Role of Oxide Stress in the Initial Growth of Self-Organized Porous Aluminum Oxide

Porous anodic oxide films are formed by electrochemical oxidation of aluminum, titanium and other metals in solutions in which the oxide is soluble. Anodic films produced at particular process conditions contain spontaneously ordered hexagonal arrangements of submicron-diameter cylindrical pores. These self-organized porous anodic oxides have been studied extensively as templates for nanostructured devices and as functional materials.^1-2 For example, much recent work has explored the use of TiO₂ nanotubes in dye-sensitized solar cells, photocatalysis and sensors. Understanding of the oxide growth and metal dissolution processes that control the porous oxide morphology has enabled the improvement of these devices. Further advancements in the design and processing of anodic oxides will be possible by development in models based on mathematical descriptions of these processes.^3,4Ultimately, models should be able to predict the bath compositions and anodizing voltages in which self-organized oxides are produced, and the scaling relations relating processing conditions to porous layer geometry. The experiments reported here focus on identifying the critical factors that govern pore initiation, so as to guide the development of anodizing models. 

Recent work has introduced pore formation mechanisms in which oxide stress plays an important role. Compressive stress is considered to be elevated near the pore base, where the current density is high. Stress-driven transport of oxide toward the pore walls is thought to assist pore formation. In the reported work, the role of oxide stress in the initiation in anodic aluminum oxide was investigated, for constant current anodizing of aluminum in phosphoric acid.^5-8 Using phase-shifting curvature interferometry, through-thickness profiles of the in-plane stress in the oxide were measured by in-situ monitoring of stress change during both oxide growth, open circuit dissolution following anodizing. During barrier oxide growth prior to pore formation, compressive stress accumulated to several GPa within a 3-5 nm thick layer at the oxide surface, while stress in the interior of the oxide was relaxed. Oxide composition measurements revealed elevated concentrations of incorporated phosphate ions in the same region, indicating that stress is generated by field-driven anion incorporation. Pore initiation occurs when surface stress reaches a maximum, and is accompanied by oxide flow establishing the pore shape. It is suggested that pores are created by a flow instability caused by spatially nonuniform near-surface compressive stress caused by anion incorporation. Anion-induced stress can explain why the acid anion type determines the length scales of self-organized porous anodic films.¹

ACKNOWLEDGMENT

This work was supported by the National Science Foundation through NSF-CMMI-100748.

REFERENCES

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Figure 1. Residual stress profiles in oxides formed to various voltages at 5 mA/cm² in 0.4 M H₃PO₄.⁸ The range of voltages corresponds to the initial stage of barrier oxide growth prior to pore initiation, which occurs at about 100 V.