Journal of Alloys and Compounds 433 (2007) 216–220 Hydrothermal synthesis and characterization of ␣-FeOOH and ␣-Fe 2 O 3 uniform nanocrystallines Xiaohe Liu a,b , Guanzhou Qiu b , Aiguo Yan b , Zhong Wang a , Xingguo Li a,∗ a College of Chemistry & Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China b Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, People’s Republic of China Received 18 April 2006; received in revised form 4 June 2006; accepted 6 June 2006 Available online 24 July 2006 Abstract Inorganic nanoparticles with controlled size and shape are technologically important due to the strong correlation between these parameters and magnetic, electrical, and catalytic properties. Herein we demonstrated that under appropriate conditions, rodlike ␣-FeOOH (goethite) and porous fusiform ␣-Fe 2 O 3 (hematite) uniform nanocrystallines could be selectively synthesized in large quantities via a facile surfactant sodium dodecyl sulfate (SDS) assisted hydrothermal synthetic route. The morphology and structure of the final products were investigated in detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and selected area electron diffraction (SAED). The probable formation mechanism of the ␣-FeOOH and ␣-Fe 2 O 3 uniform nanoparticles was discussed on the basis of the experimental results. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; Porous; ␣-FeOOH; ␣-Fe 2 O 3 ; Nanocrystallines 1. Introduction Over the past decades, inorganic nanoparticles with con- trolled size and shape have attracted vast attention because of their size and shape-dependent properties and great potential applications. Considerable effort has been devoted to the design and controlled fabrication of inorganic materials with controlled size and shape. Various synthetic methods are continually being improved to this end. Recently a variety of novel shapes such as tubes [1], rods [2], wire [3], belts [4], prisms [5], and cubes [6] have been reported through chemical reactions of precursors at room or slightly elevated temperatures. However, with the growing interest in building advanced materials using nanoscale building blocks, it remain a challenge to find simple and mild routes to control the parameters of final products to fine-tune their properties. Iron oxides have attracted enormous attention owing to their interesting electrical [7], magnetic [8], and catalytic [9] proper- ties and wide variety of potential applications in various fields ∗ Corresponding author. Tel.: +86 10 62765930; fax: +86 10 62765930. E-mail address: xgli@chem.pku.edu.cn (X. Li). such as electro-optic materials [10], sorbents [11], pigments [12], ion exchangers [13], and magnetic resonance imaging (MRI) [14], particularly in the field of catalysis [15]. Amongst the readily available carbon monoxide oxidation catalysts, iron oxide-based materialshave beenfound tobe especiallyattractive candidates as cheap and efficient catalysts [16]. Various pro- cedures including wet chemical [17–20], electrochemical [21], thermal decomposition techniques [22], and chemical oxidation in polymer [23] have been successfully employed for the syn- thesis of iron oxides nanocrystallines. As is well known, the properties of iron oxides nanocrystallines sensitively depend on their size and shape. In order to improve the functional proper- ties such as catalytic activity, it is significant challenge to control the size and shape of iron oxides nanocrystallines. Porous iron oxides nanoparticles may provide some immediate advantages over their solid counterparts because of their relatively low den- sities and large surface areas for their applications. In recent years, Oca ˜ na and co-workers have synthesized uniform iron oxides nanoparticles via aerial oxidation and forced hydroly- sis methods [24], however, the products of such synthesis often involved complicated process or produced in low quantities. In this paper, we demonstrated that rodlike goethite (␣- FeOOH) and porous fusiform hematite (␣-Fe 2 O 3 ) iron