Mechanism of Charge Separation and Frontier Orbital Structure in Graphitic Carbon Nitride and Graphene Quantum Dots

Abstract

Graphene quantum dots (GQDs) and carbon nitride quantum dots (CNQDs), the latest addition to the carbon material family, are promising materials for numerous novel applications in optical sensing, photocatalysis, biosensing, and photovoltaics. However, understanding the photocatalytic capability of CNQDs compared to GQDs requires investigations of the charge behavior on the excited state energy surface. In this work, through time-dependent density functional tight binding (TD-DFTB) calculations, we show that CNQDs exhibit superior ground state frontier orbitals (FOs) localization. Strong localization of the FOs and excited state charge separation observed in the first excited state are caused by the relaxation of the structure. Excited energy surface investigations reveal spatial confinement of FOs to the stretched C-N bonds due to excited state structural relaxation. On the other hand, no such localized FOs structure was found for GQDs, presumably caused by its strong π-conjugated configuration not allowing large structural changes upon excitation. The optical absorption and emission of CNQDs is sensitive to size and does not show large variations with the shape of the QD. Our approach provides an explanation for the origin of the enhanced photocatalytic performance of CNQDs over GQDs and their characteristic FOs localization.

From Triazine to Heptazine: Origin of Graphitic Carbon Nitride as a Photocatalyst

“…On the contrary, the extended heptazine skeleton in melem results in uniformly distributed and well complementary HOMOs and LUMOs (Figure 2e,f), which would promote the dissociation of the photoinduced charge carriers and thus suppress their recombination (Figure 6b). Note that the centered N 3C atom shows a notable contribution to the LUMO state because of its pronounced sp 2 hybridization feature, 51,52 which could facilitate the chargetrapping process. Meanwhile, the terminal amine groups are defined to contribute LUMOs solely, which are favorable to produce electrons and thus serve as primary reduction sites for the possible photocatalytic reactions.…”

Controllable Synthesis of Carbon Nitride Films with Type-II Heterojunction for Efficient Photoelectrochemical Cells

“…The water-splitting photoelectrochemical cell (PEC) is being considered as a clean and cost-effective approach to produce hydrogen. − In the last years, metal-free polymeric carbon nitride (CN) has emerged as a promising semiconductor material for photoanodes, owing to its light-harvesting properties, suitable energy band positions, stability under a wide pH range, and low price. − However, despite the progress both in synthesis and application of CN as a powder, more controllable synthesis of continuous and uniform CN films, which directly and intimately couple with a conductive substrate, is still needed for its realization in PEC cells. ,− …”

Preparation of the heterojunction catalyst N-doping carbon quantum dots/P25 and its visible light photocatalytic activity

“…Carbon quantum dots (CDs) are zero dimensional nanocarbon material less than 10 nm in diameter, formed by crystalline graphitic sp 2 -bonded carbon atoms, which have a quantum confinement effect 15–18 . Apart from its unique electronic attributes, the quasi-spherical nanocarbon material possess several other attractive advantages, such as the large specific surface area, high stability, low toxicity, simple synthetic routes and the strong and tunable fluorescence emission properties 19–22 .…”