Evolution of Nanoscale SnO2 Grains, Flakes, and Plates into Versatile

Jan 26, 2005 - Evolution of Nanoscale SnO2 Grains, Flakes, and Plates into Versatile Particles and Films through Crystal Growth in Aqueous Solutions...
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Evolution of Nanoscale SnO2 Grains, Flakes, and Plates into Versatile Particles and Films through Crystal Growth in Aqueous Solutions Hirotoshi Ohgi, Takahiro Maeda, Eiji Hosono, Shinobu Fujihara, and Hiroaki Imai*

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1079-1083

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received October 19, 2004

ABSTRACT: Hierarchically structured porous particles and films consisting of nanocrystalline SnO2 were spontaneously grown by gradual oxidation of tin(II) in a simple aqueous system at a low temperature. The nanoscale shape and macroscopically assembled architecture of SnO2 crystallites were totally controlled by preparation conditions for crystal growth. Spherical and prickly particles exhibiting a high specific surface area in the range of 120-230 m2/g were produced by organized growth of nanoscale SnO2 grains and flakes, respectively. Porous SnO2 films consisting of the nanograins and nanoflakes were directly grown on a glass substrate through heterogeneous nucleation promoted by addition of urea. Cellular aggregates and films composed of platy subunits were constructed in the solutions under an oxygen-deficient condition. Amorphous and monoxide phases contained in as-deposited particles and films were easily transformed into SnO2 crystals without deformation of the macroscopic architecture by subsequent hydrothermal treatment at 150 °C in water and calcination at 500 °C in air, respectively. The easyto-handle nanocrystalline SnO2 with hierarchical and porous architectures would be utilized for various practical applications. Introduction Recently, nanoscale materials have attracted much attention because of their potential for excellent and fascinating properties. Consequently, nanoparticles and thin films of metals and metal oxides have been prepared by various physical and chemical techniques. In the progressive stage, sophisticated assembly of the nanoscale units into a hierarchical architecture would be required to obtain higher functionality and performance. Self-assembly and self-organization have become an important fashion for exquisite fabrication of sophisticated architectures. For instance, amphiphilic and ionic organic molecules are utilized for tailoring elaborate structures through self-assembly of nanoscale units.1-3 Whereas various architectures of biogenetic and nanocomposite materials were reported,4-6 morphological studies on the nanofabrication of practical and functional metal oxides7-10 have been limited. Transparent and semiconducting tin dioxide (SnO2) is widely used for various devices, such as transparent electrodes, gas sensors, photosensors, photocatalysts, antistatic coatings, and dye-sensitized solar cells, because of its excellent optical and electrical properties and chemical stability. The performance of these devices is influenced by the nanoscale structure of SnO2 crystals, including their size, specific surface area, and surface condition. Various kinds of SnO2 particles and films were prepared using pyrolysis,11,12 laser ablation,13,14 and sol-gel15-17 and homogeneous precipitation methods.18 A mesoporous structure of SnO2 was successfully produced by a templating technique with self-assembly of surfactants.19 However, the nanoscale * Fax: +81 45 566 1551. Phone: +81 45 566 1556. E-mail: [email protected].

control of the morphology is still a challenging task for advanced applications of SnO2 and SnO2-based materials. In recent years, aqueous solution processes mimicking the synthesis of inorganic materials in life have been noteworthy as a new nanofabrication technique of functional materials.20 The biomimetic techniques enable the construction of a hierarchical architecture including macro- and nanostructures through the crystal growth of inorganic materials with coexisting species at ambient temperature and pressure. The control of the nucleation sites, the degree of supersaturation, and coordination states of coexisting species in the aqueous system is essential for the processes. However, experimental trials for fabrication of SnO2 crystals are inadequate, whereas simply planar films were prepared from solutions containing SnF221 and SnCl2.22,23 In this work, we found a specific route for fabrication of versatile architectures of nanocrystalline SnO2 with organized crystal growth in a solution system. Tailored morphologies including particles and thin films obtained through self-organization was highly controllable by adjustment of the preparation conditions. Moreover, we show the preparation of thermally stable porous architectures of SnO2 nanocrystals by subsequent hydrothermal treatment. Thus, the new types of SnO2 particles and films are applicable to various practical fields including electrodes for solar cells and gas sensors. Experimental Section Preparation conditions for SnO2 particles were based on those for thin films.21 A certain amount of SnF2 (Kanto Chemical) was dissolved in purified water. After the solutions were stirred for 1 h at room temperature, the tin concentration and pH of the solutions were adjusted to be in the range of 10-300 mM and 3.10-3.45 using 100 mM HCl and NH4OH

