Science in China Series B: Chemistry © 2009 SCIENCE IN CHINA PRESS Springer Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 www.scichina.com chem.scichina.com www.springerlink.com Thermal oxide synthesis and characterization of Fe 3 O 4 nanorods and Fe 2 O 3 nanowires JIAO Hua 1,2† & YANG HeQing 1 1 Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, China; 2 Department of Chemistry and Chemical Engineering, Weinan Teacher’s University, Weinan 714000, China Fe 3 O 4 nanorods and Fe 2 O 3 nanowires have been synthesized through a simple thermal oxide reaction of Fe with C 2 H 2 O 4 solution at 200－600℃ for 1 h in the air. The morphology and structure of Fe 3 O 4 nanorods and Fe 2 O 3 nanowires were detected with powder X-ray diffraction, scanning electron mi- croscopy and transmission electron microscopy. The influence of temperature on the morphology de- velopment was experimentally investigated. The results show that the polycrystals Fe 3 O 4 nanorods with cubic structure and the average diameter of 0.5－0.8 µm grow after reaction at 200－500℃ for 1 h in the air. When the temperature was 600℃, the samples completely became Fe 2 O 3 nanowires with hexagonal structure. It was found that C 2 H 2 O 4 molecules had a significant effect on the formation of Fe 3 O 4 nano- rods. A possible mechanism was also proposed to account for the growth of these Fe 3 O 4 nanorods. thermal oxide process, nanorods, nanowires, C 2 H 2 O 4 , iron sheet 1 Introduction Fe 3 O 4 is an important magnetite material having cubic inverse spinel type structure, which has been widely used as magnetic fluid and magnetic recording materials, due to its unique electrical and magnetic properties [1,2] . Nanoscale Fe 3 O 4 has been applied in magnetic ink [3] , electronics and bio-sensitive materials [4,5] , high density magnetic recording media and biomedical fields [6 － 9] , because of its good compatibility with organism and its electrical and magnetic characteristics of its size and morphology. Therefore, the preparation of Fe 3 O 4 nanos- tructures and its properties research are extremely active in recent years. At present, different kinds of Fe 3 O 4 nanostructures have been successfully synthesized via various physical and chemical methods. For example, the monodisperse Fe 3 O 4 nanoparticles were prepared by solvothermal and high temperature organic liquid reflux method [10 － 13] , on the basis of which three-dimensional superlattice has been assembled [14] . Recently, Yu et al. prepared the structure of octahedron by reflux method [15] . The Fe 3 O 4 nanorods, nanowires, branch-like nanowires, nanochains, octahedral structure, nanoflakes, peanut-like Fe 3 O 4 , nanotubes and nanopyramid arrays were prepared by hydrothermal method [16 － 23] , electroprecipitation meth- od [24] , ultrasound irradiation [25] , PLD-assisted VLS [26,27] , and microwave plasma chemical vapor deposition tech- nique (MWCVD) [28] , respectively. Recently, some researchers focused on the investiga- tion of the synthesis of Fe 3 O 4 nanorods. Wan et al. [29] obtained Fe 3 O 4 nanorods with an average diameter of 25 nm, length of 200 nm via hydrothermal reaction of FeSO 4 ·7H 2 O and FeCl 3 at 120℃ for 20 h. Kumar et al. [25] prepared the Fe 3 O 4 nanorods with acetic ferrous and the stabilizer of cyclodextrin under Ar atmosphere Received May 5 2008; accepted November 18, 2008 doi: 10.1007/s11426-009-0092-1 † Corresponding author (email: jiaohua0106@yahoo.com.cn) Supported by the Fund of Weinan Teacher’s University (Grant No. 08YKZ008), the National Natural Science Foundation of China (Grant No. 20573072) and the Doc- toral Fund of Ministry of Education of China (Grant No. 20060718010) 600 JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 of 0.15 MPa. These synthesized methods of nanorods usually require organic solvents and complex operation. In the present investigation, iron sheet as source and dripping acid solution on the surface of iron sheet were adopted to prepare Fe 3 O 4 nanorods. The Fe 3 O 4 nanorods with the rectangular cross-section and approximate 0.5－0.8 μm length were obtained by oxidizing at low temperature for 1 h. Subsequently, Fe 2 O 3 nanowires in the range of 100－300 nm were obtained at 600℃. 