发表文章毕业论文2012-Sun-CEJ-One-step synthesis of monodisperse AuAg alloy nanoparticles in a microreaction system

Chemical Engineering Journal 189 190 (2012) 451 455 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal j ourna l ho mepage: One-step synthesis of monodisperse AuAg alloy nanoparticles in a microreaction system Li Suna, Weiling Luana, Yuejin Shanb, Shan-tung Tua aThe Key Laboratory of Safety Science of Pressurized System (MOE), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China bDepartment of Applied Chemistry, Faculty of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan a r t i c l e i n f o Article history: Received 25 November 2011 Received in revised form 3 February 2012 Accepted 14 February 2012 Keywords: One-step synthesis Rapid injection AuAg alloy nanoparticles Surface plasmon resonance Microreaction system a b s t r a c t Monodisperse AuAg alloy nanoparticles were synthesized by a one-step rapid injection method in a capillary microreaction system with a short residence time (less than 3 min). AuAg alloy NPs possessed homogenous composition that revealed a linear relationship with surface plasmon resonance absorption peak. The optimal size distribution of AuAg alloy NPs with a standard deviation of 6% could be obtained from decreasing the molar ratio of Au3+:Ag+from 1:10 to 1:20 or enhancing the synthesis temperature from 140 to 160C. Transmission electron micrographs showed as-obtained AuAg alloy nanoparticles were sphere-like with the smallest average size of only 2.7 nm. Crown Copyright 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction AuAg alloy nanoparticles (NPs) have attracted much attention due to their unique optical 1, electronic 2 and catalytic prop- erties 3. Up to now, several methods have been conducted on the synthesis of AuAg alloy, including laser ablation 4, phase- transfer 5, digestive ripening 6, co-reduction of Au and Ag salts 7 and galvanic replacement reaction 8. However, it is still diffi - cult to synthesize uniform and stable NPs without strong ligands such as thiol which is hard to be removed from products. Recently, Sun et al. 9 reported the monodisperse AuAg alloy NPs using a weak ligand (oleylamine) as both reducing agent and surfactant through slowly heating the ligand and metal precursors together in fl ask. Then Swihart et al. 10 also applied oleylamine to obtain monodisperse Au and Ag NPs in fl ask and utilized a rapid injection method. Through this rapid injection method, highly monodis- perse NPs could be obtained, which was attributed to the initial nucleation burst occurred in precursor injection 11. Once the nucleation burst creates, the concentration of precursor is depleted enough so that hinders the further nucleation. The uniform NPs are formed from the slow growth of nuclei that come from the initial burst. Swihart considered it was impossible to synthesize AuAg alloy NPs in fl ask via the rapid injection method. According to his Corresponding author. Tel.: +86 21 6425 3513; fax: +86 21 6425 3513. E-mail address: luan (W. Luan). report, a mixture of Ag, Ag-rich alloy, Au or Au-rich alloy NPs was obtained instead of AuAg alloy NPs because both Au and Ag NPs might nucleate simultaneously. Au or Ag monomers might pref- erentially deposit on its own nuclei, which brought the mixture. Therefore, the key to synthesize AuAg alloy NPs by rapid injection method should be the avoidance of Au or Ag nuclei formation. In our opinion, the microreaction system might be suitable to synthesize AuAg alloy NPs by the rapid injection method. Compared with the fl ask synthesis, the microreaction system has higher control accuracy, superior size- and shape-selectivity, no inert atmosphere protection and the faster heat and mass transfer 12 which makes it possible to generate AuAg alloy nuclei instead of Au or Ag nuclei formation. The microreaction system has been used to synthesize the semiconductor NPs, metal oxide NPs, hybrid inorganic NPs, and metal NPs 13. Especially in the