Synergy between quantum confinement and chemical functionality of graphene dots promotes photocatalytic H2 evolution

Abstract

Reducing the photocatalyst size to induce quantum confinement increases the interactive overpotential of photogenerated charges, but the consequent bandgap widening is detrimental to light harvesting. This study exploits quantum confinement and the chemical modification advantages of graphene-based materials to produce photocatalysts, nitrogen-doped graphene-oxide dots (NGODs) of 3–12 nm, for the hydrogen evolution reaction (HER). The NGODs exhibit quantum confinement characteristics by emitting size-dependent photoluminescence. The amino/amide functionalities at the peripheries of the NGODs donate their nitrogen lone-pair electrons to conjugate with graphitic π-orbital electrons, thereby lifting the valence band maximum to narrow the bandgap. Additionally, the orbital conjugation delocalizes the photogenerated charges and prolongs their lifetimes. The smallest (3 nm) NGOD, which has a high amino/amide content and a high conduction band minimum, generates high HER overpotential and high apparent quantum yield (25%) under monochromatic 420 nm illumination. Large NGODs, which have a small bandgap, can be used in a tandem design to expand the applicable solar spectrum. Our study demonstrates the superiority of graphene-based materials as photocatalysts by presenting the synergy between quantum confinement and chemical modification that improves photoenergy conversion.