Carbon-Encapsulated Core−Shell Spheres

Mar 26, 2008 - Silica@carbon core−shell spheres have been synthesized via a hydrothermal carbonization procedure with glucose as the carbon precurso...
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Langmuir 2008, 24, 5024-5028

Synthesis of Silica/Carbon-Encapsulated Core-Shell Spheres: Templates for Other Unique Core-Shell Structures and Applications in in Situ Loading of Noble-Metal Nanoparticles Yong Wan, Yu-Lin Min, and Shu-Hong Yu* DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, School of Chemistry and Materials, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed NoVember 16, 2007. In Final Form: February 1, 2008 Silica@carbon core-shell spheres have been synthesized via a hydrothermal carbonization procedure with glucose as the carbon precursor and silica spheres as the cores. Such SiO2@C core-shell spheres can be further used as templates to produce SiO2@C@SiO2, and SiO2@SiO2 spheres with a vacant region in two SiO2 shells, noble-metal nanoparticle loaded SiO2@C core-shell spheres, and hollow carbon capsules through different follow-up processes. The obtained core-shell materials possess remarkable chemical reactivity in reducing noble-metal ions to nanoparticles, e.g., platinum. These unique core-shell spherical composites could find applications in catalyst supports, adsorbents, encapsulation, nanoreactors, and reaction templates.

Introduction Nanostructured carbon materials, e.g., carbon nanotubes (CNTs) or mesoporous carbons, because of their extraordinary properties of high-temperature stability, resistance to acid, base, and solvents, large pore volumes, and very special surface properties, hold extensive applications including filter separation technology,1 catalyst supports,2 electrode materials in lithium batteries,3 fuel cells,4 electrochemical double-layer capacitors,5 templates for the preparation of other nanostructured inorganic materials,6 flat panel displays,7 energy storage,8 and chromatographic packing.9 Most approaches for the synthesis of nanostructured carbon materials are operated under rather harsh conditions, such as arc discharge,10 chemical vapor deposition (CVD),11 carbonization of organic compounds,12 and ultrasonic treatment.13 Recently, * To whom correspondence should be addressed. Fax: + 86 551 3603040. E-mail: [email protected]. (1) (a) Shiflett, M. B.; Foley, H. C. Science 1999, 285, 1902. (b) Suda, H.; Haraya, K. Chem. Commun. 1997, 93. (c) Han, S.; Sohn, K.; Hyeon, T. Chem. Mater. 2000, 12, 3337. (2) (a) Rodrı´quez-Reinoso, F. Carbon 1998, 36, 159. (b) Yu, J. S.; Kang, S.; Yoon, S. B.; Chai, G. J. Am. Chem. Soc. 2002, 124, 9382. (3) Zhou, H.; Zhu, S.; Hibino, M.; Honma, I.; Ichihara, M. AdV. Mater. 2003, 15, 2107. (4) (a) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (b) Guay, P.; Stansfield, B. L.; Rochefort, A. Carbon 2004, 42, 2187. (5) Lee, J.; Kim, J.; Lee, Y.; Yoon, S.; Oh, S. M.; Hyeon, T. Chem. Mater. 2004, 16, 3323. (6) (a) Han, W. Q.; Zettl, A. Nano Lett. 2003, 3, 681. (b) Lu, A.-H.; Schmidt, W.; Taguchi, A.; Spliethoff, B.; Tesche, B.; Schuth, F. Angew. Chem., Int. Ed. 2002, 41, 3489. (7) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (8) (a) Dahn, J. R.; Zheng, T.; Liu, Y. H.; Xue, J. S. Science 1995, 270, 590. (b) Isao, M.; Cha-Hun, K.; Yozo, K. Carbon 2001, 39, 399. (c) Prabhuram, J.; Zhao, T. S.; Tang, Z. K.; Chen, R.; Liang, Z. X. J. Phys. Chem. B 2006, 110, 5245. (9) Li, Z.; Jaroniec, M. Anal. Chem. 2004, 76, 5479. (10) (a) Iijima, S. Nature 1991, 354, 56. (b) Satyanarayan, B. S.; Robertson, J.; Milne, W. I. J. Appl. Phys. 2000, 87, 3126. (11) (a) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362. (b) Boskovic, B. O.; Stolojan, V.; Khan, R. U. A.; Haq, S.; Silva, S. R. P. Nat. Mater. 2002, 1, 165. (c) Bajpai, V.; Dai, L. M.; Ohashi, T. J. Am. Chem. Soc. 2004, 126, 5070. (12) Tang, C.; Qi, K.; Wooley, K. L.; Matyjaszew, K.; Kowalewski, T. Angew. Chem., Int. Ed. 2004, 43, 2783.

