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261 results on '"Beranek, Radim"'

1. Structural Influence on Exciton Formation and the Critical Role of Dark Excitons in Polymeric Carbon Nitrides

Author

Im, Changbin, Beranek, Radim, and Jacob, Timo

Subjects
Condensed Matter - Materials Science
Abstract

Polymeric carbon nitrides (PCNs) exhibit intriguing optical properties and exceptional performance in (photo)catalysis, optoelectronics, and energy storage. Nevertheless, the intricate phenomena involving light absorption extending to the visible range, long-lived excitons, photo-charging, and photochemical processes observed in PCN derivatives remain poorly understood. Our theoretical investigation elucidates the origin of distinct dark and bright excitons, their stability and lifetimes, and their correlation with the microstructural attributes of PCNs. Furthermore, we identify the decisive role of dark excitons in catalytic reactivity, which underlies the differences in the photocatalytic performance of different PCN derivatives.

Published
2024

2. Unraveling the Optical Signatures of Polymeric Carbon Nitrides: Insights into Stacking-Induced Excitonic Transitions

Author

Im, Changbin, Kirchhoff, Björn, Mitoraj, Dariusz, Krivtsov, Igor, Farkas, Attila, Beranek, Radim, and Jacob, Timo

Subjects
Condensed Matter - Materials Science
Abstract

Two-dimensional (2D) materials have attracted considerable attention due to their unique physicochemical properties and significant potential in energy-related applications. Polymeric carbon nitrides (PCNs) with 2D stacked architecture show promise as photocatalysts for solar-to-fuel conversion and as versatile 2D semiconductors. However, the lack of a clear definition of the exact structural model of these materials limits our fundamental understanding of their unique properties. Here, we investigate the structure-induced optical properties of PCNs through \textit{ab initio} calculations. Our study on the electronic and optical properties of PCNs highlights the significant influence of structure on their behavior, especially near band edges. The analysis reveals that the degree of condensation and corrugation influences the electron/hole localization and the energy levels of $\pi$ electrons, which are crucial for the optical behavior. In addition, the microstructures of 2D configurations lead to divergent optical properties in 3D configurations, with characteristic peaks identified at 350 nm and interlayer interactions ranging from 400 to 500 nm, depending on the specific microstructures. Through observations over 2D and 3D structures, we elucidate exciton photophysical processes in PCN materials. This highlights the substantial differences in optical properties between actual 2D and 3D structures, while also demonstrating the potential for carrier and energy transport mechanisms to occur perpendicular to the plane. Finally, our results provide deep insights into the understanding of previously hidden microstructural, electronic, and optical properties of PCNs, paving the way for further performance and property enhancements in this class of materials.

Published
2024

3. Structure and optical properties of polymeric carbon nitrides from atomistic simulations

Author

Im, Changbin, Kirchhoff, Björn, Krivtsov, Igor, Mitoraj, Dariusz, Beranek, Radim, and Jacob, Timo

Subjects
Condensed Matter - Materials Science, Physics - Chemical Physics
Abstract

Detailed understanding of the structural and photophysical properties of polymeric carbon nitride (PCN) materials is of critical importance to derive future material optimization strategies towards more desirable optical properties and more photocatalytically active materials. However, the wide range of structural motifs found in synthesized PCNs complicates atomistic simulations that rely on well defined models. Performing hybrid DFT studies, we systematically investigate formation energy trends and optical properties of PCNs as a function of dimensionality, going from molecular oligomers over periodic sheet models to stacked crystals. Thermochemical calculations that take into account vibrational enthalpy and entropy contributions predict that a mixture of structural motifs including the melon string structure, poly(heptazine imide), and g-C3N4 motifs is stable under typical synthetic conditions. The degree of lateral condensation as well as stacking can reduce the bandgap while out-of-plane corrugation of the material increases both stability and the optical gap. The key result of this work is that already small domains of strongly condensed PCN are calculated to give rise to favorable optical properties. This result reconciles conflicting literature reports indicating that the thermodynamically favorable melon motif has a too large bandgap compared to experiments, while the g-C3N4 structure, for which bandgap calculations are in better agreement with experiments, does not agree with measured chemical compositions of PCNs. Finally, we postulate a new computational model for carbon nitride materials that encompasses the most important structural motifs and shows a bandgap of ca. 2.9 eV.

