1. Introduction Catalysts enable molecular transformations to be carried out, while mitigating the inputs of thermal energy and other resources (in many cases solvents), and simultaneously curb the creation of residues that require later environmental disposal. A photocatalyst is a material that is able to harvest photons from light (ideally sunlight) and convert them to useful chemical energy, for which there are various green applications. Photocatalytic water splitting is the dissociation of water into its component elements, hydrogen and oxygen, to form [H.sub.2] and [O.sub.2], driven by the energy from light [Eqn (1)]. [H.sub.2] is one of the principally sought (1), in which energy from sunlight might be stored, thus overcoming the issue of inconstant supply, which is an implicit limitation to renewable energy sources such as solar-power or windpower. Since water is a cheap and renewable resource, it appears very attractive as a solar fuel precursor, requiring only a suitable photocatalyst to accomplish the task. Capturing sunlight and converting it to chemical fuels is sometimes referred to as artificial photosynthesis (Figure 1). In principle, solar fuels might provide an alternative to the fossil fuels, serving the dual purpose of reducing carbon emissions and conserving declining fossil resources (2): conventional crude oil production is expected to peak imminently, while the production of both (2) natural gas and coal is expected to peak around 2020. An independent analysis concludes that 90% of the world's reserves of coal will be used-up by the year 2070 (3). [FIGURE 1 OMITTED] Water is most efficiently split using sunlight, on a semiconductor surface, Eqn (1). 2[H.sub.2]O + hv [right arrow] 2[H.sub.2]+ [O.sub.2] (1) An electric potential difference of at least 1.23 V is required to split water into hydrogen and oxygen. Typically, a cathodic overpotential of 100 mV and an anodic overpotential of 200 mV are also necessary, meaning that a band gap of at least 1.53 eV is required for splitting water (4,5). As the band gap increases, the fraction of the solar spectrum the semiconductor can absorb decreases (6). Appropriate energetic requirements must also be met, in terms of the valence and conduction band edges at the solution interface, since the energy bands must encompass the potentials at which the following half-reactions occur: 2[H.sup.+] + 2[e.sup.-] [right arrow] [H.sub.2] -0.56 V (vs Ag/AgC1) (2) 2[H.sub.2]O [right arrow] [O.sub.2] + 4[e.sup.-] + 4[H.sup.+] +0.67 V (vs Ag/AgC1) (3) In fact, the potentials associated with Eqns (2) and (3) will vary according to the Nernstian dependence on solution pH, and those values given above are for an electrolyte at pH = 6. Semiconductors in which the majority of charge carriers are electrons are classified as n-type, whereas those in which the majority of charge carriers are holes are designated as p-type (5). The evolution of [O.sub.2] occurs on the surface of p-type materials while the evolution of [H.sub.2] occurs on the surface of n-type materials. To make a preliminary characterisation of semiconductors, open circuit potential measurements, photocurrent measurements, and Mott-Schottky analysis are applied (5). The Fermi level of the material, which for n-type materials lies just below the conduction band, and for p-type materials lies slightly above the valence band, must be factored-in when estimating the location of the band edges (5). Titanium dioxide (Ti[O.sub.2]) is one semiconductor which has an appropriate band structure to function as a photocatalyst for water splitting but, because of its relatively positive conduction band, the driving force for [H.sub.2] production is weak. The rate of [H.sub.2] production is enhanced when a co-catalyst such as Pt is introduced, and it is a common practice to add co-catalysts to accelerate [H.sub.2] evolution in photocatalytic systems, in consequence of the conduction band placement. …