Structure evolution at the gate-tunable suspended graphene-water interface.

Graphitic electrode is commonly used in electrochemical reactions owing to its excellent in-plane conductivity, structural robustness and cost efficiency 1,2 . It serves as prime electrocatalyst support as well as a layered intercalation matrix 2,3 , with wide applications in energy conversion and storage 1,4 . Being the two-dimensional building block of graphite, graphene shares similar chemical properties with graphite 1,2 , and its unique physical and chemical properties offer more varieties and tunability for developing state-of-the-art graphitic devices 5-7 . Hence it serves as an ideal platform to investigate the microscopic structure and reaction kinetics at the graphitic-electrode interfaces. Unfortunately, graphene is susceptible to various extrinsic factors, such as substrate effect 8-10 , causing much confusion and controversy 7,8,10,11 . Hereby we have obtained centimetre-sized substrate-free monolayer graphene suspended on aqueous electrolyte surface with gate tunability. Using sum-frequency spectroscopy, here we show the structural evolution versus the gate voltage at the graphene-water interface. The hydrogen-bond network of water in the Stern layer is barely changed within the water-electrolysis window but undergoes notable change when switching on the electrochemical reactions. The dangling O-H bond protruding at the graphene-water interface disappears at the onset of the hydrogen evolution reaction, signifying a marked structural change on the topmost layer owing to excess intermediate species next to the electrode. The large-size suspended pristine graphene offers a new platform to unravel the microscopic processes at the graphitic-electrode interfaces.