**4. Three-dimensional CNT arrays by post-processing with liquids**

The above-described method can be used to produce 'planar', drawing-like arrays of the verti‐ cally-aligned carbon nanotubes on silicon surfaces. When a need in a complex three-dimen‐ sional array arises, post-processing of the uniform array (array-precursor) can be used. Among others, the post-treatment with a liquid is the most cheap and convenient [29-38]. Nevertheless, this technique still lacks controllability. In this section we show several possible ways of en‐ hancing controllability of the fabrication of three-dimensional structures of the verticallyaligned carbon nanotube arrays. Specifically, we show that the array structure can be a key factor of the resultant structure fabricated by immersing the CNT array into liquid.

Figure 11a is an SEM image of the cross-section of the array of vertically-aligned CNTs grown using the CVD technique. This array exhibits super hydrophobic properties and thus, it cannot be wetted by water. After immersing into water, only weakly-collapsed irregular structure was produced (Figure 11b). In contrast, this array does not show super-hydrofobicity to acetone, and thus, highly-regular completely collapsed pattern was produced by immersing this array into acetone (Figures 11c, 11d). As one can see in this figure, this pattern exhibits very high surface area of the 'sponge', produced by carbon nanotubes (and hence, the walls of this sponge can be highly-conductive or semi-conductive). Such structures could be very useful for the fabrication of gas and bio-sensors, gas storage devices, as well as energytransforming applications requiring very high levels of the light absorbance.

It is apparent that the control over the resultant structure of such patterns is a key issue for the above applications. Using different growth conditions, we have grown a similar CNT array with denser structure (see Figure 12a), which does not exhibit super-hydrophobic properties.

**Figure 11.** Treatment of super-hydrophobic sample with water and acetone. (a) Cross-section of the arrays of vertical‐ ly-aligned nanotubes. (b) Irregular structure was produced after treating with water. (c, d) Regular structure (com‐ pletely collapsed pattern) was produced by acetone, low and high magnifications.

**Figure 12.** Treatment of weakly hydrophobic CNT array (a) with ethanol (b) and acetone, application of 5 drops (c)

Large Arrays and Networks of Carbon Nanotubes: Morphology Control by Process Parameters

http://dx.doi.org/10.5772/52674

31

**Figure 13.** Treatment of weakly hydrophobic sample with water. Smaller islands demonstrate the sponge-like

structure.

and 10 drops (d). Acetone produces a patterned sponge-like structure consisting of fine-porous island.

Post-treatment of this array with ethanol and acetone has produced the sponge-like structure consisting of separated fine-porous island (Figures 12b, c, d). This structure is significantly different of that fabricated using super-hydrophobic sample (Figure 11). Moreover, variation of the dosage of liquid (we used 5 and 10 droplets of acetone, applied in sequence after complete drying of the preciously-applied drop) can be used to slightly change the structure. A comparison of the structures produced by 5 and 10 drops (Figures 12c and d) reveals a slight change in the pore sizes.

The use of water to treat this weakly hydrophobic CNT array produces slightly different pattern (Figure 13) consisting of smaller islands, which still demonstrate the sponge-like structure, i.e., each island is not a solid, intact array of the vertically-aligned carbon nanotubes but also consists of collapsed CNTs forming fine pores. One can expect that the fine-sponged structures produced using weakly hydrophobic CNT arrays can be very promised for the gas storage applications, whereas the highly-collapsed patterns may be more promising for sensing and other applications requiring control of the electrical resistivity of the surface. Thus, different internal structure of the vertically-aligned CNT array, together with the type of liquid and dosage, can be control parameters for the production of CNT patterns with a high level of the controllability.

Large Arrays and Networks of Carbon Nanotubes: Morphology Control by Process Parameters http://dx.doi.org/10.5772/52674 31

**Figure 12.** Treatment of weakly hydrophobic CNT array (a) with ethanol (b) and acetone, application of 5 drops (c) and 10 drops (d). Acetone produces a patterned sponge-like structure consisting of fine-porous island.

**Figure 11.** Treatment of super-hydrophobic sample with water and acetone. (a) Cross-section of the arrays of vertical‐ ly-aligned nanotubes. (b) Irregular structure was produced after treating with water. (c, d) Regular structure (com‐

Post-treatment of this array with ethanol and acetone has produced the sponge-like structure consisting of separated fine-porous island (Figures 12b, c, d). This structure is significantly different of that fabricated using super-hydrophobic sample (Figure 11). Moreover, variation of the dosage of liquid (we used 5 and 10 droplets of acetone, applied in sequence after complete drying of the preciously-applied drop) can be used to slightly change the structure. A comparison of the structures produced by 5 and 10 drops (Figures 12c and d) reveals a slight

The use of water to treat this weakly hydrophobic CNT array produces slightly different pattern (Figure 13) consisting of smaller islands, which still demonstrate the sponge-like structure, i.e., each island is not a solid, intact array of the vertically-aligned carbon nanotubes but also consists of collapsed CNTs forming fine pores. One can expect that the fine-sponged structures produced using weakly hydrophobic CNT arrays can be very promised for the gas storage applications, whereas the highly-collapsed patterns may be more promising for sensing and other applications requiring control of the electrical resistivity of the surface. Thus, different internal structure of the vertically-aligned CNT array, together with the type of liquid and dosage, can be control parameters for the production of CNT patterns with a high level

pletely collapsed pattern) was produced by acetone, low and high magnifications.

30 Syntheses and Applications of Carbon Nanotubes and Their Composites

change in the pore sizes.

of the controllability.

**Figure 13.** Treatment of weakly hydrophobic sample with water. Smaller islands demonstrate the sponge-like structure.
