Artificial photosynthesis is one of the most promising forms of renewable fuel production, due to the abundance of water, carbon dioxide, and sunlight. However, the water oxidation reaction remains a significant bottleneck due to the high thermodynamic and kinetic requirements of the four-electron process. While significant work has been done on the development of catalysts for water splitting, many of the catalysts reported to date operate at high overpotentials or with the use of sacrificial oxidants to drive the reaction. Here, we present a catalyst embedded metal-organic framework (MOF)/semiconductor composite that performs photoelectrochemical oxidation of water at a formal underpotential. Ru-UiO-67 (where Ru stands for the water oxidation catalyst [Ru(tpy)(dcbpy)OH₂]²⁺ (tpy = 2,2':6',2''-terpyridine, dcbpy = 5,5-dicarboxy-2,2'-bipyridine)) has been previously shown to be active for water oxidation under both chemical and electrochemical conditions, but here we demonstrate, for the first time, incorporation of a light harvesting n-type semiconductor as a base photoelectrode. Ru-UiO-67/WO₃ is active for photoelectrochemical water oxidation at a thermodynamic underpotential (η ≈ 200 mV; E_onset = 600 mV vs. NHE), and incorporation of a molecular catalyst onto the oxide layer increases efficiency of charge transport and separation over bare WO₃. The charge-separation process was evaluated with ultrafast transient absorption spectroscopy (ufTA) and photocurrent density measurements. These studies suggest that a key contributor to the photocatalytic process involves a hole transfer from excited to Ru-UiO-67. To our knowledge, this is the first report of a MOF-based catalyst active for water oxidation at a thermodynamic underpotential, a key step towards light-driven water oxidation.

Benjamin Thomas

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