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Does the Catalyst Always Assist the Methylene Blue Photodegradation or Does It Sometimes Hinder It?

  • Chiara Alessandrello 1,   
  • Zainab Dahrouch 1,   
  • Salvatore Maida 1,   
  • Moreno d'Ambrosio 2,   
  • Salvatore Patanè 2,3,*,   
  • Saveria Santangelo 1,3,*,   
  • Claudia Triolo 1,3

Received: 17 Nov 2025 | Revised: 23 Dec 2025 | Accepted: 04 Jan 2026 | Published: 02 Mar 2026

Abstract

Lattice defects, particularly oxygen vacancies, are commonly considered beneficial to photocatalytic performance, as they can reduce the bandgap, broaden the light absorption range, and suppress electron/hole recombination. In this perspective, high-entropy oxides (HEOs) could be considered “ideal” materials for photocatalysis because the random distribution of multiple metal cations within the crystalline lattice generates intrinsic structural disorder, which can be modulated by varying their composition and synthesis conditions. This work studies the photocatalytic activity of electrospun Cr–Mn–Fe–Co–Ni-based HEOs for the degradation of methylene blue in aqueous solution. The selected preparation conditions (calcination in air at 400 °C for 2 h) yield nanofibers consisting of ultra-small grains (≤9 nm) with copious oxygen vacancies on their surface. The presented results raise an important question: does the catalyst always promote the dye photodegradation or does it sometimes hinder it? A critical discussion is proposed, highlighting the case in which defects can induce competitive recombination pathways, negatively impacting the material’s photocatalytic performance.

References 

  • 1.

    Khan, I.; Saeed, K.; Zekker, I.; et al. Review on Methylene Blue: Its Properties, Uses, Toxicity and Photodegradation. Water 2022, 14, 242. https://doi.org/10.3390/w14020242.

  • 2.

    Shah, D.I.; Santani, D.D.; Goswami, S.S. A Novel Use of Methylene Blue as a Pharmacological Tool. J. Pharmacol. Toxicol. Methods 2006, 54, 273–277. https://doi.org/10.1016/j.vascn.2005.12.003.

  • 3.

    Kathi, S.; El Din Mahmoud, A. Trends in Effective Removal of Emerging Contaminants from Wastewater: A Comprehensive Review. Desal. Water Treat. 2024, 317, 100258. https://doi.org/10.1016/j.dwt.2024.100258.

  • 4.

    Siong, V.L.E.; Lee, K.M.; Juan, J.C.; et al. Removal of Methylene Blue Dye by Solvothermally Reduced Graphene Oxide: A Metal-Free Adsorption and Photodegradation Method. RSC Adv. 2019, 9, 37686–37695. https://doi.org/10.1039/C9RA05793E.

  • 5.

    Jiang, G.; Zheng, X.; Wang, Y.; et al. Photo-Degradation of Methylene Blue by Multi-Walled Carbon Nanotubes/TiO2 Composites. Powder Technol. 2011, 207, 465–469. https://doi.org/10.1016/j.powtec.2010.11.029.

  • 6.

    Berehe, B.A.; Desalew, A.A.; Derbe, G.W.; et al. Enhanced Photocatalytic Degradation of Methylene Blue Dye via Valorization of a Polyethylene Terephthalate Plastic Waste-Derived Metal–Organic Framework-Based ZnO@Co-BDC Composite Catalyst. Nanoscale Adv. 2025, 7, 3834–3845. https://doi.org/10.1039/D4NA01071J.

  • 7.

    Shah, S.S.; Ramos, B.; Otubo, L.; et al. Deep Removal and Photodegradation of Methylene Blue Dye Using Superabsorbent Polymer Hydrogel Composite with Activated Charcoal and TiO2 Nanoparticles. ACS Omega 2025, 10, 26441–26457. https://doi.org/10.1021/acsomega.4c11428.

  • 8.

