synthesis and growth of hematite nanodiscs through a facile hydrothermal approach

RESEARCH PAPER Synthesis and growth of hematite nanodiscs through a facile hydrothermal approach X. C. Jiang Æ A. B. Yu Æ W. R. Yang Æ Y. Ding Æ C. X. Xu Æ S. Lam Received: 28 November 2008 / Accepted: 7 April 2009 Ó Springer Science+Business Media B.V. 2009 Abstract This study reports a facile hydrothermal method for the synthesis of monodispersed hematite (a-Fe 2 O 3 ) nanodiscs under mild conditions. The method has features such as no use of surfactants, no toxic precursors, and no requirements of high-temper- ature decomposition of iron precursors in non-polar solvents. By this method, a-Fe 2 O 3 nanodiscs were achieved with diameter of 50 ± 10 nm and thickness of *6.5 nm by the hydrolysis of ferric chloride. The particle characteristics (e.g., shape, size, and distribu- tion) and functional properties (e.g., magnetic and catalytic properties) were investigated by various advanced techniques, including TEM, AFM, XRD, BET, and SQUID. Such nanodiscs were proved to show unique magnetic properties, i.e., superparamag- netic property at a low temperature (e.g., 20 K) but ferromagnetic property at a room temperature (*300 K). They also exhibit low-temperature (\623 K) catalytic activity in CO oxidation because of extremely clean surfaces due to non-involvement of surfactants, compared with those spheres and ellip- soids capped by PVP molecules. Keywords Hematite nanoparticles Á Nanodiscs Á Hydrothermal synthesis Introduction Hematite (a-Fe 2 O 3 ) nanoparticles have been widely studied because of their attractive properties, includ- ing stability in air, n-type semiconducting, non- toxicity, and corrosion-resistance. These properties have driven them for potential applications in catal- ysis, gas sensing, pigment, nonlinear optic, and field- effect transistor (Shin et al. 2004; Schertmann and Cornell 1991; Wang and Willey 1998, 1999). A variety of synthesis methods have been employed for shape and size control, such as ball milling, co-precipitation, sol-gel method, micelle template, thermal decomposition of precursors in non-polar solvents, and hydrothermal methods (Ozaki et al. 1984; Matijevic ´ 1985; Matijevic ´ and Hamada 1982; Matijevic ´ and Scheiner 1978; Woo et al. 2003; Vayssieres et al. 2005; Cao et al. 2005; Yin et al. 2005; Raming et al. 2002). Some methods focused on X. C. Jiang Á A. B. Yu (&) School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia e-mail: a.yu@unsw.edu.au W. R. Yang Australian Key Centre for Microscopy and Microanalysis (AKCMM), Electron Microscopy Unit, University of Sydney, Sydney, NSW 2006, Australia Y. Ding Á C. X. Xu School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, China S. Lam CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, NSW 2070, Australia 123 J Nanopart Res DOI 10.1007/s11051-009-9636-8 the thermal decomposition of organometallic precur- sors (e.g., Fe(CO) 5 , Fe(acac) 3 , Fe(oleate) 3 , or their dual source systems) in non-polar solvents for generating monodispersed iron oxide nanoparticles, which, however, may limit their applications in aqueous system (Yin and Alivisatos 2005; Casula et al. 2006; Cheon et al. 2004). For example, the iron oxide nanoparticles obtained by thermal decomposi- tion in non-polar solvents are difficult to be transferred directly into aqueous solution because of the surface- coated surfactants. The removal of such surfactants may lead to particle aggregation, and hence affect the covalently binding other surfactants such as poly(eth- ylene)glycol (PEG) spacer with hydrophilic groups for further dextran coating that is targeted toward solid tumor treatment (Sonvico et al. 2005). Among the achieved hematite nanocrystals obtained, non-spherical particles have become attrac- tive because of their anisotropic properties. To date, one-dimensional (1D) iron oxide nanoparticles (e.g., rods and wires) have been widely studied, but only a few studies were reported on two-dimensional (2D) ones (e.g., plates and discs). Their growth mecha- nisms are not well understood. A representative example in this area was reported by Casula et al. (2006) who demonstrated the preparation of iron and/ or iron oxide nanodiscs through a thermal decompo- sition (at *293°C) of iron pentacarbonyl in the presence of an oxidizer