J Electroceram (2006) 17:895–898 DOI 10.1007/s10832-006-7670-9

Preparation of SnO2 whiskers via the decomposition of tin oxalate Ki-Won Kim · Pyeong-Seok Cho · Jong-Heun Lee · Seong-Hyeon Hong

Received: 24 June 2005 / Revised: 24 June 2005 / Accepted: 8 February 2006 C Springer Science + Business Media, LLC 2006 

Abstract A new method for preparing SnO2 whiskers by the decomposition of SnC2 O4 is suggested. A Whisker-like morphology of a SnC2 O4 precipitate was attained via the gradual addition of an oxalic acid solution to a hot SnCl2 aqueous solution (T > 50◦ C). In comparison, when the solution temperature was either lower than 50◦ C or when ethanol was used as the solvent, the SnC2 O4 precipitate showed an angular and relatively isotropic morphology. The morphology of the SnC2 O4 precipitate remained even after its thermal decomposition into SnO2 at 400◦ C indicating that SnC2 O4 precipitation is a key step in preparing the whiskers. The formation mechanism of SnO2 whiskers was explained by the supersaturation during the precipitation of SnC2 O4 . Keywords SnO2 whisker . SnC2 O4 . Orthorhombic phase . Gas sensor

1 Introduction Tin dioxide (SnO2 ) is a key material in the applications of gas sensors [1], transparent electrodes [2], catalyst supports [3] and dye-sensitized photocells [4]. There has been extensive research on the preparation of SnO2 in the form of nano powders and thin films. In comparison, research on oneK.-W. Kim · P.-S. Cho · J.-H. Lee () Division of Materials Science and Engineering, Department of Materials Science & Engineering, Korea University, 1, 5-ka, Anam-dong, Sungbuk-ku, Seoul 136-701, South Korea e-mail: [email protected] S.-H. Hong School of Materials Science and Engineering and Nano Systems Institute-National Core Research Center, Seoul National University, Seoul 151-744, Korea

dimensional SnO2 structures such as whiskers, nanowires, and nanorods are in the initial stages. Considering the preferred orientation and high surface-to-volume ratio, SnO2 whiskers and nanorods might be a promising platform for enhancing the gas sensitivity and achieving selective gas sensing [5, 6]. One-dimensional SnO2 structures have been prepared via the heat treatment of (SnC2 O4 + NaCl) at 800◦ C [7], the heat treatment of a mixture of SnCl4 , NaCl, and Na2 CO3 [8], solvothermal heating of a mixture containing Sn(OH)2− 6 and NaOH [9] and the refluxing of SnC2 O4 with additives at 195◦ C [10]. However, most studies required Nacontaining flux materials such as NaCl, Na2 CO3 , and NaOH [7–9]. This makes it difficult to prepare highly pure SnO2 whiskers or nanostructure. This paper reports that phase-pure SnO2 whiskers can be synthesized from the decomposition of whisker-like SnC2 O4 , which was precipitated at high solution temperatures. From this preliminary study, it was found that precipitation is the key step in determining the particle morphology. Therefore, the formation mechanism of the SnO2 whiskers was investigated as a function of the solution temperature, solvent system, the sequence of precipitant addition, and the heat-treatment schedules.

2 Experimental High purity SnCl2 ·2H2 O (GR grade, Kanto Chemical Co., Ltd, Japan) and (COOH)2 (GR grade, Junsei Chemical Co., Ltd, Japan) were used as the raw material and precipitant, respectively. A 0.5 M SnCl2 aqueous solution was heated at a constant temperature (30, 50, 60, 70, 80, 90◦ C) and 0.5 M (COOH)2 aqueous solution was then added gradually. This process will henceforth be denoted as the ‘forward strike’ in this paper. For the investigation of the reaction mechanism, the precipitant 0.5 M (COOH)2 aqueous solution was Springer

896 Table 1 Experimental conditions and the morphology of the SnC2 O4 and SnO2

J Electroceram (2006) 17:895–898 Solution temp. (◦ C)

Precipitation method

Solvent

Morphology of SnC2 O4

Morphology of SnO2

30 50 60 70 80 90 70 80

Forward Forward Forward Forward Forward Forward Reverse Forward

Water Water Water Water Water Water Water Ethanol

Angular Angular Whisker Whisker Whisker Whisker Angular Angular

Irregular Irregular Whisker Whisker Whisker Whisker Irregular Irregular

heated to 70◦ C and a 0.5 M SnCl2 aqueous solution was added slowly. (denoted as the ‘reverse strike’) In order to determine the effect of the solvent, C2 H5 OH was also employed. In every experiment, a white SnC2 O4 precipitate was formed, which was washed with distilled water 5 times to remove the residual Cl− ions and dried at 90◦ C for 24 h. The SnC2 O4 precipitates were heated to 400◦ C for thermal decomposition into SnO2 and then cooled without temperature holding at 400◦ C. Two different heating rates (3.3◦ C/min and 1◦ C/min) were employed. Table 1 summarizes the experimental conditions and the morphology of the SnC2 O4 and SnO2 before and after thermal decomposition. The morphology and phase of the SnC2 O4 and SnO2 were examined by scanning electron microscopy (SEM, S-4300, Hitachi, Japan) and X-ray diffraction (XRD, MAX-IIA, RIGAKU, Japan), respectively.

