Distance engineering between $TiO_2$ nanoparticles and graphene quantum dots to enhance photocatalytic activity

Abstract

Titanium dioxide ($TiO_2$) is the most widely used semiconductor photocatalyst due to its strong oxidation power and photochemical stability. However, since pristine $TiO_2$ has critical limitations such as a narrow absorption spectrum and a high recombination rate, it is still difficult to use itself as a photocatalyst. Graphene quantum dots (GQDs), which intentionally open the bandgap by reducing the size of graphene, have arisen as a candidate for sensitizer of $TiO_2$ photocatalyst via band structure modulation. In particular, GQDs fabricated from graphite intercalation compounds (GICs) have a wide and discrete bandgap due to low defects and oxidation content, which can cause effective ligand-to-metal charge transfer (LMCT) with $TiO_2$. GQDs decorated $TiO_2$ form proper band alignment, leading to the charge transfer from the HOMO level of GQDs to the conduction band of $TiO_2$. However, strong orbital coupling induced by a direct chemical bonding of GQDs and $TiO_2$ hinders the delocalization of excited electrons transferred from GQDs to $TiO_2$, resulting in fast back electron transfer (BET). Herein, the distance between $TiO_2$ and GQDs was engineered using the organic linkers to improve the photocatalytic efficiency by suppressing BET. As a result, TA6G with a computational distance of about 1.7 nm between GQDs and $TiO_2$ shows the most effective methylene blue degradation, which can be expected as a significant improvement in the inhibition of BET and charge separation at that distance.