Ionic Carbon Nitrides for Photocatalysis, Trendbericht Physikalische Chemie 2026 1/3

Ionische Kohlenstoffnitride eignen sich als Materialien, um Ladung zu speichern und lichtgetrieben Wasserstoffperoxid herzustellen.

Ionic Carbon Nitrides for Photocatalysis

One of the key scientific challenges of the 21^st century is the development of green technologies that conserve resources by utilizing efficient catalysts capable of driving chemical transformations with low energy and cost input. In this context, photocatalysis has become an attractive strategy because it enables redox reactions to proceed by supplying energy in the form of light, for example from sunlight or low-cost LEDs powered by renewable electricity sources.^1,2) Such reactions are often carried out under mild conditions: at ambient temperature and pressure, and by using abundant, sustainable reagents such as water or oxygen from the air. While homogeneous photoredox catalysis based on molecular light absorbers has already demonstrated several synthetic applications,^3–5) now heterogeneous photocatalysts are gaining more attention.⁶⁾ Solid photocatalytic materials offer possibilities difficult to achieve with molecular systems alone: products are easily separated, the catalysts degrade slower, and they offer the possibility to realize demanding multi-electron processes, for example the splitting of water into hydrogen and oxygen.

Ionic Carbon Nitrides

Among the most intensively investigated photocatalytic materials are polymeric carbon nitrides, which have low toxicity, are chemically and thermally stable, and easy to prepare.^7–9) Compared to classical semiconductor photocatalysts such as TiO₂, carbon nitrides offer advantages, including a red-shifted optical absorption edge extending into the visible region (bandgaps of around 2.7 – 2.9 eV, corresponding to 430 – 460 nm) and high activity in reactions such as photocatalytic hydrogen evolution or light-driven hydrogen peroxide production from dioxygen. They are easy to synthesize by thermal polymerization of inexpensive nitrogen-rich precursors such as melamine, urea, thiourea, cyanamide, or dicyandiamide. The synthesis itself is so simple that carbon nitrides can even be prepared in a school laboratory using little more than an espresso maker and a hot plate.¹⁰⁾ Interestingly, the structural, photophysical, and catalytic properties depend strongly on the synthetic route employed, including precursor composition, additives, heating protocol, and post-synthetic treatment.

Within this family of materials, poly(heptazine imide)(PHI)-based photocatalysts have recently emerged as a particularly intriguing subclass known as ionic carbon nitrides (Figure 1).^11–14) In contrast to conventional non-ionic PCNs, PHIs consist of negatively charged two-dimensional heptazine-based frameworks balanced by alkali-metal cations and/or protons located between the layers. These ionic materials are typically synthesized by introducing alkali-metal salts or hydroxides during the high-temperature condensation of the organic precursor. Surface functional groups, such as cyanamide and cyamelurate moieties, increase wettability and dispersibility and often leads to higher photocatalytic activity. Furthermore, the type of incorporated cation influences the photophysical and catalytic behavior of the material. Consequently, one of the major strategies for tuning the performance of PHIs is to vary the cation composition during synthesis. Recent spectroscopic and theoretical studies suggest that these cation-dependent effects are linked to changes in exciton dynamics and interlayer interactions.^15,16) Different cations alter the structural corrugation and packing of the PHI layers, thereby modifying charge separation, transport, and recombination processes (Figure 2).

Unique Properties

Ionic carbon nitrides exhibit properties that distinguish them from conventional polymeric carbon nitrides. Remarkable is their ability to accumulate and store photogenerated electrons in long-lived trap states. This phenomenon enables delayed, also called “dark” photocatalysis. After illumination is switched off, the stored electrons can still participate in chemical transformations.^17–19) For example, in the presence of co-catalysts such as platinum, accumulated electrons continue to reduce protons to hydrogen even in the dark. Such behavior is unusual for polymeric photocatalysts and has stimulated interest in the possibility of temporally decoupling light absorption from catalytic turnover.

Even more recently, the complementary accumulation of photogenerated holes in PHIs has also been demonstrated with strong electron acceptors present, such as fluoranil.²⁰⁾ The possibility to stabilize both electrons and holes opens opportunities for studying complex multi-electron and multi-hole processes that are otherwise difficult to realize. In particular, the controlled accumulation of holes may give access to oxidative transformations that require several consecutive oxidative equivalents.

