H3BO3-Induced Formation of Metal Oxide Hollow Spheres in Flowing

Mar 13, 2009 - Hugo M. Galindo , Yadira Carvajal , Eric Njagi , Roger A. Ristau and Steven L. Suib. Langmuir 2010 26 (16), 13677-13683. Abstract | Ful...
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J. Phys. Chem. C 2009, 113, 5455–5459

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H3BO3-Induced Formation of Metal Oxide Hollow Spheres in Flowing Aerosols Xiao Song, Xing Ding, Pengna Li, Zhihui Ai, and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: December 12, 2008; ReVised Manuscript ReceiVed: February 09, 2009

We report that hollow spheres of transition metal oxides (TiO2, ZrO2, SnO2, Fe2O3, and Co3O4) could be formed in flowing aerosols. The formation of hollow structures was attributed to the in situ emission of gas phase HBO2 from the inner part of the metal oxide microspheres during the pyrolysis of flowing aerosols. We interestingly found that the addition of H3BO3 could not only result in the formation of hollow spheres but also tailor the property of the metal oxides. The resulting TiO2 and Co3O4 hollow spheres exhibited enhanced photocatalytic and magnetic properties compared to the counterpart solid spheres, respectively. We believe this study provides a simple and convenient method for industrial production of transition metal oxide hollow spheres with enhanced properties. 1. Introduction Hollow nanospheres of metal oxides show some unique advantages such as low effective density, high specific surface area, and distinct optical, electrical, and magnetic properties;1-3 hence, they have great potential applications in many fields including catalysis, biomedical diagnosis, drug delivery, chromatography, separation and photonic devices, and so on.4 The unique advantages of hollow interiors have been the motivation behind intense research efforts to develop specific and general strategies for synthesizing hollow nanostructures and for functionalizing their interior and exterior spaces with desirable chemistry.5-7 Among the methods employed for the preparation of hollow nanostructured materials, hard or soft templates have been widely utilized. Various types of hard templates, such as polystyrene, carbon, and silica spheres,8,9 and soft ones, such as micelles, liquid drops, and bubbles, are used to prepare hollow spheres.10-12 However, these template-based approaches involve a multistep process13 and could also generate pollutants from the oxidation/decomposition of the sacrificial templates. Therefore, it is necessary to explore more economical and efficient processes for large scale synthesis of hollow spheres with the tailored properties. The aerosol-assisted flowing synthesis method is a versatile technique for producing ceramic materials on the industrial scale with a wide variety of particle morphologies, sizes, and compositions.14 A distinctive feature of these sprayed powders is the homogeneous distribution of constituents throughout all of the particles because all of the constituents are formed from a solution.15 In the aerosol-assisted flow synthesis process, the solution was first atomized by a nebulizer, and the resulting droplets were then passed through a hightemperature tube by the transportation of carrier gases, where several reactions, such as solvent evaporation and atomic rearrangement, take place in a continuous flow process. For example, Suslick’s group utilized an aerosol-assisted flowing synthesis method to prepare porous, hollow, and ball-in-ball metal oxide microspheres with postetching procedures.16 Our group successfully obtained a series of core-shell microspheri* To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn. Phone/Fax: +86-27-6786 7535.

