Abstract

Light-driven water splitting on semiconducting metal oxides is an attractive technology for generating hydrogen fuel in a sustainable way. The efficiency of this process is, however, far below theoretical predictions due to different loss mechanisms in photoelectrode materials, including a high overpotential for the overall water splitting reaction. In the first part, we will describe recent strategies towards creating metal oxide nano-morphologies based on the assembly of ultrasmall mixed metal oxide building blocks, biotemplating using nanocrystalline cellulose, and hierarchical structures aimed at understanding and optimizing light harvesting, charge transport and electrocatalysis in such systems.

For example, zinc ferrite (ZnFe2O4) was recently recognized as a promising photoanode material, but understanding of its intrinsic semiconductor properties and surface reaction kinetics has been lacking. Using well-defined thin films of zinc ferrite, prepared for the first time by atomic layer deposition (ALD), we find a considerably higher charge transfer efficiency of ZnFe2O4 compared to benchmark hematite (Fe2O3). This is the result of a significantly lower rate of surface recombination and a similar rate of charge transfer compared to hematite. By integrating such films into porous, transparent conductive scaffolds we obtain hierarchical electrodes combining the low onset potential of zinc ferrite thin films with a much higher photocurrent.

In the second part, we will explore the opportunities offered by spatially integrating photoactive molecular building blocks into a crystalline lattice based on the paradigm of covalent organic frameworks (COFs), thus creating models for organic bulk heterojunctions. We will address means of controlling the morphology and packing order of COFs through additives, in thin films, and with spatially locked-in building blocks. We will discuss different strategies aimed at creating electroactive networks capable of light-induced charge transfer. For example, we have developed a COF containing stacked thienothiophene-based building blocks acting as electron donors with a 3 nm open pore system, which showed light-induced charge transfer to an intercalated fullerene acceptor phase. Contrasting this approach, we have recently designed a COF integrated heterojunction consisting of alternating columns of stacked donor and acceptor molecules, promoting the photo-induced generation of mobile charge carriers inside the COF network. Due to the great structural diversity and the large degree of morphological precision that can be achieved with COFs, these materials are viewed as intriguing model systems for organic heterojunctions.