Another feature of PHIs is their peculiar behavior in photoelectrochemical systems. Ionic carbon nitrides can be processed into stable photoanodes capable of oxidizing alcohols at applied bias potentials that are in some cases even below 0 V versus the reversible hydrogen electrode (RHE).^21,22) This property enables efficient and potentially bias-free photoelectrochemical reforming reactions coupled to hydrogen production. Recent spectroscopic investigations indicate that the extensive electron trapping occurring in PHI photoanodes results in photodoping of the material.²³⁾ The accumulated electrons increase electronic conductivity and facilitate charge transport through the otherwise relatively disordered polymeric framework. As a consequence, PHI photoanodes exhibit quicker charge extraction and electron transport compared to many other organic semiconductors.

Light-driven H₂O₂ Production

Particularly attractive among reactions photocatalyzed by carbon nitrides is the light-driven two-electron selective reduction of dioxygen to hydrogen peroxide (H₂O₂) (Figure 3). Hydrogen peroxide is an environmentally benign oxidizing agent used in bleaching, disinfection or chemical synthesis. However, it is currently manufactured predominantly via the energy-intensive anthraquinone process.

Both conventional and ionic carbon nitrides are highly active photocatalysts for H₂O₂ production.^13,24) In principle, the reaction can proceed using only water, dioxygen, and light. Although, in practice, purely water-driven systems are usually slow,²⁵⁾ making the addition of sacrificial electron donors such as aliphatic or aromatic alcohols almost indispensable for efficient H₂O₂ generation. And, an added benefit of this is when the oxidation product obtained from the alcohol is worth more than the starting substrate, as is often the case for biomass-derived feedstocks.

Nevertheless, the development of photocatalytic H₂O₂ synthesis was limited by a kinetic paradox. The same photocatalysts capable of reducing O₂ to H₂O₂ also catalyze the decomposition of the generated product back into water and oxygen, greatly hindering their performance. As H₂O₂ concentration increased, the decomposition pathway prevents the accumulation of peroxide, affording dilute H₂O₂ solutions. A major breakthrough overcoming this hurdle was the development of biphasic photocatalytic reactor systems using hydrophobized ionic PHI photocatalysts.²⁶⁾ In this approach, PHI materials are integrated with functionalized hydrophobized silica nanoparticles. As a result, the photocatalyst resides within a hydrophobic organic phase, for example 1-octanol, which simultaneously acts as the electron donor (Figure 3). Upon formation, the generated H₂O₂ is rapidly extracted into a separate adjacent aqueous phase. This spatial separation is crucial because it effectively protects the product from further photocatalytic decomposition. In essence, the aqueous phase hides the formed H₂O₂ from the photocatalyst located in the organic phase. Consequently, H₂O₂ accumulates in water at concentrations previously considered unattainable for photocatalytic systems. The biphasic strategy represents how reactor engineering and materials design overcomes intrinsic kinetic limitations of photocatalytic processes. Such systems may enable practical production of concentrated aqueous H₂O₂ solutions as a green oxidant suitable for disinfection technologies or subsequent chemical transformations. More broadly, these developments demonstrate how ionic carbon nitrides can not only serve as photocatalysts, but as platforms for engineering new reaction concepts.

Challenges and Outlooks

Many challenges remain before ionic carbon nitrides can realize their technological potential. One priority is improving the quantum efficiencies in photocatalytic transformations and the long-term operational stability, particularly in photoelectrochemical systems.^21,27) At the same time, several opportunities are emerging: crystallinity engineering, rational tuning of ionic composition, and coupling with suitable co-catalysts have recently enabled overall water splitting using PHI-based systems.²⁸⁾ Progress will therefore depend on the development of new synthetic approaches combined with advanced mechanistic investigations integrating spectroscopy, photoelectrochemistry, and theoretical modeling. Better understanding how structure, charge dynamics, catalytic selectivity, and long-term stability relate to each other will be needed to rationally design new ionic carbon nitride photocatalysts. Such insights may open the way toward economically viable emerging photocatalytic technologies for sustainable chemical production, solar fuel generation, and environmental remediation.

Three questions for the author: Radim Beránek

Your research in 140 characters?

My research focuses on design and characterization of novel interface-dominated materials for functional light-driven architectures and devices.

What are your current main research projects?

They are centered around interfacial engineering for photo(electro)catalysis, the physical chemistry of carbon nitrides, and the design and mechanistic understanding of selectivity in photo(electro)catalytic systems, as well as autonomous, stimuli-responsive and chemically active micromotors and systems operating far from thermodynamic equilibrium.

