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J. Phys. Chem. C 2007, 111, 5034-5038
Growth and Properties of Single-Crystalline γ-Fe2O3 Nanowires Qin Han,† Zhenghui Liu,† Yingying Xu,† Ziyu Chen,‡ Tianmin Wang,‡ and Han Zhang*,† Materials Physics Laboratory, State Key Laboratory for Mesoscope Physics, Department of Physics, Peking UniVersity, Beijing 100871, China, and Department of Physics, Beijing UniVersity of Astronautics & Aeronautics, Beijing, 100083, China ReceiVed: NoVember 26, 2006; In Final Form: January 27, 2007
By carefully controlling the reacting conditions, including atmosphere, temperature, and time, we directly acquire the nanowires of γ-Fe2O3 from the nanowires of R-Fe2O3 in a reduced atmosphere. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectrum, and transmission electronic microscope analyses demonstrate that the nanowires are single-crystalline γ-Fe2O3. The nanowires have a diameter ranging from 50 to 90 nm and their typical lengths are in the range of 10∼20 µm. The optical property of the nanowires is observed by photoluminescence spectrum. The magnetic behavior of it is investigated by a magnetic property measurement system. The blocking temperature is found to be about 200 K. In addition, the mechanism of the transformation from R-Fe2O3 nanowires to γ-Fe2O3 nanowires is preliminarily studied by ab initio approach. It is found that the role of H2 is to change the Fe-O bonds in R-Fe2O3 nanowires, and then R-Fe2O3 is transformed into γ-Fe2O3.
Introduction
Experimental Section
To date, various approaches have been developed for the synthesis of nanostructured materials, such as metals and oxides, because of their peculiar properties1,2 and potential applications in functional nanodevices.3,4 For example, anodic aluminum oxide (AAO) templates can be effectively employed for electrochemical deposition of metals and alloys.5 However, this method usually acquires polycrystalline nanowires. So far, a method is still lacking for preparation of the single-crystalline nanowires especially for the magnetic materials. γ-Fe2O3 is a magnetic material which has been widely used as semiconductors,6 recording materials,7 and photocatalyst.8 A few reports9,10 have been published about the preparation of γ-Fe2O3 nanowires. Zhang et al.9 employed AAO template through electrodeposition followed by a heat-treating process to acquire nanowires-like iron oxides (R-Fe2O3, Fe3O4, and γ-Fe2O3) of diameter about 200 nm and length up to 8 µm. However, the wires have polycrystalline structures and consist of fine particles. Xiong et al.10 prepared single-crystalline γ-Fe2O3 nanowires with diameters of about 20 nm and lengths of 3∼6 µm from FeCl3 solution. Here, we report a completely different method to achieve single-crystalline γ-Fe2O3 nanowires, which have typical diameters of 50-90 nm and lengths of 10∼20 µm or even longer. Following our previous work,11,12 the nanowires of R-Fe2O3 are prepared by direct oxidation of iron, and the properties are studied in some aspects.13 Then, the R-Fe2O3 nanowires are heated in a reduced atmosphere which is a mixture of hydrogen and nitrogen. Some papers9,14 report that the reduction of R-Fe2O3 usually leads to Fe3O4. However, in our work, the single-crystalline γ-Fe2O3 nanowires are directly acquired from the original R-Fe2O3 nanowires. The appropriate temperature should be a key factor for the reaction because Fe3O4 nanowires are found at a higher temperature.
