Mechanism of Nanotube Growth

For better control over the morphology and the degree of ordering in nano-tubes, it is vital to understand the underlying principles and mechanism for the formation of aligned nanotubes under anodic conditions. The growth of nanotubes by anodising titanium can be described as a selective etching, and the method can be related to a top down approach. In the simplest approach, such nanotube growth can be described in terms of a competition between several electrochemical and chemical reactions, including: anodic oxide formation:

chemical dissolution of the titanium oxide as soluble fluoride complexes, e.g.: TiO2 + 6F~ + 4H+ ! [TiF6]2~ + 2H2O (2.7)

and direct complexation of Ti41 ions migrating through the film:

Reaction (2.6) describes the oxide growth on an anodized metal surface in a fluoride-free electrolyte. Firstly, a layer of anodically formed oxide is formed. Further oxide growth is controlled by the migration of O2 and Ti41 ions through the growing oxide film. As the system is under a constant applied voltage, the electric field within the oxide is progressively reduced by the increasing oxide thickness, the process is self-limiting. The question is why under certain conditions, does the formation of cylindrical pores organized in hexagonally symmetrical arrays occur?

In aqueous electrolytes and at constant potential, most valve metals give rise to current time curves with an exponential decay shape, due to the passivation of the electrode surface as a result of the formation of a barrier layer of low-conductivity metal oxide (reaction 2.6 and Figure 2.12, top left hand side). In contrast, the addition of HF or another source of fluoride ions, may result in an

Cavity Side Mechanism

Figure 2.12 Schematic representation of the film formation of a TiO2 nanotubular array under anodic conditions. Top left-hand side: typical current-time plot for the oxidation of Ti in an electrolyte with, or without, F- ions. Diagrams a), b) and c) show the morphology of the coating during the corresponding phases of the process. Right-hand side: details of ion transport occurring in phases b) and c).

Figure 2.12 Schematic representation of the film formation of a TiO2 nanotubular array under anodic conditions. Top left-hand side: typical current-time plot for the oxidation of Ti in an electrolyte with, or without, F- ions. Diagrams a), b) and c) show the morphology of the coating during the corresponding phases of the process. Right-hand side: details of ion transport occurring in phases b) and c).

initial exponential decrease of current (phase a) followed by an increase (phase b) to the quasi steady-state level (phase c). The steady-state level and the rate of the current recovery are increased with an increase in fluoride concentration.74 Typically, such behaviour of the current can be ascribed to different stages in the pore formation process, as schematically illustrated in Figure 2.12 (where drawings a, b and c correspond to the phases a, b and c in the current-time curve for fluoride-containing electrolyte). In the first stage, a barrier oxide is formed, leading to a decay in current (phase a) due to the reduced electro-conductivity of the layer.

Due to the roughness of the barrier layer and the barrier layer-metal interface, different parts of the film have different film thicknesses L1 and L2 (see Figure 2.12, right hand side at points 1 and 2). Such non-uniformity results in an uneven distribution of the electric field within the film, causing faster ion migration in the thinner areas (point 1) than in the thicker areas (point 2). This effect can be particular pronounced due to the high mobility of fluoride ions in TiO2 film. The differences in ion transport result in differences in the local current densities and, as a consequence, in the local dissolution rates. During this stage, the surface is locally activated and pores start to grow randomly (phase b). This is usually accompanied by a rise in current due to an increase in the available surface area.

After some time, many pores have been initiated and a tree-like growth takes place. The individual pores start interfering with each other and competing for the available current. This leads under optimised conditions to a redistribution of local current density, resulting in an equal sharing of the current between pores, and accompanied by a self-assembling of the pores. The current passing through the electrode is stabilised and the steady-state growth of nanotubes occurs (phase c). During this phase, the rate of titanium oxide formation is almost equal to the rate of [TiF6]2~ formation and dissolution. In this situation, the nanotube oxide cap continuously moves through the titanium substrate without thickening nanotubes walls (see Figure 2.12, bottom left hand side). The typical current efficiency of TiO2 nanotube formation in an acidic electrolyte is relatively low (3-10%)74 and is still decreasing at the end of the process. The overall rate of the process in steady-state phase is limited by the transport (diffusion) of F_ inside the channel from bulk solution towards the growing TiO2 cap, and the transport of [TiF6]2~ in the opposite direction. Both effects can limit the total current.

During the electrochemical growth of TiO2 nanotubes, slow chemical dissolution of nanotube walls also occurs in the acidic environment. This results in the nanotube end which faces the electrolyte having a thinner wall compared with that of the nanotube end facing the substrate, as can be seen by comparing Figure 2.10a and Figure 2.10d. This is a consequence of the variation in electrolyte exposure time of the different nanotube ends.87 However, the reason for the separation of pores into tubes (unlike the case of AAO) is not yet clear.

Since the difference in densities of TiO2 from metal titanium is significant, during TiO2 growth there is an internal stress, which pushes the metal oxide up, resulting in an increase in nanotube length of approximately 20%.87

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  • Adelmio
    What do titanate nanotubes do?
    1 year ago

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