Langmuir 2004, 20, 9775-9779
9775
Formation and Photoluminescence of Silver Nanoparticles Stabilized by a Two-Armed Polymer with a Crown Ether Core Junpeng Gao,† Jun Fu,† Cuikun Lin,‡ Jun Lin,*,‡ Yanchun Han,*,† Xiang Yu,§ and Caiyuan Pan§ State Key Laboratory of Polymer Physics and Chemistry, and Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China, and Department of Polymer Science & Engineering, University of Science & Technology of China, Hefei 230026, People’s Republic of China Received March 28, 2004. In Final Form: August 14, 2004 Silver nanoparticles were synthesized by the use of a two-armed polymer with a crown ether core [poly(styrene)]-dibenzo-18-crown-6-[poly(styrene)] based on the flexibility of the polymer chains and the complex effect of crown ether with Ag+ and Ag. The size of silver nanoparticles could be tailored by controlling the initial concentrations of the polymer and Ag+, and the molecular weight of the polymer. The emission of silver nanoparticles was blue-shifted, and the intensity of the photoluminescence of silver nanoparticles stabilized by the polymer was significantly increased due to the complex effect between the crown ether embedded in the polymer and the silver nanoparticles.
Introduction Recent development in nanoparticles and nanostructural materials has opened new opportunities for building up functional nanostructures.1 Due to their small sizes and large specific surface areas, the nanoparticles exhibit novel properties which differ significantly from those of the bulk materials.2,3 Because of their vast number of important applications such as in catalysis,4 electronics,5 high-density information storage,6 photoluminescence, and electroluminescence devices,7 many efforts have been devoted to the preparation of the functional nanoparticles. Very recent examples include the growth of nanoparticles in hydrophilic block copolymers,8 the use of microphase separated morphologies of block copolymers to sequester metal particles on a surface,9,12 the use of functional * To whom correspondence should be addressed. Phone: 086431-5262175. Fax: 086-431-5262126. E-mail: (Y.H.) ychan@ ciac.jl.cn; (J.L.)
[email protected]. † State Key Laboratory of Polymer Physics and Chemistry, Chinese Academy of Sciences. ‡ Key Laboratory of Rare Earth Chemistry and Physics, Chinese Academy of Sciences. § University of Science & Technology of China. (1) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M. Langmuir 2000, 16, 407. (2) Ozin, G. A. Science 1996, 271, 920. (3) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843. (4) Ohde, H.; Wai, C. M.; Kim, J.; Ohde, M. J. Am. Chem. Soc. 2002, 124, 4540. (5) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (6) Ross, C. Annu. Rev. Mater. Res. 2001, 31, 203. (7) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. D. Nature 1994, 370, 354. (8) Qi, L. M.; Colfen, H.; Antonietti, M. Nano. Lett. 2001, 1, 61. (9) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (10) Sun, S.; Anders, S.; Hamann, H. F.; Thiele, J.; Baglin, J. E.; Thomson, T.; Fullerton, E. E.; Murray, C. B.; Terris, B. D. J. Am. Chem. Soc. 2002, 124, 2884. (11) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (12) Misner, M. J.; Skaff, H.; Emrick, T.; Russell, T. P. Adv. Mater. 2003, 15, 221.
