Selective Oxidation Synthesis of MnCr2O4 Spinel Nanowires from Commercial Stainless Steel Foil Yongjun Chen,*,† Zongwen Liu,‡ Simon P. Ringer,‡ Zhangfa Tong,† Xuemin Cui,† and Ying Chen§ School of Chemistry and Chemical Engineering, Guangxi UniVersity, Nanning, Guangxi, 530004, P.R.China, Australian Key Centre for Microscopy and Microanalysis, The UniVersity of Sydney, NSW 2006, Australia, Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National UniVersity, Canberra, ACT 0200, Australia
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2279–2281
ReceiVed June 6, 2007; ReVised Manuscript ReceiVed August 14, 2007
ABSTRACT: For the first time, MnCr2O4 spinel single-crystalline nanowires were simply synthesized by heating commercial stainless steel foil (Cr0.19Fe0.70Ni0.11) under a reducing atmosphere. The nanowires have an average diameter of 50 nm and a length of about 10 µm. Some nanowires are sheathed with a thin layer of amorphous silicon oxide. Photoluminescence measurements revealed that the nanowires exhibit an emission band at 435 nm, which resulted from the oxygen-related defects in the silicon oxide sheath. It was found that the reducing atmosphere plays a key role for the nanowire growth. In the reducing atmosphere, the Mn and Cr elements in the stainless steel could be selectively oxidized because of their higher affinity for oxygen than the Fe and Ni elements. The Fe and Ni elements in the stainless steel, however, acted as the catalyst for the vapor–liquid–solid (VLS) growth of the MnCr2O4 nanowires. One-dimensional (1D) nanostructured materials have attracted great interest in recent years because of their unique physical and chemical properties and potential application in nanoscale electronic, photonic, and sensing devices.1,2 As a result, research in the production of 1D nanostructures has gained increasing momentum.3–7 Those spinels (AB2O4) with cations that occupy the tetrahedral and octahedral holes in a dense cubic packing of oxygen anions are one of the most important and interesting oxides because of their wide range of potential applications arising from their excellent electrical and magnetic properties.8,9 MnCr2O4 is an important member of the spinel family and has the normal spinel structure (space group Fd3m). The divalent Mn2+ and trivalent Cr3+ occupy tetrahedral and octahedral sites, respectively. The MnCr2O4 spinel has been widely investigated because of its excellent magnetic and electric properties.10–12 For example, in solid oxide fuel cells (SOFCs), MnCr2O4 layers were readily formed on the surface of stainless steel, which has been found to be very useful in improving the electric conductivity of the interconnects.13 More recently, nanostructured spinels have also attracted increasing attention and found important applications in nanodevices, sensors, photoelectronics, electronics, and catalysts. For example, ZnCr2O4 and CoFe2O4 nanoparticles have been widely used as novel magnetic materials;14,15 ZnFe2O4 nanoparticles have been found to exhibit excellent sensitivity and selectivity to alcohol.16 However, much less attention has been paid to 1D spinel nanostructures.17–19 Indeed, to the best of our knowledge, there has been no research reported on 1D MnCr2O4 nanostructures to date. Herein, we report the synthesis of single-crystalline nanowires of MnCr2O4 on a large scale by a very facile and economical route, the direct heating of commercial stainless steel foil, for the first time. It is very interesting that the products are MnCr2O4 nanowires rather than FeCr2O4 or other Fe- and Ni-containing compound nanowires, although EDS results show that Fe, Cr, and Ni are the main elements in the stainless steel foil (see the Supporting Information, Figure S1). The growth mechanism was investigated in detail in the present study. MnCr2O4 nanowires were synthesized from commercial stainless steel foil at an elevated temperature by a horizontal tube furnace. The stainless steel foil acted as both the precursor and substrate for growth of the MnCr2O4 nanowires. The foil was loaded into an Al2O3 boat that was placed at the center of a ceramic tube (length * Corresponding author. E-mail:
[email protected]. † Guangxi University. ‡ The University of Sydney. § The Australian National University.
Figure 1. (a) Low-magnification and (b) high-magnification SEM images of the nanowires..
