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Hydrothermal Routes To Prepare Nanocrystalline Mesoporous SnO2 Having High Thermal Stability Shinobu Fujihara,*,† Takahiro Maeda,† Hirotoshi Ohgi,† Eiji Hosono,† Hiroaki Imai,† and Sae-Hoon Kim‡ Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan, and Department of Ceramic Engineering, Kangnung National University, 123 Jibyeon-Dong, Gangneung, Gangwon-Do, 210-702, Korea
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Received March 17, 2004. In Final Form: May 13, 2004 We report simple hydrothermal routes to prepare thermally stable SnO2 particles having high specific surface areas and mesoporosity. The preparation method includes a new combination of synthetic processes: hydrolysis of tin(IV) chloride at 95 °C in the absence of alkaline solutions (aqueous NH3 or NaOH), formation of nanocrystalline SnO2, and subsequent hydrothermal treatments at temperatures between 100 and 200 °C. After annealing treatments of the hydrothermally treated SnO2 particles at 400 or 500 °C, their crystallite sizes remained smaller than 7.7 nm and their specific surface areas were still higher than 110 m2/g, indicative of the high thermal stability against particle growth and sintering. Furthermore, mesoporosity evolved with a relatively narrow pore size distribution typically in the range of 3.0-4.3 nm. The effects of the hydrothermal treatment were explained by uniformization of the particle size that was beneficial to the suppression of particle growth.
Introduction Semiconducting metal oxides such as tin dioxide (SnO2), zinc oxide (ZnO), nickel oxide (NiO), etc. have found potential or practical applications in various types of solidstate gas sensors.1-5 In the presence of flammable and/or toxic gases, surface physicochemical processes including oxygen adsorption/desorption can take place, changing the electrical conductivity of metal oxides. The use of nanocrystalline oxide particles has been shown to enhance sensor performance because of their characteristic microstructural as well as electronic properties.6 High porosity and specific surface areas can lead to an increase in active surfaces for sensing. The size of the nanoparticles can be compatible with the thickness of the electron depletion layer, resulting in neck- or grain-controlled sensing action and hence enhanced sensitivity.7 A variety of chemical techniques have been proposed to prepare nanocrystalline SnO2 particles using tin(IV) chloride pentahydrate,6,8 homoleptic tin(II) amides,9 tin(II) citrates,10 tin(IV) isopropoxide,11 dialkoxydi(β-dike* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81 (0)45-566-1581. Fax: +81 (0)45-566-1551. † Keio University. ‡ Kangnung National University. (1) Weimar, U.; Schierbaum, K. D.; Gopel, W.; Kowalkowski, R. Sens. Actuators, B 1990, 1, 93-96. (2) Katsuki, A.; Fukui, K. Sens. Actuators, B 1998, 52, 30-37. (3) Baik, N. S.; Sakai, G.; Miura, N.; Yamazoe, N. Sens. Actuators, B 2000, 63, 74-79. (4) Xu, J. Q.; Pan, Q. Y.; Shun, Y. A.; Tian, Z. Z. Sens. Actuators, B 2000, 66, 277-279. (5) Neubecker, A.; Pompl, T.; Doll, T.; Hansch, W.; Eisele, I. Thin Solid Films 1997, 310, 19-23. (6) Kudryavtseva, S. M.; Vertegel, A. A.; Kalinin, S. V.; Oleynikov, N. N.; Ryabova, L. I.; Meshkov, L. L. S.; Nesterenko, N.; Rumyantseva, M. N.; Gaskov, A. M. J. Mater. Chem. 1997, 7, 2269-2272. (7) Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N. Sens. Actuators, B 1991, 3, 147-155. (8) Davis, S. R.; Chadwick, A. V.; Wright, J. D. J. Mater. Chem. 1998, 8, 2065-2071. (9) Nayral, C.; Ould-Ely, T.; Maisonnat, A.; Chaudret, B.; Fau, P.; Lescouze`res, L.; Peyre-Lavigne, A. Adv. Mater. 1999, 11, 61-63.
