Photoluminescence Mechanisms in Anatase and Rutile TiO2

Photoluminescence (PL) represents a sensitive tool for probing molecular adsorption and surface reactions in photocatalytic materials. Titanium dioxide (TiO2) is one of the most widely used photocatalysis, and clarifying its basic PL mechanism can give important information. However, differently from other electronic and surface processes, the actual PL mechanisms of TiO2 are not extensively studied. In this work, we address the topic by focusing our investigation on which are the different states that trigger the PL activity and on identifying the specific recombination pathways acting in the two stable TiO2 polymorphs (rutile and anatase). On the basis of our experimental results on PL emission, PL excitation, and oxygen-induced and photoinduced PL modifications, we sketch an interpretative scheme for both the polymorphs. Excitation-resolved PL and recombination quenching caused by molecular oxygen evidence distinct contributions to anatase PL, originating from different kinds of hole-trapping and electron-trapping defects that we ascribe to surface and subsurface oxygen vacancies, respectively. Two possible mechanisms are discussed for rutile PL, involving self-trapped holes located at oxygen atoms or trapped electrons occupying midgap states positioned below the Fermi level. We argue that the validity of the former mechanism would imply that self-trapped holes are efficiently formed far from the rutile surface, while the latter mechanism seems more plausible although the very nature of the involved midgap electron state still has to be clarified.