Methods to Control the Morphology of Nanotubes

For practical purposes, it is important to know what methods exist for adjusting the morphological properties of anodic TiO2 nanotubes, including: tube diameter and length, wall thickness and roughness, as well as the degree of ordering of the aligned nanotubes.

Control of nanotube diameter. Anodic oxidation of titanium in fluoride-free electrolyte usually leads to the growth of a compact layer of TiO2. The thickness of the layer is linearly proportional to the applied potential to the electrode, and the proportionality coefficient is in range of 1 to 5 nm V"1 (ref. 88). In contrast, the increase in electrode potential during the growth of TiO2 nanotubes in fluoride-containing electrolytes, usually results in an increase in the nanotube diameter. This has been confirmed, for example, by SEM images of TiO2 nanotubes grown in a water-glycerol electrolyte89 at various applied potentials (see Figure 2.13, top). The functional dependence of the average nanotube diameter on applied potential is linear across a wide range of applied potential (up to 40 V). Figure 2.13 (bottom) shows the change in nanotube diameter with increasing potential in electrolytes of differing composition,

Figure 2.13 Top: SEM images of TiO2 nanotubes grown in glycerol-water (50:50 vol.%) electrolyte containing NH4F (0.27moldm-3) at various applied potentials. (Images are reproduced with kind permission from ref. 89). Bottom: TiO2 nanotube internal diameters shown as a function of the applied potential during anodising in aqueous electrolyte containing (□) glycerol-water (50:50 vol.%) with NH4F (0.27moldm"3; ref. 89), and (O) H3PO4 (1 moldm"3) with 0.3 wt% HF (ref. 90).

Figure 2.13 Top: SEM images of TiO2 nanotubes grown in glycerol-water (50:50 vol.%) electrolyte containing NH4F (0.27moldm-3) at various applied potentials. (Images are reproduced with kind permission from ref. 89). Bottom: TiO2 nanotube internal diameters shown as a function of the applied potential during anodising in aqueous electrolyte containing (□) glycerol-water (50:50 vol.%) with NH4F (0.27moldm"3; ref. 89), and (O) H3PO4 (1 moldm"3) with 0.3 wt% HF (ref. 90).

including a phosphoric acid electrolyte.90 Nanotube diameters of up to 250 nm can be achieved without a significant deterioration in the ordering of nano-tubes. A similar dependence of nanotube diameter on applied voltage is also true for titanium alloys.91

Control of nanotube lengths. The length of anodised TiO2 nanotubes is determined by the ratio between the rates of pore growth from the electrochemical oxidation-etching of titanium substrate, and the nanotube top end dissolution resulting from chemical reaction with electrolyte components. The latter can be decreased by the use of less corrosive electrolytes [e.g., NaF (ref. 69) or NH4F (ref. 88) as a source of fluoride ions instead of HF], which can prevent the rapid dissolution of nanotube walls. The rate of pore electrochemical etching can be facilitated by applying conditions which favour a thinner barrier layer. A thinner barrier layer may result in faster ion transport, allowing an increased current density, leading to a higher rate of nanotube growth and a shorter exposure time to aggressive electrolyte. A recent approach uses polar organic electrolytes with minimal water content, which can inhibit the donation of oxygen atoms, decrease the tendency to form an oxide, and reduce the thickness or lower the quality of the barrier layer. A thinner barrier layer can enhance the transport of ions.89 The use of quaternary ammonium ions can also inhibit the formation of a thick barrier layer.92

If the rate of titanium substrate dissolution is rapid during nanotube growth, then the length of the nanotubes becomes limited by the transport of fluoride ions inside their channels. In general, the longer the tube, the slower the ion transport in the channel. When the nanotube length exceeds a certain value, the overall rate of nanotube growth becomes equal to the rate of their dissolution. This determines the length of the nanotubes. The rate of diffusion can be increased to some extent by increasing the fluoride ion concentration in the electrolyte, by increasing the gradient of concentration between the bulk solution and the surface of the electrode. This usually results in an increase in nanotube length.72

Control of nanotube ordering. Under certain conditions, anodic TiO2 nano-tubes are assembled into an array with a high degree of hexagonal ordering. Several factors strongly influence the degree of ordering:93 the anodising cell voltage (the highest possible voltage just below dielectric breakdown appears to be the optimal voltage), and the purity of the material (certain ordering faults can be eliminated by using a high-purity Ti). In addition, repeated anodising (as in the case of Al) can clearly improve ordering. By using this approach, the bottom imprints of a first tube layer in the underneath Ti act as "pre-ordering" guides for subsequent anodic tube initiation and growth - an in situ templating effect.

Control of wall thickness. The typical thickness of the TiO2 nanotubes wall is a few tens of nanometres. Despite the importance of this parameter for some practical applications including hydrogen sensing,72 there is only limited data available regarding the control of nanotube wall thickness. One possible approach would be to use the recently observed correlation between nano-tube wall thickness and the concentration of fluoride ions in solution; an increase in fluoride ion concentration can result in a decrease in nanotube wall thickness.94

The type of organic additive in the electrolyte may also affect the smoothness of the nanotube walls. For example, the addition of acetic acid to electrolyte can significantly decrease the roughness of nanotube walls.68 A decrease in the water content of a glycerol electrolyte also increases the smoothness of nano-tube walls.89

At the present time, by adjusting the anodising conditions (including: cell voltage, reaction time, electrolyte composition, addition of additives or the use of an organic electrolyte), it is possible to control the internal diameter of nanotubes from 12 nm to 242 nm, wall thickness from 5 nm to 34 nm (ref. 72) and the length of nanotubes up to 260 mm.70

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