Self-ordered nanospike porous alumina fabricated by an anodizing process in alkaline media

The fabrication of high-aspect ratio, self-ordered porous anodic aluminum oxide (AAO) structures with numerous nanospike pores was demonstrated by anodizing Al in an alkaline sodium tetraborate solution. otentiostatic anodizing of Al in 0.2-0.8 M sodium tetraborate solutions resulted in the self-ordering of porous AAO structures with a wide interpore distance of 260-590 nm at an applied voltage of 90-190 V. The interpore distance was proportional to the applied voltage with a new proportionality constant, k = 3.0 nmV-1, which is different from that when anodizing in typical acidic electrolyte solutions. Highly ordered porous AAO structures could be successfully fabricated via two-step anodizing and nanoimprinting before anodizing. Anodizing Al in sodium tetraborate solution enabled a stable process and the subsequent formation of ordered AAO structures without the occurrence of oxide burning on complicated three-dimensional substrates. The nanopores of the AAO had a unique nanospike morphology measuring several tens of nanometers in periodicity through the top surface to the bottom interface. The AAO structure could be used as a template for the fabrication of nanomaterials with a large surface area.