the adsorption of phosphate ions on faces parallel to the long dimension of elongated R-Fe2O3 nanoparticles (c axis) during nanocrystal growth, and the hollow structure is given by the preferential dissolution along the c axis due to the strong coordination of the sulfate ions. Moreover, the as-synthesized hematite (R-Fe2O3) SNTs can be converted to magnetite (Fe3O4) and maghemite (γ-Fe2O3) ferromagnetic SNTs by a reducing atmosphere annealing process while preserving the same morphology. The structures and magnetic properties of these iron oxide SNTs were characterized by various analytical techniques. 1. Introduction Concomitant with the rapid development of nanoscience and nanotechnology, the simple pursuit of individual nanoparticles no longer satisfies general scientific interests and specific highend applications. The facile synthesis and fabrication of 1D, 2D, and 3D nanoparticle assemblies with highly controlled structures, uniform morphologies, and novel properties is now emerging as a new field of significance. In recent years, magnetic nanostructures have attracted significant and growing amounts of attention not only because of their fascinating physicochemical properties but also because of their potential uses in a range of applications, including biomedicine, magnetic fluids, magnetic recording, and spin electronics.1-5 Among the various magnetic materials, the iron oxides (such as hematite, magnetite, and maghemite) represent an important class of magnetic transitionmetal oxide materials. As the most stable iron oxide and n-type semiconductor properties under ambient conditions, hematite (R-Fe2O3) is widely used in catalysts,6 pigments,7-9 and gas sensors10,11 owing to its low cost and high resistance to corrosion, and it can be used as the starting material for the synthesis of magnetite (Fe3O4) and maghemite (γ-Fe2O3), which have been intensively pursued for both fundamental scientific interest and technological applications in the last few decades. Because of their excellent properties and good application prospects, much attention has been directed to the controlled synthesis of iron oxide nanomaterials. For example, Itoh and Sugimoto have prepared and systematically controlled uniform * To whom correspondence should be addressed. Tel: +86-27-68752567. Fax: +86-27-68753587. E-mail:
[email protected]. † Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education, Wuhan University. ‡ Center for Electronic Microscopy and Department of Physics, Wuhan University. § Wuhan University of Science and Technology.
iron oxide nanoparticles with controlled sizes and shapes by a sol-gel method.12 Though enormous progress has been made in fabricating magnetic iron oxide nanoparticles, nanowires, and ultrathin films with controllable morphologies, it is still a key goal in modern materials chemistry and has attracted substantial interest in recent years, and the search for novel geometries continues to be an important aspect of nanotechnology.13-16 An emerging area is the synthesis of tubular magnetic nanostructures, which has been pioneered in inorganic chemistry. Therefore, magnetic SNTs have particularly been the focus of interest because of their well-defined, reproducible magnetic states that result from their unique geometry, and such states can be easily detected and manipulated either in a magnetic field or with a spin-polarized current in industrial applications.17 Recently, Jia and co-workers reported a solution-based route to the production of iron oxide nanorings employing a doubleanion-assisted hydrothermal method (involving phosphate and sulfate ions). They found that nanotubes are eventually formed with the increase in the number of phosphate ions.17 Fan et al. reported a general thermal transformation approach to synthesizing magnetic iron oxide nanotubes with controlled size and shape by simple adjustments of reactant concentration and molar ratio.18,19 Nevertheless, a gap in knowledge remains with regard to the distinct parameters of the hydrothermal growth of hematite SNTs as a function of time, precursor, and additive concentration. Herein, we report a facile solution-phase route to the mass synthesis of iron oxide SNTs with different morphologies in the presence of phosphate and sulfate. In our system, the morphological evolution from nanoclusters to hollow nanoparticles, nanocapsules, and SNTs has been systemically investigated by using different amounts of ferric ions and additives (phosphate and sulfate), which may be attributed to the selective interaction of supersaturated ferric ions with additives during the reaction. In addition, the effect of the reaction parameters
10.1021/jp1010154 2010 American Chemical Society Published on Web 09/07/2010
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TABLE 1: Hydrothermal Synthesis Processing Variables and a Summary of the Reaction Products, Morphologies, Dimensions, and Aspect Ratios of r-Fe2O3 Nanoparticles samples no.
