Journal of Alloys and Compounds 454 (2008) 261–267 Synthesis and characteristics of Fe 3+ -doped SnO 2 nanoparticles via sol–gel-calcination or sol–gel-hydrothermal route L.M. Fang a , X.T. Zu a,b,∗ , Z.J. Li a ,S.Zhu c , C.M. Liu a , W.L. Zhou e , L.M. Wang c,d a Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b International Center for Material Physics, Chinese Academy of Sciences, Shengyang 110015, PR China c Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USA d Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104, USA e Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148, USA Received 15 October 2006; received in revised form 3 December 2006; accepted 5 December 2006 Available online 29 December 2006 Abstract Fe 3+ -doped SnO 2 nanoparticles were prepared by sol–gel-calcination and sol–gel-hydrothermal routes, respectively, and their microstructure as well as physical and chemical properties have been characterized and compared. Based on XRD, TEM, and Fourier transform infrared (FT-IR) anal- yses, the SnO 2 crystallites with the tetragonal rutile structure formed directly during a hydrothermal process. Compared with the sol–gel-calcination route, sol–gel-hydrothermal route led to better dispersed nanoparticles with a narrower size distribution and a larger Brunauer–Emmett–Teller (BET) surface area. Also, the Fe 3+ -doped SnO 2 nanoparticles prepared by sol–gel-hydrothermal route had a better thermal stability against agglomeration and crystalline grain size growth than those prepared by the sol–gel-calcination route. XRD, EDS, and diffuse reflectance spectra (DRS) analyses proved that the Fe 3+ and SnO 2 formed a solid solution in the nanoparticles with both processing routes. A significant red shift in the UV absorbing band edge was observed with the increasing Fe 3+ content. © 2006 Elsevier B.V. All rights reserved. Keywords: SnO 2 ; Sol–gel; Hydrothermal; Calcination; Doping; Nanoparticles 1. Introduction Nano-sized SnO 2 particles, as an n-type semiconductor with a wide band gap (E g = 3.6 eV, at 25 ◦ C), have been attracting much attention due to their transparency and sensitivity to reduc- ing gases. They have a wide range of applications in gas sensors [1–3], optoelectronic devices [4–5], dye-base solar cells [6], sec- ondary lithium batteries electrode materials [7], and catalysts [8]. One area of primary importance is the field of solid state gas sensors for environmental monitoring, such as for CO, NO, and C 2 H 5 OH, where SnO 2 has been established as the predom- inant sensing material. It is generally accepted that increasing the surface/bulk ratio by decreasing the grain size of rutile SnO 2 nanoparticles is crucial for achieving high-sensitivity in gas sen- ∗ Corresponding author at:Department of Applied Physics, University ofElec- tronic Science and Technology of China, Chengdu 610054, PR China. Tel.: +86 28 83201939; fax: +86 28 83201939. E-mail address: xiaotaozu@yahoo.com (X.T. Zu). sors. One of the other most common ways to modify the charac- teristics of the material is introducing dopants into the structure. Many results have shown that several additives (cations: Fe, Cu, Co, Cr, Al, Mn, Mg; anions: P, S) can lead to an increase of the surface area of SnO 2 -based powders [3,9–14]. The added active elements stabilize the SnO 2 surface, and promote a decrease in grain size. It has been found that Fe 3+ -doped SnO 2 nanopar- ticles with lower crystallinity and high surface area have the higher catalytic activity and sensor signal than both pure SnO 2 and Fe 2 O 3 [15,16]. Thus, nanocrystalline Fe 3+ /SnO 2 have been investigated as a material for oxygen, ethanol, hydrogen sulfide, carbon monoxide, and water vapor gas sensors [3,9,17]. The properties of metal ions (including transition metal ions and rare-earth elements)-doped SnO 2 can be altered to differ- ent states due to many factors during the preparation process [18,19]. Thus, the final products are related to both composi- tion and the processing method used for synthesize the material. To take advantage of the properties of metal ions-doped SnO 2 nanoparticles, a number of attempts have been made by devel- oping effective synthetic techniques in the preparation of metal 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.12.014 262 L.M. Fang et al. / Journal of Alloys and Compounds 454 (2008) 261–267 ions-doped SnO 2 nanoparticles, such as mechanical alloying [20], sol–gel [3,11,14,18], co-precipitation [13,16], and chemi- cal vapor deposition [21,22]. The sol–gel method is the most effective