a green methodology for the preparation of hard carbon spheres was reported.14,15 The carbon spheres, with the diameter ranging from 100 nm to several micrometers, can be synthesized from saccharide compounds under hydrothermal conditions at 160200 °C. Silica is a valuable material due to its excellent thermal stability, low thermal expansion, excellent chemical durability, purity, and good optical transmission. In the mesoporous carbon preparation methodology, multiform mesoporous silica acts as a template, such as MCM-48,16 SBA-15,17 MCM-41,18 MSUH,19 aluminosilicate,20 monoliths,21 silica nanoparticles,22 and sol-gel-polymerized silica gel.23 To achieve an aligned array or well-separated individual carbon nanotubes, submicrosized silica spheres were used as substrates.24 Recently, some research groups have reported the latest progress on carbon/silica composites.25 Correa-Duarte26 encapsulated carbon nanotubes inside wormlike silica shells. The shells could be dissolved gradually to create hollow capsules. CNTs coated with silica can resist aggregation and ambient influence, and maintain the desirable mechanical, electronic, and optical (13) Wang, Z. X.; Yu, L. P.; Zhang, W.; Zhu, Z. Y.; He, G. W.; Chen, Y.; Hu, G. Phys. Lett. A 2003, 307, 249. (14) Wang, Q.; Li, H.; Chen, L. Q.; Huang, X. J. Carbon 2001, 39, 2211. (15) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 597. (16) (a) Solovyov, L. A.; Zaikowskii, V. I.; Shmakov, A. N.; Belousov, O. V.; Ryoo, R. J. Phys. Chem. B 2002, 106, 12198. (b) Yoon, S. B.; Kim, J. Y.; Yu, J. -S. Chem. Commun. 2002, 1536. (17) Lu, A.; Kiefer, A.; Schmit, W.; Schuˆth, F. Chem. Mater. 2004, 16, 100. (18) We, G. C.; Bein, T. Science 1994, 266, 1013. (19) Kim, S. -S.; Pinnavaia, T. J. Chem. Commun. 2001, 2418. (20) (a) Ma, Z.; Kyotani, T.; Tomita, A. Chem. Commun. 2000, 2365. (b) Lee, J.; Sohn, K.; Hyeon, T. J. Am. Chem. Soc. 2001, 123, 5146. (21) Yang, H. F.; Shi, Q. H.; Liu, X. Y.; Xie, S. H.; Jiang, D. C.; Zhang, F. Q.; Yu, C. Z.; Tu, B.; Zhao, D. Y. Chem. Commun. 2002, 2842. (22) (a) Li, Z.; Jaroniec, M. J. Am. Chem. Soc. 2001, 123, 9208. (b) Han, S.; Hyeon, T. Chem. Commun. 1999, 1955. (23) Li, Z.; Jaroniec, M. Carbon 2001, 39, 2080. (24) (a) Chen, J.; Lee, S. W.; Grebel, H. Carbon 2006, 44, 587. (b) Huang, S. M. Carbon 2003, 41, 2347. (c) Lan, A.; Iqbal, Z.; Aitouchen, A.; Libera, M.; Grebel, H. Appl. Phys. Lett. 2002, 81, 433. (25) (a) Ji, L. J.; Ma, J.; Zhao, C. G.; Wei, W.; Ji, L. J.; Wang, X. C.; Yang, M. S.; Lu, Y. F.; Yang, Z. Z. Chem. Commun. 2006, 1206. (b) Wang, Z. M.; Shishibori, K.; Hoshinoo, K.; Kanoh, H.; Hirotsu, T. Carbon 2006, 44, 2479. (c) Agrawal, S.; Kumar, A.; Frederick, M. J.; Ramanath, G. Small 2005, 1, 823. (26) Grzelczak, M.; Correa-Duarte, M. A.; Liz-Marza´n, L. M. Small 2006, 2, 1174.

