Shell Structured Nanoparticles: Controlled Synthesis

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Ag@C Core/Shell Structured Nanoparticles: Controlled Synthesis, Characterization, and Assembly Xiaoming Sun and Yadong Li* Department of Chemistry, Tsinghua University, Beijing, 100084, People’s Republic of China, and National Center for Nanoscience and Nanotechnology, Beijing, 100084, People’s Republic of China Received January 24, 2005. In Final Form: April 8, 2005 Silver nanoparticles with tunable sizes were encapsulated in a carbonaceous shell through a green wet chemical routesthe catalyzed dehydration of glucose under hydrothermal condition. In this one-pot synthesis, glucose was used as the reducing agent to react with Ag+ or Ag(NH3)2+, and it also served as the source of carbonaceous shells. The effects of hydrothermal temperature, time, and the concentrations of reagents on formation of the final nanostructures were systematically studied. The presence of competitive molecules poly(vinyl pyrrolidone) was found to be able to relieve the carbonization process, to incorporate themselves into carbonaceous shell, and to make the carbonaceous shell colorless. All these approaches provided diverse means to tailor the Ag@C nanostructures. By evaporation of the solvents gradually in a moist atmosphere, the monodispersed nanoparticles could self-assemble into arrays. Transmission electron microscopy, scanning electron microscopy, and UV-vis extinction spectra and surface-enhanced Raman spectra were used to characterize the core/shell nanostructures. These Ag@C core/shell nanoparticles have hydrophilic, organic-group-loaded surfaces and characteristic optical properties, which indicated their promising applications in optical nanodevices and biochemistry.

Introduction Core/shell structured nanoparticles are now attracting more and more investigation interest since these composite nanoparticles are constructed of cores and shells of different chemical compositions, which endow us with the possibility to combine the advantages or distinctive properties of varied materials together and, especially, to manipulate the surface functions to meet diverse application requirements.1-3 Encapsulation of nanoparticles into shells of particular chemical compositions through surface precipitation has been a general “postsynthesis” approach to get the core/ shell structured composite nanoparticles.4-16 Oxides (e.g., * To whom correspondence may be addressed. Fax: (+86) 106278-8765. E-mail: [email protected]. (1) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293-346 and references therein. (2) Caruso, F. Adv. Mater. 2001, 13, 11-22 and references therein (3) Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. Adv. Mater. 2000, 12, 693-713 and references therein. (4) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329-4335. (5) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740-3748. (6) Graf, C.; Vossen, D. L. J.; Imhof, A.; von Blaaderen, A. Langmuir 2003, 19, 6693-6700. (7) Lu, Y.; Yin, Y. D.; Li, Z. Y.; Xia, Y. N. Nano. Lett. 2002, 2, 785788. (8) Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 2003, 107, 7312-7326. (9) Oldfield, G.; Ung, T.; Mulvaney, P. Adv. Mater. 2000, 12, 15191522. (10) Mayya, K. S.; Gittins, D. I.; Caruso, F. Chem. Mater. 2001, 13, 3833-3836. (11) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462-463. (12) Clark, H. A.; Campagnola, P. J.; Wuskell, J. P.; Lewis, A.; Loew, L. M. J. Am. Chem. Soc. 2000, 122, 10234-10235. (13) Raula, J.; Shan, J.; Nuopponen, M.; Niskanen, A.; Jiang, H.; Kauppinen, E. I.; Tenhu, H. Langmuir 2003, 19, 3499-3504. (14) (a) Mayya, K. S.; Schoeler, B.; Caruso, F. Adv. Funct. Mater. 2003, 13, 183-188. (b) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846-6852. (15) Ohno, K.; Koh, K.-moo; Tsujii, Y.; Fukuda, T. Macromolecules 2002, 35, 8989-8993.

SiO2,4-8 SnO2,9 or TiO2 10), which are transparent and chemically stable, have been chosen as shell materials for metal nanoparticles cores to tailor their optical properties.4-10 As formed nanostructures have been demonstrated or proposed as building blocks for construction of photonic crystals or as model particles to study phase behavior, rheology, and diffusion.9-10 Polymers, such as polyelectrolytes or dendrites, can be alternatively used for postsynthesis encapsulation.11-16 Caruso and coworkers have developed a layer-by-layer process to deposit oppositely charged polyelectrolytes onto Au nanoparticles.14 Fukuda et al. have applied the surface-initiated living radical polymerization to form polymer brushes on Au nanoparticles.15 Chuang et al. have developed an electrochemical pathway to prepare Au@polypyrrole nanocomposites.16 As formed hybrid nanostructures are especially welcome for biological applications since their rich surface functional groups can be tailored with ease to serve as conjugates for biological applications.11,12,14 Coupled synthesis and encapsulation in one step is another choice to form core/shell nanostructures, which takes advantage of the reaction sequences and thus exhibits higher efficiency and facility. However, up to now, this method has not been as well developed as the postsynthesis methods, and relative reports are rare.17-20 Recently, we have reported a “green” wet route to obtain core/shell or sandwich structured composite nanoparticles of carbon and noble metal nanoparticles, using water as environmentally benign solvent and glucose as reducing (16) Liu, Y.-C.; Chuang, T. C. J. Phys. Chem. B 2003, 107, 1238312386. (17) Gao, J. P.; Fu, J.; Lin, C. K.; Lin, J.; Han, Y. C.; Yu, X.; Pan, C. Y. Langmuir 2004, 20, 9775-9779. (18) Cassagneau, T.; Fendler, J. H. J. Phys. Chem. B 1999, 103, 17891793. (19) Tom, R. T.; Nair, A. S.; Singh, N.; Aslam, M.; Nagendra, C. L.; Philip, R.; Vijayamohanan, K.; Pradeep, T. Langmuir 2003, 19, 34393445. (20) Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2000, 16, 2731-2735.