    Nasrollahpour, A.; Moradi, S.E. Photochemical Degradation of Methylene Blue by Metal Oxide-Supported Activated Carbon Photocatalyst. Desal. Water Treat. 2016, 57, 8854–8862. https://doi.org/10.1080/19443994.2015.1035675.

  • 9.

    Chatterjee, D.; Dasgupta, S. Visible Light Induced Photocatalytic Degradation of Organic Pollutants. J. Photochem. Photobiol. C 2005, 6, 186–205. https://doi.org/10.1016/j.jphotochemrev.2005.09.001.

  • 10.

    Petrovičová, B.; Dahrouch, Z.; Triolo, C.; et al. Photocatalytic Degradation of Methylene Blue Dye by Electrospun Binary and Ternary Zinc and Titanium Oxide Nanofibers. Appl. Sci. 2021, 11, 9720. https://doi.org/10.3390/app11209720.

  • 11.

    Pantò, F.; Dahrouch, Z.; Saha, A.; et al. Photocatalytic Degradation of Methylene Blue Dye by Porous Zinc Oxide Nanofibers Prepared via Electrospinning: When Defects Become Merits. Appl. Surf. Sci. 2021, 557, 149830. https://doi.org/10.1016/j.apsusc.2021.149830.

  • 12.

    Guillén, C. Determination of the Band Gap Energy of SnO2 and ZnO Thin Films with Different Crystalline Qualities and Doping Levels. Electron. Mater. 2025, 6, 3. https://doi.org/10.3390/electronicmat6010003.

  • 13.

    Khattab, E.-S.R.; Abd El Rehim, S.S.; Hassan, W.M.I.; et al. Band Structure Engineering and Optical Properties of Pristine and Doped Monoclinic Zirconia (m-ZrO2): Density Functional Theory Theoretical Prospective. ACS Omega 2021, 6, 30061–30068. https://doi.org/10.1021/acsomega.1c04756.

  • 14.

    Wetchakun, N.; Chainet, S.; Phanichphant, S.; et al. Efficient Photocatalytic Degradation of Methylene Blue over BiVO4/TiO2 Nanocomposites. Ceram. Int. 2015, 41, 5999–6004. https://doi.org/10.1016/j.ceramint.2015.01.040.

  • 15.

    Girotto, G.Z.; Thill, A.S.; Matte, L.P.; et al. Ni/SrTiO3 Nanoparticles for Photodegradation of Methylene Blue. ACS Appl. Nano Mater. 2022, 5, 13295–13307. https://doi.org/10.1021/acsanm.2c03007.

  • 16.

    Tahir, M.N. Synthesis of Hierarchically Organized α-Fe2O3 Nanostructures for the Photocatalytic Degradation of Methylene Blue. Emergent Mater. 2020, 3, 605–612. https://doi.org/10.1007/s42247-020-00127-9.

  • 17.

    DePuccio, D.P.; Botella, P.; O’Rourke, B.; et al. Degradation of Methylene Blue Using Porous WO3, SiO2–WO3, and Their Au-Loaded Analogs: Adsorption and Photocatalytic Studies. ACS Appl. Mater. Interfaces 2015, 7, 1987–1996. https://doi.org/10.1021/am507806a.

  • 18.

    Bai, S.; Zhang, N.; Gao, C.; et al. Defect Engineering in Photocatalytic Materials. Nano Energy 2018, 53, 296–336. https://doi.org/10.1016/j.nanoen.2018.08.058.

  • 19.

    Senthamizhan, A.; Balusamy, B.; Aytac, Z.; et al. Grain Boundary Engineering in Electrospun ZnO Nanostructures as Promising Photocatalysts. CrystEngComm 2016, 18, 6341–6351. https://doi.org/10.1039/C6CE00693K.

  • 20.

    Sarkar, A.; Wang, Q.; Schiele, A.; et al. High-Entropy Oxides: Fundamental Aspects and Electrochemical Properties. Adv. Mater. 2019, 31, 1806236. https://doi.org/10.1002/adma.201806236.

