Optimizing P25-rGO composites for pesticides degradation: Elucidation of photo-mechanism

The junction of graphene oxide with TiO2 particles can help develop more efficient photocatalysts capable to harvest radiation in a wider range of the electromagnetic spectrum for real photocatalytic applications. The synthesis procedure of TiO2 P25-rGO composites was optimized to photodegrade a selected mixture of pesticides classified by EU as priority pollutants (alachlor, diuron, atrazine and isoproturon). The influence of temperature and time of hydrothermal method, as well as the effect of graphene oxide (GO) percentage added in the synthesis, was studied to obtain the nanocomposite that showed the highest photoactivity. Long time and moderate temperature have offered the best interaction between TiO2 P25 and rGO. GO was quantitatively reduced to rGO during the hydrothermal treatment, but maintains a higher level of disorder. The optimal GO loading was found around 0.25 wt. %, which allowed the photocatalyst achieve high photocatalytic performance both in phenol and pesticides photodegradation. Finally, in order to try to elucidate the photocatalytic mechanism of the selected mixture of pesticides three scavengers were employed: methanol to scavenge hydroxyl radicals, formic acid for the photogenerated holes, and copper (II) nitrate to quench the electrons of the conduction band. In conclusion, all these pesticides were mostly photodegraded by the hydroxyl radicals (HO%) produced from the photo-induced holes (h(+)); given that the oxidant species generated from electrons or mediated by direct mechanism were not relevant.