The optical behaviour of those latter hybridised specimens (Cu-G0.5/450, and Cu-G1.0/450) upon visiblelight irradiation is quite different, as shown in Figure . In them, photocatalytic induced photochromism made a swift and continuous reduction of Cu(II) to Cu(I) possible, leading to a regular red-shift in the Eg upon visible-light irradiation. Then, the interface between TiO2 and Cu2O is aligned as a staggered gap (type-II) heterojunction, thus favouring a spatial charge separation of the photogenerated exciton (Figure ). Besides, graphene in the hybrids allowed for an increase in both electric transport and thermal conductivity . This is thermodynamically favourable, as graphene work function (WF) is at 4.5 eV (with respect to the vacuum level) . This explains the visible-light induced photocatalytic activity of TiO2 specimens modified with CuxO and graphene. ; the value for graphene WF is from Liang and Ang . The Eg of TiO2 is from specimen Cu-G1.0/450. EA and IP stand for electron affinity, and ionisation potential, respectively. Plots drawn with the MacroDensity python script .