2304
J. Phys. Chem. C 2008, 112, 2304-2307
Formation of Short In2O3 Nanorod Arrays Within Mesoporous Silica Shih-Chieh Chang and Michael H. Huang* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: October 16, 2007; In Final Form: December 3, 2007
We report the formation of arrays of short In2O3 nanorods inside of the nanoscale channels of mesoporous silica SBA-15. In(NO3)3 dissolved in methanol was incorporated into the SBA-15 powder, followed by heat treatment under a nitrogen flow at 700 °C for 4 h to generate densely aligned In2O3 nanorods. The nanorods have been characterized by XRD patterns, TEM images, nitrogen adsorption-desorption isotherm measurements, and optical spectroscopy. They possess a cubic In2O3 crystal structure. The free-standing In2O3 nanorods obtained after silica framework removal with a 2.0 M NaOH solution showed diameters of 6-7 nm and lengths of largely less than 50 nm. Some nanorods can be as short as less than 10 nm in length. The nanorods exhibit an absorption band at ∼300 nm. The observation of this quantum-confined effect is likely induced by the small nanorod diameter. A fluorescence peak centered at 387 nm was recorded. No other oxygen-vacancyrelated emission signals in the blue to green light region were present, suggesting a high crystalline quality of the short In2O3 nanorods synthesized.
Introduction Indium oxide has a wide direct band gap of 3.6 eV, although a band gap energy in the range of 3.55-3.75 eV has also been reported.1,2 Indium oxide nanowires and nanoparticles have recently been demonstrated as sensitive gas sensors for NO2, CO2, oxygen, and ethanol vapor.3-7 Research interests to prepare size- and shape-controlled In2O3 nanoparticles continued in the past few years. Several reports have described the organic-phase syntheses of monodispersed spherical In2O3 nanoparticles and nanocubes with fine size control in the range of 4-20 nm.2,8,9 These nanoparticles readily form ordered two-dimensional selfassembled patterns. In addition, indium oxide octahedral nanoparticles, large aggregated nanostructures, and nanofibers have been made by solution- and vapor-phase approaches.10-13 Long In2O3 nanorod and nanowire arrays with lengths of several hundred nanometers to microns have been prepared using anodic aluminum oxide membrane templates.14 Despite these efforts, the growth of arrays of short In2O3 nanorods with diameters of a few nanometers and lengths of tens of nanometers still has not been reported. Mesoporous silica SBA-15 possesses hexagonally ordered pore channels of a few nanometers in diameter and represents as a suitable candidate for the confined growth of rodlike In2O3 nanostructures. One-step direct nanocasting growth of In2O3 nanowire arrays during mesostructured silica synthesis has been presented, but the formation of extensive In2O3 nanowire arrays using calcined SBA-15 template was not successful.15 Very recently, In2O3 nanowires with lengths of ∼200 nm were produced using the SBA-15 template.7 However, the focus of that study was on CO2 sensing after loading the nanowires with CaO. No optical characterization of the products was performed. Here, we describe the extensive formation of short In2O3 nanorod arrays within the pore channels of mesoporous silica SBA-15 via a convenient infiltration method. In(NO3)3 dissolved in methanol was introduced into the interior of SBA-15. Subsequent heat treatment at 700 °C under a nitrogen flow * To whom correspondence should be addressed.
