FUNCIONES DEL SISTEMA GESTIVO

The thickness of the barrier layer plays an important role in the anodic formation of aluminium oxide, as it is established above that the requirement of uniform electric-field distribution, i.e. constant thickness barrier layer at the entire barrier/metal interface is the main internal driving force for the self-organisation of hexagonal pattern. In the following section we try to study the impact factors on the thickness of the barrier layer.

As it is known that the effective electric field strength across the barrier layer determines the incorporation rate of the oxygen-containing anions which is equal to the dissolution rate of barrier layer at the electrolyte/oxide interface, and the dissolution rate of the barrier layer can be increased by decreasing the pH value of the electrolyte or elevating the temperature. If the local temperature at the anode surface increases or the pH of the electrolyte decreases, leading to a faster dissolution rate, a higher electric field will be required to maintain the equilibrium between the dissolution and oxidation of the barrier layer. Hence, the effective electric field strength across the barrier layer,i.e., the value ofU/t=Ein the produced anodic alumina should increase as pH of the electrolyte decreases or the local temperature at the dissolution interface rises. This coincides with the work by O’Sullivanet al25shown in Table 3-1.

For AAO produced at a constant voltage, raising the temperature of the electrolyte will lead to an increase of the effective electric field strength across the barrier layer, i.e., a smaller nm/V ratio for the barrier thickness. A higher concentration of the electrolyte corresponds to a lower pH value and faster dissolution rate of aluminium oxide. Therefore, the nm/V ratio for the barrier thickness decreases with increasing concentration of the electrolyte.

In contrast, for AAO produced at a constant current density, the nm/V ratio for the barrier thickness increases with increasing concentration of the electrolyte and the solution temperature. As discussed above, the current density can be divided into two parts: one from the dissolution of oxide joxide and the other from the field enhanced dissociation of the water jwater. Increasing the

concentration of the electrolyte or the temperature will increase the part of current density from the dissolution of oxide joxide. Since the total current density remain a constant, jwater from the

strength across the barrier layer should decrease and the nm/V ratio of the barrier layer increases. Previous work showed that for phosphoric acid, the cell dimension/voltage ratio was 2.77 nm/V at 100 V;25and 2.5 nm/V at 195 V; for oxalic acid, the ratio was 2.5 nm/V at 40 V and 2 nm/V at 140 V;70 and for the sulfuric acid, it was 2.5 nm/V at 25 V and 1.8 nm/V at 70 V.91 The corresponding cell dimension/voltage ratio decreases fastest in the sulfuric acid and slowest in the phosphoric acid. As the barrier layer thickness/voltage ratio decreases with increasing current density, this difference could be attributed to the different increasing rate of the current density in the three electrolytes: sulfuric > oxalic > phosphorus..

It has been discussed above that higher current density and consequent increasing of the local temperature will cause a smaller nm/V anodic ratio for the barrier layer. If the curvatureWof the barrier layer kept at the same value or changed slightly, the proportion fortbarrier/twallwill retain a

constant value approximately. The pore diameter and the thickness of the wall will also have a decreased nm/V ratio as the tbarrier, subsequently decreasing the cell dimension at a certain

voltage. So the local heating induced by increasing current density has an impact on the cell dimension which could not be neglected especially for a rather high current density. As reported by Leeet al,70for the anodic alumina prepared in 0.3 M oxalic acid with an anodising voltage of 120 V, the anodic ratio for the barrier layer is about 1 nm/V, compared to the typical 1.3 nm/V ratio of the barrier layer thickness for AAO anodised at 40 V. This could be ascribed to the local heating effect caused by much higher current density for hard anodisation.

Table 3-1. Variation of barrier layer nmV-1 ratio with acid concentration and temperature at constant current density or constant voltage. (By O’Sullivanet al, 1970)25

4 Formation of Self-ordered ATO Nanotubular Arrays

Although the fabrication of porous aluminium oxide layers via anodisation of aluminium has a long history,11only in recent years was this process achieved for titanium.28Since the formation of porous anodic oxide films could only be achieved in electrolytes in which the oxide could be dissolved slightly, usually fluoride containing electrolytes were chosen for the anodisation of titanium. Compared to the metal substrates, nanoarchitectured porous anodic titanium oxide films are expected to have specific functional properties, which may be promising in applications in dye-sensitized solar cells,23 photocleavage of water,22 photocatalyst,240 and in application in biology.21

As shown in Figure 4-1, highly ordered anodic titanium oxide nanotube arrays could be achieved via anodisation at 60 V in ethylene glycol containing 0.3 wt % NH4F and 2 wt % H2O. While the anodising voltage drops, the mean diameter of the corresponding ATO nanotube cells decreases as well as the ordering of the nanotube arrays.

Figure 4-1.Morphologies of ATO films produced in ethylene glycol containing 0.3 wt % NH4F and 2 wt % H2O under different voltages: 60 V (a), 25 V (b), 12 V (c), and 5 V (d), respectively.

The most significant difference between ATO and AAO is that the former contains separated nanotubes while the latter is a continuous film with a pore array (Figure 4-2). The mechanism of this difference has not been well established. The microstructures of ATO are obviously more complicated than those in AAO. Even for AAO, the formation mechanism is still not fully understood. A widely accepted model for the hexagonal ordering in AAO is based on mechanical stress associated with volume expansion during the oxidation of aluminium.27

Figure 4-2.(a) Typical scanning electron microscopic (SEM) image (top view) of an AAO film prepared by two-step anodisation: an aluminium plate was anodised in 0.3 M oxalic acid at 40 V for 3 h, the porous oxide layer was removed by a mixed solution of chromic and phosphorus acid, and the plate was anodised again for 10 h. The inset is a profile view of an AAO film prepared at 120 V in 2 wt% H3PO4 aqueous solution for 60 min. (b) SEM image of an ATO film prepared with the anodising voltage of 60 V and anodisation time of 16 h. The inset shows the bottom of the film. Some regions with enlarged distorted pores are marked by a square.

However, it is difficult to use this model to elucidate the self-ordering process in ATO since the nanotubes are separated by at least a few nanometers. As discussed above, we have proposed an equifield strength model for explaining the formation of parallel pores and geometry of the pores in AAO.78,225 We believe this model can also be used in ATO and other porous metal oxides.

The equifield strength model has been used to elucidate the formation of the pores in ATO, the self-ordering and the geometry (e.g., hemispherical pore bottom) of the pores. TEM and SEM were applied to reveal the microstructures of ATO nanotubes, including double-layer wall and

periodic O-ring like pattern on the outer surface of the nanotubes, therefore understanding the reason of the separation of the nanotubes. In addition, the porosity of ATO films was found to be governed by the relative dissociation rate of water which is dependent on anodisation conditions, such as electrolyte, applied voltage, current density, and electric field strength. With these achievements, the fabrication of ATO films can now be controlled more precisely.