ZrO2–SnO2 nanocomposite film containing superlattice ribbons

Journal of Molecular Structure 975 (2010) 47–52

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ZrO2–SnO2 nanocomposite film containing superlattice ribbons Hongjun Ji, Xiaoheng Liu *, Xin Wang ** Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China

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Article history: Received 3 March 2010 Received in revised form 25 March 2010 Accepted 25 March 2010 Available online 2 April 2010 Keywords: Superlattice ZrO2–SnO2 Nanofilm Self-assemble Ribbon

a b s t r a c t An air–water interfacial ZrO2–SnO2 nanocomposite film has been self-assembled with sodium dodecyl sulfonate (SDS) as template and metal alkoxide as precursor. Results show that a number of superlattice ribbons with a periodic spacing of 3 nm have been successfully assembled in the ZrO2–SnO2 film. The related element dispersion analysis and the self-assembly mechanism of these ribbons are carefully discussed. After heat-treatment, these superlattice ribbons convert to porous nanorods due to the removal of SDS template. Both the original ZrO2–SnO2 nanocomposite film and the heat-treated product may have a potential to apply on gas sensors. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Layered oxides with a periodic spacing of several nano-meters have attracted much attention because these materials with regular structures that resemble superlattices [1,2] have taken rise to a variety of physical and chemical properties. For instance, Paulsen 4þ et al. [3] reported layered O2-type A2=3 ½M02þ 1=3 M2=3 —O2 (A = Li, Na; 0 M = Ni, Mg; and M = Mn, Ti) and Ida et al. [4] investigated interesting photoluminescence properties of layered Eu/TiO composite. It is well-known that the metal oxide composite organized by zirconium dioxide (ZrO2) and tin dioxide (SnO2) has been widely used as catalysts and gas sensors due to the special surface acidity of ZrO2 and the excellent electrical property of SnO2. In order to obtain high performance on catalysts and gas sensors, a great many synthesis methods [5–8] have been applied on the preparations of ZrO2–SnO2 composites, e.g. co-precipitation method and sol–gel technique. However, few works have reported well-ordered structural ZrO2–SnO2 nanocomposite through those conventional methods. Herein, we will propose a self-assembly method to prepare ZrO2–SnO2 nanocomposite film with regular nanostructures. In the previous, White et al. took interesting research on the selfassembly of well-ordered SiO2 film [9–11], TiO2 film [12,13] and ZrO2 film [14] with surfactants as template and our group prepared a series of (surfactant–protein composite)-templated metal oxide

* Corresponding author. Tel./fax: +86 25 8431 5054. ** Corresponding author. Tel./fax: +86 25 8431 5054. E-mail addresses: [email protected] (X. Liu), (X. Wang).

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films [15–18], e.g. target-like multiring ZrO2 film, mesoporous TiO2 film and lamellar ZrO2–TiO2 complex film. Accordingly in this work, an air–water interfacial ZrO2–SnO2 nanocomposite film containing superlattice ribbons will be firstly self-assembled with sodium dodecyl sulfonate (SDS) as template and zirconium butoxide and tin butoxide as precursor. Then, the detailed chemical organizations and the formation mechanism of these superlattice ribbons are discussed. Finally, the structural transformation of ZrO2–SnO2 product after heat-treatment will be investigated. 2. Experimental 2.1. Chemicals Zirconium (IV) n-butoxide (Zr(OC4H9)4, 28%) and sodium dodecyl sulfonate (SDS, 99%) were purchased from Alfa–Aesar. Tin (IV) tert-butoxide (Sn(OC4H9)4, 99.99%) was purchased from Sigma–Aldrich. Chlorohydric acid (36–38%) was purchased from Nanjing chemical reagent Co. Ltd. All the chemicals were used without any further purification. 2.2. Preparations Preparation for ZrO2–SnO2 nanocomposite film was as followed. ZrO2–SnO2 nanocomposite film was prepared by a self-assembly method. First, 0.15 g SDS was mixed with 18.4 ml deionized water in a 25 ml beaker by stirring magnetically at 40 °C for 10 min to prepare the template solution. At the same time, the precursor solution was prepared by stirring 0.84 g

0022-2860/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.03.077

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Zr(OC4H9)4, 0.15 g Sn(OC4H9)4, 0.64 ml HCl and 2 ml deionized water in a 10-ml beaker for 5 min. Then, the precursor solution was transferred into a Petri dish with a diameter of 90 mm and a depth of 10 mm, and subsequently, the template solution was added as a coating. Over a reaction time 12 h at room temperature (21 °C), the white ZrO2–SnO2 composite film could be clearly observed at the air–water interface. Finally, by calcination of the white film at 500 °C for 6 h, the gray ZrO2–SnO2 particles were obtained. 2.3. Characterizations

Fig. 1. XRD pattern of ZrO2–SnO2 nanocomposite film with SDS as template.

