Self-Organizing Anodization in Pure Molten O-H3PO4

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In 1995 Masuda et al. reported on a straightforward anodization approach that allowed for the growth of self-organized vertically-aligned alumina nanopores.1 Since then, electrochemical anodization has been widely investigated as a most suitable way to fabricate one dimensional (1D) functional oxide structures.2 Highly-ordered architectures of several metal oxides were successfully formed by anodizing in aqueous acidic solutions (e.g., Al2O3),1 fluoride containing aqueous or organic solvents (e.g., TiO2, Ta2O5, ZrO2, HfO2, etc.),3,4 and hot glycerol electrolytes (e.g., Al2O3, Ta2O5, TiO2).5-7 With this contribution we introduce a class of anodizing electrolytes, based on pure ortho-phosphoric acid (i.e., o-H3PO4), that provides high aspect-ratio highly-ordered oxide nanostructures on some of the most challenging elements such as W, as well as on more common elements such as Al, Ti, Nb.8 We describe how to tailor the structure of the formed anodic oxides by adjusting a set of key electrochemical parameters such as the electrolyte temperature, its water content, the applied voltage, and the anodization time. We show that under suitable conditions, an ideal equilibrium is established between field-assisted passivation of the metal and oxide dissolution, this leading to the growth of the anodic oxide layers in a highly ordered fashion.Not only pure molten ortho-phosphoric acid but also other phosphorus-containing acids, namely, hot pure pyro- and poly-phosphoric acids, are able to establish the required self-organizing electrochemical conditions for the growth of ordered oxide structures. We propose that the role of the molten phosphate electrolyte is two-fold: i) it provides an environment with controlled and limited water content (that is the key oxidant), and ii) it provides phosphate ions that protect the anodic oxide from rapid dissolution, this allowing for a controlled growth of the metal oxide nanostructures.(1) Masuda, H.; Fukuda, K. Science 1995, 268, 1466; (2) Roy, P.; Berger, S.; Schmuki, P. Angew. Chem. Int. Ed. 2011, 50, 2904; (3) Assefpour-Dezfuly, M. J. Mater. Sci. 1984, 19, 3626; (4) Kowalski, D.; Kim, D.; Schmuki, P. Nano Today 2013, 8, 235; (5) Melody, B.; Kinard, T.; Lessner, P. Electrochem. Solid-State Lett. 1999, 1, 126; (6) Lu, Q.; Alcalá, G.; Skeldon, P.; Thompson, G.; Graham, M. .; Masheder, D.; Shimizu, K.; Habazaki, H. Electrochim. Acta 2002, 48, 37; (7) Kim, D.; Lee, K.; Roy, P.; Birajdar, B.; Spiecker, E.; Schmuki, P. Angew. Chemie Int. Ed. 2009, 48, 9326; (8) Altomare, M.; Pfoch, O.; Tighineanu, A.; Kirchgeorg, R.; Lee, K.; Selli, E.; Schmuki, P. J. Am. Chem. Soc. 2015, 137, 5646. Figure 1