Tailoring the Band Gap of Carbon Nanoparticles in Carbon Soot by Ball Milling

Carbon nanoparticles (CNPs) in carbon soot have been ball milled in air for time durations of 1, 3, and 5 hours. A reduction of the average height of CNPs, measured using atomic force microscopy, with increase in ball milling time has been observed with values about 22.54 nm, 11.09 nm, 9.48 nm, and 7.25 nm for un-milled and ball milled samples for durations of 1, 3, and 5 hours, respectively. The optical band gaps of the CNPs have been estimated from the optical absorption data with values of about 5.06 eV, 3.77 eV, 2.08 eV, and 1.55 eV for the as-synthesized CNPs and 1, 3, and 5 hours of ball milled CNPs, respectively. Such observations of reduction of height and decrease in band gap of the CNPs are due to the mainly increase in in-plane graphitic ordering in the CNPs and simultaneous attachment of oxygen from the air ambient. The ball milling provided the required energy to acquire the in-plane graphitic order in the CNPs reducing the disorders in it. Also changes in the functional groups (formation of carbon and oxygen bonds) in the surfaces of the in-plane graphite in the CNPs can lead to the formation of graphene oxide (GO) leading to change in the surface states which can result in decrease in band gap of the CNPs. With increase in ball milling time, amount of GO has been increased resulted in further decrease in the band gap of the CNPs.