ZrO2–SnO2 nanocomposite film was transferred to a glass substrate and dried for 24 h, and then the substrate was directly detected by X-ray diffraction (XRD) with a Bruker D8 advance diffractometer, using monochromatic Cu Ka radiation (k = 1.5406 Å) operated at an accelerating voltage of 40 kV and an emission current of 40 mA. The fresh sample dropped to a copper grid after ultrasonic dispersion for 5 min was observed by a JEOL-2100

Fig. 2. TEM (a and b) and HRTEM (c and d) observations of ZrO2–SnO2 nanocomposite film with SDS as template.

Fig. 3. (a) A typical TEM image of ZrO2–SnO2 ribbon for the EDX analysis of selected areas: ‘a’ area is the bright superlattices with the composition of Sn0.6Zr0.4O2; ‘b’ area is the black pieces covered on superlattice with the composition of Sn0.7Zr0.3O2; ‘c’ area is the supporter for the superlattices with the composition of Sn0.7Zr0.3O2 and (b) an architecture model for ZrO2–SnO2 ribbon carrying superlattice structures and supporter.

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Fig. 4. EDX analysis for ZrO2–SnO2 ribbon with selected areas marked in Fig. 3A. (a) The bright superlattices with the composition of Sn0.6Zr0.4O2; (b) the black pieces covered on the bright superlattices with the composition of Sn0.7Zr0.3O2; and (c) the supporter for the superlattices with the composition of Sn0.7Zr0.3O2.

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transmission electron microscopy (TEM) at 200 kV and a Gatan 794 charge-coupled device (CCD) camera. Scanning transmission electron microscopy in high angle annular dark field mode (STEMHAADF) equipped with energy dispersive X-ray analysis (EDX, model: Genesis2000) was applied to detect element dispersions of the samples. In addition, the obtained grey ZrO2–SnO2 particles after heat-treatment were characterized by XRD and TEM.

3. Results and discussion In the 2h angle range 2–15°, XRD pattern of ZrO2–SnO2 nanocomposite film with SDS as template is shown in Fig. 1. Note that no obvious diffraction peaks are presented between 15° and 80°. According to some literature [12–14,16,17], the strongest diffraction peak appeared at low 2h angle (<10°) suggests a well-ordered structure consisted in the as-synthesized metal oxide films and the corresponding d value means the spacing between lamellas or the average diameter of high-ordered mesopores. In this ZrO2–SnO2 film, the predominant peak is appeared at 2h = 2.8° with a d spacing of 3.2 nm. TEM and HRTEM observations of ZrO2–SnO2 nanocomposite film are displayed in Fig. 2. A number of ribbons and some piecelike structures are observed in Fig. 2a. It is interesting that these ribbons are composed of high-ordered superlattice structures (Fig. 2b). Note that many black pieces are covered on the bright superlattice regions and some irregular pieces are well distributed around these ribbons, acting as supporters. From the inset of Fig. 2b, a typical ribbon has a width 100 nm and the length/width radio is estimated 6. The extremely periodic order in these superlattice ribbons is further confirmed by the HRTEM observations in Fig. 2c and d. The periodic d spacing for the superlattices is measured 3.5 nm, corresponding with the XRD result of 3.2 nm in Fig. 1. In fact, the black layers in these superlattice ribbons indicate ZrO2–SnO2 complex layers and the SDS micelles existed between the black layers are much brighter. Remarkably, it is found that even exposed to the intensive light of TEM for a long time, these superlattice structures were not broken. In the following, the element dispersion of these superlattice ribbons will be analyzed in detail. In order to analyze the detailed organizations of ZrO2–SnO2 superlattice ribbons, three main areas have been labeled on a typical ribbon in Fig. 3A, including the bright superlattice area with the composition of Sn0.6Zr0.4O2 (Fig. 3Aa), the black pieces covered on the bright superlattice area with the composition of Sn0.7Zr0.3O2 (Fig. 3Ab) and the supporter for the superlattice ribbons with the composition of Sn0.7Zr0.3O2 (Fig. 3Ac). The architecture model in Fig. 3B shows that these ribbons are mainly composed of ZrO2– SnO2 superlattices and ZrO2–SnO2 supporters. During the growth of superlattice ribbons, ZrO2–SnO2 supporters first formed, and subsequently, the ZrO2–SnO2 superlattices started to self-assemble above those supporters in the pathway of ‘layer by layer’. All the EDX images in Fig. 4 shows that the elements C, O, S, Sn, and Zr are clearly observed, indicating that SDS, ZrO2, and SnO2 have been successfully assembled in these ribbons. In comparison with the EDX analysis of the bright superlattices in Fig. 4a and that of the black pieces covered on the bright superlattices in Fig. 4b, the element S (1.9%) seems a little lower of the supporter in Fig. 4c, suggesting that the SDS content is relatively a little higher in these superlattices. No significant difference can be observed for the S content in Fig. 4a and b. This means that well distributed SDS micelles took good conditions for the facilitating of high-ordered superlattice structures. In addition, the Sn/Zr molar ratio (2.3) for the supporter is much higher than that of the bright superlattice area (1.4) and similar to that of the covered black pieces (2.1). This confirms that these black pieces are mainly composed

