The effect of the mesoporosity on the crystalline growth of hematite films used as photoelectrodes in water splitting

The environmental effect of atmospheric CO2 associated with other geopolitical and economic problems linked to fossil fuels have urged the research and the development of new and clean sources of energy. The photoelectrolysis of water is an interesting way to produce clean fuel by converting solar energy into hydrogen.
Hematite is a particularly attractive material for the photoelectrolysis of water thanks to its abundance, high stability in water and absorption in the visible part of the solar spectrum. Furthermore, the position of its valence band affords the oxidation of water into O2. Nevertheless, hematite has some drawbacks like a too low conduction band to reduce water, a bad conduction of electrons and a very short diffusion length of holes (2-10 nm). The application of a bias resolves the positioning of the conduction band, while the conduction of electrons is improved by adding a dopant (SiO2 or TiO2). The short diffusion length, for its part, is improved by reducing the distance electrode-electrolyte.
For this purpose, we developed mesoporous hematite films doped by TiO2 by spin coating, based on the EIMP principle (Evaporation Inducing Micelles Packing). For the sake of comparison, a non mesoporous film was prepared. The crystallinity, the mesostructure and the performance in water splitting of the films were then characterized.
Due to the high temperature of crystallization of the hematite, we had to proceed to the fine tuning of the thermal treatment to obtain a crystallized film and to preserve its mesoporosity, which was confirmed by TEM. Based on XRD analysis, it comes out that the mesoporosity leads to smaller grains than without mesoporosity.
The water splitting performances were evaluated by measuring the photocurrent density under illumination (AM 1.5G, 100mW/cm²) and the mesoporous film showed promising efficiencies (1.9mA/cm²).