Please cite this article in press as: A.A. Firooz, et al., Effects of flower-like, sheet-like and granular SnO 2 nanostructures prepared by solid-state reactions on CO sensing, Mater. Chem. Phys. (2008), doi:10.1016/j.matchemphys.2008.11.028 ARTICLE IN PRESS G Model MAC-13055; No.of Pages4 Materials Chemistry and Physics xxx (2008) xxx–xxx Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Effects of flower-like, sheet-like and granular SnO 2 nanostructures prepared by solid-state reactions on CO sensing Azam Anaraki Firooz a , Ali Reza Mahjoub a,∗ , Abbas Ali Khodadadi b a Department of Chemistry, Tarbiat Modares University, 14115-175 Tehran, Iran b School of Chemical Engineering, University of Tehran, 11155-4563 Tehran, Iran article info Article history: Received 19 August 2008 Received in revised form 29 October 2008 Accepted 17 November 2008 Keywords: SEM XRD Semiconductor Nanostructures abstract Nanostructured SnO 2 with different morphologies of flower-like, sheet-like and granular have been suc- cessfully prepared viaa solid-state reaction in the presence of NaBr, NaCl, and NaF, respectively. The added salts not only prevent a drastic increase in the size of the tin species but also provide suitable conditions for the oriented growth of primary nanoparticles. The formation mechanisms of these materials by solid- state reaction at ambient temperature are proposed. The gas sensitivity experiments have demonstrated that the as-synthesized SnO 2 especially, flower-like morphology, exhibit high sensitivity to CO, which may of fer potential applications in gas sensors. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Hierarchical self-assemblies of nano-/micro-crystallites with specific morphology are of great interest in areas of chemistry and materials science because of their unique and exciting properties [1]. Some reports indicate that the catalytic selectivity and sensitiv- ity of these structures are significantly improved, compared with other shapes [2,3]. SnO 2 is an n-type semiconductor with a long band gap [4], and is well known for its applications in gas sensors [5,6], dye-base solar cells [7], optoelectronic devices [8], electrode materials [9] andcatalysts [10]. Thus, designing andpreparing SnO 2 materials with novel morphology are of significant importance in meeting the scientific and technological applications. There are many methods to synthesize of different morphology of this mate- rial. Ohgi et al. reported the evolution of nanoscale SnO 2 flakes into hierarchically structures by subsequent hydrothermal treatment [11]. Xie and co-workers prepared 2D hierarchical SnO 2 flower- like nanostructures without post-treatment of calcinations, taking advantage of slow oxidation of tin foil by the solution of KBrO 3 and NaOH [12]. Mu and co-workers synthesized the flowerlike SnO 2 quasi-square submicrotubes by reaction between SnCl 2 and oxalic acid in ethanol solution, followed by calcination in air[13]. All these methods to dioxide nanoparticles are in general complicated and expensive. There are many advantages in the solid-state reaction approach such as: (a) simple, cheaper and convenient; (b) involve ∗ Corresponding author. Tel.: +98 21 82883442; fax: +98 21 88007930. E-mail address: mahjouba@modares.ac.ir (A.R. Mahjoub). less solvent and reduce contamination; (c) give high yields of prod- ucts [14]. In this paper, we synthesize SnO 2 nanostructures with differ- ent morphologies by solid-state reaction method. Our studies show that this method is not only a simple process but also gives as uniform and monodisperse products as those by other lucrative methods.Wehavealso investigated theeffect ofhalogen saltsonthe morphology and explained in light of the proposed mechanisms. We found that the as-prepared SnO 2 materials with flower-like morphology exhibit higher sensitivity to CO gas and thus are expected to be useful in industrial applications such as gas sensors. 2. Experimental 2.1. Preparation A mixture of SnCl 2 ·2H 2 O (0.01mol, 3.51 g) and NaOH (0.038 mol, 2.13 g) pow- ders was ground for 30 min. Then, each of NaBr, NaCl, and NaF halogen salt with a weight ratio of 2:1 was added to the system and ground for another 30 min at room temperature. The reaction began immediately during the mixing process (accom- panying an emission of water vapor from the system). The products dried in air to yield black SnO powder. The powder was calcined at 40 0 and 600 ◦ C for 2h in air and washed with distilled water for removing the halogen salt and dried in 80 ◦ C. 2.2. Characterization The morphology of tin oxide powders was determined by using scanning electron microscopy (SEM) of a Holland Philips XL30 microscope. X-ray powder diffraction (XRD) patterns of the powders were recorded in ambient air with using a Holland Philips Xpert X-ray powder diffraction (Cu K␣, = 1.5406 Å), at scanning speed of 2 ◦ min −1 from 20 ◦ to 80 ◦ (2Â). Specific surface area of SnO 2 nanoparticles were determined by nitrogen adsorption, after degassing at 300 ◦ C for 2 h, using a surface area analyzer (CHEMBET 3000) and BET method. 