Enhancing the Alkaline Hydrogen Evolution Reaction of Graphene Quantum Dots by Ethylenediamine Functionalization.

Developing metal-free photocatalyst for water splitting is one of the important rising research topics. Although graphene quantum dots (GQDs) have already been investigated as a water splitting photocatalyst several times, studies on modification and design are still needed for an efficient hydrogen evolution reaction (HER) rate particularly in alkaline solutions with an aim to realize overall water splitting. We have synthesized covalently functionalized GQDs with ethylenediamine (EDA) by an amide coupling reaction. It was found that EDA-functionalized GQDs generally exhibited much higher HER activity than bare GQDs. Importantly, the HER activity of EDA-functionalized GQDs increased in proportion to the pH and peaked at pH = 10, which is in stark contrast to the simple decreasing HER rate with the pH of bare GQDs. Through linear sweep voltammetry measurement and electrochemical impedance spectroscopy analysis, it was verified that covalently bonded EDA acts as water dissociation sites to enhance the photocatalytic HER in alkaline medium.