The γ-Fe2O3 nanowires are prepared via the following process: the first step is to prepare R-Fe2O3 nanowires by oxygenating pure iron, which has been detailed in ref 11. Then, the R-Fe2O3 nanowires are heated at 375-390 °C for 6 h in a flowing reduced atmosphere (60 mL/min) which is a 10:1 mixture of N2:H2. The prepared samples are characterized and analyzed by X-ray diffraction (XRD) (Philips X’PERT with Cu KR radiation), X-ray photoelectron spectroscopy (XPS) (VG ESCA Lab 5 with Mg KR radiation), field emission scanning electron microscope (SEM) (Amray-1910FE), and transmission electron microscope (TEM) (HRTEM, Tecnai F30). MicroRaman spectrum (Renishaw-inVia) is measured at room temperature under the excitation of the 514.5-nm wavelength of an Ar+ laser. The laser power of 0.7 mW is focused into a spot of about 10 µm in diameter. During the micro-Raman spectrum measurement, the local temperature of the spot may be increased to hundreds of degrees under the laser beam with a high power density or a long acquisition time. It will cause phase transition15,16 from γ-Fe2O3 to R-Fe2O3. So, in our experiment, the laser power is limited to 0.7 mW which cannot only prevent such a transition but can also acquire the characteristic spectrum of our samples. To eliminate possible false signals from noise, different places on the same sample and different samples are measured, and finally we achieved similar Raman spectra. The photoluminescence (PL) (Jobin-Yvon-Spex) study is carried out at room temperature using a Xe arc lamp as the excitation source. The excitation wave length for the PL measurement was 399 nm. The nanowires are removed from the substrates and put together to measure the magnetic property on a magnetic property measurement system (Quantum Design MPMS-XL7). Because the dimension of one single substrate is about 0.5 square centimeters, wires of only one substrate are not enough to get strong signals for magnetic measurements. So, the nanowires are carefully stuck off several substrates by nonmagnetic adhesive tape and then are put together for measurement. We can conclude that the results are reproducible for the
* To whom correspondence should be addressed. Telephone: 86-1062754233. Fax: 86-10-62751615. E-mail:
[email protected]. † Peking University. ‡ Beijing University of Astronautics & Aeronautics.
10.1021/jp067837m CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007
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Figure 1. XRD pattern of the γ-Fe2O3 nanowires prepared under a reduced atmosphere.
wires from different substrates are prepared in separate processes. In fact, we have an average signal of group nanowires with similar appearances. The mechanism of the transformation from R-Fe2O3 phase to γ-Fe2O3 phase is studied by ab initio calculations, which are performed by the computer program CASTEP.17 We employ the pseudopotential plane wave total-energy method on the basis of the density functional theory within the generalized gradient approximation. The surfaces are represented by a repeated-slab model containing 12 atomic layers. The spacing between the slabs is approximately 10 angstroms. The plane-wave kinetic energy cutoff is 380 eV and the k-points for Brilliouin-zone sampling are 3 × 3 × 1 using the Monkhorst-Pack scheme. Spin polarization is considered for Fe atoms. Results and Discussion Figure 1 represents the XRD pattern of the nanowires, which is identical with that of γ-Fe2O3. However, because of the very similar patterns between γ-Fe2O3 and Fe3O4, the XRD pattern cannot provide enough evidence to confirm that the sample is γ-Fe2O3. Further investigations are carried out as following. First, the colors of our samples are the characteristic color of γ-Fe2O3 (red-brown) which is significantly different from that of Fe3O4 (black). Second, as shown in Figure 2A, the microRaman spectra of the nanowires has two peaks centered at the wavelengths of around 1430 and 1580 cm-1, which are attributed to γ-Fe2O3 phase.15,16 The broadened peak over the wavelength 600-800 cm-1 is also a characteristic band of γ-Fe2O3. There is no peak at 663 cm-1 which is typical of Fe3O4.15 Third, as shown in Figure 2C, the centers of electronbinding energy of Fe 2p3/2 and Fe 2p1/2 are 710.72 and 724.15 eV, respectively. The shakeup satellite structures at the higher binding energy sides of the main peaks are the fingerprints of the electronic structure of Fe3+ and indicate that Fe2+ is absent.18 All these experimental results confirm that the sample is γ-Fe2O3 rather than Fe3O4. As a supplement, the micro-Raman spectrum of the original R-Fe2O3 nanowires is shown in Figure 2B. Figure 3A is the SEM image of the original R-Fe2O3 nanowires before the heat treatment while Figure 3B is the image of the γ-Fe2O3 nanowires. The γ-Fe2O3 nanowires keep wirelike nanostructures, and their sizes are close to that of the R-Fe2O3 nanowires. It is also shown that the nanowires cover the surface of the substrate with a high surface density up to 108∼109 cm-1. Figure 3C and 3D is images of γ-Fe2O3 nanowires with higher magnification which reveal the smoothness and uniformity of the nanowires. The diameter and length of the typical nanowires are 50-90 nm and 10-20 µm, respectively. The TEM image of a single γ-Fe2O3 nanowire is shown in Figure 4A. The electron diffraction (ED) pattern (the inset of
Figure 2. Micro-Raman spectra of (A) the γ-Fe2O3 nanowires and (B) the original R-Fe2O3 nanowires. (C) XPS spectra of the γ-Fe2O3 nanowires for Fe (2p).
