Decoration of Nanotubes using the Ion Exchange Method

The ion-exchange properties of nanostructured titanates can be utilized in the deposition of the dispersed nanoparticles of other materials onto the surface of titanates. Such an approach is schematically illustrated in Figure 4.5. The precursor of the required nanoparticles in cationic form is ion-exchanged with the protons of the nanostructured titanates, which are localised on both the concave and convex nanotube surfaces, as well as in the interlayer space in the nanotube wall. Ideally, such ion-exchange processes should allow a uniform atomic distribution of the precursor on the nanotube surfaces by adsorption, as well as between the layers of the walls by intercalation. The overall process follows chemical reaction (4.13), where M1 is the cationic precursor. Such ion-exchanged materials are thoroughly washed with solvent to remove the impurities of precursors retained in the pores between the nano-tubes, preventing the formation of nanoparticles in the bulk material during next step.

The second step of the procedure is a growth of nanoparticles of the required materials on the nanotube surface, using a chemical reaction between the intercalated precursor and added chemical reagents. During this stage, the precursor diffuses to the surface of nanotubes, where it reacts with chemicals added to the solution to form nanoparticles which adhere to the nanotube surface (see Figure 4.5). Due to the suppression of nanoparticle crystallisation in the bulk material and an agglomeration of particles on the surface, the composites obtained are characterised by a high loading of nanoparticles, which are evenly distributed on the surface of the titanate nanotubes.

Figure 4.5 Ion-exchange assisted deposition of Pd nanoparticles onto the surface of titanate nanotubes.

The ionic form of the precursor is important for the successful distribution of nanoparticles and for a high material loading. The effect of speciation of the metal complex on its ability to participate in ion-exchange with titanate nano-tubes, was studied using the the polyamine complexes of gold.14 The adsorption

Figure 4.6 Isotherms for the adsorption of three gold complexes on titanate nano-tubes from an aqueous suspension at 22°C: (■) [Au(en)2]Cl3, (K) [Au(dien)Cl]Cl2 and (■) H[AuCl4].

isotherms of three different complexes, namely [Au(en)2]Cl3, [Au(dien)Cl]Cl2 and H[AuCl4], from aqueous solution onto titanate nanotubes at room temperature (22 °C) are shown in Figure 4.6. The highest molar exchange ratio (0.081) was observed for the gold complex, [Au(en)2]Cl3, a cation with a charge of +3. The [Au(dien)Cl]Cl2 complex, which has a charge of +2, adsorbs onto the titanate nanotubes with a lower value for the maximum molar exchange ratio (ca. 0.045). These values of molar exchange ratio are achieved when the concentration of gold complexes is in the range of several mmol dm-3. A further increase in the concentration of precursor in solution results in saturation of the ion-exchange sites in the titanate nanotubes, establishing an equilibrium distribution of metal between aqueous solution and the solid nanotubes.

The negatively-charged tetrachloride complex of gold demonstrates very poor ion-exchange properties with the negatively-charged titanate nanotubes, showing the significance of electrostatic interactions for the adsorption of metal salts from aqueous solution onto the titanate nanotubes.

Suitable precursors include any species characterised by a strong affinity to titanate nanotubes, such as: cations of precious metals,15 transition metals,16,17 or nitrogen-containing organics.18 The process of nanoparticle growth can be stimulated by various chemical processes, including: the reduction of metal cations to metal nanoparticles14,15 (see Figure 4.7d), the sol-gel hydrolysis of metal cations to oxides19 (see Figure 4.7a), the recipitation of insoluble solids5,16 (see Figure 4.7c), or the oxidative polymerization of monomers18 (see Figure 4.7b).

Figure 4.7 TEM images of nanostructured titanates decorated with coatings using the ion-exchange method: a) TiO2/TiNF, b) PANI/TiNT, c) RuOOH/TiNT and d) Au/TiNT composites. In a) the arrow shows the crystallographic direction of the nanofibre, and in b) arrows 1 and 2 show the crystallographic directions of the polymer and nanotubes, respectively. (Images a) and b) are reproduced with kind permission from ref. 19 and 18, respectively).

Figure 4.7 TEM images of nanostructured titanates decorated with coatings using the ion-exchange method: a) TiO2/TiNF, b) PANI/TiNT, c) RuOOH/TiNT and d) Au/TiNT composites. In a) the arrow shows the crystallographic direction of the nanofibre, and in b) arrows 1 and 2 show the crystallographic directions of the polymer and nanotubes, respectively. (Images a) and b) are reproduced with kind permission from ref. 19 and 18, respectively).

The resultant composites are characterised by a good adhesion between the titanate substrate and coatings; a uniform distribution of the coating material on the surface of the nanostructured titanates; and a high dispersity of nano-particles even at very high loading of supported materials (see Figure 4.7). This method of ion-exchange is perspective for preparation of dispersed catalysts on the surface of mesoporous nanotubular supports and particular examples are considered in Chapter 5.

4.2.3 Decoration of Substrates with Nanotubes

One of the key advantages of TiO2 nanotube arrays produced by anodization of titanium is that they are already deposited as a film on the surface of the substrate, and can be consequently used immediately in various engineering applications. In contrast, the powdered titanate nanotubes require further processing for their immobilization on various supports.