oxide 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.029 X. Liu et al. / Journal of Alloys and Compounds 433 (2007) 216–220 217 uniform nanocrystallines could be selectively synthesized through a facile hydrothermal synthetic route in large quantities under mild conditions. When only prolonging the hydrothermal time from 12 to 24 h, the rodlike ␣-FeOOH nanocrystallines could be transfered to porous fusiform ␣-Fe 2 O 3 nanocrys- tallines. In this synthetic system, surfactant sodium dodecyl sulfate (SDS) was used as a structure-directing agent, and simple compounds of the hydrated ferrous chloride and sodium boro- hydride as reactants directly. The high yields, simple reaction apparatus and low reaction temperature give this novel method a good prospect in the future applications. 2. Experimental All chemicals in this work, such as hydrated ferrous chloride (FeCl 2 ·4H 2 O), sodium borohydride (NaBH 4 ), and sodium dodecyl sulfate (SDS) were of ana- lytical grade, and which were used without further purification. 2.1. Preparation of iron oxides nanocrystallines In a typical procedure, pure sodium dodecyl sulfate (0.001 mol) and hydrated ferrous chloride (FeCl 2 ·4H 2 O, 0.001 mol) were firstly dissolved in distilled water to form a salmon pink micellar solution under vigorous stirring at room temperature. Then, sodium borohydride solution (NaBH 4 , 0.5 mmol) was added to the salmon pink solution. With the introduction of sodium borohydride solu- tion, the color of mixed micellar solution turned immediately from salmon pink to black. Next, the mixture was transferred to a Teflon-lined stainless steel auto- clave of 50 mL capacity. Finally, the autoclave was filled with distilled water up to 75% of the total volume, sealed and maintained at 140 ◦ C for 4, 8, 12, and 24 h, respectively. After the heating treatment, the autoclave was allowed to cool down to room temperature naturally. The resulting products were filtered, washed with distilled water and absolute ethanol, and finally dried in vacuum at 50 ◦ C for 6 h. 2.2. Characterization The rodlike ␣-FeOOH (goethite) and porous fusiform ␣-Fe 2 O 3 (hematite) nanocrystallines were characterized using various techniques. X-ray powder diffraction (XRD) patterns were obtained on a Bruker D8-advance X-ray diffrac- tometer with graphite-monochromatized Cu K␣ (λ = 1.54178 ˚ A) radiation. The operation voltage and current were kept at 40 kV and 40 mA, respectively. TEM patterns were recorded on a Hitachi Model H-800 transmission electron micro- scope at an accelerating voltage of 200 kV. The samples were dispersed in absolute ethanol in an ultrasonic bath. Then the suspensions were dropped onto Cu grids coated with amorphous carbon films. Selected area electron diffraction (SAED) was further performed to identify the crystallinity. The scanning elec- tron microscopic (SEM) images were obtained using a LEO 1530 field-emission scanning electron microscope (FE-SEM), under typical working conditions of 10 kV. 3. Results and discussion Fig. 1 shows typical X-ray diffraction (XRD) patterns (2θ scan) of the goethite (␣-FeOOH) iron oxide nanocrystallines obtained at 140 ◦ C for different reaction time. All the reflections of the XRD pattern can be finely indexed to an orthorhombic phase [space group: Pbnm(62)] of ␣-FeOOH with cell param- eters a = 4.64 ˚ A, b = 10.0, c = 3.03 ˚ A (JCPDS file Card, No. 03- 0249). The XRD patterns of Fig. 1 from a to c correspond to the goethite (␣-FeOOH) iron oxide nanocrystallines obtained for 4, 8, and 12 h, respectively. The XRD pattern (Fig. 1a) of sample shows that the goethite (␣-FeOOH) iron oxide nanocrystallines Fig. 1. The evolution of the XRD patterns of the ␣-FeOOH nanocrystallines obtained at 140 ◦ C for different reaction time: (a) 4 h; (b) 8 h; (c) 12 h. ( * ␣- Fe 2 O 3 ). obtained for 4 h were poorly crystallized. With the elongation of reaction time, the XRD patterns of samples at the whole process show that the crystallinities of the samples were continuously