Published
2022

12. Enhancing Photocatalysis: Understanding the Mechanistic Diversity in Photocatalysts Modified with Single‐Atom Catalytic Sites

Author

Kruczała, Krzysztof, primary, Neubert, Susann, additional, Dhaka, Kapil, additional, Mitoraj, Dariusz, additional, Jánošíková, Petra, additional, Adler, Christiane, additional, Krivtsov, Igor, additional, Patzsch, Julia, additional, Bloh, Jonathan, additional, Biskupek, Johannes, additional, Kaiser, Ute, additional, Hocking, Rosalie K., additional, Caspary Toroker, Maytal, additional, and Beranek, Radim, additional

Published
2023
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14. Structure and optical properties of polymeric carbon nitrides from atomistic simulations

Author

German Research Foundation, Research and Art Baden-Württemberg, Alexander von Humboldt Foundation, Im, Changbin, Kirchhoff, Björn, Krivtsov, I. V., Mitoraj, Dariusz, Beranek, Radim, Jacob, Timo, German Research Foundation, Research and Art Baden-Württemberg, Alexander von Humboldt Foundation, Im, Changbin, Kirchhoff, Björn, Krivtsov, I. V., Mitoraj, Dariusz, Beranek, Radim, and Jacob, Timo

Abstract

Computational investigation of the structural and photophysical properties of polymeric carbon nitride (PCN) materials can help to establish understanding-driven material optimization strategies. However, the plethora of structural motifs found in synthesized PCNs complicates atomistic simulations. Performing hybrid DFT studies, we systematically investigate formation energy trends and optical properties of PCNs as a function of dimensionality. Thermochemical calculations predict that a mixture of structural motifs including the melon string structure, poly(heptazine imide), and g-C3N4 motifs is stable under typical synthetic conditions. Lateral condensation reduces the bandgap while out-of-plane corrugation increases both stability and the bandgap. The key result of this work is that already small domains of strongly condensed PCN embedded in a less condensed framework can give rise to desirable optical properties. This result reconciles conflicting literature reports indicating that the calculated bandgap of the thermodynamically favorable melon structure is too large compared to experiments, while the g-C3N4 structure, for which bandgap calculations are in better agreement with experiments, does not agree with the measured chemical composition of synthesized PCNs. Finally, we postulate a new computational model for PCNs which combines the most important structural motifs and for which we calculate a bandgap of ca. 2.9 eV.

Published
2023

15. Supporting Information Available for Structure and optical properties of polymeric carbon nitrides from atomistic simulations

Author

Im, Changbin, Kirchhoff, Björn, Krivtsov, I. V., Mitoraj, Dariusz, Beranek, Radim, Jacob, Timo, Im, Changbin, Kirchhoff, Björn, Krivtsov, I. V., Mitoraj, Dariusz, Beranek, Radim, and Jacob, Timo

Abstract

Illustrations of the model system, the calculated chemical potential of ammonia, phase diagram of ammonia chemical potential, the optical property of the structures, the calculated phase diagram, the free energy of formation, the free energy of reaction.

Published
2023

20. Hydrophobized poly(heptazine imide) for highly effective photocatalytic hydrogen peroxide production in a biphasic fatty alcohol–water system

Author

Krivtsov, Igor, primary, Vazirani, Ashish, additional, Mitoraj, Dariusz, additional, Elnagar, Mohamed M., additional, Neumann, Christof, additional, Turchanin, Andrey, additional, Patiño, Yolanda, additional, Ordóñez, Salvador, additional, Leiter, Robert, additional, Lindén, Mika, additional, Kaiser, Ute, additional, and Beranek, Radim, additional

Published
2023
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22. Hydrophobized Poly(Heptazine Imide) for Highly Effective Photocatalytic Hydrogen Peroxide Production in a Biphasic Fatty Alcohol-Water System