  • 21.

    Sun, Y.; Wu, T.; Bao, Z.; et al. Defect Engineering of Ceria Nanocrystals for Enhanced Catalysis via a High-Entropy Oxide Strategy. ACS Cent. Sci. 2022, 8, 1081–1090. https://doi.org/10.1021/acscentsci.2c00340.

  • 22.

    Jia, D.; Chigan, T.; Li, X.; et al. Photocatalytic Degradation Performance for High-Entropy Oxide (La0.2Ce0.2Gd0.2Zr0.2FeX)O2 Enriched with Defects. J. Alloys Compd. 2024, 982, 173808. https://doi.org/10.1016/j.jallcom.2024.173808.

  • 23.

    Pradhan, S.; Barick, K.C.; Anuraag, N.S.; et al. Development of Ferrimagnetic (Mn0.6Cr0.6Co0.6Fe0.6Al0.6)O4 High Entropy Oxide for Photo-Fenton Degradation of Organic Dye. Appl. Surf. Sci. 2025, 688, 162343. https://doi.org/10.1016/j.apsusc.2025.162343.

  • 24.

    Zhang, F.; Zhou, W.; Zhang, Y.; et al. Spectroscopic Analyses and Photocatalytic Properties of Transition Group Metal Oxide Films with Different Entropy Values. Mater. Sci. Semicond. Process. 2024, 169, 107928. https://doi.org/10.1016/j.mssp.2023.107928.

  • 25.

    Du, M.; Liu, S.; Ge, Y.; et al. Preparation and Effect of Grain Size on the Thermal Stability, Phase Transition, Mechanical Property, and Photocatalytic Property of Pyrochlore (La0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 High-Entropy Oxide. Ceram. Int. 2022, 48, 20667–20674. https://doi.org/10.1016/j.ceramint.2022.04.046.

  • 26.

    Triolo, C.; Moulaee, K.; Ponti, A.; et al. Spinel-Structured High-Entropy Oxide Nanofibers as Electrocatalysts for Oxygen Evolution in Alkaline Solution: Effect of Metal Combination and Calcination Temperature. Adv. Funct. Mater. 2024, 34, 2306375. https://doi.org/10.1002/adfm.202306375.

  • 27.

    Vezzù, K.; Triolo, C.; Moulaee, K.; et al. Interplay Between Calcination Temperature and Alkaline Oxygen Evolution of Electrospun High-Entropy (Cr1/5Mn1/5Fe1/5Co1/5Ni1/5)3O4 Nanofibers. Small 2025, 21, 2408319. https://doi.org/10.1002/smll.202408319.

  • 28.

    Triolo, C.; Moulaee, K.; Bellato, F.; et al. Interplay between Alkaline Water Oxidation Temperature, Composition and Performance of Electrospun High-Entropy Non-Equimolar (Cr,Mn,Fe,Co,Ni) Oxide Electrocatalysts. J. Power Sources 2025, 654, 237887. https://doi.org/10.1016/j.jpowsour.2025.237887.

  • 29.

    Patriarchi, A.; Triolo, C.; Minnetti, L.; et al. Evaluation of High-Entropy (Cr, Mn, Fe, Co, Ni)-Oxide Nanofibers and Nanoparticles as Passive Fillers for Solid Composite Electrolytes. Electrochim. Acta 2025, 512, 145425. https://doi.org/10.1016/j.electacta.2024.145425.

  • 30.

    Triolo, C.; Patriarchi, A.; Darjazi, H.; et al. Electrospun Spinel-Structured High-Entropy Oxide Nanofibers as Fillers for Solid Polymer Electrolytes: The Pivotal Role of Oxygen Vacancies. IL Nuovo Cimento C 2025, 48, 246. https://doi.org/10.1393/ncc/i2025-25246-3.

  • 31.