Figure 4A) obtained along this typical individual nanowire confirms the γ-Fe2O3 nanowire to be single crystalline. The high-resolution transmission electron microscopy (HRTEM) image (Figure 4B) shows that the lattice fringes are about 2.9 nm, which agrees well with the separations between the (110) lattice planes. Combined with the ED pattern, it can be concluded that the nanowires grow along [110] direction. For the original R-Fe2O3 nanowires, the growth direction is [110] which has been referred to in our previous work.12 It is interesting to know how R-Fe2O3 nanowires are transformed to γ-Fe2O3 nanowires. Some experimental results demonstrate that the reduced air and the appropriate temperature play important roles. Under 375-390 °C, oxygen atoms could not gain enough energy to escape from inside the singlecrystalline nanowires and hydrogen acts as the catalyzer to push the transformation from R-Fe2O3 to γ-Fe2O3 phase: H2/380°
R-Fe2O3 98 γ-Fe2O3 When the temperature is increased to about 410 °C, Fe3O4 nanowires are found. Such an acquisition of Fe3O4 from R-Fe2O3 has been reported by some earlier works.9,14 Up to about 440 °C, the nanowires break and decompose into particles which are finally deoxidized to Fe particles. We have performed ab initio calculations about the interaction between the hydrogen and the surface of hematite to explore
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Figure 3. SEM images of nanowires with different magnification. (A) Low-magnification image of the original R-Fe2O3 nanowires. (B) The image of the γ-Fe2O3 nanowires with the same magnification as A. (C) The image of the γ-Fe2O3 nanowires with a higher magnification, showing the growing orientation of γ-Fe2O3 nanowires. (D) Magnified image which shows the smoothness and uniformity of the nanowires.
Figure 4. (A) The TEM image of a single γ-Fe2O3 nanowire; the inset in A shows an ED pattern of the nanowire. (B) HRTEM image of the nanowire.
the mechanism of the transformation from R-Fe2O3 to γ-Fe2O3. Because the (001) surface is the most stable surface of hematite in existence,19 we consider the reactions that happen on the (001) surface of the R-Fe2O3 nanowires. As a result of growth in an oxidized atmosphere, according to ref 19, the (001) surface of the R-Fe2O3 nanowires would be completely covered with oxygen. Then, in the reduced air, hydrogen interacts first with
the fully oxidized surface, denoted by Figure 5A. The result of the geometry optimization (Figure 5B) shows that the molecule of hydrogen disassociates and forms a water molecule with oxygen, which gives an energy gain of -2.1 eV per H2 molecule. After the water molecules escape from the surface, hydrogen would interact with the surface covered by Fe atoms, denoted by Figure 5C. As we know, the growth directions of
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Figure 5. (A) Chemisorption of a H2 molecule on the fully oxidized surface and (B) after the geometry optimization. H2 molecule forms H2O combined with the oxygen atom on the surface. (C) Chemisorption of a H2 molecule on the fully reduced Fe-terminate surface and (D) after geometry optimization. (E) The Fe-O bonds in R-Fe2O3; (F) changed Fe-O bonds which are extracted from D; (G) the Fe-O bonds in γ-Fe2O3. Detailed angles and lengths of these bonds are shown in Table 1.
TABLE 1: The Changed Lengths and Angles of Chemical Bonds after the Adsorption of H2 Are Compared with r-Fe2O3 and γ-Fe2O3 Nanowiresa angle (deg) Fe1-O-Fe2 Fe1-O-Fe3 Fe1-O-Fe4 Fe2-O-Fe3 Fe2-O-Fe4 Fe3-O-Fe4
R-Fe2O3 93 120 132 132 87 93
after the adsorption of H2 117 122 123 103 92 92
γ-Fe2O3 125 125 125 90 90 90
Length (Å) Fe1-O Fe2-O Fe3-O Fe4-O a
1.93 2.13 1.93 2.13
1.80 2.03 1.91 2.11
1.88 2.05 2.05 2.05
The related structures are shown in figure 5E-G.
