Size of electron polarons in anatase TiO2 and their role in photocatalysis

Experiments detect small electron polarons in rutile-${\mathrm{TiO}}_{2}$, but only large ones in anatase. By using an accurate hybrid functional, we show that the ``large polaron'' in anatase is actually of medium size and can be obtained in a static calculation. These states are two-dimensional in the (001) plane and can get trapped at the (101) surface (where small polarons may exist too). The calculated binding energy and the simulated scanning tunneling microscopic image are in good agreement with the experimental observation. The fact that holes are trapped in small polaron states in the bulk while electrons are only at the surface means an efficient separation of photoexcited carriers, and is a factor in the excellent photocatalytic behavior of anatase.