Creating reactive interfaces via anion-guided plasma electrolytic oxidation and tryptophan–TEOS hybrid coating for enhanced photocatalytic performance

Abstract

Organic–inorganic hybrid coatings offer a promising strategy to enhance photocatalytic activity by engineering surface chemistry and interfacial properties. In this work, hybrid coatings were fabricated on AZ31 magnesium alloy through a two-step process combining plasma electrolytic oxidation (PEO) and hydrothermal treatment with L-tryptophan (Trp) and tetraethyl orthosilicate (TEOS). Three different electrolytes, namely phosphate, aluminate, and silicate, were used during PEO to form MgO layers enriched with Mg₃(PO₄)₂, MgAl₂O₄, and Mg₂SiO₄, respectively. These oxide layers acted as chemically active platforms for subsequent hybrid layer formation. Among all samples, MgO-Si-LT exhibited the most favorable characteristics, notably reduced porosity (~ 4.47%) and the highest average pore diameter (~ 1.69 µm). It also showed the strongest Si signal, confirming dual silica incorporation from both the electrolyte and TEOS. In parallel, MgO-Si-LT revealed the most chemically diverse nitrogen states, suggesting stronger Trp coordination and enhanced interfacial stabilization. Photocatalytically, MgO-Si-LT significantly achieved 98.01% degradation of crystal violet (CV) under visible light within 120 min and notably retained 96.2% efficiency after five cycles. DFT calculations revealed that the Mg₂SiO₄ surface forms the strongest interaction with the Trp-TEOS complex, supporting enhanced charge transfer and photocatalytic efficiency. These results highlight the crucial role of anion-guided oxide chemistry in stabilizing hybrid layers and demonstrate a scalable strategy for designing high-performance photocatalytic surfaces.