Facile Synthesis of Porous α-Fe2O3 Nanorods and Their Application

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J. Phys. Chem. C 2008, 112, 17804–17808

Facile Synthesis of Porous r-Fe2O3 Nanorods and Their Application in Ethanol Sensors Yan Wang, Jianliang Cao, Shurong Wang, Xianzhi Guo, Jun Zhang, Huijuan Xia, Shoumin Zhang, and Shihua Wu* College of Chemistry, Nankai UniVersity, Tianjin 300071, P.R. China ReceiVed: June 2, 2008; ReVised Manuscript ReceiVed: September 12, 2008

A facile solution approach was employed to synthesize R-FeOOH nanorods by using FeSO4 · 7H2O and CH3COONa without templates at low temperature (40 °C). The porous R-Fe2O3 nanorods were successfully obtained by calcining the R-FeOOH precursors at 300 °C for 2 h. The as-prepared products were characterized by thermogravimetry-differential thermal analysis, X-ray powder diffraction, transmission electron microscopy (TEM), high-resolution TEM, and N2 adsorption-desorption analysis techniques. The as-prepared porous R-Fe2O3 nanorods have a tiny crystal size (5 nm) and a pore size distribution of 1-10 nm, resulting in a high specific surface area of 221.9 m2 · g-1. A possible growth mechanism of the porous R-Fe2O3 nanorods was proposed. The gas-sensing measurement results demonstrated that the porous R-Fe2O3 nanorods presented a much higher response than the R-Fe2O3 nanoparticles and showed excellent selectivity and stability to ethanol vapor. Due to the fact that it has exciting gas-sensing properties and can be obtained easily, the as-prepared porous R-Fe2O3 nanorod would be an ideal candidate for application in ethanol sensors. 1. Introduction In recent years, the fabrication of nanostructure materials with a desired size, morphology, and porosity has received steadily growing interest owing to their special electrical, optical, magnetic, and physicochemical properties that are superior to those bulk materials.1-4 Currently, one-dimensional (1-D) nanostructures, such as nanorods, nanowires, nanobelts, and nanotubes, have become the focus of intensive research not only for their peculiar properties but also for many potential applications in catalysis, electronics, photonics, drug delivery, medical diagnostics, sensors, and magnetic materials.5-8 Hematite (R-Fe2O3) is the most stable iron oxide with n-type semiconducting properties (Eg ) 2.2 eV) under ambient conditions. It has been intensively investigated because of its wide applications in catalysts, pigments, magnetic materials, gas sensors, and lithium ion batteries.9-15 For its excellent properties, much attention has been directed to the controlled synthesis of one-dimensional (1-D) R-Fe2O3, such as nanospindles,16,17 nanofibers,18,19 nanorods,20,21 nanowires,22,23 nanobelts,24 and nanotubes25,26 by a variety of techniques and methods. Wang et al. prepared R-Fe2O3 nanobelts and nanowires via a gas-solid reaction process under 700 and 800 °C.20 Mann et al. synthesized R-Fe2O3 nanotubes by using biomacromolecules as templates.27 Yi-Xie et al. and Bo-Tong et al. prepared R-Fe2O3 nanorods through a hydrothermal process at 120 and 100 °C, respectively.28,29 The preparation of R-Fe2O3 nanotubes with alumina membranes as the substrates was also employed by many researchers.30-33 However, the gas-solid reaction usually requires special equipment and high temperatures, the methods employing templates or substrates often suffer from disadvantages related to the high cost and the removal of impurities, and the hydrothermal process usually needs tedious reaction times. It is still a challenge to develop simple, low-cost, and environmentally friendly approaches for the synthesis of 1-D structural R-Fe2O3. * Corresponding author. Phone: +86 22 2350 5896. Fax: +86 22 2350 2458. E-mail: [email protected].