3 Results and discussion Figure 1 shows the scanning electron micrographs of the SnC2 O4 precipitated at 30 and 70◦ C. The SnC2 O4 prepared at 30◦ C showed an angular morphology. In comparison, when the solution temperature for precipitation was increased to 70◦ C, the precipitate showed a whisker-like morphology. The whiskers were 0.5–2 μm thick and 40–100 μm long. The marked difference in morphology emanated from the change in the solution temperature. The solution temperature was systematically changed from 30 to 90◦ C (Table 1). Below 50◦ C, the SnC2 O4 precipitate had an angular shape. However, a whisker morphology began to develop at solution temperatures above 60◦ C. The thermal decomposition behavior of the SnC2 O4 precipitates was examined using differential thermal calorimetry (DSC) and thermogravimetry (TG). The results were shown in Fig. 2. The abrupt decrease in mass from ∼270 to 370◦ C with the large exothermic peaks could be assigned to the decomposition of SnC2 O4 into SnO2 , CO, and CO2 . Accordingly, the SnC2 O4 precipitate was calcined at 400◦ C to obtain the SnO2 phase. Figure 3(a) and (b) show the SEM photos of the SnO2 prepared via the calcination of the SnC2 O4 precipitate prepared at 30 and 70◦ C, respectively. Note that the heatSpringer

Fig. 1 Scanning electron micrographs of the SnC2 O4 precipitated at 30 and 70◦ C by the forward strike method. (water solvent system)

ing rate to the calcination temperature was 1◦ C/min. The morphology of SnO2 was quite similar to the precursor SnC2 O4 . This suggests that SnO2 whiskers can be prepared by increasing the precipitation temperature to >60◦ C (Table 1). The heating rate to the calcination temperature is another important parameter that affects the particle morphology.

Fig. 2 DSC/TG curves of SnC2 O4 precursor. (precipitated at 30◦ C by the forward strike method (water solvent system)

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Fig. 4 XRD patterns of the SnO2 powders at different heating rates to the decomposition temperature. The precursor (SnC2 O4 ) was precipitated at 70◦ C in a water solvent system. (a) heating rate to 400◦ C: 3.3◦ C/min (b) heating rate to 400◦ C: 1◦ C/min

Fig. 3 Scanning electron micrographs of SnO2 prepared by the calcination of SnC2 O4 precipitate at 400◦ C. (a) precipitation temperature: 30◦ C, heating rate to 400◦ C: 1◦ C/min, (b) precipitation temperature: 70◦ C, heating rate to 400◦ C: 1◦ C/min, (c) precipitation temperature: 70◦ C, heating rate to 400◦ C: 3.3◦ C/min (All the precipitates were prepared in water solvent system)

Figure 3(c) shows SEM photographs of a SnO2 whisker when the heating rate was 3.3◦ C/min. Although the whisker-like morphology was retained, the whisker surface was relatively rough and small particles were also found. Figure 4 shows the XRD pattern of the SnO2 powders at different heating rates. When the heating rate was slow (1◦ C/min), all the peaks could be indexed to tetragonal SnO2 (Fig. 4(b)). According to the JCPDS file (77-0450), the (110) peak is larger than the (101) peak. However, Fig. 4(b) shows that the (101) peak was larger than the (110) peak, which suggests a preferred orientation of the SnO2 whisker. When the heating rate was fast (3.3◦ C/min), the (110) peak become the main peak and an orthorhombic phase was formed (Fig. 4(a)). The orthorhombic SnO2 is a metastable phase that can be formed at the high pressure condition (150 kbar) [11]. However, this can be prepared in ambient pressure conditions via the oxidation of a nano-sized tin powder [12, 13], a

