J. Phys. Chem. C 2008, 112, 11645–11649
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Synthesis of Crystalline Microporous SnO2 via a Surfactant-Assisted Microwave Heating Method: A General and Rapid Method for the Synthesis of Metal Oxide Nanostructures Guangcheng Xi,* Yanting He, Qing Zhang, Haiqing Xiao, Xing Wang, and Chao Wang Inspection and Research Center for Nanomaterials and Nanoproducts, Chinese Academy of Inspection and Quarantine, Beijing 100025 People’s Republic of China ReceiVed: March 12, 2008; ReVised Manuscript ReceiVed: May 28, 2008
Although synthetic investigations of inorganic nanomaterials had been carried out extensively over the past decade, the rapid synthesis of crystalline microporous nanostructures with a high surface-volume ratio is still a significant challenge. In this work, by combining the excellent properties of both microwave heating and surfactant, we developed for the first time a novel surfactant-assisted microwave heating route for the rapid synthesis of crystalline microporous SnO2 with a very high sensitivity to ethanol. The formation mechanism of the as-prepared microporous SnO2 was been investigated and discussed in this paper. Furthermore, this synthetic route was shown to be a general and rapid method for the shape-controlled synthesis of metal oxide nanostructures. Introduction As an important member in the family of wide-gap semiconductors, tin dioxide (SnO2) has been extensively investigated. SnO2 is among the oldest and probably the most important material used for gas sensing.1 At the same time, it has been widely used in catalysis,2 optoelectronic devices,3 and lithium batteries.4 It is well known now that the size and morphology of the materials greatly affect their properties as well their further applications. Particularly for SnO2 materials, early studies demonstrated that their gas sensing performance is affected by several structural parameters such as particle size, surfacevolume ratio, and crystallinity. For example, small particle size, a large surface-volume ratio, and high crystallinity are required to enhance the sensitivity of gas sensors.5 For industrial applications of SnO2 nanomaterials, one of the most important requirements is reducing the material preparation period. However, the preparation of SnO2 nanomaterials with high crystallinity and a high surface-volume ratio generally needs long reaction time. Physical methods such as thermal evaporation and laser ablation have been used to prepare various SnO2 nanostructures with excellent crystallinity.6 However, the size of SnO2 nanostructures obtained by these methods is generally larger than 20 nm, which limits their applications in some areas and especially as gas sensors. Recently, chemical solution-phase methods such as hydrothermal,7 solvothermal,8 and gel-sol methods9 have been developed to synthesize SnO2 nanostructures with a high surface-volume ratio. Several SnO2 nanostructures such as nanoparticles,10 nanorods,11 nanotubes,12 and mesoporous nanostructures9 have been synthesized via various solution phase methods. However, one obvious disadvantage of these above-mentioned solution phase synthetic methods is the long reaction time (generally 15 h to several days). This is untreatable since the solution phase synthetic route generally is carried out at relatively low reaction temperatures. As a rapid and simple synthetic method, microwave chemistry has been widely research since the first reports of microwave-assisted * To whom correspondence should be addressed. E-mail: xgch001@ mail.ustc.edu.cn. Phone: +86-010-85757002. Fax: +86-010-85772625.