cal Ti1-xZrxO2 solid solution photocatalysts by an ultrasonic spray pyrolysis method.17 In this study, we report that hollow spheres of transition metal oxides (TiO2, ZrO2, SnO2, Fe2O3, and Co3O4) could be formed in flowing aerosols. The formation of hollow structures was attributed to the in situ emission of gas phase HBO2 from the inner part of the metal oxide microspheres during the pyrolysis of flowing aerosols. This general aerosol-assisted flowing synthesis method is able to produce metal oxide hollow spheres on a large scale. The addition of H3BO3 could not only result in the formation of hollow spheres but also tailor the property of the metal oxides. The resulting TiO2 and Co3O4 hollow spheres exhibited enhanced photocatalytic and magnetic properties compared to their counterpart solid spheres, respectively. 2. Experimental Section 2.1. Preparation of Metal Oxide (TiO2, ZrO2, SnO2, Fe2O3, and Co3O4) Hollow Spheres. All chemicals used in this experiment were of analytical grade and used as received without further purification. The water used was deionized water. The metal oxides hollow spheres were all synthesized by an aerosol-assisted flow synthesis method. The schematic illustration of the experimental setup for the aerosol-assisted flow synthesis is shown in Figure S1 (Supporting Information). For the TiO2 hollow spheres, TiCl4 (10 mmol) and H3BO3 (10 mmol) were dissolved into 80 mL of distilled water at 0 °C under stirring. The resulting solutions were nebulized using an ultrasonic nebulizer at 1.7 MHz ( 10% (YUYUE402AI, Shanghai) and then carried by an air flow with a suction pump through a quartz tube surrounded by a furnace thermostatted at 600 °C for 1 h. The quartz reaction tube with the diameter of 3.5 cm was 1 m long. The products were collected in a percolator with distilled water, then filtered by a fritted glass funnel, washed thoroughly with distilled water and ethanol, and finally dried in an oven at 60 °C. For the SnO2, Co3O4, Fe2O3, and ZrO2 hollow spheres, SnCl4 · 5H2O (10 mmol), Co(NO3)2 · 5H2O (10 mmol), Fe(NO3)3 · 9H2O (10 mmol), ZrOCl5 · 5H2O (10 mmol), and H3BO3 (10 mmol) were dissolved into 80 mL of distilled water at 25 °C under stirring, respectively. The subsequent pyrolysis processes for the preparations of SnO2, Fe2O3, Co3O4, and ZrO2 hollow spheres

10.1021/jp810967y CCC: $40.75  2009 American Chemical Society Published on Web 03/13/2009

5456 J. Phys. Chem. C, Vol. 113, No. 14, 2009 were the same as that of TiO2 hollow spheres. The pyrolysis temperature for SnO2 and ZrO2 was 600 °C, but for the Co3O4 and Fe2O3 hollow sphere the pyrolysis temperature was 800 °C. 2.2. Characterization. The powder X-ray diffraction (XRD) measurements were carried out using a Rigaku D/MAX-RB diffractometer with monochromatized Cu KR radiation (λ ) 0.154 18 nm). Transmission electron microscopy (TEM) images were taken with a JEOL JSM-2010 electron microscopy instrument. The samples for TEM were prepared by dispersing the final powders in ethanol with ultrasonic, and the dispersion was then dropped on carbon-copper grids. Furthermore, the obtained powders deposited on a copper grid were observed by high-resolution TEM (HRTEM). A Varian Cary 100 Scan UV-visible system equipped with a labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts over a range of 200-800 nm. Laboratory sphere USRS-99-010 was employed as a reflectance standard. A nitrogen-adsorption system (Micrometritics ASAP2010) was employed to record the adsorption-desorption isotherms at the liquid-nitrogen temperature of 77 K. The magnetic properties of the Co3O4 samples were investigated using a vibrating sample magnetometer (VSM) from Ade (model 4 VSM). 2.3. Photocatalytic Activity Test. The photocatalytic activities of the samples were evaluated by the degradation of RhB in an aqueous solution. A 500-W tungsten halogen lamp was positioned inside a cylindrical vessel and surrounded by a circulating water jacket to cool it. We used the 500-W tungsten halogen lamp with UV-visible light irradiation to stimulate the solar light for the photocatalytic activity measurements because most of the solar light is in the UV-visible region. Therefore, the photocatalysts with good performance under this stimulate solar light would also be highly active under real solar light irradiation. For the photocatalytic activity measurements, a 0.1 g amount of photocatalyst was suspended in a 100 mL of aqueous solution of 5 mg/L RhB. The solution was continuously stirred forabout1htoensuretheestablishmentofanadsorption-desorption equilibrium among the photocatalyst, RhB, and water before irradiation, and then the solution was shined with artificial solar light from the tungsten halogen lamp. The distance between the light source and the bottom of the solution was about 15 cm, and the temperature of the RhB solution stirred by a dynamoelectric stirrer in an open reactor was about 25 °C. The concentration of RhB was monitored by colorimetry with a U-3310 UV-vis spectrometer (HITACHI). 3. Results and Discussion 3.1. XRD Analysis. X-ray diffraction (XRD) was used to investigate the phase structures of the resulting powders. Figure 1 shows the XRD patterns of the as-prepared samples obtained by the aerosol-assisted flow synthesis method. These patterns can be easily indexed to phase-pure TiO2 (JCPDS, file no. 211272), ZrO2 (JCPDS, file no. 37-31), SnO2 (JCPDS, file no. 77-452), Fe2O3 (JCPDS, file no. 84-310), and Co3O4 (JCPDS, file no. 74-1656), respectively. The broad reflections of the patterns corresponding to the respective metal oxides reveal their nanocrystalline nature. By application of the Debye-Scherrer formula on the strongest diffraction peaks, the average crystallite sizes of the TiO2, ZrO2, SnO2, Fe2O3, and Co3O4 hollow spheres were estimated to be 10.9, 5.4, 2.7, 21.6, and 25.9 nm, respectively. 3.2. TEM Images. The size and morphology of the products were analyzed by TEM measurements. Figure 2 shows the TEM images of the as-prepared samples. The strong contrast between the dark edge and bright center suggests the existence of hollow

