**3.2 The problem of visible-light response of TiO2**

The use of TiO2 in remediation of oil spills is sufficient enough because of its impressive photocatalytic and material properties, but one of the major technical challenges that restrict its large-scale application and its use in oil spill remediation technologies is that TiO2 has a relatively wide bandgap (3.2 eV, which falls in the UV range of the solar spectrum), and therefore it is unable to harness visible light thus ruling out sunlight as the energy source of its photo activation [52–55]. Because TiO2 nanoparticles are only responsive to UV light of the solar spectrum, it means could only convert or utilise less than 5% of the total solar radiation (**Figure 3**), which is quite a small proportion of the solar spectrum. On the other hand, the solar spectrum that is mainly composed of about 95% visible light does not have enough energy to excite TiO2 to be photocatalytically active (**Figure 4**). In view of that, one of the major challenges that hold the throat of scientists, government and entrepreneurial organisations is the development of material(s) using "clean and renewable" energy applications based on the sounding calls and/or demands of Green and Sustainable Science, to relieve the environmental burden due to pollution. In the quest to improve the photocatalysis ability of TiO2 by responding to visible light or solar, several attempts have been made and shown that visible light responsive modified-TiO2 based materials for environmental applications are sufficiently promising. Significant progresses have been made in the synthesis of novel materials and nano-structures of TiO2 meant for efficient processes for the degradation of pollutants, particularly organics. As such, photocatalysis of TiO2 can be considered as a well understood field of study; yet, immense challenges and opportunities exist in realising this technology for large scale practical applications in the decontamination of the environment, particular in relation to oil spill remediation [57]. Fortunately, fabrication of photo-active TiO2 in a wide range of solar spectrum plus coating turns its engineered surface into a new smart and environmentally resilient

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

**Figure 3.** *Absorption region of TiO2 in solar spectrum.*

**Figure 4.** *Absorption of solar spectrum against bang gap of TiO2. Sourced from Linsebigler et al. [56].*

material that once exposed to solar light will be able to function well for the designated purpose.

Undoubtedly, TiO2 is an efficient photocatalyst in the UV region, which corresponding to an absorption threshold of 390 nm. This restrains its utilisation in the visible range (400–800 nm) for practical applications using solar radiation as the light source. Therefore, for it to be use for oil spill clean-up whose prominent superiority is *in-situ* remediation under visible light irradiation, the surface of the material must be re-engineered. The technological application of TiO2

photocatalysis in oil spills remediation processes require the development of TiO2 containing materials that are efficiently responsive to sunlight, since sunlight is the only free source of photons that can yield the desirable clean-up of huge volume of oil spills in a bearable cost. On the account that greater part of the solar radiation that reaches Earth is comprised of visible light couple with a minute fraction of ultraviolet radiation, several improvements have been made to overcome the limitation of solar spectrum to initiate photoexcitation over TiO2 and optical responsiveness to the visible light region after modification has been reported feasibly [58–61]. Accordingly, it becomes apparent that with such ground-breaking discoveries, TiO2 is a suitable and an excellent candidate for oil spills remediation and that in addition can pave ways and increase interest for its incorporation within commercial oil spill remediation products in coming years, because of this unique and superior optical property.

