**3.1 The problem of dispersion and aggregation of TiO2 nanoparticles**

On the issue of whether TiO2 nanoparticles remains dispersed or forms much larger-sized aggregates or clusters that affect its photocatalytic activity in soil and surface water environments has already been documented and is strongly influenced by the ionic strength and pH of the aqueous suspensions in which TiO2 nanoparticles is placed [46]. This clustering of TiO2 nanoparticles is consistent with the principles of colloidal chemistry of other metal oxide nanoparticles, which rate of formation of nanoparticle aggregates in aqueous suspensions is a function of ionic strength and of the nature of the electrolyte in a moderately acid to circumneutral pH range typical of soil and surface water conditions [46–48]. It is true that clustering of TiO2 nanoparticles has important repercussions for its practical uses in soils and surface waters. However, such problem can be overcome during architectural fabrication of nano-engineered surface of TiO2-containing material suitable for oil spill remediation by making sure that all the environmental parameters considered fall within ranges likely to be encountered in nature, specifically in situations where TiO2 nanoparticles enters into contact with surface waters and soils. In another words, the wettability of the flat surface of TiO2-containing material can be engineered to assume the natural hydrophobicity of butterfly wings or lotus leaves, which forms solid-water or solid-soil solution static wetting mechanism that can enhance distribution by creation of evasion for the influence of ionic strength and pH of aqueous suspensions. With the intention to artificially make TiO2-containing photocatalyst as hydrophobic surfaces by introducing environmentally acceptable bulk material such as organo-clayed material to create roughness and reduced surface energy, the relationships between the water contact angle on a rough surface (*θ*rough) and that on a flat surface (*θ*flat) for homogeneous and heterogeneous wettings can be described by the Wenzel and the Cassie–Baxter equations, respectively. These two equations are shown as follows:

Wenzel's equation:

$$\mathbf{Cos}\,\theta\_{\text{rough}} = \mathbf{r}\,\mathbf{Cos}\,\theta\_{\text{flat}}\tag{1}$$

Cassie–Baxter's equation:

*Titanium Dioxide – A Missing Photo-Responsive Material for Solar-Driven Oil Spill… DOI: http://dx.doi.org/10.5772/intechopen.98631*

$$\text{Cost}\,\theta\_{\text{rough}} = \text{q}\_{\text{S}}\text{Cost}\,\theta\_{\text{flat}} - (\mathbf{1} - \mathbf{q}\_{\text{S}}) \tag{2}$$

where r is the roughness factor, defined as the ratio of the actual surface area to the geometrical one, and ⱷ<sup>S</sup> is the area fraction of the solid surface that comes into contact with water. Both the theories pointed out that a rough surface is essential for enhancing hydrophobicity and they are commonly used to explain the wetting behaviour on rough hydrophobic surfaces.

In addition, the wettability behaviour that occurs at the interface of the solid, air, water and oil can also be described on the value of the contact angle alone, where surface properties are usually categorised as hydrophilic, hydrophobic and superhydrophobic. If the water contact angle (*θ*) is less than 90°, the surface is described as hydrophilic, if *θ* is between 90° and 150° then hydrophobic and if *θ* is above 150°, the surface is described as superhydrophobic. The water contact angle (CA) *θ* is usually used to measure the wettability of a flat surface of the nanocomposites, which will depend on the solid–vapour, solid–liquid, and liquid– vapour surface tensions, and can be expressed by Young's equation:

$$\text{Cost}\,\Theta = \frac{\chi\_{\text{SV}} - \chi\_{\text{SL}}}{\chi\_{\text{LV}}} \tag{3}$$

where γSV, γSL and γLV are the interfacial tensions between solid and vapour, solid and liquid, and liquid and vapour, respectively, as shown in **Figure 2**.

Such material modifications may be of particular importance in saline environments where high ionic strength could promote coagulation and precipitation of dispersed TiO2. In a positive note, for example, TiO2 photocatalysts modified with hydrophobic coatings remain dispersed within organic target pollutants and do not lose photo-degradation efficiency after salt addition [49]. Hence, this result can help to delineate such potential limitation for *in-situ* application of TiO2 nanoparticles in oil spill remediation, and can also provide insight to guide future material development effort in relation to controlling the buoyancy, hydrophobicity and other desired surface properties of TiO2-containing photocatalyst to enable oxidative radicals generation in proximity to floatable hydrophobic compounds such as oil spills.

Another difficulty that perhaps has not given TiO2 edge in oil spill remediation involves separation and recovery of suspensions containing nanoparticles and/or microparticles after use, which leading to inevitable secondary pollution and low reusability. To address this problem, immobilisation of TiO2 on a support material would render nanoparticles recovery unnecessary. Besides, immobilisation of TiO2 unto a support is thus an advantage because it allows reusability of material in a number recycles. Therefore, for oil spill photo-remediation in surface waters, fabrication of a floatable photocatalytic material by immobilising TiO2 on

**Figure 2.** *The interfacial tensions between solid and vapour, solid and liquid, and liquid and vapour.*

environmentally acceptable bulk material can serve as a relevant brand of solution to the problem. For example, a floatable photocatalytic material by immobilising TiO2 on expanded perlite, a siliceous rock of volcanic origin was fabricated and reported [50]. Similarly, TiO2 nanotube films was successfully anchored on the surfaces of levees for use in degradation of low-level oil spills, e.g., oil spills in harbours [51]. A floatable TiO2-containing photocatalyst can not only maximise the illumination/light utilisation process, especially in a system with solar irradiation, but could also maximise addition of oxygen (oxygenation) to the photocatalytic system by the proximity with the air/water interface, especially for non-stirred reactions [51]. Accordingly, proximity of TiO2-containing material with the air/ water interface can result in high concentrations of surface oxygen molecules to act as the primary electron acceptor, which can therefore trap electrons resulting in prevention of recombination of the electron–hole pairs and increasing the rate of electron scavenging by O2 resulting in the formation of an increased yield of superoxide radicals (O2 •–) that can directly or indirectly contribute to the degradation and mineralisation of spilled oils. This can increase the rate of oxidation of spilled oil during photo-remediation, as floatable TiO2-containing photocatalyst can have the ability to interact with floating oil. The potential benefit of engineering of the surface of TiO2-containing photocatalyst that is sufficiently buoyant to maintain close proximity with floating oil and highly hydrophobic to act at the oil–water interface is to facilitate interaction with the powerful oxidative radicals generated in the process. Therefore, such proximity can be an advantage against low quantum efficiency due to the low rate of electron transfer to oxygen resulting in a high recombination of the photogenerated electron–hole pairs.
