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Facile Synthesis and Characterization of Iron Oxide Semiconductor Nanowires for Gas


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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 ef?cient 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 ef?cient synthesis technique combining the solvothermal method and organic
* Corresponding author. E-mail: gwang@uow.edu.au.

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 ?ammable 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 ?rst 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 Te?on lined autoclave and hydrothermally treated at 180 °C for 24 h. The resultant white ?occules 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 ?nal 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

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Figure 1. (a) SEM image of FeNTA precursor nanowires, exhibiting the formation of 100% nanowire 1D nanostructures. (b) Magni?ed 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 (<5 ?m, Sigma-Aldrich) were also fabricated and tested. Results and Discussion We employed a two-step strategy for the synthesis of iron oxide nanowires. Initially, Fe3+ ions reacted with nitrilotriacetic acid (NTA) in isopropyl alcohol solvent to form chelating compounds. Then, they were hydrothermally treated in an autoclave to form 1D long-chain polymers, in which Fe3+ ions were bonded and anchored to amino groups or carboxyl groups. In the second step, the (-Fe-NTA-)n long-chain ligands were heat-treated at 350 °C to convert to R-Fe2O3 nanowires. Figure 1a and b shows typical scanning electron microscope (SEM) images of the precursor nanowires and ?nal R-Fe2O3 nanowires. A low magni?cation image of the precursor nanowires is shown in Figure 1a, indicating that the products are entirely nanowires. The lengths of those precursor nanowires extend to several tens of micrometers. We examined the products from several different synthesis batches, and all of them exbibited the same morphology. No other morphologies, such as nanopowders or nanoplates, were formed. This demonstrates that the current synthetic technique can ef?ciently produce 100% 1D nanowires. Figure 1b further shows a magni?ed view of the precursor nanowires, clearly illustrating that the nanowires have a smooth surface and a homogeneous distribution of diameters and lengths. When we focused the electron beam on individual precursor nanowires for a few minutes at high magni?cation, they were easily broken, indicating the unstable nature of the (-Fe-NTA-)n ligands under the electron beam. Figure 1c shows a low magni?cation TEM image of (-Fe-NTA-)n precursor nanowires. They tend to stick together. Those precursor nanowires are amorphous in nature (as con?rmed by the HRTEM image shown in Figure 1d, which has a diameter of about 120 nm).

After heat-treating the (-Fe-NTA-)n precursor nanowires, they were completely converted to R-Fe2O3 nanowires. X-ray diffraction (XRD) and Raman spectroscopy were performed on the sintered product to identify the phase purity. Figure 2 shows the XRD pattern and Raman spectra of R-Fe2O3 nanowires. All diffraction lines can be readily indexed to the R-Fe2O3 phase (hematite) with a rhombohedral crystal structure (ICDD-JCPDS Card No. 33-0664). The features of Raman spectra of R-Fe2O3 nanowires are essentially the same as that of previously reported R-Fe2O3 micrpcrystalline powders except for the shift of their peaks to the higher frequency region by 2-5 cm-1 (blue shift), which could be originated from nanocrystalline.14a Both XRD pattern and Raman spectra con?rmed the pure phase of R-Fe2O3 nanowires. We also successfully synthesized (-Mn-NTA-)n and (-Ni-NTA-)n nanowires using this technique, demonstrating that this synthetic approach can also be generally applied to prepare other transition metal oxide nanowires (Figure S-2, Supporting Information). The detailed crystal structures of R-Fe2O3 nanowires were further analyzed by TEM, high resolution TEM (HRTEM), and electron diffraction. Low magni?cation TEM and high magni?cation TEM images of R-Fe2O3 nanowires are shown in Figure 3a and b, elucidating the polycrystalline nature of the R-Fe2O3 nanowires. After sintering, the dense and smooth precursor nanowires were converted to the porous polycrystalline R-Fe2O3 nanowire structure. The single R-Fe2O3 nanowire shown in Figure 3b has a diameter of 100 nm, indicating a slight shrinkage in the sintering process due to evaporation of organic components. Figure 3c presents a HRTEM image of R-Fe2O3 nanocrystals, which are interconnected to form nanowires. The individual R-Fe2O3 nanocrystals have an average crystal size of 4-5 nm. The corresponding selected area electron diffraction (SAED) pattern is shown as the inset in Figure 3d. All diffraction rings can be readily indexed to the rhombohedral crystal structure, con?rming the R-Fe2O3 hematite phase. Figure 3d

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Wang et al. are two exothermic peaks at 233 and 268 °C, respectively, which are related to the decomposition and burning of organic NTA in FeNTA precursor nanowires. Figure 6 shows the UV-vis optical absorption spectrum of the R-Fe2O3 nanowires. The band gap of hematite varies in the range of 1.9-2.2 eV, depending on its crystal size and preparation technique. The band gap Eg can be calculated by the following equation:

(Rhν)2 ) B × (hν - Eg)

(1)

Figure 2. (a) X-ray diffraction pattern of R-Fe2O3 nanowires. (b) Raman spectra of R-Fe2O3 nanowires.