2 Experimental 2.1 Experimental materials All the chemical reagents in our experiments are ana- lytical grade and they are used without further purifica- tion. Iron (Fe, 99.6%) was obtained from Shaanxi Huaou Industry Ltd, and oxalic acid (C 2 H 2 O 4 ·2H 2 O, analytical grade) was purchased from Xi’an reagent factory. 2.2 Syntheses of Fe 3 O 4 nanorods In a typical experiment, a sheet of iron with size of 1×1 cm was polished by the sand paper and dealt with alco- hol in ultrasonic for 15 min. Then, it was placed in quartz boat and a drop of oxalic acid (0.75 mol·L −1 ) so- lution was taken onto the iron surface. After that, the quartz boat was placed in the oven and maintained at 200, 300, 400, 500 and 600℃ for 1 h with heating rate of 10℃·min −1 before being naturally cooled to room temperature. There was a red and black thin film on the surface layer of the iron sheet. 2.3 Characterization of products X-ray powder diffraction (XRD) patterns of the products were obtained on a Japan Rigaku D/Max-ⅢC diffrac- tometer at a voltage of 60 kV and a current of 80 mA with Cu Kα radiation (λ=1.5406 Å), employing a scan- ning rate of 8° min −1 in the 2θ ranging from 10° to 70°. Scanning electron microscopy (SEM) images were ex- plored on a Holand model FEI Quanta 200 microscope. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-3010 transmission electron mi- croscope at an accelerating voltage of 200 kV. 3 Results and discussion 3.1 SEM analysis Figure 1(a)－(f) showed the SEM images of samples synthesized by reactions of C 2 H 2 O 4 with Fe at 300℃ for 1 h and Figure 2(a)－(d) showed the SEM images of the samples synthesized at 200, 400, 500 and 600℃ for 1 h, respectively. It can be seen clearly that samples were nanorods with rectangular cross-section and the size between 0.5－1.0 μm at 200℃, as seen in Figure 2(a). When the reaction temperature was increased to 300℃, the different magnification of the front SEM im- ages were shown in Figure 1(a), (c)－(f) and the side SEM image was shown in Figure 1(b). As seen in Figure 1(c) and (e), the shape of nanorods samples were the appearance of rectangular cross-section and with the length range of 0.5－0.8 μm. A number of rods were split along the same axis. The results showed that the smaller rods and lines were split by the relatively coarse rods, as seen in Figure 1(d). Figure 1(f) was the cross-section image of single nanorod with the length of 0.8 μm and width of 0.6 μm under the high multiple. When the reaction temperature was increased to 400℃, the samples were a small amount of nanolines Figure 1 SEM images of samples synthesized by reactions of C 2 H 2 O 4 with Fe at 300℃ for 1 h. JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 601 expect for the nanorods of rectangular cross-section, as shown in Figure 2(b). When the temperature was 500℃, the morphology of the sample was changed continuously from nanorods with the size range of 0.3－0.5 µm to nanowires with the size range of 100－300 nm in Figure 2(c). When the reaction temperature was increased to 600℃, the morphology of the sample was nanowires with the size range of 100－300 nm, as shown in Figure 2(d). From the analysis of the reaction kinetic, a part of lower energy molecules became activated as the tem- perature increased. Later, the increasing chance of the effective collisions made the reaction rate (ν) increase. As the temperature was increased, the decomposition rate of oxalic acid (ν d ) increased with the gas-liquid in- terface of the oxalic acid solution contacting with air. At the same time, the reaction rate (ν r ) was also increased with the liquid-solid interface of the oxalic acid solution contacting with Fe. When the temperature was up to 600℃, ν d >ν r , the sample morphology was mainly de- cided by the reaction of water vapor and Fe [30] . Figure 2 SEM images of samples synthesized by reactions of C 2 H 2 O 4 with Fe at different temperatures for 1 h. (a) 200℃; (b) 400℃; (c) 500℃; (d) 600℃. 