A Study in Red: The Overlooked Role of Azo‐Moieties in Polymeric Carbon Nitride Photocatalysts with Strongly Extended Optical Absorption

Abstract

The unique optical and photoredox properties of heptazine‐based polymeric carbon nitride (PCN) materials make them promising semiconductors for driving various productive photocatalytic conversions. However, their typical absorption onset at ca. 430–450 nm is still far from optimum for efficient sunlight harvesting. Despite many reports of successful attempts to extend the light absorption range of PCNs, the determination of the structural features responsible for the red shift of the light absorption edge beyond 450 nm has often been obstructed by the highly disordered structure of PCNs and/or low content of the moieties responsible for changes in optical and electronic properties. In this work, we implement a high‐temperature (900 °C) treatment procedure for turning the conventional melamine‐derived yellow PCN into a red carbon nitride. This approach preserves the typical PCN structure but incorporates a new functionality that promotes visible light absorption. A detailed characterization of the prepared material reveals that partial heptazine fragmentation accompanied by de‐ammonification leads to the formation of azo‐groups in the red PCN, a chromophore moiety whose role in shifting the optical absorption edge of PCNs has been overlooked so far. These azo moieties can be activated under visible‐light (470 nm) for H2 evolution even without any additional co‐catalyst, but are also responsible for enhanced charge‐trapping and radiative recombination, as shown by spectroscopic studies.

Structure and Optical Properties of Polymeric Carbon Nitrides from Atomistic Simulations

“…In light of these trends, the present results suggest that in order to achieve a desirable bandgap, a material with only small connected patches of the same structural motif, thereby avoiding corrugation, is desirable. A recent study by Mitoraj et al has used this principle to good effect . The group employed functionalization with a small fraction of azo-linkers in the inner domain to trigger local flattening of the material, thereby redshifting the absorption spectrum.…”

“…This result suggests that larger structures corrugate more easily. Section 19 The group employed functionalization with a small fraction of azo-linkers in the inner domain to trigger local flattening of the material, thereby redshifting the absorption spectrum. We furthermore note that we found linear molecular systems to be more corrugated than angled systems; see Figure S8 for an exemplary comparison between tetramers.…”

“…Typical PCN materials obtained by straightforward thermal synthesis are semicrystalline and contain a multitude of microstructural motifs. − This lack of crystallinity complicates structural characterization. However, if controlled reaction conditions are enforced, certain motifs can become more prominent.…”

A Guide to Chemical Reactions Design in Carbon Nitride Photocatalysis

“…The community of materials scientists is focused on optimizing structure of g‐CNs to improve efficiency of photon absorption – to shift the onset of absorption more into the visible range of the electromagnetic spectrum, i. e. narrow the band gap, exciton separation, carrier diffusion and carrier transport (gears one‐to‐four). Typically, this is achieved by doping of g‐CN structure with heteroelements, [96,97] introducing a defined number of defects, such as carbon vacancies, [98,99] creating heterojunction composites with other semiconductors, [100] and adding co‐catalysts [101,102] . These strategies have been reviewed [103,20,104,105] .…”

Polymeric carbon nitride-based photocathodes for visible light-driven selective reduction of oxygen to hydrogen peroxide

“…Polymeric carbon nitrides (PCN) are sustainable, tunable, non-toxic and chemically stable materials, [1][2][3][4] that have been utilized in the construction of various photocatalytic systems for a wide range of light-driven reactions, including hydrogen evolution, [5][6][7][8][9][10] water oxidation, [6,11,12] CO2 reduction, [13,14] organic pollutant degradation, [15][16][17] or selective chemical conversions [18][19][20][21]. Notably, a vast majority of the studies on PCN-based photocatalytic systems encompassed investigations of suspensions of PCN powders, whereas studies of photoelectrocatalytic systems utilizing PCN-based photoelectrodes are much less frequent.…”