Investigation On The Large-Scale Synthesis And Properties Of Nano Onion-Like Fullerenes

Nano Onion-Like Fullerenes (NOLFs), which were discovered as a new form of carbon element in recent decades, have become a frontier topic and hotspot in carbon field and condensed matter physics. NOLFs are a kind of carbon atom cluster with C60 core. The innermost C60core is surrounded by concentric graphitic shells, the carbon atoms of each shell increase by the law of 60n2, where n is the sequence number of the shell from the core. Because of their mesoscopic scale and unique physical and chemical properties, NOLFs are considered to be of very importance in theoretical study and application exploration.In this paper, the physical properties of NOLFs were discussed following the synthesis in large quantities. The influencing factors, optimized preparing parameters, and the mechanisms for the formation ofNOLFs in arc discharge and catalytic pyrolysis method were studied; the morphologies, microstructures and graphitization degree of NOLFs were characterized by FESEM, HRTEM, XRD and Raman spectra; the growth models and mechanisms, and the magnetic and optical properties of NOLFs synthesized by different methods were also investigated.The main conclusions are as follows:1.During the self-sustained arc-discharge mode, using graphite as raw materials, metal nano particles as catalyst, under the current of 120-130A, large quantity of NOLFs have been successfully synthesized. The advantage of this method is that in the state of self-sustained arc-discharge mode, adding metal nanoparticles as catalysts, not only arc pole can be obtained with good conductance in the process of discharging, but the large-scale synthesis of NOLFs can also be realized.It can be concluded from FESEM and HRTEM observation that NOLFs mainly grow inside the cathode deposit, with diameters of 20-50 nm and high graphitization degree, as also proved by XRD and Raman analysis. It is experimentally found that four main factord, the current, metal nanoparticles catalysts, amortizing gases and the wideth between electrode, affect the morphologies and yieldof NOLFs indirectly. Through series of comparisons and studies. The optimum conditions were set up as follows: current of discharge: 120-130A; catalystic effect in order: Cu, Fe, Ni, Al. quantity of mixed: 4wt%; Buffer gas: 2×104 Pa, distance ofelectrode: 1-4mm. Under this condition the products can be prepared in half an hour about 1.0g.On the analysis of experimental result, the formation mechanism of NOLFs was discussed. The growths of NOLFs could not be completely explained by theories of intensive electric field and bitype speed distribution. The analysis of the physical process of arc-discharging revealed that under certain conditions, the increase in C-atoms benefits the formation of NOLFs. A mode of growth was established for pure NOLFs and metal-encapsulating NOLFs catalyzed by metal nanoparticles during arc-discharging.2. With acetylene as carbon source, argon as carrier gas, ferrocene as catalyst, and thiophene catalyst promoter, metal/NOLFs were synthesized in large quantities at 1173-1373K by catalytic pyrolysis method. This method has many advantages: low cost, low reaction temperatures, simple operations and semicontinuous or continuous production. From the microstructure observation of the metal/NOLFs, it can be found that their diameters are mainly distributed within a range of 20-50nm and the Fe particles are encapsulated in NOLFs. The optimum conditions were set up as follows:Acetylene:200-300ml/min; Argon gas flow rate:150-250ml/min; Ferrocene;0.3-0.6g;Thiophene:3-5wt%; Reaction time:20-30min; Reaction temperature: 1273-1373K.It is reasonable to think that NOLFs grow in the way of 'dissolution-diffusion- precipitation' in the presence of catalyst. It was indicated that the growth rate of NOLFs is accelerated with the increase of carbon source. However, the catalyst particles would become Fe3C and the growth of NOLFs would be cut down if carbon density on the surface of catalyst nanoparticles reach to a critical value. The average growth rate of NOLFs is inversely proportional to the size of catalyst nanoparticles. It was also showed that the refinement of the catalyst can help increasing the yield of NOLFs. The growth model was established for catalytic pyrolysis method; some nano carbon materials with special structures(Y-type, bamboo-shape, and spire-type) were also discussed.3. The magnetic and optical performances of NOLFs were analyzed, and Raman-spectra of NOLFs prepared by two kinds of methods were compared.l).With the magnetization-curve and hysteretic-loop, the magnetic performances of metal/NOLFs form catalylic pyrolysis method were analyzed. The results show that metal/NOLFs are in accordance with the features of the ferromagnetic materials and the process of magnetization consists of two steps: steep region and flat region, and the former are primary magnetization region. The metal/NOLFs can be deseribed as a soft magnetic material from hysteretic-loop. 2). With the results of fluorescence and UY-spectrum testing, the opticalproperties of NOLFs prepared by arc-discharge and catalytiv pyrolysis methods were analyzed and calculated. The results show both NOLFs and metal/NOLFs have fluorescence performance, several emission-peaks exist in PL spectra, and the peaks of UV-spectra shift compared with that of graphite. The calculation on the band gap of NOLFs gave the results of 1.04eV and 0.4eV for NOLFs and metal/NOLFs, respectively. 3) The first-order and the second-order peaks of Raman spectra of the products were studied. The results indicated that the structure and graphitization degree of NOLFs are similar to those of the HOPG. The shift of G-peak is probably due to the pull stress correspond to the curved carbon layers of NOLFs. The D-peak of NOLFs is stronger than that of HOPG due to the more amorphous carbons in NOLFs. From the Raman spectra, it can be concluded that NOLFs produced by arc discharge method have higher degree of graphitization than that by catalytic pyrolysis method.