tonato)tin complexes,12 or tin(II) halides13 as starting materials. Basically, these techniques are carried out on the basis of low-temperature synthetic processes because higher temperatures are responsible for undesired particle growth and sintering, which decrease surface areas and increase particle sizes. For practical applications in sensor devices, however, SnO2 particles need to be printed on substrates and annealed at temperatures as high as 400600 °C. The operation temperature of the sensors should also be high enough (typically 400 °C) to obtain good response to gases. Efforts have therefore been made to suppress particle growth of SnO2 by adding cationic dopants such as Y3+, La3+, Ce4+, Nb5+, Cu2+, or Fe3+.8,10,14 The particle growth rate can be reduced through formation of metastable solid solutions or a change of the diffusion kinetics. A problem arising from the use of such dopants is that their migration and segregation can possibly occur during the heat treatment or the operation, leading to irreproducibility and aging of the sensors.8 It is therefore of prime importance to develop synthetic methods for pure SnO2 particles that are stable against heat. Here we investigate new hydrothermal routes to prepare nanocrystalline mesoporous SnO2 having specific surface areas higher than 110 m2/g after heating at 400 or 500 °C. Nanocrystalline SnO2 particles were first precipitated in a transparent homogeneous aqueous solution of tin(IV) chloride pentahydrate (SnCl4‚5H2O). Hydrolysis of SnCl4 was achieved without using aqueous ammonia to avoid formation of coarse particles.15 Hydrothermal treatments (10) Leite, E. R.; Weber, I. T.; Longo, E.; Varela, J. A. Adv. Mater. 2000, 12, 965-968. (11) de Monredon, S.; Cellot, A.; Ribot, F.; Sanchez, C.; Armelao, L.; Gueneau, L.; Delattre, L. J. Mater. Chem. 2002, 12, 2396-2400. (12) Toupance, T.; Babot, O.; Jousseaume, B.; Vilac¸a, G. Chem. Mater. 2003, 15, 4691-4697. (13) Deng, H.; Lamelas, F. J.; Hossenlopp, J. M. Chem. Mater. 2003, 15, 2429-2436. (14) Leite, E. R.; Maciel, A. P.; Weber, I. T.; Lisboa-Filho, P. N.; Longo, E.; Paiva-Santos, C. O.; Andrade, A. V. C.; Pakoscimas, C. A.; Maniette, Y.; Schreiner, W. H. Adv. Mater. 2002, 14, 905-908. (15) Mulvaney, P.; Grieser, F.; Meisel, D. Langmuir 1990, 6, 567572.
10.1021/la0493060 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/24/2004
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were then introduced for the SnO2 particles, expecting that we could modify their structure and surface properties. The resultant particles exhibited high thermal stability; they maintained their high specific surface areas even at elevated temperatures. This enhanced thermal stability was attributed to uniformization of the particle size during the hydrothermal treatment and evolution of the mesoporous structure during the annealing treatment. Experimental Methods Materials. SnCl4‚5H2O with 98% purity was obtained from Wako Pure Chemicals, Japan. Deionized water was used for preparing an aqueous SnCl4 solution. Silver nitrate (AgNO3) was used to test residual chlorine species. Precipitation of Nanocrystalline SnO2. SnCl4‚5H2O was dissolved in deionized water at room temperature with a concentration of 0.05 mol/dm3. In contrast to the previous reports,3,6,8 hydrolysis of SnCl4 was achieved without using aqueous ammonia. That is, the resultant SnCl4 solution was heated at 95 °C for 20 min under refluxing conditions to promote hydrolysis of SnCl4 and formation of SnO2. Precipitates, which were identified as cassiterite (the rutile-type SnO2), were then centrifuged and washed with deionized water until the absence of chloride ion. We confirmed the removal of chloride ion by testing supernatants with a few drops of an aqueous AgNO3 solution (0.05 mol/dm3), following the method reported in the literature.6 White AgCl precipitates were no longer observed after the SnO2 precipitates were washed typically 10 times. Hydrothermal Treatments and Annealing. The SnO2 precipitates were dispersed in deionized water, and then placed in Teflon-lined autoclaves. Hydrothermal treatments (denoted as HTT, hereafter) were then carried out at 100, 150, or 200 °C for 24 or 72 h. The resultant SnO2 particles were centrifuged and washed with deionized water, performing the AgNO3 test. After drying, the particles were annealed at 400 or 500 °C for 5 h in air to examine thermal stability. Precipitates that were not hydrothermally treated were also tested for comparison. Characterization. The crystal structure was identified by X-ray diffraction (XRD) analysis with a Rigaku RAD-C diffractometer using Cu KR radiation. The particle morphology was observed by high-resolution transmission electron microscopy (HRTEM) with a Philips TECNAI F20 microscope. Images were obtained by deposition of a drop of the SnO2-dispersed water onto a carbon-covered copper grid. The particle size distribution was determined by directly measuring the size shown in the HRTEM images. X-ray photoelectron spectroscopy (XPS) was carried out with a JEOL JSP-9000MX spectrometer using Mg KR radiation for elemental analysis. The phase purity was also confirmed by Fourier transform infrared (FT-IR) spectroscopy with a BIO-RAD FTS-165 spectrometer using the KBr method. The specific surface area was estimated by the BrunauerEmmett-Teller (BET) method on the basis of the nitrogen adsorption isotherm (77 K) with a Shimadzu Tristar 3000 Micrometrics analyzer. The pore size distribution was also analyzed with the same apparatus. The Barret-Joyner-Halenda (BJH) method was applied to the adsorption branch of the adsorption-desorption isotherms. To check the validity of adopting this method, the standard reduced adsorption, Rs, was calculated and plotted against the volume adsorbed. The detailed methodology of the “Rs plot” has been described in the literature.16
Results and Discussion Characterization of As-Prepared and Hydrothermally Treated SnO2. During refluxing of the aqueous SnCl4 solution at 95 °C, white precipitates started to be observed, indicative of promotion of hydrolysis and formation of SnO2. XRD analysis revealed that a tetragonal SnO2 phase was formed after refluxing for 20 min as shown in Figure 1a (marked with “as-prepared”). No diffraction peaks related to secondary phases such as SnO were (16) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 54105413.
