On The Characterization of Periodicity of Pores in Nanoporous Anodic Alumina

Abstract

Small angle X-ray scattering (SAXS) has been performed on porous anodic alumina prepared at non-ordering conditions. These were prepared by two-step anodizing to have uniform through-thickness structure. The thickness of the membranes was of the order of 100-200 μm. The scattering has been explained based on structure factor; the magnitude of scattering vectors (q) corresponding to well-established peaks being multiples of 2 π /d, where d is the inter-pore distance. The usually studied form factor has not been found to play any profound role to determine the scattering response of anodic alumina. The scattering from PAA samples has been extremely anisotropic, where q always lies at about 90° with respect to the sample’s normal. A deviation from this angle which is limited to about ± 2° has been due to scatter of the pore axis with respect to sample’s normal, which is caused by occasional pore turning, branching or annihilation events. The analysis and methodology adopted in this study has also been suggested as extremely useful for determining the orientation of high-aspect ratio objects in an anisotropic sample along with assessing the degree of misalignment. Theoretically calculated / simulated structure factor (corresponding to interpore distance and interplanar distance) have been sucessfully fitted to SAXS spectra of PAA prepared by variety of conditions such as using single-step anodizing for various durations and double-step anodizing at various voltages. From these fittings useful quantitative informamtion such as lattice constant, interpore distance, long-range ordering (average domain size) and strctural disorder have been obtained, which previously was not possible through direct imaging techniques. PAA, prepared at ordering conditions by single-step and double-step anodizing, exhibit SAXS intensity peaks that correspond to two-dimensional hexagonal lattice. At all of the ordering voltages, long-range hexagonal pore ordering increases and structural disorder decreses with increase in time of anodizing during first-step of anodizing. PAA samples prepared at non-ordering voltages do not have hexagonal pore ordering, however, they exhibit regular interpore distance. SAXS analysis on the doubly-anodized samples prepared at ordering voltages reveal that the variation of pore axes in the ordered PAA is smaller than non-ordered PAA. Fitting of the simulated 2-D hexagonal lattice to the SAXS spectrum of PAA prepared (by both single step and two-step) at 60 V reveals that the sample possesses 2-D hexagonal nanoporous structure. Pore nucleation on surfaces with different pre-texture has been studied in detail. Electropolishing Aluminum surface in Perchloric acid-alcohol solution leads to nano stripes with Protrusions. These troughs in the nano stripes was found to have predominant role as pore nucleation sites in the subsequent anodizing at smaller voltages as well as larger voltage. The role of pre-texture was further investigated and confirmed by preparing a pre-textured surface which was prepared by anodizing at low voltage the electropolished aluminum followed by dissolution of the oxide grown which leaves nanopit surface for the subsequent anodizing at higher voltage. Anodizing at higher voltages on a pre-textured surface leads to the idea of pore exclusion zone that excludes the neighboring pore nucleating sites due to oxide flow in the subsequent anodizing. The study helps to understand pore nucleation at ordering at non-ordering voltages.