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.11.028 Please cite this article in press as: A.A. Firooz, et al., Effects of flower-like, sheet-like and granular SnO 2 nanostructures prepared by solid-state reactions on CO sensing, Mater. Chem. Phys. (2008), doi:10.1016/j.matchemphys.2008.11.028 ARTICLE IN PRESS G Model MAC-13055; No.of Pages4 2 A.A. Firooz et al. / Materials Chemistry and Physics xxx (2008) xxx–xxx Fig. 1. XRD patterns of flower-like (NaBr) calcined at: (a) 400 ◦ C (NaBr400); (b) 600 ◦ C (NaBr600) for 2h. 2.3. Gas sensing measurement A paste of tin oxide powder was applied on an aluminatube with gold electrodes already deposited on it. The sample was dried and calcined at 400 ◦ C. Thus obtained sensor was placed in a glass holder immersed in a molten salt bath, temperature of which was accurately controlled by a PID temperature controller. The sensor was connected to an electrical circuit using platinum electrodes. The DC electrical mea- surement was made by using an applied voltage of 4.0 V onto a known resistance in series with the sensor. The DC voltage across the sensor was read out using an A/D converter and data was transferred to the computer for further processing. The sen- sor response was measured at different temperatures in the presence of 1000 ppm CO in air. 3. Result and discussion 3.1. Tin oxide morphology The structure of tin oxide powders was determined by XRD, as shown in Figs. 1–3. Major SnO 2 cassiterite structure (JCPDS no. 41-1445) is observed in all XRD patterns. The marked peaks Fig.2aand3a are attributed to SnOphase(JCPDS no.6-395), which is stable up to 400 ◦ C. The ratio of (1 01) SnO peak to (1 10) SnO 2 peak intensities, as a semiquantitative measure of SnO/SnO 2 ratio, are included in Table 1. After calcination at600 ◦ C, the (10 1) diffraction peak intensity of SnO is reduced, as shown in Fig. 2b and 3b. All XRD patterns show that, when the calcination temperature increases, the intensity of the diffraction peaks increases, indicating a high degree ofcrystallinity andgrain sizes ofthenanoparticles. The crys- tal grain sizes were calculated from the FWHMin XRD pattern using the Debye–Scherrer’s equation and listed in Table 1. Scanning electron microscopy was employed to study the mor- phologies of the tin oxide samples. Fig. 4a–c shows that flower-like, sheet-like, and granular morphologies of tin oxide are formed, when NaBr, NaCl, or NaF halogen salts are used in the salt-assisted solid-state synthesis, respectively. The flower-like morphology reveals that the structure is built up many sheet-like nanoparticles. Fig. 2. XRD patterns of sheet-like (NaCl) calcined at: (a)40 0 ◦ C (NaCl400); (b) 600 ◦ C (NaCl600) for 2h. Fig. 3. XRD patterns of nanoparticle (NaF)calcined at: (a) 400 ◦ C (NaF400); (b)600 ◦ C (NaF600) for 2h. 3.2. Growth mechanism of SnO 2 nanostructures Li et al. have proposed that in the first step of the solid-state reaction, SnO fine particles are formed [15]. The reaction is often self-initiated and self-sustained with H 2 O vapor releasing after grinding of the mixture of the precursors. After the calcination at 400or600 ◦ C, the SnO is mostly converted to SnO 2 nanoparticles. It is well known that the structure of products by a solid-state reac- tion depends on the rate of nucleationand growth. It isalsothought that adding inorganic salts causes to reduce the overall reaction rate and broaden the distribution of product. NaBr, NaCl, and NaF as salt-assisted additives are expected to cause cage-like shells surrounding the SnO particles, preventing their growth. Adjacent nanoparticles rotate to find the low-energy configuration repre- sented by a coherent particle–particle interface [16]. As a result, the added salts help to form of suitable morphology with high yields. Table 1 The sizes, sensitivity, semiquantitative measure of SnO/SnO 2 ratio and physical properties of the as prepared materials. Samples Color Morphology Crystallite size (nm) Sensitivity Surface area (m 2 g −1 ) Grain size (nm) SnO/SnO 2 (ratio of peaks intensity) NaBr400 Pale Flower-like 16.78 71.5 45 19.18 0 NaBr600 White Flower-like 20.52 9.88 25 34.53 0 NaCl400 Gray Sheet-like 19.26 7.46 34.6 24.95 1/3 NaCl600 Pale Sheet-like 25.72 17.7 14.09 61.27 1/7 NaF400 Brown Nanoparticle 10.59 4.181 49 17.6 1/4 NaF600 Brown Nanoparticle 15.95 16.34 30 28.77 1/8 Please cite this article in press as: A.A. Firooz, et al., Effects of flower-like, sheet-like and granular SnO 2 nanostructures prepared by solid-state reactions on CO sensing, Mater. Chem. Phys. (2008), doi:10.1016/j.matchemphys.2008.11.028 ARTICLE IN PRESS G Model MAC-13055; No.of Pages4 A.A. Firooz et al. / Materials Chemistry and Physics xxx (2008) xxx–xxx 3 Fig. 4. The SEM images of (a) NaBr (flower-like), (b) NaCl (sheet-like), and (c) NaF (granular). 