Sensors and Actuators B 141 (2009) 381–389 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Controlled synthesis and gas-sensing properties of hollow sea urchin-like ␣-Fe 2 O 3 nanostructures and ␣-Fe 2 O 3 nanocubes Fenghua Zhang, Heqing Yang ∗ , Xiaoli Xie, Li Li, Lihui Zhang, Jie Yu, Hua Zhao, Bin Liu Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, China article info Article history: Received 4 September 2008 Received in revised form 28 June 2009 Accepted 30 June 2009 Available online 9 July 2009 Keywords: ␣-Fe 2 O 3 Hollow sea urchin-like nanostructure Nanocubes Gas sensors abstract Hollow sea urchin-like ␣-Fe 2 O 3 nanostructures were successfully synthesized by a hydrothermal approach using FeCl 3 and Na 2 SO 4 as raw materials, and subsequent annealing in air at 600 ◦ C for 2 h. The hollow sea urchin-like ␣-Fe 2 O 3 nanostructures with the diameters of 2–4.5 ␮m consist of well-aligned ␣-Fe 2 O 3 nanorods with an average length of about 1 ␮m growing radially from the centers of the nanos- tructures, have a hollow interior with a diameter of about 2 ␮m. ␣-Fe 2 O 3 nanocubes with a diameter of 700–900 nm were directly obtained by a hydrothermal reaction of FeCl 3 at 140 ◦ C for 12 h. The response S r (S r = R a /R g ) of the hollow sea urchin-like ␣-Fe 2 O 3 nanostructures reached 2.4, 7.5, 5.9, 14.0 and 7.5 to 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone, and 42ppm ethanol, respectively, which was excess twice that of the ␣-Fe 2 O 3 nanocubes and the nanoparticle aggregations. Our results demonstrated that the hollow sea urchin-like ␣-Fe 2 O 3 nanostructures were very promising for gas sensors for the detection of flammable and/or toxic gases with good-sensing characteristics. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Ammonia and various volatile organic compounds have been released in high quantities into the atmosphere as a result of human activities, and have generated environmental risks. Chemical sen- sors for the detection of these flammable and/or toxic gases play a very important role in chemical industries, environmental pro- tection, public safety and human health. Metal oxides [1–4], such as SnO 2 , ZnO, Fe 2 O 3 and V 2 O 5 , function as gas-sensitive materials by changing their resistance due to exposure to oxidizing or reduc- ing gases. Nevertheless, there are still some critical limitations to be overcome for the commercial sensors based on particulate or thin-film semiconductor metal oxides, such as limited maximum sensitivity, high working temperatures and lack of long-term sta- bility [5]. Recently, nanorods, nanowires and nanoribbons of metal oxides were used to fabricate sensors; the results indicate that one- dimensional (1-D) nanomaterials are promising for highly sensitive chemical sensors [5]. Therefore, the fabrication of the metal oxide nanomaterials with a defined size and shape for gas sensor applica- tion is currently a major focus of nanoscience and nanotechnology. Hematite (␣-Fe 2 O 3 ), the most stable iron oxide with n-type semiconducting properties (E g = 2.1eV) under ambient conditions, is widely used as catalysts, pigments, gas sensors, and electrode materials [3], owing to its low cost, high resistance to corro- ∗ Corresponding author. Fax: +86 29 85307774. E-mail address: hqyang@snnu.edu.cn (H. Yang). sion, and environmentally friendly properties. The previous studies mainly focused on the ␣-Fe 2 O 3 films [6–11] and powders [12,13]. Stimulated by both the promising applications of iron oxides and the novel chemical and physical properties of nanoscale mate- rials, considerable efforts have been made in the synthesis of ␣-Fe 2 O 3 nanostructured materials with different morphologies. Up to now, a variety of ␣-Fe 2 O 3 nanostructured materials