polymers to mediate the assembly of magnetic and gold nanoparticles10 (where the particles and polymers carry complementary functionality),11 controlling particle spatial distribution based on dendrimer generation,12-14 and the utilization of functional block copolymer containing two structurally different blocks to fabricate nanowires consisting of [111] oriented crystalline silver.15 Undoubtedly, these methods are highly successful in creating ordered nanostructures with specific arrangements and properties, and the size and structures of the nanoparticles can be manipulated within a fairly wide range. However, the cost and complicated procedures for some systems are the emerging problems. Therefore, many recent research efforts have been focused on the development of an alternative cost-effective and simple method to fabricate nanoparticles for both industrial applications and fundamental studies.16,17 Here, we reported a simple and novel method without using any templates to fabricate Ag nanoparticles through a functional two-armed polymer with a crown ether core. In the solution containing Ag+ and the designed polymer, due to the complexing effect of crown ether embedded in the polymer with the Ag+, some aggregations composed of Ag+ and polymer were formed. Subsequently, the Ag+ ions in the aggregations were photochemically reduced by visible light to form Ag nanoparticles. The resulted Ag nanoparticles were also surrounded by the polymer for the complexing effect of crown ether with Ag. Thus, the polymer stabilized the Ag nanoparticles. It showed that the molecular weight of polymer, Ag+ concentration, and polymer concentration had a great effect on the nanoparticle size. Furthermore, the intensity of the photolu(13) Crooks, R. M.; Zhao, M. Q.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (14) Donners, J. J. M.; Hoogenboom, R.; Schenning, A. P. H. J.; van, Hal, P. A.; Nolte, R. J. M.; Meijer, E. W.; Sommerdijk, N. A. J. M. Langmuir 2002, 18, 2571. (15) Cornelissen, J. J. L. M.; Heerbeek, R.; Kamer, P. C. J.; Reek, J. N. H.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Adv. Mater. 2002, 14, 489. (16) Husein, M.; Rodil, E.; Vera, J. Langmiur 2003, 19, 8476. (17) Biffis, A.; Orlandi, N.; Corain, B. Adv. Mater. 2003, 15, 1551.
10.1021/la049197p CCC: $27.50 © 2004 American Chemical Society Published on Web 09/30/2004
9776
Langmuir, Vol. 20, No. 22, 2004
Gao et al.
Scheme 1. Chemical Structure of the Two-Armed Polymer with a Crown Ether Core
minescence of silver nanoparticles stabilized by the polymer was significantly increased upon photoexcitation at 343 nm due to the complexing effect between the crown ether and silver nanoparticles, as compared to that of photoexcitation at 408 nm, the characteristic absorption of silver nanoparticles. The advantage of the proposed method in the present work was that the introduction of crown ether substitutes, as realized for the two-armed polymer, greatly simplified the process of fabrication and stabilization of silver particles and increased significantly the photoluminescence property of silver particles. Therefore, it may be a very interesting and promising approach to fabricate metal nanoparticles. Experimental Section The designed two-armed polymer (Scheme 1), [poly(styrene)]dibenzo-18-crown-6-[poly(styrene), with different molecular weights (Mw ) 5700, Mw/Mn ) 1.25, and Mw ) 13 600, Mw/Mn ) 1.25) was synthesized by atom transfer radical polymerization (ATRP) as described previously.18 Silver acetate was obtained from Shanghai Chemical Regent Factory (C. R. grade). Tetrahydrofuran (A. R. grade) was supplied by Beijing Chemical Reagent Co. Deionized (DI) water was obtained using the Millipore water purification system. All glassware was washed with aqua regia solution (H2SO4:K2CrO4) by sonication for 10 min in a normal ultrasonic bath (50 Hz) and was rinsed thoroughly with DI water. The silicon substrates with a 2 nm thick layer of native SiOX were cleaned by immersion in piranha solution (3:1 concentrated H2SO4/H2O2) and sonication for 30 min. Subsequently, the substrates were rinsed repeatedly with DI water and blown dry in a N2 flow. In the experiment, an aqueous solution of silver acetate and a solution of polymer in THF of desired concentration were freshly prepared separately, and then mixed and stirred for 72 h at room temperature. The polymer and silver acetate concentrations were changed from 0.25 to 1 wt % and from 0.02 to 0.2 M, respectively, to investigate their effects on the size of the resulted Ag nanoparticles. The solutions were then spin coated onto the freshly cleaned silicon wafers to produce thin films. The film was then calcined to remove the polymer component. The obtained products were characterized with atomic force microscopy (AFM) (SPI3800N Probe Station, Seiko Instruments Inc., Japan) in tapping mode (silicon tips on silicon cantilevers with a spring constant 2 N/m were used; the set point was about 0.8-0.9), transmission electron microscopy (TEM, JEOL at 200 kv), X-ray photoelectron spectroscopy (XPS; VG ESCALAB MK, VG Co., UK), luminescence spectra (F-4500 spectrophotometer), and Zetasizer (1000 HSA, Malvern, UK).
Results and Discussion First, a reaction route and a mechanism of Ag nanoparticles stabilization by the two-armed polymer with a crown ether core were proposed. Subsequently, the influence of several parameters on the size and size distribution of the nanoparticles was discussed. Finally, the luminescence of Ag nanoparticles stabilized by the polymer was investigated. Nanoparticle Synthesis. Since the crown ethers’ accidental discovery by Pedersen in 1967,19,20 they have (18) Feng, X. S.; Yan, L. F.; Wen, J.; Pan, C. Y. Polymer 2002, 43, 3131.