synthesis in 1986.13 Due to the higher reaction rate and selectivity, microwave chemistry has recently been shown to be an effective and fast method for the synthesis of nanomaterials.14 On the other hand, more and more studies have revealed that the addition of a small quantity of a surfactant to a solution-phase reaction can strongly affect the morphologies of the obtained nanomaterials.15 However, the application of microwave heating to nanomaterial synthesis in conjunction with surfactants has hardly been exploited until now. By combining the excellent properties of both microwave heating and surfactants, we developed for the first time a novel surfactant-assisted microwave heating route for the rapid synthesis of crystalline microporous SnO2 nanostructures. This synthesis method offers the following four advantages: (1) The full reaction time is only 10 min; (2) no toxic or dangerous chemicals are used during the whole reaction process; (3) the present microporous SnO2 synthesized via the surfactant-assisted microwave heating method is high crystalline and exhibits a very high sensitivity to ethanol; and (4) ZnO and Fe3O4 nanostructures also can be prepared by this method, which suggests that the surfactant-assisted microwave heating method can be used as a general method to synthesize various metal oxides nanostructures. Experimental Section Synthesis of Microporous SnO2. In a typical procedure, 10 mL of ethanolamine, 0.6 g of 5-hydrated tin tetrachloride (SnCl4 · 5H2O), and 1.45 g of cetyltrimethylammonium bromide (CTAB) were added to 30 mL of distilled water with vigorous stirring, forming a transparent solution. The transparent solution was transferred into a Teflon-lined double-walled digestion vessel with a capacity of 60 mL. The digestion vessel was sealed and maintained at 160 °C for 10 min using a microwave digestion system (CEM-MARS-5, America). Finally, the vessel was allowed to cool to room temperature naturally. The resulting white product was retrieved by centrifugation and washed several times with distilled water and absolute ethanol and then dried in vacuum at 50 °C for 4 h. Characterization. The X-ray powder diffraction (XRD) pattern of the products was recorded on a Rigaku (Japan)
10.1021/jp802180z CCC: $40.75 2008 American Chemical Society Published on Web 07/10/2008
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Figure 1. A typical XRD pattern of the as-synthesized microporous SnO2.
D/max-γA X-ray diffractometer equipped with graphite monochromatized Cu Ka1 radiation (λ )1.54178 Å). The transmission electron microscope (TEM) images, energy-dispersive X-ray spectra (EDS), fast Fourier transform (FFT) pattern, and highresolution transmission electron microscope (HRTEM) images were recorded on a JEOL 2010 microscope. Nitrogen adsorption measurements were performed using a BelSorp Max system utilizing Brunauer-Emmett-Teller (BET) calculations for surface area. The X-ray photoelectron spectrum (XPS) was recorded on a VGESCALAB MKII X-ray photoelectron spectrometer using a nonmonochromatized Mg KR X-ray as the excitation source. Adsorption Activity Test. The adsorption activity of the assynthesized microporous SnO2 was evaluated by adsorption of acid fuchsine molecules in a dark environment. A conical flask (capacity ca. 50 mL) was used as the experimental vessel. Microporous SnO2 (8 mg) was added as sorbent to the aqueous acid fuchsine solution (C20H17N3O9S3Na2) (1.0 × 10-4 M, 20 mL). UV-vis absorption spectra were recorded at different intervals to monitor the adsorption process using a Shimadzu2550 spectrophotometer.
Figure 2. XPS spectra of the as-synthesized microporous SnO2.
Results and Discussion Powder XRD and XPS of Microporous SnO2. Figure 1 shows a typical power X-ray diffraction (XRD) pattern from the as-synthesized microporous SnO2. All of the diffraction peaks can be indexed as tetragonal rutile phase SnO2 with lattice parameters comparable to that of the JCPDS card (41-1445). The relatively broad XRD peaks reveal the small size of the SnO2 nanocrystalls. No other crystalline impurities were detected by XRD, which indicates the pure SnO2 was obtained via the present surfactant-assisted microwave heating route. XPS was carried out to investigate the surface compositions and chemical state of the sample, and the results are shown in Figure 2. The Sn3d appeared as a spin-orbit doublet at ca. 486.9 eV (3d5/2) and ca. 495.4 eV (3d3/2) (Figure 2a), which is in agreement with the reported values in the literature.16 The O1s binding energy of 530.9 eV (Figure 2b) indicates that the oxygen atoms exist as O2- species in the compounds. Consequently, from the results of the XRD and XPS measurements the as-synthesized products could be determined to be pure SnO2. Morphology and Microstructure of the Microporous SnO2. The size, morphology, and microstructure of the microporous SnO2 were explored by TEM and HRTEM. Figure 3ab shows the typical TEM images of the product, which indicate the product is composed of a mass of interconnected spherelike nanoparticles 3.5 ( 0.5 nm in diameter. The fast Fourier transform (FFT) pattern of the sample (Figure 3c) reveals
Figure 3. Shape and microstructure images of the as-synthesized microporous SnO2. (a) Low-magnification TEM image; (b) highmagnification TEM image; (c) the corresponding SAED pattern; (d) HRTEM image.
