Role of glycine in porous alumina growth and barrier layer properties in oxalic acid electrolytes

Porous anodic alumina with controlled morphology and enhanced corrosion resistance is of high practical importance in nanotechnology. However, the effect of simple amino acid additives on anodic oxide growth in conventional anodizing electrolytes, including oxalic, sulfuric, and phosphoric acids, remains poorly understood. In this study, we demonstrate that glycine (Gly)-modified anodizing of aluminum in 0.3 M oxalic acid containing 0.1-0.5 M Gly enables control over the morphology of porous alumina without altering its composition. The addition of Gly shifts oxide formation toward higher potentials, reduces the anodizing current density, and prolongs the initial stage of oxide growth. SEM analysis indicates that Gly suppresses oxide dissolution near the surface, thereby significantly reducing pore diameter while maintaining internal pore ordering and film thickness. The barrier layer was characterized using a re-anodizing technique in a boric acid electrolyte, revealing a lower density of electron traps in films formed in electrolytes containing 0.5 M Gly. The addition of Gly does not affect the film composition, as the carbon content remains unchanged (ca. 2.8 mass%), and no nitrogen incorporation was detected, indicating that Gly does not incorporate into the oxide matrix. Instead, Gly acts at the oxide/electrolyte interface by complexing with Al3+ ions, thereby modifying interfacial processes and ion transport, which in turn suppresses oxide dissolution near the surface. These results demonstrate that glycine is an effective and environmentally friendly additive for controlling anodic alumina growth, surface morphology, and barrier layer properties.