Self-assembled photonic mesostructures for water splitting photoanodes

Solar water splitting is a relevant principle for the production of green hydrogen fuel. A wealth of different designs has been envisioned to produce hydrogen using sunlight. Among those designs photoelectrochemical water splitting offers possible advantages regarding components integration and costs. This technology requires blending many materials requirements in a single component, such as solar light absorption, high electric conductivity, resistance to photocorrosion, and electrocatalytic properties. To achieve this goal it is necessary to build materials with emerging properties by discovering complex architectures at the micrometric and nanometric scales that can overcome bulk material limitations.
Materials of interest for application as photoanode for photoelectrochemical water splitting are metal oxides because of their resistance to corrosion. In this thesis I focused on two of these oxides, namely hematite (alpha-Fe2O3) and monoclinic tungsten oxide (mWO3) since these materials have a relatively narrow band gap allowing absorption of a significant part of sun's irradiance. In a photoanode they were implemented as thin films on a conductive substrate. I proposed to investigate inexpensive and upscalable structuration processes for the formation of such photoanodes thin films with a controlled microstructure and studied the impact of such structures on the film photoelectrochemical performance.
Self-assembly strategies are bottom-up approaches which allow to grow structures with original morphologies at a low cost compared to top-down techniques such as lithography. I was particularly interested in strategies that would grant a fine control of the feature sizes. Two different processing techniques were implemented, a polymer templated sol-gel route and electrohydrodynamic lithography. Both techniques allowed to obtain metal oxides structures at the meso- to nanoscale. The polymer templated sol-gel route was the most successful strategy. It allowed to produce microspheroids with a tungsten oxide core and a hematite nanometric overlayer with control on the structure dimensions.
In addition to an in depth understanding of the different bottom-up approaches investigated, I proposed a complete description of the relationship between form and function in the film composed of tungsten oxide / hematite microspheroids. These films have significant photonic features linked to their original morphology and I discussed how their photoactivity is influenced by light trapping in these films.