in various geometrical morphologies have been successfully fabri- cated, such as nanoparticles [14], nanotubes [15], nanowires [16], nanobelts [16], nanocubes [17], nanorods [18], spindles [19], hollow spheres [20,21], nanoplates [22], nanorings [23], rhombohedra [24] and complex hierarchical structures constructed with nanoscale building blocks [25–30]. In particular, three-dimensional (3-D) superstructures assembled with one-dimensional nanorods have attracted much attention because of their unique properties and potential applications [28–30]. However, to our knowledge, the synthesis of hollow sea urchin-like ␣-Fe 2 O 3 nanostructures has not been reported until now. The hollow nanostructures have widespread potential applications in catalysts, gas sensors, drug delivery, etc., owing to their higher specific surface area and lower density. Recently, the nanorods [3], nanotubes [15], hollow spheres with a mesoporous shell [20], porous nanospheres [21], nanorings [23] and flutelike porous nanorods and hexapod-like nanostructures [31] of ␣-Fe 2 O 3 have been used to fabricate gas sensors for the detection of ethanol, acetone, 92 # gasoline, heptane, hydrogen, formaldehyde, toluene, acetic acid and ammonia. However, they did not investigate the stability of the sensors even though stability 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.06.049 382 F. Zhang et al. / Sensors and Actuators B 141 (2009) 381–389 is one of the most important parameters of sensors. In addition, the sensing properties of hollow sea urchin-like ␣-Fe 2 O 3 nanostruc- tures and ␣-Fe 2 O 3 nanocubes have not been studied until now. In this paper, we report on controlled synthesis of hollow sea urchin-like ␣-Fe 2 O 3 nanostructures and ␣-Fe 2 O 3 nanocubes. Con- trol over the both ␣-Fe 2 O 3 nanostructures were achieved by adding different anions in the Fe 3+ –H 2 O hydrothermal system. To the best of our knowledge, this is the first report of the selective synthesis of ␣-Fe 2 O 3 superstructures and nanocubes. The gas- sensing properties of the ␣-Fe 2 O 3 superstructures, nanocubes and nanoparticle aggregations with irregular morphology in detect- ing ammonia, formaldehyde, triethylamine, acetone, ethanol and hydrogen were studied. The sensitivities of the as-prepared ␣- Fe 2 O 3 superstructures are higher than that of nanocubes and nanoparticle aggregations of ␣-Fe 2 O 3 with irregular morphology. 2. Experimental 2.1. Synthesis Hollow sea urchin-like ␣-Fe 2 O 3 nanostructures: In a typical syn- thesis, 10 mL of 0.1 M sodium sulfate(Na 2 SO 4 ) aqueous solutionwas added to 2 mL of 0.5 M iron chloride (FeCl 3 ) solution under mag- netic stirring. After stirring for 10 min, 8 mL of deionized water was added under constant stirring to form a homogeneous solution. The mixed solution was sealed into a Teflon-lined stainless steel auto- clave of 50 mL capacity and heated at 140 ◦ C for 12 h. After reaction, the autoclave was cooled to room temperature naturally. The yel- low product was isolated by centrifugation, washed with deionized water and absolute ethanol several times, and finally dried in air at room temperature. The as-prepared product was heated to 600 ◦ C witharateof1.0 ◦ Cmin −1 and then was maintained at 600 ◦ Cfor 2 h in air. The red powder was obtained, which was used for further analysis and characterization. ␣-Fe 2 O 3 nanocubes and ␣-Fe 2 O 3 nanoparticle aggregations with irregular morphology: 10 mL of deionized water was employed instead of 0.1 M Na 2 SO 4 aqueous solution, ␣-Fe 2 O 3 nanocubes were directly obtained in the same hydrothermal con- ditions. When Fe(NO 3 ) 3 was used as Fe source instead of FeCl 3 , ␣-Fe 2 O 3 nanoparticle aggregations with irregular morphology were directly obtained in the same hydrothermal conditions. 