Figure 1. TEM micrograph of the synthesized silver nanoparticles. The initial concentration of Ag+ and the polymer (Mw ) 5700) was 0.02 M and 0.5 wt %, respectively.
proved to be enormously popular and extremely useful ligands (hosts) for a startling range of metal ions. It is known that crown ethers exhibit an extremely versatile range of interactions with Ag+ and other metal cations in the solid state that goes far beyond simplistic “size fit” considerations.21 In this study, the two-armed polymers with a crown ether core [poly(styrene)]-dibenzo-18-crown6-[poly(styrene)] were used to fabricate Ag nanoparticles due to the complex effect of crown ether with Ag+ and the flexibility of polymer chains. The Zetasizer results revealed that in water/THF mixtures at concentrations larger than 0.025 wt %, the two-armed polymer with a crown ether core (Mw ) 13 600) associated into the aggregates with the average diameter of most of the aggregates of ca. 909.0 nm because of the peripheral benzene units’ π-π interaction in the twoarmed polymer.22 When the aqueous solution of silver acetate was added, the aggregations composed of Ag+ and the polymer formed at the very beginning of the reaction as a result of the complexing effect between Ag+ and the polymer with crown ether core. The Ag+ could not choose to bind within the interior of the macrocycle because of the large but unsuitable diameter.23 The Zetasizer results showed that the size of the aggregations decreased from 389.0 to 355.4 nm when the concentration of Ag+ increased from 0.02 to 0.06 M. Thus, the smaller aggregations of Ag+ and polymers were formed. In such solutions, the Ag+ ions in the aggregations were photochemically reduced by visible light to form Ag. Visible light is presumably absorbed by the existing Ag, and the subsequent photooxidization of water resulted in further Ag+ reduction.24 Colloids prepared via chemical reduction characteristically have anions adsorbed to the surface of the particles. The resulting negative surface charges provide the repulsive forces between the particles that keep them suspended in solution. In this experiment, after the reduction of Ag+ into nanoparticles, they were surrounded by polymers through the complexing effect between Ag and crown ethers embedded in the polymer. The Ag particles dispersed in the solution because they were surrounded by the long chains of polymers. Therefore, the polymers controlled the aggregations of the metal atoms in solution and stabilized the nanoparticles. Figure 1 shows the TEM photograph of silver nanoparticles synthesized with the (19) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 2495. (20) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017. (21) Steed, J. W. Coord. Chem. Rev. 2001, 215, 171. (22) Nostrum, C. F.; Picken, S. J.; Schouten, A.; Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 9957. (23) Christensen, J. J.; Hill, J. O.; Izatt, R. M. Science 1971, 174, 459. (24) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528.
Formation and Photoluminescence of Ag Nanoparticles
Figure 2. XPS spectrum of Ag nanoparticles spin-coated from the solution on the Si wafer.
Figure 3. AFM images of the silver particles synthesized with different molecular weights (MW) of the two-armed polymer with a crown ether core. (a) MW ) 5700; (b) MW ) 13 600. The concentration of Ag+ and polymer was 0.1 M and 0.25 wt %, respectively.
