Generation of radical species in CVD grown pristine and N-doped solid carbon spheres using H2 and Ar as carrier gases

Abstract

Solid carbon spheres (CSs, d ≈ 200 nm) were synthesized (yield, <40%) in a vertically oriented chemical vapor deposition (CVD) reactor using acetylene as a carbon source and Ar or H₂ as the carrier gas. The CSs synthesized in the presence of H₂ exhibited a broader thermal gravimetric derivative curve and a narrower paramagnetic signal than the CSs synthesized in Ar. Post synthesis doping of both types of CSs with nitrogen was achieved by passing acetonitrile at 800 °C for 1 h over the CSs in a CVD reactor. The N-doped CSs (NCSs) synthesized under both H₂ and Ar displayed an increase in I_D/I_G ratios as obtained from Raman spectroscopy and showed an increase in the paramagnetic signal due to the presence of nitrogen induced defects compared to the undoped CSs. The NCSs synthesized in H₂ had less graphitic-N (22%) than those produced in Ar (50%). The presence of a higher percentage of pyridinic-N and pyrrolic-N for the NCSs prepared with H₂ as carrier gas suggested H₂ etching effects on the CSs. Further, the N-doped carbon spheres obtained in the presence of H₂ gave a higher N/C ratio (5.0) than in the presence of Ar (3.7). The introduction of edge defects and paramagnetic centers in CSs in the presence of H₂ gas without the aid of a metal catalyst opens up a platform for regulating surface and catalytic reactions of CSs.