2.2. Characterization The as-obtained samples were characterized by X-ray diffrac- tion (XRD, Rigaku D/MAX-IIIC X-ray diffractometer with Cu K␣ radiation,  = 1.5406Å), scanning electron microscopy (SEM, FEI Quanta 200, 20kV), and transmission electron microscopy (TEM, JEOL JEM-3010, 300 kV). The samples for TEM were prepared by dispersing ␣-Fe 2 O 3 powders on carbon-coated copper grids. The Brunauer–Emmett–Teller (BET) specific surface area was per- formed by N 2 gas adsorption using a ST-03A surface analytical instrument (Beijing Analysis Instrument Factory, China). 2.3. Gas-sensing properties test Measurements on gas sensitivity were performed with a WS-30A system (Weisheng Instruments Co., Zhengzhou, China). Schematic diagram of the typical gas sensor system is shown in Fig. 1. First, the as-prepared ␣-Fe 2 O 3 nanostructures were mixed with terpineol to form a slurry, the slurry was coated as a thin film on a ceramic tube with a pair of previously printed gold electrodes that were connected by four platinum wires (the outer diameter of the ceramic tube is 1.34 mm, and the distance between the both gold electrodes is 1.40 mm). The thickness of the sensing thin film is about 0.06 mm. After drying at 60 ◦ C in air, the ceramic tube was Fig. 1. Schematic diagrams of the gas sensor measurement system. heated to 600 ◦ Catarateof1 ◦ Cmin −1 in air and kept for 2 h. A Ni–Cr resistor wire was crossed through the ceramic tube as a heater allowing us to control the working temperature by adjusting the heating voltage (V heating ). Then the electrical contact was made through connecting the four platinum (or Ni–Cr resistor) wires with the instrument base by Sn paste. The as-prepared ␣-Fe 2 O 3 sensors were aged at 350 ◦ C for 7 days toimprove the stability of the devices. Finally,a reference resistor was put in series with the sensor to form a complete measurement circuit. Test gases were injected into the 18 L-testing chamber directly by a microinjector and mixed with air immediately. The air was used as a reference gas. In the test pro- cess, the voltage (V output ) across the reference resistor changes with the sensor’s resistance, which responds to the types and concentra- tions of the test gases. Thus, the response of the sensor in clean air or in the test gas can be measured by monitoring V output. Here, the sensor response (S r ) 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. Our gas-sensing measurements were carried out at a working temperature of 350 ◦ C and relative humidity of 5–38%. 3. Results and discussion 3.1. Microstructures of hollow sea urchin-like ˛-Fe 2 O 3 nanostructures and ˛-Fe 2 O 3 nanocubes Fig. 2 shows the XRD patterns of the ␣-FeOOH precursors pre- pared by the hydrothermal reaction of FeCl 3 with Na 2 SO 4 at 140 ◦ C for 12 h and ␣-Fe 2 O 3 products obtained by calcining the precursors at 600 ◦ C in air for 2 h. As can be seen in Fig. 2a, all the diffrac- tion peaks of ␣-FeOOH precursors can be indexed to the pure orthorhombic ␣-FeOOH, which are consistent with the valuesinthe literature (Joint Committee on Powder Diffraction Standards, JCPDS No: 81-0462). All the strong and sharp diffraction peaks shown in Fig. 2b can be indexed to ␣-Fe 2 O 3 with a hexagonal structure, which are consistent with the values in the literature (JCPDS No: F. Zhang et al. / Sensors and Actuators B 141 (2009) 381–389 383 Fig. 2. XRD patterns of(a) the precursors prepared by the hydrothermal process and (b) the products obtained by calcining the precursors at 600 ◦ Cinairfor2h. 33-0664). In addition, no peaks from other phases are found, sug- gesting high purity of the as-synthesized ␣-Fe 2 O 3 . The SEM images of the ␣-FeOOH precursors prepared by the hydrothermal process are shown in Fig. 3a and