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A Study in Red: The Overlooked Role of Azo‐Moieties in Polymeric Carbon Nitride Photocatalysts with Strongly Extended Optical Absorption
Author(s) -
Mitoraj Dariusz,
Krivtsov Igor,
Li Chunyu,
Rajagopal Ashwene,
Im Changbin,
Adler Christiane,
Köble Kerstin,
Khainakova Olena,
Hniopek Julian,
Neumann Christof,
Turchanin Andrey,
Silva Ivan,
Schmitt Michael,
Leiter Robert,
Lehnert Tibor,
Popp Jürgen,
Kaiser Ute,
Jacob Timo,
Streb Carsten,
Dietzek Benjamin,
Beranek Radim
Publication year - 2021
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202102945
Subject(s) - chromophore , absorption edge , photochemistry , photocatalysis , absorption (acoustics) , materials science , moiety , carbon nitride , graphitic carbon nitride , visible spectrum , nitride , chemistry , optoelectronics , nanotechnology , organic chemistry , catalysis , band gap , composite material , layer (electronics)
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 H 2 evolution even without any additional co‐catalyst, but are also responsible for enhanced charge‐trapping and radiative recombination, as shown by spectroscopic studies.