Iron Oxide Nanopropellers Prepared by a Low-Temperature Solution

Nguyen Duc Cuong , Dinh Quang Khieu , Tran Thai Hoa , Duong Tuan Quang , Pham Hung Viet , Tran Dai Lam , Nguyen Duc Hoa , Nguyen Van Hieu. Materials ...
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J. Phys. Chem. B 2006, 110, 14087-14091

14087

Iron Oxide Nanopropellers Prepared by a Low-Temperature Solution Approach Wan-Hua Yang,† Chi-Fang Lee,† Horng Yi Tang,‡ Dar-Bin Shieh,§ and Chen-Sheng Yeh*,† Department of Chemistry and Center for Micro/Nano Science and Technology, National Cheng Kung UniVersity, Tainan 701, Taiwan, Department of Applied Chemistry, National Chi Nan UniVersity, Nantou, Puli, 545, Taiwan, and Institute of Oral Medicine and Department of Stomatology and Center for Micro/Nano Science and Technology, National Cheng Kung UniVersity, Tainan 701, Taiwan ReceiVed: April 18, 2006; In Final Form: June 2, 2006

The R-Fe2O3 (hematite) nanopropellers were synthesized via a low-temperature solution-based method using FeCl2 as a precursor in the presence of urea and glycine hydrochloride. The formation of R-Fe2O3 nanopropellers is strongly depended on the addition of glycine hydrochloride, which serves as a pH modulator and affects the oxidation rate of Fe2+. The structural evolution of the propeller-structured hematite was found to follow dissolution and recrystallization processes. For the structural conformation, each nanopropeller presents a hexagonal central column closed by six equivalent surfaces of {1h100} and the six arrays of the nanopropeller structure are a result of growth along ([1h100], ([1h010], and ([01h10]. Preliminary results show that the magnetic maghemite (γ-Fe2O3) nanopropellers could also be prepared by a reduction and reoxidation process from the R-Fe2O3 (hematite) nanopropeller precursors.

Introduction Functional metal oxide nanocrystals have been extensively investigated in the recent decade for their outstanding new properties suitable for a broad spectrum of downstream applications.1-3 Iron oxide is one of the most widely investigated nanomaterials for both biological and industrial applications. Iron oxides (Fe2O3) have four phases: R-Fe2O3 (hematite); β-Fe2O3; γ-Fe2O3 (maghemite); -Fe2O3.4 Magnetic nanocrystals of γ-Fe2O3 have been applied in information storage, magnetic refrigeration, bioprocessing, controlled drug delivery, and ferrofluids,5,6 while R-Fe2O3 is environmentally friendly and of great interest for potential applications as a gas sensor, lithiumion battery, catalyst, and pigment.7-9 Iron oxide nanodevices hold great potential to serve as essential elements in nano- or micrometer scale motors, actuators, generators, or sensors. For example, nanometer size propellers could be the core for a nanomotor or nanogenerator depending on the input energy and design. For the variety of nanomaterials, only a handful of propeller-typed structures were fabricated among the different morphologies including spheres, rods, tubes, wires, belts, cubes, starlike, flowerlike, and other hierarchical architectures by various approaches. Wang et al. has reported the synthesis of ZnO nanopropeller arrays through a high-temperature solidvapor deposition process.10 Meanwhile, Gao et al. performed hydrothermal decomposition to prepare propeller-typed ZnO nanostructures.11 In this contribution, we demonstrate a simple synthetic route to prepare iron oxide nanopropellers starting from FeCl2 as a precursor in the presence of urea and glycine hydrochloride for the first time. Bottom-up techniques using a solution-phase approach provide great benefits in control of the size, morphology, and * Corresponding author. E-mail: [email protected]. † Department of Chemistry and Center for Micro/Nano Science and Technology, National Cheng Kung University. ‡ Department of Applied Chemistry, National Chi Nan University. § Institute of Oral Medicine and Department of Stomatology and Center for Micro/Nano Science and Technology, National Cheng Kung University.

