ABSTRACT
The near-infrared light (NIR) absorption of nitrogen-doped graphene quantum dots (NGQDs) containing different N-doping sites is systematically investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations with Perdew-Burke-Ernzerhof (PBE) functionals. The results show that the ultra-small HOMO-LUMO gaps (0.3-1.0 eV) of various N-doping structures (graphitic, amino, and pyridinic at center, and graphitic at edge) are attributed to the spin-polarization of the energy states, which effectively enhances the NIR absorption for NGQDs. Overall, the graphitic N-doping structure exhibits the best NIR absorption. Moreover, the electron attraction effect of the different N-sites is found to be crucial for the LUMO level, where stronger electron attraction lowers the LUMO energy. This work provides critical insight in further design of NGQDs for NIR absorption.
ABSTRACT
It was recently discovered that the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can be completed on the {110} and {001} facets, respectively, of a 18-facet SrTiO3 mono-crystal. The effective charge separation is attributed to the facet junction at the interface between two arbitrary anisotropic crystal planes. Theoretical estimation of the built-in potential at the facet junction can greatly improve understanding of the mechanism. This work employs density functional theory (DFT) calculations to investigate such potential at the (110)/(100) facet junction in SrTiO3 crystals. The formation of the facet junction is verified by a calculated work function difference between the (110) and (100) planes, which form p-type and n-type segments of the junction, respectively. The built-in potential is estimated at about 2.9 V. As a result, with the ultra high built-in potential, electrons and holes can effectively transfer to different anisotropic planes to complete both photo-oxidative and photo-reductive reactions.