Another issue of concern that also limits all day(s) of full application of TiO2 in oil spills remediation is one major drawback associated to all the traditional photocatalysts like TiO2 that they can only work under illumination. Surface chemistry and engineering has provided a solution to this limitation, where visible-lightdriven energy storage photo-responsive TiO2-containing photocatalysts have been developed and have been widely used in photocatalysis in dark in recent years [62–72]. Upon advancement, energy storage photocatalysts that are full-sunlightdriven made up of UV–visible-NIR with possession of long-lasting energy storage ability have also been advanced technologically. The materials exhibit a strong absorption at full-sunlight spectrum (300–1,000 nm) that cut-across UV–visible-NIR with a super-long energy storage time. In a system like this, the material system is composed of two kinds of composite materials [65, 71], namely light harvesting material and energy storage material. The light harvesting material is the material capable of absorbing light to generate electron–hole pairs while the energy storage material is the material in charge of trapping and saving the electrons or holes transferred from light harvesting centres during illumination, and releasing them in dark. In the architectural design of such new materials, hydrogen-treated (because hydrogen treatment can extend the light absorption threshold of TiO2 to NIR) [73] and bulk surface modified-TiO2 functions as the light harvesting material and also serves as a candidate in charge of the electrons or holes generation simultaneously, while a co-catalyst such NaxMoO3 is mainly made to display self-photochargeability effect by trapping and saving electrons [74]. The extraordinary full-spectrum absorption effect and long persistent energy storage ability make such material a potential solar-energy storage and an effective photocatalyst in practice, as such the material has dual functions by harnessing solar energy to excite electrons, store electrons and when light is over in a time there is no sunlight can still do the remediation reaction by allowing the stored electrons to go back to the photocatalyst and initiate the generation of oxygen superoxide radicals (O2 •–) for the degradation and mineralisation of pollutants under treatment. The oil molecules adsorbed over the surface or in the pores of TiO2-containing photocatalyst can be directly oxidised by the O2 •– during the night operational process. The possible reaction pathways could be presented as shown below in Eqs. (4) and (5). The participation of crucial active species of O2 •– in the photocatalytic remediation of diesel oil was detected under visible light illumination [75]. Accordingly, the nonstopped generation and the intensity of O2 •– species in the remediation environment, it simultaneously expands the photocatalytic capacity of TiO2-containing photocatalyst with the increase of time. It means that long-time illumination can further enhance the photocatalytic remediation effect both in the day time and night. Therefore, the drawback of TiO2-containing photocatalysts that they can only function under illumination has been overcome and the long persistent energy

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

storage ability of the photocatalysts allows not only be used during daytime, but also be used during the night. Consequently, TiO2-containing photocatalyst endowed with this new optical and electronic properties still presents TiO2 as a missing material for its potential application using solar energy utilisation for oil spills remediation in all day(s) operational process.

$$\text{e} \text{e} + \text{O}\_2 \rightarrow \text{O}\_2 \text{"} \tag{4}$$

$$\text{Spilled oil} + \text{O}\_2\text{}^{\bullet-} \rightarrow \text{Degradation products} \ (\text{CO}\_2 + \text{H}\_2\text{O}) \tag{5}$$

In addition to above, to further enhance the solar-driven activity of TiO2, upconversion luminescence agent was coupled with TiO2 to transform the unused near-infrared (NIR) sunlight tail into UV–vis radiation available for photoreaction activation and the result demonstrated promising contribution suggesting that it can be used for treating surface water and soil pollution problems using solar light [74]. This is an alternative approach of enhancing solar absorption ability of TiO2 and the process is of considerable interest for photocatalytic processes because it produces UV–visible range from infrared light sources through multi-photon and energy transfer mechanisms. In the up-conversion photonic processes, materials such as rare-earth (RE) doped materials appear as one of the most promising candidates for efficient up-conversion luminescence that assist in the longwavelength light harvesting of solar irradiation [76–78]. In fact, this technological advancement has already been applied in agricultural production by improving the sunlight conversion efficiency of the photosynthetic process. For that reason, when applied in TiO2 photocatalysis, transforming the incoming infrared light into UV– visible light provides extra photons for absorption by TiO2 and therefore, the process cannot only optimise TiO2 photocatalytic remediation process, but also the incident radiation can lead to an endless range of possibilities. Interestingly, amongst the possibilities is that the process improves the photocatalytic activity of TiO2 even when solar radiation intensity is low. Although the solar irradiance to be received by a body of water or soil in a particular location depends on its position in the Earth. The locations on the equator of the Earth receive solar radiation at a higher intensity (irradiance) than the norther and southern hemispheres (**Figure 5**). This means that more solar radiation reaches the surface at these altitudes. In other words, all locations receives visible light in the same wavelengths, but the brightness and intensity are very different. However, with a system of TiO2

comprised of up-conversion luminescence agent, the problem of solar intensity in different locations of the Earth would no longer be an issue of challenge. This means that the perceived disadvantage of the location of the North and South poles with smaller solar exposition than the equator when it comes to application of photoremediation is now a false impression. Besides, the process also decreases the irradiation time needed for decontamination by solar light. As a result, when such technological process is applied in oil spills remediation, the rare-earth doped materials amalgamated with TiO2 would facilitate increased solar absorption and higher energy conversion efficiency. This serves as a clear testimony that oil spills remediation can be driven by sunlight using TiO2-containing photocatalyst, making the remediation process a zero-cost of energy and resulting in considerable economic savings.