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Figure 1. XRD patterns of the as-prepared samples.

Figure 2. TEM images of the as-prepared samples: (a) TiO2, (b) ZrO2, (c) Fe2O3, (d) SnO2, and (e) Co3O4.

microspheres composed of a hollow inner cavity and thin outer shell. The thickness of the microspheres shells was very thin, and therefore we can observe some obvious shrinkage on the surface of the samples. In addition, a few fragments could be found during the TEM observation, indicating that some of spheres may be destroyed by intensive postsonication or inner expansibility. The diameters of the TiO2 (Figure 2a), Fe2O3 (Figure 2c), and SnO2 (Figure 2d) hollow spheres were about

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Figure 4. TEM images of the TiO2 samples synthesized with different H3BO3 concentrations: (a) 0 M, (b) 0.05 M, (c) 0.1 M, and (d) 0.2 M.

SCHEME 1: Schematic Illustration of the H3BO3-Induced Formation Processes of the TiO2 Hollow Microspheres in the Flowing Aerosola

a

Figure 3. HRTEM images of the as-prepared hollow spheres: (a) TiO2, (b) ZrO2, (c) Fe2O3, (d) SnO2, and (e) Co3O4.

1-1.5 µm, while the sizes of the ZrO2 (Figure 2b) and Co3O4 (Figure 2e) hollow spheres were much smaller, with the diameters in the range from 200 nm to 1 µm. Therefore, TEM analysis reveals that hollow metal oxide spheres could be obtained with this aerosol-assisted flowing synthesis method. Without the addition of H3BO3, only solid microspheres were obtained and no hollow sphere was found in the as-prepared samples. We conclude that the metal oxide hollow spheres are formed with the help of H3BO3. 3.3. HRTEM Images. High-resolution TEM (HRTEM) analysis was further used to characterize the samples. As shown in Figure 3, clear lattice fringes can be observed, confirming the well-crystalline natures of hollow spheres. Furthermore, the HRTEM images of the metal oxide hollow spheres are in good accordance with the results of XRD patterns shown in Figure 1. For example, the lattice spacing is about 0.35 nm between adjacent lattice planes of anatase TiO2 (Figure 3a), which is consistent with the d-spacing of its [101] reflection. The image of a lattice fringe in Figure 3b shows an interplanar distance of d ) 0.30 nm of ZrO2, agreeing well with the [003] lattice planes. As shown in Figure 3c, the interlayer distance is calculated to be about 0.27 nm, corresponding to the [104] lattice planes of Fe2O3. The image of a lattice fringe in Figure 3d shows an interplanar distance of d[110].) 0.33 nm, corresponding to SnO2.

(1) The formation of TiO2 solid spheres via the hydrolysis of TiCl4 in the water droplet; (2) the decomposition of H3BO3 and the emission of HBO2 gas from the solid spheres; (3) the formation of TiO2 hollow spheres in the flowing aerosol.

