Traditional surface science studies have been quite successful in determining several surface structures for alumina films, in particular those grown on NiAl, where the bulk structure consists of a combination of several building blocks but the surface Al atoms are found to occupy sites with either tetrahedral or pyramidal coordination [1]. However, the naturally forming ‘native oxide’ layers are found to be largely amorphous. The thickness of this layer can be increased electrochemically, i.e. by the industrial process of anodization, and to some extent the degree of crystallinity found depends on the applied voltage [2]. This type of oxide layer is called an anodic barrier layer and forms in neutral type electrolytes. In acidic electrolytes a porous oxide film is found, in which case the oxide has many nanometer sized pores and forms the base of most industrial aluminum coloring processes. Furthermore, under certain conditions these pores are found to be self-ordering [3]. They form hexagonal arrays and the pore diameter can be tuned by changes in potential, making excellent nanoscale templates.

We have investigated the formation and self-ordering behavior of porous type anodic alumina films in situ using Grazing Transmission Small-Angle X-Ray Scattering (GTSAXS) [5]. We observe the that the in-plane arrangement of the nanopores is independent of substrate crystallographic orientation whereas the oxide growth rate is not. The self-ordering behavior is studied in a variety of electrolytes and at several potentials in-order to explore the dynamics of self-ordering both below and above the breakdown potential (U_B) at which optimal ordering is achieved [4]. It is also possible to follow the chemical etching of the oxide (pore widening) and subsequent metal deposition within the nanopores. The experimental approach presented can be applied to the study of a large variety of electrochemically produced materials such as magnetic nanowires, novel solar cell designs and catalysts.

[1] G. Prevot, et al. Phys. Rev. B, 85, 205450 (2012)

[2] J.W. Diggle, et al. Chem. Revs., 69(3), 365 (1969)

[3] H. Masuda, K. Fukuda, Science. 268, 1466–1468 (1995)

[4] Chu S.Z, et al. A. J. Electrochem Soc., 153, B384 (2006)

[5] Vinogradov, N.Am et al. ACS Applied Nano Materials, Article ASAP, (2018)

Figure 1: GTSAXS pattern during a 2^nd anodoization of PAA. The oscillations correspond to the height of the pores.