synthesis of the metal NPs, var- ious high-quality metal NPs such as Ag, Au, Co and Cu 1417 have been prepared. Even the bimetal AuAg with a coreshell structure was also obtained via a microreactor 18. However, the monodisperse and controllable AuAg alloy NPs have not been reported. Here, we tried to synthesize monodisperse AuAg alloy NPs using oleylamine/octadecene by a one-step rapid injection method in a microreaction system. The parameters including synthesis tem- perature, the molar ratio of Au3+:Ag+and the residence time were investigated systematically. AuAg alloy NPs were characterized by EDS, ICP, UVvis and HR-TEM. 1385-8947/$ see front matter. Crown Copyright 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.02.043 452L. Sun et al. / Chemical Engineering Journal 189 190 (2012) 451 455 Fig. 1. Set-up of the capillary microreaction system for the synthesis of AuAg NPs. 1. syringe pump; 2. capillary; 3. oil bath; 4. blender; 5. microreactor; and 6. collector. 2. Experimental Oleylamine (OLA, Aladdin, AR), octadecene (ODE, Aladdin, AR), silver nitrate (AgNO3, Aladdin, GR), chloroauric acid (HAuCl43H2O, Aladdin, AR), ethanol (SCR, AR) and chloroform (SCR, AR) were used directly as the raw materials without further treatment. In our experiment, AuAg alloy NPs were prepared via a cap- illary microreaction system shown in Fig. 1. The syringe pump supplied a different fl ow rate, which led to a variety of residence time. The blender and oil bath supported a stable temperature con- dition. The PTFE microreactor capillary (ID: 750 ?m; L: 100 cm, : 100 ?m), which is stable in the chemical reagents and can with- stand moderate temperatures (up to 350C), was coiled in oil bath. The structure of capillary enhanced the mass and heat transfer and moreover saved the space. AuAg colloid was collected in collector and then ethanol was added to precipitate the NPs. The suspension was centrifuged (8000 rpm, 10 min) for two times. The supernatant was discarded. The NPs were re-dispersed in chloroform. Typically, 0.01 mmol silver nitrate was dissolved in 4 ml OLA and 1 ml ODE via ultrasonic machine at 60C. And 0.001 mmol chloroauric acid was dissolved in 4 ml OLA and 1 ml ODE, which was fi xed in our experiment. The two dissolved solutions were mixed with mag- netic stirrer. Then the mixture was sucked in the injector that was fi xed on syringe pump. When the temperature of oil bath was stable on reaction temperature, the reaction solution was injected rapidly into the capillary. The temperature was changed from 140 to 160C. The molar ratio of Au3+:Ag+in the raw material was adjusted from 1:10 to 1:20. The residence time was tuned by different fl ow rate supplied by syringe pump. Energy-dispersive spectrum (EDS) was applied to determine the Au molar ratio of various AuAg alloy NPs using a scanning electron microscope (JSM-6360LV, JEOL) equipped with EDX (Fal- con, EDAX). The concentrations of Au and Ag were measured by means of Varian 710 inductively coupled plasma spectrometry (ICP-MS). The samples were solubilized in aqua regia (HCl/HNO3) in advance. Ultraviolet visible spectrum (UVvis) analysis was car- ried out on a spectrophotometer (Cary 50, Varian) using chloroform as the dispersing agent. The relationship between the wavelength corresponding to surface plasmon resonance (SPR) absorption peak against the Au molar ratio of various AuAg alloy NPs was investigated by UVvis absorption spectra and EDS. High resolu- tion transmission electron microscope (HR-TEM, JEM-2100F, JEOL) operated at 200 kV was used to observe the morphology. The sam- ple for TEM was prepared by dipping an amorphous carbon-copper grid in a chloroform solution dispersed homogeneously AuAg NPs by sonication for 5 min, then the sample was left to evaporate in air at room temperature. For each sample, the diameter and standard Fig. 2. (a) Normalized UVvis spectra of Ag, AuAg alloy and Au NPs. (b) Plot of the wavelength corresponding to SPR absorption peak against Au molar ratio of