Introduction

In recent years, one-dimensional (1D) nanostructured materials have attracted great interest due to their superior performance in applications such as electronics,optoelectronics, energy storage and conversion, sensing and drug delivery [1]. Up to now, lots of approaches have been reported to synthesize 1D nanostructures, such as laser ablation, chemical vapor deposition (CVD), thermal evaporation, hydrothermal/solvethermal process, chemical reaction, template/precursor-directed methods, and so on [2–9]. More recently, current existing 1D nanostructures have been considered as useful precursors to generate other 1D nanostructures that might be difficult to directly synthesize as uniform samples [10–15]. For example, Wang and Qi reported the synthesis of Ag2S, Ag2Se, and Ag nanofibers by using Ag2C2O4 as a general precursor, employing both anioic exchange and redox reactions [11]. Fe3O4 nanotubes have been prepared by epitaxial coating of Fe3O4 layer on MgO nanowires and subsequently wet-chemical etching [12]. Single-crystalline ZnAl2O4 spinel nanotubes could be fabricated through a spinel-forming interfacial solid-state reaction of core–shell ZnO–Al2O3 nanowires as precursors [13]. More recently, we have used carbon nanotubes as templates to synthesize metal oxide and composite nanotubes via layer-by-layer technique [14, 15].

As typical anisotropic magnetic nanomaterials, 1D iron oxide nanostructures are of great interest because of their interesting magnetic properties caused by shape anisotropy [16–18]. However, it is difficult for magnetic iron oxides to form 1D nanostructures due to their spinel crystal [19]. Recently, great efforts have been made to synthesize 1D iron oxides nanostructures by means such as oxidation of iron plates [20, 21], utilization of various templates [22, 23] and surfactant-assisted hydrothermal and solvothermal [24, 25]. However, it remains challenge to selective synthesize 1D iron oxides nanostructures with controllable morphology.

Herein, as precursors, FeC2O4·2H2O nanowires were synthesized by a simple microemulsion-based method. Ultralong and uniform α-Fe2O3 and Fe3O4 nanowires could be selectively fabricated by annealing of FeC2O4·2H2O nanowires under different temperatures and atmospheres. Co3O4 nanowires were also prepared to confirm the versatility of the method for metal oxide nanowires.

Experimental

All the chemicals are analytical grade without further purification. The experiment details were as follows: 2.5 g CTAB (cetyltrimethylammonium bromide) was dissolved into a mixture of 75 ml of cyclohexane and 2.5 ml of n-pentanol. After 20 min of stirring, 3.75 ml of 0.1 M H2C2O4 aqueous solution was introduced into the above resulting solution, and the mixture was stirred for an additional 25 min. Finally, 1.25 ml of 0.1 M FeSO4 was added to the above microemulsion and stirred for 24 h at room temperature. After the reaction was completed, the resulting products were centrifugalized, washed with deionized water and ethanol to remove the ions possibly remaining in the final product, and finally dried at 80°C in air. The obtained powders were annealed under O2 (550/700°C) and Ar/H2 (400°C), respectively.

The obtained samples were characterized by X-ray powder diffraction (XRD) using a Rigaku D/max-ga x-ray diffractometer with graphite monochromatized Cu Kα radiation (γ = 1.54178 Å). The morphology and structure of the samples were examined by field emission scanning electron microscopy (FESEM, Hitachi S-4800), transmission electron microscopy (TEM, JEM-200 CX, 160 kV) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010) with an energy-dispersive X-ray spectrometer (EDX). The infrared (IR) spectra were measured with a Nicolet Nexus FTIR 670 spectrophotometer.

Result and Discussion

Figure 1 shows the X-ray diffraction (XRD) patterns of the precursors and the subsequent products after annealing processes. It can be seen that all the peaks of the precursors can be assigned to pure ferrous oxalate dehydrate (FeC2O4·2H2O) (JCPDS: 72-1305). No diffraction peaks from impurities were found in the sample. When the precursors were annealed in air at 550/700°C, all the diffraction peaks of the products can be readily indexed to the pure rhombohedral phase of α-Fe2O3 (JCPDS: 33-0664). When the precursors were annealed in Ar/H2 (5% H2) at 400°C, all the diffraction peaks of the products can be readily indexed to a pure cubic phase of Fe3O4 (JCPDS: 65-3107). The results of XRD demonstrate that FeC2O4·2H2O can be transformed to α-Fe2O3 and Fe3O4 during the annealing processes under different atmosphere.

