Pyrolytic Synthesis of Spherical Carbon Nanoshells

Carbon nanostructured materials have attracted the great interest due to their potential in a variety of applications in different areas of techniques, biology and medicine, such as adsorption, capacitive deionization, catalysis, electrocatalysis, photocatalysis, energy storage, sensory and biosensory systems, drug delivery, etc. A special type of carbon material are the hollow microand nanoparticles (shells, boxes, spheres) having low density, high values of conductivity and specific surface. The better way to form such three-dimensional (3D) nanostructures is synthesis over specific nanoparticles, viz., template growth. Metals, crystalline and amorphous oxides, carbides, silicides, and fluorides can be used as a template material. Various templating techniques have been developed to synthesize carbon core-shell microand nanoparticles, such as vacuum sputtering (Hayashi, 1996), molecular beam epitaxy (Jerng, 2011 a; Jerng, 2011 b), sol-gel method (Zhu, 2009), hydrothermal approach (Xuan, 2007; Liu, 2012; Li, 2013), electrostatic interaction (Yang, 2010; Chen, 2012 a), aerosol spraying (Wen, 2013) and catalytic chemical vapor deposition (CVD).

Certain transfer-free methodologies for flat substrates have been also proposed (Ismach, 2010;Yan, 2011;Wang, 2013).Direct growth of graphene ribbons on sapphire and SiO2 dielectric substrates using chemical vapor deposition has been studied by Ismach et al. (2010) and Wang et al. (2013).The process includes deposition of Cu or Ni films, chemical vapor deposition using hydrocarbons and metal etching.Graphene ribbons up to few millimeters long were formed along the periphery of pre-patterned Ni films (Wang, 2013).
Many oxides can encourage the carbon graphitization.Different hydrocarbons were used for the pyrolysis and covering the oxide surface.An impressive series of works on developing, testing and use of 3D composite materials has been done in Japan by M. Inagaki et al. (1998,2004,2006).
As a rule, the materials were produced by metal-catalyst-free pyrolysis of poly(vinyl chloride), poly(ethylene terephtalate), hydroxyethylcellulose or poly(vinyl alcohol) in contact with ceramic particles at temperatures between 700 and 1000 o C in inert atmosphere.At a definite thickness of carbon shells the values of their specific surface and porosity can be significantly increased.Here we report on the formation of hollow carbon nanoshells by pyrolysis of methane on spherical SiO2 nanoparticles.

Sample Preparation
Nanoparticles of SiO2 produced by plasma pyrolysis of SiF4 have been used as a matrix.The size distribution of SiO2 particles is shown in Fig. 1.It is seen that the most part of them has diameter 30-90 nm with average value of 50-70 nm.The carbon-encapsulated SiO2 nanoparticles are synthesized in a horizontal tube furnace (Fig. 2).The reactor consists of a sectioned quartz tube 55 mm in outer diameter.The tube has working area heated by electric furnace, and cold area, with a sealing leg between the areas.We introduce methane into quartz tube 1 h before the pyrolysis to achieve the full displacement of air from the reactor.The boat with 250 mg of SiO2 was put into reactor by means of a special stick after heating the working area.The boat is held for a definite time in the hot area, then moved to the cold area, cooled and drawn out.The product is weighted and analyzed.Gas flow (100-800 mL/min) is preliminary calibrated at room temperature.The flow rate is controlled precisely by volume flow controller.

Product Characterization
For the determination of deposited carbon mass, the samples were calcined at 750-800 о С during 3 h in air, cooled in closed exiccator and weighted.The thickness of carbon shells is estimated using total mass gain of deposited carbon and the mean size of the encapsulated SiO2 particles.Scanning electron microscopy (Chem JEOL, JSM-6510LV) and transmission electron microscopy (FEI Tecnai G 2 30 ST) were used to study the morphology of the starting SiO2 particles, encapsulated nanoparticles and hollow shells.The specific surface of start materials and the products is measured by BET method (Sorbi MS).