various NPs. deviation were determined by averaging measurements of more than 100 NPs. 3. Results and discussion The color of AuAg NPs dispersions is a refl ection of its compo- sition and microstructure 19. Fig. S1 shows the photographs of Ag, AuAg and Au NPs dispersions prepared in the molar ratio of Au3+:Ag+as 1:10 at 140C. The colors of AuAg dispersions grad- ually change from purple to yellow, which matches well with the reported results 19,20. As shown in Fig. 2a, each UVvis absorp- tion spectra exhibits only one peak which is blue shifted from 495 to 432 nm with the increase of the residence time from 30 to 320 s. When a mixture of Au and Ag NPs is obtained, two peaks between 522 and 410 nm will present in the spectra 21. Generally, AuAg alloy NPs with a homogenous composition can be con- fi rmed through examining a single SPR absorption peak and a linear fashion of the absorption peak and the Au molar ratio of AuAg NPs 6,20,22. Fig. 2b manifests the linear relationship between the absorption peaks and Au molar ratios of NPs corresponding to Fig. 2a. It indicates that AuAg alloy NPs with homogenous composition were obtained by the rapid injection method in a microreaction system. The TEM images of AuAg alloy NPs, refl ecting to Fig. 2, are shown in Fig. 3. It clearly displays that AuAg alloy NPs with the SPR absorption peak from 495 to 432 nm are sphere-like with uni- form distribution. In Fig. 3d, the HR-TEM image of AuAg alloy is exhibited, which further confi rms that such obtained AuAg NPs are of alloy structure. The size of AuAg alloy NPs changes from 4.0 to 6.9 nm (in Fig. S2) with the increasing residence time. There is no report found on such small size of AuAg alloy NPs obtained using the same reactants via fl ask synthesis. The size of AuAg alloy NPs synthesized in fl ask was more than 8 nm 9. The synthesis L. Sun et al. / Chemical Engineering Journal 189 190 (2012) 451 455453 Fig. 3. TEM images of AuAg alloy NPs synthesized at 140C in the molar ratio of Au3+:Ag+as 1:10 at different SPR absorption peak (a) 495 nm, (b) 456 nm, (c) 432 nm) and residence time (a) 30 s, (b) 120 s, (c) 320 s). (d) The HR-TEM image of AuAg alloy NP (c). temperature of 140C was higher than the reduction tempera- ture of Au ions (65C) and lower than that of Ag ions (180C) 23,24. Thus, the formation of AuAg alloy NPs was decided by the reductive amount of Ag ions which intensively depended on the reduction temperature and the residence time. In fl ask, long residence time was required because of its low heat and mass trans- fer effi ciency. Usually, the reaction time of the fl ask synthesis was about 1 h, however, here the residence time was shortened to less than 15 min. The longer growth time responded for the big size of AuAg alloy NPs. The molar ratio of Au3+:Ag+and the synthesis temperature were investigated to prepare more monodisperse AuAg alloy NPs and take a shorter residence time. Decreasing the molar ratio of Au3+:Ag+to 1:20 at 140C, AuAg alloy NPs with the same SPR absorption peak could be obtained in less than 5 min. Compared with the molar ratio of Au3+:Ag+to 1:10, the residence time was only a third because the high concentration of Ag ions led to the relative increased nucleation rate. According to Fig. S3, homoge- nous AuAg alloy NPs are obtained in the molar ratio of Au3+:Ag+ as 1:20. Fig. 4a shows the TEM image of AuAg alloy NPs with the SPR absorption peak at 495 nm synthesized in the molar ratio of Au3+:Ag+as 1:20 at 10 s, which displays a much smaller sphere- like AuAg alloy NPs. In Fig. 4c, the average diameter and standard deviation are presented. As-obtained NPs have a more monodis- perse distribution with a standard deviation of about 7% and smaller average size nearly 2.7 nm. It is diffi cult to obtain so monodisperse and small-sized AuAg alloy NPs by the reported methods in fl ask. When the molar ratio of Au3+:Ag+was kept as 1:10, the effect of synthesis temperature was further studied. With the synthesis temperature