3.2 XRD analysis Figure 3(a)－(e) showed the XRD patterns of the sam- ples prepared from 200－600℃ for 1 h. The samples were obtained at 200℃ with the two diffraction peaks corresponding to the cubic structure of Fe (110) (200) crystal plane (JCPDS No. 06-0696), as shown in Figure 3(a). It indicated that the crystal sample was not com- plete at 200℃. When the temperature was increased to 300－500℃, the diffraction peak corresponding to the cubic phase of Fe 3 O 4 (111), (220), (311), (222), (331), (511) crystal plane (JCPDS No. 65-3107) and hexagonal phase of Fe 2 O 3 (012), (104), (311), (113), (024), (116), (214) crystal plane (JCPDS No. 33-0664) became wide and weak, which indicated that the product was multi- crystalline structure. Further, the Fe 3 O 4 diffraction peaks disappeared gradually. When the temperature was up to 600℃, the Fe 3 O 4 diffraction peaks were not obvious, which indicated that Fe 2 O 3 could be obtained in higher temperatures. Figure 3 XRD images of samples synthesized by reactions of C 2 H 2 O 4 with Fe at different temperatures for 1 h. (a) 200℃; (b) 300℃; (c) 400℃; (d) 500℃; (e) 600℃. 3.3 TEM analysis In order to determine the detailed crystalline structure, TEM measurements were employed to investigate the samples prepared at 300℃ for 1 h. A typical TEM image of the single Fe 3 O 4 nanorod was shown in Figure 4(a). The size of the nanorod with the length of 2.4 µm and the width of 0.5 µm was in good agreement with the above SEM image shown in Figure 1. Figure 4(b) is the top TEM image of Figure 4(a). It can be seen from the Figure 4(b) that the samples of nanorods were fibri- form-like structure self-assembly. A selected area elec- tron diffraction (SAED) pattern was presented in Figure 4(c) according to the rectangular frame of Figure 4(b), 602 JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 Figure 4 TEM images of samples synthesized by reactions of C 2 H 2 O 4 with Fe at 300℃ for 1 h. (a)－(c)TEM; (d) SAED. indicating the cubic phase of Fe 3 O 4 (311), (400), (220), (511) crystal plane diffraction. Meanwhile, the energy dispersive spectrometer (EDS) was used to analysis the chemical composition of the sample, and the results can be seen in Figure 4(d). It can be clearly identified that the nanorods were composed of Fe and O elements, and the ratio of the number of atoms Fe and O was about 3︰4. The TEM, SAED and EDS analyses revealed that Fe 3 O 4 nanorods were of polycrystalline cubic phase structure. 3.4 The influence of oxalic acid on the morphology of samples In order to study the role of oxalic acid, the morphology of the products from water reacting with Fe sheet at 300℃ and 600℃ for 1 h in the air were investigated, respectively, as shown in Figure 5. From Figure 5(a), it can be observed that, when the reaction temperature was 300℃, the surface of the Fe sheet had not shaped regu- larity morphology, only sporadic small particles. When the reaction temperature was up to 600℃, the surface of Fe sheet was nanowires with the size of 100－300 nm in Figure 5(b). It was obvious that the addition of oxalic acid was benefit for the formation of nanorods in the temperature range of 200－500℃. Figure 6 showed the XRD patterns of the samples above. It can be seen from the patterns, the two diffrac- tion peaks were corresponding to the cubic phase Fe (110) (200) crystal plane (JCPDS No. 06-0696) under Figure 5 SEM images of samples synthesized by reactions of H 2 O with Fe at 300℃ (a) and 600℃(b) for 1 h in air. Figure 6 XRD images of samples synthesized by reactions of H 2 O with Fe at 300℃ (a) and 600 ℃(b) for 1 h in air. 300℃. When the temperature was up to 600℃, the sam- ples of the diffraction peaks corresponding to the hex- agonal phase of the Fe 2 O 3 (JCPDS No. 33-0664) indi- cated that dropping water on the iron surface did not react at 300℃, and the pure Fe 2 O 3 products were ob- tained at 600℃. The results of SEM and XRD indicated that Fe 3 O 4 nanorods on the surface of Fe sheet were complexation reaction of oxalic acid and iron at a rela- tively low temperature. In order to determine the detailed crystalline