From Figure 3e, the lattice spacing is about 0.47 nm, consistent with the d-spacing of [111] reflection of Co3O4. 3.4. Formation Mechanism of Hollow Spheres. In order to find the effect of H3BO3 on the formation of metal oxide hollow spheres, we selectively synthesized TiO2 with the addition of different amounts of H3BO3. We found that the concentration of H3BO3 was crucial for the formation of hollow TiO2 spheres. Figure 4 shows the typical TEM images of TiO2 synthesized in the presence of different amounts of H3BO3. It reveals that all the spheres are solid in the sample synthesized in the absence of H3BO3 (Figure 4a). Figure 4b shows a TEM image of the sample synthesized when the concentration of H3BO3 was increased to 0.05 M. We still observed solid spheres. When the concentration of H3BO3 was 0.1 M (Figure 4c), the resulting samples completely consisted of hollow structured microspheres. When the concentration of H3BO3 was increased to 0.2 M (Figure 4d), broken hollow spheres and some fragments were obtained (Figure 4d). On the basis of characterization results, we proposed possible formation processes of the hollow structured microspheres in the flowing aerosols by taking the formation of TiO2 hollow spheres as an example (Scheme 1). It is known that the aerosolassisted flowing synthesis process generally involves several stages: atomization, solvent evaporation and solute precipitation, drying, precursor decomposition, calcinations, and particle shaping. It is widely believed that the drops, when sprayed into

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Figure 5. Pseudo-first-order degradation of rhodamine B in the presence of TiO2 hollow spheres, TiO2 solid spheres, Degussa TiO2 photocatalyst P25 (P25), and self-degradation (SD) of rhodamine B under simulated solar light irradiation.

a tubular reactor under pyrolysis conditions, serve as microreactors and yield one particle per drop during the aerosol flowing.14 According to the one particle per drop rule, in our study each drop served as a microreactor containing TiCl4 precursor and H3BO3 as well as water. The temperature of the drop varied as it traveled with the flowing aerosol along the high-temperature furnace tube. As the water evaporated, the diameter of the precursor drop decreased, and the titanium precursor concentration increased.18 Eventually, the titanium precursor drop was completely dehydrated, and the hydrolysis of TiCl4 produced hydrous TiO2; thus, TiO2 spheres were subsequently formed in the flowing aerosol. During the formation of TiO2 spheres, the decomposition of H3BO3 would proceed simultaneously and/or subsequently in the droplet under high temperature, producing HBO2 which is volatile at 600 °C. The resulting HBO2 gas would escape from the TiO2 spheres to create hollow inner structures, as revealed by Scheme 1. 3.5. Photocatalytic Activity of TiO2. Titanium dioxide is widely used as pigment, catalyst, and photocatalyst. As the most studied photocatalyst, titanium dioxide can convert solar energy to electricity and split water for the production of hydrogen, as well as decompose environmental pollutants by using solar light. Here, we first studied the photocatalytic activity of the TiO2 hollow spheres by the degradation of RhB in an aqueous solution under simulated solar light from a 500-W tungsten halogen lamp without cutoff filter. We compared their activity with those of TiO2 solid spheres and the famous TiO2 photocatalyst (Degussa P25). The self-degradation of RhB was negligible, while the degradation of RhB was obvious in the presence of TiO2 photocatalysts (inset of Figure 5). The degradation of RhB on the photocatalysts was found to follow the pseudo-first-order decay kinetics. The degradation constant (k) for RhB was calculated to be 0.1493, 0.0987, and 0.6843 h-1 for P25, TiO2 solid spheres, and hollow spheres, respectively (Figure 5). Therefore, TiO2 hollow spheres possessed the highest photocatalytic activity. Its activity was about 4.6 times of that of P25 and 7 times of that of TiO2 solid spheres, respectively. The reasons for the high photocatalytic activity of TiO2 hollow spheres were analyzed by studying the TiO2 hollow spheres and solid spheres with nitrogen sorption, X-ray photoelectron spectrometry (XPS), and UV-vis diffuse reflectance spectrometry. The N2 adsorption-desorption isotherms and pore size distribution curves of TiO2 solid spheres and hollow spheres are shown in Figure 6. The Brunauer-Emmett-Teller (BET)

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Figure 6. Nitrogen adsorption-desorption isotherm and pore size distribution curve (inset) of TiO2 solid and hollow spheres.