of SnO2, SDS and a relatively less amount of ZrO2. Considering the factor that the precursors Zr(OC4H9)4 and Sn(OC4H9)4 were hydrolyzed in the air atmosphere, the Sn/Zr molar ratio for the raw was estimated 1, and in contrast, this ratio was much higher in the superlattice ribbons. This should be attributed to the aggregation of ZrO2 and SnO2 in superlattice ribbons with the help of SDS dimeric micelles. The formation mechanism for the self-assembled superlattice structures in ZrO2–SnO2 ribbons is purposely proposed, as shown in Fig. 5. First, by van der Waals force between alkyl groups (– C12H25), the surfactant SDS in deionized water formed dimeric micelles with negative headgroups (—SO 3 ). It should mention that the SDS dimeric structure under special concentrations has been proposed and proved by a great deal of literature [13,15,18,20]. Through strong electrostatic interactions between —SO 3 in dimeric 2nþ SDS micelles and —ðZrOÞ2mþ m —ðSnOÞn — that derived from the hydrolysis of ZrO(C4H9)4 and Sn(OC4H9)4 in strong acidic conditions, small superlattice ribbons began to form, slowly grew up and rose up. The dimer micelles played a key role in determining the superlattice structures as seen in Figs. 2 and 3. According to

Fig. 5. A self-assembly mechanism for the superlattices in ZrO2–SnO2 ribbons.

Fig. 6. XRD pattern of ZrO2–SnO2 product after calcination at 500 °C for 6 h.

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Fig. 7. TEM observations of ZrO2–SnO2 product after calcination at 500 °C for 6 h.

the previous research [17], the d spacing analysis was determined by the equation d = (L  2  X)  cos(a), where L/Å was the critical length of surfactant molecule, X/Å was the van der Waals radii of the coupled surfactant headgroup and a/deg was the angle of the titled surfactant. It is assumed that L was similar to that of the sodium dodecyl sulfate (2.2 nm), the a was 5°, 15°, 40°, and 45° provided by Smith et al. [19] and the X was a positive value. In the aqueous environment, SDS polar group would combine some water in the form of SDS1/8H2O (a = 5°), SDS1/2H2O (a = 40°) or SDSH2O (a = 45°). By calculations, two possible results were produced: one was that X = 12 Å when a = 5° and the other was X = 2 Å when a = 40°. However, when X was 2 Å, the interactions between SDS molecules were easily to be broken. So SDS1/8H2O (X = 12 Å) dimers directed stable lamellas for the formation of ZrO2–SnO2 superlattice ribbons. XRD pattern of ZrO2–SnO2 product after heat-treatment is shown in Fig. 6. The predominant diffraction peaks could be indexed to ZrO2, corresponding to the JCPDS 50-1089. The peaks appeared at 2h = 30°, 50°, 60° are assigned to the diffractions of (0 1 1), (1 1 2) and (1 2 1) of the tetragonal ZrO2. Only a small amount of orthorhombic ZrO2 can be found (JCPDS 33-1483). Furthermore, the diffraction peaks situated at 2h = 26°, 33°, and 37° are assigned to the diffractions of (1 1 0), (1 0 1) and (2 0 0) of the cassiterite, as reported by Gu et al. [21]. A few unidentified diffraction peaks may be resulted from the pyrolysis of ZrO2–SnO2 composite. TEM observations of ZrO2–SnO2 products after heat-treatment are shown in Fig. 7. A number of well-dispersed rod-like structures are observed in Fig. 7a. An average length/width ratio (6) of the aggregated rods in Fig. 7b is similar to that of the typical superlattice ribbon in Fig. 2b. Fig. 7c shows a typical rod with a variety of porous nanostructures and the average diameter of these pores