Author

Krivtsov, Igor, primary, Vazirani, Ashish, additional, Mitoraj, Dariusz, additional, Elnagar, Mohamed M., additional, Neumann, Christof, additional, Turchanin, Andrey, additional, Patiño, Yolanda, additional, Ordóñez, Salvador, additional, Leiter, Robert, additional, Lindén, Mika, additional, Kaiser, Ute, additional, and Beranek, Radim, additional

Published
2022
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24. On the importance of catalysis in photocatalysis: Triggering of photocatalysis at well-defined anatase TiO2 crystals through facet-specific deposition of oxygen reduction cocatalyst.

Author

Adler, Christiane, Mitoraj, Dariusz, Krivtsov, Igor, and Beranek, Radim

Subjects
PHOTOCATALYSIS, OXYGEN reduction, PHOTOCATALYSTS, CATALYSIS, SURFACE reconstruction, PHOTODEGRADATION, QUANTUM efficiency, CRYSTALS
Abstract

Well-defined anatase TiO2 crystals with co-exposed {101} and {001} facets represent a promising platform for fundamental studies in photocatalysis and for the development of novel photocatalytic systems exhibiting higher than usual quantum efficiencies. Herein, we present protocols enabling the photoreductive deposition of Pt nanoparticles onto anatase TiO2 micro-sized (1–3 µm) crystals prepared by hydrothermal growth in fluoride-containing solutions to be carried out either facet-selectively (on {101} facets only) or facet-non-selectively (on both {101} and {001} facets). The photocatalytic behavior of the resulting photocatalysts is studied using investigations of oxidative photodegradation of a test pollutant [4-chlorophenol (4-CP)], photocurrent measurements, and kinetic analysis of the open-circuit photopotential decay. We demonstrate that the deposition of Pt nanoparticles effectively triggers the photocatalytic degradation of 4-CP at anatase crystals that are otherwise completely inactive. The role of Pt in triggering the photocatalysis is demonstrated to consist chiefly in the catalytic enhancement of the reaction rate of oxygen reduction by the photogenerated electrons. Only the platinized {101} facets contribute to photocatalysis, whereas the {001} facets, in the literature often referred to as "highly reactive", are even after platinization completely inactive, most likely due to (1 × 4) surface reconstruction upon the heat treatment necessary to decrease the amount of surface fluorides. Based on our results, we highlight the eminent role of efficient surface catalysis for effective charge separation and provide specific design rules for further development of photocatalysts with high quantum efficiencies. [ABSTRACT FROM AUTHOR]

Published
2020
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25. Corrigendum: A Study in Red: The Overlooked Role of Azo‐Moieties in Polymeric Carbon Nitride Photocatalysts with Strongly Extended Optical Absorption

Author

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

Published
2022
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30. Benzaldehyde‐Promoted (Auto)Photocatalysis under Visible Light: Pitfalls and Opportunities in Photocatalytic H2O2 Production**.

Author

Krivtsov, Igor, Vazirani, Ashish, Mitoraj, Dariusz, and Beranek, Radim

Subjects
VISIBLE spectra, ALCOHOL oxidation, BENZYL alcohol, PHOTOCATALYSIS, ELECTRON donors, AROMATIC aldehydes, BENZALDEHYDE
Abstract

Selective photooxidation of aromatic alcohols to corresponding aldehydes is a widely used model reaction for evaluation of performance of heterogeneous photocatalysts. A top example is the photocatalytic production of hydrogen peroxide via reduction of dioxygen with concomitant photooxidation of benzyl alcohol to benzaldehyde. Herein we demonstrate the ability of benzaldehyde to promote the auto‐photocatalytic oxidation of benzyl alcohol and produce large quantities of H2O2 in solvent‐free (no water) or biphasic (with water) systems even under nominally visible (>420 nm cut‐off filter) light irradiation. These results shed critical light on the plethora of research reports on photocatalytic H2O2 production in which benzyl alcohol was employed as electron donor. Since the autocatalytic pathway based on the photocatalytic activity of benzaldehyde formed during photocatalysis under such conditions cannot be neglected, the interpretations of photocatalytic performance are likely contentious and distorted in such reports. The use of benzyl alcohol as a model electron donor in photocatalytic studies should be therefore definitely discouraged. On the other hand, as we found that benzaldehyde‐mediated photocatalysis of H2O2 production proceeds also using alternative electron donors (e. g. ethanol), our results highlight the possible prospects for a more general use of benzaldehyde and other aromatic aldehydes in visible light‐driven photocatalysis. [ABSTRACT FROM AUTHOR]