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reagent.21 Since the reaction took place in water and no toxic reagents were added, products showed promising applications in clinic diagnostics, therapeutics, molecular biology, and bioengineering. Among the as formed core/ shell nanoparticles, Ag@C nanospheres were considered the most promising ones, since with silver nanoparticles inside and a polysaccharide-like surface outside, they not only provided the opportunity to tailor the optical properties for detection but also made possible the loading of other functional molecules (e.g., enzymes, antigens) for clinic diagnostics. Herein, the approaches to tailor the size, shape, and optical properties of the Ag@C nanoparticles are reported in detail. Conversion of these composite nanospheres to hollow structures was also investigated through thermal and chemical routes. Assembly of these core/shell structured nanospheres was realized at room temperature, which provides a possibility to construct future nanodevices. Experimental Section Reagents. The reagents (glucose, AgNO3, and aqueous ammonia solution) used were all A.R. grade in purity, bought from Beijing chemical reagent factory and used as received. Poly(vinyl pyrrolidone) (PVP, A.R. grade, mol wt 1 300 000) was bought from Acros Organics Factory. Synthesis of Ag@C Nanospheres (Standard Synthesis). In a typical procedure, 4 g of glucose was dissolved in 35 mL of water to form a clear solution. A 0.5 mL portion of AgNO3 aqueous solution (0.1 M) was added drop by drop into the above-mentioned solution under vigorous stirring. After being stirred for more 10 min, the solution was transferred and sealed in a 40 mL Teflonsealed autoclave. The autoclave was kept at 180 °C for 4 h before being cooled in air naturally. The thick suspensions as formed were usually deeply brown in color. Such a suspension was stable even after storage for 2 days in the absence of surfactants. The final products were separated from the reaction medium by centrifuging at 4500 rpm for 30 min. A rinsing process including three cycles of centrifugation/washing/redispersion procedure in deionized water and in alchohol was required before oven drying at 80 °C for 4 h. As formed nanoparticles were marked using hydrothermal temperature and time as 180-4. Synthesis of Ag@C Core/Shell Structured Nanoparticles with a Colorless Shell. When the hydrothermal temperature was lowered to 160 °C (3-4 h), or the hydrothermal duration was shortened to 2-2.5 h (180 °C), as formed suspensions were usually yellow in color, implying the transparency of the shell. However, the reproducibility is not satisfactory. To steadily obtain colorless carbonaceous shells with silver cores, a certain amount of PVP was added. In a typical procedure, 4 g of glucose and 0.1 g of PVP were dissolved with stirring to form a clear solution before subsequent addition of AgNO3 (0.5 mL, 0.1 M). The following hydrothermal and rinse procedure was the same as that used in the standard synthesis except that the hydrothermal time should not be longer than 5 h, or a further carbonization process would make the final sample brown in color. As formed samples were encoded with a PVP amount, hydrothermal temperature, and time, for instance, addition of 0.1 g of PVP, at 180 °C, for 4 h was named P01-180-4. Synthesis of Small Cored Ag@C Nanoshperes. For this synthesis, the concentration of silver ions was reduced from ∼10-3 to 10-5 M and ammonia was added. In a typical synthesis, 4 g of glucose was dissolved in 35 mL of water. A 0.5 mL portion of AgNO3 aqueous solution (0.001 M) and ammonia was respectively added drop by drop into above-mentioned solution under vigorous stirring. The final pH value reached 8. The solution was transferred into autoclaves after stirring for 10 min. The consequent hydrothermal and rinse procedure was the same as the standard synthesis. Conversion of These Composite Nanospheres to Hollow Structures. The conversion underwent thermal and chemical (21) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 597-601.

