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What is Anodic Aluminum Oxide (AAO)?

Nanoporous Anodic Aluminum Oxide or AAO  (also known as Porous Aluminum Oxide – PAO, or NanoPorous Alumina Membranes – NPAM) is a self-organized material with a highly uniform structure resembling nanoscale “honeycomb”.  AAO is a popular template for bottom-up nanofabrication and is widely used in nanotechnology, life science, separation and other research areas. Read more about unique features and benefits of AAO  or visit InRedox’ Nanofabrication Toolbox to review AAO products.

What are Free-Standing AAO Wafers?

Nanoporous AAO Wafers are free-standing disks or rectangles of AAO that are thick enough to handle (20 to 200 µm).  The nanopores in these products go all the way from one face to another, without a normally present dense barrier layer at the bottom of the pores.  This form factor is the most common choice for many applications.  Visit the product page for AAO wafers or AAO wafer variety packs to purchase these products:

What form factors are possible for AAO?

The most common form factors for AAO are:

  1. AAO films attached to Al substrates.
    Conformal nanoporous AAO coatings are formed on the surface of Al in standard anodization processes, when Al is electrochemically oxidized in certain acidic electrolytes.  Resulting AAO films are firmly adhered to the substrate, which cold be Al foil, sheet, tube or any other shape or components.  Anodization is commonly used to provide corrosion and mechanical protection to Al products, as we  as for decorative purposes.  AAO films on Al are also popular as nanotemplates, especially when thin AAO, which is too fragile to be available as AAO wafer, is required.  At the bottom of the pores, a dense layer of alumina (aka the barrier layer) is inherently present. This barrier layer is electrically isolating and could be “thinned” to facilitate electrochemical deposition.
  2. AAO films on different (non-Al) substrates.  
    AAO films could be created on other substrates (glass, sapphire, silicon wafers, plastic and many other materials) by first depositing thin film of Al onto such substrate, followed by anodization process to convert Al into AAO.  Non-conductive substrates
  3. AAO Wafers.
    AAO wafers are relatively thick free-standing films of AAO that have been separated from Al substrate.  These wafers could be as thin as 20 µm (and for 5 mm wafers as low as 5-10 µm) and as thick as 200-500 µm.  Due to their application primarily as a template for deposition of materials inside the pores, the dense barrier layer is removed to open the pores on both faces of the AAO.   AAO Wafers have architecture identical to AAO Membranes with symmetric pore structure.
  4. AAO Membranes.
    AAO Membranes are similar to AAO Wafers in that they are also free-standing films of AAO.  However, the structure and chemistry of nanopores in AAO membranes reflect the requirements of diverse separation applications and might include: anisotropic pores with branched and tapered shape for size based filtration, different surface chemistry in the pore walls for chemical affinity separation, thin dense layer of alumina on one facr as a support for other materials (i.e., graphene), and other separating materials deposited inside the pores.

Read more about AAO properties and applications.

What is the difference between AAO Wafer and AAO Membrane?

Both terms refer to a well-defined and free-standing piece of AAO, normally of round or rectangular shape and sufficient thickness (typically 20 to 200 micrometers) to allow manual handling.   However, there are differences in the architecture and chemistry of the nanopores, primarily driven by the requirements of the different end uses:

AAO Wafers:

  • Typically used as nanotemplates for deposition of materials inside the pores of AAO.
  • The barrier layer is removed to provide access to the pores from both faces.
  • Pores are uniform and aligned cylinders that propagate all the way through AAO thickness
  • Pore structure is typically isotropic, meaning the pore diameter and pore density do not change along the pore length.
  • As an option, pore diameter could be modulated along the pore length.
  • Metal contact is applied to AAO wafers as an option for templated electrodeposition.

AAO Membranes:

  • Application primarily involve separation of species by size, diffusion or chemical affinity.
  • The pore structure could be either isotropic as in AAO wafers (for transport studies or diffusion based deparation) or anisotropic (for size-based filtration).
  • In anisotropic membranes, to enable higher permeance in size-based separation, up to 98% of the AAO thickness has larger pores (100-200 nm) and porosity (25-40%).  These larger pores branch and/or taper into smaller pores in a thin layer on one face of the membrane to provide specific size cutoff.
  • Other type of membranes can be realized with AAO: (a)  the pore chemistry could be modified to provide affinity-based separation, (b) plugs of other materials could be deposited in the pores for separation based on selective  permeance (i.e., Pd alloys for H2 separation) or sub-nm pores (i.e,  zeolites for molecular sieving).

What is the smallest pore size in AAO?

The smallest pore size that could be obtained in AAO through anodization is ~2-3 nm. Such pores are formed at very low anodization voltages and temperatures, which results in very slow growth rate, and thus could be realized only in very thin AAO films.  Reducing anodization voltage further does not result in organized porous structure.  Increasing AAO thickness beyond 1-5 µm requires faster growth conditions (e.g., greater voltage, higher temperature),  which raises the lower limit of the pore diameter.  As a result, the smallest pore diameter available for free-standing AAO wafers is ~5 nm for up to 20 µm thickness and ~10 nm  for up to 50 µm thickness.

Small pore size could be also achieved via conformal deposition of aluminum oxide or other materials on the walls of larger pores using processes such as Atomic Layer Deposition (ALD), sol-gel deposition or solution polymerization.  InRedox has capabilities to implement all of these processes as a custom order – contact us with your requirements.

What is the largest pore size for AAO?

The largest pore size that could be obtained in AAO through anodization is 250-300 nm. Such pores are formed at very high (hundreds of volts) anodization voltages in specially formulated electrolytes and could only be realized in AAO films  at least 5 µm in thickness or AAO wafers of 20 µm and up in thickness.

Further increase of the pore size (often referred to as “pore widening”) could be achieved via conformal etching of aluminum oxide in the walls using carefully controlled etch conditions.  InRedox uses such processes to expand the upper limit of the pore diameter up to 500 nm. Contact us with your requirements and to for availability.

Do you provide technical support?

InRedox offers technical and application support to our customers free of charge.   Our team has decades of experience with using nanostructured materials in research and product development and  is always ready to help you achieve the results you need. Please do not hesitate to contact us if you any questions regarding using our products in your work.