The core problem the researchers set out to solve is one that has haunted the clean energy and water treatment fields for years. Electrochemical water splitting, which uses electricity to pull water molecules apart into hydrogen and oxygen, is one of the most promising routes to green hydrogen, but it demands large amounts of extra energy because the half-reactions at the electrodes are sluggish. Meanwhile, photocatalytic advanced oxidation processes, which use light-activated catalysts to generate reactive molecules that shred organic pollutants, are hampered by the fact that excited electrons and holes tend to recombine almost instantly, wasting the absorbed light energy before it can do useful chemistry. Catalysts also tend to degrade over time, undermining long-term operation.

To attack both problems simultaneously, the team synthesized a family of composite materials by coupling vanadium pentoxide, a well-known transition metal oxide, with graphitic carbon nitride in two forms: an unmodified version abbreviated CN and a sulfur-modified version abbreviated SCN. By varying the amount of the sulfur-modified carbon nitride, they produced three composites containing 10, 20 and 30 weight percent of the modified polymer, labeled VSCN-1, VSCN-2 and VSCN-3 respectively. The idea behind this interfacial engineering is that when two semiconductors with different electronic structures are brought into intimate contact, the junction between them creates an internal electric field that sweeps charge carriers apart, keeping electrons and holes separated long enough for them to drive chemical reactions.

The star performer was VSCN-2, the composite with 20 weight percent sulfur-modified carbon nitride. When deposited on nickel foam, a common conductive support for electrodes, and tested in a concentrated potassium hydroxide electrolyte, the material required an overpotential of just 250 millivolts to drive the oxygen evolution reaction at a current density of 10 milliamperes per square centimeter. For the hydrogen evolution reaction, the overpotential was an impressively low 89 millivolts at the same current density. Overpotentials are the extra voltage beyond the thermodynamic minimum that must be applied to make a reaction proceed at a useful rate, so lower values translate directly into lower energy consumption and cheaper hydrogen.

Equally important for real-world deployment is how the catalyst behaves over time, and here the results were equally striking. The measured Tafel slopes, which describe how quickly current rises as voltage is increased, came in at 68 millivolts per decade for the oxygen reaction and 109 millivolts per decade for the hydrogen reaction, indicating favorable reaction kinetics on the modified surface. The electrode also operated stably for 60 hours without significant loss of activity, a duration that rules out the rapid degradation that plagues many experimental catalysts. When two identical VSCN-2 electrodes were paired into a full water-splitting electrolyzer, the device needed only 1.55 volts to sustain 10 milliamperes per square centimeter and kept running steadily for 80 hours, edging close to the 1.48-volt thermodynamic benchmark that defines an ideal electrolyzer.

The same material proved remarkably adept at a completely different task: destroying tetracycline, a widely used antibiotic that escapes into waterways through agricultural runoff and pharmaceutical wastewater. Residual antibiotics in the environment are a serious concern because they promote the evolution of drug-resistant bacteria, so finding efficient ways to degrade them is a growing research priority. Under irradiation from ordinary light-emitting diodes, the VSCN-2 composite achieved 99 percent removal of tetracycline through a process called sonophotocatalysis, which combines ultrasonic agitation with light activation. The ultrasound generates microscopic cavitation bubbles in the water that collapse violently, producing additional reactive species and improving mass transport, while the illuminated catalyst generates electron-hole pairs that attack the pollutant. The synergy between the two energy inputs pushes degradation rates well beyond what either light or sound alone can achieve.

Durability carried over to the photocatalytic application as well. The composite retained its degradation activity across repeated use cycles, suggesting that the interfacial structure is robust rather than a fleeting artifact of fresh synthesis. To understand which reactive species were actually doing the work of shredding the tetracycline molecule, the researchers performed radical-scavenging experiments, in which specific chemicals are added to selectively neutralize particular reactive species, alongside electron spin resonance spectroscopy, which detects short-lived radicals directly. Both approaches pointed to superoxide radicals and hydroxyl radicals as the dominant destructive agents, which in turn supported the proposed charge transfer mechanism operating across the vanadium pentoxide and sulfur-modified carbon nitride interface.

The team went further than simply confirming that the antibiotic disappeared. Using liquid chromatography coupled with mass spectrometry, they identified the intermediate products formed as the tetracycline molecule was progressively broken apart, and then used the ECOSAR predictive software to estimate the toxicity of those fragments. Encouragingly, the analysis indicated that the degradation pathway leads toward products with lower predicted toxicity than the parent antibiotic, meaning the process genuinely detoxifies the water rather than merely converting one harmful compound into another. This combination of mechanistic spectroscopy, product identification and toxicity prediction represents the kind of thorough evidence chain that regulators and engineers need before such technologies can move from the laboratory toward practical water treatment systems.

