Tunable Onset of Hydrogen Evolution in Graphene with Hot Electrons.

Abstract

Here, we show that the turn-on voltage for the hydrogen evolution reaction on a graphene surface can be tuned in a semiconductor-insulator-graphene (SIG) device immersed in a solution. Specifically, it is shown that the hydrogen evolution reaction (HER) onset for the graphene can shift by >0.8 V by application of a voltage across a graphene-Al2O3-silicon junction. We show that this shift occurs due to the creation of a hot electron population in graphene due to tunneling from the Si to graphene. Through control experiments, we show that the presence of the graphene is necessary for this behavior. By analyzing the silicon, graphene, and solution current components individually, we find an increase in the silicon current despite a fixed graphene-silicon voltage, corresponding to an increase in the HER current. This additional silicon current appears to directly drive the electrochemical reaction, without modifying the graphene current. We term this current "direct injection current" and hypothesize that this current occurs due to electrons injected from the silicon into graphene that drives the HER before any electron-electron scattering occurs in the graphene. To further determine whether hot electrons injected at different energies could explain the observed total solution current, the nonequilibrium electron dynamics was studied using a 2D ensemble Monte Carlo Boltzmann transport equation (MCBTE) solver. By rigorously considering the key scattering mechanisms, we show that the injected hot electrons can significantly increase the available electron flux at high energies. These results show that semiconductor-insulator-graphene devices are a platform which can tune the electrochemical reaction rate via multiple mechanisms.