Solution Growth and Cathodoluminescence of Novel SnO2 Core

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J. Phys. Chem. C 2010, 114, 8235–8240

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Solution Growth and Cathodoluminescence of Novel SnO2 Core-Shell Homogeneous Microspheres Yutao Han,† Xiang Wu,*,†,‡ Guozhen Shen,*,§ Benjamin Dierre,| Lihong Gong,† Fengyu Qu,† Yoshio Bando,‡ Takashi Sekiguchi,| Fabbri Filippo,| and Dmitri Golberg‡ College of Chemistry and Chemical Engineering, Harbin Normal UniVersity, Harbin 150025, People’s Republic of China, International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Wuhan National Laboratory for Optoelectronics and College of Optoelectronic Science and Engineering, Huazhong UniVersity of Science and Technology, Wuhan 430074, People’s Republic of China, and AdVanced Electronic Materials Center, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: February 1, 2010; ReVised Manuscript ReceiVed: April 2, 2010

Large-scale novel core-shell microspheres of SnO2 have been synthesized through a simple solution method at 200 °C. Morphologies, microstructures, and compositions of the products are investigated by X-ray powder diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Results indicate that the SnO2 microspheres are core-shell structures with an average diameter of 6 µm and an average shell thickness of 200 nm. A growth mechanism and the cathodoluminescence properties of these novel structures are presented. 1. Introduction Compared with bulk counterparts, nanoscale materials possess rich morphologies and widely tunable physical and chemical properties. They have potential important applications in catalysis, gas sensing, optoelectronics, and dye-sensitive lithiumion batteries.1-6 The development of novel synthetic routes of nanostructures, exploration underlying growth mechanisms, and control of their shapes are very important issues for understanding relationships between the materials’ morphologies/structures and their properties and smart assembling of nanomaterials toward their effective applications. SnO2 is a very important semiconductor with a band-gap energy of 3.6 eV at room temperature. So far, 1D SnO2 nanostructures have been used to fabricate field effect transistors,7,8 gas sensors,9-11 lithium-ion batteries,12-14 and solar cells.15,16 Specially, as a gas sensor material, SnO2 has bright prospects due to its good physical and chemical stability, reversibility, and short adsorption time. It is worth noting that morphologies and sizes of SnO2 can bring dramatic effects on their properties and applications. A variety of synthesis methods have been introduced, including thermal evaporation,17 hydrothermal growth,18,19 solvothermal growth,20 pulsed laser deposition,21 template induced growth,22 and so on to synthesize versatile structures of SnO2, such as nanowires,23,24 nanobelts,25,26 and heterostructures.27,28 However, there have been fewer reports in regard to core-shell structures of SnO2.29-31 This special structure can enrich the SnO2 properties. In this work, we synthesized novel core-shell microspheres of SnO2 at a large scale using a simple hydrothermal method at 200 °C with the assistance of hexamethylenetetramine (HMT). Microstructures * To whom correspondence should be addressed. E-mail: wuxiang05@ gmail.com (X.W.), [email protected] (G.S.). † Harbin Normal University. ‡ International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS). § Huazhong University of Science and Technology. | Advanced Electronic Materials Center, National Institute for Materials Science (NIMS).

of a product were characterized through XRD, SEM, TEM, and Raman spectroscopy. A growth mechanism of as-grown SnO2 microspheres was proposed. The cathodoluminescence properties were finally investigated. 2. Experimental Section All reagents were of analytical grade and were used without further purification. In a typical experimental procedure, 1.05 g of SnCl4 · 5H2O and 1.40 g of NaOH were added into 40 mL of deionized water with 0.1 g of hexamethylenetetramine (HMT). After stirring for 20 min, the solution was transferred into a PTFE-lined autoclave (a volume of 50 mL). The autoclave was sealed and kept at 200 °C for 14 h. The system was then cooled naturally to room temperature. The product was washed with deionized water and ethanol several times and dried in a chamber at 70 °C for 5 h. As-synthesized products were characterized by XRD (Rigaku Dmax-rB, Cu KR radiation, λ ) 0.1542 nm, 40 kV, 100 mA), Raman spectroscopy (HR800), SEM (Hitachi S-4800), and TEM (JEOL-3000F). Cathodoluminescence properties were investigated by means of a scanning electron microscope (Hitachi S4200) equipped with a CL system. The CL spectra and images were taken at 5 kV and 500 pA at room temperature. 3. Results and Discussion The products were first characterized by SEM. Figure 1a shows that large-scale microsphere structures are formed. Detailed observations confirm that most of them have spherical shapes. There are also some with flat parts, as shown in Figure 1b. Figure 1c illustrates a single microsphere with a diameter of 6 µm. Interestingly, there are several holes on the microsphere surface; that is, a part of the sphere is wiped off. From Figure 1d, one can see that a part of the sphere shell breaks off, giving a view of the shell with a thickness of ∼200 nm. The core has a concave shape, and the surface is rather coarse. Some other microspheres are shown in Figure 1e,f. Figure 2 presents an

10.1021/jp100942m  2010 American Chemical Society Published on Web 04/13/2010

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Figure 1. SEM images of SnO2 core-shell microspheres: (a) low-magnification SEM image, (b) medium-magnification SEM image, (c-f) SEM images of a single microsphere.