What makes this study notable in a crowded field of catalyst papers is the deliberate dual-function design. Most catalysts are optimized for a single reaction and a single application, but the underlying physics that limits both technologies, namely the wasteful recombination of charge carriers and sluggish interfacial charge transfer, is shared. By engineering a heterostructure in which the sulfur modification tunes the electronic structure of the carbon nitride and the intimate junction with vanadium pentoxide accelerates charge separation, the researchers addressed both limitations with one material. The sulfur atoms in the carbon nitride framework are thought to adjust the band positions and introduce active sites, while the oxide-polymer interface acts as a highway for electrons migrating in one direction and holes in the other, depending on whether the driving force is an applied voltage or absorbed light.

The implications stretch across the clean energy and environmental technology landscape. A catalyst that can lower the voltage needed for hydrogen production could reduce the cost of green hydrogen, which remains a key stumbling block for the hydrogen economy, while the same chemistry applied to polluted water offers a low-energy route to eliminating pharmaceutical contaminants using nothing more than LEDs and ultrasound. The work was supported by the National Research Foundation of Korea through its Basic Science Research Program, with additional funding from King Saud University in Saudi Arabia. As laboratories worldwide race to design multifunctional materials that squeeze more value from every input of energy, this sulfur-modified heterostructure stands as a compelling demonstration that clever interfacial engineering can turn two hard problems into one elegant solution.

Subject of Research: Sulfur-modified g-C3N5/V2O5 heterostructure catalysts for electrochemical water splitting and sonophotocatalytic antibiotic degradation

Article Title: Interfacial engineering of sulfur-modified g-C3N5/V2O5 heterostructures for enhanced overall water splitting and sonophotocatalytic tetracycline degradation

Article References: Kumaravel, S., Durai, M., Gnanasekaran, L., Kumaravel, S., Erusappan, E., Shanmugapriya, D., Afzal, M., Paskalis, S. M. K., & Lee, D. S. (2026). Interfacial engineering of sulfur-modified g-C3N5/V2O5 heterostructures for enhanced overall water splitting and sonophotocatalytic tetracycline degradation. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02085-y

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02085-y

Keywords: electrocatalysis, water splitting, hydrogen evolution, oxygen evolution, photocatalysis, sonophotocatalysis, tetracycline degradation, g-C3N5, V2O5, heterostructure, sulfur modification, advanced oxidation processes

Indazolones are bicyclic nitrogen-containing heterocycles that appear repeatedly in pharmaceutical research, and attaching an alkyl group to the ring nitrogen is one of the most common ways chemists tune their biological properties. The classic ways of doing this, however, often rely on precious transition metals such as palladium, ruthenium or gold, on stoichiometric oxidants, or on harsh conditions that generate substantial waste. The research team, Kumar Aravindraj and Selvaraj Mohana Roopan of the Department of Chemistry in the School of Advanced Sciences, set out to replace those expensive and environmentally burdensome approaches with a heterogeneous, light-driven process that operates under mild conditions and recycles its catalyst batch after batch.

The heart of the innovation lies in the pairing of two semiconducting materials with complementary properties. Iron tungstate, FeWO4, is an iron-based inorganic semiconductor that has attracted attention for electrocatalysis, water treatment and energy storage, but on its own it suffers from the limitations typical of narrow-band metal oxides, including rapid recombination of the charge carriers generated when light is absorbed. Graphitic carbon nitride, and in particular the nitrogen-rich variant g-C3N5, is a metal-free polymeric semiconductor with a smaller band gap than the widely studied g-C3N4, which allows it to harvest a larger share of the visible spectrum. When the two are combined into a single nanocomposite, the interface between them promotes the separation of photogenerated electrons and holes, extending the lifetime of the energetic charge carriers that ultimately do the chemical work.

Under blue LED irradiation, the nanocomposite absorbs photons and promotes electrons from valence to conduction bands, leaving behind positively charged holes. These electrons and holes participate in a photocatalytic cycle that activates the benzyl chloride partner and the nitrogen of the indazolone, enabling the formation of the new C–N bond that defines the N-alkylated product. The authors report that the process delivers benzyl-indol-yl-phenyl-indazolones in remarkable yields across a variety of substrate combinations, demonstrating that the method tolerates different benzyl chlorides and different indol-yl-phenyl-indazolone frameworks. The use of a simple blue LED rather than ultraviolet lamps or high-intensity light sources keeps the energy input low and the setup inexpensive, features that matter for any reaction intended to be scaled beyond a research laboratory.

