Controllable fabrication of PS/Ag core-shell-shaped nanostructures
© Zhang et al.; licensee Springer. 2012
Received: 30 August 2012
Accepted: 11 October 2012
Published: 23 October 2012
In this paper, based on the previous steps, a facile in situ reduction method was developed to controllably prepare polystyrene/Ag (PS/Ag) core-shell-shaped nanostructures. The crucial procedure includes surface treatment of polystyrene core particles by cationic polyelectrolyte polyethyleneimine, in situ formation of Ag nanoparticles, and immobilization of the Ag nanoparticles onto the surface of the polystyrene colloids via functional group NH from the polyethyleneimine. The experimental parameters, such as the reaction temperature, the reaction time, and the silver precursors were optimized for improvement of dispersion and Ag coat coverage of the core-shell-shaped nanostructures. Ultimately, the optimum parameters were obtained through a series of experiments, and well-dispersed, uniformly coated PS/Ag core-shell-shaped nanostructures were successfully fabricated. The formation mechanism of the PS/Ag core-shell-shaped nanostructures was also explained.
Recently, considerable effort has been devoted to controllable fabrication of nanostructured materials with tunable functional properties. One example is the fabrication of core-shell-shaped nanostructures. Such nanocomposite structures have many applications in different technological fields like bio-sensor, chem-sensor, electronics, catalysis, drug delivery, diagnostics, antibacterial agent, etc. [1–7]. Especially, fabrication of noble metal-coated latex core-shell-shaped nanostructures (CSSNs) is currently an attractive area of investigation. The noble metal nanoshells grown on dielectric core particles are of great interest due to their tunable optical properties from ultraviolet to near-infrared regions of the electromagnetic spectrum [8–13]. In this sense, fabrication of Ag-coated CSSNs is particularly of significance .
The properties of the Ag-coated CSSNs are dependent on metal coverage. Thus, control of metal coverage is important to the application of this kind of core-shell-shaped nanostructure . Until now, various approaches have been reported to fabricate the Ag-coated CSSNs, such as electroless plating [16, 17], layer-by-layer self-assembly , sonochemical methods , and so on. These methods have been attempted to coat uniform metal on the colloid cores. However, some disadvantages exist during the synthesis. For example, in the electroless plating, metals or compounds such as gold, palladium, and SnCl2 are used to activate the core surface, but they remain as an impurity in the final product. The layer-by-layer self-assembly is too complicated and time-consuming. In the sonochemical method, the high-intensity ultrasound and removal of oxygen are necessary; otherwise, impurity like Ag2O can be observed.
In this paper, based on the previous investigations , an advanced, rapid, and low-cost in situ reduction method with optimized fabrication parameters was developed for the fabrication of polystyrene/Ag (PS/Ag) CSSNs. In the first step, monodispersed PS colloids as core particles were prepared by emulsion polymerization and then modified with polyethyleneimine (PEI) several times. Subsequently, Ag seeds were formed in situ and immobilized on the surface of PS colloids by adding AgNO3 or silver ammonia solution into the PEI-modified PS colloids. Finally, sodium citrate was added in the dispersion to increase thickness of the Ag shell. In this work, key experimental parameters were optimized to improve the dispersion and Ag coverage of PS/Ag CSSNs. Finally, well-dispersed and uniformly coated PS/Ag CSSNs were obtained. The thickness and coverage of the Ag shells can be easily controlled by changing the temperature, time, and silver precursors by which the properties of PS/Ag CSSNs can be tailored.
Styrene, potassium pyrosulphate (K2S2O3), sodium citrate (C6H5O7Na3·2H2O), ethanol (C5H6OH), and sodium lauryl sulfate (CH3(CH2)SO4Na) were purchased from Shantou Dahao Fine Chemical Co., Ltd. (Shantou, Guangdong, China); silver nitrate (AgNO3), ammonia, PEI (MW600000-1000000) were purchased from Shanghai Chemicals Co. Ltd. (Shanghai, China). Water used during the experiments was distilled twice.
The monodispersed PS colloids for the coating cores were prepared by emulsion polymerization in aqueous alcohol system . Potassium pyrosulphate (0.0490 g) and sodium lauryl sulfate (0.0538 g) were dissolved in 70-ml aqueous alcohol solution (volume ratio is 2:5) in a glass vial. After sealing in nitrogen gas, 2.2-ml styrene was added under nitrogen atmosphere and stirred rapidly. Then, the vial was submerged in a thermostatic oil bath and heated at 343 K for 8 h. The as-obtained PS particles with a diameter at about 450 nm were washed extensively with ethanol in centrifuge and dried in air at room temperature.
For the surface treatment, 0.5 ml of the PS colloids were dispersed into 30 ml of the deionized water containing 1 ml of 1% PEI under stirring and then the suspension was ultrasonically treated for 10 min. Following that, a further stirring was carried out for another 15 min to ensure the adsorption of PEI on the surface of the PS spheres. After that, the excess PEI was removed by centrifuging at 10,000 rpm for 10 min and then washed with water. The above procedure was repeated three times to obtain completely the PEI-modified PS colloids.
Finally, quantitative AgNO3 or silver ammonia solution was mixed with the dispersed solution containing PEI-modified PS colloids. The mixture was heated until the color changed from white to yellow. After cooling the mixture to certain temperature, sodium citrate was added and stirred under this temperature. The obtained composite was washed with the deionized water to remove excess PEI.