Recently, the concern over environmental protection and increasing demands to monitor hazardous gases in industry and the home has attracted considerable attention to developing gas sensors for various polluting and toxic gases. Due to its low cost, good stability, and reversibility, R-Fe2O3 has been proved to be an important semiconductor gas sensor. The gas sensors based on R-Fe2O3 nanoparticles have been widely investigated by many researchers in the past decades.11,12 However, so far, there are only a few reports on the gas-sensing properties of 1-D nanostructural R-Fe2O3. Generally, the properties of a gas sensor are strongly dependent on its surface area. The relatively low ratio of surface to volume of the conventional bulk R-Fe2O3 materials leads to their poor gas-sensing properties. Hence, developing the 1-D nanostructure R-Fe2O3 with high surface area is very important for increasing their applications on gas sensors. Herein, we report a facile route for the preparation of porous R-Fe2O3 nanorods without any templates via a low-temperature (40 °C) solution approach. First, the precursor of R-FeOOH nanorods was prepared by using FeSO4 · 7H2O as the iron source material in the presence of CH3COONa in an aqueous solution. The CH3COONa was used as a source of hydroxide ions during the hydrolysis of iron salts to form iron oxyhydroxide (FeOOH). Then the porous R-Fe2O3 nanorods were obtained by the calcination of as-prepared R-FeOOH at 300 °C for 2 h. The as-obtained porous R-Fe2O3 nanorods have a tiny crystal size (5 nm) and a high surface area (221.9 m2 · g-1). The gas-sensing properties of the sensor based on the porous R-Fe2O3 nanorods to ethanol were systematically investigated. Meanwhile, the gassensing properties of the porous R-Fe2O3 nanorods were compared with those of R-Fe2O3 nanoparticles. 2. Experimental Section. All chemicals were of reagent grade and used without further purification. In a typical synthesis procedure of the R-FeOOH nanorods, 2.78 g of FeSO4 · 7H2O and 3.28 g of CH3COONa were dissolved in 50 mL of deionized water under magnetic stirring. After stirring vigorously for a period at 40 °C, a yellow slurry was formed. The products were collected and washed with

10.1021/jp806430f CCC: $40.75  2008 American Chemical Society Published on Web 10/23/2008

Porous R-Fe2O3 Nanorods distilled water several times by vacuum extraction filtering with two sheets of medium speed qualitative filter paper (pore diameter 30-50 µm) and then dried at 40 °C under vacuum for 2 h. The porous R-Fe2O3 nanorods were obtained by calcining the as-prepared R-FeOOH nanorods precursor at 300 °C for 2 h in air. The color of the samples changed from yellow to red. The whole preparation process for the porous R-Fe2O3 nanorods can be finished in no more than 6 h. The short production process would be helpful for the large-scale industrial manufacture of porous R-Fe2O3 nanorods. Thermogravimetry-differential thermal analysis (TG-DTA) of the as-prepared R-FeOOH precursor was conducted on a ZRY2P thermal analyzer. Ten milligrams of an R-FeOOH sample was heated from room temperature to 600 °C in air at a heating rate of 10 °C min-1. X-ray diffraction (XRD) analysis was performed on a D/MAX-RAX diffractometer with Cu KR radiation (λ ) 0.154 18 nm) operating at 40 kV and 100 mA. Diffraction peaks of crystalline phases were compared with those of standard compounds reported in the JCPDS data file. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis were carried out on a Philips-T20ST electron microscope operating at 200 kV. N2 adsorption-desorption isotherms were collected at liquid nitrogen temperature using a Quantachrome Nova 2000e sorption analyzer. The pore diameter and the pore size distributions were determined by the Barret-Joyner-Halenda (BJH) method. The specific surface areas (SBET) of the samples were calculated following the multipoint Brunauer-EmmettTeller (BET) procedure. The gas-sensing performance was systematically investigated by a HW-30A gas-sensing measurement system (Henan Hanwei Electronical Technology Co., Ltd.). The fabrication and testing principle of the gas sensor are similar to that described in our previous reports.34,35 The porous R-Fe2O3 nanorod samples were mixed with terpineol to form a paste and then coated onto the outside surface of an alumina tube 4 mm in length. The thickness of the coated sensing layer is around 50 µm. A small Ni-Cr alloy coil was placed through the tube to supply the operating temperatures from 100 to 500 °C. Electrical contacts were made with two platinum wires attached to each gold electrode. To improve their stability and repeatability, the gas sensors were sintered at 300 °C for 10 days in air. Here, the sensing properties of the gas sensors were measured under a steady-state condition in a chamber with a volume of 15 L at a working temperature of 250 °C and 40% relative humidity (RH). An appropriate amount of ethanol vapor was injected into the closed chamber by a microinjector, and the sensor was exposed to air again by opening the chamber when the test was completed. 3. Results and Discussion TG-DTA measurement was performed to study the conversion process of the as-prepared R-FeOOH during calcination in air, and the result is shown in Figure 1. From the TG curve of Figure 1, it can be seen that the total weight loss is about 12%, which is a little larger than the theoretical value (10.1%), indicating that about 2% adsorbed water is present in the asprepared R-FeOOH. The abrupt weight loss (about 10.5%) that occurred at the temperature range of 250-300 °C is attributed to the decomposition of R-FeOOH precursors. Correspondingly, there are an endothermic peak and an exothermic peak on the DTA curve which may be ascribed to the removal of the structural water molecules and the crystallization process of R-Fe2O3, respectively. Above 300 °C, the weight of the precursor no longer changes, which indicates that the stable residue

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Figure 1. TG-DTA curves of as-prepared FeOOH nanorods.