3.3. CO sensing The CO sensitivity is defined as S =R air /R CO where R air and R CO are resistances of the tin oxide in air and in CO, respectively [17]. Figs. 5–7 show the changes in sensitivity of the tin oxide materi- als calcined at 400 and 600 ◦ C, when their thick-film sensors were exposed to CO at various temperatures. Maximum sensitivities occur at about 275–300 ◦ C for all sensor materials at the two calci- Fig. 5. Temperature-dependent sensitivity of SnO 2 flower-like (NaBr). (a) NaBr 400; (b) NaBr 600. nation temperatures. The highest maximum sensitivity is observed for the flower-like morphology calcined at 400 ◦ C. Calcination at the higher temperature of 600 ◦ C leads to an increase in the grain sizes, as indicated by XRD and BET results (Table 1). Lower sensitiv- ities are observed for the sheet-like and granular morphologies. As the calcination temperature increases from 400 to 60 0 ◦ C, the max- imum sensitivities of sheet-like and granular tin oxides increase, while their grain sizes increase (see Table 1 and Figs. 6 and 7). The Fig. 6. Temperature-dependent sensitivity of SnO 2 sheet-like (NaCl). (a) NaCl 400; (b) NaCl 600. Please cite this article in press as: A.A. Firooz, et al., Effects of flower-like, sheet-like and granular SnO 2 nanostructures prepared by solid-state reactions on CO sensing, Mater. Chem. Phys. (2008), doi:10.1016/j.matchemphys.2008.11.028 ARTICLE IN PRESS G Model MAC-13055; No.of Pages4 4 A.A. Firooz et al. / Materials Chemistry and Physics xxx (2008) xxx–xxx Fig. 7. Temperature-dependent sensitivity of SnO 2 granular (NaF). (a) NaF 400; (b) NaF 600. XRD patterns of the same samples show the presence of stannic suboxide (i.e. SnO), which decreases as the calcination temperature increases. The sensitivity to a target gas strongly depends on the ease of diffusion ofgas molecules insidethe sensor[18–20]. Thus,the struc- ture and morphology of materials can be correlated with the sensor performance. Flower-like SnO 2 consists of numerous nanoparticles joined together into flower-like structure, resulting in much more active exposed sites for gas chemisorptions. Thus, the realization of a high sensitivity of flower-like morphology may be explained in terms of rapid gas diffusion onto the entire sensing surface due to the specific morphology of this material. Usually the gas sensitivity of metal oxide semiconductors decreases, as their sizes increase, due to lower surface areas and defect density [21]. It sounds that, the presence of tin with lower oxidation state in tin oxide sensors diminishes their sensitivity to reducing gases such as CO [22,23]. On the other hand, the presence of SnO causes a decrease in oxygen vacancies in the samples. Therefore, not only the morphology but also the presence of SnO in sheet-like and granular tin oxides, contributes to their lower sensitivity to CO. 4. Conclusion We report the preparation of flower-like, sheet-like and granu- lar morphologies of SnO 2 by solid-state reactions in the presence of NaBr, NaCl, and NaF salts, respectively. The salts added are expected to cause cage-like shells surrounding the SnO particles, prevent- ing their growth of nanoparticles. Adjacent nanoparticles rotate to find the low-energy configuration represented by a coherent particle–particle interface, resulting in particular morphologies. The gas sensitivity experiments showed that the SnO 2 flower- like offered higher sensitivity than SnO 2 sheet-like and granular. The high sensitivity may be explained in terms of rapid gas dif- fusion onto the sensing surface. In addition, the XRD results reveal the presence of a stannic suboxide, which explains in part the lower sensitivity to CO shown by the sheet-like and granular nanostruc- tures. Acknowledgments Supports for this investigation by Tarbiat Modares University and University of Tehran are gratefully acknowledged. References [1] C. Sun, J. Sun, G. Xiao, H. Zhang, X. Qiu, H. Li, L. Chen, J. Phys. Chem. B 110 (2006) 13445. [2] X. Li, Y. Xiong, Z. Li, Y. Xie, Inorg. Chem. 45 (2006) 3493. [3] Y. Zhang, X. He, J. Li, Z. Miao, F. Huang, Sens. Actuators B: Chem. 132 (2008) 67. [4] A.A. Firooz, A.R. Mahjoub, A.A. Khodadadi, Mater. Lett. 62 (2008) 1789. [5] F. Pourfayaz, A. Khodadadi, Y. Mortazavi, S.S. Mohajerzadeh, Sens. Actuators B: Chem. 108 (2005) 172. [6] G.X. 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