generated crystalline In2O3 nanorods. The products have been characterized with several typical techniques and electron microscopy. Optical properties of these In2O3 nanorods were also examined. Experimental Section Mesoporous silica SBA-15 was synthesized following our previously described procedure with slight modification.16 First, 2.0 g of triblock copolymer Pluronic P-123 surfactant (Aldrich) was added to a solution containing 15 g of deionized water and 60 g of 2 M HCl at 40 °C with stirring until the copolymer was completely dissolved. Then, 4.25 g of tetraethyl orthosilicate (TEOS, Acros Organics) was added and stirred for 12 h at 40 °C. The mixture was transferred to a sealed container and heated to 100 °C in an oven for 20 h. The resulting white precipitate was filtered, washed with water and ethanol, and dried at 65 °C overnight. Finally, the copolymer was removed by calcination in air at 550 °C for 3 h. Next, 0.5 g of In(NO3)3‚xH2O (Aldrich, 99.99%) was added to 3 mL of methanol and stirred until dissolved. The mixture was drop-by-drop added to a 100 mL round-bottom flask containing 0.1 g of the SBA-15 powder. After stirring for 24 h, the powder was filtered, washed with 1 mL of methanol, and dried at 65 °C overnight. The precursor-impregnated SBA-15 powder was placed in a ceramic boat, which was inserted into the center of a 1 in. quartz tube in a tube furnace. The quartz tube was purged with nitrogen at a flow rate of 90 sccm for 1 h. After that, the furnace temperature was increased to 700 °C at a speed of 10 °C/min and held at 700 °C for 4 h. At the end of this process, the furnace was slowly cooled down to room temperature. Pale yellow products were obtained. Low- and high-angle X-ray diffraction (XRD) patterns were obtained using a Shimadzu XRD-6000 diffractometer with Cu KR radiation. Transmission electron microscopy (TEM) images were taken using JEOL JEM-1200CX, JEM-2010, and JEM3000F transmission electron microscopes. TEM samples of In2O3 nanorods incorporated within SBA-15 were prepared by ultramicrotoming the SBA-15/epoxy mixture to thin slices of
10.1021/jp710032w CCC: $40.75 © 2008 American Chemical Society Published on Web 01/26/2008
Formation of Short In2O3 Nanorod Arrays
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2305
Figure 1. (a) Low-angle XRD patterns of (a) calcined SBA-15 and (b) In2O3-incorporated SBA-15.
Figure 3. (a) Low-magnification TEM image of an ultramicrotomed In2O3-incorporated SBA-15 sample. Inset shows the selected-area electron diffraction pattern of the sample. (b and c) High-magnification TEM images of different regions of the sample.
Figure 2. XRD pattern of the In2O3 nanostructures formed within SBA15.
50 nm in thickness. To perform TEM characterization of freestanding In2O3 nanorods, after silica template removal with a 2.0 M NaOH solution overnight and centrifugation at 8000 rpm for 30 min, In2O3 nanorods dispersed in deionized water were added to an amorphous carbon-coated TEM grid. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Quantachrome Nova2000e analyzer. Diffuse reflectance spectra of the In2O3-incorporated SBA-15 powder in the range of 240-800 nm were recorded on a Hitachi U-3310 spectrophotometer equipped with an integrating sphere. A Hitachi F-4500 spectrophotometer was used to take photoluminescence spectra. Results and Discussion The structure of mesoporous silica SBA-15 and the formation of In2O3 nanostructures were characterized by XRD patterns. Figure 1 gives the low-angle XRD patterns of calcined SBA15 and In2O3-incorporated SBA-15. The diffraction patterns indicate that the framework still possesses a long-range ordered hexagonal structure after the formation of In2O3 nanostructures. The (100) peak is shifted slightly to 1.00° 2θ, corresponding to a d spacing of 88.3 Å (from 92.0 Å for calcined SBA-15), possibly due to a higher degree of silicate condensation after the heat treatment to form the In2O3 nanostructures. Figure 2 shows the high-angle XRD pattern of In2O3-incorporated SBA15. This diffraction pattern matches well with that of bixbyite-
Figure 4. (a) TEM image of the free-standing In2O3 nanorods after silica framework removal. (b) High-resolution TEM image of an In2O3 nanorod showing clear lattice fringes. (c) Enlarged view of the rectangular region in panel b.
type cubic In2O3 (JCPDF No. 06-0416), confirming the successful formation of In2O3 nanostructures. Direct evidence of the formation of In2O3 nanorods within mesoporous silica SBA-15 was obtained by TEM characterization. Figure 3a is a low-magnification TEM image of SBA-15 powder loaded with rodlike In2O3 nanostructures. Extensive arrays of In2O3 nanostructures were formed and followed the
2306 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Chang and Huang
Figure 6. Diffuse reflectance spectra of calcined SBA-15 and In2O3incorporated SBA-15.
Figure 5. (a) Nitrogen adsorption-desorption isotherms of calcined SBA-15 and In2O3-incorporated SBA-15. (b) Pore diameter distribution of the samples calculated from the desorption branch of the isotherms using the BJH algorithm.