and surfactants (tridecanoic acid or 3-Chloro peroxybenzoic acid). Niederberger et al. (2002) reported the fabrication of hematite disclike particles with outer diameter of *1 lm and the thickness of *250 nm through a hydrolysis and subsequent hydrothermal approach. Chen and Gao (2004) prepared the crystalline a-Fe 2 O 3 nanodiscs by a hydrothermal method at 150°C and through aging for 24 h in the presence of surfactants. The above synthesis methods suggested that the utilization of surfactants is necessary and important for shape control. This is also evidenced by other wet-chem- istry methods in the past. However, drawbacks resulting from surface-adsorbed surfactants have the unpredictable influence on the surface functionality of nanoparticles and the diminished accessibility to particle surface. A small amount of residual may significantly reduce the functionalities in catalysis or gas sensing. For example, the pyrolysis of oleic acid could introduce some reducing agents such as carbon (C), carbon monoxide (CO) and hydrogen (H 2 ), which can cause a negative effect on the particle performances (Kim et al. 2007). Therefore, to develop facile synthesis methods to produce mon- odispersed iron oxide nanodiscs with extremely clean surfaces is still a challenging task. In this study, we demonstrate a facile hydrother- mal approach to generate monodispersed a-Fe 2 O 3 nanodiscs in the absence of surfactants under mild conditions. The particle characteristics (e.g., shape, size, crystallization) and physicochemical properties (e.g., magnetic, catalytic properties) of the as- prepared nanoparticles are investigated by various advanced techniques. The influence of a few exper- imental parameters (e.g., pH, temperature, time, and concentration of Fe 3? ) on the particle growth in the surfactant-free system is then investigated. The catalytic CO oxidation, as one of typical functional- ities of hematite nanoparticles, is also examined. Experimental work Synthesis of iron oxide nanoparticles The iron oxide nanoparticles could be prepared by the hydrolysis of FeCl 3 salt in an acid solution under mild conditions. This approach is similar to the previous studies (Matijevic ´ and Scheiner 1978; Raming et al. 2002), and the modification of experimental para- meters has been adopted to prepare plate-like nano- particles. In a typical procedure, three steps were involved. First, 0.5 g FeCl 3 Á 6H 2 O (Sigma-aldrich, 99.9%) was put in 10 ml of water, followed by vigorous stirring to ensure that all the powders got dissolved completely. Second, the transparent yellow- brown solution was quickly injected into a conical flask containing 90 ml of hot water (*90°C) and 0.75 ml of dilute HCl (1.0 M), followed by vigorous stirring to ensure that the reaction system was homogeneous. Finally, the mixed solution was refluxed heating at 90°C for around 5 min before being transferred into an oven for heating at 90°C. In order to avoid water evaporation, the flask was sealed by aluminum foil and a glass lid. After heating for 48 h, the solution turned deep red color. The particles were found homogeneously dispersed in this solution. In the surfactant-assisted synthesis, poly(vinyl pyr- rolidone) (PVP, M w = 55,000, Sigma-aldrich, 99.9%) was used to control particle shape and size, but other J Nanopart Res 123 parameters were kept constant. Ultra-pure water was used in all the synthesis processes. All the glasswares were cleaned with aqua regia, thoroughly rinsed with ultra-pure water and alcohol prior to use. Characterization Various techniques were used to characterize the particle characteristics and properties in this study, as described below: (i) Particle characteristics such as shape, size, and size distribution were checked using Philips CM200 field emission gun transmission electron microscope (TEM) operated at an accelerated voltage of 200 kV. The specimen was prepared by dropping the solution onto a Formvar-coated copper grid and dried in air naturally. The data for particle-size distribution were collected based on TEM analysis, and also assisted by Image Processing and Analysis Program (ImageJ 1.37v, 2006); (ii) The composition of the as-synthesized sample was identified by powder X-ray diffraction (XRD), and recorded using Siemens D5000 at a scanning rate of 0.5°/min in the 2h range of 