The influence of acceptor and donor doping on the protonic surface conduction of TiO2.

The transport of protonic charge carriers along and within the pore surfaces of porous oxide matrices is of significant importance for many catalytic and electrochemical applications, with porous TiO2 being a candidate material both for photocatalytic applications and as an electronically conducting support for polymer-based electrochemical cells. This work investigates the effect of acceptor (Cr and Fe) and donor (Nb) doping on protonic surface conduction in porous TiO2 over a wide range of relative humidity, temperature and oxygen activity. Generally, we find that acceptor dopants on the surface counteract dissociation and reduce the mobility of protons, while donor dopants give rise to enhanced dissociation making protonic surface conduction the highest for donor-doped samples, contrary to conventional bulk proton conductors. Moreover, protonic surface conduction in Cr-doped TiO2 is significantly higher under oxidising conditions compared to reducing conditions, which we relate to the presence of a higher valent species such as Cr6+ on the surface under oxidising conditions, again emphasising that protonic surface conduction increases with higher-valent (donor) and more acidic cations present on the surface.