b. Fig. 3a and b shows that the as-obtained ␣-FeOOH precursors consist of a large quantity of microspheres with typical diameters in the range of 2–4.5 ␮m. The SEM image at high magnifications (inset of Fig. 3b) reveals that the micro- sphere is constructed from one-dimensional nanorods with the diameters of about 150 nm. Fig. 3c and d shows SEM images of ␣-Fe 2 O 3 products obtained by calcining the ␣-FeOOH precursors. As can be seen in Fig. 3c and d, when ␣-FeOOH precursors were decomposed to form ␣-Fe 2 O 3 , the spherical morphologies of the products were almost maintained. Interestingly, the solid micro- spheres became into the hollow ones. The hollow sea urchin-like nanostructures with a diameter of about 2␮m are built from a sin- gle layer of radially oriented nanorods with a diameter of about 120 nm,self-wrapping to form hollow interior(inset ofFig. 3d). TEM is employed to study the structural characteristics of the hollow sea urchin-like ␣-Fe 2 O 3 nanostructures in details, and the results are given in Fig. 4. Fig. 4a shows a typical TEM image of an individual ␣-Fe 2 O 3 superstructure. The central portion of the superstructure is lighter than that of the edge, further confirming the hollow inte- riors of the unique self-wrapped nanorod arrays. Interestingly, an important phenomenon is found in the TEM observations: a sheaf of tiny nanorods with diameters of about 15nm are attached side- by-side into the external sharp end of the constituent “mother” nanorods, as shown in Fig. 4b. The high-resolution transmission electron microscopy (HRTEM) image of the constituent nanorod is displayed in Fig. 4c. We observed that the constituent nanorod is assembled fromnanorodswithdiametersof13–20 nm inFig.4c,the measured lattice spacings of 0.25, 0.37 and 0.22 nm are consistent with the d values of the (1 1 0), (0 1 2) and (1 1 3) planes of ␣-Fe 2 O 3 with a hexagonal structure, respectively. These results suggest that hollow sea urchin-like ␣-Fe 2 O 3 nanostructures constructed with ␣- Fe 2 O 3 nanorods can be fabricated by the hydrothermal reaction of FeCl 3 and Na 2 SO 4 and subsequent annealing. In addition, we can clearly see some brighter areas on the HRTEM image of Fig. 4c, which indicates that there are some pits on the nanorod surfaces. The diameter of these pits is 5–9 nm. The presence of the pits may be due to the decomposition of ␣-FeOOH and release of H 2 O. Fig. 5a and b show SEM image and XRD pattern of the prod- ucts obtained via a hydrothermal reaction of FeCl 3 with H 2 Oat 140 ◦ C for 12 h. The SEM and XRD results indicated that FeCl 3 reacts with H 2 O in the absence of Na 2 SO 4 to form monodispersive Fig. 3. Typical SEM images of (a and b) ␣-FeOOH precursors synthesized by the hydrothermal process and (c and d) ␣-Fe 2 O 3 products prepared by calcining of ␣-FeOOH precursors at 600 ◦ Cinairfor2h. 384 F. Zhang et al. / Sensors and Actuators B 141 (2009) 381–389 Fig. 4. (a and b) Typical TEM and (c) HRTEM images of the hollow sea urchin-like ␣-Fe 2 O 3 nanostructures. ␣-Fe 2 O 3 nanocubes with a hexagonal structure (JCPDS No: 33- 0664) instead of sea urchin-like ␣-FeOOH nanostructures. The edges of the nanocubes are 700–900 nm. Fig. 5c and d displays SEM image and XRD pattern of the products obtained via a hydrothermal reaction of Fe(NO 3 ) 3 with H 2 Oat140 ◦ C for 12 h. The SEM image and XRD pattern reveal that the as-synthesized products consist of ␣- Fe 2 O 3 with a hexagonalstructure(JCPDSNo:33-0664) nanoparticle aggregations with irregular morphology. It is well known that an aggregation process involving the for- mation of larger crystals by greatly reducing the interfacial energy of small primary nanocrystals is energetically favored. However,the interactionbetween unprotected building units