20–80°; (iii) The atomic force microscope (AFM) image was obtained by a Molecular Imaging Picoscan II instrument in tapping mode. The sample was prepared by depositing a few drops of a dilute solution of the nanoparticles onto a mica disc and then dried in air naturally. Analysis of the AFM image was performed using the WSxM software (version 3, Nanotec Electronica S.L., Spain); (iv) The Brunauer–Emmett–Teller (BET) surface area of the as-prepared particles was measured at 77 K (liquid nitrogen) on a Quantachrome Autosorb-6B Surface Area & Pore Size Ana- lyzer. Before BET measurements, the sample was degassed at 150°C for 3–4 h to ensure that no gas molecules adsorbed on the particle surfaces; (v) The magnetic properties were investigated on a Quantum Design MPMS XL-5 (SQUID) magnetometer. The sample was put in a low- susceptibility plastic sample holder for mea- surements. The magnetic moment from the sample holder was found to be at least three orders of magnitude smaller than the signal from the sample and thus can be ignored; (vi) The catalytic oxidation of CO gas was per- formed in a home-built fixed bed microreactor. A reactant gas containing CO (6,000 ppm) in O 2 atmosphere (CO/O 2 = 1/10) was buffered with N 2 gas and with a total flow rate of 60 ml/ min. An appropriate amount of hematite pow- ders (*50 mg) was used in the measurement. The heating and cooling cycles were monitored in a temperature range of 30–450° C. Results and discussion Microstructure of iron oxide nanoparticles The microstructure of the as-prepared nanoparticles obtained by the proposed synthesis strategy was checked by TEM technique. Figure 1a shows the TEM image of one representative sample. The particles were found nearly monodispersed with diameters of 50 ± 10 nm based on their size distri- bution (Fig. 1b). In order to confirm the crystalliza- tion, the selected area electron diffraction (SAED) was carried out under TEM operations. Several clear diffraction rings doped with spots were recorded and shown in a pattern (inset of Fig. 1a), suggesting that the nanoparticles are of crystalline structure. The further confirmation on single crystalline or poly- crystalline particles needs other techniques like XRD and HRTEM. These diffraction rings could be assigned to (104), (110), (113), (024), (116), and (300) crystallographic planes of rhombohedral phase a-Fe 2 O 3 , respectively, based on the standard JCPDS card (No. 02-915) (Cullity and Stock 2001). A close inspection on the particle shape and crystallization was conducted by various techniques. Figure 2a shows a magnification TEM image reveal- ing that the as-prepared particles are spherical parti- cles, and some of them overlapped, as pointed by arrows. Further evidence could be directly obtained from the AFM image shown in Fig. 2d. The curve plotted in Fig. 2d reveals that the particle is of plate- like structure with a thickness of *6.5 nm. Combin- ing the structural analysis of TEM and AFM, the as-prepared hematite particles are of nanodiscs in J Nanopart Res 123 shape. The lattice fringes of the individual nanodisc could be clearly seen in the high-resolution TEM (HRTEM) image (Fig. 2b), indicating that the nano- discs are well crystallized under the reported condi- tions. Measuring the distance between two adjacent planes gives a value of *0.411 nm, corresponding to the lattice spacing of {110} facets of rhombohedral a-Fe 2 O 3 . The electron diffraction (ED) pattern could be indexed to the [001] zone of rhombohedral hematite (inset of Fig. 2b). The crystallization of the nanodiscs was also evidenced by the well-resolved peaks in XRD pattern (Fig. 2c), in which all the diffraction peaks could be assigned to rhombohedral phase a-Fe 2 O 3 (a = b = 5.028 A ˚ and c = 13.728 A ˚ , JCPDS 02-915) (Cullity and Stock 2001). This is also supportive to the indexed diffraction rings in the SAED pattern (inset of Fig. 1a). These results revealed that the as-prepared nanodiscs are pure rhombohedral a-Fe 2 O 3 with single crystalline structure. Particle nucleation and growth The particle nucleation and growth occurred while the ferric ions solution was mixed with hot water (90°C), accompanied by a rapid color change from yellow to red. In order to trace the growth, the colloids were isolated from the heated suspension at different times for statistic