phosphate/mg
sulfate/mg
ferric salt/g
process time/h
major morphology
average dimension (l/w)/ nm
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17
7 7 7 7 7 7 7 7 7 7 7 7 0 7 0 7 14
19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 0 0 19.5 39 19.5
0.135 0.135 0.135 0.2025 0.2025 0.2025 0.27 0.27 0.27 0.27 0.3375 0.3375 0.27 0.27 0.27 0.27 0.27
3 6 12 3 6 12 1.5 3 6 12 6 12 12 12 12 12 12
nanoclusters hollow nanoclusters nanorings nanoclusters nanocapsules nanocapsules nanocapsules nanocapsules SNTs SNTs nanospindles + nanocapsules nanospindles + nanocapsules nanostars nanospindles nanoneedles nanocapsules nanorings
84.4 82.9 94.1 91.3 162.0/96.9 198.2/109.2 258.1/138.3 249.1/142.6 201.3/124.3 429.4/218.2 542.0/232.6 529.2/242.9 118.3 419.2/176.1 >200 nm 308.0/147.2 115.8
aspect ratiosa 1b 1 1 1c 1.7 1.8 1.8 1.7 1.6d 2.0 2.3 2.2 1 2.4 e
2.1 1
a Determined that the aspect ratio is equal to the average length diameter divided by the average outer diameter. b Assumed that the aspect ratio of quasi-sphere-like nanoparticles is equal to 1. c Some particles’ aspect ratio is >1.5. d The slight decrease in the aspect ratio should be due to the change in the projected scale. e Statistics for the aspect ratios of nanoneedles are not available.
on the final morphologies and sizes of iron oxide nanocrystals is also explored in detail, and the results may provide insight into the growth mechanism of iron oxide nanoparticles. Finally, the magnetic properties of these iron oxide SNTs are obtained to determine how particle shape affects their magnetic characteristics. 2. Experimental Section 2.1. Materials. Iron(III) chloride hexahydrate (FeCl3 · 6H2O, AR), anhydrous sodium sulfate (Na2SO4, AR), and sodium dihydrogen phosphate dihydrate (NaH2PO4 · 2H2O, AR) were purchased from Sinopharm Chemical Reagent CO., Ltd. and used as received without further purification. A MagneticSphere Technology magnetic separation stand (MSS), purchased from Promega (Z5333), was used to separate magnetic particles during washing and selecting steps. 2.2. Synthesis of Hematite SNTs. The hematite (R-Fe2O3) SNTs were prepared by the hydrothermal treatment of iron(III) chloride with sulfate and phosphate additives. In a typical experimental procedure, 0.27 g of FeCl3 · 6H2O, 7 mg of NaH2PO4 · 2H2O, and 19.5 mg of Na2SO4 aqueous solutions were mixed together and then doubly distilled water was added to the mixture to keep the final volume at 25 mL. After ultrasonic dispersion, the mixture was transferred to a Teflon-lined stainless steel autoclave with a capacity of 30 mL for hydrothermal treatment at 220 °C for 12 h. After the autoclave was allowed to cool to room temperature, the precipitate was separated by centrifugation, washed with doubly distilled water, and dried under vacuum at 120 °C. In a single batch of experiments, more than 0.1 g of hematite SNTs could be prepared. 2.3. Formation Mechanism Investigation. Specific amounts of FeCl3 · 6H2O, NaH2PO4 · 2H2O, and Na2SO4 aqueous solutions were mixed together, and then doubly distilled water was added to the mixture to keep the final volume at 25 mL. After ultrasonic dispersion, the mixture was transferred to a Teflonlined stainless steel autoclave with a capacity of 30 mL for hydrothermal treatment. The autoclave was sealed and inserted into a temperature-controlled furnace at a reaction temperature of 220 °C. The controlling parameters of ferric ion concentration (0.02-0.05 M), time (1.5-12 h), phosphate concentration