10.1021/la703578u CCC: $40.75 © 2008 American Chemical Society Published on Web 03/26/2008

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Figure 1. (a) TEM image of silica microspheres and (b, c) TEM images and (d) SEM images of representative SiO2@C core-shell micropheres.

properties, which was attributed to the wide band gap of silica with a small, positive electron affinity (0.6-0.8 eV) together with the relatively high electronic inertness of the noncovalent SiO2-CNT interaction.27 Carbon/silica composites possess potential applications in adsorbents,28 biosensors,29 templates,30 and solar absorbers.31 Our group has prepared uniform carbon nanofibers embedded with noble-metal nanoparticles32 and Ag@C nanocables via a mild hydrothermal process.33 Recently, Titirici et al. investigated the relationship between the surface polarity of silica templates and carbon coating in detail.34 Herein, we report the synthesis of silica/carbon spherical coreshell structures via a hydrothermal carbonization procedure with glucose as the carbon precursor and silica spheres as the cores. The obtained core-shell materials possess outstanding chemical reactivity in reducing noble-metal ions to nanoparticles, e.g., platinum. Such SiO2@C core-shell spheres can act as templates to be further converted to SiO2@C@SiO2 and SiO2@SiO2 with a vacant region between two SiO2 shells, noble-metal nanoparticle loaded SiO2@C core-shell spheres, and hollow carbon capsules through different follow-up processes.

Figure 2. TEM images of (a) SiO2@C@SiO2 core-shell microspheres and (b) core-shell silica microspheres (SiO2@SiO2) formed by calcination at 800 °C. Scheme 1. Schematic Illustration of the Fabrication of Silica/Carbon Core-Shell Composites

Experimental Section Materials. Tetraethoxysilane (TEOS; 28.4 wt % in silica), absolute ethanol (95 wt %), ammonia hydroxide (25 wt %), glucose (D(+)-type) and poly(vinylpyrrolidone) (PVP; k30, 95 wt %) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used without further purification. Doubly deionized water was used through all the processes. Synthesis of SiO2@C Microspheres. Nanoporous, monodisperse silica microspheres were prepared by the sol-gel progress using ammonia as catalyst.35 In a typical case, at room temperature, 1 mL of ammonia hydroxide and 30 mL of absolute ethanol were poured into a 50 mL glass conical tube, and then 0.5 mL of TEOS was injected at a time interval of 2 h under vigorous magnetic stirring. After 6 h, monodisperse silica microspheres, ∼100 nm in diameter, were dispersed in ethanol for further experiments. For carbon coating, 4 g of glucose and 0.5 g of PVP were added to a Teflon-lined stainless steel autoclave containing 20 mL of doubly deionized water, and the mixture was stirred vigorously for a while. Then 0.75 mL of as-prepared silica microsphere colloidal suspension was added to the above transparent solution. The autoclave was (27) Wojdel, J. C.; Bromley, S. T. J. Phys. Chem. B 2005, 109, 1387. (28) Shi, Z. G.; Feng, Y. Q.; Xu, L.; Da, S. L. Carbon 2003, 41, 2668. (29) Li, J.; Chia, L. S.; Goh, N. K.; Tan, S. N. J. Electroanal. Chem. 1999, 460, 234. (30) Kawashima, D.; Aihara, T.; Kobayashi, Y.; Kyotani, T.; Tomita, A. Chem. Mater. 2000, 12, 3397. (31) Matsai, Y.; Polarz, S.; Antonietti, M. AdV. Funct. Mater. 2002, 12, 197. (32) Qian, H. S.; Antonietti, M.; Yu, S. H. AdV. Funct. Mater. 2007, 17, 637. (33) Yu, S. H.; Cui, X. J.; Li, L.; Li, K.; Yu, B.; Antonietti, M.; Co¨lfen, H. AdV. Mater. 2004, 16, 1636. (34) Titirici, M. -M.; Thomas, A.; Antonietti, M. AdV. Funct. Mater. 2007, 17, 1010. (35) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.