improved. When the samples maintained for 8 h, the XRD pat- tern of the sample is shown in Fig. 1b. The main diffraction peaks of ␣-FeOOH are clear observed in the patterns. When the reaction time prolonged to 12 h (in Fig. 1c), the peaks of ␣-FeOOH become slightly weak, along with the weakening of ␣-Fe 2 O 3 peaks, which indicates the transition from ␣-FeOOH to ␣-Fe 2 O 3 . Fig. 2 shows the X-ray diffraction pattern of the sample of hematite (␣-Fe 2 O 3 ) iron oxide nanocrystallines that were obtained at 140 ◦ C for 24 h. The composition could be expressed as rhombohedral phase ␣-Fe 2 O 3 [space group: R ¯ 3c(167)] with lattice constants a = 5.0356, c = 13.7489 (JCPDS file Card, No. 33-0664), since the main diffraction peaks of rhombohedral phase ␣-Fe 2 O 3 , 012, 104, 110, 113, 024, 116, 018, 214, and 300, are clearly observed in the patterns. No impurity peaks are Fig. 2. The XRD pattern of the porous fusiform ␣-Fe 2 O 3 nanocrystallines obtained at 140 ◦ C for 24 h. 218 X. Liu et al. / Journal of Alloys and Compounds 433 (2007) 216–220 Fig. 3. (A) TEM image of the sample produced at 140 ◦ C for 4 h. (B) TEM image of the sample produced at 140 ◦ C for 8 h. The insets of (A and B) show the SAED pattern of the rodlike ␣-FeOOH nanocrystallines taken on a mass of ␣-FeOOH nanocrystallines. observed, indicating the hematite (␣-Fe 2 O 3 ) iron oxide success- fully synthesized under current experimental conditions. The size and morphology of the rodlike ␣-FeOOH (goethite) and porous fusiform ␣-Fe 2 O 3 (hematite) nanocrystallines were further examined by transmission electron microscopy (TEM). Fig. 3A shows the typical TEM photograph of ␣-FeOOH nanocrystallines obtained at 140 ◦ C for 4 h through a surfac- tant SDS assisted hydrothermal synthetic route, and its select area electron diffraction pattern indicates the sample A is poorly crystallized, being in good agreement with the XRD patterns. In contrast to the sample processed for 4 h, the sample prepared at 140 ◦ C for 8 h is mostly rodlike form, as can be observed more clearly from Fig. 3B. The inset showing a select area elec- tron diffraction pattern of the rodlike ␣-FeOOH nanocrystallines taken on a mass of ␣-FeOOH nanorods reveals the satisfactory crystallinity of the sample, which can be indexed to the pure Fig. 4. TEM image of the sample of produced at 140 ◦ C for 12 h. (B) TEM image of the sample produced at 140 ◦ C for 24 h. The inset shows the SAED pattern of the individual fusiform ␣-Fe 2 O 3 (hematite) nanocrystallines. X. Liu et al. / Journal of Alloys and Compounds 433 (2007) 216–220 219 Fig. 5. (A) Low- and (B) high-magnification SEM images of the fusiform ␣-Fe 2 O 3 nanocrystallines synthesized with the surfactant SDS process at 140 ◦ C for 24 h. orthorhombic phase of goethite ␣-FeOOH. When the reaction time prolonged to 12 h, the previous nanorods appeared agglom- erate (in Fig. 4A), which is very critical for the formation of porous fusiform ␣-Fe 2 O 3 (hematite) nanocrystallines. Fusiform ␣-Fe 2 O 3 (hematite) nanocrystallines were obtained when the reactants were treated at 140 ◦ C for 24 h, as shown in Fig. 4B. TEM observations indicates that about 100% of the products are fusiform ␣-Fe 2 O 3 nanocrystallines whose diameter ranges from about 50 to 70 nm with a length of up to ∼200 nm. There was an interesting change in the morphology of the sample of ␣-Fe 2 O 3 nanocrystallines. With careful observation, the fusiform ␣- Fe 2 O 3 nanocrystallines can be made up from many porous struc- tures. The inset is an electron diffraction pattern of an individual fusiform ␣-Fe 2 O 3 nanocrystallines, which can be indexed to rhombohedral phase of hematite ␣-Fe 2 O 3 and exhibits that each porous fusiform ␣-Fe 2 O 3 nanocrystalline was a single crystal. Fig. 5A and B shows scanning electron microscope images of a typical sample of fusiform ␣-Fe 2 O 3 nanocrystallines obtained at 140 ◦ C for 24 h and indicate the large quantity and good uniformity ␣-Fe 2 O 3 nanocrystallines were achieved using this