Solar energy harvesting with carbon nitrides: Do we understand the mechanism?

TL;DR

This paper explores the mechanism of solar water splitting using carbon nitrides, proposing an alternative photochemical pathway involving hydrogen-bonded water molecules and aromatic N-heterocycles, supported by first-principles calculations.

Contribution

It introduces a novel mechanistic paradigm for water splitting with carbon nitrides focusing on photochemistry of N-heterocycles in aqueous environments.

Findings

Hydrogen-bonded water molecules can be decomposed into radicals via simple photochemical reactions.

First-principles calculations support the proposed alternative mechanism.

The mechanism emphasizes the role of aromatic N-heterocycles in water splitting.

Abstract

The photocatalytic splitting of water into molecular hydrogen and molecular oxygen with sunlight is the dream reaction for solar energy conversion. Since decades, transition-metal-oxide semiconductors and supramolecular organometallic structures have been extensively explored as photocatalysts for solar water splitting. More recently, polymeric carbon nitride materials consisting of triazine or heptazine building blocks have attracted considerable attention as hydrogen-evolution photocatalysts. The mechanism of hydrogen evolution with polymeric carbon nitrides is discussed throughout the current literature in terms of the familiar concepts developed for photoelectrochemical water splitting with semiconductors since the 1970s. We discuss in this perspective an alternative mechanistic paradigm for photoinduced water splitting with carbon nitrides, which focusses on the specific features of the photochemistry of aromatic N-heterocycles in aqueous environments. It is shown that a water molecule which is hydrogen-bonded to an N-heterocycle can be decomposed into hydrogen and hydroxyl radicals by two simple sequential photochemical reactions. This concept is illustrated by first-principles calculations of excited-state reaction paths and their energy profiles for hydrogen-bonded complexes of pyridine, triazine and heptazine with a water molecule.