dispersion of nanocrystals in nanofabrication. The related studies have reported on the synthesis of hematite (R-Fe2O3) with different conformations such as nanorods, nanowires, nanotubes, nanobelts,andotherthree-dimensionalhierarchicalarchitectures.12-16 Obtaining R-Fe2O3 on the basis of the solution approach has all started from Fe3+ systems. One recent work introduced FeCl2 salt to prepare FeOOH nanowires,17 and we have used FeCl2 accompanied with urea and glycine hydrochloride to directly synthesis R-Fe2O3 nanopropellers in aqueous solution. Urea serves a source of hydroxide ions during the hydrolysis of iron salts for the formation of iron oxide. Addition of glycine hydrochloride in this work strongly affects the oxidation rate of Fe2+; it plays the crucial role in determining propeller formation and is treated as a pH modulator. This strategy could enrich the related iron oxide chemistry and sheds light on the synthetic method in the preparation of R-Fe2O3 starting from Fe2+ systems. Furthermore, the low-temperature synthesis in aqueous solution employed here presents an environmentally and user-friendly approach. This synthesis has yielded the exclusive formation of uniform propeller-like nanocrystals consisting of 6-fold symmetrical branches with central nanorods along the c axis. Experimental Section Hematite (R-Fe2O3) nanopropellers were prepared by a reaction of FeCl2‚4H2O in a reflux system at 100 °C. A typical approach employed is as follows: 0.096 g of FeCl2‚4H2O, 2.4 g of (NH2)2CO, and 0.1 g of glycine hydrochloride were mixed in 40 mL of distilled water with constant magnetic stirring until a clear solution was obtained. Then the solution was transferred to a flask with a reflux condenser, which was maintained at 100°C for 95 min. The solution color gradually changed from colorless to orange-red. The orange-red solution was then cooled to room temperature. The resulting precipitates were collected by centrifugation and were washed several times with distilled water.

10.1021/jp062371t CCC: $33.50 © 2006 American Chemical Society Published on Web 07/04/2006

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Figure 1. XRD pattern of R-Fe2O3 nanopropellers synthesized in the presence of glycine hydrochloride (0.1 g) and urea (2.4 g) at 100 °C for 95 min.

Electron micrographs using transmission electron microscopes (JEOL 3010, at 300 kV, and Philips CM-200, at 200 kV) were performed by placing one drop of the sample on a copper mesh coated with an amorphous carbon film, followed by evaporation of the solvent in a vacuum desiccator. Scanning electron microscopic (SEM) images of the as-synthesized materials on the copper substrates were carried out with a Hitachi S4200 field emission scanning electron microscope. X-ray diffraction (XRD) results were collected on a Rigaku D-Max IIIV diffractometer using Cu KR radiation (λ ) 1.540 56 Å) at 30 kV and 30 mA. The magnetites magnetization was carried out at room temperature using a Quantum Design MPMS-7 SQUID magnetometer. Results and Discussion In a typical experimental procedure, the nanopropellers were obtained by a reflux treatment of FeCl2 (0.096 g) solution in the presence of glycine hydrochloride (0.1 g) and urea (2.4 g) at 100 °C for 95 min. The resulting orange-red precipitates were analyzed by a powder X-ray diffraction (XRD) analysis that determined the chemical composition and crystal structure. The resolved diffraction peaks (Figure 1) reveal a pure hexagonal structure, which perfectly matches standard R-Fe2O3 (hematite). Figure 2 shows the typical size and morphology of the R-Fe2O3 nanopropellers. Figure 2a-c presents SEM images of the as-synthesized nanopropellers. The low-magnification image shown in Figure 2a indicates the exclusive formation of the homogeneous nanopropeller structure and presents the high yield and good uniformity achieved by this approach. The highmagnification image in Figure 2b shows a top view of a single nanopropeller. It reveals a clear and well-defined propeller structure with a prominent protruded central column and six blades of high ordered layered structures symmetrically extended perpendicular to the central column. The length and diameter of the central column range 400-500 and 200-300 nm, respectively, while the length of a single blade is 300-400 nm with the thickness of 5-10 nm. Figure 2c presents a side view of a single R-Fe2O3 nanopropeller; the 6-fold arrays of blades are made of stacks of platelets with a well-controlled orientation and which intersect each other at an angle of ca. 60°, corresponding to a 6-fold symmetry. A close examination of the nanopropeller showing the pillarlike structures grown out of the central nanorod suggests that each blade is made of the assembly of the parallel aligned pillars. In the course of growing hexagonal symmetrical arrays of the Fe2O3 nanopropeller, the iron oxide crystal gradually grew from both poles of the central nanorod into a longer rod, while

Figure 2. (a) Low-magnification SEM images of the nanopropellers. (b) High-magnification SEM image of a single nanopropeller. (c) Side view of a single nanopropeller.