method due to its capability in controlling the tex- tural and surface properties of metal oxides. This method is mainly based on the hydrolysis and polycondensation of a metal salt, which ultimately yields hydroxide or oxide under cer- tain conditions [19]. Generally, sol–gel-derived precipitates are amorphous in nature. So calcination in air is needed for the trans- formation from amorphousto rutile phase to complete. However, the calcinationprocess generally lead to serious particle agglom- eration, grain growth, and thus, a small total surface area, which decreases the sensitivity to a number of reducing gases. The hydrothermal processing represents an alternative to the calci- nation for the crystallization of SnO 2 under mild temperatures. In the hydrothermal treatment, grain size, particle morphol- ogy, crystallinity, and surface chemistry can be controlled via processing variables such as sol composition, pH, reaction tem- perature and pressure, aging time, and nature of solvent and additive. It provides a facilitated route to prepare uniform, well- dispersed, and well-crystallized SnO 2 nanoparticles. However, the hydrothermal methodhas not been usedfor the preparation of Fe 3+ -doped SnO 2 nanoparticles. It is well known that the method of preparing Fe 3+ -doped SnO 2 nanoparticles can determine the physico-chemical, optoelectronic properties and sensitivity for gasses, and the effects of Fe 3+ doping on SnO 2 can be quite different according to the synthetic procedures, so a compara- tive study on the properties of Fe 3+ -doped SnO 2 nanoparticles prepared by different routes is necessary. In the present work, Fe 3+ -doped SnO 2 nanoparticles were prepared by sol–gel calcination and sol–gel-hydrothermal routes, respectively, and the physico-chemical and optoelec- tronic properties were compared. The results demonstrate that Fe 3+ -doped SnO 2 nanoparticles prepared by sol–gel- hydrothermal route are better dispersed and have a narrower size distribution with larger specific surface area than that prepared by the normal sol–gel route. 2. Experimental 2.1. Synthesis of samples SnCl 4 ·5H 2 O and FeCl 3 ·6H 2 O were used as tin andiron sources, respectively. First, citric acid was added to 96 ml of H 2 O until the pH 1.5 at 50 ◦ C with magnetic stirring; then, 17.529 g of SnCl 4 ·5H 2 O were added and dissolved. Secondly, FeCl 3 ·6H 2 O (the Fe molar content in samples was 0, 1.0, 5.0, 10.0, 15.0, and 20.0mol%) and 10 ml polyglycol were added to the above solution. It was stirred for 10 min and a sol formed. Thirdly, 30 ml NH 3 ·H 2 O (15mol/l) was added dropwise to the above sol under magnetic stirring within 30 min. Then, the hydrolysis product was stirred for 30 min to form a gel. Finally, the gel was treated by the following two routes, respectively. 2.1.1. The calcination process The gel was filtered, and dried at 110 ◦ C for 12 h. After that, the powders were further calcined at 350 ◦ C in air for 1 h. Then, the Fe 3+ -doped SnO 2 nanoparticles were obtained. The products were denoted as SGC-series. 2.1.2. The hydrothermal process The gel was transferred into a Teflon-lined autoclave for hydrothermal reac- tion at 150 ◦ C (the pressure in the hydrothermal process should about 0.4 MPa) for 12 h. After that, the hydrothermal product was filtered and dried at 110 ◦ C for 10 h. Then, the Fe 3+ -doped SnO 2 nanoparticles were obtained. The products were marked SGH-series. According to the Fe molar content in samples (0, 1.0, 5.0, 10.0, 15.0, and 20.0 mol%), the products were designed as SGC0, SGC1, SGC5, SGC10, SGC15, and SGC20 for SGC series and SGH0, SGH1, SGH5, SGH10, SGH15, and SGH20 for SGH series, respectively. To compare the effect of temperature on the nanoparticle properties, both the SGC and SGH series of samples were calcined at 350, 600, and 800 ◦ C in air for 1 h. 2.2. Characterization of samples The crystalline phase of Fe 3+ -doped SnO 2 particles were determined by X-ray diffraction (XRD, Cu K␣, 40 kV, 60 mA, Rigaku D/max-2400). The mor- phologies of samples were characterized using a JEOL 2010 F field emission gun electron microscope (TEM) with an accelerating voltage of 200 keV. The specific surface area of the prepared samples was calculated from the adsorp- tion isotherm of nitrogen at 77 K on the basis of the Brunauer–Emmett–Teller method (BET; Tristar3000, Micromeritics). The chemical structure information of the particles was collected by FT-IR spectra (Nicolet 560 Spectrom- eter). Diffuse reflectance spectra (DRS) was recorded by a Shimadzu UV-2101 apparatus, equipped with an integrating sphere, using BaSO 4 as reference. 