that the microporous SnO2 is crystalline. Furthermore, the HRTEM image (Figure 3d) shows that the nanoparticles have well-defined lattice fringes, which further demonstrats that the microporous SnO2 is highly crystalline. Figure SI-1 in Supporting Information shows a representative EDS spectrum of the as-synthesized microporous SnO2. The tin and oxygen peaks can be clearly observed in this spectrum. The Cu and C peaks in the EDS spectrum arise from the copper TEM grid used in the measurements. By combining the results of both HRTEM and EDS, it can be concluded that pure and highly crystalline
Synthesis of Crystalline Microporous SnO2
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11647
TABLE 1: Experimental Conditions for the Preparation of SnO2 Samples sample no. 1 2 3 4 5 6
starting regents 10 mL ethanolamine 30 mL H2O 10 mL ethanolamine 30 mL H2O 10 mL ethanolamine 30 mL H2O 10 mL ethanolamine 30 mL H2O 10 mL ethanolamine 30 mL H2O 10 mL ethanolamine
samples
+ 0.6 g SnCl4 · 5H2O + 1.6 g CTAB +
microporous structures
+ 0.6 g SnCl4 · 5H2O + 1.45 g CTAB +
microporous structures
+ 0.6 g SnCl4 · 5H2O + 0.9 g CTAB +
microporous structures
+ 0.6 g SnCl4 · 5H2O + 0.6 g CTAB +
short nanorods
+ 0.6 g SnCl4 · 5H2O + 0.3 g CTAB +
nanoparticles and short nanorods (see Figure SI-4 in Supporting Information) spherelike nanoparticles
+ 0.6 g SnCl4 · 5H2O + 30 mL H2O
microporous SnO2 were synthesized via the rapid surfactantassisted microwave-heating method. Formation Mechanism of the Microporous SnO2. The experimental parameters have a great effect on the formation of the microporous SnO2. When the experiment was carried out without the CTAB surfactant while keeping other experimental conditions unchanged, only spherelike SnO2 nanocrystals (2.5 ( 0.5 nm in diameter) were obtained (Figure 4a). If the mass of the CTAB surfactant was increased to 0.6 g and other experimental conditions were kept unchanged, SnO2 short nanorods were obtained (Figure 4b). The nanorods are 8 ( 3 nm in length and 4 ( 1 nm in diameter. If the mass of the CTAB surfactant was further increased to 0.9 and 1.6 g, microporous SnO2 with different degrees of aggregation were obtained (Figure 4c,d). The relations between CTAB concentrations and SnO2 structures have been summarized in Table 1. The experimental results indicate that the CTAB surfactant acts as the shape-controlling agent in formation of the SnO2 nanostructure. CTAB has been shown to be an excellent softtemplate for the synthesis of Ag and Au nanorods and nanowires.17 On the basis of the previous work and our experimental observation, one possible function for CTAB in the present synthetic method would to be to form sphere- or rodlike micelles in aqueous solution at low concentrations which act as a soft template for the formation of 0D or 1D nanostructures. When the concentration of CTAB is high, micelles with
Figure 4. TEM images of the as-synthesized SnO2 nanostructures prepared under different CTAB concentrations: (a) no CTAB; (b) 0.6 g; (c) 0.9 g; (d) 1.6 g.