Figure 2. XRD patterns of (a) R-FeOOH and (b) R-Fe2O3 nanorods.

can reasonably be ascribed to the pure R-Fe2O3 phase. This result can also be confirmed by the following XRD analysis results. As expected, porous R-Fe2O3 nanorods have been prepared by the calcination of the as-prepared R-FeOOH precursors at 300 °C in air. Figure 2 shows the XRD patterns of the samples. The deflection peaks of the as-prepared precursor (Figure 2a) can be perfectly assigned to the standard value of the R-FeOOH phase (JCPDS No. 29-0713). When the R-FeOOH precursor were calcined in air at 300 °C for 2 h, all the deflection peaks of the product (Figure 2b) were in agreement with the standard data of R-Fe2O3 (JCPDS No. 33-0664). No characteristic peaks are observed for impurities such as γ-Fe2O3 and Fe3O4, indicating that the R-FeOOH precursor was completely transformed into hematite at 300 °C, which is also consistent with the results of TG-DTA.

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Wang et al.

Figure 4. Schematic diagram of the formation mechanism of the porous R-Fe2O3 nanorods.

Figure 3. (a, b) TEM and HRTEM images of R-FeOOH nanorods. (c, d) TEM and HRTEM images of porous R-Fe2O3 nanorods. (e) The corresponding ED pattern and (f) HRTEM image of a single R-Fe2O3 nanorod.

The morphologies of the as-prepared R-FeOOH and R-Fe2O3 nanorods were further investigated by TEM and HRTEM. Figure 3a and b shows the representative TEM and HRTEM micrographs of the as-prepared R-FeOOH sample, respectively. The images clearly demonstrate that the sample has a smooth, rodlike morphology with average diameter of about 10-15 nm and a length of about 200 nm. After being calcined at 300 °C for 2 h, the sample still maintains the rodlike 1-D morphology, as is shown in Figure 3c and d. Compared with the smooth surface of the as-prepared R-FeOOH nanorods, it is interestingly found that the calcined sample possesses a pore structure. These pores are 1-10 nm in size, open to the outer surface, and almost isolated from each other. The formation of the pores may be due to the removal of H2O from the as-prepared R-FeOOH nanorods during the calcination process. Some shorter rods were also found in Figure 3c, which may be the rudiments of the nanorods or the broken ones destroyed by high temperature. Figure 3e presents the corresponding electron diffraction pattern of 300 °C calcined products (R-Fe2O3 nanorods); the shape diffraction ring indicates the product is highly crystallized. The HRTEM image of a typical R-Fe2O3 nanorod (Figure 3f) shows regular lattice fringes with a spacing of 0.37 nm, which corresponds to the (012) plane of R-Fe2O3. To investigate the formation processes of the R-FeOOH nanorods and the porous R-Fe2O3 nanorods, time-dependent experiments were carried out, and the resultant products were investigated by TEM (see Figure S1 in the Supporting Information). At a shorter reaction time of only 5 min, there are almost no nanorods formed, and the average diameter of the nanoparticles is about 5 nm. As the reaction time increased to 10 min, part of the nanoparticles began to combine with each other, and the rodlike structure appeared. Upon prolonging the reaction time to 30 min, the products were totally transformed to rodlike nanostructures. If the reaction time was further increased to 2 h, as seen in Figure 3, well-structured nanorods were obtained, and the length of the nanorods increased with the increase in the reaction time. On the basis of the above results, a growth mechanism of the porous R-Fe2O3 nanorods can be proposed. The schematic diagram of the process is described in Figure 4. In the first stage, the R-FeOOH crystal nucleus formed by the reaction of Fe2+ with O2 and OH- produced by the hydrolysis of CH3COO-. Then these R-FeOOH particles further assembled into rodlike structures by combining together with OH groups.

Figure 5. N2 adsorption-desorption isotherm and BJH pore-size distribution plot (inset) of porous R-Fe2O3 nanorods.