3. Results and discussion 3.1. XRD analyses The XRD patterns of Fe 3+ -doped SnO 2 nanoparticles are shown in Fig. 1a and b, respectively. It clearly shows that the rutile phase formed after calcined at 350 ◦ C in the SGC series of samples (see Fig. 1a). However, rutile structured SnO 2 crystallites have also been obtained by the SGH route with- out calcination (see Fig. 1b). The Fe-doping not only makes the degradation of the crystallinity but also makes effects on the size of the nanoparticles, similar results had been reported [23–25]. The crystallite grain size of SnO 2 can be estimated according to Scherrer formula L = Kλ/(β cos θ), where λ is the wavelength of the X-ray radiation (Cu K␣ = 0.15406 nm), K the constant taken as 0.89, β the line width at half maximum height and θ is the diffracting angle. While the crystalline grain sizes of SnO 2 of the SGH series decreased from 5.98 to 3.85 nm when the Fe 3+ content increased from 0.0 to 20% (see Fig. 2), the crystalline grain size of the SGC-series decreased from 17.55 to 7.20 nm (see Fig. 2). It is clear that the addition of Fe pre- vented the growth of crystalline grains of SnO 2 , and the grain size for the SnO 2 by the sol–gel-calcination route with the same Fe 3+ content was larger than that by the sol–gel-hydrothermal route. In addition, no characteristic peaks for the ferric oxide crystal phase were observed in all samples derived from both preparing routes. At the same time, the addition of Fe 3+ significantly influ- enced the crystallinity of SnO 2 . Fig. 1 suggested a decrease of crystallinity in the Fe 3+ -doped SnO 2 nanoparticles in compar- ison with the undoped sample by the decrease in the intensity of SnO 2 peaks. This means that a portion of Fe 3+ formed stable solid solutions with SnO 2 , it could occupy regular lattice site in SnO 2 and that may cause the introduction of point defects and change in stoichiometry due to charge imbalance. The effect leads to a distortion of the crystal structure of the host com- L.M. Fang et al. / Journal of Alloys and Compounds 454 (2008) 261–267 263 Fig. 1. XRD patterns of Fe 3+ -doped SnO 2 nanoparticles: (a) SGC series and (b) SGH series. pound. Because of the interference of Fe 3+ with SnO 2 lattice, the crystallinity of the Fe 3+ -doped SnO 2 nanoparticles were worse than that of pure SnO 2 . But when the Fe 3+ content was high (for example, 15 and 20%), a portion of Fe 3+ should exist as amorphous ferric oxide aggregates. The amorphous ferric oxide Fig. 2. The crystalline grain sizes of SnO 2 calculated from Scherrer formula. aggregates also retarded the growth of SnO 2 nanoparticles. This means that a portion of Fe 3+ formed stable solid solutions with SnO 2 . In addition, no diffraction peaks of ferric oxide could be detected even after calcination at 800 ◦ C. The above results suggested that the grain sizes of the Fe 3+ - doped SnO 2 nanoparticles prepared by both methods depended on the Fe 3+ content, and Fe 3+ could prevent the growth of crys- tal grains of SnO 2 . Moreover, with sol–gel-hydrothermal route, rutile phase was directly synthesized and the addition of Fe 3+ was more effective in preventing the growth of crystalline grain size, suggesting that sol–gel-hydrothermal route was a more effective process for preparing Fe 3+ -doped SnO 2 nanoparticles compared with the sol–gel-calcination route. It is well known that calcination can improve the crys- tallinity of the samples, but increase the crystallite grain size of SnO 2 at the same time. In order to investigate the possible crystallization process of SnO 2 by increasing the calcinations temperature, XRD analysis was carried out on the samples cal- cined at 350, 600, and 800 ◦ C, respectively. Typical diffraction patterns obtained from samples of pure SnO 2 and 10% Fe 3+ - doped SnO 2 are shown in Figs. 3 and 4. The grain sizes of SnO 2 calculated by Scherrer formula are showed in Table 1. Compared to pure SnO 2 , the crystalline grain size of 10% Fe 3+ - Fig. 3. XRD patterns of samples from sol–gel-calcination route calcined at different temperature: (a) SGC0 and (b) SGC10. 