Direct synthesis of hollow carbon nanofibers on bio-char during microwave pyrolysis of pine nut shell

Hollow carbon nanofibers (HCNFs) were formed on the bio-char surface during microwave pyrolysis of pine nut shell in the temperatures range of 400-700 degrees C without the use of any additional catalyst, except activated carbon added as a microwave absorber. Scanning electron microscopy (SEM) analysis showed that HCNFs were only formed on microwave pyrolysis chars and not fixed-bed chars, suggesting that microwave irradiation had a major influence on their formation. High resolution transmission electron microscope (HRTEM) results showed that the synthesized HCNFs at 500 degrees C and 600 degrees C had a diameter of about 400 nm and length of 1400-5000 nm. HCNFs had multi-walled structure with a d-spacing of about 0.35 non. Unlike fixed-bed bio-chars, the X-ray diffraction (XRD) analysis of the microwave bio-chars showed typical graphite peak at around 2 theta = 26.3 degrees with the strongest peak observed in 600 degrees C bio-char. Raman spectroscopy analysis revealed that the highest degree of carbon order of HCNFs was achieved at 600 degrees C, which agreed well with XRD analysis results. Detailed analysis of the volatiles evolved during microwave pyrolysis suggested that hydrocarbons in bio-oil such as benzene and alkenes and CO, CO2, methane, and ethane in bio-gas acted as the carbon source during formation of HCNFs.