Finally, the porous R-Fe2O3 nanorods formed with the removal of H2O after being calcined at 300 °C in air. The equations of the reactions in the synthetic process are as follows:

CH3COO-+H2O f CH3COOH + OH-

(1)

4Fe2++8OH-+O2 f 4FeOOH + 2H2O

(2)

2FeOOH f Fe2O3+H2O

(3)

The porosity of the porous R-Fe2O3 nanorods was further confirmed by nitrogen adsorption-desorption analysis, and the results are shown in Figure 5. The isotherm indicates that the R-Fe2O3 nanorods have a porosity of type IV with a distinct hysteresis loop in the range of 0.5-1.0 P/P0.36 The curve of pore size distribution of the porous R-Fe2O3 nanorods is shown in the inset figure. The curve exhibits that the sample has relatively small pores with a size distribution of 1-10 nm and centered at 2 nm. This is in good agreement with the TEM images. Calculated by multipoint the BET method, the porous R-Fe2O3 nanorods have a high surface area of 221.9 m2 · g-1, whereas, the surface area of the R-Fe2O3 nanoparticles is only 18.31 m2 · g-1, which is reported in our previous work.37 The high surface area of the porous R-Fe2O3 nanorods may be attributed to their tiny crystal size and porosity structure. Prompted by the high specific surface area, we forecast that the sensor based on the as-prepared porous R-Fe2O3 nanorods should have enhanced gas sensitivity. As an n-type semiconductor, one of the most important applications of R-Fe2O3 material is in gas sensors. It has been reported by many researchers that an R-Fe2O3 sensor exhibits an excellent gas-sensing property to some combustible or toxic gases.38-42 It is generally accepted that the sensing mechanism of the R-Fe2O3-based sensor belongs to the surface-controlled type. The gas-sensing properties of an R-Fe2O3-based sensor are coherent with its surface area. The higher the surface area the sensor has, the more test gas and oxygen molecules it adsorbs, and the better sensitivity it exhibits. Therefore, the design of sensing materials with a high specific surface area should be useful for enhanced gas-sensing performance. In addition, it has been demonstrated that a decrease in the size of

Porous R-Fe2O3 Nanorods

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Figure 6. Responses of porous R-Fe2O3 nanorods and R-Fe2O3 nanoparticles to ethanol of different concentrations.

TABLE 1: Sensitivities of the Two Sensors to Ethanol of Different Concentration sample

SBET (m2/g)

S to ethanol (ppm) 50

100

200

500

1000

porous R-Fe2O3 nanorods 221.9 43.6 60.7 82.8 127.3 174.9 R-Fe2O3 nanoparticles 18.31 1.9 2.2 2.9 4.8 11.8

the crystallites in the sensing layer can result in a considerable increase in sensitivity.43 Thus, the as-prepared porous R-Fe2O3 nanorods, which possess a tiny particle size (5 nm) and a high surface area (221.9 m2 · g-1), are expected to have a good gassensing performance. Figure 6 illustrates the typical response-recovery characteristics of the porous R-Fe2O3 nanorods to ethanol vapor with concentrations of 50, 100, 200, 500, and 1000 ppm. The sensing properties of R-Fe2O3 nanoparticles with an average particle size of about 30 nm and a surface area of 18.31 m2/g, reported in our previous work,37 is also shown in Figure 6 for comparison purposes. It can be seen from Figure 6 that the response of the sensor based on the porous R-Fe2O3 nanorods increases dramatically with the increase in the ethanol vapor concentration and is much higher than that of the R-Fe2O3 nanoparticles under the same ethanol concentration. This result indicates that the gas-sensing property of the as-prepared porous R-Fe2O3 nanorods is much better than that of the previously reported R-Fe2O3 nanoparticles. A comparison study between the nanorods and the nanoparticles in sensitivities to ethanol of different concentrations is shown in Table 1. From Table 1, we can see that the sensitivities of the porous R-Fe2O3 nanorods are almost several decade times greater than that of R-Fe2O3 nanoparticles for all the ethanol vapor of different concentrations. The gas sensitivity is defined as the resistance ratio Rair/Rgas, where Rair and Rgas are the electrical resistance for the sensor in air and in gas. When the sensor is in air, the surface of R-Fe2O3 is covered by plenty of oxygen adsorbates, such as O2-, O-, and O2-. The formation of the oxygen adsorbate layer leads to a decrease in the electron density on the sensor surface due to the transfer of electrons from the sensor surface to the adsorbate layer. When the sensor is exposed to ethanol vapor, the ethanol gas reacts with the oxygen ions on the surface, which results in the release of free electrons to the sensor. This leads to the change in resistance of the R-Fe2O3 sensor. The amount of oxygen and test gas on the surface of materials is strongly dependent on the microstructure of the materials; namely, the specific area, particle size, and the porosity. The main reason for the above result is that the conventional R-Fe2O3 nanoparticle sensor has a poor surface

Figure 7. Sensitivities of porous R-Fe2O3 nanorods to various gases of 50-1000 ppm.