with nanoscale size is generally not competent to form stable and uniform microstruc- tures [29], such as the sea urchin-like ␣-FeOOH nanostructures and ␣-Fe 2 O 3 nanocubes discussed here. According to Korchef et al. [32], Xie et al. [33] and Sun et al. [34], in the presence of SO 4 2− ions the iron precipitates obtained from aqueous solution are ␣-FeOOH instead of ␣-Fe 2 O 3 . The SO 4 2− ions play an impor- tant role in the formation and self-assembly of ␣-FeOOH nanorods into sea urchin-like nanostructures. They serve as ligand to Fe 3+ , and may adsorb on the facets parallel to the c-axis of ␣-FeOOH nuclei by a monodentate structure (Fe–O–SO 3 ) to obtain ␣-FeOOH nanorods [34]. These nanorods gradually assemble into 3-D urchin- like congeries because that bidentate (Fe–O–SO 2 –O–Fe) structure is formed between FeOOH nanorods [34]. The ␣-FeOOH nanos- tructures changed into hollow urchin-like ␣-Fe 2 O 3 nanostructures by calcining in air. The presence of Cl − ions was believed to be crucial for the formation of Fe 2 O 3 nanocubes [34]. The primary ␣- Fe 2 O 3 nanocrystals with a hexagonal structure were capped with Cl − ions acting as ligand. According to Zheng et al. [17], the den- sity of the iron atom on the low-index crystal planes of {110}, {111}, and {100} of␣-Fe 2 O 3 with a hexagonal structureis10.1, 7.9, and 4.1atoms/nm 2 , respectively.The {110} planes with a relatively higher density of iron atom adsorb more Cl − ions, thus, they would grow slower during the oriented attachment of primary nanocrys- tals than the other planes, tending to form nanocubes enclosed by {110} exposure planes [17].However,NO 3 − ions do not serve as ligand to Fe 3+ , primary ␣-Fe 2 O 3 nanocrystals randomly aggregate into aggregations with irregular morphology. 3.2. Gas-sensing properties The transient response characteristics towards ammonia, formaldehyde, triethylamine, acetone, ethanol and hydrogen of the sensors based on the hollow sea urchin-like nanostructures, nanocubes and nanoparticle aggregations were investigated at F. Zhang et al. / Sensors and Actuators B 141 (2009) 381–389 385 Fig. 5. (a) SEM image and (b) XRD pattern of the products synthesized via a hydrothermal reaction of FeCl 3 at 140 ◦ C for 12h, (c) SEM image and (d) XRD pattern of the products prepared by a hydrothermal reaction of Fe(NO 3 ) 3 at 140 ◦ C for 12 h. 350 ◦ C and relative humidity of 5–18%, and the results are dis- played in Fig. 6. Fig. 6a–f shows typical response curves on cycling between increasing concentration of ammonia, formalde- hyde, triethylamine, acetone, ethanol and hydrogen and ambient air, respectively.It can be seen that V output values increased abruptly with the injection of ammonia, formaldehyde, triethylamine, ace- tone or ethanol then decreased rapidly and recovered their initial value after the test gas was released. In particular, the change of V output values for the sensor based on hollow sea urchin-like ␣-Fe 2 O 3 nanostructures is the sharpest. However, the V output val- ues hardly change with the injection and release of hydrogen. From Ohm’s law, the electric resistance of the sensors accord- ingly underwent a decreasing and increasing process when the test gas was injected and released, respectively, which is consis- tent with the sensing behavior of n-type semiconductor sensors. After many cycles between the test gas and clean air, the voltage of the reference resistor and the resistance of the sensor could recover their initial states, which indicates that the sensors have good reversibility. The response time (defined as the time required to reach 90% of the final equilibrium value) of the hollow sea urchin-like nanostructure-based sensor is 5–8, 17–50, 8–18, 5–10 and 7–21 s towards ammonia, formaldehyde, triethylamine, ace- tone