(0-14 mg), and sulfate concentration (0-39 mg) were varied in a systematic fashion, as summarized in Table 1. The autoclave, once removed from the furnace, was allowed to cool to room temperature naturally. 2.4. Synthesis of Magnetite and Maghemite SNTs. Magnetite (Fe3O4) SNTs were prepared by a reduction process with the above hematite products as starting materials. The dried R-Fe2O3 powders were annealed in a tubular furnace at 300 °C under continuous hydrogen flow for 5 h. The furnace was allowed to cool to room temperature while still under continuous hydrogen gas flow. Then, the as-obtained magnetite samples was subjected to a series of isochronal annealings at 400 °C for 2 h in an oxygen atmosphere to prepare the maghemite (γFe2O3) SNTs; the heating rate was 5 °C/min. 2.5. Characterization. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 (HT) transmission electron microscope at 200 kV, and the samples were dissolved in water and dropped onto copper grids. High-resolution transmission electron microscopy (HRTEM) images were performed on a JEOL JEM-2010 FET (UHR) transmission electron microscope at 200 kV, and the samples also dissolved in water and were dropped onto copper grids. Field-emission scanning electron microscopy (FSEM) studies were carried out using an FEI Sirion FEG operated at 25 keV, and samples were sprinkled onto the conductive substrate. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a D8 Advance X-ray diffractometer (Germany) operated at 40 kV and 40 mA at a scan rate of 0.05° 2θ s-1 using Cu KR radiation (λ ) 0.1542 nm). Magnetic measurements were performed on a Quantum Design physical property measurement system (PPMS) equipped with a vibrating sample magnetometer (VSM). A diamagnetic plastic tube was filled with the powder sample, and then the packed sample was put into a diamagnetic plastic straw and packed into a minimal volume for magnetic measurements. Background magnetic measurements were checked for the packing material. In the zero-field-cooled (ZFC) measurements, the samples were cooled from 300 to 10 K without applying an external field. After the samples reached 10 K, a field of 1000 Oe was applied and the magnetic moments were
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Figure 1. (a, b) SEM at different magnifications of hematite SNTs. (c) Size histograms of the length and outer diameter of hematite SNTs. (d) XRD patterns of hematite SNTs.
recorded as the temperature increased. For field-cooled (FC) measurements, the samples were cooled from 300 K under an applied field of 1000 Oe; then the magnetic moments were recorded as the temperature increased. 3. Results and Discussion 3.1. Hematite SNTs. Hematite SNTs were synthesized by a solution-phase approach. In a typical experimental procedure, the SNTs were obtained by the hydrothermal treatment of 0.04 M ferric ions in the presence of phosphate and sulfate ions (sample S10, see Table 1). The morphology of the typical hematite SNTs obtained was studied by electron microscopy. Figure 1a shows an FSEM image of the as-prepared hematite SNTs, and the corresponding magnified image is shown in the Figure 1b; the results illustrate that large-scale R-Fe2O3 SNTs with a uniform size have been fabricated by this facile hydrothermal method. It is noteworthy that some SNTs have one end open and the other end closed. As can be seen in Figure 1c, the average length and outer diameter of these SNTs are 429.4 and 218.2 nm, respectively. (The result was statistically analyzed by JEOL SmileView software, with more than 150 resoluble SNTs.) The structure and phase purity of the asobtained products were examined by PXRD, as shown in Figure 1d, and all of the peaks can be well indexed to a pure rhombohedral structure with an R3cj space group for hematite (JCPDS PDF 33-0664). To provide further insight into the nanostructures of hematite SNTs, TEM and HRTEM investigations were also performed. Figure 2a shows the TEM image of hematite SNTs at low