structure, TEM measurements were employed to investigate the samples prepared at 600℃ for 1 h in air, on the surface of which water dripped, as shown in Figure 7. A typical TEM image of single Fe 2 O 3 nanowire was shown in Figure 7(a). The size of the nanowire was in good agreement with the above SEM image shown in Figure 5(b) with the length of about 100 nm. Figure 7(b) is the high resolution TEM image of Figure 7(a). It can be seen from the image that the crystal plane spacing was 0.37 nm, corresponding to the distance of hexagonal JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 603 Figure 7 TEM images of samples synthesized by reactions of H 2 O with Fe at 600℃ for 1 h. (a),(d)TEM; (b) HRTEM; (c) SAED. phase of Fe 2 O 3 (012) crystal plane. A SAED pattern was presented in Figure 7(c) according to Figure 7(a), indi- cating the hexagonal phase of Fe 2 O 3 [0001] zone axis diffraction. The growth of nanowires was from the rough to the fine in Figure 7(d). The TEM and HRTEM analyses revealed that Fe 2 O 3 nanowires were of single crystalline hexagonal phase structure. 3.5 Mechanism Based on the above results, the reaction process was: the oxalic acid solutions contacted with air and formed an interface of the gas-liquid phase after oxalic acid drip- ping on iron surface, which occurred as reaction of ox- alic acid decomposition (ν d ). Meanwhile, the reaction of the oxalic acid solution and Fe happened on the liq- uid-solid interface (ν r ). When the Fe sheet with a drop of acid was placed in the oven, before reaching the de- composition temperature of 190℃, ν d <ν r , the oxalic acid occurred complexation reaction with iron and obtained ferrous oxalate, as seen eq. (1); latter, the unstable fer- rous oxalate decomposition became FeO (eq. (2)); fer- rous oxide was oxidized to Fe 3 O 4 by oxygen in the air (eq. (3)); the ν d >ν r was increasing with the temperature increasing at the same time. When the temperature was up to 600℃, ν d >ν r , Fe 2 O 3 nanowires were the result of Fe reacted with water vapor (eq. (4)). The chemical reactions can be expressed as: 2 224 24 2 2Fe O 2H C O 2FeC O 2H O + +→+ (1) 24 2 FeC O FeO CO + CO→+ (2) 234 6FeO + O 2Fe O→ (3) 22 232 4Fe + O + 2H O Fe O H O→⋅ (4) Firstly, at a relatively low temperature, Fe 3 O 4 nano- rods were obtained in situ with oxalic acid solution dripped on. The ferrous oxalate was obtained via heat treatment in the air (Figure 8(b)). FeO was obtained from the unstable ferrous oxalate decomposition. Whereafter, FeO was oxidized to Fe 3 O 4 grains by oxy- gen in the air (Figure 8(c)). The nanorods grew from saturation Fe 3 O 4 grains as the reaction going on (Figure 8(d)). When the reaction temperature was up to 600℃, the product was only the nanowires due to the high reac- tion temperature. Actually, the reaction happened be- tween the water vapor and iron, and the growth process was depicted in Figure 9. Fe 2 O 3 ·nH 2 O grains were gained in the air under high temperature (Figure 9(a) and (b)). Fe 2 O 3 ·nH 2 O grain began to decompose and became Fe 2 O 3 nanocrystals as the temperature increased (Figure 9(c)). The nanowires grew from saturation Fe 2 O 3 grains as the reaction going on (Figure 9(d)). Figure 8 Schematic diagram of the growth process of Fe 3 O 4 nanorods. Figure 9 Schematic diagram of the growth process of Fe 2 O 3 nanowires. 4 Conclusions In summary, we successfully prepared Fe 3 O 4 nanorods and Fe 2 O 3 nanowires via a simple thermal oxide process. We investigated the influence of reaction temperature on the samples morphology. A possible mechanism was 604 JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 also proposed to account for the growth of these samples. 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Chem Phys Lett, 2001, 350: 491 － 494 . www.springerlink.com Thermal oxide synthesis and characterization of Fe 3 O 4 nanorods and Fe 2 O 3 nanowires JIAO Hua 1,2† & YANG HeQing 1 1 Key Laboratory of. of the synthesis of Fe 3 O 4 nanorods. Wan et al. [29] obtained Fe 3 O 4 nanorods with an average diameter of 25 nm, length of 200 nm via hydrothermal