shows the lattice resolved HRTEM image of the R-Fe2O3 nanocrystal marked in Figure 3c, in which two d spacings (110) and (006) crystal planes are indexed and illustrated. Nitrilotriacetic acid (NTA) is an organic chelating agent, which itself is a tetradentate ligand. Therefore, hexacoordinated metals, i.e., iron, can form ternary complexes with NTA. When NTA is mixed into a FeCl3 solution, it forms the FeNTA coordination compound. We observed the light yellow precipitates at room temperature. We analyzed those precipitates and found that they were powders. Under solvothermal conditions, those FeNTA monomers were further reacted and formed longer chain products. The individual polymer chains could selfassemble into nanowire structures due to van der Waals forces. The infrared (IR) spectra of NTA and the precursor nanowires, are shown in Figure 4. The C-H stretching frequencies induce IR bands in the 3000 - 2800 cm-1 region.16 We observed C-H bands for NTA, but not for FeNTA nanowires. This could be caused by the introduction of Fe3+ coordination.17 The band at 1716.42 cm-1 for NTA in Figure 4a should be assigned to carboxyl groups due to the CdO stretching vibration. On chelating, this CdO band disappears, as in Figure 4b. Instead, the coordinated carboxyl bands at 1681.71 cm-1 and 1581.42 cm-1 appeared, indicating the formation of -COOFe coordination groups.18 The total weight loss of the precursor nanowires in heat treatment was determined to be 68.2 wt% by thermogravimetic analysis (TGA, Figure 5). This value is very close to the calculated value (67.25 wt.%) based on the molecular formula of N(CH2COO)3Fe, suggesting one chelating NTA per unit. The corresponding DTA curve is also presented in Figure 5. There

where hν is the photon energy, R is the absorption coef?cient, B is a constant. The (Rhν)2 ? hν curve is shown as the inset in Figure 6, from which the bandgap of R-Fe2O3 nanowires was determined to be 2.46 eV by extrapolating eq 1. This indicates a blue shift of 0.26-0.56 eV. Such a substantial bandgap increase may be ascribed to the quantum con?nement due to the small crystal size of R-Fe2O3 nanowires.15a The temperature dependence of the magnetic moment M of the R-Fe2O3 nanowires is shown in Figure 7a, measured under zero ?eld cooled (ZFC) and ?eld cooled cooling (FCC) conditions with a ?eld of 500 Oe. There is an apparent difference between the ZFC and the FCC curves. The magnetic moment M decreases steadily with increasing temperature in the FCC curve. However, in the ZFC curve, M initially increases with increasing temperature, followed by a decrease above ?155 K. This is different from a previous report that R-Fe2O3 showed a decrease in the ZFC and FC magnetic moments at the same temperature, corresponding to the Morin temperature, TM.13 Our polycrystalline R-Fe2O3 nanowires consist of crystallites with a size of 4-5 nm. They agglomerate together and thus form the 1D nanowire shape. Because of small size of the crystallites, thermal excitations will tend to misalign them at high temperatures, giving the observed decrease of magnetic moment with temperature in the case of FCC. In the ZFC case, the magnetic moments are oriented randomly at high temperatures in zero ?eld and this arrangement of moments is frozen-in upon cooling. After applying the ?eld at low temperatures, the net moment is still small because the ?eld is not strong enough to align the frozen-in magnetic moments of individual nanocrystallites. It is unclear why there is no anomaly due to the Morin transition, which is observed for larger crystallites. It may be that, for this surface to volume ratio, surface defects disturb the usual mechanism responsible for the Morin transition.13 Magnetic hysteresis loops measured at 10, 200 and 298 K are shown in Figure 7b. The values of the coercive ?eld and remanent magnetization for the loops at 200 and 298 K are negligibly small, as shown in the inset. This is not a surprise because the FCC and ZFC curves in Figure 7a overlap at these temperatures, showing that there is no magnetic irreversibility at these temperatures for H ) 500 Oe (i.e., 39.8 kA/m). The hysteresis loop at 10 K reveals a magnetic saturation moment of 40 Am2/kg, a coercive ?eld of 38 kA/m, and a remanent moment of 16 Am2/kg. The saturated moment is about 40% lower than the one reported by Kim et al. for R-Fe2O3 nanowire arrays at 5 K.14 The coercive ?eld of our samples is about 3 times higher and the remanent moment is about 2.5 times higher than for the single crystalline nanowire array samples.14 This could be explained by increased surface defects in our samples due to smaller crystallite size. The gas sensing performance of the as-prepared polycrystalline R-Fe2O3 nanowires was systematically investigated toward a variety of ?ammable, toxic, and corrosive gases such as ethanol, acetone, gasoline, heptane, formaldehyde, toluene, acetic acid, and ammonia. Figure 8a and b show the sensitivity of R-Fe2O3 nanowires toward ethanol and acetic acid gas at a

Iron Oxide Semiconductor Nanowires for Gas Sensing

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Figure 3. (a) Low magni?cation TEM image of R-Fe2O3 nanowires. (b) High magni?cation TEM image of a single R-Fe2O3 nanowire, illustrating the porous nature of the iron oxide nanowire. (c) HRTEM image of an R-Fe2O3 nanowire. The inset contains the corresponding selected area electron diffraction pattern. (d) Atomic resolution lattice image of the Fe2O3 crystal marked with the circle in c.