magnification, where the obvious electron-density differences between the dark edge and pale center further confirm the hollow, tubular structure clearly. The results of the length and outer diameter were in agreement with the FSEM images. Figure 2b shows typical HRTEM images that were taken from the tube wall and open-end region of a single hematite SNT (the lowresolution TEM image). The clear lattice image indicates the high crystallinity of the hematite SNTs. A lattice spacing of 0.274 nm for the (104) planes and 0.207 nm for the (202) planes of the rhombohedral hematite structure can be readily resolved. The above results illustrate that the hematite SNTs have been successfully fabricated by hydrothermal treatment at this ratio of reactants. 3.2. Morphology and Growth Mechanism of Hematite SNTs. The hydrothermal processing of aqueous FeCl3 solution enabled the synthesis of R-Fe2O3 nanoparticles with different morphologies, depending on the reaction time, ferric ion concentration, and phosphate and sulfate additive concentrations. The range of hydrothermal experimental conditions investigated and the reaction products produced are summarized in Table 1, the evidence for which is now presented in detail. 3.2.1. Effects of Ferric Ion Concentration and Reaction Time. To understand the growth process of R-Fe2O3 SNTs, the time-dependent evolution of the morphology has been studied at 220 °C with different ferric ion concentrations. At first, experiments were conducted with a fixed mass of phosphate ions (7 mg) and sulfate ions (19.5 mg) but various quantities of ferric ions, and a series of R-Fe2O3 nanostructures including nanoclusters, hollow nanoparticles, nanorings, and hollow
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Figure 2. (a) TEM image of hematite SNTs at low magnification. (b) HRTEM images of a single hematite SNT, where the scale bar of inset HRTEM image is 2 nm.
SCHEME 1: Schematic Illustration of the Shape Evolution for Hematite Nanostructures at Different Reaction Times and Different Ferric Concentrations
capsules were prepared. Moreover, the shape evolution map for hematite nanostructures at different reaction times and different ferric concentrations is depicted in Scheme 1. Figure 3 shows both TEM images of hematite nanocrystals at different reaction times when the concentration of ferric ions
is equal to 0.02 M (samples S1-S3). From the TEM images of the products taken as the reaction time increased (Figure 3a-c), it can be seen that R-Fe2O3 undergoes an evolution from clusterlike nanoparticles to hollow nanoclusters and then to hollow nanoparticles. As shown in Figure 3a, the products are
Figure 3. Morphology evolution of the hollow hematite nanoparticles with reaction time (CFe obtained after (a) 3, (b) 6, and (c) 12 h.
ions
) 0.02 mol/L). TEM images of the products
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Figure 4. Morphology evolution of the hollow hematite nanoparticles with reaction time (CFe obtained after (a) 3, (b) 6, and (c) 12 h.
well dispersed and have rough surfaces; the inset TEM image at high magnification shows the quasi-nanowire array nature of the nanoparticles, and the results indicate that each nanoparticle is composed of a large number of smaller nanoparticles with diameters ranging from 3 to 6 nm. For reaction time extending to 6 h, the obvious electron-density differences between the dark edge and pale center confirm that the previous nanoclusters have begun to appear in the hollow structure (Figure 3b). Moreover, the inset TEM image at high magnification also shows that the hollow nanocluster is composed of a large number of smaller nanoparticles with increasing diameter ranging from 6 to 10 nm. When the reaction time further extend to 12 h, as shown in Figure 3c, the central holes of the nanoclusters become larger and finally become ringlike hollow nanoparticles with an average diameter of 94.3 nm. The above results illustrate that a low concentration of ferric ions will promote the formation of spherical hollow nanoparticles. When the concentration of ferric ions increases to 0.03 M (samples S4-S6), from the TEM images of the products taken as the reaction time increased (Figure 4a-c), it can be seen that R-Fe2O3 undergoes a direct evolution from