## 1 Introduction  
Fabrication of porous aluminum oxide (PAA) has been extensively investigated owing to both theoretical [1; 2; 3] and applicative interest [4]. The key feature distinguishing PAA is the oxide morphology consisting of self-assembled ordered arrays of hexagonal cells with a central cylindrical channel vertically oriented throughout oxide thickness. The porous layer overlays the initial formed compact aluminum oxide (barrier layer) [5]. Under continuing anodizing, pore develops on the barrier film as evidenced by the change in the current vs. time plot [3]. Figure 1 shows a scheme of the ordered porous structure of PAA where the underlying barrier layer is clearly visible. All the structural parameters, such as the thickness of the barrier layer, diameter of the pores and size of the cells correlate with the anodizing voltage, with values of the order 1 nm V\({}^{-1}\). The precise values depend upon parameters such as current density and electrolyte composition, while the porous layer thickness depends on the charge passed during anodizing [6]. Over the years, several models have been proposed on the formation mechanism of the porous structure [7; 8; 9; 10]. Structural parameters of the PAA layer are dependent on anodizing conditions (such as voltage, current, bath temperature, surface finishing of aluminum, and so on), different reaction rates between pore bottom and pore walls, the time evolution of the current density, and scalloped shape of the barrier layer were successfully predicted by modeling and simulating self-ordering process in PAA [11; 12; 13; 14; 15]. The key issue for PAA fabrication is the easy control of morphological features by adjusting the anodizing conditions of aluminum so that tailored porous layers can be fabricated according to the desired application [16; 17; 18; 19; 20; 21; 22; 23]. Some procedures were shown above leading to well-ordered structures such as either multi-step or pre-textured aluminum anodizing [24; 22; 25; 26]. Fabrication of such a highly ordered porous structure was investigated for applications in the field of optoelectronics and sensors [27; 28; 29; 30]. Other anodizing conditions lead to less ordered structures that can be used for the fabrication of templates because the pore size is more relevant than the order degree for many applications in the field of nanotechnology [31; 32; 33]. For instance, materials for either energy storage or catalysis require high surfaces exposed to the reaction environment rather than highly ordered structures [34; 35; 36; 37; 38; 39; 40; 41; 42; 43]. The aluminum anodization can be conducted in only one step, adjusting the pore size from 18-20 nm by 20 \(\div\) 25 V anodizing in 1.5 M H\({}_{2}\)SO\({}_{4}\) to 180 \(\div\) 220 nm by 160 \(\div\) 200 V anodizing in 0.4 M H\({}_{3}\)PO\({}_{4}\). Intermediate pore diameters (\(\sim\)70 nm) can be achieved by anodizing in 0.45 M oxalic acid. Additionally, the pore aspect ratio (length/width) can be easily controlled by adjusting the passed electrical charge, while the inter-pore distance can be varied in dependence on anodizing voltage, typically, in the interval 2.5 \(\div\) 2.8 nm V\({}^{-1}\)[44; 45]. One simple way for controlling the PAA morphological parameters consists in adjusting the composition of the anodizing bath. Therefore, in the following, the attention will be focused on the fabrication of PAA in 0.4 M H\({}_{3}\)PO\({}_{4}\) aqueous solutions added with different amounts of Al(OH)\({}_{3}\). In comparison with PAA grown in pure 0.4 M H\({}_{3}\)PO\({}_{4}\)[46], different morphologies were found due to the specific influence of Al(OH)\({}_{3}\) concentration and electrical charge of anodization. Therefore, it is possible to modify the PAA morphology by a simple variation of both bath composition and anodizing time, so widening the effective use as a template. To our best knowledge, aluminum anodization in pore-forming acid added with aluminum hydroxide is investigated for the first time.