Mixed-acid intercalation for synthesis of a high conductivity electrochemically exfoliated graphene

Abstract

Precise control of electrochemical intercalation and exfoliation conditions provides a method for tuning graphene properties. A systematic evaluation of the defects induced in graphene for varying intercalation times and intercalant composition is done using a sulphuric and phosphoric acid mixtures. Intercalation was performed in a concentrated mixture of sulphuric and phosphoric acid, followed by anodic exfoliation in aqueous ammonium sulphate solution. Multi-atom-doped graphene with primarily “boundary-type” defects is obtained, and the ratio of the defect-activated Raman modes showed a unique characteristics at 1600 s intercalation, irrespective of the intercalant composition. Careful selection of intercalation time is required to control the graphene defects. Phosphorus and nitrogen-doping increased with phosphoric acid content, influencing graphene properties. Interestingly, extending intercalation times beyond 1600 s deteriorated the electrical conductivity but enhanced the thermal stability with higher phosphoric acid content. We attribute this to loss of the sp2 graphitic structure at higher intercalation times which dominates over contributions from the dopants. The highest electrical conductivity of ∼14,000 S m−1 was two orders of magnitude higher than graphene obtained with pure sulphuric-acid intercalation. By adding phosphoric acid into the intercalant, the energy consumption and exfoliation time was reduced by 34% and by a factor of 3 respectively.