The study, which paired the experimental work with electromagnetic simulations in COMSOL Multiphysics, was motivated by a persistent environmental problem. Industries that manufacture textiles, paper, leather and related products discharge large volumes of colored wastewater containing residual dyes, which pose hazards to aquatic ecosystems in lakes, rivers and oceans. Photocatalysis has attracted attention as a remediation strategy because it can degrade and mineralize pollutants under ambient conditions, using light-activated semiconductors to generate reactive species that attack organic molecules. The efficiency of any such photocatalyst, however, is constrained by the recombination of photogenerated charge carriers, and much of modern photocatalyst research focuses on tuning band-gap energy, surface area and morphology to suppress that recombination.

All three oxide nanoparticles were prepared through the same co-precipitation route. Zinc nitrate hexahydrate, magnesium chloride hexahydrate and copper nitrate hexahydrate were each dissolved in distilled water and treated dropwise with sodium hydroxide while the pH was held between 10 and 11. The resulting precipitates were washed with deionized water and ethanol, dried at 80 degrees Celsius and calcined at 550 degrees Celsius for two hours. The choice of co-precipitation reflects its reputation as a reliable and cost-effective route to phase-pure oxide powders, and using an identical synthesis protocol for all three materials allowed the team to attribute differences in performance to the intrinsic properties of each oxide rather than to preparation effects.

Scanning electron microscopy revealed distinct morphological personalities among the three powders. The ZnO nanoparticles were fairly uniform, highly dispersed spheres with an average particle size of 83.5 nanometers and closely packed, smooth surfaces suggesting good crystallinity. MgO particles, averaging 43 nanometers, were also roughly spherical but showed a greater tendency to agglomerate, a common consequence of the high surface-to-volume ratio and surface energy of nanoparticles. CuO produced a 73-nanometer average particle size but displayed the most aggregated and irregular distribution, with lower dispersion and surface uniformity than either competitor. Energy-dispersive X-ray spectroscopy confirmed the presence of the expected constituent elements in each sample, with the carbon signals detected in the spectra traced to the carbon tape used for mounting rather than to the nanoparticles themselves.

X-ray diffraction pinned down the crystal structures and crystallite dimensions. The ZnO pattern matched the hexagonal wurtzite structure, with lattice parameters of a = b = 3.2568 angstroms and c = 5.2125 angstroms, and a mean crystallite size of 30.6 nanometers calculated from the (101) reflection using the Debye-Scherrer equation. MgO adopted the face-centered cubic rock-salt structure with a lattice parameter of 4.2200 angstroms and the smallest crystallite size of the set at 15.4 nanometers. CuO crystallized in the monoclinic phase, with lattice parameters a = 4.6927, b = 3.4283 and c = 5.1370 angstroms and a crystallite size of 28.6 nanometers. No unassigned diffraction peaks appeared in any pattern, supporting the formation of single crystalline phases, although the authors note that the absence of extra peaks does not exclude trace impurities or amorphous material.

Optical measurements proved central to explaining the performance gap. UV-visible spectroscopy combined with Tauc-plot analysis yielded direct band-gap energies of 3.0 electronvolts for ZnO, 4.9 electronvolts for MgO and 5.5 electronvolts for CuO. ZnO, with the lowest band gap, also showed the highest optical absorbance in the ultraviolet region. Photoluminescence spectroscopy, in which a stronger emission signal indicates faster radiative recombination of photogenerated electron-hole pairs, showed that ZnO had significantly lower intensity than the other two oxides, suggesting a reduced tendency for radiative recombination. The authors are careful to point out that photoluminescence alone cannot conclusively establish enhanced charge separation or longer carrier lifetimes, since defect trapping, surface properties and crystallinity also influence emission, and that direct confirmation would require techniques such as time-resolved photoluminescence or photocurrent measurements. Fourier-transform infrared spectroscopy confirmed the characteristic metal-oxygen stretching vibrations at 531, 543 and 520 wavenumbers per centimeter for ZnO, MgO and CuO respectively, along with surface hydroxyl groups, adsorbed water and carbonate species.

