Metal-organic framework supported metal-oxide semiconductor hetero-nanostructures for efficient photoelectrochemical water splitting (MOFMOX)

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Photoelectrochemical water splitting (PEC-WS) is a very attractive strategy to convert solar energy into chemical energy. Porous Metal– Organic Frameworks (MOFs) are promising photocatalysts owing to their inherent structural and physio-chemical properties. By suitable choices of organic linkers metal ion nodes and/or incorporation of photoactive metal-organic complexes (1) the reactant adsorption, local enrichment and activation, (2) the light absorption and (3) the effective charge separation can be modified and leading to enhanced and even superior photocatalytic performance. However, there are still significant challenges for improving the MOFs’ intrinsic photocatalytic catalytic efficiency for practical application due to still limited visible light absorption, energy loss associated to fast recombination of photogenerated charge carriers and low materials stability. Similar drawbacks are also attributed to the metal-oxides as the by far more widely studied materials for PEC-WS. Within this collaborative research project of the research groups at TU Munich and Palacky University Olomouc we introduce a novel strategy to enhance the current PEC-WS efficiencies by simultaneous addressing the key limitations of both these groups of photoactive materials via their combination to yield an advanced photoactive hetero-nanostructure. The exploitation of one-dimensional (1D) metal oxide semiconductor morphology and the multifunctional properties provided by the MOFs opens doors to the development of entirely new class of highly active photoelectrode hybrid materials for efficient PEC-WS (see Figure 1). Metal oxide semiconductors (MOx) like TiO2 and α-Fe2O3 in the form of 1D nanostructures (nanotubes, nanorods, nanowires) show fast and long-distance electron transport, larger surface area and pore volume, as well as enhanced light absorption and scattering capabilities in comparison with bulk counterparts. Thus, we propose fabrication of hybrid hetero-nanostructures by surface selective growing MOF thin films over the 1D metal-oxide nanostructures to yield MOF@MOx systems (MOx = TiO2 and α- Fe2O3). The multifunctional capacity of MOFs in general and the specific co-catalytic effect induced by Ruthenium modified MOFs (Ru- MOFs) in conjunction with intimate interfacial integration between the photoanode components afforded by MOF thin film deposition strategies is anticipated to synergistically contribute to enhance the PEC-WS efficiency. Furthermore, the porous MOF thin films will serve as the template/host for various guest (G) nanoparticles (NPs) such as Au, Pt, Ag nanoparticles and/or carbon quantum dots (CQDs, e.g. various doped carbon or graphene) and we anticipate a further enhancement of the efficiency.