enhanced to 160C, the residence time was less than 3 min which was decreased to a fi fth. The result could be explained through the improvement of the relative nucleation rate of Ag ions with the increase of the synthesis temperature. Although the res- idence time is very short, Fig. S4 also shows a linear relationship, which indicates homogenous AuAg alloy NPs can be obtained at this synthesis condition. Fig. 4b and d exhibit the TEM image and the size distribution of AuAg alloy NPs synthesized at 160C, respec- tively. AuAg alloy NPs are about 5 nm in diameter with a standard deviation of 6%, which are more monodisperse comparing with Fig. 3b. A possible mechanism of our experiment is shown schemati- cally in Fig. 5. The whole synthesis process can be divided into two stages: nucleation and diffusion. When the reactants are rapidly injected into capillary microreactor, both Au and Ag ions may be reduced simultaneously, accompanying with AuAg bimetal nuclei generated owing to the fast heat and mass transfer 12. In the period of nucleation, the Au precursor is depleted, but the Ag pre- cursor just has a low consumption because of its relative slow reduction rate (see Table S1). Based on the reason, Au should be rich in the surface of the initial nuclei. During the diffusion stage, few Au NPs are further generated while Ag NPs can be obtained continu- ously due to its higher concentration. As a result, SPR absorption peak of AuAg alloy NPs is blue shifted with the extending of the residence time. Meantime, AuAg alloy NPs get further into 454L. Sun et al. / Chemical Engineering Journal 189 190 (2012) 451 455 Fig. 4. TEM images and size distributions of AuAg alloy NPs. (a) abs: 495 nm, synthesized at 140C in the molar ratio of Au3+:Ag+as 1:20; (b) abs: 456 nm, synthesized at 160C in the molar ratio of Au3+:Ag+as 1:10; (c) and (d) are the size distribution of (a) and (b), respectively. Fig. 5. Drawing of possible mechanism of producing AuAg alloy NPs. homogeneity with a simultaneous mutual diffusion between Au and Ag. In nature, the structure of AuAg NPs is depended on the relative rate of both nucleation and diffusion. Only is the rate of diffusion between Au and Ag higher than that of their nucle- ation, AuAg homogenous alloy NPs can be synthesized. Otherwise, AuAg gradient alloy NPs will be produced 25. 4. Conclusions We have synthesized monodisperse and homogenous AuAg alloy NPs by a one-step rapid injection method achieved in a microreaction system. Sphere-like AuAg alloy nanoparticles with the smallest average size of only 2.7 nm were obtained at 140C in the molar ratio of Au3+:Ag+as 1:20. The size distribution was narrowed and the residence time was shortened through increas- ing the synthesis temperature from 140 to 160C or decreasing the molar ratio of Au3+:Ag+from 1:10 to 1:20. A uniform size distribu- tion of AuAg alloy NPs was displayed with a standard deviation of about 6%. The process took less than 3 min that was much shorter than the fl ask synthesis. The facile and rapid approach is potentially useful for preparing other kinds of metal and alloy NPs. Acknowledgments Authors appreciated the fi nancial supports from the Fundamen- tal Research Funds for National Nature Science Foundation of China (51172072), the Central Universities (WJ0913001), the Focus of Scientifi c and Technological Research Projects (109063), and the State Key Laboratory of Chemical Engineering at ECUST (SKL-ChE- 08C09). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2012.02.043. References 1 M. Broyer, E. Cottancin, J. Lerme, M. Pellarin, N.D. Fatti, F. Vallee, J. Burgin, C. Guillon, P. Langot, Optical properties and relaxation processes at femtosecond scale of bimetallic clusters, Faraday Discuss. 138 (2008) 137145. 2 K.S. Lee, M.A. E.I-Sayed, Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition, J. Phys. Chem. B 110 (2006) 1922019225. 3 T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki, K. 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