15220
J. Phys. Chem. C 2008, 112, 15220–15225
Facile Synthesis and Characterization of Iron Oxide Semiconductor Nanowires for Gas Sensing Application Guoxiu Wang,* Xinglong Gou, Josip Horvat, and Jinsoo Park School of Mechanical, Materials and Mechatronic Engineering and Institute for Superconducting and Electronics Materials, UniVersity of Wollongong, NSW 2522, Australia ReceiVed: May 2, 2008; ReVised Manuscript ReceiVed: July 30, 2008
We report a facile and efficient synthesis technique for the preparation of iron oxide semiconductor nanowires in large quantity by using nitrilotriacetic acid (NTA) as a chelating agent to form polymeric chains, followed by heat treatment. This technique can also be applied for preparing other transition metal oxide nanowires such as MnO2 nanowires and NiO nanowires. The as-prepared R-Fe2O3 nanowires have exhibited a blue shift of bandgap, peculiar magnetic properties, and high sensitivities toward ethanol and acetic acid gases. 1. Introduction One-dimensional (1D) nanomaterials, including nanowires, nanotubes, nanorods, and nanoribbons, have peculiar and intriguing chemical and physical properties. They have been extensively investigated as building blocks for many functional applications, ranging from molecular nanosensors and nanoscale electronics to nanocomputing.1-6 On the basis of the bottomup paradigm for nanotechnology, controllable synthesis of 1D nanostructures is an initial and critical step toward developing functional nanodevices.7 One-dimensional nanostructures can be effectively prepared by many techniques, such as templatedirected synthesis,8 vapor deposition, either vapor-solid (VS) or vapor-liquid-solid (VLS) growth,9 hydrothermal (or solvothermal) methods,10 and the solution-liquid-solid method.11 Among them, only the soft-chemistry approach can produce large quantities of materials at low cost. Hematite (R-Fe2O3) is the most stable iron oxide, with band gaps in the range of 1.9-2.2 eV (depending on the crystallinity and preparation method). R-Fe2O3 tends to be an n-type semiconductor in the presence of oxygen vacancies. However, it can change to p-type semiconductivity under doping due to its narrow bandgap.12 The magnetic properties of R-Fe2O3 are strongly related to its particle size, shape, and morphology. Bulk R-Fe2O3 has a Morin transition at TM ≈ 263 K. Below TM, it is in an antiferromagnetic state with spins aligned along the crystalline c-axis; above TM, spins are ordered antiferromagnetically within the basal plane. The spins are canted out of the basal plane, and this out-of-plane component of the spins is coupled ferromagnetically, making the R-Fe2O3 weakly ferromagnetic.13 However, the Morin temperature TM depends on the particle size of the R-Fe2O3. On the basis of those unique physical properties, hematite has many intriguing technological applications in such areas as magnetic materials, gas sensors, catalysts, drug delivery, and biomedical therapies. R-Fe2O3 nanowires and nanobelts have been synthesized by VS or VLS growth on Fe foils.14 R-Fe2O3 nanorods and nanotubes have been prepared by a chemical synthesis route. 15 However, it is still a big challenge to develop a general synthesis strategy to prepare R-Fe2O3 1D nanostructures on a large scale at low cost. In this article, we report a facile and efficient synthesis technique combining the solvothermal method and organic * Corresponding author. E-mail:
[email protected].
chains as a mediating template for preparing iron oxide (RFe2O3) nanowires in large quantities. The as-prepared R-Fe2O3 nanowires exhibited high gas sensitivity toward flammable and corrosive gases when tested as nanosensors, together with unique magnetic properties. We also demonstrated that this novel approach can also be applied to synthesize other transition metal oxide nanowires, such as manganese oxide nanowires and vanadium oxide nanowires. 2. Experimental Section Nanowire Synthesis. R-Fe2O3 nanowires were synthesized via two steps. FeNTA precursor nanowires were prepared in the first step. In a typical synthesis, 0.15 M FeCl3 aqueous solution was mixed with isopropanol, to which 3 mmol nitrilotriacetic acid (NTA) was added. After thorough stirring, the mixture was transferred into a Teflon lined autoclave and hydrothermally treated at 180 °C for 24 h. The resultant white floccules were collected by centrifugation, washed with deionized water and absolute ethanol, and vaccuum-dried at 60 °C. In the second step, the precursors were sintered at 350 °C for 1 h to be converted to R-Fe2O3 nanowires. The synthesis of MnO2 and NiO nanowires followed the same procedure by using Mn(NO3)2 · 6H2O and NiCl2 · 6H2O. Structural, Optical, and Magnetic Characterization. Field emission scanning electron microscopy (FESEM) was performed to observe the morphologies of the precursors and final products, using a JEOL JSM 6460A SEM. X-ray diffraction patterns (XRD) were recorded on a Phillips 1730 X-ray diffractometer with Cu KR radiation. Crystal structures were analyzed by transmission electron microscopy (JEOL JEM 2011). Raman spectra of R-Fe2O3 nanowires were recorded on a JOBIN Yvon Horiba Confocal Micro Raman spectrometer model HR 800 with 632.8 nm diode laser excitation on a 300 lines/mm grating at room temperature. The bandgap energy of R-Fe2O3 nanowires was determined via UV-vis spectra, recorded on a Shimadzu UV-1700 spectrophotometer. Magnetic measurements were performed with a Quantum Design MPMS 5 T SQUID magnetometer. Gas Sensing Measurement. Gas sensing properties of R-Fe2O3 nanowires were measured on a WS-30A gas sensing measurement system. The schematics of the device and operating principles are shown in Figure S-1 (Supporting Information).
10.1021/jp803869e CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
Iron Oxide Semiconductor Nanowires for Gas Sensing
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15221
Figure 1. (a) SEM image of FeNTA precursor nanowires, exhibiting the formation of 100% nanowire 1D nanostructures. (b) Magnified view of the precursor nanowires, showing their smooth surface. (c) TEM image of FeNTA precursor nanowires. (d) HRTEM image of a single FeNTA precursor nanowire, showing its amorphous nature.
For comparison, the sensors using commercial microcrystalline R-Fe2O3 powders (
