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

Dariusz Mitoraj,Carsten Streb,Timo Jacob,Chunyu Li,Ute Kaiser,Julian Hniopek,Kerstin Köble,Benjamin Dietzek,Changbin Im,Robert Leiter,Andrey Turchanin,Michael Schmitt,Christiane Adler,Tibor Lehnert,Igor Krivtsov,Christof Neumann,Jürgen Popp,Olena Khainakova,Ashwene Rajagopal,Iván Da Silva ,Radim Beránek

doi:10.1002/chem.202102945

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.

Highlights

Heptazine-based polymeric carbon nitride (PCN) materials[1] stand out from a range of other typical, mostly metal oxidebased photocatalysts mainly by their unique optical (i. e., bandgap and intra-gap states)[2] and photoredox (i. e., quasiFermi levels of electrons and holes) properties.[3]
We report a simple post-synthetic approach involving a brief high-temperature treatment under vacuum that effectively turns the typically yellow PCN into a red PCN by extending significantly its visible light absorption to > 600 nm
We provide evidence that the extended optical absorption edge of PCNs subjected to high-temperature treatment is due to the formation of azomoieties that act as linkages between the heptazine units inherent to PCN structure and/or between heptazine and triazine units formed upon partial fragmentation of PCN

Summary

Introduction

Heptazine-based polymeric carbon nitride (PCN) materials[1] stand out from a range of other typical, mostly metal oxidebased photocatalysts mainly by their unique optical (i. e., bandgap and intra-gap states)[2] and photoredox (i. e., quasiFermi levels of electrons and holes) properties.[3]. E., quasiFermi levels of electrons and holes) properties.[3] the typical absorption onset of PCN is around 430–450 nm, which is far from making it an ideal absorber of solar radiation. Heptazine-based polymeric carbon nitride (PCN) materials[1] stand out from a range of other typical, mostly metal oxidebased photocatalysts mainly by their unique optical This motivated significant research efforts directed towards shifting the optical absorption and the corresponding photocatalytic activity to the red. Post-synthetic approaches comprise chemical surface modification of conventional PCN, for instance, by hydrogenation or oxidation,[9] sensitization with light harvesting compounds such as organic dyes,[10] carbon dots or fullerenes,[11] or various forms of post-synthetic thermal treatment.[12]

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Conclusion