and ethanol, respectively. The recovery time (taken as the time necessary for the sensor to attain a conductance 10% above the original value in air) is 10–21, 12–20, 10–20, 9–20 and 11–14s, respectively. Fig. 7a shows the response of the sensors based on the hol- low sea urchin-like nanostructures, nanocubes and nanoparticle aggregations to 56ppm ammonia, 32ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone and 42 ppm ethanol at 350 ◦ C. The responses of the ␣-Fe 2 O 3 superstructures are about twice, five- fold, twice, twice and three-fold higher than that of the ␣-Fe 2 O 3 nanocubes, twice, six-fold, twice, twice and four-fold higher than that of the ␣-Fe 2 O 3 nanoparticle aggregations, respectively. The results indicate that the response performance of the hollow sea urchin-like nanostructures is better than that of the nanocubes and nanoparticle aggregations, and the performance of nanoparticle aggregations is the worst, whichever gas was tested. The response of the ␣-Fe 2 O 3 gas sensors can be empirically represented as R =1+A g (P g ) ˇ , where P g is the target gas partial pressure, which is in direct proportion to the gas concentration, A g is a prefactor, and ˇ is the exponent on P g . Generally, ˇ has an ideal value of either 0.5 or 1, which is derived from surface interac- tion between chemisorbed oxygen and reducing gas to the n-type semiconductors [35]. So, logarithm of the response should be lin- ear with logarithm of gas concentration. Fig. 7b–f displays chart of the logarithm of response of the three kinds of sensors versus the logarithm of gas concentration. The linear equations and correla- tion factor, R, were given in Table 1. The value of ˇ for the hollow sea urchin-like nanostructures towards ammonia, formaldehyde, triethylamine, acetone and ethanol, is about 0.28, 0.44, 0.69, 0.72 and 0.75, respectively, determined by the fit using the empirical formula. The deviation of the ˇ may be due to the disorder and some insensitive area (vacancy between the hollow microspheres) existing in the sensors [35]. The lowest detection limit for ammo- nia, formaldehyde, triethylamine, acetone and ethanol is about 17, 2, 2, 3 and 5 ppm, respectively. The response of the sensor based on ␣-Fe 2 O 3 superstructures towards 560 ppm ammonia, 160 ppm formaldehyde, 90 ppm tri- ethylamine, 170 ppm acetone and 210 ppm ethanol at different working temperatures were studied, and the results are given in Fig. 8. It can be seen that the responses of the sensor to these gases 386 F. Zhang et al. / Sensors and Actuators B 141 (2009) 381–389 Fig. 6. Typical transient response curves of the sensors based on (I) the hollow sea urchin-like ␣-Fe 2 O 3 nanostructures, (II) nanocubes and (III) nanoparticle aggregations to (a) ammonia, (b) formaldehyde, (c) triethylamine, (d) acetone, (e) ethanol and (f) hydrogen of different concentrations at 350 ◦ C and relative humidity of 5–18%. Table 1 The linear equations and correlation factor for the chart of the logarithm of response of the three kinds of sensors versus the logarithm of gas concentration. Sensor Hollow sea urchin-like nanostructures Nanocubes Nanoparticle aggregations Gas Ammonia y =0.2803x − 0.1183 y =0.0959x − 0.0107 y =0.0336x +0.0373 R =0.9930 R =0.9783 R =0.9894 Formaldehyde y =0.4379x +0.1440 y = 0.2311x − 0.1416 y =0.2509x − 0.2971 R =0.9927 R =0.9526 R =0.9823 Triethylamine y =0.6753x − 0.1765 y =0.4258x − 0.0829 y =0.3175x +0.0083 R =0.9914 R =0.9915 R =0.9703 Acetone y =0.7156x − 0.4811 y =0.3319x − 0.2199 y =0.3282x − 0.2461 R =0.9989 R =0.9966 R =0.9984 Ethanol y =0.5916x − 0.0789 y =0.2823x +0.0106 y =0.225x − 0.1158 R =0.9990 R =0.9877 R =0.9698 F. Zhang et al. / Sensors and Actuators B 141 (2009) 381–389 387 Fig. 7. (a) The response