Sun and Li

Figure 1. (A) The schematic illustration on formation mechanism of Ag@C core/shell structured nanospheres. (B) Schematically illustration on Ag@C nanospheres. (C) IR spectrum recorded on Ag@C nanospheres routes, respectively. A chemical dissolution approach was used to minimize rather than eliminate the cores inside the nanoparticles. The Ag@C nanoparticles were dispersed in 5 M ammonia solution, which was sealed in autoclaves, followed by hydrothermal treatment at 180 °C for 18 h. Products were centrifuged and rinsed with deionized water. The core can be completely eliminated through a thermal route at 850 °C. During the procedure, the Ag@C nanospheres were accumulated in an alumina boat, which was set in the hot zone of an alumina tube furnace. The content was heated at 850 °C for 2 h in a high-purity argon atmosphere under ambient pressure. Then the furnace was naturally air cooled to room temperature under the protection of argon. The sample for transmission electron microscopy (TEM) observation was scraped from the alumina boat and dispersed in alcohol via the assistance of ultrasonication. Assembly of Ag@C Nanospheres. The nanospheres obtained through standard synthesis after rinse were redispersed in water to form very thick suspensions (∼5 wt %) by ultrasonication. Then the suspension was dropped on the surface of a piece of silicon wafer. The wafer was covered and isolated from outside with a 100 mL glass vessel and quietly aged for more than 2 days. The array formed as the suspension was naturally dried (see Supporting Information). Characterization of Samples. The size, shape, and structure of Ag@C nanospheres were measured by using a Hitachi H-800 transmission electron microscope operated at 200 kV. A LEO 1530 scanning electron microscope gave the pictures of assembled nanospheres. Raman (Renishaw RM1000 Raman spectrometer, excitation wavelength 514 nm) spectroscopy was used to characterize the structures of samples. The UV-vis extinction spectra were taken at room temperature on a Hitachi F-4500 spectrometer using a quartz cuvette with a 1 cm optical path. All the solutions had been diluted near 50 times with water before taking spectra.

Results and Discussion The formation of Ag@C core/shell structured nanospheres involved two primary steps (Figure 1A): nucleation of inner silver nanoparticles, and consequent epitaxial growth and thickening of carbonaceous shell on the silver core. Initially, both AgNO3 and glucose are dissolved in deionized water and results in clear and homogeneous solution. In the absence of ammonia, silver ions do not react with glucose under ambient condition. As the solution is sealed in autoclaves and heated to 160180 °C, AgNO3 is deoxidized by glucose and Ag nanoparticles nucleate. These nanoparticles are small in size (∼80 nm) and evenly dispersed in solution, exposing a reactive surface outside, which catalyzes following carbonization of glucose and leads to in situ deposition of carbonaceous products around the Ag nanoparticle surface to form a

Silver Nanoparticles in a Carbonaceous Shell

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Figure 2. (A-D) Ag and Ag@C nanoparticles prepared at 180 °C for varied times: (A) 2.5 h; (B) 3 h; (C) 3.5 h; (D)4 h; (E) the function curve of shell thickness versus hydrothermal time; (F, G) Ag@C nanoparticles prepared at 160 °C, 4 h and 6 h; (H) Ag@C nanoparticles prepared at 160 °C, 4 h.

carbonaceous shell. There exists a large amount of functional groups such as -OH or dCdO covalently bonded onto the carbon framework in the carbonaceous component (Figure 1B) as demonstrated by Fourier transform infrared study (Figure 1C), which is in accord with previous study of core-free carbonaceous spheres.21 The Effects of Hydrothermal Time and Temperature (Mechanism Demonstration). To demonstrate the mechanism and elucidate the effects of hydrothermal time and temperature at the same time, batches of samples were prepared at 180 °C in half hour intervals. It should be noted that the hydrothermal temperature was critical for formation of these nanoparticles. As the temperature was higher than 190 °C, the carbonization took place in a violent way and was hard to control. However, if temperature was less than 140 °C, the carbonization did not start even after 12 h of treatment. The favored temperature was 160-180 °C. We chose 180 °C as a model and found that the shell thickness could be manipulated in the range 15-200 nm by adjusting the time and temperature. Typical TEM images of sample prepared at 180 °C for 150-240 min are shown in Figure 2A-D. Repeated experiments indicated that at a given concentration and temperature (e.g., [glucose] ) 0.5 M, [Ag+] ) 1.25 × 10-3 M, T ) 180 °C), the core size remained constant (∼80 nm), while the shell thickness increased with time. As the hydrothermal time was less than 2 h, the resultant solutions were usually clear. Solutions turned thick as the reaction time reached 2-2.5 h, indicating the formation of Ag nanoparticles. Both samples prepared at 2 and 2.5 h appeared the same under TEM observation (Figure 2a). It is believed that nanoparticles formed at 2 h were not under effective protection of surface organic or carbonaceous compositions since they turned gray in air during the process of centrifugation because of surface oxidization. The existence of such Ag nanoparticles implied that the carbonization of glucose was not simultaneous with reduction of AgNO3 but occurred at a consequent step in time. Meanwhile, the samples prepared at 2.5 h were stable in air with color always the same as those freshly prepared at 180 °C, 2 h, implying they were under effective protection of a transparent shell. It is believed that the shell must be very thin (