Published
2023
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32. A Study in Red: The Overlooked Role of Azo‐Moieties in Polymeric Carbon Nitride Photocatalysts with Strongly Extended Optical Absorption

Author

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

Published
2021
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37. Photodriven charge accumulation and carrier dynamics in a water‐soluble carbon nitride photocatalyst

Author

Li, Chunyu, Hofmeister, Elisabeth, Krivtsov, Igor, Mitoraj, Dariusz, Adler, Christiane, Beranek, Radim, and Dietzek, Benjamin

Subjects
DDC 540 / Chemistry & allied sciences, Carbon nitride, Photochemistry, Energiewandler, ultrafast transient absorption spectroscopy, ddc:540, charge trapping, Fotochemie, Energy conversion
Abstract

Taking a (very) quick look: A water-soluble carbon nitride reveals excellent storage capacity for photogenerated charges. The photoinduced dynamics in this high-performance water-soluble carbon nitride photocatalyst are investigated by ultrafast time-resolved spectroscopy. Charge accumulation in photoactive molecules and materials holds great promise in solar energy conversion as it allows for decoupling solar-driven charging from (dark) redox reactions. In this contribution, light-driven charge accumulation was investigated for a recently reported novel water-soluble carbon nitride [K,Na-poly(heptazine imide); K,Na-PHI] photocatalyst, which exhibits excellent activity and stability in highly selective photocatalytic oxidation of alcohols and concurrent reduction of dioxygen to H2O2 under quasi-homogeneous conditions. An excellent charge storage ability of the K,Na-PHI material was demonstrated, showing an optimal density of accumulated electrons (32.2 μmol of electrons per gram) in the presence of 10 vol % MeOH as a sacrificial electron donor. The long-lived electrons accumulated under anaerobic conditions as K,Na-PHI.− radical ions were utilized in interfacial electron transfer to O2 or methyl viologen in a subsequent dark reaction. Ultrafast time-resolved spectroscopy was employed to reveal the kinetics of charge-carrier recombination and methanol oxidation. Geminate recombination of electrons and holes within approximately 100 ps was followed by trap-assisted recombination. The presence of methanol as a sacrificial electron donor accelerated the decay of the transient absorption signal when a static sample was used. This behavior was ascribed to the faster charge recombination in the presence of the radical anions generated after hole extraction. The work suggests that photodriven electron storage in the water-soluble carbon nitride is enabled by localized trap states, and highlights the importance of the effective electron donor for creating long-lived photo-generated carbon nitride radicals., publishedVersion

Published
2021
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38. Sol−gel processing of water-soluble carbon nitride enables high-performance photoanodes

Author

German Research Foundation, Ministerio de Economía y Competitividad (España), Principado de Asturias, Agencia Estatal de Investigación (España), Alexander von Humboldt Foundation, Projekt DEAL, Adler, Christiane, Krivtsov, I. V., Mitoraj, Dariusz, Santos-Gómez, Lucía dos, García-Granda, Santiago, Neumann, Christof, Kund, Julian, Kranz, Christine, Mizaikoff, Boris, Turchanin, Andrey, Beranek, Radim, German Research Foundation, Ministerio de Economía y Competitividad (España), Principado de Asturias, Agencia Estatal de Investigación (España), Alexander von Humboldt Foundation, Projekt DEAL, Adler, Christiane, Krivtsov, I. V., Mitoraj, Dariusz, Santos-Gómez, Lucía dos, García-Granda, Santiago, Neumann, Christof, Kund, Julian, Kranz, Christine, Mizaikoff, Boris, Turchanin, Andrey, and Beranek, Radim

