Assembly of Silver Nanoparticles into Hollow Spheres Using Eu(III) Compound based on Trifluorothenoyl-Acetone
© to the authors 2008
Received: 25 October 2007
Accepted: 9 January 2008
Published: 26 February 2008
The preparation of luminescent silver hollow spheres using Eu(III) compound based on trifluorothenoyl-acetone is described. The structure and size of silver hollow spheres were determined by TEM images. The result shows the formation of hollow structure and average size of the silver hollow spheres (0.9 μm). The silver hollow spheres were further characterized by UV absorption spectrum, SNOM and SEM images, suggesting them to be formed by self-assemble of some isolated silver nanoparticles. The luminescent properties of them were also investigated and they are shown to be high emission strength; moreover, they offer the distinct advantage of a lower packing density compared with other commercial luminescent products.
Inorganic hollow spheres of nanometer to micrometer dimensions represent an important class of materials, and are attended for wide potential applications , such as catalysts, fillers, coatings, and lightweight structural materials owing to their low density, large specific area, and surface permeability [2–5]. Especially, noble metal hollow spheres have attracted lots of attention for their remarkable optical properties [6, 7]. However, there are few works to report preparation of noble metal hollow spheres. Only, previous efforts to prepare noble metal hollow spheres have been focused on polymer-surfactant compels micelles  and using template methods . The nanometer silver hollow spheres are difficult to be obtained and should be removed of the core, resulting in breaking of shell by these methods. Moreover, the functional metal hollow spheres cannot be obtained. In the design of multicompositional materials with spatially defined arrangements of the different components, block copolypeptides may be highly useful as structure-directing agents for nanoparticle assembly . It is well-known that noble metals like gold and silver are capable of existing in the unoxidized state at the nanoscale and offer a unique surface chemistry that allows them to be used as platforms for self-assembly layers of organic molecules [11–14]. So, it is expected to prepare the nanometer noble metal hollow spheres by crystal self-assemble method under functional organic molecules assistant, which is easy to prepare and control. Furthermore, the hollow spheres containing functional molecules are expected to be functional properties.
So, here, a new route of synthesis silver hollow spheres is developed. The silver hollow spheres are formed by the self-assemble of silver nanoparticles assisted functional molecules of Eu(TTA)3 · 2H2O. The Eu(III) organometallic compounds of Eu(TTA)3 · 2H2O as the dispersion and bridge of silver nanoparticle results in the self-assemble of them, along a certain axis in thexy-plane and the curl and extension of Eu(III) organometallic in a mixed solvent microenvironments for confining the 3D growth of silver hollow spheres. In other way, the fluorescence of silver hollow spheres is further observed, which is expected to apply in optical materials.
Synthesis of Rare-earth Complexes
Preparation of Silver Hollow Spheres
Silver hollow spheres were prepared according to the process as shown in Scheme 1. The first step is to synthesize the Ag colloidal solution in the presence of Eu(TTA)3 · 2H2O complex according to the literature . The morphology and size of silver nanoparticles and the surface plasma on resonant absorption peak are determined to be sphere with an average size of 21.5 and 425.2 nm by transmission electron microscope (TEM) and UV–Vis absorption spectrum, respectively. In the second step, the silver colloidal TFH solution with a concentration of 6.34 × 10−4 M was obtain and added to be 1 mmol free Eu(TTA)3 · 2H2O complex. After this, centrifuging (3,000 rpm) gave a brown acetone/water precipitate, and supernatant solution containing excess Eu(TTA)3 · 2H2O was extracted. The precipitates were again dissolved to acetone. The purification procedure was repeated for three times. Morphology and size of the sample was obtained by using TEM, scanning electron microscopy (SEM), and scanning near-field optical microscopy (SNOM). The samples were also characterized by UV–Vis spectroscopy and fluorescence spectroscopy.
Results and Discussions
The silver/Eu(TTA)3 · 2H2O composite nanoparticles were prepared by the interaction between Ag nanoparticles and thiophene chromophores group of Eu(TTA)3 · 2H2O, and the CF3 groups of Eu(TTA)3 · 2H2O extend away from the Ag nanoparticle to provide solubility of the nanoparticles, which has been discussed in previous work . So it is not discussed in detail here. It is further found that if the concentration of silver/Eu(TTA)3 · 2H2O composite nanoparticles is kept at more than 6.34 × 10−4 M and 1 mmol free Eu(TTA)3 · 2H2O is present in the solution, silver hollow spheres are formed by self-assemble of silver/Eu(TTA)3 · 2H2O composite nanoparticles as shown in Scheme 1. Free Eu(TTA)3 · 2H2O is as bridge of silver/Eu(TTA)3 · 2H2O composite nanoparticles by the interaction between Ag nanoparticles and thiophene chromophores, too.
In conclusion, silver hollow spheres have been successfully synthesized using two-step approach. This radiation synthetic pathway provides an important example of well-ordered and functional silver hollow spheres with designed morphology. The unique silver shell structure obtained here may be promising candidates for both fundamental research and application, and it is believed that assembling synthesis based on functional molecules represents a novel route to prepare functional inorganic hollow sphere, which is a topic of intense interest. Moreover, the silver hollow spheres have high luminescent property at 614.3 nm, which is to be applied in optical materials.
This work was supported by the National Natural Science Foundation of China (No: 50025309, and No: 90201016), Youthful Science Foundation of Shanxi province (No: P20072185 and No: P20072194), and Youthful Science Foundation of North University. The authors are grateful for the financial support and express their thanks to Hui Zhao for helpful discussions and Wan Qun Hu for IR measurements.
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