Fig. 2. FM images of Gr-coated NCA particles and the bonding configuration between Gr and Ni-rich oxide.

a–c Optical and d–f FM images of NCA particles that were subjected to Gr coating process (one time) a, d without the DSPE-mPEG surfactant and using the surfactant of b, e 0.1 wt% and c, f 0.2 wt%, placed on a carbon adhesive tape. The scale bars in a–f indicate 20 μm. g Schematic illustration of the bonding configuration between Gr and Ni-rich oxide. The Gr layer, DSPE-mPEG, and rhodamine B are illustrated using a ball-and-stick model (drew by Marvin Sketch), where red, gray, ivory, blue, and orange colors indicate oxygen, carbon, hydrogen, nitrogen, and phosphorus, respectively. The DSPE-mPEG could bind to the oxide surface by the hydrophilic head via “hydrogen bonding” with the hydroxyl groups of oxide surface, while retaining “π–π interactions” between the basal plane of Gr and the hydrophobic tails. In addition, the surfactant could bind to Gr edges by the hydrophilic head via “hydrogen bonding” with the hydroxyl functional groups of Gr, consequently contributing to conformal Gr coating in a face-to-face manner. The orange dotted boxes highlight the possible reactions between the cationic rhodamine B and the anionic phosphate group of DSPE-mPEG.