in Fig. 7d is measured 4 nm. Overall, the well-ordered ZrO2– SnO2 superlattice ribbons converted to porous nanorods after heat-treatment, due to the removal of SDS template and other physical and chemical reactions. The gas sensing performance of these as-prepared ZrO2–SnO2 products will have a preliminary discussion. For SnO2 based materials, the gas sensing performance can be explained by the wellknown space-charge layer model. It has been found that SnO2 nanostructure performs better sensitivity when its size is reduced to a scale close to or smaller than the space-charge layer of SnO2 (6 nm) [22]. This has been demonstrated by the high gas sensitivity of SnO2 materials with various nanostructures, e.g. nanotubes [23], nanowires [24,25] and mesopores [22,26,27]. Accordingly, these self-assembled ZrO2–SnO2 superlattice ribbons with a periodic d spacing of 3.2 nm could have a high gas sensing performance at room temperature. In addition, the obtained porous ZrO2–SnO2 nanorods after heat-treatment may have additional advantages in enhancing the sensor, due to the high surface area and more efficient access of target gases to the active sites.

4. Conclusion By a self-assembly method, a high-ordered ZrO2–SnO2 nanocomposite film containing superlattice ribbons with a d spacing of 3.2 nm has been prepared using SDS as template and Zr(OC4H9)4 and Sn(OC4H9)4 as precursor. These as-synthesized superlattice ribbons are composed of the bright superlattices (Sn0.6Zr0.4O2), the black pieces covered on the bright superlattices (Sn0.7Zr0.3O2) and the supporters (Sn0.7Zr0.3O2). The proposed formation mechanism is attributed to the strong electrostatic interactions between the negative —SO 3 headgroups in SDS dimeric micelles and the po-

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2nþ sitive precursor —ðZrOÞ2mþ m —ðSnOÞn —. After heat-treatment, these ZrO2–SnO2 superlattice ribbons converted to porous nanorods with an average diameter measured 4 nm. Next, we will continue the studies on the applications of these ZrO2–SnO2 products containing superlattice ribbons or porous nanorods on high sensitive gas sensors.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50772048), the Natural Science Foundation of China and China Academy Engineering Physics (No. 10776014) and the innovation fund from the Graduate School of Nanjing University of Science and Technology (No. 200903009). References [1] J.R. Williams, A.L.E. Smalley, H. Sellinschegg, C.D. Hafer, J. Harris, M.B. Johnson, D.C. Johnson, J. Am. Chem. Soc. 125 (2003) 10335–10341. [2] S.Y. Hong, R.P. Biro, Y. Prior, R. Tenne, J. Am. Chem. Soc. 125 (2003) 10470– 10474. [3] J.M. Paulsen, R.A. Donaberger, J.R. Dahn, Chem. Mater. 12 (2000) 2257–2267. [4] S. Ida, U. Unal, K. Izawa, O. Altuntasoglu, C. Ogata, T. Inoue, K. Shimogawa, Y. Matsumoto, J. Phys. Chem. B 110 (2006) 23881–23887. [5] G.B. Han, N.K. Park, J.D. Lee, S.O. Ryu, T.J. Lee, Catal. Today 111 (2006) 205–211. [6] D.R. Burri, K.M. Choi, D.S. Han, Sujandi, N.Z. Jiang, A. Burri, S.E. Park, Catal. Today 131 (2008) 173–178. [7] M.S. Selim, Sensors Actuators, A 84 (2000) 76–80. [8] G.J. Fang, Z.L. Liu, Z.C. Zhang, K.L. Yao, Phys. Status Solidi (a) 156 (1996) 81–85.

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