the shorter iron oxide pillars also grew along six preferred crystal orientations out of the surface of the central nanorod. Considering a time formation sequence for six arrays of propeller-typed blades, the equatorial region of the central nanorod has the longest time for the deposition of the iron oxide pillars while both poles are the last sites formed, which results in the UFOshaped nanopropellers. In other word, the newly grown nanoblades on the side walls near both poles are shorter than those blades near the center of the central rod where the blades have grown for a longer period of time. These results from repeated experiments show excellent reproducibility. The resulting structures are highly stable, and no morphological and compositional changes are detectable even after 2 months of storage in air. To provide further detailed structural information about the nanopropellers, high-resolution TEM (HRTEM) investigations were performed. The TEM image of a single R-Fe2O3 nanopropeller is illustrated in Figure 3a. Figure 3b displays the selected-area electron diffraction (SAED) pattern as corresponding to the single R-Fe2O3 nanopropeller of Figure 3a and clearly indicates a single-crystalline structure. Figure 3c presents the corresponding SAED of the central column and has an identical pattern with hexagonal diffraction spots. The diffraction pattern shows that the central column of the nanopropeller is oriented along [0001]. The six side surfaces of the column are {1h100}

Iron Oxide Nanopropellers

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Figure 3. (a) TEM image of a single nanopropeller. (b) Selected electron diffraction pattern recorded from the corresponding nanopropeller. (c) SAED pattern taken from the central column. (The inset shows the directions of the six side blades.) (d) High-resolution TEM image recorded from the enclosed circle shown in (a).

faces, and the six blade arrays are grown out of the column’s six side surfaces in a perpendicular direction. Thus, the six arrays of the nanopropeller structure are a result of growth along ([1h100], ([1h010], and ([01h10]. The lattice structure shown in Figure 3d was taken from the tip of one blade corresponding to the enclosed circle in Figure 3a. The other five blades were also determined, giving the same lattice fringes for the nanopropeller. The lattice spacing of 2.51 Å between adjacent lattice planes in the image corresponds to the distance between two {1h100} crystal planes. To obtain a better understanding of the formation and shape evolution of the R-Fe2O3 nanopropellers, the products at various reaction stages were collected for SEM investigations. Figure 4a-c shows the representative SEM images of the samples prepared at 100 °C for 40, 66, and 75 min. respectively. In the course of collection of samples at different reaction stages, the reaction time may have varied time to time. However, the reaction time has distributed in a range of (5 min for the temperatures specified at different stages shown in Figure 4. At a shorter reaction time (40 min) as shown in Figure 4a, the aggregates contained iron oxide nanorods and many of them aligned side-by-side in the form of a bundle (inset of Figure 4a). By an increase of the reaction time from 40 to 66 min, the aggregated rodlike structures appeared in the molten state and exhibited as a dissolution morphology (Figure 4b). Following this state, the propeller-typed nanostructures began to form after 75 min, as seen in Figure 4c. If the reaction time was further increased to 95 min, the nanocrystals evolved with a well-

Figure 4. SEM images of the hematite nanopropellers as a function of reaction time: (a) 40 min (inset: TEM view); (b) 66 min; (c) 75 min.

defined propeller-shaped morphology showing single crystallinity, as seen in Figure 2. These time-dependent morphology results suggest that the formation process of the nanopropellers occurs via dissolution of iron oxide nanorods, and subsequent recrystallization leads to well-defined R-Fe2O3 nanopropellers. The dissolution is possibly due to a high proportion of structural defects being produced in the iron oxide nanorods (Figure 4a), which display low crystallinity as observed by XRD. Once the recrystallization begins, the intrinsic hexagonal crystal nature of R-Fe2O3 is proposed to determine the final symmetry features, where the central rod of the nanopropellers is grown along [0001], followed by epitaxial growth of nanoblades perpendicular to the central rod along the six radial directions. Prolonging the reaction did not result in either more nanopropeller products or an increase in the propeller size, where the supply of the iron oxide seeds (particles) was exhausted resulting in crystal growth ceasing. In this approach, FeCl2 and urea, which provides hydroxide ions during the hydrolysis of iron salts, accompanied with the addition of glycine hydrochloride are used to prepare R-Fe2O3. Glycine is the simplest amino acid and displays both acidic and basic functionalities, treated as zwitterions, having two pKa values of 2.2 and 9.2. Herein, we have found the resulting iron

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Figure 5. SEM and TEM images showing iron oxide morphology after reaction time of 95 min at various dosages of glycine hydrochloride: (a) 0 g; (b) 0.05 g; (c) 0.2 g. (d) Image from using 0.2 g of glycine hydrochloride for reaction time up to 150 min.