sealed and heated at 160 °C in an oven for 12-30 h to attain the desired shell thickness. The obtained samples were centrifuged to remove the supernatant liquid, and the solid was redispersed in ethanol. The centrifugation/redispersion cycles were repeated three times at least. Synthesis SiO2@C@SiO2 Core-Shell Microspheres. To 30 mL of ethanol were added 1.5 mL of ammonia hydroxide and a certain amount of SiO2@C core-shell composites. A 0.1 mL volume of TEOS (0.28 wt % in silica) was dropped into the dispersion in 10 h and the mixture further stirred for 6 h. To remove the carbon layer, the as-prepared products were heated at a rate of 1 K/min to 800 °C and maintained for 8-12 h. Synthesis SiO2@C Microspheres Loaded with Metal Nanoparticles. The SiO2@C core-shell microspheres were transferred to a 100 mL reaction flask containing 30 mL of doubly deionized water and several drops of noble-metal ion solution (10 mmol/L H2PtCl6‚6H2O aqueous solution), and then the mixture was refluxed at 130 °C for 3-5 h. The obtained SiO2@C core-shell composites attached to Pt particles were collected for characterization. Characterization. The morphology was examined with field emission scanning electron microscopy (FE-SEM; JEOL JSM-

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Figure 3. TEM images of SiO2@C microspheres obtained at different hydrothermal reaction times: (a) 12 h, (b) 18 h, (c) 24 h. The carbonization temperature is 160 °C for these samples. 6700F), transmission electron microscopy (TEM; Hitachi H-800), and high-resolution transmission electron microscopy (HRTEM; JEOL-2010) at an acceleration voltage of 200 kV. The samples for the SEM measurements were prepared by dropping a few drops of the sample suspension with ethanol as the solvent onto a copper grid, and the solvent was allowed to evaporate to dryness and sputtercoated with gold before analysis. X-ray diffraction (XRD) detection was carried out on a Philips X’Pert Pro Super X-ray powder diffractometer equipped with graphite-monochromatized Cu KR radiation (λ ) 1.54187 Å). FTIR spectra were measured in transmission mode using KBr tablets by Fourier transform infrared spectroscopy (Bruker, EQUINOX55). Raman spectra were obtained using an excitation wavelength of 514.32 nm on a JobinYvon (France) Labram-HR confocal laser micro-Raman spectrometer.

