Surface chemistry of ferroelectric photocatalysts.

Ferroelectrics with a switchable spontaneous polarization are considered highly promising for catalysis beyond the Sabatier principle, i.e., switching their polarization can modulate adsorption energies of reaction intermediates and accelerate catalyzed reactions. The surfaces of ferroelectrics are often described as either electron or hole doped, depending on the polarization direction, thus favoring oxidation or reduction reactions. Here, we show that the polarization of typical ferroelectrics is too weak to yield free electrons doped into the conduction band when adsorbates form on a ferroelectric surface that is not in ultra-high vacuum but in contact with, e.g., an aqueous environment. In such a situation, the charge present to cancel the depolarizing field is accommodated not in the form of free electrons in the conduction band but trapped on the electronic state of the adsorbates. This significantly reduces the oxidation capacity of negative carriers and can explain the rather weak effect of ferroelectric switching on the catalytic activity.