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Mapping the charge distribution in nano scale systems still is a difficult task, but is important to provide fundamental insights into the properties of materials. We demonstrate how in-line holography in transmission electron microscopy can be used to extract the charge distribution in the nanowire in a quantitative way. This technique can realize a fast acquisition of delicate charge variations. By taking advantage of the possibilities of in-situ electron microscopy, variations of the external field can be used to modulate the charge distribution. Because of the fast response to charge variations, this method provides an efficient probing tool for detecting dynamic charge redistribution.

Carbon-based materials exhibit high electrocatalytic activity for the oxygen reduction reaction (ORR), which is an essential step for several renewable and clean energy systems. To make them alternatives to Pt-based catalysts for the ORR, a deep understanding on the ORR active sites on the carbon surface is needed but remains a major challenge. In nitrogen (N)-doped carbons, the carbon atoms next to pyridinic N are suggested to be the active sites for the ORR. Here we show that non-doped carbon materials (from carbon dots by annealing treatment at 1000 °C) create surface defects by removing the carbonate groups from the carbon surface under alkaline conditions (KOH/NaOH). Subsequently, the K+ (Na+) attached to the defects (e.g., hydroxyl group) to form the adjacent C–O−–K+ (Na+) groups (C–O−–K+ (Na+) pairs), which are suggested to be the real catalytic active sites for the ORR, similar to those of Pt catalysts.