mechanochemical reaction between SnCl2 and Na2 CO3 [14], and high temperature thermal oxide synthesis [15]. Two main explanations for the orthorhombic SnO2 phase are the large effective internal pressure of the nano-sized particles [13, 14] and the presence of oxygen deficiencies due to incomplete oxidation [12, 15]. According to the Scherrer’s equation, the crystallite sizes for the powders in Fig. 3(b) and (c) were calculated to be 11 and 17 nm, respectively. Because the SnC2 O4 precursor is divalent one and CO gas evolved during decomposition can produce a reducing atmosphere, the fast passing of the decomposition region can induce incomplete oxidation. Both explanations for the orthorhombic phase should be taken into account. By combining the XRD patterns in Fig. 4 with the SEM photos (Fig. 3(b) and (c)), the rough surface morphology of the whisker and small particles in Fig. 3(c) can be attributed to the orthorhombic SnO2 phase. The results clearly suggest that the whisker-like morphology is determined at the stage of precipitation. The reason why a whisker-like precipitate can be prepared might be due to the solubility of SnCl2 and the degree of supersaturation during precipitation. Generally, when the solubility of a cation is very low or when the large amount precipitant is added abruptly, a powder-like morphology develops as a result of the high degree of supersaturation. In comparison, the degree of supersaturation should be minimized to develop a whisker-like morphology. The increase in solution temperature usually increases the solubility of a salt, which can decrease the degree of supersaturation. In order to test this hypothesis, the SnCl2 aqueous solution was added to a (COOH)2 aqueous solution at 70◦ C (reversestrike method). The degree of supersaturation at the initial stage of reverse-strike precipitation is very high. Indeed, the SnC2 O4 precipitate and resulting SnO2 did not show a Springer

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4 Conclusions A facile route for preparing SnO2 whiskers without a Nacontaining flux was developed. The whisker-like SnC2 O4 was precipitated when adding a (COOH)2 aqueous solution to a SnCl2 aqueous solutions heated to 60–90◦ C, and the high aspect ratio of the precipitate was retained after thermal decomposition into SnO2 at 400◦ C. In comparison, the morphology of SnC2 O4 became angular and isotropic at low solution temperatures (≤50◦ C), in ethanol, or when using the reverse-strike method. The whisker-like morphology of the SnC2 O4 and SnO2 was explained in terms of the supersaturation at the stage of precipitation. References

Fig. 5 Scanning electron micrographs of SnC2 O4 precipitated at 70◦ C with varying the sequence of precipitation and solvent system. (a) reverse-strike method, water solvent system, (b) forward-strike method, ethanol solvent system

whisker morphology (Fig. 5(a)). This suggests that a low supersaturation is very important for developing a whiskermorphology precipitate. In order to decrease the solubility of a salt, other solvents such as ethanol, propanol, and butanol are employed. Among the solvents used, only ethanol was found to dissolve SnCl2 . This indicates that the solubility of SnCl2 in alcohol solvent is not high. Figure 5(b) shows SEM photos of the SnC2 O4 precipitated by the forward-strike method at 70◦ C using an ethanol solvent. No whiskers were found, which supports this hypothesis. Throughout all the experiments, the morphology of the SnO2 was always similar to that of the SnC2 O4 precipitate. This suggests that a skeleton of SnC2 O4 remains even after the active decomposition reaction and that the morphology should be controlled at the stage of oxalate precipitation.

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1. C.C. Wang, S.A. Akbar, and M.J. Madou, J. Electroceram., 2(4), 273 (1998). 2. M. Okuya, S. Kaneko, K. Hiroshima, I. Yagi, and K. Murakami, J. Euro. Ceram. Soc., 21, 2099 (2001). 3. J. Oi-Uchisawa, A. Obuchi, R. Enomoto, S. Liu, T. Nanba, and S. Kushiyama, Applied Catalysis B., 26, 17 (2000). 4. S. Chappel and A. Zaban, Solar Energy Materials & Solar Cells, 71, 141 (2002). 5. A. Kolmakov, Y. Zhang, G. Cheng, and M. Moskovits, Adv. Mater., 15, 997 (2003). 6. M. Egashira and Y. Shimuzu, In: N. Yamazoe (ed), Chemical Sensor Technology (Kodansha & Elsevier, Tokyo, 1992), p. 1. 7. C. Xu, G. Xu, Y. Liu, X. Zhao, and G. Wang, Scripta Materialia, 46, 789 (2002). 8. Y. Liu, C. Zheng, W. Wang, C. Yin, and G. Wang, Adv. Mater. 13, 1883 (2001). 9. D.-F. Zhang, L.-D. Sun, J.-L. Yin, and C.-H. Yan, Adv. Mater., 15, 1022 (2003). 10. Y. Wang, X. Jiang, and Y. Xia, J. Am. Chem. Soc., 125, 16176 (2003). 11. K. Suito, N. Kawai, and Y. Masuda, Mater. Res. Bull., 10, 677 (1975). 12. C.H. Shek, J.K.L. Lai, G.M. Lin, Y.F. Zheng, and W.H. Liu, J. Phys. Chem. Solids, 58, 13 (1997). 13. M.-Y. Huh, S.-H. Kim, J.-P. Ahn, J.-K. Park, and B.-K. Kim, Nanostructured Materials, 11, 211 (1999). 14. L.M. Cukrov, P.G. McCormick, K. Galatis, and W. Wlodarski, Sensors and Actuators B., 77, 491 (2001). 15. Z.R. Dai, J.L. Gole, J.D. Stout, and Z.L. Wang, J. Phys. Chem. B., 106, 1274 (2002).