Visible-Light-Induced Photosplitting of Water over γ′-Fe4N and γ′-Fe4N/α-Fe2O3 Nanocatalysts

The application of gamma'-Fe4N, a noble-metal-free, low-cost catalyst, in the photosplitting of neat water into stoichiometric amounts of H-2 and O-2 under visible-light irradiation is reported for the first time. The catalyst showed optical absorption and photoluminescence emission bands in the entire visible region. The photocatalytic water-splitting activity was wavelength-dependent, the quantum efficiency for H-2 evolution being ca. 1.7 and 0.7% at excitation wavelengths of 450 and 500 nm, respectively. Addition of electron donor/acceptor sacrificial reagents considerably affected the yield and stoichiometry of H-2 and O-2. At the same time, the product yield increased in a composition-dependent manner for (gamma'-Fe4N)(x) + (alpha-Fe2O3)(1-x) nanocomposites. This activity augmentation is ascribed to the better dispersion of the active component gamma'-Fe4N and also to the availability of more active surface sites at Fe4N/Fe2O3 contacts. Moreover, the proximity of the valence band potential of the component photosystems promotes the preferential transfer/entrapment of photoexcited hole carriers. We envisage that the defect/impurity-induced interband-gap energy states may play a vital role in these charge-transfer processes, leading thereby to more effective e(-)-h(+) separation and the enhanced rate of the water-splitting reaction. First-principles electronic structure analysis suggests that the extraordinary photocatalytic and optical properties of intermetallic gamma'-Fe4N may arise from the particle-size-dependent changes in electronic structure.