more complicated 3D structure will be formed.18 At the same time, Sn4+ ions join the surface of the 3D structural micelles via intermolecular forces. As a result, a well-organized arrangement of Sn4+ ions can be achieved in the solution in the presence of CTAB 3D micelles. When microwave heating is initiated, this arrangement of Sn4+ ions around the CTAB micelles directly becomes the SnO2 microporous framework via the hydrolyzation of the Sn4+ ions. After the CTAB micelles were removed with ethanol and distilled water, pure microporous SnO2 structures were obtained. Furthermore, when SnCl4 was replaced by ZnCl2 and FeCl3, a controlled synthesis of various crystalline ZnO (Figure 5, EDS spectrum see Figure SI-2 in Supporting Information) and Fe3O4 (Figure 6, EDS spectrum see Figure SI-3 in Supporting Information) nanostructures was possible with this method (for the preparation process, see the Supporting Information), indicating that the present surfactantassisted microwave heating method may be used as a general method to synthesize various metal oxide nanostructures. The formation processes of the SnO2 microporous structures are summarized in Figure 7. Surface Area and Pore Volume of the Microporous SnO2. The nitrogen adsorption-desorption isotherm curves of the microporous SnO2 nanocrystals are shown in Figure 8. Accord-
Figure 5. (a) TEM and HRTEM (inset) images of the spherelike ZnO nanocrystals (2 ( 0.5 nm in diameter) obtained when there is 0.5 g CTAB in the reaction mixture. (b) TEM and HRTEM (inset) images of the small diameter (3 ( 0.5 nm in diameter and 6 ( 2 nm in length) ZnO nanorods obtained when there is 1.0 g CTAB in the reaction mixture. (c,d) TEM and HRTEM images of the microporous ZnO obtained when there is 1.45 g CTAB in the reaction mixture.
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Figure 6. Typical TEM and HRTEM images of the as-synthesized Fe3O4 nanostructures: (a) TEM and HRTEM (inset) images of the spherelike Fe3O4 nanocrystals (2 ( 0.5 nm in diameter) obtained when there is 0.5 g CTAB in the reaction mixture. (b and c) TEM and HRTEM images of the microporous Fe3O4 obtained when there is 1.5 g CTAB in the reaction mixture.
Figure 9. Absorption spectrum of a solution of acid fuchsine in the presence of the microporous SnO2 (a) intial solution; (b-h) after adding microporous SnO2 (b) 3, (c) 6, (d) 9, and (e) 15 min. Figure 7. Schematic illustration of the formation mechanism of microporous SnO2.
Figure 8. Nitrogen adsorption/desorption isotherms obtained at 77 K and pore size distribution (inset) of the as-synthesized microporous SnO2.
ing to the IUPAC nomenclature, this curve is a characteristic of the different process between adsorption onto and desorption from the micropores.19 The result confirmed that most of the pores in the porous SnO2 nanocrystals are microporous. To further analyze the pore structures of the as-synthesized sample, the pore-size distribution curve was investigated. As displayed in the inset of Figure 8, the peak of the pore size from the microporous SnO2 was centered at 1.1 nm. On the basis of the nitrogen adsorption-desorption results, the microporous SnO2 exhibited a BET surface area of 122.5 m2 g-1 and a total pore volume of 0.22 cm3 g-1.