the γ-Fe2O3 nanowires and the original R-Fe2O3 have the same index [110], but the γ-Fe2O3 has a cubic crystal cell while the R-Fe2O3 has a trigonal crystal cell. The transformation of the crystal structure means a change of angles and lengths of chemical bonds. Here, we consider a H2 molecule that adsorbs on the Fe-terminate surface. After the geometry optimization (Figure 5D), we find the process gains adsorption energy of -0.7 eV per H2 molecule and the angles and lengths of bonds are changed (Figure 5F) which are compared with those of R-Fe2O3 (Figure 5E) and γ-Fe2O3 (Figure 5G) in Table 1. The results clearly show that after the adsorption of H2, the chemical bonds in R-Fe2O3 have a tendency to those of γ-Fe2O3, implying that hydrogen should play an essential role in changing the bonds to transform R-Fe2O3 into γ-Fe2O3. Because the result of the geometry optimization is the theoretical ground state at 0 K, the temperature factor is not considered in our calculations. The actual process may be more complicated. After the preparation of the nanowires, some elementary physical properties are studied. The PL spectra of γ-Fe2O3 nanowires and the original R-Fe2O3 nanowires are shown in Figure 6. The prominent peak can be identified as the band gap emission of the nanowires. The band gap of the original R-Fe2O3 nanowires is 578 nm (2.14 eV) which is consistent with that of bulk R-Fe2O3 (2.10 eV).20 However, the band gap of the γ-Fe2O3 nanowires is 558 nm (2.22 eV) which shows a noticeable red
Figure 6. The photoluminescence spectra of (A) γ-Fe2O3 nanowires and (B) R-Fe2O3 nanowires.
shift of about 48 nm, compared with that of the bulk γ-Fe2O3 (510 nm, 2.43 eV).21,22 The shift can be attributed to the lattice strain developed as a result of the particle size reduction. A reduction in particle size may have two opposing effects in the band structure thus causing a shift in the optical spectra:22 one leads to a blue shift because of a simple kinetic effect of electrons confined in a smaller volume; the other leads to a red shift because of the increased lattice strain generated by the surface tension. The net result will depend on the relative magnitude of these two effects. For the prepared γ-Fe2O3 nanowires, the stress effect is predominant and results in a red shift of the PL spectra. The red shift of our samples is about half the amount of that of the nanoparticles prepared by Vassiliou et al. which have an average diameter of 10 nm.22 Because our nanowires have relative large diameters (50-90 nm), the red shift seems too large. Considering that there may be small nanoparticles trapped in the samples, a certain amount of the red shift is possibly contributed by the nanoparticles. Figure 7A shows the temperature dependence of magnetization for the γ-Fe2O3 nanowires. The curves are acquired between 6 and 380 K using zero-field-cooling (ZFC) and field-cooling
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Han et al. Oe and remanence ratio Mr/Ms (Mr the remnant magnetic moment, Ms the saturation magnetic moment) of 0.5. Conclusion In summary, R-Fe2O3 nanowires can be transformed into γ-Fe2O3 nanowires in a reduced atmosphere. Single-crystalline nanowires of γ-Fe2O3 with diameters of 50-90 nm and lengths of about 10∼20 µm are formed. HRTEM and ED results confirm that the single-crystalline nanowires grow along [110] direction. The transformation of R-Fe2O3 into γ-Fe2O3 is catalyzed by H2 on the Fe-terminate surface. The PL spectra of the γ-Fe2O3 nanowires show a red shift of band edge compared with the bulk. The magnetic nanowires have a blocking temperature of about 200 K. Acknowledgment. The project was supported by National Natural Science Foundation of China under grant Nos.10374003 and 10434010. The computation made use of the HP highperformance computer cluster provided by Center for Computational Science and Engineering of Peking University. The project is also supported by the Engineering Research Institute, Peking University (204031). References and Notes
Figure 7. (A) Temperature dependence of ZFC and FC magnetization for γ-Fe2O3 nanowires under an applied field of 100 Oe. (B) Magnetic hysteresis curves measured at 300 and 30 K for the γ-Fe2O3 nanowires.
(FC) procedures under an applied magnetic field of 100 Oe. The blocking temperature of the γ-Fe2O3 nanowires is found to be about 200 K. This result is larger than the blocking temperature of the γ-Fe2O3 nanowires in ref 10 (120 K), which can be attributed to the size effect: our nanowires have diameters of 50-90 nm while the nanowires in ref 10 have diameters of about 20 nm, and it is known that the blocking temperature increases with the increase of the particle size. We also find that the blocking temperature of the γ-Fe2O3 nanowires is larger than the origin R-Fe2O3 nanowires whose blocking temperature is about 120 K.13 It can be explained by the stronger magnetic interaction between the γ-Fe2O3 wires than R-Fe2O3 because γ-Fe2O3 is ferrimagnetic material while R-Fe2O3 is antiferromagnetic material. Figure 7B shows the field-dependent magnetization above and below TB (300 and 30 K), respectively. At 300 K, the γ-Fe2O3 nanowires exhibit a superparamagnetic behavior and no coercivity or remanence is observed. At 30 K, magnetic hysteresis loop is present with coercivity (Hc) of 360
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