10.1021/cg049644z CCC: $30.25 © 2005 American Chemical Society Published on Web 01/26/2005

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Figure 1. Variation of morphology of particles grown in 24 h. Filled circles, triangles, and squares indicate formation of spheres, prickly particles, and aggregates of plates, respectively. aqueous solutions. Precipitation in the precursor solutions was induced within several hours by heating at 60 °C in an electric oven. Resultant precipitates, which were separated from the solutions by centrifugation and decantation, were washed with purified water and then dried in air at 60 °C for 24 h. Subsequent hydrothermal treatment of the precipitates was performed in purified water at 100-200 °C using a Teflonlined stainless steel autoclave for 1-7 days. Calcination was conducted in an electric furnace in air at 500 °C. Porous films were prepared using solutions dissolving a certain amount of SnF2 and urea. The concentrations of SnF2 and urea were adjusted to 10 and 0-50 mM, respectively. Glass slides, which were previously washed with nitric acid and acetone, were immersed as a substrate for heterogeneous nucleation. The surface for the deposition was placed downward to prevent sedimentation of particles produced through the homogeneous nucleation. Resultant films on the substrate in the precursor solutions maintained at 60 °C were washed with purified water and then dried in air at 25 °C for 24 h. The deposition on the substrate was repeated by using a freshly prepared solution with the same concentrations of reagents when the thickness of the films was increased. The crystallinity of resultant powders and films was identified by X-ray diffractometry (XRD, Rigaku RAD-C) using Cu KR radiation by a 2θ/θ scanning mode. The macroscopic and microscopic morphologies of the products were observed by field-emission scanning electron microscopy (SEM, Hitachi S-4700) and field-emission transmission electron microscopy (TEM, Philips TECNAI F20). X-ray photoelectron spectroscopy (XPS) was carried out with a JEOL JPS-9000MC using Mg radiation to analyze the content of residual fluorine in the powders. The specific surface area and pore-size distribution were calculated by the BET and BJH methods, respectively, based on the N2 adsorption and desorption isotherms at 77 K with a Micromeritics TriStar 3000. Thermogravimetry (TG) was performed with a Seiko Instruments TG/DTA6200.

Results and Discussion Figure 1 shows variation of the morphology of precipitates with the SnF2 concentration and pH of the precursor solution. Precipitation was not observed at a low pH condition, below 3.0, and a low SnF2 concentration, below 10 mM. On the other hand, rapid precipitation occurred even at room temperature above pH 3.4 and 100 mM. Spherical particles as shown in Figure 2a were produced in 24 h in the precursor solutions of 10200 mM at pH 3.10, 10-100 mM at pH 3.20, or 10 mM at pH 3.30-3.45. Figure 2b indicates that the spherical particles consisted of small grains with a diameter below 10 nm. We observed nanoscale crystallites (ca. 5 nm in diameter) exhibiting lattice images of the (101) and (110) planes of rutile-type SnO2 (cassiterite) in a TEM pho-

Figure 2. SEM images of spheres (a, b), prickly particles (c, d), and aggregates of plates (e, f) grown in 24 h at pH 3.20 with 10, 150, and 200 mM of SnF2 concentration, respectively.

Figure 3. Typical TEM images of spheres (a) and prickly particles (b) shown in Figure 2.

tograph of the spherical particles (Figure 3a). The formation of crystalline SnO2 was also suggested by an XRD pattern for the spherical particles (Figure 4a) although the diffraction peaks were fairly broadened. The broad peaks were assigned to SnO2 crystals accompanied with water17 and an amorphous phase.23 The results of XPS analysis revealed that a large amount (ca. 10 mol %) of fluorine derived from the raw material remained on as-deposited spherical particles. The peak positions of the X-ray diffraction for SnO2 were not shifted, and the residual fluorine atoms were easily removed with the improvement of the crystallinity by heating at 500 °C for 3 h (Figure 4c). Thus, fluorine atoms are deduced to be located with water in the surface layer of the nanoscale SnO2 crystallites. The results of TG indicated that a weight of ca. 10% was lost upon calcination up to 500 °C with removal of water and fluorine from the surface layer. According to nitrogen adsorption-desorption isotherms, the spherical particles had a high specific surface area above 120 m2/g and contained micropores below 2 nm. The maximum value, 230 m2/g, was obtained at 150 mM of SnF2 concentration and pH 3.10. The high specific surface area with the micropores is associated with the presence of the nanoscale grains observed in the SEM and TEM images. However, heating at 500 °C for 3 h considerably

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Figure 5. SEM images of particles grown at pH 3.20 and 150 mM SnF2 for various periods: 3 (a), 18 (b), 72 (c), and 144 h (d).

