Control of Pore Nucleation and Rearrangement Kinetics During Aluminium Anodizing in Phosphoric Acid Electrolyte

Anodic aluminium oxide (AAO) porous films with an interpore distance of several hundred nanometers are of great interest due to their unique interaction with visible and near-infrared light, and high thermal stability up to 1500 °C. These porous films are prepared by aluminium anodizing at high voltages in weak acids, leading to a slow kinetics of initial stages of porous structure formation. Here, we propose an approach to accelerate AAO formation in electrolytes based on weak acids such as phosphoric acid. Aluminium foils, pre-patterned using first anodizing under different conditions and subsequent selective dissolution of a sacrificial AAO layer, were utilized as substrates. The morphology of the aluminium surface, including surface roughness and height of pyramidal spikes, plays a crucial role in the pore nucleation and rearrangement process during the second anodizing. In particular, by first anodizing in strong acid electrolytes at low voltages (such as 0.3 M sulfuric acid at 25 V), it is possible to double the rate of pore nucleation and subsequent reach of the steady-state regime during second anodizing in phosphoric acid. As a result, about 2 hours can be saved during the two-step anodizing process in phosphoric acid if a strong acid electrolyte is used for the first anodizing to pre-pattern aluminium surface.