Porous Anodic Oxide (PAO) Films

PAO 101

Anodic oxide films are formed electrochemical oxidation of metals such as aluminum and titanium in a solution where oxide is moderately soluble.  These films undergoes four distinct morphological stages during anodizing;

  1. Formaiton of barrier oxide films upon anodic oxidation ( upon anodizing)
  2. Perturbations in the the film/electrolyte interface (low potential)
  3. Initiation of pores (around potential peak)
  4. Formation of fully-grown porous anodic oxide films (once potential becomes constant)

Image reference: Ö. Özgür Çapraz et al. , Electrochim. Acta, 2017, link
PAO films have been used extensively to design numerous devices for optical, catalytic, and biological and energy related applications, due to their vertically aligned-geometry, high-specific surface area and tunable geometry by adjusting process variables.  However, specific processes leading to porous film formation include the mechanism of surface instability, which leads to pore initiation, and the influence of the experimental conditions on PAO morphology are not well understood.

Phase-Shifting Curvature Interferometry 
A new in situ high-resolution curvature interferometry technique was developed to measure stress caused by electrochemical processes. 

Unlike other curvature measurement techniques, phase-shifting curvature interferometry can be applied to both thin and bulk samples due to high-resolution and this feature of the technique increased the applicability of the stress measurements.
Ö. Özgür Çapraz, Kurt Hebert and Pranav Shrotriya,  J. Electrochem. Soc. 2013,  160, 11, D501-D506, doi: 10.1149/2.025311jes. Link

Novel Method to Reveal Stress Distribution in Oxide 
A new experimental method was developed to reveal the stress distribution in barrier anodic oxide film ​ in order to discriminate the potential stress mechanisms during anodizing.
Measuring the stress profiles using the method provided remarkable information about transport processes and driving forces governing the interface motion in anodic films.

Ö. Özgür Çapraz, Kurt Hebert and Pranav Shrotriya, J. Electrochem.  Soc.  2014,  161, 5, D256-D262, doi: 10.1149/2.057405jes. Link
Factors Controlling Stress Generation in Porous Oxide Films   
Stress generation was correlated with interfacial volume change due to reactions and transport processes: oxide or interface stress is compressive when interfacial volume is created, and vice versa.

A simple mathematical criterion was derived specifying the conditions for compressive stress and pore formation in terms of parameters governing film composition, ionic transport and interfacial reaction kinetics.

Ö. Özgür Çapraz, Pranav Shrotriya, Peter Skeldon, George E.Thompson, and Kurt R.Hebert,  Electrochim. Acta,  2015, 159, 22,  doi: 10.1016/j.electacta.2015.01.183, link
Role of Stress in the Initial Growth of Porous Oxide Films    
Stress profiles reveal buildup of compressive stress in the oxide near the oxide-solution interface.

The surface compressive stress appears to coincide with enhanced concentrations of incorporated anions close to the solution interface.
Pores initiate 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 non-uniform near-surface compressive stress.  

Ö. Özgür Çapraz, Pranav Shrotriya, Peter Skeldon, George E.Thompson, and Kurt R.Hebert, Electrochim. Acta, 2015, 167, 404-411, doi: 10.1016/j.electacta.2015.03.017,  link
Anion Incorporation in Porous Oxide Films

Combination of in situ stress measurements and X-ray photoelectron spectroscopy showed the evidence for anion incorporation-induced stress in Al oxide films, and accumulation of such stress during anodic oxidation.

Anion incorporation before anodizing can stimulate large tensile stress relaxations during the initial formation of pores, can be associated with the formation of metal vacancies.

Ö. Özgür Çapraz, Quentin Van Overmeere, Pranav Shrotriya and Kurt Hebert, Electrochim. Acta, 2017, 238, 368-374, doi: 10.1016/j.electacta.2017.04.014, link
Role of Oxide Defects on Pore Formation

Stress measurements revealed that pore initiation introduces tensile residual stress into the oxide at the pore base, suggesting the formation of vacancy-type defects in the film. 

The defects induced large current increase can be apparently responsible for growth of concave scalloped features on the metal-oxide interface at the pore base.
Shinsuke Ide, Ö. Özgür Çapraz, Pranav Shrotriya and Kurt Hebert, Electrochim. Acta, 2017, 238, 368-374, doi: 10.1016/j.electacta.2017.02.113, link