area and a relatively large particle size, whereas the sensor based on porous R-Fe2O3 nanorods has a high surface area and tiny crystal size, which can provide more adsorption-desorption sites for gas molecules. Moreover, the abundant pores on the surface of the R-Fe2O3 nanorods can facilitate the diffusion of the gas molecules and enable them to access all surfaces of the nanoparticles contained in the sensing unit. As is known, response and recovery times, which are defined as the time required to reach 90% of the final resistance, are the basic parameters for gas sensors. It can also be seen from Figure 6 that the porous R-Fe2O3 nanorod sensor still shows a short response/recovery time, even to high-concentration ethanol vapor, indicating a good response/recovery capability for practical application. For practical use, the selectivity of the sensor is a necessary consideration. Hence, we also examined the gas-sensing of the same sensor on the basis of the response of the porous R-Fe2O3 nanorods to methanol, NH3, H2S, H2, and CO. The results are shown in Figure 7. It can be seen clearly from Figure 7 that the sensor exhibits the highest response to ethanol and very low responses to other gases. In addition, the sensor was totally insensitive to CO and H2. According to the experimental results, the as-prepared porous R-Fe2O3 nanorod sensor can selectively detect ethanol gas with the interference of other gases. The effect of humidity on the gas sensitivity of the sensor has also been investigated. The sensitivity of the sensor to ethanol at different relative humidities is shown in Figure S2 in the Supporting Information. The result reveals that it is no problem for the sensor of porous R-Fe2O3 nanorods to detect ethanol under 60% relative humidity. Furthermore, the sensor exhibited a nearly constant response to ethanol under the same conditions, even after 6 months, illustrating the good reversibility of the porous R-Fe2O3 nanorod sensor. 4. Conclusions In summary, we have presented a facile route for preparing porous R-Fe2O3 nanorods via a template-free solution approach at low temperature (40 °C). This method is feasible for largescale industrial manufacture of porous hematite nanorods due to the advantages of the simple production process, low cost, and environmental friendliness. The as-prepared porous R-Fe2O3 nanorods have a tiny crystal size (5 nm) and a porosity structure, resulting in a high surface area of 221.9 m2 · g-1. On the basis of the experimental results, a possible growth mechanism of the porous R-Fe2O3 nanorods has been proposed. The gassensing measurements demonstrated that the sensor based on

17808 J. Phys. Chem. C, Vol. 112, No. 46, 2008 porous R-Fe2O3 nanorods exhibited a much higher sensitivity to ethanol vapor than the sensor based on R-Fe2O3 nanoparticles. This is possibly due to the fact that the porous R-Fe2O3 nanorods have a high surface area and plentiful pores to adsorb and react with gas molecules. Moreover, the sensor also presented excellent selectivity to ethanol and good stability for a rather long time (6 months). Hence, it is expected that this facile route prepared porous R-Fe2O3 nanorods would be an ideal candidate for applications in ethanol sensors. Other properties and applications, such as catalysts and fuel cells, may also be found. Acknowledgment. The authors thank the National Nature Science Foundation of China (20871071), the 973 Program (2005CB623607), and the Applied Basic Research Programs of Science and Technology Commission Foundation of Tianjin (08JCYBJC00100) for financial support. Supporting Information Available: TEM images of the products obtained at different reaction times and the sensitivities of porous R-Fe2O3 nanorods to ethanol at different relative humidity. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (2) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. J. Science 2000, 287, 622. (3) Hamley, I. W. Nanotechnology 2003, 14, 39. (4) Wan, Y.; Shi, Y. F.; Zhao, D. Y. Chem. Comm. 2007, 897. (5) Ding, Y.; Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2004, 126, 2066. (6) Liu, J. F.; Wang, X.; Peng, Q.; Li, Y. D. AdV. Mater. 2005, 17, 764. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) Wang, X.; Li, Y. D. J. Am. Chem. Soc. 2002, 124, 2880. (9) Zhang, Y. P.; Chu, Y.; Dong, L. H. Nanotechnology 2007, 18, 435608. (10) Weiss, W.; Zsccherpel, D.; Schlogl, R. Catal. Lett. 1998, 52, 215. (11) Fukazawa, M.; Matuzaki, H.; Hara, K. Sens. Actuators, B 1993, 13, 521. (12) Neri, G.; Bonavita, A.; Galvagno, S.; Siciliano, P.; Capone, S. Sens. Actuators, B 2001, 82, 40. (13) Bondioli, F.; Ferrari, A. M.; Leonelli, C.; Manfredini, T. Mater. Res. Bull. 1998, 33, 723. (14) Mitra, S.; Das, S.; Mandal, K.; Chaudhuri, S. Nanotechnology 2007, 18, 275608.

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