Figure 5. TGA and DTA curves of FeNTA precursor nanowires.

Figure 4. Infrared (IR) spectra of (a) NTA and (b) FeNTA precursor nanowires.

working temperature of 150 °C and 30% relative humidity (RH). For comparison, the gas sensitivities of commercial microcrys-

talline R-Fe2O3 powders (1 ?m) are also presented in Figure 8. The gas sensitivity is de?ned as the ratio of the stationary electrical resistance of the sensor in air (Rair) and in the test gas (Rgas), i.e., S ) Rair/Rgas. The R-Fe2O3 nanowires displayed a much higher sensitivity than that of the microcrystalline R-Fe2O3 powders. We noted that R-Fe2O3 nanowires also exhibited a very impressive sensing response toward ethanol and acetic acid gases, even at the low concentration of 5 ppm. The corresponding real-time response curves are shown as insets in Figure 8a and b. The response time and recovery time (de?ned as the time required to reach 90% of the ?nal equilibrium value) of R-Fe2O3 nanowire based sensors were only 1-3 s. After many cycles between the test gas and fresh air, R-Fe2O3 nanowire sensors can still quickly recover to their initial states, indicating excellent reversibility. The gas sensing curves in Figure 8 clearly indicate a sensing mechanism based on surface chemisorption of gas molecules

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Wang et al.

Figure 6. UV-vis spectra of R-Fe2O3 nanowires.

Figure 8. Gas sensitivities of R-Fe2O3 nanowire based sensors as a function of the gas concentrations of (a) ethanol gas and (b) acetic acid gas. The insets are real-time gas-sensing curves. For comparison, the gas sensing performances of commercial R-Fe2O3 powders are also presented.

Figure 7. Magnetic properties of the as-prepared R-Fe2O3 nanowires. (a) Temperature dependences of the ZFC and FCC magnetic moments under a magnetic ?eld of 500 Oe. (b) Magnetic hysteresis loops at 10 K, 200 K, and 298 K.

together into 1D porous nanostructures, resulting in many more active sites for gas chemisorption. This should contribute to the high gas sensing performance of R-Fe2O3 nanowire sensors. The sensitivity of R-Fe2O3 nanowire sensors toward other gases such as formaldehyde, acetone, gasoline, toluene, ammonia and heptane, are shown in Figure S-3 (Supporting Information). The as-synthesized R-Fe2O3 nanowires always show much higher sensitivity than that of the commercial powder. We noticed that the sensitivities of R-Fe2O3 nanowire sensors toward gasoline, ammonia, heptane, and toluene gases were relatively low, indicating that the R-Fe2O3 nanowire based sensors have a degree of selectivity to different gases. 3. Conclusions In summary, R-Fe2O3 nanowires can be facilely synthesized by using nitrilotriacetic acid (NTA) as a chelating agent to form polymeric chains. This synthesis technique can also be used for the synthesis of other transition metal oxide nanowires, such as MnO2 nanowires and NiO nanowires. The as-prepared R-Fe2O3 nanowires have exhibited a blueshift of 0.26-0.56 eV in bandgap and peculiar magnetic properties. When used as sensors, R-Fe2O3 nanowires have high sensitivities toward ethanol and acetic acid gases. Acknowledgment. Financial support by research grants (DP0559891 and DP0772999) from the Australian Research Council (ARC) is gratefully acknowledged. Supporting Information Available: Schematic of the gas sensing device, SEM images of Mn-NTA and Ni-NTA

and electron donation, resulting in a decrease in the sensor resistance. Fe2O3 is an n-type semiconductor, with the free carriers originating from oxygen vacancies. In the ambient environment, Fe2O3 nanocrystals are expected to adsorb both oxygen and moisture, in which moisture may be adsorbed as hydroxyl groups. The adsorbed O2- and OH- groups trap electrons from the conduction band of the Fe2O3 nanocrystals, inducing the formation of a depletion layer on the surface of the Fe2O3 nanocrystals. When exposed to test gases such as ethanol and acetic acid, gas molecules are chemi-adsorbed at the active sites on the surface of the Fe2O3 nanocrystals. These molecules will be oxidized by the adsorbed oxygen and lattice oxygen (O2-) of Fe2O3 at the sensor working temperature (150 °C). During this oxidation process, electrons will transfer to the surface of the Fe2O3 nanocrystals to lower the number of trapped electrons, inducing a decrease in the resistance. The R-Fe2O3 nanowires consist of tiny nanocrystals (4-5 nm) joined

Iron Oxide Semiconductor Nanowires for Gas Sensing nanowires, and gas sensitivities of R-Fe2O3 nanowires and commercial powders toward (a) formaldehyde, (b) acetone, (c) gasoline, (d) toluene, (e) ammonia, and (f) heptane. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
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