analysis. Owing to the limitations in in-site observing the nucleation of colloids, several representative samples were chosen here to illustrate the growth process. After heating for *1 min, small colloids formed, but they are difficult to be clearly distinguished in shape and size as shown in Fig. 3a. After heating for *5 min, the solution turned a bit dark red. The colloids isolated from this dark-red solution were checked by TEM technique. Figure 3b shows the TEM image that the shapes of colloids are still difficult to distinguish, but the particle size becomes larger (20 ± 10 nm) than those obtained at the reaction time of *1 min. This result suggested that the nucleation is fast, as the so-called ‘‘burst-nucleation’’ happens in this reaction system. Although the nucleation and the growth may be overlapping each other at the initial stage, the particle size increasing with time could be clearly observed after 5 min (Fig. 3b–f). The fast nucleation and the subsequent slow growth obtaining well-crystallized nanodiscs could also be confirmed by the relationship between reaction time and particle sizes as described in Fig. 4, the corresponding data for which were collected and compared on the basis of TEM images obtained at different times. This is different from the nucleation-delayed mechanism that occurred in the formation of iron oxide nanodiscs through the thermal decomposition of Fe(CO) 5 precursor in non-polar solvent (Casula et al. 2006). After 5-min heating at 90°C, the reaction system was transferred into an oven with the heating continued further. In order to further understand the particle growth, the particles produced at different times (e.g., 1, 6, 12, and 24 h) were separately isolated for TEM characterization. Figure 3c shows the TEM image that 1-h heating merely resulted in the formation of irregular-shaped particles, but par- ticle size increased with time. The 6-h heating was found to result in small particles to grow to a diameter of 50–70 nm, and some of them were still irregular in shape (Fig. 3d). Again, the aggregation of small particles or the clusters could be observed in Fig. 1 a TEM image of hematite (a-Fe 2 O 3 ) nanodiscs with inset of SAED pattern; and b The size distribution of the a-Fe 2 O 3 nanodiscs J Nanopart Res 123 this sample. With continuous heating up to 12 h, more well-shaped particles formed with diameter of *70 nm, although the size distribution was a bit wide. In the meantime, it was found that almost the smaller irregular particles disappeared at this stage (Fig. 3e). This process could be consistent with Ostwald ripening, i.e., smaller particles continue to shrink, while larger particles continue to grow (Ostwald 1896). The 24-h heating could lead to the formation of nearly monodispersed particles with a mean diameter of 55 nm (Fig. 3f). A close look at the particles reveals that they have a slight shrinkage in 20 30 40 50 60 70 80 (306) (217) (1010) (208) (300) (214) (018) (116) (024) (113) (110) (104) (012) Intensity (a.u.) 2 θ (degree) C 200 300 400 500 600 0 2 4 6 8 10 Thickness (nm) Diameter (nm) D Fig. 2 a A high magnification TEM image of a-Fe 2 O 3 nanodiscs with overlapping as pointed by arrows; b HRTEM image showing the lattice fringe of {110} planes with spacing between two adjacent planes of 0.411 nm; c XRD pattern of the nanodiscs showing that the particles are of rhombohedral phase; d AFM image of an individual nanodisc and the curve showing that the thickness is *6.5 nm J Nanopart Res 123 size relative to those obtained by heating for a short time (e.g., 12 h). This was probably due to by atomic reconstruction on particle surfaces to minimize surface energy. Moreover, the electron diffraction rings recorded in the SAED patterns become clear, indicating that the particles crystallized better with longer heating duration. In order to clearly describe the time-dependent growth, the relationship between particle size and heating time was plotted and shown in Fig. 4.It could be seen from Fig. 4 that the nucleation is fast but the particle size increase slowly with time. The particle size increases up to the maximum *70 nm around 12 h. After that, more and more well-shaped Fig. 3 Time dependence of iron oxide nanoparticles formed in aqueous solution: a 1 min; b 5 min; c 1h; d 6h;e 12 h; f 24 h J Nanopart Res 123 