Figure 1. (a) XRD patterns of the as-prepared SnO2 and the hydrothermally treated SnO2 at 100, 150, or 200 °C for 24 h. (b) XRD patterns of the hydrothermally treated SnO2 at 100, 150, or 200 °C for 72 h.
observed. The well-known Scherrer crystallite size calculated using the XRD pattern was 4.2 nm, which agreed with the average crystallite size of 4.0 nm observed with HRTEM as described later. Figure 1a also compares XRD patterns of the asprepared and the hydrothermally treated SnO2 (at 100, 150, or 200 °C for 24 h). Broad diffraction peaks around 2θ ) 26.6°, 33.9°, and 51.8° are ascribed to the (100), (101), and (211) planes of the tetragonal SnO2, respectively. At the higher HTT temperatures of 150 and 200 °C, a peak due to (200) also appears around 2θ ) 38.0°. It can also be seen that the peak intensity increases and the width decreases upon increases in the HTT temperature. The Scherrer crystallite size was calculated to be 4.5, 5.4, and 6.0 nm with HTT temperatures of 100, 150, and 200 °C, respectively, indicating that crystal growth can be slightly promoted. A similar behavior was also observed for the SnO2 hydrothermally treated for 72 h, as indicated by XRD patterns in Figure 1b. The crystallite size increased from 4.6 to 5.8 and 6.0 nm upon increases in the HTT temperature from 100 to 150 and 200 °C. The XRD analysis also suggested that prolonging the HTT duration from 24 to 72 h had almost no influence, at least on the size of the SnO2 crystallites. Typical HRTEM images are shown in Figure 2 for the SnO2 particles (as-prepared or hydrothermally treated at 150 or 200 °C for 24 or 72 h). A lattice image can be clearly seen for all the samples due to phase contrast. The distance between the adjacent lattice fringes corresponds to the interplanar distance of the tetragonal SnO2 (110), which is d110 ) 0.335 nm. Considering that each domain having the parallel lattice fringes is a single crystal of SnO2, the average crystallite size is observed to be 4.0 (as-prepared), 4.6 (HTT at 150 °C for 24 h), 5.4 (HTT at 200 °C for 24 h), and 5.8 (HTT at 200 °C for 72 h) nm. These data support the slight crystal growth indicated by the XRD analysis. A careful look at the images also reveals that the present
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Fujihara et al. Table 1. Scherrer Crystallite Size, BET Surface Area, and Average Pore Diameter of SnO2 Particles Prepared under Various Conditions HTT temp (°C) no HTTa no HTTa no HTTa 100 100 100 150 150 150 200 200 200 100 100 100 150 150 150 200 200 200
HTT duration (h)
annealing temp (°C)
24 24 24 24 24 24 24 24 24 72 72 72 72 72 72 72 72 72
no annealing 400 500 no annealing 400 500 no annealing 400 500 no annealing 400 500 no annealing 400 500 no annealing 400 500 no annealing 400 500
pore crystallite BET surface diameter 2 size (nm) area (m /g) (nm) 4.2 10.4 17.4 4.5 4.6 5.1 5.4 5.6 6.2 6.0 6.5 7.3 4.6 5.1 5.1 5.8 6.0 6.7 6.0 7.0 7.7
195 87 36 218 159 145 225 145 111 209 140 115 216 163c 134 220 147 136 180 141 133