264 L.M. Fang et al. / Journal of Alloys and Compounds 454 (2008) 261–267 Fig. 4. XRD patterns of samples from sol–gel-hydrothermal route calcined at different temperature: (a) SGH0 and (b) SGH10. Table 1 The crystalline grain sizes of SnO 2 calcined at different temperature Samples Crystalline grain sizes (nm) 110 ◦ C 350 ◦ C 600 ◦ C 800 ◦ C SGC0 – 17.55 34.14 55.68 SGC10 – 10.03 14.85 45.62 SGH0 5.98 9.73 19.69 33.26 SGH10 5.02 6.43 8.18 17.71 doped SnO 2 was smaller. On the other hand, for the SGH series, the addition of Fe 3+ showed more significant inhibiting effect on the increase of crystallite size. After calcination at 800 ◦ C, the grain size of SGH10 was 17.71 nm, but it was 45.62 nm for the SGC10. It is clear that the Fe 3+ -doped SnO 2 nanopar- ticles prepared by sol–gel-hydrothermal process has a better thermal stability against grain growth than that prepared by the sol–gel-calcination process. This result suggested that sol–gel- hydrothermal route led to a better atomically mixed structure between Fe 3+ and SnO 2 , which retarded agglomeration of grains due to the strong Fe–O–Sn linkage. In addition, no diffraction peaks of ferric oxide could be detected even after calcination at 800 ◦ C. 3.2. TEM and BET surface areas The bright-field TEM and high resolution TEM (HRTEM) images of pure SnO 2 nanoparticles and Fe 3+ -doped SnO 2 nanoparticles prepared by SGC or SGH routes are shown in Figs. 5 and6, respectively. It could be seenthat all SnO 2 nanopar- ticles were not larger than 6 nm in diameter for SGH1, SGH5, SGH15, and SGC15. Selected-area diffraction exhibited the polycrystalline rings that were all from the rutile SnO 2 struc- ture as indexed in Fig. 5a. These rings are in agreement with the peaks in the power XRD spectra as shown in Fig. 1. More- over, there are no extra peaks from Fe or ferric oxides. Thus, the Fe 3+ ions are believed to be dissolved in the SnO 2 structure. The HRTEM image showed the crystallinity of the nanoparticles are well-developed. The Fe 3+ -doped SnO 2 nanoparticles prepared by sol–gel-hydrothermal route were uniform in size with narrow size distribution, and were well dispersed. The average grain size decreased when the Fe 3+ content increased. It decreasedfrom 6.0 to 3.0 nm when the Fe 3+ content increased from 1 to 15%. This result is similar to that obtained from the XRD analysis. But for the SGC15 nanoparticles derived from sol–gel-calcination route, it could be seen that the average grain size was not uni- form, range from 2 to 6 nm. And unfavorable agglomeration occurred (see Fig. 6). The BET surface areas of the samples doped with differ- ent Fe 3+ content are listed in Table 2. It could be seen that the surface areas were strongly dependent on the preparation route and the Fe 3+ content. It increased with the increasing of Fe 3+ content, and it was clear that the BET surface areas of samples obtained from sol–gel-hydrothermal method were remarkably higher than that from sol–gel-calcination route. That is because the samples prepared by sol–gel-hydrothermal route were well-dispersed and less size with narrow distribution, but the samples prepared by sol–gel-calcination route were agglom- erated and larger in grain size. This result indicated that the addition of Fe 3+ into SnO 2 matrix suppressed the reduction of surface area and the suppression was more effective with higher Fe 3+ content. Compared with the sol–gel-calcination route, the sol–gel-hydrothermal route was a more effective process to pro- duce the SnO 2 with large surface areas. Table 2 The BET surface area of samples with different Fe 3+ content Samples SGC SGH SGC0 SGC1 SGC5 SGC15 SGH0 SGH1 SGH5 SGH15 Fe content (mol%) 0 1.0 5.0 15.0 0 1.0 5.0 15.0 BET surface area (m 2 /g) 30.2 34.6 47.0 68.9 206.1 211.7 237.1 259.8 L.M. Fang et al. / Journal of Alloys and Compounds 454 (2008) 261–267 265 Fig. 5. Bright field TEM and HRTEM micrographs of samples: (a) SGH1, (b) SGH5, and (c and d) SGH15. SnO 2 nanoparticles owned much greatersurface-to-bulk ratio than that of bulk materials. A large fraction of the atoms pre- sented at the surface of SnO 2 nanoparticles. An important effect is associated with the depth of the surface space charge region that is affected by gas adsorption in relation to the particle size. When the particle size gets into the nano-range, the depletion layer takes over the bulk, and it becomes difficult to distinguish surface from bulk conduction. So the narrow size distribution, excellent dispersibility and large surface areas are all favorable for application in gas sensors and optoelectronic devices. The characteristics of Fe 3+ -doped SnO 2 nanoparticles prepared by sol–gel-hydrothermal route make the processing route particu- larly appealing. 