Conference-IWAMN 2009-Synthesis and Gas-Sensing Properties of Hollow Sea Urchin-Like α-Fe 2 O 3 Nanostructure

Hollow sea urchin-like α-Fe2O3 nanostructures has been successfully prepared by a hydrothermal method using Fe(NO3)3·9H2O and Na2SO4 as starting materials. The structure and morphology of α-Fe2O3 were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM). The hollow sea urchin-like α-Fe2O3 nanostructures with the diameters of 2-5 μm consist of well-aligned α-Fe2O3 nanorods growing radically from the centers of the nanostructures. Gas sensing properties of the hollow sea urchin-like α-Fe2O3 nanostructures were measured to ammonia, LPG, ethanol, acetone. The experimental results showed that the α-Fe2O3 nanostructures are promising gas sensing material for the detection of flammable and toxic gases with good-sensing characteristics. [DOI: 10.1380/ejssnt.2011.508]


I. INTRODUCTION
Nowadays, chemical sensors for the detection of these flammable and toxic gases play a very important role in chemical industries, environmental protection, public safety and human health.Metal oxides, such as SnO 2 , Fe 2 O 3 , WO 3 , TiO 2 , function as gas sensitive materials by changing their resistance due to exposure to oxidizing or reducing gases.
Recently, α-Fe 2 O 3 nanostructures have been used to fabricate gas sensors for the detection of acetone, ethanol, hydrogen, ammonia and formaldehyde [13,14].Generally, the properties of gas sensor depend strongly on the microstructure of sensor device, which is characterized by such parameters as oxide grain size, pore size and specific surface area [15].Hence, three-dimensional superstructures assembled with one-dimensional nanorods have attracted much attention because of their unique properties and potential gas sensing application.
In this paper, we report on synthesized of hollow sea urchin-like α-Fe 2 O 3 by a hydrothermal method using Fe(NO 3 ) 3 and Na 2 SO 4 as starting materials.The gassensing properties of the α-Fe 2 O 3 superstructures in detecting ammonia, LPG, ethanol, acetone were investigated.

II. EXPERIMENTAL
The hollow sea urchin-like α-Fe 2 O 3 nanostructures were synthesized by a hydrothermal method.In a typical procedure, 3 ml of 0.5M iron (III) nitrate Fe(NO 3 ) 3 aqueous solution was added to 3 ml of 0.5M sodium sulfate (Na 2 SO 4 ) solution under magnetic stirring.After stirring at room temperature for 30 min, 14ml of deionized water was added under constant stirring to form a homogeneous solution.After being stirred for 3h, the mixed solution was transferred into a teflon-lined stainless steel autoclave of 30ml capacity and treated at 120 • C for 24h.After that, the autoclave was cooled down to room temperature naturally.The product was collected by filtration and washed with deionized water and absolute ethanol several times, finally dried at 100 • C for 6h in air.The as-prepared product was annealed at 600 • C for 2h in air.The obtained powder was characterized by x-ray diffrac- The gas sensing properties of hollow sea urchin-like α-Fe 2 O 3 nanostructures were measured using a static test system.First, the as-prepared α-Fe 2 O 3 nanostructures were mixed with ethanol to form a slurry, the slurry was coated as a thin film on silicon substrate attached with interdigitated platinum electrodes (electrode spacing = 30 µm).After drying at 120 • C for 4h, the sample was heated to 600 • C with rate of 1 • Cmin −1 in air and kept for 2h.The gas sensing properties of α-Fe 2 O 3 nanostructures were carried out at operating temperature in the range of 200-370 • C with ammonia, LPG, ethanol and acetone.The sensor response (S) to a test gas is defined as R a /R g , where R a and R g are the resistance of the sensor in clean air and in the test gas, respectively.The first, α-FeOOH are formed by the chemical reactions [16,17,20]:

