## 1 Introduction  
In recent years, the application of titanium dioxide based photocatalytic systems has gained interest within the scientific community due to its promising implementation in the removal of pollutants and microorganisms from air and water systems [1; 2]. This is attributed to several reasons, including the high photocatalytic activity, excellent long-term physical and chemical stability, cost effectiveness, nontoxicity, and abundance of starting material. The strong redox activity of photoexcited titanium dioxide was first discovered by [PERSON] et al. [3] in 1972, who reported the photoinduced decomposition of water on titanium dioxide electrodes. This finding led to the term 'Honda-Fujishima effect' being coined. Titanium dioxide exists in three crystalline forms, anatase, rutile and brookite, which differ in chemical stability and photocatalytic activity [4]. Of the three crystal structures, anatase and rutile are the most common. In view of this, most of the photocatalytic research is conducted on anatase and rutile rather than on brookite [5]. The anatase phase is the most photochemically active of the three; however, it is metastable as it transforms back to rutile when heated [5; 6; 7]. Ruelle alone has a very low photocatalytic activity; however, mixtures of anatase and rutile have in some cases been shown to have a higher activity when compared to the single phases [5; 6; 7; 8]. In the presence of enough light energy, titanium dioxide nanotubes (TiO\({}_{2}\)-NT) can promote an electron from the valence band into the conduction band. The promotion of electrons leaves an electron void inside the valence band (photohole) and an excess of negative charge (electron) in the conduction band. The hole and electron couple are referred to as charge carriers. The point of excess charge in the conduction band causes the surface to become prone to oxidation, while the positive charge left behind inside the valence band causes the surface to become prone to reduction. This redox duality of the TiO\({}_{2}\)-NT surface enables the surface to act both as an oxidizing and reducing agent. The use of titanium dioxide as photocatalytic surfaces is not without its limitations. The most significant being the charge carrier recombination rate. On a photocatalyst surface, the excited electron and the photohole coexist at the same time, introducing the possibility of the recombination of the charge carriers. The recombination of electrons and photoholes decreases the rate of the photocatalytic reaction with respect to the light energy absorbed [9; 10]. The promotion of electrons to the conduction band is also limited by the amount of energy required to promote an electron from the valence band to the conduction band. To activate anatase, UVA irradiation (\(\lambda\)\(\leq\) 387 nm) is required as this exceeds the band gap of this TiO\({}_{2}\) polymorph (\(E\)bg = 3.2 eV). Visible light, having a longer wavelength, is unable to activate anatase. Given that only 5% of the solar spectrum is composed of UV light, the possible application of such surfaces for photocatalysis using solar radiation is questionable [11; 12]. Furthermore, titanium dioxide surfaces tend to be non-porous polar surfaces, thus exhibiting very low absorption of organic compounds [11; 13; 14], which is partially overcome by using TiO\({}_{2}\)-NT. Electrochemically prepared TiO\({}_{2}\)-NT generally forms vertically oriented tube-like structures with circular nanotubular openings that offer a large specific surface area and increased porosity [15]. Whilst the application of a nano structural framework increases the surface area and the porosity, the effect of the specific surface morphology cannot be discounted. In general, thicker nanostructured photocatalyst layers are more active, and wider nanotubes provide better access for pollutant to diffuse through the nanostructure. The thin walls of the nanotubes ensure that the charges generated can easily reach the surface and partake in the electrochemical reactions before these can recombine [16].