Formation of iron oxide nanoparticles for the photooxidation of water: Alteration of finite size effects from ferrihydrite to hematite.

Iron oxide nanoparticles represent a promising low-cost environmentally-friendly material for multiple applications. Especially hematite (α-FeO) nanoparticles demonstrate great possibilities in energy storage and photoelectrochemistry. A hydrothermal one-pot synthesis can be used to synthesise hematite nanoparticles. Here, the particle formation, nucleation and growth of iron oxide nanoparticles using a FeCl precursor over time is monitored. The formation of 6-line ferrihydrite seeds of 2-8 nm which grow with reaction time and form clusters followed by a phase transition to ~15 nm hematite particles can be observed with ex situ X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman and UV/Vis spectroscopy. These particles grow with reaction time leading to 40 nm particles after 6 hours. The changes in plasmon and electron transition patterns, observed upon particle transition and growth lead to the possibility of tuning the photoelectrochemical properties. Catalytic activity of the hematite nanoparticles can be proven with visible light irradiation and the use of silver nitrate as scavenger material. The generation of elementary silver is dependent on the particle size of iron oxide nanoparticles while only slight changes can be observed in the oxygen generation. Low-cost nanoscale hematite, offers a range of future applications for artificial photosynthesis.