On the Texturing and Nanostructuring of Iron Oxide Photoanodes for Solar Water Splitting

The world needs to think about the after fossil fuel era and while the sun baths the earth with a tremendous amount of energy, man must learn how to harvest and store it in order to dispose of a continuous supply meeting his needs. Water photo-electrolysis is a route toward the conversion of solar energy and its storage into chemical energy. Iron oxide, in the form of hematite, has been identified as a promising material, showing an excellent bandgap coupled with a high stability and abundance. However, its use is impeded by a very short charge carrier lifetime and poor oxygen evolving reaction kinetics. The former point creates the incentive for a fine nanostructuring, allowing the absorption of light close enough to the semiconductor-liquid interface for the photo-generated hole to be able to diffuse and react with water. The kinetics limitations can be overcome by mean of catalysis. In this thesis, I use a technique called atmospheric pressure chemical vapor deposition (APCVD), developed by Kay et al. in 2006 in order to fabricate hematite photoanodes that are highly nanostructured and show record solar-to-hydrogen conversion efficiencies. My work focuses on the improvement of the nanostructure in order to reduce the energy loss by deep light absorption in the material. First, the deposition parameters of the APCVD are optimized in order to improve the crystallinity of the photoanodes. The key parameter modified is the residence time of the precursor in the gas stream above the substrate. The process results in an improved overall solar energy conversion and a better electric conductivity of the material. The further application of the known best oxygen evolving catalyst – Iridium oxide – results in a record photocurrent of 3.0 mA cm−2 at 1.23 V bias vs. RHE for this class of material. In a second phase, the electrodes prepared using the newly developed conditions are analyzed by X-Ray diffraction and I find that the reduction of the residence time leads to an increased orientation of the iron oxide crystal planes. Since hematite presents a strong conduction anisotropy, and the better conducting basal planes are found perpendicular to the substrate, a correlation is made between the improved conductivity of the photoanodes prepared by reducing the residence and the preferential orientation. In order to better control the morphology of the hematite photoanodes, I present strategies of directed nucleation for the particle-assisted chemical vapor deposition. A flat model substrate is prepared as a platform for the elaboration of directing patterns. Gold nanoparticles are homogeneously deposited on the substrate with a controlled surface density. However, the chosen material reveals to be unable to initiate the iron oxide precursor decomposition and the nanoparticles show no effect on the morphology. Another patterning technique – the nanosphere lithography – is employed to create highly ordered nano-islands of a FePt alloy on the flat substrate. The dimensions of the patterning nanospheres used for the lithography controls the size and spacing of the imprints. The deposition of hematite by APCVD is successfully affected by the pattern and the size of the iron oxide nanostructures is controlled by the imprinted substrate. Finally, I combine the knowledge acquired on the directed nucleation and modified deposition conditions in order to use the APCVD in the so-called host-guest strategy. A transparent, conductive and macroporous host is built using templating TiO2 nanoparticles and a conductive niobium doped tin oxide scaffold. The host is then infiltrated by hematite using different combinations of parameters in the APCVD setup. A maximum photocurrent – equivalent to the state-of-the-art hematite host-guest strategy – is reached with a hybrid nanostructure. A dendritic formation growing atop of the macroporous host while Nb-SnO2 is conformally coated with a thin layer of hematite. By mean of parameter tuning in the APCVD system, a selective improvement of the conformal hematite layer is achieved, showing a better performance when the sample is illuminated from the substrate side as well as a modified surface morphology.