The environmental stakes are considerable. Rhodamine B carries a xanthene ring system substituted with two di-ethylamino groups and a carboxyphenyl group, and it ranks among the more problematic dye pollutants in aquatic systems. Conventional wastewater treatments exist, but photocatalytic degradation powered by abundant, clean solar energy has emerged as a green, pollution-free and comparatively inexpensive alternative. What makes the new work distinctive is not just the catalyst itself but the way it is made: instead of preparing the porphyrin-semiconductor composite in advance, purifying it, drying it and storing it, the team simply mixed the two components into the dye solution and let weak physical interactions assemble the composite on the spot.

That in-situ strategy, the authors argue, sidesteps the energy inputs and extra solvent consumption that ex-situ composite fabrication demands. It also avoids the shelf-life problem: pre-made composites can degrade over time through moisture, heat, contamination or self-degradation. The trade-off, which the researchers candidly acknowledge, is that the components are not strongly bonded to each other, since P1 lacks strong anchoring groups, and much of the catalyst was lost through the filter paper during recovery after the first photocatalytic cycle. They propose functionalizing the porphyrin with carboxylic acid anchoring groups or inducing self-assembly on the ZnO surface as future fixes.

Every step of the synthesis was designed with sustainability in mind. The porphyrin was made by an acid-catalyzed condensation of pyrrole and 4-methoxybenzaldehyde in a water-methanol mixture at room temperature, using just 10 milliliters of hydrochloric acid rather than the large quantities of harsher catalysts and chlorinated solvents that traditional Adler-type methods require. A single modification to the published procedure proved decisive: instead of stirring the DMF solution overnight after reflux, the team left it undisturbed. That small change yielded well-separated, lozenge-shaped violet crystals of high quality, suitable for single-crystal X-ray diffraction, while avoiding the mechanical disturbance that disrupts molecular ordering during crystal growth and cutting the energy cost of overnight stirring.

The crystallography revealed a monoclinic P21/c structure with an asymmetric unit containing half the molecule, bifurcated hydrogen bonds within the porphyrin core, and a three-dimensional supramolecular framework stitched together by non-covalent C-H···π interactions. The crystallinity matters for catalysis: ordered porphyrin nanocrystals favor enhanced electron transport and photostability, so the crystalline P1 is expected to outperform amorphous forms of the same molecule.

The zinc oxide nanoparticles were equally green in their origins. Fresh Morus alba leaves, collected on the college campus in April, were steeped in distilled water at 70 degrees Celsius for half an hour, releasing phytochemicals such as alkaloids, phenols and flavonoids into solution. When this extract was added to zinc sulfate under alkaline conditions and stirred at 45 degrees Celsius, the biomolecules reduced zinc ions to metallic zinc, which reacted with dissolved oxygen to form ZnO nuclei, while other phytochemicals capped the growing particles and kept them from clumping. Notably, the team skipped the energy-intensive calcination step that most green syntheses employ, arguing that high-temperature treatment often fails to remove organic impurities anyway and promotes agglomeration that enlarges the particles and shrinks their surface area.

Skipping calcination did not compromise quality. Powder X-ray diffraction confirmed a phase-pure hexagonal wurtzite structure matching the standard reference card, with an average crystallite size of about 20 nanometers, smaller than typically reported for calcinated green-synthesized ZnO. Scanning electron microscopy showed flower-like agglomerates and, at higher magnification, the hexagonal prisms characteristic of wurtzite crystals. Energy-dispersive X-ray spectroscopy detected only zinc, oxygen and carbon, the latter attributable to the sample grid and residual capping phytochemicals, with no foreign elements such as potassium, calcium or sodium that often contaminate calcinated phytosynthesized samples. Zinc and oxygen accounted for 56.48 and 23.15 weight percent respectively. Optical measurements placed the ZnO band gap at 3.5 electronvolts, with the characteristic absorption peak at 350 nanometers confirming nano-scale rather than bulk particles, while the porphyrin’s sharp Soret band at 418 nanometers corresponded to a 2.9 electronvolt gap.

The photocatalytic tests were conducted outdoors under real subtropical sunlight in Delhi between late April and early May, during the pre-monsoon peak of solar intensity. Dye solutions of 10 micromolar rhodamine B were monitored by the decay of the absorption peak near 550 nanometers. As irradiation proceeded, the peak intensity fell and shifted toward shorter wavelengths, a signature of stepwise de-ethylation: reactive oxygen species strip the N-ethyl groups one by one, reducing electron density on the conjugated chromophore with each removal. The solutions faded progressively but did not turn completely colorless, indicating that the central xanthene ring largely survived under these conditions even as the ethyl groups were stripped away.

The performance hierarchy was clear. Porphyrin alone managed 50.87 percent degradation, hampered by fast recombination of its photoinduced electron-hole pairs. Bare ZnO nanoparticles reached 67.42 percent, benefiting from their large surface area and abundant reactive sites. The ZnO@P1 composite led at 76.12 percent, 1.13 times better than ZnO and 1.5 times better than P1. Pseudo-first-order kinetic analysis gave rate constants of 5.93 x 10-3 per minute for P1, 9.36 x 10-3 for ZnO and 1.20 x 10-2 for the composite. The mechanism behind the synergy is a classic heterojunction story: the lower-band-gap porphyrin absorbs visible photons, and its excited electrons migrate to the conduction band of the higher-band-gap ZnO while holes flow the opposite way, achieving spatial separation of charge carriers. With recombination suppressed, more electrons and holes survive to react with water and oxygen, generating the hydroxyl radicals and superoxide anion radicals that actually dismantle the dye molecules toward carbon dioxide and water.

Compared with published ZnO-porphyrin composites, the new system looks modest at first glance. Cardanol-based H2Pp-ZnO and CuPp-ZnO composites degraded 98 percent of rhodamine B in 30 minutes under tropical sunlight, a ZnO/TAPPI-CoTPPS system achieved complete degradation in 120 minutes under simulated light, and a ZnO/TCPP composite reached 97 percent in 180 minutes. But the dosage comparison changes the picture: the Delhi composite ran at 0.1 grams per liter, roughly ten times less catalyst than the cardanol systems and twenty times less than the TAPPI-CoTPPS system for the same dye concentration, and it did so under natural rather than simulated illumination, where passing clouds can interrupt the light supply. The authors suggest the composite could achieve further degradation with extended irradiation. As a proof of concept for on-demand, low-energy composite formation from plant-derived nanomaterials and green-synthesized porphyrins, the work points toward a future where wastewater treatment catalysts are grown, not manufactured, and assembled exactly when and where they are needed.

Subject of Research: Green synthesis of a ZnO-porphyrin composite for solar-light photocatalytic degradation of rhodamine B dye in water

Article Title: Eco-friendly ZnO nanoparticles coupled with porphyrin for enhanced solar light driven degradation of rhodamine B

Article References: Anushikha, & Dechan, P. (2026). Eco-friendly ZnO nanoparticles coupled with porphyrin for enhanced solar light driven degradation of rhodamine B. Results in Chemistry, 31, Article 103903. https://doi.org/10.1016/j.rechem.2026.103903

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103903

Keywords: zinc oxide nanoparticles, porphyrin, photocatalysis, rhodamine B, green synthesis, Morus alba, solar light, dye degradation, wastewater treatment, heterojunction, reactive oxygen species, sustainable chemistry