Silver Nanoparticles and Graphitic Carbon Through Thermal Decomposition of a Silver/Acetylenedicarboxylic Salt
© to the authors 2009
Received: 24 March 2009
Accepted: 20 July 2009
Published: 17 September 2009
Spherically shaped silver nanoparticles embedded in a carbon matrix were synthesized by thermal decomposition of a Ag(I)/acetylenedicarboxylic acid salt. The silver nanoparticles, which are formed either by pyrolysis at 300 °C in an autoclave or thermolysis in xylene suspension at reflux temperature, are acting catalytically for the formation of graphite layers. Both reactions proceed through in situ reduction of the silver cations and polymerization of the central acetylene triple bonds and the exact temperature of the reaction can be monitored through DTA analysis. Interestingly, the thermal decomposition of this silver salt in xylene partly leads to a minor fraction of quasicrystalline silver, as established by HR-TEM analysis. The graphitic layers covering the silver nanoparticles are clearly seen in HR-TEM images and, furthermore, established by the presence of sp2carbon at the Raman spectrum of both samples.
Acetylenedicarboxylic acid (ACD) as carboxylic acids with short aliphatic chains  is well known to form complexes with transition metals such as Cd(II) , Cu(II) , Mn(II)  or even lanthanide cations  either in single crystal or in powder form. The metal cations are coordinated with both carboxylate groups in a chelating mode, thus forming metal-organic chains. Interestingly, the triple bond centered between the carboxylate units of acetylenedicarboxylic acid provides new design parameters for the synthesis of novel structures since the distance between the ligands can be decreased enough to succeed polymerization leading to conjugated materials as demonstrated by Skoulika et al. . As such, acetylenedicarboxylic acid is a promising candidate for the synthesis of novel metal-organic networks with interesting properties. Nonetheless, the derived carbon materials obtained after thermal decomposition of such complexes are yet to be the target of intense research, especially considering that the acetylene unit provides an excellent source for carbon, whereas the central metal cation may act as a catalyst.
On the other hand, in another research field, the field of nanoscience, applications of noble-metal nanoparticles, especially silver, have recently grown exponentially. Silver nanoparticles display unique physical, chemical [7–9], and biologic properties such as high antibacterial activity toward a large number of bacterial strains [10, 11] and furthermore they have been incorporated in various natural , conductive  or dendritic  polymer matrices toward the synthesis of advanced nanocomposite materials. Besides the above mentioned colloidal nanocrystals and polymer nanocomposites, carbon-supported silver metal nanoparticles exhibit a wide range of applications in catalysis, antibacterial activity, thermal conductivity, and electronic materials [15, 16]. These hybrid materials are usually obtained by impregnation of a presynthesized carbon support with silver salts and subsequent reduction to silver metal (i.e., a multistep process). Accordingly, the one-step fabrication of silver–carbon hybrids would be much recommended and is highly anticipated.
Recently, an interesting procedure has been proposed describing the catalytic growth of crystalline graphite through thermal decomposition of an organometallic iron complex in solution . This process leads to the catalytic graphitization of the organic component and simultaneously to the formation of magnetic iron oxide nanoparticles. This synthetic route seems to be of high importance since the graphitization process usually demands high temperatures, typically in the range 500–1,000 °C [18–20]. To that direction, herein we report an entirely different but conceptually relevant case of catalytic graphitization based on the thermal decomposition of the silver acetylenedicarboxylate salt, which leads to the reduction of silver cations to metallic nanoparticles and the simultaneous formation of a carbon coating. Two different processes have been employed involving either thermolysis of the silver salt or thermal decomposition in the solid state. Given the dramatic effect of several metal nanoparticles on the growth and morphology of a series of intriguing carbon nanostructures, the direct thermal decomposition of suitable organometallic precursors may give an easy access to metal-carbon nanocomposites as well as carbogenic nanostructures with emergent morphologies.
Experimental Section (Scheme 1)
Synthesis of Silver/Acetylenedicarboxylic Salt
Thermolysis of Ag/ACD in Xylene
The white Ag/ACD powder (200 mg) was suspended in xylene (30 mL) and refluxed for 1 h. Within few minutes the color of the suspended solid changed from white to black. The reaction is completed in much lower temperatures than the boiling point of xylene (140 °C) as evidenced by DTA analysis of the Ag/ACD salt (Fig. 6a). After reaction accomplishment, the black powder was isolated by centrifugation, washed with alcohol and acetone several times, and dried at 50 °C for 24 h. Sample name: Ag/sol.