Gas-Sensing Properties of the Microporous SnO2. To demonstrate the potential applicability of the as-prepared microporous SnO2 in fabricating gas sensors, we first investigated their adsorption activity by following the adsorption of acid fuchsine as reference. Figure 9 shows the absorption spectra of an aqueous solution of acid fuchsine in the presence of microporous SnO2. As shown in Figure 9, the absorbance intensity of the peak corresponding to the acid fuchsine molecule at 545 nm decreased very quickly once the microporous SnO2 was added. With time increasing, the typical sharp peak at 545 nm decreased and has almost completely vanished after 15 min, indicating that the microporous SnO2 has a very high adsorption activity. Since the as-synthesized microporous SnO2 has high crystallinity, a large surface area, and excellent absorption activity, it can be expected that the microporous SnO2 possesses an enhanced gas-sensing performance at room temperature. To test this hypothesis, we investigated the C2H5OH gas sensing properties of the as-synthesized microporous SnO2. The sensitivity (S) is defined as S ) Ra/Rg, where Ra is the resistance in atmospheric air (its relative humidity is about 25%) and Rg is the resistance of the microporous SnO2 in ethanol-air mixed gas. Ra is around 4.7 MΩ in atmospheric air at the working temperature of 270 °C. The experimental results demonstrated that the microporous SnO2 has a very high sensitivity, as shown in Figure 10. The sensitivities of the microporous SnO2 are 7.4, 14.1, 28.4, 40.6, and 50.5 to 10, 50, 100, 150, and 200 ppm ethanol vapor, respectively. Compared with the ethanol gassensing properties of SnO2 nanorods (31.4 to 300 ppm),20 hollow spheres (7.5 to 50 ppm),21 nanotubes (8 to 20 ppm),22 and Sbdroped SnO2 nanowires (2.5 to 200 ppm)23 reported by others, those of the as-synthesized microporous SnO2 are superior. At the same time, compared with the ethanol gas-sensing properties of SnO2 nanoparticles and nanorods shown in Figure 4 panels
Synthesis of Crystalline Microporous SnO2
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11649 croporous SnO2 displays very high sensitivity to ethanol gas. Different SnO2 nanostructures can be obtained by adjusting the concentration of the CTAB surfactant in the reaction mixture. Furthermore, this surfactant-assisted microwave heating method represents a general and rapid strategy to prepare oxide nanostructures. Acknowledgment. We would like to thank the Dean Foundation of Chinese Academy of Inspection and Quarantine (2007JK014 and 2007JK015) for financial support. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 10. The linear relationship between the sensor sensitivity and the ethanol concentration.
a and b, respectively, those of the microporous SnO2 are also superior (see Figures SI-5 and SI-6 in Supporting Information). As for the sensing mechanism of microporous SnO2, the mechanism involves the interaction between ethanol and the chemisorbed oxygen ions, such as O2-, O-, and O2-, on the surface of the microporous SnO2. 10 As we known, a spacecharge layer will form on the surface of the microporous SnO2 when an electron from semiconducting SnO2 is trapped by adsorbed oxygen species. Because the ethanol reacts with the ionic oxygen species, the electrons trapped by the oxygen adsorbents are released to the metal oxides. As a result, the conductivity of the microporous SnO2 will increase. Earlier studies found that SnO2 nanostructure exhibits better gas-sensing properties when its size is reduced to a scale close to or smaller than the space-charge thickness of SnO2 (for SnO2, this value is about 6 nm). In the present work, the size of the spherelike nanoparticles composed of the microporous structure is about 3-4 nm, which is obviously smaller than the space-charge thickness of SnO2. Therefore, as a result, the as-synthesized microporous SnO2 displays very high sensitivity to ethanol gas. In addition, we believe that the higher sensitivity of the micorporous SnO2 might be attributed to the porous structures and tunnels of the sample, which has more chance to adsorb and desorb the gas molecules. 24 Furthermore, the sensitivity of the microporous SnO2 is directly proportional to the concentration of ethanol gas as shown in Figure 10, which is consistent with previous reports.20,25 The linear relation between sensitivity and gas concentration can be expressed as
S ) A[C]N
(1)
where S is the sensitivity of the sensor, A is a constant, and C is the gas concentration. The value of N is 1 or 1/2. Generally, the value of N is 1 when the size of the SnO2 nanoparticle is comparable with the space-charge thickness of SnO2 (6 nm), while the value is 1/2 when the size of the SnO2 nanoparticle increases to more than 20 nm. Here, such a linear dependence of the sensitivity on the ethanol concentration further suggests that the microporous SnO2 can be used as a promising material for gas sensors. Conclusion Microporous SnO2 was successfully synthesized using an aqueous solution containing SnCl4, CTAB, and ethanolamine via a surfactant-assisted microwave heating method. By using this method, microporous SnO2 with high crystallinity and high surface to volume ratio could be synthesized after only 10 min in the microwave heating process. The as-synthesized mi-
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