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, 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)-O-center dot radicals and produces the strongly reducing (CH3COHCH3)-O-center dot 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 (sp(2)), 3.4% C-C (sp(3)), 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.