A critical part of the study was the characterization of the catalyst itself. The team deployed an extensive battery of analytical techniques to confirm the structure, morphology and electronic properties of the nanocomposite. X-ray diffraction established the crystalline phases present, while field emission scanning electron microscopy and high-resolution transmission electron microscopy, together with selected area electron diffraction, revealed the nanoscale architecture and the intimate contact between the iron tungstate and the carbon nitride components. Energy dispersive X-ray spectroscopy confirmed the elemental composition, and X-ray photoelectron spectroscopy probed the chemical states of the constituent elements at the surface.

Optical measurements provided the mechanistic insight that underpins the catalyst’s performance. Diffuse reflectance spectroscopy was used to determine the light-absorption characteristics and band structure of the composite, while photoluminescence spectroscopy served as a window onto charge-carrier behavior: a quenched photoluminescence signal relative to the individual components indicates that electrons and holes are being separated efficiently at the heterojunction rather than recombining and releasing their energy as light. This suppression of recombination is precisely what allows the absorbed blue photons to be funneled into productive chemistry, and it explains why the composite outperforms what either semiconductor could achieve alone.

The products themselves were rigorously verified. The researchers used nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry to confirm the identity and purity of the N-alkylated indazolones, and single crystal X-ray diffraction, reported with an ORTEP thermal ellipsoid plot, provided unambiguous structural proof for at least one representative product. Such crystallographic confirmation is particularly valuable in heterocycle synthesis, where regioisomeric products can complicate assignment, and it demonstrates the analytical thoroughness of the study. Control experiments with radical traps such as TEMPO, 2,2,6,6-tetramethyl-1-piperidinyloxy, were consistent with the mechanistic picture of a light-driven process involving reactive intermediates generated at the catalyst surface.

Perhaps the most practically significant result is the recyclability of the catalyst. Heterogeneous photocatalysts often degrade, leach metal into solution, or lose activity through fouling, which undermines their green credentials. In this work, the FeWO4/g-C3N5 nanocomposite was recovered after each reaction and redeployed for five consecutive cycles of N-alkylation without any loss in catalytic activity. That durability means the cost and environmental footprint of catalyst preparation are amortized over multiple batches, and it simplifies product purification because the solid catalyst can simply be filtered off at the end of the reaction. The authors also note that no specific funding was received for the study and that no datasets were generated or analysed beyond those reported in the paper.

The new report builds on the same group’s earlier demonstration of a blue LED assisted indazolone preparation using a reusable CuWO4/g-C3N5 nanocomposite, published in the Journal of Photochemistry and Photobiology A: Chemistry in 2024. By swapping copper tungstate for its iron analogue, the team has moved to an even more earth-abundant and inexpensive metal while retaining the recyclability and efficiency of the previous system. Iron is among the most benign and plentiful elements in the periodic table, and iron-based heterogeneous catalysts have become a major focus of sustainable chemistry research precisely because they avoid the toxicity and supply concerns associated with palladium, ruthenium, gold and platinum systems.

The broader context makes the advance timely. Visible-light photocatalysis has transformed synthetic organic chemistry over the past decade by allowing reactions to be triggered under exceptionally mild conditions, and carbon nitride materials have emerged as robust, metal-free light absorbers for applications ranging from hydrogen peroxide production to pollutant degradation and fine chemical synthesis. Meanwhile, N-alkylation remains a workhorse transformation in medicinal chemistry, and methods that couple it to abundant catalysts and low-energy light sources are exactly what industrial and academic laboratories are seeking. By demonstrating that a FeWO4/g-C3N5 heterojunction can deliver N-alkylated indazolones in remarkable yields under a simple blue LED, and that the catalyst survives five reuse cycles intact, Aravindraj and Mohana Roopan have added a genuinely reusable, iron-based option to the photocatalysis toolbox. The work suggests that carefully engineered interfaces between metal tungstates and nitrogen-rich carbon nitrides could serve as a general platform for light-driven C–N bond formation, and it brings the vision of sustainable, low-cost photochemical manufacturing of pharmacologically relevant heterocycles a step closer to routine practice.

Subject of Research: A reusable FeWO4/g-C3N5 nanocomposite photocatalyst for blue LED-driven N-alkylation of indazolones via C–N bond formation

Article Title: FeWO4/g-C3N5 Nanocomposite: A Reusable Photocatalyst for N-Alkylation of Indazolones Under Blue LED

Article References: FeWO4/g-C3N5 Nanocomposite: A Reusable Photocatalyst for N-Alkylation of Indazolones Under Blue LED. (n.d.). https://doi.org/10.1007/s10562-026-05523-0

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05523-0

Keywords: photocatalysis, FeWO4, g-C3N5, nanocomposite, indazolone, N-alkylation, blue LED, C–N bond formation, heterogeneous catalysis, iron tungstate, graphitic carbon nitride, green chemistry