III. RESULTS AND DISCUSSION
After calcinations at high temperature, the as-prepared   As is known, an aggregation process involving the formation of larger crystals by greatly reducing the interfacial energy of small primary nanocrystals is energetically favored.However, the interaction between unprotected building units with nanoscale size is generally not com- petent to form stable and uniform microstructures, such as the sphere-like superstructures discussed here.Moreover, the building blocks would a ways randomly aggregate into disordered crystals rather than single crystals in the absence of sufficiently strong surface-protecting layers.Therefore, the presence of Na 2 SO 4 was believed to be crucial for the formation of the unique α-FeOOH microspheres constructed with nanorods.The SO 4 2− ions play an important role in the formation and self-assembly of FeOOH nanorods into sphere-like superstructures.SO 4 2− ions serve as ligand to Fe 3+ , and may adsorb on the facets parallel to the c-axis of FeOOH nuclei by a monodentate structure to obtain α-FeOOH nanorods.The nanorods gradually assemble into 3D urchin-like and spherical congeries because that bidentate (Fe-O-SO 2 -O-Fe) structure is formed between FeOOH nanorods.After annealing at 600 • C in air, the α-FeOOH microspheres changed into α-Fe 2 O 3 nanorods-based hollow microspheres [18,19].
The gas sensing properties of the hollow sea urchin-like α-Fe 2 O 3 nanostructures were measured to various concentrations of ammonia, LPG, ethanol and acetone in air at operating temperature in the range of 200-370 • C. It can be seen from Fig. 3 that α-Fe 2 O 3 material is an ntype semiconductor and the sensor response to LPG gas increases dramatically with the increase in the LPG concentration from 500 ppm to 4000 ppm.The decrease the resistance of sensor based on the reaction of target gas to surface oxygen species such as O 2− , O − , O 2 − on the material and release of free electron to the sensor, led to the change in resistance of α-Fe 2 O 3 sensor.
Figure 4 shows the dependence of sensor responses on operating temperature in the range of 200-370 • C to 2000 ppm of acetone vapor, LPG, alcohol vapor and NH 3 gas in air.It can be seen that sensor response values to acetone increase with a raise of operating temperature and attains a maximum at ∼350 • C, followed by a decrease with a further increase of operating temperature.
For practical use, the sensitivity of the sensor is a necessary consideration.In our experiments, It can be seen clearly from Fig 5 that the sensor exhibits the high response to acetone, LPG and alcohol and low response to NH 3 .Therefore, hollow sea urchin-like α-Fe 2 O 3 nanostructures materials are promising gas sensing material in detecting acetone, LPG or alcohol vapor.

IV. CONCLUSION
The hollow sea urchin-like α-Fe 2 O 3 nanostructures with a diameter in the range of 2-5 µm consist of well-aligned α-Fe 2 O 3 nanorods growing radically from the centers of the nanostructures have been successfully prepared by using hydrothermal method using Fe(NO 3 ) 3 •9H 2 O and Na 2 SO 4 as starting materials.Sensor based on the α-Fe 2 O 3 material shows high sensitivity to LPG, ethanol, acetone and low sensitivity to NH 3 .It indicates that hollow sea urchinlike α-Fe 2 O 3 nanostructures could be promising candidates for the fabrication of gas sensor.

Figure 1
Figure 1 shows typical X-ray diffraction patterns of hematite (α-Fe 2 O 3 ) after annealing at 600 • C for 2h.All the reflections of the XRD pattern can be finely indexed to a hexagonal phase (JCPDS No 01-089-0596) with lattice constants of a = 5.04 Å; b = 5.04 Å; c = 13.77Å and angles α = 90 • ; β = 90 • ; γ = 120 • .No peaks from other phases are found, suggesting high purity of the as-synthesized α-Fe 2 O 3 .This result is the same some papers published [16-18].The morphology and structure of the α-Fe 2 O 3 nanostructures were further studied by SEM, the result is shown in Fig. 2. The spherical morphologies of the products were almost obtained.The hollow sea urchin-like α-Fe 2 O 3 nanostructures with a diameter in the range of 2-5 µm consist of well-aligned α-Fe 2 O 3 nanorods growing radically from the centers of the nanostructures.The growth mechanism of hollow sea urchin-like α-Fe 2 O 3 nanostructures can be described as follows:The first, α-FeOOH are formed by the chemical reactions[16,17,20]:

FIG. 4 :
FIG. 4: Sensor responses of α-Fe2O3 nanorod sensor to 2000 ppm of target gases as correlated with operating temperature.

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FIG. Response plots of α-Fe2O3 nanorod sensor towards ammonia, LPG, ethanol and acetone concentration in the ranges 500-4000 ppm at 350 • C Figure 3 illustrated the typical transient response curve of α-Fe 2 O 3 nanorods to various concentration of tested gases.