Figure 4. Typical XRD profiles for particles grown for 24 h at pH 3.20 with 100 (a) and 200 mM (b) SnF2 concentration. Heat treatment at 500 °C for 3 h (c) and subsequent hydrothermal treatment at 150 °C for 7 days (d) were performed for particles of profiles b and a, respectively.

decreased the specific surface area to ca. 25 m2/g. On the other hand, subsequent hydrothermal treatment at 150 °C for 7 days increased the specific surface area to 240 m2/g and the interparticle spacing to ca. 3 nm. The desirable change is associated with the shrinkage of the nanograins with the removal of fluorine atoms and an increase in the intensity of XRD peaks (Figure 4d). Thus, the hydrothermal treatment is inferred to induce the densification of the amorphous layer surrounding the crystalline core without sintering between the particles. The weight loss of the products upon calcination was obviously decreased after the hydrothermal treatment. Thus, the densification occurred with removal of water and fluorine from the surface layer. Because sensing devices of SnO2 usually work at a relatively high temperature, a low thermal stability of the nanostructure is a serous problem for the application. We found that the specific surface area of the hydrothermally treated particles was maintained above 150 m2/g even after heating at 400 °C. Although hydrothermal treatment was already reported to suppress the grain growth of SnO2 particles, the detailed mechanism was not clarified.24 Here, a high thermal stability is ascribed to a high crystallinity of the SnO2 nanograins achieved with the densification of the amorphous part. In consequence, submicron-sized porous SnO2 particles were directly produced by organized growth of nanocrystals in an aqueous solution of SnF2 at a low temperature and subsequent hydrothermal treatment. Prickly particles (Figure 2c) were grown in the SnF2 solutions of 250 mM at pH 3.10, 150 mM at pH 3.20, or 50 mM at pH 3.30. In these conditions, spherical particles were initially prepared and were gradually transformed into prickly and flaky forms (Figure 5). Then, cellular particles were produced with an out-

Figure 6. SEM images of films grown for 24 h at 10 mM SnF2 without urea. The deposition procedure was conducted once (a, b) and four times (c, d). The thicknesses of films in images a and c were ca. 300 nm and 1.4 µm, respectively.

growth of nanoscale flakes and projections (Figure 2d). A lattice image of the (110) plane (Figure 3b) indicates that the nanoflakes mainly exhibited the (001) face of the SnO2 crystals. Another type of aggregates of plates (Figure 2e,f) was obtained at a relatively high pH and SnF2 concentration shown in Figure 1. Although diffraction peaks due to SnO were mainly observed for the particles (Figure 4b), calcination at 500 °C for 3 h in air transformed the aggregates into pure SnO2 (Figure 4c) without deformation of the macroscopic morphology. Crystalline SnO2 films were successfully formed through heterogeneous nucleation on a glass substrate immersed in 10 mM SnF2 solution. The pH value of this solution naturally settled into 3.45 with the dissolution of the raw materials, and the spherical particles consisting of nanocrystalline SnO2 were precipitated through homogeneous nucleation. The surface morphology consisting of nanograins of 10-20 nm in diameter (Figure 6a,b) was almost the same as that of the spherical particles grown through homogeneous nucleation. Repetition of the deposition using a freshly prepared solution (2-4 times) (Figures 6c,d) or addition of urea (1040 mM) (Figure 7a,b) produced prickly and platy

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Figure 7. SEM images of films grown for 24 h at 10 mM SnF2 and 40 mM urea. The deposition procedure was conducted once (a, b) and four times (c, d).

Figure 8. SEM images of films grown for 24 h at 10 mM SnF2 and 50 mM urea.