initial concentrations of Ag+ and the polymer (MW ) 5700) of 0.02 M and 0.5 wt %, respectively. To obtain further information on the nature of the reaction product, XPS spectra had been taken (Figure 2). The Ag 3d5/2 peak appears at 368.0 ev, confirming the presence of Ag.25 Size Control of the Silver Nanoparticles. To demonstrate the influence of the polymer chain length on the size of Ag nanoparticles, different molecule weights (MW) of polymer (5700 and 13 600) were used. Figure 3 illustrates the AFM images of silver nanoparticles synthesized with the polymer of different MW. As the MW of the polymer increased from 5700 to 13 600, the average size of particles decreased from 10 to 3 nm (the concentration of Ag+ and polymer was 0.1 M and 0.25 wt %, respectively; the ratio of Ag+ to polymer was 500). In fact, the longer
Langmuir, Vol. 20, No. 22, 2004 9777
polymer chain means a larger spatial block of the aggregations. Thus, the number of polymers in the larger aggregations is less than that of polymers in the small aggregations while reaching to the critical conditions that the polymers cannot move to the aggregations anymore because of the spatial block. Therefore, the number of Ag+ in the aggregations decreased with the increasing of the polymer chain length during the formation process of the aggregations through the complexing effect between Ag+ and the functional polymers with a crown ether core. This result also supported the process of aggregations formed in the solution by Ag+ and polymers. The AFM images of Ag particles synthesized with different concentrations of Ag+ are shown in Figure 4. The concentration of polymer (MW ) 13 600) was fixed at 0.25 wt %. The concentration of Ag+ was changed from 0.02 to 0.1 M; that is, the molar ratio of Ag+ to polymer was from 100 to 500. With increasing the concentration, the average particle size decreased from 8 to 3 nm. Yet when the concentration was further increased to 0.2 M (the ratio of Ag+ to polymer was 1000), the average size of the particles increased to 7 nm. The increasing of the amount of Ag+ would cause Ag+ and the polymer to have more of an opportunity to complex. Thus, it accelerated the formation process of aggregations and resulted in more aggregations in the solution of high Ag+ concentration at the very beginning of the reaction. Therefore, it resulted in less Ag+ in each aggregation. From the Zetasizer results, when the concentration of Ag+ was increased from 0.02 to 0.06 M, the average diameter of aggregates formed of Ag+ and the two-armed polymer decreased from 389.0 to 355.4 nm. In other words, the speed to form the aggregations was larger than that of Ag+ to be arrested by the polymer with crown ether core through the complexing effect between them. From the AFM images, we can also see that the density of silver particles increased with increasing the concentration of Ag+. Therefore, the size of Ag particles surrounded by polymer decreased. Yet when the concentration of Ag+ was sharply increased, the amount of aggregations formed at the very beginning of the reaction reached the limit because of the fixed concentration of polymer. There was residual Ag+ in the solution because the concentration of Ag+ was very high. The residual Ag+ was attached to the aggregations as a result of the complex effect between Ag+ and the polymer with a crown ether core. Thus, the size of the Ag particles increased (Figure 4d). Yet the complexation no longer continued until it reached the equilibrium state of the complex reaction. When the Ag+ concentration increased to 0.2 M, deposition occurred on the glass wall of the vessel. It became initially yellow, then brown, and eventually shiny and metal-like. Observation of the surfaces with a
Figure 4. AFM images of the silver particles synthesized with different concentrations of Ag+: (a) 0.02 M, (b) 0.06 M, (c) 0.1 M, (d) 0.2 M. The molecular weight and the concentration of the two-armed polymer with a crown ether core were fixed at13 600 and 0.25 wt %, respectively.
9778
Langmuir, Vol. 20, No. 22, 2004
Gao et al.
Figure 5. AFM images of the silver particles synthesized with different concentrations of the two-armed polymer with a crown ether core (the molecular weight is 5700): (a) 0.25 wt %, (b) 0.5 wt %, (c) 1 wt %. The concentration of Ag+ was fixed at 0.02 M.