3.3. Fourier transform infrared (FT-IR) spectroscopy analyses The FT-IR transmission spectra of samples are given in Fig. 7. The absorption peaks at 3400 and 1640 cm −1 were attributed to vibration of hydroxyl due to the fact that SnO 2 retained cer- tain adsorbed water. Additionally, bands at 2930, 2850, and 1390 cm −1 were assigned to C–H vibrations. The C–H could be attributed to the organic trace residuals, which remained in nanoparticles even after calcinations for the sol–gel-calcination route. The band at approximately 2365 cm −1 resulted from the adsorption and interaction of atmospheric carbon dioxide with water according to literature [26]. The bands observed in the Fig. 6. HRTEM micrographs of SGC15. 266 L.M. Fang et al. / Journal of Alloys and Compounds 454 (2008) 261–267 Fig. 7. FT-IR transmission spectra of samples: (a) SGC series and (b) SGH series. range of 1059 cm −1 were assigned to the vibration of different types of surface hydroxyl groups. Lenaerts [27] had observed that the bands at 1068 and 970 cm −1 most strongly appeared in the spectra of SnO 2 taken in O 2 at room temperature versus 450 ◦ C spectra. They ascribed these absorptions to Sn O and Sn O modes of surface cation–oxygen bonds. The Sn–O–Sn vibration appeared in the range of 400–700 cm −1 as the result of condensation reaction. The peaks at 660 and 560 cm −1 are attributed to the Sn–O–Sn antisymmetric vibrations. Compared with the SGC series samples, the absorption peaks of 3400 cm −1 were higher for the SGH series samples, suggesting that there were more O H bonds on the Fe 3+ -doped SnO 2 nanoparticle prepared by sol–gel-hydrothermal route than that prepared by sol–gel-calcination route. 3.4. UV–vis diffuse reflectance spectroscopy (DRS) analyses The diffuse reflectance spectra of samples with different Fe 3+ content are depicted in Fig. 8. The spectra of Fe 3+ -doped SnO 2 nanoparticles displayed a red shift in the band gap transition with the increasing dopant content. It was reported that similar shift was resulted from the incorporation of Fe 3+ into the SnO 2 Fig. 8. The diffuse reflectance spectra of samples: (a) SGC series and (b) SGH series. nanoparticles [28–30]. Red shifts of this type could be attributed to the charge-transfer transitions between the Fe 3+ d-electrons and the SnO 2 conduction or valence band. On the other hand, there was no separated phase of ferric oxide because its broad band centered at 500 nm was not present [31]. This is another evidence showing the Fe 3+ were doped in theSnO 2 nanoparticles rather than existed as ferric oxide. After comparing the DRS spectra, it can be seen that the SGH-series had larger band gap than SGC-series. The larger band gap was resulted from the well-known quantum size effect of semiconductors [32]. The results of XRD and HRTEM have proved that with same Fe 3+ content, the crystalline grain size of SnO 2 from sol–gel-hydrothermal route was more uniform and smaller than that prepared with the sol–gel-calcination route. According to the size quantization, an increase in the band gap happened with the decrease in particle dimensions [33]. 4. Conclusion Fe 3+ -doped SnO 2 nanoparticles were prepared by sol–gel- calcination and sol–gel-hydrothermal routes, respectively. The SnO 2 crystallites with the tetragonal rutile structure could form directly during the hydrothermal process without calcination. L.M. Fang et al. / Journal of Alloys and Compounds 454 (2008) 261–267 267 Fe 3+ formed stable solid solutions in SnO 2 nanoparticles. Com- pared with sol–gel-calcination route, sol–gel-hydrothermal route led to better dispersed spherical Fe 3+ -doped SnO 2 nanoparticles with narrower size distribution and larger specific surface area. The composite nanoparticles prepared by the sol–gel-hydrothermal route have better thermal stability against agglomeration and particle growth than those prepared by sol–gel-calcination route. The BET surface area of the SnO 2 sample with 15% Fe 3+ prepared by sol–gel-hydrothermal route was increased to 259.8 m 2 /g from 206.1 m 2 /g of SGH0. Narrow size distribution nanoparticles, excellent dispersibility and large surface areas of Fe 3+ -doped SnO 2 nanoparticle prepared by sol–gel-hydrothermal route make it particularly appealing in applications for gas sensors and optoelectronic devices. Acknowledgements This study was supported financially by the NSAF Joint Foundation of China (10376006) and by the Program for New Century Excellent Talents in University (NCET-04-0899) and by the Ph.D. Founding Support Program of Education Ministry of China (20050614013) and by Program for Innovative Research Team in UESTC. References [1] A. Teeramongkonrasmee, M. Sriyudthsak, Sens. 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