× diameter ) 600 × 42 mm2). The tube was then inserted into the furnace with the stainless steel foil being placed at the center of the furnace. The furnace chamber was first flushed with pure N2 flow (1000 ml/min) for 30 min The furnace was then heated to 1100 °C at a rate of 15 °C/min under a 200 ml/min mixed-gas flow of N2 + 5%H2 (vol) and held at this temperature for 90 min. Finally, the furnace was allowed to cool naturally to room temperature under a N2 flow. After removed from the furnace, the stainless steel foil was found to be covered with a layer of green fluffy film on both sides. The product film was characterized by field-emission scanning electron microscopy (FE-SEM; Hitachi S4500), high-resolution field-emission transmission electron microscope (HRTEM, JEM-3000F), energy-dispersive X-ray (EDX) spectroscopy, electron energy-loss spectroscopy (EELS), X-ray diffraction (XRD) with cobalt KR radiation (λ ) 0.178897 nm), Raman spectroscopy (Renishaw 1000; 514.5 nm green line of an Ar-ion laser excitation), and photoluminescence (PL) spectroscopy (Hitachi F-4500; 408 nm excitation) at room temperature. Figure 1a is a SEM image of the film, showing the high production of nanowires with lengths of several to 10 µm. Figure 1b reveals that the nanowires have a pillar shape and rectangular cross-section. The nanowire surface is very smooth and clean and has a typical diameter of ∼50 nm. An irregular particle can also be observed at one end of the nanowires. TEM characterization shows that some of the nanowires are sheathed with a thin layer of amorphous material (Figure 2a). The top-right inset is the EDX spectrum of the nanowire, revealing that the nanowire is composed of O, Cr, and Mn. The Si peak should come from the sheath, whereas Cu comes from the TEM copper grid. The EDX spectrum of the sheath reveals the sheath composition of Si and O (not shown here). Therefore, it can be seen that the synthesized nanowires are composed of Mn, Cr, and O, and some nanowires have a sheath of silicon oxide, which will be further confirmed by the EELS results later in the paper. Figure 2b clearly shows a particle attached to
10.1021/cg070514a CCC: $37.00 2007 American Chemical Society Published on Web 10/18/2007
2280 Crystal Growth & Design, Vol. 7, No. 11, 2007
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Figure 2. High-angle annular dark-field (HAADF) images showing (a) a nanowire that has a sheath and (b) a nanowire with a particle capped at one of its end. The inset in (a) is the EDX spectra of the nanowire body. The inset in (b) is the EDX spectrum of the capped particle.
Figure 5. XRD patterns of the nanowires and the substrate. The peaks labelled with “SS” belong to the stainless steel sbustrate.
Figure 3. EELS spectrum taken from an individual nanowire without a sheath. The nanowire contains O, Cr, and Mn only.
Figure 6. Raman spectrum of the nanowires taken at room temperature.
Figure 4. High-resolution TEM image of one of the nanowires and its corresponding microbeam electron diffraction pattern (inset). The pattern can be indexed as being in the [110] zone axis of MnCr2O4 spinel. The marked spacing is the atomic plane spacing between the {111} planes.
the nanowire, which is consistent with the SEM result shown in Figure 1b. The inset EDX spectrum indicates that the particle mainly contains Fe and S with a small amount of Ni, Cr, P, and Fe. Ni and Cr originate from the main components of the foil, whereas S and P are from the microcomponents of the foil. As discussed later in the paper that it is believed those particles were initially in a liquid state and acted as the catalyst during the vapor–liquid–solid (VLS) growth of the nanowires. The nanowire composition of Mn, Cr, and O is further verified by the EELS spectrum taken from an individual wire without a silicon oxide sheath (Figure 3). Further quantitative analysis reveals that the atomic ratio of Mn:Cr:O is close to 1:2:4. A high-resolution image of one of the nanowires in [110] orientation and its corresponding electron diffraction (ED) are shown in Figure 4. It can be observed that the lattice fringes are welldefined, suggesting that the crystallinity of the nanowires is perfect. The interlayer space is approximately 0.487 nm and consistent with (111) plane lattice parameter of spinel MnCr2O4. The inset ED patterns also verifies the good crystallinity of the
nanowires. The MnCr2O4 spinel structure was further verified by the XRD result (Figure 5). The peaks located at the 2θ angles of 51.1, 59.7, and 89.3° can be attributed to the cubic Cr0.19Fe0.70Ni0.11 phase of the stainless steel foil (JCPDS 33–0397) and are indicated by the symbol “SS”. The other peaks belong to the face-centered cubic (fcc) structure of MnCr2O4 spinel with a ) 8.437 Å (JCPDS 75–1614). Therefore, all the above results indicate that the synthesized nanowires are single-crystalline MnCr2O4 nanowires and that some nanowires have a silicon oxide sheath. Figure 6 shows the Raman spectrum of the synthesized MnCr2O4 nanowires. It is very similar to the Raman spectrum of FeCr2O4 spinel,20 providing further evidence for the spinel structure of the synthesized nanowires. The strongest peak at 685 cm-1 can be assigned to the A1g mode, which was presumably generated by the bonds in Cr3+O6 octahedral, whereas the peaks at 600 and 511 cm-1 can be assigned to Eg and F2g symmetries. However, compared with the Raman spectrum of FeCr2O4, some peak shifts were observed in the current spectrum of the MnCr2O4 nanowires: 690 cm-1 f 685 cm-1 and 520 cm-1 f 511 cm-1. Another distinct feature of the MnCr2O4 Raman spectrum is the lack of peaks at 650 and 445 cm-1 that appeared in the FeCr2O4 spectrum. In fact, consistent with these observations, previous factor-group analyses have suggested that five Raman-active vibrational modes are allowable for oxide spinels: A1g + Eg + 3F2g.21 The variation in chemical composition (cation substitution), structure, and site occupancies of cations in the spinels may cause shifting of the peak and even additional peaks. The cation substitution of Fe by Mn should be responsible for these peak shifts and shortening of some peaks. Furthermore, size effects may also have effect on the peak shifts because of the nanometer scale of the MnCr2O4 nanowires.