scanning electron microscope showed that metallic silver particles were indeed attached onto the glass. The full account of these phenomena was discussed elsewhere.26 It was believed that the particles attached to the glass wall were formed by the reduction of residual Ag+ in the solution. These phenomena also supported the reason the size of the nanoparticles increased when Ag+ was sharply increased. In some cases, a bimodal distribution of the Ag nanoparticles was observed. It may be due to the fact that the possibility of Ag+ and polymer being attached to the aggregates was different and stochastic. This caused the distribution of Ag nanoparticles to be inhomogeneous. Figure 5 shows that the particle size decreased with the concentration of the polymer at the fixed concentration of Ag+. When the concentration of polymer (MW ) 5700) increased from 0.25 to 1 wt % (the ratio of Ag+ was from 100 to 25), the average particle size decreased from 21 to 3 nm. The smaller particle size at higher concentration of polymer can be attributed to more aggregations being formed at the very beginning of the reaction. In the solution of high concentration of polymer, Ag+ can be arrested and surrounded easily by the polymer. Therefore, the more aggregations were formed at the very beginning of the reaction, the smaller particles were resulted. Optical Properties of Silver Nanoparticles Stabilized by the Polymers with a Crown Ether Core. There is growing literature on the optical and electronic properties of silver nanostructures, particularly silver nanoparticles.27 The luminescence of silver metal and that of noble metals is generally attributed to electronic transitions between the upper d band and conduction sp band.28 Luminescence of silver can be induced by irradiating the metal surface or film with electron,29 photon,30 or laser beams,31 with reported emission peak positions distributed over a wide range, from 320 to 520 nm. The plasmon absorption peak at about 400 nm is characteristic of nanosized silver particles.32-35 The luminescence spectrum of the silver particles stabilized by the polymer with a crown ether core in solutions (25) Kumar Kaushik, V. J. Electron Spectrosc. Relat. Phenom. 1991, 56, 273. (26) Pastoriza-Santos, I.; Serra-Rodry´guez, C.; Liz-Marzan, L. M. J. Colloid Interface Sci. 2000, 221, 236. (27) Huang, T.; Murray, R. W. J. Phys. Chem. B 2003, 107, 7434. (28) Mooradian, A. Phys. Rev. Lett. 1969, 22, 185. (29) Eesley, G. L. Phys. Rev. B 1981, 24, 5477. (30) Zivitz, M.; Thomas, E. W. Phys. Rev. B 1976, 13, 2747. (31) Von Raben, K. U.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1981, 79, 465. (32) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (33) Zhang, R.; Liu, J.; Han, B.; He, J.; Liu, Z.; Zhang, J. Langmuir 2003, 19, 8611. (34) Cason, J. P.; Khambaswadkar, K.; Roberts, C. B. Ind. Eng. Chem. Res. 2000, 39, 4749. (35) Retic, C.; Linxon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974.
Figure 6. (A) Photoluminescence spectra of the silver particles stabilized by the two-armed polymer with a crown ether core (MW ) 13 600): (a) excited at 408 nm (initial concentration of the polymer: 0.25 wt %), (b) excited at 343 nm (initial concentration of the polymer: 0.25 wt %), (c) excited at 343 nm (initial concentration of the polymer: 0.5 wt %), (d) excited at 343 nm (initial concentration of the polymer: 0.75 wt %). The concentration of Ag+ was fixed at 0.02 M. (B) Excitation spectra of emission detected at 442 nm of the silver particles stabilized by the two-armed polymer with a crown ether core (MW ) 13 600). The concentration of Ag+ and the polymer was fixed at 0.02 M and 0.25 wt %, respectively.
shows an emission peak at 486 nm upon photoexcitation at 408 nm (the characteristic absorption of Ag nanoparticles, Figure 6A: a). The emission of silver particles was blue-shifted to 442 nm upon photoexcitation at 343 nm (metal-ligand charge-transfer absorption, Figure 6A: b,c,d), and the intensity increased sharply. Most significantly, when increasing the initial concentration of the
Formation and Photoluminescence of Ag Nanoparticles
polymer from 0.25 to 0.75 wt %, the intensity of emission upon photoexcitation at 343 nm also increased (Figure 6A: b,c,d). Excitation spectra (Figure 6B) for the silver particles fluorescence (detected at 442 nm) exhibit maxima at 341 nm. This clearly showed that the substitution of crown ether greatly improved the photoluminescence property of silver particles due to the complexing effect between the crown ether embedded in the polymer and silver particles. Conclusions Silver nanoparticles were synthesized by the use of the two-armed polymer with a crown ether core [poly(styrene)]-dibenzo-18-crown-6-[poly(styrene)] based on the flexibility of the polymer chains and the complexing effect of crown ether with Ag+ and Ag. The size of silver nano-
Langmuir, Vol. 20, No. 22, 2004 9779
particles decreased with the increasing of the polymer molecular weight, and the polymer and the Ag+ concentration, respectively. The emission of silver particles was blue-shifted, and the intensity of the photoluminescence of silver particles stabilized by the polymer was significantly increased due to the complexing effect between the crown ether embedded in the polymer and silver particles. Acknowledgment. This work is subsidized by the National Natural Science Foundation of China (50125311, 20334010, 20274050, 50390090, 50373041, 50225205), the Ministry of Science and Technology of China (2003CB615600, 2002CCAD4000, 2003CB314707), and the Chinese Academy of Sciences (Distinguished Talents Program, KJCX2-SW-H07). LA049197P