oxides highly influenced by the amount of glycine hydrochloride added. In the absence of glycine hydrochloride, only R-Fe2O3 spherical nanoparticles were obtained instead of propeller structures after a reaction time of 95 min (Figure 5a). If glycine hydrochloride was reduced from 0.1 to 0.05 g, spherical-like shapes with only a single layer of six blades were observed (Figure 5b). On the contrary, when glycine hydrochloride was increased up to 0.2 g, a mixture of nanorods and irregular particles with significant aggregation were generated after 95 min (Figure 5c). If the reaction time was prolonged to 150 min, the resulting products were found to have epitaxial growth but without well-defined shapes such as hexagonal symmetry (Figure 5d).

the formation of R-Fe2O3 can be described as the following reactions:

Furthermore, we monitored pH changes at various dosages of glycine hydrochloride. Without the addition of glycine hydrochloride, the solution had pH 2.7 to begin with and the value increased to 8.3 after a reaction time of 95 min. If different amounts of glycine hydrochloride were introduced to prepare iron oxide, the pH changes were 2.6 f 7.9, 2 f 5.8, and 1.8 f 3.8 for 0.05, 0.1, and 0.2 g, respectively. As can be seen, glycine hydrochloride strongly affects the pH of the solution in the course of the reaction. Addition of more glycine hydrochloride leads to a narrower range of pH changes and results in a more acidic solution in the final products. It is well-known that the time required for ferrous ion (Fe2+) to undergo oxidation to the ferric state (Fe3+) is dependent on pH. The lower the pH, the longer the time for completion of the oxidation reaction. Meanwhile, the protons released by glycine hydrochloride also play a role to neutralize hydroxide ions provided by urea. Therefore, the 6-fold symmetry of the propeller-typed structure is formed in a kinetic control of a relative lower oxidation rate of Fe2+, where the process occurs in an acidic solution, and glycine hydrochloride has acted as a pH modulator. Overall,

Although the acidic environment does not favor the oxidation of Fe2+, as shown in eq 4, the reaction temperature also influences the oxidation rate. In this approach, the synthesis takes place at 100 °C, which should be considered as well. The detailed kinetics involving in the oxidation rate of Fe2+ and the related chemistry aspects need to be further studied. Finally, the presence of glycine hydrochloride is also important in the intermediate state of nanorod formation, as seen in Figure 4a. The formation of the nanorods can be attributed to the glycine acting as a capping agent and initiating hematite growth along [0001]. Previous investigation of the synthesis of hematite has shown that selective adsorption of phosphate ions on the surfaces of hematite caused tubular structure growth along [0001] direction.16 Regarding the hematite structure, adsorption is considered to involve only the singly coordinated surface hydroxyl groups, and both the doubly and triply coordinated hydroxyl groups are comparatively unreactive.18 Since the majority of the double surface hydroxyls are on the [0001] face,19 the less adsorption affinity on (0001) means that phosphate is preferentially adsorbed on the other planes and favors the direction of morphological elongation along the crystallographic c axis. Therefore, we have determined the ζ

(NH2)2CO + H2O f 2NH3 + CO2

(1)

NH3 + H2O f NH4+ + OH-

(2)

H+ + OH- f H2O

(3)

2Fe2+ + 1/2O2 + 2H+ f 2Fe3+ + H2O

(4)

Fe3+ + 3OH- f Fe(OH)3 f R-Fe2O3

(5)

Iron Oxide Nanopropellers potential of the rodlike structures (Figure 4a) to be about +30 mV. ζ-potential was carried out with a ζ potential analyzer (Zetasizer 3000HS-Advanced). Samples were collected by centrifugation and washed several times before being redispersed in water for taking measurements. As mentioned early, glycine has two pKa values (2.2 and 9.2). +NH3CH2COO- should be the dominant form in the course of the reaction and has a positive charge outward adsorbing on the side walls of the nanorods. Meanwhile, the ζ potential measurements show the surface charge of the final nanopropeller structures (Figure 2) is near neutral, which suggests the absence of glycine adsorption on the surface of the final products. Maghemite (γ-Fe2O3), especially in its nanometer size range, is of scientific interest and technological importance as a class of magnetic material. Through a reduction and reoxidation process, our preliminary results have shown that maghemite propeller-shaped structures can be obtained from the precursors, hematite nanopropellers (Supporting Information). Maghemite (γ-Fe2O3) nanopropellers were obtained by the following procedures. The dried hematite powders were annealed, and the temperature was slowly increased to 360 °C over 6 h under an H2/Ar (5%/95%) mixture condition and kept at this temperature for an additional 3 h. Subsequently the gas flow was changed to 100% O2 with the furnace temperature decreased to 200 °C over 4 h and kept at this temperature for 4 h. However, the XRD patterns from the resulting nanopropellers indicate that the aforementioned process did not fully transform R-Fe2O3 to γ-Fe2O3, but a mixture of both compositions was obtained. We have also conducted the SAED measurements from a single nanopropeller and characterized both R-Fe2O3 and γ-Fe2O3 compositions containing one nanopropeller (Supporting Information). Although the transformation is not perfect for R-Fe2O3 f γ-Fe2O3, the magnetic nanopropellers still could be measured by a superconducting quantum interference device (SQUID) magnetometer. The hysteresis loop of the γ-Fe2O3 nanopropellers exhibits a ferromagnetic behavior with saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) values of ca. 50 emu/g, 8.8 emu/g, and 150.3 Oe, respectively. This phenomenon encourages further work to integrate the magnetic nanopropellers in the nanomotor, nanoelectric generator, and sensor devices. Conclusion Single-crystalline hematite nanopropellers were fabricated for the first time by a low-temperature solution-based method. The experimental conditions, including modulation of the glycine hydrochloride and the reaction time, were evaluated for the growth of hematite nanopropellers. The formation process of the propeller-structured hematite was found to yield rodlike