comparison between the three kinds of sensors; (b–f) the logarithm of response of hollow sea urchin-like ␣-Fe 2 O 3 nanostructures (--), nanocubes (--) and nanoparticle aggregations (-᭹-) sensors versus the logarithm of ammonia, formaldehyde, triethylamine, acetone and ethanol concentration at 350 ◦ C and relative humidity of 5–18%. increase with an increase in the working temperature, and reaches to maximum at 350 ◦ C. It is possibly related to the chemical reac- tion kinetics between gas molecules and oxygen ions adsorbed on the surface of the ␣-Fe 2 O 3 superstructures. The chemical reaction rate is lower at lower temperature, leading to a lower response of the sensor. Stability is also one of the most important characteristics for the sensors. To investigate the time stability of the hollow sea urchin- like ␣-Fe 2 O 3 nanostructure-based sensor, the sensor was stored in air and kept working at350 ◦ C for subsequent sensingproperty tests after the first measurement. A series of tests were carried out at the times of 1, 5, 6, 7, 17 and 47 days after the sensor fabrication and aging for 7 days, with a 42 ppm ethanol concentration at a working temperature of 350 ◦ C and relative humidity of 5–18%. The chart of the sensor response versus the storing time is shown in Fig. 9.It was found that the response of the sensor to ethanol is lower during the initial 5 days and reached subsequently a nearly constant value, showing thatthesensorexhibited good long-term stability afterthe sensor storing for 6 days in air. It is generally accepted that for metal oxide-based sensors the change in resistance is mainly caused by the adsorption and des- orption of gas molecules on the surface of the sensing structure. To understand the origin of the difference in the sensing perfor- mance of the three kinds of sensors, the BET surface area of the nanocubes, hollow sea urchin-like nanostructures and nanoparti- cle aggregations of ␣-Fe 2 O 3 was measured. It was found that their BET surface area is 3.63, 18.8 and 28.0 m 2 g −1 , respectively. Further- more, the sensor response is also determined by the quantity of active sites on the surfaces of ␣-Fe 2 O 3 gas sensors. Fig. 10 shows the XRD patterns of the nanocubes, hollow sea urchin-like nanostruc- tures and nanoparticle aggregations of ␣-Fe 2 O 3 . The XRD indicates that their crystallinity reduces in file. Although surface area of the ␣-Fe 2 O 3 nanoparticle aggregations is the maximal, their crys- tallinity is the worst. According to Li et al. [36] and Hyodo et al. [37], 388 F. Zhang et al. / Sensors and Actuators B 141 (2009) 381–389 Fig. 8. Response of the sensor based on the as-prepared hollow sea urchin-like ␣- Fe 2 O 3 nanostructures to ammonia (560 ppm, --), formaldehyde (160 ppm, --), acetone (170 ppm, --), triethylamine (90 ppm, --) and ethanol (210ppm, -᭹-) at different temperatures and relative humidity of 23–38%. Fig. 9. Variations in response of the sensor based on the hollow sea urchin-like ␣- Fe 2 O 3 nanostructures to 42ppm ethanol at 350 ◦ C and relative humidity of 5–18% after storage in air for different time periods. Fig. 10. XRD patterns of (a) nanoparticle aggregations, (b) hollow sea urchin-like nanostructures and (c) nanocubes of the ␣-Fe 2 O 3 . grain-boundaries or grain-junctions are considered as the active sites and they act positively on the sensor response, whereas sec- ondary grains, in which many grain-boundaries have disappeared during their formation, act negatively on the sensor response. The ␣-Fe 2 O 3 nanoparticle aggregations have serious agglomeration, which was typical large secondary grains with a fewer amount of grain-boundaries. So the gas response of the sensor based on ␣-Fe 2 O 3 nanoparticle aggregations is the lowest. The amount of