clusterlike nanoparticles to capsulelike hollow nanoparticles. When the reaction is carried out for 3 h, it can be seen that all of the obtained particles appear with clusterlike morphology (Figure 4a). The as-obtained product contains some oval-shaped nanoparticles with an average diameter of 91.3 nm and a length of 150-200 nm. By prolonging the reaction time to 6 h, the products are mainly hollow capsules with an average diameter of 96.9 nm and an average length of 162.0 nm (Figure 4b). We found that the wall of a capsule in the perpendicular direction is 40.0 nm. When the reaction time further increases to 12 h (Figure 4c), the wall thickness of a capsule in the perpendicular direction is decreased to 37.6 nm. Compared with S5, the wall thickness of other hollow capsules of as-prepared S6 is clearly thinner. The results demonstrate that the current ferric ion concentration still cannot generate tubularly shaped hematite nanomaterials. Then, the concentration of ferric ions is further elevated to 0.04 M (samples S7-S10) to investigate the functions of reaction time. The bright-field TEM images of Figure 5a-d
ions
) 0.03 mol/L). TEM images of the products
illustrate the effect of increasing reaction times of 1.5, 3, 6, and 12 h, respectively. As shown in Figure 5a, corresponding to sample S7 growth after 1.5 h, the presence of capsulelike nanoparticles with a wall thickness of 51.5 nm was confirmed. Such thick walls lead to some particles with solid structures. The inset TEM image at high magnification indicates that the nanoparticle is composed of a large number of smaller nanoparticles with diameters ranging from 5 to 8 nm. When the reaction proceeds for 3 h, certain numbers of well-defined capsulelike nanoparticles with an obviously hollow structure begin to appear, and the wall thickness decreases to 28.4 nm (Figure 5b). However, if the reaction time is further increased to 6 h, then the particle size and morphology remain unchanged while the wall thickness further decreases to 24.6 nm (Figure 5c). After the reaction time increases to 12 h, it is clearly found that a higher proportion of tubelike particles is formed, as shown in Figure 5d. The wall thickness further decreases to 16 nm and the particle size increases. Furthermore, we also found the amount of end opening increases from S10 to S9. The above results reveal that the reaction time will decrease the wall thickness. Compared with the inset images in Figure 5a (S7) and Figure 2b (S10), the diameter of small nanoparticles has grown from ca. 8 to ca. 20 nm. As we know, a typical Ostwald ripening process involving the formation of larger crystals by greatly reducing the interfacial energy of small primary nanocrystals is energetically favored. Finally, when the ferric ion concentration further increases to 0.05 M (samples S11-S12), Figure 6 shows the TEM images of the obtained product. Figure 6a shows a general image of the S11 obtained sample at reaction time of 6 h and clearly indicates that as-prepared products are composed of solid and hollow capsulelike nanoparticles (nanospindles and nanocapsules) with an average diameter of 232.9 nm and an average length of 542.0 nm. After the reaction time is increased further to 12 h, the particle size and morphology remain unchanged but the proportion of solid nanoparticles is decreased. Compared with CFe2+ ) 0.04 M, the average particle length and diameter have been increased. From the TEM images, we found that some particles have one end open with the other end closed.
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Figure 5. Morphology evolution of the hematite SNTs with reaction time (CFe 1.5, (b) 3, (c) 6, and (d) 12 h.
ions
) 0.04 mol/L). TEM images of the products obtained after (a)
Figure 6. Morphology evolution of hollow hematite nanoparticles with reaction time (CFe ions ) 0.04 mol/L). TEM images of the products obtained after (a) 6 and (b) 12 h.
