Photocatalytic water splitting is one of the major routes towards sustainable energy
conversion and storage based on hydrogen gas. The electrodes which absorb light from
the solar spectrum and finally inject carriers into the water are the key elements of the
photochemical cell [1].
The electrodes have to fulfill a number of requirements like large light absorption
coefficients, efficient carrier transport to the surface, and stability in aqueous
environment. Cuprous oxide, Cu2O, is a prime candidate due to the small, direct
bandgap of 2.1 eV and due to abundant and cheap constituents, but turned out to be
unstable towards Cu reduction in contact with water. Moreover, even when capped with
protective layers, the photochemical conversion efficiencies are well below the
theoretically possible figures [2].
Using surface science techniques and, in particular time-resolved photoelectron
spectroscopy, we studied the (111)-surface of Cu2O. Beside strong band bending at the
surface-vacuum interface [3] we found that the electron dynamics strongly depend on
the density of oxygen vacancies. In particular for the (√3x√3)R30° surface reconstruction
which is indicative of the presence of an oxygen-deficient surface layer, the
photogenerated carriers are found to be trapped very efficiently [4]. Using density
functional theory we investigated the electronic structure and in presence of various
types of charged defects and computed the carrier capture coefficients [5].
In this talk I will present the experimental studies undertaken so far and compare the
outcome with theoretical simulations. The latter yield strong evidence for charged
oxygen vacancies being responsible for the poor electrode efficiencies, and the capture
coefficients found are in agreement with trapping time scales obtained from the
experiments.
[1] See e.g. https://www.lightchec.uzh.ch/en.html
[2] W. Niu et al., Extended Light Harvesting with Dual Cu2O-Based Photocathodes for
High Efficiency Water Splitting, Adv. Energy Mater. 8, 1702323 (2018).
[3] D. Leuenberger et al., Atomically Resolved Band Bending Effects in a p-n
Heterojunction of Cu2O and a Cobalt Macrocycle, Nano Lett. 17, 6620 (2017).
[4] L. Grad et al., Influence of surface defect density on the ultrafast hot carrier relaxation
and transport in Cu2O photoelectrodes, Sci. Rep. 10, 10686 (2020).
[5] C. Ricca et al., Importance of surface oxygen vacancies for ultrafast hot carrier
relaxation and transport in Cu2O, submitted (2021); arXiv:2103.03167v2 [cond-mat.mtrlsci].