Turning Nano Porous Aluminium Black: An in Situ High-Energy X-Ray Diffraction and X-Ray Fluorescence Study

Abstract

Anodized aluminum, commonly used everyday applications, is effectively coloured by electro chemical deposition of materials into the nano-porous anodized aluminum oxide (NP-AAO). The colouring process is sensitive to the details of the nano-porous structure, and increased knowledge is essential to increase the effectiveness of the process to prevent cost, time and environmental losses. To study the electro deposition of material into nano-scale objects and coloring as it happens in a harsh electrolyte is notoriously difficult. As a consequence, knowledge about the coloring process has been obtained from time consuming ex-situ studies, in which the composition of the sample may change upon exposure to ambient environments. We have recently performed in situ studies of the formation of ordered NP-AAO using Grazing Transmission Small Angle X-ray Scattering (GTSAXS) [1]. The small x-ray beams produced by modern synchrotron radiation facilities provides the means for non-intrusive X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF) on a micron and sub-micron scale, allowing for high temporal and spatial resolution as the coloring process is occurring. In the present study, we have used micro XRD and XRF to study the electro deposition of Sn into the NP-AAO. An X-ray beam of 5 microns and an energy of 70 keV was used to penetrate the electrolyte and focused on the 20 micron thick layer of NP-AAO. Prior to deposition, the anodized alumina was pre-treated electrochemically in situ using a selected number of chemical procedures. The Sn deposition, which colours the aluminum black, was followed by XRF and initiates at the pore bottom and crystalizes as Sn metal nanowires along the pore height according to XRD.