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

## TL;DR

Researchers found that aligning carbon nitride layers improves charge transfer at their interface, enhancing catalytic performance for oxygen reduction.

## Contribution

A workflow was developed to correlate charge behavior with chemistry and structure at organic interfaces, revealing nanoscale mechanisms in carbon nitride heterojunctions.

## Key findings

- PHI crystallites grow on PTI layers with crystallographic alignment, promoting charge transfer.
- Quaternary nitrogen atoms in the heterojunction aid O2 adsorption and 2e– reduction to H2O2.
- Terminal and bridging nitrogen atoms promote charge separation during the oxygen reduction reaction.

## 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.