Results and Discussion The monodisperse silica spheres were synthesized by hydrolysis and polymerization of TEOS in a mixture of ammonium hydroxide and ethanol.35 The as-prepared silica spheres are hydrophilic because of hydroxyl groups on the surface. In contrast with the literature,34 we prepared exquisite silica@carbon coating structures. The synthesis procedure is schematically illustrated in Scheme 1. Using the silica spheres as the cores for hydrothermal carbonization coating, a mixture of glucose and PVP was added to a colloidal suspension of silica microspheres and allowed to react. An adequate amount of TEOS and ammonia was introduced to the dispersion of SiO2@C particles in ethanol, and another shell of silica was formed. After calcination at elevated temperature, the carbon shell was removed and an interesting SiO2@SiO2 core-shell composite formed. Scheme 1 also reveals that the noble-metal nanoparticles can be loaded onto the carbon shell because of the reducing ability of carbon shells on noblemetal ions, i.e., Pt4+. Dissolution of the silica cores by using a 25 wt % ammonia aqueous solution will result in the formation of hollow carbon capsules. The silica micropheres were coated with uniform shells of carbon from glucose via a hydrothermal procedure at 160 °C. Figure 1 shows representative TEM and SEM images of SiO2@C micropheres. The carbon shells are about 23 nm in thickness, and no individual separated carbon spheres are observed in the sample, indicating that all the carbon-included materials are loaded onto the surfaces of all silica spheres. The outside surfaces of silica microspheres with a smooth surface become coarse after the carbon coating process (Figure 1a,c), which could be caused by the strong acidic and high-pressure environment during the

hydrothermal carbonization of glucose. The pH of the puce solution containing as-prepared products is about 2.8. A surprising feature of the product is that, between the carbon shells and silica cores, there is a vacant region (the white-gray region between the silica cores and carbon shells in Figure 1c), which is observed in all obtained samples under different hydrothermal conditions. Another shell of silica coating onto the SiO2@C spheres was achieved by the modified Sto¨ber process. As seen from Figure 2a, a thin shell of silica covers the SiO2@C microspheres and no compact silica nanoparticles appear in the sample. If the concentrations of the silica precursor (TEOS) and ammonia hydroxide are double or more, some individual silica microspheres appear around the SiO2@C microspheres (data not shown). After the SiO2@C@SiO2 multilayer spheres are calcined at 800 °C for 8-12 h with a heating rate of 1 K/min, the carbon shells can be removed and the as-formed SiO2@SiO2 spheres have a vacant region 10 nm in thickness between the cores and shells, which is much larger than that of the original SiO2@C micropheres (Figure 2b). The carbon shells are thin, and the surfaces of the silica spheres from SiO2@C are rough; therefore, some protuberant silica cores connect with the silica shells. The silica cores almost stay at the center of the silica shells as a result of the fact that no centrifugal process is applied throughout the synthesis procedure. In fact, some noble-metal nanoparticles, oxide nanoparticles, and semiconductor quantum dots or biomaterials could be introduced into such a vacant region of the SiO2@SiO2 spheres to make them functionalized. Both the silica microspheres synthesized by the Sto¨ber method and the reactive carbon coating from hydrothermal decomposition can be or are organically functionalized; thus, a material could be loaded onto the silica spheres or carbon coating, this structure could be further coated with silica, and then the carbon could be removed, resulting in the formation of an encapsulated material in the vacant region of the SiO2@SiO2 spheres. Detailed work needs to be done in future. The thickness of the carbon shells can be tuned by controlling the reaction time, carbonization temperature, and concentration of the carbon source. As one dominant parameter, the reaction time is examined. When the reaction time is 12 h, but the carbonization temperature, the amount of silica spheres, and the carbon precursor concentration are kept constant, the carbon shells are about 25 nm in thickness (Figure 3a). If the reaction time is prolonged to 18 and 24 h, the thickness of the carbon shells can increase up to ∼38 and ∼50 nm, respectively (Figure

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Figure 6. TEM images of SiO2@C microspheres from dried silica powder.

Figure 4. TEM image of carbon nanoshells after removal of silica spheres through dissolution.

Figure 7. TEM images showing noble-metal Pt nanoparticles loaded onto the surfaces of SiO2@C microspheres.

Figure 5. FTIR spectrum of SiO2@C microspheres.