nanodiscs formed, and the particle size gradually decreased to *55 nm due to the possible recon- struction of the surface atoms with extended heating duration. In order to understand the formation and growth mechanism of nanodiscs, a few possibilities have been proposed previously. A typical example was reported by Casula et al. (2006) who supposed a delayed nucleation mechanism for the formation of iron oxide nanodiscs during the high-temperature decomposition process, which results in the occurrence of nanopar- ticle crystallization well separated in time from the injection of the precursors. They suggested a burst- like nucleation at a certain delayed time and subsequent fast nanocrystal line growth at a high iron monomer concentration that promoted the kinetically induced formation of anisotropic discs. The retarda- tion of the nucleation was induced by the surfactant (e.g., fatty acid) used as a coordinating agent, which strongly stabilizes the monomer in solution. On the other hand, Redl et al. (2004) and Hyeon et al. (2001) reported that when the reaction was carried out under conditions that favor gradual and slower monomer release into the solution, thermodynamically sta- ble iron oxide nanospheres were produced. For those nanodiscs obtained in aqueous solution, Niederberger et al. (2002) supposed that the use of an iron–polyolate complex of [N(CH 3 ) 4 ] 2 –[OFe 6 (H -3 thme) 3 (OCH 3 ) 3 Cl 6 ] Á MeOH as a precursor mate- rial can produce disclike hematite particles by a procedure involving the hydrolysis and subsequent hydrothermal treatment at 150°C over 24 h. They found that each of the large particles (outer diameter *1 lm and thickness *250 nm) was made up of many small plate-like particles. Chen and Gao (2004) also suggested that the presence of surfactants, such as poly(oxyethylene)(20)-sorbitan monooleate (Tween 80) and pluronic amphiphilic triblock copolymer (P123), played an important role in the formation of crystalline a-Fe 2 O 3 nanodiscs during hydrothermal treatment at 150°C for a period of 24 h. The abovementioned methods revealed that the surfactants played a key role in formation and growth of iron oxide nanodiscs. However, these proposed mechanisms seem to be unsuitable for our case. On the one hand, no delayed nucleation occurred on the basis of the time-depen- dent nucleation and growth processes (Figs. 3, 4). That is, the burst-like nucleation and the subsequent anisotropic growth do not significantly occur in this case under the reported conditions. On the other hand, no surfactants or complex precursors were used in our synthesis, and thus the surfactant-assisted growth by selective face adsorption could not be considered. Therefore, it is believed that the particle morphology may be determined by other factor(s) in this system. Let us now consider chloride (Cl - ) ions first. In the reaction solution, the Cl - ions are excessive due to the addition of HCl for pH adjusting below 2 (it was measured that the pH is *2 without addition of acid in this case), which is believed to cause the Cl - ions to play dual possible roles: to retard hydrolysis of the ferric ions and to reduce surface energy on a certain crystal plane to promote preferential growth. Some investigators have also studied the particle preferential growth in the systems without surfactants. For exam- ple, Wang et al. (2008) suggested that the forced hydrolysis of ferric chloride under acidic pH could result in the direct transformation from amorphous iron oxide to crystalline hematite when aged at 100°C for a period of 48 h, and these hematite nanocrystals could assemble into large-size disclike particles via solvent evaporation. Moreover, Matijevic ´ and Scheiner (1978) and Matijevic ´ (1985) investigated the influ- ence of inorganic ions such as chloride, nitrate (NO 3 - ), and perchlorate (ClO 4 - ) on the shape and size of hematite in the hydrothermal reaction carried out at 100°C. They found these inorganic ions could lead to different shapes and sizes of hematite particles. Raming et al. (2002) reported a similar approach by using iron chloride salt and being carried out at 01020304050 0 20 40 60 80 Particle diameter (nm) Reaction time (h) Fig. 4 The plotted curve showing the time-dependent nucle- ation and growth of the colloids J Nanopart Res 123 90–100°C for different