Figure 1: Schematic view of different TiO2-assisted photoprocesses. (a) Direct photocatalysis initiated by excitation of an electron from the valence band (VB) to the conduction band (CB) of TiO2. (b) Mechanism of “photocurrent multiplication” at an irradiated TiO2 electrode in the presence of methanol. (c) Sensitization of TiO2 by a dye: the dye is photoexcited with visible light from its ground state S 0 to excited state S , injects an electron to the conduction band of TiO2, and is thereby oxidized to S+. All recombination pathways are omitted for the sake of clarity. Whereas the common convention in solid state physics relates the positions of band edges with respect to the vacuum level, in photoelectrochemistry and photocatalysis, the potentials are usually given with respect to the normal hydrogen electrode (NHE; 𝐸 N H E = 0 V ). On the energy scale, the NHE is reported to lie at 4 . 4 4 ± 0 . 0 2  eV (at 298.15 K) with respect to the vacuum level [122]; for more details, see [123, 124].