The above results reveal that the concentration of ferric ion can promote growth in the direction normal to the length direction (c axis). Additionally, different concentrations of ferric chloride would result in different concentrations of Cl- ions, which plays a key role in shape control, consistent with the discussion in previous studies.20 From the viewpoint of thermodynamics, it is the crystal structure rather than the additive that determines the final morphology. Because the current hydrothermal synthesis system involves a highly nonequilibrium environment originating at higher pressure and higher temperature, further investigation is needed to understand the formation of these unique materials better. 3.2.2. Effects of Phosphate Ion and Sulfate Ion Concentrations. The roles that phosphate and sulfate ions play in the formation of the SNTs may be different. To elucidate the roles of these two additives, the reactions (i) without any salt additives, (ii) only phosphate or sulfate ions, and (iii) one of these in excess were investigated separately. The formation process for hematite nanostructures mediated by different amounts of sulfate and phosphate additives is illustrated in Scheme 2. First, TEM images of a typical batch of synthesized R-Fe2O3 nanostars, extracted from sample S13 originally added without any phosphate or sulfate ions, are shown in Figure 7a. The highyield nature of the stars and their complex morphology and homogeneous distribution can be clearly interpreted. The
average diameter of as-obtained product nanoparticles is about 118.3 nm. Figure 7b is presented at this magnification to detail the star morphology for an enhanced perspective of their 3D structure. The results suggest that there is no preferential anisotropic growth without two additives. Unlike the SNTs, either solid spindlelike nanoparticles (sample S14) or nanoneedles (sample S15) were formed with only phosphate or only sulfate ions as an additive, respectively (Figure 7c,d). Figure 7c indicates that the phosphate ions are mainly attributed to the selective adsorption on surfaces parallel to the c axis. In comparison with phosphate, the adsorption affinity of sulfate to hematite is much weaker. The TEM image of the products prepared only with sulfate ions is shown in Figure 7d. The products were composed of nanoneedles with obvious anisotropic growth, and the measured interplanar spacings of in situ SAED patterns (inset in Figure 7d) were found to be in excellent agreement with database values for R-Fe2O3. The needlelike morphology appears to be caused by the preferential adsorption of foreign species on the {110} or {104} planes.21 The result confirms that sulfate also gives elongated crystals along the c axis. By increasing the ratio of sulfate ions to ferric ions, a mixture of nanocapsules and double nanocapsules (labeled with an arrow) was obtained, as shown in Figure 8a (sample S16). In comparison with the R-Fe2O3 SNTs (sample S10), the average length of the nanocapsules has shortened by about 100 nm.
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SCHEME 2: Schematic Illustration of the Formation Process for Hematite Nanostructures Mediated by Different Amounts of Sulfate and Phosphate Additives
Conversely, by increasing the ratio of phosphate ions to ferric ions, nanorings were obtained, as shown in Figure 8b (sample S17). The morphology of nanorings is similar to those identified in sample S3 (Figure 3c) synthesized at CFe2+ ) 0.02 M, but the average diameter has increased from 94.1 to 115.8 nm. The result reveals that increasing the ratio of sulfate ions will cause the end to close and the length to decrease, and most of the phosphate ions act as a shape controller to induce anisotropic growth, as shown in Scheme 2. Therefore, phosphate ions play a much more crucial role than sulfate ions in the formation of
capsule-shaped nanostructures because of their stronger adsorption effect. As a ligand, sulfate ions favor the dissolution of hematite because of their coordination effect with ferric ions, resulting in the final formation of a hollow structure; these results are in agreement with other reports.17,22,23 3.2.3. Proposed Growth Mechanism. It is well known that phosphate and sulfate ions can adsorb on the surface of R-Fe2O3, and the selective adsorption to R-Fe2O3 planes is similar. The presence of phosphate anions is found to increase the aspect ratio of the R-Fe2O3 nanoparticles, resulting in single-crystal
Figure 7. TEM images of the products prepared (a, b) without any salt additive, (c) with only phosphate, and (d) with only sulfate. (The inset shows in situ SAED patterns.)
Figure 8. TEM images of the products prepared by adding (a) twice the amount of sulfate and (b) twice the amount of phosphate.
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Figure 9. (a) High-magnification TEM image of the particles in Figure 4c, (b, c) HRTEM of the edge area, and (d) central area.