grain-boundaries on the surface of ␣-Fe 2 O 3 nanocubes is the most due to its best crystallinity, thus its response is higher than that of ␣-Fe 2 O 3 nanoparticle aggregations. The as-synthesized ␣-Fe 2 O 3 superstructures with a large specific area are composed of many small well-aligned nanorods, and many nanorods/nanorods grain- boundaries could be formed. The hollow interiors and interspaces between the nanorods can facilitate the diffusion of the test gas and improve the kinetics of both the reaction of the test gas with surface-adsorbed oxygen and the replacement of the latter fromthe gas phase [21]. In addition, SO 4 2− ions on the surface of ␣-Fe 2 O 3 nanorods in the superstructures may contribute to the enhance- ment of gas sensitivity [38]. Therefore, the sensor based on the ␣-Fe 2 O 3 superstructures exhibits excellent sensing performances in detecting ammonia, formaldehyde, triethylamine, acetone and ethanol. 4. Conclusions In summary, hollow sea urchin-like ␣-Fe 2 O 3 nanostructures have been successfully fabricated via the hydrothermal reaction of FeCl 3 and Na 2 SO 4 and subsequent annealing in air. Sensor based on the ␣-Fe 2 O 3 superstructures shows high gas-sensing responses, short response and recovery time and long-term stability in detect- ing ammonia, formaldehyde, triethylamine, acetone and ethanol, indicating that these hollow sea urchin-like ␣-Fe 2 O 3 nanostruc- tures could be promising candidates as the building blocks for the fabrication of gas sensors for the detection of ammonia and various flammable and/or toxic volatile organic compounds in air. 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Heqing Yang received his PhD degree from Xi’an Jiaotong University in 1999. He did his post-doctorial research in Fudan University for two years from 2000 to 2002. Now, he is doing research in the School of Chemistry and Materials Science, Shaanxi Normal University. He is currently involved in research on using nanostructured materials as gas sensors, catalysts and biosensors. Xiaoli Xie received her MS degree in physical chemistry in 2008 from Shaanxi Normal University, Xi’an, China. Li Li received her MS degree in inorganic chemistry in 2002 from Northwest Uni- versity, China. She is now a PhD student at the School of Chemistry and Materials Science under the supervision of prof. Heqing Yang. Lihui Zhang received her MS degree from the School of Chemistry and Materials Science, Shaanxi Normal University, in 2004. She is now a PhD student at the School of Chemistry and Materials Science under the supervision of prof. Heqing Yang. Jie Yu was born in 1982, and he is now a master student at the School of Chemistry and Materials Science under the supervision of prof. Heqing Yang. Hua Zhao was born in 1982, and he is now a master student at the School of Chem- istry and Materials Science under the supervision of prof. Heqing Yang. Bin Liu received his MS degree from the School of Chemistry and Materials Sci- ence, Shaanxi Normal University, in 2004. He is now a PhD student at the School of Chemistry and Materials Science under the supervision of prof. Heqing Yang. . Sensors and Actuators B 141 (2009) 381–389 is one of the most important parameters of sensors. In addition, the sensing properties of hollow sea urchin- like ␣-Fe 2 O 3 nanostruc- tures and ␣-Fe 2 O 3 nanocubes. 2009 Keywords: ␣-Fe 2 O 3 Hollow sea urchin- like nanostructure Nanocubes Gas sensors abstract Hollow sea urchin- like ␣-Fe 2 O 3 nanostructures were successfully synthesized by a hydrothermal approach using FeCl 3 and. the best of our knowledge, this is the first report of the selective synthesis of ␣-Fe 2 O 3 superstructures and nanocubes. The gas- sensing properties of the ␣-Fe 2 O 3 superstructures, nanocubes and nanoparticle