The size and morphology of the particles were characterized by field-emission scanning electron microscopy (FE-LEO-1530 SEM) operating at 20 kV. The chemical compositions of particles were examined by X-ray energy-dispersive spectra (EDS). Zetasizer Nano Essentials (Malvern Instruments, Westborough, MA, USA) was used to measure the size and potential of particles.
Results and discussion
Since these PS particles are covalently bonded whereas the nature of bonds in Ag is metallic, it is difficult to bond Ag atoms directly onto the PS surface. Therefore, surface modification with functional groups is essential for the purpose. Thus, cationic polyelectrolyte PEI was used to attach functional group of NH on the PS surface as shown in Figure 2b. The functional group of NH here mainly acts as a linker that easily coordinates with Ag ions in the solution. The polyelectrolyte PEI adsorption on the PS particles occurs spontaneously due to the driving force provided by electrostatic attraction, rendering a reverse of the surface charge of the PS particles. After the surface modification, the Zeta potential of the PEI-modified PS particles was measured to be 25.1 mV.
Figure 6d,e,f shows the SEM images of PS/Ag CSSNs seeded with the same nucleation temperature at 100°C for 1 h but reduced or grew for 30 min with the sodium citrate at different temperatures (70°C, 90°C, and 100°C). We can see that the size of the Ag particles gradually increases with the reducing temperature. It indicates that the key factor affecting the crystal growth via reduction is the diffusion of ions in the solution. With the increase in temperature, the diffusion process becomes facile, and the size of Ag nanoparticles becomes larger. Nonetheless, the size of Ag nanoparticle is too large as reduced at temperature higher than 90°C which effects homogeneity of the PS/Ag CSSNs. As a consequence, the best coating process is seeding at 100°C and then reducing at 80°C.
The experimental results showed that the stirring of the PEI-modified PS colloids with AgNO3 at 100°C at least for 1 h followed by the reduction via sodium citrate at 80°C for 30 min leads to the well-dispersed, uniformly coated PS/Ag CSSNs. During the in situ reduction, the Ag seeds immobilized on the surface of the PEI-modified PS colloids through the linkage of functional group N-H. The dispersion and the Ag shell coverage of the PS/Ag CSSNs can be easily controlled by changing the temperatures of seeding and reduction, the seeding time, and the kind of silver ions. This optimized process is crucial to fabrication of PS/Ag CSSNs and their applications.
CZ is an M.S. candidate in materials science and engineering in the China-Australia Joint Laboratory for Functional Nanomaterials at Xiamen University with her research interest focused on fabrication of nanomaterials. XZ is a Ph.D. in electronic materials engineering at the Australian National University and one of the earliest scientists who initialized nanoresearch in China. He is presently the director of the China-Australia Joint Laboratory for Functional Nanomaterials, an adjunct professor at The University of Queensland in Australia, and a full-time professor in the School of Physics and Mechanical and Electrical Engineering at Xiamen University in China, as well as the chief scientist for the AMAC International Inc., USA. He is the editor-in-chief of the Scientific Journal of Physical Science, associate editor of the International Journal of Molecular Engineering and is on the editorial board of several journals such as the Chinese Science Bulletin, etc. He is also an active referee for several top international journals such as Applied Physics Letters, Journal of Physical Chemistry, Crystal Engineering Communications, Journal of Materials Chemistry, and Chinese Physics Letters. His current research interests are focused on nanoinstabilities of low-dimensional nanostructures under external excitations, energetic beam nanoprocessing, controllable fabrication and growth of low dimensional nanostructures, controllable assembling and construction of large scale arrays of zero-dimensional nanostructures, organic and inorganic hybrid at nanoscale, functionalization of nanostructure, etc. He has authored and co-authored over 100 publications, filed 8 patents, chaired, co-chaired, or served as committee or advisory board member at over 20 international or national conferences, and presented over 60 invited lectures and talks at universities, research institutes, and major international conferences worldwide.HL is an engineer in the College of Materials at Xiamen University with her research interest focused on advanced energy materials.IK is a Ph.D. student in condensed matter physics in the China-Australia Joint Laboratory for Functional Nanomaterials at Xiamen University with his research interest focused on processing of nanomaterials. MI is a Ph.D. candidate in condensed matter physics in the China-Australia Joint Laboratory for Functional Nanomaterials at Xiamen University with his research interest focused on fabrication of nanomaterials. LW is a professor and research director in the ARC Centre of Excellence for Functional Nanomaterials at University of Queensland and a visiting professor in the China-Australia Joint Laboratory for Functional Nanomaterials at Xiamen University. His current research interest is concentrated on advanced functional nanomaterials. JB is an associate professor in the State Key Laboratory of Polymer Materials Science and Engineering at Sichuan University and a visiting associate professor in the China-Australia Joint Laboratory for Functional Nanomaterials at Xiamen University with his research interest in polymer materials. XC is a professor in the College of Materials at Xiamen University and a deputy director of the China-Australia Joint Laboratory for Functional Nanomaterials at Xiamen University with her research interest focused on advanced functional nanomaterials.
This work was supported by the China-MOST International Science and Technology Cooperation and Exchange Project under grant number 2008DFA51230, National Key Basic Science Research Program (973 Project) under grant number 2007CB936603, NSFC projects under grant number 11074207, and China Ministry of Education Special Scientific Research Fund for Doctor Discipline of Institution of Higher of Learning under grant number 20100121110023.
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