times (e.g., 1–6 days) to prepare hematite colloids; however no disclike parti- cles were formed. In other systems, the effect of inorganic ions on particle growth was also studied. Both Livage et al. (1988) and Reeves and Mann (1991) groups have demonstrated the influence of inorganic ions such as chloride, phosphate (PO 4 3- ), sulphate (SO 4 2- ), and perchlorate on the shape and size of hematite and other transition metal oxides. They reported that the presence of Cl - ions could result in rhombohedral hematite crystals comprising 10 " 14 faces that exhibited relatively high energy. The formation of such high-energy 10 " 14 faces indicates that the Cl - anion has a profound influence on the stability of these faces because the 10 " 14 face has an open structure that may be able to accommodate Cl - (ionic radius 1.8 A ˚ ) and thereby stabilize the bonding within the surface plane. The selective surface effect of Cl - anion could finally affect the morphology and size of particles. Similar effects have been observed for nanocrystals grown in the presence of fluoride (F - ), Cl - , and bromide (Br - ) for copper nanorods (Filankembo et al. 2003; Filankembo and Pileni 2000), hydroxide (OH - ) for silver nanowires (Caswell et al. 2003), as well as F - and Cl - anions for titania nanosheets (Yang et al. 2008; Penn and Banfield 1999), through selective surface adsorption. As a further confirmation, the replacement of HCl by dilute HNO 3 , HClO 4 , and H 2 SO 4 was carried out to adjust solution pH in this study. Figure 5 shows the corresponding TEM images of nanoparticles obtained by addition of various acids as mentioned above. When NO 3 - ions were added, the irregular-shaped particles were obtained (Fig. 5a). The precise size of particles was difficult to measure. The addition of SO 4 2- ions could result in flocculation rapidly (B3 min) during heating at 90°C. The particles obtained under such conditions were unshaped, and the size is difficult to estimate (Fig. 5b). While ClO 4 - ions were used, cube-like particles formed with edge lengths of 30–70 nm (Fig. 5c), consistent with the previous literature. Unfortunately, the addition of these acids could not produce disclike hematite particles under the reported conditions. This sug- gested that the Cl - ions indeed played a crucial role in the formation of hematite nanodiscs under the conditions considered, which is consistent with the observations reported by Matijevic ´ and Scheiner (1978) that the inorganic ions could lead to shape and size change of hematite particles. Effects of experimental parameters In order to better understand, the effects of other experimental parameters were also investigated including pH, reaction temperature, and concentra- tion of Fe 3? ions, as discussed in the following context. pH The hydrolysis of Fe 3? ions is closely related to the pH of the solution. The pH is normally adjusted by addition of acid or base. Here the effect of pH on particle growth was further investigated. Figure 6 shows the TEM images of the particles obtained at different pH values. At pH = 1.5, the particles were found to be well crystallized with an average Fig. 5 Effect of the inorganic anions on the shape and size of nanoparticles: a NO 3 - ; b SO 4 2- ; c ClO 4 - J Nanopart Res 123 diameter of 77 nm (Fig. 6a), confirmed by their size distribution shown in Fig. 6d. When the pH value was increased to *2.5 by addition of an appropriate amount of NaOH solution (5 M), the average size of nanoparticles decreased a bit to *70 nm in diameter (Fig. 6, panels b and e). On further increasing pH to *3.0, the particles continuously reduced in average size to *63 nm (Fig. 6, panels c and f). However, when pH was further altered by adding HCl or NaOH, it was found that a low pH (\1.0) could not produce any particle but a clear solution, whereas a high pH ([3.5) could lead to some precipitates (e.g., Fe(OH) 3 ) prior to any reflux heating. Further inspec- tion of the particles (Fig. 6, panels a–c) suggested that the slight shrinkage in size with pH increasing was probably caused by the fast hydrolysis rate at a high pH (*3.0). The diffraction rings in the SAED patterns (inset of Fig. 6, panels a–c) revealed that the as-prepared nanoparticles are of crystalline structure. In this system, the H ? ions may have two functions: to slow