Paper AS+BI+SA+SS-ThM12
Co-Pyrphyrin on Cu₂O(111) and TiO₂(110): Properties and Stability under Near Operando Conditions

Thursday, November 2, 2017, 11:40 am, Room 13

The pyridine-based macrocycle Co-pyrphyrin (Co-Pyr) is a promising molecular water reduction catalyst recently synthesized at the University of Zurich [1]. We investigated Cu₂O(111) and TiO₂(110) substrates covered with a complete monolayer of Co-Pyr at pressures spanning from ultra-high vacuum (UHV) up to near ambient pressures of 1 mbar of water vapor. To study the surface photovoltage (SPV) effect, samples were illuminated with UV laser light through the electron spectrometer lens system. Both under UHV and water pressures up to 1 mbar, SPV-induced shifts of the order of ΔE_k = +120 meV were observed in case of Cu₂O(111), while for TiO₂(110), much smaller SPV shifts of -0.12 meV were observed. X-ray absorption spectroscopy (XAS) of the Co L3-edge in dependence of illumination and water exposure was used to monitor the electronic structure of the Co metal center of Co-Pyr molecules. Comparison to simulated XAS spectra reveals that on the TiO₂(110), the Co centers partially transform from a +2 to +1 oxidation state upon exposure to water, while on the Cu₂O(111), the Co remains in the +2 oxidation state irrespective of the water exposure. Our measurements provide insights into the stability and behavior of the Co-Pyr molecules studied under near operando conditions, further stimulating the use of these molecular catalysts in the next-generation of solar fuel cells.

[1] Joliat, E et al., Dalton Transactions 2016,45 (4), 1737-1745.