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Crystal Growth & Design, Vol. 7, No. 11, 2007 2281 However, further work is needed to clarify the detailed growth mechanism and explain the uneven formation of silicon oxide sheath. The possible reactions for the formation of MnCr2O4 nanowires are described as eqs 1–4 2Mn(s) + O2(g) f 2MnO(s)
(1)
4Cr(s) + 3O2(g) f 2Cr2O3(s)
(2)
S(s) + Fe(s) + Ni(s) + Cr(s) + P(s) f S–Fe–Ni–Cr–P(1) (3) MnO(s) + Cr2O3(s) f MnCr2O4(s)
Figure 7. PL spectrum of the nanowires taken at room temperature, showing an emission band at 435 nm.
Generally, MnCr2O4 spinel does not possess good photoluminescence (PL) properties because the defect number in the spinel structure is small and the luminescence caused by the electronic transfer between Mn cations and O anions should be very weak. Considering that some of the MnCr2O4 nanowires have an amorphous silicon oxide sheath and similar PL emission features of silica or silica-sheathed silicon nanowires reported in the literature,22,23 the PL emission band at about 435 nm of the current study (Figure 7) can be assigned to the oxygen-related defects in the silicon oxide sheath of the MnCr2O4 nanowires. It is very interesting to note that the products are MnCr2O4 nanowires rather than FeCr2O4 or other Fe- and Ni-containing compound nanowires, although Fe, Cr, and Ni are the most predominant elements in the stainless steel. That is because the alloying elements in steels such as Mn, Si, Cr, V, Al, and Ti have a higher affinity for oxygen than Fe and Ni and hence the formal elements are more readily oxidized than the later ones. In a reducing atmosphere of N2 + 5%H2, Fe and Ni could not be oxidized but Mn and Cr could still be oxidized provided that trace amounts of oxygen remained in the chamber. In fact, the growth of MnCr2O4 spinels has already been reported before when stainless steel was heated in a reducing atmosphere.24–26 For comparison, we also heated the stainless foil in the air and found that no MnCr2O4 nanowires formed except the formation of some spinel and iron oxide particles. Therefore, we believe that the reducing atmosphere (with a trace amount of oxygen) plays a key role in the growth of MnCr2O4 nanowires. In other words, in the reducing atmosphere of N2 + 5%H2, these MnCr2O4 nanowires grew through a selective oxidation of Mn and Cr elements in the stainless steel foil. The associated growth process is proposed as follows. First, on the surface of the stainless steel foil, Cr and Mn elements reacted with the residual oxygen (trace amount) in the chamber and formed MnO and Cr2O3 compounds. The elements of S, P, Ni, Cr, and Fe also interacted and formed eutectic alloy liquid on the surface of the foil at high temperature. MnO and Cr2O3 then diffused into the liquid droplets, reacted and created MnCr2O4 crystal nucleus. MnCr2O4 crystal precipitated from the liquid droplets once the concentrations of species were greater than the saturation threshold. The adsorption of oxygen from the surrounding atmosphere led to the formation of MnO and Cr2O3 and the continuous growth of MnCr2O4 crystal. However, the gas flow in the experiments further leads to a very low oxygen partial pressure and then a low supersaturation of MnCr2O4 crystal, which favors the growth of the 1D nanowires. Hence, the growth mechanism is still in the framework of VLS model. The particles at nanowire ends provided further evidence for this supposition. In addition, the microcomponent element of Si in the stainless steel foil was also oxidized and silicon oxide phase formed in some areas. It was then deposited onto the surface of the growing or already grown MnCr2O4 nanowires, and finally, the silicon oxide sheath was generated.
(4)
In summary, MnCr2O4 nanowires have been successfully prepared by heating commercial stainless steel foil at 1100 °C under a reducing atmosphere of N2 + 5%H2 (vol). The nanowires are generally covered with a silicon oxide sheath, which led to a PL emission band at 435 nm. The nanowires were formed via a selective oxidation of Mn and Cr elements in the stainless steel and the reducing atmosphere plays an important role for the VLS growth of the nanowires. These nanowires may find potential application in the fields of electrical, magnetic materials, and nanodevices.
Acknowledgment. Y.J.C. thanks financial support from Guangxi University (Grant DD040042). Supporting Information Available: Figure S1 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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