TABLE 1: Nitrogen Adsorption-Desorption Isotherm Dataa and the Wall Thicknesses for the Two SBA-15 Samples nitrogen adsorption isotherms sample
SBET/m2 g-1
Vt/cm3 g-1
DBJH/nm
wall thickness/nm
SBA-15 In2O3/SBA-15
544 400
0.83 0.57
6.0 5.7
4.6 4.5
a SBET, BET specific surface area; Vt, total pore volume; DBJH, pore diameter calculated using the BJH method. Wall thickness is calculated from A0 - DBJH, where A0 ) (2/x3)d(100).
curvature of the pore channels over long distances. Practically no In2O3 nanoparticles can be found outside of the channels. A selected-area electron diffraction image of the sample gives a ring pattern which matches well with that of cubic In2O3. Figure 3b shows a high-magnification image of a portion of the sample. The nanorods have a uniform diameter limited by the channel diameter (that is, ∼6 nm). When viewing the nanorods directly in the pore channels (Figure 3c), many channels are filled with In2O3 nanorods, but the nanorods appear to have diameters slightly larger than the pore diameter. This result is attributed to the possible shearing effect introduced during ultramicrotoming of the SBA-15 sample. To further observe the formation of these nanorods, the mesoporous silica template was removed with a 2.0 M NaOH solution. Figure 4a displays the densely aligned free-standing In2O3 nanorod arrays. Individual nanorods can be more clearly seen. The image shows that most of the nanorods are short rods that are just several tens of nanometers in length (less than 50 nm for many of the rods). Some nanorods can be as short as 10 nm in length. A high-resolution TEM image of a short In2O3
nanorod is given in Figure 4b. The nanorod has a diameter of ∼7 nm, suggesting that slight indium oxide crystal growth into the microporous region of SBA-15 is possible. SBA-15 possesses additional micropores because of the templating agent Pluronic P-123 used; the hydrophilic ethylene oxide chains can penetrate into the silica walls during the synthesis.17 Two sets of well-resolved lattice fringes at an angle of 70° can be discerned with the same d spacing of 2.90 Å (Figure 4c), which should correspond to the (222) and (222h) planes of cubic In2O3. The nitrogen adsorption-desorption isotherms and pore diameter distribution plots for the calcined SBA-15 and In2O3incorporated SBA-15 are given in Figure 5. Table 1 summarizes the BET surface areas, total pore volumes, pore diameters, and wall thicknesses of these samples. Both isotherms show typeIV hysteresis loops, confirming that these samples possess mesoporous structures. After the formation of arrays of In2O3 nanorods, the surface area and total pore volume of SBA-15 decrease substantially from 544 to 400 m2 g-1 and 0.83 to 0.57 cm3 g-1, respectively. Thus, a reasonably high degree of In2O3 loading in the pore channels has occurred. The pore diameter and wall thickness exhibit only a slight shrinkage after the nanorod growth process. Optical properties of the synthesized In2O3 nanorods have been characterized. Figure 6 is the diffuse reflectance spectrum of In2O3-incorporated SBA-15. A band at ∼300 nm was recorded (or ∼4.13 eV). As compared to the band gap energy of bulk In2O3 material, these short nanorods exhibit a clear quantum-confined effect. A similar effect has also been observed for the mesostructured In2O3 framework (∼310 nm) and nanoparticles with diameters of 11.5-20 nm (∼305 nm).8,15 Figure 7a presents a photoluminescence spectrum of the isolated In2O3 nanorods after the removal of the mesoporous silica framework. An emission peak centered at 387 nm was observed using an excitation wavelength of 300 nm. Figure 7b shows an excitation spectrum of the isolated In2O3 nanorods monitored at 387 nm. Higher emission intensities can be obtained by exciting the nanorods from 270 to 340 nm than those at wavelengths below 250 nm. An excitation wavelength of 300 nm was used because it is not too close to the emission peak. No other fluorescence bands were measured, even when lower excitation wavelengths were used. Thus, the emission peak is attributed to the near-band-edge emission of these short nanorods, although the band position may have been effected by the presence of donor levels introduced by oxygen vacancies during the nanorod synthesis.14c Emission signals in the blue to green light region are generally observed for In2O3 nanostructures with oxygen vacancies.9,12,14b The lack of these signals
Formation of Short In2O3 Nanorod Arrays
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2307 and lengths of largely less than 50 nm. XRD and TEM characterization confirmed that the nanorods possess a cubic In2O3 crystal structure. These In2O3 nanorods show an absorption band at ∼300 nm. The observation of this quantum-confined effect is possibly induced by the small rod diameter. An emission band centered at 387 nm was recorded for the freestanding In2O3 nanorods. No other additional oxygen-vacancyrelated emission signals in the blue to green light region were recorded, indicating the high crystalline and relatively defectfree quality of the nanorods synthesized. It is believed that other short oxide nanorods may be prepared by using a similar synthetic strategy to that described here, which allows the investigation of their novel physical properties. Acknowledgment. This work was supported by the National Science Council of Taiwan (Grant NSC 95-2113-M-007-031MY3). References and Notes
Figure 7. (a) Photoluminescence spectra of the isolated In2O3 nanorods after silica framework removal with NaOH solution and calcined SBA15 powder dissolved in NaOH solution. The excitation wavelength was 300 nm. A 310 nm filter was used. (b) Excitation spectrum of the isolated In2O3 nanorods monitored at 387 nm.