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of large plates with a thickness of ca. 10 nm (Figures 7c,d and 8a,b). This type of films contained SnO crystals with SnO2 (Figure 9c). The peak of the (002) plane in the XRD pattern suggests that the [002] of the SnO structure was perpendicular to the surface. The cellular films containing SnO plates were easily converted into pure SnO2 films (Figure 9d) without deformation of the macroscopic morphology by calcination at 500 °C in air for 3 h. Generally, antimony doping is required to decrease the electric resistivity for transparent electrodes. Doping of antimony into SnO2 particles was successfully achieved by adding SbF3 into the precursor solutions. Actually, Sb-doped SnO2 (Sb/Sn ratio 8 mol %) particles and films were prepared in the solutions by addition of 15 mol % SbF3 to SnF2. Since the specific morphologies were not influenced by the doping of antimony, we obtained various kinds of Sb-doped SnO2 particles including spherical and prickly particles. However, the presence of antimony was not effective for a decrease in the resistivity of as-deposited SnO2 crystals. Heating at 500 °C was needed for a sufficient decrease in the resistivity of the products. The formation of SnO2 from SnF2 involves an oxidation process of tin(II) in an aqueous solution. The yield of the SnO2 particles was only 2-3% for a 100 mM SnF2 solution contained in a closed vessel. Because their solubility in water is 1.4 mM at 20 °C, the oxidation reaction is deduced to be mainly caused by oxygen molecules dissolving in the precursor solution. This assumption is supported by the fact that the yield was drastically increased by introduction of air bubbles into the precursor solution. Since dissolution of SnF2 decreased pH of the solutions, SnF2 is presumed to be transformed into SnO32- through oxidation by oxygen in water as expressed by eq 1,

SnF2 + 1/2O2 + 2H2O f SnO32- + 2H+ + 2HF (1) Usually, Sn(OH)4 is immediately produced from SnO32in acidic water as expressed by eq 2,25

SnO32- + H2O + 2H+ f Sn(OH)4

Figure 9. Typical XRD profiles for films consisting of nanograins (a), prickly flakes (b), and large plates (c, d). Heat treatment at 500 °C for 3 h was performed on films in profile d.

subunits on the films with an increase in the thickness. An intense diffraction peak due to the (110) plane (Figure 9a,b) indicates that the SnO2 crystals grew along the [110] direction in these films on the substrate. The repetition of deposition with 30-40 mM urea or a further increase in the amount of urea (>50 mM) promoted the formation of a cellular structure consisting

(2)

Here, we directly obtained crystalline SnO2 instead of Sn(OH)4 from SnO32- in the acidic solutions, whereas the precipitated grains were accompanied with an amorphous phase and water. Rapid precipitation of amorphous Sn(OH)4 was observed when an oxidizing agent, such as hydrogen peroxide, was added into the precursor solution. Thus, the essence of the direct preparation of crystalline SnO2 is ascribed to a moderate reaction rate involving the oxidation of tin(II) into tin(IV) by oxygen molecules dissolving in water. We found that SnCl2 was utilized as a precursor similar to SnF2. This fact implies the importance of the oxidation reaction of tin(II) for the formation of crystalline SnO2. In this case, however, crystalline SnO2 particles were obtained in the solution at around pH 2.0. The preparation condition is influenced by the reactivity of the halide species in water. The preparation of SnO2 particles and films having a hierarchical architecture is associated with self-assembled growth of nanoscale crystalline units in an aqueous solution. However, crystalline SnO2 nanograins

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were enveloped with an amorphous layer containing fluorine originating from the precursor and water. Then, higher-order spherical particles would be produced through stepwise growth of the enveloped nanograins. The formation of prickly and platy SnO2 particles was fundamentally achieved with the progress of crystal growth (Figure 5). The specific crystallographic orientation in the films deposited on a substrate (Figure 9a,b) suggests that the crystal elongated in the [110] direction as a rod (Figure 2d). In the latter stage of the reaction, a decrease in the oxygen concentration due to the consumption for the oxidation reaction reduced the growth rate. A steady growth of the crystals would weaken the coverage of the envelope and promote the evolution of rods to flakes exhibiting the (001) face. In the final stage of the reaction, SnO was formed on the previously deposited SnO2 crystals because of the deficiency of oxygen molecules in water for the oxidation of tin(II) into tin(IV). On the other hand, precipitation of SnO has never observed in the absence of SnO2 crystals. The platy morphology may be derived from a layered structure of SnO crystal. Especially, the plates were also grown along a certain direction ([200] of SnO) (Figure 9c). Thus, the formation of a SnO layer on the (001) face of SnO2 with a specific crystallographic interaction could induce the expansion of the grains along the [110] direction in the solution containing a small amount of oxygen. As a consequence, the morphological evolution from spherical particles with nanograins into macrocellular forms with nanoplates is fundamentally ascribed to the progress of the crystal growth with a decrease in oxygen concentration. As shown in Figure 1, the morphological change was also observed with variation of the growth conditions, such as pH and SnF2 concentration. A relatively high SnF2 concentration and a high pH condition produced the platy morphology similar to the products obtained in the final stage of the crystal growth as shown in Figure 5c,d. These results are explainable by assuming promotion of the crystal growth with the precursor concentration and pH. An increase in the SnF2 concentration and a decrease in the proton concentration (an increase of pH) promote the oxidation reaction expressed by eq 1 and then forward to the stage of the morphological evolution in a certain period (24 h). Thus, prickly and platy morphologies were produced in 24 h at high pH and a high SnF2 concentration through a rapid consumption of oxygen in the solution with the progress of the reaction. On the other hand, spherical particles with nanograins were still produced in 24 h at a low pH and a low concentration because the growth stage slowly progressed with a gentle reaction. Urea molecules performed two roles in the formation of SnO2 films through heterogeneous nucleation. Initially, the coordination of urea to dissolved tin species decreased the reaction rate and induced heterogeneous nucleation on a specific surface, such as a glass plate.26 Then, SnO2 nanograins and nanoflakes were assembled on a substrate rather than precipitated through homogeneous nucleation. Another role of urea was the promotion of crystal growth with increasing the pH through the generation of ammonia by gradual decomposition. Consequently, the platy units were constructed

