Heterogeneous electron transfer kinetics of defective graphene investigated by scanning electrochemical microscopy

We report the controllable fabrication of defective single-layer graphene and the investigations on kinetic rates (k0) of heterogeneous electron transfer (HET). Electron beam lithography (EBL) and Ar+ plasma treatment were employed to fabricate graphene patterns with different defect density. Raman spectroscopy, scanning electrochemical microscopy (SECM) and finite element simulations were performed to correlate the HET kinetic rate to the defect density (nD) and defect distance (LD) of single-layer graphene. The results showed that k0 is linearly increased with nD initially and then increased rapidly with nD, which would be attributed to the density dependent interactions between defects. The facilitated HET results from the increased electronic density of states near the Dirac point of graphene, and the enlarged overlap of electronic states between graphene and redox. Increasing the defect density larger than 7.73 × 1012 cm−2 (LD < 2.03 nm), k0 was rapidly decreased because the disordered structures of defects decrease the conductivity. The optimal nD (7.17 × 1012 cm−2) and LD (2.11 nm) were obtained, which leaded to 60-fold enhancement of k0. SECM imaging combined with Raman imaging provides a powerful method for the combinatorial screening and defect engineering of other 2D materials to realize the optimization of electrochemical activity.