The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics

A systematic study of metal ion doping in quantum (Q) -sized (2-4 nm) TiO₂ colloids is performed by measuring their photoreactivities and the transient charge carrier recombination dynamics. The presence of metal ion dopants in the TiO₂ crystalline matrix significantly influences photoreactivity, charge carrier recombination rates, and interfacial electron-transfer rates. The photoreactivities of 21 metal ion-doped colloids are quantified in terms of both the conduction band electron reduction of an electron acceptor (CCl₄ dechlorination) and the valence band hole oxidation of an electron donor (CHCl₃ degradation). Doping with Fe^ (3+), Mo^ (5+), Ru^ (3+), Os^ (3+), Re^ (5+), V^ (4+), and Rh^ (3+) at 0. 1-0. 5 at. % significantly increases the photoreactivity for both oxidation and reduction while Co^ (3+) and Al^ (3+) doping decreases the photoreactivity. The transient absorption signals upon laser flash photolysis (λ_ (ex) = 355 nm) at λ = 600 nm are extended up to 50 ms for Fe^ (3+) -, V^ (4+) -, Mo^ (5+) -, and Ru^ (3+) -doped TiO₂ while the undoped Q-sized TiO₂ shows a complete "blue electron" signal decay within 200 μs. Co^ (3+) - and Al^ (3+) -doped TiO₂ are characterized by rapid signal decays with a complete loss of absorption signals within 5 μs. The quantum yields obtained during CW photolyses are quantitatively correlated with the measured transient absorption signals of the charge carriers. Photoreactivities shown to increase with the relative concentration of trapped charge carriers. The photoreactivity of doped TiO₂ appears to be a complex function of the dopant concentration, the energy level of dopants within the TiO₂ lattice, their d electronic configuration, the distribution of dopants, the electron donor concentration, and the light intensity.