Time-resolved microwave conductivity. Part 2.—Quantum-sized TiO2 and the effect of adsorbates and light intensity on charge-carrier dynamics

Abstract

Charge-carrier recombination dynamics after a pulsed laser excitation are investigated by time-resolved microwave conductivity (TRMC) for quantum-sized (Q-) TiO₂ and P25, a bulk-phase TiO₂. Adsorbed scavengers such as HNO₃, HCl, HClO₄, isopropyl alcohol, trans-decalin, tetranitromethane, and methyl viologen dichloride result in different charge-carrier recombination dynamics for Q-TiO₂ and P25. The differences include a current doubling with isopropyl alcohol for which electron injection into Q-TiO₂ is much slower than into P25 and relaxation of the selection rules of an indirect-bandgap semiconductor due to size quantization. However, the faster interfacial charge transfer predicted for Q-TiO₂ due to a 0. 2 eV gain in redox overpotentials is not observed. The effect of light intensity is also investigated. Above a critical injection level, fast recombination channels are opened, which may be a major factor resulting in the dependence of the steady-state photolysis quantum yields on l^–1/2. The fast recombination channels are opened at lower injection levels for P25 than for Q-TiO₂, and a model incorporating the heterogeneity of surface-hole traps is presented.