    Altomare, A.; Cuocci, C.; Giacovazzo, C.; et al. QUALX: A Computer Program for Qualitative Analysis Using Powder Diffraction Data. J. Appl. Crystallogr. 2008, 41, 815–817. https://doi.org/10.1107/S0021889808016956.

  • 32.

    Altomare, A.; Corriero, N.; Cuocci, C.; et al. QUALX2.0: A Qualitative Phase Analysis Software Using the Freely Available Database POW_COD. J. Appl. Crystallogr. 2015, 48, 598–603. https://doi.org/10.1107/S1600576715002319.

  • 33.

    Heger, D.; Jirkovský, J.; Klán, P. Aggregation of Methylene Blue in Frozen Aqueous Solutions Studied by Absorption Spectroscopy. J. Phys. Chem. A 2005, 109, 6702–6709. https://doi.org/10.1021/jp050439j.

  • 34.

    Sur, S.; Vekariya, N.; Jha, S.K.; et al. High Entropy Spinel Oxide for Natural Sunlight-Driven Photocatalytic Degradation of Methylene Blue: A Sustainable Water Remediation Process. Results Surf. Interfaces 2025, 19, 100549. https://doi.org/10.1016/j.rsurfi.2025.100549.

  • 35.

    Kalaycıoğlu, Z.; Özuğur Uysal, B.; Pekcan, Ö.; et al. Efficient Photocatalytic Degradation of Methylene Blue Dye from Aqueous Solution with Cerium Oxide Nanoparticles and Graphene Oxide-Doped Polyacrylamide. ACS Omega 2023, 8, 13004–13015. https://doi.org/10.1021/acsomega.3c00198.

  • 36.

    Talebian, N.; Nilforoushan, M.R. Comparative Study of the Structural, Optical and Photocatalytic Properties of Semiconductor Metal Oxides toward Degradation of Methylene Blue. Thin Solid Films 2010, 518, 2210–2215. https://doi.org/10.1016/j.tsf.2009.07.135.

  • 37.

    Zafar, Z.; Yi, S.; Li, J.; et al. Recent Development in Defects Engineered Photocatalysts: An Overview of the Experimental and Theoretical Strategies. Energy Environ. Mater. 2022, 5, 68–114. https://doi.org/10.1002/eem2.12171.

  • 38.

    Tsubota, H.; Jitianu, A.; Kawamura, G. Recent Advances in High-Entropy Oxides for Photocatalytic Applications. ACS Mater. Lett. 2025, 7, 1042–1056. https://doi.org/10.1021/acsmaterialslett.4c02323.

  • 39.

    Ali, M.A.; Maafa, I.M.; Qudsieh, I.Y. Photodegradation of Methylene Blue Using a UV/H2O2 Irradiation System. Water 2024, 16, 453. https://doi.org/10.3390/w16030453.

  • 40.

    Kato, K.; Uemura, Y.; Asakura, K.; et al. Role of Oxygen Vacancy in the Photocarrier Dynamics of WO3 Photocatalysts: The Case of Recombination Centers. J. Phys. Chem. C 2022, 126, 9257–9263. https://doi.org/10.1021/acs.jpcc.2c01662.

  • 41.

    Rezaei, M.; Nezamzadeh-Ejhieh, A.; Massah, A.R. A Comprehensive Review on the Boosted Effects of Anion Vacancy in the Heterogeneous Photocatalytic Degradation, Part II: Focus on Oxygen Vacancy. ACS Omega 2024, 9, 6093–6127. https://doi.org/10.1021/acsomega.3c07560.

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Supplementary Materials
DOI
10.53941/ps.2026.100007
Issue
Volume 2, Issue 1
How to Cite
Alessandrello, C.; Dahrouch, Z.; Maida, S.; d’Ambrosio, M.; Patanè, S.; Santangelo, S.; Triolo, C. Does the Catalyst Always Assist the Methylene Blue Photodegradation or Does It Sometimes Hinder It?. Photochemistry and Spectroscopy 2026, 2 (1), 7. https://doi.org/10.53941/ps.2026.100007.
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