Figure 7. Plots of the (ahV)1/2 vs the energy of absorbed light and UV-vis diffuse reflectance spectra (inset) of TiO2 solid and hollow spheres.

specific surface areas were 39 and 106 m2 g-1 for TiO2 solid spheres and hollow spheres, respectively. The pore volume of TiO2 hollow spheres (0.16 cm3/g) was also significantly larger than that (0.11 cm3/g) of TiO2 solid spheres. The larger surface area and pore volume of TiO2 hollow spheres should be caused by the in situ emission of HBO2 gas from the decomposition of H3BO3 during the formation of TiO2 spheres. This large specific surface area could enhance the photocatalytic activity of TiO2 hollow spheres because a larger specific surface can absorb more hydroxyl radicals and other reactive species.19 However, after normalization by their surface areas, the photocatalytic activity of TiO2 hollow spheres was still 2.6 times that of solid spheres. So there is another factor for the activity enhancement of TiO2 hollow spheres. Further XPS analysis revealed that the addition of H3BO3 resulted in boron doping in the TiO2 hollow spheres (Figure S1, Supporting Information), existing in the form of Ti-O-B structure as previously reported by Chen and coworkers.20 UV-vis diffuse reflectance spectrometry analysis revealed that this boron doping could enlarge the band gap of TiO2 hollow spheres (Figure 7). The band gaps of the TiO2 solid spheres and hollow spheres were found to be 2.91 and 3.03 eV, respectively. Normally, band gap enlargement would make the photocatalyst less active under visible light because of less

Metal Oxide Hollow Spheres in Flowing Aerosols

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5459 in flowing aerosols. The formation of hollow structures was ascribed to the in situ emission of gas phase HBO2 from the inner part of the metal oxide microspheres during the pyrolysis of flowing aerosols. The addition of H3BO3 could not only result in the formation of hollow spheres but also tailor the property of the metal oxides. We believe this study provides a simple and convenient method for industrial production of transition metal oxide hollow spheres with interesting properties. Acknowledgment. This work was supported by the National Basic Research Program of China (973 Program) (Grant 2007CB613301), the National Science Foundation of China (Grants 20673041 and 20777026), the Program for New Century Excellent Talents in University (Grant NCET-07-0352), the Key Project of Ministry of Education of China (Grant 108097), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

Figure 8. Magnetization curves of the as-prepared Co3O4 solid and hollow spheres measured by VSM at room temperature (300 K).

light absorption. However, in this study the photocatalytic activity of the B-doped TiO2 hollow spheres was measured by the degradation of RhB under simulated solar light containing both UV and visible light. Although the band gap of B-doped TiO2 hollow spheres was enlarged to 3.03 eV, they could still be activated under the simulated solar light. We believe the enlargement of band gap by boron doping is the second important factor for the enhancement of photocatalytic activity of TiO2 hollow spheres because a larger band gap corresponds to a more powerful redox ability21 and a higher energy level of the conduction band, which could favor the production of active oxygen radicals (e.g., O2•-,•OOH, •OH)22 and thus enhance the degradation of RhB. 3.6. Magnetic Property of Co3O4. The magnetic properties of the as-prepared Co3O4 solid and hollow spheres were further compared. Figure 8 shows their magnetization curves measured by a vibrating sample magnetometer at room temperature (300 K). Typical “S”-like shape of hysteresis loops were observed, indicating a superparamagnetic property of both Co3O4 solid and hollow spheres. This superparamagnetic property is evident by zero coercivity and remanance on the magnetization loop.22-25 These S-like shape loops can be divided into two parts: curvature parts and linear parts. The linear parts are attributed to the antiferromagnetic parts of the samples, while the curvature parts may arise from the change of the inversion parameter as the particle size decreases to nanoscale. We found that the hysteresis loops of the resulting Co3O4 solid and hollow spheres could not be saturated with the available maximum field of 15 kOe. This is an indication of the presence of large anisotropy in the materials. It was very interesting to find that the Co3O4 hollow spheres possessed enhanced magnetization compared to their solid counterparts. The magnetization enhancement may also be attributed to the incorporation of B into the crystal lattice of Co3O4, switching on the superexchange interaction, and thus giving rise to ferrimagnetic ordering.26 Therefore, the addition of H3BO3 could not only result in the formation of hollow spheres but also tailor the property of the resulting metal oxides. 4. Conclusions In summary, crystalline hollow spheres of transition metal oxides (TiO2, ZrO2, SnO2, Fe2O3, and Co3O4) could be formed