down the hydrolysis rate of Fe 3? ions and to stabilize the oxygen-terminated crystal planes such as a-Fe 2 O 3 {0001} (Cotton and Wilkinson 1988). At a low pH (1–3), a particle prefers to grow along other planes such as a-Fe 2 O 3 {01ı¯1}, beneficial for the anisotropic growth of particles (Schertmann and Cornell 1991; De Leeuw and Cooper 2007; Goldschmidt 1913/1923). After many tests, we found that the most suitable pH value for disc formation is 1–3. Lower pH (\1.0) could not produce any particle due to their rapid dissolution, whilst higher pH ([3.5) could lead to precipitates (e.g., Fe(OH) 3 ) directly under the reported conditions. This is in agreement with those previously reported that a-Fe 2 O 3 was Fig. 6 Effect of solution pH on the shape and size of iron oxide nanoparticles and the corresponding size distributions: a, d pH = 1.5; b, e pH = 2.5; c, f pH = 3.0 J Nanopart Res 123 merely obtained at pH \4 (Weiser and Milligan 1935; Mackenzie and Meldau 1959). Due to the complicated processes involving hydrolysis (Fe 3? )- nucleation and (Fe(OH) 3 )-phase transformation (from b-FeOOH to a-Fe 2 O 3 ), it is believed that a further study needs to be performed to understand the particle growth. Reaction temperature The particle growth is also affected by the reaction temperature. Figure 7 shows the TEM images of the prepared nanoparticles at different temperatures (e.g., 70, 80, and 100°C). Other experimental parameters were maintained the same as those for the temperature of 90°C. When heated at 70°C for 48 h, small particles formed with irregular shape (Fig. 7a). Particle size was in the range of 5–10 nm, and most of the particles aggregated together. The weak diffraction rings in the SAED pattern suggested that these particles were not well crystallized (inset of Fig. 7a). When heated at 80°C, some bigger particles were formed (diameter of 40–80 nm), along with some smaller ones (Fig. 7b). The diffraction rings (Fig. 7b) became clear, indicat- ing that the particles crystallized further with increas- ing temperature. While at 100°C, the as-produced particles were spindle or multi-armed nanostructures with diameters of 20–50 nm and length up to several hundred nanometers (Fig. 7c). Similar scenarios were observed as reported by Raming et al. (2002) con- firming that the same particles were present after heating at 100°C for 1 day and for 1 week if the ferric chloride salt was added directly into the preheated hydrochloric acid solution (method 1). They also described that a mixture of two particle types (i.e., spindle and oval shapes) was produced if the ferric chloride was not added directly to the preheated hydrochloric acid solution, which, however, first dissolved in cold water before heating to 100°C (method 2). In particular, the XRD analysis from Raming et al. (2002) showed the presence of two phases, hematite and akagane ´ ite, if the reaction was carried out by method 2. Moreover, such spindle or multi-armed nanostruc- tures show rough surfaces and no well-defined crystalline faces, similar to those prepared by addi- tion of phosphate ions during the hydrolysis and growth processes. Reeves and Mann (1991) sug- gested that the interaction of phosphate with hematite crystals was not specific to a single set of symmetry- related faces in forming spindle-shaped iron oxide particles. Our observations are also consistent with the previous studies that were performed at a high temperature (e.g., 100°C), although the shape and the size distribution of particles are slightly different. The precursor of [Fe(OH) 2 (OH 2 ) 5 ] ? does not form a polycation but nucleates directly into a-Fe 2 O 3 parti- cles, which, however, may result in various morphol- ogies (Matijevic ´ and Scheiner 1978). Concentration of Fe 3? ions In order to investigate the effect of concentration of ferric salt ([Fe 3? ]) on particle shape and growth, the concentrations tuned from 0.038 to 5.55 mM were tested. During all the tests, the reaction temperature and heating time were kept constant. The pH value of the solution was adjusted carefully and kept around 2. Fig. 7 Temperature dependence of iron oxide nanoparticles formed in aqueous solution: a 70°C; b 80°C; c 100°C J Nanopart Res 123 [...]