Abstract

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol−gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol−gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm−2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a ‘soft matter’-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from sol−gel processing of water-soluble PCN thus constitute a new paradigm for high-performance ‘soft matter’ photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

Published
2021

41. Cover Feature: Sol−Gel Processing of Water‐Soluble Carbon Nitride Enables High‐Performance Photoanodes (ChemSusChem 10/2021)

Author

Adler, Christiane, primary, Krivtsov, Igor, additional, Mitoraj, Dariusz, additional, Santos‐Gómez, Lucía, additional, García‐Granda, Santiago, additional, Neumann, Christof, additional, Kund, Julian, additional, Kranz, Christine, additional, Mizaikoff, Boris, additional, Turchanin, Andrey, additional, and Beranek, Radim, additional

Published
2021
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43. Water‐soluble polymeric carbon nitride colloidal nanoparticles for highly selective quasi‐homogeneous photocatalysis

Author

German Research Foundation, Ministerio de Economía y Competitividad (España), Universidade de Aveiro, Principado de Asturias, Fundação para a Ciência e a Tecnologia (Portugal), Ministério da Educação e Ciência (Portugal), University of Iceland, Ministério da Ciência, Tecnologia e Ensino Superior (Portugal), Alexander von Humboldt Foundation, Krivtsov, I. V., Mitoraj, Dariusz, Adler, Christiane, Ilkaeva, Marina, Sardo, Mariana, Mafra, Luís, Neumann, Christof, Turchanin, Andrey, Li, Chunyu, Dietzek, Benjamin, Leiter, Robert, Biskupek, Johannes, Kaiser, Ute, Im, Changbin, Kirchhoff, Björn, Jacob, Timo, Beranek, Radim, German Research Foundation, Ministerio de Economía y Competitividad (España), Universidade de Aveiro, Principado de Asturias, Fundação para a Ciência e a Tecnologia (Portugal), Ministério da Educação e Ciência (Portugal), University of Iceland, Ministério da Ciência, Tecnologia e Ensino Superior (Portugal), Alexander von Humboldt Foundation, Krivtsov, I. V., Mitoraj, Dariusz, Adler, Christiane, Ilkaeva, Marina, Sardo, Mariana, Mafra, Luís, Neumann, Christof, Turchanin, Andrey, Li, Chunyu, Dietzek, Benjamin, Leiter, Robert, Biskupek, Johannes, Kaiser, Ute, Im, Changbin, Kirchhoff, Björn, Jacob, Timo, and Beranek, Radim

Abstract

Heptazine‐based polymeric carbon nitrides (PCN) are promising photocatalysts for light‐driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom‐up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi‐homogeneous conditions. The superior performance of water‐soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4‐methoxybenzyl alcohol and benzyl alcohol or lignocellulose‐derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re‐dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

Published
2020

48. CVD grown GaSbxN1-x films as visible-light active photoanodes

Author

Zywitzki, Dennis, primary, Mitoraj, Dariusz, additional, Vilk, Yury, additional, Mendoza Reyes, Oliver, additional, Schleuning, Markus, additional, Friedrich, Dennis, additional, Sadlo, Alexander, additional, Rogalla, Detlef, additional, Eichberger, Rainer, additional, Beranek, Radim, additional, and Devi, Anjana, additional

Published
2021
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50. Polymeric Carbon Nitride Coupled with a Molecular Thiomolybdate Catalyst: Exciton and Charge Dynamics in Light-Driven Hydrogen Evolution

Author

Rajagopal, Ashwene, primary, Akbarzadeh, Elham, primary, Li, Chunyu, primary, Mitoraj, Dariusz, primary, Krivtsov, Igor, primary, Adler, Christiane, primary, Diemant, Thomas, primary, Biskupek, Johannes, primary, Kaiser, Ute, primary, Im, Changbin, primary, Heiland, Magdalena, primary, Jacob, Timo, primary, Streb, Carsten, primary, Dietzek, Benjamin, primary, and Beranek, Radim, primary

Published
2020
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