How Eosin Y/Graphene Oxide-Based Materials Can Improve Efficiency of Light-Driven Hydrogen Generation: Mechanistic Aspects

We have demonstrated that the photocatalytic system, containing Eosin Y (EY) as a sensitizer, triethanolamine (TEOA) as a sacrificial electron donor, CoSO4 as a catalyst, and graphene oxide (GO), exhibited a 9-fold increase in hydrogen production rate compared to the analogous system in the absence of graphene oxide. Interaction of Eosin Y (EY) with graphene oxide (GO) in the ground state and excited state was probed by steady-state and time-resolved absorption and emission measurements. Analysis of the emission quenching of EY by GO revealed that the measured decrease of the fluorescence in the presence of GO was solely attributed to inner filter effects I and II and an absorbance change of EY itself at the excitation wavelength in the presence of GO. Femtosecond and nanosecond transient absorption spectroscopy experiments pointed to a lack of electron transfer from the excited states of EY, neither singlet nor triplet excited states, to GO sheets. It was demonstrated that the electron donor triethanolamine (TEOA) participates in the primary photochemical reaction, and the electron transfer to GO occurs from the EY radical anion and not directly from the excited state of EY. GO sheets were photochemically reduced using EY and TEOA under ambient conditions. After illumination of GO in the presence of TEOA and EY, the formation of reduced graphene oxide (rGO) was confirmed through optical absorption, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopies (XPS). This indicates that the electron transfer to GO is not followed by back electron transfer and thus can be further transferred to hydrogen evolution catalysts. The existence of a stable charge-separation state explains the role of graphene in the improvement of photocatalytic efficiency in the Eosin Y-based systems.