Figure 2. XRD pattern of SnO2 core-shell microspheres. Figure 3. Raman spectrum of SnO2 core-shell microspheres.

XRD pattern of a product. All diffraction peaks can be attributed to the rutile-type tetragonal phase of SnO2, in accord with the standard JCPDS card (No. 41-1445). No impurity phases were detected during XRD measurements, confirming the superb purity of the product. Figure 3 shows a Raman spectrum of a SnO2 microsphere at room temperature under illumination with a 488 nm light. The Raman peaks located at 507, 627, and 767 cm-1 can be attributed to E1g, A1g, and B2g vibration modes of tetragonal SnO2, respectively. These peaks show that the SnO2 microspheres actually possess the main optical characteristics

of a tetragonal rutile structure.32 However, an additional new peak at 541 cm-1 has also been observed in the spectrum. The latter has not been observed in the Raman spectra of SnO2 crystals but is close to the vibrational mode (566 cm-1) of an amorphous SnO2 film, as reported by Kar et al.33 The microstructures of the SnO2 microspheres were further investigated by TEM. Figure 4a shows a part of the core of a microsphere with a coarse surface; the diameter of the broken core is ∼5 µm. The HRTEM image taken from the framed area

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Figure 4. TEM images of SnO2 core-shell microspheres. (a) TEM image of the core part. (b) HRTEM image taken in the framed area in (a); the inset is the corresponding SAED pattern. (c) TEM image of the shell part. (d) HRTEM image taken in the framed area in (c).

in Figure 4a is depicted in Figure 4b. The clear lattice image indicates the single-crystalline nature of the SnO2 microsphere shell. A lattice spacing of 0.32 nm peculiar to the (011) plane of a tetragonal SnO2 structure is apparent. An SAED pattern (inset) in Figure 4b is consistent with the HRTEM analysis. Figure 4c is a low-magnification TEM image of the shell of a SnO2 microsphere. The edge is transparent due to its small thickness, in agreement with the SEM observations. Figure 4d is a HRTEM image from the framed segment in Figure 4c, implying again that the shell is single-crystalline. The formation of the micro/nanostructures is a complicated process in which both thermodynamics and kinetics should be taken into account. To better understand the growth mechanism of the present core-shell microspheres, a series of test experiments were performed through changing several growth parameters, such as reaction temperature, reaction time, and a dose of the alkali solution. Figure 5a-c shows the synthetic-timedependent SEM images of the products. When the reaction time is 10 h, microplates made of microrod ensembles appeared. When the time increases to 16 h, hollow microspheres are formed, as shown in Figure 5c. The optimal time for the formation of core-shell microspheres was set at 14 h (Figure 5b). We then investigated the effect of temperature on the structure morphologies. When the temperature was set at 160 °C, only some rodlike microcrystals were found, as shown in Figure 5d. With an increase of temperature to 200 °C, core-shell microspheres appeared. A further increase to 220 °C leads to some microsphere shell degradation (see the inset in Figure 5f). At last, the effects of solvent on the product morphology were investigated. Urchin-like structures were formed when we, in part, substituted ethanol for the deionized water, and only irregular spherelike structures were obtained under additions of HMT. This indicated that the kinds of solvent

and surfactants play important roles for the formation of the present core-shell microstuctures. Previously, Deng et al. synthesized SnO2 core-shell hollow microspheres by a solvothermal method and postsynthesis calcination with the assistance of D-glucose monohydrate.29 They thought that C mesospheres loaded with SnO2 nanoparticles were formed first. Under calcination in air, carbon was oxidized to CO2 and escaped from a microsphere, giving its final hollow shape. Wang et el reported CdMoO4 core-shell microspheres synthesized by an aqueous solution route with the assistance of sodium dodecyl sulfate at room temperature.34 Their results indicated that the Ostwald ripening process induced the formation of the core-shell microspheres. Chen et al. synthesized core/shell PbWO4/WO3 microstructures with different morphologies by immersing PbWO4 microstructures in HNO3 solution for a desired time.35 A modified Kirkendall effect was proposed to explain the formation mechanism. On the basis of these results, we suggest a possible growth mechanism for our SnO2 microspheres. Under alkali condition, Sn4+ reacts with OH- via Sn4+ + OH- f Sn(OH)4 to form small Sn(OH)4 particles as the nucleus, which is then slowly transformed into SnO2 nanoparticles. At the same time, organic molecules from the HMT are adsorbed onto the surface of the newly formed SnO2 nanoparticles. Their aggregates further induce the formation of large SnO2 microspheres with HMT molecules loaded inside. At the stage of the calcination in air at 70 °C, HMT inside the microspheres partially spills over, inducing the appearance of holes, as shown in Figure 1c,f. A possible growth schematic illustration of SnO2 core-shell microspheres is shown in Figure 6. Optical properties of the SnO2 microspheres were studied by CL. Figure 7 shows the CL spectrum of as-obtained SnO2 core-

