The complex interplay between adsorption and photoactivity in hybrids rGO/TiO2

The kinetic of photocatalytic degradation of 2,4-DCP and 1-naphthol on hybrids TiO2/reduced-Graphene Oxide (TiO2/rGO) obtained with two commercial TiO2 specimens and different % rGO loadings was investigated under different irradiation conditions (UV only, UV+Vis and Vis only). 2,4-DCP adsorbs strongly on the rGO phase. With the increment of the rGO loading no increment on the overall reaction rate under UV and UV+Vis irradiation is observed, while a very low but not null photoactivity that increases with the increment of %rGO is observed under Vis only irradiation. It is here proposed for the first time a quantitative kinetic model able to predict the photocatalytic rate of hybrid photocatalysts composed of two phases (whatever the nature of the phases is) with different light absorption and intrinsic reactivity, diverse ability to adsorb the substrate and to partition charge carriers depending on their band alignment. The model is able to decouple the adsorption effect from the kinetic ones. From the analysis of the adsorption and kinetic data at different rGO loadings emerges that: 2,4-DCP partitions preferentially on the rGO phase; the operational photocatalytic mechanism is based on the band-to-band transition promoted by the UV absorption of the semiconductor; rGO shows a negligible but not null visible photoactivity toward 2,4-DCP; the photo-promoted TiO2 conduction band electrons are not injected into rGO where 2,4-DCP is mainly adsorbed. In this light some hypothesis are proposed to justify the negative effect of rGO on the overall oxidative photoactivity.