... still reveal that they are of crystalline structure Functional properties of hematite nanodiscs The properties of nanoparticles are heavily dependent on the morphology and size, particle surface, and crystallinity As characterized above, the hematite nanodiscs achieved by a surfactant-free hydrothermal method show a few features including monodisperse in shape and size, extremely clean surfaces, and wellcrystallized... (i.e., Morin temperature) between the weak ferromagnetic and the uniaxial antiferromagnetic states (Amin and Arajs 1987) Such a difference is probably caused by unique morphology and dimension of hematite nanodiscs (e.g., diameter of 50 ± 10 nm and J Nanopart Res Catalytic property Characterized with extremely clean surfaces, the hematite nanodiscs produced have great potential in catalysis In order to... Functional Nanomaterials, Natural Science Foundation of China (NSF50671019), and China Postdoctoral Science Foundation (No 2005038252) X J gratefully thanks Miss K.Y Koh at UNSW for her help in measurement of BET surface area, and Mr S Gnanarajan for his technical assistance in using the SQUID Magnetometer in CSIRO (Australia) References Amin N, Arajs S (1987) Morin temperature of annealed submicronic a- F2O3... our cases The high surface area of nanodiscs than that of PVP-capped particles is also a J Nanopart Res possible reason why the a- Fe2O3 nanodiscs have low temperature (\623 K) catalytic ability in CO oxidation Here, the PVP molecules may occupy some active sites on the particles surface to block or reduce the adsorption of CO and/ or O2 molecules and then reduce the catalytic ability On the other hand,... this, the catalytic activity of a- Fe2O3 nanodiscs in CO oxidation was evaluated and examined under the conditions where CO concentration is 6,000 ppm and CO/O2 = 1:10 buffered with N2 gas, with a total flow rate of 60 ml/min Figure 1 0a (triangle spotted line) shows the temperature dependence of catalytic CO oxidation by a- Fe2O3 nanodiscs It was found that the catalytic CO oxidation mainly took place below... these nanodiscs exhibit weak ferromagnetic behavior that may be contributed from both the canting of the sublattice magnetization directions and the uncompensated spins as observed for a- Fe2O3 nanodiscs; whereas at a low temperature, they show superparamagnetic behavior with the Morin temperature (TM) of *20 K This is quite different from other hematite nanoparticles that have a clear transition temperature... superantiferromagnetic behaviors of ‘ferritin’ with a spherical protein shell of external diameter 13 nm surrounding an antiferromagnetic iron oxyhydroxide core of 123 Fig 9 a Magnetization as a function of field for a- Fe2O3 nanodiscs at temperatures of 5 and 300 K; the inset showing the data around zero field with an expanded scale ranging from -20 to ?20 kOe; and b low-field susceptibility as a function... performance in catalysis In addition, the nearly overlapped heating– cooling cycles (Fig 1 0a, triangle spotted lines) suggested that the a- Fe2O3 nanodiscs are stable in structure during the catalytic test In order to verify the availability and applicability of the proposed synthesis approach, we prepared PVP-capped iron oxide particles for catalytic property examination This approach was carried out by slightly... heating process might also affect the catalytic property of nanoparticles (Millan et al 2002; Kim et al 2007) Although these nanodiscs show relatively high surface area, there is always room for improvement, which will be directly related to the catalytic activity enhancement Nonetheless, the features such as high surface area, extremely clean surface, and wellcrystallized structure will make hematite. .. the similar composition as that of hematite nanodiscs a- Fe2O3 nanodiscs with extremely clean surfaces might result in lower temperature (\623 K) catalytic activity than those of ellipsoids or spheres capped by PVP A further confirmation involves measuring the surface areas of those nanoparticles by BET technique, as shown in Fig 10b The measured curves for calculating BET surface area of particles were . RESEARCH PAPER Synthesis and growth of hematite nanodiscs through a facile hydrothermal approach X. C. Jiang Æ A. B. Yu Æ W. R. Yang Æ Y. Ding. structural analysis of TEM and AFM, the as-prepared hematite particles are of nanodiscs in J Nanopart Res 123 shape. The lattice fringes of the individual nanodisc could