capsulelike nanoparticles along the c axis with stabilized faces. Sulfate behaved in a similar manner, but the effect was less pronounced.24 It is considered that the phosphate is preferentially adsorbed onto the faces parallel to the c axis of R-Fe2O3, in view of the better matching of the O-O interatomic distance of PO43- anions (2.50 Å) with the Fe-Fe spacing parallel (2.29 Å) rather than perpendicular (2.91 Å) to this axis, thereby promoting the development of spindlelike or capsulelike nanoparticles.25 As a ligand, sulfate ions favor the dissolution of R-Fe2O3 because of their coordination effect with ferric ions, resulting in the final formation of a hollow structure.17 It is noted that Sugimoto and Wang26 also suggest that sulfate ions were actually found to be most strongly adsorbed to crystal faces parallel to the c axis of the hexagonal crystal system from the adsorption isotherms of sulfate to hematite particles of different crystal habits, in accord with the retardation of their growth in the direction normal to the c axis. The functions of phosphate and sulfate ions have been confirmed by samples S14 (Figure 7c) and S15 (Figure 7d). Additionally, it is noteworthy that the ferric ion concentration plays a key role in the formation of SNTs; at a high PO43-/ Fe3+ mass ratio, the reaction process would produce ringlike nanoparticles, and at a low PO43-/Fe3+ mass ratio, the reaction process would produce tubelike nanoparticles. Accordingly, a high SO42-/Fe3+ mass ratio will cause some particles with a double capsulelike structure and the end of some particles cannot open, and a low SO42-/Fe3+ mass ratio will produce hematite nanoneedles. Only with the two appropriate additives can the hematite precursors grow into SNTs. Conversely, the hematite precursors grow into polygonal-like nanoparticles without the two additives as a result of anisotropic growth. Moreover, on the basis of the above condition-dependent morphology evolution experiments, we analyzed some particles of sample S6 by HRTEM to study the detail of the mechanism, which was an
intermediate in the formation process of SNTs from the viewpoint of ferric ions. Figure 9a shows three capsulelike particles in sample S6. Figure 9b,c shows HRTEM images of the different edge area from one of the particles (labeled in Figure 9a), and the clear lattice fringes reveal the crystalline nature of the edge area. The distance between the adjacent lattice fringes is 0.207 nm, which is very close to the interplanar distance of the (202) plane. Figure 9d is the HRTEM image of the center area of the particle in Figure 9a. The clear lattice fringes and diffraction spot reveal the crystalline nature of the edge area. The distances between adjacent lattice fringes are 0.219, 0.184, and 0.126 nm, which are very close to the interplanar distances of the (113), (024), and (220) planes, respectively. On the basis of these characterization results, we realized that the dissolution process occurred along the [001] direction that is perpendicular to the disk face and the recrystallization process occurred at the edge of the particles on the (202) plane. Therefore, in the presence of phosphate and sulfate ions, the growth of the prism planes, such as (110) and (100), is restrained, and the R-Fe2O3 nanoparticles grow along the length axis direction, which generates the appearance of a capsule-shaped structure.17 This is consistent with the previous TEM results. Moreover, from the viewpoint of the reaction time, we believe that the formation of hematite SNTs can be rationally expressed as a classical Ostwald ripening process. Thus, the whole reaction process of preparing the hematite SNTs can be classified as three stages following these reactions to describe the generalized precipitation and dissolution and subsequent precipitation of R-Fe2O3. First, the ferric ions will hydrolyze and form an FeOOH precipitate because of the high temperature and pressure, Fe3+ + 2H2O f FeOOH + 3H+. Owing to the protonation effect, FeOOH will dissolve and change back to ferric ions. Moreover, phosphate and sulfate ions can accelerate the dissolution by their coordination effects
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Figure 10. TEM images and SAED patterns (inset) of the products of (a) Fe3O4 SNTs and (b) γ-Fe2O3 SNTs. (c) XRD patterns of (curve a) Fe3O4 SNTs and (curve b) γ-Fe2O3 SNTs.
with detached ferric ions in this stage. For instance, some “tubein-tube” particles were found in sample S16 (the arrow labeled in Figure 8a), and that might be recognized as an important evidence of the dissolution mechanism. Figure S1a,b (Supporting Information) shows a high-magnification TEM image of the tube-in tube structure in which the inner tube and the outer tube are still connected to each other, indicating that the tubes had been a single particle. The formation of this structure can be attributed to the equal opportunity for dissolution on multiple sides of the ellipsoids and the different dissolution speeds, which were caused by microsurroundings, for example, the adsorption concentration of sulfate or phosphate ions.27,28 A SAED investigation (Figure S2, Supporting Information) of sample S7 after 90 min of processing demonstrated the development of much larger R-Fe2O3 hollow spindles alongside many smaller (