Results and discussion
The CH4 pyrolysis is observed at the temperature 500-900 o C. The typical dependencies of mass change on the process duration and gas flow rate at relatively high temperatures are represented by the curves with saturation (Fig. 3 and 4).By analogy with the results of hydrocarbon pyrolysis over Cu, Ni and other metals, one can suppose that the surface of oxides exerts a catalytic influence on pyrolysis reaction.With increasing deposit thickness and process duration this influence weakens and finally ceases.Ceramic materials also has been used as catalysts of methane decomposition, e. g. by Hussain and Iqbal (2011).The activity of SiO2 was found to be much lower than MgO/SiO2 or Ni doped MgO/SiO2.It would be noted that at the highest temperatures, along with the carbon deposition on the tested samples, it takes place on the walls of quartz reactor.This uncontrolled deposition follows by the increasing of hydrogen content in reaction gases and some distortion of calculated data.The increasing of H2 concentration in reaction gases delays the pyrolysis reaction too.
The final mass gain due to formation of carbon deposit on SiO2 depends on the temperature (Fig. 5), and at 850-900 о С rises up to 72 mass %.   1.

Table 1. The calculated thickness of the shells at different experimental Conditions
* Gas flow rate 466 mL/min, in other cases 330 mL/min.
As it is seen, the value of the shell thickness can be controlled by alteration of pyrolysis temperature and duration.SiO2 matrixes covered by carbon shells can be easily dissolved in HF aqueous solution, thus the shells are microporous and permeable to solutions.Therefore, the calculated values are approximate and underestimated by 15 -20 %.The real thickness of shells produced at 600 o C and pyrolysis duration of 30 min, measured in micrographs (Fig. 6), is 1-12 nm.This interval coincides with calculated values.At lower temperature and process duration, the thickness less than 1 nm can be achieved.The carbon spherical shells with the thickness of some nanometers become not strong and do not keep their habitus.Therefore destroyed spheres are seen on some of the pictures (Fig 6 d), but its number is negligible.Coalescent particles are also observed.
In The lower is the pyrolysis temperature, the higher are porosity and specific surface values of shells.The specific surface of shells synthesized at 600 o C was 175 m 2 /g, whereas specific surface value of a matrix SiO2 particles was equal to 12-13 m 2 /g.Carbon shells having thickness lower than 5-7 nm in the case of individual particles must be transparent or semitransparent, as it was shown earlier by Kaplas and Svirko (2012).Our shells are transparent in electron beam, however as a whole they retain black color.The unique spherical carbon shells synthesized here have high electrical conductivity and can be used as electrode material for capacitive deionization (electrosorption) of waste water effluents.

Conclusion
A simple method for the preparation of 3D architectures is developed via a pyrolytic process.
The methane pyrolysis at temperatures of 500-900 о С on the spherical nanoparticles of SiO2 allows to get a соre-shell hybrid composites, and the consequent dissolution of SiO2 matrix leads to formation of isolated hollow carbon shells.The main part of these shells have diameters of 40 -80 nm with a shell thickness of 1-12 nm.The values of shell density and specific surface are temperature dependent: The lowering of synthesis temperature leads to the reduction of density and to the increasing of specific surface.
and E. G. Rakov (2014), Journal of Research in Nanotechnology, Vol.2014 (2014), DOI: 10.5171/2014.717173rather thick.The number of carbon layers can be tuned by changing the growth time and the temperature.

Fig. 2 .
Fig. 2. Reactor for pyrolysis: 1 -metal stick for introducing of the boat to hot area; 2 -boat with SiO2 nanoparticles in cold area; 3 -sealing leg; 4 -tube for exhausted gases; 5 -electric heating furnace; 6 -quartz tube; 7 -tube for gas input; 8 -equipment for measuring and adjustment of temperature.

Fig. 3 .
Fig. 3.The influence of pyrolysis time on mass of carbon deposit on SiO2 at СН4 flow rate of 330 mL/min and temperature of 800 o C (1) and 700 o C (2).

Fig. 4 .Fig. 5 .
Fig. 4. The influence of СН4 flow rate on mass of carbon deposit on SiO2 at temperature of 850 о С and process duration of 60 min.