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A Study in Red: The Overlooked Role of Azo-Moieties in Polymeric Carbon Nitride Photocatalysts with Strongly Extended Optical Absorption |
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| Authors: | Dr Dariusz Mitoraj Dr Igor Krivtsov Chunyu Li Dr Ashwene Rajagopal Changbin Im Christiane Adler Kerstin Köble Olena Khainakova Julian Hniopek Dr Christof Neumann Prof Andrey Turchanin Dr Ivan da Silva Prof Michael Schmitt Robert Leiter Dr Tibor Lehnert Prof Jürgen Popp Prof Ute Kaiser Prof Timo Jacob Prof Carsten Streb Prof Benjamin Dietzek Prof Radim Beranek |
| Institution: | 1. Institute of Electrochemistry Chemistry, Ulm University, Albert-Einstein-Allee 47, 89081 Ulm, Germany;2. Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Lessingstr. 10, 07743 Jena, Germany
Department Functional Interfaces, Leibniz Institute of Photonic Technology (IPHT), Albert-Einstein-Str. 9, 07745 Jena, Germany;3. Institute of Inorganic Chemistry I, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany;4. Department of Organic and Inorganic Chemistry, University of Oviedo, 33006 Oviedo, Spain;5. Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Lessingstr. 10, 07743 Jena, Germany;6. ISIS Neutron and Muon Source, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX UK;7. Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Lessingstr. 10, 07743 Jena, Germany Department Spectroscopy/Imaging, Leibniz Institute of Photonic Technology (IPHT), Albert-Einstein-Str. 9, 07745 Jena, Germany;8. Electron Microscopy of Materials Science, Central Facility for Electron Microscopy, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany |
| 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. |
| Keywords: | carbon nitrides hydrogen photocatalysis solar energy conversion visible light |