Thermal Decomposition of Ag/ACD in the Solid State
Ag/ACD white powder (1 g) was loaded in Teflon equipped stainless steel autoclave and the sealed system was heated at 300 °C for 2 h at a heating rate of 10 °C min−1. The black powder was washed numerous times with water and acetone prior to drying. Sample name: Ag/pyr.
XRD patterns were recorded on powder samples using a Siemens 500 Diffractometer. Cu Kα radiation was used with a scan rate 0.03 s−1. Thermogravimetric and Differential thermal analysis measurements were recorded on a Perkin–Elmer Pyris TGA/DTA under airflow with a heating rate 10 °C min−1. Infrared spectra were taken on KBr (Aldrich, 99%, FT-IR grade) pellets with a FT-IR spectrometer of Bruker, Equinox 55/S 123 model. The UV–visible spectrum was recorded on a Shimadzu 2100 spectrometer using ethanol suspensions in quartz cuvettes. The Raman spectra were recorded using a Raman microscope system (Renishaw, System 1000) consisting of an optical microscope (Leica) coupled to a Raman spectrometer (532 nm).
Results and Discussion
Synthesis, FT-IR and Raman Spectroscopy
Further structural information based on the acetylene triple bond was not possible to be collected due to the absence of characteristic IR signals, something that is expected in a symmetric molecule like ACD. Lastly, in a blank experiment, when neat ACD was refluxed in xylene a light yellow-brown colored solution was obtained, meaning that the graphitization is not possible in the absence of silver.
Structural and Morphological Study: XRD Analysis and Electron Microscopy
Secondly, in the Ag/pyr sample, curved graphitic filaments are revealed in the HR-TEM images (Fig. 5) forming a matrix where the silver nanoparticles are hosted. The curvature of the carbon filaments is more pronounced near the edges and can be ascribed to the previously reported catalytic effect of silver nanoparticles on the growth of carbon onions . The silver nanoparticles seem to be the core areas of the composite, which are interconnected by the carbon layers. This is in accordance with the reaction steps that we propose, where the formation of silver nanoparticles is the catalytic step for the polymerization of the central acetylene units. And in fact, the pyrolytic process is much closer to this mechanism than the solvothermal, most probably due to the low reaction time and violent conditions that are taking place inside the autoclave.
The weight percentages of carbon and silver in both samples were obtained with thermogravimetric analysis under airflow. The TGA/DTA diagrams for the two composites are presented in Fig. 6. The traces of the Ag/pyr sample present a weight loss due to the thermal decomposition of the carbon layer, starting at 300 °C and completed at 400 °C. A sharp exothermic peak in the DTA diagram, which is centered at 349 °C, also marks this thermal decomposition. Accordingly, the calculated weight percentage of the silver nanoparticles is about 94% wt and remains a 6% wt which can be assigned to the carbon coating. A similar thermogravimetric analysis curve is obtained for the Ag/sol sample with the weight percentage of carbon being significantly higher (~13% wt) most probably due to the lower reaction temperature in refluxing xylene. The corresponding DTA exothermic peak is quite the same with that of the Ag/pyr sample and it is centered at 332 °C. It should be noted that during the thermogravimetric analysis measurements and the exposure of the samples to oxygen, most probably a minor percentage of silver is oxidized to silver oxide (Ag2O) near the surface of the nanoparticles. Therefore, it is difficult to establish precisely the silver content of the composites by TGA. However, since silver is significantly heavier than oxygen and the oxidation takes place exclusively near the surface of the nanoparticles, any formation of silver oxide should be considered negligible and without seriously affecting our calculations regarding the silver content.
An insoluble, white, Ag(I) salt with acetylenedicarboxylic acid was synthesized and used for the preparation of two silver–carbon nanocomposites via different synthetic routes. As it is indicated from the XRD patterns and TEM images both reactions lead to the formation of silver nanoparticles embedded in a carbon matrix. The graphitization proved to be much better in the solid-state reaction than in solution, however, the carbon yield is relatively lower, the reaction temperature is higher and the interesting fivefold symmetry in the silver nanoparticles is absent. As a future step toward expansion of this procedure, the violent reaction between a functional molecule like ACD and coordinated metal ions can lead to various interesting morphologies as well as nanostructures.
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