How do you manage to combine family life with a career?

Honestly, “managing” is a polite euphemism for what my wife and I do. With two careers and four children, we spent years feeling like we were failing – until we decided to make failure our baseline. Once we embraced Donald Winnicott’s idea of “good enough parenting”, we discovered an unexpected amount of creative freedom. We are still failing, mind you – we’re just doing it with much better comedic timing now.

Radim Beránek has been a professor at the Institute of Electrochemistry at Ulm University since 2015. Previously, he served as a junior professor at the University of Bochum from 2010 to 2015. He earned his PhD in 2007 under Horst Kisch at the University of Erlangen-Nuremberg. Before that, he studied chemistry at the University of Chemistry and Technology Prague.

• ¹ H. Kisch, Semiconductor Photocatalysis: Principles and Applications, Wiley-VCH, Weinheim, 2015
• ² H. Kisch, R. Perutz, Light: From Photochemistry, Photocatalysis and Photobiology to Visual Arts, De Gruyter, 2024
• ³ C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev. 2013, 113, 5322–5363
• ⁴ B. König, Chemical Photocatalysis, De Gruyter, 2020
• ⁵ E.K. Taskinen, B. König, J. Nat. Prod. 2025, 88, 2822–2848
• ⁶ J. Strunk, Heterogeneous Photocatalysis: From Fundamentals to Applications in Energy Conversion and Depollution, Wiley, 2021
• ⁷ X. Wang, K. Maeda, A. Thomas et al., Nat. Mater. 2009, 8, 76–80
• ⁸ F. K. Kessler, Y. Zheng, D. Schwarz et al., Nat. Rev. Mater. 2017, 2
• ⁹ V.W.-h. Lau, B.V. Lotsch, Adv. Energy Mater. 2022, 12, 2101078
• ¹⁰ M. Petersen, J. Bauschulte, A. Lange et al., CHEMKON 2026, 33, 36–43
• ¹¹ V.W. Lau, I. Moudrakovski, T. Botari et al., Nat. Commun. 2016, 7, 12165
• ¹² A. Savateev, M. Antonietti, ChemCatChem 2019, 11, 6166–6176
• ¹³ I. Krivtsov, D. Mitoraj, C. Adler et al., Angew. Chem. Int. Ed. 2020, 59, 487–495
• ¹⁴ C.M. Pelicano, M. Antonietti, Angew. Chem. Int. Ed. 2024, 63, e202406290
• ¹⁵ C. Im, R. Beranek, T. Jacob, Adv. Energy Mater. 2025, 15, 2405549
• ¹⁶ A. Konar, J. Liessem, C. Im et al., Adv. Sci. 2025, 12, e09312
• ¹⁷ V.W. Lau, D. Klose, H. Kasap et al., Angew. Chem. Int. Ed. 2017, 56, 510–514
• ¹⁸ Y. Markushyna, P. Lamagni, C. Teutloff et al., J. Mater. Chem. A 2019, 7, 24771–24775
• ¹⁹ H. Schlomberg, J. Kröger, G. Savasci et al., Chem. Mater. 2019, 31, 7478–7486
• ²⁰ M. Isobe, M. Hattori, C. Miyazaki et al., Chem. Mater. 2025, 37, 7484–7491
• ²¹ C. Adler, I. Krivtsov, D. Mitoraj et al., ChemSusChem 2021, 14
• ²² C. Pulignani, C.A. Mesa, S. A. J. Hillman et al., Angew. Chem. Int. Ed. 2022, 61, e202211587
• ²³ C. Adler, S. Selim, I. Krivtsov et al., Adv. Funct. Mater. 2021, 31, 2105369
• ²⁴ Y. Shiraishi, S. Kanazawa, Y. Sugano et al., ACS Catal. 2014, 4, 774–780
• ²⁵ Y. Shiraishi, S. Kanazawa, Y. Kofuji et al., Angew. Chem. Int. Ed. 2014, 53, 13454–13459
• ²⁶ I. Krivtsov, A. Vazirani, D. Mitoraj et al., J. Mater. Chem. A 2023, 11, 2314–2325
• ²⁷ C. Pulignani, A. Bin Mohamad Annuar, S. J. Cobb et al., J. Am. Chem. Soc. 2026, 148, 12839–12848
• ²⁸ C. Zhang, X. Liang, W. Xing et al., J. Am. Chem. Soc. 2026, 148, 17272–17282

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