Photocatalytic process in TiO2/graphene hybrid materials. Evidence of charge separation by electron transfer from reduced graphene oxide to TiO2

Different amounts of graphene oxide were chemically reduced with hydrazine in the presence of nano-metric TiO2and SiO2. The photocatalytic performance of the resulting hybrid materials was comparedwith pristine supports using phenol and methylene blue (MB) under two different irradiation conditions(UV–vis and Vis only light). MB is strongly adsorbed on the hybrid materials. Significant MB degradationrates were observed on pristine TiO2and hybrid TiO2-reduced graphene oxide (rGO) material under bothirradiation conditions. In the presence of the hybrid catalyst, the degradation of MB under Vis is due tothe dye-sensitized mechanism, while under UV–vis there is an additional semiconductor-based photo-catalytic mechanism. Conversely, the presence of rGO reduces the rate of photocatalytic transformationfor the poorly adsorbed phenol under UV irradiation, and a negligible degradation rate was observedunder Vis.The UV–vis absorption spectra of aqueous suspensions of hybrid materials with different rGO load-ing indicate a strong interaction of the two materials and a reduction of the light absorption due to thepresence of rGO. Among many mechanisms reported on the role of rGO, it is inferred that the work-ing mechanism involves electron transfer from photoexcited states of rGO onto the titania, and holesmigration from titania to rGO, where adsorbed substrates are oxidized. This oxidation is possible only ifthe substrate HOMO has higher energy (less positive standard redox potential) than the empty states ofexcited rGO, supposedly for MB and not for phenol. Then, reduced graphene is advantageous when sub-strates are adsorbed and when the charge separation is possible (coupled with a proper semiconductorlike TiO2). Alone, or coupled with low work function oxides like SiO2, rGO could be ineffective.