During the coating of surfaces with nanostructured titanates, the following properties of the deposits are controlled: the adhesion to the substrate; the durability and hardness of the coating; the uniformity of material distribution; and the thickness and density of the films, as well as their composition. A particular challenge is the ability to control the orientation of nanotubes, to be either perpendicular or parallel relative to the substrate surface. Brief descriptions of various methods used in nanotube immobilisation follow.

The doctor blade technique is commonly used for the preparation of the nanotube composite electrodes used in electrochemical studies.20,21 In general, the nanotube powder is dispersed in a minimal volume of solvent (typically 1-3 g in 10 cm3 of solvent) using a shear blade mixer of an ultrasonic dispergator, to form a dense slurry. The slurry is mixed with dissolved polymer binder and any other optional additives (e.g. carbon nanoparticles for improving electro-conductivity). The prepared mixed slurry is deposited on the flat surface of the substrate using metal bars to ensure an even distribution of material, followed by evaporation of the solvent and a curing of the film at elevated temperature. The film thickness varies from a fraction of a millimetre to several millimetres.

A variation on the above method is the spin coating technique, in which the slurry is deposited into the centre of the spinning substrate, so that it flows towards the edge under centrifugal force. The film thickness, uniformity and quality are controlled by the rate of slurry introduction, the viscosity of the slurry and the spinning rate. Typically, the films produced are thinner than those prepared by the doctor blade technique.22

Titanate nanotubes can be immobilised onto the surface of the titanium substrate during their synthesis under alkaline hydrothermal conditions, using the method of in situ growth. In this technique, the metal titanium is oxidised to intermediate TiO2, which undergoes further transformation to titanate nano-tubes which are immobilised onto the surface of the dissolving titanium. The nanotubes produced are usually randomly oriented,23 however, some reports24 suggest a small degree of orientation in such films. The thickness of these films can reach the value of several hundreds of microns. Preliminary vacuum spattering of titanium onto the substrate, followed by the in situ alkaline hydrothermal growth of titanate nanotubes, also allows films to be deposited onto non titanium substrates. In this case, the choice of substrates is restricted by the durability of the support in the alkaline environment of the hydrothermal conditions.

In an aqueous suspension, titanate nanotubes develop a relatively large negative zeta potential due to acid-base dissociation. Under a potential gradient, titanate nanotubes experience an electrophoretic motion towards a positively-charged electrode. Due to their close proximity to the electrode, the nanotube concentration increases resulting in precipitation onto the electrode surface.25 The method of electrophoretic deposition (EPD) of ceramic nano-particles onto the surface of an electrode, is based on this principle of particle

Figure 4.8 Schematic representations and SEM images of: a) electrophoretic, and b) Langmuir-Blodgett deposition of titanate nanotubes on a planar substrate. (SEM images a) and b) are reproduced from ref. 26 and 29, respectively).

migration, followed by their coagulation onto the electrode surface (see Figure 4.8a; ref. 26,27). The SEM image in Figure 4.8a, shows the typical morphology of a titanate nanotube film obtained by the EPD method. Note that the nanotubes are randomly oriented and packed in a dense film layer. The thickness of the layer depends on the time of EPD and can be greater than several microns. The films are often consolidated by subsequent heat treatment.

The typical electrolyte for EPD is the stable suspension of titanate nanotubes in alcohol or an alcohol-water mixture. During deposition, the distance between electrodes is minimized in order to increase the electric field, to typically a few centimetres. The EPD is carried out at a typical potential range of 10 to 40 V for a period of time of up to 60 min.20,27 The addition of various stabilizing agents (e.g. polyvinyl butyral; Mw: 19 000,28), can result in a significant change in the zeta potential, even switching it to positive values and allowing tthe deposition of nanotubes on the surface of the cathode.

The Langmuir-Blodgett technique for film manufacture provides a better control of layer structure and the potential for nanotubes to self assemble with a preferential orientation. The method also enables the deposition of a monolayer of inorganic nanoparticles. The principle of this method is shown schematically in Figure 4.8b. Certain amphiphilic surfactant molecules are preferentially localised at the solvent-air interface, such that the hydrophilic part of the molecule is submerged in the liquid phase, whereas the hydrophobic part is removed from the liquid phase. The decrease in interface area usually results in the formation of a condensed layer with self-organised surfactant. If the hydrophilic part of the surfactant has a cationic form (e.g. cetyltrimethyl ammonium), it can attract negatively-charged titanate nanotubes to the surface. Slow withdrawal of the substrate allows nanotubular titanates to be trapped between substrate and surfactant layer, this can result in the formation of a monolayer of nanotubes.

Recent experiments have shown that a monolayer of titanate nanotubes can be formed on the surface of solid substrate by the Langmuir-Blodgett technique, using cetyltrimethyl ammonium chloride as surfactant.29 An SEM image of a monolayer of nanotubes is shown in Figure 4.8b. The increase in surface pressure during deposition (from 1 to 18 mNm-1), results in a dense monolayer film of nanotubes which consists of isolated grains with parallel tubes. The size of these grains is as large as several microns (see Figure 4.8b). The task of forming larger grains would require a narrower distribution of nanotube length and diameter.

For several applications using electron transport in titanate nanotubes, there would be the additional requirements of a perpendicular orientation of nano-tubes relative to the substrate surface. This problem is very challenging, however, and requires a better understanding of the nanotube surface chemistry in order to successfully control the nanotube self-assembling process in the liquid phase.

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