The photocatalytic tests were run under tightly matched conditions. A 20 milligram-per-liter methyl red solution was prepared, and 100-milliliter aliquots were mixed with 0.002 grams of each catalyst, a loading of 0.02 grams per liter. Suspensions were stirred in the dark for 30 minutes to establish adsorption-desorption equilibrium before being irradiated for 120 minutes by a 450-watt continuous-wave xenon arc lamp positioned 15 centimeters away at an intensity of 1.0 milliwatt per square centimeter. Absorbance at the dye’s maximum absorption wavelength of 520 nanometers was monitored at 30-minute intervals. Fitting the concentration data to a pseudo-first-order kinetic model gave apparent rate constants of 0.0147 per minute for ZnO, 0.00957 per minute for MgO and 0.00842 per minute for CuO, with coefficients of determination between roughly 0.93 and 0.99. The authors stress that these constants describe behavior under the specified conditions and should not be read as intrinsic material properties.

Additional experiments probed the mechanism and reusability of the best performer. Zeta-potential analysis of ZnO gave a principal peak at approximately +9.92 millivolts, indicating a positive electrokinetic potential under the measurement conditions. Over five successive photocatalytic cycles, ZnO’s degradation efficiency declined from 85 percent to 82.7, 77.0, 74.2 and finally 70.9 percent, a gradual decay the team attributes tentatively to partial catalyst recovery losses, loss of active sites or accumulation of intermediates, none of which was independently investigated. Radical-scavenging tests showed that adding methanol or isopropanol cut degradation sharply to 30.1 percent and 47.0 percent respectively, while ascorbic acid produced a maller drop to 70.6 percent, supporting the involvement of reactive species in the degradation while stopping short of establishing the individual contributions of hydroxyl radicals, superoxide radicals and photogenerated holes. The proposed reaction sequence, in which photogenerated holes and electrons produce hydroxyl and superoxide radicals that break the azo bond and aromatic structures of the dye, is presented as a qualitative interpretation rather than definitive proof of every pathway.

The modeling component distinguished this study from most prior single-catalyst reports. A two-dimensional COMSOL Multiphysics RF model simulated a stack of glass substrate layers, a 100-nanometer active layer containing the metal oxide nanoparticles, and dye-solution domains, with wavelength-dependent complex optical constants, periodic lateral boundaries and a perfectly matched layer to suppress artificial reflections. The simulated optical responses followed the order ZnO greater than MgO greater than CuO, mirroring the experimental degradation efficiencies and the kinetic rate constants. The authors emphasize that the simulated quantity is an electromagnetic response rather than a direct measurement of dye concentration, so the simulation was used to establish a consistent trend rather than to calculate degradation numbers directly. Band-edge positions estimated from the optical band gaps placed the conduction and valence bands of ZnO at -0.21 and +2.79 volts versus normal hydrogen electrode, with corresponding estimates of -1.75/+3.15 volts for MgO and -1.44/+4.06 volts for CuO.

The practical takeaway is a controlled benchmark rather than a recipe for immediate deployment. The team notes that methyl red removal figures reported across the literature vary widely because studies differ in catalyst composition, loading, dye concentration, irradiation source and duration, so cross-study comparisons serve as benchmarks rather than direct rankings. What this work adds is the rare combination of three pure metal oxides tested side by side under identical conditions, quantified kinetics, radical-scavenging evidence and a common electromagnetic modeling framework, all pointing the same way: among ZnO, MgO and CuO prepared by co-precipitation, zinc oxide’s favorable optical properties, uniform spherical morphology and lower radiative recombination tendency translate into the fastest and most complete degradation of methyl red under the conditions tested. Long-term stability and the causes of the cycle-to-cycle decline remain open questions for future work.

Subject of Research: Comparative photocatalytic degradation of methyl red dye using ZnO, MgO and CuO nanoparticles with COMSOL electromagnetic modeling

Article Title: Comparative analysis of ZnO, MgO and CuO nanoparticles (NPs) in the photocatalytic degradation of methyl red: An experimental and COMSOL-based study

Article References: Waqar, A., Fiaz, S., Iqbal, T., Seemab, M., & Hussain, A. (2026). Comparative analysis of ZnO, MgO and CuO nanoparticles (NPs) in the photocatalytic degradation of methyl red: An experimental and COMSOL-based study. Results in Chemistry, 31, Article 104012. https://doi.org/10.1016/j.rechem.2026.104012

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.104012

Keywords: zinc oxide nanoparticles, magnesium oxide, copper oxide, photocatalysis, methyl red, dye degradation, wastewater treatment, co-precipitation synthesis, band gap energy, COMSOL simulation, reaction kinetics, radical scavenging