suggests that the synthesized In2O3 nanorods are highly crystalline and relatively defect-free. Conclusion In summary, we have demonstrated a facile method to form short indium oxide nanorod arrays within the pore channels of mesoporous silica SBA-15. In(NO)3 dissolved in methanol was loaded into the mesopores, followed by heat treatment at 700 °C under a nitrogen flow for 4 h to generate densely aligned In2O3 nanorods. The nanorods have a uniform diameter of 6-7 nm
(1) Murali, A.; Barve, A.; Leppert, V. J.; Risbud, S. H.; Kennedy, I. M.; Lee, H. W. H. Nano Lett. 2001, 1, 287. (2) Seo, W. S.; Jo, H. H.; Lee, K.; Park, J. T. AdV. Mater. 2003, 15, 795. (3) Zhang, D.; Liu, Z.; Li, C.; Tang, T.; Liu, X.; Han, S.; Lei, B.; Zhou, C. Nano Lett. 2004, 4, 1919. (4) Soulantica, K.; Erades, L.; Sauvan, M.; Senocq, F.; Maisonnat, A.; Chaudret, B. AdV. Funct. Mater. 2003, 13, 553. (5) Neri, G.; Bonavita, A.; Micali, G.; Rizzo, G.; Galvagno, S.; Niederberger, M.; Pinna, N. Chem. Commun. 2005, 6032. (6) Zhuang, Z.; Peng, Q.; Liu, J.; Wang, X.; Li, Y. Inorg. Chem. 2007, 46, 5179. (7) Prim, A.; Pellicer, E.; Rossinyol, E.; Peiro´, F.; Cornet, A.; Morante, J. R. AdV. Funct. Mater. 2007, 17, 2957. (8) Liu, Q.; Lu, W.; Ma, A.; Tang, J.; Lin, J.; Fang, J. J. Am. Chem. Soc. 2005, 127, 5276. (9) Lee, C. H.; Kim, M.; Kim, T.; Kim, A.; Peak, J.; Lee, J. W.; Choi, S.-Y.; Kim, K.; Park, J.-B.; Lee, K. J. Am. Chem. Soc. 2006, 128, 9326. (10) Zhao, Y.; Zhang, Z.; Wu, Z.; Dang, H. Langmuir 2004, 20, 27. (11) Zhang, Y.; Ago, H.; Liu, J.; Yumura, M.; Uchida, K.; Ohshima, S.; Iijima, S.; Zhu, J.; Zhang, X. J. Cryst. Growth 2004, 264, 363. (12) Yang, J.; Lin, C.; Wang, Z.; Lin, J. Inorg. Chem. 2006, 45, 8973. (13) Liang, C.; Meng, G.; Lei, Y.; Phillipp, F.; Zhang, L. AdV. Mater. 2001, 13, 1330. (14) (a) Zheng, M. J.; Zhang, L. D.; Li, G. H.; Zhang, X. Y.; Wang, X. F. Appl. Phys. Lett. 2001, 79, 839. (b) Kuo, C.-Y.; Lu, S.-Y.; Wei, T.-Y. J. Cryst. Growth 2005, 285, 400. (c) Cao, H.; Qiu, X.; Liang, Y.; Zhu, Q.; Zhao, M. Appl. Phys. Lett. 2003, 83, 761. (15) Yang, H.; Shi, Q.; Tian, B.; Lu, Q.; Gao, F.; Xie, S.; Fan, J.; Yu, C.; Tu, B.; Zhao, D. J. Am. Chem. Soc. 2003, 125, 4724. (16) (a) Yang, C.-T.; Huang, M. H. J. Phys. Chem. B 2005, 109, 17842. (b) Hsueh, H.-S.; Yang, C.-T.; Zink, J. I.; Huang, M. H. J. Phys. Chem. B 2005, 109, 4404. (17) Yang, C.-M.; Lin, H.-A.; Zibrowius, B.; Spliethoff, B.; Schu¨th, F.; Liou, S.-C.; Chu, M.-W.; Chen, C.-H. Chem. Mater. 2007, 19, 3205.
