Is surface doping or bulk doping more beneficial to the photocatalytic activity of TiO₂

Doping has been demonstrated to inhibit charge recombination and improve photocatalytic activity of photocatalysts. Conventional doping incorporates hetero-ions into the bulk lattice. In the presence of the defects, few charge carriers are trapped to suppress band-to-band recombination. Because most of trapped carriers annihilate with the counterparts, a low doping level is required, and the optimal doping concentration decreases with increasing particle size. Surface doping, on the other hand, only introduces dopants into the surface lattice. Surface-doped ions create an internal electric field to drive charge carriers to quickly drift to the surface. In addition, they trap the carriers and mediate charge transfer to the chemisorbed species. Compared to bulk doping, surface doping not only inhibits both bulk and surface recombination but also improves interfacial charge transfer, thus is more effective to improve photocatalytic activity.