J. Phys. Chem. B, Vol. 110, No. 29, 2006 14091 precursors, followed by dissolution and recrystallization mechanisms leading to the single crystallinity of 6-fold symmetrical feature. The presence of glycine hydrochloride in this synthesis is crucial in determining the oxidation of Fe2+ and modulates pH in the reaction. An acidic environment is suited to prepare R-Fe2O3 exhibiting hexagonal symmetry. In addition, we have utilized a reduction and reoxidation process to convert hematite into magnetic maghemite nanopropellers. However, detailed studies are required and underway to completely transform R-Fe2O3 to γ-Fe2O3. Overall, these results not only provide a novel approach for potentially large-scale synthesis of the functional inorganic nanopropellers but also promise a rich, broad spectrum of advanced applications in nanorobots and functional NEMS and MEMS devices due to their endogenous magnetic property. Acknowledgment. We thank the National Science Council of Taiwan for financially supporting this work. Supporting Information Available: XRD, TEM, and SQUID results of γ-Fe2O3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Monge, M.; Kahn, M. L.; Maisonnat, A.; Chaudret, B. Angew. Chem., Int. Ed. 2003, 42, 5321. (2) Jun, Y. W.; Casula, M. F.; Sim, J. H.; Kim; S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. (3) Shin, H. C.; Dong, J.; Liu, M. AdV. Mater. 2004, 16, 237. (4) Cornell, R. M.; Schwertmann, U. The iron oxide; VCH: Weinheim, Germany, 1996; p 464. (5) Wang, J.; Chen, Q.; Zeng, C.; Hou, B. AdV. Mater. 2004, 16, 137. (6) Hyeon, T. Chem. Commun. 2003, 927. (7) Sun, Z. Y.; Yuan, H. Q.; Liu, Z. M.; Han, B. X.; Zhang, X. R. AdV. Mater. 2005, 17, 2993. (8) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. AdV. Mater. 2005, 17, 582. (9) Brown, A. S. S.; Hargreaves, J. S. J.; Rijniersce, B. Catal. Lett. 1998, 53, 7. (10) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883. (11) Gao, X. P.; Zheng, Z. F.; Zhu, H. Y.; Pan, G. L.; Bao, J. L.; Wu, F.; Song, D. Y. Chem. Commun. 2004, 1428. (12) Wang, H. Z.; Zhang, X. T.; Liu, B.; Zhao, H. L.; Li, Y. C.; Huang, Y. B.; Du, Z. L. Chem. Lett. 2005, 34, 184. (13) Wen, X. G.; Wang, S. H.; Dong, Y.; Wang, Z. L.; Yang, S. H. J. Phys. Chem. B 2005, 109, 215. (14) Xiong, Y. J.; Li, Z. Q.; Li, X. X.; Hu, B.; Xie, Y. Inorg. Chem. 2004, 43, 6540. (15) Vayssieres, L. N.; Sathe, C.; Butorin, S. M.; Shuh, D. K.; Nordgren, J.; Guo, J. H. AdV. Mater. 2005, 17, 2320. (16) Jia, C. J.; Sun, L. D.; Yan, Z. G.; You, L. P.; Luo, F. L.; Han, X. D.; Pang, Y. C.; Zhang, Z.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 4328. (17) Xiong, Y.; Li, Z.; Li, X.; Hu, B.; Xie, Y. Chem.sEur. J. 2003, 9, 4991. (18) Barrnj, V.; Torrent, J. J. Colloid Interface Sci. 1996, 177, 407. (19) Huang, X. J. Colloid Interface Sci. 2004, 271, 296.