Nitrogen-doped graphene quantum dots for extensive production of hydrogen: A hybrid density functional study

Photocatalytic water splitting has emerged as a promising approach for storing solar energy in the form of chemical energy, which can solve the energy crisis and environmental issues. Herein we have explored the potentiality of nitrogen-doped graphene quantum dots for solar-driven hydrogen evolution. The motivation behind applying quantum dots as a photocatalyst is to utilize a major portion of the solar energy spectrum to enhance hydrogen yield. The alignment of frontier molecular orbitals (FMOs) in comparison to the redox potential of water reveals that all the studied quantum dots are eligible for photocatalytic hydrogen evolution. The energy gap between FMOs varies with the size of quantum dots from 3.01 to 2.32 eV, showing absorption of light in the visible range. The Gibbs free energy calculation shows that the formation of intermediate species is energetically unfavorable without exposure to light at pH=7, except for hydrogen adsorption. However, the photogenerated electron and hole drive the steps of oxygen and hydrogen evolution by making those a downhill process. The optical absorption spectra show that all the studied quantum dots absorb visible light.