Site Defects and Structural Alignment Enhance Interfacial Charge Mobility in Heterostructured Carbon Nitride Catalysts

Abstract

Engineering interfaces between organic semiconductors is an effective way to tailor organic electronic device performance, as charge transport and light interaction efficiency are strongly influenced by electronic coupling at molecular interfaces. Scanning transmission electron microscopy is routinely used to analyze interfaces at the atomic scale; however, its use for organic materials is limited due to the electron beam sensitivity of organic molecules, buried interfaces, and the semicrystalline nature of organics. In this work, we developed a workflow to correlate charge behavior at organic interfaces with their chemistry and structure, even when interface components are chemically and structurally similar and mixed at the nanoscale. We used this workflow to reveal the nanoscale mechanism behind enhanced charge transfer at the heterojunction between two-dimensional carbon nitride catalysts (poly heptazine imide (PHI) and poly-triazine imide (PTI)) during the oxygen reduction reaction. We found that PHI crystallites grow on PTI layers formed at the gas−liquid interface in the salt melt, following the [001]PTI/[001]K‑PHI orientation. This crystallographic alignment promotes the charge transfer from PTI to PHI and creates an electron-rich interface. Electron energy loss spectroscopy showed quaternary N atoms in the heterojunction, which aid O2 adsorption and 2e− reduction to H2O2, as well as a higher proportion of terminal and bridging N atoms, promoting charge separation during the reaction.