3b,c). As shown in Figure 3c, some individual bulk carbon particles exist in the SiO2@C composites and the carbon shells link each other. Moreover, plentiful carbon floccules appear when the hydrothermal time is prolonged to 30 h. It is obvious that the longer reaction time can produce more carbon materials in the final product. Moreover, upon dissolution of silica from the SiO2@C microspheres using ammonium hydroxide, remnant carbon capsules can be obtained, as clearly shown in Figure 4, and the silica cores are completely removed. This result provides an easy way to fabricate uniform carbon nanoshells. The hydrothermal temperature, 160 °C, is higher than the normal glycosidation temperature and thus leads to aromatization and carbonization.36 The FTIR spectrum reveals bands at 10001300 cm-1, which are attributed to the C-OH stretching and OH bending vibrations (Figure 5). The peaks appearing at 1642.77 (36) (a) Sakaki, T.; Shibata, M.; Miki, T.; Hirosue, H.; Hayashi, N. Bioresour. Technol. 1996, 58, 197. (b) Liujkx, G. C. A.; van Rantwilk, F.; van Bekkum, H.; Antal, M. J., Jr. Carbonhydr. Res. 1995, 272, 191.

Figure 8. XRD pattern of the Pt-loaded SiO2@C microspheres.

and 2919.16 cm-1 arise from CdO and CH vibrations, respectively. It is obvious that the as-prepared SiO2@C microspheres are highly organically functionalized on the basis of the FTIR data. If the silica microspheres from modified Sto¨ber method are dried at room temperature and then used for carbon coating on their surfaces by hydrothermal carbonization of glucose, the derived carbon shells are loose and the vacant region between the silica cores and carbon shells becomes not so clear (Figure 6).

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To demonstrate the chemical reactivity of as-prepared SiO2@C microspheres, noble-metal salt solution is mixed with the microspheres and dispersed in doubly deionized water. H2PtCl6‚ 6H2O is chosen as the noble-metal platinum source. Figure 7 shows that Pt nanoparticles can be in situ loaded onto SiO2@C microspheres by simply refluxing at 140 °C in an oil bath for 3 h without using any reducing agents due to the highly reductive ability of the carbon-rich coatings formed on silica by hydrothermal carbonization of glucose.32 The XRD pattern in Figure 8 for the obtained sample reveals the presence of a Pt phase, and the diffraction peaks can be indexed to the cubic phase of Pt and are in good agreement with the standard literature value reported for Pt (JCPDS card no. 04-0802). The Pt nanoparticles with sizes in the range from 17 to 40 nm can be sparsely decorated on the surface of SiO2@C spheres with a low density. The Raman spectrum of the SiO2@C microspheres shows no obvious peaks because of the poor crystallinity (not shown here). In contrast, noble-metal nanoparticle loaded carbon spheres have two strong peaks at 1585 and 1390 cm-1, which are due to the vibrations of crystalline graphite and disordered amorphous carbon, respectively.15

Conclusions In summary, uniform and functionalized SiO2@C microspheres have been prepared by hydrothermal carbonization of a carbon

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precursor (e.g., glucose) with silica spheres as the cores. These SiO2@C microspheres display excellent chemical reactivity and are able to in situ reduce noble-metal ions to nanoparticles and load them onto their surfaces without using any reducing agents. Such SiO2@C core-shell spheres can be further converted to SiO2@C@SiO2 and SiO2@SiO2 with a vacant region between the two SiO2 shells, noble-metal nanoparticle loaded SiO2@C core-shell spheres, and hollow carbon capsules through different synthetic processes. These unique core-shell spherical composites could find applications in catalyst supports, adsorbents, encapsulation, nanoreactors, and reaction templates. Acknowledgment. This work was supported by the National Science Foundation of China (Grant Nos. 50732006, 20325104, 20621061, and 20671085), the 973 project (Grant 2005CB623601), special funding support from the Centurial Program of the Chinese Academy of Sciences, the Anhui Development Fund for Talent Personnel and Anhui Education Committee (Grants 2006Z027 and ZD2007004-1), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Specialized Research Fund for the Doctoral Program (SRFDP) of the Higher Education State Education Ministry, and the Partner-Group of the Chinese Academy of Sciences-Max Planck Society. LA703578U