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High-resolution neutron imaging: a new approach to characterize water in anodic aluminum oxides

During the growth of anodic Al oxide layers water incorporates in the film and therefore influences the intrinsic properties of the oxide formed. In this study, we propose a new approach, based on the use of high-resolution neutron imaging, to visualize and quantify the water content in porous Al oxides as a function of anodizing conditions. Water in these porous films is either incorporated directly in the oxide structure (structural) and/or fills the pores (morphological). This preliminary study demonstrates that the differences in water content of porous anodic Al oxide layers are strongly related to the oxide growth parameters but interestingly cannot be directly correlated to a specific change in the amorphous oxide structure or in the pore morphology. Due to the high sensitivity of high-resolution neutron imaging to small changes in the water content, we furthermore show that the morphological water content in Al oxides formed in sulfuric acid as well as in phosphoric acid is partially reversible upon heat treatment and immersion. High-resolution neutron imaging is also found to be highly sensitive to structural disorder and crystallographic orientations, allowing to identify different crystalline Al oxide samples based on their structural and morphological defect content. This offers new perspectives to study the effect of the hydrogen and/or water incorporation as well as oxide-related structural modifications on Al oxihydroxides in relation to growth parameters, stability, functionalization as well as properties tuning, highly relevant to surface protection and their use as templates.