Unusual Reduction of Graphene Oxide by Titanium Dioxide Electrons Produced by Ionizing Radiation: Reaction Products and Mechanism

The research concerns the reduction of graphene oxide (GO) by excess electrons on TiO₂ nanocrystallites, e_TiO2^–, produced with the aid of ionizing radiation in the presence of 2-propanol at acidic pH prior to mixing with a GO solution. Under these conditions, 2-propanol reacts with the radiation-produced ^•OH radicals and produces the strongly reducing CH₃C^•OHCH₃ free radicals. The latter, together with the radiation-produced hydrated electrons, reacts with the TiO₂ nanoparticles by electron transfer, producing up to 60 excess electrons per colloid particle. The reaction of e_TiO2^– with GO takes place after mixing the two sols. The reaction kinetics shows a multistage reduction, extending from seconds to many minutes. Simulations of the time profile of e_TiO2^– based on the complex kinetics involving four types of reactive GO segments reacting with e_TiO2^– agree with the observed rate of electron decay. The multireaction kinetics is expected in view of several reducible segments of GO (CC, C–O–C, C–OH, and CO) and the trapping energy distribution of e_TiO2^–. GO used in the present study had 48.8% CC (sp²), 3.4% C–C (sp³), 29.6% C–O bonds (as C–OH and C–O–C), 12.6% CO, and 5.6% O–CO. XPS analysis along the reaction time shows that the reduction of the oxygen-containing segments is the fastest process, while the saturation of CC double bonds is considerably slower. The latter involves the formation of C–H and C–C bonds. High-resolution transmission electron microscopy (HRTEM) shows the formation of nanodiamond islands within the amorphous carbon backbone.