Figure 1
figure 1

XRD patterns of as-synthesized products

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The FeC2O4·2H2O nanowires used as the precursors for the synthesis of α-Fe2O3 and Fe3O4 nanowires were prepared by a simple microemulsion-based method. Figure 2 shows the SEM and the TEM images of the as-synthesized FeC2O4·2H2O nanostructures. As observed, the obtained FeC2O4·2H2O nanostructure exhibits the morphology of the nanowires with the diameter of about 100 nm and the length of tens of micrometers. Moreover, the surface of the FeC2O4·2H2O nanowires seems smooth, and no defects can be observed. When the FeC2O4·2H2O nanowires were annealed in air at 550°C, porous α-Fe2O3 nanowires were fabricated. Figure 3 shows the SEM, TEM, and HRTEM images of the as-synthesized α-Fe2O3 nanowires. It can be seen that the as-synthesized α-Fe2O3 nanowires retain the morphology of the FeC2O4·2H2O nanowires (Fig. 3a–3d). However, many nanopores with diameters of about 10–30 nm appear because of the release of CO2 from FeC2O4·2H2O during the heat treatments, resulting in the porous nanowires (Fig. 3e). Such the porous nanowires may exhibit superior performance in Li-battery and gas sensors because of the large surface area. Figure 3f shows the HRTEM image of an α-Fe2O3 nanowire. As can be seen, two nanocrystals connect together with a grain boundary between them. There are two kinds of lattice fringes with lattice spacings of about 0.37 and 0.25 nm, corresponding to the {1012} plane and {1120} plane of α-Fe2O3, respectively, which indicates their polycrystalline nature. Figure 4 shows the SEM and the TEM images of the products when the annealing temperature was raised up to 700°C. Compared with the products following a 500°C annealing process, this sample shows a similar morphology of nanowires but the nanocrystals that composed of nanowires shows better fusion between them due to the higher annealing temperature. Moreover, quasi-singlecrystalline α-Fe2O3 nanowires can be observed (Fig. 4c, 4d). Figure 5 shows the morphology and the structure characterizations of the products when the FeC2O4·2H2O nanowires were annealed in Ar/H2 (5% H2) at 400°C. The SEM and the TEM images indicate that the as-synthesized products show the morphology of nanowires, which retains the morphology of the precursors (Fig. 5a–5d). However, the diameters of the nanocrystals and the nanopores that composed of the nanowires are about 5 and 2 nm (Fig. 5e, 5f), respectively, which are smaller than the α-Fe2O3 nanowires due to the lower annealing temperature. The lattice fringe with a lattice spacing of 0.25 nm corresponds to the {311} plane of Fe3O4 (Fig. 5f).

Figure 2
figure 2

SEM images (a), (b) and TEM images (c), (d) of FeC2O4 nanowires

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Figure 3
figure 3

Morphological and structural characterization of Fe2O3 nanowires synthesized at 550°C: a, b SEM images; c–e TEM images; f HRTEM images

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Figure 4
figure 4

SEM images (a), (b) and TEM images (c), (d) of Fe2O3 nanowires synthesized at 700°C

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Figure 5
figure 5

Morphological and structural characterization of Fe3O4 nanowires: a, b SEM images; c–e TEM images; f HRTEM images

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IR analysis was employed to further confirm the transformation from FeC2O4·2H2O nanowires to α-Fe2O3 and Fe3O4 nanowires during the thermal treatments. As can be seen from Fig. 6a, the peaks at 3,358, 1,629, 1,318, and 496 cm−1 are attributed to O–H, C=O, C–O, and Fe–O functional groups, respectively, indicating the formation of FeC2O4·2H2O [26]. In order to clarify the difference of IR spectra between Fe2O3 and Fe3O4, the magnified IR spectra of Fe2O3 and Fe3O4 were analyzed (Fig. 6b). It can be seen that there is only one peak at 574 cm−1 for Fe3O4, while α-Fe2O3 shows two or three peaks which is related with its structure and size. In addition, γ-Fe2O3 also exhibit three peaks between 500 and 700 cm−1, which is different from Fe3O4[27, 28]. The IR analysis combined with TEM images and XRD pattern can confirm the synthesis of Fe3O4 nanowires.

Figure 6
figure 6

FTIR spectra of as-synthesized products

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As a result, ultralong and uniform α-Fe2O3 and Fe3O4 nanowires were selectively synthesized by the annealing of FeC2O4·2H2O nanowires under different temperatures and atmosphere. Figure 7 shows the schematic illustration diagram for the growth mechanism. As can be seen, when two microemulsion solutions containing FeSO4 and H2C2O4 are mixed, FeC2O4 nucleation and irreversible micellar fusion may be coincident. During the nucleation, the surfactant molecules of side surfaces of the cylindrical droplets may adsorb on surface planes of the formed FeC2O4 nucleus, which may result in the one-dimensional growth of FeC2O4. Finally, α-Fe2O3 and Fe3O4 nanowires can be obtained after the calcinations of FeC2O4 nanowires under different conditions. The morphology, the structure, and the phase of as-synthesized products could be determined by the precursors and the annealing conditions.

Figure 7
figure 7

Schematic illustrations for LBL synthesis of α-Fe2O3 and Fe3O4 nanowires

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Based on the above-mentioned analysis, the mechanism for the formation of α-Fe2O3 and Fe3O4 nanowires can act as a guideline for the synthesis of the other metal oxide nanowires. Herein, Co3O4 nanowires were also synthesized through the similar procedures. Figure 8 shows the as-synthesized precursor and Co3O4 nanowires. It can be seen that porous Co3O4 nanowires with diameters of about 200 nm and lengths of several micrometers can be obtained by the calcinations of CoC2O4 nanowires, which confirms the versatility of the microemulsion-based method.

Figure 8
figure 8

a SEM image of CoC2O4 nanowires; b SEM image of Co3O4 nanowires; c TEM image of Co3O4 nanowires; d XRD of Co3O4 nanowires

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Conclusions

In summary, we have developed a microemulsion-based method in combination of the calcinations to selectively synthesize α-Fe2O3 and Fe3O4 nanowires with the diameter of about 100 nm and the length of tens of micrometers. The FeC2O4·2H2O nanowires used as the precursors were prepared by a simple microemulsion-based method. α-Fe2O3 and Fe3O4 nanowires can be synthesized by the calcinations of FeC2O4·2H2O nanowires under air and H2/Ar at different temperatures, respectively. Moreover, it is believed that the approach presented here can be extended to synthesize other 1D metal oxide nanostructures. Co3O4 nanowires have been fabricated to confirm the versatility of the method for metal oxide nanowires.