Anodic Alumina in Graphene Synthesis

Graphene is a 2-dimensional  allotrope of carbon in sheet form one atom thick. Extraordinary mechanical, thermal, electrical and optical properties of graphene stimulate ever growing body of research with this material.  Most experiments with and applications of  graphene require a stable and controllable substrate on which graphene is synthesized or transferred.

Anodic Aluminum Oxide (AAO) has been used in synthesis or modification of graphene in a number of ways:

• As a substrate to either directly grow or to transfer graphene from one substrate to another.
• As a shadow mask to create holes in graphene sheets (‘holey graphene’) or deposit materials onto graphene sheets, generally with a goal of modifying zero bandgap electronic structure of graphene. 
• As a filter to extract exfoliated graphene from solutions

AAO as a support for graphene / graphene oxide

• Using polished AAO substrates as a support, MIT Karnik group, demonstrated the first molecular sieving across atomically thin nanoporous graphene on a centimeter scale.
• Graphene Oxide (GO) hydrogel was formed by dipping AAO film on Al foil into an aqueous dispersion of GO. After formation, lyophilization was used to create a sponge like macroform (SGO), which was shown to have potential for electrochemical energy storage.
• Graphene was grown directly onto AAO via chemical vapor deposition (CVD), which created an ‘interconnected macroporous framework of graphene sheets’. Graphene conformally coated AAO pores (~100nm diameter), and the structure was proposed as a highly efficient heat sink.
• AAO was used as a substrate for introducing periodic strain into graphene sheets, thus creating a bandgap. (Periodic strain in graphene can alter its electronic structure, similarly to holes in the graphene like the AAO as a shadow mask technique.) Here the graphene was grown via CVD on Cu foil, and transferred to AAO using the PMMA transfer technique.

AAO as a Mask

• A Graphene Nano Mesh (GNM) was created using AAO as mask. A thin film of AAO grown on Al foil was spin coated with a PMMA layer, the underlying Al was etched away, and the AAO was placed over a reduced graphene oxide (rGO) layer on a Si/ SiO₂  After removing the PMMA layer, an O₂ plasma etch was used to create holes in the rGO layer.  Holes in the rGO correspond to the AAO pores, while the rGO covered by the non-porous portion of the AAO mask the rGO.  The defects or holes in the rGO result in a raised bandgap.
• A highly uniform Nano Patterned Graphene (NPG) was created via O₂ plasma etch without the PMMA layer by using a ‘floating in water’ technique. Here, a thin film of AAO was separated, kept in solution, and a Si/SiO2 graphene substrate was raised up underneath the AAO, then annealed and the graphene was etched through the AAO pores.
• AAO was used as a mask to create nanoporous graphene, which was then in turn used to chemically etch an Si wafer. The researchers dub this technique ‘GaCE’ (Graphene assisted Chemical Etch) and present it as an alternative to the traditional MaCE (Metal assisted Chemical Etch).

AAO as a Filter

• One common route to produce graphene oxide is the Hummers Method, where potassium permanganate is added to a mix of graphite, sodium nitrate, and sulfuric acid. Often, the resulting graphene oxide is filtered on nanoporous alumina to form a thin film, which can then be transferred to another substrate.
• Novel synthesis techniques to directly produce graphene, such as the exfoliation of graphite by sonication, also involve filtering a final product using nanoporous alumina filters.
• The importance of using alumina filters has recently drawn attention when researchers at Northwestern University in the US and Tianjin University in China reported that stability of graphene oxide membranes in water could be attributed to the Al³⁺ ions that leached out of the Whatman alumina membranes to form cross linking layers that hold planar graphene oxide (GO) layers together. (GO membranes created from Teflon filters rapidly dissolved in water, while GO membranes filtered with alumina were robust in water and 340% stiffer, among other evidence reported.)

In most of the published research, however, relatively little attention is given to the alumina membranes, in particular to the potential of improving yields and engineering graphene / graphene oxide properties by varying nanoporous alumina geometry and crystal structure.

InRedox anisotropic AAO membranes offer very tight pore size distribution in a broad range of pore sizes (from 2-4 to 100 nm) that are not available with filters from other vendors. Additionally, InRedox membranes can be annealed to change the crystal structure from amorphous alumina to polycrystalline alumina, dramatically extending the pH range.

Contact us for more information about anisotropic AAO membranes.