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Figure 5. SEM images of the products at different growth conditions. (a-c) Reaction-time-dependent SEM images: (a) 10, (b)14, and (c) 16 h. (d-f) Reaction-temperature-dependent SEM images: (d) 160, (e) 200, and (f) 220 °C. (g-i) Solvent dependence SEM images: (g) SEM images of a product when the solvent is water with 50% ethanol, (h) SEM images of a product with the addition of HMT, and (i) SEM images of as-obtained products without the addition of HMT.

Figure 6. Growth schematic illustration of SnO2 core-shell microspheres.

shell microspheres at 5 kV at room temperature and the corresponding SEM images. The CL spectrum consists of a peak at 491 nm with a shoulder at 453 nm. These peaks are related to the defects in SnO2. Calestani et al. thought that an emission peak at 452 nm for SnO2 nanowires originates from the oxygen vacancies.36 Wu et al. proposed that the peak of 463 nm could be ascribed to the formation of Sn interstitials during the crystal growth.37 As for the peak at 490 nm, Chen et al. attributed it to an optical transition from the oxygen interstitials to intrinsic surface states.38 It should be mentioned that the near-band-edge (NBE) emission of SnO2 was not detected for the present core/ shell spheres, which may be caused by the dipole-forbidden nature of the first transition in semiconducting SnO2, according

Figure 7. (a) CL spectrum of as-obtained SnO2 core-shell microspheres at 5 kV at room temperature. (b-c) the corresponding SEM images.

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J. Phys. Chem. C, Vol. 114, No. 18, 2010 8239 average diameter of 6 µm and the shell thickness of 200 nm. CL spectra reveal emissions at 453 and 491 nm; the latter originates on the flat parts of the spheres. The small particles show a broad emission at 630 nm. The present SnO2 core-shell microspheres can be used for building blocks to fabricate optoelectronics micro/nanodevices. Acknowledgment. This work was supported by the Doctor Start-up Fund of Harbin Normal University (KGB200802), the National Natural Science Foundation of China (20871037), the Natural Science Foundation of Heilongjiang Province (B2007-2), the Science Technology and Research Project of Education Bureau, Heilongjiang Province (11531229 and 12531236), the High-level Talent Recruitment Foundation of Huazhong University of Science and Technology, the Basic Scientific Research Funds for Central Colleges (Q2009043), the Natural Science Foundation of Hubei Province (2009CDB326), and the Research Fund for the Doctoral Program of Higher Education (20090142120059). References and Notes

Figure 8. SEM image (a) of SnO2 small particles and spheres and CL spectra (b) taken from local spots indicated on the SEM image.

to previous reports.36-38 The CL image recorded by us shows that the luminescence from the present spheres is uniform but that the flat areas (indicated by yellow arrows in Figure 7b,c) show a weaker 491 nm emission. It may be noted that spectra taken on these flat areas are similar to those on the spheres. It may be related to the formation of some nonradiative defects, such as oxygen vacancies or tin interstitials resulting from the core/shell SnO2. Figure 8 shows an SEM image (panel a) of SnO2 small particles and spheres and the CL spectra (panel b) taken in local spots. The CL spectra were taken in the positions indicated on the SEM images. The CL spectra show the emissions at 450 and 490 nm for SnO2 core-shell hollow microspheres, whereas those taken on the small particles show an additional broad peak at 630 nm. Previously, Chen et al. studied the luminescence properties of SnO2 nanoribbons and they attributed the emission peak at 625 nm to the combination of oxygen-vacancy centers.38 We think the emission at 630 nm for our sample here may be explained in a similar way, which may also be attributed to the combination of oxygen-vacancy centers or the incorporation of impurities related to our experiments. Detailed studies on the explanations of the CL properties of our sample should be given and will be reported in due time. 4. Conclusion In summary, SnO2 core-shell microspheres have been synthesized by a simple solution route. The products have an

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