Capsule-embedded reduced graphene oxide: synthesis, mechanism and electrical properties

Abstract

We report the synthesis of capsule-embedded reduced graphene oxide (rGO) by photocatalytic reduction in the presence of peanut shaped α-Fe₂O₃ particles under visible light. This process is able to produce highly stabilized water dispersible rGO without the help of a stabilizer, which is suitable to make thin films on a glass substrate by the drop casting method. In addition, the evolution of the morphology with different visible light exposure times ranging from 5 to 120 minutes reveals that the catalyst leads to transformation of a few rGO sheets into capsules via a core–shell structure. These capsules are stable on the rGO sheet at a vacuum of 10⁻⁹ Torr, but unstable under vacuum annealing at 400 °C and 10⁻⁵ Torr that created impressions on the rGO film. The bandgap of the capsule-embedded rGO decreases non-linearly with the visible light exposure time; from 2.47 eV at 5 min to 1.73 eV at 120 min. The vacuum annealed rGO films on glass substrates show an electrical conductivity of 2000 S m⁻¹ at 300 K and the conduction is dominated by two-dimensional variable range hopping. The formation of capsules is described based on electrostatic interactions in association with the dipole force of the monodispersed peanut shaped α-Fe₂O₃ particles, whereas, the bandgap opening and high conductivity are attributed to the residual oxygen functional groups such as carbonyl (>CO) and epoxy (C–O–C) on the rGO, in addition to the modulated surface morphology.