Carbon onions within silica nanoshells

Carbon onions have been known since Iijima first observed them in 1980 [1]. In 1992 Ugarte et al. made carbon onions by irradiating carbon soot by an intense electron beam in a transmission electron microscope [2]. There are reports in the literature on the production of carbon structures in water medium by generating an electric arc to create high temperatures [3,4]. It was found that several metals including gold catalyze carbon onion growth from various precursor species. Multiwalled carbon nanotubes have also been coated with silica (SiO2) [5,6]. It has previously been demonstrated that gold and silver nanoparticles react with halocarbons leading to the complete leaching of the metals along with deposition of carbon in an amorphous form [7]. This method, if used with core–shell nanoparticles such as Au@SiO2 [8] makes possible the deposition of amorphous carbon species inside ceramic nanoshells. The nano dimension of the cavity may force a reaction between the carbon species in the solution phase itself, with or without catalysis by the metals or their ions. With this conjecture, we investigated the metal oxide nanoshells after leaching the metal core of core–shell nanoparticles. Carbon onion structures within silica shells were detected in these experiments. It may be noted that the main objective of this work is not to introduce a new method for the synthesis of carbon onions, but to introduce a new material where carbon onions have been made inside the silica shell. Chloroauric acid and trisodium citrate were purchased from CDH fine chemicals, India. (3-amino) propyl trimethoxysilane (APS) and tetra methoxysilane (TMS) were purchased from Aldrich and were used without additional purification. Ethanol and 2-propanol were purchased from E. Merck. CCl4 was purchased from Ranbaxy Chemicals, India. Triply distilled water was used for all the experiments. Gold nanoparticles of size 15 nm were prepared using the Turkevich reduction method [9]. In order to cover this gold particle with silica, a method adopted by Makarova et al. [10] was followed. Further growth of the silica shell was achieved by following the Stober method [11]. To 50 ml of the gold sol, 0.25 ml of millimolar solution of freshly prepared solution of 3-aminopropyl trimethoxysilane (APS) was added with vigorous stirring. This mixture was allowed to stand for around 15 min for complete complexation. A solution of active silica was prepared by adjusting the pH to 10–11 of a 0.54 wt.% of sodium silicate solution by progressive addition of a cation exchange resin, Dualite C 225–Na 14–52 mesh. Two milliliters of active silica thus prepared was added to 50 ml of the surface modified gold sol. The resulting mixture was allowed to stand for one day so that the active silica polymerizes on the surface of the gold particle to form Au@SiO2. The solution thus obtained was centrifuged for around 1 h and the particles were collected and re-dispersed in about 50 ml of 2propanol. To this solution, around 5 ml of CCl4 solution was added. The reaction between themetal core and CCl4 happens over a period of several days and was studied by UV/Vis spectroscopy in a time dependent manner. It was found that the reaction between the gold nanoparticle and CCl4 is quite slow and the reason for this slow rate may be the thickness of the silica shell. It is reported that silica covering of Au@SiO2 is porous in nature, and gold core is accessible for halocarbons. Earlier studies including ours onAu@SiO2 shows that after leaching the core with CN ion a well-defined shell exists showing that shell is permeable for ions [10] and possibly, molecules. Same way clear shells were seen in the case of mineralization of silver core of Ag@ZrO2 using CCl4 [12]. The plasmon absorption of the Au@SiO2 nanoparticles at 524 nm progressively reduces in intensity as the reaction occurs. Once the reaction is complete, a peak corresponding to Au3þ appears at around 320 nm in the UV/Vis spectrum of the solution. Chlorine is removed as Cl . At this point, the solution is pale yellow in