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on a substrate with the progress of morphological variation, and a cellular morphology was ultimately obtained. Conclusions We prepared hierarchically structured SnO2 particles and films by self-assembled growth of nanoscale grains and flakes in aqueous solutions dissolving SnF2. The macroscopic morphology, the microscopic shapes of the crystalline subunits, and the thermal stability were successfully controlled by changing pH, SnF2 concentration, growth period, and subsequent treatments. Thus, the new types of crystalline SnO2 particles and films with a porous architecture and high specific surface area are easy to handle and would be applicable for electrodes for solar cells and gas sensors. Acknowledgment. This work was supported by Grant-in-Aid for Scientific Research (No. 15560587) and 21st Century COE program “KEIO Life Conjugate Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References (1) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (2) Antonietti, M.; Ozin, G. A. Chem.sEur. J. 2004, 10, 28. (3) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. (4) Pohnert, G. Angew. Chem., Int. Ed. 2002, 41, 3167. (5) Ba¨uerlein, E. Angew. Chem., Int. Ed. 2003, 42, 614. (6) Mann, S., Angew. Chem., Int. Ed. 2000, 39, 3392. (7) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (8) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (9) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (10) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (11) Leite, E. R.; Weber, I. T.; Longo, E.; Varela, J. A. Adv. Mater. 2000, 12, 965. (12) Lee, J. H.; Park, S. J. J. Am. Ceram. Soc. 1993, 76, 777. (13) Williams, G.; Coles, G. S. V. J. Mater. Chem. 1998, 8, 1657. (14) Willett, M. J.; Burganos, V. N.; Tsakiroglou, C. D.; Payatakes, A. C. Sens. Actuators B 1998, 53, 76. (15) Zhang, J.; Gao, L. Chem. Lett. 2003, 32, 458. (16) de Monredon, S.; Cellot, A.; Ribot, F.; Sanchez, C.; Armelao, L.; Gueneau, L.; Delattre, L. J. Mater. Chem. 2002, 12, 2396. (17) Furusaki, T.; Takahashi, J.; Takaha, H.; Kodaira, K. J. Ceram. Soc. Jpn. 1993, 101, 451 (in Japanese). (18) Song, K. C.; Kang, Y. Mater. Lett. 2000, 42, 283. (19) Hyodo, T.; Nishida, N.; Shimizu, Y.; Egashira, M. Sens. Actuators B 2002, 83, 209. (20) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48. (21) Tsukuma, K.; Akiyama, T.; Imai, H. J. Non-Cryst. Solids 1997, 210, 48. (22) Unuma, H.; Takabatake, H.; Watanabe, K.; Ogata, T.; Sugawara, M. J. Mater. Sci. Lett. 2002, 21, 1241. (23) Shirahata, N.; Shin, W.; Murayama, N.; Hozumi, A.; Yokogawa, Y.; Kameyama, T.; Masuda, Y.; Koumoto, K. Adv. Funct. Mater. 2004, 14, 580. (24) Baik, N. S.; Sakai, G.; Miura, N.; Yamazoe, N. Sens. Actuators B 2000, 63, 74. (25) Nagaoka, S.; Yoshimoto, A.; Okazaki, N.; Harada, T.; Majima, H. J. Surf. Finish. Soc. Jpn. 1996, 47, 338 (in Japanese). (26) Yamabi, S.; Imai, H. Thin Solid Films 2003, 434, 86.

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