Supporting Information Available: Schematic illustration of experimental setup for the aerosol-assisted flow synthesis (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Murphy, C. J. Science 2002, 298, 2139. (2) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (3) Peng, Q.; Dong, Y. J.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3027. (4) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (5) Lou, X. W.; Yuan, C.; Zhang, Q.; Archer, L. A. Angew. Chem., Int. Ed. 2006, 45, 3825. (6) Cheng, D.; Zhou, X.; Xia, H.; Chan, H. S. O. Chem. Mater. 2005, 17, 3578. (7) Kamata, K.; Lu, Y.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 2384. (8) Salgueirino-Maceira, V.; Spasova, M.; Farle, M. AdV. Funct. Mater. 2005, 15, 1036. (9) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206. (10) Buchold, D. H. M.; Feldmann, C. Nano Lett. 2007, 7, 3489. (11) Chu, M. Q.; Liu, G. J. Mater. Lett. 2006, 60, 11. (12) Zhou, J.; Wu, W.; Caruntu, D.; Yu, M. H.; Martin, A.; Chen, J. F.; Zhou, W. L. J. Phys. Chem. C 2007, 111, 17473. (13) Li, X. H.; Zhang, D. H.; Chen, J. S. J. Am. Chem. Soc. 2006, 128, 8382. (14) Messing, G. L.; Zhang, S.; Jayanthi, G. V. J. Am. Ceram. Soc. 1993, 76, 2707. (15) Li, D.; Ichikuni, N.; Shimazu, S.; Uematsu, T. Appl. Catal. A,. 1999, 180, 227. (16) Suh, W. H.; Jang, A. R.; Suh, Y. H.; Suslick, K. S. AdV. Mater. 2006, 18, 1832. (17) Huang, Y.; Zheng, Z.; Ai, Z.; Zhang, L.; Fan, X.; Zou, Z. J. Phys. Chem. B 2006, 110, 19323. (18) Tsal, S. C.; Song, Y. L. J. Mater. Sci. 2004, 39, 3647. (19) Wang, Y. W.; Zhang, L. Z.; Deng, K. J.; Chen, X. Y.; Zou, Z. G. J. Phys. Chem. C 2007, 111, 2709. (20) Chen, D.; Yang, D.; Wang, Q.; Jiang, Z. Ind. Eng. Chem. Res. 2006, 45, 4110. (21) Yu, J. C.; Zhang, L. Z.; Yu, J. G. Chem. Mater. 2002, 14, 4647. (22) Bo, S.; Panagiotis, G. S. Catal. Today 2003, 88, 49. (23) Gao, F.; Chen, X. Y.; Yin, K. B.; Dong, S.; Ren, Z. F.; Yuan, F.; Yu, T.; Zou, Z. G.; Liu, J. M. AdV. Mater. 2007, 19, 2889. (24) Mathur, S.; Veith, M.; Ruegamer, T.; Hemmer, E.; Shen, H. Chem. Mater. 2004, 16, 1304. (25) Li, F. S.; Wang, H. B.; Wang, L.; Wang, J. B. J. Magn. Magn. Mater. 2007, 309, 295. (26) Chung, C. C.; Chung, T. W.; Yang, T. C. K. Ind. Eng. Chem. Res. 2008, 47, 2301.

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