approach. These fusiform ␣-Fe 2 O 3 nanocrystallines have a mean diameter of 60 nm and length of up to ∼200 nm, which agree well with the TEM results. Fig. 5B is a higher magnifi- cation SEM image obtained from a selected area of Fig. 5A. Herein, fusiform ␣-Fe 2 O 3 nanocrystallines with many porous structures can be clearly observed. The chemical compositions of the as-prepared fusiform ␣-Fe 2 O 3 nanocrystallines have been investigated by means of EDS. Results from EDS spectra (Fig. 6) show that the fusiform ␣-Fe 2 O 3 nanocrystallines contain Fe and O, and no contamination elements are detected. The atomic ratio of Fe and O matched their stoichiometries quite well. Associating all those results, the whole reaction to form the ␣-FeOOH and ␣-Fe 2 O 3 uniform nanoparticles can be expressed as the following equation: 4Fe 2+ + BH 4 − + 3H 2 O → 4Fe ↓+H 3 BO 3 + 7H + (1) Fe + 2H + → Fe 2+ + H 2 ↑ (2) 4Fe 2+ + O 2 + 6H 2 O → 4FeOOH ↓+8H + (3) 2FeOOH → Fe 2 O 3 ↓+H 2 O (4) Soluble ferrous chlorides will firstly dissociate in water into fer- rous ions, which then react with sodium borohydride to form ultrafine iron particles. The reduction of transition metal ions for the production of ultrafine metal particles by BH 4 − is a ubiquitous reaction. The ultrafine iron particles may be redis- solved under acidic conditions. Subsequently, rodlike ␣-FeOOH (goethite) nanocrystallines will gradually form under surfactant SDS assisted hydrothermal conditions. With the reaction time prolonged, rodlike ␣-FeOOH nanocrystallines appear agglom- erate and form fusiform nanocrystallines. Finally, the ␣-FeOOH Fig. 6. The EDS spectra of the as-prepared porous fusiform ␣-Fe 2 O 3 nanocrys- tallines. 220 X. Liu et al. / Journal of Alloys and Compounds 433 (2007) 216–220 nanocrystallines maybe dehydrate and form porous fusiform ␣- Fe 2 O 3 nanocrystallines. 4. Conclusion In summary, we have successfully synthesized the rodlike goethite (␣-FeOOH) and porous fusiform hematite (␣-Fe 2 O 3 ) iron oxide uniform nanocrystallines via a facile surfactant sodium dodecyl sulfate assisted hydrothermal synthetic route at mild conditions. The influence of reaction time on size and shape was investigated. Results show the hydrothermal time was dominant. When maintained at 140 ◦ C for 4 h, the sample of goethite (␣-FeOOH) iron oxide is poorly crystallized, however, the sample of ␣-FeOOH prepared at 140 ◦ C for 8 h is mostly nanorodlike form with satisfactory crystallinity. With increas- ing the hydrothermal time, goethite (␣-FeOOH) iron oxide nanorods appeared agglomerate, and a part of the ␣-FeOOH phase transformed to ␣-Fe 2 O 3 phase. When the reaction time prolonged to 24 h, we successfully synthesized porous fusiform hematite (␣-Fe 2 O 3 ) iron oxide nanocrystallines with good uni- formity. The synthetic strategy presented here may have a good prospect in the future application and provide an effective route to synthesize other metal oxide nanocrystallines. Owing to the excellent physical properties of the iron oxides, it is expected the rodlike ␣-FeOOH (goethite) and porous fusiform ␣-Fe 2 O 3 (hematite) uniform nanocrystallines exhibit some important applications in, e.g., sensors, magnetic media, catalytic fields, etc. Acknowledgements Financial support of this work by National Natural Science Foundation of China (Grant no. 50504017) and Hunan Provin- cial Natural Science Foundation of China (Grant no. 05JJ30104) is gratefully acknowledged. References [1] O.Y. Min, J L. Huang, C.L. Cheung, C.M. Lieber, Science 292 (2001) 702. [2] X.H. Liu, Mater. Chem. Phys. 91 (2005) 212. [3] H. Yu, J. Li, R.A. Loomis, L W. Wang, W.E. Buhro, Nat. Mater. 2 (2003) 517. [4] X.Y. Kong, Y. Ding, R.S. Yang, Z.L. Wang, Science 303 (2004) 1348. [5] R.C. Jin, Y.C. Cao, E. Hao, G.S. M ´ etraux, G.C. Schatz, C.A. Mirkin, Nature 425 (2003) 487. [6] F. Dumestre, B. Chaudret, C. Amiens, P. Renaud, P. Fejes, Science 303 (2004) 821. [7] M.F. Haquea, S. Arajs, Int. 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